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 sampl e cell under constant irradiance excitation is described and tested. T he model is developed with and without the assumption that the sample cell does not change temperature over the irradiation time, In both ca ses, the photothermal lens is predicted to be parabolic in form with a strength that is independent of sample cell radius. The predicted irr adiance independence suggests that incoherent illumination can be used to perform photothermal lens spectroscopy in low-volume cells, Experi mental evidence is obtained using a Xe are lamp to perform phototherma l lens spectroscopy in a 6 mu L cylindrical spectrophotometric cell. O ptical filters are used to reduce the power at IR and UV wavelengths o f the Xe lamp emission spectrum. This pseudo-white-light source enable s indirect optical absorbance measurement independent of the absorptio n spectrum of the analyte. The preliminary data reported show that pho tothermal lens signals can be obtained using wide-spectral-bandwidth, incoherent excitation sources, Although the theoretical enhancement fa ctor is found to be only similar to 0.01 for these experiments, limits of detection of the order of 30 to 300 pM pseudoisocyanine dye in eth anol solution are found. This corresponds to spectral integrated absor ption detection limit from 10(-4) to 10(-6) au in the centimeter path length cell. These low detection limits are found even with low enhanc ement factors because the factors that affect the noise in the phototh ermal lens and conventional transmission spectroscopy signals are not the same in these experiments. The major sources of uncertainty in the se detection limit estimates are knowledge of the excitation source sp ectrum and periodic chaotic behavior of the diode laser used as a prob e of the photothermal lens. Examination of the response time of the si gnal reveals that thermal conductivity of the sample cell influences t he 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 ce ll walls. The sample cell thermal conductivity determined by this meth od is consistent with ferrous materials. (C) 1997 Society of Photo-Opt ical Instrumentation Engineers.