Selection of Portable Spectrometers for Planetary Exploration: A Comparison of 532 nm and 785 nm Raman Spectroscopy of Reduced Carbon in Archean Cherts

Effect of LIBS-Induced Alteration on Subsequent Raman Analysis of Iron Sulfides

Mineral alteration is a possible side effect of spectroscopic techniques involving laser ablation, such as laser-induced breakdown spectroscopy (LIBS), and is related to the interaction of the generated plasma and ablated material with samples, dust, or ambient atmosphere. Therefore, it is essential to understand these interactions for analytical techniques involving laser ablation, especially for space research. In this combined LIBS-Raman analytical study, pyrite (FeS₂) and pyrrhotite (Fe_1-x S) samples have been consecutively measured with LIBS and Raman spectroscopy, under three different atmospheric conditions: ∼10^-4 mbar (atmosphereless body), ∼7 mbar, and Martian atmospheric composition (Martian surface conditions), and 1 bar and Martian atmospheric composition. Furthermore, a dust layer was simulated using ZnO powder in a separate test and applied to pyrite under Martian atmospheric conditions. In all cases, Raman spectra were obscured after the use of LIBS in the area of and around the formed crater. Additional Raman transitions were detected, associated with sulfur (pyrite, 7.0 mbar and 1.0 bar), polysulfides (all conditions), and magnetite (both minerals, 1.0 bar). Magnetite and polysulfides formed a thin film of up to 350-420 and 70-400 nm in the outer part of the LIBS crater, respectively. The ZnO-dust test led to the removal of the dust layer, with a similar alteration to the nondust pyrite test at 7.0 mbar. The tests indicate that recombination with the CO₂-rich atmosphere is significant at least for pressures from 1.0 bar and that plasma-dust interaction is insignificant. The formation of sulfur and polysulfides indicates fractionation and possible loss of volatile elements caused by the heat of the LIBS laser. This should be taken into account when interpreting combined LIBS-Raman analyses of minerals containing volatile elements on planetary surfaces.

In Situ Raman Spectroscopy Monitoring of Material Changes During Proton Irradiation

Organic molecules are prime targets in the search for life on other planetary bodies in the Solar System. Understanding their preservation potential and detectability after ionic irradiation, with fluences potentially representing those received for several millions to billions of years at Mars or in interplanetary space, is a crucial goal for astrobiology research. In order to be able to perform in situ characterization of such organic molecules under ionic irradiation in the near future, a feasibility experiment was performed with polymer test samples to validate the optical configuration and the irradiation chamber geometry. We present here a Raman in situ investigation of the evolution of a series of polymers during proton irradiation. To achieve this goal, a new type of Raman optical probe was designed, which documented that proton irradiation (with a final fluence of 3.10¹⁴ at·cm^-2) leads to an increase in the background level of the signal, potentially explained by the scission of the polymeric chains and by atom displacements creating defects in the materials. To improve the setup further, a micro-Raman probe and a temperature-controlled sample holder are under development to provide higher spectral and spatial resolutions (by reducing the depth of field and laser spot size), to permit Raman mapping as well as to avoid any thermal effects.

Raman Characterization of the CanMars Rover Field Campaign Samples Using the Raman Laser Spectrometer ExoMars Simulator: Implications for Mars and Planetary Exploration

The Mars 2020 Perseverance rover landed on February 18, 2021, and has started ground operations. The ExoMars Rosalind Franklin rover will touch down on June 10, 2023. Perseverance will be the first-ever Mars sample caching mission-a first step in sample return to Earth. SuperCam and Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals (SHERLOC) on Perseverance, and Raman Laser Spectrometer (RLS) on Rosalind Franklin, will comprise the first ever in situ planetary mission Raman spectroscopy instruments to identify rocks, minerals, and potential organic biosignatures on Mars' surface. There are many challenges associated when using Raman instruments and the optimization and quantitative analysis of resulting data. To understand how best to overcome them, we performed a comprehensive Raman analysis campaign on CanMars, a Mars sample caching rover analog mission undertaken in Hanksville, Utah, USA, in 2016. The Hanksville region presents many similarities to Oxia Planum's past habitable conditions, including liquid water, flocculent, and elemental compounds (such as clays), catalysts, substrates, and energy/food sources for life. We sampled and conducted a complete band analysis of Raman spectra as mission validation analysis with the RLS ExoMars Simulator or RLS Sim, a breadboard setup representative of the ExoMars RLS instrument. RLS Sim emulates the operational behavior of RLS on the Rosalind Franklin rover. Given the high fidelity of the Mars analog site and the RLS Sim, the results presented here may provide important information useful for guiding in situ analysis and sample triage for caching relevant for the Perseverance and Rosalind Franklin missions. By using the RLS Sim on CanMars samples, our measurements detected oxides, sulfates, nitrates, carbonates, feldspars, and carotenoids, many with a higher degree of sensitivity than past results. Future work with the RLS Sim will aim to continue developing and improving the capability of the RLS system in the future ExoMars mission.

Confocal Raman spectroscopy at different laser wavelengths in analyzing stratum corneum and skin penetration properties of mixed PEGylated emulsifier systems

Emulsifier mixtures are widely used in cosmetics and pharmaceutics and thus, brought extensive studies for their performances on skin applications. PEG-20cetyl ether (C20) is recently proposed to induce skin irritation and is of interest to study its skin interactions when mixed with other emulsifiers. PEG-2oleyl ether (O2) and PEG-20stearyl ether (S20) are selected and in specific, 50 mM of C20, O2, S20 as well as Mix1 (50 mM C20 mixed with 50 mM O2) and Mix2 (50 mM C20 mixed with 50 mM S20) solutions were applied on skin samples. Confocal Raman spectroscopy (CRS) analyses of stratum corneum (SC) thickness and SC lipid content were performed after 4 h skin treatments. In parallel, skin penetration properties were also evaluated via CRS by applying procaine solutions with/without emulsifiers on skin samples for 24 h. In terms of the CRS measurements, two excitation wavelengths of 532 nm and 785 nm are both utilized in this study and we secondly aimed to compare their results and suitability in SC and skin analyses. Based on the experimental observations, comparable results are obtained by using both excitation wavelengths of 532 nm and 785 nm demonstrating their suitability in analyzing SC and skin samples. Thereinto, 785 nm laser wavelength shows the advantage of deeper skin penetration and allows the measurements of fluorescent skin samples; 532 nm laser wavelength enables simple measurement performance without substrate and coverslip interference. With regards to the results of emulsifier mixtures, the addition of S20 and O2 reduced the skin interactions and penetration enhancing ability of C20, giving us the hint to build milder systems with emulsifier mixtures. Besides, the CRS results of stronger skin interruption were also correlated with the higher critical micelle concentration (CMC) values of emulsifiers and their mixtures, which may provide evidence in explaining the interactions between emulsifiers and skin.

Dual-wavelength excitation combined Raman spectroscopy for detection of highly fluorescent samples

As fluorescence is the major limitation in Raman scattering, near-infrared excitation wavelength (>780nm) is preferred for fluorescence suppression in Raman spectroscopy. To reduce the risk of fluorescence interference, we developed a dual-wavelength excitation combined Raman spectroscopy (DWECRS) system at 785 and 830 nm. By a common optical path, each laser beam was focused on the same region of the sample by a single objective lens, and the dual-wavelength excitation Raman spectra were detected by a single CCD detector; in addition, 785 and 830 nm excitation Raman spectra can be directly constructed as combined Raman spectrum in the DWECRS system. The results of pure peanut oil and glycerol indicate that the combined Raman spectrum cannot only reduce fluorescence interference but also keep a high signal-to-noise ratio in the high-wavenumber region. The results of dye-ethanol solutions with different concentrations show that the handheld DWECRS system can be used as a smart method to dodge strong fluorescence. Furthermore, we developed a peak intensity ratio method with the DWECRS system to distinguish different types of edible oils. The peak intensity ratio distribution chart of edible oils showed each oil normalized center was relatively independent and nonoverlapped, which can be used as the basis of edible oil classification analysis. In the future, the DWECRS system has potential to be used as a tool for more complex applications.

Click-Reaction-Triggered SERS Signals for Specific Detection of Monoamine Oxidase B Activity

Human monoamine oxidases (MAOs) play important roles in maintaining the homeostasis of biogenic amines. One of its isoforms, monoamine oxidase B (MAOB), is thought to be involved in several neurodegenerative diseases, which make the selective detection of MAOB activity essential. In this work, a novel surface-enhanced Raman scattering (SERS) sensor was fabricated and the MAOB activity was specifically determined by detecting the SERS signals of an enzyme-catalyzed reaction product via an amine-aldehyde click reaction. This process was simply achieved by coating core-shell gold-silver nanoparticles (Au@Ag NPs) on 3-aminopropyl aminopropyl triethoxysilane (APTES)-modified glass, and then, a monolayer of cysteamine (CA) was attached to the nanoparticle surface as a linker through Ag-S bonds. Using phenethylamine (PA) as a specific substrate of MAOB, the enzyme product phenylacetaldehyde (PAA) will produce significant Raman signals via the amine-aldehyde click reaction with CA, while other molecules, such as MAOB and PA, have no signal output because they cannot form close interaction with nanoparticles due to the existence of a CA layer. This sensor was further used for the specific determination of MAOB activity in clinical blood samples and the MAOB inhibitor assessment successfully. Meanwhile, by changing the click reaction types and taking advantage of the SERS fingerprint peaks for the specific click reaction products, this strategy offers huge potential to detect multiple enzyme activities simultaneously and can be used for new click reaction screening, enzyme-related disease diagnosis, drug screening, and clinical diagnosis.

Formation of micro-spherulitic barite in association with organic matter within sulfidized stromatolites of the 3.48 billion-year-old Dresser Formation, Pilbara Craton

The shallow marine and subaerial sedimentary and hydrothermal rocks of the ~3.48 billion-year-old Dresser Formation are host to some of Earth's oldest stromatolites and microbial remains. This study reports on texturally distinctive, spherulitic barite micro-mineralization that occur in association with primary, autochthonous organic matter within exceptionally preserved, strongly sulfidized stromatolite samples obtained from drill cores. Spherulitic barite micro-mineralization within the sulfidized stromatolites generally forms submicron-scale aggregates that show gradations from hollow to densely crystallized, irregular to partially radiating crystalline interiors. Several barite micro-spherulites show thin outer shells. Within stromatolites, barite micro-spherulites are intimately associated with petrographically earliest dolomite and nano-porous pyrite enriched in organic matter, the latter of which is a possible biosignature assemblage that hosts microbial remains. Barite spherulites are also observed within layered barite in proximity to stromatolite layers, where they are overgrown by compositionally distinct (Sr-rich), coarsely crystalline barite that may have been sourced from hydrothermal veins at depth. Micro-spherulitic barite, such as reported here, is not known from hydrothermal systems that exceed the upper temperature limit for life. Rather, barite with near-identical morphology and micro-texture is known from zones of high bio-productivity under low-temperature conditions in the modern oceans, where microbial activity and/or organic matter of degrading biomass controls the formation of spherulitic aggregates. Hence, the presence of micro-spherulitic barite in the organic matter-bearing Dresser Formation sulfidized stromatolites lend further support for a biogenic origin of these unusual, exceptionally well-preserved, and very ancient microbialites.

Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) with Raman Imaging Applied to Lunar Meteorites

Lunar meteorites provide a more random sampling of the surface of the Moon than do the returned lunar samples, and they provide valuable information to help estimate the chemical composition of the lunar crust, the lunar mantle, and the bulk Moon. As of July 2014, ∼96 lunar meteorites had been documented and ten of these are unbrecciated mare basalts. Using Raman imaging with multivariate curve resolution-alternating least squares (MCR-ALS), we investigated portions of polished thin sections of paired, unbrecciated, mare-basalt lunar meteorites that had been collected from the LaPaz Icefield (LAP) of Antarctica-LAP 02205 and LAP 04841. Polarized light microscopy displays that both meteorites are heterogeneous and consist of polydispersed sized and shaped particles of varying chemical composition. For two distinct probed areas within each meteorite, the individual chemical species and associated chemical maps were elucidated using MCR-ALS applied to Raman hyperspectral images. For LAP 02205, spatially and spectrally resolved clinopyroxene, ilmenite, substrate-adhesive epoxy, and diamond polish were observed within the probed areas. Similarly, for LAP 04841, spatially resolved chemical images with corresponding resolved Raman spectra of clinopyroxene, troilite, a high-temperature polymorph of anorthite, substrate-adhesive epoxy, and diamond polish were generated. In both LAP 02205 and LAP 04841, substrate-adhesive epoxy and diamond polish were more readily observed within fractures/veinlet features. Spectrally diverse clinopyroxenes were resolved in LAP 04841. Factors that allow these resolved clinopyroxenes to be differentiated include crystal orientation, spatially distinct chemical zoning of pyroxene crystals, and/or chemical and molecular composition. The minerals identified using this analytical methodology-clinopyroxene, anorthite, ilmenite, and troilite-are consistent with the results of previous studies of the two meteorites using electron microprobe analysis. To our knowledge, this is the first report of MCR-ALS with Raman imaging used for the investigation of both lunar and other types of meteorites. We have demonstrated the use of multivariate analysis methods, namely MCR-ALS, with Raman imaging to investigate heterogeneous lunar meteorites. Our analytical methodology can be used to elucidate the chemical, molecular, and structural characteristics of phases in a host of complex, heterogeneous geological, geochemical, and extraterrestrial materials.

Spatial and spectral resolution of carbonaceous material from hematite (α-Fe2O3) using multivariate curve resolution-alternating least squares (MCR-ALS) with Raman microspectroscopic mapping: implications for the search for life on Mars

We report a novel application of multivariate analysis with Raman microspectroscopic mapping to enhance the search for life on Mars.

The preservation of archaeological brain remains in a human skeleton

The identification of biomass within the cranial cavity of a waterlogged human skeleton inside a fish-tailed wooden coffin from a nineteenth century burial has been confirmed as brain tissue. A comparison is made between the Raman spectra obtained in the current study with those from an Iron Age brain found in an isolated cranium dating from about 500 years BCE, the only other Raman spectroscopy study made of human brain recovered from waterlogged, archaeological excavations. The spectra give some surprisingly detailed information about the state of preservation of brain tissue in both burials, especially when it is realized that, unlike preserved bog bodies, no other soft tissue has survived. The biosignatures of proteinaceous brain material are well characterized. The presence of spectral signatures from extraneous cyanobacterial colonization in the depositional site of the Iron Age brain had been construed to be responsible in part for the unusual preservation of brain tissues in the waterlogged environment, but they were not detected in the current study of the nineteenth century brain. The challenges for Raman spectroscopic analysis of biomaterials under these conditions are reviewed in the light of the successful outcome of the experiments.This article is part of the themed issue 'Raman spectroscopy in art and archaeology'.

Challenges Analyzing Gypsum on Mars by Raman Spectroscopy

Raman spectroscopy can provide chemical information about organic and inorganic substances quickly and nondestructively with little to no sample preparation, thus making it an ideal instrument for Mars rover missions. The ESA ExoMars planetary mission scheduled for launch in 2018 will contain a miniaturized Raman spectrometer (RLS) as part of the Pasteur payload operating with a continuous wave (CW) laser emitting at 532 nm. In addition, NASA is independently developing two miniaturized Raman spectrometers for the upcoming Mars 2020 rover mission, one of which is a remote (stand-off) Raman spectrometer that uses a pulse-gated 532 nm excitation system (SuperCam). The other is an in situ Raman spectrometer that employs a CW excitation laser emitting at 248.6 nm (SHERLOC). Recently, it has been shown with analyses by Curiosity that Gale Crater contains significantly elevated concentrations of transition metals such as Cr and Mn. Significantly, these transition metals are known to undergo fluorescence emission in the visible portion of the electromagnetic spectrum. Consequently, samples containing these metals could be problematic for the successful acquisition of fluorescence-free Raman spectra when using a CW 532 nm excitation source. Here, we investigate one analog environment, with a similar mineralogy and sedimentology to that observed in martian environments, as well as elevated Cr contents, to ascertain the best excitation wavelength to successfully collect fluorescence-free spectra from Mars-like samples. Our results clearly show that CW near-infrared laser excitation emitting at 785 nm is better suited to the collection of fluorescence-free Raman spectra than would be a CW laser emitting at 532 nm.