Hands-on Soil Infrared Spectroscopy Training Course Getting the
best out of light 11 15 November 2013 Fundamentals of infrared
spectroscopy Elvis Weullow
Slide 2
INFRARED SPECTRSCOPY Vibrational spectroscopy (or IR
spectroscopy): measures transitions from one molecular vibrational
energy level to another, and requires radiation from the IR portion
of the ER spectrum. Ultraviolet-visible spectroscopy: (also called
electronic absorption spectroscopy) involves transitions among
electronic energy levels of the molecule, which require radiation
from the UV visible portion of the electromagnetic spectrum. Such
transitions alter the configuration of the valence electrons in the
molecule. Increasing Frequency Increasing Wavelength X-RayVisNIRMIR
FIR, Microwave UV 200 nm380 nm780 nm 2,500 nm25,000 nm 50,000 cm -1
12,820 cm -1 4,000 cm -1 400 cm -1
Slide 3
IR PRINCIPLES Each molecular vibrational motion occurs with a
frequency characteristic of the molecule and of the particular
vibration. The energy of the vibration is measured by its amplitude
(the distance moved by the atoms during the vibration), so the
higher the vibrational energy, the larger the amplitude of the
motion. According to quantum mechanics, only certain vibrational
energies are allowed to the molecule (this is also true of
rotational and translational energies), so only certain amplitudes
are allowed. For a vibrational mode to absorb IR radiation, the
vibrational motion associated with that mode must produce a change
in the dipole moment of the molecule
Slide 4
Vibrations Modes of vibration CH Stretching Bending COH
Symmetrical 2853 cm -1 Asymmetrical 2926 cm -1 Scissoring 1450 cm
-1 Rocking 720 cm -1 Wagging 1350 cm -1 Twisting 1250 cm -1
Stretchingfrequency Bendingfrequency
Slide 5
Infrared radiation VNIR 350nm to 2500nm, NIR 12500cm -1 to 8000
cm -1 MIR 8000 cm -1 to 4000 cm -1 Bonds subject to vibrational
energy changes => continually vibrate in different ways: Energy
absorption in IR region then occurs & translated into
absorption spectrum. Antisymmetrical stretching Symmetrical
stretching Rocking Wagging Twisting Scissoring Source :
www.wikipedia.org/ 5 IR PRINCIPLES
Slide 6
IR SPECTROSCOPY Foundation The IR region spans the wavelength
range of 12500 -400 wavenumbers, in which absorption bands
correspond mainly to overtones and combinations of fundamental
vibrations. The vibration of molecules can be described using the
harmonic oscillator model, by which the energy of the different,
equally spaced levels can be calculated from E vib = (+1/2)h/2(k/)
Where is the vibrational quantum number, h the plancks constant K
the force constant and the reduced mass of the bonding atoms. Only
those transitions between consecutive energy levels (=1) that cause
a change in dipole moment are possible. E=E rad =h Where is the
fundamental vibrational frequency of the bond that yields an
absorption band in the mid IR region.
Slide 7
IR Principles For soils Different clay types have very distinct
spectral signatures in the near infrared region because of strong
absorption of the overtones of SO 4 2-, CO 3 2- and OH -, and
combinations of fundamental features of, for example, H 2 O and CO
3 2 Absorption due to charge transfer and crystal field effects in
Fe 2+ and Fe 3+ is particularly evident at 0.35 to 1.0 nm. Soil
spectra are a product of many overlapping absorption features of
organic and mineral materials they generally have few distinct
absorption features. This makes qualitative interpretation of
individual features of limited value, but even subtle differences
in shape can yield quantitative information on soil
properties.
Slide 8
IR INSTRUMENTATION IR equipment can incorporate a variety of
devices depending on the characteristics of the sample and the
particular analytical conditions and needs (such as speed, sample
complexity and environmental conditions), so the technique is very
flexible. Classification of Modern IR Instruments Filter based
instruments: -Here filters are used as wavelength selectors,
commercially available for dedicated applications. For example, an
instrument for determination of the quality parameters of gasoline
(Zeltex Inc.) employs 14 interference (Fabri-Perrot) filters and 14
LED (Light Emitting Diode) sources in the NIR region. LED based
instruments:Using Light Emitting Diodes (LED) in the field, price
and size of the instrument can be reduced, produce NIR radiation
with a band width of about 30-50 nm. LEDs function as both the
light source and the wavelength selection system, typically cover
the range 4001700 nm. They have the advantages that the measurement
is very fast (e.g. one spectrum per second) and noninvasive. These
features are particularly useful where a high sample throughput or
ultra-rapid on-line measurements are required.
Slide 9
IR INSTRUMENTATION Dispersive optics-based instruments: These
are instruments based on grating monochromators, offer advantage of
longer life, relatively low cost, when compared with other scanning
instruments employing modern technologies. The main limitations are
the slow scan speed and a lack of wavelength precision, which
deteriorates for long term operation due to mechanically driven
mechanism fatigue. Also, the presence of moving parts limits the
use of dispersive instruments in the field and in more aggressive
environments. It is mainly employed in the control of sugar and
alcohol production, also on a truck to monitor the content of
protein, oil and humidity in real time during grain harvest.
Slide 10
IR INSTRUMENTATION AOTF based instruments Acousto-Optical
Tunable Filters (AOTF) are modern scan spectrophotometers offer
advantage of constructing instruments with no moving parts, very
high scan speeds, broad NIR spectral region, random access to any
number of wavelengths. The scan speed is usually limited by the
detector response time. An AOTF comprises a crystal of TeO 2
through which a plane traveling acoustic wave is generated at right
angles to the incident light beam
Slide 11
IR INSTRUMENTATION Fourier-transform based instruments: These
instruments are based on the use of interferometers and Fourier
transform to recover the intensities of individual wavelengths in
the IR region are, undoubtedly, the instruments combining most of
the best characteristics in terms of wavelength precision and
accuracy, high signal-to-noise ratio and scan speed.Typical
wavelength accuracy is better than 0.05 nm and the resolution can
achieve values below 1 nm in the NIR region.
Slide 12
COMMON NIR/MIR DETECTORS USED
Slide 13
To separate IR light, a grating is used. Grating Light source
Detector Sample Slit To select the specified IR light, A slit is
used. Dispersion Spectrometer In order to measure an IR spectrum,
the dispersion Spectrometer takes several minutes. Also the
detector receives only a few % of the energy of original light
source. Fixed CCM B.S. Moving CCM IR Light source Sample Detector
An interferogram is first made by the interferometer using IR
light. The interferogram is calculated and transformed into a
spectrum using a Fourier Transform (FT). FTIR In order to measure
an IR spectrum, FTIR takes only a few seconds. Moreover, the
detector receives up to 50% of the energy of original light source.
(much larger than the dispersion spectrometer.) Comparison Between
Dispersion Spectrometer and FTIR
Slide 14
FOURIER TRANSFORM IR Fourier transform (FT) relates to a
mathematic way to convert rapidly collected time domain data to
spectra that we can interpret. The FT spectrometer works by
splitting an IR beam into two. When these recombine, the light
waves interfere constructively or destructively, creating
theinterferogram which is the time domain signal.
Slide 15
Interferometer interferogram Output of a Laser interferometer
Primary interferometer interferogram that was sampled Optical path
difference x Sampling of an actual interferogram
Slide 16
Instrumentation Dispersive VNIR FT-NIR FT-MIRHandheld NIR/MIR
Portable Repeatability? External service No validation Benchtop
Repeatability *** Self serviceable Validation in-built ISO
compliant Industry proven Multipurpose Benchtop Repeatability*** No
gas purging Some servicing Robotic Validation in-built ISO
compliant Outperforms NIR Liquid Nitrogen required Handheld Sample
homogeneity? Variable moisture? Repeatability? Still expensive
Rapidly developing Need to prepare by developing soil reference
libraries FT-MIR Benchtop Miniaturized MIR instrument
Repeatability*** No gas purging Some servicing Validation in-built
ISO compliant Outperforms NIR ATR accessory can be attached to it.
Does not use liquid nitrogen
Slide 17
Advantages of FTIR Speed: analysis done in a very short time.
Sensitivity: detectors used are more sensitive and offer high
optical throughput. Mechanical simplicity: Few moving parts (only
interferometer) hence rugged. Internal calibration: HeNe laser used
as an internal wavelength calibration standard hence no need for
external calibration.
Slide 18
MIR and NIR Spectra Mid-Infrared Absorption Soil Spectrum
Near-Infrared Absorption Soil Spectrum
Slide 19
EXAMPLES OF APLICATION OF INFRARED SPECTROSCOPY Soils
high-throughputs soil analysis, mapping and monitoring of soil
properties, remote sensing, digital soil mapping, precision
agriculture, monitoring schemes for good agricultural practice and
environmental services payments schemes, sediment analysis, soil
pollution, mobile rural IR spectroscopy soil testing services. Crop
agronomy/breeding/plant sciences Tissue testing and crop response,
Germplasm screening /breeding, seed viability/treatment,
metabolomics relating plant tissue biochemical fingerprints to
ecological factors Crop and livestock products quality and
processing grain quality and storage, cash crops- tea and coffee
etc, fruits and vegetables, Beverages and juice, Dairy products
butter, cheese, Meat, Wood and paper, Biofuels. Water quality Long
term monitoring of aquatic systems and Heavy metals pollution of
fresh water sediments e.g. Cd, Cu, Zn, Pb, Mn, Fe in sediments
Etc.