Double-beam grating spectrometer

The workhorse infrared instrument used for routine characterization of materials in the undergraduate organic laboratory is the optical-null double-beam grating spectrometer (Fig. 8.3a). For a discussion of double-beam spectrometers, see the UV-vis instrumentation discussion (p. 604). Although many... 

Figure 2.3a The optical path of a double-beam infrared spectrometer with a grating monochromator...

Figure 8.2. A schematic diagram of a typical double-beam infrared spectrometer. The symbols Ml, Ml,... indicate mirrors SI and SI indicate slits and Gl and G2 indicate gratings. Courtesy of the Perkin-Elmer Corporation.

Figure 2.1 Schematic of the optical path of a double-beam infrared spectrometer with a grating monochromator. Reproduced from Brittain, E. F. H., George, W. O. and Wells, C. H. J., Introduction to Molecular Spectroscopy, Academic Press, London, Copsnight (1975), with permission from Elsevier.

Fourier transform spectrometer or double-beam spectrophotometer incorporating prism or grating monochromator, thermal or photon detector, alkali halide cells. 

The levitated laser dye droplet was optically pumped by a pulsed (pulse length 5 ns, repetition rate 10 Hz), frequency-doubled Nd YAG laser (2 = 532 nm) in free-space optical setup. Droplet light emission was collected by a multimode optical fiber placed at an angle of approximately 50° relative to pump laser beam. Collected light was analyzed in a fixed-grating spectrometer with a resolution of FWHM 0.15 nm. 

The design of a conventional atomic absorption spectrometer is relatively simple (Fig. 3.1), consisting of a lamp, a beam chopper, a burner, a grating monochromator, and a photomultiplier detector. The design of each of these is briefly considered. The figure shows both single and double beam operation, as explained below. 

Smaller values of are obtained for interferometers operated in a double-beam mode, since the moveable mirror must be left stationary for a fraction of the cycle time to allow the detector to stabilize each time the beam is switched from the sample to the reference position. With an optical null grating spectrometer the chopper is used not only to modulate the beam but also to alternate the beam between sample and reference channels. Thus, it takes approximately the same time to measure a transmittance spectrum using a double beam optical null spectrometer as it takes to measure a single-beam spectrum with the same S/R. Hence, for this type of spectrometer may be assigned a value of 2. 

Spectrometers that use phototubes or photomultiplier tubes (or diode arrays) as detectors are generally called spectrophotometers, and the corresponding measurement is called spectrophotometry. More strictly speaking, the journal Analytical Chemistry defines a spectrophotometer as a spectrometer that measures the ratio of the radiant power of two beams, that is, PIPq, and so it can record absorbance. The two beams may be measured simultaneously or separately, as in a double-beam or a single-beam instrument—see below. Phototube and photomultiplier instruments in practice are almost always used in this maimer. An exception is when the radiation source is replaced by a radiating sample whose spectrum and intensity are to be measured, as in fluorescence spectrometry—see below. If the prism or grating monochromator in a spectrophotometer is replaced by an optical filter that passes a narrow band of wavelengths, the instrument may be called a photometer. 

IR spectroscopy became widely used after the development of commercial spectrometers in the 1940s. Double-beam monochromator instruments were developed, better detectors were designed, and better dispersion elements, including gratings, were incorporated. These conventional spectrometer systems have been replaced in the last decade by FTIR instrumentation. This chapter will focus on FTIR instrumentation and applications of IR spectroscopy. In addition, the related techniques of near-IR (NIR) spectroscopy and Raman spectroscopy will be covered. 

Until the early 1980s, most IR spectrometer systems were double-beam dispersive grating spectrometers, similar in operation to the double-beam system for UV/VIS spectroscopy described in Chapter 2. These instruments have been replaced almost entirely by FTIR spectrometers because of the advantages in speed, signal-to-noise ratio, and precision in determining spectral frequency that can be obtained from a modern multiplex instrument. There are NIR instruments that are part of double-beam dispersive UV /VIS/NIR systems, but many NIR instruments are stand-alone grating instruments. 

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