LC-GC Europe 16, 2-5 (2003). Overview of utility of the technique for the identification and quantification of pharmaceutical compounds and related substances such as UK-137,457 (C31H31NO5) and UK-124,912 (C27H25NO3). Several of the issues that have arisen in the development of TLC-MALDI-MS methods for the successful analysis of pharmaceuticals were adressed in this article. Electrospray deposition method, which was found to be superior to other methods studied and was successfully applied to a range of compounds presented, concerns a method for the deposition of the MALDI matrix onto the TLC plate. Post-source decay analysis can be performed directly on the TLC spots, to aid in structural evaluation of the analyzed compounds. The generation of quantitative data using a structural analogue as internal standard and incorporation into the mobile phase has also been demonstrated. Outlook for next step development of TLC-MALDI-MS in pharmaceutical analysis for more widespread use in industry: enhance sensitivity, mass resolution and reproducibility, availability of commercial instruments that allow the scanning of whole TLC plates rapidly with data-imaging software.

Anal. Chem. 81, 9274-9281 (2009). TLC of dye standards (FD&C Green No. 3, FD&C Red No. 3, eriochromcyanin R), drug standards (3,4-methylene-dioxy-N-methamphetamine, lysergic acid diethylamide, flunitrazepam), and rosemary essential oil on 1) RP-18 with 65 % acetone containing 1.5 % formic acid and 2) on silica gel with chloroform - methanol 9:1. Rosemary essential oil was also separated on silica gel with toluene - ethyl acetate 9:1. Detection under UV 254 nm and in daylight. The combination of laser-induced acoustic desorption and electrospray ionization mass spectrometry (LIAD/ESI/MS) can be used to rapidly characterize chemical compounds separated on a TLC plate.

Quim. Nova. 34, 330-334 (2011). TLC of rosmarinic acid in preparations of Rosmarinus officinalis on silica gel with acetone – formic acid – methylene chloride 50:17:170. Detection under UV 366 nm. Quantitative determination by solid state differential pulse voltammetry (DPV). Linearity was between 0.694x10-3 to 0.526x10-3 mol/L. The limits of detection and quantification were 1.2x10-5 and 3.6x10-5 mol/L, respectively. The intermediate/interday/intraday precisions were 3.03 % and 2.2 %, respectively. Recovery (by standard addition) was 96.3 % for rosmarinic acid. The method presented high recovery levels compared to an HPLC method.

J. AOAC Int. 96, 1167-1174 (2013). The review reports various applications of bioautography tests for determination of wide spectrum of antimicrobials. Based on two direct tests using Bacillus subtilis and Escherichia coli, the review discusses different applications for the screening of analytes in various biological samples. For example, the antibacterial activity of essential oils in conifers is estimated on the basis of the measured area of inhibition zones.

Food Chem. 210, 200-211 (2016). Review of the principles and applications of matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) in food science and related fields. The review described the combination of MALDI-MSI and HPTLC, including the visualization of phospholipids in chicken egg yolk on silica gel with acetonitrile – water 1:1.

J. Chromatogr. A 1460, 181-189 (2016). Development of an online simultaneous TLC-MS analysis system, liquid thin layer chromatography–mass spectrometry (LTLC-MS), using regular TLC plates combined with a newly developed ambient ionization method, glow discharge-matrix assisted infrared desorption ionization (GD-MAIRDI), and forced-flow TLC (FFTLC) technique. This guarantees that the MS detection process does not disturb the TLC separation process throughout the analysis, but successfully synchronizes the TLC separation and MS detection processes like GC–MS and HPLC–MS do. Demonstration by applying to the analysis of the two main components of a pain relief pills proved that the technique opens up new possibilities of combining TLC with other techniques like MS, etc.

J. Chromatogr. A 1560, 97-103 (2018). Proposal of a simple and convenient on-spot derivatization approach for the modification of hydroxyl-containing compounds for their analysis by TLC/MALDI. Post-chromatographic acylation of separated analytes with 3-bromopropionyl chloride with simultaneous quaternization of pyridine, resulted in derivatives with permanent positive charges, which, in contrast to the initial alcohols not ionizable under TLC/MALDI conditions, reveal intense peaks of their cationic moieties in MALDI mass spectra recorded directly from TLC plates. Demonstration of the method by applying to a series of mammalian and plant sterols, phenols and terpene alcohols.

Anal. Chem. 56, 2935-2939 (1984). Presentation of a method for obtaining IR information from mg level of material in situ on TLC plates using diffuse reflectance FTIR spectrometry with reference laser aiding in spot alignment. Subtraction of plate contributions to background from the resultant IR spectra by the FTIR data system. Test of a variety of different materials on various plate types. The technique allows for both spot identification and functional group information useful in the subsequent characterization of unknown impurity.