Purdue University graduate student Santosh Soparawalla, left, and postdoctoral researcher Fatkhulla Tadjimukhamedov demonstrate a miniature mass spectrometer used to detect chemicals on store produce.
Purdue News Service / Mark Simons
A team of scientists at Purdue University is perfecting a portable device that can quickly detect both chemical residues and bacterial contamination on fresh fruits and vegetables without complicated laboratory analysis.
In a recent trial in a supermarket, university graduate students armed with the machine were able to identify oranges with residues of the fungicide benzimidazole and apples that had been treated with the scald inhibitor diphenylamine (DPA).
The device is a miniature mass spectrometer that weighs 22 pounds compared to the 500-pound versions that are used in laboratories to identify chemical compounds.
The small size is one aspect of the new technology Dr. R. Graham Cooks and Dr. Zheng Ouyang and their team have created. Cooks is the Henry Bohn Haas Distinguished Professor of Chemistry and cofounder of the Purdue Center for Analytical Instrumentation Development in West Lafayette, Indiana. Ouyang is an assistant professor of biomedical engineering.
In an interview with Good Fruit Grower, Cooks said another facet combined with the small size could make this instrument a useful tool in food safety in the future. “The method works without complicated sample collection and preparation,” he said. “That means it can be used in situ, like when walking around in a supermarket.”
It’s also fast. The test takes only a few seconds.
The field trial focused on finding chemical residues, but Cooks said that in laboratory trials it was also able to detect bacteria like Escherichia coli (E. coli) and salmonella.
The work at Purdue dates back nearly a decade. From the start, researchers envisioned a Star Trek-like “tricorder,” a small instrument that, when pointed at an object, could reveal its chemical composition. Now, instead of tricorder, they use words like iPhone apps—something an ordinary person might use while shopping to scan an apple for chemical residue or bean sprouts for bacteria.
Mass spectrometry was developed more than a century ago and is a commonly used analytical method, but mass spectrometers cost $100,000 and up, are installed in laboratories, and require special sample preparation and analysis in a vacuum chamber. Cooks and his team developed “ambient ionization,” in which a handheld wand dislodges ionized molecules from a surface and vacuums them into the equipment for analysis. It works in open air.
The small mass spectrometer works like the larger laboratory version, although it is less precise, Cooks said. It can identify the presence of a compound, but not tell exactly how much it found. It could be used for screening, but more careful analysis would be needed to quantify the finding.
Two methods tested
The team tested two ambient ionization methods, according to information in a news release from Purdue. Both involve ionizing molecules on a sample’s surface. This ionization step gives charge to the molecules and allows them to be identified by the mass spectrometer.
In one method, called paper spray ionization, a sample is wiped with a common lens wipe wet with alcohol. A small piece is cut from the wipe and placed on a special attachment of the miniature mass spectrometer where a high voltage is applied. The mixture of alcohol and residues from the sample’s surface become fine droplets containing ionized molecules that pop off the wipe and are vacuumed into the mass spectrometer for analysis.
In the other method, called low-temperature plasma ionization, a special probe sprays a collection of charged particles onto the sample’s surface using a slow stream of helium gas. The charged particles ionize molecules on the sample’s surface, which then bounce off the surface and are vacuumed into a mass spectrometer for analysis.
While this new device has application in food safety, it is much more widely applicable. It has applications in medical diagnostics and airport security, Cooks said.
The machine has been used to identify $50 bills containing traces of cocaine, to sniff luggage for bombs, and to analyze fingerprints for evidence of explosives. It can scan blood samples from patients to determine whether drugs administered for illness are circulating at therapeutic levels and detect molecular disease markers in urine samples.
Mass spectrometry produces a graph showing peaks that are unique to every chemical compound, Cooks said. For the machine to identify a chemical, it must compare this to a known pattern. Over the years, a library of chemical fingerprints has been developed, and computers scan and match them automatically.
A number of patents have been applied for or granted, and work is under way with companies interesting in commercializing the technology, Cooks said. In food safety applications, it could be used in testing all along the food chain, from producers to shoppers. It could be used by producers, in packing houses, or by regulators, but the price tag will probably be high initially.