Automated online and off-line solid-phase extraction (SPE) methods coupled to isotope dilution–high-performance liquid chromatography–tandem mass spectrometry for measuring four isoflavones (daidzein, genistein, equol, and O-desmethylangolensin)

methods coupled to isotope dilution–high-performance liquid chromatography–tandem mass spectrometry for measuring four isoflavones (daidzein, genistein, equol, and O-desmethylangolensin) and two lignans (enterolactone and enterodiol) in urine are developed. The SPE recoveries for the online SPE method are excellent for most analytes (83–94%) and somewhat lower for enterolactone (61%). The recoveries for all analytes with the offline SPE method are also very good (65–80%). The limit of detection is lower for the online method (0.1–0.7 ng/mL) than for the off-line method (0.4–3.3 ng/mL). Similarly, the reproducibility is generally better for the online method [coefficient of variation (CV) of 4–12%) than for the off-line method, except for enterolactone, which has a higher CV (18–19%) that is consistent with its lower online SPE recovery. Both methods are adequate for analyzing a large number of samples for epidemiological studies to assess the prevalence of human exposure to isoflavones and lignans.