Page 269 - Handbook of Instrumental Techniques for Analytical Chemistry

Infrared Spectroscopy 259

Figure 15.10 The “double-beam” IR spectrum of dibutyl
phthalate, produced by ratio of the corresponding single-beam
sample spectrum against the single-beam background
spectrum.

An FTIR instrument can achieve the same signal-to-noise (S/N) ratio of a dispersive spectrom-
eter in a fraction of the time (1 sec or less versus 10 to 15 min). The
S/N ratio is proportional to the square root of the total number of measurements. Because multi-
ple spectra can be readily collected in 1 min or less, sensitivity can be greatly improved by in-
creasing S/N through coaddition of many repeated scans.
• Increased optical throughput (Jaquinot advantage). Energy-wasting slits are not required in the
interferometer because dispersion or filtering is not needed. Instead, a circular optical aperture
is commonly used in FTIR systems. The beam area of an FT instrument is usually 75 to 100
times larger than the slit width of a dispersive spectrometer. Thus, more radiation energy is
made available. This constitutes a major advantage for many samples or sampling techniques
that are energy-limited.
• Internal laser reference (Connes advantage). The use of a helium neon laser as the internal ref-
erence in many FTIR systems provides an automatic calibration in an accuracy of better than
–1
0.01 cm . This eliminates the need for external calibrations.
• Simpler mechanical design. There is only one moving part, the moving mirror, resulting in less
wear and better reliability.
• Elimination of stray light and emission contributions. The interferometer in FTIR modulates all
the frequencies. The unmodulated stray light and sample emissions (if any) are not detected.
• Powerful data station. Modern FTIR spectrometers are usually equipped with a powerful, com-
puterized data system. It can perform a wide variety of data processing tasks such as Fourier
transformation, interactive spectral subtraction, baseline correction, smoothing, integration,
and library searching.
Although the spectra of many samples can be satisfactorily run on either FTIR or dispersive instru-
ments, FTIR spectrometers are the preferred choice for samples that are energy-limited or when in-
creased sensitivity is desired. A wide range of sampling accessories is available to take advantage of
the capabilities of FTIR instruments.