We present a new scheme for a pump-probe mode-mismatched thermal lensing experiment aimed at extracting the value of low-absorbing components in the presence of a high-absorbing background. The method allows
for the use of large path-length samples. Changes in the background absorption of a column of ethanol of 20 cm of 1% and a pollutant concentration detection limit of 0.04 ppm are obtained when using a pump
light power of 34 mW.
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