Dispersive instruments, infrared

In-situ Fourier transform infrared spectroscopy. The final technique in this section concerns the FTIR approach which is based quite simply on the far greater throughput and speed of an FTIR spectrometer compared to a dispersive instrument. In situ FTIR has several acronyms depending on the exact method used. In general, as in the EMIRS technique, the FTIR-... 

Barbillat, J. and Da Silva, E., Near infrared Raman spectroscopy with dispersive instruments and multichannel detection, Spectrochim. Acta A, 53, 2411, 1997. 

Let us cite an example to help us judge the equivalence between Fourier and dispersive instruments. A grating spectrometer employing a four-passed 8 x 104-line grating in the first order has a resolving power of 4 x 8 x 104 = 3.2 x 105. At 3200 cm -1 in the near infrared, this instrument has a Rayleigh resolution of 10" 2 cm- L The same resolution can be achieved by a Fourier... 

All of the usual sampling techniques used in infrared spectroscopy can be used with FT-IR instrumentation. The optics of the sampling chamber of commercial FT-IR instruments are the same as the traditional dispersive instruments so the accessories can be used without modification for the most part. To make full use of the larger aperature of the FT-IR instrument, some accessories should be modified to accomodate the larger beam. The instrumental advantages of FT-IR allow one to use a number of sampling techniques which are not effective using dispersive instrumentation. Transmission, diffuse reflectance and internal reflectance techniques are most often used in the study of epoxy resins. 

The overall simplicity of an FT-IR compared to a dispersive instrument is also an advantage. For example, a single instrument can be easily converted to study the near, mid or far-infrared frequency region whereas with the dispersive method, three totally different instruments are required. To improve resolution with an FT-IR instrument, the basic design is only slightly modified while for a dispersive instrument different optical components are required. 

This instrument has evolved from ihe laboratory spectrophotometer to satisfy the specific needs of industrial process control. While dispersive instruments continue to be used in some applications, the workhorse infrared analyzers in process control are predominantly nondispersive infrared (NDIR) analyzers. The NDIR analyzer ean be used for either gas or liquid analysis. For simplicity, the following discussion addresses the NDIR gas analyzer, hut it should be recognized that the same measurement principle applies to liquids. The use of infrared as a gas analysis technique is certainly aided by the fact that molecules, such as nitrogen (N ) and oxygen tO , which consist of two like elements, do not absorb in the infrared spectrum. Since nitrogen and oxygen are the primary constituents of air. it is frequently possible to use air as a zero gas. 

The introduction of commercial Fourier transform (FT) spectrometers in the early 1960 s has made it possible, in part, to overcome the limitations associated with dispersive instruments and has helped to broaden the scope of problems amenable to investigation by infrared spectroscopy. The purpose of this review is to compare the performance of FT and dispersive spectrometers and to illustrate areas of application in which FT spectroscopy has proven advantageous for the study of adsorbed species. In view of these objectives only a limited treatment of the theory underlying FT spectroscopy will be presented here. 

Fourier transform infrared spectroscopy (FTIR) has provided support to a number of areas in Diamond Shamrock s pesticide program. Commercially available FTIR spectrometers offer a number of advantages over dispersive instruments. Although some of the advantages are related to the ability to perform computerized data manipulations, the basic design of the FTIR system does provide superior capabilities in infrared spectroscopy (1). ... 

The relative absorbance changes AA/A in the infrared as a result of protein activity are on the order of 10 - to lO underneath a high background absorbance of up to 1. FT instruments have a number of distinct advantages over dispersive instruments. Dispersive instruments such as those shown in Fig. 6.6-2, which measure wavenumbers sequentially are not sufficiently stable during the entire measuring time to provide complete, high quality difference spectra. 

Between the source and the detector is put either monochromators used in dispersive instruments or interferometers used in Fourier transform infrared (FT-IR) instruments. In a dispersive instrument the intensity at each wavenumber is measured one by one in sequence and only a small spectral range falls on the detector at any one time. In a FT-IR instrument the intensities of all the wavenumbers are measured simultaneously by the detector. Fourier transform infrared spectroscopy offers some advantages compared to dispersive instruments, namely (i) higher signal-to-noise ratios for spectra obtained under conditions of equal measurement time, and (ii) higher accuracy in frequency for spectra recorded over a wide range of frequencies. Therefore we will give below a brief picture of the principle of FT-IR spectroscopy, based on a Michelson interferometer (Fig. 2). 

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. 

The interferometer used in a non-dispersive instrument is a device that divides the beam of radiation into two paths and recombines the two beams after a path difference has or has not been introduced. The basic concept of the interferometer was introduced by Michelson almost a century ago (Fig. 2). It consists of a stationary mirror, a moving mirror, and a beam splitter. The radiation from the infrared source is divided at the beam splitter half the beam is passed to a fixed mirror and the other half is reflected to the moving mirror. The two beams are later recombined at the beam splitter and passed through the sample to the detector. For any particular wavelength, the... 

When Fourier transform infrared (FTIR) spectrometers first appeared on the market in the early 1970s, they were bulky and expensive (more than 100,000) and required frequent mechanical adjustments. For these reasons, their use was limited to special applications in which their unique characteristics (great speed, high resolution, high sensitivity, and excellent wavelength precision and accuracy) were essential. Currently, however, FTIR spectrometers have been reduced to benchtop size and have become very reliable and easy to maintain. Furthermore, the simple models are now priced similarly to simple dispersive spectrometers. Hence, FTIR spectrometers are largely displacing dispersive instruments in most laboratories. 

Fourier transform IR instruments contain no dispersing element, and all wavelengths are detected and measured simultaneously. Instead of a monochromator, an interferometer is used to produce interference patterns that contain the infrared spectral information. The. same types of sources used in dispersive instruments are used in FTIR spectrometers. Transducers are typically triglycine sulfate—a pyroelectric transducer—or mercury cadmium telluride—a photoconductive trans-... 

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