Infrared spectrometer monochromator

Fourier transform infrared spectrometers first appeared in the 1970s. These single beam instruments, which differ from scanning spectrometers, have an interferometer of the Michelson type placed between the source and the sample, replacing the monochromator (Figs 10.9c and 10.11). 

An infrared spectrometer measures the frequencies of infrared light absorbed by a compound. In a simple infrared spectrometer (Figure 12-4), two beams of light are used. The sample beam passes through the sample cell, while the reference beam passes through a reference cell that contains only the solvent. A rotating mirror alternately allows light from each of the two beams to enter the monochromator. 

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. 

Detailed experimental procedures for obtaining infrared spectra on humic and fulvic acids have been reported previously 9,22,25-26) and will be briefly described here. Infrared spectra were taken on the size-fractionated samples by using a Fourier transform infrared spectrometer (Mattson, Polaris) with a cooled Hg/Cd/Te detector. Dried humic and fulvic materials were studied by diffuse reflectance infrared spectroscopy (Spectra Tech DRIFT accessory) and reported in K-M units, as well as by transmission absorbance in a KBr pellet. Infrared absorption spectra were obtained directly on the aqueous size-fractioned concentrates with CIR (Spectra Tech CIRCLE accessory). Raman spectra were taken by using an argon ion laser (Spectra-Physics Model 2025-05), a triple-grating monochromator (Spex Triplemate Model 1877), and a photodiode array detector system (Princeton Applied Research Model 1420). All Raman and infrared spectra were taken at 2 cm resolution. 

The crystal structure of as prepared samples was identified by using a powder X-ray diffractometer equipped with CuKa radiation (30kV, 20mA) and a monochromator. An infrared spectrometer was used for the chemical structure analysis. Chemical composition of samples was determined by EDX analysis. To determine the content of organic species in the composites, thermal gravimetric (TG) analysis was carried out at a heating rate of 10 °C/min in air. The BET surface area was determined by measuring N2 adsorption isotherms at 77 K. The microstructure of samples was observed by FE-SEM. Diffuse reflectance spectra were recorded with a UV-vis spectrometer. 

Infrared spectra are measured by special instruments called infrared spectrometers. These instruments measure the differences in the intensity of the infrared light of a certain wavelength that penetrates into the sample and goes out from the sample. The most important parts of the infrared spectrometer are light source, which produces an intensive infrared radiation monochromator detector... 

The next important part of an infrared spectrometer is the sample chamber. The sample chamber is used for placing the cuvette that contains the sample or for placing any other accessory that contains the sample. The sample chamber is installed between the infrared light source and the monochromator. 

Modern monochromators consist of a rift system, the optics and the infrared radiation splitting system, which is usually presented by prism or diffraction grid. The following two types of monochromators are most popular in modern infrared spectrometers ... 

Most of the component parts used in Raman spectroscopy such as the monochromator and sample chamber have the same functioning principle as in infrared spectrometers. All these were described in detail in section 2.2.1. 

All the component parts used in photometers have the same working principle as those already described in other spectrometers, for example, the infrared spectrometer. The prism and refraction grids are used as monochromators. The detector is usually made of different types of photoresistors depending on the instrument type. 

Conventional infrared spectrometers are known as dispersive instruments. With the advent of computer- and microprocessor-based instruments, these have been largely replaced by Fourier transform infrared (Fllk) spectrometers, which possess a number of advantages. Rather than a grating monochromator, an FTIR instrument employs an interferometer to obtain a spectrum. 

The dispersive element is contained within a monochromator. Figure 2.3a shows the optical path of an infrared spectrometer which uses a grating monochromator. Dispersion occurs when energy falling on the... 

Figure 2.3a The optical path of a double-beam infrared spectrometer with a grating monochromator...

Also shown in Fig. 1 is the location of the reflectance infrared spectrometer relative to the vacuum chamber. The spectrometer consists of a double beam goniometer which accurately controls the angle of incidence on the reflectance sample and reference, and a Spex Industries Incorporated Model 1701, 3/4 meter monochromator. 

The basic component of most Fourier Transform Infrared spectrometers is the Michel son interferometer. This is not the only interferometer used in FT-IR, but it is employed more often than other designs. A treatment of many other interferometer designs is available. The Michel son interferometer in a Fourier Transform Infrared spectrometer replaces the monochromator in a dispersive instrument, although the functions cannot be correlated. A monochomator divides a continuous bandwidth into its component frequencies, whereas an interferometer produces interference patterns of the bandwidth in a precise and regulated manner. It should be noted that this type of interferometer is not restricted to the infrared region and its use can be extended to the visible and millimeter regions of the electromagnetic spectrum. 

Commercial infrared spectrometers operate either on the dispersive or, less frequently, the interferometric principles. In the dispersive instruments, a source of infrared radiation passes through a sample, is dispersed into its frequencies by a monochromator, and the relative intensities of individual frequencies measured by a detector are displayed on a stripchart recorder. Gratings, rather than prism monochromators, are used nowadays as dispersive devices in the IR region (Kemp, 1975). 

Perkin-Elmer introduced in 1975 the first microprocessor-controlled commercial dispersion infrared spectrometer, and advantage was taken of this facility for lipid studies a few years later (Chapman et aL, 1980). The monochromator is synchronized with the recorder drive by the abscissa microprocessor for accurate reproduction of wavenumber settings. The source radiation is split in two, one beam passing through the sample, whilst the other serves as a... 

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