摘要:

Determination of rare earth elements U and Th in environmental samples by inductively coupled plasma double focusing sectorfield mass spectrometry (ICP-SMS) Thomas Prohaska,* Stephan Hann, Christopher Latkoczy and Gerhard Stingeder Institute of Chemistry, Vienna University of Agricultural Sciences – BOKU Wien, A-1190 Vienna, Austria Received 27th August 1998, Accepted 2nd November 1998 The excellent capability of high resolution inductively coupled plasma sectorfield mass spectrometry for measurements of the rare earth elements Th and U is demonstrated by investigating diVerent materials such as geological matrices (sediments, soils), plant tissues and marine animal tissues.Appropriate digestion of the samples resulted in complex matrices, especially in the case of silicate containing samples. The elemental loss in silicate residues of plant material was found to be up to 30% and therefore required HF-containing digestion methods. The high concentration of matrix elements leads to spectral interferences, which are investigated by measuring the elements with diVerent mass resolution. High mass resolution is shown to be a prerequisite for accurate determination of Sc and Y, respectively.Furthermore, eVects of non spectral interferences are investigated and could be properly corrected for by using 115In as internal standard. Moreover, the capability of a microconcentric nebulizer in combination with a membrane desolvation unit compared to a conventional microconcentric nebulizer is discussed with respect to suppression of spectral interferences. Oxide interferences could be reduced to a negligible amount, whereas it could be observed that high salt freight leads to a blockage of the membrane. Nowadays there is an increasing demand for rare earth interferences14–16 and even since the first investigations by ICP-MS numerous interference eVects have been reported.17,18 elements (REE) in new technology applications such as, e.g., Isobaric interferences, polyatomic ions and to a small extent electronics, optoelectronics, superconductors, supermagnets, (for Sc and Y) multiple charged ions are the most important lasers, computers, rechargeable hydride batteries, artificial spectral interferences, which are overcome usually by math- diamonds, glass and ceramics.Hence, REE technological ematical correction procedures.15,16,19 Obviously, one pos- materials are increasingly produced by industry and consesibility to deal with the problem of several spectral interferences quently also released into the environment where REE traces in ICP-MS is to separate the analyte peak and interference are bio-accumulated by organisms. Their bioavailability and peak in the mass spectrum by using high mass-resolution toxic properties are currently under investigation and REE instruments.20–22 However, there are several other instrumental are known to activate or inhibit metabolism or enzyme approaches which allow us to partially overcome this problem.activity.1–3 As a consequence, REE are already part of national One possibility to reduce spectral interferences originating and EU legislation. from the solvent is the formation of dry aerosols by means of In recent years attention has been drawn to the development a membrane desolvation unit. Membrane desolvation transfers and steady introduction of analytical methods suitable for solvents (e.g., H2O, HNO3) and dissolved gases (e.g., HF, quantification of Th and U traces.In general, spectrometric HCl, BF3, SiF4) into the gaseous phase by evaporation and determinations of REE in any matrices are characterized by separates them from the dried aerosol by diVusion through a several complicating features such as low REE concentrations membrane. Thus, mainly the analyte of interest is transported in most samples relative to the instrumental detection limits, into the plasma, whereas the concentration of possibly interfer- high concentration of matrix elements, often including major ing matrix-elements in the plasma is significantly reduced.elements (e.g., Fe, Mg, Si) and other minor and trace elements, Furthermore, enhanced signal intensities are observed comwhich can result in high levels of chemical or spectral inter- pared to conventional pneumatic introduction systems. ferences on the analyte determination. In order to overcome Another way to deal with spectral interferences is to separate some of these limitations, extensive sample pretreatment pro- the interfering matrix from the analyte of interest prior to cedures have been utilized to separate and preconcentrate REE ICP-MS analysis either in a batch process or by means of prior to analysis.4–7 oZine or online coupling of column separation methods.6,23–25 For many years atomic absorption spectrometry, X-ray Besides spectral interferences, systematic errors occur due fluorescence and inductively coupled plasma atomic emission to non-spectral interferences which are often referred to as spectrometry were the principal reference techniques to deter- ‘matrix-induced signal variations’ (suppression or enhancemine REE.8 Other important techniques are instrumental ment) or ‘matrix eVects’. These matrix eVects can be caused neutron activation analysis9,10 and isotope dilution mass by irreversible processes (e.g., clogging the nebulizer, cones, spectrometry.11 The introduction of inductively coupled torch) or reversible processes occurring only during the plasma mass spectrometry in the 1980s brought a remarkable measurement of the sample (e.g., change of nebulization change in REE analysis because of its very low detection eYciency and transport, ionization in the plasma, ion extraclevels, high sensitivity, large dynamic range, limited inter- tion).26 These signal variations have to be corrected for ference eVects and simplified sample preparation together with instrumental fluctuations by (i) internal stanprocedures. 8,12,13 However, ICP-MS analysis of REE is dardization,27–29 (ii) standard addition30 or (iii) isotope dilution,31 respectively. known to be hampered by unwanted spectral and non-spectral J. Anal. At. Spectrom., 1999, 14, 1–8 1A crucial point in ICP-MS analysis of REE is to find the acid) calibration solutions ranging from 0.01–2 ng g-1. appropriate digestion method in order to avoid incomplete Calibration standards were prepared by diluting a 10 mg g-1 digestion and too complex matrices causing severe interference REE multielement standard (SPEX, Metuchen, USA). 115In problems.Digestion of plants or animal tissues in was used as internal standard element and was prepared by HNO3–H2O2, for example, is known to lead to considerable diluting a 1000 mg g-1 standard solution (Merck, Darmstadt, formation of oxide interferences in ICP-MS.32 Especially in Germany).Other internal standard solutions (101Ru, 103Rh, the case of the determination of REE in silicate containing 187Re) were also prepared by diluting a 1000 mg g-1 standard materials, sample preparation is not straightforward. solution. The accuracy of the standards were cross checked by Incomplete digestion or loss of volatile species are two prob- single element standards if available. All standards were lems that face the analyst.12,33,34 DiVerent dissolution methods prepared by weighing.in open systems, closed vessels or in fusion are employed.35 For the preparation of HPLC eluents, lithium hydroxide Conventionally, the filtered silicate residue is dissolved in a Pt monohydrate pro analysi (Fluka, Buchs, Switzerland), oxalic crucible by using HF to dissolve SiO2 by volatilizing silicon acid pro analysi (Merck), diglycollic acid 98% (Acros Organics, as SiF4. Subsequently, the remaining metal fluorides are New Jersey, USA), tetramethylammonium hydroxide pentahyconverted (i) into nitrates by HNO3 and (ii) into chlorides by drate 97% (Fluka, Buchs, Switzerland) and purified water HCl and finally recombined with the filtrate.36 Alternatively, were used.a mixture of HF and HNO3 is used to digest the silicates. For digestion double sub-boiled HNO3 (Janssen Chimica, H3BO3 is added in a second step in order to prevent partial Geel, Belgium), H2O2 30% pro analysi (Merck), single subcoprecipitation of REE with CaF2 and other fluorides37 by boiled HCl pro analysi (Merck), H3BO3 pro analysi and HF forming a (BF4)- complex.As a main disadvantage, new 48% pro analysi (Fluka) were used. elements (B, F) are added to the matrix in high concentration The following samples were investigated: sediment material levels and consequently changing matrix-properties influence (Standard reference material ‘Canadian Stream Sediment both the occurrence of spectral and non-spectral interferences STSD1, STSD2’) and soil material (Standard reference matein ICP-MS analysis.Microwave induced decomposition was rial ‘Chinese Soil GBW07403’), aquatic plant material (‘Lemna found to be most eVective for rapid dissolution both of plant minor’) and mussel tissue (‘Mytilus edulis’) (both latter matematerial38 and geological material.39 The following work rials were provided by the ‘Mermayde Institute for monitoring demonstrates the potential of using a high mass resolution of water, sediment and biota’, Beregen, Netherlands, for an instrument in combination with two diVerent introduction intercomparison study within a certification campaign of the systems for accurate measurement of REE in various matrices EU).containing HNO3, HCl, HF and H3BO3. Sample preparation Experimental All digestions were performed in a closed vessel (PTFE vessels) Instrumentation microwave unit (MLS 1200mega, Leutenkirch, Germany). For method development the system was equipped with a pressure Measurements were carried out by means of a Finnigan MAT and temperature control vessel. ELEMENT (Finnigan MAT, Bremen, Germany) high reso- Sediments and soils were digested using (i) HNO3–HCl lution inductively coupled plasma sectorfield mass specand (ii) HNO3–HCl–HF and subsequently H3BO3 as trometer (ICP-SMS).The instrument is equipped with a F--complexing agent. double focusing mass analyzer consisting of a magnetic and electric sector field of reversed Nier–Johnson geometry. The Plant materials were digested with (i) HNO3–H2O2 and (ii) system allows three predefined nominal mass resolutions of HNO3–HF using again H3BO3 as F--complexing agent. 300, 3000 and 7500 by means of preselectable slits. The actual Marine animal tissues were digested using HNO3–H2O2. mass resolutions vary depending on the optimization of param- Table 1 shows the microwave program for sediments, soils, eter settings between 300–500 for low, 3500–4500 for medium plants and animal tissues, respectively.and 7500–8500 for high mass resolution. The system is operated under standard operating conditions, which are discussed elsewhere.40 As sample introduction systems the instument Results and discussion was equipped (i) with a microconcentric nebulizer (MCN 100, Selection of isotopes and isobaric interferences Cetac Techn. Inc., Omaha, Nebraska, USA) in combination with a double-pass Scott type spray chamber cooled at 4 °C Many isotopes of the REE are known to give rise to isobaric and (ii) with a microconcentric nebulizer in combination with interferences.Nevertheless, all these elements have at least one a heated Teflon spray chamber and a membrane desolvation isotope free of isobaric interferences which is suYciently system (MCN 6000, Cetac). abundant and hence suitable for quantitative measurements All measurements were performed in class 10 000 clean room by ICP-MS.6,15,25,35,39,41 laboratory. Isotope-pairs with isobaric overlap require mass resolutions much higher than obtainable with high resolution instruments Samples, standards and reagents (from m/Dm=75 227 for 144Nd/144Sm to m/Dm=5 854 614 for 164Dy/164Er). Therefore, an isobaric overlap cannot be All chemical preparations were carried out in class 100 metal resolved.Nevertheless, it can be corrected for by measuring free clean benches. In all analytical work analytical reagent the intensity of an isotope of the interfering element, which is grade nitric acid (Janssen Chimica, Geel, Belgium) was itself free from interferences. In case of unknown matrices, the additionally cleaned by double sub-boiling distillation where isotopic pattern additionally allows one to estimate other the solvent is distilled by surface evaporation below the boiling possible spectral interferences.However, it is important to point in an ultrapure quartz apparatus (Milestone-MLS point out that even correct isotopic patterns for at least two GmbH, Leutkirch, Germany).diVerent isotopes are not an unambiguous indicator for not Water purified by (i) reverse osmosis subsequently passes having spectral interferences as shown later for Dy, e.g., where through a (ii) laboratory-reagent grade water system (F+L both investigated isotopes (161Dy and 163Dy) have a spectral GmbH, Vienna, Austria) and is finally purified by (iii) a interference of approximately the same extent. Table 2 lists the sub-boiling system (Milestone-MLS).External calibration was performed by aqueous (1% nitric isotopes selected for further quantification. 2 J. Anal. At. Spectrom., 1999, 14, 1–8Table 1 Digestion operation parameter Aquatic Sediment or Aquatic plant Sediment or soil Material plant (HF–digestion) soil (HF–digestion) Mussel tissue Sample dry weight/g 0.15 0.15 0.10 0.10 0.25 Digestion mixture 3 mL HNO3/ 3 mL HNO3/ 3 mL HNO3/ 3 mL HNO3/ 3 mL HNO3/ 0.5 mL H2O2 0.75 mL HF 2 mL HCl 2 mL HCl/ 0.5 mL H2O2 1 mL HF Final dilution weight/mL 20 20 20 20 20 Additional dilution factor None None 100 100 None Total dissolved solids (%) 0.75 1.55 0.005 0.015 1.25 Microwave digestion program [Time (Power)/min (W)] Step 1 1 (250) 1 (250) 1 (250) 1 (250) 1 (250) Step 2 2 (0) 2 (0) 2 (0) 2 (0) 2 (0) Step 3 15 (250) 15 (250) 15 (250) 20 (250) 8 (250) Step 4 5 (400) 5 (400) 5 (400) 6 (600) 5 (400) Step 5 5 (650) 5 (650) 5 (650) 5 (650) 5 (650) Step 6 2 (0) 2 (0) H3BO3 (40 g L-1) after digestion/mL None 4 None 5 None Step 7 15 (250) 15 (250) Table 2 Isotopes selected for analysis diVerent samples.Therefore, single element standard solutions were analyzed which contained the element that was expected Required to form interfering polyatomic oxide in the same concentration resolution to as found in the matrices. Since most of the oxide interferences Abundance Main Abundance separate cannot be resolved even by high mass resolution,43 correction Isotope (%) interferences (%) interference factors were determined by means of these correction solutions. 45Sc 100.00 28Si16O1H 91.99 1893 Additionally, the influence of sample introduction on oxide 89Y 100.00 178Hf++ 27.30 1348 formation was evaluated using two diVerent introduction 139La 99.90 123Sb16O 42.65 10 000 systems, the MCN100 and the MCN6000, respectively. The 140Ce 88.50 124Sn16O 5.78 57 000 oxide formation rate of both nebulizers is significantly diVerent 141Pr 100.00 (0.3% CeO/Ce for the MCN100 and 0.01% for the MCN6000). 143Nd 12.18 127I16O 99.76 13 701 145Nd 8.30 Generally, it has been observed that oxide interferences at 146Nd 17.19 130Te16O 34.41 12 177 the selected isotopes were negligible within the analyzed 147Sm 15.00 133Cs14N 99.63 22 954 samples and resulted in systematic errors<5% for the geologi- 149Sm 13.80 133Cs16O 99.76 8839 cal material using the MCN100 as nebulizer. REE determi- 135Ba14N 6.57 17 620 nation in plant material is known to pose an analytical 151Eu 47.80 135Ba16O 6.57 7828 challenge since problems arise due to high levels of Ba and 157Gd 15.65 141Pr16O 99.76 7334 158Gd 24.84 142Nd16O 27.03 7356 extremely low levels of REE .Thus, only for 151Eu the high 159Tb 100.00 143Nd16O 12.11 7709 levels of Ba (about 650 ng g-1) resulted in a significant BaO+ 161Dy 18.90 145Nd16O 8.27 8961 interference whereas a systematic error of only <5% was 163Dy 24.90 147Sm16O 15.03 8613 calculated for other isotopes, although significant oxide inter- 165Ho 100.00 149Sm16O 13.81 9049 ferences could have been expected (147Sm, 149Sm, 158Gd, 159Tb, 167Er 22.95 151Eu16O 47.66 9656 163Dy, 167Er and 174Yb).The BaO+ interference on 151Eu is 169Tm 100.00 153Eu16O 52.11 9349 171Yb 14.30 155Gd16O 14.69 9094 comprehensively discussed elsewhere15,16 and leads to a system- 172Yb 21.90 156Gd16O 20.42 8886 atic error of 45% for the investigated plant material. It had to 173Yb 16.12 157Gd16O 15.64 8941 be corrected for mathematically, although the separation of 174Yb 31.80 158Gd16O 24.81 8763 the 151Eu peak and the 135Ba16O+ peak in the mass spectrum 175Lu 97.41 159Tb16O 99.76 8524 is possible with a mass resolution of 7828, which is available 232Th 100.00 using the high resolution mode.Fig. 1 shows a high resolution 238U 99.27 spectrum (m/Dm=7500) of 151Eu in the plant matrix. It can Multiple charged ions The elements causing spectral interferences at m/2 were not found to be present at high levels in the samples under investigation. 45Sc and 89Y can be interfered with by double charged Zr2+ and Hf2+ ions requiring a mass resolution of 12 644 or 1348, respectively. A screening of the samples for Zr and Hf revealed that the the concentration of these elements and consequently the mass peaks at m/2 are negligible. However, in the investigated plant material high levels of Ba were observed resulting in a Ba3+ interference at m/3 for 45Sc.The problem is discussed in more detail later. Oxide interferences on mass 139–175 Oxide interferences by REE, well discussed in the Fig. 1 Mass spectrum (m/z=150.87–150.95) of 151Eu in high literature,15,16,42 were investigated considering the diVerent mass resolution (m/Dm=7500) of a plant material digested in HNO3–HF–H3BO3. c(Eu)=0.06 ng g-1; c(Ba)=650 mg g-1. concentration levels of the interfering parent element in the J. Anal. At. Spectrom., 1999, 14, 1–8 3be seen that a possible BaN+ interference is present on the shoulder between the BaO+ and the Eu+ peak which would require a mass resolution of 13 757.However, the low concentration of Eu in plant material makes the quantification in the high mass resolution mode problematic due to low sensitivity and high relative standard deviation (RSD) of the signal. In the case of the marine animal tissue all isotopes including 151Eu led to deviation of the accurate value of <5%. The interferences were also observed to be negligible in case of the sediment and the soil sample even if significant oxide interferences were observed for other ICP-MS instruments.41 Although several elements also form hydroxide molecules within this mass range, the levels of formation are usually 10% of the oxide formation and are evidently negligible in the present study.41 Oxide formation strongly depends on the plasma and operating conditions and both vary with the diVerent types of Fig. 2 Mass spectrum (m/z=44.94–45.03) of 45Sc in medium mass ICP mass spectrometer; it is usually subject to changing resolution (m/Dm=3000) of a 1 ng g-1 standard solution (1% nitric operating parameters. Therefore, the oxide ratio (CeO/Ce) was acid), blank solution (1% nitric acid), digested plant material measured prior to analysis to recalculate the correction factors (HNO3–HF–H3BO3—matrix) and a HNO3–HF–H3BO3—blank with respect to the changed oxide ratio which varied from 0.1 digestion solution.to 0.4% (for CeO/Ce) in case of the MCN100 operating parameters. The use of the MCN6000 significantly reduced the oxide Table 3 Possible interferences on 45Sc by the matrix elements Ba, Si, interferences to a negligible amount due to membrane desolv- O, H, N, F and B ation. Oxide formation rates of about 0.01% (CeO/Ce) were usually observed. Therefore, correction for oxide interferences Element Mass Dm (Sc) Resolution was not necessary by using the MCN6000 as the introduction system.Nevertheless, since the high salt transfer to the nebul- 45Sc+ 44.95591 135Ba3+ 44.96856 0.01265 3555 izer, especially in case of the undiluted digestion solution of 29Si16O+ 44.97141 0.01550 2901 the plant material (salt concentration #1.6%), leads to a 28Si17O+ 44.97606 0.02015 2232 blockage of the membrane after about 10–20 consecutive 28Si1H16O+ 44.97967 0.02376 1893 measurements and subsequently to an increase of the oxide 12C1H16O2+ 44.99765 0.04174 1078 ratio from 0.01% to 0.3%, a continuous check of the oxide 12C14N19F+ 45.00147 0.04556 988 ratio is necessary.Therefore, we checked the ratio (CeO/Ce) 28Si1H314N+ 45.00349 0.04758 946 16O10B19F+ 45.00625 0.05034 894 continuously during data acquisition. 1H14N216O+ 45.00888 0.05297 850 12C21H219F+ 45.01406 0.05815 774 Interferences on 45Sc 1H216O211B+ 45.01479 0.05888 765 45Sc was expected to be influenced by a significant SiOH+ 1H14N11B19F+ 45.01861 0.06270 718 12C1H314N16O+ 45.02147 0.06556 687 interference due to high levels of Si in the sediment, the soil 12C210B11B+ 45.02225 0.06634 679 and also the plant matrices.Furthermore, the use of HF as 1H316O210B+ 45.02625 0.07034 640 digestion agent leads to an increased background of Si released 1H214N10B19F+ 45.03007 0.07416 607 from the quartz torch by remaining free HF (even if the major 12C1H311B19F+ 45.03120 0.07529 598 part is complexed by H3BO3) in sediment, soil and plant 1H314N3+ 45.03270 0.07679 586 matrices.Since the required resolution is 1894 for the main 1H414N16O11B+ 45.03861 0.08270 545 12C1H410B19F+ 45.04266 0.08675 519 abundant 28Si16O1H+ interference, 2233 for 28Si17O+ and 2902 1H411B219F+ 45.04834 0.09243 487 for 29Si16O+, 45Sc was measured in medium resolution mode (m/Dm=3000). The spectrum of the plant material in Fig. 2 shows unequivocally that besides the above listed interferences additional interferences are present. The major matrix elements interfering signal is changing significantly with respect to the H, B, C, N, O, F and Si lead to numerous complex molecular sample material.This clearly proves that measurement in low ions. Table 3 shows a list of possible interferences of the prior resolution mode or by means of quadrupole based instruments listed elements considering only the major isotopes. The prodwith subsequent blank correction solely, leads to systematically ucts of all these main matrix elements are usually di- or wrong values for Sc.The presence of a variety of spectral triatomic ions. It is very diYcult, however, to determine their interferences makes a correction even more diYcult. Therefore, probability and intensities, and this can only be done experit is evident that high mass resolution is a key prerequisite for imentally under typical operating conditions.45 The influence separation of the spectral interferences from the 45Sc signal. of the triple charged Ba3+ ion at mass 44.97 was studied in When using the MCN6000 as introduction system, we could more detail for the plant material.By investigating single observe an increase of the signal intensity for 45Sc by a factor element standards it could be proved that 1 mg g-1 Ba leads of 8 and a reduction of spectral interferences even if still to signal intensities corresponding to about 0.1 ng g-1 Sc. The significantly present. The Si–OH interference was reduced Ba level in the investigated plant material was about 10-fold, whereas the Si, B, and F containing interferences were 600 ng g-1.Furthermore, the Ba3+ signal in Fig. 2 is overonly reduced by a factor of 5. The Ba3+ interference remained lapped with 29Si16O+. nearly unaVected. The results showed clearly that even the use It is noteworthy that even after correction by internal of a nebulizing system with membrane desolvation could not standardization, the interfering signal of the method-blank is reduce the interferences to a negligible amount for 45Sc and significantly lower than that originating from the digested therefore requires high mass resolution for accurate sample.Even the interference of the standard-blank is lower than for the standard itself. Moreover the intensity of the quantification. 4 J. Anal. At. Spectrom., 1999, 14, 1–8Fig. 3 Mass spectrum (m/z=88.86–89.04) of 89Y in medium mass resolution (m/Dm=3000) of a 1 ng g-1 standard solution (1% nitric acid), blank solution (1% nitric acid), digested plant material (HNO3–HF–H3BO3—matrix) and a HNO3–HF–H3BO3—blank digestion solution.The 1% nitric acid blank solution is <20 cps and therefore not visible in this spectrum. Interferences on 89Y, 141Pr, 161Dy, 163Dy and 165Ho Since significant variations of the concentration levels in the plant material digested in HNO3–HF–H3BO3 were found for 89Y, 141Pr, 161Dy, 163Dy and 165Ho in low and high mass resolution, these isotopes were investigated with respect to possible spectral interferences. Fig. 3 shows the spectra of 89Y in the HNO3–HF–H3BO3-matrix and in 1% HNO3, respectively (medium resolution, m/Dm=3000). By calculating the accurate mass of all possible element combinations (about 150 for the major isotopes, only) in a mass range from 88.90–89.03 u, most of the interfering signals could be identified as complex combinations of H, O, B, F, N, Cl and Ar. The same eVect can be observed for 141Pr, 161Dy, 163Dy and 165Ho.Again as a consequence we proved that blank correction solely leads to erroneous quantification of these elements. It is furthermore important to point out that even the measurement of diVerent isotopes does not give evidence of spectral interferences if not measured in high mass resolution, as demonstrated for the element Dy. Both Dy isotopes 161Dy and 163Dy which were chosen for quantification were found to be interfered with to the same extent. Fig. 4 Comparison of the quantified concentration levels in the When using the MCN6000 as introduction system, the above digestion solution of a plant material for 89Y, 141Pr, 161/163Dy and mentioned spectral interferences could be reduced to a negli- 165Ho measured by means of the MCN100 in low resolution (m/Dm= gible amount.Accordingly, these isotopes could be measured 300), medium resolution mode (m/Dm=3000) and by means of the nearly free of interferences in low mass resolution when using MCN6000 in low resolution (m/Dm=300), respectively.the MCN6000 as introduction system. Again, the high salt transfer to the nebulizer is blocking the membrane and leads to a deterioration of the reduction of spectral interferences. an isotope, which is (i) not present in the sample, (ii) has no Due to the high concentration levels of REE in soil and obvious spectral interferences and (iii) shows the same eVect sediment the digested samples were diluted 100-fold, reducing of signal variation as the analyte of interest.In this study the eVect of spectral interferences. Thus, ICP-MS analysis 101Ru, 103Rh, 115In and 187Re were tested regarding their revealed only negligible production of molecular ions even suitability as internal standard by investigating the matrix when using the MCN100. suppression of both the single isotopes of interest and the Fig. 4 summarizes the eVect of spectral interferences on the internal standard. For that purpose a multielement REE result of the above mentioned elements and shows clearly the solution was digested using HF–H3BO3 and a 110 ng g-1 good agreement of the results obtained with the MCN100 in internal standard solution was added 1+9 in one experiment mass resolution m/Dm=3 000 and with the MCN6000 in mass prior to digestion and in another experiment after digestion resolution m/Dm=300.to avoid systematic errors caused by elemental loss during digestion. The analysis was performed using the MCN100 as Non-spectral interferences introduction system.Since no elemental loss could be observed, the deviation of the signal intensity could be ascribed unam- Since the use of an internal standard is the most used and biguously to non-spectral interferences. Comparing the signal most attractive method for the correction of non-spectral intensities of the digested standard to those of an aqueous interferences in routine analysis, several internal standards multielement solution showed that all investigated isotopes have been investigated with respect to their applicability in were subject to the same signal suppression as 115In.For all REE analysis by ICP-MS according to previous studies.41,46,47 The method involves the addition of a specified amount of elements under investigation the signal intensities of the J. Anal. At. Spectrom., 1999, 14, 1–8 5digestion agent. Within our investigations digestion was performed both with and without HF in order to estimate elemental loss within the silicate residues.Indeed, REE-concentrations measured in the HNO3–HCl-matrix compared to the HNO3–HCl–HF–H3BO3-matrix show significant losses of the investigated elements amounting to about 50% for U, 35% for Yb and Lu, 30% for Tm and Sc, 25% for Y and Er, 15% for La, Ce, Dy, Ho and Th and less than 10% for Nd, Sm, Eu, Gd and Tb. The same investigation was performed for the plant material. When comparing the two digestion procedures for the plant material significant losses of Sc (40%), Y (30%), Ce (10%), Nd (10%), Gd (15%), Er (35%) and Th (10%) were observed within the silicate residues when using only HNO3–H2O2 as digestion agents.For the other elements Fig. 5 Signal supression of 115In in a HNO3–HF–H3BO3—matrix. the diVerence was <5%. Since it can be expected that silicates The intensity is normalized to the maximum intensity obtained for an do not come from the plant tissues directly but are subject to aqueous standard solution (infinite dilution). The dilution represents the dilution of a matrix consisting of 3 mL HNO3, 0.75 mL HF and contamination of roots by soil or of leaves by mud in case of 4 mL H3BO3 (40 g L-1) by ultrapure water.(The final dilution to aquatic plant material, it is important to trace the sources of 20 g for the measurement of the plant material corresponds to a silicate residues in plant material. Therefore, future analysis dilution of 152 .The dilution of 0 represents an infinite dilution and of plant material has to focus on the investigation of silicate corresponds to an aqueous standard solution.) residues to find out if the elemental content in silicate residues is significant for the actual plant tissues or remaining contami- digested sample were reduced to 45–55% compared to the nation and if it is possible to separate remaining silicate signal intensities in an aqueous matrix.Only 238U showed a contaminants from the plant tissue by sequential digestion. slightly less pronounced signal suppression (from about 60–65%). Therefore it is evident that 115In is an appropriate Memory eVects internal standard. 238U had to be corrected further for the less pronounced matrix suppression. Final recovery rates were An overestimation of elemental concentration can result from found to be 100±5% after internal standardization for all elements adsorbed within the system (tubing, membrane) isotopes under investigation. Ru, Rh and Re turned out to be which are washed out in subsequent samples.REE were less suitable for internal standardization, since these elements observed not to show a significant memory eVect. For the showed lower signal suppression than the REE, but these MCN100 washout times of <30 s and of 1 min were required internal standards revealed better results for 238U. As for the MCN100 and MCN6000, respectively. Only Th showed expected,48 the signal suppression clearly depends on the a significant memory eVect, which would necessitate washout matrix concentration amounting to only 10% for the diluted times of about 20 min in case of the MCN6000 when using sediment and soil digestion samples.the conventional operation parameters (see Fig. 6). To over- Fig. 5 shows as an example that the matrix supression on come this problem, a washing solution of 10% HNO3 was 115In is not linearly related to the matrix (HF–H3BO3) concen- used with significantly increased pump speed to reduce washout tration.The same eVect was found for the other isotopes of times to 4 min for Th using the MCN6000. interest. Furthermore, the nebulizer gas flow rate was observed to be the parameter, which aVects most severely the occurrence Limits of detection of non-spectral interferences and is studied in more detail Limits of detection (DL) are calculated for mg kg-1 (ppm) elsewhere.49,50 The observed constant matrix supression indidry sample considering spectral and non-spectral interferences: cates according to Vanhaecke et al.51 that space eVects in the plasma are not solely the origin of the observed signal decrease.DL=(3×s/k)×DW/(SW×1000) Moreover, nebulizer eVects caused by changing nebulizing s, standard deviation (n=25) of the digestion blank signal eYciency with respect to viscosity and salt concentration are (cps); k, slope of the blank-corrected calibration curve expected to have a strong impact on non-spectral interferences. (cps ppb-1); DW, dilution weight (=20 g); SW, sample weight The nebulizing eYciency is again strongly influenced by the (=0.20 g).nebulizer gas flow rate when using a microconcentric nebulizer. When using the MCN6000 a reproducible reversed eVect could be observed. The signal intensities were up to 30% higher for all elements in the digested matrix compared to the aqueous standard solutions. This eVect is not fully explainable, yet.In general the main problem of the MCN6000 is an irreversible non-spectral interference and arises in the analysis of samples with high salt concentrations (e.g., digested plant material: salt concentration=1.55%). The high salt concentration leads to a blocking of the membrane and deterioration of the oxide formation ratio from 0.01% to about 0.3% (comparable to the conventional MCN100) within a sequence of about 20 consecutive measurements.Also the signal intensity of the element of interest was decreasing by a factor of 5–8, leading to a signal intensity comparable to the conventional MCN100. Silicate residues Fig. 6 Washout curve of an aqueous standard containing 5 ng g-1 Sediments and soils contain a significant amount of silicates 232Th under standard operating parameter using the MCN100 and MCN6000, respectively. and therefore require a digestion procedure with HF as 6 J. Anal. At. Spectrom., 1999, 14, 1–8Table 4 Limits of detection (mg kg-1 dry weight) for the investigated isotopes using the MCN100 and the MCN6000 in low resolution mode (LR) and the MCN100 in medium resolution (MR) for diVerent digestion methods (HNO3–H2O2 and HF) Isotope MCN100 (LR) MCN100 (LR) MCN6000 (LR) MCN100 (MR) MCN100 (MR) (HNO3/H2O2) (HF) (HF) (HNO3/H2O2) (HF) 45Sc — — 9E- 03 6E-03 5E-02 89Y 7E- 05 5E-01 8E-04 2E-03 1E-03 139La 1E-04 3E-03 1E-03 2E-03 2E-03 140Ce 2E-04 2E-03 2E-03 9E-03 3E-03 141Pr 7E-05 7E-03 3E-04 8E-04 1E-03 145Nd 3E-04 3E-03 6E-04 1E-02 4E-03 146Nd 1E-04 2E-03 1E-03 1E-02 3E-03 147Sm 1E-04 3E-03 1E-03 1E-03 5E-04 149Sm 8E-05 3E-03 2E-03 4E-03 3E-03 151Eu 6E-05 7E-04 8E-05 4E-03 4E-04 157Gd 1E-04 2E-03 1E-03 5E-03 1E-03 158Gd 9E-05 7E-04 5E-04 6E-03 2E-03 159Tb 1E-05 5E-04 2E-04 1E-03 4E-04 161Dy 1E-04 6E-03 4E-04 1E-03 9E-04 163Dy 5E-05 2E-02 8E-04 3E-03 4E-04 165Ho 2E-05 5E-03 2E-04 9E-04 2E-04 167Er 5E-05 4E-03 5E-04 2E-03 2E-03 169Tm 2E-05 2E-04 6E-05 6E-04 1E-04 171Yb 6E-05 5E-04 3E-04 3E-03 1E-04 172Yb 3E-05 6E-04 2E-04 NDa ND 173Yb 3E-05 2E-03 2E-04 ND ND 174Yb 2E-05 6E-04 3E-04 2E-03 1E-03 175Lu 1E-05 2E-04 7E-05 6E-04 9E-05 232Th 9E-04 1E-03 1E-03 5E-03 3E-03 238U 2E- 05 2E-04 2E-04 4E-04 3E-04 aNot determined.The detection limits were calculated on the basis of 25 behavior of an element, this element is usually part of additional analytical investigations to determine the accuracy of the applied independent analysis (including blank digestion in diVerent vessels by microwave digestion and subsequent analysis) method before drawing conclusions of an anthropogenic impact of REE on the sample.Thus chondrite normalization can be performed on three diVerent days. Table 4 shows the LOD for diVerent matrices and diVerent an additional assessment of analytical accuracy. Fig. 7 depicts the normalized distribution of the elemental nebulizers, respectively. content of REE in STSD1 of the certified values, values of Sen Gupta and Bertrand39 and the values obtained within this Chondritic pattern study by using the MCN100 and MCN6000, respectively.Normalization of the measured REE pattern to a known The normalization shows that the certified values for standard allows a better insight into geological processes. Yb and certainly for Lu might be too high. The slope of the Additionally, anthropogenic REE-contaminations or pollution plot from La to Ce seems also to be slightly too steep.The can be identified. Chondritic meteorites are used as standards, latter may also concern values obtained by Sen Gupta and because they represent primitive solar material that may be Bertrand.39 The MCN100-values behave discretely, but part of parental earth (values for chondritic meteorites used the MCN6000-values for Sm and Eu are slightly too high, for normalization; see Table 5).52 If the chondrite-normalized which could be caused by unresolved interferences.patterns of REE-concentrations result in a smooth curve, These results indicate that for Sm and Eu systematically anthropogenic impact can be excluded, although curves may wrong values could be found within the sediment matrix. vary in their slopes. Furthermore, Ce and Eu may display Comparable results were found for the other sediment and anomalous behavior because of their diVerent oxidation states. soil samples. Also the chondritic pattern of the animal and When the analytical chemist is faced with an anomalous Table 5 Values for chondritic meteorites used for normalization Element Concentration/mg kg-1 La 0.329 Ce 0.865 Pr 0.122 Nd 0.630 Sm 0.203 Eu 0.077 Gd 0.276 Tb NDa Dy 0.343 Ho 0.076 Er 0.226 Tm ND Fig. 7 Chondrite normalized plot of results for sediment STSD1 for Yb 0.220 Lu 0.034 the MCN100 in medium resolution (m/Dm=3000) mode, the MCN6000 in low resolution mode (m/Dm=300), the certified values aNot determined in this publication.46 and literature values.36 J.Anal. At. Spectrom., 1999, 14, 1–8 74 P. Henderson, Rare Earth Element Geochemistry, Elsevier, plant tissue indicate too high values for Lu. This is proved Amsterdam, 1984. and properly corrected for by reference measurements. Eu 5 H. Haraguchi, A. Itoh, C. Kimata and H. Miwa, Analyst, 1998, showed a significant deviation from the expected smooth curve 123, 773. in the case of plant tissue for the values obtained by low 6 X. Cao, G.Zhao, M. Yin and J. Li, Analyst, 1998, 123, 1115. resolution mode and subsequent correction for BaO+. 7 B. Li, Y. Zhang and M. Yin, Analyst, 1997, 122, 543. 8 I. B. Brenner and H. E. Taylor, Crit. Rev. Anal. Chem., 1992, 23(5), 355. Conclusion 9 R. A. Schmitt, R. H. Smith, J. E. Jasch, A. W. Mosen, D. A. Olehy and J. Vasilevskis, Geochim. Cosmochim. Acta, 1963, 27, 577. Spectral interferences are mainly important when encountering 10 P. Voldet, Trends Anal. Chem., 1984, 1, 262.low concentrations of REE in undiluted complex matrices 11 C. C. Schnetzler, H. H. Thomas and J. A. Philpotts, Anal. Chem., (e.g., digested plant material ), since a part of these inter- 1967, 39, 1888. 12 K. E. Jarvis, Chem. Geol., 1988, 68, 31. ferences are reduced to a negligible amount if the solution can 13 V. Balaram, Trends Anal. Chem., 1996, 15(9), 475. be diluted 100 fold, which is easily possible for digested 14 K. Shinotsuka and M. Ebihara, Anal. Chim. Acta, 1997, 338(3), sediment and soil samples containing high levels of REE.For 237. Sc and Y even the diluted digestion solution of soil and 15 P. Dulski, Fresenius’ J. Anal. Chem., 1994, 350, 194. sediment require correction for spectral interferences as is 16 M. A. Vaughan and G. Horlick, Appl. Spectrosc., 1990, 44, 587. obvious in high resolution spectra. It is evident that spectral 17 R. S. Houk, V. A. Fassel, G. D. Flesch, H. J. Svec, A. L. Gray and C. E. Taylor, Anal. Chem., 1980, 52, 2283.interferences are very complex, depending on the matrix, and 18 A. L. Gray, Analyst, 1975, 100, 289. simple mathematical correction procedures lead inevitably to 19 J. Goossens, L. Moens and R. Dams, Talanta, 1994, 41/2, 187. erroneous results. The varying amount of matrix induced 20 U. Gießmann and U. Greb, Fresenius’ J. Anal. Chem., 1994, 350, spectral interferences in blanks, calibrants and samples, 186. respectively, makes as a consequence high mass resolution a 21 L.Moens and N. Jakubowski, Anal. Chem., 1998, 70(1), 251A. prerequisite for accurate determination of REE in a variety of 22 B. Schnetger, Fresenius’ J. Anal. Chem., 1997, 359, 468. 23 J. A. Barrat, F. Keller, J. Amosse�, R. N. Taylor, R. W. Nesbitt diVerent matrices. Therefore, also in the case of technological and T. Hirata, Geostandard Newslett., 1996, 20(1), 133. materials ( like oxides with very low REE concentration) 24 V. K. Panday, K. Hoppstock, J. S. Becker and H.J. Dietze, At. spectral interferences remain an analytical challenge in future Spectrosc., 1996, 17(2), 98. applications. 25 M. Bettinelli and S. Spezia, At. Spectrosc., 1995, 16(3), 133. The MCN6000 was observed to eliminate the matrix solvent 26 R. F. J. Dams, J. Goossens and L. Moens, Mikrochim. Acta, 1995, and volatile species very eYciently and is therefore a powerful 119, 277. 27 C. Vandecasteele, M. Nagels, H. Vanhoe and R. Dams, Anal. tool to reduce spectral interferences (includes O, H, B, N, F Chim.Acta, 1988, 211, 91. containing species) to a negligible amount without using high 28 F. Vanhaecke, H. Vanhoe, R. Dams and C. Vandecasteele, mass resolution, except for 45Sc and also partially 89Y. Talanta, 1992, 39, 737. Moreover, the signal intensity of the analyte of interest is 29 J. J. Thompson and R. S. Houk, Appl. Spectrosc., 1987, 41, 801. increased by a factor of 5–8 and the higher sensitivity leads 30 Z. Peng, H. Klinkenberg, T.Berren and W. Van Borm, Spectrochim. Acta, 1991, 46, 1051. to lower LOD. However, higher RSD values of the signal 31 K. Y. Patterson, C. Veillon, P. B. Moser-Veillon and G. F.Wallau, (about 4–6%) compared to the MCN100 (1–3% RSD) are Anal. Chim. Acta, 1992, 258, 317. observed. As a drawback, blocking of the membrane by high 32 S. Chenery, T. Williams, T. A. Elliott, P. L. Forey and salt concentration as found in undiluted digestion solutions L. Werdelin, Mikrochim. Acta, 1996, 13, 259.decreases the performance of the desolvation unit and conse- 33 R. C. Hutton and A. N. Eaton, J. Anal. At. Spectrom., 1988, quently the reduction of polyatomic interferences. This has to 3, 547. 34 M. Totland, I. Jarvis and K. E. Jarvis, Chem. Geol., 1992, 95, 35. be checked for by, e.g., determining the oxide ratio (Ce/CeO) 35 V. Balaram, S. L. Ramesh and K. V. Anjaiah, Fresenius’ J. Anal. during the measurement. Since the sensitivity is higher with Chem., 1995, 353, 176.the MCN6000, it is possible to dilute the digestion samples 36 U. R. Kunze, Grundlagen der quantitativen Analyse, Thieme, five-fold and still obtain the detection limits comparable to Stuttgart, 1990, p. 220. the MCN100. This reduces the blockage of the membrane; 37 F. Panholzer, Spezielle Aufschlußverfahren fu�r anorganische however, a thorough rinsing is recommended after 10–20 Materialien, script Moderne Aufschlußtechnik, Vienna, 1995. 38 J. S. Alvarado, T. Neal, L. L. Smith and M. D. Erickson, Anal. consecutive measurements. Since isobaric interferences cannot Chim. Acta, 1996, 322, 11. be separated even when using high mass resolution, presepara- 39 J. G. Sen Gupta and N. B. Bertrand, Talanta, 1995, 42, 1595. tion of the REE by chromatographic methods is the only 40 G. Stingeder, T. Prohaska, Ch. Latkoczy and W. W. Wenzel, solution, e.g., when undisturbed isotope ratio measurements Proceedings of the Fourth International Conference on the are the point of investigation. Biogeochemistry of Trace Elements, Berkeley, USA, 1997, p. 391. 41 S. Z. Zhang, X. Q. Shan and H. Z. Zhang, At. Spectrosc., 1997, 115In was furthermore shown to be an appropriate isotope 18(5), 140. for the correction for non-spectral interferences for the equip- 42 H. Longerich, Can. J. Spectrom., 1986, 31, 111. ment used. It is highly recommended for the accuracy of 43 S. Augagneur, B. Me�dina, J. Szpunar and R. Lobinski, R., J. Anal. analytical data to test the applicability of an internal standard At. Spectrom., 1996, 11, 713. for all elements of interest with the used ICP mass 44 A. R. Date, Y. Y. Cheung and M. E. Stuart, Spectrochim. Acta, spectrometer, especially if a new matrix is investigated. Part B, 1987, 42, 3. 45 A. R. Date and A. L. Gray, Applications of inductively coupled plasma mass spectrometry, Blackie, New York, 1989. We gratefully acknowledge Alexandra Pederzolli and Gunda 46 K. Shinotsuka and M. Ebihara, Anal. Chim. Acta, 1997, 338, 237. Ko�llensperger for their support. 47 J. J. Thompson and R. S. Houk, Appl. Spectrosc., 1987, 41, 801. 48 H. Falk, R. Geerling, B. Hattendorf, K. Krengel-Rothensee and K. P. Schmidt, Fresenius’ J. Anal. Chem., 1997, 359, 352. References 49 J. A. Olivares and R. S. Houk, Anal. Chem., 1986, 58, 20. 50 I. Rodushkin, T. Ruth and D. Klockare, J. Anal. At. Spectrom., 1 Y. Nakamura, Y. Tsumura, Y. Tonogai, T. Shibata and Y. Ito, 1998, 13, 159. Fund. Appl. Toxicol., 1997, 37(2), 106. 51 F. Vanhaecke, J. Riondato, L. Moens and R. Dams, Fresenius’ 2 S. R. Higsmith and M. R. Head, J. Biol. Chem., 1983, 258, 52. J. Anal. Chem., 1996, 355, 397. 3 H. R. Wen and R. S. Chen, in Metal Ions in Biology and Medicine, 52 K. E. Jarvis and I. Jarvis, Geostandard Newslett., 1988, 12(1), 1. ed. P. Collery, P. Bra�tter, V. N. Bra�tter, L. Khassanova and J. C. Etienne, John Libbey Eurotext, Montrouge, 1998, vol. 5, ch. 5, p. 199. Paper 8/06720A 8 J. Anal. At. Spectrom., 1999, 1

ISSN:0267-9477    

DOI:10.1039/a806720a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Gas dynamics of the ICP-MS interface: impact pressure probe measurements of gas flow profiles Terry N. Olney, Wei Chen and D. J. Douglas* Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T 1Z1 Received 6th August 1998, Accepted 13th November 1998 A versatile ICP-vacuum interface was constructed to investigate the gas dynamics and gas flow profiles of the ICP-MS interface. This new interface combines a low interface pressure,0.24 Torr, with the ability to accommodate a variety of skimmer designs at sampler to skimmer spacings up to 80 mm.Plasma beams were extracted by placing a 0.9 or 2.0 mm diameter skimming orifice at distances of 6.7–47 mm behind the sampling orifice. Gas flow profiles of each resulting plasma beam were measured using an impact pressure probe placed at distances of 35–105 mm behind the skimmer. The centreline flux and width of the gas beam were compared with those calculated for an ideally skimmed beam.The results for the 0.9 mm diameter skimmer orifice showed that placing the skimmer at 17.0 and 27.3 mm downstream of the sampler formed the highest intensity and narrowest beams. In contrast, by placing the skimmer closer to the sampler, as in common interface designs, the beam profile is less intense on the centreline and much wider. Using a larger 2.0 mm diameter skimming orifice at 16.5–26.8 mm downstream of the sampler produced a more intense beam than any arrangement using the 0.9 mm skimming orifice.However with the 2.0 mm diameter skimmer, centreline intensities were still about half those of an ideally skimmed beam. These eVects are consistent with the formation of a shock Introduction wave or some other disturbance at the skimmer tip. In the The ion sampling interface that is used on nearly all inductively most extreme case the supersonic expansion stops, a shock coupled plasma mass spectrometer (ICP-MS) systems was first wave forms across the skimmer tip and there is a secondary described by Douglas and French1 for use with a microwave expansion through the skimmer.It is also possible that shock induced plasma. The design is based on molecular beam waves can form inside the skimmer, partially scattering and techniques developed by Campargue.2–6 In this interface, the disrupting an otherwise ideal beam. In this case an attenuated plasma expands from atmospheric pressure into a region at a beam can be formed.pressure of several Torr. A molecular beam skimmer is placed The ion extraction process at the skimmer base is ineYcient. in the free jet expansion and the centreline flow passes through In part, this has been attributed to space charge eVects in the skimmer into a region at a pressure of about 10-4 Torr. which mutual charge repulsion causes the ion beam to ‘blow At this pressure electric fields from the first ion optic elements up’.8b,c,13 However, if a molecular beam does not pass cleanly extract ions from the rarefied plasma. The theory of the gas through the skimmer tip, there will be an additional contridynamics of this interface has been described in detail.7–9 bution to the beam ‘blow-up’.The local density of gas within Formation of a molecular beam was considered desirable the skimmer will be higher and this will lead to increased because the gas would then flow through the skimmer undis- scattering of ions in the extraction process.Thus, if a true turbed. In this case there is a minimum number of collisions beam can be formed it may help improve the eYciency of the of ions and neutrals in the expanding gas and hence the least ion extraction process. Further, Tanner et al.14 have described chance for reaction or recombination. The gas flow down- a three aperture gas dynamic interface that relies on formation stream of the skimmer behaves as though it comes from a of a molecular beam through the skimmer.In this interface, point source very close to the sampling orifice and the density downstream of the skimmer, the beam hits a flat plate where in the beam decreases as 1/x2, where x is the distance from a shock wave is formed. In this shock wave the directed gas the sampler. flow stops and the gas stagnates at a temperature near that of Recent results10–12 suggest that in common ICP-MS the ICP. However, the density in the shock wave is orders of interfaces a molecular beam is not being formed by the magnitude lower than in the atmospheric pressure source.Ions skimmer. Niu and Houk10 measured electron densities in the can then be extracted through a small orifice in the plate under interface with a Langmuir probe. They found that in the near collisionless conditions. The total ion current is reduced region between the sampler and skimmer the electron density but, because there are no collisions in the ion extraction decreased as 1/x2, as expected for a free jet.However, down- region, scattering is eliminated and a brighter ion source can stream of the skimmer the centreline electron density decreased be formed. Ying and Douglas15 used this approach for highmuch more rapidly than that of an ideally skimmed beam. resolution quadrupole ICP-MS where the brightest possible Also, laser induced fluorescence measurements11 have shown ion source is required. Tanner et al.14 found that measured a broader distribution of ions and neutrals than expected for ion intensities from this source were less than expected from a skimmed beam.Preliminary impact pressure probe measure- an ideal beam forming a shock wave on the plate. This again ments have shown lower gas densities and a wider gas flow can be attributed to formation of an attenuated or perturbed beam with a conventional sampler–skimmer arrangement. profile than expected for a beam behind the skimmer.12 J. Anal. At. Spectrom., 1999, 14, 9–17 9In the ICP interface the plasma expansion from atmospheric The probe was moved across the gas flow at diVerent distances from the sampler and skimmer.Calibration of the probe with pressure through the sampler forms a free jet with supersonic flow, terminating at the Mach disk.8 The distance (xM) from a free jet showed that the measured pressure was linearly proportional to the gas flux into the probe within a few the sampler to the Mach disk is given by per cent.The gas flow profiles were measured for an interface xM Do =0.67SP0 P1 (1) arrangement with a first stage pressure, P1, of 3.3 Torr and a sampler–skimmer spacing of 6.4 mm. This interface is similar where Do is the sampling orifice diameter and P0 and P1 are to that used on commercial ICP-MS systems and is referred the source and interface background pressures, respectively to here as a ‘conventional’ interface. Furthermore, we con- (see Table 1 for a summary of symbol definitions).In most structed a new interface in which the pump speed on the current ICP-MS interfaces the pump speed on the region region behind the sampler has been increased to about 110 l s-1 behind the sampler is about 5–15 l s-1. This gives a back- in order to lower the interface pressure, P1, to 0.24 Torr. This ground pressure, P1, of about 2–5 Torr, which, in the absence causes the Mach disk to move to 37.7Do (approximately of a skimmer, gives rise to a Mach disk at a distance 8–13Do 42 mm with a 1.12 mm diameter sampler) downstream of the downstream of the sampler (where Do is often about 1 mm).sampler. Hence, the skimmer can be placed much further For beam formation the skimmer must be placed in the downstream from the sampler while remaining inside the free directed flow upstream of the Mach disk. The relatively small jet. Measurements of gas flow profiles under these conditions sampler–skimmer separation (6–12 mm) in standard ICP allowed us to investigate the eVects of moving the skimmer interfaces means that the gas density is relatively high at the further away from the sampler into lower gas density regions skimmer tip and this may give rise to a disturbance in front of the free jet. In addition, a skimmer with a larger diameter of the skimmer.Furthermore, the close proximity of the Mach orifice could be placed in the region of lower gas density in disk to the sampler leaves little room to explore the eVect of the free jet and still produce a suYciently low gas flow such moving the skimmer to diVerent positions in the free jet.that high vacuum (about 10-4 Torr) could be maintained In this work we measured gas flow profiles downstream of behind the skimmer with a modest pump speed. For comparia skimmer using an impact pressure probe. The probe consisted son, flow profiles were measured with the skimmer replaced of a small 0.254 mm diameter orifice in a flat plate at the end by an orifice in a flat plate.Under these conditions no beam of a 6.35 mm od tube that was connected to a pressure gauge. is expected. The gas flow profiles were characterised by (i) their absolute Table 1 Definitions of symbols centreline flux compared with that expected for an ideal beam and (ii) their measured full width at half maximum compared Symbol Units Description with the geometric width of an ideal beam. It was necessary to correct the measured impact pressure for scattering between x mm Distance downstream from the source the skimmer and gas probe to obtain a true measure of the xM mm Distance of the Mach disk downstream from the source beam intensity produced by the skimmer. The results show xs mm Sampler–skimmer separation that the skimming of the free jet using a small diameter xp mm Sampler–impact probe separation on the skimmer (0.9 mm) produces a highly perturbed beam for all centreline sampler–skimmer separations (xs).This eVect is particularly xsk–p mm Skimmer–impact probe separation on the pronounced for separations commonly used in conventional centreline interface designs (xs12 mm).In contrast, the larger diameter Do mm Sampler orifice diameter Ds mm Skimmer orifice diameter skimmer (2.0 mm) produces beams much closer to ideal for P0 Torr Source pressure 16.5 mmxs<xM mm. However, even with the 2.0 mm diam- P1 Torr Interface pressure eter skimmer orifice the centreline intensity of the beam from Pi Torr Impact probe pressure the skimmer is about half that of an ideally skimmed beam.PMax Torr Impact probe pressure on the centreline G1 s-1 Gas flow out of the probe Gx s-1 Gas flow through an orifice a distance x from Experimental the source k erg K-1 Boltzmann constant The ICP and extraction interface used in this work, shown in Kn — Knudsen number Fig. 1, were constructed at the University of British Columbia. n cm-3 Number density The ICP was formed using a centre tapped load coil, quartz n0 cm-3 Gas number density in the source torch (SCP Science, Montreal, PQ, Canada Cat.No. 020– nx cm-3 Gas number density at a distance x from the 050–0710) and Meinhard nebulizer (Meinhard Associates, source v: cm s-1 Average thermal speed Santa Ana, CA, USA), and was operated at 27.7 MHz and vx cm s-1 Flow velocity at a distance x from the source 1000 W. The plasma, auxiliary and nebulizer argon gas flow A cm2 Orifice area rates were 15, 1 and 0 l min-1, respectively.Gas flow profiles m g Molecular or atomic mass were initially recorded using nebulizer flow rates of 0 and Trs — Skimmer transmission 1.0 l min-1. No significant diVerences in the flow profiles were T0 K Source temperature found. Therefore, a nebulizer gas flow rate of 0 l min-1 was Tp K Probe temperature FWHM mm Full width measured at half the maximum used for all the gas flow profile measurements. The plasma peak height expanded into vacuum through a sampler and skimmer.The FWHMG mm Full width of a geometric projection of a sampler, which had a 1.12 mm diameter aperture, was mounted point source at the sample through the in a water cooled plate. Skimmers with orifice diameters of skimmer 0.9 and 2.0 mm, termed the ‘small’ and ‘large’ diameter r mm Distance from source to impact probe skimmers, respectively, were used. The height and the internal r0 mm Free jet source orifice radius w Polar angle between the source and pressure and external half angles of the small diameter skimmer were probe 19.1 mm, 25.8° and 30.3°, respectively, and those of the large l(xs) mm Mean free path at the skimmer tip diameter skimmer were 37.1 mm, 30.3° and 34.3°, respectively.zp mm Distance from the centreline to the pressure The skimmer was mounted in a water cooled flange which probe was positioned on spacer rings such that the distance between 10 J. Anal. At. Spectrom., 1999, 14, 9–17rate given by G1= 1 4 nv:A (4) where n is the number density in the probe and v: is the average thermal speed of the gas in the probe, given by v: =A8kTp pm B1/2 (5) where Tp is the probe temperature.At the steady state, G1=Gx so that 1 4 nv:A=nxvxA (6) and so Fig. 1 Schematic diagram of the interface and impact pressure probe. n=nxvxA4 v: B (7) Ip, moveable impact pressure probe; Sa, sampler; Sk, skimmer; Sm, skimmer mount; Sp, spacers. Typically, the probe reached a steady state pressure within 1 min of being placed in the gas flow.The number density in the probe, and hence the pressure, are proportional to the gas flux in the beam (nxvx). Because vx is essentially constant for the sampler and skimmer (xs) was varied from 6.7 to 47 mm x>5Do,8 the probe pressure is proportional to the gas density with the small diameter skimmer and from 11.5 to 36.8 mm in the beam. Substituting vx from Eqn. (3) and v: from Eqn. (5) with the large diameter skimmer. In addition, a flat plate with gives a 2.0 mm diameter orifice was used in place of the skimmer to model a system which is known not to produce a molecular beam when placed inside of a free jet. The plate was placed between 26.7 and 47.0 mm from the sampler.n nx = vx v: 4 =A5kT0/m 8kTp 16pm B1/2 =A10p T0 TpB1/2 (8) The region between the sampler and skimmer was pumped by a mechanical booster or roots pump (Leybold WSU 501, 168 l s-1; backing rotary pump, Leybold D40B, 13.3 l s-1; Taking T0=Tp gives n/nx=5.605 and taking T0=5000 K and Leybold Vacuum Products, Export, PA, USA) to a pressure Tp=295 K gives n/nx=23.07.of 0.24 Torr, measured with a capacitance manometer Each gas flow profile was obtained from pressure (Baratron Model 122AA, MKS. Instruments, Andover, MA, measurements made at several points as the probe was scanned USA). The eVective pump speed at the interface was 110 l s-1, radially across the gas beam at a fixed centreline distance limited by the conductance of the line to the pump.The region behind the sampler (xp) and skimmer (xsk–p). The gas flow downstream of the skimmer was evacuated with a 345 l s-1 profiles were recorded at several sampler–probe distances, xp, turbomolecular pump (Leybold, Turbovac 361). The backfor each skimmer position xs (small diameter skimmer, xs= ground pressure, which was dependent on the skimmer diam- 6.7, 12.0, 17.0, 27.3, 37.3, 47.0 mm; large diameter skimmer; eter and the sampler–skimmer separation, was measured with xs=11.5, 16.5, 26.8 and 36.8 mm).In addition, gas flow an ionisation gauge (Model 0571, Varian Vacuum Products, profiles were obtained using a flat plate (2.0 mm diameter Lexington, MA, USA). Background pressures varied between orifice) at distances from the sampler of 26.7, 37.0 and 5.0×10-5 and 1.4×10-3 Torr. 47.0 mm. An impact pressure probe tip is shown in the inset of Fig. 1. The flow out of the probe is modelled as eVusive [Eqn.(4)]. The probes had a 0.254 mm diameter orifice in a 0.051 mm This requires that the mean free path, l, within the probe be thick plate mounted on the end of a 6.35 mm od stainless steel substantially greater than the orifice diameter. In measure- tube. The tube was connected to a 0.00–1.00 Torr capacitance ments of gas flow profiles from the interface, the highest probe manometer (Baratron Model 120A, MKS Instruments; manupressure encountered was 30 mTorr. For an assumed, but facturer’s stated accuracy±0.12% of reading). To change the reasonable, collision cross-section for Ar of 50 A° 2, the smallest sampler–probe distance, tubes of diVerent length were used.mean free path is 0.20 cm, which is about eight times greater For an impact probe placed in a free jet at a distance x from than the orifice diameter. Hence the probe remained under the source, the directed gas streams into the probe at a flow eVusive flow conditions in this work. rate Gx given by To verify the calibration of the pressure probes, the ICP Gx=nxvxA (2) and interface were replaced with the flow system of Fig. 2. The pressure of Ar behind a 0.313 mm diameter orifice in a where A is the area of the probe orifice and nx and vx are the 0.051 mm thick plate was controlled by a needle valve. The number density and gas velocity, respectively, in front of the pressure was measured with a 0–100 Torr capacitance man- orifice. The gas velocity of Ar expanding from the source is ometer (Baratron Model 122AA, MKS Instruments; manufac- given by the terminal speed of the expansion: turer’s stated accuracy±0.5% of reading).Argon expanding through the orifice produced a free jet with well defined vx=A5kT0 m B1/2 (3) centreline and angular density distributions. The density on the centreline, nx, at a distance x from the orifice is given by where k is Boltzmann’s constant, T0 is the source temperature (approximately 5000 K in the ICP) and m is the mass of argon.nx n0 =0.161 ADo x B2 (9) Gas can leave the probe and pressure gauge only by flowing back out through the orifice. Provided that the probe pressure is suYciently low, gas leaves the probe in eVusive flow with a where n0 is the density behind the orifice with diameter Do. J. Anal. At. Spectrom., 1999, 14, 9–17 11Fig. 2 Flow apparatus used to calibrate the impact pressure probe. Fig. 4 Free jet profile. Filled circles show the profile of a free jet Ip, impact pressure probe; Nv, needlevalve; So, source orifice; Sp, formed from 101.4 Torr of Ar expanding through a 0.313 mm source spacers. orifice and measured using an impact probe with a 0.254 mm orifice at a distance of 13.0 mm from the source.The dotted line represents the free jet density profile calculated from Eqn. (10). OV the centreline the density is given by16 n(r,w) n0 =B cos2 Apw 2CBAr r0B-2 (10) increases over that of Eqn. (4). Hence, for a given flux into the probe, a lower steady state pressure is reached. Data sets recorded under the same conditions on two diVerent days where w is the polar angle between the centre axis and the (shown on Fig. 3 as open circles and filled squares and as point of interest, r0 is the orifice radius, r is the distance of open inverted triangles and open squares) agreed within 2%. the probe from the source (r2=xP2+zp2, where zp is the The solid line in Fig. 3 is the calculated impact pressure, which distance from the probe to the centreline) and B and C are has a slope of 3.09×10-21 Torr cm2 s.This is within 6% of constants, 0.643 and 1.365, respectively, for Ar. the linear regression of all the combined data sets shown in Fig. 3, which gives a slope of 2.92×10-21 Torr cm2 s. This Results agreement is considered more than adequate for this work. Fig. 4 shows the free jet gas flow profile obtained from Probe calibration impact pressure measurements made at several points as the Fig. 3 shows the measured impact pressure versus the impact probe was scanned radially across the gas beam at a corresponding calculated flux on the centreline of the free jet. fixed centreline distance (xp=13.0 mm) behind the free jet This calibration curve is linear for gas fluxes less than orifice. This profile is compared in Fig. 4 with the density field 1×1019 cm-2 s-1 and the measured free jet flux density agrees of Eqn. (10). In Fig. 4 the density field of Eqn. (10) was with the calculated free jet flux density [Eqns.(8) and (9)] to multiplied by an additional factor cosw to account for the within 2%. For higher fluxes, which give probe pressures decrease in the apparent area of the probe (solid angle) viewed greater than 30 mTorr, the measured pressures are slightly less from the source orifice as the probe moves oV-axis. When the than expected from extrapolation of the linear low pressure probe is moved oV-axis there is a decrease in the apparent data.This may be caused by a slight deviation from purely area because the probe orifice remains perpendicular to the eVusive flow from the probe. As the flow moves from purely centreline. The agreement between the measured profile and eVusive to transition flow (mean free path comparable to the calculated profile is good. The measured FWHM is 0.93 orifice diameter), the flow out of the probe for a given pressure of the FWHM calculated from Eqn. (10). The small diVerence may be due to edge eVects of the probe.As the impact probe is moved oV-axis the finite thickness of the orifice plate can cause scattering of the incoming gas flow so that the flux into the probe is no longer simply described by Eqn. (2). Measurements of the free jet profile were also carried out using tube probes. These were stainless steel tubes (id 2.3 mm, od 3.0 mm) of various lengths (to give diVerent sampler–probe separations). The measured gas flow profiles and centreline impact pressures of the interface were qualitatively similar to the orifice impact probe results.However, after calibration with a free jet it was found that the tube probe response was not linear with the calculated centreline flux of the free jet. Also, the gas flow profile of the free jet measured with the tube probe was much narrower than the profiles either recorded using the orifice impact probe or calculated from Eqn. (10). The gas flow into and out of a tube is complex and cannot be described simply by Eqns.(2) and (4).17 Conventional interface Fig. 3 Measured impact pressure versus calculated free jet flux density. Shown are six individual data sets for sampler orifice–probe separa- Radial gas flow profiles were recorded downstream of the tions of 13.0–58.8 mm. Linear regression (not shown) of a composite small diameter skimmer in an interface typical of many of all the data produced a slope of 2.92×10-21 Torr cm2 s. The solid line represents the impact pressure calculated from Eqns.(8) and (9). ICP-MS instruments. This interface, which employed a 12 J. Anal. At. Spectrom., 1999, 14, 9–17Table 2 Results from gas flow profiles produced using a conventional interface (0.9 mm diameter orifice) xs/mm xp/mm FWHM/FWHMG Centreline/ideal pressure 6.4 41.0 2.8 0.062 51.0 2.8 0.053 71.0 2.8 0.041 112.0 2.8 0.030 sampler, the centreline gas flux of an ideally skimmed beam is the same as the intensity of the free jet measured at the same distance xp from the source.At xp=41.0, 51.0, 71.0 and 112.0 mm, the centreline impact pressures, Pi, measured using the conventional interface were 0.062, 0.053, 0.041 and 0.030 times, respectively, that expected from an ideally skimmed beam, Pideal (Table 2). The ratio of the measured centreline pressure to the pressure of an ideally skimmed beam shows a systematic decrease as xp increases. This decrease is Fig. 5 Gas flow profiles produced from a conventional ICP-MS attributed to scattering of the gas beam between the skimmer interface.Do=1.12 mm; DS=0.9 mm; xs=6.4 mm; P1=3.3 Torr. The and probe, which attenuates the gas flow and causes the probe horizontal lines indicate the FWHM for each curve. pressure to decrease. Scattering is expected to attenuate the beam by an additional factor exp (-nsxsk–p), where n is the 1.12 mm diameter sampler separated by 6.4 mm from a 0.9 mm number density of the background gas and s is the collision diameter skimmer, was evacuated to a pressure of 3.3 Torr cross-section.To determine the intensity of the unscattered using a rotary pump (Leybold S25B, 8.5 l s-1). The resulting beam at the skimmer tip a plot of log (Pi/Pideal) versus xp was flow profiles, recorded at four distances behind the sampler extrapolated back to xp=xs to estimate the ratio that would (xp), are shown in Fig. 5. The full width measured at half of be observed at the skimmer tip without scattering.For the the maximum height (FWHM) of each experimental gas flow data of the conventional interface, this is shown in Fig. 7. In profile is indicated on Fig. 5. In Fig. 6, the experimental this case at xs=xp=6.4 mm the ratio is 0.0836. That is, the FWHMs are compared with the width of a geometric projecskimmer transmits a beam with an intensity 8.36% that of an tion of a point source, through the skimmer, to the probe ideal skimmer. We refer to this as the ‘skimmer transmission,’ position. This width, FWHMG, is given by Trs.The same procedure was used to correct for scattering and to calculate the skimmer transmission for both skimmers FWHMG=Ds Axp xsB (11) and the flat plate at all positions, xs, of the new interface. where Ds is the skimmer orifice diameter. The measured Small diameter skimmer FWHMs were 2.8 times the widths based on the geometric The small diameter skimmer used in the conventional interface projection. The laser-induced fluorescence measurements of was then mounted on the apparatus of Fig. 1 with an interface Duersch et al.11 showed beam widths about twice those pressure of 0.24 Torr. Gas flow profiles were recorded at expected for a skimmed beam. several skimmer–probe separations (xsk–p#35, 45, 65 and 105 mm) for four sampler–skimmer positions (xs=6.7, 17.0, Skimmer transmission 27.3 and 37.3 mm). Additional profiles were recorded at xsk–p= Under conditions of ideal skimming, the centreline intensity 39.0, 49.5, 66.5 and 109.5 mm for xs=12.0 mm and at xsk–p= of the beam is unaVected by the presence of the skimmer.The 34.5 mm for xs=47.0 mm. Typical gas flow profiles recorded gas flows undisturbed through the skimmer and no Mach disk at xs=17.0 mm are shown in Fig. 8 and the FWHM/FWHMG is formed on the centreline. Thus, at a distance xp from the and centreline Pi/Pideal ratios for all of the above experiments are summarised in Table 3. Fig. 7 The measured centreline pressure with the conventional Fig. 6 The FWHM versus impact probe distance behind the sampler interface divided by the pressure for an ideally skimmed beam for diVerent probe positions xp. The straight line is an exponential fit to from the data in Fig. 5. Also shown are the widths (FWHMG) of a geometric projection from the sampler, through the skimmer, to the the data. The intercept at xp=6.4 mm gives the skimmer transmission Trs=0.083. impact probe position. J. Anal. At.Spectrom., 1999, 14, 9–17 13were about 1.6–1.7 times the widths of a geometric projection and the centreline pressures were 0.194–0.128 times the pressures of an ideally skimmed beam. The skimmer transmission increased to Trs=0.23. A comparison of the gas flow profiles obtained at xp=52 mm for various values of xs showed that the FWHM decreased from 38.3 to 6.7 to 4.3 mm as xs was increased from 6.7 to 12.0 to 17.0 mm, respectively. In addition, the centreline probe pressure was observed to increase from 3.4 to 7.1 to 13.6 mTorr as xs was increased from 6.7 to 12.0 to 17.0 mm, respectively.Thus, as the skimmer is pulled back from 6.7 to 17.0 mm behind the sampler, the gas flow profile becomes 8.8 times narrower and the centreline intensity becomes 4.0 times greater. At xs=27.3 mm the gas flow profile FWHM were even narrower, typically 1.3–1.7 times the geometric projected widths. In addition, the measured centreline pressures were about 0.19–0.087 times that of an ideally skimmed beam.The skimmer transmission was Trs=0.30. At a common xp of Fig. 8 Gas flow profiles using the small diameter skimmer. Do= 61.5 mm the FWHM for xs=27.3 mm was narrower than that 1.12 mm; DS=0.9 mm; xs=17.0 mm; P1=0.24 Torr. The horizontal for 17.0 mm, as expected, and the centreline intensities were lines indicate the FWHM for each curve. comparable. Increasing the sampler–skimmer separation to xs=37.3 mm Table 3 Results from gas flow profiles produced using the small produced gas flow profiles with FWHM of 1.4–1.7 times that skimmer (0.9 mm diameter orifice) of the geometric projected width and measured centreline pressures of 0.084–0.047 times the intensities of an ideally xs/mm xp/mm FWHM/FWHMG Centreline/ideal intensity skimmed beam.The skimmer transmission decreased to Trs= 6.7 41.3 5.33 0.051 0.093. When compared with skimmer positions of xs=17.0 52.1 5.47 0.043 and 27.3 mm, at a common distance downstream of the 71.5 4.37 0.039 sampler, the beam profiles were narrower, as expected, but the 111.5 4.00 0.019 centreline intensities dropped to half of the intensity of the 12.0 51.5 1.79 0.090 profiles of the smaller sampler–skimmer separations of 17.0 61.5 1.99 0.085 and 27.3 mm. 78.5 1.83 0.084 121.5 1.85 0.071 When the skimmer was placed downstream of the Mach 17.0 52.1 1.56 0.194 disk, at xs=47.0 mm (recall xM=42 mm), the FWHM again 61.5 1.72 0.177 became twice the width of a geometric projection.Also, the 81.5 1.71 0.155 centreline intensity suddenly dropped to 0.02 times that of an 121.5 1.74 0.128 ideally skimmed beam in the absence of the Mach disk. At a 27.3 61.5 1.33 0.190 common xp of 81.5 mm, when the skimmer was placed down- 71.5 1.44 0.187 91.5 1.52 0.157 stream of the Mach disk, the centreline intensity dropped to 131.5 1.75 0.087 0.25 times the intensity when xs=36.8 mm and 0.12 times the 37.3 71.5 1.39 0.084 intensity when xs=17.0 mm. 81.5 1.65 0.061 101.5 1.55 0.056 141.5 1.99 0.047 Large diameter skimmer 47.0 81.5 1.92 0.020 The small skimmer was replaced with the large diameter skimmer having a 2.0 mm orifice and the interface pressure was maintained at 0.24 Torr. Gas flow profiles were recorded The gas flow profiles recorded for xs=6.7 mm were broader than the profiles obtained using the conventional interface at xsk–p#35, 45, 65 and 105 mm for skimmer position xs= 16.5, 26.8 and 36.8 mm and at xsk–p=41.0, 50.0, 67.0 and arrangement. The FWHM were about 4–5.5 times larger than the geometric projected widths.The centreline pressures for 110.0 mm for xs=11.5 mm. Typical gas flow profiles recorded at xs=16.5 mm are shown in Fig. 9 and the results are xs=6.7 mm at xp=41.3, 52.1, 71.5 and 111.5 mm were 0.051, 0.043, 0.039 and 0.019 times, respectively, the pressures of an summarised in Table 4. At a sampler–skimmer separation of 11.5 mm the gas flow ideally skimmed beam. The skimmer transmission was 0.0825, very similar to that of the conventional interface at xs= profiles had FWHM which were typically only 1.1–1.2 times the width obtained from a geometric projection.The measured 6.4 mm (T=0.0836). The measured FWHM and centreline intensities were about 1.7 and 0.8 times, respectively, those of centreline pressures were 0.278–0.161 times the pressures of an ideally skimmed beam. The skimmer transmission was 0.39. the conventional interface. As the skimmer was moved back to a position of xs= These data can be compared with the profiles produced by the small skimmer at xs=12.0 mm where the measured FWHM 12.0 mm, the FWHM became much narrower than at xs= 6.7 mm, but were still about 1.8–2.0 times the widths based were about 1.8–2.0 times the geometric projection and the skimmer transmission was 0.102.on a geometric projection. The centreline pressures for xs= 12.0 mm were 0.090–0.071 times the intensity of an ideally The gas flow profiles measured for xs=16.5 mm had FWHM which were 1.0–1.1 times the widths of a geometric projection skimmed beam.The skimmer transmission was 0.102. At a common xp of about 52 mm, comparison of the gas flow and had measured centreline pressures which were about 0.365–0.233 times the intensity calculated for an ideally profiles at xs=12.0 mm and 6.7 mm shows that the FWHM becomes 5.5 times narrower and the centreline intensity skimmed beam. The skimmer transmission increased to 0.536.These centreline pressures were about 1.4–1.5 times the press- becomes 2.1 times greater as the skimmer is moved further from the sampler. ures obtained at xs=11.5 mm for the same values of xp. For xs=17.0 mm the centreline impact pressure at xp=51.5 mm At a sampler–skimmer spacing of 17.0 mm the FWHM 14 J. Anal. At. Spectrom., 1999, 14, 9–17Table 5 Results from gas flow profiles produced using the flat plate (2.0 mm diameter orifice) xs/mm xp/mm FWHM/FWHMG Measured/ideal intensity 26.7 61.0 7.77 0.016 71.5 7.60 0.017 91.5 7.22 0.013 132.5 7.56 0.009 37.0 71.5 6.10 0.016 81.5 7.03 0.014 101.5 7.11 0.008 141.5 0.007 47.0 81.5 8.50 0.016 91.5 11.40 0.013 111.5 13.60 0.009 149.0 0.005 Flat plate Fig. 9 Gas flow profiles using the large diameter skimmer. Do= Finally, flow profiles at xsk–p#35, 45, 65 and 105 mm were 1.12 mm; DS=2.0 mm; xs=16.5 mm; P1=0.24 Torr. The horizontal lines indicate the FWHM for each curve. obtained using a flat plate with a 2.0 mm diameter orifice for each sampler–plate separation of 26.7, 37.0 and 47.0 mm.The results are summarised in Table 5. At xs=26.7 mm the gas flow profiles had centreline intensities of 0.013–0.017 times Table 4 Results from gas flow profiles produced using the large that of an ideally skimmed beam. The flat plate transmission skimmer (2.0 mm diameter orifice) was 0.024. This can be compared with the transmission of the xs/mm xp/mm FWHM/FWHMG Measured/ideal intensity small and large skimmers of about 0.3 and 0.5, respectively, at similar xs.The FWHM of the profiles produced using the flat 11.5 52.5 1.12 0.278 plate were 7.2–7.8 times the widths from a geometric 61.5 1.10 0.262 projection. 78.5 1.20 0.240 Moving the flat plate back to a position of xs=37.0 mm 121.5 1.38 0.161 produced gas flow profiles similar to those obtained at xs= 16.5 51.5 1.02 0.415 61.5 1.06 0.365 26.7 mm. The intensities ranged from 0.016–0.007 times that 79.0 1.13 0.326 of an ideally skimmed beam and the transmission was 0.022. 121.5 1.18 0.233 The FWHM, which were 6.1–7.1 times the widths from a 26.8 61.5 1.07 0.354 geometric projection, were marginally smaller than the profiles 71.5 1.07 0.350 produced using the flat plate at xs=26.7 mm. 92.0 1.06 0.327 When the flat plate was placed behind the Mach disk, 131.5 1.18 0.230 36.8 71.5 1.13 0.164 47.0 mm from the sampler, the centreline intensities remained 81.5 1.10 0.134 at about 0.013–0.017 times that of an ideally skimmed 101.5 1.21 0.127 beam.The transmission was found to be Trs=0.030. The 141.0 1.24 0.104 FWHM of the profiles obtained at xp=81.5, 91.5 and 149.5 mmwere 8.5, 11.4 and 13.6 times, respectively, the widths from a geometric projection. The impact pressure at xp= 81.5 mm was 0.51 mTorr, which is similar to the intensity of was 33 mTorr, which is much greater than the centreline 0.48 mTorr obtained by placing the small skimmer behind the impact pressures of 3.4, 7.1 and 13.6 mTorr produced at the Mach disk at xs=47.0 mm.same xp using the small skimmer at xs=6.7, 12.0 and 17.0 mm, respectively. Discussion At a sampler–skimmer separation of 26.8 mm the FWHM of the gas flow profiles were only about 1.1 times the widths The skimmer transmissions for all cases are given in Table 6. of a geometric projection. The centreline pressures were The wide, low intensity, gas flow profiles associated with the 0.354–0.230 times those of an ideally skimmed beam and were conventional interface design show that a highly perturbed approximately equal to those at the same xp for an xs of 17.0 mm.The skimmer transmission was 0.462. At xp= Table 6 Skimmer transmission and inverse Knudsen numbers 61.5 mm the centreline pressure of the gas flow profile using the large skimmer for xs=26.8 mm was 3.4, 2.0 and 2.2 times Interface xs/mm Trs Kn-1 larger than the pressures with the small skimmer for xs=12.0, 17.0 and 27.3 mm, respectively.Conventional 6.4 0.0836 5.47 Small diameter skimmer 6.7 0.0825 5.09 Moving the skimmer further from the sampler, to xs= 12.0 0.102 2.05 36.8 mm, produced gas flow profiles which had FWHM about 17.0 0.230 1.20 1.1–1.2 times larger than the widths from a geometric projec- 27.3 0.304 0.58 tion and centreline intensities 0.164–0.104 times those from 37.3 0.083 0.36 an ideally skimmed beam. The skimmer transmission was Large diameter skimmer 11.5 0.393 4.57 Trs=0.187.The centreline intensities for xs=36.8 mm, at 16.5 0.536 2.66 26.8 0.462 1.29 common values of xp, were only about 0.35 and 0.45 times 36.8 0.187 0.79 those at xs=17.0 and 26.8 mm, respectively. However, the Flat plate 26.7 0.024 1.31 intensities were more than twice the centreline intensities 37.0 0.022 0.79 produced at common xp using the small skimmer at nearly 47.0 0.031 0.55 the same position (xs=37.3 mm). J. Anal. At. Spectrom., 1999, 14, 9–17 15Beijerinck et al.7 have assessed the transmission probability of an expansion through a skimmer (experimental/calculated flow through skimmer) for a Campargue type source as a function of the inverse Knudsen number (Kn-1).The Knudsen number is given by Kn= l(xs) Ds (12) where l(xs) is the mean free path at the skimmer tip. They found that for a skimmer with a 0.5 mm diameter orifice the highest transmission (about 100%) was observed at the lowest inverse Knudsen numbers (Kn-1#1) and that the transmission decreased exponentially with increasing Kn-1 from about 0.8 at Kn-1=2 to about 0.1 at Kn-1=6. The Kn-1 calculated as in ref. 7, for the operating conditions used in this work, are given in Table 6. The transmissions found for the small skim- Fig. 10 Skimmer transmissions for the small (0.9 mm) and large mer show a decrease from Kn-1=5 to 0.6, i.e., for xs from (2.0 mm) skimmers and the flat plate (2.0 mm) at diVerent sam- 6.7 to 27.0 mm, in qualitative agreement with the data of pler–skimmer spacings, xs.The open circle shows the transmission Beijerinck et al.7 At larger xs (i.e., Kn-1< 0.5) background for a conventional interface. xs=6.4 mm; Ds=0.9 mm. penetration due to the encroaching Mach disk causes the transmission to decrease below the expected transmission beam is formed from this type of interface arrangement. Even probability based on Kn-1 calculated for a free jet. For the with a lower interface pressure (i.e., a higher speed pump) a large diameter skimmer the calculated Kn-1 at xs from 11.5 small sampler–skimmer separation produces a highly per- to 26.8 mm are 4.6–1.3.The large skimmer transmissions are turbed beam with essentially the same skimmer transmission. also similar to those reported by Beijerinck et al.7 for the same At larger sampler–skimmer separations and lower interface Kn-1. However, in this work, the small diameter and large pressures the FWHM and centreline pressure measurements diameter skimmers have diVerent transmissions for the same show that the beam quality improves significantly over the Kn-1.Apparently, for the ICP source the transmission depends conventional interface arrangement. Fig. 10 shows the skimmer not just on the inverse Knudsen number but also on other transmission versus xs. For the small skimmer the transmission parameters such as edge eVects at the skimmer. increases as xs is increased from 6.7 to 27 mm, although the For sampler–skimmer positions less than the optimum postransmission is still substantially lower than 1.From this we ition the beam is probably attenuated by shock formation in conclude that the beam formed using the small diameter front of the skimmer tip. For sampler–skimmer positions skimmer is less than ideal at all xs. greater than the optimum position the beam intensity should The FWHM, centreline pressures and skimmer remain unchanged. However, attenuation of the skimmed transmissions measured for beams produced using the large beam will occur as the skimmer approaches the Mach disk.diameter skimmer are generally much closer to those expected The distance of closest approach of the skimmer to the Mach from an ideally skimmed beam. The transmission reaches disk will depend on the finite thickness of the Mach disk and about 0.5 at xs=16.5 and 26.8 mm. At xs=36.8 mm the on the height of the skimmer cone.7,18 When the skimmer is skimmer transmission drops to 0.187.At this xs, the close too close to the Mach disk, penetration of the background proximity of the skimmer to the Mach disk may give rise to gas into the free jet results in scattering of the beam and loss penetration of the background gas into the free jet, leading to in intensity of the skimmed beam. increased beam scattering in front of the skimmer.4,5,7 Photographs of the shock wave around a skimmer in a free Conclusion jet can be found in ref. 18. The flat plate corresponds to a situation where the supersonic The most common interface design produces poor quality gas beams in the ion extraction region. The poor quality arises flow of the free jet forms a shock wave in front of the orifice and this arrangement is known not to form a beam. At xs= from a combination of a high gas density at the skimmer tip and a small sampling orifice. Increasing the distance between 27 mm the centreline pressures were 10% of the pressures with the small diameter skimmer.This demonstrates that although the sampler and skimmer decreases the degree of scattering, but does not completely alleviate the problem because further the beam is perturbed with the small diameter skimmer, some beam-like qualities remain. For example, ion kinetic energies scattering occurs due to the small size of the orifice. The data here show that using a larger diameter skimmer placed at a match those expected from a skimmed beam.8,9,19 It is apparent that a disturbed beam is formed using the small skimming sampler–skimmer spacing greater than most conventional instruments employ eliminates the problem.This is only poss- orifice and modelling the gas flow downstream of the skimmer based on a simple extrapolation of the free jet flow upstream ible by lowering the interface pressure by about an order of magnitude. The resulting beam appears to be closer to the of the skimmer is not possible. The centreline gas flux increased as the small skimmer was moved from xs=6.7 mm to 17.0 mm conditions expected from the ideal skimming of a free jet.In contrast to Langmuir probe and laser induced because the density at the skimmer tip was being lowered. Any disturbance at the skimmer tip, such as shock waves or fluorescence measurements, impact probe measurements are relatively simple to implement and interpret. Impact probe scattering from the edges of the skimmer, produces less of a perturbation of the flow through the skimmer at the lower gas measurements have been used here to confirm the poor beam quality from a conventional interface, to elucidate the problems densities.With the larger diameter skimmer placed at xs= 11.5 mm or greater, the measured FWHM and centreline associated with the conventional interface design and to demonstrate the higher quality beam produced using a large orifice intensities indicate that a much better beam is formed. In this case, the eVects of a lower gas density at larger xs and a larger diameter and a large sampler–skimmer separation.In the future it would be of interest to measure electron and orifice diameter are such that, even if a shock is formed at the edges of the skimmer, the edges are further from the centreline ion densities with the large skimming orifice under conditions that appear to produce a much better quality beam. The and the amount of centreline scattering is reduced. 16 J.Anal. At. Spectrom., 1999, 14, 9–17VCH, New York, 1992, p. 613; (c) S.D. Tanner and D. J. Douglas, implications of these results for improving the eYciency of ion in Inductively Coupled Plasma Mass Spectrometry, ed. sampling are currently under investigation. A. Montaser, Wiley-VCH, New York, 1998. 9 H. Niu and R. S. Houk, Spectrochim. Acta, Part B, 1996, 51, 779. 10 H. Niu and R. S. Houk, Spectrochim. Acta, Part B, 1994, 49, 1283. Acknowledgement 11 B. S. Duersch, Y. Chen, A. Ciocan and P. B. Farnsworth, This work was supported through a Natural Sciences and Spectrochim. Acta, Part B, 1998, 53, 569. 12 D. J. Douglas, paper presented at the Annual Meeting of the Engineering Research Council (NSERC) SCIEX Industrial Federation of Analytical Chemistry and Spectroscopy Societies, Chair. Cincinnati, OH, October 1995, paper 216. 13 (a) G. R. Gilson, D. J. Douglas, J. E. Fulford, K. W. Halligan and References S. D. Tanner, Anal. Chem., 1988, 60, 1472; (b) S. D. Tanner, Spectrochim. Acta, Part B, 1992, 47, 809. 1 D. J. Douglas and J. B. French, Anal. Chem., 1981, 53, 37. 14 (a) S. D. Tanner, L. M. Cousins and D. J. Douglas, Appl. 2 R. Campargue, Rev. Sci. Instrum., 1964, 35, 111. Spectrosc., 1994, 48, 1367; (b) S. D. Tanner, D. J. Douglas and 3 R. Campargue, Entropie, 1969, 30, 15. J. B. French, Appl. Spectrosc., 1994, 48, 1373. 4 R. Campargue, J. Chem. Phys., 1970, 52, 1795. 15 J.-F. Ying and D. J. Douglas, Rapid Commun. Mass Spectrom., 5 R. Campargue, Thesis, Faculty of Sciences, University of Paris, 1996, 10, 649. 1970. 16 J. B. French, in Molecular Beams for Rarefied Gas Dynamic 6 R. Campargue, in Proceedings of the 6th International Symposium Research, ed. W. D. Nelson, AGARDograph 12, NATO-AGARD of Rarefied Gas Dynamics, ed. G. Trilling and H. Y. Wachman, Fluid Dynamics Panel, Paris, 1966. Academic Press, New York, 1969. 17 K. R. Enkenhus, PhD thesis, University of Toronto, 1957. 7 H. C. W. Beijerinck, R. J. F. Van Gerwen, E. R. T. Kerstel, 18 (a) G. E.McMichael and J. B. French, Phys. Fluids, 1966, 9, 1419; J. F. M. Martens, E. J. W. Van Vliembergen, M. R. Th. Smits and (b) A. L. Gray, J. Anal. At. Spectrom., 1989, 4, 371. G. H. Kaashoek, Chem. Phys., 1985, 96, 153. 19 J. E. Fulford and D. J. Douglas, Appl. Spectrosc., 1986, 40, 971. 8 (a) D. J. Douglas and J. B. French, J. Anal. At. Spectrom., 1988, 3, 743; (b) D. J. Douglas, in Inductively Coupled Plasmas in Analytical Spectrometry, ed. A. Montaser and D. W. Golightly, Paper 8/06205F J. Anal. At. Spectrom., 1999, 14, 9–17 17

ISSN:0267-9477    

DOI:10.1039/a806205f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Analysis of soil and sediment samples by laser ablation inductively coupled plasma mass spectrometry Scott A. Baker,† Melody Bi, Ricardo Q. Aucelio, Benjamin W. Smith and James D. Winefordner* Department of Chemistry, University of Florida, Gainesville, FL 32611, USA Received 1st June 1998, Accepted 8th October 1998 The analysis of soil and sediment samples using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was studied. Solution-based calibration was used for the quantification of trace elements in these samples. In most cases, the measured concentrations were within ±20% of the certified values using 60Ni or 107Ag as the internal standard.Measurements with Ag were carried out to investigate whether an internal standard could be spiked into soils for quantification purposes. The influence of particle size on the applicability of sample spiking was briefly studied, and it was demonstrated that particle size could significantly influence measurements if only the surface constituents of the particle were ablated.Use of 43Ca or 44Ca as an internal standard produced poorer results owing to interferences at these masses. In some cases, such as with Sr, Ba, Y and Rb, the measured concentrations were low by a factor of 2–3. This could be remedied by using one of these elements as an internal standard for the others. The eVects of elemental speciation, organic content and particle size were investigated.Elemental speciation and organic content of the soils did not appear to significantly aVect the LA-ICP-MS measurements. Particle size, however, was found to influence the precision and sensitivity of the measurements. Samples with smaller particle sizes yielded higher signal levels and better precision. The elemental analysis of soils is important for several reasons, matched standardization in the analysis of complex matrices, such as soils, but reported no data for quantitative measure- one of the most important being the identification of contaminants and the establishment of levels of toxic elements present ments in soil samples.As an alternative to direct analysis of soils, fused mixtures of soil and diluent have been employed in the soil. A complicating factor in the determination of elemental constituents in soils is the refractory nature of these for LA-ICP-MS analyses.14 The production of fused beads oVers improved matrix matching; however, a limitation of this materials.1 Traditional methods of analysis rely on decomposition of the soil, typically through microwave or acid diges- methodology is the loss of volatile species as a result of the high fusion temperatures and increased sample preparation tion, and analysis of the resulting solution by flame or furnace AAS2 or ICP-AES.2,3 Direct methods of analysis are advanta- times. This paper reports on the use of LA-ICP-MS for the analysis geous because of the elimination of time-consuming sample preparation steps and the risk of sample contamination from of soil and sediment samples.Solution-based calibration was investigated for obtaining accurate quantitative measurements chemical reagents used in the digestion process. Several solid sampling techniques have been utilized for analyzing soil and in these complex matrices. The influences of sampling strategy, internal standards, instrumental parameters, sample particle sediment samples, including ETV-ICP-MS,4,5 GDMS,6 dc Arc-AES7 and XRF.2,8 XRF is commonly used to analyze sizes and composition were studied in order to characterize the strengths and limitation of this approach.soils and other environmental samples; however, the technique lacks the sensitivity to determine many low-level species of interest. Experimental Recently, laser sampling techniques have gained popularity Instrumentation for analyzing soil samples, since little to no sample preparation is required.Laser induced breakdown spectroscopy (LIBS) is The experiments were performed with a Finnigan MAT (San well suited for the field based analysis of soil samples, as Jose, CA, USA) SOLA ICP-MS and Finnigan MAT System evidenced by the number of recent publications on this appli- 266 laser ablation accessory. Typical operating conditions for cation.9–11 The technique, however, suVers from more severe the ICP-MS are given in Table 1. In these studies, a combined matrix eVects and poorer sensitivity than LA-ICP-MS.flow of nebulized solution and carrier gas from the ablation Relatively few applications of LA-ICP-MS for analyzing soils chamber was introduced to the ICP-MS at all times.15–17 This have been reported.12–14 Durrant and Ward12 analyzed several allowed the use of solutions in optimizing the ICP-MS Chinese reference soils by LA-ICP-MS using elemental operating conditions and provided a convenient means of responses derived from a single soil in the series and reported calibration (discussed later).that determined concentrations for about 80% of the elements The laser used was an Nd5YAG with an output beam at studied were within a factor of two of concentrations measured 266 nm. It was operated at 5 Hz with typical pulse energies of by INAA, with many being significantly closer. HoVmann 0.7 mJ and pulse widths of 10 ns. The System 266 ablation et al.13 discussed the importance of internal standardization accessory was modified by using a separate computer to and the grinding of soil samples for achieving acceptable levels control the x–y–z translation stage. This allowed for transof precision.They also mentioned the importance of matrix- lation of the sample at approximately 15 mm s-1 while the laser was repetitively fired. Single spot and translation sampling were both used in this work and a comparison of the two will †Present address: United States Department of Agriculture, be made.Typical analysis times were 60–80 s (signals measured Agricultural Research Service, Beltsville Human Nutrition Research Center, Food Composition Laboratory, Beltsville, MD 20705, USA. over 300–400 laser shots). J. Anal. At. Spectrom., 1999, 14, 19–26 19Table 1 Typical ICP-MS operating conditions of particle sizes, sand (about 10 g) was ground in an alumina grinding vial (Spex Model 8003) for 1–2 min. The ground Rf power 1200 W material was then transferred to a sieve and four particle size Coolant gas flow rate 15 L min-1 fractions (<35, 35–60, 60–80 and >80 mm) were obtained.Auxiliary gas flow rate 0.9 L min-1 Samples of 1 g of the diVerent particle size fractions of sand Nebulizer gas flow rate 0.7 L min-1 Ablation chamber flow rate 0.3 L min-1 were placed in glass vials, spiked with solutions containing Solution uptake rate 1.0 mL min-1 Co, Ag, Y, Rb, W, Ba and Pb, and dried at 110 °C for several hours. The sand samples were then mixed with 10% (m/m) Scan conditions— Faraday scans (major elements)— cellulose binder (Spex CertiPrep) for 30 min and pressed into Scan range per isotope 1 u pellets for analysis.Number of passes 32 To determine whether diVerences in measured composition, Number of channels per u 16 precision and ablation yield were observed for diVerent particle Dwell time per channel 64 ms size fractions, NIST SRM 2704 and HPS Sandy Soil B samples Multiplier scans (minor and trace elements)— were sieved into two diVerent size fractions (<35 and Scan range per isotope 0.25 u 35–60 mm).Samples of 1 g of each fraction were pressed into Number of passes 128 pellets for analysis. Number of channels per u 16 ms The eVect of the organic content of the sample on the Dwell time per channel 2–4 ms LA-ICP-MS results was studied by adding cellulose binder to NIST SRM 2709 soil in diVerent proportions (10 and 20% Solution-based calibration m/m). The 1 g soil and binder samples were mixed for 30 min and then pressed into pellets.Additionally, an inorganic sample The solution-based calibration approach for quantitative (NIST SRM 1633 Coal Fly Ash) was studied for comparison. measurements with LA-ICP-MS, which utilizes dual sample With the coal fly ash reference material, a sample mixed with introduction or a ‘wet plasma’ configuration, has been cellulose binder (10% m/m) was prepared, and also one mixed described previously.15–17 Briefly, it involved measuring the with high-purity graphite (20% m/m).The 1 g samples were intensity of one or several isotopes for each element and an pressed into pellets at 35 MPa. internal standard and determining relative sensitivity factors Because elements exist in a variety of forms (e.g., carbonates, (RSFs) for each analyte. The RSFs (analyte/internal standard) silicates, oxides) in soils, the eVect of speciation on the measured from standard solutions were then used to determine LA-ICP-MS results was studied.Samples containing various the concentration of analytes in the solid samples based on compounds of Ba were prepared in a sand matrix. Compounds the expression of Ba (nitrate, oxide, chloride, carbonate, and sulfate) (Aldrich, Milwaukee, WI, USA) were added to 10 g of sand to produce concentration (analyte)solid= intensity (analyte)solid intensity (internal standard)solid samples with a total Ba concentration of about 750 ppm.In addition, Ni (as sulfate) was added at the same concentration (about 500 ppm) in each sample for use as an internal standard. × concentration (internal standard)solid RSFsolution Each of the mixtures was ground and homogenized in an alumina grinding vial for 30 min. A 0.9 g portion of the sample The success of this methodology depended on achieving identwas then mixed with 0.1 g of cellulose binder for 30 min before ical plasma conditions whether an ablated solid or nebulized being pressed into a pellet.To study the matrix dependence of solution was being analyzed, and that the ablated material speciation eVects, Ba (and Ni as internal standard) samples was representative of the bulk solid sample. were prepared in a graphite matrix. Direct grinding and mixing of the Ba and Ni compounds in graphite, however, did not Samples produce homogeneous samples. It was necessary to prepare concentrated mixtures (10% m/m) of Ba and Ni in sand and Several soil and sediment standard reference materials were analyzed to determine the applicability of solutions for cali- then dilute this mixture in graphite to give final concentrations of about 750 ppm for both analytes.bration of these materials. These materials included NIST SRM 2704 (BuValo River Sediment), NIST SRM 2709 (San Joaquin Soil ), NIST SRM 2710 (Montana Soil ), NIST SRM Results 2711 (Montana Soil ), High Purity Standards (HPS) (Charleston, SC, USA) Sandy Soil B and HPS Loam A.For Single spot versus translational sampling analyses, the samples were pressed into pellets without binder at a pressure of 35 MPa. For the determination of RSFs, Single spot and translational sampling were studied for the analysis of soils and sediments. Signals were typically a factor solutions were prepared by dilution of a 10 ppm multi-element standard (High Purity Standards) with de-ionized water and of 4–5 larger when the sample was translated. This signal enhancement was greater than what has been observed in Optima HNO3 (Fisher Scientific, St.Louis, MO, USA) to a final acid concentration of 2%. work involving glass and ceramic samples, most likely because a deep crater was formed more quickly in these compacted A limitation of the use of solutions for calibration was that a knowledge of the concentration of the internal standard in particulate samples. A comparison of the results obtained with both sampling strategies is shown in Fig. 1 for NIST SRM the solid was required. To address this limitation, Ag was spiked into the soils as an internal standard. This was done 2704. These plots represent the analyte responses for (a) Ni and (b) Ba relative to Ca from a total of 25 scans obtained by adding 2.5 mL of a 100 ppm Ag standard to 1 g of soil (250 ppm Ag in soil ) and drying the samples at 110 °C for from 25 diVerent spots on the sample, or at diVerent lines (five scans per line) produced from translation of the sample.several hours. The soil was then transferred into a plastic vial and mixed with a Spex (Metuchen, NJ, USA) Mixer/Mill Copper and cobalt were also studied and produced similar results. The relative analyte responses from both sampling Model 8000 for 15 min to ensure homogeneity. The influence of particle size on the applicability of sample methods were similar in most cases; however, significant deviations from the mean (as much as a factor of five) were spiking was investigated by spiking sand samples (Mallinckrodt Baker, Paris, KY, USA) of diVerent particle observed in some scans when sampling at a fixed location.This must have resulted from local inhomogeneity in the soil size ranges with a multi-element solution. To generate a range 20 J. Anal. At. Spectrom., 1999, 14, 19–26have been related to diVerences in the particle size distribution for the NIST SRM 2704 sample compared with the other soils. The eVects of particle size on measurement precision will be discussed later.Selection of the internal standard was found to be an important factor in obtaining accurate results for soils. Initially, Ca was used as the internal standard because it was present at significant levels in all of the samples and possessed minor isotopes that could be measured with the multiplier detector. In addition, it has been successfully used in this laboratory for the analysis of glass samples by LA-ICP-MS with solution-based calibration. This was not the case with the soil samples.A comparison of the results for several elements (V, Co, Ni, Cu, Zn, Sr, Ag, Ba and Pb) in (a) NIST SRM 2709 and (b) NIST SRM 2704 soils using 43Ca as the internal standard is shown in Fig. 2. These plots have been normalized to the certified or information values; therefore, the accuracy of the solution calibration method can be directly assessed by comparing the normalized value with unity (a value of unity would signify that the measured and certified concentrations were identical ).For NIST SRM 2709, the measured concentrations of V, Co, Ni, Cu, and Zn were between 40 and 60% higher than the certified values for these elements, those for Ba and Sr were about 60% lower than the certified values and that for Pb was within 10% of the certified value. For NIST SRM 2704, diVerent results were obtained. Fig. 1 Relative analyte signals for (a) Ni and (b) Ba from NIST SRM There was a systematic increase in the measured concentration 2704 sediment sample using both single spot (A) and translational for all of the analytes studied in this sample.Systematic (B) sampling. changes were also observed for the other soils. Because errors in the measured concentrations were largely systematic, rather than random, it is believed that they were the result of an samples, and clearly indicated the need to sample a large interference by aluminum oxide at m/z 43.Based on these enough portion of the soil to ensure accurate and precise observations, 43Ca was deemed a poor choice for internal measurements. Measurement of these local inhomogeneities is standardization since the levels of aluminum in the sample required for accurate measurement of the bulk composition; aVected the signal at this mass. The use of 44Ca was also however, their presence can significantly aVect the precision investigated for internal standardization. Similar systematic of analyses.From the results in Fig. 1, it appeared that diVerences in the measurement accuracy were also observed translation of the sample eVectively averaged out the inhomowith this isotope, most likely resulting from interferences due geneities during a typical 1 min analysis, which should result in more precise measurements. The absence of large deviations in the relative analyte signal (analyte/Ca) was almost certainly due to the larger mass ablated with sample translation.The average precision (RSD) of the relative intensities for the four analytes studied was 8.8% with translation sampling and 37% with single spot sampling. Since higher sensitivity and better precision were obtained with translation of the sample, this mode of sampling was used for all subsequent measurements. Solution-based calibration Solution-based calibration required the selection of an appropriate internal standard with a known concentration in the sample.Because of inhomogeneity in the soil samples, measurement of a minor isotope of a major matrix constituent (Ca) was initially used to provide an acceptable level of precision. Using an isotope of Ca (43Ca or 44Ca), which was present at levels of 1–3% in the soils studied, as the internal standard resulted in typical precision values (RSD, n=10) of <10%. The precision values were not consistent among the soils analyzed. Measurements on the NIST SRM 2704 sample exhibited the poorest level of precision, with RSD values consistently around 10–12%.With the other soils, the RSD was typically around 6–7%. Measurement precision is obviously dependent on the concentration of analyte in the sample and the amount of material ablated; however, this could not explain the poorer precision observed with the NIST SRM 2704 sample. This sample contained similar levels of trace Fig. 2 Results for the analysis of NIST SRM 2709 soil and NIST elements and Ca as the other samples.In addition, a similar SRM 2704 sediment samples using 43Ca as the internal standard. mass (about 50 ng per shot) was ablated for all of the soils Measured concentrations for analytes have been normalized by their certified or information values. studied. The diVerences in precision that were observed might J. Anal. At. Spectrom., 1999, 14, 19–26 21to SiO+ or CO2+ at m/z 44. Using an isotope of silicon (28Si, 29Si or 30Si) as the internal standard worked well for the analysis of minor elements in soil; however, none of the silicon isotopes could be measured on the multiplier detector, which was required for measuring trace elements, because of saturation of the detector at these masses.The inability to measure both large (>106 counts s-1) and small (<106 counts s-1) ion signals during a measurement is a limitation of the present detection system, which employs a dual detector (electron multiplier and Faraday cup) design in which only one detector can be selected at a time.It is important to mention that this limitation is due to the present ICP-MS instrumentation and that silicon isotopes for internal standardization could be used with other ICP-MS systems. Because of the interferences on Ca isotopes and the lack of any other suitable internal standard that could be used over the whole series of soils, a minor sample constituent was investigated as an internal standard. The use of trace elements as internal standards produced more accurate measurements for the soil samples.This is illustrated in Table 2, which compares the measured concentrations for several analytes in NIST SRM 2704, 2709 and 2711 soils using the solution calibration method with both 43Ca and 60Ni as internal standards. The RSF values using Ni as the internal standard were determined by dividing the analyte/Ca RSF by the Ni/Ca RSF. Significantly more accurate results were obtained using Ni as the internal standard, indicating that solution calibration was useful even for a complex matrix like soil.A limitation of the technique, however, was that the concentration of an element in the sample that could serve as the internal standard had to be known. This could be addressed by using a complementary technique to determine the concentration of an element in the sample or by spiking in a known amount of some element that was not in the sample at an appreciable concentration relative to the amount added.A suitable element for spiking could be chosen by doing a survey scan over the sample of interest. The utility of sample spiking for quantification purposes was investigated by adding Ag (initially present at levels of <5 ppm) to NIST SRM 2709 and HPS Loam A soils to produce final concentrations of 250 ppm. Results from the use of solution-based calibration with Ag as the internal standard are provided in Table 3 and demonstrate that reasonable accuracy could be obtained in most cases.The potential influence of particle size on the applicability of sample spiking will be discussed later. For the analytes studied, consistent patterns concerning the accuracy of solution calibration were observed. Most analytes (V, Cr, Mn, Co, Ni, Cu and Zn) could be determined with reasonable accuracy (typically ±20%) using a single solution for calibration; however, the measured concentrations for several analytes (Rb, Sr, Ba and Y) were consistently lower (by around a factor of 2–3) than their certified values in the soils.More accurate measurements of the latter elements could be obtained if one element in this group was used as the internal standard for the other elements. The low results obtained for Rb, Sr, Ba and Y might be partially due to matrix eVects resulting from high levels of eYciently ionized elements (EIEs), such as Na and Al. EIEs generally cause a decrease in ICP-MS intensities, and are most severe for elements with low ionization energies.18 Since Rb, Sr, Ba and Y possessed the lowest ionization energies of the elements studied, the EIE eVect could help explain the reduced sensitivity observed for these elements in soil and sediment matrices.In general, matrix eVects in ICP-MS are diYcult to measure and quantify. They can often be minimized for particular analytes through optimization of the ICP-MS operating conditions or selection of appropriate internal standards;19 however, for multi-element determinations, this is often not feasible Table 2 Measured concentrations (ppm) in soils using solution calibration with 43Ca or 60Ni as the internal standard Soil V Cr Co Cu Zn Sr Ag Ba Pb NIST SRM 2704 Measureda (43Ca) 210±36 326±48 33.6±5.2 249±31 940±140 119±12 351±48 570±100 Measureda (60Ni) 87±15 135±22 13.9±2.6 103±16 390±72 49.0±8.2 146±34 235±58 Certifiedb 95±4 135±5 14.0±0.6 98.6±5.0 438±12 (130) 414±12 161±17 NIST SRM 2709 Measureda (43Ca) 160±12 146±12 21.6±1.3 52.3±3.9 150±9 107±5 0.39±0.10 369±27 21.1±4.2 Measureda (60Ni) 116±10 106±11 15.6±1.4 37.9±3.8 109±9 77.5±5.8 0.28±0.08 268±23 15.3±3.2 Certified 112±5 130±4 13.4±0.7 34.6±0.7 106±3 231±2 0.41±0.03 968±40 18.9±0.5 NIST SRM 2711 Measureda (43Ca) 106±9 59.8±6.7 18.4±2.3 182±18 471±56 139±9 7.2±1.3 439±40 3150±210 Measureda (60Ni) 57.8±4.7 32.5±4.8 9.2±1.0 110±16 255±38 75.6±5.6 3.9±0.8 240±30 1570±140 Certifiedb 81.6±2.9 (47) (10) 114±2 350.4±4.8 245.3±0.7 4.63±0.09 726±38 1162±31 aConfidence intervals at the 95% level, n=5 measurements.bValues in parentheses are non-certified information values provided by NIST. since all elements do not behave identically in the ICP-MS. 22 J. Anal. At. Spectrom., 1999, 14, 19–26Table 3 Measured concentrations (ppm) for analytes in NIST SRM 2709 and HPS Loam A soil samples using Ag as the internal standard. Ag was spiked into each of the soils at 250 ppm Sample Co Ni Cu Zn Sr Ba NIST SRM 2709 Measureda 12.0±0.5 73.0±4.9 33.2±1.0 103±4 80.9±4.2 371±34 Certified 13.4±0.7 88±5 34.6±0.7 106±3 231±2 968±40 HPS Loam A Measureda 14.7±2.7 21.4±2.1 19.6±2.4 20.6±3.5 164±26 Certified 14.4±0.4 20.9±1.0 12.4±1.4 60.3±2.6 448±28 aConfidence intervals at the 95% level, n=5 measurements. Therefore, improvements with respect to one analyte may adversely aVect the measurement of another analyte.In the analysis of steels by solution nebulization ICP-MS, Vaughan and Horlick20 reported that matrix eVects could be minimized by a slight decrease in the nebulizer flow rate corresponding to the maximum analyte signal.This strategy was investigated in this work to determine whether more accurate results could be obtained for Rb, Sr, Y and Ba. Decreasing the nebulizer flow rate from 0.7 L min-1, the flow that produced the highest sensitivity, aVected the RSFs for both nebulized solutions and laser ablated solids in a similar manner (Fig. 3). Both (a) Co, an element for which relatively accurate results could be obtained, and (b) Sr are included for comparison. Several other analytes were studied and the same trends were observed. In Fig. 3, the data have been normalized to the solutiondetermined RSF at 0.7 L min-1. Results from these measurements indicated that adjustment of the nebulizer flow rate could not be used to improve the accuracy of the measurements, since the analytes were aVected similarly, regardless of sample introduction method or matrix (soil or 2% HNO3).Similarly, adjustment of the ICP rf power was investigated to determine if this variable could be selected to improve the accuracy of measurements for all analytes in the soils. The results for (a) Co and (b) Sr are presented in Fig. 4. In these plots, Ni was used as the internal standard and the RSF values were normalized to the solution-based RSF value at 1200 W Fig. 4 EVect of rf power on relative sensitivity factors (RSFs) for (a) Co and (b) Sr with solution nebulization of a 10 ppb solution (B) and laser ablation (A) of NIST SRM 2709 soil.RSF values have been normalized to the solution-based RSF at 1200 W, which represented the rf power producing maximum analyte signals. (optimum sensitivity) of rf power. Changing the rf power aVected the RSF values slightly; however, the changes were similar for both the solution and the laser ablated NIST SRM 2709 soil sample. Therefore, no improvement in the accuracy of the measurements was observed as the rf power was changed over the range 1050–1350 W.Although accurate measurements were not obtained in all cases, solution calibration provided a relatively simple means of estimating analyte concentrations in the sample. In all cases, the measured analyte concentrations were within a factor of three of their certified concentrations using 60Ni as the internal standard and, in most cases, much better accuracy was achieved.Lower concentrations were consistently measured for Rb, Sr, Y and Ba; however, the use of one of these elements as an internal standard for the rest of the group produced more accurate results. To investigate potential sources of inaccuracy in soil samples, element speciation, sample organic content and particle size eVects were studied. Speciation eVects Since metals exist as a variety of compounds in soil samples, Fig. 3 EVect of nebulizer flow rate on relative sensitivity factors the eVect of speciation on LA-ICP-MS measurements was (RSFs) for (a) Co and (b) Sr with solution nebulization of a 10 ppb examined.In a study involving the detection of Ba and Pb in solution (B) and laser ablation (A) of NIST SRM 2709 soil. RSF soils by LIBS, it was determined that the chemical form (oxide, values have been normalized to the solution-based RSF at 0.7 L min-1, carbonate, sulfate, chloride, nitrate) of the analyte aVected the which represented the nebulizer flow rate providing maximum analyte signals.sensitivity of LIBS measurements.11 To determine if this might J. Anal. At. Spectrom., 1999, 14, 19–26 23help explain the reduced sensitivities observed for Ba, Sr, Rb the level corresponding to the maximum for a 1% HNO3 solution. and Y in this work, the eVect of speciation on Ba in a sand matrix was examined. The results for these studies are provided Results for the analysis of NIST SRM 2709 soil which contained 1.2% carbon, with no binder added, 10% cellulose in Table 4.The first column of data compares the RSF (Ba/Ni) values obtained from the five sand samples. The BaO sample binder added and 20% cellulose binder added were compared (Table 5). No significant changes in the measured analyte produced a lower value (statistically significant at the 95% confidence level ); however, the decrease was minor in compari- concentrations were observed with the addition of cellulose binder at levels up to 20%.Similarly, a coal fly ash sample son with the factor of 2–3 lower concentration measured with solution calibration. To determine if the observed decrease was studied to determine if diVerent results were obtained when 10% cellulose or 20% graphite was used to prepare the could be attributed to the formation of Ba2+ or BaO+, the signals for these isotopes were included in determining RSFs sample pellet. The results for these analyses (Table 6) indicated that the addition of an inorganic (graphite) or organic (cellu- (column 2 in Table 4).The same trend was observed; BaO produced a slightly lower RSF than the other compounds. lose) binder produced similar results. The accuracy of these measurements, based on solution calibration with Ni (98 ppm The RSF values given in the last column of Table 4 were measured from the sand pellets by solution nebulization to in sample) as the internal standard, was around 10–20% for all analytes studied.ensure that the samples contained the same levels of Ba and Ni. For the solution measurements, a 5 mg portion of each sand sample was diluted in 20 mL of 5% HNO3 and then Analysis of particle size fractions heated to dissolve the Ba and Ni species. The results indicated Two diVerent particle size fractions (<35 and 35–60 mm) of that the pellets contained reproducible levels of Ba and Ni. NIST SRM 2704 and HPS Sandy Soil B were analyzed to The sulfate pellet was not included because BaSO4 was insoldetermine whether analytes were distributed similarly in both uble in the dilute HNO3 solution.fractions, and to determine how particle size aVected the The eVects of speciation were also examined in a graphite precision of LA-ICP-MS measurements when analyzing par- matrix. Only the oxide, sulfate and carbonate forms of Ba ticulate samples. The results for NIST SRM 2704 indicated were studied, with Ni (as sulfate) as the internal standard.No that the concentrations of the eight trace elements studied significant diVerences between the RSFs (Ba/Ni) were were identical, within the experimental uncertainty, in both observed. The RSF (Ba/Ni) values were 1.45±0.19 for the particle size fractions. The ablated mass was slightly higher oxide, 1.48±0.25 for the sulfate and 1.54±0.27 for the carbon- (about 20%) for the pellet consisting of particles less than ate. Based on speciation studies with Ba in both sand and 35 mm compared with the 35–60 mm pellet.Also, the precision graphite matrices, it appeared that the form of the analyte did of the measurements was significantly better for the smaller not significantly aVect the accuracy of LA-ICP-MS particle sample (average RSD=7.3%) than the larger particle measurements. sample (average RSD=14%). The unsieved sample was analyzed for comparison, and the precision of measurements on EVect of organic content this sample was intermediate between the two fractions, with an average RSD of 10%.The original material consisted of To determine whether the organic content of the sample aVected the measurements, both soil and coal fly ash samples about 75–80% (by mass) of particles less than 35 mm and the remainder in the 35–60 mm range. were studied. The organic content was studied not only to determine if this might help explain the observed discrepancies The HPS Sandy Soil B sample produced diVerent results in terms of particle composition.The results indicated that the for Ba, Sr, Rb and Y, but also to determine if the addition of cellulose binder, used in pellet preparation, aVected the accu- smaller particles (<35 mm) had approximately three times more Pb and Ag than the larger particles (35–60 mm). All of racy of results. Previous studies using solution nebulization ICP-MS have indicated that the addition of small amounts of the other elements studied contained identical levels of trace elements.These results are not easily explained, but might be organic solvents could either increase or decrease analyte signals. Allain et al.21 found that the addition of glycerol or related to the preparation of this standard reference material. In this soil, the concentrations of most trace elements were methane significantly enhanced the signals for some analytes (As, Au, Se, Te and Hg), whereas other analytes were essen- enriched 10–100-fold by spraying the sample with an aerosol mist.The samples were then dried, ground, sieved and blended. tially unaVected. These workers attributed the signal enhancements with addition of carbon to a modification of ionization If, for instance, the elements were added sequentially rather than simultaneously, preferential adsorption of Pb and Ag on equilibrium over a limited energy range (9–11 eV). The ionization energy of carbon (11.2 eV) is slightly above this range. the smaller particles might have occurred.The behavior of these samples, in terms of measurement precision, was similar Longerich22 reported ICP-MS signal suppression for several elements when various organic solvents were added to a 1% to that observed with the NIST SRM 2704 sample. Measurements on the smaller particle sample were charac- HNO3 solution. The decrease in sensitivity could be recovered, however, when the nebulizer gas flow rate was reduced from terized by higher precision (average RSD=5.5%) than the Table 4 Measured relative sensitivity factors (RSFs) for Ba species in a sand matrix using Ni as the internal standard.Ba and Ni concentrations in the sand were approximately 750 and 500 ppm, respectively LA-ICP-MS ICP-MS: Ba species RSF (Ba/Ni)a,b RSF (Batotal/Ni)a,c RSF (Ba/Ni)a,d Oxide 0.89±0.09 1.18±0.10 1.66±0.02 Nitrate 1.12±0.16 1.64±0.17 1.70±0.02 Carbonate 1.14±0.07 1.50±0.09 1.68±0.04 Chloride 1.33±0.09 1.75±0.09 1.56±0.01 Sulfate 1.19±0.05 1.58±0.05 aConfidence intervals at the 95% level, n=5 measurements.bLaser ablation results using Ba+ signal only. cLaser ablation results using Ba+, Ba2+ and BaO+ signals. dSolution results using Ba+ signal only. 24 J. Anal. At. Spectrom., 1999, 14, 19–26Table 5 Measured relative sensitivity factors (RSFs) for analytes in NIST SRM 2709 with and without the addition of cellulose binder RSFa V Cr Mn Co Ni Cu Zn Rb Ba No cellulose 32.8±2.7 3.24±0.30 35.6±4.1 36.5±2.1 7.04±0.36 16.4±0.9 3.48±0.17 11.6±0.9 1.41±0.16 10% cellulose 34.3±1.0 3.45±0.38 35.9±1.9 37.4±1.0 7.01±0.25 16.7±0.8 3.51±0.25 12.1±0.7 1.40±0.11 20% cellulose 36.2±1.4 3.55±0.26 36.3±2.3 40.4±3.6 7.69±0.50 18.6±1.4 3.73±0.33 14.0±0.6 1.39±0.08 aConfidence intervals at 95% level, n=5 measurements, 43Ca used as the internal standard.Table 6 Measured concentrations (ppm) from NIST SRM 1633 (Coal Fly Ash) using solution calibration with graphite and cellulose as binders V Cr Mn Cu Zn Cd Certified 214±8 131±2 493±7 128±5 210±20 1.45±0.06 Measureda (20% graphite) 239±12 153±14 571±21 143±3 223±9 1.65±0.27 Measureda (10% cellulose) 244±9 160±8 579±43 153±10 235±15 1.83±0.26 aConfidence intervals at the 95% level, n=5 measurements. larger particle sample (average RSD=12%).The RSD of the was close to the maximum output of the laser system. In addition, particle sizes at which preferential vaporization measurements for the unsieved sample was, on average, 7.1%. Based on these studies, the precision of measurements was became significant would depend on the type of sample being analyzed.For more refractory materials, diVerences in relative significantly aVected by sample particle size. Samples consisting of larger particles produced poorer precision and lower analyte signals would be expected to occur at even smaller particle sizes. Based on the observed diVerences in relative ablation yields than samples consisting of smaller particles.To investigate whether particle size influenced the relative analyte signals for the spiked sand samples, it was concluded that particle sizes in the sample could significantly aVect the signals for analytes deposited on the surface of particles with respect to an analyte in the particle, several spiked sand accuracy of LA-ICP-MS results when using sample spiking to introduce an internal standard into the sample. If the particles samples were analyzed. The same amount of each analyte was added to four diVerent sized fractions of sand (<35, 35–60, are too large to be eYciently ablated, only the surface concentration will be measured and the use of a spiked internal 60–80 and >80 mm).All analyte signals were normalized to 44Ca, which was present in the sand matrix. This was necessary standard will lead to systematically low results for elements present in the bulk particles. because analyte signals from the pellet consisting of particles <35 mm were more than twice as large as those obtained from the pellet containing particles >80 mm because of the larger Conclusions ablation yield.The results (Fig. 5) indicated that the relative Reasonably accurate (±20%) results for most trace elements signals for all analytes increased significantly for particles in soil and sediment samples could be obtained using a single larger than 35 mm. This was due to the ineYciency of the solution standard for calibration. Using a spiked internal ablation process at the laser energies used (0.7 mJ) in vaporizstandard was demonstrated to be useful for particulate samples ing larger particles of sand. The deposited analytes were and enhances the utility of the LA-ICP-MS technique, since concentrated on the surface of the particles; therefore, they potential internal standards can be rapidly identified with a were preferentially vaporized relative to the bulk of the particle survey scan over the sample of interest. When using a spiked and produced higher relative intensities.The onset of this element as the internal standard, care must be taken to ensure process should be shifted to larger particle diameters as the that particle sizes are suYciently small so that they are laser energy is increased; however, the energy used (0.7 mJ) eYciently ablated relative to the deposited spike. In the analysis of soils and sediment, systematically low results were obtained for Rb, Sr, Y and Ba when using Ni or Ag as the internal standard.This was probably due, in part, to matrix eVects associated with large amounts of easily ionizable elements present in the soils. The eVects of elemental speciation and sample organic content were investigated and no significant diVerences in the accuracy of the measurements were observed as a result of these factors. Particle size was demonstrated to aVect the precision of measurements and influenced the ablation yield. Measurements performed on samples composed of smaller particles were more precise and produced higher signal levels.This work was supported by an Air Force OYce of Scientific Research–University Research Initiative (AFOSR–URI) grant (F49620–93–1-0349). References 1 J. C. Van Loon, Selected Methods of Trace Metal Analysis: Biological and Environmental Samples, Wiley, New York, 1985. 2 S. M. Pyle, J. M. Nocerino, S. N. Deming, J. A. Palasota, J. M. Fig. 5 EVect of particle size on relative analyte signals for a sand Palasota, E. L. Miller, D. C. Hillman, C. A. Kuharic, W. H. Cole, P. M. Fitzpatrick, M. A. Watson and K. D. Nichols, Environ. Sci. matrix. Analytes were spiked into the sand as solutions and 44Ca was used as the internal standard. Technol., 1996, 30, 204. J. Anal. At. Spectrom., 1999, 14, 19–26 253 C. J. Warren, B. Xing and M. J. Dudas, Can. J. Soil. Sci., 1990, 13 E. HoVmann, C. Lu�dke and H. Stephanowitz, Fresenius’ J. Anal. Chem., 1996, 355, 900. 70, 617. 14 X. Guo and F. E. Lichte, Analyst, 1995, 120, 2707. 4 G. Galbacs, F. Vanhaecke, L. Moens and R. Dams, Microchem. 15 S. Chenery and J. M. Cook, J. Anal. At. Specom., 1993, 8, 299. J., 1996, 54, 272. 16 E. F. Cromwell and P. Arrowsmith, Anal. Chem., 1995, 67, 131. 5 M. J. Liaw and S. J. Jiang, J. Anal. At. Spectrom., 1996, 11, 555. 17 S. A. Baker, M. J. Dellavecchia, B. W. Smith and J. D. 6 J. Teng, C. M. Barshick, D. C. Duckworth, S. J. Morton, D. H. Winefordner, Anal. Chim. Acta, 1997, 355, 113. Smith and F. L. King, Appl. Spectrosc., 1995, 49, 1361. 18 J. W. Olesik, Anal. Chem., 1996, 68, 469A. 7 C. A.Mahan, J. Anal. At. Spectrom., 1997, 12, 247. 19 Handbook of Inductively Coupled Plasma Mass Spectrometry, ed. 8 S. J. Goldstein, Environ. Sci. Technol., 1996, 30, 2318. K. E. Jarvis, A. L. Gray and R. S. Houk, Blackie, Glasgow, 1992. 9 R. Barbini, F. Colao, R. Fantoni, A. Palucci, S. Ribezzo, H. J. L. 20 M.-A. Vaughan and G. Horlick, J. Anal. At. Spectrom., 1989, van der Steen and M. Angelone, Appl. Phys. B, 1997, 65, 101. 4, 45. 10 K. Y. Yamamoto, D. A. Cremers, M. J. Ferris and L. E. Foster, 21 P. Allain, L. Jaunault, Y. Mauras, J.-M. Mermet and T. Appl. Spectrosc., 1996, 50, 222. Delaporte, Anal. Chem., 1991, 63, 1497. 11 A. S. Eppler, D. A. Cremers, D. D. Hickmott, F. J. Ferris and 22 H. P. Longerich, J. Anal. At. Spectrom., 1989, 4, 665. A. C. Koskelo, Appl. Spectrosc., 1996, 50, 1175. 12 S. F. Durrant and N. I. Ward, Fresenius’ J. Anal. Chem., 1993, 345, 512. Paper 8/04060E 26 J. Anal. At. Spectrom., 1999, 14, 19–26

ISSN:0267-9477    

DOI:10.1039/a804060e

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Theoretical calculation of the standard deviation in inductively coupled plasma mass spectrometry† Evgeniy D. Prudnikova and Ramon M. Barnes*b aEarth’s Crust Institute, State University of St. Petersburg, St. Petersburg, 199034, Russia bChemistry Department, Lederle Graduate Research Center Towers, University of Massachusetts, Amherst, MA 01003–4510, USA Received 22nd January 1998, Accepted 27th October 1998 The theoretical calculation of the standard deviation in inductively coupled plasma mass spectrometry (ICP-MS) was investigated.The theoretical dependence of the standard deviation on instrument parameters and element concentration was examined. The connection between the standard deviation and the detection limit was demonstrated. The theoretical results were compared with experimental data and showed good agreement. The theory proposed may be used for the improvement of the detection limit in ICP-MS. The method of inductively coupled plasma mass spectrometry detection limit.The theoretical calculation of the standard deviation in ICP-MS was the aim of this work. (ICP-MS) is widely used in the determination of trace elements.1 Study of the theoretical and practical aspects of ICP-MS permits improvements of the instrument and method Theory parameters.1–5 The calculation of the standard deviation and detection limit theoretically is useful for future developments Prudnikov and co-workers13–15 showed the possibility of the in ICP-MS analysis.In analytical practice, an empirical linear theoretical calculation of the standard deviation in AES and approximation has been used for the characterization of the AAS. This approach was based on classical papers concerning connection between element concentration and standard the theoretical calculation of the SBR and detection limit in deviation:6–8 AES and AAS.16–18 The theory allows one to examine the noise in analytical chemistry, to propose practical methods for sC=a+bc (1) the study of standard deviation and detection limit, and to where sC is the standard deviation of a single analytical calculate theoretically the standard deviation and detection determination in units of concentration, c is the element limit in ICP-AES.19–21 These investigations allow one to give concentration in units of concentration (ng ml-1 or others) a mathematical description of the process of an analytical and a and b are coeYcients (a in concentration units and b measurement.22 They may be the basis for the theoretical dimensionless). In the theory of measurements a determines calculation of the standard deviation and the detection limit the additive error and bc determines the multiplicative error.in ICP-MS. In practical analysis, the standard deviation of the analytical According to previous papers,19–22 five kinds of random method is calculated by methods of mathematical statistics.8–10 errors may be distinguished in diVerent analytical systems.For the calculation of the detection limit Kaiser’s statistical These fluctuations depend on the system parameters and the criterion can be used.7 DiVerent methods are used for the element concentration. Their values are proportional to the calculation of the detection limit from experimental data. In power 0, 1/2, 1, 3/2 and 2 of the mentioned parameters. The inductively coupled plasma atomic emission spectrometry standard deviation of the analytical measurement may be (ICP-AES), Boumans11 proposed the determination of the written in the following manner: detection limit by the ‘SBR–RSDB approach’ using the signals=[ sbl2+(kd+2RDsbl)Svc+D2Sv2c2+kkSv3c3+knSv4c4]1/2 to-background ratio (SBR) and the relative standard deviation (3) of the background (RSDB) as the objective criteria for instrumental performance. Boumans12 also proposed an empirical where s is the instrumental value of the standard deviation in relationship for the connection between standard deviation arbitrary units (counts s-1 or others), sbl is the blank fluctuand detection limit: ations (counts s-1), kd is the coeYcient of shot noise if produced by the detector (counts s-1), R is the coeYcient of sC=(cm2k-2+D2c2)1/2 (2) correlation (cross-correlation) of the net and interfering signals, where cm is the detection limit in concentration units, k is a Sv is the instrumental sensitivity of the apparatus numerical factor chosen according to the confidence level (counts s-1 ng-1 mL) and kk and kn are the coeYcients of the desired and D is the instability coeYcient of the apparatus, non-linear noise.The first three terms on the right-hand side otherwise this is the relative standard deviation (RSD) of a characterize the measurements in the linear range of the strong signal which is determined by flicker noise. calibration function, whereas the following two terms deter- The theoretical calculation of the standard deviation and mine the non-linearity in the analytical conditions.The term detection limit may be helpful for the improvement of practical including the correlation coeYcient R concerns the correlation methods and for characterization of standard deviation and fluctuations between the net and interfering signals. The nonlinear processes break the theoretical relationship between the element concentration and the analytical signal. For a high concentration and strong signal, non-linearity can occur during †Presented at the 1996 Winter Conference on Plasma Spectrochemistry, Fort Lauderdale, FL, January 8–13, 1996. the processes of nebulization, ionization and ion counting and J.Anal. At. Spectrom., 1999, 14, 27–31 27processes in the mass spectrometer. For a small signal, the According with this comment, we have in the denominator of eqn. (7) the single element concentration 1 g mL-1, and also non-linearity can be due to the matrix interferences.For the theoretical calculation of the standard deviation in in the numerator of this equation the ion detector eYciency is 1 count s-1 for a single ion. ICP-MS, we do not consider the non-linearity in the calibration function and ignore the correlation moment for the net and Eqns. (4)–(7) represent the process of analytical determinations by ICP-MS theoretically. These expressions interfering signals, because this moment plays a major role for considerable matrix interferences.The examination of these include the parameters of the apparatus, the sensitivity of measurement, the blank fluctuations, the measured element factors may be the subject of more detailed investigation. The expression for the theoretical standard deviation in concentration and the standard deviation of a measurement. These expressions are also connected with the detection limit ICP-MS taking into account the method theory1 and the theoretical eqn.(1) is given as of the measurement. Taking into account the statistical Kaiser’s criterion and the data in previous papers,20,21 one can write s=(sbl2+Svc+D2Sv2c2)1/2 (4) cm=kSv-1sbl where s is the instrumental standard deviation in ICP-MS (counts s-1), sbl is the blank fluctuations (counts s-1), Sv is and for the 3s criterion (k is the confidence level coeYcient) we have the instrumental sensitivity of the ICP mass spectrometer (counts s-1 ng-1 mL), c is the element concentration (ng ml-1) cm=3Sv-1sbl (8) and D is the instability coeYcient of the ICP mass spectrometer.Eqn. (4) is a particular case of eqn. (3) and shows the general Using eqn. (8), we can include the detection limit in eqns. (4) and (5): principle of the theoretical calculation of the standard deviation. The value of the standard deviation is the sum of blank s=[cm2Sv2k-2+(1+2RDcmSvk-1)Svc+D2Sv2c2]1/2 (9) fluctuations and the shot and flicker noise of the useful net signal. Taking into consideration also the co-correlation Eqn.(9) includes all the parameters of the analytical method of ICP-MS without the non-linear noise. between net and interfering signals, eqn. (4) may be rewritten as For practical use, the relative standard deviation (RSD= sr) is more convenient than the absolute standard deviation. s=[sbl2+(1+2RDsbl)Svc+D2Sv2c2]1/2 (5) The instrumental RSD is sr=s/Svc, and for the calculation of RSD in concentration units we have sr=sc/c. The theoretical The blank fluctuations in eqns.(4) and (5) include the blank counts from the ion detector, the ICP source and mass relative standard deviation (sr) of the ICP-MS method can be obtained from the eqn. (4): spectrometer and also the blank counts from spectral and matrix interferences. For the blank standard deviation, we can sr=(sbl2Sv-2c-2+2RDsblSv-1c-1+D2)1/2 (10) write a relationship without matrix interferences: Eqn. (10) will be used for the calculation of the instrumental sbl2=nbl i.d.+nbl ICP+D2nbl ICP2 (6) RSD in ICP-MS.For the detection limit, one can use eqns. (8) and (9). where nbl i.d. is the number of the blank ions from the ion detector (counts s-1) and nbl ICP is the number of blank ions from the ICP source and mass spectrometer (counts s-1). The Experimental blank fluctuations and the standard deviation also depend on the measurement time as a power of a number sbl=f (t-1/2). The experimental results utilize archived data.The experiments were performed an Perkin-Elmer SCIEX (Thornhill, ON, We accept the measurement time constant for all theoretical and experimental data as t=1 s. For a change in measurement Canada) Elan 250 ICP-MS system. The conditions of the measurements were as follows: plasma gas (argon) flow rate, time, the results can be recalculated by use of the abovementioned equation. 11.9 L min-1; rf power, 1.25 kW; nebulizer gas flow rate, 1 L min-1; auxiliary gas flow rate, 1.41 L min-1; and solution The instrumental sensitivity of ICP-MS can be estimated using the results of the calculations for the ICP-AES method:21 flow rate, 1 mL min-1.The standard deviations for the ICP-MS determinations of Sv=6×1023arTsFsabbi/ATICPVgnT/298×109 (7) 89Y, 52Cr, 139La, 142Nd, 140Ce, 90Zr were measured experimentally. The concentrations of these elements in solution were where 6×1023 is Avogadro’s number, ar is the natural relative abundance of various element isotopes, Ts is the introduced 0.1, 0.5, 1.0, 2.0, 5.0, 20.0 and 100.0 ng mL-1 in a 2% solution of HNO3.Standard methods for calibration curve construction sample temperature (K), Fs is the solution flow rate (ml min-1), a is nebulization eYciency, b is the ionization were used. The computer-processed results for the standard deviation values for above element concentrations were taken. eYciency, bi is the coeYcient of the mass spectrometer processing eYciency (or the ion use eYciency), A is the element Table 1 gives the blank values and their fluctuations for the listed elements. The experimental data for the standard devi- atomic mass, TICP is the ICP source temperature (K), Vg is the nebulizer gas flow rate (mL min-1), nT/298 is the number ation of the ICP-MS determination of the listed elements are given in Tables 2–7.of moles present or the change in the number of moles of gas during the transition from the sample temperature to the The value of the blank count due to the ion detector is ni.d.=20, the nebulization eYciency is a=0.02,1,7,21 the tem- plasma temperature (for Ar this coeYcient is equal to 1) and 109 is the coeYcient for the recalculation of concentration perature of the sample is Ts=300 K, the temperature of the ICP source is TICP=8000 K1,21 and the number of moles from g to ng.Eqns. (4) and (5) have been considered for a time registration of t=1 s. For any registration time, it is present for argon is nT/298=1.The other parameters for the theoretical calculations are examined further. necessary to apply the coeYcient 1/t1/2. According to IUPAC recommendations (Inf. Bull. No. 27, November 1972), SV is determined by the value of the first derivative of the calibration Results and discussion function at a given concentration of an element, or SV is the slope of the calibration function. In practice, the instrumental First we will calculate theoretically the minimum value of the detection limit for ICP-MS with sample nebulization.We sensitivity may be determined like the value of an analytical signal (V, in ICP-MS counts s-1) for an element concentration apply the ICP-MS instrument parameters. To predict theoretically the blank count and the eYciency of ion use is diYcult; c=1 (in g mL-1, or for ICP-MS more preferably in ng mL-1). 28 J. Anal. At. Spectrom., 1999, 14, 27–31Table 1 Blank value and standard deviation of the blank in ICP-MS Experimental data/counts s-1 Theoretical/counts s-1 Blank value Absolute standard Absolute standard Element 1 and 2 measured deviation of blank Blank value deviation of blank 89Y 1035; 984 26; 22 1000 32 52Cr 592; 633 47; 37 600 25 139La 666; 669 45; 20 670 26 142Nd 60; 75 10; 15 65 8 140Ce 77; 81 12; 8 81 9 90Zr 2107; 1182 216; 23 1600 40 Table 5 Theoretical and experimental RSDs and detection limits for Table 2 Theoretical and experimental RSDs and detection limits for 89Y in ICP-MS 142Nd in ICP-MS RSD (%) RSD (%) Concentration/ng mL-1 Theoretical Experimental Concentration/ng mL-1 Theoretical Experimental 0.1 0.99 0.45 0.1 0.65 0.71 0.5 0.14 0.08 0.5 0.24 0.18 1.0 0.14 0.075 1.0 0.075 0.05 2.0 0.045 0.037 2.0 0.089 0.043 5.0 0.051 0.011 5.0 0.024 0.005 20.0 0.011 0.007 20.0 0.025 0.013 100.0 0.011 0.01 100.0 0.0068 0.008 Detection limit 0.2 0.2 Detection limit 0.27 0.25 Table 6 Theoretical and experimental RSDs and detection limits for Table 3 Theoretical and experimental RSDs and detection limits for 52Cr in ICP-MS 140Ce in ICP-MS RSD (%) RSD (%) Concentration/ng mL-1 Theoretical Experimental Concentration/ng mL-1 Theoretical Experimental 0.1 0.37 0.42 0.1 0.36 0.64 0.5 0.085 0.38 0.5 0.10 0.067 1.0 0.067 0.029 1.0 0.05 0.27 2.0 0.031 0.086 2.0 0.045 0.019 5.0 0.027 0.01 5.0 0.018 0.038 20.0 0.010 0.018 20.0 0.014 0.011 100.0 0.008 0.008 100.0 0.007 0.01 Detection limit 0.1 0.5 Detection limit 0.1 0.12 Table 7 Theoretical and experimental RSDs and detection limits for Table 4 Theoretical and experimental RSDs and detection limits for 139La in ICP-MS 90Zr in ICP-MS RSD (%) RSD (%) Concentration/ng mL-1 Theoretical Experimental Concentration/ng mL-1 Theoretical Experimental 0.1 1.56 0.16 0.1 0.84 1.0 0.5 0.17 0.074 0.5 0.32 0.16 1.0 0.16 0.106 1.0 0.099 0.05 2.0 0.055 0.037 2.0 0.089 0.13 5.0 0.042 0.036 5.0 0.03 0.004 20.0 0.014 0.007 20.0 0.017 0.047 100.0 0.010 0.011 100.0 0.007 0.007 Detection limit 0.25 0.25 Detection limit 0.46 0.25 ni.d.Y 10–100, for example, for a mass spectrometer with a loss of ions occurs at the sampler and skimmer. For plasma diameters up to 15–20 mm and inner sampler and skimmer medium resolving power up to 1000–2000. For the Perkin- Elmer SCIEX Elan 250 the blank ion detector count is about diameters up to 0.8–1 mm, the coeYcient of ion use is <2×10-3–5×10-3. Further, the intermediate nebulizing gas 20, and we used this value for the calculation.The ionization eYciency may be calculated theoretically.1,2,7 The value for (argon) is diluted by the outer argon flow, and for argon intermediate flow rates up to 0.8–1 L min-1 and outer argon many elements is approximately 0.5–1 for ICP. The most diYcult problem is the theoretical characterization of ion use flow rates up to 10–15 L min-1 the loss of ions can reach up to 0.1–0.05. The loss of ions in the mass spectrometer must eYciency (bi) (or the mass spectrometer processing eYciency).We cannot calculate theoretically the loss of ions in the mass also be considered. Tanner et al. have shown1,2 that losses immediately after the skimmer based space charge are very spectrometer. This is a problem of theory in mass spectrometry. 1–5 We tried to estimate the eYciency of ion use with significant. For the product of all these coeYcients of ion losses, we can use the value bi=1×10-4 for the calculation the application of some general parameter in ICP-MS.A large J. Anal. At. Spectrom., 1999, 14, 27–31 29of the standard deviation and detection limits in ICP-MS. The combination of this theory with a computer simulation program may be useful for the more precise and rapid charac- This coeYcient is in keeping with literature data.1,2 Then for 89Y we have: terization of the possibility of instrumental improvements. However, because the theory proposed in eqns. (4)–(10) sbl=4.5 describes all the parameters of ICP-MS determination, it may be used for the indication of means for the improvement of Sv=6×1023×300×1×0.02×10-4/89×8000×1000×1×109 the characteristics of the ICP-MS method.According to eqns. =5×102 (8) and (9), the detection limit in ICP-MS and any other cm=3×4.5/5×102#2.5×10-2 ng ml-1=25 ng L-1 analytical method decreases with increase in the instrumental (11) sensitivity and decrease in the blank signal fluctuations. This corresponds to a decrease in the noise of the trace element The theoretical results obtained for the detection limit in ICP-MS correspond to the literature data for the instrument concentration determinations.According to eqn. (7), the instrumental sensitivity in ICP-MS may be increased by used.1 In real measurement conditions, the blank ion count improvements in the nebulization eYciency, ionization eYciency and ion use eYciency, and also the solution flow increases owing to the influence of the ICP source and mass spectrometer blank value.In this case the overall blank count rate. The application of the method of direct introduction of the sample into the plasma1 allows one to increase the absolute also increases and may reach a value much more than 100–1000. The maximum contribution for real samples with a detection limit more than 10-fold in comparison with the sample nebulization method.24–27 The theory permits the esti- complicated composition make matrix interferences of a diVerent type.1,2 The experimental data in Table 1 for the mation of this possibility exactly.A decrease in the blank fluctuations and matrix interferences is the other way to blank confirm this theoretical calculation. The pure theoretical calculations of the detection limits give results which are lower decrease the detection limit and improve the accuracy and instrumental parameters in ICP-MS. This approach is also than those of measurements under real conditions. Because we cannot predict theoretically the influence of diVerent factors widely used in practice.1 Hence the theory shows a direct way to improve the characteristics of the ICP-MS method.in the determination of the real blank value, we use the real conditions of the measurement. For the theoretical calculation The demonstration of the possibilities of the practical use of this theory is also valuable. Practical methods for the we take the real experimental results as the blank count, but the value of the fluctuation of the blank is calculated characterization of the standard deviation and detection limit under conditions of real analysis of diVerent samples were theoretically (see Table 1).The theoretical calculation of the standard deviation in given in a previous paper.20 With the theoretical eqns. (4)–(10) and the practical standard deviations for minimum numbers ICP-MS was realized with the use of experimental and theoretical measurement parameters. For the theoretical calculations of samples, one can calculate the standard deviation for any other concentration and the detection limit in real analysis.20 we applied eqn.(10). All the necessary parameters for the calculations are given above. The results of the theoretical This permits one to control the results of the analysis and the operation of the equipment. The results in Tables 2–7 calculation are given in Tables 2–7. Good agreement between the theoretical and experimental data is observed. The diVer- demonstrate these possibilities.Under real analysis conditions non-instrumental errors play ences may be explained by the errors in the experimental standard deviation calculation and the theoretical data calcu- an important role. In Fig. 1 and 2 the theoretical and experimental results for real samples are shown, with application lation. The random error for the standard deviations of our experiments may be equal to 0.5 of the values given in Tables of archive data for 52Cr and 89Y, respectively.The higher values of RSD for the experimental data do have a place 2–7 for the experimental data. According to the mathematical statistics, the random error of RSD determination in our within the theory, particularly for Cr and for trace concentrations (see also Table 3). We can see that the non- experiments can be calculated using the equation: relative random error of RSD=RSD1/2.8,9 For concentrations near instrumental errors are prevalent in the analysis of real samples.For Cr the importance of the non-instrumental errors the detection limit the value of this error is about 0.5. For Cr we can see also the influence of the non-instrumental blank, is particularly clear. The proposed theory permits us to examine and study also the non-instrumental errors. The next step resulting from the presence of Cr in the solution and the other systems of the mass spectrometer. The molecular ion overlap in the progress of the theory may be the theoretical study of non-instrumental errors and a search for ways to eliminate can also increase the blank and RSD for Cr.In Tables 2–7 are also given the theoretical and experimental results of the the influence of diVerent non-instrumental errors in ICP-MS analysis. calculation of the detection limits of the ICP-MS method. Using the Kaiser 3s criterion, the detection limits were determined like the concentration with the standard deviation s= 0.33 (see the data in Tables 2–7). The theoretical detection limits are controlled also by eqn.(8). These results agree with the limits of the experimental error of the detection limit determination and the errors of the theoretical parameter characterization. Hence the proposed theory shows the possibility of the theoretical study of the standard deviation and detection limit in ICP-MS. One can demonstrate the viability of this theory with Voigtman’s computer simulation program for analytical instruments.23 First it is necessary to emphasize that this theory gives a strict mathematical description of the process of the analytical measurement in ICP-MS. Some problems arise only because it is diYcult to give the theoretical value of some parameters of the analytical instrument and measurement.In this case the theory requires some assumptions and Fig. 1 Theoretical and experimental RSDs for the determination of 52Cr in practical samples.Line, theory; ×, experimental data. practical data. 30 J. Anal. At. Spectrom., 1999, 14, 27–31References 1 Inductively Coupled Plasmas in Analytical Atomic Spectrometry, ed. A. Montaser and D. W. Golightly, VCH, New York, 1992. 2 G. Jarvis and B. Houk, Handbook of ICP-MS, Chapman and Hall, London, 1992. 3 J. W. Olesik, in Abstracts of Winter Conference on Plasma Spectrometry, San Diego, California, January 10–15, 1994, IL1, 43 and ThP8, 227. 4 K. G. Heumann and B. Laser, in Abstracts of Winter Conference on Plasma Spectrometry, San Diego, California, January 10–15, 1994, TP44, 114. 5 J. W. McLaren, in Abstracts of Winter Conference on Plasma Spectrometry, San Diego, California, January 10–15, 1994, PL6, 308. 6 G. I. Kavalerov and S. M. Mandelshtam, Introduction to Fig. 2 Theoretical and experimental RSDs for the determination of Informative Theory of Measurements, Energy, Moscow, 1974. 89Y in practical samples. Line, theory; ×, experimental data. 7 Spectral Analysis of Pure Substances, ed. Kh. I. Silberstein, Khimiya, Leningrad, 1971 and 1994. 8 V. V. Nalimov, Application of Mathematical Statistics in Analysis of Substances, Fizmatgiz, Moscow, 1960. Conclusions 9 K. DoerVel, Statistik in der Analytischen Chemie, Leuna- Merseburg, Leipzig, 1966. The theoretical calculation of the standard deviation and 10 Y. L. Pliner, E. A. Svechnikova and V. M. Ogurzov, Control of detection limit in ICP-MS has been presented and theoretical Quality of Chemical Analysis in Metallugy, Metallurgiya, expressions for the standard deviation in ICP-MS have been Moscow, 1979.proposed. These expressions include all the parameters of the 11 P. W. J. M. Boumans, Spectrochim. Acta, Part B, 1991, 46, 641. 12 P. W. J. M. Boumans, Fresenius’ Z. Anal. Chem., 1979, 299, 337. ICP-MS instrument, the sensitivity of measurements, the 13 E. D. Prudnikov, Fresenius’ Z. Anal. Chem., 1981, 308, 339. blank fluctuations and the element concentration. The pro- 14 E. D. Prudnikov, Spectrochim. Acta, Part B, 1981, 36, 385. posed theory may be useful for the further development of 15 E. D. Prudnikov, H. Bradaczek and H. Labischinski, Fresenius’ Z. the instrumental and analytical characteristics of ICP-MS. A Anal. Chem., 1981, 308, 324. comparison of theoretical and experimental results showed 16 S. L. Mandelshtam, Zh. Prikl. Spectrosk., 1964, 1, 5. good agreement. The proposed theory may be used in 17 J. D. Winefordner and T. J. Vickers, Anal. Chem., 1964, 36, 161 and 1939. practice for the characterization of the standard deviation 18 E. D. Prudnikov, Zh. Anal. Khim., 1972, 27, 2327. and detection limit in the analysis of diVerent samples by 19 E. D. Prudnikov and Y. S. Shapkina, Analyst, 1984, 109, 305. ICP-MS. The possibilities of using this theory for the 20 E. D. Prudnikov, Fresenius’ J. Anal. Chem., 1990, 337, 412. examination of non-instrumental errors in analysis have been 21 E. D. Prudnikov, J. W. Elgersma and H. C. Smit, J. Anal. At. demonstrated. Spectrom., 1994, 9, 619. 22 E. D. Prudnikov and Y. S. Shapkina, Vestn. Sankt Peterburgskovo Univ., 1992, 25, 38. Acknowledgement 23 E. Voigtman, Anal. Chem., 1993, 65, 1029A. 24 G. Settembre and S. A. Koch, ICP Inf. Newsl., 1996, 22, 104. This study was supported by a Collaboration Research Grant 25 L. Moens, M. Van Holderbeke, F. Vanhaecke and R. Dams, ICP (No. CRG 950859) from the NATO International Scientific Inf. Newsl., 1996, 21, 815. Exchange Programme. 26 U. Vollkopf, ICP Inf. Newsl., 1996, 22, 487. 27 H. P. Longerich, D. Gunther, L. Forsythe and S. E. Jackson, ICP Inf. Newsl., 1995, 20, 784. Paper 8/00628H J. Anal. At. Spectrom., 1999, 14, 27–31 31

ISSN:0267-9477    

DOI:10.1039/a800628h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Inductively coupled plasma mass spectrometry analysis of wines Mercedes Yolanda Pe�rez-Jorda�n, Jose Soldevila, Amparo Salvador, Agustý�n Pastor and Miguel de la Guardia* Department of Analytical Chemistry, Faculty of Chemistry, Doctor Moliner St. 50, Burjassot, 46100-Valencia, Spain Received 8th May 1998, Accepted 22nd October 1998 An inductively coupled plasma mass spectrometry (ICP-MS) procedure has been developed for the determination of major elements, such as Mg, Na, K, Ca and Fe, minor elements, such as Al, Cr, Mn, Cu, Zn, Se, Sr, Br and Rb, and trace elements, such as Li, Ti, Ni, As, I, Ba, Pb, Sc, V, Co, Y, Zr, Mo, Sn, Cs, Ga, Nb, Pd, Cd, Sb, Hf, W, Hg, Tl, Th and U, in wines.The results obtained for Na, Mg, Al,Mn, Fe, Co, Zn, Rb and Cs were compared with those found by neutron activation analysis (NAA). Two ICP-MS calibration methodologies were used and results evaluated from spike recovery studies from which an average recovery of 102±20% was found for quantitative mode measurements.Multi-determination, using Be, Ge, In and Bi for the calibration of the ICP-MS sensitivity in the whole mass range and Rh as the internal standard, provided fast and accurate results, whereas the quantitative mode, using a series of external standard solutions, needs more time and consumes more reagent. A great number of articles can be found in the analytical Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Th, U and literature on the chemical characterization of wines, including Pb being monitored using In as internal standard, with limits investigations of their adulteration, studies of changes that of detection from 0.005 to 8 mg l-1 and recovery percentages take place during vinification and other researches focused on for all reported elements within 100±25%.The rest of the clearly establishing wine origin. papers were focused on the determination of Pb,15,18,19 with Mineral contents of wines depend on several factors, includdetection limits of 0.2 mg l-1.18 ing soil, area of production, type of grape, weather or environ- The accuracy of ICP-MS determinations has been evaluated mental conditions and viticultural practices.1–3 The by intercomparison using diVerent nebulization systems,2 or determination of the concentration of some elements is also from recovery studies,16,20 or by the analysis of reference of interest due to their toxicological or physiological materials of a diVerent nature than wines,15 which entails a characteristics.1 correct evaluation of the ICP-MS methodology through com- In the period from 1980 to 1997, 217 papers have been parison with a reference method also applied to the analysis published on trace element determination in wines.Atomic of natural samples of wine. spectrometric techniques were the most commonly used but The main objectives of this paper were the development of also other techniques such as molecular spectrometry, eleca methodology suitable for routine analysis of trace elements troanalytical methods or chromatography were employed for in wines by ICP-MS using a semi-quantitative procedure, this purpose.The main body of articles were focused on the based on an internal calibration, and the development of analysis of only a single element or just a few, because no another methodology using multi-calibration for quantitative multi-element techniques were used.However, modern analytanalysis, involving the ICP-MS data of Na, Mg, Al, Mn, Fe, ical methods, such as those based on inductively coupled Co, Zn, Rb and Cs validated by a comparison with those plasma atomic emission spectrometry (ICP-AES),4–14 ICPfound by NAA, and those obtained for additional elements MS2,15–20 or NAA,21,22 were also used to carry out multievaluated from spike recovery studies. element analysis of wines. Neutron activation analysis is a multi-elemental technique widely used in reference material certification, which has several advantages for direct sample Experimental measurement, although it suVers serious interferences caused Apparatus by major activation products.23 ICP-AES permits multi-element analysis but ICP-MS pro- An Elan 5000 ICP-mass spectrometer from Perkin-Elmer vides higher selectivity and sensitivity and a lower limit of SCIEX (Thornhill, Ontario, Canada), with a cross-flow nebudetection than ICP-AES, so that it could be preferable for the lizer from Perkin-Elmer (Norwalk, CT, USA) and a Minipuls multi-elemental analysis of wines.peristaltic pump from Gilson (Middleton, WI, USA), was In the literature concerning ICP-MS analysis of wines some used for ICP-MS measurements. The experimental conditions papers focused on the evaluation of rare element concen- employed in both the semi-quantitative and quantitative mode tration,2,16,17 but a few also studied other trace elements.17,20 are summarized in Table 1.Pb, Cd, Cr, V, Bi, Li, Ba, Rb, Mn, Fe, Cu, Ni, Sr, B, Cs, As The irradiation of samples was carried out in the swimming and Se were determined in wines, obtaining detection limits pool reactor Berlin II of the Hahn–Meitner Institut (Berlin, of the order of 0.3–3 ng l-1 and a recovery percentage of Germany), and detection was made by gamma-ray spec- 99–109% for the analysis of spiked samples. On the other trometry using an HPGe (Berlin, Germany) detector, the hand the determination of the authenticity of wine has been experimental conditions indicated in Table 1 being employed.carried out from its elemental composition,20 Li, Be, Al, Sc, Polyethylene vials (3 cm height and 1 cm id) and quartz Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Rb, ampoules (4 cm height and 0.5 cm id) were used to introduce samples into the nuclear reactor. An AE240 analytical balance Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, I, Cs, J.Anal. At. Spectrom., 1998, 13, 33–39 33Table 1 Operating conditions for inductively coupled plasma mass spectrometry analysis and neutron activation analysis. Isotopes monitored by ICP-MS were 7Li, 23Na, 24Mg, 27Al, 39K, 44Ca, 55Mn, 56Fe, 58Ni, 59Co, 63Cu, 64Zn, 69Ga, 75As, 85Rb, 88Sr, 114Cd, 133Cs, 138Ba, 202Hg, 208Tl, 209Pb in the quantitative mode, all isotopes being measured in the mass range from 6 to 238 in the semi-quantitative mode.Isotopes monitored by NAA were 24Na, 27Mg, 28Al, 42K, 56Mn, 59Fe, 60Co, 65Zn, 86Rb and 134Cs ICP-MS operating conditions Parameter Plasma conditions RF power 1100 W Plasma gas flow 14–15 l min-1 Nebulizer gas flow 0.85–0.95 l min-1 Auxiliary gas flow 0.75–0.85 l min-1 Sample flow rate 1 ml min-1 Mass spectrometer settings Parameter Semi-quantitative mode Quantitative mode Vacuum pressure 9.26×10-6 Torr 9.26×10-6 Torr Dwell time 250 ms 75 ms Sweeps/reading 1 5 Readings/replicate 1 1 Number of replicates 2 3 Total time 103 s 135 s NAA conditions at Berlin II nuclear reactor Parameter Long-time irradiation Short-time irradiation Irradiation time 42 h 2 min Waiting time 18 d – Measuring time 2 h 4 min Thermal neutron flux 2×1014 n cm-2 s-1 1×1013 n cm-2 s-1 Epithermal neutron flux 6.1×1012 n cm-2 s-1 1×1011 n cm-2 s-1 Fast neutron flux 5.7×1013 n cm-2 s-1 6.2×1010 n cm-2 s-1 from Mettler (Zu� rich, Switzerland) was used for weighing Dorset, UK), and ethanol 95–96% (v/v), Prolabo R.P.Normapur (Fontenay Sous Bois, France), were added to samples inside the irradiation containers. Polystar 100GE (Rische-Herfurth, Berlin, Germany) welding scissors and a standard and blank solutions in order to provide a final concentration of 1% (v/v) and 6% (v/v), respectively. home-made N2O–O2 welding torch were used to seal the containers. Argon C-45, high purity 99.995%, provided by Carburos Meta�licos (Barcelona, Spain), was used as the plasmogen.A freeze-drier Lyovac GT2 from Leybold Heraeus and an electric heater from Heraeus Instruments (Hanau, Germany) High purity water with a conductivity higher than 18.3 MV cm-1 was obtained using a Milli-Q water system were used in the initial treatment of samples. (Millipore Inc., Paris, France). A 10 000 mg ml-1 Ti amg ml-1 V standard solu- Reagents and samples tion, both prepared in 5% (v/v) HNO3, and an Mg, Al, Ti, A multi-elemental 1000 mg ml-1 standard solution containing K, Na and Mn multi-elemental standard solution containing Li, B, Al, Cr, Mn, Fe, Ni, Co, Cu, Zn, Sr, Ag, Cd, Be, Tl and 28, 28, 2.4, 20, 7 and 0.4 g l-1, respectively, were used as Pb, dissolved in 13% (v/v) HNO3 (Alfa, Karlsruhe, Germany), standards for short irradiation experiments. A Cs, Co, Fe, Rb was used to prepare diluted solutions of these elements, and a and Zn multi-elemental standard solution containing 2, 20, multi-elemental 1000 mg ml-1 standard solution containing Sc, 2000, 250 and 200 mg l-1, respectively, was used as a standard As, Se, Rb, Hg and Cs prepared from Sc2O3, As2O3, H2SeO3, for long irradiation experiments using the operating conditions RbCl, HgO and CsCl from Merck (Darmstadt, Germany), indicated in Table 1.dissolved in HCl 0.3 M, HCl 0.6 M, a mixture of 0.18 M HCl No reference wine materials can be found for trace element and 0.08 M HNO3, water, HCl 5 M and water, respectively, analysis in wines.Therefore, a biological reference sample was also used to prepare diluted standards. Seronorm Second Generation from Nycomed Pharma Sero Standard solutions (1000 mg ml-1) of Na, K, Mg and Ca A/S (Oslo, Norway), with a similar level of concentration to were prepared from their corresponding salts NaCl, KCl, that found in wines for the elements to be studied, was used Mg(NO3)2 and CaCO3, all from Merck, dissolved in water, in NAA experiments as a reference.except CaCO3 which was dissolved in 1 M HCl, and diluted Three red and three white Spanish bottled wines from standards were then prepared from these. diVerent areas (Rioja, Utiel-Requena and Valencia) were Rh 10 mg ml-1 standard solution prepared from RhCl3, employed as real samples. Merck, dissolved in 5 M HNO3, was used as the internal standard. Standard solutions (1000 mg ml-1) of Ge and Bi were pre- General procedures pared from GeO2 (Fluka, Buchs, Switzerland) and Bi(NO3)3 (Merck) dissolved in 1 M HCl and 0.5 M HNO3, respectively.ICP-MS analysis. For the semi-quantitative mode 5 ml of sample were introduced into a flask, and then 100 ml of the A 996 mg ml-1 Be standard solution was prepared from Be(NO3)2·4H2O (Merck) dissolved in 0.5 M HNO3 and a 10 mg ml-1 standard solution of Rh, 100 ml of the 10 mg ml-1 multi-elemental standard solution of Be, Ge, In and Bi and 1000 mg ml-1 standard solution of In was dissolved in 5% nitric acid (Alfa).These solutions were used to prepare a 150 ml of 70% HNO3 were added, before a final dilution to 10 ml with distilled water was made. 10 mg ml-1 solution for calibration of the whole mass range for the measurements made in the semi-quantitative mode. Blanks were prepared under the same conditions as samples but including 600 ml of ethanol and without adding Be, Ge, Nitric acid, BDH AnalaR 69.0–70.5% (Merck Ltd., Poole, 34 J. Anal. At. Spectrom., 1998, 13, 33–39In and Bi because blank corrections were made after adjusted to be at the same intensity level while that of 208Pb was set as low as possible.measurement. The parameters employed for data acquisition in the semi- For the quantitative mode, samples were prepared by 50% quantitative mode were fixed to reduce the number of sweeps (v/v) dilution with distilled water and addition of Rh and per reading and readings per replicate and to improve the HNO3. Standards of all elements to be determined were dwell time, without increasing excessively the time of analy- prepared in the concentration range from 0.050 ng ml-1 to sis,25 whereas those used in the quantitative mode were fixed 3 mg ml-1.The same concentrations of Rh, nitric acid and to maintain a short measuring time but averaging several ethanol as employed in the semi-quantitative mode were used replicates in order to be able to obtain the standard deviation for this kind of measurement. of the ion counting process.The operating conditions selected Samples, standards and blanks were aspirated into the throughout this study are summarized in Table 1. ICP-MS system by a peristaltic pump under the experimental Previous studies on wine analysis by ICP-MS reported that conditions shown in Table 1. the best sample dilution level, in order to obtain a low organic The data acquisition system allows one to obtain the concenmatter content, corresponded to 10% (v/v).18 However, in the tration of samples from the ratio between the signals provided paper of Baxter et al.,20 a 151 dilution was recommended.To by each element considered and the Rh internal standard, be able to determine elements at very low concentrations, using, in the semi-quantitative mode, the signals of Be, Ge, In throughout this study a 50% (v/v) dilution of samples was and Bi to calibrate the sensitivity of the instrument over the used. In the aforementioned conditions, and taking into whole mass range considered.For the use of external caliaccount that the wine samples analyzed contain between 11 bration the appropriate interpolation was carried out for each and 12.5% (v/v) alcohol, standards were prepared with 6% element in the corresponding calibration line using at least 20 (v/v) ethanol. standard solutions, covering the concentration range from 0.05 Table 2 shows, as an example, the eVect of the presence of to 3000 ng ml-1. ethanol on the slope of calibration lines obtained for some of the elements considered.As can be seen, it is absolutely Neutron activation analysis. Trace element analysis was necessary to match standards with a concentration of ethanol carried out using long- and short-time irradiation conditions, to avoid systematic errors, which can occur when aqueous according to the neutron cross section of the parent nuclides solutions are employed as standards for direct analysis of low of the elements to be determined.Thus, Cs, Cr, Co, Fe, Sc, diluted wine samples. Rb, Se and Zn were determined by lengthy irradiation of In fact, it can be seen from the data in Table 2 that for samples, whereas Al, Mg, Mn, K, Na and V were determined elements with a mass number lower than that of Rh, the using a short irradiation time. For long irradiation analyses presence of a 6% (v/v) concentration of ethanol reduces the 100 ml of each wine sample were weighed into a quartz value of the slope of calibration lines from 60%, in the case ampoule, which was heated at 50 °C for 24 h inside an oven, of Li, Co and Zn, to 90% in the case of Cd.On the contrary, and this procedure was repeated until a final mass of sample for elements with a mass number higher than Rh, like Pb, a of the order of 20 mg was obtained for approximately 1000 ml sensitivity increase of the order of 130% was found, this value of wine. Ampoules were then sealed with an N2O–oxygen being increased until reaching 170% for Bi.This eVect could flame. Two empty quartz ampoules, treated in the same way be overcome by using multiple internal standards, but it creates as those containing samples, were used as blanks. Standards the need for a series of elements absent in natural samples, were prepared in ampoules by the introduction of 2.5 mg of a thus increasing the complexity of these analyses. Because of stock standard solution of Cs, Cr, Co, Fe, Rb, Sc, Se and Zn, that it is preferable to match samples and standards with an using the same procedure as described above, or from a freeze- appropriate level of ethanol.dried serum. Two blanks, two reference materials and four The eVect of varying the ethanol concentration, from 4 to standards, prepared from inorganic salts, were introduced with 8% (v/v), on the relative signal of a 100 ng ml-1 Bi standard 16 samples into an aluminium container in two steps, with 12 produced variations of ±5%, thus validating the methodology quartz ampoules in each, and were then irradiated for 42 h employed for the range of alcohol contents of the wines under the conditions indicated in Table 1.After irradiation, analyzed. elements were determined by c-spectrometry, after a cooling time established as a function of the half-lives of the activation Comparative study of results obtained by semi-quantitative and products generated. quantitative mode in ICP-MS For short-time irradiation, samples, previously freeze-dried Semi-quantitative calibration is a very rapid methodology for until a viscous solution was obtained, were introduced into the determination of a large number of elements in the same polyethylene vials and then dried at 50 °C.A final mass of the order of 50 mg was employed. Two standards, one containing Table 2 Calibration lines obtained by ICP-MS in the quantitative a mixture of K, Al, Mg, Mn and Na, and another with V, mode for the diVerent elements considered with and without 6% were prepared in polyethylene vials from aqueous solutions of (v/v) ethanol inorganic salts.Four vials, two with samples and two with Calibration lines y=a+bx (r2) standards, were packed in a plastic bag and introduced in a polyethylene capsule, which was irradiated under the con- Elements Without ethanol With 6% (v/v) ethanol ditions indicated in Table 1, and then the elements were determined by c-spectrometry. Li y=-0.0388+0.0058x y=-0.0615+0.0038 x Additional details about the irradiation conditions can be r2=0.9983 r2=0.999 Co y=0.1141+0.0068x y=-0.0027+0.0042 x found in previously published documents.24 r2=0.9976 r2=0.9996 Zn y=0.0138+0.0015x y=-0.0007+0.0009 x Results and discussion r2=0.9988 r2=0.9999 Cd y=0.014+0.0019x y=0.0103+0.0018 x Experimental conditions employed in ICP-MS determination r2=0.9993 r2=0.9997 Pb y=0.0336+0.0028x y=0.0253+0.0038 x ICP-MS experimental conditions were investigated in order to r2=0.9983 r2=0.9992 obtain the maximum sensitivity and as short a measuring time Bi y=0.0723+0.0037x y=0.053+0.0062 x as possible.The 103Rh ion signal was adjusted to be as high r2=0.9967 r2=0.9984 as possible and, at the same time, 24Mg and 207Pb were J. Anal. At. Spectrom., 1998, 13, 33–39 35sample with an appropriate precision and accuracy for routine for pattern recognition analysis based on ICP-MS data, as has been explored by Baxter et al.20 analysis. To achieve this, Be, Ge, In and Bi, which are absent in the samples to be analyzed, were used in order to calibrate Concerning the repeatability of trace metal contents, it can be seen that an average coeYcient of variation of 6% was the signals in the whole mass range considered.To evaluate the applicability of this strategy in the analysis of natural wine obtained for all elements in all of the samples with just a few exceptions, these being elements with an extremely low concen- samples, these were analyzed also by using an external calibration and employing Li, Na, Mg, Al, K, Ca, Mn, Fe, Co, tration, such as Pd, Hg, W and Th, for which variation coeYcients of the order of 20% or more were found in some Ni, Cu, Zn, Ga, As, Rb, Sr, Cd, Cs, Ba, Hg, Tl and Pb as test elements.samples, and other elements, such as Sc, Ti, Cr or Se, for which molecular interferences have been reported.26 The results obtained for trace element determination in natural wine samples analyzed by both semi-quantitative and The results found for a selected number of 22 common elements, for which standards are easily available, also show quantitative ICP-MS modes are indicated in Tables 3 and 4 respectively.The standard deviation for 3 replicates is also (see Table 4) that major, minor and trace elements could be determined with an average coeYcient of variation of 5% by indicated as an estimation of the precision of these results. As can be seen in Table 3, 40 elements, at major, minor and ICP-MS using long concentration range calibration lines obtained from 0.05 to 3000 ng ml-1.trace levels, could be determined in a sample in less than 2 min, having observed that Na, Mg, K, Ca and Fe are, in A comparative study between the results obtained by both semi-quantitative and quantitative ICP-MS modes showed general, at concentrations higher than 1000 mg ml-1, Al, Cr, Mn, Cu, Zn, Se, Br, Rb and Sr between 100 and 1000 ng ml-1, that for 18 of the 22 elements determined by both methodologies, comparable results were found.For As, a systematic and the other elements at levels of a few ng ml-1, or parts of ng ml-1. average diVerence of +200% was found between the results obtained by the semi-quantitative and quantitative modes, In general, the content of trace metals varies with the nature and the origin of samples, thus opening up exciting possibilities while for Ga and Hg systematic errors of the order of -70% Table 3 Results obtained by ICP-MS analysis of wines using the semi-quantitative mode, expressed in ng ml-1 (*expressed in mg ml-1). Concentration values correspond to the average of 3 replicates±the corresponding standard deviation Samples (ethanol percentage content) 1 2 3 4 5 6 Red/Valencia Red/Utiel-Req.Red/Rioja White/Valencia White/Utiel-Req. White/Rioja Element LODb (11.5%) (12%) (12.5%) (11.5%) (12.5%) (11%) Li 3 13±1 13.73±0.01 28.8±0.2 13.0±0.5 10.1±0.8 29±1 Na* 3 18±0.3 13±1 17± 0.1 15±1 15± 1 12.9±0.8 Mg* 0.3 44±3 92± 4 89± 2 69± 3 21.3±0.4 66±1 Al 0.2 960±30 537±30 968±10 810±10 304±7 620±10 K* 2 460±40 1140±50 487±6 400±100 221±3 420±10 Ca* 0.6 109±6 62± 1 85± 3 103.2±0.9 68±1 92± 3 Sc 0.02 2.2±0.3 2.23±0.08 2.0±0.3 2.12±0.03 1.4±0.3 1.3±0.4 Ti 0.4 30±20 27±30 13±7 11± 2 9±6 7.1±0.2 V 0.3 7.3±0.3 12.4±0.4 55.1±0.9 12.9±0.2 13.6±0.5 36.5±0.7 Cr 0.04 300±100 290±20 340±20 290±40 220±10 220±20 Mn 0.1 186±5 540±20 700±40 280±10 274±9 662±5 Fe 18 2010±40 3490±40 4000±200 1720±60 900±20 2990±40 Co 0.03 2.04±0.07 2.4±0.1 3.3±0.15 1.65±0.03 2.70±0.04 1.90±0.08 Ni 2 13±1 13.3±0.2 18.9±0.5 9.1±0.3 11.7±0.9 13±1 Cu 0.05 130±1 170±10 79±4 183±5 490±10 28.4±0.6 Zn 0.7 390±10 350±20 470±10 194±9 627±10 231±6 Ga 0.008 0.76±0.03 1.4±0.1 1.5±0.2 1.2±0.1 2.60±0.06 0.69±0.06 As 0.6 24.4±0.3 12.2±0.4 20.3±0.6 12.2±0.3 15.6±0.3 12.4±0.7 Se 4 130±7 36± 9 40± 20 45±7 43± 5 20± 10 Br 40 400±50 300±10 280±10 92±4 115±9 150±10 Rb 0.06 185±5 571±3 653±6 189±4 299±1 475±3 Sr 0.1 1000±10 860±20 1320±40 720±20 780±20 930±20 Y 0.005 1.07±0.04 0.37±0.03 2.00±0.07 0.57±0.04 0.37±0.06 0.75±0.05 Zr 0.03 2.2±0.1 3.7±0.3 4.2±0.3 5.20±0.08 2.7±0.1 3.4±0.1 Nb 0.001 0.13±0.01 0.29±0.04 0.20±0.02 0.08±0.02 0.30±0.02 0.39±0.01 Mo 0.04 1.6±0.1 5.6±0.3 1.68±0.05 2.3±0.1 8.2±0.3 1.6±0.4 Pd 0.02 NDa 0.03±0.01 ND ND ND ND Cd 0.06 ND ND 0.53±0.06 ND ND 0.35±0.06 Sn 0.08 1.7±0.2 3±1 0.6±0.1 6.6±0.3 15.5±0.2 0.99±0.09 Sb 0.002 0.9±0.09 0.8±0.1 0.60±0.02 1.31±0.04 1.9±0.2 1.27±0.03 I 0.2 22±2 10.0±0.3 8.0±0.6 13±1 8.8±0.4 6.9±0.4 Cs 0.01 0.21±0.002 1.31±0.02 3.51±0.04 0.04±0.004 0.81±0.02 3.45±0.05 Ba 0.06 66±2 87± 2 87± 1 40.9±0.7 47±1 39.4±0.5 Hf 0.005 0.04±0.02 0.15±0.01 0.16±0.02 0.13±0.01 0.16±0.02 0.11±0.05 W 0.01 0.25±0.06 0.09±0.03 0.55±0.05 0.25±0.01 0.11±0.01 0.6±0.4 Hg 0.005 0.29±0.04 0.20±0.04 0.19±0.01 0.17±0.01 0.31±0.04 0.31±0.08 Tl 0.001 ND 0.08±0.02 0.15±0.02 ND ND 0.08±0.02 Pb 0.02 43.8±0.3 52±2 39± 2 40± 2 61.5±0.8 26.2±0.8 Th 0.007 0.1±0.02 ND 0.06±0.02 0.03±0.01 ND 0.03±0.01 U 0.005 0.83±0.06 0.19±0.03 0.6±0.1 1.21±0.05 1.12±0.07 0.62±0.02 aND, not detectable values.bLOD, limit of detection values in ng ml-1. 36 J. Anal. At. Spectrom., 1998, 13, 33–39Table 4 Results obtained by ICP-MS analysis of wines using the quantitative mode, expressed in ng ml-1 (*expressed in mg ml-1). Concentration values correspond to the average of 3 replicates±the corresponding standard deviation Samples (ethanol percentage content) 1 2 3 4 5 6 Red/Valencia Red/Utiel-Req.Red/Rioja White/Valencia White/Utiel-Req. White/Rioja Element LODb (11.5%) (12%) (12.5%) (11.5%) (12.5%) (11%) Li 0.2 18.9±0.5 19.8±0.9 44±1 20.0±0.2 14.9±0.2 39.5±0.5 Na* 10 18±4 17.6±0.9 26±1 18.5±0.5 13.4±0.9 19.8±0.6 Mg* 100 77±2 94± 4 98± 2 69± 2 67± 2 73.7±0.7 Al 0.6 1360±30 800±30 1210±20 1000±20 240±10 840±10 K* 90 770±92 460±40 700±200 620±43 470±10 300±200 Ca* 30 81±3 57± 2 73± 2 99± 2 68.4±0.6 97±1 Mn 0.05 200±20 690±80 940±20 360±20 251±7 800±200 Fe 6 2500±100 3600±400 4680±80 1460±20 1001±50 3000±40 Co 0.004 1.99±0.001 2.5±0.1 4.0±0.1 1.56±0.03 2.79±0.04 2.14±0.08 Ni 0.003 9.6±0.2 13.9±0.6 17.8±0.3 11.0±0.2 13.66±0.09 16.2±0.4 Cu 0.003 112±2 190±2 76.0±0.9 229±4 609.2±0.2 36.6±0.5 Zn 0.6 320±2 309±9 385±5 199±4 575±1 330±80 Ga 0.02 NDa 5.4±0.2 5.4±0.2 3.05±0.07 3.13±0.03 2.79±0.05 As 0.5 ND 3.87±0.06 4.8±0.2 4.9±0.4 5.00±0.08 3.9±0.1 Rb 0.06 186±5 600±10 690±20 190±2 322±12 685±8 Sr 0.005 840±20 890±20 1280±30 810±9 889±6 1120±10 Cd 0.03 0.21±0.01 0.30±0.02 0.64±0.04 0.20±0.03 0.38±0.03 0.43±0.02 Cs 0.007 0.23±0.05 1.43±0.03 3.5±0.1 0.054±0.004 0.911±0.03 3.82±0.02 Ba 0.003 77±2 100±3 106±3 53± 1 59.8±0.3 51.2±0.6 Hg 0.5 13±9 1.7±0.4 0.7±0.1 4±1 1.0±0.1 0.58±0.08 Tl 0.0005 0.077±0.003 0.160±0.003 0.26±0.01 0.052±0.004 0.12±0.01 0.18±0.01 Pb 0.02 47±1 60± 1 48± 1 50.7±0.6 76.7±0.8 32.2±0.3 aND, not detectable values. bLOD, limit of detection values in ng ml-1.and -80%, respectively, were found. On the other hand, mode (x) after removing As, Ga, Hg and K for which noncomparable results were found. These points provide a results for K were diVerent without any systematic bias. These diVerences are probably due to the lack of an appropriate regression line, y=-0.00907+0.9959 x, with a regression coeYcient r=0.982 by weighted least squares fitting. Two calibration in the semi-quantitative mode, but additionally to problems found in the quantitative mode when extremely high statistical tests were applied to compare the aforementioned regression line with an ideal line y=0+1x.In order to ensure concentration ranges were employed for standardization, because the use of highly concentrated standards modifies the the comparability of the two data populations, values obtained for the intercept (a) and the slope (b) were compared with 0 confidence interval of the calibration line by changing the middle point of the line, thus aVecting the repeatability of and 1, respectively, using two tests: |b-1| must be lower than the product of tsb, t being the statistical parameter of Student signals and increasing the errors in the quantitative determination of trace elements.for a probability level of 95% (1.96) and sb the standard deviation (0.0024) of the slope of the regression line; on the Fig. 1 shows the plot of the mean values (in ng ml-1) obtained by use of the semi-quantitative mode (y), for each other hand |a-0| must be lower than the product of tsa, where sa is the standard deviation (0.0058) of the intercept of the element and sample, against those found by the quantitative regression line.The application of these tests shows that the obtained line is statistically comparable with the ideal one, thus demonstrating that both ICP-MS measurement modes give similar results for the elements and samples considered, therefore providing, with the semi-quantitative ICP-MS measurements, a good idea of the concentration level of as many elements as possible in wine samples in a short analysis time. Validation of ICP-MS procedure by comparison with results obtained by NAA Average values and the corresponding standard deviations of 3 replicate analyses obtained by NAA for Na, Mg, Al, K, Mn, Fe, Co, Zn, Rb and Cs are shown in Table 5.The only elements determined were those with a concentration level which could be determined by NAA using the procedure indicated in the experimental part.The results obtained by quantitative mode ICP-MS (y) and those obtained by NAA (x) have been compared for Na, Mg, Al, Mn, Fe, Co, Zn, Rb and Cs in the natural wine samples analyzed. K was removed for this comparison because of a severe lack of repeatability. Fig. 2 shows the regression between these two data populations. A correlation coeYcient of r= Fig. 1 Comparison between quantitative and semi-quantitative data obtained by ICP-MS in the analysis of 18 elements in six wine samples. 0.983 was obtained for a regression line, established by weighed J. Anal. At. Spectrom., 1998, 13, 33–39 37Table 5 Results obtained by neutron activation analysis of wine samples, expressed in ng ml-1 (*expressed in mg ml-1). Concentration values correspond to the average of 3 replicates±the corresponding standard deviation Samples (ethanol percentage content) 1 2 3 4 5 6 Red/Valencia Red/Utiel-Req.Red/Rioja White/Valencia White/Utiel-Req. White/Rioja Element LODa (11.5%) (12%) (12.5%) (11.5%) (12.5%) (11%) Na* 800 11.3±0.4 15.2±0.7 24±2 17.2±0.7 8.7±0.3 16.2±0.4 Mg* 50 40±2 81± 2 92± 7 61± 2 28± 5 59± 1 Al 6 681±0 730±60 1312±7 1010±80 230±40 810±20 K* 700 440±50 600±50 500±50 400±100 200±60 400±100 Mn 0.2 172±0.007 650±20 880±70 360±10 231±5 670±40 Fe 1 2400±100 3320±70 4600±600 1810±20 890±50 3100±100 Co 0.001 2.01±0.06 2.80±0.07 3.8±0.4 1.77±0.02 2.8±0.2 2.00±0.09 Zn 0.03 335±12 316±5 390±60 189±3 530±40 300±10 Rb 0.2 210±10 660±20 730±20 226±7 360±20 610±40 Cs 0.003 0.24±0.04 1.58±0.06 4.0±0.5 0.05 0.98±0.05 4.4±0.2 aLOD, limit of detection values in mg ml-1.Fig. 3 Relative accuracy errors obtained by comparison between quantitative ICP-MS and NAA data found for diVerent elements and samples. Fig. 2 Comparison between neutron activation analysis and quantitative ICP-MS determination of 9 elements in six wine samples.Table 6 Recovery percentages obtained for the analysis of spiked wine samples by ICP-MS in the quantitative mode. Recovery percentages minimum least squares fitting, of y=0.0014+1.008 x with sa= were established from data found on diVerent wine samples spiked with known amounts of elements from 50 to 200 ng ml-1 0.0039, sb=0.006 and t=1.96. It can be concluded that the results obtained by both techniques are similar and thus the Red wine Sweet wine White wine same accuracy level can be obtained for the elements evaluated, recovery recovery recovery providing a validation of the measurement process by ICP-MS Element percentage±s percentage±s percentage±s for these wine elements.In order to evaluate the comparability of the results found Li 143±4 119±7 130±6 Sc 108±5 99± 2 107±3 for each particular element and sample, relative accuracy error Mn 150±30 103±9 87± 8 percentages were calculated from the diVerence between Ni 118±3 118±2 111±4 ICP-MS and NAA.Fig. 3 shows the experimental results Co 120±2 120±2 116±5 obtained and it can be seen that for all the elements, except Cu 120±10 115±2 107±4 for Na and Mg in samples 1 and 5 and for Al in sample 1, Zn 130±10 134±4 108±5 for which the errors found were higher than 30%, the average Ga 106±2 111±1 105±4 As — 95±3 115±4 diVerences are in general lower than ±10%, with only a few 77Se 106±7 83± 6 — data with relative diVerences between ±10 and ±20%. 82Se 108±6 82± 4 — For the evaluation of the accuracy of the data obtained for Rb 110±10 60±10 110±7 those elements not determined by NAA, recovery studies on Sr 80±10 88±6 70± 10 spiked wine samples were carried out. Cd 97±3 96± 2 96± 3 Natural samples of red, white and sweet wines were spiked Cs 100±2 107±2 92± 3 Ba 96±5 107±1 94± 3 with known concentrations of Li, Sr, Mn, Ni, Co, Cu, Zn, Hg 75±2 78± 3 63± 3 Ga, As, Se, Rb, Sr, Cd, Cs, Ba, Hg, Tl, Pb and Bi of Tl 94±6 97± 1 88± 3 50–200 ng ml-1, and the samples analyzed by ICP-MS using Pb 89±7 94± 1 83± 3 the quantitative mode.The average results found are summa- Bi 86±8 103±3 85± 4 rized in Table 6, from which it can be concluded that all values 38 J. Anal. At. Spectrom., 1998, 13, 33–393 I. E. Frank and B. R. Kowalski, Anal. Chim. Acta, 1984, 162, 241. were within 102±20% in the case of quantitative 4 G. Thiel and K. Danzer, Fresenius J. Anal. Chem., 1997, 357, 553. measurements. 5 M.Bodyne-Szalai and P. Fodor, Magy. Kem. Foly., 1996, 102, 411. 6 M.Lo� pez-Artiguez, A. M. Camean and M. Repetto, J. AOAC. Conclusions Int., 1996, 79, 1191. 7 J. Yang, X. Zeug and B. Huang, Fenxi Huaxue, 1991, 19, 362. The procedure proposed for the ICP-MS analysis of wines is 8 H. Eschnauer, L. Jakob, H. Meierer and R. Neeb, Mikrochim. based on a direct 151 dilution with distilled water and does Acta, 1989, III, 291. not require any additional pre-treatment of samples.However, 9 S. Zhu, Z. Chen and L. Huang, Fenxi Huaxue, 1988, 16, 556. standards and blank solutions must be prepared in order to 10 L. Gu, Z. Zhou, R. Shen and H. Shi, Fenxi Huaxue, 1987, 15, compensate for the ethanol concentration of diluted samples 1140. to avoid systematic errors. 11 A. Voulgaropoulos and T. Soulis, Connaiss. Vigne Vin, 1987, 21, 23. It has been shown that semi-quantitative and quantitative 12 J. Zihlmann, Labor Betr., 1987, February, 15. ICP-MS oVer valuable alternatives for the multi-elemental 13 F.S. Interesse, G. D3Avella, V. Alloggio and F. Lamparelli, Z. determination of major, minor and trace components in wines. Lebensm.-Unters.-Forsch., 1985, 181, 470. Both methodologies can provide useful values, the semi- 14 F. S. Interesse, F. Lamparelli and V. Allogio, Z. Lebensm.- quantitative mode being a fast way of obtaining information Unters.-Forsch., 1984, 178, 272. about as many elements as possible in the same sample, thus 15 L.Moens, H. Vanhoe, F. Vanhaeke, J. Goossens, M. Campbell R. Dams, J. Anal. At. Spectrom., 1994, 9, 187. providing a good methodology for quality control, and the 16 A. Stroh, P. H. Brueneckner and U. Voellkopf, At. Spectrosc., quantitative mode requiring a careful standardization with 1994, 15, 100. appropriate solutions of the elements to be determined at 17 E. McCurdy, D. Potter and M. Medina, Lab. News, 1992, concentration levels comparable to those present in the September, 10. samples. 18 J. Goossens, T. De Smaell, L. Moens and R. Dams, Fresenius On the other hand, the comparability of data found for Na, J. Anal. Chem., 1993, 347, 119. 19 J. Goossens, L. Moens and R. Dams, Anal. Chim. Acta, 1994, Mg, Al, K, Mn, Fe, Co, Zn, Rb and Cs by ICP-MS and by 293, 171. NAA, employed as a reference procedure, indicates the accu- 20 M. J. Baxter, H. M. Crews, M. J. Dennis, I. Goodall and D. racy of the methodology employed, which has also been Anderson, Food Chem., 1997, 60, 443. validated for the other elements through spike recovery studies, 21 A. R. Byrne, M. Dermelj, L. Kosta and M. Tusek Znidaric, obtaining an average recovery of 102±20%. Mikrochim. Acta, 1984, I, 119. 22 S. May, H. Leroy, D. Piccot and G. Pinta, J. Radioanal. Chem., 1982, 72, 305. Acknowledgements 23 P. Bra�tter, Radiochim. Acta, 1983, 34, 85. 24 D. Gawlik and T. Robertson, Irradiation Devices at the Upgraded Authors acknowledge the financial support of the EU Program Research Reactor BER II, Hahn-Meitner Institutut, Berlin, ‘Human Capital and Mobility’ for the access to NAA in Berlin Germany, 1992. and are grateful to Dr. Virginia Negretti, Dr. Peter Bratter 25 J. Soldevila, M. El-Himri, A. Pastor and M. de la Guardia, and Dr. Dieter Gawlik for their help in the NAA. ICP-MS J. Anal. At. Spectrom., 1998, 13, 803. 26 S. E. Long and T. D. Martin, ICP Inf. Newsl., 1991, 16, 460. determinations have been supported by the project of the Generalitat Valenciana 2232/94. Paper 8/03476A References 1 C. Baluja-Santos and A. Gonza�lez-Portal, Talanta, 1992, 39, 329. 2 S. Augagneur, B. Me�dina, J. Szpunar and R. £obin� ski, J. Anal. At. Spectrom., 1996, 11, 713. J. Anal. At. Spectrom., 1998, 13, 33&n

ISSN:0267-9477    

DOI:10.1039/a803476a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Seasonal and regional variations of iodine in Danish dairy products determined by inductively coupled plasma mass spectrometry Erik H. Larsen,* Pia Knuthsen and Marianne Hansen Institute of Food Research and Nutrition, Danish Veterinary and Food Administration, 19 Mørkhøj Bygade, DK-2860 Søborg, Denmark. E-mail: ehl@vfd.dk Received 24th August 1998, Accepted 11th November 1998 The content of iodine in 72 samples of Danish dairy products was determined by direct flow injection (FI ) sample introduction of whole milk into the ICP-MS instrument, or by bomb ashing of cream and cheese samples prior to the ICP-MS measurement. The performance of the FI-based method was superior to the bomb ashing method in terms of the limit of detection which was 9 ng g-1 and 60 ng g-1 for the two methods, respectively, and in repeatability which was 8.4 and 45 ng g-1, respectively.Both methods of analysis were accurate as demonstrated by analyses of the CRM 063R Skim Milk Powder.The iodine content of the milk samples varied between 42–162 ng g-1. A geographical diVerence in the iodine content showed that milk from Jutland contained less iodine than milk from Sealand. This can be explained by the lower natural iodine content in the drinking water resources in Jutland. A temporal diVerence showed a general increase in iodine concentration in milk from all regions during the winter months over the summer months. This can be explained by the use of iodine-enriched fodder during the winter months.The poorer repeatability of the analyses of cream and cheese samples obscured any possible geographical and temporal variation of iodine in these samples. Iodine is an essential element to man, and a constituent of the Experimental thyroid hormones. Deficiency of iodine causes goitre.1 Like Samples and sampling many other countries Denmark is a naturally iodine-deficient region and the average daily intake of iodine2 is about The sampling was planned to illustrate the average contents 50–100 mg, which is less than the Nordic recommendation3 at of a range of nutrients including iodine in dairy products 150 mg.Because iodine enrichment of food or table salt has sold in Denmark, and also to include possible regional and been allowed only recently, the occurrence of non-toxic goitre seasonal variations in the iodine content of the samples.10 A among the Danes is relatively high.2 total of 24 samples of each of whole milk (3.5% fat), cream In Denmark, milk products (27%), water and other (38% fat) and Danbo cheese (firm cheese, 45% fat in dry beverages (26%), fish (16%) and eggs (10%) are the main matter) were taken throughout several months in 1995 from contributors to the dietary iodine intake.2 The Danish Food three dairies which are located in Northern and Central Monitoring Programme4,5 has shown that the iodine content Jutland (Hjørring, Hobro and Bjerringbro), in Southern of dairy products sampled in 1985 and 1990 varied according Jutland (Tyrstrup, Christiansfeld and Ribe) and, in the case to the season of the year and according to the geographical of milk and cream, additionally on the island of Sealand origin of the milk.The dietary iodine intake was lowest in (Slagelse). All dairies received the raw milk predominantly Jutland and highest on Sealand.2 This variation could be from regional farmers, and the samples were taken from the ascribed to geographical diVerences mainly in the iodine ordinary productions at the dairies.The samples were frozen content of potable water. Therefore, in order to be able to immediately after arrival in the laboratory and kept at assess the variation in the iodine intake with the diet in the -18 °C until the time of analysis. late 1990s, data on the geographical and seasonal variation in the iodine content of particularly milk products and water are important. Standard substances and chemicals A variety of analytical methods including spectrophotometry6 and neutron activation analysis7 have been used for An aqueous standard stock solution at 1000 mg ml-1 of iodine was prepared from potassium iodate Volumetric Standard the determination of iodine in food.Owing to its high selectivity and sensitivity, ICP-MS is a useful modern detector for (Merck, Darmstadt, Germany). Working standard solutions at 10 mg ml-1 were prepared daily from the stock solution by the determination of iodine in biological samples.Furthermore, sample preparation and manipulations can be dilution with water. Water (>18 MV cm-1) was produced in a Millipore Super-Q apparatus (Millipore, Milford, MA, reduced to a minimum8 or completely omitted if direct analysis of liquid samples using flow injection (FI ) sample introduction USA). Nitric acid, pro analysi, which was sub-boil distilled in an all-quartz apparatus (Hans Ku� rner, Rosenheim, Germany) into the ICP-MS instrument is employed.9 The aims of this paper are two-fold.Firstly, the figures of and perchloric acid, Suprapur (Merck, Darmstadt, Germany), were used for the wet ashings of the cream and cheese samples. merit obtained during routine use of a recently developed method of analysis for iodine in food8 are given. Secondly, An aqueous solution containing 0.07 mol l-1 of tetramethylammonium hydroxide (TMAH) and 0.05 mol l-1 of potassium the iodine content in Danish milk, cream and cheese are reported and discussed with emphasis on geographical and hydroxide was used to dilute the milk samples prior to the direct iodine analysis.seasonal variations. J. Anal. At. Spectrom., 1999, 14, 41–44 41Sample preparation sample introduction, a Meinhard TR-30-K3 glass concentric nebulizer (Meinhard, Santa Ana, CA, USA) was used through- The samples of milk (0.5 g) were mixed with 9.5 ml of the out in combination with a low dead-volume cyclonic glass alkaline diluent in the ICP-MS autosampler vials and were spray chamber (Glass Expansion, Victoria, Australia).This analysed with no further pretreatment. nebuliser and spray chamber assembly improved the ICP-MS The cream and cheese samples (0.3 and 0.5 g, respectively) signal-to-noise ratio for iodine by approximately 50% comwere dissolved by pressurised ashing using a mixture of 3.5 ml pared with a conventional Scott type double-pass spray nitric and 1.5 ml perchloric acid as described in detail by chamber.8 Larsen and Ludwigsen.8 The PTFE-lined steel bombs were heated at 160 °C for 4 h.The acid mixture oxidised potentially Contamination control and quality assurance volatile iodine species present in the sample to non-volatile species such as iodate. In contrast to iodide these oxidised Prior to using new PTFE liners in the pressure bombs for analytical work, the iodine content therein was reduced by species did not exhibit any memory and adhesion eVects in the ICP-MS instrument. The wet-ashed residue was diluted to treatment by the nitric acid–perchloric acid mixture at 160 °C for 4 h.The acid residue was discarded and the liners rinsed 50 ml by water prior to determination of iodine. three times with water. During routine analytical work the remaining iodine contamination of the PTFE material was ICP-MS determination of iodine kept to a minimum by extraction with base between each The diluted milk sample was pumped from the AS-90 analytical run.For this purpose an aqueous solution of 1% autosampler to fill the 500 ml sample loop of the FIAS 400 FI (m/v) TMAH was run into the liners which were heated at unit. The alkaline diluent was used as carrier solution for 90 °C in a laboratory oven for 12 h. Following this treatment introduction of the milk samples with no further pretreatment the base was discarded and the clean liners were air dried. into the ELAN 5000 ICP-MS instrument (Sciex Perkin-Elmer, On each day of analysis the 14 analytical places available Concord, Ontario, Canada).9 The determination of the iodine comprised eight unknown samples of similar composition, one content in the cream and cheese sample digests was carried of which was determined in duplicate, four blanks which were out by conventional continuous sample aspiration into the taken through the entire procedure and one determinatioof ICP-MS.The instrumental settings used are given in Table 1.the CRM 063R Skim Milk Powder11 (Commission of the The method of standard additions was used for European Communities, Community Bureau of Reference, quantification of the iodine. Three spikes of iodine (as iodate) Brussels, Belgium). were added at 2, 4 and 8 ng ml-1 to the diluted milk samples and at 1, 2 and 4 ng ml-1 to the diluted wet-ashed sample Results and discussion residues. Iodate was used because iodine presumably occurred as this species following the wet ashing procedure, and was Method performance and quality assurance added at 100%, 200% and 400% of the natural iodine content to achieve optimum precision of the results.In order to assure the quality of the analyses a set of acceptance criteria was applied to the raw data. The criteria included the To minimise the risk of losses and memory eVects during magnitude and variation of the blank (contamination control ), the relative standard deviation (RSD) of the recorded ICP-MS Table 1 ICP-MS and sample introduction settings signal for each sample (conversion of volatile to non-volatile iodine species during bomb-ashing), and the correlation ICP-MS instrument— coeYcient of the standard additions calibration curve Rf power 1200 W (precision).8 Sampler and skimmer cones Platinum The results obtained for iodine in the blank solutions from Argon flow rates the analytical work are plotted in Fig. 1. Values which exceeded Outer 15 l min-1 Intermediate 0.8 l min-1 the average plus three standard deviations of the preceding Nebulizer 1 l min-1 (variable) values were not accepted for analysis and reflected incompletely Mass-to-charge ratio detected m/z 127 Conventional sample aspiration Dwell time 1000 ms Sweeps per reading 3 Readings per replicate 1 Number of replicates 10 Flow injection sample introduction Dwell time 60 ms Sweeps per reading 1 Readings per replicate 250 Number of replicates 1 Scanning mode Peak hop Spray chamber and nebuliser Glass cyclonic with a Meinhard assemblies (type TR-30-K3) concentric nebuliser Data acquisition mode Quantitative Sample introduction systems— Conventional Wash time between samples 120 s Read delay 80 s Peristaltic pump speed 1.4 ml min-1 Flow injection Injection loop volume 500 ml Carrier solution 0.07 mol l-1 Tetramethyl- Fig. 1 Control chart for procedural blank values in the diluted sample ammonium hydroxide and solution. The average value is marked as X, and the average value 0.05 mol l-1 potassium plus 3 standard deviations of the blank (X+3s) is the upper hydroxide in water rejection limit. 42 J. Anal. At. Spectrom., 1999, 14, 41–44Table 2 Figures of merit for iodine determinations by ICP-MS in cream and cheese using wet ashing or in milk using flow injection (FI ) sample introduction Wet ashing FI Blank value, x±s/ng ml-1 0.32±0.21 n.d.a (n=20) Limits of detection Samples/ng g-1 60 or 100b 9c Sample solutions/ng ml-1 0.62 0.45c Sensitivity (standard additions) Mean value/counts s-1 per ng ml-1 1477 4530 (n=9) (n=7)d Within-day variation, RSDe(%) 4.0 4.1 Between-day variation, RSD(%) 8.6 n.d.a Repeatabilityf Standard deviation, sr/ng g-1 16 3.0 Repeatability, 2.8 sr/ng g-1 45 8.4 Fig. 2 Control chart with upper and lower warning and rejection Reproducibility/ng g-1 limits for the CRM 063R Skim Milk Powder reference material. Standard deviation, sR/ng g-1 35 n.d.a Reproducibility, 2.8 sR/ng g-1 98 n.d.a Accuracy (CRM 063R Skim Milk Powder) the optimisation of the ICP-MS instrument on diVerent days Found, x±s/mg g-1 0.81±0.04 0.76±0.02 of analysis.Calibration curves must therefore be constructed (n=11) (n=12)c Certified/mg g-1 0.81±0.05 0.81±0.05 daily. The coeYcient of correlation of the standard additions calibration curve were in most cases better than 0.999. aNot determined. bFor 0.5 and 0.3 g sample intakes, respectively. cData from Stu�rup and Bu� chert. dIntegrated value, counts per ng ml-1.Otherwise a new standard additions calibration curve was eRelative standard deviation (RSD) pooled and weighed average of constructed. RSD values from separate days. fBased on six double determinations Based on double determinations, the repeatability and of samples at the 100 ng g-1 concentration level on separate days. reproducibility (both at the 95% confidence level ) were estimated (Table 2) according to guidelines given by the International Organization for Standardization.12 Expressed cleaned PTFE bomb inserts.If the four blanks exceeded this in relative terms, the values given for cream and cheese upper limit, the results obtained for the samples were discarded. correspond to a relative repeatability of 45% and a relative After several bomb ashings had been carried out in the same reproducibility of 98% at the 100 ng g-1 iodine concentration PTFE liner, the iodine contamination tended to increase in level. The analysis of iodine in milk by the FI-ICP-MS method that liner.This was especially pronounced when samples high provided a relative repeatability of 8.4% also at the 100 ng g-1 in iodine had been wet-ashed in the liners. In this case the concentration level. The reproducibility however, was not extraction treatment by the 1% solution of TMAH was not determined for the FI-based method. suYcient to keep the blanks at a low concentration level. Finally, the accuracy of the bomb-ashing method as well as Therefore the treatment by the acid mixture used for new that of the FI-based method of analysis was assured by PTFE inserts as mentioned in the Experimental section was analysing theCRM 063R Skim Milk Powder reference material applied for satisfactory reduction of the blank value. in parallel with the samples.The results have been shown in The limit of detection (LOD) given in Table 2 for the wet Fig. 2 and the warning and rejection limits defined as ±2 and ashing and the FI-based method was determined as three ±3 standard deviations, respectively, are also indicated.The standard deviations of all accepted blank values. The corre- standard deviation of the distribution of means which was sponding LOD values for the samples depended on the sample derived from the results of the laboratories participating in intake and the dilution factor. When using wet ashing the high the certification campaign of the CRM11 was used to establish fat content of the cream and cheese samples limited the these limits.Using this control chart it became possible to maximum possible sample intake whereby the LOD value accept or reject a single determination13 of iodine in the CRM. became relatively high. In comparison, the FI-based analytical If the value for the CRM was accepted, the analyses of the method used for milk was superior in LOD value (Table 2) unknown samples which were carried out in parallel with the due to less variation of the blank and less sample dilution.CRM were also accepted. The results given for the CRM in If conversion of volatile iodine species contained in the the control chart in Fig. 2 show that the analyses were accurate samples to non-volatile species had not occurred completely although the standard deviation of the values determined by during the bomb ashing, the RSD value of the recorded the bomb-ashing method was twice of that for the FI-based ICP-MS signal intensity would increase markedly owing to method as indicated in Table 2.memory eVects. Earlier investigations showed that a RSD value below 1.5% was acceptable as accurate results using The content and variation of iodine of dairy products CRMs were obtained.8 Consequently, if this value was exceeded the results were rejected and the analysis repeated. Whole milk. The results for iodine in milk shown in Fig. 3 varied between 42 and 162 ng g-1 with an average value of The method of standard additions was used for calibration of all samples and provided information on the within-day 101 ng g-1.The results reflect regional as well as seasonal diVerences. Milk from Sealand contained more iodine than and between-day variation of the sensitivity. The within-day RSD value of approximately 4% (Table 2) is relatively modest milk from Jnd which can be explained by the higher iodine content of potable water on Sealand. Typical contents of and allows for a more cost-eYcient addition calibration procedure in future analytical work.Using this procedure, stan- iodine in tap water were 2–8 ng ml-1 in Jutland and 10–30 ng ml-1 on Sealand.14 Within Jutland, higher iodine dards may be spiked to only one out of every 10 unknowns of similar composition. However, the somewhat higher contents were found in milk from the dairy in Southern rather than in Central Jutland. During the winter period the iodine between-days RSD in sensitivity was caused by diVerences in J.Anal. At. Spectrom., 1999, 14, 41–44 43Cream and cheese. The iodine content in cream varied from <100–217 ng g-1 with an average content at 110 ng g-1. The results for iodine in cream were close to the LOD value of 100 ng g-1. Consequently, the repeatability at 45 ng g-1 (Table 2) obscured possible regional and seasonal variations in the iodine content. The iodine content of cheese varied from <60 to 164 ng g-1 with an average value at 121 ng g-1.All samples were taken from three dairies in Jutland because the production of cheese is centralised in this part of the country. Any possible regional diVerence in the iodine content of the cheese samples was obscured by the relatively high repeatability. Consequently, the average iodine content of cheese from the three dairies has been plotted against the month of production. The results indicate that no significant seasonal diVerences can be demonstrated.Fig. 3 Regional and seasonal variations in iodine contents of milk. The repeatability has been indicated by error bars as ± one standard References deviation. y axis: Iodine content/ng g-1; x axis: Month of production. —#—, Average of the three dairies; - -×- -, Slagelse 1 B. S. Hetzel and J. T. Dunn, Ann. Rev. Nutr., 1989, 9, 21. (Sealand); - -%- -, Tyrstrup (Southern Jutland); - -6- -, Hobro 2 L. B. Rasmussen, G. Andersson, J. Haraldsdottir, E. Kristiansen, (Central Jutland).K. Molsted, P. Laurberg, K. Overvad, H. Perrild and L. Ovesen, Int. J. Food Sci. Nutr., 1996, 47, 377. 3 Nordic Nutrition Recommendations 1996, Scand. J. Nutr./ Nœringsforskning, 1996, 4/96, 40, 161. 4 National Food Agency of Denmark, Food Monitoring 1988–1992, Publication no. 232 (English version), December 1995, DK-2860 Søborg, Denmark. 5 J. S. Jacobsen and T. Leth, Food Monitoring System for Nutrients, Dairy Products, 2nd Cycle (In Danish with English summary). Report CLA 92004, 1992, National Food Agency of Denmark, Søborg. 6 P. W. Fischer, M. R. L’Abbe� and A. Giroux, J. Assoc. OV. Anal. Chem., 1986, 69, 687. 7 M. Dermelj, Z. Slejkovec, A. R. Byrne, P. Stegnar, V. Stibilj and M. Rossbach, Fresenius’ J. Anal. Chem., 1990, 338, 559. 8 E. H. Larsen and M. B. Ludwigsen, J. Anal. At. Spectrom., 1997, 12, 435. 9 S. Stu�rup and A. Bu� chert, Fresenius’ J. Anal. Chem., 1996, 354, 323. 10 J. S. Jacobsen and P. Knuthsen, Food Monitoring System for Nutrients, Dairy Products, 3rd. Cycle (in Danish with English summary). Report IFE 1998.2, 1998, Danish Veterinary and Food Administration, Søborg. Fig. 4 Average iodine contents of cheese against time of the year. The 11 P. Quevauviller, D. Van Renterghem, B. Griepink, S. T. Sparkes repeatability has been indicated by error bars as ± one standard and G. N. Kramer, The certification of the contents (mass deviation. Month 12 of production designates December of the year fractions) of Ca, Cu, Cl, I, Fe, K, Mg, P, Pb, N, Na and Zn in Skim prior to the year of sampling. y axis: Iodine content/ng g-1; x axis: Milk Powder (CRM 063R), Report EUR 15021 EN, DG XIII, Month of production. —#—, Average of the three dairies. L-2920 Luxembourg, 1993. 12 International Standard ISO 5725, 2nd edn., 1986, International content of milk from all regions was significantly higher than Organization for Standardization, Geneva, Switzerland. 13 E. H. Larsen, G. A. Pedersen and J. W. McLaren, J. Anal. At. during summer as indicated in Fig. 3. The marked decrease in Spectrom., 1997, 12, 963. iodine content from April to May coincides with a change to 14 L. B. Rasmussen and E. H. Larsen, 1998, unpublished results. outdoors grazing during the summer, while the cows’ fodder during the winter months is enriched in iodine. Paper 8/06642F 44 J. Anal. At. Spectrom., 1999, 14, 41&nd

ISSN:0267-9477    

DOI:10.1039/a806642f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Two-cascade glow discharge ion source† G. G. Sikharulidze and A. E. Lezhnev Institute of Microelectronics Technology and Highpurity Materials RAS, 142432, Chernogolovka, Moscow, Russia Received 23rd July 1998, Accepted 22nd September 1998 The mechanism of the transformation of a solution into a low-temperature plasma is considered. The analyzed solution enters a hollow cathode directly from the atmosphere through a capillary. As it enters the hollow cathode the solution acquires a linear speed greater than 1–2 mm s-1 in a narrow gap between a ball or wire and the capillary walls.After evaporation of the solvent, the impurity remains on the walls of the hollow cathode. On the basis of these considerations, a two-cascade glow discharge ion source for solution analysis was developed. The first cascade of the source consists of a capillary, with a wire or ball inserted into it, and a cylindrical anode. The anode of the first cascade is, in turn, the hollow cathode of the second cascade.The discharge current of the first cascade is 1–3 mA at a voltage of 1–3 kV, and that of the second cascade is 10–30 mA at a voltage up to 4 kV. The sensitivity of the proposed source is comparable to that of an ICP source. The direct transformation of a solution into a low-temperature to eqn. (1) the thickness of a water layer in a vacuum decreases at a rate of 1–2 mm s-1 at a temperature close to zero. If an plasma in a glow discharge ion source for mass spectral analysis is diYcult because of the high vapor pressure of the aqueous solution is introduced into a vacuum through a capillary, the linear speed of the water flow in the capillary solvent.At the same time, the analysis of the solvent itself is of no interest (for example, in aqueous solutions the solvent should exceed 1–2 mm s-1, irrespective of the capillary diameter. Proceeding from this condition, a design of the cathode contains only H2O and complexes of these molecules). Of analytical interest are impurities in the solution, their absolute unit can be chosen.Fig. 1 illustrates several variants of the cathode design. and relative concentrations. A method for the elemental analysis of solutions has been Once the aqueous solution has been introduced through an ordinary capillary a, the solution will constantly be at the top proposed,1–4 which consists in the analysis of the residue extracted from the solution by evaporation.The residue was of the capillary, if the input rate mr n s, where r is the density of the liquid, s is the area of the capillary section, and placed on the cathode and atomized in a glow discharge. For certain elements, the sensitivity was as high as several ppb. A n is the linear flow of the liquid at the top of the capillary. If the inner capillary diameter is 0.3 mm, the water flow will be major disadvantage of the method was the fast atomization of the residue. Electrochemical deposition of analyte elements about 0.2 mm3 s-1=12 mm3 min-1.Such a water flow will overload the vacuum system of a mass spectrometer. Therefore, (Pt and Ir) on a substrate with subsequent atomization in a glow discharge has been suggested,5 but this method has the the rate of water input should not exceed 1–2 mm3 min-1. The use of capillaries of a smaller diameter is inexpedient since same disadvantage. For a highly sensitive multi-elemental analysis of solutions, the residue on the cathode should be they easily become blocked and are hard to clean.It is possible to alter the cathode unit design. Fig. 1 presents diVerent renewed continuously during the analysis. It has been shown that this problem can be solved variants of the design. The variants b and c consist of a capillary with an inner diameter of about 0.3–0.5 mm and an successfully.6 The basic idea of the considered source is as follows. inserted wire of similar diameter. In the variant d, a metal ball is inserted into the capillary.The gap between the capillary A liquid is introduced directly from the atmosphere into a hollow cathode through a capillary without any losses. The walls and the ball or wire does not exceed 5–15 mm. Roughness is of no importance. What is important is to reduce the liquid continuously moistens the internal walls of the hollow cathode, delivering the impurities to be analyzed onto its evaporation area as much as possible and to increase the surface.There, the liquid and impurities therein are sputtered by the discharge plasma ions and the sputtered atoms are ionized. The process is carried out without argon or other gas. In the present work we consider the mechanism of this process. In a vacuum the open surface of a liquid evaporates at a rate:6 m$0.06ÓM ps ÓT (g s-1 m-2 Torr K) (1) where M is the molecular weight of the solvent, p is the vapor pressure at temperature T and s is the square of the liquid surface. At a surface temperature of about 0 °C (273K), the water vapor pressure is about 6 Torr.For instance, according Fig. 1 DiVerent variants of cathode unit. a, Capillary with hollow cathode; b, c, capillary with wire inserted; d, capillary with ball †Presented at the 1998 Winter Conference on Plasma Spectrochemistry, Scottsdale, AZ, USA, January 5–10, 1998. inserted. J. Anal. At. Spectrom., 1998, 13, 45–47 45resistance to the liquid flow. If the resistance to the liquid flow solution then penetrates into the gap and two processes proceed simultaneously.is not increased, the liquid from the capillary will enter the hollow cathode in an extremely non-uniform manner. The ball 1. The liquid arrives at the hollow cathode non-uniformly, in small portions. The molecules of the liquid are adsorbed does not need to be centered (neither does the wire). Centering is eVected by heterogeneities on the walls. Certainly, the ring on the surface of the cathode, and the liquid spreads on this surface.In the absence of complete wetting of the surface of gap will not be ideal. The cathode unit is inserted into the discharge chamber via the cathode the boundary angle is not established, it tends to zero, and the liquid spreads on the surface of the cathode as an insulator. A solution is introduced into the capillary directly from the atmosphere. The inner diameter of the capillary is a polymolecular layer. The time of formation of this layer is 10-2–10-3 s, at velocities of a spreading liquid of 100– 0.5 mm.The solution vapors fill the capillary and through the gap between the capillary walls and the ball penetrate into the 200 cm s-1. The wetting heat is related to the heat of adsorption and latent heat of vaporization of the liquid. The ball on hollow cathode. The evaporation rate of the liquid in the capillary is governed by the conductivity of the gap and the top of the capillary tube allows the liquid to enter the hollow cathode more uniformly.8 the solvent vapor pressure.The gap conductivity is given by:7 2. The solvent evaporates into the vacuum. Rapid evaporation of the solvent is stimulated by ion bombardment G= 3.27×10-2 g p0(D2+D1) (D2-D1)3 L (2) of the inner surface of the hollow cathode. In this case, a deposit of impurities, which were dissolved in the liquid, and part of the solvent, which is strongly bound to the surface and where g is the viscosity of the solvent vapor, p the vapor pressure, D1 the ball diameter, D2 the inner diameter of the the dry residue, are left on the cathode surface.The calculations show that the inner surface of the hollow hollow cathode, and L the gap length. The width of the gap between the ball and the capillary walls is 15–20 mm, and the cathode becomes covered with a monomolecular layer of a deposit every second for an impurity concentration in the length is about 0.5 mm. As soon as the solution is in the capillary, it begins to evaporate.Through the capillary, vapors solution of about 100 ppm and a rate of solution introduction into the capillary of 1 mm3 min-1. of the solution pass into the hollow cathode which is under an applied voltage. If the ball is removed from the top of the The solution continuously penetrates into the hollow cathode and a deposit continuously precipitates on its walls. capillary, the rate of vapor penetration into the hollow cathode becomes fairly high, and a glow discharge immediately ignites The solvent plays a dual role here: it introduces dissolved substances into the hollow cathode and continuously supports in the vapors.For the solution to reach the hollow cathode, the rate of liquid input should be fairly high (more than the a glow discharge in the vapors. Therefore, the deposit, formed on the walls of the hollow cathode, is continuously sputtered linear rate of liquid evaporation). Under this condition, the load on the vacuum system of the mass spectrometer is too and acts in the discharge.A similar picture is also observed in ICP mass spectrometry, high. Inserting a ball at the top of the capillary limits the conductivity. According to eqn. (2), the width and length of where two impurity carriers are used: argon transfers liquid drops, which, in turn, transfer the impurities. the gap define the conductivity. However, the vapors of the liquid penetrate through the gap and a discharge appears in A knowledge of the mechanism of the transformation of a solution into a low-temperature plasma allows one to develop these vapors.First, the liquid in the hollow cathode behaves very non-uniformly, because of the large pressure diVerence an optimum design of a glow discharge source for the analysis of solutions. between the atmosphere and the discharge chamber. This diVerence breaks the liquid flow in several places. This can be These considerations and previous work on the mechanism of the transformation of a liquid into a plasma suggest the seen visually if a transparent polyethylene capillary is used for the ICP mass spectrometer. The discharge ignites and dies requirements for the design of an ion source. On transformation of a liquid into a glow discharge, the capillary must not away. It usually takes several minutes for the process to stabilize, and the number of breaks in the liquid column be heated.Heating of the capillary leads to an abrupt increase in the pressure of the solvent vapor, and, consequently, decreases. The discharge becomes stable after 2–3 min. The gap limits the conductivity. The rate of solution impurity-free vapor rather than a solution would enter the hollow cathode. For example, heating an aqueous solution to introduction into the capillary should always exceed the rate of solvent vapor penetration through the gap. The hollow 40 °C increases the vapor pressure to 60 Torr. Therefore, the discharge current should be a minimum.On the other hand, cathode is established on the top of the capillary. The inner diameter of the hollow cathode can be varied from 1.5 to at a small discharge current the degree of plasma ionization drops abruptly, which leads to a loss of sensitivity. To avoid 3 mm. The liquid enters the hollow cathode at diVerent intervals. After penetration into the hollow cathode, the liquid this diYculty, the processes of atomization and ionization should be separated in space.This can be achieved by a two- spreads over the surface of the cathode and moistens it. More than 90% of the liquid evaporates from the hollow cathode cascade ion source design. This source is shown schematically in Fig. 2. The first cascade of the source consists of a capillary rather than from the capillary. A high voltage of 3–4 kV is applied between the cathode and the discharge chamber (1) with a wire inserted into it, and an anode (2) in the form of a cylinder.Anode 2 of the first cascade is, in turn, the through a ballast resistor. A glow discharge is generated in the hollow cathode. The depth of plasma penetration into the hollow cathode of the second cascade. The anode of the second cascade is the discharge chamber. The first and second hollow cathode depends on the cathode diameter, vapor pressure and amplitude of the discharge voltage. At a rate of hollow cathodes are cooled with water.The inner diameter of the first hollow cathode is 2 mm, that of the second is 5 mm. solution introduction up to 1–2 mm3 min-1, the hollow cathode generates a plasma jet with a divergence angle of about A small gap between the first and second cathodes serves to shield the external surface of the first cathode from the 5°, the discharge voltage is 2–3.5 kV and the discharge current is 2–5 mA; at a greater rate, the plasma jet transforms into a discharge.The electrical power to the two-cascade source is supplied plasma torch, the discharge voltage decreases to 0.4–0.6 kV and the discharge current increases to 20–30 mA. The plasma from two independent adjustable power sources. The discharge current of the first cascade is 1–3 mA at a voltage of 1–3 kV jet mode is preferable, because it aVords the maximum ion current. The ions formed in the discharge bombard the inner and 10–30 mA at a voltage up to 4 kV in the second cascade.In the hollow cathode on the top of the capillary (2), the walls of the hollow cathode and clean them of pollutants. The 46 J. Anal. At. Spectrom., 1998, 13, 45–47Fig. 3 Mass spectrum of pure water. Region from 15 to 19 Da has been removed. Fig. 2 Two-cascade glow discharge ion source. 1, Capillary with ball inserted; 2, first hollow cathode; 3, first anode–second hollow cathode; 4, discharge chamber; 5, 6, power supplies; 7, extraction aperture. liquid passing through the capillary and its components are atomized to form a low-ionized plasma jet, which is ionized more eYciently in the hollow cathode (3).The resulting ions are withdrawn through the aperture (7). The discharge chamber (4) is evacuated through the same aperture. The continuous input of a solution into the hollow cathode facilitates the accumulation of an impurity on the walls of the hollow cathode, which is particularly convenient for the analy- Fig. 4 Mass spectrum of water with impurity. Region from 6 to 70 sis of solutions with low impurity concentrations.The use of Da has been removed. a pulse mode of glow discharge enables one to increase the absolute and relative sensitivities of the analysis by an order of magnitude. The pulse mode of feeding a glow discharge is Table 1 Count rate of ions (impulses s-1) for selected elements also preferable from the viewpoint of the temperature regime introduced into water. The concentration of each element was 1 ppm in the first cascade of the source.Element Count rate Element Count rate The impurity concentration in the solution is not important. In experiments it was possible to introduce salt solutions Li 3 500 000 Co 400 000 (NaCl, KBr) with concentrations up to 20% into the hollow Mg 1 600 000 Cu 400 000 cathode without any diYculties. What is necessary is to match Si 1 000 000 Rb 1 200 000 the rate of impurity atomization with the rate of impurity Fe 800 000 Cs 1 000 000 input into the hollow cathode.Usually, the rate of impurity atomization at these concentrations is insuYcient and the impurity accumulates in the hollow cathode. This problem More detailed analytical results obtained in the elemental needs to be solved in due course. analysis of solutions, using the proposed glow discharge ion The proposed glow discharge source was used for solution source, will be presented in a subsequent paper. analysis on a PlasmaQuad quadrupole mass spectrometer, in which the ICP source was replaced by the two-cascade source.References The mass spectrum of pure water thus obtained is shown in Fig. 3. The lines of argon and complexes associated with argon 1 W.W. Harrison and C. W. Magee, Anal. Chem., 1974, 46, 461. 2 E. H. Daughtrey, Jr. and W. W. Harrison, Anal. Chem., 1975, are absent from the mass spectrum—the glow discharge is 47, 1024. excited in the vapors of the solvent alone. The intense line of 3 D. L. Donohue and W. W. Harrison, Anal. Chem., 1975, 47, 1528. doubly charged oxygen implies a high temperature of the 4 W. A. Mattson, B. L. Bentz and W. W. Harrison, Anal. Chem., plasma. The mass spectrum of an aqueous solution, to which 1976, 48, 484. the elements Cu, Zn, Rb, Ag, In, Cs, Pb and Bi were added 5 N. Jakubovski, D. Stuewer and G. To� lg, Sperochim. Acta, Part B, at a level of 100 ppb, is shown in Fig. 4. The sensitivity of the 1991, 46, 155. 6 G. G. Sikharulidze and A. E. Lezhnev, presented at the 1996 Winter proposed source is comparable to that of an ICP source. As Conference on Plasma Spectrochemistry, Fort Lauderdale, FL, an example, Table 1 presents the count rates of ions of selected January 8–13, 1996. impurities at a level of 1 ppm in the analyzed solution. The 7 J. Groszkowski, Technika Wisokiey Prozni,Warszawa, 1972, p. 622. results show that the two-cascade glow discharge ion source 8 B. D. Summ and Ju. V. Gorunov, Physico-Chemical Fundamentals can be eYciently used, particularly in combination with a high of Wetting and Spreading, Chemistry, Moscow, 1976, p. 232. resolution double focusing mass analyzer, which allows one to diVerentiate multiple signals on the analytical lines. Paper 8/05758C J. Anal. At. Spectrom., 1998, 13, 45–47 47

ISSN:0267-9477    

DOI:10.1039/a805758c

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Evaluation of a high-pressure, high-temperature microwave digestion system Keith E. Levine, James D. Batchelor, Charles B. Rhoades, Jr. and Bradley T. Jones Department of Chemistry, Wake Forest University, Winston-Salem, NC 27109, USA Received 22nd May 1998, Accepted 5th November 1998 A high-pressure, high-temperature focused microwave digestion system was evaluated and compared with a conventional, closed-vessel digestion system. A suite of 18 test elements was determined in six Standard Reference Materials (Peach Leaves, Orchard Leaves, Oyster Tissue, Bovine Liver, Bituminous Coal, and River Sediment) by inductively coupled plasma atomic emission spectrometry and electrothermal atomic absorption spectrometry.Observation of the graphite furnace background absorbance for the digestates, and their high-performance liquid chromatograms, demonstrate that the high-pressure, high-temperature system results in a more complete destruction of the sample matrix.The high-temperature system also shows higher accuracy for the test metals in the reference materials (average error 7.1%) than the conventional system (11.8%). Finally, the high-temperature system has a per-sample analysis time (7.5 min) that is 10 min faster than the conventional system. On the negative side, the high-temperature system is not capable of monitoring temperature during a digestion program. As a result, vessel rupture is common during method development. This results in complete destruction of the vessel, and the emission of potentially dangerous exhaust fumes into the laboratory environment.however, reveal that some species are resistant to complete Introduction oxidation. Aromatic proteins, such as phenylalanine, are Many modern instrumental techniques require complete diYcult to decompose because of the additional stability sample dissolution prior to an analysis. In general, dry ashing provided by the ring structure. The incomplete decomposition or wet digestion methods can successfully decompose a variety of such proteins can result in the formation of several resistant of sample matrices.Although dry ashing procedures are eVec- organic decomposition products. These non-volatile species tive, they are time consuming and can often result in the loss have been identified as nitrobenzoic acids (NBA) for both of volatile analyte species.1 Conventional wet digestion pro- conventional heated and microwave-assisted nitric acid cedures employing hot-plates at atmospheric pressure often decompositions.10–13 suVer from similar diYculties.In 1975, Abu-Samra et al. Nitric acid resistant decomposition products can result in described the use of microwave energy as a means to complete elevated residual carbon content (RCC) values for digested acid-assisted wet digestions rapidly.2 Since that time, micro- samples. Although the RCC of a digestate is not critical for wave heating for sample decomposition has become a well- many instrumental techniques, such as inductively coupled established technique.Open-vessel,3,4 closed-vessel,5,6 and plasma atomic emission spectrometry (ICP-AES), it can cause focused techniques7 are all well documented in the literature. serious problems for other methods. Residual NBA species Although open-vessel microwave wet digestions are popular, can compete favorably for electrode surface sites, making they do suVer from some drawbacks.6 Because samples are electrochemical determinations diYcult.12–14 The p-NBA exposed to the atmosphere, volatile elements can be lost during isomer in particular introduces a serious electrochemical interthe digestion process, the risk of sample contamination is ference.The increased background current and interferences increased, and laboratory equipment and personnel can be that result from this and other nitric acid resistant organic exposed to corrosive acid vapors.Further, samples digested in materials in solution can greatly hinder trace metal voltamopen vessels are limited in temperature to the boiling-point of metric stripping analyses.15,16 Preconcentration methods that the employed mineral acid. In contrast, samples digested in employ extraction or hydride generation are also vulnerable closed-vessel environments are protected from the atmosphere to the presence of RCC.17 In addition, matrix constituents and undergo accelerated decomposition reactions as a conse- surviving nitric acid decomposition can cause interferences for quence of increased pressure and temperature.In addition, the electrothermal atomic absorption spectrometry (ETAAS). To elevated temperatures that can be attained in a closed-vessel reduce the risk of losing volatile species such as Cd, ashing procedure increase the oxidizing power of mineral acids and temperatures are often reduced. Organic matrix componallow for the decomposition of matrix components, which ents surviving the reduced ashing temperatures can cause would not be possible in open-vessel environments.background absorption signals during furnace atomization.18 In 1988, Kingston and Jassie described the specific In order to circumvent the diYculties associated with RCC decomposition temperatures for three basic matrix constituents in nitric acid digestates, the use of additional oxidants is of biological and botanical samples:8 140 °C for carbohydrates, heavily reported in the literature.Perchloric acid was fre- 150 °C for proteins and 160 °C for lipids. Since closed-vessel quently used to eliminate any organic material that survived microwave dissolution with only nitric acid can routinely nitric acid dissolution.13,14,19 In addition to adding a potential achieve temperatures in excess of 160 °C,9 biological and source of trace metal contamination, the use of perchloric acid botanical matrices should be completely destroyed by such can be extremely dangerous.As a result, many other combinations of nitric acid and additional oxidants have been procedures. Studies of nitric acid decomposition products, J. Anal. At. Spectrom., 1999, 14, 49–59 49explored. Sulfuric acid has been employed with mixed results. These vessels can withstand 350 °C temperatures at pressures up to 160 bar. Elements that form sparingly soluble sulfates have been reported to precipitate from solution when sulfuric acid was Atomic spectrometric analyses of digestates were carried out using several instruments.A Perkin-Elmer 3000 XL induc- used in a digestion procedure.20 The use of hydrogen peroxide and hydrochloric acid has also been reported in the literature tively coupled plasma atomic emission spectrometer was used for the analyses of digested reference materials (Norwalk, CT, to destroy residual NBA compounds.1,21 As with perchloric acid and sulfuric acid, acceptable trace metal recoveries USA).This axially viewed plasma system is equipped with a segmented-array charge-coupled device (CCD) for detection obtained from the use of additional oxidants must be weighed against the potential contamination that these species intro- and an autosampler to facilitate sample introduction. A Polyscan Thermo Jarrell Ash (TJA) 61E ICP system was duce into a sample and the additional risk that the analyst undertakes. employed to determine recoveries from pure elemental standard solutions (Franklin, MA, USA).This instrument utilizes The ideal closed-vessel sample dissolution technique would be simple, rapid, inexpensive, and safe. The exclusive use of an autosampler to aid in sample introduction and a series of photomultiplier tubes that are arranged around a Rowland nitric acid as an oxidizing agent meets these criteria. However, Wu� rfels et al. demonstrated that a temperature of 300 °C is Circle for detection. ETAAS was used to determine several elements in the reference materials when ICP-AES lacked the necessary for the complete destruction of all kinds of organic matter.22 The Teflon@ vessels employed by conventional micro- necessary sensitivity.Two ETAAS instruments were employed during this investigation. Elemental recovery data in digested wave digestion systems cannot withstand such extreme reaction conditions. A digestion method that is capable of achieving materials werobtained using a Perkin-Elmer Model 5100Z system with Zeeman-eVect background correction.A GBC 300 °C and up to 100 bar of pressure is high-pressure ashing (HPA). Large amounts of biological and botanical samples 906/GF3000 unit (Dandenong, Australia) with continuum source background correction was used to study the have been completely digested with only nitric acid in an HPA system.23 The prohibitive cost of HPA technology and the background surviving nitric acid decomposition procedures.SRM were kept in clean Berghof/America 400 mL time required to complete a digestion procedure somewhat limit the utility of this technique. poly(tetrafluoroethylene) (PTFE) evaporation bowls (Concord, CA, USA) and stored in a Sanpla Drykeeper Recently, a closed-vessel digestion system was developed that utilizes focused microwave radiation to wet-ash samples desiccator cabinet with automatic silica gel regenerator (Sanplatec, Japan). Reference materials were massed using a eYciently.24 Patented tetrafluorometoxil polymer bomb vessels (TFM-PTFE) were employed which were capable of with- Mettler AT400 analytical balance with a 450 g capacity (Switzerland).An EndStat 2100 static control device was standing 350 °C temperatures. This is more than suYcient to destroy completely all organic matrix constituents with only employed to minimize diYculties while weighing the materials of interest (East Berlin, CT, USA). At the end of the digestion nitric acid.This technology recently became commercially available.25 The objective of this investigation was to evaluate procedures, samples were quantitatively transferred into clean 100 mL Nalge poly(propylene) calibrated flasks, filtered, and the practical performance of this technology through the digestion of a variety of Standard Reference Materials (SRM). stored in clean 15 mL autosampler vials with caps (Norcross, GA, USA). Poly(propylene) tanks, used for acid-cleaning all The digested biological, botanical, and geological samples were then analyzed by HPLC, ICP-AES and ETAAS.The high- digestion and volumetric labware, were also obtained from Nalge. The distilled, de-ionized water used for sample dilution pressure, high-temperature microwave digestion unit was compared with a conventional commercially available closed-vessel and cleaning procedures was provided by a Nanopure Ultrapure Water system (Dubuque, IA, USA). This system microwave system.was directly connected to the laboratory water supply and delivered 2.0 L min-1 of 18 MV resistivity water. Class M 3.5 Experimental clean air laminar flow stations were also employed in this investigation to reduce the risk of contamination. The Clean Instrumentation Room Products systems were equipped with High EYciency Particulate Air (HEPA) filters capable of maintaining class M Conventional closed-vessel microwave digestion procedures were conducted with a CEM MDS-2100 (Matthews, NC, 3.5 conditions (Ronkonkoma, NY, USA).All HPLC analyses were conducted using a Hewlett-Packard USA). This system is capable of delivering a maximum power of 950 W and provides constant feedback control of reaction 1090 Series II system (Wilmington, DE, USA). A void sealing column from Whatman was employed to reduce undesirable conditions through temperature and pressure monitoring of a control vessel. The perfluoroalkoxy (PFA) Teflon@ lined dead volume resulting from compaction of the stationary phase at the column inlet (Clifton, NJ, USA).The 25 cm long decomposition vessels and caps used in this investigation were also obtained from CEM. These vessels are capable of sustain- end-capped column had a C18 bonded phase and a 4.6 mm internal diameter. Whatman PartiSphere@ 5 mm spherical ing temperatures of 200 °C and pressures of 14.0 bar. Recently, CEM introduced a closed-vessel system capable of 300 °C and packing media were also used.A photodiode array detector capable of monitoring the 200–400 nm spectral region was 100 bar, in a conventional, non-focused oven. This system may demonstrate capabilities similar to the focused system used to measure eluate absorption. evaluated below, but an instrument was not available for evaluation. Reagents A BM-1S/II system from Plazmatronika (Wroclaw, Poland) was used for high-pressure, high-temperature, focused, closed- Several SRM from the National Institute of Standards and Technology (NIST) were employed during this investigation vessel microwave digestion procedures.This two-stand unit is capable of delivering 150W of focused power to each vessel. (Gaithersburg, MD, USA). These included botanical (SRM 1547 Peach Leaves, SRM 1571 Orchard Leaves), biological The system also requires a single-phase 220 V, 50 Hz power supply and a 1.5 dm3 min-1 flow of cooling water at 0.3 MPa. (SRM 1577a Bovine Liver, SRM 1566a Oyster Tissue), and geological (SRM 1645 River Sediment, SRM 1632b The pressure generated during decomposition reactions can also be continuously monitored.The high-pressure/tempera- Bituminous Coal ) substances. The calibration standards used for all atomic spectrometric determinations and the chemical ture tetrafluorometoxil TFM-PTFE@ digestion vessels and caps required by this system were supplied by Plazmatronika. modifiers employed with the Perkin-Elmer 5100 graphite fur- 50 J. Anal.At. Spectrom., 1999, 14, 49–59Table 1 Microwave decomposition procedures nace were obtained from SPEX Industries (Edison, NJ, USA). The high-purity acids used in this study for sample digestion Conventional closed-vessel digestion procedure— and cleaning procedures were Fisher Scientific Optima Grade Stage Power (%) Pressure/bar Temperature/°C Time/min concentrated nitric and hydrochloric acids (Pittsburgh, PA, USA). Baxter Micro concentrated cleaning solution (McGaw 1 25 4.1 80 10 25 4.8 90 10 Park, IL, USA) was also used during the labware cleaning 30 6.9 100 20 procedure.Burdick and Jackson Brand high-purity solvent 30 10.3 125 20 methanol (McGaw Park, IL, USA) was used for both labware 2 30 6.9 125 20 cleaning and liquid chromatographic procedures. Phosphoric 35 8.6 125 20 acid (HPLC grade) was obtained from Fisher Scientific 35 10.3 125 20 (Pittsburgh, PA, USA) and was used exclusively during separa- 3 40 8.6 150 30 tion procedures. The NBA standards employed for HPLC 50 10.3 150 30 analyses were purchased from Aldrich (Milwaukee, WI, USA). 4 50 10.3 175 15 60 12.1 175 15 Labware preparation High-pressure, high temperature microwave procedures— All labware used in this investigation was subjected to a Sample name Power (%) Pressure/bar Time/min vigorous cleaning procedure. Digestion vessels and calibrated flasks were rinsed with distilled, de-ionized water and placed Peach Leaves and 100 25.3 5 Orchard Leaves 0 25.3 5 in a 1% v/v soap cleaning solution overnight prior to use.The labware was then rinsed with hot tap water to remove all soap Oyster Tissue and 60 25.3 2 Bovine Liver 70 25.3 2 residue. A methanol soaked cloth was used to remove any 90 25.3 3 residual material remaining in the Teflon@ digestion vessels. 0 25.3 8 All labware was again rinsed with distilled, de-ionized water Bituminous Coal 60 35.4 3 and allowed to soak in a poly(propylene) tank containing a and River Sediment 0 35.4 2 solution of trace metal grade nitric acid, trace metal grade 100 35.4 5 hydrochloric acid, and distilled, de-ionized water (1+2+9) 0 35.4 5 for at least 4 h.When removed from the acid solution, labware was rinsed with distilled, de-ionized water and stored under a clean room station in plastic Ziplock storage bags until use. was achieved. A closed-vessel digestion was considered to be complete when a temperature of 175 °C was maintained for Sample preparation approximately 10 min.This was to ensure that the basic components of biological and botanical samples (carbo- A general procedure was followed for microwave sample hydrates, proteins, and fats) would be decomposed. The closed- digestions. Approximately 0.3 g of reference material was vessel digestion program used for all of the sample types under weighed directly on an analytical balance into a digestion investigation was that employed by Rhoades et al. and is vessel. Prior to weighing, each vessel was momentarily held presented in Table 1.26 At the conclusion of each digestion up to an antistatic device to reduce static electricity.A 5 mL stage, samples were allowed to cool to room temperature and aliquot of trace metal grade nitric acid was then added to each were manually vented. This venting procedure ensured that vessel and was allowed to decompose the sample at atmosthe pressure limit of the vessels was not exceeded in subsequent pheric pressure under a clean room station.After 1 h, samples program stages. If the maximum desired temperature for a were capped and subjected to closed-vessel microwave digesparticular stage was not achieved as a consequence of pressure tion (see procedures below). At the conclusion of the digestion build-up, the vessels were vented after cooling to below 50 °C procedure, samples were quantitatively transferred into 100 mL (approximately 15 min) and the stage was repeated. The cool- poly(propylene) calibrated flasks and were diluted to volume ing step could be shortened if the vessels were placed in an with distilled, de-ionized water. Acid-cleaned poly(propylene) ice-bath.A final 15 min cooling step was allowed at the end caps were added and each flask was inverted several times to of each digestion before the vessels were opened. Blank ensure complete homogenization. Digested samples were then solutions for this digestion method were treated in an identical poured into disposable 0.45 mm vacuum-assisted filtration fashion to samples.units. When filtration was completed, samples were poured into clean storage tubes, capped, and stored under a clean Method II: High-pressure, high-temperature microwave room station until analysis. digestions Method I: Conventional closed-vessel microwave digestions Caps were placed over two Teflon@ vessels after 1 h of room temperature acid decomposition. A rupture membrane was After 1 h of room temperature acid decomposition, the vessels used for conventional closed-vessel microwave digestions were then placed on top of each cap before loading vessels into the instrument.The safety membranes provided were designed to sealed according to the manufacturer’s specifications. Twelve vessels can be placed at a time in the sample carousel. For rupture when pressures exceeded 160 bar. Two vessels were then enclosed inside a pressure actuator which was sealed with safety purposes, the vessels were also equipped with membranes that were designed to rupture if the internal pressure a wrench provided by the instrument manufacturer.Poly(propylene) protective jackets were placed over each bomb exceeded 14 bar. Five vessels containing approximately the same amount of an identical reference material were then and exhaust fume hoses were attached to trap corrosive decomposition gases. Other instrumental parameters were set loaded into a carousel.The vessels were arranged in a fashion so as to balance the 12-position carousel as much as possible. to values recommended by the product manufacturer in the user’s manual.25 Recommended starting-point digestion par- Acid-resistant tubes were attached to the top of each vessel so that corrosive vapors could be trapped during venting. ameters were also provided in the user’s manual. These procedures were modified for safety and digestion completion Microwave heating was controlled by pressure and temperature feedback of a monitor vessel.If the maximum value for purposes during this investigation. The employed decomposition parameters are subsequently presented in Table 1. Post- either of these control parameters was exceeded in the monitor vessel, microwave energy was cut oV until an acceptable value digestion cooling times are listed in Table 1 as the final stage J. Anal. At. Spectrom., 1999, 14, 49–59 51Table 2 ICP-AES operating parameters (0% power).After this short period, the vessels could be safely opened. At the conclusion of the digestion, samples remained Thermo Jarrell Ash Polyscan 61E— undisturbed until they cooled to room temperature. The Parameter Setting pressure actuator nut was then loosened and matrix decomposition gases were allowed to escape. Rf power 1350 W Auxiliary Ar flow 1 L min-1 Nebulizer flow 1 L min-1 Plasma flow ‘High flow’ Sample appearance Pump rate 100 rpm Wash time 50 s At the conclusion of the digestion procedures, samples were Processing mode Area poured into clean, transparent 15 mL centrifuge tubes and Background correction Manual selection of points capped for storage.Each reference material decomposed in Replicate measurements 3 the high-pressure, high-temperature digestion system resulted Perkin-Elmer 3000 XL — in a clear, colorless solution, indistinguishable from water. Parameter Setting The same was observed for the Peach Leaves, Oyster Tissue, Bovine Liver, and River Sediment materials digested with the Rf power 1360 W described conventional closed-vessel procedures. All of the Auxiliary Ar flow 0.5 L min-1 Nebulizer flow 0.70 L min-1 Orchard Leaves digestate solutions had a very pale yellow Plasma flow 15 L min-1 color which could be observed when holding a vial up to the Sample flow rate 1.60 mL min-1 light.The Bituminous Coal samples digested by a conventional Wash time 30 s closed-vessel digestion procedure all had an obvious deep Processing mode Area yellow color on first appearance.Background correction Manual selection of points Replicate measurements 3 Preliminary ICP-AES analysis of pure elemental standard solutions ETAAS analysis of reference material digestates Prior to the digestion of reference materials, several pure A Perkin-Elmer 5100 ETAAS system was used in several elemental standard solutions were digested in the high- instances where the sensitivity of ICP-AES was not suYcient pressure, high-temperature microwave system.This was per- to determine elemental recoveries in the decomposed reference formed to determine the eVects of elevated temperature and materials. Data were collected for the volatile As, Cd, and Pb pressure on the recoveries of several test elements. Rhoades atomic species. The furnace temperature programs used to et al. demonstrated acceptable elemental recoveries for the determine these elements are presented in Table 3.Zeemandescribed conventional closed-vessel digestion procedure.26 As eVect background correction and an AS-60 autosampler were a result, only the high-pressure, high-temperature system’s used for all measurements. Coated graphite tubes with preelemental recoveries were investigated. A 10 mL aliquot of a inserted platforms were also obtained from Perkin-Elmer. multi-element ICP grade standard was added to a Teflon@ Hollow cathode lamps were used for Cd and Pb determinations vessel.Two vessels with identical contents were sealed in the while an electrodeless discharge lamp was employed for As. instrument and treated with the program listed in Table 1. All of the lamps were obtained from Perkin-Elmer and were Four replicates were completed for standard 1 (Ag, Al, As, run at the recommended currents. A 20 mL sample volume Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Se, V, Zn), standard 2 (Ba, was injected into the graphite tube for all analyses and the Ca, K, Mg, Na, Sr), and standard 3 (B, P, S).Working resulting peak area was processed. In addition, a 5 mL aliquot standard solutions were prepared by the serial dilution of the of chemical modifier was added to the tube by the autosampler same multi-element stock standards. Sample blanks were pre- after the sample had been injected. A 0.2 mg NH4H2PO4 and pared in an identical fashion. This preliminary ICP-AES 0.01 mg Mg(NO3)2 modifier was used for Pb while a 0.015 mg analysis was conducted with a Thermo Jarrell Ash Polyscan Pd and 0.01 mg Mg(NO)2 modifier was used for all of the 61E ICP system.The operating parameters employed with this other elements. The slit-width used for the As 193.7 nm line, instrument are presented in Table 2. Cd 228.8 nm line, and Pb 283.3 nm line was 0.7 mm. ETAAS background ICP-AES analysis of reference material digestates A GBC 906/GF3000 ETAAS instrument was used to study the residual matrix content in the reference materials after Elemental recoveries for the six reference materials digested by conventional closed-vessel and high-pressure, high- nitric acid decomposition.Two volatile atomic species (Cd and Pb) normally require a chemical modifier to prevent temperature microwave procedures were determined with a Perkin-Elmer Optima 3000 XL ICP system. The instrument analyte losses during an ashing step. In order to compare the eYciency of the described digestion methods for destroying was calibrated by diluting multi-element standards to appropriate concentrations and matrix-matching them to the samples organic and geological matrices, no ashing step was employed in these ETAAS procedures.As a consequence, no chemical with a 5% v/v trace metal grade nitric acid concentration. All analyses were performed using Yb and Rb as internal standards modifiers were employed during this part of the investigation. The GBC 906/GF3000 system uses deuterium lamp back- (Yb 369.419 nm, Rb 780.040 nm).A separate internal standard solution was prepared such that the final concentrations of ground correction and an autosampler for sample introduction. For each analysis, a 20 mL sample volume was injected Yb and Rb were 5 and 30 mg L-1, respectively. Rb was chosen as the internal standard for easily ionized species, such as K into a coated tube without a platform. Hollow cathode lamps provided by the manufacturer were run at their recommended and Na, and Yb was used for all of the other test elements.The internal standard and sample solutions were pumped currents for both Cd and Pb. Absorbance was measured for Pb at 283.3 nm using a 0.5 mm slit-width and for Cd at separately through a peristaltic pump. The sample streams were merged with a PTFE ‘Y’-piece between the pump and 228.8 nm using a 1.0 mm slit-width. The GBC 906/GF3000 furnace temperature programs for both Cd and Pb are the plasma. Operating parameters for the Optima analyses are presented in Table 2.presented in Table 3. 52 J. Anal. At. Spectrom., 1999, 14, 49–59Table 3 Parameters and temperature programs for ETAAS analyses Step Ramp time/s Hold time/s Final temperature/°C Ar flow/mL min-1 Perkin-Elmer 5100— Arsenic: Dry 35 20 200 300 Char 1 10 1300 300 Cool 1 10 20 0 Atomize 0 3 2100 0 Clean 1 5 2650 300 Cadmium: Dry 35 20 200 300 Char 1 10 850 300 Cool 1 10 1650 300 Atomize 0 3 20 0 Clean 1 5 2650 300 Lead: Dry 35 20 200 300 Char 1 10 850 300 Cool 1 10 20 300 Atomize 0 3 1650 0 Clean 1 5 2650 300 GBC 906/GF3000— Cadmium: Dry 10 70 100 200 Cool 1 5 20 0 Atomize 1 3 1000 0 Clean 1 5 2650 200 Lead: Dry 10 70 100 200 Cool 1 5 20 0 Atomize 1 3 1400 0 Clean 1 5 2650 200 Table 4 Detection limits HPLC analysis of reference material digestates Both conventional closed-vessel and high-pressure, high- Wavelength/ IDL/ Element nm ng mL-1 MDL (ppm) MQL (ppm) temperature microwave digestates for each of the reference materials employed in this investigation were analyzed by Al 396.2 10 3 10 HPLC.For each analysis, the injection volume was 20 mL and Asa 193.7 2 0.7 2 the oven temperature was set to 50 °C. A gradient mobile B 249.8 5 2 6 phase consisting of diVerent concentrations of distilled, Ba 233.5 1 0.3 1 de-ionized water, methanol, and 1% v/v phosphoric acid was Ca 317.9 5 2 6 Cda 228.8 0.5 0.2 0.6 used. Initially, the mobile phase constitution was 0% methanol, Cu 324.8 2 0.7 2 40% phosphoric acid solution, and 60% water.At 24 min, the Fe 259.9 1 0.3 1 mobile phase was 60% methanol, 40% phosphoric acid solu- K 766.5 20 7.0 20 tion, and 0% water. This was changed to 0% methanol, 40% Mg 279.1 10 3.0 10 phosphoric acid solution, and 60% water at 25 min. The Mn 257.6 1 0.3 1 mobile phase was returned to its original constitution after Na 589.6 20 7.0 20 P 177.4 60 20 70 30 min. Prepared ortho-, meta-, and para-NBA standards were Pba 283.3 1 0.3 1 analyzed using the same HPLC gradient procedure.Since the S 180.7 100 30 100 optimum absorbance for these standards occurred at 260 nm, Sr 407.8 0.1 0.03 0.1 this wavelength was monitored for all samples. V 292.4 1 0.3 1 Zn 213.9 1 0.3 1 Detection limits aDenotes ETAAS detection limits. Instrument detection limits (IDL), method detection limits (MDL), and method quantification limits (MQL) determined for each of the test elements are presented in Table 4. A 5% v/v to ten times the blank standard deviation.The MDL and nitric acid mixture was used as a blank solution. Ten replicate MQL are reported in ppm units (mg analyte per g sample). aliquots of this solution were measured for each test element with the Perkin-Elmer 3000 XL ICP system. The IDL for each Results and discussion element was defined as the concentration of analyte solution giving rise to a signal equal to three times the standard Atomic spectrometric determinations deviation of these readings.The IDL for test elements determined with the Perkin-Elmer 5100 ETAAS system were based Data for the ICP-AES analysis of pure elemental standard solutions treated in the high-pressure, high-temperature system on three times the standard deviation of four replicates. The MDL is defined as the concentration of analyte in the sample are shown in Table 5. Results for the test elements are presented in order of the multi-element standard solution used. matrix resulting in a signal equal to three times the standard deviation of the blank signal.The MQL is the concentration Recoveries for elements contained in the first two solutions are all within 5% of their expected values. The values for B, of analyte in the sample matrix giving rise to a signal equal J. Anal. At. Spectrom., 1999, 14, 49–59 53Table 5 TJA ICP-AES multi-element standard recoveries for the 15 min. The conventional closed-vessel digestion program is high-pressure, high-temperature digestion unit significantly slower (210 min), and in addition it requires timeconsuming vessel venting between program stages.This vent Element Wavelength/nm Standard number Recovery (%) procedure ensured that the pressure limit of 14 bar was not exceeded in subsequent program stages. The time advantage Al 396.152 1 102 As 188.979 1 103 of the high-temperature digestion system is oVset, however, if Cd 226.502 1 103 many multiple sample digestions are required.The conven- Cu 324.754 1 102 tional system has a 12-vessel carousel, resulting in a digestion Fe 259.94 1 102 time of 17.5 min per sample. The high-temperature system, Mn 257.61 1 102 which can handle only two samples at a time, has a digestion Pb 220.353 1 103 time of 7.5 min per sample. V 292.402 1 102 Zn 213.856 1 104 Ba 233.527 2 101 2. Extent of digestion. The extent of digestion of the reference Ca 317.933 2 104 materials was examined by ETAAS and HPLC analysis of the K 766.491 2 102 digestates.Results obtained from the GBC 906/GF3000 Mg 279.079 2 102 Na 589.592 2 103 ETAAS instrument are presented in Fig. 1. Since a char step Sr 407.771 2 102 was not employed during the furnace procedures, non-volatile B 249.773 3 105 carbon material surviving the digestion procedures should P 177.428 3 108 appear as background absorption. The profiles in Fig. 1(a) are S 180.669 3 107 for background and Pb absorption in the Bituminous Coal SRM digested by the high-temperature microwave system.The same profiles are presented in Fig. 1(b) for a coal sample P and S, present in the third pure elemental standard solution, decomposed by the conventional microwave digestion pro- are approximately 5, 8 and 7% higher than expected, respectcedure. A comparison of these data reveals that the high- ively. The results from this preliminary study indicate that temperature digestion procedure is more eYcient in decompos- acceptable elemental recoveries can be attained for pure ing this particular matrix.Similar profiles are presented for elemental standards under the high temperature and pressure the high-temperature [Fig. 1(c)] and conventional [Fig. 1(d)] conditions produced by the high-pressure, high-temperature digestions of the Bovine Liver SRM. The ETAAS analysis of digestion unit. these digestates for Cd reveals that matrix destruction is not Tables 6–8 report the metal recoveries observed for the six complete for either mode of digestion.Two background SRM with the two digestion methods. Only metals with absorption peaks appear in the conventional digestate while a certified values for each reference material are listed. Those single peak remains in the high-temperature digestate. metals that were determined at levels below the MQL are The destruction of sample matrices was also investigated by reported as ‘<MQL value.’ the described HPLC procedure. Collected chromatograms for both digestion techniques and each sample matrix are pre- Comparison of the microwave digestion systems sented in Fig. 2. Similar chromatograms are observed for both of the botanical samples decomposed in this investigation. 1. Sample preparation time. Perhaps the most significant advantage of the high-temperature digestion system is the Peach Leaves [Fig. 2(a)] and Orchard Leaves [Fig. 2(b)] digested by the conventional procedure both have three strong speed at which a sample can be digested.Even organic samples that produce copious decomposition gases when treated with absorption signals. The retention and absorbance behavior of these peaks matches that of pure ortho-, meta- and para-NBA nitric acid can be rapidly digested. Botanical samples are decomposed and cooled to room temperature in 10 min while standards. These decomposition species result from the incomplete destruction of aromatic amino acids during a nitric acid biological and geological materials are digested and cooled in Table 6 Comparison of digestion methods for botanical samples High- Hightemperature Conventional temperature Conventional Wavelength/ Peach Leaves Peach Leaves/ Peach Leaves/ Orchard Leaves Orchard Leaves/ Orchard Leaves/ Element nm certified/mg g-1 mg g-1 mg g-1 certified/mg g-1 mg g-1 mg g-1 Al 396.152 249±8 265±5 225±2 270±10 213±9.0 Asa 188.979 0.060±0.018 <2 <2 10± 1 10.2±0.6 10.0±0.2 B 249.773 29±2 25± 1 24± 1 33± 3 31± 1 30± 1 Ba 233.527 124±4 120±2 117±1 (44) 43.0±0.5 42.0±0.3 Ca 317.933 15600±200 15600±200 14990±80 20900±300 20000±300 19600±300 Cda 226.502 0.026±0.003 <0.6 <0.6 0.11±0.01 <0.6 <0.6 Cu 324.754 3.7±0.4 2.5±0.2 <2 12± 1 12± 1 10.0±0.2 Fe 259.94 218±14 207±4 191±2 300±20 200±100 250±10 K 766.491 24300±300 22900±400 22500±500 14700±300 14000±100 12900±600 Mg 279.079 4320±80 4330±80 4060±20 6200±200 6070±40 5720±50 Mn 257.61 98±3 95± 1 88.0±0.3 91±4 87± 1 82± 1 Na 589.592 24±2 26± 1 27± 11 82±6 50± 4 58± 5 P 177.428 1370±70 1470±30 1410±10 2100±100 2060±70 2030±20 Pba 220.353 0.87±0.03 <1 <1 45± 3 43± 1 38± 1 S 180.669 (2000) 1860±30 2200±20 (1900) 2200±10 2600±10 Sr 407.771 53±4 56± 1 54.1±0.4 37±1 36.0±0.4 34.0±0.4 V 292.402 0.37±0.03 <1 <1 <1 <1 Zn 213.856 17.9±0.4 17.7±0.4 18.6±1 25± 3 26± 3 32± 8 aAs, Cd and Pb were determined by ETAAS. 54 J. Anal. At. Spectrom., 1999, 14, 49–59Table 7 Comparison of digestion methods for biological samples High- Hightemperature Conventional temperature Conventional Wavelength/ Oyster Tissue Oyster Tissue/ Oyster Tissue/ Bovine Liver Bovine Liver/ Bovine Liver/ Element nm certified/mg g-1 mg g-1 mg g-1 certified/mg g-1 mg g-1 mg g-1 Al 396.152 202.5±12.5 149±3 136±5 (2)< 10 <10 Asa 188.979 14.0±1.2 12.0±0.4 12.4±0.3 0.047±0.006 <2 <2 B 249.773 <6 <6 <6 <6 Ba 233.527 <1 <1 <1 <1 Ca 317.933 1960±190 1860±70 1840±50 120±7 129±5 130±7 Cda 226.502 4.15±0.38 4.3±0.2 4.21±0.09 0.44±0.06 <0.6 <0.6 Cu 324.754 66.3±4.4 66±2 64± 2 158±7 160±10 165±8 Fe 259.94 539±15 400±200 500±20 194±20 190±10 187±9 K 766.491 7900±470 7800±300 7800±200 9960±70 10000±500 9700±400 Mg 279.079 1180±170 1160±40 1130±20 600±15 650±40 620±30 Mn 257.61 12.3±1.5 11.7±0.4 11.4±0.3 9.9±0.8 10.3±0.6 10.0±0.4 Na 589.592 4170±130 4200±100 5500±700 2430±130 2500±100 3400±100 P 177.428 6230±180 5900±100 5900±100 11100±400 11000±800 11000±300 Pba 220.353 0.371±0.014 <1 <1 0.135±0.015 <1 <1 S 180.669 8620±190 9100±300 10900±300 7800±100 8300±500 9600±300 Sr 407.771 11.1±1.0 10.9±0.3 10.6±0.3 0.138±0.003 0.17±0.01 0.20±0.06 V 292.402 4.68±0.15 4.7±0.2 4.4±0.1 0.099±0.008 <1 <1 Zn 213.856 830±57 890±30 870±20 123±8 132±9 129±5 aAs, Cd and Pb were determined by ETAAS.digestion procedure.13 In contrast, the overlaid chromatograms digested by the conventional, closed-vessel procedure reveals a complicated matrix of nitric acid decomposition products.obtained from the high-temperature digestates show relatively small absorption peaks. Using only nitric acid as an oxidant, The NBA decomposition peaks present in the organic samples are obviously absent. Although the high-temperature digestion the higher temperatures and pressures attained in the focused system destroyed these representative botanical matrices much procedure results in a much ‘cleaner’ matrix, two large absorption signals are present after approximately 19 and 23 min of more completely than the conventional closed-vessel procedures.Similar results were obtained from chromatograms column retention. As was expected, the River Sediment chromatograms [Fig. 2(f )] for both digestion modes are virtually of biological reference material digestates. Oyster Tissue [Fig. 2(c)] and Bovine Liver [Fig. 2(d)] digestate chromatog- identical in appearance. These HPLC data correlate with the collected atomic spectrometric numbers.The lack of a rams have a similar appearance to the HPLC data collected from the botanical matrices just described. Non-volatile high carbon-content matrix results in only minor (if any) improvement in elemental recoveries for the high-pressure, organic decomposition products which are present in the conventional digestate solutions are absent or are significantly high-temperature microwave digestion procedure. mitigated by the high-temperature digestion procedure.As expected, the chromatograms obtained from the HPLC analy- 3. Metal recovery. Fig. 3 compares the per cent. recovery for metals in the SRM from the high-pressure, high- sis of the representative geological samples used in this study are vastly diVerent from those collected for organic matrices. temperature system with that from the conventional microwave digestion system. Only elements that were certified in the The sample chromatogram for Bituminous Coal [Fig. 2(e)] Table 8 Comparison of digestion methods for geological samples High- Hightemperature Conventional temperature Conventional Wavelength/ Bituminous coal Bituminous coal/ Bituminous coal/ River Sediment River Sediment/ River Sediment/ Element nm certified/mg g-1 mg g-1 mg g-1 certified/mg g-1 mg g-1 mg g-1 Al 396.152 8550±190 8900±300 6000±1000 22600±400 8170±90 6900±300 Asa 188.979 3.72±0.09 3.9±0.6 3.8±0.1 (66) 47±4 40± 10 B 249.773 80±1 104±5 261±4 620±30 Ba 233.527 67.5±2.1 63±1 60± 2 50± 3 40± 10 Ca 317.933 2040±60 2010±40 2000±30 (29000) 28060±40 28300±400 Cda 226.502 0.0573±0.0027 <0.6 <0.6 10.2±1.5 10.0±0.7 10.1±0.3 Cu 324.754 6.28±0.30 4.4±0.8 2.5±0.8 109±19 110±6 100±4 Fe 259.94 7590±450 7300±100 7000±200 113000±1200 69000±1000 90000±4000 K 766.491 748±28 750±40 570±80 12600±500 940±50 840±90 Mg 279.079 383±8 394±7 349±6 7400±200 6700±300 6900±200 Mn 257.61 12.4±1.0 11.4±0.8 10.8±0.1 785±97 710±10 676±8 Na 589.592 515±11 477±9 600±30 5400±100 1090±10 1440±30 P 177.428 <70 <70 510±10 150±50 <70 Pba 220.353 3.67±0.26 3.2±0.3 2.6±0.4 714±28 700±10 660±10 S 180.669 18900±600 19700±300 21700±600 (11000) 9000±2000 14000±3000 Sr 407.771 (102) 99±1 98± 1 770±10 770±10 V 292.402 (14) 13.0±0.4 12.0±0.3 23.5±6.9 <1 <1 Zn 213.856 11.89±0.78 11±1 12± 3 1720±170 1730±30 1630±30 aAs, Cd and Pb were determined by ETAAS.J. Anal. At. Spectrom., 1999, 14, 49–59 55account for the abnormally high recovery for Zn in Orchard Leaves with the conventional method.On three occasions, the Fe level determined after high-temperature microwave digestion is significantly lower than that determined following conventional digestion (Orchard Leaves, Oyster Tissue, River Sediment). These data suggest that Fe is more eYciently extracted from the sample matrices by the conventional method. If the Fe is bound tightly in an inorganic or silicate matrix, then increased exposure times to cooler acid (below 180 °C) may provide higher extraction eYciency than short exposure times to very hot acid (300 °C).Previous workers have reported low recoveries for Na in Orchard Leaves, stating the need for hydrofluoric acid in the digestion process.9 This poor recovery for Na in Orchard Leaves is also observed in the present work, for both digestion methods. The recovery data for the coal sample [Fig. 3(d)] show a significantly lower error for the high-temperature microwave system (6.9%) than for the conventional system (15.9%, Table 9).If the potential contamination elements (Na, S, Zn) are omitted for the coal sample, then the per cent. recovery for the focused system is always higher than that observed with the conventional system. This may be attributed to the higher degree of matrix decomposition observed for the hightemperature system. The chromatograms observed for the coal digestates support this interpretation [Fig. 2(e)].The metal recovery from the River Sediment sample is approximately the same for both digestion techniques [Fig. 3(e)]. Many elements had very low recovery for this sample (P, Na, K, and Al for example, Table 8). This indicates that the River Sediment sample requires hydrofluoric acid for complete digestion. These low recovery elements were eliminated from Fig. 3(e) for clarity. The overall per cent. error in the recovery data for the six reference materials is lower for the high-temperature microwave system (7.1%) than for the conventional system (11.8%, Table 9).This diVerence is slightly reduced if the suspected contamination elements (Na, S) are eliminated from the calculation. Certainly, additional contamination–prevention steps could be incorporated into the conventional method. Also, the use of hydrofluoric acid would improve recovery for several elements, but HF digestions cannot be safely performed with the present, high-pressure, high-temperature microwave system.Fig. 1 ETAAS background, Cd and Pb absorption profiles for: 4. Safety concerns. The most serious drawback for the (a) Bituminous Coal decomposed by the high-pressure, highhigh- pressure, high-temperature system was that the instru- temperature procedure; (b) Bituminous Coal decomposed by the conventional procedure; (c) Bovine Liver decomposed by the high- ment experienced unexpected vessel ruptures, even when pressure, high-temperature procedure; (d) Bovine Liver decomposed operating under conditions recommended by the manufacby the conventional procedure.turer. As a consequence, the experimental procedures provided in the instrument manual were modified to ensure safe operation. Four vessel ruptures were observed for the high- reference materials at levels above the MQL for the analysis temperature system. All occurred during the method develop- method are included. A few conclusions can be drawn from ment phase of the study.No membranes were ruptured with Fig. 3. the conventional system, but the digestion methods for this In all cases the Na and S levels determined after conventional system had already been prepared for the sample types investi- digestion are higher than those determined after digestion with gated.26 The extreme temperature and pressure conditions the high-temperature microwave system. In several cases achieved in the high-pressure unit inevitably result in the (Bovine Liver, Oyster Tissue, Bituminous Coal ) the S and Na complete destruction of digestion vessels when a sample values determined after conventional digestion were signifi- explodes.This is in contrast to the loss of an inexpensive cantly above the certified amount (greater than two standard rupture membrane for the conventional closed-vessel pro- deviations above 100% recovery). These data suggest that cedures. Also, the large pressure release from such an exother- contamination may have occurred for these samples.Such an mic reaction would always eject the exhaust hoses during a explanation is plausible considering the increased opportunity vessel rupture. As a consequence, the use of hydrofluoric acid for contamination with the conventional digestion procedure: is impossible. This will inevitably result in low metal recoveries the procedure employs a much longer sample preparation time with several venting steps. A similar explanation may also for inorganic sample matrices. 56 J. Anal. At. Spectrom., 1999, 14, 49–59Fig. 2 HPLC analysis of reference material digestates prepared by the high-pressure, high-temperature procedure, and by the conventional method: (a) Peach Leaves, (b) Orchard Leaves, (c) Oyster Tissue, (d) Bovine Liver, (e) Bituminous Coal, (f ) River Sediment. In all cases, the conventional digestate chromatogram is shifted up by 0.01 absorbance units. 5. System monitoring. The high-temperature digestion system Conclusion lacks the ability to monitor vessel temperature during the digestion process.The user therefore has very limited knowl- The high-pressure, high-temperature microwave digestion edge concerning the extent of reaction during the process. This system exhibits several advantages over conventional closedinevitably leads to vessel rupture during the method develop- vessel microwave digestion. The sample digestion time is much ment procedure. Most conventional systems are equipped with shorter if only a small number of samples need to be prepared.Even if multiple samples are analyzed, the high-temperature both pressure and temperature feedback systems. Table 9 Per cent. error in metal recovery from reference materials Focused system Conventional system Reference material All elements No Na or S All elements No Na or S Peach Leaves 5.2 4.7 8.0 7.4 Orchard Leaves 8.5 5.6 13.6 10.6 Oyster Tissue 7.0 7.6 10.3 7.4 Bovine Liver 5.8 6.1 12.5 8.2 Bituminous Coal 6.9 7.1 15.9 15.9 River Sediment 9.4 8.3 10.3 8.2 Average 7.1 6.6 11.8 9.6 J.Anal. At. Spectrom., 1999, 14, 49–59 57Fig. 3 Metal recoveries observed for the high-pressure, high-temperature procedure (striped bars), and by the conventional method (solid bars): (a) Peach Leaves, (b) Orchard Leaves, (c) Oyster Tissue, (d) Bovine Liver, (e) Bituminous Coal, (f ) River Sediment. system is approximately 10 min faster per sample. The high- Company and Mr. Tommy White for invaluable suggestions and contributions during this investigation.temperature system also provides more complete matrix destruction than the conventional method. The ETAAS background absorbance was lower, and HPLC traces were cleaner for the high-temperature digestates. Metal recoveries observed References for the high-temperature system were better than those for the conventional system for the six SRM tested. The high- 1 G. Damkro� ger, M. Grote and E. Janßen, Fresenius’ J. Anal. Chem., 1997, 357, 817. temperature system does have some drawbacks. The system 2 A. Abu-Samra, J. S. Morris and S. R. Koirtyohann, Anal. Chem., has no means for monitoring temperature during a digestion. 1975, 47, 1475. Exceeding the pressure limit during a digestion procedure 3 R. T. White Jr. and G. E. Douthit, J. Assoc. OV. Anal. Chem., results in complete destruction of the vessel, with subsequent 1985, 68, 766. emission of the digestion gases into the laboratory environ- 4 D. Chakraborti, M. Burguera and J. L. Burguera, Fresenius’ ment. The use of hydrofluoric acid to assist in the digestion J. Anal. Chem., 1993, 347, 233. 5 H. M. Kingston and L. B. Jassie, Anal. Chem., 1986, 58, 2534. of inorganic and geological samples is therefore impossible. 6 G. Heltai and K. Percsich, Talanta, 1994, 41, 1067. 7 J. Liu, R. E. Sturgeon and S. N. Willie, Analyst, 1995, 120, 1905. Acknowledgements 8 H. M. Kingston and L. B. Jassie, J. Res. Natl. Bur. Stand. (U.S.), 1988, 93, 269. This work was supported by a grant from the NSF-GOALI 9 C. B. Rhoades Jr., J. Anal. At. Spectrom., 1996, 11, 751. Program (NSF-CHE 9710218). The authors also gratefully 10 M. Wu� rfels, E. Jackwerth and M. Stoeppler, Anal. Chim. Acta, 1989, 226, 1. acknowledge the support of the R. J. Reynolds Tobacco 58 J. Anal. At. Spectrom., 1999, 14, 49–5911 M. Wu� rfels, E. Jackwerth and M. Stoeppler, Anal. Chim. Acta, 19 K. Lamble and S. J. Hill, Analyst, 1995, 120, 413. 1989, 226, 17. 20 E. J. M. TemminghoV and I. Novozamsky, Analyst, 1992, 117, 23. 12 M. Wu� rfels, E. Jackwerth and M. Stoeppler, Anal. Chim. Acta, 21 I. Equiarte, R. M. A. Lonso and R. M. Jime�nez, Analyst, 1996, 1989, 226, 31. 121, 1835. 13 K. W. Pratt, H. M. Kingston, W. A. MacCrehan and W. F. Koch, 22 M. Wu� rfels, E. Jackwerth and M. Stoeppler, Fresenius’ Z. Anal. Anal. Chem., 1988, 60, 2024. Chem., 1987, 329, 459. 14 H. J. Reid, S. Greenfield and T. E. Edmonds, Analyst, 1995, 120, 23 R. T. White, J. Assoc. OV. Anal. Chem., 1989, 72, 387. 1543. 24 H. Matusiewicz, Anal. Chem., 1994, 66, 751. 15 J. Wang, in Stripping Analysis: Principles, Instrumentation and 25 R. Parosa and E. Reszke, in BM-1SII Technical User’s Manual, Applications VCH, Deerfield Beach, FL, USA, 1985 ch. 4, Plazmatronika, Wroclaw, 1995. pp. 104–107. 26 C. B. Rhoades Jr., K. E. Levine, A. Salido and B. T. Jones, Appl. 16 J. Golimowski and K. Golimowska, Anal. Chim. Acta, 1996, 325, Spectrosc., 1998, 52, 200. 111. 17 A. Krushevska, R. M. Barnes and C. Amarasiriwaradena, Analyst, 1993, 118, 1175. 18 R. Chakraborty, A. K. Das, M. L. Cervera and M. de la Guardia, Fresenius’ J. Anal. Chem., 1996, 355, 43. Paper 8/03895C J. Anal. At. Spectrom., 1999, 14, 49

ISSN:0267-9477    

DOI:10.1039/a803895c

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Evaluation of several commercially available spray chambers for use in inductively coupled plasma atomic emission spectrometry Salvador Maestre, Juan Mora, Jose�-Luis Todolý� and Antonio Canals* Departamento de Quý�mica Analý�tica, Universidad de Alicante, 03071 Alicante, Spain Received 19th August 1998, Accepted 5th November 1998 Four diVerent spray chambers were compared for the elemental analysis of liquid samples by ICP-AES: a doublepass Scott-type spray chamber made from Ryton and three cyclonic spray chambers manufactured from various materials [i.e., glass, poly(propylene), PP, and poly(tetrafluoroethylene), PTFE].A glass concentric pneumatic nebulizer was used in conjunction with all four chambers. The parameters evaluated were: the characteristics of the aerosols at the exit of each chamber (i.e., tertiary aerosols); the solvent (Stot) and analyte (Wtot) transported through each chamber; and the ICP-AES analytical parameters (i.e., net emission intensities, limits of detection, LOD, and background equivalent concentrations, BEC).The interference produced by the presence of a widely used matrix (i.e., acids) was also evaluated for the four chambers. The results indicated that the cyclonic glass and PP spray chambers gave rise to coarser tertiary aerosols, higher solution transport rates, higher emission signals and lower LOD and BEC values than the other two spray chambers. For the cyclonic spray chambers, the position of the nebulizer proved to be of critical importance.With regard to the acid eVects, these were more pronounced as the tertiary aerosols became finer. Hence, for the Scott-type spray chamber, the signal reduction induced by the presence of acids was enhanced compared with the cyclonic spray chambers. The introduction of the sample is a critical step in atomic as for example the vortex type,3,8–10,14 Sturman Masters type15 and vertical rotary;16,17 and (iii) the single-pass or cylindrical spectrometric analysis.1,2 The characteristics of the analytical type8,21–23 or direct spray chamber.8 signal finally obtained depend, to a great extent, on the quality Among the designs mentioned above, the most widely used (i.e., eYciency and reproducibility) of the sample introduction in ICP-AES are the double-pass (Scott-type) and the cyclonic system.Usually, in inductively coupled plasma atomic emission designs.On comparing both designs, it has been reported that spectrometry (ICP-AES), the sample is introduced as a liquid the former gives rise to lower analyte transport eYciencies and solution.In most cases, the main components of the higher memory eVects than the latter.1,4,7,11,16,24–26 liquid sample introduction system are: (i) a nebulizer, that Another aspect to take into account is the matrix eVect (i.e., spreads out the liquid bulk generating an aerosol; (ii) a spray the change in performance induced by the main sample chamber, that filters the aerosol and selects the maximum components). Acids are among the most commonly present droplet size entering the plasma; and (iii) an injector tube, matrices.Several workers have reported that acids cause an used to introduce the aerosol into the plasma. ICP-AES signal depression with respect to water,27–38 although The spray chamber is a component of major interest.3,4 It at low acid concentration a signal enhancement has been is recognized that, when less of 5% of the analyte nebulized is observed.27 All these eVects are complex and diVerent reasons transported to the atomizer, the spray chamber, rather than have been proposed in order to explain them, such as modifi- the nebulizer (i.e., pneumatic nebulizers), determines the cations in the nebulization process28,32 due to a change in the characteristics (fineness and amount) of the aerosol injected solution viscosity and/or surface tension; changes in the aerosol into the plasma.4 The processes taking place within the spray transport;27,29,32 and deterioration in the plasma thermal chamber are many and complex in nature.Hence, the aerosol characteristics.30–32 In addition, acids induce increases in suVers from solvent evaporation, agglomeration, impact losses memory eVects through ‘transient’ mechanisms.39,40 Therefore, due to turbulence and/or inertia.5 Also, the aerosol thermal it is obvious that the spray chamber plays a very important and charge equilibria are established and the turbulence associrole also from the point of view of matrix eVects, as has been ated with the nebulization process is reduced inside the spray recently anticipated.37 chamber.The aim of this work was, thus, to evaluate the behavior of Some characteristics of an ideal spray chamber are: (i) it several spray chambers in ICP-AES. In this study, a Scott- should be able to transport as much analyte mass to the type spray chamber and three cyclonic spray chambers built atomization cell as possible without deteriorating its excitation from diVerent materials [i.e., glass, poly(propylene) and poly- properties; (ii) the aerosol entering the plasma should be as (tetrafluoroethylene)] were used.A systematic investigation of fine as possible, and less turbulent than the aerosol generated the characteristics of the aerosol exiting each chamber (i.e., in the nebulization step. As a consequence, the analyte will be tertiary) the transport of solution, analytical parameters and more eYciently excited; (iii) the spray chamber should be matrix (acid) eVects was carried out.robust (i.e., minimum variation of its performance with samples of very diVerent nature and composition); (iv) the memory eVects should be minimal in order to increase the Experimental sample throughput; and (v) it should have mechanical simplicity and low production cost. A pneumatic concentric nebulizer (Model AR-35–07-C2, Glass To this end, several spray chamber designs have been Expansion, Australia) was used throughout.Its gas outlet proposed in the literature:6–26 (i) the double-pass (so-called cross-section area was 3.1×10-2 mm2, giving a back-pressure of 19 psig for a 0.8 l min-1 gas flow rate. The aerosol was, Scott-type) or reverse flow type;6–9 (ii) the cyclone3,8–10,14–20 J. Anal. At. Spectrom., 1999, 14, 61–67 61Table 1 Plasma instrumental conditions Rf power/kW 1.3 Integration time/ms 100 Sampling time/ms 1000 Outer gas flow rate/l min-1 15 Intermediate gas flow rate/l min-1 0.5 Central gas flow rate/l min-1 Variable Viewing height above load coil/mm From 10 to 12 mm, optimized in each case Torch Fassel-type Injector id/mm 2 Sample uptake rate/ml min-1 Variable Table 2 lists the element wavelengths as well as the ionic line Fig. 1 Schematic diagram and critical dimensions of the cyclonic energy sum values, Esum (i.e., sum of ionization and excitation spray chambers used.The parameters A, B, C, D and E are expressed energies). Solutions of two acids (i.e., nitric and sulfuric) at in mm, while a is given in degrees. concentrations ranging from 0.4 to 3.0 mol l-1 were also prepared. therefore, generated by the exposure of the liquid sample to a high-velocity gas stream. The four spray chambers used were: Results and discussion a Ryton Scott-type (Perkin-Elmer, U� berlingen, Germany) with an inner volume of 100 cm3 and three cyclonic spray chambers Spray chamber characterization with water made from diVerent materials: glass, poly(propylene) (PP), First, the performance of the four spray chambers was and poly(tetrafluoroethylene) (PTFE) (Glass Expansion); compared with water solutions. To this end, the characteristics their inner volumes were 47.5, 62 and 57.3 cm3, respectively.of the tertiary aerosols, amount of solution transported and Fig. 1 shows an outline of the cyclonic spray chamber design analytical figures of merit in ICP-AES were evaluated.as well as the critical dimensions of the three cyclonic spray chambers employed. The nebulizer was positioned tangentially Aerosol characterization. Fig. 2 shows the variation of the to the wall of the cyclonic spray chamber. Sauter mean diameter (D3,2) of the tertiary aerosols versus the The liquid sample was supplied to the by means nebulizing gas, Qg [Fig. 2(a)] and liquid, Ql [Fig. 2(b)] flow of a Gilson Minipuls 3 peristaltic pump ( Villiers-Le-Bel, rates for water and the four spray chambers evaluated.As can France). The gas flow, in turn, was varied by means of a mass be derived from Fig. 2(a), an increase in the gas flow rate led flow controller, Model FC260 (Tylan, Torrance, CA, USA). to finer tertiary aerosols for the four chambers tested. This Argon was employed as the nebulizer gas in all cases. might be because the nebulizer generated finer primary aerosols Drop size distributions (DSD) of the aerosols generated by on increasing Qg, since the amount of available kinetic energy the nebulizer (primary) and those exiting the spray chamber of the gas stream became higher.5,43,44 Hence, at Ql= (tertiary) were characterized by means of a Model 2600c 0.6 ml min-1, D3,2 values of the primary aerosols were 12.8 Fraunhofer laser diVraction system (Malvern Instruments, and 5.8 mm on switching from Qg=0.4 to 0.8 l min-1. Malvern, Worcestershire, UK) equipped with a lens of 63 mm In addition to the mean diameters of the DSD, the laser focal length that allowed for the measurement of drop diamdi Vraction instrument provides some parameters related to the eters ranging from 1.2 to 118 mm.The software employed was aerosol liquid volume contained in the sampling zone. In this the B.OD version. A model-independent algorithm was used work, the volume concentration (VC) was taken as an indi- to calculate the whole DSD from the energy data.The primary cator of this aerosol property. The VC is referred to the aerosols were measured at 10 mm from the nebulizer tip, percentage of the laser measurement volume that is occupied whereas, for the tertiary aerosols, the spray chamber exit was by droplets.45 The results indicated that VC for tertiary placed at 5 mm from the laser beam. A set of three replicates aerosols increased with Qg. Thus, for instance, for the double- was performed in each case; the precision (RSD) reached was pass spray chamber the VC values were 0.009 and 0.027% always lower than 2%. when Qg was raised from 0.4 to 0.8 l min-1.This fact was due The solvent transport rate (Stot) was measured, only for to both the generation of finer primary aerosols and the water, by means of a direct method (i.e., by the adsorption of increase in the carrying capability of the gas stream.21,44 the aerosol in a U-tube filled with silica gel for 10 min).40,41 On increasing Ql from 0.4 to 0.8 ml min-1, at Qg= By weighing the tube before and after aerosol exposure, the 0.5 l min-1, a slight growth in the D3,2 of the primary aerosol Stot values were easily derived.Analyte transport rate values were obtained by a direct method (i.e., by collecting the aerosol on a glass-fiber filter, Type A/E, 47 mm diameter, 0.3 mm pore Table 2 Elements and lines used, wavelengths and energy sum values size, Gelman Sciences, Ann Arbor, MI, USA).5,41,42 A Element Wavelength/nm Esum/eVa 500 mg ml-1 Mn solution was nebulized.The Mn retained after 10 min was extracted by washing the filters with 1.0% Al I 396.152 — (v/v) hot nitric acid solution. The total solution volume was Ba II 455.403 7.93 made up to 100 ml in a calibrated flask. Finally, the Mn Cd II 214.438 14.77 concentration in each solution was determined by flame atomic Cr II 205.560 12.80 absorption spectrometry (FAAS). The precision of transport Cu I 324.754 — Mg II 280.270 12.25 experiments was always lower than 5% (RSD from three Mg I 285.213 — replicates).Mn II 257.610 12.25 Emission signals were measured with a Perkin-Elmer Optima Ni II 221.647 14.27 3000 ICP-AES instrument. Table 1 summarizes the plasma Sr II 407.771 8.73 instrumental conditions. Samples containing ten elements Zn I 213.856 — (1 mg ml-1 each) were prepared in water from an ICP multi- aEsum (only for ionic lines)=ionization energy+excitation energy. element standard solution (IV, Merck, Darmstadt, Germany). 62 J. Anal. At. Spectrom., 1999, 14, 61–67results. Therefore, neither the inner volume of the chamber nor its geometry seems to be responsible for the observed behavior. A more complete study on this subject is necessary. A factor that has proved to be critical is the relative position of the nebulizer–spray chamber (i.e., the distance between the nebulizer tip and the wall of the cyclonic spray chamber). Each cyclonic spray chamber accepted a given (fixed) position for the nebulizer and the latter could not be introduced beyond this point (Fig. 1). However, a conspicuous variation in the tertiary aerosol characteristics was observed as a function of the nebulizer position. In this way, with the PTFE cyclonic spray chamber, the D3,2 values of the tertiary aerosols varied from 2.3 to 2.9 mm when the nebulizer was moved backward around 5 mm from its initial (fixed) position. As regards the VC, this parameter changed from 0.025 to 0.034% for the reported situations.These results clearly demonstrate that the extent of the aerosol transport processes ( likely impaction losses) was modified on changing the nebulizer position. According to the VC data, the established nebulizer position was not the optimum and more liquid solution was allowed to leave the spray chamber on bringing the nebulizer out by 5 mm. Therefore, it seems that the nebulizer position is an important factor that should be optimized for cyclonic spray chambers.In summary, the nebulizer position for each chamber might partially explain the diVerences found for the three cyclonic spray chambers (Fig. 1). The spray chamber material might play a role in the aerosol transport and filtering process, but, at this stage, this was not verified. Furthermore, small diVerences in some critical dimensions (i.e., angle of the Fig. 2 EVect of the gas flow rate, Qg (a), Ql=0.6 ml min-1, and the top and/or bottom of the spray chamber, inner diameter or liquid flow rate, Ql (b), Qg=0.5 l min-1, on the Sauter mean diameter height) might induce diVerences in the behavior of these (D3,2) of tertiary aerosols for the spray chambers studied. (A) cyclonic chambers (Fig. 1). glass, (B) cyclonic PP, (C) cyclonic PTFE and (D) double-pass. Transport parameters. Fig. 3 shows the variation of the solvent transport rate (Stot) with Qg [Fig. 3(a)] and Ql from 9.7 to 10.5 mm was observed. This fact was due to the drop in the gas kinetic energy to liquid (sample) mass ratio as [Fig. 3(b)] for the four spray chambers tested. As expected from the VC data, Stot increased on increasing both variables, Ql was raised.44,46,47 In spite of this, D3,2 values of tertiary aerosols did not show noticeable variations with Ql, irrespec- its variation with Qg being more pronounced than with the liquid flow rate. Again, on comparing the diVerent spray tive of the spray chamber employed [Fig. 2(b)].Therefore, from Fig. 2(b) it seems that each spray chamber imposed its chambers, a correlation between Stot and VC was found (i.e., Stot followed the order: Scott#PTFE<PP<glass). own cut-oV diameter, masking the variations produced in the primary aerosols.4 In contrast, the VC values slightly increased, The results concerning the analyte transport rate (Wtot) confirmed the variations observed in Stot (i.e., the analyte although the magnitude of the increase was lower than for Qg.For the double-pass spray chamber, at Qg=0.5 l min-1, transport rate became higher on increasing both gas and liquid flow rates, irrespective of the spray chamber used). Hence, for VC was 0.012 and 0.019% for Ql=0.4 and 0.8 ml min-1, respectively. The slight increase in VC values might be due to the Scott-type spray chamber at Ql=0.6 ml min-1, Wtot was 0.04 and 0.18 mg s-1 when Qg was switched from 0.4 to the intensification of solution losses due to the generation of coarser primary aerosols, that counterbalanced, at least in 0.8 l min-1.The increase in the analyte transported was less pronounced when the liquid flow rate was raised. Thus, at part, the sample uptake rate increase. From the results discussed above, it can be anticipated that Qg=0.5 l min-1 for the Scott-type spray chamber, Wtot was 0.06 and 0.07 mg s-1 for Ql=0.4 and 0.8 ml min-1, an increase in both Qg and Ql should produce an increase in the amount of solution exiting the spray chambers.respectively. As regards the spray chamber used, at Qg=0.5 l min-1 and With regard to the spray chamber design and material, Fig. 2 reveals that the tertiary aerosols obtained with the Ql=0.6 ml min-1, Wtot was 0.15, 0.11, 0.05 and 0.06 mg s-1 for the glass, PP, PTFE and Scott-type spray chambers, double-pass Scott-type spray chamber had lower D3,2 values than those generated by the glass and PP cyclonic spray respectively, which agreed with the results obtained for Stot (Fig. 3). As was observed for tertiary aerosols,Wtot also varied chambers but slightly higher values than those for the PTFE cyclonic spray chamber. Among the cyclonic spray chambers, on changing the nebulizer position. Hence, for the PTFE chamber, at Qg=0.5 l min-1 and Ql=0.6 ml min-1, Wtot D3,2 values followed the order: glass>PP>PTFE. The VC values, in turn, varied as follows: VC(Scott)# values of 0.06 and 0.07 mg s-1 were obtained on moving the nebulizer from the fixed position to 5 mm behind this point VC(PTFE)<VC(PP)<VC(glass).The reasons that might be argued to explain the diVerent behavior are based on changes (Fig. 1). in the inner volume and geometry of each spray chamber.9,16,22,23 Nevertheless, for the PP and PTFE chambers Analytical figures of merit. Fig. 4 shows the variation of the net emission signal of Mn when the gas flow [Fig. 4(a)] and the D3,2 values were very diVerent, whereas their inner volumes were similar.In addition, the inner volume of the PTFE sample uptake rate [Fig. 4(b)] were increased. From Fig. 4(a) it can be seen that the emission signal increased with Qg up chamber was smaller than that of the Scott-type chamber by a factor of 0.56, whereas both chambers gave rise to similar to a value of 0.5 l min-1. Further increases in the gas flow J. Anal. At. Spectrom., 1999, 14, 61–67 63resulted in a sharp signal decrease. This last fact was explained by the reduction in the analyte residence time inside the plasma as well as a deterioration in its thermal characteristics.48–50 On increasing the sample uptake rate a slight growth in emission signal was obtained [Fig. 4(b)]. These results were predictable from the variation in VC and transport parameters. As can be seen in Fig. 4, the cyclonic spray chamber made from glass aVorded higher emission signals than the remaining spray chambers. An interesting point is that, under the optimum conditions of Fig. 4(a), the maximum signal increase factor for the glass cyclonic spray chamber with respect to the Scott-type chamber (e.g., 2.3) was lower than the corresponding Wtot (e.g., around 3). This eVect could be accounted for by a reduction in the plasma thermal capability. In order to evaluate whether the plasma was deteriorated, the Mg II-to- Mg I (see Table 2) emission intensity ratio was measured. This parameter has been considered a good indicator of the plasma excitation properties.49–51 Actually, this ratio was lower for the glass cyclonic spray chamber than for the double-pass spray chamber.Hence, at Qg=0.5 l min-1 and Ql= 0.6 ml min-1, this ratio was 6.3 and 8.8 for the glass cyclonic and Scott-type spray chambers, respectively. When a sample introduction system is evaluated, another interesting analytical parameter is the signal stability. In this study, the so-called short-term stability was evaluated by means of the RSD of 20 replicates of the ICP-AES emission intensity.The results indicated that the RSD values were, in general, higher for the Scott-type spray chamber than for the cyclonic spray chambers. Hence, at Qg=0.5 l min-1 and Ql= 0.6 ml min-1, this parameter was 1.7, 1.1, 1.7 and 2.9% for the glass, PP, PTFE and Scott-type chambers, respectively. Fig. 3 EVect of the gas flow rate, Qg (a), Ql=0.6 ml min-1, and the Note that these values represent the mean of the RSD for the liquid flow rate, Ql (b), Qg=0.5 l min-1, on the solvent mass transport rate (Stot) for the spray chambers studied.(A) Cyclonic glass, (B) ten elements evaluated (Table 2). These results indicated that, cyclonic PP, (C) cyclonic PTFE and (D) double-pass. in relative terms, cyclonic spray chambers exhibited more stable emission signals than the Scott-type chamber. Fig. 5 summarizes the limits of detection (LOD) obtained for the diVerent spray chambers and elements studied. In this case, LOD values were calculated according to the 3sB criterion, where sB is the standard deviation of 20 replicates of the background.As can be observed, the glass cyclonic chamber gave the lowest LOD values for all the elements tested and, depending on the element, the PTFE cyclonic or Scott-type chamber aVorded the highest LOD values. These data mainly reflected the behavior of the emission signal. Table 3 lists the background equivalent concentration (BEC) values for the diVerent elements and spray chambers tested.It is interesting that, as happened with the LOD values (Fig. 5), the cyclonic spray chamber manufactured from glass gave rise to the lowest BEC values, whereas the Scott-type and the PTFE chambers provided the highest BEC values. Fig. 4 EVect of the gas flow rate, Qg (a), Ql=0.6 ml min-1, and the liquid flow rate, Ql (b), Qg=0.5 l min-1, on the net emission signal Fig. 5 Limits of detection (LOD) for the elements studied with all the for a 1 mg ml-1 Mn solution.(A) Cyclonic glass, (B) cyclonic PP, (C) cyclonic PTFE and (D) double-pass spray chamber. spray chambers evaluated. Qg=0.5 l min-1; Ql=0.6 ml min-1. 64 J. Anal. At. Spectrom., 1999, 14, 61–67Table 3 Background equivalent concentration (BEC) values for the elements and spray chambers tested BEC/mg ml-1 Element Glass PP PTFE Scott Al 0.83 1.56 3.10 2.30 Ba 0.02 0.04 0.07 0.05 Cd 0.09 0.11 0.21 0.19 Cr 0.11 0.16 0.30 0.24 Cu 0.17 0.27 0.52 0.43 Mg 0.01 0.02 0.03 0.02 Mn 0.03 0.05 0.08 0.07 Ni 0.22 0.31 0.60 0.47 Sr Saturation 0.02 0.03 0.02 Zn 0.040 0.06 0.10 0.10 The wash-out times (i.e., the time spent by the system to achieve 1% of the steady-state signal value) were measured for a 1mg ml-1 multi-elemental aqueous solution.The results indicated that the wash-out times were, in general, lower for the cyclonic than for the Scott-type chamber. As a consequence, at Qg=0.5 l min-1 and Ql=0.6 ml min-1, 20, 24, 22 and 30 s were required to diminish the signal to 1% of its initial value for the glass, PP, PTFE and double-pass spray chambers, respectively.This eVect has been discussed previously by other workers and can be explained in terms of the smaller inner volume and simpler geometry of the aerosol path for the cyclonic spray chambers.4,7,16,25 The study of the memory eVects was carried out with Pd. This element seems to be preferentially adsorbed on some Fig. 6 EVect of acid concentration on Sauter mean diameter of the polymer surfaces. It was found that the wash-out times were tertiary aerosol (D3,2).(A) Cyclonic glass, (B) cyclonic PP, (C) 26, 36, 38 and 44 s for the glass, PP, PTFE and Scott-type cyclonic PTFE and (D) double-pass spray chamber. (a) Nitric acid; spray chambers, respectively. These results showed an import- (b) sulfuric acid. Qg=0.5 l min-1; Ql=0.6 ml min-1. ant diVerence based on the spray chamber material; the lowest memory eVect was observed for the glass spray chamber, the The trends in D3,2 values of the tertiary aerosol versus acid memory eVect being more severe for the polymer-based spray concentration were the same irrespective of the spray chamber chambers.Other workers found Pd adsorption on the inner material and design and acid type used. Among the two acids walls of polystyrene digestion vessels.52 Therefore, from the studied, sulfuric acid generated the finer tertiary aerosols. The point of view of memory eVects, it seems that glass is the best Sauter mean diameters of the tertiary aerosols obtained with choice as the spray chamber material.the diVerent spray chambers followed the same order as those reported for pure water in Fig. 2. Spray chamber characterization with acid solutions The VC values of the tertiary aerosols increased with acid concentration. This growth was more pronounced for the Acids are one of the most commonly present matrices in ICP-AES analysis. Based on the results reported in the litera- Scott-type spray chamber than for the cyclonic spray chambers.Hence, with the former chamber, the VC value for water ture, there are two groups of inorganic acids:29,31,32,35,36 (i) those that interfere with the aerosol transport processes; and (0.011%) was improved by a factor of around two when 3 mol l-1 solutions of either nitric (0.022%) or sulfuric (ii) acids that, in addition, deteriorate aerosol production during the nebulization process.In this work, two acids (0.023%) acid were employed. In contrast, for the glass cyclonic spray chamber the VC values varied only slightly (0.073, 0.078 representative of each of these two groups were used (i.e., nitric and sulfuric) in solutions of diVerent concentration. and 0.074% for water, and 3 mol l-1 nitric and sulfuric acid, respectively). In summary, on increasing the acid concentration there was Aerosol characterization. Fig. 6 shows the variation of the D3,2 of the tertiary aerosols with the acid concentration for a higher (or at least the same) amount of aerosol in liquid form at the exit of the spray chamber and it was contained in nitric acid [Fig. 6(a)] and sulfuric acid [Fig. 6(b)]. The corresponding points for pure water are also shown. From Fig. 6 smaller droplets. These findings agreed well with those previously reported in the literature.32,35,36 it can be seen that the presence of acids in solution led to a decrease in the D3,2 values relative to pure water, the reduction being more important on switching from pure water to the Transport parameters.It was diYcult to measure the solvent transport rate by means of a continuous method with concen- 0.4 mol l-1 concentration. According to the data shown in Fig. 6, further acid concentration increases did not produce trated acid solutions, since a leak of acid (white vapors) was observed at the exit of the U-shaped tube even when two tubes significant variations in the D3,2 values.In contrast, no appreciable variations were found in the primary aerosol D3,2 values. were placed in series at the exit of the spray chamber. For this reason, some experiments were performed by applying an At Qg=0.5 l min-1 and Ql=0.6 ml min-1, the primary aerosol D3,2 values were 10.9, 10.1 and 10.9 mm for pure water and indirect method.53,54 Both solution and drain containers were continuously weighed. Then, the mass loss was plotted versus 3 mol l-1 nitric and sulfuric acid, respectively.All these results revealed that, as it was stated in a previous study, there is a time, giving a straight line, the slope of which corresponded to the Stot value. From these experiments no important vari- diVerent aerosol transport mechanism for water and for acids.35 ations in the solvent transport rate were observed between J. Anal. At. Spectrom., 1999, 14, 61–67 65pure water and acid solutions. Hence, at Qg=0.5 l min-1 and Ql=0.6 ml min-1 and with the PP cyclonic spray chamber, the relative Stot values, i.e., (Stot)acid/(Stot)water, were 0.97 and 1.09 for 3 mol l-1 nitric and sulfuric acid solutions, respectively.Note that the precision of this method was poorer than that of the direct method.40 Similar results were obtained for the other chambers. As regards the analyte transport values, the results indicated that when an acid was present the Wtot values decreased with respect to water. Nonetheless, as was mentioned previously, the aerosol liquid fraction at the exit of the spray chamber became greater as the acid concentration increased and Stot did not vary significantly.These results support the existence of the so-called aerosol ionic redistribution (AIR) phenomenon. 35,55,56 As has been demonstrated,35,56 this eVect contributes to a decrease in the analyte concentration in aerosol droplets exiting the spray chamber when ionic species are present. On comparing the diVerent spray chambers employed, it was observed that, for both nitric and sulfuric acid, the reduction in Wtot was, in general, less pronounced for the cyclonic spray chambers than for the double-pass Scott-type spray chamber.Hence, for nitric acid at Qg=0.5 l min-1 and Ql=0.6 ml min-1, the ratios (Wtot)nitric/(Wtot)water were 0.95 and 0.87 for the glass cyclonic and Scott-type spray chambers, respectively. The ratios for sulfuric acid were 0.80 and 0.70, respectively.Note that 3 mol l-1 acid solutions were employed. This result indicates that acid eVects are expected to be less severe for the cyclonic spray chamber than for the Scott-type spray chamber. Fig. 7 EVect of acid concentration on the relative net emission intensity, defined as the mean of the ratio Iwith acid/Iwithout acid, obtained Emission signal. Fig. 7 shows the variation of the relative for the ionic lines; 1 mg ml-1 solution concentration. (A) Cyclonic net emission intensity (Irel) versus the acid concentration for glass, (B) cyclonic PP, (C) cyclonic PTFE and (D) double-pass spray the four spray chambers evaluated and nitric [Fig. 7(a)] and chamber. (a) Nitric acid; (b) sulfuric acid. Qg=0.5 l min-1; Ql= 0.6 ml min-1. sulfuric [Fig. 7(b)] acid. The parameter Irel refers to the mean of the ratio Iwith acid/Iwithout acid obtained for each ionic line. In addition to the mean points, in Fig. 7 the maximum and minimum Irel values have also been represented as bars.In chambers (those that aVorded the lowest Mg II/Mg I ratios), the acid eVects were less severe than for the Scott-type spray this way, the bar width indicates the degree of variability of the acid eVect for the diVerent ionic lines tested. As can be chamber. Therefore, the acid eVects found in this study were mainly of a physical nature (i.e., aerosol transport through the derived, no significant variations in Irel were found as a function of the ionic line. spray chamber, rather than analyte excitation deterioration, was the reason for the acid interference indicated in Fig. 7). From Fig. 7, it can be seen that, as expected, the higher the acid concentration, the lower the Irel values. As was anticipated by the Wtot measurements, for a given acid, the greatest Conclusions depressive eVect (i.e., lowest Irel value) was found with both acids for the Scott-type spray chamber, whereas a lower signal Cyclonic spray chambers manufactured from glass and PP generate coarser tertiary aerosols and higher solution transport reduction was produced for the glass cyclonic spray chamber.For a given spray chamber, sulfuric acid [Fig. 7(b)] provoked rates than the double-pass and PTFE spray chambers. As a consequence, higher ICP-AES emission intensities and lower the strongest matrix depressive eVect. From the results discussed above, it seems that there is a LOD are obtained with the former two spray chambers. Cyclonic spray chambers aVord a better short-term stability correlation between the aerosol drop size and the matrix eVect caused by acids; the finer the tertiary aerosols, the lower the of the signal (in relative terms) than the Scott-type spray chamber.The wash-out times for pure water solutions are, in Irel values (i.e., the more severe the acid eVects). This observation agrees with others in which the acid eVect was studied general, lower for the cyclonic spray chambers. The chamber material can influence the spray chamber for diVerent tertiary aerosol fractions.35 Deterioration of the plasma thermal characteristics has also performance, mainly in terms of memory eVects when elements that can be adsorbed on the walls (i.e., Pd) are present. In been suggested as a reason to explain the acid eVect, mainly if the plasma is working under so-called non-robust con- this instance, polymeric materials seem to be less appropriate than glass.ditions.30,31,40,57 These latter conditions prevail at low rf power levels and high gas flow rates.In this study, no appreciable Cyclonic spray chambers are less sensitive to acid eVects than double-pass spray chambers. Among the cyclonic spray variations in the Mg II-to-Mg I emission intensity ratio with the acid concentration were observed. The robustness of the chambers, that made from glass oVers the lowest matrix eVect and that made from PTFE the greatest matrix eVect. This plasma was also confirmed by the small variation in the Irel values versus the ionic line energy sum (Table 2 and Fig. 7),58 fact, previously observed for other ionic matrices (i.e., Na),59 seems to be related to the proportion of fine droplets present i.e., under the experimental conditions employed in this work, the acid matrix eVect might be mainly attributed to a decrease in the tertiary aerosol. The position of the nebulizer inside the chamber has a in the amount of analyte reaching the atomization cell.Another fact that supports this reasoning is that, for the cyclonic spray noticeable eVect on the performance of cyclonic spray cham- 66 J. Anal. At. Spectrom., 1999, 14, 61–6726 R. L. Lawrence and F. E. Lichte, Anal. Chem., 1982, 54, 638. bers. This is a parameter that must be optimized in order to 27 M. Marichy, M. Mermet and J. M. Mermet, Spectrochim. Acta, obtain the best performance in terms of ICP-AES sensitivity. Part B, 1990, 45, 1195. The nebulizer plume shape and spray chamber geometries 28 S.Greenfield, H. Mc. D.McGeachin and P. B. Smith, Anal. Chim. need to be matched. The matrix eVects should also be Acta, 1976, 84, 67. dependent on the nebulizer position, as it determines the 29 J. Farino, J. R. Miller, D. D. Smith and R. F. Browner, Anal. Chem., 1987, 59, 2303. characteristics of the tertiary aerosols. 30 E. Yosimura, H. Suzuki, S. Yamazaki and S. Toda, Analyst, 1990, 115, 167. The authors thank Mr. Alan Eastgate (Glass Expansion 31 A.Ferna�ndez, M. Murillo, V. Carrio�n and J. M. Mermet, J. Anal. Europe, Switzerland) for the loan of the nebulizer and spray At. Spectrom., 1994, 9, 217. chambers and for the suggestions given during the preparation 32 A. Canals, V. Hernandis, J. L. Todolý� and R. F. Browner, of the manuscript. Also, the authors acknowledge the DGICyT Spectrochim. Acta, Part B, 1995, 50, 305. 33 I. B. Brenner, J. M. Mermet, I. Segal and G. L. Long, Spectrochim. (Spain) for the financial support of this work (Project Acta, Part B, 1995, 50, 323.PB95–0693). 34 H. Ishii and K. Satoh, Talanta, 1983, 30, 111. 35 J. L. Todolý�, J. M. Mermet, A. Canals and V. Hernandis, J. Anal. At. Spectrom., 1998, 13, 55. References 36 M. Carre�, K. Lebas, M. Marichy, M. Mermet, E. Poussel and J. M. Mermet, Spectrochim. Acta, Part B, 1995, 50, 271. 1 R. F. Browner and A. V. Boorn, Anal. Chem., 1984, 56, 786A. 37 J. M. Mermet, J. Anal. At. Spectrom., 1998, 13, 419. 2 Sample Introduction in Atomic Spectroscopy, ed.J. Sneddon, 38 R. L. Dalhquist and J. W. Knoll, Appl. Spectrosc., 1978, 32, 1. Elsevier, New York, 1990, p. 1. 39 I. I. Stewart and J. W. Olesik, J. Anal. At. Spectrom., 1998, 13, 843. 3 A. Gustavsson, Spectrochim. Acta, Part B, 1984, 39, 85. 40 F. J. M. J. Maessen, J. Balke and J. L. M. de Boer, Spectrochim. 4 B. L. Sharp, J. Anal. At. Spectrom., 1988, 3, 939. Acta, Part B, 1982, 37, 517. 5 A. Canals, V. Hernandis and R. F. 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ISSN:0267-9477    

DOI:10.1039/a806550k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Determination of tellurium in indium antimonide by slurry sampling electrothermal atomic absorption spectrometry M. Y. Shiue, Y. C. Chan, J. Mierzwa and M. H. Yang* Department of Nuclear Science, National Tsing-Hua University, 30043 Hsinchu, Taiwan Received 17th August 1998, Accepted 2nd November 1998 A method for the determination of tellurium dopant concentration in indium antimonide (InSb) by Zeeman eVect electrothermal atomic absorption spectrometry using the slurry sampling technique was developed.The eVects of chemical modifier type and mass on the absorbance-peak characteristics of tellurium in InSb slurried samples were studied. The atomization behavior of tellurium in InSb slurries could be greatly improved by the use of palladium nitrate as a chemical modifier. The detection limit of the optimized procedure was 0.4 mg g-1. In the determination of tellurium at the concentration level of 16 mg g-1, a relative standard deviation of 7% was obtained.Good agreement of the results obtained by the slurry sampling technique with those obtained by solution electrothermal atomic absorption spectrometry and inductively coupled plasma mass spectrometry was found. dissolution. Unfortunately, a significant decrease in sensitivity 1. Introduction was obtained owing to the interference caused by the parent The III–V semiconductor compounds, e.g., gallium arsenide matrix. Moreover, this method also has the disadvantage of a (GaAs), indium phosphide (InP) and indium antimonide higher acid concentration of sample solution contacting the (InSb), are very important materials in the microelectronics graphite tubes, thus decreasing their lifetime.The third techand optoelectronics industries.1 Indium antimonide has nique is based on a prior preconcentration and/or separation become the material of choice in the fabrication of photocond- procedure.5,12,20 A typical example based on reductive copreuctors, magnetoresistors and infrared detectors since it has the cipitation with palladium using ascorbic acid for the determihighest electron mobility and maximum drift velocity.2 It is nation of trace amounts of tellurium in nickel-based known that the performance of the final devices is markedly superalloys and several other metals was reported.5 A detection influenced by the presence of defects on the substrates.3 limit as low as 0.01 mg g-1 was found to be achievable.Another Tellurium-doped InSb single crystals are grown by the possibility based on in situ preconcentration via volatilization Czochralski method, by adding elemental Te to the undoped of tellurium hydride compounds from the sample matrix polycrystalline InSb in the starting charge.The concentration followed by trapping and atomization in a graphite atomizer, of Te through the ingot depends on its concentration in the coated with silver or palladium, has also been reported.21,22 liquid phase in front of the growing crystal face.This concen- The sensitivity of determination reported in terms of tration is not constant if the distribution coeYcient of Te in characteristic mass was about 20 pg.21,22 the crystal is not unity. Generally, only the chemical analysis The purpose of this study was to develop a relatively simple of the wafers is conclusive in determining the distribution and rapid method for the determination of tellurium at mg g-1 coeYcients. concentration levels in tellurium-doped InSb with a slurry- Electrothermal atomic absorption spectrometry (ETAAS) sampling (SS) ETAAS method.The important features of this has long been used for the determination of trace impurities technique are its simplicity even compared with direct solid in Group III–V semiconductors.4 Methods for the determi- sample analysis and that it is less prone to the contamination nation of Te in a wide variety of real matrices including heat- that is frequently encountered in sample dissolution and resisting alloys,5–8 environmental,9,10 biological11–13 and geo- mineralization procedures.23,24 This technique also oVers adequate sensitivity.In this paper, a method of slurry prep- logical samples have been reported.14 The determination of Te aration and direct injection into the electrothermal atomizer in GaAs and InP has also been described.15,16 Although some for the determination of tellurium in InSb without sample fundamental studies on the tellurium atomization eYciency in dissolution is reported.The eVects of palladium nitrate and ETAAS from aqueous solution have been reported,17,18 there palladium nitrate–magnesium nitrate chemical modifiers on appear to have been no studies on the determination of Te the atomization of Te from slurried samples were also investi- in InSb. gated. The quality of the analyte peak shape, precision, Up to now, three main techniques for tellurium accuracy and limit of detection achievable by the proposed determination by ETAAS have generally been used.The first method were evaluated and are discussed. is based on tellurium determination by direct atomization of the analyte from solid samples. Headridge and Nicholson19 analyzed a nickel-base alloy using solid sampling ETAAS. However, this method needs calibration with standard alloys 2. Experimental and it is diYcult to obtain a series of well characterized 2.1. Apparatus standard alloys.The second technique is based on the determination of tellurium by ETAAS after acid dissolution of solid AAS measurements were made on a Perkin-Elmer (Norwalk, samples.6,7,16 Taddia et al.16 determined tellurium in indium CT, USA) Zeeman/5100 PC atomic absorption spectrometer equipped with an HGA-600 graphite furnace atomizer, an phosphide by ETAAS after hydrochloric acid and nitric acid J. Anal. At. Spectrom., 1999, 14, 69–74 69Table 1 Experimental conditions for the determination of tellurium 2.4.ETAAS procedure by slurry sampling ETAAS A 20mL aliquot of the slurry sample solution followed by Temperature/ Ramp Hold Argon flow 20 mL of chemical modifier solution was injected into the Step °C time/s time/s rate/mL min-1 furnace. Two modifiers including Pd and Pd–Mg (with a mass ratio of 251) were used. The thermal cycle in Table 1 was Temperature program— applied and the peak absorbance was read. The method of Drying 120 10 40 300 standard additions based on spiking the slurries with an Pyrolysis 1100a 10 30 300 aqueous standard solution was used to determine the sample Cooling 20 1 15 300 Atomization 2200 or 2600 0 5 0 concentration.Clean out 2650 1 5 300 Sample volume 20 mL 3. Results and discussion Instrumental parameters— Wavelength 214.3 nm 3.1. Optimization of experimental conditions Slit width 0.2 nm Radiation Te electrodeless discharge lamp Pyrolysis temperature. The optimization of the furnace Read time 5 s pyrolysis temperature was carried out for both 5 mg mL-1 Signal mode Peak area InSb slurry and 2 ng of Te standard aqueous samples with USS 100 device power 40% for 25 s and without the presence of a chemical modifier.The results output setting of these experiments are shown in Fig. 1. As can be seen in a800 °C in the absence of Pd. Fig. 1(a) and (d), in the absence of modifier, for both the slurry and aqueous standard volatilization losses occur if the samples are pyrolyzed above 800 and 600 °C, respectively. The AS-60 autosampler and a USS-100 slurry sampler.A tellurium fact that the maximum pyrolysis temperature can be increased electrodeless discharge lamp (EDL) and pyrolytic graphite- by 200 °C in the presence of a slurry sample (as compared coated tubes with integrated platforms (Perkin-Elmer, with its absence) may imply that the matrix of slurries, U� berlingen, Germany) were used throughout. The optimized probably indium, can work as a chemical modifier. A similar heating program and instrumental parameters are given in result was reported by Taddia et al.16 in their study of the Table 1.For the preparation of the powdered InSb sample, a determination of Te in the sample solution containing indium Retsch Mixer Mill MM 2000 (F. Kurt Retsch, Germany) trichloride. In the presence of palladium modifier as shown in equipped with tungsten carbide grinding jars and bas Fig. 1(b) and (e), tellurium is thermally stabilized without was used.Micro-weighing was performed on a Mettler significant losses up to about 1100 and 1300 °C, respectively. (Hightstown, NJ, USA) AT 201 electronic microbalance. Moreover, in the presence of Pd–Mg modifier (both as nitrates), as shown in Fig. 1(c), tellurium is also stable up to 2.2. Reagents and sample about 1100 °C in a slurry sample. The results presented above show that in the presence of All reagents were of the analytical reagent grade, unless stated either Pd or Pd–Mg as a chemical modifier a pyrolysis tempera- otherwise.High-purity water, which was purified by deminture of 1100 °C is equally applicable without significant loss eralization and two-stage quartz distillation, was used throughof Te and the sensitivities are almost the same in both cases. out. Nitric acid and hydrochloric acid were prepared by sub- However, the background absorbance in the presence of boiling distillation in quartz stills.A stock standard solution Pd–Mg modifier is slightly higher than that in the presence of of Te (1000 mg L-1) from Aldrich (Milwaukee, WI, USA) Pd modifier. On the basis of the above results, palladium as a was diluted to the desired concentrations with high-purity chemical modifier and a pyrolysis temperature of 1100 °C were water containing 0.2% nitric acid. Palladium (10 000 mg L-1, employed throughout the following study. as nitrate) and magnesium nitrate (10 000 mg L-1) solutions were purchased from Inorganic Ventures (Lakewood, NJ, Atomization temperature.The influence of atomization USA). Triton X-100 was obtained from Merck (Darmstadt, temperature, as shown in Fig. 2, on the tellurium signal from Germany). Tellurium-doped indium antimonide slices were a 5 mg mL-1 indium antimonide slurry was compared with supplied by the MCP Wafer Technology (UK). 2.3. Slurry preparation procedure A slice of InSb was washed with acetone to remove traces of grease, dipped in 1 M hydrochloric acid for a few minutes and then rinsed with high-purity water and air-dried in a class 100 clean bench.For the preparation of slurries, approximately 1 g of InSb slice was ground in Retsch Mixer Mill MM2000 at a 50% power output setting for 20 min. The particle diameter of the InSb powder so obtained did not exceed 3 mm as examined on several scanning electron microscope micrographs. A portion of sample (from 2.5 to 40 mg) was weighed into a 2.5 mL polyethylene vial and 2 mL of 0.2% HNO3 containing 0.005% of Triton X-100 surfactant were added. Shortly before analysis, the suspensions were pre-treated for 2 min in an ultrasonic bath to disintegrate larger particle agglomerates.Fig. 1 Pyrolysis curves of tellurium obtained for (a) slurry sample The vials containing the slurry were then directly transferred without Pd modifier, (b) slurry sample with addition of 200 mg of Pd, into the autosampler tray. Prior to taking each slurry aliquot (c) slurry sample with addition of 200 mg of Pd+100 mg ofMg(NO3)2, by the sampling capillary, it was homogenized by ultrasonic (d) aqueous tellurium standard (2 ng) without Pd modifier and agitation using a USS-100 slurry sampler at a 40% power (e) aqueous tellurium standard (2 ng) with addition of 20 mg of Pd modifier.Atomization temperature, 2200 °C. output setting for 25 s. 70 J. Anal. At. Spectrom., 1999, 14, 69–74palladium is an eVective chemical modifier for the ETAAS determination of tellurium in slurried InSb.EVect of amount of palladium. A few papers have reported that some of the analyte signal is often reduced by the presence of larger masses of Pd modifier.26,27 Qiao and Jackson28 suggested a physical mechanism of the eVect of Pd on analyte modification. Thus, if the mass of Pd is increased too much, larger droplets of Pd are formed and the analyte diVuses more slowly out of these larger droplets and consequently result in lower peak heights and greater signal tailing, i.e., the analyte signal is reduced.Moreover, Frech et al.27 reported that there Fig. 2 Atomization curves of tellurium obtained (a) for slurry sample existed not only the eVects of analyte release from the palwith addition of 200 mg of Pd and (b) aqueous tellurium standard ladium modifier, but also the eVects of analyte adsorption and (2 ng) with addition of 20 mg of Pd modifier. Pyrolysis temperature, desorption at the cooler ends of the tube in the presence of 1100 °C.increasing modifier mass. In this study, a similar eVect of Pd modifier on Te was also observed. A detailed study using electrothermal vaporization inductively coupled plasma that from a 2 ng of Te aqueous solution in the presence of mass spectrometric (ETV-ICPMS), laser ablation-ICP-MS palladium modifier. It can be seen clearly that no plateau appears with either atomization curve. The low atomization eYciency of Te at lower atomization temperatures resulted in broader tellurium peaks and a lower integrated absorbance.At higher temperatures, owing to the sudden atomization of Te, the diVusion losses were significantly higher, hence a lower integrated absorbance and narrower peak profile were observed. For the curve obtained with aqueous standard solution, the maximum sensitivity was obtained at an atomization temperature of around 2000 °C, but, a relatively broader peak was also observed.In order to obtain adequate sensitivity and a better profile, an optimum atomization temperature of 2200 °C was selected for both slurry and aqueous solution. 3.2. EVect of palladium modifier on tellurium absorbance The tellurium atomization profiles with and without palladium chemical modifier were investigated. The results indicate that when tellurium is atomized from an InSb slurry without the addition of palladium modifier, the background absorbance is too high (&2) and seriously overlaps the Te signal.The irregular shape of the Te peak shown could be attributed to the extremely high background absorbance that is beyond the capacity of the Zeeman-eVect background corrector. With the addition of palladium modifier, the background absorbance signal suddenly decreases from &2 to around 1.1 and shifts to a later appearance time, leading to only partial overlap with the Te signal. Therefore, a better tellurium peak shape is obtained.The pronounced eVect of palladium modifier on tellurium absorbance mentioned above may be explained on the basis of co-expulsion of the analyte element with the volatilized matrix.25 As described previously, in the absence of chemical modifier only a relatively lower pyrolysis temperature, i.e., 800 °C, can be used in order to prevent volatilization loss of Te. This may consequently result in the retention of a substantial amount of InSb matrix in the graphite tube at this lower temperature.In the subsequent atomization stage Te is expected to be carried into the absorption volume by co-explusion with the rapidly expanding vapors of the InSb matrix that remained in the graphite tube. The large background absorbance and the irregular shape of the tellurium peak as previously described can be attributed to a gas-phase interference caused by the matrix substance. On the other hand, in the presence of palladium modifier a higher pyrolysis temperature (1100 °C) can be used to expel the sample matrix more eYciently and can therefore result in a less significant Fig. 3 Absorption signals of tellurium in 5 mg mL-1 InSb slurries influence of the matrix on the atomization stage. The lower with diVerent additions of Pd modifier: (a) 100 mg, atomic absorption background absorbance and better peak shape of tellurium (AA)=0.119, background (BG)=1.37; (b) 200 mg, AA=0.239, BG= obtained in the presence of palladium modifier can be 1.10; (c) 300 mg, AA=0.122, BG=0.78; and (d) 400 mg, AA=0.129, BG=0.73.explained on this basis. It can therefore be concluded that J. Anal. At. Spectrom., 1999, 14, 69–74 71Table 2 Tellurium partitioning in 5 mg mL-1 InSb slurry Concentration of Concentration of liquid fraction/ HNO3 (% v/v) concentration of slurry (%) 0.2 0 1 0 5 0 20 50 Table 2 gives the Te partitioning data for four diVerent concentrations of HNO3, 0.2, 1, 5 and 20% v/v. The results show that the amount of Te present in the liquid phase was below 1% (close to 0%) when the liquid phase contained 0.2, 1 and Fig. 4 EVect of amount of palladium on the absorbance of tellurium 5% HNO3. However, the amount of Te present in the liquid obtained for (a) 0, (b) 5, (c) 10 and (d) 20 mg mL-1 InSb slurries. phase increased to 50% at 20% HNO3. These results indicate that Te in the InSb slurry is not easily extracted into the liquid phase by dilute HNO3. However, as the concentration of (LA-ICP-MS) and scanning electron microscopic (SEM) techniques to investigate the mechanism of tellurium atomization HNO3 increases to 20%, some chemical reactions may occur during the ultrasonic mixing.This may be clearly indicated by in the presence of palladium nitrate modifier in ETAAS is currently in progress. the color change of the slurry sample from black to gray–white and finally to a white precipitate, most probably in the form The eVect of palladium modifier mass on the tellurium signal was first investigated.Fig. 3 shows the eVect of various of Sb2O3. This shows that InSb can be partially dissolved by 20% HNO3 and about half of the Te goes into a readily soluble palladium masses on the Te absorption profiles for a 5 mgmL-1 InSb slurried sample with the furnace heating form. The total dissolution of the sample is achieved in concentrated nitric acid. program in Table 1. Fig. 3(a)–(d) represent the absorbance profiles for Te in the presence of 100, 200, 300 and 400 mg of Replicate aliquot precisions in the 3–4% range were obtained for 5 mg mL-1 InSb slurries prepared in 0.2, 1, 5 and 20% v/v palladium, respectively.Comparison of these figures indicates a trend of decreasing peak height and increasing signal tailing HNO3. The good precision of measurements obtained for both 0 and 50% of Te extracted into the liquid phase indicates that with increasing addition of Pd to the samples. However, when the amount of Pd added is too small, as in Fig. 3(a), a higher the slurry is very homogeneous if the proposed method of slurry preparation is used. background absorbance is observed, presumably due to co-expulsion of analyte with the volatile matrix as described previously. In the present study, the maximum sensitivity was 3.4. Standardization and sample analysis achieved for this specific sample (5 mg mL-1 InSb) with In general, the use of solid standards with certified addition of 200 mg of Pd as modifier.concentrations of the elements of interest and matrices corre- In order to find a general guideline to be followed for the sponding to those of the samples is the most accurate stan- optimum amount of Pd to be added to an InSb slurried sample dardization method in solid sampling ETAAS, including the of specific concentration, a series of experiments were carried slurry sampling technique.31 Unfortunately, such standard out as follows. To slurry samples containing 0, 5, 10 and materials are rare and are not available for all matrices.The 20 mg mL-1 InSb, various additions of Pd modifier were made next choice, considering the accuracy achievable, is calibration and the eVect on the absorbance was investigated. The results using a calibration graph based on aqueous standard solutions. obtained are shown in Fig. 4. As can be seen, in the absence However, in this standardization technique, a similar behavior of InSb, i.e., the Te standard solution, the maximum of the analyte element in the standard and in the slurry during absorbance was not appreciably changed in the presence of the pyrolysis and atomization stages is prerequisite for good Pd amounts in the range 10–80 mg, whereas for the InSb accuracy.32 slurried samples, a certain range of maximum absorbance was In this study, the pyrolysis was studied for a 5 mg mL-1 obtained upon specific addition of palladium modifier.These InSb slurry and an aqueous Te standard using 200 and 20 mg results clearly indicate that the choice of the optimum mass of palladium modifier, respectively.The results shown in of palladium modifier is dependent on the slurry concentration. Fig. 1(b) and (e) clearly indicate a similar trend between these From the practical point of view, it means that a higher mass two pyrolysis curves. This may serve as an evidence for a of palladium modifier must be used to obtain the best sensisimilar behavior of Te atoms in both the aqueous standard tivity when a more concentrated InSb slurry is to be analysed.and InSb slurry during the pyrolysis step. The absorption signals of tellurium in the slurried samples, spiked slurry and 3.3. Tellurium partitioning in slurries aqueous solution were further tested and the results are shown in Fig. 5(a)–(c). As can be seen, the atomization behavior of The distribution of analyte in the slurry is of particular interest in the characterization of the precision of the slurry sampling Te is also very similar in all three cases, despite the appearance of peak maxima that slightly deviate from each other.approach. When no analyte is found in the liquid phase, the limiting source of measurement variability from the replicate Furthermore, the characteristic masses of tellurium for the aqueous solution and indium antimonide slurry using a pal- aliquots of a single slurry will be related to slurry mixing and the heterogeneity of the analyte in the insoluble solid fraction.ladium modifier were calculated and the results show that as little as 19 and 34 pg, respectively, can be achieved. The slopes When large percentages of analyte are extracted into the liquid phase, replicate aliquot precision may approach those of pure of the calibration curve method and standard addition method are 0.004 and 0.0026, respectively, indicating a significant liquid digests.29,30 To test Te partitioning, slurries were agitated and then left undisturbed for 90 min to ensure that settling suppression of the tellurium signal due to the presence of InSb matrix.Hence the method of standard additions based on would occur. After this time interval, the top portion of the slurry was carefully sampled and the concentration of the spiking the slurries with aqueous standard solutions is needed for quantification purposes. dissolved Te was determined by ETAAS. The slurry was re-suspended and the Te concentration was determined again.The applicability of the method was tested for the analysis 72 J. Anal. At. Spectrom., 1999, 14, 69–74Fig. 6 Dependence of Te integrated absorbance on InSb slurry concentration (mg mL-1). method, measurements of Te absorbance with respect to diVerent slurry concentrations were conducted, and for each slurry concentration the amount of palladium modifier was adjusted accordingly. As is evident from Fig. 6, for InSb slurry concentrations up to 20 mg mL-1, there is a linear relationship between the slurry concentration and absorbance.It was observed for the tested sample that the contribution of the sample inhomogeneity starts to be significant at InSb slurry concentrations below about 1.2 mg mL-1, indicating that slurry concentrations lower than this level could not be recommended with this analytical technique. 3.5. Method detection limit The method detection limit is defined as the analyte concentration that gives a signal which is three times the standard deviation of the procedure blank (n=7).The method detection limits were found to be 0.4 and 0.8 mg g-1 for slurry ETAAS and solution ETAAS, respectively. In this study, the evaluation of the blank and limits of detection were based on the analysis of a 5 mg mL-1 undoped InSb. As can be seen, Fig. 5 Absorption signals of tellurium for (a) 5 mg mL-1 InSb slurry the limit of detection obtained by SS-ETAAS is two-fold better (with about 1.74 ng of Te) with the addition of 200 mg of Pd, than that achievable by solution ETAAS.The detection limit (b) 5 mg mL-1 InSb slurry spiked with 3.75 ng of Te with the addition obtained by SS-ETAAS is good enough to determine typical of 200 mg of Pd and (c) aqueous tellurium standard (2 ng) with the addition of 20 mg of Pd. dopant concentrations of tellurium in InSb. of a tellurium-doped InSb and the results are presented in 4. Conclusion Table 3. Since no standard samples with known reference A method for the direct determination of tellurium in InSb by values are available, the evaluation of the reliability of the SS-ETAAS was developed.Palladium nitrate was used as an data can alternatively be conducted by using diVerent analyte Vective chemical modifier to overcome otherwise strong ical methods. In Table 3 the results of two independent matrix interferences. The amount of modifier must be carefully methods, i.e., solution ETAAS and ICP-MS, are also preoptimized and calibration must be performed with the standard sented.As in slurry ETAAS, a significant suppression of the additions method. The good analytical reliability achievable tellurium signal due to the presence of the InSb matrix was with this technique was confirmed by comparison with solution also observed in solution ETAAS and therefore the method ETAAS and ICP-MS. The detection limit is suYcient to of standard additions was used for quantification purposes. determine typical dopant concentrations of tellurium in InSb.As can be seen, the concentrations of Te in Te-doped InSb The proposed method has been proved to be relatively simple determined by the present method, solution ETAAS and and rapid. Another advantage is the possibility of avoiding ICP-MS are 16.2, 16.4 and 17.0 mg g-1, respectively. From the sample decomposition, which reduces time-consuming good agreement with results obtained by solution ETAAS and sample preparation procedures and the use of hazardous ICP-MS and its good reproducibility (RSD 7.5%), the concentrated acids.analytical reliability of the established method is confirmed. To establish the linear working range for the proposed Acknowledgement Table 3 Comparison of the results of tellurium determination by The authors gratefully acknowledge the financial support of SS-ETAAS, solution ETAAS (Sol-ETAAS) and solution ICP-MS the National Science Council of Taiwan. Method Te concentration/mg g-1 RSD (%) References SS-ETAAS 16.2±1.2 (n=5) 7 Sol-ETAASa 16.4±0.7 (n=4) 4 1 S.M. Sze, Semiconductor Devices Physics and Technology, Wiley, New York, 1985. ICP-MSa 17.0±0.8 (n=3) 5 2 O. Sugiura and M. Matsumura, J. Appl. Phys., 1985, 24, 25. aDetermination after acid dissolution of the sample. 3 K. Tada, M. Tatsumi, M. Morioka, T. Araki and T. Kawase, in J. Anal. At. 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ISSN:0267-9477    

DOI:10.1039/a806462h

出版商:RSC    

年代:1999

数据来源: RSC