Page 487 - Tandem Techniques

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The acoustic waves were detected with a Brüel and Kjoer 4165 microphone, which was exposed to the
helium in the cell, through a pipe 10 mm long and 1 mm I.D. By comparison with the spectra obtained
from a KBr pellet sample, reasonably fine structure was disclosed. However, as might be expected, the
signal to noise was not very good, and consequently the absolute sensitivity was also not as good as that
obtained by other scanning procedures.

Alternative TLC/IR Interfaces

Other analysts have tackled the problem of linking TLC with IR spectroscopy in different ways. Shafer
et al. [7] adopted a sample transfer approach to the problem, in which each separated component is
removed simultaneously from the TLC plate to an IR transparent substrate, and then examined by
diffuse reflectance IR spectrometry. The transfer is achieved with the minimum sample loss,
decomposition or contamination, and spectra have been obtained from as little as 40 µg of material for
identification. The separations were performed on EMR TLC plates, 8 cm x 2 cm, carrying silica gel 60
and cut from plates 50 cm x 50 cm. The plates were cleaned prior to use in a bath of acetone, and then
reactivated by heating at 200°C for 2 hours. Each plate was spotted with 1 µl of sample, and placed
about 1 cm from the side of the plate, the reason for which will be given later.

The separation was developed along the length of the plate. After the separation was complete, and the
plate dried, it was turned sideways and attached to an aluminum strip, in which a row of cups had been
drilled. The transfer system is diagramatically depicted in Figure 12.11. Each cup was 4 mm deep and
1.2 mm I.D., and packed with finely ground infrared-transmitting glass, composed of germanium,
antimony and selenium. At the mouth of the holes, a bundle of glass fibers was placed which allowed a
solution to be transferred by surface tension forces up to the powdered glass packing. The fibers were
arranged to be in contact with the TLC plate, which was then developed at right angles to the original
development with dichloromethane. This second development transferred the spot contents through the
fibers and into the glass packing which took less than 2 minutes. After sample transfer, the aluminum
block was placed