Abstract: This invention concerns isotopically coded or non-isotopically coded affinity-tags for analysis of certain target molecules in complex samples, in particular for mass spectrometric analysis of proteomic samples. The affinity-tags have the following general formula X-SPACER-OPO3H2, wherein X is a functional group or moiety capable of reacting with a functional group of a protein, peptide, DNA, lipid, sugar and/or steroid. These phosphate affinity tags (‘PTAG’) are capable of high but reversible binding to metal-oxides like TiO2. Due to this property, tagged sample fractions can be isolated from non-tagged sample fraction by affinity chromatography. The binding of organophosphate to metal-oxides remains intact during multiple washings of preferably acidic solutions to remove non-specifically bound components. PTAG's are also envisaged wherein X is selected such that it is capable of binding proteins, peptides, nucleic acid molecules, lipids, carbohydrates, steroids and the like.