Nontargeted Analysis – A Revolution in PFAS Data Collection
We are only scratching the surface …
When analyzing for environmental contaminants, the usual quest is to accurately quantify a relatively short list of targeted compounds and trace elements. Mass spectrometry (MS) environmental methods focus on the US EPA Target Compound List or a project-specific list of contaminants of concern, capturing only those compounds with established cleanup benchmarks. These analyses only cover perhaps 200-300 of the 65 million chemicals identified in the Chemical Abstracts Service database, a catalog that increases by 15,000 chemicals per day!
Per- and polyfluoroalkyl substances (PFAS) include a wide variety of head groups, chain lengths and branching, so thousands of such chemicals may be present in the environment. However, because regulatory methods are targeted, the MS data are typically acquired in a filtered mode (e.g., multiple reaction or transition monitoring), such that only specific analyte types can be “seen.” This severely limits the ability to distinguish among types and sources of contamination.
Power tools …
With the advent of high-resolution tandem (and hybrid) MS and chemical informatics, we have new tools to strengthen and broaden the application of non-targeted analysis (NTA) methods so we can extract much more chemical information from the same samples, and perform rapid characterization of thousands of chemicals in environmental media and their sources.
In 2017, a team of researchers from Oregon State University, the Colorado School of Mines, Duke University, and the University of Guelph, used a tandem MS set up to capture “all” the MS data and found 40 previously unreported classes of PFAS present in samples impacted by aqueous film-forming foams (AFFFs).1 The Society of Environmental Toxicology and Chemistry (SETAC) has recognized the expansive data collection needed to assess risk for the mushrooming list of PFAS. SETAC held a special conference in 2019, and plans another one in May of 2020, focused solely on NTA for environmental assessment. At the recent North American Meeting of SETAC in Toronto, Canada, there were more than 50 presentations on NTA – most of which were focused on PFAS.
The US EPA has recognized the importance of NTA and has launched a significant effort titled, EPA’s Non-Targeted Analysis Collaborative Trial (ENTACT). ENTACT is designed to determine how measurement data generated from NTA methods can be used to direct high-throughput screening (HTS) research and strengthen chemical safety evaluations, and to demonstrate how resources procured for HTS research in support of chemical safety evaluations can be used to advance NTA methods. ENTACT is applying US EPA’s ToxCast library of approximately 4700 compounds and is conducting studies to identify the most accurate NTA methods and workflows. ENTACT involves more than 25 government, academic, and private/vendor laboratories internationally.
Expanding the forensic repertoire …
Environmental Standards, and a few others, have long used instrument data files to perform NTA for environmental forensic analysis. In this process, Environmental Standards uses the laboratory’s instrument files from the full‑scan gas chromatography-mass spectrometry (GC-MS) volatile and semivolatile organic compound methods to do library match searches and MS interpretation. In this manner, we find (or eliminate) co‑contaminant markers that are characteristic for a source. We are sometimes able to use this technique to characterize a source by its chemical pattern, without having to identify the exact isomer for each component. Once the patterns for sources are defined, we calculate dilution curves and construct mixing models to estimate the contributions of each source. The addition of the growing availability of high-resolution liquid chromatography-tandem mass spectrometry (LC-MS/MS) instruments in commercial laboratories has given us an opportunity to broaden the application of these techniques to PFAS and other analyte groups.
Researchers from the University of Washington (Tacoma and Seattle) and the Center for Urban Waters (Tacoma) have demonstrated using high mass resolution LC-MS/MS data in an analytical and data‑reduction technique to estimate source contributions based on NTA data. They report, “Relying solely on the richness of this data and avoiding the need for individual targeted contaminants, we developed a novel method to quantitatively estimate chemical source contributions to complex mixed systems that generated accurate estimates … even in multisource systems with < 1% source contributions.”2 The graphic above summarizes the work approach.
Getting more from your sampling and analysis …
At Environmental Standards, we import routine PFAS instrument files from the laboratory to assess isomer patterns of targeted PFAS homologs. We also work with laboratories to provide them the instrument acquisition parameters needed for NTA. We have demonstrated a variety of software tools and workflows to visualize and perform MS/MS interpretation and library searches. It can be of great value to capture all the data for suspected sources when sampling to perform nature and extent studies. This can be as simple as having the laboratory run a second analysis in a “data independent” mode and archiving the file or conducting an NTA screen to determine whether evidence for multiple sources can be identified.
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- Discovery of 40 Classes of Per- and Polyfluoroalkyl Substances in Historical Aqueous Film-Forming Foams (AFFFs) and AFFF-Impacted Groundwater. Krista A. Barzen-Hanson, et al., Sci. Technol. 2017, 51, 2047−2057
- Application of Nontarget High Resolution Mass Spectrometry Data to Quantitative Source Apportionment. Katherine T. Peter et al., Sci. Technol. 2019, 53, 12257−12268