Page 472 - Tandem Techniques

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Chapter 12
Thin Layer Chromatography/ Spectroscopy (TLC/S) Tandem Systems

Tandem combinations, involving the in-line association of a spectroscopic instrument with a TLC plate,
is not possible in the generally accepted meaning of the term. The separation must be completed before
the plate can be examined. IR spectra, mass spectra and, if sufficient material is available, even NMR
spectra can all be obtained from TLC fractions by scraping the material contained in the spot from the
plate, and extracting the solute with an appropriate solvent, and using the solution for spectroscopic
examination. The technique requires a little dexterity, but is not difficult, and by employing modern
spectroscopic equipment, good spectra can usually be obtained. This off-line procedure is still that most
commonly used in the contemporary examination of TLC fractions. There are, however, a number of
tandem systems where the plate is used as a transport medium, which allows the spots to be examined
in situ, by an appropriately modified spectrometer. One of the first of these methods to be developed
was the technique called scanning densitometry. Scanning densitometry and other TLC tandem systems
involve relatively complex equipment, which is perhaps incongruous in view of the intrinsic simplicity
and low cost of the TLC analysis. Nevertheless, despite the apparent conflict between the unpretentious
and inexpensive TLC plate and the complex and costly associated spectroscopic equipment necessary to
scan the plate, TLC tandem systems have been actively developed over recent years. In view of the
great similarity between the techniques of