Determination of amoxicillin in human plasma using cephalexin as an internal standard and HPLC-MS/MS for the application of bioequivalence studies

A method for determining amoxicillin in human plasma by ESI-LC-MS/MS and using cephalexin as an internal standard was validated. The chromatographic separation was performed in a C18 Shim-pack CLC-ODS(M) analytical column (150 mm x 4.6 mm i.d. x 5 μm), which provided satisfactory separation of amoxicillin and cephalexin peaks, with analysis time of 3.1 minutes. A yield of extraction of 96.6% for amoxicillin and 98.7% for cephalexin was obtained, limit of quantification of 90 ng mL -1 , limit of detection of 500 pg mL -1 , and acceptable linearity for the analytical curve and calibration. Hence, the results show that the present method is appropriate for application in bioequivalence studies of amoxicillin, and it can also act as basis for the study of cephalexin.