Densitometer, thin-layer chromatography

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. 

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. 

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]. 

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. 

Normal and reverse-phase thin-layer chromatography (TLC) chromatograms are quantitated using a dual-wavelength scanning densitometer. Trace levels of organics in a mixture are separated and detected using UV-visible, reflectance, or fluorescence modes. Hydrazine, for example, can be analyzed for trace components on polymeric materials, determined to a 50-ppb or even lower level via established official (i.e., ASTM, USP) analytical procedures. TLC plates can be scanned unattended and quantitated automatically. The technique has been found to be particularly useful for compositional analysis, since individual component spots can be collected for subsequent spectral analysis or other available analytical techniques. 

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. 

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