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Vertical flow apparatus for enhancement and efficient collection of Raman signal



To improve the signal-to-noise (S/N) ratio of Raman spectra, we developed a sample flow system in a vertical flow apparatus (VFA). The VFA consists of a brass plate with a drilled pinhole, cover slip on the brass plate, circulation pump, and reservoir. The sample solution flows through a gap between the brass plate and cover slip, spouts from the pinhole, and forms a laminar flow column. The sample column and surrounding air, respectively, serve as the core and cladding of an optical fiber that confines both excitation beam and scattered light owing to the total reflection at the core–cladding interface. The VFA enabled the enhancement and efficient collection of the Raman signal. The VFA with a 0.2-mm pinhole produced a 12.0-fold enhancement of the non-resonant Raman signal of H2O compared to that measured using a capillary, and the S/N ratio exhibited a 4.1-fold enhancement. The signal count was inversely proportional to the square of the pinhole diameter (φ), possibly owing to the dependence on the density of the electromagnetic energy of the excitation beam in the sample column. The S/N ratio was inversely proportional to φ because the signal enhancement was accompanied by an increase in the photon shot noise. The VFA also produced a 4.9-fold enhancement of the resonance Raman signal of Congo red, although the enhancement factor was smaller than that in the non-resonant Raman measurement. Copyright © 2014 John Wiley & Sons, Ltd.