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Determination of U and Th at ultra-trace levels by isotope dilution inductively coupled plasma mass spectrometry using a geyser-type ultrasonic nebulizer: application to geological samples
SAB 1747
Spectrochimica Acta Part B 52 (1997) 1783–1789
Determination of U and Th at ultra-trace levels by isotope dilution inductively coupled plasma mass spectrometry using a geyser-type ultrasonic nebulizer: application to geological samples Sylviane Joannon a,b, Philippe Telouk1b, Christian Pin a a
De´partement de Ge´ologie, U.R.A. 10 C.N.R.S., Centre Re´ gional de Mesures Physiques, Universite´ Blaise Pascal, 5, rue Kessler 63038, Clermont-Ferrand, France b Service Central d’Analyse du C.N.R.S., Echangeur de Solaize, BP 22 69390 Vernaison, France Received 9 January 1997; accepted 19 May 1997
Abstract In this report, the suitability of an ultrasonic nebulizer (USN) used in batch-type sampling mode is evaluated for the isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) analysis of U and Th at the sub-mg g −1 level in geological materials. Compared to conventional pneumatic nebulizers, the system is characterized by high efficiency, highlighted by count rates in excess of 5 × 10 5 cps for 1 ng g −1 of 238U, and good sample usage, with about 60% of the solution converted into a usable aerosol. Although the RSD values observed for the isotopic ratios 230Th/ 232Th and 235U/ 238U were four to five times higher than the theoretical precision expected from counting statistics, within-run precisions are in the range typical for quadrupole-based ICP-MS instruments. The U and Th concentration data obtained for two separate solutions of six international silicate rock reference materials differ by only 1–3%, despite their very low concentrations (between 0.01 and 0.4 mg g −1 of U and between 0.03 and 0.7 mg g −1 of Th), which made them difficult or even impossible to analyse precisely by ID-ICP-MS using conventional pneumatic nebulization. Although one analysis including system cleaning takes about 30 min, the method outlined is still superior to ID-TIMS in terms of throughput. q 1997 Elsevier Science B.V. Keywords: ICP-MS; Ultrasonic nebulizer; Isotope dilution; U; Th; Geostandard
1. Introduction Isotope dilution mass spectrometry (ID-MS) is a highly sensitive method, capable of high precision and high accuracy [1–3]. In this technique, chemical manipulations are carried out on a direct weight basis and the mass spectrometric determinations involve only isotopic ratio data. Therefore, ID-MS can be 1 Present address: De´partement des Sciences de la Terre, Ecole Normale Supe´rieure de Lyon, 46, alle´e d’Italie 69634 Lyon 07, France.
directly related to fundamental units of measurement, and it can be considered a definitive method for quantitative elemental analysis [4]. Compared to conventional thermal ionization mass spectrometry (TIMS), inductively coupled plasma mass spectrometry (ICPMS) has two major advantages: direct liquid sampling at atmospheric pressure and the capability of measuring isotope ratios without extensive chemical separation of the elements analysed, which makes routine application of ID-MS much easier. In a previous study, the potential of ID-ICP-MS for the analysis of U and Th in geological standard reference materials
0584-8547/97/$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 7 )0 0 07 2 -4
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was evaluated [5]. Although most of the results were in excellent agreement with recommended values, it was noted that the most depleted standards (with concentrations below 0.5 mg g −1) could not be measured satisfactorily. This limitation of the ICP-MS approach to isotope dilution analysis at a very low concentration level was largely caused by the poor efficiency of the conventional pneumatic nebulizer used. In an attempt to overcome this problem, the possibilities offered by an ultrasonic nebulizer (USN) used in the batch-type sampling mode were explored. In this design, used in the early days of ICP spectrometry [6,7], the sample solution is contained in a nebulizing cell coupled to the ultrasonic transducer by a water transmitting bath. These batch- or geyser-type USNs have some disadvantages [8], including rather difficult sample change and poorer long-term precision than their continuously fed counterparts which have been more extensively developed (see, for example, Ref. [9]) and are commercially available. However, where long-term precision is not of vital importance and sample solutions do not need to be changed frequently, as is the case for isotope ratio measurements, batch-type USNs could provide an interesting nebulizing system because of dense aerosol production and efficient sample usage, since no waste solution is generated. This paper describes the experimental set-up on this basis and its application to the analysis of ultra-trace amounts of U and Th in a set of silicate rock standard reference materials.
2. Experimental 2.1. Reagents 18 MQ cm water purified through a Milli-Q system (Millipore S.A., Molsheim) was used throughout. Reagent grade perchloric acid (Prolabo, Paris) was used without further purification. Nitric (7 M) and hydrochloric (6 M) acids, both of reagent grade (Prolabo, Paris), were distilled in sub-boiling silica glass stills (Model PB5, Quartex, Paris). Hydrofluoric acid (29 M) was purified using a subboiling distillation system made of two Teflont FEP bottles (Nalgene, Rochester, NY) connected by a homemade Teflont PTFE block following the design of Mattinson [10].
2.2. Geyser-type ultrasonic nebulizer The batch-type USN basically follows the design of Mermet and coworkers [11,12] and was built by modifying a commercially available [13] continuous flow USN (Applied Research Laboratories, Ecublens, Switzerland). It consists of a piezo-electric crystal (transducer) energized at 1.4 MHz by a radiofrequency generator. In contrast with the original ARL instrument, the transducer is positioned horizontally and coupled by a water bath to an interchangeable sample cell made of a silica glass tube (20 mm i.d.) with a Mylar sheet (6 mm thickness) at the bottom sealed with epoxy. The transducer is cooled by circulating water (temperature controlled to 1 6 0.18C) to improve stability of the aerosol generation rate. A chimney-like borosilicate glass spray chamber with an axial impact bead is held on a Teflont PTFE base containing the carrier gas inlet (Fig. 1). This argon flow sweeps the aerosol cloud into the desolvation unit of the ARL USN, which is used without modification, except that it is held vertically. This consists of a heated (1508C) tube followed by a condenser (18C) which separates the dried aerosol from most of the solvent. This batch-sampling mode USN allows large drops and any condensation on the walls of the spray chamber to be recycled in the sample cell, instead of being directed to waste through a drain. The exit of the desolvation unit is connected to the ICP by a 1 m long 3/16 inch i.d. Tygon tubing fitted with a three-way valve enabling the plasma to be isolated when opening the nebulization system for changing the sample cell. An argon sheath gas [14], obtained from Jobin-Yvon S.A. (Longjumeau, France) and placed just before the inlet into the plasma torch, was found to be useful for absorbing pressure fluctuations produced in the desolvating system and stabilize the ICP-MS signal. Operating parameters of the nebulizer–desolvation system are listed in Table 1. For sample changeover, the three-way valve is switched to isolate the plasma torch from the nebulizing system. The spray chamber is opened and the sample cell and the spray chamber, including its Teflont base, are changed (at least two sets should be available). The connection between the spray chamber and the desolvation unit is rinsed with 10 M HNO 3 followed by deionized water. 3 ml of 5 M HNO 3 are nebulized in order to clean out the
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Fig. 1. Schematic diagram of the sample introduction system used in this study. 1, interchangeable nebulizing cell; 2, sample solution and geyser-like aerosol plume; 3, piezo-electric transducer; 4, ultrasonic generator feed; 5, transducer cooling circuit; 6, PTFE base with argon carrier gas inlet; 7, spray chamber; 8, heated tube (1508C); 9, condenser (18C); 10, 3-way valve; 11, argon sheath gas.
desolvation unit and the tubing to the torch injector. This is accompanied by a strong increase of the U and Th signals which subsequently decrease. Next, 2 ml of a blank solution of 1 M HNO 3 are nebulized until a stable signal , 100 cps is reached for m/z = 238. The entire cleaning procedure takes about 15 min; subsequently, the next sample can be analysed.
settings were optimized in order to maximize signal intensity for 238U. Count rates in excess of 5 × 10 5 cps ppb −1 were obtained, whereas the typical sensitivity observed on this ICP-MS instrument ranged between 3 × 10 4 and 4 × 10 4 cps ppb −1 using a Meinhard
2.3. ICP-mass spectrometry
(a) Settings
A Plasmaquad PQII + Turbo was used, with the operating parameters given in Table 2(a). Lens
RF forward power Reflected power Argon flow rates outer auxiliary (b) Parameters
Table 1 Operating parameters for the geyser-type USN and the coupled desolvation unit Sample solution volume USN transducer current Sample consumption rate Argon carrier gas flow Argon sheath gas flow Desolvation unit Heating tube Condenser
3 ml 5 mA 0.2 ml min −1 0.18 l min −1 1.2 l min −1 1508C 18C
Table 2 (a) ICP settings and (b) data acquisition parameters
Acquisition mode Detection mode Dead time Selected peaks Number of points per peak Mass step between points Dwell time per point Settle time between peaks Number of sweeps Number of repeat integrations
1350 W ,5W 13.5 l min − 1 1.5 l min − 1 Peak hopping pulse counting 20 ns 230, 232, 235, 238 3 0.02 u 5.12 ms 5 ms 368 (30 s) 10
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concentric pneumatic nebulizer. The data acquisition parameters were selected with the aim of increasing the precision of isotopic ratio measurements [15] using a solution of the isotopic standard SRM U-500 from N.I.S.T. The best precision (0.21 6 0.04%, based on 10 measurements) was obtained using the conditions listed in Table 2(b). Compared with conventional thermal ionization mass spectrometers, ICP-sources exhibit relatively large instrumental mass bias. These are generally interpreted as reflecting space charge effects in the ion optics (between the sampling interface and the quadrupole mass filter) with mutual repulsion of positive ions and preferential loss of the lighter isotopes compared with the heavier isotopes [16]. In order to monitor this mass bias, a 2.5 mg l −1 solution of the isotopic standard SRM U-500 was analysed. This reference material has a certified 238U/ 235U ratio of 1.0003, with an accuracy better than 0.1%. It allows an optimal assessment of mass bias, because both isotopes have nearly identical abundances. This eliminates any possible cause of bias other than the three mass unit difference. Under the conditions used in this study, the mass bias varied between 0.5 and 0.8% per amu, at the expense of the lighter isotope. In contrast with uranium, no reference material is available for thorium. Based on the relatively small mass interval separating these two elements, the mass bias factor measured on SRM U-500 was used to correct raw thorium data [5]. During isotope dilution analyses, mass fractionation effects were monitored by measuring the SRM U-500 solution every four unknown samples. An average mass bias factor was obtained from each pair of data of the isotopic standard, and the raw 230Th/ 232Th and 235U/ 238U isotope ratios measured on the four intervening samples were corrected using this factor. 2.4. Analytical procedure The powdered SRMs were used as received. Subsamples of ca. 200 mg were accurately weighed on a Mettler AE 163 balance and spiked with a weighed aliquot of a mixed tracer solution of 230Th (83.8% enriched, from Oak Ridge National Laboratory, USA) and 235U (93.3% enriched, prepared from SRM U-930 of NIST). This tracer solution had been calibrated by reverse isotope dilution and TIMS and
contained 1.20 mg g −1 Th and 0.17 mg g −1 U. The rock powder was dissolved using a mixture of 2 ml 14 M HNO 3, 2 ml 29 M HF and 1 ml 12 M HClO 4 in closed Teflont PFA 15 ml vessels (Savillex, Minnetonka, Mi, USA) on a hot plate at ca. 708C overnight. After evaporation of excess acids and volatile SiF 4, the sample was taken to HClO 4 fumes in order to expel F and promote isotopic equilibration between the tracers and the U and Th contained in the rock samples. The residue was dissolved in 2 ml 7 M HNO 3 and evaporated to dryness. This was repeated twice. Finally, the sample was dissolved in 7 M HNO 3 for separation of U and Th from the matrix, in order to overcome the limited tolerance of ICP-MS to total dissolved solids, that may be particularly troublesome when using very efficient sample introduction systems such as the USN. Silica glass columns (10 mm i.d.) containing 4 ml of AG1X8 (200–400 mesh) resin (Bio-Rad S.A., Ivry-sur-Seine, France) were used. The sample solution was centrifuged, then loaded onto the column that had been preconditioned with 7 M HNO 3. Among common silicate rock-forming elements, only Th and U are retained by strongly basic anion-exchangers under these conditions [17]. The matrix elements were washed off with 7 ml of 7 M HNO 3, then U and Th were eluted sequentially with 5 ml H 2O and 5 ml of 6 M HCl, respectively. The U + Th fraction was evaporated to dryness then converted to nitrates by addition of 2 ml of 7 M HNO 3 followed by evaporation. Prior to ICP-MS measurement, the U + Th fraction was dissolved in 6 ml of 1 M HNO 3, of which 3 ml were transferred to the sample cell. After isolation of the plasma torch with the three-way valve located between the desolvation unit and the plasma, the cell was placed into the ultrasonic bath. The spray chamber was closed and the carrier gas flow allowed to purge residual air. Then, the nebulizing system was connected to the ICP using the three-way valve, and the transducer switched on. It took at least 2 min for the ICP-MS signal to reach steady state conditions enabling isotopic ratios to be measured over the following 8–10 min. During the last 3–4 min of the approximately 15 min cycle, the aerosol production became unstable due to the advanced stage of solution consumption in the sample cell, and the resulting ICP-MS signal was unusable.
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Six international rock standards have been analyzed in duplicate following the method outlined in Section 2. The standards were selected on the basis of their extremely low concentrations of U (between 0.01 and 0.4 mg g −1) and Th (between 0.03 and 0.7 mg g −1), which made them difficult or even impossible to analyze precisely by ID-ICP-MS using conventional pneumatic nebulization [5]. The results are given in Table 3, along with working values compiled by Govindaraju [18]. For comparison, data obtained on the same batches of rock powder using isotope dilution thermal ionization mass spectrometry [19], are provided for standards BIR-1, UBN-1 and JB-2.
sample consumption rates and concomitant steady aerosol production. In fact, the sample usage is not optimal because of the time delay before reaching a stable signal (at least 2 min) and late stage instabilities due to the reduced solution level in the nebulizing cell. Because of these early and late unsuitable stages, only about 60% of the sample solution can be effectively used for isotope ratio measurements. Despite this limitation, usable aerosol production is superior to those typical for most other nebulizer systems, that is a few percent for conventional pneumatic nebulizers and up to about 20–30% for continuous flow USNs and glass frit nebulizers (see, for example, Refs. [20,21]). Only direct injection nebulizers achieve superior efficiency [22].
3.1. Sample usage
3.2. Within-run precision
In contrast with most other nebulizers, including continuous flow USN, no waste is generated in the batch sampling mode, and most of the sample volume can be converted into a fine aerosol. The carrier gas flow-rate was kept low (0.18 l min −1). This was beneficial for signal stability and assured reduced
The within-run precision of sample analysis, expressed by the relative standard deviation (RSD) calculated from 10 repeat measurements of each standard studied, was between 0.24 and 0.85% for 235 U/ 238U and between 0.13 and 0.71% for 230 Th/ 232Th ratios. These RSD values are comparable
3. Results and discussion
Table 3 Results obtained on six international standard reference materials Sample
Measured concentrations of U
Working values for U
Measured concentrations of Th
Working values for Th
BIR-1
0.0117 6 2 0.0115 6 1
0.011 0.010 0.012 0.070 0.052 0.053 (0.1)
0.0273 6 14 0.0283 6 20
0.031 0.029 0.030 0.070 0.070 0.067 (0.2)
UB-N
DNC-1 JB-2
SRM 688 JA-1
0.0509 6 2 0.0521 6 2 0.0549 6 2 0.0560 6 4 0.150 6 1 0.153 6 1 0.298 6 2 0.305 6 2 0.343 6 6 0.353 6 2
0.16 0.15 a 0.15 a (0.37) 0.34
a a
a
0.0896 6 20 0.0877 6 12
a
0.238 6 2 0.241 6 4 0.248 6 1 0.251 6 1 0.346 6 2 0.351 6 4 0.713 6 8 0.704 6 4
a a
a a
0.33 0.25 a 0.25 a 0.33 6 0.02 0,82
a Ref. [19]. Concentrations are given in mg g −1, together with experimental uncertainty at the 95% confidence level. Working values (Ref. [18]), or certified concentration (for SRM 688), are given in bold, along with ID-TIMS results obtained on the same batches of powder for standards BIR-1, UB-N and JB-2 (Ref. [19]). Concentrations within brackets are information values only Sources of standards: JA-1 and JB-2, Geological Survey of Japan, Ibaraki, Japan; BIR-1 and DNC-1, U.S. Geological Survey, Reston, VA, U.S.A.; UB-N, A.N.R.T., Paris, France; SRM 688, N.I.S.T. (formerly N.B.S.), Gaithersburg, MD, USA.
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to that obtained during the analysis of the SRM U-500 solution (0.21 6 0.04%) and well in the range typical for quadrupole-based ICP-MS instruments used with conventional pneumatic nebulization. However, the observed RSD values were four to five times higher than the theoretical precision expected from counting statistics. This ratio of determined RSDs over counting statistics RSDs is about twice as large as that observed when using conventional pneumatic nebulization [15], showing that the batch-type USN evaluated in this study is noisier. 3.3. External precision and accuracy The reproducibility, or external precision of the method is acceptable, as shown by the U and Th concentration data obtained for the two separate dissolutions, which differ by only 1 to 3%, despite the very low concentration levels of the elements analyzed. The accuracy can be judged from the overall fair agreement of our data with working values [18] and with some ID-TIMS results obtained on the same standard batches [19]. For UB-N, both the TIMS and ICP-MS data for U cluster at 0.052 6 0.01 mg g −1, suggesting that the currently admitted working value (0.07 mg g −1) is overestimated. While our U data for SRMs JA-1 and JB-2 are in agreement with working values, the Th concentrations measured on these standards are 15–30% lower than expected. For JB-2, the consistency of ID-ICP-MS and ID-TIMS values lends support to the suggestion that working values for Th in these standards might be biased. In contrast, our Th concentrations in UB-N are about 25% higher than both the working values and IDTIMS results. The reason for this discrepancy is not yet clear, but it might reflect imperfect isotopic equilibration between the tracer and the sample because of fluorides formed during dissolution in such an Mgrich rock (MgO = 35 wt.%). Alternatively, the too high ICP-MS data might result from memory effects which are known to be a severe problem for Th (see, for example, Refs. [23,24]). 3.4. Blanks and memory effects The sample solution is in direct contact only with the nebulizing cell and spray chamber. These are changed for each sample and can be thoroughly
cleaned with appropriate reagents, i.e. HNO 3 containing 0.2% HF as a complexing agent [23]. However, the desolvation unit, the tubing, the torch and the sample interface can all contribute to memory effects that can be reduced only by means of the nebulization of a blank acid solution. This is time-consuming, and not necessarily the most efficient procedure, as may be inferred from the too high Th values found for the UB-N standard. As emphasized by Heumann [1], blank problems are a major limitation to the measurement of ultra-traces of elements by ID-MS. This problem is enhanced when ICP-MS is used, because the sample introduction system has a large volume that cannot be kept as clean as the disposable filaments used for sample loading in TIMS. 3.5. Suggestions for further improvement Several aspects of the described introduction system could be improved in order to make it more attractive for routine analyses. Firstly, the sample usage could be improved by extending the duration of stable nebulization and decreasing the minimum volume of solution needed. This could be achieved by using a smaller internal diameter nebulizing cell in order to reduce sample volume and by keeping the solution level as high as possible above the Mylar sheet constituting the bottom of the cell. Secondly, a smaller spray chamber would allow the dead volume of the system to be reduced. This might also provide more efficient entrainment of the aerosol using a small carrier gas flow rate, which might make the rise of the signal more rapid. Thirdly, the commercial ARL ultrasonic generator was used far below its nominal power, which may have caused some of the instabilities observed. The use of a dedicated, less powerful generator could possibly improve the overall stability of the system. Also, the use of degassed water in the transmitting bath [25] might reduce the instabilities due to the formation of air bubbles beneath the bottom of the nebulizing cell. Lastly, the length of time required to rinse the system between samples is as long as the analysis itself, mainly because rinsing the desolvation unit requires nebulization of acid for an extended time. To reduce the time between analyses it would be better if two separate desolvation units could be used alternately, with one in use while the other is cleaned.
S. Joannon et al./Spectrochimica Acta Part B 52 (1997) 1783–1789
4. Conclusion The geyser-type ultrasonic nebulizer is characterized by good sample usage compared to conventional pneumatic nebulizers and continuous flow USNs. Combined with isotope dilution ICP-MS, it permits the analysis of samples with very low concentrations of U and Th, which cannot be measured satisfactorily using conventional sample introduction. Although still in the development stage, the method outlined is superior to ID-TIMS in terms of throughput, since one analysis takes about 30 min, including system cleaning. This compares favourably with a typical TIMS run which requires filament preparation and loading, pumping down the mass spectrometer source then sequential measurement of U and Th isotopic ratios lasting for at least 2 h. This advantage is somewhat reduced, at least for Th, by possible memory effects of the desolvation unit, which constitute the major limitation of the method for ultra-trace analysis. Other highly efficient sample introduction systems such as the direct injection nebulizer [22] should be more attractive for this reason.
Acknowledgements We are grateful to Michel Condomines for providing the 230Th– 235U tracer and to Jeanne Se´range for help with sample preparation. Malcolm Roberts is thanked for correcting the english. The constructive reviews of Dr Epstein and an anonymous referee allowed significant improvements of the manuscript.
References
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(eds.), Inorganic Mass Spectrometry, Wiley, New York, 1988, p. 301. J.D. Fassett and P.J. Paulsen, Anal. Chem., 61 (1989) 643A. P. de Bie`vre, Anal. Proc., 30 (1993) 328. J.R Moody and M.S. Epstein, Spectrochim. Acta, 46B (1991) 1571–1575. C. Pin, S. Lacombe, Ph. Telouk and J.L. Imbert, Anal. Chim. Acta, 256 (1992) 153. R.H. Wendt and V.A. Fassel, Anal. Chem., 37 (1965) 920. C.D. West and D.N. Hume, Anal. Chem., 36 (1964) 412. M.B. Denton, J.M. Freelin and T.R. Smith, in: J. Sneddon (ed.), Sample introduction in atomic spectroscopy, Elsevier Science, Amsterdam, 1991, p. 73. V.A. Fassel and B.R. Bear, Spectrochim. Acta, 41B (1986) 1089. J.M. Mattinson, Anal. Chem., 44 (1972) 1716. J.M. Mermet and J.P. Robin, Anal. Chem., 40 (1968) 1918. J.M. Mermet, C. Trassy and P. Ripoche, in: R.M. Barnes (ed.), Developments in Atomic Plasma Spectrochemical Analysis, Heyden and Sons, 1981, p. 245. T.J. Johnson, P. Cassagne and D. Rupp, Intern. Lab., Sept. (1988) 30. C. Trassy and J.M. Mermet, Les applications analytiques des plasmas HF, Technique and Documentation Lavoisier, Paris, 1984, 328 pp. I.S Begley and B.L. Sharp, J. Anal. Atom. Spectrom., 9 (1994) 171. G.R. Gillson, D.J. Douglas, J.E. Fulford, K.W. Halligan and S.D. Tanner, Anal. Chem., 60 (1988) 1472. J.P. Faris and R.F. Buchanan, Anal. Chem., 36 (1964) 1158. K. Govindaraju, 1994 Compilation of Working Values and sample description for 383 geostandards. Geostandards Newsletters, Spec. Issue 18, 1994, 158 p. C. Pin and J.F. Santos Zalduegui, Analytica Chimica Acta, 339 (1997) 79–89. E. Michaud-Poussel and J.M. Mermet, Spectrochim. Acta, 41B (1986) 49–61. T.R. Smith and M.B. Denton, Applied Spectrosc., 44 (1990) 21–24. D.R. Wiederin, F.G. Smith and R.S. Houk, Anal. Chem., 63 (1991) 219. H. Bauman and J. Pavel, Mikrochim. Acta III, (1989) 413. C. Vandecasteele, G. Wauters, R. Dams and W. de Jonghe, J. Anal. Atom. Spectrom., 4 (1989) 461. R.H. Clifford, A. Montaser, S.P. Dolan and S.G. Capar, Anal. Chem., 62 (1990) 2745.
Spectrochimica Acta Part B 52 (1997) 1783–1789
Determination of U and Th at ultra-trace levels by isotope dilution inductively coupled plasma mass spectrometry using a geyser-type ultrasonic nebulizer: application to geological samples Sylviane Joannon a,b, Philippe Telouk1b, Christian Pin a a
De´partement de Ge´ologie, U.R.A. 10 C.N.R.S., Centre Re´ gional de Mesures Physiques, Universite´ Blaise Pascal, 5, rue Kessler 63038, Clermont-Ferrand, France b Service Central d’Analyse du C.N.R.S., Echangeur de Solaize, BP 22 69390 Vernaison, France Received 9 January 1997; accepted 19 May 1997
Abstract In this report, the suitability of an ultrasonic nebulizer (USN) used in batch-type sampling mode is evaluated for the isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) analysis of U and Th at the sub-mg g −1 level in geological materials. Compared to conventional pneumatic nebulizers, the system is characterized by high efficiency, highlighted by count rates in excess of 5 × 10 5 cps for 1 ng g −1 of 238U, and good sample usage, with about 60% of the solution converted into a usable aerosol. Although the RSD values observed for the isotopic ratios 230Th/ 232Th and 235U/ 238U were four to five times higher than the theoretical precision expected from counting statistics, within-run precisions are in the range typical for quadrupole-based ICP-MS instruments. The U and Th concentration data obtained for two separate solutions of six international silicate rock reference materials differ by only 1–3%, despite their very low concentrations (between 0.01 and 0.4 mg g −1 of U and between 0.03 and 0.7 mg g −1 of Th), which made them difficult or even impossible to analyse precisely by ID-ICP-MS using conventional pneumatic nebulization. Although one analysis including system cleaning takes about 30 min, the method outlined is still superior to ID-TIMS in terms of throughput. q 1997 Elsevier Science B.V. Keywords: ICP-MS; Ultrasonic nebulizer; Isotope dilution; U; Th; Geostandard
1. Introduction Isotope dilution mass spectrometry (ID-MS) is a highly sensitive method, capable of high precision and high accuracy [1–3]. In this technique, chemical manipulations are carried out on a direct weight basis and the mass spectrometric determinations involve only isotopic ratio data. Therefore, ID-MS can be 1 Present address: De´partement des Sciences de la Terre, Ecole Normale Supe´rieure de Lyon, 46, alle´e d’Italie 69634 Lyon 07, France.
directly related to fundamental units of measurement, and it can be considered a definitive method for quantitative elemental analysis [4]. Compared to conventional thermal ionization mass spectrometry (TIMS), inductively coupled plasma mass spectrometry (ICPMS) has two major advantages: direct liquid sampling at atmospheric pressure and the capability of measuring isotope ratios without extensive chemical separation of the elements analysed, which makes routine application of ID-MS much easier. In a previous study, the potential of ID-ICP-MS for the analysis of U and Th in geological standard reference materials
0584-8547/97/$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 7 )0 0 07 2 -4
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was evaluated [5]. Although most of the results were in excellent agreement with recommended values, it was noted that the most depleted standards (with concentrations below 0.5 mg g −1) could not be measured satisfactorily. This limitation of the ICP-MS approach to isotope dilution analysis at a very low concentration level was largely caused by the poor efficiency of the conventional pneumatic nebulizer used. In an attempt to overcome this problem, the possibilities offered by an ultrasonic nebulizer (USN) used in the batch-type sampling mode were explored. In this design, used in the early days of ICP spectrometry [6,7], the sample solution is contained in a nebulizing cell coupled to the ultrasonic transducer by a water transmitting bath. These batch- or geyser-type USNs have some disadvantages [8], including rather difficult sample change and poorer long-term precision than their continuously fed counterparts which have been more extensively developed (see, for example, Ref. [9]) and are commercially available. However, where long-term precision is not of vital importance and sample solutions do not need to be changed frequently, as is the case for isotope ratio measurements, batch-type USNs could provide an interesting nebulizing system because of dense aerosol production and efficient sample usage, since no waste solution is generated. This paper describes the experimental set-up on this basis and its application to the analysis of ultra-trace amounts of U and Th in a set of silicate rock standard reference materials.
2. Experimental 2.1. Reagents 18 MQ cm water purified through a Milli-Q system (Millipore S.A., Molsheim) was used throughout. Reagent grade perchloric acid (Prolabo, Paris) was used without further purification. Nitric (7 M) and hydrochloric (6 M) acids, both of reagent grade (Prolabo, Paris), were distilled in sub-boiling silica glass stills (Model PB5, Quartex, Paris). Hydrofluoric acid (29 M) was purified using a subboiling distillation system made of two Teflont FEP bottles (Nalgene, Rochester, NY) connected by a homemade Teflont PTFE block following the design of Mattinson [10].
2.2. Geyser-type ultrasonic nebulizer The batch-type USN basically follows the design of Mermet and coworkers [11,12] and was built by modifying a commercially available [13] continuous flow USN (Applied Research Laboratories, Ecublens, Switzerland). It consists of a piezo-electric crystal (transducer) energized at 1.4 MHz by a radiofrequency generator. In contrast with the original ARL instrument, the transducer is positioned horizontally and coupled by a water bath to an interchangeable sample cell made of a silica glass tube (20 mm i.d.) with a Mylar sheet (6 mm thickness) at the bottom sealed with epoxy. The transducer is cooled by circulating water (temperature controlled to 1 6 0.18C) to improve stability of the aerosol generation rate. A chimney-like borosilicate glass spray chamber with an axial impact bead is held on a Teflont PTFE base containing the carrier gas inlet (Fig. 1). This argon flow sweeps the aerosol cloud into the desolvation unit of the ARL USN, which is used without modification, except that it is held vertically. This consists of a heated (1508C) tube followed by a condenser (18C) which separates the dried aerosol from most of the solvent. This batch-sampling mode USN allows large drops and any condensation on the walls of the spray chamber to be recycled in the sample cell, instead of being directed to waste through a drain. The exit of the desolvation unit is connected to the ICP by a 1 m long 3/16 inch i.d. Tygon tubing fitted with a three-way valve enabling the plasma to be isolated when opening the nebulization system for changing the sample cell. An argon sheath gas [14], obtained from Jobin-Yvon S.A. (Longjumeau, France) and placed just before the inlet into the plasma torch, was found to be useful for absorbing pressure fluctuations produced in the desolvating system and stabilize the ICP-MS signal. Operating parameters of the nebulizer–desolvation system are listed in Table 1. For sample changeover, the three-way valve is switched to isolate the plasma torch from the nebulizing system. The spray chamber is opened and the sample cell and the spray chamber, including its Teflont base, are changed (at least two sets should be available). The connection between the spray chamber and the desolvation unit is rinsed with 10 M HNO 3 followed by deionized water. 3 ml of 5 M HNO 3 are nebulized in order to clean out the
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Fig. 1. Schematic diagram of the sample introduction system used in this study. 1, interchangeable nebulizing cell; 2, sample solution and geyser-like aerosol plume; 3, piezo-electric transducer; 4, ultrasonic generator feed; 5, transducer cooling circuit; 6, PTFE base with argon carrier gas inlet; 7, spray chamber; 8, heated tube (1508C); 9, condenser (18C); 10, 3-way valve; 11, argon sheath gas.
desolvation unit and the tubing to the torch injector. This is accompanied by a strong increase of the U and Th signals which subsequently decrease. Next, 2 ml of a blank solution of 1 M HNO 3 are nebulized until a stable signal , 100 cps is reached for m/z = 238. The entire cleaning procedure takes about 15 min; subsequently, the next sample can be analysed.
settings were optimized in order to maximize signal intensity for 238U. Count rates in excess of 5 × 10 5 cps ppb −1 were obtained, whereas the typical sensitivity observed on this ICP-MS instrument ranged between 3 × 10 4 and 4 × 10 4 cps ppb −1 using a Meinhard
2.3. ICP-mass spectrometry
(a) Settings
A Plasmaquad PQII + Turbo was used, with the operating parameters given in Table 2(a). Lens
RF forward power Reflected power Argon flow rates outer auxiliary (b) Parameters
Table 1 Operating parameters for the geyser-type USN and the coupled desolvation unit Sample solution volume USN transducer current Sample consumption rate Argon carrier gas flow Argon sheath gas flow Desolvation unit Heating tube Condenser
3 ml 5 mA 0.2 ml min −1 0.18 l min −1 1.2 l min −1 1508C 18C
Table 2 (a) ICP settings and (b) data acquisition parameters
Acquisition mode Detection mode Dead time Selected peaks Number of points per peak Mass step between points Dwell time per point Settle time between peaks Number of sweeps Number of repeat integrations
1350 W ,5W 13.5 l min − 1 1.5 l min − 1 Peak hopping pulse counting 20 ns 230, 232, 235, 238 3 0.02 u 5.12 ms 5 ms 368 (30 s) 10
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concentric pneumatic nebulizer. The data acquisition parameters were selected with the aim of increasing the precision of isotopic ratio measurements [15] using a solution of the isotopic standard SRM U-500 from N.I.S.T. The best precision (0.21 6 0.04%, based on 10 measurements) was obtained using the conditions listed in Table 2(b). Compared with conventional thermal ionization mass spectrometers, ICP-sources exhibit relatively large instrumental mass bias. These are generally interpreted as reflecting space charge effects in the ion optics (between the sampling interface and the quadrupole mass filter) with mutual repulsion of positive ions and preferential loss of the lighter isotopes compared with the heavier isotopes [16]. In order to monitor this mass bias, a 2.5 mg l −1 solution of the isotopic standard SRM U-500 was analysed. This reference material has a certified 238U/ 235U ratio of 1.0003, with an accuracy better than 0.1%. It allows an optimal assessment of mass bias, because both isotopes have nearly identical abundances. This eliminates any possible cause of bias other than the three mass unit difference. Under the conditions used in this study, the mass bias varied between 0.5 and 0.8% per amu, at the expense of the lighter isotope. In contrast with uranium, no reference material is available for thorium. Based on the relatively small mass interval separating these two elements, the mass bias factor measured on SRM U-500 was used to correct raw thorium data [5]. During isotope dilution analyses, mass fractionation effects were monitored by measuring the SRM U-500 solution every four unknown samples. An average mass bias factor was obtained from each pair of data of the isotopic standard, and the raw 230Th/ 232Th and 235U/ 238U isotope ratios measured on the four intervening samples were corrected using this factor. 2.4. Analytical procedure The powdered SRMs were used as received. Subsamples of ca. 200 mg were accurately weighed on a Mettler AE 163 balance and spiked with a weighed aliquot of a mixed tracer solution of 230Th (83.8% enriched, from Oak Ridge National Laboratory, USA) and 235U (93.3% enriched, prepared from SRM U-930 of NIST). This tracer solution had been calibrated by reverse isotope dilution and TIMS and
contained 1.20 mg g −1 Th and 0.17 mg g −1 U. The rock powder was dissolved using a mixture of 2 ml 14 M HNO 3, 2 ml 29 M HF and 1 ml 12 M HClO 4 in closed Teflont PFA 15 ml vessels (Savillex, Minnetonka, Mi, USA) on a hot plate at ca. 708C overnight. After evaporation of excess acids and volatile SiF 4, the sample was taken to HClO 4 fumes in order to expel F and promote isotopic equilibration between the tracers and the U and Th contained in the rock samples. The residue was dissolved in 2 ml 7 M HNO 3 and evaporated to dryness. This was repeated twice. Finally, the sample was dissolved in 7 M HNO 3 for separation of U and Th from the matrix, in order to overcome the limited tolerance of ICP-MS to total dissolved solids, that may be particularly troublesome when using very efficient sample introduction systems such as the USN. Silica glass columns (10 mm i.d.) containing 4 ml of AG1X8 (200–400 mesh) resin (Bio-Rad S.A., Ivry-sur-Seine, France) were used. The sample solution was centrifuged, then loaded onto the column that had been preconditioned with 7 M HNO 3. Among common silicate rock-forming elements, only Th and U are retained by strongly basic anion-exchangers under these conditions [17]. The matrix elements were washed off with 7 ml of 7 M HNO 3, then U and Th were eluted sequentially with 5 ml H 2O and 5 ml of 6 M HCl, respectively. The U + Th fraction was evaporated to dryness then converted to nitrates by addition of 2 ml of 7 M HNO 3 followed by evaporation. Prior to ICP-MS measurement, the U + Th fraction was dissolved in 6 ml of 1 M HNO 3, of which 3 ml were transferred to the sample cell. After isolation of the plasma torch with the three-way valve located between the desolvation unit and the plasma, the cell was placed into the ultrasonic bath. The spray chamber was closed and the carrier gas flow allowed to purge residual air. Then, the nebulizing system was connected to the ICP using the three-way valve, and the transducer switched on. It took at least 2 min for the ICP-MS signal to reach steady state conditions enabling isotopic ratios to be measured over the following 8–10 min. During the last 3–4 min of the approximately 15 min cycle, the aerosol production became unstable due to the advanced stage of solution consumption in the sample cell, and the resulting ICP-MS signal was unusable.
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Six international rock standards have been analyzed in duplicate following the method outlined in Section 2. The standards were selected on the basis of their extremely low concentrations of U (between 0.01 and 0.4 mg g −1) and Th (between 0.03 and 0.7 mg g −1), which made them difficult or even impossible to analyze precisely by ID-ICP-MS using conventional pneumatic nebulization [5]. The results are given in Table 3, along with working values compiled by Govindaraju [18]. For comparison, data obtained on the same batches of rock powder using isotope dilution thermal ionization mass spectrometry [19], are provided for standards BIR-1, UBN-1 and JB-2.
sample consumption rates and concomitant steady aerosol production. In fact, the sample usage is not optimal because of the time delay before reaching a stable signal (at least 2 min) and late stage instabilities due to the reduced solution level in the nebulizing cell. Because of these early and late unsuitable stages, only about 60% of the sample solution can be effectively used for isotope ratio measurements. Despite this limitation, usable aerosol production is superior to those typical for most other nebulizer systems, that is a few percent for conventional pneumatic nebulizers and up to about 20–30% for continuous flow USNs and glass frit nebulizers (see, for example, Refs. [20,21]). Only direct injection nebulizers achieve superior efficiency [22].
3.1. Sample usage
3.2. Within-run precision
In contrast with most other nebulizers, including continuous flow USN, no waste is generated in the batch sampling mode, and most of the sample volume can be converted into a fine aerosol. The carrier gas flow-rate was kept low (0.18 l min −1). This was beneficial for signal stability and assured reduced
The within-run precision of sample analysis, expressed by the relative standard deviation (RSD) calculated from 10 repeat measurements of each standard studied, was between 0.24 and 0.85% for 235 U/ 238U and between 0.13 and 0.71% for 230 Th/ 232Th ratios. These RSD values are comparable
3. Results and discussion
Table 3 Results obtained on six international standard reference materials Sample
Measured concentrations of U
Working values for U
Measured concentrations of Th
Working values for Th
BIR-1
0.0117 6 2 0.0115 6 1
0.011 0.010 0.012 0.070 0.052 0.053 (0.1)
0.0273 6 14 0.0283 6 20
0.031 0.029 0.030 0.070 0.070 0.067 (0.2)
UB-N
DNC-1 JB-2
SRM 688 JA-1
0.0509 6 2 0.0521 6 2 0.0549 6 2 0.0560 6 4 0.150 6 1 0.153 6 1 0.298 6 2 0.305 6 2 0.343 6 6 0.353 6 2
0.16 0.15 a 0.15 a (0.37) 0.34
a a
a
0.0896 6 20 0.0877 6 12
a
0.238 6 2 0.241 6 4 0.248 6 1 0.251 6 1 0.346 6 2 0.351 6 4 0.713 6 8 0.704 6 4
a a
a a
0.33 0.25 a 0.25 a 0.33 6 0.02 0,82
a Ref. [19]. Concentrations are given in mg g −1, together with experimental uncertainty at the 95% confidence level. Working values (Ref. [18]), or certified concentration (for SRM 688), are given in bold, along with ID-TIMS results obtained on the same batches of powder for standards BIR-1, UB-N and JB-2 (Ref. [19]). Concentrations within brackets are information values only Sources of standards: JA-1 and JB-2, Geological Survey of Japan, Ibaraki, Japan; BIR-1 and DNC-1, U.S. Geological Survey, Reston, VA, U.S.A.; UB-N, A.N.R.T., Paris, France; SRM 688, N.I.S.T. (formerly N.B.S.), Gaithersburg, MD, USA.
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to that obtained during the analysis of the SRM U-500 solution (0.21 6 0.04%) and well in the range typical for quadrupole-based ICP-MS instruments used with conventional pneumatic nebulization. However, the observed RSD values were four to five times higher than the theoretical precision expected from counting statistics. This ratio of determined RSDs over counting statistics RSDs is about twice as large as that observed when using conventional pneumatic nebulization [15], showing that the batch-type USN evaluated in this study is noisier. 3.3. External precision and accuracy The reproducibility, or external precision of the method is acceptable, as shown by the U and Th concentration data obtained for the two separate dissolutions, which differ by only 1 to 3%, despite the very low concentration levels of the elements analyzed. The accuracy can be judged from the overall fair agreement of our data with working values [18] and with some ID-TIMS results obtained on the same standard batches [19]. For UB-N, both the TIMS and ICP-MS data for U cluster at 0.052 6 0.01 mg g −1, suggesting that the currently admitted working value (0.07 mg g −1) is overestimated. While our U data for SRMs JA-1 and JB-2 are in agreement with working values, the Th concentrations measured on these standards are 15–30% lower than expected. For JB-2, the consistency of ID-ICP-MS and ID-TIMS values lends support to the suggestion that working values for Th in these standards might be biased. In contrast, our Th concentrations in UB-N are about 25% higher than both the working values and IDTIMS results. The reason for this discrepancy is not yet clear, but it might reflect imperfect isotopic equilibration between the tracer and the sample because of fluorides formed during dissolution in such an Mgrich rock (MgO = 35 wt.%). Alternatively, the too high ICP-MS data might result from memory effects which are known to be a severe problem for Th (see, for example, Refs. [23,24]). 3.4. Blanks and memory effects The sample solution is in direct contact only with the nebulizing cell and spray chamber. These are changed for each sample and can be thoroughly
cleaned with appropriate reagents, i.e. HNO 3 containing 0.2% HF as a complexing agent [23]. However, the desolvation unit, the tubing, the torch and the sample interface can all contribute to memory effects that can be reduced only by means of the nebulization of a blank acid solution. This is time-consuming, and not necessarily the most efficient procedure, as may be inferred from the too high Th values found for the UB-N standard. As emphasized by Heumann [1], blank problems are a major limitation to the measurement of ultra-traces of elements by ID-MS. This problem is enhanced when ICP-MS is used, because the sample introduction system has a large volume that cannot be kept as clean as the disposable filaments used for sample loading in TIMS. 3.5. Suggestions for further improvement Several aspects of the described introduction system could be improved in order to make it more attractive for routine analyses. Firstly, the sample usage could be improved by extending the duration of stable nebulization and decreasing the minimum volume of solution needed. This could be achieved by using a smaller internal diameter nebulizing cell in order to reduce sample volume and by keeping the solution level as high as possible above the Mylar sheet constituting the bottom of the cell. Secondly, a smaller spray chamber would allow the dead volume of the system to be reduced. This might also provide more efficient entrainment of the aerosol using a small carrier gas flow rate, which might make the rise of the signal more rapid. Thirdly, the commercial ARL ultrasonic generator was used far below its nominal power, which may have caused some of the instabilities observed. The use of a dedicated, less powerful generator could possibly improve the overall stability of the system. Also, the use of degassed water in the transmitting bath [25] might reduce the instabilities due to the formation of air bubbles beneath the bottom of the nebulizing cell. Lastly, the length of time required to rinse the system between samples is as long as the analysis itself, mainly because rinsing the desolvation unit requires nebulization of acid for an extended time. To reduce the time between analyses it would be better if two separate desolvation units could be used alternately, with one in use while the other is cleaned.
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4. Conclusion The geyser-type ultrasonic nebulizer is characterized by good sample usage compared to conventional pneumatic nebulizers and continuous flow USNs. Combined with isotope dilution ICP-MS, it permits the analysis of samples with very low concentrations of U and Th, which cannot be measured satisfactorily using conventional sample introduction. Although still in the development stage, the method outlined is superior to ID-TIMS in terms of throughput, since one analysis takes about 30 min, including system cleaning. This compares favourably with a typical TIMS run which requires filament preparation and loading, pumping down the mass spectrometer source then sequential measurement of U and Th isotopic ratios lasting for at least 2 h. This advantage is somewhat reduced, at least for Th, by possible memory effects of the desolvation unit, which constitute the major limitation of the method for ultra-trace analysis. Other highly efficient sample introduction systems such as the direct injection nebulizer [22] should be more attractive for this reason.
Acknowledgements We are grateful to Michel Condomines for providing the 230Th– 235U tracer and to Jeanne Se´range for help with sample preparation. Malcolm Roberts is thanked for correcting the english. The constructive reviews of Dr Epstein and an anonymous referee allowed significant improvements of the manuscript.
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