Chemical ionization over a wide range of pressures. 1. Mass spectra of dimethyl esters of maleic and fumaric acids

Abstract

By using chemical ionization over a wide range of pressures, from 0.01 torr to atmospheric pressure, and also by selecting the reagent gas, different mass spectra of isomers can be obtained, which are suitable for their reliable identification. Under ionization conditions at atmospheric pressure in helium (reagent ions [H(H2O)n]+), peaks of cluster ions [MGH]+ and [2M+1]+ are observed in the spectrum of dimethyl fumarate, which are absent in the case of the cis-isomer. Under the conditions of chemical ionization at a normal pressure (0.4-0.2 torr) of the reagent gases Me3CH, n-C7H16 and at an ionic source temperature of 50°C, a stereospecific fragmentation of dimethyl maleate [MH]+-MeOH is observed, which is absent in the case of the the trans-isomer. In the chemical ionization spectra at reduced pressure of the reagent gases MeOH, EtOH, i-PrOH (0.01 torr), a peak of the cluster ion [MGH]+ is observed for dimethyl fumarate, which is absent in the spectra of the cis-isomer.