nir spectroscopy - ku · • needs longer absorption path than mid-ir (small absorption...
TRANSCRIPT
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NIR spectroscopyPros and cons
Christian B. Zachariassen, PhD, Post.Doc.
Quality and Technology, IFV, UCPH
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Sir Frederick William Herschel (1738 – 1822)
History of NIR, 1800-2008
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Electromagnetic spectrum
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NIR is repeating IR
C-HO-HC-HO-HC-H
800 1000 1200 1400 1600 1800 2000 2200 2400
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
1/Lo
gT
nm
O-H C-H
28003000320034003600380040000.0
0.2
0.4
0.6
0.8
1.0
1.2
cm-1
Abs
orba
nce
Ethanol
Reproduced from D.K. Pedersen, and S.B. Engelsen, Monitoring Industrial Food Processes Using Spectroscopy & Chemometrics, New Food, 2 (2001), 9-13
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NIR characteristics
• From 4000 cm-1 (2500 nm) to 12.000 cm-1 (800 nm)• Overtones and combinations of fundamental
vibrations which involve at least one hydrogen atom, or C=O
• Needs longer absorption path than mid-IR (small absorption coefficients)
• Each peak has a well defined frequency, band shape and height which gives us information about the group of atoms that vibrate, their immediate environment and the number of particular groups of atoms in a sample
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The exact position of NIR bands depend upon…
• Hydrogen bondingLowers frequency of X-H stretch, raises frequency of X-H bendBroadens and changes intensity of all X-H bands
• Temperature• Moisture content• Crystallinity
Crystal structure gives sharp peaks; some peaks may disappear by grindingAmorphous structure gives broad peaks
• Particle size• …
• Changes can be up to 50 nm
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System of NH3, IPA and Water
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Penetration of light in semi-solids
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Examples of vibrations
SymmetricalStretching
AsymmetricalStretching
Scissoring
Rocking Wagging
+ +
Twisting
+ -
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Examples of vibrations
SymmetricalStretching
AsymmetricalStretching
Scissoring
Rocking Wagging Twisting
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NIR absorption bands
14286 12500 11111 10000 9091 8333 7692 7143 6667 6250 5882 5556 5263 5000 4762 4545 4348 4167 4000(cm-1)
Reproduced from http://www.asdi.com/nir-chart_grid_rev-3.pdf
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NIR assignments
NIR absorption regions of functional groups of molecules found in meat and fish samples. Spectra of pork meat samples are inserted.
J. Brøndum, D.V. Byrne, L.S. Bak, G. Bertelsen and S.B. Engelsen, Warmed-over flavour in porcine meat – a combined spectroscopic, sensory and chemometric study, Meat Science, 54 (2000), 83-95.
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Advantages
• Analysis time in seconds• Multicomponent analysis
(physical, chemical, structural and biological properties)
• No sample preparation• Non-invasive / non-destructive• Relative low cost• Automatic corrections by computer algorithms
(MSC, EISC/EMSC, Derivatives, SNV, …)
• Global and robust calibrations available• Detection limit low• Sample size from nanograms to planets
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Applications of NIR
• Agriculture• Food• Chemistry• Polymers• Petrochemistry• Pharmaceuticals• Medical• Environment• Geophysics• Astronomy
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Basic components
• Light sources: Tungsten-halogen lamps, LED’s• Wavelength selector
(pre-dispersive, post-dispersive, non-dispersive): Grating, prism, AOTF, optical filters , interferometer
• Detectors: Silicon, PbS, InAs, InGaAs, Diode arrays
Monochromator
Light source
Sample
Transmission
ItI0
IrReflection
Ia
Absorbtion
Conservation of energy: I0 = Ia + It + Ir
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Transmission and reflection spectroscopy
Transmission
Source
MonochromatorSample
Detector
Diffuse Reflection
0IIT =
0IIR =
cbaII
TA ⋅⋅=== 0log1log
sca
II
R⋅
≈= 0log1log
T: TRANSMITTANCEA: ABSORBANCER: REFLECTANCEa: ABSORPTIVITYb: SAMPLE THICKNESSc: CONCENTRATIONs: SCATTERING COEFFICIENTI0: INT. OF REFERENCE BEAMI: INT. OF SAMPLE BEAM
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NIR Detectors
• Silicon detector: Up to 1100 nm, stable, rapid, reliable, inexpensive
• Lead sulphide: PbS, 900-2600 nm, common NIR detector, established, slow response
• InGaAs:, 800-1700 nm, 1300-2200 nm, 1500-2500 nm, sensitive, expensive
Reproduced from FT-IR reference manual, Revision 1.1, August 2001, ABB Inc.
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Dispersive instrument design
Lamp(Source) Entrance
Slit
ConcaveHolographicGrating
CoherentLight
First SurfaceMirror
ExitSlit
Sampling Area
WavelengthReference
“Order”Sorter
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A dispersive instrument
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Interior of a dispersive instrument
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Michelson Interferometer principle
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FT-NIR instrument design
Reproduced from FT-IR reference manual, Revision 1.1, August 2001, ABB Inc.
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Interior of a FT-NIR instrument
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Acoustic Optical Tunable Filter (AOTF)
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Diode Array Prism based instrument
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Integrating sphere LED based instrument
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NIR instrument principles: Pros and cons
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Optical fibers and NIR
NIR
λ(nm)
%T
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Sample modes
• Transmittance• Transflectance• Reflectance• Remote sensing, sample containers, probes, …
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Flexible sampling
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Cuvette thickness by transmittance (Water)
0
1
2
3
4
5
600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 10_20C 04_20C 01_20C 30_23,5C
1 2
3
4
Wavelength (nm)
Abs
orba
nce
1 mm4 mm
10 mm
30 mm
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Influence of particle size
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Effect of particle size (NIR, flour)
Coarse
Medium
Fine
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Effect of particle size
1.0-2.0 mm0.25-0.50 mm< 0.125 mm
Pulverized green bottle glass
Reproduced from D. Dahm and K. Dahm, The Physics of Near-Infrared Scattering, in Near-Infrared Technology (Eds. P Williams and K. Norris) , 2nd Edition, American Association of Cereal Chemists, St. Paul, 2004, 1-17
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Effect of refractive index
Mineral oil n1 = 1.48
Watern1 = 1.33
Airn1 = 1.48
Pulverized greenbottle glass
< 0.125 mm
Reproduced from D. Dahm and K. Dahm, The Physics of Near-Infrared Scattering, in Near-Infrared Technology (Eds. P Williams and K. Norris) , 2nd Edition, American Association of Cereal Chemists, St. Paul, 2004, 1-17
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Sampling and instrumental advantages
• Quartz, glass and Teflon do not absorb NIR light• Brighter sources are available than in MIR• Very sensitive detectors are available• Higher energy photons (yet non-destructive) than
MIR• Less absorption, thus longer path lengths• Less water absorption• Sensitive to particle size (could be a disadvantage)
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Sucrose vibrations - NIR and IR
4000cm-1
NIR
8000 7000 6000 5000
1.80
1.90
2.00
2.10
2.20
2.30
2.40
cm-1 0
A
MIR
4000 3000 2000 1500 1000 500
0.4
0.8
1.2
1.6
2.0
2.4
4000cm-1
NIR
8000700060005000
1.80
1.90
2.00
2.10
2.20
2.30
2.40
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NIR is repeating IR
C-HO-HC-HO-H
1/Lo
gT
nm800 1000 1200 1400 1600 1800 2000 2200 2400
0.1
0.2
0.3
0.4
0.5O-H C-H
cm-1
Abs
orba
nce
2800300032003400360038000
0.4
0.8
1.2
1.6
2
2.54Å 2.31Å
2.53Å
O4
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Measuring crystallinity
Intensity and resolution highly dependent on chemical environment.
Note – less H-bonding in the crystalline sucrose
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Effect of particle size (NIR, Sucrose)
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 45 63-71 71-100 150-200 200-400 400-500 500-700
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Comparison with other spectroscopies
P.W. Hansen, Spectroscopic Analyses on Dairy Products, PhD-thesis, Royal Veterinary and Agricultural University of Denmark, Copenhagen, 1999
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Pros and cons
• Pros– Relatively inexpensive– Known technology– Often non-invasive and
non-destructive– Often no need for sample
preparation– Flexible sampling– Large penetration depth– Relatively large samples – Remote sensors with fibers– Reproducible– Excellent for ppt and %
quantification– Excellent for bulk analysis– Can be combined with
microscopy
• Cons– Chemical interferences– Physical interferences– Not suited for ppm and ppb
analysis in bulk materials– Not well suited for molecular
or physical distribution in bulk materials
– Not well suited for metal/salt analysis
– Intricate visual interpretation difficult
– Relies on chemometrics
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Acknowledgements
• Q&TSøren Balling Engelsen
• CP Kelco• The Internet ;-)