Page 473 - Tandem Techniques

Page 458

TLC and LC, and that the main advantage of TLC is its simplicity and low cost, the need to develop
both TLC and LC tandem instruments might appear unnecessary and perhaps wasteful.

Scanning Densitometry

Scanning densitometry can serve a number of purposes. It can be employed to precisely identify the
position and size of the spot, for the accurate measurement of Rf values, and it can also provide
transmission, reflectance or adsorption spectra to confirm spot identification. In addition, the density of
the spot can also be employed to estimate the quantity of solute present in the spot. If, for some reason,
the technique of TLC is chosen as preferential to LC for a particular analysis, then in situ scanning of
the TLC plate, employing optical instrumentation, is now considered essential for both the accurate
location of a spot and the precise quantitative estimation of its content. The surface of the plate can be
examined using either reflected light, transmitted light, or fluorescent light. In addition, the incident
light can be either adsorbed, diffusely scattered or transmitted.

Figure 12. 1
Single and Double Beam Densitometers
It is usual to measure the light scattered, reflected, or generated by fluorescence from the spot, and
compare it electronically with light from