Photothermal deflection spectroscopy of an aqueous sample in a narrow bore quartz capillary

Abstract

An apparatus to perform photothermal deflection spectroscopy on liquid samples within a cylindrical capillary is described. A tunable dye laser, modulated by an optical chopper, serves as an excitation source. The resolution of a spectral absorption peak near 575 nm, of a 1×10−3 M Nd3+ aqueous solution, demonstrates the effectiveness of the system. The sample is contained within a 75 μm internal diameter quartz capillary, typical of those used for capillary electrophoresis. Across the middle of the probed section of the capillary, the magnitude of the resolved peak is 5.2×10−5 absorbance units. A helium–neon laser, focused to a 1/e2 waist diameter of 40 μm, provides an optical probe beam across the center of the sample, overlapping the excitation beam at an angle of 3°. Maximum signal-to-noise ratio is achieved with the apparatus when the excitation beam is modulated at a frequency near 205 Hz. The deflection responsivity of the probe beam at this frequency is 650 nrad per μW of absorbed excitation radiation, with an internal noise level in the system of 0.6 nrad Hz−1/2. The shot noise from probe beam radiation upon the photodiodes in the position sensitive detector exceeds noise from other sources.