Development of sensitive derivatization method for aldosterone in liquid chromatography–electrospray ionization tandem mass spectrometry of corticosteroids

A highly sensitive quantification method of aldosterone by liquid chromatography–electrospray ionization tandem mass spectrometry (LC–ESI-MS/MS) was investigated in a positive mode using recently developed picolinyl derivatization. Aldosterone was smoothly and quantitatively converted to the ethyl ether–picolinyl derivative by treatment with HCl–ethanol followed by the esterification with picolinic acid in the presence of 2-methyl-6-nitrobenzoic anhydride and 4-dimethylaminopyridine. The positive ion-ESI mass spectrum of the ethyl ether–picolinyl derivative was characterized by an appearance of protonated molecule ([M+H]+) as a base peak. The ethyl ether–picolinyl derivatization gave a successful result in a separation of aldosterone from corticosterone, dehydrocorticosterone and cortexolone, and also provided an approximately 10-fold higher ESI response in the positive-LC–ESI-MS/MS (selected reaction monitoring; SRM) when compared to that of underivatized molecule (negative mode). The limit of quantification of aldosterone by SRM using ethyl ether–picolinyl derivatization (m/z 494 → m/z 448) was 1 pg/0.2 ml serum with accuracy and precision of 92.6% and 5.6%, respectively.