Optimization strategies for the reduction of non-spectroscopic interferences in inductively coupled plasma mass spectrometry

05&t-8547/92 $5 00 + .OO 0 1992 Pergamon Press Ltd

Spectrochimico Acre, Vol. 47B. No. 8. pp IWl-1012. 1992 Printed in Great Bntain.

Optimization strategies for the reduction of non-spectroscopic interferences in inductively coupled plasma mass spectrometry E. HYWEL EVANS* and ~CISEI’H A. Cmusot University of Cincinnati, Department

of Chemistry, Cincinnati, OH 45221-0172, U.S.A.

(Received 23 September 1991; accepted 10 February 1992) Abstract-Simplex optimization has been used to optimize ion lens conditions for a VG PlasmaQuad inductively coupled plasma mass spectrometer for the reduction of non-spectroscopic interferences due to a uranium matrix at a concentration of 10000 pg g- ‘. The extraction lens voltage was shown to have the greatest effect on the extent of analyte signal suppression caused by the matrix. The use of a 0.4 mm orifice sampler (smaller than the 1.0 mm orifice sampler normally used) resulted in a large reduction in analyte suppression at all extraction lens voltages, whereas for the 1.0 mm and 0.7 mm orifice sampler a judicious choice of extraction lens voltage was necessary to ensure minimal analyte suppression. The main disadvantages of the 0.4 mm orifice sampler were the propensity for a secondary discharge to form in the orifice, resulting in elevated levels of doubly charged ions, and the tendency for the orifice to rapidly block in the presence of the 10000 pg g-* uranium matrix. A reduction in the expansion stage pressure also decreased the degree of analyte suppression in some experiments, although the effects were not reproducible from day to day.

1. INTR~DU~N~N CONFLUXING

reports exist on the influence of the sample matrix on the analytical signal in ICP-MS [l-13]. In general, the most serious matrix effects are those caused by an excess of a heavy, easily ionizable element (EIE) or elements in the matrix, which cause suppression of analyte signal, or in some cases an enhancement. For instance, TAN and HORLICK [5] observed suppression of analyte signal in the presence of various EIEs at low nebulizer gas flows, but enhancements at high flows. GREGOIRE [3, 41 observed only suppression, while BEAUCHEMIN et al. [2] mainly observed enhancements. However, several important conclusions result: (i) (ii) (iii) (iv)

heavy matrix elements with low ionization potentials cause the most severe effects; light analyte elements with high ionization potentials are most severely affected; plasma operating conditions have a great influence on the magnitude of these effects; and the matrix effect is dependent on the absolute amount of matrix element rather than on the molar ratio to analyte, hence the effects can be reduced by dilution of the sample.

Several theories have been proposed to account for these effects. One theory is that of ionization suppression in the plasma, whereby the large excess of a matrix element with a low first ionization potential results in a large excess of electrons and positive ions after ionization [l]. This excess forces the equilibrium for the analyte towards atom formation, resulting in a suppression of analyte ion formation. However, this theory lacks credibility considering that the proportion of ions and electrons contributed by even a large quantity of matrix will be small compared to the vast excess contributed by argon and nebulized water molecules in the plasma, and cannot alone explain the severity of the matrix effects observed in ICP-MS. Indeed, this mechanism has largely been discounted as the explanation for matrix effects in ICP-AES [14-161. *Present address: Elemental Research Inc., 309-267 West Esplanade, V7M lA5. tAuthor to whom correspondence should be addressed. 1001

N. Vancouver,

B.C., Canada

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E. H. EVANS andJ. A. CARUSO

GREGOIRE[4] has proposed that “ambipolar diffusion” may be a possible explanation. The mechanism is such that the presence of a large excess of a high mass EIE in the plasma gives rise to an electrical field caused by the diffusion of electrons, at a greater rate than ions, out of the central channel. The electrical field results in the diffusion of lighter analyte ions towards the annular region of the plasma, thereby resulting in a decrease in the number of ions that can be sampled from the central channel. TAN and HORLICK[5] have suggested that mass separation effects, in the expansion region and subsequently in the ion beam, may play a role. They mention two effects, namely pressure diffusion and Mach-number focusing, whereby the heavier ions are focused towards the axis of the beam, and the lighter ions diffuse away to a greater extent, though the theories were originally developed for neutral molecular beams. The most popular mass separation theory is that of space-charge effects in the ion beam [5, 81. This is caused by the loss of electrons from the ion beam, due to the nature of the ion optics which focus only positively charged species, and results in coulombic repulsion between ions. In the presence of an excess of relatively heavy matrix ions, which have greater translational energy, the lighter analyte ions are repelled from the ion beam to the greatest extent. It has been postulated that mass separation effects are particularly critical in the expansion region since the skimmer cone samples from only a relatively narrow portion of the expansion gases around the axis behind the sampling orifice, and indeed, matrix effects have been shown to be less severe when a skimmer with a larger orifice than normal was used [9]. Reports of optimization studies for ICP-MS have mainly concentrated on optimization of plasma operating conditions [17-211. Relatively little attention has been paid to the effect of ion lens voltages, particularly with respect to their effects on non-spectroscopic matrix effects [22, 231. It is also important to optimize the instrument for maximum analyte response while at the same time minimizing potential interferences which may be spectroscopic in nature. Several workers have optimized plasma operating conditions or ion lens voltages for two commercially available systems, namely the Sciex Elan [17, 201 and the VG PlasmaQuad [18, 191. They optimized the systems for maximum signal using univariate optimization techniques, and subsequently chose operating conditions which gave large analyte signals but also minimal signals due to doubly charged and polyatomic ions. In general they concluded that suitable compromise operating conditions could be found for different elements with widely differing masses, ionization potentials and chemistries, and that these conditions yielded tolerably low levels of doubly charged and oxide ions. However, it is quite possible that other workers may find completely contrary results, since it has been our experience that there is considerable variability in the levels of polyatomic ions from day to day with the same instrument, let alone between different instruments. Such variability may depend to a great extent on the condition of the sampling and skimmer cones, which gradually deteriorate with time. The trends observed for the two instruments were broadly similar, though differences have been noted in the behavior of doubly charged ions [19], probably due to important differences in design of the load coil, interface and ion lenses, which have a great influence on the ion energies of extracted ions and their subsequent transmittal into the quadrupole analyzer. All workers identified the most important parameters to be nebulizer gas flow, forward power and sampling depth. The optimization studies mentioned above have been applied to ICP-MS systems operating with standard components, and the preferred optimization technique has been to perform a series of univariate searches. However, a more rigorous optimization regime is necessary for instrumental systems that have operating parameters that are interdependent variables. More recently, SCHMITand CHAUVE~TE[22] and EVANS ef al. [21, 231 have demonstrated the applicability of a multivariate simplex optimization technique for the optimization of ICP-MS. Since the operating variables are known to interact, univariate methods alone are unsuitable for system optimization. Therefore, a multivariate optimization technique such as simplex optimization is necessary to locate the true optimum.

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Fig. 1. Schematic diagram of the interface and lens stack showing: Sa, sampler; Sk, skimmer; E, extraction lens; P, photon stop; C, collector; Ll, L2, L3 and L4 lenses; DA, differential aperture.

2. EXPERIMENTAL 2.1. Instrumentation using an inductively coupled Experiments were performed PlasmaQuad, VG Elemental, Winsford, Cheshire, U.K.).

plasma-mass

spectrometer

(VG

2.2. Reagents and standards

Multi-element solutions (10 pg g-l) of Be, Co, In, Ba and Pb were prepared from 1000 p.g ml-’ stock solutions (Fisher Scientific, Fair Lawn, NJ, U.S.A.), and used for subsequent dilution in 2% (v/v) nitric acid (Reagent grade, Fisher Scientific). Two test solutions were prepared for comparison, both containing 100 ng g-l of Be, Co, In, Ba and Pb, with the addition of 10000 pg g-l uranium as uranyl nitrate (Fluka Chemicals, Ronkonkoma, NY, U.S.A.), to one solution to serve as a synthetic matrix. Respective blank solutions were also prepared. All solutions were prepared by weight. 2.3. Flow injection The flow injection instrumentation comprised a programmable electronic controller (Electronics Shop, University of Cincinnati), a pneumatically actuated, solenoid switch (Model 7163, Rheodyne, Cotati, CA, U.S.A.), a six-way valve (Mode1 5701, Rheodyne) and a 100 ~1 sample loop. Flow injection was used as the method of sample introduction in order to minimize the effects of nebulizer and cone blockage. This was particularly important since the matrix chosen for the study was a solution of 10000 kg g-’ uranium, which is particularly prone to forming refractory oxides on the sampling cone. Hence, a relatively small sample loop of 100 pl was used. Due to the rapid scanning nature of the ICP-MS quadrupole, it was possible to obtain data for a number of elements for each flow injected sample, rather than monitor a single element in the traditional manner. In practice, the quadrupole scan was initiated immediately before the initial rise of the flow injection signal, each scan comprising 400 sweeps, each of 0.16 s duration. In this way the data could be treated in the same way as that obtained for a steady state signal. The carrier stream was 2% nitric acid for all experiments. 2.4. Simplex optimization The extraction, collector, and Ll and L3 ion lenses were included in the simplex optimization, since preliminary experiments had indicated that these lens elements had the greatest influence on non-spectroscopic matrix effects. A schematic diagram of the lens stack is shown in Fig. 1. The boundary conditions for each lens element were determined by roughly tuning the lenses for the maximum In signal at 115 m/z, and then taking the ion lens voltage which resulted in 10% of the signal obtainable at the optimum as the boundary condition. The ranges were -100 to -289 V for the extraction lens, -32 to +17 V for the collector, -2.5 to +13 V for Ll, and -4 to +lO V for L3. These boundary conditions were merely used as a starting point for the simplex algorithm, which could subsequently explore the factor space outside these conditions as the optimization progressed.

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E. H. EVANSand J. A. CARUSO

Simplex optimization experiments were performed using a software package developed in Ebdon’s group [24] and run on a microcomputer. The algorithm used was based on that of YARBROand DEMING [25], and initially covered the whole of the factor space, followed by contraction in the step-size as the optimum was approached. The optimum was deemed to have been reached when the relative standard deviation of the response factors for the vertices retained in the simplex was within the medium term precision of the technique, which was given as 5% for this experiment. After each optimization was completed, univariate searches were performed for each lens element in turn while holding the others at the optimum established by the simplex procedure. Other operating conditions held constant during the simplex optimization experiment were: nebulizer gas, 0.66 1 mini; auxiliary gas, 1.0 1 min-‘; coolant gas, 15 1 min-‘; forward power, 1350 W; sampler orifice, 0.7 mm; expansion pump rate, 400 1 min-’ (resulting in an expansion pressure of approximately 1.5 mbar). Data acquisition parameters were: mass range, 5-211 m/z; sweeps, 400; channels, 2048; dwell time, 80 ps. For the simplex optimization experiment, 100 l.~lof the solutions were injected in the following order: (i) (ii)

100 ng gg’ Be, Co, In, Ba and Pb, in 2% nitric acid; and 100 ng gg’ Be, Co, In, Ba and Pb, and 10000 p,g gg’ U in 2% nitric acid.

It was not necessary to perform a blank subtraction for this experiment because only data for In was used, and there was no In present in either the 2% nitric acid or the uranyl nitrate. The sample introduction system was flushed with the carrier stream for 75 s between injections (i) and (ii) and for at least 120 s between sets of conditions defined by the simplex. The response factor used as the criterion of merit for the simplex optimization was formulated as follows:

Response factor =

J

3s,+s, 2

R

(1)

where S, is the l151nf signal intensity for the solution containing a 10000 p,g g-i uranium matrix; S, is the l151nf signal intensity for a solution without uranium; and R is the ratio of S, and S, such that R was always less than or equal to 1 (i.e. the smaller of the two values S, and S, was always the numerator). The rationale behind the choice of response factor was as follows. In order to optimize the system for a minimum In suppression, the R term was incorporated. In the ideal case (i.e. no analyte suppression) R would be equal to one. Whenever enhancement or suppression of the In signal occurred, due to the uranium matrix, then R would be a fraction, resulting in a decrease in the magnitude of the response factor given that the cube root term remained constant. The cube root term was incorporated to prevent the optimization from moving towards lens conditions which yielded zero suppression at the expense of greatly reduced analyte signal. This term was simply the cube root of the mean of the In signals with and without the uranium matrix. The cube root weighting was arrived at empirically by preliminary experiment. 2.5. Effect of extraction voltage and expansion pressure Three different sampling cones were used in this study, with orifices of 1.0 mm, 0.7 mm and 0.4 mm respectively. The 0.4 mm sampler was machined from aluminum, while the other two were machined from nickel. The pumping rate for the expansion stage could be varied by using a combination of two rotary vacuum pumps, with pumping capacities of approximately 400 1 mine1 (Model E2M-18, Edwards High Vacuum, Crawley, UK) and 1500 1 min-’ (Model ElM-80, Edwards High Vacuum). Both pumps were connected to the expansion stage such that the pumps could be operated individually, or together to achieve a maximum pumping capacity of 1900 1 min-‘. In combination with the sampling cones, various expansion pressures could be obtained, as shown in Table 1. One hundred microliters of the solutions were flow injected in the following order: (i) (ii) (iii) (iv)

2% nitric acid blank; 3 repeats of 100 ng g-r Be, Co, In, Ba and Pb, in 2% nitric acid; 10000 p,g g-l U in 2% nitric acid matrix blank; and 3 repeats of 10000 p,g g-l U and 100 ng gg’ Be, Co, In, Ba and Pb, in 2% nitric acid.

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Table 1. Expansion pressures obtained with different combinations of pump rate and sampler orifice Expansion pressure (mbar) Expansion pump rate (1 min-I) 400 1500 1900

0.4 mm sampler

0.1 mm sampler

1.0 mm sampler

0.7 0.5 n/d

1.5 n/d 0.5

2.3 n/d 0.7

n/d = not determined. The sample 120 s between final data.

introduction each change

system was flushed with carrier for 46 s between injections, in instrumental conditions. Blank subtraction was performed

and for on the

3. RESULTSAND DISCUSSION 3.1.

Simplex optimization The optimization was completed in 41 iterations and approximately three hours, including repeats of some of the vertices which were necessary due to instrumental drift. This may seem a long time, but the univariate search performed subsequently for confirmation required an equal number of iterations. Furthermore, if a univariate search had been employed as the sole method of optimization, then several repeat cycles would have been necessary, with no guarantee that the dependent nature of the variables would have been deconvoluted. In the context of this experiment, the univariate search was not a necessary part of the optimization and was performed only to confirm the effectiveness of the simplex optimization, and to illustratk the relative importance of each of the varibles. Results of univariate searches performed using the optimal conditions are illustrated in Figs 2(a)-2(d). The results in each graph are represented in the form of plots of lens voltage vs three different criteria, namely response factor, lX51n+ signal without the matrix, and 9n+ signal with the matrix present. The optimal condition and range for each parameter, determined by the final simplex vertices, is indicated on each plot. The extraction lens voltage was a critical parameter, as shown in Fig. 2(a). The simplex optimization successfully located an extraction lens setting at which matrix induced analyte suppression was eliminated. The plots for analyte signal indicate that this was at the expense of approximately 50% of the analyte sensitivity in the matrixfree solution, but only a 20% decrease of the analyte signal in the matrix solution was observed, compared to the maximum signal achievable. It is also evident that analyte suppression was much less pronounced at an extraction lens setting which was optimal for the matrix solution, i.e. a crossover point occurred between the plots for the matrix-free and matrix solutions, which coincided with the optimum for the matrix solution. This confirms results obtained in previous studies [12, 261 in which matrix induced suppression was reduced by tuning the ion lenses in the presence of the matrix. Comparatively, the other three lenses studied had a much less critical influence on the degree of analyte suppression caused by the matrix [Figs 2(b)-(d)], with a much smaller deviation occuring for the analyte signal in the matrix solution compared to the matrix-free solution. For each of these lenses, it is evident that the simplex procedure successfully located a setting at which suppression was minimal, but analyte signal was still reasonable. The optimal setting determined for the collector lens was not confirmed by the univariate search (Fig. 2(b)). However, during the course of the experiment, some drift occurred in the optimal ion lens settings, which not only delayed completion of the simplex optimization, but also resulted in a discrepancy between the optimal condition determined by the simplex experiment, and that indicated by the

E. H. EVANSand J. A. CARUSO

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100

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mh Fig. 3. Mass response curves obtained for two extraction lens voltages: solid line: -172 V; dashed line: -244 V; 0: matrix-free solution; l : matrix solution (10000 pg g-l U). Other ion lenses were held at the optimal values determined by the simplex procedure.

univariate search. Such instrumental drift is unavoidable, especially considering the nature of the matrix being introduced into the system, and it is surprising that the effects were not even more pronounced. The fact that the extraction lens was the most critical parameter, with respect to analyte suppression, can be explained by consideration of the effects occurring in the ion beam. Evidently, the presence of a large quantity of matrix ions will increase the ion flux in the plasma and subsequent ion beam, albeit by a minor extent compared to the total ion flux, so it might be expected that a slightly more negative voltage would be necessary to extract ions through the skimmer. If an increased extraction voltage also had the effect of accelerating the extracted ions, and thereby increasing their ion energies somewhat, this might increase or decrease space-charge effects in the ion beam depending on the resultant spread in ion kinetic energies. Figure 3 shows the mass response curves for the matrix-free and the matrix solutions, without correction for degree of ionization, at two extraction lens voltages. It is evident that the relative transmission of ions of different m/zwas essentially unaltered by changing the extraction lens voltage, hence leading to the conclusion that space-charge effects were undiminished. However, the suppression caused by the matrix was severe at -172 V, but negligible at -244 V which was the optimal value determined by the simplex procedure, and therefore a more likely explanation may be that the particular set of ion lens conditions arrived at by the optimization was simply fortuitous in that it allowed a similar degree of ion transmission with and without the matrix present. VAUGHANand HORLICK[27] have shown, using a computer simulation of ion transmission through the ion optics of a commercial ICP-MS instrument, that a variety of ion lens settings can result in similar ion throughput, which lends support to the argument that it is possible to find a compromise set of tuning conditions to eliminate non-spectroscopic interferences. In this work, the usefulness of simplex optimization to achieve this was particularly important, since it took account of the interdependent nature of the variables and located the “true optimum”. The term “true optimum” should be used with qualification, since it is possible that several sets of ion lens conditions could yield very similar results. What is significant, however, is the ability of the simplex optimization to quickly optimize the system (relative to other optimization methods such as factorial experiments) for several criteria at once. It is debatable whether such an optimization strategy could be used on a regular basis for routine analysis, though it was certainly useful in identifying the most significant variables. Furthermore, even though instrumental drift caused the optimal lens settings to drift over the course of the experiment, this was not so great as to

E. H. EVANSand J. A. CARUSO

1008 120

(a) 2.3 mbar

“t 40

c

“t

OL

0

Extraetlon lens voltsge, V Fig. 4. Effect of extraction lens voltage and expansion pressure for a sampler with a 1.0 mm orifice: 0, Be; n , In; A, Pb (each at 100 ng g-l). Solid line: matrix free solution; dashed line: matrix solution (loo00 wg g-’ U).

require constant retuning, of cones was used.

and the conditions remained valid as long as the same set

3.2. Effect of extraction voltage, sampler orifice and expansion pressure Having identified the extraction lens voltage as the parameter which had the greatest influence on the magnitude of analyte suppression, it was decided to investigate the interaction between the extraction lens, diameter of the sampler orifice and expansion stage pressure. Figures 4(a) and 4(b) show the effect of extraction lens voltage at two different expansion pressures, and using a 1.0 mm orifice sampler. For these experiments, the ion lenses were initially tuned to obtain maximum IsIn+ signal for a 100 ng g-l solution, and were thereafter maintained at a constant value, with the exception of the extraction lens. As can be seen there was extreme analyte suppression due to the 10000 pg g-l U matrix, at both expansion pressures, relative to the maximum signal obtainable for the matrix-free solution. The analyte signals in the matrix and matrix-free solutions were comDarable at extraction voltages of less than -200 V. This setting was optimum for the matrix solution, but resulted in an analyte signal that was approximately 20% of that at the optimum for the matrix-free solution. Such results illustrate the main problem associated with the analysis of solutions with a high concentration of matrix. While it is possible to arrive at ion lens conditions which result in zero suppression, a sacrifice in sensitivity is usually necessary to achieve this, and perhaps more importantly a narrow band exists over which the lenses can be varied before either suppression or enhancement again occurs. The ideal condition would be the elimination of suppression over the whole range of extraction lens voltages, rather than just at one particular setting. When a 0.7 mm orifice sampler was used, the plots shown in Figs 5(a) and 5(b) were obtained. Although the degree of matrix induced analyte suppression in Fig. 5(a) is similar to that observed for the 1.0 mm sampler, when the expansion presure was reduced further by pumping at an increased rate, the resultant plot (Fig. 5(b)) exhibited much less suppression, of the order of 50%. A further reduction in the orifice diameter,

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120 (a) 1.5 mbar ‘00 t

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(b) 0.5 mbar

P’W u) SOdo4020 -

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Ionr sattlng, V Fig. 5. Effect of extraction lens voltage and expansion pressure for a sampler with a 0.7 mm orifice: 0, Be; n , In; A, Pb (each at 100 ng g-l). Solid line: matrix free solution; dashed line: matrix solution (10000 pg g-l U).

to 0.4 mm, resulted in a further reduction in analyte suppression (Figs 6(a) and 6(b)). For the 0.4 mm sampler at an expansion pressure of 0.7 mbar the suppression was only approximately 20%. At 0.5 mbar very little suppression, and in the case of Pb an enhancement, was observed. The above results illustrate that the degree of suppression caused by the 10000 u,g

120

(a) 0.7 mbar 100

00

00

1” = a20

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0

loo

(b) 0.5 mbar

20

00

OL

900

-260

ktractlon Ions Wtlng, V Fig. 6. Effect of extraction lens voltage and expansion pressure for a sampler with 0.4 mm orifice: 0, Be; m, In; A, Pb (each at 100 ng g-i). Solid line: matrix free solution; dashed line: matrix solution (10000 pg g-l U).

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E. H. EVANSand J. A. CARUSO Table 2. Signals obtained for singly and doubly charged barium ions using different sampler orifices and expansion pressures, for the matrix-free solution, and ion lens conditions which gave maximum ion transmission for the respective samplers

Sampler orifice (mm) 1.0 1.0 0.7 0.7 0.4 0.4

13RBa+

Expansion pressure (mbar)

integral (counts)

*Ba*+ integral (counts)

2.3 0.7 1.5 0.5 0.7 0.5

2215 2370 2780 1024 741 50

44 55 117 11 229 203

69Ba*+,13XBa+ ratio 0.02 0.02 0.04 0.01 0.31 4.1

g-l matrix was reduced by reducing the size of the sampling orifice from 1.0 to 0.4 mm. However, the 0.4 mm sampler resulted in greatly increased levels of doubly charged ions compared to the 1.0 and 0.7 mm samplers (Table 2) suggesting that a secondary discharge may have formed in the region of the sampler orifice. This explains the reduction in analyte suppression to some extent, since a secondary discharge could cause a substantial increase in ion energies which may have resulted in less spacecharge in the ion beam. However, the mass response curve obtained when using the 0.4 mm sampler at an expansion pressure of 0.7 mbar (Fig. 7), indicates that mass bias effects were still pronounced, and that space-charge effects were still prevalent. Early work on ICP-MS was primarily undertaken using samplers with orifices less than 100 pm in diameter [28, 291, and was characterized by severe non-spectroscopic matrix interferences. Subsequently, orifices of between 0.4 and 0.5 mm were found to reduce the extent of this type of interference [30-321, though no comprehensive study on non-spectroscopic interferences was performed using this size orifice, so comparisons are difficult. Signal intensities obtained for several elements, using different combinations of sampler orifice and expansion pressure, are shown in Table 3. As can be seen, a reduction in the size of the sampling orifice had an unpredictable effect on signal. However, it is clear that the signals were reduced considerably when using the 0.4 mm sampler, for an expansion pressure of 0.5 mbar in particular. This can be explained to some extent by the build-up of refractory uranium salts on the sampler, which caused the 0.4 mm orifice to block much sooner than the 0.7 mm or 1.0 mm orifices. Other

Fig. 7. Mass response curves obtained using a 0.4 mm orifice sampler and an expansion pressure of 0.7 mbar: 0, matrix-free solution; n , matrix solution (10008 pg gg’ U).

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Table 3. Signals obtained using different sampler orifices and expansion pressures for the matrix-free solution at ion lens settings which gave maximuth ion transmission

Sampler orifice (mm) 1.0 1.0 0.7 0.7 0.4 0.4

Peak integral (counts)

Expansion pressure (mbar)

9Be

=co

‘isIn

i3sBa

208pb

2.3 0.7 1.5 0.5 0.7 0.5

265 191 443 129 135 148

1655 1652 4485 1624 1482 801

3121 3073 3913 1763 1097 356

2275 2370 2780 1024 741 40

2607 2333 1749 930 398 96

contributing factors were the reduced volume of plasma gas sampled per unit time, and the adverse effect of a secondary discharge on the sampling process. Probably the most significant of these effects was that caused by salt build-up on the sampler, despite the fact that only 100 pl injection volumes were used. From the results shown in Figs 4-6, the conclusion could be drawn that the expansion pressure had some effect on the degree of analyte suppression. However, this effect was not reproducible in so far as the degree of suppression varied between 75% and 50% for the 0.7 mm sampler from day to day. Such variations may have been linked to the unpredictable existence of a secondary discharge in the 0.7 and 0.4 mm sampler orifices, though this was only definitely the case for the 0.4 mm sampler, as can be seen in Table 2, which shows the amount of doubly charged barium ions formed with the different samplers. The amount of doubly charged barium was significantly greater only for the 0.4 mm sampler.

4. CONCLUSIONS Multivariate simplex optimization was successfully used to optimize the ion lenses of the VG Plasma&ad, inductively coupled plasma mass spectrometer. In this way, non-spectroscopic matrix effects due to 10000 kg g-l were effectively eliminated. The diameter of the sampling orifice and the extraction lens voltage were shown to have the most significant influence on the extent of analyte suppression caused by the matrix. Analyte suppression could be eliminated by judicious choice of extraction voltage for 1.0 mm and 0.7 mm orifice samplers; however, the use of a 0.4 mm sampler resulted in the elimination of analyte suppression at all extraction voltages. The main disadvantages of the 0.4 mm orifice sampler were the tendency for the orifice to rapidly block in the presence of the 10000 kg g-l uranium matrix and the propensity for a secondary discharge to form in the orifice, resulting in elevated levels of doubly charged ions, though this effect may well have been the reason for the decrease in analyte suppression using the 0.4 mm orifice sampler. While a reduction in the expansion stage pressure seemed to decrease the degree of analyte suppression in some experiments, the effects were not reproducible from day to day. Acknowledgemenrs-The authors wish to acknowledge the National Institute for Environmental Health Sciences for providing research support through grants numbered ES-03221 and ES-04908, and the NIHBRS Instrument Program for providing the VG PlasmaQuad through grant SlORRO2714.

REFERENCES [l] J. A. Olivares and R. S. Houk, Anal. Chem. 58, 20 (1986). [2] D. Beauchemin, J. W. McLaren and S. S. Berman, Spectrochim. Acta [3] D. C. Gregoire, Appl. Specfrosc. 41, 897 (1987).

42B, 467 (1987).

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Acta 38B, 39 (1983).