Influence of fulvic acids and copper ions on thiram determination in water

The literature concerning the application of solid-phase extraction (SPE) to the concentration of thiram (bis(dimethyldithiocarbamoyl) disulfide) from natural waters is scarce, the available results being contradictory or with no analytical significance. To clarify these contradictory results, a C18-SPE procedure combined with HPLC-UV was applied to thiram analysis in river water, and the influence of several factors on recoveries was studied. This procedure gave thiram recoveries of about 100% when applied to thiram standard solutions. However, when the same procedure was applied to river water samples spiked with thiram, the recoveries depended on the equilibration time after spiking. The influence of river fulvic acids (FAs) and Cu(II) on thiram recoveries from standard solutions was studied as a possible interference for such a result. In the presence of FA, thiram recoveries were always higher than 85%. In the presence of Cu(II), thiram recoveries decreased significantly, due to complexation, but the addition of an excess of EDTA before C18-SPE eliminated that interference, and thiram was completely recovered. However, in river water samples the addition of EDTA had to be done before thiram spiking to obtain a recovery >90%. Thiram standard solutions containing both river FA and Cu(II) showed a behavior similar to the one observed in river water samples. On the basis of these results, the catalytic effect of Cu(II) on the degradation of thiram by FA, with formation of a Cu(II)-dimethyldithiocarbamate complex, was hypothesized.