23 Inline and Online Process Analytical Technology with an Outlook …
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23.3 Robustness in PAT Applications with a Focus on NIR
Spectroscopy: About Sensitivity, Selectivity,
and Signal-to-Noise
23.3.1 General Approach to Optical Spectroscopy in PAT
and Their Advantages
Optical spectroscopy describes the interaction of molecules or matter with photons
[1, 4]. The inelastic interaction leads to the transition of the molecule from a lower to
a higher energy level. This spectral property is called wavelength-dependent absorption. The elastic interaction, on the other hand, leads to a scattering of the photon in
different directions and is also wavelength-dependent. Optical spectroscopy detects
absorption and scattering simultaneously and thus provides the following information
as summarized in [1]:
• The chemical composition of the molecule or matter by measuring the wavelengthdependent (inelastic) absorption of light.
• The morphological information (nanoscopic substructure) by measuring the
wavelength-dependent (elastic) scattering of light (e.g., Mie scattering).
• The texture of the heterogeneous system by combining spectroscopy and image
analysis. This is called hyperspectral or chemical imaging.
Optical spectroscopy is currently the workhorse in PAT. Within optical spectroscopy, near-infrared spectroscopy is the most commonly used technique [1, 31–
34]. Online and inline NIR optical process spectroscopy is well-established and
widely used. It is also frequently used to characterize materials or to control chemical reactions in industry at the molecular level, e.g., to monitor the concentration
of a chemical component in a mixture. The spectral information is often evaluated
by multivariate data analysis (MVA) and correlated with quality parameters of the
material or finished product [4].
There are three key parameters that are important for the functionality of spectroscopic methods in PAT solutions: sensitivity, selectivity, and robustness of the
method [1, 33].
23.3.2 Sensitivity: Definition at Molecular Level
and Classification of NIRS Within the Spectroscopic
Toolbox
23.3.2.1 Definition
Analytical sensitivity is the smallest amount of a substance in a sample that can be
accurately determined using a specific technology. In general, this is the concentration
563
23.3 Robustness in PAT Applications with a Focus on NIR
Spectroscopy: About Sensitivity, Selectivity,
and Signal-to-Noise
23.3.1 General Approach to Optical Spectroscopy in PAT
and Their Advantages
Optical spectroscopy describes the interaction of molecules or matter with photons
[1, 4]. The inelastic interaction leads to the transition of the molecule from a lower to
a higher energy level. This spectral property is called wavelength-dependent absorption. The elastic interaction, on the other hand, leads to a scattering of the photon in
different directions and is also wavelength-dependent. Optical spectroscopy detects
absorption and scattering simultaneously and thus provides the following information
as summarized in [1]:
• The chemical composition of the molecule or matter by measuring the wavelengthdependent (inelastic) absorption of light.
• The morphological information (nanoscopic substructure) by measuring the
wavelength-dependent (elastic) scattering of light (e.g., Mie scattering).
• The texture of the heterogeneous system by combining spectroscopy and image
analysis. This is called hyperspectral or chemical imaging.
Optical spectroscopy is currently the workhorse in PAT. Within optical spectroscopy, near-infrared spectroscopy is the most commonly used technique [1, 31–
34]. Online and inline NIR optical process spectroscopy is well-established and
widely used. It is also frequently used to characterize materials or to control chemical reactions in industry at the molecular level, e.g., to monitor the concentration
of a chemical component in a mixture. The spectral information is often evaluated
by multivariate data analysis (MVA) and correlated with quality parameters of the
material or finished product [4].
There are three key parameters that are important for the functionality of spectroscopic methods in PAT solutions: sensitivity, selectivity, and robustness of the
method [1, 33].
23.3.2 Sensitivity: Definition at Molecular Level
and Classification of NIRS Within the Spectroscopic
Toolbox
23.3.2.1 Definition
Analytical sensitivity is the smallest amount of a substance in a sample that can be
accurately determined using a specific technology. In general, this is the concentration