Table 4. Comparison of IS performances for each C18 analyte, arranged by proximity to analyte peak. RSDs and accuracies shown are averages for all male and female spiked replicates. Mass fractions shown for each sample (Male, Female, and SRM) represent the mean of endogenous replicates. The best performing IS for each analyte is shown in bold. Values exceeding thresholds for accuracy (< 70 or > 120 %) and RSD (> 15) are in gray.