Page 273 - Handbook of Instrumental Techniques for Analytical Chemistry

Infrared Spectroscopy 263

in a small cup inside a small chamber containing an IR-transparent gas such as helium or nitrogen. IR
radiation absorbed by the sample converts into heat inside the sample. The heat diffuses to the sample
surface, then into the surrounding gas atmosphere, and causes expansion of a boundary layer of gas next
to the sample surface. Thus, the modulated IR radiation produces intermittent thermal expansion of the
boundary layer and generates pressure waves. A sensitive microphone is used to detect the resulting
photoacoustic signal.
PAS spectra are generally similar to conventional IR spectra except for some minor differences:
Absorbance peaks appear at the same frequency locations, but truncation of strong absorbance bands
due to photoacoustic signal saturation is often observed. However, the presence of such truncated bands
does not limit the practical use of PAS. Spectral search against standard commercial spectral libraries
can be satisfactorily performed. FTIR PAS technique also offers a unique capability for examining
samples at various depths from 1 to 20 µm. The acoustic frequencies depend on the modulated frequen-
cy of source: The slower the modulation frequency, the greater depth of penetration. Thus, samples
such as multilayer polymers can be studied at various depths by simply varying the scan speed of the
FTIR spectrometer.
Emission spectroscopy is another technique used with difficult samples such as thin coatings and
opaque materials. The sample is normally heated to an elevated temperature, emitting enough energy to
be detected. The sample acts as the radiation source, so the normal IR source is turned off. The ability of
FTIR instruments to obtain spectra from weak signals makes it possible to study emisssion in the infrared
region, even when the sample is at low temperatures such as 50 to 100 °C. Emission spectral bands occur
at the same frequencies as absorption bands. The spectra from thick samples can be complicated when
radiation from the interior of the sample is self-absorbed by the outer part of the sample.
Infrared microspectroscopy has become a popular technique for analyzing difficult or small sam-
ples such as trace contaminants in semiconductor processing, multilayer laminates, surface defects, and
forensic samples. Infrared microscopes are energy-inefficient accessories that require the signal-to-
noise advantages of FTIR to obtain spectra from submilligram samples. Using a liquid nitrogen cooled
mercury cadmium telluride (MCT) detector, samples in the size range of 10 µm can be examined on IR
microscopes.
The primary advantages of the IR microscope relate not only to its improved optical and mechan-
ical design, but also to its manipulative capability. In many cases, the major problem in microsampling
is focusing the spectrometer beam on the sample. The computerized/motorized control of microscope
functions of IR microscope instruments permit these extremely small samples to be moved in the
field of view to isolate the portion from which spectra are obtained.
Fiberoptic accessories deliver unique flexibility and diversity in sampling. They are particularly
useful in acquiring IR spectra when samples are situated in a remote location or when the unusual size
or shape of samples prevents them from fitting well in a standard sample compartment. Many analyses
in hazardous or process environments used these devices.
Fiberoptic sample probes or flow cells are coupled to standard FTIR spectrometers with two fi-
beroptic cables and an optic interface that transfers IR radiation from spectrometer to fiberoptic cables.
A variety of probes are available for ATR, specular reflectance, diffuse reflectance, and transmittance
measurements. Chalcogenide (GeAsSeTe), a mid IR–transmitting material in the range of 4000 to 900
–1
cm , was recently developed by Spectra-Tech and used to make the fiberoptic cables.

Hyphenated Methods Involving Infrared

Gas chromatography/Fourier transform infrared (GC/FTIR) spectroscopy is a technique that uses a gas
chromatograph to separate the components of sample mixtures and an FTIR spectrometer to provide
identification or structural information on these components. The real potential of GC-IR instrumenta-