Raman Hyperspectral Imaging of Microfossils: Potential Pitfalls

Abstract

Initially, Raman spectroscopy was a specialized technique used by vibrational spectroscopists; however, due to rapid advancements in instrumentation and imaging techniques over the last few decades, Raman spectrometers are widely available at many institutions, allowing Raman spectroscopy to become a widespread analytical tool in mineralogy and other geological sciences. Hyperspectral imaging, in particular, has become popular due to the fact that Raman spectroscopy can quickly delineate crystallographic and compositional differences in 2-D and 3-D at the micron scale. Although this rapid growth of applications to the Earth sciences has provided great insight across the geological sciences, the ease of application as the instruments become increasingly automated combined with nonspecialists using this techique has resulted in the propagation of errors and misunderstandings throughout the field. For example, the literature now includes misassigned vibration modes, inappropriate spectral processing techniques, confocal depth of laser penetration incorrectly estimated into opaque crystalline solids, and a misconstrued understanding of the anisotropic nature of sp 2 carbons. Key Words: Raman spectroscopy-Raman imaging-Confocal Raman spectroscopy-Disordered sp 2 carbons-HematiteMicrofossils. Astrobiology 13, 920-931.

Evaluating the Effects of Autofluorescence during Raman Hyperspectral Imaging

“…Previous studies have noted that this method can be problematic if materials contain Raman bands in similar locations, for example the haematite 1320 cm −1 band and the ‘D’ band in carbonaceous material (Marshall et al . , Marshall and Marshall , Marshall and Olcott Marshall ). As a result, it has been suggested that intensity images representing the ‘G’ band at 1600 cm −1 should be used to identify carbonaceous material (Marshall and Marshall ).…”

“…, Smith and Godard , Zinin et al . , Marshall and Olcott Marshall ). Because fifth‐order polynomials' baseline correction has been widely used on hyperspectral data sets a priori and linear models were the most common in geological studies, a hyperspectral data set from the Apex chert was baseline‐corrected using linear and fifth‐order polynomials with two different mask region scenarios (Table ).…”

“…Samples from the Apex chert dyke were collected from fourteen localities every 10 m along the outcrop including the original microfossil locality (Marshall et al . , , Marshall and Olcott Marshall ). Thin section blanks were cut using a water‐cooled diamond saw.…”

EPR of Primitive Organic Matter: A Tool for Astrobiology

“…Among all the possible spectroscopic methods to study the primitive IOM in the context of the origin of life, Raman spectroscopy is one of the most emblematic one, since its first use in paleobiology in the seventies [19,20]. This technique is now sufficiently mature so that ESA/NASA ExoMars planetary mission (proposed for launch in 2018) will contain a miniaturized Raman spectrometer [21].…”

“…This technique is now sufficiently mature so that ESA/NASA ExoMars planetary mission (proposed for launch in 2018) will contain a miniaturized Raman spectrometer [21]. However, in addition to some pitfalls such as autofluorescence and laser heating of IOM by the incident laser beam, the latter is highly absorbed by the brown or dark rocks containing carbon (usually cherts), so that Raman spectroscopy is limited to the surface, and 3D Raman hyperspectral imaging is limited to depth smaller than 20 μm in samples [20]. In the perspective of combining complementary techniques for the analysis of fossilized IOM, EPR spectroscopy presents interesting characteristics that are not shared with other techniques.…”