Photothermal Spectrometry in Small Liquid Channels

Abstract

A novel apparatus for performing photothermal lens spectroscopy is described. The apparatus uses a low-volume cylindrical sample cell, a chopped or pulsed excitation laser, and a continuous probe laser. The full volume of the sample is irradiated with constant, e.g., non-Gaussian, irradiance beam produced by the excitation laser. Constant irradiance excitation source does not directly produce the photothermal lens element in the sample. The lens element is formed by thermal diffusion from the irradiated sample volume, through the sample cell walls. Under continuous irradiation, thermal diffusion results in a parabolic temperature change profile. The apparatus has been found to work with cells designed to contain sample volumes from 6 μL down to 24 nL. Larger and smaller volume cells are practical but signals produced in the smallest sample cells exhibit deviation from that expected based on the theory. This is attributed to probe laser diffraction by the circular aperture of the sample cell. (Received on August 9, 2001, Accepted on......) Introduction Photothermal lens spectrometry is a novel means to measure optical absorption. It is one of several ultra-sensitive photothermal spectroscopy methods applicable to trace analysis. The applicability to trace analysis is due to the high sensitivity and small sample volumes needed for an analysis. There are several physical effects that limit sensitivity and influence the accuracy of measurements obtained using photothermal lens spectrometry. First, the photothermal lens signal is related to the lens formed the sample as a consequence of light absorption and subsequent energy transfer to the matrix. The optical element produced when laser beams are focussed into a homogeneous sample is not a simple lens. The aberrant nature of the photothermal lens results in signal magnitudes that are somewhat smaller than expected and also more difficult relate to sample absorbance. Second, sample irradiation can produce large temperature changes. In theory, the on-axis temperature change approaches infinity at long times for any excitation power when using continuous laser excitation sources. In practice, large temperature changes distort the thermal lens perturbation due to density differences and convection heat transfer or even boiling may occur. The later distorts the signal and ruins the analytical utility. Other factors resulting in inaccurate absorbance determination are related to the photophysics of sample excitation and solvent (matrix) relaxation. Nonlinear optical absorption and bleaching, transient volume changes, and excited state analyte absorbance can occur when using high irradiance laser excitation sources. These effects may generally be avoided only by using low irradiance. This is often accomplished by choosing continuous over pulsed lasers for sample excitation. An apparatus utilizing a two-laser photothermal lens apparatus that is immune to many problems associated with photothermal lens spectrometry and capable measuring optical absorbance in very small samples is described in this work. The apparatus uses a nanoliter-volume sample cell with a visible laser used as the excitation source. The sample cell is constructed from high thermal conductivity metal and has a large surface area and relatively large heat capacity. A simplified version of the theory for describing the photothermal lens developed in a cylindrical sample cell is given. A model for relating signals to sample absorbance is given and experimental evidence is presented. Timedependent photothermal signals are detected The technique exhibits high sensitivity and can be used for ultra-sensitive detection. Analyte excitation is often performed using laser light sources. The resulting photothermal lens is measured using a probe laser. Excitation and probe lasers may be the same. Laser light sources produce a small spot size at a focus. The small spot size allows one to probe extremely small sample volumes. Theory Theoretical time-dependent temperature changes and inverse photothermal lens focal lengths for both standard and cylindrical sample cells are described in the literature. The cylindrical sample cell used here has a large thermal conductivity compared to the solvent. Little temperature change occurs at the sample cell wall because of the high thermal conductivity ratio. The solution to the thermal diffusion equation for a zero temperature change at the cell wall temperature change is