We are research about extraction methods for drugs in blood serum.

We were speaking with Greg Kovac about how to denaturalize protein and we have a question:
How do we use AcN or any solvent for precipitate proteins in SPE methods:
in eluting solution or in a previous step.
Best regards friends

I do not see why you want to precipitate proteins with acetonitrile in a SPE method. You load the sample, wash the proteins out, retain the analyte, and then do sample clean-up.
Here is what we typically do: the plasma (or serum) sample is acidified to break the interaction of the analytes with the protein. If necessary, it is centrifuged to remove precipitates. The supernatant is loaded onto a SPE cartrige. Then proteins, sugars and salt are washed out with an acidic solution, preferentially using the same acid as was used for acidifying the sample. We use nearly exclusively Oasis cartridges. The remainder of the SPE depends on the type of analyte and the type of packing material chosen. You can find well established methods for hydrophobic cartridges and mixed mode ion-exchange cartridges on the web side or I can explain them in detail here.

Check the discussion "SPE for drugs in blood plasma from Feb. 18. In the ref. I gave there it was shown that the acid disruption doesn´t always work (there in the case of an ultrafiltration pre-purification). You will have to do a recovery check (absolute yields, not relative!!).