What is FTIR spectroscopy?
FTIR spectroscopy (short for Fourier transformation infrared spectroscopy) is a special form of infrared spectroscopy used both for the determination of the structure of molecules and for the qualitative and quantitative measurement of substances. Unlike dispersive IR spectroscopes, the spectrum is not determined directly, but instead using a Fourier transformation from the interferogram acquired.
Table of contents
FTIR spectroscopy at Quality Analysis
Technical progress has made spectrometers increasingly compact and powerful in recent decades. Modern FTIR spectroscopy requires sound specialist knowledge during the selection of the methodology, the preparation of the samples and, of course, the evaluation and interpretation of the data obtained.
As an accredited test laboratory, we at Quality Analysis can guarantee that these requirements are met in full. We use FTIR spectroscopy, e.g. in plastics analytics. There it is an important method for the identification of polymers. Along with the material characteristics, we are also able to determine and quantify decomposition products, additives and filler materials. For this purpose, we use not only state-of-the-art FTIR spectrometers, we are also able to combine this measurement method with others such as RAMAN spectroscopy or TG pyrolysis.
For the analysis of particulate contamination, we combine FTIR spectroscopy with light microscopic and other methods and, as a consequence, are able to make statements about the type of particle, their quantity and size class, their definition and the damage they can cause.
We are also able to analyse filmic contamination samples with the aid of FTIR spectroscopy. Here we can determine the contamination both qualitatively and quantitatively directly on the surface of the component.
Using FTIR spectroscopy, we identify deposits, residues and inclusions.
Are you uncertain whether an analysis using FTIR spectroscopy would be suitable for your samples? Then simply contact us, we would be pleased to advise you – of course without any obligation.
Design of a FTIR spectrometer
A FTIR spectrometer is basically a Michelson interferometer: it uses a black body as the radiation source in the IR spectrum (not in the optical spectrum). The beam generated is steered continuously along a beam path to a beam splitter that splits the beam into two separate beams. One of the two beams is incident on a moving mirror and is reflected by this mirror, the other beam is reflected by a fixed mirror. In this way the light is directed back to the beam splitter. The two beams are superimposed on each other behind the beam splitter producing interference. The interference is greatest if both mirrors are at the same distance from the beam splitter.
The light must then be routed to the sample to be analysed. There the molecules absorb part of the light. The part that is not absorbed is then acquired by a detector and depicted as an interferogram. The interferogram produced in this manner must then be converted into a spectrum. This is not a technical procedure, but instead a mathematical procedure during which the eponymous Fourier transformation is used.
Which measuring modes are possible on a FTIR spectrometer?
A differentiation is made between three different measuring modes for FTIR spectroscopy:
Transmission method
The transmission method does not require any further preparation of the spectroscope. The only possible preparation is that it is often necessary to prepare the sample in film, gauze or similar. Then the sample is placed in the IR beam and the measurement undertaken as described above.
This type of measurement is suitable for, among others, thermoplastic and water-soluble polymers, polymer films, powders, gases and liquids. Because the transmission method is also the most traditional form of measurement, there exist many standard work instructions with the aid of which it is easy to implement quantitative measurements.
Attenuated total reflection (ATR)
For an ATR measurement, an opaque crystal is required as an accessory to the standard design of the spectroscope. The IR beam is reflected inside this crystal when it is incident on the crystal. These internal reflections generate evanescent waves that are in turn absorbed by the sample. The consequence is an attenuation of the evanescent waves in the area of the spectrum absorbed by the sample. The attenuated beam is incident on the detector on the other side of the crystal.
Because the intensity of the evanescent waves drops with increasing distance from the surface of the crystal, the thickness of the sample is irrelevant for the ATR method; for this reason it is the method of choice for thick and/or heavily absorbing samples. This is particularly the case for solids such as laminates, paints, plastics or rubber as well as liquids and biological samples.
Reflection
With the reflection method (real directed reflection or reflection/absorption) the energy that passes through the sample is not measured, instead the energy that is reflected by the sample is measured. Each sample is characterised by a specific refractive index; the rate of change of this index during reflection varies in the different frequency bands. By testing the bands with a large change in the refractive index, conclusions can be drawn as to the absorption of the sample.
This very sensitive method provides high-quality data and is therefore used, for example, for analysing metallic surfaces, silicon wafers and in general for reflecting surfaces.
When is which measuring mode most suitable?
The transmission method is the method of choice if the issue is to assess very fine peaks optically because these are clearer using transmission. The ATR measurement, on the other hand, has the advantage that the spectra are linear in relation to the concentration, which makes it particularly suitable for quantitative analyses.
Five advantages of FTIR spectroscopy
Compared to other methods, FTIR spectroscopy offers a series of advantages; for this reason it has become the standard method for IR spectroscopy in recent decades.
1
Throughput advantage
Compared to dispersive spectrometers, the light throughput is significantly higher because round apertures can be used in FTIR spectrometers instead of slit apertures. As a consequence the light output increases by up to 200 times, which in turn results in a significant improvement of the signal to noise behaviour.
2
Greater accuracy
Because for FTIR spectroscopy a helium-neon laser is used as a reference, the accuracy of the frequency and wavelength axes is significantly higher than for classic dispersive infrared spectroscopy.
3
Multiplexing advantage
Because an interferometer is used during FTIR spectroscopy instead of a diffraction grating, all wavelengths in the pre-defined spectral range are measured at the same time. This situation again improves the signal to noise ratio, also thanks to this characteristic so-called fast scanning FTIR spectrometers can be designed that permit significantly faster measurements.
4
Higher speed
Thanks to the advantage of multiplexing, scans with the FTIR spectrometer can be undertaken in fractions of a second. This feature permits not only the quick analysis of numerous samples one after the other, it also permits the study of dynamic processes in realtime.
5
Robustness
Modern FTIR spectrometers are comparatively robust, compact units, for this reason they are also suitable for mobile use.
FTIR spectroscopy in practice
Due to its specific advantages, FTIR spectroscopy is used above all in the chemical analytics of organic substances because it makes possible statements about the presence and concentration of functional groups.
Pharmaceutical and plastics industry
Both in the pharmaceutical and in the plastics industry, the determination of the structure of the molecules in the product plays a major role. Also an assessment of the constituents as well as the distribution of solids in mixtures is possible. However, FTIR spectroscopy is also used for the detection of contamination and inclusions or foreign objects in tablets. For this purpose the spectrum of the sample is compared to databases and references. Mapping measurements then permit the determination of the distribution of an active substance in the sample of the medicine.
In short: along with the determination of the structure of unknown substances, FTIR spectroscopy plays a vital role in the quality control of chemical and pharmaceutical products.
Automotive industry
FTIR spectroscopy plays a major role in the automotive sector during the measurement of exhaust emissions from combustion engines and other sources. The reason for this usage is the possibility of analysing exhaust gases for all relevant parameters (NO, NO2, NH3, CO2, etc.) at the same time. Only the concentration of hydrocarbons cannot be determined using this method.
Closely related to the analysis of exhaust gases in general is the specific measurement of the formaldehyde content of automotive exhaust gases. For this purpose the exhaust gas is allowed to flow through a measuring cell on which the spectrometer is aligned. From the attenuation of specific wavelengths it is possible to determine the formaldehyde content directly - this action is not possible with any other physical or chemical method for the measurement of formaldehyde.
Foodstuff monitoring
The determination of the genus and to some extent even the species of microorganisms in samples is an important application for FTIR spectroscopy. But how can microorganisms be identified with the aid of infrared light? Direct identification is of course not possible, instead the spectrum measured is compared to the spectra of known samples from special databases. The genus or species of the microorganism in the sample analysed can be identified from matching spectra.
This method is used widely above all in foodstuff monitoring, in particular if routes of infection need to be identified for food intoxication.
Primary applications at Quality Analysis
FTIR spectroscopy is suitable for numerous applications. Primarily we use this spectroscopy for the analysis of filmic contamination, particle analysis and in plastics analytics.
Filmic contamination
FTIR spectroscopy is often used for the verification of filmic contamination such as greases, oils, cleaning agents, lubricants or corrosion inhibitors on the surface of components. Here the presence of the contamination can not only be verified, but also unambiguously identified with the aid of reference databases.
Particle analysis
Particle analysis is these days able to undertake fully automatically a spectrum comparison that provides information about the type of particle (fibre, plastic, etc.), the quantity, size and distribution of the particles and also the chemical composition and the damage the particles can cause. For this purpose spectroscopic methods are combined with microscopic analytical methods.
Plastics analytics
The specific vibration of a molecule produces a spectrum that is so characteristic that it can be termed a "molecular fingerprint". These characteristic spectra are again compared to a database in plastics analytics such that it is possible to identify unknown substances (e.g. plastics, fibres, particles). In addition, quantitative determination and verification of contamination are possible.
