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Spectroscopic methods are based on the interaction of radiation with matter. In the case of infrared spectroscopy the radiation is in the infrared region of the electromagnetic spectrum, as seen in Figure 1, with wavelengths between 0.78 and 300 µm.
This region can be further separated into near (0.78-3 µm), mid (3-30 µm), and far (30-300 µm) infrared. This article focuses on mid-infrared radiation, which can be absorbed by the sample upon interaction with molecular vibrations. The exact frequency that is required for this interaction depends on numerous factors: the type of vibration, such as stretching or bending; the strength of the bonds, such as single, double or triple bonds; the atoms and isotopes involved; the symmetry of the molecule; and the chemical environment.
Consequently, every infrared active molecule has an unique infrared fingerprint; however, not all vibrations can absorb mid-infrared radiation. The number of fundamental vibrations in a molecule consisting of N atoms can be calculated as 3N-6 for non-linear molecules, or 3N-5 in linear molecules. With H2O as an example molecule, there exist three fundamental vibrations which are comprised of two stretching vibrations and one bending vibration, as seen in Figure 2.
There are two different types of stretching vibrations: symmetrical and asymmetrical. Types of bending vibrations include deformation, rocking, wagging and twisting. Symmetry and group theory can be used to determine which of the possible vibrations are infrared active, but in short, vibrations are only infrared active if they result in a change of the dipole moment. Molecules such as H2O having a large dipole moment can easily be seen using infrared spectroscopy, while molecules such as N2 and O2 cannot, and therefore will not, interfere with the analysis. While infrared spectroscopy is an old method, it was recently revolutionised and revitalised by the introduction of Fourier transformation, with the first commercial instruments made available in the early 1980s.
As opposed to traditional dispersive techniques, absorption of light at all frequencies is monitored simultaneously based on interference, and recorded as an interferogram. This interferogram is subjected to Fourier transformation, which calculates the contributions of the constituent frequencies and plots the spectrum as intensity versus wavelength, or wavenumbers (cm-1), as is commonly used in infrared spectroscopy.
Samples
One of the major advantages of Fourier transform infrared (FTIR) spectroscopy is that it can give detailed qualitative and quantitative chemical information without destroying the sample. In most cases, the samples used for FTIR spectroscopic investigations can be completely recovered and used for further analysis elsewhere. Exceptions include cases when samples are too concentrated, practically absorbing all infrared radiation, and thus must be diluted by mixing with infrared transparent diluters, such as KBr for diffuse reflectance measurements.
While the sample is not destroyed in this procedure, it is intimately combined with KBr making it unrecoverable; however, samples in most cases can easily be recovered. This is particularly important when examining the chemical composition of unique, non-reproducible samples such as in forensic analysis, or in the analysis of artwork and historical artefacts. It would hardly be worth ruining Mona Lisa’s smile in order to determine its chemical components. The same is true with most experiments in the laboratory, where further experimentation or characterisation needs to be done on a small quantity of sample.
An additional advantage of FTIR spectroscopy is its versatility. It is a tool that can be used on almost any kind of sample, regardless of the physical state. Samples may be organic, inorganic, biological, polymers, or any combination of these. While other techniques such as nuclear magnetic resonance (NMR) spectroscopy or mass spectrometry (MS) may be able to produce more specific or detailed data than FTIR spectroscopy, they are often considerably more expensive, have lower throughput, require more sample, labour and training, and can be more difficult to automate.
In addition, they may not be as versatile or allow for spatially resolved studies, such as microspectroscopy. Ultimately, FTIR spectroscopy can provide key data at low cost and high speed.
Applications
With a host of techniques available, FTIR spectroscopy can be used to examine almost any problem thrown its way. This may sound arrogant, but the benefits of FTIR are extremely far reaching. The techniques described in this article are currently being employed to investigate topics ranging from doping tests in sports and quality control of Formula One car tyres, to medical diagnostics and pharmaceutical identification; quality control and monitoring in the food, semiconductor, solar energy and pulp and paper industries; identification and preservation of art and historical artefacts; and the tracking of toxins to determine how long they stay in groundwater.
Measurement techniques
As stated previously, one of the greatest strengths of FTIR spectroscopy is its versatility. Measurements are fully customisable to suit the samples, needs and preferences of the user.
Transmission
Transmission is the oldest and perhaps most straightforward infrared spectroscopic method: the sample is simply placed in the beam path of the infrared radiation. Samples can be gases, liquids or solids, with different sample cells available for each case, such as sealed for gases and volatile liquids studied under different pressures, or open containers for fixed or variable path lengths.
The advantages of the technique are simplicity, practically no sample preparation and a superior speed of more than 100 spectra per second. In addition to this, the flow through setup means that continuous, real-time monitoring is possible. The disadvantages include the sometimes difficult handling procedures and maintenance of the sample cells; for example, cleaning can be challenging as windows must be made of infrared transparent material and not glass.
In addition, the useful concentration range for this technique can be limited by the very short path lengths required for highly absorbing materials, such as aqueous solutions and the need to dilute highly absorbing samples with infrared transparent materials. Application areas of the technique include: continuous or batch gas analysis, which can be used for environmental monitoring such as car exhaust testing, and continuous or batch water quality monitoring, which can be used to study or monitor wastewater, groundwater and crude oil quality.
Reflectance
Reflectance methods are based on the interactions of reflected infrared light with the sample. These methods are often used instead of transmission techniques when transmission is too difficult to implement, or when only specific areas of the sample need to be analysed, such as surfaces or thin layers. The three most common reflection methods described independently below are diffuse reflectance, attenuated total reflectance and grazing angle FTIR spectroscopy.
Diffuse reflectance FTIR spectroscopy
In diffuse reflectance measurement (DRIFTS) the infrared light interacts with small solid particles and is consequently reflected in all directions. The signal is collected as diffusely reflected light, which filters off the interactionless surface reflections. DRIFTS is only applicable for solid samples or samples that can be made solid by, for example, freezing, solvent evaporation or adsorption onto solid particles.
The major advantages of the technique include speed, reproducibility and easy automation. While advantageous in some respects, the sensitivity of the technique can also be a disadvantage. Small amounts of impurities can disturb the signal and highly absorbing samples often need to be mixed with infrared transparent diluters, such as KBr, meaning they do not remain intact. In addition, grinding is often needed to reach the required small particle size. This is laborious and can affect the sample via the heat generated or the bonds broken by the grinding process.
Application areas of DRIFTS include high throughput monitoring, screening and compositional analysis of solid samples – from soils and sediments to plants and wood.
Attenuated total reflectance FTIR spectroscopy
During attenuated total reflectance (ATR) measurements, the sample is placed on an internal reflection element (IRE) made of high refractive index material, such as ZnSe, Ge or diamond. The infrared radiation enters the IRE at a certain angle and reflects as it passes through. Each time the infrared radiation reaches the surface of the IRE, a so-called evanescent wave penetrates into the sample.
The penetration depth depends on a number of factors, including the refractive index differences between the sample and the IRE, but never exceeds a few micrometres. It is critically important that the sample forms a perfect contact with the IRE. This is evidently the case for liquids, but for solids either some sort of pressure may need to be applied or the solid material should be deposited directly onto the IRE. Due to the low penetration depth, this technique is inherently surface sensitive and generally yields lower signals than transmission techniques or DRIFTS.
On the other hand, the low penetration depth also means that highly absorbing bulk materials do not disturb the signal. Consequently, aqueous solutions can be measured conveniently using this method, as the infrared light does not need to pass through the entire sample volume. Other advantages of the technique include the minimal sample preparation and the speed, as well as the completely non-invasive and non-destructive nature of ATR, which allows for rapid in situ chemical compositional analysis.
A splendid illustration of the versatility and capability of the technique, is the possibility of coupling the FTIR spectrometer with an ATR accessory to a potentiometric titration setup, or simultaneous infrared and potentiometric titrations (SIPT). This coupling could create a flow through setup which would continuously monitor changes in situ as a function of pH, concentration, temperature and addition of chemicals. Typical applications include the study of surface reactions, absorption and desorption phenomena of various chemicals on, for example, mineral surfaces to assess the bioavailability, speciation and potentially hazardous effects of those chemicals, such as heavy metals or other pollutants reaching groundwater through various soils.
Grazing angle FTIR spectroscopy
The reflection method using grazing incident angle is particularly suited for reflective materials or thin layers, such as coatings or films deposited onto a reflective material. As the name of the technique suggests, the infrared radiation reaches the thin layer of sample at a low angle, maximising the path it takes for the light to pass through the sample.
Much like ATR, grazing angle FTIR spectroscopy is an inherently surface sensitive technique, allowing for very thin layers to be studied, such as bacterial biofilms and polymer monolayers. The advantages of this method are similar to that of ATR, with minimal to no sample preparation, and the non-destructive and non-invasive nature of the technique enabling high throughput and in situ measurements.
The disadvantages are also similar to those of ATR, where weak signals require more sensitive detectors and more rigorous setup and optimisation of the measurements for individual samples if they are markedly different. Typical applications can relate to quality control in industries that investigate semiconductors, catalysis and solar panels.
Microspectroscopy
By attaching a microscopy accessory to an FTIR spectrometer, spatially resolved chemical compositional analysis becomes possible, producing high quality qualitative and quantitative chemical maps of the sample. The highest achievable spatial resolution depends on a number of factors: the intensity of the infrared light source, the wavelength of the light and the type of detector, such as single element or focal plane array (FPA).
Modern FPA detectors are capable of recording images at diffraction limited spatial resolution, or, in other words, the spatial resolution depends on the wavelength of the light, which in the case of mid-infrared radiation means a spatial resolution in the micron range, consisting of thousands of spectra within a minute. Different accessories are available for the microscopes as well, allowing for transmission, reflection, ATR or grazing angle measurements.
Analysis can be done on single samples, multiple spots on a single sample or on multiple samples. A flow through setup for continuous spatial and chemical monitoring can also be implemented. Samples can be solids or liquids, suspensions or colloids. The obvious advantage of microspectroscopy is the possibility to investigate the spatial distribution of chemical changes and sample heterogeneity, and to follow reaction fronts and reactions at certain sites, as well as precise sampling, such as when only measuring a certain area in a sample. Compared to these gains, the disadvantages are minor. Although faster than many other microscopy techniques, FTIR microspectroscopy is slower than FTIR spectroscopy.
Microscopy is also slightly more expensive than standard FTIR spectroscopy; however, considering the amount of information gained it is still inexpensive. FTIR microspectroscopy can be harder to automate and requires more training than other FTIR methods, especially for data analysis, which is not always straightforward and may require additional multivariate techniques. Microspectroscopy has been developed mostly in connection with biomedical research, which is at the forefront of the technique; however, other application areas are emerging quickly, ranging from art to forensics, and nanomaterials to polymers.
Concluding remarks
The aim of this article is to raise awareness of FTIR spectroscopy in connection with industrial and environmental applications. Specificity and interpretability may be limited due to overlapping or non-diagnostic bands, and detection limits depend largely on the compounds studied; however, FTIR spectroscopic techniques are inexpensive and provide non-destructive chemical compositional analysis in situ, which is easy to perform and automate. FTIR spectroscopy is a fast and sensitive technique which is extremely versatile and customisable to suit the task, the sample and the user.
Due to speed and easy automation, ‘on the fly’ analyses, such as for quality control, are possible. FTIR spectroscopy is, in addition, easy to combine with many other complementary techniques, such as X-ray absorption spectroscopy and Raman spectroscopy, or to perform as well as other more laborious, expensive and time consuming techniques, such as NMR spectroscopy.
Published: 01st Sep 2012 in AWE International
