The team estimated a concentration of 15 parts per million of DPA was present on an apple, but the margin of error for the test is large enough that the concentration could easily be within the regulated limits, Soparawalla said.

"These tests of apples demonstrate how this technology could be a part of a larger regulatory system. The experiments were not a robust scientific examination of the levels of chemicals present on produce," Cooks said. "The test is what's called a factor of two test, meaning the actual concentration could be half or could be twice as much as the approximation. The results were not statistically above the legal limit, but it is food for thought."

The team also examined the distribution of the chemical in a cross section of the apple and found DPA throughout, with the greatest concentrations in the skin and near the core of the apple, Cooks said.

"It appears that washing or peeling an apple may not reduce one's exposure to the chemical much," Cooks said. "If the approximate levels held true, eating one apple a day would bring a person to the daily limit of exposure to diphenylamine."

The team found that benzimidazole was limited only to the skin of the orange.

The team tested two ambient ionization methods. 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 the first method, called paper spray ionization, a sample is wiped with a common lens wipe wet with alcohol. A small triangle is then 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 of the wipe and are vacuumed into the mass spectrometer for analysis.

In the second 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.

This work was presented on Tuesday (June 7) at the American Society for Mass Spectrometry annual meeting in Denver. Funding from the National Science Foundation and Thermo Fisher Scientific supported this work.

The paper spray ionization method is licensed to the Alfred Mann Institute for Biomedical Development at Purdue University, and the low temperature plasma ionization method is licensed to Thermo Fisher Scientific Inc.

A video of the miniature mass spectrometer is available on YouTube at