  We report multipass broadband photoacoustic spectroscopy of trace gases in
the mid-infrared. The measurement principle of the sensor relies on
supercontinuum-based Fourier transform photoacoustic spectroscopy (FT-PAS), in
which a scanning interferometer modulates the intensity of a mid-infrared
supercontinuum light source and a cantilever microphone is employed for
sensitive photoacoustic detection. With a custom-built external Herriott cell,
the supercontinuum beam propagates ten times through a miniature and
acoustically non-resonant gas cell. The performance of the FT-PAS system is
demonstrated by measuring the fundamental C-H stretch bands of various
hydrocarbons. A noise equivalent detection limit of 11 ppb is obtained for
methane (40 s averaging time, 15 $\mu$W/cm$^{-1}$ incident power spectral
density, 4 cm$^{-1}$ resolution), which is an improvement by a factor of 12
compared to the best previous FT-PAS systems. High linearity and good stability
of the sensor provide reliable identification of individual species from a gas
mixture with strong spectral overlap, laying the foundation for sensitive and
selective multi-species detection in small sample volumes.
