Application of Densitometry for the Evaluation of the Separation Effect of Nicotinic Acid Derivatives. Part II. Nicotinic Acid and its Esters

Abstract

The nicotinic acid (1) and its esters, namely methyl nicotinate (2), ethyl nicotinate (3), isopropyl nicotinate (4), butyl nicotinate (5), hexyl nicotinate (6), and benzyl nicotinate (7) were investigated by NP‐TLC, and RP‐HPTLC. The RF values were obtained from the densitometric analysis. The separation factors ΔRF, RF α and selectivity α were calculated from the RF values. The comparison and characteristic of chromatographic bands of the examined compounds were presented on the basis of calculated resolutions: RS(c), RS(b), RS(h), and RS(a). The resolutions of substances were determined by visual method (RS(c)) and densitometric method (RS(b), RS(h), and RS(a)). It was affirmed that the densitometric method is correct and is 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 RP18WF254 plates, and by use of dioxane‐water in a volume composition of 50:50. However, butyl nicotinate from benzyl nicotinate cannot be separated by RP‐HPTLC. Good separations of isopropyl nicotinate from ethyl nicotinate, as well as ethyl nicotinate from methyl nicotinate, were also not obtained by RP‐HPTLC. It was affirmed, that adsorption thin‐layer chromatography (NP‐TLC) in the system of a neutral aluminum oxide 60F254 and the acetone‐n‐hexane mobile phase in a volume composition of 20:80 provided the optimum conditions for the complete separation of butyl nicotinate from benzyl nicotinate. These chromatographic conditions also allow better separations of methyl nicotinate from ethyl nicotinate, as well as ethyl nicotinate from isopropyl nicotinate, in relation to the separations of these compounds by RP‐HPTLC.