Breath Test

Breath analysis usually measures just the volatile components of breath. Swiss scientists now show that mass spectrometry can be used to measure semivolatile and even nonvolatile compounds in breath. Such a method could be used for metabolic studies or diagnostic tests.

Renato Zenobi and coworkers at the Swiss Federal Institute of Technology, Zurich, use an unmodified electrospray ionization source to deliver the breath straight into the mass spectrometer (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200602942). Instead of mixing the sample with solvent first, they use the electrospray source's sheath-gas line—which usually assists in breaking up solvent droplets—to bring the breath to the charged solvent. The electrical charge is transferred directly from the solvent to the analytes in breath. The method, called extractive electrospray ionization (EESI), is a variant of desorption electrospray ionization (DESI).

Zenobi was surprised at what they could see. "We thought with breath you would just see the volatile compounds, but breath is in fact an aerosol," he says. "We think that some of the small droplets in the breath carry sugars and other nonvolatiles." For example, they were able to detect urea. They also measured nicotine and its metabolites in a female smoker's breath and compared alcohol metabolism in Asian and European males.

They simulated bad breath by eating garlic and then breathing into the instrument. At first, they had difficulty detecting the thiol compounds that give garlic its characteristic odor. But by using silver nitrate in the electrospray solvent, they were able to ionize the thiols. The silver converts the sulfur-containing molecules to cations "very easily," Zenobi explains, "so you see them right away."

"This study is interesting because the sample "breath' has been much studied by MS for its volatile constituents but not for its nonvolatiles, which presumably carry more biological information," says R. Graham Cooks of Purdue University, the original inventor of both DESI and EESI.