Raman spectroscopy measures the minute vibrations of atoms within the molecules of a given material. The bonds between atoms vibrate at a frequency specific to each chemical bond; as they vibrate Raman spectroscopy allows scientists to identify key aspects of a sample’s chemical/molecular composition. The data output—a Raman spectrum—represents the vibration of a molecule or atom in response to laser light. A typical spectrum for a sample containing organic matter appears as a curve with two main peaks—one wide peak, and a sharper, more narrow peak. The wide peak is called the D, or disordered, band as vibrations in this region correlate with carbon atoms that have a disordered makeup, bound to any number of other elements. The second more narrow peak is the G (graphite) band, which is typically related to more ordered arrangements of carbon.
But if you look closely, you’ll notice the two dominant peaks each have small “bumps” on them, referred to as substructures. It’s these bumps that lead MIT Professor Roger Summons and research scientist Nicola Ferralis to discover “hidden features” of Raman spectra that have applicability in everything from space science to paleontology.
Ferralis, working with ancient sediment samples being investigated in the Summons Laboratory, identified substructures within the main D band that are directly related to the amount of hydrogen in a sample. Specifically, D4 and D5 showed a linear relationship to the hydrogen content of the organic matter. The higher the substructures, the more hydrogen is present—an indication that the sample has been relatively less altered, and its original chemical makeup better preserved.
“As rocks become older and older, they are subjected to higher and higher temperatures,” Summons explained to Laboratory Equipment. “The rocks become corrupted by the natural forces of geology, and make the organic signatures indistinct.”
For example, a rock that is 10 million years old usually has a perfectly preserved fossil and organic matter. Researchers like Summons and Ferralis are able to extract a variety of information from the “young” rock, including the environment in which it was formed and the organisms that contributed organic manner to the sediment. However, the further back in time you go, the harder it is to obtain information.
“I can [obtain information from rocks that are] 100 million years old, as well as 600 million years old, to a certain extent; but once I get into the billions of years, which is the part of the record that is more obscure, these things get harder and harder,” Summons said.
To test Ferralis’ discovery, the Summons laboratory applied Raman spectroscopy and their analytic technique to samples of sediments whose chemical composition was already known. They obtained additional samples of ancient kerogen—fragments of organic matter in sedimentary rocks—from a team based at UCLA, who, in the 1980s, used meticulous chemical methods to accurately determine the ratio of hydrogen to carbon.
Summons’ team quickly estimated the same ratio, first using Raman spectroscopy to generate spectra of the various kerogen samples, then using their new method to interpret the peaks in each spectrum. The team’s ratios of hydrogen to carbon closely matched the original ratios.
“This means our method is sound, and we don’t need to do an insane or impossible amount of chemical purification to get a precise answer,” Summons said.
Previously, to obtain the hydrogen to carbon ratio in a rock sample, researchers would have to isolate the organic matter, purify it and then send it off to a laboratory to have the elemental hydrogen and elemental carbon content measured. Not only could this take days or even weeks, the sample and resulting data were prone to errors along the way.
But, with validation of the new technique, researchers can now quickly measure the amount of hydrogen to carbon in a sample of organic matter, and do so at a very fine spatial scale. They can also do it without causing any damage to the sample, something that was not previously possible.
Space and paleontology
Summons and Ferralis immediately identified two research areas that could benefit from their new Raman technique—the search for previous life on Mars, as well as Earth’s own biological evolution.
The MIT researchers think their technique will be especially useful for NASA’s Mars 2020 Mission, which is set to launch a rover to the Red Planet in July/August 2020 to search for signs of habitable conditions in the ancient past, as well as signs of past microbial life. The rover will do this with the assistance of the SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals) instrument, mounted on the rover’s robotic arm.
Designed by NASA’s Jet Propulsion Laboratory, SHERLOC is a deep UV (DUV) resonance Raman and fluorescence spectrometer that relies on a 248.6-nm DUV laser and <100 micron spot size. It enables non-contact, spatially resolved and highly sensitive detection and characterization of organics and minerals on the Martian surface and near subsurface. One of the instrument’s goals is to collect samples of rocks and soil and store them on the Martian surface for a future mission to retrieve and subsequently analyze.
This is where the new MIT Raman technique could come in handy. With its ability to quickly and non-invasively measure the hydrogen to carbon ratio, the technique could identify ideal Martian rocks/samples—in other words, the better-preserved rocks that will give scientists the best analytical chance of finding signs of life.
Space may be the final frontier for humans, but it’s certainly not the only one for Summons and Ferralis. Taking their work a step further, the researchers wondered whether they could use their technique to map the chemical composition of a microscopic fossil, which ordinarily would contain so little carbon that it would be undetectable by traditional chemistry techniques.
The team used Raman spectroscopy to measure the atomic vibrations throughout a microscopic fossil of a protist, at a sub-micron resolution, and then analyzed the resulting spectra using their new analytic technique. They then created a chemical map based on their analysis.
“The fossil has seen the same thermal history throughout, and yet we found the cell wall and cell contents have higher hydrogen than the cell’s matrix or its exterior,” Summons said. “That, to me, is evidence of biology. It might not convince everybody, but it’s a significant improvement over what we had before.”
Ultimately, Summons feels this technique will assist paleontologists in understanding the original nature of their fossils to a much greater extent than they can now.