J. Planar Chromatogr. 36, 121-127 (2023). HPTLC of genistein in Flemingia procumbens on silica gel with toluene - ethyl acetate - acetone - formic acid 20:4:2:1. Quantitative determination by absorbance measurement at 271 nm. The hRF value for genistein was 49. Linearity was in the range of 50-300 ng/zone. Intermediate precisions were below 2 % (n=6). LOD and LOQ were 2 and 71 ng/zone, respectively. Average recovery was 99.6 %.

J. Planar Chromatogr. 36, 169-178 (2023). HPTLC of ellagic acid (1) and gallic acid (2) in Terminalia species (Terminalia arjuna, T. bellirica, T. chebula, and T. catappa) on silica gel with methanol - water - formic acid - acetic acid 20:25:1:1. Quantitative determination by absorbance measurement at 279 and 254 nm. The hRF values for (1) and (2) were 33 and 82, respectively. Linearity was in the range of 100 and 700 ng/zone. Intermediate precisions were below 1 % (n=3). LOD and LOQ were 80 and 240 ng/zone for (1) and 75 and 225 ng/zone for (2). Average recovery was between 95.5 and 98.4 % for (1) and (2).

Front. Nutr. 9, 887992 (2022). HPTLC of 13 mogrosides, 1 flavonoid, and 3 sugars in monk fruit products on silica gel with n-butanol - water - ethanol - acetic acid 70:10:10:2. Detection by spraying with 10 % sulfuric acid in ethanol solution, followed by heating at 105 °C for 10 min. Orthogonal partial least square-discriminant analysis (OPLS-DA) was subsequently conducted allowing the identification mogroside V, 11-oxo-mogroside V, isomogroside V, mogroside IV and sucrose to be the characteristic compounds to distinguish the two types of monk fruits.

J. Sep. Sci. 46, 2200921 (2023). Review of the techniques for the analysis of sodium-glucose linked transporter-2 (SGLT-2) inhibitors, including HPTLC for the determination of canagliflozin, dapagliflozin and empagliflozin.

J. Planar Chromatogr. 36, 63-70 (2023). HPTLC of quercetin (1), berberine (2), rutin (3) and curcumin (4) in a polyherbal formulation on silica gel with toluene - ethyl acetate - methanol - formic acid 10:6:4:1. Quantitative determination by absorbance measurement at 366 nm for (1) to (3) and 425 nm for (4). The hRF values for (1) to (4) were 57, 30, 9 and 66, respectively. Linearity was in the range of 500-3000 ng/zone for (1), 200-700 ng/zone for (2), 1000-6000 ng/zone for (3) and 100-350 ng/zone for (4). Intermediate precisions were below 2 % (n=6). LOD and LOQ were 119 and 362 ng/zone for (1), 33 and 100 ng/zone for (2), 248 and 750 ng/zone for (3) and 14 and 42 ng/zone for (4). Recovery was between 95 and 102 % for (1) to (4).

J. Planar Chromatogr. 35, 579-584 (2022). HPTLC of neohesperidin from Citrus aurantium peel extract on silica gel with ethyl acetate - methanol - water - formic acid 71:14:10:5. Quantitative determination by absorbance measurement at 254 nm. The hRF value for neohesperidin was 54. Linearity was between 1000 and 3000 ng/zone. Intermediate precisions were below 2 % (n=9). Recovery was between 99.6 and 101.8 %. 

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.

J Chromatogr A 1638, 461597 (2021). Samples were Isatis tinctoria (= I. indigotica) root extracts (Brassicaceae) and their fractions. Standards were oseltamivir acid (OA), a neuraminidase (NA) inhibitor; pinoresinol (PR, a lignan), β-sitosterol (SS, a sterol), and dihydro-neoascorbigen (DHNA, an alkaloid). HPTLC / TLC on silica gel with (1) petroleum ether – ethyl acetate – acetic acid 48:8:1 for petroleum ether extracts and SS, or 30:40:1 for ethyl acetate extracts, or 10:30:1 for PR; (2) with toluene – ethyl acetate – methanol – formic acid 16:3:1:2 or 10:4:1:2 also for ethyl acetate extracts and DHNA; (3) with n-butanol – acetic acid – water 25:4:3 for butanol extracts. OA was applied but not developed. RP-18, polyamide, cellulose, alumina layers were tested, but the resolution was lower. Derivatization by spraying with sulfuric acid (10 % in ethanol). Enzymatic assay by immersion of the plates into neuraminidase solution (6 U/mL), followed by 1 h incubation at 37 °C and by immersion into chromogenic substrate solution (1.75 mM 5-bromo-4-chloro-3-indolyl-α-D-N-acetylneuraminic acid). After 5 min, NA inhibitors were seen as white zones on blue background. The experiment was previously improved for the following parameters: incubation times, substrate and enzyme concentrations, followed by statistical evaluation and calculations using Box-Behnken design. Quantification by absorbance measurement (detection wavelength 605 nm, reference wavelength 420 nm). In optimal conditions, OA had LOD 300 ng/zone. Zones of interest on underivatized plates were directly submitted to MS, using EFISI (electrostatic-field-induced spray ionisation), as follows. Chromatograms were immersed 1–3 s into dimethicone – n-hexane 1:1 to form a hydrophobic film, and dried 30 min at room temperature; on the analyte spot, a hydrophilic droplet was formed with 5 µL methanol – water 1:1, extracting the analyte from the layer; the analyte was further attracted through a capillary tube (3–4 cm long, made of non-deactivated fused silica) under a strong electrostatic field, into the in-let orifice of the triple-quadrupole ­– linear ion-trap MS (induction voltage 4 kV; capillary voltage 40 V; tube lens voltage 100 V; capillary temperature 200 °C). Full-scan spectra were recorded in m/z range 50 – 1000, helium was used for collision-induced dissociation. 11 active compounds were identified in the extract: SS, 6 alkaloids (including cycloanthranilylproline, DHNA, hydroxy-indirubin, isatindigodiphindoside, isatindinoline A and), 3 lignans (including PR and isolariciresinol), 1 fatty acid (trihydroxy-octadecenoic acid).