Mass spectra of organic acids and their derivatives

In the following pages the mass spectra of organic acids and their common derivatives are discussed to give some indication of the fragmentation 

A comprehensive collection of spectra from a number of sources of trimethylsilyl derivatives of organic acids of biological interest was compiled by Markey et al (1972) and subsequently included in a larger reference collection (Markey et al., 1974b) and in other library files (e.g. Mass Spectrometry Data Centre Collection, The University, Nottingham, England). In addition to spectra of trimethylsilyl (TMS) derivatives are spectra of free acids, methyl esters, methyl ester-trimethylsilyl ethers (hydroxy acids), and trimethylsilyl esters-trimethylsilyl ethers-methoximes (oxo acids or oxo hydroxy acids). 

The availability of these data collections is of considerable value, permitting the identification of organic acid spectra by comparison with reference spectra in the collection. As variations in ion intensities can arise when spectra are acquired on different instruments with differing ion source conditions and inlet systems, many laboratories prefer to obtain authentic reference spectra on their own instruments for matching purposes. Such personalized collections are usually limited and do not have the benefit of input of less commonly encountered acid spectra from contributors working in related but different areas. 

Acids in the free form (with the exception of some low-molecular-weight acids) are too polar to permit GC introduction into the MS, and most spectra of free compounds have been obtained from samples introduced from a reservoir inlet or a direct insertion probe. This has limited their study to pure samples or very simple mixtures. Nevertheless a surprising number of acids have sufficient vapour pressure and thermal stability to be volatilized and, when coupled with a purification procedure where necessary, MS analysis may be satisfactory. 

Interpretation of the mass spectra of short-chain acids (C1-C5) was made by Happ and Stewart in 1952. The molecular ion is present in all spectra, although it decreases in intensity with increasing molecular weight. Useful diagnostic ions formed from the molecular ion are present due to loss of 45 mass units, equivalent to the CO2H group, and of 17 mass units resulting from loss of the carboxyl OH. 

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