Desorption electrospray ionization has taken mass spectrometry to new places
Desorption electrospray ionization (DESI) was the original ambient sample ionization method for mass spectrometry (Science 2004, DOI: 10.1126/science.1104404). In the 10 years since R. Graham Cooks, Zoltán Takáts, and coworkers at Purdue University first described DESI, many other ambient ionization methods have joined it, but none have managed to supplant it.
Ambient ionization “represents a huge advance in chemical analysis because it so simplifies sample preparation,” says Richard N. Zare, a chemistry professor at Stanford University. Zare’s lab uses DESI to characterize short-lived reaction intermediates.
DESI works by bombarding a sample surface with charged solvent droplets. The droplets in turn desorb charged microdroplets that proceed to fission and evaporate to yield gas-phase ions that are transported into a mass spectrometer. The sample typically is left out in the open and requires little to no sample prep. Mass spec sampling is normally done under vacuum.
The proliferation of ambient ionization methods has been “the second revolution in mass spec,” says Renato Zenobi, a chemistry professor at the Swiss Federal Institute of Technology (ETH), Zurich. The first revolution was the development of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) sampling methods, he notes. Zenobi expects eventual consolidation of the various ambient methods. “There are too many different acronyms, which is confusing,” he says.
The original understanding of DESI was purely empirical, Cooks adds. “We coined the term ‘droplet pickup’ as an explanation for the mechanism,” he says. “The real explanation only came about with careful measurement of droplet sizes and velocities using laser scattering experiments.” But Cooks believes there’s still more to learn about the mechanism before it will be fully understood.
At first, people thought of DESI and its cousins as being qualitative methods at best, Cooks says. “That’s just not true. The ambient methods are turning out to be excellent quantitative methods.”
DESI technology is commercially available from Prosolia, an Indianapolis-based company headed by Justin M. Wiseman, a coauthor on the original DESI paper. Besides routine sampling, DESI has proven useful for imaging applications. For example, DESI has been in the operating room. Both Cooks and Takáts have developed methods for analyzing tissue to help establish tumor boundaries during cancer surgery.
“The future of ambient ionization mass spectrometry is extremely bright,” Zare says, “especially as mass spectrometers become less expensive and more portable.”