Transcript or IR spectroscopy - World Agroforestry Centre

Fundamentals of infrared spectroscopy
Elvis Weullow
Hands-on Soil Infrared
Spectroscopy Training Course
Getting the best out of light
11 – 15 November 2013
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
50,000 cm-1
X-Ray
UV
200 nm
12,820 cm-1
Vis
380 nm
4,000 cm-1
NIR
780 nm
Increasing Wavelength
2,500 nm
400 cm-1
FIR, Microwave
MIR
25,000 nm
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
Vibrations
Stretching
frequency
Bending
frequency
O
Modes of vibration
Stretching
Bending
C—H
C
H
H
H
H
H
Wagging
1350 cm-1
Scissoring
1450 cm-1
H
H
H
Symmetrical
2853 cm-1
H
Asymmetrical
2926 cm-1
H
H
Rocking
720 cm-1
H
H
Twisting
1250 cm-1
IR PRINCIPLES
• 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:
Symmetrical
stretching
Antisymmetrical
stretching
Scissoring
Rocking
Wagging
Twisting
• Energy absorption in IR region then
occurs & translated into absorption
spectrum.
Source: www.wikipedia.org/
5
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
Evib= (ν+1/2)h/2Π√(k/μ)
Where ν is the vibrational quantum number,
h the planck’s constant
K the force constant and
μ the reduced mass of the bonding atoms.
Only those transitions between consecutive energy levels (Δν=±1) that cause a change
in dipole moment are possible.
ΔE=ΔErad=hν
Where ν is the fundamental vibrational frequency of the bond that yields an
absorption band in the mid IR region.
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 SO42- , CO32and OH-, and combinations of fundamental features of, for example, H2O and
CO32 Absorption due to charge transfer and crystal field effects in Fe2+ and Fe3+
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.
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 400–1700 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.
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.
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 TeO2 through
which a plane traveling acoustic wave
is generated at right angles to the
incident light beam
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.
COMMON NIR/MIR DETECTORS USED
Comparison Between Dispersion Spectrometer and
Dispersion
FTIR
To separate IR light, a grating is used.
Grating
Detector
Slit
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.
Sample
To select the specified IR light,
A slit is used.
Light source
An interferogram is first made by
the interferometer using IR light.
Fixed CCM
Moving CCM
Detector
B.S.
Sample
The interferogram is calculated and
transformed into a spectrum using a Fourier
Transform (FT).
IR Light source
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.)
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
the“interferogram” which is the time domain signal
.
Sampling of an actual interferogram
Interferometer interferogram
Output of a Laser interferometer
Primary interferometer
interferogram that was
sampled
Optical path difference x
Instrumentation
Dispersive
VNIR
• Portable
• Repeatability?
• External service
• No validation
FT-NIR
• Benchtop
• Repeatability ***
• Self serviceable
• Validation in-built
• ISO compliant
• Industry proven
• Multipurpose
FT-MIR
Benchtop
• Repeatability***
• No gas purging
• Some servicing
• Robotic
• Validation in-built
• ISO compliant
• Outperforms NIR
• Liquid Nitrogen
required
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
Handheld NIR/MIR
• Handheld
• Sample homogeneity?
• Variable moisture?
• Repeatability?
• Still expensive
• Rapidly developing
• Need to prepare by
developing soil
reference libraries
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.
MIR and NIR Spectra
Mid-Infrared Absorption Soil
Spectrum
Near-Infrared Absorption Soil
Spectrum
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.
