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OPTICS

A light in the field

São Carlos group builds a laser-equipped robotic device to analyze chemicals in soil and plant leaves

INFOGRAPHIC ANA PAULA CAMPOS ILLUSTRATION ALEXANDRE AFFONSOSmall beams of laser light focused onto a plant leaf or patch of soil could become commonplace in agriculture within a few years. Lasers are becoming a reliable means of analyzing chemical elements in plants and obtaining important data on fertilizer use, for example. The best news is that the test can be done in real time, right in the field within a few minutes, even with the help of a GPS. This technology, known as laser-induced breakdown spectroscopy (LIBS), is being used on Mars by NASA’s Curiosity robot to test for elements such as iron, carbon and aluminum in rocks on the Martian surface. A similar apparatus has been developed at Embrapa Instrumentação in São Carlos, in inland São Paulo State. “It is the first LIBS system built in Brazil,” says researcher Débora Milori of Embrapa (the Brazilian Agricultural Research Corporation), who heads the project at the Optics and Photonics Research Center, one of FAPESP’s Research, Innovation and Dissemination Centers (RIDCs). FAPESP is also providing research support through a post-doctoral grant to physicist Jader Cabral.

A more sophisticated version of the LIBS technique uses a double-pulse system. In this case, there is a delay between the two laser pulses, lasting from microseconds to nanoseconds. The advantage over the single-pulse system, more widely used by the scientific community, is the possibility of enhancing signal intensity several times over and thus improving the technique’s detection limit for quantifying elements. Embrapa has also recently built a double-pulse LIBS system with collaboration from physicist Gustavo Nicolodelli, a FAPESP post-doctoral grant recipient.

“There are several studies showing that the double-pulse LIBS system results in better analytical performance, and provides greater sensitivity and detection limits 2 to 20 times higher than those obtained with a single-pulse laser,”  says Professor Francisco Krug of the Center for Nuclear Energy in Agriculture (Cena) at the University of São Paulo (USP). He headed a FAPESP-funded project from 2005 to 2009 that advanced our understanding and development of leaf and soil analysis using laser-induced breakdown spectrometry. Milori and other researchers at Embrapa also participated in the project. “Generally speaking, double-pulse LIBS requires instrumentation that is a little more complex, but both systems are relatively simple. The portable systems currently available on the market are based on measurements generated by a laser pulse,” Krug says.

“In any case, double-pulse technology incorporated into portable devices will no doubt expand the number of analytical applications even further,” he says. “Commercial units that explore other areas of spectroscopy are already installed on agricultural machinery, making it possible to analyze soils in real time, and we anticipate that LIBS will very soon be instrumental in assessing the nutritional status of agricultural crops in real time as well.” The Atomic Spectroscopy Group at Cena – USP, headed by Krug, specializes in the development and validation of quantitative methods for direct analysis of agronomically and environmentally important samples.

“The quantification can be done through calibration models using reference samples. The emission intensity of an element is proportional to its concentration in the sample. The calibration is heavily dependent on the  matrix, meaning that you have to build one model to quantify carbon in soil and a different one for carbon in plants,” says Milori. Laser analysis is also advantageous because it takes less time compared to conventional tests conducted in laboratories, where the sample must be prepared with chemical reagents. Used without these products, LIBS helps reduce waste. “It’s a clean technique,” Milori says.

The objective of the Embrapa researchers was to build a portable device and adapt it to a kind of trolley resembling a robot that can be taken into the field. “We put together a robotic system to demonstrate the concept, in collaboration with professors Marcelo Becker and Daniel Magalhães of the São Carlos School of Engineering at USP (EESC – USP).

Milecular breakdown
A pulsed laser on the robot is focused onto samples of leaves or soil. The spot heats up, reaching a temperature of up to 50,000 degrees Kelvin (K). The thermal effect causes the molecules in the material to break down and evaporate into a plasma, i.e., a dense, gaseous cloud of atoms, ions and electrons. After a few microseconds, the plasma cools to temperatures on the order of 5 to 15,000K, and light emission lines characteristic of each chemical element in the sample appear. That luminosity is captured by a set of lenses on the apparatus and is focused onto a spectrometer.

In the spectrometer, the light will be detected by an optoelectronic system such as those found on digital cameras to capture images. Depending on the light spectrum emitted, the elements present in the sample—such as phosphorous, carbon and copper, for example—can be identified in the apparatus. “The emissions produced by the atoms and ions represent the fingerprint of each chemical element,” says Milori. Laser analysis falls within the concept of precision agriculture, which increasingly uses IT instruments and resources such as computers, GPS and wireless networks to implement improvements in agricultural production.

Projects
1.
Development and evaluation of a double-pulse LIBS system: Application for soil characterization (nº 2012/24349-0); Grant mechanism Post-doctoral research grant Recipient Gustavo Nicolodelli/Embrapa; Coord. Débora Milori/Embrapa; Investment R$163,082.88 (FAPESP).
2. Analysis of soils using photonic techniques for the development of portable equipment for in situ measurements (nº 2012/22196-1); Grant mechanism Post-doctoral research grant; Recipient Jader de Souza Cabral/Embrapa; Coord. Débora Milori/Embrapa; Investment R$163,082.88 (FAPESP)
3. CEPOF – Optics and Photonics Research Center (nº 2013/07276-1); Grant mechanism Research, Innovation and Dissemination Center (RIDC) Program; Coord. Vanderlei Bagnato/USP; Investment R$1,000,000.00 per year (FAPESP)

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