Giebelmann R.

University of Greifswald, Institute of Legal Medicine, Greifswald, Germany

The Rf values of basic drugs like atropine, codeine, strychnine, diphenhydramine under condition of ion association on silica gel may be regulated by the type and concentration of acid and the polarity of the solvent in the mobile phase.
Methods. TLC of basic drugs on silica gel with ethanol, dioxan, acetone, tetrahydrofuran and hydrochloric, sulfuric, nitric or perchloric acid (0.1 to 0.5 Mol/L).
Results. In liquid-solid TLC of protic ammonium salts on silica gel with organic-aqueous acid solutions (HA), the partition number as "capacity factor" is inversely proportional to the anion concentration c(A) up to the defined limit value:

(1 - Rf)/Rf = c(0)/c(A)        (Eq. 1)

With monovalent ammonium ions and monovalent anions A, c(0) represents the (hypothetical) counter-ion concentration for Rf = 0.5 or for partition number = 1.
For large counter-ions (perchlorate) the concentration limit for the validity of the Eq. l is reached sooner (below 0.3 Mol/L) than it is for small counter-ions (chloride; 0.5 Mol/L).
The term c(0) expresses the adsorption constant of ion-pair LSTLC, while the partition of analytes between the mobile and stationary phase is expressed by the inverse value K (L/Mol). Growth in the counter-ion diameter is accompanied by decreasing retention of the drug cation.
Conclusions. The (largest) counter-ion poorest in hydrophily associated with the drug cation in the least polar mobile phase will stay shortest time on the stationary phase. The chromatography of amines with acids in the mobile phase can be interpreted a reversible reactive separation in a wider context.