Raman spectroscopic investigations on soil using a 785 nm dual-wavelength diode laser
Precision agriculture is becoming increasingly important on a global level, for example in terms of meeting the food demand of a steadily increasing world population. Site-specific fertilizer application not only ensures the sustainable use of limited resources. It is, at the same time, crucial for environmental protection by avoiding excess fertilizer usage. With this goal in mind, the Ferdinand-Braun-Institut evaluates the potential of Raman spectroscopy. The non-destructive optical method can be used to characterize soil, thus paving the way for efficient nutrient management.
Shifted excitation Raman difference spectroscopy (SERDS) is applied to overcome interferences resulting e.g. from daylight or fluorescence and to extract the characteristic Raman signals. Such disturbing signals are common for a wide range of natural samples including soil. As SERDS requires an excitation light source with two distinct emission lines, an in-house developed Y-branch dual-wavelength 785 nm diode laser is used. The measurement principle has already been successfully demonstrated for in situ outdoor investigations in an apple orchard  and is now transferred to soil analysis.
To this end, confocal Raman microscopy measurements were performed in conjunction with our SERDS experiments to generate a better understanding of soil properties, e.g. the heterogeneous distribution of target substances. As the investigated area for each measured point is about 1 µm2, spatially resolved Raman spectra of individual soil constituents can be obtained. Fig. 1 shows a 3D false-color Raman image of the sample surface. Contributions of rutile, anatase, quartz, feldspar and our target analyte hydroxyapatite can be seen.
For our SERDS investigations, the sample was initially excited at the first laser wavelength (784.47 nm) of the dual-wavelength diode laser , and the Raman spectrum was recorded (red curve in Fig. 2). Here, a larger excitation spot size (diameter approx. 20 µm) was applied with respect to future field applications of portable Raman systems to obtain spectroscopic information from a larger, i.e. more representative area of the sample. The larger spot size also causes pronounced fluorescence interference and only one single Raman signal at 961 cm-1 is visible in this case. Subsequently, the sample was excited at the second laser wavelength (785.05 nm), the corresponding Raman spectrum is shown as green curve in Fig. 2. The spectrally narrow Raman signals shift with the shift in excitation wavelength, i.e. 10 cm-1, while the spectrally broad fluorescence remains unchanged. A subtraction of both spectra (and further mathematical processing) can therefore effectively separate the Raman signals from fluorescence interference as shown in the blue curve in Fig. 2. Due to the larger excitation spot size contributions from three individual soil components can be identified in the SERDS spectrum: quartz, feldspar and hydroxyapatite. These results highlight the large potential of SERDS as a promising tool for soil analysis in precision agriculture to improve soil nutrient management .
This study was funded by the Federal Ministry of Education and Research (BMBF) under contract 031A564C through the funding measure BonaRes (Soil as a Sustainable Resource for the Bioeconomy) within the consortium I4S (Intelligence for Soil).
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