Some systems incorporate a compensating mechanism that automatically adjusts the orientation of one mirror to maintain the alignment. Arrangements that avoid this problem include using cube corner reflectors instead of plane mirrors as these have the property of returning any incident beam in a parallel direction regardless of orientation.
Fourier Transform Infrared Spectroscopy of Polymers—Theory and Application
Systems where the path difference is generated by a rotary movement have proved very successful. One common system incorporates a pair of parallel mirrors in one beam that can be rotated to vary the path without displacing the returning beam. Another is the double pendulum design where the path in one arm of the interferometer increases as the path in the other decreases. A quite different approach involves moving a wedge of an IR-transparent material such as KBr into one of the beams. Increasing the thickness of KBr in the beam increases the optical path because the refractive index is higher than that of air.
One limitation of this approach is that the variation of refractive index over the wavelength range limits the accuracy of the wavelength calibration. The interferogram has to be measured from zero path difference to a maximum length that depends on the resolution required.
In practice the scan can be on either side of zero resulting in a double-sided interferogram. Mechanical design limitations may mean that for the highest resolution the scan runs to the maximum OPD on one side of zero only.
The interferogram is converted to a spectrum by Fourier transformation. This requires it to be stored in digital form as a series of values at equal intervals of the path difference between the two beams. To measure the path difference a laser beam is sent through the interferometer, generating a sinusoidal signal where the separation between successive maxima is equal to the wavelength of the laser typically a nm HeNe laser is used. This can trigger an analog-to-digital converter to measure the IR signal each time the laser signal passes through zero.
Alternatively, the laser and IR signals can be measured synchronously at smaller intervals with the IR signal at points corresponding to the laser signal zero crossing being determined by interpolation.
Fourier Transform Infrared Analysis (FTIR)
The result of Fourier transformation is a spectrum of the signal at a series of discrete wavelengths. The range of wavelengths that can be used in the calculation is limited by the separation of the data points in the interferogram. The shortest wavelength that can be recognized is twice the separation between these data points. For example, with one point per wavelength of a HeNe reference laser at 0.
Because of aliasing any energy at shorter wavelengths would be interpreted as coming from longer wavelengths and so has to be minimized optically or electronically. The wavelengths used in calculating the Fourier transform are such that an exact number of wavelengths fit into the length of the interferogram from zero to the maximum OPD as this makes their contributions orthogonal.
This results in a spectrum with points separated by equal frequency intervals. The separation is the inverse of the maximum OPD. This is the spectral resolution in the sense that the value at one point is independent of the values at adjacent points. Most instruments can be operated at different resolutions by choosing different OPD's. Instruments for routine analyses typically have a best resolution of around 0.
The point in the interferogram corresponding to zero path difference has to be identified, commonly by assuming it is where the maximum signal occurs.
How Does FTIR Work?
This so-called centerburst is not always symmetrical in real world spectrometers so a phase correction may have to be calculated. The interferogram signal decays as the path difference increases, the rate of decay being inversely related to the width of features in the spectrum. If the OPD is not large enough to allow the interferogram signal to decay to a negligible level there will be unwanted oscillations or sidelobes associated with the features in the resulting spectrum. To reduce these sidelobes the interferogram is usually multiplied by a function that approaches zero at the maximum OPD.
This so-called apodization reduces the amplitude of any sidelobes and also the noise level at the expense some reduction in resolution. For rapid calculation the number of points in the interferogram has to equal a power of two. A string of zeroes may be added to the measured interferogram to achieve this. More zeroes may be added in a process called zero filling to improve the appearance of the final spectrum although there is no improvement in resolution. Alternatively, interpolation after the Fourier transform gives a similar result.
There are three principal advantages for an FT spectrometer compared to a scanning dispersive spectrometer. Another minor advantage is less sensitivity to stray light, that is radiation of one wavelength appearing at another wavelength in the spectrum. In dispersive instruments, this is the result of imperfections in the diffraction gratings and accidental reflections. In FT instruments there is no direct equivalent as the apparent wavelength is determined by the modulation frequency in the interferometer.
The interferogram belongs in the length dimension. Much higher resolution can be obtained by increasing the maximal retardation. This is not easy, as the moving mirror must travel in a near-perfect straight line. The use of corner-cube mirrors in place of the flat mirrors is helpful, as an outgoing ray from a corner-cube mirror is parallel to the incoming ray, regardless of the orientation of the mirror about axes perpendicular to the axis of the light beam.
In Connes measured the temperature of the atmosphere of Venus by recording the vibration-rotation spectrum of Venusian CO 2 at 0. The throughput advantage is important for high-resolution FTIR, as the monochromator in a dispersive instrument with the same resolution would have very narrow entrance and exit slits. FTIR is a method of measuring infrared absorption and emission spectra.
For a discussion of why people measure infrared absorption and emission spectra, i. The output is similar to a blackbody. Mid-IR spectrometers commonly use pyroelectric detectors that respond to changes in temperature as the intensity of IR radiation falling on them varies. These detectors operate at ambient temperatures and provide adequate sensitivity for most routine applications.
To achieve the best sensitivity the time for a scan is typically a few seconds. Cooled photoelectric detectors are employed for situations requiring higher sensitivity or faster response. Liquid nitrogen cooled mercury cadmium telluride MCT detectors are the most widely used in the mid-IR. With these detectors an interferogram can be measured in as little as 10 milliseconds.
To ensure that the highest quality of data is obtained several alternative FTIR sample presentation techniques including FTIR microscopy are available covering an extensive range of applications. Get in touch. Find out more. Typical Applications Identification of solid or liquid organic and inorganic compounds Multi-component mixture analysis of solids liquids and gels using spectral subtraction and data mining Identification of polymers, polymer blends, rubbers and filled rubbers, adhesives, coatings, promoters and hardening agents Confirmation of consistency of raw and finished manufacturing materials Surface modification and sample weathering studies Multi-component quantitative analysis of complex mixtures by Partial Least Squares PLS analysis Solvent extraction and identification of manufacturing impurities, metabolites and contaminants Analysis of unknown solvents, cleaning agents and detergents FTIR microscopy for examination of microstructures and manufacturing defects Mapping the consistency of raw and finished products and investigation of product irregularities Mapping coating thickness and cross sectional imaging of cut sections.
Each molecule or chemical structure will produce a unique spectral fingerprint, making FTIR analysis a great tool for chemical identification. A change in the characteristic pattern of absorption bands clearly indicates a change in the composition of the material or the presence of contamination. If problems with the product are identified by visual inspection, the origin is typically determined by FTIR microanalysis.
This technique is useful for analyzing the chemical composition of smaller particles, typically 10 microns, as well as larger areas on the surface. FTIR Spectograph. We have shown that FTIR spectroscopy is a very powerful tool with many applications, however data interpretation is not straightforward.
By nature, the total spectrum generated is a series function of absorbed energy response hence the Fourier Transform portion of the name. The absorbed bands presented in the spectrum are only somewhat discrete and degenerative. The spectrum must be interpreted as a whole system and therefore probably demands the most experienced analysts in all of the spectrographic techniques in correctly characterizing the functionality presented.
Yes there are libraries which can yield lookup information but these libraries are limited in scope and depth compared to the millions of industrial chemicals used, and also will not account for mixtures of chemicals which can yield erroneous search information. Although typically a qualitative tool for material identification, FTIR analysis can also be used as a quantitative tool to quantify specific functional groups, when the chemistry is understood and standard reference materials are available.
The intensity of the absorbance will correlate to the quantity of functionality present in the sample. For instance, we utilize FTIR for quantitative analysis for characterizing the amount of water in an oil sample and the degree of oxidation and nitration of an oil. We have even developed a method for characterizing how paraffinic or naphthenic an oil sample is.