Spectroscopy detects toxins in veggies

Krista D. Zanolli, krista.zanolli@photonics.com

For as long as picky eaters have resorted to hiding vegetables or feeding them to the dog under the table, they also have sought excuses not to eat them. But “They’re icky” doesn’t hold up against the advice of nutritionists. And now neither does the health concern, “What if the soil is polluted?”

A new high-resolution, time-resolved spectroscopy system is helping researchers directly measure trace pollutants in fresh vegetables that are intended for public consumption. There goes the “pollutant” excuse.

The research, led by Jin Yu, a professor at the University of Lyon, used a system composed of Andor Technology’s iStar ICCD camera and Mechelle 5000 spectrograph to detect optical emissions from plasma using a technique called laser-induced breakdown spectroscopy (LIBS).

LIBS is useful for detecting trace elements because it is both sensitive and reliable. In the LIBS process, a high-power laser pulse is focused onto a sample – in this case, a vegetable – to create plasma, a gas made up of electrons and ions; a spectrograph then analyzes its optical emission. The LIBS technique has appeal because little or no sample preparation is required to obtain useful results, opening up the possibility of field applications.

Yu and his team observed the emission spectrum of the plasma generated by focusing a laser pulse on a potato skin. Within a fixed time window, the emission from the plasma plume was collected and evaluated by the detection system, which determines electron density and plasma temperature. From those readings, the trace element concentrations were determined. This technique is the first step toward studying the link between soil pollution and food impurities.

“The real challenge for the LIBS technique is to get quantitative measurements of trace elements contained in a complex matrix, such as a fresh vegetable, because we don’t know in detail the property of the plasma generated by a laser on it,” Yu said. “The Andor system is important to this work because we can make time-resolved observations of the plasma.”

Time-resolved observations are important because the plasma plumes expand with time. After about 1 µs from the incident laser pulse, discrete spectral lines start to become apparent. The spectral lines and the timing vary depending upon the sample, the distance from the center of the plasma and the wavelength of the laser light, but the evolution of the changes within the plasma plume occurs on a microsecond timescale.

Yu also noted the importance to the study of the system’s high resolution. “The Andor system also has a high resolution. When we combine these attributes, we can simultaneously measure a large number of elements. It’s a truly multiple-element detection system,” he said. The spectrograph also allows measurements over a very large spectral range – from the ultraviolet to the infrared.

“There are several metals that are harmful or beneficial to your health. One example is copper, which is toxic if you absorb too much of it. You can prevent vegetables grown in environments with too much copper from reaching the food chain and clean up those places, if you have a sensitive and reliable detection technique,” added Dr. Matthieu Baudelet, a member of Yu’s team who is now at the University of Central Florida in Orlando. “LIBS is a good technique for this kind of analysis because the hot plasma can excite every element in a vegetable – even if it’s present at low concentrations.”

Results of the study were published in the November 2009 issue of Spectrochimica Acta, Part B.