Choosing between atmospheric pressure chemical ionization and electrosprayionization interfaces for the HPLC/MS analysis of pesticides

An evaluation of over 75 pesticides by high-performance liquid chromatograp hy/mass spectrometry (HPLC/MS) clearly shows that different classes of pest icides are more sensitive using either atmospheric pressure chemical ioniza tion (APCI) or electrospray ionization (ESI). For example, neutral and basi c pesticides (phenylureas, triazines) are more sensitive using APCI (especi ally positive ion). While cationic and anionic herbicides (bipyridylium ion s, sulfonic acids) are more sensitive using ESI (especially negative ion). These data are expressed graphically in a figure called an ionization-conti nuum diagram, which shows that protonation in the gas phase (proton affinit y) and polarity in solution, expressed as proton addition or subtraction (p K(a)), is useful in selecting APCI or ESI. Furthermore, sodium adduct forma tion commonly occurs using positive ion ESI but not using positive ion APCI , which reflects the different mechanisms of ionization and strengthens the usefulness of the ionization-continuum diagram. The data also show that th e concept of ",wrong-way around" ESI (the sensitivity of acidic pesticides in an acidic mobile phase) is a useful modification of simple pK(a) theory for mobile-phase selection. Finally, this finding is used to enhance the ch romatographic separation of oxanilic and sulfonic acid herbicides while mai ntaining good sensitivity in LC/MS using ESI negative.