Propofol

We dissolved propofol (97%, Sigma Aldrich, Munich, Germany) in HPLC-grade water (VWR, Darmstadt, Germany) to a 50 µg/mL propofol stock solution. The stock solution was gravimetrically diluted in 50 mL flasks to 5, 10, 20, and 50 µg/mL samples. 2 µL from each standard or HPLC-grade water for blanks, respectively, was directly pipetted onto Tenax sorbent tubes as quadruplicate per sample. The mass of each injection was determined on a Cubis analytical scale (Sartorius, Göttingen, Germany) to determine the exact amount of propofol injected in ng. The resulting samples contained final propofol masses of 10, 20, 50, and 100 ng. For an even distribution of propofol, each tube was flushed after injection for 30 seconds with 1 bar of 99.999% pure synthetic air (Air Liquide, Düsseldorf, Germany) (20.5% O₂ purity [4.5], 79.5 % N₂ purity [5.0]). To test whether propofol is lost during loading, a second Tenax sorption tube was used to investigate the synthetic air exiting the end of the loaded tube. We calibrated on the day of shipment and immediately after their return, with eight calibration standards (0, 10, 20, 30, 40, 50, 60, and 100 ng) prepared on the day of calibration, which were pipetted as stated above.