The NH_4^ + (H2O) Reagent Ion: Mechanism of Increasing the Specificity of Ion Mobility Spectrometry Based Devices for Toxic Substances Detection in the Presence of Alkanes

In toxic substance detection devices based on ion mobility spectrometry with atmospheric pressure chemical ionization, hydrated ammonium NH_4^ + (H2O)n is increasingly used as a reagent ion instead of the commonly used hydrated hydroxonium H3O+(H2O)n ion and ions preceding and accompanying its formation. The use of hydrated ammonium makes it possible to increase the specificity of devices in the presence of such problematic interferences as exhaust gases of internal combustion engines and motor fuel vapors. In this paper, the mechanism of increasing specificity in the presence of alkanes, significantly predominant in these interferences, is justified. In the absence of ammonia, which is used to produce hydrated ammonium, the chemical ionization of alkanes occurs as a result of fast exergonic reactions with ions preceding ( O_2^ + ∙ , N_2^ + ∙ , N_4^ + ∙ , H2O+ ^∙ , and H3O+) and accompanying (NO+ and NO+(H2O)) the formation of hydrated hydroxonium. When ammonia is added, these ions react with it and are almost completely consumed in fast exergonic reactions. Thereby ammonia suppresses the chemical ionization of alkanes by precursor and accompanying ions. The direct chemical ionization of alkanes with the help of hydrated ammonium does not occur due to the endergonicity of the reactions and the almost complete shift of their equilibrium towards the formation of the initial reagents. As a result, the devices do not give false-positive signals when exposed to alkanes. In addition, the devices do not produce false-negative signals when analytes are detected in a mixture with alkanes, since hydrated ammonium is practically not consumed in reactions with alkanes.