Instrument Designs for Infrared Absorption
The simplest instrument for IR ab- sorption spectroscopy is a filter photometer similar to that shown in Figure 10.24 for UV/Vis absorption. These instruments have the advantage of portability and typically are used as dedicated analyzers for gases such as HCN and CO.
Infrared instruments using a monochromator for wavelength selection are con- structed using double-beam optics similar to that shown in Figure 10.26. Double- beam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than that for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and H2O vapor when using double-beam optics. Resolutions of 1–3 cm–1 are typical for most instruments.
In a Fourier transform, infrared spectrometer, or FT–IR, the monochromator is replaced with an interferometer (see Figure 10.13). Because an FT–IR includes only a single optical path, it is necessary to collect a separate spectrum to compen- sate for the absorbance of atmospheric CO2 and H2O vapor. This is done by collect- ing a background spectrum without the sample and storing the result in the instru- ment’s computer memory. The background spectrum is removed from the sample’s spectrum by ratioing the two signals. In comparison to other IR instruments, an FT–IR provides for rapid data acquisition, allowing an enhancement in signal-to- noise ratio through signal averaging.
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 mir- rors to pass the beam of radiation through the sample several times.
Liquid samples are analyzed in one of two ways. For nonvolatile liquids a suit- able sample can be prepared by placing a drop of the liquid between two NaCl plates, forming a thin film that typically is less than 0.01 mm thick. Volatile liquids must be placed in a sealed cell to prevent their evaporation.
The analysis of solution samples is limited by the solvent’s IR-absorbing prop- erties, with CCl4, CS2, and CHCl3 being the most common solvents. Solutions are placed in cells containing two NaCl windows separated by a Teflon spacer. By changing the Teflon spacer, pathlengths from 0.015 to 1.0 mm can be obtained. Sealed cells with fixed or variable pathlengths also are available.
The analysis of aqueous solutions is complicated by the solubility of the NaCl cell window in water. One approach to obtaining infrared spectra on aqueous solu- tions is to use attenuated total reflectance (ATR) instead oftransmission. Figure 10.31 shows a diagram of a typical ATR sampler, consisting of an IR-transparent crystal of high- refractive index, such as ZnSe, surrounded by a sample of lower-refractive index. Radiation from the source enters the ATR crystal, where it undergoes a series of total internal reflec- tions before exiting the crystal. During each reflection, the ra diation penetrates into the sample to a depth of a few microns. The result is a selec- tive attenuation of the radiation at those wavelengths at which the sample absorbs.
ATR spectra are similar, but not identical, to those obtained by measuring the transmission of radiation.
Transparent solid samples can be analyzed directly by placing them in the IR beam. Most solid samples, however, are opaque and must be dispersed in a more transparent medium before recording a traditional transmission spectrum. If a suit- able solvent is available, then the solid can be analyzed by preparing a solution and analyzing as described earlier. When a suitable solvent is not available, solid samples may be analyzed by preparing a mull of the finely powdered sample with a suitable oil. Alternatively, the powdered sample can be mixed with KBr and pressed into an optically transparent pellet.
Solid samples also can be analyzed by means of reflectance. The ATR sampler (see Figure 10.31) described for the analysis of aqueous solutions can be used for the analysis of solid samples, provided that the solid can be brought into contact with the ATR crystal. Examples of solids that have been analyzed by ATR include polymers, fibers, fabrics, powders, and biological tissue samples. Another re- flectance method is diffuse reflectance, in which radiation is reflected from a rough surface, such as a powder. Powdered samples are mixed with a nonabsorbing mate- rial, such as powdered KBr, and the reflected light is collected and analyzed. As with ATR, the resulting spectrum is similar to that obtained by conventional transmission methods.