At the beginning 50 mg cn cartridges were tested out, due to their very fast procedure.however,problems arose when trying to pre-concentrate plasma samples as it was found that a volume of 200 uL was necassary to redissolve the analyte after the spe procedure. To reach a three-fold pre-concentration, it was thus necessary to load 600 uL of plasma. In this case, lower extraction yields were obtained due to overloading of the sorbent. For this reason finally 100 mg cn cartridges were used that allowed very good extraction yields.