ON THE PRECISE ESTIMATION OF R(M) VALUES IN REVERSED-PHASE THIN-LAYERCHROMATOGRAPHY INCLUDING ASPECTS OF PH-DEPENDENCE

In order to improve the application of reversed-phase thin-layer chrom atography (RP-TLC) for the chromatographic estimation of drug lipophil icity, some aspects of measuring true R(M) values [log (1/R(F) - 1)] a re considered in the present investigation: (1) An optimization of exp erimental conditions, including the importance of temperature and humi dity, as well as densitometric evaluation of spots is presented. (2) T he estimation of thermodynamically true R(M) values is described; it i s shown that in case of high modifier contents preloading effects indu ce pronounced deviations of R(F) from R(F)' values. Only the latter al low the calculation of true R(M) values. (3) The influence of solvent pH on R(M) values is negligible for pure partition chromatography in t he case of low modifier contents; with increasing modifier contents po lar adsorption becomes more prominent; under these conditions an influ ence of pH on R(M) in the case of strong bases is detected.