Dual-wavelength thermal-lens spectrometry as a sensitive and selective method for trace gas analysis

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Abstract
A novel dual-wavelength thermal-lens spectrometer with pump/probe configuration that is capable of simultaneously measuring thermal-lens signals at two different wavelengths has been developed. In this instrument, the two excitation beams are derived from the same argon-ion laser operating in a multiline mode. The sample is excited by these two wavelengths alternately and the corresponding thermal-lens signals are monitored by a He-Ne probe laser. Advantages of this dual-wavelength apparatus include its inherent sensitivity and enhanced selectivity. The selectivity is improved because the ratio of the thermal-lens signals at the two different wavelengths provides fingerprints and allows identification of the sample. Therefore, it can be used not only to detect and identify trace chemical species at very low concentration but also to analyse two-component mixtures. Using this apparatus, the detection limit for NO2 using 30 mW excitation beams modulated at 4.6 Hz is estimated to be 300 PPB.