Eliminating Non-linear Raman Shift Displacement Between Spectrometers via Moving Window Fast Fourier Transform Cross-Correlation.

Hui Chen,Yan Liu,Zhi-Min Zhang,Yongbing Cao,Feng Lu

doi:10.3389/fchem.2018.00515

Abstract

Obtaining consistent spectra by using different spectrometers is of critical importance to the fields that rely heavily on Raman spectroscopy. The quality of both qualitative and quantitative analysis depends on the stability of specific Raman peak shifts across instruments. Non-linear drifts in the Raman shifts can, however, introduce additional complexity in model building, potentially even rendering a model impractical. Fortunately, various types of shift correction methods can be applied in data preprocessing in order to address this problem. In this work, a moving window fast Fourier transform cross-correlation is developed to correct non-linear shifts for synchronization of spectra obtained from different Raman instruments. The performance of this method is demonstrated by using a series of Raman spectra of pharmaceuticals as well as comparing with data obtained by using an existing standard Raman shift scattering procedure. The results show that after the removal of shift displacements, the spectral consistency improves significantly, i.e., the spectral correlation coefficient of the two Raman instruments increased from 0.87 to 0.95. The developed standardization method has, to a certain extent, reduced instrumental systematic errors caused by measurement, while enhancing spectral compatibility and consistency through a simple and flexible moving window procedure.

Highlights

Over the last few decades, the use of Raman spectroscopy in combination with chemometric methods has increased significantly for analysis of pharmaceutical products (Sacré et al, 2010; Dégardin et al, 2011; Loethen et al, 2015), detection of food adulteration (Zou et al, 2009; Cheng et al, 2010), and other applications (Mrozek et al, 2004; Taleb et al, 2006; Muehlethaler et al, 2011)
When a drug sample is measured on a secondary instrument to obtain an average spectrum for synchronization, its corresponding standard spectrum contained in the existing spectral library can be certainly used as the reference to correct shift displacements
Methods for the synchronization of spectra are indispensable for successful applications using different spectrometers

Summary

Introduction

Over the last few decades, the use of Raman spectroscopy in combination with chemometric methods has increased significantly for analysis of pharmaceutical products (Sacré et al, 2010; Dégardin et al, 2011; Loethen et al, 2015), detection of food adulteration (Zou et al, 2009; Cheng et al, 2010), and other applications (Mrozek et al, 2004; Taleb et al, 2006; Muehlethaler et al, 2011). Raman Shifts Correction by MWFFT it has been extensively applied in pharmaceutical analysis by constructing multivariate calibration models These models will be invalid if an existing calibration model is applied to spectra that are collected on a different occasion or a separate instrument, or when the response of an old instrument suffers from variations (Du et al, 2011; Brown, 2013). These variations may, if left untreated, dominate the calibration models, thereby making analysis of samples impractical. Chemometric techniques have been used to circumvent these problems through instrumental transfer or standardization so as to isolate and compensate for any instrumental and environmental variations

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