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

• Published in: Sensors (Basel, Switzerland) Switzerland), 2015-09, Vol.15 (9), p.23110-23125
• Main Authors: Rupp, Simone, Off, Andreas, Seitz-Moskaliuk, Hendrik, James, Timothy M, Telle, Helmut H
• Format: Article
• Language: English
• Subjects: Capillarity; capillary; Fluorescence; fluorescence reduction; Gas analysis; Gases; instrument development; Lasers; Light; Process control; Process controls; Raman spectroscopy; Real time; real-time monitoring; Sensors; Spectrum analysis
• ISSN: 1424-8220; 1424-8220
• DOI: 10.3390/s150923110

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.