Near Infrared (NIR) Spectroscopy Instrumentation

Paul Geladi

Paul Geladi

Head of Research NIRCEChairperson NIR Nord

Unit of Biomass Technology and ChemistrySwedish University of Agricultural SciencesUmeåTechnobothniaVasa

paul.geladi @ btk.slu.se paul.geladi @ uwasa.fi

Content

• Spectroscopy?• Instrumentation• Modes of measurement

Content

• Spectroscopy?• Instrumentation• Modes of measurement

Content

• Spectroscopy?• Energy levels in atoms, molecules, crystals• Example IR-NIR calculations• Related techniques

Content

• Spectroscopy?• Energy levels in atoms,molecules, crystals• Example IR-NIR calculations• Related techniques

Spectroscopy

• Interaction of radiation and matter

• Electromagnetic radiation

• Gases, liquids, solids, mixtures

• Heterogeneous materials

Electromagnetic radiation

Cosmic Gamma Xray UV VIS NIR IR Micro Radio

Electromagnetic radiation• Cosmic > 2500 KeV• Gamma 10-2500 KeV• Xray 0.1-100 KeV• Ultraviolet 10-400 nm• Visible 400-780 nm• Near Infrared 780-2500 nm• Infrared 2500-15000 nm• Microwave GHz• Radio MHz-KHz

Why interaction?

• Photon energy matches some energy level

• E = h• E = hc/• Planck’s constant 6.63 10-34

Some useful constants

• qe= 1.602176462*10-19 As

• me = 9.10938188*10-31 Kg

• c = 2.99792458*108 m/s

• h = 6.62606876*10-34 Js

• 1 Joule to Electronvolt 6.241506363094028*1018

Units

• Joule (energy)

• Electron volt (KeV)

• Wavelength (nm, m, mm)

• Inverse cm (cm-1)

• Frequency (GHz,MHz,KHz)

Content

• Spectroscopy?• Energy levels in atoms,molecules, crystals• Example IR-NIR calculations• Related techniques

HCl molecule (no true sizes)

HCl

UV,VISXray

UV,VIS

NIR,IR

Gamma ray

= electron

Photon-matter interaction

• Atomic nucleus = gamma ray

• Inner electron = Xray

• Outer electron, chemical single bond = UV

• Chemical double, triple bond = UV,VIS

• Molecular vibration overtone = NIR

• Molecular vibration = IR

• Molecular rotation = Micro

E

h

Ground level

First excited level

Quantized energy levels

What can be measured?

• Emission

• Absorption

• Fluorescence

E

h

Ground level

First excited level

Emission

Thermal

E

h

Ground level

First excited level

Absorption

Thermal

E

h

Ground level

First excited level

Fluorescence

h out

Techniques?

• Gamma spectrometry• Instrumental neutron activation analysis• Xray spectrometry• UV-VIS spectrometry (AES,AAS,ICP...)• NIR spectrometry• IR spectrometry• Raman spectrometry• Microwave spectrometry

What can be used?

Intensity

Energy

Position

Intensity, integral

Width

Special topics

• Polarization

• Time resolved spectroscopy

Content

• Spectroscopy?• Energy levels in atoms,molecules, crystals• Example IR-NIR calculations• Related techniques

Vibrational spectroscopy

Morse curves

The Morse curve describes the potential energy V of a diatomic molecule as a function of interatomic distance x.

V = De [1-exp(-bx)]2

-2 -1 0 1 2 3 4 5 6 70

5

10

15

De = 5 b = 0.5

• If the atoms go far apart the bond breaks.

• It is impossible to press the atoms close together. Enormous amounts of energy are needed.

-2 -1 0 1 2 3 4 5 6 70

2

4

6

8

10

12

14

16

De = 10 b = 0.4

Zero = equilibrium distance

-2 -1 0 1 2 3 4 5 6 70

2

4

6

8

10

12

14

16

Quantum levels = discrete

F

O1

O2

F FundamentalO1 First overtoneO2 Second overtone

This was diatomic molecules

• Polyatomic molecules:

M=3N-6 quantized vibration modes

M=3N-5 linear molecules (N=1)

• N=3 , M=3 H2O, H2S, SO2

• N=4 , M=6 etc

Triatomic molecules

• G(a,b,c)=v1(a+1/2) + v2(b+1/2) + v3(c+1/2)

• Energy levels

• a=b=c=0 (0,0,0)

• a=1 b=c=0 (1,0,0)

• a=2 b=c=0 (2,0,0)

• a=0 b=1 c=0 etc (0,1,0)

a cb

Combination band

Overtone

Groundlevel

Hot band

Fundamental

(0,0,0)

(1,0,0)

(2,0,0)

(0,1,0)

(0,2,0)

(0,0,1)

(0,0,2)

Intensity

• Some transitions are more probable

• Gives more intense bands

• Fundamentals in Gas phase

• Overtones in liquid,solid

• Combination bands in liquid, solid

Hot bands

• Only exist because of thermal excitation

• Boltzmann

• Ne = No exp(-E/kT)

• Ne number excited, No number ground

• k Boltzmann constant 1.3806503*10-23 J/K

• E energy difference

Why cm-1?

Additive

S02

wavenumber band

519 v2

606 v1-v2

1151 v1

1361 v3

1871 v2+v3

2296 2v1

2499 v1+v3

Thermal radiation

• Planck’s law

• W() = c1-5[exp(c2-1 T-1)-1]

• T °K

• c1 = 1.91*10-12

• c2 = 1.438*104

• m

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

1

2

3

4

5

6

7x 10-14

m

Radiance

4000 K (Tungsten melts)

3500 K

3000 K

2500 K2000 K

Planck curves

• More total energy for high temperature

• More UV for high temperature

• More flat curve for low temperature

Content

• Spectroscopy?• Energy levels in atoms,molecules, crystals• Example IR-NIR calculations• Related techniques

Energy supply

• Photon

• Thermal

• Electron -

• Proton +

• Ion + -

Optics

• Electron optics

• Ion optics

Techniques

• Electron microscopy

• Electron spectroscopy

• Mass spectrometry

• Ion microscopy

Transmission

Readoutelectronics

Detector

Sample cell

Mono-chromator

Radiation source

Transmission

Readoutelectronics

Detector

Sample cell

Mono-chromator

Radiation source

I0 It

Lambert-Beer-Bouguer law

TransmissionAbsorbance

T = It / I0

A = log10 ( I0 / It) = -log10 (It / I0)

Lambert-Beer-Bouguer law

A = klC

l = path lengthk = constantC = concentration

Reflection

Readoutelectronics

Detector(s)

Sample cell

Mono-chromator

Radiation source

Reflection

Readoutelectronics

Detector(s)

Sample cell

Mono-chromator

Radiation source

I0 Ir

Lambert-Beer-Bouguer law

ReflectionPseudoabsorbance

R = Ir / I0

A* = -log10 (Ir / I0)

Content

• Spectroscopy?• Instrumentation• Modes of measurement

What can be changed?

• Radiation source

• Monochromator

• Sample cell

• Detector

Radiation source

• Tungsten-halogen lamp (Car type)

• Coated tungsten SiC

• Laser(s)

• LEDs

• LED arrays

ln(Wavelength), m

ln(Energy flux)

3000K

1000K

0.2 1

Wavelength, m

Energy flux

1000

1150

1300

1520

LEDs

What can be changed?

• Radiation source

• Monochromator

• Sample cell

• Detector

Monochromator

• ”Glass filter”

• Interference filters

• Prism

• Grating

• Interferometer

• Electrooptical

Monochromator

• ”Glass filter” not selective

• Interference filters

• Prism too primitive, never used

• Grating

• Interferometer

• Electrooptical

Interference filter

Glass

High RI coating

Low RI coating

Multiple reflections

Tilting interference filter

Glass

High RI coating

Low RI coating

Differentpathlengths

There are also gradual interference filters

• Disk with increasing thickness

• Rotate for new wavelength bands

Filter wheel

Readoutelectronics

Detector(s)

Sample cell

Radiation source

Filter wheel

Grating

Mirror staircase

Pathlength difference

Grating

Polychromatic

Monochromatic

Rotate

Entrance slit

Exit slit

Interferometer

Fixed mirror

Moving mirror

Semitransparantmirror (50%)

Detector

Sample

Interferometer

Fixed mirror

Moving mirror

Semitransparantmirror (50%)

Detector(interferogram)

a

b

Wavelengths for whichb-a = whole cycle reachdetector

Interferometer

Interferogram

Fourier transform

Spectrum

What can be changed?

• Radiation source

• Monochromator

• Sample cell

• Detector

Content

• Spectroscopy?• Instrumentation• Modes of measurement

Modes of measurementThis is a real strong point of NIR spectroscopy. There are many modes of measurement:

• Transmission

• Diffuse reflection

• Fiber optic based

-Transflection

-Interaction

DetIntegratingsphere

Det Det

Fiberoptic Fiberoptic Mirror

Transflectance probe

Fiber bundle Sapphire mirror

Mixed solutions

• Use tunable laser instead of monochromator (more lasers?)

• Use LED’s in different wavelengths instead of monochromator

• Use array of detectors instead of scanning monochromator

DIODE ARRAY

Grating

Polychromatic

Entrance slit

Diode array

Filter wheel instrument with interference filters

Interferometricinstrument

Process NIR spectrometer based on moving grating

Transmision instrument

Sample changer for seeds (transmission)

Diffuse reflectance instrument (rotating cup)