NIR spectroscopyPros and cons

Christian B. Zachariassen, PhD, Post.Doc.

Quality and Technology, IFV, UCPH

Sir Frederick William Herschel (1738 – 1822)

History of NIR, 1800-2008

Electromagnetic spectrum

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

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

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

System of NH3, IPA and Water

Penetration of light in semi-solids

Examples of vibrations

SymmetricalStretching

AsymmetricalStretching

Scissoring

Rocking Wagging

+ +

Twisting

+ -

Examples of vibrations

SymmetricalStretching

AsymmetricalStretching

Scissoring

Rocking Wagging Twisting

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

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.

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

Applications of NIR

• Agriculture• Food• Chemistry• Polymers• Petrochemistry• Pharmaceuticals• Medical• Environment• Geophysics• Astronomy

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

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

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.

Dispersive instrument design

Lamp(Source) Entrance

Slit

ConcaveHolographicGrating

CoherentLight

First SurfaceMirror

ExitSlit

Sampling Area

WavelengthReference

“Order”Sorter

A dispersive instrument

Interior of a dispersive instrument

Michelson Interferometer principle

FT-NIR instrument design

Reproduced from FT-IR reference manual, Revision 1.1, August 2001, ABB Inc.

Interior of a FT-NIR instrument

Acoustic Optical Tunable Filter (AOTF)

Diode Array Prism based instrument

Integrating sphere LED based instrument

NIR instrument principles: Pros and cons

Optical fibers and NIR

NIR

λ(nm)

%T

Sample modes

• Transmittance• Transflectance• Reflectance• Remote sensing, sample containers, probes, …

Flexible sampling

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

Influence of particle size

Effect of particle size (NIR, flour)

Coarse

Medium

Fine

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

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

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)

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

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

Measuring crystallinity

Intensity and resolution highly dependent on chemical environment.

Note – less H-bonding in the crystalline sucrose

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

Comparison with other spectroscopies

P.W. Hansen, Spectroscopic Analyses on Dairy Products, PhD-thesis, Royal Veterinary and Agricultural University of Denmark, Copenhagen, 1999

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

Acknowledgements

• Q&TSøren Balling Engelsen

• CP Kelco• The Internet ;-)