Selective determination of thiram residues in fruit and vegetables by hydrophilic interaction LC-MS.

ABSTRACT

Thiram belongs to the most important class of dithiocarbamate (DTC) fungicides including dimethyldithiocarbamates (DMDs), ethylenebis(dithiocarbamtes) (EBDs) and propylenebis(dithiocarbamates) (PBDs). During the surface extraction of fruit and vegetables for the LC-MS determination of residues of DMDs, EBDs and PBDs, thiram is reduced by the penicillamine buffer to the DMD anion, thus resulting in false-positive findings of DMD fungicides like ziram. Therefore, an alkaline sulfite buffer was applied for surface extraction, quantitatively transforming thiram into the DMD anion and a stable DMD-sulfite adduct that was used as a selective marker for thiram. Separation was performed isocratically on a ZIC-pHILIC column with acetonitrile-10 mM ammonium hydroxide solution (85/15). Mass selective detection was carried out on a single-quadrupole mass spectrometer coupled to an electrospray ionisation interface operating in negative mode. Using d12-thiram as the internal standard, recoveries of 80-108% were obtained from apples, tomatoes, grapes and sweet peppers, spiked in the range of 0.02-1 mg kg(-1). Limits of detection and quantification were 0.6 and 2 µg kg(-1), respectively.