Raman spectroscopic identification of phthalic and mellitic acids in mineral matrices

Raman spectroscopy has many advantages for planetary exploration, and the Raman technique is currently being developed for future space missions. The Raman microspectroscopic study of organic acids (phthalic and mellitic acids) in experimentally prepared mixtures with halite and gypsum was performed using near infrared 785 nm and visible 514 nm excitation wavelengths. Gypsum and halite matrices were chosen as analogues of Martian minerals. Carboxylic acids mixed with mineral powders were also investigated through a UV-transparent crystal of gypsum and halite (approximately 2 mm, resp. 5 mm thick), thereby creating a type of artificial inclusion that could potentially be present in Martian minerals. The detection limit of phthalic acid mixed in mineral matrices and analyzed through crystals was 1 wt% using both excitation wavelengths. A Raman mellitic acid signal was obtained at a concentration as low as 1 wt% in halite matrix, and at a concentration of 5 wt% when analyzed through halite crystal. In the case of mellitic acid mixed with gypsum and for analyses through a gypsum crystal, the detection limit is 5 wt% using both excitation wavelengths.