Anal. Chem. 1997, 69, 1604-1611
Sensitive, Simultaneous Determination of Organomercury, -lead, and -tin Compounds with Headspace Solid Phase Microextraction Capillary Gas Chromatography Combined with Inductively Coupled Plasma Mass Spectrometry Luc Moens, Tom De Smaele, and Richard Dams*
Laboratory of Analytical Chemistry, Ghent University, Proeftuinstraat 86, B-9000 Gent, Belgium Paul Van Den Broeck and Pat Sandra
Laboratory of Organic Chemistry, Ghent University, Krijgslaan 281 S4, B-9000 Gent, Belgium
The use of the solid phase microextraction (SPME) technique for the simultaneous determination of organomercury, -tin, and -lead is described. The organometallic compounds were in situ derivatized with sodium tetraethylborate, sorbed on a poly(dimethylsiloxane)-coated fused silica fiber, and desorbed in the splitless injection port of the GC. The different organometallic compounds were simultaneously determined by an inductively coupled plasma mass spectrometer (ICPMS) coupled to the GC using a transfer line developed in-house. The detection limits obtained with CGC-ICPMS for monobutyl-, dibutyl-, and tributyltin are between 0.34 and 2.1 ng/L as Sn. Analysis of a PACS-1 reference material confirmed the reliability of the combination of SPME as extraction technique with CGC-ICPMS for the analysis of organotin compounds. Today, a wide range of atomic spectroscopic techniques is available for the sensitive determination of trace and ultratrace metals in all kind of samples. The growing awareness of the strong dependence of the toxicity of heavy metals on their chemical form,1 has led to an increasing interest in the quantitative determination of specific heavy metal species. Therefore, speciation has become an important topic of present day analytical research, and during the last decade various chromatographic techniques have been coupled to highly sensitive and elementspecific detectors. The most commonly used hyphenated techniques are a combination of gas chromatography (GC), highperformance liquid chromatography (HPLC), or supercritical fluid chromatography (SFC) with element-specific detection methods like atomic absorption spectrometry (AAS), microwave-induced plasma and inductively coupled plasma atomic emission spectrometry (MIP-AES and ICP-AES, respectively), and mass spectrometry (MS).2-4 In addition, the coupling of a GC to an ICPMS (1) Craig, P. J. Organometallic compounds in the environment. Principles and reactions; Harlow: Essex, England, 1986. (2) Hill, S. J.; Bloxham, M. J.; Worsfold, P. J. J. Anal. At. Spectrom. 1993, 8, 499-515. (3) Ebdon, L.; Hall, S. J.; Ward, W. R. Analyst 1987, 112, 1-16. (4) Smits, R. LC-GC Int.1994, 7, 694-697.
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has proved to yield a highly sensitive and selective method for organometal speciation.5-16 For GC, organometallic compounds need to be extracted from the sample matrix and to be derivatized to volatile species. For many years, hydride generation17-19 and especially Grignard derivatization20-22 have been used. Grignard derivatization, however, requires nonaqueous, aprotic media and, therefore, numerous handling steps. Recently, ethylation with sodium tetraethylborate was introduced by Ashby et al.23-25 for the determination of organotin compounds in environmental samples. Derivatization with NaBEt4 has been investigated for a whole range of organometallic compounds such as organolead,26 -mercury,27 -cadmium,28 (5) Van Loon, J. C.; Alcock, L. R.; Pinchin, W. H.; French, J. B. Spectrom. Lett. 1986, 19, 1125-1135. (6) Chong, N. S.; Houk, R. S. Appl. Spectrosc. 1987, 41, 66-74. (7) Peters, G. R.; Beauchemin, D. J. Anal. At. Spectrom. 1992, 7, 965-969. (8) 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. (9) Evans, E. H.; Caruso, J. A. J. Anal. At. Spectrom. 1993, 8, 427-431. (10) Pretorius, W. G.; Ebdon L.; Rowland, S. J. Chromatogr. 1993, 646, 369375. (11) Kim, A.; Hill, S.; Ebdon, L.; Rowland, S. J. High Resolut. Chromatogr. 1992, 15, 665-668. (12) Prange, A.; Jantzen, E. J. Anal. At. Spectrom. 1995, 10, 105-109. (13) De Smaele, T.; Verrept, P.; Moens, L.; Dams, R. Spectrochim. Acta Part B 1995, 50, 1409-1416. (14) De Smaele, T.; Moens, L.; Dams, R.; Sandra, P. Fresenius’ J. Anal. Chem. 1996, 354, 778-782. (15) De Smaele, T.; Moens, L.; Dams, R.; Sandra, P. LC-GC Int. 1996, 9, 138142. (16) De Smaele, T.; Vanhaecke, F.; Moens, L.; Dams, R.; Sandra, P. In Proceedings of the 18th International Symposium on Capillary Chromatography, 20-25 May 1996, Riva del Garda, Italy; Sandra, P., Ed.; Verlag: Heidelberg, 1996; p 52. (17) Martin, F. M.; Donard, O. F. X. Fresenius’ J. Anal. Chem. 1995, 351, 230236. (18) Donard, O. F. X.; Rapsomanikis, S.; Weber, J. H. Anal. Chem. 1986, 58, 772-777. (19) Cai, Y.; Rapsomanikis, S.; Andreae, O. Anal. Chim. Acta 1993, 274, 243251. (20) Dirkx, W. M. R.; Van Mol, W. E.; Van Cleuvenbergen, R. J. A.; Adams, F. C. Fresenius’ J. Anal. Chem. 1989, 335, 769-774. (21) Mueller, M. D. Anal. Chem. 1987, 59, 617-623. (22) Maguire, R. J.; Tkacz, R. J. J. Chromatogr. 1983, 268, 99-101. (23) Ashby, J.; Craig, P. J. Appl. Organomet. Chem. 1991, 5, 173-181 (24) Ashby, J.; Craig, P. J. Sci. Total Environ. 1989, 78, 219-232. (25) Ashby, J.; Clark, S.; Craig, P. J. J. Anal. At. Spectrom. 1988, 3, 735-736. S0003-2700(96)00905-5 CCC: $14.00
© 1997 American Chemical Society
Table 1. Instrumental Parameters for CGC-ICPMS gas chromatograph column
carrier gas/inlet pressure
Perkin Elmer Autosystem FSOT, poly(dimethylsiloxane), 30 m, 0.25 i.d., df ) 0.50 µm splitless 250 °C 60 °C (1 min), 20 °C/min to 200 °C (0.5 min) Xe/H2 (1/99 mixture), 30 psi
transfer line transfer line temperature
home-made heated stainless steel tube 250 °C
ICPMS rf power sampling depth carrier gas flow rate auxiliary gas flow rate plasma gas flow rate sampling cone/aperture diameter skimmer cone/aperture diameter dwell time
Perkin Elmer Sciex Elan 5000 1250 W 10 mm 1.10 -1.25 L/min 1.20 L/min 15 L/min Ni/1.125 mm
injection technique injection temperature temperature program
Ni/0.875 mm 30-50 ms (depending on number of nuclides to be measured) 10 ms (126Xe)
-tin,23,29 and -selenium.30 The advantages of this derivatization technique are that ethylation can take place in the aqueous phase and extraction can simultaneously be performed. Although ethylation has considerably reduced the sample preparation time, classical liquid/liquid extractions are still tedious and time consuming and sometimes need large amounts of highly pure organic solvents. Therefore, solid phase microextraction, developed by Pawliszyn and co-workers31-35 and designed for extraction of organic compounds from aqueous samples, has been applied to metal speciation for the determination of organotin, -mercury, and -lead compounds.36-38 We have used the SPME technique in combination with the highly sensitive and selective CGC-ICPMS hyphenated technique for the simultaneous determination of organomercury, -tin, and -lead compounds in aqueous samples. EXPERIMENTAL SECTION CGC-ICPMS. A Perkin Elmer Autosystem gas chromatograph was coupled to a Perkin Elmer Sciex Elan 5000 ICP mass spectrometer by means of an in-house-made transfer line.13 The operating conditions are summarized in Table 1. The isotopes 120Sn, 202Hg, and 208Pb were selected for simultaneous detection of Sn, Hg, and Pb, respectively. Xe, present at a concentration of 1% in the H2 gas used as carrier gas, was measured (126Xe) as an internal standard for the sensitivity of the ICPMS detection. The (26) Rapsomanikis, S.; Donard, O. F. X.; Weber, J. H. Anal. Chem. 1986, 58, 35-48. (27) Rapsomanikis, S.; Craig, P. J. Anal. Chim. Acta 1991, 248, 563-567. (28) D’Ulivo, A.; Chen, Y. J. Anal. At. Spectrom. 1989, 4, 319-322. (29) Cai, Y.; Rapsomanikis, S.; Andreae, M. O. J. Anal. At. Spectrom. 1993, 8, 119-125. (30) Clark, S.; Craig, P. J. Microchim. Acta 1992, 109, 141-144. (31) Louch, D.; Motlagh, S.; Pawliszyn, J. Anal. Chem. 1992, 64, 1187-1199. (32) Potter, D. W.; Pawliszyn, J. Environ. Sci. Technol. 1994, 28, 298-305. (33) Buchholz, K. D.; Pawliszyn, J. Anal. Chem. 1994, 66, 160-167. (34) Zhang, Z.; Yang, M. J.; Pawliszyn, J. Anal. Chem. 1994, 66, 844A-852A. (35) Zhang, Z.; Pawliszyn, J. Anal. Chem. 1993, 65, 1843-1852. (36) Morcillo, Y.; Cai, Y.; Bayona, J. M. J. High Resolut. Chromatogr. 1995, 18, 767-770. (37) Cai, Y.; Bayona, J. M. J. Chromatogr. 1995, 696, 113-122. (38) Tutschku, S.; Mothes, S.; Wennrich, R. Fresenius’ J. Anal. Chem. 1996, 354, 587-591.
raw data were further processed with the Chromafile MS software (Perkin Elmer, Lab Control GmbH, Ko¨ln, Germany). SPME Device. A SPME fiber holder for manual injections, with a fused silica fiber coated with 100 µm poly(dimethylsiloxane), was obtained from Supelco (Bellafonte, PA), and 50 mL glass vials closed with PTFE-coated silicone rubber septa were used for sampling. Proper mixing of the sample solutions during the SPME extractions was achieved with a magnetic stirrer. Reagents and Solutions. Monobutyltin trichloride (MBTCl3, 95% purity), dibutyltin dichloride (DBTCl2, 97% purity), and tributyltin chloride (TBTCl, 96% purity) were purchased from Sigma-Aldrich (Bornem, Belgium). Methylmercury chloride (MMCl, 98% purity) was obtained from Merck (Darmstadt, Germany), and trimethyllead chloride (TMLCl, p.a.) was from ABCR (Karlsruhe, Germany). Stock solutions of 1 g/L (as metal) organometallic compounds were separately prepared in ethanol (p.a., Merck). Mixed organometallic solutions were prepared in and further diluted with ethanol to concentrations varying between 2 and 100 µg/L as metal and stored at 4 °C in the dark. Tripropyltin acetate (TPTOAc, for synthesis, Merck) was used as internal standard. Sodium tetraethylborate (NaBEt4) was purchased from Strem Chemicals (Bischheim, France). Milli-Q water (Millipore Corp., Bedford, MA) was used to freshly prepare 1% (m/v) solutions immediately before the start of the analyses. To obtain buffer solutions with pH values between 4 and 5.5, appropriate volumes of 0.2 mol/L acetic acid (HOAc, p.a., Merck) and sodium acetate (NaOAc, p.a., UCB, Leuven, Belgium) solutions were mixed together. For the liquid/liquid extraction of sediment samples and for the optimization of the derivatization, isooctane (p.a., UCB) was used. Optimization of the Derivatization. First, the conditions for derivatization with NaBEt4 were optimized using liquid/liquid extraction. Into 50 mL glass vials was pipetted 25 mL of an appropriate HOAc/NaOAc buffer, and 500 µL of a 100 µg/L organometal standard, 500 µL of TPTCl (100 µg/L) as internal standard, and 1 mL of isooctane were added. Subsequently, different amounts (50-2000 µL) of a 1% aqueous NaBEt4 solution were added. The vials were closed and shaken for 10 min, and then an aliquot of the isooctane layer was transfered into GC sample vials. One microliter of the isooctane extract was manually injected in the splitless mode. Optimization of the SPME Parameters. Twenty-five microliters of an appropriate NaOAc/HOAc buffer was pipetted into a 50 mL glass sample vial, and 100 µL of a 100 µg/L organometal standard solution was added. The vial was closed with a septum, and subsequently 1 mL of a freshly prepared NaBEt4 solution was added via syringe. The reaction mixture was magnetically mixed, the SPME needle was pierced into the septum, and the fiber was exposed to the headspace. After 10 min of sampling, the SPME needle was removed and inserted in the GC injection port operated in the splitless mode for thermal desorption during 1 min. Relative Sensitivity of SPME versus Liquid/Liquid Extraction. To compare the sensitivity of SPME with liquid/liquid extraction, both methods were applied to 25 mL of HOAc/NaOAc buffer to which 500 µL of an organometal standard solution was added. The latter contained different organolead, -tin and -mercury compounds at a concentration level of 2 µg/L as metal for Analytical Chemistry, Vol. 69, No. 8, April 15, 1997
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Figure 1. Influence of the NaBEt4 concentration on the extraction efficiency. Peak areas are normalized to TPT (IS) and corrected for changes in ICPMS sensitivity via the peak area of the 126Xe signal.
SPME and 100 µg/L for liquid/liquid extraction. After derivatization with NaBEt4, the SPME fiber was exposed for 10 min to the headspace and, in an alternative experiment, was submerged in the solution. For the liquid/liquid extraction experiment, the organometals were enriched by extraction in 500 µL of isooctane, 1 µL of which was injected in the CGC instrument. Analysis of Standard Reference Material. To evaluate the reliability of the SPME-CGC-ICPMS analysis, the NRC PACS-1 marine sediment reference material was analyzed for organotin compounds. The organotin compounds were extracted with both the classical liquid/liquid extraction technique and the SPME technique. Approximately 0.2 g of sediment was weighed in a 50 mL glass vial, and 5 mL of Milli-Q water was added to moisturize the sediment. Subsequently, 5 mL of HOAc and 15 mL of methanol were added. Finally, 250 µL of 100 µg/L TPTCl, used as internal standard, was added. The samples were well shaken and ultrasonically treated for 30 min. For the SPME extraction technique, the leached sediments were centrifuged for 15 min at 4000 rpm prior to further analysis. Of the obtained supernatant, 250 µL was pipetted into a 50 mL glass vial, and 25 mL of HOAc/NaOAc buffer at pH 5.3 was added. The vial was sealed with a Teflon-coated septum, and 1 mL of 1% NaBEt4 was added in four small portions with a syringe while the sampling solution was being stirred magnetically. Subsequently, the SPME needle was inserted in the headspace of the vial, and sampling took place for 10 min. The SPME needle was then inserted in the GC injection port. For the liquid/liquid extraction technique, centrifugation of the sediment was redundant. For the classical liquid/liquid extraction, 5 mL of isooctane and subsequently four 1 mL portions of a 1% NaBEt4 solution were added to the leached sediment. The mixture was well shaken manually. Reaction and extraction were performed within 10 min. One milliliter of the isooctane layer was transferred into a GC sample vial. One microliter of the isooctane extract was manually injected in the splitless mode. RESULTS AND DISCUSSION Optimization of the Derivatization. To obtain the highest efficiency of the SPME extraction technique, the derivatization 1606
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of the organometallic compounds with NaBEt4, which is necessary in SPME as well as in liquid/liquid extraction, was optimized by using liquid/liquid extraction. One of the important parameters is the pH of the sample solution. In a series a experiments, the pH was varied between 4 and 6, and the highest derivatization yield was obtained at pH 5.3, which is in agreement with values found by other research groups.22,24,25,39 In Figure 1, the normalized peak areas of the organometals (normalized to IS and to Xe to correct for instrument instabilities14-16) are plotted versus the volume of NaBEt4 solution added to the spiked buffer solutions. As can be seen, an amount of 5001000 µL was found to be optimal for all species. In all further optimization experiments, 1000 µL of a 1% NaBEt4 solution was used. To shorten the analysis time as much as possible, the reaction time of the organometallic compounds with NaBEt4 was investigated. Reaction time and SPME sampling time were studied separately, allowing us to determine which process, reaction or extraction, limits sample throughput. The reaction time was varied between 1 and 20 min with no significant influence on the signal obtained with ICPMS detection after SPME and CGC. The extraction with SPME, taking about 10 min (see further), therefore, is the rate-determining step. Optimization of the SPME Parameters. The main difference between SPME and conventional extraction techniques is the fact that, in SPME, no exhaustive extraction of the analytes takes place, but extraction is based on an equilibrium between the analyte concentrations in the liquid phase, headspace, and the solid phase fiber coating. The number of moles of analyte sorbed by the fiber coating, n, can be calculated from the equation35
n)
C0V1V2K KV1 + K2K2 + V2
(1)
with C0 the initial analyte concentration (mol/L) in the aqueous phase; V1, V2, and V3 are the volumes of the coating, the aqueous (39) Michel, P.; Averty, B. Appl. Organomet. Chem. 1991, 5, 393-397.
a
b
Figure 2. (a) Influence of sampling time on extraction efficiency for organotin compounds. Peak areas are corrected for changes in ICPMS sensitivity via the peak area of the 126Xe signal. (b) Influence of sampling time on extraction efficiency for organomercury and -lead compounds. Peak areas are corrected for changes in ICPMS sensitivity via the peak area of the 126Xe signal.
phase, and the headspace, respectively. K1 is the partition coefficient between the fiber coating and the headspace, and K2 is the partition coefficient between the headspace and the aqueous phase. Finally, K ) K1K2 is the global partition coefficient of the analyte between the coating and the aqueous phase. The derivatized organometallics are volatile and apolar and have a greater affinity for the apolar poly(dimethylsiloxane) phase than for the polar sample matrix. Also, since diffusion through the headspace goes fast, it could be expected that equilibrium between fiber, headspace, and sample would be reached quickly. In Figure 2, the effect of the extraction time on the relative extraction yield for a number of alkyllead, -tin, and -mercury compounds is demonstrated. The curves shown in the figure level off at higher extraction times; after 10 min of extraction, 90% of the maximal extracted amount of organometals is collected. In this figure, it
can be seen that equilibrium is reached sooner for the more volatile MM than for TBT. The less volatile the compound is (lower K2 value), the lower is its concentration in the headspace and the slower the diffusion in the headspace since the concentration gradient of less volatile compounds in the headspace will be smaller.35 A sampling time of 10 min was considered to yield sufficient extraction within an acceptable sampling time. Next, it was found that, after 1 min at a temperature of 250 °C, complete desorption was obtained for all organometallic species; a 1 min desorption time was, therefore, used in all further experiments. In Figure 3, a chromatogram of a simultaneous headspace SPME extraction of organotin, -lead, and -mercury is shown. The influence of the sample temperature was investigated by sampling standard mixtures for 10 min at different temperatures. Analytical Chemistry, Vol. 69, No. 8, April 15, 1997
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Figure 3. Chromatogram of an organometal standard, extracted with headspace SPME. 120Sn, 126Xe, 202Hg, and 208Pb were simultaneously measured. 1, MM; 2, TML; 3, diethylmercury; 4, inorganic Sn (tetraethyltin); 5, inorganic Pb (tetraethyllead); 6, MBT; 7, DBT; 8, TBT; X, unknown components.
Figure 4. Influence of sampling temperature on extraction efficiency. Peak areas are corrected for changes in ICPMS sensitivity via the peak area of the 126Xe signal.
The results are plotted in Figure 4 for TML, MBT, DBT, and TBT. For DBT and TBT, heating the sample mixture to 60 °C leads to a higher sorption efficiency. The extracted amounts of the more volatile TML and MBT, however, start to decrease from 20 °C on. As mentioned before, the equilibrium between the analyte concentration sorbed by the SPME fiber coating and the concentration of the analyte in the sample solution depends on both the solubility of an analyte in the aqueous phase (Henry’s law) and its sorption affinity onto the SPME fiber coating. Increasing the temperature will increase the Henry constants of the organometallic compounds, resulting in a higher analyte partial vapor pressure in the headspace. The sorption, on the other hand, will 1608 Analytical Chemistry, Vol. 69, No. 8, April 15, 1997
decrease with increasing temperature and in relation to the analyte’s volatility. For DBT and TBT, the less volatile species, the temperature increase enhances the overall extraction process. For MBT and TML, however, partial desorption of the species from the SPME fiber coating occurs from relatively low temperatures on. Since the derivatization reaction is less efficient for MBT, and because performing extractions at higher temperatures is more tedious and time consuming, further extractions were accomplished at 25 °C. In view of the apolar nature of the derivatized organometallic compounds, increasing the ion strength of the sample solution by addition of an indifferent electrolyte such as NaCl is expected
Figure 5. Relative sensitivity for MM, TML, MBT, DBT, and TBT of SPME (direct and headspace) and liquid/liquid extraction. ICPMS peak areas normalized to the peak area of the 126Xe signal to correct for changes in ICPMS sensitivity.
Figure 6. Linear dynamic range of headspace SPME. Peak areas are normalized to TPT (IS) and corrected for changes in ICPMS sensitivity via the peak area of the 126Xe signal.
to increase the equilibrium concentration of the analytes in the apolar SPME fiber.31-35 Since the derivatization with NaBEt4 is influenced by the sample matrix, the NaCl solution was added with a syringe after 1 min of derivatization, prior to SPME extraction. The addition of 5 mL of a saturated NaCl solution, however, was found to have no effect on the extraction efficiency, probably because the derivatized organometals are highly hydrophobic and because the aqueous solution was already sufficiently polar, by the use of the 0.2 M HOAc/NaOAc buffer and the concentrated NaBEt4 (1% w/v) solution. Similar results were reported by Bayona et al.36,37 Relative Sensitivity of SPME versus Liquid/Liquid Extraction. Figure 5 shows the relative signals for different organotin, -lead, and -mercury species measured with CGCICPMS after SPME (direct and headspace) and liquid/liquid extraction. In direct SPME, the fiber is introduced in the aqueous phase. The observed signal intensities were normalized so as to
Table 2. Reproducibility and Limits of Detection of SPME-CGC-ICPMS component
RSD (n ) 10) (%)
LOD (3 s, n ) 10) (ng/L as metal)
MBT DBT TBT MM TML
5.2 8.9 14 11 8.2
0.34 2.1 1.1 4.3 0.19
correct for the different concentrations in the solutions used for liquid/liquid extraction on one hand, and SPME on the other, and for ICPMS sensitivity via the peak area of the 126Xe signal. The sensitivity of headspace SPME is higher by a factor of up to 10 (MBTCl3) when compared to direct SPME and by a factor of up to 324 (MBTCl3) when compared to liquid/liquid extraction. Headspace SPME is more advantageous for the less volatile Analytical Chemistry, Vol. 69, No. 8, April 15, 1997
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Table 3. Comparison between Extraction Procedures for Organotin Speciation in Water Samples species
sample volume (mL)
butyl-, phenyltin
100-500
butyl-, phenyltin butyltin butyltin methyltin butyltin methyltin methyllead methylmercury
50 250 500 20-25 20-25 10 10 10
extraction tropolone/C18 silica cartridge (SPE) hexane isooctane isooctane headspace SPME headspace SPME purge and trap purge and trap purge and trap
derivatization reagent
injection volume (µL)
EtMgBr
2
NaBEt4 NaBEt4 NaBEt4 NaBEt4 NaBEt4 NaBEt4 NaBEt4 NaBEt4
25 2 1
hyphenated technique
LOD (ng/L as metal)
ref
CGC-FPD
1.0-10
21, 41
CGC-MIP-AES CGC-FPD CGC-ICPMS CGC-FPD CGC-ICPMS CGC-MIP-AES CGC-MIP-AES CGC-MIP-AES
0.1 0.4 0.3-0.8 4.5-27 0.3-2.1 0.15 0.20 0.60
42 39 this work 36 this work 43 43 43
Figure 7. Chromatogram of the PACS-1 reference material. 1, Inorganic Sn (tetraethyltin); 2, MBT; 3, TPT (IS); 4, DBT; 5, TBT.
compounds (e.g., MBTCl3) which show higher concentrations in the solid phase at equilibrium. Linearity. For organolead and organotin compounds the overall analytical procedure shows a linear dynamic range (correlation coefficients between 0.9986 and 0.9994) between 10 and 1000 ng/L (see Figure 6). At low concentrations, the linear dynamic range is limited by high blanks (0.1-1 ng/L), mainly originating from the NaBEt4 used; with purer NaBEt4 the range could be extended. At concentrations >1000 ng/L, the SPME fiber coating tends to be saturated, resulting in a lower absorbed amount of analyte. Reproducibility and Limits of Detection. The reproducibility of 10 subsequent SPME extractions is summarized in Table 2 and varies between 5 and 14%. The limits of detection, LODs, were determined as 3 times the standard deviation of the background measured after 10 succesive SPME extractions of buffer and NaBEt4 and are summarized in Table 2. The LODs for MBT, DBT, and TBT range between 0.34 and 2.1 ng/L and are in the same order of magnitude (0.30-0.82 ng/L as Sn) as those obtained via classical liquid/liquid extraction and NaBEt4 derivatization. The latter method, however, requires at least 500 mL of sample, and thus a higher enrichment, when concentrations near the LOD must be determined, whereas for SPME 25 mL of sample is sufficient. The LOD for TML is somewhat lower than those for organotin species. This is due to the lower background of Pb. To the contrary, the LOD of MM is higher (4.3 ng/L as Hg). This is probably due to 1610
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the lower extraction efficiency of the headspace SPME technique for the relatively volatile MM in comparison with organotin and -lead species. In Table 3, LOD’s are summarized for different, commonly used extraction procedures combined with different hyphenated techniques. It is clear that SPME-CGC-ICPMS is at least a factor or 10 more sensitive than SPME-CGC-FPD and can compete with large-volume injection and purge-and-trap techniques43 in CGC. These techniques, in contrast with headspace SPME, preconcentrate not only the analytes but also the interfering compounds, coextracted from the sample matrix. It must, however, be emphasized that much lower detection limits most probably can be obtained with SPME-CGC-ICPMS when much purer NaBEt4 is available. This is being studied at present. Accuracy. The reliability of the SPME-CGC-ICPMS technique for the determination of MBT, DBT, and TBT was checked by the analysis of the standard reference material PACS-1 marine sediment from the National Research Council Canada (NRCC). The organotin compounds were extracted by both classical liquid/ liquid extraction and headspace SPME extraction. A chromatogram of the PACS-1 reference material is shown in Figure 7. In Table 4 the results are summarized. As can be seen, the (40) Szpunar, J.; Schmitt, V. O.; Donard, O. F. X.; Łobinj ski, R. Trends Anal. Chem. 1996, 15, 181-187. (41) Fent, K.; Mu ¨ ller, M. D. Environ. Sci. Technol. 1991, 25, 489-493. (42) Ceulemans, M.; Łobinj ski, R.; Dirkx, W. M. R.; Adams, F. C. Fresenius’ J. Anal. Chem. 1993, 347, 256-262. (43) Ceulemans M.; Adams F. C. J. Anal. At. Spectrom. 1996, 11, 201-206.
Table 4. Determination of MBT, DBT, and TBT in PACS-1 Reference Material by Liquid/Liquid Extraction- and SPME-CGC-ICPMS component MBT DBT TBT a
classical liquid/liquid headspace SPME certified extraction (ng/g) extraction (ng/g) values (ng/g) 390 ( 110a 1070 ( 140 1280 ( 230
428 ( 76 1045 ( 160 1244 ( 150
280 ( 170 1160 ( 180 1270 ( 220
Limit of 95% confidence (n ) 3).
concentrations obtained with SPME are in good agreement with the certified values as well as with the results of the classical liquid/liquid extraction. Only for MBT, significantly higher values were found when compared to the certified values. Several authors, however, have reported12,40 systematically higher values for MBT in PACS-1 after NaBEt4 derivatization. The reproducibility is of the same order of magnitude as that of classical liquid/ liquid extraction, or better. This indicates that the leaching procedure and derivatization are the mean sources of error during sample preparation, rather than the applied extraction technique. Analysis Time. With accuracy and precision comparable to those of liquid/liquid extraction, headspace SPME is characterized by a substantially shorter analysis time. In the analysis of water samples, the extraction and enrichment via liquid/liquid extraction take approximately 20-30 min or more if further enrichment by evaporation is required. This time is to be compared with 10 min contact time between the SPME fiber and headspace. SPME thus allows the CGC chromatogram to be run in about the same time as needed for the sample preparation and extraction. Sample preparation and analysis thus run “in time”. In the analysis of solid samples (e.g., sludges),
the analysis is dominated by the leaching of the organometallic compounds from the sample matrix. Subsequent sample processing, again, is simpler and faster with headspace SPME since there are no longer difficulties with phase separations and further sample cleanup is redundant. Other advantages of headspace SPME are reduced solvent consumption and waste production and possibilities to automate. CONCLUSIONS Of two methods, headspace SPME and liquid/liquid extraction, for sample preparation in organometal speciation of Sn, Pb, and Hg via CGC, headspace SPME was shown to be superior. The method is as accurate and precise as classical liquid/liquid extraction and offers practical advantages. Sample processing time is substantially shorter, and solvent consumption and waste production are lower. Moreover, SPME is, in contrast with largevolume injection techniques, cheap, and ordinary split/splitless injection systems can be used. With ICPMS as a multielement detector, detection limits can be obtained that are better than those hitherto attainable. For laboratories involved in trace element speciation, the coupling of CGC with ICPMS allows the possibilities of the laboratory to be extended to organometal speciation while maintaining full flexibility to use both the CGC and the ICPMS instrument for other applications. ACKNOWLEDGMENT Thanks are due to the Fund for Scientific Research (Flanders, Belgium) for financial support. Received for review September 10, 1996. Accepted January 20, 1997.X AC960905O X
Abstract published in Advance ACS Abstracts, February 15, 1997.
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