A new approach to detecting metal contaminants in water involves the use of laser-induced breakdown spectroscopy (LIBS) to analyze the presence of heavy metals, such as mercury, in water drops that are levitated in midair using ultrasonic waves. The sound waves produce a force strong enough to counteract gravity, allowing a droplet to hover unsupported in the air. 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. Using this method, researchers from Instituto de Ciencias Físicas UNAM detected even very low concentrations of heavy metals. For example, they detected 0.7 mg/l of cadmium and 0.2 mg/l of barium. They also showed that the acoustic levitation technique was stable enough for reproducible LIBS analysis. LIBS works by focusing a high-energy laser pulse onto a sample, which vaporizes the material and generates a plasma. The light emitted by the plasma contains the atomic fingerprints of the material. The chemical components of the sample are identified by analyzing this emitted light. 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. Courtesy of Jairo Peralta and Victor Contreras/Instituto de Ciencias Físicas UNAM. With liquid samples, creating a plasma that provides a good signal for chemical detection requires high levels of laser energy. To circumvent the need for bulky, nonportable lasers, 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. With this method, there is a chance that the laser pulse will excite atoms from the substrate and from the sample. “Acoustic levitation is a simple and inexpensive method to preconcentrate the elements of interest while avoiding contamination from the substrate surface,” said researcher Victor 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 new technique could eventually lead to instruments that perform real-time, on-site contaminant monitoring. The process, from placing the sample in the acoustic levitator to the chemical identification of the trace elements, takes only a few minutes. A water droplet was analyzed with laser-induced breakdown spectroscopy (LIBS), which uses a high-energy laser pulse to vaporize the sample and generate a plasma. The light emitted by the plasma can be detected and used to identify the chemical components of the sample. Courtesy of Victor Contreras/Instituto de Ciencias Físicas UNAM. “This technology has the 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.” Researchers are now working to improve the instrumentation. 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 would further concentrate the contaminants. This is a key step toward miniaturizing the device because it will allow the use of less sensitive, but more compact detectors. The research was published in Optics Letters, a publication of OSA, The Optical Society (doi:10.1364/OL.43.002260).