2.2. OCPs exposure assessment

During enrollment, fasting blood samples were collected, stored at −35 °C, and shipped to the National Center for Environmental Health at the Centers for Disease Control and Prevention (CDC; Atlanta, GA, USA) for analysis. Samples, including method blanks, and quality control samples were spiked with a mixture of ¹³C₁₂-labeled pesticides as internal standards. Serum analytes were isolated by C₁₈ solid-phase extraction (SPE), followed by a multicolumn automated cleanup, extraction and enrichment procedure (Sjödin et al., 2004; Turner et al., 1997). Analytes were separated using a DB-5 MS capillary column (Phenomenex, Torrance, CA, USA) and quantified using selected-ion-monitoring (SIM) high-resolution (10,000 resolving power) mass spectrometry (HRGC-ID/HRMS; Thermo Electron North America, LLC, West Palm Beach, FL, USA) (Barr et al., 2003; Patterson et al., 1987). Quantification was done by isotope dilution mass spectroscopy using calibration standards containing ¹³C₁₂-labeled and unlabeled analytes. The total serum lipid content of the sample was derived from enzymatic measurements of total cholesterol and triglycerides, then calculated using the Phillips equation (Phillips et al., 1989). Quality control sample coefficients of variation combining between/within-run reproducibility were generally between 10% and 15%. All study serum concentrations of HCB, βHCH, and p,p′-DDE were above the limit of detection. All OCP concentrations were expressed on a wet-weight basis (pg/g serum) or on a lipid-normalized basis (ng/g lipid) (division of wet-weight levels by lipid concentrations).