Ambient-Pressure Multischeme Chemical Ionization for Pesticide Detection: A MION-Orbitrap Mass Spectrometry Study

Abstract

This study explores pesticide detection with diverse ionization reagents by employing Multischeme chemical IONization inlet (MION) in conjunction with high-resolution Orbitrap mass spectrometry (MS). Various ionization schemes, specifically charging by Br^- and O₂ ^- in negative polarity, and by H₃O⁺ and C₃H₆OH⁺ in positive polarity, were investigated. The findings build on our previous work concerning pesticide detection using multischeme ionization and further demonstrate the effectiveness of the MION-MS methodology for detecting pesticides from complex standard mixtures and fruit extracts. The method successfully detected 136 compounds at a concentration of 10 ng/mL, and 447 at a concentration of 100 ng/mL, from standard solutions containing altogether 651 pesticides. The analysis of 10 fruit extracts revealed detections comparable to those obtained with validated methods. Subsequent molecular modeling provided insight into product identities observed when using protonated acetone as a reagent ion, which revealed that fragmentation into protonated pesticide and neutral acetone is energetically favored over decomposition to pesticide and protonated acetone (reactants). The current study amply underscores the versatility of the MION-MS methodology in seamlessly transitioning between different reagent ions in both polarities, enabling detection of a wider range of chemical compounds than with any single-ion-scheme instrument.

1

Characteristic spectra obtained from 369 pesticides with each ionization scheme: (a) hydronium (H₃O⁺), (b) protonated acetone (C₃H₆OH⁺), (c) bromide (Br^–), and (d) superoxide (O₂ ^–). Spectra were recorded from a 1000 ng/mL injection of solution A, captured at approximately 90 s into the measurement at 100–500 m/z.

2

Result of different volumes of injection for solution “A” and solution “B” with all ionization schemes (the neutral reagent used is presented in brackets). Solid fill columns and pattern fill columns correspond to 1 μL injection volume and 10 μL injection volume, respectively. (The notation ″+Br–″ indicates the addition of bromide anion, leading to the formation of a bromide adduct.). Detection trends vary by reagent, with nonlinear scaling observed at higher loading amounts. The error bars indicate an estimated 5% uncertainty.

3

Statistical results for different pesticide concentrations using all reagents, combination of Br^– ion scheme and hydronium ion scheme, and combination of Br^– ion scheme and protonation with acetone as reagent.

4

Triflumuron, at a concentration of 100 ng/mL, was detected in both the spiked extract and standard solution “A”.

5

Computed enthalpy of formation energies suggest fragmentation of cycloate, dimefox, trietazine, and crimidine adducts with protonated acetone toward protonated pesticide (MH ⁺ ).

6

Relationship between peak area and the imbalance of acceptor (HBA) and donor (HBD) sites for both protonated (by C3H6OH⁺ ionization scheme: blue circles) and bromide adducts (green crosses). Regions with the largest peak areas are circled in red, highlighting protonated cycloate and dimefox via C₃H₆OH⁺ ionization and bromide adducts of fipronil-desulfinyl, fipronil-sulfide, and fluxapyroxad.