Simultaneous Determination of Mono-, Di-, and Tributyltin in

Anal. Chem. 2001, 73, 3174-3180

Simultaneous Determination of Mono-, Di-, and Tributyltin in Sediments by Isotope Dilution Analysis Using Gas Chromatography-ICPMS Jorge Ruiz Encinar,† Marı´a I. Monterde Villar,‡ Vicente Gotor Santamarı´a,‡ J. Ignacio Garcı´a Alonso,*,† and Alfredo Sanz-Medel†

Department of Physical and Analytical Chemistry and Department of Organic and Inorganic Chemistry, Faculty of Chemistry, Julia´ n Claverı´a 8, 33006, Oviedo, Spain

A mixed spike containing 119Sn-enriched monobutyltin (MBT), dibutyltin (DBT), and tributyltin (TBT) was prepared by direct butylation of 119Sn-enriched tin metal using a 1:3 molar excess of butyl chloride with iodide and triethylamine as catalysts. The isotopic composition of the different tin species in the spike solution was determined by gas chromatography-ICPMS after aqueous ethylation using sodium tetraethylborate. Reverse isotope dilution analysis was used for the characterization of the spike by means of natural MBT, DBT, and TBT standards. No species transformation was evident during derivatization from the reverse isotope dilution experiments based on the measured isotope ratios both before and after spiking. The mixed spike was applied to the simultaneous analysis of MBT, DBT, and TBT in certified reference materials, PACS-2 and CRM 646, with satisfactory results. Organotin compounds in general but mainly tributyltin and triphenyltin have been extensively used as additives in antifouling paints, wood preservatives, fungicides, biocides, and polymer additives.1 Tributyltin (TBT) and its degradation products dibutyltin (DBT) and monobutyltin (MBT) have been detected in different environmental compartments both marine (waters, sediment, biota) and terrestrial (waters, soil). A recent report indicated the presence of butyltin compounds in open ocean seawaters.2 However, the toxicity of such compounds varies not only with the particular species or compound but also in relation to the organism monitored.3,4 All these reasons have led to an increased interest in the development of reliable methods for organotin speciation analysis in different environmental samples. Tributyltin, in particular, is one of the most toxic substances ever deliberately introduced into the marine environment through its use as antifouling paint on boats and ships.6 Although many countries banned and/or restricted its use,5 TBT and its degradation * Corresponding author: (e-mail) [email protected]; (fax) +34985103125. † Department of Physical and Analytical Chemistry. ‡ Department of Organic and Inorganic Chemistry. (1) Craig, P. J. Organometallic compounds in the environment, 1st ed.; Longman: London, 1986. (2) Tao, H.; Rajendran, R. B.; Quetel, C. R.; Nakazato, T.; Tominaga, M.; Miyazaki, A. Anal. Chem. 1999, 71, 4208-4215. (3) Havezov, I. Fresenius J. Anal. Chem. 1996, 355, 452-456. (4) Kot, A.; Namiesnik, J. Trends Anal. Chem. 2000, 19, 69-78.

3174 Analytical Chemistry, Vol. 73, No. 13, July 1, 2001

products, DBT and MBT, are still widespread in the marine environment. Under the influence of light and microorganisms, TBT can be broken down in water into less toxic DBT and MBT species. The problem is that this favorable decomposition takes place far more slowly when TBT has been accumulated in sediment (which acts as the ultimate sink for the organotin compounds), creating an ecotoxicological risk long after its release into a given area.6 The complexity of such matrixes (e.g., sediments) makes it necessary to ensure the quality of the speciation process used.7-9 It is well known that organotin speciation involves a number of discrete analytical steps including extraction, cleanup, preconcentration, derivatization (when a gas chromatograph is operated), separation, and detection. Every step could become a source of error.10,11 All these critical steps make speciation analysis an error-prone and difficult task favoring results of poor accuracy and reproducibility. In the search for validating speciation methodologies and techniques, the use of adequate certified reference materials (CRMs) could play a major role. At this stage of development, RMs with certified values of the desired species are urgently needed. Thus, great efforts should be directed to produce “speciated” CRMs covering a wide range of relevant species of the trace elements in a sufficient range of environmental and biological matrixes.5 Unfortunately, at present such CRMs are scarce. An alternative approach, in the search of validating trace metal speciation results, could be to resort to using highly qualified “referee” or “primary”12 methods such as those provided by isotope dilution (ID)-mass spectrometry (MS). Under adequate operational conditions, ID-ICPMS could provide such “primary” methods of analysis, and so it could play a crucial role for quality assurance in trace element chemical speciation of environmental (5) Quevauviller, P.; Astruc, M.; Morabito, R.; Ariese, F.; Ebdon, L. Trends Anal. Chem. 2000, 19, 180-188. (6) Kortlandt, E.; Stronkhorst, J. TBT in marine antifouling paints. Direct Dutch BV, National Institute for Coastal and Marine Management, Amsterdam, 1998. (7) Graupera, E.; Leal, C.; Granados, M.; Prat, M. D.; Compan ˜o´, R. J. Chromatogr., A 1999, 846, 413-423. (8) Donard, O. F. X.; Lale`re, B.; Martin, F.; Lobinski, R. Anal. Chem. 1995, 67, 4250-4254. (9) Ceulemans, M.; Slaets, S.; Adams, F. Talanta 1998, 46, 395-405. (10) Morabito, R.; Massanisso, P.; Quevauviller, P. Trends Anal. Chem. 2000, 19, 113-118. (11) Adams, F.; Slaets, S. Trends Anal. Chem. 2000, 19, 80-85. (12) De Bievre, P. Anal. Proc. 1993, 30, 328-333. 10.1021/ac010147o CCC: $20.00

© 2001 American Chemical Society Published on Web 06/02/2001

and biological samples.13 All the well-known advantages of ID (e.g., high accuracy and precision) can be implemented to correct for low derivatization yields, incomplete separation, and determination errors, and so it is likely to that a wider use of ID-ICPMS for speciation will take place in the future.13,14 This concept was first presented by Heumann.15,16 Two different approaches were described: (a) the species-unspecific spiking mode17-20 and (b) the species-specific spiking mode. This latter species-specific spiking mode can be applied when the structure of the compound is well known and adequate spikes are available or can be synthesized. In this case, the spike is added at the beginning of the analytical procedure, as in the classical isotope dilution experiment. Examples of this approach have been published for the speciation of iodine,21,22 selenium,23,24 chromium,25,26 lead,27,28 mercury,29 and tin.30 Regarding the techniques for determination of butyltin compounds in environmental samples, the preferred couplings have been GC-AAS, GC-MIP-AES, GC-FPD, GC/MS,31 and, more recently, GC-ICPMS.30,32 The extremely low concentration levels of organotin compounds to be monitored in environmental and biological samples demand highly sensitive and selective detection techniques.33 In this vein, the coupling of gas chromatography to ICPMS appears to be one of the techniques of choice to perform this type of speciation analysis due to its extremely high sensitivity.34,35 In addition, very short analysis times can be obtained using multicapillary columns.36 Moreover, the capability of measuring isotope ratios would pave the way for isotope dilution analysis (13) Sanz-Medel, A. Spectrochim. Acta, B 1998, 53, 197-211. (14) Hill, S. J.; Pitts, L. J.; Fisher, A. S. Trends Anal. Chem. 2000, 19, (2/3), 120-126. (15) Heumann, K. G. In Metal speciation in the environment; Broekaert, J. A. C., Guˆc¸ er, S., Adams, F., Eds.; NATO ASI series, Vol. G 23, Springer-Verlag: Berlin, 1990; pp 153-168. (16) Heumann, K. G. Int. J. Mass Spectrom. Ion Processes 1992, 118/119, 575592. (17) Rottmann, L.; Heumann, K. G. Fresenius. J. Anal. Chem. 1994, 350, 221227. (18) Rottmann, L.; Heumann, K. G. Anal. Chem. 1994, 66, 3709-3715. (19) Beauchemin, D. K.; Siu, K. W. M.; Berman, S. S. Anal. Chem. 1988, 60, 2587-2590. (20) Vogl, J.; Heumann, K. G. Anal. Chem. 1998, 70, 2038-2043. (21) Heumann, K. G.; Rottmann, L.; Vogl, J. J. Anal. At. Spectrom. 1994, 9, 13511355. (22) Heumann, K. G.; Gallus, S. M.; Ra¨dlinger, G.; Vogl, J. Spectrochim. Acta B 1998, 53, 273-287. (23) Gallus, S. M.; Heumann, K. G. J. Anal. At. spectrom. 1996, 11, 887-892. (24) Tanzer, D.; Heumann, K. G. Anal. Chem. 1991, 63, 1984-1989. (25) Kingston, H. M.; Huo, D.; Lu, Y.; Chalk, S. Spectrochim. Acta, B 1998, 53, 299-309. (26) Nusko, R.; Heumann, K. G. Anal. Chim. Acta 1994, 286, 283-290. (27) Brown, A. A.; Ebdon, L.; Hill, S. J. Anal. Chim. Acta 1994, 286, 391-399. (28) Ebdon, L.; Hill, S. J.; Rivas, C. Spectrochim. Acta, B 1998, 53, 289-297. (29) Evans, R. D.; Hintelmann, H. Fresenius J. Anal. Chem. 1997, 358, 378385. (30) Ruiz Encinar, J.; Garcı´a Alonso J. I.; Sanz-Medel, A. J. Anal. At. Spectrom. 2000, 15, 1233-1239. (31) Ariese, F.; Cofino, W.; Go´mez-Ariza, J. L.; Kramer, G.; Quevauviller, Ph. J. Environ. Monit. 1999, 1, 191-196. (32) Montes Bayo´n, M.; Gutierrez Camblor, M.; Garcı´a Alonso, J. I.; Sanz-Medel, A. J. Anal. At. Spectrom. 1999, 14, 1317-1322. (33) Abalos, M.; Bayona, J. M.; Compan ˜o´, R.; Granados, M.; Leal, C.; Prat, M. D. J. Chromatogr., A 1997, 788, 1-49. (34) Ritsema, R.; de Smaele, T.; Moens, L.; de Jong, A. S.; Donard, O. F. X. Environ. Pollut. 1998, 99, 271-277. (35) Rajendran, R. B.; Tao, H.; Nakazato, T.; Miyazaki, A. Analyst 2000, 125, 1757-1763. (36) Rodriguez, I.; Monicou, S.; Lobinski, R.; Sidelnikov, V.; Patrushev, Y.; Yamanaka, M. Anal. Chem. 1999, 71, 4534-4543.

procedures that could be applied to validate speciation procedures.13 Reliable single speciation of DBT, using a 118Sn-enriched spike, has been shown previously.30 A more general approach using “multispeciated” ID is described here for the accurate and simultaneous determination of MBT, DBT, and TBT in sediments, just by using a mixed 119Snenriched spike. EXPERIMENTAL SECTION Instrumentation. A Hewlett-Packard (Palo Alto, CA) gas chromatograph model 6890, fitted with a split/splitless injector and a HP-5 capillary column (cross-linked 5% phenyl methyl siloxane, 30 m × 0.32 mm i.d. × 0.25 µm coating), was used for the separation of the organotin compounds. The gas chromatograph was coupled to a Hewlett-Packard model HP-4500 inductively coupled plasma mass spectrometer (Yokogawa Analytical Systems, Tokyo, Japan) via the transfer line described by Montes Bayo´n et al.32 Analytical performance characteristics of this interface for the speciation of Hg,37 Pb,38 and Sn30,32 have been described elsewhere. Integration of the chromatographic peaks was performed using the software supplied with the ICPMS. Reagents and Materials. Stock solutions of tin (1000 µg mL-1) were obtained from Merck (Darmstadt, Germany) while tin metal was from Panreac (Barcelona, Spain). Dilutions of the stock solutions were performed with 1% v/v subboiled nitric acid. Tributyltin chloride (96%), dibutyltin dichloride (97%), and monobutyltin trichloride (95%) were obtained from Aldrich (Steinheim, Germany). Stock solutions were prepared by dissolving the corresponding salt in methanol (Merck). All organometallic standard solutions were kept in the dark at 4 °C, and diluted working solutions were prepared daily before the analysis in methanol. Ethylation of these compounds was carried out with sodium tetraethylborate (Strem Chemicals, Bisheheim, France). Beware: Butyltin compounds are toxic substances and sodium tetraethylborate is highly flammable. Also it decomposes extremely rapid in the presence of light and air. Sediment reference materials tested were PACS-2 and CRM 646 purchased from NRCC (Ottawa, ON, Canada) and BCR (Retieseweg, Geel, Belgium), respectively. Tin metal, enriched in 119Sn, was obtained from Cambridge Isotope Laboratories (Andover, MA) and different butyl halides (BuI, BuBr, BuCl) were purchased from Aldrich (Steinheim, Germany). All other reagents were of analytical reagent grade. Ultrapure water was obtained from a Milli-Q 185 system (Millipore, Molsheim, France). Procedures. Ethylation of Sn Compounds. In PFA-stoppered glass test tubes, mixed standard solutions of different organotin compounds were adjusted to pH 5.4 with 3 mL of a 1 M acetic acid/sodium acetate buffer and then 1 mL of 2% w/v sodium tetraethylborate in 0.1 M NaOH and 1 mL of hexane were added for extraction. After 5 min of manual shaking, the organic layer was transferred to a glass vial and stored at -18 °C until GCICPMS measurement. Synthesis and Analysis of 119Sn-Enriched MBT, DBT, and TBT. The procedure was as follows: a mixture of 90 mg of 119Sn tin (37) Garcı´a Ferna´ndez, R.; Montes Bayo´n, M.; Garcı´a Alonso, J. I.; Sanz-Medel, A. J. Mass Spectrom. 2000, 35, 639-646. (38) Leal Granadillo, I. A.; Garcı´a Alonso, J. I.; Sanz-Medel, A. Anal. Chim. Acta, 2000, 423, 21-29.

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Table 1. Operating Conditions of the GC-ICPMS

injection mode split time injection volume split ratio injection temperature

Injector Parameters split/splitless 0.5 min 1 µL 1:20 250 °C

GC Parameters column HP-5 (30 m × 0.32 mm × 0.25 µm) carrier gas/inlet pressure He/100 kPa GC program 50 (0.5 min) to 250 °C (2 min) at 30 °C min-1 transfer line PFA tube 80-cm length, 1.5-mm i.d. heating block temperature 250 °C ICPMS Parameters rf power 1300 W carrier gas flow rate 1 L min-1 intermediate gas flow rate 1 L min-1 outer gas flow rate 15 L min-1 Data Acquisition Parameters points per peak 1 integration time per point 66 ms isotopes selected 3 (118, 119, 120)

metal (0.768 mmol of Sn, 1 equiv), 241 µL of n-butyl chloride (2.304 mmol, 3 equiv), 32.4 µL of triethylamine (0.227 mmol, 0.06 equiv) used as transfer-phase catalyst, and 11.69 mg of iodine (0.046 mmol, 0.06 equiv) used as reaction catalyst, were heated in a sealed tube at 160° C for 14 h with magnetic stirring in a sand bath. After cooling, the brown suspension was washed with 2 mL of MeOH and 1 mL of CH2Cl2, rotavaporated, and redissolved in methanol, and the final reaction mixture was stored at -18 °C in the dark. The purity and isotopic composition of the final products was checked by adequate dilution of the stock solution with a 1 + 3 methanol/acetic acid mixture followed by ethylation, as described above. The yield and the concentration of the different tin species in the final mixture was determined by reverse isotope dilution analysis30 using natural MBT, DBT, and TBT chloride standards, after corresponding ethylation. All measurements were performed by GC-ICPMS. Extraction and Derivatization of Organotin Compounds from Sediments. Approximately 0.25 g of sediment was spiked with a diluted solution of the 119Sn-enriched mixture of MBT, DBT, and TBT and mixed with 1 mL of methanol and 3 mL of acetic acid. The resultant slurry was shaken mechanically for 12 h in stoppered glass test tubes. After centrifugation, 200 µL of the extract was ethylated as described above. Water Content Calculation for the Certified Samples. One gram of the sediment was exactly weighed in a separate experiment and heated at 105-110 °C for 24 h. It was allowed to cool to room temperature in a desiccator and weighed again. The water contents obtained were 1.40% for the PACS-2 and 1.05% the BCR 646. Thus, all final species concentration results related to dry sediment were corrected for such values. Separation and Measurement of Isotope Ratios Using GCICPMS. Typical operating conditions used for the gas chromatographic separation and the ICPMS detection are illustrated in Table 1. Daily optimization of the ICPMS conditions was performed after connection of the GC to the ICPMS, by using m/z ) 80 (40Ar2+). Integration of the chromatographic peaks was performed using the software of the ICPMS instrument. Isotope 3176 Analytical Chemistry, Vol. 73, No. 13, July 1, 2001

ratios were measured always as peak area ratios. Mass bias was also corrected using ethylated natural organotin standards. No dead time correction was necessary on the HP-450039 as previous studies indicated that dead time was lower than 10 ns. RESULTS AND DISCUSSION Optimization of the GC and ICPMS Conditions. Separation conditions described previously on a HP-5 capillary column were used.30,32 This temperature program, indicated in Table 1, provided adequate resolution of the different organotin compounds. The temperature of the heating block of the interface only influenced the peak width of the last eluting organotin peak (tributylethyltin) but did not show any noticeable effect on the other more volatile tin compounds. In fact, peak widths observed were comparable to those found using a standard GC-FID instrument where no interface is used.30 Under optimum conditions, very small peak broadening was observed for the less volatile species. This broadening increased significantly after many repeated injections of real samples (after ∼75 injections), due to some deposition of matrix components on the PFA transfer tube. However, no broadening was observed by the repeated injection of standards. This problem was easily overcome by changing the used PFA tube by a new one. No cleaning of the PFA tube was attempted. Plasma and ion lens conditions were optimized daily using the following protocol: first, the system was started with the standard Meinhard nebulizer and Scott double-pass spray chamber and a solution containing 10 ng/g Li, Y, Ce, and Tl was nebulized for system optimization and long-term stability check (sensitivity, double-charged ions, oxides, and mass calibration). Then, the plasma was switched off and the nebulizer-spray chamber assembly was substituted with the PFA tube from the GC. Finally, the plasma was ignited again, and a final optimization of the ion lenses (extraction lens, einzel lens, omega lens) was performed using m/z ) 80 (40Ar2+). No optimization of the plasma conditions were required here. Previous work in our laboratory30 demonstrated that the optimum ion lens conditions for a dry plasma differed greatly from those obtained during nebulization. Other ions tested for optimization included 13C+, 40Ar12C+, 126Xe+, and 202Hg+ (impurities in the Ar) with similar results. Compromise conditions for the data adquisition parameters had to be selected, due to the opposite influence of integration time on sensitivity and peak profiles. In general terms, the fast transient gas chromatographic peak is perfectly defined even at very short integration times. However, then an increase in the instrumental noise is detected (due to poorer counting statistics). On the other hand, using very long integration times, peak definition worsened dramatically, spectral skew was observed, and peak areas exhibited poor reproducibility.40 Eventually, isotope dilution analysis was performed using the tin isotopes 118, 119, and 120 and 66-ms integration time per isotope. Precision of Tin Isotope Ratios Using GC-ICPMS. As indicated above, peak area ratios between the tin isotopes measured (118/119 and 120/119) were computed. Isotope ratio precision was obtained for every sample and standard injected in the chromatograph throughout all measurements performed for (39) Valles Mota, J. P.; Ruiz Encinar, J.; De la Campa, M. R. F.; Garcı´a Alonso, J. I.; Sanz-Medel, A. J. Anal. At. Spectrom. 1999, 14, 1467-1473. (40) Ruiz Encinar, J.; Granadillo, I. L.; Garcı´a Alonso J. I.; Sanz-Medel, A. J. Anal. At. Spectrom. 2001, 16, 475-480.

Table 2. Tin Isotope Ratios and Relative Error for Those Ratios Using GC-ICPMSa

a

ratio

natural ratios

MBT (st dev)

measured tin isotope ratio DBT (st dev)

TBT (st dev)

∆M

average relative error

116/120 117/120 118/120 119/120 122/120 124/120

0.4463 0.2357 0.7434 0.2637 0.1421 0.1777

0.4350 (0.0034) 0.2296 (0.0020) 0.7333 (0.0054) 0.2599 (0.0001) 0.1424 (0.0008) 0.1824 (0.0027)

0.4357 (0.0025) 0.2287 (0.0027) 0.7348 (0.0022) 0.2610 (0.0030) 0.1434 (0.0006) 0.1809 (0.0032)

0.4334 (0.0065) 0.2313 (0.0026) 0.7373 (0.0067) 0.2586 (0.0040) 0.1431 (0.0017) 0.1806 (0.0039)

4 3 2 1 -2 -4

-0.025 96 -0.024 89 -0.011 10 -0.014 58 0.006 24 0.020 04

Data correspond to three independent injections of a mixture of 0.1 ng (as Sn) of each tin species (natural abundances).

this work (over 50 isotope ratio measurements). Every sample and standard was injected three times and typical relative standard deviation was ∼0.8% ranging from 0.1 to 2% depending on the peak size. It is worth stressing that this isotope ratio precision was much better than that obtained directly for peak areas after manual injection (∼8%, n ) 5 injections, 25 ng/g as tin). Simultaneous detectors such as TOF-MS and MC-MS are expected to be ideal for measuring isotope ratios in transient signals. However, only the TOF-MS has been operated for isotope ratio measurements in narrow chromatographic peaks such as those obtained by using gas chromatography and/or capillary electrophoresis. Precision reported by different authors41,42 ranged from 1.2 to 2.9%. This unexpected poor precision is related to the way it performs the measurement. Operating a TOF-MS, we have to measure the whole mass spectrum during the transient peak so the integration time per isotope has to be divided by the total number of isotopes measured. On the other hand, with a Q-MS, the integration time is only divided by the number of selected isotopes and counting statistics are better. With regard to MCMS, no data are available so far about isotope ratio precision in very fast transient signals but it could offer a much better picture as the selected masses are monitored continuously and simultaneously so counting statistics would improve in comparison with Q-MS. Accuracy of Tin Isotope Ratios by GC-ICPMS. The only factor influencing the accuracy of tin isotope ratio measurements was found to be mass bias. This effect can be ascribed to the preferential transmission of heavier ions over lighter ones through the ICPMS instrument and must be corrected when isotope ratio measurements are carried out by ICPMS.43,44 A linear mass bias was initially assumed here, as shown previously for the quadrupole ICPMS used.39 To verify this assumption for the GC-ICPMS coupling, tin isotope ratios were measured for a natural standard containing a mixture of MBT, DBT, and TBT, ethylated as described previously. For this experiment, seven tin isotopes (116, 117, 118, 119, 120, 122, 124) using 30-ms integration time per isotope were measured. Three injections of 0.1 ng of each tin species were carried out to check for isotope ratio precision. The obtained results, compared with the theoretical natural ratios, are (41) Costa-Ferna´ndez, J. M.; Bings, N. H.; Leach, A. M.; Hieftje, G. M. J. Anal. At. Spectrom. 2000, 15, 1063-1067. (42) Bings, N. H.; Costa-Ferna´ndez, J. M.; Guzowski, J. P., Jr.; Leach, A. M.; Hieftje, G. M. Spectrochim. Acta B 2000, 55, 767-778. (43) Longerich, H. P.; Fryer, B. J.; Strong, D. F. Spectrochim. Acta, B 1987, 42, 39-48. (44) Ruiz Encinar, J.; Garcı´a Alonso J. I.; Sanz-Medel, A.; Main, S.; Turner, P. J. J. Anal. At. Spectrom. 2001, 16, 315-321.

given in Table 2. As can be observed, in all cases, for the different tin species the isotope ratios were within the standard deviation of the measurements. Thus, no isotopic fractionation seems to occur during derivatization and extraction. However, those measured isotope ratios differ from the natural ratios and the relative error in the ratio is a function of the mass difference between the measured isotopes, ∆M (last two columns in Table 2). This can be expressed as

Rexp - Rtheo ) K∆M Rtheo where K is the mass discrimination factor and Rexp and Rtheo are the measured and the theoretical ratios, respectively. By adjusting a linear function through the 18 measured ratios versus the mass difference, a slope of -0.0059, the mass bias factor, was obtained and the linear regression had a correlation coefficient of 0.98. This demonstrates the validity here of the assumption of a linear model of mass bias for GC-ICPMS in a way similar to previous data obtained by nebulization.43 Synthesis of 119Sn-Enriched MBT, DBT, and TBT Spike. Previous work in this laboratory was focused on the synthesis of 118Sn-enriched butyltin species.30 Using tin metal and butyl iodide, this synthesis produced almost exclusively DBT for a wide range of experimental conditions and this material was applied for the determination of DBT in sediment reference materials by isotope dilution analysis.30 The actual synthesis of MBT and TBT proved more difficult than expected. Thus, additional experiments, under different experimental conditions, and 119Sn-enriched tin metal (for its future combined use with the previously prepared material) were used. Sisido et al.45 reported that alkyltin chlorides can be obtained in good yields by the direct reaction of alkyl chlorides with tin metal at relatively low temperatures (130-180° C) when both an organic base and a iodine compound were used as catalysts. Without either of these two substances, alkyltin compounds were scarcely obtained, recovering almost all the tin metal used (it has been proposed that iodine could work as a reaction activator leading to activated tin more susceptible to nucleophilic attack by the alkyl halides).46 Natural tin metal was used for the study of synthesis conditions. Preliminary experiments were performed using either butyl chloride, butyl bromide, and butyl iodide in the presence of small (45) Sisido, K.; Kozima, S.; Tuzi, T. J. Organomet. Chem. 1967, 9, 109-115. (46) Holland, F. S. Appl. Organomet. Chem. 1987, 1, 449-458.

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Table 3. Isotopic Composition of the Enriched Mixed Spike Solution Computed from Measured GC-ICPMS Isotope Ratios for Five Independent Injectionsa

a

Sn isotopes

inorganic tin (n ) 5)

MBT (n ) 5)

DBT (n ) 5)

TBT (n ) 5)

mean (n ) 20)

116 117 118 119 120

0.043 (0.008) 0.120 (0.008) 14.38 (0.29) 82.30 (0.34) 3.157 (0.070)

0.045 (0.001) 0.125 (0.007) 14.41 (0.34) 82.23 (0.37) 3.189 (0.052)

0.014 (0.004) 0.104 (0.002) 14.22 (0.26) 82.60 (0.31) 3.065 (0.048)

0.013 (0.009) 0.105 (0.004) 14.32 (0.32) 82.47 (0.38) 3.095 (0.065)

0.029 (0.008) 0.114 (0.005) 14.33 (0.12) 82.40 (0.15) 3.127 (0.032)

Uncertainty corresponds to 95% confidence interval.

Figure 1. Influence of the excess of BuCl over the tin metal on the distribution of the 119Sn-enriched butyltin species mixture.

amounts of triethylamine and iodine at 160 °C for 14 h in a sealed tube. It was observed that, only in the reaction using butyl chloride, measurable amounts of TBT could be obtained (a mixture of mono-, di-, and tributyltin was obtained). In the other two cases, the reaction formed preferentially DBT with small amounts of MBT. Once butyl chloride (BuCl) was selected, the molar excess of reagent used in the reaction tube was studied. For 100 mg of natural tin, metal molar ratios from 1:3 to 1:100 (Sn:BuCl) were studied. The obtained results, expressed as peak area ratios for the different synthesis assays performed, are illustrated in Figure 1. It was observed that when a large excess of BuCl was used, the synthesis proceeded almost exclusively to DBT. As can be observed in the figure, the largest relative yield of MBT and TBT was obtained for a molar ratio of 1:3. Under those conditions, the formation of TBT is favored, although DBT and MBT were also obtained. The final reaction conditions used for the species synthesis, using the 119Sn-enriched material, are summarized under Procedures. It was observed that, from the 91.4 mg of metallic tin added to the reaction tube, 20.3 mg of residual unreacted tin was recovered after 14 h. The final solution was rotavaporated to dryness and the residue redissolved in 10 mL of methanol/acetic acid (1 + 3). This stock solution was kept refrigerated at -18 °C and was characterized by GC-ICPMS. Characterization and Analysis by Reverse Isotope Dilution of the Synthesized Mixture. The isotopic analysis of the spike mixture was performed by GC-ICPMS after dilution with methanol/acetic acid (1 + 3) and ethylation, as described in the procedures. Figure 2 shows the chromatogram obtained at masses 117, 118, 119, and 120 (shifted for clarity). As can be observed, isotope 119 is the more abundant tin isotope, followed by 118 and 120 for all detected species (including tetraethylated inorganic 3178 Analytical Chemistry, Vol. 73, No. 13, July 1, 2001

Figure 2. GC-ICPMS chromatogram obtained for the enriched butyltin species mixture.

119Sn-

tin). If we assume that the derivatization yield is the same for all organotin species and that their response in ICPMS is similar, the organotin mixture will contain similar amounts of TBT and DBT and lower amounts of MBT. However, Sn(IV) should be present at much higher levels, since its ethylation yield is much lower than that for organotin species. The small peaks at 225, 280, and 325 s in Figure 2 can be identified as BuMeEt2Sn, Bu2MeEtSn, and Bu3MeSn, which are due to methyl impurities in the NaBEt4 used. The isotopic compositions of all 119Sn-enriched tin species are presented in Table 3 for those isotopes that could be detected by GC-ICPMS. Mass bias was corrected using a natural mixed standard injected before and after the injection of the enriched species, as described previously. As can be observed, all three organotin species plus inorganic tin provide isotope abundances that are within the standard deviation of the measurements for all major isotopes. This means that no isotopic fractionation effects occurred during the synthesis. For the isotope dilution calculations, the average isotopic composition indicated in Table 3 will be used. The average 120/ 119 tin ratio obtained for the species, 0.03795, differs greatly from the natural 120/119 tin ratio, 3.7928. This would allow the direct

Table 4. Characterization of the Spike by Reverse Isotope Dilution Analysisa MBT (n ) 3)

DBT (n ) 3)

TBT (n ) 3)

1 2 3

205 (1.1) 212 (1.6) 215 (0.54)

1180 (0.52) 1165 (0.48) 1138 (0.62)

1566 (0.79) 1587 (0.60) 1591 (0.69)

average recoveryb

211 (2.4) 3.6

1161 (1.9) 19.9

1581 (0.84) 27.1

replicate

a Results for MBT, DBT, and TBT in micrograms per gram as Sn (% RSD). b Recovery expressed as percentage of the reacted metallic tin.

Figure 3. Chromatogram obtained for the determination of the 119Sn-enriched TBT by reverse isotope dilution analysis.

use of the synthesized mixture as a mixed spike solution for the simultaneous isotope dilution analysis of MBT, DBT, and TBT in sediments. To determine the concentration of every individual organotin species in the spike (and so the reaction yield for the individual species), reverse isotope dilution analyses using natural organotin standards were carried out. This experiment was performed sequentially (one species at a time) to study possible rearrangement reactions during derivatization and analysis. In this sense, the synthesized mixture was first spiked with a natural TBT standard, derivatized, and analyzed by GC-ICPMS. Figure 3 shows one of the chromatograms obtained, at masses 119 and 120. The isotopic composition for TBT is clearly altered while, as can be observed, the MBT and DBT peaks exhibit relative tin abundances close to those obtained in the synthesized 119Snenriched product. This fact indicated that no rearrangement/ decomposition reactions from TBT to DBT and/or MBT occurred during derivatization and measurement by GC-ICPMS. Similar experiments were performed for the DBT and MBT species, showing again that no species transformation seemed to be taking place. Three independent reverse isotope dilution experiments were carried out for each species and each solution was injected three times to evaluate both the precision on the isotope ratios and the overall concentration uncertainty. Between each triplicate injection, a natural standard was injected to compute mass bias and evaluate possible mass bias drift. The final concentration results are summarized in Table 4. As can be observed, precisions ranged from 0.48 to 1.6% for each triplicate injection and from 0.84 to 2.4% for the triplicate isotope dilution experiments. The average concentrations in the spike solution resulted 211 µg/g for MBT, 1161 µg/g for DBT, and 1581 µg/g for TBT providing reaction yields from 3.6 to 27.1%. This mixed spike solution so characterized was applied to the simultaneous determination of all three tin species in certified reference materials by “speciated” isotope dilution analysis. Isotope Dilution Analysis of MBT, DBT, and TBT in Certified Sediments. Speciated isotope dilution analysis of the three organotin species was carried out in two recently certified

sediments: PACS-2 and BCR-646. Sample pretreatment consisted simply of the spiking of the dry sediment with the spike solution (conveniently diluted) and extraction of the organotin species with methanol/acetic acid as described in the procedures. No equilibration time of the spike with the sample was allowed before the extraction. It was assumed that quantitative extraction from the solid was required and that complete equilibration in the solid never take place under laboratory conditions. After centrifugation, 200 µL of the extract was derivatized and injected in the GC-ICPMS system. Three independent spiking experiments and a blank were performed for each certified sediment, and each spiked sediment was injected three times using a natural mixed standard for mass bias correction both before and after each sample triplicate injection. As an example of the type of the results obtained, the measured 120/119 peak area ratios for MBT, DBT, and TBT in the 14 consecutive GC-ICPMS injections required to analyze PACS-2 are illustrated in Figure 4. In this figure, the horizontal line corresponds to the theoretical 120/119 natural tin ratio.47 Injections 1, 5, 9, and 13 correspond to the MBT, DBT, and TBT mixed natural standard used for mass bias correction. A slight mass bias drift was detected during the whole analysis (the mass bias factor varied from 0.9 to 1.5%/u), but it did not affect the results as mass bias was evaluated every three sample injections. Tin isotope ratios for the blank (injection 14) turned out to be very close to the tin ratio in the spike mixture indicating, as expected, no organotin presence in the reagents used. As could also be clearly observed in Figure 4, an alteration of the natural tin abundances was detected for all the tin species and the degree of departure would depend on the concentration of each tin species present in the sample and on the mixed spike added itself. Panels a and b of Figure 5 show the type of chromatograms obtained for the PACS-2- and BCR-646-spiked sediments, respectively, at masses 119 and 120 (119 shifted for clarity). As can be observed, the sensitivity of the GC-ICPMS system is more than enough to determine those low levels of butyltin compounds in the samples and to provide precise isotope ratios. It is worth stressing here that, if the concentration of the butyltin compounds had been much lower, the required preconcentration of the organic extract by evaporation of the solvent would not have affected the results as isotope equilibration had already taken place. Final results obtained for PACS-2 and BCR-646 are shown in Table 5. An excellent agreement between the certified and found (47) Rosman, K. J. R.; Taylor, P. D. P. J. Anal. At. Spectrom. 1999, 14, 5N24N.

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3179

Table 5. Determination of MBT, DBT, and TBT in PACS-2 and BCR-646 by Isotope Dilution Analysis Using a Mixed 119Sn-Enriched Butyltin Spikea MBT (n ) 3)

Figure 4. The 120/119 peak area ratios obtained for the different organotin species in the speciated isotope dilution analysis of the PACS-2 certified material.

Figure 5. GC-ICPMS chromatograms obtained for the PACS-2 (a) and the BCR-646 (b) spiked sediments.

values was achieved for each individual organotin species and in both sediments analyzed showing the high accuracy of the proposed method. Individual precisions were always lower than 2%, being close to 1% on average. The uncertainty of the final concentrations is expressed as the 95% confidence interval of the mean to compare it with the uncertainty of the certified values. As can be observed in Table 5, the experimental uncertainties were between 1.4 and 6.4% for a 95% confidence interval which, for a speciation procedure, can be considered satisfactory. So, the speciated isotope dilution method proposed can provide results 3180 Analytical Chemistry, Vol. 73, No. 13, July 1, 2001

DBT (n ) 3)

TBT (n ) 3)

1 2 3

0.52 (1.3) 0.50 (1.7) 0.51 (0.15)

PACS-2 1.03 (1.4) 0.99 (0.72) 1.01 (0.45)

0.88 (0.36) 0.86 (0.37) 0.86 (1.5)

average certified

0.51 ( 0.02 0.45 ( 0.05

1.01 ( 0.03 1.09 ( 0.15

0.86 ( 0.03 0.98 ( 0.13

1 2 3

373 (1.2) 398 (0.8) 393 (2.0)

BCR-646 414 (1.0) 421 (0.2) 416 (0.5)

195 (0.8) 198 (1.1) 196 (1.0)

average certified

388 ( 25 410 ( 69

417 ( 6 394 ( 36

197 ( 3 195 ( 18

a Concentration expressed in micrograms and nanograms per gram as Sn, respectively (uncertainty corresponds to 95% confidence interval of the mean).

of high accuracy and precision for the simultaneous analysis of MBT, DBT, and TBT in sediments. CONCLUSIONS The experimental results obtained in this paper demonstrate the possibility of a fast and simultaneous isotope dilution analysis of MBT, DBT, and TBT in sediments. GC-ICPMS in combination with isotope dilution analysis using isotopically labeled spikes proved to be a simple, fast, and reliable method for organotin speciation in sediments. These optimum results are based on the ability of GC-ICPMS to provide isotope ratios of adequate accuracy and precision. Mass bias can be corrected by measuring a natural organotin standard between samples. Isotope ratio precision obtained were, on average, 0.8%, a value similar to the overall uncertainty of the whole isotope dilution procedure. A second conclusion to be extracted from the data presented is that no isotopic effects during synthesis, derivatization using NaBEt4, and measurement by GC-ICPMS were observed. However, the fact that accurate results were eventually obtained for both certified materials does not mean that complete extraction of the organotin compounds present in other real-life sediments will be accomplished with the methanol/acetic acid extraction mixture and mechanical shaking used here. It is clear that further studies on extraction methods for different sediment types have to be performed. In this vein, isotope dilution analysis can play a crucial role as the errors in all subsequent analytical steps are minimized. Future work in our laboratory will involve the use of a mixed enriched spike with different tin isotopes (118Sn, 119Sn) for the comparison of extraction methods and to elucidate species transformations. ACKNOWLEDGMENT The provision of a research grant for Jorge Ruiz Encinar from II Plan Regional de Investigacio´n and FICYT is gratefully acknowledged. Funding from the European Union through project SMT4CT98-2220 is also acknowledged. Received for review February 5, 2001. Accepted April 4, 2001. AC010147O