evaluation of ethene addition to the nebulizer gas in inductively coupled plasma-mass spectrometry...

Evaluation of Ethene Addition to the Nebulizer Gas in Inductively Coupled Plasma-Mass Spectrometry for the Removal of Matrix-, Solvent-, and Support-Gas-Derived Polyatomic Ion Interferences

L E S EBDON,* M I C H A E L J. FORD, ROBERT C. HUTTON, and STEVE J . H I L L Analytical Chemistry Research Unit, Department of Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, Devon PL4 8AA, U.K. (L.E., M.J.F., S.J.H.); and FI Elemental, lon Path, Road Three, Winsford, Cheshire CW7 3BX, U.K. (R.C.H.)

Ethene addition to the nebulizer gas is assessed as a means of reducing the levels of polyatomic ions in ICP-MS. It was found that, while the absolute analyte response was generally poorer with the addition of ethene, the levels of the interferences were greatly reduced, when compared to the all argon plasma. The plasma operating conditions, with the addition of ethene to the nebulizer gas, were optimized with the use of a variable-step-size Simplex procedure in order to remove or reduce the ArO +, ArCI +, CIO ÷, CeO +, ArNa ÷, SO~+/S~ +, and PO~ + interferences. Complete removal of the ArCI +, ArNa +, SOz+/S2 +, and POz + was achieved, while the levels of the remaining interferences were very much reduced. Detection limits for As, Se, and V in 10% HCI were excellent (0.27, 2.7, and 2.04 ng cm 3, respectively), as was the detection limit for Cu in 1% H3PO 4 at 6.9 ng cm -3. Cu, Ni, and Zn detection limits in the presence of H 2 S O 4 and sodium ions were also improved, and the detection limit for Fe was 0.48 ng cm -3. Levels of CeO + were reduced to just 0.2% relative to Ce. Recovery tests and the analysis of certified reference materials further demonstrated the utility of the optimal conditions. A set of compromise ethene conditions was identified and shown to be of value in reducing the interferences simultaneously.

Index Headings: Ethene addition; Mixed-gas plasmas; Polyatomic ion interference removal; Inductively coupled plasma-mass spectrometry; As, Cu, Fe, Gd, Ni, Se, V, and Zn determination.

INTRODUCTION

The occurrence of polyatomic ion species, at the same nominal mass as the analyte of interest, in inductively coupled plasma-mass spectrometry (ICP-MS) produces interferences. These interferences are most pronounced below m/z 80,1-4 originating predominantly from the support gas and the sample solvents, and are made worse by the presence of matrix elements such as C1, S, P, and Na in high concentrations. Of the many methods available to overcome these interferences (see the excellent review by Evans and Gigli@), a simple and potentially very cheap method is the use of mixed-gas plasmas. These mixed- gas plasmas are typically conventional argon ICPs into which a molecular or another inert gas is introduced, via one or more of the three gas flows or, less typically, via the argon gas inlet.

Nitrogen addition in ICP-MS for interference removal was first reported by Evans and Ebdon , 6,7 the authors finding the gas to be of use in reducing the ArC1 ÷ interference when added to the nebulizer gas flow. This ap-

Received 28 May 1993; accepted 30 August 1993. * Author to whom correspondence should be sent.

proach was later applied for improving the determination of As in high chloride matrices? Nitrogen addition has been extensively studied by a number of other groups. Lain and Horlick 9 investigated nitrogen addition to the nebulizer and outer gas flows to reduce polyatomic ion response and also found nitrogen to cause analyte enhancement. Lam and McLaren ~° further showed the effectiveness of nitrogen addition to the outer gas by com- bining it with desolvation of the analyte aerosol to achieve large reductions in the ArO + and UO + responses. Wang et al. ~ have added nitrogen to all three gas flows in order to reduce chloride-based interferences. Beauchemin and Craig 12,~3 focused their addition of nitrogen to the outer gas to improve the analysis of 56Fe and 788e and also to overcome nonspectral matrix interferences from Na. More recently Hill et al. ~4 have added nitrogen to all three gas flows and used Simplex optimization to reduce or remove the ArC1 ÷ and C10 + interferences, reporting as well on reductions in the levels of ArO ÷, MO +, and continuum background levels. The authors found that, with their VG Plasmaquad, the addition of nitrogen was most effective when added to the nebulizer gas and less so in the intermediate gas flow. Louie and Yoke-Peng Soo ~5 and Emer- tyd and Olsson 16 have reported the addition of nitrogen to the gas inlet for interference removal or analyte enhancement, the latter being interested in improving the measurement of sulfur ratios.

While nitrogen addition to the plasma has been the most popular supplementary gas, others have been studied. Hydrogen addition has been reported; 15.~7 in the latter reference it was found to be useful in reducing MO ÷ interferences but enhanced other interferences, such as ArC1 + and ArO +. This observation further indicates the different origins of these polyatomic ions. Air and oxygen additions have been u s e d , 7,9 but were not found to offer any great advantage. Xenon has been investigated by Smith et al., TM and they reported reduced polyatomic ion levels, in common with other mixed-gas plasmas. Readers will observe that this approach is a very expensive option. Hydrocarbon gases have more recently been introduced to the plasma. Allain et al. ~9 used methane addition to achieve analyte enhancement, particularly of poorly ion- ized elements. Methane has also been used to reduce interferences in ICP-MS; ~,2° the latter report employed Simplex optimization to define optimal operating parameters, and methane was found to be even more effective

Volume 48, Number 4, 1994 0003-7028/94/4804-050752.00/0 APPLIED SPECTROSCOPY 507 © 1994 Society for Applied Spectroscopy

than nitrogen addition for reducing a wide range of interferences.

Given the relative advantages of methane addition compared to hydrogen addition, it seemed logical to investigate ethene addition, since it has a greater C:H ratio (0.5) in comparison to methane (0.25). In this paper we report the addition of ethene to the nebulizer gas of a conventional argon ICP for MS. Simplex optimization was employed as a rapid means of defining plasma operating parameters to allow removal or reduction ofArO +, ArC1 +, C10 +, CeO +, ArNa +, SO2+/$2 +, and PO2 +. These defined optima were then tested by the determination of the detection limits and recoveries of the "obscured" elements in the presence of their interference precursors and also by the analysis of suitable certified reference materials. The utility ofethene as a plasma modifier was also investigated by reference to a set of compromise conditions which were used to reduce interference levels simultaneously on several analytes.

EXPERIMENTAL

Instrumentation. The instrument used was an inductively coupled plasma-mass spectrometer (PQ2, VG El- emental, Winsford, Cheshire, U.K.). The sample introduction system included a modified high-solids nebulizer (Ebdon type, PSA, Sevenoaks, Kent, U.K.), which allowed higher nebulizer gas flows than the standard high- solids nebulizer. Samples were pumped to the nebulizer with the use of a peristaltic pump at a rate of 1 cm 3 min -1 . Standard nickel sampler and skimmer cones were used with a 1-mm orifice, and these were cleaned at the start of each session with an aluminum-based cleaning agent. Ethene addition to the nebulizer gas was achieved with the use of a gas blender (Series 850, Signal, Standards House, Doman Road, Camberly, Surrey, U.K.). The ethene was 99.8% pure (ECM Ltd., Stoke on Trent, U.K.). For Simplex optimization experiments involving oxygen- based polyatomic interferences, the spray chamber temperature was maintained at around I°C by the use of ice/ water mixture circulated with a conventional water pump (Tempette TE 8A, Techne Ltd, Duxford, Cambridge, U.K.).

Materials and Chemicals. Four certified reference materials (CRMs) that had high levels of interferent precursors were analyzed: citrus leaves (NIST SRM No. 1572, National Institute of Standards and Technology, Oai- thersburg, MD, U.S.A.), lobster hepatopancreas (TORT- 1, National Research Council of Canada, Division of Chemistry, Marine Analytical Chemistry Standards Pro- gramme, Ottawa, Canada) seawater and riverine water (NASS-2 and SLRS-2, respectively, National Research Council, Canada).

Standard and calibration solutions were prepared from 1000-~g mL-1 stock solutions of As, Co, Cu, Fe, Ni, Se, and Zn (Merck, Poole, Dorset, U.K.), Be and In (Aldrich Chemical Company, Milwaukee, WI, U.S.A.), and Ce, Dy, Er, Eu, Gd, Ho, Lu, Nd, Sm, Tb, U, and V (prepared from Ce203, Dy203, Er203, Eu203, Gd203, H0203, Lu203, Nd203, Sm203, Tb203, UO2(NO3)2" 6H20, and NH4VO3, respectively). Internal standardization employing either Co, Ho, or In at 100 ng mL- ~ was used in all experiments. All standard solutions were made up in 2% HNO3 (Ar-

istar, Merck). The hydrogen peroxide used in the digestion of the CRMs was 30% (Aristar, Merck). Chloride, sulfate, and phosphate spiking of standard solutions used hydrochloric acid, sulfuric acid, and orthophosphoric acid, respectively (concentrated, Aristar grade, Merck).

Sample Preparation. Approximately 0.5-g amounts of lobster pancreas and citrus leaf CRMs were all digested with the use of microwave bomb digestion. The procedure has been reported elsewhere,~4 with a nitric acid/hydrogen peroxide mix (3:2) used for the digestions. After digestion, the solutions were quantitatively transferred to volumet- ric flasks (50 mL) and made up with deionized, distilled water. Samples were spiked with Co to a final concentration of 100 ng mL- ~ as an internal standard. The seawater and freshwater CRMs were diluted and spiked with Co as above. Five replicates of each sample were prepared. Standard solutions and calibration solutions were prepared fresh from the stock solutions as required.

Procedure. Seven separate variable-step-size Simplex optimizations of the operating parameters, with the addition of ethene to the nebulizer gas, were performed to find the optimal conditions to reduce or remove ArO +, ArC1 +, CIO +, CeO +, ArNa +, SO2+/$2 +, and PO2+; the criterion of merit for these optimizations was the cobalt response divided by the interference signal, since this approach allowed for the minimization of interferences without loss of analyte signal. Cobalt was chosen as the spike element since it is present at very low levels in natural environmental samples, is not subject to significant polyatomic ion interferences, and was within the mass range studied. For cerium, the cerium response divided by the cerium oxide signal was used, since in this case minimizing the oxide signal with good analyte signal is vital. The Simplex optimization was considered to be complete when the figure of merit of the n + 1 (i.e., five) best vertices agreed to within 5% or better. The optimum conditions for each parameter were then defined as the average of these five vertices. The optimal conditions defined by these Simplex optimizations were tested by determination of detection limits (3a) for As, Se, and V in the presence of increasing levels of chloride (0, 100, 1000, 10,000, and 33,000 #g mL-~); Gd in the presence of increasing levels of Ce (0, 0.1, 4, and 10 #g mL ~); Cu and Ni in the presence of increasing levels of Na (0, 1, 10, 100, and 1000 #g cm 3); Cu and Zn in the presence of increasing levels of S (0, 0.01 0.1, 1% H2SO4); Cu and Zn in the presence of increasing levels of P (0, 0.01 0.1, 1% H3PO4); and also for Fe. Recovery tests determined with Simplex optimized and typical all-argon plasma operating conditions of similarly spiked solutions of the above elements were also performed. Analysis of the CRMs was undertaken at the optimal conditions for As, Cu, Fe, Se, and V, and results were compared to those determined at "typical" operating parameters (see Table I and Ref. 20).

RESULTS

Simplex Optimization of the Plasma with Ethene Add- ed to the Nebulizer Gas for the Removal of Polyatomic Ion Interferences. Simplex optimization 21 has been shown to be of great value to analytical chemists for the tuning of spectroscopic instruments, 22-25 where optimal perfor-

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T A B L E I. Typical operating parameters used for the ICP-MS instrument.

Parameter Value

O u t e r g a s (din 3 rain ~) 15.0 In termedia te gas (dm 3 min ') 0.5 Nebul izer gas (dm 3 min ~) 1.0 Forward power (W) 1400 Chiller water tempera ture (°C) 18-20

mance is often dependent on a number o f interrelated variables that do not lend themselves to simple univariate optimizat ion techniques. In the field o f ICP-MS it has been used to optimize organic solvent analysis, 26 reduce the nonspectroscopic matrix interference of U, 27 and improve interference removal for mixed-gas plasmas. ~4,~7,2° For this reason, it was considered the most appropriate method for the optimizat ion ofe thene-modif ied plasmas.

A variable-step-size Simplex approach was undertaken, optimizing the nebulizer gas, auxiliary gas, forward power, and percentage ethene addition. The exclusion of the outer gas from the Simplex (it was fixed at 15 dm 3 m i n - 1) was done because previous experience had shown it to have little or no effect on the figure of merit. The figure of merit for the Simplex was the ratio Co+/interference for all except the CeO + optimization, where Ce+/CeO + was used. The Simplex was considered to have finished when the n + 1 (5) highest ratios agreed to within 5%. Simplex optimizations were performed to define conditions to remove ArO +, ArC1 +, C10 +, CeO +, ArNa +, SO2+/ $2 +, and PO2 +.

Simplex Optimization for the Removal of ArO +. This opt imizat ion used the ratio o f the response of 100 ng cm -3 Co to the ArO + response (Co+/ArO + ) as the figure o f merit. The optimizat ion was stopped after 27 steps, and the optimal conditions are given in column one of Table II. As has been found with the addit ion of nitrogen and methane, interference removal was normally favored by lowered forward power and increased nebulizer gas flow, and this was again found in the case for ethene addit ion for the removal of ArO +. Whenever a Simplex optimization was performed, the resulting conditions were checked by the use of univariate searches. These involve adjusting each o f the opt imized parameters in turn while the others are held fixed. In this paper only those searches for the ArO + are shown; however, the other optimizations showed the same trends. Figures 1A-1D show the uni- variates for the ArO + optimization, and these confirm that it is the nebulizer gas, forward power, and percentage ethene addit ion that are the critical parameters, with the intermediate gas having little effect. The sharp values defined for the other conditions are something of a dis- advantage because they make the system prone to minor

fluctuations in the settings of these parameters. Generally the opt imizat ion has, within the bounds of the error as- sociated with the stop conditions, found the optimal conditions, being only slightly off the op t imum for the nebulizer gas and ethene addition.

The detection limits for iron under these conditions are given in Table III and are compared to those found for methane addit ion and an all-argon system. 2° The detection limits for iron with the addit ion o f ethene are essentially the same as those found for the addit ion o f methane, but these still represent a marked improvement over those obtained under typical conditions with the use o f an all-argon plasma. The ArO + peak was found to be slightly bigger with ethene addit ion with a background equivalent concentrat ion (BEC) o f 4.5 ng cm 3 compared to one o f 3.5 ng cm -3 found with methane, but the difference is not considered significant and is still far below the BEC of 63 ng cm -3 reported for the typical conditions. 2° Figures 2A and 2B show calibration graphs obtained for the 56Fe and S7Fe isotopes and demonstrate that, with the use o f the optimal conditions, excellent calibrations can be obtained (in the 1-100 ng cm 3 range for iron isotopes), which are otherwise obscured by ArO + and ArOH + .

Results for the analysis o f iron in CRMs are shown in Table IV, and it can be seen that good agreement between the certificate and measured values was obtained under the opt imized conditions. Under typical all-argon plasma conditions, values for iron were significantly lower due to the increased blank levels because of the increased ArO + response.

Simplex Optimization for the Removal of ArCl +. This opt imizat ion used the ratio o f the response of 100 ng cm -3 Co to the ArC1 + response (Co+/ArC1 + ) as the figure of merit. The ArC1 + response was generated by the as- piration o f a 10,000-ug cm -3 solution of chloride (3.3 cm -3 concentrated HC1 + 96.7 cm -3 deionized water). The optimizat ion ran to completeness after 21 steps, and the optimal conditions are given in column two of Table II. These conditions showed the expected higher nebulizer gas flow but, surprisingly, not the lower forward power previously found, and this higher power was found with a number of the other optimizations. At these optimal condit ions the levels of ArC1 + at both masses 75 and 77 were equivalent to the random background response with signals on the order o f 10 ACPS. Effectively, the addit ion of ethene under these conditions was leading to a complete removal o f the ArCI + interference in the presence of a matr ix of 3.3% HCI. More recent work looking at hydride generator waste water--which, owing to the na- ture of the chemistry o f hydride generation is at approximately 18% HCl - -has revealed that, even at these ele- vated levels o f CI-, no ArC1 + is observed.

TABLE II. Simplex optimized conditions for the removal of seven polyatomic ion interferences with the addition of ethene to the nebulizer gas of an argon ICP.

Simplex

Parameter ArO* ArCI + C10 ÷ CeO ÷ NaAr + SO2+/$2 + PO2 ÷

Nebulizer gas (dm 3 m i n - ' ) 1.11 1.19 1.08 0.90 1.22 1.26 0.98 In termedia te gas (rim 3 min ~) 0.75 1.15 1.55 0.60 1.3 1.40 0.90 Forward power (W) 1390 1570 1520 1330 1540 1560 1440 Ethene (% v/v) 0.27 0.23 0.41 0.16 0.275 0.31 0.195

APPLIED SPECTROSCOPY 509

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FiG. IC. Univariate search of percentage ethene (v/v) at Simplex optimized conditions for the reduction ofArO + with the addition ofethene to the nebulizer gas, with the other operating parameters held at the optimal values shown in Table II. The dashed arrows indicate the limits of the final Simplex and the solid arrows the median of that Simplex.

Detection limits for As and Se in the presence of increasing levels of C1- are given in Table V. Also included in Table V are detection limits for 788e, which does not suffer an ArCI ÷ interference and hence is independent of C1- concentration but does suffer from an Ar2 ÷ interference, which is reduced by the addition of ethene. The As detection limit was effectively the same at 0 and 3.3% v/v chloride, with only a modest increase in the BEC, defined as the signal from the background interference expressed in terms ofanalyte concentration (0.39 ng cm -3 at 0% C1 and 0.66 ng cm -3 at 3.3% C1). Detection limits for As in 3.3% C1 compared for the various plasma modifiers that have been utilized in this laboratory are shown in Table VI. The most striking feature of these data is that, even compared to results for an all-argon system optimized for minimum interferences, the detection limit for As in this matrix is improved by two orders of mag- nitude with the ethene-modified plasma.

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FIG. 1A. Univariate search of nebulizer gas flow at Simplex optimized conditions for the reduction ofArO ÷ with the addition ofethene to the nebulizer gas, with the other operating parameters held at the optimal values shown in Table II. The dashed arrows indicate the limits of the final Simplex and the solid arrows the median of that Simplex.

Forward power (W)

FIG. lB. Univariate search of forward power at Simplex optimized conditions for the reduction ofArO + with the addition ofethene to the nebulizer gas, with the other operating parameters held at the optimal values shown in Table II. The dashed arrows indicate the limits of the final Simplex and the solid arrows the median of that Simplex.

The detection limits for Se at both the 77 and 78 isotopes are also given in Table V and show that sub and low ng cm -3 detection limits are possible for this prob- lematic element, even in the presence of a high-chloride matrix. Again, a comparison of the detection capabilities of this and other techniques is presented in Table VI for these two isotopes; once again, the improvements over the all-argon system are striking. To further emphasize the quality of these data, Figs. 3A and 3B show calibration graphs for 775e and 785e in a 10% HC1 matrix; it should be noted that the bottom standard for these calibrations is just 1 ng cm -3.

Recovery tests for As and Se were undertaken with solutions at 50 ng cm -3 As and Se with increasing CI concentrations and were found to yield quantitative recoveries for all the analytes, with the interference becoming significant only at the highest C1 concentration. These

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Auxiliary gas flow (L/min)

FIG. ID. Univariate search of intermediate gas flow rate at Simplex optimized conditions for the reduction of ArO ÷ with the addition of ethene to the nebulizer gas, with the other operating parameters held at the optimal values shown in Table II. The dashed arrows indicate the limits of the final Simplex and the solid arrows the median of that Simplex.

S10 Volume 48, Number 4, 1994

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Fro. 2. Calibration for Fe at the Simplex optimized conditions for the removal of ArO + with the addition of ethene to the nebulizer gas. (A) 56Fe, and (B) STFe.

conditions still offered a huge improvement on recoveries attained with the all-argon system? °

The analysis o f reference materials, shown in Table VII, again showed that the opt imized conditions gave excellent agreement between the measured and certificate values but, with the use o f the typical all-argon plasma, the data were poor. The successful determinat ion of Se is particularly encouraging.

Simplex Optimization for the Removal of ClO +. This opt imizat ion used the ratio o f the response o f 100 ng cm -3 Co to the C10 + response (Co+/C10 + ) as the figure of merit. The ArC1 + response was generated by the as- piration of a 10,000-#g cm -3 solution of chloride. The optimal conditions defined are given in column three of Table II, and vanadium detection limits determined in the presence of increased levels of C1 are given in Table VIII. These detection limits are similar to those found with previous mixed-gas studies 15,2° and are an improvement over results for an all-argon system. Calibration of V down to 1 ng cm 3 V in 3.3% CI was easily performed. As with the As and Se, quanti tat ive recoveries of a 50- ng cm -3 V solution were possible at up to 1% C1, the optimal conditions being a vast improvement on all-argon conditions.

The successful determinat ion of vanadium in lobster tissue is shown in Table IX, which also shows that, under

TABLE IIl. Detection limits for iron with the addition of ethene to the nebulizer gas, compared to detection limits with the addition of methane and with typical conditions, a

Operating Isotope

conditions S4Fe 56Fe ~'Fe

Ethene addition 8.5 0.48 3.4 Methane addition 5.0 0.48 6.0 Typical conditions b 380 7.3 48

" Detection limits (3a) are in ng cm 3. t' Data from Ref. 20.

TABLE IV. Iron concentration in a range of CRMs at (1) the Simplex optimized conditions for ethene addition to the nebulizer gas for the removal of ArO ÷ and (2) for typical operating parameters."

Levels found

Certified Sample (1) (2) value

Lobster pancreas (TORT- l ) (ug g - ' ) 171 _+ 6.7 89 _+ 14.8 b 186 _+ 11

Riverine water (SLRS-2) (ngcm 3) 117 + 3.0 92_+ 6.3 c 129_+ 7

" Note: n = 5; all values _+ 1 SD. The 5~Fe isotope was used. h Data from Ref. 20. " See Table I.

TABLE V. Detection limits for arsenic and selenium at increasing chloride levels determined at the Simplex optimized conditions for addition of ethene to the nebulizer gas for the removal of ArCI+."

Element Chloride level

(ug cm 3) As 77Se 78Se

0 0.21 1.4 0.9 100 0.18 1.6 0.8

1000 0.18 0.6 0.5 10,000 0.18 1.7 0.7 33,000 0.27 2.7 1.4

" Detection limits (3a) are in ng cm 3.

TABLE VI. Improvement factors in the detection limits of arsenic and selenium with Simplex optimized conditions for the addition of ethene, methane, nitrogen, and an all-argon system. ~

Element

Arsenic 77Selenium 78Selenium

Detec- Improve- Detec- Improve- Detec- Improve- tion ment tion ment tion ment

Added gas limit factor b limit factor h limit factor b

None (all argon) 30.0 -.- 90.0 .. . 60.0 . . . Nitrogen 2.1 14.3 27.0 3.3 200 0.3 Methane 0.75 40.0 14.0 6.4 6.9 8.7 Ethene 0.27 111 2.8 32.1 1.4 43.0

" Detection limits (3a) are in ng cm 3. h Improvement factor is relative to detection limits obtained for the all-

argon system.


TABLE VII. Arsenic and selenium concentration in a range of CRMs at (A) the Simplex optimized conditions for ethene addition to the nebulizer gas for the removal of ArC! ÷ and (B) for typical operating parameters:

Levels found (ug g - ' )

(A) (B) Certified value

Arsenic

Lobster pancreas (TORT-I ) 25.9 ± 1.5 40.2 ± 16.5 b 24.6 ± 2.2 Citrus leaves (NIST 1572) 2.5 _+ 0.08 . ..c.a 3.1 ± 0.3 Seawater (NASS-3) 0.0045 ± 0.00036 2.74 ± 0.11 b 0.00165 ± 0.00019

Selenium

Lobster pancreas (TORT- l ) 6.1 ± 0.4 50 ± 18 6.88 ± 0.47

~' Note: n = 5; all values ± 1 SD. Values for seawater are ug cm -3. The 78Se isotope was used. b Data from Ref. 20. c Value less than the blank. d See Table I.

typical all-argon plasma conditions, the measured vanadium value is both less accurate and less precise.

Simplex Optimization for the Removal of CeO +. This optimization used the ratio of the Ce + signal to CeO + signal as the figure of merit. Cerium was chosen because it is one of the most refractory elements. The above ratio was determined by the analysis ofa 100-ng cm -3 solution of Ce using the 14°Ce isotope and the 156CeO oxide peak. The optimal conditions for the removal of this interference are given in column four of Table II.

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These conditions were found to be somewhat different from all the other sets of conditions determined for the other interferences, the lower nebulizer gas flow reflecting the fact that, at both low and high nebulizer gas flows, CeO + response can be far greater than the Ce + parent ion response. The difference in the optimal conditions is further evidence of the different mechanisms of formation o fMO + species. At the optimal conditions, the Ce selec- tivity was improved from a typical value of 20 to 470, the latter being equivalent to a percentage oxide level of 0.21%. This level is some three times better than levels achievable with the addition of hydrogen or methane to the plasma. 16,2° The reduction in the level of the CeO + is particularly impressive when it is remembered that there was no desolvation applied to the system, only a reduction in the spray chamber cooling water (measured as I°C in the water bath). The application of desolvation/ethene addition to increase the efficacy of CeO + removal is not possible, however, because in the absence of water even the smallest amounts of ethene lead to rapid deposition of carbon on the sampling cone. The mechanism by which ethene reduces the levels of CeO + is still unclear, and research into this problem is at present being undertaken.

In an effort to test the tolerances of the conditions, detection limits were determined at the optimal and typical all-argon plasma conditions for 156Gd and lSSGd in the presence of increasing levels of Ce (0, 0.1, 4, and 10 ug cm-3). These data are given in Table X and show that, at the higher Ce concentrations, the optimized conditions are giving approximately a tenfold improvement in Gd detection limits. The recoveries for a similarly spiked 40- ng cm -3 Gd solution were also tested at the two sets of conditions; as demonstrated in Fig. 4, the optimal conditions greatly improved the recoveries of the Gd, the CeO + becoming significant only at approximately 1 ~g

TABLE VIII . Detection limits for vanadium at increasing chloride levels determined at the Simplex optimized conditions for addition of ethene to the nebulizer gas for the removal of CIO+. a

Chloride level (ug cm -3) Detection limit

10 20 30 40 50 60 70 80 90 100 0 0.15 100 0.12

Concentration (ng/ml) 1000 0.21

FIG. 3. Calibration for Se at the Simplex optimized conditions for the 10,000 1.14 removal of ArC1 + with the addition of ethene to the nebulizer gas. (A) 33,000 2.04 77 Se and (B) 78Se. " Detection limits (3a) are in ng cm -3.


Levels found (#g g- ' )

° Note: n = 5; all values ± 1 SD. b Data from Ref. 20.

Certified Sample (A) (B) value

Lobster pancreas (TORT-l) (ug g- ' ) 1.29 ± 0.07 2.54 ± 0.22 b 1.4 ± 0.3

w

cm -3 for the 156Gd and 5 ~g cm -3 for t58Gd. A final test of these conditions was to investigate percentage oxide formation again at the two sets of conditions identified above. The results of this study are demonstrated in Fig. 5, where it can be seen that the reduction in oxide levels occurs across the whole range of REE. Improvement factors range from 1.3 for Eu (0.049% oxide formation under typical conditions to 0.038% under optimal conditions) to 9.7 for Nd (1.72% to 0.178%).

The final three Simplex optimizations undertaken on ArNa +, SO2+/$2 ÷, and PO2 + had not been extensively studied before in mixed-gas work but have been recog- nized as potential problems, particularly in the analysis of biological materials which are rich in the precursors of these interferences.

Simplex Optimization for the Removal of NaAr +. The NaAr ÷ interference on 63Cu and a related sodium interference, Na20 ÷ on 62Ni, can present particular problems for the determination of these elements in both biological and marine materials. It was suspected that these interferences could easily be removed by a mixed-gas plasma technique since these species are similar to some already successfully removed. In this experiment the NaAr + interference was minimized by the use of Simplex optimization using the ratio of Co÷/ArNa + as a figure of merit. The optimization was completed in 18 steps, and the optimal conditions are given in column five of Table II and are seen to be similar to the conditions defined for the other Ar-based interferences. Detection limits for 62Ni and 63Cu were determined in increasing concentrations of Na (0, 1, 10, 100, and 1000 ug cm -3) to assess the utility of these conditions, and these are given in Table XI. It can be seen that increasing the concentration of Na has the most severe effect on the 62Ni isotope; this result is a reflection of its low abundance (3.71%) and the fact

TABLE X. Detection limits for gadolinium at increasing cerium levels determined at the Simplex optimized conditions for addition of ethene to the nebulizer gas for the removal of CeO +, and under typical all- argon plasma operating conditions."

Operating conditions

Simplex conditions Typical conditions

Cerium Gd isotope Gd isotope level

(/.tg cm -3) '56Gd ~SgGd 156Gd 158Gd

0 0.18 0.09 0.09 0.09 0.1 0.45 0.21 0.93 0.21 4 2.9 0.42 30 3.7

10 6.9 0.90 81 9.5

" Detection limits (30) are in ng cm -~.

750

675 E '~ 600

525

~ 450

375

300

225

150

<

0

TABLE IX. Vanadium concentration in a range of CRMs at (A) the Simplex optimized conditions for ethene addition to the nebulizer gas for the removal of CIO + and (B) for typical operating parameters, a

.01 .1 1 10

Cerium concentration (gg/ml)

FIG. 4. Apparent ~56Gd concentration with increasing concentrations of cerium at (A) typical operating conditions and (O) Simplex optimized operating conditions for the removal of CeO ÷ with the addition of ethene to the nebulizer gas.

that this effect is greatly reduced with the addition of ethene. The complete removal of the NaAr + interference is demonstrated by the observation that 63Cu detection limits at 0 and 1000 ug cm -3 Na are essentially the same. For completion of this set of experiments, recoveries were once again tested for 50-ng cm -3 Ni and Cu solutions with increasing levels of sodium (0, 1, 10, 100, and 1000 ug cm-3) , and it was found that recoveries were better with the optimized conditions, particularly for the Ni.

Simplex Optimization for the Removal of S02+/$2 +. The use of H 2 S O 4 as a digestion acid is common for the dissolution of a wide range of materials, and, in addition to this consideration, sulfur is ubiquitous in biological matrices. The introduction of sulfur into the plasma can, however, produce a range of quite substantial interferences based on the three isotopes of S in combination with O and H. These are most dominant at m/z 48, 49, and 50 (mostly SO÷/SOH ÷) and m/z 64, 65, and 66 (S02+/S2+/S02H+/S2H +). The latter set of interferents can obscure the determination of Cu and Zn. In this experiment the ratio of Co ÷ at m/z 64 to (SO2+/$2 +) interference was used as a figure of merit for the optimization

3.0

2.5

2.0

1.5 k

o.o

Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Lu

FIG. 5. Percentage oxide formation of a range of rare earth elements at (z~) typical all-argon plasma operating conditions and (D) Simplex optimized operating conditions for the removal of CeO + with the addition of ethene to the nebulizer gas.


TABLE XI. Detection limits for copper and nickel at increasing sodium levels determined at the Simplex optimized conditions for addition of ethene to the nebulizer gas for the removal of NaAr + and under typical all-argon plasma operating conditions."


Simplex conditions Typical conditions Sodium Element Element

level (#g cm -3) 6-'Ni 63Cu 62Ni 63Cu

0 1.5 0.36 0.27 0.03 1 1.0 0.18 2.0 0.24

10 1.6 0.09 2.0 0.18 100 2.8 0.15 1.8 0.24

1000 11.0 0.5 44 8.6

" Detection limits (3a) are in ng cm -3.

of the system. The optimum conditions are given in column six of Table II and are in keeping with the other conditions defined for non-MO + interferences. At the optimal conditions the response at m/z 64 was a few tens of ACPS as compared to several thousand under typical conditions. Table XII shows the detection limits for 64Zn, 65Cu, and 66Zn with increasing levels of H z S O 4 (0, 0 .01 , 0.1, and 1%), and it is seen that the optimized conditions offer only modest improvements over the typical conditions. The value of ethene addition was demonstrated, however, with recovery tests for Cu and Zn with increasing levels of sulfuric acid (0, 0.01, 0.1, and 1%), an ex- ample of which is shown for 64Cu in Fig. 6. In this figure it can be seen that, even though the optimized conditions for ethene addition recoveries are still far greater than 100%, they are much improved with respect to the data obtained with typical all-argon plasma conditions.

Simplex Optimization for the Removal of P02 +. As with H2SO4, H3PO4 is a common digestion acid, and phosphorus is also ubiquitous in biological matrices. Again as with sulfur, the introduction of phosphorus into the plasma can cause a number of interferences, i.e., PO2 + and PO2H +. These, as with SO2 +, etc., can obscure the determination of Cu and Zn. In this experiment the ratio ofCo+/PO2 + was used as a figure of merit. The optimum conditions for this Simplex optimization are given in col-

1200

E 1 0 0 0

c~ 8 0 0

6 0 0

8 S 4 0 0

~. 200 <

0 .01 .1 1

Sulfuric acid concentration (%v/v)

FIG. 6. Apparent 64Zn concentration with increasing concentrations of sulfuric acid at (A) typical all-argon plasma operating conditions and ([]) Simplex optimized operating conditions for the removal of SO_,+/ S~ + with the addition of ethene to the nebulizer gas.

TABLE XII. Detection limits for copper and zinc at increasing H 2 S O 4 levels determined at the Simplex optimized conditions for addition of ethene to the nebulizer gas for the removal of SO~+/$2 + and under typical all-argon plasma operating conditions."



H~SO4 Element Element level

(% W/V) 64Zn 65Cu 66Zn 64Zn 65Cu 66Zn

0 0.48 0.12 0.45 0.78 0.27 0.69 0.01 1.5 0.78 0.70 1.5 0.54 0.54 0.1 1.5 4.5 0.93 4.0 1.9 1.4 1 14 3.9 1.7 13 6.3 4.0

" Detection limits (3a) are in ng cm 3.

umn seven of Table II. At the optimal conditions, the responses ofPO2 + and HzPO3 + were reduced about 100- fold. Table XIII shows the detection limits for 63Cu with increasing levels of H3PO4 (0, 0.01, 0.1, and 1%). At the highest spike of phosphoric acid, the addition of ethene results in a 115-fold improvement in the 63Cu detection limit and a 500-fold reduction in the BEC (9700 to 21 ng c m - 3 ) . This result was further shown in the recovery tests, where the quantitative recovery of 50 ng cm -3 Cu was possible at all spikes of H 3 P O 4 with ethene addition, but not at all with typical all-argon plasma conditions (see Fig. 7).

The Use of Ethene to Remove a Suite of Polyatomic Interferences Simultaneously. Simplex optimization experiments having been successfully performed for seven common and severe polyatomic interferences, attention turned to the possibility of defining conditions which could remove or reduce all these interferences simultaneously. First, an average of each of the conditions shown in Table II was taken, and these averages were compared to the defined conditions and the univariate searches that were undertaken for each of the optimizations. This approach made it possible to estimate the effect of moving from the defined optima for each of the interferences. For ex- ample, consider the ArO + optimal conditions; if, as is shown in Fig. 1A, the nebulizer gas is reduced to 0.9 dm 3 min -~, the figure of merit falls threefold, and this would be an inappropriate setting for compromise conditions. If, however, the power for this optimization were reduced to 1300 W, then the figure of merit would remain un- changed, and this would be appropriate for a set of compromise conditions. With the application of this procedure, it was possible to define a set of compromise conditions for the removal of all the interferences, with the exception ofCeO ÷, where the optimal conditions were too far from the compromise conditions. These compro-

TABLE XIII. Detection limits for copper with increasing HaPO4 levels determined at the Simplex optimized conditions for addition of ethene to the nebulizer gas for the removal of PO2 ÷ and under typical all-argon plasma operating conditions."

H3PO 4 level (% v/v)



0 0.87 0.87 0.01 1.1 0.93 0.1 0.87 4.7 1 6.9 795

Detection limits (3a) are in ng cm 3.


900

800 E "~ 700

600

'- 500

= 400 8 = 300

~ 200

100 <

0 .01 .1

Phosphoric acid concentration (%v/v)

PIG. 7. Apparent 63Cu concentration with increasing concentrations of phosphoric acid at (A) typical all-argon plasma operating conditions and (D) Simplex optimized operating conditions for the removal ofPO2 + with the addition of ethene to the nebulizer gas.

mise conditions are given in Table XIV and were tested by determining the detection limits of all the elements that have been discussed in this paper in a matrix of 0.1% sulfuric acid, 0.1% phosphoric acid, 1.0% hydrochloric acid, and 1000 ug cm -3 sodium. These data are given in Table XV and demonstrate that the compromise conditions can greatly improve the removal of interferences for a whole suite of elements.

CONCLUSIONS

This work has demonstrated that a wide range of interferences, such as ArC1 +, ArO +, C10 +, CeO +, ArNa +, and SO2+/$2 +, may be greatly reduced or indeed com- pletely removed by the addition ofethene to the nebulizer gas. Simplex optimization of the operating parameters has once again been demonstrated to be an efficient way of rapidly identifying the best operating parameters for removal ofpolyatomic interferences. The all-around utility ofethene addition has been supported by the analysis of certified reference materials and by the improved detection limits and enhanced recoveries obtained.

The choice of gases for addition to the ICP has grown in recent years and, as previously discussed, should be determined by a number of factors. 2° Nitrogen is pref- erable since it is both inert and extremely cheap, and using hydrocarbon gases requires some simple safety measures appropriate for a flammable, gas flash-back arrestor, safe venting ofunburnt gas, and so on. Ethene has been shown to be of immense value for the removal of a suite of interferences, and it would appear that the optimal use

TABLE XIV. Compromise operating parameters used for the ICP-MS instrument with the addition of ethene to the nebulizer gas to remove polyatomic ion interferences.

Parameter Value

Outer gas (dm 3 min -~) 15.0 Intermediate gas (dm 3 min-~) 1.1 Nebulizer gas (dm 3 m i n - ' ) 1.15 Ethene (%) 0.25 Forward power (W) 1425 Chiller water temperature (°C) 0-2

TABLE XV. Detection limits in a sulfuric acid, phosphoric acid, hydrochloric acid, sodium matrix (0.1, 0.1, 1.0, and 0.1%, respectively), for various analytes determined at the compromise conditions for addition of ethene to the nebulizer gas and under typical all-argon plasma operating conditions, a


Compromise Single-element Typical Element conditions Simplex b conditions

5'V 4.3 1.14 66.5 54Ee 19.3 8.5 110 ~rFe 5.8 0.48 13.4 ~TFe 25.0 3.4 320 62Ni 2.0 11 23.0 63Cu 1.5 0.5 4.1 64Zn 4.2 14 19.0 6~Cu 1.6 3.9 7.02 66Zn 2.4 1.7 6 75As 1.1 0.18 -- 768e 11.6 ND c 127 775e 5.3 1.7 - -

78Se 2.2 0.7 65

° Detection limits (3or) are in ng cm -3. (--) Not determinable (100 ng cm 3 std < blank).

h Note the single-element Simplex detection limits are for single matrix solutions, i.e., 1% HCI for As, Se, and V; 1% H2SO4 for 64Zn, 65Cu, and 66Zn; and 1000 ug cm -3 for 62Ni and 63Cu.

c ND: Not determined in the single-element Simplex procedure.

of ethene can be simply and effectively applied to nearly all the polyatomic interferences in ICP-MS.

The real challenge that now exists in the study of mixed- gas plasmas in ICP-MS is the elucidation of the complex mechanisms that are contributing to the effectiveness of these gas additions to the ICP, in reducing polyatomic interferences. The effects that these plasma modifiers have on the plasma temperature, ionization efficiency, physical structure, and ion energies and on conditions in the in- terface region need to be fully characterized, and the role of water vapor should also be considered. This infor- mation can then be applied to the formulation of theories to explain the observed processes. Such studies are now being undertaken in this laboratory.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support through the CASE scheme of the Science and Engineering Research Council and VG Elemental to one of us (M.F.), which has made this work possible.

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evaluation of ethene addition to the nebulizer gas in inductively coupled plasma-mass spectrometry...

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