One of the most important and groundbreaking applications of Raman spectroscopy is detecting materials associated with signs of life and organic chemistry. Gypsum, a sulfate mineral found on Mars, is closely associated with water and has been known to harbor fossil life on Earth. Apatite is made of calcium phosphates, and phosphates can be evidence of life because they are part of the backbone of DNA. Scientists believe that the first life on Earth may have been formed near volcanic seeps in seawater, a sulfur-rich environment. Calcite is also important to life because L- and D-amino acids are known to adsorb to this mineral.
Below we compare the results from our instruments (blue) to the data published on the RRUFF online database (red).
This summer research project greatly expanded upon the work done previously by Brett Berger in his Senior Thesis (see work here). In Berger’s device, however, the current increased exponentially with the input voltage, making the laser quite unstable. This summer’s group came up with an op-amp circuit design that would be controlled by a potentiometer and provided a linear relationship between the current and voltage. After weeks of testing, the team found that though the current was stable, the temperature and power output of the laser greatly fluctuated with time, also making the laser unstable. This also posed a potential threat to the laser and/or sample, as they may overheat and be damaged. In the future, the lab hopes to design a temperature-control circuit that will not greatly cut down on the power output of the laser. Below is a diagram of the circuit implemented:
In order to make the instrument adaptable, the team also made changes to the instrument itself. A new stage was designed to hold a cuvette for liquid samples, a slide for solid samples, and a power meter to measure the laser power output. The team also stationed the spectrometer inside a box with the inside painted black in order to minimize background noise in the spectra and to absorb any laser light that may bounce and reflect. Below is a photograph of the adapted instrument:
Meet the summer research students of 2015! Yvonne Ban is a physics major of the class of 2017, interested in astrophysics. Viviana Bermudez is a prospective engineering major of the class of 2018. During the first few days of research, they came up with an official name for the HMC Astrobiology lab: Extraterrestrial Vehicle Instrumentation Lab, a.k.a. EVIL.
Brett Berger graduated in the Harvey Mudd College class of 2015 with a Physics degree. In his senior thesis, he presented the design, calibration, and use of an inverted lab 532 nm Raman microscope. He also devised algorithms to refine the Raman signals and separate them from fluorescence peaks.
The lab worked with a doubled Nd:YVO4 laser that lacked a built-in infrared filter, so they implemented a glass filter in order to minimize infrared heating of organic samples. Below are a diagram and picture of the final apparatus:
The device has both a signal-collection mode for taking spectra and an imaging mode for taking high-resolution photographs of the samples. The team used a power meter (Thorlabs part PM100USB) to measure the device’s output power and power delivered to sample to ensure that the laser does not damage the samples.
Spectra collected by this device can fall into one of three categories. The ideal target produces a strong Raman signal that can be easily seen even in the raw data. Other samples will exhibit exceptionally strong absorption at 532 nm emitting neither reflectance, fluorescence, nor Raman signals. These samples are simply ill-fit for a 532 nm device and need to be interrogated using other excitation wavelengths (blue, red, or near-IR). Finally, some samples generate spectra that are a mixture of fluorescence and Raman signals. These signals can be deconvolved to provide both a LIFE spectral profile and a pure Raman spectrum.
Because of its sharp Raman peaks and relatively lack of fluorescence, Teflon is used as the calibration sample. The peaks “at 289 cm−1 and 380 cm−1 arise from torsion and deformation of CF2. The strong peak at 731 cm−1 is from symmetric CF2 stretching, that at 1216 cm−1 from anti-symmetric CF2 stretching, and those at 1300 cm−1 and 1382 cm−1 from C-C stretching.”
Berger produced fluorescence and Raman spectra for carbon tetrachloride (CCl4) and compared both with the spectra obtained using Ruiz’s SERDS device as well as spectra published in literature. Berger’s and Ruiz’s spectra were consistent with the literature, down to peaks as low as 217 cm-1.
The lab analyzed a solid sample of sulfur to test the low-Raman-shift capabilities of the instrument. The raw data obtained contained no fluorescence signatures and displayed the three peaks documented in the RRUFF Raman database (link here). A Raman-active electric quadrupole vibration exists at 84 cm−1. Analysis of the transmission curve of the system’s dichroic and edge ﬁlters indicates that the majority of an 84 cm−1 peak is blocked from collection but enough of its blue-ward photons pass through to register as a modest peak with apparent center frequency shifted to the blue. From this it would seem the device is capable of reliably identifying Raman shifts as low as 100 cm−1.
The formation of calcium carbonate, CaCO3, requires the presence of water. As a result, the mineral can be used as a marker of ancient hydrological activity on other planets. Berger’s instrument was able to fully separate the fluorescence and Raman spectra in calcite, and detect the 1085 cm-1 band in the calcite variant, aragonite. The team also analyzed an unknown fluorescent mineral deposit from the Pisgah crater lava tubes collected during Ramon field trials in summer 2014. The sample exhibited strong Raman peaks at 996 cm-1 and 1064 cm-1 constant with a mixture of sulfates and phosphates such as diadochite (Fe2PO4SO4.6H20). However, the sample lacked a water signature near 3500 cm-1 suggesting that this is an anhyhdrous mineral.
Alberto Ruiz graduated from Harvey Mudd College in 2014 with a degree in Physics and is now working at ThorLabs. For his senior thesis, he explored the temperature dependence of laser emission wavelength and its utility in Shifted Excitation Raman Difference Spectroscopy (SERDS). Ruiz built an instrument that can tune its excitation wavelength to extract Raman signals from highly fluorescent samples.
His instrument collects two raw spectra of a sample, the first with the laser diode tuned, for example, to ~405.1 nm, and the second with the laser tuned to ~405. 2 nm. The Raman spectral signal shifts precisely with the shift in excitation while the fluorescence signal does not. Subtracting the two spectra produces a biphasic Raman signal and eliminates the fluorescence response. His system used a thermoelectrically cooled GlacierX spectrometer provided by Edmonds Optics using a 10µm slit and a 1800/500 diffraction grating.
The Raman system successfully generated Raman signatures for water, carbon tetrachloride (CCl4), benzene, and calcite even though it suffered from laser bleed-through because it lacked a shaping filter.
Excitation sources close to the absorption bands of a sample can achieve a state known as resonance to increase the Raman signal output by several orders of magnitude. Unfortunately, this also increases the fluorescence signal. Shifted Excitation Raman Difference Spectroscopy (SERDS) is one technique that efficiently separates Raman signal from fluorescence, making the peaks sharper and the data easier to analyze. Ruiz’s device was able to collect these signals 100% more efficiently than previous works presented in literature.
The astrobiology lab at HMC dedicated the summer of 2011 to prototyping a Laser Induced Fluorescence Emission (L.I.F.E.) imaging spectrometer “to identify the presence of living organisms within rock and ice. The system, which provides an economical way to study samples without destroying them, may one day be used to identify life on other planets.” Watch the following video for more information.
Over the summer 2013, the HMC Astrobiology lab (also known as the Extravehicular Instrument Laboratory – EVIL) partnered with HMC’s Lab for Autonomous and Intelligent Robotics (LAIR) to document a project exploring life in lava tubes in the Mojave Desert. LAIR’s Jaguar Lite robots produced a map of a lava tube, while EVIL’s Pandora scanned the walls of the desert caves in search for signs of life using Laser Induced Fluorescence Emission (LIFE) spectroscopy. The effort is part of an ongoing project to develop an autonomous system capable of exploring lava tubes on Mars and one day exoplanets beyond the reach of human exploration.