Page 268 - Handbook of Instrumental Techniques for Analytical Chemistry

258 Handbook of Instrumental Techniques for Analytical Chemistry

Figure 15.8 A single-beam IR spectrum of background,
showing contribution from trace amount of ambient water
and carbon dioxide.

3. The ratio of the single-beam sample spectrum in Fig. 15.9 against the single beam background
spectrum in Fig. 15.8 results in a “double-beam” spectrum of the sample (Fig. 15.10).
To reduce the strong background absorption from water and carbon dioxide in the atmosphere, the
optical bench is usually purged with an inert gas or with dry, carbon dioxide–scrubbed air (from a com-
mercial purge gas generator). Spectrometer alignment, which includes optimization of the beamsplitter
angle, is recommended as part of a periodic maintenance or when a sample accessory is changed.

FTIR Advantages
FTIR instruments have distinct advantages over dispersive spectrometers:
• Better speed and sensitivity (Felgett advantage). A complete spectrum can be obtained during a
single scan of the moving mirror, while the detector observes all frequencies simultaneously.
Figure 15.9 A single-beam IR spectrum of dibutyl
phthalate (a liquid sample).