Further, the solvent type, i.e., MTBE versus ethyl acetate, and solvent-to-sample ratio were examined for the best EC extraction yield. 5-Sulfosalcylic acid was added to the spiked plasma containing 1 µg/mL of EC to precipitate the proteins. The resulting supernatant was then extracted with MTBE or ethyl acetate using solvent-to-sample volume ratios of 1:1, 2:1, 4:1, 6:1, 8:1. After the mixing, 80% volume of the solvent extract was recovered and spiked with 20 µL of 10 µg/mL of EC-d5 dissolved in the corresponding solvent. Then the extracts were dried under nitrogen until about ~100 µL left. Salts were not added in this experiment.