摘要:

REVIEW Atomic absorption spectrometry as an alternate technique for iodine determination (1968–1998) Pilar Bermejo-Barrera,* Manuel Aboal-Somoza and Adela Bermejo-Barrera University of Santiago de Compostela, Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Chemistry, Avda. de las Ciencias s/n, E-15706—Santiago de Compostela (La Corun�a), Spain Received 9th November 1998, Accepted 12th May 1999 1 Introduction cases, the substance that is directly determined is a metal.The success of an indirect procedure greatly depends on whether 2 Discussion 2.1 Iodine species and optical details the selectivity of the reaction is maintained through (and ‘in spite of ’) the chemical reaction (or reactions) carried out 2.2 Atomization and procedure 2.2.1 Solvent extraction methods previously to performing the atomic absorption measurement. 2,3 Since there are many factors that can aVect the whole 2.2.2 Precipitation methods 2.2.3 Complexation methods procedure, indirect methods are often laborious, diYcult to automatize and rarely used in routine analysis.3 On the con- 2.3 Analytical figures of merit 2.4 Interferences and samples trary, in a direct method the analyte is atomized and the absorption of radiation by its atoms is recorded to determine 3 Conclusions the analyte concentration.Thus, a direct method is usually much more simple (and much less time-consuming) as well as 1 Introduction easier to automatize than an indirect one.The determination of non-metals, such as halogens, sulfur, Iodine main resonance lines are 178.3, 183.0, 184.4, 187.6 oxygen, nitrogen, phosphorus and carbon by AAS involves and 206.2 nm, all of them (except 206.2 nm) lying in the certain diYculties. Firstly, given the high energy diVerence vacuum-UV spectral region, the most intense one being between the lowest excited state and the ground state for non- 183.0 nm. This line corresponds to the electronic transition metals, short wavelength radiation is needed for the electronic 4P5/2 � 2P3/2, between the electronic states 6s and 5p, respecttransitions to occur.Thus, the analytical resonance lines of ively. As iodine main resonance lines lie in the vacuum-UV non-metals lie in the shorter wavelength regions of the electro- region of the electromagnetic spectrum, it is not usually magnetic spectrum, in the so-called vacuum-UV spectral region determined by AAS, but by other instrumental techniques.(which ranges from 10 to about 180–190 nm), as shown in Several methods involve UV-VIS spectrophotometric measure- Table 1.1–4 Unfortunately, there is a lack of commercial radi- ments: iodine species contained in the sample can be converted ation sources that provide these lines,1 especially for elements into molecular iodine, this is extracted into an organic solvent of high volatility (such as iodine), for which it is diYcult to (such as HCCl3 or CCl4) and the absorption of the pink ensure the manufacture of reproducible and stable lamps.3,5 extract measured; molecular iodine can also be complexed Moreover, the selection of the analytical line in the spectro- with starch, and the deep blue solution which results can be photometer represents another problem, because commercial analyzed for its absorption; if excess iodide is added to a atomic absorption spectrophotometers are usually equipped solution containing molecular iodine, triiodide ion results with monochromators that cover from near-UV to near-IR (I2+I- � I3-), which absorbs strongly in the UV spectral regions, that is, from about 190 to 850 nm.Therefore, the region; the catalytic eVect of iodide on the reaction: 2 vacuum-UV region lies outside the ‘commercially attainable’ Ce(IV)+As(III)�2 Ce(III)+As(V) is well-known [since Ce(III), range of wavelengths. As(III ) and As(V) are colorless cations, the decrease in the Finally, the absorption of analytical radiation3,6–8 by the absorption of the solution containing the orange ion Ce(IV) atmosphere produces a high background absorption that can can be applied to determine the iodide in the solution]; and make it diYcult to attain useful results.5 This atmospheric finally, another example is the extraction into nitrobenzene of absorption depends on the wavelength and is due to atmos- the red ion pair [Fe(1,10-phenanthroline)3]I2 formed between pheric oxygen or to the oxygen contained in flames6 (as oxygen the chelate cation [Fe(1,10-phenanthroline)3]2+ and iodide.exhibits a strong absorption at wavelengths below 200 nm9) The absorption exhibited by the extract can easily be related rather than to atmospheric nitrogen (in spite of what McGregor and co-workers state1) which can be used as a purge gas in iodine determination.5–7,10–12 Table 1 Main resonance lines of some non-metals These problems make the determination of non-metals Element Wavelength/nm virtually impossible by conventional AAS.However, in the late 1960s, procedures aimed at overcoming those diYculties C 156.1, 165.7 began to be reported, and nowadays are known as indirect N 120.0, 174.3 methods. Basically, an indirect method is based on one or P 177.5, 178.3, 178.8 several chemical reactions between the analyte and another O 130.2, 130.5 S 180.7, 182.4, 182.6 substance (or substances), one of which can be directly deter- F 95.2, 95.5 mined by AAS.Thus, if the stoichiometry of the reaction or Cl 134.7, 138.0, 139.0 reactions that take place between the analyte and the other Br 145.0, 148.9, 157.6 substance (or substances) is known, by determining the latter I 178.3, 183.0, 184.5, 187.6 (or one of them), the analyte can be determined also. In most J. Anal. At. Spectrom., 1999, 14, 1009–1018 1009to the iodide present. Other useful techniques are liquid about background correction: only twice7,11 is a deuterium lamp or the two-line method used in direct methods; and only chromatography that can be applied, for example, to determine iodide in milk, as proposed by the AOAC;13 and ICP-AES, four times is the use of deuterium lamps for background correction in indirect methods21–24 reported.which gives good results by direct nebulization of the solution that contains the iodized species. There are more instrumental techniques that can be used for iodine determination as well. 2.2 Atomization and procedure In the present bibliographical survey, the methods proposed Atomization is carried out by means of flames in most of the from January 1968 to February 1999 for iodine determination methods, both direct and indirect, but flame composition is by AAS are described and compared, where iodine is present rarely discussed.In addition, atomizers such as platinum in samples or standards as any iodized species: iodide, iodate, loops9 or cathode sputtering cells12 are used at times.periodate or molecular iodine. The original published papers In spite of the risk of radiation absorption by oxygen, air- were taken into consideration in most cases, but for other containing flames are not significantly less used than other references ( less than 30%) this was impossible (mainly due to flames. To avoid such absorption, the flame is isolated by language diYculties, as they were written in Chinese, Japanese means of a nitrogen5,6,10 or argon9 shield, or is diluted with or Russian). In these latter cases, the corresponding abstracts those non-absorbing gases.19 (printed or in CD format) in Analytical Abstracts and in It is interesting to describe briefly the experimental pro- Chemical Abstracts were the main sources of information.cedures involved, especially in indirect methods. Considering Further details not described in those abstracts were obtained the reactions or processes carried out, three groups of methods by cross references among the diVerent articles published.The can be defined: solvent extraction methods, precipitation reviews about the determination of non-metals by AAS pubmethods and complexation methods. lished, chronologically, by Pinta,14 Kirkbright and Johnson,2 Gol’dshtein and Yudelevich,15 Garcý�a-Vargas and co-workers3 2.2.1 Solvent extraction methods. Almost 50% of the pub- and, more recently,or et al.,1 are recommended to lished works involve solvent extraction of a complex.However, interested readers. Among them, the review by Garcý�a-Vargas rather few authors17,24–27 perform careful studies on extraction and co-workers3 must be highlighted, not only for its amplitude conditions. IBMK is the organic solvent most used, and the and the wide variety of topics covered but also for the great organic layer is the one usually subjected to AAS (though number of references included. some stripped-back aqueous phases are analyzed17,26). The most important details of the published procedures for iodine analysis by AAS are contained in Tables 2 (direct methods) and 3 (indirect methods). We have calculated some 2.2.2 Precipitation methods.These methods are generally of the data in those tables from the information contained in designed to determine iodide after its precipitation as silver the original papers or in their abstracts, and some cells in iodide, where silver is determined in the supernatant21,28–30 or tables 2 and 3 are empty because the information they should in the redissolved precipitate.31–34 The application of FIA contain is not explicit in the papers or in the abstracts.In the manifolds in those methods can be highlighted, unlike in following paragraphs, some points observed in tables 2 and 3 solvent extraction and in complexation methods. are highlighted due to their interest. 2.2.3 Complexation methods. Here, complexes are prepared 2 Discussion without further solvent extraction.In all these methods, iodine is determined as iodide by measuring the atomic absorption 2.1 Iodine species and optical details of mercury (usually by the cold vapour atomization technique). Iodide is the species most usually determined, both in direct These are simplier methods than methods 2.2.1 and 2.2.2, and indirect methods, though iodate6,7,16,17 and periodate,6 as since they only involve the adding of an aliquout of a Hg(II ) well as a number of iodized organic compounds (iodoform, solution to the sample solution, followed by the measurement alkyliodides)6 have been determined at times.However, of the atomic absorption of the mercury linked to according to published works,6,7 the same results are obtained iodide22,23,35,36 or of the unreacted one.37–40 Finally, Wifladt with diVerent iodine species. In indirect methods, mercury is and Lund40 and Chuchalina et al.35 provide the reader with the element most commonly chosen to combine with the iodine interesting discussions about the performance of cold vapour species (which is probably owed to its easy reaction with atomization.halogen species). For iodine, the lack of commercial lamps made the authors 2.3 Analytical figures of merit design and build a lamp in their laboratories, as they report.5,8,18,19 In addition, the use of home-made lamps is To compare the methods studied in this review on the basis of their analytical figures of merit is a very diYcult task, supposed in other cases.6,7,9,10–12,20 These lamps were EDLs, except for two research groups that designed CDLs.8,19 because not all authors study the same parameters and also because the meaning of some of those parameters has varied 183.0 nm was the iodine line generally selected, giving better sensitivities than other lines (Table 2).Moreover, to obtain from 1968 up to now. This is especially evident in the cases of the limit of detection (LOD), characteristic mass (m0), successful results using this line, the absorption of radiation by atmospheric oxygen was a serious problem to solve.Thus, characteristic concentration (c0) and sensitivity. As an attempt to overcome this problem, the current IUPAC recommended in order to keep an oxygen-free area around the radiation beam, the purging of the monochromator19 or of both the definitions of m0,41 c042 and sensitivity,43 have been applied in this review by calculating their values from the original pub- monochromator and the whole optical path with an inert, non-absorbing gas (such as argon or nitrogen) was the solution lished data when possible and necessary and when old nomenclature had been used in the original papers.usually adopted. This purging involved the design and implementation in the spectrophotometer cavity of a silica In direct methods, as is theoretically expected, electrothermal atomization is usually more sensitive than other atomization tubular system (equipped with quartz windows) filled with the gas.The alternative proposal by Klein and Heithmar,19 who procedures (flame, platinum loop, cathode sputtering cell ). Within flames, N2O–C2H2 often give better results than other kept the unpurged optical path to its minimum length, should also be pointed out. mixtures. Moreover, 206.2 nm produces poorer sensitivities than 183.0 nm, whereas the use of argon or nitrogen as Finally, rather scarce information is given in the literature 1010 J.Anal. At. Spectrom., 1999, 14, 1009–1018Table 2 Direct methods for iodine determination by AAS (blank spaces mean no information available about the subject)i Iodine l/nm, Interference Ref. species lamp Modifications Atomizer Atomizer details Analytical figures of merit and other data studies Samples 20a I- 183.0 Ar-purged light path; Graphite Atom.: 1500 °C Sensitivity: 0.12 ng-1 Aq. solns. EDL vacuum monochromator furnace Lin.range: up to 0.3–0.5 absorbance units for I- and other elements analyzed 18 I- 206.2 Graphite Atom.: 2427 °C/2 atm Ar LOD:b ca. 2 ng (0.2 mg ml-1) No Aq. solns. EDL furnace Vol. injected: 10 ml 44a I2 c 206.2 Flame Air–C2H2 flame LOD:d 600 mg ml-1 Aq. solns. Lin. range: 2–40 mg ml-1 I2 c 6 I-, IO3-, 183.0 N2-purged light path and Flame Slightly fuel-rich N2- c0: 12 mg ml-1 I- Yesf Aq. solns. IO4- EDL monochromator separated N2O-C2H2 LOD:b 25 mg ml-1 I- (both IO3- and IO4- gave flame same c0 and LOD than I-) Lin.range: 25–1000 mg ml-1 I-e 10 I- 183.0 N2-purged light path and Flame N2-separated N2O–C2H2 Aq. I- soln. is subjected to solvent extraction with Yes Aq. solns. EDL monochromator flame IBMK. The organic layer is then subjected to flame Cu2+, Fe3+, AAS V5+ c0: 0.32 mg ml-1 in aq. phase before extraction. Anal rec.: 98.3–100% within the lin. range. Lin. range: 1–25 mg ml-1 I- referred to aq. soln. 7 I-, IO3- 178.2, N2-purged light path; 1 m Graphite Atom.: 8.5 V across the 183.0: c0: 0.04 mg ml-1, i.e., 0.4 ng Yes Aq. solns. 183.0 long vacuum furnace furnace Lin. range: up to 6 mg ml-1 NaCl, EDL monochromator 178.2: c0: 0.02 mg ml-1, i.e., 0.2 ng Na2HPO4 Precision of measurements: lower at 178.2 nm, but same LOD,d 2 ng, for both wavelengths. Vol. injected (at both wavelengths): 10 ml Same figures for the two species, I- and IO3- at each wavelength 9 I- 183.0 Ar-purged light path and Platinum Dry: ca. 0.1 A dc c0: 9 mg ml-1 Yes Aq.solns. EDL monochromator loop atom.: ca. 1.9 A dc (some LOD:b 18 mg ml-1 or, 1.8 ng absolute C2O42-, F-, below 1227 °C) Vol. injected: about 0.1 ml Cr3 +, Al3+ RSD: 4% at 100 mg ml-1 level 11 I- 183.0, N2-purged light path and Graphite Dry: 60 s/125 °C 183.0: m0: 1 ng Yes Rat thyroid 206.1 monochromator (double- furnace Ash: 30 s/room temp. LOD:d 4.0 ng, or 0.4 mg ml-1 Br-, PO43-, EDL beam spectrophotometer, (sample: 30 s/400 °C) RSD: 7% at 10 mg ml-1 level Ca2+ with D2 arc background Atom.: 3 s/2100 °C Lin. range: 4–100 ng I- corrector) Samples measured at this wavelength after acid digestion (aqua regia–HNO3) 206.1: m0: 35 ng LOD:d 100 ng RSD: 4% Lin.range: up to 7.5 mg Vol. injected (at both wavelengths): 10 ml J. Anal. At. Spectrom., 1999, 14, 1009–1018 1011Table 2 (Continued) Iodine l/nm, Interference Ref. species lamp Modifications Atomizer Atomizer details Analytical figures of merit and other data studies Samples 5 I2 183.0 N2-purged light path and Flame N2-separated N2O–C2H2 c0: 14 mg ml-1 No Aq.solns. HCL monochromator flame LOD:g 22 mg ml-1 12 I2, I- 183.0 N2-purged light path and Cathode P(Ar) in cell: 4 Torr; m0: 0.06 mg of I- or I2 Yesf Aq. solns. EDL monochromator sputtering 50 mA LOD:d 0.03 mg of I- cell RSD: 4% at 1000 mg ml-of KI level (i.e. 5 mg KI, given the vol. injected) Lin. range: 0–10 mg I- Vol. injected: 5 ml 8 I2 183.0, Ar-purged light path and Flame Ar-shielded and unshi- 183.0/N2O–C2H2 flame: c0: 6 mg ml-1 (unshielded) No Aq.solns. 206.2 monochromator elded N2O–C2H2 and air- LOD:b 9 mg ml-1 (unshielded ) CDL C2H2 flames. Always very c0: 17 mg ml-1 (shielded) fuel-rich flames 183.0/air–C2H2 flame: c0: 37 mg ml-1 (unshielded) c0: 31 mg ml-1 (shielded) 206.2/N2O–C2H2 flame: c0: 380 mg ml-1 (unshielded) 206.2/air-C2H2 flame: c0: 630 mg ml-1 (unshielded) 19 I- 183.0 Monochromator purged Flame Ar–H2- and N2–H2- Aq. I- soln. is treated with oxidant (K2Cr2O7 in H2SO4 Yes Aq.solns. CDL with Ar; unpurged light entrained air flames and medium), and resulting I2 solution is aspirated into SCN-, S2-, path, but reduced to its mini- fuel-rich air–C2H2 flame. the flame. Hg2+ mum length. Fuel-rich air-C2H2 flame: c0: 5 mg ml-1 LOD:h 15 mg ml-1 RSD: 9% at 100 mg ml-1 level Ar–H2-entrained air flame: c0: 0.6 mg ml-1 LOD:h 1.8 mg ml-1 N2–H2-entrained air flame: c0: 0.8 mg ml-1 LOD:h 2.4 mg ml-1 aReferences handled as abstracts published in the journals Analytical Abstracts and/or Chemical Abstracts. bLOD defined on the basis of twice the standard deviation of the measurements of the blank.cSupposed. dLOD not defined, and referred to as ‘practical LOD’ in ref. 7. eMore data are given corresponding to the determination of other substances containing iodine, namely HCI3 and alkyl iodides, which are determined in EtOH and IBMK solutions after solvent extraction procedures. fNo information provided about the main interferent agents.gNo definition given of the LOD, but since the values obtained are compared with those of Ref. 6, by the same authors, the LOD in this Ref. 5 is assumed to be defined in the same way. hLOD defined on the basis of 3 times the standard deviation of the measurements of the blank. iAbbreviations used: anal. rec.: analytical recovery; aq.: aqueous; atom.: atomization; CDL: capillary discharge lamp; CRM: certified reference material; c0: characteristic concentration; m0: characteristic mass; EDL: electrodeless discharge lamp; HCL: hollow cathode lamp; IBMK: isobutyl methyl ketone; IR: infrared; LOD: limit of detection; LOQ: limit of quantitation; lin. range.: linear range (of concentrations); org.: organic; ref.: reference; RSD: relative standard deviation; soln./solns.: solution/solutions; temp.: temperature; and vol.: volume. 1012 J. Anal. At. Spectrom., 1999, 14, 1009–1018Table 3 Indirect methods for iodine determination by AAS (blank spaces mean no information available about the subject; for abbreviations used see Table 2) Element/ Iodine l (nm), Interference Ref.species lamp Atomizer Atomizer details Procedure Analytical figures of merit study Samples 49a I- Cd/228.8 Flame Air–C2H2 Solvent extraction with Ph–NO2 at pH 3.5–5.5 of the Lin. range: 0.6–5.1 mg ml-1 Yesb Aq. solns. HCL Ion-pair tris-(1,10-phenanthroline)Cd2+ I-. The org. phase is measured for Cd 16c I- Cr/357.9 Flame I- reduces Cr(VI) in 3M HCl medium; excess Cr(VI) No Aq.solns. is extracted into IBMK. Cr(III) is determined in the aq. layer and Cr(VI) in the org. one IO3- Fe/248.2 Flame IO3- oxidizes Fe(II ) in 9M HCl medium; Fe(III) is extracted into Et2O. The org. phase is analyzed for Fe, present as Fe(III) I- Se Flame I- reduces Se(IV) in acid soln.; Se0 is separated by filtration with Millipore filter. The filtrate is measured for excess Se(IV) 27 I- Cd/228.8 Flame Air–C2H2 oxidant The ion-pair tris-(1,10-phenanthroline)Cd2+ I- is RSD: 0.6% at 5.1 mg ml-1 Yes Aq.solns. HCL extracted into Ph–NO2 at pH ca. 5. The org. phase Lin. range: 0.5–5.1 mg ml-1 Br-, ClO3-, is analyzed for Cd ClO4-, IO4-, NO3- 31a I- Ag Flame H2–air Sample treated with Na2O2, KNO3 and saccharose to c0: 0.1 ppm Agd Org. reduce IO3- to I-, which is precipitated as AgI compounds with AgNO3 in acid medium; precipitate is redissolved with excess KI soln. and Ag is determined in the resulting soln. 29a I-e Ag Flame Air–C2H2 I- is precipitated as AgI, and excess Ag+ is RSD: 3.61% within lin. range. Aq. solns. determined by AAS Lin. range: 1–6 mg ml-1 22 I- Hg/253.7 Graphite Dry: 30 s/20 A (ca. I- is complexed with Hg2+ in acid (HNO3) medium Sensitivity:g 0.019 ml mg-1 Yes Aq. solns. HCLf furnace 150 °C) (pH ca. 4); on atomizing (ramp), two absorbance RSD: 1.6% at 2.5 mg ml-1 CN-, S2-, S2O32- atom: 30 s/4.0 A/s peaks are observed, the second one being (at about Lin. range: 0.13–6.4 mg ml-1 Ag+ 440 °C) due to the Hg–I complex. By recording this latter peak, I- is determined indirectly 35 I- Hg/253.7 Cold vapour I- is complexed with Hg2+ in acid (H2SO4) medium; LOD:h 0.07 mg ml-1 No Aq.solns. EDL the soln. is passed through a resin (KU-2×8 RSD: 2.4% within lin. range cation-exchange resin) to sorbe excess Hg2+ and Lin. range: 0.07–80.0 mg ml-1 then subjected to cold vapour (the Hg2+ bound to I- is the determined) 37 I-i Hg/253.6 Cold vapour T(cell ): 250 °C Sample is acid-digested (HNO3–H2SO4), carried to Sensitivity: ca. 0.003 mg-1 Yesb Seaweed EDL pH 7–8, its I- complexed with Hg2+ and subjected LOD:j 2.5±0.7 mg to cold vapour to measure the free Hg2+ RSD: mean within lin. range 3.0%g Accuracy: a seaweed sample (std. NGU) gave 816 mg g-1 k (RSD: 4.4%) before the 850 mg g-1 k measured by NAA Lin. range: 2.5–25.0 mg IO3- gave the same values for these figures of merit as I- J. Anal. At. Spectrom., 1999, 14, 1009–1018 1013Table 3 (Continued) Element/ Iodine l (nm), Interference Ref.species lamp Atomizer Atomizer details Procedure Analytical figures of merit study Samples 28 I- Ag/328.1 Flame Air–C2H2l After alkaline digestion (Na2CO3) of the sample, the Sensitivity:g -0.0014 Yes FoodstuVs and HCL ash is dissolved with HNO3, diluted with water Abs ml mg-1 CrO42-, ClO-, wine (final pH 1.3–6.2) and subjected to a FIA LOD:h 6.0 mg ml-1 CN-, procedure to determine Cl- and I-: both ions are RSD: 2.3% within lin.range, [Fe(CN)6]3- precipitated with Ag+ in acid medium, the and 4.9% at 100 mg ml-1 level precipitate is washed with HNO3 and partially Anal. rec.: 97.2–104.7% in redissolved with NH3 (remaining AgI precipitated). samples at diVerent levels of Ag+ concentration is continuously monitored, and concentration the diVerence in its concentration in the two steps Lin. range: 10–320 mg ml-1 (precipitation and redissolution) is proportional to I- concentration. Since Ag absorbance signal decreases when precipitation occurs, negative peaks and slope of the calibration line are observed 38 I- Hg/253.8 Cold vapour T(cell ): 180 °C The sample soln.resulting from an alkaline fusion RSD: 4.9–8.8% analyzing the Yes Cod liver oil EDL (KOH) is neutralized (H2SO4) to pH 7–8, the I- CRMs Ag+, Pt4+ and CRMs: it contains is complexed with Hg2+ and the final Accuracy: the CRMs under NBS oyster soln. subjected to cold vapour to determine the ‘Samples’ gave results inside tissue and free Hg2+ certification range NBS citrus Anal.rec: 94–103% (mean: 98%) leaves in samples 46a I- Cu/324.8 Flame Air–C2H2 The complex neocuproine-Cu+ I- is extracted into LOD:h 0.04 mg ml-1 Aq. solns. Ph–Cl, and the org. layer is analyzed for Cu by Lin. range: 0.12–4.8 mg ml-1 AAS. 26 I- Hg/253.7 Cold vapour I- contained in the soln. resulting from an acid LOD:m 0.68 mg l-1 Yes Various HCL (HNO3–H2SO4) digestion of the sample is c0: 1.48 mg l-1 Cl- in presence of seaweeds complexed to form the ion-pair 2,2’-dipyridyl-Hg2+ RSD: 2.25% at 15 ng level and Fe3+, S2O32- I-, which is extracted (pH 7.2–7.5) into AcOEt 5.53% at the same level in the Cu2+, Ag+, and back-stripped into 4 M HNO3, the final aq.sample. Ba2+ phase being analyzed by cold vapour Results verified spectrophotometrically (otolidine), with a general concordance 39a I- Hg/253.7 Cold vapour Sample pre-treated with ZnAcO2 is applied to an Anal. rec: 92–100% Yes Water ion-exchange resin type 717.To the I--containing Lin. range: 2.5–25 mg l-1 n CN-, S2-, S2O32- eluate was added an Hg2+ soln. to form an Hg-I Ag+ complex. Excess Hg2+ is reduced by shaking with (NH4)2SO4 (while that contained in the complex remains unreduced) and measured by cold vapour. The same procedure is carried out for an I--free soln. and I- is determined by diVerence 40 I- Hg/HCL Cold vapour Seaweed: To the soln. prepared after alkaline fusion Seaweed: RSD of method: 2–4% Yeso Seaweed and (NaOH), a Hg2+ soln.is added to complex Hg2+ Accuracy: Sample also analyzed table salt and I-. Unreacted Hg2+ is measured by cold by NAA: good agreement vapour between both results Table salt: the soln. resulting from dissolving the Table salt: RSD (within batch): sample is treated like that from seaweed ca. 2% RSD (between batch): 10.2% 1014 J. Anal. At. Spectrom., 1999, 14, 1009–101817 IO3- Hg/253.7 Cold vapour IO3- contained in the sample soln. complexes to LOD:m 7.5 ng Yes High purity HCL form the ion-pair 2,2’-dipyridyl-Hg2+ IO3-, which m0: 10.5 ng S2-, S2O32- Ag+ reagents: is extracted (pH 6.95–7.05) into IBMK and RSD: 2.4% at 1.0 mg level EDTA NaNO3, stripped back into 4 M HNO3, being the final aq.RSD: 6% at 1.1 mg level in a KNO3 and phase analyzed by cold vapour certain sample (NaNO3) KI Anal. rec.: 92–112% in samples at diVerent levels of concentration Lin. range: up to 1.18 mg 32 I- Ag/328.1 Flame Air–C2H2 oxidant In a FIA manifold, I- is precipitated as AgI, the LOD:p 0.63 mg ml-1 Yes Aq.solns. HCL precipitate redissolved with CN- and the resulting RSD <3% Br-, CN-, SCN- soln. analyzed for Ag by flame AAS Lin. range: 0.6–15.2 mg ml-1 33 I- Ag/328.1 Flame Air–C2H2 oxidant By means of a FIA system, I- is precipitated as AgI, LOD:p 0.13 mg ml-1 Yes Aq. solns. HCL the precipitate being redissolved with CN- and Ag RSD: 1.1% at 2.54 mg ml-1 Cl-, SCN-, determined in the resulting soln.by flame AAS CrO42- 25 I- Cu/324.7 Flame Air–C2H2 oxidant Solid sample is extracted into H3CCN and back- Sensitivity: 14.6 ml mg-1 Yes Reagents HCL stripped with water. The I- this soln. contains is LOD:q 47 mg l-1 Br-, Cl- (KBr, KIO3, used to prepare the chelate complex [Cu(bptc)]+ LOQ:q 157 mg l-1 NaCl) and I-, which is extracted at pH 6.5 into AcOBu. This RSD: 3.2% at 5 mg ml-1 level in table salt org. phase is analyzed for Cu by AAS. commercial iodized NaCl (iodized and (bptc=2-benzoylpyridine thiosemicarbazone) Anal.rec.: 97.3–101.4% in not iodized) samples at diVerent levels of concentration Results, if compared with those by a spectrophotometric method (o-tolidine), are slightly lower (attributed to interferences in the latter method) Lin. range: 0.4–10.4 mg ml-1 47a,c I- Cu/324.8 Flame Air–C2H2 After pretreatment (not described in the abstracts), LOD:h 0.104 mg ml-1 Yes Aq. solns., sample soln. is added CuSO4, and thiourea solns.RSD: 1.29% halides seaweed, to prepare an ion–pair which, after adding a Anal. rec.: 98–106% kelp and K2HPO4–KH2PO4 buVer soln., is extracted into Lin. range: 0.79–9.52 mg ml-1 laver IBMK. This org. layer is analyzed for Cu by AAS (referred to aq. phase, before (both by flame and by graphite furnace) extraction) Graphite LOD:h 0.253 ng furnace Anal. rec.: 85–109% Lin. range: 15.9–222.1 mg·l-1 (referred to aq. phase, before extraction) 50a I- Cu Flame Sample is alkaline digested (K2CO3), macerated with RSD 2.9% Kelp, laver, H3PO4, and adjusted to pH 4.5.An aliquot of the Anal. rec.: 93.3–106.7% tablets huasu resulting soln. is added KH2PO4, thiourea and Lin. range: 0–5 mg ml-1 CuSO4 solns to form a neocuproine-Cu+ I- complex that is extracted into Ph–Cl. This organic layer is analyzed for Cu by AAS 23 I- Hg/253.7 Graphite Dry: 20 s/50 °C I- contained in samples is complexed with Hg2+ in Sensitivity:g 8.02×10-4 l mg-1 Yes Tap water EDLf furnace atom. (ramp): 50 acid medium (10-4M HNO3).When subjected to (standard additions) CN-, S2-, S2O32- to 900 °C in 35 s AAS, this soln. gives two Hg peaks, the second of LOD:q 3.0 mg l-1 Pb2+, Zn2+ plus 15 s/900 °C them being due to the Hg contained in the LOQ:q 10.1 mg l-1 (hold) complex, and recording its signal, I- is determined. m0: 38.3 pg RSD: mean within lin. range: 6.7%g Anal. rec.: 94.8–104.4% within lin. range Lin. range: 3.0–20.0 mg l-1 J. Anal. At. Spectrom., 1999, 14, 1009–1018 1015Table 3 (Continued) Element/ Iodine l (nm), Interference Ref.species lamp Atomizer Atomizer details Procedure Analytical figures of merit study Samples 48a I- Cd/228.8 Flame Air–C2H2 After reducing IO3- in the sample to I- (no sample Anal. rec.: 97.2–106.7% in Yesb Rice, rice pretreatment described), the I- contained in this samples straw, soln. was used to form an ion–pair with Lin. range: 0.01–1.8 mg ml-1 spinach and tris-(1,10-phenanthroline) Cd2+ which is extracted chilli into Ph–NO2 at pH 5.2, this org.phase being analyzed for Cd 45 I- As/193.7 Hydride I- in the problem soln. is oxidized by Ce4+ in acid RSD: 2.9% at 12.8 ng level Yes Aq. solns. generator medium (HClO4) to I2, which distills and is Lin. range: 3.2–25.4 mg l-1 Br-, Mn2+ collected in a KI soln. This soln. is treated with excess As3+ to reduce I2 to I- at pH 8.0 (phosphate buVer). I- is precipitated as AgI. After centrifugation, the supernatant is analyzed for As by AAS (which gives a negative-sloped calibration line) 24 I- Hg/253.7 Graphite Dry: 15 s/100 °C and The ion-pair (2,2’-dipyridyl )Hg2+ I- is formed and Sensitivity:g 8.84×10-4 l mg-1 No Aq.solns. EDLf furnace 15 s/110 °C extracted into IBMK at pH 6.8–7.6. The org. layer (standard additions) Ash: 15 s/200 °C is analyzed for Hg by AAS RSD: mean within lin. range Atom: 7 s/1000 °Cs 7.8%g Lin. range: up to 75 mg l-1 21a I- Ag/338.3f Graphite The residue from an alkaline fusion [Na2CO3- Anal.rec.: 97.5–102.3% Yes Blood serum furnace Mg(NO3)2] of the sample is dissolved with HNO3. The I- precipitates as AgI in acid (HNO3) medium, the precipitate being separated by centrifugation and the supernatant analyzed for Ag by AAS 30a I- Ag/328.1 Flame Air–C2H2 oxidant I- from sample is precipitated as AgI and Ag is Anal. rec.: 96.6–99.6% within Seaweed, kelp, determined in the supernatant by AAS lin. range medical Lin. tange: 10–40 mg ml-1 tablets and iodine salt 36a I-e Hg Graphite I- from sample reacts with Hg(NO3)2 to form HgI2, LOD:h 1 mg l-1 Yes Water furnace and Hg is determined by AAS RSD: 3.2–6.0% CN-, S2- Anal.rec.: 85–110% Lin. range: 1–10 mg l-1 Results consistent with those of contact method. 34a I- Ag Flame I- from sample is precipitated as AgI and this Relative error: 1.52–4.17% Water extracted into IBMK in the presence of excess Ag+ in 0.8M NH3. aReferences handled as abstracts published in the journals Analytical Abstracts and/or Chemical Abstracts.bNo information provided about the main interferent agents. cSeparated conditions for the two I- and IO3- determinations. dNot specified in the abstracts if ppm means mg ml-1 or mg g-1. eSupposed. fD2 lamp background correction used. gCalculated from the data provided in the work published. hNo definition given. iThe work is developed with iodide, but several times iodate is mentioned to give the same results as iodide. jLOD defined as ‘the amount of iodine that causes an average decrease in absorbance of 100 ng of Hg2+ equal to six times the standard deviation for measurements carried out in the absence of iodine’.kSupposed, since in the work only ‘ppm’ appears. lThe flame is reported to have been ‘adjusted following standard recommendations’. mLOD is defined on the basis of 3 times the SD of the blank, according to that stated by G.L. Long and J.D. Winefordner, Anal. Chem., 1983, 55, 712A, reference cited by Chakraborty and Das.26 nSupposed, since in the abstracts only ‘ppb’ appears.oCl- is the only potential interferent agent studied. pDefined as the concentration that gives a signal 3 times the base line noise. qDefined as the concentrations of analyte that give a signal located 3 times (LOD) and 10 times (LOQ) above the standard deviation of the blank signal, as proposed by ACS in the reference cited by Nag, Chatteraj and Das:25 Anal. Chem., 1980, 52, 2242. rIn Analytical Abstracts, the method is reported to be applied to Cu analysis in these samples.sIBMK palladium soln. used as matrix modifier. 1016 J. Anal. At. Spectrom., 1999, 14, 1009–1018shielding gases for the optical system (and/or for the atomizer) the good results obtained, the reliability of these AAS methods for iodine determination is evident. Moreover, the results of does not give clear diVerences in the analytical figures of merit. Finally, both in direct and in indirect methods, the accuracy the interference studies reported by most of the authors can be a very useful tool for future applications to real samples of is usually evaluated by calculating analytical recoveries, the analysis of reference materials was rather unusual (it only these methods.appears twice37,38). The authors that compare their results with those obtained by other methods, such as NAA37,40 or UV-VIS spectrophotometry25,26 also stand out. References 1 D. A.McGregor, K.B. Cull, J. M. Gehlhausen, A. S. Viscomi,M. 2.4 Interferences and samples Wu, L. Zhang and J. W. Carnahan, Anal. Chem., 1988, 60, 1089A. Most of the authors report interference studies, but in some 2 G. F. Kirkbright and H. N. Johnson, Talanta, 1973, 20, 433. 3 M. Garcý�a-Vargas, M. Milla and J. A. Pe� rez-Bustamante, Analyst, works the interfering species are not cited at all. Additionally, 1983, 108, 1417. procedures aimed at minimizing or removing interfering eVects 4 CRC Handbook of Chemistry and Physics, 65th edn., ed.R. C. are included at times as well. Weast, CRC Press, Boca Raton, Florida, USA, 1984, p. E-228. Regarding samples, one should underline that an important 5 G. F. Kirkbright and P. J. Wilson, Anal. Chem., 1974, 46, 1414. disadvantage of the direct methods is the lack of application 6 G. F. Kirkbright, T. S.West and P. J. Wilson, At. Absorpt. Newsl., to real samples, except for the work by Kirkbright and 1972, 11, 53. 7 M. J.Adams, G. F. Kirkbright and T. S. West, Talanta, 1974, Wilson11 (who successfully analyze rat thyroid samples). 21, 573. Therefore, the applicability of those direct procedures to real 8 M. D. Lowe, M. M. Sutton and O. E. Clinton, Appl. Spectrosc., samples is not guaranteed. On the contrary, more than 50% 1982, 36, 25. of the indirect methods are applied to real samples, which are 9 J. M. Manfield, T. S. West and R. M. Dagnall, Talanta, 1974, often of marine origin (seaweed, algae, salt, .. .). 21, 787. 10 G. F. Kirkbright, T. S.West and P. J. Wilson, At. Absorpt. Newsl., 1972, 11, 113. 3 Conclusions 11 G. F. Kirkbright and P. J. Wilson, At. Absorpt. Newsl., 1974, Although iodine is not usually determined by AAS (only 40 13, 140. 12 G. F. Kirkbright, T. S. West and P. J. Wilson, Anal. Chim. Acta, works in 30 years), the studies (both direct and indirect) that 1974, 68, 462. have been published show that this element can indeed be 13 AOAC, OYcial Methods of Analysis, AOAC, Washington, 16th determined successfully by that technique.However, it is not edn., 1995, p. 29. easy to find out which is the best method: the variety of 14 M. Pinta, Methodes Phys. Anal. (GAMS), 1970, 6, 268. topics and parameters studied by each author, and the diVerent 15 M. M. Gol’dshtein and I. G. Yudelevich, Zh. Khim. Anal. (J. Anal. definitions used for parameters such as LOD, sensitivity, c0, Chem. USSR), 1976, 31, 801. 16 G. D. Christian and F.J. Feldman, Anal. Chim. Acta, 1968, 40, etc, imply such diVerences among the papers that trying to 173. compare them is a diYcult task. Even when these diVerences 17 D. Chakraborty and A. K. Das, Talanta, 1989, 36, 669. simply reflect the evolution of the analytical concepts, it would 18 B. V. L’vov and A. D. Khartsyzov, Zh. Khim. Anal. (J. Anal. be desirable to achieve a wider agreement among the analytical Chem. USSR), 1969, 27, 799. chemists regarding those concepts. 19 M. A. Klein and E. M. Heithmar, Appl. Spectrosc., 1984, 38, 590. Due to their simplicity (at least ‘theoretically’), direct 20 T. A. M. Ure, L. R. P. Butler, B. V. L’vov, I. Rubeska and R. Sturgeon, Pure Appl. Chem., 1992, 64, 253. methods can be considered to be more adequate than indirect 21 J. Guo, P. Li, S. Wang, Y. Li, Y. Yuan and R. Ma, Fenxi Huaxue, ones, but their instrumental requirements imply really serious 1997, 25, 121; Anal. Abstr., 1997, 59, 8F25; Chem. Abstr., 1997, diYculties; this explains why there are no commercial instru- 126, 197011.ments that permit AAS measurements in the vacuum-UV. 22 T. Nomura and I. Karasawa, Anal. Chim. Acta, 1981, 126, 241. However, it is important to state that the direct methods 23 P. Bermejo-Barrera, A. Moreda-Pin� eiro, M. Aboal-Somoza, reviewed show that a relatively simple technology is needed J. Moreda-Pin� eiro and A. Bermejo-Barrera, J. Anal. At. Spectrom., 1994, 9, 483. for building such instruments.In spite of this, the future of 24 P. Bermejo-Barrera, M. Aboal-Somoza, A. Moreda-Pin� eiro and these types of methods is not hopeful (note that no direct A. Bermejo-Barrera, J. Anal. At. Spectrom., 1995, 10, 227. method has been published since 1984). 25 J. K. Nag, S. Chattaraj and A. K. Das, Bull. Chem. Soc. Jpn., Unlike direct methods, commercial instruments do enable 1993, 66, 774. the performance of indirect methods but, in these, the diY- 26 D. Chakraborty and A.K. Das, At. Spectrosc., 1988, 9, 189. culties lie in the complexity of the procedures. However, that 27 T. Kumamaru, Bull. Chem. Soc. Jpn., 1969, 42, 956. 28 P. Martý�nez-Jime�nez, M. Gallego and M. Valca�rcel, Anal. Chim. complexity does not prevent achieving good results and thus, Acta, 1987, 193, 127. these indirect methods show good perspectives for the future. 29 E. Than, H. Koenig and U. Hascher, Z. Chem., 1975, 15, 288; Among them, it seems that the methods involving mercury Anal.Abstr., 1976, 30, 3B134; Chem. Abstr., 1975, 83, 187929p. determination as well as solvent extraction processes are the 30 J. Shi, K. Jiao and T. Liu, Fenxi Huaxue, 1998, 26, 122; Anal. ones providing the best performance for iodine determination. Abstr., 1998, 60, 8D109; Chem. Abstr., 1998, 128, 175507. Another interesting point is the fact that the iodine species 31 M. A. T. M. de Almeida, S. de Moraes and J. C. Barberio, Relat. Inst. 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Abstr., 1995, 57, 10D71; Chem. Abstr., 1995, 123, 81831e. 42 B. V. L’vov and A. D. Khartsyzov, Zh. Prikl. Spektrosk., 1969, 49 Y. Yamamoto, T. Kumamaru, Y. Hayashi and Y. Otani, Bunseki 10, 413; Anal. Abstr., 1971, 20, 868; Chem. Abstr., 1970, 11, Kagaku, 1968, 17, 92; Anal. Abstr., 1969, 17, 2101; Chem. Abstr., 18208v. 1968, 69, 15942g. 43 Ll. A. Currie, Pure Appl. Chem., 1995, 67, 1699. 50 G. Wang and B. Chen, Lihua Jianyan, Huaxue Fence, 1994, 30, 44 K. C. Thompson, Spectrosc. Lett., 1970, 3, 59; Anal. Abstr., 1971, 270; Anal. Abstr., 1995, 57, 3H241; Chem. Abstr., 1995, 122, 20, 1659; Chem. Abstr., 1970, 73, 31263z. 208999c. 45 I. Y. Kim, H. Nakamura, K. Yamaya and M. Yoshida, Anal. Sci., 1995, 11, 363. 46 Y. Hou, S. Lin and X. Chi, Gaodeng Xuexiao Huaxue Xuebao, 1988, 9, 118; Anal. Abstr., 1988, 50, 12B176; Chem. Abstr., 1988, Paper 8/08749K 109, 34721v. 1018 J. Anal. At. Spectrom., 1999, 14, 1009–1018

ISSN:0267-9477    

DOI:10.1039/a808749k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Reduction of metal oxides by carbon in graphite furnaces Part 1. Temporal oscillations of atomic absorption in the process of slow evaporation of Al, Bi, Cr, In, Mg, Mn, Pb, Sb, Sn and Te oxides Boris V. L’vov,a Andrey A. Vasilevich,a Alexey O. Dyakov,a Joseph W. H. Lamb and Ralph E. Sturgeonb aDepartment of Analytical Chemistry, St. Petersburg State Technical University, St. Petersburg 195251, Russia bInstitute for National Measurement Standards, National Research Council, Ottawa, Ontario K1A 0R6, Canada Received 9th March 1999, Accepted 13th May 1999 Temporal oscillations in the kinetics of carbothermal reduction of oxides of 10 elements (Al, Bi, Cr, In, Mg, Mn, Pb, Sb, Sn and Te) have been observed by electrothermal atomic absorption spectrometry.Microgram amounts of the elements, as their nitrates, were evaporated with slow heating of the graphite tube. With the exception of Al, Cr and Mn, these oscillations were observed for the first time. Some of the features of this phenomenon are described, including the eVects of sample mass, acidity of the solution, nature of the graphite substrate, the influence of the presence of Sr and the type of sheathing gas (He and Ar) on the characteristics of Al spikes.Similar results were obtained in two diVerent laboratories. The appearance of spikes on absorption signals under con- Experimental ditions of comparatively slow heating (5–20 °C s-1) of micro- Instrumentation and reagents gram masses of alumina in a graphite furnace (GF) was first reported in 1981.1 This phenomenon was subsequently sub- Measurements made in both laboratories used Perkin-Elmer jected to a number of studies, mainly by L’vov et al.,2–27 as a (Norwalk, CT, USA) Model 5000 spectrometers equipped result of which a gaseous carbide mechanism for the reduction with HGA-500 graphite furnaces and AS-40 autosamplers.of oxides by carbon (ROC) was proposed.21,22 The foundation Hollow-cathode lamps (HCL’s), electrodeless discharge lamps of this mechanism is the assumption that the oxide is directly (EDL’s) and a deuterium lamp (D2) were used as light sources.reduced by gaseous molecules of metal carbides which, in their Absorption signals were registered using personal computers turn, form by the interaction of metal vapor with carbon, e.g. and laboratory developed programs. Standard pyrolytic graphby the reactions: ite coated tubes were employed in most cases.Argon containing not more than 1×10-3 % oxygen served as a sheath gas. M(g)+2C(s)�MC2(g) (1) Helium was used for the internal purge gas in some MC2 (g)+2MO(s)�3M(g)+2 CO (2) experiments. The first of these reactions, initiated by defects in the carbon structure, occurs on the graphite surface whereas the second Procedures occurs on that of the oxide. Reactions (1) and (2) determine the boundary conditions for the fluxes of gaseous products The first stage of the study consisted of the selection of optimum experimental conditions for each of the investigated between the solid reactants.Based on this model, many specific features of the process, in particular, the periodicity of the elements in order to record the absorbance signal during the process of slow evaporation of microgram masses of metal spikes noted for Al,21 Mn,21,26 Tm,2,21,27 and Yb21,25 oxides were explained.21,22 For this reason, it is appropriate to oxides. For this purpose, secondary low-sensitivity lines were used.In some cases, such lines are absent (Mg and Mn) or consider below not only the appearance of fast single spikes, but more generally, temporal oscillations in the kinetics of are located in the visible part of the spectrum (Cr), inconvenient for the absorption measurement at high tube tempera- carbothermal reduction of oxides. In the early 90’s, a number of publications appeared,28–33 wherein various groups of tures. Because of this, a D2-lamp, in combination with a narrow slit width, was used in these cases.The second stage researchers critically discussed details of the gaseous-carbide mechanism, as applied to reduction of alumina, and questioned of the study consisted of the selection of optimum conditions for the generation of temporal oscillations. Several factors its validity. L’vov34 subsequently responded to all of the arguments presented against it. were varied: the concentration, acidity and volume of solution injected; substrate type (uncoated and pyrolytic graphite The purpose of this study was to investigate temporal oscillations for some elements from diVerent Groups of the coated, new and old tubes); wall and platform evaporation; rate of heating during the dry and atomization stages; and Periodic Table (in addition to Al, Cr and Mn) and to examine the general features of this phenomenon.One may speculate internal flow rate of argon. In some cases, 0.1–10 mg of Sr, as its nitrate, was added to the sample.All experiments were that the eVects noted should also be evident for additional elements, but are unlikely to arise for those of the iron and repeated at least 5–10 times to obtain a measure of the reproducibility of the results. platinum groups, alkali metals or Ag, Au, Cu and Hg. A second thrust was to compare the experimental results obtained The parameters for atomic absorption measurements and the heating programs used in both laboratories are given in for the same elements in two diVerent laboratories: the St.Petersburg State Technical University in Russia (lab 1) and Tables 1 and 2 (only those parameters were included which correspond to the results presented below). the National Research Council of Canada (lab 2). J. Anal. At. Spectrom., 1999, 14, 1019–1024 1019istics of these temporal oscillation profiles: mean values of spike widths and intervals between the spikes. The scatter in the values of these measured parameters is in the range 20–30% and is typical of the reproducibility of these phenomena.Activation energies The most convincing argument for the ROC origin of these temporal oscillations is the apparent value of the activation energy, Ea (summarised in Table 4), associated with the rising part of the spike22: Ea=R ln (A2/A1)/(T1-1-T2-1) (3) Here R is the gas constant and the absorbance and corresponding temperature values used for the calculation of Ea (A1 , A2 and T1 , T2) are taken from the profiles presented in Figs. 1–10. The error in calculations associated with an uncertainty in temperature calibration of the HGA-500 power supply (within ±100 °C) is not higher than 20%. As evident from Table 4, these apparent Ea values for the spikes are much higher than the theoretical values of activation energies calculated35 for the process of thermal dissociation of oxides in the Fig. 1 Oscillation profiles for Al recorded under conditions given in equimolar mode for all the elements investigated.This clearly Tables 2 (a) and 1 (b). demonstrates the accelerating character of the process, which is typical for the autocatalytic ROC mechanism. A substantial Results and discussion longitudinal non-isothermality in the temperature of the graph- Atomic absorption spike profiles ite tube occurs when HGA-type atomizers are used and thus one must entertain the possibility that the oscillations are due Figs. 1–10 contain some typical spike profiles for Al, Bi, Cr, to analyte condensation/re-vaporisation phenomena.In light In, Mg, Mn, Pb, Sb, Sn and Te. For 7 of these 10 elements of the element specificity of the eVect, however, it is unlikely (exceptions being Al, Cr and Mn), these temporal oscillations that this can be considered as correct. This problem has earlier are reported here for the first time. For all elements, the been discussed in detail by L’vov.34 oscillation profiles obtained in the two diVerent laboratories were very similar (see Figs. 3, 4, 6, 7 and 9, for illustration). Characteristics of oxides From a cursory examination of these data, some general diVerences in the shape and position of the spikes becomes It is interesting to correlate the diVerences in spike profiles for the elements investigated with any diVerences in the character- apparentr Al, Cr, In, Sb, Sn and Te, they are located on the front of the thermal dissociation pulse, but for Bi, Mg, Mn, istics of their oxides.In Table 5, the melting points and enthalpies of formation for these oxides36 are presented. As Pb, on its tail. Additionally, two diVerent types of ‘spikes’ arise: fast and sharp spikes for Al and Mn and smooth oscillations can be seen from the comparison of the melting points with the temperatures corresponding to the oscillation process, only for the other elements. In some cases (Sb, for example), spikes of both types occur. Table 3 contains some additional character- oxides of Mg, In and Cr remain in the solid state during the Table 1 Experimental conditions used in lab 1 for observation of spike profiles presented in Figs. 1–10 Heating program Heating Wave- Light Current/ Mass/ Volume/ Temperature/ Ramp time/ Hold rate/ Ar flow/ Element length/nm sourcea mA Slit/nm mg ml °C s time/s °C s-1 ml min-1 Solution Al 309.3 HCL(LSP-1) 15 0.07 11 5 130 20 40 1%HNO3 1500 1 5 2300 80 0 10 50 Bi 227.7 HCL(LSP-1) 20 0.7 4 20 150 10 40 20%HNO3 800 1 5 1700 75 0 12 50 Cr 429.0 HCL(LSP-1) 25 0.07 2 20 150 10 40 10%HNO3 1500 1 5 2300 70 0 11 300 In 410.2 HCL(LSP-1) 20 0.07 3 20 150 10 40 10%HNO3 1200 1 5 1900 60 0 12 20 Mg 285.2 D2 — 0.07 11 5 150 10 40 1%HNO3 1500 1 5 2400 60 0 15 20 Mn 222.2 HCL(P-E) 20 0.2 2 20 150 10 40 3%HNO3 1300 1 5 2100 80 0 10 10 Pb 205.3 HCL (P-E) 12 0.7 5 20 150 10 40 20%HNO3 600 1 10 1800 70 0 17 30 Sb 231.1 EDL (P-E) 400 0.2 19 20 200 1 5 20%HNO3 110 1 50 1200 5 5 1900 60 0 12 50 Sn 254.7 HCL(P-E) 20 0.2 13 20 150 10 40 5%H2SO4 1100 5 5 2200 70 0 16 30 Te 225.9 EDL(P-E) 400 0.2 6 20 130 1 3 10%HNO3 150 30 20 800 1 5 2000 60 0 20 50 aP-E: Perkin-Elmer; LSP-1: manufactured in Russia. 1020 J. Anal. At. Spectrom., 1999, 14, 1019–1024Table 2 Experimental conditions used in lab 2 for observation of spike profiles presented in Figs. 1–14 Heating program Heating Internal Wavelength/ Light Current/ Slit/ Mass/ Volume/ Temperature/ Ramp Hold rate/ gas flow/ Element nm sourcea mA nm mg ml °C time/s time/s °C s-1 ml min-1 Solution Al 309.3 HCL(H) 15 1 5 150 1 40 1%HCl 0.07 (+2 mgSr) (+5 mlSr) 1700 1 5 2100 60 0 7 20(Ar) Al 309.3 HCL(H) 15 0.2 9 20 150 1 40 20%HNO3 (+1 mgSr) (+5 mlSr) 1950 1 5 2250 50 0 6 40(He) Cr 357.9 D2 — 17 20 150 10 40 4%HNO3 0.07 1500 1 5 1900 30 0 13 300(Ar) In 410.2 HCL(C) 6 0.2 4 10 150 10 40 10%HNO3 1100 1 5 1800 60 0 12 30(Ar) Mn 279.5 D2 — 19 15 150 10 40 3%HNO3 0.07 (+1 mgSr) (+5 mlSr) 1400 1 5 2100 60 0 12 50(Ar) Pb 205.3 HCL(H) 15 0.2 5 20 150 10 40 5%HNO3 700 1 10 1700 60 0 17 30(Ar) Sn 254.7 HCL(H) 15 0.7 10 20 150 10 40 30%HNO3 1100 5 5 2200 60 0 18 50(Ar) Sn 254.7 HCL(H) 15 0.7 10 20 180 1 40 4%H2SO4 300 5 5 500 1 5 2200 50 0 34 20(He) aC: Cathoden; H: Hamamatsu.Fig. 2 Oscillation profile for Bi recorded under conditions given in Table 1. Fig. 4 Oscillation profiles for In recorded under conditions given in Table 2 (a) and 1 (b). Fig. 3 Oscillation profiles for Cr recorded under conditions given in Fig. 5 Oscillation profile for Mg recorded under conditions given in Table 1. Tables 2 (a) and 1 (b). J. Anal. At. Spectrom., 1999, 14, 1019–1024 1021Fig. 8 Oscillation profile for Sb recorded under conditions given in Table 1. Fig. 6 Oscillation profiles for Mn recorded under conditions given in Tables 2 (a) and 1 (b). Table 3 Spike characteristics Meana Meana temperature Heating spike interval Mass/ rate/ Ar flow/ width/ between Element mg °C s-1 ml min-1 °C spikes/°C Reference Al 1 10 20 12 89 Fig. 1a Bi 4 12 50 29 67 Fig. 2 Cr 2 11 300 20 57 Fig. 3b In 3 12 20 35 88 Fig. 4b Mg 11 15 20 30 60 Fig. 5b Mn 2 10 10 17 38 Fig. 6b Pb 5 17 30 47 75 Fig. 7a Sb 19 12 50 9 30 Fig. 8 Sn 13 16 30 26 72 Fig. 9b Te 10 20 50 42 107 Fig. 10 aTypical reproducibility is ±20–30% RSD. Fig. 7 Oscillation profiles for Pb recorded under conditions given in Tables 2 (a) and 1 (b). oscillation period. Oxides of Pb, Sb, Bi and Te are in a liquid state from the very beginning of the oscillations and oxides of Al, Sn and Mn are in a solid state in the beginning, and in a liquid state by the end of the oscillation period.Despite these significant diVerences in the physical states of the oxides during the oscillation process, no diVerences in the character of the oscillations can be noted. Very significant diVerences in the enthalpies of formation for these oxides also exist. For Al2O3 (the most stable) and PbO (the most unstable), the enthalpies (per one metal atom) diVer by about 4-fold.This explains, firstly, the diVerence in Fig. 9 Oscillation profiles for Sn recorded under conditions given in Tables 2 (a) and 1 (b). the appearance temperatures of the spikes for these elements (2000 K and 1100 K, respectively) and, secondly, the dramatic diVerence in associated activation energy values (Table 4). It EVect of sample mass is also possible to relate this enthalpy distinction to the diVerence in the width of the spikes (Table 3).Nevertheless, As an illustration of the eVect of analyte mass on the width of the spikes, Fig. 11 and Table 6 present the results obtained this last correlation does not appear to be very reliable or, in any case, unambiguous. As shown later, the width of the for aluminium. In all cases, the width of the first spike was utilized. It is clear that the width varies nearly proportionally spikes, to a large extent, is a function of sample mass and graphite substrate conditions.to the analyte mass in the range 10 to 200 mg Al. 1022 J. Anal. At. Spectrom., 1999, 14, 1019–1024EVect of solution acidity In some cases an increase in solution acidity stimulated temporal oscillations. Therefore, practically all solutions were acidified to contain up to 20–30% HNO3 or H2SO4 (for tin). This eVect may be associated with the activation of the graphite surface and, as a result, with formation of gaseous carbides.7 EVect of graphite substrate Graphite substrate conditions are of primary importance among other parameters which aVect the generation of temporal oscillations.Pyrolytic graphite coated tubes are preferable in all cases. For most of the elements tested, new tubes (up to 20–30 firings) promote better results than used ones. In some cases, the influence of the graphite substrate on the width of the spikes was evident. The width of spikes generated in the process of carbothermal reduction of MnO from a pyrolytic graphite platform (Fig. 12) is about 6-fold less than those from a pyrolytic graphite coated tube wall (Fig. 6). Fig. 10 Oscillation profile for Te recorded under conditions given Additionally, a diVerence in the position of spikes on the front in Table 1. (Fig. 12) and on the tail (Fig. 6) is evident. Unfortunately, temporal oscillations for samples deposited on the platform Table 4 Activation energies for the elements investigated are more diYcult to generate, likely as a result of a diVerence in the structure of the pyrolytic graphite. This diVerence in Ea a/ Ea b/ the spike shapes requires further consideration. Element T1/ K T2/K A1 A2 kJ mol-1 kJ mol-1 Al 2059 2064 0.05 1.71 25000 651 EVect of sheath gas Bi 1301 1307 0.66 0.87 650 257 Cr 1783 1848 0.05 0.55 1010 537 In 1523 1583 0.02 0.30 900 394 Experiments on the observation of temporal oscillations were Mg 1783 1938 0.02 0.81 690 489 performed using helium as a sheath gas for Al and Sn.For Mn 1400 1803 0.02 0.76 700 436 Pb 1057 1103 0.02 0.35 600 268 Al, the spike width in He (Fig. 13) is less than that in Ar Sb 1378 1403 0.04 0.66 1800 329 (Fig. 1b). In contrast, for Sn, the spike width in He (Fig. 14) Sn 1618 1626 0.13 1.28 6250 432 Te 1638 1663 0.54 0.91 470 233 is larger than that in Ar (Fig. 9b). An interesting feature of aEa : apparent activation energy; bEa : calculated for thermal dissociation (e-mode).35 this trace is the appearance of a very sharp spike at 2000 °C.In both cases, the shift of oscillations in He to about 200– 300 °C higher apparent tube wall temperature is connected Table 5 Thermal characteristics of oxides36 with the much higher thermal conductivity of He and, as a Group Oxide Tm/K -DfH°298/kJ mol-1 IIa MgO 3100 601.5 IIIa Al2O3 2327 1675.7 IIIa In2O3 2186 926.3 IVa SnO2 1903 557.6 IVa PbO 1160 218.6 Va Sb2O3 928 699.6 Va Bi2O3 1090 578.2 VIb Cr2O3 2705 1140.6 VIa TeO2 1006 322.0 VIIa MnO 2058 385.4 Table 6 Dependence of spike width on mass of Ala Mass/mg Width/s 10 1.5 20 2.8 40 6.0 80 11.0 100 14 200 26 Fig. 12 Oscillation profile for 3.5 mg Mn evaporated from a pyrolytic aTypical reproducibility is 20–30% RSD. graphite platform at 10 °C s-1 heating rate and 300 ml min-1 internal flow of Ar.37 Fig. 11 First spike of aluminium for (a) 10; (b) 40 and (c) 100 mg Al. J. Anal. At. Spectrom., 1999, 14, 1019–1024 1023tive or phenomenological investigation, with little comprehensive interpretation of the data.The primary purpose of this report was to stimulate further research into this eVect by the analytical community. Acknowledgement Financial support from the Royal Society of Chemistry to B. V. L. for his visit to the Institute for National Measurement Standards, NRCC, is gratefully acknowledged. References Fig. 13 Oscillation profile for Al recorded under conditions given in 1 B. V. L’vov, in Proceedings of the XII Mendeleev Congress on Table 2 (in He).General and Applied Chemistry, Part I, Nauka, Moscow, 1981, p. 286. 2 B. V. L’vov andA. S. Savin, Zh. Anal. Khim., 1982, 37, 2116. 3 B. V. L’vov andA. S. Savin, Zh. Anal. Khim., 1983, 38, 1925. 4 B. V. L’vov andA. S. Savin, Zh. Anal. Khim., 1983, 38, 1933. 5 B.V.L’vov,Dokl. Akad. Nauk SSSR, 1983, 271, 119. 6 B. V. L’vov and L. F. Yatsenko, Zh. Anal. Khim., 1984, 39, 1773. 7 B.V.L’vov,Zh. Anal. Khim., 1984, 39, 1953. 8 B.V.L’vov,Izv. Akad. Nauk SSSR, Ser.Metally., 1984, 5, 3. 9 B.V.L’vov, Dokl. Akad. Nauk SSSR, 1985, 283, 1415. 10 B. V. L’vov, A. S. Savin and L. F. Yatsenko, Zh. Prikl. Spektrosk., 1985, 43, 887. 11 B. V. L’vov, Izv. Vuzov, Chern.Metall., 1986, 1, 4. 12 B. V. L’vov and L. F. Yatsenko, Izv. Vuzov, Chern. Metall., 1986, 5, 1. 13 B. V. L’vov, Kinet Katal, 1986, 27, 1513. 14 B. V. L’vov, Kinet Katal, 1986, 27, 1513. 15 B. V. L’vov, J. Anal. At. Spectrom.,1987, 2, 95. Fig. 14 Oscillation profile for Sn recorded under conditions given in 16 B.V. L’vov, Khim. Zh., 1987, 5, 30. Table 2 (in He). 17 B. V. L’vov, V.G. Nikolaev, A. V. Ilyukhin and H.-Y. Stark, Khim. Tverd. Topliva, 1988, 1, 114. 18 B. V. L’vov, V.G. Nikolaev, A. V. Ilyukhin and H.-Y. Stark, Khim. result, with the diVerence in the real tube temperature in Tverd. Topliva, 1988, 1, 118. diVerent gases at the same input power. 19 B. V. L’vov and A. V. Ilyukhin, Izv. Vuzov, Chern. Metall., 1988, 9, 10. EVect of Sr addition 20 B.V. L’vov, L. K. Polzik and A. I. Yuzefovskii, Zh. Anal. Khim., 1989, 44, 794. As noted earlier,34 addition of Sr greatly stimulates temporal 21 B. V. L’vov, Spectrochim. Acta, Part B, 1989, 44, 1257. oscillations in the process of carbothermal reduction of Al 22 B. V. L’vov, L.K. Polzik, N. P. Romanova and A. I. Yuzefovskii, J. Anal. At. Spectrom., 1990, 5, 163. and Mn oxides in a spatially isothermal transverse-heated 23 B. V. L’vov and N. P. Romanova, Zh. Prikl. Spectrosk., 1990, graphite atomizer (THGA).The frequency of oscillations and 53, 664. their regularity in the presence of a small addition of Sr (as 24 B. V. L’vov and N. P. Romanova, Zh. Anal. Khim., 1990, 45, 506. its nitrate) were much higher than those in the absence of Sr. 25 B. V. L’vov, L. K. Polzik and A. I. Yuzefovskii, Zh. Anal. Khim., Such comparative experiments were repeated with all the 1990, 45, 920. 26 B. V. L’vov and N. P. Romanova, Zh. Anal. Khim., 1991, 46, 461.elements using non-isothermal HGA tubes. The same stimula- 27 B. V. L’vov, L. K. Polzik and N. P. Romanova, Zh. Anal. Khim., tion of oscillations was observed for Al (Fig. 1) and Mn 1991, 46, 837. (Fig. 6) but no eVect of Sr on oscillations for other elements 28 K. E. Ohlsson and W. Frech, Spectrochim. Acta, Part B, 1991, was evident. No reasonable explanation for this can be 46, 559. advanced at this time. 29 J. A. Holcombe, D. L. Styris and J. D. Harris, Spectrochim. Acta, Part B, 1991, 46, 629. 30 A. Kh. Gilmutdinov, Yu. A. Zakharov, V. P. Voloshin and K. Conclusions Dittrich, J. Anal. At. Spectrom., 1992, 7, 675. 31 D. A. Katskov, A. M. Shtepan, I. L. Grinshtein and A. A. The most significant result of these investigations is the obser- Pupushev, Spectrochim. Acta, Part B, 1992, 47, 1023. vation of temporal oscillations in the kinetics of carbothermal 32 K. E. Ohlsson, E. Iwamoto, W. Frech and A. Cedergren, Spectrochim. Acta, Part B, 1992, 47, 1341. reduction of oxides of diVerent elements from several Groups 33 M.M. Lamoureux, C. L. Chakrabarti, J. C. Hutton, A. Kh. of the Periodic Table. The mechanism of gaseous carbide Gilmutdinov, Yu. A. Zakharov and D. C. Gregoire, Spectrochim. reduction, which was applied to Al2O3 reduction, can thus be Acta, Part B, 1995, 50, 1847. extended to the oxides of many other elements (Bi, In, Mn, 34 B. V. L’vov, Spectrochim. Acta, Part B, 1996, 51, 533. Pb, Sb, Sn, Te, etc.), suggesting that the formation of gaseous 35 B. V. L’vov, Spectrochim. Acta, Part B, 1997, 52, 1. 36 Thermodynamic Properties of Individual Substances, ed. L. V. carbides is a much more typical phenomenon than generally Gurvich, G. A. Khachkurusov and V. A. Medvedev, Nauka, accepted. The most eYcient approach to verify this implication Moscow, 1978–1982. is the investigation of the gas phase composition by means of 37 L. F. Yatsenko, Dissertation, Leningrad State University, 1985. mass spectrometry, as so clearly demonstrated more than 10 38 D. L. Styris and D. A. Redfield, Anal. Chem., 1987, 59, 2891. years ago by Styris et al. with oxides of Al,38 Be,39 Se,40 Ga 39 D. L. Styris and D. A. Redfield, Anal. Chem., 1987, 59, 2897. 40 D. L. Styris, Fresenius’ Z. Anal. Chem., 1986, 323, 710. and In,41 and Mg, Ca, Sr and Ba.42 The observation of an 41 D. L. Styris, Personal communication, 1989. over-equilibrium concentration of gaseous carbides in the 42 L. J. Prell, D. L. Styris and D. A. Redfield, J. Anal. At. Spectrom., process of carbothermal reduction of metal oxides is a result 1991, 6, 25. of fundamental importance and deserves further and more intensive investigation. Paper 9/01850F It is recognised that this study has been primarily a descrip- 1024 J. Anal. At. Spectrom., 1999, 14, 1019–1024

ISSN:0267-9477    

DOI:10.1039/a901850f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Atomization interferences in ICP atomic absorption spectrometry† Carl E. Hensman‡ and Gary D. Rayson* Department of Chemistry and Biochemistry, Box 30001 MSC 3C, New Mexico State University, Las Cruces, NM 88003, USA Received 6th February 1999, Accepted 12th May 1999 ICP atomic absorption spectrometry (ICP-AAS) has been presented as a possible solution for the analysis of complex samples. Unfortunately, the performance of an earlier configuration was limited as to the elements that could be determined with favorable figures of merit.The present study was undertaken to ascertain those fundamental parameters responsible for these limitations. The central channel through the plasma using a torch with an enlarged sample introduction tube (6.25 mm id) was found to exhibit lower Ar and Fe excitation temperatures than predicted. The presence of water vapor was also determined to have a significant impact on analytical signals from within the central channel-viewing region.The production of atomic species for detection by absorption was found to be strongly influenced by both analyte molecular bond strengths and the first ionization potential. for inductively coupled plasma atomic absorption spec- Introduction trometry (ICP-AAS) measurements.23–27 However, the con- An inductively coupled plasma atomic absorption (ICP-AA) clusion of these early studies was that, although ICP-AAS is technique has recently been developed utilizing a novel optical feasible, the energetic plasma was generally more suitable configuration allowing spatial dispersion of the discharge for AES.image.1 The results have been very promising with figures of The ICP torch presented in this ICP-AA study has a sample merit at least rivaling those of conventional lateral viewing introduction channel of 6.25 mm id. As previously mentioned, ICP atomic emission spectrometry (ICP-AES). Unfortunately, this may exaggerate cooling eVects within the plasma discharge this level of performance was not observed for all metals.This resulting in poor response reported for some analytes. To lack of response has been proposed to be associated with a understand the possible eVects of this potential cooling probcooling of the plasma central channel, thus inhibiting the lem, physical characterization of this unique torch is required. formation of the free atoms.1 The possible cooling eVect may This paper describes the results of those investigations.also be exaggerated by the large sample introduction tube (6.25 mm id) utilized for the ICP-AA technique. This study adopts a mechanistic approach to investigate the reasons for the lack of response from certain analytes to the ICP-AA Experimental technique. Such an understanding of the possible mechanisms Calculations involved may then enhance the utility of this technique. Conventional ICP torches have been studied for many years. Atomic iron was selected as a thermometric species because it These studies have resulted in an understanding of many of demonstrated desirable characteristics.17 Although there is a the processes occurring within the discharge.Unfortunately, a significant diVerence in the number of lines used for such complete understanding of this dynamic source remains elus- studies in the literature, six lines of atomic iron were selected. ive.2–14 However, many of the physical characteristics of the It was found that the standard error encountered using these discharge (both axially and radially) have been meas- Fe emission lines for the temperature determinations became ured.2–6,15,16 The acquisition of spatial profiles of the plasma unacceptable in areas of high background emission, such as emission has enabled the assessment of optimum conditions the toroidal ring of the plasma.To allow the satisfactory for the plasma discharge as well as insight into the atomization, calculation of excitation temperature within these regions, excitation and inter-element interference mechan- argon was also chosen as a thermometric species.The paramisms. 8,13,14,17–21 Among several fundamental ICP properties eters of the corresponding lines for Fe and Ar used for these that have been studied is the spatially resolved excitation calculations are listed in Table 1. The excitation (spectroscopic) temperature of the plasma. temperatures were determined from a logarithmic form of the Historically, Wendt and Fassel first suggested the use of the Boltzman equation.29 ICP as an atom reservoir for AAS.22 Successful absorption measurements were accomplished in that study by using multipass optics perpendicular to the plasma discharge. Several Ln A Ilki ( gkAki)B=A Ek (kTEB (1) other instrumental configurations have since been investigated where I is the relative intensity of a specific line, lki is the †Presented at the 1998 Winter Conference on Plasma wavelength, gk is the statistical weight of the upper energy Spectrochemistry, Scottsdale, AZ, USA, January 5–10, 1998.level for that transition, Aki is the transition probability, k is ‡Present address: Department of Geological Sciences, 275 Mendenhall the Boltzman constant (0.694 cm-1), Ek is the energy of the Lab, 125 South Oval Mall, Ohio State University, Columbus, OH 43210, USA. excited state, and TE is the excitation temperature. A plot of J. Anal. At.Spectrom., 1999, 14, 1025–1031 1025Table 1 Parameters used for excitation temperature calculations, in study. The output of the hollow cathode lamp (HCL) is first conjunction with eqn. (1) collimated by a lens, L1 (focal length 150 mm). This collimated radiation is directed through the center of a modified ICP l/nm E/cm-1 gA19,28 torch with a 6.25 mm id sample introduction tube. The second lens, housed in a cooling mount, L2 (focal length 300 mm), Iron 371.99 26 875 1.793 373.49 33 695 4.430 then focuses the image of the hollow cathode onto the entrance 381.58 38 175 6.636 slits of a 0.3 m focal length monochromator (Varian, Techtron) 382.44 26 140 0.2210 with a bandpass of 0.5 nm.The plasma discharge was pos- 360.89 35 856 3.985 itioned at a distance from L2 much less than its focal length 358.21 34 844 13.39 (approximately 125 mm). This results in the dispersion of the Argon 425.1 116 660 0.0243 light from the plasma discharge, thus minimizing the amount 425.9 118 871 0.3540 of light from the ICP illuminating the photomultiplier tube 426.6 117 184 0.1070 (PMT) while maximizing the throughput of light from the 430.0 116 999 0.1675 HCL. 433.3 118 419 0.2515 Sample introduction into the ICP torch was achieved using a Meinhard-type concentric glass nebulizer with a concentric Scott-type spray chamber. The sample solution was introduced ln A Ilki ( gkAki)B against Ek allows the application of a best-fit at 1 mL min-1 using a peristaltic pump (Model Rabbit, Rainin Instrument Co.).straight line with a slope inversely proportional to TE. Absorbance measurements were calculated from four separate measurements. A blank sample was introduced into Materials the plasma and plasma emission (Ice) and plasma emission Stock solutions of each metal, 1000 mg L-1, were prepared plus lamp emission (Icl) were initially recorded. Each sample by dissolution of the reagent grade nitrate salt in distilled, was then introduced into the plasma and similar measurements de-ionized water with 1% trace pure nitric acid.All standard were made yielding the values with (Icls) and without (Ices) the solutions were prepared daily by serial dilution. HCL signal. Subtraction of the measurements Icl-Ice yielded the intensity of the incident radiation (I0) and Icls-Ices results Instrumentation in the calculation of the transmitted radiation intensity (I ). Applying the Beer–Lambert law, The ICP-AAS configuration has been discussed in detail elsewhere.1 Table 2 summarizes the parameters used for the Abs=-log AIcls-Ices Icl-Ice B (2) ICP.Fig. 1 depicts the instrumental configuration used in this the absorbance (Abs) by ground state analyte atoms was thus Table 2 Excitation temperature calculated using iron as a thermocalculated. metric species Emission measurements were conducted on the same Applied power/W Excitation temperature/K instrumental set-up as the absorption measurements with three modifications.The HCL and collimating lens (L1) were 500 3833 removed. An imaging lens and an adjustable iris replaced the 600 4019 focusing lens (L2) and cooling mount. Finally, the imaging 700 4176 lens (focal length 98 mm) was positioned 289 mm from the 800 4312 900 4432 top of the load coil and 149 mm from the monochromator 1000 4540 entrance slits. Magnification of the image on the entrance slit 1100 4637 was 151.94. Thus, the portion of the image passing through 1200 4726 the 100 mm entrance slit of the monochromator corresponded 1300 4807 to a region of the discharge with a width of 52 mm. All 1400 4883 emission measurements (except for those using argon as the 1500 4953 1600 5019 thermometric species) were background-corrected with the 1750 5111 introduction of a blank into the plasma.Results and discussion Viewing regions Three diVerent viewing methods of the plasma discharge were employed during the study for the calculation of excitation temperatures: radially resolved emission (temperature) measurements, integrated emission measurements, and integrated absorbance measurements.Fig. 2(a) shows a typical temperature profile of the plasma as a function of radial position. For these measurements, the torch was aligned concentrically along the optical axis and then moved in increments of 0.27 mm (significantly greater than the calculated spatial resolution of the system) across the optical axis for each set of operating conditions. Fig. 2(b) shows a typical intensity profile from integrating the central viewing region of the plasma as a function of the applied forward rf power. The Fig. 1 Schematic representation of optical configuration for solid vertical line in Fig. 2(a) indicates the viewing region of absorption measurements: HCL, hollow cathode lamp; L1, quartz the discharge. This method was also used for all analyte collimating lens (focal length 150 mm); L2, quartz camera lens (focal length 300 mm); FS, field stop; MC, monochromator.emission measurements. Fig. 2(c) shows a typical absorbance 1026 J. Anal. At. Spectrom., 1999, 14, 1025–1031Fig. 3 Calculated excitation temperatures as a function of radial position from the torch center. (a) Argon as the thermometric species. (b) Iron as the thermometric species. ($) 600 W, (,) 1000 W and Fig. 2 (a) Radial profile of argon excitation temperatures; 0 mm (&) 1400W applied rf power.indicates the center of the axially oriented torch. (b) Mg atomic emission (285.2 nm) as a function of Fe excitation temperature integrated throughout the region indicated by the solid line in (a). (c) Ag atomic absorbance (328.1 nm) as a function of Fe excitation metric species. Again, it can be seen that the measured temperature integrated throughout the region indicated by the dotted excitation temperatures with 600 and 1000 W applied power line in (a). exhibited very similar radial profiles.At 1400 W, an excitation temperature increase was observed much closer to the center profile also from the central viewing region of the plasma as of the torch than was indicated using Ar as the thermometric a function of applied power. Incident radiation from the species. If the measured temperatures within the emission HCL is passed through the area indicated by the dotted line viewing region (i.e., the center of the discharge) for the 600 in Fig. 2(a) and then focused onto the slits of the and 1400 W power conditions were integrated, a relative monochromator. excitation temperature increase of 978 K was calculated. It should be noted that the relative excitation temperature Temperature determinations increase between 600 and 1400 W for the fitted data (Table 2) was 864 K. The relationship between calculated excitation temperature A region of constant argon excitation temperatures as a and applied power using iron as the thermometric species was function of viewing position was observed in the center of the found to follow a curve-fit logarithmic relationship: plasma.This was approximately the same radius as the sample TE=1020ln (P)-2506.2 (3) introduction channel. This suggests that the sample introduction channel has created a cooler region within the plasma where P is the indicated applied rf power in Watts. It should discharge. However, the iron excitation temperature radial be emphasized that this is an empirical relationship and profiles indicated that this cooling eVect was limited.This facilitated the application of temperatures instead of applied limited cooling eVect hypothesis is also supported by the powers in subsequent discussions. The calculated excitation distribution of magnesium ion to atom emission ratios (Mg temperatures are presented in Table 2. II/I ) across the plasma discharge. Fig. 4 shows these Mg I, Mg II and Mg II/I distributions. Radial profiles As the applied power was increased, magnesium ion production within the plasma increased [Fig. 4(a)]. The mag- Fig. 3(a) shows the radial temperature profile of the discharge using argon as the thermometric species for three diVerent nesium ion intensity was observed to be highest near the inside skin of the toroidal ring and to decrease towards the center power levels (600, 1000 and 1400 W). The excitation temperature of the central portion of the discharge was found to be of the torch.As expected this decrease in Mg II was mirrored by an increase in Mg I [Fig. 4(b)]. At 600 W, the Mg I was similar in magnitude for all three powers. Unexpectedly, as the high-energy plasma of the toroidal ring is approached, the reasonably constant across the center portion of the discharge, only decreasing in the area of higher magnesium ion emission. response of 600 and 1000 W applied power continued to be similar. At 1400 W, a relative increase in the excitation tem- The application of 1000 Wpower showed a significant increase in Mg II intensity towards the center of the discharge while perature was observed closer to the plasma center.The radial profile at 1400 W also showed an overall increase in the maintaining the same Mg I intensity towards the toroidal ring inner skin as for 600 W. Applying 1400 W of power yielded excitation temperature in the region of the toroidal ring. Fig. 3(b) shows the radial temperature profile of the discharge relatively consistent atom emission intensity across the discharge center.This was similar to the atom emission intensities for each of these three power levels using iron as the thermo- J. Anal. At. Spectrom., 1999, 14, 1025–1031 1027sion intensity decreased significantly at about 3 mm from the plasma center. The Ba I and Fe I signals exhibited emission maxima at that same location. It is suggested, from the above discussion, that the sample introduction channel creates an environment of low energy at the center of the discharge.At the same time a high-energy environment is created at the inner skin of the toroidal ring. This results in a non-uniform energy environment across the central channel region, resulting in analyte-dependent regions of maximum excitation. An alternative explanation for variations in emission intensities across the torch would involve a variation in the number densities of species within this region.However, such a mass transportation explanation would predict similar behavior for diVerent analytes. This was not observed (Fig. 5). Even so, the impact of such number density variations should be considered in the interpretation of these data. Mercury emission Fig. 6(a) demonstrates relative emission intensities for Hg I using wet aerosol introduction. As the applied power is increased the emission becomes stronger and moves towards the center of the torch. The profile shape, although similar to Fe I and Ba I, is much closer to the inner skin of the toroidal ring. This suggests that a higher energy environment is required to excite mercury than with any of the previously discussed analytes.Introducing atomic mercury into the plasma as a dry aerosol removes the solvent eVects on the analyte.30–32 Clearly, Fig. 4 (a) Mg II (279.5 nm); (b) Mg I (285.2 nm); (c) Mg II/Mg I as the highest emission is seen to be at the center of the plasma a function of radial position from the torch center.($) 600 W, (,) torch [Fig. 6(b)]. Because the presence of water vapor would 1000 W and (&) 1400W applied rf power. not be predicted to impact the distribution of the independently formed Hg vapor within the discharge, this suggests that the solvent is suppressing the production of excited analyte atoms at the center at 1000 W. Combining the information from ion at the center of the torch. In other words, the solvent may be and atom emission results in the magnesium ion-to-atom ratio the primary source of the previously discussed reduced energy [Fig. 4(c)] the conditions of 600 and 1400 W applied power at the center of the plasma torch. However, this is not seen in had nearly non-variant profiles across the discharge center. An applied power of 1000 W resulted in a similar profile, as seen in Fig. 3(b). The center of the discharge had a similar ion and atom emission intensity as a lower power plasma. As the inner skin of the plasma was approached, ion and atom emission intensities increased until a value similar to that experienced with a higher power plasma was realized.Fig. 5 clearly indicates regions of emission across the plasma discharge for the radial profiles of Ag I, Cd I, Ba I, Fe I and Ar I. The Ag I and Cd I emission signals are at a maximum at the center of the channel. This indicates that a suYcient amount of energy is available for atom excitation. This emis- Fig. 6 Relative emission intensities of Hg I (253.7 nm) as a function Fig. 5 Normalized emission intensity for (,) Ag I (328.1 nm), (&) of radial position from the torch center. (a) Hg introduced as a wet aerosol. (b) Hg introduced as a dry aerosol. ($) 600 W, (,) 1000W Cd I (228.8 nm), ($) Ba I (553.5 nm), (2) Fe I (344.1 nm) and (+) Ar I (545.1 nm) as a function of radial position from the torch center. and (&) 1400W applied RF power. 1028 J. Anal. At. Spectrom., 1999, 14, 1025–1031Fig. 7 Calculated excitation temperatures as a function of radial position from the torch center, using Ar as the thermometric species.With ($) and without (,) a water aerosol at 1000W applied power. the radial excitation temperature profiles of the plasma under dry and wet vapor conditions (Fig. 7). The dry metal vapor data show an increase in excitation temperature as compared with the wet atomic vapor. It also has a significantly broader region of high excitation temperature towards the center of the plasma torch.If Hg I was to agree with the argon excitation temperature studies for the dry vapor conditions, an emission maximum should be seen at 2 mm from the torch center. This was not observed [Fig. 6(b)]. It can therefore be concluded Fig. 8 Power profiles of atomic Hg emission and absorbance that the solvent aVects the plasma by more than a simple (253.7 nm) with (,) and without ($) a water aerosol as a function reduction of energy or ‘cooling’ across the radius of the sample of integrated Fe excitation temperatures (based on applied power introduction channel.Because of the method of introduction levels). of iron into the plasma (i.e., as a solution aerosol ), it was not possible with this sample introduction system to use this Table 3 Values for the dissociation of the metal oxide (MO) bond thermometric species to measure plasma temperatures for the and first ionization potential (IP) of the metal with the previously reported1 limits of detection using ICP-AAS dry vapor condition.Use of argon as the thermometric species did not suVer from this limitation. However, power-consistent Dissociation energy Limit of measurements of temperatures in the central channel of the Element of MO/eV33 IP/eV33 IP/MO detection1,a plasma were not obtained [Fig. 3(a)]. This complex energy reduction was further experienced Ag 1.4 7.57 5.4 1.84 when viewing the emission and absorption of mercury with Al 5.0 5.98 1.2 1120 Be 4.6 9.32 2.0 respect to excitation temperature (applied power) increase Cd 3.8 6.11 1.6 2.19 (Fig. 8). Emission intensity for the wet aerosol plasma initially Co 3.7 7.86 2.1 4.51 increased with plasma temperature similarly to the dry metal Cr 4.2 6.76 1.6 14.5 vapor condition. At about 4200 K (700–800W), the emission Cu 4.9 7.72 1.6 2.47 intensity with the wet aerosol increased more significantly than Fe 4.0 7.87 2.0 14.4 for the dry aerosol plasma condition until a plateau region Mn 4.0 7.43 1.9 1.71 Ni 4.3 7.63 1.8 7.50 was exhibited by each at approximately 5100 K (1600– Pb 4.1 7.4 1.8 14.7 1750 W).Interestingly, while the dry aerosol plasma showed Tl <3.9 6.11 >1.6 67.7 a strong increase in absorbance as the excitation temperature Zn 4.0 9.39 2.4 1.88 was increased, the wet aerosol plasma remained constant. The Hg 2.29 10.44 4.6 discrepancy between the wet aerosol plasma emission and Nd 7.4 5.45 0.74 N.D.b absorbance is suggested to result from the sensitivity of U 7.7 6.1 0.79 N.D.W 7.98 7.2 0.90 N.D. mercury emission as compared with that of absorption Y 7 6.51 0.93 N.D. measurements in the plasma. aAll detection limits are indicated in units of ng mL-1. bN.D. indicates that this metal was not detected by atomic absorption in the ICP. Temperature profiles Previously, it has been indicated that certain analytes exhibited poorer limits of detection by the ICP-AAS technique.1 Table 3 with a wet aerosol as applied power was increased.There is an initial increase in absorbance followed by a decrease in lists both the first ionization potentials and metal oxide bond strengths for a set of representative analytes and their pre- absorbance resulting in a characteristic curve. The turning point of the curve has been observed to be analyte-dependent. viously reported limits of detection.1 In particular cases the metal’s oxide bond strength is greater than its first ionization This behavior is strongly indicative of an initial low-energy limited process, i.e., the conditions for atomization of the potential (further indicated by a IP/MO less than one).Interestingly, analytes with a IP/MO value of less than one analyte from its metal oxide become more favorable with increasing applied power. A high-energy limited process then correlate well with those analytes that exhibit poorer figures of merit by the ICP-AAS technique. Fig. 9 shows typical follows this, i.e., a reduction of atomic analyte as the applied power increases due to the formation of the corresponding absorbance and emission profiles for an analyte introduced J. Anal. At. Spectrom., 1999, 14, 1025–1031 1029Fig. 9 Typical atomic absorbance (,) and emission ($) profiles for Fig. 11 Normalized Cd ion (,, 214.4 nm) and atom ($, 228.8 nm) Ag (328.1 nm) as a function of integrated Fe excitation temperatures. absorbances as functions of integrated Fe excitation temperatures. ion.(It should be noted that although this discussion uses discussed assumptions, that as the applied power is increased analyte oxides as a starting component, the authors acknowl- there should also be a marked increase in ion absorption. edge that the analyte may be initially present as other molecular Fig. 11 demonstrates this increase in ion absorption (even species.) Those target analytes whose ionization potentials are though the instrument is not presently optimized for ion less than the metal oxide bond strengths (i.e., IP/MO is less absorption measurements).than one) would then be predicted to experience conditions not favorable for atomization as the applied power increases. At the point where there would be suYcient energy to ionize Summary the analyte, the metal oxide bond will not have been broken. Under applied power conditions where the metal can be An instrumental configuration for ICP-AAS has been eYciently atomized, there would then be more than suYcient described.This configuration has been noted to be unsatisfacenergy for the eYcient ionization of the analyte. In this tory for specific metals. The present study has addressed the manner, it is conceivable that there would never be a suYcient problem of these poorly responding metals, particularly in number density of analyte atoms present to yield a detectable relation to the unique sample introduction diameter of the absorption signal.[ This may account for the lack of a signal plasma torch (6.25 mm id). when viewing mercury in a wet aerosol, Fig. 7(b).] Previous Initial studies were made associating the applied power to eVorts1 to measure the atomic absorption signal for each of the spectroscopic excitation temperature for the plasma disseveral elements that are in this category (i.e., Nd, U, W and charge. These resulted in a logarithmic fit of the applied power Y) yielded no discernible signal, as would be predicted from to the temperature. Radial profile studies of the thermometric their respective IP/MO ratios of less than 1.0 (i.e., 0.74, 0.79, species (iron and argon) within the plasma discharge in 0.90 and 0.93, respectively).combination with radial studies of magnesium atom and ion Aluminum has an IP/MO ratio closer to one than most emission suggest a non-uniform energy environment within other detectable analytes (Table 3). It also demonstrates a the viewing region utilized for the atomic absorption studies.poor response to the ICP-AAS technique.1 As the applied This environment extends to an approximate 3 mm radius power was increased while monitoring aluminum, it can be from the torch/discharge center, with the center being of lowest seen (Fig. 10) that there is no significant signal response until energy and that energy increasing as the inner skin of the about 4700 K (1200 W). This response is supportive of the toroidal ring is approached.It was demonstrated that the lack above explanation of the poor sensitivity for aluminum. The of analyte atom number density in the absorption-viewing refractory nature of aluminum oxide means that a relatively region was predominant due to the analyte solvent. However, higher energy environment is required to dissociate the it is suggested that solvent is not solely responsible for the metal–oxygen bond. It also would be expected, under the ineYcient atomization; rather, the problem lies with a combination of eVects caused by the diameter of the sample introduction channel.These eVects may also be due to increased gas expansion in the plasma, ineYcient rf coupling and ineYciency of excitation species mass transport from the plasma. Further studies are required to characterize the contributions mentioned and will be the subject of future papers. It is suggested that atomic absorption measurements are limited by atom formation at lower applied powers due to the presence of molecular analyte species and ion formation limits the atom number density at higher applied powers.This would result in the inability to observe those metals whose molecular bond strengths are greater than the first ionization potential. The correlation between an ionization potential-to-metal oxide bond strength ratio less than one and the non-detectable metals was demonstrated. This was reinforced by the applied power profile of aluminum not showing a relatively reasonable signal for both emission and absorption until 4700 K (1200 W), probably due to the aluminum refractory species.Fig. 10 Al atomic absorbance (,) and emission ($, 308.2 nm) as a function of integrated Fe excitation temperatures. This is consistent with the need for the higher temperatures 1030 J. Anal. At. Spectrom., 1999, 14, 1025–103112 J. A. M. van der Mullen, A. C. A. P. van Lammeren, attainable with a N2O–C2H2 flame as compared with the more D.C. Schram, B. van der Sijde and H. J. W. Schenkelaars, common air–C2H2 flame-gas mixture. Spectrochim. Acta, Part B, 1987, 42, 1039. Finally, it is suggested that to increase the analyte response 13 S. Nowark, J. A. M. van der Mullen and D. C. Schram, either dry aerosol introduction should be used to increase Spectrochim. Acta, Part B, 1988, 43, 1235. atomization eYciency (i.e., analyte desolvation or electrother- 14 M. Huang, D. S. Hanselman, P.Yang and G. M. Hieftje, Spectrochim. Acta, Part B, 1992, 47, 765. mal vaporization), or ion absorption measurement should 15 N. N. Sesi, D. S. Hanselman, P. Galley, J. Horner, M. Huang and also be employed in conjunction with atomic absorption G. M. Hieftje, Spectrochim. Acta, Part B, 1997, 52, 83. measurements within the plasma. 16 J. M. de Regt, F. P. J. de Groote, J. A. M. van der Mullen and D. C. Schram, Spectrochim. Acta, Part B, 1996, 51, 1371. 17 D. J. Kalnicky, R.N. Kniseley and V. A. Fassel, Spectrochim. Acknowledgements Acta, Part B, 1975, 30, 511. 18 D. J. Kalnicky, V. A. Fassel and R. N. Kniseley, Appl. Spectrosc., The authors thank Sandia National Laboratories for the 1977, 31, 137. donation of the inductively coupled plasma. Also, financial 19 H. Kawaguchi, T. Ito and A. Mizuike, Spectrochim. Acta, Part B, support from the National Science Foundation (grant 1981, 36, 615. #CHE-9312219) and the New Mexico Water Resources 20 L. M.Faires, B. A. Palmer, R. Engleman and T. M. Niemczyk, Research Institute (grant USGS #14–08–0001-G2035) is grate- Spectrochim. Acta, Part B, 1984, 39, 819. 21 G. R. Kornblum and L. de Galan, Spectrochim. Acta, Part B, fully acknowledged. 1974, 29, 249. 22 R. H. Wendt and V. A. Fassel, Anal. Chem., 1966, 38, 337. 23 C. Veillon and M. Margoshes, Spectrochim. Acta, Part B, 1968, References 23, 503. 24 S. Greenfield, P. B. Smith, A. E. Breeze and N. M. D. Chilton, 1 C. E. Hensman, J. F. Mihalic and G. D. Rayson, Anal. Commun., Anal. Chim. Acta, 1968, 41, 385. 1997, 34, 355. 25 J. M. Mermet and C. Trassy, Appl. Spectrosc., 1984, 38, 876. 2 J. W. Olesik and K. R. Bradley, Spectrochim. Acta, Part B, 1987, 26 G. D. Rayson and D. Y. Shen, Spectrochim. Acta, Part B, 1991, 42, 377. 46, 1237. 3 P. B. Farnsworth, Spectrochim. Acta Rev., 1991, 14, 447. 27 M. A. Mignardi, B. T. Jones, B. W. Smith and J. D. Winefordner, 4 P. B. Farnsworth, D. A. Rodham and D. W. Ririe, Spectrochim. Anal. Chim. Acta, 1989, 227, 331. Acta, Part B, 1987, 42, 393. 28 I. Rief, V. A. Fassel and R. N. Kniseley, Spectrochim. Acta, Part 5 A. F. Parisi, G. D. Rayson, G. M. Hieftje and J. W. Olesik, B, 1973, 28, 105. Spectrochim. Acta, Part B, 1987, 42, 361. 29 S. Nakamura, J. Anal. At. Spectrom., 1995, 10, 467. 6 J. W. Olesik, Spectrochim. Acta, Part B, 1990, 45, 975. 30 I. Novotny, J. C. Farinas, W. Jia-liang, E. Poussel and 7 J. M. de Regt, F. P. J. de Groote, J. A. M. van der Mullen and J. M. Mermet, Spectrochim. Acta, Part B, 1996, 51, 1517. D. C. Schram, Spectrochim. Acta, Part B, 1996, 51, 1527. 31 J. W. Olesik, J. A. Kinzer and B. Harkleroad, Anal. Chem., 1994, 8 D. S. Hanselman, N. N. Sesi, M. Huang and G. M. Hieftje, 66, 2022. Spectrochim. Acta, Part B, 1994, 49, 495. 32 D. G. J. Weir and M. W. Blades, Spectrochim. Acta, Part B, 1994, 9 F. H. A. G. Fey, D. A. Benoy, M. E. H. van Dongen and 49, 1231. J. A. M. van der Mullen, Spectrochim. Acta, Part B, 1995, 50, 51. 33 Analytical Chemistry Handbook, ed. J. A. Dean, McGraw-Hill, 10 H. U. Eckert, Spectrochim. Acta, Part B, 1985, 40, 145. New York, 1995, pp. 7.13–7.14. 11 J. A. M. van der Mullen, I. J. M. Raaijmakers, A. C. A. P. van Lammeren, D. C. Schram and B. van der Sijde, Spectrochim. Acta, Part B, 1988, 43, 317. Paper 9/01071H J. Anal. At. Spectrom., 1999, 14, 1025–1031 1031

ISSN:0267-9477    

DOI:10.1039/a901071h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Computer-assisted SIMPLEX optimisation of an on-line preconcentration system for determination of nickel in sea-water by electrothermal atomic absorption spectrometry Ma Teresa Siles Cordero, Elisa I. Vereda Alonso, Pedro Can�ada Rudner, Amparo Garcý�a de Torres and Jose� M. Cano Pavo�n* Department of Analytical Chemistry, Faculty of Sciences, University of Ma�laga, E-29071, Ma�laga, Spain Received 22nd February 1999, Accepted 28th April 1999 An original on-line preconcentration system for the determination of nickel by electrothermal atomic absorption spectrometry is achieved by replacing the sample tip of the autosampler arm by a microcolumn packed with a silica gel chelating resin functionalised with 1-(di-2-pyridyl )methylene thiocarbonohydrazide.The modification of the autosampler in the tubing line and circuit allowed either the flow of the sample through the column or the operation of the autosampler in the normal mode, where microlitres of HNO3 2 M, which acts as the elution agent, pass through the microcolumn, eluting Ni(II), which is directly deposited in the graphite tube as a drop of a precisely defined volume.Optimum graphite furnace operating conditions were sought by software which integrates the SIMPLEX optimisation method and quality requirements such as sensitivity and precision. The optimised method has two linear calibration ranges, between 0 and 1 ng ml-1 and between 1 and 5 ng ml-1, with a detection limit of 0.06 ng ml-1 and a throughput of 36 samples h-1 for 60 s of preconcentration time.The accuracy of the method was examined by the analysis of a certified reference material and by determining the analyte content in spiked seawater and synthetic sea-water. The results show suYciently high recoveries. The accurate determination of trace and ultratrace elements resins as packing materials is simpler and less time consuming than other options. Chelating resins, such as Chelex-100,4–7 in sea-water is one of the most important and challenging tasks in analytical chemistry. The determination of heavy Muromac A-1,8–10 quinolin-8-ol,5,11–14 poly(dithiocarbamate) 15,16 and diethyl dithiophosphate on C18 bonded silica,17 metals is often required for the routine monitoring of marine environmental pollution and for ocean modelling programmes.have been used for the enrichment of natural waters and biological materials prior to their measurement by flame Although electrothermal atomic absorption spectrometry (ETAAS) has very low detection limits for trace metals in atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectrometry (ICP-AES) and oV-line aqueous solution,1 the direct determination of trace metals in sea-water by ETAAS is diYcult even with sophisticated back- ETAAS.On-line flow injection column preconcentration in atomic spectrometry was reviewed by Fang et al.18 Based on ground correction and chemical modification.This is due to the low concentrations and strong interference from the sample solid-phase extraction with C18 silica gel, Sperling and co-workers2,3,19,20 modified the on-line flow injection system matrix. ETAAS with on-line sorbent extraction separation and preconcentration can solve these problems and lead to easy of a flame atomic absorption spectrometer to achieve feasible determinations with ETAAS. Liu and Huang used C18 silica determination.Flow injection (FI ) as a microsample introduction system gel and ammonium pyrrolidine dithiocarbamate (APDC) for the determination of Cu, Cd and Pb21,22 and Muromac A-1 oVers some distinct advantages over manual batch-type procedures, such as fully automated sample management and for Cu and Mo in sea-water.23 Yan et al. employed APDC for the determination of Sb in sea-water24 and diethyl dithi- operation in a closed system. Although FI, as the name implies, handles flowing streams, FI preconcentration and ophosphate for Pb in biological and environmental samples,25 both using a knotted reactor. matrix separation on solid sorbents is a discontinuous process which is appropriate for the discrete, non-flow-through nature In developing an ETAAS method, one needs to adjust many variables (injection volumes, drying, ashing and atomisation of ETAAS.2 While with flow-through detectors the loading time is wasted in waiting for elution, this time can be fitted times and temperatures) in order to establish optimum conditions for the analysis.This can be very time-consuming if a perfectly into the cycle of a furnace programme. The closed nature of the FI system, together with the direct coupling to conventional univariate optimisation is undertaken manually and, where interactions exist between the variables, one is the sample introduction capillary of an autosampler, avoiding any collection of samples in vessels exposed to the environ- unlikely to find the true optimum.26 The problem is greater if the optimisation procedure must be in accordance with the ment, reduces contamination problems to a minimum. As the analyte element is eVectively separated from the matrix and quality policy of a particular laboratory.Of the direct optimisation methods, the SIMPLEX method (first introduced by an almost pure solution is injected into the furnace, chemical modifiers are no longer required, which further reduces con- S.N. Deming and co-workers27,28 in Analytical Chemistry) has been the most commonly used in analytical chemistry tamination problems. On-line reagent purification is another feature of FI which helps to keep blank values at a low level, applications. A deduced alternative that has been widely accepted by analytical chemists was devised by Spendley et al.29 permitting determinations in the ng l-1 range for a large number of elements.3 and modified by Nelder and Mead.30 This useful algorithm is known as the modified SIMPLEX method, MSM; however, Of these systems, column preconcentration using chelating J.Anal. At. Spectrom., 1999, 14, 1033–1037 1033Table 1 Graphite furnace temperature programme (Vi=20 ml ) it optimises a single parameter. Successful use requires the selection of a response function which weights a number of Temperature/ Ramp Hold Argon flow rate/ variables. Recently,31 the possibility of coupling a mathemat- Step °C time/s time/s mlmin-1 ical model with optimisation algorithms was evaluated following an insight into the response function.In this paper, a 1 110 1 13 250 2 130 5 21 250 methodology that can alleviate the above-mentioned diYculties 3 1060 11 22 250 is proposed for the optimisation of the graphite furnace 4 2300 0 5 0 (read 5 s) programme; it is based on the coupling of a descriptive 5 2400 1 4 250 mathematical model of the response function developed by Vereda et al.32 with a SIMPLEX algorithm, which are programmed into software, using quality criteria previously selecby soaking in 0.1 M hydrochloric acid) from coastal surface ted.With this software, seven variables were optimised towards water of La Cala del Moral beach, Ma�laga, Spain, and quality requirements such as sensitivity and precision. analysed in less than 24 h. In this paper, a simple on-line preconcentration system, achieved by replacing the sample tip of the autosampler arm Instrumentation and procedure by a microcolumn packed with a silica gel chelating resin functionalised with 1-(di-2-pyridyl )methylene thiocarbonohy- A Perkin-Elmer (Norwalk, CT, USA) Zeeman/4100 Z1 atomic drazide (DPTH-gel ), is developed for the determination of absorption spectrometer equipped with an AS-70 furnace nickel by ETAAS.The modification of the AS-70 autosampler autosampler was used throughout. Pyrolytic graphite coated in the tubing line and circuit allowed either the flow of the tubes with pyrolytic graphite platforms were used in all sample through the column or the operation of the autosampler experiments.The light source was a nickel hollow cathode in the normal mode, where microlitres of HNO3 2 M, which lamp operated at 25 mA; the wavelength was set to 232.0 nm acts as the elution agent, pass through the microcolumn, with a spectral slit width of 0.2 nm. The optimised graphite eluting Ni(II), which is directly de graphite tube furnace programme, by the SIMPLEX method, is shown in as a drop of a precisely defined volume.The accuracy of the Table 1. The software of the instrument permits the pipetting method was examined by the analysis of a certified reference speed of the pump of the autosampler to be varied from 40% material and by determining the analyte content in spiked sea- to 100%; thus a pipetting speed for the autosampler of 40% water and synthetic sea-water. The results showed good was programmed to increase the contact time of the eluent agreement with the certified value and suYciently high with the microcolumn, which produces a greater eYciency recoveries.of elution. The microcolumn containing the DPTH-gel was a glass tube (3 cm×3 mm id) packed to a height of 0.5 cm; at both ends Experimental of the microcolumn, polyethylene frits (Omnifit, Cambridge, Reagents and samples UK) were fixed to prevent material losses. On the end of this column was placed a piece of sample capillary of the sampler High-purity reagents were employed in all experiments.For arm, in imitation of the sample tip of the sampler arm. Details the synthesis of DPTH-gel, the following were used: silica gel on the design of this microcolumn are given in Fig. 2. Thus (particle size, 0.2–0.3 mm), 3-aminopropyltriethoxysilane and the sample tip of the sampler arm was replaced with this diglutaric aldehyde were purchased from Fluka (Buchs, microcolumn, permitting normal working of the sampler.Switzerland), thiocarbohydrazide and di-2-pyridyl ketone were A peristaltic pump, P (Gilson Minipuls 3, Villiers, France), supplied by Aldrich Chemie (Steinheim, Germany), ethanol fitted with a vinyl pump tube (1.65 mm id), was used for and toluene were obtained from Carlo Erba (Milan, Italy) loading of the sample. A Rheodyne (Cotati, CA, USA) Type and hydrochloric acid was supplied by Merck (Darnstadt, 50 six-port rotary valve was used as a switching valve.Germany). The synthesis and characterisation of DPTH-gel Transport lines were made using 0.5 mm id Teflon tubing. The were described in a previous paper.33 The structure of DPTHFI manifold is shown in Fig. 2. It operated as follows: during gel is shown in Fig. 1. the 1 min sample loading period, a 3.4 ml min-1 flow of sample A stock solution of Ni(II) was prepared from the nitrate (standard or blank) at pH 9.0, buVered with boric acid–NaOH, (Merck) and standardised gravimetrically; standards of is pumped (via P) through the microcolumn ( located in the working strength were made by appropriate dilution as sampler arm); the metal ion is adsorbed on the sorbent required, immediately prior to use.A pH 9 buVer was prepared by mixing boric acid (Merck) 0.2M with NaOH (Merck) until pH 9 was reached. HNO3 (Merck) 2 M was used as eluent. Glycine (Merck) 0.2M was used to mask interferent ions. De-ionised water (18 MV cm-1) was used throughout. The certified reference material was Community Bureau of Reference (BCR) CRM 505 estuarine water.The composition of the synthetic sea-water was (in g l-1): 27.9 of NaCl, 1.4 of KCl, 2.8 of MgCl2, 0.5 of NaBr and 2.0 of MgSO4. Sea-water was collected in poly(propylene) bottles (previously cleaned Fig. 2 Schematic diagram of FI-ETAAS system: P, peristaltic pump; N N N N H N N H H N Si S H O O O Fig. 1 Structure of DPTH-gel. PAAS, AS-70 autosampler pumps; Vs, switching valve. 1034 J. Anal. At. Spectrom., 1999, 14, 1033–1037microcolumn and the sample matrix is sent to waste; then, the precision and sample throughput) calculated as a function of two variables (except for the sample throughput that depends switching valve (Vs) is actuated and the pumps of the AS-70 furnace autosampler, PAAS, are connected, permitting the on a single variable): S=f(slope of the calibration curve, detection limit); C=f(consumption of sample, consumption operation of the autosampler in the normal mode; a wash step takes place with de-ionised water and, immediately after, the of reagent); A=f(relative standard deviation of the slope of the calibration curve, relative standard deviation of the deter- sampler arm lowers the sample capillary into an autosampler cup (filled with eluent) aspirating 20 ml HNO3 2 M; then, the mination); and R=f(time of analysis).Each value of the diVerent variables is subject to preliminary normalisation.sampler arm swings over to the graphite furnace and the tip of the sampler capillary is inserted into the dosing hole of the The algorithm used for the SIMPLEX optimisation was based on the modified SIMPLEX method of Nelder and graphite tube where the eluted Ni(II) is deposited as a drop; the sampler arm then returns to its initial position and the Mead.30 The convergence criterion used in the SIMPLEX algorithm stated that the standard deviation between the cycle of the furnace operation commences (Table 1); while the temperature programme is running, the switching valve is values of the response function should be less than a preset value, fixed by the user at the beginning of the optimisation again turned to start a new loading of the sample (standard or blank); thus, when the spectrometer gives the measurement, procedure.In this study, the value was fixed at 0.12, and the values of the weighting coeYcients were: s=100%; c=75%; the microcolumn is ready for a new injection of eluent.The eluent injection system (normal operation of auto- a=100%; and r=75%. The values of the response function were evaluated by sampler) provokes a flow through the column in both directions, up (when it is aspired) and down (when it is injected means of the construction of a calibration curve with four points [a blank and three standard solutions of 2, 5 and into the graphite tube), avoiding the continuous increase in column compactness. 8 ng ml-1 of Ni(II)].Six blank replicates were measured in order to calculate the detection limit and six replicates of the intermediate standard of 5 ng ml-1 for the calculation of the Software and mathematical model of response function for the optimisation of the ETAAS program relative standard deviation. For the consumption of the sample and reagent, cost (C), the value of 1.0 was given because the The system software, written in C++, is menu-driven and consumption of the sample (optimised in a univariate manner; interactive (Fig. 3). It includes a modified SIMPLEX optimis- 3.4 ml for 60 s loading) and the consumption of the reagent ation procedure and a mathematical model of our response (HNO3 2 M) (varied between 12 and 40 ml ) cannot be function that permits automated optimisation of analytical considered to be important in the cost of the analysis. methods, including all the parameters that define the overall The experimental parameters optimised were: eluent analytical quality, in a weighted way.injection volume (Vi/ml ); hold drying time 1 (td1/s); hold The response function, developed by Vereda et al.32 drying time 2 (td2/s); ashing temperature (Tm/°C); ramp ashing time (rm/s); hold ashing time (tm/s); and atomisation time A= (sS+cC) (aA+rR) 20000 (1) (ta/s). Therefore, a SIMPLEX with eight initial vertices was established. The normalisation conditions and the initial vertiis a weighted linear combination of variables, guiding the ces of the SIMPLEX were determined in preliminary experioptimisation towards diVerent compromise quality require- ments by using a number of systems with extreme values of ments, where s, c, a and r are the weighting coeYcients, called the variables to be optimised.quality coeYcients, which can vary between 0 and 100% and S, C, A and R are analytical properties (sensitivity, cost, Results and discussion Optimisation of the chemical variables In the initial study, the stability of the DPTH-gel resin was studied experimentally in acidic, neutral and basic media by observing any physical change occurring in the material; the results obtained showed that the resin was stable over a wide pH range, namely pH 0–13. Since the solution pH aVects the extent of complexation, which in turn determines the percentage of metal retained by the resin, the preconcentration of nickel from solutions buV- ered at diVerent pH was studied.The pH was adjusted from pH 2.0 to 5.0 using sodium acetate–acetic acid buVer and from pH 5.0 to 11.0 using boric acid–sodium hydroxide buVer.The results (Fig. 4) indicated that the optimum pH range was around pH 6.0 to 9.0. All subsequent studies were carried out at pH 9.0. Nitric acid was chosen as the eluent owing to its eVective elution of the adsorbed analyte complex. The eVect of eluent concentration on the emission signal of 1 ng ml-1 Ni, using a constant volume of injection of eluent of 15 ml, was examined.The signal increased as the HNO3 concentration increased up to 1 M. An HNO3 concentration of 2 M was chosen for subsequent studies. Preliminary tests showed that the sample volume was not an important factor when the mass of analyte arriving at the column was kept constant. The influence of the sample flow Fig. 3 Schematic representation of software. Boxes indicate separate rate was studied using a constant volume of injection of eluent programs; items within a box indicate functions available from that program’s menu.of 15 ml. For this purpose, 17 ng of nickel were brought to J. Anal. At. Spectrom., 1999, 14, 1033–1037 1035Table 2 Performance of the method Linear Detection Calibration range/ limit/ Determination equation r ng ml-1 ng ml-1 limit/ng ml-1 ya=0.047+0.095x 0.992 0–1 0.09 0.45 yp=0.064+0.116x 0.992 0–1 0.06 0.36 ya=-0.01+0.150x 0.998 1–5 yp=0.188+0.078x 0.999 1–5 ya, Absorbance as peak-area; yp, absorbance as peak-height; x, concentration; r, regression coeYcient.function versus the number of experiments at any time of the optimisation. The experimental responses were measured using the peakheight and peak-area. The optimum ranges optimised for the Fig. 4 Influence of pH on the preconcentration of nickel. furnace programme variables using the peak-height were: 15–30 ml for Vi; 10–20 s for td1; 21–30 s for td2; 1000–1300 °C for Tm ; 10–15 s for rm ; 20–30 s for tm; and 4–6 s for ta; using pH 9.0 and passed through the column at diVerent flow rates the peak-area: 12–30 ml for Vi; 9–25 s for td1; 20–30 s for td2; (the flow rate was varied by changing the speed of the sample 981–1300 °C for Tm; 9–15 s for rm; 19–30 s for tm and 4–6 s pump). Changes in the flow rate of the sample were studied for ta.Thus an average value of these variables was taken for between 1.8 and 6.3 ml min-1, resulting in an optimum sample subsequent experiments (Table 1).flow rate of 3.4 ml min-1 with the best signal-to-blank ratio. The eVect of the sample loading time on the emission signal Performance of the method of 1 ng ml-1 Ni was studied at a sample flow rate of Under the optimum conditions, with the use of a 60 s 3.4 ml min-1. The signal increased linearly up to a 5 min preconcentration time, a sample flow rate of 3.4 ml min-1 and preconcentration time using the peak-height signal and up to an injection volume of eluent of 20 ml, two linear calibration a 6 min preconcentration time using the peak-area signal after graphs were obtained in the ranges 0–1 and 1–5 ng ml-1 of which the slope decreased gradually.The sensitivity was Ni(II). The figures of merit of these calibration graphs are increased by increasing the sample loading time; however, a summarised in Table 2. The detection and determination limits, loading time of 60 s was selected in order to fit this time into defined as the concentrations of analyte giving signals equival- the cycle of the furnace programme and thus achieve a high ent to three and ten times, respectively, the standard deviation sampling frequency with a reasonable degree of sensitivity.A of the blank plus the net blank intensity, were calculated. longer loading time can be employed for samples with low These values are also given in Table 2. concentrations of nickel. The signal appeared 83 s after sample injection plus 15 s for washing the microcolumn before the injection, giving a sample Software and optimisation procedure for the ETAAS program throughput of about 36 h-1.The structure of the software, shown in Fig. 3, is based on a The precision of the method for aqueous standards, screen menu system, which adds great flexibility to the system evaluated as the relative standard deviation (RSD, n=6), was besides simplifying its use. Adaptation of the system to the 5.8% for 0.8 ng ml-1 of Ni(II) and 2.6% for 3 ng ml-1.quality requirements of a particular laboratory is performed The enrichment factor (EF), defined as the ratio of the by the user changing the quality coeYcients: Change quality detection limits before and after preconcentration, was 58. coeYcients from the Main menu. The values of the coeYcients The concentration eYciency (CE), defined as the product of for this work were: s=100%; c=75%; a=100%; and r=75%. the EF and the sampling frequency in number of samples The normalisation conditions were introduced from the item analysed per hour, was 35 min-1.The consumptive index, Values Max-Min. The ranges used for the normalisation were: defined as the volume of sample, in millilitres, consumed to 0.010–0.200 for the slopes; 0–3 ng ml-1 for the detection limit; achieve a unit EF, was 0.06 ml. 1.0–10.7% for the relative standard deviation of the slope of the calibration curve and for the relative standard deviation Interferences of the analysis; 1–2 for the consumption of the reagent sample; The eVect of various ions that commonly occur with nickel in and 1.0–2.5 min for the time of analysis.sea-water samples was examined under the optimum working For access to the SIMPLEX optimisation, the user has to conditions. For this study, diVerent amounts of the ionic strike the tab key and then the computer demands new species tested were added to a 3.0 ng ml-1 solution of Ni(II). experimental parameters. This process is repeated until the response function is optimised.During the optimisation pro- Table 3 Interferences cess, the software oVers the advantage to the user of modifying the values of the experimental conditions or of giving the Foreign species Tolerated ratio/m/m response function the value of 0.0. These possibilities impede the occurrence of impossible experimental conditions for the Ca(II ), Mg(II ), Ba(II), Mn(II ), K(I), system (for example, a negative injection volume) and establish Na(I ), Pb(II), Mo(VI), Se(IV), acetate, Cl-, NO3-, I-, F-, CO32-, glycine >4000 the boundaries given by the user, assigning to the response Hg(I ), Ag(I), PO43-, SO42- 3000 function the value of 0.00 when these are overcome.In this Cd(II), Al(III ), Fe(III )a, Co(II )a 2000 work, a maximum cycle time of furnace operation of 2.5 min Cu(II), Hg(II), Zn(II ), Cr(III)a, Sn(II)a 1000 was used, and the relative standard deviations of the slope of Bi(III), Mn(II ) 750 the calibration curve and of the analysis should not be higher Co(II), Fe(III), Sn(II ) 200 than 10.7%.The program also permits the user to save the aWith 2.5 ml of glycine 0.2 M. response function and display the graph of the response 1036 J. Anal. At. Spectrom., 1999, 14, 1033–1037Table 4 Determination of nickel in certified and spiked sea-water experimental parameters to optimise, adapting the system to samples his/her particular quality requirements. Certified Added/ Founda/ Recovery Sample value/ng ml.1 ng ml.1 ng ml.1 (%) Acknowledgements CRM 505 1.41¡¾0.12 1.57¡¾0.17 111.3 The authors thank the Direccio¢¥n General de Investigacio¢¥n Synthetic sea-water 0.40 0.42¡¾0.18 105.0 Cient©¥¢¥fica y Te¢¥ cnica of Spain (DGICYT) for supporting this Synthetic sea-water 0.70 0.71¡¾0.22 101.0 study (Project PB 96-0702) and also the Junta de Andaluc©¥¢¥a.Synthetic sea-water 1.00 1.02¡¾0.18 102.0 Sea-water . 2.34¡¾0.13 Sea-water 1.00 3.45¡¾0.08 111.0 References Sea-water 2.00 4.46¡¾0.07 106.0 Sea-water 4.00 6.26¡¾0.15 98.0 1 D.L. Tsalev, V. I. Slaveykova and P. B. Mandjukov, Spectrochim. Acta Rev., 1990, 13, 225. aMean¡¾standard deviation for four replicate measurements of four 2 Z. Fang, M. Sperling and B. Welz, J. Anal. At. Spectrom., 1990, individual preparations. 5, 639. 3 M. Sperling, X. Yin and B. Welz, J. Anal. At. Spectrom., 1991, 6, 295. The starting point was an interference-to-nickel ratio of 4000 4 S. Olsen, L.C. R. Pessenda, J. Ru¡Æz¢§ic¢§ka and E. H.Hansen, Analyst, m/m; if any interference occurred, the ratio was gradually 1983, 108, 905. lowered until the interference disappeared. A given species 5 Z. Fang, J. Ru¡Æz¢§ ic¢§ka and E. H. Hansen, Anal. Chim. Acta, 1984, 164, 23. was considered to interfere if it resulted in a ¡¾5% variation 6 K. Vermeiren, C. Vandecasteele and R. Dams, Analyst, 1990, 115, of the AAS signal. The results obtained are given in Table 3. 17. The interference of Fe(III), Co(II), Cr(III ) and Sn(II) can be 7 R.Rattray and E. D. Salin, J. Anal. At. Spectrom., 1995, 10, 1053. significantly lowered by adding glycine 0.2M to the medium. 8 S. Hirata, Y. Umezaki and M. Ikeda, Anal. Chem., 1986, 58, 2602. 9 S. Hirata, K. Honda and T. Kumamaru, Anal. Chim. Acta, 1989, Determination of nickel in certified and spiked sea-water 221, 65. 10 D. B. Taylor, H. M. Kingston, D. J. Nogay, D. Koller and samples R. Hutton, J. Anal. At. Spectrom., 1996, 11, 187.In order to test the accuracy and applicability of the proposed 11 V. Porta, O. Abollino, E. Mentalti and C. Sarzanini, J. Anal. At. method for the analysis of real samples, a certified reference Spectrom., 1991, 6, 119. 12 F. Malamas, M. Bengtsson and G. Johansson, Anal. Chim. Acta, material CRM 505 and spiked sea-water and synthetic sea- 1984, 160, 1. water samples were analysed. CRM 505 shows a pH of 1.6, 13 A. Fang and B.Welz, J. Anal. At. Spectrom., 1989, 4, 543.so that a previous neutralisation with diluted NaOH is rec- 14 K. Akatsuka, J. W. McLaren, J. W. H. Lam and S. S. Berman, ommended. The slopes of the calibration graphs and those J. Anal. At. Spectrom., 1992, 7, 889. using the standard additions method were very similar for all 15 X. Wang and R. M. Barnes, J. Anal. At. Spectrom., 1989, 4, 509. samples, except for CRM 505 which had to be evaluated with 16 S. Arpadjan, L. Vuchkova and E. Kostadinova, Analyst, 1997, 122, 243.the standard additions method. The results given in Table 4, 17 D. Pozebon, V. L. Dressler, J. A. Gomes Neto and A. J. Curtius, as the average of four individual preparations, show good Talanta, 1998, 45, 1167. agreement with the certified value and suYciently high 18 Z. Fang, S. Xu and G. Tao, J. Anal. At. Spectrom., 1996, 11, 1. recoveries. 19 M. Sperling, X. Yin and B. Welz, J. Anal. At. Spectrom., 1991, 6, 615. 20 M. Sperling, X. Yin and B.Welz, Spectrochim. Acta, Part B, 1991, Conclusions 46, 1789. 21 Z. Liu and S. Huang, Spectrochim. Acta, Part B, 1995, 50, 197. The most frequently stated advantage of ETAAS is its low 22 Z. Liu and S. Huang, Anal. Chim. Acta, 1993, 281, 185. detection limit. However, in certain instances, the detection 23 Y. Sung, Z. Liu and S. Huang, J. Anal. At. Spectrom., 1997, limits are still inadequate, especially when the sample has a 12, 841. complex matrix. In these circumstances, a preliminary precon- 24 X. Yan, W. VanMol and F. Adams, Analyst, 1996, 121, 1061. centration and/or separation is required. Conventional oV-line 25 X. Yan and F. Adams, J. Anal. At. Spectrom., 1997, 12, 459. 26 D. Betteridge, T. J. Sly, A. P. Wade and J. E. W. Tillman, Anal. procedures for separation and preconcentration, although Chem., 1983, 55, 1292. eVective, are usually time-consuming and tedious, and are 27 S. N. Deming and S. L. Morgan, Anal. Chem., 1973, 45, 278A. vulnerable to contamination and analyte loss. This study has 28 S. N. Deming and L. Parker, CRC Crit. Rev. Anal. Chem., 1978, shown that the FI-ETAAS method described allows the rapid 7, 187. determination of nickel. The analytical scheme of the proposed 29 W. Spendley, G. R. Hext and F. R. Himsworth, Technometrics, system is much simpler than other oV-line and on-line pro- 1962, 4, 441. 30 J. A. Nelder and R. Mead, Comput. J., 1965, 7, 308. cedures because it combines trace enrichment, derivatisation 31 M. Poch, J. L. Montesinos, M. del Valle, J. Alonso, A. N. Arau¢¥ jo and detection in a single analytical set-up. and J. L. F. C. Lima, Analusis, 1992, 20, 319. For a detailed optimisation of systems with four or more 32 E. Vereda, A. R©¥¢¥os and M. Valca¢¥rcel, Anal. Chim. Acta, 1997, variables, SIMPLEX may still provide the most viable option, 348, 129. as the computerised optimisation is faster and much less labour 33 P. Can.ada Rudner, A. Garc©¥¢¥a de Torres, J. M. Cano Pavo¢¥n and intensive. The interactive optimisation procedure can be E. Rodriguez Castello¢¥ n, J. Anal. At. Spectrom., 1998, 13, 243. applied to any analytical method because the user chooses the Paper 9/01451I J. Anal. At. Spectrom., 1999, 14, 1033.1037 1037

ISSN:0267-9477    

DOI:10.1039/a901451i

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Evaluation of helium–argon mixed gas plasmas for bulk and depthresolved analyses by radiofrequency glow discharge atomic emission spectroscopy Matthew L. Hartenstein, Steven J. Christopher and R. Kenneth Marcus* Department of Chemistry, Howard L . Hunter Chemical Laboratories, Clemson University, Clemson, SC 29634-1905, USA Received 24th February 1999, Accepted 27th April 1999 Studies were performed to determine the practical benefits of mixed discharge gases (Ar and He) in the bulk and depth-resolved analysis of solids using a radiofrequency glow discharge atomic emission source.This study examined the characteristics of analyte emission intensity and yield, sample sputter rate and crater shape as a function of added He gas, for both conductive and non-conductive sample matrices. In comparison with pure Ar plasmas, the addition of He does not ultimately improve the limits of detection in the bulk analysis of conductive (metallic) solid samples. However, non-conductive sample matrices such as glasses, which are greatly aVected by discharge gas pressure to maintain sputtering conditions (crater shape, sputter rate), may benefit from the addition of He to the plasma gas as a means of enhancing the excitation conditions.The shape of the sputtered craters was minimally aVected by the addition of He, indicating that Ar partial pressure is the parameter that most critically aVects sputtering characteristics. Overall, the addition of He to the discharge plasma has been found to enhance analyte emission intensity without significantly influencing sputtering characteristics.In terms of depth-resolved analyses, optimized discharge conditions (specifically pressure) derived for the pure Ar case, which are sometimes accompanied by a loss of analytical sensitivity, can be augmented by the addition of He to the discharge to yield improved analytical responses while retaining the desired sputtering characteristics.for MIPs employed as gas chromatographic detectors.7 Argon Introduction has been the dominant plasma gas in analytical GD devices Advances in materials development have illustrated distinct for several reasons. Argon ions are eYcient sputtering agents, advantages which may be gained from the use of coatings the plasmas generate high electron temperatures and densities (paint, PTFE) and multi-layered (optics, semiconductors) sys- and the metastable energy levels of 11.5 and 11.7 eV are tems.The radiofrequency glow discharge atomic emission suYcient to ionize a majority of the elements in the Periodic (rf-GD-AES) source approaches the ideal for characterizing Table. Recently, as applications have pushed analytical requirethese types of materials as sample composition and thickness ments beyond traditional capabilities, there has been renewed information may be generated quickly and reliably in a single interest in alternative plasma gases. For example, helium oVers run.The duality of the GD plasma as both an atomization the potential to satisfy many of the needs as it yields a higher (sputtering) and excitation source makes depth-resolved analy- energy plasma than does argon. This advantage is especially sis possible. However, these processes within the plasma are significant to the analysis of hard to excite elements such as intertwined and cannot be independently controlled by the the non-metals. parameters aVecting them, specifically the applied power and The possible advantages of the use of alternative discharge discharge gas pressure.Because the ionization potential, the gases in GD plasmas have been illustrated by earlier studies; presence of metastable energy levels and atomic (molecular) however, the potential benefits to depth profile analysis, in mass of the discharge gas are determining factors in discharge particular, have not been examined extensively. Wagatsuma atomization and excitation characteristics, it is reasonable to and co-workers have conducted several studies involving the expect that there may be instances where diVerent gases, or use of alternative gases in direct current (dc) GD-AES,8–10 mixtures thereof, may be employed to advantage. illustrating that the emission spectrum of a plasma is strongly While argon has been the dominant discharge gas employed dependent on the discharge gas employed.For example, helium across the range of spectrochemical plasmas [i.e., arcs, sparks, and neon were found to increase significantly the intensities inductively coupled plasmas (ICPs), direct current plasmas of analyte lines, both ionic and atomic, regardless of sputter (DCPs), microwave induced plasmas (MIPs) and glow dis- rate.8,9 The eVects, both positive and negative, are strongly charges], gases other than argon have been of interest for element and transition dependent.Enhancements in ionic some time.For example, helium-based ICP studies were con- emission intensities were attributed to charge transfer between ducted by Reed nearly 40 years ago.1 In the area of ICP plasma gas (Ar, He, Ne) ions and analyte atom species, spectrometry, alternative gases have been employed as means varying with the respective energetics of the noble gas and of removing spectral interferences in ICP-MS, providing analyte ion states. The advantageous use of Ne as a means of enhanced matrix volatilization, enhancing excitation con- enhancing F (I ) emission illustrates the possible analytical ditions and improving the capacity to tolerate organic solu- utility of diVerent discharge gases.10 In the field of glow tions.2–5 Nitrogen has found acceptance as the plasma gas for discharge mass spectrometry (GD-MS), Woo et al.found that the addition of even small amounts (4%) of He to the discharge high power (>500 W) MIPs,6 while helium is the standard J. Anal. At. Spectrom., 1999, 14, 1039–1048 1039plasma increased the ionization eYciency by as much as 25 conditions yield the most desirable crater shapes: for the conductive (steel ) sample, an Ar base pressure of 5 Torr, and times.11 Teo and Hirokawa12 compared depth profiles of metal coatings obtained using pure argon and an argon–helium for the non-conductive (glass) sample, an Ar base pressure of 3 Torr.Sputter rate and spectral data were collected at these mixture in a Grimm-type discharge.The presence of He in the discharge gas was found to enhance the emission intensity for pressures and each subsequent pressure of added He above this pressure (3 Torr intervals equaling total pressures of 8, elements having a simple energy level structure (Cu and Zn in this case), but not for Ni, which has a much higher density of 11 Torr, etc. for the metal, and 6, 9 Torr, etc. for the glass). In each case the range of added He was 3–15 Torr. Optical and states.The former eVect was attributed to the higher excitation and ionization potentials of He than Ar. Helium was also sputtering data were also collected at the same total pressures using pure Ar. Another set of studies involved a variation of found to reduce sputtering rates by a factor of two, while also producing broader interface transitions. A recent study pub- the He5Ar ratio at a fixed total pressure. In this case, an absolute total gas pressure of 10 Torr was maintained and the lished by this laboratory touched on the possible practical advantages for He–Ar mixed gas plasmas in rf-GD-AES.13 He5Ar ratio was varied from 100% Ar to 100% He.While involving only conductive samples, the study reported Determination of emission and sputtering characteristics increases in analyte emission intensity of up to 300% over a 0–15 Torr range of increasing He pressure. Consistent with all The following procedure was employed for the collection of of the above mentioned studies, a decrease in sputter rate of optical emission intensities and the subsequent determination up to 50% was observed over the same pressure range.of sputtering rates. After a pressure equilibration period of In this work, we set out to compare the sputtering and approximately 1 min and a 3 min pre-sputter period, the emission yield characteristics of the rf glow discharge atomic discharge was ignited and emission profiles were recorded over emission source for He–Ar mixtures, from pure Ar to pure a ~0.05 nm window about each monitored transition. While He plasmas.Since plasma characteristics may vary greatly stabilization usually required less than 1 min, the time delay from sample to sample, especially between conductive and ensured a stable emission signal prior to data collection. The non-conductive matrices, both sample types were employed in plasma was extinguished after a total burn time of 5 min, a the evaluation. Trends for several He5Ar ratios were examined suYcient sputtering period to facilitate meaningful crater at fixed and increasing total discharge gas pressure situations. (sputtered area) profile measurements.The depth and shape Benefits in depth profiling applications are demonstrated in of each crater were measured using a Tencor (Mountain View, the analysis of layered samples at selected He5Ar pressure CA, USA) Model P-10 surface profilometer. Relevant ratios. In general, the data indicate that the addition of He to operating parameters of the profilometer include a 15 mg force an Ar plasma enhances the excitation process without signifi- on a 12.5 mm diameter, diamond-tipped stylus, a horizontal cantly aVecting the sputtering characteristics with respect to scan length of 10 mm across the breadth of each crater (2 mm the pure Ar case.resolution) and a vertical (depth) resolution of 25 A° . The average crater depth was determined for each burn from the central 2000 points of the total of 5000 in the lateral scan.Experimental The corresponding sputter rate is obtained by dividing the Optical emission studies average depth by the total sputtering time of 5 min for each sample. The data reported here represent the average of the The spectrometric equipment employed in this study has been triplicate runs at each of the chosen set of conditions. The described previously,14 so only a brief description is given precision of the emission intensities, over the three runs for here.All rf-GD-AES experiments were performed on a Model each condition, was typically better than 5% RSD for all 5000 RF polychromator system (Jobin-Yvon, Longjumeau, elements monitored (extreme pressures yielded poorer France). The 0.5 m Paschen–Runge polychromator permits precision values, although <15%). the simultaneous monitoring of optical emission from the desired analyte(s) and the discharge gas elements. The source Samples emission is focused with MgF2 optics on to a 2400 grooves/mm-1 ion-etched, holographic grating. The optical Since conductive and non-conductive matrices are known to respond optimally at diVerent plasma conditions, both types path of the spectrometer is nitrogen purged and operates over the wavelength range 110–620 nm, with the practical spectral of samples were examined in this study.NIST SRM 1250 High Temperature Alloy was chosen for the study of conduc- resolution for this instrument being ~0.01 nm.The discharge gas pressure in the GD source was monitored with a silicon tive samples. The following components’ emission lines were monitored (% m/m composition): Al (I ) 396.15 nm (0.99%), diaphragm pressure transducer (Model PX811-005) and a matching controller/meter (Model DP41-S, Omega, Stamford, Co (I ) 345.35 nm (16.1%), Cr (I ) 425.43 nm (0.077%), Cu (I ) 327.39 nm (0.022%), Ni (I ) 341.47 nm (37.78%) and Ar (I ) CT, USA). The pressure transducer allowed the absolute pressures of both He and Ar gases (individually and in total ) 404.44 nm.This sample was prepared by sanding the surface with 600 grit sandpaper to an even finish and subsequently to be measured, without bias, at a resolution of ~0.2 Torr and an experimental reproducibility of better than 5% RSD wiping with a lint-free cloth. Common, 1 mm thick microscope slides (Fisher, Pittsburgh, (at 3 Torr). The source body (anode) is constructed of stainless steel PA, USA, catalog No. 12-544-1) were chosen for the study of non-conductive samples. This product is an ideal sample choice with the dimensions of 8×6.5×6.5 cm. The flat sample is sealed against a Teflon O-ring (located on the demountable for several reasons: (1) the standard deviation of thickness, from slide to slide, is very low (0.6% RSD for 10 samples), limiting orifice plate) by means of a pneumatically controlled piston. A ceramic disk around the O-ring ensures a constant (2) the sample oVers a large surface area and (3) low cost and ease of preparation.Each glass slide required only rinsing with anode-to-cathode spacing (0.5 mm). In all cases, a 4 mm id limiting (anode) orifice was employed and 30W of power were methanol before use. To ensure consistency, 10 separate glass slide samples were analyzed (4 Torr, 20 W), yielding a pre- applied using a modified Dressler (Berlin, Germany) generator. The specific pressures chosen for analysis were based on data cision of <5% RSD for background-corrected signals of Al (I ) 396.15 nm, Mg (II ) 280.27 nm and Si (I ) 288.15 nm lines.from previous studies13,15 and unpublished trials conducted in preparation for this study. Based on the criterion of producing Two very diVerent types of layered materials were employed to evaluate the possible diVerences in operating characteristics crater bottoms parallel to the sample surface, the following 1040 J. Anal. At. Spectrom., 1999, 14, 1039–1048of mixed gas plasmas.The first, a ceramic-coated steel, was undertaken here, Belkin et al. characterized He–Ar working provided by a steel manufacturer who was interested in gas systems in this laboratory by Langmuir probe and emission evaluating rf-GD-AES as a means of characterizing layer intensity studies.18 Those studies indicated that whereas elecformation processes. The non-conductive coating was made tron and ion number densities decrease with helium addition, up predominately of Si, Al and Mg oxides, with a thickness the measured average electron energy e and electron temof ~1 mm on the 1 mm thick steel substrate.This layered perature (Te) values increase. The net result of these diVerences system can be generally characterized as having a broad, was a general increase in analyte atomic emission intensities. diVusional interface rather than a very clean, finite interface. The precise degree of the enhancement in excitation eYciency The second layered system was a Cu/Cr–Ni superlattice mate- cannot be assessed without a corresponding knowledge of rial provided by IFW (Dresden, Germany).This sample changes in sputtering rate. Hence the assessment of the relative consists of 10 alternating layers of Cu and Cr–Ni, each having emission yields for diVerent gas mixtures is of both analytical a thickness of ~100 nm, on a 0.5 mm thick Si substrate. The and fundamental relevance. interfaces in this system are well structured.As listed in Table 1, an increase in emission yield is observed for all of the monitored analytes in NIST SRM 1250 High Temperature Alloy as a function of the total source pressure, Results and discussion for both pure Ar and for the mixed gas (He–Ar) conditions. Comparison of emission yield from pure Ar and mixed gas The data set for the pure Ar discharge reveals an increase in plasmas REY of up to ~25-fold over the pressure range 5–20 Torr.The increase in REY for the mixed gas plasma over the same Analyte emission intensity is a product of two factors: the pressure range is also significant, yet not nearly as great for number of analyte atoms in the plasma (atomization rate) and most elements. These eVects surely reflect enhanced excitation the emission yield (excitation eYciency). Since the discharge eYciencies due to increases in e and Te.18 The Cu 327.96 nm gas type and pressure are known to influence both atomization line exhibited approximately the same overall increase in REY and excitation processes, intensity is not a valid indication of for both cases (~400%); however, for the Cr 425.43 nm line, excitation conditions.Relative emission yield is an eVective there was a nearly 10-fold greater increase in REY as the means of assessing the eVects of plasma conditions (power, pressure was increased for pure Ar in comparison with the pressure, etc.) on analyte excitation because intensity is normixed gas condition (2600% for Ar versus 300% for He–Ar).malized by both the sample sputter rate and analyte concen- This diVerence in REY between the two analytical lines is tration. In all following discussions, the relative emission yield surely due to diVerences in the excitation energies and path- (REY) is expressed as ways for each of the respective excited states. REYx=Ix/(SDsam[X ]) (1) Comparison of the REY data for the NIST SRM 1250 alloy as a function of added discharge gas (Table 1) indicates that for element X, where I is the background-corrected analyte the pure Ar plasma excites the atomic species in the discharge emission intensity in volts, SD is the sputtered depth (mm, per plasma more eYciently than does helium.The same trend is 5 min run) for the respective sample and [X] is the concenobserved on comparing the Ar and Ar–He plasmas at a fixed tration of the element in the sample. The subject of which pressure, as Fig. 1 depicts the trend with a plot of REY versus discharge parameter(s) (current, voltage, or pressure) most He5Ar ratio at a fixed total pressure of 10 Torr. (Data for the strongly aVects the emission yield of analytes in GD plasmas pure He condition were not included with this set as a stable has been debated for some time.16 Payling et al. have published plasma could not be maintained at this pressure.) The emission data that support a theory that discharge gas pressure is yield for Cr and Cu declined sharply on addition of any He, primarily responsible for influencing this phenomenon.17 While by ~80 and ~70%, respectively, over the range of gas several studies have focused on the eVects of power (mostly mixtures.The eVect of added He on the REY of other dc) and pressure on emission yield, few have been conducted monitored emission lines was not as dramatic, but the overall to compare the relative emission yields produced by diVerent decrease was within the same range (decreases of ~70, ~80 discharge gases across a range of diVerent pressures.Since and ~50% for Ni, Co and Al, respectively). Hence it is clear each discharge gas has unique excited state (metastable) and that, on a same-pressure basis, the pure Ar plasma is more ionization energies, each can be expected to influence the emission yield to diVerent extents. As a prelude to the studies eYcient at exciting these particular transitions. Very diVerent Table 1 Relative emission yields (REY) as a function of discharge gas pressure and composition for NIST SRM 1250 High Temperature Alloy (rf power=30 W) REY/V mm-1 %-1 Ar pressure/ He pressure/ Total pressure/ Cu (I ) Co (I ) Cr (I ) Al (I ) Torr Torr Torr 327.39 nm 345.35 nm 425.43 nm 396.15 nm 5 — 5 0.126 0.006 0.022 0.014 8 — 8 0.202 0.012 0.074 0.034 11 — 11 0.278 0.021 0.160 0.075 14 — 14 0.377 0.033 0.277 0.134 17 — 17 0.448 0.042 0.352 0.185 20 — 20 0.637 0.065 0.573 0.306 Increase (%): 500 1150 2600 2150 5 — 5 0.126 0.006 0.022 0.014 5 3 8 0.096 0.010 0.022 0.031 5 6 11 0.180 0.015 0.034 0.056 5 9 14 0.268 0.019 0.041 0.081 5 12 17 0.407 0.024 0.049 0.111 5 15 20 0.675 0.036 0.070 0.177 Increase (%): 530 630 310 1240 J.Anal. At. Spectrom., 1999, 14, 1039–1048 1041at 280.27 nm are diVerent from any other line in the study as the enhancement in REY was found to be greater for the mixed gas condition (~600× vs. 1500×) over the pressure range. Clearly, this disparity in response with respect to all of the other transitions points to diVerences in excitation mechanisms.Since the energies associated with Arm* and Mg+* are 11.7 and 12.07 eV, respectively, a plausible excitation pathway in this situation is the Penning-type reaction Arm*+Mg0�Mg+*+Ar0 (2) The diVerence of ~0.4 eV (endothermic) between the two energy states can be supplied by the thermal energy of the plasma. Of course, for helium to influence Mg(II ) significantly, the number of argon metastables would also need to increase as a function of helium content.Owing to the small energy diVerence between helium metastable and argon ion excited states, collisional energy exchange between the two states influences Ar+ populations in mixed gas plasmas.9 The forma- Fig. 1 EVect of He5Ar discharge gas ratio on relative emission yields tion of Ar metastables from the neutralization of Ar ions for NIST SRM 1250 High Temperature Alloy (total pressure= completes the pathway 10 Torr, rf power=30W).Hem+Ar0�He0+Ar+ (3) from the response observed for transitions of the sputtered Ar++e�Arm* (4) atoms, Fig. 1 also depicts the raw intensity of the Ar (I ) Arm*+Mg0�Mg+*+Ar0 (2) 404.44 nm transition. In this case, the ability of the He plasma to excite this high-lying transition is dramatic as the Ar (I ) For the glass sample matrix, diVerent He5Ar pressure ratios intensity is essentially unchanged while the actual concen- (at constant absolute pressure) yielded results slightly diVerent tration is decreased by 90%.The underlying reasons for this from those for the NIST SRM 1250 sample. As can be seen diVerence in response are discussed below. in Fig. 2, Si (I ) and Mg (II ) lines yielded the greatest REY In general, rf-GD-AES analyses of glasses have shown that values at He5Ar ratios of ~555 (456 for Si). The Al (I ) line the most favorable analytical responses occur at discharge conditions of lower gas pressures (2–6 Torr Ar) than for metallic samples.14 Similar to trends observed for the alloy sample Table 2 illustrates that both the pure Ar and mixed gas plasmas exhibit a steady increase in REY over the 3–18 Torr range.The highest REY values were observed for pure Ar plasma operating at 15 Torr (an REY increase of nearly two orders of magnitude for Si 288.15 nm). The highest Ar pressure, 18 Torr, produced an unstable plasma, as indicated by REY values that do not follow the trend of the five preceding data points.A very interesting (and important) selfconsistency is borne out on inspection of the response of Al, the lone common element in the metallic and glass matrices, in Tables 1 and 2. In both data sets, the percentage increase from the lowest to highest pressures for the pure Ar and mixed gas plasmas diVers by twofold. Therefore, to a first approximation, the influence of He on the excitation eYciency is the same for conductive and non-conductive samples.Fig. 2 EVect of He5Ar discharge gas ratio on relative emission yields for glass slide samples (total pressure=10 Torr, rf power=30 W). The results observed for the monitored Mg (II ) transition Table 2 Relative emission yields (REY) as a function of discharge gas pressure and composition for glass slide samples (rf power=30 W) REY/V mm-1 %-1 Ar pressure/ He pressure/ Total pressure/ Si (I ) Mg (II ) Al (I ) Torr Torr Torr 288.15 nm 280.27 nm 396.15 nm 3 — 3 0.0004 0.0059 0.0105 6 — 6 0.0008 0.0067 0.0205 9 — 9 0.0025 0.0114 0.0404 12 — 12 0.0190 0.0260 0.0918 15 — 15 0.0295 0.0355 0.1281 18 — 18 0.0070 0.0221 0.0279 Increase (%): 7250 600 1200 3 — 3 0.0004 0.0059 0.0105 3 3 6 0.0007 0.0137 0.0156 3 6 9 0.0012 0.0304 0.0245 3 9 12 0.0021 0.0484 0.0364 3 12 15 0.0036 0.0749 0.0579 3 15 18 0.0046 0.0892 0.0737 Increase (%): 1130 1500 700 1042 J.Anal. At. Spectrom., 1999, 14, 1039–1048at 396.15 nm displayed a drastically lower REY upon He addition.As discussed previously, the addition of He has been shown to increase the average electron energy and electron temperature (e and Te) in Ar plasmas.13 At constant pressure, it reasonably follows that transitions with lower energy requirements [Al (I ) 396.15 nm, 3.15 eV ] will be favored at lower concentrations of He than those with higher energy requirements [Si (I ) 288.15 and Mg (II ) 280.27 nm, 5.09 and 12.07 eV, respectively].The response of the Ar (I ) 404.44 nm signal depicted in Fig. 2, increasing steadily with increasing He5Ar ratio (decreasing Ar partial pressure), is surprising. This trend was observed in earlier He–Ar mixed gas studies, 13,18 and supports the cascade model depicted in eqns. (3) and (4) as that transition is an iportion of the pathway from the ionic to the metastable state. In addition, the high energy requirements (14.7 eV) of the Ar (I ) 404.44 nm line Fig. 3 EVect of discharge gas pressure on sputter rates for NIST SRM may exhibit enhancements by virtue of the higher electron 1250 High Temperature Alloy. %, Pure Ar discharge; 1, mixed gas energies which may contribute to more eVective direct electron discharge, He added to 5 Torr Ar base pressure (rf power=30W). impact population of the excited state. Direct ionization is supported through previous Langmuir probe studies which show that electron energies tend to be much higher in the case of oxide (glass) sample analyses versus metals.19 Addition of He appears to accentuate this diVerence.Influence of He fraction on sample sputtering rates The mass of a sputtering ion logically influences sputter rate, and several theories on sputtering have been published.20–23 In particular, argon is a much more eYcient sputtering agent (at a given accelerating potential ) than helium owing to its greater mass. In fact, the sputtering yield for Ar at a copper target for a nominal ion energy of 400 eV is about eight times higher than that for He (~1.65 vs. 0.21).22,23 The particular argon pressure that yields maximum sputter rates is often specific to the matrix being analyzed, and above this pressure the sputter rate typically declines. These diVerences are particu- Fig. 4 EVect of discharge gas pressure on sputter rates for glass slide larly pronounced when comparing conductive and nonsamples. %, Pure Ar discharge; 1, mixed gas discharge, He added to conductive sample matrices.15 There are several factors that 3 Torr Ar base pressure (rf power=30W).explain this influence.24 At low pressures (mTorr range), the mean free path of discharge gas ions impinging the cathode surface is long, translating into high ion energies. With increas- to the initial base Ar pressure reveals optimum pressure conditions with regard to sputtering rates, whereas addition ing Ar pressure, collisional processes become significant as Ar+–Ar0 collisions result in either elastic scattering or charge of He tends to depress the sputtering rates only slightly relative to the initial (pure Ar) values.In fact, Fig. 4 demonstrates transfer reactions. Both processes reduce the average Ar ion energies, which are manifest in rf-GDs as dc-bias values that beyond the Ar optimum, the sputter rates for the mixed gas plasmas are considerably greater than those for the pure steadily decrease.25,26 To a first approximation, this would be expected to result in lower sputtering yields/rates.The diVusion Ar plasma at the same pressure (i.e., the depression in sputtering rate is greater for the pure Ar discharge). In the case of rate of sputtered species away from the sample surface is also reduced at higher pressures as mean free paths are shorter, the glass sample, it may be that the higher ionization potential (and thus bias potentials) of He helps to maintain the sputter- contributing to higher redeposition rates.When discharge pressure is increased through He addition, however, these ing rates at levels above those of the argon plasma. In general, it appears that addition of He to an Ar plasma tends to have interactions are much less significant. Ar+–He0 charge transfer reactions are not possible and, owing to its low mass, helium only minimal eVects on sputtering rates beyond those established for that pressure of Ar. This characteristic, which is does not strongly influence the trajectory of the impinging ions or the sputtered analytes.The latter point might suggest consistent throughout the studies described here, points clearly to a situation wherein Ar+ is the major positive charge carrier that smaller amounts of redeposition may be realized at the same total pressures for the mixed gas plasma versus the pure in the plasma and thus the predominant sputtering agent. This is a reasonable assumption given the much lower ionization Ar discharge.Thus helium can be expected to aVect sputtering characteristics only mildly when added to an argon plasma. potential of Ar than He. At some point, He would be expected to become ionized to a larger extent as the Ar concentration As can be seen in Figs. 3 and 4, the sputter rates for pure Ar and Ar–He plasmas yield diVerent responses in the cases is reduced, so as to maintain the overall ionization level of the plasma. of the metallic and glass sample matrices. The pressure ranges chosen here begin at Ar values below the optimum values for Fig. 5 is a plot of the sputter rates for both sample types at diVerent He5Ar ratios and fixed absolute pressure. As was pure argon discharges, hence addition of either gas allows passage through maxima in sputtering rates. For both sample observed for increasing pressures with added He, the sputter rates for NIST SRM 1250 decreased steadily with increasing types, a comparison of sputter rate data shows that the mixed gas conditions produce a much more uniform response than He5Ar ratio.As would be expected from Fig. 3, the highest sputtering rate is obtained for the pure Ar (10 Torr) plasma. does pure Ar, collected over the same absolute pressure ranges. Both Figs. 3 and 4 illustrate the fact that addition of Ar gas Note also that sputter rates do not decrease proportionally J. Anal. At. Spectrom., 1999, 14, 1039–1048 1043Fig. 5 EVect of He5Ar discharge gas ratio on sputtering rates.%, NIST SRM 1250 High Temperature Alloy; 1, glass slide samples (total pressure=10 Torr, rf power=30 W). with He dilution. For example, 5 Torr He and 5 Torr Ar yielded a sputtered depth of 6.4 mm whereas pure Ar at 5 Torr yielded a sputtered depth of 8.1 mm. A 50% dilution of Ar yielded only a 20% decrease in sputter rate. Again, this example illustrates that, with respect to Ar, He has little influence over sputtering conditions in the GD plasma. Sputter rate results for the glass analyses at constant pressure closely follow trends set by the pure Ar data in Fig. 4. The data indicate that the increased pressure from added He Fig. 6 EVect of discharge gas pressure on resultant sputtered crater (decreasing the Ar partial pressure) increases the observed shapes for NIST SRM 1250 High Temperature Alloy: (a) pure Ar sputtering rates, reaching maximum values under the con- discharge and (b) He added to 5 Torr base Ar pressure (rf power= ditions where the Ar pressure is in the range 3–4 Torr.While 30 W). added He has been shown generally to decrease sputter rates for Ar-based plasmas, these data indicate that optimum sputprobably the most desirable results for this sample, in depth tering conditions (i.e., Ar partial pressure) may require tailorprofiling terms. Note that as the discharge gas pressure is ing with respect to added He and sample matrix type. This is increased, the crater bottom becomes more curved and the consistent with the pure Ar plasmas also.As expected, there crater walls more sloped. The higher pressure conditions yield is limited correlation between sputtering trends and REYs. a crater profile that is somewhat W-shaped (convex), owing to Since REYs have been shown to increase with increases in a complex gradient of sputtering and redeposition rates across discharge gas pressure (Ar or He), the addition of He to an the analysis area. Work by Parker et al.15 and Angeli et al.27 Ar plasma can be expected to increase the analyte intensity has suggested numerical expressions to quantify better the while minimally influencing the sample sputtering conditions.overall flatness of a crater. However, it is believed that the For example, one could envision a situation wherein sputtering results of this study are best expressed by a comparison of characteristics (rates) can be optimized with respect to Ar crater profiles. Fig. 6(b) illustrates the eVect of mixed gas discharge gas pressure and then He added to improve analytical conditions on crater shapes for the sputtering of NIST SRM emission responses. 1250 alloy. Over a broad range of added He from 0 to 12 Torr (5 to 18 Torr total pressure), the craters maintain their rec- Influence of He fraction on resultant crater shapes tangular shape. Only at the highest pressure (15 Torr added He, 20 Torr total ) does the crater shape become unsuitable for DiVerent from the case of bulk analyses, the eVect of plasma conditions on the shape of sputtered craters is a crucial aspect depth profile analysis.Hence, absolute discharge pressure may be significantly increased by the addition of He, without a of the quality of depth profiling analyses. The most desirable crater is one in which the sides (walls) are perpendicular to, significant aVect on crater shape. These results are consistent with those of the sputter rate studies. and the base parallel to, the original sample’s surface.Such a sputtering profile should yield intensity information most For the glass matrix sample, discharge pressures of 3 and 6 Torr pure Ar yield crater shapes which suggest favorable representative of the sample strata. Previous studies by Parker et al. evaluated the respective roles of the discharge conditions conditions for depth profiling [Fig. 7(a)]; however all craters produced at higher pressures are undesirable for depth profiling of applied rf power and discharge gas pressure.15 In general, rf power was found simply to aVect the rate of cathodic purposes.Interestingly, the 18 Torr condition yielded a crater not nearly as deformed as those at 9, 12 and 15 Torr. The sputtering, whereas Ar discharge gas pressure tended to aVect both the rate and the shape. As with other GD sources, high uniform shape of the 18 Torr crater is probably due to the very low sputter rate at this pressure. Fig. 7(b) is an overlay pressures tend to form concave-shaped craters, whereas low pressures produce more convex craters.As would be expected, of crater profiles from the mixed gas sputtering of the glass sample, using a 3 Torr base Ar pressure. The craters obtained optimum crater shapes for non-conductive samples were achieved at lower pressures than for metallic samples (~3 Torr from this set of analyses remain more uniform as the discharge gas pressure is increased, as compared with pure Ar. The crater vs. 6 Torr Ar). Fig. 6(a) displays an overlay of crater profiles from the GD profiles produced by mixed gas conditions, up to absolute pressures of 9 Torr, appear to remain acceptable for depth sputtering of NIST SRM 1250 at increasing pure Ar pressures from 5 to 20 Torr. The 5 Torr condition yields the flattest and profile analysis. However, above 9 Torr the convexity of the 1044 J. Anal. At. Spectrom., 1999, 14, 1039–1048optimal discharge conditions for depth resolution and intensity can be mutually exclusive. For example, increasing discharge power or pressure to enhance the intensity of analyte optical emission typically increases the sputter rate and in some instances alters the sputtered crater shape, thus decreasing the depth resolution. Depth profiling applications can be envisioned to be enhanced by a process wherein optimum sputtered crater shapes are achieved by adjustment of the Ar discharge gas pressure and then He gas is added to achieve enhanced analyte emission responses.The following examples illustrate the eVect of He gas addition on the depth profile analyses of two distinctly diVerent sample types. The first sample type is a ceramic-coated steel and the second a semiconductive (Si) substrate coated with multiple metal layers (e.g., superlattice). Signal intensity data from the analyte elements were used to calculate and compare depth resolution at diVerent discharge conditions for the well structured superlattice material.Depth resolution was determined from the intensity–time profile of each sample, based on changes in analytical response as sputtering progressed from one layer to the next as described by HoVmann and co-workers.28 Table 3 presents depth resolution (Dz) data as defined by Dz=q Imax (Dt/DI) (5) where q is the sputter rate, Imax is the maximum intensity for the respective analyte line and Dt/DI is the slope of the intensity–time profile. Slope values were calculated using points at 16 and 84% of Imax for each element, in accordance with Fig. 7 EVect of discharge gas pressure on resultant sputtered crater the definition employed by HoVmann and co-workers. For the shapes for glass slide samples: (a) pure Ar discharge and (b) He added multi-layered sample, depth resolution was determined using to 3 Torr base Ar pressure (rf power=30W). data from both the topmost and deepest layers to compare resolution as a function of depth. The sputtering rate, signal crater bottom becomes severe. As indicated previously, sputter- intensity and depth resolution values for the two sample types ing characteristics for the glass sample are more sensitive to are summarized in Table 3.It should be noted that the samples discharge gas conditions than metallic samples. Although not and discharge conditions employed here were chosen to illuspursued in this study, it is believed that flat craters may be trate the eVects of He gas addition to Ar plasmas, in terms of obtained at even higher total pressures, with the proper depth profiling, rather than to define the analytical limits of optimization of operating conditions. rf-GD-AES.Since it is known that dc-bias and ion mean free path Depth profiles of the ceramic-coated, metallic sample are conditions change with increased pressure,24–26 a logical expla- pictured in Fig. 8. This sample consists of a steel substrate nation for the consistency of crater shape (as He is added) with a ~1 mm ceramic (primarily SiO2 and MgO) coating on may be based on the consistency of Ar density within the the surface.Note that this sample is not expected to yield a discharge plasma. Remember that added He does not increase step-type response of the analytes in the interface region as the frequency of charge exchange reactions as does Ar; however, this is probably a spray coated layer that is further annealed a small eVect on redeposition can be expected. It follows that at high temperature.As such, a great deal of interdiVusion of at suYciently high total pressures, the overall density of gas alloy and oxide materials is expected, as is revealed in the species will ultimately lead to deterioration of sputtering depth profiles. The sample was originally analyzed at 10 Torr characteristics as helium gas is added. Because the capacitance pure Ar and 20W power [Fig. 8(a)]. Based on previous steel manometer reflects the total gas pressure in the cell, regardless specimen sputtering data,15 a lower Ar operating pressure of the gas mixture, the gas-phase density is constant in these (e.g., ~5 Torr) would have produced a more desirable crater studies between the pure Ar and mixed gas cases.On the other profile. However, an Ar pressure of 10 Torr was chosen as a hand, there will most certainly be diVerences in total gas flow compromise value for comparing a range of Ar, He and mixed as the ratios vary.The fact that the introduction of the gases gas pressures. Analysis of the same sample at 5 Torr Ar, 5 Torr is far removed from the cathode surface suggests that any He (10 Torr total ) and the same power produced analyte convective perturbations would be small. The influence of gas signals slightly higher for Mg (II ) and three times higher for make-up is especially evident in the changes observed for the Si (I ) [Fig. 8(b)]. Increasing the He pressure to 10 Torr glass sample.Since Ar pressure critically influences sputtering (15 Torr total ) yielded greater returns in raw analyte signal conditions for the glass sample, excessive He partial pressures [Fig. 8(c)]. The net Si signal was, however, largely unchanged may require adjustments of the Ar partial pressure. Further owing to a concurrent increase in background at the Si (I ) investigation of this phenomenon is warranted in future studies, 288.15 nm line. Note that the Fe (I ) signal decreases signifi- but in general results consistent with pure Ar pressures can be cantly from the 10 Torr Ar condition to the 5 Torr He, 5 Torr expected for partial pressures of Ar in He–Ar mixed gas Ar condition, but nearly regains its original intensity under systems. the final sputtering conditions (Table 3).The decrease in Fe (I ) response is consistent with those of low-lying excited states, Assessment of depth resolution: analysis of layered samples as illustrated in previous sections of this paper, where the further increase in He pressure probably provides a gas phase The preceding discussion suggests that depth profile analysis can be enhanced by the addition of He gas to rf-GD methods atmosphere more like that of a 10 Torr Ar discharge.Very qualitative computation of the depth resolution for the analy- previously employing only pure Ar. As mentioned previously, J. Anal. At. Spectrom., 1999, 14, 1039–1048 1045Table 3 Sputtering rates, analyte emission intensities and depth resolution as a function of discharge gas composition for layered materials Imax/V Sample/ Rf power/ Ar pressure/ He pressure/ Sputtering rate/ type Fig.W Torr Torr mm s-1 Si (I ) Mg (II ) Fe (I ) Ceramic-coated steel 8(a) 20 10 — 0.017 0.45 3.95 5.26 8(b) 20 5 5 0.013 1.36 5.81 2.66 8(c) 20 5 10 0.013 1.99 9.05 4.72 Dz/mm Cu (I ) Cr (I ) Cu (I ) Cr (I ) Superlattice — Top layer 9(a) 15 3 — 0.011 1.79 0.61 0.074 0.061 9(b) 15 2 9 0.010 5.32 1.07 0.037 0.041 10 1.5 10 0.006 3.25 0.70 0.020 0.039 10 1.25 11 0.005 2.32 0.518 0.016 0.048 Bottom layer 9(a) 15 3 — 0.010 1.41 0.59 0.071 0.107 9(b) 15 2 9 0.009 4.55 1.01 0.056 0.093 10 1.5 10 0.006 2.96 0.59 0.046 0.103 10 1.25 11 0.004 2.05 0.37 0.053 0.103 ses remained largely unchanged based on Mg and Fe intensities, although, as seen in the temporal profiles in Fig. 8(b) and (c), the definition was greatly enhanced based on the Si signal following He addition.The reduction in sample sputtering rate does not account for this improvement, although improvements in crater shape would be predicted. The improvement here is simply due to the overall improvement in the Si (I ) S/N upon He addition, providing better definition in terms of DI. While the coated steel sample by nature does not contain highly defined layers, it serves as a good example of modern coated systems as the profiles clearly point to very diVerent spatial distribution of the Mg and Si in the ceramic coating. Overall, the addition of He to the plasma increased analyte emission signals while maintaining or enhancing depth resolution.The depth profiles illustrated in Fig. 9 obtained for the superlattice material reflect a sample with highly defined layers. This sample consists of 10 layers, each approximately 100 nm in thickness, deposited on a semiconductor grade silicon wafer. The ‘superlattice’ includes five layers of Cu alternating with five layers of Cr and Ni.The analysis conditions of 3 Torr pure Ar and 15 W power were chosen to best illustrate the highly defined layers of the superlattice, with the resulting depth profile shown in Fig. 9(a). As can be seen, the individual layers are well defined, with the calculated resolution values (shown in Table 3) for the topmost layers being of the order of 70 nm and an average of ~90 nm in the deepest layer, depending on the chosen element. A comparison to the resolution for the upper layer is inferior to that obtained by HoVmann and co-workers, who calculated a value of 25 nm,28 while the lowest levels in their study produced a value of ~60 nm.The addition of 9 Torr He to the base pressure of 3 Torr Ar (profile not shown) increased the sputter rate and changed the sputtering profile (crater shape) to the extent that the last few individual layers of the sample were no longer well defined in the analysis. As suggested by the crater shape study, lowering the Ar partial pressure from 3 to 2 Torr was expected to restore the sharp, well defined profile that was collected for the pure Ar condition [Fig. 9(b)]. These conditions yielded increases in intensity from 70 to 200% (Cr and Cu, respectively). In this case, the depth resolving powers were also enhanced, with the values in the uppermost layers being ~40 nm, and an average of ~75 nm. It must be stressed that this improvement represents the realization of better crater characteristics, as the actual sputtering rates are unchanged. Fig. 8 EVect of discharge gas composition on depth (temporal ) profiles While optimized depth resolution capabilities were not the of ceramic-coated (~1 mm) steel specimens: (a) 10 Torr Ar, (b) 5 Torr He–5 Torr Ar and (c) 10 Torr He–5 Torr Ar (rf power=20 W). ultimate aim of this study, the values obtained here are not 1046 J. Anal. At. Spectrom., 1999, 14, 1039–1048increased significantly with an increasing ratio of He5Ar pressure.Since the monitored Mg (II ) and Ar (I ) lines require relatively high energies for excitation, compared with the other lines monitored, He might be expected to enhance the sensitivity for diYcult to excite species and emission lines, such as non-metals. Practical application of He5Ar systems for rf-GD analysis requires further study and selection of appropriate analytical lines for all analytes of interest. If only atomic lines are considered, this study clearly indicates that Ar is the discharge gas of choice for bulk analysis, owing to the eYciency of Ar plasmas in exciting atomic transitions.Further investigation into the use of ion lines for bulk analysis using He–Ar mixed gas plasmas may well reveal that the limits of detection currently available using Ar based plasmas can be surpassed. The sputtering characteristics of mixed gas plasmas are surprising in comparison with previous published work which reported that the use of He severely compromises sputtering rates.In simple terms, He–Ar mixed gas plasmas exhibit nearly the same sputtering rates as those obtained given Ar pressure alone. Slight depressions in sputtering rate are observed with He addition, but not nearly in proportion to what might be expected based on the stoichiometry of the gas mixture. By the same token, the resultant crater shapes obtained for the case of mixed gas sputtering are analogous to those which would be obtained for a pure Ar plasma.Therefore, a scenario can be envisioned wherein an optimum Ar pressure may be established for the desired sputtering characteristics for a given sample and then He gas added beyond that point to optimize the optical emission response further. This phenomenon can be used to particular advantage in the case of depth profiling Fig. 9 EVect of discharge gas composition on depth (temporal ) profiles of Cu/Cr–Ni superlattice materials (100 nm each layer on silicon): applications.Optimum Ar pressure conditions for depth profi- (a) 3 Torr Ar; and (b) 9 Torr He–2 Torr Ar (rf power=15W). ling tend to be very specific and can restrict analyte sensitivity. However, sensitivity enhancement via added helium is not detrimental to depth resolution as He does not strongly far from those reported by others where this was the intended goal. Analyses at even higher partial pressures of helium (not influence the resultant crater shapes from GD sputtering.In summary, helium may be used as an additional parameter, shown because they are not visually diVerent) were conducted to probe the extent of signal enhancement. For each subsequent along with power and Ar pressure, to tune the glow discharge plasma for specific applications. addition of He, sputter rates decreased and, as a consequence, the analyte intensities also decreased, although not to the extent of the sputtering rates. Interestingly, depth resolution Acknowledgements determinations (Table 3) based on the uppermost layer of the sample improved with increasing He content, although the Financial support from the National Science Foundation under calculations based on the final (deepest) layers of the sample grant No.DMR-9727667 and from Jobin-Yvon, Division of remained substantially constant over the range of conditions Instruments SA, is gratefully acknowledged. (<5% RSD for Cr). Overall, depth resolution appears to be aVected only by sputtering profile (crater shape) and not by References average sputter rate.With this in mind, the 9 Torr He–2 Torr Ar condition produced the highest gains in analyte sensitivity 1 T. B. Reed, J. Appl. Phys., 1961, 32, 821. with no degradation to depth resolution. 2 J. W. H. Iam and G. Horlick, Spectrochim. Acta, Part B, 1990, 45, 1313. 3 F. G. Smith, D. R. Wiederin and R. S. Houk, Anal. Chem., 1991, Conclusions 63, 1458. 4 S. Nam, R. Masamba and A. Montaser, Anal.Chem., 1993, 65, The atomization and excitation characteristics of He–Ar mixed 2784. gas plasmas were compared with the traditionally used pure 5 A. Montaser, K. D. Ohls and D. W. Golightly, in Inductively argon discharge gas for rf-GD-AES analyses. Specifically, Coupled Plasmas in Analytical Atomic Spectrometry, ed. emission yield, sputter rate and crater shape were studied for A. Montaser and D. W. Golightly, VCH, New York, 2nd edn., 1992. both conductive and non-conductive samples as a function of 6 K.Oishi, T. Okumoto, T. Lino, M. Koga, T. Shirasaki and He and Ar partial pressures. The results from this comparison N. Furuta, Spectrochim. Acta, Part B, 1994, 49, 901. were applied to the depth profile analysis of coated and multi- 7 HP G2350A Atomic Emission Detector for Gas Chromatography, layered samples. Hewlett-Packard, Palo Alto, CA. The REY values were found generally to be lower for He 8 K. Wagatsuma and K. Hirokawa, Spectrochim. Acta, Part B, based plasmas than Ar plasmas at the same pressures, for the 1987, 42, 523. 9 K. Wagatsuma, J. Anal. At. Spectrom., 1996, 11, 957. monitored emission lines. This is surprising, as helium has 10 K. Wagatsuma, K. Hirokawa and N. Yamashita, Anal. Chim. been reported generally to enhance emission intensities. Only Acta, 1996, 324, 147. one ion line was monitored [Mg (II ) 280.15 nm], and this line 11 J. Woo, D. Moon, T. Tanaka, M. Matsuno and H. Kawaguchi, was found to have greater REYs for the mixed gas condition. Anal. Sci., 1996, 12, 459. The results agree with other studies that indicate helium aids 12 W. B. Teo and K. Hirokawa, Surf. Interface Anal., 1989, 14, 143. in more eYciently exciting ion lines than does argon. At a 13 S. J. Christopher, M. L. Hartenstein, R. K. Marcus, M. Belkin and J. A. Caruso, Spectrochim. Acta, Part B, 1998, 53, 1181. fixed total pressure, the intensity of the Ar (I ) 404.44 nm line J. Anal. At. Spectrom., 1999, 14, 1039–1048 104714 M. A. Parker, M. L. Hartenstein and R. K. Marcus, Spectrochim. 22 N. Laegreid and G. K. Wehner, J. Appl. Phys., 1961, 32, 365. 23 D. Rosenberg and G. K. Wehner, J. Appl. Phys., 1962, 33, 1842. Acta, Part B, 1997, 52, 567. 24 B. Chapman, Glow Discharge Processes,Wiley, New York, 1980. 15 M. A. Parker, M. L. Hartenstein and R. K. Marcus, Anal. Chem., 25 C. Lazik and R. K. Marcus, Spectrochim. Acta, Part B, 1992, 1996, 68, 4213. 47, 1309. 16 A. Bengtson, Spectrochim. Acta, Part B, 1994, 49, 411. 26 M. Parker and R. K. Marcus, Spectrochim. Acta, Part B, 1995, 17 R. Payling, D. G. Jones and S. A. Gower, Surf. Interface Anal., 50, 617. 1993, 20, 959. 27 J. Angeli, K. Haselgrubler, E. M. Achammer and H. Burger, 18 M. Belkin, J. A. Caruso, S. J. Christopher and R. K. Marcus, Fresenius’ J. Anal. Chem., 1993, 346, 138. Spectrochim. Acta, Part B, 1998, 53, 1197. 28 F. Prassler, F. HoVmann, J. Schumann and K. Wetzig, J. Anal. 19 Y. Ye and R. K. Marcus, Spectrochim. Acta, Part B, 1996, 51, 509. At. Spectrom., 1995, 10, 677. 20 P. Sigmund, Phys. Rev. A, 1969, 20, 855. 21 G. Carter and J. S. Colligon, Ion Bombardment of Solids, Heinemann Educational Books, London, 1968, ch. 7. Paper 9/01517E 1048 J. Anal. At. Spectrom., 1999, 14, 1039–1048

ISSN:0267-9477    

DOI:10.1039/a901517e

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Slurry sampling of silicon nitride powder combined with fluorination assisted electrothermal vaporization for direct determination of titanium, yttrium and aluminum by ICP-AES Peng Tianyou, Jiang Zucheng* and Qin Yongchao Department of Chemistry, Wuhan University, Wuhan 430072, China Received 25th November 1998, Accepted 18th May 1999 The vaporization behavior of silicon and three refractory trace elements (Al, Ti and Y) were studied in the presence and absence of a PTFE emulsion as fluorinating reagent and applying an electrothermal ICP-AES coupled system.It was found that during a 60 s ashing step at 700 °C about 90% of 100 mg of Si3N4 can be decomposed and evaporated without considerable losses of the trace elements investigated. Calibration could be carried out by the standard addition method and the calibration curve method applying spiked slurries and aqueous standard solutions with peak height intensity measurements, respectively. The detection limits varied from 0.11 mg g-1 (Al ) to 0.09 mg g-1 (Ti) with RSD 1.9–4.2%.Advanced ceramics, owing to their unique properties, are refractory carbides and eliminates memory eVects, but also markedly decreases matrix eVects and the influence of the increasingly applied in various fields of industry. For example, particle size of the samples. In the present work, we directly silicon nitride (Si3N4) ceramics have high chemical resistance determined trace amounts of refractory elements (Al, Ti and and excellent mechanical properties at high temperatures and Y) in Si3N4 ceramic powders with slurry sampling ETV-ICP- they are used as high-density, corrosion- and heat-resistant AES by using PTFE as halogenating reagent.Selective volatil- materials for turbine blades and ceramic engines. The presence ization between the analytes and the matrix was used to of metallic impurities such as Al and Ti in Si3N4 ceramics improve the sensitivity and reduce the matrix interference.adversely aVects their mechanical properties. Moreover, several substances (e.g., Y2O3) play an important role in the production of the Si3N4 ceramics. Therefore, in addition to the Experimental characterization of compact ceramics, the analysis of the basic Instrumentation and main operating conditions products is also important so as to optimize production procedures, including the exclusion of contamination at the A ca. 27 MHz ICP spectrometer with a power of 2 kW(Beijing various production stages and tailoring the properties of the Broadcast Equipment Factory, Beijing, China) and a convenmaterials by controlling the levels of impurities.Trace impurit- tional plasma silica torch were used. A laboratory-made graphite furnace vaporizer15–18 was employed as the vaporiz- ies in ceramic powder have routinely been determined by the ation device. The radiation from the plasma was focused as a use of ICP-AES1–5 and AAS.6,7 151 straight image on the entrance slit of a WDG500–1A Solid sampling ICP-AES methods can be recommended monochromator (Beijing Second Optics, Beijing, China) owing to the elimination of the time-consuming dissolution having a reciprocal linear dispersion of 1.6 nm mm-1.The procedure. Direct solid sampling techniques for ICP include evolved components were swept into the plasma excitation the direct insertion technique,8,9 slurry nebulization1–3,10 and source through a 35 cm×4 mm id polythene tube by a stream electrothermal vaporization (ETV).11,12 ETV seems to be a of argon.The transient signals were detected with an R456 very promising technique for routine applications, because of type photomultiplier tube (Hamamatsu, Tokyo, Japan) and a the inexpensive equipment, simple handling and the possibility laboratory-built direct current amplifier, and recorded with a of calibration with aqueous standards. Furthermore, selective U-135C recorder (Shimadzu, Kyoto, Japan).volatilization between the analytes and the matrix through sequential volatilization of the sample components is feasible. Standard solutions and reagents Recently, several approaches to solid sampling ETV have been described, including the application of pelletized solids,13 of Standard solutions (1 g L-1) of Al, Ti and Y were prepared from their Specpure oxides by applying a conventional method. the miniature cup technique,14 of halocarbon vapor to promote A 60% m/v PTFE emulsion (viscosity 7×10-3–15×10-3 Pa fast vaporization4 and of slurry sample injection.15–18 Of these, s) was purchased from the Shanghai Institute of Organic the last mentioned method has demonstrated its potential as Chemistry (Shanghai, China).All other reagents were of a promising method for direct solid analysis. analytical reagent grade or better. Doubly distilled, de-ionized Generally, it is diYcult to vaporize completely ceramic water and HF and HNO3 were further purified by applying powder from the ETV device because of its refractory nature.sub-boiling distillation. Si3N4 powder (d<80 mm) was provided Favorable conditions for the selective volatilization of analyte by the Northwest Iron and Steel Research Institute of China from refractory materials could be achieved with the use of a (Xi’an, China). halogenating reagent (e.g., CF2Cl2 12 and PTFE15,16). Our previous studies15–18 indicated that the direct determination of Preparation of sample trace elements in high-purity Y2O3, SiO2 and biological samples could be carried out by slurry sampling fluorination An Si3N4 powder sample (50 mg, d<80 mm) was weighed into assisted ETV-ICP-AES.The experimental results also showed calibrated flasks, 0.5 mL of 60% m/v PTFE, 0.5% agar and 0.1% Triton X-100 solutions were added and the mixture was that the addition of PTFE not only prevents the formation of J. Anal. At. Spectrom., 1999, 14, 1049–1053 1049diluted to 5 mL with water.The resulting mixture (1% m/v slurry) was dispersed with an ultrasonic wave vibrator for 20 min and the calibrated flasks were shaken prior to any sampling. Solutions containing the same amounts of Si and PTFE were prepared by dissolving the same mass of sample with HF–HNO3 (1+1) in a high pressure system at 180 °C for 12 h.2 These samples were used to study the vaporization behavior. For the standard addition method, the slurries prepared as above were spiked with appropriate amounts of aqueous multielement standard solutions; for the calibration curve method, a multi-element standard series containing PTFE (6% m/v) were also prepared from standard solutions. Analytical procedure After the plasma had stabilized, 10 mL of sample were pipetted into the furnace.After being dried and ashed, the analyte was vaporized and carried into the plasma by argon carrier gas, Fig. 1 EVect of ashing temperature on the signal intensities of Al, Ti, the emission signal from the plasma was recorded and the Y and Si (vaporization temperature: 2700 °C).Al, 0.2 mg L-1 with peak height was measured for quantification. Calibrations PTFE; Al¾, 0.2 mg L-1 without PTFE; Ti, 0.2 mg L-1 with PTFE; were carried out by the standard addition and calibration Ti¾, 0.2 mg L-1 without PTFE; Y, 0.5 mg L-1 with PTFE; Y¾, 10mg curve methods. Six replicates were analyzed by using slurry L-1 without PTFE; Si, 0.4 mg L-1 with PTFE; Si¾, 10mgL-1 and solution sampling.A similar number of experiments were without PTFE. performed using a blank solution without sample to evaluate the blank values. Results and discussion Optimization of ETV-ICP-AES system The ETV-ICP-AES operating parameters were optimized on the basis of signal-to-background ratios by using standard solutions of analytes containing 6% m/v PTFE. From a comparison with pneumatic nebulization (PN)-ICP-AES, there were no obvious diVerences in the operating parameters between PN-ICP-AES and ETV-ICP-AES with regard to rf power and observation height but the carrier gas flow rates diVered.The experimental parameters selected are given in Table 1. Typical ashing and vaporization curves for the elements of interest in standard solutions with/without PTFE are shown in Fig. 1 and 2. Our previous studies demonstrated that both the analyte and matrix could react with the pyrolysis products of PTFE in the graphite tube and be vaporized in the form of the corresponding fluorides, when the temperature in the graphite tube reaches the chemical decomposition temperature Fig. 2 EVect of vaporization temperature on the signal intensities of of PTFE (400 °C). Therefore, the addition of PTFE can Al, Ti, Y and Si (ashing temperature: 700 °C). Al, 0.2 mg L-1 with prevent the formation of refractory carbides at high tempera- PTFE; Al¾, 0.2 mg L-1 without PTFE; Ti, 0.2 mg L-1 with PTFE; tures, which has also been proved by using XRD analyses of Ti¾, 0.2 mg L-1 without PTFE; Y, 0.5 mg L-1 with PTFE; Y¾, 10mg the residues of ETV after recycling heating.16–18 As can be L-1 without PTFE; Si, 0.4 mg L-1 with PTFE; Si¾, 10mgL-1 without PTFE.seen from Fig. 1 and 2, the ashing temperatures of the same Table 1 ETV-ICP-AES operating conditions Incident power/kW 1.2 Carrier gas (Ar) flow rate/L min-1 0.5 (Al, Si); 0.6 (Ti); 0.7 (Y) Auxiliary gas (Ar) flow rate/L min-1 0.8 Coolant gas (Ar) flow rate/L min-1 16 (Al, Y, Ti); 18 (Si) Observation height/mm 12 Entrance slit width/mm 25 Exit slit width/mm 25 Wavelength/nm Al 308.215, Ti 334.941, Y 371.03, Si 251.611 Drying temperature/°C 100, ramp 10 s, holds 20 s Ashing temperature/°C 400, ramp 10 s, holds 20 s ( Ti, Si) 1200, ramp 10 s, holds 20 s (Al ) 1300, ramp 10 s, holds 20 s (Y) Vaporization temperature/°C 1500, 3 s (Si), 2340, 3 s (Ti), 2240, 3s (Al ), 2460, 4 s (Y) Clear-out temperature/°C 2700, 4 s Sample volume/mL 10 1050 J.Anal. At. Spectrom., 1999, 14, 1049–1053elements decreased significantly in the presence of PTFE. It is obvious from Fig. 2 that PTFE has a great influence on the vaporization behavior of both the analyte and matrix, and the vaporization curves reach their plateaux at lower temperature. This means that the fluorination reactions between the analytes and the pyrolysis products of PTFE are complete. In contrast, no signal plateaux were found in the tested temperature range in the absence of PTFE.Ashing time Fig. 3 shows the eVects of the ashing time on the signal intensities of the tested elements at an ashing temperature of 700 °C. It is obvious that the ashing time has no great influence on the signal intensities of Al, Ti and Y. However, the signal intensity of Si declines on prolonging the ashing time owing to the formation of gaseous SiF4 . Based on these results, there is a possibility of removing the matrix (Si) at the ashing stage by prolonging the ashing time.Fig. 4 EVect of matrix (Si) on the signal intensity of Al, Y and Ti in the presence of PTFE. Conditions: 10 mL of standard solutions with EVects of matrix concentrations of Al 0.2, Ti 0.2 and Y 0.5 mg L-1 and containing 0.6 mg of PTFE. The experimental results show that the signal intensities of the analyte and the plasma discharge were not stable with the large amounts of matrix (Si) entering the ICP with the analyte. this paper, unless stated otherwise, the PTFE concentration in Fortunately, as shown in Fig. 3, selective volatilization between the sample was 6% m/v.the matrix and the analyte could be carried out by prolonging the ashing time at an ashing temperature of 700 °C. Therefore, Comparative investigation of fluorinating vaporization behavior a 60 s ashing time was chosen to reduce the matrix influence Fig. 5 shows the typical emission profiles of Y in solution or as much as possible, and the analytical signal of Ti decreased slurry at the same concentration. In the absence of PTFE, the only slightly.Fig. 4 shows the largest tolerable matrix concensignal intensity of Y in the solution or slurry of Si3N4 was trations are 12 g L-1 for Al and Y and 20 g L-1 for Ti at an very weak, and a broad signal profile with tailing was recorded. ashing temperature of 700 °C. Obviously, the matrix inter- Furthermore, the residual signals for Y were almost the same ference observed in this work is low.A possible reason is that as the original signal. However, in the presence of the fluorin- the relative vaporization eYciency of silicon is about 90% (see ating reagent PTFE, a sharper, more intense and symmetrically Table 2) at 700 °C. The relative vaporization eYciency is shaped peak without tailing was obtained for Y, and there defined as the ratio of the signal intensity of an element at a was no memory eVect on the performance after vaporization certain ashing temperature and a 60 s ashing time to that of of Y in solution and slurry.Similar results were also observed the element at a 400 °C ashing temperature, which is discussed for Al and Ti. The possible reasons for this are that the below. Once the concentration of the matrix (Si) exceeds the temperature supplied by the conventional ETV device is not tolerance level, the signal intensity of the analyte starts to high enough to destroy and vaporize repeatedly the refractory decline.The reason may be a lack of suYcient PTFE, which ceramics, but using PTFE as a chemical modifier, the fluoride leads to incomplete vaporization or a change in the vaporizproduced from decomposition of PTFE can react very strongly ation rate of the samples. However, our previous studies17,18 with the ceramic. Hence, the addition of PTFE not only showed that the optimum concentration was 6% m/v, otherwise eliminates the diVerence in the form in which the samples the stability of the plasma decreased markedly.Therefore, in exist, but also prevents the formation of refractory carbides, promotes vaporization eYciency and improves the analytical sensitivity. Moreover, the profiles and the height of the emission signal of the analytes in the slurry are very similar to those in the solution in the presence of PTFE. Hence we can conclude that the vaporization behavior of the analytes in slurry and in solution are very similar; the standard solutions can be used for the calibration of slurry samples and the systematic errors in the analytical results can be ignored.As described above, the low level matrix eVects can be attributed to the removal of most of the matrix at the ashing stage, but once the ashing temperature exceeds 400 °C, the analyte (Ti) is partially lost owing to the formation of TiF4 with a very low sublimation temperature (284 °C). To obtain quantitative information about the vaporization processes, a solution containing PTFE (6% m/v) and Al, Ti, Y and Si (0.4 mg L-1) was prepared from the standard solutions.The signal intensities of the above elements were then determined subsequent to various ashing steps carried out at ashing temperatures of 400, 500, 700, 900, 1100 or 1300 °C for 60 s. Fig. 3 EVect of the ashing time on the signal intensities of the four The relative vaporization eYciencies of the analytes are given elements investigated in presence of PTFE at an ashing temperature in Table 2.As can be seen, when the ashing temperature is of 700 °C. Conditions: 10 mL of standard solutions with concentrations increased from 400 to 1100 °C, the losses of Al and Y are of Al 0.2, Ti 0.2, Y 0.5 and Si 0.4 mg L-1 and containing 0.6 mg of PTFE. negligible, but the relative vaporization eYciencies of Ti and J. Anal. At. Spectrom., 1999, 14, 1049–1053 1051Table 2 EVect of ashing temperature on the relative vaporization eYciency of analytes (mean±s, n=3) with an ashing time of 60 s Element 400 °C 500 °C 700 °C 900 °C 1100 °C 1300 °C Al 100.1±3.0 98.9±2.6 98.6±2.2 95.2±2.4 93.7±2.2 87.2±2.0 Ti 99.3±2.5 91.7±2.0 85.3±1.9 73.2±2.2 50.5±2.0 42.3±1.7 Y 99.8±2.7 98.9±2.3 98.3±2.1 94.2±2.3 93.1±1.9 90.9±1.3 Si 98.0±3.2 43.2±3.0 10.3±2.9 — — — Fig. 5 Typical signal profiles of Y using a standard solution or Si3N4 slurry with the same analyte concentration and with or without PTFE: Fig. 6 Calibration curves (peak height) of Al, Ti and Y determined a, Y in solution with PTFE; b, Y in solution without PTFE; c, Y in using multi-element standard solutions containing PTFE (6% m/v).slurry with PTFE; d, Y in slurry without PTFE; a¾, b¾, c¾ and d¾, residual signals detected during the second heating period. Sample analysis Si decrease. Table 2 shows that when the ashing temperature and time are chosen as 700 °C and 60 s, respectively, about The proposed method was applied to determine refractory 90% of the silicon nitride is released without significant losses elements (Al, Y and Ti) in a real sample (Si3N4).The sample of the analyte of interest. In other words, under the above was also analyzed by dissolution-based PN-ICP-AES. The conditions, the determination of Al, Ti and Y in Si3N4 powders analytical results (Table 4) are in good agreement. is reliable and accessible. Analytical characteristics Conclusions The detection limit, the lowest concentration level that can be The use of PTFE not only eVectively destroys the skeleton of determined to be statistically diVerent from a blank, is defined Si3N4, prohibits the formation of refractory carbides, removes as three times the within-batch standard deviation of a signal matrix and memory eVects and promotes the vaporization blank determination, corresponding to the 99% confidence eYciency, but eVects selective volatilization between the analevel.The blank signal was the signal obtained from the lytes and the matrix.Moreover, the experimental results also vaporization of 10 mL of 6%m/v PTFE slurry containing indicated that the use of PTFE can eliminate eVectively the 0.05% agar and 0.01% Triton X-100. Table 3 presents the diVerence in the form in which the sample exists, hence detection limits calculated for a ca. 10gL-1 Si3N4 slurry, calibration could be carried out with aqueous standards. This compared with those obtained using conventional ETV and method can be expected to become a routine method for the PN-ICP-AES. As can be seen, compared with conventional direct analysis of high purity and refractory ceramic materials.ETV and PN-ICP-AES, the detection limits for Al, Y and Ti are improved to diVerent extents. The precision (RSD) for six replicate measurements were in the range 1.9–4.2%. Acknowledgements Fig. 6 shows the calibration curves for Al, Ti and Y determined by ETV-ICP-AES using multi-element standard solu- This work was supported by the National Natural Science Foundation and the Education Commission Foundation of tions and PTFE as chemical modifier.The graphs are linear over a concentration range of three orders of magnitude. China. Table 3 Comparison of detection limits and RSDs PN-ICP-AES/mg l-1 ETV-ICP-AES/mg g-1 Wavelength/ Element nm Solution19 Slurry20 Literature21 This worka RSDa (%) Al 308.215 50.0 120.0 17 0.11 4.1 Ti 334.941 12.0 12.0 0.95 0.09 1.9 Y 371.03 10.0 12.0 — 0.10 4.2 aSampling volume 10 mL, six replicate measurements, concentrations of Al, Y and Ti 0.2, 0.2 and 0.5 mg L-1, respectively. 1052 J. Anal. At. Spectrom., 1999, 14, 1049–1053Table 4 Concentrations of Al, Y and Ti in Si3N4 powder determined by slurry sampling and fluorination assisted ETV-ICP-AES Standard addition Calibration curve Calibration curve PN-ICP-AESb/ Element methoda/mg g-1 methoda/mg g-1 methodb/mg g-1 mg g-1 Al 10.6±1.2 10.2±1.0 10.5±1.0 11.0±0.9 Y 7.20±1.1 7.42±0.79 7.50±0.55 7.80±0.49 Ti 3.02±0.40 3.27±0.27 3.25±0.21 3.32±0.3 aSlurry sampling for ETV.bDissolved in HF–HNO3 and dispensed into the ETV. 12 P. Barth, S. Haupton and V. Krivan, J. Anal. At. Spectrom., 1997, References 12, 1359. 13 V. Karanassios, J. M. Ren and E. D. Salin, J. Anal. At. Spectrom., 1 I. Varga, Gy. Zaray and J. Szepvolyi, Mikrochim. Acta, 1989,III, 1991, 6, 527. 381. 14 I. Atsaya, T. Itoh and T. Kyrotaki, Spectrochim. Acta, Part B, 2 Gy. Zaray, A. Farakas and I. Varga, Acta Chim. Hung., 1991,128, 1991, 46, 103. 489. 15 T. Y. Peng and Z. C. Jiang, Anal. Sci., 1997, 13, 595. 3 Gy. Zaray, I. Varga and T. Kantor, J. Anal. At. Spectrom., 1994, 16 T. Y. Peng, Y. C. Qin and Z. C. Jiang, J. Rare Earths, 1997, 9, 707. 15, 149. 4 T. Kantor and Gy. Zaray, Fresenius’ J. Anal Chem., 1992, 342, 927. 17 Z. C. Jiang, B. Hu, Y. C. Qin and Y. Zeng, Microchem. J., 1996, 5 S. Mann, D. Geilenberg, J. A. C. Broekaert and M. Jansen, 53, 326. J. Anal. At. Spectrom., 1997, 12, 975. 18 B. Hu, Z. C. Jiang, Y. C. Qin and Y. Zeng, Anal. Chim. Acta, 6 K.-Ch. Friese and V. Krivan, Anal. Chem., 1989, 335, 637. 1996, 319, 255. 7 T. Nakamura, Y. Noike, Y. Koizami and J. Sato, Analyst, 1995, 19 M. Franek and V. Krivan, Fresenius’ J. Anal Chem., 1992, 342, 120, 89. 118. 8 Gy. Zaray, J. A. C. Broekaert and F. Leis, Spectrochim. Acta, Part 20 R. Lobinski, W. Van Brom, J. A. C. Broekaert, P. Tschopel and B, 1988, 43, 241. G. Tolg, Fresenius’ J. Anal Chem., 1992, 342, 563. 9 M. Reisch, H. Nickel and M. Mazurkiewicz, Spectrochim. Acta, 21 Gy. Zaray, T. Kantor, G. WolV, Z. Zadgorska and H. Nickel, Part B, 1989,44, 307. Mikrochim. Acta, 1992, 107, 345. 10 B. Raeymaekers, T. Graule, J. A. C. Broekaert, F. Adams and P. Tschopel, Spectrochim. Acta, Part B, 1988, 43, 1923. 11 L. Ebdon and P. Goodall, Spectrochim. Acta, Part B, 1992, 47, 1247. Paper 8/09221D J. Anal. At. Spectrom., 1999, 14, 1049–1053 1053

ISSN:0267-9477    

DOI:10.1039/a809221d

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Determination of cadmium in fly ash and metal alloy reference materials by inductively coupled plasma mass spectrometry and chemometrics Ludvig Moberg,a Karin Pettersson,b Ingemar Gustavssonb and Bo Karlberg*a aDepartment of Analytical Chemistry, Stockholm University, S-106 91 Stockholm, Sweden. E-mail: bo.karlberg@anchem.su.se bSwedish Institute for Metals Research, Drottning Kristinas va�g 48, S-114 28 Stockholm, Sweden Received 22nd February 1999, Accepted 29th April 1999 Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine Cd in five certified reference materials consisting of fly ashes and metal alloys following dissolution.Spectral overlap is observed for all analytically important Cd isotopes in both matrices, being particularly severe with the latter matrix. To overcome this problem, chemometric methods were applied. Metals incorporated in this study were Cd, Mo, Zr, Pd and Sn, while Rh was used as internal standard, the recorded mass numbers (m/z) ranging from 90 to 124.Regression models were based on data derived from hundreds of standards, and it was shown that it is possible to use results of several ICP-MS runs in the same model. Fly ash and metal alloy matrices were evaluated separately. The root mean square error of cross-validation (RMSECV) for the fly ash domain was 0.64 mg l-1 and 1.42 mg l-1 for the metal alloy domain. The Cd concentrations in the reference materials were predicted accurately, and since the same sample was wet-digested and analysed several times and in diVerent runs, figures of reproducibility are reported.Multivariate and univariate regression approaches were also compared. Univariate models were comparable in terms of predictive ability for the fly ash domain, but not for the metal alloy domain. calibrate for Cd in two diVerent matrices, Mo–Cd and Introduction Zr–Mo–Ru–Cd–In. The calibration models were based on About ten years ago, ICP-MS was introduced as a new approximately 20 samples.The Cd concentration was varied analytical technique to determine trace and ultra-trace elements on three levels: 0, 50 and 100 mg l-1. The authors concluded in diVerent kinds of sample materials routinely.1–4 It then that ‘multivariate calibration schemes do tend to require a seemed to be a straightforward technique, but due to inter- large number of standards, which is disadvantageous in terms ferences many samples were shown to be troublesome to of analysis time’.Variations in the plasma conditions resulted analyse.5–8 For instance, metal alloys yield complex matrices in varying MO+/M+ ratios.18 However, the prediction results since the major elements will be left in the sample solution were not greatly influenced. They also addressed the advantage after the wet-digestion procedure. Ions in the acids used for of having a calibration model that can be used for diVerent the wet digestion may react to give polyatomic ions and the operating conditions or even on diVerent instruments.In a metal oxides/hydroxides formed in the plasma can cause paper by Venth et al.,19 multivariate calibration was applied spectral overlap. The major metal alloy matrix elements are for several trace metals, including Cd in a Mo–Zr matrix Fe, Ni, Cr and Mo, and they can all react with Ar in the corresponding to a Mo–Zr alloy. The Cd concentration range plasma, forming argides, ArMe.Finally, isobaric overlap was 0–400 mg l-1. Analysts working with ICP-MS perform caused by elements may occur. Consequently, few ICP-MS calibration and analysis in the same run, since there is a drift based applications for metal alloy samples9–16 have been in the system implying day-to-day variations. For more complireported. cated matrices presenting several interferences, many standards are required as well as frequent recalibration. Cadmium is of great environmental concern, currently being In this study, ICP-MS data collected for two diVerent the subject of considerable attention in regulations and restricsample types, metal alloys and fly ashes, were subjected to tions. The concentration of Cd in metal alloys is extremely multivariate analysis.The properties of various calibration low, at mg kg-1 levels or lower, making the analysis diYcult models based on several runs for low levels of Cd were and demanding. In particular, for stainless steel, the problems investigated.Up to 25 variables, i.e. mass numbers, were become severe, since the element Mo is present at a concenincluded in the projection to latent structures (PLS) models. tration level of several per cent. The fraction of the formed These models were validated with certified reference materials. oxide ion, MoO+, can, in the plasma, amount to 0.1% of the total Mo+ concentration. These MoO+ ions then overlap all the Cd peaks in the mass spectrum. One exception, however, is the 106Cd isotope, but the relative abundance of this naturally Experimental occurring isotope is only 1.2%.Instrumentation There are several papers dealing with chemometrics applied to ICP-MS, a few of which focus on the calibration for Cd. The ICP-MS instrument, a VG Elemental PlasmaQuad 2 Plus In one of the first papers,17 multiple linear regression (MLR) (Fisons, Winsford, UK), was equipped with a Gilson sample exchanger, and a thermostatically controlled bath (model and principal component regression (PCR) were used to J.Anal. At. Spectrom., 1999, 14, 1055–1059 1055Table 1 Operating conditions for the ICP-MS instrument to those of the samples. All results were evaluated with internal standardisation. The standards contained 1, 2, 3, 4 or 5 of the Plasma power 1.25 kW components of interest, namely Cd, Zr, Mo, Sn and Pd. In Argon gas flow— standards with two or more constituents, the concentrations Coolant 12.2–12.7 l min-1 were varied in an experimental design fashion in order to Auxiliary 0.40–0.45 l min-1 Nebuliser 1.0–1.2 l min-1 avoid covariance between the metal concentrations.The con- Spray chamber system Teflon centration intervals were chosen to correspond to those of the Nebuliser V-groove metals of concern in fly ash and metal alloys, namely: Cd: Sample cone Ni, 1.0 mm orifice 0–50; Zr: 0–800; Mo: 0–50 000; Sn: 0–30 000; Pd: 0–50 mg l-1. Skimmer cone Ni, 0.75 mm orifice Altogether 460 samples, standards and blanks were analysed Vacuum— in 17 runs, 385 of these being standards.At the beginning of Intermediate stage 1.0×10-4 mbar Expansion stage 2.4–3.0 mbar each run some standards with only Cd were analysed, normally Analyser stage 4.0×10-7–2.1×10-6 mbar 8–10, in the interval 0–50 mg l-1. Reference material. The reference materials chosen for this investigation were iron and nickel base alloys and coal fly F3CH, Haake, Karlsruhe, Germany). The operating conashes, see Table 2.Cadmium concentrations, as well as interfer- ditions are given in Table 1. Solutions were introduced into ing element concentrations, are shown in Table 2. Some of the the ICP-MS instrument by a peristaltic pump, at an uptake interfering elements have not been certified but estimated by rate of 1.0 ml min-1. The spray chamber system was thermoan appropriate analytical method; certified and estimated stated to 6 °C by water-cooling. The instrument was operated concentrations are given in bold and italic type, respectively.in peak jumping mode, with 20 sweeps and 3 points per peak, If known, the 95% confidence interval is given. using a dwell time of 10.24 ms, duplicate measurements being made for each solution. The blank, the standards and the sample solutions were aspirated for 90 s, allowing the system Data: acquisition and analysis to equilibrate before data acquisition. A washing time of 90 s The results are based on measurements of the following was applied between each solution.isotopes: Prior to analysis, the analytical signal was optimised by 90Zr, 91Zr, 92Zr, 94Zr and 96Zr aspirating an indium solution (100 mg l-1), while adjusting the 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo and 100Mo torch position and the lens-settings until the ximum count 104Pd, 105Pd, 106Pd, 108Pd and 110Pd rate was achieved. 103Rh 106Cd, 108Cd, 110Cd, 111Cd, 112Cd, 113Cd, 114Cd and 116Cd Reagents 113In Hydrochloric, nitric and hydrofluoric acids used for the 112Sn, 114Sn, 116Sn, 117Sn, 118Sn, 119Sn, 120Sn, 122Sn and 124Sn preparation of samples and standards were of suprapur grade The relative abundances of the naturally occurring isotopes (Merck, Darmstadt, Germany).For washing, hydrochloric of these metals, as well as the masses of potentially overlapping and nitric acids of pro analysi grade were used (Merck). oxides, are given in Table 3. Furthermore, other interferences Ultrapure water was acquired from a Milli-Q water exist such as metal hydroxides (e.g. 96Mo16OH), argides (e.g. purification system (Millipore, Bedford, MA, USA), which 10Ar58Fe), chlorides (e.g. 56Fe35Cl ) and nitrides (e.g. was supplied with cation–anion exchanged tap water. The 97Mo14N). However, usually in metal alloy matrices, these following stock standard solutions (Ventures Inc., Lakewood, interferences are of minor importance when compared with NJ, USA), 1000 mg l-1, were used: Cd, Zr, Mo, Sn, Rh and Fe.the oxides formed. There are several chemometric methods for analysing multivariate data; in the context of this work, Washing multivariate data refer to the fact that each sample/standard is represented by several mass numbers. A commonly applied To prevent contamination, all the laboratory ware and the multivariate calibration technique is projections to latent struc- sample introduction system of the ICP-MS instrument were tures (PLS), and a variant of the PLS-algorithm which can soaked in 7 M HNO3 for at least 24 h and then rinsed carefully handle several responses, here metal concentrations, is called with ultrapure water.PLS2. PLS does not require fully selective detectors. A thorough description of these techniques is given in Martens and Samples and standards Naes.20 If potential interferences, at appropriate levels, are present in the calibration set, a multivariate model can predict Sample preparation. About 100 mg of the sample were weighed accurately and transferred into a Teflon pressure the concentration of the analyte in future unknown samples with interferences present.This is sometimes referred to as the vessel and dissolved in 3 ml of HCl, 1 ml of HNO3 and 0.5 ml of HF using a microwave oven (MDS 2000, CEM, Matthews, ‘first order advantage’. Thereby it is important to span the domain, but for more complicated matrices with several inter- NC, USA) and applying the following program: 10% power for 10 min, 40% power for 10 min, 80% power for 10 min.ferences, many standards are required making this calibration approach tedious. In addition, there is a drift in the system, Full power (100%) corresponds to 600 W. Rh was used as internal standard at a concentration of 10 mg l-1. After which implies day-to-day variations. Analysts working with ICP-MS perform calibration and analysis in the same run. To addition of internal standard, the mixture was diluted to 100 ml in a poly(propylene) flask by weighing.Blank, stan- circumvent the need for recalibration, the possibility to build up the calibration set on several runs was investigated. dards and samples were prepared in a clean bench. Micropipettes were used for preparation of standards and Raw data were exported from the instrumental software as ASCII files, reorganised in MATLAB and finally imported to sample solutions. UNSCRAMBLER 7.01.21 All chemometric models were calculated with this program.The multivariate models were based Standards. Standards used were prepared from synthetic stock standard solutions and matrix matched with 1000 mg l-1 on full cross-validation and autoscaled X-data. The univariate models were based on full cross-validation, unscaled, Fe, which corresponds to 100% Fe (m/m) in the solid sample. HCl, HNO3 and HF were added in concentrations equivalent uncentered X-data and linear regression (LR). A measure of 1056 J.Anal. At. Spectrom., 1999, 14, 1055–1059Table 2 Reference materials, their origin and element concentrations Concentration/mg kg-1 Type of reference material Identification Cd Zr Mo Sn High alloy steel GBW 01620a 4.6±0.4 300 20 000 53±4 High alloy steel GBW 01622a 1.9±0.2 300 20 000 1040±70 Nickel base alloy T31b 0.52±0.13 Coal fly ash BCR CRM 038c 4.6±0.3 Coal fly ash NIST SRM 1633bd 0.784±0.006 aCentral Iron and Steel Research Institute, Beijing, China. bAB Sandvik Steel, Sandviken, Sweden.cCommunity Bureau of Reference, Brussels, Belgium. dNational Institute of Standards and Technology, Gaithersburg, MD, USA. Table 3 Relative abundances of the isotopes and masses of potentially interfering oxides (marked with a) Relative isotope abundances (%) and interfering oxides m/z Cd Zr Mo Sn Pd Rh 90 51.4 91 11.2 92 17.1 14.8 93 94 17.5 9.3 95 15.9 96 2.8 16.7 97 9.6 98 24.1 Fig. 1 Count rate for 103Rh versus sample number. 100 9.6 103 100 106 1.25 a 27.3 models based on six standards and 113Cd as X-variable.By 107 a 108 0.9 a a 26.5 applying internal standardisation the within-run variation is 109 reduced and was therefore applied throughout this work. 110 12.5 a a 11.7 To evaluate the between-run variation, least-squares models 111 12.8 a based on all but one run were calculated and, finally, standards 112 24.1 a a 0.97 from the run left out were predicted. Predicted versus measured 113 12.2 a value plots for two diVerent runs are shown in Fig. 2, where 114 28.7 a 0.65 115 0.36 the ideal line is also drawn. As can be seen, the slopes and 116 7.5 a 14.5 intercepts deviate from the ideal values. These diVerences are 117 7.7 unsatisfactory and, obviously, internal standardisation fails to 118 24.2 eliminate between-run variation. To eliminate this variation, 119 8.6 runs 2–17 were adjusted to run 1. This was accomplished by 120 32.6 regressing values, arbitrary units, at m/z 113 (after internal 122 4.6 124 5.8 standardisation) for two standards, 0 and 10 mg l-1Cd, from each run to the corresponding standards in run 1.The regression coeYcients obtained were then used to adjust the remaining samples and standards in each run. In Fig. 3, the the predictive ability of a multivariate model is the root mean same standards as in Fig. 2 are predicted after data adjustment. square error of prediction (RMSEP); if based on full Compared with the data shown in Fig. 2, the diVerences in cross-validation it is called the root mean square error of slope and intercept are reduced.cross-validation (RMSECV). Fly ash domain Results and discussion Univariate and multivariate calibration models were calculated Within- and between-run variation for standards in the fly ash domain. The variable used for In Fig. 1, count rate for 103Rh is depicted versus sample number, for a run with 44 samples/standards. As can be seen, the signal decreases with increasing sample number (over time).This is a well known phenomenon, and as a consequence, samples analysed at the end will have apparently lower concentrations than the true values. To overcome this problem with drift, internal standardisation is applied, simply by dividing every count rate in each mass spectrum by that of an appropriate internal standard, here 103Rh. At this mass number, 103Rh is the only occurring isotope. The eYciency of this standardisation is demonstrated by replicates with 10 mg l-1 Cd and no other metal, sample numbers 10 and 43.These replicates were predicted as containing 10.1 and 8.6 mg l-1 Cd Fig. 2 Predictions of Cd standards: for data marked with 2 the without internal standardisation and 10.6 and 10.3 mg l-1 Cd intercept and slope are 0.84 and 1.06, respectively; for points marked with + the corresponding values are 0.08 and 0.91. after drift correction. The models used were least-squares J. Anal. At. Spectrom., 1999, 14, 1055–1059 1057Table 5 Results based on a PLS2 model using seven components and 321 standards Analyte r RMSECV Maximum concentration/mg l-1 Cd 0.990 1.34 50 Zr 0.989 15 800 Mo 0.996 739 50 000 Sn 0.987 311 30 000 Pd 0.980 3.23 50 Fig. 3 Predictions of Cd standards after data adjustment: for data marked with 2 the intercept and slope are 0.97 and 0.28, respectively; for points marked with + the corresponding values are 1.01 and -0.021. univariate calibration was m/z 113, which was found to give the lowest calibration errors.The results of these calibration models are presented in Table 4; the correlation coeYcient is between predicted and true concentrations and the number of components is as determined by the UNSCRAMBLER program default criteria. By including more variables the prediction error is reduced, even though information about the analyte of interest is lacking. The optimum number of components was found to be three, i.e., a large part of the variation in the data originates from the Cd concentration and the interferents do not cause extensive disturbances.The prediction Fig. 4 Explained Y-variance versus number of components. error is increased and the correlation coeYcient decreases for the univariate model, although it still appears to constitute a also Mo and Sn occur at the highest concentrations, giving useful model. The original concentration unit given for the rise to the largest variation in X-data. However, predictions reference materials is mg kg-1; hence, weighing 100 mg of of Mo in particular, which is present at relatively high levels, sample and then diluting to 100 ml makes this unit equivalent are uncertain and should be regarded as approximations.This to mg l-1 as used for the standards. is probably because the applied data adjustment was performed with standards in the narrow range 0–10 mg l-1. A minor Metal alloy domain error in slope at this level can still imply large diVerences at concentrations several decades higher.The metal alloy domain was analysed analogously to the fly ash domain. Results for univariate and multivariate models are also presented in Table 4. Standards with high residuals in Reference materials X or Y were deleted (outliers). From Table 4 it is obvious that For the prediction of concentrations in the reference materials, multivariate models manage to predict Cd concentrations it was generally found that including more variables than the accurately, in contrast to the univariate model.The multivari- Cd isotopes increased the deviation for the predicted values. ate models for the metal alloys require more components than Some variables were not well described by the regression the model for the fly ash domain. model and had high residuals. These samples contain elements that might form polyatomic ions with Ar, for example Ni, Simultaneous determination of several analytes which is likely to be present in some samples and NiAr+ can then interfere.This eVect was more evident for metal alloy To investigate the possibilities to determine all metals simultaneously, a PLS2 model was calculated based on all samples than for fly ash samples. Hence, models used for prediction of concentration in the reference samples are based runs and all five elements, the results being presented in Table 5. It is indeed possible to determine all five elements on the Cd isotopes.Univariate and multivariate predictions and 95% confidence limits for the reference samples are from a single analysis in one model. In Fig. 4, the explained Y-variance is depicted for the diVerent metals. Sn and Mo are presented in Table 6 together with certified concentrations. These predictions are averages based on 3–6 analyses in well described/predicted with three components, Zr and Pd with four components and finally Cd requires seven compo- diVerent runs, i.e., diVerent wet digestions, so they provide a measure of the reproducibility of the method.The best univari- nents to be accurately predicted. This is expected since some of the Mo and Sn isotopes do not suVer from interference and ate predictions for fly ash samples are as good as the multivari- Table 4 Regression models and results Number of Domain Regression model r RMSECV Components Standards Variables Fly ash PLS 0.997 0.64 3 181 25 PLS 0.995 0.96 3 181 8 LR 0.995 1.03 — 181 1 Metal alloy PLS 0.986 1.42 9 145 25 PLS 0.968 1.66 5 145 8 LR 0.503 7.71 — 145 1 1058 J.Anal. At. Spectrom., 1999, 14, 1055–1059Table 6 Predicted concentrations for Cd applying univariate models (at m/z 110, 111 and 113) and a multivariate model Concentration/mg kg-1 Univariate Reference material Certified m/z 110 m/z 111 m/z 113 Multivariate BCR CRM 038 4.6 5.7 4.5±0.8 5.1 4.60±0.5 NIST SRM 1633b 0.784 1.9 0.9±0.2 1.0 0.56±0.2 GBW 01620 4.6 27.2 38.1 27.1 4.64±0.7 GBW 01622 1.9 24.6 35.5 23.7 2.24±1 T31 0.52 6.8 0.9 0.5±0.1 0.62±0.2 11 I.Gustavsson and H. Larsson, Inductively Coupled Plasma Mass ate predictions. However, there is no straightforward method Spectrometry. Application to Steel, Other Metals and Metal Alloys. to tell which m/z to use if several univariate calibrations are Part II, Technical report, Swedish Institute for Metals Research, available. The choice of m/z is therefore based on the experi- Stockholm, IM-2794, 1992.ence and skill of the operator. By performing multivariate 12 I. Gustavsson and H. Larsson, Inductively Coupled Plasma Mass calibrations this choice is omitted. For GBW 01620 and GBW Spectrometry. Application to Steel, Other Metals and Metal Alloys. Part III, Technical report, Swedish Institute for Metals Research, 01622, the univariate predictions are far too high since the Stockholm, IM-2920, 1992. samples contain high levels of severely interfering Mo. 13 I. Gustavsson and K.Pettersson, Determination of Micro and Multivariate calibration is therefore the method of choice. Trace Elements in Steel by ICP-MS. Application to Steel Certified Reference Materials (CRMs), Technical report, Swedish Institute for Metals Research, Stockholm, IM-3323, 1996. References 14 K. Pettersson and I. Gustavsson, in Plasma Source Mass Spectrometry, ed. G. Holland and S. D. Tanner, Royal Society of 1 D. Beauchemin, J. W. McLaren, S. N. Willie and S. S. Berman, Chemistry, Cambridge, 1997. Anal. Chem., 1988, 60, 687. 15 H. M. Kuss, M. Mueller and D. Bossman, Chem. Listy, 1992, 2 D. Beauchemin, J. W. McLaren, S. N. Willie and S. S. Berman, 86, 523. J. Anal. At. Spectrom., 1988, 3, 305. 16 H. M. Kuss, D. Bossman and M. Mu� ller, At. Spectrosc., 1994, 3 H. T. Delves and M. J. Campbell, J. Anal. At. Spectrom., 1988, 15, 148. 3, 343. 17 M. E. Ketterer, J. J. Reschl and M. J. Peters, Anal. Chem., 1989, 4 K. E. Jarvis, Chem. Geol., 1988, 68, 31. 61, 2031. 5 A. R. Date, Y. Y. Cheung and M. E. Stuart, Spectrochim. Acta, 18 M. E. Ketterer and D. A. Biddle, Anal. Chem., 1992, 64, 1819. Part B, 1987, 42, 3. 19 K. Venth, K. Danzer, G. Kundermann and K. H. Blaufuss, 6 J. J. Thompson and R. S. Houk, Appl. Spectrosc., 1987, 41, 801. Fresenius’ J. Anal. Chem., 1996, 354, 811. 7 S. H. Tan and G. Horlick, J. Anal. At. Spectrom., 1987, 2, 745. 20 H. Martens and T. Naes, Multivariate Calibration, Wiley, New 8 M. A. Vaughan and G. Horlick, Appl. Spectrosc., 1987, 41, 523. York, 1991. 9 A. G. Coedo and M. T. Dorado, Appl. Spectrosc., 1995, 49, 115. 21 UNSCRAMBLER, Camo AS, Trondheim, Norway. 10 H. Ekstro�m and I. Gustavsson, in Applications of Plasma Source Mass Spectrometry, ed. G. Holland and A. Eaton, Royal Society of Chemistry, Cambridge, 1993. Paper 9/01440C J. Anal. At. Spectrom., 1999, 14, 1055–1059

ISSN:0267-9477    

DOI:10.1039/a901440c

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Determination of nickel in water samples by isotope dilution inductively coupled plasma mass spectrometry with sample introduction by carbonyl vapor generation Chang J. Park* and Seong A. Yim Inorganic Analytical Group, Korea Research Institute of Standards and Science, Taejon, Korea 305–600 Received 12th April 1999, Accepted 17th May 1999 Nickel was determined in natural water samples by inductively coupled plasma mass spectrometry (ICP-MS) with sample introduction by on-line carbonyl vapor generation.The isotope dilution method was used for quantification with 60Ni and 62Ni as the major and minor isotopes. With sample introduction by the common pneumatic nebulization, the two isotopes may suVer from spectral interferences arising from molecular ions such as CaO, ArMg and Na2O. In order to obtain accurate results free from spectral interferences, nickel in the water samples was reduced with sodium borohydride to its elemental form, and the mixed solution of sample and sodium borohydride subsequently met a stream of 10% CO gas to form the volatile nickel carbonyl.The generated carbonyl vapor was directed by a stream of argon carrier gas into the ICP after separation in a liquid–gas separator made of a polytetrafluoroethylene membrane tube. Optimum conditions for carbonyl generation were investigated. The analyte transport eYciency of the carbonyl generation method was estimated to be >50%. Good agreement was achieved with the certified values in the analysis of two water reference materials, SLRS-3 riverine water and CASS-3 sea-water. The uncertainty of the analytical results was estimated to be 4.56% for CASS-3 and 1.64% for SLRS-3 with the coverage factor k=2.Inductively coupled plasma mass spectrometry (ICP-MS) has applied method of sample introduction by vapor generation. Elements that form volatile covalent hydrides such as As, Bi, attracted widespread interest because of its analytical figures Ge, Pb, Sb, Se, Sn and Te were determined with hydride of merit such as the excellent power of detection and the generation.11,12 Osmium tetraoxide vapor generation was used ability to measure isotope ratios.The most common sample to separate 187Os from 187Re without prior chemical separa- introduction method for ICP-MS is the pneumatic nebulization tion.13 Recently B14, Cd15 and Cu16 were also successfully of solutions. The pneumatic nebulization technique is simple determined following the generation of some volatile species to use and relatively inexpensive, but it is also a very ineYcient in the ICP.method, only about 2–3% of the sample being introduced into Nickel, in its elemental form, is known to react with carbon the plasma.1 Despite such salient features, ICP-MS suVers monoxide to form volatile nickel carbonyl vapor at room from both spectral and the non-spectral interferences in most temperature.17 Lee18 determined nickel in sea-water by the applications.2,3 The non-spectral interference problem or the carbonyl generation.He trapped the carbonyl vapor in liquid matrix eVect can be solved by using isotope dilution,4,5 stannitrogen- cooled glass-wool and subsequently stripped it to dard addition6 and internal standard7 methods. However, the detect nickel by atomic absorption spectrometry. In this work, spectral interference can be eliminated only by somehow a continuous-flow nickel carbonyl vapor generation device was separating the analyte of interest from the concomitant developed to determine sub-ppb amounts of nickel in natural elements.The accurate determination of nickel in natural waters by ICP-MS. The carbonyl vapor generation method waters is extremely diYcult because the concentration of nickel provided excellent analyte transport eYciency (>50%) and is below the ppb level and also because the detection of nickel spectral interference-free determination of nickel.by ICP-MS suVers from spectral interference arising from polyatomic ions such as CaO, ArMg and Na2O.2,6 Separation of analytes from matrix elements is generally Experimental accomplished before the sample introduction into the ICP by Instrumentation ion exchange or solvent extraction. Recent advances in ICP-MS technology has allowed the separation of analyte ions All measurements were carried out on an ELEMENT highfrom interfering molecular ions after the sample introduction resolution ICP-MS instrument (Finnigan MAT, Bremen, into the ICP at an increased resolution (high-resolution Germany).The instrument provides three fixed resolution ICP-MS).8,9 Measurements at the increased resolution settings (m/Dm=300, 3000 and 8000). All data were acquired (m/Dm=3000), however, suVer from reduced sensitivity (about in the low-resolution mode (m/Dm=300). The continuousone tenth of the sensitivity at m/Dm=300)10 and relatively flow carbonyl vapor generation system is illustrated in Fig. 1. poor reproducibility of signals. Analytes can be also separated A sample solution and sodium borohydride solution are mixed from matrix elements through the selective volatilization of in a Teflon tube (id=0.07 cm and length=30 cm) and nickel the analytes in sample solutions during the sample introduc- in the sample solution is reduced to its elemental form. The tion. This vapor-phase sample introduction technique oVers mixed solution of the sample and sodium borohydride suban enhanced analyte transport eYciency and avoidance of sequently meets a stream of CO gas to form nickel carbonyl contamination of the system in addition to the elimination of vapor.The carbonyl vapor is separated from the mixture solution in a liquid–gas separator made of a Teflon membrane spectral interferences. Hydride generation is the most widely J. Anal. At. Spectrom., 1999, 14, 1061–1065 1061argon base was obtained from the Standard Gas Laboratory of the Korea Research Institute of Standards and Science (Taejon, Korea).The 10% CO gas was passed through an activated charcoal filter (50 mesh) (Junsei Chemical Co., Tokyo, Japan) to eliminate nickel carbonyl impurity in the gas. Results and discussion Optimization The eYciency of the carbonyl vapor generation is extremely sensitive to both the pH and the matrix content of the sample solution. The eYciency was found to be high when the pH of the final solution mixed with sodium borohydride was >7.Fig. 1 Schematic diagram of apparatus for nickel carbonyl vapor generation. This result agrees with that reported by Lee.17 In this work, in order to minimize contamination from the reagents, a buVer solution was not added to the sample solution before the tube and introduced into the plasma by argon carrier gas. The carbonyl generation, but various amounts of 8 M ammonia CO gas flow was controlled with a flow meter made by Dwyer solution were added to the sodium borohydride solution (Michigan City, IN, USA).A sampler cone made of platinum depending on the acidity of the sample solutions. Fig. 2 shows had to be used to withstand the atomic oxygen in the plasma the eVect of ammonia solution concentration on the carbonyl from the CO gas. Details of the instrument component, typical generation eYciency for a typical sample solution acidity of operating conditions and data acquisition parameters are given 1% HNO3.Depending on the matrix composition of the in Table 1. sample solution, the optimum concentration of the sodium borohydride solution was found to be diVerent. Fig. 3 shows Reagents the eVect of sodium borohydride concentration for three A stock standard solution of nickel was prepared by dissolution diVerent kinds of samples but in 1% HNO3 (1 ng g-1 Ni of pure nickel powder (99.999%, Aldrich, Milwaukee, WI, standard solution, sea-water spiked with 1 ng g-1 Ni, and USA).Working standard solutions were prepared by serial 1 ng g-1 Ni containing 30 mg g-1 Ca). The 30 mg g-1 Ca in dilution of the stock standard solution. Sodium borohydride Fig. 3 was chosen as a typical matrix amount in river water solution (0.4% m/v) was prepared daily by dissolving 0.4 g of and ground water. As can be expected, the optimum concensodium borohydride powder (99%, Aldrich) in 100 ml of tration of the sodium borohydride solution is the lowest for de-ionized water. Depending on the pH of the sample solu- 1 ng g-1 Ni standard solution.However, it is peculiar that the tions, diVerent amounts (1.0–2.5 ml ) of 8 Mammonia solution optimum sodium borohydride concentration for the sea-water (electronic grade) (Dongwoo Pure Chemicals, Iksan, Korea) is lower than that for the 1 ng g-1 Ni solution containing only were added to the sodium borohydride solution. This solution 30 mg g-1 Ca. Fig. 3 also shows that the carbonyl generation was purified by precipitation of nickel on the addition of 1.2 g eYciency is reduced to about half with the matrix of 30 mg g-1 of 1% La in the nitrate hexahydrate form (Aldrich).De-ionized water obtained from a Milli-Q Plus water purifier (Millipore, Bedford, MA, USA) was further purified by sub-boiling distillation. High-purity HNO3 was prepared in the laboratory by sub-boiling distillation of electronic grade HNO3 purchased from Dongwoo Pure Chemicals. The enriched isotope 62Ni (97% enrichment) was purchased from ISOTEC (Miamisburg, OH, USA) and nickel isotopic standard (SRM 986) for mass bias correction was bought from the National Institute of Standards and Technology (Gaithersburg, MD, USA).Water reference materials (SLRS-3 and CASS-3) were purchased from LGC (Teddington, Middlesex, UK). A 10% CO gas in Table 1 Operating conditions and data acquisition parameters of high- Fig. 2 EVect of ammonia solution concentration on relative intensity resolution ICP-MS from 1 ng g-1 Ni in 1% HNO3 .ICP— Rf power/W 1000 Sample uptake rate/ml min-1 0.7 Argon gas flow rates/l min-1: Coolant 13.4 Auxiliary 0.7 Carrier 0.55 CO gas flow rate/l min-1 0.02 Sampler cone Platinum, 1.1 mm orifice diameter Skimmer cone Copper, 0.8 mm orifice diameter Data acquisition— No. of passes 3000 Mass window(%) 5 Search window(%) 100 Integration window(%) 100 Samples per peak 200 Fig. 3 EVect of sodium borohydride concentration on relative intensit- Sample time/s 0.002 ies from 1 ng g-1 Ni solutions containing various matrix amounts (2, Settling time/s 0.001 no matrix; $, sea-water; and &, 30 mg g-1 Ca). 1062 J. Anal. At. Spectrom., 1999, 14, 1061–1065Ca or sea-water. Such a degree of the signal suppression is mild considering that the sea-water sample was directly analyzed without dilution. The carbonyl vapor generation eYciency is also very sensitive to the CO gas flow rate, as shown in Fig. 4. At first 50% CO gas in Ar was used, but the optimum CO gas flow rate was too low (<5 mlmin-1) to carry the mixture solution of the sample and sodium borohydride to the liquid–gas separator. It was also very diYcult to control such a low CO gas flow rate.The CO concentration in Ar was thereafter lowered to 10%. Fig. 5 shows the eVect of the carrier argon flow rate on the Ni signal. The optimum Ar carrier flow rate for the sample introduction by the carbonyl generation was found to Fig. 6 Calibration curve for Ni with carbonyl vapor generation.be 0.55 l min-1, which is slightly lower than the normal carrier flow rate of about 0.8 l min-1 for the common pneumatic ibility, as is the case with the nickel carbonyl vapor generation. nebulization. This diVerence can be explained as follows. The The ID method is based on addition of a known amount of optimum carrier flow rate for the carbonyl generation is an enriched isotope to a sample. After equilibration of the determined only on the basis of the optimum time to pass spike isotope with the analyte in the sample, the altered isotope through the plasma, whereas that for the pneumatic nebulizratio of the mixture solution is measured to calculate the ation is determined on the basis of the most eYcient generation analyte concentration by the ID equation given below.19,20 In of droplets sacrificing the optimum residence time in the the ID equation, the spike concentration is determined by plasma.reverse ID with a primary standard solution, and isotopic compositions of Ni in the sample and primary standard Calibration curve solution are assumed to be equal.The two isotopes of Ni employed for the ID are 60Ni and 62Ni. Fig. 6 shows a calibration curve of 60Ni for sample introduction by carbonyl vapor generation. The major isotope 58Ni gives a higher sensitivity, but the detection limit calculated from the Cs=Cp msp ms mp mspr ARsp-R Rsp-Rr Rr-Rs R-RsB (1) 60Ni calibration curve is lower than that from the 58Ni calibration curve owing to the presence of the background ion where Cs=analyte concentration in the sample (ng g-1), Cp= 40Ar18O at mass 58.The calibration curve in Fig. 6 is not concentration of the primary standard solution (ng g-1), mp= linear enough for quantitative analysis. The unstable signal is mass of the primary standard solution (g) used for the spike– believed to be due to the unsteady CO gas flow controlled by primary standard mixture; ms=mass of sample (g) used for the flow meter.It seems that a mass flow controller in the CO the sample–spike mixture; msp=mass of the spike (g) used for gas flow would improve the reproducibility of the nickel signal. the sample–spike mixture; mrsp=mass of the spike (g) used for the spike–primary standard mixture; R=isotope ratio Isotope dilution method (60Ni/62Ni) measured from the sample–spike mixture, Rr= isotope ratio (60Ni/62Ni) measured from the spike–primary All the analytical data reported here were quantified by the standard mixture, Rs=isotope ratio (60Ni/62Ni) in the sample, isotope dilution (ID) method, which provides accurate results and Rsp=isotope ratio (60Ni/62Ni) in the spike.even when the analyte signal is measured with poor reproduc- Interferences The analytical results of the ID method are critically dependent on the accuracy and precision of the isotope ratio measurements of the spiked mixture solutions. One of the ways to check the spectral interferences in the ID method is to compare the measured isotope ratio of the sample solution with that of the isotopic standard solution.In order to assess the magnitude of the spectral interferences for sample introduction by pneumatic nebulization, the 60Ni/62Ni isotope ratio of 10 ng g-1 Ni isotopic standard solution was measured in the low resolution mode and compared with that of 10 ng g-1 Ni isotopic Fig. 4 EVect of 10% CO gas flow rate on relative intensity from standard solution containing matrix elements such as 20 mg g-1 1 ng g-1 Ni standard solution.Na, 8 mg g-1 Mg and 30 mg g-1 Ca. Table 2 shows that the isotopic standard solution containing the matrix elements exhibits a much higher ratio than the isotopic standard solution without the matrix. The higher ratio for the standard contain- Table 2 EVect of matrix elements on Ni isotope ratios measured in low resolution mode by pneumatic nebulization Measured 60Ni/62Ni ratio Solution (mean±s)a 10 ng g-1 isotopic standard 6.82±0.03 10 ng g-1 isotopic standard +20 mg g-1 Na, 8 mg g-1 Mg and 30 mg g-1 Ca 8.08±0.03 Fig. 5 EVect of carrier argon flow rate on relative intensity from aBased on five measurements. 1 ng g-1 Ni standard solution. J. Anal. At. Spectrom., 1999, 14, 1061–1065 1063ing the matrix indicates that a significant part of the spectral Analysis of water reference materials interference can be caused by the matrix elements.In Table 3 The carbonyl vapor generation method was applied to the are listed the interfering molecular ions and the required determination of nickel in two certified reference materials, as resolution to separate the two isotopes from the molecular riverine water (SLRS-3) and sea-water (CASS-3). For each ions. Hence with pneumatic nebulization it is necessary to use reference material, four sample aliquots were separately spiked the medium resolution mode or to employ the matrix separaand analyzed using the ID method.Analytical results for the tion procedure before sample introduction. In this work, reference materials are presented in Table 5 together with the carbonyl vapor generation in the low resolution mode was certified values. The measurement reproducibility (as relative used to solve the spectral interference problem because it standard deviation) of the concentrations from the four diVer- oVered advantages such as high sensitivity, lower uncertainty ent sample–spike mixtures is 0.96% for CASS-3 and 0.41% for of analytical results and direct nebulization of sea-water with- SLRS-3. Since the mass of a solution, instead of its volume, out contaminating the torch, cones and ion optics parts with is measured in the ID method, the determined concentrations the matrix elements.In high-resolution ICP-MS, sensitivity is are in ng g-1. The certified values are, however, in ng ml-1. generally decreased by an order of magnitude when the In order to compare the determined concentrations with the resolution is increased from low to medium.Since the carbonyl certified values, the densities of the two reference materials vapor generation gives about a 25-fold sensitivity improvement were measured with a density bottle (5 ml, BRAND, compared with pneumatic nebulization, the carbonyl vapor Wertheim, Germany). The density of CASS-3 was about generation in the low resolution mode provides about a 1.023 g ml-1 and that of SLRS-3 was about 1.001 g ml-1.The 250-fold sensitivity enhancement compared to pneumatic nebudetermined concentrations in Table 5 agree with the certified lization in the medium resolution mode. Such a high sensitivity values fairly well. The detection limit of this method, deter- enhancement results in a lower uncertainty of the isotope ratio mined as the concentration corresponding to three times the measurements at sub-ppb levels.standard deviation of the blank, was calculated to be In order to confirm that the carbonyl vapor generation 0.004 ng g-1. provides interference-free measurement of Ni isotope ratios, The uncertainty of the analytical results was estimated using 60Ni/62Ni ratios of 1 ng g-1 Ni isotopic standard solutions the EURACHEM Guide.21 The ID eqn. (1) was modified to containing various amounts of Ca were measured in the low eqn. (2) to take into account the mass bias correction (Kr for resolution mode.Table 4 shows that 60Ni/62Ni ratios measured R; Krr for Rr; Ks for Rs; and Ksp for Rsp): with carbonyl vapor generation are not influenced by the presence of Ca matrix up to 3000 mg g-1. Fe metal is also Cs=Cp msp ms mp mspr AKspRsp-KrR KspRsp-KrrRr KrrRr-KsRs KrR-KsRs B (2) known to form the carbonyl, but the reaction can proceed only at relatively high temperature (250 °C) and high press- For each parameter in eqn. (2), the relative standard uncer- ure.17 It is, therefore, almost impossible that Fe(CO)5 is formed tainty contribution to the final concentration was estimated under the present experimental conditions.Although 58Ni is and the combined standard uncertainty was calculated by not the isotope used in the ID, the 58Ni/60Ni ratio was taking the square root of the sum of squares. The evaluated measured to show that the present carbonyl generation method uncertainty budget is listed in Table 6 for the two reference is also useful for the separation of Ni analyte from the Fe materials.The combined relative uncertainty was found to be matrix. As could be expected, no appreciable diVerence can 0.0228 for CASS-3 and 0.0082 for SLRS-3. The combined be found in Table 4 between Ni ratios of 1 ng g-1 Ni solutions uncertainty was obtained by multiplying the average of the with and without 100 ng g-1 Fe matrix. determined concentrations in Table 5 with the relative standard uncertainty. Finally, the expanded uncertainty was obtained by multiplying the combined uncertainty with the coverage Table 3 Interfering molecular ions and resolution required for factor k=2.Thus the expanded uncertainty for the determined separation concentration in Table 5 is estimated to be 0.0168 ng g-1 for CASS-3 and 0.0126 ng g-1 for SLRS-3. Isotope Molecular ion Resolution required 60Ni 44Ca16O 3057 Conclusions 36Ar24Mg 2750 23Na37Cl 2409 Carbonyl vapor generation is very sensitive to pH and the 62Ni 36Ar26Mg 2842 matrix content of the sample solutions, CO gas flow rate and 38Ar24Mg 3188 the sodium borohydride concentration.For each sample, there- 23Na216O 1343 fore, the sodium borohydride concentration and the amount 46Ti16O 3226 of ammonia solution added to the sodium borohydride solution has to be optimized. The linearity of the external standard calibration curve is not good enough for quantitative analysis Table 4 Ni Isotope ratiosa measured with various amounts of matrix elements Table 5 Ni concentrations (ng g-1) determined by the ID method Matrix concentration/mg g-1 Isotope ratio Spiked sample No.CASS-3 SLRS-3 Ca Fe 58Ni/60Ni 60Ni/62Ni 1 0.368 0.769 2 0.366 0.770 0 0 2.611±0.010 6.941±0.020 0 0.1 2.601±0.012 — 3 0.365 0.770 4 0.373 0.776 30 0 — 6.934±0.019 300 0 — 6.966±0.045 Mean±s 0.368±0.0035 0.771±0.0032 (0.376±0.0036)a (0.772±0.0032)a 3000 0 — 6.923±0.041 Certified: 2.596061 7.215007 Measurement reproducibility (%) 0.96 0.41 Certified 0.386±0.062a 0.83±0.08a aPrecision expressed as the standard deviation (n=5); concentration of Ni is 1 ng ml-1.aNi concentration in ng ml-1. 1064 J. Anal. At. Spectrom., 1999, 14, 1061–1065Table 6 Uncertainty budget for the determined Ni concentrations according to the EURACHEM guide21 CASS-3 SLRS-3 ID parameter Typical Final Typical Final value SUa RSUb value SUa RSUb Cp/ng g-1 10.50 0.031 0.003 100.4 0.301 0.0030 Ms/g 20.396 0.001 4.9×10-5 20.316 0.001 4.9×10-5 Msp/g 0.409 0.001 0.0024 0.331 0.001 0.0030 Mp/g 4.134 0.001 0.0002 0.501 0.001 0.0020 Mspr/g 2.144 0.001 0.0005 1.018 0.001 0.0010 R 0.887 0.006 0.0078 0.873 0.002 0.0026 Rsp 0.01164 0.0010 0.00012 0.01164 0.0010 3.9×10-5 Rs 6.921 0.017 4.8×10-5 6.921 0.017 1.4×10-5 Rr 0.988 0.003 0.0035 0.903 0.003 0.0038 Kr 1.04 0.003 0.0033 1.04 0.003 0.0033 Krr 1.04 0.003 0.0034 1.04 0.003 0.0033 Ks 1.04 0.003 5.6×10-5 1.04 0.003 1.6×10-5 Ksp 1.04 0.003 4.0×10-6 1.04 0.003 1.3×10-6 Combined RSU 0.0228 0.0082 Determined concentration/ng g-1 0.368 0.771 Combined SU/ng g-1 0.0084 0.0063 Expanded SU (k=2) 0.0168 0.0126 aStandard uncertainty.bRelative standard uncertainty contribution to the final concentration. 7 M. A. Vaughan and G. Horlick, J. Anal. At. Spectrom., 1989, because of signal fluctuations, probably due to the unsteady 4, 45. flow rate of the CO gas. Since the ID method is not aVected 8 A. T. Townsend, K. A. Miller, S. McLean and S. Aldous, J.Anal. by the signal fluctuations, carbonyl vapor generation coupled At. Spectrom., 1998, 13, 1213. with the ID method provides accurate results without the need 9 C. Sariego Muniz, J. M. Marchante-Gayon, J. I. Garcia Alonso for a time-consuming chemical separation. Comparing the and A. Sanz-Medel, J. Anal. At. Spectrom., 1999, 14, 193. 10 I. Rodushkin and T. Ruth, J. Anal. At. Spectrom., 1997, 12, 1181. nickel signal counts obtained by carbonyl generation with 11 X.Wang, M. Viczian, A. Lasztiny and R. Barnes, J. Anal. At. those obtained by pneumatic nebulization, the analyte trans- Spectrom., 1988, 3, 821. port eYciency of the carbonyl generation method is estimated 12 M. R. Cave and K. A. Green, J. Anal. At. Spectrom., 1989, 4, 223. to be higher than 50%. Quantification by ID-ICP-MS following 13 G. P. Russ, J. M. Bazan and A. R. Date, Anal. Chem., 1987, sample introduction by carbonyl generation is an ultra- 59, 984. 14 T. Wilke, H. Wildner and G. Wunsch, J. Anal. At. Spectrom., sensitive and interference-free method especially useful for the 1997, 9, 1083. determination of sub-ppb levels of nickel in natural waters. 15 J. P. Valles Mota, M. R. Fernandez de la Campa, J. I. Garcia Alonso and A. Sanz-Medel, J. Anal. At. Spectrom., 1999, 14, 113. 16 R. E. Sturgeon, J. Liu, V. J. Boyko and V. T. Luong, Anal. Chem., 1996, 68, 1883. References 17 A. Yamamoto, Organotransition Metal Chemistry, Wiley, New 1 B. L. Sharp, J. Anal. At. Spectrom., 1988, 3, 613. York, 1986, p. 131. 2 H. Vanhoe, J. Goosens, L. Moens and R. Dams, J. Anal. At. 18 D. S. Lee, Anal. Chem., 1982, 54, 1182. Spectrom., 1994, 9, 177. 19 R. L. Jr. Watters, K. R. Eberhardt, E. S. Beary and J. D. Fassett, 3 G. R. Gilson, D. J. Douglas, J. E. Fulford, K. W. Halligan and Metrologia, 1997, 34, 87. 20 I. Papadakis, P. D. P. Taylor and P. De Bievre, Metrologia, 1998, S. D. Tanner, Anal. Chem., 1988, 60, 1472. 35, 715. 4 E. S. Beary, P. J. Paulsen and J. D. Fassett, J. Anal. At. Spectrom., 21 Quantifying Uncertainty in Analytical Measurement, 1994, 9, 1363. EURACHEM, London, 1995. 5 C. J. Park and J. K. Suh, J. Anal. At. Spectrom., 1997, 12, 573. 6 D. Beauchemin, J. W. McLaren, A. P. Mykytiuk and S. S. Berman, Anal. Chem., 1987, 59, 778. Paper 9/02882J J. Anal. At. Spectrom., 1999, 14, 1061–1065 1065

ISSN:0267-9477    

DOI:10.1039/a902882j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Determination of sulfur isotope ratios and concentrations in water samples using ICP-MS incorporating hexapole ion optics Paul R. D. Mason,* Karsten Kaspers and Manfred J. van Bergen Faculty of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands. E-mail: mason@geo.uu.nl Received 15th March 1999, Accepted 12th May 1999 Sulfur isotope ratios are diYcult to determine by quadrupole ICP-MS due to interfering O2+ and NO+ molecular ions of high signal intensity at isotopes 32S and 34S.Rf-only hexapole devices have recently been introduced into ICP-MS instrumentation to facilitate ion transfer from interface to analyser. By introducing a mixture of ‘reactive’ gases into the hexapole, a series of ion–molecule reactions can be induced to reduce or remove interfering polyatomic species. The eVects of various gas mixtures (He, H2 and Xe) on the transfer of sulfur ions through the hexapole and the breakdown of interfering O2+ and NO+ molecular ions at m/z=32 and m/z=34 were investigated. A rapid charge transfer reaction between O2+ and Xe gives at least a factor of 10 improvement in the S+/O2+ ratio.A further reduction in O2+ is achieved by the addition of H2. d34S variations were investigated in crater-lake waters and waters obtained from springs and rivers on the flanks of volcanoes in Java, Indonesia. Under optimum conditions (S=10–50 mg l-1), the 34S/32S measurement precision for standards and samples was <0.3% RSD.Mass bias errors were corrected by using a concentration-matched in-house standard of average North Atlantic sea-water (d34S=20.5‰). Results compare favorably against published data measured by standard gas source mass spectrometric techniques. The proposed technique is potentially useful as a survey tool due to the large d34S variation (±20‰) encountered in nature and the accuracy and reproducibility of the technique (±3–5‰). The determination of S isotope abundance and S concentration ture [eqn.(1)] is essential in many environmental and geological studies. The 32SO42-(aq)+H234S(g)< 34SO42-(aq)+H232S(g) (1) S-cycle aVects all living organisms and depending on chemical form S can be beneficial, useful or hazardous to man. Sulfur Anaerobic bacteria in ocean floor sediments promote this isotopes are usually measured to high precision on gaseous reaction and can cause significant shifts in 34S/32S due to the SO2 by gas source mass spectrometry or on solids by secondary large relative mass diVerence. 34S/32S variations in nature are ionization mass spectrometry (SIMS). Sulfur concentrations large and are usually reported as d34S relative to the Canyon are commonly determined independently by isotope dilution Diablo Troilite (CDT) [eqn. (2)] MS, X-ray techniques or inductively coupled plasma atomic d34S ‰=[(34S/32Ssample-34S/32Sstandard)/34S/32Sstandard]×1000 emission spectrometry. Sample preparation for isotopic analy- (2) sis of natural waters is time-consuming and more than one analytical technique is usually required for combined isotop- Oxidation usually increases d34S whilst reducing conditions ic–elemental investigations. A single, more rapid and easy to can significantly lower d34S.use technique would be beneficial for large-scale environmental Sulfur isotope ratios in natural water samples are typically or geological chemical surveying. Inductively coupled plasma measured by gas source mass spectrometry.Sulfate in solution mass spectrometry (ICP-MS) is a technique that could poten- is initially reduced to sulfide and precipitated as BaS or CdS. tially fulfil these requirements if current problems related to Sulfides are then converted to SO2 by reactions with CuO, instrumental spectroscopic interferences could be overcome. V2O5 or O2 at high temperature. SO2 gas is introduced into ICP-MS is a versatile technique as many types of sample the inlet of the mass spectrometer and isotope ratios can be introduction method can be utilized (e.g.direct analysis of measured to high degrees of accuracy and precision. solutions, introduction of solids by laser ablation), potentially Disadvantages of this technique include slow sample throughopening up new applications. put and the possibilities of errors due to large amounts of Sulfur is widely distributed in the environment by volcanism, sample handling. volatile emissions, precipitation, acid mine drainage and Sulfur isotope ratios are not routinely measured by ICP-MS, anthropogenic activity.Sulfate is released into the atmosphere as in a solution-loaded plasma there is a significant contrias SO2 by volcanic and biogenic activity. Sulfur can be bution from O-, N- and H-based polyatomic ions that on a concentrated and stored in its reduced form as sulfide in typical instrument with a quadrupole mass spectrometer often metallic minerals that occur in many natural rock types or saturate the counting system due to their prevalent abundance. which are the products of industrial processes.Sulfur concen- However, if these spectroscopic interferences can be reduced trations have a limited use when defining the S-cycle and a or eliminated then there is scope for using ICP-MS to investistudy of S isotopes enables sources, sinks and natural mixing gate this isotope system. Sulfur isotopes are strongly fractionprocesses to be characterised.Sulfur isotopes have been used ated in nature, exhibiting a relatively large 34S/32S ratio to investigate magma de-gassing and the influence of toxic variation and thus a high precision technique may not be crater-lake waters in drainage systems.1–5 essential. The most important isotopes of S are free from Sulfur isotopes are fractionated from one another by elemental isobaric overlap and isotopes 32S and 34S are suYcchanging redox conditions. 32S and 34S are most significantly iently abundant to be measured in the same acquisition by the counting system of a typical ICP-MS instrument.A limitation fractionated by a biogenic chemical reaction at low tempera- J. Anal. At. Spectrom., 1999, 14, 1067–1074 1067Table 1 Instrument operating parameters that remains however is the poor ionization eYciency of S in an Ar plasma due to its high first ionization energy. In Rf forward power 1300W addition, S is relatively light and thus is not transmitted by Cool gas flow rate 13.00 l min-1 typical instrumental ion optics as eVectively as heavier masses.Intermediate gas flow rate 1.00 l min-1 As a result, ICP-MS is a trace rather than an ultra-trace Carrier gas flow rate 0.85 l min-1 Sample uptake rate 0.5 ml min-1 method for determination of S, with expected detection limits Sampling depth 12 mm in the parts-per-billion range. Another potential problem to Sampling/skimmer cones Nickel consider is that S ions may be prone to matrix-induced space Extraction lens -650 kV charge eVects, as observed with light B and Li isotope systems.6 Intermediate lens 400 V Sulfur has been measured successfully by ICP-MS by reducing or eliminating the solvent load of the nebulised solution, Hexapole gases— Optimum He gas flow rate 1.0 ml min-1 through alternative sample introduction techniques, e.g.elec- Optimum H2 gas flow rate 4.0 ml min-1 trothermal vaporization.7 However, residual O2+ has been Optimum Xe gas flow rate 0.15 ml min-1 significant due to continuous air entrainment in the plasma Signal measurement parameters— leading to problems in the determination of 32S.Attempts to Acquisition mode Single ion measuring (peak hopping) measure S isotopes by high resolution ICP-MS8 have been Dwell time 30 ms per peak successful giving excellent precision (<0.2% RSD) and low Points per peak 1 detection limits down to 100 ng l-1. A relatively low resolution Total analysis time 10 min setting of 2000 gives adequate separation of 32S and O2+, conditions under which sensitivity and peak shape are only partially compromised.Multiple collector magnetic sector field Experimental instrumentation is expected to give further improvements in measurement precision for S isotopes but is expensive to install. Instrumentation Recent developments in ICP-MS technology have intro- The ICP mass spectrometer used for this study was a duced ‘collision’ or ‘reaction’ cells for ion focusing9–11 and for Micromass (Manchester, UK) Platform ICP.14 This instru- filtering of selected ions by reactions with gases during ment utilizes an rf-only hexapole to extract ions from the transmission from plasma to analyzer.12–15 H2, Xe, N2, O2, region behind the skimmer cone and to transport them into NH3 and various other gases have been introduced into the the quadrupole mass analyzer (Fig. 1). Electrostatic ion lenses rf-only multipole ‘reaction’ cell in small volumes to promote are used to guide ions into and out of the hexapole, which is ion–molecule reactions.16–19 mounted at an angle to the instrument axis, and for guiding This study was undertaken to investigate the feasibility of ions into the oV-axis quadrupole.A photomultiplier ‘Daly- using an rf-only hexapole ion-focusing device as a tool to type’ detector is used in configuration with an ion–electron remove suYcient O2+, O2H+ and NO+ polyatomic ions to be conversion dynode and a photon producing phosphor plate.20 able to measure S isotope ratios by ICP-MS.A series of In other respects the configuration is similar to that of a ion–molecule reactions between Xe, H2 and O2+ were induced standard commercial ICP-MS instrument. The instrument was to remove the interfering species and thus enable accurate operated with a standard sample introduction configuration measurement of 34S/32S ratios. To test the validity of the of Fassel quartz torch, Scott double pass quartz spray chamber technique some natural volcanic crater-lake and spring waters, and Meinhard nebuliser.Operating conditions are given in previously analyzed by gas source MS, were chosen, which Table 1 and discussed in more detail below. contained high concentrations of S that remained in the mg l-1 range after matrix dilution. Natural isotopic variation in the Samples and reagents sample set was large, enabling meaningful data to be acquired within the resolution of a quadrupole counting system.This Water samples were collected in the field and filtered through paper discusses the possible reactions and processes that lead 0.45 mm Millipore filters.4 They were acidified with HNO3 and to polyatomic ion reduction and comments on the potential transported to the laboratory in polyethylene bottles. Samples of this new technique for routine S isotopic and elemental were diluted by volume with Millipore ultrapure water between 10 and 100 times prior to ICP-MS analysis.For the waters analysis in the earth and environmental sciences. Fig. 1 Schematic diagram of mass spectrometer used in this study. 1068 J. Anal. At. Spectrom., 1999, 14, 1067–1074from Patuha, West Java, identical aliquots of the same samples have been analyzed by ICP-AES, ICP-MS and gas source MS with data reported in the literature.4 The sample from Ijen, a large and chemically homogeneous crater lake in the Kawah Ijen volcano, East Java, was collected in the same month as a sample analyzed by gas source MS reported in a separate study.21 Sulfur isotopic standards are not readily available in solution form.Most internationally recognized isotopic standards are prepared in an insoluble BaS, AgS or ZnS matrix as a consequence of the sample preparation procedures involved in gas source MS. Thus, in this study, it was necessary to find an inexpensive, readily available and well characterized solution standard.Sea-water contains abundant S (#1000 mg l-1) Fig. 2 EVect of addition of Xe into hexapole on ion transmission. Figure shows relative response of a 10 mg l-1 multi-element solution and has a relatively uniform isotopic composition. d34S in before and after the addition of 0.2 ml min-1 Xe to 5.0 ml min-1 He. average sea-water is 20.5‰.22 A 200 ml volume of open ocean sea-water collected from the North Atlantic Ocean near Madeira was stripped of metal cations using a SaYdex ion- multipole device.The number of ions transmitted by the exchange column and collected in a dilute HCl matrix for use multipole will therefore increase as gas partial pressure as an isotopic standard. increases until a maximum is reached and ion transmission starts to decay once more. Collisional focusing is dependent Analysis procedure on the mass of the focused ion and the mass of the collision gas. The energy distribution of ions of a given mass is reduced Diluted SPEX and Johnson-Matthey standard solutions were by over 90%, enabling improved performance of a subsequent used for instrument optimization and calibration for quantitatmass analyser.14 ive analysis.The solutions are certified for elemental concen- In this study, an optimum amount of gas for maximum tration but not for isotope ratios of S. The instrument was focusing of S+ ions corresponded to a He or H2 gas inlet rate optimized on 32S using a 10 mg l-1 single element standard of approximately 5 ml min-1 (partial pressure in the region solution and a blank acid solution. All gas lines leading into 1×10-3 mbar).The use of Xe in the hexapole had a dramatic the hexapole were carefully cleaned prior to analysis and the eVect on transmission of all masses due to the influence of its highest available commercial grade gases were used in all relatively heavy mass on axial ion kinetic energy. Relative cases. Gas input into the hexapole was controlled by mass transmission into the quadrupole was mass-dependent (Fig. 2) flow controllers. and for the addition of 0.2 ml min-1 (#5×10-5 mbar) only To determine the isotope ratio in an unknown sample, the 30% transmission for U and 5% transmission for Be was sample and standard were first diluted to the same concenmaintained. The transmission of S ions was also reduced to tration (usually in the range 20–50 mg l-1) using published 10–20% of the amount measured with only He in the hexapole, ICP-AES data for S concentrations in the samples.4 Two whilst background interferences (mostly O2+) were reduced to blanks were acquired before each block of sample and stanat least <1% of their original abundance (which was close to dards.A sample blank consisting of water and acids used to saturating the counting system). Thus, the improvement factor dilute and acidify the samples and a standard blank using was better than 10 for the S+/O2+ ratio. reagents from the preparation of the standard solutions were run.Standards were run immediately before and after each Ion–molecule reactions unknown sample. For each sample and standard, ten 1 min scans were acquired and statistics calculated for the ten repeat The ease with which ions can react with molecules has been measurements. All data were collected in single ion measuring known since the earliest history of mass spectrometry.26 (peak hopping) mode using one point per peak. Quadrupole Electron or proton transfer reactions (described in dwell and settle times were optimized to take account of Knewstubb27) were induced in this study by focusing the ion dominant instrumental noise frequencies leading to superior beam, extracted from the plasma, through a partial volume of short-term precision.23 The data for the standards, after blank a ‘reactive’ gas in the hexapole (e.g.H2, Xe). The reactions correction, were used to calculate a correction factor, which that are most likely to occur are those that are exothermic was then applied to the ratio measured for the unknown with large reaction rate constants.Reactions have been extensample to overcome mass bias error. sively studied under experimental thermal conditions where the reaction energy is defined as the diVerence in the sums of Results and discussion heats of formation of the products and neutrals (or diVerence in ionisation potentials for charge transfer reactions).Reaction The processes enabling the relatively interference-free rate and cross-section data available in the literature28–30 may measurement of the 32S and 34S isotopes are initially discussed thus provide some guidance as to which reactions take place. and the results of optimisation work for accurate and precise However, the hexapole used in this study was operated under isotope ratio measurements are shown. Data for crater-lake non-thermal conditions where the kinetic energy of the ions and spring waters from Java are presented and discussed in must also be taken into account.Non-thermal conditions tend terms of day-to-day precision and accuracy. to reduce the rate of exothermic and increase the rate of endothermic reactions. In addition, the ions entering the Ion focusing in a hexapole hexapole have high kinetic energies, much of which is lost in energy-damping collisions, but of which a substantial pro- An rf-only multipole device24,25 when filled with inert gas at relatively high operating pressures (10-3–10-4 mbar) can portion remains as ions are focused into the quadrupole.Thus, reaction rate data are used here simply as a qualitative guide focus ions through a process termed collisional focusing.9 Significant amounts of axial kinetic energy are lost, corre- to indicate which reactions are possible. Scanned spectra for the mass region from m/z=30 to m/z= sponding to the energy losses predicted from collision crosssections, and ions are focused towards the centre of the 36 are shown in Fig. 3. Fig. 3(a) shows the case where the J. Anal. At. Spectrom., 1999, 14, 1067–1074 1069abundance of O2+ increased slightly, whilst many other polyatomic ions (including ArO+ and O+) decreased and Ar+ decreased dramatically [Fig. 3(b)]. The slight increase in O2+ may be explained by O2 impurities (ionised by charge transfer with Xe+) in the H2 gas or gas lines. Under routine, solutionloaded plasma conditions, S isotopic measurements are not possible due to a saturation of the counting system at m/z=32.The addition of small volumes of Xe gas into the hexapole cell had a dramatic eVect on signal response [Fig. 3(c)]. Count rates at all masses were significantly reduced, with the exception of the large 14N16O+ peak that remained at m/z=30. With a He–Xe mixture a peak at m/z=32 of the order of 500 000 counts s-1 remained, reflecting a contribution from residual 16O16O+ and 14N18O+.The addition of H2 to the He–Xe mixture promoted a further reduction in background count rates to <100 000 counts s-1 at m/z=32. When a solution containing 10 mg l-1 S was introduced [Fig. 3(d)] a spectrum for S, significantly above the background (>10 times), was observed. In order to assess, qualitatively, the reactions that had been taking place, other parts of the mass spectrum were investigated before and after the addition of Xe as a reactive gas. By observing the peaks produced during the addition of Xe it is possible to suggest a series of reactions that may have been important.Table 2 summarises a suggested reaction series in the hexapole cell involving Xe and H2 gases and some abundant ions. Thermal reaction rate data from the literature are shown, where available, indicating that such reactions are tentatively supported by experimental work.29,30 Xe is reactive, undergoing endothermic charge transfer with O2+ in experimental work at 290 K.32 The non-thermal environment of the hexapole in this study may contribute suYcient kinetic energy to the reactant ion to increase dramatically the rate of this reaction above the rate predicted by experimental thermal data.The Xe background was typically very low when Xe was not used in the hexapole, showing that plasma Xe contamination, a problem frequently encountered with Ar plasma gases, was minimal. Major products of reactions with Xe included Xe+ and XeH+ accompanied by more minor XeO+, XeOH+ and XeH2O+ (Fig. 4, Table 2). The formation of Xe+ and XeH+ has been observed in previous studies12 and attributed to charge and proton transfer reactions between Xe and Ar+ and ArH+. Much of the Xe+ and XeH+ was also the result of charge transfer reactions with O2+ and our data show an identical isotopic abundance with the results produced by Rowan and Houk.12 When H2 was substituted for He with Fig. 3 EVects of addition of various gases into hexapole on response between m/z=30 and m/z=36.The spectra are for a 2% v/v ultrapure the Xe in the hexapole, the overall response for Xe+ and HNO3 solution unless stated otherwise. (a) Introduction of He only XeH+ fell and proportionally more XeH+ was formed. H2 (inlet flow rate of 5 ml min-1). (b) Introduction of a 1.0 ml min-1 appears to have promoted the reduction of O2+. This is not He–4.0 ml min-1 H2 mixture. (c) Addition of Xe to the 1.0 ml min-1 supported by thermal reaction rate data, but in the non- He–4.0 ml min-1 H2 mixture.(d) As in (c) but with aspiration of 10 mg l-1 S single element solution in 2% v/v HNO3. Table 2 Suggested reaction series when using Xe and/or H2 as a reactive gas. Thermal reaction rate constant data, where available, are from Anicich and Huntress29 and Anicich30 eVects of ion–molecule chemistry have been minimized. He, which does not react with most charged species (exceptions Thermal reaction rate constant k are H2+, D2+ and He+), was the only gas introduced into Reaction at 290–300 K/cm3 s-1 the hexapole. Impurities that can react with the ion beam, such as N2 and O2, were present in the high grade He at the O2++XeAXe++O2 5.5×10–11 O2++XeAXeO++O — 0.1 ml l-1 level (Grade 6.0, Air Products) and could have made OH++XeAXeH++O 9.2×10–10 some small but relatively insignificant modifications to this H2O++XeAXe++H2O 8.0×10–10 part of the mass spectrum.Large peaks were observed at all Xe++H2AXeH++H <2.0×10–11 masses, but most notably at m/z=30, 32, 33 and 34 correspond- XeO++H2AH2O++Xe — ing to NO+ and O2+ polyatomic ions.This is similar to a XeOH++H2AH3O++Xe — background spectrum for a dilute HNO3 solution, observed Ar++XeAXe++Ar 4.3×10–13 ArH++XeAXeH++Ar — on ICP-MS instruments with standard electrostatic ion optics, Ar++H2AArH++H 8.6×10–10 where ion–molecule reactions do not take place.31 ArH++H2AH3++Ar 8.9×10–10 H2 gas has previously been used in collision cells as a OH++H2AH2O++H 8.6×10–10 reactant to remove ArX+ species13 (where X=O, H, N, Ar, H2O++H2AH3O++H 8.3×10–10 Cl, etc.).As H2 was added to the He in our experiments, the 1070 J. Anal. At. Spectrom., 1999, 14, 1067–1074Table 3 Stability of S standard solutions in various matrix types and at diVerent concentrations Counting Acid matrix 34S/32S 1s RSD (%) statistics 10 mg l-1 S standard— De-ionised water 0.044 0.00018 0.41 0.22 2% v/v HNO3 0.042 0.00012 0.30 0.25 2% v/v aqua regia 0.046 0.00014 0.29 0.34 2% v/v HCl 0.047 0.00009 0.19 0.22 5% v/v HCl 0.047 0.00009 0.18 0.25 50 mg l-1 S standard— 5% v/v HCl 0.050 0.00008 0.16 0.18 Optimization of ICP-MS Plasma parameters such as rf forward power, torch positioning and carrier gas flow were carefully optimized prior to each experiment.Minor fine-tuning was carried out to the ion lens settings, but the adjustments were relatively insignificant. Fig. 4 Some products of ion–molecule reactions with Xe.Count rates for both S+ and background ions were optimized to give the highest S+/background ion signal, alternately using a 10mg l-1 S single element standard and a blank, both in a 2% v/v ultrapure HNO3 matrix. thermal environment of the hexapole the rate of this reaction Experiments were performed with a sheathing bonnet34 may have been substantially increased. between the outer end of the plasma torch and the ion XeO+, XeOH+ and XeH2O+ were observed in the mass extraction aperture to observe if any further gain in reduction region m/z=140–156.A close assessment of the isotopic of the O2+ polyatomic ion could be achieved due to a reduction abundance of the observed peaks reveals that XeOH+ and in entrainment of atmospheric gases. No real gain in perform- XeH2O+ were formed more readily or removed less readily ance was observed, suggesting that much of the O2+ may be by subsequent reactions than XeO+. When H2 was added to originating from the solvent carrying the sample into the the He–Xe gas mixture, the XeO+, XeOH+ response was plasma.dramatically reduced. Meanwhile, H3O+ increased to a much The eVects of diVerent acid matrix types and analyte greater degree than would be expected if this light polyatomic concentrations on S isotope response and ratio measurement ion was only the product of reactions between H2, OH+ and precision were investigated (Table 3). Optimum S+ response H2O+. Thus, we postulate reactions between XeO+, XeOH+ and precision on the 34S/32S ratio, measured on the synthetic and H2 as shown in Table 2.However, similar eVects may be S standard solution, was achieved with a HCl matrix, although caused by reactions between Xe and contaminant H2O or O2 the eVects were not very significant. Slightly higher S+ count in the H2 gas. rates in the HCl matrix were found to give marginally improved A disadvantage of adding H2 is increased production of detection limits. 34S/32S ratio precision was improved further SH+ at m/z 33 and 34, which could be detrimental to accurate by increasing the HCl concentration from 2 to 5% v/v. The isotope ratio measurements. 33S/32S and 33S/34S ratios are HCl matrix was also considered suitable for standards and larger than expected assuming that the single element S samples as it matched the matrix of the cation-stripped sea- standard (Johnson-Matthey) has a close to natural average water isotopic standard.The production of NO+ polyatomic isotopic abundance. The contribution of 33S1H on 34S is ions was subdued in HCl with respect to HNO3, due to a diYcult to assess as ratio accuracy was aVected by many lowering of N loading in the core of the plasma, leading to sources of instrumental mass bias (discussed below) but slightly reduced background count rates at m/z=32. appeared to occur at the 3–5% level. Assuming that the rate Residual background signals during washout periods, after of production of 33S1H is constant and only a minor compoprolonged aspiration of S-bearing solutions, were no poorer nent of the response at m/z=34, it should be adequately than routinely experienced for many elements by ICP-MS.corrected for against the reference standards used for cali- Washout times of 2–3 min proved suYcient to return to bration. The ease of production of SH+ is surprising as no background count rates. reaction was recorded in experimental studies of ion–molecule Calibration lines for 32S and 34S are shown in Fig. 5. A high reactions.33 However, it is possible that a reaction was facilidegree of linearity was observed when using Xe in the hexapole tated under the non-thermal conditions encountered in this cell. The lower limit of detection (3s) for S when using H2 in study or that S+ reacted with H2O impurities in the H2. the gas mixture was of the order of 20–50 mg l-1 and with Other reaction products were observed including complexes only a He and Xe mixture was 100–300 mg l-1.Sulfur concen- with Cl+ made possible by the use of HCl as a sample trations of the order of 50 mg l-1 were measured routinely to introduction matrix. Most notable were XeCl+ and ClH2+. an internal precision (repeatability of ten measurements) of The production of these new polyatomic ions appeared to be better than 2% RSD using either 34S or 32S for calibration. of little significance for the modification of background count Reproducibility of S concentration measurements performed rates for the measurement of S isotopes.on diVerent occasions with diVerent tuning conditions was The majority of the reaction products were not transmitted noticeably poorer (6–15% RSD). into the mass analyser due to a potential barrier applied between the hexapole and the quadrupole. The potential Precision of S isotope ratio measurements barrier could be adjusted manually to obtain the most favourable transmission of S+ analyte ions and reaction products.Good precision is a critical parameter for isotope ratio data In this manner, ion–molecule reactions were used as a tool to to be usefully applied in environmental or geological studies. In quadrupole ICP-MS, sequential measurements are suscep- filter out interfering molecular ions. J. Anal. At. Spectrom., 1999, 14, 1067–1074 1071variations, (iv) incorrect blank subtraction, (v) concentrationdependent mass discrimination, (vi) detection system nonlinearity, and (vi) short-term variation in magnitude of background polyatomic ion transmission.Some of these eVects are discussed in detail below. Three of the most critical tuning parameters that aVect mass bias were adjusted whilst other instrumental parameters were set at their optimum settings (Fig. 6). Nebuliser flow rate aVects the region of the plasma sampled by the ion extraction orifice. A higher flow rate changes the temperature of the portion of sampled plasma and consequently changes the ionization eYciency of S+ and the rate of production of O2+. 34S/32S ratios can vary by as much as 25% through small changes in plasma conditions. Lens potential was generally found to have a relatively minor eVect on mass bias unless set below -600 V. At such a setting, sensitivity was considerably poorer and 34S response fell below the detection limits of the technique, thus modifying the 34S/32S ratio. Therefore, it was critical to ensure that 34S remained significantly above the background count rates whilst 32S did not saturate the detection system.The type and amount of gases introduced into the hexapole Fig. 5 Calibration lines for 32S and 34S. cell had the greatest eVect on mass bias. As discussed above, the addition of H2 can significantly aVect the transmission of tible to instabilities of ionization in the plasma, ion extraction background polyatomic species at both m/z=32 and m/z=34.and operation of the quadrupole.34 For isotope ratios involving In addition, SH+ may increase the signal at m/z=34 relative isotopes of contrasting abundance, a precision of between 0.2 to m/z=32. The addition of H2, whilst further reducing and 1.0% RSD is typical.31 For the S isotope ratios in this background count rates at m/z=32, was detrimental to 34S/32S study it was necessary to achieve the lowest possible RSD, at accuracy. Measured 34S/32S approached the isotopic composithe lower end of this range, in order to be able to resolve tion of the standard most closely with no H2 in the hexapole diVerences in the water samples to be tested.With careful cell. Large deviations in 34S/32S by as much as 40% were optimization of instrument parameters23 it was possible to observed as H2 was added. It was necessary to ensure prior to obtain isotope ratio precision approaching values predicted by each experiment that no added impurities were present in the counting statistics.Xe gas supply due to leaks or insuYcient evacuation of gas Single ion measuring or peak hopping mode was used as lines when changing bottles. Owing to the high cost of Xe gas, more time was proportionally spent in the accumulation of the Xe bottle was isolated from the ICP-MS instrument when counts on the isotopes of interest. 32S and 34S were the only not in use. Each time the gas lines were used it was necessary isotopes measured in order to maximise counting time.Mass to flush for several minutes to remove residual atmospheric scale shift resulting from instabilities in the quadrupole was not found to be a major problem with the instrument used in this study, but calibration across the mass range was checked on a regular basis. No significant gain in short-term precision was observed with the sheathing bonnet, suggesting acoustic noise and pump pulsation peaks to be a minor source of noise.34 Laboratory temperature stability and cleanliness was high (Class 10 000 clean room).The dwell time selected for each isotope had a major eVect on the short-term precision that was obtainable. Instrumental noise frequencies were observed at diVerent dwell time settings and these were optimized at 30 mS per peak after all other sources of noise (e.g. peristaltic pump noise) had been reduced as far as possible. In this study, 34S/32S short-term repeatability was generally excellent, with RSDs often approaching values predicted by counting statistics (Table 3).An important factor in the routinely high degrees of precision could have been the detection system, as it has been shown in previous studies that analogue ‘Daly’-type detectors can give improved isotope ratio repeatability at high count rates.35 Better results were also obtained after the instrument had been conditioned by continuous aspiration of the acid matrix for several minutes prior to analysis. Accuracy of S isotope ratio and S concentration measurements Accurate isotope analysis may be aVected by incorrect calibration, instrumental mass bias or matrix-induced mass bias.Sources of error are often cumulative and include: (i) short-term drift in instrument response, (ii) poor characteriz- Fig. 6 EVect of tuning parameters and analyte concentration on measured 34S/32S isotope ratio. ation of reference standards, (iii) matrix-dependent response 1072 J. Anal. At. Spectrom., 1999, 14, 1067–1074with ultrapure water as discussed above.Thus, we have eliminated the eVects of sample handling as far as possible. Accuracy and precision of data collected for these real samples (which have a complex matrix) more closely represent the data quality that can be expected in analysis of routine samples than if reference standards of a simple matrix had been chosen. The samples were analyzed repeatedly, on diVerent occasions, with some subtle changes to tuning conditions during routine optimization for each experiment.This approach gives an indication of the reliability and reproducibility of the technique over time. d34S variations and S concentrations for crater-lake and Fig. 7 Linear dynamic range of analogue ‘Daly-type’ scintillation detector. spring and river waters from Patuha volcano (West Java) and Kawah Ijen volcano (East Java) are shown in Table 4 and Fig. 9. Under optimum conditions (S=10–50 mg l-1), the gases and to prevent introduction into the hexapole cell. 34S/32S measurement precision or repeatability for samples Fluctuating levels of H2 in the hexapole cell clearly have a during a 10 min scan was <0.3% RSD, close to data achieved large eVect on isotope ratio accuracy and must be avoided for matrix-free standard solutions. External reproducibility where possible. (1s) of mean data for the water samples from day-to-day for Concentration-dependent mass bias is evident from Fig. 6. d34S was typically±2–3‰.d34S for BRT1/7 in experiment 5 34S/32S ratios increased significantly as the concentration of (Table 4) was outside this range and the reason for this the S standard solution was increased. A 1–2% relative increase spurious result is not clear. Fig. 9 shows within-run repeat- in transmission eYciency was observed for 34S over 32S from ability, within-day reproducibility and inter-experiment 10 to 50 mg l-1. The measured 34S/32S ratio deviated from the reproducibility.natural isotopic composition with increasing concentration. It The data demonstrate that d34S can be measured at best to is diYcult to explain this eVect as related to ion-beam space a precision of 2‰ in real samples, approaching what can be charge eVects, which would be expected to have a reverse expected from counting statistics. However, some samples can eVect on relative transmission with increasing concentration.6 be problematic, as demonstrated by sample ALN1/5, which The eVect necessitates careful concentration matching of was measured with an internal precision of only 6‰.Accuracy standards and samples. was variable and results agree to between 1 and 6‰ of Experiments were performed to establish the linear dynamic previously published data. Mean results for five experiments, range of the detector using single element Tm solutions in conducted on separate occasions, smooth out the noise, and 2% v/v HNO3 (Fig. 7). The analogue detection system appears mean data for samples BT1/5 and ALN 1/5 agree to within to be not as prone as pulse counting systems to counting loss 1‰ of published results. However, the result for sample at high count rates.However, some counting loss was observed BRT1/7 diVers significantly from the gas source MS result. above 1×108 counts s-1, most probably related to the specifi- This may be due to a calibration error caused by a matrix cations of the counting electronics, and all data collected at component in BRT1/7, contamination or isotopic modification count rates above this value were discarded.Dead time correcin one of the two aliquots used by the two techniques, or an tions were not applied to the isotope ratio data collected in error during gas source MS. The result for sample IJM1/1 this study. agrees well with published data, although the sample analyzed Mass bias errors were corrected using the in-house standard in this study was not from the same aliquot analyzed in the of matrix-stripped North Atlantic sea-water (d34S=20.5‰).published work.5 The reproducibility of standards was acceptable but sometimes d34S variations between samples from Patuha volcano can erratic, especially after the introduction of samples with an be related to the dynamics of the local drainage system. The apparently heavier matrix (e.g. some spring waters) and was springs at Barutunggul and Alun-alun are the main source for a major limitation to accurate analysis (Fig. 8). However, the crater-lake waters in local streams and rivers. Diluted acid mean results from this study compare favorably against published data measured by gas source MS. Results for crater-lake and spring waters Sulfur isotope ratios were determined in some natural volcanic crater-lake and spring water samples using the methods outlined above. The chosen samples have all been previously measured for d34S by gas source MS and were re-analyzed to test the accuracy of the proposed ICP-MS technique.No sample preparation was necessary other than simple dilution Fig. 9 Results for crater-lake and spring waters from Patuha volcano showing within-run, within-day and external reproducibility and Fig. 8 Typical reproducibility of S isotopic standards. accuracy. J. Anal. At. Spectrom., 1999, 14, 1067–1074 1073Table 4 d34S and S concentration results for volcanic crater-lake and spring waters from Patuha and Kawah Ijen volcanoes, Java, Indonesia d34S (‰) Sample Source Published Mean Expt 1 Expt 2 Expt 3 Expt 4 Expt 5 PT1/5 Crater-lake acid water 19.0a 18 17 21 19 15 19 ALN1/5 Spring water 1.6a 2.1 0.5 5.8 2.8 0.6 1.0 BRT1/7 Spring water 15.5a 12 10 12 11 10 17 IJM1/1 Crater-lake acid water 22.5b 21 21 S/mg l-1 Sample Source Published Mean Expt 1 Expt 2 Expt 3 Expt 4 Expt 5 PT1/5 Crater-lake acid water 4192a 3489 3802 4074 3566 3896 2108 ALN1/5 Spring water 239a 198 180 178 164 193 276 BRT1/7 Spring water 210a 182 166 170 158 158 260 aData from Sriwana et al.4 bData from Delmelle et al.21 6 D.C.Gre� goire, B.M. Acheson and R. P. Taylor, J. Anal. At. waters from the crater-lake transport many toxic elements and Spectrom., 1996, 11, 765. deposit them further downstream into the local environment. 7 H. Naka and D. C. Gre�goire, J. Anal. At. Spectrom., 1996, 11, 359. The S isotope variations measured in this study show that the 8 T. Prohaska, C. Latkoczy and G. Stingeder, European Winter crater-lake waters have been modified during percolation Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999.through the caldera walls of the volcano. Water–rock inter- 9 D. J. Douglas and J. B. French, J. Am. Soc. Mass Spectrom., 1992, action or the precipitation of sulfate phases has led to S 3, 398. isotopic fractionation. This percolation process has impli- 10 V.I. Baranov and S.D. Tanner, European Winter Conference on cations for transport processes of heavy elements and can Plasma Spectrochemistry, Pau, France, January 10–15, 1999.provide constraints on dilution eVects with meteoric water. 11 E. R. Denoyer, S. D. Tanner and U. Voellkopf, Spectroscopy, 1999, 14, 2. 12 J. T. Rowan and R. S. Houk, Appl. Spectrosc., 1989, 43, 976. Conclusions 13 G. C. Eiden, C. J. Barinaga and D. W. Koppenaal, J. Anal. At. Spectrom., 1996, 11, 317. Interfering O2+ background ions at m/z=32 m/z=34 can 14 P. Turner, T. Merren, J.Speakman and C. Haines, in Plasma Source Mass Spectrometry. Developments and Applications, ed. be dramatically reduced through ion–molecule reactions with G. Holland and S. D. Tanner, Royal Society of Chemistry, Xe gas in a hexapole ion focusing device, confirming the Cambridge, Spec. Publ., 1997, vol. 202, p. 28. previous work of Rowan and Houk.12 The addition of H2 gas 15 N. Jakubowski, I. Feldmann and D. Stuewer, European Winter further promotes the reduction of O- and N-based background Conference on Plasma Spectrochemistry, Pau, France, January polyatomic interferences. However, H2 addition leads to 10–15, 1999. 16 J. Batey and S. Nelms, European Winter Conference on Plasma greater degrees of instrumental mass bias for measured 34S/32S Spectrochemistry, Pau, France, January 10–15, 1999. ratios. The transmission of S+ analyte ions was simultaneously 17 R. S. Houk, European Winter Conference on Plasma reduced, but by a factor of 10 less than the reduction of O2+.Spectrochemistry, Pau, France, January 10–15, 1999. Under these conditions, the transmission of S+ is suYcient 18 S. D. Tanner and V. I. Baranov, European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999. for accurate and precise concentration isotope ratio measure- 19 U. Voellkopf, V. I. Baranov and S. Tanner, European Winter ments in the 10–50 mg l-1 range. Conference on Plasma Spectrochemistry, Pau, France, January Sulfur isotope ratios, determined for Indonesian crater-lake 10–15, 1999. and spring water samples, showed acceptable degrees of 20 J.Speakman, P. J. Turner, A. N. Eaton, F. Abou-Shakra, internal precision (<0.3% RSD measured on ten repeats of Z. Palacz and R. C. Haines, European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999. 34S/32S ratio) and accuracy (34S within 1–6‰ of published 21 P. Delmelle, A. Bernard, M. Kusakabe, T. Fischer and B. Takano, data). Accuracy was limited by short- to medium-term repro- J. Volcanol. Geotherm. Res., 1999, in the press. ducibility of standards and samples and some sample matrix 22 K. Bruland, Chem. Oceanogr., 1983, 8, 157. eVects. ICP-MS incorporating hexapole ion optics is a 23 I. S. Begley and B. L. Sharp, J. Anal. At. Spectrom., 1997, 12, 395. 24 I. Szabo, Int. J. Mass Spectrom. Ion Processes, 1986, 73, 197. potentially useful survey tool for d34S determinations in 25 D. Gerlich, Adv. Chem. Phys., 1992, LXXXII, 1. environmental and geological samples. 26 J. J. Thomson, Rays of Positive Electricity, Longmans, London, 1933. Norbert Jakubowski is thanked for a preliminary review. We 27 P. F. Knewstubb, Mass Spectrometry and Ion-Molecule Reactions, are grateful to Jeroen Kraan and Gijs Nobbe for assistance Cambridge University Press, Cambridge, 1969. during laboratory work. The Utrecht ICP-MS laboratory is 28 E. W. McDaniel, V. Cerma�k, A. Dalgarno, E. E. Ferguson and jointly funded by the Netherlands Organization of Scientific L. Friedman, Ion-Molecule Reactions,Wiley, New York, 1970. Research (NWO/GOA) and the University of Utrecht. 29 V. G. Anicich and W. T. Huntress, Astrophys. J. Suppl. Ser., 1986, 62, 553. 30 V. G. Anicich, J. Phys. Chem. Ref. Data, 1993, 22, 1469. 31 K. E. Jarvis, A. L. Gray and R. S. Houk, A Handbook of References Inductively Coupled Plasma Mass Spectrometry, Blackie, 1 Y. Kiyosu and M. Kurahashi, Geochim. Cosmochim. Acta, 1983, Glasgow, 1992. 47, 1237. 32 E. E. Ferguson, D. Smith and N. G. Adams, Int. J. Mass 2 S. N. Williams, N. C. Sturchio, V. M. L. Calvache, F. R. Mendez, Spectrom. Ion Processes, 1984, 57, 243. C. A. Londozo and P. N. Garcia, J. Volcanol. Geotherm. Res., 33 D. Smith, N. G. Adams and W. Lindinger, J. Chem. Phys., 1981, 75, 3365. 1990, 42, 53. 34 A. L. Gray, J. G. Williams, A. T. Ince and M. Liezers, J. Anal. At. 3 B.W. Robinson and S. H. Bottrell, Appl. Geochem., 1997, 12, 305. Spectrom., 1994, 9, 1179. 4 T. Sriwana, M. J. Van Bergen, J. C. Varekamp, S. Sumarti, 35 L-Q. Huang, S-H. Jiang and R. S. Houk, Anal. Chem., 1987, B. Takano, B. J. H. Van Os and M. J. Leng, J. Volcanol. Geotherm. 59, 2316. Res., 1999, in the press. 5 P. Delmelle and A. Bernard, J. Volcanol. Geotherm. Res., 1999, in the press. Paper 9/02037C 1074 J. Anal. At. Spectrom., 1999, 14, 1067–10

ISSN:0267-9477    

DOI:10.1039/a902037c

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

A calculation method based on isotope ratios for the determination of dead time and its uncertainty in ICP-MS and application of the method to investigating some features of a continuous dynode multiplier Andrea Held and Philip D. P. Taylor* Institute for Reference Materials and Measurements, European Commission-JRC, B-2440 Geel, Belgium. E-mail: taylor@irmm.jrc.be Received 20th November 1998, Accepted 27th April 1999 A method for the determination of dead time (and its uncertainty) of the ion-counting detection system of ICP-MS instruments is presented.It is based on isotope ratio measurements, which are more precise than ion current measurements and will result in values for the dead time with a measurable uncertainty. The suggested method allows us to estimate the uncertainty associated with the dead time. Typical relative uncertainties of the dead time values were found to be in the range of 10–20%. This method has been applied to monitoring the dead time for a detection system over the lifetime of a particular electron multiplier.The dead time was found to be an important criterion for replacing an old electron multiplier when optimum isotope ratio measurements are required. The dependence of the dead time on the mass of the measured isotopes has been investigated. No mass dependence of the dead time could be concluded within the measurement uncertainty on this particular instrumental set-up. It is therefore easily applicable for routine monitoring of dead Introduction time.Furthermore it can also deliver a reliable estimate of the The dead time of the ICP-MS detection system is an important combined uncertainty associated with the dead time value parameter for the reliable determination of isotope ratios when according to published guides on uncertainty evaluation.12,13 using an ion counting detection system1–4 If not corrected for, This method has been applied to monitoring dead time during it deteriorates the linearity of the instrument over a range of the lifetime behaviour of a Channeltron electron multiplier isotope ratios, the eVect being greater for larger isotope ratios.and to investigate a possible dependence of the dead time on Furthermore, the uncertainty associated with the determi- the mass of the incident ions. nation of the dead time aVects the uncertainty related to ratio measurements, as has been shown by Hayes and Schoeller,1 Instrumentation and reagents and consequently the uncertainty of isotope dilution analysis results.5,6 Ageing of the electron multiplier, which is part of A VG Instruments PQ2+ ICP-MS (VG Elemental, Winsford, the ion counting detection system, significantly changes the Cheshire, UK), equipped with a Channeltron continuous apparent dead time.Continuous monitoring of the dead time dynode electron multiplier (Galileo Electro-Optics Corpis therefore necessary. Frequently applied methods for the oration, Stunbridge, MA, USA), was used.Typical operdetermination of dead time for ICP-MS instrumentation ating conditions are given in Table 1. Further information on involve checking the linearity of the relation between measured Channeltron electron multipliers can be found in the ion current and concentration of the measured solution as a literature.14,15 function of dead time, deducing dead time from isotope ratio Water and nitric acid used for preparation of solutions in measurements by ‘trial-and-error’ methods, in which usually this work have been purified by sub-boiling distillation.the dead time is varied until the observed isotope ratio does A solution of NIST SRM 982 Pb was prepared by dissolving not vary with the concentration any more,3,6,7 or using the an appropriate amount of the metal in dilute nitric acid and eVect of dead time on isotope ratios measured at diVerent further dilution to the required concentrations with 0.14 M concentration levels systematically to calculate a dead time, which has been used by Vanhaecke et al.,2 Hayes et al.8 and in this work.Table 1 Typical operating conditions for the ICP-MS used in this work, equipped with a V-groove nebuliser In this article a simple method for the determination of dead time is described. It is based on isotope ratio measure- Parameter Typical setting ments as these are very sensitive to dead time eVects and can be measured with small uncertainties.Similar methods have Gas flows (Ar): Cooling 15 L min-1 been applied for the determination of dead time for secondary Auxiliary 2 L min-1 Nebuliser 0.8 L min-1 ion mass spectrometry (SIMS).9,10 We suggest applying this Forward power ICP generator 1400W method to ICP-MS. It can easily be implemented as a spread- Reflected power <5 W sheet using commercially available spreadsheet software as it Sample solution flow 1 mL min-1 is entirely based on straightforward calculations.It does not Spray chamber temperature 4 °C require iterative procedures as other workers have suggested.11 J. Anal. At. Spectrom., 1999, 14, 1075–1079 1075HNO3. Note that SRM 982 is slightly radioactive and neces- the certified 204Pb5208Pb ratio.) sary precautions have to be taken when working with this material. R(1E/2E)dt= Idt(2E) Idt(2E) = [1-Idt(1E)·t]·Itrue (1E) [1-Idt(2E)·t]·Itrue(2E) For the other elements investigated in this work, commercially available standard solutions from diVerent suppliers were used.= [1-Idt(1E)·t] [1-Idt(2E)·t] ·E(1E/2E)true (2) Bottles for sample storage were cleaned by leaching with 5% HNO3 for a minimum of one day. Various types of PE The equation is further rearranged to yield a linear expression and FEP bottles were used. R(1E/2E)true R(1E/2E)dt = (1-Idt(2E)·t (1-Idt(1E)·t) (3) Theoretical background R(1E/2E)true[1-Idt(1E)·t]=R(1E/2E)dt[1-Idt(2E)·t] (4) It is well known that ion counting systems show dead time R(1E/2E)true-R(1E/2E)true·R(1E/2E)dt·Idt(2E)·t eVects.This means that a minimum time diVerence between the impact of two ions is necessary to identify those as two =R(1E/2E)dt-R(1E/2E)dt·Idt(2E)·t (5) individual events. This eVect is partly due to processes taking Division of equation (5) by R(1E/2E)dt and rearranging yields place in the electron multiplier and is partly caused by eVects equation (6). in the counting electronic device connected to the multiplier.This leads to an observed count rate that is smaller than the true one. Counting systems can be divided into two fundamen- R(1E/2E)true R(1E/2E)dt =1+[R(1E/2E)true-1]·t·Idt(2E) (6) tally diVerent types, ‘paralysable’ and ‘non-paralysable’ counters. 1,16 Paralyzable counters will not count an incoming ion Plotting R(1E/2E)true/R(1E/2E)dt over Idt(2E) then yields a B that arrives within the dead time t of a previous ion A, but straight line, the slope being m¾=[R(1E/2E)true-1]·t.The dead the counter will be ‘dead’ for a new time span of t after the time can thus be calculated from the slope of such a plot. As arrival of ion B. Non-paralyzable counters would completely a basic requirement for linear regression it is assumed that the ignore a second ion B arriving within the dead time of ion A, relative uncertainty on the abscissa values is non-existent.22 In without any extension of the time that it will be ‘dead’ for practice, it is assumed to be negligible compared to the relative other incoming ions.Fortunately, both types can be described uncertainty of the ordinate, when the uncertainty of a typical using the same formula to a good approximation [eqn. (1)]. ordinate value compared to the range of the ordinate values Therefore, count losses occurring in any real counting system is bigger than the uncertainty of a typical abscissa value which might be in between those two types can be described compared to the full range of the abscissa values.23 This using Equation (1).relationship has been checked for the current problem and this criterion is fulfilled. Idt=(1-Idt·t)·Itrue (1) For any real measurement, the measured isotope ratio R(1E/2E)dt is not only aVected by dead time, but also by mass with Idt being the observed rate of events (i.e., aVected by bias eVects. A correction has to be applied to take these eVects dead time losses), Itrue the true rate of events and t the into account.In order to achieve lowest possible uncertainties apparent dead time. on the resulting dead time values, the mass bias correction is More refined models for the description of real counting carried out for each measured data point. When measuring a systems have been developed, including models where series suitable element or isotopic reference material, a ratio of paralysable and non-paralysable counters are con- R(3E/4E)mb of a second pair of isotopes 3E and 4E can be sidered.17,18 Sometimes, dead time is modelled for diVerent measured and used for the mass bias correction (indicated by parts of the counting system and pulse overlap is taken into the subscript mb) if that ratio is close to unity and therefore account17,19 or where dead time is assumed to have a distri- not aVected by dead time eVects.(In the example of measurebution itself.20,21 Here, only the simplest model [eqn. (1)] ments of NIST SRM 982, the isotopes 206Pb and 208Pb can be is used.used as 3E and 4E, respectively.) We define a mass bias For isotope ratio measurements with a combined relative correction factor k(3E/4E) as the ratio of the known to certified uncertainty uc<0.5%, the impact of dead time eVects on isotope ratio of the sample R(3E/4E)true and the measured one isotope ratios is significant. As the eVect increases with the R(3E/4E)mb, which is aVected by mass bias intensity of the ion beam hitting the counter, it causes a nonlinear response of the counting system.For isotope ratio k(3E/4E)= R(3E/4E)true R(3E/4E)mb (7) measurements this means that the measured ratio changes with the intensity of the ion beams measured. This unwanted eVect can, on the other hand, be used to determine the dead By applying, for example, the linear law,24 which is suYcient to correct for mass bias eVects within the measurement uncer- time, allowing a correction of all measurement data. A similar approach has been used by Hayes et al.,8 who also used tainties achievable on a quadrupole based ICP-MS, we can calculate from the mass bias k(3E/4E) the mass bias per mass isotope ratio measurements for the determination of dead time, but applied a more complex model, taking into account unit, e, [eqn.(8)] and further derive the mass bias for the ratio R(1E/2E) [eqn. (9)]. discriminator eVects for overlapping pulses. The present work focuses on a simple but nevertheless suYciently accurate k(3E/4E)=1+e·Dm(3E/4E) (8) method for the determination of dead time, as well as on the estimate of the uncertainty associated with the dead time.k(1E/2E)=1+e·Dm(1E/2E) (9) Applying eqn. (1) to a dead time aVected isotope ratio R(1E/2E)dt of two isotopes 1E and 2E (2E is the higher with Dm(1E/2E) and Dm(3E/4E) being the mass diVerence between the isotopes 1E–2E and 3E–4E, respectively. abundant isotope) yields eqn. (2). (As an example, in this work the NIST SRM 982 Pb isotopic reference material was In eqn.(6) we can replace R(1E/2E)dt by the actually measured ratio of 1E and 2E, R(1E/2E)meas, which is aVected used for the measurements. In this case 1E would refer to 204Pb and 2E would refer to 208Pb, Rtrue would correspond to by dead time and mass bias, multiplied by the mass bias 1076 J. Anal. At. Spectrom., 1999, 14, 1075–1079correction factor k(1E/2E), yielding eqn. (10) This lead material was chosen because it oVers a unity ratio of 206Pb5208Pb for the mass discrimination correction and a 1520 (204Pb5207Pb) and a 1540 ratio (204Pb5208Pb).In this R(1E/2E)true k(1E/2E)·R(1E/2E)meas =1+[R(1E/2E)true-1]·t·Imeas(2E) case the 204Pb5208Pb ratio was used for dead time determi- (10) nation. A set of solutions was prepared from this material with concentrations ranging from 5 ng g-1 to 100 ng g-1. Of As without dead time correction, the dead time is calculated these solutions a set of 4–10 were selected according to the from the slope m¾=[R(1E/2E)true-1]·t of a linear regression instrument’s sensitivity, such that for the major isotope, count line of R(1E/2E)true/[k(1E/2E)·R(1E/2E)meas] versus Imeas(2E).rates between 50 000 and 800 000 s-1 were obtained. It was The resulting dead time is given by eqn. (11) found that a set of four solutions was suYcient, if each solution was measured repeatedly. For each sample, the iso- t= m¾ [R(1E/2E)true-1] (11) topes 204Pb, 206Pb and 208Pb were measured.Further, the contribution of 204Hg to the measured ion current on m/z= This equation can also be used to estimate the uncertainty 204 needed to be taken into account. Therefore the 202Hg ion associated with the dead time. As the biggest source of current was monitored, and if any mercury above blank levels uncertainty in eqn. (11) is the slope, m¾, of the regression line, was detected, the respective ion current originating from the combined uncertainty12,13 of the dead time is estimated mercury on m/z=204 was calculated using the respective from the uncertainty on that slope m¾.The uncertainty of the abundances of 202Hg and 204Hg and subtracted from the ratio R(1E/2E)true is negligible compared with the uncertainty measured ion current on m/z=204. of the slope, and is therefore disregarded. The uncertainty of The software of most of the commercially available the dead time can then be included in uncertainty budgets for instruments automatically applies a dead time correction. isotope ratio measurements.Furthermore, the uncertainty of Therefore, it is necessary to set the dead time in the software the dead time can be used to calculate optimum measurement set-up to zero before starting the measurements. In the case conditions, such as the maximum admissible count rate and of the instrument used for this work this is not possible, as minimum measurement time required for a minimal contrithe software does not allow any value smaller than 1 ns for bution of the dead time, and counting statistics to the combined the dead time.During the dead time determinations the dead uncertainty of a ratio measurement1 or to optimise measuretime is thus set to 1 ns. This introduces a deviation of the ments by isotope dilution mass spectrometry.5 dead time values calculated, which was found to be negligible within the uncertainty of the determination, typically in the Experimental order of 2–5 ns. To achieve a variation in the intensity of the major isotope, Possible errors diVerent concentrations of an element in solution are used.In general any poly-isotopic element with 3 isotopes could be As the dead time in this work has been determined by used for dead time determinations: in practice, however, a measuring the 204Pb5208Pb ratio, the possible influence of solution with a suYciently small isotope ratio, Rt<0.05, is background, blanks and interferences (e.g., 204Hg) needs to be preferably used.If the isotope ratio is too close to unity, the discussed. The elimination of background contributions in the eVect of the dead time will be very small and hidden in the determination of dead time by an iterative procedure has been scatter of the measured ratios. Furthermore, the quality of the described by Hayes et al.8 In ICP-MS measurements, such a measurements can be improved by choosing another isotope procedure is not necessary as background can usually be ratio of the same element to calculate a mass bias correction measured and appropriately corrected for.In any case, such a within each sample rather than determining the mass bias background would be visible from a larger spread of measured correction in a separate measurement. Mass bias correction is ratios and therefore result in an increased uncertainty of the also important as it changes the ratio R(1E/2E)true/R(1E/2E)dt dead time. Another relevant source of error could also be a in eqn.(6) by a multiplicative factor that would consequently significant contamination of the solutions used to measure also change the value for the dead time [see eqn. (10)]. The dead time by material of diVerent isotopic composition. When isotope ratio for the mass bias correction should then be as material with natural isotopic composition is used to prepare close as possible to unity, as dead time has a minimal influence these solutions this risk is, of course, eliminated.on unity ratios. This limits the suitable elements to those with Contamination would result in a significant error in the at least three isotopes, with two isotopes having approximately determined dead time without being accounted for by an the same abundance. Possibilities are solutions of, e.g., Mg, increased uncertainty of the dead time and should therefore Zr, Ru, Cd, Dy, Os or Pt, of natural isotopic composition. be avoided at all cost. Another option is the use of isotopic reference materials, such as the IRMM-072 series, which oVers a 235U5238U ratio very Results close to unity and a 233U5238U ratio ranging from unity to 2×10-6, certified to an uncertainty of 0.03%.(This material Fig. 1 shows a typical example of the data resulting from a is radioactive and therefore not easily accessible for every dead time determination on an ICP-MS. The dead time as user.) In this case a Pb isotopic reference material, NIST calculated from the regression line for the dead time uncor- SRM 982, was used.This reference material has an isotopic rected data using eqn. (11) corresponds to 17.4±1.9 ns. The composition as given in Table 2. uncertainty of the dead time has been estimated from the uncertainty of the slope of the regression line. Using the instruments software the same data has been reintegrated Table 2 Isotopic composition of NIST SRM 982 using this dead time value (Fig. 2). A regression line fitted with these dead time corrected data points no longer shows a Isotope Relative isotope abundance (×100) significant variation of the isotope ratios with the intensity of 204Pb 1.0912±0.0012 the major isotope.This is also a verification of the dead time 206Pb 40.0890±0.0072 correction algorithm used in the instruments software. 207Pb 18.7244±0.0032 There is a residual oVset in the regression lines of Fig. 1 208Pb 40.0954±0.0077 and Fig. 2. The regression lines in both graphs should cross J.Anal. At. Spectrom., 1999, 14, 1075–1079 1077Fig. 4 Dead time determined using diVerent elements in solution (Mg, Fig. 1 Typical graph obtained for dead time determination on the Zr, Dy, Pb SRM 982) on one day. Uncertainty bars represent the ICP-MS. The resulting dead time is 17.4±1.9 ns, X= estimated uncertainty of the dead time (coverage factor k=1), broken R(204Pb/208Pb)true/[k(204Pb/208Pb)·R(204Pb/208Pb)meas]. lines represent the standard uncertainty (k=1) of the average value (continuous line).even showing apparently negative dead time (on day 329, indicated by the arrow in Fig. 3), although the gain was still high. Adjusting the voltage (increased to -2500 V from initially -2375 V on day 235, further increased to -2625 V on day 328) applied accross the multiplier did not improve its behaviour. The multiplier was finally replaced on day 353. As can be seen from Fig. 3, the dead time of the detection system should have been monitored more frequently in order have a valid value available at any time.The useful life of this particular multiplier ended when the observed dead time values started to scatter unpredictably. It should have been replaced earlier than was actually done, although its gain (the usual criterion for replacing a multiplier) was still unaVected. Regular monitoring of the multiplier’s dead time can give Fig. 2 Same data as displayed in Fig. 1, after correction for 17.4 ns valuable information to help decide when a multiplier needs dead time, X=R(204Pb/208Pb)true/[k(204Pb/208Pb)·R(204Pb/208Pb)meas]. replacing.Another concern regarding the dead time was the possible mass dependence of the dead time as the multiplier gain can the abscissa at a value of 1 if mass bias has been corrected for depend on the energy and m/z of a given ion.14,15,25 Vanhaecke appropriately. This residual oVset was due to an instrumental et al.2 have observed an increase of the dead time by 50% problem which was only discovered very recently and can be when going from Mg to Pb as the element used for dead time ignored for discussion of the proposed method as such.The determination using a Perkin-Elmer SCIEX ELAN 5000 determination of the dead time was carried out at regular ICP-MS equipped with a Channeltron continuous dynode intervals on this instrument to assess whether the dead time electron multiplier, whereas they did not observe a significant would stay constant over time.Fig. 3 shows the behaviour of variation of dead time with analyte mass on a Finnigan-MAT one multiplier over its lifetime. Although only a few determi- Element ICP-MS equipped with a conversion dynode and a nations of the dead time have been carried out, it shows a secondary electron multiplier with discrete dynodes. On our rather uniform increase over the first 220 days; after that, the instrument, which is equipped with a Channeltron multiplier dead time of the multiplier started to be rather unpredictable, similar to the Elan 5000 set-up used by Vanhaecke et al., no significant trend of the dead time with m/z could be observed within the uncertainty of the dead time measurement.In this work, dead time was determined using the Pb SRM 982 and Mg, Zr and Dy of natural isotopic composition to cover a wide mass range (Fig. 4). These results show that a similar type of multiplier can exhibit diVerent behaviour of dead time depending on the type of instrument in which it is installed.Therefore, dead time as a function of analyte mass needs to be determined on diVerent types of instrumentation. Conclusion A straightforward method for the determination of dead time and its associated uncertainty on ICP-MS instrumentation was presented. It can easily be implemented in commercial spreadsheet software for routine application. Compared to other methods it requires only few calculations and is not based on Fig. 3 Behaviour of dead time over the lifetime of the multiplier, the arrow indicates a negative dead time value. ‘trial-and-error’ methods. The dead time can be calculated 1078 J. Anal. At. Spectrom., 1999, 14, 1075–10797 I. S. Begley and B. L Sharp, J. Anal. At. Spectrom., 1997, 12, 395. with suYciently small uncertainties, typically in the range of 8 J. M. Hayes, D. E. Matthews and D. A. Schoeller, Anal. Chem., 10–20% relative. The dead time of the continuous dynode 1978, 50, 25.multiplier system used in this work varies constantly over time 9 Y. A. Liu and R. H. Fleming, Rev. Sci. Instrum., 1993, 64, 1661. and needs to be monitored frequently to ensure best possible 10 A. J. Fahey, Rev. Sci. Instrum., 1998, 69, 1282. isotope ratio measurements. The usual criterion for replacing 11 J. I. Garcia Alonso, F. Sena, Ph. Arbore, M. Betti and L. Koch, J. Anal. At. Spectrom., 1995, 10, 381. a multiplier, its loss in gain, should not be the only criterion 12 Guide to the Expression of Uncertainty in Measurement, when one is interested in isotope ratio measurements.It is International Organization for Standardization (ISO), Geneva, advisable to replace a multiplier when it shows unpredictable Switzerland, 1st edn., 1993. changes in dead time. 13 Quantifying Uncertainty in Analytical Measurement, Furthermore, a mass dependence of the dead time as EURACHEM Working Group on Uncertainties in Chemical observed by Vanhaecke et al.2 for a similar type of electron Measurement, Teddington, UK, 1st edn., 1995. 14 ChanneltronA Electron Multiplier Handbook for Mass multiplier could not be supported by results presented in this Spectrometry Applications, Galileo Electro-Optic Corporation, work. This should be considered when using ICP-MS for Sturbridge, MA, USA, 1991. isotope ratio measurements and renders this method for dead 15 E. A. Kurz, Am. Lab., March 1979, 67. time determination very useful. 16 G. F. Knoll, Radiation Detection and Measurement, John Wiley & Sons, Chichester, Sussex, UK, 1979, pp. 95–103. 17 J. A. Williamson, M. W. Kendall-Tobias, M. Buhl and M. Seibert, Acknowledgement Anal. Chem., 1988, 60, 2198. 18 J. W. Mu� ller, Nucl. Instrum. Meth., 1973, 112, 47–57. The authors would like to thank C. Quetel, who greatly helped 19 J. D. Ingle and S. R. Crouch, Anal. Chem., 1972, 44, 777. to refine this manuscript. 20 T. Stephan, J. Zehnpfennig and A. Benninghoven, J. Vac. Sci. Technol., 1994, A12, 405. References 21 S. K. Srinivasan, J. Phys. A: Math. Gen., 1978, 11, 2333. 22 K. DoerVel, Statistik in der analytischen Chemie, Deutscher Verlag 1 J. M. Hayes and D. A. Schoeller, Anal. Chem., 1977, 49, 306. fu� r GrundstoYndustrie, Leipzig, Germany, 1990, pp. 159–164. 2 F. Vanhaecke, G. de Wannemacker, L. Moens, R. Dams, 23 P. R. Bevington, Data Reduction and Error Analysis for the Ch. Latkoczy, T. Prohaska and G. Stingeder, J. Anal. At. Physical Sciences, McGraw-Hill Inc., New York, USA, 1969, Spectrom., 1998, 13, 567. p. 98. 3 S. R. Koirtyohann, Spectrochim. Acta, Part B, 1994, 49, 1305. 24 P. D. P. Taylor, P. De Bie`vre, A. J. Walder and A. Entwistle, 4 P. G. Russ and J. M. Bazan, Spectrochim. Acta, Part B, 1987, J. Anal. At. Spectrom., 1995, 10, 395. 42, 49. 25 E. Zinner, A. J. Fahey and K. D. McKeegan, Springer Ser. Chem. 5 A. G. Adriaens, W. R. Kelly and F. C. Adams, Anal. Chem., 1993, Phys., 1986, 44 (Second. Ion Mass Spectrom., SIMS 5), 170. 65, 660. 6 A. A. van Heuzen, T. Hoekstra and B. van Wingerden, J. Anal. At. Spectrom., 1989, 4, 483. Paper 8/09098J J. Anal. At. Spectrom., 1999, 14, 1075–1079

ISSN:0267-9477    

DOI:10.1039/a809098j

出版商:RSC    

年代:1999

数据来源: RSC