The effect of various additives (ammonium acetate, ammonium formate, triethyl-and tripropylammonium formate and chloroacetonitrile) on the sensitivity and selectivity in liquid chromatography-thermospray-mass spectrometry (positive and negative ion mode) is studied for a group of 55 pesticides including phenoxy and carboxylic acids, hydroxy-keto-lactones, substituted amides, triazines, (thio)carbamates, phenylureas, substituted phenols, organo-phosphorus compounds, thiocyanates and anilines.

In the positive ion mode the base peaks correspond to [M + H]⁺ and [M + NH₄]⁺ for the majority of the test compounds. Fragmentation is observed in some isolated cases only. In the negative ion mode processes like (dissociative) electron capture and ion-molecule reactions take place. Fragment ions such as [M-H-CONCH₃]⁻ for the carbamates, [M-C₃H₆O]⁻ for the hydroxy-keto-lactones and [M-H]⁻ for the chlorinated phenoxy acids are observed. Depending on the additive used, adduct ions such as [M + CH₃COO]⁻, [M + HCOO]⁻ and [M + CI]⁻ are formed. Adducts with fragmentions are also observed quite often. The use of triethylammonium formate and tripropylammonium formate instead of ammonium formate increases the sensitivity and selectivity of the method in the NI mode, because their high proton affinity enhances deprotonation.