Anal. Chem. 1998, 70, 3703-3707
Complete Elimination of Interferences in the Organotin Determination by Oxidation with Dimethyldioxirane Combined with Alumina Cleanup Inma Ferna´ndez-Escobar,† Mariona Gibert,‡ A Å ngel Messeguer,‡ and Josep M. Bayona*,†
Environmental Chemistry Department and Biological Organic Chemistry Department, C.I.D.-C.S.I.C., Jordi Girona Salgado, 18-26, E-08034 Barcelona, Spain
Most of the analytical procedures used in organotin (OT) speciation from sediment involves the Grignard derivatization reaction followed by a cleanup step and a desulfuration reaction since sulfur and/or sulfur species interfere with OT determination by GC/MS or GC-FPD. However, alkyl sulfides are generated from the coextracted elemental sulfur, and they are not removed by conventional desulfurization procedures. We propose here a method based on the oxidation of all the sulfur species with dimethyldioxirane (DMD) to sulfones or sulfur oxides. While sulfones are easily eliminated by alumina adsorption chromatography because they have higher polarity than OTs, the sulfur oxides are spontaneously evaporated. The DMD chemoselectivity favors the oxidation of sulfur compounds to sulfones in a few minutes, whereas OTs remain unreacted. In addition, the excess DMD is easily removed by evaporation under a nitrogen stream before the Al2O3 cleanup step. The effectiveness of the desulfurization reaction combined with the cleanup step is demonstrated for a variety of sediment samples containing up to 3.1% of elemental sulfur, which is completely removed by adding 0.6 molar equiv of DMD. No statistical differences in the OT distribution pattern throughout the DMD intermediate oxidation steps were observed. Organotin compounds (OTs) constitute a broad class of compounds used in many technical applications due to their versatility. Although they are introduced into the marine environment by different pathways, antifouling paints are the major source, important input and they are responsible for toxic effects at different trophic levels.1 However, the toxicity of OTs in the aquatic ecosystems depends on their substitution pattern2 (e.g., number and type of substituents), which makes their speciation necessary in risk assessment studies. In particular, di- and tributyl- and triphenyltin compounds are included in the European Union (EU) list of priority pollutants. * Corresponding author. Tel.: (34-93) 400 61 00. Fax: (34-93) 204 59 04. E-mail:
[email protected]. † Environmental Chemistry Department. ‡ Biological Organic Chemistry Department. (1) Francois, R.; Short, F. T.; Weber, J. H. Environ. Sci. Technol. 1989, 23, 191-196. (2) Fent, K. CRC Crit. Rev. Toxicol. 1996, 26, 1-117. S0003-2700(98)00259-5 CCC: $15.00 Published on Web 07/22/1998
© 1998 American Chemical Society
Elemental sulfur occurring in sediments is coextrated with OT, and it is alkylated in the Grignard derivatization prior to GC determination, leading to the formation of dialkyl mono-, di-, and trisulfides.3 Whether using a band-pass filter at 610 nm fitted to GC-FPD or the SIM mode in GC/MS, high concentrations of sulfur-containing compounds may affect the quantification of propylated, pentylated,4 or hexylated3 OT derivatives due to the concurrent GC retention time with OTs. Atom-selective detection techniques (e.g., AAS, ICP, AED) coupled to GC do not suffer such sulfur interferences due to their increased selectivity, but since the OT identification relies on their retention time, the coelution of high amounts of organosulfur components leads to stationary-phase overloading, which is known to shift retention time,5 leading to missidentification. Therefore, an effective method for the quantitative elimination of sulfur and alkylsulfur compounds is necessary for accurate OT determination. Until now, desulfuration methods for the OT determination, i.e., reduction of sulfur to sulfide with activated copper6 or tetrabutylammoniun sulfite7,8 and sorption of sulfur by mercury,9 have not been efficient in removing alkyl sulfides. Only argentation chromatography on a AgNO3-coated silica column is able to retain alkyl sulfides quantitatively from organic extracts.10 However, this method is not completely useful for OT speciation since phenyltin compounds are irreversibly adsorbed on the AgNO3-silica.11 Sulfur elimination prior to the Grignard reaction gives poor OT recoveries, since desulfurization agents are reactive with the underivatized OTs.12 One approach that is frequently used to isolate organosulfur compounds from oil or other fossil fuels is the oxidationreduction procedure originally reported by Drushell and Som(3) Cai, Y.; Alzaga R.; Bayona J. M. Anal. Chem. 1994, 66, 1161-1167. (4) Marr, L.; White, C.; Ristau, D.; Wardell, J. L.; Lomax J. Appl. Organomet. Chem. 1997, 11, 11-19. (5) Poole, C. F.; Poole, S. K. Chromatography Today; Elsevier: New York, 1991; Chapter 2. (6) Smith, L. M.; Stalling, D. L.; Johnson, J. L. Anal. Chem. 1984, 56, 18301842. (7) Jensen, S.; Renberg, L.; Reutergårdh, L. Anal. Chem. 1977, 49, 316-318. (8) Donard, O. F. X.; Lare`re, B.; Martin, F.; Lobinski, R. Anal. Chem. 1995, 67, 4250-4254. (9) Goerlitz, D. F.; Law, L. M. Bull. Environ. Contam. Toxicol. 1971, 6, 9-10. (10) Joyce, W. F.; Uden, P. C. Anal. Chem. 1983, 55, 540-543. (11) Schubert, P.; Ferna´ndez-Escobar, I.; Rosenberg, E.; Bayona, J. M. J. Chromatogr. A, in press. (12) AÄ balos, M.; Bayona, J. M.; Compan ˜o´, R.; Granados, M.; Leal, C.; Prat, M. D. J. Chromatogr. A 1997, 788, 1-49.
Analytical Chemistry, Vol. 70, No. 17, September 1, 1998 3703
mers.13 This method involves a chemical oxidation of sulfides to sulfones in the aqueous phase with H2O2 followed by LLE. In this work, we have adopted an elegant approach to eliminate sulfur compounds by complete oxidation to the corresponding sulfones with dimethyldioxirane (DMD). Indeed, its application to routine analysis is advantageous, i.e., easy preparation in acetone solution,14 high reactivity in the organic phase (no LLE needed), high selectivity vs that for the sulfur compounds, and simple reagent excess elimination by evaporation. Accordingly, numerous applications involving the oxidation reaction with this reagent have been recently developed in organic synthesis,15,16 but it has not yet been used for analytical purposes. Thus, the objectives of this work are (1) to assess OT stability in the presence of DMD at the reaction conditions (time and temperature), (2) to evaluate the DMD efficiency as oxidant reagent to remove the interfering organosulfur compounds, and (3) to develop efficient cleanup procedures to remove the sulfur oxidation products. EXPERIMENTAL SECTION Reagents and Materials. Marine sediments were collected at western Mediterranean commercial harbors (i.e., BC, SCR-3) and marinas (i.e., SCR-4 and MS). A freshwater sediment from the Amsterdam channel (i.e., AC-1) was also included since it contains larger amounts of elemental sulfur (Table 1). The highest commercially available grade of monobutyl(MBT), dibutyl- (DBT), tributyl- (TBT), tripropyl- (TPrT), tetrabutyl- (TeBT), monophenyl- (MPhT), diphenyl- (DPhT), triphenyl(TPhT), and tricyclohexyltin (TCyT) as OT chlorides were obtained from different commercial sources. Neutral aluminum oxide (Al2O3, 70-230 mesh) and elemental sulfur were obtained from Merck and activated overnight at 120 °C before using. DMD solution (80 mM in acetone) was obtained as shown below and described in detail elsewhere.17 The stability of the DMD solution at -20 °C in the darkness was found to be longer than 3 months.
Extraction and Cleanup Procedures. Sediments were freeze-dried, sieved through 100 µm, extracted, and derivatized as described elsewhere.18 The DMD-organic extract mixture (0.5-8 mL) was manually shaken for 3 min at room temperature. The solvent and the excess reagent were then evaporated under nitrogen stream to 1 mL and filtered through a glass funnel containing 1 g of Na2SO4. The cleanup step is carried out using 1 g of activated Al2O3 and eluting with 4 mL of hexane in order to remove the sulfones formed in the oxidation reaction. TeBT at 100 ng was the IS, and TPrT and TCyT were the surrogates. The former compound (TeBT) was spiked prior to the determination and the latter (TPrT and TCyT) prior to the extraction. Safety (13) (14) (15) (16) (17) (18)
Drushell, H. V.; Sommers, A. L. Anal. Chem. 1981, 53, 1819-1829. Murray, R. W.; Jeyaraman, R. J. Org. Chem. 1985, 50, 2847-2853. Adam, W.; Curci, R.; Edwards, J. O. Acc. Chem. Res. 1989, 22, 205-211. Murray, R. W. Chem. Rev. 1989, 89, 1187-1201. Adam, W.; Bialas, J.; Hadjiarapoglou, L. Chem. Ber. 1991, 124, 2377. AÄ balos, M.; Bayona, J. M. Appl. Organomet. Chem., in press.
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Table 1. Sample Characteristics and DMD Needed for Complete Removal of Sulfur Interferences sample total sulfurb elemental sulfurc DMD codea (%) (mg/g) (mmol) SC-1 SC-3 SC-4 SC-5 BC-1 BC-2 MS-2 AC-1
