Ammonia gas: an improved reagent for chemical ionization mass spectrometry of bile acid methyl ester acetates

The ammonia chemical ionization mass spectra of 28 methyl ester acetate derivatives of bile acids and related compounds have been determined by gas-liquid chromatography-mass spectrometry. Advantages of ammonia ionization over the previously studied isobutane ionization include a 130-270% enhancement in the sensitivity of base peak monitoring, and direct determination of molecular weight from the base peak (M + NH4+) in the mass spectrum of any of the derivatives. Minor ions in the ammonia spectra also allow selective detection of 3-keto compounds and can indicate unsaturation or double bond conjugation in the molecule. The significance of these studies for the detection and quantitation of bile acids is discussed.