DEPTH PROFILING OF TLC PLATES BY PHOTOACOUSTIC-SPECTROSCOPY

Photoacoustic spectroscopy (PAS) has been used to investigate the in-d epth distribution of compounds on the adsorbent of a TLC plate. The te chnique was also used to obtain unknown values of the thermal diffusiv ity of the plates, values essential for determination of thermal diffu sion lengths and layer thicknesses that are probed at a certain modula tion frequency of the laser excitation beam. The PAS results revealed nonuniform concentration distribution of compounds inside the sorbent which arise mainly from secondary chromatography occurring inside the sorbent during drying. By use of PAS measurements across the entire so rbent laver such effects were eliminated. This represents an important Improvement in the evaluation of TLC plates compared with conventiona l reflectance densitometry which can only probe the surface of a TLC p late.