We have established the use of photothermal deflection or mirage effect spectroscopy for the use of trace gas detection. This technique has roughly the same order of sensitivity as that of PA spectroscopy, but is more flexible, and has been used in (solid state) materials characterization, liquid phase diffusion studies, and in gas phase chemistry. With transparent media, the sample to be studied is heated by absorption of a modulated (pump) laser source. The thermal gradient induces a time dependent refractive index gradient within the heated region, and a second (probe) laser beam is deflected through an angle directly related to the thermal gradient and frequency of modulation. The periodic beam deflection is measured either with a single photodiode and mask or with a pair of photodiodes to reduce laser intensity noise. In a gas, the laser power that is absorbed, and therefore the thermal gradient, and corresponding deflection signal is linearly proportional to gas concentration. And as the measured signal is generated in proportion to the absorbed energy, sensitivity may approach the shot noise limit, whereas in traditional absorption studies, the signal depends on the typically weak difference between the direct and transmitted flux.
We have further extended the technique to selective trace gas detection of multiple species by the use of a tunable laser. A frequency resonant with a vibrational mode of the species to be detected is chosen, taking care to avoid resonance with other molecules in the mixture. The resonant gas species absorbs the excitation, and is heated through kinetic quenching. We measure the deflection signal, and either deduce absolute detection limits for the species of interest, or calibrate the instrument by the introduction of premixed gas samples. Detection has species selectivity equal to direct absorption spectroscopy, with much higher sensitivity.
Current applications include measurements of gas produced by various solid and semi-solid samples, including polymers and live plants. The composition and quantity of gas produced permits us to perform analysis on internal processes which cannot be measured by other means. The image below shows a homemade oven integrated with the detection cell which permits measurements of controlled coffee roasting. We wish to evaluate the production of chemicals thought to be carcinogenic in over-roasted coffee by evaluating the roasting process in real time.
more information describing Photothermal spectroscopy can be found HERE
