Application of densitometry for the evaluation of the separation effect of nicotinic acid derivatives. Part III. Nicotinic acid and its derivatives

Nicotinic acid (1) and its derivatives, namely nicotinamide (8), 3-pyridinecarbaldehyde (11), 3-pyridinecarbonitrile (12), 3-pyridylmethanol (13), and methyl 3-pirydyl ketone ( 14) were investigated by NP-TLC, and RP-HPTLC. The RF values were obtained from the densitometric analysis. The separation factors Delta R-F, R-F(alpha), and selectivity a were calculated from the R-F values. The comparison and characteristic of chromatographic bands of the examined compounds were presented on the basis of calculated resolutions: R-S(c), R-S(b), R-S(h), and R-S(a). The resolutions of substances were determined by visual method (R-S(c)) and densitometric method (R-S(b), R-S(h), and R-S(a)). It was affirmed that the densitometric method is correct and the standard method to determine the above mentioned parameters. Furthermore, the R-S parameter determined by the visual method for two adjacent substances is always larger than determined by the densitometric method. It was affirmed, that the best separation of the studied compounds was obtained by the RP-HPTLC technique on RP18WF(254) plates, and by use of dioxane-water in a volume composition of 20 : 80. However, 3-pryridinecarbaldehyde (11) from 3-pryridinecarbonitrile (12) cannot be separated by the RP-HPTLC technique. It was also affirmed, that adsorption thin-layer chromatography (NP-TLC) in the system of a silica gel 60F(254) and the acetone-n-hexane mobile phase in a volume composition of 80 : 20 provided the optimum conditions for the complete separation of 3-pyridinecarbaldehyde (11) from 3-pryridinecarbonitrile (12).