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C. W. Huck
past decades, a relatively modest attention has been paid to the potential that NIR
spectroscopy is able to bring to the latter field. Recent years have demonstrated that
NIR spectroscopy may be used in a number of similar applications concurrently
with IR or Raman spectroscopy. To better present the strengths and limitations of
the eponymous method, it is useful to briefly summarize the essential similarities
and differences existing between these approaches. The chemical specificity of IR
or Raman spectra is relatively superior to that of NIR spectra, and they are more
straightforward in a direct interpretation. The observed bands are broader because of
strong overlapping, reducing the potential for linking the spectra with the structural
information. This becomes particularly significant in the case of chemically complex
samples of biological origin. However, recent years have witnessed rapid progress in
our ability to understand NIR spectra of such samples, with combined use of spectral
imaging, novel chemometrics or theoretical methods of spectra calculation [1, 2].
Relevant examples will be discussed in this chapter that demonstrate the progress
recently achieved at this direction.
IR and NIR spectroscopy differ significantly in the typical sampling depth, or in
other words, the information on the sample is collected from distinctively different
sample volumes. This fact has a notable influence, as it is common in bioscience to
investigate highly inhomogeneous, often micro-structured samples, such as cells and
tissues of either plant or animal origin. For example, the consequences of that difference is well exemplified in medical application, in which the optimal sample thickness
for IR transmission measurements conveniently matches the typical configuration of
microtomed tissue specimen used in conventional medical diagnosis. However, the
absorptivity of organic matter is up to two orders of magnitude lower in NIR than
in IR region. Consequently, no useful NIR signal can be obtained from such specimen. On the other hand, this permits NIR radiation to reach deeper and measure
the spectrum of the sample beneath its surface. This enables, for instance, sensing
the information from beneath human skin or examining entire organisms such as
fish embryo. Deep tissue sampling is a key advantage for bioanalytical applications. Moreover, for the same reason, a larger sample volume is permissible in NIR
spectroscopy, making it better suited for the analysis of bulk materials essential for
bio-related studies (e.g., natural products). Moreover, NIR spectroscopy is relatively
better suited for examination of samples with high water content. Measurements in
transmission or diffuse reflection mode without sample preparation are more feasible
than in IR spectroscopy, which requires attenuated total reflection (ATR) approach is
such cases. Compared with Raman, which is suitable for examining moist samples,
NIR technique is applicable to specimen with high content of fluorophores; those
most typically encountered in bioscience are, e.g., chlorophyll or proteins. Because
of that, NIR spectroscopy is easily applicable for examination of plants and plantrelated materials, as well as protein-rich samples. Better suitability of the principal
features of NIR instrumentation in certain applications may be mentioned. Availability of fiber probes makes in vivo diagnosis easier. Miniaturized instrumentation
is readily available for NIR spectroscopy. In contrast, such IR sensors are practically
limited to ATR, and their application faces difficulties, e.g., because of the stability
C. W. Huck
past decades, a relatively modest attention has been paid to the potential that NIR
spectroscopy is able to bring to the latter field. Recent years have demonstrated that
NIR spectroscopy may be used in a number of similar applications concurrently
with IR or Raman spectroscopy. To better present the strengths and limitations of
the eponymous method, it is useful to briefly summarize the essential similarities
and differences existing between these approaches. The chemical specificity of IR
or Raman spectra is relatively superior to that of NIR spectra, and they are more
straightforward in a direct interpretation. The observed bands are broader because of
strong overlapping, reducing the potential for linking the spectra with the structural
information. This becomes particularly significant in the case of chemically complex
samples of biological origin. However, recent years have witnessed rapid progress in
our ability to understand NIR spectra of such samples, with combined use of spectral
imaging, novel chemometrics or theoretical methods of spectra calculation [1, 2].
Relevant examples will be discussed in this chapter that demonstrate the progress
recently achieved at this direction.
IR and NIR spectroscopy differ significantly in the typical sampling depth, or in
other words, the information on the sample is collected from distinctively different
sample volumes. This fact has a notable influence, as it is common in bioscience to
investigate highly inhomogeneous, often micro-structured samples, such as cells and
tissues of either plant or animal origin. For example, the consequences of that difference is well exemplified in medical application, in which the optimal sample thickness
for IR transmission measurements conveniently matches the typical configuration of
microtomed tissue specimen used in conventional medical diagnosis. However, the
absorptivity of organic matter is up to two orders of magnitude lower in NIR than
in IR region. Consequently, no useful NIR signal can be obtained from such specimen. On the other hand, this permits NIR radiation to reach deeper and measure
the spectrum of the sample beneath its surface. This enables, for instance, sensing
the information from beneath human skin or examining entire organisms such as
fish embryo. Deep tissue sampling is a key advantage for bioanalytical applications. Moreover, for the same reason, a larger sample volume is permissible in NIR
spectroscopy, making it better suited for the analysis of bulk materials essential for
bio-related studies (e.g., natural products). Moreover, NIR spectroscopy is relatively
better suited for examination of samples with high water content. Measurements in
transmission or diffuse reflection mode without sample preparation are more feasible
than in IR spectroscopy, which requires attenuated total reflection (ATR) approach is
such cases. Compared with Raman, which is suitable for examining moist samples,
NIR technique is applicable to specimen with high content of fluorophores; those
most typically encountered in bioscience are, e.g., chlorophyll or proteins. Because
of that, NIR spectroscopy is easily applicable for examination of plants and plantrelated materials, as well as protein-rich samples. Better suitability of the principal
features of NIR instrumentation in certain applications may be mentioned. Availability of fiber probes makes in vivo diagnosis easier. Miniaturized instrumentation
is readily available for NIR spectroscopy. In contrast, such IR sensors are practically
limited to ATR, and their application faces difficulties, e.g., because of the stability