Infrared-transmitting windows

In this technique the sampling area of the spectrometer is a special type of microscope. The sample is placed on an infrared transmitting window and examined with a conventional visible light microscope. The selected microsample area is masked off as desired. Then the optical path is changed and the infrared spectrum of the selected microsample area is recorded. 

As implied by Eq. 15.2, all the materials in Table 15.1 must have higher refractive indices than the material with which they are in contact. Materials that are commonly used as windows in the mid-infrared [e.g., KBr n = 1.53) and KCl ( i = 1.45)] are not included in this list, as their refractive indices are too low for use as IREs. Because the refractive indices of KBr and KCl are roughly equal to the refractive index of organic compounds, total internal reflection will not be observed. In this case, radiation passes directly through the IRE and sample without regard to the angle of incidence. In other words, these materials are better suited for use as infrared-transmitting windows than as internal reflection elements. 

In contrast to flowcell interfaces, solvent-elimination approaches lead to spectra free of solvent interferences. Various sampling techniques are possible the sample can be deposited on a flat ZnSe plate, on a smooth metallic substrate or a thin layer of powdered alkali halide salt, whereas the spectrum can be taken in conventional transmission, external-reflection or diffuse-reflection arrangement. In one of the first applications a synthetic mixture of three quinones was separated on a microbore column packed with silica, using a mobile phase of 5% methanol in supercritical carbon dioxide. The peaks were collected on a plate on which a layer of KCl powder was deposited, and then spectra were measured by a diffuse-reflection accessory. Test measurements on acenaphthenequi-none (AQ) showed later that conventional transmission spectra of samples on flat infrared-transmitting windows give the best compromise between high sensitivity, correct relative band intensities and adherence to the Lambert-Beer law. 

Using matched cuvettes for solvent and analyte is seldom practical for infrared measurements because it is difficult to obtain cells with identical transmission characteristics. Part of this difficulty results from degradation of the transparency of infrared cell windows (typically polished sodium chloride) with use due to attack by traces of moisture in the atmosphere and in samples. In addition, pathlengths are hard to reproduce because infrared cells are often less than 1 mm thick. Such narrow cells are required to permit the transmission of measurable intensities of infrared radiation through pure samples or through very concentrated solutions of the analyte. Measurements on dilute analyte solutions, as is done in ultraviolet or visible spectroscopy, are usually difficult because there are few good solvents that transmit over appreciable regions of the infrared spectrum. 

The transition metal ion-containing films were prepared carefully by ion-exchange as described elsewhere (10-16). These films were mounted in appropriate stainless steel or pyrex glass reactors/spectroscopic cells. Both reactors, described in detail elsewhere, had infrared (KBr) or uv-vis (quartz) transmitting windows for spectroscopic studies. They were equipped with vacuum valves for evacuation and gas admission, a heating system and a temperature monitor. 

Table 16.6 hsts the properties of several infrared transmitting materials. The short pathlengths required in infrared spectrometry are difficult to reproduce, especially when the windows must be repolished, and so quantitative analysis is not... 

Typically the thin films are made by deposition of a known amount of protein dissolved in a relatively high vapor pressure solvent such as water on a circular window of infrared-transmitting material (see below). Frequently, the next step is to remove some fraction of the solvent in order... 

Given these special properties, this high-performance material has been successfully used for a variety of applications such as telescope mirror blanks in precision optics and infrared-transmitting range tops (Section 4.3.1). This glass-ceramic has also been successfiilly mass produced for household applications such as cookware and woodstove windows (Section 4.2). 

Cells for holding samples, or windows within the spectrometer must be made of infrared transmitting material. Table 2.1 lists approximate low wavenumber transmission limits for some optical materials used in infrared spectroscopy. All the materials listed transmit in the mid-IR above the transmission limit except for polyethylene, which is used below 600 cm" only, as it has absorption bands at higher frequencies. The transmission limits are not sharply defined and depend somewhat on the thickness of the material used. For example, a typical cell for liquid samples which is made of NaCl... 

Applications include transparent cookware (Visions , thermal expansion = 7 x 10 V°C), optical materials (e.g., ring-laser gyroscopes), telescope mirror blanks (Zerodur ), infrared-transmitting electric range tops, wood-stove windows, fire door glazing... 

Optical windows of highly purified magnesium fluoride which transmit light from the vacuum ultraviolet (140 nm) into the infrared (7) are recommended for use as ultraviolet optical components for use in space exploration. 

Optical Properties. Teflon FEP fluorocarbon film transmits more ultraviolet, visible light, and infrared radiation than ordinary window glass. The refractive index of FEP film is 1.341—1.347 (74). 

Infrared optics is a fast growing area in which CVD plays a maj or role, particularly in the manufacture of optical IR windows. 1 The earths atmosphere absorbs much of the infrared radiation but possesses three important bandpasses (wavelengths where the transmission is high) at 1-3 im, 3-5 im and 8-17 pm. As shown in Table 16.2, only three materials can transmit in all these three bandpasses single crystal diamond, germanium, and zinc selenide. 

The major absorbers of infrared radiation in the atmosphere are HjO, CO2, and O3. The infrared wavelength ranges of 2-2.5 pm, 3-5 pm, and 8-12 pm are known as atmospheric windows. The radiative energies transmitted from the Sun to the Earth s surface within these atmospheric windows are shown in Table 12.3. 

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