Tunable deep-UV Raman spectroscopy reveals nitrate photolysis

Abstract

Deep-UV Raman spectroscopy is a promising method for the analysis of nitrates and nitrites in water at ppm (mg/l) concentrations. In addition to the high sensitivity, the tunability of the laser source allows to deeper investigate the photoinduced reactions taking place under deep-UV illumination. Under these conditions, nitrate ions decompose into oxygen and nitrite through different reaction pathways. Analysis of the evolution of nitrate and nitrite Raman modes as a function of the excitation wavelength allows for estimating the photo-energy dependent quantum yield of the photolysis process. The results highlight the limits and capabilities of deep UV Raman as a on-line nitrate and nitrite monitoring method.

Short-wavelength Raman spectroscopy for in-situ water analysis: a feasibility study

“…7 top), exhibits a clearly visible N O − 3 peak at 1050 cm −1 , which corresponds to the ν symmetric stretching mode. The related LOD, as demonstrated in our previous study, 31 is approximately 2 mg/l and it is achievable by simply increasing the integration time from 20 to 60 seconds. Notably, a broader Raman mode at around 1330 cm −1 is assigned to a N O − 2 vibration.…”

“…This study is based on the prototype deep-UV Raman spectrometer described in Ref. 31 The system is further reduced in dimensions and cost and is adapted for measurements of liquid samples in 180 • backscattering geometry (Fig. 2).…”

“…A single aluminum 90 • off-axis parabolic mirror is used to focus the beam and collect the scattered light. 31 The complete instrument with the compact light source fits on a 45 x 30 cm breadboard. The advantages of the chosen excitation wavelength include a spectral separation of the fluorescence and Raman signals and availability of the optical components.…”

“…35 However, their lifetime is relatively short and their peculiar annular beam profile may impede a proper beam focusing. Tunable light sources covering the deep-UV range exist in the form of tunable lasers 31,36 and UV synchrotron radiation, 37 enabling measurement of analytes in the resonance condition. However in both cases they cannot be implemented in view of developing a portable device.…”

“…the separation of the Rayleigh from the Raman light, is even more challenging. 31 Additional deep-UV range effects potentially include optical self-absorption, photo-decomposition of the sample and resonances, practically affecting the measured spectrum. Those "intrinsic-processes", which are seldom present in standard Raman spectroscopy, while boosting the sensitivity (resonance enhancement) increase the complexity of the measurement and thus impede an extensive application.…”