A novel dual-wavelength photothermal refraction spectrometer that is capable of simultaneously measuring photothermal refraction at two different wavelengths has been developed. In this instrument, the two excitation wavelengths were provided by an argon-ion (488 nm) and a He-Ne (632.8 nm) laser. These two pump beams were sequentially modulated at the same frequency by means of two mechanical choppers. The photothermal refraction signals produced by the sample absorption of these two pump beams were monitored by a probe beam which was derived from the same He-Ne laser by a beamsplitter. Compared to the single-wavelength techniques, this dual-wavelength apparatus has advantages that include its ability to provide fingerprints and identification of the analyte (as the ratio of the two signals at the two different excitation wavelengths) and to simultaneously determine two-component samples. With this apparatus, the detection limit for two-component samples containing oxohydroxy bis-(8-hydroxy quinoline)-vanadium and 1,2-diamino-anthraquinone in methanol with the use of 25-mW excitation beams modulated at 3.4 Hz is estimated to be 10−7 M. This corresponds to the absorbance of 10−6 for a probe volume of 10−11 L.
Chieu D. Tran and Minren Xu, "Dual-Wavelength Photothermal Refraction Spectrometry for Small-Volume Samples," Appl. Spectrosc. 43, 1056-1061 (1989)