Analysis of organophosphorus pesticides by gas chromatography–mass spectrometry with negative chemical ionization: a study on the ionization conditions

The present paper is aimed to study the optimal ionization conditions of 19 organophosphorous pesticides to be analyzed by gas chromatography (GC) with mass spectrometric (MS) detection in negative chemical ionization (NCI). Isobutane, methane, ammonia in methane, and pure ammonia were used as ionizing gases. The effect of the source temperature on the mass spectrum was evaluated for each pesticide considered in this study and for each ionizing gas. The influence of the electron energy, of the emission intensity and of the gas pressure was also studied for ammonia and ammonia in methane. The MS profiles have shown that NCI fragmentation induced by methane, isobutane and ammonia, generally leads to fragmentation by electron capture. Furthermore, significant differences in the mass spectra of several pesticides were recorded by changing the ionizing gas. Finally, the use of pure ammonia leads to a marked reduction of the background noise, with a parallel improvement of the overall sensitivity of the method, with values of the limit of detection (LOD) lower ( −1 ) than those obtained by methane or isobutane.