Abstract: A method for determining an analyte is provided, wherein a sample is prepared in a form useful for chemical inonization mass spectrometry and analyzed. Included in the sample is an analog of the compound which differs in having an unnatural isotopic distribution. The combined analyte and analog are derivatized as appropriate and then subjected to chemical protonation in the gaseous phase to provide charged species. The resulting charged species are then analyzed mass spectrometrically and the amount of analyte is determined by determining the ratio of one or more of the major peaks of the analyte and its analog.The method finds particular use in the determination of physiologically active compounds as found in physiological fluids, particularly blood. With blood, the analyte is extracted with an appropriate extraction solvent freed of protein, combined with its isotopically different analog, and the mixture then derivatized, if necessary.