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Determination of arsenic in plant samples by inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization: a complex problem
Spectrochimica Acta Part B 56 Ž2001. 223᎐232
Research note
Determination of arsenic in plant samples by inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization: a complex problem Emilia Vassilevaa , Michel Hoenig b,U b
a Uni¨ ersity of Sofia, Faculty of Chemistry, J. Bourchier A¨ . 1, BG-1126 Sofia, Bulgaria Veterinary and Agrochemical Research Centre (CERVA), Leu¨ ensesteenweg 17, B-3080 Ter¨ uren, Belgium
Received 25 October 2000; accepted 18 December 2000
Abstract Under well-defined conditions, the analysis of most trace elements by inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization ŽICP-AES-USN. leads to accurate results for environmental matrices usually studied. Due to differences in matrix composition between standards and samples, ICP-AES-USN determinations of arsenic are interfered with by changes that take place mainly within the desolvation stage of the USN device. In this work, effects of plant matrices on the determination of As in six arsenic species have been investigated. Firstly, interferences were simulated by measuring analyte Žspecies. signals in solutions containing variable concentrations of the main matrix elements encountered in mineralized plant samples ŽK, Ca, Mg, P and Na.. Secondly, the influence of real plant matrices on emission signals of arsenic species was also studied. In this case, the observed effects were different than for individual matrix elements considered separately: Ca and Mg always present in real samples efficiently compensate the undesirable effects. Validation of this statement has been performed using mineralized plant reference materials. In addition, ICP-AES-USN results have been compared with those obtained by Zeeman electrothermal atomic absorption spectrometry. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Arsenic; Plant matrices; ICP-AES; Ultrasonic nebulization
U
Corresponding author. E-mail address: [email protected] ŽM. Hoenig..
0584-8547r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 1 . 0 0 1 5 6 - 2
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1. Introduction Inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization is now a widely utilized instrumental technique for the determination of a large number of analytes. The main advantage of this technique is the high sample throughput that ensures lower detection limits in comparison to ICP-AES with pneumatic nebulization ŽPN. w1᎐4x. The ICP-AES-USN analysis has been successfully applied to various sample types including petroleum samples w5x, biological materials w6,7x, high-salt content solutions w8x and plants w9,10x. Compared to the ICP-AES analysis with PN as a means of sample introduction, some additional problems were reported for USN, however, their mechanisms have not yet been clarified and are often contradictory w10᎐14x. As already quoted in the earlier literature concerning ICP-AES-USN, differences in matrix composition between standards and samples may often lead to changes in efficiency of aerosol formation and transport but they may also be responsible for different evolution of phenomena occurring in the plasma itself. The drawbacks observed with USNs appear to a significantly lesser extent with conventional PNs and consequently they may be more easily controlled during the analysis. Most of the severe interferences observed with earlier USNs have been overcome by some recent improvements in their design, particularly by the commercial introduction of auto-tuned power supplies for RF generators in 1996. They allow a large degree of compensation for changes in the aerosol formation brought about by the variable matrix composition. In previous work w10x we have shown that changes in the acid concentration are really compensated for with auto-tuned transducers. The influence of the main matrix elements is also considerably reduced however, it is not totally eliminated. In such conditions, the only remaining alternative is the use of an internal standard ŽIS. correction which compensates for variation in analyte signals intensities affected by phenomena taking place during aerosol formation and transport, but also by changes occurring in the plasma.
With an objective to replace the time-consuming electrothermal atomic absorption spectrometry ŽETAAS. with the ICP-AES-USN technique for routine analyses Ženvironmental monitoring: plant and fish samples. in our laboratory we have previously shown the relative efficiency of the IS correction of environmental matrices using an Ar-emission line for trace element analysis w10,15x. However, such a correction may compensate for changes occurring in the plasma only. It has been concluded that Ar-correction may sometimes be satisfactory however, for the analysis of high-salt matrices it is not sufficiently powerful. From the same work w15x it also appears that the nebulization efficiency of the USN is approximately 8᎐10 times higher than that of the PN. This means that not only is 8᎐10 times more analyte transported to the plasma, but also of the matrix. Compared to pneumatic nebulizers that apparently behave very well in terms of matrix effects, these are much more pronounced in the analysis with an USN. To decide upon the behavior of the analyte signal in the presence of heavy matrices, several cases may be postulated: Ži. differences in the aerosol formation due to changes in the viscosity or in the salt content ŽUSN: transducer .; Žii. changes in the aerosol transport ŽUSN: principally condensation stage.; and Žiii. changes in analyte dissociation andror excitation Žplasma loading.. All of these phenomena result in a similar observation ᎏ a progressive suppression of the analyte emission signal with increasing matrix. In principle, a classical internal standard correction should make it possible to compensate for all of the above-mentioned phenomena. For most matrices Žplant and animal tissues . and analytes studied, matrix effects can be compensated for using internal standard correction. Unfortunately, some unpredictable drawbacks remain: these mainly concern the determination of boron, copper and arsenic, but also probably other volatile elements such as Se and Hg. The particular behavior of boron in ICP-AESUSN is well known w16᎐18x; even without interferent, its emission intensity is approximately 10 times lower with USN than with PN. For this particular case an explanation exists: in the presence of nitric acid, always added to standards and
E. Vassile¨ a, M. Hoenig r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 223᎐232
samples Žstabilization, mineralization., the formation and condensation of boric acid occurs. This happens largely during the cooling stage of the USN prior to its evacuation under this chemical form to the waste. This phenomenon has been confirmed by analysis of the waste from the USN condensation stage where more than 85% of boron Ž"30% for all other analytes . has been recovered w15x. In the same USN study we also observed that the emission intensities of copper are drastically and unusually depressed by the presence of nitric acid or Ca. Once again, this may be entirely attributable to particular physico᎐chemical mechanisms occurring in the USN condensation stage and which are different than in the case of other analytes w19x. The fact that the strong interference on Cu disappeared when the USN condensation stage was switched off is sufficient proof of this statement. During a multi-element run in the same work we observed that arsenic also shows some drawbacks with the use of USN. At first sight, these shortcomings are strongly associated with the arsenic form present in the sample. Although the determination of species is an increasingly essential task in environmental studies, the total analyte content in samples is still useful for a global evaluation of the situation. Despite the numerous analytical approaches for arsenic in recent years, there remains a need for reliable sensitive techniques that allow the determination of arsenic in some environmental samples. To reach very low As-concentrations in environmental samples, the hydride generation ŽHG-ICP-AES. may be of use, however, the unavoidable change of the introduction device necessitates that this analysis be performed in a separate run, thereby complicating the routine multi-element aspect of the analysis. Because the USN also allows the attainment of significantly lower concentrations than PN, the aim of this work is to evaluate the consistency of the As-determination in plant samples using ICP-AES-USN as well as to evaluate the behavior of six Asspecies ŽAsBet, AsŽIII., AsŽV., MMAs, PhAs and DMAs. during this determination. We have oriented this study towards plant matrices because other solid environmental samples such as sedi-
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ments and soils generally exhibit higher As levels that do not necessitate the high sensitivity of analytical techniques such as ETAAS, HG-ICPAES or USN-ICP-AES.
2. Experimental 2.1. Apparatus Measurements were performed using a Liberty 200 vacuum ICP-AES system ŽVarian, Australia. equipped with an U-5000ATq ŽCETAC Technologies, USA. ultrasonic nebulizer. Some control measurements were also performed using a pneumatic nebulizer ŽV-groove q cyclonic spray chamber, Glass Expansion, Australia.. Optimized ICP-AES and USN parameters are given in Table 1. The ETAAS control determinations were performed using a SpectrAA-400 Zeeman atomic absorption spectrometer equipped with a GTA-96 graphite furnace and an autosampler ŽVarian, Australia.. The primary As-source was a boosted hollow cathode lamp ŽPhotron Superlamp, Australia.. Pyrolytically coated graphite tubes with forked solid pyrolytic platforms ŽRingsdorff, Germany. were used throughout. The electrothermal program used is given in Table 2. 2.2. Reagents and reference materials All main matrix element test solutions were prepared from pure salts ŽCa, K, Na and Mg: nitrates, Specpure, Johnson Matthey, UK, P: dihydrogen phosphate, Merck, Germany.. Disodium hydrogen arsenate Žs AsŽV., Na 2 HAsO4 7H 2 O, ŽFluka, Switzerland., phenylarsonic acid Žs PhAs., ŽAldrich, USA., arsenic trioxide Žs AsŽIII., NaAsO 2 , ŽMerck, Germany., di-sodium monomethyl arsonate Žs MMAs. CH 3 AsOŽONa. 2 , ŽArgus Chemicals, Italy., sodium dimethyl arsinate Žs DMAs., ŽCH 3 . 2 AsŽO.ONa 3H 2 O, ŽAldrich, USA. and arsenobetain Žs AsBet., CH 3 . 3 Asq CH 2 COOy, ŽFluka, Germany. were used without further purification. All Ascompounds were dissolved in de-mineralized redistilled water and NaAsO 2 in 1% Žwrw. NaOH. For the ICP-AES calibration, a 2% nitric acid
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Table 1 Instrumental conditions for ICP-AES-USN study of arsenic Instrumental conditions Wavelength Žnm. Background correction Observation height Žmm. Integration time Žs. Number of replicates Generator power ŽkW. PMT voltage ŽV. Plasma Ar-flow Ž1 miny1 . Auxiliary Ar-flow Ž1 miny1 . Pump speed Žrev. miny1 . Rinse time Žs. Nebulizer pressure ŽkPa. USN: heating stage Ž⬚C. USN: cooling stage Ž⬚C.
blank solution and a 100 g ly1 -As working standard were used. All standard and sample solutions studied were acidified to 2% with nitric acid. For dilutions, calibrated glass pipettes or adjustable micropipettes ŽGilson, France. were used. Plant reference materials were from the National Institute for Standards and Technology ŽNIST, Gaithersburg, MD, USA.: SRM-1572 Citrus leaves, or from the International Atomic Energy Agency ŽMonaco.: IAEA-140 Sea-plant and IAEA-393 Algae. Three CERVA laboratory control samples validated for arsenic by ETAAS were also analyzed ŽGrasses CERVA 1, 2 and 3.. 2.3. Sample preparation procedure For the plant samples studied, a wet digestion
As 188.98 Dynamic mode 6 Žabove the coil. 5 3 1.2 700 16.5 0.75 15 Žsample intake " 2 ml miny1 . 40 Žfast pump on. 140 140 3
procedure with elimination of silicon was applied Žmodified CII method. w20x. Half a gram of the sample was weighed in a PTFE flask and 5 ml of concentrated nitric acid was added. The mixture was heated on a sand bath Žtemperature approx. 120⬚C. for 30 min and 5 ml of hydrogen peroxide was added slowly to avoid possible strong foaming that can occur with some samples. The heating was carried out during 1 h at the same temperature. After the decomposition of organic matter, 5 ml of hydrofluoric acid was added. The evaporation step was continued overnight and the dry residue finally leached with 2 ml of nitric acid and dissolved by a short heating. After cooling the sample, the solution was made up to 50 ml. Validation of this method for As has been presented in previous work w20,21x.
Table 2 Electrothermal program for As-determinationa,b,c Step
Temperature Ž⬚C.
Time Žs.
Ar-flow Ž1 miny1 .
1 2 3 5 6 7 8 9
300 1200 1200 100b 100 2500c 2500c 110
30.0 ramp 4.0 ramp 10.0 hold 5.0 rampx 1.0 hold 0.7 ramp 4.0 hold 12.0 rampx
3.0 3.0 3.0 3.0 0 0 0 3.0
a
The sample is introduced with IrrMg modifier on the preheated platform Ž110⬚C.. Cool-down step. c Signal integration. b
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3. Results and discussion 3.1. Effect of nitric acid In plasma spectroscopic methods where sample introduction is ensured by a nebulization device, the effect of acids is attributed to physical changes in the aerosol generation process or to modifications of plasma conditions resulting from physico᎐chemical mechanisms. In most dissolution procedures, environmental samples are decomposed using single nitric acid or a mixture of other mineral acids. To simulate sample mineralization conditions, six arsenic species at the same As-concentrations were measured by ICP-AES-USN and PN in the presence of 2% HNO3 after heating aliquots of these solutions in a domestic microwave oven or on a sand bath. In addition, initial aliquots Žone without acid and one with acid but without heating. were measured as control samples. For the following text, reference recoveries Žs 100% for each species. were values given by the ETAAS analysis for which the accuracy has been validated in previous studies w21᎐23x. Fig. 1a demonstrates that with a conventional PN device the recoveries of all As-species, regardless of sample preparation procedure were approximately 100%. However, significant differences were observed when the same solutions were aspirated using USN ŽFig. 1b., confirming thus the above-mentioned assumption that the main analytical problem is located within the desolvation stage of the USN system. A similar conclusion has been previously postulated for copper, which exhibited different behavior to all other trace elements studied w15,24x. Using the USN, recoveries for As were systematically less than 50% for the species studied. Indeed the recovery of the As-species in pure aqueous solutions were minimum; they were increased when nitric acid was present in solution. Maximum recoveries were obtained when the solutions were treated with microwaves ᎏ this resulted in phase transformation of As-species. 3.2. Effect of main matrix elements Solutions of six arsenic species at the same
Fig. 1. Average As-recoveries Ž100 g ly1 As. for six arsenic species after different sample treatments. The uncertainty of measurements is inferior to 5%. a, pneumatic nebulizer; b, rrr ultrasonic nebulizer; I 2% nitric acid; I 2% nitric acid aaa 2% nitric acid boiled in a microwave boiled on sand bath; I oven; B initial aqueous solution of As-species.
As-concentrations were measured by ICP-AESUSN and PN in the presence of increasing matrix elements. Calibrations were performed against As-standard solutions that also contain an average level of main elements representing plant matrices ŽTable 3, test solution B.. All other test solutions have been measured as samples. Obtained values were recalculated in order to display recoveries for all species in comparison with the calibration standard solution. As already specified in previous work w10x, the plant matrix may be highly variable. Main matrix elements are Ca, K, Na, Mg and P. K and Ca exhibit the highest concentrations Žusually up to 5% in dry matter.. Mg, P and Na contents are generally lower in these media, up to 0.5%. Mineralization procedures are usually applied to a 1-g sample, made up to 100 ml. In this way, the initial content of matrix elements is lowered by a factor of 100 in the sample solution analyzed.
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Table 3 Main matrix element composition of plant samples Žexpressed on dry matter and after mineralization. usually encountered in environmental studies a Element
K Ca Na Mg P a
Concentrations Žg gy1 dry matter.
Concentrations Žg mly1 . for a mineralisation 1 g in 100 ml
Synthetic matrices Žg mly1 . A
B
C
5000᎐30 000 8000᎐50 000 100᎐5000 1000᎐5000 100᎐3000
50᎐300 80᎐500 1᎐50 10᎐50 1᎐30
50 50 50 10 10
100 100 100 20 20
500 500 500 100 100
Corresponding synthetic matrix test solutions ŽA᎐C. were prepared.
Fig. 2 gives mean As-recoveries for different species in the presence of increasing individual matrix components. With the USN, the greatest changes in the arsenic emission intensity occurred in the presence of calcium and magnesium matrices. Compared to intensities obtained in the absence of a matrix, a significant improvement of recoveries was observed for all arsenic species studied: mean values for AsŽV., AsBet, MMAs, DMAs and PhAs were close to 100% in the presence of relatively low concentrations of Ca or Mg. The recovery for AsŽIII. was close to 80% in the presence of 100 g mly1 Mg and to 40% in the presence of 400 g mly1 Ca. The influence of K, P and Na matrices was less pronounced. For AsŽV. and AsBet, a quantitative recovery was obtained in the presence of Na only. From Fig. 2 it is clear that the presence of phosphorus at low concentrations Žapprox. 10 g mly1 . has a great impact on AsBet recovery. At higher concentrations of P, recoveries for other Asspecies increase significantly in comparison to solutions without concomitants. Effects due to a composite matrix Žsimulation of real plant solutions. are shown in Fig. 3a. This clearly indicates that in the presence of these matrices, generally good recoveries are observed. The cases of DMAs and particularly of AsŽIII. excepted, As-recoveries for all matrix concentrations ŽA, B and C. are practically quantitative ŽAsŽV., MMAs, AsBet and PhAs.. On the other hand, recoveries were approximately 80% for DMAs and only 40% for AsŽIII.. In comparison to using the pneumatic nebulization, all of the
observed effects using the USN are negligible ŽFig. 3b.. This indicates that observed shortcomings are mainly due to phenomena taking place within the USN desolvation stage. These phenomena give rise to interferences that do not occur in pneumatic nebulizers. An accurate analysis requires that any changes in the plant matrix composition do not result in a significant variation in the analyte signal. However, matrix effects presented in this work for USN cannot be simply explained on the basis of known parameters such as changes in the analyte transport rate and efficiency, electron number density or excitation temperatures in the plasma. High amounts of relatively volatile matrix elements introduced to the USN desolvation stage may induce other complementary dominant mechanisms, such as changes in the volatilization processes of analyte species within this device. Since such analyte-specific matrix effects result in both changes in the analyte transport Žmain cause. and possibly within the plasma, the internal standardization is, in any case, an inadequate means of correction for practical analysis when dealing with samples with a variable and unknown matrix composition. Fig. 3a indicates that a matrix matching with an average matrix could be effective for analysis of four As-species: AsŽV., MMAs, AsBet, PhAs. This is true for samples presenting low and average matrices only ŽTable 3, matrices A and B, and Fig. 3a., not for the high matrix ŽC.. For the latter, a new appropriate matrix matching should be envisaged. On the other hand, matrix matching does not allow quantitative recoveries
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for AsŽIII. and DMAs to be reached. Fortunately, it may be supposed that during the mineralization, AsŽIII. will be oxidized to AsŽV.. In this case, the recovery becomes quantitative ŽFig. 4.. In addition, the poor recovery of DMAs is not very important from the practical aspect because this As-form is usually present at very low concentrations in plants in comparison with other species Žgenerally AsŽV. and AsBet.. For DMAs, 80% recovery has been reached and the error realized on the total As-content may be neglected. Generally excessive recoveries of As in the presence of highest matrix concentrations ŽFig. 3a, matrix C. may be due, in the presence of easily ionizable elements, to a shift in the atomsrions ratio toward atoms by ion᎐electron recombination reactions, which result in an enhancement in atomic emission intensities w25᎐27x. In any case, the problems described are very complex and the probable synergy of phenomena
Fig. 2. Influence of main matrix components on As-recoveries Ž100 g ly1 As. for arsenic species studied. Concentrations of matrix elements added Žsquare symbols. are in g mly1 . The uncertainty of measurements is inferior to 5%.
Fig. 3. Average As-recoveries Ž100 g ly1 As. for six arsenic species in the presence of different matrices Žfor composition see Table 3.. The uncertainty of measurements is inferior to 5%: a, ultrasonic nebulizer; b, pneumatic nebulizer; I no rrr aaa matrix B; B matrix C. matrix; I matrix A; I
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230
Fig. 4. Average As-recoveries Ž100 g ly1 As. for six arsenic species after different sample treatments. The uncertainty of measurements is inferior to 5%: I initial aqueous solution of rrr aaa 12% nitric acid boiled on As-species; I 12% nitric acid; I sand bath; B 12% nitric acid q matrix B Žsee Table 3. boiled on sand bath.
does not permit us to postulate neither a general explanation nor a recommendation at this point.
satisfies the determination of As in plant samples and if shortcomings observed for model solutions influence results in a real analysis. The best means is a method validation using reference materials. Unfortunately, reference plant samples certified for arsenic at expected concentrations are not readily available and consequently we were only able to analyze three reference standards. For this reason, we added three laboratory control samples previously validated by the slurry sampling ETAAS method. The calibration of ICPAES-USN was performed using a usual working standard containing 1 g mly1 As together with 100 g mly1 Ca, K and 10 g mly1 Mg, P and Na. Results presented in Table 4 are in good agreement with expected values.
3.3. As-analysis of real samples
4. Conclusions
From previous paragraphs, it may be concluded that determination of arsenic in complex matrices by ICP-AES-USN is not a simple task and that several factors may invalidate expected results. We have attempted to simplify investigations by a separate study of the influence of matrix elements on the most common arsenic species. On the basis of these experiments, no useful conclusions may be postulated for the analysis of real samples, in which arsenic species are present and the matrix compositions are unknown. However, in some cases, matrix matching allowed consistent recoveries to be reached. It was therefore interesting to check if a common matrix matching
The analysis of reference samples shows that for a routine As-analysis of plant samples, the composite matrix is able to compensate for severe undesirable effects observed for main elements considered separately. As it has been already mentioned, the elements responsible for this beneficial leveling are calcium and magnesium, fortunately always present in these media. Compared to ICP-AES with a pneumatic nebulization, an ultrasonic nebulizer allows concentrations approximately ten times lower to be reached. With the instrumentation used and matrices studied, the limit of quantitation ŽLOQ. for arsenic was approximately 10 g ly1 . For a mineralization of
Table 4 Results for arsenic in plant reference samples a,b RMs
NIST-1572 IAEA-140 IAEA-393 CERVA 1 CERVA 2 CERVA 3 a b
Type
Citrus leaves Sea plant Algae Grass Grass Grass
Found Žg gy1 . Slurry ETAAS
ICP-AES-USN
Expectedb Žg gy1 .
3.23" 0.40 41.8" 2.50 106 " 1.00 3.48" 0.12 9.45" 0.07 12.1" 1.60
3.32" 0.27 44.5" 0.30 110 " 1.00 3.42" 0.10 10.2" 0.72 11.6" 0.82
3.10" 0.30 Ž44.2᎐46.4. 109 " 4.80 Ž3.40᎐3.70. Ž9.52᎐12.9. Ž11.0᎐14.2.
Each value is a mean of nine determinations Ž3 sub-samples, 3 measurements .. Expected: certified or Žrecommended. values
E. Vassile¨ a, M. Hoenig r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 223᎐232
a 1-g sample in 100 ml, this means that an Asconcentration of 1 g gy1 in a real sample may be quantified ᎏ a level often sufficient for environmental monitoring studies. On the other hand, taking into account numerous and pronounced phenomena occurring within the condensation stage of the USN Žand also in the plasma itself., some uncertainties exist regarding accuracy during a routine As-analysis. Severe problems observed in this work for individual As-species are generally significantly less pronounced in analyses of real samples. AsBet excepted, the species studied are supposed to be transformed during the mineralization procedure to AsŽV.. In this case, both forms remaining in the sample solution ŽAsŽV. and possibly AsBet. display a consistent analytical response. For our analytical applications however, in spite of the rapidity of the ICP-AES-USN method, the ETAAS determination of arsenic remains more sensitive ᎏ approximately three times lower Asconcentrations can be easily measured. In addition, ETAAS determinations of As may be simply calibrated against aqueous standards because possible matrix effects are better controlled by the signal integration and an appropriate chemical modifier. However, it must be also specified that the ICP-AES apparatus used in this work was approximately ten years old. Due to technological improvements resulting in lower noise, modern simultaneous multi-element ICP-AES instrumentation allows the determination of concentrations of approximately one order lower. Nevertheless, the observed shortcomings associated with the USN principle will remain present also in this case.
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Research note
Determination of arsenic in plant samples by inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization: a complex problem Emilia Vassilevaa , Michel Hoenig b,U b
a Uni¨ ersity of Sofia, Faculty of Chemistry, J. Bourchier A¨ . 1, BG-1126 Sofia, Bulgaria Veterinary and Agrochemical Research Centre (CERVA), Leu¨ ensesteenweg 17, B-3080 Ter¨ uren, Belgium
Received 25 October 2000; accepted 18 December 2000
Abstract Under well-defined conditions, the analysis of most trace elements by inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization ŽICP-AES-USN. leads to accurate results for environmental matrices usually studied. Due to differences in matrix composition between standards and samples, ICP-AES-USN determinations of arsenic are interfered with by changes that take place mainly within the desolvation stage of the USN device. In this work, effects of plant matrices on the determination of As in six arsenic species have been investigated. Firstly, interferences were simulated by measuring analyte Žspecies. signals in solutions containing variable concentrations of the main matrix elements encountered in mineralized plant samples ŽK, Ca, Mg, P and Na.. Secondly, the influence of real plant matrices on emission signals of arsenic species was also studied. In this case, the observed effects were different than for individual matrix elements considered separately: Ca and Mg always present in real samples efficiently compensate the undesirable effects. Validation of this statement has been performed using mineralized plant reference materials. In addition, ICP-AES-USN results have been compared with those obtained by Zeeman electrothermal atomic absorption spectrometry. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Arsenic; Plant matrices; ICP-AES; Ultrasonic nebulization
U
Corresponding author. E-mail address: [email protected] ŽM. Hoenig..
0584-8547r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 1 . 0 0 1 5 6 - 2
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1. Introduction Inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization is now a widely utilized instrumental technique for the determination of a large number of analytes. The main advantage of this technique is the high sample throughput that ensures lower detection limits in comparison to ICP-AES with pneumatic nebulization ŽPN. w1᎐4x. The ICP-AES-USN analysis has been successfully applied to various sample types including petroleum samples w5x, biological materials w6,7x, high-salt content solutions w8x and plants w9,10x. Compared to the ICP-AES analysis with PN as a means of sample introduction, some additional problems were reported for USN, however, their mechanisms have not yet been clarified and are often contradictory w10᎐14x. As already quoted in the earlier literature concerning ICP-AES-USN, differences in matrix composition between standards and samples may often lead to changes in efficiency of aerosol formation and transport but they may also be responsible for different evolution of phenomena occurring in the plasma itself. The drawbacks observed with USNs appear to a significantly lesser extent with conventional PNs and consequently they may be more easily controlled during the analysis. Most of the severe interferences observed with earlier USNs have been overcome by some recent improvements in their design, particularly by the commercial introduction of auto-tuned power supplies for RF generators in 1996. They allow a large degree of compensation for changes in the aerosol formation brought about by the variable matrix composition. In previous work w10x we have shown that changes in the acid concentration are really compensated for with auto-tuned transducers. The influence of the main matrix elements is also considerably reduced however, it is not totally eliminated. In such conditions, the only remaining alternative is the use of an internal standard ŽIS. correction which compensates for variation in analyte signals intensities affected by phenomena taking place during aerosol formation and transport, but also by changes occurring in the plasma.
With an objective to replace the time-consuming electrothermal atomic absorption spectrometry ŽETAAS. with the ICP-AES-USN technique for routine analyses Ženvironmental monitoring: plant and fish samples. in our laboratory we have previously shown the relative efficiency of the IS correction of environmental matrices using an Ar-emission line for trace element analysis w10,15x. However, such a correction may compensate for changes occurring in the plasma only. It has been concluded that Ar-correction may sometimes be satisfactory however, for the analysis of high-salt matrices it is not sufficiently powerful. From the same work w15x it also appears that the nebulization efficiency of the USN is approximately 8᎐10 times higher than that of the PN. This means that not only is 8᎐10 times more analyte transported to the plasma, but also of the matrix. Compared to pneumatic nebulizers that apparently behave very well in terms of matrix effects, these are much more pronounced in the analysis with an USN. To decide upon the behavior of the analyte signal in the presence of heavy matrices, several cases may be postulated: Ži. differences in the aerosol formation due to changes in the viscosity or in the salt content ŽUSN: transducer .; Žii. changes in the aerosol transport ŽUSN: principally condensation stage.; and Žiii. changes in analyte dissociation andror excitation Žplasma loading.. All of these phenomena result in a similar observation ᎏ a progressive suppression of the analyte emission signal with increasing matrix. In principle, a classical internal standard correction should make it possible to compensate for all of the above-mentioned phenomena. For most matrices Žplant and animal tissues . and analytes studied, matrix effects can be compensated for using internal standard correction. Unfortunately, some unpredictable drawbacks remain: these mainly concern the determination of boron, copper and arsenic, but also probably other volatile elements such as Se and Hg. The particular behavior of boron in ICP-AESUSN is well known w16᎐18x; even without interferent, its emission intensity is approximately 10 times lower with USN than with PN. For this particular case an explanation exists: in the presence of nitric acid, always added to standards and
E. Vassile¨ a, M. Hoenig r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 223᎐232
samples Žstabilization, mineralization., the formation and condensation of boric acid occurs. This happens largely during the cooling stage of the USN prior to its evacuation under this chemical form to the waste. This phenomenon has been confirmed by analysis of the waste from the USN condensation stage where more than 85% of boron Ž"30% for all other analytes . has been recovered w15x. In the same USN study we also observed that the emission intensities of copper are drastically and unusually depressed by the presence of nitric acid or Ca. Once again, this may be entirely attributable to particular physico᎐chemical mechanisms occurring in the USN condensation stage and which are different than in the case of other analytes w19x. The fact that the strong interference on Cu disappeared when the USN condensation stage was switched off is sufficient proof of this statement. During a multi-element run in the same work we observed that arsenic also shows some drawbacks with the use of USN. At first sight, these shortcomings are strongly associated with the arsenic form present in the sample. Although the determination of species is an increasingly essential task in environmental studies, the total analyte content in samples is still useful for a global evaluation of the situation. Despite the numerous analytical approaches for arsenic in recent years, there remains a need for reliable sensitive techniques that allow the determination of arsenic in some environmental samples. To reach very low As-concentrations in environmental samples, the hydride generation ŽHG-ICP-AES. may be of use, however, the unavoidable change of the introduction device necessitates that this analysis be performed in a separate run, thereby complicating the routine multi-element aspect of the analysis. Because the USN also allows the attainment of significantly lower concentrations than PN, the aim of this work is to evaluate the consistency of the As-determination in plant samples using ICP-AES-USN as well as to evaluate the behavior of six Asspecies ŽAsBet, AsŽIII., AsŽV., MMAs, PhAs and DMAs. during this determination. We have oriented this study towards plant matrices because other solid environmental samples such as sedi-
225
ments and soils generally exhibit higher As levels that do not necessitate the high sensitivity of analytical techniques such as ETAAS, HG-ICPAES or USN-ICP-AES.
2. Experimental 2.1. Apparatus Measurements were performed using a Liberty 200 vacuum ICP-AES system ŽVarian, Australia. equipped with an U-5000ATq ŽCETAC Technologies, USA. ultrasonic nebulizer. Some control measurements were also performed using a pneumatic nebulizer ŽV-groove q cyclonic spray chamber, Glass Expansion, Australia.. Optimized ICP-AES and USN parameters are given in Table 1. The ETAAS control determinations were performed using a SpectrAA-400 Zeeman atomic absorption spectrometer equipped with a GTA-96 graphite furnace and an autosampler ŽVarian, Australia.. The primary As-source was a boosted hollow cathode lamp ŽPhotron Superlamp, Australia.. Pyrolytically coated graphite tubes with forked solid pyrolytic platforms ŽRingsdorff, Germany. were used throughout. The electrothermal program used is given in Table 2. 2.2. Reagents and reference materials All main matrix element test solutions were prepared from pure salts ŽCa, K, Na and Mg: nitrates, Specpure, Johnson Matthey, UK, P: dihydrogen phosphate, Merck, Germany.. Disodium hydrogen arsenate Žs AsŽV., Na 2 HAsO4 7H 2 O, ŽFluka, Switzerland., phenylarsonic acid Žs PhAs., ŽAldrich, USA., arsenic trioxide Žs AsŽIII., NaAsO 2 , ŽMerck, Germany., di-sodium monomethyl arsonate Žs MMAs. CH 3 AsOŽONa. 2 , ŽArgus Chemicals, Italy., sodium dimethyl arsinate Žs DMAs., ŽCH 3 . 2 AsŽO.ONa 3H 2 O, ŽAldrich, USA. and arsenobetain Žs AsBet., CH 3 . 3 Asq CH 2 COOy, ŽFluka, Germany. were used without further purification. All Ascompounds were dissolved in de-mineralized redistilled water and NaAsO 2 in 1% Žwrw. NaOH. For the ICP-AES calibration, a 2% nitric acid
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Table 1 Instrumental conditions for ICP-AES-USN study of arsenic Instrumental conditions Wavelength Žnm. Background correction Observation height Žmm. Integration time Žs. Number of replicates Generator power ŽkW. PMT voltage ŽV. Plasma Ar-flow Ž1 miny1 . Auxiliary Ar-flow Ž1 miny1 . Pump speed Žrev. miny1 . Rinse time Žs. Nebulizer pressure ŽkPa. USN: heating stage Ž⬚C. USN: cooling stage Ž⬚C.
blank solution and a 100 g ly1 -As working standard were used. All standard and sample solutions studied were acidified to 2% with nitric acid. For dilutions, calibrated glass pipettes or adjustable micropipettes ŽGilson, France. were used. Plant reference materials were from the National Institute for Standards and Technology ŽNIST, Gaithersburg, MD, USA.: SRM-1572 Citrus leaves, or from the International Atomic Energy Agency ŽMonaco.: IAEA-140 Sea-plant and IAEA-393 Algae. Three CERVA laboratory control samples validated for arsenic by ETAAS were also analyzed ŽGrasses CERVA 1, 2 and 3.. 2.3. Sample preparation procedure For the plant samples studied, a wet digestion
As 188.98 Dynamic mode 6 Žabove the coil. 5 3 1.2 700 16.5 0.75 15 Žsample intake " 2 ml miny1 . 40 Žfast pump on. 140 140 3
procedure with elimination of silicon was applied Žmodified CII method. w20x. Half a gram of the sample was weighed in a PTFE flask and 5 ml of concentrated nitric acid was added. The mixture was heated on a sand bath Žtemperature approx. 120⬚C. for 30 min and 5 ml of hydrogen peroxide was added slowly to avoid possible strong foaming that can occur with some samples. The heating was carried out during 1 h at the same temperature. After the decomposition of organic matter, 5 ml of hydrofluoric acid was added. The evaporation step was continued overnight and the dry residue finally leached with 2 ml of nitric acid and dissolved by a short heating. After cooling the sample, the solution was made up to 50 ml. Validation of this method for As has been presented in previous work w20,21x.
Table 2 Electrothermal program for As-determinationa,b,c Step
Temperature Ž⬚C.
Time Žs.
Ar-flow Ž1 miny1 .
1 2 3 5 6 7 8 9
300 1200 1200 100b 100 2500c 2500c 110
30.0 ramp 4.0 ramp 10.0 hold 5.0 rampx 1.0 hold 0.7 ramp 4.0 hold 12.0 rampx
3.0 3.0 3.0 3.0 0 0 0 3.0
a
The sample is introduced with IrrMg modifier on the preheated platform Ž110⬚C.. Cool-down step. c Signal integration. b
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3. Results and discussion 3.1. Effect of nitric acid In plasma spectroscopic methods where sample introduction is ensured by a nebulization device, the effect of acids is attributed to physical changes in the aerosol generation process or to modifications of plasma conditions resulting from physico᎐chemical mechanisms. In most dissolution procedures, environmental samples are decomposed using single nitric acid or a mixture of other mineral acids. To simulate sample mineralization conditions, six arsenic species at the same As-concentrations were measured by ICP-AES-USN and PN in the presence of 2% HNO3 after heating aliquots of these solutions in a domestic microwave oven or on a sand bath. In addition, initial aliquots Žone without acid and one with acid but without heating. were measured as control samples. For the following text, reference recoveries Žs 100% for each species. were values given by the ETAAS analysis for which the accuracy has been validated in previous studies w21᎐23x. Fig. 1a demonstrates that with a conventional PN device the recoveries of all As-species, regardless of sample preparation procedure were approximately 100%. However, significant differences were observed when the same solutions were aspirated using USN ŽFig. 1b., confirming thus the above-mentioned assumption that the main analytical problem is located within the desolvation stage of the USN system. A similar conclusion has been previously postulated for copper, which exhibited different behavior to all other trace elements studied w15,24x. Using the USN, recoveries for As were systematically less than 50% for the species studied. Indeed the recovery of the As-species in pure aqueous solutions were minimum; they were increased when nitric acid was present in solution. Maximum recoveries were obtained when the solutions were treated with microwaves ᎏ this resulted in phase transformation of As-species. 3.2. Effect of main matrix elements Solutions of six arsenic species at the same
Fig. 1. Average As-recoveries Ž100 g ly1 As. for six arsenic species after different sample treatments. The uncertainty of measurements is inferior to 5%. a, pneumatic nebulizer; b, rrr ultrasonic nebulizer; I 2% nitric acid; I 2% nitric acid aaa 2% nitric acid boiled in a microwave boiled on sand bath; I oven; B initial aqueous solution of As-species.
As-concentrations were measured by ICP-AESUSN and PN in the presence of increasing matrix elements. Calibrations were performed against As-standard solutions that also contain an average level of main elements representing plant matrices ŽTable 3, test solution B.. All other test solutions have been measured as samples. Obtained values were recalculated in order to display recoveries for all species in comparison with the calibration standard solution. As already specified in previous work w10x, the plant matrix may be highly variable. Main matrix elements are Ca, K, Na, Mg and P. K and Ca exhibit the highest concentrations Žusually up to 5% in dry matter.. Mg, P and Na contents are generally lower in these media, up to 0.5%. Mineralization procedures are usually applied to a 1-g sample, made up to 100 ml. In this way, the initial content of matrix elements is lowered by a factor of 100 in the sample solution analyzed.
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228
Table 3 Main matrix element composition of plant samples Žexpressed on dry matter and after mineralization. usually encountered in environmental studies a Element
K Ca Na Mg P a
Concentrations Žg gy1 dry matter.
Concentrations Žg mly1 . for a mineralisation 1 g in 100 ml
Synthetic matrices Žg mly1 . A
B
C
5000᎐30 000 8000᎐50 000 100᎐5000 1000᎐5000 100᎐3000
50᎐300 80᎐500 1᎐50 10᎐50 1᎐30
50 50 50 10 10
100 100 100 20 20
500 500 500 100 100
Corresponding synthetic matrix test solutions ŽA᎐C. were prepared.
Fig. 2 gives mean As-recoveries for different species in the presence of increasing individual matrix components. With the USN, the greatest changes in the arsenic emission intensity occurred in the presence of calcium and magnesium matrices. Compared to intensities obtained in the absence of a matrix, a significant improvement of recoveries was observed for all arsenic species studied: mean values for AsŽV., AsBet, MMAs, DMAs and PhAs were close to 100% in the presence of relatively low concentrations of Ca or Mg. The recovery for AsŽIII. was close to 80% in the presence of 100 g mly1 Mg and to 40% in the presence of 400 g mly1 Ca. The influence of K, P and Na matrices was less pronounced. For AsŽV. and AsBet, a quantitative recovery was obtained in the presence of Na only. From Fig. 2 it is clear that the presence of phosphorus at low concentrations Žapprox. 10 g mly1 . has a great impact on AsBet recovery. At higher concentrations of P, recoveries for other Asspecies increase significantly in comparison to solutions without concomitants. Effects due to a composite matrix Žsimulation of real plant solutions. are shown in Fig. 3a. This clearly indicates that in the presence of these matrices, generally good recoveries are observed. The cases of DMAs and particularly of AsŽIII. excepted, As-recoveries for all matrix concentrations ŽA, B and C. are practically quantitative ŽAsŽV., MMAs, AsBet and PhAs.. On the other hand, recoveries were approximately 80% for DMAs and only 40% for AsŽIII.. In comparison to using the pneumatic nebulization, all of the
observed effects using the USN are negligible ŽFig. 3b.. This indicates that observed shortcomings are mainly due to phenomena taking place within the USN desolvation stage. These phenomena give rise to interferences that do not occur in pneumatic nebulizers. An accurate analysis requires that any changes in the plant matrix composition do not result in a significant variation in the analyte signal. However, matrix effects presented in this work for USN cannot be simply explained on the basis of known parameters such as changes in the analyte transport rate and efficiency, electron number density or excitation temperatures in the plasma. High amounts of relatively volatile matrix elements introduced to the USN desolvation stage may induce other complementary dominant mechanisms, such as changes in the volatilization processes of analyte species within this device. Since such analyte-specific matrix effects result in both changes in the analyte transport Žmain cause. and possibly within the plasma, the internal standardization is, in any case, an inadequate means of correction for practical analysis when dealing with samples with a variable and unknown matrix composition. Fig. 3a indicates that a matrix matching with an average matrix could be effective for analysis of four As-species: AsŽV., MMAs, AsBet, PhAs. This is true for samples presenting low and average matrices only ŽTable 3, matrices A and B, and Fig. 3a., not for the high matrix ŽC.. For the latter, a new appropriate matrix matching should be envisaged. On the other hand, matrix matching does not allow quantitative recoveries
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for AsŽIII. and DMAs to be reached. Fortunately, it may be supposed that during the mineralization, AsŽIII. will be oxidized to AsŽV.. In this case, the recovery becomes quantitative ŽFig. 4.. In addition, the poor recovery of DMAs is not very important from the practical aspect because this As-form is usually present at very low concentrations in plants in comparison with other species Žgenerally AsŽV. and AsBet.. For DMAs, 80% recovery has been reached and the error realized on the total As-content may be neglected. Generally excessive recoveries of As in the presence of highest matrix concentrations ŽFig. 3a, matrix C. may be due, in the presence of easily ionizable elements, to a shift in the atomsrions ratio toward atoms by ion᎐electron recombination reactions, which result in an enhancement in atomic emission intensities w25᎐27x. In any case, the problems described are very complex and the probable synergy of phenomena
Fig. 2. Influence of main matrix components on As-recoveries Ž100 g ly1 As. for arsenic species studied. Concentrations of matrix elements added Žsquare symbols. are in g mly1 . The uncertainty of measurements is inferior to 5%.
Fig. 3. Average As-recoveries Ž100 g ly1 As. for six arsenic species in the presence of different matrices Žfor composition see Table 3.. The uncertainty of measurements is inferior to 5%: a, ultrasonic nebulizer; b, pneumatic nebulizer; I no rrr aaa matrix B; B matrix C. matrix; I matrix A; I
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230
Fig. 4. Average As-recoveries Ž100 g ly1 As. for six arsenic species after different sample treatments. The uncertainty of measurements is inferior to 5%: I initial aqueous solution of rrr aaa 12% nitric acid boiled on As-species; I 12% nitric acid; I sand bath; B 12% nitric acid q matrix B Žsee Table 3. boiled on sand bath.
does not permit us to postulate neither a general explanation nor a recommendation at this point.
satisfies the determination of As in plant samples and if shortcomings observed for model solutions influence results in a real analysis. The best means is a method validation using reference materials. Unfortunately, reference plant samples certified for arsenic at expected concentrations are not readily available and consequently we were only able to analyze three reference standards. For this reason, we added three laboratory control samples previously validated by the slurry sampling ETAAS method. The calibration of ICPAES-USN was performed using a usual working standard containing 1 g mly1 As together with 100 g mly1 Ca, K and 10 g mly1 Mg, P and Na. Results presented in Table 4 are in good agreement with expected values.
3.3. As-analysis of real samples
4. Conclusions
From previous paragraphs, it may be concluded that determination of arsenic in complex matrices by ICP-AES-USN is not a simple task and that several factors may invalidate expected results. We have attempted to simplify investigations by a separate study of the influence of matrix elements on the most common arsenic species. On the basis of these experiments, no useful conclusions may be postulated for the analysis of real samples, in which arsenic species are present and the matrix compositions are unknown. However, in some cases, matrix matching allowed consistent recoveries to be reached. It was therefore interesting to check if a common matrix matching
The analysis of reference samples shows that for a routine As-analysis of plant samples, the composite matrix is able to compensate for severe undesirable effects observed for main elements considered separately. As it has been already mentioned, the elements responsible for this beneficial leveling are calcium and magnesium, fortunately always present in these media. Compared to ICP-AES with a pneumatic nebulization, an ultrasonic nebulizer allows concentrations approximately ten times lower to be reached. With the instrumentation used and matrices studied, the limit of quantitation ŽLOQ. for arsenic was approximately 10 g ly1 . For a mineralization of
Table 4 Results for arsenic in plant reference samples a,b RMs
NIST-1572 IAEA-140 IAEA-393 CERVA 1 CERVA 2 CERVA 3 a b
Type
Citrus leaves Sea plant Algae Grass Grass Grass
Found Žg gy1 . Slurry ETAAS
ICP-AES-USN
Expectedb Žg gy1 .
3.23" 0.40 41.8" 2.50 106 " 1.00 3.48" 0.12 9.45" 0.07 12.1" 1.60
3.32" 0.27 44.5" 0.30 110 " 1.00 3.42" 0.10 10.2" 0.72 11.6" 0.82
3.10" 0.30 Ž44.2᎐46.4. 109 " 4.80 Ž3.40᎐3.70. Ž9.52᎐12.9. Ž11.0᎐14.2.
Each value is a mean of nine determinations Ž3 sub-samples, 3 measurements .. Expected: certified or Žrecommended. values
E. Vassile¨ a, M. Hoenig r Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 223᎐232
a 1-g sample in 100 ml, this means that an Asconcentration of 1 g gy1 in a real sample may be quantified ᎏ a level often sufficient for environmental monitoring studies. On the other hand, taking into account numerous and pronounced phenomena occurring within the condensation stage of the USN Žand also in the plasma itself., some uncertainties exist regarding accuracy during a routine As-analysis. Severe problems observed in this work for individual As-species are generally significantly less pronounced in analyses of real samples. AsBet excepted, the species studied are supposed to be transformed during the mineralization procedure to AsŽV.. In this case, both forms remaining in the sample solution ŽAsŽV. and possibly AsBet. display a consistent analytical response. For our analytical applications however, in spite of the rapidity of the ICP-AES-USN method, the ETAAS determination of arsenic remains more sensitive ᎏ approximately three times lower Asconcentrations can be easily measured. In addition, ETAAS determinations of As may be simply calibrated against aqueous standards because possible matrix effects are better controlled by the signal integration and an appropriate chemical modifier. However, it must be also specified that the ICP-AES apparatus used in this work was approximately ten years old. Due to technological improvements resulting in lower noise, modern simultaneous multi-element ICP-AES instrumentation allows the determination of concentrations of approximately one order lower. Nevertheless, the observed shortcomings associated with the USN principle will remain present also in this case.
w4x w5x
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