PFP-PEG-Azido is a Azido labeling reagent that reacts with primary and secondary amines. The pentafluorophenyl (PFP) ester-activated PEG linker is less subject to hydrolysis than NHS esters, resulting in more efficient reactions. PFP-PEG-Azido must be first dissolved in a minimal amount of an organic solvent, such as dimethylsulfoxide (DMSO) or dimethylformamide (DMF) and then added to the buffer containing the protein or other molecule. The reagent forms an emulsion that allows the reaction to proceed.