Special issue: Thin-layer chromatography
This is the 19th special issue of the Journal of Liquid Chromatography & Related Technologies on thin-layer chromatography (TLC) that we have guest edited beginning in 1999. The biennial review written by Sherma in the Journal of AOAC International (volume 99, No. 2, pp. 323–331, 2016) reports the continuing high level of research activity involving TLC, with more than 3000 references found in Chemical Abstracts during the review period November, 1, 2013 to November 1, 2015. The papers in this special issue are examples of many of the most important current methods, materials, instrumentation, and application areas of TLC as documented in that review and subsequent literature searches covering November 2015 to January 2017 through Chemical Abstracts and ISI Web of Science.
The first three papers in this special issue are reviews, and the other 10 report experimental research studies. All were invited from authors who are among the most highly renowned and productive researchers in the field.
The measurement of compound lipophilicity, which is usually performed by TLC, is important because this physicochemical parameter is crucial in quantitative structure-activity relationship studies related to drug and pesticide design. Paper 1 by Wicha-Komsta and Komsta reviews the use of cyano-silica (CN), amino-silica (NH2), DIOL, normal phase silica gel, and oil-impregnated, reversed phase (RP) silica gel plates as alternatives to the most commonly used RP C18 octadecylsilyl chemically bonded silica gel plates for determination of lipophilicity.
Paper 2 by Sherma is a review of the use of TLC in pesticide analysis for the period from November 2014, to November 2016. It continues the series of biennial reviews on this topic begun in 1981. Publications on a variety of pesticide classes and sample types such as residue analysis, chromatographic retention, identification and characterization of natural pesticides, metabolism, bioactivity, degradation, soil mobility, and lipophilicity are included.
Paper 3 by Hosu and Cimpoiu reviews the TLC analysis of fermented and nonfermented fruit beverages (juices and wines) for assessment of quality and authenticity. An overview of the determination of different compounds (e.g., polyphenols, dyes, carboxylic acids, biogenic amines, and vitamin C) is included.
The next three papers deal with retention and separation studies for selected compounds using a variety of layer-mobile phase systems. In Paper 4, Obradović et al. determined the influence of 19 different mobile phases composed of 1–4 components on the separation of the antipsychotic drug ziprasidone and its five impurities on Merck silica gel 60 F254 aluminum plates, which along with their glass-backed counterparts are clearly the most used stationary phases for TLC worldwide. A CAMAG Linomat 5 was used for initial zone application, a CAMAG twin trough chamber for plate development, and a CAMAG TLC Scanner II for zone detection at 250 and 320 nm.
Paper 5 by Petruczynik et al. reports the effect of TLC conditions on the one- and two-dimensional separation of 10 alkaloids as standards and from plant extracts on Merck CN layers using nonaqueous and aqueous mobile phases containing various free silanol blocking agents. A Chromdes horizontal Teflon chamber was used for plate development and JSC SorbPolymer (Sorbfil TLC Videodensitometer) computer program for obtaining densitograms.
Włodarczyk and Zarzycki in Paper 6 studied the behavior of 18 dyes on 5 × 5 cm Merck silica gel 60 WF254s (W designates a water wettable layer and s means the presence of an acid stable indicator) and cellulose plates using methanol–water and methanol–dichloromethane (0–100%) mobile phases in a horizontal micro-TLC chamber thermostated at 30°C. Chromatograms were acquired in visible light by a Plustek OpticPro USB scanner with Image Folio software. The reported retention database is designed to help select proper conditions for solid-phase extraction devices, optimization of TLC separations, and application as internal standards for planar/column chromatography or microfluidic devices.
The next five papers are in the most active current application areas of TLC, analysis of drug formulations, and natural medicines. In Paper 7, Armour and Sherma used a previously reported model process for transfer of field screening methods contained in the Global Pharma Health Fund (GPHF) Minilab manual or US Food and Drug Administration (FDA) compendium of unofficial methods for rapid screening of pharmaceuticals by TLC to HPTLC densitometry for clarithromycin, azithromycin, and amodiaquine + artesunate with detection by simple heating of the silica gel layer to produce fluorescence quenching zones. This so-called reagentless thermochemical activation detection was first described in the 1950s for detection of certain compounds as fluorescent zones on NH2 plates, but Sherma’s group was the first to show this approach on silica gel F plates in 2001 to detect creatine by fluorescence quenching under 254 nm UV light. They have since 2011 shown that this detection method is suitable for many drugs while using their model process for transfer of TLC screening methods as well as modification of GPHF Minilab and FDA compendium screening methods that otherwise require use of chemical detection reagents such as iodine vapor, ninhydrin, and sulfuric acid–ethanol. The transferred quantitative assay methods in this paper were validated for linearity, accuracy, precision, and peak identity and purity.
Paper 8 by Lotz et al. from the research group of Bernd Spangenberg, the esteemed Editor-in-Chief of the Journal of Planar Chromatography-Modern TLC, illustrated the measurement of hypericin from St. John’s work on Merck spherical particle LiChrosper silica gel 60 HPTLC plates using a pipeJet piezoelectric application system, ethyl acetate–water–formic acid–methyl tert-butyl ether–cyclohexane (180:14:14:80:30) mobile phase, chemiluminescence detection after reaction with bis(2,4,6-trichlorophenyl)oxalate solution, chemiluminescence measurement with a Biostep Celvin S charge coupled device camera, and fluorescence evaluation at 366 bm by a Biostep ProViDoc system DD70 with a Canon EOS600 camera.
In Paper 9, Jesionek et al. used silica gel TLC-direct bioautography with Bacillus subtilis to evaluate the antibacterial properties of Thymus vulgaris L. and Salvia officinalis L. essential oils produced by three different manufacturers. Merck silica gel TLC plates were developed the optimized mobile-phase chloroform stabilized with amylene in a Chromdes horizontal chamber, chemical detection was with anisaldehyde–sulfuric acid spray reagent, and chromatogram documentation was by use of a CAMAG TLC Visualizer.
The development and validation of a method for determination of amygdalin from fruit kernels were reported by Radoičić et al. in Paper 10. Samples were extracted with a Dionex ASE 100 accelerated solvent extraction instrument and applied with standards using a CAMAG Linomat 5 onto Merck C18 HPTLC plates that were developed in a horizontal chamber with acetronitrile–water (1:1) mobile phase. Densitometric scanning was performed at 210 nm with a CAMAG TLC Scanner 3. The method was validated with respect to sensitivity, specificity, linearity, precision, and accuracy.
Paper 11 by Szeremeta et al. presents the TLC fingerprinting of the nonvolatile fraction from 12 traditional medicinal herbal samples of the Cistus incanus L. species. Extraction was carried out using a Dionex ASE 200 instrument, and chromatograms on Merck silica gel 60 plates produced by ethyl acetate–formic acid–acetic acid–water (100:11:11:13) or ethyl acetate–dichloromethane–formic acid–acetic acid–water (100:10:10:10:11) mobile phases were visualized and quantified by scanning fluorescence at 366 nm with a Desaga CD 60 densitometer equipped with ProQuant software. Antioxidant potential of separated compounds was assessed with 2,2-diphenyl dipicrylhydrazyl (DPPH) spray reagent.
As an example of the active area of food analysis, Skorek et al. in Paper 12 reported a study aimed at detecting pelargonidin, cyanin, keracyanin, and delphinidin in selected fruit juices by means of TLC–densitometry and TLC–mass spectrometry (MS) and to evaluate the antioxidant properties of the preparations. Plant infusions of cosmetic importance were also analyzed. Merck microcrystalline cellulose plates were developed with 80% formic acid–water–n-butanol (16:19:65) or conc. HCl-80% formic acid–water (9:46:90), and chromatograms were scanned densitometrically at 580 nm with a Desaga CD 60 densitometer equipped with ProQuant software. Individual chromatogram bands were directly introduced into an electrospray ionization Varian 500-MS mass spectrometer using a CAMAG TLC–MS interface with dicholormethane–methanol (2:8) eluent. Antioxidant activity of the plant samples was assessed with the DPPH test. Instrumentation and applications of coupled MS is undoubtedly the main methodology research area of TLC at this time.
Pressurized planar electrochromatography (PPEC) is a method in which a mixture applied to a thin layer is separated by mobile-phase flow driven by electro-osmotic flow. Solutes migrate according to their degree of partitioning between the stationary and mobile phases, and if they undergo electrolytic dissociation, an additional electrophoretic effect is involved in the separation. Dzido et al. show in Paper 13 that PPEC performed with TLC and ultraviolet/visible enhances the results of toxicological chemical analysis. Merck TLC silica gel 60 F254 and HPTLC C18 W F254 plates, a CAMAG Automatic TLC Sampler 4, a Chromdes horizontal DS-II chamber for 10 × 10 cm plates, a CAMAG TLC Scanner 4, and a CAMAG TLC Visualizer were used for TLC. For PPEC, a special chamber, high voltage power supply, and sealed plate under pressure were used.
We will begin to solicit papers for our guest-edited 2018 TLC Special Issue during the summer of 2017. In that issue, we will especially welcome review papers on important research areas of TLC as well as papers reporting new research results. We invite readers to send us comments on this and our past special issues, as well as suggestions for topics and contributors for future issues. We also encourage the submission of papers on TLC and HPTLC for regular issues of this journal, which under the editorship of Dr. Nelu Grinburg is one of the world’s leading sources of research in the field of liquid chromatography.