Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods569blanksampleshuttersourceλ 1 λ2λ 3choppersampleP TmirrorsignalprocessormonochromatortemperaturemirrorsamplecontrolcompartmentFigure 10.27 Schematic diagram of a scanning, double-beam spectrophotometer. A chopper directs the source’sradiation, using a transparent window to pass radiation to the sample and a mirror to reflect radiation to theblank. The chopper’s opaque surface serves as a shutter, which allows for a constant adjustment of the spectrophotometer’s0% T. The photographic insert shows a typical instrument. The unit in the middle of the photois a temperature control unit that allows the sample to be heated or cooled.alternating it between the sample, the blank, and a shutter. The signalprocessor uses the chopper’s known speed of rotation to resolve the signalreaching the detector into the transmission of the blank, P 0 , and the sample,P T . By including an opaque surface as a shutter, it is possible to continuouslyadjust 0% T. The effective bandwidth of a double-beam spectrophotometeris controlled by adjusting the monochromator’s entrance and exitslits. Effective bandwidths of 0.2–3.0 nm are common. A scanning monochromatorallows for the automated recording of spectra. Double-beaminstruments are more versatile than single-beam instruments, being usefulfor both quantitative and qualitative analyses, but also are more expensive.Diode Array Spectrometer. An instrument with a single detector canmonitor only one wavelength at a time. If we replace a single photomultiplierwith many photodiodes, we can use the resulting array of detectorsto record an entire spectrum simultaneously in as little as 0.1 s. In a diodearray spectrometer the source radiation passes through the sample and isdispersed by a grating (Figure 10.28). The photodiode array is situated atthe grating’s focal plane, with each diode recording the radiant power overa narrow range of wavelengths. Because we replace a full monochromatorwith just a grating, a diode array spectrometer is small and compact.blankP 0semitransparentmirrordetector