Spectroscopy of Levitated Droplets

Researchers used sound waves to levitate droplets of water. This approach allows the water to evaporate, which concentrates the sample for spectroscopic detection of harmful heavy metal contaminants such as lead and mercury in water. (Source: J. Peralta & V. Contreras, UNAM)

In a new study, researchers showed that using sound waves to levi­tate droplets of water in midair can improve the detec­tion of harmful heavy metal conta­minants such as lead and mercury in water. Detecting small amounts of heavy metals in water is important because these conta­minants are harmful to human health and the environ­ment. The new technique could even­tually lead to instru­ments that perform real-time, on-site contaminant moni­toring, which could help prevent future lead conta­mination problems.

“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 specia­lized labora­tory analysis,” said the research team leader Victor Contreras from Insti­tuto de Ciencias Físicas UNAM, Mexico. “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.”

The researchers used laser induced break­down spectro­scopy (LIBS) to analyze heavy metals present in levi­tating drops of water. Levi­tating the water droplets allows the water to evaporate in a controlled position, which increases the mass concen­tration of conta­minants in the sample and makes it easier to perform LIBS analysis. The researchers showed that their new approach can reliably detect very low levels of the heavy metals like barium, cadmium and mercury with analysis times of just a few minutes.

The researchers used LIBS because it offers a fast and straight­forward way to identify several elements simul­taneously. LIBS works by focusing a high energy laser pulse onto a sample, which vapo­rizes the material and generates a plasma. Because the light emitted by the plasma contains the atomic finger­prints of the material, it is possible to identify the chemical components of the sample by analyzing the emitted light. It is a straight­forward process to use LIBS analysis on solid samples. In fact, several commercially available handheld devices are available for this type of analysis. However, it is difficult to use this method to directly analyze liquids because the plasma formed in liquids cools down faster and lasts a very short time. In addition, producing a plasma on a liquid surface produces water splashes that directly affect the spectro­scopy reading.

With liquid samples, creating a plasma that provides a good signal for chemical detection requires high levels of laser energy, which can only be provided by bulky, non-portable lasers. To circum­vent this problem, liquid samples are typi­cally analyzed by placing a drop on a substrate and waiting for it to dry in order to concen­trate the elements of interest in the sample. Although depo­siting the sample on a substrate is quite simple, the laser pulse excites atoms from elements in the sample as well as from the substrate. Besides, water evaporation could lead to inhomo­geneous distri­bution of the impurities on the substrate, compro­mising its signal repro­ducibility.

Instead of depo­siting the droplets onto a substrate, the researchers used intense sound waves to levi­tate single droplets of water. The sound waves produce a force strong enough to counter­act gravity, allowing a droplet to hover unsupported in the air. “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 researchers showed that using acoustic waves to levi­tate a single drop of water allowed them to detect very low concen­trations of heavy metals. For example, they detected 0.7 milli­grams per liter of cadmium and 0.2 milli­grams per liter of barium. They also showed that the acoustic levi­tation technique they used is stable enough for repro­ducible LIBS analysis. “This technology has a potential to simul­taneously detect heavy metals and other elements in water in a fast and cost-effective way,” said Contreras. “An online analyzer based on our tech­nology could one day help prevent environ­mental disasters and contri­bute to improved water quality control.”

The researchers are now working to improve the instru­mentation. For example, they want to optimize the mechanical design of 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 stably levi­tating smaller drops, which further concen­trates the conta­minants. This is a key step toward minia­turizing the device because it will allow the use of less sensi­tive, but more compact detectors. (Source: OSA)

Reference: V. Contreras et al.: Chemical elemental analysis of single acoustic-levitated water droplets by laser-induced breakdown spectroscopy, Opt. Lett. 43, 2260 (2018); DOI: 10.1364/OL.43.002260

Link: Laboratorio de Espectroscopia, Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Chamilpa, Mexico

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