Dual-beam thermal lens spectrophotometry in flowing samples with chopped continuous wave laser excitation

Abstract

The thermal lens response for flowing samples following chopped continuous wave laser excitation has been studied with respect to the chopping frequency and flow-rate. The use of two types of flow cell and distinct configurations of flow direction with respect to beam propagation enabled the main ways by which the flow may disturb the thermal lens signal to be considered. With a cell in which the flow is perpendicular to the beam axis, the signal was mainly degraded by the bulk flow the effect of which was related to the chopping period and the residence time of the sample in the probe region as defined by the beam spot size. When the flow was parallel to the beam propagation and the optical path length not sufficiently long the decrease in signal amplitude was found to no longer depend on the bulk flow, but rather to originate from turbulence and mixing in the channel cell. With a chopping frequency of 10 Hz and flow-rate of 1 ml min–1, the signal was degraded by a factor of 2 by the bulk flow and by less than a factor of 1.2 by mixing compared with a static sample signal. At 80 Hz, the signal for continuously flowing samples was nearly independent of flow for flow-rates as high as 1.2 and 2 ml min–1 in the transverse and axial configurations, respectively. However, for flow–injected samples the signal was found to depend on the time response of the detection system.