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.