Using sub-microliter cylindrical sample cells for photothermal
lens spectrometry of stable and photo-labile species

Stephen E. Bialkowski and Agnès Chartier

Department of Chemistry and Biochemistry
Utah State University
Logan, UT 84322-0300
USA

Poster: Area 10, New instrumentation and methodology

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 several advantages over the conventional photothermal lens using a relatively large volume sample cell. Obviously, the small volume of the sample cell, 0.25 mL in the experiments performed here, requires less sample. This is advantageous in micro-analysis. Second, photothermal lens signals produced by thermal diffusion through the sample cell walls are less susceptible to the bulk heat transfer effects due to convection. Third, although the refractive index change may depend on the partial refractive index and partial molar volume of transient species, these changes do not result in a photothermal lens element. By monitoring both the central portion and the full probe laser beam, the apparatus can compensate for transmission changes due to bulk density, refractive index, or absorbance changes. Fourth, the maximum temperature change is finite. The maximum on-axis temperature change produced by continuous, Gaussian laser excitation of a homogeneous sample is, in theory, infinite. Even though the temperature change doesn't actually reach infinity, temperature changes can be high enough to boil solvents and produce signal instabilities due to turbulent convection heat transfer. The cylindrical sample cell circumvents the large temperature changes by heat transport to the surrounding. Fifth, the parabolic-form photothermal lens produced by thermal diffusion is aberration free. Subsequently, the beam propagation theory that only approximately describes the effect of the photothermal lens produced from Gaussian laser excitation exactly describes the effect of the cylindrical cell lens.

The present experimental apparatus uses a 514.5 nm Ar⁺ laser as an excitation source and a HeNe laser as the probe laser. The cylindrical sample cell is a commercial micro HPLC absorption spectrophotometry cell. The cell volume is 0.25 mL, has a 2 mm optical pathlength, and a cell radius of 200 mm. It is equipped with fused silica entrance and exit windows. Samples are introduced through narrow-bore HPLC tubing. The total volume, including the sample introduction tubes, is under 0.5 mL. The Ar⁺ laser beam has a cylindrical profile. The beam is attenuated and imaged to nearly 200 m m radius at the entrance window. The maximum excitation laser power used in 10 mW. The 2 mW 632.8 nm probe laser beam is independently focused to optimize response to the photothermal lens produced within the sample cell. After passing through the sample cell, the probe beam is split and directed to a large area Si photodiode, and through a pinhole aperture to a second photodiode. Photodiode outputs are processed with transimpedence amplifiers, and an operational divider is used to obtain the signal that is independent of the probe laser power.

Experiments to verify the operation of the apparatus are performed with dicyclopentadienyl iron (FeCp₂) in ethanol and acetonitrile solvents. Photothermal lens signals are processed in the usual, F (0)/F (t)-1, fashion. The resulting signals are found to be relatively linear and reproducible. The experimental photothermal lens enhancement is found to be that predicted from theory, within experimental error.

Experiments are also performed with tris-bipyridyl Ru(II) chloride and perchlorate salts (Ru(II)bpy₃), ferric chloride and sulfate, xanthene dyes, and methyl viologen. Ethanol is used as the solvent in these experiments. Results for methyl viologen show that there is no substantial change in absorbance with irradiation time indicating that photoelectron production from the stainless steel sample cell walls does not occur to a great extent. Results obtained for Ru(II)bpy₃ show that there is an increase in the optical absorbance of these samples with irradiation time. Since sample cell wall irradiation does not produce free electrons, electron capture to produce the Ru(I)bpy₃ cannot be responsible. Another possibility for the apparent change in absorbance is the Soret effect. However, experiments in which the excitation source was blocked for several hours after the sample had been irradiated show no change in the absorbance over that just before blocking the beam. Mass concentration changes due to the Soret effect would have dispersed in this time. These results suggest that photolysis is occurring, even at moderate irradiances. This is not expected since Ru(II)bpy₃ is reportedly stable toward photolysis. It is possible that these photolysis effects are observed for the first time in these experiments because the irradiated sample volume cannot mix with a larger volume of nascent solution. Experiments preformed with ferric chloride also exhibit anomalous effects. In this case, the short term, chopped signal does not follow the simple behavior observed with FeCp₂.

In summary, it is likely that photolysis products are being observed in these experiments because of the small static volumes used. The irradiated analyte solution cannot mix with a reservoir of nascent solution as in the normal photothermal lens experiments. The apparatus and methods open up new areas of application where only small sample quantities can be obtained. We plan to exploit the other advantages of the cylindrical sample cell in the future for various applications in photodynamics and to deduce the specific volumes of metastable states