Selective ion-molecule reaction chemistry to enhance detection in ion mobility spectrometry

Ion mobility spectrometry (IMS) is a rugged, portable, sensitive, field instrumental technique capable of trace organic analysis for environmental pollutants, pesticides, explosives, drugs, and other analytes. Even though it has many advantages, IMS does have some limitations. Notable problems have been resolution of similar analytes, masking by interfering compounds, and unpredictable responses. These limitations have generally been associated with a lack of understanding of the chemical ionization mechanisms at the atmospheric pressure conditions which determine IMS responses. The research presented here investigates the formation of reactant ions, considers the ionization mechanism of analytes, and addresses the pathways that can lead to misidentification and masking in IMS. Fundamental data are provided for the improvement of analyses made by IMS.; The foundation of the research leading to this dissertation is that the formation of product ions in IMS can be controlled to achieve preferred results. It is demonstrated here, through the judicious choice of reagent chemicals, that reactant ions with a desired gas phase acidity or electron affinity can be formed which can preferentially ionize an analyte. Selected reagent chemicals are also used to promote adduct formation reactions to form detectable analyte ions that can be resolved from interfering compounds. The effect of reagent chemical concentration on formation of reactant ions and the subsequent change in effectiveness of the reactant ions are also investigated. The chemical ionization of 2,4,6-trinitrotoluene (TNT), a common explosive, is explored by making modifications to the flows of gases within the IMS instrument, that allow independent evaluation of the mechanisms leading to ion formation.