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