Near Infrared Spectroscopy Introduction Instrumentation Near infrared NIR

Near Infrared Spectroscopy Introduction Instrumentation • Near infrared (NIR) spectroscopy is a rapid non-destructive technique for analysing the chemical composition of materials. It is widely used in many pharmaceutical and industrial applications for quality control. Scanning soil samples with a Fourier Transform Near Infrared spectrometer. • A sample is illuminated and diffuse reflected light (electromagnetic radiation) is measured in narrow wavebands over the range from about 780 nm to 2, 500 nm (Fig. 1). Air-dried 2 -mm sieved soils are loaded into glass Petri dishes and scanned in 30 seconds. Increasing Frequency 50, 000 cm-1 X-Ray UV 200 nm 12, 820 cm-1 4, 000 cm-1 Vis NIR 380 nm 780 nm 2, 500 nm 400 cm-1 FIR, Microwave MIR 25, 000 nm Spectral analysis Increasing Wavelength Figure 1: The electromagnetic spectrum • • The resulting spectral signature summarize how much energy was Multivariate (multiple wavelength) calibration techniques (e. g. partial least squares regression) are used to calibrate standard reference analyses to NIR spectra (Fig. 4). Absorption (Log 1/R) absorbed at each wavelength (Fig. 2). Figure 4. Calibration of nitrogen concentration in a wide range of organic manures. • The statistical model is then used to predict the composition of unknown samples that are part of the sample population. • Samples that fall outside the population can be analyzed by traditional means and included in the new model. • Careful development of calibration libraries is essential for reliable use of NIR methods. Wavenumber (cm-1) Figure 2. NIR spectra of a soil (red) and a plant (green) sample. • Spectral signatures respond to the soil organic and mineral composition. • A wide range of agricultural inputs and outputs can be analyzed (soils, sediments, organic manures, feed and fodder, plant tissue, grain, tree products). • NIR provides a rapid, versatile, low cost, high throughput analytical technique for a wide range of agricultural and environmental applications. Working Principles • Materials are composed of molecules consisting of atoms linked together by bonds (e. g. C-H, O-H, N-H), which are constantly vibrating. • Irradiation of materials by light energy excites molecules to change their vibrations from one energy level to another. • Molecules that absorb near-infrared energy vibrate in two modes: Stretching and bending (Fig. 3). The resulting absorbance of light at different frequencies produces a characteristic spectrum of a substance. Key Advantages/Limitations • Large sample required • Multipurpose analysis: soil, plant tissue, wood, fruits, oils. • Cannot detect quartz in soils • Benchtop, portable • Direct spectral interpretation limited • Validation in-built, ISO compliant • Little or no sample preparation. • Rapid and easy technique. • High repeatability and reproducibility • Self serviceable Applications • Prediction of soil properties (e. g. soil organic carbon, exchangeable Ca, cation exchange capacity, P sorption) and soil fertility capability. • Digital soil mapping. • Nitrogen content and decomposition characteristics of manures/composts. Leaf N concentration. Figure 3: Stretching and bending vibrations • Feed/fodder quality. • Grain moisture, protein and germination rate. • Wood density, moisture and carbon content. • Biofuel moisture, ash and calorific value. Contact: World Agroforestry Centre (ICRAF), P. O. Box 30677 -00100 Nairobi, Kenya. Tel: +254 020 722 4000. www. worldagroforestry. org