Search

Sampled d.c. and differential pulse polarography are used, in batch mode, to determine sulfur in methanol/0.1 M ammonium acetate (pH 5.0). A two-electron reaction (reduction of sulfur to sulfide) is shown to be involved. Differential pulse polarography is sensitive for the determination of sulfur in relatively clean solutions; the detection limit is 7.2 μg l−1. The interference of heavy metals (Pb and Cd) is avoided by addition of EDTA. For complex matrices, such as extracts of wheat and grapes, matrix effects are serious. For such samples, reversed-phase liquid chromatography with amperometric detection (dropping mercury electrode) gives excellent results. A relatively simple procedure is described for the determination of sulfur residues in wheat and grapes at levels ⩾ 0.5 mg kg −1; linear response is obtained up to ca. 7 mg kg−1.

An analytical procedure is described for the determination of residues of the fungicide fentin (triphenyltin acetate) from potatoes and celery. The method utilises a simple sample extraction procedure involving maceration with hexane followed by a methylation step using a Grignard reaction with methylmagnesium chloride. Samples are determined by packed-column gas chromatography with flame photometric detection. The limit of determination is 2 µg kg–1, which is well below the maximum concentration of fentin allowed in the substrates concerned (0.1 and 1 mg kg–1 for potatoes and celery, respectively). At spike levels of 0.1 mg kg–1, the recovery from potatoes was 84 ± 2%(n= 5); at 0.01 mg kg–1 this value was 82 ± 7%(n= 7). The blank value for potatoes, bought in a grocery store, was 2.8 µg kg–1.