Page 481 - Tandem Techniques

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The sensitivity of the system was studied by spotting different solutions of various compounds on to the
disks, and it was found that most substances were easily distinguishable from their spectra, even when
as little as one microgram of material was present on the plate. As an example of the sensitivity of the
silver halide plate measurement technique, 1 µg of methylene blue, eluted 5 cm along the plate with
90:90:4 mixture of chloroform:methanol:acetic acid gave a spectrum in which all the major absorption
bands could be clearly discerned.

There have been a number of other approaches to the problem of scanning the TLC plate to obtain IR
spectra of the solutes, including the use of diffuse reflectance IR Fourier transform spectrometry and
photo acoustic infrared spectrometry (PAS). Both these techniques will be briefly discussed.

Diffuse Reflectance IR Fourier Transform Spectrometry (DRIFTS)

The principle of diffuse reflectance IR Fourier transform spectrometry is depicted in Figure 12.7. When
incident light strikes a surface, the light that penetrates is reflected in all directions and this is called
diffuse reflectance. As the light that leaves the surface has passed through a thin layer of the reflecting
material, its wavelength content will have been modified by the optical properties of the matrix.
Consequently, the wavelength and intensity distribution of the reflected light will contain structural
information on the substrate.

Figure 12.7
The Principle of Diffuse Reflectance
IR Fourier Transform Spectrometry