摘要:

Method Performance Studies for Inorganic Analysis: Examples of Results Obtained by Plasma Spectrochemical Techniques in the Frame of Community Bureau of Reference (BCR)-Certification Campaigns† Plenary Lecture PH. QUEVAUVILLER European Commission, DGXII/C/5, SM&T Programme (MO75 3/09), rue de la L oi 200, B-1049 Brussels, Belgium The Community Bureau of Reference (BCR) (now renamed This paper describes organizational aspects of BCR-interlaboratory studies. A set of examples taken from recent Standards, Measurements and Testing Programme) of the European Commission has actively contributed to the certification campaigns are used to discuss the performance of plasma spectrochemical techniques as compared with alter- improvement of analytical methods used for inorganic determinations in various matrices through the organization of native techniques. The examples focus on inorganic determinations in various matrices, e.g., human hair, fish tissue, interlaboratory studies, the final aim of which was to certify reference materials.In the frame of these collaborative plankton, plant matrices (white clover, lichen), soil matrices and estuarine water. exercises, it was possible to compare dierent techniques and to draw conclusions on their performance (accuracy and precision). This paper briefly describes the organization of BCR-interlaboratory studies and discusses the performance of METHOD PERFORMANCE STUDIES— plasma spectrochemical techniques in comparison with GENERAL PRINCIPLES alternative techniques as a result of various certification The best way to make progress in a particular field is to collaborcampaigns on environmental and biological reference ate with colleagues who work in similar areas of analytical materials.sciences. By sharing expertise, many diculties encountered in Keywords: Interlaboratory studies; method performance; some laboratories may be solved and improvement may be plasma spectrochemistry; reference materials; European obtained in a collaborative manner.The dissemination of infor- Commission; Community Bureau of Reference (BCR) mation through the literature, workshops or international conferences, and the exchange of ideas during meetings, is important but is not sucient to improve drastically the state-of-the-art of The Community Bureau of Reference (known as its French a given analytical field. Therefore, in addition to the transfer of acronym, BCR) has been active for more than 20 years in the knowledge via the ‘classical’ route, many analysts have started to organization of interlaboratory studies with the aim to improve exchange samples, calibrants, reference materials, in order to test the quality control of measurements performed within the their techniques.This approach has been structured in a system- European Union (EU) and to produce certified reference atic way by the BCR which has conducted a considerable number materials (CRMs).Within the Fourth Framework Programme of interlaboratory studies aimed at evaluating and possibly (1994–98) of the European Commission, the Standards, improving analytical methods. With respect to environmental Measurements and Testing Programme (SM&T) is pursuing analysis, the projects focused mainly on inorganic, organic and this aim of contributing to the harmonization and improvement speciation analysis of a wide variety of matrices (water, sediment, of methods of measurement and analysis within the EU by soils, plants, etc.).Nearly all projects enabled the methods to be supporting four types of shared-cost actions: (i) interlaboratory improved to such a degree of reliability that they could be applied studies, (ii) production of CRMs, (iii) pre-normative research to the certification of reference materials which, in addition to and (iv) development of new measurement methods.1 their use for the validation of methods, represents a tool for the In the field of chemical analysis, interlaboratory studies and dissemination of the developed knowledge.certification campaigns involve EU laboratories covering vari- Two basic parameters should be considered when discussing ous analytical fields (environment, health and safety, food and analytical results: accuracy and uncertainty as caused by random agriculture); these collaborative projects provide an excellent errors and random variations in the procedure (see definitions means to test the performance of various analytical methods in the CIPM Guide to the Expression of Uncertainty in as applied to dierent matrices, e.g., plasma spectrochemical Measurement, 1993).In this context, accuracy is of primary techniques. In the frame of these exercises, dierent laboratories importance. However, if the uncertainty in a result is too high, it may compare dierent analytical methods and discuss the cannot be used for any conclusion concerning, e.g., the quality of possible sources of error linked to a specific laboratory (e.g., the environment or of food.An unacceptably high uncertainty error in manipulation) or a specific method (e.g., systematic renders the result useless. In the performance of an analysis, all error related to one or several steps of a technique). basic principles of calibration, of elimination of sources of contamination and losses, and of correction for interferences should † Presented at the 1997 European Winter Conference on Plasma Spectrochemistry, Gent, Belgium, January 12–17, 1997.be followed.2,3 These concepts are in principle well known to all Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 (871–879) 871analysts; they imply that the methods used are applied following the organizer, but also a good scientific background, in order to design properly the exercises, and evaluate their results. The good quality control principles (statistical control using control charts, knowledge of the method performance, i.e., accuracy and choice of the determinands and matrices should be based on the feasibility of preparation of homogeneous and stable reproducibility, etc.).A good within-laboratory quality control does not always samples with composition as similar as possible to real matrices; for candidate CRMs, a full verification should be enable a systematic error that is present from the moment of introduction of the method into a laboratory to be detected.carried out of the (within- and between-vial) homogeneity and long-term stability.5 Results should hence be verified by other methods. All methods have their own particular source of error, e.g., for some techniques It should be stressed that laboratories should have set-up all necessary quality assurance and quality control systems where sample dissolution is required, errors may occur due to an incomplete digestion or poor recovery.For other techniques, such prior to participation in an interlaboratory study, i.e., the method(s) used should be validated and performed under as instrumental neutron activation analysis (INAA), sample digestion would not be necessary, but other errorsmay be encountered statistical control for the particular matrix concerned by the study, i.e., interlaboratory studies should not serve the purpose as a result of shielding eects. An independent method should be used to verify the results of routine analysis.If the results of both of evaluating and/or optimizing a method in the course of its development. methods are in good agreement, it can be concluded that the results of the routine analysis are unlikely to be aected by a In all cases, the organization will involve: the supply of clear information to the participants; the production and distri- contribution of a systematic nature (e.g., insucient extraction).This conclusion is stronger when the two methods dier widely, bution of the samples [giving evidence that they are representative for the analytical problem(s) studied and that they are such as acid digestion–inductively coupled plasma spectrometry and INAA. If the methods have similarities, such as an extrac- homogeneous and stable]; the collection of results; the presentation and technical evaluation of the data (discussed, if tion step, a comparison of the results will most likely lead to conclusions concerning the accuracy of the method of final possible, with all the participants); and the statistical treatment of the results which have been accepted on technical grounds.determination, and not as regards the analytical result as a whole. Interlaboratory certification studies are organized following the same basic principles as other interlaboratory studies but INTERLABORATORY STUDIES involve only highly specialised actors. All participants should have demonstrated their quality in prior exercises.When all necessary measures have been taken in the laboratory to achieve accurate results, the laboratory should demonstrate This approach has been used by the BCR for all reference materials where new property values were to be certified for its accuracy in interlaboratory studies, which are also useful to detect systematic errors.4 In general, in addition to the the first time in matrix materials. In all studies, detailed protocols and reporting forms were prepared, requesting each sampling error, three sources of error can be detected in all analyses: sample pre-treatment (e.g., extraction, separation, participant to demonstrate the quality of the measurements performed, in particular the validity of calibration (including clean-up, preconcentration); final measurements (e.g., calibration errors, spectral interferences, peak overlap, baseline calibration of balances, volumetric glassware and other tools of relevance, use of calibrants of adequate purity and known and background corrections); and the laboratory itself (e.g., training and educational level of workers, care applied to the stoichiometry, proper solvents and reagents).The magnitude of contamination has also to be controlled by procedural work, awareness of pitfalls, management, clean bench facilities). Interlaboratory studies are organized in such a way that blanks and chemical reaction yields should be known accurately and demonstrated. All precautions should be taken to several laboratories analyse a common material which is distributed by a central laboratory responsible for the data avoid losses (e.g., formation of insoluble or volatile compounds, incomplete extraction and clean-up).collection and evaluation. When laboratories participate in an interlaboratory study, dierent sample pre-treatment methods and techniques of separation and final determination are Evaluation of Results compared and discussed, as well as the performance of these laboratories.If the results of such an intercomparison are in Protocols accompanying the test samples should be provided to the participants. The instructions should specify the number good and statistical agreement, the collaboratively obtained value is likely to be the best approximation of the truth. of samples and replicates to be analysed, and provide necessary information on, e.g., sample preservation, sub-sampling and Before conducting an interlaboratory study the aims should be clearly defined.An intercomparison can be held:4 (i) to sample pre-treatment. A full description of methods is an essential element when the detect the pitfalls of a commonly applied method and to ascertain its performance in practice, or to evaluate the per- performance of a laboratory or a method has to be evaluated. The BCR has elaborated reporting forms for the description of formance of a newly developed method; (ii ) to measure the quality of a laboratory or a part of a laboratory (e.g., audits various types of analytical methods for the certification of inorganic and organic parameters (intended to remind the partici- for accredited laboratories); (iii ) to improve the quality of a laboratory in collaborative work with mutual learning pro- pants of important parameters to be taken into consideration).They serve the purpose of preparing summaries of method cesses; and (iv) to certify the contents of a reference material.In the ideal situation, where the results of all laboratories descriptions including information on the most critical steps in the analytical procedures, e.g., calibration, pre-treatment, are under control and accurate, intercomparisons of types (ii) and (iv) will be held only. For the time being, however, types extraction and clean-up, separation and final detection. The technical evaluation of the data consists of a scientific (i) and (iii ) play an important role.In this paper, organizational aspects of interlaboratory studies organized prior to certifi- scrutiny of the reports submitted by the participants. To facilitate the discussion in a meeting, the data should be cations will be dealt with; this organization also stands for the certification campaigns. presented in a visualized manner and the results should be ear-marked to the laboratories (laboratory codes and method abbreviations).A classical presentation of results is to present Organization them in the form of bar-graphs (Fig. 1). Technical meetings are necessary to allow participants to The organization of interlaboratory studies (including certifi- cations) not only requires a good management capability from share expertise and extract information by comparing and 872 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Fig. 1 Example of a bar-graph showing the results for zinc (mg kg-1) in CRM 482 (Lichen).possibly discussing their performance and method with other the overall consistency of the variance values obtained in the participating laboratories (Bartlett test), and to detect ‘outly- participants using similar or dierent procedures. A statistical analysis of data from an interlaboratory study ing’ values in the laboratory variances (si2) (Cochran test). One-way analysis of variance (F-test) may be used to compare cannot explain deviating results nor can give alone any information on the accuracy of the results. Statistics only treat a and estimate the between- and within-laboratory components of the overall variance of all individual results.population of data and provide information on the statistical characteristics of this population. The results of the statistical treatment may lead to discussions on particular data not Case Studies: Performance of Plasma Spectrochemical belonging to the remainder of the population, but outlying Techniques data can sometimes be closer to the true value than the bulk of the population.6 If no systematic errors aect the population Examples have been selected from recent certification campaigns to illustrate the performance of plasma spectrochemical of data, various statistical tests may be applied to the results which can be treated either as individual data or as means of techniques in comparison with alternative techniques.Four trace elements were considered, As, Hg, Pb and Zn, in various laboratory means. When dierent methods are applied, the statistical treatment is usually based on the mean values of environmental and biological matrices. These results correspond to the final data accepted for certification, i.e., they may replicate determinations. The statistical treatment involves tests, e.g., to assess the conformity of the distributions of be considered to represent the state-of-the-art of plasma spectrochemistry for the elements and matrices of concern.The individual results and of laboratory means to normal distributions (Kolmogorov–Smirnov–Lilliefors tests), to detect discussion will be based on the relative standard deviation (RSD) obtained by the dierent laboratories and techniques ‘outlying’ values in the population of individual results and in the population of laboratory means (Nalimov test), to assess for each element in the dierent types of matrices; all results Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 873listed in the tables correspond to data included in the range ashing, or microwave digestion (Tables 1–3); for As, the pretreatment included in some cases hydride generation whereas of the certified values, i.e., they are all considered to be accurate. Some sources of systematic error detected during the technical some techniques for Hg determination also involved a reduction step (by NaBH4 or SnCl2).scrutiny meetings are also discussed. Some results of a systematic study on microwave digestion used in connection with ICP-AES or ICP-MS for the analysis Pre-treatment methods of various environmental CRMs for a range of trace elements are also reported; the dierent programmes tested are described The pre-treatment methods used in the certifications were in Table 4. Results of this study are presented in Table 5 and based on digestion with mixtures of acids, e.g., HNO3, HClO4, are discussed below.H2SO4, HF, in a pressurized or atmospheric mode, pressure Table 1 Pre-treatment methods used for the certification of trace elements in Human Hair, White Clover, Plankton and Lichen reference materials Material Pre-treatment Mercury— Human Hair, CRM 397 Pre-digestion overnight with HNO3; further digestion with HF–H2O2; reduction with NaBH4 (ICP) L ead and Zinc— Pressurized digestion with HNO3–HClO4 with or without addition of HF, or pre-digestion with HNO3 followed by microwave digestion with HF–H2O2 (ICP) White Clover, CRM 402 Arsenic— Digestion with mixture of acids (HNO3, HClO4, H2SO4) followed by hydride generation Plankton, CRM 414 Arsenic— Digestion with HNO3–HClO4–H2O2 followed by hydride generation (ICP), or microwave digestion (HNO3–H2O2) followed by ICP-MS Mercury— Pressurized digestion with HNO3–HClO4–H2SO4, or microwave digestion with HNO3, followed by cold vapour ICP L ead— Pressurized digestion (HNO3–H2O2) followed by ICP-MS Zinc— Pressurized digestion with HNO3–HCl (ICP and DCP), or microwave digestion with HNO3 (ICP), or digestion with HNO3 and HClO4–HF (ICP-MS) Lichen, CRM 482 Arsenic— Microwave digestion with HNO3–HF, ion exchange to eliminate ArCl interference Mercury— Pressurized digestion with HNO3 (ICP-MS), or microwave digestion with HNO3 (ID-ICP-MS) L ead and zinc— Pressurized digestion with HNO3–HF (ICP-MS) or microwave digestion with HNO3–HF (ICP-MS and ID-ICP-MS) Table 2 Pre-treatment methods used for the certification of Estuarine Water, Tuna Fish and Cod Muscle reference materials Material Pre-treatment Zinc— Estuarine Water, CRM 505 Direct measurement (ICP), or pH adjustment with gas phase neutralizer, adsorption on reversedphase column and elution with HNO3–HCl (ICP-MS) Tuna Fish, CRMs 463 and 464 Mercury— Microwave digestion with HNO3 Cod Muscle, CRM 422 Arsenic— High pressure ashing and addition of HNO3, HClO4 and H2SO4 (HG-ICP), or digestion with HNO3–HClO4 (ICP), or microwave digestion with HNO3 (ICP), or digestion under reflux with HNO3 and treatment of the residue with H2O2 (ICP-MS) Mercury— Microwave digestion with HNO3 and reduction with SnCl2 (ICP-MS and ID-ICP-MS), or microwave digestion with HNO3 and addition of HCl (ICP) L ead— Digestion with HNO3–HClO4 (ICP-MS and ID-ICP-MS) Zinc— Digestion with HNO3–HClO4 (ICP), or microwave digestion with HNO3–H2O2 or HNO3 (ICP), or pressure ashing and HNO3 addition (ICP and DCP), or digestion with HNO3 under reflux and treatment of the residue with H2O2 (ICP-MS) Table 3 Pre-treatment methods used for the certification of Soil and Sewage Sludge reference materials Material Pre-treatment Mercury— Calcareous Soil and Sewage Sludges, CRMs 141R, 144R and 146R Digestion with HNO3–HClO4 under reflux, or pressurized digestion with HNO3 and H3BO4 followed by SnCl2 reduction L ead and zinc— Pressurized digestion with HNO3 and H3BO4 (ICP and ICP-MS), or open digestion with HNO3, HClO4 and HF (ICP), or programmed dry ashing followed by digestion with HF–HNO3–HClO4 (ICP-MS) 874 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Table 4 Programmes tested for the evaluation of microwave digestion applied to the analysis of various environmental matrices (adapted from ref. 7) Programme Reagents Time/min Power/W I (1) 10 ml HNO3 5 10 10 30 10 60 (2) 10 ml HNO3 10 60 (3) 2 ml H2O2 5 60 (4) 5 ml H2O 5 50 II (1) 7 ml HCl+3 ml HNO3 5 40 10 50 (2) 7 ml HCl+3 ml HNO3 10 54 (3) 1 ml H2O2 5 40 (4) 5 ml H2O 5 50 III (1) 5 ml HNO3+2 ml H2SO4 5 20 10 40 10 100 (2) 5 ml HNO3 10 100 (3) 1 ml H2O2 5 100 (4) 1 ml H2O2 5 100 (5) 5 ml H2O 5 80 Arsenic Table 5 Results obtained from microwave-based methods for the Various types of systematic errors are likely to occur in As analysis of Cod Muscle, Plankton and Human Hair reference materials determination using plasma spectrochemical techniques, e.g., (adapted from ref. 7). All values in mg kg-1 related to sample digestion or to interferences at the detection Hg Pb Zn step. RSDs obtained for As determination in biological materials (Cod Muscle, Plankton, Lichen and White Clover) are Cod Muscle: given in Table 6. Programme I 0.440±0.05 ND* 19.5±0.5 Programme II ND ND 19.9±0.2 The results of the microwave digestion study7 have shown Programme III 0.488±0.05 ND ND that, for Cod Muscle analysis, no major influence of organic Certified value 0.559±0.016 19.6±0.9 As compounds on total As determination could be demon- Plankton: strated.HPLC–ICP-MS determination of As compounds gave Programme I 0.33±0.44 ND 112±4 a clearer picture: with Programmes I and II (Table 4), As was Programme II ND 3.73±0.38 112±2 Certified value 0.276±0.018 3.97±0.19 112±3 mostly present in the form of arseno-betaine (more than 80% Human Hair: of the total As present in the matrix) whereas with Programme Programme I 11.9±0.7 32.9±2.9 197±16 III, As was mineralized in the form of AsV.The results showed Programme II 10.6±0.7 31.2±0.6 191±10 that the digestion procedures were adequate to recover the Certified value 12.3±0.5 33.0±1.2 199±5 total As content present in the matrix in all cases with ICP- * ND=Not determined. AES and ICP-MS. However, in Programmes I and II, analyses by hydride generation (HG)-ICP-AES did not result in a quantitative recovery of As since the arseno-betaine does not Table 6 RSDs (%) obtained for various techniques used in the certification of arsenic in biological reference materials.RSDs obtained by independent laboratories for the dierent techniques are given Material Arsenic Cod Muscle, CRM 4228 HGAAS: 1.5–1.2 Certified value: ZETAAS:* 5.7–2.2 21.1±0.5 mg kg-1 HG-ICP: 2.2 ICP: 2.0–3.9 ICP-MS: 3.1 INAA: 2.5 RNAA: 1.8–3.9–6.9 Plankton, CRM 4149 HGAAS: 2.3–2.3 Certified value: ZETAAS: 3.3–4.2 6.82±0.5 mg kg-1 HG-ICP: 3.9 ICP-MS: 3.0 RNAA: 1.6–3.2–3.1–9.0 SPEC:† 2.4 Lichen, CRM 48210 HGAAS: 1.4 Certified value: ICP-MS: 7.7 0.85±0.07 mg kg-1 INAA: 3.5–2.6 RNAA: 18.2–5.5 White Clover, CRM 40211 HGAAS: 13.2–10.2–13.3–9.6–7.9 Certified value: HG-ICP: 19.5–22.0–25.0–10.3–31.6–27.4 0.093±0.010 mg kg-1 RNAA: 4.4–11.6–7.7–4.2 * ZETAAS=Zeeman-eect background corrected electrothermal atomic absorption spectrometry. † SPEC=UV/VIS spectrometry.Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 875Fig. 2 Bar-graph showing the results for mercury (mg kg-1) in Cod Muscle.Table 7 RSDs (%) obtained for various techniques used in the certification of mercury in biological reference materials. RSDs obtained by independent laboratories for the dierent techniques are given Material Mercury Human Hair, CRM 39713 CVAAS: 2.8–2.9–2.6–7.4–7.3 Certified value: CVAFS: 2.1 12.3±0.5 mg kg-1 ZETAAS:* 5.7 ICP: 2.1–10.0 INAA: 2.4–3.8–10.4–4.3 RNAA: 1.6 Cod Muscle, CRM 4228 CVAAS: 2.9–2.5–4.1 Certified value: CVAFS: 8.3 0.559±0.016 mg kg-1 HG-ICP: 1.2 ICP-MS: 4.4 ID-ICP-MS: 1.9 Tuna Fish, CRM 46314 CVAAS: 2.6–2.7–1.6 Certified value: CVAFS: 1.7–8.4 2.85±0.16 mg kg-1 ICP-MS: 3.7 INAA: 7.8 RNAA: 6.4 Tuna Fish, CRM 46414 CVAAS: 2.5–2.1–2.6 Certified value: CVAFS: 2.5–5.7 5.24±0.10 mg kg-1 ICP-MS: 5.1 INAA: 4.4 RNAA: 3.1 Plankton8 CVAAS 1.8–5.1–1.9 Certified value: HG-ICP: 2.2–1.5 0.276±0.018 mg kg-1 RNAA: 7.7 Lichen10 CVAAS 3.1–2.9 Certified value: CVAFS: 13.0 0.48±0.02 mg kg-1 ICP-MS: 3.0–5.1 ID-ICP-MS: 3.5 INAA: 8.1 RNAA: 12.4 * ZETAAS=Zeeman-eect background corrected electrothermal atomic absorption spectrometry. 876 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12generate hydride forms, i.e., As was solubilized but not min- injection to remove the high chlorine contents; this set displayed a slightly higher RSD than other techniques (Table 6). eralized, which limited the eciency of the hydride generation; with Programme III, the hydride technique could be used A low precision was observed for HG-ICP-AES analysis of White Clover, which was inherent to the low As content successfully since all the As was converted into inorganic AsV.No particular problem was experienced for Plankton analy- (Table 6). sis, i.e., the precision obtained by HG-ICP-AES and ICP-MS was comparable to the precision of alternative techniques Mercury (Table 6). For Lichen analysis, the spread of raw As results indicated relatively high ICP-MS results in comparison with For Hg determination in Cod Muscle, low values were first data obtained by radiochemical separation neutron activation suspected to be due to the diculty of destroying organic analysis (RNAA).Eects of ArCl interference on the determi- matter by microwave irradiation (Fig. 2); a verification made nation of 75As (particularly with the high chlorine content by CVAAS and CVAFS has, however, shown that a good found in the material) obviously caused such systematic inter- agreement could be obtained with the certified value, using ference to occur.This risk of systematic error was not accept- similar microwave digestion techniques to those shown in able at the stage of certification and the ICP-MS data sets Fig. 2. Additional checking by the laboratories demonstrated were recommended to be withdrawn; the only set of data that a calibration error occurred for the CVAAS set (Lab. 05 accepted was based on an ion-exchange treatment with a CVAAS in Fig. 2), whereas the lower recovery found by ICPchromatographic column which was used prior to ICP-MS AES (Lab. 01 HG-ICP in Fig. 2) was assumed to be due to unexplained interferences (e.g., ionization interferences due to the high methylmercury content, representing almost 100% Table 8 RSDs (%) obtained for various techniques used in the of the total Hg present in the sample) rather than to the certification of (total and aqua regia soluble) mercury in Soil and digestion step; this assumption was confirmed by systematic Sewage Sludge reference materials.RSDs obtained by independent comparisons of CVAAS, ICP-AES and ICP-MS analysis of laboratories for the dierent techniques are given. (t) stands for total Cod Muscle,12 as well as by the results obtained in the contents whereas (a) is for aqua regia soluble contents microwave digestion study shown in Table 4.7 A similar slightly lower value of Hg obtained in the Human Hair material with Material Mercury Programme II (Table 4) could be due to a similar source of Calcareous Soil, CRM 141R CVAAS (t): 4.0 error since the amount of methylmercury was assessed to be Certified value (total): CVAAS (a): 1.6–7.9–5.7 0.25±0.02 mg kg-1 ICP-MS (t): 3.5–2.6 ICP-MS (a): 4.4 Certified value (aqua regia) ID-ICP-MS (a): 8.8 Table 10 RSDs (%) obtained for various techniques used in the 0.24±0.03 mg kg-1 RNAA (t): 7.9 certification of (total and aqua regia soluble) lead in Soil and Sewage Sewage Sludge, CRM 144R CVAAS (t): 3.7 Sludge reference materials.RSDs obtained by independent laboratories Certified value (total): CVAAS (a): 3.8–5.0–2.7 for the dierent techniques are given. (t) stands for total contents 3.14±0.23 mg kg-1 ICP-MS (t): 4.7 whereas (a) is for aqua regia soluble contents ICP-MS (a): 4.0 Certified value (aqua regia): ID-ICP-MS (a): 3.2 Material Lead 3.11±0.18 mg kg-1 RNAA (t): 10.0 Calcareous Soil, CRM 141R16 FAAS (t): 4.2 Sewage Sludge, CRM 146R CVAAS (t): 2.9 Certified value (total ): FAAS (a): 2.3–0.6 Certified value (total): CVAAS (a): 2.5–3.1–1.5 57.2±1.2 mg kg-1 ETAAS (t): 1.3 8.62±0.25 mg kg-1 ICP-MS (t): 3.1–6.5 ETAAS (a): 3.9 ICP-MS (a): 2.0 Certified value (aqua regia) ICP (t): 5.8–3.3 Certified value (aqua regia): ID-ICP-MS (a): 2.3 51.3±2.0 mg kg-1 ICP (a): 3.5–6.6 8.39±0.25 mg kg-1 RNAA (t): 0.6 ICP-MS (t): 3.0–3.6 ICP-MS (a): 0.6–2.3 ID-ICP-MS (a): 1.6 ID-MS (a): 0.5 Table 9 RSDs (%) obtained for various techniques used in the DPASV* (t): 3.2–3.7 certification of lead in biological reference materials.RSDs obtained Sewage Sludge, CRM 144R16 FAAS (t): 2.4–3.6 by independent laboratories for the dierent techniques are given Certified value (total ): FAAS (a): 4.0–0.2 106±4 mg kg-1 ETAAS (t): 2.6–5.2 Material Lead ETAAS (a): 1.7 Certified value (aqua regia): ICP (t): 6.7–4.2 Human Hair, CRM 39713 FAAS: 2.8–4.7–4.5–7.2 Certified value: ETAAS: 3.9–3.2–5.3 96.0±1.5 mg kg-1 ICP(a): 2.0–4.3 ICP-MS (t): 6.1–1.2 33.0±1.2 mg kg-1 ICP: 2.8–2.2–5.6 ID-MS: 1.5–1.0 ICP-MS (a): 1.0–4.4 ID-ICP-MS (a): 1.2 DPASV:* 3.5–1.7–2.7–2.3 Cod Muscle, CRM 4228 ICP-MS: 12.2 ID-MS (a): 0.6 DPASV (t): 5.2 Certified value: ID-ICP-MS: 20.0 0.085±0.015 mg kg-1 ID-MS: 9.5 EDXRF† (t): 0.9 Sewage Sludge, CRM 146R16 FAAS (t): 0.7–2.6 ZETAAS:† 10.2–9.1–4.3–22.1 Plankton9 ZETAAS: 11.0–4.5–3.2–4.3 Certified value (total ): FAAS (a): 2.0–0.7 609±14 mg kg-1 ETAAS (t): 1.7 Certified value: ICP-MS: 6.7 3.97±0.19 mg kg-1 ID-MS: 1.6–0.9 ETAAS (a): 2.4 Certified value (aqua regia): ICP (t): 2.9–0.9 DPASV: 3.8–10.5–4.1 Lichen10 ICP: 3.8 583±17 mg kg-1 ICP (a): 2.3–1.3 ICP-MS (t): 1.4–2.3 Certified value: ICP-MS: 3.7–6.3–1.4–1.1 40.9±1.4 mg kg-1 ID-ICP-MS: 1.7 ICP-MS (a): 1.8–1.7 ID-ICP-MS (a): 1.0 ID-MS: 0.2 ETAAS: 1.7 ID-MS (a): 0.6 DPASV (t): 3.6 DPASV: 2.7 EDXRF (t): 3.0 * DPASV=Dierential-pulse anodic stripping voltammetry. † ZETAAS=Zeeman-eect background corrected electrothermal * DPASV=Dierential-pulse anodic stripping voltammetry.† EDXRF=Energy dispersive X-ray fluorescence spectrometry. atomic absorption spectrometry. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 87720% of the total Hg. For matrices where Hg was likely to be found with other techniques for all the matrices tested. Results obtained for total and aqua regia soluble contents for Soil in an inorganic form (e.g., Sediment), an excellent agreement was obtained with the certified value when microwave digestion analysis were not significantly dierent.This tends to show, for the dierent elements tested, that the use of a standardized was applied.7 For Tuna Fish, Plankton and Lichen analysis, no particular digestion procedure (aqua regia) did not, in this case, enhance the method precision. problems were experienced, i.e., the precision of HG-ICP-AES, ICP-MS and ID-ICP-MS was of the same order of magnitude as that of the other techniques (Table 7).Similar conclusions CONCLUSIONS AND TRENDS can be drawn for Soil analysis (Table 8); in addition, it should be noted that no significant dierences in precision were Plasma spectrochemical techniques present the obvious advanobserved between total and aqua regia digestions. tage of multi-element determination capabilities. These techniques are, however, not without problems. For ICP-MS, spectroscopic interferences by polyatomic species are particu- L ead larly severe for elements below mass 80.In this context, the A large standard deviation was found in the Cod Muscle necessity to have two interference-free isotopes for ICP-MS analysis by ID-ICP-MS which was assumed to be due to a rather than the single isotope required for conventional calihigh level of spectroscopic interference at this level of Pb; this bration is considered to be a limitation of the method. Means eect was not observed for Lichen or Soil analysis owing to a to overcome this problem are the use of high resolution much higher total Pb content (Table 9) for which a smaller ICP-MS, chemical separations (e.g., hydride generation) or standard deviation was obtained. chromatography, modification of the plasma chemistry, and No particular problems were generally experienced for electrothermal vaporization methods for removal of the sample ICP-AES and ICP-MS for the other matrices tested, i.e., matrix prior to the atomization of the analytes.ICP-MS is the precision was generally not significantly dierent from certainly a method of choice for routine measurements in that of other techniques (Tables 9 and 10). As observed for comparison with, e.g., ETAAS. There is, however, a clear need Hg, no significant dierences were noted in the precision for education and training of personnel in the use of this obtained for total and aqua regia soluble contents for technique.In addition, interlaboratory studies are needed to Soil analysis. investigate the limitations of this technique, which is often wantonly applied to the whole Periodic Table of elements whereas it is not without problems for the determination of a Zinc range of elements (e.g., Cr, V). Hence, ICP-MS should be supplemented by techniques such as ETAAS in specific As shown in Tables 11 and 12, the RSDs of ICP-AES, DCPAES and ICP-MS were generally comparable to the precision cases.Table 11 RSDs (%) obtained for various techniques used in the certification of zinc in biological reference materials. RSDs obtained by independent laboratories for the dierent techniques are given Material Zinc Human Hair, CRM 39713 FAAS: 0.8–1.6–2.3–2.9 Certified value: ICP: 1.1–2.4–2.8–6.1 199±5 mg kg-1 ID-MS: 1.0–1.4 DPASV*: 3.3 INAA: 2.6–2.1–3.1 Cod Muscle, CRM 4228 FAAS: 4.1–1.8–0.6–2.8–1.4–5.2 Certified value: SS-ZETAAS†: 7.2 19.6±0.5 mg kg-1 ICP: 3.5–1.0–4.1–4.0 DCP: 2.4 ICP-MS: 2.1 ID-MS: 1.1 INAA: 3.7–3.5–1.1 Plankton, CRM 4149 FAAS: 0.9–4.6–4.0–1.3 Certified value: ICP: 1.0–6.9 112±3 mg kg-1 DCP: 1.2 ICP-MS: 2.3–3.5 ID-MS: 0.9 DPASV: 2.7–2.1 INAA: 2.1–3.5 Lichen, CRM 48210 ICP: 2.4–2.2–1.8–0.4 Certified value: DCP: 1.9 100.6±2.2 mg kg-1 ICP-MS: 2.3–6.4–1.9 ID-MS: 1.3 DPASV: 2.2 INAA: 1.3–2.8 RNAA: 1.9 Estuarine Water15 FAAS: 1.1–2.3–1.6–7.6–2.4–7.3 Certified value: ZETAAS‡: 2.7–4.2 172±11 nmol kg-1 ICP: 3.4–7.1 ICP-MS: 3.2 DPASV: 4.2–8.1–2.8 DPCSV§: 1.5 * DPASV=Dierential-pulse anodic stripping voltammetry.† SS-ZETAAS=Solid sampling Zeeman-eect background corrected electrothermal atomic absorption spectrometry. ‡ ZETAAS=Zeeman-eect background corrected electrothermal atomic absorption spectrometry. § DPCSV=Dierential-pulse cathodic stripping voltammetry. 878 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Table 12 RSDs (%) obtained for various techniques used in the pressure nebulization, which are rather dedicated to water certification of (total and aqua regia soluble) zinc in Soil and Sewage analysis; the development of a direct nebulization system would Sludge reference materials.RSDs obtained by independent laboratories be necessary to overcome problems due to the dierent density for the dierent techniques are given. (t) stands for total contents or viscosity of the analysed solutions. whereas (a) is for aqua regia soluble contents Plasma spectrochemical techniques have proved to be Material Zinc methods of choice for the determination of chemical species, owing to the ease of coupling separation devices (GC or Calcareous Soil, CRM 141R16 FAAS (t): 1.4–0.7–3.8 HPLC) with various detectors, e.g., microwave-induced plasma Certified value (total): FAAS (a): 2.3–0.4–2.1 283±5 mg kg-1 ICP (t): 3.4–1.2 atomic emission spectrometry (MIP-AES) or ICP-MS.17 ICP (a): 2.9–0.7 Certified value (aqua regia) ICP-MS (t): 2.9 270±8 mg kg-1 ICP-MS (a): 2.3–3.1 REFERENCES ID-MS (t): 0.8 ID-MS (a): 1.8 1 Measurements and Testing Newsletter, European Commission, DPASV* (t): 2.2 DG XII, SM&T Programme, Brussels.INAA (t): 1.1–2.1 2 Maier, E. A., in Quality Management in Chemical L aboratories, INAA (a): 2.2 ed. Cofino, W. P., and Griepink, B., Elsevier, Amsterdam, EDXRF† (a): 2.0 submitted for publication. Sewage Sludge, CRM 144R16 FAAS (t): 1.3–1.0–2.2 3 Prichard, E., in Quality in the Analytical Chemistry L aboratory, Certified value (total): FAAS (a): 3.0–3.7–0.7 ed.Newman, E. J., Wiley, Chichester, 1995. 932±23 mg kg-1 ICP (t): 4.3–1.2 4 Maier, E. A., Quevauviller, Ph., and Griepink, B., Anal. Chim. ICP (a): 0.8–0.6 Acta, 1993, 283, 590. Certified value (aqua regia): ICP-MS (t): 2.2–2.2 5 Quevauviller, Ph., Mikrochim. Acta, 1996, 123, 3. 919±16 mg kg-1 ICP-MS (a): 2.2–2.2 6 Griepink, B., Quevauviller, Ph., Maier, E. A., and ID-MS (t): 1.6 Vandendriessche, S., Fresenius’ J.Anal. Chem., 1993, 346, 530. ID-MS (a): 1.5 7 Quevauviller, Ph., Imbert, J. L., and Olle�, M., Mikrochim. Acta, DPASV (t): 4.4 1993, 112, 147. INAA (t): 1.6–2.0 8 Quevauviller, Ph., Kramer, G. N., and Griepink, B., Mar. Pollut. EDXRF (a): 1.5 Bull., 1992, 24, 601. Sewage Sludge, CRM 146R16 FAAS (t): 1.8–0.7–2.0 9 Quevauviller, Ph., Vercoutere, K., Muntau, H., and Griepink, B., Certified value (total): FAAS (a): 1.8–1.8–3.2 Fresenius’ J. Anal. Chem., 1993, 345, 12. 3061±59 mg kg-1 ICP (t): 1.4–1.4 10 Quevauviller, Ph., Herzig, R., and Muntau, H., Sci. T otal Environ., ICP (a): 1.2–1.0 1996, 187, 143. Certified value (aqua regia): ICP-MS (t): 0.7 11 Quevauviller, Ph., Vercoutere, K., and Griepink, B., Anal. Chim. 3043±58 mg kg-1 ICP-MS (a): 1.1–3.5 Acta, 1992, 259, 281. ID-MS (t): 2.5 12 Campbell, M. J., Vermeir, G., Dams, R., and Quevauviller, Ph., ID-MS (a): 2.0 J. Anal. At. Spectrom., 1992, 7, 617. DPASV (t): 4.2 13 Quevauviller, Ph., Maier, E. A., Vercoutere, K., Muntau, H., and INAA (t): 1.2–1.3 Griepink, B., Fresenius’ J. Anal. Chem., 1992, 343, 335. INAA (a): 1.4 14 Quevauviller, Ph., Drabaek, I., Muntau, H., Bianchi, M., EDXRF (a): 2.0 Bortoli, A., and Griepink, B., T rAC T rends Anal. Chem., (Pers. Ed.), 1996, 15, 160. * DPASV=Dierential-pulse anodic stripping voltammetry. 15 Quevauviller, Ph., Kramer, K. J. M., and Vinhas, T., Fresenius’ † EDXRF=Energy dispersive X-ray fluorescence spectrometry. J. Anal. Chem., 1996, 354, 397. 16 Quevauviller, Ph., Maier, E. A., Griepink, B., Fortunati, U., Vercoutere, K., and Muntau, H., T rAC T rends Anal. Chem., (Pers. For ICP-AES, the development of new software for fast Ed.), 1996, 15, 504. screening analysis is needed for qualitative measurement of 17 Quevauviller, Ph., Maier, E. A., and Griepink, B., in Element unknown samples. Trends are focusing on the development of Speciation in Bioinorganic Chemistry, ed. Caroli, S., Wiley, New multi-channel detectors combined with high resolution optical York, 1996, pp. 195–220. systems and conventional photomultipliers with new dynamic electronics to improve the present linear range. The analysis Paper 7/00463J of complex matrices remains dicult using ultrasonic or high Received January 21, 1997 Accepted April 14, 1997 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 8

ISSN:0267-9477    

DOI:10.1039/a700463j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Double-focusing Sector Field Inductively Coupled Plasma Mass Spectrometry for Highly Sensitive Multi-element and Isotopic Analysis† Invited Lecture J. SABINE BECKER* AND HANS-JOACHIM DIETZE Zentralabteilung fu� r Chemische Analysen, Forschungszentrum Ju�lich GmbH, D-52425 Ju�lich, Germany The dierent areas of application in double-focusing sector charged atomic ions (e.g., 92Zr+ and 92Mo+: m/Dm#52 000) requires a mass spectrometer with high mass resolution (e.g., field ICP-MS are described, such as the determination of trace and ultratrace elements in environmental and materials Fourier transform or ion trap mass spectrometers,10–12 interferences of singly charged with doubly charged atomic ions research and for the characterisation of long-lived radionuclides in environmental and radioactive waste samples.(e.g., 92Mo+ and 184W2+: m/Dm#1300) can be separated in many cases at the required mass resolution by using a double- Analytical methods using double-focusing sector field ICP-MS allow the determination at low mass resolution of ultratrace focusing sector field mass spectrometer. The apparent interferences of atomic ions of analyte and disturbing molecular elements (e.g., rare earth elements) and some radionuclides in the mg l-1 and pg l-1 concentration ranges (e.g., for U, Th, Pu ions at the same nominal mass in the mass spectra (e.g., 41K+ and 40ArH+: m/Dm#5000) in all mass spectrometric methods and 129I ) in aqueous solutions and at the ng g-1 level and lower in solid samples after a digestion step.Some examples are of the greatest importance, as demonstrated by the numerous investigations that have been reported.13–20 Therefore, of trace analysis of solid samples after matrix separation are discussed. For the determination of spallation nuclides and overcoming these interference problems of atomic ions of the analyte with molecular ions is important for accurate and impurities in irradiated tantalum (with an 800 MeV proton beam) from a spallation neutron source, double-focusing sector precise trace, ultratrace and isotopic analysis of inorganic materials by ICP-MS. The rate of formation of molecular ions field ICP-MS was used after liquid–liquid extraction of the tantalum matrix.Further, the results of the analysis of high- (such as MOn+, MOnH+, CnOm+, NnOm+; MN+, MCl+, MAr+ with M=matrix element; ArnXm+ with X=H, O, N, Cl purity GaAs by double-focusing ICP-MS after dissolution with and without matrix separation are compared with those of and others) in an inductively coupled plasma is dependent on the stability of the ionic species (bond dissociation energy) and quadrupole ICP-MS.varies as a function of rf power, nebulizer gas flow rate and Keywords: Double-focusing sector field mass spectrometry; sampling depth, as demonstrated in many papers.14–16,18,20 A environmental samples; high-purity GaAs; inductively coupled mass spectrometric separation of such interferences applying plasma; long-lived radionuclides; molecular ions; tantalum; a double-focusing sector field mass spectrometer at higher ultratrace analysis mass resolution in comparison with low-resolution quadrupole ICP-MS (ICP-QMS) is often useful for the determination of elements abundant in nature in the mass range up to 75 u.21–26 Among the dierent methods for the determination of trace and ultratrace elements and isotopic composition in inorganic Reed et al.19 reported these possible interferences of molecular ions and atomic ions from 24Mg+ to 76Ge+ and discussed the materials, ICP-MS is well established as a universal, powerful and very sensitive multi-element method applicable in all fields determination of elements in dierent matrices using a doublefocusing sector field ICP-MS with a maximum mass resolution of modern science and technology: in materials research, e.g., for the characterisation of high-purity materials, in the semi- of 10 000.The mass resolution required for the separation of oxide conductor industry and microelectronics, in environmental and biological research, in medical science and in geology and ions (MO+) from the atomic ions of the analyte as a function of mass is demonstrated in Fig. 1. With a maximum mass mineralogy.1–4 The determination of elements in the trace and ultratrace concentration range is often dicult owing to poss- resolution of 10 000 for commercial double-focusing ICP mass spectrometers, the isobaric interferences of atomic and oxide ible matrix eects, which could be avoided by matrix separation or selective preconcentration of the trace elements of interest.5–9 ions in the mass range with the most abundant elements in nature can be separated.Maximum mass resolution Furthermore, possible contamination during sample preparation and high blank values, especially from elements abun- (m/Dm#4×106) is required for the separation of interferences of 90Zr18O+ and 108Pd+.All other points in the figure relate dant in nature, lead to incorrect analytical results. Blank values and possible contamination should be minimised by careful to interferences of M16O+ (M=metal or non-metal) with the most abundant analyte ions at a mass which is 16 u higher working under ultraclean conditions and using ultrahighpurity chemicals. In addition to these general problems, trace than mass of M. It is clearly seen that possible interferences of MO+ and analyte ions with mass higher than 70 u are dicult and ultratrace analysis is often disturbed by possible interto separate using, e.g., double-focusing sector field ICP-MS ferences in mass spectra.Whereas due to the low mass dierwith a maximum mass resolution (m/Dm) of 7500. In practice, ence the separation of the interferences of isobaric singly the theoretically required mass resolution is insucient if the intensity of molecular ions is significantly higher than that of † Presented at the 1997 European Winter Conference on Plasma Spectrochemistry, Gent, Belgium, January 12–17, 1997.the analyte ions. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 (881–889) 881Further molecular ion formation in the ICP and by the expansion of plasma in the low-pressure interface of the ICP-MS system can be reduced using the cool plasma technique in combination with a shielded ICP torch.33,34 This cool plasma technique leads to a significant improvement in the determination of some elements important in the environment, e.g., Fe, Ca, Na and K, with detection limits in the low ng l-1 concentration range, as was demonstrated by Tanner et al.35 and Georgitis et al.36 using quadrupole ICP-MS.An interesting development proposed by Speakman et al.37 in quadrupole ICP-MS is the use of a collision cell filled with helium in combination with a hexapole ion lens behind the skimmer cone for the thermalisation of ions in order to decrease the energy spread and to dissociate disturbing molecular ions.COMPARISON OF DOUBLE-FOCUSING SECTOR FIELD ICP-MS INSTRUMENTS Some experimental parameters and properties of the commercial double-focusing sector field ICP mass spectrometers Element (Finnigan MAT, Bremen, Germany), PlasmaTrace 2 (Micromass, Manchester, UK) and JMS-Plasma X2 (Jeol, Tokyo, Japan)38–41 are compared in Table 1. All these mass spectrometers are double-focusing sector-field instruments with reverse Nier–Johnson geometry.A double-focusing sector field mass spectrometer combines a magnetic sector field for direction focusing and an electric sector field for energy focusing of ion beams. Double focusing is reached at the point where the two image curves for focusing of energy and direction cross. Fig. 1 Mass resolution required for separation of oxide ions from The performances of commercial double-focusing sector field atomic ions of the analyte as a function of mass.ICP-MS systems are similar with respect to the achievable high sensitivity (n×107 ions s-1 per mg l-1 at In), very low noise (<0.2 ions s-1) and very low detection limits (in the The mass resolution required for the separation of argon molecular ions (ArX+) from atomic ions of the analyte has low pg l-1 concentration range determined at low mass reon in ideal aqueous solutions). A dierence is in the slit been discussed.16 It is possible to separate most ArX+ from atomic ions of the analyte at a mass resolution of 10 000, apart system (and in the price).The slit system of the PlasmaTrace 2 with up to five resolution values allows a variable mass from some metal argide ions in the mass range 80–100 u. In contrast to the curves for the mass resolution required resolution by automatically changing the resolution in order to maximise the transmission of ions for each element. for the separation of MO+ and ArX+ from atomic ions of the analyte as a function of mass with the maximum at a mass of Compared with the fixed slit system of Element, this fully variable slit system has the advantage that the mass resolution about 100 and 90 u, respectively, the mass resolution required for separation of hydride ions (MH+) increases with increasing required for the separation of interferences of atomic and molecular ions can be adjusted.mass, where at mass>89 u a mass resolution (m/Dm) of >10 000 is necessary for the separation of interferences with A double-focusing sector field ICP-MS system with Mattauch–Herzog geometry and multi-channel ion detection atomic ions. This means that in many cases a decrease in molecular ion formation is required for the determination of (photodiode-array detector) is used for the simultaneous detection of ions, especially for the detection of transient signals, as ultratrace elements, especially when a complicated matrix is to be analysed. described by Cromwell and Arrowsmith.42 This simultaneous ion detection is important for precise isotope measurements.The ways of decreasing molecular ion formation in order to improve the detection limits in double-focusing ICP-MS, by The major limiting factor on the precision of isotopic ratio measurements by ICP-MS is the instability of the argon applying the same as used in quadrupole ICP-MS, i.e., by applying special sample introduction systems, matrix separa- plasma.Therefore, a multi-collector ion detection system is realised by the double-focusing ICP-MS Plasma 54 prototype tion, enrichment of trace impurities, suppression of molecular ion formation, etc. The combination of electrothermal vaporis- (VG Elemental, Winsford, Cheshire, UK) with nine Faraday detectors.43 Recently, a single-focusing magnetic sector field ation or a hydride generator with ICP-MS (e.g., for the sensitive determination of Se or As,27–30) which is applied in order to ICP-MS instrument with multi-collector (Iso-PlasmaTrace, Micromass)44 was introduced for the determination of precise reduce some of the disturbing interferences by selective separation of the analyte, leads to a loss of the multielement capability isotopic ratios.Multi-collection using the fully adjustable Faraday cup permits the measurement of isotopic ratios down of ICP-MS. Furthermore, using an ultrasonic nebulizer, the microconcentric nebulizer (both with desolvator), direct injec- to a precision of better than 0.002%.As an alternative to double-focusing sector field ICP-MS, a tion nebulizer or thermospray nebulizer–membrane separator31,32 for small sample volumes, multielement analysis can quadrupole ICP-MS system was recently developed by Ying and Douglas45 with a maximum mass resolution of 9000. At a be carried out in the ultratrace concentration range with detection limits at the ng l-1 level.The separation of matrix mass resolution of 5000 the sensitivity is 106 ions s-1 per mg l-1 of element in solution. This sensitivity is comparable to that elements or of the organic matrix or enrichment of selected trace elements, such as by extraction, ion chromatography or of commercial double-focusing sector field ICP-MS systems operated at the same mass resolution. In the low-resolution HPLC, can be performed before mass spectrometric measurement or on-line coupled to ICP-MS.5–9 mode the sensitivity is 108 ions s-1 per mg l-1 of element in 882 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Table 1 Comparison of commercial double-focusing ICP mass spectrometers Element PlasmaTrace 2 JMS-Plasma X2 (Finnigan MAT) (Micromass) (Jeol) MS configuration Reverse Nier–Johnson Reverse Nier–Johnson Reverse Nier–Johnson Rm=16 cm Rm=20 cm Rm=31 cm Re=10.5 cm Re=26.5 cm Re=22.3 cm Accelerating voltage UB=1–8 kV UB=1–6 kV UB=1–5 kV Mass range 2–260 Da 2–300 Da 2–500 Da Mass resolution setting 3 resolutions 5 resolutions Continuous Maximum mass resolution 7500 10 000 10 000 (m/Dm) Ion detection Analogous; Analogous; Analogous; (single ion collector) ion counting ion counting ion counting Noise <0.2 counts s-1 <0.2 counts s-1 <0.2 counts s-1 Sensitivity 2×107 counts s-1 per mg l-1 at In 5×107 counts s-1 per mg l-1 at In 2×107 counts s-1 per mg l-1 at In (m/Dm=300) (m/Dm=400) (m/Dm=500) solution.The continuum background is significantly higher 20 mg l-1 for Zn.Most elements are measured with high sensitivity at low mass resolution. (about 1000 ions s-1) compared with that of a commercial double-focusing sector field ICP-MS instrument. A decrease ICP-MS is used in some applications for the determination of only one ultratrace element. Takaku et al.48 described the in the background and clarification of the commercial production of such mass spectrometers are major tasks in further determination of silicon in ultrahigh-purity water for microelectronics and investigated the instrumental blank due to the development work.The application of double-focusing sector field ICP-MS in quartz plasma torch, nebulizer and spray chamber. For the determination of silicon the contamination problems are most trace and ultratrace analysis is not restricted to the determination of some ‘dicult’ elements (Cr, Cu, Ni, Fe, V, As, Se, important. The silicon contamination from the original quartz plasma torch was determined to be 5–20 mg l-1. The back- P, Al, Si, S, Co, Ti, Mn, Sc) in environmental samples, highpurity metals or semiconductors, ceramics, etc., by the mass ground of the PlasmaTrace double-focusing ICP-MS system could be decreased by about one order of magnitude using a spectrometric separation of molecular ions from the atomic ions of the analyte.The largest application field in dierent platinium torch and Teflon nebulizer.The determination of Si using the PlasmaTrace double-focusing sector field ICP-MS laboratories is the highly sensitive determination of many other elements (e.g., Li, Be, B, Na, noble metals, lanthanides, actin- system in order to separate the interferences with 12C16O+ and 14N2+ at a mass resolution of about 2000 in semiconductor- ides) carried out at low mass resolution, which will be discussed in the following sections. grade water yielded <1 mg l-1 in comparison with the Si concentration in Milli-Q-purified and de-ionized water of 30 and 35 mg l-1, respectively.For the determination of Si by DETERMINATION OF IMPURITIES OF TRACE double-focusing sector field ICP-MS in the ng l-1 concen- AND ULTRATRACE ELEMENTS IN WATER tration range, a preconcentration step by evaporation in a SAMPLES clean bench is necessary. A similarly important analytical task for the microelectronics ICP-MS using an ecient nebulizer in order to introduce industry is the determination of P in high-purity water or aqueous solutions is ideal for the highly sensitive multi-element acids.The determination of phosphorus (mass spectrometric analysis of water samples for the microelectronics industry separation of the analyte ions 31P+ from interfering molecular (high-purity water) and environmental research (terrestrial ions 15N16O+ and 14N16O1H+) was performed in our labora- water, drinking or rain water, waste water or snow). The tory at a fixed mass resolution of 3000 using an Element detection limits of most elements attained by double-focusing double-focusing sector field ICP-MS system with detection ICP-MS in the low-resolution mode were in the extremely limits in the 10 ng l-1 concentration range.low pg l-1 concentration range for high-purity water, as dem- Narasaki and Cao41 developed a method for determining onstrated by Yamasaki et al.46 using direct sample introduction As and Se by hydride generation ICP-MS using a JMS-Plasma in an ultrasonic nebulizer combined with a PlasmaTrace X2 double-focusing sector field ICP-MS instrument (Jeol), double-focusing sector field ICP-MS system.The detection which allows a high sample introduction eciency owing to limits for elements which were measured at a higher mass matrix separation. The detection limits were 30 ng l-1 for As resolution compared with ICP-QMS varied for dierent and 60 ng l-1 for Se. Although the mass resolution required elements from 0.019 ng l-1 for Sc through 2.09 ng l-1 for Fe for the separation of interferences of 82Se+ and 40Ar2H2+ is to 360 ng l-1 for Si.They determined around 40 ultratrace about 3500, the measurements were carried out at m/Dm= elements in terrestrial water. In further work, Yamasaki et al.47 10 000 owing to the relatively high disturbance by 40Ar2H2+. determined especially the lanthanide and actinide concen- The method developed using hydride generation ICP-MS was trations in terrestrial water in Japan with a precision of 5% applied to the determination of trace levels of As and Se in (RSD) with the same experimental arrangement.Although the river water in the low mg l-1 to ng l-1 concentration range. isobaric interference of 153Eu+ with 137Ba16O+ ions at a mass resolution of 7500 could be separated, they used the mathematical correction of interferences of BaO+ ions on dierent DETERMINATION OF ULTRATRACE isotopes of lanthanide ions in order to avoid a serious decrease ELEMENTS IN ENVIRONMENTAL AND in sensitivity.The detection limits for lanthanide and actinide BIOLOGICAL SAMPLES elements in terrestrial water samples without sample pretreatment are in the low pg l-1 concentration range and The application of double-focusing ICP-MS is demonstrated for the determination of elements dicult to determine in <1 pg l-1, respectively. The element concentrations varied in rainwater samples over a concentration range of about six biological and medical research in the ultralow concentration range, e.g., by the groups of Moens23 and Duneman.26 orders of magnitude, from 10 pg l-1 for some lanthanides to Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 883Riondato et al.49 measured the ultratrace elements P, S, Si, Al, coupled to an Element double-focusing sector field ICP-MS system.51 Iodide in the sample solution was oxidised to gaseous Cr, Mn, and Ti in human serum by flow injection coupled to an Element double-focusing sector field ICP-MS system at a iodine with concentrated perchloric acid.By desolvation of the initial aqueous solution, an increase in ion intensity of about fixed mass resolution of m/Dm=3000. Owing to numerous disturbing interferences the determination of precious metals two orders of magnitude was achieved in comparison with a conventional Meinhard nebulizer with a detection limit of (Rh, Pd, Ag and Pt) at the ultratrace level in biological materials is dicult, as demonstrated by Begerow et al.26 100 ng l-1 in solutions.An improvement of the detection limit in the determination In our laboratory, some environmental standard reference materials (SRMs, e.g., apple leaves, NIST 1515; pine needles, of 129I is dicult owing to interference with the 129Xe isobar. Furthermore, for determination of 129I, on-line isotope dilution NIST 1575; and mussel tissue, BCR 278) were investigated in which the rare earth elements are incompletely certified.For with stable 127I using the flow injection principle in order to improve the accuracy of results has been applied.51 their selective determination after digestion of the samples the rare earth elements were separated from the matrix using Another task in our institute is the nuclide analysis of an irradiated tantalum target. Tantalum was used as the target liquid–liquid extraction.8 The results of the determination of rare earth elements in dierent SRMs using quadrupole material in a spallation neutron source with 800 MeV protons.The determination of the concentration of spallation nuclides ICP-MS without and with isotope dilution and doublefocusing sector field ICP-MS (using an Element system) agree in a highly radioactive solid matrix in the concentration range from 1 ng g-1 to 50 mg g-1 should serve to verify the theoretical well with one another and also with the certified values, if available.All rare earth elements could be determined only by results of spallation yields of tantalum. For the determination of spallation nuclides in irradiated tantalum, an Element double-focusing ICP-MS after matrix separation. The determination of As and Se in biological SRMs after double-focusing sector field ICP-MS system52 can be used after liquid–liquid extraction of the tantalum matrix in order acid digestion of sample and cation exchange (using a Chelex-100 column) was described by Narasaki and Cao.41 to reduce the high 182Ta activity.The method for the determination of trace impurities after matrix separation was developed using high-purity inactive tantalum. In Table 2, the results ULTRATRACE DETERMINATION OF of the trace analysis for inactive tantalum after matrix separa- LONG-LIVED RADIONUCLIDES tion by double-focusing sector field ICP-MS are compared with those of neutron activation analysis (NAA). It can be The special area in which double-focusing sector field ICP-MS is advantageous concerns the ultratrace determination of long- clearly seen that in comparison with NAA, ICP-MS allows more elements to be determined in low concentration ranges.lived radionuclides in environmental and radioactive waste samples at very low concentration ranges. Conventional radio- The main problem in determining long-lived radionuclides in irradiated tantalum is possible interferences of radioactive metric methods are, however, time consuming at low concentration levels and possess a low specific activity and poor spallation nuclides with stable isotopes at the same mass with a dierent atomic number.Therefore, in order to separate precision. The correct and precise determination of long-lived radionuclides is required for the determination of enrichments isobars for rare earth elements, such as the long-lived 151Sm from stable 151Eu, HPLC was coupled with double-focusing of radioactive nuclides due to nuclear weapons testing or fallout from, e.g., the Chernobyl accident in biological samples, ICP-MS in our laboratory.53 The method of separation of rare earth elements with natural isotopic abundances and enriched waters and geological materials.Further, the determination of long-lived radionuclides in radioactive wastes from nuclear stable isotope HPLC–ICP-MS has been described;53 investigations on irradiated Ta samples after matrix separation are reactors for recycling and final storage of radioactive waste is important. In addition to the required isotopic analysis for U, in progress.The determination of 99Tc in environmental samples is Th, Cm, Am and Pu, especially mass spectrometric methods for the sensitive determination of the long-lived radionuclides disturbed by isobaric interference with 99Ru. Yamamoto et al.54 avoided this interference by the separation of 99Tc using 237Np, 129I, 99Tc, 79Se, 107Pd, 135Cs, 93Mo, 93Zr, 151Sm, etc., in the ultralow concentration range are of interest, where other dierent solvent extraction and purification techniques using anion exchange after leaching of the soil sample.The soil radiometric methods experience diculties. Kim et al.50 determined the detection limits for the long- sample was spiked with a 99mTc nuclide before leaching and no loss of 99Tc was observed during this treatment. Yamamoto lived radionuclides 99Tc, 226Ra, 232Th, 237Np, 238U, 239Pu and 240Pu with half lives of 103–1010 years in standard solutions.et al. determined 99Tc with an absolute detection limit of 0.25 pg, corresponding to 0.16 mBq, and analysed sediment Using a PlasmaTrace double-focusing ICP-MS system with an ultrasonic nebulizer, the detection limits ranged from 2 to samples from the Irish Sea. The same group determined 237Np in a similar way after leaching of the soil sample, solvent 20 pg l-1. The sensitivity of double-focusing ICP-MS with the application of an ultrasonic nebulizer was 10 times better than the same arrangement without the ultrasonic nebulizer.Table 2 Determination of impurities in inactive tantalum after matrix Double-focusing sector field ICP-MS has also been used for separation (concentrations in mg g-1) the selective and sensitive determination a single long-lived Element ICP-MS NAA radionuclide, e.g., the determination of 79Se.28 In this case, hydride generation was used in order to improve the sample Al 1.52±0.05 — V <0.006 — introduction eciency and to avoid possible interferences with Fe 0.88±0.09 — isobaric singly charged ions (79Br+), doubly charged atomic Co 0.023±0.006 0.032±0.003 ions (158Gd2+ and 158Dy2+) and dierent molecular ions Zn 0.45±0.15 <0.5 (39K40Ar+, 38Ar40ArH+ and 63Cu16O+).Most of the possible Zr 0.34±0.1 <8 interferences, but not the 38Ar40ArH+ ions, were eliminated. Mo 17.6±0.6 35±3 The detection limit of 79Se using a special hydride generator Ba 0.06±0.02 <7 La <0.006 <0.001 coupled to an Element double-focusing sector field ICP-MS Hf <0.11 0.12±0.2 system was about 100 ng l-1.Similarly to the hydride generator W 208±20 332±30 described by Hoppstock et al.,28 the sample introduction Ir 0.49±0.04 — equipment for the highly sensitive determination of iodine and Pb 0.48±0.05 — also the stable isotope 127I and the long-lived nuclide 129I is 884 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Table 3 Comparison of some results obtained on GaAs after selective extraction and anion exchange in order to evaluate the fallout volatilization of the matrix (concentrations in ng g-1) radionuclides from nuclear weapons tests. Whereas in the determination of selected long-lived radio- Element ICP-QMS Double-focusing ICP-MS nuclides the high sensitivity of double-focusing ICP-MS, but Li <10 3.2±0.8 not the multi-element capability, is used, the multi-element Be <5 <1 capability is important for the characterisation of high-purity Mg 190±45 225±4 materials.Al 1076±108 850±10 V 24±8 <5 Sn <20 3.8±1 DETERMINATION OF TRACE AND Te <10 4±1 ULTRATRACE IMPURITIES IN HIGH-PURITY Pb 102±5 32±2 SOLID MATERIALS AFTER DIGESTION Bi <50 25±3 Trace analysis of high-purity materials is important especially in the microelectronics industry. With increasing purity of (with interchangeable ICP and rf GD ion sources) for trace metals, semiconductors and insulators, an improvement of and ultratrace analysis of GaAs has been described.55 ultratrace analytical techniques is required.Takaku et al.25 The applications of double-focusing sector field ICP-MS in determined trace impurity rare earth elements in high-purity trace and ultratrace analysis are summarized in Table 4. Gd2O3 and Y2O3 using double-focusing ICP-MS. Because the mass resolution required for the separation of rare earth element ions and disturbing molecular ions of Gd is higher ISOTOPIC RATIO MEASUREMENTS than was practically possible with ICP-MS, doubly charged An important advantage of double-focusing sector field rare earth element ions were used as analyte ions for the ICP-MS compared with quadrupole ICP-MS is in the area of detection of trace impurities in lanthanide oxides.The ratios isotopic ratio measurements. The precision of the isotopic of doubly charged to singly charged rare earth element ions abundance ratios has typically been limited to 0.1–0.5% with varied from 0.2% for Lu to 11% for Ce.The detection limits the quadrupole ICP-MS systems commercially available. In for rare earth elements were 80 pg l-1–3 ng l-1 using doubly general, isotopic ratio measurements using double-focusing positively charged ions, compared with 6–30 pg l-1 using sector field ICP-MS yield more accurate and precise isotopic singly charged ions by analysing an ideal aqueous solution. ratios owing to better counting statistics and lower noise in The real concentrations of trace impurities of rare earth comparison with quadrupole ICP-MS. elements in high-purity solid Gd2O3 and Y2O3 were measured The correct and precise isotopic analysis of radioactive in the ng g-1 concentration range.elements, such as U and Pu, is required for the determination A method with selective volatilisation of the matrix was of the enrichment of radioactive nuclides due to nuclear developed in our laboratory for the ultratrace analysis by weapons testing in biological samples, waters and geological ICP-MS of GaAs, which is important for microelectronics.materials. Kim et al.50 compared the results of the isotopic This selective volatilisation of GaAs is achieved by converting analysis of a Pu isotopic standard solution containing 20 ng l-1 the matrix elements into their chlorides in a stream of argon Pu by double-focusing sector field and quadrupole ICP-MS.and gaseous chlorine. Owing to the low volatility of the The accuracy and precision of Pu isotopic analysis could be chlorides of gallium and arsenic (melting points, GaCl3 78 °C improved by double-focusing ICP-MS with an ultrasonic and AsCl3 -20 °C; boiling points, GaCl3 201 °C and AsCl3 nebuliser compared with quadrupole ICP-MS by about one 130 °C), it is possible to separate the matrix elements, leaving order of magnitude. behind those impurities which do not readily form volatile Isotopic standard solutions of uranium have been analysed chlorides.A schematic diagram of the experimental arrangeto determine the mass discrimination eect (the mass discrimi- ment for matrix separation is shown in Fig. 2. The GaAs nation was determined to be about 0.1%) during the measure- sample was transferred into the heating chamber in a quartz ment and the dead time eect on the ion detector (about boat. After purge treatment, 0.5 g of sample was completely 26 ns).56 In Table 5, the isotopic ratios of uranium measured volatilized (30 min) in an argon–chlorine stream (Ar5Cl#551) on a digested soil sample are compared with the values from at 220 °C.The small amount of white residue in the quartz an isotopic table.57 An agreement with the natural isotopic boat was dissolved in hot HNO3. abundance of uranium was measured. The precision of The results of trace impurity determination in high-purity determining the 235U/238U isotopic ratio of uranium in a real GaAs after matrix separation using quadrupole and doublegeological sample was approximately 0.6%.The possible inter- focusing ICP-MS are summarised in Table 3. In comparison ferences of molecular ions in the mass spectra of dissolved soil with the results without matrix separation, the detection limits samples in the mass range of actinides are discussed in the were improved from 500 ng g-1 by over two orders of magninext section. tude in the low ng g-1 concentration range.These detection Vanhaecke and co-workers58,59 demonstrated the capability limits are comparable to those of direct solid analysis on GaAs of double-focusing ICP-MS at low mass resolution in determin- by rf GDMS. A comparison of ICP-MS and rf GDMS using ing the isotopic ratios of magnesium and lead. At higher an Element double-focusing sector field mass spectrometer element concentrations isotopic ratios with an RSD of 0.04% were measured. The determination of the 63Cu/65Cu isotopic ratio at a mass resolution of 3000 yielded a precision of about 0.2%.The use of a multiple collector system in double-focusing ICP-MS (Plasma 54 prototype combined with a desolvator), as demonstrated for the determination of the 176Hf/177Hf ratio by Walder et al.61 allows isotopic ratio measurements at precisions of down to 0.002% (RSD) (concentration of Hf, 50 mg l-1). In their investigations of geological, environmental and nuclear materials, Walder and co-workers,60–62,65 showed that it is possible to achieve a precision for isotopic analysis Fig. 2 Experimental set-up for separation of GaAs. comparable to that of thermal ionisation mass spectrometry. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 885Table 4 Application of double-focusing sector field ICP-MS in trace and ultratrace analysis Sample Equipment Elements Concentration range Limit of detection Ref. Terrestrial water PlasmaTrace 40 ultratrace elements 20 mg l-1 (Zn) – At m/Dm=400: Yamasaki et al.46 and USN 15 pg l-1 (Tm) 0.5–5 pg l-1 (lanthanides) At m/Dm=3000: 0.02 ng l-1 (Sc) 2 ng l-1 (Fe) Terrestrial water PlasmaTrace Lanthanides and 1–40 ng l-1 0.5–12 pg l-1 Yamasaki and and USN actinides Tsumura47 Ultrahigh-purity water PlasmaTrace Si (after preconcentration) 0.3–35 mg l-1 0.1 mg l-1 Takaku et al.48 (restricted by blank) Biological samples JMS-Plasma X2 and As, Se Biological SRM: In solution: Narasaki and Cao41 (SRM) hydride generator 0.4–26 mg g-1 As 30 ng l-1 (As) 0.4–7 mg g-1 Se 60 ng l-1 (Se) River water River water: low mg l-1 Standard solutions PlasmaTrace 99Te, 226Ra, 232Th, 1–6 ng l-1 0.002–0.02 ng l-1 Kim et al.50 and USN 237Np, 238U, 239Pu, 240Pu Human serum Element V, Fe, Cu, Zn, Ag 0.8 ng g-1 (V)– In solution: Moens et al.23 24 mg g-1 (Fe) 4.3 ng l-1 (Ag) Human serum Element and P, S, Si, Al, Cr, Mn, 0.7 ng l-1 (U) 0.05 mg l-1 (Ti) Riondato et al.49 flow injection Ti, Ag, Cd, Sn, K 226 ng l-1 (Cd) 0.66 mg l-1 (Al ) (restricted by blanks) Human urine Element Rh, Pd, Ag, Pt 0.5–7.6 ng l-1 (Pt) 0.24 ng l-1 (Pt) Begerow et al.26 0.17 ng l-1 (Au, Pd) Biological samples Element Lanthanides after 0.014–20 ng g-1 0.01 ng g-1 Panday et al.8 (SRM) liquid–liquid extraction Biological samples Element and Se(79Se) Biological samples: In solution: Hoppstock et al.28 (SRM), radioactive hydride generator 0.05–1.7 mg g-1 100 ng l-1 waste Biological samples Element and special 129I, 127I Biological samples: In solution: Kerl et al.53 (SRM), radioactive sample introduction 0.3–1.8 mg g-1 50 ng l-1 waste (0.8 ml sample volume) Tantalum Element 39 elements after 0.008–200 mg g-1 1 ng g-1 (Lu, Tm) Becker et al.52 matrix separation 6 ng g-1(V) Environmental samples PlasmaTrace 99Tc after solvent extraction Sediments: In solution: Yamamoto et al.54 (geological samples) 0.8–3.5 ng g-1 0.25 ng l-1 High-purity solids: PlasmaTrace lanthanides Gd2O3: In solution: Takaku et al.25 Y2O3, Gd2O3 2–3200 ng g-1 6–30 pg l-1 Al2O3 Element prototype V, Cr, Mn, Fe, Ga, Co, 0.04–3.7 mg g-1 At m/Dm=300: Jakubowski et al.24 Ni, Cu, Zn, Ce 0.4–400 ng g-1 At m/Dm=3000: 8–1400 ng g-1 Concrete, radioactive Element and 99Tc, 232Th, 233U, 235U 30ngg-1–6.1 mg g-1 In synthetic concrete Gastel et al.68 waste samples laser ablation 238U, 237Np laboratory standard: 0.5 ng g-1 (237Np) 4 ng g-1 (99Tc) 886 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Table 5 Results of isotopic analysis of uranium molecular ions in the high mass range of a digested soil sample is shown in Fig. 3. In the mass spectrum of a soil sample Uranium of natural isotopic digested with an acid mixture containing HCl, dierent molecu- Isotope ratio composition57 Uranium in soil sample lar ions in the mass range of actinides were observed. The 233U/238U — <10-6 relative ion intensities of disturbing PbCl+ and PbAr+ molecu- 234U/238U 5.5×10-5 5.6×10-5 lar ions, some radionuclide ions disturbed by molecular ions 235U/238U 7.26×10-3 7.25×10-3 in the investigated mass range and their half-lives are summar- 236U/238U — <10-6 ized in Table 7.Owing to the positive mass defect for actinides, the required mass resolution for the separation of disturbing molecular ions from long-lived actinide ions is relatively low, The determination of possible isotopic variations in nature m/Dm#2000. A systematic study of possible interferences in due to radioactive decays of unstable nuclides will be used in the determination of ultralow concentrations of actinides is geochronology for age determination (e.g., Rb–Sr method necessary. In order to exclude molecular ion formation in this based on the b-decay of 87Rb; Sm–Nd method; Re–Os age mass range for the determination of ultratraces levels of determination or U–Pb method which uses the fission of 238U) actinides, the method of choice will be a selective actinide where precise isotopic analysis is necessary.For geochronology, separation. thermal ionisation mass spectrometry (TIMS) is the most established method for precise isotope measurements. With the improvement in the precision of isotopic ratio measure- SOLID STATE MASS SPECTROMETRY ments by double-focusing sector field ICP-MS, this method In order to analyse solid samples by mass spectrometric also allows such applications if the elements of parent and methods without dissolution steps, a laser ablation (LA) system daughter nuclides are separated chemically.was coupled to double-focusing sector field ICP-MS. The applications of double-focusing sector field ICP-MS in LA–ICP-MS has been realized by dierent groups, e.g., by isotopic analysis are summarized in Table 6 Walder et al.66 Christensen et al,67 using the Plasma 54 doublefocusing ICP-MS prototype. A non-commercial laser ablation FORMATION OF MOLECULAR IONS system was coupled to an Element double-focusing sector field A systematic study of cluster ion and molecular ion formation in an ICP can be used to measure interferences with analyte ions in the determination of trace and ultratrace elements in inorganic materials.14–16,18,20 The knowledge of the formation, abundance distribution and electronic stability of molecular and cluster ions is of great analytical significance if mass spectrometric systems with low mass resolution (e.g., quadrupole ICP-MS) are applied for trace and ultratrace analysis.A correlation has been found for metal argide (MAr+) ion intensities in ICP-MS and binding dissociation energies.16 From such a linear correlation curve, unknown dissociation energies, e.g., CdAr+ and MnAr+, were estimated. A comparison of theoretically investigated bond dissociation energies of argon molecular ions for elements of the second and third periods of the Periodic Table with measured ion intensities by double-focusing ICP-MS yielded a qualitative correlation.Higher intensities of non-metal argon molecular ions in comparison with the species with lower intensities in ICP-MS can be interpreted owing to the higher stability.16 Besides the general interest of molecular ion formation Fig. 3 Part of a mass spectrum in the mass range of actinides. investigated by ICP-MS, a practical example of disturbing Table 6 Application of double-focusing sector field ICP-MS in isotopic measurements Precision of isotopic analysis Sample Equipment Isotopic ratio (at concentration) Ref.Standard solutions PlasmaTrace and USN 240Pu/239Pu 2.0% (20 ng l-1) Kim et al.50 Standard solutions Element 25Mg/26Mg At m/Dm=300: Vanhaecke et al.50 206Pb/207Pb 0.04% (Mg 5 mg l-1; Pb 100 mg l-1 Sediment digests, Element 63Cu/65Cu At m/Dm=3000: Vanhaecke et al.59 human serum (SRM) 0.096% (1 mg l-1) Standard solutions, Element 235U/238U 0.07% (10 mg l-1) Kerl et al.56 uranium metal, and USN 0.23% (100 ng l-1) waste sample Standard solutions Plasma 54 prototype 233U/238U 0.03% (1 mg l-1) Taylor et al64 235U/238U Standard solutions Plasma 54 prototype 176Hf/177 0.002% (Hf 1 mg l-1) Walder et al.63 (Mistral) 208Pb/204Pb 0.05% (Pb 1 mg l-1) Standard solutions Plasma 54 prototype 87Sr/88Sr 0.008% (Sr 1 mg l-1) Walder and Freedman61 235U/238U 0.014% (U 1 mg l-1) Glass (SRM) Plasma 54 prototype 208Pb/204Pb; 206Pb/204Pb; 208Pb/204Pb:~0.1% Walder et al.66 and laser ablation 207Pb/204Pb (426 mg g-1) Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 887Table 7 Relative molecular ion intensities and possible interferences with actinide ions Half-life Relative ion Possible of actinide intensity interference by Disturbed nuclide Molecular ion (MX+/M+) molecular ion actinide ion years PbCl+ 8.0×10-6 206Pb35Cl+ 241Am+ 4.3×102 207Pb35Cl+ 242Pu+ 3.8×105 208Pb35Cl+ 243Am+ 7.4×103 207Pb37Cl+ 244Pu+ 8.3×107 208Pb37Cl+ 245Cm+ 8.5×103 PbAr+ 4.3×10-6 206Pb40Ar+ 246Cm+ 4.7×103 207Pb40Ar+ 247Cm+ 1.6×107 208Pb40Ar+ 248Cm+ 3.4×105 13 Shao, Y., and Horlick, G., Appl.Spectrosc., 1991, 45, 143. mass spectrometer in our laboratory,68 where a method for 14 Nonose, N. S., Matsuda, N., Fudagawa, N., and Kubota, M., the determination of some radioactive nuclides (e.g., 99Tc, 129I, Spectrochim. Acta, Part B, 1994, 49, 955. 232Th, 233U, 237Np and 238U) in non-conducting radioactive 15 Nui, H., and Houk, R. S., Spectrochim. Acta Part B, 1996, 51, 779.waste samples such as cement or concrete was evaluated. The 16 Becker, J. S., Seifert, G., Saprykin, A. I., and Dietze, H.-J., J. Anal. detection limits are more than one order of magnitude lower At. Spectrom., 1996, 10, 643. 17 Becker, J. S., and Dietze, H.-J., J. Anal. At. Spectrom., 1995, 10, 637. using double-focusing sector field ICP-MS in comparison with 18 Kobota, M., Fudagawa, N., and Kawase, A., Anal. Sci., 1989, LA–ICP-QMS and reach values <1 ng g-1 (e.g., 233U 5, 701. 0.6 ng g-1; 237Np 0.5 ng g-1). 19 Reed, N. M., Cairns, R. O., Hutton, R. C., and Takaku, Y., J. Anal. At. Spectrom., 1994, 9, 881. 20 Tanner, S. D., J. Anal. At. Spectrom., 1995, 10, 905. CONCLUSIONS 21 Yamasaki, S.-I., and Tsumura, A., Anal. Sci., 1991, 7, 1135. 22 Yamasaki, S.-I., Tsumura, A., Takaku, Y., Microchem. J., 1995, Double-focusing ICP-MS is a useful tool for the ultrasensitive 49, 305. multi-element determination of trace impurities in environmen- 23 Moens, L., Verrept, P., Dams, R., Greb, U., Jung, G., and tal materials and for the characterization of high-purity Laser, B., J.Anal. At. Spectrom., 1994, 9, 1075. samples. The high sensitivity and the very low detection limits 24 Jakubowski, N., Tittes, W., Pollmann, D., Stuewer, D., and achieved, in the pg l-1 concentration range, used in the low- Broekaert, J. A. C., J. Anal. At. Spectrom., 1996, 11, 797. resolution mode, are the most important features. In many 25 Takaku, Y., Masuda, K., Takahashi, T., and Shimamura, T., J.Anal. At. Spectrom., 1994, 9, 1385. cases, if the mass resolution required for the separation of 26 Begerow, J., Turfeld, M., and Dunemann, L., J. Anal. At. analyte ions and disturbing molecular ions is higher than was Spectrom., 1996, 11, 913. practically possible, a further decrease in molecular ion forma- 27 Boonen, S., Vanhaecke, F., Moens, L., and Dams, R., Spectrochim. tion and matrix eects by special sample introduction methods Acta, Part B, 1996, 51, 271.or preparation of sample is required for the determination of 28 Hoppstock, K., Becker, J. S., and Dietze, H.-J., J. Anal. At. ultratrace elements. The precision of isotopic analysis by Spectrom., submitted for publication. 29 Becotte-Haigh, P., Tyson, J. F., Denoyer, E., and Hinds, M. W., double-focusing ICP-MS is about one order of magnitude Spectrochim. Acta, Part B, 1996, 51, 1823. better than that of quadrupole ICP-MS and can be improved 30 Laly, S., Nakagawa, K., Arimura, T., and Kimijima, T., to become comparable to that of TIMS using a multiple Spectrochim.Acta, Part B, 1996, 51, 1393. collector for the simultaneous detection of ion beams of 31 Montaser, A., Tan, H., Ishii, I., Nam, S.-H., and Cai, M., Anal. isotopes. Chem., 1991, 63, 2660. 32 Vanhaeke, F., Van Holderbeke, M., Moens, L., and Dams, R., J. Anal. At. Spectrom., 1996 11, 543. The authors would like to thank V. K. Panday (New Delhi, 33 Sakata, K., and Kawabata, K., Spectrochim.Acta, Part B, 1994, India) and R. S. Soman (Cincinnati, USA) for investigations 10, 1027. on matrix separation carried out at this laboratory. 34 Kawabata, K., Yamanaka, K., and Kishi, Y., paper presented at the 1997 European Winter Conference on Plasma Spectrochemistry January 12–17 1997, Ghent, Belgium. REFERENCES 35 Tanner, S. D., Paul, M., Beres, S. A., and Denoyer, E. R., At. Spectrosc., 1995, 16, 16. 1 Houk, R.S., Acc. Chem. Res., 1994, 27, 33. 36 Georgitis, S., Stroh, A., and Tyler, G., paper presented at the 5th 2 Inductively Coupled Plasma Source Mass Spectrometry, ed. International Conference on Plasma Source Mass Spectrometry, Montaser, A. VCH, New York, 1996. September 1996, Durham, UK. 3 Hieftje, G. M., J. Anal. At. Spectrom., 1996, 11, 613. 37 Speakman, J., Turner, P. J., Haines, R. C., and Merren, T. O., 4 Barnes, R. M., Fresenius’ J. Anal. Chem., 1996, 355, 433. paper presented at the 1997 European Winter Conference on 5 Chen, Z., Fryer, B.J., Longerich, H. P., and Jackson, S. E., Plasma Spectrochemistry, January 12–17, 1997, Gent, Belgium. J. Anal. At. Spectrom., 1996 11, 805. 38 Bradshaw, N., Hall, E. F. H., and Sanderson, N. E., J. Anal. At. 6 Nelms, S. M., Greenway, G. M., and Koller, D., J. Anal. At. Spectrom., 1989, 4, 801. Spectrom., 1996, 11, 907. 39 Morita, M., Ito, H., Uehiro, T., and Otsuka, K. Anal. Sci., 1989, 7 Panday, V. K., Becker, J.S., and Dietze, H.-J., Anal. Chim. Acta, 5, 609. 1996, 329, 153. 40 Giessman, U., and Greb, U., Fresenius’ J. Anal. Chem., 1994, 8 Panday, V. K., Hoppstock, K., Becker, J. S., and Dietze, H.-J, At. 350, 186. Spectrosc., 1996, 17, 98. 41 Narasaki, H., and Cao, J. Y., Anal. Sci., 1996, 12, 623. 9 Taylor, D. B., Kingston, H. M., Nogay, D. J., Koller, D, and 42 Cromwell, E. F., and Arrowsmith, P., J. Am. Soc. Mass Spectrom., Hutton, R., J. Anal. At. Spectrom., 1996, 11, 187. 1996, 7, 458. 10 Barinaga, C. J., Eiden, G. C., Alexander, M. L., and Koppenaal, 43 Halliday, A. N., Lee, D.-C., Christensen, J. N., Walder, A. J., D. W., Fresenius’ J. Anal. Chem., 1996, 355, 487. Freedman, P. A., Jones, C. E., Hall, C. M., Yi, W., and Teagle, D., 11 McLaerty, F. W., Senko, M. W., Little, D. P., Wood, T. D., Int. J.Mass Spectrom. Ion Processes, 1995, 146/147, 21. O’Connor, P. B., Speit, J. P., Chorush, R. A., and Kelleher, N. L., 44 Turner, P., and Wortel, N., 1997 European Winter Conference Adv.Mass Spectrom., 1995, 13, 115. on Plasma Spectrochemistry, January 12–17, 1997, Gent, Belgium, 12 Kluge, H.-J., Bollen, G., Carlberg, C., Moore, R. B., and Quint Q. W., Adv. Mass Spectrom., 1995, 13, 420. Exhibition, Information Note on Iso-PlasmaTrace. 888 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 1245 Ying, J.-F., and Douglas, D. J., Rapid Commun. Mass Spectrom., 57 Isotopic Composition of the Elements, Pure Appl. Chem., 1991, 1996, 10, 649. 63, 991. 46 Yamasaki, S.-I., Tsumura, A., and Takaku, Y, Microchem. J., 58 Vanheacke, F., Moens, L., and Dams. R., Anal. Chem., 1996 68, 567. 1994, 49, 305. 59 Vanheacke, F., Moens, L., Dams, R., Papadakis, I., and Taylor, P., 47 Yamasaki, S.-I., and Tsumura, A., Anal. Sci., 1991, 7, 1135. Anal. Chem., 1997, 69, 268. 48 Takaku, Y., Masuda, K., Takahashi, T., and Shimamura, T., 60 Walder, A. J., Koller, D., Reed, N. M., Hutton, R. C., and J. Anal. At. Spectrom., 1993, 8, 687. Freedman P. A., J. Anal. At. Spectrom., 1993, 8, 1037. 49 Riondato, J., Vanhaeke, L., Moens, L., and Dams, R., J. Anal At. 61 Walder, A. J., Platzner, I., and Freedman, P. A., J. Anal. At. Spectrom., in the press. Spectrom., 1993, 8, 19. 50 Kim, C.-K., Seki, R., Morita, S., Yamasaki, S. I, Tsumura, A., 62 Walder, A. J., Platzner, I., and Freedman, P. A., Int. J. Mass Takaku, Y., Igarashi, Y., and Yamamoto, M., J. Anal. At. Spectrom. Ion Processes, 1993, 8, 19. Spectrom., 1991, 6, 205. 63 Taylor, P. D. P., DeBie`vre, P., Walder, A. J., and Entwistle, A., 51 Kerl, W., Becker, J. S., Dietze, H.-J., and Dannecker, W., J. Anal. J. Anal. Atom. Spectrom., 1995, 10, 395. At. Spectrom., 1996, 10, 723. 64 Lee, D.-C., Halliday, A. N., Int. J. Mass Spectrom. Ion Processes., 52 Becker, J. S., Ku� ppers, G., Carsughi, F., Kerl, W., Schaal, W., 1995, 146/147, 35. Ullmaier, F., and Dietze, H.-J., Ber. Forschungszentrum Ju�lich, 65 Walder, A. J., Furuta, N., Anal. Sci., 1993, 9, 675. 1996, No. 3272, 143. 66 Walder, A. J., Abell, I. D., Platzner, I., and Freeman, P. A., 53 Kerl, W., Becker, J. S., Dietze, H.-J., and Dannecker, W., Ber. Spectrochim. Acta Part B, 1993, 48, 397. Forschungszentrum Ju�lich, 1996, No. 3272, 176; paper presented 67 ChristeJ. N., Halliday, A. N., Lee, D. C., and Hall, C. M., at the 1997 European Winter Conference on Plasma Spectrochemistry, January 12–17, 1997, Gent, Belgium. Earth Planet. Sci. L ett., 1996, 136, 79. 54 Yamamoto, M., Syarbaini, Kofuji, K., Tsumura, A., Komura, K. 68 Gastel, M., Becker, J. S., and Dietze, H.-J., Spectrochim. Acta, Ueno, K., and Assinder, D. J. J. Radioanal. Nucl. Chem., 1995, Part B, submitted for publication. 197, 185. 55 Becker, J. S., Saprykin, A. I., and Dietze, H.-J., Int. J. Mass Paper 7/02178J Spectrom. Ion Processes, in the press. Received April 1, 1997 56 Kerl, W., Becker J. S., Dietze, H.-J., and Dannecker, W., Fresenius’ J. Anal. Chem., 1997, 359, 4. Accepted June 3, 1997 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 889

ISSN:0267-9477    

DOI:10.1039/a702178j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Temporal Considerations With a Microsecond Pulsed Glow Discharge† Invited Lecture W. W. HARRISON*, WEI HANG, XIAOMEI YAN, KRISTOFOR INGENERI AND CYNTHIA SCHILLING Department of Chemistry, University of Florida, Gainesville, FL 32611, USA A high instantaneous power microsecond pulsed glow discharge abbreviated duty cycle that result in advantageous analytical performance. Both the plasma physics and the plasma chemistry reveals many interesting features as an ion source for atomic mass spectrometry and as a photon source for atomic emission are aected.Owing to the higher discharge voltage, the argon ions bombarding the cathode surface are more energetic and spectroscopy. Its advantages over a conventional continuous source are high signal intensity, temporal profile resolution, create a higher sputtering rate. In addition, greatly increased discharge currents produce higher plasma particle energies that additional sputtering control and high sample utilization eciency.It is demonstrated how these features may be useful cause more ecient excitation and ionization of the sputtered atoms. Polyatomic species are subject tomore energetic collisions in solids elemental analysis by mass spectrometry, including thin-layer analysis. In addition, the complementary possibilities that aid in their dissociation, reducing potential spectral interferences. Hence pulsing the GD results in a system that produces of a pulsed atomic emission source are shown for a hollowcathode lamp and for a glow discharge cell.more sputtering, excitation and ionization, even at the same or lower average power, compared with dc operation. Keywords: Glow discharge mass spectrometry; glow discharge Increased signal output is not the only advantage of a pulsed emission spectroscopy; microsecond pulsed glow discharge; GD. By use of gated detection systems, a possibly even larger atomization; excitation; ionization advantage is revealed, namely the temporal resolution of analytical species from concomitant species in the discharge plasma.We have reported this previously with a millisecond Pulsed devices are not uncommonly employed in various areas pulsed GD,3 showing that post-discharge time regimes are of science (e.g., lasers, magnets, furnaces). Operating at a pulsed particularly advantageous for sputtered analytical species. or intermittent duty cycle requires auxiliary control equipment Coupling the GD with a time-of-flight (TOF) mass spec- and the justification for such added expense is normally some trometer and reducing the discharge to the microsecond range desired modified output of the device.Often this is simply creates additional temporal separation opportunities.4,5 Even additional output power. For example, magnets can generate within the pulse discharge envelope, discharge processes occur extremely intense fields by passing high currents for short at dierent times, permitting selective discrimination.Pulsed periods of time; likewise, briefly applied high currents may glow discharges also allow several additional plasma control drive electrothermal vaporization sources to high atomization factors of potential benefit. In addition to the plasma current rates, and light sources can produce high spectral emission and the applied voltage, the pulse width and frequency are when the driving power supply need only provide these intense adjustable.There are therefore compelling reasons to explore conditions over a small fraction of the total time period. In the use of pulsed discharges in analytical spectroscopic systems. this way, devices that normally operate at lower power can be transformed into high-power devices, if only for brief intervals. Glow discharges are commonly considered as low-power EXPERIMENTAL sources. For glow discharge mass spectrometry (GDMS), Pulsed GD–TOFMS typical wattages are in the single digit range.1 Glow discharge atomic emission sources may operate in the tens of watts The ion source assembly for the TOFMS system is constructed range.2 At these low-power conditions, the sputter rate and from a modified four-way cross (MDC, Hayward, CA, USA) the excitation and ionization rates are correspondingly low.Increasing the voltage and current to the GD results in more atoms, more photons and more ions, but the cathode may soon overheat or even melt under such conditions.The average power must be controlled within the thermal limitations of the GD. However, it is possible to create very high peak powers in a GD if the duty cycle is adjusted to maintain a satisfactory average power. Fig. 1 shows a comparison of typical operating conditions in pulsed and dc operation. In this manner, a lowpower source is converted into a high-power source, with attendent capabilities, on a short time basis. However, if the average power over the measurement period is identical for pulsed and continuous wave glow discharges, there might appear to be no obvious gain by this method.In fact, intense plasma conditions may be created within the Fig. 1 Comparison of power considerations for a continuous versus † Presented at the 1997 European Winter Conference on Plasma Spectrochemistry, Gent, Belgium, January 12–17, 1997. microsecond pulsed glow discharge. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 (891–896) 891A small roughing pump (Varian SD-90) is connected with the chamber to attain the necessary vacuum (5×10-3 Torr). A monochromator (Model 1680A, Spex Industries, Edison, NJ, USA) with a resolution of 0.2 nm is employed. The signal is collected by a power supply gated boxcar (Model 250, Stanford Research Systems, Sunnyvale, CA, USA), further amplified by a signal processor (Model 235, Stanford Research Systems) and digitized by an A/D converter (Model 245, Stanford Research Systems) before being sent to a PC for data manipulation. The pulsed discharge is run at 100 Hz, with 10 pulses collected per spectral data point.RESULTS AND DISCUSSION The analytical advantages oered by a pulsed GD may be Fig. 2 Schematic diagram of the pulsed glow discharge time-of-flight considered as a function of the species measured. Photons oer mass spectrometer. (and require) an immediacy of response and experimental simplicity that are attractive. Ions require a considerably more equipped with 2.75 in conflat-type flanges.An in-house con- complex physical configuration and involve mass transport of structed direct insertion probe is used to move the sample into sample species, but permit selective separations over time that the GD chamber without degrading the vacuum. A 500 V, 5W more than compensate for the experimental diculties. The resistor is placed between the insertion probe and the pulsed two methodologies require the ability to extract information power supply (M3k-20, Instrument Research, Columbia, MD, from a transient signal, but the measurement modes are USA) to prevent arcing.Ultrahigh-purity argon gas (99.999%) suciently dierent, both in equipment and in fundamental (Liquid Air, San Francisco, CA, USA) is used. A detailed approach, that they will be discussed separately here. description of this source can be found elsewhere.5 A representation of the TOFMS system is shown in Fig. 2; Pulsed Glow Discharge Mass Spectrometry it includes enhancements over an earlier reported version.5 The Varian 80 L/S turbo pump has been replaced with a To acquire mass spectra from a transient signal ion source, Balzers 330 L/S turbo pump (TPH 330S, Balzers, Fremont, several types of mass spectrometers may be used, in principle, CA, USA), which is backed by a Balzers roughing pump as shown in Fig. 4. A rapid scanning (or peak hopping) (DUO-016B). The vacuum of the last stage reaches quadrupole mass filter may be suitable, depending on the 5×10-6 Torr (formerly 1.5×10-5 Torr).Therefore, the mean length of the transient signal and the extent of the information free path has extended from 3 to 10 m, which increases the desired from the signal. A magnetic sector instrument equipped transmission. with multiple fixed detectors permits the simultaneous monitor- A deflector and an energy filter are added to the system. In ing of selected species or, when fitted with a wide-range this orthogonal structure instrument, the energy filter reduces detector such as a photoplate or its electronic equivalent (diode the background noise resulting from the leakage ions.6 By array/charge-coupled device), a given m/z range is available.applying a pulse into the deflector, intense matrix ions can be Trapping spectrometers (ion trap or ion cyclotron resonance) deflected from their original flight path and will not reach the can accept a limited number of ions from a transient source detector, so that saturation of the microchannel detector is for later sorting and analysis.Finally, the spectrometer selected avoided. A 100×pre-amplifier7 follows the detector to intensify for this study, a TOF system, couples conveniently with a the signal. transient source, as its operational mode is based on the introduction of a pulse of ions for separation during a specific flight path. The TOF system has the advantage of facilitating Pulsed GD–AES the display of a full elemental mass range for each pulsed signal.The emission source, outlined in Fig. 3, is constructed from a standard six-way cross on to which four quartz windows are Temporal Separations mounted. The six-way cross sits on an XYZ translational stage for position adjustment. The direct insertion probe is coaxial Two ion transit regimes should be considered: (1) within the with the spectrometer slit. The GD is powered by a pulsed ‘high’-pressure GD source and (2) through the low-pressure power supply (Model 350, Velonex, Santa Clara, CA, USA). TOF flight tubes.The 10 ms discharge pulse can be considered as an atom injection source, producing a planar source of Fig. 4 Examples of mass analyzers coupled with a transient ion Fig. 3 Schematic diagram of glow discharge atomic emission system. source. 892 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12atoms that then undergo a diusion-controlled process, relative atom densities from a planar diusion model, which yields the results shown in Figs. 7 and 8. This model suggests resulting in the movement of sputtered atoms toward the ion sampling orifice about 5–7 mm distant. At the collision rate that no well defined sample injection volume traverses the GD. Instead, as shown in Fig. 7, atom densities change only slowly produced by a 1 Torr environment, several hundred microseconds are required before significant sputtered material with respect to time at calculated distances from the cathode. Fig. 8 confirms a time-dependent sputtered cathode atom reaches the sampler. Fig. 5 shows an oversimplified representation of the sputtered population migration after the discharge concentration at the sampler, but it does not reflect the much more sharply defined ion response actually measured (Fig. 6). has fired and terminated. In this model, the argon fill gas ionizes during the first 10 ms when the discharge is on, produc- Clearly, other factors must be involved. Either the flow eect is larger than predicted, or the concentration of another factor, ing argon ions throughout the GD cell and sputtering sample atoms from the cathode surface.critical to sample ionization, is changing at a more rapid rate than is the atom density. The most likely possibility is that the According to this model, TOF mass spectra taken shortly after discharge termination should show a mass spectrum of metastable argon atom population, often implicated as a prime ionization species in a GD,9 may drop o rapidly in the multi- argon and any other discharge gases, such as residual water vapor or nitrogen.Only after a sucient delay of several hundred microsecond period to cause the decrease in sample ions observed in Fig. 6. We have begun other measurements hundred microseconds should the collected spectra begin to show ions from the cathode sample. Fig. 6 shows that TOF in an optical GD configuration to determine the response of argon metastable species with time after discharge termination.spectra taken over a wide range of delay times support this model and reveal a significant advantage in discrimination against potentially interfering discharge gases. By acquiring GD–TOFMS Measurements mass spectra at a 350 ms delay, a spectrum dominated by the sample ions is obtained. The pulse injection model and the GDMS lends itself readily to solids sample analysis. The most resultant spectra follow the same general considerations as common and the most convenient materials analyzed by found in flow injection analysis.8 GDMS are bulk solids.A 4.5 mm diameter by 1 mm thick disc In reality, the mass transport dynamics in the glow discharge sample was taken from a NIST brass standard. This was are likely to be more complex than the model shown in Fig. 5. subjected to a 10 ms 2 kV pulse discharge and examined in the Assuming that the gas flow through the GD (several ml min-1) TOF mass spectrometer.10 Fig. 9 shows the response of trace is low enough to ignore mass flow eects, we can calculate lead components, where 204Pb is present at the sub-ppm level.At this point, detector noise serves as the limiting step in reaching lower detection limits. Powdered materials require an additional preparation step of sample compaction, including a base matrix conductor, if the sample is a non-conductor. There is also the complication produced by the high surface area of the powdered material, which adsorbs significant amounts of water vapor, air and other potential impurities.The purity of the matrix binder is also critical. To demonstrate the ability of the TOF instrument, we show here spectra taken from a very complex mixture of 49 elements in various chemical combinations. Fig. 10 represents a full mass range scan of the resultant ions taken from a pulsed dc discharge.Note that the TOF instrument provides adequate resolution to display all the elements with relative sensitivity factors that are generally within a factor of five, Fig. 5 Simplified schematic diagram of sputtered particle migration in a microsecond pulsed GD source. except for mercury, which is enhanced owing to thermal eects. Fig. 6 Time-resolved mass spectra of a silver disc sample at dierent repelling pulse delays. GD pulse, 10 ms, 2 kV, 100 Hz; cathode–orifice distance, 5 mm; source pressure, 1 Torr.Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 893Fig. 9 Mass spectrum of trace levels of lead (3.25 ppm) and bismuth (500 ppb) in NIST unalloyed copper II 495. GD pulse, 10 ms, 2 kV, 100 Hz; source pressure, 1 Torr; cathode–orifice distance, 5 mm; 10 000 average. Fig. 7 Theoretical atom number densities with respect to the time after the termination of the microsecond pulsed GD as a function of distance from the cathode.Based on a planar diusion model, n(r,t)= [N/(4pDt)1/2][ exp(-r2/4Dt)], where n is the atom number density, N is the original atoms originating from the cathode, D (=300 cm2 s-1) is diusion coecient and r is the distance from the cathode. (a) Atom number variation versus time at distances of 3, 5, 7 and 9 mm from the cathode; (b) atom distribution versus distance at 100, 200, 500 and Fig. 10 Complete mass spectrum of a 49-element commercial mixture 1000 ms after the termination of the GD.(Spex Mix 1000, Spex Industries, Edison, NJ, USA) mixed with highpurity copper powder (90% of the total mass). GD pulse, 10 ms, 2 kV, 100 Hz; source pressure, 1 Torr; cathode–orifice distance, 5 mm; 500 average. done by GD–OES,11,12 but mass spectrometric studies are also well known.13 Because of the relative high pressure conditions of a GD, redeposition of sputtered atoms is an unavoidable eect that limits depth resolution. However, applications in the tens of nanometers range have been demonstrated by careful control of the conditions.14 The GD does have the advantage in analyzing thicker layers (micrometer range), where the high sample ablation rate of the gas discharge becomes important.The pulsed GD oers a dierent approach to thin-layer analysis by permitting a controlled application of the sputtering step. As depicted in Fig. 11, the layered sample is subjected to a succession of pulses, each of which will remove a specific amount of the extant surface, depending on the pulse length, Fig. 8 Experimental ion signal variation at indicated distances after energy and number of bombarding species (current). For the termination of the microsecond-pulsed GD. Silver cathode; GD example, a 10 ms pulse at 14 mA would direct a packet of 1012 pulse, 10 ms, 2 kV, 100 Hz; source pressure, 1 Torr. ions on to the sample surface (ignoring for the moment all charge exchange considerations in the cathode dark space).At A comparison spectrum run in the dc mode shows a much a pulse frequency of 200 Hz, 15 min (or 1.8×105 pulses) are larger heating eect for mercury, reflecting the overall lower required to remove the 2500 A ° layer of silver. Based on a silver thermal eects in the pulse mode. volume of 1017 atoms, we can estimate that the net sputter rate is roughly 6×1011 atoms per pulse. On this basis, each Thin-layer Samples pulse removes approximately 0.01 atomic layer. Obviously, there are many oversimplifications in this example, which is Another type of solid sample that finds considerable GDMS application is thin-layer materials.Most of this work has been designed more to indicate the control possibilities of the pulsed 894 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Fig. 11 Illustration of microsecond-pulsed GD for sputtering thin film samples (see text). Fig. 13 Emission spectra of a copper hollow-cathode lamp (WL 22603,Westinghouse) using (a) continuous (20 mA) and (b) pulse (4.0 A, 10 ms, 100 Hz) modes.Fig. 12 Sequential mass spectra of 250 nm silver coated on a copper disc GD pulse, 10 ms, 1.5 kV, 250 Hz; source pressure, 1 Torr. discharge than any precise picture of actual steps. The sample erosion rate may be varied by control of the pulse length, repetition rate, current and voltage. An advantage of the TOF system is that a complete mass spectrum may be obtained for any given pulse at any time during the analysis procedure, which would be particularly Fig. 14 Time-resolved spectra of the hollow-cathode lamp using the pulsed conditions shown in Fig. 15. advantageous for the monitoring of multi-element layers. Fig. 12 shows sequential examples of the mass spectra taken during an examination of a silver layer on base copper. These were taken only in the axial mode, so only limited time spectra reflect selected time-dependent ion signals. In one resolution is possible. sense, the entire process can be thought of as pulses of current Our initial studies were made using a commercial hollow applied to the sample to ‘dissolve’ it into the argon ‘solvent.’ cathode as the glow discharge source.Fig. 13 shows a compari- The object of the experiment is to determine the time at which son of one wavelength segment taken from a much longer the silver analyte has become exhausted. The pulsed GD oers spectral range. For similar average powers the pulsed GD interesting and as yet only partially tested opportunities for spectrum features lines not visible in the dc counterpart and layered sample analysis.also exhibits dierences in emission intensity ratios. The much more intense plasma creates populated energy levels that are not observed in the lower energy dc mode. Note also the many Pulsed Glow Discharge Atomic Emission copper ion lines, which arms the pulsed GD potential as an ion source. It should not be surprising that any discharge mechanism that produces enhanced ion populations would also aect favorably Although the opportunity for temporal separation of discharge signals is much more limited compared with the the excited state levels.We have shown15 that the pulsed GD yields emission signals up to two orders of magnitude higher TOFMS method, some resolution is possible. Fig. 14 shows the eect of setting increasing delays for signal accumulation than those observed from dc operation.In addition, some temporal advantages may be gained, although not to the same after discharge initiation. A large signal for copper atom emission is observed even at the first delay setting of 2 ms, at degree as observed in the TOF mass spectrometric approach. For the measurement of photons, compared with ions, no which time very little copper ion emission can be seen. This latter signal does build, however, and maximizes at about invasive procedures or material transport phenomena are necessary. Strategically placed optical windows permit the 15 ms for most of the displayed ion lines.Much remains to be learned about these relative atomic states under the conditions measurement of GD-emitted photons either axially, which eectively integrates over the entire discharge volume, or employed in a microsecond pulsed discharge. Turning to a GD source that we have used for many years for GDMS, a orthogonally, which allows sampling of discrete discharge regions ranging from the sample surface, through the dark time-dependent response is shown in Fig. 15. The argon discharge gas lines appear very slightly before the sputtered atom space and well into the negative glow. The data shown here Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 895We are grateful to the Department of Energy, Basic Chemical Sciences, and Hewlett-Packard Laboratories for their support of this research. REFERENCES 1 Harrison, W.W., J. Anal. At. Spectrom., 1992, 7, 75. 2 Broekaert, J. A. C., in Glow Discharge Spectroscopies, ed. Marcus, R. K., Plenum Press, New York, 1993, ch. 4. 3 Klingler, J. A., Barshick, C. M., and Harrison, W. W., Anal. Chem., 1991, 63, 2571. 4 Hang, W., Yang, P., Wang, X., Yang, C., Su, Y. X., and Huang, B., Rapid. Commun. Mass Spectrom., 1994, 8, 590. 5 Harrison, W. W., and Hang, W., J. Anal. At. Spectrom., 1996, 11, 835. 6 Mehoney, P. P., Ray, S. J., and Hieftje, G. M., Appl.Spectrosc., 1997, 51, 16A. 7 Myerholtz, C., Li, G., and Doherty, T., Hewlett-Packard Fig. 15 Temporal emission profiles of microsecond-pulsed GD for a Laboratories, personal communication. copper disc sample. Source pressure, 2.4 Torr Ar. 8 Kerlberg, B., Flow Injection Analysis: a Practical Guide, Elsevier, Amsterdam, 1989. 9 Chapman, B., Glow Discharge Processes, Wiley, New York, 1980. lines, with maxima of the ionic lines from the sputtered atoms 10 Hang, W., Baker, C., Smith, B. W., Winefordner, J. D., and delayed by a few microseconds beyond discharge termination. Harrison, W. W., J. Anal. At. Spectrom., 1997, 12, 143. 11 Bengston, A., Spectrochim. Acta, Part B, 1994, 49, 411. 12 Pra�bler, F., Homann, V., Schumann, J., and Wetzig, K., J. Anal. CONCLUSION At. Spectrom., 1995, 10, 677. 13 Coburn, J. W., Eckstein, E. W., and Kay, E., J. Appl. Phys., 1975, The pulsed GD creates an energetic discharge plasma that 46, 2828. yields significant changes in atomic processes compared with 14 Jakubowski, N., and Stuewer, D., J. Anal. At. Spectrom., 1992, the dc counterpart. While the enhanced intensities are the most 7, 951. obvious result, full utilization of the pulsed GD capabilities 15 Hang, W., Walden, W. O., and Harrison, W. W., Anal. Chem., calls for a recognition of the temporal events that occur during 1996, 68, 1148. and after the microsecond regime pulse. In TOFMS, the reduced duty cycle is no longer an experimental limitation. Paper 7/01641G Instead, the transient nature of the pulsed GD complements Received March 7, 1997 the intrinsic need for pulsed sample introduction for TOFMS. Accepted May 20, 1997 The exploration of time-dependent analytical signals and their relevance to other atomic spectrometric techniques may lead to valuable adjustments in measurement modes for emission, absorption and fluorescence methods. 896 Journal of Analytical Atomic Spectrometry, September 1997, Vol.

ISSN:0267-9477    

DOI:10.1039/a701641g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Comparison of Axially and Radially Viewed Inductively Coupled Plasmas for Multi-element Analysis: Effect of Sodium and Calcium† I. B. BRENNER‡a, A. ZANDERa, M. COLEb AND A. WISEMANc aGinzton Research Center, Varian Associates, 3075 Hansen Way, Palo Alto, CA, USA bVarian Instruments, 3073 Hansen Way, Palo Alto, 94304, CA, USA cVarian Instruments, 679 Springvale Road, Mulgrave, V ictoria, 3170, Australia Conventional figures of merit such as LODs, interferences due Kornblum,10 among others, compared the axial configuration to Ca and Na, the MgII 280.270 nm/Mg I 285.213 nm plasma with the radial set-up.They concluded that axial (end-on) robustness ratio, precision and accuracy were employed to viewing of the ICP had the advantage of enhanced analyte compare the analytical performance of radially and axially signal and reduced background intensity. On the other hand, viewed ICPs. The cool region in the plasma, where EIE Ivaldi and Tyson16 observed that both the analyte and backinterferences predominate, was symmetrically peeled away ground were higher for axial viewing, but the S/B values were using an end-on stream of argon as opposed to a 90 ° shear higher owing to the larger increase in the signal relative to the gas flow, thus leaving the central axial zone free of the background.However, these workers also noted a decrease in interfering sheath. LODs were enhanced by factors varying dynamic range and an increase in interference eects.Demers9 from about 2 to 20 and could further be improved by and Faires et al.11 reported up to a 10-fold improvement in optimizing the nebulization pressure. Additional enhancements LODs for the axially viewed ICP. The decrease in dynamic by factors of 2–16 were obtained using ultrasonic nebulization. range in the axial configuration was attributed to cool absorb- It was observed that interferences due to Na and Ca were ing atoms, while matrix interferences were attributed to solute relatively small and were similar in the axial and conventional vaporization and ionization eects due to the presence of alkali radial configurations.Interferences due to Ca were larger than and alkaline earth elements in the cool plasma fringe.12 those caused by Na, probably owing to the larger amount of Recently, Dubuisson et al.17 compared the S/B values of several energy required to dissociate the matrix. The extent of matrix commercially available ICP-AES systems.They showed that eects, which did not exceed 10–20%, is attributed to the up to 3-fold improvements could be obtained for axial viewing, robust plasma as evidenced by MgII 280.270/Mg I 285.213 nm and observed that matrix eects due to Na (Ca was not ratios of 7 at 0.9 kW and #12 at 1.4 kW using normal evaluated) could be minimized by using higher rf power and aerosol flow rates. The use of internal standards such as a low carrier gas flow rate. They emphasized the influence of Sc II 361.384 and Cd II 226.502 nm, measured sequentially, the torch injector diameter on energy exchange between the improved the accuracy of determination using conventional and plasma and the central channel. ultrasonic nebulization.The major, minor and trace element According to Milburn,18 even the presence of 10% HNO3 contents of several geological CRMs, varying widely in and HCl in an axially viewed ICP caused an average 6% composition, compared favorably with the recommended data.intensity depression. These acid eects were compensated by It was concluded that the ICP operating conditions and the use of an internal standard. Interferences due to concomiperformance for quantitative multi-element analysis using the tant elements were considered to be more complex in nature present axial configuration do not dier significantly from and were related to the pathlength of the emission. Attempts those of the conventional radial configuration.have been made to minimize these eects by sweeping the tail flame where these interferences predominate. Keywords: Axial and radial configurations; easily ionized de Loos-Vollebregt et al.15 examined the performance of an element eect; geoanalysis axially viewed low-flow ICP. In contradiction to the results previously cited, they found that in comparison with a conven- Until recently, the inductively coupled plasma with atomic tional plasma, the linearity of the calibration graphs was emission detection has routinely been viewed radially at 90 ° similar and there was a remarkable reduction of interferences.to the central channel of the plasma. In this mode of operation Recently, it was shown16 that easily ionized elements (EIEs) the detection capability has been enhanced by increasing the such as Na resulted in a large suppression, by 65% (relative eciency of aerosol production and transport using ultrasonic to an aqueous solution), of the analyte intensities.This was nebulization,1,2 thermospray,3 by analyte preconcentration4 accompanied by a large decrease in plasma robustness, reflected and improving spectral resolution.5 Further enhancements in the MgII 280.270/ MgI 285.213 nm ratio which decreased have also been achieved by using mixed gas plasmas containing from 5.9 to 2. These values were obtained even after an air hydrogen6 and by creating a pinch plasma.7 shear gas was aimed at 90 ° to the plasma axis to deflect the It is now recognized that the detection capability of ICP- interfered plasma zone and to protect the entrance optics.On AES can be improved substantially by axial (end-on) viewing the other hand, when Demers9 directed an air shear gas flow of the plasma.8–17 Lichte and Koirtyohann,8 Demers9 and towards the apex of the plasma, similar linearity ranges for the axial and radial configurations were reported. The disadvantage of an air flow is the possibility of air entrainment † Presented at the 1997 European Winter Conference on Plasma and the presence of a UV-absorbing medium limiting the Spectrochemistry, Gent, Belgium, January 12–17, 1997.spectral range. Visual inspection of cross-flow configurations ‡ Present address: Environmental Analytical Laboratories, Faculty showed that the plasmas were deformed in the areas of contact. of Engineering Sciences, Ben Gurion University, 9 Dishon Street, Malkha, Jerusalem, Israel, 96956.In the present investigation the cool plasma fringe, directed to Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 (897–906) 897Table 1 ICP operating conditions for radial and axial viewing the spectrometer, was peeled away symmetrically by an endon argon gas flow, leaving the central axial zone free of the Generator 40.86 MHz, free-running interfering sheath. Power/kW 0.9 with axial viewing; 1.2 for radial While detection enhancement is important for trace element viewing determinations in environmental and geological samples, from Torch injectors Axial, 2.3 mm diameter; radial, 1.4 mm diameter the above discussion, however, the use of axially viewed ICPs Observation height Radial, 6 mm above the work coil may not be straightforward for the analysis of these samples.Auxiliary gas/l min-1 1.5 This is due to changes in excitation conditions in the plasma Sample delivery/ 1.5 caused by the presence of high concentrations of alkali and ml min-1 alkaline earth elements in the samples or in solutions prepared Nebulizer Concentric with the axial configuration by alkali fusion (e.g., lithium metaborate, sodium peroxide).Varian V-groove with the radial configuration In a previous report,19 preliminary data on the eect of Na Nebulizer pressure 180 kPa for both configurations on spectral line intensities in these types of axial and radial Spray chamber Glass cyclone with the axial configuration configurations were given. Cr ion and atom spectral line Sturman–Masters with the radial set-up intensities decreased by about 25% in the presence of USN Cetac 5000 AT.Desolvation temperature, 5000 mg l-1 Na using the radially viewed plasma. The 140 °C; chiller temperature, 0 °C depression in the axial position was only about 5% larger Integration time/s 1 Number of repetitions 3 than in the radial view, implying that the dierences between the configurations were small with respect to Na.The MgII 280.270/MgI 285.213 nm ratio has been used as plasma. The extended one-piece torch with a 2.3 mm injector a criterion for plasma robustness and excitation conditions20,21 was positioned in the holder so that the distance between the using conventional and ultrasonic nebulization (USN). This intermediate tube and the load coil was about 2 mm. The ratio is related to electron density (ne) by the Saha–Eggert distance between the coil and the top of the outer tube of the equation, assuming LTE, and to the ionization temperatorch was 3 mm, and the distance between the top of the outer ture.21,22 Mermet21 determined MgII 280.270/MgI 285.213 nm tube of the torch and the cone was 1.4 mm. The axial obser- intensity ratios in plasmas produced in torches with dierent vation zone was focused onto a computer-controlled mirror central injector diameters. He concluded that energy transfer for optimization of the S/B.The optimum horizontal and was most ecient (when MgII 280.270/MgI 285.213 nm ratios vertical positions were determined using an aqueous solution were close to LTE values) when the inner injector diameter containing 1 mg l-1 Y and monitoring the YII 371.030 nm line exceeded 2 mm.However, in the study of Dubuisson et al.,17 using the x–y profiling routine in the software. the Mg ratios were not cited. A glass concentric nebulizer was employed for both the axial In the present study, we have measured the eects caused and radial studies.In the former, a glass cyclone spray chamber by solutions containing up to 1000 mg l-1 Ca and 1000 mg l-1 (Glass Expansion, Camberwell, Victoria, Australia) with a Na on the ion and atom line intensities of 15 element waverapid wash-out time was used; in the latter, a Sturman–Masters lengths diering in energy potentials with axially and radially chamber was used. This is a vertical cyclonic spray chamber. viewed ICPs. These concomitants occur in high and variable The advantage of the glass cyclone spray chamber is the small concentrations in environmental and geological samples, and void volume of approximately 50 ml.With lateral and axial have variable eects on the intensities of the trace elements of viewing using a narrow injector, the optimum nebulizer press- interest. The eect of nebulization pressure and power on ure was about 120 kPa. However, when using 1.4 and 2.3 mm spectral intensity was examined.Interference eects due to Na injectors, a nebulizer pressure of 180 kPa was employed. This were also examined using USN. Internal standards were evaluis equivalent to about a 0.75 l min-1 carrier flow rate using ated for minimizing the interference eects. The analytical the axial configuration and about 0.8–0.9 l min-1 for the radial performance of the axially viewed ICP was evaluated by system. A Cetac U 5000AT (Cetac Technologies, Omaha, NE, analyzing various geological CRMs.Sc II 361.384 nm has been USA) ultrasonic nebulizer was employed to determine LODs widely employed as an internal standard for simultaneous and eects due to Na. The aerosol from the unit was coupled multi-element analysis.23–25 In the present work it was used to directly to the base of the torch. The desolvation temperature improve the accuracy of the sequential determination of the was 140 °C, the chiller was set at 0 °C. Details of ICP operating major, minor and trace elements (Si, Ca, Mg, Fe, Ti, Al, Be, conditions are listed in Table 1. Li, Pb, Sr, V and Zn) in a wide variety of geological CRMs.This mode of measurement was successfully applied previously using a single-channel slew-scanning sequential Reagents spectrometer.26,27 Concomitant solutions containing variable Ca and Na concentrations were prepared from ultrapure Spex stock solutions. Solutions containing up to 1000 mg l-1 Ca and Na were spiked EXPERIMENTAL with 1 mg l-1 of a multi-element stock solution (Spex Instrumentation Industries, Edison, NJ, USA).The blank was a 5%v/v solution of HCl. All solutions contained 5% v/v HCl and 5 mg l-1 Sc A Varian (Palo Alto, CA, USA) Liberty Series II sequential as the internal standard and 5 mg l-1 Mg for the determination ICP atomic emission spectrometer was employed. This disperof the MgII 280.270/MgI 285.213 nm ratio. sion system operates in any one of four grating orders of diraction for maximum resolution.In the 170–470 nm region (in the second order), where most of this work was conducted, Wavelength Selection the resolution is approximately 0.009 nm. As a result, spectral interferences were minimal. In the axial configuration, the The wavelengths used are listed in Table 2 together with their excitation and first ionization potentials. Spectral line inter- plasma is aligned horizontally and viewed end-on through a water-cooled cone interface (CCI). The outer fringe of the ference corrections were only performed for the interference of Al on Pb II 220.353, MoII 202.030 and Cd II 226.502 nm plasma was symmetrically stripped away from the optical path by an argon sheath gas that flows end-on through the CCI, (1000 mg l-1 Al produced perturbations of 280, 240 and 20 mg l-1, respectively).Sc II 361.384 nm was used as an leaving the central axial channel isolated from the surrounding 898 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Table 2 Spectral lines employed for the examination of Ca and Na interferences, and their excitation (EP) and ionization potentials (IP) Wavelength/ nm EP IP PMT/V Order Be I 234.861 5.28 9.32 700 2 Cd II 226.502 5.47 9.0 700 3 Co II 238.892 5.6 7.86 700 2 Cr II 267.716 6.18 6.76 700 2 Cu I 324.754 3.82 7.72 700 2 Fe II 259.94 4.77 7.87 700 2 Li I 670.784 1.85 5.39 700 1 MgI 285.213 4.34 7.64 600 2 MgII 280.270 4.42 7.64 600 2 MoII 202.03 6.13 7.1 700 1 Ni II 231.604 6.39 7.63 700 2 Pb I 220.353 7.37 7.42 700 3 Fig. 2 Eect of power on net intensity and S/B. Axial viewing, Sc II 361.384 3.45 6.54 600 1 YII 371.03 nm. Si I 212.412 6.62 8.14 600 3 Sr II 407.771 3.04 5.69 500 1 Ti II 337.280 3.74 6.82 700 1 Eect of Power With Axial Observation VII 292.402 4.63 6.74 700 2 Zn II 213.856 5.80 9.39 700 3 The eect of power on the net signal and the S/B of YII 371.0 nm is shown in Fig. 2. At a constant nebulizer pressure of 180 kPa, the net signal increased with increasing power approximately by a factor of 3; however, the S/B decreased by a factor of 6.This behavior is typical of the internal standard using conventional nebulization; analyte spectral lines studied. Thus, the use of low power Cd II 226.502 nm was evaluated for USN. In both cases they for trace element determinations is more favorable, provided were measured in sequential mode. The ratio of MgII 280.270 that the plasma is suciently robust. Using the to MgI 285.213 nm was applied as a criterion for plasma MgII 280.270/MgI 285.213 nm ratio as a criterion for plasma robustness.Background corrections were performed on one or robustness, a value of 7.6 was obtained using an rf power of both sides of the line, as appropriate. 0.9 kW (see under MgII 280.270/MgI 285.213 nm), indicating that energy transfer even to a low power plasma was ecient. RESULTS Eect of Nebulization Pressure With Axial Viewing General Observations of the Axially Viewed ICP The eect of nebulizer pressure on net intensity and S/B is At a low nebulizer pressure of 100 kPa and power of 1 kW, illustrated in Figs. 3 and 4, respectively. With the exception of the plasma was relatively unstable, in particular near the Al and K, intensities increased slightly with increasing nebulizinjector; the initial radiation zone (IRZ) was poorly-formed, ation pressure, passing through a maximum at about 160– and the intensity was low. At 120 kPa, the top of the IRZ 180 kPa, after which intensities decreased.At this pressure the reached up to the second coil, and the intensity increased apex of the IRZ was located about 2 mm above the upper coil. sharply. At 180 kPa, the IRZ was located between the second The similar variations of ion lines (Fig. 3) indicate that any and third coil. Under these conditions, the plasma was stable one of them could be used as an internal standard to compeneven when the sample delivery tube was withdrawn from the sate for variations in the nebulization pressure.Noteworthy sample solution and air permitted to be sucked into the was the dierent behavior of KI 766.49 and Al I 396.152 nm, discharge. Fig. 1 shows a symmetrical axially viewed plasma the S/B values of which increased with increasing pressure of using the conditions listed in Table 1. This photograph dis- 180 kPa, remaining constant up to 280 kPa (Fig. 4). tinctly shows the various zones of the plasma as described by The S/B values of the ion lines increased only moderately Koirtyohann et al.28 and is in sharp contrast to other crosswith increasing nebulization pressure, passing through a maxi- flow gas configurations where plasma deformation can be mum at a higher pressure of 240 kPa.The S/B values of observed. Al I 396.152 and KI 766.49 nm continued to increase at a high Fig. 1 Structure of the plasma using the axial configuration, with end-on argon sheath gas which symmetrically strips the cool plasma fringe.Note absence of plasma deformation and the location of the Fig. 3 Variation of net intensity as a function of nebulizer pressure. plasma plume in close proximity to the CCI. On the right, plasma with 100 mg l-1 Y. Note well-formed IRZ. Axial viewing. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 899increase with further pressure increase. Thus, in axially and radially viewed ICPs, nebulization pressure can be optimized for a particular analytical task.For trace element determinations, a pressure of about 160–180 kPa can be used to obtain maximum sensitivities, whereas the major elements can be determined at higher pressure. Nebulization pressure did not aect the horizontal and vertical positions of the central axis of the plasma, which varied by ±2% for pressures varying from 120 to 220 kPa. This indicated that the central channel of the plasma was not deformed as a result of the end-on argon gas flow.With increase in the auxiliary gas flow rate from 0 to 0.75 l min-1, the net intensity increased by a factor of 20. Limits of Detection Fig. 4 Eect of nebulization pressure on the S/B. Axial viewing. Three sigma (3s) LODs for the axially and radially configurations with conventional and ultrasonic nebulization (USN) are listed in Table 3. Relative to the radial view, detection pressure of 280 kPa. Fig. 5 shows that the nebulizer pressureenhancement factors for the axial configuration using conven- induced intensity depressions and excitation and ionization tional nebulization varied from about 2 to 20 with a mean of potentials are significantly correlated.Intensity depressions about 7. Improvements varying from 2 to 16 were obtained can be divided into two groups: (a) low energy ‘soft’ lines such when USN and desolvation was employed; i.e., a mean factor as Al I 396.152 and KI 766.49 nm; (b) ‘hard’ atom lines such of about 50 over that obtained with a conventional radial as As I 189.042, As I 193.759 and Si I 212.412 nm and ionic lines.observation. In the presence of 5% NaCl the LODs were In the radially viewed plasma, maximum net intensities were degraded approximately by a factor of 5. also obtained at about 160 kPa, corresponding to an IRZ Fig. 7 shows that the LODs of several ‘hard’ ion lines were position of about 2 mm above the load coil. improved in the axial view when the nebulization pressure In the axially viewed ICP, the RSDs of the analyte signals decreased; for example, As I 189.042, As I 193.759 and also varied with nebulization pressure, decreasing from 1.5% Cd II 226.502 nm, whereas the ‘soft’ lines such as Al I 396.152 at 140 kPa to about 0.1–0.4% at 200–220 kPa (Fig. 6), and and KI 766.49 nm were significantly enhanced at high pressure. increasing again with increasing pressure. In the radially viewed These enhancements can be used with advantage for specific plasma, the RSDs also decreased with increasing nebulizer applications, for example, in the case of Al for the determination pressure from about 1 to 0.2% at 160–180 kPa followed by an of low mg l-1 concentrations in environmental and biological materials.The close dependence of line intensities and LODs on nebulization pressure and the correlation with excitation and ionization potentials can be explained by changes in temperature and energy conditions in the plasma as discussed by Mermet.21 Eect of Na and Ca The eect of Na and Ca, varying from 0 to 1000 mg l-1, on several spectral lines of dierent excitation and ionization potentials (EP and IP, respectively) (Table 2), is illustrated for the axial view in Fig. 8 and for the radial observation in Fig. 9. The intensities of the blank matrices were subtracted from the respective spiked concentrations. Relative intensities (expressed as the ratio of the intensities obtained in an aqueous solution Fig. 5 Relationship between nebulizer pressure-induced intensity to those obtained in 1000 mg l-1 Na and Ca) decreased with depressions (NPID) versus excitation+ionization potentials. Axial increasing Na and Ca concentrations. (The closer the value to viewing. 1, the smaller the eect.) It is worth noting the pronounced decrease in intensity at 100 mg l-1 and the dissimilar responses for most of the analytes studied. As a consequence of these variations, there are several questions to be addressed: (a) Are the Na and Ca interferences in the present axially viewed plasma configuration larger or smaller than those observed in the radial configuration? (b) Are the variations due to Na similar to those of Ca? (c) Are the observed variations a function of the energy potentials of the spectral line? Figs. 10–12 depict several trends: (a) Matrix eects due to Ca and Na are, with few exceptions, similar with axial viewing, although the interferences due to Ca exceeded those from Na (Fig. 10). (b) The correlation of the matrix eect due to 1000 mg l-1 Ca in the axially and radially positions was more significant that that of Na (Figs. 11 and 12) and, with only a few exceptions, there is a trend of higher matrix eects induced Fig. 6 Eect of nebulization pressure on the % RSDs. Axial viewing. by Ca. 900 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Table 3 Comparison of 3s LODs (mg l-1) for radially (RP) and axially (AP) viewed ICPs, with conventional and ultrasonic nebulization (USN) LOD/ngml-1 Enhancement factor Wavelength/ APUSN– Element nm RP RP+USN AP AP+USN AP–RP RPUSN–RP APUSN–AP RPUSN Al I396.152 5 0.5 1 0.15 5.0 10.0 6.7 3.3 As I188.979 12 5 10 2 1.2 2.4 5.0 2.5 As I193.759 10 5 4 1 2.5 2.0 4.0 5.0 Ba II455.403 0.1 0.04 0.01 0.005 10.0 2.5 2.0 8.0 Be II234.861 0.15 0.01 0.05 0.005 3.0 15.0 10.0 2.0 Cd II226.502 2 0.3 0.15 0.05 13.3 6.7 3.0 6.0 Co II238.892 4 0.2 0.25 0.05 16.0 20.0 5.0 4.0 Cr II267.716 4 0.2 0.2 0.03 20.0 20.0 6.7 6.7 Cu I324.754 2 0.25 0.6 0.05 3.3 8.0 12.0 5.0 Fe II259.940 1.5 0.15 0.3 0.04 5.0 10.0 7.5 3.8 K I766.490 10 3 3 0.6 3.3 3.3 5.0 5.0 Li I670.784 1 0.2 0.12 0.06 8.3 5.0 2.0 3.3 Mn II257.610 0.2 0.03 0.06 0.005 3.3 6.7 12.0 6.0 Mo II202.030 4 0.2 0.6 0.04 6.7 20.0 15.0 5.0 Ni II231.604 5 0.5 0.75 0.06 6.7 10.0 12.5 8.3 Pb II220.353 12 1.5 1.5 0.2 8.0 8.0 7.5 7.5 Se I196.090 30 4 3 0.8 10.0 7.5 3.8 5.0 Sr II407.771 0.03 0.01 0.01 0.005 3.0 3.0 2.0 2.0 Ti II334.941 0.6 0.1 0.07 0.03 8.6 6.0 2.3 3.3 V II292.402 2 0.3 0.8 0.05 2.5 6.7 16.0 6.0 Zn I213.856 1 0.06 0.4 0.1 2.5 16.7 4.0 0.6 Mean enhancement 5.1 1.0 1.3 0.3 6.8 9.0 6.9 4.7 is in accordance with previous observations.16 In contrast to the present data, however, these workers found that ion signal suppression was more severe with the axial configuration.Role of the Internal Standard A single internal standard did not compensate for the dierent responses of the analyte spectral lines to various concentrations of Ca and Na. Nevertheless, the potential of Sc II 361.384 nm as an internal standard to compensate for some of these depressive eects was evaluated.Solutions containing up to 1000 mg l-1 Ca and Na, 5 mg l-1 Sc and 1 mg l-1 trace elements were analyzed using aqueous solutions as the calibration standards (also containing 1 mg l-1 Sc). A significant improvement in accuracy was obtained for all the spectral lines studied for various Na and Ca concentrations in both the axially and radially viewed plasmas.The extent of compensation attained at 1000 mg l-1 Ca and Na for all the spectral lines is illustrated in Figs. 14 and 15. In the axial position, the uncorrected concentrations varied from 0.85 to 0.92 and from 0.78 to 0.92 mg l-1 for the Na and Ca solutions, respectively, while with Sc II 361.384 nm as the internal standard, the concentrations varied from 1 to 1.07 and from 0.92 to 1.1 mg l-1, respectively.Similarly, in the radially viewed plasma, the uncompensated concentrations varied from 0.78 to 0.92 and from 0.73 to 0.95 for Na and Ca solutions, respectively, and from 0.88 to 1 and from 0.88 to 1.12 with compensation. Thus, the maximum negative bias of up to about 15% was reduced to <10% in both plasmas. Since the behavior of low energy Fig. 7 Eect of nebulization pressure on the LODs of the axially Li I 670.7 nm diered from that of Sc II 361.384 nm (spectral viewed ICP.intensity variations as a function of Na and Ca concentrations were small), overcompensation was registered. With USN, the depressive eect of 1000 mg l-1 Na in the Fig. 13, which compares the interference factors, shows that, axially viewed ICP amounted to no more than 10%. When with very few exceptions, the eects in the axial situation were Cd II 226.502 nm was employed as an internal standard this smaller than those observed in the radial position, using the eect was reduced to no more than 2–3%.(Fig. 16) and the conditions listed in Table 1. Coupled with the enhanced LODs, RSDs were less than 1%. this advantageous behavior is in sharp contrast to other reports emphasizing that interferences in the axial configuration are substantially larger than those observed in radially viewed MgII 280.270/Mg I 285.213 nm systems.16 A broad correlation of these interference factors with total excitation and ionization potentials indicates that The MgII 280.270/MgI 285.213 nm ratio has been used as a criterion for energy transfer, residence times and plasma robust- higher energy lines are more susceptible to interference.This Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 901Fig. 8 Eect of Na and Ca on net intensities. Axially viewed ICP. (a), Na; (b), Ca. ness.16,20,21 This ratio is related to the excitation temperature viewed ICP and a narrow-bore central torch injector. They reported significantly lower ratios of 5.2 and 5.9 for 0.95 kW, via the Saha–Eggert equation.21,22 Robustness has been associated with the internal diameter of the torch injector and when for an aqueous solution, which increased to 7.3 and 5.9 at 1.45 kW and an aerosol flow of 0.6 l min-1, for the radially a wide injector is employed the velocity is reduced and the residence time increases.21 It should be noted that in the and axially viewed ICPs, respectively. However, with high power, LODs were degraded by factors of 2–6 owing to the present investigation, a wide torch injector was employed and the ratio was used to express the response of the plasma decreased S/B.In the presence of 10% NaCl, the ratio was suppressed by up to a value of 2, whereas in the present to changes in operating conditions and major element composition and acid concentration. configuration it was 6–7. In comparison with the axially and radially configurations employed by Ivaldi and Tyson,16 the The influence of plasma operating conditions such as nebulization pressure and power on the MgII 280.270/ presently configured torch appears to be more robust.MgI 285.213 nm ratio is given in Table 4 and Fig. 17. With axial viewing, at 0.9 kW, the ratio increased with increasing Accuracy nebulization pressure, passing through a maximum of about 7.6 at 160–180 kPa. MgII 280.270 and MgI 285.213 nm inten- The accuracy was determined by analyzing a wide range of geological CRMs listed in a previous publication.29 These sities decreased with increasing nebulization pressure, but the rates diered.This ratio increased with increasing power; for samples were decomposed using a sodium peroxide sintering procedure. A 0.5 g sample was sintered with 3 g of sodium example, at 180 kPa, from 7.5 at 0.9 kW to about 12 at 1.4 kW. In both configurations, at low power, the ratio was about 10.5 peroxide in a Zr crucible for 45 min at about 500 °C.The fused cake was dissolved in HCl and made up to 100 ml. The for an aqueous solution and 6–7 for solutions containing up to 5% Na (10% NaCl) (Table 5). recommended values quoted by Govindaraju30 were correlated with the net intensities and with the intensity ratios using Using USN, the ratio also varied as a function of nebulizer pressure and power. In an aqueous solution at 1.2 kW, the Sc II 361.384 nm as the internal standard. Examples of these graphs for major and trace elements are given in Figs. 18 and ratio amounted to 8 and 7 in the radially and axially viewed ICPs, respectively. In the presence of 1000 mg l-1 Na at 1 kW, 19. The correlation coecients, describing the goodness-of-fit and the concentration ranges, are listed in Table 6. In all cases, the ratio varied from 7 to 5.5, and at 1.4 kW the ratio amounted to 8 and 7, respectively. the accuracy of the determinations improved with internal standard compensation (which was measured sequentially).In A comparison of the present data with those reported in the literature is given in Table 5. Ivaldi and Tyson16 measured the this analytical situation, a significant amount of the interferences could be due to physical eects in the aerosol gener- MgII 280.270/MgI 285.213 nm ratio for a radially and axially 902 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Fig. 9 Eect of Na and Ca on net intensities. Radially viewed ICP.(a), Na; (b), Ca. Fig. 11 Comparison of interference eects due to 1000 mg l-1 Na in Fig. 10 Comparison of interference eects in the axially viewed ICP axially and radially viewed plasmas. due to 1000 mg l-1 Ca and 1000 mg l-1 Na. ation and transport system, since the presence of large amounts depression of intensity depending on the excitation and ionizof Na in solution might have had a ‘buering’ eect in the ation energies of the analyte, concomitant element and the plasma.It should be noted that calibration graphs were linear observation zone in the plasma have been reported.21,32–34 for at least 3–4 orders of magnitude. The variable response of These workers have documented that the addition of an EIE the analyte lines to dierent Na and Ca concentrations can be enhances analyte emission in the lower region of the plasma, further compensated by using the generalized internal standard while depressing it higher in the plasma. method (GIRM).31 This is currently under evaluation.While the spatial emission behavior can account for the interferences in the radially viewed ICPs, spatial inhomogeneities may have less influence in axially viewed ICPs where the CONCLUSIONS field of observation is large. Davies et al.14 showed that, by using the correct optics, the spatial variations of ‘soft’ and Matrix eects due to low ionization potential EIEs and Ca in the radially viewed ICP have been correlated with dierent ‘hard’ lines were essentially negated with axial viewing owing to optical integration of emission from all the regions of spatial distributions of ion and atom spectral lines in the normal analytical zone, and are the result of several processes observation.Thus, the interferences caused by varying the concentrations of Na and Ca may be due to the energy such as nebulization, excitation, shifts in ionization equilibrium and collisional processes.32–34 Both enhancement and withdrawal required to dissociate the concomitant matrix.Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 903Fig. 12 Correlation of interference eects due to 1000 mg l-1 Ca in the axially and radially viewed ICP. Fig. 14 Compensation of interference eects due to 1000 mg l-1 Na and 1000 mg l-1 Ca using Sc II 361.384 nm as the internal standard. Axial viewing. Fig. 13 Comparison of interference eects of axially and radially viewed ICPs. (a), 1000 mg l-1 Na; (b), 1000 mg l-1 Ca.Thompson and co-workers35,36 noted that Ca had a large eect on the sensitivity of the atom and ion lines in the radially viewed ICP. These interferences were correlated with the excitation and ionization potentials of the analytes, lowering of the excitation temperature and dissociation energies. Kovacic et al.34 observed matrix eects from Mg and Li in a radially viewed ICP. Matrix eects increased with carrier gas flow rates and amounted to about 10% for 1000 mg l-1 Mg.Although the EIE-induced depressions observed in the present axial configuration are generally similar to those reported in the literature for radially viewed ICPs, further decreases in the magnitude of the interferences might be possible by optimizing plasma operating conditions. In this respect it should be emphasized that the interference eects observed here for the Fig. 15 Compensation of interference eects due to 1000 mg l-1 Na axially viewed ICP were obtained at a relatively low power and 1000 mg l-1 Ca using Sc II 361.384 nm as the internal standard.of 0.9 kW. Radial viewing. For the radially viewed ICP, the moderately large interferences of up to 15–20% (depending on the excitation and ionization potentials), were probably due to the relatively small diameter of the injector (1.4 mm), and the high nebulization 904 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Table 4 Influence of nebulizer pressure and rf power on the MgII 280.270/MgI 285.213 nm ratio.Aqueous solution Nebulizer pressure/kPa 0.9 kW 1.0 kW 1.2 kW 1.4 kW 140.0 6.2 9.5 10.5 11.3 160.0 7.2 10.1 11.1 11.7 180.0 7.6 10.0 11.4 11.9 200.0 7.2 9.4 10.9 11.8 220.0 6.4 8.1 9.9 11.0 240.0 5.4 6.8 8.7 10.1 260.0 4.4 5.5 7.4 9.0 280.0 3.4 4.4 6.3 7.9 300.0 2.8 3.4 5.3 6.8 Fig. 16 Compensation of interference eects due 1000 mg l-1 Na in Table 5 MgII280.270/MgI285.213 nm ratios for axially and radially the axially viewed ICP using USN.Cd II 226.502 nm was used as the viewed ICPs as a function of power with comparison of data from internal standard. Analyte concentration, 1 mg l-1. the literature Power/ kW Solution Radial Axial Reference 1.0 Aqueous 10.5 10 This work 1.0 1000 mg l-1 Na 10.5 7 This work 1.0 1000 mg l-1 Ca 10.1 6.95 This work 1.0 5% Na 6–7 6–7 This work 1.40 Aqueous 12 11.9 This work 1.2 Aqueous, USN 8 7 This work 1.0 1000 mg l-1 Na, USN 7 5.5 This work 1.4 1000 mg l-1 Na, USN 8 7 This work 0.95 Aqueous 5.2 5.9 16 0.95 10% NaCl 3.6 2 16 1.45 Aqueous 7.3 5.9 16 Fig. 17 Variation of MgII 280.270 and MgI 285.213 nm in the axially viewed ICP as a function of nebulization pressure and power.Aqueous solution. Fig. 19 As in Fig. 18. Determination of V and Zn. pressure of 180 kPa. Thus, these interference eects will be minimized by increasing the injector diameter and further optimizing the ICP operating conditions. A point of interest is the relatively high sensitivity of the ‘soft’ lines to changes in nebulization pressure. An increase in aerosol flow rate resulted in a change in the position of the IRZ (at a high pressure the apex was located higher up, close to the upper coil) and the region of observation of the low energy spectral lines could be more favorably located.Fig. 18 Intercalibration graphs for Al2O3 and CaO using Al I 308.22 Therefore, although the field of focus was relatively large, such and Ca II 317.93 nm, in geological CRMs with and without changes in position could account for changes in S/B.In this Sc II 361.384 nm as the internal standard, using the axially viewed discussion the physical eects of aerosol generation have been ICP. Sc was measured sequentially. CRMs were decomposed using a sodium peroxide fusion. Solutions contained approximately 5% NaCl. neglected and it is feasible to assume that part of the depression Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 905Table 6 Summary of statistics of intercalibration graphs for geological REFERENCES CRMs using axially viewed ICP. Correlation coecients are listed for 1 Brenner, I. B., Bremier, P., and Le Marchand, A., J. Anal. At. data with and without Sc II 361.384 nm as the internal standard. Spectrom., 1992, 7, 819. Samples were decomposed using a sodium peroxide sinter (0.5 g with 2 Fassel, V. A., and Bear, B. R., Spectrochim. Acta, Part B, 1986, 2.5 g Na2O2 into 100 ml).IS, Internal standard 41, 1089. 3 Koropchak, J. A., and Winn, D. H., T rAC, T rends Anal. Chem Wavelength/nm Without IS With IS Range (Pers. Ed.)., 1987, 6, 171. Al I 308.215 0.9716 0.9915 5–15% 4 Horvath, Z., Lasztity, A., and Barnes, R. M., Spectrochim. Acta Ca II 317.933 0.9917 0.9985 0.5–15% Rev., 1991, 14, 45. Fe II 259.94 0.9799 0.9968 1–20% 5 Boumans, P. W. J. M., and Vrakking, J. J. A. M., Spectrochim. Si I 251.612 0.9892 0.9957 30–70% Acta, Part B, 1987, 42, 4 and 553.Ti II 337.280 0.9950 0.9975 0.1–2.5% 6 Murillo, M., and Mermet, J. M., Spectrochim. Acta, Part B, 1989, 44, 359. Be I 234.861 0.9990 0.9998 1–300 mg kg-1 7 Lim, B. H., Carney, K. P., Edelson, M. C., Brenner, I. B., and Li I 670.784 0.9978 0.9985 10–500 mg kg-1 Houk, R. S., Spectrochim. Acta, Part B, 1993, 48, 1617. Pb II 220.353 0.9736 0.9930 10–200 mg kg-1 8 Lichte, F. E., and Koirtyohann, S. R., presented at FACSS, Sr II 407.771 0.9853 0.9975 10–1000 mg kg-1 Philadelphia, PA, 1972, paper 26.VII 292.402 0.9706 0.9960 5–200 mg kg-1 9 Demers, D. R., Appl. Spectrosc., 1979, 33, 584. Zn II 231.856 0.9882 0.9935 5–300 mg kg-1 10 Kornblum, G. R., 2nd ICP Conf., Noordwijk Aan Zee. Report by Danielsson, A., ICP Inf. Newsl., 1978, 4, 147. 11 Faires, L. M., Bieniewski, T. M., Apel, C. T., and Niemczyk, eects are due to physical changes in nebulization as the salt T. M., Appl. Spectrosc., 1985, 39, 5. concentration increases. 12 Nakamura, Y., Takahashi, K., Kujirai, O., Okochi, H., and A principal dierence in the analytical performance of the McLeod, C.W., J. Anal. At. Spectrom., 1994, 9, 751. present configuration, in comparison with those described in 13 Abdallah, M. H., Diemiaszonek, R., Jarosz, J., Mermet, J. M., the literature, is the high robustness of the ICP. In the axially Robin, J., and Trassy, C., Anal. Chim. Acta, 1976, 84, 271. 14 Davies, J., Dean, J. R., and Snook, R. D., Analyst, 1985, 110, 535.viewed plasma, high MgII 280.270/MgI 285.213 nm ratios, 15 de Loos-Vollebregt, M. T. C., Tiggelman, J. J., and de Galan, L., varying from 7 to 12, were observed using conventional Spectrochim. Acta, Part B, 1988, 43, 773. nebulization conditions. For an aqueous solution, using ‘robust’ 16 Ivaldi, J. C., and Tyson, J. F., Spectrochim. Acta, Part B, 1995, conditions17 and 1.4 kW rf, a ratio of 12 was obtained, whereas 50, 1207. in the configuration decribed by Ivaldi and Tyson,16 the ratio 17 Dubuisson, C., Poussel, E., and Mermet, J.-M., J.Anal. At. was only 5.9 at 1.45 kW. Unfortunately, Dubuisson et al.,17 in Spectrom., 1997, 12, 281. 18 Milburn, J. W., At. Spectrosc., 1996, Jan/Feb, 10. citing that these conditions were ‘robust’ in the two systems 19 Varian Spring Seminar Lecture Series, 1995, Varian Optical they evaluated, did not state the MgII/MgI ratios. Moreover, Spectroscopy Instruments, Woodale, IL. the ratio for a solution containing 5% Na (approximately 10% 20 Poussel, E., Mermet, J.M., and Samuel, O., Spectrochim. Acta, NaCl) was about 6, compared with the value of 2 obtained by Part B, 1993, 48, 743. Ivaldi and Tyson.16 21 Mermet, J. M., Spectrochim. Acta, Part B, 1989, 44, 1109. The robustness of the present configurations is probably a 22 Tripkovic�, M. R., and Holclajtner-Antunovic�, I. D., J. Anal. At. Spectrom., 1993, 8, 349. result of several factors: (a) The use of a relatively wide torch 23 Watson, A.E., and Russell, G. M., ICP Inf. Newsl., 1979, 4, 447. injector tube of 2.3 mm in the axial torch, as recommended by 24 Brenner, I. B., Watson, A. E., Russell, G. M., and Goncalves, M., Mermet co-workers.17,20,21 (The use of a narrow t injector Chem. Geol., 1980, 28, 321. reduces the MgII 280.270/MgI285.213 nm ratios as a result 25 Marshall, J., Rodgers, G., and Campbell, W. C., J. Anal. At. of degraded energy transfer as demonstrated by Mermet.21) Spectrom., 1988, 3, 241.(b) High eciency of energy transfer in the plasma (the 26 Brenner, I. B., Zander, A., and Shkolnik, J., ICP Newsl., 1995, 20, 738. MgII 280.270/MgI 285.213 nm ratio was also high in the radi- 27 Brenner, I. B., Zander, A., Kim, S., and Shkolnik, J., J. Anal. At. ally viewed ICP, even though a narrow injector was employed). Spectrom., 1996, 11, 91. (c) The end-on gas dynamics which symmetrically removes the 28 Koirtyohann, S. R., Jones, J. S., Jester, C. P., and Yates, D. A., cool plasma fringe, leaving the central axial zone free of the Spectrochim. Acta, Part B, 1981, 36, 49. interfering sheath. 29 Brenner, I. B., Zander, A., Kim, S., and Holloway, C., Spectrochim. In the sequential mode the internal standard compensated Acta, Part B, 1995, 50, 562. 30 Govindaraju, K., Geostand. Newsl., 1994, 18, 1. mainly for transport eects which result from physical vari- 31 Lorber, A., Goldbart, Z., Harel, A., Sharvit, E., and Eldan, M., ations in the aerosol generation system.25,26 On the other Spectrochim. Acta, Part B, 1986, 41, 105. hand, the variable eects of EIE-induced processes in the 32 Blades, M. W., and Horlick, G., Spectrochim. Acta, Part B, 1981, plasma were only partly compensated using Sc II 361.384 and 36, 881. Cd II 226.502 nm as internal standards. Even though the action 33 Davies, J., and Snook, R. D., J. Anal. At. Spectrom., 1986, 1, 325. of Sc was compromised as a result, the need to matrix-match 34 Kovac¡ic�, N., Budic¡, M., and Hudnik, V., J. Anal. At. Spectrom., 1989, 4, 33. and to use tedious standard additions procedures was avoided. 35 Thompson, M., and Ramsey, M. H., Analyst, 1985, 110, 1413. The accuracy of the determination of major, minor and trace 36 Ramsey, M. H., and Thompson, M., J. Anal. At. Spectrom., 1986, elements in complex geological samples was significantly 1, 185. improved, the goodness-of-fit between recommended values increasing with Sc signal compensation. Paper 7/00465F Hence, with an optimized torch injector and ICP operating Received January 21, 1997 conditions such as rf power and nebulization, a robust axial Accepted June 25, 1997 ICP configuration could be universally employed to solve a wide range of applications, thus overcoming the need for a radial set-up or for configurations where both axial and radial systems might be used to solve analytical tasks. 906 Journal of Analytical Atomic Spectrometry, September 1997, Vol.

ISSN:0267-9477    

DOI:10.1039/a700465f

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Inductively Coupled Plasma Cavity Ringdown Spectrometry† G. P. MILLER* AND C. B. WINSTEAD Diagnostic Instrumentation and Analysis L aboratory (DIAL ), College of Engineering,Mississippi State University, Mississippi State,MS 39762 USA Cavity ringdown spectrometry (CRS) diers from standard increases], reducing the decay time of the cavity. Therefore, an absorption spectrum is simply obtained by monitoring the atomic absorption methods in that it is a measurement of the rate of light absorption by a sample within a closed optical decay time constant as a function of wavelength.This, in turn, is easily represented as optical loss per pass through the cavity cavity. The ringdown technique yields a very high sensitivity achieved from a combination of long eective sample versus laser wavelength. Since only the decay time constant is measured, this pathlengths and relaxed accuracy constraints on the measurement of the decay rate of light in the cavity. While technique is insensitive to the power fluctuations common to pulsed lasers.Eqn. (1) does appear to neglect some forms of there has been rapid scientific recognition of the potential of cavity ringdown for molecular spectroscopy, there has been no optical loss from the cavity, such as Rayleigh and Mie scattering. However, these losses are accounted for in the ‘eective’ systematic attempt to incorporate the advantages of CRS into an instrument for analytical atomic spectrometry.In this mirror reflectivity in eqn. (1), as they simply contribute a small component to the ‘background’ cavity loss.5 The sensitivity of paper, the application of the cavity ringdown method to analytical atomic spectrometry is discussed. In particular, the the technique is clearly enhanced by using highly reflective mirrors to extend the ringdown time of the cavity and by first theoretical and experimental results concerning the use of CRS for trace analysis in an ICP are presented.precise time constant measurement. High quality mirrors (R#99.9%) are now commercially available at operating wave- Keywords: Inductively coupled plasma; cavity ringdown lengths down to almost 200 nm. Typical ringdown times range spectrometry; trace analysis from tenths to tens of microseconds (depending on mirror reflectivity), corresponding in some experiments to kilometers of eective sample path length. In many cases, 1% accuracy O’Keefe and Deacon1 introduced cavity ringdown spectroscopy (CRS) in 1988 as a new, highly sensitive method for in time measurement is all that is required in order to determine a change in per pass optical absorbance of a few ppm.6 absorption spectrometry using pulsed lasers. This initial CRS work demonstrated the high sensitivity of the technique for The utility of ringdown spectroscopy has been demonstrated in a variety of experiments.For example, it has been used to spectroscopic applications by measuring doubly forbidden electronic transitions in molecular oxygen.This innovative study species present in environments ranging from molecular beams6–8 and gas cells9,10 to atmospheric pressure flames.4 technique, being based upon the measurement of the time required for a laser pulse to decay inside an optical cavity, is Investigations of several molecular transitions have been carried out with unprecedented precision,1,2,11–13 demonstrating a distinctly new variant for absorption spectroscopy.Although non-linear optical techniques have provided pulsed laser sys- that the sensitivity of CRS rivals or even outperforms that available with photoacoustic spectroscopy (widely recognized tems with a significantly wider wavelength operating range than continuous lasers, the use of pulsed lasers has generally as one the most sensitive absorption spectroscopic techniques). Ringdown studies have also been used for kinetics studies,3,14,15 been avoided in absorption spectroscopy owing to their inherent shot-to-shot power fluctuations.CRS avoids this gain measurements in chemical laser systems,16 concentration measurements in a laser desorption molecular beam17 and diculty as follows. A small percentage of the laser pulse energy is injected into an optical cavity formed from two have even been proposed for the spectrometry of ions trapped via ion cyclotron resonance.18 highly reflective dielectric mirrors (see Fig. 1). The laser pulse is introduced into the cavity through one of the end mirrors, Use of cavity ringdown for the detection and measurement of molecular trace species has also been clearly demonstrated. where it subsequently becomes trapped between the mirror surfaces.The intensity of the light in the cavity decays exponen- For example, Zalicki et al.9 used ringdown spectrometry to measure the absorbance and spatial profile of methyl radicals tially with time at a rate determined by the round trip loss experienced by the laser pulse.This decay time (t), measured in a heated flow reactor at a column density at least an order of magnitude smaller than that required by the more traditional by a photomultiplier tube located behind the opposing end mirror, is given approximately by1–4 laser multipass absorption techniques.19 The experiments of Jongma et al.15 also provided extensive evidence of the potential for using CRS to achieve extremely low detection limits for t= Tr 2[(1-R)+als] (1) trace species.The absolute concentration of OH radicals was measured in heated air as a function of temperature, and NH3 where Tr is the round trip time for the pulse in the cavity, R is the eective reflectivity of the cavity mirrors, a is the wavelength dependent absorption coecient of a sample in the cavity and ls is the length of the optical path through the sample. When the laser wavelength is tuned through a resonance with an atomic or molecular absorption line, the absorption experienced by the laser pulse increases [i.e., a in eqn.(1) † Presented at the 1997 European Winter Conference on Plasma Fig. 1 Basic optical configuration for CRS. Spectrochemistry, Gent, Belgium, January 12–17, 1997. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 (907–912) 907was detected at the 10 ppb level in NH3–air flows. A detection eqn. (1)] of approximately 1 ppm,1,5,7 while per pass absorbances as small as 0.1 ppm have been observed.2,13 limit for Hg in air of less than 1 ppt (number concentration units) was also achieved.8 While discussion continues Accounting for the lower mirror reflectivities available for UV wavelengths (R#99.9% maximum) leads to a 10 ppm per pass concerning the fundamentals of laser pulse behavior in an optical cavity,10,20–24 it has been convincingly demonstrated absorbance estimate for ICP-CRS detection.This estimate is based on a 1% variation in baseline ringdown times, although that CRS is one of the most promising and most sensitive new spectrometric tools available.time constant stability better than 0.1% has been previously demonstrated.17 Rayleigh scattering constitutes only a fraction of the per pass loss when using R=99.9% mirrors and thus is CRS AND ANALYTICAL ATOMIC ABSORPTION neglected. The absorbance per pass (at a 10 ppm per pass limit) is given by1 Methodology 10-5<sCls (2) The results obtained in studies using CRS have clearly demonstrated ringdown’s potential as an analytical technique.To where s is the photon absorption cross-section (cm2 atom-1), utilize the potential of CRS for trace element detection, it is C is the minimum detectable concentration (atom cm-3) and necessary to couple the ringdown cavity to an appropriate ls is the sample pathlength per pass (cm). Thus, in order to atomization source. To this end, two possibilities were focused calculate the minimum detectable concentration C, it is neceson: the ICP and graphite furnace.Investigations of these two sary to estimate both the absorption cross-section for a given methods hold the promise of providing a full evaluation of the atomic transition and the sample pathlength. The absorption potential of CRS as a powerful new technique for atomic trace cross-section for a line broadened atomic transition may be analysis. The discussion that follows concerns preliminary estimated as follows.For a predominantly Doppler broadened results on the coupling of the ICP and CRS. line, the absorption cross-section is given by27 ICP-CRS sij= gj gi l4 8pc Aji Dl (3) There is currently widespread interest in the development of where l is the transition wavelength, Dl is the transition analytical instrumentation for online characterization of induslinewidth, Aji is the spontaneous emission transition rate, gj trial processes and emissions. For example, important concerns and gi are the upper and lower state degeneracies and c is the for the development and implementation of remediation prospeed of light [when pressure broadening eects of the plasma cesses for hazardous and radioactive wastes include characare dominant, the 8p in eqn.(3) is replaced by 4p2]. terizing the waste prior to treatment and monitoring the safety Using compilations of linewidth and line shape data26,28 and eciency of the treatment process. Central to these issues and Aki data,29,30 and assuming a sample path length of 5 mm, is the need for very sensitive detection of hazardous atomic the detection limits for selected elements in an ICP-CRS system and molecular species in the waste stream, treatment ogas were estimated.These are listed in Table 1. (NOTE—This and final treated product. In particular, a critical need exists table is not intended to serve as an exact statement of ICP- for techniques to monitor heavy metal concentrations in CRS detection limits, nor does it list every absorption line for remediation ogases at a ppb level or lower in a near realthe elements compiled.It is merely intended to depict the time manner; see for example ref. 25. However, for reliable order of magnitude of performance that may be expected from monitoring of atomic species in ogas systems, ogas samples an ICP-CRS system.) must be eciently atomized. Metals will be present in a given While eqn. (2) yields detection limits in atoms per unit ogas in a variety of molecular forms (e.g., oxides or chlorides) volume, the values in Table 1 have been converted into the or as particles.Methods that employ less than complete more standard units of mass per unit volume. Estimated ultra- atomization techniques may underestimate the actual concensonic nebulized sample eciencies (ppb#2.5×mg m-3) were tration of metallic species. Thus, the importance of eective then incorporated to provide solution detection limits in sample atomization cannot be overstated.By using CRS for ppb.31,32 The variation in detection limits between a strong and species detection in conjunction with an ICP (air or argon) weak atomic transition is illustrated by the Cd 229 nm (Dl= atomization system (Fig. 2), detection limits comparable to 1.5 pm,28 Aki=6×108 l s-1 29 yields ICP s#4.36×10-12 cm2) ICP-MS26 may be realized in a simple, robust and cost-eective and Cd 326 nm (ICP s#6.15×10-15 cm2) transitions. Note system that avoids the complications inherent in using ICP-MS that even the weak Cd (5 1S0–5 3S1) 326 nm transition is pre- in such hostile environments. dicted to yield ppb detection at a level better than the 5 mg m-3 Estimates of expected detection limits using ICP-CRS may level required by proposed US environmental regulations be performed in a straightforward manner.CRS typically for continuous emission monitors. The Hg 283 nm line is also detects changes in optical absorbance per cavity pass [als in Table 1 Theoretical ICP-CRS detection limits Detection limit Element Wavelength/nm mg m-3 ppb Cd 229 0.0008 0.002 Cd* 326 0.6 1.5 Hg 254 0.28 0.7 Pb 283 0.016 0.04 Sr 461 0.00028 0.0007 Mn 403 0.012 0.03 Tl 378 0.012 0.03 Cr 425 0.0036 0.009 Cs* 852 0.00024 0.0006 Al* 394 0.0028 0.007 Fig. 2 Experimental arrangement for performing ICP-CRS. * Estimated linewidth. 908 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12fairly weak, yet the predicted Hg limit of 0.3 mg m-3 is still ICP-CRS OPERATION AND OPTIMIZATION lower than proposed requirements. The strong Cd 229 nm line Instrumentation exhibits a predicted ppt level of detection, nearly three orders of magnitude better than presently proposed requirements. A schematic diagram of the experimental setup is given in Fig. 3. For the initial feasibility studies investigating the use of In general, even if these estimates are still one or two orders of magnitude overly optimistic, performance comparable to CRS in an ICP, the following experimental system was used.ICP-MS can be achieved for many elements. These ICP-CRS estimates are obtained based on the assumption that the ICP ICP is operated in a fashion similar to that used for ICP atomic fluorescence (ICP-AFS) studies, namely as an atomization A 1.6 kW, 27.12 MHz argon ICP equipped with a standard source producing predominantly ground state atomic species. Fassel torch was used throughout this initial study.An ultra- As suggested in Fig. 2, these conditions have often been opti- sonic nebulizer was employed for sample injection. mally achieved in ICP-AFS studies several centimeters above the ICP coil.27 However, pulsed laser induced fluorescence has L aser system also yielded excellent sensitivity when pumping ground state atoms only 20 mm above the ICP coil.33 While the present A Quanta Ray DCR-1A Nd:YAG laser was used to pump a preliminary ICP-CRS studies have focused on the detection of tunable dye laser (Quanta-Ray PDL-2).The dye laser output ground state species, the ICP may also be operated as an was frequency doubled (Spectra-Physics WEX) to obtain UV excitation or ionization source, and selected excited states or laser wavelengths. The dye laser output has a specified lineionized species detected. width of 0.2 cm-1, which when doubled yields a final UV linewidth of approximately 0.3 cm-1. The laser pulse duration is approximately 10 ns and the laser repetition rate is 10 Hz.Cavity Design Cavity optics It has been previously demonstrated that dye lasers can operate Inherent in the application of CRS is the need for high quality with an ICP located inside the laser cavity to probe absorbing cavity mirrors. Recent improvements in optical coatings have species within the plasma (intracavity absorption specyielded commercially available mirrors with specified reflectiv- trometry).34–36 Although the technique employed was signifi- ity of 99.9% to wavelengths around 200 nm.Various radius of cantly more limited than CRS, particularly at UV wavelengths, curvature mirrors coated for dierent wavelength ranges have it suggested that plasma stability would not be a problem for been used, although no mirrors with reflectivity higher than a ringdown cavity. This fact is confirmed by the present initial 99.6% have been tested to date with the ICP.Various spatial studies with both flames and the ICP (see later discussion). filtering and mode matching optical configurations for the A number of potentially important issues must be addressed input laser beam are also currently under evaluation. A narrow- in designing a cavity for an ICP-CRS system. An important band interference filter or small spectrometer centered on the consideration in achieving optimum detection limits with such laser wavelength is placed in front of the detection electronics a system is to maximize the stability of the ringdown time to reject background radiation.measurement, i.e., to maximize the stability of the cavity with respect to index of refraction variations introduced by the plasma. In this manner, the standard deviation of the baseline Electronics ringdown times will be reduced, allowing a lower detection A photomultiplier tube (PMT) is placed behind the cavity limit. While it is known that symmetric confocal cavities (i.e., output mirror to detect the light leaking from the cavity.The cavity length equals mirror radius) are fairly stable with respect signal from the PMT is digitized by a 200 MHz digital to mirror misalignment and provide a good starting point in oscilloscope (Tektronix TDS420A) and transferred via a GPIB traditional laser cavity design,26 the frequencies of the eigeninterface to a personal computer. The ringdown decay time modes for a true confocal cavity are degenerate.It has been constant is calculated by fitting the waveform to a single suggested that such a mode structure may introduce uncertaintexponential decay. These time constants are displayed on the ies into the interpretation of ringdown spectra.4,20–23 For this computer monitor along with the most recent ringdown wave- reason, stability experiments were carried out with an ICP form. Typically 10–100 laser shots are averaged for a given using both a very stable nearly confocal cavity and an identical time constant calculation.A dual channel boxcar integrator length cavity constructed with long radius of curvature mirrors (nearly planar configuration). Another issue relevant to the overall design and optimization of the ICP cavity ringdown system is the spatial resolution that is required. This is determined by the cavity length and the radius of curvature chosen for the cavity mirrors which, together with the operational wavelength, determine the beam waist dimensions (minimum beam spot size) in the cavity.26 The only real limitation imposed to ensure optical stability is that the cavity length must be less than twice the mirror radius of curvature.Symmetric confocal cavities yield small beam spot sizes at the cavity center for the lowest cavity eigenmodes. This can be used to increase the spatial resolution of the ICPCRS measurement. However, the small spot size also reduces the volume of the sample with which the beam interacts.An almost planar cavity configuration will possess a larger spot size than that associated with a confocal cavity. Spot sizes of a few tenths of a millimeter are typical near the center of the Fig. 3 Experimental configuration used for preliminary ICP-CRS ringdown cavities used, adequate for optimizing the operating investigations. parameters such as observation height, within the ICP. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 909was also employed in place of the oscilloscope and fitting software to obtain cavity ringdown spectra as suggested by previous investigators.2 However, for the present purposes it was found that the software method allowed a more rapid diagnosis of the quality of the cavity alignment, and provided a quick method of determining if the ringdown waveform was a true single exponential decay.PRELIMINARY RESULTS AND DISCUSSION While the theoretical predictions suggest that ICP-CRS could under ideal conditions achieve detection limits superior to ICP-MS, practical demonstration is required. Therefore, a study was undertaken to establish the viability of ICP-CRS for the detection of trace metals.Prior to obtaining definitive Fig. 5 Cavity ringdown transient recorded both with and without a detection limits, a considerable amount of research must be burner flame at the cavity center. conducted in the following areas. (1) Operating conditions and analytical performance: (a) influence of ICP torch design and operation, atomization or ionization? (b) observation height length allows significant variations in cavity design to be tested within the ICP; (c) gases and gas flow for the ICP torch; with only these two mirror sets.Each set was evaluated for (d) wavelength selection; and (e) calibration. (2) Interference stability and ease of alignment both with and without the ICP eects: (a) light scatter; (b) sample transport; (c) matrix eects; operating at the cavity center.With either the near confocal and (d) spectral interferences. or near planar cavity design, it was possible to achieve standard The necessary first concern for developing an ICP-CRS deviations in ringdown time of slightly less than 1% of the system is interfacing the ICP and optical cavity systems, that total time constant. For example, with a ringdown time of is, the eect on the cavity stability, and ringdown time, of approximately 370 ns, the standard deviation of the measured introducing an atomization source into the cavity must be ringdown times was about 3 ns.This level of stability was determined. An initial 1 m long cavity was built using 5 m relatively unchanged by the introduction of the ICP into the radius of curvature mirrors with coatings having 99.5% cavity. While it seems reasonable that the ICP should lead to reflectivity (at 460 nm) as a prototype. The ringdown waveform a slight decrease in time constant stability, it is believed that associated with the empty cavity upon the injection of 460 nm the current level of stability is determined primarily by the light is depicted in Fig. 4. The accuracy of the ringdown performance of the laser system used. Thus, at present, it can technique for mirror reflectivity determination is demonstrated, be stated that instabilities caused by the introduction of the as the manufacturer’s specified reflectivity is easily confirmed ICP into the optical cavity are less than the present stability by the ringdown measurement.An initial evaluation of the limits of detection. This implies that the ICP induced instability, stability of this cavity was carried out by introducing a when quantified, will be less than 1% of the total ringdown ‘turbulent’ atomization source into the cavity, namely a stan- time, allowing for a small baseline variation and hence low dard laboratory Bunsen burner. That the presence of the flame limits of detection for trace elements.has little eect on the ringdown decay is clearly demonstrated Although the ringdown stability does not appear to be a in Fig. 5. This stability of the ringdown method was confirmed significant problem, the ICP plasma does appear to aect the with the introduction of the ICP plasma as described below. magnitude of the cavity ringdown time. As shown in Fig. 6, A systematic study was begun to determine the relative the ignition of the plasma, without the introduction of a stability of dierent cavity designs when used in conjunction sample, decreases the ringdown time by approximately 40 ns.with an ICP. These studies were performed using a cavity The cause of this increase in absorbance is presently still under length of 0.58 m with two sets of mirrors coated for optimal investigation but could, for example, be a result of increased reflectivity at 280 nm. The first set of mirrors had a 0.5 m scattering or the presence of an absorbing species.The latter radius of curvature, which when used in the 0.58 m long cavity cause is the most probable, noting the inherent sensitivity of represented a near confocal cavity configuration. The second CRS to the presence of trace species and the slight variation mirror set had a 5 m radius of curvature, allowing the investi- in ringdown time with laser wavelength. Careful alignment of gation of cavities closer to the planar limit.This allows the the cavity prior to and during ICP operation ensures that the evaluation of the time constant stability during ICP operation for two very distinct cavity cases. Adjustment of the cavity Fig. 4 Cavity ringdown signal obtained using an empty 1 m long cavity with 5 m radius of curvature mirrors. An exponential fit yields Fig. 6 Ringdown transient recorded before and after ignition of the a 676 ns time constant, corresponding to a measured average mirror reflectivity of 99.51% (99.5% specified by the manufacturer at 460 nm).ICP at the cavity center. 910 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12observed time decrease does not arise from alignment errors the added advantages of (i ) detection limits that theoretically could exceed those achieved by ICP-MS and (ii ) sensitivity caused by the presence of the plasma. Taking into account both equipment restraints and possible that increases as sample absorption decreases; (2) based on mature ICP technology; (3) potentially applicable in hostile ICP-CRS applications, investigations of trace species were initiated using two metals, Pb and Mg.To this end, mirrors environments such as on-line monitoring of industrial processes; and (4) little secondary contamination relative to centered at 280 nm were selected for the detection of Pb atoms at 283.3 nm and Mg positive ions at 279.6 nm. Ringdown time ICP-MS when used in radioactive applications.However, the disadvantage is that true simultaneous elemen- measurements without the ICP imply a reflectivity for these mirrors of approximately 99.5%. Preliminary experiments were tal determination is not possible, although a rapid multielement capability is possible with the use of OPO lasers, where performed using the Pb (283.3 nm) absorption line to confirm the viability of ICP-CRS for trace analysis. Shown in Fig. 7 is tunability over a wide wavelength range is not a problem. With the rapid technological advances being made in the a measure of the relative absorbance that occurs as the dye laser is tuned across the 283.3 nm Pb line, for a blank and a development of tunable solid state lasers, diode lasers and continuous wave CRS systems,37–39 an ICP cavity ringdown 100 ppm Pb nebulized solution.A region approximately 70 mm above the ICP coil was probed for this scan. device could within the near future provide detection limits comparable to ICP-MS in a simple and cost eective real- As the laser linewidth of the present system at this time is approximately twice that of the absorption feature, the appar- time system.ent absorption linewidth in Fig. 7 is a combination of the The authors thank P. R. Jang at Mississippi State University actual absorption linewidth and the laser linewidth. However, for writing the software used for data acquisition and control as first described by Zalicki and Zare20 and recently demonin these experiments, and also wish to acknowledge many strated by Hodges et al.37 and Jongma et al., 5 an important helpful discussions with Anthony O’Keefe of Los Gatos consideration for ICP-CRS (and other atomic absorption Research.This work is supported by United States Department methods as well) is the use of a light source with a linewidth of Energy grant number DE-F602-93CH-10575. narrower than the absorption profile to be probed. This can be understood as follows.If the laser linewidth is narrower than the absorption feature then the decay is dominated by REFERENCES absorption losses. If, however, the laser linewidth is broader 1 O’Keefe, A., and Deacon, D. A. G., Rev. Sci. Instrum., 1988, than the absorption line, then only the loss near the absorption 59, 2544. line center is due to absorption loss. O center, the decay is 2 Romanini, D., and Lehmann, K. K., J. Chem. Phys., 1993, 99, 6287. dominated by losses at the mirror surface.This results in 3 Yu, T., and Lin, M. C., J. Am. Chem. Soc., 1993, 115, 4371. reduced sensitivity and accuracy. Thus, the system used cur- 4 Meijer, G., Boogaarts, M. G. H., Jongma, R. T., Parker, D. H., rently suers a reduced sensitivity due to the relatively broad and Wodtke, A. M., Chem. Phys. L ett., 1994, 217, 112. 5 Jongma, R. T., Boogaarts, M. G. H., Holleman, I., and Meijer, G., bandwidth. Once the current laser system upgrades have been Rev. Sci. Instrum., 1995, 66, 2821.completed, the eect of the ICP operating conditions and ICP- 6 O’Keefe, A., Scherer, J. J., Cooksy, A. L., Sheeks, R., Heath, J., CRS detection limits will be reported. While atomic lines may and Saykally, R. J., Chem. Phys. L ett., 1990, 172, 214. have natural linewidths somewhat less than a typical narrow- 7 Scherer, J. J., Paul, J. B., and Saykally, R. J., Chem. Phys. L ett., band dye or OPO laser system, the broadening present in an 1995, 242, 395.atmospheric pressure ICP plasma will allow the use of currently 8 Scherer, J. J., Paul, J. B., Collier, C. P., and Saykally, R. J., J. Chem. Phys., 1995, 102, 5190. available standard commercial systems. 9 Zalicki, P., Ma, Y., Zare, R. N., Wahl, E. H., Dadamio, J. R., Owano, T. G., and Kruger, C. H., Chem. Phys. L ett., 1995, 234, 269. CONCLUSION 10 Scherer, J. J., Voelkel, D., Rakestraw, D. J., Paul, J. B., Collier, C. P., Saykally, R. J., and O’Keefe, A., Chem. Phys. L ett., 1995, Preliminary results indicate that coupling CRS to an ICP has 245, 273.the potential to become a valuable asset in analytical atomic 11 Huestis, D. L., Copeland, R. A., Knutsen, K., Slanger, T. G., Jongma, R. T., Boogaarts, M. G. H., and Meijer, G., Can. J. Phys., spectrometry. At this preliminary stage of research, all indi- 1994, 72, 1109. cations are that sub-ppb detection limits are readily attainable, 12 Jongma, R. T., Boogaarts, M. G. H., and Meijer, G., 1994, J.Mol. with sub-ppt levels possible. Spectrosc., 1994, 165, 303. The advantages of the ICP-CRS technique for trace analysis 13 Romanini, D., and Lehmann, K. K., J. Chem. Phys., 1995, 102, 633. are briefly summarized as: (1) absorption spectrometry with 14 Yu, T., and Lin, M. C., Int. J. Chem. Kinet., 1993, 25, 875. 15 Yu, T., and Lin, M. C., J. Phys. Chem., 1994, 98, 9697. 16 Benard, D. J., and Winker, B. K., J. Appl. Phys., 1991, 69, 2805. 17 Boogaarts, M. G. H., and Meijer, G., J.Chem. Phys., 1995, 103, 5269. 18 Huang, Y., Jackson, G., Kim, H. S., Guan, S., and Marshall, A. G., Phys. Scr., 1995, T59, 387. 19 Air Monitoring by Spectroscopic T echniques, ed. Sigrist, M. W., Wiley, New York, 1994. 20 Zalicki, P., and Zare, R. N., J. Chem. Phys., 1995, 102, 2708. 21 Martin, J., Paldus, B. A., Zalicki, P., Wahl, E. H., Owano, T. G., Harris, J. S., Jr., Kruger, C. H., and Zare, R. N., Chem. Phys. L ett., 1996, 258, 63. 22 Lehmann, K. K., and Romanini, D., J.Chem. Phys., 1996, 105, 10263. 23 Hodges, J. T., Looney, J. P., and van Zee, R. D., J. Chem. Phys., 1996, 105, 10278. 24 Scherer, J. J., Paul, J. B., O’Keefe, A., and Saykally, R. J., Chem. Rev., 1997, 97(1), 25. 25 U.S. Department of Energy Announcement Number DE-R021-96MC33204, Washington, DC. 26 Inductively Coupled Plasmas in Analytical Atomic Spectrometry, Fig. 7 ICP-CRS wavelength scan recorded 70 mm above the ICP ed. Montser, A., and Golightly, G. W., VCH, New York, 2nd edn., 1992, and references therein. load coil for blank and 100 ppm Pb solutions. See text for discussion. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 91127 Siegman, A. E., L asers, University Science Books, Mill Valley, 35 Downey, S. W., and Nogar, N. S., Anal. Chem., 1985, 57, 13. CA, 1986. 36 Downey, S. W., Keaton, G. L., and Nogar, N. S., Spectrochim. 28 Boumans, P. W. J. M., and Vrakking, J. J. A. M., Spectrochim. Acta, Part B, 1985, 40, 927. Acta, Part B, 1986, 41, 1235. 37 Hodges, J. T., Looney, J. P., and van Zee, R. D., Appl. Opt., 1996, 29 Radzig, A. A., and Smirnov, B. M., Reference Data on Atoms, 35, 4112. Molecules, and Ions, Springer-Verlag, New York, 1985. 38 Engeln, R., von Helden, G., Berden, G., and Meijer, G., Chem. 30 CRC Handbook of Chemistry and Physics, ed. Weast, R. C., and Phys. L ett., 1996, 262, 105. Astle, M. J., CRC Press, Boca Raton, FL, 1978. 39 Romanini, D., Kachanov, A. A., Sadeghi, N., and Stoeckel, F., 31 Baldwin, D. P., Zamzow, D. S., and D’Silva, A. P., J. Air & Waste Chem. Phys. L ett., 1997, 264, 316. Manage. Assoc., 1995, 45, 789. 32 Trivedi, A., Qian, S., and Monts, D. L., Appl. Phys. B, in the press. Paper 7/01523B 33 Human, H. G. C., Omenetto, N., Cavalli, P., and Rossi, G., Received March 4, 1997 Spectrochim. Acta, Part B, 1984, 39, 1345. 34 Downey, S. W., and Nogar, N. S., Appl. Spectrosc., 1984, 38, 876. Accepted May 15, 1997 912 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a701523b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Evaluation of an Inductively Coupled Air– Argon Plasma as an Ion Source for Mass Spectrometry† HIROSHI UCHIDA*a AND TETSUMASA ITOb aKanagawa Industrial Research Institute, 705–1, Shimoimaizumi, Ebina, Kanagawa 243–04, Japan bSeiko Instruments, 36–1, Takenoshita, Oyama-cho, Sunto-gun, Shizuoka 410–13, Japan An inductively coupled air–argon plasma (air–Ar ICP) has mass discrimination by Xiao and Beauchemin.10 The addition of N2 to the outer, intermediate and aerosol carrier Ar flows been developed using a modified torch and a 40.68 MHz generator (maximum rf power 4 kW), and evaluated as an ion has also been studied separately, mainly to reduce polyatomic ion interferences.11–13 Luie and Soo14 also investigated the source for mass spectrometry (MS).The outer and aerosol carrier Ar flows were completely replaced with air; however, addition of N2 and H2 to the main inlet for the three types of Ar flow and discussed its analytical characteristics. Further, 1.5 l min-1 of Ar was used as the intermediate gas to maintain a stable plasma discharge at low rf power.The optimized Lam and Horlick15 illustrated the eects of varying the sampler –skimmer spacing in a mixed gas ICP with N2 in the outer sampling depth and the aerosol carrier air flow rate for maximum analyte signals were found to be 10 mm above the flow. The addition of O2 to the aerosol carrier flow has frequently been used in order to decompose organic samples load coil and 0.9 l min-1, respectively.The analyte signal increases with rf power, but 2 kW was sucient for the stable in an Ar ICP, an example of which has recently been reported for the determination of lead by ICP-AES.16 The developments discharge and the acceptable analytical sensitivity. In the mass spectra obtained under the optimized conditions, N+, O+ and of molecular gas and mixed gas ICPs in AES have been comprehensively reviewed by Montaser.17 NO+ were clearly observed, but the signal for Ar+ was weak, which is similar to that for an N2 ICP.The ion signals for There have been a few reports on attempts to generate a molecular gas ICP for MS with the addition of a small amount N2+ and O2+ were relatively large, compared with N2 and O2 ICPs operated with Ar added to the outer gas. The analytical of Ar. Tanaka et al.18 reported N2 and O2 ICPs, where both the outer and aerosol carrier Ar flows were completely replaced sensitivity of the proposed air–Ar ICP is superior to an Ar ICP using the same equipment for elements with low first by N2 and O2, respectively.However, Ar remained in the intermediate flow. Another type of N219 or O220 ICP assisted ionization potentials (IP) of <6.5 eV, but inferior for elements with high first IPs (>6.5 eV). The secondary discharge by adding Ar to the outer gas has been achieved using a 40.68 MHz rf generator and a modified torch21 by Uchida and increases the average kinetic energy of the analyte ions, the distribution of which is wider in the air–Ar ICP than in the Ar Ito.Typical analytical characteristics of molecular gas ICP-MS were found and the analytical sensitivity was compared with ICP. The ratios of monoxide ion to singly charged ion remain almost constant along the plasma axis. Space charge eects Ar ICP-MS. In the present work, an attempt was made to generate an from co-existing elements are also discusssed. air–Ar ICP, because of the low running costs, using the same Keywords: Inductively coupled plasma mass spectrometry; air– equipment as for N219 and O220 ICPs, which was then evaluated argon plasma; analytical characteristics; comparison with as an ion source for MS.Mass spectra were obtained under other plasmas optimized plasma conditions, and the analytical sensitivity, kinetic energy distribution, ratios of doubly charged and monoxide ions and matrix eects are also discussed, and Inductively coupled plasma mass spectrometry (ICP-MS) has been used to study the determination of ultratrace elements in compared with Ar19,22, N219 and O220 ICPs.various types of samples.1 The Ar ICP is the most useful ion source, because of its stable discharge at low rf power, high sensitivity for many elements and relative freedom from inter- EXPERIMENTAL ferences. However, mass spectral interferences resulting from Instrumentation and Operating Conditions the plasma-support gas Ar2,3 cannot be avoided, especially in the m/z range from 50 to 80.A high resolution mass spec- The system used consisted of a quadrupole mass spectrometer trometer has been developed to remove mass spectral inter- (SPQ8000A, Seiko Instruments) and a specially constructed ferences caused by polyatomic ions, but the initial cost is high 40.68 MHz rf generator with a maximum power of 4 kW and it requires a large space. Another problem for an Ar ICP (Nippon Koshuha). A description of the load coil, quartz torch is the high running costs because of the Ar consumption.and sampler has been given elsewhere,19,22 including details Mixed molecular gas ICPs have been investigated for the about the equipment used. A shielding system for reduction of reduction of spectroscopic interferences and other matrix the plasma potential23–25 could not be used, because the metal eects. The eects of the addition of N2 and O2 to the central plate inserted between the quartz torch and the load coil Ar flow have been discussed for the reduction of polyatomic melted at an rf power of more than 1.8 kW.ion interferences by Evans and Ebdon4,5 and the addition of The actual operating conditions for the proposed air–Ar N2 by Laborda et al.6 The addition of N2 to the outer Ar flow ICP are listed in Table 1. An Ar ICP was used as a starter for has been studied for the reduction of oxide interferences by the air–Ar ICP and also for a comparative study. The equip- Lam and McLaren,7 the elimination of the co-existing Na8 ment used was the same as for the air–Ar ICP, and the and K9 by Beauchemin and Craig, and for the reduction of operating conditions are also given in Table 1.Analytical characteristics of this Ar ICP are almost equivalent to those of a conventional Ar ICP; however, the detection limit (DL) † Presented at the 1997 European Winter Conference on Plasma Spectrochemistry, Gent, Belgium, January 12–17, 1997. is approximately one order of magnitude inferior.The ion Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 (913–918) 913Table 1 Operating conditions for the air–Ar and Ar ICPs. The same power in the proposed ICP having outer and aerosol carrier modified torch was used in both ICPs without shielding air flows. On the other hand, the intermediate Ar flow could be replaced by air when a certain amount of Ar was left in the Parameter Air–Ar ICP Ar ICP outer flow, which had previously been adopted in N218 and O219 ICPs using the same equipment.The flow rate of the Forward power/kW 2.0 1.0 Outer gas flow rate/l min-1 Air 20.0 Ar 16.0 intermediate or outer Ar flow in each case could be reduced Intermediate gas flow rate/l min-1 Ar 1.5 Ar 1.5 at high rf power, but the damage to the top of the sampler Aerosol carrier gas flow rate/l min-1 Air 0.9 Ar 0.65 was more pronounced than at low power. Consequently, a Sampling depth/mm above the work coil 10 13 flow of 1.5 l min-1 of Ar was used as the intermediate flow in subsequent experiments, because the Ar consumption was lower and the gas replacement was carried out more easily signal counts obtained from the Ar ICP using the present and rapidly, compared with the addition of Ar to the outer equipment were found to be less than those for conventional flow.19,20 use of an Ar ICP,22 because a 0.5 mm gap between the outer A bright discharge was clearly observed around the top of and intermediate tubes was adopted in the modified torch, and the Cu sampler, which seemed to be caused by a secondary a Cu sampler and skimmer with a thick top were used to discharge. In the MS spectra ( later shown in Fig. 4), 63Cu+ prevent damage caused by the secondary discharge. and 65Cu+ were strongly observed, and these ion signals could not be decreased in the present mass spectrometer without a Gases shielding system and with a load coil grounded at one end. The air used in this work was provided by a compressor with a maximum generation of 2.9 m3 min-1 (Hitachi HISCREW22 Distribution of Analyte Ions Along the Plasma Axis OSP-22E5AR II), and passed three times through an ordinary The spatial distribution of analyte ion signals along the plasma air filter and once through a charcoal filter (Nippon Pall axis should be discussed when optimizing sampling depth. The Filter), and then distributed at a pressure of 7.5 kgf cm-2 to optimized sampling position for the analyte signal was found each laboratory in Kanagawa IRI.The Ar gas used was of to be 6–7 mm above the load coil in N219 and O220 ICPs, and 99.99% purity (Nippon Sanso). 12–14 for the Ar ICP.22 The ion signal clearly decreased in the outer part of the plasma in the N219 and O220 ICPs, and Reagents might depend on cooling by the molecular gas. The results for Co+, Y+, Cd+ and Pb+ obtained using the Stock solutions (10 mg l-1) of the analytes and the matrix for air–Ar ICP are indicated in Fig. 1. The plasma discharge the discussion of co-existing Li, Co and Tl were prepared from became unstable in sampling positions greater than 15 mm commercially available 1000 mg l-1 solutions for AAS (Wako above the load coil. Each element has its own distribution, Pure Chemical or Kanto Chemicals). Working solutions were and the ion signals do not clearly decrease in the outer part diluted from the stocks using pure water from a Milli-Q system of the plasma.Maximum signals were found in a region of (Millipore) in a matrix of 3% ultrapure nitric acid from 9 to 12 mm. The maximum signal position and the (Tamapure-100, Tamakagaku Kogyo). Ultrapure water distribution obtained from the air–Ar ICP seem to be similar (Tamakagaku Kogyo), where the concentration of trace to those of the Ar ICP.22 A sampling position of 10 mm was impurities was less than 10 ng l-1 was used for the measureused subsequently. ments of mass spectra profiles and actual background ion counts.Eect of Rf Power on the Analyte Ion Signal RESULTS AND DISCUSSION An rf power of 2.5 kW was required in the N2 ICP,19 and the plasma discharge was maintained in a range of from 1.8 to Air–Ar Plasma Discharge 2.5 kW in the O2 ICP.20 In the air–Ar ICP, the plasma The Ar ICP was generated as a starter under the conditions discharge could be maintained at lower rf power, but became given in Table 1. After optimization of the ion lens parameters unstable below 1.8 kW.The eects of rf power on the analyte in the mass spectrometer, which were found to be almost ion signals are indicated in Fig. 2. The ion signals of almost equivalent to those for the proposed air–Ar ICP, the sampling all elements gradually increase with an increase in rf power. depth was set at 10 mm above the load coil. Before replacement with air, the outer Ar flow rate was increased to 20 l min-1 and then the rf power to 2.0 kW. The air was introduced to the outer flow with a reduction in the reflected power until the air flow rate reached 20 l min-1, and the Ar flow rate was thus decreased to zero. Shrinkage of the plasma was observed when the air was introduced to the outer flow, and the outer edge of the shrunken plasma was brilliant, which seemed to depend on the Ar in the intermediate or aerosol carrier flow.When the aerosol carrier Ar flow was replaced by air the plasma with the outer flow of air expanded again and the outer edge emission disappeared.The emission from the proposed air–Ar ICP, operated under the conditions shown in Table 1, was observed to be less brilliant than the Ar ICP. An entirely air plasma operated at 2.75 kW rf power with a 40.68 MHz generator was used as an emission source for an air pollution study,26 and a method for the approximation of the electron temperature was applied to the entirely air ICP at 2.2 kW using a 64 MHz generator.27 However, it was very Fig. 1 Distributions of the relative analyte ion signals along the axis: rf power, 2 kW; air carrier flow rate, 0.9 l min-1. dicult to replace the intermediate Ar with the air at low rf 914 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12of 0.9 l min-1 was used for the carrier gas flow in the air–Ar ICP. Air–Argon ICP Mass Spectra Mass spectra from the proposed air–Ar ICP, obtained under the optimized analytical conditions, are shown in Fig. 4 over the m/z range from 5 to 85.Fig. 4(a) and (b) was obtained by nebulization of ultrapure water with low sensitivity, where analyte ions were loosely focused on the ion detector with a low applied voltage. Signals for N+, O+ and NO+ were clearly observed, and the profile basically seems to be similar to that of an N2 ICP.19 The signal counts for O+, some of which might be caused by the introduced water, were larger than an N2 ICP. The signal counts for N2+ and O2+, clearly observed in Fig. 4(a), were 1.3 and 1.4% N+ and O+, respectively. These Fig. 2 Eect of the rf power on the relative ion signals: air carrier signal ratios of the polyatomic ion to the atomic ion are larger flow rate, 0.9 l min-1; sampling depth, 10 mm above the load coil. than in the N2 (0.25%)19 and O2 (0.19%)20 ICPs, and seem to indicate that the concentrations of N2 and O2 in the air–Ar ICP might be higher than in the N2 and O2 ICPs. The higher However, the top of the sampler was badly damaged by the concentration of molecules could depend on the lower rf power secondary discharge at higher power.The signal ratios of and also on the Ar in the intermediate gas. The Ar+ signal, as doubly charged ions to singly charged ions slightly increased shown in Fig. 4(b), is weakly observed, because the first IP of with increasing rf power, but the ratios of monoxide ions to Ar is relatively high (15.76 eV). The actual concentration of singly charged ions were kept almost constant.An rf power of Ar could be higher. 2.0 kW was used in the latter experiment, since the ion signals Fig. 4(c) was also obtained from nebulization of ultrapure obtained were sucient to detect trace amounts of analyte. water with relatively high sensitivity, where analyte ions were tightly focused on the ion detector with a high applied voltage. Eect of the Aerosol Carrier Gas Flow Rate on the Analyte Ion The detector was masked in the m/z range 14–18, 29–31, 40 Signal and 41 in order to avoid damaging the detector.Several types of polyatomic ions of the component elements of air, e.g., The aerosol carrier (central ) gas flow rate is one of the most N2O+ and NO2+, were observed. The sampler might be more significant factors aecting the analytical characteristics of an damaged by the secondary discharge than in an N2 ICP19, ICP. A higher carrier gas flow rate gives a higher eciency since CuN+ and CuO+ were observed with the addition of for the introduction of the sample solution into the plasma, strong Cu+ signals.but sometimes causes chemical and ionization interferences. 28,29 Further, the analyte cannot acquire sucient energy for excitation or ionization at higher flow rates. The Sensitivity and Detection Limit maximum signals were found at 0.5–1.0 l min-1 in both Ar The sensitivities and DL of the air–Ar ICP were compared ICP-AES and ICP-MS, but higher carrier flow rates of 1.6 with the Ar ICP. The signals in both ICPs were measured and 1.4 l min-1 were adopted in N219 ICP and O220 ICP-MS, under the operating condition shown in Table 1, and using the respectively, in order to extend the hot region available for the same nebulizer, quartz torch, sampler, skimmer and mass ion source.The results obtained are shown in Fig. 3. Maximum signals were found at 0.9 l min-1 for Cd+, Co+ and Y+, and at 1.2 l min-1 for Pb+. These flow rates are a little higher than in an Ar-ICP,22 but lower than in N219 and O220 ICPs.The distributions are similar to those of an Ar ICP. The small hot region available for the ion source assumed in N2 and O2 ICPs does not seem to be distinct in the air–Ar ICP. A rate Fig. 4 Mass spectra of the air-Ar ICP under optimized conditions: (a) and (b) obtained under low sensitivity conditions; (c) obtained under Fig. 3 Eect of the air aerosol carrier flow rate on the relative ion high sensitivity conditions with masked detector in the m/z ranges 14–18, 29–31, and 40–41.signals: rf power, 2 kW; sampling depth, 10 mm above the load coil. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 915spectrometer. The sensitivity was obtained as counts per second measurements for 56Fe+, 75As+ and 80Se+ could be easily carried out, because of the lower concentration of Ar in the per ppb in each ICP. The Ar ICP sensitivities were almost at the same level as previous work with N219 and O220 ICP-MS air–Ar ICP.On the other hand, signal detection for 77Se+ was dicult because of the formation of 77CuN+, as shown in arrangements. Ultrapure water was used for the measurement of the actual background signal for each element. The DL was Fig. 4(c). The DLs calculated for 30 elements were found to be approximately 1–2 orders of magnitude inferior to those in calculated as the concentration corresponding to a signal-tobackground noise ratio of three. Calculated DLs in the air–Ar the Ar ICP.ICP are listed in Table 2. The ratios of the sensitivity, background signal and calculated DL obtained from both ICPs Kinetic Energy Distribution of the Analyte Ions are also summarized in Table 2, together with first and second IP values30 and also oxide dissociation energy values.31 Ion kinetic energy measurements yielded interesting information about the ICP-MS characteristics.32,33 The ion kinetic The sensitivity seems to depend on the first IP,30 and to be higher than in the Ar ICP for elements with a low IP, energy can be discussed by use of the retarding dc quadrupole rod bias, since the ion lens system in the present mass particularly for Sr, Y, In, Ba and rare earth elements.However, the sensitivity was found to be lower for elements with a high spectrometer does not act as an energy filter. The normalized ion counts for Be+, Co+, Y+, Cd+ and Pb+ versus the IP, especially Pt, Au and Hg. These results are typical of molecular gas ICPs.19,20 Furthermore, the sensitivity seems to retarding voltage in the air–Ar ICP are shown in Fig. 5, together with that for Pb+ in the Ar ICP (broken line) for decrease slightly with increasing m/z values, which was observed in an O2 ICP with the same equipment.20 Although comparison. The energy distributions of the other elements in the Ar ICP were almost equivalent to that of Pb+. the oxide dissociation energy is high, the sensitivity for an element with a low first IP was found to be high, as shown The average energy values, obtained at a relative signal of 50 for five elements, were found to be 18–28 eV in the air–Ar for Ce+ and La+.The background signals were found to be higher in the ICP and 24 eV for Pb+ in the Ar ICP. These higher values in both ICPs seem to depend on the secondary discharge, since air–Ar ICP than the Ar ICP, but the RSD values were calculated to be in the range 0.5–2%, which is almost equal an average of 6 eV was found in the Ar ICP with a shielding system.22 Even if the average kinetic energy values are almost to those of the Ar ICP.One of the reasons might be the increase in pressure in the ion lens chamber, when the Ar is at the same level in both ICPs, the width of the energy distribution for each ion, calculated as the dierence between replaced with air. The increase in the background signal does not seem to depend on contamination of the air used, since the two retarding voltage values at relative ion signals of 100 and 0, was found to be approximately 1.5 times larger in the the background signal was found to be almost at same level when commercially available purified air (Nippon Sanso) was air–Ar ICP than in the Ar ICP, as shown in Fig. 5. In addition, each ion distribution curve shifts to a higher voltage with used instead of the original air used in this study. Ion signal Table2 Comparison of the sensitivities, background and DLs for the air–Ar and Ar ICPs DL Oxide† Sensitivity‡ Background Abundance First* Second* dis.en./ ratio signal ratio Air–Ar ICP Ratio Element m/z (%) IP/eV IP/eV eV Air–Ar:Ar Air–Ar:Ar (ppt)§ Air–Ar:Ar Be 9 100 9.32 18.21 4.6 0.40 170 20 290 Ti 48 73.8 6.82 13.57 7.2 1.1 11 60 10 V 51 99.8 6.74 14.65 6.4 1.0 4.8 1 1.7 Cr 52 83.8 6.76 16.49 4.4 0.92 4.0 10 3.3 Mn 55 100 7.43 15.64 4.2 0.48 21 80 40 Fe 56 91.7 7.86 16.18 4.3 0.42 0.33 1000 0.50 57 5.8 0.45 0.14 70 0.78 Ni 58 67.8 7.63 18.15 <4.2 0.21 0.53 10 1.3 Co 59 100 7.86 17.05 – 0.29 130 20 220 Zn 64 27.9 9.39 17.96 2.8 0.15 3.1 100 20 Ga 69 60.1 6.00 20.51 3.0 1.1 20 1 10 As 75 100 9.81 18.63 4.9 0.092 13 40 40 Se 80 49.6 9.75 21.5 4.3 0.045 0.99 700 1.0 Sr 88 87.9 5.69 11.03 4.2 3.0 55 0.6 6.0 Y 89 100 6.38 12.33 7.31 2.0 400 4 50 Zr 90 51.4 6.84 13.13 7.8 0.85 10 2 5 Mo 98 24.1 7.10 16.15 5.0 0.35 150 50 100 Pd 106 27.3 8.33 19.42 – 0.17 61 20 200 Ag 107 51.8 7.57 21.48 2 0.25 140 20 100 Cd 114 28.7 8.99 16.90 <3.8 0.097 12 20 40 In 115 95.7 5.79 18.66 3.3 1.7 39 0.9 9.0 Ba 138 71.7 5.79 10.00 5.75 3.3 60 2 10 La 139 99.9 5.58 11.43 8.2 2.2 6.8 0.4 2.0 Ce 140 88.5 5.47 10.00 8.03 1.7 39 0.6 6.0 Pt 195 33.8 9.0 18.56 – 0.015 10 100 250 Au 197 100 9.22 20.5 – 0.0038 5.5 400 700 Hg 200 23.1 10.43 18.75 – 0.010 1.1 300 100 Pb 208 52.4 7.42 15.03 3.87 0.22 1.9 10 10 Bi 209 100 7.29 16.68 3.1 0.16 23 5 71 Th 232 100 6.95 – 8.5 0.13 15 3 60 U 238 99.3 6.1 – 7.8 0.081 5.9 4 40 * IP, ionization potential from ref. 30. † Oxide dis. en., oxide dissociation energy from ref. 31. ‡ Sensitivity was obtained as counts per second per ppb. § Ppt: parts per 1012. 916 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Fig. 7 Signal ratio distributions of monoxide ions to singly charged Fig. 5 Distributions of the analytical ion kinetic energy in the air–Ar ions: continuous line, air–Ar ICP; broken line, Ar ICP. and Ar ICPs: continuous line, air–Ar ICP; broken line, Ar ICP.Signal Ratio Distribution of Monoxide Ions increasing m/z values, which was observed in the Ar ICP with The distributions of the signal ratios of monoxide ions to the shielding system,22 but not clearly observed in the Ar, N2 singly charged ions are shown in Fig. 7. The ratio seems to and O2 ICPs without shielding.19,20 depend on the oxide dissociation energy value,31 also listed in Table 2, in the air–Ar and Ar ICPs. The ratio increases with Signal Ratio Distribution of Doubly Charged Ions decreasing sampling position above the load coil in the Ar ICP, which could depend on the eect of the introduced water.The concentrations of doubly charged and monoxide ions are However, the ratios for three elements remain constant along also interesting with respect to analytical characteristics, in the plasma axis in the air–Ar ICP. The ratios are almost at spite of the interferences encountered in ultratrace element the same level in both ICPs for Ce, which has a higher oxide determinations by ICP-MS.The distributions of the signal dissociation energy. The ratio for Pb, with a lower dissociation ratio of doubly charged ion to singly charged ion for Ba, Y energy, is much higher in the air–Ar ICP than the Ar ICP, as and Pb in the air–Ar and Ar ICPs are shown in Fig. 6. The shown in Fig. 7. values of the second IPs30 are also listed in Table 2. The ratio in the Ar ICP clearly increases with increasing sampling depth above the load coil, however, it seems to be relatively constant Matrix Eects along the axis of the plasma in the air–Ar ICP.The ratio for Matrix eects on the analyte ion signals were investigated with the three elements clearly depends on the second IP values in co-existing Li (m/z=9, first IP=5.39 eV), Ag (listed in Table 2) the Ar ICP, but the ratio for Pb (second IP=15.03 eV) seems and Tl (200, 6.11 eV). Results obtained for Co+ and Pb+ as to be almost the same as that of Y (12.23 eV) in the air–Ar ICP.the analytes are shown in Fig. 8. As a 10 mg l-1 solution of Co The ratio for Ba and Y, whose second IP values are relatively and Pb was used, the matrix concentration was 1000–100 000 low, is lower in the air–Ar ICP than in the Ar ICP. In contrast, times that of the analyte. The analyte signal decreases with an the ratio is higher in the air–Ar ICP for Pb, which has a increase in the matrix concentration. As shown in Fig. 8, relatively higher second IP value.These phenomena were also greater signal suppression with co-existing Li was found, observed in N219 and O220 ICPs. As shown in Fig. 5, the compared with Ag or Tl. The signal for Pb+ was suppressed kinetic energy distribution in the air–Ar ICP is wider than in more than Co+ with co-existing Li, Ag and Tl. the Ar ICP. A high ratio of doubly to singly charged ions A decrease in the sample introduction rate to the plasma, could be linked to a high concentration of higher energy ion caused by an increase in the matrix concentration, will prob- species.Similar results were observed for Mo which also has ably not aect the signal suppression significantly, since the a higher second IP (16.15 eV). emission intensity obtained was reduced less when the same Fig. 6 Signal ratio distributions of doubly charged ion to singly Fig. 8 Eects of the co-existing elements, Li, Ag and Tl, on the ion signals of Co+ and Pb+; elements in parentheses, co-existing.charged ion: continuous line, air-Ar ICP; broken line, Ar ICP. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 9177 Lam, J. W., and McLaren, J. W., J. Anal. At. Spectrom., 1990, introduction system was used in ICP-AES. Another inter- 5, 419. ference is the disturbance of the ion beam path through the 8 Beauchemin, D., and Craig, J. M., Spectrochim. Acta, Part B, ion optics and mass spectrometer, which is known as the space 1991, 46, 603. charge eect.It has been reported that this eect is readily 9 Craig, J. M., and Beauchemin, D., J. Anal. At. Spectrom., 1992, observed in the determination of light elements or in the 7, 937. 10 Xiao, G., and Beauchemin, D., J. Anal. At. Spectrom., 1994, 7, 509. presence of co-existing heavy elements.34 Suppression with 11 Lam, J. W., and Horlick, G., Spectrochim. Acta, Part B, 1990, co-existing Ag and Tl seems to be almost at the same level as 45, 1313. Li or less, if mass concentration is converted into atom 12 Hill, S.J., Ford, M. J., and Ebdon, L., J. Anal. At. Spectrom., concentration. Mass concentrations of 800 mg l-1 of Li, Ag 1992, 7, 719. and Tl correspond to atom concentrations of 6.9×1019, 13 Wang, J., Evans, E. H., and Caruso, J. A., J. Anal. At. Spectrom., 4.5×1018 and 2.4×1018 l-1, respectively. As the atom concen- 1992, 7, 929. 14 Louie, H., and Soo, S. Y.-P., J. Anal. At. Spectrom., 1992, 7, 557. tration of Co is also calculated to be 3.5 times that of Pb, the 15 Lam, J.W., and Horlick, G., Spectrochim. Acta, Part B, 1990, space charge eect should be more severely observed for Pb+ 45, 1327. in the presence of Li, Ag and Tl, as shown in Fig. 8. Ionization 16 Brenner, I. B., Zander, A., Kim, S., and Shkolnik, J., J. Anal. At. interference with co-existing Li seems to contribute less to the Spectrom., 1996, 11, 91. signal suppression, since the carrier flow rate of the air is 17 Montaser, A., and Gilightly, D.W., Inductively Coupled Plasmas in Analytical Atomic Spectrometry, VCH, New York, 1987. moderate in the air ICP.28,29 18 Tanaka, T., Yonemura, K., Obara, K., and Kawaguchi, H., Anal. Sci., 1993, 9, 765. 19 Uchida, H., and Ito, T., J. Anal. At. Spectrom., 1995, 10, 843. CONCLUSIONS 20 Uchida, H., and Ito, T., Anal. Sci., 1997, 13, 391. 21 Yang, P., Barnes, R. M., Vechiarelli, J., and Uden, P. C., Appl. An economical air–Ar mixed ICP (1.5 l min-1 of Ar in the Spectrosc., 1990, 44, 531.intermediate gas flow) was achieved at the relatively low rf 22 Uchida, H., and Ito, T., J. Anal. At. Spectrom., 1994, 9, 1001. power of 2 kw using a 40 MHz generator and a modified 23 Gray, A. L., J. Anal. At. Spectrom., 1986, 1, 247. torch. Analytical sensitivity depends on the IP value of the 24 Nonose, N. S., Matsuda, N., Fudakawa, N., and Kubota, M., analyte, and the DLs calculated were approximately 1–2 orders Spectrochim. Acta, Part B, 1994, 49, 955. 25 Sakata, K., and Kawabata, K., Spectrochim. Acta, Part B, 1994, of magnitude inferior to those of an Ar ICP.The analytical 49, 1027. characteristics are basically the same as those of molecular gas 26 Baldwin, D. P., Zamzow, D. S., and D’Silva, A. P., J. Air Waste ICPs, however, some characteristics were as for an Ar ICP, Manage. Assoc., 1995, 45, 789. i.e., the stable plasma discharge, analyte ion signal distribution, 27 Gomes, A.-M., Sarrette, J. P., Madon, L., and Epifanie, A., J. Anal. the eect of carrier gas flow rate and the mass spectra. The At. Spectrom., 1995, 10, 923. 28 Kalnicky, D. J., Fassel, V. A., and Kniseley, R. W., Appl. proposed plasma should be termed an air–Ar mixed ICP as Spectrosc., 1977, 31, 137. regards its analytical characteristics. A study of the application 29 Kosinsky, M. A., Uchida, H., and Winefordner, J. D., Anal. of an air–Ar ICP to practical analyses is in progress. Chem., 1983, 55, 688. 30 Moore, C. E., Atomic Energy L evels, Circular No. 467, National Bureau of Standards, Washington DC, 1949. REFERENCES 31 Gaydon, A. G., Dissociation Energy and Spectra of Diatomic Molecules, Chapman and Hall, London, 1968. 1 Date, A. R., and Gray, A. L., Application of Inductively Coupled 32 Olivares, J. A., and Houk, R. S., Appl. Spectrosc., 1985, 39, 1070. Plasma Mass Spectrometry, Blackie, Glasgow, 1989. 33 Gray, A. L., and Williams, J. G., J. Anal. At. Spectrom., 1987, 2, 599. 2 Tan, S. H., and Horlick, G., Appl. Spectrosc., 1986, 40, 445. 34 Tan, S. H., and Horlick, G., J. Anal. At. Spectrom., 1987, 2, 745. 3 Vaughan, M. A., and Horlick, G., Appl. Spectrosc., 1986, 40, 434. 4 Evans, E. H., and Ebdon, L., J. Anal. At. Spectrom., 1989, 4, 299. Paper 7/01269A 5 Evans, E. H., and Ebdon, L., J. Anal. At. Spectrom., 1990, 5, 425. Received February 24, 1997 6 Laborda, F., Baxter, M. J., Crews, H. M., and Dennis, J., J. Anal. At. Spectrom., 1994, 9, 727. Accepted June 10, 1997 918 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a701269a

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Measurements of 44Ca543Ca and 42Ca543Ca Isotopic Ratios in Urine Using High Resolution Inductively Coupled Plasma Mass Spectrometry† STEFAN STU� RUP*ab , MARIANNE HANSENc AND CHRISTIAN MØLGAARDc aPlant Biology and Biogeochemistry Department, Risø National L aboratory, P.O. Box 49, DK-4000 Roskilde, Denmark bDepartment of Chemistry, T he T echnical University of Denmark, Building 207, DK-2800 Lyngby, Denmark cResearch Department of Human Nutrition, T he Royal Veterinary & Agricultural University, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark A method is described for the measurement of calcium isotopic the same nominal masses as the analyte ions are often seen using FABMS.Many of those interferences can be resolved ratios (42Ca+543Ca+ and 44Ca+543Ca+) in human urine using high resolution inductively coupled plasma mass spectrometry with a higher resolution setting. Increasing the resolution reduces the sensitivity owing to a lower ion transmission (HR-ICP-MS).Relative standard deviations (RSD) of 0.33 and 0.41% were found for 44Ca+543Ca+ and 42Ca+543Ca+, through the instrument. For the measurement of calcium isotopic ratios in plasma and urine by FABMS, a resolution respectively. Using a mass spectrometric resolution setting of 4000, the calcium peaks were resolved from interfering setting of 5000 has been used to separate the calcium ions from other interfering ions.12 Less frequently, secondary ion polyatomic ions (40Ar2H+ and 40ArH2+ on 42Ca+ and 28Si16O+ and 12C16O2+ on 44Ca+).Interferences from Sr2+, mass spectrometry (SIMS),13 resonant ionization mass spectroscopy (RIMS)14 and laser desorption time-of-flight mass which cannot be resolved even using high resolution, were corrected for mathematically using the 43.5Sr2+ peak. The spectrometry15 have been used for the measurement of calcium isotopic ratios. The use of quadrupole inductively coupled sample preparation step consisted of a simple 50-fold dilution of the urine sample with 0.14 M HNO3, which ensured a plasma mass spectrometry (Q-ICP-MS) for measurement of calcium isotopic ratios is very limited due to the fact that the relatively high sample throughput of 6 samples per hour. signals from the calcium isotopes are severely interfered with Keywords: Calcium isotopic ratios; urine; high resolution by polyatomic ions and doubly charged ions.The 44Ca+543Ca+ inductively coupled plasma mass spectrometry; polyatomic ions; and the 48Ca+543Ca+ ratios have, however, been successfully doubly charged ions measured in human urine using Q-ICP-MS with RSD below 1% following a preconcentration step in which calcium was precipitated as oxalate.16,17 In high resolution inductively Calcium is the fifth most abundant element in the earth’s crust, the calcium concentration is approximately 3%.1 Calcium coupled plasma mass spectrometry (HR-ICP-MS), a doublefocusing magnetic sector mass spectrometer is used instead of is essential to humans and a daily intake of 800 mg is recommended for adults in order to maintain calcium homeo- the quadrupole filter of ICP-MS.The mass spectrometric resolution of the HR-ICP-MS can be varied up to a maximum stasis.2 Calcium is, however, one of the nutrients for which requirements are least agreed on. The ability of the human of 10000. The resolution is defined as m/Dm; Dm being the mass dierence between two ions of an average mass, m, which body to adapt to lower calcium intakes3 and, in turn, the influence of various dietary factors on calcium availability4 is give rise to equally intense peaks which overlap at 10% of the maximum peak height.A high resolution setting can sub- currently being debated. Radioisotope methods have been used successfully to determine calcium absorption in humans,5 sequently be used to separate analyte peaks from otherwise overlapping polyatomic interferences, e.g., 42Ca+ can be however, this technique is less suitable for children and pregnant/ lactating women as radioisotopes are a potential hazard separated from 40ArH2+ using a resolution setting of 2300.HR-ICP-MS has previously been used for the measurement of because of internal radiation exposure. By a double stable isotope method calcium absorption can be estimated from the the 25Mg+526Mg+ and 206Pb+5207Pb+ ratios with RSD below 0.1% using a resolution setting of 300.18 This is similar to or ratio of the isotopes in urine.6 A number of mass spectrometric techniques are potentially useful for the measurement of stable even better than RSD obtained by TIMS.With a resolution setting of 3000, 63Cu+ and 65Cu+ peaks were separated from calcium isotopes. Thermal ionization mass spectrometry (TIMS) is the technique that oers the best precision [relative interfering polyatomic ions and the 63Cu+565Cu+ ratio was measured with RSD of 0.3–0.6% in human serum and in standard deviations (RSD) in the range of 0.2–0.5%]7 for the measurement of calcium isotopic ratios and is the most com- Antarctic sediments.19 This paper presents the development of an HR-ICP-MS method for the measurement of calcium monly used technique.8–11 One disadvantage of the TIMS technique is that the analyte has to be separated from the isotopic ratios in human urine.The impact of signal suppression from sample matrix and interferences from polyatomic sample matrix.The sample preparation is therefore often very tedious and time-consuming. Another technique frequently and doubly charged ions on the method performance are also discussed. Throughout this paper the term precision means used for the measurement of calcium isotopic ratios is fast atom bombardment mass spectrometry (FABMS).6,11,12 The relative standard deviation (RSD) unless otherwise stated. precision achieved by this technique is poorer than for TIMS, but the technique has the advantages that liquid samples, like EXPERIMENTAL urine and serum, can be applied directly without prior matrix Instrumentation separation.7 Interference from other ions (often hydrides) with All measurements were performed on a PlasmaTrace2 High Resolution Inductively Coupled Plasma Mass Spectrometer † Presented at the 1997 European Winter Conference on Plasma Spectrochemistry, Gent, Belgium, January 12–17, 1997.(Micromass Ltd, Manchester, England) in operation at Risø Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 (919–923) 919National Laboratory, Denmark.This instrument was equipped investigated. The aim was to use the lowest possible dilution factor in order to obtain the largest possible calcium signal with a double-focusing magnetic sector mass spectrometer of reverse Nier–Johnson geometry. The resolution can be varied and thereby also the best theoretical precision. Using dilution factors higher than 200, the experimental precision was limited between 400 and 10000.Throughout this study, a resolution of 4000 was used. At this resolution setting, the ion transmission by the theoretical precision. Using dilution factors lower than 50, particulate matter was deposited inside the injector tube is approximately 20% of that at low resolution (resolution setting 400). All measurements were made with the standard after analysis of only a few samples resulting in decreasing sensitivity and an eventual extinction of the plasma. A dilution configuration of the Plasmatrace2, that is, with a Minipuls 2 Gilson peristaltic pump (Gilson, Villers, France), a concentric factor of 50 was chosen.Before a measurement sequence was started, the instrument was carefully mass calibrated using a nebulizer (AR35–1-F04, Glass Expansion Pty., Australia) and a Scott-type spray chamber maintained at 5 °C. The instrument 2 mg l-1 Ca standard solution. For every six human urine samples, a 2 mg l-1 Ca standard solution was analysed.The settings are summarised in Table 1. analysis time was 9 min per sample solution leading to a sample throughput of approximately six samples per hour. Standard Solutions and Reagents The isotopic ratios measured in the standard solutions were used to correct for instrumental mass bias and to check for All samples and standard solutions were prepared by dilution instrumeal drift. The counted ions under the whole peak with 0.14 M nitric acid.The nitric acid was prepared from 65% were integrated and used for the further calculations. The nitric acid, pro analysi, (Merck, Darmstadt, Germany) further integrated data were transferred to a spread sheet programme purified by sub-boiling (Berghof BSB-939-IR, Germany) and all calculations including correction for detector dead diluted with ultrapure water (>18.2 MV cm) from an Elgastat time, correction for instrumental mass bias and overlap from Maxima Analytical System (Elga, Blocks, England).Calcium doubly charged ions were performed in the spreadsheet. standard solutions were prepared from a 1000 mg l-1 commercially available standard (Merck, Darmstadt, Germany) by dilution with 0.14 M nitric acid. A 2 mg l-1 Ca standard Dead Time Correction solution was used for instrument optimisation and calibration. The 42Ca+543Ca+ and 44Ca+543Ca+ isotopic ratios decreased with increasing calcium concentration. This error was caused Samples by dead time in the ion counting circuitry, which showed a relatively lower sensitivity of the most abundant isotopes with Human urine samples with elevated concentrations of 42Ca increasing calcium concentration. All isotope ratio measure- and 44Ca were sampled following a double-label stable isotopes ments were corrected for detector dead time.The detector experiment (a double-isotope procedure for the measurement dead time was calculated by the following equation21 using Ca of calcium absorption20) at the Research Department of standard solutions of 0.5, 1, 2, 3, 4 and 5 mg l-1: Human Nutrition, at The Royal Veterinary & Agricultural University, Copenhagen, Denmark.All collected urine samples Rc=Rm/(1-Rmt) (49 ml ) were acidified with 1 ml of HNO3 (65%) after sampling where Rm and Rc are the measured and corrected counts in and kept frozen (-18 °C) until analysis. the integrated analyte peak, and t is the detector dead time(s).A detector dead time of #10 ns was found, which is somewhat Sample Preparation, Measurements and Calculations lower than values reported for Q-ICP-MS instruments.21 The sample preparation step was very simple. The acidified human urine was diluted 50 times with 0.14 M nitric acid and Mass Bias thereafter aspirated directly into the HR-ICP-MS instrument. The instrument mass bias was calculated from the following The use of dierent dilution factors (between 10 and 400) was equation:22 (A/B)m=(A/B)t(1+a)n Table 1 Instrumental operating conditions and signal measurement parameters where (A/B)m is the measured isotopic ratio, (A/B)t is the true ratio, n is the mass dierence between the two isotopes and a Rf power 1350 W is the bias per mass unit.The instrument mass bias (%) was Plasma gas flow 14 l min-1 Auxiliary gas flow 1.0 l min-1 found to vary between 1–2 u-1. Bias values of #0.3% u-1 Nebulizer gas flow 0.95 l min-1 (optimised daily) are more typical, but higher values are often found for the Sample uptake rate 0.6 ml min-1 lighter elements due to mass discrimination.22 Ion sampling depth Optimised daily for max.intensity Ion lens settings Optimised for max. intensity and optimum resolution RESULTS AND DISCUSSION Sampler/skimmer cone nickel Polyatomic Ions Resolution 4000 Sweeps 80 All the calcium isotopes are overlapped by polyatomic ions at Scans 1 low resolution. Since the 40Ca+ peak can not be separated Peak widths 3 Points per width 30 from the 40Ar+ peak even at high resolution (a resolution of Dwell times/ms— approximately 190 000 is needed), it cannot be measured by 42Ca+ 4 argon plasma ICP-MS.The 46Ca and 48Ca isotopes have a 43Ca+ 8 low natural abundance (0.004 and 0.187%, respectively) and 44Ca+ 2 the signals are isobaric overlapped by titanium isotopes. 43.5Sr++ 5 Consequently the 42Ca, 43Ca and 44Ca isotopes were chosen Magnet rest mass 41 u Magnet scan region 41–45 u for this study.These isotopes are also interfered with by Hysteresis settle times 10000 ms polyatomic ions, but can be resolved using a resolution setting Large jump settle time 100000 ms >2700. Table 2 shows the polyatomic ions seen when analysing Small jump settle time 100 ms human urine as well as the resolution needed to resolve them Sampling time 9 min per sample from the analyte peaks. According to Table 2, the calcium 920 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Table 2 Polyatomic interferences found in human urine conditions.23 For the PlasmaTrace2 HR-ICP-MS instrument run under standard conditions we found formation rates well Natural Polyatomic above 1%. The formation rates for barium, strontium and Isotope abundance (%) interference Resolution* cerium, measured in standard solutions, are shown in Table 3. 42Ca+ 0.647 40Ar2H+ 2350 Even by very careful optimisation of the argon gas flows, it 40Ar1H2+ 2165 was dicult to obtain formation rates lower than 3% for 43Ca+ 0.135 — — strontium, for which the doubly charged ions interfere with 44Ca+ 2.086 12C16O2+ 1281 the analysis of calcium isotopes.Under these conditions the 28Si16O+ 2688 calcium signals were reduced by approximately 30%, so these * The resolution, calculated as m/Dm, needed to separate the poly- gas flow settings were not used for the measurement of calcium atomic ion from the analyte signal. isotopic ratios. The reason for this higher formation rate is unknown, but it might be due to the presence of a secondary signals can be resolved from the polyatomic interferences using discharge behind the skimmer cone.a resolution setting of 2700. However, since baseline separation Strontium has four isotopes at 84, 86, 87 and 88 u and is required in order to achieve the best possible precision, a doubly charged strontium ions interfere with the 42Ca+, 43Ca+ resolution setting of 4000 was used. The mass spectra for the and 44Ca+ signals in urine samples.The strontium concencalcium isotopes in urine are shown in Fig. 1. The most severe tration in human urine is approximately 0.2 mg l-1.24 The polyatomic interferences are 40Ar1H2+ and 40Ar2H+ on 42Ca+. peaks from doubly charged strontium ions cannot be resolved It can be seen from Fig. 1 that the signal from these inter- from the calcium peaks even at high resolution (resolution= ferences give rise to a signal equal to that of approximately 10000) and must therefore be corrected for mathematically. 1.5 mg l-1 calcium in urine.Since the 2H to 1H ratio in nature Like strontium, rubidium also has an isotope at 87 u, but since is approximately 156500 it might be suggested that the inter- the second IP of rubidium is very high (27.5 eV) it does not ference from 40Ar2H+ is insignificant. The actual ratio between form doubly charged ions in the ICP. As seen in Table 3 we the two interferences cannot be measured, since they are very did not observe any doubly charged rubidium ions when close in mass and cannot be separated even using a resolution analysing standard solutions.The signal from doubly charged setting of 10 000 (a resolution setting of approximately 25 000 strontium at m/z=43.5 can then be used directly to correct for is needed). But the 2H+ to 1H2+ ratio can be measured at overlaps from Sr2+ on 42Ca+, 43Ca+ and 44Ca+. m/z=2. This ratio was found to be approximately 151 in both standard solutions and urine samples, i.e., the formation rate Non-spectral Interferences of 1H2+ from 1H atoms/ions in the plasma and/or interface region must be low given that the 1H concentration is relatively HR-ICP-MS instruments are prone to interference from the high in the plasma.Suggesting that 2H+ and 1H2+ combine sample matrix in the same way as Q-ICP-MS instruments. with 40Ar+ at the same rate, the 40Ar1H2+ to 40Ar2H+ ratio When calcium isotopic ratios are measured in urine, matrix would be approximately 151.Following this argument it is interferences have to be considered since urine contain high likely that both polyatomic ions contribute significantly to the levels of the major elements, (especially sodium) in concencombined peak. trations of approximately 2200 mg l-1.24 The average total calcium concentration in the urine sampled in the present study was 135 mg l-1. In order to investigate the eect of a Doubly Charged Ions high sodium content on the calcium isotopic ratio measurements, these were measured in 3 mg l-1 standard solutions Elements with a low second ionization potential (IP) can form containing 0, 20, 40, 60, 80 and 100 mg l-1 of sodium.These doubly charged ions in the ICP ion source. In Q-ICP-MS, the concentrations cover the concentration range in which sodium formation rate is generally <1% under normal operation is found in the sample solutions, since the urine was diluted 50 times before analysis.The measured 44Ca+543Ca+ ratios and the intensity of the 44Ca+ signal are shown in Fig. 2. As expected, the calcium sensitivity decreased with increasing sodium concentration. In the presence of 100 mg l-1 of sodium the calcium signal was depressed by 20%. The 44Ca+543Ca+ ratio was not aected by the increasing sodium concentration. It can therefore be concluded that matrix interferences from major elements do not aect the measurement of calcium isotopic ratios.The only eect is that signal intensities are depressed, which in theory can degrade the precision of the measurements owing to poorer counting statistics. Similar results were found for the 42Ca+543Ca+ ratio. Precision of the Isotopic Measurements The precision of measurements of isotopic ratios by ICP-MS is limited by counting statistics (Poisson statistics) but other Table 3 The formation rate of doubly charged ions found under standard conditions of the PlasmaTrace2 in standard solutions Element Second IP/eV M2+5M+ (%) Ba 10.0 14.9 Ce 10.9 6.5 Sr 11.0 6.5 Fig. 1 Mass spectra of (a) 42Ca+; (b) 43Ca+; and (c) 44Ca+ in a urine Rb 27.5 0 sample containing approximately 2 mg l-1 of calcium. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 921Table 4 The long-term RSD of the 42Ca+543Ca+ and 44Ca+543Ca+ ratios in human urine measured over 8 days Days 42Ca+543Ca+ 44Ca+543Ca+ 1 4.7734 15.4598 1 4.7500 15.4889 2 4.8140 15.5268 2 4.7579 15.3960 3 4.7966 15.5383 3 4.7754 15.3975 3 4.7907 15.5054 4 4.7806 15.4824 4 4.7628 15.5184 5 4.8256 15.5486 5 4.7722 15.4495 6 4.7987 15.5469 6 4.8113 15.6925 7 4.7830 15.6515 Fig. 2 Eect of sodium on the isotopic ratio measurements. All 8 4.7779 15.5065 solutions contain 3 mg l-1 of calcium. The 44Ca+543Ca+ ratio is 8 4.8086 15.5112 shown as the mean±standard deviation (s), s=0.33%. The 44Ca+ intensity is set to 100% in the standard solution with no sodium added.Average 4.7862 15.5138 Standard deviation 0.0216 0.0775 Relative standard deviation (%) 0.45 0.50 sources of error, like plasma flicker noise, noise from the Expected ratio 4.7926 15.4518 peristaltic pump during sample uptake and changes in the nebulization, ionization and extraction processes also contribute to the overall method precision. Precisions of 2–3 times of that imposed by counting statistics are most often found using ICP-MS.25,26 The minimum error in the ion counting process calculations, e.g., the calculation of human calcium absorption rates in nutritional studies.with the Plasmatrace2 is, as mentioned above, set by Poisson statistics. The RSD of a measurement is 1/ÓN, where N is the A precision of 0.33% corresponds with that of <1% found for the measurement of the 44Ca+543Ca+ and the 48Ca+543Ca+ total number of ions observed. The theoretical RSD of an isotopic ratio is therefore Ó1/N1+1/N2, if the precision is ratios following a sample preparation step in which calcium was isolated from the sample matrix by precipitation as limited only by counting statistics, where N1 and N2 are the number of ions observed for isotopes 1 and 2, respectively. oxalate.16,17 Comparable precisions (0.2–0.5%) have been obtained by FABMS for the measurement of the 42Ca+540Ca+ When the two isotopes are not equally abundant, longer measuring time should be spent on the less abundant isotopes.and 44Ca+540Ca+.7 As in HR-ICP-MS analysis, the sample preparation in FABMS analysis is very simple and plasma and The optimum ratio of counting times are t1/t2=ÓA2/A1, where t1, A1 and t2, A2 are the dwell time and natural abundance for urine can be applied directly if the elemental concentration is suciently high in the samples.HR-ICP-MS and FABMS isotopes 1 and 2, respectively. This equation was used to optimise the dwell times of 42Ca+, 43Ca+ and 44Ca+. methods show similar figures of merit with regard to sample preparation, need of interference corrections and precision.The theoretical precision function (calculated from Poisson statistics) and the experimentally measured RSD values of the One distinct advantage of HR-ICP-MS is that this technique has a higher sensitivity than FABMS. TIMS, on the other 44Ca+543Ca+ ratio are shown in Fig. 3. The short-term RSD (10 replicates in one day) was found to be in the range of hand, shows better precision for the measurement of calcium isotopic ratios, often >0.2%.7 Yet, the TIMS technique 0.3–0.4% (0.33% on average) in urine samples. The RSD found is approximately 3 times that of the theoretical RSD, requires a full separation of the analyte from the sample matrix.This sample preparation is demanding and often very time- hence there is a considerable contribution from other sources than counting statistics to the RSD. A similar RSD of 0.41% consuming and consequently TIMS leads to a considerably lower sample throughput than HR-ICP-MS methods.No was found for the 42Ca+543Ca+ ratio. The long-term RSDs (16 replicates in 8 days) in human urine are approximately other reports on the measurement of calcium isotopic ratios by HR-ICP-MS have been published previously, but in a 0.45–0.50%, as shown in Table 4 where also the day to day results are shown. Short-term RSDs of 0.33 and 0.41% corre- similar study the 63Cu+565Cu+ ratio in human serum and Antarctic sediments has been measured with RSDs in the spond to standard deviations of the mean of 0.10% and 0.13% (s/Ón, n=10, where n is the number of replicates).Thus, the range of 0.3–0.6% using a resolution setting of 3000.19 That study and the present report indicate that the HR-ICP-MS overall precision of the method can be improved by replication of single measurements. The RSD will be improved by a factor technique provides results with a precision better than 0.6%, for the measurement of isotopic ratios of elements in the lower of 1/Ón.Even though the overall analysis time will increase, if all samples have to be measured in replicates or triplicates, it mass region (<80 u), where the analyte signals often are interfered with by polyatomic ions using Q-ICP-MS. might be useful if high precision data are used for further Fig. 3 Short-term precision for the 44Ca+543Ca+ ratio in human urine: (—) theoretical precision induced by counting statistics; (D) precision measured for 10 replicates in one day for a human urine sample. 922 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Table 5 Isotopic ratios measured in urine samples before and after enrichment with 42Ca and 44Ca: isotopic ratios are given as the measured value ±2 s, where s is the method standard deviation of 0.33% and 0.41% for the 44Ca+543Ca+ and 42Ca+543Ca+ ratio, respectively. A1 and A2 were given the low calcium isotope doses, B1 and B2 the high doses Volunteer 42Ca+543Ca+ (before) 42Ca+543Ca+ (after) 44Ca+543Ca+ (before) 44Ca+543Ca+ (after) A1 4.7966±0.0393 4.9837±0.0409 15.5383±0.1026 15.6955±0.1036 A2 4.7806±0.0392 4.9441±0.0405 15.4824±0.1022 15.8035±0.1043 B1 4.7722±0.0391 5.0915±0.0418 15.4495±0.1020 15.7845±0.1042 B2 4.7987±0.0393 5.0357±0.0413 15.5469±0.1026 15.7548±0.1040 Kastenmayer, P., Luten, J.B., and McGaw, B. A., Analyst, 1994, Analysis of Urine Samples Enriched in 44Ca and 42Ca 119, 2491. In the double-label stable isotopes experiment, volunteers were 8 Yergey, A.L., Vieira, N. E., and Hansen, J. W., Anal. Chem., 1980, 52, 1811. given 8.9 or 16.7 mg 44Ca orally with a milk-based meal 9 Yergey, A. L., Vieira, N. E., and Covell, D. G., Biomed. Environ. containing 400 mg calcium. Subsequently 1.7 or 3.4 mg 42Ca Mass Spectrom., 1987, 14, 603. was injected intravenously. Urine samples were collected before 10 Price, R. I., Kent, G. N., Rosman, K. J. B., Gutteridge, D. H., and after isotope administration. Table 5 shows the isotopic Reeve, J., Allen, J.P., Stuckey, B. G. A., Smith, M., Guelfi, G., ratios measured in the base and enriched urine samples from Hickling, C. J., and Blakeman, S. L., Biomed. Environ. Mass Spectrom., 1990, 19, 353. 4 of the volunteers [two that were given the low doses (A1 11 Sandstro�m, B., Fairweather-Tait, S. J., Hurrel, R., and Van and A2) and two that were given the high doses (B1 and B2)]. Dokkum, W., Nutr. Res.Rev., 1993, 3, 71. The 42Ca+543Ca+ ratios in the enriched urine samples were 12 Smith, D. L., Anal. Chem., 1983, 55, 2391. significantly larger than that of the base urine for volunteers 13 Roy, S., Gillen, G., Conway, W. S., Watada, A. E., and Wergin, given both the low and high calcium doses, whereas the W. P., Protoplasma, 1995, 189, 163. 14 Nicolussi, G. K., Pellin, M. J., Calaway, W. F., Lewis, R. S., 44Ca+543Ca+ ratio in the urine samples from volunteers given Davis, A.M., Amari, S., and Clayton R. N., Anal. Chem., 1997, the low 44Ca doses are not significantly larger than that of the 69, 1140. base urine. The conclusion is therefore that in future experi- 15 Koumenis, I. L., Vestal, M. L., Yergey, A. L., Abrams, S., Deming, ments volunteers should be given the low 42Ca dose and the S. N., and Hutchens, T. W., Anal. Chem., 1995, 67, 4557. high 44Ca dose. Alternatively, all measurements should be 16 Luten, J. B., Muys, T., and Dokkum, W., Proceedings of Bioavailability ’93, Part II, ed.Schlemmer, U., Bundes- repeated (e.g., replicates or triplicates) and the mean used for forschungsanstalt fu� r Erna�hrung, Karlsruhe, 1993, pp. 161–168. further calculations. This would reduce the overall standard 17 Van Dokkum, W., De La Gueronniere, V., Schaafsma, G., Bouley, deviation by a factor of Ón. In this case, the low dose of 44Ca C., Luten, J., and Latge, C., Brit. J. Nutr., 1996, 75, 893. would probably be sucient to create a statistical significant 18 Vanheacke, F., Moens, L., Dams, R., and Taylor, P., Anal.Chem., enrichment of the urine samples. Even though each sample 1996, 68, 567. 19 Vanhaecke, F., Moens, L., Dams, R., Papadakis, I., and Taylor, should be analysed more than once this would reduce the total P., Anal. Chem., 1997, 69, 268. cost of future experiments, since stable isotopes are very 20 Van Dokkum, W., Fairwether-Tait, S. J., Hurrell, R., and expensive. Sandstro�m, B., in Stable Isotopes in Human Nutrition, Inorganic Nutrient Metabolism, ed. Mellon, F. A., Sandstro�m, B., Academic Press. London, 1996. pp. 23–42. 21 Koirtyohann, S. R., Spectrochim. Acta, Part B., 1994, 49, 1305. REFERENCES 22 Price Russ III, G., in Application of Inductively Coupled Plasma Mass Spectrometry, ed. Date, A. R., Gray, A. L., Blackie. Glasgow 1 Dilena, B. A., Larsson, L., and O� hman, S., in Handbook on metals and London, 1989. pp. 90–110. in clinical and analytical chemistry, ed. Seiler, H. G., Sigel, A., and 23 Jarvis, K. E., Gray, A. L., and Houk, R. S., in Handbook of Sigel, H., Marcel Dekker, Inc, New York, 1994, pp. 299–310. Inductively Coupled Plasma Mass Spectrometry, ed. Jarvis, K. E., 2 Nordiske Na�ringsrekommendationer (1996). Nord 1996528 © Gray, A. L., and Houk, R. S., Blackie, Glasgow and London, Nordiske Ministerra°det, Ko� penhamn 1996. ISBN 92 9120 930 9 1992, pp. 143–145. (Swedish). 24 Caroli, S., Alimonti, A., Coni, F., Petrucci, F., Senofonte, O., and 3 Lee, W. T., Leung, S. S., Fairweather-Tait, S. J., Leung, D. M., Violante, N., Crit. Rev. Anal. Chem., 1994, 24, 363. Tsang, H. S., Eagles, J., Fox, T., Wang, S. H., Xu, Y. C., and 25 Ting, B. T. G., and Janghorbani, M., J. Anal. At. Spectrom., 1988, Zeng, W. P., Brit. J. Nutr., 1994, 72, 883. 3, 325. 4 Miller, D. D., Adv. Food Nutr. Res., 1989, 103. 26 Begley, I. S., and Sharp, B. L., J. Anal. At. Spectrom., 1994, 9, 171. 5 Shipp, C. C., Maletskos, C. J., and Dawson-Hughes, B., Calcif. T issue Int., 1987, 41, 307. Paper 7/04079B 6 Fairweather-Tait, S. J., Johnson, A., Eagles, J., Ganatra, S., Received June 11, 1997 Kennedy, H., and Gurr, M. I., Brit. J. Nutr., 1989, 62, 379. 7 Crews, H. M., Ducros, V., Eagles, J. E., Mellon, F. A., Accepted July 22, 1997 Journal of Analytical Atomic Spectrometry, September 1

ISSN:0267-9477    

DOI:10.1039/a704079b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Direct Determination of Heavy Metals at Picogram per Gram Levels in Greenland and Antarctic Snow by Double Focusing Inductively Coupled Plasma Mass Spectrometry† CARLO BARBANTE*‡a, TANIA BELLOMIa , GIUSEPPE MEZZADRIa , PAOLO CESCONa‡, GIUSEPPE SCARPONIa , CHRISTINE MORELb , STEPHAN JAYb, KATJA VAN DE VELDEb, CHRISTOPHE FERRARIb§ AND CLAUDE F. BOUTRONb¶ aDipartimento di Scienze Ambientali, Universita` Ca’ Foscari di Venezia, Dorsoduro 2137, I-30123 Venezia, Italy bL aboratoire de Glaciologie et Ge�ophysique de l’Environnement du CNRS, 54, rueMolie`re, Domaine Universitaire, BP 96, 38402 Saint Martin d’He� res, France The potential of a double focusing ICP-MS instrument in of sample contamination during collection, storage, treatment terms of high sensitivity, sample throughput and low volume of and analysis.As an example, the very few reliable data sets sample consumed was investigated for the direct, simultaneous available for Cd, Pb, Zn and Cu show that their concentrations determination of Co, Cu, Zn, Mo, Pd, Ag, Cd, Sb, Pt, Pb, Bi range from tenths of pg g-1 (10-12 g g-1) in Antarctic and U at the low and sub-pg g-1 level in polar snow.The Holocene ice4,5 up to tens–hundreds of pg g-1 for presententire analytical procedure, including cleaning of material, day Greenland surface snow.6 field sampling, sample handling, determination of the blanks The ideal analytical technique to be used in the challenging and instrumental analysis, is described.The mean task of heavy metal determination in polar snow should present concentrations detected in snow samples collected in Central extremely low detection limits, multi-element capability and Greenland (2.7 m deep pit) are (in pg g-1): Co 5.8, Cu 4.6, low sample consumption and should avoid, as far as possible, Zn 47, Mo 1.6, Pd 1.1, Ag 0.60, Sb 0.86, Pt 0.61, Bi 2.5 and any preconcentration step which is time consuming and could U 1.8. The Cd, Pb and U concentrations in a snow core be the source of contamination.section collected in East Antarctica are: Cd 0.39, Pb 5.0, Various instrumental methods have been used in the U 0.04 pg g-1. Repeatability of measurements ranges between past, i.e., laser excited atomic fluorescence spectrometry 8 and 25% depending on the element considered. For some of (LEAFS),7–11 thermal ionisation mass spectrometry the elements investigated these results constitute the first (TIMS),5,12–15 instrumental neutron activation analysis available for polar snow.The results of direct analysis by (INAA),16,17 graphite furnace atomic absorption spectrometry double focusing ICP-MS on Cd and Pb in the Antarctic snow (GFAAS),4,18–20 dierential pulse anodic stripping voltamsamples and on Zn and Cu in Greenland samples are metry (DPASV),21–25 atomic fluorescence spectrometry consistent with those obtained by dierential pulse anodic (AFS)26,27 and inductively coupled plasma mass spectrometry stripping voltammetry (DPASV) and graphite furnace atomic (ICP-MS).28–30 Of these only LEAFS and DPASV have demabsorption spectrometry (GFAAS), respectively.onstrated enough sensitivity for a direct determination at the required levels.7–11,21–25 However, skilful operators and time Keywords: Double focusing inductively coupled plasma mass consuming procedures are required in both cases and DPASV spectrometry; trace elements; snow; Greenland; Antarctica requires a large amount of sample, which is not always available. The other techniques are less sensitive and require The Greenland and Antarctic snow and ice caps are among dierent preconcentration or extraction methods,5,31,32 which the best preserved and most detailed archives for the recon- in addition to slowness, require fairly large volumes of samples struction of past and recent variations in the chemical composi- and need very great care to avoid contamination.tion of the earth’s atmosphere.1–3 These archives consist of As regards ICP techniques, a quadrupole ICP-MS instrusuccessive, datable snow layers accumulated during the past ment, after preconcentration of samples by non-boiling evaporhundreds of thousands of years up to the present.They are ation, has already been used for the determination of trace depositories of hemispheric-scale events that happened in the elements in snowfall of the remote Scottish Highlands at atmospheres of the past.the ng g-1 level.28,29 By coupling a quadrupole ICP-MS instru- Heavy metals occur in polar snow and ice in such a minute ment with an electrothermal vaporisation system (ETVamount that their detection poses real problems for analytical ICP-MS) trace elements in Arctic snow (Ellesmere Island) chemists in terms of very high instrumental sensitivity and risk have been determined.30 In spite of the considerable improvement in terms of sensitivity achieved through the use of ETV sample introduction, the detection limits of this technique † Presented at the 1997 European Winter Conference on Plasma could be too high for direct determination of trace elements Spectrochemistry, Gent, Belgium, January 12–17, 1997.in the snow and ice of remote polar regions. ‡ Also at Centro di Studio sulla Chimica e le Tecnologie per l’Ambiente-CNR, Dorsoduro 2137, I-30123 Venezia, Italy. A valuable improvement for the direct determination of a § Also at Institut des Sciences et Techniques de Grenoble, Universite� wide range of trace elements in polar snow is potentially Joseph Fourier, 28 Avenue Benoit Frachon, BP 53, 38041 Grenoble, oered by double focusing ICP-MS which, thanks to the very France.low background signal and the high ion transmission, oers ¶ Also at U.F.R. de Me�canique, Universite� Joseph Fourier de extreme sensitivity at and below the pg g-1 level. The high Grenoble (Institut Universitaire de France), Domaine Universitaire, BP 68, 38041 Grenoble, France.purity of the matrix to be analysed is very attractive for direct Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 (925–931) 925trace metal determination by double focusing ICP-MS, third acid bath (NIST HNO3 diluted in ultrapure water, 1+1000, 50 °C, 2 weeks); finally, bottles are rinsed several although spectral interferences should be carefully considered. This paper presents the first results of a new approach to times with ultrapure water, filled with a diluted ultrapure HNO3 fresh solution (1+1000) and stored inside double the simultaneous direct determination of several trace elements in Greenland and Antarctic snow by double focusing ICP-MS, polyethylene acid clean bags, while other tools remain in the last bath until use.which shows the great potential of the technique in terms of high sensitivity, sample throughput and low volume of sample A slightly modified multi-step procedure23 was used for the cleaning of LDPE hammers and LDPE home made scrubbers, required.For comparison purposes additional measurements were carried out using GFAAS and DPASV, and results are used to decontaminate the pit walls and snow core section at the DSA in Venice. presented. EXPERIMENTAL Sampling and Datation Laboratories and Chemicals Shallow snow samples were collected in June 1995 at a remote site located a few kilometres East South East of the The clean chemistry laboratory used at Laboratoire de US-European ATM sampling site (72°20¾N; 38°45¾W; elevation Glaciologie et Ge�ophysique de l’Environnement (LGGE) of 3270 m; mean snow accumulation rate 23 g H2O cm-2 y-1) the CNRS (Grenoble, France) is a non laminar flow class in central Greenland.35 During sampling, operators wore clean 10 000 clean room, inside which two class 100 laminar flow room garments, masks, polyethylene gloves and boot covers clean benches were installed for the cleaning of bottles, handling to prevent contamination.and acidification of Greenland samples.33 A total of 68 shallow snow samples were collected in a 2.7 m Clean chemistry laboratories with class 100 vertical laminar deep hand-dug pit, inserting cylindrical LDPE containers (id flow areas were available at the Dipartimento dim, length 40 cm) horizontally in the carefully decontami- Ambientali (DSA) of the University of Venice (Italy) for the nated upwind pit wall and then transferring the contents cleaning of plastic items used in the decontamination of an to clean, wide-mouth LDPE 1 l bottles (Nalgene, Nalge Antarctic snow core section and for acidification of the stan- Company, Rochester, NY, USA).After collection the con- dards. A cold room (-20 °C) equipped with a laminar flow tainers were sealed inside double polyethylene bags and trans- bench (class 100) was used for the storage of samples and for ported frozen to LGGE, where they were stored at -20 °C.the decontamination of frozen core sections. Researchers fol- Inside the clean laboratory, samples were allowed to melt lowed strictly the clean room procedures in these clean at room temperature and sub-aliquots of 15 ml were transferred environments. to 15 ml LDPE clean bottles and 75 ml of ultrapure NIST Ultrapure water was obtained by coupling a reverse osmosis HNO3 was added. These sub-aliquots were then transported system, Milli-RO, with a four column ion-exchange system, frozen to the DSA in Venice.Milli-Q (both from Millipore, Bedford, MA, USA) or by Snow core samples were collected in January 1994 in passing tap water through a succession of activated charcoal the Hercules Ne�ve�, Victoria Land, East Antarctica (73°06¾S; and mixed bed ion-exchange resins from Maxy (St Remy les 165°28¾E; elevation 2960 m, mean snow accumulation rate Chevreuses, France). 16 g cm-2 y-1).36 A stainless steel auger (id 10.4 cm) was used Ultrapure concentrated HNO3 (70%) doubly distilled at the to collect samples down to a depth of 10 m.After collection, National Institute of Standards and Technology (NIST), snow core sections were sealed in double polyethylene bags Gaithersburg, MD, USA34 was used for the acidification of and transported frozen to the DSA in Venice. In this work samples (1+200) and for the final steps of the cleaning a 37 cm length core section (depth at the middle 956 cm, procedures of plastic items (see below).Chloroform (Merck, estimated year of deposition, 196836) was used. Darmstadt, Germany) and Suprapur grade HNO3 (65%, Despite the great care taken in the field to avoid contami- Merck) were used during the first steps of the cleaning nation, the external layers of the snow core collected in procedures. Antarctica were more or less susceptible to contamination by Acidified (NIST HNO3, 1+200) multi-element synthetic metal impurities due to the auger tube and the long storage standard solutions were prepared through successive dilutions period.For this reason a special decontamination procedure37 [with ultrapure Maxy water, in ad hoc used low density was carried out in the laminar flow area of the cold room. polyethylene (LDPE) bottles (1 l and 250 ml)] of ICP-MS This method allows four concentric layers of snow (radius stock solutions (1000 mg l-1, Merck, and 1000 mg l-1, Spex 0–1.5, 1.5–2.5, 2.5–3.5, 3.5–5.0 cm) to be separated and stored Certiprep, Metuchen, NY, USA).These standards contained for analysis. Melted snow samples were then acidified (NIST 14 elements according to the typical elemental concentrations HNO3, 1+200) and the trace element concentrations measured expected in recent Greenland and Antarctic snows. Whenever from the outside to the centre in order to obtain a radial not used the standard solutions were stored frozen. concentration profile. Acidified (NIST HNO3, 1+200) blank solutions were pre- The age of the Greenland pit samples was estimated from pared using Milli-Q and Maxy ultrapure waters.the Na and Al concentration profiles obtained by GFAAS in dierent sub-aliquots of the same samples. As usual the Na Cleaning of Materials and Al late winter/spring concentration maxima were used to reconstruct the age of snow layers. LDPE bottles for storage of samples and plastic tools were acid cleaned in the class 100 environment of the LGGE clean The age of the Antarctic snow core was roughly estimated from the hydrogen peroxide and d18O vertical profiles meas- chemistry laboratory, following a five-step procedure previously described in detail.33 ured in cores drilled at the same sites in the 1991–92 season.36 Briefly, items were cleaned as follows: rough rinse with tap water to remove dust; degrease with chloroform and rinsing Analytical Instrumentation with ultrapure water; immersion in a first acid bath (Merck ‘Suprapur’ HNO3 in ultrapure water, 1+3, 50 °C, 2 weeks) Measurements were carried out with a Finnigan MAT Element (Finnigan MAT, Bremen, Germany) High Resolution and rinsing with ultrapure water; immersion in a second acid bath (NIST HNO3 diluted in ultrapure water, 1+1000, 50 °C, Inductively Coupled Plasma Mass Spectrometer (double focusing ICP-MS).38–40 The instrument is equipped with a fast scan, 2 weeks) and rinsing with ultrapure water; immersion in a 926 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12laminated magnetic sector field analyser and an electrostatic analyser (ESA) in reverse Nier–Johnson geometry.This particular geometry, with the magnet in front of the ESA, make it possible to reduce the background signal intensity and simultaneously increase the abundance sensitivity. Furthermore the sampling interface, operating at the ground potential, is easily accessible, thus facilitating the coupling with several sample introduction systems and hyphenation.Moreover the pre definite resolution settings available of 300, 3000 and 7500 M/DM allow most spectral interferences caused by polyatomic species to be separated. The ion detection unit consisted of a conversion dynode in front of a secondary electron multiplier (SEM). Because of the low concentrations expected for the analysed elements, the Fig. 1 Optimisation of aerosol carrier gas flow rate with a radiocounting mode was chosen instead of the analog mode.frequency generator power of 1.3 kW. A 10 pg g-1 solution of Co (+), The sample introduction system consisted of a Spetec In ($) and Pb (2) was used. (Erding, Germany) peristaltic pump, a Meinhard nebulizer Type A (Meinhard, Santa Ana, CA, USA) and a home made, Table 2 Measurement parameters for the Finnigan MAT Element quartz, double-pass spray chamber, cooled at 5 °C by a closed circuit refrigerating system. The same quartz Fassel torch was E-scan; electric scanning over Acquisition mode small mass range used throughout the experiment. The tube of the peristaltic No.of scans 25 pump was cleaned by allowing a solution of ultrapure acid Dwell time per acquisition 10 ms (NIST HNO3, 1+100) to flow for at least two hours before point each measurement session. Sampling and skimmer cones, with No. of acquisition points per 50 an orifice diameter of 1.0 and 0.75 mm, respectively, were made mass segment of nickel. Total acquisition time 0.5 s per mass segment and per scan Washing time 3 min Instrumental Parameters Resolution required 59Co, 64Zn, 63Cu M/DM=3000 The instrumental conditions used throughout the work are 98Mo, 106Pd, 107Ag, 109Ag, M/DM=300 reported in Table 1. 112Cd, 114Cd, 195Pt, 208Pb, The ion intensities recorded by the detection system are a 209Bi, 238U function of several parameters, the most important of which are sampling distance, lens voltage, nebulizer gas flow rate and the radio-frequency generator power.except 59Co, 63Cu and 64Zn, for which severe interference In particular the eect of the nebulizer gas flow rate on the mainly due to 40Ar18OH, 43Ca16O, 40Ar19F, 23Na40Ar, signal intensity was carefully investigated on the snow matrix. 24Mg40Ar, 32S16O2, 48Ca16O and 38Ar12C14N could aect the Considering the purity of the polar snows, which can be accuracy of analytical determinations. considered as among the purest natural materials in the earth’s surface, an acidified (NIST HNO3, 1+200) synthetic snow sample was prepared by successive dilution in ultrapure water Calibration Curves and Quantification of Co, In and Pb ICP stock solutions.The concentration of Considering that the more direct the analytical procedure is, elements in the tuning solution was 10 pg g-1. Cobalt, In the more appropriate it will be for contamination control, an and Pb were selected because they represent a wide range of external calibration curve was used for the quantification of relative masses, which encompasses masses of several of the concentrations in the real samples, avoiding the use of internal analytes investigated. This solution was measured at dierent standards.Owing to this procedure, to avoid problems due to gas flow rates and signal behaviour plots (signal intensity the drift of the signal intensity with time, a special sequence of versus nebulizer gas flow rate) were recorded. A nebulizer gas standards and samples was adopted.A blank (acidified Maxy flow rate corresponding to the peak maximum was selected water), 4 standards and 6–8 samples were analysed during for the analysis (see Fig. 1). each session of measurement and the results immediately The ion transmission at low resolution (300 M/DM) was evaluated before proceeding with another session. 50 000–100 000 counts s-1 per ng g-1 of In, depending on For each isotope the intensity of the blank was then sub- tuning conditions. Optimisation of instrumental parameters tracted from the intensities of the standard solutions and the was carried out daily in order to maximise the performance of signal increment plotted against the nominal concentrations the instrument in terms of sensitivity and signal stability.The of the standards. The linear calibration curves so obtained acquisition parameters selected for all measurements are briefly were used for trace element quantification in the real samples summarised in Table 2.measured in the same session. Low resolution (M/DM=300) was used for all the isotopes A total of nine sessions was used for the whole analysis. Table 1 Instrumental conditions for the Finnigan MAT Element DPASV and GFAAS Measurements Forward power 1300 W Since no reference materials exist for polar snow and ice, we Gas flow rates measured Cd and Pb in the Antarctic snow core sample and Plasma 14.5 l min-1 Intermediate 0.7 l min-1 Zn and Cu in the Greenland snowpit samples by independent Nebulizer Optimised to obtain maximum signal intensity techniques in order to check the analytical quality of the data.Sample uptake rate 0.8 ml min-1 DPASV had already been used with success for the direct Ion sampling depth Optimised to obtain maximum signal intensity simultaneous determination of Cd and Pb in Antarctic Ion lens settings Optimised to obtain maximum signal intensity snow.23 The principle of the technique is based on electrolytic Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 927Table 3 Trace elements concentration of ultrapure Milli-Q and deposition of analytes on a thin mercury film electrode (TMFE) Maxy waters and subsequent stripping by means of a dierential pulse potential scan with measurement of the diusion current Element concentration/pg g-1 involved in the process. The concentrations of metals in the sample are then determined through the heights of the current Maxy water Milli-Q Detection peaks developed during the stripping phase, using the standard water This work Literature data limit*/pg g-1 additions method.Because of the very high sensitivity of the technique, samples are analysed directly after melting, without Co 1.55 (0.04)† 1.47(0.03)† 0.09 Cu 0.9 (0.3) 0.6 (0.2) <0.1‡ 0.6 any preconcentration step.23 Zn 2.1 (0.3) 1.8 (0.3) 0.3‡ 0.9 GFAAS was used for the determination of Zn and Cu in Mo 0.32 (0.09) 0.64 (0.07) 0.21 the Greenland samples.The relatively high Zn concentration Pd 0.20(0.02) 0.24 (0.03) 0.09 allowed the metal to be determined, with a precision of ±10%, Ag 0.07 (0.01) 0.06 (0.01) 0.03 directly without any preconcentration.31 Multiple injections Cd 0.8 (0.2) 0.6 (0.1) <0.05§ 0.3 (up to 10) of a 50 ml sample in the graphite furnace were used. Sb 0.15 (0.05) 0.15 (0.02) 0.06 Pt 0.05 (0.01) 0.043 (0.003) 0.009 The Cu concentration in Greenland snow often falls below the Pb 1.2 (0.2) 1.04 (0.03) 0.27d; 0.28d 0.09 detection limit of the technique; in this case a volume (about Bi 0.03 (0.04) 0.02 (0.01) 0.03 30 ml) of the sample was preconcentrated to about 1 ml by U 0.12(0.03) 0.06 (0.03) 0.09 non-boiling evaporation inside FEP Teflon beakers, according to a procedure already reported,31 and then analysed by * From Maxy water data, according to the 3s criterion (see text).41 GFAAS.For Cu the repeatability of measurements is about † In brackets, standard deviations, n=5.‡ From Boutron.33 30% (RSD). External calibration curves were used for the § From Bolshov et al.10 quantification of Zn and Cu concentrations. dFrom Bolshov et al.11 Maxy water.33 No considerable dierences are noted between RESULTS AND DISCUSSION the Milli-Q and Maxy ultrapure waters, except for Mo, which Blank Determinations is more concentrated in Maxy water, and U, which shows higher concentrations in Milli-Q water. Considering the high purity of the analysed matrix, particular The discrepancies observed for the results obtained by care was taken in the choice of reagents, storage bottles, plastic ICP-MS and other techniques for the Maxy water could be items used for sampling and decontamination of snow cores imputed to a possible release from the sampling injection and in the evaluation of the blanks. interface.This possible contribution of a procedural blank is Possible, although low, contributions to the trace element presently under investigation. concentrations could come from several sources: snowpit sampling, melting of samples in LDPE bottles, handling, sub-aliquots preparation, decontamination procedure for the Repeatability and Detection Limits Antarctic snow core samples and introduction of samples in Repeatability of measurements was determined on a real the sampling injection interface of the instrument.Particular Greenland sample collected at the depth of 257 cm, for which care has been devoted to the estimation of the possible blank the available sample volume was unusually large.Ten distinct coming from the acidification of the samples and from the use consecutive measurements were carried out within 140 min. of ultrapure waters. The contribution of the acid was evaluated The trace element concentrations determined in the sample by measuring ultrapure Maxy water solutions with increasing (in pg g-1) and the relative standard deviations (% in brackets) content of HNO3 NIST (from 1+1000 to 1+50).No signal are: Co 5.4 (10), Cu 4.6 (8), Zn 39 (8), Mo 2.6 (16), Pd 1.2 intensity variations were observed that were significantly (14), Ag 1.4 (15), Sb 0.7 (25), Pt 0.4 (16), Bi 1.3 (12), U 0.8 (21). dierent from the blank standard deviations (see below) for all From these findings it is possible to observe that the the measured elements, except for Zn. For this element the repeatability of measurements ranges typically between 8 and contribution due to acidification was estimated to be 0.4 pg g-1 25% depending on the element considered.in the final (1+200 acidified) sample. Detection limits were calculated as three times the standard The amount of metal impurities introduced with the acidifi- deviation of the blank.41 Five aliquots were analysed five times cations was also calculated from literature data,34 considering each and the standard deviation calculated from the five the 1+200 dilution of samples.The obtained concentration averages obtained. The results are reported in Table 3. increments due to acidification were: Co 1, Cu 15, Mo 3.5, Pd Considering that the estimated instrumental detection limit 4, Cd 25, Sb 5, Pt 40, Pb 20 and U 1 fg g-1 (10-15 g g-1), was reported as about 8 fgml-1,40 blanks coming from dierent while it was 0.31 pg g-1 for Zn. It is to be noted that concensteps of the analytical procedure can be considered the real trations are always below the detection limit of the technique limiting factor for the determination of trace elements at the (see below).In most cases the calculated contribution of low and sub pg g-1 level. acidification (with respect to both samples and ultrapure waters) can be considered negligible except for Zn and Pt. However, while the Zn contribution has been detected in Antarctic Snow Samples approximately the same amount in the acid used in this work, no signal for Pt has been observed, which shows that the With the aim of intercalibration between double focusing ICP-MS and DPASV, Cd and Pb concentrations were deter- present acid batch is of better quality than the one analysed at NIST in the past.34 For these reasons only the measured mined in the four concentric layers of the snow core collected in Antarctica.Uranium concentrations were also determined Zn contribution has been subtracted from the results of measurements carried out in samples and ultrapure water.in these samples by double focusing ICP-MS. Results obtained for Cd and Pb are reported in Table 4 together with compara- The concentrations of trace elements in the ultrapure Milli-Q and Maxy waters were determined from the four-point cali- tive data obtained by DPASV. The radial concentration profile for U is reported in Fig. 2. bration curves used as in the standard additions method. In Table 3 the concentration values for the two ultrapure waters The radial profiles for the three metals exhibit a clear concentration plateau in the two–three internal layers, showing are reported together with previously published data for the 928 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12Table 4 Comparative determination of Cd and Pb in Antarctic snow by double focusing ICP-MS and DPASV. Analysis of radial layers of a snow core collected in the Hercules Ne�ve�, Victoria Land, East Antarctica Cd concentration/pg g-1 Pb concentration/pg g-1 Layer (from outside) double focusing ICP-MS DPASV double focusing ICP-MS DPASV 1st 8.8* ( 9.2, 8.9)* 80* (77, 65)* 2nd 1.6* ( 1.3, 1.9)* 11.0* ( 8.8, 12.6)* 3rd 0.43 0.45 5.0 4.9 Central 0.38 0.44 5.0 4.4 Mean 0.40 0.45 5.0 4.6 * Values not considered in the mean calculation.Greenland Snow Samples The 68 Greenland snow samples were analysed directly after melting and acidification (NIST HNO3, 1+200), to determine the concentration of Co, Cu, Zn, Mo, Pd, Ag, Sb, Pt, Bi and U.A few statistics of the results obtained are reported in Table 5. The wide dispersion of concentration values is highly dependent upon variations due to the strong seasonal influence on the mechanism of atmospheric transport from remote continents to the Greenland ice sheet. As an example Zn and Cu depth concentration profiles are reported in Fig. 3(a) and (b), respectively, together with the estimated period of deposition of the snow. Comparative results obtained by GFAAS are also reported.As regards Zn, the agreement between the two independent techniques used is good: slope, double focusing ICP-MS versus GFAAS=1.16±0.13 (95% confidence interval), corr 0.916. For Cu the values measured by double focusing ICP-MS are systematically lower than those measured by GFAAS: slope, double focusing ICP-MS versus GFAAS=0.53±0.08 (95% Fig. 2 Uranium radial concentration profile for an Antarctic snow confidence interval), corr 0.860. The cause of this systematic core determined by double focusing ICP-MS (Hercules Ne�ve�, Terra dierence for Cu is presently under investigation by means of Victoria, summer 1993–94, depth 956 cm). further intercalibration exercises on real samples.However, it is to be observed that, in spite of the dierences in the absolute concentrations found for Cu, consistent temporal changes are observed with both techniques for the two that the external contamination of snow cores did not diuse metals, which show a similar seasonal pattern.In fact both to the central part of the section. The plateau concentration concentration profiles reveal spring and summer maxima, values, therefore, represent the real concentrations of the core. which can be attributed to the transport and subsequent Concentration values for Cd and Pb are in good agreement scavenging of polluted Arctic air masses from lower latitude with reliable data reported for recent Antarctic snows.10,20,25,42 countries during these periods of the year.The transition from Results of the intercomparison show good agreement between winter to spring is often associated with enhanced vertical the two independent techniques for all the snow layers. mixing correlated to the northward retreat of the polar front, As regards U, the mean value obtained from data of the while the summer layers peaks may reflect short, but intense central core and the two inner layers is 0.04 pg g-1. The input phenomena closer or within the Arctic circle.44 present data constitute, to the authors’ knowledge, the first Uranium concentrations in Greenland snow are much higher report for uranium concentration in Antarctic snow.For this than those found in the Antarctic and also with respect to reason, in order to test the reliability of our value for U, a calculated values due to rock and soil dust input, but they possible natural background concentration has been estimated, agree with the value reported for recent Greenland snow although with large uncertainty.5,20,25 The metal contribution (about 3 pg g-1).45 This dierence may be imputed to the from rock and soil dust is calculated from the Al concentration dierent extent of pollution of the Greenland snow, which is measured in the same sample (2 ng g-1).Combining this value closer to the highly industrialized countries of North America, with theU5Al mass ratio in bulk crustal material (1.1×10-5),43 Europe and Western Asia.an average rock and soil dust contribution of 0.022 pg g-1 of Mean contributions to each metal from rock and soil dust uranium is obtained. This contribution is close to the U are tentatively estimated, for the present-day climatic con- concentration in the snow, which indicates that U probably ditions, by combining the Al concentration values, measured originates from rock and soil dust. The possible excess in the by GFAAS in each of the 68 samples (mean 7.3 ng g-1, SD U concentration found in the Antarctic snow sample could be 10.7 ng g-1), with the metal5Al mass ratio in bulk crustal attributed to other natural (e.g., marine, volcanic or biogenic) material,43,46 according to the usual procedure.Results are or unknown anthropogenic origin. However, the use of mean reported in Table 5. The excess in the metal concentrations crustal values listed in literature43 could not be correct, since found in the Greenland snow samples could be attributed to the field site could be under the influence of a particular other natural (e.g., marine, volcanic or biogenic) or unknown source.In this respect the calculation of rock and soil dust anthropogenic origin. A detailed geochemical interpretation of contribution must be considered as order of magnitude estimations. seasonal variations in the concentration of trace elements in Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 929Table 5 Summary statistics for concentration of trace elements in 68 Greenland snow samples covering a four year time period (Winter 1990 to Spring 1995).Crustal contributions are calculated combining Al concentration measured in the samples by GFAAS with the metal5Al mass ratio in bulk crustal material.43,46 Element concentration/pg g-1 Crustal contribution/pg g-1 Element Median (min–max) Mean (SD) Mean (SD) Co 5.3 (0.65–17.2) 5.8 (3.6) 2.5 (3.7) Cu 3.7 (0.78–13.6) 4.6 (3.0) 6.5 (9.6) Zn 30 (2.0–207) 47 (40) 6.9 (10.1) Mo 1.6 (0.08–7.0) 1.6 (1.2) 0.09 (0.13) Pd 0.9 (0.05–4.2) 1.1 (0.81) 0.09 (0.13) Ag 0.38 (0.08–5.0) 0.60 (0.75) 0.006 (0.009) Sb 0.72 (0.21–4.3) 0.86 (0.60) 0.02 (0.03) Pt 0.55 (0.08–1.5) 0.61 (0.34) 0.001 (0.001) Bi 1.7 (0.29–21.2) 2.5 (3.2) 0.02 (0.03) U 0.8 (0.21–15.4) 1.8 (2.7) 0.08 (0.12) origin of airborne deposited material, making it possible to obtain valuable new information on sources and long range transport pathways from mid-latitudes to the polar troposphere.This study was performed in the framework of projects on ‘Environmental Contamination’ and ‘Glaciology and Paleoogy’ of the Italian Programma Nazionale di Ricerche in Antartide and financially supported by ENEA through co-operation agreements with the Universities of Venice and Milan, respectively. The financial contribution of the European Project for Ice Coring in Antarctica (EPICA) supported by the European Union and coordinated by the European Science Foundation is gratefully acknowledged.The Greenland samples were collected within the European program TAGGSI (EC EV5V-0412) as part of the joint US-European ATM program. We would like to thank the PICO, GISP-2 and GRIP personnel for general support in the field. This work was supported by the French Ministry of the Environment (grant 92095 and 94074), the Institut Universitaire de France, the Institut National des Sciences de l’Univers and the University Joseph Fourier of Grenoble. Special thanks are due to Jean-Luc Jarezo and Vale�rie Hoyau for their valuable work in the collection of the Greenland snowpit samples and to the researchers of the ‘Environmental Contamination’ group for the skilful sampling activity in Antarctica.Fig. 3 (a) Zn and (b) Cu depth profiles at Summit (Central Greenland) REFERENCES obtained by double focusing ICP-MS (——). GFAAS results (- - - - -) are shown for comparison. Datation is also reported: W=Winter; 1 Peel, D.A., in T he Environmental Record in Glaciers and Ice Sp=Spring; Su=Summer. Sheets, eds. Oeschger, H., and Langway, C. C., Jr, Wiley, New York, 1989, pp. 207–223. terms of changing sources and/or associated transport path- 2 Wol, E. W., Antarc. Sci., 1990, 2, 189. 3 Boutron, C. F., Environ. Rev., 1995, 3, 1. ways is presently under consideration and will be discussed in 4 Batifol, F., Boutron, C., and de Angelis, M., Nature, 1989, 337, 544.a future publication. 5 Boutron, C. F., and Patterson, C. C., Nature, 1986, 323, 222. 6 Savarino, J., Boutron, C. F., and Jarezo, J.-L., Atmos. Environ., 1994, 28, 1731. CONCLUSIONS 7 Apatin, V. M., Arkhangelskii, B. V., Bolshov, M. A., Ermolov, V. V., Koloshnikov, V. G., Kompanetz, O. N., Owing to the low signal background and the high sensitivity, Kuznetsov, N. L., Mikheilov, E. L., Shishkovskii, V. S., and the detection limit obtained by double focusing ICP-MS can Boutron, C.F., Spectrochim. Acta, Part B, 1989, 44, 253. be considered adequate for the direct determination of trace 8 Bolshov, M. A., Boutron, C. F., and Zybin, A. V., Anal. Chem., elements down to sub pg g-1 level in polar snow. Furthermore 1989, 61, 1758. the multi-element capability of the technique allows the simul- 9 Bolshov, M. A., Rudniev, S. N., Candelone, J.-P., Boutron, C. F., taneous determination of several elements in just a few ml of and Hong, S., Spectrochim. Acta, Part B, 1994, 49, 1445. 10 Bolshov, M. A., Boutron, C. F., Ducroz, F. M., Go� rlach, U., samples, which is of paramount importance in glacio-chemical Kompanetz, O. M., Rudniev, S. M., and Hutch, B., Anal. Chim. investigations, particularly when deep ice cores are to be Acta, 1991, 251, 169. analysed. 11 Bolshov, M. A., Koloshnikov, V. G., Rudnev, S. N., Boutron, Most of the elements considered here and others which C. F., Go� rlach, U., and Patterson, C. C., J. Anal. At. Spectrom., could potentially be determined by double focusing ICP-MS 1992, 7, 99.have never been detected in polar snow; indeed they could 12 Ng, A., and Patterson, C. C., Geochim. Cosmochim. Acta, 1981, 45, 2109. constitute important tracers of the natural and anthropogenic 930 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 1213 Chisholm, W., Rosman, K. J. R., Boutron, C. F., Candelone, J.-P., 30 Sturgeon, R. E., Willie, S. N., Zheng, J., Kudo, A., and Gre�goire, D.C., J. Anal. At. Spectrom., 1993, 8, 1053. and Hong, S., Anal. Chim. Acta, 1995, 311, 141. 14 Rosman, K. J. R., Chisholm, W., Boutron, C. F., Candelone, J.-P., 31 Go� rlach, U., and Boutron, C. F., Anal. Chim. Acta, 1990, 236, 391. and Go� rlach, U., Nature, 1993, 362, 333. 32 Suttie, E. D., and Wol, E. W., Anal. Chim. Acta, 1992, 258, 229. 15 Rosman, K. J. R., Chisholm, W., Boutron, C. F., Candelone, J.-P., 33 Boutron, C. F., Fresenius’ J. Anal. Chem., 1990, 337, 482. and Patterson, C. C., Geophys.Res. L ett., 1994, 21, 2669. 34 Paulsen, P. J., Beary, E. S., Bushee, D. S., and Moody, J. R., Anal. 16 Murozumi, M., Chow, T. J., and Patterson, C. C., Geochim. Chem., 1988, 60, 971. Cosmochim. Acta, 1969, 33, 1247. 35 Dibb, J. E., and Jarezo, J.-L., J. Geophys. Res, 1996, in the press. 17 Boutron, C. F., Leclerc, M., and Risler, N., Atmos. Environ., 1984, 36 Barbolani, E., Dini, M., Maggi, V., Piccardi, G., Serra, F., Stenni, 18, 1947. B., and Udisti, R., T erra Antarct., 1997, in the press. 18 Boutron, C. F., Patterson, C. C., and Barkov, N. I., Earth Planet. 37 Candelone, J.-P., Hong, S., and Boutron, C. F., Anal. Chim. Acta, Sci. L ett., 1990, 101, 248. 1994, 299, 9. 19 Boutron, C. F., Go� rlach, U., Candelone, J.-P., Bolshov, M. A., 38 Feldmann, I., Tittes, W., Jakubowski, N., Stuewer, D., and and Delmas, R. J., Nature, 1991, 353, 153. Giessmann, U., J. Anal. At. Spectrom., 1994, 9, 1007. 20 Wol, E. W., and Suttie, E. D., Geophys. Res. L ett., 1994, 21, 781. 39 Giessmann, U., and Greb, U., Fresenius’ J. Anal. Chem., 1994, 21 Mart, L., T ellus, 1983, 35B, 131. 350, 186. 22 Volkening, J., and Heumann, K. G., Fresenius’ J. Anal. Chem., 40 Moens, L., Vanhaecke, F., Riondato, J., and Dams, R., J. Anal. 1988, 331, 174. At. Spectrom., 1995, 10, 569. 23 Scarponi, G., Barbante, C., and Cescon, P., Analusis, 1994, 41 Long, G. L., and Winefordner, J. D., Anal. Chem., 1983, 55, 712A. 22, M47. 42 Go� rlach, U., and Boutron, C. F., J. Atmos. Chem., 1992, 14, 205. 24 Scarponi, G., Barbante, C., Turetta, C., Gambaro, A., and 43 Taylor, S. R., and McLennan, S. M., T he Continental Crust: its Cescon, P., Microchem. J., 1997, 55, 24. Composition and Evolution. An Examination of the Geochemical 25 Barbante, C., Turetta, C., Capodaglio, G., and Scarponi, G., Int. Record Preserved in Sedimentary Rocks, Blackwell Scientific J. Environ. Anal. Chem., 1997, in the press. Publications, Oxford, U.K., 1985, pp. 1–300. 26 Vandal, G. M., Fitzgerald, W. F., Boutron, C. F., and Candelone, 44 Candelone, J.-P., Jarezo, J.-L., Hong, S., Davidson, C. I., and J.-P., Nature, 1993, 362, 621. Boutron, C. F., Sci. T otal Environ., 1996, 193, 101. 27 Vandal, G. M., Fitzgerald, W. F., Boutron, C. F., and Candelone, 45 Koide, M., and Goldberg, E. D., Earth Planet. Sci. L ett., 1983, J.-P., in Ice Core Studies of Global Biogeochemical Cycles, eds. 65, 245. Delmas, R. J., Hammer, C., Kley, D., and Mayewski, P., Kluwer, 46 Mason, B., Principles of Geochemistry, Wiley, New York, 3rd Dordrecht, 1995, pp. 401–415. edition, 1978, pp. 1–300. 28 Davies, T. D., Tranter, M., Jickells, T. D., Abrahams, P. W., Landsberger, S., Jarvis, K., and Pierce, C. E., Atmos. Environ., Paper 7/01686G 1992, 26A, 95. ReceivedMarch 11, 1997 29 Jickells, T. D., Davies, T. D., Tranter, M., Landsberger, S., Jarvis, K., and Abrahams, P. W., Atmos. Environ., 1992, 26A, 393. Accepted June 16, 1997 Journal of Analytical Atomic Spectrometry, September 1

ISSN:0267-9477    

DOI:10.1039/a701686g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Trace and Ultratrace Elements in Human Serum With a Double Focusing Magnetic Sector Inductively Coupled Plasma Mass Spectrometer† JO� RGEN RIONDATO, FRANK VANHAECKE, LUC MOENS AND RICHARD DAMS* L aboratory of Analytical Chemistry, Ghent University, Institute for Nuclear Sciences, Proeftuinstraat 86, B-9000 Ghent, Belgium The purpose of this study was to evaluate the potential of gases introduced into the plasma, water and the reagents used. In addition, biological samples with a heavy matrix can ICP-MS with a double focusing instrument for the determination of both spectrally and not spectrally interfered contribute to an important extent to the formation of polyatomic ions. Consequently, a significant number of elements ultratrace elements in human serum after minimised sample pre-treatment and without using any separation or cannot be determined without preliminary work, aimed at a reduction or elimination of spectral interferences.The latter preconcentration techniques.Because of the heavy matrix (6–8% proteins and 1% inorganic compounds) of the material can, for example, be obtained by reducing the amount of solvent reaching the plasma1 or by applying electrothermal investigated, a higher resolving power than is available with quadrupole instruments was necessary for the determination of vaporisation for sample introduction.2–5 The most elegant and direct solution, however, is to use a double focusing magnetic some elements in order to eliminate spectral interferences.This was achieved by using ICP-MS apparatus equipped with a sector field mass spectrometer instead of a quadrupole filter. The ICP-MS instrument used in this study, the Element double focusing magnetic sector MS. This type of instrument also permits lower detection limits when compared with from Finnigan MAT (Bremen, Germany) allows operation in three dierent resolution modes (m/Dm=300, 3000 and 7500, traditional quadrupole ICP-MS instruments, mainly owing to the low instrumental background values.Determinations were 10% valley definition). In this study, the potential of the apparatus has been proved by analysing the Second Generation carried out using external calibration and, for most of the elements, also single standard additions as calibration Human Serum Reference Material.6 At the low resolution setting, a combination of high sensitivity and an extremely low techniques.Both the non-spectrally interfered (Cd, Sn, Ag and U) and the spectrally interfered (Al, Si, P, S, Ti, Cr, Mn, Fe, background permits detection limits significantly lower than those obtained by quadrupole-based ICP-MS. In order to Cu and Zn) elements were determined in a Second Generation Human Serum Reference Material. Sample preparation determine those elements for which the nuclides of interest are subject to spectral overlap at low resolution, a higher resolution consisted in reconstitution of the freeze-dried material or in microwave digestion, both followed by dilution.In addition to setting is necessary. Although this implies a decrease in sensitivity, the resulting detection limits are generally suciently continuous nebulization, flow injection was also applied as an introduction technique for the determination of some elements low.7 Previous analysis of the reference material by quadrupole in reconstituted serum. The results for the certified elements were all within the certified range except for Cr for which a ICP-MS8 enabled the determination of the concentrations of up to 15 trace and ultratrace elements.Because of the aforemen- significantly higher value was obtained. The results of the remaining elements were compared with literature values tioned limitations of this technique, some of the elements of interest could not be determined. For the serum analysis where they existed. Very low concentrations could be determined: for instance a concentration as low as 98 ng l-1 described in the present work, chemical separation or other preliminary sample preparation was not required except for a was determined for Ag with a standard deviation of less than 10%.The major diculty encountered in this study was to four- or five-fold dilution with 0.14 M HNO3, which seemed to be sucient to minimise signal suppression and other matrix keep the blank values suciently low, which could only be achieved by using ultra-pure reagents and by working in a eects.9 Some elements, however, have also been determined in microwave digested samples and results were compared.clean environment. Finally, a flow injection system was used to introduce the Keywords: Ultratrace elements; spectral interference; double (diluted) reconstituted serum samples for some of the elements focusing inductively coupled plasma mass spectrometry; human of interest. serum; flow injection; blank value EXPERIMENTAL ICP-MS is a powerful technique for trace and ultratrace High Resolution ICP-MS Instrumentation analysis of biological samples and especially of body fluids, combining rapid analysis with excellent detection limits and Measurements were carried out on an Element ICP mass multi-element capabilities.However, most instruments are spectrometer from Finnigan MAT. Details about the instruequipped with a quadrupole mass filter which implies that mentation used and its capabilities are to be found elsespectral interferences often jeopardise accurate trace element where.7,9–11 The instrumental operating conditions are determination, especially at mass-to-charge ratios of 80.Most summarised in Table 1. of these interferences are caused by polyatomic ions originating from a combination of elements from the plasma, atmospheric Flow Injection Instrumentation The flow injection system used is a modular system purchased † Presented at the 1997 European Winter Conference on Plasma Spectrochemistry, Gent, Belgium, January 12–17, 1997.from EVA (EVA product line is a joint venture of FIAtron Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 (933–937) 933Table 1 Instrumental operating conditions 10 mg l-1, except for P and S, at the 100 mg l-1 level ). However, 115In was used as an internal standard at a 1 mg l-1 level Rf power 1370 W for the determination of Ag, Cd and Sn and finally 205Tl Gas flow rates (1 mg l-1) acted as an internal standard for U.Outer plasma 14 l min-1 Standard solutions were prepared containing 1–1000 mg l-1 Intermediate 0.7 l min-1 Aerosol carrier 0.7–0.9 l min-1 depending on the isotopic abundance and the ionisation energy Sample uptake rate 1 ml min-1 of the nuclide of interest. The concentration level chosen also Ion sampling depth Adjusted in order to obtain maximum varied with the mass-to-charge ratio (m/z) depending on signal intensity the sensitivity of the instrument.7 In addition, the standard Ion lens settings Adjusted in order to obtain maximum solutions of spectrally interfered elements contain at least signal intensity and resolution 10 mg l-1 since a resolution of 3000 implies a decrease in sensi- Sampling cone Nickel, 1.1 mm orifice diameter Skimmer Nickel, 0.8 mm orifice diameter tivity by approximately a factor of 20 compared with low Torch Fassel type resolution (300).7 Nebulizer Meinhard TR-30-A3 Spray chamber Scott-type (‘double pass’), cooled to 5 °C Calibration Techniques All determinations were carried out using external calibration Systems, Oconomowoc, WI, USA and Eppendorf-Netheleras a calibration technique.For some of the elements which Hinz, Hamburg, Germany). The sample loop with a volume were determined in the five-fold diluted samples the method of 200 ml was controlled by a switchable valve allowing two of standard additions was applied as well.positions (loading the loop or injecting the sample). The carrier solution (0.14 M HNO3) was pumped at 1.2 ml min-1 while the introduced sample solution was pumped at 0.7 ml min-1. Samples and Sample Preparation The Second Generation Human Serum Reference Material analysed contains 14 trace elements with certified concen- Acquisition trations.6 This reference material has been prepared under The elements for which determination was not hampereas a rigorously controlled conditions to avoid any contamination result of spectral interferences were determined using a resoor losses of trace elements and the trace element concentrations lution setting of 300.The other elements required a resolution therefore closely approximate those in normal human serum. setting of 3000 to separate the signals of the analyte and the Some of the elements determined in this work were present at interfering polyatomic ions. All measurements were performed very low concentrations and were not certified; for U no in the electric scan (E-scan) acquisition mode.literature values have been reported until now. As previously As indicated in Table 2, for at least three independently mentioned, the freeze-dried serum was prepared for analysis prepared sample solutions, three replicate measurements each in two dierent ways. Simple reconstitution was obtained by of 1 min per isotope were carried out. However, for determiadding 6 ml of Millipore Milli-Q water to 0.5 g of the lyonation with flow injection, five replicate acquisitions of only philised serum which was transferred quantitatively into a 30 s each were carried out, owing to the transient nature of calibrated flask (25 ml for five-fold dilution) and adjusted to the signals obtained (signal duration typically 10–15 s).Data volume with 0.14 M HNO3. This sample preparation method were acquired during a short period of time half way through has the advantages of being less time-consuming and introducthe transient signal, where this is more or less constant.Blanks, ing less contamination. On the other hand, the heavy matrix samples and standards were measured in that order. was seen to cause a rapid and irreversible clogging of the torch. Therefore, for some of the elements, the reference material was also prepared for analysis by microwave digestion Reagents and Standard Solutions and the results obtained were compared. For this digestion, a In order to reduce the risk of contamination, all work was commercially available microwave oven (Amana Radarange, carried out under clean room conditions.model RS 560A, Amana Refrigeration International, IA, USA) All standard solutions used were prepared by successively was used. First, 5 ml of HNO3 (purified by sub-boiling distildiluting commercially available mono-element standard solulation) were added to #0.5 g of serum in a PFA digestion tions (1 g l-1). For these dilutions, 100-fold diluted (0.14 M) bomb.The destruction was achieved by subjecting the bomb HNO3 was used. The HNO3 (14 M) was purified by sub-boiling to a four-step heating programme, during which an increasingly distillation and diluted with Millipore Milli-Q water. In order higher power was applied. Again, the solution obtained was to avoid contamination, only thoroughly precleaned polyethyltransferred quantitatively into a 25 ml calibrated flask and ene vials were used and dilutions were carried out using adjusted to volume with 0.14 M HNO3.Attention should be micropipettes. paid to avoid contamination from vessels and especially from For the determination of Al, Si, P, S, Ti, Cr, Mn, Fe, Cu the HNO3 used. and Zn, 59Co was used as an internal standard (at a level of In Table 3 blank serum values for several elements are compared with their concentrations in the measured diluted Table 2 Measurement parameters Table 3 Average serum blank concentrations and concentrations in Acquisition mode Electric scanning the diluted Human Serum Reference Material (blank corrected) Number of samples 3 Number of replicate measurements per sample 3* Element Serum blank Diluted serum 5† Number of scans per replicate 60* Al* 0.73 mg l-1 0.34 mg l-1 Al† 0.85 mg l-1 0.30 mg l-1 300† Time per scan 1 s* Si* 15 mg l-1 35 mg l-1 Ti* 0.05 mg l-1 0.22 mg l-1 0.1 s† Resolution setting m/Dm 300 and 3000 Ag† 5 ng l-1 18 ng l-1 * Continuous nebulization.* Sample preparation: reconstitution only. † Sample preparation: microwave digestion. † Flow injection. 934 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12human serum (Reference Material). As can be observed, blank Ag values for some elements largely exceed the instrumental Determination of the element Ag (not certified in the reference background making it impossible to take full advantage of the material ) has also been carried out and published in a previous extremely low instrumental detection limits of the Element publication.12 The 107Ag nuclide (51.8%) was used for the system.determination. Since no spectral interferences occur at a massto- charge ratio of 107, a resolving power of 300 was sucient. The resulting value is substantially lower than the only value RESULTS AND DISCUSSION reported for this material and obtained by RNAA13 (see Determination of Trace Elements in Diluted Reconstituted Table 6).The relatively high standard deviation is caused by Human Serum a high blank value of 38.5 ng l-1. The Ag concentration measured in the four-fold diluted serum corresponds to about As mentioned before, some of the elements of interest (Ti, Fe, 15 ng l-1, which is below the detection limit attainable with a Cu, Zn, Ag, Cd, Sn and U) were determined using both quadrupole ICP-MS. external calibration and single standard additions as calibration techniques. Excellent agreement between the results (within 5%) from both techniques was observed, implying that Mn and Cr internal standardisation is sucient to overcome matrix eects.The concentrations of Mn and Cr in the reference material are Indeed, for all the elements measured only a slight or even no low (<1 mg l-1) and in addition, all nuclides involved are signal suppression was observed in the diluted serum. spectrally interfered, making determination with quadrupole ICP-MS impossible. In this work, Cr was determined at a Fe, Cu and Zn resolution of 3000 via the main 52Cr nuclide; the peak of 52Cr+ had to be measured next to a 1000 times more intense ArC+ Although in the measurement of diluted human serum all the peak.Manganese is mono-isotopic (55Mn) but polyatomic ions isotopes of these elements are interfered to a significant extent, with a mass-to-charge ratio of 55 (Table 4) can easily be determination via quadrupole ICP-MS has been carried out separated from the Mn+ ion signal at R=3000.The Mn result previously at this laboratory.8 Matrix matching of the blank was in excellent agreement with the certified value.6 The Cr after a thorough study of the composition of the serum matrix result, however, seemed to be high, probably due to and an appropriate choice of the (less interfered) nuclides to contamination. be measured made accurate quadrupole determinations possible. With a double focusing instrument, set at a resolution of 3000, the peaks of the isotopes 56Fe, 63Cu and 66Zn were P and S separated from the interferences listed in Table 4.These In order to determine these elements present at a concentration elements were determined using a prototype instrument with of up to 100 mg l-1, 50-fold diluted samples were measured. limited instrumental and software possibilities. The results In spite of the fact that P and S are highly concentrated and obtained have already been reported in a previous publiconsequently are not considered as trace elements, until now cation12 and are shown in Table 5.Good agreement with determination with ICP-MS was problematic or impossible certified values was obtained for Fe and Cu whereas the Zn because of large spectral interferences. A spectrum at a mass- result was in reasonable agreement. to-charge ratio of 32 is presented in Fig. 1 for the Human Serum Reference Material showing the 32S+ signal next to the Table 4 Nuclides used for element determination and the most 16O2+ peak.important polyatomic ions, jeopardising accurate determination at low resolution in human serum Determination of Trace Elements in Microwave-digested Element Nuclide Interfering species Human Serum Aluminium 27Al (100%) 13C14N, 12C14N1H Silicon 28Si (92.2%) 12C16O, 14N2 To overcome the rapid clogging of the torch when analysing Phosphorus 31P (100%) 14N16O1H, 15N16O simply diluted (human) serum, freeze-dried serum was also Sulfur 32S (95.0%) 16O2 microwave digested prior to the measurement.Titanium 47Ti (7.3%) 31P16O, 12C35Cl 49Ti (5.5%) 35Cl14N, 32S16O1H, 12C37Cl Chromium 52Cr (83.8%) 40Ar12C, 35Cl16O1H Fe, Cu, Zn and Ag Manganese 55Mn (100%) 39K16O, 40Ar14N1H Iron 56Fe (91.7%) 40Ar16O, 40Ca16O To allow comparison between both methods of sample prep- Copper 63Cu (69.1%) 40Ar23Na, 31P16O2 aration (digestion and simple dilution) Fe, Cu, Zn and Ag were Zinc 64Zn (48.9%) 32S16O2, 32S2 determined. The results for Fe and Zn in digested samples 66Zn (27.9%) 34S16O2, 34S32S were in agreement but the Cu result was 4% lower.Silver has Table 5 ICP-MS results (with standard deviations) for some certified elements in the Human Serum Reference Material in microwave digested and simply diluted (five-fold unless indicated otherwise) samples Nuclide used Resolution Digested Reconstituted Certified range 27Al 3000 1.49 (0.61) mg l-1 1.70 (0.54)* mg l-1 1.59–2.12 mg l-1 52Cr 3000 — 135 (25) ng l-1 61–79 ng l-1 55Mn 3000 — 701 (22) ng l-1 672.7–727.3 ng l-1 56Fe 3000 2.08 (0.14) mg l-1 2.153 (0.074)† mg l-1 2.21–2.49 mg l-1 63Cu 3000 0.932 (0.012) mg l-1 0.970 (0.021)† mg l-1 0.97–1.05 mg l-1 64Zn 3000 0.850 (0.053) mg l-1 — 0.85–0.89 mg l-1 66Zn 3000 — 0.750 (0.074)† mg l-1 0.85–0.89 mg l-1 111Cd 300 226.4 (4.5) ng l-1 — 154.5–227.3 ng l-1 * Determined with flow injection. † Eight-fold diluted; from ref. 12. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 935Table 6 ICP-MS results (with standard deviation) for some non-certified elements in the Human Serum Reference Material in microwave digested and simply diluted (five-fold unless indicated otherwise) samples Nuclide used Resolution Digested Reconstituted (Lowest) literature values Reference 28Si 3000 — 175 (20)* mg l-1 170 mg l-1 14 31P 3000 — 122.7 (1.5)† mg l-1 115–163 mg l-1 15 32S 3000 — 1.113 (0.021)† g l-1 1.12–1.27 g l-1 15 47Ti 3000 — 0.99 (0.24)* mg l-1 1.3±0.3 mg l-1 16 0.7±0.1 mg l-1 16 49Ti 3000 — 1.09 (0.17)* mg l-1 1.3±0.3 mg l-1 16 0.7±0.1 mg l-1 16 107Ag 300 98.2 (2.7) ng l-1 53.6 (6.4)‡ ng l-1 84.5 (3.6) ng l-1 13 120Sn 300 601 (26) ng l-1 — 690.9 (127.3) ng l-1 17 238U 300 — 0.77 (0.14)* ng l-1 — * Determined with flow injection.† 50-Fold diluted. ‡ Four-fold diluted; from ref. 12. lution, indicating that the applied microwave destruction was not yielding complete mineralisation which was confirmed by earlier experiences in this laboratory.Analysis of Reconstituted Human Serum Using Flow Injection for Sample Introduction Finally, for the determination of Al, Si, Ti and U, flow injection was used as an introduction system. It allows small amounts of sample solution to be injected and the system to be rinsed with 0.14 M HNO3 in between injections. Thus deposition of matrix components in the torch, especially proteins, can be avoided. Also memory eects are reduced to a significant Fig. 1 Sulfur signal in 50-fold diluted human serum at a resolution extent.The only drawback of this technique is that the analyses setting of 3000. are somewhat more time-consuming. There is no significant dierence between the results of determinations via both sample been determined with a precision better than 10%; a signifi- introduction techniques. cantly higher concentration was however found than the value obtained in the reconstituted samples under non-optimal Al conditions.12 Results are shown in Tables 5 and 6.The result of the determination of Al in reconstituted samples (1.70 mg l-1) was in good agreement with that obtained in the Cd and Sn microwave-digested serum. A detection limit of 0.14 mg l-1 (3s- Cadmium and Sn are present at a low concentration criterion) was obtained for the serum. (<1 mg l-1). For the determination of Cd and Sn, the ion signals of 111Cd+ and 120Sn+ were monitored. The Cd result Si (see Table 5) was in excellent agreement with the certified range.6 The Sn value obtained (see Table 6) confirmed the Few papers on the determination of Si in serum or blood have lowest literature value of 691 ng l-1 with a standard deviation been published.19 Silicon is one of the most abundant elements of 127 ng l-1 (determined with ICP-MS)17 but had a much in the lithosphere19 and is omnipresent; consequently the risk better precision.Since no spectral interferences are involved of contamination is not fictitious.In addition, the nebulizer and concentrations are not too low, Cd has already previously and spraychamber used in this work were made out of borosilbeen determined via quadrupole ICP-MS.8 icate glass and hence also contributed to the high blank levels. The concentration in the serum reference material analysed in this work may be higher than the actual concentration in Al normal serum. Indeed, the blood used to prepare the serum reference material had been collected into quartz vessels,6 This certified element is mono-isotopic (27Al) and resolution settings of 1454 and 919 are theoretically sucient to separate which undoubtedly provoked a certain degree of contamination. the 27Al+ signal from that of the most important interfering species 12C15N+ and 12C14N1H+,18 respectively.The major Silicon determinations were carried out via the major 28Si isotope and a resolving power of 3000 was used, which is problem to deal with is the high blank, originating from reagents and airborne dust particles19 and yielding an Al largely sucient to avoid spectral interferences from 14N2+ and 12C16O+.In Fig. 2, spectra of the blank, the standard concentration of the same order of magnitude as the five-fold diluted samples. This explains the relatively high standard (20 mgl-1) and of five-fold diluted serum are shown. As can be seen, the 14N2+ and 12C16O+ peaks are clearly separated from deviation. Nevertheless, the result of 1.5 mg l-1 was in good agreement with the certified range.6 Since C is responsible for the Si ion signal.From the blank spectrum, the high degree of Si contamination can be estimated. In Table 3, blank values the major spectral interferences (see Table 4) in quadrupole ICP-MS, this determination was also carried out at low for Si are compared with those in undiluted human serum. The Si blank values are as high as 35% of the concentration resolution in order to check whether the digestion removed C suciently, a partial contribution to which may come from levels in five-fold diluted samples, which explains the relatively high standard deviation.A value of 175 mg l-1 with a standard dissolved CO2 in the solution. The result obtained (6.1 mg l-1) is four times higher than the one obtained with higher reso- deviation of about 10%, has been determined in the reference 936 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12CONCLUSION The double focusing ICP-MS instrument used allowed precise and accurate determination of the certified elements Al, Mn, Fe, Cd, Cu, Zn and Cd in the Human Serum Reference Material.In addition, Si, P, S, Ti, Ag and U were also determined, at a concentration as low as 770 pg l-1 and with a standard deviation of 140 pg l-1 for U. Reconstitution followed by dilution, as sample preparation, combined with flow injection as an introduction technique seemed to be the most successful approach for determination of ultratrace elements in this type of matrix.A resolution setting of 3000 was found to be sucient to eliminate all spectral interferences encountered. Fig. 2 Determination of silicon in human serum at a resolution As illustrated in Table 4, detection limits for many elements, setting of 3000. especially in matrices such as serum, are restricted by blank levels. Hence, additional research will be necessary to reduce these blank levels further, as was illustrated by the biased result for Cr.material, which is comparable to the result of 170 mg l-1 in In the near future, in addition to U, the presence of rare plasma (for 15 normal individuals) from Gitelman and earth elements such as Y, Ce, Lu and Th will be examined in Alderman14 and the values reported by Huang.20 the reference material. Ti J.R. is a Research Assistant and F.V. is a Senior Research Assistant of the Fund for Scientific Research, Flanders. Very little has been published on the Ti levels in serum or blood.21,22 The interest in this element, however, has increased as a result of the increased use of prosthetic devices made out REFERENCES of Ti and the awareness that metallic total joint replacement 1 Alves, L. C., Wiederin, D.R., and Houk, R. S., Anal. Chem., 1992, devices could interact with the surrounding body fluids and 64, 1164. tissues, as was confirmed by Skipor et al.21 2 Gregoire, D. C., J. Anal. At. Spectrom., 1988, 3, 309.Titanium has five isotopes: 46Ti (8.0%), 47Ti (7.3%), 48Ti 3 Tsukahara, R., and Kubota, M., Spectrochim. Acta, Part B, 1990, (73.8%), 49Ti (5.5%) and 50Ti (5.4%). Since the Ca concen- 45, 779. 4 Carey, J. M., Evans, E. H., Caruso, J. A., and Shen, W.-L., tration in the Human Serum Reference Material is about Spectrochim Acta, Part B, 1991, 46, 1711. 90 mg l-1,23 Ti determination via the most abundant isotope 5 Carey, J. M., and Caruso, J. A., Crit. Rev. Anal.Chem., 1992, 48Ti is excluded because of an overwhelming isobaric inter- 23, 397. ference by 48Ca (0.2%). In contrast to ion signals from most 6 Versieck, J., Vanballenberghe, L., De Kesel, A., Hoste, J.,Wallaeys, of the interferences caused by polyatomic ions, isobaric ion B., Vandenhaute, J., Baeck, N., Steyaert, H., Byrne, A. R., and signals cannot be separated from one another with the double Sunderman, F. W. Jr., Anal. Chim. Acta, 1988, 204, 63. 7 Moens, L., Vanhaecke, F., Riondato, J., and Dams, R., J.Anal. focusing mass spectrometer currently used in ICP-MS since At. Spectrom., 1995, 10, 569. the required resolution greatly exceeds 10 000. The second 8 Vandecasteele, C., Vanhoe, H., and Dams, R., J. Anal. At. isotope 46Ti is also excluded for determination due to the Spectrom., 1993, 8, 781. presence of 46Ca (0.03%). Finally, overlap with the 50V+ and 9 Vanhaecke, F., Riondato, J., Moens, L., and Dams, R., Fresenius’ 50Cr+ ion signals made determination using the 50Ti isotope J.Anal. Chem., 1996, 355, 397. impossible. The remaining isotopes, 47Ti and 49Ti, are suitable 10 Giessmann, U., and Greb, U., Fresenius’ J. Anal. Chem., 1994, 350, 186. for measurements. Many polyatomic ions such as 31P16O+ 11 Feldmann, I., Tittes, W., Jakubowski, N., Stuewer, D., and and 12C35Cl+ at m/z=47 and 35Cl14N+, 32S16O1H+ and Giessmann, U., J. Anal. At. Spectrom., 1994, 9, 1007. 12C37Cl+ at m/z=49, however, hamper accurate determina- 12 Moens, L., Verrept, P., Dams, R., Greb, U., Jung, G., and Laser, tions of Ti at R=300 and consequently a resolution of B., J.Anal. At. Spectrom., 1994, 9, 1075. 3000 is required. The low concentrations demand extreme 13 Xilei, L., Van Renterghem, D., Cornelis, R., and Mees L., Anal. care in order to avoid contamination. A detection limit of Chim. Acta, 1988, 211, 231. 14 Gitelman, H. J., and Alderman, R., J. Anal. At. Spectrom., 1990, 0.008 mg l-1 has been reached in human serum and in the 5, 687. reference material a value of 1.1 mg l-1 with a standard devi- 15 Vanhoe, H., Vandecasteele, C., Versieck, J., and Dams, R., ation of 15% has been obtained using the 49Ti isotope. This Mikrochim. Acta, 1989, III, 373. value was confirmed by the result obtained using the 47Ti 16 Yu, L., Koirtyohann, S. R., Rueppel, M. L., Skipor, A. K., and isotope: 0.99 (0.24) mg l-1. Cantone et al. reported a value of Jacobs, J. J., J. Anal. At. Spectrom., 1997, 12, 69. 90 mg l-1 (standard deviation 10 mg l-1) in the serum of five 17 Baumann, H., 1989, personal communication. 18 Reed, N. M., Cairns, R. O., Hutton, R. C., and Takaku, Y., healthy individuals22 but this seems improbably high.17 Skipor J. Anal. At. Spectrom., 1994, 9, 881. et al.21 found a value of 3.3 mg l-1 in serum with a method 19 Versieck, J., and Cornelis, R., T race Elements in Human Plasma detection limit (MDL) of 2.1 mg l-1 by ETAAS. Recently, or Serum, CRC Press, Bacon Raton, FL, 1989. values of 1.3±0.3 and 0.7±0.1 mg l-1 were quoted by Yu et al. 20 Huang, Z., Spectrochim. Acta, Part B, 1995, 50, 1383. using ETV-ICP-MS as technique for which a detection limit 21 Skipor, A. K., Jacobs, J. J., Schvocky, J., Black, J., and Galante, of 0.4 mg l-1 was obtained.16 J. O., At. Spectrosc., 1994, 15, 131. 22 Cantone, M. C., Molho, N., and Pirola, L., J. Radioanal. Nucl. Chem., 1984, 91/1, 197. U 23 Vanhoe, H., Ph. D. Thesis, University of Gent, 1992. No values of U in serum have been reported up till now. Paper 7/01652B Measurements were made at low resolution since no spectral ReceivedMarch 10, 1997 interferences are to be found at an m/z of 238. As was to be Accepted June 13, 1997 expected, an extremely low value (0.8 ng l-1) was found. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 937

ISSN:0267-9477    

DOI:10.1039/a701652b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Capabilities of an Argon Fluoride 193 nm Excimer Laser for Laser Ablation Inductively Coupled Plasma Mass Spectometry Microanalysis of Geological Materials† DETLEF GU� NTHER*a , ROLF FRISCHKNECHTa , CHRISTOPH A. HEINRICHa AND HANS-J. KAHLERTb aSwiss Federal Institute of T echnology (ETH) Zurich, Institute of Isotope Geology and Mineral Resources, CH-8092 Zurich, Switzerland bMicroL as L asersystem, D-37079 Go�ttingen, Germany Recent developments in laser ablation inductively coupled for trace element analysis of geological samples and other environmental materials, owing to the increased sensitivity of plasma mass spectrometry (LA-ICP-MS) have demonstrated its potential for in situ microanalysis for major, minor and ICP-MS and the more ecient interaction of UV lasers with solid samples.The use of UV laser beams has led to a more trace elements in solids, such as minerals. With the low backgrounds and high sensitivity of new ICP-MS instruments, controlled ablation process.1 With new generation ICP-MS instruments, modifications of the sample cone geometry and limits of detection of 1–10 ng g-1 in a 40 mm ablation pit for many elements can be reached. Fractionation eects due to changes in the torch box configuration, limits of detection for ten selected rare earth elements (REE) of less than 10 ng g-1 dierent ablation rates of various elements have prevented quantification without matrix-matched standards with 1064 nm in a 35–40 mm pit can now be reached.2,3 Earlier work by Hirata and Nesbitt,4 Fryer et.al.5 and Jeries et al.6 demon- Nd5YAG lasers. These eects have been reduced but not eliminated using shorter UV wavelengths (e.g. a quadrupled strated the limitations of LA-ICP-MS analysis because of significant fractionation eects observed during the ablation Nd5YAG 266 nm). Excimer lasers with wavelengths below 200 nm are expected to reduce fractionation eects further, but for some elements, especially Zn, Pb and U, which are of major interest in geological samples.Stix et al.7 reported matrix they present a serious challenge to the design of optical systems, especially if high-resolution UV ablation needs to be eects for mineral analysis using synthetic glass standards. Various strategies applied to LA-ICP-MS to minimise these combined with high quality visual observation, which is essential for the study of complex materials, such as geological eects have been reported, including moving the stage during ablation,4 spraying water onto the ablation site or the use of samples.An LA system was developed using an homogenized UV laser beam (193 nm, Argon Fluoride excimer) with a several internal standards for specified groups of elements of geochemical interest.8 However, none of these approaches can common UV–visual objective on a modified petrographic microscope with reflected and transmitted light illumination, in be used for routine LA analysis or match the improved ablation characteristics oered by the 193 nm excimer system described.combination with a Perkin-Elmer Elan 6000 ICP-MS instrument. The optical system allows imaging of both visible and UV laser light onto the sample surface at the same time. Laser operating parameters and their influence on the ablation process were investigated using NIST SRM 612/610. EXPERIMENTAL Fractionation eects due to dierential ablation of various ICP-MS Instrumentation elements as a function of time can be reduced to interelement correlation coecients of r=0.9 or better and have become The ICP-MS instrument, an Elan 6000 (Perkin-Elmer, insignificant within the precision of quadrupole ICP-MS using Norwalk, CT, USA), has a sensitivity for La of 100×106 this new optical design.Energy densities and repetition rates counts s-1 per mg g-1 abundance when used with solution need to be kept within limited ranges for accurate and sample introduction using a standard concentric nebuliser, and reproducible determinations of trace elements such as Zn, U 1000–2000 counts s-1 per mg g-1 for a 40 mm (10 Hz) ablation and Pb, which have previously presented strong fractionation pit.Quantitative determination of trace elements in mineral problems. LA-ICP-MS determinations on natural hornblende, grains with LA depends on the determination of at least one augite, and garnet, calibrated against NIST SRM 612 using major element of known concentration which can be used as any major element as an internal standard, agree well with an internal standard element to correct for fluctuations in the independent literature data.These experiments with the Argon rate of sample ablation. The Elan 6000 is supplied with an Fluoride 193 nm excimer system demonstrate a greatly analogue and digital detector system, which eectively allows reduced matrix dependence of the ablation process, which simultaneous detection of concentrations up to 3×106 facilitates in situ analysis of unknown samples.counts s-1 in the pulse-counting mode and at signal intensities above this threshold in the analogue mode. The analogue Keywords: L aser ablation; excimer laser; inductively coupled mode is limited in speed by a detector-settling time of about plasma mass spectrometry; geological material 3 ms, which leads to a 70% measurement eciency (the eective time fraction used for measurement). Ultraviolet laser ablation inductively coupled plasma mass ICP-MS instrument optimisation for laser induced aerosol spectrometry (LA-ICP-MS) is an increasingly important tool introduction is slightly dierent from that used for solution nebulisation. However, no single parameter can be used to explain the dierences between wet and dry plasma conditions.† Presented at the 1997 European Winter Conference on Plasma Spectrochemistry, Gent, Belgium, January 12–17, 1997. An in-house machined Al cone with an orifice diameter of Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 (939–944) 9390.7 mm and two additional rotary pumps in parallel with the illumination with a central observation to comply with the Schwarzschild mirror objective performance. The condenser standard supplied pumps (Edwards 18 Two Stage Vacuum Pump, Edwards High Vacuum, Oakville, Ontario L6K 2H4, lens behind the homogeniser collimates the parallel light into a field of 0.5 x 0.5 cm in front of the field lens.The field lens Canada) were used, to reduce pressure in the expansion chamber between the sampler and skimmer cones.2 The provides the optically processed laser light to the Schwarzschild imaging objective. A masking aperture behind the field lens reduced pressure lowers the background by 1–2 orders of magnitude for most of the elements and increases the peak consists of round holes and was varied from 100 mm to obtain an ablation pit of 4 mm diameter to 2.5 mm to obtain a 100 mm intensity of elements with m/z>85 by a factor of three.pit. A 45° dielectric mirror which reflects UV while transmitting visible light, reflects the UV beam onto the reflecting objective Laser Ablation Sample Introduction System lens (25× magnification), with 1.35 cm focal length and 35–40% transmission, which images the aperture onto the An excimer laser (Compex 110i, Argon Fluoride 193 nm, sample (Fig. 1). The objective thus images the aperture onto Lambda Physik, Go� ttingen, Germany) with a gas mixture the sample surface, which permits even illumination at constant containing 5% F2 in Ar with small amounts of He and Ne energy densities, independent of the size and shape of the was used.The maximum output energy of the laser is sample spot. The mirror objective has a high numerical aper- 200 mJ per pulse with a beam size of 2×1 cm (homogeneous ture of 0.4, to obtain high image resolution and to avoid a beam energy). The repetition rate can be varied from 1 to high energy density on the window of the sample chamber. 10 Hz with 200 mJ per pulse using air cooling, and up to Pulse energy can be varied from 50 mJ to 1.5 mJ using 100 Hz at 145 mJ per pulse using water cooling. The beam dierent laser output energies and by a beam splitter in front path is shown in Fig. 1. The inner maximum of the beam of the first prismpulse energy can be measured after the profile is converted into an outer maximum with prism 1 aperture (Laser Energy Meter EM1, EDL-500A, GMP, and prism 2 (telescope) and homogenised by two crossed Renens, Switzerland) or at the sample site by inserting a small lens arrays (9×9 lenses, 3 mm thick, MicroLas, Go� ttingen, Germany).The prism set up is necessary to produce the active area joulemeter (ED-100A, GMP). The stability of the Fig. 1 Schematic representation of the optical beam path for an excimer laser (193 nm, ArF) in combination with a petrographic microscope. 940 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12laser pulse energies measured for 1000 pulses showed a relative RESULTS AND DISCUSSION standard deviation of less than 1%. The low energy density of the large excimer beam profile The sample is placed in a Plexiglas cell with a volume of (2×1 cm) was increased for LA using a homogeniser in combi- 20 cm3, from which the ablated material is carried into the nation with a condenser and a field lens to produce energy ICP-MS by an Ar gas stream.A fused silica window 0.4 mm densities of 2–20 J cm-2 corresponding to pulse energies of thick is anti-reflection coated to minimise UV reflection and 50 mJ to 1.5 mJ. In addition, the divergence of the excimer to reduce the formation of multiple laser beam images. The laser beam was limited using the two arrays transferring the sample can be observed through the petrographic microscope beam into a homogeneous field of 0.5×0.5 cm, which is using eye pieces and a range of low magnification objective produced right in front of the aperture.Only small areas lenses. During ablation, the optical image can be continously (2.5 mm in diameter or less) of this homogenised field are monitored through the Schwarzschild objective and a TV imaged on the sample surface for pit sizes of 4–100 mm, hence camera. The microscope is an assembly of a microscope head allowing constant energy densities on the sample surface.with a changeable mirror-holder (Axiotech, Zeiss) and a micro- Owing to the homogeneous beam profile, the intensity is a scope body (Axioplan, Zeiss) to extend the working distance linear function of the energy density or energy per pulse. The between the microscope table and the objective and to allow sensitivity over a small part of the attainable range is shown sample observation with transmitted and reflected light. All for a 20 mm pit [Fig. 2(a)]. The pulse energy per shot can be optical elements are anti-reflection coated and mounted on an increased up to 1.5 mJ per shot. Deviations from a linear optical bench. A Plexiglas box covering the bench protects the function are caused primarily by measurement uncertainties. user from potentially dangerous UV light and shields the The pulse energy is a linear function of the aperture size optical parts from dust. Flushing of the box with N2 further and allows sample ablation within the whole range of pulse reduces laser beam absorption. energies for a given pit size [Fig. 2(b)]. The ablation depth per pulse is a linear function of the pulse energy and can be varied between 0.05 and 0.5 mm per pulse. This low ablation rate and homogenous illumination of the laser beam onto the sample Data Acquisition surface allows a very well defined ablation process independent The 20 elements listed in Table 1 were measured using a 10 ms of the spatial resolution.A signal for ablation through a 30 mm dwell time and 3 ms quadrupole settling time. Backgrounds thin section of quartz mounted on a plate of Na bearing glass were measured for 30 s (laser not firing) and the transient is shown in Fig. 3. The ablation rate per pulse calculated from signals from the analytes were acquired for approximately 30 s. the Si signal is 0.075 mm. Background corrected signals were integrated. The internal Owing to the high numerical aperture of the objective, standard used for glass and mineral analyses was Ca.required for high-resolution imaging, clean cylindrical pits can Calibration for the minerals was carried out with NIST SRM only be drilled to a depth of about 0.8 pit diameters. However, 612 glass as the external standard. Limits of detection were apertures can be switched during the ablation process, which calculated as three times the standard deviation of the back- permits a stepwise ‘zooming-in’ to target, for example, a small ground normalised to the volume of sample ablated (counts s-1 inclusion deep inside the sample, as illustrated by Fig. 4. In per mg g-1). The data acquisition parameters used are listed addition, prior to data acquisition, very low pulse energies of in Table 1. 50 mJ can be used to remove very thin layers of the sample Table 1 LA-ICP-MS working conditions ICP–MS— Instrument Elan 6000 (Perkin Elmer) Intermediate gas 0.75 l min-1 flow Aerosol carrier 1.3 l min-1 gas flow Outer gas flow 16.4 l min-1 Detector mode Dual, pulse counting and analogue mode Rf power 1200W Vacuum pressure 1.45×10-5 Torr* 7.6×10-6 Torr (two additional rotary pumps, Al cone with 0.5 mm diameter) Isotopes 29Si, 42Ca, 66Zn, 88Sr, 90Zr, 93Nb, 137Ba, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 151Eu, 159Tb, 157Gd, 163Dy, 165Ho, 167Er, 169Tm, 173Yb, 175Lu, 178Hf, 181Ta, 208Pb, 232Th, 238U Excimer laser— Compex 110I (Lambda Physik) ArF 193 nm Output energy 200 mJ at 193 nm Pulse duration 15 ns Repetition rate 1–100, 5, 10, 20, 50, 100 Hz Aperture 0.075–2.5 mm (8 dierent hole sizes) Objective Schwarzschild objective (25× magnification), 1.35 cm focal length Ablation cell Plexiglas, 20 cm3 Gas inlet Nozzle (0.1 mm) Fig. 2 (a), Sensitivity and energy per pulse as a function of the energy Window Fused silica, 0.4 mm thick, anti-reflection coated density for a given pit size of 20 mm; (b), linear function between pulse energy and pit size, obtained by the imaging optics of an excimer laser configuration.* 1 Torr=133.322 Pa. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 941sample uptake visible in SEM pictures taken for a 100 Hz ablation (picture not shown). Dierent pulse energies (2–20 J cm-2) and repetition rates (5–100 Hz) were used for the analysis of the NIST SRM 612. Listed in Table 2 are only two mean concentrations determined, using 10 and 100 Hz. The higher relative standard deviation and the overestimated concentrations (based on Ca as the internal standard) for Zn and Pb (given in bold in Table 2) using repetition rates above 20 Hz demonstrated that fractionation eects are occurring during the ablation.The correlation of the signal intensity of Ca with Pb and Ca with Zn give correlation coecients of about 0.8, even for a 193 nm laser, whereas other elements are not influenced by the high repetition rate. The mean concentrations from analysis using 10 Hz deviate from the certified values by less than 5% (see Table 2).Transient signal correlation for Ca with Zn gave a Fig. 3 Signal (30 s) for drilling through a 30 mm thin section of quartz correlation coecient between these signals of r=0.96, which (approximately 0.1 mm ablation depth per pulse using 10 Hz). is an indication of a controlled ablation process, minimising element fractionation as a function of time to a statistically insignificant value. The pulse energy, responsible for the sample uptake, shows less influence on elemental fractionation, however, the best interelement correlation coecients have been calculated for energy densities between 8.5 and 14 J cm-2.Owing to the greatly reduced fractionation eects, other calibration strategies such as direct solution ablation using the 193 nm can be used in LA, as has been reported elsewhere.9 Limits of detection are a function of the background noise and the sensitivity.10 The dependence of the limits of detection on pit size (constant repetition rate 10 Hz) is shown in Fig. 5.In this experiment, 26 isotopes were measured with a total of 30 s integration of the gas background and 30 s integration for the signal. The linear function of pulse energy and aperture size allows limits of detection below 10 mg g-1, even for 4 mm pits. Uncertaties in the limits of detection pattern for 88Sr Fig. 4 SEM picture of a ‘drill cascade’ into the NIST SRM 612 using sequentially smaller aperture sizes to sample a 10 mm pit about 100 mm below the sample surface.with a larger diameter, in order to remove all contamination from the sample surface. This is important for the determination of the trace element distributions in very small samples. Signal intensities can be changed by using dierent repetition rates. Experiments using 10, 20, 50 and 100 Hz showed increased sensitivity with higher repetition rates, through higher intensities over a shorter time. Unfortunately, the relationship between the repetition rate and signal intensity is not a linear function.Measurements of the output energy show that the laser is not able to maintain the same pulse energy at maximum repetition rates at 100 Hz, as was obtained at 10 Hz. The energy losses above 20 Hz are approximately 10%. The signal intensities for 5–100 Hz dier by more than 10%, which Fig. 5 Relation between limits of detection and pit size (constant frequency and energy density). can be explained by more particle losses owing to the fast Table 2 Improvements in accuracy using selected ablation parameters Dierence Dierence Reference Low frequency (10 Hz) relative to High frequency (100 Hz) relative to value/ measured concentration reference value measured concentration reference value Element m/z mg g-1 mg g-1 (%) mg g-1 (%) Zn 66 49.4 47.9 3.0 60.4 21.2* Sr 88 75.3 74.6 0.9 75.0 0.2 Ba 137 38.9 39.1 0.3 39.4 0.6 Ce 140 37.9 37.8 0.4 38.0 0.2 Tb 159 36.9 37.1 0.4 36.4 1.3 Lu 175 37.2 36.6 1.5 36.1 3.3 Pb 208 36.6 35.3 3.5 44.0 19.1 U 238 36.7 37.1 1.2 37.5 2.6 * See text for explanation of bold figures. 942 Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12and 141Pr (see Fig. 5) are caused by single spikes in the reference material (NIST SRM 612 Trace Elements in Glass) was used for the quantification of trace elements in these background leading to an increased standard deviation. The high background on Pb, caused by ubiquitous Pb contami- minerals, using Ca as the internal standard. The reproducibility of homogeneous samples is for most elements better than 4%.nation in a normal laboratory environment, leads to higher limits of detection compared with all other elements shown in However, variations due to mineral heterogeneity were not investigated in detail. The comparison of the LA-ICP-MS data this figure. The limits of detection calculated for the ablation rate of 0.1 mm per pulse indicate high transportation eciency with literature12,13 values shows satisfactory agreement for most of the elements (Tables 3–5). (90%) from the ablation cell into the plasma.The relatively high count yield per ablated material is probably due to a The amount of ablated material measured as Ca sensitivity (counts s-1 per mg g-1) was in all minerals within 20% of that predominance of smaller particles obtained by ablation with the 193 nm excimer laser, as indicated by particle-counting of the NIST SRM 612, which demonstrates that the ablation rate with excimer system is relatively matrix-insensitive. The studies currently in progress.11 Analyses with optimised laser and ICP-MS conditions have same ablation behaviour was observed for zircon (data not shown).been carried out on the natural hornblende, garnet and augite. For the demonstration of the accuracy an external calibration CONCLUSION Table 3 Trace element determinations of Kakanui hornblende, in The high quality petrographic microscope laser ICP-MS comparison with literature data system, designed for the 193 nm excimer laser, allows sample observation using transmitted and reflected light.The use of a Reference Reference beam homogeniser leads to an improved beam quality and a value/ value/ homogeneous imaging of the laser beam onto the sample Element mg g-1 This work Element mg g-1 This work surface, and thus to a controlled ablation process and a very Rb 15* 21±1.2 Sm 4.4† 4.4±0.2 reproducible pit structure.The energy density is independent Sr 480* 483±9.2 Eu 1.6† 1.56±0.03 of the pit size and of the pit geometry due to the homogeneous Y 10* 9.6±0.5 Gd 4.2† 3.7±0.12 lateral and angular illumination, such that energy density is a Zr 58* 51.8±1.7 Dy — 2.6±0.2 Nb 24* 25.5±0.2 Yb — 0.34±0.06 linear function of pulse energy. The improved beam quality Ba 260* 295±5.1 Hf 1.9† 2.2±0.1 allows pits of less than 4 mm diameter with limits of detection La 5.0† 5.0±0.01 Th — 0.06±0.02 of less than 10 mg g-1 for selected elements between 85Rb and Ce 15.9† 17.6±0.2 Cr 5† 7.6±1.7 238U.The adjustable ablation rate of 0.05–0.5 mm per pulse is Nd 15.2† 15.9±0.2 Sc 18† 18±0.9 suitable for vertical depth profiling and allows controlled removal of surface contamination prior to analysis. As a result * Ref. 12. † Ref. 13. of the improved pulse to pulse stability of the excimer laser, Table 4 Trace and major element determinations in Kakanui augite Reference Reference value/ value/ Element mg g-1 This work Element mg g-1 This work Rb — 0.7±0.2 Eu 0.79† 0.8±0.1 Sr 58* 58.8±0.5 Gd 2.3† 1.7±0.3 Y 8.5* 7.8±0.3 Dy — 1.9±0.1 Zr 24* 20.7±1.1 Yb 0.67† 0.48±0.17 Nb — 0.28±0.09 Hf — 1.1±0.2 Ba — 0.41±0.16 Th — 0.03±0.01 La 1.76† 1.21±0.05 Sc 29† 37±1.5 Ce 5.7† 5.6±0.2 SiO2 (50.73% m/m) (53.4±2.5% m/m) Nd 6.7† 5.7±0.2 — — — Sm 2.14† 2.17±0.2 — — — * Ref. 12. † Ref. 13. Table 5 Trace and major element determinations in Kakanui garnet Reference Reference value/ value/ Element mg g-1 This work Element mg g-1 This work Rb — N.d.* Eu 0.65† 0.62±0.02 Sr — 0.82±0.3 Gd 3.5† 2.9±0.4 Y 55† 44.8±0.7 Dy 8.5† 6.9±0.6 Zr 48‡ 43.8±0.5 Yb 9.1† 7.8±0.9 Nb — 0.62±0.02 Hf 1† 0.8±0.18 Ba — 0.22±0.05 Th — N.d.La 0.02† 0.03±0.01 Sc 132† 169±2 Ce 0.2† 0.12±0.07 SiO2 (41.4% m/m) 43.3±0.7% m/m) Nd 1.14† 0.34±0.23 — — — Sm 1.15† 0.85±0.1 — — — * N.d.=not detected. † Ref. 13. ‡ Ref. 12. Journal of Analytical Atomic Spectrometry, September 1997, Vol. 12 943the precision of analyses for homogeneous materials such as REFERENCES glass, are less than 4% even for concentrations below 1 mg g-1. 1 Jackson, S. E., Longerich, H. P., Dunning, G. R., and Fryer, B. J., Elemental fractionation eects are insignificant, if a carefully Can. Mineral, 1992, 30, 1049. selected combination of intermediate energy density depending 2 Gu� nther, D., Longerich, H. P., Jackson, S. E., and Forsythe, L., on the material and relatively low repetition rates (<20 Hz) is Fresenius’ J.Anal. Chem., 1996, 355, 771. used, which minimise local melting of the sample. Increasing 3 Jackson, S. E., Longerich, H. P., Horn, I., and Dunning, G. R., Abstract, VM Goldschmidt Conference, 1996, p. 283. the pulse repetition rate to between 10 and 100 Hz improves 4 Hirata, T., and Nesbitt, R. W., Geochim. Cosmochim. Acta, 1995, count rate, but the improvement is less linear and leads to 59, 2419. increased element fractionation in silicates above 20 Hz. 5 Fryer, B. J., Jackson, S. E., and Longerich, H. P., Can. Mineral, Ablation rate was found to be largely matrix-independent 1995, 33, 303. amongst the NIST SRM 612, hornblende, augite and garnet, 6 Jeries, T. E., Pearce, N. J. G., Perkins, W. T., and Raith, A., which may in future open the possibility of direct LA-ICP-MS Anal. Commun., 1996, 33, 35. analysis of trace elements without a pre-determined internal 7 Stix, J., Gauthier, G., and Ludden, J. N., Can. Minal, 1995, standard element. However, calibration without an internal 33, 435. 8 Longerich, H. P., Jackson, S. E., and Gu� nther, D., Fresenius’ standard needs more studies on dierent minerals and other J. Anal. Chem., 1996, 355, 538. solid samples to be carried out. A direct comparison between 9 Gu� nther, D., Frischknecht, R., Mu�schenborn, H. -J., and Heinrich, excimer 193 nm and Nd5YAG lasers (266 nm) is not possible C. A., Fresenius’ J. Anal. Chem., 1997, 357, 358. due to the dierences in the beam profile, the wavelength and 10 Longerich, H. P., Jackson, S. E., and Gu� nther, D., J. Anal. At. the optical components necessary to transfer the beam onto Spectrom., 1996, 11, 899. the sample surface. 11 Frischknecht, R., Garbe-Scho� nberg, D., Cousin, H., and Gu� nther, D., unpublished data. The authors acknowledge the financial support of the ETH 12 Czamanske, G. K., Sisson, T. W., Campbell, J. L., and Teesdale, Zurich and the support of the Swiss National Science W. J., Am. Mineral, 1993, 78, 893. 13 Mason, B., and Allen, R. O., J. Geol. Geophys., 1973, 16, 935. Foundation (Project 2100–045548.95/1). GMP (Renens) also provided support of the optical components. Expert technical help by Urs Menet was essential for the successful construction Paper 7/01423F of the laser ablation system, for which we are most grateful. Received February 28, 1997 Critical reading of the manuscript by H. Longerich and two anonymous reviewers is also greatly appreciated. Accepted June 3, 1997 944 Journal of Analytical Atomic Spectrometry, September 1

ISSN:0267-9477    

DOI:10.1039/a701423f

出版商:RSC    

年代:1997

数据来源: RSC