Infrared instruments

Infrared instruments using a monochromator for wavelength selection are constructed using double-beam optics similar to that shown in Figure 10.26. Doublebeam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than that for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and 1420 vapor when using double-beam optics. Resolutions of 1-3 cm are typical for most instruments. 

Beyond the complexities of the dispersive element, the equipment requirements of infrared instrumentation are quite simple. The optical path is normally under a purge of dry nitrogen at atmospheric pressure thus, no complicated vacuum pumps, chambers, or seals are needed. The infrared light source can be cooled by water. No high-voltage connections are required. A variety of detectors are avail-... 

Collecting optics, radiation detectors and some form of indicator are the basic elements of an industrial infrared instrument. The optical system collects radiant energy and focuses it upon a detector, which converts it into an electrical signal. The instrument s electronics amplifies the output signal and process it into a form which can be displayed. There are three general types of instruments that can be used for predictive maintenance infrared thermometers or spot radiometers line scanners and imaging systems. 

This type of infrared instrument provides a single dimensional scan or line of comparative radiation. While this type of instrument provides a somewhat larger field of view, i.e. area of machine surface, it is limited in predictive maintenance applications. 

Point-of-use infrared thermometers are commercially available and relatively inexpensive. The typical cost for this type of infrared instrument is less than 1,000. Infrared imaging systems will have a price range between 8,000 for a black and white scanner without storage capability to over 60,000 for a microprocessor-based, color imaging system. 

One very important group of infrared instruments consists of spectrometers used for quantitative measurements either as part of a continuous industrial monitoring process or for environmental studies. These instruments are normally purpose-made, dedicated machines designed to run virtually automatically, and are normally intended only to measure a single compound or family of compounds. 

We will begin with the next chapter with an analysis of the effect of one of the most common cases constant detector noise, typical of mid-infrared and near-infrared instruments. 

The pharmaceutical industry comprises the largest segment, roughly 15 to 20%, of the infrared (IR) market. Modern mid-infrared instrumentation consists almost exclusively of Fourier transform (FT) instruments. Because of its ability to identify molecular species, FT-IR is routinely used as an identification assay for raw materials, intermediates, drug substances, and excipients. However, the traditional IR sample preparation techniques such as alkali halide disks, mulls, and thin films, are time-consuming and not always adequate for quantitative analysis. 

C is present in 1.1% natural abundance and so approximately n% of a M (CO) complex is isotopically labelled. General infrared instrumental improvements have made studies on such natural-abundance species perfectly feasable, for most i>(CO) infrared peaks are rather narrow in hydrocarbon solvents. The commercial availability of 13 CO and C180 (substitution by either of which has very similar frequency perturbation consequences) has led to studies of isotopically enriched species. Perhaps because of the emphasis on studies of v(CO) vibrations, little work has been reported... 

The principal reasons for choosing Fourier transform infrared spectroscopy are first, that these instruments record all wavelengths simultaneously and thus operate with maximum efficiency and, second, that Fourier transform infrared spectroscopy spectrometers have a more convenient optical geometry than do dispersive infrared instruments. These two facts lead to the following advantages. 

Fourier transform infrared instruments provide a more convenient beam geometry—circular rather than slit shaped—at the sample focus. 

D.L. Wetzel, Contemporary near-infrared instrumentation, in Near-Infrared Technology in the Agricultural and Food Industries, P. Williams and K. Norris (eds), 2nd edn, American Association of Cereal Chemists, Inc., St. Paul, MN, 2001. 

In laboratory tests conducted at the Technical Research Center of Finland (6) comparisons were made between the 3M Monitor and charcoal tubes (150 mg). The tests were monitored by both gas chromatography and Miran IA infrared instruments. 

Since the article by Spedding1 on infrared spectroscopy and carbohydrate chemistry was published in this Series in 1964, important advances in both infrared and Raman spectroscopy have been achieved. The discovery2 of the fast Fourier transform (f.F.t.) algorithm in 1965 revitalized the field of infrared spectroscopy. The use of the f.F.t., and the introduction of efficient minicomputers, permitted the development of a new generation of infrared instruments called Fourier-transform infrared (F.t.-i.r.) spectrophotometers. The development of F.t.-i.r. spectroscopy resulted in the setting up of the software necessary to undertake signal averaging, and perform the mathematical manipulation of the spectral data in order to extract the maximum of information from the spectra.3... 

Most available infrared instruments use the Littrow mount for the prism, the beam being reflected from a plane mirror behind the pnsm and thus returning it through the prism a second time. This doubles the dispersion produced. Actually, a double-pass system is also used so that the beam goes through the pnsm four times. Other design modifications include those with single beam and double monochromator, double beam and double monochromator, and related combinations, See also Infrared Radiation. 

Here, / and 70 represent the transmission of the infrared instrument with and without the sample in place, respectively. It is convenient to define the dimensionless ratio of the VCD absorbance to the ordinary absorbance as ... 

As in the case of the infrared instrument described in the preceding subsection, multiple points can be hooked to one analyzer and sampled periodically.11... 

Searching the spectrum of an unknown chemical against a spectral library is a routine method used to identify chemicals. Most of the commercial infrared instruments include library search software that has several search algorithms to choose from. The search algorithm can sometimes have a strong effect on the library search result. This is due to the different ways the actual comparison between the spectra is done. Especially when the library and the unknown spectra have been measured differently (e.g. using solid KBr disk and cryodeposition GC/FTIR), the... 

Feam, T., Standardization and calibration transfer for near infrared instruments a review, J. Near Infrared Spectrosc., 9, 229-244, 2001. 

In considering the three most prominent analytical techniques for quartz, i.e. colorimetric, infrared (IR) and x-ray diffraction, it was possible to immediately exclude the x-ray diffraction procedure for use because the instrument was not available and the cost of purchasing such a unit was considered to be prohibitive in view of the relatively small number of samples anticipated. The Talvite (1) colorimetric procedure has previously been employed without particular success. This method was generally considered to be unacceptably tedious, time-consuming and of questionable accuracy. For these reasons and because the infrared instrumentation was available, it was decided to focus our preliminary efforts on the development of an infrared procedure. 

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