Sorption of analyte on the column

[shah_sunil]

Recently we have been trying to determine an antiallergic compound from it's tablet formulation and found that we get assay values to be lower by ~3-4 % of the labelled claim. Only when we use Trisodium phosphate for sample preparation, we are able to get assay values which are close to 100 % of label claim and hence most likely true values. We tried understanding this phenomenon by postulating that the reason for getting lower assay may be due to the fact that the part of the drug may be getting sorbed on the RP18 stationary phase and Trisodium phosphate may be helping to overcome this sorption process and hence yield better recovery. Does any body has such experinces and can there be any explaination for such phenomenon?

[tom jupille]

If the TSP is used in sample prep and not added to the mobile phase, then the hypothesis that it is interfering with sample sorption on the stationary phase is quite a stretch (the TSP and your analyte will quickly separate from each other).

A more likely explanation is that part of your analyte is sorbed/bound/conjugated/... to something in the formulation and is released by the TSP (high pH? Na+ concentration?).

[Rob Burgess]

Tom's answer makes a lot of sense here. What are the pKa's of your compound or is it a neutral species. What pH is your extraction solvent at then? Can you prepare a different extraction solvent at the same pH to see if that works, say perhaps just NaOH solution?

[shah_sunil]

We had tried sodium hydroxide as well, beside many other solvents. but, that has not helped. Only TSP seemed to be working. I too beleive that what Tom is saying is making sense. But, can there be any explaination for this phenomenon. The pH of sample solution with TSP is ~11 and the formulation is having routine excipeints used for a solid oral dosage.

[tom jupille]

At this point, you have eliminated Na+ and high pH as possibilities. Any chance that your compound is interacting with the phosphate? or that your compound is anionic and the phosphate is competing to liberate it?