Typical calibration curve for absolute quantification. Dilution series of the St peptides (5, 10, 50, 100, 500 and 1000 fmol) and 500 fmol IS peptides were subjected to LC-MS/MS and analyzed in SRM/MRM mode under optimized analytical conditions. The calibration curves were prepared using every transition set by plotting the peak area ratios of the St and IS peptides (y-values) against the St peptide amounts (x-values). This figure represents the calibration curve for mdr1a. R2, correlation coefficient.