2.4. Spectral data processing
The FTIT-ATR and LIBS spectra were pre-processed with baseline
correction and a Savitzky–Golay smoothing filter (Savitzky & Golay,
1964), to reduce the baseline float/noise and improve the
signal-to-noise ratio. Matlab software (Mathworks, USA) was used to
smooth the data and for all other statistical analyses performed.
The 2DCOS analysis was performed using the method established by Noda
and Ozaki (Noda, 2018; Noda & Ozaki, 2005). The intensity of the
synchronous 2D correlation spectrum, Ф (λ1,
λ2), represents the simultaneous or coincidental changes
in the spectral intensity at wavenumbers λ1 and
λ2 with an external perturbation. The peaks on the
diagonal line are called auto peaks, while the peaks on the off-diagonal
line are called crosspeaks. The asynchronous spectrum, Ψ
(λ1, λ2), represents the out-of-phase or
sequential changes in spectral intensities with external perturbation.
There is no diagonal peak, but there is a cross peak in off-diagonal
areas (Noda, 2018). The red and blue colors on the map represent
positive and negative intensities, respectively.
In this study, different salt contents were applied as the external
perturbation for the FTIR spectra, while the different vertical
detecting points were used as external perturbations for the LIBS
spectra. In our study, the spectral synchronous and asynchronous
correlation maps were generated from the analysis of the ATR and LIBS
spectra, respectively.