J Chromatogr A 1666, 462863 (2022). Theoretical discussion on the factors determining the RF value of a given substance in a chromatographic system: A) the stationary phase (SP); B) the mobile phase (MP), the composition of which can be different from the solvent mixture prepared because of evaporation, saturation and liquid or gas adsorption effects over migration time; C) the difference of the free energies for the analyte transfer from SP to MP; D) external parameters like temperature and humidity. The universal HPTLC mixture (UHM) is a mixture of reference compounds that can be used for the system suitability test (SST) for the full RF range in all HPTLC experiments. Its composition is: thioxanthen-9-one (0.001 %), guanosine (0.05 %), phthalimide (0.2 %), 9-hydroxyfluorene, octrizole, paracetamol, sulisobenzone and thymidine (each 0.1 %), in methanol. The purpose was to study the potential of UHM to replace SST (described with specific markers in European Pharmacopoeia monographs) and to assess the quality of HPTLC results. TLC and HPTLC silica gel on different support (aluminium, glass) or with different granulometries and binders (classic, Durasil, Adamant), of the UHM, an acetonitrile extract of Abelmoschus manihot flowers (Malvaceae), a methanol extract of Sambucus canadensis flowers (Adoxaceae), and essential oils of Lavandula angustifolia, of Mentha × piperita (Lamiaceae) and of Myristica fragrans (Myristicaceae), as well as the following specific markers (standards): borneol, bornyl acetate, linalool, linalyl acetate (terpenoids), isoeugenol, isoeugenol acetate, chlorogenic acid (phenylpropanoids), gossypin (flavone), gossypetin-glucuronide, hyperoside (flavonol heterosides). Development (after 20 min plate conditioning with a saturated MgCl2 solution) with one of the following mobile phases: (MP1) toluene – ethyl acetate 19:1, especially for essential oils; (MP2) ethyl acetate – butanone – formic acid – water 5:3:1:1, especially for S. canadensis; (MP3) ethyl acetate – acetic acid – formic acid – water 100:11:11:26, especially for A. manihot. Documentation in UV 254 nm and 350 nm, and with white light (reflection + transmission), before and after derivatization. RF values were determined by scanning densitometry at 254 nm in absorption mode (for octrizole, at 366 nm in fluorescence mode with mercury lamp and optical filter K400 nm). For each HPTLC condition, intra-laboratory precision assay of UHM separation was performed (at least 5 analyses) with average RF values and 95 % prediction intervals, and calculating RF differences between pairs of UHM constituents and 95 % confidence intervals, which were max. +/-0.012 of the RF values for all UHM and markers. The sensitivity of UHM, and thus its usefulness as generic SST was demonstrated by repeating the HPTLC experiments with modifying by 10 % the quantity of one of the solvent each time. There were always significant changes in RF values of UHM components and/or in RF differences between pairs of UHM bands; it was often but no always the case with the official specific markers. UHM underwent also significant changes (although less than A. manihot extract) when several silica gel phases were compared under the same HPTLC conditions. This property is crucial to verify the right stationary phase before doing any RF correlations, and could make UHM a universal tool to identify discrepancies between different analyses. Finally, the use of UHM for a computer-supported evaluation of HPTLC results was discussed, either for zone identification and RF corrections (within confidence intervals), or for correlations of entire fingerprints as first step to implement machine learning algorithms.

Drug Standards of China 22 (3), 259-264 (2021). Gentianella acuta (Michx.) Hulten is a herbal traditional Chinese medicine, containing mainly efficacy components like diphenylpyrione, cycloether terpenoids, flavonoids, and triterpenoids. It has liver protection, hypoglycemic, anti-inflammatory and other pharmacological activities, and is used clinically to treat jaundice, headache, fever, dry mouth and bile fever etc. To establish a quality standard of the herb, TLC was used for the investigation of the chemical composition and fingerprints. TLC of methanolic extracts of 10 batches of Gentianella acuta  collected from different regions (A) for cycloether terpene components (gentiopicroside and swertimarin), on silica gel with ethyl acetate - methanol - water 4:1:1, detection under UV 254 nm, identification by comparison of the fingerprints with those of the standards gentiopicroside and swertimarin; (B) for terpenoids (oleanolic acid), on silica gel with chloroform - methanol - ammonium hydroxide 20:6:1, detection under UV 254 nm, identification by comparison of the fingerprints with those of the oleanolic acid standard; (C) for aqueous extracts (water-soluble components such as flavonoids and phenolic acids), on silica gel with 1-butanol - acetic acid - water 9:3:2, detection by spraying with 5 % aluminium trichloride solution and evaluation under UV 366 nm, identification by comparison of the fingerprints with those of the oleanolic acid standard. The results showed that the TLC profiles of 10 batches were very similar, and well consistent with the HPLC fingerprint results. In addition, gentiopicroside, swertimarin and oleanolic acid were identified by TLC in the medicine, thus can be used as the target components of the identification. Therefore, the results of this study can be used as the basis for the authenticity identification and quality evaluation of the medicine.

J. Planar Chromatogr. 34, 113-120 (2021). HPTLC of thymol (1), carvacrol (2), α-terpineol (3), t-anethole (4), safrole (5), p-anisaldehyde (6), fenchone (7), quercetin (8), kaempferol (9) and quercitrin (10) in common thyme essential oil, star anise essential oil and acerola fruit extract on silica gel with toluene - ethyl acetate 17:3 and methanol - water 1:1. The hRF values for (1) to (10) were 24, 22, 3, 79, 83, 21, 46, 53, 27, 18 and 44, respectively. The OPLC methods were  time- and solvent-saving in comparison with conventional TLC.

J. Planar Chromatogr. 32, 205-210 (2019). HPTLC fingerprint of essential oils from five sandalwood species, namely, Santalum album, Santalum spicatum, Santalum austrocaledonicum, Santalum paniculatum, Santalum lanceolatum on silica gel with toluene - ethyl acetate 17:3. Detection by spraying with p-anisaldehyde sulfuric acid reagent. Qualitative identification under UV light at 254 and 366 nm. The hRF value of α-bisabolol was 48. 

J. Planar Chromatogr. 30, 121-125 (2017). Overpressured TLC of essential oil components of clove, rosemary, eucalyptus, tea tree, spearmint, thyme, and cinnamon on silica gel with toluene. The conditions in infusion mode were as follows: 50 bar, external pressure; 350 μL, rapid mobile phase flush; 500 μL/min, mobile phase flow rate; 3900 μL, mobile phase; 475 s, development time. The hRF values were 8 for α-terpineol, 12 for borneol, 17 for terpinen-4-ol, 20 for 1,8-cineole, 22 for R(–)-carvone, 31 for trans-cinnamaldehyde, 36 for eugenol and 43 for thymol. Antibacterial and antioxidant activities were also detected by infusion–transfusion OPLC hyphenated with Aliivibrio fischeri assay and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical assay, respectively.

J. of Natural Products 51, 314-318 (1988). Centrifugal preparative TLC of sesquiterpenes on silica with benzene - acetone 4:1 and chloroform - methanol 19:1.

(Leaves of sage und three-lobed leaves of sage). Deutsche Apotheker Zeitung 130, 169-170, (1990). TLC of a,ß-thujon, cineol, campher, borneol and bornylacetate (leaves of sage), cineol, a,ß-thujon, campher, borneol, bornylacetate, caryophyllen, epoxy-dihydrocaryophyllen (three-lobed leaves of sage) on silica with toluene – ethyl acetate 95:5. Detection by spraying with anisaldehyde – sulfuric acid reagent, heating at 100-105°C for 5 min, observation under day light or UV 365 nm.

J. Liquid Chromatogr. 18, 105-115 (1995). TLC of the title compounds on silica with benzene - isopropyl ether 1:1. Detection under UV 254 nm. Identification of the eluted spots by GC-MS. Quantification by densitometry at 254 nm.