Thermal lens calorimetry is applied to the measurement of infrared absorption of condensed phase samples. The unique trade offs encountered with longer wavelength lasers in thermal lens measurements are discussed. As a demonstration of the technique, the determination of hydrocarbons using a helium-neon laser operated at 3.39 μm is reported. The minimum detectable sample has an absorbance, A = 5 × 10−4, corresponding to 8 ng of 2,2,4-trimethylpentane within the beam volume in the sample. Interpretation of absorbance at the helium-neon laser transition is aided by the determination of functional group molar absorptivities from a series of normal, cyclic, and branched-chain hydrocarbons. The average specific absorption coefficient for hydrocarbons at this wavelength is found to be 2.09 (±0.09) 1 g−1 cm−1.

Laser-induced thermal lens calorimetry as applied to small absorbance determinations has traditionally relied upon the approximation that the sample behaves optically as a thin lens. The choice of a cell length which is not ideal for theory may be necessary for increased sensitivity. Two models for treating long samples of variable absorbance and enhancement are presented. The simpler of these is based on an approximation that such samples will behave according to the integrated position dependence of a thin lens. A more complex model, requiring numerical evaluation, includes the effects of sample refractive index and lens element coupling. The numerical model is verified experimentally and used to establish the limits of accuracy of the thin lens and integrated position dependence models. The consequences of using long path length samples in analytical thermal lens measurements are discussed in light of the numerical model results.