Alternative analytical procedures chemical substances

TLC is useful both as an analytical and a preparative technique, and substances tentatively identified by TLC may be further characterized by various analytical techniques such as nuclear magnetic resonance spectrometry, mass spectrometry, or gas liquid chromatography. Moreover, many specific chemical detection tests are available to help identify substances separated by TLC. TLC is a microanalytical procedure and provides for separations and at least tentative identification of substances in the milligram (mg), microgram (/ig), and nanogram (ng) range. TLC can provide the biochemist with a method of eluting separated substances from plates for quantitative analyses. Recent studies indicate that elution techniques may not be the best alternative for quantitative analyses of many substances separated by TLC and that the preferred method may involve quantitative in situ densitometric analysis [1,2]. 

Procedures suitable for the incorporation of chemical or biochemical substances or ion-exchange sites onto an electrode surface have been developed in various laboratories. " Such modified surfaces can then be used to perform a variety of functions. They have, for instance, been employed to preconcentrate analytes prior to voltammetric analysis and hence impove sensitivity. For example, dimethylglyoxime has been used to preconcentrate nickel and EDTA to preconcentrate silver." If this preconcentration can be achieved in an environment conducive to rapid electron transfer, then sensitivity will be enhanched even further. Alternatively electrocatalysts have been attached to electrode surfaces, and by speeding up what would otherwise be a sluggish electron transfer process, increased sensitivity has been attained. However, this approach yields no added selectivity and a large background must usually be tolerated. 

Immediate application of a direct instrumental method (e.g.. atomic spectroscopy in one of its many variants) usually represents the most economical approach to elemental analysis provided the procedure in question is essentially unaffected by the sample matrix, or if one has access to appropriate reference materials similar in composition to the substance under investigation [23]-[26]. The alternative is an analytical method consisting of multiple operations. separated by either space or time, often referred to as a multi-step procedure, as indicated on the left in Figure 5. The possibility of combining two or more discrete techniques adds a whole new dimension to chemical analysis, although there is a long tradition of observing a formal distinction between sep-... 

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