Transcript NIR SPECTROSCOPY: AN ADVANCED ALTERNATIVE

NIR SPECTROSCOPY:
AN ADVANCED ALTERNATIVE
Electromagnetic spectrum
INFRARED
X-RAY
0,2 nm
ULTRA-VIOLET
VISIBLE
2 nm
400-800 nm
ʎ, cm (wavelength)
3x10-3
3x10-3 to 3x10-2
ʎ, cm-1 (wavenumber)
400 to 33
MICROVAWE
3 mm-20 cm
NEAR MID FAR
7.8x10-5 to 3x10-4
12820 to 4000
3x10-4 to
4000 to 400
RADIO
10 m-30 Km
NIR Facts
 NIRS provides rapid, chemical-free, flexible
analysis
 NIRS is used globally for food and feed analysis
 NIRS has enormous potential for agroenvironmental applications, including soil carbon
assessment
Near-infrared Spectroscopy
 Utilizes the absorbance of NIR light (780 - 2500
nm) by vibrating bonds between atoms in
molecules
 O-H, C-H, C-N, C-O, P-O, S-O
 Molecular spectroscopy - analyzes intact samples
 NIR absorbances obey the Beer/Lambert law
The Work of Doing NIR Analysis
 Compositional information on samples (n ~>100)
is correlated with the spectral information to
develop statistical calibration models
 The calibrations “train” the instrument to analyze
future unknown samples
Features





does not destroy the sample
is rapid, < 2 min/test
analyzes many constituents simultaneously
analyzes compositional and functional
properties
field portable
Lab and Field Instrument: Zeiss
Corona
Organic Matter Compositional
Parameters
 Organic matter/organic C
 % OM, % OC
 Total C (LECO)
 %C HUMUS
 Humic acid fractions
 Humic and Fulvic
 Fulvic acid fractions
 Lignin content
 Cellulose content
r2
Performance
0.81-0.97
good – exc.
0.93-0.96
v.good - exc.
0.94
v.good
0.95
v.good
0.91
v.good
0.63
poor
0.77-0.83
good
0.81
good
Compositional Parameters cont’d
 % Clay
 Total N
 % moisture
 CEC
r2
0.81-0.87
0.86-0.96
0.93-0.98
0.9
performance
good
good - v.good
v.good – exc.
v.good
 Miniota area
 Newdale Soil





Assoc.
Dried, ground
samples (2mm)
N = 267
1100 - 2500 nm
r2 = 0.78
SEP = 0.33 %
NIR-predicted Org C (%)
Organic Carbon
5
4
3
2
1
0
0
1
2
3
4
LECO-determined Org C (%)
5
“Field-moist” applications
 Moisture corrected




calibration
0.033 and 1.5 MPa
moisture tension
r2 = 0.89
SEP = 0.23 %
Range = 0.45 – 3.16 %
OC
Sudduth, K.A. and J.W. Hummel (1993). Soil organic matter, CEC and moisture
sensing with a portable NIR spectrophotometer. Trans of the ASAE 36:1571-1582
Example of On-site Soil Testing
Method
 Soil cores - grid or stratified sampling
 Cores sliced on-site
 Presentation of static, “as is”, field moist samples
 Multiple constituents simultaneously
NIRS Benefits
 COST !
 LECO OC = $27/sample
 NIR OC = $6/sample
 Minimal sample preparation
 Dried and ground (2mm mesh)
 Potential for “as is” or “field moist” determinations
 Timeliness
 Potential for immediate analysis
NIRS Benefits, cont.
 Precision
 Precision of NIR equal or better than reference
 Does not destroy the sample
 The same sample can be analyzed many times over
 Positive implications for long term and/or incubative
studies
NIRS Limitations
 Site to Site Bias
 Potential for bias in predictions of samples from one
site using calibrations derived from samples from
another site.
 Affects absolute accuracy
 Does not affect precision
This can be corrected by “incorporating” a
small number of samples from the “new”
site into the calibration.
 At present, this means that NIRS is not
practical for small sample groups
How can NIRS work for you?
 Objective sample selection1
 NIRS can be used to select sample sets from a large
group of samples which:
 Retain a maximum representation of overall sample
population variability
 Samples selected better than random because:
 Greater recovery of range
 Higher variance
 Better Kurtosis (more even distribution)
1Stenberg, B. et
al. (1995) Use of near infrared reflectance spectra of soils for objective selection of
samples. Soil Science. 159:109-114.
Objective Sample Selection, cont.
 Using NIR for selecting analytical samples
reduces cost directly by lowering the number of
samples that need to be analyzed to encompass
soil variability.
 Stenberg, et al. estimated a 70% reduction in cost
for their study using this method
 For their study, the overall n = 144 samples,
selected n = 20 samples
Calibration and Prediction
 Calibrations are developed on a selected set of
samples (ie. using the NIR selection method)
 These calibrations can be used to predict the
remaining samples.
 Requires large sample sets
 ncalibration :100 samples recommended
Calibration and Prediction, cont.
 Extra cost of calibration and accompanying wet
chemistry is offset by a large economy of scale
 Once a calibration is developed, it only requires
updating with a much smaller number of QA/QC
samples
 Calibrations will eventually exist for various soils,
bringing initial costs down
Monitoring and Long-term Soil
Quality Assessment
 NIR spectra contain information for both
carbon quantity, and carbon quality in soil
 High precision plus lower cost of NIR results
make large scale assessments of soil carbon
flux much more feasible, both:
 Over time
 Under varying management practices.
Monitoring and Long-term Soil
Quality Assessment, cont.
 Non destructive nature of NIR, coupled with “as-
is” and/or “on-site” assessment potential mean
that:
 The same sample could be analyzed indefinitely
over time.
 Could reduce potential subsampling error
 Could increase relevance of results
Sensing Soil Quality
Large Area Surveillance of Soil Condition and Trend
http://www.worldagroforestrycentre.org/sites/program1/specweb/home.htm
The classical physics considers the atoms as particles with a
given mass in the IR absorption process, and the vibrations of
diatomic molecule described as follows (e.g., HCl):
equilibrium bond length
Spring force
Spring force
stretched
compressed
Mechanical model of a vibrating diatomic molecule
Courtesy Bruker
Optics
Modes of vibrations
Region
antisymmetric
R
R
R
R
R
symmetric
H
R
H
R
H
R
R
H
scissoring
Origin of the absorption
R
H
R
H
R
H
H
rocking
in-plane
bending
H
H
Overtones and combination
bands of fundamental
molecular vibrations
MIR
fundamental molecular
vibrations
FIR
molecular rotations
stretching
H
H
NIR
bending
Molecule
Degrees of freedom
Non linear
Linear
3N -6
3N- 5
Far-IR
 The region below 400 cm-1, is now generally classified as the far
infrared, characterized by low frequency vibrations typically assigned
to low energy deformation vibrations and the fundamental stretching
modes of heavy atoms. There is only one IR-active fundamental
vibration that extends beyond 4000 cm-1, and that is the H-F stretching
mode of hydrogen fluoride.
J. Coates, “Vibrational Spectroscopy:
Instrumentation for Infrared and Raman
Spectroscopy”, Applied Spectroscopy
Reviews, 1998, 33(4), 267 – 425.
Mid-IR
 Today, the mid-infrared region is normally defined as the
frequency range of 4000 cm-l to 400 cm-1. The upper limit is more
or less arbitrary, and was originally chosen as a practical limit
based on the performance characteristics of early instruments. The
lower limit, in many cases, is defined by a specific optical
component, such as, a beam splitter with a potassium bromide
(KBr) substrate, which has a natural transmission cut-off just
below 400 cm-1.
J. Coates, “Vibrational Spectroscopy: Instrumentation
for Infrared and Raman Spectroscopy”, Applied
Spectroscopy Reviews, 1998, 33(4), 267 – 425.
Near-IR
 The original NIR work was with extended UV-Vis spectrometers.
 Indicates that mid and NIR should be considered the same field. The
NIR overtones are derived from the fundamental bands observed in
the mid-IR. Mid-IR also has a number of overtones. Furthermore he
states, “To strengthen this position, it must be realized that more than
half of the mid-infrared spectrum contains overtones and combination
frequencies of fundamental absorptions occurring below 2000 cm-1.”
J. Coates, “Vibrational Spectroscopy: Instrumentation
for Infrared and Raman Spectroscopy”, Applied
Spectroscopy Reviews, 1998, 33(4), 267 – 425.
NIR past and present
The history of near infrared (NIR) begins in
1800 with Frederick William Herschel.
He was trying filters to observe sun spots
and when he used a red one, he noticed
that a lot of heat was produced, which was
of a higher temperature than the visible
spectrum. After further studying, he
concluded that there must be an invisible
form of light beyond the visible spectrum.
http://coolcosmos.ipac.caltech.edu/cosmi
c_classroom/ir_tutorial/discovery.html
NIR past and present
MIR
NIR
DIFFICULT?
MIR spectra obtained by ATR and NIR spectra obtined by Diffuse Reflectance
NIR spectroscopy was neglected by spectroscopist who, for long time,
could not find any additional attractive information in that spectral region
which was occuped by broad, superimposed and weak absortion bands
(see reference 1).
NIR past and present
Why NIR now?
1. Optical fibers
2. Computing power
Improvements in the
fields of
3. Chemometrics
4. Interest in
procces analysis
Sample preparation is not required leading to significant reductions in
analysis time.
Waste and reagents are minimized (non-destructive testing).
Online for process applications
Excellent analytical method for the study of solids. (For example, in
the analysis of minerals)
Lepidolite rock
Spectra may be obtained in non-invasive manner.
Totally non-invasive analysis of blood glucose by NIR
Remote sampling is possible (good for hazardous materials).
Source
Detector
By Raúl E. Gómez Perez,
NIR allows us to create calibration models for predicting concentrations of the
pharmaceutical industry in real time (during the manufacturing process)
* - M. Blanco, J. Coello, A. Eustaquio, H Iturriaga, and S. Maspoch,
Development and Validation of a Method for the Analysis of a
Pharmaceutical Preparation by Near-Infrared Diffuse Reflectance
Spectroscopy, Journal of Pharmaceutical Sciences, 1999, 88(5), 551 –
556.
Possibility of using it in a wide range of applications (physical and chemical),
and viewing relationships difficult to observe by other means.
Milled sugar
Granular sugar
Identification Testing of Raw Materials and Finished Products.
Determination of Water Content.
Determination of Particle Size
Drug Content in Tablets and Powder Mixtures.
Evaluation of Blend Uniformity (in-line monitoring)
Thickness of Film Coating.
Quantitating
and
tracking
pharmaceutical processing.
polymorphic
changes
during
Overlapping bands (combination), not easy to
interpret.
Differences in spectra are often very subtle.
Usually not for trace level analysis.
Basic Principles of Vibrational
Spectroscopy
Scattering technique
Absorption technique
Raman
V
Stoke
s
Near-Infrared
Mid-Infrared
V
V
Anti-Stokes
r
Fundamentals
4000 – 50 cm-1
Source
Monochromatic radiation
Laser VIS - NIR
n=3
n=2
n=1
n=0
n=3
n=2
n=1
n=0
r
Fundamentals
4000 – 200 cm-1
r
Overtones-Combinations
12500 – 4000 cm-1
Source
(Dispersed) Polychromatic radiation
Globular tungsten
Mid Infrared Spectroscopy and Near
Infrared Spectroscopy
MIR alkanes.
NIR alkanes.
Observationss


The intensities of absorption bands decrease from the MIR to the vis.
The most intense MIR absorptions = polar groups.

Overtones and combination bands in the NIR are fundamental bands in the MIR.

The wavenumber positions of the overtones stray with increasing multiplicity from the exact multiples
of their fundamentals due to the anharmonicity of the vibrations.
The Absorption Techniques of MIR and NIR
Spectroscopy
The Harmonic Oscillator
The simplest classical model employed to have a didactic
insight on the interaction of radiation and matter in the NIR
spectral region depicts a diatomic molecule as two spherical
masses (m1 and m2) connected with a spring with a given
force constant (k). Hook´s law states that the energy (E) of
this system is given by:
V
-A
0
E = (h/2π)√(k/µ)
+A
where μ is the reduced mass.
The molecular vibration can be described by a simplified
model supposing a harmonic oscillator for which the
potential energy (V), as a function of the displacement of the
atoms (x), is given by:
-A
0
Displacement
+A
V = ½ kx2
The potential energy curve of such an oscillator is parabolic
in shape and symmetrical about the equilibrium bond length.
Harmonic Oscillator prepared by Carlos Ortega
The Absorption Techniques of MIR and NIR
Spectroscopy
For the harmonic oscillator the energy levels are equidistant and transitions are only allowed between
neighboring energy levels with:
Δn = ±1
According to the Boltzmann distribution, most molecules at room temperature populate the ground level n
= 0, and consequently the allowed, so-called fundamental, transitions between n = 0 and n = 1 dominate
the vibrational absorption spectrum.
For the harmonic oscillator Δn = ±1 and Ep = hv, which matches the predicted equal energy difference
between one state and the other of immediately higher energy. The figure at right shows the effect of
photon absorption on the energy and amplitude of vibration.
The Absorption Techniques of MIR and NIR
Spectroscopy
A quantum mechanical treatment by the Schrödinger equation shows that the vibrational energy has only certain discrete
values that are energy levels are expressed in wave number units (cm−1) given by:
where h is Planck’s constant, ν0 is the vibrational frequency defined above and n is the vibrational quantum number that
can only have integer values 0, 1, 2, 3, ... and so on. And c is the speed of light and .ν-0 is the wave number
corresponding to the frequency ν0.
Interaction of infrared radiation with a vibrating molecule, however, is only possible if the electric vector of the radiation
oscillates with the same frequency as the molecular dipole moment, μ.
The requirement of a dipole moment change during the vibration makes MIR spectroscopy specifically sensitive to polar
funcionalities.
The Absorption Techniques of MIR and NIR
Spectroscopy
The Anharmonic Oscillator
The picture of the harmonic oscillator cannot be retained at larger amplitudes of vibration
owing to:
• Repulsive forces between the vibrating atoms.
• The possibility of dissociation when the vibrating bond is strongly extended.
Accordingly, the allowed energy levels for an anharmonic oscillator have to be modified:
where χ is the anharmonicity constant.
The potential energy curve
asymmetric Morse function.
is
represented
by
an
Fundamentals and Overtones
In the case of the anharmonic
oscillator,
the
vibrational
transitions no longer only obey
the selection rule n = 1. This
type of vibrational transition is
called fundamental vibration.
Vibrational transitions with n =
2, 3, ... are also possible, and
are termed overtones. Called
first, second, and so on,
overtones.
The Absorption Techniques of MIR and NIR
Spectroscopy
a)
re
C
l
H
C
l
b)
H
C
l
V
Coulomb attraction
H
Nuclear repulsion
n=3
n=2
n=1
n=0
c)
r
V
n=6
n=5
n=4
n=3
n=2
n=1
n=0
r
•(a) Vibration of diatomic molecule of HCl,
(b) potential energy of an ideal harmonic
oscillator, and (c) an anharmonic oscillator
described by the Morse function.
•The minimum in the Morse potential is not
the minimum in the actual energy of the
diatomic molecule.
Diatomic molecules
vibrate; diatomic molecules rotate. Thus,
within
the
Morse
potential
are quantitized levels of vibration and
rotation.

The frequency of a combination is approx. the sum of the frequencies of the
individual bands.

Combinations of fundamentals with overtones are possible as well as well as
fundamentals involving two or more vibrations.

The vibrations must involve the same functional group and have the same
symmetry.
Combination bands for water speciation in
hydrated Na2O·6SiO2 (NS6) glasses
Shigeru Yamashita, Harald Behrens, Burkhard C. Schmidt, Ray
Dupree. Water speciation in sodium silicate glasses based on NIR
and NMR spectroscopy. Chemical Geology 256 (2008) 231–241.
The Calculation of Overtones and
Anharmonicities
With the wave number position of the
fundamental vibration ν1 or an overtone νn (n =
2, 3, 4, ...) of the anharmonic oscillator can be
given by:
The intensities of overtone absorption bands depend on the anharmonicity, and it has been
shown that vibrations with low anharmonicity constants also have low overtone intensities.
X−−H stretching vibrations, for example, have the largest anharmonicity constants and
therefore dominate the spectra in the NIR region.
Fermi Resonance, Darling–Dennison Resonance,
and the Local Mode Concept
Fermi resonance A overtone or combination band that has the same
symmetry and nearly the same frecuency as that of a fundamental vibration is
called Fermi resonance.
Famous example: Fermi resonance is observed in the Raman spectrum of CO2
Fundamental vibrational modes of the CO2 group
v2
v1
(667 cm-1)
(1300 cm-1)
v3
(2350 cm-1)
.
2x (667 cm-1) = 1334 cm-1
Kazuo Nakamoto “Infrared and Raman Spectra of Inorganic and Coordination Compounds: Theory and
Applications in Inorganic Chemistry (Volume A)” John Wiley, 1997. ISBN 0-471-16394-5
Robert M. Silverstein, Francis X. Webster, David Kiemle “Spectrometric Identification of Organic
Compounds”Edition: 7th ed., John Wiley & Sons, 2005. ISBN 0471393622.
Fermi Resonance, Darling–Dennison Resonance,
and the Local Mode Concept
A resonance that is of importance in the NIR spectra of water has been
discussed by Darling and Dennison, but can also occur in other molecules
containing symmetrically equivalent X−−H bonds. Thus, of the three normal
modes of water — ν2 bending vibration (1595 cm−1), ν3 antisymmetric
stretching (3756 cm−1), and ν1 symmetric stretching (3657 cm−1)—the two
stretching vibrations absorb at similar wave number positions but
belong to different symmetry species and therefore cannot interact
directly. However, energy levels of these vibrations associated with
specific vibrational quantum numbers n1, n2, and n3 can interact if
they belong to identical symmetry species and have similar energies.
These interactions then lead to several pairs of NIR absorption bands with
appreciable intensities.
Fermi Resonance, Darling–Dennison Resonance,
and the Local Mode Concept
The main idea of the local mode model is to treat a molecule as if it
were made up of a set of equivalent diatomic oscillators, and the reason
for the local mode behavior at high energy (>8000 cm−1) may be understood
qualitatively as follows. As the stretching vibrations are excited to high energy
levels, the anharmonicity term χ.ν0 tends, in certain cases, to overrule the
effect of interbond coupling and the vibrations become uncoupled vibrations
and occur as “local modes.”
The absorption bands in the spectrum can thus be interpreted as if they
originated from an anharmonic diatomic molecule. This is the reason why NIR
spectra are often said to become simpler at higher energy. Experimentally, it is
found that the inversion from normal to local mode character occurs for high
energy transitions corresponding to n ≥ 3.
A comparison of the basic instrumentation
of RAMAN, MIR, and NIR spectroscopy
RAMAN
No sample preparation
MIR/ATR
Sample preparation required
(except ATR)
Small sample volume (μL)
or sample thickness (μm)
NIR
No sample preparation
Large sample thickness
(Up to cm)
Fiber optics
Quartz
Light-fiber optics ( > 100 m)
Chalcogenide or AgCl
light-fiber optics (<10 m)
Quartz
Light-fiber optics ( > 100 m)
A comparison of the basic instrumentation
of RAMAN, MIR, and NIR spectroscopy
RAMAN
MIR/ATR
NIR
Type of acquiring spectra
AT-line/In-line probes
ATR-probes
Transmission, transflection,
diffuse-reflection probes
Instrument Design
NIR-Raman (FD)
VIS-Raman (CCD)
FT-IR
Grating, FT-NIR, AOTF, Diodearray, discrete filter
NIR reflectance vs. NIR transmission
NIR
Reflectance
NIR
Transmission
NIR
Absorption
NIR Refelectance
NIR Transmission (NIT)
Detector
Detector
IR Beam
Detector
Position
Tablet
R.J. Romañach and M.A. Santos, “Content Uniformity Testing with
Near Infrared Spectroscopy”, American Pharmaceutical Review,
2003, 6(2), 62 – 67.
Reflectance is termed diffuse where the angle of reflected light is independent of
the incident angle
Spectra Affected by:

Particle size of sample.

Packing density of sample, and
pressure on sample.

Refractive index of sample.

Crystalline form of sample.

Absorption
sample.

Characteristics of the sample’s
surface.
coefficients
J.M. Chalmers and G. Dent, “Industrial Analysis with Vibrational
Spectroscopy”, Royal Society of Chemistry, 1997, pages 153 -162.
of
Particle Size and Scattering
High scattering
Smaller particle sizes
More remission, less
transmission
Low Scattering
Absorbing power (absence of
scattering)
Absorption coefficient (includes
effects of voids, surface reflection,
distance traveled)
Larger particle sizes
Less remission, more
transmission
References
1. H.W. Siesler, “Basic Principles of Near Infrared Spectroscopy”, In
Handbook of Near Infrared Analysis Ed. D.A. Burns and E.W. Ciurczak, 3rd
ed., CRC Press, Boca Raton, FLA.
2. Miller CE. 2001. Chemical Principles of Near Infrared Technology. In
Williams P, Norris K, editors. Near Infrared Technology in the Agricultural
and Food Industries, 2nd ed., Saint Paul: American Association of Cereal
Chemists, p 19-37.
3. A.S. Bonanno, J. M. Olinger, and P.R. Griffiths, in Near Infra-Red
Spectroscopy, Bridging the Gap Between Data Analysis and NIR
Applications, Ellis Horwood, 1992.
4. Dahm DJ, Dahm KD. 2001. The Physics of Near-Infrared Scattering. In
Williams P, Norris K, editors. Near Infrared Technology in the Agricultural
and Food Industries, 2nd ed., Saint Paul: American Association of Cereal
Chemists, p 19-37.
Applications
Applications
Applications
Conclusion
Over the past years MIR, NIR, and Raman spectroscopy have been further
developed to a point where each technique can be considered a potential
candidate for industrial quality-control and process-monitoring
applications. However, adding up the specific advantages and
disadvantages of the individual techniques, NIR spectroscopy is certainly
the most flexible and advanced alternative.