Comprehensive-Trace Level Determination of Methyltin

With the development of industry and agriculture, organotin compounds are widely used as stabilizers, catalysts, and biocides.1 The use of organotin compounds as antifouling paints has led to deleterious effects for nontarget aquatic life even at low levels below several ng L–1. The fate of organotin compounds in the environment involves biotic and abiotic dealkylation processes leading to the formation of inorganic tin, which can be methylated under certain chemical and biological circumstances.2 In fact, the anthropogenic inputs of methyltin compounds do not account for their ubiquitous presence in the environment.3 Environmental methylation of tin undoubtedly contributes to the widely occurrence of methyltin compounds in natural waters. Modeling of the fate of methyltin ions required a highly sensitive analytical technique due to their low concentrations. Several researchers reported the occurrence of methyltin compounds in the aquatic environment. The concentrations of monomethyltin (MMT), dimethyltin (DMT) and trimethyltin (TMT) were depicted in the range of 10 – 1000 ng L–1.4 Analytical methods used in methyltin compounds speciation analysis generally involved a chromatographic separation technique, such as gas chromatography (GC), coupled with selective detectors which included atomic absorption spectrometry (AAS),5 atomic emission spectrometry (AES),6 mass spectrometry (MS),7 flame photometric detector (FPD)1 and inductively coupled plasma mass spectrometry (ICP-MS).8 As most organotin compounds existed as water soluble mono-, di-, and tri-alkylated species, they might interact strongly with the stationary phase of GC, which would seriously damage their separation. They were normally converted into non-polar, volatile corresponding compounds by complete alkylation, e.g., hydride generation with NaBH4, ethylation with NaBEt4, or derivatization with Grignard reagents.11 For direct in situ derivatization, hydride formation was usually the preferable method.12–14 Because of low levels of methyltin compounds in the environment and possible disturbance of the sample matrix, a sample preparation step including concentration and purification was of paramount importance. The conventional sample preparation method combined extraction by organic solvent, derivatization by Grignard reagent and purification, which was usually multistep and time-consuming. Recently, a commercially available purge and trap injector was applied to speciation analysis of organometallic compounds.6,15 With the usage of PTI, sample preparation and separation could be easily carried out on-line. The purged analytes were first trapped in a cryogenic fused-silica capillary, then desorbed by heating and forced into a gas chromatography capillary column for separation by carrier gas. This solvent-free sample preparation technique was preferred for application in the analysis of volatile and semi-volatile compounds, primarily because of the solution of the problems associated with those traditional sample preparation methods, such as use of toxic solvents and multistep procedures. This paper describes a comprehensive method for the sensitive and fast determination of trace levels of methyltin compounds in aqueous samples by a purge-and-trap injector (PTI) on-line coupled with gas chromatography–flame photometric detection (GC-FPD). The experimental results showed the feasibility of the combination of PTI with GC-FPD in the analysis of methyltin compounds in water samples. 1279 ANALYTICAL SCIENCES NOVEMBER 2001, VOL. 17 2001 © The Japan Society for Analytical Chemistry