Densitometer, thin-layer

Thin-layer chromatography (TLC) is used both for characterization of alcohol sulfates and alcohol ether sulfates and for their analysis in mixtures. This technique, combined with the use of scanning densitometers, is a quantitative analytical method. TLC is preferred to HPLC in this case as anionic surfactants do not contain strong chromophores and the refractive index detector is of low sensitivity and not suitable for gradient elution. A recent development in HPLC detector technology, the evaporative light-scattering detector, will probably overcome these sensitivity problems. 

A densitometer is a photometer that is designed to measure tho intensity of coloured stain on a thin layer preparation. 

Planar chromatography, also known as Thin Layer Chromatography (TLC), is a technique related to HPLC but with its own specificity. Although these two techniques are different experimentally, the principle of separation and the nature of the phases are the same. Due to the reproducibility of the films and concentration measurements. TLC is now a quantitative method of analysis that can be conducted on actual instruments. The development of automatic applicators and densitometers has lead to nano-TLC, a simple to use technique with a high capacity. 

The Vitatron TLD-100 thin-layer densitometer (Vitatron, Dieren, The Netherlands) is a single-beam instrument (Fig.3.11) which has been designed for reflectance, transmission... 

Scanning the developed colored spot on the thin layer chromatographic plate with the help of a photoelectric densitometer. 

The universal TLC facilities are utilized plates, adsorbents, microcapillaries, or micropipettes for sample application, development tanks, detection spray reagents, devices for spraying, and densitometers for quantification. Plates are either commercially precoated or handmade. Silica gel G (G, for gypsum as a binding substance), silica gel H (no binding substance) and, rarely, alumina and kieselguhr, form the thin-layer stationary phases. Complete sets of devices necessary for the preparation of handmade plates are commercially available. After the silica gel slurry is spread on the plates, they are left to dry in the air for at least 24 hr and shortly in an oven at 110°C. The plates are then ready for either direct use or for modification of the layer. From the great variety of precoated plates, which are commercially available and preferred nowadays, silica gel plates and plates with layers... 

In order to achieve the optimum accuracy, precision, and sensitivity, most quantitative analyses are performed by using high-performance thin-layer chromatography (TLC) plates and direct quantification by means of a modern optical densitometic scanner with... 

Petrovic, M., M. Kastelan-Macan, K. Lazaric, and S. Babic, Vahdation of thin layer chromatography quantitation with CCD camera and slit-scanning densitometer, / AOACInt. 82 25-39 (1999). 

The exact chemical composition of a plant extract is not always completely known. Many articles published in recent years attempt to identify the compounds structure by coupling chromatography with spectro-metric methods. Modern densitometers are able to record the in situ ultraviolet-visible (UV-vis) spectra of a separated substance on a TLC plate [6]. Thin-layer chromatography can be also coupled with other methods in order to enhance the identification of compounds, such as mass spectrometry (MS) or nuclear magnetic resonance (NMR). There are devices able to record the in situ spectra on the TLC plate, or the separated substance is removed from the plate together with the layer, then extracted in a small volume of an adequate solvent, and the sample can be used for obtaining the spectra [6,7]. 

In these densitometers, a small bundle of light images a square or round spot on the thin-layer plate. 

When using other kinds of chromatography, it is often necessary to clean and to replace the column. The thin-layer plate, however, is independent of the measuring instrument and is used only once. Both the sample application system and the transport of the mobile phase are independent of the densitometer. The latter can cause many problems and require a lot of maintenance particularly in high performance liquid chromatography. 

As already mentioned, the densitometers require little maintenance and cleaning unlike the instruments in gas and high performance liquid chromatography. This is advantageous as less personnel is needed. Laboratory assistants working in thin-layer chromatography have to be able to work precisely, but they do not need a special technical knowledge. 

A system is as weak as its weakest link. In thin-layer chromatography today the densitometers are not anywhere near the weakest link in the off-line system. 

Thin layer chromatography is a rapid and inexpensive method that is often used to separate surfactants into classes, i.e. to separate ionic surfactants from nonionic ones. Typical stationary phases used in TLC are usually unmodified or modified (e.g. hydrophobized) silica or alumina. Quantitative analysis using TLC is based on one s ability to isolate the spot on the TLC plate and determine the concentration of active material in it. For example, the surfactant can be extracted into an appropriate solvent, and then analyzed by a suitable technique. Alternatively, one can develop the chromatogram with the suitable reagent to produce colored compound and then analyze the colored spot with optical densitometer. 

Petrovic, M. Kastelan-Macan, M. Lazaric, K. Babic, S. Validation of thin layer chromatography determination with CCD camera and sUt-scanning densitometer. J. AOAC Int. 1999,82 (1), 25-30. 

Kawazumi, H. Yeung, E.S. Resonant cell laser-hased photoacoustic densitometer for thin-layer chromatography. Appl. Spectrosc. 1988, 42 (7), 1228-1231. 

Sherma, J. A field guide to instrumentation Quantitative thin layer chromatography densitometers. Inside Lab. Manag. 2000, 4 (10), 5-9. 

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