Succinimidyl 4-[3,5-Dimethyl-4-(4-nitrobenzyloxy)phenyl]-4-oxobutyrate (1) is a photocleavable molecular tag developed by Maki et al, which generates an MS detectable ion selectively and reproducibly upon laser irradiation. For example, 1 reacts with amino acids, short peptides, and sugar derivatives to generate the adducts, which, when followed by negative mode LDI-MS detection, enables easy detection without assistance of a matrix. Quantitative monitoring of very small-scale reactions by TOF-MS is possible using 1 as a molecular tag.