We work on drugs in human plasma. Sometimes we have to use silanized test tubes and silanized sample vials for sample preparation. If regular test tubes are used, sample recovery is lower. What causes this kind of problems? If the drugs are strongly absorbed onto glass surface? What surface chemistry does regular test tubes have? Those drugs are not always very polar. Do silanol or residual metals on glass surface cause sample decomposition?

Thanks a lot for your help!

First, although one might expect that a glass surface is hydrophilic and thus not have any affinity for nonpolar analytes, the fact is that glass is always contaminated from the environment unless the glass is chemically cleaned within a few hours prior to use. A clean glass surface will have a contact angle of less than 10 degrees but most glass articles used in the lab have a contact angle of greater than 90 degrees unless it has recently been chemically cleaned. So, nonpolar analytes may well be retained on this contaminated surface.

Second, although you don't say so explicitly, it sounds like you are observing this problem exclusively in conjunction with proteinaceous samples. If this is so, your analytes may be sticking to adsorbed proteins on the surface. Certain proteins such as serum albumin are well documented regarding their tendency to stick to glass surfaces (along with a number of other surfaces). Furthermore, such "carrier proteins" are especially good at binding molecules to their surface. So the problem you describe may be caused by your nonpolar analytes binding to proteins which have adsorbed on the surface of your glass. Silanized surfaces are supposed to prevent this problem by preventing surface wetting although one would think that this would also increase the nonpolar interaction between your analytes and the surface.