Quantitative analysis of inorganic ions in soil employing diffuse reflectance Fourier transform infrared spectroscopy (DRS-FTIR)

摘要

Nitrogen and sulfur are among the various important macronutrients present in the soil which continuously remain in dynamic equilibrium in the soil-system while participating in essential biogeochemical processes. The levels of ammonium (NH4+), nitrate (NO_3~-) and sulphate (SO_4~(2-)) also indicate soil fertility status. The approach of conventional analytical techniques for quantitative determination of inorganic ions restricts their use for large number of samples and in situ soil analysis. The objective of this study was to develop a fast and non-destructive method for quantitative determination of inorganic cationic and anionic species, i.e. NH4+, NO_3~-and SO_4~(2-) in soil employing diffuse-reflectance Fourier transform infrared spectroscopy (DRS-FTIR) with potassium bromide (KBr) matrix as background. The basis of determination of these ions is the selection of non-interfering quantitative vibrational peaks among the various observed peaks for the different symmetry types of the selected multi-atomic ionic units. The peaks at 3132 cm~(-1) (asymmetric stretching vibration, v_3), 1385 cm~(-1) (asymmetric stretching vibration, v_3} and 617 cm~(-1) (bending vibration, v_4) were selected for quantitative determination of NH_4~+, NO_3~-and SO_4~(2-) respectively, due to the simple reason of strong, sharp signals and avoiding the interfering peaks due to presence of other possible ions in soil. The quantitative analysis has been done by preparing calibration curves by obtaining the peak areas for a wide range of concentration at selected peaks for respective ions. The concentration of NH_4~+, NO_3~-and SO_4~(2-) were determined, by the present method, to be in the ranges 1.2-5.6, 16.3-23.8 and 11.4-24.3 μg g~(-1) respectively in 8 different mixed featured loam type soil samples collected randomly and freshly from the surrounding areas of the city. The uncertainty in the determination, in terms of rel. std. dev. (n = 10), were found to be in the range 2.16-4.82, 1.08-3.73 and 1.97-4.12%, respectively for NH4+, NO_3~-and SO_4~(2-) ions. The present method has relatively very high sample throughput as compared to ion-chromatographic (IC) technique. The work proposes a rapid, free of interferences, low cost and non-destructive method employing DRS-FTIR technique for characterization of key components of soil viz, NH_4~+, NO_3~-and SO_4~(2-).