Dual-wavelength Rapid Excitation Raman Difference Spectroscopy System for Direct Detection of Ethanol in Illegal Beverages

A dual-wavelength rapid excitation Raman difference spectroscopy (DWRERDS) system for separation of Raman signals from background interference was developed. In the DWRERDS system, a dual-wavelength diode laser at 784.59 and 785.22 nm with volume Bragg gratings was used as the excitation light source. For each laser line, optical power could reach as high as 550 mW. Under dual-wavelength excitation mode, the DWRERDS system could switch each laser instantaneously and the excitation time of each laser was as short as 1 s. Two Raman spectra were measured with slightly shifted excitation wavelengths and subtracted from each other, resulting in a background-free difference spectrum. The DWRERDS system was applied to complex matrix background suppression, which could improve a very good signal-to-noise ratio (SNR) for Raman spectra. Although ethanol in Coca-Cola could not be detected by conventional Raman spectroscopy, the DWRERDS system could be used for the direct detection of ethanol in illegal alcoholic beverages. The experimental results showed that the limit of detection (LOD) of ethanol in Coca-Cola was volume percentage of 4.76% and the correlation coefficient for the linear regression was as high as 0.984. The DWRERDS system could be used to determine the concentration of ethanol in illegal alcoholic beverages within 1 min without any sample pretreatment.