Biomedical Engineering Reference
In-Depth Information
where A
v
is the absorbance at wavenumber v (no units), I
v
is the intensity of light
emitted from the sample at wavenumber v (arbitrary units), I
v,0
is the intensity of
light emitted from the background at wavenumber v (arbitrary units), e is the molar
absorption coefficient (M
-1
cm
-1
), b is the path length (cm), and c is the con-
centration (M).
Raman spectroscopy is a complementary technique to infrared spectroscopy
and is based on the scattering of light [
101
,
102
].
To transform spectral data into meaningful results, it is necessary to develop a
calibration model which relates the spectra to a process parameter e.g. concen-
tration values of a substrate. Chemometric techniques are exploited to extract the
relevant data (see Chap. 7).
2.3.1 Development of MIR Spectroscopy
Infrared radiation was discovered in 1800 by Sir William Herschel, and following
this, the first mid-infrared spectrometer was constructed by Melloni in 1833. The
first half of the twentieth century saw little development in FT-IR spectroscopy,
and its potential as an analytical tool remained largely untapped until the late
1950s and early 1960s. Since the commercial debut of the FT-IR system in the
1970s, the technology has been embraced by manufacturing industries and
research communities alike. Instruments have been adapted and improved to meet
the specific needs of the end user.
MIR immersion probes have been available since the late 1980s. At the early
stages of development MIR, optic fibres suffered from high material absorption
and scattering and poor mechanical and chemical stability, therefore ''fixed'' arm
probes with parallel light pipes using internal reflection spectroscopy were found
to be more suitable. However, when placed in a process environment, this design is
far from ideal. These probes need to be precisely aligned and are highly sensitive
to vibrations in the surrounding area, which can result in alignment changes and
hence spectral differences [
103
-
105
]. There have been major advances in the
development of fibre-optic materials over the last 10 years, and these improve-
ments have had far-reaching consequences. In the case of MIR instrumentation,
this has resulted in flexible, more robust immersion probes which address many of
the problems encountered with the rigid conduit probes. However, regardless of
probe type, process disturbances will regularly impact the spectra collected, and
these disturbances need to be accounted for when developing multivariate cali-
bration models. The short path length of MIR, when compared with that of NIR,
means that from a sampling perspective MIR does not penetrate as far into the
material and may not be as representative of the sample as NIR would be; how-
ever, in the presence of particulate matter the shorter path length of MIR reduces
light scattering, which is commonly experienced when NIR is used in a similar
situation [
106
].
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