Page 254 - Tandem Techniques

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It is clear from Figure 6.6, that the minimum detectable mass of the overall apparatus can be as low as 2
x 10-13 g of organotin present in the injected sample. This level of sensitivity for assaying an inorganic
environmental contaminant can be extremely useful. Lobinski et al.[4]examined the speciation of tin in
water and sediments. The sample preparation process involved the extraction of the methyl and butyl tin
compounds into pentane, as diethyl dithiocarbamate complexes at a pH of 5. The solvent was then
removed by evaporation under reduced pressure, after which the residue was derivatized with an n-
pentyl Grignard reagent in n-octane. This produced derivatives of the form R SnPe (4-n) . Test samples
n
showed recoveries of 95-100%. The separation of the extracted and derivatized components monitored
at the emission wavelength of carbon is shown in the upper chromatogram of Figure 6.7.

It is seen that the extract is exceedingly complex, and it would be extremely difficult, if not impossible,
to locate and measure the specific peaks for the tin compounds. In contrast, by monitoring the
separation on a wavelength specific for tin, the tin compounds are clearly and unambiguously selected
for detection. The sensitivity of the analysis can be assessed, for example, from peak 10
(dibutyldipentyl tin), which is present in the sediment sample at about 1.8 ng/g, while that in the water
sample was about 0.6 ng/L. The GC/AS tandem instrument is clearly very useful for inorganic element
identification, and can easily accommodate the very low concentrations of contaminants that can occur
in environmental samples.

Liu et al. [8] also used a GC/AS tandem system for determining the speciation of organotin compounds
in soils and sediments, but in addition used a complexing/supercritical fluid extraction procedure to
isolate the tin derivatives. The soil or sediment sample was amended with a complexing agent
(diethylammonium diethyldithiocarbamate) and was then extracted with supercritical carbon dioxide,
carrying 5% of methanol. The extraction was carried out at 450 atm. and at 60°C. The extracted
material, after the carbon dioxide and methanol had been removed, was then treated with
pentylmagnesium bromide, which converted the ionic organotin compounds to their neutral derivatives.