Gas chromatographic–mass spectrometric determination of sulfamethazine in animal tissues using a methyl/trimethylsilyl derivative

A method is described for the determination of sulfamethazine in swine tissues by GC-MS. Samples are extracted with chloroform-acetone, followed sequentially by two solid-phase clean-up steps using silica gel and SCX ion exchange. The extracts are then partitioned between sodium dihydrogenphosphate (0.1 mol l-1) and methyl tert-butyl ether, the organic phase is evaporated to dryness and the sulfamethazine subjected to a double derivatization via methylation and silylation and determined by GC-MS in the selected-ion monitoring mode. Quantification is achieved by measuring the ratio of the abundances of the M-65 (-HSO2) ions of the derivatives of sulfamethazine and the internal standard, [phenyl-13C6]sulfamethazine, at m/z 299 and 305, respectively. The presence of sulfamethazine can be confirmed using the abundance ratios of the ions of m/z 299, 300 (-SO2) and 349 (-CH3). Recovery values from muscle, kidney and liver spiked at 0.05, 0.2 and 0.4 ppm ranged from 86 to 114% with RSDs between 2.8 and 9.0%. The limit of detection for the assay is 0.01-0.02 ppm. The methyl/trimethylsilyl derivatives exhibited better chromatography than the commonly used N1-methyl derivatives; for the same conditions, the peak was sharper and tailing was significantly reduced.