4 Spectral Analysis in the NIR Spectroscopy
73
Amide I’
Amide II’
Amide III’
Fig. 4.9 Amide I’, Amide II’, and Amide III’ modes of deuterated N-methyacetamide. Reproduced
from Ref. [16] with permission
stretching vibration, yields a very small isotope shift [15]. It is noted with respect to
isotope shifts of polyatomic molecules that vibrational modes vary more or less with
isotopic substitution, which can be evidently understood from the comparison of
Fig. 4.9 and Fig. 2.11. In studies of IR, Raman, and NIR spectra, [15] N-substitution,
[13] C-substitution, and the like are often used in addition to deuterium substitution.
Although an isotope shift is small when such a heavy atom is replaced, a variation
in a vibrational mode associated with the isotopic substitution is also small.
4.3 Pretreatment Methods in NIR Spectroscopy
NIR spectra often encounter the problems of unwanted spectral variations and
baseline shifts [1–3]. They come from the following sources.
1. Various kinds of noises such as those from a detector, an amplifier, and an AD
converter.
2. Light scattering from cloudy liquids or solid samples.
3. Changes in temperature, density, and particle size of samples.
4. Poor reproducibility of NIR spectra caused by, for example, path length
variations.
5. The use of optical fiber cable may cause baseline shifts.
The above interferences may become obstacles for conventional spectral analysis
and 2D-COS. More importantly, they may easily violate the assumptions on which
chemometrics equations are based [3]. For instance, the simple linear relationship
stated by Beer’s law does not hold any more, and the additivity of individual spectral
responses is not guaranteed. Accordingly, data pretreatment is often necessary [1–3].
(Chap. 7) Whenever one attempts to improve SN ratio or to correct baseline fluctuations, one should explore the cause of poor SN ratio and that of baseline changes.
Otherwise, one cannot find proper pretreatment methods. One interesting example
of studies of baseline changes was reported by Geladi et al. [17] They modeled the
reflectance spectra of milk by optical effects and chemical light absorption effects.
The former induces variations in the direction of the light, and the latter is concerned
73
Amide I’
Amide II’
Amide III’
Fig. 4.9 Amide I’, Amide II’, and Amide III’ modes of deuterated N-methyacetamide. Reproduced
from Ref. [16] with permission
stretching vibration, yields a very small isotope shift [15]. It is noted with respect to
isotope shifts of polyatomic molecules that vibrational modes vary more or less with
isotopic substitution, which can be evidently understood from the comparison of
Fig. 4.9 and Fig. 2.11. In studies of IR, Raman, and NIR spectra, [15] N-substitution,
[13] C-substitution, and the like are often used in addition to deuterium substitution.
Although an isotope shift is small when such a heavy atom is replaced, a variation
in a vibrational mode associated with the isotopic substitution is also small.
4.3 Pretreatment Methods in NIR Spectroscopy
NIR spectra often encounter the problems of unwanted spectral variations and
baseline shifts [1–3]. They come from the following sources.
1. Various kinds of noises such as those from a detector, an amplifier, and an AD
converter.
2. Light scattering from cloudy liquids or solid samples.
3. Changes in temperature, density, and particle size of samples.
4. Poor reproducibility of NIR spectra caused by, for example, path length
variations.
5. The use of optical fiber cable may cause baseline shifts.
The above interferences may become obstacles for conventional spectral analysis
and 2D-COS. More importantly, they may easily violate the assumptions on which
chemometrics equations are based [3]. For instance, the simple linear relationship
stated by Beer’s law does not hold any more, and the additivity of individual spectral
responses is not guaranteed. Accordingly, data pretreatment is often necessary [1–3].
(Chap. 7) Whenever one attempts to improve SN ratio or to correct baseline fluctuations, one should explore the cause of poor SN ratio and that of baseline changes.
Otherwise, one cannot find proper pretreatment methods. One interesting example
of studies of baseline changes was reported by Geladi et al. [17] They modeled the
reflectance spectra of milk by optical effects and chemical light absorption effects.
The former induces variations in the direction of the light, and the latter is concerned