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 67, 1356-1364 (1984). TLC of synthetic oligodeoxyribonucleotides a) on silica with dichloromethane- methanol 9:1, b) on RP-18 with acetone - Et 3 NH(HCOO3 ) (pH 7) 3:1. Detection by UV. Also TLC of 5'-doxyribonucleotides on polyethyleneimine coated plates with 0.1 M Na-phosphat (pH 6.8) - (NH4)2S04-ethanol 100:60:2.

J. Planar Chromatogr. 8, 98-102 (1995). HPTLC of nucleotides and bases (adenine, adenosine, cytidine, cytosine, thymidine, thymine, uridine, uracil, and methyl uracil) on silica and cyanopropyl-, aminopropyl-, and diol-bonded silica with methanol - dichloromethane mixtures, with or without addition of aqueous ammonia, in different ratios. Detection under UV 254 nm. Identification by fast atom bombardment (FAB) and tandem mass spectrometry (MS-MS).

J. Liq. Chromatogr. Relat. Technol. 40, 50-57 (2017). HPTLC of six nucleobases, guanosine (1), guanine (2), cytosine (3), adenine (4), uracil (5), and thymine (6) in sea buckthorn leaves on silica gel with dichloromethane – methanol – formic acid 160:45:16. Detection at UV 254 nm. The hRF values for (1-6) were 17, 23, 32, 38, 64 and 73, respectively. The bands were eluted using a TLC–MS interface and subjected to electrospray ionization-mass spectrometry (ESI-MS)._x000D_

(Chinese): Chinese J. Pharm. Anal. (Yaowu Fenxi Zazhi) 8, 316-317 (1988). TLC of propylthiouracil on silica with 1) chloroform - isopropanol - acetic acid 50:6:1, 2) cyclohexane - ethyl acetate 2:8, 3) benzene - ethyl acetate 1:1, 4) hexane - acetone - ethanol 60:20:2. Detection under UV 254 nm and by exposing to iodine vapor.

Chinese J. Adv. in Biochem. & Biophys. (Shengwu Huaxue Yu Shengwu Wuli Jinzhan) 20, 134-136 (1993). TLC of oligonucleotides on silica with isopropanol - water - conc. NH3 6:3:1. Quantification by densitometry (absorbance) at 260 nm. Comparison of results with those obtained by UV spectrophotometry.

J. Planar Chromatogr. 12, 63-65 (1999). Determination and quantification of impurities and synthetic by-products in theophylline by videodensitometry and classical TLC scanning densitometry, in each case using equipment from two different manufacturers. No significant differences of the performance of the two scanners were found in terms of detection limit (approx. 24 ng for Scanner I and 23 ng for Scanner II) and precision (variation coefficients 3.2 and 4.1% resp.) Detection by videodensitometry is possible for concentrations larger than 25 ng/spot; quantification requires approx. 100 ng/spot. The limits of impurities in pharmaceutical drugs set by Ph Eur - usually 0.1% - could not be quantified by videodensitometry at its current stage of development. - Regression curves show average variation coefficients of approx. 17.4 for videodensitometer I and 5.9% for videodensitometer II. The use of videodensitometry for impurity testing is nevertheless useful in special cases where higher deviations are considered acceptable for assays, and when higher concentrations are available for quantitation of the main component.