Page 264 - Handbook of Instrumental Techniques for Analytical Chemistry

254 Handbook of Instrumental Techniques for Analytical Chemistry

devices (Golay detectors). They measure the heating effect produced by infrared radiation. A variety of
physical property changes are quantitatively determined: expansion of a nonabsorbing gas (Golay de-
tector), electrical resistance (thermistor), and voltage at junction of dissimilar metals (thermocouple).
Photon detectors rely on the interaction of IR radiation and a semiconductor material. Nonconducting
electrons are excited to a conducting state. Thus, a small current or voltage can be generated. Thermal
detectors provide a linear response over a wide range of frequencies but exhibit slower response times
and lower sensitivities than photon detectors.

Spectrometer Design
In a typical dispersive IR spectrometer, radiation from a broad-band source passes through the sample
and is dispersed by a monochromator into component frequencies (Fig. 15.3). Then the beams fall on
the detector, which generates an electrical signal and results in a recorder response.
Most dispersive spectrometers have a double-beam design. Two equivalent beams from the same
source pass through the sample and reference chambers respectively. Using an optical chopper (such as a
sector mirror), the reference and sample beams are alternately focused on the detector. Commonly, the
change of IR radiation intensity due to absorption by the sample is detected as an off-null signal that is
translated into the recorder response through the actions of synchronous motors.

Fourier Transform Spectrometers
Fourier transform spectrometers have recently replaced dispersive instruments for most applications
due to their superior speed and sensitivity. They have greatly extended the capabilities of infrared spec-
troscopy and have been applied to many areas that are very difficult or nearly impossible to analyze by
dispersive instruments. Instead of viewing each component frequency sequentially, as in a dispersive
IR spectrometer, all frequencies are examined simultaneously in Fourier transform infrared (FTIR)
spectroscopy.

Spectrometer Components
There are three basic spectrometer components in an FT system: radiation source, interferometer, and
detector. A simplified optical layout of a typical FTIR spectrometer is illustrated in Fig. 15.4.
The same types of radiation sources are used for both dispersive and Fourier transform spectrom-
eters. However, the source is more often water-cooled in FTIR instruments to provide better power and
stability.
In contrast, a completely different approach is taken in an FTIR spectrometer to differentiate and
measure the absorption at component frequencies. The monochromator is replaced by an interferometer,
which divides radiant beams, generates an optical path difference between the beams, then recombines
them in order to produce repetitive interference signals measured as a function of optical path difference
by a detector. As its name implies, the interferometer produces interference signals, which contain infra-
red spectral information generated after passing through a sample.
The most commonly used interferometer is a Michelson interferometer. It consists of three active
components: a moving mirror, a fixed mirror, and a beamsplitter (Fig. 15.4). The two mirrors are per-
pendicular to each other. The beamsplitter is a semireflecting device and is often made by depositing a
thin film of germanium onto a flat KBr substrate. Radiation from the broadband IR source is collimated
and directed into the interferometer, and impinges on the beamsplitter. At the beamsplitter, half the IR
beam is transmitted to the fixed mirror and the remaining half is reflected to the moving mirror. After
the divided beams are reflected from the two mirrors, they are recombined at the beamsplitter. Due to