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Spectroscopy can be difficult to carry out outside a controlled laboratory environment. Imagine, then, the hurdles that would accompany performing spectroscopy in the extreme conditions of deep space or the ocean floor. Mike Angel, a professor of chemistry at the University of South Carolina, has taken on those challenges, working on new types of instruments for remote and in- situ laser spectroscopy, with a focus on deep-ocean, planetary, and homeland security applications of deep ultraviolet (UV), Raman and laser-induced breakdown spectroscopy (LIBS), to develop the tools necessary to work within these extreme environments. A key development is the spatial heterodyne Raman spectrometer, a fixed grating Fourier transform Raman spectrometer. For this development and other work, Angel has been awarded the Lester W. Strock Award from The New England section of the Society of Applied Spectroscopy, in recognition of a selected publication of substantive research in, or application of, analytical atomic spectrochemistry in the fields of earth science, life sciences, or stellar and cosmic sciences.
A past awardee of the 2012 and 2018 Society of Applied Spectroscopy William F. Meggers Award (for published work on spatial heterodyne Raman spectroscopy) as well as their 2018 NASLIBS Award (for remote LIBS using spatial heterodyne spectrometer), Angel, who will receive the Strock award this fall at SciX 2019 in Palm Springs, California, took the time to speak to us about his recent work. This interview is part of a series of interviews with the winners of awards that are presented at the SciX conference.
A recent paper you co-authored discusses the initial demonstration of a deep-ultraviolet (UV) 244 nm excitation spatial heterodyne Raman spectrometer (SHRS), and you mentioned that one of the motivations for developing such a device is planetary exploration using a lander or rover (1). Later in the paper, you discuss the limitations of Raman spectroscopy in such an environment. Would you briefly summarize these limitations, and how this device will overcome them?
Raman issues include poor sensitivity, 0.1% in many cases, and luminescence interference. Deep UV excitation (< ~250 nm) gets around the fluorescence issue by shifting the Raman spectrum to a wavelength range where most things don’t fluoresce. Using 244 nm excitation, for example; even CH stretch bands are below 265 nm, and most things don’t fluoresce at such short wavelengths. You can also avoid a lot of fluorescence using near-infrared excitation, at say 785 nm, but in this case, the Raman signal can be much lower, because of the smaller Raman cross section at red wavelengths. The Raman scattering efficiency goes as the inverse 4th power of wavelength, so a 1000 cm-1 Raman band, using 244 nm excitation, has more than two orders of magnitude larger cross section than the same band using 244 nm excitation. This doesn’t mean you will necessarily get 100 times more signal using 244 nm excitation though, since there are other factors to consider, such as sample penetration depth. Deep-UV also offers another signal advantage: resonance enhancement. In this case, resonance can provide another several orders of magnitude larger Raman cross section. So, the potential is there to get much higher sensitivity using deep-UV excitation. Traditionally, UV spectrometers are large, because of the need for very high spectral resolution, which typically requires a high dispersion path. The SHRS technique allows high spectral resolution in the deep-UV, in a relatively small footprint.
Another paper you co-authored focuses on combining remote LIBS and Raman spatial heterodyne spectrometers to measure a variety of organic and mineral samples, including materials from deep ocean hydrothermal vents, at a distance of 10 m, using a ~100 mm diameter Fresnel lens as the collection optic (2). What would be the expected discoveries? Who do you feel would be the beneficiaries of this work?
The discovery of organisms that can survive in environments like deep-ocean hydrothermal vents has changed our way of thinking about where life can evolve. These kinds of places might be similar to where life got started on Earth, or might have started on other planets. Studying these places is important to understanding ocean chemistry, but also to understanding the extremes in which life can thrive. Hydrothermal vents are believed to have existed once on Mars, and such locations would be good candidates to explore in the search for signs of past life there. Making chemical measurements at deep-ocean vents is hard, because of the depth, and extreme temperature and pressure, and because chemical sensors that can be used under these extreme conditions are rare. The ability to use remote LIBS and Raman to analyze the elemental composition of hydrothermal vent fluids would greatly expand the amount of chemical information that could be obtained in a limited time.
You recently co-authored a paper on coupling a spatial heterodyne Raman spectrometer (SHRS) with millimeter-sized optics with a standard cell phone camera as a detector for Raman measurements (3). What prompted you to address this development? Would you explain what is unique about this development and its purpose?
The driver for this study was planetary exploration, to places like the outer Jovian moons, where spacecraft instruments size and weight are at a premium. I wanted to demonstrate that a truly miniature Raman SHRS spectrometer was feasible. It occurred to me that a cell phone already has a high resolution CMOS CCD detector, though uncooled, and a miniature collection lens of very high quality, built in. So, all we had to do to make a miniature Raman spectrometer was mask down the gratings on our existing SHRS, to a couple mm in this case, and add a relatively low power laser. It worked so well in large part because the throughput of the SHRS was very high, and CMOS detector technology is really good and relatively low noise, even uncooled. I found out when doing background research for the paper that a cell-phone Raman spectrometer had not yet been published, so that was fortunate for us.
I’m struck by the range of research work your group is involved with. Are the projects you are working on very similar to each other, or diverse? How do you choose the projects? What are the biggest challenges you tend to encounter when working within these topics?
To me, all of our work is very closely related. I’ve been working for years to make spectrometers and chemical sensors that can be used to make measurements in hostile environments. Whether it's the deep ocean, the surface of Mars, or a battlefield, the challenges are similar. Most projects come about by talking to people about their measurement needs and problems. It almost always starts with the need. The biggest challenge is long term funding. I have been very lucky to have had long term support from NSF and NASA.
What does being awarded the Lester W. Strock Award mean to you?
I feel incredibly honored, especially looking at the list of previous award winners. This award is traditionally for work in atomic spectroscopy, and even though I do a lot of work using LIBS, an atomic spectroscopy technique, I think of myself mostly a molecular spectroscopist. So, I was very surprised by the award, and I am very thankful.
(1) N. Lamsal and S.M. Angel, Appl. Spectrosc. 2015, 69 (5), 525-534.
(2) A. Allen and S. M. Angel, Spectrochim. Acta, Part B 2018, 149, 91-98 (2018).
(3) P.D. Barnett and S. M. Angel, Appl. Spectrosc. 2017, 71 (5), 988-995.