Optimization of protein precipitation based upon effectiveness of protein removal and ionization effect in liquid chromatography–tandem mass spectrometry

Four categories of protein precipitation techniques (organic solvent, acid, salt and metal ion) were tested in plasma using spectrophotometry to assess protein removal efficiency across a range of volumes, species and lots. Acetonitrile, trichloroacetic acid (TCA) and zinc sulfate were found to be optimal at removing protein in their categories (>96, 92 and 91% protein precipitation efficiency at a 2:1 ratio of precipitant to plasma, respectively). A post-column infusion LC–MS/MS system was used to assess ionization effect of a protein-bound drug caused by the endogenous components remaining after using various protein precipitants. The extent of ionization effect varied with mobile phase (−20 to 93%), protein precipitant (0.3–86%), but only slightly with species (86–93%). The optimal bioanalytical methodologies for removal of plasma proteins and minimal ionization effect for the probe molecule in positive ion turboionspray LC–MS/MS involve the use of TCA for precipitation with mobile phases consisting of either pure organic solvents (methanol:water or acetonitrile:water) or precipitation with all of the mass spectrometer compatible precipitants evaluated with a methanol:aqueous 0.1% formic acid mobile phase.