Page 482 - Tandem Techniques

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This has obvious application to scanning TLC plates and the possibilities were investigated by Zuber et
al. [2]. In the system developed by Zuber et al. the TLC plates were inserted directly into the
spectrometer sample chamber and the spectrum obtained by reflectance directly from the plate surface.
A reference laser aids in spot alignment, and the plate contributions to the background are subtracted
from the spectrum obtained in the usual manner. A number of different TLC plates were examined in
this way, and it was found that spot identification was possible, providing reference spectra were
available that had been obtained under the same operating conditions. The quality of the spectra and the
useful size of the IR window available varied between different types of plate. The technique that was
employed was an extension of some earlier work by Fuller and Griffiths [3], but smaller plates were
used and great care was taken to ensure surface uniformity.

The preparation and handling of both the sample and background plates were be the key to the success
of the method. It was found that although the selection of the solvent to achieve the separation was
critical, providing the plate was completely dried before IR measurements were made, the solvent had
no effect on the quality of the spectra that were produced. The spectrometer used was the Nicolet
(Madison, WI) 6000 FTIR, which was equipped with a nitrogen cooled cadmium telluride detector, and
a diffuse reflectance attachment was used to run the spectra. The interferometer was run at a mirror
velocity of 0.586 cm/s and 2000 scans or less were found necessary to produce a good quality
TLC/FTIR spectrum. In most experiments, small plates, (2.5 x 10 or 2.5 x 7.5 cm), were employed, so
that they would fit easily into FTIR sample chamber. All plates were thoroughly dried before
examination. The TLC spot diameter varied between 2 and 8 mm, and the diameter of the infrared
beam, focused on each plate, was 1 mm. The sample and background spectra were run, and the
spectrum of the sample obtained by difference. The analysis time was typically 15 minutes for the TLC
separation, and about 30 minutes were needed to obtain the IR spectra. Examples of the results obtained
from the tandem system are shown in Figure 12.8. It is seen that there is a distinct difference in the form
of the spectra taken from the TLC plate compared with that from the KBr pellet (Figure