Thesis(M.Sc.)--Chulalongkorn University, 2005

In this study, a challenging method for simultaneous extraction and determination of six sulfonamides, three tetracyclines, and pyrimethamine residue in cow milk was developed. The sample preparation method include the acidic deprotenization of milk and dechelation by metals with McIlvain buffer, followed by enrichment and clean-up using a co-polymeric solid phase extraction cartridge (SPE), Oasis HLB. Reversed-phase LC-ESI-MS with positive ion mode was used for separation and detection of ten target analytes. Various factors affecting the sensitivity, separation, and extraction efficiency were optimized. Satisfactory recoveries for all compounds were obtained greater than 70%. Estimated method detection limits ranged from 0.48 to 2.64 ng/g which is far below the MRLs regulated by both EU and U.S. FDA. Method quantitation limits ranged from 0.97 to 8.64 ng/g. Linearity (R2> 0.99990) for quantitative analysis was obtained over dynamic range from MQL values up to 300 ng/g. Intra-assay and intermediate precision were acceptable with RSD at MQL and 5-fold MQL levels less than 8.42 and 9.97%. The method demonstrated satisfactory accuracy within an acceptable range of AOAC for trace analysis. The method was found to be robust and showed the pH of sample solution is a critical point for simultaneous extraction all compounds.