Concentration Effects

In our experience, one of the most important factors in making IP-TLC effective is the concentration of the

Figure 1 The distribution of IP reagent (tetrabutylammonium bromide) following chromatography as detected using iodine vapour. Shaded areas show the highest concentrations of iodine and thus ion-pair reagent. 2.3, 2.4, 2.5, 2.6, 3.4 and 3.5 are the dihydroxy-benzoic acids used as model compounds.

Concentration (mol L )

Figure 2 Effect of the concentration of tetrabutylammonium bromide used to impregnate C2-bonded reversed-phase TLC plates on the RF values of a range of dihydroxybenzoic acids.

Concentration (mol L )

Figure 2 Effect of the concentration of tetrabutylammonium bromide used to impregnate C2-bonded reversed-phase TLC plates on the RF values of a range of dihydroxybenzoic acids.

reagent employed. This is shown in Figure 2 for the reagent tetrabutyl ammonium bromide used for a range of dihydroxybenzoic acids on C2-bonded reversed-phase (RP)-TLC plates. As this figure shows, with methanol-water (1 : 1v/v) as mobile phase when no reagent is present the test compounds all have Rf values in the range 0.8-0.9. Prechromato-graphic impregnation of the phase with a solution of the reagent in dichloromethane-ethanol (85 : 15 v/v) at 0.001 mol L_1 was virtually without effect. However, increasing the concentration of the impregnating solution to 0.05, 0.01, 0.5 and 0.1 mol L^1 produced significant reductions in RF that increased with increasing reagent concentration. Above 0.01 mol L_1 the reduction in RF observed does not dramatically increase with increasing concentration.

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