Suspect screening for trace pesticides in wet deposition using liquid and gas chromatography with high-resolution mass spectrometry

Pesticides released into the air can undergo atmospheric transport and return to the biosphere through wet deposition. Here, both liquid chromatography (LC) and gas chromatography (GC) were coupled with high-resolution mass spectrometry to characterise pesticides in rainwater collected at a semi-urban site in the central United States. The extraction procedure was optimised for efficient recovery of semivolatile current-use pesticides present in rainwater at trace levels of ~1 part per trillion. Suspect screening was used to identify 22 pesticides and related compounds in rainwater extracts. All 22 identifications were validated against retention times, exact masses, and mass spectra of reference standards. The 21 compounds detected by LC were quantified against surrogates added prior to extraction, and the compound detected by GC (benzothiazole) was semi-quantified against an injection standard added after extraction. To the best of our knowledge, we report the first quantification of the fungicide azoxystrobin in wet deposition from North America and the first quantification of triadimefon and trifloxystrobin in wet deposition globally. Total pesticide levels in rainwater samples ranged from 14–716 ng L−1 for fungicides (n = 7) and 13–2362 ng L−1 for herbicides (n = 13), contributing total wet deposition fluxes of 45 μg m−2 and 375 μg m−2, respectively, from May-September 2022. Rainwater samples from May and June showed elevated wet deposition of herbicides in comparison to samples from July and August, which showed elevated wet deposition of fungicides. This evidence suggests that pre-emergent herbicides and post-emergent fungicides might not linger in the atmosphere beyond their period of application to crops near the sampling site.