The Ionization-Continuum Diagram: A Concept for the Selection of APCI and ESI Conditions for HPLC/MS Analysis of Pesticides

Abstract

The evaluation of 30 pesticides from six classes (triazines, organophates, phenylureas, chlorinated phenols, phenoxyacids, and quaternary amines) by high-performance liquid chromatography/mass spectrometry (HPLC/MS) clearly shows that different classes of pesticides are detected more sensitively under either atmospheric pressure chemical ionization (APCI) or electrospray ionization (ESI) conditions. For example, neutral pesticides in solution often are more sensitive in APCI (especially positive ion), and ionic pesticides in solution are more sensitive in ESI (especially negative ion). These data are expressed graphically in a figure called an ionization-continuum diagram that shows that polarity expressed as proton addition or subtraction (pKa) is useful to predict sensitivity under either APCI or ESI conditions. The importance of protonation and de-protonation within the microdroplet is discussed, as well as the role that this process plays in enhancing sensitivity under APCI or ESI conditions.

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Thurman, E.M., Ferrer, Imma, and Barcelš, Damić, 2000, The Ionization-Continuum Diagram: A Concept for the Selection of APCI and ESI Conditions for HPLC/MS Analysis of Pesticides [abst.]: 17th Montreux Symposium on Liquid Chromatography/Mass Spectrometry, Montreux, Switzerland, November 8-10, 2000, p. 31.

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