Dual modulation Faraday rotation spectroscopy of HO2 in a flow reactor.

The technique of dual modulation Faraday rotation spectroscopy (DM-FRS) has been applied to achieve technical-noise-limited detection of HO2 at the exit of an atmospheric pressure flow reactor. This was implemented by combining direct current modulation at 51 kHz of an external cavity quantum cascade laser system with 610 Hz modulation of the magnetic field generated by a Helmholtz coil. The DM-FRS measurement had a 1.5 times better signal-to-noise ratio than a conventional FRS measurement acquired under identical flow reactor conditions. High harmonic detection of the FRS signal also eliminated the substantial offset associated with electromagnetic interference pickup from the Helmholtz coils that is observed in the conventional FRS spectrum. A noise equivalent angle of 8.9×10(-9)  rad Hz(-1/2) was measured for the DM-FRS measurement, corresponding to a 3σ detection limit for HO2 of 0.35  ppmv Hz(-1/2).