Session: Parallel session 10 - Lipidomics, Metabolomics, and Small molecules

Ion Mobility-Mass Spectrometry for small molecules, a focus on emerging environmental contaminants

The well-known pollutants such as legacy persistent organic pollutants (POPs) have been voluntarily phased out due to environmental and health concerns, and hundreds to thousands of emerging and unknown contaminants were introduced as replacements. POPs are ubiquitous in our environment and they bioaccumulate when they are passed from species to species through the trophic chain, ending up and, ultimately, to human. Traditional analytical methods target a limited number of regulated POPs by gas chromatography (GC) or liquid chromatography (LC) coupled with high-resolution mass spectrometry (HRMS) in selected ion monitoring mode (SIM) or tandem mode (MS/MS), known as GC-HRMS, GC-MS/MS or LC-MS/MS. These techniques are selective and sensitivity enough to provide accurate results on ultra-trace levels of regulated contaminants but produce limited results on many families not routinely screened in the environment.

Further selectivity can be achieved with the inclusion of ion mobility (IM) after chromatographic separation and before mass spectrometry. IM is a gas phase separation technique that separates ions based on their size, shape and charge state and offers two main advantages. First, it adds an additional dimension of separation to retention time and mass-to-charge ratio, enabling the removal of some co-eluting interferences and leading to cleaner MS and MS/MS spectra. Second, the experimentally measured ion mobility can be converted to a collision cross-section (CCS) value, which can be compared to experimentally derived or in silico calculated values available in libraries and serve as an additional analyte identification point along with accurate mass, ion ratios, retention time, and tandem mass spectrum in targeted, suspected and nontargeted approaches.

During this keynote, I will showcase how the addition of ion mobility spectrometry to existing GC-MS workflow has facilitated the separation of highly similar molecular structures of legacy and emerging halogenated POPs. Specifically, the integration of Trapped Ion Mobility Spectrometry (TIMS) coupled with time-of-flight mass spectrometer equipped with a GC-APCI source for sample separation and ionization provides a powerful tool to monitor a broad range of suspected and nontargeted contaminants. TIMS separations are performed using the new concept of Sliding Window in Ion Mobility (SWIM) mode to enhance the ion mobility resolving power (Muller et al., 2023).

The proposed nontargeted workflow encompasses feature detection (retention time, drift time, m/z and isotopic patterns), feature filtering (mass defect and Kendrick mass defect filtering, CCS trends filtering, and homologous series) and feature characterization (exact mass and molecular formula assignment, open-source database queries and ultimately structural elucidation). Through different examples of complex matrix samples, the methodology is conducted for various classes of legacy (e.g., PCBs, PBDEs, OCPs) as well as emerging (e.g., PXBs) persistent organic pollutants (POPs). We show that among the features developed, drift time filtering is of great help in obtaining clean mass spectra in order to get rid of most matrix background signal as well as other coeluting isobaric interferences, greatly improving the quality of the spectra and the identification confidence, especially for low signal intensity features.