Multi-Modal Structure Characterization of Synthetic Batch Impurities with Liquid Chromatography Coupled to Infrared Ion Spectroscopy

Abstract

Assessing the safety of agrochemical products requires a thorough structural identification of synthetic batch impurities. The traditional method, relying on liquid chromatography paired with high-resolution tandem mass spectrometry, is often insufficient for complete structural determinations. Infrared ion spectroscopy has emerged as a novel analytical approach for structural characterisation in mass spectrometry. This technique integrates infrared spectroscopy with mass spectrometry and obtains infrared spectra of mass-isolated ions within the spectrometer and can be combined with liquid-chromatography separation methods. The infrared spectral patterns provide unique fingerprints for molecules with identical masses but different structures, and can be predicted through quantum chemistry, eliminating the need for reference standards. We demonstrate this methodology's application in analysing agrochemical batch impurities by successfully identifying five impurity structures, including two previously undocumented compounds that are identified based on the match to their computed spectra. Additionally, we showcase the detection and structural determination of a trace impurity that undergoes reconversion to the parent active compound. Here, Born-Oppenheimer molecular dynamics calculations are used to predict and assign the infrared spectrum, describing the experimental broadened absorption line shapes resulting from hydrogen bonding.