Liquid chromatography high-resolution tandem mass spectrometry (LC-HRMS2) is used for detection of trace organic compounds (TrOCs) in the environment because it is sensitive, selective, and enables simultaneous detection of many TrOCs. A priori analyte selection is also not required, so users can screen for unknowns, e.g., transformation products (TPs), that are formed when TrOCs are transformed in environmental processes. With HRMS2, formulas are proposed based on accurate mass and, in some cases, structures can be proposed based on fragmentation. Ideally, reference standards are needed for final structure confirmation; however, many TPs are not commercially available and therefore other strategies need to be considered. With in silico fragmentation, HRMS2 can be calculated for known structures. These theoretical HRMS2 spectra are compared to measured MS2 spectra to assess likelihood of a correct assignment. To date, possible structures are retrieved from chemical repositories; however, many TPs are not present in these repositories. Therefore, including predicted structures in in silico fragmentation can increase the possibility of TP identification. Demonstrated are two examples of the application of in silico fragmentation with MetFrag (Ruttkies et al. 2016) for tentative identification of TPs. First, the presence of pesticide TPs was assessed in groundwater in Switzerland through a “smart” suspect screening. Over 1000 pesticide metabolites known from the European pesticide registration process were screened for and over 8400 accurate mass/retention time (AMRT) features were annotated as possible hits. Comparison of the isotopic pattern and the predicted fragments from MetFrag resulted in 300 plausible suspect hits, 11 of which were later confirmed. Second, a “broad” screening approach was applied to assess the formation of ozonation TPs (OTPs) in advanced wastewater treatment. With a prediction system (Lee et al. 2017), plausible OTPs were calculated for 87 known TrOCs, yielding over 70,000 possible structures. After screening and prioritization, 3380 possible OTPs remained. Of the 356 suspects with MS2 spectra recorded, 10 had higher scores for the OTP structure compared to any candidate structures retrieved from ChemSpider. These examples demonstrate that even without reference standards, additional confidence can be achieved in candidate structure assignment through the use of in silico fragmentation for the assessment of TPs in the environment.

LC-HRMS data and in silico fragmentation for identifying transformation products in environmental matrices

2019 Fall National Meeting

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