Laser desorption of quaternary alkaloids can be used to generate the intact cations which can be mass selected and then identified by recording a spectrum of fragment ions generated by collision-induced dissociation. The resulting MS/MS spectra show much less chemical noise than do the corresponding laser desorption mass spectra. This procedure allows Identification of the quaternary alkaloid candicine chloride (N,N,N-trimethyltyramlne chloride, 1) in crude extracts of cactus material at the level of 1 part per thousand (dry weight). Procedures for rapidly surveying plant material for quaternary alkaloids are given and tested for candicine in six species of cacti. Results agree with those from a much less specific chromatographic method. Secondary ion mass spectrometry provides similar capabilities and two new quaternary alkaloids, O-methylcandiclne and N,N,N-trlmethyl-β-methoxyphenylethylamine, were discovered with the aid of this procedure. © 1983, American Chemical Society. All rights reserved.