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A sensitive and selective liquid chromatography-electrospray ionization-tandem mass spectrometry method was developed and validated for the quantification of beclomethasone 17-monopropionate (BMP), the main active metabolite of beclomethasone dipropionate (BDP) , in human plasma, using diazepam as an internal standard (IS).Beclomethasone 17-monopropionate and diazepam in plasma were extracted by sample liquid-liquid extraction with ethyl acetate.The analyte and IS were separated on a C18 reversed phase column, eluted with mobile phase of methanol-water (containing 33 mmol·L-1 ammonium acetate and 0.5% formic acid) (85∶15, V/V), at flow-rate of 0.8 ml·min-1, and ionized by positive ion pneumatically assisted electrospray and detected in the multi-reaction monitoring mode using precursor→product ions of m/z 465.1→279.2 for BMP and 285.1→193.1 for the IS, respectively.The calibration curve was linear (r2 ≥ 0.99, n =5) over the concentration range of 0.02-2 ng· ml-1 and with lower limit of quantitation of 0.02 ng· ml-1.The specificity, matrix effect, recovery, sensitivity, linearity, accuracy, precision, and several of stabilities have been validated for BMP in human plasma.In conclusion, the validation results showed that this method is sensitive, economical and less toxic and it can successfully fulfill the requirement of clinical pharmacokinetic study of beclomethasone dipropionate in Chinese healthy volunteers.