Page 44 - Tandem Techniques

Page 26

usually either silica or silanized silica, is spread on the surface of a glass plate which is spotted at one
end with the sample. As a rule, the end of the plate where the sample was placed is then dipped in a
trough of mobile phase and the separation developed. As the solvent rises up the plate, the solutes are
eluted to different distances along the plate, and a separation is achieved. Inherently the technique has
limited resolving power and restricted sample size, although it is very inexpensive to operate. The latter
advantage is its main cause for survival. Perhaps the last comment is a little severe, as there are certain
types of sample, and certain monitoring procedures, where the simplicity and economy of TLC make it
the separation technique of choice.

Various modifications of the basic system have been developed such as the circular plate, where the
sample is placed in the center and the mobile phase is introduced by a siphoning procedure at the same
point. The separation results in the individual solutes forming concentric rings, which can be made
visible by the usual procedures (e.g. charring and staining etc.). Another modification of the technique
is two-dimensional TLC development. After initial development along one axis of a rectangular plate,
the plate is dried and then the complementary edge of the plate immersed in a different solvent and the
separation developed perpendicular to the previous axis. This procedure increases not only the
resolution obtainable, but also the component capacity, and thus allows the technique to be used for the
separation of more complex mixtures.

TLC is not normally employed as a separation technique for tandem operation with spectroscopic
instruments. The reason for this is fairly obvious. Unless the plate itself is spectroscopically scanned,
the solute bands must be located by a non-destructive means and the solutes removed by scraping the
plate and subsequent extraction. Only then can the material separated be examined. UV and
fluorescence scanning of TLC plates has been successfully carried out [20-21]. However, as will be
discussed later, UV and fluorescence spectra, although helping to confirm the identity of a suspected
compound, provide very limited information if the solutes of interest are completely unknown. Mass
spectra, NMR spectra and even IR