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The utility of FT-IR PAS depends significantly on the performance of both
the photoacoustic detector and the FT-IR spectrometer. Optimization of both can be critical in demanding
applications because of the thermal-wave reflection problem mentioned in the
previous section, which leads to low acoustic signal levels and vulnerability to
noise interference. Optimization is
especially necessary when shallow sampling depths are required.
In this case, the amount of detected energy is low because of the
thinness of the layer being probed, and this leads to very low signals.
The
two major noise sources in FT-IR PAS measurements are structure-borne vibration,
which couples to the microphone, and electronic noise.
These can be controlled by design considerations and careful choice of
the microphone and preamplifier components, but there are practical limitations. Consequently, in terms of the FT-IR spectrometer, a high
intensity infrared source, low f-number
optics, a stable mirror velocity to reduce interferometer-induced noise in the
IR beam modulation, and control of mechanical vibrational resonances associated
with the optical bench are all very important.
The
main PAS detector design considerations can be divided into two categories:
signal enhancement and noise suppression. The
former includes an optical design to focus all of the available IR energy
onto the sample, restricting the gas volume to just suffice for signal
generation
with minimal excess, provision for helium gas purging of the sample chamber to
enhance the transfer of heat to the gas and boost the signal generation
efficiency by approximately a factor of 3 over signal generation in air, and use
of a high sensitivity microphone with a typical value of 50 mV/Pa.
The
noise suppression factors include both avoiding and damping mechanical
resonances in the detector housing and support structure as much as possible,
supporting the detector housing on vibration isolators, using limp cables and
purge tubing to reduce propagation of vibration along these paths to the
detector, employing seals and a window design that make the detector as immune
as possible to air-borne noise, and use of a low noise preamplifier.
Most
commercial FT-IR PA detectors currently in use are products of MTEC
Photoacoustics, Inc.,15 manufacturer of the Model 100, 200, and
currently the Model 300 instruments. The
newest MTEC Model 300 units reflect the present trend in FT-IR accessories by
mounting on a pre-aligned FT-IR-specific baseplate, as shown in Figure 12, and
having, in the case of the system shown, a computerized FT-IR PAS tutorial to
instruct the user. The detector has
an optical path leading to the detector that is sealed from the room atmosphere
on purged FT-IR systems. The
spectral range of a PAS system is defined by the transmission range of the
detector’s sample-chamber window and by the spectral range of the FT-IR.
Most FT-IR PAS measurements are made in the mid- and near-infrared
spectral regions.