Principles of Fourier Transfer Infrared Spectroscopy

 

The application of traditional infrared spectroscopy to low concentration measurements, such as ambient air measurements, is limited by several factors. First is the significant presence of water vapour, CO 2 and methane, which strongly absorb in many regions of the infrared (IR) spectrum. Consequently, the spectral regions that can easily be used to search for pollutants are limited to 760-1300cm -1 , 2000-2230 cm -1 , and 2390-3000 cm -1 . Another problem is that the sensitivity is not enough to detect very small concentrations in the sub-ppm level. Finally, spectral analysis was difficult since subtraction of background spectra had to be carried out manually.

 

The development of Fourier Transform InfraRed spectroscopy (FTIR) in the early 1970s provided a quantum leap in infrared analytical capabilities for monitoring trace pollutants in ambient air. This technique offered a number of advantages over conventional infrared systems, including sensitivity, speed and improved data processing.

 

The basic components of an FTIR are shown schematically in Figure 1 . The infrared source emits a broad band of different wavelength of infrared radiation. The IR source used in the Temet GASMET FTIR CR-series is a SiC ceramic at a temperature of 1550 K. The IR radiation goes through an interferometer that modulates the infrared radiation. The interferometer performs an optical inverse Fourier transform on the entering IR radiation. The modulated IR beam passes through the gas sample where it is absorbed to various extents at different wavelengths by the various molecules present. Finally the intensity of the IR beam is detected by a detector, which is a liquid-nitrogen cooled MCT (Mercury-Cadmium-Telluride) detector in the case of the Temet GASMET FTIR CR-series. The detected signal is digitised and Fourier transformed by the computer to get the IR spectrum of the sample gas.

Figure 1: Basic components of FTIR

 

The unique part of an FTIR spectrometer is the interferometer. A Michelson type plane mirror interferometer is displayed in Figure 2 . Infrared radiation from the source is collected and collimated (made parallel) before it strikes the beamsplitter. The beamsplitter ideally transmits one half of the radiation, and reflects the other half. Both transmitted and reflected beams strike mirrors, which reflect the two beams back to the beamsplitter. Thus, one half of the infrared radiation that finally goes to the sample gas has first been reflected from the beamsplitter to the moving mirror, and then back to the beamsplitter. The other half of the infrared radiation going to the sample has first gone through the beamsplitter and then reflected from the fixed mirror back to the beamsplitter. When these two optical paths are reunited, interference occurs at the beamsplitter because of the optical path difference caused by the scanning of the moving mirror.

Figure 2: Michelson interferometer

 

The optical path length difference between the two optical paths of a Michelson interferometer is two times the displacement of the moving mirror. The interference signal measured by the detector as a function of the optical path length difference is called the interferogram. A typical interferogram produced by the interferometer is shown in Figure 3 . The graph shows the intensity of the infrared radiation as a function of the displacement of the moving mirror. At the peak position, the optical path length is exactly the same for the radiation that comes from the moving mirror as it is for the radiation that comes from the fixed mirror.

Figure 3: A typical interferogram

 

The spectrum can be computed from the interferogram by performing a Fourier transform. The Fourier transform is performed by the same computer that ultimately performs the quantitative analysis of the spectrum.

 

The degree of absorption of infrared radiation at each wavelength is quantitatively related to the number of absorbing molecules in the sample gas. Since there is a linear relationship between the absorbance and the number of absorbing molecules, multicomponent quantitative analysis of gas mixtures is feasible.

 

To perform multicomponent analysis we start with the sample spectrum. In addition, we need reference spectra of all the gas components that may exist in the sample, if these components are to be analysed. A reference spectrum is a spectrum of one single gas component of specific concentration. In multicomponent analysis we try to combine these reference spectra with appropriate multipliers in order to get a spectrum that is as close as possible to the sample spectrum. If we succeed in forming a spectrum similar to the sample spectrum, we get the concentration of each gas component in the sample gas using the multipliers of the reference spectra, provided that we know the concentrations of the reference gases.

 

For example, suppose we have a sample spectrum and reference spectra like those shown in Figure 4 . In this case, we know that the sample gas consists of gases Reference 1 and Reference 2. We have the reference spectra available and we know that these reference spectra represent concentrations of 10 ppm and 8 ppm respectively. To find out the concentration of each component in the sample gas, we try to form the measured sample spectrum using a linear combination of the reference spectra. We find out that if we multiply the spectrum Reference 1 by 5 and the spectrum Reference 2 by 2, and combine these two spectra, we get a spectrum that is similar to the sample spectrum. Accordingly, the sample gas contains reference gas 1 at five times the amount in the reference spectrum 1, and reference gas 2 at two times the amount in the reference spectrum 2. The analysis indicates that the sample indeed consists of these two reference gases. The concentration of the reference gas 1 in the sample is found to be 50 ppm, and the concentration of the reference gas 2 in the sample is 16 ppm.

Figure 4: An example of spectra for multicomponent analysis

 

This multicomponent ability of FTIR means that theoretically, any spectrum obtained with the FTIR can be reprocessed at a future date to determine the concentration of any newly calibrated gases. Therefore it is worth saving the spectra obtained from FTIR since they potentially contain so much information about the sample gas.

More information from the manufacturer's website can be found here where you can download a pdf document all about the theory of FTIR spectroscopy.