Acetic acid methyl ester, mass spectrum

Halogenated aiyloxyphenoxypropionic acids are a new class of herbicides used for the selective removal of grass species. In commercial preparations, they are present as alkyl esters. In negative-ion ESI, haloxifop, fluazifop, and diclofop all show similar behaviour. The deprotonated molecule is the base peak in the spectrum. Weak formate and acetate adducts occur, and a fragment due to the loss of the propionate part [45]. The analysis of fluazifop and its butyl ester, fenoxaprop, quizalofop and haloxyfop and their ethyl esters, and diclofop and its methyl ester was reported. The free acids were analysed in negative-ion mode, and the esters in positive-ion mode. The esters showed sodium and potassium adducts next to the protonated molecules. The adduct formation was suppressed by the addition of 25 mmol/1 formic acid to the mobile phase. The influence of the orifice potential on the appearance of their mass spectra was studied [46-47]... 

Esters are carboxylic acid derivatives, and the spectrum of ethyl acetate is shown in Figure 14.19D. The carbonyl absorption does not distinguish this compound from an aldehyde or a ketone, but there is the C-O absorption at about 1200 cm 1. Because this is in the fingerprint region, however, its position can be difficult to identify. This is clearly the case for methyl pentanoate, where the C-0 absorption can easily be missed or misidentified. Based only on the infrared, it may be difficult to distinguish an ester from an aliphatic aldehyde or ketone. If the formula is known, however (from mass spectrometry), the identification is easier because an ester has two oxygen atoms, whereas the aldehyde or ketone has only one. 

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