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H. M. Heise
composition and properties of materials and products such as polymers, pharmaceutical, agricultural products or beverages to list a few. The use of optical spectroscopy
within the near-infrared regime in particular is widespread in analytical chemistry,
and chemical process monitoring has received much attention in the past for the
aim of optimization and quality control in the aforementioned application fields,
especially in industrial processes.
Vibrational spectroscopy is based on molecular vibrations, which are localized
in molecular substructures or larger molecular entities. The quantized energies for
exciting vibrations within molecules start with longest wavelengths in the far-infrared
range, with wavelengths of up to 1000 μm, and reach down to the short-wave NIR
(SW-NIR) with a minimum wavelength of 780 nm by definition. NIR spectroscopy is
mostly restricted to the study of substances with OH, NH and CH group absorptions,
providing spectral patterns with much band overlap, but multivariate chemometric
techniques render these best suited for quantitative and qualitative analysis as for
raw material identification. The NIR bands arise from combination and overtone
vibrations and for reaching reasonable absorbances in transmission measurements
or for –log (reflectance) values of powders and tissues, the optical path in samples
can be millimeters or with higher overtones even centimeters.
This part of NIR spectroscopy was for a long time considered a “sleeping giant”
before its rapid implementation in applications for the forage and food industry, for
petrochemicals, polymers and pharmaceuticals took place especially for quantitative assays. In the last few years, process analytical chemistry, also called process
analytical technology (PAT), has been implemented with robust process NIR spectrometers, especially when online data were made available for reaction controlling
and product quality monitoring. Other areas and applications within life sciences
have also been dealt with in earlier chapters of this handbook.
Similarly, clinical chemistry applications also evolved for the analysis of blood
and other body fluid constituents. The late emergence of near-infrared spectroscopy
into the clinical chemistry field is mainly due to the complexity of the body fluids
under investigation. These contain many analytes of rather low concentrations in the
per mille and even lower range apart from total protein or albumin quantification
with lower percentage values. Advantageously has been the high reproducibility for
the recording of spectra and their high signal-to-noise ratios, which could be accomplished with sensitive photodetectors within Fourier transform or excellent dispersive
spectrometers. Early clinical chemistry applications focused on the analysis of blood
and blood plasma with blood glucose as the most promising analyte. This was also
due to expectations that this technique could finally be used for non-invasive blood
glucose monitoring.
Tissue spectroscopy for skin cancer detection or wound healing has also been
investigated using the so-called combination band region, showing most spectral
information for discrimination and identification. With shorter wavelengths band,
half-widths become broader and with less structure, so that the selectivity for analytical assays is usually reduced compared to the combination band region. Within
the short-wave near-infrared (SW-NIR) regime, also other phenomena give rise to
absorptions: these are biological molecules such as hemoglobin, myoglobin and
H. M. Heise
composition and properties of materials and products such as polymers, pharmaceutical, agricultural products or beverages to list a few. The use of optical spectroscopy
within the near-infrared regime in particular is widespread in analytical chemistry,
and chemical process monitoring has received much attention in the past for the
aim of optimization and quality control in the aforementioned application fields,
especially in industrial processes.
Vibrational spectroscopy is based on molecular vibrations, which are localized
in molecular substructures or larger molecular entities. The quantized energies for
exciting vibrations within molecules start with longest wavelengths in the far-infrared
range, with wavelengths of up to 1000 μm, and reach down to the short-wave NIR
(SW-NIR) with a minimum wavelength of 780 nm by definition. NIR spectroscopy is
mostly restricted to the study of substances with OH, NH and CH group absorptions,
providing spectral patterns with much band overlap, but multivariate chemometric
techniques render these best suited for quantitative and qualitative analysis as for
raw material identification. The NIR bands arise from combination and overtone
vibrations and for reaching reasonable absorbances in transmission measurements
or for –log (reflectance) values of powders and tissues, the optical path in samples
can be millimeters or with higher overtones even centimeters.
This part of NIR spectroscopy was for a long time considered a “sleeping giant”
before its rapid implementation in applications for the forage and food industry, for
petrochemicals, polymers and pharmaceuticals took place especially for quantitative assays. In the last few years, process analytical chemistry, also called process
analytical technology (PAT), has been implemented with robust process NIR spectrometers, especially when online data were made available for reaction controlling
and product quality monitoring. Other areas and applications within life sciences
have also been dealt with in earlier chapters of this handbook.
Similarly, clinical chemistry applications also evolved for the analysis of blood
and other body fluid constituents. The late emergence of near-infrared spectroscopy
into the clinical chemistry field is mainly due to the complexity of the body fluids
under investigation. These contain many analytes of rather low concentrations in the
per mille and even lower range apart from total protein or albumin quantification
with lower percentage values. Advantageously has been the high reproducibility for
the recording of spectra and their high signal-to-noise ratios, which could be accomplished with sensitive photodetectors within Fourier transform or excellent dispersive
spectrometers. Early clinical chemistry applications focused on the analysis of blood
and blood plasma with blood glucose as the most promising analyte. This was also
due to expectations that this technique could finally be used for non-invasive blood
glucose monitoring.
Tissue spectroscopy for skin cancer detection or wound healing has also been
investigated using the so-called combination band region, showing most spectral
information for discrimination and identification. With shorter wavelengths band,
half-widths become broader and with less structure, so that the selectivity for analytical assays is usually reduced compared to the combination band region. Within
the short-wave near-infrared (SW-NIR) regime, also other phenomena give rise to
absorptions: these are biological molecules such as hemoglobin, myoglobin and