Infrared spectrometer optical components

A typical IR spectrometer consists of the following components radiation source, sampling area, monochromator (in a dispersive instrument), an interference filter or interferometer (in a non-dispersive instrument), a detector, and a recorder or data-handling system. The instrumentation requirements for the mid-infrared, the far-infrared, and the near-infrared regions are different. Most commercial dispersive infrared spectrometers are designed to operate in the mid-infrared region (4000-400 cm ). An FTIR spectrometer with proper radiation sources and detectors can cover the entire IR region. In this section, the types of radiation sources, optical systems, and detectors used in the IR spectrometer are discussed. 

By far the most common use of mid-infrared radiation for process analysis is in the non-dispersive infrared analysers that are discussed below. The widespread use of FTIR spectrometers in the mid-lR has yet to be fully realized in process analytical apphcations. The requirements for the optical components and the wavelength sta-bihty of the instraments available have, until recently, detracted from the use of this region of the spectrum in on-line process analysis. Optical fibers that provide such a benefit to the apphcations of NIR (see below) are not available for the mid-IR in robust forms or forms that are capable of transmitting over more than a few tens of metres. Improvements and developments to sample cells, particularly designs of attenuated total reflectance (ATR) cells, for use with mid-lR are being made and will influence the application of the technique. An impressive list of apphcations including both FTIR and the NDIR approaches has been compiled (2, 3]. 

Fourier Transform-Near infrared (FT-NiR). Only within the last 20 years has FT-NIR instrumentation (Fig. 4.1.14) become available. Even then, the first commercial instmments had a distinct disadvantage compared to grating-based scanning instruments. FT-NIR spectrometers employ an entirely different method for producing spectra. There is no dispersion involved. Energy patterns set up by an interaction with a sample and a reference and moving mirrors (or other optical components) produce sample and reference interferograms that are used to calculate the absorbance spectrum of the sample. 

Because the lower wavenumber limit covered by a conventional FT-IR spectrometer operating in the mid-infrared region is typically 400 cm it is necessary, in order to measure far-infrared spectra, to exchange a few of the main optical components of an FT-IR interferometer these are described here ... 

Optical Components Used in Infrared Spectrometers Specially Designed for External Reflectance Spectroscopy... 

During the 1960s further improvements made infrared spectroscopy a very useful tool used worldwide in the analytical routine laboratory as well as in many fields of science. Grating spectrometers replaced the prism instruments due to their larger optical conductance (which is explained in Sec. 3 of this book). The even larger optical conductance of interferometers could be employed after computers became available in the laboratory and algorithms which made Fourier transformation of interferograms into spectra a routine. The computers which became a necessary component of the spectrometers made new powerful methods of evaluation possible, such as spectral subtraction and library search. 

A diagram of an experimental set-up used for PM IRRAS experiments is shown in Fig. 9.26 and a photograph of the spectroelectrochemical cell in Fig. 9.27. The set-up is built from components purchased from Newport and custom-machined parts in an external tabletop optical mount (TOM) box. A convergent infrared beam from the spectrometer enters a port of the TOM box where it is deflected by a flat mirror and focused onto the working electrode by a parabolic mirror (f=6 in, Nicolet). Before entering the cell, the beam passes through a static polarizer (diameter 1 in, with an anti-reflective coating -... 

Infrared spectrophotometry of ozone based on the principal absorption band near 9.5 pm is relatively free from interference by the bands of other atmospheric constituents. However, a long optical path is necessary for detection of atmospheric ozone. A White cell (multiple reflection cell, 10-1000 m path) combined with a Fourier-transform infrared (FTIR) spectrometer (spectral resolution of lcm or better) with a HgCdTe detector is often used in multi-component air-monitoring and smog chamber experiments. 

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