  We present a dual-modulation Faraday rotation spectrometer with in-line
optical subtraction for differential measurement of nitric oxide (NO)
isotopologues. In-situ sample referencing is accomplished via differential
dual-cell measurements, with 3.1 ppbv/rt(Hz) (15NO) sensitivity through 15 cm
optical path length. Our system operates at 1.9x the shot-noise limit, with a
minimum fractional absorption of 1.8e-7/rt(Hz). Differential measurement of
both 14NO and 15NO are shown, yielding ~20 dB magneto-optical suppression.
Noise analysis demonstrates stability of the differential signal up to ~500 s,
with normalized ratiometric precision of 3.0 permil/rt(Hz) using 1 ppmv 15NO
(or 272 ppmv 14NO at natural abundance). We rigorously model our differential
method and demonstrate the utility of in-line calibration for precise isotopic
ratiometry.
