PHOTOTHERMAL DEFLECTION SPECTROSCOPY OF AN AQUEOUS SAMPLE IN A NARROW-BORE QUARTZ CAPILLARY

An apparatus to perform photothermal deflection spectroscopy on liquid samples within a cylindrical capillary is described. A tunable dye la ser, modulated by an optical chopper, serves as an excitation source. The resolution of a spectral absorption peak near 575 nm, of a 1x10(-3 ) M Nd3+ aqueous solution, demonstrates the effectiveness of the syste m. The sample is contained within a 75 mu m internal diameter quartz c apillary, typical of those used for capillary electrophoresis. Across the middle of the probed section of the capillary, the magnitude of th e resolved peak is 5.2x10(-5) absorbance units. A helium-neon laser, f ocused to a l/e(2) waist diameter of 40 mu m, provides an optical prob e beam across the center of the sample, overlapping the excitation bea m at an angle of 3 degrees. Maximum signal-to-noise ratio is achieved with the apparatus when the excitation beam is modulated at a frequenc y near 205 Hz. The deflection responsivity of the probe beam at this f requency is 650 nrad per mu W Of absorbed excitation radiation, with a n internal noise level in the system of 0.6 nrad Hz(-1/2). The shot no ise from probe beam radiation upon the photodiodes in the position sen sitive detector exceeds noise from other sources; [S0034-6748(98)03506 -0].