Multischeme Chemical Ionization Orbitrap Mass Spectrometry for Comprehensive Pesticide Detection

The detection and screening of pesticide residues remain analytically challenging due to the wide chemical diversity of active substances and their occurrence in complex matrices. This study evaluates the performance of an ambient-pressure Multischeme Chemical Ionization inlet (MION) coupled to high-resolution Orbitrap mass spectrometry for comprehensive pesticide detection. The MION inlet enables rapid switching between multiple reagent ion chemistries and polarities, allowing complementary ionization pathways to be exploited within a single analytical platform.

Pesticide detection was investigated using four ionization schemes: bromide (Br⁻) and superoxide (O₂⁻) in negative polarity, and hydronium (H₃O⁺) and protonated acetone (C₃H₆OH⁺) in positive polarity. Measurements were performed using a thermal desorption unit coupled to the MION inlet (TD-MION-MS), enabling direct analysis of liquid samples without chromatographic separation. A total of 651 pesticide standards were analyzed across a range of concentrations, along with ten real fruit and vegetable extracts, and results were compared to validated reference methods.

The results demonstrate reagent-dependent selectivity, with individual ionization schemes detecting distinct subsets of pesticides. No single reagent ion could detect all compounds; however, combining results from multiple ionization schemes substantially increased detection coverage. At a concentration of 100 ng/mL, 447 pesticides were detected, while 218 and 136 compounds were detected at 20 ng/mL and 10 ng/mL, respectively. Protonated acetone ionization yielded the highest overall number of detections, while bromide ionization provided robust detection for compounds forming stable adducts. Measurements of fruit extracts showed detection performance comparable to conventional GC-MS/MS and LC-MS/MS methods.

Overall, this study highlights the versatility and effectiveness of multischeme chemical ionization combined with high-resolution mass spectrometry for rapid pesticide screening. The ability to seamlessly switch between reagent ions and polarities enables broader chemical coverage than single-ionization approaches, demonstrating the potential of the MION-Orbitrap methodology for comprehensive pesticide analysis in food and environmental applications. In the subsequent study, the TD-MION inlet was coupled to a high-resolution Orbitrap Exploris 120 mass spectrometer, representing an advancement compared to the LTQ Velos Pro used in earlier work. A systematic comparison of three reagent ion schemes, bromide (Br⁻), uronium ([(NH₂)₂COH]H⁺), and nitrate (NO₃⁻), was performed using X-ray ionization. The performance of these schemes was evaluated using five individual pesticides and comprehensive pesticide solutions comprising 651 compounds from the previous study. The expanded instrumental capability and additional ionization modes enabled a broader assessment of reagent-dependent pesticide detection.