Transcript PRINCIPLES OF NIR and mid-IR SPECTROSCOPY

NIR SPECTROSCOPY:
AN ADVANCED ALTERNATIVE
University of Puerto Rico - Mayagüez
Nataly Galan Freyle
Jenny Vargas Irizarry
Carlos Ortega Zuñiga
QUIM 8995 – Special Topics in Solid State Vibrational Spectroscopy
ENGINEERING RESEARCH CENTER FOR
STRUCTURED ORGANIC PARTICULATE SYSTEMS
RUTGERS UNIVERSITY
PURDUE UNIVERSITY
NEW JERSEY INSTITUTE OF TECHNOLOGY
UNIVERSITY OF PUERTO RICO AT MAYAGÜEZ
10/11/2005
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Electromagnetic spectrum
INFRARED
X-RAY
0,2 nm
ULTRA-VIOLET
VISIBLE
2 nm
400-800 nm
MICROVAWE
3 mm-20 cm
NEAR
ʎ, cm (wavelength)
3x10-3
3x10-3 to 3x10-2
ʎ, cm-1 (wavenumber)
MID
7.8x10-5 to 3x10-4
12820 to 4000
RADIO
10 m-30 Km
FAR
3x10-4 to
4000 to 400
400 to 33
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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
stretched
Spring force
compressed
Mechanical model of a vibrating diatomic molecule
Courtesy Bruker Optics
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Modes of vibrations
Region
antisymmetric
R
R
R
symmetric
H
R
H
R
H
R
R
H
scissoring
R
R
H
R
H
R
R
Origin of the absorption
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
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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.
5
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 beamsplitter 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.
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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 midIR. 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.
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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
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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 atractive information in that spectral region
which was occuped by broad, superimposed and weak absortion bands
(see reference 1).
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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
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Sample preparation is not required
significant reductions in analysis time.
leading
to
Waste and reagents are minimized (non-destructive
testing).
Online for process applications
11
Excellent analytical method for the study of solids.
(For example, in the analysis of minerals)
Lepidolite rock
12
Spectra may be obtained in non-invasive
manner.
Totally non-invasive analysis of blood glucose by NIR
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Remote sampling is possible (good for hazardous materials).
Source
Detector
By Raúl E. Gómez Perez, MS, 2000
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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.
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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
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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
polymorphic
changes
during
pharmaceutical processing.
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Overlapping bands (combination), not easy to
interpret.
Differences in spectra are often very subtle.
Usually not for trace level analysis.
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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
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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.
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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
+A
E = (h/2π)√(k/µ)
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
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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.
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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.
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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
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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.
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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
•(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.
r
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
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.
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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.
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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.
v1
(1300 cm-1)
v3
(2350 cm-1)
v2
(667 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.
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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.
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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.
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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)
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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
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NIR reflectance vs. NIR
transmission
NIR
Reflectance
NIR
Transmission
NIR
Absorption
NIR Refelectance
NIR Transmission (NIT)
Detector
Detector
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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.
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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
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Particle Size and Scattering
High scattering
Smaller particle sizes
More remission, less
transmission
Absorbing power (absence of
scattering)
Absorption coefficient (includes
effects of voids, surface reflection,
distance traveled)
Low Scattering
Larger particle sizes
Less remission, more
transmission
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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.
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Applications
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Applications
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Applications
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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 qualitycontrol 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.
ENGINEERING RESEARCH CENTER FOR
STRUCTURED ORGANIC PARTICULATE SYSTEMS
RUTGERS UNIVERSITY
PURDUE UNIVERSITY
NEW JERSEY INSTITUTE OF TECHNOLOGY
UNIVERSITY OF PUERTO RICO AT MAYAGÜEZ
10/11/2005
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