Coupled Methods for Substance Identification

TLC is an ideal sample preparation method prior to further analysis for substance identification. Using suitable TLC systems, especially those involving optimal precoated layers, it is possible not only to separate mixtures of unknown substances but also to use various spectroscopic evaluation methods directly on the layers. By this combination of two different analytical methods, it is often possible to perform an imequivo-cal identification of a substance even when the samples are extremely small. These spectroscopic evaluation methods include the following  

Raman spectroscopy is a type of molecular spectroscopy similar to FTIR, but in this case there are no interfering bands even when sUica gel precoated layers are used. HPTLC-aluminium sheets sUica gel F254 (Merck no. 1.05548) are suitable ready-to-use plates for this piupose. TLC/Raman coupling is described in [88,89], and a new publication describes the routine use of TLC-Raman, taking the analysis of hy-droxybenzenes as the example [89a]. 

Compared with normal Raman spectroscopy, the SERS technique gives a considerably lower limit of determination of the substance to be analyzed. This is achieved by bringing colloidal noble metal atoms into close proximity to the sample molecules after they have been separated by TLC. Further information can he found in [90, 91]. 

MS is a widely used analytical method which enables a substance to be identified from the size of its molecules and characteristic molecular fragments. It has so far been mainly used in combination with GC, but attempts have been made for some time to couple TLC with this spectroscopic method [92, 93]. 

Many publications exist in the field of food analyses. Gerda Morlock described in 2004 the determination of heterocyclic aromatic amines after a 6-step AMD-separation. She used an HPTLC-MS online extractor, developed by Luftmarm [93a]. In 2004 a patent application was made for the mode of operation of this so-called ChromeXtrakt device [93b]. In her latest paper (2006), the quantification of isopropylthioxanthon (ITX) in food using HPTLC/FLD coupled with ESI-MS and DART-MS was reported. The prepared samples were separated on a HPTLC-plate and determined by a fluorescence detector (FLD). Positive results have been verified by ESI-MS (Electrospray-Ionisation-Mass-Spectrometry) and DART-MS (Direct Analysis in Real Trme-Mass-Spectrometry) [93c]. 

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