Matrix Assisted Laser Desorption Ionization(MALDI)mass spectrometry is limited to analysis of small metabolites due to extensive background peaks in the low mass range generated by organic matrix materials.It was demonstrated that photoinduced heterogeneous electron transfer across the semiconductor-molecule interface is not only conceptually new but also practically important as a reliable soft ionization technique [1].Irradiation of UV laser(355nm)on nanoparticles of semiconductors such as ZnO excites electrons from valent bonds to conduction bonds.Electrons with enough kinetic energies can tunnel away from the surface of nanoparticles and are eventually captured by charge deficient atoms of adsorbed organic molecules.Resultant unpaired electrons subsequently initiate cleavages of nearby chemical bonds.LAET has been applied to in situ microanalysis of endogenous metabolites in rat brain tissues.