I am studying the adsorption of lincomycin on activated carbons and zeolites. I have carried out a preliminary stage for the determination of equilibrium time, For this purpose I take samples every day and they are analyzed by UV-Vis spectrometry. The trend was good and I thought I have found the equilibrium at 7 days. However, after this time I have continued measuring the concentration of my adsorbate into the solution and now it is increasing. I mean, a kind of desorption process is being carried out after 8 days. Do you have any idea why is it happening or what parameters can be affecting the process?
As adittional data, I have running blanks (only lincomycin) that shows the same concentration every day, besides temperature of samples is controlled in a controlled temperature chamber, so these parameters are not affecting the process. Thank you for your valuable help!
Dear Oscar,
I have not worked with this compound, but I worked with other organic compounds with complex chemical structure. In the systems investigated by me - temperature, pH, light radiation and the stability of the compounds influenced significantly the adsorption process.
In my opinion there are some hypotheses:
1. It's possible to be an desorbtion process, it depends about the diferences betwen concentration Ct at t=7 days and Ct at t=8 days!
2. The characteristics of the lincomycin are very important - it is stable in aqueous solutios? It is photosensible? . If the experiments are performed in the presence of visible light it 's posible to take place a photodegradation process of the compund (considering time = 8 days), and in the solution can be some intermediates that adsorb at the same wavelength . In this case is mandatory to perform the experiments in the dark. And to investigate the behaviour of the lincomycin.
3. Try to see what happens in a blank system adsorbent - water, wich running in the same conditions!
A suggestion is to analize the concentration in samples by HPLC (if is possible).
I wish you luck!
Camelia
Especially for activated carbon there is suspicion that compounds are released that add to UV-absorption. So it seems to me detection method might be the problem. So you would need a second independent analytical procedure.
Totally agree with Titus. UV spectrometry is not specific and many other things can be "seen" by UV and wrongly assigned to the molecule under study. It already happened to be with plastic caps that were impossible to clean further: some molecules were desorbing from them during the experiments until the calculated concentration was higher at the end that what was introduced at the beginning !!
The problem was solved by using new caps, i.e., never used before.
Alain
Thank you for your valuable answers. I will try HPLC analysis because, as you mentioned, I agree it can be the source of the problem.