Cantilever-enhanced photo-acoustic spectroscopy, as shown by Hershbach et al.10, is a relatively unexplored configuration of photo-acoustic spectroscopy. Cantilever-enhanced photo-acoustic spectroscopy uses a cantilever as both a piezo-electric transducer and a very sensitive cantilever-enhanced photo-acoustic detector. The cantilever vibrates when irradiated with a laser pulse, thus converting the laser pulse into an acoustic wave which is detected by the cantilever via its piezoelectric properties. This works in a similar fashion to the piezoelectric detection principle of QEPAS. Despite the use of a cantilever as a detector, it is not directly connected to a transducer, thus circumventing the need of connecting an additional transducer to the cantilever. It is, however, somewhat limited by the inherent low mechanical Q of the cantilevers that are typically used in acoustic detection, which translates into a low resonance frequency and high mechanical Q of the cantilevers.
In the case of CEPAS, we use a high-Q cantilever as the detector and a mid-infrared optical parametric oscillator as the excitation source for photo-acoustic spectroscopy. This paper is to demonstrate the use of this configuration for trace gas detection at room temperature with improved sensitivity compared to the more conventional resonance mode. The limit of detection is demonstrated to be 650 ppq for SF6 in ultrahigh purity nitrogen as well as sub-ppt level sensitivity for NO2 in ambient air.
In an earlier work, we demonstrated the use of a cantilever-enhanced photo-acoustic detector for remote gas sensing at room temperature and atmospheric pressure.10 Here we extend the work to consider the possible use of the spectroscopic setup for detecting trace gas concentrations in situ in a laboratory environment, specifically detecting hydrogen fluoride (HF) in ultrapure N2 at concentrations as low as 120 parts-per-million. In this work, in addition to the cantilever-enhanced photo-acoustic spectroscopy, we also introduce a diamond-enhanced photo-acoustic spectroscopy, where a diamond window is used to enhance the absorption cross-section of the gas in an ambient pressure environment. The ability to detect HF remotely and in situ is presented with detection limits of c.f. ∼ 200 ppb. The detection limit can be improved to c.f. 200 ppq using the quartz-enhanced photo-acoustic spectroscopy13,14 method if necessary.
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