Analytical Chem istry - DePauw University

572 Analytical Chemistry 2.0of fiber-optic cable transmits the nonabsorbed radiation to the wavelengthselector. Another design replaces the flow cell shown in Figure 10.31 witha membrane containing a reagent that reacts with the analyte. When theanalyte diffuses across the membrane it reacts with the reagent, producinga product that absorbs UV or visible radiation. The nonabsorbed radiationfrom the source is reflected or scattered back to the detector. Fiber opticprobes that show chemical selectivity are called optrodes. 6In s t r u m e n t De s i g n s f o r In f r a r e d Ab s o r p t i o nFilter Photometer. The simplest instrument for IR absorption spectroscopyis a filter photometer similar to that shown in Figure 10.25 for UV/Vis absorption.These instruments have the advantage of portability, and typicallyare used as dedicated analyzers for gases such as HCN and CO.Double-beam spectrophotometer. Infrared instruments using a monochromatorfor wavelength selection use double-beam optics similar to thatshown in Figure 10.27. Double-beam optics are preferred over single-beamoptics because the sources and detectors for infrared radiation are less stablethan those for UV/Vis radiation. In addition, it is easier to correct for theabsorption of infrared radiation by atmospheric CO 2 and H 2 O vapor whenusing double-beam optics. Resolutions of 1–3 cm –1 are typical for mostinstruments.Fourier transform spectrometer. In a Fourier transform infrared spectrometer,or FT–IR, the monochromator is replaced with an interferometer(Figure 10.13). Because an FT-IR includes only a single optical path, itis necessary to collect a separate spectrum to compensate for the absorbanceof atmospheric CO 2 and H 2 O vapor. This is done by collecting abackground spectrum without the sample and storing the result in the instrument’scomputer memory. The background spectrum is removed fromthe sample’s spectrum by ratioing the two signals. In comparison to otherinstrument designs, an FT–IR provides for rapid data acquisition, allowingan enhancement in signal-to-noise ratio through signal-averaging.Sample Cells. Infrared spectroscopy is routinely used to analyze gas, liquid,and solid samples. Sample cells are made from materials, such as NaCl andKBr, that are transparent to infrared radiation. Gases are analyzed usinga cell with a pathlength of approximately 10 cm. Longer pathlengths areobtained by using mirrors to pass the beam of radiation through the sampleseveral times.A liquid samples may be analyzed using a variety of different samplecells (Figure 10.32). For non-volatile liquids a suitable sample can be preparedby placing a drop of the liquid between two NaCl plates, forming athin film that typically is less than 0.01 mm thick. Volatile liquids must beplaced in a sealed cell to prevent their evaporation.6 (a) Seitz, W. R. Anal. Chem. 1984, 56, 16A–34A; (b) Angel, S. M. Spectroscopy 1987, 2(2),38–48.