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). 

IPA Convention, 2010, RA-PO 26. HPTLC of risedronate sodium on silica gel with water - methanol - 25 % ammonia 20:3:3. Densitometric evaluation at 262 nm. The method was linear in the range of 3-6 ng/band, recovery was 98.6 %. Results were comparable with HPLC results. The advantage of the HPTLC method is the simultaneous analysis of several samples.

Chromatographia 20, 683-684 (1985). TLC of some purines on silica with iso-butanol MEK - NH3 - water 8:5:4:3 and propanol - NH3 water 16:3:1. Visualization by spraying with 1 % aqueous uranyl acetate and irradiating with UV at 254 or 366 nm. Detection limit 0.01/mg.

J .Chromatogr. Sci., 29, 210-216 (1991). Determination of the retentions of 21 5-substituted deoxyuridine derivatives on cyano-, diol- and amino-bonded silica using normal phase (dichloroethane – methanol and acetonitrile – ethyl cellosolve mixtures) and reverse-phase (water – methanol mixtures). Separation of the retention strengths (potencies) and selectivities by the spectral mapping techniques, and correlation of the potencies of the solutes with the physico-chemical parameters by using stepwise regression analysis. Discussion of the retention strength and retention selectivities of the systems for nucleoside derivatives.

J. Planar Chromatogr. 9, 31-34 (1996). HPTLC of adenine, AMP, hypoxanthine, IMP, adenosine, cAMP, inosine, xanthine on silica with water - methanol - NH3 (25%) - trichloromethane 2:9:1:9. Detection by in-situ FTIR spectroscopy.

J. Sep. Sci. 33, 3794-3799 (2010). HPTLC of caffeine (1), theobromine (2) and theophylline (3) in different types of tea on silica gel with chloroform - dichloromethane - isopropanol 4:2:1. Quantitative determination by absorbance measurement at 254 nm. The hRF of (1), (2) and (3) was 65, 45 and 56, respectively. Limits of detection and quantification were 22 and 45 ng for (1), 23 and 46 ng for (2) and 22 and 43 ng for (3), respectively. The intra-day and inter-day precisions had a %RSD lower than 2.55 % (n=6) for all substances. Recoveries (by standard addition) were between 95.1-101.5 % for all the three methylxanthine derivatives. The values of LOD and LOQ obtained are similar with those obtained by HPLC.

J.A.O.A.C. 69, 499-503 (1986). Improvement of a spray reagent for TLC identification of uric acid: aqueous solution of ferric chloride and potassium ferricyanide, requiring neither heating nor pH control. Comparison of specificity and sensitivity to 5-10 ng of uric acid with those of other sprays. The false negative rate (at 95 % confidence interval) is 0-9.7 % for this spray compared with 0.7-18.7 % for the others.