Positive ion chemistry of esters of carboxylic acids in air plasma at atmospheric pressure

Ionization of esters of carboxylic acids RCOOR' (R = H, alkyl; R' = alkyl) within the air plasma of the Atmospheric Pressure Chemical Ionization (APCI) source occurs largely via H+-transfer and, to a minor extent, via NO+ association. The protonated ester MH+ is normally observed as M2H+ and as higher aggregates (M3H+, M3H+(H2O)) also at high source temperature. The behavior of M2H+ upon collisional activation is consistent with the reported dissociation of proton-bound dimers to MH+ species that, in turn, fragment according to the known paths of lowest energy. In addition, other important product ions form within the plasma, some in very high relative abundance, which are attributed to ion-molecule condensation reactions between neutral M and either MH+ or M2H+ resulting in the elimination of CO, R'OH, alkene from the alkoxy moiety of the ester and HCOOH. A general scheme is proposed to account for the experimental observations, which suggest that the encounter complex formed between MH+ and M or between M2H+ and M may either collisionally relax to the protonated dimer or trimer, respectively, or react via covalent bond forming and cleaving steps to eliminate stable neutral molecules. The proposed scheme is supported by both the observed concentration dependence and the temperature dependence of the products relative abundances within the plasma. Such reactions can be the dominant process, as in the case of formate esters. A second significant ionization route involves addition of NO+ to form MnNO+ (n = 1, 2, 3). An additional product corresponding to [M2NO+ - CO2] is also observed with iso- and n-butyl formate esters. Copyright (c) 2005 John Wiley & Sons, Ltd.