Alternative Thermodiffusion Interface for Simultaneous Speciation of

Jun 25, 2008 - Chemical Engineering, Xiamen University, Xiamen 361005, China, and State Key Laboratory of Marine Environmental. Science, Xiamen ...
0 downloads 0 Views 594KB Size
Download PDF
Anal. Chem. 2008, 80, 6104–6109

Alternative Thermodiffusion Interface for Simultaneous Speciation of Organic and Inorganic Lead and Mercury Species by Capillary GC-ICPMS Using Tri-n-propyl-lead Chloride as an Internal Standard Dong Yan,† Limin Yang,† and Qiuquan Wang*,†,‡ Department of Chemistry and the MOE Key Laboratory of Modern Analytical Sciences, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China, and State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China An alternative thermodiffusion interface (TDI) was designed and constructed for the effective online coupling of capillary gas chromatography (cGC) and inductively coupled plasma mass spectrometry (ICPMS). Pb2+, (CH3)3Pb+, (C2H5)3Pb+, Hg2+, CH3Hg+ and C2H5Hg+ were derived as Pb(C4H9)4, (CH3)3PbC4H9, (C2H5)3PbC4H9, (C4H9)2Hg, CH3HgC4H9, and C2H5HgC4H9 when butyl magnesium bromide was employed as a derivatization reagent for a proof-of-concept study, avoiding the loss of their species specific information. All these derivatives together with the neutral fully saturated (CH3)4Pb and (C2H5)4Pb could be quantitatively separated within 7 min using a 15 m long capillary column, allowing the determination and speciation of organic and inorganic Pb and Hg species in a single run. The method detection limits (3σ) for Me4Pb, Et4Pb, Me3Pb3+, Pb2+, MeHg+, EtHg+, and Hg2+ are 0.07, 0.06, 0.04, 7.0, 0.09, 0.1, and 0.2 pg g-1, respectively. Moreover, tri-n-propyl-lead chloride was synthesized and used as an alternative internal standard for the accurate and simultaneous speciation analysis of Pb and Hg in complicated environmental and biological samples for the first time. This cGC-TDI-ICPMS method was validated by analyzing Pb and Hg species in certified reference materials and then was applied to simultaneous speciation analysis of Pb and Hg in reallife samples. It is expected that these approaches can be extended to the speciation of other organometallic compounds after suitable modifications and so will aid in monitoring the occurrence, pathways, toxicity, and/or biological effects of these compounds in the environment and in organisms. Pb and Hg are very toxic elements, which both come mainly from anthropogenic activities but follow different pathways of species transformation in the environment. Their toxicity and/or * Corresponding author. Phone: +86 592 218 1796. Fax: +86 592 218 1796. E-mail: [email protected]. † Department of Chemistry and the MOE Key Laboratory of Modern Analytical Sciences. ‡ State Key Laboratory of Marine Environmental Science.

6104

Analytical Chemistry, Vol. 80, No. 15, August 1, 2008

bioavailability, in addition to their mobility and their impact on the environment, depend not only on their total concentration but also significantly on their chemical forms.1 In general, the monopositive cations behave as maximal toxic species that are derived by loss of one organic group from the neutral fully saturated organometal, e.g. (C2H5)3Pb+, and CH3Hg+. Interest in the environmental pathways of organolead and organomercury compounds has been growing in order to elucidate and understand the biogeochemical cycles and the toxicity and/or bioavailability of Pb and Hg for decades. To this end, novel and sophisticated analytical methods have played a crucial role, and it is now recognized worldwide that one has to rely on a combination of chromatographic separation techniques with structure-selective and/or element-specific detectors in most cases of real-life analytical speciation.2–5 The techniques most frequently applied for the speciation of Pb or Hg species are gas or liquid chromatography or capillary electrophoresis coupled to electron-impact mass spectrometry or element-specific detection systems such as atomic absorption spectrometry, inductively coupled plasma atomic emission spectrometry, microwave-induced plasma atomic emission spectrometry, and atomic fluorescence spectrometry6–10 as well as, especially, inductively coupled plasma mass spectrometry (ICPMS) and species-specific isotope dilution ICPMS.11–15 Although such approaches have enabled the detection of specific (1) World Health Organization (WHO). Environmental Criteria 101: Methylmercury; WHO: Geneva, Switzerland, 1995; 165. (2) Dean, J. R.; Butler, O.; Fisher, A.; Garden, L. M.; Cresser, M. S.; Watkins, P.; Cave, M. J. Anal. At. Spectrom. 1998, 13, 1R–56R (3) Quevauviller, Ph.; Ebdon, L.; Harrison, R. M.; Wang, Y. Analyst 1998, 123, 971–976. (4) Sa´nchez Ur′y´a, J. E.; Sanz Medel, A. Talanta 1998, 47, 509–524. (5) Lobinski, R.; Dirkx, W. M. R.; Szpunar-Lobinska, J.; Adams, F. C. Anal. Chim. Acta 1994, 286, 381–390. (6) Cai, Y.; Jaffe, R.; Jones, R. Environ. Sci. Technol. 1997, 31, 302–305. (7) Liang, L. N.; Jiang, G. B.; Liu, J. F.; Hu, J. T. Anal. Chim. Acta 2003, 477, 131–137. (8) Yan, X. P.; Yin, X. B.; Jiang, D. Q.; He, X. W. Anal. Chem. 2003, 75, 1726– 1732. (9) Yin, Y. M.; Liang, J.; Yang, L. M.; Wang, Q. Q. J. Anal. At. Spectrom. 2007, 22, 330–334. (10) Yin, Y. M.; Jiu, J. H.; Yang, L. M.; Wang, Q. Q. Anal. Bioanal. Chem. 2007, 388, 381–386. (11) Fernandez, R. G.; Bayon, M. M.; Alonso, J. I. G.; Sanz-Medel, A. J. Mass Spectrom. 2000, 35, 639–646. 10.1021/ac800347j CCC: $40.75  2008 American Chemical Society Published on Web 06/25/2008

Scheme 1. Synthesis of Tri-n-propyl-lead Chloride

organometal compound at the picogram level on a routine basis, few have been reported for simultaneous speciation of the organometallic species Sn, Pb, and Hg.16–20 A limitation for the simultaneous speciation of Pb and Hg species rests with the coextraction and consequent derivatization of Pb and Hg species into suitable chemical forms without losing their species-specific information for a high resolution separation when gas chromatography is employed using sodium tetraethylborate (NaBEt4) as a derivatization reagent. Derivatization of Pb2+ and (Et)3Pb+, and/ or Hg2+ and EtHg+ by deuterium NaBEt4 into Pb(C2D5)4 and (Et)3Pb(C2D5) and/or (C2D5)2Hg and EtHg(C2D5) yield mass differences but are still difficult to quantitatively separate.21 On the other hand, an interface for effectively coupling GC and ICPMS is still a challenge for avoiding the condensation of the volatile derivatized analytes in some cases when the effective interface is not available in hand and/or use a specific ICPMS instrument, especially in the case that the condensation effect from ambient or cool Ar gas (from a pressured or liquid Ar bottle) on the hot volatile derivatized analytes from the GC column before entering into the ICP torch is considered. For the first time, in this study, we synthesized tri-n-propyllead chloride (Pr3PbCl) as an alternative internal standard for simultaneous quantification and speciation of Pb and Hg species and used butyl magnesium bromide (BuMgBr) as a derivatization reagent for a proof-of-concept study to maintain the species specific information of organic and inorganic Pb and Hg species. Moreover, a novel thermodiffusion interface (TDI) was designed and constructed for high efficient transportation of the derivatives of the Pb and Hg species from capillary gas chromatography (cGC) to ICPMS. This methodology was validated and applied to the simultaneous speciation analysis of Pb and Hg in certified reference materials as well as environmental and biological samples and showed great potential for the multielemental speciation of toxic metals. EXPERIMENTAL SECTION Chemicals. All reagents used were of at least analytical-grade reagent purity. Ultrapure water (18 MΩ) was used throughout (12) Martı´n-Doimeadios, R. C. R.; Krupp, E.; Amouroux, D.; Donard, O. F. X. Anal. Chem. 2002, 74, 2505–2512. (13) Qvarnstro ¨m, J.; Lambertsson, L.; Havarinasab, S.; Hultman, P.; Frech, W. Anal. Chem. 2003, 75, 4120–4124. (14) Demuth, N.; Heumann, K. G. Anal. Chem. 2001, 73, 4020–4027. (15) Poperechna, N.; Heumann, K. G. Anal. Chem. 2005, 77, 511–516. (16) Prange, A.; Jantzen, E. J. Anal. At. Spectrom. 1995, 10, 105–109. (17) Smaele, T. D.; Moens, L.; Dams, R.; Sandra, P.; Van der Eycken, J. J. Chromatogr., A 1998, 793, 99–106. (18) Bayon, M. M.; Camblor, M. G.; Alonso, J. I. G.; Sanz-Medel, A. J. Anal. At. Spectrom. 1999, 14, 1317–1322. (19) Jitaru, P.; Infante, H. G.; Adams, F. C. J. Anal. At. Spectrom. 2004, 19, 867–875. (20) Poperechna, N.; Heumann, K. G. Anal. Bioanal. Chem. 2005, 383, 153– 159. (21) Yu, X. M.; Pawliszyn, J. Anal. Chem. 2000, 72, 1788–1792.

this study and was obtained from the PEN-TUNG SAH MEMS Research Center of Xiamen University, China. Pb (purity >99.99%), and HgCl2 (Guaranteed Reagent) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Me3PbCl (98%), Et3PbCl (98%), MeHgCl (98.5%), and EtHgCl (98.5%) were obtained from Dr. Ehrenstorfer GmbH (Augsburg, Germany); Me4Pb (65%) and Et4Pb (50%) were obtained from Sigma-Aldrich. Tetramethylammonium hydroxide (TMAH, 25%) and sodium diethyldithiocarbamate (NaDDTC) used for sample pretreatment were purchased from the Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). BuMgBr and Pr3PbCl were synthesized in our own laboratory for Pb and Hg species derivatization and used as an internal standard for quantification, respectively. Synthesis of Pr3PbCl. The synthesis of (C3H7)3PbCl was carried out according to Scheme 1. Briefly, appropriate amounts of Na granules and Pb powder were mixed and pressed into a die under a no water and oxygen atmosphere in a Laboratory master 100 glovebox (MBRAUN, Germany). The die was subjected to vacuum conditions so as to allow Na diffusion into the Pb and then heated at about 400 °C for 3 h under an argon atmosphere in a canal-style furnace in order to obtain Pb-Na alloy containing 59% Pb as identified using scanning electron microscope and energy dispersive X-ray spectrometer (SEM/EDS 1530, LEO, Germany), as shown in Figure S-1. The resulting Pb-Na alloy was ground into a powder and placed in a 250 mL round-bottomed flask fitted with a condenser of CaCl2 drying tube, stirring rod, and dropping-funnel. Appropriate amounts of n-propyl iodide and pyridine were poured down through the condenser, and the whole mixture was stirred on an ice-water bath, with the reactants kept in an N2 atmosphere. The progress of the reaction was controlled over a period of 5-6 h by the addition of H2O every 45 min. The tetra-n-propyl-lead synthesized was separated out by steam-distillation. Petroleum ether (bp 60-90 °C) was added to dissolve the tetra-n-propyl-lead, and it was washed with 5% NaOH then 5% H2SO4, and finally H2O. After phase separation, the petroleum ether phase was dried using CaCl2, and then a stream of dry hydrogen chloride (produced by the mixture of NaCl and concentrated H2SO4) was passed through the petroleum ether phase containing tetra-n-propyl-lead for about 30 min, and then the petroleum ether quickly evaporated on a water-bath (40 °C) under reduced pressure until a small quantity of a milk-white precipitate appeared in the round-bottomed flask. Finally, hexane was added to dissolve the resulting product prior to GC-EI-MS (Shimadzu GC/MS QP2010, Japan) identification, which revealed that the product was (C3H7)3PbCl, and the purity is more than 99% as shown in Figure S-2. The yield of (C3H7)3PbCl was more than 60%. The long-term stability of the (C3H7)3PbCl hexane solution (1 g L-1) was investigated. No significant change in its concentration was found during this study after more than 2 years of storage at 4 °C in the dark. Analytical Chemistry, Vol. 80, No. 15, August 1, 2008

6105

Figure 1. Schematic diagram of the thermodiffusion interface (TDI). Table 1. Optimization Parameters of the cGC-TDI-ICPMS System

radio frequency power plasma gas flow rate nebulizer gas flow rate auxiliary gas flow rate dwell time per point isotope determination numeric controlled heater temperature

ICPMS Parameters 1200 W 15.5 L min-1 0.79 L min-1 1.08 L min-1 100 ms 208 Pb, 206Pb, 202Hg, 160 °C

200

Hg

cGC Parameters DB-17 (50% phenyl-50% dimethyl polysiloxane) carrier gas/flow rate 2 mL min-1 He held for 3 min and then ramped to 40 mL min-1 at 4.75 mL min-1. oven temperature program 80 °C ramped to 100 °C at 10 °C min-1 and then ramped to 260 °C at 45 °C min-1, finally to 270 °C at 5 °C min-1. Injection mode splitless injection volume 1 µL injection temperature 220 °C column

Sample Pretreatment. CRM 463 (tuna fish), NIST1946 (Lake Superior fish tissue), codfish, and solen meat samples were each ground to a slurry in a mortar. Approximately 2 g of the slurry spiked with 0.1 mL of 400 ng mL-1 (C3H7)3PbCl was digested in 5 mL of 25% TMAH solution in a water bath at 45 °C for 2 h until the tissue completely dissolved to form a pale yellow solution. After cooling, the solution was neutralized with 50% hydrochloric acid to pH 7, and then 5 mL of ammonium acetate buffer (pH 7.0) was added. The mixture was extracted with 4 mL of n-hexane for 2 h on a mechanical shaker after the addition of 3 mL of 0.5 6106

Analytical Chemistry, Vol. 80, No. 15, August 1, 2008

Figure 2. Influence of the temperature of the numeric control heater on the sensitivity of the Pb and Hg species.

mol L-1 NaDDTC and 2 g of NaCl to maintain the ion balance of the water solution. After phase separation by centrifugation, 2 mL of the hexane layer was transferred into a glass-capped vial for butylation with 0.2 mL of 2.0 mol L-1 BuMgBr in ether with occasional mixing for 20 min. The mixture was washed with 2 mL of 0.5 mol L-1 H2SO4 to destroy the excess BuMgBr, and then the organic layer was collected in a capped vial and dried with anhydrous Na2SO4. Finally, 1 µL of the resulting solution was injected under splitless injection mode into the cGC-TDIICPMS system for analysis. Tree bark samples were shredded into thin pieces before the TMAH digestion, since they generally

Figure 3. Simultaneous speciation of a model solution of known concentrations of Pb and Hg species by cGC-TDI-ICPMS using Pr3PbCl as an internal standard.

were optimized in order to obtain maximum resolution and efficiency as well as detection sensitivity of the butyl derivatives of the Pb and Hg species. The optimization parameters are summarized in Table 1.

Figure 4. pH dependence of the extraction of Pb and Hg species from the sample solution.

dissolved much more slowly than fish and shellfish meat samples. It was suggested to leave the mixture of thin pieces of bark samples and TMAH in a capped tube in a water bath (45 °C) overnight. In the case of CRM580 (estuarine sediment) as well as urban soil sample, 2 g was extracted in a capped vial with 4 mL of hexane after the addition of 10 mL of H2O, 6 g of NaCl, 1 g of KI, and 3 mL of 0.5 mol L-1 NaDDTC as well as 2 g of glass beads (20-40 mesh) for 2 h on a mechanical shaker. After phase separation, 2 mL of the hexane layer was taken for butylation as described above. Instrumentation. A gas chromatograph (Clarus 500 PerkinElmer SCIEX, Canada) fitted with a DB-17 capillary column (crossbond 50% phenyl-50% dimethyl polysiloxane; 0.53 mm i.d. × 15 m in length × 0.25 µm film thickness) was coupled to an ICPMS (DRC II, PerkinElmer, SCIEX, Canada) via a novelly designed TDI as shown in Figure 1. The interface consisted of a copper tube (thermodiffusion heating part) and a numeric control heater, in which the capillary column passed directly through the ICP torch, for the highly efficient transportation of the butyl derivatives of Pb and Hg species. The cGC and ICPMS conditions

RESULTS AND DISCUSSION Thermodiffusion Interface for Coupling cGC and ICPMS. The interface between cGC and ICPMS should maintain the volatile derivatives of Pb and Hg species in a gaseous state during the transportation process from cGC to ICPMS and avoid any condensation effect.16,18,22–26 In this regard, two points should be considered to be responsible for the possible condensation of the derivatives for cGC through ICPMS. One is the temperature of the transfer line between the cGC column oven to the ICP and another the cooling effect of the ambient or cool Ar gas on the separated gaseous derivatives before entering the ICP. In this study, a TDI was designed using a copper tube (1.68 o.d. × 1.44 i.d. × 16 cm in length) for maintaining the temperature over the part of the capillary column out of the cGC column oven. The copper tube was connected to the column oven, making use of the thermodiffusion effect for having the same temperature as that in the oven and wrapped by an asbestos attemperation tube (4.4 o.d. × 1.7 i.d. × 16 cm in length) (Figure 1). Such a design can be thought as the cGC column oven is enlarged. A stainless bushing (0.32 o.d. × 0.26 i.d. × 30 cm in length) was used for avoiding possible breakage of the capillary column, and more important, the Ar gas was preheated by a numeric controlled heater and was introduced to make up the nebulizer gas at an angle of 45° for eliminating any possible condensation of the separated volatile derivatives of Pb and Hg species in the interface (Figure 1). A short flexible PTFE tube (0.32 o.d. × 0.18 i.d. × 10 cm in length) was used to connect the interface to the ICP torch; the time for passing through this tube was calculated to be 19 ms ([(0.18/ 2)2π(10 cm3)]/[(0.79 × 103 cm3)/60 s] ) 0.019 s) when the preheated nebulizer gas flow rate was 0.79 L min-1 (Figure 1). Results obtained indicated that ICPMS intensities of all the (22) Kim, A. W.; Foulkes, M. E.; Ebdon, L.; Hill, S. J.; Patience, R. L.; Barwise, A. G.; Rowland, S. J. J. Anal. At. Spectrom. 1992, 7, 1147–1149. (23) Pretorius, W. J.; Ebdon, L.; Rowland, S. J. J. Chromatogr. 1993, 646, 369– 375. (24) De Smaele, T.; Moens, L.; Dams, R.; Sandra, P.; Vandereycken, J.; Vandyck, J. J. Chromatogr., A 1998, 793, 99–106. (25) Gallus, S. M.; Heumann, K. G. J. Anal. At. Spectrom. 1996, 11, 887–892. (26) Poehlman, J.; Pack, B. W.; Hieftje, G. M. Am. Lab. 1998, 30, C50.

Analytical Chemistry, Vol. 80, No. 15, August 1, 2008

6107

2.83 75.5 ± 3.7c 0.39 ± 0.02 2.85 132 ± 3 0.43 ± 0.01 4.39 ± 0.16c,15 NAb NAb NAb NAb 0.7 2892 ± 190 128 100 ± 5600 424 ± 30 13.1 ± 0.5 4.4 ± 0.3 866 ± 20 88.2 ± 5.5 11.8 ± 1.2 128 000 ± 5600 59.6 ± 1.0 13.1 ± 0.5 4.4 ± 0.3 78.8 ± 0.9 3.5 ± 0.2