The present invention provides a novel procedure of preparing samples for analysis by way of mass spectrometry, preferably LC-MS/MS. Accordingly, functionalized magnetic particles with a hydrophobic surface were used for extracting low molecular weight compounds from complex liquid biological samples such as plasma, serum, whole blood or hemolyzed blood. The method of the invention includes (a) contacting the sample with an amount of functionalized magnetic particles with a hydrophobic surface, (b) incubating the sample and the particles, thereby adsorbing the compound to the hydrophobic surface, (c) separating the particles by applying a magnetic field and removing the liquid, (d) optionally washing the particles, (e) eluting the compound from the particles.