John Fenn developed electrospray ionization for analyzing large biological molecules.
Electrospray ionization, usually abbreviated ESI, converts molecules in a liquid into charged ions that can be measured by a mass spectrometer. Fenn showed that the technique could gently transfer large molecules such as proteins into the gas phase without destroying them. This made mass spectrometry far more useful for biological research.
Fenn shared half of the 2002 Nobel Prize in Chemistry with Koichi Tanaka; Kurt Wüthrich received the other half for developing nuclear magnetic resonance methods for determining three-dimensional structures of biological macromolecules. Electrospray ionization is now central to proteomics, pharmaceutical analysis, and biomolecule research. It is not the same as electron ionization, an older technique that often fragments molecules more extensively.