The use of thermospray (TSP) for small molecule LC-MS applications also facilitated familiarity with the concept of generation of ions through cation attachment (e.g.,[M + NH4]⁺) and the use of negative ion mode for acidic compounds. TSP found tremendous use as an interface for higher flow-rate LC/MS, including for pesticide residue analysis.

ES ionization and APCI (atmospheric pressure) are now among the most commonly used techniques for creating ions from pesticides in solution.

The application of the more recent atmospheric pressure photoionization (APPI) to pesticide residue analysis is less common.

Although still at the development stage, there has been growing interest in EI for LC-MS to obtain library-matchable, readily interpretable, mass spectra, to aid the confirmation of identity of targeted pesticides and, more importantly, for the characterization of unknowns such as metabolites or transformation products.

p78, Analysis of Pesticides in Food and Environmental Samples, CRC Press, 2008, edited by José L. Tadeo