The invention provides methods and apparatus for the selective isolation of phosphorylated and glycosylated proteins and their fragments. A lanthanide metal cation is used to precipitate proteins or protein fragments containing phospho groups and/or glyco groups. The sample preparation method can be used for many types of biological samples, including HeLa cells, food, and human cerebrospinal fluid. The proteins are isolated, recovered and ready for analysis by mass spectrometry or other analytical methods allowing detection limits down to the femtomole level. The method and apparatus are valuable tools in the field of protein analysis and diagnostics.