J Chromatogr A 1653, 462442 (2021). Samples were peptides obtained through tryptic hydrolysis of the 5 most abundant milk proteins: α-lactalbumin (α-LA), β-lactoglobulin (β-LG), α-, β- and κ-casein (CA). As standards, synthetic whey and pea (Pisum sativum, Fabaceae) peptides (selected based on the in silico tryptic digest of α-LA, β-LG, legumin A, and vicilin with one or zero miscleavages) were only used in the last assay for prediction of the RF values of peptides with known amino-acid (AA) sequences. Two-dimensional HPTLC on silica gel (pre-washed with methanol and activated 10 min at 100°), first with basic mobile phase sec-butanol – pyridine – ammonia – water 39:34:10:26, and (after 12h drying) in the orthogonal direction with acidic mobile phase sec-butanol – pyridine – acetic acid – water 11:8:2:5. Derivatization for peptides and proteins by immersion into fluorescamine (0.05 % in acetone); visualization under UV 254 nm and 365 nm. Computer-assisted determination of the x- and y-coordinates of the derivatized zones. Repeatability (n=8) of the 2D-HPTLC was statistically tested with the Kolmogorov-Smirnov test for normal distribution and with Dixon’s Q test for outliers. Relative standard deviation (RSD) for the RF values was 12.9 % for the first dimension (y-coordinates) and 16.5 % for the second dimension (x-coordinates). According to their higher intensity and sharpness, 15 – 20 detected zones from each protein hydrolyzate were selected, manually scraped from the derivatized layer, dissolved in formic acid solution (0.1 % in acetonitrile – water 3:2), mixed with an equal volume of matrix (dihydroxybenzoic acid 2 % in acetonitrile – water 3:7), crystallized on air on a ground steel target, before being desorbed by the laser beam of the MALDI-TOF-MS/MS (matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry). Direct hyphenation of HPTLC to MS was not performed, to avoid zone diffusion during plate coating with the matrix and to circumvent the stronger binding of polar peptides on the layer.  The MS spectra were acquired in positive reflector mode in m/z range 340 – 4000 (10 – 2500 for fragments), using an external peptide as calibration standard. Identification of 51 from the 85 selected peptides according to AA sequences was performed, using software programs allowing m/z calculation of protein fragments and estimation of cleavage sites. Correlation of the retention behaviour of the peptides with their properties (molecular weight MW, isoelectric point IEP, charges, polarity) was tested with Student’s two-sided t-test after calculation of Pearson’s correlation coefficients. The correlation was significant with IEP, percentages of anionic AA and of non-polar AA; but not with the following properties: MW, percentages of cationic AA and of uncharged polar AA. Finally, based on the correlation results, regression formulas were found to calculate the x- and y-coordinates of any known peptide from the percentage of non-polar AA (or vice-versa). The prediction power of these formulas was verified by repeating the complete 2D-HPTLC-MS experiment with the standard peptides of whey and of peas, and measuring the absolute and relative deviations between the actual x- and y-coordinates and the predicted values. The absolute deviations were higher in the lower RF zones. The average, relative RF value deviations (range 22.1 – 25.7 %) were not different between whey and pea peptides.

J. Liq. Chromatogr. Relat. Technol. 44, 820-828 (2021). HPTLC of Gentiana extracts from in vitro cultures on silica gel with acetate - methanol - water 4:1:1 in sandwich mode. Detection under UV light at 254 nm and fluorescence at 312 nm (emission above 370 nm). Principal component analysis (PCA), hierarchical cluster analysis and the novel proposal—nonnegative PCA was performed to identify common and distinct features.

J Chromatogr Sci, 60 (4), 372-386 (2022). Investigation of the dual retention mechanism in TLC taking place on three stationary phases of different polarity (C-18, silica gel and DIOL)  using binary mobile phases composed of acetonitrile as the main component and water, or methanol as a modifier. The test analytes were 12 compounds of pharmaceutical importance and considerably different chemical structure, i.e. the imidazoline and serotonin receptor ligands, and their related compounds. Determination of retention of each analyte in each investigated chromatographic system in a wide enough range of the mobile phase composition, with volume fraction of the mobile phase modifier ranging from 0.10 to 0.90. Calculation of the exact turning point values as a proof of occurrence of the reversed-phase hydrophilic interaction chromatography (HILIC/RP) retention mechanism based on the multimodal retention model. Analysis of the dual retention mode with the use of the volume fraction of the mobile phase modifier, the total polarity and the total solubility models, allowing the dual (HILIC/RP) retention mechanism for the DIOL, C-18 and silica gel stationary phase to be confirmed. The observed retention mechanism was more complicated than the dual HILIC/RP one in the case of the DIOL stationary phase and acetonitrile/methanol mobile phase.

Acta Chromatographica 24 (4), 563-573 (2008). TLC of nine alkaloids on RP-18 with mixtures of aqueous acetone or aqueous dioxane with various mobile phase additives (ammonia, diethylamine, or tetrabutylammonium chloride) in order to suppress ionization of the alkaloids and/or reduce ionic interactions with surface silanol groups. Ion-pair TLC on RP-18 with aqueous acetone mixtures and various mobile phase additives (pentane sulfonic acid, octane sulfonic acid, or di-(2-ethylhexyl)orthophosphoric acid). Investigation of relationships between RM values and modifier concentration using a linear semilogarithmic equation for experimental data to calculate lipophilicity values RMW (RM for pure water), the slope, and the intercept with the x -axis. Comparison with retention data of standards with known lipophilicity (log P).

J. Chromatogr. A 1571, 223-230 (2018). Evaluation of lipophilicity of 11 representative drugs, including six proton pump inhibitors (omeprazole, pantoprazole, rabeprazole, lansoprazole, ilaprazole, and tenatoprazole), an anti-vertigo drug, betahistine, nonsteroidal anti-inflammatory drug, ibuprofen, anti-malarial drug, atovaquone, an anti-HIV agent, atazanavir and a hormonal drug, calcitriol, by employing NP and RP separation modes. Investigation of the effect of different organic modifiers for the estimation of lipophilicity. Estimation of the quantitative descriptor of lipophilicity, the partition coefficient (logP) by suitably optimizing the solvent systems for normal and reversed phase TLC. The best mobile phase pairs were toluene – acetonitrile and water – methanol, respectively. Analysis of the dominant pattern in the data by using principal component analysis, hierarchical cluster analysis, as well as non-parametric methods like sum of ranking differences and generalized pair wise correlation. The results obtained from both separation modes were comparable and in good agreement with the computational data for all the drugs.

Kontakte (Darmstadt) 36-39 (1990). Explanation of physical aspects of TLC with reasonable approximations. Discussion of differences between observed and true retention values. Proposal of a general approach for the estimation of correction coefficients (including some real examples). Explanation of physical aspects of TLC with reasonable approximations.

J. Planar Chrom. 4, 402-405 (1991). Study of the dependence of the eluent front migration velocity in TLC on the pore volume and particle size distribution of the sorbent. Proposition and experimental confirmation of an equation which relates the velocity to the parameters of the chromatographic system.

J. Chromatogr. 673, 113-124 (1994). TLC determination of lipophilicity parameters on the basis of thermodynamically true RM values. Discussion of the superiority of methanol as compared with other modifiers and its physico-chemical behavior. Influence of the solvent pH on the silanophilic effect. Extrapolation to modifier-free condition (RMW value) to diminish the contribution of polar interactions. Comparison of the advantages and disadvantages of non-linear extrapolation procedures with linear regression.