Ultrasonic levitation helps detect water contamination

(Cedit: Jairo Peralta and Victor Contreras, Instituto de Ciencias Físicas UNAM)

Laser induced breakdown spectroscopy (LIBS) is commonly employed to identify what chemicals are present in a particular sample of a substance. While this is a straightforward process with solids, it is more difficult with liquids, as the plasma created by the laser pulse lasts such a short time, making the chemical fingerprint hard to detect. Typically, samples of water have to be sent to laboratories where high-powered, non-portable lasers perform the task.

Using sound waves, the researchers suspended a single droplet of water that allowed them to detect very low concentrations of heavy metals using LIBS. In their paper, published in the journal Optics Letters, they detected 0.7mg/L of cadmium and 0.2mg/L of barium. It’s hoped that the work will form the basis of a portable or handheld device that can perform vital water analysis on-site.

"Despite the large variety of water sensors that offer continual monitoring, detection of multiple heavy metals dissolved in water can only be performed by sending samples off for specialised laboratory analysis," said the research team leader Victor Contreras, from Instituto de Ciencias Físicas at the National Autonomous University of Mexico (UNAM).

"Our new technique is one step toward the development of a simpler analysis approach that could be applied on-site and in real time. This type of water analysis could be used by agricultural, pharmaceutical, water purification and other industries to monitor water for contaminants."

Currently, analysing water samples on-site requires drops of water to be evaporated on a substrate. However, spectral analysis can then contain elements of the substrate, as well as inconsistent readings due to changes in concentration caused by the evaporation. By levitating the water drops, these problems are avoided, providing more accurate readings that can be consistently reproduced.

"Acoustic levitation is a simple and inexpensive method to preconcentrate the elements of interest while avoiding contamination from the substrate surface," said Contreras. "Moreover, it does not require the sample to have any type of electric or magnetic response like some other methods used to achieve levitation."

The team is now working to optimise the acoustic trap to achieve more stable levitation conditions, which will improve the reproducibility of the LIBS readings. They also want to increase the sensitivity by levitating smaller drops, which further concentrates the contaminants and will ultimately make it easier to develop a handheld device.

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