The Infrared Spectrometer

Until the last twenty years, an infrared spectrometer meant a dispersive instrument this had a grating or prism to split light into its component frequencies, slits to provide the required resolution, and usually a dual sample beam to provide internal referencing of transmission. Today, Fourier transform infrared has almost completely replaced dispersive machines in the analytical laboratory. There are several reasons for this, which to be understood first require a brief introduction to the principles and technology involved. 

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Coleman and Sivy also used an infrared transmission cell to undertake degradation studies under reduced pressure on a series of poly(acrylonitrile) (ACN) copolymers [30-33]. Thin films prepared from a polymer were mounted in the specially designed temperature-controlled cell mounted within the infrared spectrometer. The comparative studies were made on ACN copolymers containing vinyl acetate [30,32], methacrylic acid [30,31] and acrylamide [30,33]. The species monitored was the production of the cyclised pyridone structure. This was characterised in part by loss of C=N stretch (vC = N) intensity at 2,240 cm-1 accompanied by the appearance and increase in intensity of a doublet at 1,610/1,580 cm-1. 

The half-scale response time of the modified infrared electronics was 0.02 s, as opposed to the half-scale response time of about 0.5 s for the infrared spectrometer before the above-described modifications. 

Over the last decade there have been great developments of the infrared spectrometers used in surface science. One has moved from simple, single... 

Describe the apparatus and experiment in your own words. Include a description of the infrared spectrometer and its mode of operation. 

In contrast to previous work (12), in the present paper the D2 exchange is followed continuously with the infrared spectrometer at reaction temperature. Samples were selected to compare the ability for deuteration of all the types of hydroxyl groups reported in synthetic faujasites. 

Here the infrared spectra of a thin film on a silicon wafer is obtained first. Then the absorbance at the Si-H and N-H bond regions is measured by estimating the area within each absorption band. As shown in Figure 34, the absorbance seen by the infrared spectrometer correlates well with the hydrogen concentration. 

Auxiliary Instruments. Auxiliary instruments can be used on the fly as special detectors, or analytes can be trapped and taken to other instruments. Instruments that have been used with chromatography include the mass spectrometer (MS), the infrared spectrometer (IR), the nuclear magnetic resonance spectrometer (NMR), the polarograph, the fluorescence spectrophotometer, and the Raman spectrometer, among others. The two most popular ones are MS and IR, and they will be discussed in more detail in Chapter 11. In the beginning of this chapter we noted the utility iof GC/MS and LC/MS. 

Fig. 7.1. Layout of the infrared spectrometer showing the Michelson Interferometer Optical System. An FTIR spectrometer s optical system requires two mirrors, an infrared light source, an infrared detector and a beamsplitter. The beamsplitter reflects about 50% of an incident light beam and transmits the remaining 50%. One part of this split light beam travels to a moving interferometer mirror, while the other part travels to the interferometer s stationary mirror. Both beams are reflected back to the beamsplitter where they recombine. Half of the recombined light is transmitted to the detector and half is reflected to the infrared source.

In general, tandem systems involving the combination of the liquid chromatograph in-line with the infrared spectrometer have not been very successful and most IR spectra of LC eluents are obtained by what are, in effect, off-line procedures, as in the example given above. The FTIR spectrometer, in its present form, demands too large a sample size and is too insensitive for successful in-line association with modem high-efficiency microbore LC columns. Fortunately, the demand for in-line production of IR spectra from LC eluents is not great and, in most cases, the off-line methods are quite satisfactory for the majority of LC/IR applications. 

Voyager s radio occultations, the infrared spectrometer and the ultraviolet spectrometer experiments, all gave us information about the atmosphere. These data are all consistent with a nitrogen atmosphere in what is called vapor pressure equilibrium. In vapor pressure equilibrium, the gas in the atmosphere comes from the sublimation of ice for the same material frozen on the surface. The amount of gas in the atmosphere is controlled by the temperature of the ice, and the atmosphere acts to keep the ice at a constant temperature by the transport and condensation of the gas from warm to colds areas. Mars primarily carbon dioxide atmosphere is in a similar equilibrium with its polar carbon dioxide caps. 

Fashion a number of mounting brackets from cardboard. The brackets should be of such size that they will fit into the slot in the infrared spectrometer s sample compartment. Cut a rectangular hole that is about 1 inch by 0.5 inch, in the center of each bracket. This is the window through which the infrared light beam will pass and where the polymer film will be located. 

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... 

It is very important to make the right choice of the cuvette material for liquid and gas samples. This material must be transparent to the infrared light. Sodium chloride is the most often used material for the cuvettes and the optics of the infrared spectrometer. Other material such as special types of glass, quartz, aluminum oxide, calcium chloride, potassium bromide and so on are also used for special purposes. 

The next component part in the infrared spectrometer is the detector. The most important types of detectors used in infrared spectroscopy are the thermal detectors. In this type of detector, radiation energy is first absorbed and then converted into heat energy. The actual measured value is an electrical voltage, which is produced or changed by the heating. Despite their higher sensitivity, photo electric detectors have a lower popularity due to the limits they have of the ana-lyzable wavelength area. 

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. 

One approach has been the use of a low-volume, internally reflecting, heated lightpipe (flow cell) interface between the gas chromatography and the infrared spectrometer (see Figure 3.37). The lightpipe has IR-transparent alkali halide windows at each end so the IR gas phase spectra of the eluent are recorded as they emerge from the column. 

The development in 1940 of the first commercial ultraviolet-risible spectrometer, the Beckman DU, and its marketing in 1941 (see chapter 4) revolutionized analytical chemistry and had significant impact on biology. During the early years of World War II another laboratory workhouse, the infrared spectrometer was developed in order to help solve a strategic national problem rubber. 

Two modes of operation of the spectroelectrochemical setup (including both the infrared spectrometer and the devices for electrode potential control) are possible in order to obtain the requested surface sensitivity and to remove unwanted absorption contributions from solution, gas phase in sample chamber, etc. ... 

Two types of infrared spectroscopic analysis have been applied. The first is to follow the changes in the evolved gas product infrared spectra during heating. The precursor polymer is heated in a thermal gravimetric analyzer (TGA) and the evolved gases are directed into a gas cell in the infrared spectrometer. This TGA-IR method enables us to characterize the composition of the species evolved during the thermal elimination reaction. 

The Combination of the Infrared Spectrometer with the Liquid Chromatograph... 

P. A. Wilks. How the infrared spectrometer reached the bench chemist. Spectroscopy 16(12) 14-15,2001. 

All spectra obtained on the infrared spectrometer were taken under the following conditions 100 scans, 4 cm resolution, boxcar apodization TGS detector. 

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