Obtaining Raman spectra of minerals and carbonaceous matter using a portable sequentially shifted excitation Raman spectrometer – a few examples Jan Jehlička,* Adam Culka and Filip Košek We present examples of application of a recently developed portable sequentially shifted excitation Raman spectrometer for iden- tifying three series minerals and carbonaceous matter. Those include compounds of relevance for the fields of geobiology and exobiology: sulfates and carbonates, organic minerals, and carbonaceous matter. It is demonstrated that unambiguous obtaining of Raman spectra can be achieved fast and with the gain in eliminating potential fluorescence features. Copyright © 2017 John Wiley & Sons, Ltd. Keywords: portable sequentially shifted excitation Raman spectrometer; minerals; geobiology; exobiology Introduction Portable Raman instruments are nowadays considered as useful tools for identifying minerals, [1–4] microbiological pigments, [5,6] as well as pigments used in different art disciplines. [7,8] The use of transportable and portable Raman spectrometers in different disci- plines was recently reviewed by Colomban [9] and Vandenabeele et al., [10] and critical aspects adressed as well. [11,12] It was shown that whitish carbonates of similar structure and slightly different compo- sition can be unambiguously discriminated using the DeltaNu system with near infrared excitation. [13,14] Dedicated studies have demonstrated how handheld instruments allow detection of min- erals directly onsite under cave conditions [15] or in the frame of old mining works: secondary as minerals in earthy alteration crusts at Mikulov. [16] In the group of green/brown minerals, the Raman spectra are frequently not easy to obtain or characterize using the common infrared diode lasers excitation of portable instruments. [9] The major problem consists in the observation of a strong fluorescence emission background that can mask completely the Raman signatures as well as absorption of the red laser radiation. However, these problems are also common when using red excitation under laboratory conditions even with performant Raman microspectrometric systems. Black minerals are also not easy specimens, and Raman spectra are difficult to obtain when using the handheld instruments with near infrared excitation. [9] This extends also to highly reflective minerals – graphite, haematite, magnetite, wolframite, or molybdenite. Critical issues for practical use of portable Raman devices can be summarized as follows: (1) positioning, (2) illumination/collection problems (common also in the case of laboratory systems) related to the laser wavelength, and (3) precision, related to the generally lower performance and spectral resolution of the handheld instruments. The portable, handheld and palm-size systems can also be used in geobiology, microbiology areas to search for Raman spectro- scopic signals of biosignatures. [5,6] Those studies are more or less directly relevant for the next exobiology missions planned by NASA and ESA in the frame of next missions to Mars. In fact, Raman spec- troscopy is planned to be one of the techniques on-board on Exomars II mission by ESA. [17] It was suggested to carry-out training on Earth, in the frame of environments suggested as Martian analogues. [18] A new type of portable Raman systems appeared re- cently on the market. An alternative and relatively recent method to mitigate the fluorescence with benchtop instruments is so-called shifted excitation Raman difference spectroscopy. In simplistic terms, this method is based on the changing of the excitation laser wavelength during Raman spectral acquisition. [19,20] In sequentially shifted excitation Raman spectroscopy the diode lasers operate at different temperatures, providing slightly shifted wavelengths; in this case, the location of Raman intensities in spectral space changes with the excitation wavelength, while unwanted spectral intensities corresponding to fluorescence, stray light, fixed pattern detector noise, and so on remain unchanged in spectral space. This difference allows extracting the Raman spectrum separated from the fluorescence spectrum. BRAVO uses a new patented technol- ogy (SSE ™ , patent number US8570507B1) to mitigate fluorescence and is equipped with two excitation lasers with wavelengths (DuoLaser ™ ) ranging from 700 to 1100 nm. Each laser is tempera- ture shifted over a small wavelength range; for example, the distrib- uted Bragg reflector diode laser emits single-mode 785 nm radiation at 25 °C. A typical measurement consists of collecting Raman spectra at distributed Bragg reflector laser temperatures of 20, 23, 26, and 29 °C (i.e., four sequential excitations). This yields * Correspondence to: Jan Jehlička, Faculty of Science, Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University, Albertov 6, 128 43, Prague 2, Czech Republic. E-mail: jehlicka@natur.cuni.cz Faculty of Science, Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University, Albertov 6, 128 43, Prague 2, Czech Republic J. Raman Spectrosc. (2017) Copyright © 2017 John Wiley & Sons, Ltd. Research article Received: 14 October 2016 Revised: 5 December 2016 Accepted: 23 December 2016 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/jrs.5105