Thin-layer chromatography sensitivity

Thin-layer chromatography (tic) (16) is frequently used. The procedure allows for rapid screening for most dmgs of abuse using simple, inexpensive technology. A drawback to tic, however, is that the technique is not especially sensitive and low levels of dmgs may be missed. 

Just like the physical and microchemical methods of detection, the indirect, biological-physiological detection procedures are very selective when apphed to thin-layer chromatography. Here it is not chemical functional groups or particular physical properties that are selectively detected but effects on highly sensitive biodetectors . The following detection techniques have been employed ... 

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. 

The first part of the book consists of a detailed treatment of the fundamentals of thin-layer chromatography, and of measurement techniques and apparatus for the qualitative and quantitative evaluation of thin-layer chromatograms. In situ prechromatographic derivatization techniques used to improve the selectivity of the separation, to increase the sensitivity of detection, and to enhance the precision of the subsequent quantitative analysis are summarized in numerous tables. 

Mendoza CE, Shields JB. 1971. Esterase specificity and sensitivity to organophosphorus and carbamate pesticides Factors affecting determination by thin layer chromatography. J Assoc Off Anal Chem 54 507-512. 

Not only in HPLC, but also in modem thin-layer chromatography, the application of reversed-phase stationary phases becomes increasingly important. The advantage of the hydrophobic layers in comparison with the polar, surface-active stationary phases is the additional selectivity and a reduced hkehhood of decomposition of sensitive substances. 

Table 1 provides the urinalysis test results for the 4,847 arrestees, While PCP was tested for by an EMIT test only, cocaine, opiates and methadone were tested for by both EMIT and thin layer chromatography (TLC). (The EMIT test for opiates is not specific to morphine, the metabolite of heroin, and can detect the recent use of a variety of opiates. A specimen positive for opiates is most likely to indicate the use of heroin in this population, however.) Our analyses will use only the results from the EMIT tests, because we have learned that the TLC general drug screen is less sensitive for detecting recent use of these illicit street drugs (Wish et al. 1983 Wish et al. 1984). 

The latest innovation is the introduction of ultra-thin silica layers. These layers are only 10 xm thick (compared to 200-250 pm in conventional plates) and are not based on granular adsorbents but consist of monolithic silica. Ultra-thin layer chromatography (UTLC) plates offer a unique combination of short migration distances, fast development times and extremely low solvent consumption. The absence of silica particles allows UTLC silica gel layers to be manufactured without any sort of binders, that are normally needed to stabilise silica particles at the glass support surface. UTLC plates will significantly reduce analysis time, solvent consumption and increase sensitivity in both qualitative and quantitative applications (Table 4.35). Miniaturised planar chromatography will rival other microanalytical techniques. 

Most often the sterols have been collected by liquid-liquid extraction using petroleum ether and ethyl acetate [408], chloroform and methanol [409], -hexane [410,411] or chloroform [412,413]. After concentration, gas chromatography was generally used for the final separation and determination, although thin-layer chromatography has also been employed. The extra sensitivity of the electron capture detector could be used by reacting the concentrated sterols with bromomethyldimethylchlorosilane (BMDS) before separation and measurement [414],... 

Thin-layer chromatography (TLC) is one of the most popular and widely used separation techniques because it is easy to use and offers adequate sensitivity and speed of separations. Furthermore, multiple samples can be run simultaneously. It can be used for separation, isolation, identification, and quantification of components in a sample. The equipment used for performance of TLC, including applications of TLC in drug discovery process, is covered in Chapter 13. This technique has been successfully used in biochemical, environmental, food, pharmacological, and toxicological analyses. 

High-performance thin-layer chromatography (HPTLC) has become widely used and while it follows the same principles as TLC, it makes use of modern technology including automatic application devices and smaller plates, which allow for better sensitivity. 

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