Page 263 - Handbook of Instrumental Techniques for Analytical Chemistry

Infrared Spectroscopy 253

Dispersive Spectrometers

Dispersive spectrometers, introduced in the mid-1940s and widely used since, provided the robust in-
strumentation required for the extensive application of this technique.

Spectrometer Components
An IR spectrometer consists of three basic components: radiation source, monochromator, and detector.
A schematic diagram of a typical dispersive spectrometer is shown in Fig. 15.3.
The common radiation source for the IR spectrometer is an inert solid heated electrically to 1000
to 1800 °C. Three popular types of sources are Nernst glower (constructed of rare-earth oxides), Globar
(constructed of silicon carbide), and Nichrome coil. They all produce continuous radiations, but with
different radiation energy profiles.
The monochromator is a device used to disperse a broad spectrum of radiation and provide a con-
tinuous calibrated series of electromagnetic energy bands of determinable wavelength or frequency
range. Prisms or gratings are the dispersive components used in conjunction with variable-slit mecha-
nisms, mirrors, and filters. For example, a grating rotates to focus a narrow band of frequencies on
a mechanical slit. Narrower slits enable the instrument to better distinguish more closely spaced fre-
quencies of radiation, resulting in better resolution. Wider slits allow more light to reach the detector
and provide better system sensitivity. Thus, certain compromise is exercised in setting the desired slit
width.
Most detectors used in dispersive IR spectrometers can be categorized into two classes: thermal
detectors and photon detectors. Thermal detectors include thermocouples, thermistors, and pneumatic

Figure 15.3 Schematic diagram of a commercial dispersive IR instrument, the Perkin-Elmer Model 237B Infrared
Spectrometer. (Reprinted by permission of the Perkin-Elmer Corporation.)