Use of plasma desportion mass spectrometry for pesticide analysis: The s‐alkylaminotriazines

The 2‐substituted‐4, 6‐di‐(alkylamino)‐1,3,5‐triazines (s‐triazines) are well known pesticides. With the dramatic increase in usage patterns of pesticides, an urgent need has arisen to develop analytical techniques to assist in identification of such compounds. A comprehensive understanding of the major fragmentations of these molecules, after ionization, could aid in this task. The use of conventional ionization techniques such as electron impact or chemical ionization is limited because of the relatively low volatility of these compounds, or because of thermal limitations. Recently, there have been promising developments in desorption ionizaion techniques for analysis of thermolabile and low volatility compounds such as environmental compounds. Among these techniques, the desorption ionization technique called ‘plasma desorption’ (PD) leads to more informative mass spectra than those obtained with other ionization techniques. A systematic investigation of ion formation and decomposition processes of three families of 2‐X substituted‐s‐triazines (X = Cl, SCH₃ and OCH₃) under PD conditions is presented. Eight pesticides were studied in this work. The influence of the X and alkyl substituents on the fragmentation processes under PD conditions is investigated in detail. Specific molecular ions (adduct ion, substitution ions …) allow the differentiation of each family, while characteristic fragment ions lead to the identification of the 1,3,5‐triazine ring and/or of its substituents. The fragmentation processes are obtained by combining nominal masses of ions determined in positive‐ion mass spectra and metastable transitions of low internal energy ions. This study is extended to analysis of s‐triazine mixtures. The results obtained lead to the establishment of a desorption order of these compounds under PD conditions. The experimental results are supported by semi‐empirical molecular orbital calculations. Finally, the proton affinity of Ametryn (2‐methylthio‐4‐(isopropylamino)‐6‐(ethylamino)‐s‐triazine) was determined to be close to that of histidine i.e. 970 kJ/mol.