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). 

Helv. Chim. Acta 65, 2372-2393 (1982). TLC of numerous protected deoxynucleotide intermediates on silica with chloroform - methanol 9:1. Detection by UV and by exposure to chlorhydric acid vapors to visualize 4,4-dimethoxy-tritil-containing products.

J. Chromatogr. 348, 296-298 (1985). TLC of the title compounds on silica with 2-propanol -25 % NH3 - ethanol 6:3:1. Detection by UV 254 nm. Nucleotides with 2 negative charges did not migrate.

(Dünnschichtchromatographie mittels Dioden-Array Detektion). GIT Spezial Separation I/2001, 37-39 (2001). Development of a HPTLC scanner for the measurement of spectra between 198 and 612 nm using a diode array detector. Quantitative HPTLC of caffeine on silica gel with isopropanol - cyclohexane - 25% NH3 7:2:1 and of uric acid, cytosine, guanine, caffeine, adenine, uracil and thymine on silica gel with isopropanol - toluene - 25% NH3 6:3:1. Measurement of 450 single spectra between 200 and 400 nm within 225 s.

J. Chromatogr. 325, 317-322 (1985). TLC of tritiated adenosine and its derivatives on silica with butanol- acetone - acetic acid -5 % NH3 - water 9:3:2:2:4, butanol - acetone - acetic acid - water 11:3:3:3 and numerous other similar solvent systems. Scintillation counting after scraping the sample from the plate or direct radio scanning of the plate.

J. of Medicinal Chemistry 32, 1307-1313 (1989). TLC of phosphonate analo- gues an silica with dichloromethane - acetone 3:2, methanol - chloroform 1:9 and isopropanol - NH3 - water 7:1:2. Detection under UV and by spraying with 10% sulfuric acid in methanol followed by heating.

J. Planar Chromatogr. 8, 145-147 (1995). 2D-OPLC of 16 barbiturate derivatives including the twelve which are subject to international control on HPTLC silica with 7 different mobile phases. Densitometry at 250 nm (absorbance).