An ultraviolet-laser excitation microscopic photothermal lens imaging system for observing chemical compounds in biological cells has been designed and constructed, in which a reflection objective lens is used for focusing an excitation laser beam at 261 nm and a probe laser beam at 784 nm. Photothermal amplitude and phase images of yeast fungus cells are successfully obtained; together with a transmitted light image at 784 nm. The amplitude image represents photo-absorption at 261 nm by the cells. The phase image seems to represent local thermal properties around the cells.

A measurement system for ultraviolet-laser excitation photothermal lens spectroscopy has been designed and constructed for measuring dilute amino acids in liquid solutions. An ultraviolet laser beam is generated as the fourth harmonic of a Ti:sapphire laser in the wavelength range of 212 to 220 nm. Photothermal lens spectra of alanine, phenylalanine, serine, and tryptophan are observed. Photothermal lens spectra of these samples are coincident with their absorption spectra. It is demonstrated that photothermal lens signals of the amino acids can be amplified by simultaneous excitation with a visible laser. This ultravioletexcitation visible-enhancement system is applicable to photoacoustic detection.

Abstract. Thermal lensing (TL) permits ultrasensitive measurements of optical absorption of analytes in very small volumes. Separation-detection conditions of non-labeled amino acids with micro-HPLC/UV absorption detector are optimized, and direct determination of nonlabeled amino acids by micro-HPLC/UV-excitation TL detection in a gradient elution is successfully demonstrated. Non-labeled amol-level amino acids is detectable with the TL detection system, which has thousand times better sensitivity than a conventional UV detector.

A photothermal signal from a liquid sample in a capillary channel flow is detected and analyzed to optimize experimental conditions of microfluidic devices used for separation. A theoretical model for photoinduced temperature increase and photothermal signal intensity generated by intensity-modulated cw excitation beams at a crossed-beam configuration is proposed. Four experimental parameters (probe beam offset, excitation beam chopping frequency, linear flow velocity, and excitation beam size) and physical properties of solvent mainly dominate the signal. The model well explains the photothermal signal obtained experimentally under low velocity and high chopping frequency conditions. The obtained results make it possible to optimize the experimental conditions for the highly sensitive detection of chemicals under flow conditions.