Application of model systems in TLC

The separation of synthetic red pigments has been optimized for HPTLC separation. The structures of the pigments are listed in Table 3.1. Separations were carried out on silica HPTLC plates in presaturated chambers. Three initial mobile-phase systems were applied for the optimization A = n-butanol-formic acid (100+1) B = ethyl acetate C = THF-water (9+1). The optimal ratios of mobile phases were 5.0 A, 5.0 B and 9.0 for the prisma model and 5.0 A, 7.2 B and 10.3 C for the simplex model. The parameters of equations describing the linear and nonlinear dependence of the retention on the composition of the mobile phase are compiled in Table 3.2. It was concluded from the results that both the prisma model and the simplex method are suitable for the optimization of the separation of these red pigments. Multivariate regression analysis indicated that the components of the mobile phase interact with each other [79], 

The hydrophobicity of some monoazoic dyes was determined by RP-TLC. The chemical structure of analytes are depicted in Fig. 3.1. RP-TLC plates were prepared by impregnating silica plates for 24 h in a hexane-paraffin oil (90 10, v/v) mixture. Mobile phases consisted of methanol-0.5 M HC1 mixed in various volume ratios. Methanol concentration varied between 30-60 per cent in steps of 6 and 3 per cent. The RM value characterizing molecular hydrophobicity was calculated by 

The RP-TLC behaviour of some common food dyes was investigated in detail. The chemical structure of dyes are listed in Fig. 3.2. Measurements were carried out on RP-18 silica plates using aqueous ammonium sulphate (0.1 0.5 1.0 M), ethanol and acetone in various volume ratios. Developments were performed at room temperature (22 2°C) in chambers previously saturated with the vapours of the mobile phase. It was found that the presence of dissociable anorganic salt modifies markedly the RP retention behaviour of dyes. The retention of dyes generally decreases with increasing concentration of the organic modifier in the mobile phase. It was further concluded that RP-TLC can be successfully used for the separation of this class of synthetic food dyes [81]. 

Similar investigations were carried out using 2-propanol as the organic modifier instead of 1-propanol. The parameters of linear regression analyses are compiled in Table 3.5. It has been concluded from the results that the retention on both stationary phases is of mixed character hydrophilic, electrostatic and steric molecular parameters are equally involved in the retention [83], 

The same RP-TLC investigations were carried out by using ethanol, THF and dioxane as organic modifiers. The parameters of regression analyses are compiled in Table 3.6. It was established that the type of organic modifier exerts a negligible effect on the lipophilicity but influences considerably the specific hydrophobic surface area of the dyes. 

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