Concentrations based on an external calibration curve were used for trend analysis where available (for a portion of the "confirmed" compounds); peak area was used for all other trend analyses (for all other compounds). Detailed statistical methods and results for trend determination are available in sections S.1.8 and S.2.2. Table also includes the relative standard deviation (RSD) of each compound concentration or peak area (from replicate unspiked samples, n ≥ 3) as an estimate of measurement error.