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IR Spectroscopyand FTIR

Henry Buijs

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Molecular Vibrations

Material is made up of atomsA group of atoms linked together by chemical bonds is called a molecule

Atoms of hydrogenand oxygen

Molecule of water

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Molecular Vibrations

The atoms of a molecule are always in motionThese motions are called vibrations

symmetrical vibrationH-O-H

asymmetrical vibrationH-O-H

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Properties of Molecular Vibrations

The rate of vibration is millions of cycles per second (it never stops)Each vibration has a unique frequencyThe frequency of a vibration depends on:

The strength of the chemical bond between atomsThe mass of each atom

So we can learn a lot about a molecule when we study its vibrations!

Infrared spectroscopy is a method for the analysis of molecular vibrations

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IR Spectroscopy

Analysis based on the absorption at different wavelengths of an infrared beam by a sample

Sample

Source radiation Transmitted radiation

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transmittance spectrum of the sample

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Relative intensity of IR beam

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Principles of Infrared Spectroscopy

When a frequency of light corresponds to a molecular vibration it is absorbed by the sample

The fraction of light transmitted by the sample compared with the light incident as a function of frequency gives the infrared spectrum of the sample

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Infrared Spectrum of a Polystyrene Film Wavelength is expressed in number of waves per cm.

(e.g. 1000 /cm is the same as wavelength λ=1/1000th cm)Vertical scale is percent remaining intensity

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transmittance spectrum of the sample

Wavenumbers (cm-1)

%

InfraredUV

Visible

Near IR Mid IR

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Properties of an IR Spectrum There is much detail in a spectrum.Much of it is unique to a sample

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A band as narrow as 6 cm-1 at3100 cm-1 (0.2% of wavelength)

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But What Is Really Happening Here?

In fact recording a spectrum is a little like tuning in to radio stations...We turn the button (change the frequency)Sometimes we catch a signal, and we hear music (we “absorb” the signal at this frequency)We know which stations to find, because we know the frequency where they are (we know which vibrations to pickup)Sometimes the signal is strong, sometimes weaker (the peak is higher or lower)Sometimes we hear nothing at all!

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Absorbing and Non-Absorbing Species

Absorbing:Any molecular vibration which displaces an electric charge will absorb infrared radiation.Diatomic molecules with dissimilar atoms HCl, COAsymmetric vibrations in polyatomic moleculesFunctional groups

Non-absorbing:Atomic Species: He, Ne, Ar, KrAll Atomic SpeciesMolecular Species: H2, N2, O2, F2, Cl2, Br2These cannot be analyzed by infrared

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Mid IR SpectroscopyWavelength range 2500 nm to 25000 nmFrequency range 4000 to 400 cm-1Mid IR spectra provide distinctive patterns for many compoundsPermits distinction of several hundred thousand different compoundsMost compounds have high absorptivity in mid IR

Condensed phase compounds>50% transmittance for sample thickness in 10 to 100 micrometer rangeNeed sample accessories that assure short light path through sample

Capillary liquid transmission cell, ATR and diffuse reflectance

Gas phase compounds>50% transmittance for sample thickness in 10 to 100 cm rangeMid IR is suitable for quantitative gas phase analysis

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IR Spectrum of a Liquid Vegetable Oil (path: 12 microns)

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IR Spectra of Hand Cream and Water (path: 12 microns)

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Frequency (cm )

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water

hand cream

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-Infrared Spectrum of Gaseous HCl (10-cm path cell, resolution 1 cm-1)

3200 3000 2800 2600 Frequency (cm )

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2970 2960 2950 2940

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Near IR SpectroscopyWavelength range 700 nm to 2500 nmFrequency range 14000 to 4000 cm-1Near IR spectra have less distinctive patterns than Mid IR spectra

Harmonics and combination frequencies of mid IR bandsPrincipally harmonics of H-C, H-N, H-O vibrations

Absorptivity is lower than in mid IRLow absorptivity is offset with longer path sampling

Sampling with pathlength of several millimeters is much easier and permits greater reproducibility

Disposable glass vial samplingSampling of flowing sample in process pipe

Quantitative analysis using chemometrics

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Overtones (Harmonics) and Combinations

Infrared spectrum of H2O:OH stretching: 3400 cm-1

OH bending: 1630 cm-1

First overtone: 2 x 3400 = 6800 cm-1

Second overtone: 3 x 3400 = 10200 cm-1

Combination: 3400 + 1630 = 5030 cm-1

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The pathlength is increased by a factor of 150 in the NIR to get intensity similar to the mid-IR

Mid and Near Infrared Spectra of Isopropanol

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Isopropanol, mid-IR

Isopropanol, NIR2 mm cuvette

Horizontal ATR

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NIR Spectra of Surfactants

9000 8000 7000 6000 5000 Frequency (cm )

Nor

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Abs

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From the bottom to the top:

- OH value = 217.6, sap value = 145.7

- OH value = 185.6, sap value = 153.2- OH value = 107.4, sap value = 159.5

Hydrocarbon region

OH region

Hydrocarbon region

moisture

OH region

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Fundamentals and Overtones

The C-H Stretching Vibration for CHCl3 Liquid

Anharmonic Observed oscillator Absorbance Pathlength

Transition wavenumber calculation for a forυ = 0 to υ= (cm-1) (cm-1) 1 cm cell measurement

1 3019 3019 170 5 µm2 5912 5912 17 0.5 mm3 8677 8679 0.55 1.0 cm4 11318 11320 0.02 5.0 cm5 13850 13835 0.001 N/A6 16270 16224 0.0001 N/A

Note the reduction in absorbance. This requires very short path at 3019cm-1, convenient 0.5 mm and 10 mm path at 5912 and 8677 respectively.

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FTIR principlesHenry Buijs

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Dispersive Infrared Spectroscopy

Geometric wavelength separation

Prism dispersionDiffraction grating

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The Michelson Interferometer Optical part of FT-IR2 flat mirrors and a beamsplitter

One mirror movesChanges optical path of split light beamsRecombine light and get variable interference

Wavelength separation by modulation

Each wavelength hasa unique modulation frequency

Moving mirror

Fixed mirror

Beamsplitter

Compensator

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FTIR technologyMany Interferometer types are used by different FT-IR manufacturers. Why?

To achieve more stable and reproducible operation

The original Michelson interferometer is sensitive to misalignment

Tilt of 1 micrometer across the IR beam (25 micro radians) will changeinterferogram by >10%

The scan mechanism can wear out or become irratic

1 micro meter1 micro meter

Scan MechanismScan Mechanism

fixed mirrorfixed mirror

moving mirrormoving mirror

compensatorcompensator

beamsplitterbeamsplitter

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The ABB Michelson FT-IR

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FT-IR with Laser Controlled Measurement

Modulation frequencies of all wavelengths are locked to modulation frequency of an internal He-Ne laser

Provides ultra precise measurement of spectral frequenciesProvides reproducible measurement of spectral frequencies

fixed mirrorfixed mirror

Scan MechanismScan Mechanism

moving mirrormoving mirror

compensatorcompensator

beamsplitterbeamsplitter

IR beam

Laser beam

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Vol

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Interferogram / Arbitrary X

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TypicalTypical interferograminterferogram4 volts at4 volts at centerburstcenterburst

0.004 volts at end of scan0.004 volts at end of scan

Measurement with FTMeasurement with FT--IRIR

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Vol

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Interferogram / Arbitrary X

Two interferograms (near end of scan)Two interferograms (near end of scan)difference difference

Measurement with FTMeasurement with FT--IRIR

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Vol

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31400 31300 31200

Interferogram / Arbitrary X

Difference between 2 interferograms (16Difference between 2 interferograms (16 coaddedcoadded scans )scans )(peak signal (4volts)/p. to p. noise =40000:1)(peak signal (4volts)/p. to p. noise =40000:1)

Measurement with FTMeasurement with FT--IRIR

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Vol

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Interferogram / Wavenumber (cm-1)

Interferogram of blocked beam Interferogram of blocked beam (3 levels of ADC)(3 levels of ADC)

Measurement with FTMeasurement with FT--IRIR

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S/N achievableTable 1figure RMS noise in

Micro AAt 4000 cm-12500nm

At 5000 cm-12000nm

At 5800 cm-11725nm

At 6600cm-11515nm

A 0A 4.3 2.6 3.25 4.6B 1.634A 168 160 148 177C 1.634A +gain 11 6 8.5 14D 2.683A +gain 125 83 105 145

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-50E-060

50E-06

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2.6832.6835

2.6842.6845

5400 5200 5000 4800 4600 1.633

1.63351.634

1.63451.635

5400 5200 5000 4800 4600

1.6341.6345

1.6351.6355

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CB

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Interferometer Features

Patented Michelson type interferometerCube corner retroreflectors mounted on a "wishbone" swing arm.

Factory prealigned interferometerand input/output optics. Does not require any adjustment by user.

Scanning by rotating the swing arm on a flex pivot, driven by an induction motor

This provides smooth, constant-velocity, perturbation-free scanning without wear.

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The ABB Michelson FT-IR

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Acquisition of an Infrared Spectrum

The light coming from the source goes through the interferometer; the movement of the mirrors causes an interference pattern which is called the interferogramNext the light passes through the sample; theinterferogram of the source is modified as the light is absorbed by the sampleThe interferogram is measured by the detectorThe interferogram is transformed into a spectrum by a mathematical operation called the Fourier Transformalgorithm

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Acquisition of an Infrared Spectrum

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Interferogram of the source & sample

Interferogram of the source Spectrum of the source

Spectrum of the source & sample

FT

FT

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Acquisition of an Infrared Spectrum

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transmittance spectrum of the sample( polystyrene film )

Spectrum of source only(no sample present)

Spectrum of source & sample

divided by

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Advantages of FT-IR Over DispersiveOptical efficiencyMeasurement efficiencyPrecision of measurement

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Optical Efficiency of FT-IRA dispersive spectrometer has much greater light restriction than FT-IR for the same size and resolution spectrometer

A Dispersive spectrometer has a slit width of 0.25mm by 5mm highFT-IR has a round aperture of 7.5mm diameter

Ratio of areas for FT-IR/dispersive is 35

Dispersive spectrometerDispersive spectrometerSlit 0.25x 5mmSlit 0.25x 5mm

1.25mm21.25mm2

FTFT--IRIRJacquinotJacquinot stop 7.5 mm stop 7.5 mm diamdiam

44 mm244 mm2

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Optical efficiency of FT-IR

FT-IR has sufficient throughput to simultaneously illuminate 8 sampling accessories via fiber optic cables

The ABB FTPA2000-200 (Networkir)

modulated outputs8 channels

Direct FO coupled Detectors8 channels

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Measurement Efficiency of FT-IRMultiplexing

Grating observes 1 spectral element at a time1000 to 2500 nm with 4 nm resolution = 375 elements

FTIR observes all spectral elements at the same time4000 to 10000 cm-1 at 16 cm-1 =375 elementsTypical advantage 20x

All wavelengths are modulated simultaneouslyA single detector measures all wavelengths continuously

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Precision Requirements in IR Spectroscopy

For quantitative analysis via IR spectroscopy, the spectra of a sample should be reproducible

For a reproducibility of 1%,The absorbance scale must be reproducible to <1%

Requires <1% error in transmittance when transmittance is >50%Requires <0.1% error in transmittance when transmittance is near10%

Frequency scale must be reproducible to <1% of resolutionAt 4 cm-1 resolution error must be <0.04 cm-1

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Precision: Dispersive Spectrometry vs. FT-IRDispersive Spectrometers

Wavelength is established by mechanical tolerance on slit position

Best mechanical tolerance is

1 micrometer1 part in 25000

for 2.5 cm structure

+/-0.3 cm-1 at 7500 cm-1

Any detected light not passing through spectrometer adds offset to transmittance of sample

Absorbance scale reproducibility limited by stray light

Reproducibility limited to low resolution and low absorbance applications only

FT-IRWavelength is established by internal He-Ne Laser

1 part in 1 000 000repeatability <+/-0.008 cm-1 at 7500 cm-1

For ABB FT-IRsGuaranteed Absolute Accuracy of frequency scale

+/-0.04 cm-1 at 7300 cm-1 Any detected light not modulated by FT-IR does not cause offset to transmittance of sample

There is no stray light in FT-IR

Better Reproducibility over a wider range of resolution and absorbance

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Repeatability and Stability of ABB’s FT-IRs

Internal laser controlled mirror scanLaser coaxial with IR beam provides frequency scale repeatability of +/-0.001 cm-1 (2 σ )Permanent factory alignment of interferometer provides

100% line repeatability < 0.1% short term100% line repeatability < 2% over 16 hour intervalLong-term 100% line repeatability of spectral structure <0.1%

Linear baseline shift and tilt < 2%

It is common to use the same spectral response reference for as long as a year.

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Identical Spectral Response

All units have identical spectral responseReproducibility of spectral response approaches repeatabilityCalibration transfer without chemometric correction

From analyzer to analyzerAfter repair of an analyzer

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Reproducibility of ABB’s FT-IRs

Spectrometer validation using standard samples provides

Frequency scale reproducibility within +/- 0.04 cm-1 (2σ )For FT-NIR, narrow water vapor line at 7299.85 cm-1For FT-IR, narrow water vapor line at 1917.65 cm-1

Absorbance reproducibility within +/-0.002A (2σ )High purity Toluene in temperature controlled liquid cell or sealed vial, 0.5 mm path at 28ºC

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Frequency Validation Water Spectrum

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Validation bandValidation bandH2OH2O

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Frequency Reproducibility

Result of VALID test

7299.4

7299.44

7299.48

7299.52

7299.56

7299.6

7299.64

7299.68

7299.72

7299.76

7299.8

7299.84

7299.88

7299.92

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Absorbance Reproducibility: Toluene

Toluene SpectrumPeak absorbance = 1.2 at 4050 cm-1Average peak heights = 0.5 at 4200 to 4700 and near 6000 cm-1Maximum absorbance deviation <0.002 A from 4200 to 7500 cm-1 ( 2 cm-1 resolution, 50 scans)

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Typical Toluene Reproducibility

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Absorbance / Wavenumber (cm-1)

Toluene Difference Spectrum

MB160 (s/n SZM480CW)

MB160 (s/n SZM480CM)

16 cm-1 resolution

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Toluene Reproducibility Statistic

Toluene Validation

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An Example: OH Value Calibration

OHvalue (Actual v.s. Predicted)

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OH Value Repeatability

OH value

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cal samplesValidationLinear (cal samples)

Repeatability RMSD 0.05

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OH Value Repeatabilitycontrol chart 10 weeks

y = 0.0002x + 57.202

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OH Value Reproducibility

System to System Reproducibility

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Advantages of No Drift and Identical Spectral Response

Less effort is required to develop a calibrationThere are no analyzer variations to model

A robust calibration for OH value for a polyol requires 10 to 15 reference samples.

Less calibration maintenanceOnce a calibration is developed it can be used indefinitely as long as the product does not change

Some customers are working more than 5 years with the same calibration without any need for adjustment

Can do applications that are difficult or costly to calibrateCostly reference methodsProcesses that are difficult to vary.

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The ABB Spectrometer Line

The infrared band can be divided into three regions: The Far infrared, the Mid infrared and the Near infrared.In the Far Infrared (< 400 cm-1), we particularly study the structure of complex solids. This region is covered by the FTLA2000-102In the Mid Infrared (4000 - 400 cm-1), we study the structure of many natural products and gas phase samples. This region is covered by the FTLA2000-100, FTLA2000-104, FTLA2000-154The Near Infrared (12000 - 4000 cm-1) region is used particularly for quantitative analysis. It is covered by the FTLA2000-154, and FTLA2000-160

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The ABB Spectrometer Line

On-line analyzers:FTPA2000-300 (Workir)

Single-point analyzer existing in Mid IR and Near IR versions.

FTPA2000-200 (Networkir)Near IR fiber-optic coupled multipoint analyzer. Can monitor up to eight sampling points.

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Summary

With FT-NIRs from ABB expectAnalyzers that do not drift over timeAnalyzers that have identical spectral response

Calibrations give the same results on any analyzerCalibrations require no correction after repair of analyzer