116
M. A. Czarnecki and S. Morita
coordinate. Hence, the presence of the positive asynchronous peak at (ν 1 , ν 2 ) means
that the spectral changes at ν 1 occur earlier/faster than those at ν 2 . The negative sign
of the asynchronous peak means the opposite behavior. Selective correlation of the
peaks in 2DCOS spectra allows for establishing of the origin of the peaks and easy
the band assignment. Particularly useful are hetero-correlation spectra, which show
the selective correlation between known and unknown spectral features.
The rules for interpretation of 2DCOS spectra are straightforward [4, 5]; however,
correct interpretation of the real-world data is not always easy. Firstly, an application
of the external perturbation is often accompanied by side effects. To obtain a ‘net’
information on the effect of interest, at first, one has to remove these side effects. The
procedure, which removes these side effects depends on their specific nature, but in
many cases, the normalization of the spectra significantly improves the quality of 2D
contour plots [9]. Secondly, 2DCOS spectra, particularly the asynchronous ones, are
very sensitive to noise, baseline fluctuation and other distortions. Besides, interpretation of 2DCOS spectra is complicated by band position and/or width variations. All
these effects may generate artifacts in the synchronous and asynchronous spectra.
Therefore, the systematic studies were undertaken to recognize and eliminate (where
possible) these effects from 2D correlation spectra [10–13].
Sometimes, the spectral changes of interest are obscured by the noise, baseline
fluctuation or the other effects. The proper pretreatment of the experimental spectra
may significantly improve the quantity and quantity of the information obtained from
2DCOS [7, 9, 10]. An extensive baseline fluctuation generates long streaks observed
in the synchronous and asynchronous contour plots. In many cases, a simple offset
of the spectra at selected reference point can significantly reduce this effect [7].
Sometimes are necessary more advanced corrections by using polynomial functions.
In an extreme case, one can use the second derivative spectra, instead of the original
data, for the analysis [12]. The high level of noise will produce a lot of artifacts,
especially in the asynchronous spectrum. The most popular methods of smoothing
are based on Savitzky–Golay algorithm. The more advanced methods employ Fourier
or wavelet filtering, or principal component analysis (PCA). Also, normalization of
the spectra is often used as a pretreatment method. This way, one can eliminate the
effects of varying concentration, temperature, pressure or sample thickness on the
2DCOS spectra [9].
2D correlation spectroscopy offers a significant simplification of the complex NIR
spectra. However, the most important advantage of using 2DCOS in NIR region is
the ability of resolving of highly overlapped peaks. Besides, selective correlation
between MIR (or Raman) and NIR spectra allows for reliable band assignment in
NIR region and obtain information on the molecular structure and interactions [6, 7].
M. A. Czarnecki and S. Morita
coordinate. Hence, the presence of the positive asynchronous peak at (ν 1 , ν 2 ) means
that the spectral changes at ν 1 occur earlier/faster than those at ν 2 . The negative sign
of the asynchronous peak means the opposite behavior. Selective correlation of the
peaks in 2DCOS spectra allows for establishing of the origin of the peaks and easy
the band assignment. Particularly useful are hetero-correlation spectra, which show
the selective correlation between known and unknown spectral features.
The rules for interpretation of 2DCOS spectra are straightforward [4, 5]; however,
correct interpretation of the real-world data is not always easy. Firstly, an application
of the external perturbation is often accompanied by side effects. To obtain a ‘net’
information on the effect of interest, at first, one has to remove these side effects. The
procedure, which removes these side effects depends on their specific nature, but in
many cases, the normalization of the spectra significantly improves the quality of 2D
contour plots [9]. Secondly, 2DCOS spectra, particularly the asynchronous ones, are
very sensitive to noise, baseline fluctuation and other distortions. Besides, interpretation of 2DCOS spectra is complicated by band position and/or width variations. All
these effects may generate artifacts in the synchronous and asynchronous spectra.
Therefore, the systematic studies were undertaken to recognize and eliminate (where
possible) these effects from 2D correlation spectra [10–13].
Sometimes, the spectral changes of interest are obscured by the noise, baseline
fluctuation or the other effects. The proper pretreatment of the experimental spectra
may significantly improve the quantity and quantity of the information obtained from
2DCOS [7, 9, 10]. An extensive baseline fluctuation generates long streaks observed
in the synchronous and asynchronous contour plots. In many cases, a simple offset
of the spectra at selected reference point can significantly reduce this effect [7].
Sometimes are necessary more advanced corrections by using polynomial functions.
In an extreme case, one can use the second derivative spectra, instead of the original
data, for the analysis [12]. The high level of noise will produce a lot of artifacts,
especially in the asynchronous spectrum. The most popular methods of smoothing
are based on Savitzky–Golay algorithm. The more advanced methods employ Fourier
or wavelet filtering, or principal component analysis (PCA). Also, normalization of
the spectra is often used as a pretreatment method. This way, one can eliminate the
effects of varying concentration, temperature, pressure or sample thickness on the
2DCOS spectra [9].
2D correlation spectroscopy offers a significant simplification of the complex NIR
spectra. However, the most important advantage of using 2DCOS in NIR region is
the ability of resolving of highly overlapped peaks. Besides, selective correlation
between MIR (or Raman) and NIR spectra allows for reliable band assignment in
NIR region and obtain information on the molecular structure and interactions [6, 7].