Principle of the GC-TLC-Coupling Analysis

The sample under investigation is subjected first to gas chromatography. As known, only compounds which volatilise without decomposition or which decompose reproducibly, or products of p3nrolysis, are suited to this procedure (all gases, many liquids and solids with vapour pressures down to IQ- mm at 200° C). 

The sample is vaporised in an inert carrier gas at a temperature which can be selected between 20 and 500° C. If the whole sample cannot be vaporised, a measured amount of a known internal standard can be added beforehand. 

The vaporised sample is now swept through a so-called separating column by means of a suitable carrier gas at a definite temperature. Separation into the individual components occurs in the column as a result of specific retention. Analogous to TLC, a qualitative characteristic of each substance is provided-here it is the retention time f-which can be used for identification or structure elucidation. It is unnecessary to discuss further details of GC here see, for example, Bayer [53], Kaiser [337] or Keulemans-Cremer [345]. 

The separated single compounds leave the column as vapour clots (peaks) in the carrier gas. The vapour concentration is usually ca. 10 to 10 i g/ml and the carrier gas flow about 1—4 1/h. In direct GC - TLC coupling, the carrier gas leaving the apparatus is directed straight on to the thin layer placed some tenths of a millimetre from the column exit. This brings 10 to 10 g substance per sec on to the layer (Fig. 60). From 30—80% substance is retained, depending on temperature, gas flow, polarity-, concentration- and adsorption conditions. The yield can approach 100% in especially favourable instances. 

The substances thus transferred from GC to a TLC start line can then be developed with a solvent in the usual way. Mixtures perhaps not separated in the GC stage may then be separated. 

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