Evaluation of the influence of laser irradiation on Raman spectra of low-grade carbonaceous materials

<p>We evaluated the influence of laser irradiation for Raman spectroscopic analysis of low-grade carbonaceous materials (CMs) using three samples with peak metamorphic temperatures of approximately 200-300 °C. In the analysis, spectra were acquired using a 532 nm laser wavelength and a ×50 objective lens, which are common analytical conditions in CM Raman geothermometry. As CM is opaque, it is easily heated and damaged when irradiated with a high-power laser. However, an excessively low laser power results in a poor signal-to-noise ratio (SNR), making accurate parameter evaluation difficult. First, we investigated the minimum laser power conditions required to obtain a desirable SNR. The full width at half maximum (FWHM) of the D1-band, which is used as a parameter for metamorphic temperature estimation in CM Raman geothermometry, exhibited a considerably large standard deviation and small SNR at a surface laser power of <0.5 mW. Next, we focused on the influence of high irradiation power on the CM Raman spectra. The results show that the FWHM of the D1-band is sensitive to ‘damage’, i.e., the irreversible change in the structure of CM by laser irradiation. On the other hand, the peak position of the D1-band is sensitive to ‘heat’, meaning i.e., the increased surface temperature of CM. In particular, the FWHM of the D1-band increased as the laser power increased to >2.0 mW, which significantly affected the metamorphic temperature estimation. Similar results were obtained for measurements using a ×100 objective lens and different types of micro-Raman spectrometers. When the exposure time exceeded 30 s with sufficient laser power (>0.5 mW), the SNR became almost constant. More than three accumulations were efficient at eliminating cosmic-ray interference. Based on these results, we propose that a laser power of 0.5-2 mW and measurements of 10 s × 3 measurements are the optimum laser irradiation conditions for Raman spectroscopic analysis of low-grade CM.</p>