Rapid and simple clean-up and derivatizaton procedure for the gas chromatographic determination of acidic drugs in plasma

A rapid and simple clean-up and derivatization procedure that can be generally applied to the gas chromatographie (GC) determination of acidic drugs of various chemical and therapeutic classes is described. The drugs are extracted from acidified plasma with chloroform containing 5% of isopropanol, which is then evaporated. The residue is dissolved in toluene, then the drugs are back-extracted into a small volume of a methanolic tetramethylammonium hydroxide solution. The solution obtained is added to N,N-dimethylacetamide and the drugs are treated with n-butyliodide and chromatographed as their n-nutyl esters. Retention times are given for three different columns; the results show that in all but three instances one derivative is formed. The recoveries in the extraction steps were determined by GC and UV spectrophotemetry. The recovery of the back-extraction and of the extraction from plasma with chloroform-isopropanol is fairly good. For some drugs toluene can be used as the extraction solvent, which has the advantage that no evaporation of the extact is necessary, and it can be extracted directly with the tetramethylammonium hydroxided solution. Chromatograms of blank plasma treated according to the described procedure do not show interfering peaks from endogenous compounds. The advantages of the proposed method are as follows. It is a standard procedure that can be used for different types of drugs, using one standard column, with no interference from endogenous compounds. Separation is achieved between drugs that differ only in the number of position of the methyl groups and between a drug and its demethylated metabolite(s). No decomposition of the products occurs during the alkylation process and injection of the resulting neutral solution is not detrimental to the column material.