Effects of drug—protein binding on trace enrichment of drugs in blood plasma on short precolumns

A reversed-phase liquid chromatographic precolumn venting plug technique was used for the determination of drugs by direct injection of large volume (500 μl) blood plasma samples. Enrichment of the drugs from large plasma samples, as well as clean-up from less retained plasma components, such as proteins, was obtained on a short precolumn. Strong drug—protein binding resulted in losses of the drug during the enrichment step. The recovery was found to be inversely proportional to the degree of drug—protein binding in the sample solution. Techniques to increase the recovery were studied. These include methods to decrease the degree of drug—protein binding in the sample solution by dilution and changes of the pH, as well as methods to increase the residence time of the drug on the precolumn by increasing the precolumn length and precolumn hydrophobicity.