Direct Photothermal Measurement of Optical Absorption in a Flow System

We describe here a simple photothermal detection scheme in a flow stream based on the temperature difference upstream and downstream of the point of illumination. We use a single, two-junction 25 μm diameter thermocouple to measure the temperature change. The baseline standard deviation in the dark is ∼0.001 °C that increases up to 0.0016 °C depending on the illumination source. We demonstrate the detection of several chromatographically separated dyes both with a 1.5 mm and a 0.1 mm i.d. detection cell, respectively, with a white LED and a solid-state laser source. With an inexpensive 660 nm, 19 mW laser as the light source, the estimated detection limit for methylene blue (MB) was 30 nM, corresponding to 120 amol in the illuminated volume. The dimerization constant of MB and the quantum efficiency of the monomer was determined.