Your comment lead me down an interesting rabbit hole. Stumbled across this which might count - "Photoacoustic spectroscopy has become a powerful technique to st

Your comment lead me down an interesting rabbit hole. Stumbled across this which might count - "Photoacoustic spectroscopy has become a powerful technique to study concentrations of gases at the part per billion or even part per trillion levels." https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Photoacoustic_Spectroscopy https://chem.libretexts.org/Bookshelves/Physical_and_Theoret...