Twenty new alkaloids, of which 13 are new natural products, have been discovered in Pachycereus weberi by tandem mass spectrometry using a mass-analyzed Ion kinetic energy spectrometer. These observations have been confirmed by chromatography and, in many cases, by simple synthetic Interconverslons from known alkaloids. Particularly significant is the discovery of several alkaloids having the same molecular formula. Isomer distinctions such as these, which are difficult to make on pure compounds by mass spectrometry, were made by utilizing daughter spectra recorded successively during the evaporation of the material from the probe and/or from spectra recorded on different types of plant extracts. This study suggests that many natural materials may contain trace amounts of compounds of potential biomedical or synthetic interest and, in particular, that the number of known alkaloids might be greatly increased by experiments of this type. © 1985, American Chemical Society. All rights reserved.