On-site shifted excitation Raman difference spectroscopy for soil investigations

Shifted excitation Raman difference spectroscopy (SERDS) has been successfully applied for on-site soil analysis. Here, a portable SERDS sensor system is developed for in-field investigations. A dual-wavelength diode laser emitting at 785 nm is integrated into an in-house realized turnkey laser system as the excitation source. For our experiments, an operating point for the two laser lines with a spectral distance of 10 cm^-1 and 36 mW excitation power at the sample is selected. The sensor system allows a rapidly alternating operation between the two excitation wavelengths in the millisecond range and thus provides background-free SERDS spectra immediately after the measurement. This enables real-time evaluations, e.g., for quick on-site decisions. On-site soil analysis is carried out with the developed portable sensor system and SERDS extracts Raman signals of soil substances from background interferences with a 15-fold improvement of the signal-to-background-noise ratio. Besides others, closely neighbored Raman signals at 1084 cm-1 of calcite and at 1095 cm^-1 of dolomite are identified and mixtures of both soil carbonates are successfully discriminated by using SERDS. The presented results demonstrate the capability of the portable SERDS sensor system for on-site soil analysis and furthermore show the potential for applications such as geological investigations.