Officials examining recent cases of E. coli infections linked the pathogen to contaminated green onions, lettuce, and other foods. Timely identification of these and other potentially deadly pathogens is key to heading off widespread outbreaks. A typical way of analyzing bacteria and other microorganisms is with a mass spectrometer. But the process takes several hours and requires that samples be specially prepared in a lengthy series of steps.

Now, a technique from researchers at Purdue University rapidly detects and precisely identifies bacteria, including dangerous E. coli, without time-consuming pretreatment.

The technique called desorption electrospray ionization (DESI) could help build a new class of fast, accurate detectors for applications ranging from food safety to homeland security, says the group. The method can detect living, untreated bacteria, including E. coli and Salmonella typhimurium, both of which cause potentially fatal infections in humans. The ability to analyze living systems is a plus because bacteria retain their original properties.

Mass spectrometry works by turning molecules into ions inside the instrument's vacuum chamber. Once ionized, the equipment analyzes molecules based on their mass. DESI, in contrast, performs the ionization step in air or directly on surfaces outside of the mass spectrometer's vacuum chamber.

Water sprayed in the presence of an electric field generates positively charged hydronium ions that contain an extra proton. When the positively charged droplets touch the sample under test, the hydronium ions transfer their extra proton to sample molecules, ionizing them. Vacuum then transports the ionized molecules from the surface into the mass spectrometer for analysis.

The equipment can detect one nanogram of a particular bacterium down to its subspecies, a level of accuracy needed to detect and track infectious pathogens. The Purdue group is also working on a shoebox-sized mass spectrometer that weighs just 22 lb, about one-thirtieth that of conventional models. Combining the portable mass spectrometers with DESI could make possible a new class of compact detectors. Prosolia Inc. in Indianapolis plans to commercialize DESI.