Infrared Spectroscopy (IR) - Oulun yliopisto

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Infrared Spectroscopy (IR)

Characterisation methods of inorganic materials

Eetu-Pekka Heikkinen, Tommi Kokkonen & Pekka Tanskanen

[email protected] / [email protected] / [email protected]

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Goal of the lecture To learn the main operating principle of the


- What is measured?

To learn how IR is used in (metallurgical)

R&D

- Areas of application

- Examples

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Contents Infrared Spectroscopy - IR

- Main principle

- What is measured?

- Different type of IR device

- Sample requirements

- Strengths and limitations

Examples

- Application areas

Figure from:

D Skoog & J Leary: Principles of instrumental analysis. 4th ed. 1992. 700 p.

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Main operating principle

- Based on the absorption of (IR) radiation on the sample

- Radiation is focused on the sample

- Radiation is absorbed if the frequency of the IR radiation is the

same as the frequency of a bond (or collection of bonds)

- Absorption is characteristic for each compound

- Compounds of the sample are recognized based on the IR

radiation that has transmitted the sample (Qualitative analysis)

- Intensity of absorption depends on the amount of compounds

in the sample (Quantitative analysis)

- Transmitted radiation is presented as a function of either:

- Wavelength of radiation

- Wave number (= 1/Wavelength)

Different kind of techniques

- Dispersive IR

- Sequential scanning of each wave number

- Single or double beam (sample + reference)

- Non-Dispersive IR

- Filters for wavelength selection

- Cheaper, less complex, less accurate ”for rough use”

- Fourier Transform IR (FTIR)

- Collects all wavelengths simultaneously

- Widely used

Figure from:

D Skoog & J Leary: Principles of instrumental analysis. 4th ed. 1992. 700 p.

IR spectra of atmospheric water vapour and carbon dioxide

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Solid, liquid and gaseous samples can be

analysed

- For gaseous samples the detection limit depends on the

size (length) of the sample cell

- Long path length is needed to compensate dilute samples

- Glass tube of 5...10cm (equipped with IR-transparent windows

at both ends) is used for concentration levels down to few

hundred ppms

- White’s cells (in which IR is led with mirrors through the gas)

can have optical pathlengths up to 100m (ppm-level detection)

- Liquid samples are set between two plates of some salt

- Plates are transparent to IR radiation (do not show in the

spectra)

- Solid samples can be prepared in many ways

- e.g. crushing/grinding with oily mulling agent (mineral oil) and

applying thin film of the mull on salt plates

- IR is used as an analysis method in combustion analyzers

in which sample is heated/combusted and the products of

burning reactions are analysed with IR

- e.g. burning C and S into CO2 and SO2

Figure from: chem.libretexts.org

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Strengths (and weaknesses) depend on the

technique used

- Dispersive IR is cheaper, slower, less accurate, less sensitive

and studies only a narrow frequency range at the time

- FTIR is more expensive, faster, more accurate, more

sensitive and studies all frequencies simultaneously

Restrictions on the analyses

- Only compounds which are “IR active” can be detected

- Molecules can vibrate in various ways (= vibrational modes)

- Absorption of IR radiation is related to frequency of vibration

- Vibrational mode is “IR active” if it is associated with changes in

the dipole moment

- Simple diatomic molecules (e.g. CO, N2) have only one bond and

therefore only one vibrational band

- Symmetrical diatomic molecules (e.g. N2, O2, H2) are not

observed in the IR spectra (might be detectable with Raman-

spectroscopy)

- Asymmetrical diatomic molecues (e.g. CO) absorp radiation

in the IR spectrum and are detectable

- More complex the molecule, more complex the vibrational spectra

and more peaks in the IR spectra

Symmetric Antisymmetric

Radial

Latitudinal

Longitudinal

Examples of different vibration modes

using CH2X2 as an example

Figure from: Wikipedia.

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Areas of application in R&D

- Widely used in the recognition of functional groups in

organic compounds

- Different bonds vibrate with different wave numbers

- Often used together with MS and NMR-spectroscopy in

structure analyses

- IR is sufficient alone for more simple molecules

- Polymers and organic chemistry

- Quality and product control

- Pharmaceutical industry

- Food industry

- Atmospheric measurements

- Metallurgy and materials engineering

- Characterisation of samples containing silicates (e.g.

solidified slags, geopolymers, ...)

- Used to analyse contents of light elements in combustion

analyzers

Figure from: Wikipedia.

Wave number [cm-1]

Near IR

Mid IR

Far

IR

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Example IR is widely used to analyse organic

compounds

- Analysis of evolved gases during thermal degradation of

vegetable oils in N2 atmosphere as an example

- Coupled analysis with FT-IR and MS

- Analysis revealed the degradation occuring in many steps

- Determination of kinetic parameters of degradatation based

on the results from experiments with different heating rates

Source: N Tudorachi & F Mustata: Journal of thermal analysis and calorimetry. 119(2015)3,1703-1711.

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compounds


CO CSO

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compounds


CO CSO

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Example Analysis of inorganic samples

- Study of using waste cathode ray tube (CRT) glass as a

raw material for fly ash/slag geopolymer mortar

- Coupled analysis of FTIR with leaching tests, XRD, TGA

and SEM

Source: W-J Long, X Zhang, J Xie, S Kou, Q Luo, J Wei, C Lin & G-L Feng: Construction and Building materials. 322(2022)126454.

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and SEM


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and SEM


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Example IR analysis as a part of combustion analysis

- Analysis of carbon and sulfur contents

- C and S are burned into CO2 and SO2

- CO2 and SO2 are analysed from the gas with IR

- Focus only on the wavelengths (wave numbers) related to

these compounds

- Intensity is measured as measurement proceeds

- Visualised as a function of time (rather than wave number)

- Total content is a cumulative sum of measurements

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Summary Infrared spectroscopy (IR) is based on the

absorption of radiation on the sample

- Absorption is characteristic for each compound

- Compounds must be ”IR active” to be detectable with IR

- e.g. O2, N2 and H2 are non-detectable

Different kind of techniques

- Each have their own strengths and limitations

- Fourier Transform IR (FTIR) is most widely used

- Collects all wavelengths simultaneously

- Widely used

Solid, liquid and gaseous samples can be

analysed

Areas of application in R&D

- Widely used in the analysis of organic compounds

- Characterisation of samples containing silicates

- Used to analyse contents of light elements in combustion

analyzers

Infrared Spectroscopy (IR) - Oulun yliopisto

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