Fourier Transform Infrared spectroscopy (FTIR). FTIR is an advanced type of IR technique which measures all infrared frequencies simultaneously and can be used for both quantitative and qualitative measurements. Fourier transform infrared (FTIR) spectrometers are cheaper than conventional spectrometers because building of interferometers is easier than the fabrication of a monochromator. Measurement of a single spectrum is faster for the FTIR technique because the information at all frequencies is collected simultaneously. This allows multiple spectral absorption samples to be collected and averaged together resulting in an improvement in sensitivity. Today, virtually all modern IR spectrometers are FTIR instruments favoured for their speed, high resolution, sensitivity, and unparalleled wavelength precision and accuracy. The majority of commercially available Fourier transform infrared instruments are based on the Michelson interferometer shown in Figure 4-2. Michelson interferometers provide a significant sensitivity advantage over grating, prism, and circular variable filter (CVF) spectrometers, as the interferometer is not limited in aperture (slit width or height) as severely as dispersive or CVF instruments, giving a higher light gathering capability and a larger throughput.
Encoding of the infrared spectra is here accomplished by splitting the source into two beams whose path lengths can be varied periodically to give interference patterns. The interferogram is a measurement of the temporal coherence of the light at each different time delay setting. With the use of Fourier transformations it is possible to convert a signal in the time domain to the frequency domain (i.e. the spectrum). The fundamental measurement obtained by an FTIR is made in the time domain, which is Fourier transformed to give a spectrum of absorbance plotted against wavenumbers. Figure 4-3 shows the basic compounds of an FTIR instrument. After passing through the interferometer, the IR radiation passes through the sample before entering the detector from which the signal can be encoded into the resulting spectrum.
The path length of the sample cell is proportional to the measured absorbance of the resulting spectrum. The dependencies of the measured absorbance is: The measured absorbance of the FTIR-spectrum is the logarithm of the inverse transmittance T and the absorption from a single compound is typically linearly dependent on the concentration c, the length l, and the absorption coefficient of the compound, which characterizes the capacity of the sample to absorb infrared radiation. Above absorbance units of 1 (10 % of radiation transmitted through the sample), the compounds in general do not obey Beer-Lambert law as the molar absorptivity is no longer a linear function of concentration.
The most common method for online measurements of gas by FTIR is performed by an extractive method, where gas is sampled through a heated sampling line for gas transfer to a sample cell for analysis. In order to avoid condensation of the gas, the sample line and the gas cell is heated to a temperature of typically 180°C. Non-extractive methods for gas analysis are also possible, like the open-path FTIR method which is an optical remote sensing technique which consists of transmitting an infrared beam from less than 10 and up to several hundred meters across the atmosphere prior to the detector. Figure 4-3. The principles for an open-path instrument is the same as shown in , but the closed sample cell is here replaced by a light path through open air, often via a remote reflector. Open path FTIR has the advantage of potentially very low detection limits due to the long path length available for IR absorption. The forming of mist is a limitation for the open path method, as the IR beam depends on a clear line of sight to the detector.