Pulsed-laser crossed-beam thermal lens spectrometry for detection in a microchannel: Influence of the size of the excitation beam waist

Crossed-beam thermal lens spectrometry is especially designed for the detection of very small samples in capillary tubes and more generally in microfluidic devices. In this work, the effect of the size of the excitation beam with respect to the size of the sample microchannel has been investigated. Although the signal is inversely proportional to the size of the excitation waist into the sample, the use of large waists may provide greater sensitivities when short-pulse excitation lasers are used and allows easier optimization of the optical design. On the contrary, the use of small beam waists reduces the edge effects that can arise depending on the nature and thickness of the walls of the sample holder. Moreover, small beams provide better spatial resolution and have allowed the measurement of flow velocities as low as 1 mm s(-1).