Behaviour of arsenic and selenium in an ICP-QMS with collision and reaction interface

Transcription

1 TECHNICAL NOTE Journal of Analytical Atomic Spectrometry Behaviour of arsenic and selenium in an ICP-QMS with collision and reaction interface Catarinie D. Pereira, a Edivaldo E. Garcia, b Fernando V. Silva, c Ana Rita A. Nogueira d and Joaquim A. Nobrega* a Received 16th February 2010, Accepted 6th August 2010 DOI: /c003100c The performance of a collision reaction interface (CRI) for minimization of spectral interferences on analytical signals of As + and 76 Se +, Se +, Se +, Se + and 82 Se + isotopes was evaluated by using H 2 and He gases introduced through the sampler or skimmer cones of an inductively coupled plasma quadrupole mass spectrometer (ICP-QMS). Different gas flow rates were tested employing reference solutions prepared either in hydrochloric acid or in water soluble tertiary amines (CFA-C) media as carbon source. A hydrogen gas flow rate at 60 ml min 1 introduced through skimmer cone was effective for minimizing polyatomic interferences on As + and Se isotopes, except for Se +. The use of He introduced through sampler or skimmer cones had a minor effect on the minimization of interferences when compared to the use of H 2 gas. Moreover, it was observed that the gas introduction through the sampler cone had no effect on interferences when using gas flow rates up to ml min 1 for both gases evaluated. Based on these results, it can be recommended to use H 2 gas introduced through skimmer cone of the CRI for correcting polyatomic interferences on As + and Se isotopes in matrixes containing up to 2% v v 1 chloride. Introduction The interest in arsenic determination has been gaining importance because a large amount of arsenic compounds are released constantly into the environment due to its use for agricultural and industrial purposes. 1 Selenium is another element of analytical interest because it is both toxic and essential depending on its concentration and chemical form. 2 Several spectroanalytical techniques are employed for determination of total concentrations of arsenic and selenium at trace levels, such as graphite furnace-atomic absorption spectrometry (GF-AAS), 3,4 hydride generation-atomic absorption spectrometry (HG-AAS), 5,6 inductively coupled plasma-optical emission spectrometry (ICP-OES), 7,8 and inductively coupled plasmaquadrupole mass spectrometry (ICP-QMS). 9,10 This latter technique has been successfully applied for routine analysis, mainly because of advantages associated with its multielement capacity, analytical throughput, and detection limits at ultratrace levels. 11 However, as with any other sensitive analytical technique, ICP- QMS determinations can be hampered due to limitations associated with spectral and non spectral interferences. For many elements, these processes can be eliminated by using alternative isotopes and internal standards, respectively. In the case of arsenic determination, which is monoisotopic, the formation of a Departamento de Quımica, Universidade Federal de São Carlos, PO Box 676, São Carlos, SP, Brazil. djan@terra.com.br; Tel: ; b Departamento de Quımica, Universidade Estadual de Maringa, Maringa, PR, Brazil c Nestle Brasil Ltda, São Paulo, SP, Brazil d Embrapa Pecuaria Sudeste, São Carlos, SP, Brazil This article was presented at the 2010 Winter Conference on Plasma Spectrochemistry. 40 Ar 35 Cl + strongly affects its determination in matrices with relatively high concentrations of chloride, such as biological and seawater samples. 12 This type of interference can also be severe for many types of samples which require hydrochloric or perchloric acid for sample digestion. Additionally, isobaric interferences caused by 40 Ar 37 Cl +, 40 Ar 38 Ar + and 40 Ar 40 Ar + also affect the measurement of Se +, Se + and Se + isotopes, respectively. However, for Se it is possible to select another isotope non-affected by isobaric interferences, such as 82 Se +, but detection limit is deteriorated due to its smaller natural abundance. 13 Consequently, it is important to develop ICP-QMS procedures for arsenic and selenium determinations without interferences and with low limits of detection. Depending on the analytical task, different strategies may be employed for correcting spectral interferences in elemental determinations when using an ICP-QMS, such as mathematical correction equations, cool plasma, matrix matching approach and collision/reaction cells. 14,15 However, mathematical equations generally fail for correcting spectral interferences when they are found or formed in relatively high concentrations, 16 and the cool plasma strategy is not feasible for correcting spectral interferences for arsenic and selenium because both have high ionization potentials (9.82 and 9. ev, respectively). 17 The use of ICP-MS equipped with a high-resolution mass analyzer is an alternative for circumventing spectral interferences, but such instruments are more expensive. 12,16 Townsend demonstrated the accurate determination of arsenic and selenium in standard reference materials using sector field ICP-MS in high resolution mode. 18 Recently, collision reaction interface (CRI) technology was introduced for correcting polyatomic interferences using collision and reaction gases. Different to dynamic reaction cells (DRC), gases in the CRI are introduced directly through sampler and/or skimmer cones and there is no pressurization chamber. This journal is ª The Royal Society of Chemistry 2010 J. Anal. At. Spectrom.

2 Moreover, the CRI technology uses the gas expansion region of the plasma to promote collisions and reactions between polyatomic ions and introduced gases, enabling the use of a more energetic environment for promoting processes involved with polyatomic interference correction. 9 No enclosed multipole (quadrupole, hexapole or octapole) is employed for mass or kinetic discrimination on CRI technology. All interference discriminations are made along ion beam transport in a double off-axis quadruple spectrometer. 19 Some procedures proposed in the literature for correcting isobaric interference caused by 40 Ar 35 Cl + on As + isotope adopt mathematical equation correction, but this strategy usually fails when high chloride-matrix concentrations are considered. The same is observed for polyatomic interferences on selenium isotopes, mainly because most of them are argon-based interferences. An ICP-QMS equipped with DRC technology was also employed for correcting polyatomic interferences on As +, Se +, Se +, and Se +. Most analytical applications with collision reaction cell technology relied on H 2 and/or He gases for correcting polyatomic interferences. 12,20,21 In addition to spectral interferences, arsenic and selenium determinations by ICP-QMS may be affected by matrix interferences when carbon-rich matrices are analysed. The presence of carbon in the argon plasma may affect arsenic and selenium isotopes determinations by increasing their analytical signals due to charge transfer reactions from C + species to referred element atoms. 22,23 The study here described focused on the evaluation of a CRI for correcting spectral interferences on arsenic and selenium measurements by ICP-QMS. The introduction of hydrogen or helium through sampler or skimmer cones of a CRI was evaluated for correcting spectral interferences on As + and Se isotopes. Experimental Instrumentation The experiments were carried out using an ICP-QMS (Varian 820-MS, Mulgrave, Australia) equipped with CRI, and an ion focusing system configured in a 90 degree array for plasma and quadrupole/detector positions. The CRI interface is composed of modified cones which allow direct introduction of gases on a plasma expansion zone located in the vacuum region. The gas introduction can be performed through the sampler and/or skimmer cones. The ion focusing system uses a ring assembly divided in four segments which allows the creation of an electrical parabolic field. All photons and neutral species entered in the vacuum zone pass through the ion mirror being collected on the mechanical pump oil. The charged species make a 90 degree curve directed to a focal point positioned at quadrupole entrance. Maximum throughput for ion beam is achieved by setting the voltage of focusing system lens in order to generate an electrical parabolic field that minimizes ion dispersion created on transport of analytes with different masses. The sample introduction system used was composed by a concentric Seaspray nebulizer and a double-pass Scott type spray chamber, which is enclosed in a Peltier cooled device operated at 3 C to condensate excess of solvent and thus produce a plasma with lower formation of oxides. An automatic sampler (Varian SPS3, Australia) was also coupled to the spectrometer for sample presentation. The ICP-QMS operating conditions are summarized in Table 1. These conditions were kept in all experiments unless any change was stated. A closed-vessel microwave digestion system (ETHOS-1600, Milestone-MLS, Sorisole, Italy) equipped with fiber optic temperature and pressure sensors was used for sample digestion. Reagents and reference solutions A multi-element tuning solution (SpecSol, Jacareı, SP, Brazil) containing 5 ng ml 1 Ba, Be, Ce, Co, In, Mg, Pb, Th and Tl was used for instrument optimization daily (torch alignment and mass calibration). Ultrapure water (18.2 MU cm) obtained from a Milli-Q water purification system (Millipore, Billerica, MA, USA), and HNO 3 and HCl obtained by sub-boiling distillation (Milestone) were used to prepare all solutions. A mixture of water soluble tertiary amines (CFA-C, Spectrasol, Warwick, NY, USA) with ph adjusted at 8.0 by using concentrated HNO 3 was used to prepare diluted CFA-C solutions. This reagent was used as a carbon source. Stock solutions of 1000 mg L 1 As and Se were purchased from Quemis (São Paulo, SP, Brazil) and they were used to prepare 10 ng ml 1 arsenic and 5 ng ml 1 selenium reference solutions in 0.1, 0.5, 1.0 and 2.0% v v 1 HCl with or without 10% v v 1 CFA-C. Solutions prepared in these same media without As and Se were used as analytical blanks. Liquid argon for plasma formation, and H 2 and He gases introduced through sampler or skimmer cones of the CRI were % purity (White Martins, Sertãozinho, SP, Brazil). All polypropylene containers were soaked in 10% v v 1 HNO 3 for 24 h and rinsed thoroughly with ultrapure water before use. Two standard reference materials, oyster tissue (NIST 1566a, National Institute of Standard and Technology, Gaithersburg, MD, USA) and mussel tissue (GBW 08571, NRC-National Research Centre CRM, Beijing, China) were used to evaluate the accuracy of the analytical method. Effect of CRI gases on spectral interferences and net signal intensities of As and Se isotopes The effect of H 2 and He gas flow rates (0, 20, 40, 60 and ml min 1 ) introduced through sampler or skimmer cones of the CRI Table 1 Operating conditions for ICP-QMS ICP-QMS instrument Varian 820-MS Generator frequency/mhz 27 RF power/kw 1.4 Torch inner diameter/mm 2.0 Plasma gas flow rate/l min 1 18 Auxiliary gas flow rate/l min Nebulizer gas flow rate/l min Sheath gas flow rate/l min Points per peak 2 Scans/replicates 20 Replicates/Samples 5 Dwell time/ms 10 Sampler cone Ni Skimmer cone Ni Nebulizer Seaspray Spray chamber Scott-type Spray chamber temperature 3 C Isotopes monitored As +, 76 Se +, Se +, Se +, Se +, 82 Se + J. Anal. At. Spectrom. This journal is ª The Royal Society of Chemistry 2010

3 on polyatomic ions and net signal intensities of As +, 76 Se +, Se +, Se +, Se +, and 82 Se + isotopes was evaluated by employing solutions of 10.0 ng ml 1 As with 0.1, 0.5, 1.0 and 2.0% v v 1 HCl and solutions of 5.0 ng ml 1 Se with 2.0% v v 1 HCl. Solutions prepared in these same media without As and Se were used as analytical blanks. For all experiments, all default correction equations of the control software were deleted when running under CRI conditions. Effect of CFA-C on net signal intensities of As and Se isotopes The effect of different concentrations of CFA-C (0, 5.0, 7.5, 10.0 and 12.5% v v 1, ph 8.0) on net signal intensities of As +, 76 Se +, Se +, Se +, Se +, and 82 Se + isotopes was evaluated. Solutions of 10 ng ml 1 As with 0.1, 0.5, 1.0 and 2.0% v v 1 HCl and 5 ng ml 1 Se with 2.0% v v 1 HCl were employed in this study. Again, solutions prepared in these same media without As and Se were used as analytical blanks. Sample preparation Sample masses of 250 mg of Oyster and Mussel were microwaveassisted digested using 5 ml of 7.0 mol L 1 HNO 3 solutions and 3 ml of 30% vv 1 H 2 O 2 in perfluoroalkoxy copolymer resin (PFA) closed vessels. The heating program was performed in six steps: (1) 3 min at 300 W; (2) 1 min at 0 W; (3) 4 min at 500 W; (4) 3 min at 650 W; (5) 3 min at 1000 W; (6) 15 min under ventilation. Temperature and pressure sensors were used in all digestions. After digestion, samples and blank solutions were transferred to 15.0 ml volumetric flasks and the volume was made up with deionized water. Further dilution was performed to ensure a maximum 0.1% m v 1 dissolved solids. Results and discussion Effect of the CRI on As and Se interferences Skimmer and sampler cones. Experiments were carried out by introduction of either H 2 or He gases through skimmer because the sampler cone had no effect on the minimization of spectral interferences evaluated using gas flow rates up to ml min 1 (Fig. 1a). However, additional experiments showed that the introduction of H 2 or He at flow rates greater than 1000 ml min 1 through the sampler cone minimized the occurrence of spectral interferences on As + isotope (Fig. 1b), but this is not attractive considering the best performance of the skimmer is at significantly lower flow rates. It must be pointed out that it is not possible to work simultaneously with H 2 and He gases. Based on these results, it was selected the introduction of either H 2 or He through the skimmer cone of the CRI for minimization of spectral interferences on As + and Se isotopes. The reduction of polyatomic interferences occurs due to plasma conditions at the interface zone that still contains high electron density and high temperature. Consequently, reactions and collision processes are favoured before interferences and analyte ions are extracted into the ion optics. The lowest values of background equivalent concentrations (BEC) were obtained for As + and Se isotopes when using CRI compared to standard mode operation without CRI (Table 2). The best BECs were This journal is ª The Royal Society of Chemistry 2010 Fig. 1 (a) Effect of He or H 2 introduced through sampler or skimmer cones on m/z. (b) Effect of H 2 gas flow rates introduced through sampler on m/z. All acid solutions did not contain As. obtained using H 2 gas, except for 82 Se + which showed the lowest BEC values when using He. It may be supposed that the interference caused by 40 Ar 2 H 2+ on 82 Se + was less effectively controlled because the introduction of H 2 favoured the formation of this polyatomic interference. It can also be concluded that the use of H 2 gas is more effective than the use of He gas. It may be inferred that the use of hydrogen may promote the following reactions that help to decrease background signals: 2 40 Ar 35 Cl + (g) +H 2(g) / 2 40 Ar + (g) + 2 HCl (g) 40 Ar 40 Ar + (g) +H 2(g) / 40 ArH + (g) + 40 ArH (g) Hydrogen gas flow rates greater than 60 ml min 1 introduced through the skimmer cone reduced spectral interferences on As + and Se + isotopes for solutions containing HCl (Fig. 2 and 3). Best conditions were reached at ml min 1 of H 2 based on compromise conditions taking into account sensitivity and minimization of interferences. Finally, although the CRI had minimized spectral interferences, losses of signal intensities were observed for As + and Se isotopes. It was experimentally observed that increasing the H 2 and He gas flow rates decreased the net signal intensities for J. Anal. At. Spectrom.

4 Table 2 Background equivalent concentrations (BEC) obtained for solutions of 10.0 ng ml 1 As and 5.0 ng ml 1 Se in 2.0% v v 1 HCl a BEC/mg ml 1 H 2 /ml min 1 BEC/mg ml 1 He/mL min 1 Isotope As Se Se + ND Se Se + ND ND ND Se a ND Not Determined. Fig. 2 Effect of H 2 gas introduced through the skimmer cone of CRI on net analytical signals of 10 mg L 1 As in different HCl media. either H 2 or He gases, that negatively affects the focusing of these isotopes. Effect of a carbon source on net signal intensities of As and Se isotopes Based on the decrease of net signal intensities observed for all isotopes evaluated when introducing either H 2 or He gases through the skimmer cone, and on the well-known information in the literature that carbon may enhance signals for As and Se, 22,23 the use of CFA-C for attenuating the negative effect of the CRI on As and Se sensitivities was investigated. Solutions containing several concentrations of CFA-C solutions (5.0, 7.5, 10 and 12.5% v v 1 ) were measured. The effect of CFA-C on signal intensities of As +, Se +, Se +,and 82 Se + was measured by using standard mode ICP-QMS operation without using the CRI. Net signal intensities varied up to two-fold and maximum gain was reached at 10% v v 1 CFA-C for all isotopes. Net signal intensities obtained for solutions prepared in CFA-C medium containing As +, Se + and 82 Se + were 212, 50, and 33% higher, respectively, at 60 ml min 1 of H 2 gas flow rate, when compared with those obtained for solutions without CFA-C. It means that this reagent may be used for attenuating signal losses caused by CRI and, consequently, it contributed to improving the detection capability for As + and Se isotopes. Since CFA-C medium improved signals for As and Se isotopes, the use of CRI for correcting interferences was investigated. These measurements were carried out by introducing Table 3 Limits of detection (LOD, ng ml 1 ) for As in different acid media with and without CFA-C. Hydrogen gas at ml min 1 flow rate was introduced through the skimmer cone of the CRI H 2 gas flow rate/ml min 1 Fig. 3 Effect of H 2 gas introduced through the skimmer cone of the CRI on analytical signals of 5 mg L 1 Se in 2% v v 1 HCl. Solution Without CFA-C With CFA-C 0 0 As + and Se isotopes. The losses of analytical signals were higher than 90% for all isotopes investigated when inserting gas flow rates higher than 60 ml min 1 through the skimmer cone of the CRI. It may be supposed that these losses of sensitivities may be caused by collision processes between As + and Se isotopes and 1% v/v HNO % v/v HCl % v/v HCl % v/v HCl % v/v HCl J. Anal. At. Spectrom. This journal is ª The Royal Society of Chemistry 2010

5 Table 4 Limits of detection (LODs, ng ml 1 ) for Se in 2% v v 1 HCl with and without CFA-C solution (ph 8.0). Hydrogen gas at ml min 1 flow rate was introduced through the skimmer cone of the CRI Isotope H 2 gas flow rate/ml min 1 Without CFA-C 60 ml min 1 H 2 through the skimmer cone using solutions containing 10 ng ml 1 As and 5 ng ml 1 Se in chloride medium plus 10% v v 1 CFA-C. Tables 3 and 4 show limits of detection (LOD) for As and Se isotopes, respectively, obtained with and without CFA-C solutions. Data obtained without using the CRI are also shown for comparison purposes. It may be concluded that best LODs are reached without CRI due to the effects already discussed, but losses of sensitivities caused by the interface can be partially attenuated by preparing solutions in CFA-C medium. However, the use of CFA-C as a carbon source may only be recommended when needed for improving sensitivity or when sample preparation is carried out in alkaline medium. 24,25 Analysis of certified reference materials With CFA-C Se + ND a 0.35 ND a 0.18 Se Se Se + ND a 0.97 ND a Se a ND Not determined. Arsenic and selenium were determined in two CRMs and the results are shown in Table 5. The equipment was operated in standard mode and with ml min 1 H 2 introduced through the skimmer cone. Despite some apparent trend towards positive errors observed for As, according to a t-test all but one determined values for As and Se are in agreement with certified values at a 95% confidence level with or without using the CRI. The only value that was not in agreement was the As content determined in mussel without CRI. The proper performance without CRI can be explained considering that the chloride concentration Table 5 Determination of arsenic and selenium (mg g 1, mean and standard deviations, n ¼ 3) in certified reference materials (CRM) using ICP-MS operated in standard mode and with collision-reaction interface. Hydrogen gas at ml min 1 flow rate was introduced through the skimmer cone of the CRI c Isotope Oyster Tissue a Standard Oyster Tissue a CRI Mussel Tissue b Standard Mussel Tissue b CRI As Se + ND ND ND Se Se Se + ND ND Se a Certified values: As and Se b Certified values: As and Se c ND Not determined. This journal is ª The Royal Society of Chemistry 2010 in diluted digests was not enough for causing interferences. For instance, according to the certified value and the adopted procedure for sample digestion, the chloride concentration in oyster tissue digest is around 6.9 mg L 1 which is below the threshold level to cause appreciable formation of ArCl +. A similar effect was previously observed by Townsend for this same sample. 18 On the other hand, selenium determination required the use of the CRI for destroying polyatomic interferences. Best results were obtained for Se + and Se + isotopes. Results obtained for 82 Se + isotope presented positive errors due to the formation of 40 Ar 2 H 2+ which was more intense when H 2 was introduced through the skimmer cone. Conclusions Introduction of gas through the skimmer cone was more effective than through the sampler cone for correcting interferences caused by polyatomic ions in As and Se determinations by ICP- QMS. It was also experimentally demonstrated that hydrogen was better than helium for destroying these interferences. It may be supposed that reaction processes promoted by H 2 were more effective than collision processes promoted by both gases for overcoming interferences. 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