Improving the Detection Limit in a Capillary Raman System for In Situ Gas Analysis by Means of Fluorescence Reduction

Abstract

Raman spectroscopy for low-pressure or trace gas analysis is rather challenging, in particular in process control applications requiring trace detection and real-time response; in general, enhancement techniques are required. One possible enhancement approach which enjoys increasing popularity makes use of an internally-reflective capillary as the gas cell. However, in the majority of cases, such capillary systems were often limited in their achievable sensitivity by a significant fluorescence background, which is generated as a consequence of interactions between the laser light and optical glass components in the setup. In order to understand and counteract these problems we have investigated a range of fluorescence-reducing measures, including the rearrangement of optical elements, and the replacement of glass components—including the capillary itself—by metal alternatives. These studies now have led to a capillary setup in which fluorescence is practically eliminated and substantial signal enhancement over standard Raman setups is achieved. With this improved (prototype) setup, detection limits of well below 1 mbar could be obtained in sub-second acquisition times, demonstrating the potential of capillary Raman spectroscopy for real-time, in situ gas sensing and process control applications, down to trace level concentrations.

Fiber-Enhanced Raman Spectroscopy for Trace-Gas Sensing in the High-Concentration Gas Background With an Anti-Resonant Hollow Core Fiber

“…It not only requires a small gas volume of microliters but also has a flexible layout. Compared to hollow-core capillary [20] with high-transmission loss [21], hollow-core fiber has lower transmission loss [22]. Therefore, FERS combined with hollow core fiber can achieve higher Raman signal enhancement.…”

Development of a gas‐phase Raman instrument using a hollow core anti‐resonant tubular fibre

“…These include methods where the gas sample is introduced in to (a) an optical cavity where a series of mirrors guides the Raman excitation light through an increased pathlength [ 7 ] and (b) hollow core waveguide methods, where a gas sample is introduced into the hollow region of a waveguide which guides the light through an increased interaction path through the gas sample. These include glass capillary or metal ‘light‐pipe’ methods [ 8 ] ; and hollow core fibre optic based methods which operate via novel light guiding mechanisms. [ 9–11 ]…”

“…These include methods where the gas sample is introduced in to (a) an optical cavity where a series of mirrors guides the Raman excitation light through an increased pathlength [7] and (b) hollow core waveguide methods, where a gas sample is introduced into the hollow region of a waveguide which guides the light through an increased interaction path through the gas sample. These include glass capillary or metal 'light-pipe' methods [8] ; and hollow core fibre optic based methods which operate via novel light guiding mechanisms. [9][10][11] Despite the well documented advantages that Raman based gas detection systems potentially offer, no single method appears to have been widely adopted by the gas sensing community, this may be related to sample handling constraints, and/or complex optical alignment requirements which limit use to static, bench-top based environments.…”

Optimization of parabolic cell for gas Raman analysis

“…Hanf et al inserted pinhole at the focal plane to spatially separate the silica Raman signal . Rupp et al proposed a system based on full‐metal capillary whose limit of detection (LOD) is seven times better than the system based on a standard glass capillary …”