NOVELTY - The plate has electrically conductive substrate (1) that is covered with light sensitive matrix (2). The matrix comprises light absorber, charge carrier, probe molecule and photo-sensitizer (3) arranged to oxidize probe molecule by irradiation of light from light source (4). The absorber and carrier comprise crystalline acid. The matrix comprises hybrid organic-inorganic gel, and xerogel. USE - Plate for matrix-assisted laser desorption ionization (MALDI) mass spectrometry for analyzing biomolecules such as peptide by structurally determining oxidation products of probe molecules (claimed) and the products of successive reactions involving the oxidation products of the probe molecules, and for screening and evaluation of anti-oxidants and drugs, and studying metabolic pathway in biological processes. ADVANTAGE - The structural determination of oxidation product of probe molecule can be enabled. DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method for preparing plate. DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of photo-reactive MALDI plate. Substrate (1) Light sensitive matrix (2) Photo-sensitizer (3) Light source (4) Portion of matrix (5)