LC detection

[dattasubhra]

I have a question on adsorption chromatography (LC) and would deeply appreciate any pointers :

Is it possible to have a detector that detects ONLY the signals from the solute molecules residing in the mobile phase? I presume that techniques like fluorescence will respond to solute molecules irrespective of it resides in the column packing solid or the mobile phase flowing through its interstices.

Because of this, I am intrigued why band broadening theories base their calculations on the mobile phase concentration distribution, say as in the book called Dynamics of Chromatography written by Giddings. Thanks in advance.

[Uwe Neue]

The phenomena of bandbroadening are independent if the analyte is measured inside the packed bed (e.g. on the surface) or in the mobile phase. Nevertheless, you are right that things are much simpler if one can look inside the column. All bands have about the same width...

Actually reading your post again, I realized that you asked about measuring molecules in the mobile phase. This is what we are doing anyway, aren't we?

[dattasubhra]

Thank you very much...I realized after thinking over your reply (+ doing some math) that the bands in both phase do move and spread in unison even according to theory...

[dattasubhra]

For fluorescence I thought that the molecule fluoresces even when it lis staying in the packing material at the moment of its detection...I may be way off the mark ? Because i work on models and that too a novice, I am naive about instrumentation..thanks again..

[tom jupille]

Is it possible to have a detector that detects ONLY the signals from the solute molecules residing in the mobile phase?

All routine HPLC detection is done post-column, hence always done in the mobile phase.

[AA]

Your molecule will not fluoresce untill it reaches an excited stated, this usually happens in the detector, not in the column. As your sample molecules enter the detector, they are exposed to energy (light at a specific wavelength), become excited, and emit light (at a different wavlength) which is detected by a PMT.

[dattasubhra]

Thanks a lot to Tom...the picture made it clear ...I now realize that there is no packing in the detector ...Thanks to AA to for explaining the physics...
May be there is a rare circumstance where signal may be noisy : if
the molecules have the unnatural ability to stick even to the walls of detection cell...this happens in case of some proteins ..