Suspect screening and non-targeted analysis of drinking water using point-of-use filters

Abstract

Monitored contaminants in drinking water represent a small portion of the total compounds present, many of which may be relevant to human health. To understand the totality of human exposure to compounds in drinking water, broader monitoring methods are imperative. In an effort to more fully characterize the drinking water exposome, point-of-use water filtration devices (Brita^® filters) were employed to collect time-integrated drinking water samples in a pilot study of nine North Carolina homes. A suspect screening analysis was performed by matching high resolution mass spectra of unknown features to molecular formulas from EPA's DSSTox database. Candidate compounds with those formulas were retrieved from the EPA's CompTox Chemistry Dashboard, a recently developed data hub for approximately 720,000 compounds. To prioritize compounds into those most relevant for human health, toxicity data from the US federal collaborative Tox21 program and the EPA ToxCast program, as well as exposure estimates from EPA's ExpoCast program, were used in conjunction with sample detection frequency and abundance to calculate a "ToxPi" score for each candidate compound. From ∼15,000 molecular features in the raw data, 91 candidate compounds were ultimately grouped into the highest priority class for follow up study. Fifteen of these compounds were confirmed using analytical standards including the highest priority compound, 1,2-Benzisothiazolin-3-one, which appeared in 7 out of 9 samples. The majority of the other high priority compounds are not targets of routine monitoring, highlighting major gaps in our understanding of drinking water exposures. General product-use categories from EPA's CPCat database revealed that several of the high priority chemicals are used in industrial processes, indicating the drinking water in central North Carolina may be impacted by local industries.

Keywords: Drinking water; Exposome; High resolution mass spectrometry; Non-target analysis; Suspect screening.

Fig. 1.

Workflow for processing data and categorizing candidate compounds.

Fig. 2.

Kernel density plots of mass, peak area, and mass defects for negative, positive, matched (i.e., formula assigned), unmatched features, and the entire DSSTox_V2 database.

Fig. 3.

ToxPis of all A1α compounds (bottom left) with the top 20 enlarged (top left) and their corresponding ToxPi scores (right).

Fig. 4.

A) First (x-axis) and second (y-axis) principal components in a principal component analysis using summed peak areas for all compounds within a category; B) box and whisker plots representing the range of peak areas for compounds within a category; C) box and whisker plots representing the range of peak areas for compounds within each sample; and D) heat map showing the number of compounds that fall into each category by sample. Blank squares indicate no A1α compound was present for a category in a sample.

Fig. 5.

(2-chloroethyl)-bis[2,2,2-trichloro-1-(1-methylethoxy)ethyl] ester phosphonic acid (CAS 71039–43-5), the only discovered structure matching the generated formula of C₁₂H₂₀Cl₇O₅P for a large unknown peak at m/z 518.8796.