Photothermal lens spectrometry of homogeneous fluids with incoherent white-light excitation using a cylindrical sample cell
A model for photothermal lens signal generation in a cylindrical sample cell under constant irradiance excitation is described and tested. The model is developed with and without the assumption that the sample cell does not change temperature over the irradiation time. In both cases, the photothermal lens is predicted to be parabolic in form with a strength that is independent of sample cell radius. The predicted irradiance independence suggests that incoherent illumination can be used to perform photothermal lens spectroscopy in low-volume cells. Experimental evidence is obtained using a Xe arc lamp to perform photothermal lens spectroscopy in a 6 (mu) L cylindrical spectrophotometric cell. Optical filters are used to reduce the power at IR and UV wavelengths of the Xe lamp emission spectrum. This pseudo-white-light source enables indirect optical absorbance measurement independent of the absorption spectrum of the analyte. The preliminary data reported show that photothermal lens signals can be obtained using wide- spectral-bandwidth, incoherent excitation sources. Although the theoretical enhancement factor is found to be only approximately 0.01 for these experiments, limits of detection of the order of 30 to 300 pM pseudoisocyanine dye in ethanol solution are found. This corresponds to spectral integrated absorption detection limit from 10-4 to 10-6 au in the centimeter path length cell. These low detection limits are found even with low enhancement factors because the factors that affect the noise in the photothermal lens and conventional transmission spectroscopy signals are not the same in these experiments. The major sources of uncertainty in these detection limit estimates are knowledge of the excitation source spectrum and periodic chaotic behavior of the diode laser used as a probe of the photothermal lens. Examination of the response time of the signal reveals that thermal conductivity of the sample cell influences the characteristic signal rise and decay time constants. The radiative heat transfer model is applied to interpret measured time constants in terms of the cell thermal conductivity and thickness of the sample cell walls. The sample cell thermal conductivity determined by this method is consistent with ferrous materials.