Non-Destructive Material Characterization by Infrared Spectroscopy

Huy Hien Phan, Summer Semester 2020

Introduction

Spectroscopy is based on the interaction of electromagnetic radiation with matter. For this purpose, radiation is absorbed or emitted by matter. Typical parameters for electromagnetic radiation are amplitude, period of oscillation, frequency and wavelength. ^[1]
The range of the infrared extends over a wavelength λ from 780nm to 1mm between visible light [Fig.1] and microwaves. A problem is the radiation in the infrared range used (λ = 2.5 μm - 25 μm or 𝜈̅ = 4000 cm-1 - 400 cm-1), because it has only a low energy potential.
Therefore an external radiation source is needed, which excites or amplifies the sample and thus the natural oscillations of the molecules. Consequently, it is possible to change between oscillation and rotation states of a molecule.
Certain wave ranges are absorbed by the molecules, depending on the chemical and physical properties of the substance, and an attenuated signal can be emitted. A CCD detector then picks up the incoming radiation and converts it into electrical pulses.
The pulses are then converted and displayed as a spectrum. ^[2]


Figure 1. Electromagnetic spectrum ^[1]

The excitation of the molecules using infrared radiation increases the distance between the individual atoms. As a result of the molecular bond, a recoil force is created, which causes the components to begin to vibrate. In addition, small molecules rotate around the binding axis. The change between states of motion causes a change in energy. The radiation can be absorbed if the dipole moment of the molecule changes and thus a field can be generated "which can interact with the electric field of the IR radiation".^[2]

The prerequisite for this is an asymmetric structure of the atoms, i.e. the charge distribution is polar. This is true for most molecules. In a symmetrical structure (linear arrangement of the atoms) with even distribution of the electrons (non-polar), there would be no change in the dipole moment and the motions would be infrared inactive. This means that almost all compounds can absorb IR radiation except the so-called homonuclear molecules like O2. A Raman spectrometer is used to study IR-inactive compounds like CO2.^[2]

A Raman spectrometer indirectly detects the scattered radiation and is therefore used for less polar or non-polar compounds. The reason for this is that an IR device cannot detect them or can detect them very poorly due to the direct measurement of the energy absorption. Another advantage of the Raman spectrometer is the possibility to use water as solvent, because water is inactive and always available. If water is used as solvent for IR spectroscopy, distinct absorption bands for water occur. These can cover peaks of other substances that are in the same wave range.^[2]

IR radiation always absorbs frequencies, causing the molecule to vibrate. These frequencies are called natural frequencies.  Due to the excitation of different movements of the atoms, several wavelengths are absorbed, whereby different bands occur. This is due to the fact that movements of the atoms are detected by the spectrometer and their frequencies are dependent on the type and strength of the bonds. Therefore, the method is a very good possibility to infer the structure.^[2]

IR spectroscopy can be divided into the near, middle and far wave range.  Usually, devices are used for IR spectroscopy in the medium wave range. The reason for this is that the energy for transitions between states of motion is in this range. Possibilities for the application of IR devices are quantitative and qualitative analysis of substances. For this purpose, compositions of gases or the degree of contamination for air can be determined. Further possibilities for application are quality control by detecting defects or contamination, in criminology, examination of surfaces or analysis of multi-component substances.^[2]

For IR spectroscopy there are three relevant parameters that are related.
The wavelength 𝜆 is the reciprocal of the wavenumber and is given in micrometers [𝜇𝑚], more rarely also in millimeters.^[1]

                                                                   $//<![CDATA[ \begin{array}{l}ν̃ = \frac{ 1 }{\lambda}\end{array} //]]>$

Another parameter represents the wavenumber 𝜈̃ uses. This corresponds to the number of waves per distance and is the reciprocal of the wavelength. The wavenumber is directly proportional to the energy and frequency of the radiation.^[1]
                                                                   $//<![CDATA[ \begin{array}{l}c= \lambda* f\end{array} //]]>$                                                            

Additionally, the frequency f is used, which defines the number of periods per second. It can also be determined by the ratio of the speed of light to the wavelength.^[1]

Table 1. Infrared spectral ranges^[2]

Signal-To-Noise Ratio (S/N Ratio)

One way to evaluate the quality of a spectrum is the signal-to-noise ratio (S/N ratio). The signal (S) itself contains information about the sample. However, noise (N) is present in each measurement regardless of the intensity of the signal. This noise is disturbing because it overlays a measured signal, making a signal less clear. However, the strength of the noise is constant on average and does not depend on the signal strength. Consequently, the influence of N decreases with increasing S, which increases the S/N ratio.^[2]

The possible causes of noise are various and can be of chemical/instrumental nature. If there is a demonstrable change in the temperature or pressure of the surrounding medium, the chemical equilibrium changes. When considering the instrumental nature, interfering signals can always be present in any component of a device used, such as the energy source or the detector (signal processing component) for the spectrometer.^[2]

To reduce the influence of noise on the signal, a large S/N ratio is aimed for. This can be realized by additional hard- and software. With additional hardware the noise is at least reduced or eliminated. This includes the installation of filters, shielding or additional detectors. Another possibility is the use of software with algorithms which filter out signals from measured data. In this case, a coarse averaging is applied, where several data sets are obtained and then averaged. Furthermore, there is interval averaging consisting of the formation of several neighboring points to an average value. For most infrared spectrometers the Fourier Transformation is integrated as a kind of digital filtering, which is described in the following section.^[2]

Two-Beam-Device

Various devices can be used to examine samples with IR spectroscopy. For this purpose, single-beam and double-beam devices are briefly introduced.^[3]

In two-beam devices, which were frequently used in the past, the radiation used for excitation is divided into two beam paths which are as far as possible equivalent in terms of energy and geometrical-optical properties. The sample to be examined is placed in a measuring channel. For comparison, a suitable reference is introduced in the second channel with the aim of compensating the influence of the cuvette and solvent by absorption during the measurement.^[3]

The advantage of this measurement variant is the reduced effort for measurements, because no two separate measurements must be made.^[3]

However, a dual-beam instrument must be operated with a monochromator. As a result, the measurement times are relatively long with a poor S/N ratio. At certain times only a small part of the radiation is used for this purpose.^[3]

The already mentioned disadvantages are so serious that nowadays the Fourier Transformation Spectrometer is used. For the Fourier Transformation Spectrometer, the whole spectral range of the light source is needed for the measurement. This has the disadvantage that it is a single-beam device.^[3]

Principle of operation Fourier-IR spectrometer

An interferometer is used for this purpose, as shown as a principle in the figure below and known as a Michelson interferometer.^[3]


Figure 2. Schematic diagram of an interferometer ^[4]

Figure 3: Schematic layout IR spectrometer ^[4]

Starting from an energy source, an IR beam is always emitted onto the sample. When reaching the sample there are two possibilities. One possibility is reflection, if the sample cannot be penetrated. The other possibility is the penetration of the radiation in the specific wave range with attenuation by absorption. After the sample, the beam reaches a semi-transparent mirror, which acts as a beam splitter. There the beam is split, one half of it reaching the fixed mounted mirror and the rest reaching a mirror that is movable in the distance to the beam splitter. The beams are then reflected and meet again at the semi-transparent mirror. The two beams overlap, causing interference. The interference itself depends on the position of the moving mirror. This can have the consequence that the phases constructively overlap, i.e. the amplitudes are added. On the other hand, a destructive superposition up to the extinction of the beam is possible. ^[4]

At the detector a beam arrives after the interference and is detected there. The generated signals are subtracted from each other and the resulting difference is converted and output as a spectrum. The steps after the detection by the detector can now be calculated by software. The prerequisite for this is the determination of the zero value with measurement of the background. Thus, the spectrum is preserved and can be subtracted from the sample spectrum. ^[4]

Fourier Transformation Infrared Spectrometer (FTIR)

Description

With an FTIR, samples in liquid or solid state can be analyzed. For a liquid sample, a few drops are added to an ATR crystal. Before the measurement the background must be determined. Thereby a background absorption against air is measured, whereby the absorption by the crystal is detected. The measured values are used to subtract the influence of the absorption of the crystal from the sample measurement. For the measurement of solid samples, it is necessary to establish a contact between sample and crystal, because this contact is not automatically present. For this purpose, the substance is placed on the crystal and pressed from above onto the measuring window with a stamp. For the analysis, the material only must cover 10mm² of the crystal. A device for this is the FT-IR spectrometer with microscope from the company Perkin Elmer. ^[4]

Figure 4 FTIR diagram by Perkin Elmer ^[4]

Then the wavenumber is plotted linearly to an abscissa in reciprocal centimeters. The ordinate describes the percentage of transmitted radiation. An attenuated total reflection is selected for the image because this is an equivalent to reflection. The total radiation is deflected by the sample, so that we can cover a spectrum of 100% while neglecting fluctuations due to noise. Depending on the material of the sample, the radiation is absorbed at a certain wavenumber. This causes the value to drop and a peak in the spectrum is created. ^[4]

After sample collection, the substance must be completely removed from the crystal. A solvent is used for this purpose. The motivation behind this is to avoid possible mixing with different substances. Thus, the evaluation and graphic representation is not distorted. ^[4]

Manufacturer of FTIR spectrometer

• Perkin Elmer^[4]
• ThermoFischer Scientific (Nicolet iS50 and iS50R series) ^[6]
• Bruker (Invenio series) ^[7]

Application example for non-destructive testing

An example of application in non-destructive testing is the examination of paints and mineral substances in combination, as in this case with paint and concrete. ^[4]

In this example, a sample of concrete and a thin layer of paint is observed as shown in Figures 5 and 6. ^[4]

Figures 5 and 6: Exemplary representation of a thin layer of paint on concrete ^[4]

Prism viewing is difficult in this case because the paint / concrete is often crumbled off and applied very thinly. Figure 6 shows a gap between the paint and the concrete because the paint layer has detached from the concrete during preparation.

Figure 7.8 Absorption at wave number 1409 cm^{-1  [4]}

Figure 7 shows that the layer boundary can be seen in the light blue or green color range. In addition, the sloping curve at 500 micrometers also shows that the sample is somewhat crooked. In figure 8, the concrete and the color as well as the layer boundary are clearly identifiable.

Valuation Method

The examination of the samples with infrared thermography by an FTIR allows the examination of real color samples on concrete. Different materials can be recognized and distinguished from each other. If the contents materials are to be examined, then this is only conditionally possible. This has the consequence that peaks / bulges are not / chewy. ^[4]

The use of a spectrometer allows an improved exclusion of ingredients. However, the Statements on the proportions of individual substances are very vague. When observing materials and constituents with the spectrometer, it depends mainly on an appropriate and careful sample preparation, because the informative value is significantly influenced by the spectrum. ^[4]

Alternative measurement methods

In addition to FTIR, there is the possibility to examine samples with Attenuated Total Reflection (ATR) spectroscopy. This method is non-destructive and requires only a small amount of equipment with little effort to examine the surface. ^[5]

Different types of samples can be used for ATR spectroscopy:

• Colors
• Plastics
• Coatings
• Rubber

In contrast to FTIR, rubber and black samples can also be examined.

Functionality

ATR spectroscopy is based on a change in measurement that occurs in the internally reflected IR beam when the beam and sample come into contact. The IR beam is directed onto an optically dense crystal, which contains a high refractive index. The internal IR beam does not lead to evanescent waves without restrictions. The resulting evanescent waves interact with the sample via the crystal surface. The sample absorbs energy, which weakens the waves. The attenuated beam returns to the crystal and exits at the opposite end and is then passed to the detector. There, the beam is recorded as an interferogram signal, producing an IR spectrum.  ^[7]

Possible applications

ATR spectroscopy can be used for dense/strongly absorbing samples that produce intense peaks. It is insensitive to the sample thickness, because the intensity of the evanescent waves decreases exponentially with increasing distance of the crystal from the surface.  ^[8]

Literature

[1] T.Hecht, Physikalische Grundlagen der IR-Spektroskopie: Von mechanischen Schwingungen zur Vorhersage und Interpretation von IR-Spektren P.1-2
[2]  D. A. Skoog, F. J. Holler und S. R. Crouch, Instrumentelle Analytik: Grundlagen -
     Geräte - Anwendungen, 6. Aufl. Berlin: Springer Spektrum, 2013
     P.8,10,26,27,49,50,51,53,58, 274
[3]  Dr. Martin Badertscher, ANALYTISCHE CHEMIE für Biologen und Pharmazeuten:  Einführung in spektroskopische Methoden der Strukturaufklärung organischer Verbindungen [Online] P.49-50 Avaiable at: http://www.analytik.ethz.ch/vorlesungen/biopharm/Spektroskopie/Skript.pdf
[4]  K.Göhring: Infrarotspektroskopische Untersuchung von Farben auf mineralischen Baustoffen P.97,98,99,106
[5] M.Fülleborn, Zeitaufgelöste FTIR Transmissions- und ATR-Spektroskopie von Flüssigkristallen im elektrischen Feld P.27

[6]  https://www.thermofisher.com/de/de/home/industrial/spectroscopy-elemental-isotope-analysis/molecular-spectroscopy/fourier-transform-infrared-ftir-spectroscopy/ftir-spectrometer-selection-guide.html?gclid=Cj0KCQjwwOz6BRCgARIsAKEG4FWfV2AIIyYpW6bS_2bP1bb2hOWQXwJQNVdB9NxpQ9u6yfde5TaDnOAaAv_lEALw_wcB&cid=7010z000000v8XP&s_kwcid=AL!3652!3!254361630922!b!!g!!atr%20spektroskopie&ef_id=Cj0KCQjwwOz6BRCgARIsAKEG4FWfV2AIIyYpW6bS_2bP1bb2hOWQXwJQNVdB9NxpQ9u6yfde5TaDnOAaAv_lEALw_wcB:G:s&s_kwcid=AL!3652!3!254361630922!b!!g!!atr%20spektroskopie (Zugriff: 11.9.2020, 11:30 Uhr)
[7] https://www.bruker.com/de/products/infrared-near-infrared-and-raman-spectroscopy/ft-ir-research-spectrometers/invenio.html (Zugriff: 11.9.2020, 11:30 Uhr)
[8] https://www.thermofisher.com/de/de/home/industrial/spectroscopy-elemental-isotope-analysis/spectroscopy-elemental-isotope-analysis-learning-center/molecular-spectroscopy-information/ftir-information/ftir-sample-handling-techniques/ftir-sample-handling-techniques-attenuated-total-reflection-atr.html (Zugriff: 11.9.2020, 12:30 Uhr)

List of Tables

Table 1 Infrared spectral ranges  ^[2]

List of Figures

Figure 1 Electromagnetic Spectrum  ^[1]

Figure 2 Schematic diagram of an interferometer ^[4]

Figure 3: Schematic structure of IR spectrometer ^[4]

Figure 4 FTIR diagram by Perkin Elmer ^[4]

Figure 5: Example of a thin layer of paint on concrete ^[4]

Figure 6: Example of a thin layer of paint on concrete ^[4]

Figure 7 Absorption at wave number 1409 cm^-1 [4]

Figure 8 Absorption at wave number 1409 cm^-1 [4]