Commercially available IR instruments

As noted, IR instruments fall into several categories that range from simple photometers (single or multiple filter devices) to relatively complex full-spectrum devices, such as FTIR instruments. Today s process engineers often prefer optical methods of measurement rather than the traditional chromatographs because of perceived lower maintenance and ease of implementation. However, the final selection is often based on the overall economics and the practicality of the application. 

For the measurement of a single chemical entity, analyzers in the form of NDIR analyzers have been in use for decades for both industrial and environmental monitoring applications. These featnre one or more wavelength specific devices, usually optical filters, customized for the specific analyte, snch as CO, CO2, hydrocarbons, moisture, etc. This type of instrument is normally inexpensive (on average around 5000) and relatively easy to service, dependent on its design. This class of instrument is by far the most popular if one reviews the instrumentation market in terms of nsage and the number of analyzers installed. 

Filter-based instruments are often limited to applications where there is simple chemistry, and where the analytes can be differentiated clearly from other species or components that are present. Today, we may consider snch analyzers more as sensors or even meters, and the analytical instrument community does typically not view them as trne instraments. Since the late 1980s a new focns on instrumentation has emerged based on the use of advanced measnrement technologies, and as such is considered to be more of the con-seqnence of an evolution from laboratory instruments. Some of the first work on full-spectrum analyzers started with an initial interest in NIR instruments. The natnre of the spectral information obtained in the NIR spectral region is snch that an analyzer capable of measnring multiple wavelengths or preferably a fnll spectrnm is normally reqnired. 

In the ensuing sections the basic instrument technologies will be discussed as used in commercial instrumentation. A section will be included that reviews some of the emerging technologies that are used either as replacements or substitutes for the standard technologies, often for very specific applications. 

Traditional optical spectrometers for both mid-IR and NIR were based on a scanning monochromator. This design features a single source and detector, and a mechanically scanned dispersion element in combination 

Conventional wisdom leans toward a simple measurement system featuring one or two optical elements and with a simple construction. In practice there are many forms of filter assemblies that can be used. These include simple interference filters, filter clusters or 

Tunable filters in the form of AOTF devices, liquid crystal tunable filter (LCTF) and also tunable cavity Fabry-Perot etalon (FPE) devices have been considered in non-moving part instrument designs for many years. Today, the AOTF and the LCTF devices are used in the NIR spectral region.9,10 Originally, designs were also proposed for mid-IR AOTF devices, but these have not become available, mainly because of fabrication issues (cost and material purity). Tunable FPE devices, which are really just variable cavity interference filters, have been developed for the telecommunications industry. While these have been primarily used in the NIR, in most cases they can be fabricated to work also in the mid-IR, the latter being only an issue of material/substrate selection. 

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