Liquid chromatographic determination of multiple sulfonamide residues in bovine milk: collaborative study.

A collaborative study involving 8 laboratories was conducted on the determination of 8 sulfonamide residues in raw bovine milk using a liquid chromatographic (LC) method. The sulfonamides are extracted with chloroform-acetone, the organic phase is evaporated, the residues are dissolved in an aqueous potassium phosphate solution, and the fatty residues are removed by washing with hexane. The aqueous layer is collected, filtered, and injected onto an LC system, and the analyte is detected by ultraviolet (UV) absorption at 265 nm. To quantitate all 8 sulfonamides isocratically, 2 chromatographic conditions are required: 12% methanol in the mobile phase for 5 sulfonamides, and 30% methanol in the mobile phase for 4 sulfonamides. Sulfamethazine (SMZ), the most widely used sulfonamide, is detected by both systems. Collaborators were instructed to analyze 3 replicates each of control milk and control milk fortified at 3 levels. They were also provided with 20 blind incurred samples (10 samples in duplicate) to analyze. For 10 ppb fortified milk, the average interlaboratory recovery for the 8 sulfonamides ranged from 56.2% for sulfaquinoxaline (SQX) to 82.7% for SMZ in the 12% methanol mobile phase (SMZ12). Also at this level, Sr ranged from 3.2 for SQX to 8.9 for SMZ12, and SR ranged from 6.9 for sulfadimethoxine to 17.2 for SMZ in the 30% methanol system (SMZ30). At 10 ppb, RSDr and RSDR ranged from 5.7% for SQX to 10.8% for SMZ12, and 10.1% for sulfamerazine to 20.9% for SMZ30, respectively. These results demonstrate that the method is suitable for the determination of the 8 sulfonamide residues in milk at 10 ppb. However, the identification of positives by this procedure needs additional confirmation by procedures comparable to the specificity achievable by liquid or gas chromatography combined with mass spectrometry.

Liquid chromatographic method for determination of sulfamethazine residues in milk: collaborative study.

Seven laboratories participated in a collaborative study of a liquid chromatographic (LC) method for determination of sulfamethazine (SMZ) residues in raw milk that were previously frozen. The milk is extracted with chloroform, the chloroform is evaporated, and the residue is suspended in hexane and extracted with 0.1M KH2PO4 (PDP) solution. The PDP extract is analyzed by LC on a C18 column with methanol-0. 1M PDP (30 + 70) as mobile phase. Individual laboratories were instructed to analyze 5 replicates each of control milk, fortified control milk at 2 levels, and 3 blind samples. Blind samples included raw milk fortified with SMZ at 10 and 20 ppb and 1 sample containing SMZ residue from a dosed cow. For blind fortified samples containing 10 ppb SMZ, average recovery and relative standard deviations for repeatability and reproducibility (RSDr and RSRR) based on the results from 6 of the 7 participating laboratories were 8.21 ppb, 7.16%, and 23.16%, respectively. Similar data, including results from a seventh participant who reported instrumental problems but was not eliminated by the Dixon outlier test, were 9.13 ppb, 8.38%, and 31.94%, respectively. These results demonstrate that the method is suitable for the determination of SMZ residues in milk at 10 ppb and above. The method was adopted first action by AOAC International.

Liquid chromatographic determination of multiple sulfonamide residues in bovine milk.

A liquid chromatographic method has been developed for simultaneous determination of residues of 10 sulfonamide drugs at 10 ppb and above in raw bovine milk. The method is based on a chloroform-acetone extraction, evaporation of organic phase, dissolution of residues in an aqueous potassium phosphate solution, and extraction of fatty residue into hexane. The aqueous layer is collected, filtered, injected onto an LC system, and detected by ultraviolet absorption at 265 nm. To elute all 10 sulfonamides isocratically, 2 chromatographic conditions are required. Seven sulfonamides can be quantitated with 12% methanol in the mobile phase; 4 sulfonamides can be quantitated with 30% methanol. Sulfamethazine, the most widely used sulfonamide, is detected on both systems. Recoveries are 44-87% for individual sulfonamides, with only 2 below 60%. Coefficients of variation are 3-13% at 10 ppb.

Liquid chromatographic determination of sulfamethazine in milk.

A simple, relatively rapid liquid chromatographic method has been developed for the determination of sulfamethazine (SMZ) in milk at levels in the low ppb range. The method is based on extracting SMZ from milk with chloroform, evaporating the chloroform, dissolving the residues in hexane, extracting into buffers, and chromatographing the buffer solution. The method has been shown to determine levels as low as 5 ppb reliably. Levels greater than or equal to 7 ppb have been confirmed by gas chromatography/mass spectrometry after derivatization of extracts from fortified, incurred, and shelf milk. Intralaboratory recoveries and percent coefficients of variation are satisfactory. Sulfadimethoxine and sulfaquinoxaline can also be determined by the method. Application of the method to other dairy products is being investigated.