Evaluation of eluents in thermospray liquid chromatography-mass spectrometry for identification and determination of pesticides in environmental samples.

The influence of different eluents in positive and negative ion mode thermospray liquid chromatography-mass spectrometry was studied with several groups of pesticides, including carbamates, chlorotriazines, phenylureas, phenoxy acids and organophosphorus and quaternary ammonium compounds, and the corresponding degradation products. Using the positive ion mode in combination with reversed-phase eluents the base peaks generally corresponded either to [M + H]+ for the chlorotriazines and their hydroxy metabolites or to [M + NH4]+ for the carbamates, the phenylureas, the organophosphorus pesticides and their oxygen analogues. In the negative ion mode different processes such as (dissociative) electron-capture and anion attachment mechanisms occurred. Fragment ions such as [M - CONHCH3]- for the carbamates, [M - H]- for the chlorotriazines, phenylureas and chlorinated phenoxy acids and [M].-, [M - R]- (R being a methyl or ethyl group) for organophosphorus pesticides were usually formed. Depending on the eluent additive used (ammonium acetate, ammonium formate and/or chloroacetonitrile), three different adduct ions were formed: [M + CH3COO]-, [M + HCOO]- and [M + Cl]-. Normal-phase eluents with cyclohexane, n-hexane and/or dichloromethane provided more structural information and enhanced the response of several compounds. The positive ion mode was useful for the detection of chlorinated phenoxy acids and chlorophenols which could not be detected in the positive ion mode using reversed-phase systems. The base peaks generally corresponded to [M].+, [M + H]+ or [M - Cl]+. For the characterization of difenzoquat, a quaternary ammonium pesticide of which trace level analysis is troublesome, a post-column ion-pair extraction system was used. An aqueous mobile phase with a sulphonate-type counter ion was applied and an extraction solvent containing cyclohexane-dichloromethane-n-butanol (45:45:10) was used in thermospray liquid chromatography-mass spectrometry. Illustrative examples of the determination of residue levels of pesticides in soil matrices are shown.