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). 

Anal. Biochem. 409 (2), 249-259 (2011). Presentation of a less time-consuming, more sensitive, and more precise method for the quantitative determination of nucleoside triphosphates (NTPs), 5-ribosyl-1-pyrophosphate (PRPP), and inorganic pyrophosphate (PPi) in cell extracts by TLC: Separation of an acid extract of L. lactis by charcoal filtration into a filtrate and an eluate, which then was separated by TLC either in the Cashel solvent (0.85 M potassium phosphate, pH 3.4) or in the AFC solvent (3 M ammonium formate [pH 2.4] and 0.7 M ammonium chloride). Two-dimensional separation of 18 µL of the eluate sample using the AFC solvent in the first dimension and using 0.75 M LiCl in 7.5 % lithium borate (pH 6.8, borate solvent) in the second dimension.

Helv. Chim. Acta. 67, 1316-1327 (1984). TLC of CAMET and CASET active esters and their nucleotide derivatives on silica with a) acetone - water 8:2, b) chloroform - isopropanol - acetic acid 80:15:15, c) chloroform -methanol -pyridine -water 32:8:4:1, d) chloroform -methanol - triethylamine 90:5:5 and many other solvent systems. Detection by UV

Chromatographia 50, 547-552 (2000). Multi-dimensional ion-exchange TLC on PEI-cellulose TLC plate with sodium phosphate buffer (1M, pH 6.0) in direction D1 over night. After washing with water, ammonium formate, water and drying, elution in direction D2 (in the opposite to D1) with 3.5 M lithium formate, 8.5 M urea, pH 3.5. After washing with 1.3 mM tris, water and drying, elution with bugger 1.2 M sodium phosphate pH 6.0 over night. Detection by autoradiography at 80°C by use of intensifying screens. Comparison with HPLC.

J. Planar Chromatogr. 31, 57-60 (2018). HPTLC of azathioprine, caffeine, theobromine, theophylline and acyclovir on RP-18 W with acetonitrile - TRIS–HCl buffer pH 8.0 (1 mM) 1:4. It was neither possible to separate azathioprine from caffeine, nor theobromine from theophylline. Under the same conditions, pressurized planar electrochromatography with a polarization voltage of 2.5 kV showed better results, as all five purine derivatives could be separated. Detection under UV 280 nm._x000D_

Anal. Chem. 60, 2692-2694 (1988). Application of fluorescence line-narrowing spectrometry (FLNS) to a polycyclic aromatic hydrocarbon-nucleoside adduct sorbed on a TLC plate. Discussion of the effects of non-photo chemical hole burning and interfacing FLNS with the 32p-postlabeling procedure to provide for high sensitivity and selectivity analysis of TLC spots associated with fluorescing nucleoside adducts. Presentation of a calibration curve over >5 decades of concentration. Detection limit, 10 f mol.

Chinese J. Adv. in Biochem. & Biophys. (Shengwu Huaxue Yushengwu Wuli Jinzhan) 20, 234-237 (1993). TLC of synthetic oligonucleotides on silica with isopropanol - water - NH3 6:3:1, and 57:33:10. Visualization under UV 254 nm. Quantification after elution.