Measurement of phthalate metabolites
Phthalate metabolite concentrations were determined by gas chromatography/tandem mass spectroscopy.14 Reported phthalate concentrations were expressed relative to urinary creatinine (µg/g UCr) to control for urine dilution. A total of 455 urine samples were analyzed for the following 7 phthalate metabolites: mono-(iso-butyl) phthalate (MiBP), mono-isononyl phthalate (MNP), mono-benzyl phthalate (MBzP), mono-(3-carboxypropyl) phthalate (MCPP), mono-(2-ethyl-5-hydroxyhexyl) phthalate (MEHHP), mono-(2-ethyl-5-oxohexyl) phthalate (MEOHP), and mono-(2-ethyl-5-carboxypentyl) phthalate (MECPP). (Appendix S1: Fig. E1). Phthalate metabolites were grouped based on molecular weight and reported as the sum of 4 major high-molecular weight phthalate (Σ4HMWP) metabolites (MEHP, MEOHP, MECPP, and MCPP), or the sum of the 3 major low-molecular weight phthalate (Σ3LMWP) metabolites (MiBP, MNP, and MBzP). They were further divided into quartiles according to concentration (lowest to highest,Q1 to Q4). Previous research indicated these two groups of metabolites have different physicochemical properties.14,15