Laboratory for Clinical and Forensic Toxicology, University Hospital and Centre of Pharmaceutical Sciences, University of Groningen, P.O.Box 30.001, 9700 R B Groningen, The Netherlands

During the last TIAFT meeting in Padua (1997) we previously mentioned the importance of the shaking, evaporation and redissolving steps during the analysis. In this lecture the problems of unexpected adsorption on tubing of the pipettor and on the wall of plastic or glass vessels during the preparation of the stock solutions will be discussed on the basis of some practical examples.

The quality of the reagents might also have a dramatic influence on the results. For instance, the recovery of tricyclic antidepressants depends on the purity of the dichloromethane used during extraction. We noticed the same phenomenon in the reconstitution of the stock solution of a doxyrubicine-analogue using different qualities of methanol. An other point of concern could be the difference between calves' serum and human serum as an useful matrix for the calibration of several drugs.