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, <i>A</i> = 5 × 10<sup>−4</sup>, 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<sup>−1</sup> cm<sup>−1</sup>.
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