Analysis of aromatic pesticides by mass spectrometry. Advantage of plasma desorption over electron impact and NH₃ or CH₄ positive chemical ionization: The competitive formation of M⁺· and MH⁺ ions

Electron impact (EI) using ionizing electrons of different kinetic energies, ammonia and methane positive chemical ionization (PCI) and plasma desorption (PD) mass spectrometry were used for the analysis of aryl pesticides of the carbamate and phenylurea families. Under PD conditions, competitive formation and decomposition of M⁺· and MH⁺ ions was observed. It is shown that PD spectra give more information than do EI or PCI spectra, confirming PD as a method of choice for the analysis of small polyfunctional molecules. Rules are given for the identification of these families of pesticide. A thermochemical desorption ionization model defines the conditions required for the observation of both M⁺· and MH⁺ ions.