No Fuss, No Muss Mass Spec

Imagine doing a mass spectrometric analysis directly on your skin. With a new ionization method called desorption electrospray ionization (DESI), such an analysis could soon be possible on a routine basis.

In DESI, an ionized stream of solvent--produced by an electrospray source--is sprayed on the surface of a sample at ambient temperature. No prior sample preparation is needed. The solvent clusters in the beam act as projectiles, knocking ions from the sample, which are then propelled to the mass spectrometer through a hose [Science, 306, 471 (2004)]. Thus, DESI takes sampling for mass spectrometry out of the vacuum chamber and moves it into the lab and, in principle, beyond.

"The fact that you can examine material on ordinary surfaces--cloth, leather--without any preparation and do this in ambient air makes this a striking new method," says Purdue University chemistry professor R. Graham Cooks, who led the study. Cooks predicts that the technique will be a "knockout winner" in forensic and public safety applications, including the analysis of dried blood, detection of explosives, and monitoring of chemical warfare agents. He also sees applications in drug metabolism studies and in two-dimensional surface analysis to locate the position of analytes in a sample matrix.

Using an electrospray source to produce the ionized solvent that acts as the primary-ion beam is "a completely novel concept," says Renato Zenobi, an analytical chemistry professor at the Swiss Federal Institute of Technology, Zurich. "Although the ultimate sensitivity and the mechanism for ion production are still being worked out, I believe this method will have great impact for the application of mass spectrometry to real-world samples."

Cooks and his colleagues, including postdoctoral associate Zoltán Takáts, have taken mass spectra of a variety of compounds, ranging from the small, nonpolar lycopene in tomatoes to large, polar peptides and proteins. They have also shown that drug molecules can be measured on the surface of the skin.

Although not necessary to obtain spectra, the solvent system--water or a water/methanol mixture--can be adjusted to optimize the signal for particular analytes. Changing the nature of the spray solution is simply like changing the reagent gas in a chemical ionization experiment, Cooks says. For example, they optimized detection of the explosive RDX by adding trifluoroacetic acid to the solvent.

Cooks and his colleagues are moving forward on several fronts with the DESI technology. They are doing experiments to understand the fundamental ionization mechanism. They also plan to couple DESI with the miniature mass spectrometer that Cooks has developed and start doing experiments outside the laboratory.