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UV Raman ImagingsA Promising Tool for Astrobiology: Comparative Raman Studies with Different Excitation Wavelengths on SNC Martian Meteorites Torsten Frosch,† Nicolae Tarcea,† Michael Schmitt,† Hans Thiele,‡ Falko Langenhorst,§ and Ju 1 rgen Popp*,†,|
Institut fu¨r Physikalische Chemie, Friedrich-Schiller-Universita¨t Jena, Helmholtzweg 4, D-07743 Jena, Germany, Kayser-Threde GmbH, Wolfratshauser Strasse 48, D-81379 Mu¨nchen, Germany, Institut fu¨r Geowissenschaften, Friedrich-Schiller-Universita¨t Jena, Burgweg 11, D-07743 Jena, Germany, and Institut fu¨r Physikalische Hochtechnologie e.V., Albert-Einstein-Strasse 9, D-07745 Jena, Germany
The great capabilities of UV Raman imaging have been demonstrated on the three Martian meteorites: Sayh al Uhaymir, Dar al Gani, and Zagami. Raman spectra without disturbing fluorescence and with high signal-tonoise-ratios and full of spectral features were derived. This result is of utmost importance for the development of powerful instruments for space missions. By point scanning the surfaces of the meteorite samples, it was possible for the first time to construct UV-Raman images out of the array of Raman spectra. Deep-UV Raman images are to the best of our knowledge presented for the first time. The images were used for a discussion of the chemicalmineralogical composition and texture of the meteorite surfaces. Comparative Raman studies applying visible and NIR Raman excitation wavelengths demonstrate a much better performance for UV Raman excitation. This comparative study of different Raman excitation wavelengths at the same sample spots was done by constructing a versatile, robust sample holder with a fixed micro-raster. The overall advantages of UV resonance Raman spectroscopy in terms of sensitivity and selectivity are demonstrated and discussed. Finally the application of this new technique for a UV Raman instrument for envisaged astrobiological focused space missions is suggested. Raman spectroscopy1 is a nondestructive and extraordinary powerful method to determine the chemical-mineralogical composition of unprepared samples. In combination with a confocal optical microscope, Raman micro-spectroscopy can resolve sample sizes (grains, inclusions, microorganisms) less than 1 µm2 and can image the texture of the surfaces. Due to its high specificity and versatility, the method has successfully been applied for * Corresponding author. Phone: +49-3641-948320. E-mail: juergen.popp@ uni-jena.de. † Institut fu ¨ r Physikalische Chemie. ‡ Kayser-Threde GmbH. § Institut fu ¨ r Geowissenschaften. | Institut fu ¨ r Physikalische Hochtechnologie e.V. (1) Popp, J.; Kiefer W. Fundamentals of Raman Spectroscopy, Encyclopedia of Analytical Chemistry; Wiley: New York, 2000; pp 13104-13142. 10.1021/ac0618977 CCC: $37.00 Published on Web 01/05/2007
© 2007 American Chemical Society
analyzing major and minor phases in Martian meteorites, their polytypes, and their relative proportion and chemical zoning.2 With the help of this information, it is possible to classify the meteorites and discuss their alteration and evolutionary history. Therefore, Raman spectroscopy has advantages as compared to methods like R proton X-ray spectroscopy (APXS) and electron microprobe analysis and can contribute to resolving various questions in the field of planetary research.2-11 Nowadays a Raman microscope is considered a valid technique for space applications as a stand-alone instrument or in combination with laser-induced plasma spectroscopy (LIPS). The envisaged rover-based planetary missions (e.g., EXOMARS from European space agency (ESA)) will allow for an in situ analysis on the Martian surface and subsurface with a Raman sensor in a ground penetrating mole.12 This mission will be focused on astrobiology and will bring the interesting question about the past or present possibilities of the development of life on Mars in the center of worldwide interest. Fortunately, Raman spectroscopy is equally suited and well applicable for the detection of minerals, organic and inorganic substances, and biological molecules. Even water can be detected easily. (2) Wang, A.; Jolliff, B. L.; Haskin, L. A. J. Geophys. Res. 1999, 104, 85098519. (3) Isreal, E. J.; Arvidson, R. E.; Wang, A.; Pasteris, J. D.; Jolliff, B. L. J. Geophys. Res. 1997, 102, 28705-28716. (4) Haskin, L. A.; Wang, A.; Rockow, K. M.; Jolliff, B. L.; Korotev R. L.; Viskupic, K. M. J. Geophys. Res. [Planets] 1997, 102, 19293-19306. (5) Korotev, R. L.; Wang, A.; Haskin, L. A.; Jolliff, B. L. Lunar and Planetary Science XXIX. Lunar Planet. Sci. Conf. 1998, No. 29, 1797-1798. (6) Edwards, H. G. M.; Farwell, D. W.; Grady, M. M.; Wynn-Williams, D. D.; Wright, I. P. Planet Space. Sci. 1999, 47, 353-363. (7) Popp, J.; Tarcea, N.; Kiefer, W.; Hilchenbach, M.; Thomas, N.; Hofer, S.; Stuffler, T. ESA 2001, SP-496, 193-196. (8) Estec, P. A.; Kovach, J. J.; Waldstein, P.; Karr, C., Jr. Proc. Lunar Sci. Conf. 1977, No. 3, 3047-3067. (9) Wang, A.; Kuebler, K.; Jolliff, B. L.; Haskin, L. A. J. Raman Spectrosc. 2004, 35, 504-514. (10) Popp, J.; Schmitt, M. J. Raman Spectrosc. 2004, 35, 429-432. (11) Rull, F.; Martinez-Frias, J.; Sansano, A.; Medina, J.; Edwards, H. G. M. J. Raman Spectrosc. 2004, 35, 487-503. (12) Ellery, A.; Wynn-Williams, D.; Parnell, J.; Edwards, H. G. M.; Dickensheets, D. J. Raman Spectrosc. 2004, 35, 441-457.
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However, the performance of a flight-ready instrument has to fulfill constrains like very limited mission budget of mass/volume and energy.7 Furthermore, the Raman signals must be derived unambiguously, in a short measuring time and with high signalto-noise-ratio (S/N). Because the sensitivity of conventional Raman spectroscopy is rather poor and the spectra are often obscured by strong fluorescence signals, it is necessary to circumvent these drawbacks and to increase the signals by the application of special signal enhancing effects. The most promising approach is the resonance Raman effect, where the scattering cross-section can be enhanced as much as 108 if the Raman excitation wavelength lies within an electronic resonance band of the material. Nowadays the favored strategy for the search for past or present life on Mars is the direct in situ identification of biomarkers (amino acids, nucleic acids, fossilized residues of sunscreen pigments of cyanobacteria, etc.) in the subsurface levels.12 These small aromatic molecules can be studied very selectively by tuning the resonance excitation wavelength across the absorption bands of the molecules in the ultraviolet (UV) spectral range.13-21 With this technique of UV resonance Raman spectroscopy it is, for example, possible to distinguish easily between amino acids and nuclide acids (absorption between 220 and 235 nm and between 240 and 250 nm, respectively) or even between specific molecular vibrations in quinolines.19-21 Therefore, it might be even possible to explore the genotype of microorganisms within less than 1s of measuring time. In this paper, we report about the great capabilities of UV Raman microscopic investigations of extraterrestrial material. UV resonance Raman spectra of the Martian meteorites Dar al Gani, Sayh al Uhaymir, and Zagami have been derived with very high signal-to-noise-ratios, without any disturbing fluorescence and full of spectral features. By employing the point measurement technique and the point counting procedure,4 it was possible to identify the mineral phases on the surfaces of the rocks and for the first time to construct UV Raman images of extraterrestrial samples. Several advantages of UV resonance Raman microscopy are discussed, and the results are compared to investigations with Raman excitation wavelengths in the visible (VIS) and nearinfrared (NIR) spectral regions. MATERIALS AND METHODS UV Micro-Raman Spectroscopy. UV Raman microscopy was performed with a micro-Raman setup (HR800 LabRam, Horiba/ Jobin-Yvon) equipped with an Olympus BX41 microscope, a UVsensitive video camera, and a liquid N2-cooled CCD detector. For UV microscopy, a UV achromatic fused silica-CaF2 micro spot objective (LMU-20x-UVB, NA ) 0.4, OFR) with broadband UVB (13) Rava, R. P.; Spiro, T. G. J. Am. Chem. Soc. 1984, 106, 4062-4064. (14) Rava, R. P.; Spiro, T. G. J. Phys. Chem. 1985, 89, 1856-1861. (15) Copeland, R. A.; Dasgupta, S.; Spiro, T. G. J. Am. Chem. Soc. 1985, 107, 3370-3371. (16) Asher, S. A.; Ludwig, M.; Johnson, C. R. J. Am. Chem. Soc. 1986, 108, 3186-3197. (17) Song, S.; Asher, S. A. J. Am. Chem. Soc. 1989, 111, 4295-4305. (18) Chi, Z.; Chen, X. G.; Holtz, J. S. W.; Asher, S. A. Biochemistry 1998, 37, 2854-2864. (19) Frosch, T.; Schmitt, M.; Popp, J. Anal. Bioanal. Chem 2006, published online. (20) Frosch, T.; Schmitt, M.; Bringmann, G.; Kiefer, W.; Popp, J. J. Phys. Chem. B (in press). (21) Frosch, T.; Schmitt, M.; Noll, T.; Bringmann, G.; Schenzel, K.; Popp, J. Anal. Chem. (in press).
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coating was chosen. Validation of the wavenumber axis was performed between measurements using the Raman signals from polytetrafluoroethylene. The excitation wavelengths 244 and 257 nm are derived from an intracavity frequency doubled argonion laser (Innova 300, MotoFreD, Coherent Inc.). The output power at the laser head was approximately 40 mW, with 3 mW reaching the sample. The spectral resolution of the spectrometer was 5 cm-1. For the x/y scans, the meteorites were moved relative to the fixed laser spot with help of a motorized stage. VIS/NIR Micro-Raman Spectroscopy. The comparative VIS/NIR Raman spectra were aquired with a micro-Raman setup (HR LabRam, Horiba/Jobin-Yvon) equipped with an Olympus IX70 microscope, a video camera, and an air-cooled CCD detector operating at 220 K. A Nikkon LPlan 20x/0.35 objective focused the laser light on the meteorites. Validation of the wavenumber axis was performed between measurements using the Raman signals from TiO2 (anatase). As excitation wavelengths the 633 nm of a HeNe laser, the 532 nm line of a frequency doubled Nd:YAG laser (Coherent Compass) and the 830 nm of external cavity semiconductor laser (TEC100 Raman, Sacher Lasertechnik) were used. For the x/y scans, the meteorites were moved relative to the fixed laser spot with help of a motorized stage. Samples. The SNC (Shergottiten-Naklithen-Chassigniten) meteorites are the only available genuine Martian material. Raman studies have been performed on rough cut surfaces of the three meteoritessSAU 008, DAG 735, and Zagami. These meteorites are named after their finding locations: Sayh al Uhaymir in Oman (SAU), Dar Al Gani in Libya (DAG), and Zagami in Nigeria. Primarily, all three Martian meteorites belong to the group of shergottites, that is, they are basalts that are mainly composed of pyroxenes (augite, pigeonite) and plagioclase with minor phosphates and opaques (ilmenite, chromite, sulfide). In addition, SAU 008 and DAG 735 contain porphyric olivine megacrysts. All three Martian meteorites are heavily shocked, resulting in the complete conversion of crystalline plagioclase into diaplectic glass, the socalled maskelynite. For comparison of the performance of the Raman measurements, five different laser wavelengths (244, 257, 532, 633, and 830 nm) have been used for excitation. With each wavelength and each meteorite, 441 Raman spectra (organized in a 21 × 21 sampling matrix, with a sampling step size of 10 µm) were recorded from the same 200 × 200 µm2 areas of the meteorite surfaces. Point studies have been performed to find the appropriate laser power and experimental parameters of sample handling. Also control scans with only 1-5 s accumulation times were performed before and after the Raman mappings to exclude any degradation effects of the samples due to the laser illumination. The depth of the laser focus as well as the surface roughness of the sample is approximately 10 µm. RESULTS AND DISCUSSION In the following, a detailed discussion of physically based advantages of UV resonance Raman spectroscopy as compared to conventional Raman spectroscopy is presented. Then, the great capabilities of this technique are demonstrated with help of UV Raman images and comparative Raman studies with different excitation wavelengths in the UV, VIS, and NIR spectral regions. The three Martian meteoritessSayh al Uhaymir, Zagami, and Dar Al Ganisthat are known to contain compounds exhibiting strong
Figure 1. Raman image, in the spectral range (930-1120) cm-1, on Martian meteorite DAG 735. The image was constructed from Raman spectra under excitation wavelength 257 nm. The surface area has a range of approximately 250 × 250 µm2. The Raman intensity is given using a color scale. Points of interest are marked by numbers and arrows. The Raman spectra recorded at these points of interest are shown in Figures 2 and 3, respectively.
Figure 2. Raman spectra of calcite, whilockit, and pyroxenestaken at points of interest on the DAG 735 surface as indicated in Figure 1. The distribution of the three minerals calcite, whilockit, and pyroxene is displayed in the 3D Raman images on the left side.
fluorescence and are of relevance for space missions have been chosen to demonstrate the strength of UV excited Raman spectroscopy. Discussion of Advantages of UV Resonance Raman Spectroscopy. The Raman process (inelastic scattering of photons) is a weak process that has to compete with other physical
processes (e.g., fluorescence) when light interacts with matter. Because the fluorescence signals are generally much stronger than the Raman signals, the Raman spectra can often only be derived with very poor signal-to-noise-ratio, or they are even completely obscured. For a classic Raman measurement, the only way of getting rid of this main hindrance is to tune the laser Analytical Chemistry, Vol. 79, No. 3, February 1, 2007
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Figure 3. Raman spectra of olivine, quarz, maskelynit and pyroxene, and orthopyroxenestaken at points of interest on the DAG 735 surface as indicated in Figure 1.
excitation wavelength to a spectral region where the probability of having the fluorescence signal interfering with the Raman spectrum is minimal. Two approaches are normally used. The first and most widely used approach is to lower the energy of the incoming photon such that the excitation of the molecule in an electronically excited state does not take place. Therefore, the Raman excitation wavelengths are tuned in the NIR region (from 785 nm up to 1064 nm) by applying such NIR excitation wavelengths for most of the samples (especially the biological samples) that the fluorescence excitation is avoided. However, avoiding fluorescence via such an approach proves not very efficient for the minerals since minerals always contain a certain amount of rare-earth element impurities having excited electronic levels at relatively low energies. The incoming Raman excitation photon energy cannot be chosen to be arbitrarily low since by applying an excitation wavelength of 830 nm the Raman spectrum already can no longer be fully recorded with a standard CCD camera based on silicon technology (cutoff wavelength at approximately 1100 nm). The second approach used for minimizing the interference of fluorescence with the Raman signal is to move the Raman excitation wavelength into the deep UV region. For these wavelengths fluorescence is excited, but for Raman excitation wavelengths below 250 nm, the fluorescence does not interfere with the Raman signals any longer. This is because a typical Raman spectral range of 4000 cm-1 occurs in less than 30 nm above the excitation wavelength at 250 nm, and the fluorescence takes place spectrally well-shifted from the first excited electronic state. Moreover the observation of fluorescence emission of any material at wavelengths below approximately 280 nm is also very rare. This provides complete spectral separation of Raman and 1104
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fluorescence emission bands resulting in high signal-to-noise Raman measurements. In addition to having the Raman and fluorescence signals spectrally well-separated, if the Raman excitation occurs within an highly excited electronic resonance band of a material, the scattering cross-section can be improved as much as 108 (resonance Raman enhancement). Diamond, nitrites and nitrates, and many other organic and inorganic materials have strong absorption bands in the deep UV and exhibit resonance enhancement of Raman bands when excited in the deep UV. These is especially true for the already discussed biomarkers, making UV resonance Raman spectroscopy a promising tool for the search for past or present extraterrestrial life. Furthermore, when comparing the Raman signal obtained for UV excitation with those for NIR excitation, the Raman signals will increase in intensity for decreasing excitation wavelengths since the Raman cross-section itself is dependent on the excitation wavelength to the inverse fourth power. Thus an increase of a factor of approximately 360 in magnitude of the amount of Raman scattered photons can be obtained by moving from NIR (at 1064 nm) to the UV spectral region (244 nm)! Also the (diffraction limited) minimal possible size of the sampling spot for microRaman experiments is proportional to the wavelength of the laser beam; therefore, a better spatial resolution for Raman mapping experiments is achieved when the Raman excitation laser is having a shorter wavelength. However in our study with a point step size of 10 µm, we did not work at the diffraction limit and applied a laser spot of approximately 1 µm diameter. UV Raman Imaging. UV Raman imaging studies were performed on the rough cut surfaces of SAU 008, Zagami, and DAG 735. An exemplary Raman image of DAG 735 obtained with a Raman excitation wavelength of 257 nm is shown in Figure 1. By employing the point measurement technique, a surface area of approximately 250 × 250 µm2 has been scanned with a step size of 10 µm, and Raman spectra have been taken at any point. The image in Figure 1 was constructed from this array of Raman spectra using the integrated intensity in the spectral range from 930 to 1120 cm-1. The Raman intensity is given with a linear color scale. With help of this kind of Raman images one can derive information about the chemical-mineralogical composition and the texture of the surface section. The chosen wavenumber range in Figure 1 selects the surface area with amounts of calcite, pyroxene, and whitlockit. The array of Raman spectra is full of spectral features, and the signal-to-noise-ratios of the measured spectra are very good. Some points of interest on the surface area with different mineralogical composition are marked with arrows and numbers in Figure 1. In Figure 2, the information of Figure 1 is further separated into the individual distributions of calcite (1070-1120 cm-1), whitlockit (930-990 cm-1), and pyroxene (990-1020 cm-1). The three 3D images are shown (in Figure 2) together with the corresponding Raman spectra. One can observe an extended vein with high amounts of calcite. While the Zagami meteorite is the only observed fall (1962), the meteorites SAU 008 and DAG 735 were found and show therefore some degree of terrestrial alteration and secondary mineralization such as the formation of calcite along cracks. Thus the calcite vein was assigned to terrestrial alteration and verified as extended crack on the sample surface by inspection with optical microscopy. Occurrences of whitlockit were detected localized at the edge of
Figure 4. Comparative UV Raman spectra (mainly pyroxene) of the Martian meteorites SAU 008, Zagami, and DAG 735 with excitation wavelengths 244 and 257 nm on arbitrary fixed points.
Figure 5. Comparative Raman spectra (mainly pyroxene) of the Martian meteorites SAU 008, Zagami, and DAG 735 with excitation wavelengths 532, 633, and 830 nm on arbitrary fixed points.
the sample area, confirming a small amount of phosphate in the meteorite. The whole sample rectangle is covered by an even distribution of pyroxene. This result verifies pyroxene to be a mayor compound in Dar Al Gani. However, a wealth of spectral features has been detected within the sample area. Some identified Raman spectra are assigned to olivine, quartz, or maskelynit and are shown in Figure 3, respectively. The Raman spectra shown in Figure 3 were measured at the points marked in Raman image of Figure 1. However, while the UV Raman studies of the meteorites seem to be extremely promising, Raman measurements of the same meteorite samples applying Raman excitation wavelengths in the VIS or NIR are dissatisfactory in case of fluorescent compounds. Many spots at the sample surfaces showed strong fluorescence, making the mineralogical Raman information inaccessible. The high fluorescence background is covering the Raman bands and quickly saturating the detector, making detection of the weak Raman features by using longer integration time intervals impos-
sible. Also the possibility of enhancing the S/N of the Raman signals by averaging more accumulations is excluded because the read-out-noise of the CCD detector is dominating. The efforts of gathering useful VIS/NIR-Raman images were mostly unsuccessful. The application of the point measurement technique applying the same measurement period at any point is ruled out because the CCD is saturated at many sample points due to the fluorescence background, while the S/N of the Raman signals at other points is still rather poor. Because these results seem to be very important for the future study of relevant extraterrestrial material or even the development of a Raman instrument for space mission, we studied this finding more quantitatively in comparative Raman measurements applying different Raman excitation wavelength. Comparative Raman Studies with Different Excitation Wavelengths. For comparative Raman studies with different excitation wavelengths and therefore using different Raman setups, it is a necessity to always find the same sample spots on the meteorites with a spatial precision better than 1 µm. Therefore Analytical Chemistry, Vol. 79, No. 3, February 1, 2007
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Figure 6. Number of fluorescing and number of unsuccessfully measured points counted for each meteorite sample (SAU 008, Zagami, and DAG 735). Each sample was investigated with the chosen five laser wavelengths (244, 257, 532, 633, and 830 nm).
tailor-made holders were built individually for the meteorite samples to place their surfaces perpendicular to the laser beam for the xy-resolved Raman investigations. A micro-raster (with a raster-grid of 50 × 50 numbered fields with sizes of 200 × 200 µm2 brought up on a 0.4 mm thin fused silica wafer) was fixed to the sample. This mask was placed within the sample holder 2 mm above and parallel to the investigated surface of the meteorites. This distance was chosen to ensure that no quartz signal from the mask will be excited due to the depth of the laser-focus. The transmittance of the mask was verified by a measurement with a UV-VIS-NIR spectrophotometer (Cary 5000, Varian). Any influence or strong absorbance of the mask at all applied excitation wavelengths and setups was further excluded by test measurement with reference samples. Fields of 200 × 200 µm2 on the DAG 735, SAU 008, and Zagami meteorite samples were scanned with a step size of 10 µm resulting in a sampling matrix of 21 × 21 measured points. From each of the 441 points within this matrix, a Raman spectrum has been measured for each of the 1106
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five applied Raman excitation wavelengths. The available laser wavelengths were 244 and 257 nm in the deep UV spectral region, 532 and 633 nm in the VIS, and 830 nm in the NIR range. The technique of applying a fixed micro-raster to locate the same samples spots on a surface (which is rather unstructured) and investigate these spots on different Raman setups is, to best of our knowledge, unique to this contribution. The UV data gathered with Raman excitation wavelengths at 244 and 257 nm on fixed points on the surfaces of SAU 008, Zagami, and DAG 735 are presented in Figure 4 and show in general very low backgrounds. The comparative Raman spectra at the same sample spots obtained for excitation wavelengths in the VIS (532 and 633 nm) and the NIR (830 nm) are displayed in Figure 5. One can clearly see that the UV Raman spectra of all three meteorites (Figure 4) show much better signal-to-noise-ratios (S/N) in the range of 30-100 as compared to the VIS/NIR Raman spectra (Figure 5) with S/N in the range of 2-15 at this specific sample spot with high amount of pyroxene, while the differences
Figure 7. General fluorescence behavior for the investigated samples (DAG 735, SAU 008, and Zagami) when Raman excitation is made at different wavelengths: (a) 244, (b) 257, (c) 532, (d) 633, and (e) 830 nm.
in S/N are even more pronounced at other spots, as discussed in the following. Overall we observed that the Raman spectra measured with an excitation in the VIS and NIR are worse than the ones measured in UV. For each Raman scan on the 200 × 200 µm2 sample areas the numbers of spectra which present no information (no Raman bands) as well as those which do present significant fluorescent features were determined. For each measured spectrum, a baseline was manually created using a model-free baseline generation algorithm.22 The baseline created in this way was considered as the background of the Raman measurement. The Raman signal is then the difference between the measured spectrum and this baseline. We choose to define a Raman spectrum as having a fluorescent background if the intensity under the baseline was larger than approximately 10-fold of the dark current expected for a certain integration time interval. Added to this level we considered an additional amount directly proportional to the overall utile Raman signal (area of the observed Raman bands). This added amount is in general linked to the instrument used for measurement and is accounting for the (primarily) signal photons, which are ending up (reach the detector) as stray light. Each spectrum was evaluated on whether Raman bands are recognizable and whether fluorescence is interfering with that specific measurement. The statistic of this evaluation is presented in Figure 6. For all three samples (DAG 735, SAU 008, and Zagami) we observed the same general behavior. The number of unsuccessful measurements is smaller when employing Raman excitation lasers in the deep UV. Also the number of spectra which suffer from fluorescent background interference is significantly smaller when using these UV wavelengths for Raman experiments. The reason for the high number of failed measurements on Zagami when using VIS-NIR excitation (∼50% in Figure 6) is due to an extremely fluorescent shock vein, which according to recent investigations is glassy.23,24 Measurements with 244 and 257 nm laser indicate a high contribution of feldspar component to the melt vein. Figure 7 shows the general fluorescence behavior for the samples here in discussion (surface regions of DAG 735, SAU (22) Friedrichs, M. S. J. Biomol. NMR 1995, 5, 147-153. (23) Langenhorst, F.; Poirier, J.-P. Earth Planet. Sci. Lett. 2000, 176, 259-265. (24) Langenhorst, F.; Poirier, J.-P. Earth Planet. Sci. Lett. 2000, 184, 37-55.
008, and Zagami). The fluorescence is excited by the employed measuring laser beam, and its behavior is shown here only for a spectral window of roughly 2000 cm-1 red-shift relative wavenumbers from the excitation laser wavelength. It can be seen that for 244 nm excitation in the relative wavenumber region up to 1300 cm-1 no fluorescence interference is observed. The distribution of minerals on the 200 × 200 µm2 scanned surface area of DAG 735 (the small surface section might be unrepresentative for the whole meteorite surface) is determined by the UV Raman experiments to 80% pyroxene, 65% calcite, 10% olivine, and 10% phosphates. The results from VIS/NIR measurements differ significantly, while many points at the surface have been inaccessible as shown in Figure 6. The biggest weight in accounting for the statistical differences is the possibility to distinguish faint Raman spectral features when employing the excitation in UV. The higher S/N achievable in this case leads to more accurate assignments of Raman bands contrary to the case of measurements plagued by strong fluorescence. From this perspective, the higher number of spectra attributed to a specific mineral for measurements made at 244 and 257 nm when compared with the scans made at 532, 633, and 830 nm is justified. CONCLUSION AND OUTLOOK While a Raman instrument is already considered as a valid technique for space application, in this contribution it has been demonstrated that Raman UV excitation has a much better performance when compared with VIS/NIR excitation. This conclusion is based on several physical facts: (i) the interference of obscuring fluorescence is avoided; (ii) the Raman signals are strongly improved due to the ω4 dependency and the resonance enhancement; and (iii) the selectivity and the application for astrobiological questions is much improved due to resonance condition. These advantages have been applied to derive UV Raman spectra of the Martian meteorites Sayh al Uhaymir, Dar al Gani, and Zagami with very good S/N ratios. By scanning the sample surfaces, it was possible to derive an array of Raman spectra and to construct UV Raman images of meteorites for the first time. With the help of these Raman images, it was possible to discuss the texture of the surface of DAG 735. It was possible to visualize a vein with high amounts of calcite that was assigned to terrestrial alteration along an extended crack. The occurrence of whitlockit and an even pyroxene distribution as well as a small amount of minor components (olivine, quartz, and maskelynit) have been located. Comparative Raman studies on the same surface area of the three meteorites with different excitation wavelengths were performed by developing the technique of a fixed micro-raster. The quantitative comparison shows the impressive benefit of the UV Raman studies. While almost all sample spots have shown useful Raman spectra, when illuminated with deep UV laser excitation, 50% or more of the spots have been inaccessible by VIS/NIR excitation. The envisaged future planetary missions require space-born instruments, which are very sensitive, selective, and capable of unambiguously deriving spectral information from minerals as well (25) Wang, A.; Haskin, L. A.; Cortez, E. Appl. Spectrosc. 1998, 52, 477 (26) Wang, A.; Haskin, L. A.; Lane, A. L.; Wdowiak, T. J.; Squyres, S. W.; Wilson, R. J.; Hovland, L. E.; Manatt, K. S.; Raouf, N.; Smith, C. D. J. Geophys. Res. 2003, 108, 1
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as from biomarkers. With this paper, we demonstrate the great capabilities of a UV Raman spectrometer for use in rover-based astrobiological missions. The miniaturization of flight-ready Raman devices is an ongoing research topic, and several designs in VIS25,26 and NIR27,28 spectral ranges have been suggested by different groups. The technological development of highly miniaturized UV (27) Popp, J.; Tarcea, N.; Schmitt, M.; Kiefer, W.; Hochleitner, R.; Simon, G.; Hilchenbach, M.; Hofer, S.; Stuffler, T. In Proceedings of the Second European Workshop on Exo/Astrobiology; 2002; p 339. (28) Dickensheets, D. L.; Wynn-Williams, D. D.; Edwards, H. G. M.; Schoen, C.; Crowder, C.; Newton, E. M. J. Raman Spectrosc. 2000, 31, 633-635 (29) Storrie-Lombardi, M. C.; Hug, W. F.; McDonald, G. D.; Tsapin, A. I.; Nealson, K. H. Rev. Sci. Instrum. 2001, 72, 4452-4458. (30) Riesenberg, R.; Nitzsche, G.; Wuttig, A.; Harnisch, B. Micro spectrometer and MEMS for space. In Smaller Satellites: Bigger Business; Kluwer Academic Publisher: New York, 2002; pp 403-406.
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Raman instruments that fulfill the tight constrains for application in envisaged planetary missions is currently underway in our laboratories by taking advantage of new, compact NeCu hollow cathode lasers29 and the hadamard transform spectrometer principle.30 ACKNOWLEDGMENT The authors thank Albrecht Lerm and Wolfgang Fa¨hndrich for the preparation of the micro-structured masks and the customized sample holders. We gratefully acknowledge financial support by the German Aerospace Centre (Project DLR 50OW0502). Received for review October 9, 2006. Accepted November 15, 2006. AC0618977
