拉曼光谱基线校正

2 Spectroscopy 29 2 February R aman spectroscopy is used worldwide in materials characterization for its ability to obtain ination 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 s 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 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 based on theory 6 Mathematical s 7 10 include the first and second order derivatives wavelet trans me dian filter and manual polynomial fitting These s 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 can be differentiable in the high and low frequency components of the signals however it is difficult to choose a decomposition Manual polynomial fittings re quire the user to identify the non Raman locations manu ally 11 and afterwards the baseline curve is ed by fitting these locations Consequently the result involves the inevitable subjective factors and in addition the workload is always heavy Therefore it is important to choose an op timal decomposition Piecewise linear fitting based on critical point seeking was proposed in this study The 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 the Raman spectrum drifted baseline was automatically eliminated leaving only the corrected spectrum Theory and Basis of Qualitative and Quantitative Raman Analysis A 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 Kuo Sun Hui Su Zhixiang Yao and Peixian Huang The correction of baseline drift is an import part for data preprocessing An interval linear fitting 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 catalytic 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 After baseline drifts caused by fluorescence are removed the differences of character istic aspirin peaks proved the efficiency of this Baseline Correction for Raman Spectra Based on Piecewise Linear Fitting February 2014 Spectroscopy 29 2 of the source and the concentration of the active group The power of Raman emission increases with the fourth power of the frequency of the source Raman intensities are usually directly proportional to the concentration of the active species which is the basis of quantitative analysis 13 14 Equation 1 shows the factors that determine the Raman scattering cross section I K v A v v4 I0 J v C 1 where I is the intensity of the Raman line K v is the overall spectrometer response A v is self absorption of the medium v is the frequency of scattered radiation I0 is the intensity of the inci dent radiation J v is a molar scattering parameter and C is the concentration of the sample The v4 term dominates if the other terms do not differ appre ciably and a higher frequency laser beam yields a stronger Raman signal Step 1 Smoothing We used smoothing to preprocess the data for Raman spectroscopy with frequency shifts and high sensitivity Moving average smoothing can effec tively lower the frequency and sensi tivity as shown in equation 2 smooth 1 21 m k x k x ki x im 2 where is the interval number of the moving average smoothing window which must be an odd number Step 2 Locating Local Extrema Suppose that the function f x has an extreme value at point x x0 in a certain neighborhood x0 x0 where the derivative of the fu