Liquid Chromatography–Electron Capture Negative Ionization–Tandem Mass Spectrometry Detection of Pesticides in a Commercial Formulation

Abstract

Negative chemical ionization (NCI) and electron-capture negative ionization (ECNI) are gas chromatography–mass spectrometry (GC–MS) techniques that generate negative ions in the gas phase for compounds containing electronegative atoms or functional groups. In ECNI, gas-phase thermal electrons can be transferred to electrophilic substances to produce M –• ions and scarce fragmentation. As a result of the electrophilicity requirements, ECNI is characterized by high-specificity and low background noise, generally lower than EI, offering lower detection limits. The aim of this work is to explore the possibility of extending typical advantages of ECNI to liquid chromatography–mass spectrometry (LC–MS). The LC is combined with the novel liquid-EI (LEI) LC–EIMS interface, the eluent is vaporized and transferred inside a CI source, where it is mixed with methane as a buffer gas. As proof of concept, dicamba and tefluthrin, agrochemicals with herbicidal and insecticidal activity, respectively, were chosen as model compounds and detected together in a commercial formulation. The pesticides have different chemical properties, but both are suitable analytes for ECNI due to the presence of electronegative atoms in the molecules. The influence of the mobile phase and other LC- and MS-operative parameters were methodically evaluated. Part-per-trillion (ppt) detection limits were obtained. Ion abundances were found to be stable with quantitative linear detection, reliable, and reproducible, with no influence from coeluting interfering compounds from the sample matrix.

Exploring Negative Chemical Ionization of Per- and Polyfluoroalkyl Substances via a Liquid Electron Ionization LC-MS Interface

“…A recent study by Cappiello et al demonstrated the influence of three acidic modifiers, TFA, PA, and FA, on ionization performance by adding them to the solvents used in the UHPLC-MS studies of pesticides. 37 The study observed that FA did not interact with low-energy electrons, preventing analyte dissociation. Furthermore, PFASs have very low dissociation constants, and adding FA to the mobile phase keeps the analytes undissociated, preserving the peak shape in reversed-phase chromatographic separation.…”

“…As demonstrated in earlier studies, ACN was preferred as the nonpolar solvent over MeOH due to prior experimental evidence that it is best suited to the LEI interface. 37,38,57 All optimized parameters are listed in section 2.5. The base peaks observed in the scan spectra of PFASs were selected as m/z ions for SIM analysis (Table 2).…”

“…Cappiello and co-workers successfully determined two pesticides, dicamba and tefluthrin, in a single analysis, present at ultratrace levels in a commercial formulation using LC-LEI-MS/MS in NCI mode. 37,40 LEI, in combination with CI, was also applied by Vandergrift and co-workers to analyze polycyclic aromatic hydrocarbons (PAHs) in aqueous and soil samples 41 and phthalates in house dust. 42 Given that LEI can be employed to couple UHPLC and NCI for the analysis of PFASs, this study focused on evaluating how various chromatographic and MS parameters would influence PFASs' ionization under this specific condition.…”

“…LEI has been amply demonstrated to allow the proper vaporization of LC-amenable compounds before reaching an under-vacuum ionization source, such as EI or CI. Several articles have provided a detailed description of the LEI setup. − In summary, a tee-connector is used to direct the reduced LC eluate as a liquid flow inside the inlet capillary. On the opposite end, the inlet capillary releases the liquid flow into the hot zone/vaporization microchannel (VMC), a coaxial capillary heated by the transfer line heater where it is inserted.…”

“…Cappiello and co-workers successfully determined two pesticides, dicamba and tefluthrin, in a single analysis, present at ultratrace levels in a commercial formulation using LC-LEI-MS/MS in NCI mode. , …”