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Solid sampling electrothermal vaporization for sample introduction in inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry
Spectrochimica Acta, Vol. 50B, Nos. 4--7, pp. 463-475, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0584-8547/95 $9.50 + .00
Pergamon 0584-8547(94)00153-7
Solid sampling electrothermal vaporization for sample introduction in inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry* L. MOENSt, P. VERREPT, S. BOONEN, F. VANHAECKE and R. DAMS Laboratory of Analytical Chemistry, Ghent University, Proeftuinstraat 86, B-9000 Ghent, Belgium (Received 24 July 1994; accepted 27 October 1994) Abstract--Solid sampling using electrothermal vaporization is an attractive sample introduction method for atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). For AAS, the method is well established. The techniques needed to apply SS-ETV in ICP-based methods are described, with the emphasis on the coupling of different types of ETV-devices to the inductively coupled plasma torch and on the requirements for the spectrometer and the data acquisition and handling system. Though standardization is not straightforward, it is shown that standard addition and external calibration with solid standards yield accurate results. The latter is demonstrated by the analysis of standard reference materials. Figures of merit for SS-ETV-ICP-AES and SS-ETV-ICP-MS are presented. The literature concerning ICP-AES and ICP-MS (methods and applications) is briefly reviewed and new results of SS-ETV-ICP-MS analysis of SRMs are presented.
1. INTRODUCTION THOUGH pneumatic nebulization is widely used and generally accepted as a most convenient sample introduction m e t h o d in inductively coupled plasma atomic emission spectrometry ( I C P - A E S ) and inductively coupled plasma mass spectrometry ( I C P - M S ) , it is far f r o m an ideal method. It should indeed be possible to introduce solid, liquid and gaseous materials without any sample p r e t r e a t m e n t and m o r e o v e r 100% of the amount of analyte entering the sample introduction system should reach the plasma whereas interfering matrix elements should be removed. Obviously such a system will probably never be produced. Pneumatic nebulization falls short of this ideal in that only liquid samples or sample solutions can be introduced and that transport efficiency is p o o r ( 1 - 2 % ) . In addition, the solvent and the sample matrix can reach the plasma and give rise to spectral and non-spectral interferences. Several methods have b e e n developed to improve the transport efficiency of sample aerosols and/or to r e m o v e part of the solvent before the aerosol reaches the plasma: e.g. ultrasonic nebulization [1, 2], high pressure nebulization [3], t h e r m o s p r a y [4], direct insertion nebulization [5] and total desolvation [6]. Direct ICP analysis of solid materials is possible using for instance slurry nebulization [7-10], laser ablation [11-13], direct sample insertion [14-19], direct powder introduction [20-23] and electrothermal vaporization ( E T V ) [24]. In this work the t e r m solid sampling E T V (SS-ETV) will refer to solid sampling, in which a small sample is weighed, as such transferred to the E T V unit and vaporized. Solid samples can also be m a d e into slurries that can be pipetted into the E T V unit. Slurry E T V has been used with A A S [25-29], I C P - A E S [30, 31] and I C P - M S [32, 33], but will not be discussed here. With E T V , small volumes of both solids and solutions can be sampled and the transport of the sampled materials is very efficient. The technique m o r e o v e r allows the solvent and the matrix material to be fully or partly r e m o v e d thus keeping these * This paper was published in the Special Issue on Sample Introduction in Atomic Spectrometry. t Author to whom correspondence should be addressed. 463
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from entering the plasma and causing unwanted effects. A fundamental advantage of E T V is that the vaporization of the sample is separated in time and space f r o m the atomization, excitation and ionization processes, allowing separate optimization. E T V being the m e t h o d of choice for sample introduction of solutions in atomic absorption spectrometry (AAS), some of the vast experience obtained in this field can be transferred to ICP methods [34]. This implies methods to r e m o v e matrix elements and enhance the volatilization of analyte elements. Also SS-ETV has been widely applied in A A S and ample information is available [35]. However, solid sampling in general and SS-ETV in particular has a n u m b e r of drawbacks hampering its m o r e general application. A m a j o r p r o b l e m is standardization. Indeed, the physical and chemical situation of the analyte elements in different solid samples can be very different. T h e r e f o r e the evaporation process can proceed at a largely different rate and with different efficiency and standards should strongly resemble samples in matrix composition, which is an obvious limitation to the flexibility and the general applicability of the method. In this p a p e r the technique of SS-ETV for I C P - A E S and I C P - M S will briefly be described. Major developments and applications will be reviewed, advantages and limitations will be discussed and illustrated and recent developments will be presented.
2. APPARATUS AND TECHNIQUES 2.1. The S S - E T V furnace
For ETV of solutions, several types of device can be used, such as metal strips or coils [36, 37] and graphite rods or furnaces. SS-ETV however requires that relatively large amounts of sample powder (up to several mg) can be loaded conveniently into the vaporization device and that the latter can be heated in such a way that the sample powder is vaporized rapidly and completely. Therefore only graphite furnaces have been used for SS-ETV combined with ICP-AES. A first type of graphite furnaces suited for SS-ETV was designed to fit directly under the plasma torch (an example is shown in Fig. 1). An obvious advantage of these furnaces is the excellent efficiency of the aerosol transport. For loading, the furnace block can be shifted sideways or downwards while an auxiliary Ar gas flow keeps air from entering the torch. The design has been used and improved by several researchers [38, 39]. With this type of furnace, sample handling is rather difficult. The second type of SS-ETV furnace is the flow through type (an example is shown in Fig. 2). The oven consists of a graphite tube that is placed at a distance from the ICP torch and samples are transferred by an Ar gas flow through the oven. A transfer tube made out of glass or a plastic material (e.g. tygon) connects the furnace to the ICP torch. Dry aerosols, as produced by an SS-ETV oven, can efficiently be transported over long distances (0.5 m to 20.5 m) [40, 41]. Since the samples are loaded in sample boats that can easily be inserted in the furnace, this type of furnace is also called the "boat-in-tube" type. Tarred sample boats are used to replace weighing flasks, thus eliminating errors due to incomplete transfer of the weighed mass to the furnace. Moreover, sample boats act as L'vov platforms during evaporation. As in atomic absorption spectrometry (AAS), special temperature programs can be used for drying, ashing and evaporating the samples and cleaning the furnace after each sample. The solvent and the matrix elements set free during desolvation and ashing, respectively, can be kept from entering the plasma and are vented via a venting valve between the furnace an the plasma [42]. This is especially important in ICP-MS, but also in ICP-AES large amounts of solvent or matrix material can cause memory effects. Matrix modifiers can be used to enhance the volatilization during ashing of matrix elements, otherwise causing matrix effects. Results reported in this paper were obtained with the aid of the Griin Optics furnace (Griin Analytische Mess-Systeme G.m.b.H., Wetzlar, Germany), schematically shown in Fig. 2. This device was designed for AAS and required some adaptations to make it useful for ICP-AES and ICP-MS [43]. One side had to be closed with a shutter that can be opened to introduce the sample boat. Gas flows had to be changed in such a way that, during the loading of the furnace, Ar is flowing in the opposite direction of the carrier gas flow to keep air from entering the furnace and destabilising the plasma. A similar furnace was used by KANTORet al. [44] who succeeded in improving the transport efficiency by adding a cold Ar flow in counterflow with
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the transport gas during vaporization [45]. Rapid cooling of the atoms formed enhances the formation of condensation nuclei and yields a better transportable aerosol [46, 47]. In view of temperature gradients in the furnace, reproducible positioning of the boat in the furnace is crucial. Therefore a pair of tweezers slides on a rail rigidly mounted in front of the furnace. A glass, PVC or tygon transfer line of about 50 cm and a diameter of about 0.5 cm was used to couple the oven to the ICP. Typical operation parameters for the furnace used with I C P - A E S and I C P - M S are included in Table 1. 2.2. Signal handling ETV produces short (3-10 s) transient signals. In E T V - I C P - A E S the spectral background is transient as well, requiring simultaneous recording of the peak and background signals during the actual measurement. According to BOOMANS [48] accurate background information "must be derived from wavelength scans centred about the analysis line and extended over a wavelength interval of at least several times the spectral bandwidth of the apparatus". Several devices have been used for this purpose; three of these will be mentioned here. A wavelength scan over a narrow wavelength interval can be performed by oscillating the grating over a small angle. In common spectrometers however, such movement is relatively slow and the method is not suited for fast transient signals. A n obvious solution to this problem is to use a fast scanning device that allows quasi-simultaneous measurements of peak and background emission intensities. Presently, spectrometers are available that are equipped with magnetically or galvanometrically driven gratings, scanning over a range of 600 nm in less than 20 ms, which satisfies the above mentioned requirements. Another approach consists in positioning an oscillating refractor plate behind the entrance slit of the spectrometer. The refractor plate acts as a wavelength modulator and, when mounted on a high speed galvanometric motor, small wavelength intervals of around 0.2 nm can be scanned at a rate of up to 80 Hz. The scan speed will depend on the number of acquisition points within the interval, which is typically about 80, and the dwell time. These conditions allow a transient ETV signal (3 to 10 s long) to be scanned at a rate of 1-80 Hz and to collect tens of spectra while analyte atoms and ions are present in the plasma. Results shown in this work were obtained with the oscillating refractor plate technology [49, 50], which can be implemented at low cost in almost every spectrometer. Excellent devices for handling fast transient signals are solid state array detectors such as photodiode array detectors and charge transfer devices (charged coupled devices, CCD, and charge injection devices, CID). With this equipment, which is rather expensive, 30 frames Can be read out each second, which is a largely sufficient for E T V - I C P - A E S . Data handling should consist in performing appropriate background correction for each individual spectrum. For background correction of wing interferences a quadratic fit of the background in general will yield better results than a linear one [43]. The net signal can be plotted versus time and integrated [50]. For I C P - M S , background correction is less problematic because the continuous photon background is low and hardly influenced by the matrix of the sample. Moreover the scanning speed of quadrupoles is largely sufficient to register the signal at and around the mass peak and multi-element analysis is possible. 2.3. Special procedures As mentioned before, some of the experience gained in E T V - A A S can be transferred to E T V - I C P methods. S S - E T V - A A S , though much less applied than E T V - A A S of solutions, has been intensively used as well [35]. Selective vaporization of the matrix in the ashing step is possible if the volatility of matrix and analyte elements is largely different. However, in view of the complexity of solid samples, this condition is not obviously fulfilled, Pyrolysis of samples consisting of organic compounds [51, 52] is one example of selective matrix decomposition and it is also possible to add matrix modifiers to transform the matrix into more volatile compounds. To allow less volatile analyte elements to be vaporized or to decompose refractory materials both gaseous and solid matrix modifiers have been proposed. The addition of halocarbon gases to the A r carrier flow has become a widely applied method in S S - E T V - I C P - A E S . KIRI~BPaGNr and SNooK [53] introduced this method for the determination of refractory elements such as Zr, B, Cr, Mo and W. They added 0.1% of CHF3 to the Ar. Other halogenating gases have been used as well and for instance CC14 and Freon 12 were added to determine trace elements in ceramic powders and SiC, respectively [54, 55]. Different types of solid, halogenating matrix modifiers were successfully used as well. OnLS and Ht~rscn [38] added Teflon powder to lubricating oils to improve the vaporization of
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refractory elements. To decompose refractory matrices solid modifiers were frequently used. SiO2-, A1203- and MgO-based ceramics were analyzed for Fe after addition of AgCl + BaCI2 [56], for the analysis of graphite, (C2F4)n + NaF was added [57] and B4C, SiC and SiaN4 were analyzed with SS-ETV-ICP-AES after addition of CoF2 + BaO [58]. The latter modifier was also used to determine carbide forming elements. 2.4. Calibration Like for most solid sampling techniques, standardization is a problem for SS-ETV-ICP analysis and different methods have been applied. External calibration with standard solutions is unsuited for most matrices since the vaporization, transfer and plasma conditions for the solid sample and the standard solution are very different [59, 60]. External calibration with solid standards yields accurate results on condition that the composition of sample and standard are similar [43, 61]. This limits the applicability of the method since standard reference materials are not available for every possible type of sample. However this method can be very useful in routine analysis of large numbers of very similar samples where a large batch of a typical sample material can be analyzed with independent methods to serve as a standard material. Standard addition with standard solutions is probably the most versatile method available. The difference in vaporization and transport efficiencies between solid sample and standard solution largely disappears when the sample solution is transferred to the sample holder and dried before the solid sample is loaded into the holder [59]. In SS-ETV-ICP-AES both the single standard addition method and the generalized simplified standard addition method (GSAM), developed for AAS [62, 63], have been applied successfully [59].
3. RESULTS AND DISCUSSION
3.1. Figures of merit in 1 C P - A E S 3.1.1. Precision. The mass of the sample that can be loaded into an SS-ETV unit is limited to 0.5-10 mg depending on the matrix composition. This is due to the limited space of the sample boat and/or the furnace and to the fact that in SS-ETV a lot of material is transported to the plasma in a very short time, which can lead to plasma instability or even to the extinction of the plasma. The inhomogeneity of the material to be analyzed is a limiting factor for the precision of SS-ETV and it is necessary to analyze a large n u m b e r of subsamples. In the analysis of, for instance, standard reference materials an overall precision of the order of 5 - 1 5 % is feasible. T h o u g h disadvantageous for most analyses, the small sample sizes of SS-ETV can be very useful in homogeneity studies [64]. 3.1.2. Detection limits. For elements with a relatively high concentration, absolute determination limits are determined by the lowest sample mass that can accurately be weighed. For low concentrations, limits of detection should be calculated according to the I U P A C definition [65], stating that the limit of detection is given by ksB where sB is the concentration equivalent of the standard deviation on a large n u m b e r of m e a s u r e m e n t s of the blank signal and k is a constant (for a 95% confidence level, k = 3). This definition requires that an appropriate blank can be measured which is a p r o b l e m in solid sampling. Indeed in general it is almost impossible to mimic solid samples except for the analyte elements. A possible way of circumventing this p r o b l e m is to replace the blank signal by the zero-mass signal [43], which is the signal obtained when measuring without a sample whatsoever. This signal can be converted to a mass using the signal per mass value measured by E T V of standard solutions. The m i n i m u m concentrations that can be determined in a solid material will also depend on the m a x i m u m mass of sample that can be loaded in the ETV-unit and will be different for different materials. Using the experimental p a r a m e t e r s summarized in Table 1, the determination limits shown in Table 2 were obtained using the above procedure. 3.1.3. Transport efficiency. One of the advantages of solid sampling over pneumatic nebulization is the improved transport efficiency. It was indeed shown that for I C P - A E S SS-ETV is significantly m o r e efficient (up to a factor of 30 for some elements) [50, 66].
469
Solid sampling using electrothermal vaporization Table 2. Determination limits for SS-ETV- I C P - A E S under optimum conditions of the experimental setup described in Table 1
Element Cu
Pb
Cd
As
BCR Certified Reference Material 142 062 320 277 060 038 142 176 146 277 320
Light Sandy Soil Olive Leaves River Sediment Estuarine Sediment Aquatic Plant Fly Ash Light Sandy Soil City Waste Incineration Ash Sewage Sludge Industrial Origin Estuarine Sediment River Sediment
Determination limit (wg/g) 0.032 0.16 0.059 0.83 1.83 0.62 0.39 0.10 0.30 0.40 0.34
3.1.4. Accuracy. The accuracy of a method can be tested by analysing reference materials. Unbiased testing requires that the composition of the test material is not known in advance and thus is possible only when participating in a round robin or certification exercise. Results of such analyses have not been reported for SSETV-ICP-AES. However, relying on the results of the analysis of standard reference materials (such as for instance shown in the next paragraph), it can be concluded that the method is potentially accurate. Nevertheless, SS-ETV-ICP-AES is not sufficiently developed for all sources of error to be understood and appropriate correction procedures to be known. Therefore the method requires experience from the scientist and careful testing and optimization when a new type of material is going to be analyzed.
3.2. Applications of S S - E T V - I C P - A E S 3.2.1. Standard reference materials. Like several other researchers (e.g. ATSUYA et al. [67]) we have used standard reference materials in our laboratory to test SSETV-ICP-AES; results have been published in earlier papers [43, 59]. Typical analytical parameters used for the SS-ETV-ICP-AES determination of trace elements in biological and environmental standard reference materials using different standardization methods are shown in Table 1. The analytical results summarized in Table 3 demonstrate that, except for external calibration with standard solutions, all standardization methods yield accurate results. Standard deviations depend on the homogeneity of the material and the number of replicates. In favourable cases a standard deviation of less than 5% is possible. 3.2.2. Other applications. Numerous applications of SS-ETV-ICP-AES have been reported and a complete review can be found elsewhere [47]. Suffice it to summarize here by mentioning the major fields of application. Many applications concern biological and environmental materials, where the success of the method can be explained by the fact that these materials in general can easily be ashed and that trace elements are efficiently set free during vaporization. Geological materials are analyzed only exceptionally [68] because volatilization and standardization are hampered by the refractory nature of several minerals and by the specificity of the conditions required to vaporize trace elements from the latter. On the other hand, a lot of research has been done for the analysis of technically important materials such as ceramics that are hard to dissolve and therefore are not commonly analyzed with common methods such as AAS or ICP with pneumatic nebulization. Since these materials resist high temperatures as well, halogenation, using solid or gaseous matrix modifers is necessary (e.g. [54, 56]). For the analysis of organic materials such as lubricating oil [38] or resins [51, 52], SS-ETV has the advantage that the matrix can be eliminated to a large extent by pyrolysis prior to the actual measurement. For the analysis of steels and
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alloys [69], SS-ETV avoids lengthy and difficult dissolution procedures. applications are the analysis of coal [60] and graphite [57].
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3.3. Results o f S S - E T V - I C P - M S Up to now, little research has been done in the field of S S - E T V - I C P - M S [60, 32, 70]. The large amount of material, set free during vaporization, is not a real problem in I C P - A E S but in I C P - M S it may seriously contaminate the interface and the lens stack. In our experience the use of a venting valve and suited temperature programs are absolutely necessary to avoid contamination of cones and lenses as well as rapid deterioration of the quality of the oil of the mechanical interface pump. Coupling an SS-ETV device to an I C P - M S is not different from coupling it to an I C P - A E S . Moreover the data acquisition and handling system of an I C P - M S is perfectly suited for multi-element analysis of fast transient signals. The higher sensitivity of I C P - M S and the protection of the detector against overload and rapid deterioration, will make it necessary to limit the sample mass more than usual in S S - E T V - I C P - A E S . VOELLKOPF et al. [32] coupled a boat-in-tube type ETV device to an ICP-mass spectrometer but could not obtain reproducible results when directly analyzing a powdered standard reference material with SS-ETV. They concluded that slurry E T V - I C P - M S was to be preferred over direct solid sampling. In recent work by WANG et al. [60], the feasibility of S S - E T V - I C P - M S was demonstrated. A graphite furnace was used for the analysis of NBS SRM 1633a (coal fly ash) and NBS SRM 1548 (total diet). The influence of a number of experimental parameters was studied and different methods for standardization (external calibration with standard solutions, external calibration with solid standard reference materials and single standard addition) were tested. The authors obtained relatively precise (RSD% on three replicates of 11%) and accurate results when using different masses of a reference material to construct a calibration line to be used for the analysis of the same reference material. They conclude that external calibration using a solid SRM with a composition similar to that of the sample provides the best performance. After several years of experience with S S - E T V - I C P - A E S in our laboratory, more recently efforts were made to combine the advantages of solid sampling with those of I C P - M S (extremely low limits of detection, multi-element capabilities and the possibility to obtain isotopic information on the elements determined). The Griin Optics graphite furnace, previously used as a sample introduction system for I C P - A E S , was coupled via a 60 cm long 10 mm internal diameter silicone rubber tubing to a Perkin-Elmer Sciex Elan 5000 ICP-mass spectrometer. A three-way valve was used to vent vapours generated during drying, ashing and cleaning steps. As a result, only vapours generated during the vaporization step of the heating program were allowed to reach the plasma so that contamination and overloading risks could be reduced to a minimum. During a first phase of optimization, the carrier gas flow rate and both the ashing and vaporizing temperatures were optimized for the determination of As in material of plant origin (tomato leaves), leading to the operation conditions summarized in Table 1. The signal profile obtained for As in tomato leaves was observed to be to some extent of bimodal form. Both As (added as a standard for calibration purposes) and Sb (added as an internal standard to correct for fluctuations of the transport and vaporization processes and/or instrument instability) spikes however were observed to show an analogous profile on condition that these liquid spikes were previously dried (using an IR-lamp) before the solid sample was introduced into the sample boat. On plotting the 75As signal intensity as a function of the sample mass, no linear curve is obtained (Fig. 3). Since however for Sb, added as an internal standard, the signal intensity is suppressed to the same extent as As when increasing the sample mass, a plot of the signal ratio (75As/121Sb) as a function of the sample mass shows a linear curve (Fig. 4). Moreover, since linear calibration curves were obtained on the addition of increasing amounts of As standard solution to the solid samples, the potential of
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the t e c h n i q u e c o u l d b e a s s e s s e d by e s t a b l i s h i n g t h e figures o f m e r i t a n d a n a l y z i n g some standard reference materials. L i m i t s o f d e t e c t i o n for A s w e r e c a l c u l a t e d a p p l y i n g t h e 3 s - c r i t e r i o n using " e m p t y b o a t s " as b l a n k s (10 m e a s u r e m e n t s ) a n d s t a n d a r d r e f e r e n c e m a t e r i a l s with k n o w n A s c o n t e n t as e x t e r n a l s t a n d a r d s . F o r A s , t h e a b s o l u t e limit o f d e t e c t i o n was e s t a b l i s h e d to be a b o u t 1 pg. F o r t h e analysis o f five solid s a m p l e s , a r e p e a t a b i l i t y o f a b o u t 10% is typical. I n o r d e r to e v a l u a t e t h e p o t e n t i a l o f t h e t e c h n i q u e , A s was d e t e r m i n e d in s e v e r a l s t a n d a r d r e f e r e n c e m a t e r i a l s o f p l a n t origin. S e v e r a l m e t h o d s o f c a l i b r a t i o n
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Table 4. Results of As determination in two standard reference materials using SS-ETV-ICP-MS Material (NBS SRM) Tomato leaves (NBS SRM 1573) Citrus leaves (NBS SRM 1572)
Certified concentration and 95% conf. int. (l~g/g)
This work (conc. and st. dev. in ~rdg)
0.27 -+ 0.05 3.1 -+ 0.3
0.29 (0.05) 3.3 (0.8)
were studied, including external calibration using both liquid and solid (standard reference materials with a c o m p a r a b l e matrix and As content) standards, the simplified generalized standard addition m e t h o d ( G S A M ) and single standard addition. Preliminary experiments showed that even on addition of large amounts of CI (up to 50 Ixg and exceeding the C1 content of the samples), no interference on 75As+(a°mr35Cl+) could be established. Although several methods offered possibilities for accurate determination, single standard addition was assessed to be the most practicable and straightforward method. Results for the As content in a n u m b e r of standard reference materials of plant origin obtained using single standard addition as calibration method showed very good a g r e e m e n t with the certified values. Moreover, the concentration range in which the technique can be used, can be extended by the use of the " O m n i r a n g e " device, which enables selective and reproducible reduction of sensitivity and hence allows work at higher concentration ranges. As an illustration, Table 4 shows an extract of results that will be discussed in detail elsewhere [70]. The most important disadvantage of electrothermal vaporization as a means of sample introduction for solid samples in I C P - M S , was observed to be the blank value, which was seen to increase slowly during an analysis sequence ( m e m o r y effects) and hence has to be carefully corrected for. Future efforts will mainly focus on (i) the determination of other elements and (ii) a m o r e p r o n o u n c e d exploitation of the multi-element capabilities of I C P - M S .
4. CONCLUSIONS SS-ETV has been successfully used for sample introduction in I C P - A E S and I C P - M S . The hard- and software requirements are fulfilled and the m e t h o d was used for trace element determination in a variety of materials, but predominantly for the analysis of biological and environmental samples and of ceramics and other hard to dissolve materials. Accurate standardization can be p e r f o r m e d using standard addition or external calibration with standard reference materials with a composition similar to that of the sample. The precision of the m e t h o d is typically around 10% ( R S D % ) but is limited by the sample inhomogeneity since the m a x i m u m sample weight should not exceed 10 mg. On the other hand this low sample weight is advantageous for homogeneity studies. Though not all sources of error and variability are fully understood, S S - E T V - I C P methods can produce accurate results after careful study and optimization. Because it is a fast and simple to use method, SS-ETV will probably turn out to be most useful in routine analysis of large numbers of similar solid samples. In this case the analyst can produce a suited solid standard in which concentrations can be certified using independent methods of analysis. REFERENCES [1] K. W. Olson, W. J. Haas Jr and V. A. Fassel, Anal. Chem. 49, 632 (1977). [2] J. M. Mermet and C. Trassy, in Developments in Atomic Plasma Spectrochemical Analysis, Ed. R. M. Barnes, p. 245. Heydon, London (1981). [3] H. Berndt, Fres. Z. Anal. Chem. 331, 321 (1988).
474 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
[36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47]
[48]
[49] [50] [51] [52] [53] [54] [55] [56] [57] [58]
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S. Boonen, P. Verrept, L. J. Moens and R. F. J. Dams, J. Anal. At. Spectrom. 8, 711 (1993). J. Wang, J. M. Carey and J. A. Caruso, Spectrochim. Acta 49B, 193 (1994). P. Verrept, G. Galb~tcs, L. Moens, R. Dams and U. KurfiJrst, Spectrochim. Acta 48B, 671 (1993). S. Pieponnen, T. Alanko and P. Minkkinen, Anal. Chim. Acta 5, 1031 (1979). D. C. Baxter, J. Anal. At. Spectrom. 4, 415 (1989). U. Kurfiirst, Pure & Appl. Chem. 63, 1205 (1991). J. D. Winefordner and G. L. Long, Anal. Chem. 55, 712 (1983). K. Dittrich and A. Walther, Untersuchungen zur direkten Feststoffanalyse durch E T V - I C P - A E S , in C A N A S '93, Eds K. Dittrich and B. Welz, p. 793. Universit~it Leipzig, Leipzig (1993). I. Atsuya, T. Itoh and T. Kurotaki, Spectrochim. Acta 46, 103 (1991). P. G. Mitchell and J. Sneddon, Talanta 34, 849 (1987). K. D. Ohls, Mikrochim. Acta [Wien] III, 337 (1989). F. Vanhaecke, S. Boonen, L. Moens and R. Dams, J. Anal. Spectrom., 10, 81 (1995).
Pergamon 0584-8547(94)00153-7
Solid sampling electrothermal vaporization for sample introduction in inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry* L. MOENSt, P. VERREPT, S. BOONEN, F. VANHAECKE and R. DAMS Laboratory of Analytical Chemistry, Ghent University, Proeftuinstraat 86, B-9000 Ghent, Belgium (Received 24 July 1994; accepted 27 October 1994) Abstract--Solid sampling using electrothermal vaporization is an attractive sample introduction method for atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). For AAS, the method is well established. The techniques needed to apply SS-ETV in ICP-based methods are described, with the emphasis on the coupling of different types of ETV-devices to the inductively coupled plasma torch and on the requirements for the spectrometer and the data acquisition and handling system. Though standardization is not straightforward, it is shown that standard addition and external calibration with solid standards yield accurate results. The latter is demonstrated by the analysis of standard reference materials. Figures of merit for SS-ETV-ICP-AES and SS-ETV-ICP-MS are presented. The literature concerning ICP-AES and ICP-MS (methods and applications) is briefly reviewed and new results of SS-ETV-ICP-MS analysis of SRMs are presented.
1. INTRODUCTION THOUGH pneumatic nebulization is widely used and generally accepted as a most convenient sample introduction m e t h o d in inductively coupled plasma atomic emission spectrometry ( I C P - A E S ) and inductively coupled plasma mass spectrometry ( I C P - M S ) , it is far f r o m an ideal method. It should indeed be possible to introduce solid, liquid and gaseous materials without any sample p r e t r e a t m e n t and m o r e o v e r 100% of the amount of analyte entering the sample introduction system should reach the plasma whereas interfering matrix elements should be removed. Obviously such a system will probably never be produced. Pneumatic nebulization falls short of this ideal in that only liquid samples or sample solutions can be introduced and that transport efficiency is p o o r ( 1 - 2 % ) . In addition, the solvent and the sample matrix can reach the plasma and give rise to spectral and non-spectral interferences. Several methods have b e e n developed to improve the transport efficiency of sample aerosols and/or to r e m o v e part of the solvent before the aerosol reaches the plasma: e.g. ultrasonic nebulization [1, 2], high pressure nebulization [3], t h e r m o s p r a y [4], direct insertion nebulization [5] and total desolvation [6]. Direct ICP analysis of solid materials is possible using for instance slurry nebulization [7-10], laser ablation [11-13], direct sample insertion [14-19], direct powder introduction [20-23] and electrothermal vaporization ( E T V ) [24]. In this work the t e r m solid sampling E T V (SS-ETV) will refer to solid sampling, in which a small sample is weighed, as such transferred to the E T V unit and vaporized. Solid samples can also be m a d e into slurries that can be pipetted into the E T V unit. Slurry E T V has been used with A A S [25-29], I C P - A E S [30, 31] and I C P - M S [32, 33], but will not be discussed here. With E T V , small volumes of both solids and solutions can be sampled and the transport of the sampled materials is very efficient. The technique m o r e o v e r allows the solvent and the matrix material to be fully or partly r e m o v e d thus keeping these * This paper was published in the Special Issue on Sample Introduction in Atomic Spectrometry. t Author to whom correspondence should be addressed. 463
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from entering the plasma and causing unwanted effects. A fundamental advantage of E T V is that the vaporization of the sample is separated in time and space f r o m the atomization, excitation and ionization processes, allowing separate optimization. E T V being the m e t h o d of choice for sample introduction of solutions in atomic absorption spectrometry (AAS), some of the vast experience obtained in this field can be transferred to ICP methods [34]. This implies methods to r e m o v e matrix elements and enhance the volatilization of analyte elements. Also SS-ETV has been widely applied in A A S and ample information is available [35]. However, solid sampling in general and SS-ETV in particular has a n u m b e r of drawbacks hampering its m o r e general application. A m a j o r p r o b l e m is standardization. Indeed, the physical and chemical situation of the analyte elements in different solid samples can be very different. T h e r e f o r e the evaporation process can proceed at a largely different rate and with different efficiency and standards should strongly resemble samples in matrix composition, which is an obvious limitation to the flexibility and the general applicability of the method. In this p a p e r the technique of SS-ETV for I C P - A E S and I C P - M S will briefly be described. Major developments and applications will be reviewed, advantages and limitations will be discussed and illustrated and recent developments will be presented.
2. APPARATUS AND TECHNIQUES 2.1. The S S - E T V furnace
For ETV of solutions, several types of device can be used, such as metal strips or coils [36, 37] and graphite rods or furnaces. SS-ETV however requires that relatively large amounts of sample powder (up to several mg) can be loaded conveniently into the vaporization device and that the latter can be heated in such a way that the sample powder is vaporized rapidly and completely. Therefore only graphite furnaces have been used for SS-ETV combined with ICP-AES. A first type of graphite furnaces suited for SS-ETV was designed to fit directly under the plasma torch (an example is shown in Fig. 1). An obvious advantage of these furnaces is the excellent efficiency of the aerosol transport. For loading, the furnace block can be shifted sideways or downwards while an auxiliary Ar gas flow keeps air from entering the torch. The design has been used and improved by several researchers [38, 39]. With this type of furnace, sample handling is rather difficult. The second type of SS-ETV furnace is the flow through type (an example is shown in Fig. 2). The oven consists of a graphite tube that is placed at a distance from the ICP torch and samples are transferred by an Ar gas flow through the oven. A transfer tube made out of glass or a plastic material (e.g. tygon) connects the furnace to the ICP torch. Dry aerosols, as produced by an SS-ETV oven, can efficiently be transported over long distances (0.5 m to 20.5 m) [40, 41]. Since the samples are loaded in sample boats that can easily be inserted in the furnace, this type of furnace is also called the "boat-in-tube" type. Tarred sample boats are used to replace weighing flasks, thus eliminating errors due to incomplete transfer of the weighed mass to the furnace. Moreover, sample boats act as L'vov platforms during evaporation. As in atomic absorption spectrometry (AAS), special temperature programs can be used for drying, ashing and evaporating the samples and cleaning the furnace after each sample. The solvent and the matrix elements set free during desolvation and ashing, respectively, can be kept from entering the plasma and are vented via a venting valve between the furnace an the plasma [42]. This is especially important in ICP-MS, but also in ICP-AES large amounts of solvent or matrix material can cause memory effects. Matrix modifiers can be used to enhance the volatilization during ashing of matrix elements, otherwise causing matrix effects. Results reported in this paper were obtained with the aid of the Griin Optics furnace (Griin Analytische Mess-Systeme G.m.b.H., Wetzlar, Germany), schematically shown in Fig. 2. This device was designed for AAS and required some adaptations to make it useful for ICP-AES and ICP-MS [43]. One side had to be closed with a shutter that can be opened to introduce the sample boat. Gas flows had to be changed in such a way that, during the loading of the furnace, Ar is flowing in the opposite direction of the carrier gas flow to keep air from entering the furnace and destabilising the plasma. A similar furnace was used by KANTORet al. [44] who succeeded in improving the transport efficiency by adding a cold Ar flow in counterflow with
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the transport gas during vaporization [45]. Rapid cooling of the atoms formed enhances the formation of condensation nuclei and yields a better transportable aerosol [46, 47]. In view of temperature gradients in the furnace, reproducible positioning of the boat in the furnace is crucial. Therefore a pair of tweezers slides on a rail rigidly mounted in front of the furnace. A glass, PVC or tygon transfer line of about 50 cm and a diameter of about 0.5 cm was used to couple the oven to the ICP. Typical operation parameters for the furnace used with I C P - A E S and I C P - M S are included in Table 1. 2.2. Signal handling ETV produces short (3-10 s) transient signals. In E T V - I C P - A E S the spectral background is transient as well, requiring simultaneous recording of the peak and background signals during the actual measurement. According to BOOMANS [48] accurate background information "must be derived from wavelength scans centred about the analysis line and extended over a wavelength interval of at least several times the spectral bandwidth of the apparatus". Several devices have been used for this purpose; three of these will be mentioned here. A wavelength scan over a narrow wavelength interval can be performed by oscillating the grating over a small angle. In common spectrometers however, such movement is relatively slow and the method is not suited for fast transient signals. A n obvious solution to this problem is to use a fast scanning device that allows quasi-simultaneous measurements of peak and background emission intensities. Presently, spectrometers are available that are equipped with magnetically or galvanometrically driven gratings, scanning over a range of 600 nm in less than 20 ms, which satisfies the above mentioned requirements. Another approach consists in positioning an oscillating refractor plate behind the entrance slit of the spectrometer. The refractor plate acts as a wavelength modulator and, when mounted on a high speed galvanometric motor, small wavelength intervals of around 0.2 nm can be scanned at a rate of up to 80 Hz. The scan speed will depend on the number of acquisition points within the interval, which is typically about 80, and the dwell time. These conditions allow a transient ETV signal (3 to 10 s long) to be scanned at a rate of 1-80 Hz and to collect tens of spectra while analyte atoms and ions are present in the plasma. Results shown in this work were obtained with the oscillating refractor plate technology [49, 50], which can be implemented at low cost in almost every spectrometer. Excellent devices for handling fast transient signals are solid state array detectors such as photodiode array detectors and charge transfer devices (charged coupled devices, CCD, and charge injection devices, CID). With this equipment, which is rather expensive, 30 frames Can be read out each second, which is a largely sufficient for E T V - I C P - A E S . Data handling should consist in performing appropriate background correction for each individual spectrum. For background correction of wing interferences a quadratic fit of the background in general will yield better results than a linear one [43]. The net signal can be plotted versus time and integrated [50]. For I C P - M S , background correction is less problematic because the continuous photon background is low and hardly influenced by the matrix of the sample. Moreover the scanning speed of quadrupoles is largely sufficient to register the signal at and around the mass peak and multi-element analysis is possible. 2.3. Special procedures As mentioned before, some of the experience gained in E T V - A A S can be transferred to E T V - I C P methods. S S - E T V - A A S , though much less applied than E T V - A A S of solutions, has been intensively used as well [35]. Selective vaporization of the matrix in the ashing step is possible if the volatility of matrix and analyte elements is largely different. However, in view of the complexity of solid samples, this condition is not obviously fulfilled, Pyrolysis of samples consisting of organic compounds [51, 52] is one example of selective matrix decomposition and it is also possible to add matrix modifiers to transform the matrix into more volatile compounds. To allow less volatile analyte elements to be vaporized or to decompose refractory materials both gaseous and solid matrix modifiers have been proposed. The addition of halocarbon gases to the A r carrier flow has become a widely applied method in S S - E T V - I C P - A E S . KIRI~BPaGNr and SNooK [53] introduced this method for the determination of refractory elements such as Zr, B, Cr, Mo and W. They added 0.1% of CHF3 to the Ar. Other halogenating gases have been used as well and for instance CC14 and Freon 12 were added to determine trace elements in ceramic powders and SiC, respectively [54, 55]. Different types of solid, halogenating matrix modifiers were successfully used as well. OnLS and Ht~rscn [38] added Teflon powder to lubricating oils to improve the vaporization of
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refractory elements. To decompose refractory matrices solid modifiers were frequently used. SiO2-, A1203- and MgO-based ceramics were analyzed for Fe after addition of AgCl + BaCI2 [56], for the analysis of graphite, (C2F4)n + NaF was added [57] and B4C, SiC and SiaN4 were analyzed with SS-ETV-ICP-AES after addition of CoF2 + BaO [58]. The latter modifier was also used to determine carbide forming elements. 2.4. Calibration Like for most solid sampling techniques, standardization is a problem for SS-ETV-ICP analysis and different methods have been applied. External calibration with standard solutions is unsuited for most matrices since the vaporization, transfer and plasma conditions for the solid sample and the standard solution are very different [59, 60]. External calibration with solid standards yields accurate results on condition that the composition of sample and standard are similar [43, 61]. This limits the applicability of the method since standard reference materials are not available for every possible type of sample. However this method can be very useful in routine analysis of large numbers of very similar samples where a large batch of a typical sample material can be analyzed with independent methods to serve as a standard material. Standard addition with standard solutions is probably the most versatile method available. The difference in vaporization and transport efficiencies between solid sample and standard solution largely disappears when the sample solution is transferred to the sample holder and dried before the solid sample is loaded into the holder [59]. In SS-ETV-ICP-AES both the single standard addition method and the generalized simplified standard addition method (GSAM), developed for AAS [62, 63], have been applied successfully [59].
3. RESULTS AND DISCUSSION
3.1. Figures of merit in 1 C P - A E S 3.1.1. Precision. The mass of the sample that can be loaded into an SS-ETV unit is limited to 0.5-10 mg depending on the matrix composition. This is due to the limited space of the sample boat and/or the furnace and to the fact that in SS-ETV a lot of material is transported to the plasma in a very short time, which can lead to plasma instability or even to the extinction of the plasma. The inhomogeneity of the material to be analyzed is a limiting factor for the precision of SS-ETV and it is necessary to analyze a large n u m b e r of subsamples. In the analysis of, for instance, standard reference materials an overall precision of the order of 5 - 1 5 % is feasible. T h o u g h disadvantageous for most analyses, the small sample sizes of SS-ETV can be very useful in homogeneity studies [64]. 3.1.2. Detection limits. For elements with a relatively high concentration, absolute determination limits are determined by the lowest sample mass that can accurately be weighed. For low concentrations, limits of detection should be calculated according to the I U P A C definition [65], stating that the limit of detection is given by ksB where sB is the concentration equivalent of the standard deviation on a large n u m b e r of m e a s u r e m e n t s of the blank signal and k is a constant (for a 95% confidence level, k = 3). This definition requires that an appropriate blank can be measured which is a p r o b l e m in solid sampling. Indeed in general it is almost impossible to mimic solid samples except for the analyte elements. A possible way of circumventing this p r o b l e m is to replace the blank signal by the zero-mass signal [43], which is the signal obtained when measuring without a sample whatsoever. This signal can be converted to a mass using the signal per mass value measured by E T V of standard solutions. The m i n i m u m concentrations that can be determined in a solid material will also depend on the m a x i m u m mass of sample that can be loaded in the ETV-unit and will be different for different materials. Using the experimental p a r a m e t e r s summarized in Table 1, the determination limits shown in Table 2 were obtained using the above procedure. 3.1.3. Transport efficiency. One of the advantages of solid sampling over pneumatic nebulization is the improved transport efficiency. It was indeed shown that for I C P - A E S SS-ETV is significantly m o r e efficient (up to a factor of 30 for some elements) [50, 66].
469
Solid sampling using electrothermal vaporization Table 2. Determination limits for SS-ETV- I C P - A E S under optimum conditions of the experimental setup described in Table 1
Element Cu
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BCR Certified Reference Material 142 062 320 277 060 038 142 176 146 277 320
Light Sandy Soil Olive Leaves River Sediment Estuarine Sediment Aquatic Plant Fly Ash Light Sandy Soil City Waste Incineration Ash Sewage Sludge Industrial Origin Estuarine Sediment River Sediment
Determination limit (wg/g) 0.032 0.16 0.059 0.83 1.83 0.62 0.39 0.10 0.30 0.40 0.34
3.1.4. Accuracy. The accuracy of a method can be tested by analysing reference materials. Unbiased testing requires that the composition of the test material is not known in advance and thus is possible only when participating in a round robin or certification exercise. Results of such analyses have not been reported for SSETV-ICP-AES. However, relying on the results of the analysis of standard reference materials (such as for instance shown in the next paragraph), it can be concluded that the method is potentially accurate. Nevertheless, SS-ETV-ICP-AES is not sufficiently developed for all sources of error to be understood and appropriate correction procedures to be known. Therefore the method requires experience from the scientist and careful testing and optimization when a new type of material is going to be analyzed.
3.2. Applications of S S - E T V - I C P - A E S 3.2.1. Standard reference materials. Like several other researchers (e.g. ATSUYA et al. [67]) we have used standard reference materials in our laboratory to test SSETV-ICP-AES; results have been published in earlier papers [43, 59]. Typical analytical parameters used for the SS-ETV-ICP-AES determination of trace elements in biological and environmental standard reference materials using different standardization methods are shown in Table 1. The analytical results summarized in Table 3 demonstrate that, except for external calibration with standard solutions, all standardization methods yield accurate results. Standard deviations depend on the homogeneity of the material and the number of replicates. In favourable cases a standard deviation of less than 5% is possible. 3.2.2. Other applications. Numerous applications of SS-ETV-ICP-AES have been reported and a complete review can be found elsewhere [47]. Suffice it to summarize here by mentioning the major fields of application. Many applications concern biological and environmental materials, where the success of the method can be explained by the fact that these materials in general can easily be ashed and that trace elements are efficiently set free during vaporization. Geological materials are analyzed only exceptionally [68] because volatilization and standardization are hampered by the refractory nature of several minerals and by the specificity of the conditions required to vaporize trace elements from the latter. On the other hand, a lot of research has been done for the analysis of technically important materials such as ceramics that are hard to dissolve and therefore are not commonly analyzed with common methods such as AAS or ICP with pneumatic nebulization. Since these materials resist high temperatures as well, halogenation, using solid or gaseous matrix modifers is necessary (e.g. [54, 56]). For the analysis of organic materials such as lubricating oil [38] or resins [51, 52], SS-ETV has the advantage that the matrix can be eliminated to a large extent by pyrolysis prior to the actual measurement. For the analysis of steels and
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471
Other
3.3. Results o f S S - E T V - I C P - M S Up to now, little research has been done in the field of S S - E T V - I C P - M S [60, 32, 70]. The large amount of material, set free during vaporization, is not a real problem in I C P - A E S but in I C P - M S it may seriously contaminate the interface and the lens stack. In our experience the use of a venting valve and suited temperature programs are absolutely necessary to avoid contamination of cones and lenses as well as rapid deterioration of the quality of the oil of the mechanical interface pump. Coupling an SS-ETV device to an I C P - M S is not different from coupling it to an I C P - A E S . Moreover the data acquisition and handling system of an I C P - M S is perfectly suited for multi-element analysis of fast transient signals. The higher sensitivity of I C P - M S and the protection of the detector against overload and rapid deterioration, will make it necessary to limit the sample mass more than usual in S S - E T V - I C P - A E S . VOELLKOPF et al. [32] coupled a boat-in-tube type ETV device to an ICP-mass spectrometer but could not obtain reproducible results when directly analyzing a powdered standard reference material with SS-ETV. They concluded that slurry E T V - I C P - M S was to be preferred over direct solid sampling. In recent work by WANG et al. [60], the feasibility of S S - E T V - I C P - M S was demonstrated. A graphite furnace was used for the analysis of NBS SRM 1633a (coal fly ash) and NBS SRM 1548 (total diet). The influence of a number of experimental parameters was studied and different methods for standardization (external calibration with standard solutions, external calibration with solid standard reference materials and single standard addition) were tested. The authors obtained relatively precise (RSD% on three replicates of 11%) and accurate results when using different masses of a reference material to construct a calibration line to be used for the analysis of the same reference material. They conclude that external calibration using a solid SRM with a composition similar to that of the sample provides the best performance. After several years of experience with S S - E T V - I C P - A E S in our laboratory, more recently efforts were made to combine the advantages of solid sampling with those of I C P - M S (extremely low limits of detection, multi-element capabilities and the possibility to obtain isotopic information on the elements determined). The Griin Optics graphite furnace, previously used as a sample introduction system for I C P - A E S , was coupled via a 60 cm long 10 mm internal diameter silicone rubber tubing to a Perkin-Elmer Sciex Elan 5000 ICP-mass spectrometer. A three-way valve was used to vent vapours generated during drying, ashing and cleaning steps. As a result, only vapours generated during the vaporization step of the heating program were allowed to reach the plasma so that contamination and overloading risks could be reduced to a minimum. During a first phase of optimization, the carrier gas flow rate and both the ashing and vaporizing temperatures were optimized for the determination of As in material of plant origin (tomato leaves), leading to the operation conditions summarized in Table 1. The signal profile obtained for As in tomato leaves was observed to be to some extent of bimodal form. Both As (added as a standard for calibration purposes) and Sb (added as an internal standard to correct for fluctuations of the transport and vaporization processes and/or instrument instability) spikes however were observed to show an analogous profile on condition that these liquid spikes were previously dried (using an IR-lamp) before the solid sample was introduced into the sample boat. On plotting the 75As signal intensity as a function of the sample mass, no linear curve is obtained (Fig. 3). Since however for Sb, added as an internal standard, the signal intensity is suppressed to the same extent as As when increasing the sample mass, a plot of the signal ratio (75As/121Sb) as a function of the sample mass shows a linear curve (Fig. 4). Moreover, since linear calibration curves were obtained on the addition of increasing amounts of As standard solution to the solid samples, the potential of
472
L. MOENS et
360
al.
As signal intensity (arbitrary units)
300
+
260
:4_
+
200 160 100 5O I
0 0
1
I
I
2 3 sample mass (mg, tomato leevee)
I
4
5
Fig. 3. As-signal versus solid sample mass for NBS SRM 1573 (Tomato leaves) measured using SS-ETV- ICP- MS.
signal ratio (76Ae/121Sb)
5
3!
2;
× JX
0 0
1
2 3 sample mass (rag, tomato ieeves)
4
5
Fig. 4. Ratio of the As and Sb signals as a function of solid sample mass; Sb was added as an internal standard. SS-ETV-ICP-MS results for NBS SRM 1573 (Tomato leaves).
the t e c h n i q u e c o u l d b e a s s e s s e d by e s t a b l i s h i n g t h e figures o f m e r i t a n d a n a l y z i n g some standard reference materials. L i m i t s o f d e t e c t i o n for A s w e r e c a l c u l a t e d a p p l y i n g t h e 3 s - c r i t e r i o n using " e m p t y b o a t s " as b l a n k s (10 m e a s u r e m e n t s ) a n d s t a n d a r d r e f e r e n c e m a t e r i a l s with k n o w n A s c o n t e n t as e x t e r n a l s t a n d a r d s . F o r A s , t h e a b s o l u t e limit o f d e t e c t i o n was e s t a b l i s h e d to be a b o u t 1 pg. F o r t h e analysis o f five solid s a m p l e s , a r e p e a t a b i l i t y o f a b o u t 10% is typical. I n o r d e r to e v a l u a t e t h e p o t e n t i a l o f t h e t e c h n i q u e , A s was d e t e r m i n e d in s e v e r a l s t a n d a r d r e f e r e n c e m a t e r i a l s o f p l a n t origin. S e v e r a l m e t h o d s o f c a l i b r a t i o n
Solid sampling using electrothermal vaporization
473
Table 4. Results of As determination in two standard reference materials using SS-ETV-ICP-MS Material (NBS SRM) Tomato leaves (NBS SRM 1573) Citrus leaves (NBS SRM 1572)
Certified concentration and 95% conf. int. (l~g/g)
This work (conc. and st. dev. in ~rdg)
0.27 -+ 0.05 3.1 -+ 0.3
0.29 (0.05) 3.3 (0.8)
were studied, including external calibration using both liquid and solid (standard reference materials with a c o m p a r a b l e matrix and As content) standards, the simplified generalized standard addition m e t h o d ( G S A M ) and single standard addition. Preliminary experiments showed that even on addition of large amounts of CI (up to 50 Ixg and exceeding the C1 content of the samples), no interference on 75As+(a°mr35Cl+) could be established. Although several methods offered possibilities for accurate determination, single standard addition was assessed to be the most practicable and straightforward method. Results for the As content in a n u m b e r of standard reference materials of plant origin obtained using single standard addition as calibration method showed very good a g r e e m e n t with the certified values. Moreover, the concentration range in which the technique can be used, can be extended by the use of the " O m n i r a n g e " device, which enables selective and reproducible reduction of sensitivity and hence allows work at higher concentration ranges. As an illustration, Table 4 shows an extract of results that will be discussed in detail elsewhere [70]. The most important disadvantage of electrothermal vaporization as a means of sample introduction for solid samples in I C P - M S , was observed to be the blank value, which was seen to increase slowly during an analysis sequence ( m e m o r y effects) and hence has to be carefully corrected for. Future efforts will mainly focus on (i) the determination of other elements and (ii) a m o r e p r o n o u n c e d exploitation of the multi-element capabilities of I C P - M S .
4. CONCLUSIONS SS-ETV has been successfully used for sample introduction in I C P - A E S and I C P - M S . The hard- and software requirements are fulfilled and the m e t h o d was used for trace element determination in a variety of materials, but predominantly for the analysis of biological and environmental samples and of ceramics and other hard to dissolve materials. Accurate standardization can be p e r f o r m e d using standard addition or external calibration with standard reference materials with a composition similar to that of the sample. The precision of the m e t h o d is typically around 10% ( R S D % ) but is limited by the sample inhomogeneity since the m a x i m u m sample weight should not exceed 10 mg. On the other hand this low sample weight is advantageous for homogeneity studies. Though not all sources of error and variability are fully understood, S S - E T V - I C P methods can produce accurate results after careful study and optimization. Because it is a fast and simple to use method, SS-ETV will probably turn out to be most useful in routine analysis of large numbers of similar solid samples. In this case the analyst can produce a suited solid standard in which concentrations can be certified using independent methods of analysis. REFERENCES [1] K. W. Olson, W. J. Haas Jr and V. A. Fassel, Anal. Chem. 49, 632 (1977). [2] J. M. Mermet and C. Trassy, in Developments in Atomic Plasma Spectrochemical Analysis, Ed. R. M. Barnes, p. 245. Heydon, London (1981). [3] H. Berndt, Fres. Z. Anal. Chem. 331, 321 (1988).
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