title: 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.
authors.affiliation: Institute for Technical Physics, Tritium Laboratory Karlsruhe, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany. Simone.Rupp@kit.edu.
authors.forename: Simone
authors.initials: S
authors.lastname: Rupp
authors.affiliation: Institute for Technical Physics, Tritium Laboratory Karlsruhe, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany. Andreas.Off@kit.edu.
authors.forename: Andreas
authors.initials: A
authors.lastname: Off
authors.affiliation: Institute for Technical Physics, Tritium Laboratory Karlsruhe, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany. Hendrik.Seitz-Moskaliuk@kit.edu.
authors.forename: Hendrik
authors.initials: H
authors.lastname: Seitz-Moskaliuk
authors.affiliation: Institute for Technical Physics, Tritium Laboratory Karlsruhe, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany. tjames@laserquantum.com.
authors.forename: Timothy M
authors.initials: TM
authors.lastname: James
authors.affiliation: Instituto Pluridisciplinar, Paseo Juan XXIII-1, Universidad Complutense, 28040 Madrid, Spain. helmut@telle-online.eu.
authors.forename: Helmut H
authors.initials: HH
authors.lastname: Telle
keywords: Raman spectroscopy; capillary; fluorescence reduction; gas analysis; instrument development; process control; real-time monitoring