Analytical Chem istry - DePauw University

556 Analytical Chemistry 2.0dynodehνelectronsopticalwindowanodephotoemissive cathodeFigure 10.14 Schematic of a photomultiplier.A photon strikes the photoemissivecathode producing electrons, which acceleratetoward a positively charged dynode.Collision of these electrons with thedynode generates additional electrons,which accelerate toward the next dynode.A total of 10 6 –10 7 electrons per photoneventually reach the anode, generating anelectrical current.If the retina in your eye is a transducer,then your brain is a signal processor.Photon Transducers. Phototubes and photomultipliers contain a photosensitivesurface that absorbs radiation in the ultraviolet, visible, or nearIR, producing an electrical current proportional to the number of photonsreaching the transducer (Figure 10.14). Other photon detectors use a semiconductoras the photosensitive surface. When the semiconductor absorbsphotons, valence electrons move to the semiconductor’s conduction band,producing a measurable current. One advantage of the Si photodiode isthat it is easy to miniaturize. Groups of photodiodes may be gathered togetherin a linear array containing from 64–4096 individual photodiodes.With a width of 25 mm per diode, for example, a linear array of 2048 photodiodesrequires only 51.2 mm of linear space. By placing a photodiodearray along the monochromator’s focal plane, it is possible to monitorsimultaneously an entire range of wavelengths.Thermal Transducers. Infrared photons do not have enough energy to producea measurable current with a photon transducer. A thermal transducer,therefore, is used for infrared spectroscopy. The absorption of infrared photonsby a thermal transducer increases its temperature, changing one ormore of its characteristic properties. A pneumatic transducer, for example,is a small tube of xenon gas with an IR transparent window at one endand a flexible membrane at the other end. Photons enter the tube and areabsorbed by a blackened surface, increasing the temperature of the gas. Asthe temperature inside the tube fluctuates, the gas expands and contractsand the flexible membrane moves in and out. Monitoring the membrane’sdisplacement produces an electrical signal.Signal ProcessorsA transducer’s electrical signal is sent to a signal processor where it isdisplayed in a form that is more convenient for the analyst. Examples ofsignal processors include analog or digital meters, recorders, and computersequipped with digital acquisition boards. A signal processor also is usedto calibrate the detector’s response, to amplify the transducer’s signal, toremove noise by filtering, or to mathematically transform the signal.10BSpectroscopy Based on AbsorptionIn absorption spectroscopy a beam of electromagnetic radiation passesthrough a sample. Much of the radiation passes through the sample withouta loss in intensity. At selected wavelengths, however, the radiation’s intensityis attenuated. This process of attenuation is called absorption.10B.1 Absorbance SpectraThere are two general requirements for an analyte’s absorption of electromagneticradiation. First, there must be a mechanism by which the radiation’selectric field or magnetic field interacts with the analyte. For ultra-