Infrared sample cells materials

Another issue with liquid water is that it dissolves some of the materials used in IR sample preparation. Materials such as KBr and NaCl are transparent in the mid-infrared and are used to make windows and cells to hold infrared samples. These materials are highly water soluble, aud any liquid water present in a sample will damage these cells or windows. There are infrared transparent materials that are not water soluble that can be used (see Chapter 4), but they tend to be more expensive than KBr and NaCl. 

Infrared spectroscopy is routinely used for the analysis of samples in the gas, liquid, and solid states. Sample cells are made from materials, such as NaCl and KBr, that are transparent to infrared radiation. Gases are analyzed using a cell with a pathlength of approximately 10 cm. Longer pathlengths are obtained by using mirrors to pass the beam of radiation through the sample several times. 

The unique appearance of an infrared spectrum has resulted in the extensive use of infrared spectrometry to characterize such materials as natural products, polymers, detergents, lubricants, fats and resins. It is of particular value to the petroleum and polymer industries, to drug manufacturers and to producers of organic chemicals. Quantitative applications include the quality control of additives in fuel and lubricant blends and to assess the extent of chemical changes in various products due to ageing and use. Non-dispersive infrared analysers are used to monitor gas streams in industrial processes and atmospheric pollution. The instruments are generally portable and robust, consisting only of a radiation source, reference and sample cells and a detector filled with the gas which is to be monitored. 

The most straightforward method for analyzing a solid material by infrared spectrometry is to dissolve it in a suitable solvent and then to measure this solution using a liquid sampling cell such as one of the several described in Section 8.8. Thus it becomes a liquid sampling problem, the experimental details of which have already been discussed (Section 8.8). It is the only method of solid sampling suitable for quantitative analysis because it is the only one that has a defined and reproduced pathlength. 

Remote multicomponent air samples can be drawn into a centrally located analyzer (under computer control) and then into the gas cell of an infrared spectrometer. Within the spectrometer, a system of lenses and mirrors passes an infrared beam in a predetermined path through the sample. The amount of energy absorbed by the sample is compared against a standard beam, and the difference is related to the concentration of the gas of interest. By changing the wavelength of the infrared beam, additional materials may be checked for in the gas sample and their concentration levels determined the same way. If the concentration of the compound of interest exceeds a predetermined level, an alarm is activated. 

Spectra of Xe-PtFt adducts.— The infrared spectrum of material deposited on silver chloride windows in a nickel-bodied gas cell was recorded. The composition of the adduct was Xe(PtF6)i,72. Only two peaks in the region 400-4000 cm. were assignable to the adduct 652 vs, 550 s. cm. h The visible and ultraviolet spectrum of material deposited on the windows of a silica gas cell was recorded. A single peak at 3825 A. was observed. The material absorbed strongly beyond 4000 A. No differences in the absorption spectra were noted for several separate adduct samples. 

HPLC sample flow-cells have internal volumes of less than 20 pi, with a typical path length of 1mm and window area of 2-3 mm (Figure 7.13c). NaCl or KBr window materials may be used with many organic solvents and PTFE and polyethylene windows are available for both organic and aqueous based solvents. Narrow bore or microcolumns (2-3 mm i.d.) have lower flow volumes than normal columns, typically 0.3-0.5mlmin and are therefore ideally suited for infrared detectors. Micro-HPLC-FTIR techniques may employ a direct flow cell or solvent elimination techniques [16 18]. Considerable care is required to match the chromatographic system with the sample cell to avoid loss of resolution. 

Additional technical details, including a critical comparison of the dispersive and interferometric techniques and details on sample preparation, may be found in the review by Amey and Chapman (1983). We shall only comment on the sample cells for examining liquids and solutions. These cells are limited by a pair of windows made of various, more or less transparent, materials (Table 9.1). In the mid-infrared region there is no clear-cut best choice for window material CaF2 has the least reflection loss but is expensive KBr is the cheapest. 

As with other types of absorption spectroscopy e.g. UVA IS) the basis of quantitative analysis in transmission IR spectroscopy is Beer s law. This requires few components and no peak overlap. Although deviations from Beer s law exist, these can usually easily be dealt with. The challenge in FTIR quantitation for polymers is sample thickness. In infrared, sample concentration and optical pathlength can seldom be controlled as tightly as in UVAHS spectrometry. This is primarily due to the absence of suitable materials (solvents and cuvets) that are transparent over a sufficiently wide frequency range. Use of peak ratios standardises the absorbance signal and eliminates the thickness variable. Alternatively, use can be made of sealed cells with constant path-length. 

To measure an infrared absorption spectrum from a liquid or solution sample, it is necessary to use windows (polished plates of certain crystals) or a cell for containing the sample. The material used for the windows must be transparent to the infrared radiation and appropriate for the purpose of the measurement to be performed. Some characteristics of representative infrared transparent materials are given in Table 2.1. More information is available elsewhere [1 -3]. The windows are commercially available usually in the form of polished plates varying in size and thickness. 

Make sure your sample components do not react with each other. This can be checked by obtaining spectra of the sample over time and looking for changes. Also, be aware of the materials the sample cell is made of and how they might react with any components in the sample. For example, KBr is a commonly used cell and window material in infrared spectroscopy, but it is water soluble. Prior knowledge of the properties of cell and window materials can prevent this type of mistake. 

Be consistent in the use of cells, windows, crystals, and sampling accessories for your standards and unknowns. For example, KBr and NaCl are both commonly used window materials in infrared spectroscopy, but have different optical properties. If a calibration is developed with a sample cell containing KBr windows, and unknown spectra are obtained with a cell with NaCl windows, the unmodeled spectral difference may be enough to introduce error into the predicted concentrations. 

In practice, infrared spectra can be obtained with gaseous, liquid, or solid samples. The sample containers (cells) and the optical parts of the instrument are made of rock salt (NaCl) or similar material that transmits infrared radiation (glass is opaque). 

I. Spectroscopic Determinations. Gas-phase infrared spectra provide a useful adjunct to vapor pressure measurements in the identification of volatile materials. The cell illustrated in Fig. 9.15 allows the sample to be quantitatively returned to the vacuum line after the spectrum has been obtained, so the process is completely nondestructive. The primary problem with a gas cell is to obtain a vacuum-tight seal between the window material and the cell body this may be accomplished with Glyptal paint or with wax- If the latter is used, it is necessary to warm and cool the alkali halide windows slowly to avoid cracking them due to thermal stress. For this purpose an infrared lamp is handy. The most satisfactory method of attaching windows is O-rings because this allows the easy removal of the windows for cleaning and polishing. 

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