Pulsed-laser mode-mismatched crossed-beam thermal lens spectrometry within a small capillary tube: effect of flow rate and beam offset on the photothermal signal.

Pulsed-laser crossed-beam thermal lens spectrometry within a small capillary tube has been investigated using static as well as flowing samples. It is shown that when both the excitation and probe beams are perfectly aligned, time-resolved and peak signals are adequately described by the theoretical relations previously reported for infinite samples. Depending on flow velocity and/or the lateral offset between excitation and probe beam axis, the optical element formed by the excitation beam may have a diverging or converging effect on the probe beam. However, optimization of the photothermal signal with pulsed excitation and flowing samples does not require offsetting the pump and probe beams and the peak signal is almost independent on the flow rate. The performance of the method has been checked with samples injected in a flowing stream. The response is linear over three orders of magnitude and the limit of detection obtained for cobalt nitrate in ethanol corresponds to an average absorbance of 5 x 10(-7) inside the capillary.