Determination of organophosphorus acids by thermospray liquid chromatography-mass spectrometry

Abstract

The determination of thirteen organophosphorus acids, hydrolysis products of nerve agents and pesticides, by a combination of ion-pair liquid chromatography on a reversed-phase C 18 column and thermospray mass spectrometry was investigated. Ammonium acetate and three tetraalkylammonium salts with different alkyl groups (methyl, ethyl and n-butyl) were applied as ion-pair reagents. All the organophosphorus acids could be eluted using water or water-methanol mixtures. Capacity factors ( k′) were measured for some selected acids using water as eluent. The recorded thermospray mass spectra using ammonium acetate as electrolyte gave the [M + NH 4] + ion as a predominant peak, whereas with the tetraalkylammonium salts cluster ions were found. This difference in ionization mechanism was also reflected in the sensitivity. An amount of 100 pg of dimethylthiophosphoric acid could be detected by selected ion monitoring using ammonium acetate, whereas with tetramethylammonium hydroxide the amount was 5 ng. To obtain lower detection levels preconcentration could be achieved with a Sep-Pak C 18 cartridge pretreated with a tetra- n-butylammonium salt.