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 Optimized Spectra
Solid and semi-solid samples are challenging for
FTIR analysis because of their diverse properties:
micro to macro sizes, transmitting to opaque
optical densities, specular to light-scattering
reflectances. Photoacoustic detection provides
a cost-efficient, single-accessary approach that
accomodates diverse properties and signifi-
canlty simplifies FTIR analysis of solids. The
MTEC Model 300 handles all samples with little
or no sample preparation. It performs infrared
absorbance, diffuse reflectance and transmittance
measurements as required depending on sample
properties by simply exchanging the accessory's
sampling heads.
Photoacoustic Detection
Infrared absorbance of samples is measured
directly with photoacoustic detection via an
acoustic signal that evolves from the sequence
of infrared absorption, heat production and
thermal expansion. Alternatively, when a black
absorber is positioned near the front of or behind
the sample, the sample's diffuse reflectance or
transmittance spectrum, respectively, is measured
as shown schematically on the Sampling
Modes
page.
SH003
(Macro) |
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Controlled Sampling Depth
When the photoacoustic detector is operated in
the absorbance mode, the sampling depth and
effective optical density of the sample are inde-
pendent of the physical sample thickness. The
sampling depth is proportional to the squre root
of (sample thermal diffuusivity / FTIR mirror ve-
locity due to the signal generation role of the
absorption-excited thermal oscillations which de-
cay during propagation through the sample. The
net result is that depth-varying information can
be obtained by varying the mirror velocity or
phase angle of detection.
Multisampler Operation
Tne MTEC Model 300 has two sampling options
which, by the use of interchangeable sampling
heads, extend the accessory's capabilities beyond
the standard capability of measuring macro-
sample absorbance spectra. Options Sh003 and
SH004 allow the MTEC Model 300 to switch in
several seconds between macrosample diffuse
reflectance, absorbance, and transmittance
measurements and microsample measurements,
respectively. The sampling heads are illustrated
schematically on the Sampling Modes
page.
SH004
(Micro) |