Laser plasma spectroscopy provides measurements of trace pollutants in vegetables

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A high-resolution, time-resolved spectroscopy system has helped researchers make the first quantitative measurements of trace elements in fresh vegetables using a technique based on the detection of optical emission from a laser-induced plasma. Laser-induced breakdown spectroscopy (LIBS) is a sensitive, reliable and well-established technique for detecting trace elements. This study is the first to use LIBS to measure potentially harmful trace element concentrations directly in fresh vegetables, rather than just detecting the presence or absence of these elements.

It represents the first step towards a new, faster and more sensitive tool for measuring trace pollutants in fresh vegetables intended for public consumption, and by extension studying the link between soil pollution and food impurities. No sample preparation was needed for the analysis; the laser plasma was induced on a potato skin, which opens up the possibility of field applications of the technique.

The researchers, led by Professor Jin Yu from the University of Lyon, France, used a time-resolved spectroscopy system composed of Andor iStar ICCD and Mechelle spectrometer to observe the emission spectrum of a transient plasma – with a lifetime in the order of the microseconds - generated by focusing a laser pulse at a potato skin.

'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 reaching the food chain, and clean up those places, if you have a sensitive and reliable detection technique,' said Dr Matthieu Baudelet, a member of Professor Yu's team, now at the University of Central Florida. '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.'

The iStar camera was used to record emissions within a fixed time window with a precision of tens of nanoseconds. A suitably time-resolved detection enabled the determination of the electron density, the plasma temperatures and, from these, the trace element concentrations.

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

Based around the Andor Mechelle 5000, the system allows the measurements over a very large spectral range - from UV to infrared. According to Professor Yu: '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.'