拉曼光谱基线校正解读

Kuo Sun, Hui Su, Zhixiang Yao, and Peixian Huang
Rcharacterization for its ability to obtain information on vibrations from samples. It can also be used for on-line monitoring using a fiber-optic Raman probe (1,2). The Raman spectra show the characteristics for species in sharp and dense peaks. However, during the application of Raman spectroscopy, fluorescence of organic compounds in the samples, which are sometimes several orders of magnitude more intense than the weak Raman scatter, can interfere with the Raman signals (3). A phenomenon of baseline drift shows up, making the resolution and analysis of Raman spectra impractical.Both instrumental (4) and mathematical methods have been developed to reduce the drifted baseline caused by fluorescence. The use of an excitation wavelength such as 785–1064 nm lasers, which does not eliminate fluorescence (5), is the most traditional instrumental method. Raman scattering is directly proportional to the fourth power of frequency; as the excitation wavelength increases, the sen-sitivity of the Raman becomes severely reduced. The use of anti-Stokes Raman spectroscopy is another method, based on theory (6). Mathematical methods (7–10) include the first and second order derivatives, wavelet transform, me-dian filter, and manual polynomial fitting. These methods are useful in certain situations, but still have some limita-tions. For example, derivatives are effective, but as a result the shape of the Raman spectrum is changed; wavelet trans-form can be differentiable in the high- and low-frequency components of the signals; however, it is difficult to choose a decomposition method. Manual polynomial fittings re-quire the user to identify the “non-Raman” locations manu-ally (11), and afterwards the baseline curve is formed by fitting these locations. Consequently, the result involves the inevitable subjective factors and, in addition, theworkload is always heavy. Therefore, it is important to choose an op-timal decomposition method.Piecewise linear fitting based on critical-point-seeking was proposed in this study. The method determines an op-timum corrected spectrum by correlation analysis, which can conquer these limitations. A Raman spectrum from the sulfamic acid catalytic reaction of an aspirin system was used as a study subject. By using this method, the Raman spectrum drifted baseline was automatically eliminated, leaving only the corrected spectrum. Basis of Qualitative and Quantitative Raman AnalysisA Raman spectrum is a plot of the intensity of Raman scattered radiation as a function of its frequency differ-ence from the incident radiation (usually in units of wave-numbers, cm-1). This difference is called the Raman shift, which is the basis of qualitative analysis (12). The intensity or power of a normal Raman peak depends in a complex way upon the polarizability of the molecule, the intensity Theory and Method
reaction of the aspirin system, which consisted of different proportions of aspirin. A simulated base-line with different interval values of moving average smoothing determined setting parameters in this method. After baseline drifts caused by fluorescence are removed, the differences of character-istic aspirin peaks proved the efficiency of this method.
2 Spectroscopy 29(2) February 2014www.spectroscopyonline.comBaseline Correction for Raman Spectra Based on Piecewise
Linear Fitting
The correction of baseline drift is an import part for data preprocessing. An interval linear fitting method based on automatic critical-point-seeking was improved, which made it possible for the baseline to drift automatically. Experimental data were acquired from the sulfamic acid catalyBiblioteka Baiduic