An HPLC system (Shimadzu LC-20A) equipped with a UV−
VIS detector tuned to 269 − 270 nm was used to monitor the
OTC. A C
18 analytical column (150 mm × 4.6 mm × 3.5 μm)
was used but other similar columns may also generate
satisfactory data. An isocratic flow rate of 0.75 mL/min was
used for elution. The sample loop (Shimadzu) in the
instrument was 20 μL but other sizes may also suffice. The
SPE extraction can be sped up with use of a vacuum manifold
(Visiprep-DL, Supelco). However, extraction by gravity is also
possible with extended analysis time.