A pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at $970{\mathrm{cm}}^{-1}$ was used in combination with an off-axis cavity enhanced absorption (CEA) spectroscopic technique for the detection of ammonia and ethylene. Here, the laser is coupled into a high-finesse cavity with an optical path length of $\sim 76\mathrm{m}$ . The cavity is installed into a $53\mathrm{cm}$ long sample cell with a volume of $0.12\mathrm{L}$ . The laser is excited with short current pulses ( $5–10\mathrm{ns}$ ), and the pulse amplitude is modulated with an external current ramp, resulting in a $\sim 0.3{\mathrm{cm}}^{-1}$ frequency scan. A demodulation approach followed by numerical filtering was utilized to improve the signal-to-noise ratio. We demonstrated detection limits of ~15 ppb and $\sim 20\mathrm{ppb}$ for ammonia and ethylene, respectively, with less than $5\mathrm{s}$ averaging time.