Possible Interference with the

404 CLINICAL CHEMISTRY, Vol. 29, No. 2, 1983 the particular advantages that “dirty” extracts may be easily processed by repeated “washing” and that the stationary phase is disposable. In our experience, however, the main drawback to the technique has been the difficulty of transferring an appreciable proportion of drug in a dried extract to the chromatographic plate. It is not easy to dissolve such an extract completely, even with use of a vortex mixer, in less than 100 zL and, of this, no more than 5 L can be reproducibly spotted at each application. Repeated spotting is time-consuming and very subject to operator error. We have found that only 5 to 10 L of solvent need be added to the extract, if this is sited in a round-bottomed tube. The solvent is then brought into contact with all the extract by using a relatively thick, round-bottomed glass rod, which traps the solvent between itself and the inside wall of the tube by surface tension. The solution of extract in solvent, which then becomes spread over the wall of the tube, can then be collected by brief centrifugation into the bottom of the tube, from whence it can be drawn into the capillary used for spotting. The idealsolventor solventmixture should have a low volatility, should dissolve most or all of the dried extract, and should be chosen so that the compound to be assayed has a low Rf value were the solvent to be used to develop it on a thin-layerchromatographic plate. For the strongly-basic /3-blockers we have found that an equivolume mixture of butan-1-ol and acetic acid mixture fits these conditions. Although the mixture is not too volatile, it evaporates readily from a thin-layer chromatographic plate, especially if the plate is conditioned by exposure to reduced pressure before elution.