Non-invasive analysis in micro-reactors using Raman spectrometry with a specially designed probe

Abstract

An optical interface has been designed to maximise the sensitivity and spatial resolution required when Raman spectrometry is used to monitor a reaction in a micro-reactor, revealing advantages over a conventional commercial probe. A miniature aspheric lens was shown to be better than microscope objectives to focus the probing laser beam onto the sample. The diameters of the exciting and collection optical fibres were also shown to have a significant influence on sensitivity and the signal-to-background ratio, with 62.5 µm diameter 0.28 numerical aperture (NA) fibres found to be best for analysis of liquids in the 150 µm deep channel in the micro-reactor used. With a spectral measurement time of 2 s, it was shown that the probe could monitor the progress of an esterification reaction in real time and quickly optimise the reagent flow rates. The fast response time revealed features related to short-term pump instabilities and micro-reactor rheology effects that would not have been identified without rapid real-time measurements.