Coupling mass spectrometers in series provides a new technique that has many advantages for the analysis of specific organic compounds in complex mixtures. Sensitivity to picograms of targeted compounds can be achieved with high specificity and nearly instantaneous response. The targeted compound is selectively ionized, and its characteristic ions are separated from most others of the mixture in the first mass spectrometer. The selected primary ions are then decomposed by collision, and from the resulting products the final mass analyzer selects secondary ions characteristic of the targeted compound. Tandem mass spectrometry can achieve specificities and sensitivities equivalent to those of methods such as radioimmunoassay and gas chromatography/mass spectrometry, while performing analyses in much shorter times.