摘要:

Isotope Dilution as a Calibration Method for Solid Sampling Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry FRANK VANHAECKE, SYLVIE BOONEN, LUC MOENS AND RICHARD DAMS L aboratory of Analytical Chemistry, Ghent University, Institute for Nuclear Sciences, Proeftuinstraat 86, B-9000 Ghent, Belgium The present paper reports on the use of isotope dilution as a continuous improvements in instrumentation LA-ICP-MS has method of calibration for solid sampling ETV-ICP-MS.The become a well established solid sampling technique, predompossibilities and limitations of this calibration strategy were inantly used for geological applications,12–16 but also for the evaluated by determining the Cd or Se content in solid CRMs analysis of metals,17–19 glasses20,21 and plastics22. of different origin. It was shown that since isotope ratios are In addition to LA-ICP-MS,ETV-ICP-MS also offers interesonly slightly affected or not affected at all by (i) matrix ting possibilities for the direct determination of trace and ultraeffects, (ii) signal drift and instrument instability and trace elements in solid samples.Both Voellkopf et al.23 and (iii) variations in the vaporization and/or transport efficiency, Gre�goire et al.24 have reported on the analysis of solid samples isotope dilution allows accurate analyses to be carried out using ETV-ICP-MS, but both research groups preferred (mean deviation between solid sampling ETV-ICP-MS results the analysis of slurries rather than of dry solid samples.and certified values <10%). The precision attainable is Wang et al.25 explored the feasibility of ‘real’ solid sampling, determined by the sample homogeneity and is hence but encountered some difficulties that hampered the practical comparable to that obtained using other calibration techniques, use of solid sampling ETV-ICP-MS. Argentine and Barnes,26 such as (i) external calibration with either a solid standard or however, successfully used ETV-ICP-MS for the determination an aqueous standard solution or (ii) standard additions.An of non-volatile impurities in semiconductor-grade organometimportant advantage of isotope dilution over the allic materials and process chemicals. Finally, earlier work aforementioned calibration techniques for solid sampling ETV- carried out in our laboratory, concerning the accurate determi- ICP-MS, however, is that the use of an elemental internal nation of As and Se in CRMs of both plant and environmental standard is no longer required.For some materials, accurate origin using solid sampling ETV-ICP-MS, has been reported analytical results could not be obtained as at least one of the in earlier publications.27–29 isotopes involved was observed to be subject to spectral As for all solid sampling techniques, also for solid sampling interference. The use of several parameters allowing spectral ETV-ICP-MS, accurate calibration is not obvious.Wang interferences to be detected is discussed. Finally, solid et al.25 concluded that in order to obtain optimum accuracy, sampling ETV-ICP-MS was used for the determination of the external calibration with a CRM with a composition as similar Cd content in tobacco as a ‘real-life’ sample and the results as possible to that of the sample should be used. The problem obtained using isotope dilution and single standard addition for of calibration in solid sampling ETV-ICP-MS, however, has calibration were compared with one another and with the been studied extensively in our laboratory and it has been result obtained (after taking the sample into solution) using demonstrated that different strategies allow accurate analyt- pneumatic nebulization ICP-MS.ical results.27 Keywords: Inductively coupled plasma mass spectrometry; The application of a CRM with a similar matrix composition electrothermal vaporization; solid sampling; isotope dilution ; and analyte content as a solid standard indeed permitted calibration excellent results to be obtained, although only provided that a suitable elemental internal standard (added to both the samples and the standard) was used.Although the results Since its commercial introduction in 1983, ICP-MS has aroused obtained for As and Se in solid materials of different origin great interest, and during the past decade it has proven its using Sb as an internal standard27,29 clearly illustrate the utility for the determination of trace and ultra-trace elements potential of this calibration strategy, the latter also shows in a variety of matrices.In its standard configuration, ICP-MS important drawbacks. A consequence of this approach is that is mainly intended for the analysis of aqueous samples, solid sampling ETV-ICP-MS cannot be considered to be an although at present there is an increasing interest in the direct independent method, as one has to rely on certified values, analysis of solid samples.based on results obtained by other analytical techniques. Direct analysis of solid samples is of course of great impor- Moreover, the uncertainty of a certified analyte content is tance for materials that cannot or only with great difficulty be always much larger than the uncertainty of the concentration brought into solution. Moreover, in general, solid sampling in an aqueous standard solution. In addition, some information limits the necessary amount of often laborious and time- on the sample matrix is required and, finally, suitable reference consuming sample pre-treatment, leading to a reduced risk of materials are not available for all sample types.contamination and/or analyte losses, and as samples are Although when using an appropriate elemental internal analysed without dilution, also lower LOD are to be expected. standard, even external calibration with aqueous standard The possibilities of LA for the introduction of solid samples solutions was observed to provide acceptable results,27 single in ICP-MS have been reported extensively in the literature.1–5 standard addition was considered to be the most straight- Next to the general advantages of solid sampling mentioned forward and practicable method.27,29 Since matrix-induced above, LA-ICP-MS also offers the possibility for both lateral signal suppression was established to be strongly dependent and in-depth profiling of solid samples of different origin.6–11 LA-ICP-MS is somewhat expensive, but as a result of on the sample mass,27,29,30 accurate results could only be Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 (125–130) 125Table 1 Operating conditions for the ETV system and the ICP mass obtained with the latter calibration method when using an spectrometer appropriate internal standard or taking the sample masses as close together as is practically possible.The latter approach is ET V system— not only fairly wearisome and time-consuming, but the blank Type SM-30, Gru�n Analytische Mess- is also less accurately corrected for and there is no correction Systeme for signal drift, instrument instability and fluctuations in the Temperature program Multi-step temperature program vaporization and transport processes, such that the first consisting of: approach is to be preferred. 1. ‘Drying’ step (30 s at#120 °C) 2. Boosting step (1 s), during which It is obvious from the above-mentioned results that the use the power applied is 25% of the of an elemental internal standard was observed to be advanta- maximum power, allowing the geous (standard addition methods) or even imperative (external heating rate of the furnace to be calibration methods), depending on the calibration method increased at moderate temperatures used.However, the selection of a suitable elemental internal in order to obtain a stable ashing standard is not obvious.Most importantly, an appropriate temperature rapidly 3. Ashing step (30 s at#200–250 °C) internal standard should show an analogous (furnace) chemis- 4. Intermediate step (12 s at the try as the analyte element(s) and should of course only be ashing temperature), to switch the present at negligible levels in the samples under consideration.27 valve to the &lsquosuring’ position Although only of secondary importance with solid sampling and allow the plasma to stabilize ETV-ICP-MS, in general, internal standardization for correc- 5.Vaporization step (15 s) tion of matrix-induced signal suppression or enhancement and 6. Intermediate step, to switch the valve to the ‘venting’ position (after for improving the precision has been observed to be most the end of the measurement) efficient if the mass number of the internal standard is chosen 7. Cleaning step (2×3 s at#2700 °C) fairly close to that of the analyte element(s).30–32 Since a suitable internal standard should fulfil all three conditions ICP mass spectrometer— simultaneously, it is clear that selection of such an internal Type Perkin-Elmer SCIEX ELAN 5000 standard sometimes poses an unsurmountable problem.Rf power 1300 W As a result, more recently efforts have been made to circum- Sampling depth 10 mm vent the necessity of using such an elemental internal standard. Aerosol carrier gas flow rate 0.750 1 min-1 In an earlier publication,33 it was demonstrated that at least Intermediate gas flow rate 1.0 1 min-1 Outer gas flow rate 12 1 min-1 in some cases (determination of As in a solid CRM of plant Lens voltages Tuned using pneumatic nebulization; origin), the argon dimer (Ar2+) could be used as an internal no further tuning required when standard.Since isotope dilution (ID) as a calibration method switching to solid sampling ETV- only involves determination of an isotope ratio in the sample, ICP-MS the tracer and a mixture of both, matrix effects, signal drift Sampling cone Nickel; 1.0 mm orifice diameter and instrument instability and fluctuations in the vaporization Skimmer cone Nickel; 0.75 mm orifice diameter and transport processes should not have an adverse effect on the results obtained,34 and application of an elemental internal standard would hence no longer be required.The feasibility of Germany) via a 10 mm id silicone rubber tubing. In order to the latter calibration strategy for solid sampling ETV-ICP-MS (i) reduce the amount of deposition of evaporated sample and was investigated and the results obtained are reported in the furnace material on the torch, the interface and the lens stack present paper.and (ii ) avoid degradation of the interface pump oil, a threeway valve was used to vent vapours generated during the drying, ashing and cleaning steps. Operating conditions are EXPERIMENTAL summarized in Table 1. Instrumentation Measurements The ETV system used is a commercially available graphite furnace of the boat-in-tube type (SM-30, Gru�n Analytische Measurement parameters are summarized in Table 2.Fast Mess-Systeme, Ehringhausen, Germany). Although this device hopping between the nuclides monitored is necessary to obtain was originally designed for solid sampling Zeeman-effect AAS, a representative image of the corresponding signal profiles. On some simple modifications (described elsewhere35) sufficed to the other hand, an efficient use of the total measuring time make it compatible for use with both ICP-AES and ICP-MS.requires a high ratio of actual measuring time to mass spec- Graphite sample holders (‘boats’) can be easily and reproduci- trometer settling time and hence, relatively large dwell times. bly loaded into the cylindrical graphite furnace with the aid of As a compromise, 30 ms was used as the dwell time per a pair of tweezers, sliding on a rail, which is rigidly mounted measuring point.36 The three-way valve was switched manually in front of the furnace.After loading the sample, one end of to the ‘measuring position’ 12 s before the start of the vaporiz- the furnace is closed using a shutter kept in position by a ation stage, while the measurement itself was started 2 s before catch-spring. During operation, a flow of argon, the flow rate the beginning of the vaporization stage (Table 1). Each determi- of which is controlled by a mass-flow controller (Model 5876, nation consisted of three measurements of the blank (empty Brooks Instruments, Veenendaal, The Netherlands) is swept through the furnace, transporting the sample aerosol, formed Table 2 Measurement parameters by condensation of the vaporized sample, into the central channel of the ICP.The multi-step temperature program of Dwell time 30 ms the furnace (Table 1) is controlled by a computer program Scanning mode Peak hop transient Sweeps per reading 1 developed in-house, while the temperature can be monitored Readings per replicate 800 divided by the number of nuclides moni- using an optical pyrometer (PY20, Gru�n Optik, Ehringhausen, tored Germany), specially designed for use with this type of ETV Points per spectral peak 1 system.The ETV system was coupled to a Perkin-Elmer Total measurement time ca. 30 s SCIEX ELAN 5000 ICP mass spectrometer (U� berlingen, 126 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12boat) and five measurements of the isotope ratio under con- accurately the number of atoms present of both isotopes involved) of the tracer obtained as described above was sideration in the sample, the tracer and the mixtures (consisting of a given amount of sample, to which an appropriate amount determined by inverse ID (the tracer was considered as the sampleand the standard of natural composition was considered of tracer was added), respectively. In some instances, the Omnirange device was used.This option permits a selective as the ‘tracer’) using pneumatic nebulization ICP-MS. Finally, for use with solid sampling ETV-ICP-MS, the tracers obtained and reproducible reduction of the sensitivity of the mass spectrometer by varying the ion transmission efficiency at the were diluted to an appropriate concentration level, depending on the analyte concentration in the sample. exact time that a given mass-to-charge ratio is being measured and hence allows measurement at higher concentration levels. As reported on in an earlier publication,27 identical behaviour of an analyte in the sample and in the standard can only be guaranteed if the standard solution (in this instance, the Samples tracer) is inserted into the graphite sample holder (10 ml using a micropipette) and dried under an IR lamp before loading of In order to evaluate the accuracy of the results obtainable using ID as a calibration technique in solid sampling ETV- the solid sample (typically 1–2 mg).In all cases, the amount of tracer added to the solid sample was selected such that the ICP-MS, efforts were made to determine Cd or Se in a number of CRMs of biological or environmental origin. For the isotope ratio for the mixture was close to the average of the corresponding isotope ratios in the sample and the tracer. determination of Cd, Aquatic Plant (BCR CRM 060), Light Sandy Soil (BCR CRM 142) and Sewage Sludge of Industrial Finally, for the determination of Cd in the tobacco sample solutions obtained as described above, an external calibration Origin (BCR CRM 146) were selected as samples.For Se, Estuarine Sediment (BCR CRM 277), Sea Lettuce (BCR CRM solution was prepared by appropriate dilution of a commercially available Cd standard solution (1 g l-1); In was added 279) and Wheat Flour (NIST SRM 1567A) were used. No sample pre-treatment except for homogenization (by shaking) as an internal standard.and weighing was required before analysis. The moisture content of these materials was determined according to the Calculations ‘drying instructions’, given in the certificates, and correspond- All calculations were carried out according to the formulae ingly corrected for. presented by Longerich.37 In solid sampling ETV-ICP-MS, After having demonstrated the possibilities and limitations calculation of the standard deviation of the results obtained of ID as a calibration method in solid sampling ETV-ICP-MS, is, however, somewhat more complicated than for aqueous Cd was determined in Johnson heavy cigarette-tobacco as a solutions, since for each replicate measurement of a mixture ‘real-life’ sample. In order to evaluate the accuracy of this of (solid) sample and trace amount of sample taken was analysis, tobacco samples were also taken into solution and of course slightly different.However, since the uncertainty due the solutions obtained were analysed using pneumatic nebuliz- to inhomogeneity of the samples clearly exceeded other contri- ation ICP-MS with external calibration as a calibration tech- butions (e.g., the uncertainty in the isotope ratio measurement nique.To #1.2 g of tobacco, 5 ml of 14 mol l-1 HNO3 and for the sample and the tracer), the analyte concentration was 1 ml of 10 mol l-1 HCl (both purified by sub-boiling distil- calculated for each replicate measurement of a mixture of lation) were added and the mixtures obtained were subjected sample and tracer, respectively, and the standard deviation for to a step-wise temperature program (15 min at 80°C, 15 min the n individual results was calculated subsequently and was at 110 °C and 2×90 min at 240 °C) in a high-pressure asher considered as the standard deviation on the final result.(HPA, Ku� rner, Rosenheim, Germany). Since a fine white precipitate was observed in the digestion vessels, HF was added and the sample was subjected a second time to the RESULTS AND DISCUSSION multi-step temperature program.The solutions obtained were Preliminary Study diluted to 25 ml and In was added as an internal standard to correct for matrix effects, signal drift and instrument instability. During a preliminary study, the precision of the isotope ratio measurements and the mass discrimination observed using ETV-ICP-MS were compared with the corresponding values Standards obtained using pneumatic nebulization ICP-MS.For series of five successive measurements of a 100 mg l-1 For the determination of Cd, the tracer available for these experiments was enriched in 110Cd (93.63%) and was purchased Cd standard solution with pneumatic nebulization ICP-MS, the average RSD for the 110Cd5113Cd isotope ratio was from Campro Scientific (Veenendaal, The Netherlands). About 5 mg of this material, consisting of CdO, were taken into observed to be 0.20%, while the mass discrimination factor K (defined as the ratio of the ‘measured 110Cd5113Cd isotope solution by adding #2 ml of 14 mol l-1 HNO3 (purified by sub-boiling distillation).A stock solution (5 mg l-1) was ratio to the true value’) was established to be 0.94. For a series of five successive ETV-ICP-MS measurements of aliquots of obtained by dilution with Millipore Milli-Q water to a volume of 1 l. For the determination of Se, a standard enriched in 82Se 10 ml of a 200 mg l-1 Cd tracer solution, the RSD for the isotope ratio was observed to vary from 0.77 to 2.2%, with an (Se, 92.2% in 82Se) from Euriso-top (Saint Aubin, France) was used.Approximately 10 mg of the metallic Se standard were average value of 1.5%. Also for ETV-ICP-MS, the average mass discrimination factor was observed to be 0.94. taken into solution using 30 ml 14 mol l-1 HNO3 and subsequently diluted to 1 l using Millipore Milli-Q water. In order For a 100 mg l-1 Se standard solution, the 82Se577Se isotope ratio could be measured with an RSD of 0.75% using pneu- to allow accurate determination (the signal intensity for both isotopes involved significantly exceeding the blank level) of matic nebulization ICP-MS (n=5).With pneumatic nebulization ICP-MS, the mass discrimination factor (defined as the the isotope ratios (110Cd5113Cd and 77Se582Se) in the tracers using ETV-ICP-MS, these standards were ‘diluted’ with a ratio of the ‘measured 82Se577Se isotope ratio to the true value’) was established to be 1.14.For ETV-ICP-MS on the standard of natural composition. For Cd, the tracer finally used was obtained by mixing ‘enriched Cd’ with ‘natural Cd’ other hand, the RSD for five successive measurements of a 10 ml aliquot of a 100 mg l-1 Se tracer solution varied from in a 151 (m/m) ratio, giving a 110Cd5113Cd ratio of 6.37 instead of 195.1, while for Se, two portions of ‘enriched Se’ were mixed 0.93 to 1.8%, with an average value of 1.4%.The average value for the mass discrimination factor was found to be with three portions of ‘natural Se’ (m/m) giving an 82Se577Se ratio of 8.65 instead of 144.3. The concentration (or more 1.16. By means of a t-test (95% confidence level), it was Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 127demonstrated that this value did not differ significantly from instrument instability and (iii ) variations in the vaporization and/or transport efficiency,34 it could be expected that ID as the value obtained using pneumatic nebulization ICP-MS.The precision for the isotope ratio measurements attainable a calibration technique allows accurate analysis with solid sampling ETV-ICP-MS, while the application of an elemental using ETV-ICP-MS is clearly somewhat poorer than that obtained using pneumatic nebulization ICP-MS. This is prob- internal standard is no longer required. Of course, a prerequisite for accurate analysis by ID is the ably to be attributed to the transient nature of the signals involved.The precision of the isotope ratios obtained, however, availability of two ‘free’ isotopes, i.e., two isotopes which are not spectrally interfered by isobaric M+ ions, doubly charged were assessed to be sufficiently good to warrant an investigation of the possibilities of using ID as a calibration method in solid ions, oxide or other polyatomic ions. Therefore, in all cases, it should be carefully checked that this condition is fulfilled.This sampling ETV-ICP-MS. is especially the case in solid sampling ETV-ICP-MS, since as a result of the ‘nature’ of the technique (e.g., dry plasma Use of Parameters Allowing Spectral Interferences to be conditions, introduction of relatively large amounts of C into Detected the ICP), unforeseen spectral interferences can occur. Several ‘tools’ can, however, be used to check fulfilment of All relevant figures and analytical results obtained for the this condition.Firstly, the experimentally determined Cd or CRMs investigated are summarized in Tables 3–5. In order to Se isotope ratio can be compared with the value expected on evaluate these results thoroughly, several parameters were used the basis of the natural isotopic abundances of the isotopes as indicators for spectral interferences. These ‘tools’ will be involved. The difference between these values should be in introduced in the following paragraphs and thereafter, the agreement with a mass bias in favour of the higher mass ion.results obtained will be systematically discussed. In order to determine if the magnitude of the difference between As has already been discussed at the beginning of this paper, the experimentally determined isotope ratio and the true value when using external calibration or standard additions the can still be considered to be solely the result of this mass selection of a suitable (elemental) internal standard in solid discrimination effect, a mass discrimination factor (defined as sampling ETV-ICP-MS can sometimes pose an unsurmountthe ratio of the ‘measured isotope ratio to the true value’) can able problem. Since isotope ratios are not affected at all, or only slightly affected by (i ) matrix effects, (ii ) signal drift and be calculated for both the sample (Ks) and the tracer (Kt).A Table 3 Results for the determination of the 110Cd5113Cd and 82Se577Se isotope ratios in the solid samples investigated 110Cd5113Cd ratio (uncorrected for mass discrimination effects)— Material investigated 110Cd:113Cd* RSD (%) Sewage Sludge Industrial Origin (BCR CRM 146) 0.943 0.45 Aquatic Plant (BCR CRM 060) 1.013 3.8 Light Sandy Soil (BCR CRM 142) 1.034 2.9 Tobacco 1.005 0.78 82Se577Se ratio (uncorrected for mass discrimination effects)— Material investigated 82Se577Se† RSD (%) Wheat Flour (NIST SRM 1567A) 1.34 0.79 Estuarine Sediment (BCR CRM 277) 1.20 0.64 Sea Lettuce (BCR CRM 279) 1.58 12 *Value expected on the basis of the natural isotopic abundances of the isotopes involved, 1.022.†Value expected on the basis othe natural isotopic abundances of the isotopes involved, 1.144. Table 4 Mass discrimination factors obtained for the sample (Ks) and the tracer (Kt) and ratio Ks/Kt observed during the determination of Cd or Se using solid sampling ETV-ICP-MS CRM Ks* Kt Ks/Kt 110Cd5113Cd— Sewage Sludge Industrial Origin (BCR CRM 146) 0.92 0.92 1.00 Aquatic Plant (BCR CRM 060) 0.99 0.94 1.05 Light Sandy Soil (BCR CRM 142) 1.01 0.90 1.12 Tobacco 0.98 0.94 1.04 82Se:77Se— Wheat Flour (NIST SRM 1567A) 1.17 1.17 1.00 Estuarine Sediment (BCR CRM 277) 1.05 1.08 0.97 Sea Lettuce (BCR CRM 279) 1.38 1.24 1.11 *Values obtained for Ks obtained using pneumatic nebulization ICP-MS using a Cd or a Se standard solution, 0.94 and 1.16, respectively. Table 5 Solid sampling ETV-ICP-MS results (mg g-1) for the Cd or Se content in the materials analysed; calibration by ID Material investigated This work, mean±95% CL Certified value±95% CL Cd— Sewage Sludge Industrial Origin (BCR CRM 146) 80±10 77.7±2.6 Aquatic Plant (BCR CRM 060) 2.184±0.079 2.20±0.10 Se— Wheat Flour (NIST SRM 1567A) 1.204±0.031 1.102±0.088 Estuarine Sediment (BCR CRM 277) 1.664±0.093 2.04±0.18 128 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12substantial difference between the mass discrimination factor abundances of the isotopes involved by #38% (Table 3).(ii) This would lead to a mass discrimination factor for the for the sample and the tracer, respectively, indicates the presence of a spectral interference. Finally, a large RSD for sample of 1.38 and although for this determination the mass discrimination factor for the tracer was also seen to be fairly the experimentally observed isotope ratio can also be indicative of the presence of a spectral overlap. The latter is especially high (Table 4), the discrimination factor for the sample exceeded that for the tracer by >10%.Finally, (iii ) also the expected in the case of overlap with C-containing polyatomic ions, since the amount of C introduced into the plasma can large RSD of the 82Se577Se isotope ratio results is indicative of the occurrence of a spectral interference. Hence, no further differ strongly from one firing of the furnace to another. It should be noted that the first two ‘tools’ imply that it is attempts were made to obtain an analysis result.assumed that the analyte in the sample is of natural isotopic composition. However, this is also the case if external cali- Determination of Cd in Tobacco bration or standard additions are used and only one nuclide is monitored. Of course, care has to be taken for elements for For the tobacco sample investigated, the 110Cd5113Cd signal which it is known that the isotopic composition could change ratio could be measured with an RSD of only 0.78% (Table 3).from one sample to another, such as Pb. A 4% difference could be established between the mass discrimination factors for the sample and the tracer, respectively (Table 4). This difference is comparable to or even somewhat Determination of Cd in CRMs smaller than that observed for Aquatic Plant (BCR CRM For Sewage Sludge of Industrial Origin (BCR CRM 146), an 060), and for the latter CRM an excellent result was obtained. excellent RSD (Table 3) was obtained for the 110Cd5113Cd Hence the Cd content was determined using solid sampling isotope ratio.Since the mass discrimination factors for the ETV-ICP-MS using both ID and single standard addition sample and the tracer were also in excellent agreement (nuclide monitored, 110Cd) for calibration and using pneumatic (Table 4), it could be assumed that both Cd isotopes involved nebulization ICP-MS, after taking the sample into solution, as were interference-free and hence, the result obtained is in described under Experimental.All results (corrected for the excellent agreement with the certified value (Table 5). Although moisture content) are summarized in Table 6. The result the RSD of the 110Cd5113Cd ratio was observed to be <1%, obtained using ID compares excellently with that obtained the 95% confidence limit of the final result was seen to increase using standard addition (both using solid sampling ETV- readily to >10%.This is to be attributed to a certain level of ICP-MS). The deviation between these solid sampling results inhomogeneity of the analyte element in the CRM under and the pneumatic nebulization ICP-MS result is somewhat investigation. This inhomogeneity is of course not corrected larger (#12%), although no significant difference could be for using ID and is the determining factor for the precision of established. the results obtained. Hence, the level of precision attainable is comparable to that obtained when using external calibration or standard addition as a means of calibration.27 For Aquatic CONCLUSIONS Plant on the other hand, the RSD obtained for the 110Cd5113Cd The results obtained for the determination of Cd or Se in ratio was significantly higher (Table 4) and there was a 5% CRMs of different origin indicate that ID allows accurate difference between the mass discrimination factors for the analyses to be carried out (the mean deviation between the sample and the tracer.Nevertheless, the result obtained for solid sampling ETV-ICP-MS results and corresponding certi- the Cd content in Aquatic Plant is in excellent agreement with fied values is<10%). In addition, this calibration strategy can the certified value. For Light Sandy Soil, however, no accurate be applied for all elements that show (at least) two interference- result could be obtained. This is probably to be attributed to free isotopes. The precision attainable is determined by the a spectral interference at m/z 110 since the experimentally sample homogeneity and is hence comparable to that obtained observed 110Cd5113Cd ratio (Table 3) exceeds the theoretical using other calibration techniques, such as external calibration value (which is in contradiction with a mass bias in favour of using either a solid standard or an aqueous standard solution the higher mass ion, generally observed in ICP-MS), while the or standard addition. The most important advantages of ID mass discrimination factor for the sample exceeds that for the over the forementioned calibration techniques for use with tracer by more than 10% (Table 4).solid sampling ETV-ICP-MS are: (i) the use of an elemental internal standard is no longer required and (ii) the possibility Determination of Se in CRMs of erroneous (biased) results is reduced as a result of the systematic check on the presence of spectral interferences.For the determination of Se in Wheat Flour (NIST SRM Since the availability of tracers is continuously increasing and 1567A), both the low RSD for 82Se577Se (Table 3) and the the price of these materials correspondingly decreasing, ID can excellent agreement between the mass discrimination factors be considered to be a viable alternative to other calibration for the sample and the tracer (Table 4), indicated the absence techniques, and its use could lead to an extension of the of important spectral interferences on the isotopes involved.application range of solid sampling ETV-ICP-MS. Hence, the result obtained compares favourably with the certified value (Table 5). Although for Estuarine Sediment F. V. is a Senior Research Assistant of the Belgian National (BCR CRM 277) an excellent RSD for the 82Se577Se ratio Fund for Scientific Research. could be obtained (Table 3), and only a 3% difference between the mass discrimination factor for the sample and that for the tracer (Table 4) could be established, the deviation between the solid sampling ETV-ICP-MS result and the certified value Table 6 Results (s) in mg g-1 for the determination of Cd in tobacco.is larger than for Wheat Flour (Table 5). This could possibly be attributed to a 40Ar37Cl+ interference on the 77Se+ signal, Solid sampling ETV-ICP-MS PN-ICP-MS explaining the lower Ks factor found (Table 4). Finally, for Sea ID, 1.39 (0.32)* External calibration, 1.23 (0.03)† Lettuce, several factors indicated the occurrence of a spectral Standard addition, 1.36 (0.19)* interference on 82Se. (i) The experimentally determined 82Se577Se ratio (uncorrected for mass discrimination effects) *n=5.†n=3. exceeds the value expected on the basis of the natural isotopic Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 12922 Marshall, J., Franks, J., Abell, I., and Tye, C., J. Anal. At. REFERENCES Spectrom., 1991, 6, 145. 1 Gray, A.L., Analyst, 1985, 110, 551. 23 Voellkopf, U., Paul, M., and Denoyer, E. R., Fresenius’ J. Anal. 2 Arrowsmith, P., Anal. Chem., 1987, 59, 1437. Chem., 1992, 342, 917. 3 Denoyer, E. R., Fredeen, K. J., and Hager, J. W., Anal. Chem., 24 Gre�goire, D. C., Miller-Ihli, N. J., and Sturgeon, R. E., J. Anal. 1991, 63, 445A. At. Spectrom., 1994, 9, 605. 4 van de Weijer, P., Baeten, W. L. M., Bekkers, M. H. J., and 25 Wang, J., Carey, J. M., and Caruso, J. A., Spectrochim. Acta, Part Vullings, P.J. M. G., J. Anal. At. Spectrom., 1992, 7, 599. B, 1994, 49, 193. 5 Paul, M., At. Spectrosc., 1994, 15, 21. 26 Argentine, M. D., and Barnes, R. M., J. 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Acta, Part B, 1989, 44, 263. 1990, 11, 205. 32 Vanhaecke, F., Vanhoe, H., Dams, R., and Vandecasteele, C., 13 Imai, N., Anal. Sci., 1990, 6, 389. T alanta, 1992, 39, 737. 14 Imai, N., Anal. Chim. Acta, 1990, 235, 381. 33 Vanhaecke, F., Galba�cs, G., Boonen, S., Moens, L., and Dams, R., 15 Perkins, W. T., Fuge, R., and Pearce, N. J. G., J. Anal. At. J. Anal. At. Spectrom., 1995, 10, 1047. Spectrom., 1991, 6, 445. 34 McLaren, J. W., Beauchemin, D., and Berman, S. S., Anal. Chem., 16 Perkins, W. T., Pearce, N. J. G., and Jeffries, T. E., Geochim. 1987, 59, 610. Cosmochim. Acta, 1993, 57, 475. 17 Yasuhara, H., Okano, T., and Matsumura, Y., Analyst, 1992, 35 Verrept, P., Dams, R., and Kurfu�rst, U., Fresenius’ J. Anal. Chem., 117, 395. 1993, 346, 1035. 18 Kogan, V. V., Hinds, M. W., and Ramendik, G. I., Spectrochim. 36 Denoyer, E. R., At. Spectrosc., 1994, 15, 7. Acta, Part B, 1994, 49, 333. 37 Longerich, H. P., At. Spectrosc., 1989, 10, 112. 19 Watling, R. J., Herbert, H. K., Delev, D., and Abell, I. D., Spectrochim. Acta, Part B, 1994, 49, 205. Paper 6/04133G 20 Imbert, J. L., and Telouk, P., Mikrochim. Acta, 1993, 110, 151. Received June 12, 1996 21 Moenke-Blankenburg, L., Schumann, T., Gu�nther, D., Kuss, H.-M., and Paul, M., J. Anal. At. Spectrom., 1992, 7, 251. Accepted August 7, 1996 130 Journal of Analytical Atomic Spectrometry, February 1997,

ISSN:0267-9477    

DOI:10.1039/a604133g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Direct Determination of Volatile Elements in Nickel Alloys by Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry MICHAEL W. HINDS*a, D. CONRAD GRE� GOIREb AND ELISA A. OZAKIc aRoyal CanadianMint, 320 Sussex Drive, Ottawa, Ontario, Canada K1A 0G8 bGeological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8 cV illares Metals SA, Rod. Anhangvera Km 113, Nova Veneza, Sumare�, CEP 13177–900, SP, Brazil A method is described for the direct determination of Bi, Pb owing to the co-vaporization of analyte with matrix components.This concept was studied in greater detail by Vanhaecke and Te in solid Ni alloys by ETV-ICP-MS. Samples are introduced into the graphite tube as small filings or chips et al.9 who used the argon dimer as both a tool for detecting matrix interferences and for use as an internal standard for weighing up to 3 mg. Using diluted sea water as a physical carrier, both Bi and Pb could be determined in solid Ni using direct solids analysis by ETV-ICP-MS. Fonesca and Miller- Ihli10 reported on the use of Pd as a physical carrier and external calibration with aqueous samples although results for Pb were biased low.Better results in terms of accuracy and chemical modifier and oxygen ashing for the analysis of biological RMs by slurry sampling ETV-ICP-MS. Vanhaecke precision were obtained when solid RMs (Ni) were used for calibration. LODs of 14 and 44 ng g-1 were obtained for Bi et al.11 used solid sampling ETV-ICP-MS for the direct determination of As in RMs of plant origin.A detection limit for and Pb, respectively, using a reduced sensitivity mode (OmniRange). Based on signals obtained for solution standards As of 1 ng g-1 was reported. Ren et al.12 reported on the analysis for Cd in solid samples compressed into graphite measured at the highest sensitivity, LODs of 0.002 and 0.004 ng g-1 are possible for Bi and Pb, respectively. The pellets. The graphite pellet was mounted in a custom-made ETV device and heated electrothermally to produce a signal.determination of Te by this technique was not successful using either solution or solids calibration. Tellurium did not show a The requirements for successful direct solids analysis were outlined by Moens et al.13 along with a review of the current linear instrument response with concentration, which was probably due to an interaction between the Te and one or more literature on the subject.A second review on solid sampling using electrothermal devices interfaced with ICP atomic emis- matrix components in the solid phase that alters the release mechanism(s) for Te from those observed for Pb and Bi. sion and mass spectrometers was recently published by Darke and Tyson.14 Keywords: Electrothermal vaporization; inductively coupled The objective of the present study was to extend work plasma mass spectrometry ; solid sample; nickel alloys; volatile completed on the direct analysis of solid Ni using ETAAS to elements ETV-ICP-MS and to demonstrate the applicability of the technique for the analysis of metals for volatile trace elements.Trace levels (mg g-1) of Bi, Pb, Te and other volatile elements are known to alter the mechanical properties of steels and EXPERIMENTAL nickel alloys.1 Rapid and accurate determinations of these Instrumentation elements during the melting process are required to maintain both material quality and the mechanical properties of the A Perkin-Elmer SCIEX ELAN 5000 ICP mass spectrometer alloy.Sample dissolution is slow due to the corrosion resistant equipped with an HGA-600MS electrothermal vaporizer was properties of these alloys. The determination of trace elements used. The electrothermal vaporizer system was equipped with in Ni alloys by direct weighing solid sample ETAAS has been aModel AS-60 autosampler. Pyrolytic graphite coated graphite shown to be effective using calibration with solid RMs2,3 and tubes were used throughout.The experimental conditions for with aqueous standards.4,5 More recently, Te and Sb were the ELAN 5000 and the HGA-600MS are given in Table 1. determined in Ni alloys by solid sample LEAFS in a graphite Optimization of plasma and mass spectrometer conditions was furnace.6 It was considered that ETV-ICP-MS would also be accomplished using solution nebulization sample introduction applicable to the solid sampling problem.and aqueous standards (High Purity Standards, Charleston, Sample introduction using ETV is well suited to the direct analysis of solid samples. Along with providing femtogram levels of detection, ETV-ICP-MS allows for in-tube sample Table 1 Instrumental operating conditions and data acquisition pre-treatment with possible elimination of interfering species parameters and the use of chemical modifiers to vaporize either analyte ICP mass spectrometer— or matrix components selectively. In this way, both spectral Rf power/W 1000 and non-spectral interferences can be avoided.Outer argon flow rate/l min-1 15.0 Solid sampling for analysis using ETV-ICP-MS has been Intermediate argon flow rate/ml min-1 850 reported by Voellkopf et al.7 who used a ‘cup-in-tube’ technique Carrier argon flow rate/ml min-1 900 and slurry sampling for the analysis of coal. Gre�goire et al.8 Sampler/skimmer Nickel used slurry sampling for the direct analysis of biological Data acquisition— materials and coal.LODs ranged from 0.07 ng g-1 for Co to Dwell time (ETV) 20 ms 3.2 ng g-1 for Cr using 2 mg samples. Gre�goire et al.8 used the Scan mode Peak hopping argon dimer at m/z=80 as a tool to monitor the analyte signal Points per spectral peak 1 for possible interference effects such as signal suppression Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 (131–135) 131NC, USA). The HGA-600MS was interfaced to the argon valve switching.No pyrolysis step was required. The optimized atomization temperature was chosen to volatilize the analytes plasma via an 80 cm length of 6 mm (id) PTFE tube. The operation of the HGA-600MS was completely computer con- but also to vaporize a minimum amount of the metal matrix, which avoided saturating the detector and causing analyte trolled. During the dry and pyrolysis stages of the temperature program, opposing flows of argon gas (300 ml min-1) originat- signal suppression.This temperature (1300 °C) is close to the melting-point (1453 °C) of Ni metal. ing from both ends of the graphite tube removed water and other vapours through the dosing hole of the graphite tube. Prior to and during the high temperature or vaporization step, Reference Materials and Solutions a graphite probe was pneumatically activated to seal the dosing hole. Once the graphite tube was sealed, a valve located at Samples were Ni-based high-temperature alloy RMs: one end of the HGA-600MS workhead directed the carrier Tracealloys A and B, SRMs 897 and 898 (NIST, Gaithersburg, argon flow originating from the far end of the graphite tube MD, USA) and BCS CRMs 345 and 346 (Bureau of Analysed directly to the argon plasma at a flow rate of 900 ml min-1.Samples, Middlesbrough, Cleveland, UK). Metal RMs came in the form of fine turnings which did not require any further treatment. Samples (0.3–1.5 mg) were weighed on a Model Electrothermal Vaporizer Temperature Program UM3 balance (Mettler Instruments, Greifensee, Switzerland).Once each sample had been weighed, it was wrapped in a The optimized temperature program used for the electrothermal vaporizing unit (HGA-600MS) is shown in Table 2. A small piece of pre-cut white paper and labelled with the RM number and the mass in milligrams. The calibration of the clean-out step begins the program to remove any residual analytes present in the graphite furnace.For conventional balance was verified with a 1 mg weight traceable to the Canadian prototype of the kilogram, K74 (Institute of National temperature programs used in ETAAS, a clean-out step often immediately follows the atomization step. The current con- Measurement Standards, National Research Council of Canada, Ottawa, Ontario, Canada). figuration of the HGA-600MS ETV-ICP-MS system does not allow the sealing probe to be raised and for vapours to be ion solutions were prepared by serial dilution from 1000 mg ml-1 stock solutions (High Purity Standards) with vented through the dosing hole, once the high-temperature vaporization step is engaged.A cleaning step placed at the end 1% HNO3 (Seastar Chemical, Sidney, British Columbia, Canada). Distilled, de-ionized water (Millipore, Bedford, MA, of the heating cycle would unnecessarily contaminate the valve system, transport tube, plasma torch and interface with large USA) was used throughout. amounts of Ni.The second temperature program step allows adequate time RESULTS AND DISCUSSION for manually placing the small solid sample into the graphite tube. For the analysis of liquid samples, an autosampler Vaporization of Analyte injection volume of 20 ml plus 10 ml of NASS-3 (diluted A typical transient signal is shown for all three analytes and 500-fold) Open Ocean Seawater (National Research Council the Ni matrix in Fig. 1. The peak shapes for each analyte of Canada, Ottawa, Ontario, Canada) was used.The addition returned to the baseline well before the end of the heating of diluted sea water provides for a physical carrier used cycle. This observation is consistent with complete analyte primarily to equalize the transport of analyte be it from vaporization from the solid sample, although very little of the aqueous standards or solid samples.15 For solid metal samples, matrix was vaporized. It was also observed that for Pb and Bi a disposable glass pasteur pipette (which was cut to an outside the determined concentrations were in agreement with RM diameter of 2 mm, 5 mm from the tip) was placed into the concentrations.This could not have occurred unless complete dosing hole of the graphite furnace to act as a funnel. A pre- analyte vaporization was achieved. Higher vaporization tem- weighed sample was picked up with curved nosed forceps and peratures were tried but too much Ni matrix was vaporized dropped into the funnel.The funnel was then removed. This and could potentially alter the sampling orifice to the mass was followed by the addition of a 10 ml aliquot of NASS-3 sea spectrometer. Solid Ni samples remained in the graphite tube water (diluted 500-fold) as for the solution samples. The time virtuallyunchangedafterseveral high-temperature heating cycles. required to complete this step may be reduced from that No analyte signal was observed when the remaining Ni sample suggested (Table 1) as one becomes more skilled in handling was re-heated to 1300°C following the initial (measurement) the funnel and solid samples.The argon gas flow was stopped during this step to prevent the metal chips from being blown back out of the vaporizer. Two drying steps were used: one to dry the samples and a second to prevent pressure build-up and to allow the argon plasma to stabilize prior to the high-temperature measurement step. During the second drying step, the dosing hole of the graphite tube is sealed and the switching valve is activated.This occurs 5 s prior to the measurement step. The additional 5 s at zero internal flow prevents a build-up of pressure during Table 2 ETV temperature program Temperature/ Gas flow/ Step °C Ramp/s Hold/s ml min-1 Flow Clean out 2700 1 3 300 Vent Sample in* 20 1 90 0 Vent Dry I 120 120 45 300 Vent Dry II 120 1 10 900 ICP Vaporize 1300 0 8 900 ICP/read Cool 20 1 15 900 ICP Fig. 1 ETV-ICP-MS peak profiles for Ni, Bi, Pb and Te from a solid sample of SRM 898 Nickel Alloy. * Sample+10 ml NASS-3 (diluted 500-fold). 132 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Fig. 3 Ni ETV-ICP-MS background signals for: A, maximum power heating to 1300°C; B, 1 s ramp heating to 1300°C. Line C is the 13C signal recorded during maximum power heating to 1300 °C, which correlated with the surface temperature of the graphite (ETV). Fig. 2 ETV-ICP-MS peak profiles for Bi, Pb and Te from a solid sample of Nickel Alloy BCS 346. heating cycle.It was visually observed that the metal sample changed from an irregular shape to a smoother and flatter shape indicative of melting (without the clean-out step at 2700 °C). This observation is consistent with those reported in the literature,16 where it was found that Si was completely removed from a solid Au sample without vaporization of the metal sample. A follow-up study by Hinds et al.17 showed that the movement of the analyte from the bulk to the surface could not occur owing to diffusion but rather through convective currents within the sample bulk moving to the surface, where the analyte is vaporized.These currents in the molten Fig. 4 Ni ETV-ICP-MS signals obtained during A, the first sample sample are believed to be induced by surface tension inhomo- vaporization (new vaporizer) and B, subsequent sample vaporization. geneities which occur as a result of a continuous increase in the temperature during the early stages of analyte vaporization.re-condensed metal from within the graphite tubes or contact Multiple analyte peaks, as shown in Fig. 2, were commonly cones rather than from a fresh sample. The Ni signal obtained observed and could be indicative of the presence of different using a new graphite tube containing a fresh Ni sample is physical forms of analyte species, as has been suggested from shown in Fig. 4 (curve A). No large transient signal was ETAAS solid sampling studies.Multiple peaks did not occur observed. However, a second temperature cycle using the same for every sample, however, double peaks were observed for sample (no other sample added) showed a Ni signal (curve B) each analyte studied. Impurity elements (such as Pb, Bi, and similar to that obtained in Fig. 3 (curve A). The Ni contributing Te) in Ni and Ni alloys could be located on the outside surface to this signal originated from metal which had vaporized, of the sample particle, in the grain boundaries within a metal re-distributed and condensed throughout the graphite tube sample and within the bulk of the metal.There is, however, and contact cones during the clean-out step beginning the insufficient information concerning the forms of analyte species second cycle. The first cycle also began with a clean-out cycle that are present in the complex alloys used in this investigation but without solid Ni sample present.with which to correlate the observed multiple peaks. During the high-temperature vaporization step, the amount of Ni vaporized was insufficient to result in analyte signal suppression. No change in the argon dimer signal at m/z=80 Vaporization of Matrix was observed during this study indicating that analyte signal It is of interest to note that the Ni signal, along with those of was unaffected by the vaporization of Ni.8,9 the trace analytes, also returned to the baseline during the vaporization step.The Ni signal generated using two heating ramp rates is shown in Fig. 3. The 1 s (curve B) ramp rate Analytical Results (time to a pre-set maximum temperature) produced little signal Aqueous solution calibration whereas a substantial Ni peak (curve A) resulted from maximum power heating (‘0’ s ramp). It is probable that the A comparison of analyte concentrations determined in solid samples by ETV-ICP-MS with calibration by aqueous solu- transient nature of the Ni signal is due to the temperature over-ramp (heating above the maximum pre-set temperature) tions is given in Table 3.The determination of Pb by this calibration scheme gives concentration values that are close to which occurs in maximum power heating. An isotope of carbon, 13C, was recorded during a maximum power ramp the reference concentration values for BCS 346 and for NIST SRM 897, but are systematically biased low. Samples with low and is also shown in Fig. 3 (curve C). A correlation between the 13C signal and the graphite surface temperature has been concentrations of Pb such as BCS 345 and NIST SRM 898 gave Pb concentrations that are about half of the reported shown by Gilmutdinov et al.18 The 13C signal followed a similar transient pattern as observed for Ni, which confirms values with aqueous calibration. This could occur because of differences in the mechanism of vaporization of the Pb in the that analyte and matrix vaporization occurs as a result of the temperature over-shoot.Ni sample and the Pb in the aqueous sample when vaporized at 1300 °C. Complete vaporization–atomization has been The observed Ni signal probably originates from Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 133Table 3 Determined concentration of analytes (mg g-1) in Ni and steel RMs by different calibration schemes Calibration method Reference Sample Aqueous Solid sample* value Pb— BCS 346 16.1±1.9 19.3±3.7 21±2 BCS 345 0.11±0.02 0.14±0.03 0.2 NIST SRM 897 9.29±1.19 11.7±1.5 11.7±0.8 NIST SRM 898 1.02±0.22 2.2±0.4 2.5±0.6 Bi— BCS 346 9.47±1.07 9.87±1.03 10±1 BCS 345 0.0083±0.0028 0.0075±0.0022 <0.2 NIST SRM 897 0.53±0.08 0.56±0.08 <0.5 NIST SRM 898 1.02±0.22 1.07±0.23 1 T e— BCS 346 0.50±0.23 38±10 12±1 NIST SRM 897 0.20±0.03 8.6±1.3 1.05±0.07 NIST SRM 898 0.09±0.01 4.6±1.4 0.54±0.02 * Signals from the four highest mass BCS 346 samples were used as solid sample calibration standards.observed from the same RMs by ETAAS5 at higher vaporization –atomization temperatures. Unfortunately, heating at Fig. 5 Analyte mass versus peak area response from solid Ni alloy these temperatures would also vaporize large amounts of the samples by ETV-ICP-MS (a) x Pb and & Bi; (b) 2 Te. Ni alloy, which would have ultimately caused analyte signal suppression during ICP-MS measurements. Despite the slightly possible that there is an interaction between the Te and one low bias of the results, solution calibration is convenient and or more matrix components in the solid phase, which alters could best be used as a rapid screening method where one the release mechanism(s) for Te from those observed for Pb seeks to identify samples that have analyte levels above or and Bi.This variability in the release mechanism for Te from below certain concentration levels. More accurate determi- different alloys makes it impossible for the method of standard nations, if required, can be completed on only those samples additions to be used because there is a considerable difference flagged by rapid screening.in vaporization for Te from aqueous standards as well as from Results for Bi showed that the reference values are included different alloys. It may be possible to use an internal standard within the data spread from the determinations of each RM but this must have the same release mechanisms and also be examined.This verifies that calibration with solution standards present in known concentrations which would be extremely is an accurate method for determining Bi from solid Ni and difficult to find in a solid sample. steel alloy pieces. As shown in Table 3 the Pb and Bi concentration determined It should be noted that accurate results using aqueous for each RM overlapped with the corresponding reference standards were only obtained after the graphite tube had been value.In general, this was as expected. However, as noted treated with a solid Ni sample. It is probable that the residual previously, the calibration was carried out with solid samples amount of Ni remaining in the graphite tube acts as a physical of one RM (BCS 346), which has a different composition from carrier along with added diluted sea water. the other NIST SRMs used in this study (Table 4). This Tellurium in solid Ni alloys (discussed below) did not show suggests that for all the metal alloys examined in the present a linear instrument response to analyte mass, which indicated study the release of Pb and Bi is virtually complete after one that Te was a poor candidate element for this analytical vaporization cycle.This is supported by the observation that method. The results displayed in Table 3 confirm these the analytical signal for each analyte returns to the baseline observations. Neither aqueous calibration nor solid sample during vaporization (Fig. 2) and no significant analyte signal calibration resulted in Te concentration values that matched was observed during a second vaporization cycle. the RM values. Table 4 Elemental composition (%) of the RMs used in the study Solid sample calibration Element BCS 345/346 NIST SRM 897/898 Plots of the integrated signal versus the analyte mass in a solid sample (sample mass multiplied by the certified concentration) C 0.15 0.12 Si — — are shown for Bi and Pb [Fig. 5(a)] and Te [Fig 5(b)].Different Mn — — RMs were used to cover a wide analyte mass range. To a first Cr 10 12 approximation, a direct correlation is shown for both Bi and Mo 3 — Pb, which indicates that the analyte mass can be directly Al 5.5 2 related to instrument response over two orders of magnitude. Co 15 8.5 This may also be an indication that the processes involved in Ti 5 2 V 1 — analyte release from the solid and in analyte vaporization are W — 1.7 similar for the different metal alloy RMs used in this experi- Nb — 0.9 ment.However, Te did not show this correlation [Fig. 5(b)]. Ta — 1.7 It is thus unlikely that Te can be determined in this case by Hf — 1.2 direct solid sampling because a linear correlation is not Ni 60.35 69.88 observed for the analyte response and analyte mass. It is 134 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Table 5 Analytical figures of merit (n=10, k=3) to the determination of volatile elements in other samples heated, as for Ni, almost to the melting-point of the sample.Reduced High Alternatively, changing the physical form of the sample to a Analyte LOD sensitivity* sensitivity more highly divided state may make the low-temperature Bismuth Absolute 13 pg 0.002 pg removal of a volatile analyte possible. Research in these areas Relative† 13 ng g-1 0.002 ng g-1 of study are currently underway in these laboratories. Lead Absolute 44 pg 0.004 pg The authors are grateful to S.Gilbert for technical assistance Relative† 44 ng g-1 0.004 ng g-1 during the course of this work. GSC publication No. 1996134. * OmniRange settings were used to attenuate analyte and sample signal to avoid detector saturation for most samples determined. REFERENCES † Based on a 1 mg solid sample. 1 ASM International Committee, Metals Handbook, ASM International, Materials Park, OH, 10th edn., 1990, vol. 1, p. 950. L imits of detection 2 Marks, J. Y.,Welcher, G. G., and Spellman, R.J., Appl. Spectrosc., 1977, 31, 9. The LODs are summarized in Table 5 for Pb and Bi. These 3 Ba� ckman, S., and Karlsson, R. W., Analyst, 1979, 104, 1017. values were estimated from ten replicate determinations of a 4 Headridge, J. B., and Riddington, I. M., Mikrochim. Acta, 1982, single Ni alloy sample (after an initial ETV firing that removed 11, 457. analytes from the sample). Two sensitivity settings were used 5 Irwin, R. L., Mikkelsen, A., Michel, R.G., Dougherty, J. P., and Preli, F. R., Spectrochim. Acta, Part B, 1990, 45, 903. for these estimates. Under normal operation with the Ni alloys 6 Liang, Z., Lonardo, R. F., and Michel, R. G., Spectrochim. Acta, used in this study, the sensitivity was attenuated using the Part B, 1993, 48, 7. OmniRange facility on the mass spectrometer system in order 7 Voellkopf, U., Paul, M., and Denoyer, E. R., Fresenius’ J. Anal. to bring the signals on scale. This is achieved automatically Chem., 1992, 342, 917.by changing an applied voltage on the mass spectrometer 8 Gre�goire, D. C., Miller-Ihli, N. J., and Sturgeon, R. E., J. Anal. reducing ion throughput at any selected m/z. Relatively low At. Spectrom., 1994, 9, 605. 9 Vanhaecke, F., Galba�cs, G., Boonen, S., Moens, L., and Dams, R., LODs were observed even with the detector attenuation. At J. Anal. At. Spectrom., 1995, 10, 1047. maximum sensitivity, the LOD drops by a factor of 6000. This 10 Fonseca, R.W., and Miller-Ihli, N. J., Appl. Spectrosc., 1995, indicates that very low levels of Pb and Bi can be determined 49, 1403. in individual samples. However, the homog of these trace 11 Vanhaecke, F., Boonen, S., Moens, L. and Dams, R., J. Anal. At. elements in the bulk material at very low concentrations is not Spectrom., 1995, 10, 81. known but can perhaps be estimated through solid sampling 12 Ren, J. M., Rattray, R., Salin, E. D., and Gre�goire, D. C., J. Anal. At. Spectrom., 1995, 10, 1027. ETV-ICP-MS. 13 Moens, L., Verrept, P., Boonen, S., Vanhaecke, F., and Dams, R., Spectrochim. Acta, Part B, 1995, 50, 463. 14 Darke, S. A., and Tyson, J., Microchem. J., 1994, 50, 310. CONCLUSIONS 15 Hughes, D. M., Chakrabarti, C. L., Goltz, D. M., Gre�goire, D. C., ETV-ICP-MS is applicable to direct solid sample determi- Sturgeon, R. E., and Byrne, J. P., Spectrochim. Acta, Part B, 1995, nation of Pb and Bi in Ni alloys. Calibration with RMs of 50, 425. 16 Hinds, M. W., and Kogan, V. V., J. Anal. At. Spectrom., 1994, similar alloy composition gives the most accurate results for 9, 451. Pb. Either aqueous solution standards or solid RMs result in 17 Hinds, M. W., Brown, G. N., and Styris, D. L., J. Anal. At. accurate determinations for Bi in the Ni alloys studied. Spectrom., 1994, 9, 1411. Tellurium was not successfully determined probably because 18 Gilmutdinov, A. Kh., Staroverov, A. E., Gre�goire, D. C., of variable interactions between the analyte and the matrix, Sturgeon, R. E., and Chakrabarti, C. L., Spectrochim. Acta, which prevent complete analyte vaporization. The very low Part B, 1994, 49, 1007. LODs observed indicate that this technique can be used for the determination of volatile elements in solid samples either Paper 6/03313J representative of the bulk material or in micro-samples. ReceivedMay 13, 1996 It is possible that the proposed technique can be extended Accepted August 27, 1996 Journal of Analytical Atomic Spectrometry, February 1997, Vol.

ISSN:0267-9477    

DOI:10.1039/a603313j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Impurities in Boron Nitride Powder by Slurry Sampling Electrothermal Atomic Absorption Spectrometry VILIAM KRIVAN* AND THOMAS RO » MMELT Sektion Analytik und Ho » chstreinigung, Universita » t Ulm, D-89069 Ulm, Germany A slurry sampling ETAAS method for the determination of Al, for a modiÆer beaker (autosampler location 0). Furthermore, the original PTFE sampling capillary of the autosampler was Ca, Cd, Co, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Si and Zn in powdered boron nitride is described.The main aspects in the replaced by one with a larger internal diameter (0.8 mm) to avoid Æltration effects. Pyrolytic graphite coated graphite development of the method included preparation of slurry samples, pyrolysis and atomization parameters, minimization ringed tubes with fork-shaped platforms (Part No. BO 505057) and pyrolytic graphite coated graphite tubes (Part No. of the matrix effect by chemical modiÆcation and calibration. The accuracy was checked by comparison of the results with BO 091504) were used for platform and wall atomization, respectively.Suspensions were pre-treated in a Sonorex RK those obtained by analysis of sample digests by AAS and ICPAES. The LODs achieved were of the order of 2 (Cd) and 400 255 H (Bandelin Electronic, Berlin, Germany) ultrasonic bath. For the determination of Ca, Mg and Na by the Øame (Si) ng g-1. technique, a Perkin-Elmer Type 400 atomic absorption spec- Keywords: Electrothermal atomic absorption spectrometry; trometer was used.boron nitride matrix; slurry sampling; trace analysis A Jobin-Yvon JY-24 spectrometer (Longjumeau, France) extended with a JY-50P polychromator (15 elements) was used Boron nitride, mainly because of its outstanding thermal and for sequential (elements Na and K) and simultaneous (all other electrical properties, has become an important material in elements) measurements. The JY-50P allows wavelength scan- various technological Æelds.1±3 In many instances, the relevant ning by moving the entrance slit of the polychromator. Control properties are inØuenced by trace impurities.Therefore, there of the spectrometer and data collection were performed using is a need for adequate analytical methods for the trace charac- a computer system and the ISA/Jobin-Yvon software package. terization of this material. Scanning electron micrographs of the boron nitride powders ETAAS is one of the most powerful routine methods for the were obtained with a Zeiss DSM 962 microscope (Zeiss, determination of trace impurities in a large variety of materials.Oberkochen, Germany) with magniÆcation ranging from 100 For analysis by solution ETAAS, boron nitride can be decom- to 10000 times at an accelerating voltage of 20 kV. posed by fusion with alkali salts.4,5 However, the application of these procedures to boron nitride samples of higher purity Sample and Reagents grades is seriously limited by the blank, and often the introduction of high salt concentrations can become a problem for the The boron nitride powder examined (Elektroschmelzwerke, analysis.Digestion procedures using acid mixtures containing Kempten, Germany) was a commercially available material. It hydroØuoric acid in an autoclave have proved to be more was a Æne powder with typical particle size in the range useful for this purpose.6±8 Nevertheless, these methods are time 0.2±0.7 mm, with the largest particle size not exceeding 1.5 mm.consuming and also limited by the blank values. The particle size was estimated by SEM. For the estimation Slurry sampling ETAAS has proved to be a very advanta- of the LODs of some elements, a sample was prepared by geous method, especially for the analysis of high-purity mate- grinding a high-purity boron nitride sheet obtained from rials that are available in powdered form, as it requires no Ringsdorffwerke (Bonn, Germany). The resulting powder con- sample decomposition and only little sample pre-treatment sisted of particles with a maximum thickness of 5 mm and and therefore the contamination risk is essentially lower.The length of between 10 and 100 mm. method has already been successfully applied to the analysis All reagents and standard solutions used were obtained from of silicon based ceramics,9,10 zirconium dioxide,11 graphite12 Merck (Darmstadt, Germany). Laboratory-reagent grade etha- and molybdenum oxide13 and to the determination of silicon nol and nitric acid of pro analysi quality were puriÆed by sub- in boron nitride,14 titanium dioxide and zirconium dioxide.15 boiling distillation. HydroØuoric acid as well as magnesium In the present paper, slurry sampling ETAAS was applied nitrate and calcium nitrate, used as chemical modiÆers, were to the determination of 14 trace impurities in powdered boron of Selectipur quality.Doubly distilled water was used for the nitride.Solution ETAAS and ICP-AES were used as indepen- preparation of slurries, standards and chemical modiÆer dent reference methods. solutions. EXPERIMENTAL Procedure for Slurry Sampling ETAAS Apparatus A volume of 15 ml of a mixture of sub-boiled ethanol and doubly distilled water (1+1) containing the appropriate All absorption measurements using the ETAAS technique were carried out with a Perkin-Elmer Zeeman 5000 atomic absorp- amount of the chemical modiÆer (see Table 1) was prepared in a cleaned polystyrene beaker and stirred using a PTFE-covered tion spectrometer equipped with an HGA-500 graphite furnace, an AS-40 autosampler and a 3600 Data Station.This system magnetic bar. Aliquots of 20 ml were taken by the autosampler to check the blank value. For the preparation of sample was adapted to slurry sampling introduction by Æxing a remote controlled rotating magnet over the position commonly used slurries, 50±350 mg of the boron nitride powder were added Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 (137±141) 137Table 1 Instrumental parameters and experimental conditions for the analysis of boron nitride by slurry sampling ETAAS Gas Øow*/ Element Wavelength*/nm Slit/nm Pyrolysis/°C Atomization/°C ml min-1 ModiÆer� Tube type� Al 256.8 (309.3) 0.7 1700 2500 200 (300) A I Ca 239.9 (422.7) 0.7 900 2700 0 (300) A II Cd 228.8 0.7 900 2200 0 A I Co 242.5 0.2 1400 2500 0 A I Cu 324.7 0.7 1300 2500 0 A I Fe 344.1 (248.3) 0.2 1400 2500 300 (0) A I K 769.9 (766.5) 1.4 900 2300 300 A II Mg 202.5 (285.2) 0.7 900 2200 100 (300) – – Mn 279.5 0.2 1400 2200 300 (0) A I Na 330.3 (589.0) 2.0 (1.4) 900 2200 0 (300) A I Ni 232.0 0.7 1400 2700 0 A I Pb 283.3 0.7 850 2200 0 A I Si 251.6 0.2 800 2700 300 (0) A, B I Zn 213.9 0.7 700 2200 300 C I * Values in parentheses were used only for estimation of LODs.� Chemical modiÆers (per atomization): A, 50 mg of Mg(NO3)2; B, 50 mg of Ca(NO3)2 ; and C, 25 mg of Mg(NO3)2.� Tube type: I, pyrolytic graphite coated ringed tube with fork-shaped platform; and II, pyrolytic graphite coated tube. to this solution. The resulting slurry was pre-treated in the the slurries. Therefore, an alternative medium was required which would effectively reduce the surface tension, could be ultrasonic bath for 15 min in order to break-up particle agglomerates. Under continuous stirring with a PTFE-coated easily puriÆed and would be miscible with water (which is important for calibration by standard additions using aqueous magnetic bar, 20 ml aliquots of the homogenized slurries were dispensed automatically into the tube by the autosampler from standard solutions).A mixture of equal volumes of sub-boiled ethanol and doubly distilled water proved to fulÆl these position 0, which is usually assigned for the vessel containing the modiÆer solution. The optimized instrumental parameters requirements well. In spite of the relatively low boiling-point of ethanol, losses in mass during 2 h of continuous stirring in and other experimental conditions used are summarized in Table 1.Integrated absorbance evaluation was used for all a closed beaker with a hole in the lid did not exceed 1.5%. However, because of ted surface tension, the maximum measurements. QuantiÆcation was carried out using the standard additions method by spiking the slurries with aqueous dispensible volume into the cavity of the platform was limited to 20 ml.standard solutions. In the slurry sampling technique, sampling errors can arise from variations in the number of particles, and the mass of Procedure for Solution AAS and ICP-AES the individual particles, uncertainties in the volume pipetted For the analysis of the boron nitride powder by solution AAS and an inhomogeneous distribution of the analyte element and ICP-AES, 1.5 g of sample were digested in 150 ml PTFE throughout the solid sample.The characteristics required for liners (Berghof System III, Berghof, Eningen, Germany) with minimization of the sampling errors include small particle size, a mixture of 10 ml of sub-boiled nitric acid and 10 ml of 40% narrow particle size distribution and high slurry concentration. hydroØuoric acid for 4 h at 200°C. The resulting digest con- However, for the boron nitride powder under investigation, tained a crystalline fraction of ammonium boroØuoride.After the applicable slurry concentration was limited by the stability decanting the liquid phase into a 50 ml calibrated Øask, the of the slurry, analyte concentration in the sample and reduced crystalline solid was dissolved in water by standing in the lifetime of the graphite tube. Using the ultrasonic pre- ultrasonic bath for 1 h, the solution was transferred into the treatment, the large particle agglomerates disintegrated, how- calibrated Øask and then diluted with doubly distilled water ever, permanent stirring was necessary to avoid coagulation.to 50 ml. This solution was used for the determinations by The linear working range of the slurry concentrations ranges FAAS, ETAAS and ICP-AES. from about 1 to 15 g l-1. With higher concentrations, no The wavelength, slit and Øame conditions, respectively, for sufficiently stable slurries could be obtained by magnetic the determination of Ca, Mg and Na by FAAS were as follows: stirring whereas lower slurry concentrations led to low Ca, 422.7 nm, 0.7 nm, C2H2±N2O; Mg, 285.2 nm, 0.7 nm, reproducibility.C2H2±air; Na, 589.0 nm, 1.4 nm, C2H2±air. For quantiÆcation, The sampling efficiency was determined by weighing the the standard additions method was used. graphite tube before and after 20-fold pipetting of 20 ml aliquots For the determination of Al, Cd, Co, Cu, Fe, K, Mn, Ni, Pb of a 15 g l-1 slurry followed by drying. From three replicates and Zn in boron nitride digests by ETAAS, 20 ml aliquots of of this experiment, a mean slurry sampling efficiency of 98±4% the sample solution were dispensed automatically into the was estimated.atomizer. The standard additions method was used for calibration. The most important experimental conditions are Matrix Behaviour and Chemical ModiÆcation summarized in Table 2. The operating parameters used in the analysis of the boron Boron nitride reacts with carbon to form boron carbide at nitride digest by ICP-AES are summarized in Table 3.Standard temperatures around 2000°C.16 The formation of boron car- solutions used for calibration contained 10 ml of sub-boiled bide is expected to take place via a solid-phase reaction on the nitric acid and 10 ml of 40% hydroØuoric acid in a volume surface of the platform. This has two advantageous effects on of 50 ml. the atomization performance. Firstly, it favours the release of the analyte elements from the matrix. Furthermore, the conversion of boron nitride into the refractory boron carbide hinders RESULTS AND DISCUSSION the occurrence of non-speciÆc spectral interferences by the Slurry Preparation and Sampling matrix during the atomization stage.However, the boron carbide remains as a residue that cannot be removed from the Owing to the low wettability of boron nitride, pure water could not be used as a dispensing medium for preparation of platform during the clean-out step. Thus, the composition and 138 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Table 3 Operating parameters for ICP-AES Spectrometer– Outer plasma gas (Ar) 14 l min-1 Intermediate plasma gas (Ar) 0.2 l min-1 Aerosol carrier gas (Ar) 0.3 l min-1 Rf power 900 W Nebulizer V-groove, HF resistant spray chamber Solution uptake 1.5 ml min-1 Emission lines used– Element Wavelength/nm Al 396.152 Ca 393.366 Cd 226.502 Co 228.616 Cu 324.754 Fe 259.940 K 766.490 and 769.896 Mg 279.553 Mn 257.610 Na 588.995 and 589.592 Ni 231.604 Pb 220.353 Zn 213.856 the structure of the atomizer surface is continuously changing.As a result of the retention of boron carbide, the availability of free carbon for the reaction with boron nitride will decrease with an increasing number of analysis cycles. This inØuences the conditions for the atomization, especially of elements trapped in the bulk, and, consequently, the resulting absorption signals. Therefore, in order to improve the performance characteristics of the method, use was made of the possibility of chemical modiÆcation.As seen from scanning electron micrographs or even observed visually, addition of magnesium nitrate to the sample slurry led to a considerable reduction in the amount of boron carbide retained on the graphite tube surface. A possible reason for this phenomenon could be seen in reactions of magnesium oxide with boron nitride by which the more volatile boron oxide is formed. However, further investigations are necessary to explain the mechanism leading to the effect of the magnesium nitrate modiÆer.From a comparison of the integrated absorbances for manganese obtained in a series of analysis cycles with a boron nitride slurry without and with the addition of 50 mg of magnesium nitrate, shown as an example in Fig. 1, it is evident that magnesium nitrate as a chemical modiÆer improves both the sensitivity and the long-term reproducibility. Fig. 1 Dependence of the integrated absorbance of manganese on the number of analysis cycles with a boron nitride slurry (200 mg of boron nitride per atomization): without modiÆer (broken line); with Table 2 Instrumental parameters and experimental conditions used in analysis of boron nitride digests by ETAAS Pyrolysis Atomization Gas Øow/ Chemical Tube Element Wavelength/nm Slit/nm Temperature/°C Ramp/s Hold/s Temperature/°C Ramp/s Hold/s ml min-1 modiÆer* type� Al 308.2 0.7 1700 15 15 2700 0 3 300 A I Cd 228.8 0.7 400 20 15 2500 1 4 0 None II Co 242.5 0.2 400 25 25 2500 0 4 0 None II Cu 324.8 0.7 400 10 15 2300 0 4 50 None II Fe 346.6 0.7 400 10 20 2500 0 4 50 None II K 769.9 1.4 900 20 10 1800 0 5 300 None II Mn 279.5 0.2 1400 25 5 2200 0 3 100 A II Ni 232.0 0.2 400 10 20 2500 0 4 50 None II Pb 283.3 0.7 850 15 10 2200 0 3 0 B I Zn 213.9 0.7 700 15 10 1900 0 3 300 C I * Chemical modiÆers (per atomization): A, 50 mg of Mg(NO3)2 ; B, 200 mg of PO43-+10 mg Mg(NO3)2 ; and C, 25 mg of Mg(NO3)2.� Tube type: I, pyrolytic graphite coated ringed tube with fork-shaped platform; and II, pyrolytic graphite coated tube with L'vov platform. 50 mg of Mg(NO3)2 per atomization (solid line). For ETAAS conditions see Table 1. Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 139In addition, it also increases the tube lifetime. With one Calibration graphite tube, up to about 200 atomizations with 150 mg of In an accurate calibration, the analyte element present in the sample could be performed.However, as is evident from the sample and in the standard has to behave similarly during the Ægure, the signal became stable only after the execution of the charring and atomization steps. This assumption is best met 10±15 atomization cycles, which were necessary to condition by using powdered standards with a matrix composition close the inside surface of the atomizer. The lifetime of the tubes to that of the sample.However, such a calibration would ended either by a mechanical breakdown or by a more or less require a relatively high Ænancial expenditure, and nandard complete coverage of the surface with boron carbide causing of this type is actually available for boron nitride anyway. poorer reproducibility of the results. These observations are in With chemical modiÆcation (see the above section), satisfac- good agreement with those made in the determination of tory similarity of the signal proÆles originating for slurries, silicon in boron nitride using a Perkin-Elmer Model 4100 ZL spiked slurries and aqueous standard solutions was observed atomic absorption spectrometer.14 Magnesium nitrate was used indicating that the use of matrix-matched solid standards for as the chemical modiÆer in the determination of all analytes quantiÆcation was not necessary.Therefore, the applicability excluding, obviously, magnesium. of the calibration curve method, which, because of its simplicity When determining Si by ETAAS, for most of the matrices and savings in time, was considered as the Ærst choice, was reported in the literature, calcium nitrate has proved to be the compared with that of the method of standard additions.This chemical modiÆer of choice.17±22 Magnesium nitrate, known was carried out by comparison of the characteristic masses as a modiÆer of universal applicability and great efficiency,23 obtained for spiked boron nitride slurries and for matrix-free has, in general, not been found to be as effective as calcium standard solutions for Æve selected elements.While for Mg, nitrate.20,22 However, when the Perkin-Elmer Model 4100 ZL Mn and Zn, the characteristic masses obtained by both atomic absorption spectrometer with a THGA graphite furnace methods were in good agreement, for Al and Ca, the character- was used, magnesium nitrate was a more effective modiÆer for istic masses obtained from slurries were lower by 33 and 25%, the determination of Si in boron nitride by slurry sampling respectively, compared with those from standard solutions.ETAAS, leading to a signiÆcantly better reproducibility com- Thus, the use of the standard additions method was uniformly pared with calcium nitrate.14 On the other hand, using the preferred for all analyte elements. same spectrometer system, calcium nitrate proved to be the most suitable chemical modiÆer for the determination of Si in Analysis of Boron Nitride and Comparison of Results titanium dioxide and zirconium dioxide.15 For titanium dioxide, magnesium nitrate even led to a reduction in sensitivity.The developed slurry sampling ETAAS method was applied Therefore, the efficiency of both calcium nitrate and magnesium to the analysis of a boron nitride material which was also nitrate as chemical modiÆers was investigated. Surprisingly, analysed by solution ETAAS and ICP-AES. The results are using the HGA-500 graphite furnace for the determination of summarized in Table 4.The contents are given as mean values Si in boron nitride, no signiÆcant difference between these two with standard deviations for each method based on Æve modiÆers was found with respect to the sensitivity, the repro- separate analyses. ducibility and the signal shape: for 50 replicate measurements For the elements Cu, Fe, K, Mg, Mn, Na and Zn, the of a boron nitride slurry with a matrix concentration of standard deviations of slurry sampling and solution AAS are 3.3 g l-1, corresponding to about 6 ng of Si per atomization, at about the same level.The considerably higher standard deviations obtained for Al and Ca with slurry sampling ETAAS mean absorbance signals of 0.028±0.004 s (RSD 15.3%) and in comparison with solution AAS and ICP-AES indicate 0.034±0.006 s (RSD 16.1%) were obtained on applying 50 mg possibly inhomogeneous distribution of these two elements in of Mg(NO3)2 and 50 mg of Ca(NO3)2, respectively, per atomizthe material.ation as chemical modiÆers. However, with magnesium nitrate, NAA is usually used as a quasi reference method for checking the tube lifetime was slightly longer than that with calcium the accuracy of a new method in this laboratory. However, nitrate. Thus, the choice of the most effective modiÆer for Si because of the extremely strong absorption of thermal neutrons is determined by both the nature of the matrix and the type by boron, NAA could not be applied to analysis of this of graphite furnace.material. Therefore, solution ETAAS and ICP-AES were used Acceptable determinations of Cr in boron nitride were hindered by serious memory effects. Formation of chromium diboride, which melts without decomposition at 2280°C,24 is Table 4 Trace elements content of boron nitride determined by slurry the most probable reason for this effect. In the determination sampling ETAAS, solution ETAAS and ICP-AES of Ti, the formation of titanium diboride causes an even more Content/mg g-1 serious difficulty: at an atomization temperature of as high as 2700°C, no proper absorption signal could be obtained.These Slurry Solution non-spectral interferences could not be reduced sufficiently by Element ETAAS ETAAS ICP-AES any of the chemical modiÆers tested. Consequently, Cr and Ti Al 23±3 21.5±0.9 23±1 were not considered as appropriate analyte elements for the Ca 169±25 170±4* 155±6 proposed method.Cd <0.002 <0.001 <0.7 For the estimation of the levels of the background attenu- Co <0.1 <0.06 <0.4 Cu 0.78±0.05 0.76±0.1 0.73±0.08 ation occurring in processing boron nitride slurries with mag- Fe 58.5±4 59±4 60±2 nesium nitrate as the chemical modiÆer, measurements were K 1.5±0.3 1.3±0.3 <3.6 performed over the wavelength range from 206 to 588 nm Mg 8.5±0.7 9.1±0.5* 6.3±0.2 using the two-line method at the resonance wavelengths of the Mn 2.4±0.2 2.5±0.1 1.95±0.05 elements Bi, Cu, Co, Cd, Ni and Mo.Atomization temperatures Na 34±3 32±1* 24±1 Ni <0.6 0.40±0.03 <0.6 were between 2000 and 2500°C and the gas stop mode in the Pb <0.09 <0.01 <5 atomization step was used to obtain the maximum amount of Si 91±19 ND� ND� reaction products in the gaseous phase. Even by applying the Zn 0.42±0.05 0.38±0.03 0.40±0.08 maximum possible amount of boron nitride of 300 mg, no signiÆcant background absorption was observed at any of the * Determined by FAAS.� ND=not determined. wavelengths investigated. 140 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12as independent methods. The deviations between the mean The LODs of Co, Cu and Pb obtained by slurry sampling are slightly higher compared with those of the solution tech- values for slurry sampling and solution ETAAS are between 0 and 10%, with the exception of K where the deviation is 16%. nique, whereas the LODs for Cd and Mn are at about the same level.Only for Ni did the slurry sampling technique, However, even in the latter case, the deviation of the means is within the standard deviations. Also, the results for ICP-AES owing to the occurrence of memory effects, provide an essentially higher LOD than the solution technique. The high LOD are in good accordance with those of the two ETAAS methods, excluding Mg, Mn and Na, for which the contents determined for Al for solution ETAAS can be explained by a signal depression caused by hydroØuoric acid which was also by ICP-AES are lower by 26, 17 and 29%, respectively, than those of slurry sampling ETAAS, and the agreement is outside observed in the analysis of zirconium dioxide11 and quartz.25 The reason for the fairly low LODs achieved by ICP-AES is the overlap of the standard deviations of the respective means.Altogether, the comparison of the results of slurry sampling because boron represents an exceptionally favourable matrix element for AES, as it gives rise to a relatively low background ETAAS with those of the two independent methods shows good agreement, proving that results of satisfactory accuracy and line-poor spectrum.Indeed, addition of appropriate amounts of high-purity boric acidank and standard can be obtained by the proposed technique. solutions did not signiÆcantly affect the signal. Limits of Detection CONCLUSIONS The LODs achievable in the analysis of boron nitride samples by slurry sampling ETAAS are shown in Table 5, along with Owing to its simplicity, rapidity, reliability and high detection power, slurry sampling ETAAS proved to be a method well those for solution ETAAS and ICP-AES.As the concentrations of Al, Ca, Fe, K, Mg and Na in the analysed sample were suited to the determination of a large number of impurities in powdered boron nitride at concentrations down to the ultra- relatively high, for the estimation of the LODs of these elements, a high-purity boron nitride sample prepared by trace level.grinding (see Sample and Reagents section) was used. For all three methods, the LODs are expressed as three standard REFERENCES deviations of the blank resulting from ten replicates, for slurry and solution ETAAS, and from seven replicates for ICP-AES. 1 Fister, D., Ceram. Eng. Sci. Proc., 1985, 6, 1305. 2 Knoch, H., and Hunold, K., T ech. Mitt. Essen, 1987, 80, 31. Also given in the table are LODs for FAAS, which was used 3 Koval'chuk, I.A., Markov, A. V., and Mil'vidskii, M. G., Izv. instead of solution ETAAS for the determination of Ca, Mg Akad. Nauk SSSR, Neorg. Mater., 1988, 24, 324. and Na, because of the high contents of these elements in 4 Belaya, K. P., and Kustova, L. V., Zavod. L ab., 1986, 52, 40. the sample. 5 Sucha, A., and Macak, J., Pokroky Praskove Metal., 1986, 2, 14. From a comparison of the LODs of slurry sampling ETAAS 6 Morikawa, H., Uwamino, Y., Iida, Y., Tsuge, A., and Ishizuka, T., and solution ETAAS, the superiority of the slurry sampling Bunseki Kagaku, 1988, 37, 218. 7 Okano, T., Fujimoto, K., Matsumura, Y., and Harimaya, S., technique for the ubiquitous analytes Al, Fe, K, Mn and Zn is Kawasaki Seitetsu Giho, 1989, 21, 113. evident. This is mainly due to the availability of high-purity 8 Martynova, L. M., Zh. Anal. Khim., 1994, 49, 444. water and ethanol as the suspension medium, and to the 9 Docekal, B., and Krivan, V., J.Anal. At. Spectrom., 1992, 7, 521. possibility of controlling the actual blank of the medium for 10 Friese, K.-C., and Krivan, V., Anal. Chem., 1995, 67, 354. each beaker prior to the preparation of the slurry, leading to 11 Schneider, G., and Krivan, V., Spectrochim. Acta, Part B, 1995, a signiÆcant reduction in the blank values. For Ca, Mg and 50, 1557. 12 Scha»ffer, U., and Krivan, V., Spectrochim. Acta, Part B, 1996, Na, a similar improvement in the LODs compared with 51, 1211. solution ETAAS can be assumed. 13 Docekal, B., and Krivan, V., J. Anal. At. Spectrom., 1993, 8, 637. 14 Hauptkorn, S., and Krivan, V., Spectrochim. Acta, Part B, 1994, 49, 221. Table 5 LODs for trace elements in boron nitride achievable by 15 Hauptkorn, S., Schneider, G., and Krivan, V., J. Anal. At. slurry sampling ETAAS, solution ETAAS, Øame AAS and ICP-AES Spectrom., 1994, 9, 463. 16 Gmelin, Handbook of Inorganic and Organometallic Chemistry, LOD/mg g-1 Boron, Syst. No. 13, Boron Compounds, Springer Verlag, Berlin, 8th edn., 1991, 4th supplement vol. 3a, p. 53. Slurry Solution Flame 17 Thomson, K. C., Godden, R. G., and Thomerson, D. R., Anal. Element ETAAS ETAAS AAS ICP-AES Chim. Acta, 1975, 74, 289. Al 0.08 2.3 – 0.4 18 Rawa, J. A., and Henn, E. L., Anal. Chem., 1979, 51, 452. Ca 0.08 – 2.8* 1 19 Lythgoe, D. J., Analyst, 1981, 106, 743. 20� 20 Taddia, M., J. Anal. At. Spectrom., 1986, 1, 437. Cd 0.002 0.001 – 0.7 21 Gitelman, H. J., and Alderman, F. R., J. Anal. At. Spectrom., Co 0.15 0.05 12� 0.8 1990, 5, 687. Cu 0.04 0.01 8� 0.3 22 Holden, A. J., Littlejohn, D., and Fell, G. S., Anal. Proc., 1992, Fe 0.06 0.6 15� 0.4 29, 260. K 0.01 0.35 – – 23 Tsalev, D. L., Slaveykova, V. I., and Mandjukov, P. B., Mg 0.005 – 0.15* 0.7 Spectrochim. Acta Rev., 1990, 13, 225. 0.8� 24 Gmelin, Handbook of Inorganic and Organometallic Chemistry, Mn 0.02 0.05 – 0.06 Chromium, Syst. No. 52, Verlag Chemie, Weinheim, 1962, part B Na 0.01 – 0.7* 0.3 vol. 8, p. 343. Ni 0.6 0.03 – 1.7 25 Hauptkorn, S., and Krivan, V., Spectrochim. Acta, Part B, 1996, Pb 0.04 0.01 – 5 51, 1197. Si 0.4 – – – Zn 0.04 0.3 2 0.4 Paper 6/04884F Received July 11, 1996 * The present work. � From ref. 8. Accepted September 3, 1996 Journal of Analytical Atomic Spectrometry, February 1997,

ISSN:0267-9477    

DOI:10.1039/a604884f

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Microsecond-pulsed Glow Discharge Time-of-flight Mass Spectrometry: Analytical Advantages WEI HANG, CYNTHIA BAKER, B. W. SMITH, J. D. WINEFORDNER and W. W. HARRISON* Department of Chemistry, University of Florida, Gainesville, FL 32611, USA A microsecond-pulsed glow discharge time-of-flight mass mass analyser for a pulsed ion source. It has arguably the highest ion transmission among all mass spectrometers. In spectrometer was constructed and evaluated for elemental analysis. Mass spectra from the instrument show significant addition, because the TOF mass spectrometer is able to operate at a high repetition rate, a large number of spectra can be advantages, including higher signal-to-noise ratios than those of a dc glow discharge source.Important temporal advantages acquired and integrated (or averaged) in a short period of time, resulting in a significant signal-to-noise enhancement. result from the pulsed discharge and pulsed mass analyser. Mass discrimination among different elements is very small. For the observation of pulsed events, TOFMS offers the distinct advantage of multiplex detection of a high percentage The instrument currently has a resolving power of 360 in linear mode and 1600 in reflectron mode (full width at half of ions of all masses during each pulsed event.Even with a continuous source, Mahoney et al.8 have shown some promis- maximum). Present detection limits are at the low ppm level, limited primarily by the detection and data acquisition system.ing results from an orthogonal ICP-TOFMS instrument, which shows results competitive with those given by commercial Because the detector is easily saturated, the present data acquisition system has limited dynamic range and sensitivity. instruments. An earlier paper has described our ms-pulsed GD-TOFMS Possibilities exist to overcome this constraint. system design considerations and preliminary results in the Keywords: Glow discharge; pulsed glow discharge; time-of- linear mode.9 In this paper, further developments are given, flight mass spectrometry ; atomic mass spectrometry including ion optics, detection system, and data from the reflectron mode.Characteristics of this system, such as mass discrimination, temporal stability, and S/N have been meas- The application of mass spectrometry to the direct trace ured. Some standard samples have been tested for the determielemental analysis of solid samples was fairly limited until nation of the spectral resolution and detection limits, which about 10 years ago.Since then, it has become a standard are currently limited to the ppm level because of detector method in this field, basically due to the introduction of constraints. commercial glow discharge mass spectrometry (GDMS) instruments. INSTRUMENTATION The GD source has existed as an active analytical and diagnostic tool for more than 70 years,1 and it has been A diagrammatic representation of the ms-pulsed GD-TOFMS assembly is shown in Fig. 1. The whole instrument has been extensively used for the elemental analysis of solids for the past 20 years.2 A continuous GD source, either in dc mode or described in detail elsewhere.9 In the existing stage, several modifications have been made to improve the performance of in rf mode, produces a steady continuous beam of sample ions, with a small energy distribution (#10 eV). These advantages the system. One important modification was the optimization of the ion permit the manufacture of a relatively simple, and therefore more cost-effective, GDMS instrument, in sharp contrast to optics before the orthogonal extraction. High negative potential is applied to both the skimmer and slit for a higher other direct solid sampling mass spectrometers, such as laser ablation ICP-MS, spark source MS or secondary ion MS transmission, and all the other lenses are also adjusted accordingly.The sampler plate (source chamber) is fixed at ground instruments.Only recently, however, have analysts begun to extract the full potential of this low power, yet effective, source. potential. The final optimized conditions are listed in Table 1. To cope with the increased transmission, two 25 mm diam- A dc GD source is routinely operated at a power level of 1–4 W, and an rf GD source usually works at 20–50W, which eter extended dynamic range microchannel plates (MCP, Galileo Electro-Optics, Sturbridge, MA, USA), mounted in gives the same order of magnitude of signal intensity as a dc source at the lower operating power.By contrast, the instan- Chevron mode, are used in the system to replace the standard MCP assembly (Galileo) as the linear TOF detector. The taneous power of the microsecond (ms)-pulsed GD can reach several hundred watts, and the sputtering rate during the standard MCP assembly was used instead as the reflectron TOF detector. pulse-on region is about two orders of magnitude higher than that of the dc mode.3 High power also results in high excitation The increased transmission greatly increased the peak intensity of the ion signals.With the operation of the normal and ionization efficiency. In an argon gas discharge with a copper cathode, such a high instantaneous power turns the potential on the detector (1.6–2 kV), the signal intensity of the matrix can reach several volts with a 50 V impedance. The fast normally observed blue glow into a distinctive green, due to the stimulation of higher lying atomic states.A 1–4 orders of preamplifier (SR445, Stanford Research System, Sunnyvale, CA, USA), which was formerly used, is not able to handle magnitude greater signal intensity, relative to the dc source, has been observed in emission, fluorescence and mass such an intense peak. Thus, the signal from the MCP detector is connected directly to the digital oscilloscope (TDS 520A, spectrometry.4–7 With the development of ms-pulsed GD studies, it became Tektronix, Beaverton, OR, USA) with a 50 V input impedance.Ultrahigh-purity grade argon gas (99.999%, Liquid Air, San clear that the combination of a pulsed GD with a time-of- flight (TOF) mass spectrometer could be an attractive combi- Francisco, CA, USA), was used throughout. The sample used in each experiment will be described later. No cryogenic nation. In many ways, the TOFMS instrument is an ideal Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 (143–149) 143Fig. 1 Schematic diagram of ms-pulsed GD-TOFMS system. Table 1 Typical pulsed GD-TOFMS operating conditions Pulsed GD: Pulse magnitude: 1–3 kV Source pressure (argon): 0.8–1.5 Torr Pulse width: 5–20 ms Cathode–orifice distance: 4–5 mm Pulse frequency: 100 Hz Pulse current: 4–200 mA T OFMS: Orifice diameter=skimmer diameter=1 mm Orifice–skimmer distance: 4 mm Potential of L1: -800 V Flight tube: -2000 V L2: -150 V Skimmer: -400 V L3: -200 V Slit: -800 V Repelling plate bias voltage: 0 to ca.-4V Repelling pulse magnitude: 84 V (linear mode), 150 V (reflection mode) Width: 1–5 ms Microchannel-plate detector: -1600 to 1900 V Second stage pressure: 1×10-4 Torr Third stage pressure: 1.5×10-5 Torr cooling was used; hence, spectral purity may still be subject to orthogonal TOFMS instrument.9,11 Thus, the sampling efficiency of a TOF mass spectrometer is up to two orders of improvement.magnitude higher than that of a quadrupole instrument. In addition to the high sampling efficiency of the ms-pulsed RESULTS AND DISCUSSION GD-TOF mass spectrometer, this system has the opportunity for temporal separation of discharge gas species from sputtered Sampling Strategy sample components. It is believed that mass spectrometry In a sequential scanning mass analyser, such as a quadrupole samples ions primarily near the vicinity of the sampler orifice.12 instrument, after the ions pass through the interface (the Gas species (argon, nitrogen, oxygen, water, etc.), owing to sampler and skimmer), only part of the specific m/z ions will their high ionization potentials, are ionized principally by the be transmitted to the detector at any given moment.Other energetic GD electrons. These species show little ionization ions simply hit the walls and are lost. Thus, the transmission after the termination of the discharge because of the fast of a scanning instrument is only 0.1–0.01%.In a TOFMS scattering loss of electrons. On the other hand, sputtered instrument with orthogonal extraction, the sampling strategy particles diffuse across the cathode–sampler orifice distance, consists of two processes. The first is the interface sampling, are ionized by long-lived metastable argon atoms through which is similar to that used in a quadrupole mass spec- Penning collisions, and then successfully sampled by the mass trometer.The second is the ion extraction in the repelling spectrometer. Thus, most of the sputtered sample ions reach region, which is the key part of an orthogonal structure the repelling region several hundred microseconds later than instrument. The transmission of this type of instrument is the gas species. By setting the delay time of the repelling pulse determined by the speed of the ions passing through the in favour of the sample ions, interferences from gas species can repelling region and by the opening of the extraction grid (grid be greatly reduced.This process is illustrated in Fig. 2. During the pulse-on region, cathode atoms are sputtered out, but they G1 in Fig. 1).10 A 20% transmission has been observed in the 144 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12without loss of analytical signal. Therefore, this advantage offers a new approach to the analysis of samples that contain small amounts of ‘troublesome’ elements, such as C, Al, Si, S, P, K, Ca and Fe.Fig. 4 demonstrates the separation of gaseous carbon (mainly from hydrocarbons in the source) and cathodecontaining carbon by varying the delay time of the repelling pulse. Mass Discrimination In ICP-MS, because of the high pressure and frequent collisions, all ions pass through the interface at virtually the same speed. There is no concern about the transport time difference for different m/z ions from the interface to the repelling region in an orthogonal TOFMS instrument.However, GD sources operate at alow pressure (#1 Torr), and the collision frequency in the interface is also much smaller than that of an ICP. Thus, apotential problem must be considered in pulsed GD-TOFMS: smaller m/z ions have a higher velocity than larger m/z ions. Thus, different m/z ions will arrive at the repelling region at different times, which will result in some degree of mass discrimination. It was necessary to determine the extent of this problem with our system.Another potential problem, which Fig. 2 Diagram of the diffusion and transport processes in ms-pulsed has been observed in an ICP-TOFMS system,13 is the response GD-TOFMS: (a) during the pulse-on region; (b) after the termination of different m/z ions to the deflector potential. Extensive of the pulse; (c) several hundred microseconds after the pulse. experiments have been conducted to identify whether these two potential problems are significant in the present system.A mixed metal powder sample was used in this experiment are very close to the cathode surface [Fig. 2(a)]. After the (mass proportions: Al5Ti5Cu5Ag5W=151515151). The most termination of the GD, gas species ions cannot be formed, due intense Ar peak is usually observed when the repelling pulse to the rapid disappearance of electrons; sputtered atoms are delay time is#20 ms, which is close to the result from SIMION diffusing to the sampler orifice, which requires several hundred simulation (if the initial kinetic energy is set at 7 eV).If we microseconds under normal GD conditions [Fig. 2(b)]. Thus, assume that Ar ions need 20 ms to travel from the sampler to sputtered particles and gas species are physically separated the repelling region [shaded region in Fig. 2(c)], then Al ions when they arrive at the repelling region. By applying a narrow need 16.5 ms, and W ions need 50 ms. This 33.5 ms difference pulse to the repelling plate, ions in front of the extraction grid would then introduce mass discrimination into the system.[shaded region in Fig. 2(c)] will be accelerated into the flight Fortunately, the packet of mass ions from the sample has a tube, and begin their TOF mass separation. large spatial distribution. That is, the repelling pulse delay Fig. 3 is a TOF mass spectrum acquired when the temporal from the time when the mass spectrum peaks of sample ions repelling window is set to be favourable for copper ions.The initially appear to the time that those peaks have completely peak intensities of Ar and ArH are at the millivolt level, which passed is about 500 ms (when the source pressure is 1 Torr and is not evident in Fig. 3. Although, in this approach, the the cathode–sampler orifice distance is 5 mm). This large interference from gas species cannot be reduced to absolute spatial distribution is due to the diffusion time of the sputtered zero (owing to their spatial and energy distribution), it is able particles from the cathode surface to the sampling orifice and to reduce all the peak intensities of the gas species by up to 3 the initial kinetic energy distribution when the atoms are orders of magnitude (depending on the source pressure, cathsputtered from the cathode surface.Thus, compared with the ode–sampler orifice distance, and the repelling pulse delay) large spatial distribution of the ion package, the flight time differences among different elements from the interface to the Fig. 3 Demonstration of the temporal resolution of ions from sputt- Fig. 4 Carbon ion signal intensity versus repelling pulse delay time, ered atoms in a ms-pulsed GD-TOF mass spectrum of a copper disc taken at a delay time of 450 ms. Source pressure: 1 Torr argon; cathode– showing the signal from background hydrocarbons at 25 ms delay and from the cathodic carbon at 250 ms. Sample: Fe5C=9555 mass ratio; sampler orifice distance: 5 mm; pulse frequency: 100 Hz; pulse width: 15 ms; pulse magnitude: 3 kV; repelling pulse delay: 450 ms.source: 0.9 Torr, 5 mm; pulse: 100 Hz, 15 ms, 1.8 kV. Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 145Fig. 6 Mass spectra of (a) dc and (b) ms-pulsed GD. Source pressure: 1 Torr; conditions for dc: 800 V, 3 mA; pulse: 2 kV, 15 ms. Optimized cathode–sampler orifice distance for maximum Cu intensity: 7 mm (dc), 4 mm (pulse).(2.5×10-6 V)15]. Under such conditions, the signals from subppm analytes are too small to be digitized by the oscilloscope, limiting the detection of small analytical signals. An improved Fig. 5 Signal intensities of Al–Ti–Cu–Ag–W mixed disc sample versus repelling pulse delay time, demonstrating the relatively small separation data readout system is now planned for acquisition. in the repeller region. Source pressure: 1 Torr; pulse: 100 Hz, 15 ms, 2 kV; cathode–sampler orifice distance: (a) 3, (b) 5, (c) 7 mm.Stability of Signal A NIST unalloyed Cu 495 disc sample was used in this repelling region become insignificant. Experimental data show experiment, where the concentrations of Cu and Fe are 99.94% that the discrimination is small (Fig. 5). Different elements are and 96 ppm, respectively. After the sample had been pre- evenly mixed when they arrive at the repelling region. Under sputtered for 10 min (under the same pulse conditions), the normal operating conditions [Fig. 5(b)], the intensities of most measurement was begun.For a period of 1 h, the temporal elements can be optimized at the same delay time. stabilities of both the matrix and the trace elements, primarily Experiments relating to the effect of deflecting potential have long-term drift, are within 10% of the starting values (Fig. 7), been carried out at different source pressures, cathode–sampler which is close to the stability of other types of GD sources.16 orifice distances, and repelling pulse delay times.Although different operating conditions need different deflecting potentials to optimize the system, all five elements studied are optimized at the same deflecting potential. This would imply that ions of different mass have approximately the same kinetic energy after being sampled, in contrast to ICP-MS, where kinetic energy is mass-dependent. The same result was also observed in another TOFMS system where an rf-GD source was used.14 S/N The ms-pulsedGD Cu signal intensity is one order of magnitude higher than that of the dc GD, as shown in Fig. 6. Increasing the pulse power will further enhance the signal intensity in the ms-pulsed mode, while further increasing the voltage/power in the dc mode will overheat the sample. The limitation at present in reaching lower detection limits with the ms-pulsed source is the large dark current observed for the TOF system. With the Fig. 7 Temporal stability of ms-pulsed GD-TOFMS system.Sample: GD off, approximately 350 mV is measured, arising primarily NIST SRM 495 Cu; source: 1 Torr, 5 mm; pulse: 100 Hz, 15 ms, 2 kV. from the input noise associated with the digital oscilloscope Measurement started 10 min after pre-sputtering under the same conditions. [the dark current of the MCP detector is only 5×10-8 A 146 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Note that the MCP detector was not working in its linear range owing to the high intensity of the Cu signal.Resolving Power A ms-pulsed GD source permits higher resolving power and sensitivity compared with a continuous source. In the repelling region, the repelling pulse only extracts a certain volume of ions, those present in front of the extraction grid [the shaded region in Fig. 2(c)], into the flight tube. These ions have been sorted to some extent according to their speed when they move from the source to the repelling region.The ion optics (including the skimmer, three cylindrical lenses, steering plates, and slit) can shape the ion beam for less spatial distribution in the repelling region. The TOF mass spectrometer actually extracts an ion packet with limited spatial and energy distribution, which then enables the deflector plates to guide the ion package to the detector. Sputtered sample ions are sampled several hundred microseconds after the termination of the plasma. Their speed passing the repelling region is probably determined by diffusion energetics in the source rather than by the plasma potential.Because ions are formed several hundred microseconds after the termination of the discharge, their slower speed of transit across the repelling region further improves the transmission in the orthogonal extraction. On the other hand, for a continuous GD source, the ion optics after the sampler cannot efficiently focus an ion beam with a larger energy distribution. In that case, the TOF mass spectrometer extracts a volume of ions with a larger spatial and energy distribution compared with a ms-pulsed GD source, which makes it more difficult for the deflector plates to make a smooth transition of this ion packet to the detector.Some Fig. 8 Mass spectra of lead disc: (a) reflectron mode, (b) linear mode. of the ions may also strike the flight tube wall, decreasing both Source: 1 Torr, 5 mm; pulse: 100 Hz, 15 ms, 2.8 kV.the resolving power and the signal intensity. The resolving power achieved to date with our TOF instruto us. This interference was also observed with other samples ment is 1600 in the reflectron mode and 360 in the linear mode when an expanded oscilloscope scale and a high MCP voltage [Fig. 8(a) and (b)]. For a continuous source, the resolving were used. The 63Cu2563Cu65Cu dimer peak ratio is also not power is about 300. It may be noted that a small peak appears matched, because the peak height of these species just exceeds in front of every major peak in the reflectron mode [Fig. 8(a)]. the limit of the oscilloscope sensitivity. There should be a peak Several factors could contribute to the production of such a for 65Cu2 at mass 130, but the signal is too small to be sampled peak. There are two grids that act as the extraction system; by the oscilloscope. Intense 63Cu and 65Cu peaks cover a larger hence, some ions may be trapped inside the grids at the mass range than just masses 63 and 65, because of the energy repelling pulse-off time.Also, a grid is installed in front of the and spatial distribution. High detector voltage (1.9 kV) is detector, where some ions may hit it and produce electrons, probably another reason for the broad peaks, if we make a which will arrive at the detector ahead of the ions due to the comparison with the Cu peaks in Fig. 6, where the detector electric field between the grid and the MCP. Experiments are voltage is 1.6 kV.currently underway to clarify the origin of the small peak. The main limitation of this instrument at present is the detection and data acquisition system. The high sampling Mass Spectra of Standard Samples efficiency of the system results in the non-linear response of the MCP detector, which reduces the ability for semi- Fig. 9(a) is a linear mode spectrum of NIST SRM 1264a Low quantitative analysis without a calibration graph. The ana- Alloy Steel taken for an average of 1000 spectra.The concenlogue- to-digital converter has a minimum sensitivity of 40 mV trations of major and trace compositions are listed in the when the y-axis scale is set at 1 mV per division (largest upper right-hand corner of Fig. 9(a). All the constituents can expanded scale). Thus, all signals below 40 mV are simply not be clearly identified. Fig. 9(b) is a part of the spectrum of the sampled. A spectrum of 1000 averaged spectra has a small same sample with the TOF mass spectrometer operating in background which cannot be observed in the oscilloscope; the reflectron mode, where the resolving power is seen to be hence, its noise level cannot be readily measured.Based on five times better than for the linear spectrum. the high signal intensities, we anticipate a power of detection NIST SRM 495 Unalloyed Copper was also tested with this significantly below the ppm level on modification of our detec- system. The reflectron mode spectrum of 1000 averaged spectra tion system.is shown in Fig. 10. At 100 Hz, 10 s are required to acquire 1000 spectra. Signals from trace elements at the ppm level are clearly apparent. CONCLUSIONS The aim of this work was to develop an instrument capable Detection Limits of reaching low detection limits (lower than those of quadrupole- based instruments). The energetic ms-pulsed discharge Referring to Fig. 10, some problems in the spectrum are readily apparent.The 107Ag5109Ag isotope ratio is not matched well, source and high transmissive TOFMS should make the goal possible. With the use of a TOF mass spectrometer, a short perhaps due to some interference at mass 109 which is unknown Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 147Fig. 9 Mass spectra of NIST SRM 1264a Low Alloy Steel in (a) linear mode and (b) reflectron mode. Source: 1 Torr, 4 mm; pulse: 100 Hz, 15 ms, 2 kV. analysis time is expected, although the maximum frequency is last stage (presently 1.5×10-5 Torr).A matrix deflector and an energy discriminator are under construction, which will be limited to 100 Hz at present, because of the maximum averaging speed of the oscilloscope. mounted at the end of the reflectron tube, thus mitigating the MCPsaturation problem, and background noise will be further In order to determine the detection limits, a high performance preamplifier will be used in the system to increase the reduced.Even with these improvements, we still require a wider dynamic range detector which can have a linear response signal. A boxcar integrator with a minimum gate width of 2 ns (Model 4402, EG&G) will also be used for determining the for matrix and trace element signals. A wider dynamic range, more sensitive, multichannel data acquisition system is also detection limits. A 330 l s-1 turbo pump will be used to replace the 80 l s-1 turbo pump to improve the poor vacuum in the needed for multi-element analysis. 148 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Fig. 10 Mass spectra of NIST SRM 495 Cu in reflectron mode. Source: 1 Torr, 4 mm; pulse: 100 Hz, 15 ms, 1.5 kV. 6 Hang, W., Walden, W. O., and Harrison, W. W., Anal. Chem., Even with the limitations at this stage, this approach shows 1996, 68, 1148. potential as a powerful analytical technique. The temporal 7 Hang, W., Yang, P. Y., Wang, C. L., Su, Y. X., and Huang, B. L., advantage inherent to the pulsed source and analyser permits Rapid Commun. Mass Spectrom., 1994, 8, 590.strong discrimination against background discharge gases. 8 Mahoney, P. P., Ray, S. J., Li, G., and Hieftje, G. M., paper Mass discrimination of this system is also very small. Further presented at the 1996 Winter Conference on Plasma studies will include instrumentation improvement, GD param- Spectrochemistry, Fort Lauderdale, FL, ThP41. 9 Harrison, W. W., and Hang, W., J. Anal. At. Spectrom., 1996, eter optimization, determination of relative sensitivity factors, 11, 835. and isotope ratio measurements. 10 Myers, D. P., and Hieftje, G. M., Microchem. J., 1993, 48, 259. 11 Myers, D. P., Li, G., Mahoney, P. P., and Hieftje, G. M., J. Am. This research is supported by US Department of Energy, Soc. Mass Spectrom., 1995, 6, 400. Division of Chemical Sciences, University of Florida Division 12 Hess, R. K., and Harrison, W. W., Anal. Chem., 1986, 58, 1696. of Sponsored Research, and Hewlett-Packard Laboratories. 13 Myers, D. P., Li, G., Yang P., and Hieftje, G. M., J. Am. Soc. Mass Spectrom., 1994, 5, 1008. 14 Heintz, M. J., Myers, D. P., Mahoney, P. P., Li, G., and Hieftje, REFERENCES G. M., Appl. Spectrosc., 1995, 49, 945. 1 Aston, F. W., Isotopes, Longmans, New York, 1924. 15 Data sheet, Galileo Electro-Optics, Sturbridge, MA, 1996. 2 Harrison, W. W., J. Anal. At. Spectrom., 1992, 7, 75. 16 Shick C. R., Jr., and Marcus, R. K., Appl. Spectrosc., 1996, 50, 454. 3 Pollmann, D., Ingeneri, K., and Harrison, W. W., J. Anal. At. Spectrom., 1996, 11, 849. Paper 6/04454I 4 Farnsworth, P. B., and Walters, J. P., Anal. Chem., 1982, 54, 885. Received June 26, 1996 5 Huang, B., Yang, P., Lin, Y., Wang, X., and Yuan, D., Fenxi Huaxue, 1991, 19, 259. Accepted October 17, 1996 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 149

ISSN:0267-9477    

DOI:10.1039/a604454i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Grimm-type Glow Discharge Ion Source for Operation With a High Resolution Inductively Coupled Plasma Mass Spectrometry Instrument NORBERT JAKUBOWSKI, INGO FELDMANN AND DIETMAR STUEWER Institut fu� r Spektrochemie und angewandte Spektroskopie, Postfach 10 13 52, D-44013 Dortmund, Germany A GD ion source of the Grimm-type design has been preparation procedures are severe limitations for a number of developed for operation with a new commercial double analytes when using lower resolution instruments.focusing ICP-MS instrument capable of higher mass A basic significantadvantage of techniques for direct analysis resolution. Instead of introducing only the sample, a complete of solids is that most of these limitations are avoided. source arrangement is introduced into the housing of the MS Additionally, sample preparation is much more straightforward system through a slide valve by a solid insertion probe. Sample without dissolution, and the total time for an analysis prochanging and source cleaning is fast and the latter reduces the cedure can be reduced considerably.Any risk of contamination risk of cross contamination to a minimum. The reproducibility from sample pre-treatment is avoided. Finally, direct analysis of the positioning of the sample and the source is verified by avoids the loss in sensitivity owing to dissolution, and considermeasuring ‘internal’ (reproducibility of the intensity ably lower detection limits can be realized.However, mention measurements) and ‘external’ (the analysis as a whole when should also be made of the main disadvantage of direct sample changing is included) reproducibilities in analytical methods. To be quantitative, techniques such as GDMS must determinations. Over the whole mass range, the sensitivity be calibrated with standards, which should be of mainly corresponds to that measured with an almost identical GD ion identical physical and chemical properties in comparison with source and a low resolution quadrupole instrument.This the samples under investigation, and these are not available demonstrates that element sensitivities are predominantly for many matrices and elements, and in particular for trace determined by the processes taking place in and the geometry elements. of the source and not by the ion optics and mass analyser ICP-MS is certainly a strong analytical tool, but for the components. analysis of solids it should be supplemented by a direct technique, which could be more demanding but can be applied Keywords: Glow discharge; ion source; inductively coupled to special problems, particular analytes or elements.plasma mass spectrometry ; high resolution Summarizing the actual state-of-the-art, GDMS is the main candidate of choice, as has been reviewed recently.8 A GDMS Inductively coupled plasma mass spectrometry (ICP-MS) is of instrument with high mass resolution (VG 9000, Fisons/VG, continuously increasing interest for trace and ultratrace analy- Winsford, UK) as well as an instrument with low mass sis of liquids and solutions in many fields of application owing resolution (GloQuad, Fisons/VG) are commercially available.to the excellent multi-element capability and extremely low The instrument with high mass resolution is particularly detection limits. Many attempts have also been made to utilize encouraging with extremely low detection limits for a great ICP-MS for the direct analysis of solids in combination variety of elements in the bulk analysis of pure metals,9–14 with several special sample introduction techniques.Laser alloys15–18 and semiconductors.19,20 Depending on the chosen ablation1–4 has not only been used for bulk analysis, but, geometry, it can additionally be applied to the analytical owing to its local resolution, is especially promising for distri- characterization of surfaces and near-to-surface layers.21 It bution analysis of trace elements, while spark ablation5–7 can should be mentioned that with an rf source, it is also promising be a low cost alternative for bulk analysis in the case of for the analysis of insulators.22–25 conducting solids.With this in mind, one can only come to the conclusion that In spite of the progress in this field, solution analysis by a most desirable approach to the elemental analysis of solids ICP-MS has mainly been favoured so far for routine bulk would be a combination instrument enabling the ICP-MS and analysis of metals, alloys and semiconductors as a result of GDMS ion sources to be exchanged and optionally be attached several obvious advantages.From a general point of view, the to the same MS equipment. Following on from this idea, two analysis of a solution is a unique approach to multi-element research groups have already published reports on the analysis. It does not suffer from limitations concerning the development of a GD source for their commercial ICP-MS matrix.Adequate solution procedures are available for nearly instruments.26,27 A commercial instrument of this type was all types of solids. Simple quantification is possible by use of introduced some time ago (TS Sola, Turner Scientific, matrix matched solutions or by standard addition procedures, Manchester, UK). Previous work during the initial stages of and solution analysis is superior with respect to sampling and the present work realized this concept through the development homogeneity problems.However, the following limitations to of two independent instruments for GDMS28 and ICP-MS,29 solution analysis should also be considered. Dissolution of based on identical MS equipment in order to demonstrate the solids is time consuming and laborious and could be critical advantages of a ‘family’ concept. However, all of these owing to the risks of contamination. In the case of ICP-MS, approaches were based on quadrupoles as the mass analysers, the matrix load is limited by instrumental drift effects or with the associated strict limitations to analytical application sensitivity changes of elements (matrix effects), so that further because of the many spectral interferences.30 dilution of the sample often cannot be avoided, even though A strong impetus to the field of elemental MS has recently the detection limits deteriorate.Contributions from a variety of spectral interferences introduced by chemicals during been given by a new generation of ICP-MS instruments which Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 (151–157) 151have high mass resolution (ELEMENT, Finnigan MAT, operation. This unit has to include the sample itself, the gas inlet, the ion exit aperture and also the cathode and anode as Bremen, Germany; JMS-Plasmax 1, Jeol, Tokyo, Japan; and PlasmaTrace I+II, Fisons/VG, Winsford, UK). In previous components.To maintain a high vacuum in the analyser system during the exchange, it became necessary to employ a work, the analytical figures of merit of a prototype of the double focusing ELEMENT instrument, which has been solid insertion probe as a carrier of the source arrangement. The idea of changing the whole discharge chamber as a unit described in detail elsewhere,31 were investigated.32,33 After demonstrating the performance as an ICP-MS instrument, it offers the additional advantage that dedicated chambers can be used for each type of matrix, ensuring extremely low cross was of course a great challenge to continue previous endeavours of several research groups26,27 to develop a GD source contamination.The final design of the GD source arrangement is shown in in order to provide the option of using an ICP source or a GD source for this double focusing instrument. Simultaneously Fig. 1, which combines a 3D technical drawing of the entire arrangement with a more detailed diagram of the discharge the aim was to utilize experience from analytical work with GDMS to design a new GD source, in particular considering chamber. In the following detailed description, numbered references to the items as described in the caption to Fig. 1 extreme trace analysis. The development of the new source was based on experiences will be interspersed. The figure corresponds to the situatiowhen the whole discharge chamber (6) has just been introduced from previous development work28 and analytical applications34,35 of a low resolution GDMS instrument with a into its operating position.Only the sample itself is omitted from the drawing of the entire arrangement to ensure that Grimm-type ion source, for which a number of advantages can be seen in comparison with other sources. This type of some of the details of the source are unobscured. In the particular drawing of the discharge chamber, the analytical source has been well investigated, and it has often been applied to the analysis of conducting and non-conducting36 materials sample (6.1) is clamped against the cathode plate (6.2).On top of the cathode plate is located the anode plate (6.3) as well as for surface analysis37,38 in AES, and different approaches have been proposed for quantification;39,40 a more carrying the anode ring (6.4), which defines the sputtering area on the sample surface. The anode plate together with the detailed review is given elsewhere.41 In the Grimm-type design, a ‘hindered’ discharge is realized anode ring is pressed into the centre hole of the cathode plate with a capton foil in between for insulation (6.5).At the outer by a special cylindrical anode tube in a position close to the surface of the cathode sample. Flat samples are mandatory, periphery, the cathode plate is provided with a screw thread sector, which has its counterpart at the inner periphery of the because the sample itself serves as the vacuum-sealing part of the ion source by use of an O-ring.It not only enables stable cathode body (5), together forming a bayonet clutch. An insulator (4) separates the cathode body from the anode body operation over 1 h or more, owing to the fact that the burning spot of the discharge is restricted to a well defined area on the (3), which in turn is also insulated (2) against the adapter flange (1), by which the whole source arrangement can be sample surface, but it is particularly convincing because of the advantage that fast surface and in-depth analysis are possible mounted at the interface of the MS system.The discharge chamber is mounted as a unit to a tripod carrier (7) on top of simultaneously by controlling the discharge to realize planar sputtering.42 the transporting rod of a turnable solid insertion probe (10). The bearing of the probe rod is carried in the centre of a It is the aim of the present paper to present the design concept for a new interchangeable Grimm-type GD source, housing (9), designed to build a pressure tight volume with the front flange of a plate valve (8).The housing carrying the which has been developed for operation with a double focusing ICP-MS instrument with high mass resolution. Additionally, probe is mounted on two stands (12, 13), and the whole arrangement can be moved on an optical bench that is aligned preliminary characterization of its analytical performance in combination with this instrument will be given.with the MS system. This transfer can be performed manually or alternatively by a computer controlled electropneumatic driving system (14). When commencing the load procedure for a discharge EXPERIMENTAL chamber, the housing is retracted, and the chamber is mounted Source Arrangement on top of the insertion probe, which, with respect to the As mentioned above, previous work on instrumental development in GDMS favoured the Grimm-type geometry43 for operation with a quadrupole MS instrument (i.e., GD-QMS), and details of the technical,28 operational44 and analytical45,46 properties have been published previously.The Grimm-type design facilitates simple access to the source for cleaning the discharge chamber and also the ion extraction orifice, but of course with the associated risk that small leaks could limit the detection of non-metals such as oxygen and nitrogen. With the previous system, no additional valves were applied, and therefore the whole instrument had to be vented for sample changing, which restricted sample throughput significantly.30 According to experience gained with the previous system, three main design postulates were followed in development of the new source: (1) in general to preserve the established principles, but (2) to reduce the time necessary for sample Fig. 1 Technical diagram of the GD source arrangement and details changing and simultaneously (3) to reduce the risk of leakage of the discharge chamber: 1, adapter flange to interface of MS system; from the surrounding atmosphere.The final concept was not 2, isolation; 3, anode body; 4, insulator; 5, cathode body; 6, discharge only to use a discharge chamber in a vacuum environment chamber (details: 6.1, sample; 6.2, cathode plate; 6.3, anode plate; 6.4, and to exchange the attached sample, but to insert (and retract) anode ring; and 6.5, isolation); 7, tripod carrier; 8, plate valve; 9, the whole discharge chamber with the mounted sample as a housing; 10, solid insertion probe rod; 11, feed through of insertion unit, so that cleaning procedures can be performed off-line, probe; 12, stand for housing; 13, stand for insertion probe bearing; 14, electropneumatic drive.keeping the instrument free for further simultaneous analytical 152 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12housing, is also in the retracted position.The whole arrange- series SRM 1261–1265 were always used as analytical samples, ment is then moved towards the outer flange of the plate valve. because these had been used in the past as references for the After realizing the necessary vacuum in the housing, the plate GD-QMS instrument. For higher concentrations, British valve can be opened and the discharge chamber is inserted by Chemical Standards sample 401 (Bureau of Analysed Samples, the probe through the valve, until it comes to the position in Middlesborough, UK) and for very low concentrations the front of the adapter flange, as is represented in Fig. 1. The very pure steel sample SUS B87 (Dillinger Hu�ttenwerke, discharge chamber is then in close contact with the screw Dillingen, Germany) were used. Some results obtained for Cu thread at the inner surface of the adapter flange, so that it samples [NIST SRM C1251–C1253 Copper ‘Benchmark’ (chip requires only a small turn to fit the discharge chamber to this and rod forms)] are also included.flange, and into a position ready for operation. After loading of the discharge chamber, the insertion probe can be retracted, RESULTS AND DISCUSSION and the plate valve can be closed again, so that the discharge can be ignited after a very short final pumping period. General Properties Unloading of the discharge chamber is achieved with the In comparison with previous experiences with GD ion sources reverse sequence of the steps.of the Grimm-type, a considerable improvement as regards the ion yield becomes obvious at a first glance of a spectrum. An Mass Spectrometer example is shown in Fig. 2 covering a range of from m/z 57.5 to 66.5 of the main components of a steel sample. The discharge For the present investigation a prototype of the ELEMENT gas used was argon with a purity of 6.0 without additional gas double focusing ICP-MS instrument with high mass resolution purification; the spectrum was registered in the LRM by was used.Simple and fast coupling of a GD source is particuscanning with a continuous increase in the magnetic field using larly favoured by a unique and essential feature of this instruthe counting mode for detection. Although the minor abundant ment. Operation of a double focusing instrument with a isotope 58Fe represents only an atomic concentration of magnetic field analyser requires a high voltage for ion acceler- 2800 mg g-1, its counting rate of 1.7×106 counts s-1, which ation, which is usually realized by connecting the ion source resulted with the lowest permissible operating voltage of the to a high potential with the spectrometer fixed to ground.multiplier, already comes close to the safety limit. This high However, in the case of this instrument the interface is fixed intensity has the consequence that in many cases no itope at ground potential, and the whole mass analyser is connected of the matrix can be taken into account for internal standardiz- to high voltage for acceleration.To change over from ICP to ation in the counting mode, so that optimization of the GD operation overnight pumping down is required. This could operating conditions had to be performed predominantly with of course be improved further. the analogue mode. If available, this limitation can, of course, From a technical point of view, the most essential difference be overcome by use of a Faraday detector.The LRM of the between the ICP source and the GD source consists in the double focusing instrument is already sufficient to achieve fact that the ICP is operated at atmospheric pressure, whereas clearly separated peaks in this m/z range. the GD is operated at a reduced pressure, which is sustained With the flat disk geometry of the GD source, a rather high in a vacuum-tight housing. Therefore, the only modification ion yield was generally achieved. The total matrix ion current to the basic instrument consisted of mounting an adapter could be raised to around 10-9 A, which, owing to the high flange to the spectrometer, to which the GD source could be transmission of magnetic field analysers, is more than an order attached after taking away the interface that is only used for of magnitude higher than with the corresponding source of a ICP operation.A small modification to extend the extraction quadrupole instrument, as can be seen from the results of a lens by about 10 mm, by use of a stainless steel tube, was previous investigation.30 necessary for maximum ion transmission.Initially the instrument was only equipped with a discrete The new design principle provides a further striking improvedynode secondary electron multiplier for operation in the ment. In comparison with the former design, the time required counting mode. Owing to the very high counting rates of for sample changing is reduced from about 10 min to less than matrix signals, an analogue mode operating at significantly 30 s, which enhances sample throughput significantly.Sample reduced multiplier voltage has additionally been provided, throughput is additionally favoured by the fact that source which could also be selected by manual switching. This ana- cleaning can be performed off-line and does not require venting logue mode was used throughout this investigation if not of the system. otherwise mentioned.Data acquisition can be performed in a scan mode as well as in a peak jumping mode. For the latter, the integration time was 0.1 s per data point, and four data points per isotope were taken at the top of the peak. In the present instrument, the mass resolution can be varied continuously, but for this investigation the instrument was always operated in a low resolution mode (LRM) with R=300 or a high resolution mode (HRM) with R=3000, which was sufficient for the elements investigated. For the latter operating mode, an accurate mass calibration must be assured, which has to be controlled daily.It should be mentioned here that several limitations are specific to this instrument as it was when the work was performed and do not correspond to the actual state-of-theart of the commercial instrument, and the same holds true for the software. Fig. 2 Mass spectrum of the main components of a steel sample with Samples logarithmic intensity scale (sample: NIST 1261; discharge pressure: For analytical assessment of the performance of the source, 390 Pa; discharge power: 3 W; certified concentration values: Co 320 mg g-1, Ni 20.000 mg g-1, Cu 420 mg g-1).NIST Low Alloy Steels SRMs (disk and rod forms) from the Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 153Table 2 Short term RSDint. Sample NIST SRM 1262; discharge Stability and Reproducibility pressure 550 Pa; discharge voltage 1.000 V; discharge current 6 mA; The most important experiment for the study of the perform- preburning time 8 min ance of an interchangeable analytical ion source is a compari- Element Isotope ccert*/mg g-1 RSDint (%) son of the reproducilibity of the intensity measurements (‘internal reproducilibity’) and that of the analysis as a whole Ti 46 850 0.57 47 0.84 when sample changing is included in the reproduced sequence 48 0.23 (‘external reproducibility’).The former reflects the stability of 49 0.45 the analytical signal when the sytem is in continuous operation Cu 63 5.100 0.33 and the latter reflects additionally the influences that are in 65 0.68 any way connected with insertion of the sample, particularly Zr 90 2.000 0.15 the reproducibility of the positioning of the sample and source. 91 0.49 W 182 2.000 0.36 For measurement of the reproducibilities represented by the 183 0.68 corresponding relative standard deviations, RSDint and RSDext, 184 0.49 the operating conditions discharge pressure and discharge 186 0.46 voltage with respect to current were chosen according to previous experience with optimization.A preburning time of * ccert=total element concentration. 8 min was appropriate for equilibration. To determine RSDint , the intensity measurements of 20 repetitive scans in the peak overcoming drift effects, which are not compensated for by use jumping mode were evaluated; for RSDext the whole procedure of an internal standard if the scan delay time between the was repeated six times, each time including sample preparation element in question and the internal standard is too long.It by grinding with SiC paper and cleaning of the source and the should be mentioned that for the concentration range covered ion exit aperture. The instrument was operated in the analogue even better precisions, down to 0.1%, could be obtained for mode in order to enable application of an internal standard, isotope ratio measurements, if special care was taken for here 58Fe, which is a prerequisite to compensate for drift effects.optimization of the operating conditions.47,48 The results are compiled in Table 1. In addition to both RSDs A discussion of these statistical results has of course to take the concentration values from the certificate are also provided. into account the question of inhomogeneities of the elements With a value of about 1% for most elements, RSDint is slightly in the sample, because GDMS is a material consuming method.improved in comparison with the quadrupole instrument, and It can only be argued that lower values of RSDint for some has comparable integration times. This demonstrates that the elements are an indication of a more homogeneous distribution short term stability over a measurement cycle is mainly deter- for a depth scale of up to 1 mm, corresponding to erosion of mined by the source and not by further instrumentation. the sample over the measurement time.On considering RSDext, Considering RSDext, almost identical values of only about 2% however, repetition of the sample preparation procedure could be realized for the majority of elements, for instance V, removes another 100 mm from the surface by turning off and Co, Ni and Ta. This demonstrates that the reproducibility of grinding, so that local micro-inhomogeneities of distinct layers, the source position is satisfactory. For some elements such as which do not become apparent in continuous sputtering, now C, Nb and W, RSDext is worse by a factor of up to three.contribute to the scatter of the results. From Table 1 it can be The measurements discussed above were performed by seen that RSDext is not satisfactory for the elements Si, S and repetitive scanning of the whole m/z range, as is usually done P, with significant deviation from RSDint. The effects of with magnetic field instruments, whereas multiple ion detection inhomogeneities are to be suspected, as they have been specifi- by peak hopping is standard for quadrupole instruments.cally discussed for the set of steel samples under investigation Therefore, RSDint was also determined in this mode of for instance by Saito,49 who with the same set of samples in operation, repeating the intensity measurement scan over the GDMS analysis with a VG 9000 instrument obtained compar- interval of a selected m/z line for an isotope and then hopping able RSDext values in the range 1–3% for most elements.Also, to the next, so that for a single element the influence of stability Si, P, S and As with RSDext values of up to 8.4% were the covers only a very short time interval. The results are compiled only exceptions and this is in good agreement with the results in Table 2. In this mode of operation, RSDint representing the presented here. Therefore it could be concluded, that for a precision that can be realized in isotope ratio measurements, number of elements the main limitations to internal and was obtained in the range 0.3–0.8% without application of an external precision are set by the sample, and not primarily by internal standard, and this is a significant improvement in the source.Inhomogeneously distributed elements are generally comparison with the scan mode as discussed previously. This a limitation with analytical techniques of low material con- demonstrates that fast peak jumping, which is usually not sumption such as GDMS, thus worsening the accuracy of the applicable with double focusing instruments, can be realized analytical technique, as has been discussed in more detail with the present instrument. This is clearly preferable for elsewhere.50,51 Table 1 Comparison of RSDint and RSDext.Sample NIST SRM 1262; Optimization discharge pressure 550 Pa; discharge voltage 1.000 V; discharge current, 6 mA; preburning time 8 min The main parameters influencing optimization of the operating conditions are the gas pressure and the choice of the electrical Element Isotope ccert/mg g-1 RSDint (%) RSDext (%) parameters of the discharge.The influence of the burning C 12 1.630 6.7 5.5 voltage has been investigated over the range 600–1200 V. It Si 30 4.000 12.6 14.5 can be summarized from the measurements that the absolute P 31 440 2.2 9.2 intensity of the matrix increases with the voltage owing to an S 32 370 1.4 10.0 increase in the erosion of the sample, but the relative intensities V 51 410 1.5 1.9 Co 59 3.000 0.9 1.0 ratioed to the matrix are almost unaffected for most elements.Ni 60 6.000 1.1 1.8 This demonstrates that higher power dissipation increases the Nb 93 3.000 1.6 4.5 sputter rate without any significant change in the contributing Ta 181 2.100 1.2 2.4 mechanisms and processes. W 184 2.000 1.2 5.7 Some elements, for which Si is a typical example, show a 154 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12decreasing sensitivity with increasing voltage.For operation in Relative Element Sensitivities the LRM, this must be ascribed to contributions from spectral Within the framework of this investigation, a variety of element interferences. This was demonstrated by also measuring the sensitivities have been measured, a survey of which is given in voltage dependence of Si in the HRM, as shown in Fig. 3 for Fig. 5 for both instruments. For all measurements, the the main isotope and its interferents at the same nominal m/z operating conditions were chosen so as to realize optimum value using a very pure Fe sample (SUS 87).Besides 28Si+, analytical performance and were only slightly different. The three additional ion species appear clearly resolved and can resulting graph as presented in Fig. 5 is also known as the be identified by their m/z values as 56Fe2+, 12C16O+ and ‘response curve’. In general both instruments show the same 14N2+.Determination of the relative intensity for Si with high response. A certain decrease in sensitivities appears for the resolution leads to the usual result of a constant value with quadrupole instrument in the upper m/z range,52,53 representing increasing voltage. The interfering species show different behav- the known transmission loss of quadrupoles for higher m/z iour: the doubly charged matrix ion 56Fe2+ as the most intense values. Our measurements indicate that the response is pre- interfering species also increases with voltage while 12C16O+ dominantly determined by ion generation and ion extraction, shows decreasing intensity and the intensity of 14N2+ is whereas the subsequent equipment, including the ion transfer constant.Decreasing the voltage again, the final intensity of optics, energy analyser and mass analyser, does not lead to 12C16O+ will always be lower, from which it must be concluded any severe differences in the mass response.that this species is sputtered from the surface of the sample. It should be mentioned that for preparation of these data, The results obtained for optimization of the pressure with Co was chosen as the internal standard. This enabled inclusion the double focusing instrument in the HRM are shown in of results from the analysis of Cu samples. As can be seen Fig. 4 for some selected elements. In this case the relative from the dependence of the RSFs on the atomic number, the sensitivity factors (RSF) have been determined (defined as in trend is the same for both matrices, although their physical ref. 34). These are the intensity ratios of an isotope of the properties, in particular the sputter rates, differ significantly. element and an isotope of the chosen internal standard with This demonstrates once more that matrix effects are not correction for the isotopic abundances. (One should be aware pronounced in GDMS. that the inverse definition can also be found in the literature.) To give an estimation of the detection limits, measurements Higher relative sensitivities can be obtained at lower pressures, with low mass resolution in the counting mode for the m/z but the variation range of RSF values is narrower in a high region of the Pb isotopes of major abundance and of Bi are pressure region.This is in agreement with results which were shown in Fig. 6. For NIST SRM 1261, the Bi content is obtained with GD-QMS in previous work.44 certified as 4 mg g-1, and the certificate contains a value of about 250 ng g-1 for Pb.Therefore, the sensitivity could be calculated from the absolute intensities as about 2 counts s-1 per ng g-1. With respect to a background noise level below Fig. 3 Analytical signals in the m/z region of 28Si for different Fig. 5 Comparison of RSFs as a function of atomic number with discharge voltages: 1, 1000; 2, 800; and 3, 600 V. Sample, SUS B 87; different matrices and different instruments: (1, Fe matrix, GD-HRMS; discharge pressure 500 Pa; mass resolution: 3.000. 2, Fe matrix, GD-QMS; 3, Cu matrix, GD-HRMS. Discharge pressure 350 Pa. Fig. 4 Pressure dependence of RSFs for selected isotopes: 1, 51V; 2, Fig. 6 Mass spectrum in the region of isotopes 206Pb, 207Pb and 208Pb and of the 209Bi isotope. Sample, NIST SRM 1261; content, Pb 98Mo; 3, 53Cr; 4, 63Cu; 5, 59Co; and 6, 31P. Sample 401; discharge voltage 1.000 V. 250 ng g-1, Bi 4 mg g-1; peak intensity of Bi 8000 counts s-1.Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 1550.1 counts s-1 for real life samples, the detection limits can be estimated to be in the sub-ng g-1 region. For a comparison of the detection limits with those reported for the VG 9000 instrument, which is well established for many analytical applications, one should keep in mind that the aim was not to compete with this dedicated single-purpose high resolution GDMS instrument, but to develop a GD source with comparable analytical figures of merit that can, if required, be coupled without major modification to a high resolution ICP-MS instrument in order to enhance its versatility considerably.In addition, it should also be mentioned that a GD ion source has recently been introduced by the manufacturer of this ICP-MS instrument. Equilibration The preburning time, which is necessary in GDMS analysis to reach sputter equilibrium, is an important parameter of operation in order to make sure that element determinations are derived from constant analytical signals.Again high mass resolution may be helpful to obtain a more detailed insight into the processes going on. As an example, Fig. 7 shows a time dependent measurement of RSFs. The measurements were performed in the peak jumping mode with an integration time of 100 ms per data point and one data point per peak. About 200 s are necessary to approximate sputter equilibrium. During the preburning phase, the intensities may deviate considerably from the final value, not only due to any surface coatings or varying surface composition but above all due to Fig. 8 Analytical signals after different preburning times. Sample SUS preferential and selective sputter processes.54 The sputter rate 87; pressure 500 Pa; resolution 3.000. (a) m/z region of 28Si (1, of Cu, for instance, exceeds that of Fe significantly, and immediately after ignition; 2, 1 min; 3, 2 min; 4, 3 min; 5, 6 min; and 6, therefore Cu shows a signal decay, whereas elements with 13 min).(b) m/z region of 52Cr (1, 5 s; 2, 20 s; 3, 35 s; 4, 50 s; 5, 65 s; lower sputter rates, such as V, show increasing sensitivity. and 6, 120 s) after ignition. However, in the case of Fig. 7, the results for the elements Si and Cr are not consistent with an interpretation on the basis decay for the nitrogen molecule is in comparison much weaker, of sputter rates.It is of course obvious to suggest that this and the signal can be attributed, at least to a certain extent, must be attributed to interferences, and this suggestion can to the bulk concentration of nitrogen in this sample, which is now easily be checked by application of HMR. Corresponding certified to about 3 mg g-1. Simultaneously a certain increase results obtained for a pure Fe sample with fast scanning are in the doubly charged species of the matrix element Fe can be shown in Fig. 8(a) for the m/z region of Si and in Fig. 8(b) for observed. This indicates that ionization is enhanced with the m/z region of Cr, representing the first 13 min after ignition increasing surface cleaning and disappearance of the signals of the discharge. for the molecular species. It should be mentioned that the These high resolution measurements clearly indicate that contributions from the molecular species arising from contami- the changes in the RSF values are indeed due to spectral nation in the source and the discharge gas can be significantly interferences which obscure the main isotopes of the elements reduced by application of cryogenic cooling of the source, in question.In the case of 28Si+, the molecular species 56Fe2+, which was not available in this investigation. Further 12C16O+ and 14N2+ once again contribute to the analytical approaches to overcome this problem for GD-QMS have been signal in the low resolution mode. In comparison with Fig. 3 discussed in the literature, of which the use of getters,55 pulsed 12C16O+ now shows a fast decay, confirming the suggestion discharges56 or source optimization53 are the most prominent. that this species arises from surface adsorption. The intensity However, the signals of interfering molecular species from contaminants are not too pronounced after the preburning phase, and application of HMR is in any case a much more effective means. The decay of the Si signal is a particular surprise, and a similar decay is observed for ArC+ as an interfering species for Cr, whereas the signal for Cr is almost constant even in this early phase of the discharge.Both observations are hints of surface contamination resulting from the sample preparation step, and indeed, as already mentioned, SiC was used as the grinding material. The SiC grains have diameters of up to 10 mm, and therefore it takes more than 10 min to remove these surface contaminations by sputtering, with erosion rates of 1 mm min-1 as chosen for this experiment.This demonstrates not only the efficiency of surface cleaning by sputter etching, but furthermore that high resolution analysis could be helpful to clarify possible contaminations from the sample preparation Fig. 7 Time dependent measurement of RSF during preburning phase procedure, which is of particular important in trace analysis. for some selected isotopes: 1, Si; 2, Cr; 3, Mo; 4, V; 5, P; 6, Cu.Discharge pressure, 500 Pa; discharge voltage, 1.000 V; sample 401. Additionally, these measurements demonstrate that the 156 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 1221 Raith, A., Hutton, R. C., and Huneke, J. C., J. Anal. At. Spectrom., advantages of GDMS with HMR are worthwhile also for 1993, 8, 867. surface and in-depth analysis, as investigated for a quadrupole 22 Coburn, J. W., Rev. Sci. Instrum., 1970, 41, 1219. instrument in previous work.42 23 Duckworth, D.C., and Marcus, R. K., Anal. Chem., 1989, 61, 1879. 24 Duckworth, D. C., and Marcus, R. K., J. Anal. At. Spectrom., 1992, 7, 711. CONCLUSION 25 Marcus, R. K., J. Anal. At. Spectrom., 1993, 8, 935. 26 Kim, H. J., Piepmeier, E. H., Beck, G. L., Brumbaugh, G. G., and A new GD ion source of the Grimm-type for operation with Farmer, O. T., III, Anal. Chem. 1990, 62, 1368. a double focusing ICP-MS instrument has been developed. 27 Shao, Y., and Horlick, G., Spectrochim.Acta, Part B, 1991, 46, 165. Fast sample changing is achieved by an insertion probe to 28 Jakubowski, N., Stuewer, D., and Toelg, G., Int. J. Mass Spectrom. which the source unit is attached. 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ISSN:0267-9477    

DOI:10.1039/a604136a

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Self-absorption in Quantitative Glow Discharge Emission Spectrometry ZDENE¡ K WEISS L ECO Corporation, 3000 L akeview Avenue, St. Joseph,MI 49085–2396, USA The emission intensities of five resonance atomic lines affected GD-OES for situations in which self-absorption lines with non-linear intensity responses are used. by self-absorption were investigated as a function of analyte concentration in a sample for a Grimm-type atomization/ excitation source operated in the dc mode in argon.Based on considerations of radiative transfer within the source, and THEORETICAL CONSIDERATIONS using the two-layer model and other approximations, the equation The model of matrix-independent emission yields used4,5 is based on two basic assumptions: (i ) the analyte atom density I E M=R E0 qMc E M exp (-bE qMc E M) in the discharge is proportional to the flux cEMqM of analyte atoms sputtered from the sample (cathode); and (ii) excitation was derived, linking the emission intensity I E M, the sputter rate conditions are independent of the sample (cathode) material, qM and the analyte concentration c E M in the matrix, where E provided that the discharge voltage and discharge current are is a particular element in a matrix M.The R E0 and bE both kept constant. constants resulting from fitting the experimental data to this For a description of intensity of a self-absorbed emission relationship were investigated as functions of GD operating line as a function of the concentration of the particular element conditions.It was shown that R E0 can be regarded as the in the sample, a simple two-layer model was used, as suggested (generalized ) emission yield. Dependence of the R E0 and bE by West and Human6 in their early investigations of line parameters on discharge operating conditions suggests that shapes originating from the Grimm-type GD source. In this they could reflect more fundamental processes occurring in the model, the light source is assumed to consist of a layer discharge.Comparison of bE factors for different lines, containing emitting as well as absorbing atoms close to the however, leads to results that are not explicable within the cathode (layer I), followed by a layer of absorbing atoms only adopted model. Methodology of quantitative GD-OES analysis (layer II). In Fig. 1, the geometry of this model is shown, was generalized to be applicable also to self-absorption lines where z and r are cylindrical coordinates, z=0 on the sample with non-linear intensity responses.surface and z=z0 on the plane separating both layers. Keywords: Glow discharge; emission spectrometry; self- Proportionality between the flux of the sputtered atoms W and absorption; excitation; emission yield; sputtering rate; multi- the atom density n(z,r) of the particular element in the discharge element calibration can be written as n(z,r)=g(z,r)W (2) Glow discharge optical emission spectrometry (GD-OES)1,2 with a conventional dc Grimm-type atomization/excitation The intensity of a specific emission line at the entrance slit source has been used extensively for bulk as well as depth of the spectrometer can be obtained by solving the equation profile analysis of conductive materials. Depth profiling appli- of radiative transfer: cations, in which very different matrices (i.e.the substrate and the coating) have to be analysed in a single analysis, have led dIv dz =evkvIv(z,r) (3) to a significant effort aimed at developing GD-OES as a quantitative multi-matrix method of analysis.3,4 Such a task requires a reliable model of the response of where v is the angular frequency, Iv(z,r) is the spectral density emission intensity as a function of sample composition.In of the beam at a distance z from the cathode and a distance r previous papers4,5 a quantification scheme based on the con- from the axis, with propagation parallel to the axis.The terms cept of matrix-independent emission yields has been described. ev, kv are the Einstein emission and absorption coefficients, In this approximation, emission intensities were believed to be linear functions of the quantity cEMqM, where cEM is the concentration of the particular element E in the matrix M, and qM is the sputtering rate of that matrix. The product cEMqM represents the flux of atoms of that element, entering the discharge: W=cEMqM (1) As mentioned in ref. 4, emission line intensity is a linear function of the atom flux cEMqM only for certain emission lines. For some frequently used resonance atomic lines, selfabsorption causes deviations from a linear intensity response. In the present paper, a theoretical description is given of the effect of self-absorption on the emission intensity of several analytically important AE lines and a model of matrixindependent emission yields is generalized accordingly, to give Fig. 1 Geometry of the two-layer model of the discharge.a methodological basis for multi-matrix analyses by dc Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 (159–164) 159respectively, defined by reciprocal atom flux and can be used as a measure of selfabsorption, i.e., for no self-absorption, b=0, and the more severe the self-absorption, the higher the value of b can be ev=hv 4p Akink(z,r)g(v, z,r) (4) expected to be. The pre-exponential factor in eqn. (10) is proportional to the line intensity at the end of the excitation kv=hv 4p Bikni(z,r)g(v, z,r) (5) zone (of layer I in the two-layer model) and, consequently, to the flux.This, together with eqn. (1), leads to the following Here Aki and Bik are Einstein A and B coefficients associated equation: with the transition k�i, ni(z,r) and nk(z,r) are the populations IEM=RE0qMcEM exp(-bEqMcEM) (11) of the levels k and i, g(v,z,r) is the line profile and h is the The proportionality constant RE0 corresponds to what was Planck constant/2p.Eqn. (5) holds for situations in which called the emission yield RE in refs. 4 and 5 for emission lines stimulated emission can be neglected, which is the case here. with linear intensity response: for bE=0, eqn. (11) will express According to the two-layer model, for the layer II the the direct proportionality between the intensity and the flux following can be written: cEMqM with the proportionality factor of RE0. Therefore, the nk(z,r)=0, ni (z,r)=n(z,r) for z>z0 (6) constant RE0 can be termed the emission yield and this definition will hold for both the ‘linear’ and the ‘self-absorption Substituting eqns.(2), (4), (5) and (6) into eqn. (3), eqn. (7) is affected’ lines. obtained: Eqn. (11) is used as a basic relationship, to describe the observed signal intensity of self-absorption lines in the present dIv dz =-hv 4p BikWg(z,r)g(v, z,r)Iv(z,r) (7) paper. A similar equation was suggested by Payling et al.,9 based on the assumption that the line has a Gaussian profile.Integrating this equation gives PIv(r) Iv(z0,r) dIv Iv =-hv 4p BikWP2 z0 g(z,r)g(v, z,r)dz (8) EXPERIMENTAL Relationship (11) was tested for five analytically important and emission lines listed below. All of the measurements were made on the LECO SA-2000 spectrometer. The LECO SA-2000 is a Iv(r)=Iv(z0,r) exp C-hv 4p BikWf (r,v)D (9) GD-OES system based on a 40 cm Paschen–Runge vacuum polychromator with a 2400 lines mm-1 holographic grating.A Grimm-type atomization/excitation source with a 4 mm where Iv(r) is the spectral density of the line in the light leaving the lamp and propagating along a distance r from the optical internal anode diameter, operating in the dc mode and using argon (99.995%) as a working gas was used for all experiments. axis. It is assumed that the focal distance of the lens or mirror imaging the source onto the spectrometer entrance slit is much The discharge voltage was stabilized electronically.The preset discharge current was maintained by changing the argon longer than the size of the area where absorption takes place. The term f (r,v) is the integral on the right-hand side of eqn. pressure, based on the feedback loop with the PID valve as a pressure control device. (8). Further integration of the spectral density in eqn. (9) is not possible because of the unknown functions f (r,v) a In each series of experiments, the intensity of the investigated emission line was recorded for a set of different samples, while Iv(z0,r).However, as functionsof v, these functions are strongly localized close to the line centre v0. As functions of the radial keeping the discharge voltage and discharge current constant. The samples were selected to give different values for the flux coordinate r, they both have maxima on the axis (r=0) because the ‘normalized’ atom density of the sputtered atoms g(r,z) of analyte atoms entering the discharge.To check the validity of eqn. (11), ln(IEM/qMcEM) was plotted against qMcEM and the also has its maximum there7,8. Moreover, contributions from larger r are suppressed by imaging a circular source onto the constants RE0 and bE were calculated as the best linear regression fits. straight entrance slit. Consequently, substantial contributions to the total intensity can be expected from a narrow range of For investigations of Fe I 371.99, Ni I 341.48 and Cr I 425.43 nm lines, the samples listed in Table 1 were used.The both r and v. This is the basis for another approximation, assuming that the exponential dependence of Iv(r) on the flux emission intensities of the atomic lines of all certified elements were recorded. Together with these samples, NIST SRMs W will lead to an exponential dependence of the total intensity I on the flux W, for a certain range of actual line profiles (k is 1761–1768 Low Alloy Steels were measured under the same conditions. From the resulting data, relative sputtering rates a constant): of the samples from Table 1 with respect to pure iron were I=kI(z0) exp(-bW) (10) determined, using the multi-element calibration algorithm5 with the Cr II 267.716, Mn I 403.449, C I 165.701 nm and The constant b, defined by eqn.(10), has a dimension of Table 1 Samples used for investigations of Fe I 371.994, Ni I 341.477 and Cr I 425.433 nm lines (concentrations in % m/m) Sample Supplier* Al C Co Cr Cu Fe Mn Mo Ni Si Ti W BS-690 1 0.26 0.025 0.076 30.1 0.28 9.49 0.2 0.16 58.5 0.39 0.32 — JK-8F 2 — 0.0389 0.125 16.91 0.0523 67.1 1.55 2.775 11.01 0.424 — — 37A 3 0.03 0.13 0.015 4.27 0.13 94.1 0.46 0.46 0.1 0.25 0.004 0.015 38A 3 0.009 0.13 0.029 8.67 0.15 88.9 0.41 0.96 0.24 0.38 0.003 0.023 53A 3 0.12 0.063 0.11 15.14 0.02 7.62 0.32 0.05 76.12 0.15 0.25 0.01 58A 3 0.53 0.076 0.1 20.71 0.03 44.9 0.65 0.17 32.01 0.26 0.49 — 59A 3 0.05 0.02 0.25 22.12 1.71 30.8 0.33 2.68 40.91 0.1 0.83 0.13 6A 3 0.023 0.046 0.19 17.41 0.18 68.5 1.95 0.37 10.16 0.57 0.36 0.03 4A 3 0.01 0.068 0.1 25.48 0.3 51.3 1.69 0.15 20.05 0.53 0.006 0.05 * 1, Brammer Standard, Houston, TX, USA; 2, Swedish Institute for Metals Research, Stockholm, Sweden; and 3, Analytical Reference Materials International, Evergreen, CO, USA. 160 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Mo I 386.411 nm lines as reference signals and NIST SRMs The experiments described above were repeated for different 1761–1768 as the calibration basis.5 Multi-element calibration discharge conditions (voltages of 700 and 1200 V and currents is a non-linear regression algorithm for calculation of sputter of 10, 20 and 50 mA).Similar experiments were carried out to rate-corrected calibrations. Sputtering rate for a given sample evaluate the intensity response of the Al I 396.15 nm (Table 3) is calculated as a common multiplicative factor to concen- and Cu I 327.39 nm lines (Table 4). Instead of low-alloy steels, trations of certain elements within the sample, which makes Zn–Al and Cu matrix samples with known sputtering rates the intensity- and sputter rate-corrected concentration data relative to an iron matrix were used as reference samples in corresponding to that sample compatible with calibration the multi-element calibration (the calibration basis approach5 ).curves of all reference elements.Actual sputtering rates were The sensitivity of the photomultiplier detectors was kept calculated by multiplying the resulting values by the actual constant for the intensity measurements made under different sputtering rate of pure iron under the same operating con- discharge operating conditions for all the lines investigated, to ditions. To determine the sputtering rate of iron, an elec- be able to compare intensities under these different discharge trodeposited Fe-on-steel (Fe-3209) thickness standard conditions.(KOCOUR, Chicago, IL, USA) was depth profiled using the same discharge operating conditions as in the measurements above. Sputtering rate was calculated based on the time needed to reach the interface (see Fig. 2). The resulting sputtering rates RESULTS AND DISCUSSION are summarized in Table 2. In this way, it was possible to calculate the constants RE0 and bE for all the three lines (Fe I The resulting ln(IEM/qMcEM) versus qMcEM plots are in Figs. 371.994, Ni I 341.477 and Cr I 425.433 nm) from one series of 3–7. From these graphs, it can be concluded that eqn. (11) is measurements. a good approximation of the intensity response over a fairly wide range of GD operating conditions for all five lines examined. Deviations exist for the highest current and voltage (50 mA, 1200 V) for the Ni I 341.48, Al I 396.15 and the Cu I 327.39 nm lines. For the Cu I 327.39 nm line, deviation from the linear dependence was also found for 700 V, 50 mA (see Fig. 7). Values of RE0 and bE constants resulting from a linear regression are summarized in Tables 5 and 6. First, the emission yield will be discussed. No absolute intensity measurements were carried out, therefore only emission yields for each line separately can be compared. Emission yield as function of discharge conditions exhibits a similar pattern for all the lines investigated: it increases with discharge current and decreases with discharge voltage.For comparison, the emission yield of the Cr II 267.716 nm line, resulting from the same series of Fig. 2 Depth profile of minor elements in the Fe-3209 thickness experiments, was added to Table 5. Similar behaviour of the standard (9.9 mm thick electrodeposited layer of iron on steel), 700 V, emission yield for all the lines suggests that emission yield 20 mA. The position of the Cu peak at the interface was used to could possibly be related to more fundamental quantities determine the sputter rate of iron at different operating conditions.describing excitation, e.g., to the electron density in the This profile corresponds to a sputter rate of 3.0 mg s-1. excitation zone. As far as the parameter bE in the exponential term of eqn. Table 2 Sputtering rates of the samples from Table 1 (mg s-1) (11) is concerned, bE is significantly higher at 700 V than at 700 V 1200 V 1200 V for all lines investigated. One possible explanation within the two-layer model could be a possible voltage-related Sample 10 mA 20 mA 50 mA 10 mA 20 mA 50 mA change in the thickness of the excitation zone [layer I, par- BS-690 2.09 4.10 8.66 4.61 7.75 18.42 ameter z0 , see eqn.(8)]. Changes of bE with discharge current JK-8F 1.80 3.36 7.82 3.53 6.19 16.22 are far less pronounced. Because of the limited accuracy of 37A 1.47 2.98 6.33 3.23 5.55 13.33 the determination of bE , its dependence on current is not 38A 1.54 3.07 6.53 3.60 5.90 13.88 discussed here. 53A 2.09 4.10 9.37 4.51 — 18.56 58A 1.83 3.60 7.82 4.07 7.17 16.36 What can be done, however, is a comparison of the bE 59A 2.10 4.07 8.98 4.67 7.75 19.52 factors for different lines. Eqn. (9) together with the subsequent 6A 1.69 3.27 7.36 3.67 6.24 15.40 assumptions suggests that bE should be proportional to Bikv0. 4A 1.79 3.54 7.62 4.00 — 15.95 Designating the corresponding proportionality constant as k, Table 3 Samples used for investigations of the Al I 396.152 nm line (concentrations in % m/m) Sample Supplier* Si Al Ni Fe Mn Cu Pb Zn Mg Sn 629 4 0.078 5.15 0.008 0.017 0.0017 1.5 0.0135 93.1 0.094 0.012 631 4 0.001 0.5 0.0005 0.005 0.0002 0.0013 0.001 99.49 0.0005 0.001 1256A-7 4 9.16 84.2 0.41 0.91 0.38 3.51 0.11 1.02 0.063 0.1 NZA-5 5 — 10.85 — 0.016 — 1.04 0.0012 88.1 0.021 0.0017 NZA-7 5 — 13.17 — 0.016 — 0.212 0.0136 86.5 0.052 0.0116 NZA-1 5 — 28.7 — 0.046 — 1.51 0.003 69.7 0.02 0.0069 43XZ3D 6 — 3.6 0.002 0.08 0.03 1.36 0.012 94.7 0.12 0.02 41X0336Zn2 6 — 1.42 — 0.015 0.02 0.25 0.51 97.4 0.11 0.06 41X0336Zn3 6 — 0.54 — 0.05 0.01 0.33 0.042 98.6 0.09 0.11 * 4, National Institute of Standards and Technology, Gaithersburg, MD, USA; 5, CANMET, Ottawa, Ontario, Canada; and 6, MBH Analytical, Barnet, UK.Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 161Table 4 Samples used for investigations of the Cu I 327.396 nm line (concentrations in % m/m) Sample Supplier* Si Al Ni Fe Mn C Pb Zn Sn Cu 17868T 6 0.014 0.081 0.025 0.02 0.019 — 0.014 0.022 0.029 99.54 73A 3 0.003 0.01 0.06 0.3 0.01 0.009 2.94 34.78 0.2 61.7 78A 3 0.005 0.01 0.038 0.083 0.01 0.009 4.26 3.68 4.35 87.5 80A 3 0.022 9.89 4.85 4.01 0.23 0.013 0.007 0.18 0.02 80.8 87A 3 0.01 0.42 0.33 0.23 0.008 0.008 0.92 37.49 0.55 60.00 92A 3 0.005 0.01 0.36 0.01 0.01 0.012 9.58 0.27 9.75 79.6 93A 3 0.11 10.39 1.15 3.77 0.37 0.016 0.06 0.18 0.05 83.9 309B 7 0.13 12.65 1.71 0.86 1.02 — 0.05 0.22 0.53 82.6 52A 3 0.01 2.99 64.3 0.04 0.74 0.17 — — 0.0005 31.16 51A 3 0.26 0.05 66.0 2.07 1.49 0.13 0.001 — — 29.87 * 7, C¡ KD Research Institute, Prague, Czech Republic. 3, 6, see Tables 1 and 3. Fig. 3 Plots of ln(IEM/qMcEM) versus qMcEM for the Fe I 371.99 nm line under different GD operating conditions (samples from Table 1). Fig. 6 Plots of ln(IEM/qMcEM) versus qMcEM for the Al I 396.15 nm line under different GD operating conditions (samples from Table 3). Fig. 4 Plots of ln(IEM/qMcEM) versus qMcEM for the Ni I 341.48 nm line under different GD operating conditions (samples from Table 1).Fig. 7 Plots of ln(IEM/qMcEM) versus qMcEM for the Cu I 327.39 nm line under different GD operating conditions (samples from Table 4). Table 5 Emission yields resulting from the described experiments. For each line, ratios of RE0/RE0 (700 V, 20 mA) are displayed. For comparison, emission yields of the Cr II 267.716 nm line are presented in the last row 700 V 1200 V Line Wavelength/ nm 10 mA 20 mA 50 mA 10 mA 20 mA 50 mA Cr I 425.43 0.54 1.00 2.39 0.29 0.57 1.61 Ni I 341.48 — 1.00 1.71 — 0.83 1.27 Fe I 371.99 0.64 1.00 2.08 — 0.69 1.00 Cu I 327.39 — 1.00 2.29 0.21 0.55 1.36 Al I 396.15 — 1.00 3.03 — 0.51 1.44 Fig. 5 Plots of ln(IEM/qMcEM) versus qMcEM for the Cr I 425.43 nm Cr II 267.72 0.62 1.00 2.08 0.28 0.61 1.32 line under different GD operating conditions (samples from Table 1). 162 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Table 6 bE factors resulting from the described experiments (s mg-1) relationship, which is the basis of the methodology described in refs. 4 and 5: 700 V 1200 V Line Wavelength/ IEM=REqMcEM+BE+.F aEFIFM (15) nm 10 mA 20 mA 50 mA 10 mA 20 mA 50 mA where BE is the background and aEF is the inter-element Cr I 425.43 0.54 0.56 0.45 0.28 0.27 0.24 Ni I 341.48 — 0.10 0.08 — 0.07 (0.04) correction coefficient used for corrections of spectral inter- Fe I 371.99 0.16 0.14 0.09 — 0.05 0.04 ferences and other possible effects causing different back- Cu I 327.39 — 0.10 (0.06) 0.05 0.05 (0.03) grounds in different matrices.An analogous relationship for Al I 396.15 — 0.36 0.29 — 0.19 0.14 self-absorption lines would be IEM=RE0qMcEM exp(-bEqMcEM)+BE+. F aEFIFM . (16) eqn. (12) can be written: As opposed to what was suggested in ref. 4, eqn. (16) itself does not seem to be a suitable basis for calibration. The reason bE=k v0Bik ME (12) is that for an unknown sample, a non-linear algebraic equation would have to be solved to obtain qMcEM.Instead, calibration where ME is the relative atomic mass of the element E, and it can be based on the inverse relationship to eqn. (16). In corrects for the fact that the flux W in the above presented practical work, it is convenient to use a quadratic approxi- experimental data is expressed as mass sputtered per second, mation of the type instead of the number of atoms sputtered per second. Considering the relationship between the A and B coefficients10 qMcEM=IEM-BE RE0 +jE(IEM-BE)2+.F aEFIFM (17) Bik=p2c3 hv03 gk gi Aki (13) where jE is another constant to be obtained by calibration. To keep the quantification scheme as described in refs. 4 where gi and gk are the statistical weights of both levels, and 5 consistent, the lines with a non-linear intensity response should not be used as a basis for additive spectral interference bE=k¾ gk gi Akil02 ME (14) corrections.In the linearized version of the multi-element calibration scheme employing the calibration basis approach,5 is obtained where k¾ is another proportionality constant and self-absorption lines should not be used for sputter-rate deter- l0=2pc/v0 which is the wavelength of the line. Comparison minations, unless the range of qMcEM is small enough to make of experimental values of bE at 700 V and 20 mA with the the deviations from linear relationship negligible. Finally, it is corresponding quantity from the right-hand side of eqn.(14) worth mentioning that even for self-absorption lines, linear is in Table 7. Transition probabilities Aij and statistical weights calibrations are satisfactory for methods designed to cover gk and gi for the transitions discussed were taken from ref. 11. only a limited range of qMcEM for the particular element. This From Table 7, it is evident that there is no agreement with happens either if this element has to be determined only in eqn.(14). A more exact method of integration of eqns. (8) and low concentrations or if the only matrix to be analysed consists (9) should be used. One possible way would be to take the mostly of this element. An example of the second case is the atom densities n(z,r) resulting from computer simulations8 and analysis of nitrided or nitrocarburized steels. For such types to calculate the f (r,v) functions from eqn. (9). Line profiles in of analyses, satisfactory results can be obtained even if the Fe I the excitation zone g(v,z0,r) needed for these calculations could 371.99 nm line is used with linear calibration.possibly be measured using a side-viewed source similar to Previous papers4,5 together with the above suggestions that described in ref. 7. Recently, anomalous line profiles have describe a complete GD-OES quantification scheme, the most been reported for certain Fe I lines excited in the dc Grimm- important features of which are as follows. 1. In calibrations, type discharge.12 Clearly, line shape and width, including any intensities are considered to be functions of the product qMcEM. hyperfine structure, will affect the amount of self-absorption. 2. For the sputtering rate-corrected calibration to be con- Another question is adequacy of the two-layer model itself. It structed, it is not necessary to know a` priori sputtering rates is implicitly assumed that the line intensity leaving the first of all the standards used. 3. This approach does not support layer is proportional to the atomic flux, although some self- voltage- and current-intensity corrections. In the case of a absorption also occurs within the first layer. The above changing matrix, this scheme relies on the dynamic pressure reported inconsistencies indicate limitations of the proposed control, making it possible to keep the discharge current and self-absorption model. voltage constant and equal to the values used for calibration.The above reported results for bE and RE are not used for developing alternative intensity corrections. However, it is CONSEQUENCES FOR THE GD-OES interesting to discuss the correspondence of the present results METHODOLOGY with the model of Bengtson and co-workers.3,13 Assuming that The consequences of the above presented results to the method- the sputtering rate is directly proportional to discharge current ology of quantitative GD-OES analysis remains to be dis- and considering only the situations in which the exponential cussed.The matrix-independent emission yield concept leads, term in eqn. (16) can be neglected (low sputtering rates, low for lines with a linear intensity response, to the following concentrations), the emission yield ratio RE0/RE0(700 V, 20 mA) from Table 5 can be expressed as (i/i0)A(m)-1 in Bengtson’s notation, where i is the discharge current, i0 is the Table 7 Comparison of bE factors for different lines (see the text) reference current of 20 mA and A(m) is the parameter defined l0/ Aik/ bE (700V, 20mA)/ l02 gkAik/(giME )/ in ref. 13. Comparing the data from Table 5 with corresponding Line nm gi gk 108 s-1ME s mg-1 109 cm2 s-1 values predicted by the Bengtson formula, it can be concluded Cu I 327.396 2 2 1.37 63.54 0.10 2.31 that the agreement is very good (see Table 8). On the other Al I 396.152 4 2 0.98 26.98 0.36 2.85 hand, self-absorption seems to be violating the validity of Cr I 425.433 7 9 0.315 51.99 0.56 1.41 Bengtson’s corrections in some cases.As an example, the Fe I 371.994 9 11 0.162 55.85 0.14 0.49 emission intensity of the Cu I 327.39 nm line was measured as Ni I 341.477 7 9 0.55 58.71 0.10 1.40 a function of discharge current for two samples with a different Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 163Table 8 Comparison of emission yields with a discharge voltage of 700 V from Table 5 with the corresponding values predicted by the model of Bengtson et al.3, 13 (see the text) RE(10mA)/RE(20 mA) RE(10 mA)/RE(20 mA) A(m) Literature Literature Literature Line value* This work value* This work value* Cr I 425.43 nm 2.1±0.2 0.54 0.47±0.06 2.39 2.74±0.5 Ni I 341.48 nm 1.6±0.1 — — 1.71 1.73±0.17 Fe I 371.99 nm 1.8±0.2 0.64 0.62±0.08 2.08 1.90±0.4 Cu I 327.39 nm 2.0±0.2 — — 2.29 2.50±0.5 Al I 396.15 nm 2.2±0.2 — — 3.03 3.00±0.6 Cr II 267.72 nm 1.6±0.2 0.62 0.66± 0.09 2.08 1.73±0.35 * From refs. 3 and 13. Jones,14 who suggested that argon pressure is the key parameter controlling the excitation of sputtered atoms. It was shown that RE0 can be regarded as the (generalized) emission yield. Emission yield as a function of discharge conditions increases with discharge current and decreases with discharge voltage. The parameter bE in the exponential term of eqn. (11), characterizing self-absorption, is significantly higher at 700 V than at 1200 V for all lines investigated.The observed dependence of the RE0 and bE parameters on discharge operating conditions suggests that they could possibly be used for comparison of the above presented theoretical description of the discharge to more sophisticated microscopic models. Limitations of the proposed simplified self-absorption Fig. 8 Ratio of emission intensities I (74A)/I(102A) produced by model are apparent from the inconsistency found when samples 74A and 102A for the Cu I 327.39 nm line as a function of comparing the bE factors for different lines.the discharge current i (4 mm anode diameter, 700 V). Sample 74A is The methodology of quantitative GD-OES analysis was CuZn38 brass with 61.3% m/m Cu and sample 102A is AlCu4.5 alloy generalized, so that it could also be applicable to self- with 4.5% m/m Cu in an Al matrix with minor concentrations of Fe, absorption lines with non-linear intensity responses. Mn and Si (all below 1% m/m). Correspondence with the traditional approach as proposed by Bengtson and co-workers3,13 was discussed briefly.matrix and the ratio of the intensities produced by both samples was plotted as a function of the discharge current The author thanks the LECO Corporation for permission to (Fig. 8). If the intensity–current relationship were matrix- publish this paper and Dr. Arne Bengtson, Swedish Institute independent, the intensity ratio would be constant (a straight for Metals Research, Stockholm, Sweden, for his comments.line parallel to the abscissa in Fig. 8). The observed decrease in this ratio with current can be explained by a stronger self-absorption for sample 74A because it has a higher Cu REFERENCES concentration and a higher sputtering rate than sample 102A. 1 Grimm, W., Spectrochim. Acta, Part B, 1968, 23, 443. 2 Boumans, P. W. J. M., Anal. Chem., 1972, 44, 1219. 3 Bengtson, A., Spectrochim. Acta, Part B, 1994, 49, 411. CONCLUSIONS 4 Weiss, Z., J. Anal. At. Spectrom., 1995, 10, 891. 5 Weiss, Z., J. Anal. At. Spectrom., 1994, 9, 351. The emission intensities of five resonance atomic lines affected 6 West, C. D., Human, H. G. C., Spectrochim. Acta, Part B, 1976, by self-absorption were investigated as a function of analyte 31, 81. concentration in the sample for the Grimm-type atomization/ 7 Ferreira, N. P., Strauss, J. A., and Human, H. G. C., Spectrochim. excitation source operated in the dc mode in argon. Based on Acta, Part B, 1983, 38, 899. simplified considerations of radiative transfer within the source 8 Hoffmann, V., and Ehrlich, G., Spectrochim. Acta, Part B, 1995, 50, 607. and using the two-layer model and other approximations, an 9 Payling, R., Marychurch, M., Jones, D. G., and Dixon, A., paper equation [eqn. (11)] was derived, linking the emission intensity presented at the CSI Post-Symposium on GDS, Dresden, IEM, the sputter rate qM and the analyte concentration cEM in Germany, September 1–4, 1995. the matrix. The RE0 and bE constants resulting from fitting the 10 Thorne, A., Spectrophysics, Chapman and Hall, London, 1974. experimental data to this relationship were investigated as 11 Fuhr, J. R., and Wiese, W. L., in CRC Handbook of Chemistry functions of GD operating conditions. All series of experiments and Physics, ed. Lide, D. R., and Frederikse, H. P. R., CRC Press, Boca Raton, FL, 75th edn., 1995. were carried out with the discharge current and discharge 12 Steers, E. B. M., and Thorne, A., Fresenius’ J. Anal. Chem., 1996, voltage constant while changing the matrix being analysed. 355, 868. The fact that the results obtained in this way show a consistent 13 Bengtson, A., Eklund, A., Lundholm, M., and Saric, A., J. Anal. pattern, suggests that the discharge voltage and the discharge At. Spectrom., 1990, 5, 563. current are good parameters to characterize the discharge if 14 Payling, R., and Jones, D. G., Surf. Interface Anal., 1993, 20, 787. the analysed matrix is changing, i.e., it is likely that more fundamental quantities controlling the excitation are primarily Paper 6/03151J dependent on the current (current density) and the voltage. ReceivedMay 7, 1996 This finding is in disagreement with the work of Payling and Accepted July 17, 1996 164 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a603151j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Calibration Studies on Dried Aerosols for Laser Ablation–Inductively Coupled Plasma Mass Spectrometry† D. GU� NTHER*a , H. COUSINb, B. MAGYARb AND I. LEOPOLDc aSwiss Institute of T echnology Zurich, Institute for Isotope Geology, Sonneggstrasse 5, CH-8092 Zu�rich, Switzerland bSwiss Institute of T echnology Zurich, Institute of Inorganic Chemistry, Universita�tsstrasse 6, CH-8092 Zu� rich, Switzerland cInstitute of Plant Biochemistry, Weinberg 3, D-06120 Halle/Saale, Germany A simultaneous dried solution aerosol (Mistral nebulizer/ EXPERIMENTAL aerosol dryer) and laser-induced aerosol introduction system Instrumentation was used to investigate the calibration capabilities of dried solutions for LA–ICP-MS.Gas flow rates for the simultaneous A VG PQII+ instrument was used in combination with a Mistral nebulizer with a conventional Meinhard nebulizer (VG systems were optimized and gave the best results with 0.5 l min-1 for the laser gas flow rate and 0.5 l min-1 for the Elemental) and a LaserLab laser ablation unit (VG Elemental) The parameters used for the experiments are listed in Table 1.solution gas flow rate, at which a sensitivity of 65–80% for LA compared with 1.05 l min-1 single gas flow LA–ICP-MS The experimental set-up used for this investigation is shown schematically in Fig. 1. The connection piece for mixing the was maintained. The optimum temperatures for the Mistral nebulizer were 143°C ( heating) and -3 °C (cooling).Oxide two aerosols, mounted directly in front of the torch, was tested with angles a of 90° and 60°. The 90° connection piece showed formation (CeO+/Ce+) under these conditions is less than 0.3%. Introduction of enriched 207Pb (as a solution) and particle deposition inside the adapter glass surface. Therefore, the experiments were carried out with a 60° adapter, to reduce natural lead (via LA) allows the optimization of both sample introduction systems separately. Analyses were performed on memory effects from deposited particles.Optimization and tuning of the Mistral nebulizer were synthetic polyethylene materials, IAEA Soil-7 and CSB-1 reference standards. The RSDs on two sample pellets with five carried out with a 20 ng g-1 standard solution of Be, Co, Rh, In, Ce, Pb, Bi and U (1000 mg g-1 tune stock solutions; Merck, replicates each were better than 10%. Quantitative analyses for all REEs were based on In as the internal standard and Rh as the reference element.Fractionation effects of the internal Table 1 Instrumentation and parameters standard relative to the REE were not observed. ICP-MS— VG PlasmaQuad II+ Keywords: L aser ablation; inductively coupled plasma mass Power 1350 W spectrometry ; calibration ; aerosols Auxiliary gas flow rate 0.8 l min-1 Cool gas flow rate 13 l min-1 Carrier gas flow rate (laser) 0.5 l min-1 The use of standard solutions for calibration in LA–ICP-AES Carrier gas flow rate (liquid) 0.5 l min-1 and LA–ICP-MS is becoming an increasingly common pro- Sample uptake (constant) 1 ml min-1 cedure because of the simplicity of solution preparation for Acquisition mode Peak jumping the different applications. Sample introduction is based on the Points per peak 1 Dwell time 10 ms simultaneous introduction of a liquid aerosol and a laser- Detector mode Pulse induced aerosol, as described elsewhere.1–4 Results for LA–ICP- Preablation 10 s AES are not influenced by the use of a wet aerosol.2 However, Acquisition time 30 s the same technique in LA–ICP-MS leads to higher back- Replicates 5 grounds and more polyatomic interferences, e.g., ArO+, and L aser— VG LaserLab higher oxide formation, which, for REEs, are caused by the Laser type Nd5YAG, pulsed solvent.Laser wavelength 1064 nm The optimization procedure for LA–ICP-MS is greatly Laser mode Q-switched improved by a simultaneous sample introduction system with Flash lamp voltage 800 V Laser energy 0.2 J per shot-1 continuous solution sample uptake.The constant signals can Aerosol path length 1.5 m be used to optimize all gas flows, lens tuning and torchbox Ablation frequency 4 s-1 alignment (x, y, z). However, the tuning parameters are slightly different for laser-induced aerosols and have to be reoptimized on laser aerosol signals using a 4 s-1 repetition rate. This paper describes the optimization of gas flows, mixing behavior and tuning parameters for a simultaneous solution aerosol and laser-induced aerosol introduction system, in combination with a drying system (Mistral, VG Elemental, Winsford, UK) so as to achieve reduced oxide formation and associated polyatomic species formation.The applicability of this calibration technique is demonstrated for the determination of REEs in polyethylene and mixed reference materials. Fig. 1 Schematic diagram of a simultaneous ‘dried’ aerosol and laser- † Presented in part at the 1994 Winter Conference on Plasma Spectrochemistry, San Diego, CA, USA.induced aerosol introduction system (dual gas flow system). Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 (165–170) 165Darmstadt, Germany). The same element solutions were mixed for Mistral nebulization were added as 0.1 M nitric acid solutions. with polyethylene, dried, homogenized in a ball-mixer/mill, pressed to a pellet (blank=2 mg g-1) and used as synthetic Experiments for the determination of the transport efficiency of individual gas flows were carried out with enriched 207Pb test samples for the optimization procedure of the laser ablation system.solution (prepared from enriched 207Pb, 90.4%; Medgenix Diagnostics, Germany). Sample Preparation Reference soil samples and synthetic test materials were pre- RESULTS pared by the following procedure. Polyethylene powder (PE) Optimization of the Dual Gas System in spectroscopy-grade Uvasol (Merck) was used as a binder to prepare the pellets.The internal standard (In) and the For the characterization of a new dual gas flow system (combining laser-induced aerosols and Mistral ‘dry’ nebuliz- reference element (Rh) were added to each of the pellets. The PE mixture was homogenized in a Mixer/Mill in polystyrene ation), each individual system was separately optimized. The optimization procedure was divided into two stages: (a) measur- vials (3 in×1 in id), containing one 1/2 in and one 3/8 in methacrylate ball (8000 Spex Mixer/Mill, Spex Industries, ement of element isotopes for the determination of the sensitivity and (b) measurements of molecular species for the Edison, NJ, USA).The vials and balls were cleaned with 20% HNO3 and rinsed with high purity water (Milli-Q system, observation of changes in the plasma conditions. The optimum intensity for the single gas flow LA–ICP-MS measured on Millipore, Bedford, MA, USA). One blank, four sample pellets (two CSB-1 and two IAEA Soil-7) and five test material pellets 115In was determined as 1.05 l min-1.The optimum intensity for single gas flow Mistral nebulization ICP-MS on 115In was were prepared as described below. A 2.5 g amount of PE was weighed into a vial, with the following additions: (a) for a in the range 0.75–0.80 l min-1, depending on the heating temperature, and for single gas flow liquid nebulization it was blank pellet, 50 mg of 1000 mg g-1 Rh stock solution; (b) for sample pellets, 50 mg of IAEA Soil-7 and CBS-1 (dried at 0.8 l min-1.The results of the optimization are summarized in Figs. 2–5. 105 °C) and 50 mg of 1000 mg g-1 Rh stock solution; (c) for standard pellets (0–500 ml of the 10-fold diluted 100 mg ml-1 The isotopes 9Be, 59Co, 103Rh, 115In, 140Ce, 208Pb, 209Bi and 238U (to represent the whole mass range) and the molecular REE; Johnson Matthey Alfa Products, Royston, Hertfordshire, UK), 0.05–2 mg g-1, and 50 mg 1000 mg g-1 Rh stock solution; species 12C16O16O+, 14N16O16O+, 40Ar12C+, 40Ar14N+, 40Ar16O+ and 40Ar12C16O+ were measured.and (d) for tune pellets (2 mg g-1), 50 mg of tune stock solution (of Be, Mg, Co, In, Ce, Pb, Bi, U, 100 mg g-1, Merck) and Fig. 2(a) shows the absolute intensities of all three sample introduction systems usigle gas flow (1.05 l min-1 50 mg of 100 mg g-1 Rh stock solution. A 1.0 ml volume of ethanol (puriss p.a. grade, Fluka, Buchs, Switzerland) was then laser, 0.8 l min-1 Mistral, 0.8 l min-1 liquid).The introduction of two different aerosol gas flows is limited by a total carrier added and the mixture was dried in a vacuum oven at 60°C and 10 kPa for 2 h. The dry mixture was then homogenized gas flow rate of 1.3 l min-1. Higher gas flow rates lead to reduced excitation, lower ion transmission or, in the worst for 1 h in the mixer/mill. Pellets of about 1 g were pressed in a conventional pellet die for IR spectrometry at 10 t for 30 s.case, instrument shutdown. The absolute intensities of all isotopes (excluding molecular species) under all single gas flow The corresponding tune and standard solutions for Mistral and liquid nebulization were prepared from the above stock conditions (laser, Mistral, liquid) were within two orders of magnitude. solutions in 0.1 M HNO3 (Suprapur grade, Merck). The blanks Fig. 2 (a) Intensities for single gas flow optimization of laser, Mistral, and liquid aerosol.(b) Intensities for dual gas flow optimization of laser, Mistral, and liquid aerosol at 0.5 l min-1 (no added gas flow). 166 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Fig. 3 Mistral aerosol gas-flow tuning (aerosol-carrier flow 0.5 l min-1). A, U-238; B, Pb-208; C, Be-9; D, In-115; E, Co-59; F, ArN-54; G, ArO-56; H, ArCO-68; I, ArC-52. Fig. 4 Liquid nebulizer aerosol gas-flow tuning (aerosol-carrier flow 0.5 l min-1). Key as for Fig. 3. Fig. 5 Laser aerosol gas-flow tuning (aerosol-carrier flow 0.5 l min-1). Key as for Fig. 3. A lower flow rate of 0.5 l min-1 aerosol carrier gas (no (corresponding to a total argon gas flow of 0.5–1.20 l min-1). Intensities using the Mistral nebulization are improved by up added gas flow) is effectively the minimum possible total gas flow in the dual flow system [Fig. 2(b)], as carrier gas flow to a factor of 100 (above 0.3 l min-1 added gas flow, Fig. 3), whereas the liquid introduction intensities are improved by a rates below 0.5 l min-1 lead to S/B of <3 and significantly higher molecular interferences.The signal intensities for all factor of only 10 (at 0.2 l min-1 added, Fig. 4). The intensities of polyatomic species such as ArC+, ArN+, optimized dual flow systems are of the same order of magnitude and the reproducibility for five replicates was less than 5%. ArO+ and ArCO+ were measured under all experimental conditions as an indicator for (a) changes in excitation due to Because of their absolute intensities and their reproducibility, it was concluded that Mistral and laser were suitable for direct the secondary gas flow, (b) the formation of O species as a result of the remaining water content (Mistral) and (c) the comparison in the dual gas mode.The signal intensity is improved by a factor of 10 by Mistral nebulization compared influence of carbon from the ablated polyethylene matrix. The results shown in Figs. 3–5 suggest an optimum dual gas flow with liquid nebulization.The sensitivity of laser induced aerosols (test material pellets with 2 mg g-1) were three orders of rate of 0.5 l min-1 for laser and Mistral nebulization as gas flow rates above 0.5 l min-1 lead to lower isotope intensities. magnitude lower compared with the Mistral nebulization intensities [solution with 0.02 mg g-1, Fig. 2(b)]. Fig. 6 demonstrates the reduced formation of oxides (e.g., CeO+/Ce+, ArO+/In+, ArO+/Co+, CoO+/Co+) from single Figs. 3–5 illustrate the effect of a progressively measured dual flow for each dual introduction system (starting at a 0.5 liquid nebulization (A), relative to single Mistral nebulization (B–D), single laser ablation (E) and the optimized dual l min-1 aerosol flow) with added gas flows of 0–0.70 l min-1 for Mistral and laser, and 0–0.3 l min-1 for liquid nebulization Mistral–laser system (F). It was found that the excitation Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 167and 115In intensities on the dual gas flow system (with laser ablation and Mistral blank introduction). The intensities of the 103Rh and 115In signals measured with the dual gas system were normalized to their respective intensities at the optimum single gas flow laser ablation conditions (1.05 l min-1). The intensities of 65% for 115In and 80% for 103Rh indicate an increased loading of the carrier gas due to the lower sample velocity and increased residence time of the aerosol in the ablation cell.Calibration Method The first stage in the calibration procedure consisted of blank Fig. 6 Description of the dryness of the aerosol by comparing the nebulization and analyte and reference element calibration ratio of CeO+/Ce+ and ArO+/In+. (Mistral). Subsequently the samples were ablated and transported into the plasma together with the blank Mistral aerosol (nitric acid, 20 pg g-1 In). The calibration was based on the conditions for laser aerosol and Mistral aerosol are of the following equation: same order of magnitude. The efficiency of the Mistral nebulizer depends upon the heating and condensation temperature CA,solid CRh,solid = CA,laser CRh,laser (1) (B–D) and has to be optimized for each application.The slightly higher oxide formation in the dual gas flow system is probably due to air entrainment and the summation of oxide where CA,solid=unknown concentration, CRh,solid=reference forming species from both individual gas flows.element concentration, CA,laser= concentration determined The evaluation of the gas flow mixing behavior was studied using Mistral calibration and CRh,laser= concentration deterusing enriched 207Pb (99.7%) for Mistral nebulization and mined using Mistral calibration. With constant working parnatural Pb (207Pb, 22.08%) for laser ablation. Based on two ameters (e.g., excitation, gas flow rates, transportation volume), extreme points (100% Mistral flow, Pb207/Pb208=10.99±0.25; the Mistral calibration functions are 100% laser flow, Pb207/Pb208=0.426±0.045; Table 2, first two rows), the actual introduction rate was calculated assuming a IA Iln =f (CA , Cln=constant) (2) linear dependence of gas mixing (from the measured isotopic ratio Pb207/Pb208). The corresponding values are shown in Table 2 (last three rows).IRh Iln =f (CRh , C1n=constant) (3) The sample introduction efficiency for the dual system is always dominated by the Mistral nebulization (59.6–66%).A The concentrations of the laser-induced aerosols [required possible explanation of this may be the particle size distribution for eqn. (1)] can be directly determined using eqns. (2) and and the particle shape of the two different aerosols, such that (3). Indium (the internal standard) was always introduced as more particles might pass through the plasma without being Mistral dried aerosol and used for corrections of the dual completely dissociated and ionized.flow system. The drawback of the dual gas flow system is the theoretical The results of this calibration after subtracting blanks are decrease in sensitivity by a factor of 2 (0.5 l min-1 Mistral and given in Table 3. Where the concentrations are below 0.5 0.5 l min-1 laser). Fig. 7 shows the dependence of the 103Rh mg g-1 they show high deviations from the theoretical values, which depend on the amount of laser ablated material introduced into the plasma; where the concentrations are higher Table 2 Sample introduction rate of single and dual gas flow systems than 0.5 mg g-1, the intensities are equivalent to concentrations Response of about 1 ng g-1 on the Mistral calibration curves.The Ar flow/ ratio, M accuracy and reproducibility can be improved by higher sensi- Mode l min-1 Tuned* Pb207/Pb208 (%) tivity. The results in Table 3 also show that Rh has the same Mistral 0.79 M 10.99 100 ablation behavior as the REEs; fractionation as described by Laser 1.05 L 0.426 0 Longerich et al.5 and Fryer et al.6 was not observed.The Mistral/Laser 0.5/0.5 M 6.3 59.6 difference in behavior could be related to dilution of the sample Mistral/Laser 0.5/0.5 L 6.7 63.5 and/or the matrix. Mistral/Laser 0.5/0.5 M/L 6.91 66 The calibration technique presented in this paper was compared with the external calibration technique using pressed * M=tuned on Mistral, L=tuned on laser sample introduction.Fig. 7 Signal reduction using a dual gas flow system (total gas flow 0.5–1.25 l min-1). 168 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Table 3 Determination of REEs (mg g-1) in synthetic test samples and 0.5 l min-1 laser) lead to better sensitivities and lower using Mistral calibration molecular interferences (e.g., oxide formation) compared with a dual system with liquid nebulization.3 The decrease in Element REE-1 REE-2 REE-3 REE-4 REE-5 intensity compared with single gas flow systems is less than Theoretical values 0.051 0.1 0.51 1.02 2.01 35%.The tuning procedure and the sample introduction Yttrium 0.056 0.1 0.5 0.99 1.8 optimization are greatly improved by a dual system owing to Lanthanum 0.042 0.09 0.5 0.95 1.8 the constant sample uptake of the Mistral nebulizer. The use Cerium 0.069 0.12 0.49 0.93 1.85 of dried aerosols for tuning the plasma is one of the most Neodymium 0.089 0.17 0.53 0.98 1.82 suitable procedures so far investigated to set up laser ablation Samarium 0.081 0.12 0.54 0.95 1.83 conditions.Europium 0.071 0.12 0.5 0.94 1.79 The higher RSDs for Mistral nebulization are mainly the Gadolinium 0.075 0.13 0.52 1.02 1.87 Terbium 0.078 0.14 0.52 1.04 1.93 result of poor stability of the nebulizer. Improvement of the Dysprosium 0.069 0.11 0.52 0.99 1.87 Mistral unit, currently being implemented by the manufacturer, Holmium 0.081 0.14 0.53 0.98 1.91 especially to the drain, should lead to increased signal stability Erbium 0.053 0.09 0.5 0.99 1.96 and, therefore, to better SBRs and, consequently, higher Thulium 0.067 0.13 0.54 1.01 2.04 sensitivities.Ytterbium 0.084 0.15 0.57 1.03 2.08 The dual gas flow technique can be extended to other Lutetium 0.079 0.14 0.61 1.12 2.2 samples, e.g., minerals with stoichiometric composition, trace element determinations in raw materials and synthetic ceramic materials. However, such applications would need more pellets with an REE standard solution as described elsewere7 detailed investigations of the fractionation process and of using the reference materials IAEA Soil-7 and CSB-1, prepared suitable reference elements.More effective desolvation units as PE pellets. The results are summarized in Tables 4 and detailed studies of air entrainment from the plasma and 5. environment (bonnet around the torch9) could lead to more sensitive dual gas flow systems. CONCLUSIONS The dual gas flow system with Mistral nebulization (dry The authors are grateful to VG Elemental (Mainz-Kastel, aerosols) is an alternative calibration method for LA–ICP-MS Germany) for the loan of the Mistral nebulization unit for solid analyses.The dual gas flow optimization carried out in this study shows that equivalent flow rates (0.5 l min-1 Mistral this study. Table 4 Determination of REEs (mg g-1) in a reference soil (IAEA Soil-7) IAEA Soil-7 Mistral calibration External calibration Element Mean CI Mean±CI95% Mean±CI95% Yttrium 21 15–27 25.7±5.3 23.8±0.4 Lanthanum 28 27–29 32.3±9.6 32.5±2.3 Cerium 61 50–63 68.9±16.8 69.2±3.4 Praseodymium 15.3±4.0 13.4±0.6 Neodymium 30 22–34 34.4±10.6 31.0±1.0 Samarium 5.1 4.8–5.5 6.6±1.5 5.5±0.5 Europium 1.0 0.9–1.3 1.39±0.28 1.02±0.07 Gadolinium 5.6±0.97 4.3±0.3 Terbium 0.6 0.5–0.9 0.92±0.17 0.66±0.01 Dysprosium 3.9 3.2–5.3 5.2±0.98 4.4±0.2 Holmium 1.1 0.8–1.5 1.09±0.24 0.87±0.04 Erbium 3.06±0.57 2.5±0.1 Thulium 0.48±0.08 0.4±0.1 Ytterbium 2.4 1.9–2.6 2.71±0.56 2.3±0.2 Lutetium (0.3) (0.1–0.4) 0.48±0.08 0.28±0.02 Table 5 Determination of REEs (mg g-1) in a reference bentonite (CBS-1)8 Bentonite CSB-1 Mistral calibration External calibration Element mean mean±CI95% mean±CI95% Yttrium 34 33±12 37±3 Lanthanum 58 52±11 63±5 Cerium 122 127±27 174±20 Praseodymium 28±6 19±4 Neodymium 53.3 52±13 61±6 Samarium 13.1 11±3 14.5±1.3 Europium 0.73 0.73±0.11 1.64±0.25 Gadolinium 11.3 8.5±2.2 10.3±0.9 Terbium 1.34 1.4±0.3 1.55±0.17 Dysprosium 8.0±2.4 9.9±0.7 Holmium 1.5±0.4 2.30±0.23 Erbium 4.3±1.5 5.4±0.9 Thulium 0.74 0.73±0.20 1.13±0.13 Ytterbium 4.1 4.0±1.6 5.6±0.8 Lutetium 0.59 0.64±0.17 0.52±0.11 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 1697 Magyar, B., and Cousin, H., Mikrochim. Acta, 1994, 113, 313. REFERENCES 8 Hosterman, J. W., and Flanagan F. J., Geostand. Newsl., 1987, 11, 1. 1 Thompson, M., Chenery, S., and Brett, L., J. Anal. At. Spectrom., 9 Ince, A. T., Williams, J. G., and Gray, A. L., J. Anal. At. Spectrom., 1989, 4, 11. 1993, 8, 899. 2 Moenke, L., Ga�ckle, M., Gu�nther, D., and Kammel, J., Spec. Publ. R. Soc. Chem., 1990, No. 85. Paper 6/04531F 3 Chenery, S., and Cook, J. M., J. Anal. At. Spectrom., 1993, 8, 299. Received July 7, 1996 4 Cromwell, E., and Arrowsmith, P., Anal. Chem., 1995, 67, 131. Accepted September 16, 1996 5 Longerich, H. P., Gu�nther, D., and Jackson, S. E., Fresenius’ J. Anal. Chem., 1996, 355, 538. 6 Fryer, B. J., Jackson, S. E., and Longerich, H. P., Can. Mineral., 1995, 33, 303. 170 Journal of Analytical Atomic Spectrometry, February 1997, V

ISSN:0267-9477    

DOI:10.1039/a604531f

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Precise Measurement of Ion Ratios in Solid Samples Using Laser Ablation With a Twin Quadrupole Inductively Coupled Plasma Mass Spectrometer LLOYD A. ALLEN, JAMES J. LEACH, HO-MING PANG AND R. S. HOUK* Ames L aboratory±US Department of Energy, Department of Chemistry, Iowa State University, Ames, IA 50011, USA Laser ablation (LA) is used with steel samples to assess the mode, a precision of 0.04% RSD was reported for the measureability of a twin quadrupole inductively coupled plasma mass ment of Pb and Mg isotope ratios.spectrometer to eliminate Øicker noise. Isotopic and internal A great deal of work has also focused on alternative mass standard ratios are measured in the Ærst part of this work. spectrometers for elemental analysis. Hieftje and co-workers4±8 Results indicate that Øicker noise cancels and signiÆcant have used a time of Øight (TOF) mass analyser for the very improvements in precision are possible. The isotope ratio rapid detection of an entire mass spectrum.This system has 52Cr+553Cr+ can be measured with a relative standard been used in both GDMS5,6 and ICP-MS4,7,8 experiments. deviation (RSD) of 0.06±0.1%, depending on the dwell time The best ratio precisions reported for this ICP-TOF-MS device and averaging method used. The level of noise above the shot are 0.46% RSD for steady state signals during solution nebuliznoise limit is greater in internal standard measurements than ation7 and 1.6% RSD for transient signals generated by LA when doing isotope ratio measurements.Nevertheless, RSDs with single shots.8 Koppenaal and co-workers9,10 introduced improve from 1.9% in the Cr+ signal to 0.12% for the ratio of an ion trap MS instrument in which ions from an ICP are 51V+ to 52Cr+ in a steel standard reference material (SRM). trapped and later detected. In the second part of this work, one mass spectrometer is Finally, Walder and co-workers11±13 described an ICP-MS scanned while the second channel measures an individual m/z instrument with magnetic sector mass analyser and multiple value.When the ratio of these two signals is calculated, the Faraday detectors. Lee and Halliday used this device to obtain peak shapes in the mass spectrum are improved signiÆcantly accurate relative atomic masses of some elements.14 This for a wide range of elements. This technique corrects for instrument is capable of simultaneous measurement of up to Øicker noise from the LA process while scanning a mass nine adjacent m/z values.Isotope ratios can be measured with spectrum for multi-element determinations. very high precision (0.01% RSD) even when a noisy sample introduction method such as LA is used. The ICP multicollec- Keywords: L aser ablation; inductively coupled plasma mass tor (MC) MS has recently been used with LA to measure spectrometry; internal standard; isotope ratio; solids analysis isotope ratios for Sr in feldspar15 and Hf in zircons.16 The precision of these isotope ratio measurements approaches that of TIMS.LA-ICP-MCMS has the additional advantages of ICP-MS has become a major force in elemental MS in the spatial resolution and minimal sample preparation. past several years. The simplicity of the spectrum and the These MC studies in ICP-MS and other ICP emission speed of analysis make it an attractive technique. ICP-MS studies with multichannel detection17 have shown that simul- does, however, have several limitations.The usual quadrupole taneous ratio measurements correct for most of the Øicker mass analyser is a sequential or scanning device, which can noise in the ICP. In the last two years, Houk and limit the precision of internal standard and isotope ratio co-workers18,19 have developed a twin quadrupole instrument measurements. It is desirable when doing these type of measure- that simultaneously detects ions produced from an ICP (Fig. 1). ments to use long dwell times in order to increase the total This instrument splits the ion beam into two parts.Each part number of ions measured, to reduce the effects of shot noise. is then sent to its own quadrupole mass analyser and detector. However, when using a typical quadrupole system, the mass The eventual objective of this project is to produce a device analyser must be scanned rapidly or peak hopped for multi- that is capable of high precision ratio measurements but is mass measurements to reduce the effects of Øicker noise much smaller and less expensive than the ICP-MCMS device introduced by the plasma and sample introduction system.and easier to use for measurements at widely different m/z Such fast scanning or peak hopping limits the amount of signal values. obtained and therefore runs the risk of propagating shot noise The present paper presents results for LA of two steel SRMs in the measurement. Begley and Sharp1 discussed these effects using the twin quadrupole ICP-MS instrument.The poor and described a detailed procedure for optimizing precision precision of LA-ICP-MS is one of its main limitations. Because for isotope ratio measurements. of the erratic nature of the ablation process, the noise level on There are presently two basic types of scanning, single- the signal is signiÆcant. For this reason, LA-ICP-MS with a channel ICP-MS devices: quadrupole or magnetic sector. Over conventional single quadrupole instrument is not commonly the last several years, the precision and stability of commercial used for isotope ratio measurements.quadrupole ICP-MS devices have been improved to the point The emphasis of much of the work that has been done using where isotope ratios can be measured with an RSD of 0.1% LA-ICP-MS for isotope ratio measurement is in the Æeld of (sometimes better), in cases where at least 106 ions are detected geochemistry. Lead isotope ratios have been measured in for the minor isotope.This level of precision is obtained for zircon grains with a precision of #0.5%.20 Using LA-ICP-MS, steady state signals resulting from continuous nebulization of a close correlation between the experimentally measured pre- solutions.1,2 Precise measurement of isotope ratios using a cision of the isotope ratio and the precision predicted from scanning, double-focusing magnetic sector instrument have recently been reported.3 Using the device in the low resolution counting statistics has been reported for the measurement of Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 (171±176) 171different signal levels while keeping shot noise to a reasonable limit. This does mean, however, that a calibration curve for accurate measurement of isotope and ion ratios would be required as is typical for isotope ratio measurements with commercial ICP-MS instruments. The absolute value of the split ratio can be altered by applying different voltages to the beam shift plates before the splitter.The ICP-MS operating conditions are listed in Table 1. Some care is taken to match the behavior of the two electron multipliers. The voltage applied to each detector is chosen such that the measured count rate does not change substantially as the detector voltage is altered slightly. This procedure ensures that small, independent Øuctuations in the voltage output of each power supply do not affect the measured ratios greatly.The sampling position, ion lens potentials and aerosol gas Øow rate were optimized to yield the best precision for 52Cr+552Cr+ for NIST SRM 1263 (Low Alloy Steel Cr-V) and 51V+551V+ for NIST SRM 1767 (Low Alloy Steel). The voltages on the beam shift plates were modiÆed slightly to increase the signal for minor isotopes or small concentrations in some cases. Fig. 1 Schematic diagram of the ICP twin quadrupole device. An Laser Ablation Conditions approximate scale is given and typical ion lens potentials, in volts, are listed.An Nd5YAG laser (Model NY 82-30, Continuum) was frequency doubled to yield a beam at 532 nm. The laser was pulsed at 15 Hz and a steady state signal was produced by the Pb isotope ratios in zircon.21 However, the precision obtained ICP-MS instrument. The pulse width was 8 ns and the energy using LA-ICP-MS was worse than similar analyses using was #70 mJ per pulse. Laser energy was measured using an solution nebulization ICP-MS,21 most probably because of the energy detector (Scientech Model PHF50) and radiometer noisy nature of the ablation process.(Vector S200). The ablation system was similar to the one With the twin quadrupole device, most of the Øicker noise depicted previously19 with two improvements. Firstly, the can be removed when the ratio of a signal between an isotope quartz window into the cell was Ætted at a 45° angle, which of the same element or that of an internal standard element is minimized back reØection.Secondly, the aerosol gas was added taken.18 The precision of these measurements more closely tangentially at the base of the ablation cell and exited tangen- follows counting statistics when an isotope ratio is measured. tially slightly above the sample. This tangential Øow of aerosol The device also provides signiÆcant improvements in precision gas minimized deposition of particles in the cell and improved for the ratios of signals for ions of different elements (i.e., for particle transport efficiency from the cell.internal standardization), although the precision of internal Steel samples were obtained from NIST. The samples were standard ratios is generally poorer than that of isotope ratios cut to Æt the dimensions of the cell and smoothed. No other of the same element. Results are also presented in which one sample preparation was required. The sample was held on the mass analyser is scanned over a mass range while the other stage of a stepper motor (AMSI Model 301SM) and rotated remains on a single m/z value.When the ratio of these two at #30 rev min-1. The laser was focused onto the sample signals is taken, peak shapes in the mass spectrum improve slightly off-center with a quartz lens ( f=10 cm). In this fashion signiÆcantly for a wide range of elements. This feature is a circular track was made in the sample during ablation. The investigated as a correction for Øicker noise during mass scans. lens was positioned to yield a maximum metal ion signal which was obtained with the focal point slightly below the surface of the sample.The ablated particles were transported to the ICP EXPERIMENTAL through Tygon tubing (#1 m long, 6.4 mm i.d.). ICP-MS Device and Conditions The ICP-MS system used in the present work has been RESULTS AND DISCUSSION described previously18 and is depicted in Fig. 1. The ion beam Isotope Ratio Measurements from the ICP is extracted in the normal fashion.22 The beam then passes through a set of beam shift plates in front of the A plot of count rate versus time is given in Fig. 2 for 52Cr+ and 53Cr+ in NIST SRM 1263 for a dwell time of 0.5 s. One beam splitter. The splitter divides the beam into two parts. Each part of the beam is then sent to its own quadrupole mass mass analyser transmits only m/z=52 and the other transmits m/z=53 for the entire experiment. The certiÆed Cr concen- analyser and detector.In this fashion, signals at two m/z values can be measured simultaneously. tration in this sample is 1.31% m/m. The RSD of the individual signals is 3%, typical of LA-ICP-MS. However, when the ratio ModiÆcations to the system include a decrease in the sampler ±skimmer separation from 10 to 8 mm. This change of the two signals is calculated, the RSD improves to 0.54%. If Æve consecutive ratios are averaged, the RSD of Æve such decreased the background from #40 to #10 counts s-1.The sensitivity of the instrument also improved #ten-fold. For averaged ratios is 0.24%. A longer dwell time (2 s) (Fig. 3) improves the RSD of the ratio to 0.29%. This is due to the elements with ionization energies below #7 eV, the total sensitivity of the device (i.e., the sum of the sensitivity of both accumulation of more counts and a decrease in the shot noise limit, as has been described previously.18 Again the RSD can channels) is #4×106 counts s-1 per ppm when ultrasonic nebulization and desolvation are used.be improved by averaging Æve consecutive ratios. In this case the RSD of Æve averaged ratios is 0.058%. The effect of Next, the voltages on the beam shift plates prior to the splitter were offset to give a split in the ion beam of about increasing dwell time on precision for the Cr isotope ratio measurements and the precision predicted from counting 451. This enabled simultaneous measurement of ions of very 172 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12Table 1 Instrumental components and operating conditions Operating conditions Component ICP– RF generator Plasma Therm Forward power 1.25 kW (now RF Plasma Products), ReØected power <8W Model HFP-2000D RF Plasma Products torchbox Aerosol gas Øow 0.8 l min-1 (modiÆed in-house for horizontal operation Outer gas Øow 16 l min-1 with laboratory-made copper shielding box) Intermediate gas Øow 0.8 l min-1 Ion extraction interface23– Ames Laboratory construction Sampler position 8 mm from load coil on center Sampler oriÆce 1 mm diameter Skimmer oriÆce 1 mm diameter Sampler±skimmer separation 8 mm Vacuum System18– Three stages differentially pumped Differential pumping oriÆce 1.5 mm diameter Welded stainless steel Operating pressure/Torr* Ames Laboratory construction Expansion chamber 1.1 Second (ion lens) chamber 4×10-4 Third (quadrupole) chamber 5×10-6 Mass analysers– From VG PlasmaQuad Mean rod bias 0 V Model SXP 300 with rf-only pre-Ælters Model SXP 603 controllers and rf generators Electron multiplier– Galileo Model 4870, pulse counting mode Bias voltage -2800 V Counting electronics– EG&G ORTEC Model 660 dual 5 kV bias supply Model 9302 ampliÆer/discriminator Model 994 dual counter/timer * 1 Torr=133.322 Pa.Fig. 2 Plot of count rate versus time during 15 Hz laser ablation of Fig. 3 Plot of count rate versus time during 15 Hz laser ablation of steel SRM 1263, dwell time=0.5 s.The RSD of each signal is 3.2% SRM 1263, dwell time=2.0 s. The RSD of each signal is 1.3% while while the RSD of the ratio is 0.54%. The concentration of Cr= the RSD of the ratio is 0.29%. The concentration of Cr=1.31% m/m. 1.31% m/m. Table 2 Effect of dwell time on precision of Cr isotope ratio statistics are shown in Table 2. The procedure for calculating the RSD expected from counting statistics is described in Precision of 53Cr+552Cr+ ratio, eqn. (1) of ref. 18. Application of the F-test24 indicates that for RSD (%) dwell times up to 2.5 s, the measured RSD and the RSD predicted from counting statistics are not signiÆcantly different. Mean ratio* Counting Five averaged Dwell time/s 53Cr+552Cr+ Measured statistics ratios Beyond 2.5 s precision deteriorates, for reasons that are unclear at this time. 0.5 0.530 0.543 0.50 0.240 It is also shown in Table 2 that the precision can be improved 1.0 0.525 0.338 0.34 0.090 1.5 0.523 0.358 0.28 0.228 by averaging every Æve ratios, as noted earlier.This averaging 2.0 0.522 0.288 0.24 0.058 procedure could be considered to be an `artiÆcial' way to 2.5 0.521 0.253 0.21 0.226 extend the dwell time and reduce the precision limits imposed 3.5 0.520 0.255 0.18 0.186 by counting statistics. 4.5 0.578 1.20 0.18 1.14 The signals for 206Pb+ and 208Pb+ from this sample (NIST 7.7 0.555 0.563 0.13 0.478 SRM 1263) are shown in Fig. 4. The dwell time is 2 s, and Pb is present at 22 ppm.The resulting signals for each isotope are * Accepted natural abundance ratio=0.113. Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 1730.12%. In this case application of the F-test indicates that the measured RSD is signiÆcantly higher than that predicted from counting statistics. It should be noted that the measurement of 51V+552Cr+ represents the easiest internal standard measurement for different elements. The masses of V and Cr are very close (Dm=1 u) and their ionization energies (IE) are low and nearly the same (6.74 and 6.77 eV, respectively), so they should behave similarly in the ionization, ion extraction and beam splitting processes.A more difficult case is demonstrated in Fig. 6. A plot of count rate versus time for 184W+ and 52Cr+ is shown again using NIST SRM 1263. The concentrations of these two elements are 0.046 and 1.31% m/m, respectively. The large difference in mass (Dm=132 u) and ionization energy (DIE= 1.21 eV) should make for a poor internal standard pair.At a Fig. 4 Plot of count rate versus time during laser ablation of SRM dwell time of 3.0 s the RSD of the individual signals is 3%. 1263, dwell time=2.0 s. The RSD of each signal is 17% while the RSD When the ratio of the two signals is taken, the RSD improves of the ratio is 2.6%. The Pb concentration is 22 ppm. to 0.63%. This value is higher than the counting statistics limit (0.39%) by a factor of only 1.6.very noisy, with an RSD of #17%. The precision of the Values of RSD for the signal ratios 51V+552Cr+ and isotope ratio 206Pb+5208Pb+ is 2.6%. If every Æve ratios are 184W+552Cr+ at various dwell times, along with the counting averaged, the RSD of the averaged ratios improves to 0.68%. statistics limit and the RSD value obtained when using the This value is essentially the same as the counting statistics Æve ratio averaging technique, are given in Table 3. The limit of 0.66%.best RSDs obtained are #0.12% for 51V+552Cr+ and 0.35% Note that the signal for 206Pb+ is higher than that for for 184W+552Cr+. These NIST steel SRMs are generally 208Pb+ in Fig. 4. In contrast, the actual sample contains roughly half as much 206Pb as 208Pb. A similar bias occurs in the measured ratios for 53Cr+552Cr+ shown in Table 2. As described under Experimental, the signals for the less abundant isotopes (206Pb+ and 52Cr+) have been enhanced artiÆcially by adjusting the voltages applied to the deØection plates (Fig. 1) to send most of the ion beam through the appropriate channel. This ability to enhance the signal for the minor isotope actually helps improve the contribution made by the counting statistics to the precision. Internal Standardization A plot of count rate versus time for 51V+ and 52Cr+ is shown in Fig. 5 for a dwell time of 7.7 s using NIST SRM 1263. The certiÆed concentration of V is 0.31%, and Cr is present at 1.31% m/m. Each signal has an RSD of 1.9%.This RSD value Fig. 6 Plot of count rate versus time for 52Cr+ and 184W+ during is fairly good for LA-ICP-MS. However, the RSD improves 15 Hz laser ablation of SRM 1263, dwell time=3.0 s. The RSD of each signal is 3% while the RSD of the ratio is 0.63%. The left axis to 0.24% when the ratio of the two signals is taken. The corresponds to the 52Cr+ signal and the right axis corresponds to the counting statistics limit for this measurement is 0.12%. Again 184W+ signal. The concentration of Cr=1.31% and W=0.046% m/m.the precision can be improved further by averaging every Æve ratio measurements. The RSD of Æve such averaged ratios is Table 3 Effect of dwell time on precision using internal standard elements RSD of ratio (%) RSD of Æve Counting averaged Dwell time/s Measured statistics ratios (%) 51V +552Cr+– 0.5 0.882 0.440 0.350 1.5 0.493 0.255 0.348 2.5 0.374 0.200 0.226 3.5 0.326 0.171 0.149 4.5 0.479 0.153 0.416 7.7 0.241 0.119 0.119 10 0.369 0.107 0.294 184W +552Cr+– 0.5 1.41 0.900 0.477 1.5 0.817 0.526 0.354 Fig. 5 Plot of count rate versus time for 52Cr+ and 51V+ during 2.0 0.964 0.470 0.721 15 Hz laser ablation of SRM 1263, dwell time=7.7 s. The RSD of 3.0 0.632 0.390 0.427 each signal is 1.9% while the ratio is 0.24%. The concentration of 5.0 1.85 0.317 1.72 Cr=1.31% and V=0.31% m/m. 174 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12considered to be homogeneous. Thus spatial changes in sample relative error of 10.4%.The value obtained for the same ratio using the corrected mass spectrum is 1.060 with a much lower composition probably do not contribute to the differences between measured precision and that expected from counting relative error (1.14%). This value represents the mass bias in just one side of the beam splitter and one of the mass analysers statistics. The precision obtained for signal ratios of different elements is generally poorer than that obtained for isotope and is comparable to the bias seen with conventional single quadrupole instruments.25 This moderate value for mass bias, ratios of the same element, regardless of whether the sample is introduced by LA or as a solution aerosol.18,19 The RSD #1% per mass unit, is somewhat surprising considering the fact that the beam splitter could act as an energy analyser and values reported above represent substantial improvements over those obtained in early work with this device.18,19 cause more mass bias than usual.A similar mass spectrum and count rate versus time plot for several elements is shown in Fig. 8. One quadrupole is scanned Improvements in Peak Shape and Precision During Scans over the region m/z=44±53 while the other measures 51V+. The dwell time used is once again 1.2 s. Irregular peak shapes The previous results were all obtained with each mass analyser are also seen in Fig. 8(a), owing to Øicker noise in the plasma set at one m/z value.A unique feature of this instrument is the and sample introduction process. As indicated by the broken ability to scan with one quadrupole, while the other measures lines, these noise spikes in the peaks correspond to similar a single m/z value. Thus, an internal standard ion can be spikes on the 51V+ signal. The RSD of the 51V+ signal is 7.4%. measured at precisely the same time as each analyte ion to The shapes of the peaks in the mass spectrum improve correct for Øicker noise during multi-element determinations.substantially when the ratio of the two signals is calculated as Such an experiment is demonstrated in Fig. 7. NIST SRM shown in Fig. 8(b). 1767 is ablated and the resulting mass spectrum and count Suppose that V and Ti are homogeneously distributed and rate versus time plots are shown in Fig. 7(a). One quadrupole have natural isotopic abundances, that selective ablation is not measures 51V+ while the other scans over the region m/z= a problem and that both elements are 100% ionized in the 89±102.Each data point in the 51V+ signal versus time plot is plasma. Under these assumptions a comparison of accuracy measured at the same time as the point directly beneath it in can be made. The certiÆed value of the atomic abundance the scanned spectrum. Also, note that the dwell time for both ratio of 51V+548Ti+ is 3.813. Following background subtrac- the scan and single m/z measurement is 1.2 s.The use of this tion, the uncorrected mass spectrum gives a value of 4.204. relatively long dwell time allows for accumulation of sufficient The corrected mass spectrum yields 4.128. Again both meas- counts to reduce the importance of shot noise, which is not ured values are higher than the actual certiÆed value. However, removed by these simultaneous ratio measurements. the relative error using the uncorrected mass spectrum In Fig. 7(a) the RSD of the 51V+ signal is 7.0%.The broken (10.25%) is higher than the relative error using the corrected lines in Fig. 7(a) indicate several places where Øicker noise has mass spectrum (8.26%). This error is somewhat higher than perturbed the 51V+ measurement and the mass scan at the that expected from mass bias. Once again, it appears that same time. The ratio of the two signals is taken and the signals from different elements do not correlate as well as corrected mass spectrum is shown in Fig. 7(b).As can be seen, signals for different isotopes of the same element.18 irregularities in the peak shapes are corrected and a smoother The proposed method could be of value in two ways. Firstly, mass spectrum results. This result is consistent with the results it yields a more representative mass spectrum since Øicker presented above in that Øicker noise cancels for a wide range noise in the plasma and sample introduction process has been of elements. removed. Secondly, since a more accurate mass spectrum is The accepted abundance ratio for 96Mo595Mo is 1.048.The obtained, the presence of a spectral interference can be deduced measured ratio, obtained using peak height after subtraction and corrected more readily. As can be seen, the correction of the background and 96Zr+ signal, is 1.157. This value has a using two different elements is not as good as an isotope ratio Fig. 7 (a) Mass spectrum and count rate versus time plot for 51V+ Fig. 8 (a) Mass spectrum and count rate versus time plot for 51V+ during 15 Hz laser ablation of SRM 1767, dwell time=1.2 s.The broken lines indicate spikes due to Øicker noise in both the mass scan during 15 Hz laser ablation of SRM 1767, dwell time=1.2 s. The broken lines indicate spikes due to Øicker noise in both the mass scan and the 51V+ signal. (b) The ratio of the mass spectrum signal to the 51V+ signal in (a) multiplied by 1000. The value for 96Mo+595Mo+ is and the 51V+ signal. (b) The ratio of the mass spectrum signal to the 51V+ signal in (a) multiplied by 1000.The value for 51V+548Ti+ in the 1.157 in the uncorrected mass spectrum. The value for the same ratio is 1.060 in the corrected mass spectrum. The accepted value of the uncorrected mass spectrum is 4.204. The value of the ratio in the corrected mass spectrum is 4.128. The accepted value is 3.813. ratio is 1.048. Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 1754 Myers, D.P., Li, G., Yang, P., and Hieftje, G. M., J. Am. Soc. correction; however, a more accurate value results in both Mass Spectrom., 1994, 5, 1008. cases. 5 Myers, D. P., Heintz, M. J., Mahoney, P. P., Li, G., and Hieftje, G. M., Appl. Spectrosc., 1994, 49, 1337. 6 Heintz, M. J., Myers, D. P., Mahoney, P. P., Li, G., and Hieftje, CONCLUSION G. M., Appl. Spectrosc., 1995, 49, 945. 7 Myers, D. P., Li, G., Mahoney, P. P., and Hieftje, G. M., J. Am. Results obtained indicate that, using a twin quadrupole Soc.Mass Spectrom., 1995, 6, 411. ICP-MS instrument, precision can be improved by simul- 8 Mahoney, P. P., Li, G., and Hieftje, G. M., J. Anal. At. Spectrom., taneous measurement of ion signals in both mass scanning 1996, 11, 401. 9 Barinaga, C. J., and Koppenaal, D. W., Rapid Commun. Mass and selected ion monitoring modes. Flicker noise from laser Spectrom., 1994, 8, 71. ablation and from the plasma are correlated and can be 10 Koppenaal, D. W., Barinaga, C.J., and Smith, M. R., J. Anal. At. cancelled. For ratios of isotopes of the same elements, measured Spectrom., 1994, 9, 1053. values of precision are only slightly above the counting stat- 11 Walder, A. J., Abel, I. D., Platzner, I., and Freedman, P. A., istics limit. Internal standard ratios improve precision values Spectrochim. Acta, Part B, 1993, 48, 397. to about 1.6 times the counting statistics limit. Thus, Øicker 12 Walder, A. J., and Freedman, P. A., J. Anal.At. Spectrom., 1992, 7, 571. noise does not cancel as effectively when two different elements 13 Walder, A. J., Platzner, I., and Freedman, P. A., J. Anal. At. are measured, compared with an isotope ratio of the same Spectrom., 1993, 8, 19. element. Mass spectra from one channel are corrected using 14 Lee, D., and Halliday, A. N., Int. J. Mass Spectrom. Ion Processes, an internal standard from the other channel. The corrected 1995, 146/147, 35. mass spectrum gives more accurate results for isotope and 15 Christensen, J.N., Halliday, A. N., Lee, D., and Hall, C. M., Earth Planet. Sci. L ett., 1995, 136, 79. elemental ratios than an uncorrected spectrum. Interestingly, 16 Thirwall, M. F., and Walder, A. J., Chem. Geol. (Isot. Geosci. a similar principle, although without simultaneous detection, Sect.), 1995, 122, 241. could be used with a single channel instrument equipped with 17 Mermet, J.-M., and Ivaldi, J. C., J. Anal. At. Spectrom., 1993, 8, 795. the appropriate controlling software to hop or scan rapidly, 18 Warren, A. R., Allen, L. A., Pang, H., Houk, R. S., and thereby minimizing Øicker noise during multi-element Janghorbani, M., Appl. Spectrosc., 1994, 48, 1360. 19 Allen, L. A., Pang, H.-M., Warren, A. R., and Houk, R. S., J. Anal. determinations. At. Spectrom., 1995, 10, 267. 20 Feng, R., Machado, N., and Ludden, J., Geochim. Cosmochim. Ames Laboratory is operated for the US Department of Energy Acta, 1993, 57, 3479. by Iowa State University under Contract No. 21 Fryer, B. J., Jackson, S. E., and Longerich, H. P., Chem. Geol., 1993, 109, 1. W-7405-ENG-82. This work is supported by the US 22 Niu, H., and Houk, R. S., Spectrochim. Acta, Part B, 1996, 51, 779. Department of Energy, Environmental Remediation andWaste 23 Olivares, J., and Houk, R. S., Anal. Chem., 1985, 57, 2674. Management, Office of Technology Development. 24 Skoog, D. A., West, D. M., and Holler, F. J., Fundamentals of Analytical Chemistry, Saunders, New York, 5th edn., 1988, p. 38. 25 Jarvis, K. E., Gray, A. L., and Houk, R. S., in Handbook of ICPREFERENCES MS, Blackie, London, 1991, p. 331. 1 Begley, I. S., and Sharp, B. L., J. Anal. At. Spectrom., 1994, 9, 171. Paper 6/03310E 2 Koirtyohann, S. R., Spectrochim. Acta, Part B, 1994, 49, 1305. ReceivedMay 13, 1996 3 Vanhaecke, F., Moens, L., Dams, R., and Taylor, P., Anal. Chem., 1996, 68, 567. Accepted September 4, 1996 176 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a603310e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Optimization and Calibration of Laser Ablation±Inductively Coupled Plasma Atomic Emission Spectrometry by Measuring Vertical Spatial Intensity Profiles XIANGLEI MAO AND RICHARD E. RUSSO* L awrence Berkeley National L aboratory, Berkeley, CA 94720, USA Vertical spatial emission intensity proÆles for ICP-AES were these studies.29,30 In this paper, the vertical spatial emission measured to optimize and calibrate laser ablation sampling. intensity proÆles of ICP-AES were measured for laser ablation Laser ablation sampling and laser ablation plus liquid and liquid nebulization sampling.The inØuence of carrier gas nebulization sampling were studied. The position of maximum Øow rate and rf power on laser ablation sampling with the ICP emission intensity above the rf load coil changes with gas ICP is discussed in the Ærst part of this paper. Øow rate for both cases, with the maximum position shifting A concern of laser ablation sampling is preferential ablation to higher regions in the plasma at higher Øow rates.The of volatile elements from multicomponent samples. Preferential maximum emission intensity occurs at a Øow rate of ablation depends on the sample properties, laser Øuence, laser approximately 0.2±0.3 l min-1 and at approximately 5±10 mm power density, and laser pulse width.30±37 To determine the above the load coil, which are signiÆcantly different to the extent of preferential ablation for various laser conditions, it values normally employed for liquid nebulization.In addition, is necessary to know the exact composition of the laser ablated by measuring the spatial emission proÆles for laser ablation mass. However, because of preferential ablation, solid stanand nebulization sampling, solutions can be used as standards dards may not provide accurate calibration of the ICP. to calibrate the composition of the laser ablated mass. Solution standards have been proposed for calibration in ICPCalibrated Zn5Cu ratios were measured using UV nanosecond MS23 and ICP-AES.38 For aqueous solution nebulization, the and picosecond laser pulses.Stoichiometric laser ablation analyte dries from liquid droplets to form very small particles sampling of a brass alloy was achieved only by using UV in the plasma. For laser ablation, larger dry particles picosecond laser pulses at high power density. (#1±5 mm) are directly introduced into the ICP. The atomization and excitation processes are expected to be different in Keywords: L aser ablation sampling; inductively coupled plasma these two cases.Because the composition of the ICP and atomic emission spectrometry; vertical spatial proÆle; excitation mechanism will be different for aqueous solution optimization; calibration nebulization and laser ablation sampling, it is prudent to characterize the ICP response for these two cases before using The intensity of an analyte line in ICP-AES is a complex solutions as standards.function of several parameters, including analyte concentration, For ICP-AES, the spectral emission intensity spatial proÆles carrier gas Øow rate, rf power, and the observation height in are governed by ICP electron temperature, electron number the plasma. For nebulized aqueous solutions and electrother- density, total amount of analyte, kinetics of vaporization and mal vaporization, optimum conditions for ICP-AES have been atomization of larger analyte particles, number of incompletely well investigated.1±14 Commonly used conditions are a nebul- evaporated droplets, and excitation mechanism.15±21,39 In a izer gas Øow rate of 0.6±1 l min-1, an rf power of 1.1±1.3 kW, previous study, we demonstrated that the emission spatial and an observation height of 14±18 mm above the load coil.proÆle remained constant for a diverse range of laser power Vertical analyte emission intensity in the ICP is dependent on densities and sample targets.40 However, the vertical spatial the rf power and gas Øow rates.15±21 Solid sample introduction proÆles are not constant for laser ablation and solution nebuliz- using laser ablation provides numerous beneÆts for chemical ation.If solutions are used to calibrate laser ablation sampling, analysis including direct characterization of any solid sample one has to keep all ICP operating conditions constant for and no sample preparation, eliminating dissolution, additional both cases; the total gas Øow, rf power and amount of water solvent waste, and personnel exposure to samples and solvents. must be constant in order to maintain similar excitation Laser ablation requires a smaller amount of sample characteristics and temperature spatial proÆles.Even so, it is (<micrograms) than that required for solution nebulization still possible that the proÆles will be different because of a (milligrams). Because there is no water in the ICP, analyte line different vaporization mechanism for nebulized solution versus intensity may be stronger and molecular background emission laser ablated particles.Solution standards can be used to weaker. The elimination of solvent is also beneÆcial to ICP-MS calibrate laser ablation±ICP-AES only if the emission intensity because of the lower interference of oxide and hydrogen spatial proÆles are the same using both laser ablation and molecular species.22 Analyte excitation behavior during direct liquid nebulization.solid sample introduction will be different to that with nebul- Spatial proÆles for laser ablation and solution nebulization ized aqueous solutions. Previous studies have addressed trans- were measured for different ICP conditions. A parameter range fer tube and ablation chamber designs.23±28 However, to the was established that provided similar emission spatial proÆles best of our knowledge, no study has addressed optimization so that liquid standards could be used to calibrate the ICP.A of the ICP for laser ablation sampling. Such a study is possible calibration graph of Zn5Cu mole ratio versus ICP Zn5Cu only when measuring the entire ICP vertical spatial emission intensity ratio was obtained. The Zn5Cu mole ratio as intensity proÆle. The use of repetitively pulsed laser ablation sampling also provides the necessary precision to carry out a function of laser power density was measured using a Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 (177±182) 177nanosecond excimer laser (30 ns, 248 nm) and a picosecond The measurement system consists of a monochromator (Spex industries; 270M) with a 1200 grooves mm-1 holo- Nd5YAG laser (35 ps, 266 nm). The inØuence of these laser conditions on accuracy is discussed in the second part of graphic grating and an entrance slit-width of 12.5 mm, and a Peltier-cooled, charged-coupled device (CCD) detector with this paper. 512×512 pixels (EG&G Princeton Applied Research; OMA VISION).The spectral emission from the ICP was imaged EXPERIMENTAL using a quartz lens (5 cm focal length) onto the monochroma- A diagram of the experimental system is shown in Fig. 1. Two tor. This spectrometer simultaneously measures a 30 nm wave- different lasers were used for ablation: a KrF excimer laser length range. The size of the CCD is 1×1 cm. The data from with l=248 nm and a Nd5YAG laser with l=266 nm. The the CCD were digitized and transferred to a microcomputer.pulse durations of the excimer and Nd5YAG lasers are 30 ns For all data reported in this paper, emission intensities are and 35 ps, respectively. Each laser was pulsed at a repetition integrated for 6 s during repetitive ablation. In a previous rate of 10 Hz. The area of the beam was reduced by using paper, the spatial emission proÆle was not inØuenced by the a 6 mm diameter aperture, then focused into the ablation integration time of the CCD detector.40 The brass sample was sample chamber using a plano-convex UV-grade quartz lens pre-ablated for 120 s before ICP intensity measurements to ( f=200 mm). The laser beam spot size at the sample surface achieve enhanced precision.For calibration studies of laser was varied by changing the lens-to-sample distance. The energy ablation with solution nebulization, the experimental pro- of the excimer laser is 25 mJ after the aperture for most of the cedure is to set the ICP power, set the gas Øow rate through experiments.The laser power density at the sample surface is the laser ablation chamber, and introduce water into the ICP 0.07±200 GW cm-2. For the Nd5YAG laser, the energy is from the spray chamber. Without laser ablation, an ICP approximately 2.5 mJ at the sample surface. The power density background emission spectrum proÆle is recorded. The laser is 1±2000 GW cm-2. The laser ablation chamber was mounted is then repetitively pulsed on the sample and the spectral on a xyz micrometre translation stage so that the sampling emission intensity proÆle is measured.The brass solution is spot could be changed after each measurement.29 then introduced into the spray chamber without laser ablation. The samples consisted of small discs (20 mm diameter and The gas Øow rate in the laser ablation chamber was kept 1 mm thickness) of brass. The composition of brass is approxi- constant. With this procedure, the Cu and Zn emission intensity mately 36% Zn and 64% Cu measured by energy dispersive spatial proÆles were obtained with laser ablation and solution X-ray spectrometry.A brass solution was prepared by dissolv- nebulization, respectively, using the same ICP conditions. The ing a portion of this brass sample in nitric acid. Standard procedure is repeated at different gas Øow rates into the laser solutions of Zn and Cu were prepared by dissolving 99.999% ablation chamber. Cu and 99.9% Zn metal, respectively, in dilute HNO3. The Hilbert space distance was used to characterize the The ICP (RFPP; ICP20P) system included a 2.2 kW rf differences between laser ablation sampling and solution nebul- generator, impedance matching network, and mass Øow con- ization emission spatial proÆles.First, the spatial proÆles were trollers (Matheson; 8274). The Øow rate of Ar plasma gas was normalized to their maximum value. Then, f(x) and g(x) were 14 l min-1 and that of the auxiliary gas was 1.0 l min-1. To obtained with maximum values equal to 1 for laser ablation study the inØuence of gas Øow rate on ICP intensity proÆles and solution nebulization, respectively. For functions f(x) and during laser ablation sampling, the carrier gas transports the g(x), the Hilbert space distance (L2 distance) was deÆned as ablated mass directly into the ICP, without a spray chamber.For calibration studies, the exit port of the ablation chamber D= SPb a [f(x)-g(x)]2 dx (1) was connected to a steel T-connector (Swagelok), so that the carrier gas could be mixed with gas from a spray chamber.The Øow rate of gas into the spray chamber was Æxed at where x is the vertical distance and a and b are the boundaries 0.4 l min-1, the lowest level at which stability could be main- for the low and high observation region in the ICP. The tained for this nebulizer. The Øow rate of carrier gas into the Hilbert distance between spatial functions of laser ablation laser ablation chamber was varied from 0.2 to 1.0 l min-1.The and solution nebulization describes the error when solutions mixed gas was introduced into the central channel of the ICP are used as standards for calibration and the spatial proÆles torch. When using laser ablation to transport the sample into do not overlap. the ICP, water was introduced into the spray chamber to maintain constant ICP conditions. RESULTS AND DISCUSSION Optimization of Laser Ablation±ICP-AES The main parameters that inØuence spectral emission intensity in the ICP are carrier gas Øow rate, rf power, and observation height above the load coil (HALC).To optimize these conditions for laser ablation sampling and laser ablation sampling with solution nebulization, the ICP spatial emission intensity proÆles were measured for different gas Øow rates and rf powers. Both sample introduction conÆgurations were used for this study. Fig. 2 shows the Cu 224.7 nm (brass sample) spatial emission intensity proÆles for different carrier gas Øow rates with an rf power of 1500 W using the conÆguration in which the carrier gas Øows through the laser ablation chamber and transports the ablated sample directly into the ICP torch.The Øow rates were 0.1, 0.15, 0.2, 0.3, 0.5 and 1.0 l min-1. Fig. 3 shows the Cu 224.7 nm spatial emission intensity proÆles for different carrier gas Øow rates through the laser ablation chamber with a 0.4 l min-1 Ar Øow rate through the nebulizer while aspirating water.The Øow rates through the laser Fig. 1 Diagram of experimental system for laser ablation±ICP-AES. ablation chamber were 0.2, 0.3, 0.4, 0.6, 0.8 and 1.0 l min-1. 178 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12improved by a factor of about 3 with a lower Øow rate. For the laser ablation plus water case (Figs. 3 and 5), the intensity is improved by almost a factor of 10. These studies also show that the best HALC for maximum emission is lower than that generally used for conventional liquid nebulization sampling.It is interesting that the maximum intensity is similar for both cases, considering that the ICP excitation conditions are different with and without water. For ICP=1500 W, a maximum intensity of #7.8×105 occurs with a Øow rate of 0.2 l min-1 in the laser ablation chamber plus 0.4 l min-1 through the nebulizer (0.6 l min-1 total), compared with #8.5×105 with a Øow rate of 0.3 l min-1 in the laser ablation chamber.The emission intensity depends on two primary factors: the amount of sample and the excitation conditions of the ICP. For this Æxed laser power density study, the amount of ablated sample was approximately constant for this homogeneous Fig. 2 Vertical spatial emission intensity proÆles of Cu in the ICP at sample; the RSD is approximately 3%. When the carrier gas different gas Øow rates (l min-1). Carrier gas through laser ablation Øow rate increases, transport efficiency improves and the chamber and directly into the ICP.Rf power, 1500 W. amount of mass delivered to the ICP increases.41 However, increasing the Øow rate decreases the excitation temperature of the ICP.17 There are more Ar atoms per unit time in the ICP for higher gas Øow rates. With increasing Ar gas Øow rate, the maximum emission intensity in the ICP decreases and the peak position shifts upward.15±17,19±21 There is a trade off in transport efficiency versus temperature that governs the intensity and spatial properties of these emission data.A complete analysis would require measurement of the ICP temperature behavior versus Øow rate. Monitoring peak intensity is important for studying changes related to Øow rate. However, peak intensity alone does not deÆne analytical sensitivity. The ratio of analyte spectral intensity to continuum emission background is important for optimization of sensitivity.11 To determine the optimum height for AES observation, it is necessary to know the spatial proÆles of both line intensity and background. The ICP background emission decreased with increasing Øow rates for both laser Fig. 3 Vertical spatial emission intensity proÆles of Cu in the ICP at ablation and laser ablation plus water. Figs. 4 and 5 show the different gas Øow rates (l min-1). Carrier gas through laser ablation spatial proÆles for the Cu intensity ratio to background for chamber with a 0.4 l min-1 gas Øow through spray chamber with laser ablation and laser ablation plus water, respectively. water nebulization.Rf power, 1500 W. Comparing Fig. 4 with Fig. 2, the maximum position (HALC) for the ratio is approximately 5 mm higher than that of absolute intensity. The absolute intensity at 0.2 l min-1 is The intensities shown in Figs. 2 and 3 were backgroundcorrected. The laser power density used in these experiments approximately three times stronger than that at 1.0 l min-1; the ratio is almost the same.The maximum ratio occurs at a was 0.9 GW cm-2 and the brass samples were pre-ablated for each measurement. Therefore, the ablated mass was approxi- Øow rate of 0.5 l min-1, whereas maximum intensity occurs at 0.2 l min-1. For laser ablation plus water, the absolute intensity mately constant for each of these proÆles. The Cu intensity was measured at Æve separate spots on the brass sample for at 0.2 l min-1 is approximately ten times stronger than that the 0.3 l min-1 Øow rate to determine the error associated with these measurements, primarily due to the laser ablation sampling.The error at this Øow rate is approximately 3% RSD for the Cu measurements and is consistent with previous studies using repetitive pulsing and pre-ablation.29 As expected, the position of maximum ICP emission intensity changes with gas Øow rate for both conÆgurations, with the maximum shifting to higher regions in the plasma at higher Øow rates.The Zn emission intensity spatial proÆles behaved similarly to those of Cu and are therefore not shown. The Cu peak intensity initially increases with gas Øow rate, reaches a maximum, and then decreases in the laser ablation conÆguration (Fig. 2). The maximum occurs at a Øow rate of approximately 0.2±0.3 l min-1. For laser ablation plus water, the Cu intensity decreases with increasing gas Øow rate above 0.2 l min-1 in the laser ablation chamber (with a constant 0.4 l min-1 gas Øow through the nebulizer) (Fig. 3).Increasing the ICP power increases the ICP intensity, but the maximum analyte intensity always occurs at 0.2±0.3 l min-1 for both cases. This Øow rate Fig. 4 Ratio of Cu line intensity to ICP background as a function of is considerably lower than that generally accepted for optimum height above load coil at different gas Øow rates (l min-1). Carrier performance using liquid nebulization alone. For the laser gas through laser ablation chamber and directly into the ICP.Rf power, 1500 W. ablation case (cf. Figs. 2 and 4), the emission intensity is Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 179of vertical height (temperature) in the ICP. For laser ablation, the analyte already exists in the solid particle or atomic form; melt ejection or sublimation occurs from the solid and there is no aqueous evaporation. Measurement of emission spatial proÆles provides a good way to determine the conditions under which standard solutions can be used for calibration.From eqn. (2), the intensity proÆle I(x) is a function of the temperature proÆle T (x), and the number density of atoms or ions g(x). If T (x) or g(x) in laser ablation and solution sampling is different, I(x) will be different. Therefore, the ICP conditions must be established to ensure that I(x) is the same for both laser ablation and solution nebulization. The carrier gas Øow rate and rf power can be optimized to minimize the difference between laser ablation and solution nebulization. For this study, the spatial proÆles for laser ablation plus water versus brass (Cu+Zn) solutions were compared.Fig. 6(a) and (b) shows the normalized Cu emission intensity spatial proÆles for Fig. 5 Ratio of Cu line intensity to ICP background as a function of height above load coil at different gas Øow rates (l min-1). Carrier laser ablation plus water at 1.0 and 0.2 l min-1 gas Øow rates, gas through laser ablation chamber with a 0.4 l min-1 gas Øow through respectively.The nebulizer Øow rate was 0.4 l min-1 in both spray chamber with water nebulization. Rf power, 1500 W. cases. The total Øow was always the Øow through the ablation chamber plus the nebulizer; 1.4 l min-1 in Fig. 6(a) and 0.6 l min-1 in Fig. 6(b). When the ablation chamber Øow rate at 1.0 l min-1; the line-to-background ratio is approximately is 1.0 l min-1, the spatial proÆles are different [Fig. 6(a)], even four times better. The maximum ratio and absolute intensity though the total water loading into the ICP is the same. The occur at the same Øow rate (0.2 l min-1 through the laser different `particle' behavior in the ICP is apparent under these ablation chamber and 0.4 l min-1 through the spray chamber). conditions. The spatial proÆles using a 0.2 l min-1 Øow rate Analyte and background intensity are increased at lower Øow in the ablation chamber are almost identical for both laser rates, and optimum sensitivity is a balance between the two.ablation plus water versus nebulized standard solutions If absolute intensity is used for optimization, a carrier gas [Fig. 6(b)]. Under these Øow conditions, the difference in Øow rate of 0.2±0.3 l min-1 is best for both laser ablation and `particle' behavior is minimized. laser ablation plus water sampling. The observation height The Hilbert space distance was used to quantify the difference should be 1±5 mm and 3±8 mm above the load coil for laser between two emission intensity proÆles; the smaller the value, ablation and laser ablation plus water, respectively.As regards the more identical the proÆles. The difference between laser both net intensity and line-to-background ratio, the best Øow rate is 0.2±0.3 l min-1 and the best observation height is 1±5 mm for laser ablation sampling. For laser ablation plus water, 0.2 l min-1 provides the best performance with our nebulizer at 0.4 l min-1.The best observation height is 5±11 mm above the load coil. All of these observations were based on Cu and Zn emission lines located between 200 and 230 nm. Other element lines are expected to have different proÆles. However, the research indicates that lower gas Øow rates and lower observation heights provide enhanced sensitivity for laser ablation sampling. Calibration of Laser Ablation Sampling with Solution Standards To calibrate laser ablation sampling, the composition of the sample introduced into the ICP has to be known.The ICP line intensity can be expressed as:42 I(x)=Ahc 4pBAigi liQ g(x) e-Ei/[kBT(x)] (2) where Ai is the Einstein transition probability for spontaneous emission, Ei the energy of the transition, gi the statistical weight of level Ei, T the excitation temperature, li the wavelength, g the emitting atom density, h Planck's constant, kB Boltzman's constant, c the speed of light and Q the internal partition function.In order to calibrate laser ablation using solution standards, the temperature proÆles have to be the same for both laser ablation sampling and solution nebulization. Therefore, for laser ablation the same amount of water and acid must be nebulized into the ICP. However, even with the same amount of Ar gas and water, it is still possible that excitation conditions and vertical spatial emission proÆles may Fig. 6 Normalized Cu emission intensity in the ICP as a function of not be similar because of different particle sizes and different height above load coil with gas Øow rates of (a) 1.0 and (b) 0.2 l m-1, vaporization behavior.For liquid nebulization, aqueous drop- in the laser ablation chamber and 0.4 l min-1 through the spray lets undergo evaporation, atomization, ionization, excitation chamber. Solid line is laser ablation sampling plus water. The broken line is for a brass solution through the nebulizer without laser ablation.and recombination processes. These processes are a function 180 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12ablation and solution nebulization of standards was calculated discussion on optimization, emission line intensity was maximum at a gas Øow rate of 0.2 l min-1. A gas Øow rate of for different gas Øow rates and rf powers. The offset in Cu and Zn spatial proÆles with carrier gas Øow rate was measured at 0.2 l min-1 also provided the optimum overlap in spatial proÆles for accurate calibration.From Fig. 7(a) and (b), it can three different ICP powers [Fig. 7(a) and (b)]. The difference (error) for Cu ranges from approximately 0.05 at 0.2 l min-1 to be seen that laser ablation and solution nebulization provided identical spatial emission intensity proÆles using a Øow rate of 0.4 at 1 l min-1. For Zn, the difference is small (#0.02) until the gas Øow rate reaches 0.8 lmin-1, and is not very sensitive 0.2 l min-1.Therefore, these conditions were used to calibrate the Zn5Cu ratio using standard solutions. An analytical to ICP power at low Øow rates. Zn intensity proÆles were not as sensitive as those for Cu when the gas Øow rate changed. A working curve was generated and used to determine the accurate ratio for the brass sample (0.56 mole ratio). possible explanation is that Zn is easier to atomize because of its lower melting- and boiling-point than Cu. Zn may be The calibrated Zn5Cu mole ratio as a function of laser power density is shown in Fig. 8 for ablation with the 30 ns completely atomized for both laser ablation and solution nebulization sampling for gas Øow rates less than 0.8 l min-1. A UV excimer laser. Zn is severely preferentially ablated at lower laser power density, below 0.3 GW cm-2. When the laser lower gas Øow rate and increased temperature may be required to atomize Cu efficiently. In general, the offset in spatial proÆles power density is higher than 0.3 GW cm-2, the ratio is close to accurate.The insert in Fig. 8 shows that even at high power will depend on the particular species, the excitation state and ICP characteristics. Better overlap was measured for both density, the composition of the ablated mass is different to the composition of the solid sample. Similar data have been species at the lower Øow rate. If two intensity proÆles are the same, the Hilbert space measured previously, but not against a calibrated ICP.43 At lower power density the laser ablation process is mainly distance is equal to zero.Because of instrumental errors and Øuctuations in laser ablation and solution nebulization, the thermal; Zn is easier to vaporize than Cu. As the power density is increased, the amount of Cu increases faster than that of Hilbert space distance cannot be zero in these experiments. To determine the instrumental and laser ablation errors, three Zn, because of a higher surface temperature.32,33,43 The stoichiometric ratio is approached as the power density reaches laser ablation and solution ICP proÆles were obtained at a gas Øow rate of 0.6 l min-1 and an rf power of 1500 W.The #0.3 GW cm-2, the roll-off region for mass ablation efficiency previously observed.32,33 There are several competing mechan- error between laser ablation spatial emission proÆles themselves is from 0.04 to 0.09. The error between repetitive solution isms inØuencing mass removal.When the laser power density reaches 0.3 GW cm-2, plasma shielding may be import- nebulization spatial emission proÆles is about 0.02. The larger values for laser ablation demonstrate the noise from this ant.32,33,43 The plasma is dense and hot enough to absorb a portion of the laser energy. The hot, high pressure plasma will sampling technology. This error represents a `jitter' in the overall ICP behavior to the particle size distribution introduced interact with the solid sample causing sputtering.There is a trade off between direct laser heating and plasma heating that during laser ablation. At a gas Øow rate of 0.2 l min-1, the difference between laser ablation and solution nebulization is will provide the most accurate analysis. A higher laser power density provides better accuracy, although accurate analysis is approximately 0.05. Therefore, for these conditions the spatial proÆles are essentially identical for laser ablation and solution not achieved for the brass sample using the UV nanosecondpulsed laser.The error is approximately 10% at 0.3 GW cm-2. nebulization, since the Hilbert space distance is approximately equal to that of laser ablation sampling alone. From the earlier Fig. 9 shows the calibrated Zn5Cu ratio as a function of laser power density using the picosecond-pulsed laser with l= 266 nm. The behavior is different to that using the nanosecondpulsed laser; the ratio increases with increasing power density and becomes stoichiometric after approximately 10 GW cm-2.The region of accurate analysis, after #10 GW cm-2 is also the region after which roll-off occurs for the picosecond-pulsed laser material interaction.32,33 These data demonstrate that the mechanisms for nanosecond- and picosecond-pulsed laser Fig. 8 Zn5Cu mole ratio from laser ablated mass as a function of Fig. 7 Hilbert space distance between laser ablation and solution laser power density.Solid line is the calibrated ratio from a brass solution. Excimer laser with l=248 nm and pulse duration=30 ns. ICP spatial intensity proÆles versus gas Øow rate in laser ablation chamber. (a) Cu and (b) Zn. Insert shows an expanded scale for the higher power density. Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 181REFERENCES 1 Sadler, D. A., Littlejohn, D., and Perkins, C. V., J. Anal. At. Spectrom., 1996, 11, 207. 2 Ebdon, L., Foulkes, M., and O'Hanlon, K., Anal.Chim. Acta, 1995, 311, 123. 3 Galley, P. J., Horner, J. A., and Hieftje, G. M., Spectrochim. Acta, Part B, 1995, 50, 87. 4 Kantor, T., and Zaray, G., Microchem. J., 1995, 51, 266. 5 Stroh, A., and Vollkopf, U., J. Anal. At. Spectrom., 1993, 8, 35. 6 Golightly, D. W., and Leary, J. J., Spectrochim. Acta Rev., 1991, 14, 111. 7 Nickel, H., Zadgorska, Z., and Wolff, G., Spectrochim. Acta, Part B, 1993, 48, 25. 8 Boumans, P. W. J. M., in Inductively Coupled Plasma Emission Spectroscopy, Part I: Methodology, Instrumentation, and Perfor- Fig. 9 Zn5Cu mole ratio from laser ablated mass as a function of mance, ed. Boumans, P. W. J. M., Wiley, New York, 1987, ch. 4. laser power density. Solid line is the calibrated ratio from a brass 9 Thompson, M., and Barnes, R. M., in Inductively Coupled Plasma solution. Nd5YAG laser with l=266 nm and pulse duration=35 ps. in Analytical Atomic Spectroscopy, ed. Montaser, A., and Golightly, D. W., VCH, New York, 2nd edn., 1992, ch. 5. 10 Dickinson, G. W., and Fassel, V. A., Anal. Chem., 1969, 41, 1021. 11 Boumans, P. W. J. M., and de Boer, F. J., Spectrochim. Acta, Part B, 1972, 27, 391. ablation are different. For brass analysis, the picosecond-pulsed 12 Scott, R. H., Fassel, V. A., Kniseley, R. N., and Nixon, D. E., laser with l=266 nm provides a wide power density region in Anal. Chem., 1974, 46, 75. which the ablated material has the same composition as the 13 GreenÆeld, S., Jones, I. L., McGeachin, H.M., and Smith, P. B., solid. In previous work, we demonstrated that the picosecond- Anal. Chim. Acta, 1975, 74, 225. 14 Fassel, V. A., and Kniseley, R. N., Anal. Chem., 1974, 46, 1110A. pulsed laser also provides better sensitivity than the nano- 15 Blades, M. W., and Horlick, G., Spectrochim. Acta, Part B, 1981, second-pulsed laser.32 36, 861. 16 Kawaguchi, H., Ito, T., Ota, K., and Mizuike, A., Spectrochim. Acta, Part B, 1980, 35, 199. 17 Kawaguchi, H., Ito, T., and Mizuike, A., Spectrochim.Acta, Part CONCLUSION B, 1981, 36, 615. 18 Koirtyohann, S. R., Jones, J. S., Jester, C. P., and Yates D. A., By measuring vertical spatial emission intensity proÆles, ICPSpectrochim. Acta, Part B, 1981, 36, 49. AES operating conditions can be established for optimum 19 Savage, R. N., and Hieftje, G. M., Anal. Chem., 1980, 52, 1267. response during laser ablation sampling. Compared with the 20 Fister, J. C., III, and Olesik, J.W., Spectrochim. Acta, Part B, ICP operating conditions for nebulized aqueous solutions, 1991, 46, 869. 21 Olesik, J. W., and Den, S. J., Spectrochim. Acta, Part B, 1990, laser ablation sampling provides its best sensitivity at lower 45, 731. carrier gas Øow rates and lower observation heights in the 22 Horlick, G., and Shao, Y. B., in Inductively Coupled Plasma in plasma. The inØuence of rf power on emission intensity was Analytical Atomic Spectroscopy, ed. Montaser, A., and Golightly, not as sensitive as gas Øow rate for the elements investigated.D. W., VCH, New York, 2nd edn., 1992, ch. 12. 23 Cromwell, E. F., and Arrowsmith, P., Anal. Chem., 1995, 67, 131. By measuring vertical spatial emission intensity proÆles, 24 Arrowsmith, P., and Hughes, S. K., Appl. Spectrosc., 1988, 42, 1231. laser ablation sampling with the ICP can be calibrated by 25 Leis, F., and Laqua, K., Spectrochim. Acta, Part B, 1978, 33, 727. using standard solutions. The gas Øow rate should be less than 26 Ishizuka, T., and Uwamino, Y., Anal.Chem., 1980, 52, 125. 0.3 l min-1 when a standard nebulizer system is used with a 27 Carr, J. W., and Horlick, G., Spectrochim. Acta, Part B, 1982, 37, 1. 28 Liu, X. R., and Horlick, G., Spectrochim. Acta, Part B, 1995, 0.4 l min-1 Ar Øow. With calibrated ICP, preferential vaporiz- 50, 537. ation is shown to exist throughout a wide power density range 29 Chan, W. T., and Russo, R. E., Spectrochim. Acta, Part B, 1991, using a UV-pulsed laser.The Zn5Cu ratio from ablation of 46, 1471. brass samples with a UV nanosecond-pulsed laser decreased 30 Arrowsmith, P., Anal. Chem., 1987, 59, 1437. 31 Cromwell, E. F., and Arrowsmith, P., Appl. Spectrosc., 1995, with increasing laser power density, and stabilized for power 49, 1652. densities higher than 0.3 GW cm-2. However, the composition 32 Russo, R. E., Appl. Spectrosc., 1995, 49, 14A. of the ablated mass was always Zn-rich. Using UV picosecond- 33 Shannon, M.A., Mao, X. L., Fernandez, A., Chan, W. T., and laser pulses, the Zn5Cu ratio increased with increasing laser Russo, R. E., Anal. Chem., 1995, 67, 4522. 34 Russo, R. E., Mao, X. L., Chan, W. T., Bryant, M. F., and Kinard, power density and stabilized at an accurate level; the composi- W. F., J. Anal. At. Spectrom., 1995, 10, 295. tion of the laser ablated mass was the same as that of the solid 35 Chan, W. T., Mao, X. L., and Russo, R. E., Appl. Spectrosc., 1992, sample when the laser power density was higher than 46, 1025. 10 GW cm-2. The picosecond-pulsed laser provided better 36 Omori, N., and Inoue, M., Appl. Surf. Sci., 1992, 54, 232. 37 Mochizuki, T., Sakashita, A., Tsuji, T., Iwata, H., Ishibashi, Y., ablation efficiency and better accuracy for chemical analysis and Gunji, N., Anal. Sci., 1991, 7, 479. than the nanosecond-pulsed laser. 38 Baldwin, D. P., Zamzow, D. S., and D'Silva, A. P., Anal. Chem., Standard solutions represent a potential methodology for 1994, 66, 1911. calibrating the ICP for laser ablation sampling. However, it 39 Mostaghimi, J., Proulx, P., Boulos, M. I., and Barnes, R. M., Spectrochim. Acta, Part B, 1985, 40, 153. would be desirable not to use solutions. Additional work is 40 Caetano, M., Mao, X. L., and Russo, R. E., Spectrochim. Acta, necessary to understand laser ablation better so that solids Part B, 1996, 51, 1473. can be used as standards without matrix effects. It may be 41 Rosner, D. E., Mackowski, D. W., Tassopoulos, M., Castillo, J., possible that operating conditions can be established for the and Garcia-Ybarra, P., Ind. Eng. Chem. Res., 1992, 31, 760. 42 Mermet, J. M., in Inductively Coupled Plasma Emission ICP and UV picosecond-laser pulses to provide stoichiometric Spectroscopy, Part II: Application and Fundamentals, ed. Boumans, analysis, in which case solid standards can be trusted for P. W. J. M., Wiley, New York, 1987, ch. 10. calibration. 43 Mao, X. L., Chan, W. T., Caetano, M., Shannon, M. A., and Russo, R. E., Appl. Surf. Sci., 1996, 96±98, 126. This research was supported by the US Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division, Paper 6/06059E Processes and Techniques Branch, under Contract No. Received September 3, 1996 Accepted October 8, 1996 DE-AC03±76SF00098. 182 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a606059e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Correction of Matrix Effects in Quantitative Elemental Analysis With Laser Ablation Optical Emission Spectrometry C. CHALE� ARD, P. MAUCHIEN*, N. ANDRE, J. UEBBING†, J. L. LACOUR AND C. GEERTSEN‡ Commissariat a` l’Energie Atomique, L aboratoire de Spectroscopie L aser Analytique, Centre d’Etudes Saclay, Ba� t. 391, 91191 Gif sur Y vette, France A new approach to the quantification of the optical emission when ablation is performed with a laser irradiance lower than 1 GWcm-2.13 signals from a laser produced plasma in air at atmospheric pressure is described.It is based on a simple analytical model The analytical procedure commonly used for the correction of matrix effects15 is based on normalization of the analyte giving intensity of the emission lines as a function of the vaporized mass and the plasma excitation temperature. Under signal by a reference signal (generally the major element of the matrix). Such a normalization procedure is of no use when no the hypothesis of a stoichiometric ablation process, these two parameters are presumed to be responsible for the matrix reference element is available.This is frequently the case for on-line analysis, where the matrix could change with time as effects observed when the composition of the sample is changed. Under the experimental conditions chosen it is the process evolves, as for microanalysis because the matrix could change from one location to the other on the solid. demonstrated that the acoustic signal emitted by the plasma is proportional to the vaporized mass and that the excitation The purpose of the present work was to identify the dominant matrix effects appearing in LA-OES and to develop an plasma temperature can be controlled using the line ratio of a given element chosen as a ‘temperature sensor’.Normalization analytical procedure enabling correction of these effects to be made for quantitative multi-matrix analysis. of the emission signals by these two parameters allows for an efficient correction of the matrix effects.Results obtained on a series of aluminium alloys, steel and brass samples EXPERIMENTAL demonstrate, for the first time, the possibility of matrix independent analysis with LA-OES with an accuracy of a few The experimental arrangement shown in Fig. 1 is similar to percent. the one described previously5 and so will be discussed only briefly here. A XeCl excimer laser (Lambda-Physik EMG 201 Keywords: L aser ablation; optical emission spectrometry; MSC, Gottingen, Germany) emitting at 308 nm with a 28 ns matrix effects; elemental analysis pulse duration was used as the ablation source.The laser beam was spatially filtered before being focused Laser ablation is becoming a very popular technique in the on the sample in order to control the energy distribution of field of analytical atomic spectrometry. This is mainly attribu- the beam impinging on the sample throughout the experiments.table to the fact that the process is applicable to a very large For this purpose, the laser beam was focused by a 1000 mm range of materials and analytical situations. The most popular focal length lens on a diaphragm, and the central part of the applications result from the fruitful combination of LA with beam, passing through this diaphragm, was focused on the ICP-MS. sample surface (10° relative to the normal incidence) by a LA-OES is also a well known technique1,2 with a very high plano-convex lens with a focal distance of 250 cm.The resulting potential for elemental analysis. Its principle is based on the focal spot diameter was 500 mm (spot size about 2×10-3 cm2), measurement of the intensity of the atomic lines emitted by giving a power density of 1.4×109 Wcm-2 at the sample the plasma initiated above the surface, during the interaction surface. Such a large spot area is required to avoid problems of a high power density laser beam with a solid sample.This with the heterogeneity of the samples.5,10 purely optical technique offers the possibility of measurements The laser beam energy was frequently measured with a without sample contact, which is obviously of great interest power meter (Scientech 372, Boulder, CO, USA) and continufor many industrial applications. Several papers have demon- ously monitored by means of a calibrated photodiode. strated the potential of the technique for on-line measurement In order to ensure that all the experiments were carried out of meltingmaterial3 and for the control of industrialsamples.4–6 with a constant power density, even when the samples were Performances obtained have been summarized in several moved or replaced, it was necessary to control carefully the reviews.7–9 In a recent paper,10 potential of the technique for quantitative microanalysis with lateral resolution of a few microns has been demonstrated for the first time.However, for this application, as for one-line analysis where LA-OES has a unique potential, the major drawback comes from matrix induced effects.11–15 Some papers have shown either the matrix dependent character of the ablation process in binary samples with IR lasers16,17 or the non-stoichiometric evaporation process † On leave from Institut fu�r Spektrochemie und Angewandte Spektroskopie, Dortmund, Germany. Fig. 1 Experimental set-up. ‡ On leave from Pechiney Centre de recherches de Voreppe, France.Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 (183–188) 183distance between the focusing lens and the sample surface. vaporized mass and of the excitation temperature in the plasma. Hence, correction of the matrix effects could be This was done by means of a laboratory-made ultrasonic emitter–receiver, allowing positioning of the surface with a performed if both the vaporized mass and the plasma temperature are measured simultaneously with the analytical signal.precision of approximately 0.1 mm, a value compatible with the Rayleigh distance, evaluated to be 5 mm. This is obviously a very simple approach if one considers the number of parameters influencing the transformation of Emission from the plasma was collected by a set of two plano-convex lenses with a focal distance of 50 mm, resulting solid matter into a plasma but it was the purpose of the present work to simplify the problem in order to point out the in a demagnification ratio of 30. In this way, the whole volume of the plasma was collected by the 200 mm entrance slit-width key parameters of the ablation process.Nevertheless, these two key parameters must also be sup- of the spectrometer. This experimental configuration, using a very small collection angle, leads to a loss in sensitivity but posed to be dependent on the power density. As a consequence, it is essential to work under closely controlled laser parameters.could improve the reproducibility of the measurements. It is in fact known that laser produced plasmas are not spatially From this point of view, the ideal situation for laser ablation experiments would be a ‘top hat’ distribution of the laser beam and temporally homogeneous emission sources18,19 and the shot-to-shot variations in the atomic densities and temperature energy. Indeed, under such experimental conditions, the power density at the surface could be known precisely by just distribution inside the plasma could affect the reproducibility of the analytical measurements when only the central part of monitoring the laser energy and measuring the diameter on the crater.This is obviously not the case when the laser exhibits the plasma is probed. Detection was performed perpendicular to the surface, i.e., an uncontrolled energy beam distribution because each point of the laser surface does not receive the same amount of emission was collected in the expansion axis of the plasma.This configuration is the most compatible with in situ measure- energy. This is of particular importance with a multi-mode laser source for which the variation is generally very high. This ments because it is single ended and it is less sensitive to changes in the plasma-to-collecting lens distance that result can induce a highly complexion process in which the ablation regime may not be the same at each point on the when several shots are fired at the same place on the sample surface.A crater is formed on the surface, leading to a ablated surface. For these reasons, an attempt was made to improve the experimental set-up, by introducing the spatial displacement of the plasma. This displacement causes minimum perturbations if the plasma is probed in its expansion direction beam filtering system described under Experimental. The efficiency is demonstrated in Fig. 2. (i.e., perpendicular to the sample surface) because of the depth of focus of the detection optics. Both the energy distribution (integrated on the pulse duration) of the focused beam imaged on a charge coupled device The f/6 spectrometer (DILOR Model XY, Lille, France) was equipped with a gated intensified photodiode array (EG&G (CCD) camera, and the corresponding appearance of the crater in an aluminium sample before spatial filtering was done are RETICON S-series, Sunnyvale, CA, USA; 1024 photodiodes, 700 intensified photodiodes).The wavelength range recorded shown in Fig. 2(a). It can be seen that, without spatial filtering, the distribution of the beam is highly disturbed leading to a simultaneously was approximately 13 nm (at 300 nm) and the spectral resolution was about 0.15 nm. Gated electronics allows very irregular crater. The main problem comes from the fact that the beam energy distribution is known to change from the adjustment of both the delay after the laser pulse and the gate duration in the range 0.1–99 ms.According to the results pulse to pulse and also with time, as a function of the number of shots performed with the gas charge. It is also suspected obtained in previous work,5 emission from the plasma was measured 2 ms after the laser pulse using a 2 ms gate duration that the distribution would appear much more disturbed if the measurement was not integrated over the pulse duration. This (these values have been determined as giving the best SNR). Measurements of the emission intensity were made by integrat- problem corresponds to the known existence of temporal modes in an excimer laser cavity.ing a series of 20 shots fired in the same crater. Six independent measurements performed at different positions, chosen arbi- The results obtained when the laser is spatially filtered are shown in Fig. 2(b). As expected, the distribution of the beam trarily on the sample, were then averaged.is much more homogeneous and its size is only dependent on the optical arrangement. The consequence is a regular and RESULTS AND DISCUSSION well defined crater size, as can be seen in the figure. From these results, one can consider that the experimental set-up Analytical Model developed allows for reproducible and well controlled ablation The analytical procedure proposed to quantify the emission conditions and will enable a quantitative study of the matrix lines is based on the hypothesis of a stoichiometric ablation effects to be carried out.process. It is a reasonable hypothesis if one considers the power density used in the experiments.13,20 Vaporized Mass Diagnostics The intensity of an emission line of a given element i can be written as: A promising approach to this problem was presented by Chen and Yeung,21 who used the acoustic wave produced by the Ii=KCiMple-E/kT (1) ablation process as an internal standard in their experiments. The measurements were done in a closed cell filled with He where K is a constant factor which takes into account the collection efficiency and the spectroscopic data for the line at 50 Torr (1 Torr=133.322 Pa).It was demonstrated that increasing the pressure leads to a complicated waveform, being considered, Mpl is the total mass of matter vaporized in the plasma plume, Ci is the concentration of the element i in resulting from reflection and mixing of the acoustic waves within the cell.The acoustic wave was also evaluated for the the plasma, equivalent to the concentration in the solid phase under stoichiometric ablation conditions, and e-E/kT is rep- normalization of signals generated by an ICP source, using laser ablation as the sampling technique.22 The results showed resentative of the excitation temperature assuming the plasma is in local thermodynamic equilibrium (k is Boltzmann’s very good correlation between the acoustic signal and the analytical signal but, as for the previous case, experiments constant).Assuming the validity of the proposed approach, eqn. (1) were performed in a closed sample cell. Finally, to our knowledge, no results so far have been shows that matrix effects observed in laser ablation experiments result only from the sample-to-sample variations of the published on the use of acoustic wave normalization at 184 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12(a) (b) Fig. 2 Effect of spatial filtering on laser beam (left) and crater (right). Left images correspond to single shot measurements while right images result from ten shots in the same crater: (a) without spatial filtering; and (b) with spatial filtering. atmospheric pressure in air, without a sample cell. This is a interest. This result contrasts with that published, for example, by Shannon and Russo23 who have shown that, in the same very interesting challenge because acoustic wave measurement is a very simple technique and well adapted to in situ measure- range of power density, two different regimes of interaction have to be considered.This difference illustrates the difficulty ments (no contact between the sample and the instrumentation is required at all), which constitutes, from our point of view, of comparing experiments carried out with different laser sources having different energy distributions. the main interest in LA-OES. A first set of experiments was carried out to evaluate the Detection of the acoustic wave was made by means of a microphone positioned approximately 20 cm in front of the feasibility of acoustic wave normalization under the present experimental conditions.In order to be quantitative, such an sample surface. The acoustic pressure resulting from the plasma expansion was converted into an electric signal whose ampli- evaluation requires controlled variation of the ablated mass to be provoked. The simplest way of doing this consists in varying tude was measured by means of a gated electronic device.The gate width and the temporal delay after the laser pulse were the laser power, but that a correlation exists between ablated mass and laser power has to be verified. adjusted to the first maximum of the signal, the delayed signals being attributed to reflection of the acoustic wave on both the The ablated mass was evaluated by weighing the sample before and after ablation experiments; 500 shots were integrated sample and the components positioned in its vicinity.The signal was recorded for each shot and stored in the computer to obtain a good measurement accuracy. The results obtained on an aluminium sample are presented in Fig. 3. It appears memory at the same time as the emission line intensities. The variation of the acoustic signal as a function of the that the ablated mass exhibits a linear dependence versus the laser irradiance incident at the surface in the range investigated ablated mass when the laser power, incident on an aluminium sample, was varied between 0.6 and 1.2 GW cm-2 is shown in (0.6–1.2 GW cm-2).This clearly demonstrates that a unique regime of interaction has to be considered in the range of Fig. 4. It can be seen that the acoustic signal (As) is proportional to the ablated mass (As=aMpl where a is a constant) in the power range investigated. The same linear behavior has been Fig. 3 Ablated mass as a function of the laser irradiance incident on Fig. 4 Detected acoustic signal as a function of the ablated mass. The laser irradiance, incident on an Al sample, was varied between an Al sample. The measurements result from integration over 500 laser shots. 0.6 and 1.2GW cm-2. Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 185observed for different matrices such as nickel, steel and glass. from Boltzmann’s law: These results validate the use of acoustic signals as a diagnostic tool for evaluating the amount of material removed by the r= E2-E1 ln I1 I2 +ln g2A2 g1A1 (2) ablation process.The low value of the slope giving the acoustic signal as a function of the ablated mass should be noted. This is obviously where E1,2 are the upper energy levels of the lines, I1,2 are the a limitation in the precision that could be expected for the measured intensities, g1,2 are the degeneracy factors of the normalization of analytical signals, but it is partially compen- atomic states and A1,2 are the transition probabilities. sated for by the good reproducibility of the acoustic measure- From eqn. (1) the normalized intensity can then be written ments (a few percent).as: Moreover, the observation of the linearity between the acoustic wave generated by the plasma and the ablated mass Ii Ase-Eu/r=1 aKCi (3) leads to the very interesting conclusion that, for the matrices tested, most of the material removed by the laser ablation where As=aMpl is the acoustic signal intensity and Eu the process is vaporized under the experimental conditions.This upper energy level of the analytical line (i ) being considered. is confirmed by the results presented in Fig. 5, which show the Owing to uncertainties existing for the values of the spectro- linear dependence between the acoustic signal and the intensity scopic parameters of the lines, the proposed procedure will of the 403.08 nm Mn line emitted by the plasma produced lead to minimum imprecision if only one element is chosen as from aluminium, when the laser power is varied as described the ‘temperature sensor’.This means that it must be present previously. As the emission signal is proportional to the atomic both in the reference samples used to produce the calibration density of the element, it is possible to conclude that the largest curve and in the unknown sample. Copper was chosen because part of the material removed by the laser is atomized.it is present in many metallic samples and it has two close This important result probably has to be attributed to the lines (510.55 and 515.32 nm) originating from two separate very high temperature of the plasma produced by laser–surface levels (30784 and 49937 cm-1, respectively) and so is well interaction in the GW cm-2 power range.24 adapted to temperature measurements. A first set of experiments was made to evaluate the normalization procedure when the laser energy was varied.The results Plasma Excitation Temperature Diagnostic shown in Fig. 6 indicate that the excitation temperature is only very weakly dependent on the laser irradiance. This is the It is known that a laser produced plasma is not a homogeneous reason why a linear dependence was previously found (see medium.18 Hence, measurement of the excitation temperature Fig. 5) between the acoustic signal, which is proportional to when the whole volume is probed leads to a value that is not the ablated mass, and the emission signal.particularly useful for the physical interpretation of the Qualitatively, these results show that, in the range measurements. For analytical applications the problem is 0.6–1.2 GW cm-2, only a small part of the laser energy is different. Quantification is always based on the use of a absorbed in the plasma by the inverse bremstrahlung effect.10 calibration curve and, as a consequence, only a relative Provided this condition is fulfilled, an increase in the laser coefficient proportional to the excitation temperature is energy produces a proportional increase in the ablated mass required for normalization of the signals.This will be an without modification of the excitation temperature. This seems integral part of the slope of the calibration curve provided the to demonstrate that, contrary to the generally accepted view, experimental conditions remain the same.vaporization–atomization and excitation can be, to a certain The procedure used in the present work was based on the extent, considered as independent phenomena. measurement of the ratio between two lines of a given element used as a ‘temperature sensor’. The two lines chosen originate from two separate energy levels in order to give a ratio that is Analytical Evaluation sensitive to variations in the excitation temperature. Assuming The proposed procedure has been evaluated under real analyt- that the populations of the atomic levels follow an exponential ical conditions by plotting calibration curves for some elements law, a temperature normalization coefficient (r) was calculated in particular matrices.For each sample, six measurements, from the intensities using the following relationship derived taken at different positions on the surface, were averaged. Each measurement corresponds to the integration of 500 laser shots in the same crater. The intensity of the analyte line, the acoustic signal and the two copper lines used for the calculation of the Fig. 5 Intensity of the Mn (403.08 nm) line as a function of the Fig. 6 Ratios of the intensity of the Mn lines (404.145403.08 nm) as acoustic signal. The laser irradiance, incident on an Al sample, was varied between 0.6 and 1.2GW cm-2. a function of the laser irradiance in the range 0.6–1.2 GW cm-2. 186 Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12excitation temperature parameter (r) were all recorded simultaneously.The first evaluation was made on a series of aluminium alloys. The calibration curve obtained by plotting the net intensity of the 510.55 nm Cu line as a function of the concentration is shown in Fig. 7. It can be seen that, even without normalization, the emission intensity is proportional to the analyte concentration. This results from the fact that both the atomized mass and the excitation temperature were the same for the samples.This was confirmed experimentally by measurement of the two coefficients. If one considers the different matrix compositions of the samples, this indicates that matrix effects are low under the experimental conditions used. Indeed, in the case of spark emission spectrometry, a widely used technique in the metallurgical industry, analysis of the same aluminium alloys requires a calibration curve to be plotted for each alloy owing to the pronounced matrix effects inherent with this technique.The same low matrix effects have been observed for Mn. The calibration curve obtained by plotting the net intensity of the 414.14 nm Mn line as a function of concentration in a series of samples (steel, aluminium and nickel) is shown in Fig. 8. According to eqn. (1), the linearity of the response indicates that both the ablated mass and the excitation temperature are the same for the three matrices. Significant matrix effects have been observed for samples with high Zn concentrations.This is illustrated in Fig. 9(a), which shows the net 510.55 nm Cu line emission intensity as Fig. 9 (a) Net intensity of Cu (510.55 nm) line as a function of the a function of the concentration in three samples (Zn, Al–Zn Cuconcentration. The measurements were performed on three different and Al–Cu alloys). As can be seen, owing to pronounced matrices: Zn, Al–Zn, Al–Cu. (b) Normalized intensity of Cu as a matrix effects, the net intensity is not proportional to the function of the Cu concentration under the same conditions as in (a).The intensity of the Cu line is normalized by both acoustic signal and concentration. excitation temperature coefficients. The values of the acoustic signal and temperature normalization coefficients (determined as previously) for the three samples are shown in Table 1. The acoustic signal, representative of the vaporized mass, is approximately the same for the two aluminium samples (Al–Zn and Al–Cu) while it is 20% higher for the Zn alloy.On the contrary, the excitation temperature coefficient is the same for the two samples with high Zn concentrations (Zn and Al–Zn samples) and significantly lower for the sample without Zn. These results confirm that the processes of sample vaporization and excitation in the plasma do not depend on the same parameters. The amount of vaporized material appears to be dependent on the major element in the matrix, while the excitation temperature depends on the presence of Zn in Fig. 7 Net intensity of Cu (510.55 nm) line as a function of the Cu the sample. concentration. The measurements were performed on different Al After normalization of the net line intensity by both acoustic alloys. signal and excitation temperature normalization coefficients, a calibration curve is obtained, as shown in Fig. 9(b). Even if one considers the low number of points, the linearity of the response demonstrates the validity of the procedure and of the parameters used to diagnose the variations induced by matrix effects.In order to complete these results, the procedure was evaluated for a wider range of materials and concentrations. Copper was selected as the analyte (concentration range 0–90%) in Al alloys, steels and brass samples. Table 1 Acoustic signal and excitation temperature normalization coefficients determined for Zn, Al–Zn and Al–Cu alloys Alloy Acoustic signal coefficient (As) Texc coefficient (r) Zn 6302 10069 Fig. 8 Net intensity of Mn (414.14 nm) line as a function of the Mn Al–Zn 5337 10095 concentration. The measurements were performed on different Al–Cu 5449 8704 matrices: steel, aluminium and nickel. Journal of Analytical Atomic Spectrometry, February 1997, Vol. 12 187the main advantage of LA-OES over existing techniques is its ability to perform in situ measurements. Pronounced matrix effects have been observed for samples with Zn concentrations exceeding a few percent.It has been observed that the major element in the sample appears to be responsible for the amount of vaporized material, while the presence of Zn increases the excitation temperature. It was demonstrated that the normalization of the net intensity by the acoustic signal, representative of the vaporized mass, and a temperature normalization coefficient, calculated from a ratio of two lines of a fixed element, allows for a multimatrix calibration curve with a satisfactory level of precision (accuracy of a few percent).It has to be pointed out that the precision of the results is not affected by the precision of the temperature determinations provided the same lines are used for the calculation of the normalization coefficient. As a result, the only limitation of the proposed procedure lies in the fact that an element used as a ‘temperature sensor’ must be present in all samples. In many cases, however, this is an acceptable analytical constraint.REFERENCES 1 Brech, F., and Cross, L., Appl. Spectrosc., 1962, 16, 59. 2 Moenke-Blankenburg, L., Spectrochim. Acta Rev., 1993, 15, 1. 3 Carlhoff, C., Lorenzen, C. J., Nick, K. P., and Siebeneck, H. J., in In-Process Optical Measurements, ed. Spring, K. H., Proc. SPIE, SPIE Publications, Bellingham, 1989. 4 Lorenzen, C. J., Carlhoff, C., Hahn, U., and Jogwich, M., J. Anal. Fig. 10 (a) Net intensity of Cu (510.55 nm) line as a function of the At. Spectrom., 1992, 7, 1029.Cu concentration for a large range of concentrations (0–90%). The 5 Andre, N., Geertsen, C., Lacour, J. L., Mauchien, P., and measurements were performed on different matrices including Al Sjo�stro�m, S., Spectrochim. Acta, Part B, 1994, 49, 12 and 1363. alloys, steels and brass samples. One can observe that both the Al and 6 Sabsabi, M., Cielo, P., Boily, S., and Chaker, M., Proc. SPIE-Int. steel samples appear on the same calibration curve.This single slope Soc. Opt. Eng., 1993, 199, 2069. indicates the absence of matrix effects for the materials concerned. (b) 7 Ready, J. F., Effects of High Power L aser Radiation, Academic Normalized intensity of Cu as a function of the Cu concentration Press, New York, 1971. under the same conditions as in (a). The intensity of the Cu line is 8 Von Allmen, M., L aser Beam Interactions WithMaterials, Springer normalized by both acoustic signal and excitation temperature Series in Material Science, Springer, Berlin, 1987.coefficients. 9 Radziemski, L. J., and Cremers, D. A., L aser-induced Plasmas and Applications, Marcel Dekker, New York, 1989. As shown in Fig. 10(a) the calibration curve obtained from 10 Geertsen, C., Lacour, J. L., Mauchien, P., and Pierrard, L., the net 510.55 nm Cu line intensity exhibits three different Spectrochim. Acta Part B, 1996, 51, 1403. slopes, illustrating the matrix effects for the series of samples. 11 Kagawa, K., Kawai, K., Tani, M., and Kobayashi, T., Appl. Spectrosc., 1994, 48, 198.However, it has to be noted that both the Al and steel samples 12 Aguilera, J. A., Aragon, C., and Campos, J., Appl. Spectrosc., appear on the same calibration curve indicating the absence 1992, 46, 1382. of matrix effects for the materials concerned. 13 Chan, W. T., and Russo, R. E., Spectrochim. Acta, Part B, 1991, Normalization of the signal by both acoustic signal and 46, 1471. excitation temperature coefficient leads to the calibration curve 14 Wisbrun, R., Schechter, I., Niessner, R., Schro�der, H., and Kompa, shown in Fig. 10(b). A single slope calibration curve is K. L., Anal. Chem., 1994, 66, 2964. 15 Wisbrun, R., Niessner, R., and Schro�der, H., Anal. Meas. Instrum., obtained indicating the compensation of matrix effects by the 1993, 1, 17. normalization factors with a precision compatible with the 16 Ko, J. B., Sdorra, W., and Niemax, K., Fresenius’ Z. Anal. Chem., reproducibility of the individual measurements (an RSD of 1989, 335, 648. about 5%). 17 Ciocan, A., Hiddemann, L., Uebbing, J., and Niemax, K., J. Anal. At. Spectrom., 1993, 8, 273. 18 Geertsen, C., Lacour, J. L., and Mauchien, P., paper presented at CONCLUSION the E-MRS 1995 Spring Meeting, Symposium F, COLA ’95, Strasbourg, France, May 22–26, 1995. The results demonstrate that spatial filtering of the laser beam 19 Autin, M., Briand, A., Mauchien, P., and Mermet, J. M., allows for better control of the laser ablation parameters. The Spectrochim. Acta, Part B, 1993, 48, 851. first consequence is a well defined crater shape and long term 20 Russo, R. E., Appl. Spectrosc., 1995, 49, 14A. stability of the experimental conditions. Very low second order 21 Chen, G., and Yeung, E. S., Anal. Chem., 1988, 60, 2258. 22 Pang, H., Wiederin, D. R., Houk, R. S., and Yeung, E. S., Anal. matrix effects have been observed during analytical measure- Chem., 1991, 63, 390. ments performed on a series of different Al alloys. This 23 Shannon, M. A., and Russo, R. E., paper presented at the E-MRS demonstrates that neither the amount of atomized material, 1995 Spring Meeting, Symposium F, COLA ’95, Strasbourg, nor the excitation temperature of the plasma are affected by France, May 22–26, 1995. some change in the matrix composition. 24 Vertes, A., Dreyfus, R. W., and Platt, D. E., in IBM Research The single slope calibration curve obtained in the case of Report, RC 18520, 1992. Mn in very different samples, i.e., Al alloys, steel and Ni samples, indicates that under the experimental conditions Paper 6/04456E chosen, even first order matrix effects can be very low. This is Received June 26, 1996 an important analytical feature, especially if one considers that Accepted September 30, 1996 188 Journal of Analytical Atomic Spectrometry, February 1997,

ISSN:0267-9477    

DOI:10.1039/a604456e

出版商:RSC    

年代:1997

数据来源: RSC