Page 493 - Tandem Techniques

Page 478

The solvent on the TLC plate serves two purposes. First it extracts the solutes into solution so that they
can come in contact with the matrix film. Second, the matrix film takes up the solvent and thus
incorporates the solutes into the matrix layer. The steel plate is then removed, leaving the matrix layer
containing the solutes on the surface of the thin layer plate. The thin layer plate is then placed in the
laser desorption apparatus shown in Figure 12.15.

Figure 12.15
The TLC Plate Sampling Device for
the Time of Flight Mass Spectrometer

The desorption device consists of a TLC plate holder, driven by a stepping motor, that can be traversed
both in the (x) and (y) axis so that the plate can be precisely scanned in two dimensions. The plate is
situated in the ion source of the time of flight mass spectrometer, and by pulsing the laser beam the
spectra can be obtained and, if necessary, accumulated in the usual manner. This technique, inevitably
causes each spot on the plate to spread to some extent, which might diminish the chromatographic
resolution. It appeared, however, that the dispersion was not too serious and would not be important in
the majority of TLC separations. Nevertheless, it must be pointed out that the TLC/MS tandem systems
are tedious to use and sometimes difficult to operate. In addition, they are