Raman spectroscopic identification of usnic acid in hydrothermal minerals as a potential Martian analogue
Abstract
Raman spectroscopy using 785 nm excitation was tested as a nondestructive method for determining the presence of the potential biomarker, usnic acid, in experimentally prepared mineral matrices. Investigated samples consisting of usnic acid mixed with powdered hydrothermal minerals, gypsum and calcite were studied.
Various concentrations of usnic acid in the mineral matrix were studied to determine the detection limits of this biomarker. Usnic acid was mixed with gypsum (respectively, calcite) and covered by a UV-transparent crystal of gypsum thereby creating artificial inclusions similar to those which could be present in Martian minerals.
A Raman usnic acid signal at the concentration level as low as 1 g kg(-1) was obtained in the powdered mineral matrix and 5 g kg(-1) when analyzed through the monocrystal.