FTIR Technology



Fourier-Transform Infra-Red spectroscopy is a full-spectrum analytical technique that allows all IR absorbing species to be detected and measured by a single instrument. Protea has been at the forefront of FTIR technology in process and emissions measurement and prides itself on making the relatively complex operation of FTIR as simple and useful as possible. The make-up of an FTIR analyser is of a modular design – infra-red source, interferometer, sample compartment, detector and control/processing electronics, including PC. Each component part can be tuned or altered giving difference performance for certain applications; however the overall principal is the same.

The infra-red light from the source is modulated by the interferometer. This device allows for the light to be split into two different paths and recombined, producing an interference wave known as an interferogram. The light is split via an optical device known as a beamsplitter. The use of a monochromatic or single wavelength light source, typically a laser, is used to provide a reference signal in the interferometer. Measurement of the interference pattern of the single wavelength allows the speed of interferometer’s mirror movement and alignment to be controlled precisely.

The light is passed through a sample compartment or other sampling accessory. In transmission/absorption measurements of liquids and gases this is often referred to as the sample cell or gas cell. Sample cells can be of various designs in order to achieve the most suitable pathlength i.e. total length that the IR light passes through the absorbing medium. For long pathlengths (of the order of meters) this usually involves the use of mirror arrangements to bounce the light through the sample medium. As the sample cell contains the extracted sample medium, care has to be taken that the sample cell is constructed of suitable materials and operates at the required temperature and pressure.

An infra-red detector and associated electronics are required to make single-point measurements of the infra-red signal as the interferometer scans. Various flavours of detector are available, some running at room temperature and others requiring cooling. Again, the most suitable detector for the application should be used.

An FTIR analyser does not directly produce a spectrum for analysis; an interferogram is produced. This is time-domain measurement of IR signal and contains the modulated wave of the entire broad band source. To extract the IR spectrum a mathematical manipulation called a Fourier Transform must be applied to the interferogram. The mathematics of this are all handled in software in real-time.

The resulting single-beam or intensity spectrum is then compared against a zero or background spectrum to produce an absorbance spectra. This absorbance spectrum is what we need to run a spectral analysis, applying Beer’s Law. Beer’s Law describes the linear relationship between IR absorbance and concentration. We have to make sure we keep other variables such as temperature, pressure and pathlength the same. With absorbance spectra collected and saved, it is a matter of applying suitable chemometric techniques to extract concentration information.

Protea will always take a fresh approach to any new application. We do not take the stance that a certain resolution, pathlength, detector etc. be better than any other. We will use all the tools and knowledge at our disposal to deliver the best measurement system for your application.

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