Positive ion mass spectra obtained by conventional electron impact and by methane chemical ionization were compared with the negative ion mass spectra of 16 typical organophosphorus pesticides using methane, a methane-oxygen mixture, and oxygen as enhancement/reagent gases. In the negative ion spectra, phosphate and phenate anions typically carry a large fraction of the ion current. Displacement of chlorine by oxygen is noted in some cases, and migration of aryl groups from oxygen to sulfur is occasionally noted. Ion-molecule reactions between two species derived from sample molecules have been observed. The sensitivity of the negative ion techniques for the pesticides examined is greater than conventional electron impact or methane positive chemical ionization techniques, and can be up to 800 times more sensitive for some compounds.
Kenneth L. Busch, Maurice M. Bursey, J. Ronald Hass, and G. Wayne Sovocool, "Comparison of Five Ionization Methods for Producing Mass Spectra of Typical Organophosphorus Pesticides," Appl. Spectrosc. 32, 388-399 (1978)