Optimisation of thermal lens microscopic measurements in a microchip Original Research Article

The thermal lens optical scheme–design was optimised for microscopic measurements in microchannels. The efficient pathlength of the sample, irradiated volume, and the diameter of the thermal lens were estimated. Experimental time curves of development of the thermal lens and periodical oscillations of the signal due to convectional heat transfer are in good agreement with the theoretically expected behaviour. Noise sources (laser noises, instrumental flicker noise, convection, and flow noise) were studied. The possible effect of probe laser power on transient and steady-state thermal lens measurements were estimated. The effect of solvent absorption on the performance characteristics is shown. Under the optimum optical scheme–design, the limits of detection of ferroin and Sunset Yellow FCF at 488.0 nm are 1×10−8 and 4×10−9 mol dm−3, respectively (corresponding quantities in the detection volume are 3×10−21 and 1×10−21 mol). The total linear calibration range is n×10−8 to n×10−4 mol dm−3, the repeatability R.S.D. for this range is 3–7%. The optimised instrument was also used for the determination of characteristic rate constants of formation and dissociation of ferroin at the level of n × 10−8 mol dm−3. Some analytical applications are discussed.