Anal. Chem. 1998, 70, 3094-3101
Determination of Organotin Compounds in Water, Sediments, and Sewage Sludge Using Perdeuterated Internal Standards, Accelerated Solvent Extraction, and Large-Volume-Injection GC/MS Ce´dric G. Arnold, Michael Berg,* Stephan R. Mu 2 ller, Urs Dommann, and Rene´ P. Schwarzenbach
Swiss Federal Institute for Environmental Science and Technology (EAWAG), and Swiss Federal Institute of Technology (ETH), CH-8600 Du¨ bendorf, Switzerland
Two new methods for the simultaneous identification and quantification of organotin compounds (OTs) including monobutyltin, dibutyltin, tributyltin, monophenyltin, diphenyltin, triphenyltin, and tricyclohexyltin (TCyT) in natural waters and sediments, respectively, are presented. For water samples, aqueous ethylation followed by liquidliquid extraction (LLE) or solid-phase extraction and largevolume-injection GC/MS (50-200 µL) were used. For sediment samples, accelerated solvent extraction at 100 °C with a methanolic mixture of sodium acetate and acetic acid was developed. By using for the first time perdeuterated OTs as internal standards, excellent precision (relative standard deviations 98%), tributyltin chloride (>97%), dibutyltin dichloride (>97%), triphenyltin chloride (>97%), tin tetrachloride (>99%), oxalic acid dihydrate (>99.5%), and tropolone (>99%) were obtained from Fluka (Buchs, Switzerland). Butyltin trichloride (>95%), and phenyltin trichloride (>95%) were purchased from Aldrich (Steinheim, Germany). Tricyclohexyltin chloride (TCyT-Cl, >90%), nitric acid (65%, p.a.), sodium acetate (>99%), acetic acid (>99.8%), and sodium chloride (99.5%) were from Merck (Darmstadt, Germany). Sodium tetraethylborate (>98%) was obtained from Strem Chemicals (Bischheim, France). Methanol, dichloromethane, toluene, ethyl acetate, and hexane (all of purity for pesticide residue analysis) were purchased from Burdick & Jackson (Muskegon, MI). Deionized water was further purified with a Nanopure water purification device (NANOpure 4, Skan, Basel, Switzerland). TCyT-Cl was recrystallized from methanol. All other chemicals were used as received. Ethylated and perdeuterated organotin compounds were synthesized in our laboratories. MBT-Et3, DBT-Et2, TBT-Et, MPT-Et3, DPT-Et2, TPTEt, and TCyT-Et were synthesized by Grignard ethylation. Perdeuterated butyl- and phenyltin compounds (MBT-d9, DBT-d18, TBT-d27, MPT-d5, DPT-d10, TPT-d15) were synthesized from tin tetrachloride and butanol-d10 or bromobenzene-d5, respectively. More details on synthesis procedures are given in ref 38. (34) Cai, Y.; Alzaga, R.; Bayona, J. M. Anal. Chem. 1994, 66, 1161-1167. (35) Liu, Y.; Lopez-Avila, V.; Alcaraz, M.; Beckert, W. F. J. High Resolut. Chromatogr. Chromatogr. Commun. 1993, 16, 106-112. (36) Morabito, R. Microchem. J. 1995, 51, 198-206. (37) Quevauviller, P.; Astruc, M.; Ebdon, L.; Kramer, G. N.; Griepink, B. Appl. Organomet. Chem. 1994, 8, 639-644.
Analytical Chemistry, Vol. 70, No. 14, July 15, 1998
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Beware: Organotin compounds are toxic substances, and sodium tetraethylborate is highly flammable. Essential safety precautions must be employed for all manipulations. All OT concentrations are reported as Sn (ng/L and ng/g for water and sediments, respectively). Organotin chlorides (0.1-1 g/L) were dissolved in methanol containing 0.01 M HCl. Ethylated standards and tetrabutyltin solutions were prepared in hexane and methanol. Solutions were kept at 4 °C in a refrigerator. The solutions were renewed after six months even though no OT degradation was noticeable. Sodium tetraethylborate aqueous solutions were prepared daily. Glassware was rinsed with methanolic HCl (0.01 M), soaked overnight in 1 M HNO3 aqueous solution, washed with soap, and rinsed with water. The extraction cells were dismounted and rinsed with tap water. All solid residues were removed with a cotton tip. The cells were then soaked in methanol and sonicated twice for 30 min. Sampling and Sample Preparation. Lake and harbor water from Lake Zurich and Lake Lucerne (Switzerland) was grab sampled in polycarbonate flasks and extracted within 24 h. Sediments were collected with a gravity corer using Plexiglas liners of 12-cm inner diameter. Before transport to the laboratory, the sediment cores were extruded on-site and sliced at 1-, 2-, or 4-cm intervals. Sediment samples were placed in 250-mL polycarbonate centrifuge bottles. In the laboratory, they were centrifuged the same day at 20000g for 30 min. The sediment pore water was decanted, filtered using a polycarbonate filtration device (Sartorius, Goettingen, Germany) and polycarbonate filters (diameter 5 cm, pore size 0.4 µm, Nuclepore, Pleasanton, CA), and analyzed within 24 h. The solid sediment samples were freezedried and kept at -25 °C until analysis (within two months). Sediments for the spike experiments were taken from Lake Lucerne at depths between 5 and 80 cm (below the sedimentwater interface). They were pooled and freeze-dried for further use. The sewage sludge sample, a mixture of sludge from four wastewater treatment plants of canton Zurich (Switzerland), was freeze-dried and homogenized prior to analysis. A 24-h composite sample of raw wastewater was collected at the wastewater treatment plant in Uster (Zurich, Switzerland). This sample was further treated as described for sediment pore water. Surface runoff water was collected in aluminum pans during the first rain event following agricultural application of commercially available Fentin acetate (triphenyltin acetate) to a potato field. The samples were filtered, acidified to pH 4 with HCl, and kept at 4 °C until analysis. Seawater samples were collected at Sharm el Sheikh harbor (South Sinai, Egypt), acidified on-site with HCl to pH 4, kept at 4 °C until air transport, and stored at -25 °C in the laboratory. Field runoff and seawater samples were all analyzed within two weeks. Reference Sediments. PACS-1 sediment with a certified content of butyltin compounds was obtained from the National Research Council of Canada. RM 424 reference material was purchased from the European Commission, Community Bureau of Reference (Brussels, Belgium). This sediment is not certified, but the butyltin concentrations have been determined in an interlaboratory test, and the results have been published.37 Both sediments were kept at -25 °C. (38) Arnold, C. Ph.D. Thesis, Swiss Federal Inst. of Technology, 1998.
3096 Analytical Chemistry, Vol. 70, No. 14, July 15, 1998
Extraction and Enrichment of Water Samples: LiquidLiquid Extraction and Solid-Phase Extraction. Extraction and enrichment of water samples was performed in 50-mL volumetric flasks. The flasks were filled with 50-mL water samples before 0.5 mL of acetic acid/acetate buffer solution (5 M, pH 5) and 1.45 g of sodium chloride were added. The samples were shaken briefly and spiked with 100 µL of deuterated standard solution mixture (12.5 ng/mL in methanol), resulting in internal standard concentrations of 25 ng/L each. If necessary, the pH was adjusted to 5.0 ( 0.1 with acetic acid or 0.1 M NaOH. The samples were again shaken prior to addition of 150 µL of a freshly prepared 1.5% (w/v) NaBEt4 aqueous solution. The thus obtained solution was then extracted by either LLE or SPE. For LLE, 1 mL of hexane was added and the volumetric flasks were closed and shaken in the dark for 12 h on a horizontal Lab shaker (A. Ku¨hner, Basel, Switzerland) at 25 °C. For some sediment pore water samples, hexane formed a stable emulsion with the water phase. In this case, clear hexane extracts were obtained by destabilizing the emulsion with 3-5 drops of H2SO4 (96%, p.a.). For SPE, 6-mL glass cartridges were filled in the laboratory with 250 mg of graphitized carbon black (GCB; Carbopack B) between two Teflon frits and mounted on a 12-fold vacuum extraction box (all products from Supelco, Bellafonte, CA). The conditioning of the cartridges and the SPE procedure were carried out similar to Berg et al.39 Briefly, the solid phase was treated with 10 mL of dichloromethane/methanol (80:20, v/v), 4 mL of methanol, and 10 mL of Nanopure water. The prepared sample (as described above) was then drawn through the cartridge at a flow rate of 2.5 mL/min. Thereafter, the solid phase was washed with 10 mL of Nanopure water and 0.5 mL of methanol and airdried for 3 min. The organotin compounds were eluted with 6 mL of ethyl acetate. The 6-mL eluates were reduced to 1 mL by evaporation under a gentle stream of nitrogen at room temperature. Prior to the analysis by large-volume-injection GC/MS, 180 µL of the hexane (LLE) or ethyl acetate (SPE) extracts was transferred to 1-mL autosampler vials and spiked with 10 µL of the surrogate standard tetrabutyltin (TeBT, 0.2 ng/µL in hexane). The extracts were analyzed within 24 h or stored at -25 °C if a delay of more than one day occurred. Sediment Analysis: Accelerated Solvent Extraction. Samples (2.5 g) of freeze-dried sediment were weighed into 25mL beakers. The sediments were homogeneously spiked (i.e., by distributing the standard solution over the sediment sample) with 500 µL of the internal standard solution, resulting in internal standard concentrations of 100 ng/g. The sediments were thoroughly mixed with 9 g of quartz sand (Merck); the mixtures were transferred to 11-mL extraction cells, set aside for 2 h, and extracted with the Dionex ASE 200 device (Sunnyvale, CA) equipped with a solvent controller. Various complexing agents and acids in methanol were tested as extracting solvent (see Results and Discussion). The optimal solvent was found to be 1 M sodium acetate and 1 M acetic acid in MeOH. The extraction cells were filled with solvent and heated within 5 min to the extraction temperature (between 80 and 120 °C, 100 °C being (39) Berg, M.; Mu ¨ ller, S. R.; Schwarzenbach, R. P. Anal. Chem. 1995, 67, 18601865.
optimal; see results); the sediments were extracted with three to five static cycles of 5 min. Between each static extraction cycle, 4 mL of solvent was renewed. At the end of the extraction, the cells were rinsed with 4 mL of solvent and purged with nitrogen. The combined extracts (16-20 mL) were then transferred to 250mL volumetric flasks containing 7.3 g of NaCl, diluted with Nanopure water, and the pH was adjusted to 5.0 ( 0.1 with 1 M NaOH. Thereafter, 1 mL of a 5% (w/v) NaBEt4 aqueous solution was added to each flask, and the bottles were filled to 250 mL with Nanopure water. Finally, 2 mL of hexane was added and the samples were shaken for 12 h. A 500-µL aliquot of the hexane extract was then transferred to 2-mL GC vials and spiked with 10 µL of surrogate standard (TeBT, 50 ng/10 µL). Cleanup was necessary for sewage sludge samples. Prior to analysis, sewage sludge hexane extracts (i.e., after derivatization) were transferred to 10-mL centrifuge tubes containing 0.9 g of deactivated silica gel and 2 mL of water. The tubes were vigorously shaken and centrifuged. Minor OT losses were associated with this cleanup procedure (absolute recoveries 90-105%, average 99%). Large-Volume-Injection GC/MS Analysis for Water Samples. The water sample extracts were analyzed with a gas chromatograph GC 8060LV and a mass detector MD 800 (both Fisons Instruments). The GC was equipped with a deactivated fused-silica precolumn (15 m × 0.53 mm) and a 25 m × 0.25 mm DB-5 capillary column (film thickness 0.25 µm). At the column head, the carrier gas helium was set to a constant pressure of 150 kPa, resulting in a gas flow velocity of 60 cm/s at 65 °C. Separation was performed using the following temperature program: 1 min at 65 °C, to 250 °C with 10 °C/min, and 5 min at 250 °C. The interface and source temperatures were set to 250 °C. Using a Fisons AS 800 autosampler, 50 µL of the LLE and SPE extracts was injected on-column at a speed of 20 µL/s with a solvent vapor exit delay of 19 s. This technique, also called partially concurrent evaporation, is described in the textbook of Grob.40 Briefly, the solvent was introduced into the uncoated GC precolumn at 65 °C, where a solvent film was formed. The evaporating solvent was then transported by the carrier gas through an open split (solvent vapor exit), mounted between the precolumn and the analytical column. After evaporation of 45 µL of the solvent, the solvent vapor exit was closed and separation of the organotin compounds was carried out on the analytical capillary column. During the evaluation period, the injection volume was varied between 50 and 200 µL.41 LLE and largevolume injections of 50 µL were the most appropriate combination with regard to extraction efficiency and the analytical instrumentation involved (see Results and Discussion). Conventional GC/MS Analysis for Sediment Samples. Sediment extracts were analyzed with a Hewlett-Packard 5890 gas chromatograph coupled to a Hewlett-Packard 5971A mass spectrometer (Avondale, PA). Samples (1 µL) were injected oncolumn, except for the sewage sludge extracts, which were injected in the splitless mode in order to avoid the formation of active sites on the precolumn after a few injections (injector temperature 200 °C). The GC was equipped with a 2 m × 0.53 mm deactivated fused-silica precolumn and a 30 m × 0.32 mm (40) Grob, K. On-line Coupled LC-GC: Hu ¨ thig: Heidelberg, 1991. (41) Berg, M.; Mu ¨ ller, S. R.; Dommann, U.; Arnold, C. G.; Schwarzenbach, R. P. Abstract of Papers, 21st International Symposium on Chromatography, Stuttgart, Germany; Gesellschaft Deutscher Chemiker, 1996; Abstr. 333.
Table 1. Investigated Compounds, Retention Times, and Mass Traces Monitored large-volume GC/MS
conventional GC/MS
compd
ret time (min)
target ion (m/z)
qualifier ion (m/z)
ret time (min)
target ion (m/z)
qualifier ion (m/z)
MBT-d9 MBT DBT-d18 DBT TBT-d27 TBT TeBT MPT-d5 MPT DPT-d10 DPT TPT-d15 TPT TCyT
8.1 8.2 10.3 10.5 12.3 12.5 14.2 12.0 12.1 17.4 17.5 22.5 22.6 22.1
244 235 279 263 217 291 231 260 253 313 303 366 351 315
242 233 281 261 215 289 233 258 251 311 301 364 349 311
7.7 7.8 9.8 10.0 11.7 11.9 13.6 11.2 11.2 16.1 16.1 20.1 20.2 20.2
244 235 281 263 318 263 235 264 251 313 303 366 351 315
242 233 279 261 316 261 233 236 193 311 301 364 349 311
DB-5 capillary column (film thickness 0.25 µm). The helium flow velocity was electronically regulated to 50 cm/s. The oven temperature was programmed as follows: 1 min at 60 °C, to 250 °C at 10 °C/min, and 4 min at 250 °C. The interface temperature was set to 300 °C. For both large-volume-injection and conventional GC/MS analysis, detection was performed in the electron impact ionization mode and single-ion recording (SIR). Correct identification and quantification of a given analyte was assured by using two compound-specific ions and a mass ratio similar to the one determined with calibration (variation
