Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods567P T and P stray may be similar in magnitude. This results is an absorbance thatis smaller than expected, and a negative deviation from Beer’s law.Problem 10.8 in the end of chapter problemsask you to explore the effect of strayradiation on the linearity of Beer’s law.10CUV/Vis and IR SpectroscopyIn Figure 10.9 we examined Nessler’s original method for matching thecolor of a sample to the color of a standard. Matching the colors was a laborintensive process for the analyst. Not surprisingly, spectroscopic methods ofanalysis were slow to develop. The 1930s and 1940s saw the introductionof photoelectric transducers for ultraviolet and visible radiation, and thermocouplesfor infrared radiation. As a result, modern instrumentation forabsorption spectroscopy became routinely available in the 1940s—progresshas been rapid ever since.10C.1 InstrumentationFrequently an analyst must select—from among several instruments of differentdesign—the one instrument best suited for a particular analysis. Inthis section we examine several different instruments for molecular absorptionspectroscopy, emphasizing their advantages and limitations. Methodsof sample introduction are also covered in this section.In s t r u m e n t De s i g n s f o r Mo l e c u l a r UV/Vis Ab s o r p t i o nFilter Photometer. The simplest instrument for molecular UV/Vis absorptionis a filter photometer (Figure 10.25), which uses an absorption orinterference filter to isolate a band of radiation. The filter is placed betweenthe source and the sample to prevent the sample from decomposing whenexposed to higher energy radiation. A filter photometer has a single opticalpath between the source and detector, and is called a single-beam instrument.The instrument is calibrated to 0% T while using a shutter to blockthe source radiation from the detector. After opening the shutter, the in-sourcefiltershutteropenclosedsampledetectorThe percent transmittance varies between0% and 100%. As we learned in Figure10.21, we use a blank to determine P 0 ,which corresponds to 100% T. Even in theabsence of light the detector records a signal.Closing the shutter allows us to assign0% T to this signal. Together, setting 0%T and 100% T calibrates the instrument.The amount of light passing through asample produces a signal that is greaterthan or equal to that for 0% T and smallerthan or equal to that for 100%T.blanksignalprocessorFigure 10.25 Schematic diagram of a filter photometer.The analyst either inserts a removable filter or thefilters are placed in a carousel, an example of which isshown in the photographic inset. The analyst selects afilter by rotating it into place.