Sample Location in the Sorbent Layer

When a sample is placed on a thin layer, where in the sorbent matrix are the separated bands during and after their movement This had always intrigued the chro-matographer from the time optical quantitation began because any reflectance densitometry that was done at the time is only able to detect compounds in the top of the layer. Likewise, if doing desorption TLC-MS, the majority of the compound being near the surface for impact would be desired. Can this be controlled and is it reproducible  

But continued PAS work found concentration differences of samples located across a single plate, and it seemed to be related to the way the layer was dried after development [9], The different drying routines investigated were those most commonly used by all chromatographers (1) simple air drying (in the hood), (2) with a stream of warm air (hair dryer or similar), and (3) in a forced air oven at 50°C. The most reproducible was the latter. This is certainly because the layer is heated more evenly than by the other drying techniques, a point to be remembered for inclusion in any new protocol. Continued work by Prosek et al. [10] on this topic allowed the 

FIGURE 2.2 Possible configurations for a sample band in a sorbent matrix. The asterisk is under the closest representation of sample hand concentrations found from the experimental data outlined in Section 2.8. 

If elution TLC-MS is being done, then the sample mass location is not important as long as the correct eluting solvent is chosen for dissolution and compatibility with the MS mode being used. 

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