Method Validation. The intra-day and inter-day repeatability of the quantification of EC in plasma measured as the coefficient of variation (CV, %) are presented in Table . All the CVs were below 6.5% with a slight increase at the low spiking level. The CV of QC samples of CWDGS (n=5) was 2.8%. The extraction yield of EC from plasma as calculated using the measured concentration divided by the spiked concentration is 86-90%. Peaks of EC and EC-d5 differed by 4.8 s on the Agilent's DBWAX-UI column but were not completely baseline separated (Figure , next page).