The following procedure describes the final optimized experimental conditions for dSPE. Plasma or serum samples (100 µL in 96-well preparation plates) were removed from the -80°C freezer and allowed to thaw at 4°C for approximately two hours prior to extraction. A two-point internal standard solution in water was spiked into the MeOH:MeCN 1:1 (v/v) extraction solvent prior to the addition of sorbent to obtain the final concentrations of 0.05 µM L-phenylalanine-13 C9, N and 0.04 µM N-benzoyl-d5-glycine. The sorbent was washed and equilibrated as previously described. MeOH:MeCN 1:1 (v/v) was added to the sorbent to create a 16 mg/mL suspension. Two 96-well plates (192 samples) require 100 mL of suspension solution (1.6 g of sorbent), prepared in cold solvent (-20°C). The suspension (325 µL) was added to each sample well, and the sample-suspension mixture were vortexed thoroughly and left to incubate for 2 hours at 4°C. After centrifugation for 10 minutes at 3214 x g and 4°C, 212.5 µL of the supernatant (1/2 of the suspension-sample volume) were collected and dried under a gentle flow of nitrogen at room temperature. Resuspension was undertaken in 100 µL of water, containing a mixture of eight method reference standards including L-glutamic acid-13 C5; L-isoleucine-13 C6, N; Lleucine-13 C6; L-tryptophan-13 C11, 15 N2; L-glutamine-13 C5; creatinine-(methyl-d3); cytidine-5,6-d2; and benzoic acid-phenyl-13 C6 (at concentrations specified previously ).