PCIS concentration optimization. To optimize the PCIS concentration, solutions of 25, 50, 100, 200 and 300 ng/mL 2F-AEA were post-column infused (at 0.05 mL/min) for 2 min using isocratic mobile phase conditions reflecting the gradient composition at 1 min (73% A, 26% B, 1% C, 0.7 mL/min). The signal of the 2F-AEA MRM transition (350.3 to 269.2) between 0.2 and 1.8 min was exported to Excel, smoothed (3-point moving average) and used to calculate the mean intensity and signal stability (relative standard deviation, RSD).