J Chromatogr A 1638, 461830 (2021). The purpose was to find the first universal HPTLC mixture (UHM), a mixture of reference compounds that could be used for the system suitability test (SST) for the full RF range in all HPTLC experiments.
(Part 1) UHM composition: First, 56 organic molecules, detectable without derivatization, were tested on HPTLC silica gel with 20 different mobile phases (MP) belonging to different Snyder’s selectivity groups and with several polarity indices. Visualization under UV 254 nm and 366 nm. Densitometry scanning at 254 nm in absorption mode, and at 366 nm in a fluorescence mode (mercury lamp 366 nm, with wavelength filter <400 nm). For selected bands, spectra were recorded in absorbance-reflectance mode (wavelength range 190 – 450 nm, deuterium and tungsten lamp). This procedure allowed 8 molecules to be selected for their better spot resolution and for their specific RF values (at least 3 different values distributed throughout the full RF range for each MP). The final composition of UHM was: thioxanthen-9-one (0.001 %), guanosine (0.05 %), phthalimide (0.2 %), 9-hydroxyfluorene, octrizole, paracetamol, sulisobenzone and thymidine (each 0.1 %), in methanol.
(Part 2) UHM validation: Afterwards, UHM was submitted again to a panel of HPTLC assays with always two MP: (A) toluene – methanol – diethylamine 8:1:1; (B) ethyl acetate – formic acid – water 15:1:1; and for each MP, the means, standard deviation and 95 % confidence intervals of the RF values were calculated. (a) UHM was validated for intermediate intra-laboratory precision, as well as for inter-laboratory reproducibility, with ΔRF 0.045. (b) The capacity of UHM to detect small variations was demonstrated by significant changes in at least some RF values, when separation was deliberately performed at different levels of relative humidity (0 %, 33 %, 75 %, 100 %), or with smaller humidity variations (7 % compared to 0–5 %, and 49 % compared to 33 %), or when performing vs. omitting the 10min chamber pre-saturation, or when modifying the MP (+/-10% of one solvent at each time). These response characteristics (the opposite of robustness) made UHM a powerful tool for SST. (c) Finally, UHM stability was studied with UHM aliquots under several storage conditions (-78 °C, -20 °C, 4 °C, room temperature, 45 °C; or 40 °C with 75 % relative humidity) and durations (2 weeks or 2 months). The densitometric peak profiles at 254 nm were compared to those of the fresh compounds, qualitatively (RF value, UV spectrum) and quantitatively (peak area). UHM was stable at room temperature or below, for 2 months (at higher temperature, guanosine, phthalimide and paracetamol degraded).

J. Liq. Chromatogr. Relat. Technol. 41, 15-16 (2019). HPTLC of thymine (1), uracil (2), adenine (3), cytosine (4), guanine (5) and guanosine (6) in Ganoderma lucidum and Cordyceps sinensis on silica gel with dichloromethane - methanol - formic acid 160:45:16. Quantitative determination by absorbance measurement at 254 nm. Identification of nucleobases in the samples was reconfirmed by hyphenated HPTLC-MS. The hRF values for (1) to (6) were 83, 73, 46, 32, 23 and 10, respectively. The intermediate precision was below 5 % (n=3). 

Phytochemistry. 119, 51-61 (2015). HPTLC of cochloxanthine (1) and dihydrocochloxanthine (2) in Cochlospermum species on silica gel with acetone - hexane 1:1. Detection by spraying with anisaldehyde reagent, followed by heating at 110 °C for 5-10 min. Identification under UV light at 254 nm and 366 nm. The hRF values for (1) and (2) were 53 and 49, respectively.

J. of Medicinal Chemistry 31, 1575-1579 (1988). TLC of 2-[(2,2-diethoxyethyl)thio]acetamide and 4-[2,3, 5-tri-O-benzoyl-ß-D-ribofuranosyl]-1,4-thiazin-3-one on silica with hexane - ethyl acetate 90:1 and hexane - ethyl acetate 85:15. Visualization under UV or by spraying with ethanol - p-anisaldehyde - sulfuric acid 100:1:1.

J. Chromatogr. 612, 57-61 (1993). TLC of nucleotide glycation reaction products on silica with propanol - methanol - NH3 - water 3:1:1:1. Detection under UV 254 nm. Also HPLC.

J. Planar Chromatogr. 10, 416-419 (1997). Complete validation of an HPTLC method for the quantitative determination of caffeine in Coca-Cola has been performed according to ICH guidelines, taking into consideration the special features of the method. The validation parameters were: selectivity, stability of the analyte, linearity, precision, (repeatability and intermediate precision), and robustness. Quantification with a slit-scanning densitometer and with an image-analyzing system. The same plates were used in both validation procedures to reveal possible differences between the different modes of quantification. The results obtained show no difference in validation parameters - because all the results are within the range of criteria recommended for TLC validation. Both systems can be used for quantitative evaluation of TLC plates.

CBS 113, 9 (2014). HPTLC of caffeine in energy drinks (applied neatly without sample preparation) on silica gel MS-grade with 2-propanol – n-heptane – water 7:3:1. Detection under UV 254 nm and by dipping into anisaldehyde reagent. Quantitative evaluation by absorbance measurement at 273 nm. The hRf of caffeine was 55. For confirmation of results, zones were eluted with the TLC-MS Interface (acetonitrile – water 19:1 with 0.1 % formic acid, flow rate 0.1 mL/min) and directly transferred to a mass spectrometer and measured in ESI(+) full scan mode (m/z 100 – 500).

Biologically active compounds from the marine sponge Geodia baretti. J. of Natural Products 51, 1277-1280 (1988). TLC of nucleosides on silica with ethanol - NH3 (25%) 8:2 or chloroform - methanol 1:1. Visualization under UV 254 nm and 366 nm; also by spraying with ninhydrin or vanillin-sulfuric acid.