In a proof-of-principle experiment, our new analytical method was applied to heparin-derived plasma samples obtained 28 days post-partum from extremely-premature infants (gestational age <28 weeks, birth weight <1 kg). Sample volumes were between 50-200 µL. For volumes <200 µL, the sample preparation procedure was adjusted accordingly. Additional spiking experiments verified the proper performance of the procedure even when done with 50 µL (see Supplementary Table ). Detailed results are reported in Supplementary Table and include hematocrit/albumin concentrations and results of three samples after enzymatic deconjugation for selected analytes (Supplementary Table ). Figure shows representative chromatograms of the transitions used to quantitate selected compounds. MS 2 fragment ion spectra are shown in addition to an authentic reference standard of similar concentration. As many as 27 xenobiotics were detected in at least one sample (Table ; Figure ); and included plasticizers/plastic components, industrial chemicals, phytoestrogens, mycoestrogens, personal-care product ingredients, food processing by-products, and markers of exposure to smoke. Although our method was initially fully validated using serum and not plasma, the matrices were extensively and critically compared to demonstrate and ensure similar analytical behaviour. Further details, including a qualitative comparison of the plasma and serum matrices for selected analytes, are described in the Supplementary Information and Supplementary Figure .