Baseline Correction of UV Raman Spectrum Based on Improved Piecewise Linear Fitting
ZHAO Man1, GUO Yi-xin1, HE Yu-qing1*, GUO Hong1, JIN Wei-qi1, REN Lin-mao1,2
1. MOE Key Laboratory of Optoelectronic Imaging Technology and System, School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China
2. Railway Police College, Zhengzhou 450053, China
Abstract：UV Raman spectroscopy has the characteristics of high Raman scattering intensity, easy fluorescence spectrum separation, little influence by environmental interference and safety to the human eye. In this paper, the ultraviolet Raman spectrometer uses a laser with a wavelength of 266 nm. The Raman spectrum and the fluorescence spectrum will partially overlap, which increases the difficulty of accurately obtaining the characteristic information of the laser Raman spectrum, and further affects the identification and analysis of the sample. Therefore, baseline corrections need to be performed prior to analyzing Raman spectroscopy to eliminate fluorescence interference. According to the distribution characteristics of the mixed spectrum of ultraviolet Raman and fluorescence, the fluorescence spectrum has a gradual increase and is close to a piecewise linear increase. Therefore, fitting a fluorescence spectral baseline using a piecewise linear function is a relatively simple method, so that the troughs of the characteristic peaks just fall on the baseline. Aiming at the problem that the traditional piecewise linear fitting baseline correction method is over-reliant on the operator and the low level of automation, improved UV-Raman spectroscopy piecewise linear fitting baseline correction method is studied. (1) First the spectral data of the original signal after different smoothing iterations is obtained. Since the peak is a high-frequency signal with respect to the baseline, the spectral intensity at the peak position gradually decreases and changes greatly, while that at the baseline portion gradually rises and the relative change is small during the multiple smoothing process. So the standard deviation (SD) of the spectral intensity at the spectral peaks and the baseline points is different after different smoothing iterations. (2) Then the position of the quasi-valid baseline points is determined by comparing the spectral intensity deviations. The quasi-valid baseline points can be extracted by appropriately setting the threshold; (3) Next the quasi-valid baseline points divide the entire Raman spectrum into N characteristic peak intervals. Comparing the lines obtained by connecting the two ends of the characteristic peak interval with the spectral intensity of the characteristic peak interval, if the characteristic peaks are all above the straight line, there is no over-fitting, otherwise the endpoints of the characteristic peaks move toward the peak direction and are connected again by straight lines. The above process is repeated until the characteristic peaks are all above the line connecting the two ends of the interval, and the valid baseline points are obtained. (4) Finally, all adjacent valid baseline points are connected in a straight line by segment to get the baseline of the entire spectrum. The corrected Raman spectrum is obtained by subtracting the baseline from the original spectrum. Baseline calibration experiments of simulated and actual measured UV and fluorescence hybrid spectra show that the method of this paper can automatically determine the position of the baseline point and obtain better baseline correction effect than the traditional method, which will provide more accurate spectral information for the next spectral analysis.
Key words：Raman spectroscopy; UV; Baseline correction; Piecewise linear fitting
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