The mouthwash i have to analyze contains a mucoadhesive polymer that i have to separate during sample preparation. A common method is diluting the sample with methanol and centrifugating. This works fine with active principle quantification but for impurities analysis the dilution is too high! I tried SPE (Oasis) but the mouthwash doesn't pass through the column (under vacuum). Has anyone an idea how to bypass this sample preparation problem??

This might be one of those few cases where I would consider a precipitation procedure. Since you know more about the polymer than we do, you might be able to work out a suitable procedure - add salt, change pH, add an organic solvent etc.

Maybe it´s possible to do a vacuum concentration step after centrifugation. Therefore You´ll need a good internal standard.

Thank you Uwe. The polymer is Noveon ... acrilic cross linked and it is insoluble at low pH. The pH of the mouthwash is around 7,5-8. Lowering the pH without adding a solvent is a problem because also the active principle precipitates (and its related substances). I tried diluiting 1:2 with methanol and the polymer precipitates. But than i have too low concentrations for impurity analysis. One way is surely a concentration after precipitation but sample preparation for routine analysis becomes too laborius.

How about restricted access columns or ultrafilters? Also,the guys who decided to put this polymer into the mouthwash should know some of the characteristics which you can use to separate it. Maybe it also adheres to some powder or beads which can be filtered out?

From your description I would try percipitation and then try to increase the sensitivity of the method.
- Do you measure ar the UV maximum (supposing you are using UV)?
- How much is your injection volume? There are methods around that load 150 µL onto a analytical column.
Can you monitor the impurity if you (just theorethically) inject pure mouthwash?

Alex

Here is another idea: SPE with pushing instead of pulling...