Thermal Lens Spectrometric Detection and Characterization of Fatty Acids

Mladen Franko*, Dane Bicanic**, Jürgen Gibkes**, Monique Bremer**, Edwin Akkermans**

*Institut Jožef Stefan, Jamova 39, 1001 Ljubljana, Slovenia (correspondence address)
**Wageningen Agricultural University, Department of Agricultural Engineering and Physics, Bomenweg 4, 6703 HD Wageningen, The Netherlands

Article history:

Received July 8, 1996 
Accepted December 18, 1996

Key words:

fatty acids, thermal lens spectrometry, IR absorption spectra, thermal conductivity

Pholothermal spectrometric techniques such as thermal lens spectrometry (TLS) are characterized by high sensitivity, which enables measurements of absorbances as low as 10-7. In this work the potentiality of TLS to measure small absorbances of liquid samples at CO (5-7 µm) and CO2 (9-11 µm) laser wavelengths was investigated. CO2 laser was applied for excitation of characteristic vibrational transitions of trans-unsaturated fatty acids, while the CO laser was used for excitation of the carbonyl group in oleic acid. Results were compared to those obtained on an FT-IR spectrometer. For trans-unsaturated fatty acids and oleic acid the limits of detection were improved by a factor of 220, compared to the IR transmission and FT-IR method, respectively. Tunability of the CO laser enabled recording of the first TLS spectra of carbonyl compounds in liquid samples. In addition to chemical characterization the TLS technique enables measurements of some physical properties of the sample. This was successfully exploited to measure thermal conductivities of fatty acids, which were not reported to date.