11 - Examples of structure elucidation by mass spectrometry

Summary

INTRODUCTION

Before some worked examples are given to illustrate the elucidation of unknown or partially known structures by mass spectrometry, some general hints are in order. These salient points have been discussed elsewhere in this book and the treatment here is intended as a helpful guide to analysis of samples of various kinds.

A molecular structure is rarely deduced solely from its mass spectrum. What the mass spectrometrist hopes to do in the first instance is to propose a structure that is consistent with the given spectrum. This first structure may need to be modified in the light of subsequent, differently obtained mass spectra or as other information becomes available. Unfortunately, it may even be possible to propose several structures that are compatible with the mass spectrum. Of course, there are degrees of probability of correctness of any structural assignment and, if the elemental composition (high-resolution mass spectrum) and metastable ions are consistent with the proposed structure, then the correctness of an assignment will be much more certain. This is especially true if the spectrum of the unknown structure matches well against a standard spectrum from a library of mass spectra. However, there are many instances of different compounds (in particular, isomers) giving rise to mass spectra having differences that are less than the day-today variation in the recording of a mass spectrum, so one should be wary of rash structural assignments. Verification of proposed structures by other techniques is recommended and may be essential.