Holding a diffraction grating in sunlight, I recently explained to a group of children how it created the rainbow on the sidewalk. That demonstration goes back to 1666, when Sir Isaac Newton first showed this phenomenon and coined the word spectrum. To do that, he built a spectroscope, which launched the field of spectroscopy. Scientists still use this technology for many applications, such as toxicity monitoring.
Although we are more than 350 years beyond Newton’s first experiments with optical spectroscopy, it remains a powerful tool, especially in monitoring environmental toxicity. “Optical spectroscopy provides the potential for a rapid, cost-effective screening method for toxicity in contaminated waters,” says David Podgorski, assistant professor of chemistry at the University of New Orleans in Louisiana. “The method is high throughput and portable, and the results are readily reproducible in different laboratories.”
With different types of spectroscopy and methods of using it, many kinds of environmental toxicity can be studied. Here are a few examples.
Into the wild
Podgorski and his colleagues are trying to design optical probes that can pick up the wavelengths associated with potentially toxic organic pollutants. This could be implemented in a field-ready device. “This development will hopefully provide us with the opportunity to screen for toxicity in real time,” Podgorski notes.
For this research, Podgorski’s team uses a kind of fluorescence spectroscopy called excitation-emission matrix spectroscopy. The organic compounds in many toxic substances, such as petroleum, strongly emit light. “We have a general understanding of the molecularlevel composition and structure of compounds with excitation and emission maxima in different regions, even for those in a complex mixture of organic compounds,” Podgorski explains. “With the help of statistical methods, such as parallel factor [PARAFAC] analysis, we can figure out which components of the organic mixture correlate with toxicity.”
Related Article: A Wireless, Wearable Toxic-Gas Detector
In a 2018 issue of Environmental Science & Technology, Podgorski and his colleagues applied optical spectroscopy and PARAFAC to samples of groundwater. The scientists were looking for dissolved organic matter from oil spills. They concluded that this methodology can be used “in assessing the spatial and temporal natural attenuation and toxicity of the [dissolved organic matter] in petroleum-impacted groundwater systems.”
As this work by Podgorski shows, it’s not just the spectroscopy technology that counts, but also how the data get analyzed. In some cases, though, the best devices for analyzing spectroscopy are in a scientist’s pocket.
Phone it in
“Smartphone spectroscopy is a potentially very valuable tool for toxicity monitoring,” says Andrew McGonigle, reader in volcano remote sensing at the University of Sheffield in the UK. For example, a sensor connected to a smartphone could capture “the absorption of light by contaminants in water samples, and the concentration of these pollutants can [then] be determined,” he says.
The capabilities of a smartphone support this approach in many ways, one of which comes from the power of these small computers. As McGonigle describes it, “The onboard processing power of the unit enables full data capture and processing.” Further, he notes that “there is also the possibility of smartphones acting as nodes in Internet of Things–type networks in order to generate spatial data arrays, then coupling to the cloud for cloud-based analytics of the acquired data.” That could create real-time toxicity monitoring across wide ranges and the ability to track the data over time.
It’s not enough, though, to make it merely possible to monitor toxicity with spectroscopy, because it must also be practical. That might be the biggest advantage of smartphone-based spectroscopy. As McGonigle points out, “The low cost of these units—in comparison to some traditionally applied laboratory instrumentation—makes smartphone-based systems particularly useful in resource-limited settings.” As he concludes, “Portability, connectivity, and low cost are the key selling points here.”
Resonating with toxicity
It doesn’t take centuries-old technology to put spectroscopy to work with toxicity. For instance, nuclear magnetic resonance (NMR) is just 80-years-old. Scientists use this methodology to study the structure of chemicals and the interactions of molecules.
In 2014, Andre Simpson, director of the Environmental NMR Center at the University of Toronto, Scarborough Campus, and his colleagues wrote in Magnetic Resonance in Chemistry: “The practices of current and previous generations have left behind a legacy of contaminated land and water…Nuclear magnetic resonance is arguably the most powerful tool in modern research, as it provides unprecedented levels of molecular information on structures and intermolecular and intramolecular interactions.”
Simpson adds that NMR can be applied in vivo. In 2018, in Metabolites, Simpson and his colleagues described using NMR to study metabolomics. In fact, Simpson says that “the living organism becomes the ‘ultimate biosensor’ responding in real time to its environment, while the NMR spectrometer interprets the biochemical changes, providing information explaining sub-lethal toxicity.” Using NMR to study in vivo toxicity, Simpson and his team explore many applications. “The ultimate goal is to understand and categorize complex stress responses such that, one day, these molecular fingerprints can be used to identify the exact cause of environmental stress in natural organisms—and even humans—permitting improved monitoring, targeted remediation, prevention, and policy measures,” he says.
Tomorrow’s scientists will surely find ways to further advance what Newton started. As I watched the children marvel at the rainbow on the concrete, I wondered whether one of those future scientists was crowded around me. Sometimes, all it takes to move science ahead is taking the time to look at nature and using that experience in new ways. As we all know, you can learn a lot from a rainbow.
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