A swept-wavelength external cavity quantum cascade laser is used to measure time-resolved NH315 and NH314 concentrations and isotope ratios in a turbulent plume source located at 10 m standoff distance. Measurements show excellent agreement with expected values of isotope ratios for samples with natural and enriched N15 abundance. Detection sensitivity of 150 ppb*m is demonstrated for a 50 ms measurement interval, improving to 10 ppb*m with 10 s averaging, with isotopic precision as good as 0.8%.