Pulsed remote Raman system for daytime measurements of mineral spectra

A remote Raman system has been developed utilizing a 532nm pulsed laser and gated intensified charged couple device (ICCD) detector in the oblique geometry. When the system is set for 50m sample distance it is capable of measuring Raman spectra of minerals located at distances in the range of 10-65m from the telescope. Both daytime and nighttime operations are feasible and the spectra of minerals can be measured in a short period of time, of the order of a few seconds. In oblique geometry, measured sampling depth is more than 30m, during which the system maintains very high performance without any adjustments. Much longer sampling depth (0.1-120m) has been observed when the system is configured in the coaxial geometry. Clear advantages of using a gated detection mode over the continuous (CW) mode of operation in reducing the background signal and eliminating long-lived fluorescence signals from the Raman spectra are presented. The performance of the pulsed Raman system is demonstrated by measuring spectra of Raman standards including benzene (C(6)H(6)) and naphthalene (C(10)H(8)), a low Raman cross section silicate mineral muscovite (KAl(2)(Si(3)Al)O(10)(OH)(2)), and a medium Raman cross section mineral calcite (CaCO(3)).     

Portable remote Raman system for monitoring hydrocarbon, gas hydrates and explosives in the environment

We report our initial efforts to use a small portable Raman system for stand-off detection and identification of various types of organic chemicals including benzene, toluene, ethyl benzene and xylenes (BTEX). Both fiber-optic (FO) coupled and a directly coupled f/2.2 spectrograph with the telescope have been developed and tested. A frequency-doubled Nd:YAG pulsed laser (20 Hz, 532 nm, 35 mJ/pulse) is used as the excitation source. The operational range of the FO coupled Raman system was tested to 66 m, and the directly coupled system was tested to a distance of 120 m. We have also measured remote Raman spectra of compressed methane gas and methane gas hydrate. The usefulness of the remote Raman system for identifying unknown compounds is demonstrated by measuring stand-off spectra of two plastic explosives, e.g. tri-amino tri-nitrobenzene (TATB) and beta-HMX at 10 m stand-off distance. The remote Raman system will be useful for terrestrial applications such as monitoring environmental pollution, in identifying unknown materials in public places in 10s or less, and for detecting hydrocarbon plumes and gas hydrates on planetary surfaces such as Mars.     

New trends in telescopic remote Raman spectroscopic instrumentation

Raman spectroscopy is a powerful analytical technique in many areas of research for several reasons. These include the sensitivity to small structural changes, non-invasive sampling capability, minimal sample preparation, narrow line widths of Raman lines, and high spatial resolution in the case of micro-Raman spectroscopy. Advancements in lasers, spectrographs and holographic optical components have made Raman spectroscopy an effective tool for analyzing natural and synthetic materials. These advances have led to the development of both in situ Raman spectroscopy and telescopic remote Raman spectroscopy for a lander or rover for planetary exploration. A telescopic Raman spectroscopic system capable of measuring Raman spectra of minerals, inorganic and organic chemicals, and biogenic materials to radial distances in the range 10-100 m has been developed. In this work, the author reviews the current status of telescopic remote Raman spectroscopic instrumentation and examines new trends in the field of remote Raman spectroscopy and its combination with time-resolved remote laser-induced native fluorescence (LINF) and laser-induced breakdown spectroscopy (LIBS), and their applications in earth and planetary science.     

Remote-Raman spectroscopic study of minerals under supercritical CO2 relevant to Venus exploration

The authors have utilized a recently developed compact Raman spectrometer equipped with an 85 mm focal length (f/1.8) Nikon camera lens and a custom mini-ICCD detector at the University of Hawaii for measuring remote Raman spectra of minerals under supercritical CO(2) (Venus chamber, ～102 atm pressure and 423 K) excited with a pulsed 532 nm laser beam of 6 mJ/pulse and 10 Hz. These experiments demonstrate that by focusing a frequency-doubled 532 nm Nd:YAG pulsed laser beam with a 10× beam expander to a 1mm spot on minerals located at 2m inside a Venus chamber, it is possible to measure the remote Raman spectra of anhydrous sulfates, carbonates, and silicate minerals relevant to Venus exploration during daytime or nighttime with 10s integration time. The remote Raman spectra of gypsum, anhydrite, barite, dolomite and siderite contain fingerprint Raman lines along with the Fermi resonance doublet of CO(2). Raman spectra of gypsum revealed dehydration of the mineral with time under supercritical CO(2) at 423 K. Fingerprint Raman lines of olivine, diopside, wollastonite and α-quartz can easily be identified in the spectra of these respective minerals under supercritical CO(2). The results of the present study show that time-resolved remote Raman spectroscopy with a compact Raman spectrometer of moderate resolution equipped with a gated intensified CCD detector and low power laser source could be a potential tool for exploring Venus surface mineralogy both during daytime and nighttime from a lander.     

Stand-off Raman detection using dispersive and tunable filter based systems

Small, transportable Raman systems are being developed for stand-off Raman measurements at intermediate ranges (e.g. <20 m) for planetary measurements. Four variations of stand-off Raman systems are described that use a small telescope for light collection that is either fiber-optic or lens-coupled to a detection system. The performance of an acousto-optic tunable filter for wavelength selection and spectral imaging is tested by comparing signal-to-noise ratio and throughput to similar measurements using a conventional spectrograph, and by measuring a variety of organic and inorganic mineral samples at distances up to 15 m. We also determine optimal ICCD gate widths for acquiring remote Raman spectra under high ambient light conditions.     

Raman efficiencies of natural rocks and minerals: performance of a remote Raman system for planetary exploration at a distance of 10 meters

Raman spectroscopy is a powerful technique for materials analysis, and we are developing and analyzing a remote Raman system for use on a planetary lander or rover. We have acquired data at a distance of 10m from a variety of geologic materials using different instrument designs. We have employed a pulsed laser with both an ungated detector and a gated detector. A gated detector can reduce long-lived fluorescence while still collecting all Raman signal. In order to design a flight instrument, we need to quantify how natural surfaces will respond to laser stimulus. We define remote Raman efficiency of natural surfaces as the ratio of radiant exitance leaving a natural surface to the irradiance of the incident laser. The radiant exitance of a natural surface is the product of the sample radiance, the projected solid angle, and the full-width-half-maximum of the Raman signal. We have determined the remote Raman efficiency for a variety of rocks and minerals. The best efficiencies are achieved for large, clear, single crystals that produce the most radiant exitance, while darker fine-grained mineral mixtures produce lower efficiencies. By implementing a pulsed laser, gated detector system we have improved the signal detection and have generally decreased the integration time necessary to detect Raman signal from natural surfaces.     

Combined remote LIBS and Raman spectroscopy at 8.6m of sulfur-containing minerals, and minerals coated with hematite or covered with basaltic dust

Combined remote laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy investigations at a distance of 8.6m have been carried out in air and under a simulated Martian atmosphere of 933Pa (7Torr) CO(2) on calcite (CaCO(3)), gypsum (CaSO(4).2H(2)O), and elemental sulfur (S), and LIBS investigations on chalcopyrite (CuFeS(2)) and pyrite (FeS(2)). Both Raman and LIBS techniques have also been used sequentially in air on hematite-coated calcite crystals and on a sample of anhydrite covered with basaltic dust. These experiments demonstrate that by using a frequency-doubled Nd:YAG pulsed laser co-radiating 1064 nm and 532 nm laser beams with a 5x beam expander, it is possible to measure simultaneously both the Raman and LIBS spectra of calcite, gypsum and elemental sulfur by adjusting the laser power electronically. The spectra of calcite, gypsum, and elemental sulfur contain fingerprint Raman lines; however, it was not possible to measure the remote Raman spectra of pyrite and chalcopyrite because of low intensities of Raman lines. In the cases of CuFeS(2), FeS(2), and elemental sulfur, S atomic emission lines in the LIBS spectra were detected only in 7Torr of CO(2) pressure and not in air. No S atomic emission lines were detected for gypsum in air or in CO(2). In the case of coated/dusted minerals, it was possible to remove the coating or dust with the focused LIBS laser and measure the Raman spectra of subsurface minerals with a 532 nm laser excitation. The complementary nature of these two techniques is highlighted and discussed.     

Stand-off Raman spectroscopy: a powerful technique for qualitative and quantitative analysis of inorganic and organic compounds including explosives

A pulsed stand-off Raman system has been built and optimised for the qualitative and quantitative analysis of inorganic and organic samples including explosives. The system consists of a frequency doubled Q-switched Nd:YAG laser (532 nm, 10 Hz, 4.4 ns pulse length), aligned coaxially with a 6″ Schmidt–Cassegrain telescope for the collection of Raman scattered light. The telescope was coupled via a fibre optic bundle to an Acton standard series SP-2750 spectrograph with a PI-MAX 1024RB intensified CCD camera equipped with a 500-ps gating option for detection. Gating proved to be essential for achieving high signal-to-noise ratios in the recorded stand-off Raman spectra. In some cases, gating also allowed suppression of disturbing fluorescence signals. For the first time, quantitative analysis of stand-off Raman spectra was performed using both univariate and multivariate methods of data analysis. To correct for possible variation in instrumental parameters, the nitrogen band of ambient air was used as an internal standard. For the univariate method, stand-off Raman spectra obtained at a distance of 9 m on sodium chloride pellets containing varying amounts of ammonium nitrate (0–100%) were used. For the multivariate quantification of ternary xylene mixtures (0–100%), stand-off spectra at a distance of 5 m were used. The univariate calibration of ammonium nitrate yielded R ² values of 0.992, and the multivariate quantitative analysis yielded root mean square errors of prediction of 2.26%, 1.97% and 1.07% for o-, m- and p-xylene, respectively. Stand-off Raman spectra obtained at a distance of 10 m yielded a detection limit of 174 μg for NaClO₃. Furthermore, to assess the applicability of stand-off Raman spectroscopy for explosives detection in “real-world” scenarios, their detection on different background materials (nylon, polyethylene and part of a car body) and in the presence of interferents (motor oil, fuel oil and soap) at a distance of 20 m was also investigated.     

Remote Raman and fluorescence studies of mineral samples

In the present study, we investigated remote laser-induced fluorescence (LIF), at a distance of 4.8 m, of a variety of natural minerals and rocks, and Hawaiian Ti (Cordyline terminalis) plant leaves. These minerals included calcite cleavage, calcite onex and calcite travertine, gypsum, fluorapatite, Dover flint and chalk, chalcedony and nephelene syenite, and rubies containing rock. Pulsed laser excitation of the samples at 355 and 266 nm often resulted in strong fluorescence. The LIF bands in the violet-blue region at approximately 413 and approximately 437 nm were observed only in the spectrum of calcite cleavage. The green LIF bands with band maxima in the narrow range of approximately 501-504 nm were observed in the spectra of all the minerals with the exception of the nephelene syenite and ruby rocks. The LIF red bands were observed in the range approximately 685-711 nm in all samples. Excitation with 532 nm wavelength laser gave broad but relatively low fluorescence background in the low-frequency region of the Raman spectra of these minerals. One microsecond signal gating was effective in removing nearly all background fluorescence (with peak at approximately 610 nm) from calcite cleavage Raman spectra, indicating that the fluorescence was probably from long-lifetime inorganic phosphorescence.     

Critical evaluation of a handheld Raman spectrometer with near infrared (785nm) excitation for field identification of minerals

Handheld Raman spectrometers (Ahura First Defender XL, Inspector Raman DeltaNu) permit the recording of acceptable and good quality spectra of a large majority of minerals outdoors and on outcrops. Raman spectra of minerals in the current study were obtained using instruments equipped with 785 nm diode lasers. Repetitive measurements carried out under an identical instrumental setup confirmed the reliability of the tested Raman spectrometers. Raman bands are found at correct wavenumber positions within ±3 cm(-1) compared to reference values in the literature. Taking into account several limitations such as the spatial resolution and problems with metallic and black and green minerals handheld Raman spectrometers equipped with 785 nm diode lasers can be applied successfully for the detection of minerals from the majority of classes of the mineralogical system. For the detection of biomarkers and biomolecules using Raman spectroscopy, e.g. for exobiological applications, the near infrared excitation can be considered as a preferred excitation. Areas of potential applications of the actual instruments include all kind of common geoscience work outdoors. Modified Raman systems can be proposed for studies of superficial or subsurface targets for Mars or Lunar investigations.     

Raman microscopy of selected tungstate minerals

A series of tungstate bearing minerals including scheelite, stolzite, ferberite, hübnerite, wolframite, russellite, tungstenian wulfenite and cuprotungstite have been analyzed by Raman microscopy. The results of the Raman spectroscopic analysis are compared with published data. These minerals are closely related and often have related paragenesis. Raman microscopy enables the selection of individual crystals of these minerals for spectroscopic analysis even though several of the minerals can be found in the same matrix because of the pargenetic relationships between the minerals. The Raman spectra are assigned according to factor group analysis and related to the structure of the minerals. These minerals have characteristically different Raman spectra. The nu1(Ag) band is observed at 909 cm(-1) and although the corresponding nu1(Bu) vibration should be inactive a minor band is observed around 894 cm(-1). The bands at 790 and 881 cm(-1) are associated with the antisymmetric and symmetric Ag modes of terminal WO2. The band at 695 cm(-1) is interpreted as an antisymmetric bridging mode associated with the tungstate chain. The nu4(Eg) band was absent for scheelite. The bands at 353 and 401 cm(-1) are assigned as either deformation modes or as r(Bg) and delta(Ag) modes of terminal WO2. The band at 462 cm(-1) has an equivalent band in the infrared at 455 cm(-1) assigned as delta(as)(Au) of the (W2O4)n chain. The band at 508 cm(-1) is assigned as nu(sym)(Bg) of the (W2O4)n chain.     

Raman spectroscopic study of mellite--a naturally occurring aluminium benzenehexacarboxylate from lignite--Claystone series of the tertiary age

Raman spectra have been obtained for crystals of the organic mineral mellite, from three different sites. Mellite occurs in the frame of the Tertiary series including lignite and coaly slates at Artern (Thuringia), Tula (Russia) and Bílina (Northern Bohemia). Mellite, Al(2)C(6)(COO)(6) x 16H(2)O, can be considered as evidence of previous biological activity in the geological record, similar to other salts of carboxylic acids such as whewellite and weddellite. Assignments of the major Raman features of mellite are proposed on the basis of comparison with the parent, mellitic acid, C(6)(COOH)(6). During diagenesis and epigenesis, mellite is formed from the reaction between organic carbon rich solutions with aluminosilicates, hence, with the current interest in the adoption of Raman spectroscopy for incorporation into robotic instrumentation for space mission landers, it is important that organic minerals be included into a spectroscopic database for the recognition of biomolecular signatures for remote life-detection experiments.     

Vibrational spectroscopy standoff detection of explosives

Standoff infrared and Raman spectroscopy (SIRS and SRS) detection systems were designed from commercial instrumentation and successfully tested in remote detection of high explosives (HE). The SIRS system was configured by coupling a Fourier-transform infrared interferometer to a gold mirror and detector. The SRS instrument was built by fiber coupling a spectrograph to a reflective telescope. HE samples were detected on stainless steel surfaces as thin films (2–30 μg/cm²) for SIRS experiments and as particles (3–85 mg) for SRS measurements. Nitroaromatic HEs: TNT, DNT, RDX, C4, and Semtex-H and TATP cyclic peroxide homemade explosive were used as targets. For the SIRS experiments, samples were placed at increasing distances and an infrared beam was reflected from the stainless steel surfaces coated with the target chemicals at an angle of ∼180° from surface normal. Stainless steel plates containing TNT and RDX were first characterized for coverage distribution and surface concentration by reflection–absorption infrared spectroscopy. Targets were then placed at the standoff distance and SIRS spectra were collected in active reflectance mode. Limits of detection (LOD) were determined for all distances measured for the target HE. LOD values of 18 and 20 μg/cm² were obtained for TNT and RDX, respectively, for the SIR longest standoff distance measured. For SRS experiments, as low as 3 mg of TNT and RDX were detected at 7 m source–target distance employing 488 and 514.5 nm excitation wavelengths. The first detection and quantification study of the important formulation C4 is reported. Detection limits as function of laser powers and acquisition times and at a standoff distance of 7 m were obtained.     

Raman spectra of organic compounds kladnoite (C6H4(CO)2NH) and hoelite (C14H8O2)--rare sublimation products crystallising on self-ignited coal heaps

As minerals, aromatic compounds occur very rarely in nature. Not more than 10 of such minerals are known and most of them were described in the coal deposits where they were formed as a result of coal bed fires or burning of coal waste heaps. Raman spectra of kladnoite C(6)H(4)(CO)(2)NH (natural phthalimide) and hoelite C(14)H(8)O(2), (natural 9,10-anthraquinone) display complex features. Raman microspectroscopy help to detect these phases non-destructively directly in the frame of rare samples. Investigated minerals are transformation products formed from gaseous phase originating in natural pyrolytical process occurring in the frame of coal heaps and coal series outcrops. It is recommended to include Raman spectroscopic characteristics of similar materials in databases for exobiological studies.     

Raman spectroscopic detection of biomolecular markers from Antarctic materials: evaluation for putative Martian habitats

The vital UV-protective and photosynthetic pigments of cyanobacteria and lichens (microbial symbioses) that dominate primary production in Antarctic desert ecosystems auto-fluoresce at short-wavelengths. A long wavelength (1064 nm) near infra-red laser has been used for non-intrusive Raman spectroscopic analysis of their ecologically significant compounds. There is now much interest in the construction of portable Raman systems for the analysis of cyanobacterial and lichen communities in the field; to this extent, Raman spectra obtained with laboratory-based systems operating at wavelengths of 852 and 1064 nm have been evaluated for potential fieldwork applications of miniaturised units. Selected test specimens of the cyanobacterial Nostoc commune, epilithic lichens Acarospora chlorophana, Xanthoria elegans and Caloplaca saxicola and the endolithic Chroococcidiopsis from Antarctic sites have been examined in the present study. Although some organisms gave useable Raman spectra with short-wavelength lasers, 1064 nm was the only excitation that was consistently excellent for all organisms. We conclude that a 1064 nm Raman spectrometer, miniaturised using an InGaAs detector, is the optimal instrument for in situ studies of pigmented communities at the limits of life on Earth. This has practical potential for the quest for biomolecules residual from any former surface or subsurface life on Mars.     

Raman spectra of dipicolinic acid in crystalline and liquid environments

Raman spectra of dipicolinic acid (DPA) are important for detection of bacterial spores, since DPA and its salts present one of their major components. The implementation of a deeply cooled CCD camera in combination with pulsed excitation at 532 nm allowed measuring well-resolved Raman spectra of the DPA in different forms. Powder preparations, crystals grown from saturated solutions and aqueous solutions of the DPA were studied. The spectral features in different environments and comparison with the spectra obtained by other methods are discussed.     

Detection of counterfeit stevia products using a handheld Raman spectrometer

Stevia is a highly appreciated natural sweetener because it can be consumed by diabetic patients. Due to the increasing popularity of stevia during the last years, counterfeit products have been making their way into the market. Raman spectroscopy is a versatile analytical technique that can be used for control tasks and handheld modern devices expand its possible applications to instant in situ measurements. The Raman spectra of six commercial stevia products (five purchased in Bolivia and one in Germany) were recorded and compared to the spectra of standards of rebaudioside A and stevioside as well as the spectra of the artificial sweeteners sodium cyclamate and sodium saccharin. Based on the Raman spectroscopic data, it was verified that three of the Bolivian products were counterfeit products and another one was rich in maltodextrin. The Raman spectra of one Bolivian product and the German one revealed rebaudioside A and stevioside as major components. Raman spectroscopy was capable of detecting contents as low as 5% (w/w) of sodium cyclamate during measurements of stevia-sodium cyclamate mixtures. The results show that Raman spectroscopy can successfully be used to detect counterfeit stevia and underline its high potential for the detection of food adulteration.     

A compact collinear AOTF Raman spectrometer

A compact, lightweight, completely packaged, uncooled, fully-automated collinear acousto-optic tunable-filter (AOTF) based spectrometer has been used to measure Raman spectra of three organic energetic materials (NQ, HMX, and TNT) using argon-ion laser excitation. Even though the resolution of the AOTF spectrometer is modest (7.4 cm(-1)) and it was not specifically designed for measuring Raman spectra, it has performed impressively. Such an instrument is specially useful for remote sensing and field measurements. In this paper, we will describe this instrument, present the measured Raman spectra and their comparison with the corresponding FT-IR spectra.     

Near-infrared spectroscopy of natural alunites

Near-infrared spectroscopy (NIR) has been used to analyse alunites of formula K(Al3+)6(SO4)4(OH)12. Whilst the spectra of the alunites shows a common pattern differences in the spectra are observed which enable the minerals to be distinguished. These differences are attributed to subtle variations in alunite composition. The NIR bands in the 6300-7000 cm(-1) region are attributed to the first fundamental overtone of both the infrared and Raman hydroxyl stretching vibrations. A set of bands are observed in the 4700-5500 cm(-1) region which are assigned to combination bands of the hydroxyl stretching and deformation vibrations. NIR spectroscopy has the ability to distinguish between the alunite minerals even when the formula of the minerals is closely related. The NIR spectroscopic technique has great potential as a mineral exploratory tool on planets and in particular Mars.     

Estimation of ground- and excited-state dipole moments of oxazine 1 in liquid and liquid crystalline media

A portable remote Raman instrument for field analysis has been developed. This instrument has been tested in the Arctic conditions during AMASE (Arctic Mars Analog Svalbard Expedition) campaigns 2007, 2008 and 2009. Besides its capability for mineral detection the remote system proved to be a very useful tool for ice structural analysis of icebergs and ice-wall in glaciers. For the first time at our knowledge Arctic ice has been analyzed in situ in different conditions and at distances ranging from 10 to 120m. The spectra obtained have enough quality to be used for quantitative spectral analysis.