566
R. W. Kessler and W. Kessler
by a longer path length. This is also shown in Fig. 23.6. The figures in mm are the
path lengths of the cuvettes used.
The assignment of the liquid water bands has been discussed for a long time.
Usually, the peaks are assigned to the symmetric (ν s ) stretching mode in combination
with the asymmetric (ν a ) stretching mode and often also together with the bending
mode (δ) of the OH groups in water [30–33]. For example, the significant water
peaks with maxima at around 970 nm (10300 cm
−1 ) are assigned to (2ν s + ν a ), at
1200 nm (8310 cm
−1 ) to (ν s + ν a + δ), and at 1450 nm (6900 cm
−1 ) to (ν s + ν a ). The
dominating strong peak at 1940 nm (5150 cm
−1 ) is suggested to be a combination of
the asymmetric stretch and bending (ν a + δ) mode of the OH-group. The absorbance
of D 2 O (heavy water) is lower and can be used as a standard reference material for
water in solutions. There are smaller additionally peaks at approximately 770 nm
(13000 cm
−1 ) and at 850 nm (11800 cm
−1 ) (both not shown in Fig. 23.6) as well
as at 1780 nm (5620 cm
−1 ) which are difficult to assign correctly. The water peaks
with maxima near 970, 1200, and 1450 nm shift with increasing temperature towards
lower wavelengths (higher wavenumbers) and also show an isosbestic point of the
optical unresolved pair of peaks [35–39].
Due to the high dipole moment change of the vibrational transitions in water, NIR
and MIR spectroscopic investigations are highly sensitive to water absorption which
makes spectroscopy in aqueous systems, e.g., biotechnology, quite challenging. On
the other hand, water in food can easily be quantified even at very low concentrations
[38, 39].
23.3.2.3 Penetration Depth of Photons in the NIR Wavelength Range
As shown in Table 23.1, the analytical sensitivity increases from the 3rd overtone to
the combination bands with several orders of magnitude. The probability of photons
penetrating an absorbing material is dominated by absorption, but scattering also
plays an important role. This means that due to the smaller cross sections in the
NIR range, a higher penetration depth of the photons can be achieved compared to
MIR or UV–Vis spectroscopy. The great advantage of the NIR in PAT applications
is therefore that no sample preparation (e.g., dilution) is required even at higher
concentrations of an analyte. In transparent liquids, the lower sensitivity of the higher
harmonics can be compensated for by longer layer thicknesses for the measurement
setup. However, in solid particle systems this is not possible or difficult to achieve.
On the other hand, the penetration depth in particle systems with lower absorption
increases significantly. This becomes particularly clear, e.g., with the 3rd overtone
[4, 13, 40]. This is an important aspect, e.g., for the quantitative determination of
a pharmaceutical active substance in a tablet or for the measurement of substances,
e.g., through the skin of a vegetable or a fruit. These aspects are discussed in detail
in the literature [1, 32–34] and in the chapter measurement of solids.
R. W. Kessler and W. Kessler
by a longer path length. This is also shown in Fig. 23.6. The figures in mm are the
path lengths of the cuvettes used.
The assignment of the liquid water bands has been discussed for a long time.
Usually, the peaks are assigned to the symmetric (ν s ) stretching mode in combination
with the asymmetric (ν a ) stretching mode and often also together with the bending
mode (δ) of the OH groups in water [30–33]. For example, the significant water
peaks with maxima at around 970 nm (10300 cm
−1 ) are assigned to (2ν s + ν a ), at
1200 nm (8310 cm
−1 ) to (ν s + ν a + δ), and at 1450 nm (6900 cm
−1 ) to (ν s + ν a ). The
dominating strong peak at 1940 nm (5150 cm
−1 ) is suggested to be a combination of
the asymmetric stretch and bending (ν a + δ) mode of the OH-group. The absorbance
of D 2 O (heavy water) is lower and can be used as a standard reference material for
water in solutions. There are smaller additionally peaks at approximately 770 nm
(13000 cm
−1 ) and at 850 nm (11800 cm
−1 ) (both not shown in Fig. 23.6) as well
as at 1780 nm (5620 cm
−1 ) which are difficult to assign correctly. The water peaks
with maxima near 970, 1200, and 1450 nm shift with increasing temperature towards
lower wavelengths (higher wavenumbers) and also show an isosbestic point of the
optical unresolved pair of peaks [35–39].
Due to the high dipole moment change of the vibrational transitions in water, NIR
and MIR spectroscopic investigations are highly sensitive to water absorption which
makes spectroscopy in aqueous systems, e.g., biotechnology, quite challenging. On
the other hand, water in food can easily be quantified even at very low concentrations
[38, 39].
23.3.2.3 Penetration Depth of Photons in the NIR Wavelength Range
As shown in Table 23.1, the analytical sensitivity increases from the 3rd overtone to
the combination bands with several orders of magnitude. The probability of photons
penetrating an absorbing material is dominated by absorption, but scattering also
plays an important role. This means that due to the smaller cross sections in the
NIR range, a higher penetration depth of the photons can be achieved compared to
MIR or UV–Vis spectroscopy. The great advantage of the NIR in PAT applications
is therefore that no sample preparation (e.g., dilution) is required even at higher
concentrations of an analyte. In transparent liquids, the lower sensitivity of the higher
harmonics can be compensated for by longer layer thicknesses for the measurement
setup. However, in solid particle systems this is not possible or difficult to achieve.
On the other hand, the penetration depth in particle systems with lower absorption
increases significantly. This becomes particularly clear, e.g., with the 3rd overtone
[4, 13, 40]. This is an important aspect, e.g., for the quantitative determination of
a pharmaceutical active substance in a tablet or for the measurement of substances,
e.g., through the skin of a vegetable or a fruit. These aspects are discussed in detail
in the literature [1, 32–34] and in the chapter measurement of solids.