Technical Note
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Highly sensitive Raman spectroscopy with low laser power for fast in-line reaction and multiphase flow monitoring Frank Braun, Sebastian Schwolow, Julia Seltenreich, Norbert Kockmann, Thorsten Röder, Norbert Gretz, and Matthias Raedle Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b01509 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016
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Analytical Chemistry
Highly sensitive Raman spectroscopy with low laser power for fast in-line reaction and multiphase flow monitoring Frank Braun,†,⊥ Sebastian Schwolow,‡,⊥ Julia Seltenreich,† Norbert Kockmann,§ Thorsten Röder,‡ Norbert Gretz,∥ Matthias Rädle†,* †
Mannheim University of Applied Sciences, Institute of Process Control and Innovative Energy Conversion, Paul-WittsackStr. 10, 68163 Mannheim, Germany ‡ Mannheim University of Applied Sciences, Institute of Chemical Process Engineering, Paul-Wittsack-Str. 10, 68163 Mannheim, Germany § TU Dortmund University, Biochemical and Chemical Engineering, Equipment Design, Emil-Figge-Straße 68, 44227 Dortmund, Germany ∥
Medical Research Center, Medical Faculty Mannheim, University of Heidelberg, Theodor-Kutzer-Ufer 1-3, 68167 Mannheim, Germany ABSTRACT: In process analytics, the applicability of Raman spectroscopy is restricted by high excitation intensities or long integration times required. In this work, a novel Raman system was developed to minimize photon flux losses. It allows specific reduction of spectral resolution to enable the use of Raman spectroscopy for real-time analytics when strongly increased sensitivity is required. The performance potential of the optical setup was demonstrated in two exemplary applications: First, a fast exothermic reaction (Michael addition) was monitored with backscattering fiber optics under strongly attenuated laser power (7 mW). Second, high-speed scanning of a segmented multiphase flow (water/toluene) with sub-µL droplets was achieved by aligning the focus of a coaxial Raman probe with long focal length directly into a perfluoroalkoxy (PFA) capillary. With an acquisition rate of 333 Raman spectra per second, chemical information was obtained separately for both of the rapidly alternating phases. The experiment with reduced laser power demonstrates that the technique described in this paper is applicable in chemical production processes, especially in hazardous environments. Further potential uses can be envisioned in medical or biological applications with limited power input. The realization of high-speed measurements shows new possibilities for analysis of heterogeneous phase systems and of fast reactions or processes.
INTRODUCTION In recent years, Raman spectroscopy has been used in an increasingly wide range of applications for process and reaction monitoring1. Because of its non-invasive nature, this technique is applicable for obtaining chemical information in process units and production lines without any sample preparation. Thus, Raman spectroscopy has strong potential for real-time monitoring in a wide range of fields, e.g., in chemical and pharmaceutical reaction control2 and in monitoring of procedures such as crystallization,3 freeze drying,4 hot-melt extrusion,5 mixing, phase separations, sedimentation, coating,6 powder blending,7 polymerization,8,9 and distillation.10 Nondestructive measurements conducted from outside the reactor without direct physical contact of the probe head with the often corrosive or unstable reactants and products can be realized for numerous reactor systems, especially because glass (wall/window material) and water (solvent) show little interference due to their low Raman sensitivity. Furthermore, because of the possibility of using a small sampling volume, Raman spectroscopy has been increasingly applied in microstructured devices11–13 where monitoring is often limited by available detection methods. The implementation of Raman spectroscopy for reaction monitoring in microreactors has
been realized by embedding fibers in a microfluidic chip14 or by focusing an objective lens onto the fluid flow for contactless detection.15–18 Concentration profiles of reactants and products in microchannels have been captured via confocal Raman microscopy with a high spatial resolution (
