Electrothermal vaporization of mineral acid solutions in inductively coupled plasma mass spectrometry: comparison with sample nebulization

Spectrochimica Acta Part B 54 Ž1999. 959]974

Electrothermal vaporization of mineral acid solutions in inductively coupled plasma mass spectrometry: comparison with sample nebulization Juan MoraU,1 Luis Gras 1, Eric H. van Veen, Margaretha T.C. de Loos-Vollebregt Laboratory of Materials Science, Delft Uni¨ ersity of Technology, Rotterdamseweg 137, NL-2628 AL Delft, The Netherlands Received 20 October 1998; accepted 14 February 1999

Abstract The analytical behaviour of an electrothermal vaporization ŽETV. device for the introduction of mineral acid solutions in inductively coupled plasma mass spectrometry ŽICP-MS. was evaluated. Water, nitric acid, hydrochloric acid, perchloric acid and sulphuric acid in concentrations within the 0.05]1.0 mol ly1 range were studied. For all the acids tested, increasing the acid concentration increases the ion signal and deteriorates the precision. The magnitude of the signal enhancement depends on the analyte and on the acid considered. Acid solutions give rise to ion signals that are between 2 and 10 times higher than those with water. Among the acids tested, sulphuric acid provides the highest signals. The addition of palladium reduces matrix effects due to the acids and increases the signal in ETV ICP-MS. In comparison with conventional sample nebulization ŽCS., the ETV sample introduction system provides higher sensitivities Žbetween 2 and 20 times higher. at the same acid concentration. The magnitude of this improvement is similar to that obtained with a microwave desolvation system ŽMWDS.. The ETV sample introduction system gives rise to the lowest background signals from matrix-induced species. Due to this fact, the limits of detection ŽLODs. obtained for the isotopes affected by any interference are lower for ETV sample introduction than those obtained with the CS and the MWDS. For the isotopes that do not suffer from matrix-induced spectral interferences, the ETV gives rise to LODs higher than those obtained with the CS. For these isotopes the lowest LODs are obtained with MWDS. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Acids; Electrothermal vaporization; Nebulization; Microwave desolvation; Spectral interferences; Modifier; Inductively coupled plasma mass spectrometry

U

Corresponding author. e-mail: [email protected] Work performed while on leave from the Department of Analytical Chemistry, University of Alicante, P.O. Box 99, 03080 Alicante, Spain.

1

0584-8547r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 9 9 . 0 0 0 2 9 - 4

960

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

1. Introduction Pneumatic nebulization is the most widespread method for liquid sample introduction in Inductively Coupled Plasma Mass Spectrometry ŽICPMS. due to its low cost, instrumental simplicity and good stability. Nevertheless, pneumatic nebulization also shows serious drawbacks w1x: Ži. low transport efficiencies Žtypically within the 1]2% range.; Žii. the limitation to liquid samples; and Žiii. the need for a relatively large amount of sample. In order to overcome these limitations, alternative nebulization systems have been developed and their behaviour evaluated in ICP-MS: microconcentric w2]4x, ultrasonic w5x, direct injection w6,7x, hydraulic high-pressure w8x, thermospray w9x, single-bore high-pressure pneumatic nebulizer w10x, etc. However, with all these systems the analyte is introduced into the plasma simultaneously with the solvent, giving rise to serious spectral and non-spectral interferences w11]13x that are one of the major limitations of the ICP-MS technique. Different approaches have been proposed in order to reduce the amount of solvent reaching the plasma. Among them, direct sample insertion w14x and monodisperse dried microparticulate injection w15x have been recently introduced. In addition, different designs of desolvation systems have also been proposed for its use in ICP-MS w5,16]21x. With these systems, oxide and hydroxide formation has been reduced w16,17,19,21x, ion intensities have been improved w5,17,20x and matrix-induced spectral interferences have been reduced w16,20x. Electrothermal vaporization ŽETV. is one of the most promising alternatives to conventional liquid sample introduction systems in ICP-MS. For solutions, ETV shows several advantages over conventional pneumatic nebulization: Ži. analysis of small sample volumes Ž20 ml or less.; Žii. higher transport efficiencies Žapprox. 80%. w22x and thus, higher sensitivity and lower limits of detection ŽLODs.; Žiii. selective volatilization of the analyte from its matrix. It can lead to a reduction of matrix-related as well as isobaric interferences Žspectral and non-spectral interferences . on the analytical signal w12,23]27x. Judicious selection

and use of appropriate temperature programmes andror chemical or transport modifiers are the key to the successful application of this technique; and Živ. direct analysis of solids w28,29x, slurries w30x and liquids in a great variety of matrices w31]33x. Acid solutions are widely used in atomic spectrometry techniques since they are often employed for solid sample digestion procedures and solution preservation. In ICP-MS the use of acid solutions gives rise to a large number of polyatomic interferences w12,14,15,20x that should have been taken into account prior to the analysis. Nitric acid is the most advisable acid for ICP-MS determinations since the species generated from it ŽH, N and O. are already present in the plasma, originated from water and atmospheric gases entrained into the argon plasma. The background spectrum of nitric acid is relatively simple compared with that of hydrochloric or perchloric acids w13x. Sulphuric acid is generally avoided since a large number of sulphur-induced interferences can occur w12,13,20x. In addition, the use of acid solutions provides several non-spectral interferences in the plasma, as it has been thoroughly studied in Inductively Coupled Plasma Atomic Emission Spectrometry ŽICP-AES. w34x. Several investigations related to the study of acid matrix effects in ETV ICP-MS have been reported. Most of them are devoted to describe analytical methods to determine ‘difficult’ elementrmatrix combinations Žsuch as V or As in a hydrochloric acid matrix.. Nevertheless, and opposite to what occurs in ICP-AES, only a few groups have investigated the behaviour of mineral acids in an ETV device and its effects on the analytical figures of merit in ICP-MS w22,35,36x. The aim of the present work was to study the effects of mineral acids solutions in ICP-MS with ETV sample introduction. To this end, the effect of the acid nature and concentration on the analytical figures of merit in ICP-MS was studied. Results were compared with those obtained with different liquid sample introduction systems: a conventional pneumatic nebulizer coupled to a double-pass spray chamber and a desolvation system based on microwave heating, called a microwave desolvation system ŽMWDS. w20,37,38x.

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

2. Experimental 2.1. Sample introduction system An HGA-600 MS electrothermal vaporizer ŽPerkin-Elmer SCIEX, Concord, Ontario, Canada. was used. The ETV system was equipped with an autosampler Žmodel AS-60, Perkin-Elmer SCIEX, Concord, Ontario, Canada.. The operation of the HGA-600MS was fully automated. Standard pyrolytically coated graphite tubes ŽPerkin-Elmer part no. 091 504. were employed throughout. The experimental conditions and temperature programme used with the ETV are given in Table 1. The ETV system was coupled to the plasma torch by means of a 140-cm length of PTFE tubing Ž0.6 cm i.d...

961

For the purpose of comparison, a pneumatic concentric-type nebulizer Žmodel AR-30-A3, J.E. Meinhard Associates, Santa Ana, CA, USA. attached to a double-pass Scott-type spray chamber constructed of polyphenylene sulphide ŽPerkinElmer SCIEX Instruments, Concord, Ontario, Canada. was used. This system will be referred to as a conventional sample introduction system ŽCS. from now on. In addition, the results were compared with those obtained with the MWDS w20x. With both the CS and the MWDS, the nebulizer gas Žargon. flow and liquid uptake rates were kept constant in all the experiments at 0.8 and 0.4 ml miny1 , respectively. 2.2. ICP-MS instrumentation Mass intensities were measured by means of an

Table 1 Instrumental operating conditions and data acquisition parameters Inducti¨ ely coupled plasma mass spectrometer Plasma forward power ŽW. Argon flow rate Žl miny1 . Plasma Auxiliary Nebulizer Ion sampling depth from load coil Žmm. Sampling cone Skimmer Electron multiplier voltage ŽV.

1000 15 0.8 0.8 3.0 Platinum, 1.1-mm aperture diameter Platinum, 0.9-mm aperture diameter y3630

Electrothermal ¨ aporizer Sample volume Žml. Internal argon flow rate Žl miny1 . Žbefore vaporization step. Temperature programme Temperature Ž8C. 90 120 250 2400a 2650

Ramp time Žs. 5 20 15 0.7 1

Data acquisition Scanning mode Dwell time Žms. Readings per replicate Points per spectral peak Sweeps per reading Signal measurement

Peak hop transient 10 80 1 1 Signal profile integrated

a

20 0.3

Probe seals the dosing hole 2 s before the vaporization step.

Hold time Žs. 10 25 14 6 5

962

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

ICP-MS instrument Žmodel Elan 5000, PerkinElmer SCIEX Instruments, Concord, Ontario, Canada.. Solution nebulization sample introduction Žvia a cross-flow nebulizer. was used to optimize the plasma and ion lenses’ settings of the mass spectrometer according to the manufacturer’s recommendations. Table 1 lists the operating conditions employed for the ICP-MS measurements. For the spectral interpretation, a software system based on data reduction of the measured total mass spectrum through multicomponent analysis was used w39x. This software was developed for continuous sample introduction, where the signal is independent of the time. When a discontinuous sample introduction system Že.g. ETV. is used, a transient signal is obtained. In this case, some considerations must be taken into account in order to better understand the data acquisition process with the survey analysis approach. Fig. 1 shows the variation of the ion signal as a function of time and of m r z for the ETV and for a continuous sample introduction system. As it can be seen in Fig. 1, for a given set

of parameters, the time required by the approach for the acquisition of the full spectrum is 24 s. With the ETV, in the conditions showed in Table 1, the signal is obtained in approximately 6 s. Hence, four consecutive measurements are needed for the coverage of the full spectrum. In addition, the transient signal at every m r z value is measured only once during 80 ms Ždwell time.. Due to this fact, the isotopes with lower m r z values are measured at the beginning of the peak and the isotopes with larger m r z values are measured at the end. For this reason, the background transient signals provided by using this software are just semi-quantitative. 2.3. Reagents Water and solutions of several mineral acids ŽHCl, HNO3 , HClO4 and H 2 SO4 . at different concentration Žup to 1.0 mol ly1 . were used. All the acids employed were of Suprapur quality ŽMerck, Darmstadt, Germany.. Millipore Milli-Q water was used for dilution. Test solutions containing 10 mg ly1 were pre-

Fig. 1. Comparison of data acquisition process using the survey analysis approach with transient and continuous signals.

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

pared by diluting aliquots from a 1000 mg mly1 reference solutions of each element ŽMerck, Darmstadt, Germany. in the appropriate solvent. A solution containing 100 mg mly1 of palladium nitrate ŽMerck, Darmstadt, Germany. was used as modifier. A volume of 10 ml of this solution was taken in all experiments. 3. Results and discussion 3.1. Effect of the acid concentration and solution nature in ETV ICP-MS 3.1.1. Ion intensity and precision Fig. 2A shows the effect of the acid concentration and nature on the intensity of 103 Rhq. Firstly, in Fig. 2A it can be seen that, for all the solutions studied, the 103 Rhq signal increases when the acid concentration is increased. This behaviour has also been observed for all other isotopes evaluated. Nevertheless, the magnitude of the improvement in the ion intensity due to the acid solutions depends on the acid nature and on the isotope considered. Thus, for instance, using 1.0 mol ly1 of nitric acid, hydrochloric acid or perchloric acid, the 103 Rhq intensities are approximately two times higher than those obtained with water. This behaviour is much more pronounced when using sulphuric acid. The ion intensities obtained with 1.0 mol ly1 of sulphuric acid are between 4 Žfor 24 Mgq and 138 Baq . and 10 times Žfor 103 Rhq . higher than those with water. The effect of the acid concentration on the ion signal is similar to results reported by Gregoire et al. ´ w36x using an HGA-600MS ETV system and opposite to those observed by Park et al. w22x when using an ETV device based on a filament of rhenium metal ribbon. Different mechanisms can be used to explain this behaviour. Firstly, it has been suggested that acids could act as physical carriers increasing the analyte transport efficiency to the plasma and, hence, the ion signal w36x. Assuming this is correct, it must be reflected in the mass spectrum on the signal at the m r z values corresponding to the acids Že.g. 35 Clq and 32 Sq . or acid-derived polyatomic species. In order to check this explanation, the background mass spectrum was mea-

963

sured for all the solutions studied, using the survey analysis approach previously described. Table 2 shows the relevant background ion intensities obtained at different m r z values for water and for the acids at 0.05 and 1.0 mol ly1 . From these results it can be seen that, in general terms, there is no significant effect of the acid concentration and nature on the background signals obtained at the m r z values corresponding to the most relevant polyatomic species, except for sulphuric acid. In fact, these signals are, in most of the cases, similar to those obtained with water. Thus, for instance, the background signals obtained for HCl and HClO4 at m r z 75, which could be increased by signals from ArClq, are, for all acids and concentrations, not detectable. The above discussed statement is true for all the acids tested with the exception of sulphuric acid. Using H 2 SO4 , the polyatomic background signals at m r z 48, 64 and 72 Žwhich show contributions q q . from SOq, Sq 2 rSO 2 and ArS , respectively increase when the acid concentration is increased. It means that a significant amount of solvent reaches the plasma when sulphuric acid is used. This behaviour is due to the fact that sulphuric acid shows the highest boiling temperature. This effect could be minimized by modifying the temperature programme actually used. At this point it is important to point out that the temperature programme used in the present work is a compromise for all the solutions and isotopes tested, including an extended drying step rather than pyrolysis. From the above discussion it seems to be clear that acids do not act as physical carriers. Another possible explanation of the effect of the acid concentration on the ion signal is the increased degradation of the graphite surface in the ETV device when the acid concentration is increased, releasing more carbon particles from the graphite surface during the high-temperature vaporization step. These carbon particles could act as physical carriers themselves or as sources of nucleation for the analytes, giving rise to an improvement in the ion signal w40]43x. In order to check this mechanism, the 12 Cq signal for the different solutions has been evaluated. From the results shown in Table 2, two different behaviours can be distinguished depending on the acid tested.

964

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

Fig. 2. Effect of the acid concentration on the ŽA. Ž n s 5..

103

Rhq intensity for all the acids tested; and ŽB. R.S.D. of the

For nitric, hydrochloric and perchloric acids, there is no effect of the acid concentration on the 12 Cq signals Ži.e. the signals at this m r z are almost the same both for 0.05 mol ly1 and 1.0 mol y1 .. For sulphuric acid, the 12 Cq signal increases when

103

Rhq signal

the acid concentration is increased. Thus, for 1.0 mol ly1 sulphuric acid, the 12 Cq signal obtained is three times higher than that for 0.05 mol ly1 . These results are in agreement with our assumption that, using sulphuric acid, more carbon parti-

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

965

Table 2 Background ion intensities obtained with the ETV at different m r z values using water and all the acids tested at different concentrations a mrz

Species

Ion intensity Žcounts sy1 . Water

q

12 32 35 48 51 52 56 64 72 75 98

C q Oq 2,S q Clq SOq, Caq, Tiq ClOq ArCq ArOq, Feq q q Sq 2 , SO 2 , Zn ArSq, Clq 2 ArClq H 2 SO4q a

90 000 110 000 2000

13 000 8000

HNO3

HCl

HClO4

H2 SO4

0.05 M

1.0 M

0.05 M

1.0 M

0.05 M

1.0 M

0.05 M

1.0 M

120 000 140 000 3000 700 500 40 000 13 000 200

130 000 130 000 2000 1200 200 17 000 12 000 200

90 000 140 000 17 000 800 400 9000 13 000

80 000 120 000 3000 1100 400 13 000 18 000

200 000 200 000 14000 1600 500 19 000 30 000

300 000 110 000 50 000 800 400 30 000 13 000

90 000 300 000 3000 800 400 11 000 15 000

140

130

190

300 000 ) 3 = 106 90 000 140 000 500 9000 30 000 11 000 30 000

Blank cells indicate that no detectable signal was found.

cles are liberated from the graphite surface and act as a physical carrier increasing the ion signal. Nevertheless, when the acid concentration is increased, the increase in the 12 Cq signal measured at the start of the peak profile is lower than the corresponding improvement in the 103 Rhq integrated signal. Moreover, the ArCq signal does not increase proportionally to the 12 Cq signal. At this point it is important to remind that the background signals and the ion signals have been measured in a different way, as has been explained in Section 2. In order to explain the different signal improvements, it would be useful to measure the carbon and analyte signals in the same way and thus, compare the temporal profiles and appearance times of both signals. It might also explain why the magnitude of the improvement in the ion signal depends, as it has been stated above, on the isotope considered. Nevertheless, the mechanism involved in the sample transport via a physical carrier is very complex w41x and it requires a more detailed study. As regards the effect of the acid concentration on the precision, Fig. 2B shows the variation of the relative standard deviation ŽR.S.D.. obtained from five replicates of the 103 Rhq signal vs. the acid concentration for all the solutions studied. For nitric, hydrochloric and perchloric acids, the

R.S.D. values slightly increase with acid concentration. For sulphuric acid at the highest concentration used Ž1 mol ly1 ., the R.S.D. obtained is approximately five times higher than for water. The trends shown in Fig. 2B for 103 Rhq have also been observed for all the ions tested. For sulphuric acid the carbon emission from the graphite furnace increases with increasing acid concentration ŽTable 2.. Random particle emission probably deteriorates the precision, increasing the R.S.D. values when the sulphuric acid concentration is increased. The presence of acids in ETV ICP-MS does not only improve the ion intensity. There is also a significant change in the peak shape and appearance time in the presence of different acids and concentrations ŽFig. 3.. Fig. 3A shows the effect of the Žhydrochloric. acid concentration on the signal of 208 Pbq. It is clear that increasing the acid concentration results in a reduction in the appearance time of the signal. The appearance time is not only affected by the acid concentration, but also by the nature of the acid. This effect is illustrated for 24 Mgq in Fig. 3B. Obviously, the mechanism of vaporization is significantly affected by the presence of the acid. The effect shown in Fig. 3 is similar to phenomena observed when a chemical modifier is added w44x.

966

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

Fig. 3. Effect of the ŽA. hydrochloric acid concentration on the signal of 208 Pbq Ž1. 0.05 mol ly1 , Ž2. 1.0 mol ly1 ; and ŽB. acid nature on the signal of 24 Mgq Ž1. HNO3 , Ž2. HCl, Ž3. HClO4 , Ž4. H 2 SO4 . Acid concentration: 1.0 mol ly1 .

3.1.2. Effect of modifier From the results shown before, it seems clear that there is an important matrix effect associated with the presence of the different acids on the ion signal in ETV ICP-MS. Therefore, a detailed knowledge of the sample matrix composition is needed.

Modifiers are widely used in order to reduce matrix effects and losses of analyte on different parts of the ETV cell or on the transfer line that connects the ETV to the ICP w36,45,46x. Physical carriers used in ETV ICP-MS include materials such as NaCl w24,40x, magnesium w24,31x, as well as mixtures of salts such as NRCC NASS-3 w36,46x.

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

In this work, palladium has been chosen to evaluate the effect of the modifier on the acid interferences in ETV ICP-MS since it is one of the most widely used modifiers, both in ETV sample introduction and GFAAS w24,31,34x. Fig. 4 shows the effect of the addition of Pd on the ion signals obtained for different isotopes with water and all the acids tested. The presence of Pd improves all ion signals obtained w45x with the exception of sulphuric acid. With hydrochloric, nitric and perchloric acids, the improvement varies between 2 and 10, depending on the acid and on the isotope tested. Using sulphuric acid, the signal obtained with and without Pd is approximately the same. Fig. 4 shows that, for a given isotope, the signal obtained with Pd is independent of the nature of the acid and its concentration. 3.2. ETV ¨ s. nebulization for acid sample introduction in ICP-MS 3.2.1. Sensiti¨ ity In order to evaluate the benefits of ETV for liquid acid sample introduction in ICP-MS, the sensitivities of several elements with different atomic masses were measured for water and all the acids tested. The ETV sensitivities were compared with those obtained with the CS and with

Fig. 4. Effect of the addition of Pd on the ion signal for concentration, 1.0 mol ly1 .

103

967

those obtained with a desolvation system based on microwave heating ŽMWDS. w20x. For all systems, sensitivity is defined as the ion intensity provided per unit of mass of analyte. For ETV, the mass of analyte has been calculated by considering the volume Ž20 ml. and concentration Ž10 mg mly1 . of the sample used. For the nebulization systems, the mass of analyte has been calculated taking into account, in addition to the concentration of the solution used Ž10 mg mly1 ., the sample uptake rate Ž0.4 ml miny1 . and considering the same time required by the ETV to read the full peak Ži.e. 6 s.. The ratio of sensitivities obtained with ETV sample introduction and conventional sample nebulization are presented in Table 3. From the results shown in Table 3, it can be concluded that the sensitivities obtained with ETV are between 2 and 20 times higher than those obtained with the CS. The improvement in sensitivity using ETV sample introduction is similar to that with the MWDS. Nonetheless, the ion intensities obtained with the ETV are between 0.5 and 5 times the corresponding signals obtained with the CS. These ratios are lower than those found for the MWDS vs. CS, where sensitivity ratios and intensity ratios are the same. The lower volume of solution Ži.e. the lower mass of analyte. used

Rhq,

138

Baq,

208

Pbq using water and all the acids tested. Acid

968

Solvent

24

Water HCl HNO3 HClO4 H2 SO4

ŽSMWDSrSCS .b

ŽSET V rSCS . Mgq

10 9 14 9 15

103

Rhq

1.9 1.9 3 1.9 3

138

6 5 7 6 7

Acid concentration s 0.05 mol ly1 . From Mora et al. w20x. c n s 5. a

b

Baq

140

Ceq

1.9 1.5 3 2 3

208

7 5 8 10 17

Pbq

24

Mgq

4 4 5 11 10

103

2 3 4 10 10

R.S.D.c Ž%. Rhq

138

3 3 3 14 14

Baq

140

3 3 4 13 13

Ceq

208

3 3 4 14 14

Pbq

ETV

MWDSb

CSb

1]7 2]6 2]5 1]5 2]10

2]3 2]3 2]3 2]5 2]5

1]2 2]3 1]2 1]3 1]3

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

Table 3 Comparison of sensitivities and R.S.D.s obtained with ETV, MWDS and with CSa

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

with ETV is the key issue for the high sensitivities. The improvement in sensitivity observed with ETV in comparison with that obtained with the CS varies with the ion tested. Thus, the highest sensitivity ratios are obtained for 24 Mgq, 138 Baq and 208 Pbq, whereas the lowest ones are obtained for 103 Rhq and 140 Ceq. The higher retention in the graphite tube of the elements with the highest boiling points Ži.e. Rh and Ce. reduces the analyte transport efficiency for these elements w40,43,45x. From the results shown in Table 3 it seems to be clear that the analyte transport efficiency obtained with the ETV is higher than that for the CS. Assuming for the CS an analyte transport efficiency of approximately 2%, and taking into account the relative increase in the sensitivity observed with the ETV, it can be concluded that analyte transport efficiencies as high as 30% are obtained with ETV sample introduction. Although this value is relatively high in comparison to CS, it is quite low taking into account that all the analyte has been vaporized in the ETV. Analyte losses during the transport to the plasma are due to heterogeneous nucleation w41,47x andror retention of the analytes in different sections of the vaporizer and transport set-up w43x. 3.2.2. Precision Table 3 also shows the R.S.D. of five replicates of the ion intensities obtained with the different sample introduction systems considered. For all the solutions and isotopes tested, the R.S.D.s obtained with the CS are within the 1]3% range. With the MWDS, the precision obtained depends on the acid tested. R.S.D. values are between 2 and 5% w20x. With ETV sample introduction the R.S.D. values vary between 1 and 10%, irrespective of the analyte or the acid solution tested. 3.2.3. Background mass spectrum The behaviour of the ETV device as an acid sample introduction system in ICP-MS has also been evaluated by measuring the background spectrum for the different solutions tested w24x. Results have been compared with those obtained with CS and with MWDS w20x. Table 4 shows the intensities of the most relevant polyatomic ions

969

detected in the background mass spectra observed for ETV and MWDS sample introduction relative to the corresponding intensities for CS sample introduction of water and acids. First of all, it should be noted that an important interference due to ArCq at m r z 52 is observed with the ETV. This ion overlaps with the major isotope of chromium. As it can be seen in Table 4, this background signal is approximately five times higher with the ETV than with the CS, irrespective of the solution considered. For the other polyatomic ion intensities observed with ETV sample introduction, the background signals obtained with the ETV are always lower than those obtained with CS and MWDS. It means that the amount of solvent reaching the plasma is lower with ETV than with sample nebulization. Thus, with the ETV sample introduction system, the interference due to ArOq at m r z 56, which overlaps the major isotope of iron, is between 10 and 50 times lower than that obtained with the CS. Nevertheless, the background signal obtained with the ETV at this m r z still remains high Žsee Table 2. and, therefore, it is difficult to determine low levels of Feq. With the MWDS, the ion intensities obtained at this m r z value are, in general terms, of the same order of magnitude as those obtained with the CS Žapprox. 340 000 counts sy1 ., irrespective of the solution tested w20x. When HCl and HClO4 solutions are introduced into the ICP-MS, interferences due to ClOq at m r z 51 and to ArClq at m r z 75, which overlap the isotopes of V and As, respectively, have been observed with both the CS and the MWDS. These chlorine-induced polyatomic ions are lower or even not detectable with the ETV system. Also a reduction of the ion intensity of the ArClq was previously reported by Gregoire and Ballinas w31x ´ when using an ETV device. At m r z 72 the contribution of Clq 2 has not been found using ETV or CS, whereas with the MWDS and using HClO4 , an enhancement factor of 5 was found w20x. Finally, from the results shown in Table 4 it can be concluded that when using the ETV sample introduction system, the contributions due to the sulphur-induced polyatomic ions at m r z 48, 64

970

Isotope

Interferent

Water IET V rICS

Tiq Vq 52 Crq 56 Feq 64 Znq 72 Geq 75 Asq 48 51

SOq ClOq ArCq ClOHq, ArOq ArOq q SOq 2 , S2 q ArS , Cl2q ArClq

5 0.02

HCl IMWDSrICS

3 1

b

HNO3

IETV rICS

IMWDSrICS

0.1 3 0.04

1 3 1

b

IETV rICS

5 0.04

3

Acid concentration s 0.05 mol ly1 ; blank cells indicate that no interference has been found. From Mora et al. w20x.

a

b

HClO4 IMWDSrICS

3 1

b

H2 SO4

IETV rICS

IMWDS rICS

0.06 7 0.09

6 4 2 5 8

b

IETV rICS

IMWDS rICS b

0.01

4

4 0.04

3 1 3 6

0.1

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

Table 4 Relative intensities of the background at different masses a

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974 q q and 72, for the species SOq, Sq 2 rSO 2 and ArS , y1 respectively, are hardly detectable at 0.05 mol l . In contrast, using MWDS these signals were between three and six times higher than those obtained with the CS w20x.

3.2.4. Limits of detection In order to evaluate the analytical performance of ETV as an acid sample introduction system in ICP-MS, the LODs of several isotopes were measured in all acid solutions. Results were compared with those obtained with CS and with MWDS w20x. LODs obtained with ETV were calculated according to the 3s b criterion, s b meaning the standard deviation from 10 replicates of the blank. LODs with both the CS and the MWDS were taken from a previous paper w20x. Table 5 shows the LODs obtained for all the solutions tested and for various isotopes using ETV, CS and MWDS sample introduction. First of all, from the results shown in this table it is important to point out that for the isotopes that suffer from interferences due to the presence of solvent in the plasma, the LODs obtained with the ETV are in all cases lower than those obtained with the CS and even lower than those obtained with the MWDS up to a factor of 20 and 10, respectively, with the exception of 64 ZnqrH 2 SO4 . These results can be explained from the low background signals obtained with ETV in comparison with the nebulization systems. It reveals the great potential of ETV for removing of solvent. With the 64 ZnqrH 2 SO4 combination, both the background intensity ŽTable 4. and the ion intensity obtained with the ETV are lower than those with the CS, giving rise to a higher LOD with the former system. For the solventrisotope combinations that are not affected by any background interference, Table 5 shows that the LODs obtained with the ETV are, for the isotopes considered Ž 103 Rhq, 140 Ceq and 208 Pbq ., somewhat higher than those obtained with the CS. Comparing the LODs obtained with the different sample introduction systems in Table 5 it can be seen that the MWDS affords, in all cases, the lowest LODs. It should

971

be noted that, in the conditions used in Table 5, the ETV sample introduction system produces the lowest ion signals and the highest R.S.D. values ŽTable 3.. The use of a transport modifier enhances the ion signal in ETV ICP-MS, as discussed above. Therefore, it can be expected that the use of transport modifiers also improves the LODs obtained with the ETV. Indeed, using Pd, the LODs obtained with the ETV are reduced between two and eight times, depending on the isotope and on the solvent used. Under such conditions, the LODs obtained with ETV are of the same order of magnitude or sometimes even lower than those obtained with CS. 4. Conclusions The results show that there is a significant matrix effect related to the presence of acids in ETV ICP-MS. Hence, the use of aqueous standards for calibration causes analytical errors, depending on the sample matrix. For a given isotope, the ion signal depends on the acid nature and on the concentration employed. Increasing the acid concentration increases the ion intensity and decreases the precision. Among the acids tested, sulphuric acid affords the highest signals. Also the vaporization mechanism of the analytes is different in the presence of acids. In comparison to sample nebulization, the ETV sample introduction system offers higher sensitivities Žbetween 2 and 20 times higher. than those obtained with the CS. The improvement is similar to what is obtained with MWDS. Using ETV it is possible to remove interferences due to the presence of the solvent. Consequently, the LODs obtained for the isotopersolvent combinations that suffer from spectral interferences are lower with ETV than with CS sample introduction and even lower than those obtained with MWDS. For the isotopes that do not suffer from solvent-induced spectral interferences, ETV gives rise to LODs values slightly higher than those obtained with the CS. For these isotopes the lowest LODs are obtained with the MWDS.

972

Isotope

LOD Žng ly1 . Water ETV

Tiq Vq 56 Feq 64 Znq 75 Asq 98 Moq 103 Rhq 140 Ceq 208 Pbq

HCl b

MWDS

b

HNO3 b

b

CS

ETV

MWDS

CS

4000

20 180

40 1800

100 4000

80

200

200

ETV

HClO4 b

MWDS

b

H2 SO4 b

b

CS

ETV

MWDS

CS

4000

20 500

200 2000

200 4000

90

200

300

48 51

900

10 50 70

2000

0.9 2 7

3 6 20

Acid concentration: 0.05 mol ly1 . From Mora et al. w20x.

a

b

10 50 70

0.8 2 9

4 6 30

300

16 60 50

1900

0.8 2 10

4 6 30

19 90 80

0.7 1 4

4 7 30

MWDSb

CSb

500

1400

2000

500 4000

1700 600

4000 1100

ETV

40 13 40 50

2 0.3 0.9 4

80 4 7 30

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

Table 5 Limits of detection ŽLODs. obtained with ETV, MWDS and CSa

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

Acknowledgements J. Mora and L. Gras thank the Laboratory of Materials Science ŽDelft University of Technology. for facilities.

w13x w14x

References w1x J. Sneddon, Sample Introduction in Atomic Spectroscopy, Elsevier, New York, 1990. w2x E. Debrah, S.A. Beres, T.J. Gluodenis, R.J. Thomas, E.R. Denoyer, Benefits of a microconcentric nebulizer for the multielement analysis of small sample volumes by inductively coupled plasma mass spectrometry. At. Spectrosc. 16 Ž1995. 197]202. w3x F. Vanhaecke, M. Van Holderbeke, L. Moens, R. Dams, Evaluation of a commercially available microconcentric nebulizer for inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 11 Ž1996. 543]548. w4x S. Augagneur, B. Medina, J. Szpunar, R. Lobinski, Determination of rare earth elements in wine by inductively coupled plasma mass spectrometry using a microconcentric nebulizer. J. Anal. At. Spectrom. 11 Ž1996. 713]721. w5x I.B. Brenner, A. Zander, M. Plantz, J. Zhu, Characterization of an ultrasonic nebulizer]membrane separation interface with inductively coupled plasma mass spectrometry for the determination of trace elements by solvent extraction. J. Anal. At. Spectrom. 12 Ž1997. 273]279. w6x D.R. Wiederin, F.G. Smith, R.S. Houk, Direct injection nebulization for inductively coupled plasma mass spectrometry. Anal. Chem. 63 Ž1991. 219]225. w7x H. Li, B.M. Keohane, H. Sun, P.J. Sadler, Determination of bismuth in serum and urine by direct injection nebulization inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 12 Ž1997. 1111]1114. w8x N. Jakubowski, I. Feldmann, D. Stuewer, H. Berndt, Hydraulic high pressure nebulization } application of a new nebulization system for inductively coupled plasma mass spectrometry. Spectrochim. Acta Part B 47 Ž1992. 119]129. w9x H. Vanhoe, L. Moens, R. Dams, Thermospray nebulization as sample introduction for inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 9 Ž1994. 815]821. w10x V. Hernandis, J.L. Todolı, ´ A. Canals, J.V. Sala, An experimental study of the behaviour of several elements in inductively coupled plasma mass spectrometry using the single-bore high-pressure pneumatic nebulizer. Spectrochim. Acta Part B 50 Ž1995. 985]996. w11x E.H. Evans, J.J. Giglio, Interferences in inductively coupled plasma mass spectrometry. A review. J. Anal. At. Spectrom. 8 Ž1993. 1]18. w12x M.A. Vaughan, G. Horlick, Oxide, hydroxide and doubly charged analyte species in inductively coupled

w15x w16x

w17x w18x

w19x

w20x

w21x

w22x

w23x

w24x

w25x

973

plasmarmass spectrometry. Appl. Spectrosc. 40 Ž1986. 434]445. S.H. Tan, G. Horlick, Background spectral features in inductively coupled plasmarmass spectrometry. Appl. Spectrosc. 40 Ž1986. 445]460. G.E.M. Hall, J.C. Pelchat, D.W. Boomer, M. Powell, Relative merits of two methods of sample introduction in inductively coupled plasma mass spectrometry: electrothermal vaporization and direct sample insertion. J. Anal. At. Spectrom. 13 Ž1998. 791]797. J.B. French, B. Etkin, R. Jong, Monodisperse dried microparticulate injector for analytical instrumentation. Anal. Chem. 66 Ž1994. 685]691. L.C. Alves, D.R. Wiederin, R.S. Houk, Reduction of polyatomic ion interferences in inductively coupled plasma mass spectrometry by cryogenic desolvation. Anal. Chem. 64 Ž1992. 1164]1169. N. Jakubowski, I. Feldmann, D. Stuewer, Analytical improvement of pneumatic nebulization in ICP-MS by desolvation. Spectrochim. Acta Part B 47 Ž1992. 107]118. D. Gunther, H. Cousin, B. Magyar, I. Leopold, Calibration studies on dried aerosols for laser ablation-inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 12 Ž1997. 165]170. H. Tao, A. Miyazaki, Decrease of solvent water loading in inductively coupled plasma mass spectrometry by using a membrane separator. J. Anal. At. Spectrom. 10 Ž1995. 1]5. J. Mora, A. Canals, V. Hernandis, E.H. van Veen, M.T.C. de Loos-Vollebregt, Evaluation of a microwave desolvation system in inductively coupled plasma mass spectrometry with low acid concentration solutions. J. Anal. At. Spectrom. 13 Ž1998. 175]181. M.G. Minnich, R.S. Houk, Comparison of cryogenic and membrane desolvation for attenuation of oxide, hydride and hydroxide ions and ions containing chlorine in inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 13 Ž1998. 167]174. C.J. Park, J.C. Van Loon, P. Arrowsmith, J.B. French, Sample analysis using plasma source mass spectrometry with electrothermal sample introduction. Anal. Chem. 59 Ž1987. 2191]2196. J.M. Carey, E.H. Evans, J.A. Caruso, W. Shen, Evaluation of a modified commercial graphite furnace for reduction of isobaric interferences in argon inductively coupled plasma mass spectrometry. Spectrochim. Acta Part B 46 Ž1991. 1711]1721. D.C. Gregoire, R.E. Sturgeon, Background spectral fea´ tures in electrothermal vaporization inductively coupled plasma mass spectrometry: molecular ions resulting from the use of chemical modifiers. Spectrochim. Acta Part B 48 Ž1993. 1347]1364. D.C. Gregoire, Determination of platinum, palladium, ´ rhutenium and iridium geological materials by inductively coupled plasma mass spectrometry with sample introduction by electrothermal vaporisation. J. Anal. At. Spectrom. 13 Ž1998. 309]314.

974

J. Mora et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 959]974

w26x N. Shibata, N. Fudagawa, M. Kubota, Oxide formation in electrothermal vaporization inductively coupled plasma mass spectrometry. Spectrochim. Acta Part B 48 Ž1993. 1127]1137. w27x R. Tsukahara, M. Kubota, Some characteristics of inductively coupled plasma-mass spectrometry with sample introduction by tungsten furnace electrothermal vaporization. Spectrochim. Acta Part B 45 Ž1990. 779]787. w28x U. Voellkopf, M. Paul, E.R. Denoyer, Analysis of solid samples by ICP-mass spectrometry. Fresenius J. Anal. Chem. 342 Ž1992. 917]923. w29x F. Vanhaecke, S. Boonen, L. Moens, R. Dams, Solid sampling electrothermal vaporization inductively coupled plasma mass spectrometry for the determination of arsenic in standard reference materials of plant origin. J. Anal. At. Spectrom. 10 Ž1995. 81]87. w30x S. Hauptkorn, V. Krivan, B. Gercken, J. Pavel, Determination of trace impurities in high-purity quartz by electrothermal vaporization inductively coupled plasma mass spectrometry using the slurry sampling technique. J. Anal. At. Spectrom. 12 Ž1997. 421]428. w31x D.C. Gregoire, M.L. Ballinas, Direct determination of arsenic in fresh and saline waters by electrothermal vaporization inductively coupled plasma mass spectrometry. Spectrochim. Acta Part B 52 Ž1997. 75]82. w32x L. Yu, R. Koirtyohann, M.L. Rueppel, A. Skipor, J.J. Jacobs, Simultaneous determination of aluminium, titanium and vanadium in serum by electrothermal vaporization-inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 12 Ž1997. 69]74. w33x R.E. Sturgeon, S.N. Willie, J. Zheng, A. Kudo, Determination of ultratrace levels of heavy metals in Arctic snow by electrothermal vaporization inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 8 Ž1993. 1053]1058. w34x J.L. Todolı, ´ J.M. Mermet, Acids interferences in atomic spectrometry: analyte signal effects and subsequent reduction, Spectrochim. Acta Rev. in press. w35x B. Fairman, T. Catterick, Simultaneous determination of arsenic, antimony and selenium in aqueous matrices by electrothermal vaporization inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 12 Ž1997. 863]866. w36x D.C. Gregoire, D.M. Goltz, M.M. Lamoureux, C.L. ´ Chakrabarti, Vaporization of acids and their effect on analyte signal in electrothermal vaporization inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 9 Ž1994. 919]926.

w37x V. Hernandis, A. Canals, J. Mora, L. Gras, Patent Number P9500810, Spain, 1995. w38x L. Gras, J. Mora, J.L. Todolı, ´ A. Canals, V. Hernandis, Behaviour of a desolvation system based on microwave radiation heating for use in inductively coupled plasma atomic emission spectrometry. Spectrochim. Acta Part B 52 Ž1997. 1201]1213. w39x E.H. van Veen, S. Bosch, M.T.C. de Loos-Vollebregt, Spectral interpretation and interference correction in inductively coupled plasma mass spectrometry. Spectrochim. Acta Part B 49 Ž1994. 1347]1361. w40x C.M. Sparks, J. Holcombe, T.L. Pinkston, Particle size distribution of sample transported from an electrothermal vaporizer to an inductively coupled plasma mass spectrometer. Spectrochim. Acta Part B 48 Ž1993. 1607]1615. w41x T. Kantor, Interpreting some analytical characteristics ´ of thermal dispersion methods used for sample introduction in atomic spectrometry. Spectrochim. Acta Part B 43 Ž1988. 1299]1320. w42x R.W. Fonseca, N.J. Miller-Ihli, C. Sparks, J.A. Holcombe, B. Shaver, Effect of oxygen ashing on analyte transport efficiency using ETV-ICP-MS. Appl. Spectrosc. 51 Ž1997. 1800]1806. w43x U. Schafer, V. Krivan, A graphite furnace electrother¨ mal vaporization system for inductively coupled plasma atomic emission spectrometry. Anal. Chem. 70 Ž1998. 482]490. w44x J.P. Byrne, D.C. Gregoire, M.E. Benyounes, C.L. ´ Chakrabarti, Vaporization and atomization of the platinum group elements in the graphite furnace investigated by electrothermal vaporization-inductively coupled plasma-mass spectrometry. Spectrochim. Acta Part B 52 Ž1997. 1575]1586. w45x R.D. Ediger, S.A. Beres, The role of chemical modifiers in analyte transport loss interferences with electrothermal vaporization ICP-mass spectrometry. Spectrochim. Acta Part B 47 Ž1992. 907]922. w46x D.M. Hughes, C.L. Chakrabarti, D.M. Goltz, D.C. Gregoire, R.E. Sturgeon, J.P. Byrne, Seawater as a ´ multi-component physical carrier for ETV-ICP-MS. Spectrochim. Acta Part B 50 Ž1995. 425]440. w47x W.C. Hinds, Aerosol Technology: Properties, Behaviour and Measurement of Airborne Particles, Wiley, New York, 1982, Ch. 13, p. 249.