Applications of two-dimensional thin-layer chromatography

Planar chromatography - Current practice and future prospects

Planar chromatography, in the form of thin-layer or high-performance thin-layer chromatography (TLC, HPTLC), continues to provide a robust and widely used separation technique. It is unrivaled as a simple and rapid qualitative method for mixture analysis, or for finding bioactive components in mixtures. The format of TLC/HPTLC also provides a unique method for preserving the separation, enabling further investigation of components of interest (including quantification/structure determination) separated in both time and space from the original analysis. The current practice of planar chromatography and areas of development of the technology are reviewed and promising future directions in the use of TLC/HPTLC are outlined.

Nuclear Magnetic Resonance (NMR) Study for the Detection and Quantitation of Cholesterol in HSV529 Therapeutic Vaccine Candidate

This study describes the NMR-based method to determine the limit of quantitation (LOQ) and limit of detection (LOD) of cholesterol, a process-related impurity in the replication-deficient Herpes Simplex Virus (HSV) type 2 candidate vaccine HSV529. Three signature peaks from the 1D ¹H NMR of a cholesterol reference spectrum were selected for the identification of cholesterol. The LOQ for a cholesterol working standard was found to be 1 μg/mL, and the LOD was found to be 0.1 μg/mL. The identity of cholesterol, separated from the formulation of growth supplement by thin layer chromatography (TLC), was confirmed by 1D ¹H NMR and 2D ¹H-¹³C HSQC NMR. The three signature peaks of cholesterol were detected only in a six-times concentrated sample of HSV529 candidate vaccine sample and not in the single dose HSV529 vaccine sample under similar experimental conditions. Taken together, the results demonstrated that NMR is a direct method that can successfully identify and quantify cholesterol in viral vaccine samples, such as HSV529, and as well as in the growth supplement used during the upstream stages of HSV529 manufacturing.

Importance of purity evaluation and the potential of quantitative ¹H NMR as a purity assay

In any biomedical and chemical context, a truthful description of chemical constitution requires coverage of both structure and purity. This qualification affects all drug molecules, regardless of development stage (early discovery to approved drug) and source (natural product or synthetic). Purity assessment is particularly critical in discovery programs and whenever chemistry is linked with biological and/or therapeutic outcome. Compared with chromatography and elemental analysis, quantitative NMR (qNMR) uses nearly universal detection and provides a versatile and orthogonal means of purity evaluation. Absolute qNMR with flexible calibration captures analytes that frequently escape detection (water, sorbents). Widely accepted structural NMR workflows require minimal or no adjustments to become practical ¹H qNMR (qHNMR) procedures with simultaneous qualitative and (absolute) quantitative capability. This study reviews underlying concepts, provides a framework for standard qHNMR purity assays, and shows how adequate accuracy and precision are achieved for the intended use of the material.

Separation orthogonality in temperature-programmed comprehensive two-dimensional gas chromatography

In a comprehensive two-dimensional gas chromatograph, a thermal modulator serially couples two columns containing dissimilar stationary phases. The secondary column generates a series of high-speed secondary chromatograms from the sample stream formed by the chromatogram eluting from the primary column. This series of secondary chromatograms forms a two-dimensional gas chromatogram with peaks dispersed over a retention plane rather than along a line. The method is comprehensive because the entire primary column chromatogram is transmitted through the secondary column with fidelity. One might expect that a two-dimensional separation in which both dimensions are basically the same technique, gas chromatography, would be inefficient because the two dimensions would behave similarly, generating peaks whose retentions correlate across dimensions. Applying a temperature program to the two columns, however, can tune the separation to eliminate this inefficiency. The temperature program reduces the retentive power of the secondary column as a function of progress of the primary chromatogram such that the retention mechanism of the primary column is eliminated from the second dimension. Retention of a substance in the second dimension is then determined by the difference in its interaction with the two stationary phases. Retention times in the second dimension then fall within a fixed range, and the whole retention plane is accessible. In a properly tuned comprehensive two-dimensional chromatogram, retention times in the two dimensions are independent of each other, and the two-dimensional chromatogram is orthogonal. Orthogonality is important for two reasons. First, an orthogonal separation efficiently uses the separation space and so has either greater speed or peak capacity than nonorthogonal separations. Second, retention in the two dimensions of an orthogonal chromatogram is determined by two different and independent mechanisms and so provides two independent measures of molecular properties.

A new procedure for separation of complex mixtures of pesticides by multidimensional planar chromatography

The objective of this communication is to report a new procedure for the separation of complex mixtures of pesticides by a combination of different modes of multidimensional planar chromatography. Complete separation of a mixture of pesticides was carried out by multidimensional planar chromatography on silica gel plates. The plates were videoscanned to show a real picture of the experimental results. Complete separation of all compounds was not possible by a single development on silica gel. The separation can be characterized as [PC x (nPC + PC + PC + PC)]. With the aid of this new procedure a complex mixture of fourteen compounds was separated on a 10 x 10 cm TLC plate.

Thin-layer chromatography: challenges and opportunities

The purpose of this article is to identify core technologies with the potential to influence the development of thin-layer chromatography over the next decade or so. Core technologies are identified as: (i) methods to provide a constant and optimum mobile phase velocity (forced flow and electroosmotically-driven flow), (ii) video densitometry for recording multidimensional chromatograms, (iii) in situ scanning mass spectrometry, and (iv) bioactivity monitoring for selective detection. In combination with two-dimensional, multiple development and coupled column-layer separation techniques these core technologies could dramatically increase the use of thin-layer chromatography for the characterization of complex mixtures. It is also demonstrated that thin-layer chromatography has strong potential as a surrogate chromatographic model for estimating biopartitioning properties. To convert these opportunities into practice the current state-of-the-art of the core technologies is described and the principle obstacles to progress identified.

Correlation of retention parameters of pesticides in normal- and reversed-phase systems and their utilization for the separation of a mixture of 14 triazines and urea herbicides by means of two-dimensional thin-layer chromatography

The selectivities of TLC systems were compared by use of correlations between RF(II) and RF(I) (by analogy with two-dimensional TLC). The greatest spread of points, indicative of individual selectivity, was obtained for nonaqueous mobile phases on silica and aqueous mobile phases on octadecyl silica adsorbent wettable with water (RP-18 W). The correlation of RF values in normal- and reversed-phase systems was utilized in the practical separation of a mixture of 14 triazines and urea herbicides using two-dimensional thin-layer chromatography on a Multi-K CS5 dual phase (3 cm strip of octadecyl silica parallel to silica layer). The plate was videoscanned showing the real picture of the plate.

Planar chromatography at the turn of the century

An overview of the state-of-the-art of modern thin-layer chromatography (planar chromatography) is presented with emphasis on the complementary features of thin-layer and column liquid chromatographic separations. The reasons for selecting thin-layer chromatography for a particular analysis are identified by its attributes: a disposable stationary phase; simultaneous parallel separations; static detection free of time constraints; storage device for chromatographic information; all sample components are observed in the chromatogram. Future prospects for improved separation performance in TLC using zone refocusing, forced flow and electroosmotic flow methods are discussed as well as increasing zone capacity by using two-dimensional development and coupling to column chromatographic methods. Advances in coupling thin-layer chromatography with spectroscopic methods for structural elucidation are also considered. Finally, some predictions are made for how thin-layer chromatography will be practiced in the future.

Chromatographic analysis of cis/trans carotenoid isomers

Cis/trans configurations of carotenoids are known to effect the biochemistry of carotenoids in certain situations. Methodology for separating carotenoid cis/trans isomers is of importance to nutritionists and food scientists because cis isomers of provitamin A carotenoids have lower provitamin A activities than the all-trans form. Traditional food processing and preservation methods, especially thermal treatments, induce the formation of cis isomeric forms. However, many challenges, are apparent for identifying and analyzing cis/trans isomers present in foods and other biological tissues. The development of current chromatographic methods for the separation of carotenoid cis/trans isomers is reviewed. For the separation of beta-carotene isomers, most procedures employ either Ca(OH)2 or Vydac C18 columns. In general, polymeric C18 columns allow for the detection of cis carotenes, while monomeric C18 columns provide for some separation of certain xanthophylls. The main cis isomers detected in foods are the 13-cis and 9-cis forms, although other forms have also been found (mainly 15-cis and various di-cis isomers). More studies involving the metabolism and physiological consequences of cis/trans isomers in the diet are needed. However, due to limitations in current techniques, further method development in the area of separation, detection and quantitation of cis/trans carotenoid isomers will be required.

Two-dimensional field-flow fractionation

Multidimensional field-flow fractionation (FFF) is described in two major forms: one in which different separative stages are coupled together and one in which two independent displacements, at least one of them FFF, are carried out in a generally planar channel structure. The latter, the subject of this paper, is relatively promising for implementation in FFF systems because in most cases the geometry of the FFF channel is already planar; the channel structure needs mainly to be broadened along the second dimension and modified with different inlets and outlets for this two-dimensional use. The large number of potential two-dimensional FFF systems is discussed. These systems are described at greater length in four categories: (1) FFF displacement used in both dimensions, (2) FFF along one axis and chromatography along another, (3) FFF along one axis and a field-induced displacement along the other, and (4) FFF separation combined with bulk or flow displacement at right angles. Finally, theoretical equations are obtained for the deflection of the trajectories away from the main flow axis z. The sensitivity of deflection to component properties is described in terms of the deflection selectivity. Several examples are discussed in which the deflection selectivity is remarkably high.

Planar chromatographic techniques in biomedicine: current status

In planar chromatography (PLC), the solvent flows through a layer either by means of capillary forces [conventional thin-layer chromatography (TLC)] or by a forced-flow system (over-pressured layer chromatography). Phases and instrumentation currently available are briefly examined. The main applications in biomedicine are reviewed. Although silica gel TLC plates still predominate, interest in other phases is increasing. Unique detection features such as immunostaining are emphasized. Although gas chromatography and high-performance liquid chromatography have superseded TLC in the analysis of carbohydrates, amino acids and indole derivatives, interest in PLC continues to be high in lipid analysis.

Effect of the mobile phase composition on the retention behaviour of diphenylsilica pre-coated plates

The retention of some rifamycins and steroids on diphenyl bonded pre-coated silica gel plates, in relation to the mobile phase used, was examined by thin-layer chromatography. Neat organic solvents, non-aqueous and aqueous binary mixtures were tested as eluents. By comparison of retention data for rifamycins and steroids, respectively, under non-aqueous and aqueous conditions, a dual retention mechanism on this diphenyl phase was found. Interactions with the residual silanol groups seemed to prevail when employing as mobile phase the more lipophilic solvents tested, such as chloroform or dichloromethane, whereas interactions with the aryl groups of the bonded phase prevailed when using high polarity alcohols or aqueous mixtures. As a consequence, by changing the mobile phase, a large variation in selectivity with a concomitant change in retention order of the test compounds was observed.

Thin-layer chromatography of mycotoxins

TLC has become an extremely powerful, rapid and in most instances inexpensive separation technique in mycotoxicology. This review presents achievements of its applications in this field. General technical aspects of the TLC of mycotoxins that are discussed include extraction and clean-up procedures, adsorbents and solvent systems, detection methods, two-dimensional TLC, high-performance TLC (HPTLC), quantitation and preparative TLC (PLC). Special applications of TLC deal with multi-mycotoxin analyses and with structurally related or individual mycotoxins (aflatoxins, sterigmatocystins, versicolorins, ochratoxins, rubratoxins, patulin, penicillic acid, mycophenolic acid, butenolide, citreoviridin, trichothecenes, cytochalasans, tremorgenic toxins, epipolythiopiperazine-3,6-diones, hydroxyanthraquinones, zearalenone, citrinin, secalonic acids, cyclopiazonic acid, PR toxin, roquefortine, xanthomegnin, viomellein and naphtho-gamma-pyrones).