Progress in thermal lens spectrometry and its applications in microscale analytical devices

Development of thermal lens spectrometry (TLS) as a micro space-compatible photothermal technique and its applications for analysis of different chemical compounds in micro space and particularly in microfluidic systems are reviewed. Theoretical treatment of TLS in micro space has evolved from simply following conventional theory and predictions in macro space to employing a more accurate theory where impacts of the excitation source (Gaussian laser, top-hat incoherent light source, beam divergence, power density), detection scheme (probe beam waist, mode-mismatching degree), sample flow, and sample cell (top/bottom layers, side wall) on the TL signal are included. Noise sources (light sources, sample status, detector) in TLS systems have been analyzed, and optimum pinhole-to-beam radius ratio is suggested for the maximum signal-to-noise ratio. With different excitation light sources from ultraviolet, to visible, to near-infrared regions and coupled with microfluidic devices, these TLS instruments with good temporal and spatial resolution have found many applications for highly sensitive and/or high-throughput detection of chemical or biochemical analytes, for cell imaging or single particle/molecule detection, and for characterization of molecular diffusion in single- or two-phase systems. Prospects and challenges of TLS for future applications in microchemical analysis are discussed.