  Breath analysis enables rapid, non-invasive diagnostics, as well as long-term
monitoring, of human health through the identification and quantification of
exhaled biomarkers. Here, for the first time, we demonstrate the remarkable
capabilities of mid-infrared (mid-IR) cavity-enhanced direct frequency comb
spectroscopy (CE-DFCS) applied to breath analysis. We simultaneously detect and
monitor as a function of time four breath biomarkers - CH$_3$OH, CH$_4$, H$_2$O
and HDO - as well as illustrating the feasibility of detecting at least six
more (H$_2$CO, C$_2$H$_6$, OCS, C$_2$H$_4$, CS$_2$ and NH$_3$) without
modifications to the experimental apparatus. We achieve ultra-high detection
sensitivity at the parts-per-trillion level. This is made possible by the
combination of the broadband spectral coverage of a frequency comb, the high
spectral resolution afforded by the individual comb teeth, and the sensitivity
enhancement resulting from a high-finesse cavity. Exploiting recent advances in
frequency comb, optical coating, and photodetector technologies, we can access
a large variety of biomarkers with strong carbon-hydrogen bond spectral
signatures in the mid-IR.
