Characterization of Thermal Modulation of Electrical Conductivity for Small Volume Absorbance Measurements

Laser-induced thermal modulation of electrical conductivity is a new photothermal method for the measurement of minute absorbance in sub-nanoliter liquid samples. In this paper, a model is presented for the technique. This model is based upon the interaction of a Gaussian laser beam with a cylindrical, homogeneous electrolytic resistor. Particularly simple results are produced if the laser beam spot size is much smaller than the diameter of the electrodes. The theory is verified with respect to the electrode spacing and to the laser power, spot size, and chopping frequency; the highest sensitivity occurs with low chopping frequency, high laser power, and small electrode spacing. The signal is independent of laser spot size, as long as the beam does not illuminate the electrode surface. The precision of the measurement is dominated by shot noise in the current flow through the electrolytic resistor. One point absorbance detection limits of 5.5 × 10−6 measured across the 76-μm diameter electrode wires are obtained in aqueous solution with a 5- mW helium–neon laser. With this instrument, the difference in the absorbance of water and deuterated water is easily determined at 632.8 nm. Keywords: thermal modulation of electrical conductivity.