Thermal degradation of organic material by portable laser Raman spectrometry

Raman spectrometry has been established as an instrument of choice for studying the structure and bond type of known molecules, and identifying the composition of unknown substances, whether geological or biological. This versatility has led to its strong consideration for planetary exploration. In the context of the ExoGeoLab and ExoHab pilot projects of ESA-ESTEC & ILEWG (International Lunar Exploration Working Group), we investigated samples of astrobiological interest using a portable Raman spectrometer lasing at 785 nm and discuss implications for planetary exploration. We find that biological samples are typically best observed at wavenumbers >1100 cm^-1, but their Raman signals are often affected by fluorescence effects, which lowers their signal-to-noise ratio. Raman signals of minerals are typically found at wavenumbers <1100 cm^-1, and tend to be less affected by fluorescence. While higher power and/or longer signal integration time improve Raman signals, such power settings are detrimental to biological samples due to sample thermal degradation. Care must be taken in selecting the laser wavelength, power level and integration time for unknown samples, particularly if Raman signatures of biological components are anticipated. We include in the Appendices tables of Raman signatures for astrobiologically relevant organic compounds and minerals.