  We present a highly sensitive hydrogen fluoride detector based on simple
cantilever-enhanced photo-acoustic spectroscopy. A noise equivalent
concentration of 5 parts-per-trillion was achieved in 1 s and 650
parts-per-quadrillion in 32 min. The limits were reached with an average
optical power of 950 mW using a new continuous-wave optical parametric
oscillator at 2.476 \mu m. The achieved normalized noise equivalent absorption
coefficient was 2.7$\times$10$^{-10}$ Wcm$^{-1}$ Hz$^{-1/2}$.
