Abstract
Perhaps the greatest influence that isotopic labelling experiments have had on organic mass spectrometry is that reaction mechanisms originally borrowed from the chemistry of neutral counterparts have proved to be inadequate for explaining the results. It was therefore necessary to devise completely new types of fragmentation mechanisms and unconventional structures for organic gas‐phase cations. In most cases the labelling technique allows one to discover the positions at which the label atoms are found in both the charged and neutral products of an ion's dissociation. These experimental results are often difficult to rationalize by any simple mechanism, but they nearly always indicate how chemical computations should be directed in order for the latter to be able to provide a better mechanistic understanding. This short article describes some significant studies involving D and 18‐O labelling that well support the above assertions, using as examples the behaviour of some quite simple organic molecules. Copyright © 2007 John Wiley & Sons, Ltd.