]) values at different temperatures have been compared for aromatic hydrocarbons with different functional groups in reversed-phase thin-layer chromatography with aqueous binary mobile phases containing methanol, acetonitrile, or tetrahydrofuran. Considerable retention and selectivity variations were observed among the systems investigated. Separation of the solutes at different temperatures can be advantageous, because of these selectivity changes and the shortening of the development time. A horizontal DS-chamber has been developed for performing chromatogram development with temperature control.
Reversed-phase TLC of aromatic hydrocarbons with different functional groups has been performed on RP18 of high and low coverage density, and on silanized silica, with binary mobile phases comprising mixtures of methanol, acetonitrile, or tetrahydrofuran with water. Comparison of the RM values obtained revealed distinctly diverse selectivity among the systems investigated. For all the stationary phases investigated the selectivity obtained with tetrahydrofuran-containing mobile phases was very different from those containing acetonitrile and methanol. With methanol- and acetonitrile-containing mobile phases and high coverage-density RP18 plates similar selectivity was obtained for solutes with proton-donor and proton-acceptor properties; the same mobile phases resulted in greater selectivity differences when used with lower coverage-density (RP18W) or silanized silica plates. The use of different modifiers can be advantageous for optimization of TLC separations with binary and ternary mobile phases, especially when tetrahydrofuran is used in place of methanol or acetonitrile, or vice versa.
Preparative thin-layer chromatography has been performed with silica gel as adsorbent and binary mobile phases (ethyl acetate-heptane and methyl ethyl ketone-heptane) in a horizontal DS chamber adapted for temperature control. The investigations were performed in the temperature range 20–40°C. A very significant increase in the efficiency of separation of test dyestuffs was achieved at higher temperature. The data are presented as densitograms.
Two-dimensional (2D) separation of eight amino acids has been achieved by HPTLC in the first dimension and pressurized planar electrochromatography (PPEC) in the second, orthogonal, dimension. Separation selectivity in PPEC was very different from that in HPTLC. This attribute of these systems enabled complete separation of the amino acids. Chromatography was performed on reversed-phase plates with acetonitrile-buffer mobile phase. PPEC was performed after complete prewetting the plate, including the sample zones.
Separation of 2,4-dinitrophenyl-5-l-valine amide derivatives of some amino acids (leucine, isoleucine, valine, asparagine, cysteine, tryptophane) L and D-enantiomers with high-performance thin-layer chromatography (HPTLC) and pressurized planar electrochromatography (PPEC) is presented. The separation mechanism in the PPEC technique involves two effects (partition and electrophoresis) whereas HPTLC system employs only one (partition). The separation process in both modes was carried out with the HPTLC RP-18 plates. Influence of the aqueous-organic mobile phase composition (acetonitrile concentration, buffer type, its pH) on migration distance of the solute zones is investigated and compared for both techniques. The PPEC system turns out to be more sensitive to buffer type and pH than HPTLC one. Additionally, the electrophoretic effect is responsible for different separation selectivity in PPEC system in comparison to that in HPTLC one. The retention of DL-diastereoisomer derivatives of amino acids is stronger in comparison with its LL-antipodes for both separation techniques. The statistic evaluation of the migration distance in PPEC and TLC systems shows similar RSD.
A horizontal DS chamber for TLC has been adapted for planar electrochromatography in closed system. A test dye mixture was separated on pre-wetted RP-8 and RP-18 chromatographic plates with acetonitrile-buffer as mobile phase. A potential of 2 kV was applied to a 10-cm plate to create the electric field. Two modes of electrochromatogram development were applied — in open and closed (pressurized) systems. The disadvantages of the open system — evaporation of the mobile phase from the plate and excessive flow of mobile phase to the surface of the adsorbent layer during development — were eliminated by use of the closed system. Highly reproducible retention was achieved during planar electrochromatography in the closed system when the adsorbent layer of the plate was pre-wetted and equilibrated with a solution of the mobile phase. Examples are presented of electrochromatograms obtained from the test dye mixture under different operating conditions. Separation efficiency in planar electrochromatography was higher than in conventional HPTLC.
A general equation for determination of relative position, Rpg
, of solutes chromatographed under conditions of stepwise gradient elution with one void volume of mobile phase in reverse-phase thinlayer chromatography has been applied to separation of ten component dye mixture. A satisfactory agreement between calculated (by computer program) and experimental values of Rpg
has been demonstrated. A device with the new horizontal developing chamber has been applied to the investigations. This device showed practical advantages in comparison to other chambers especially in respect of its application to stepwise gradient elution.