A pulsed, distributed feedback (DFB) quantum cascade laser centered at $957{\mathrm{cm}}^{-1}$ was used in combination with a wavelength modulation spectroscopic technique for the detection of acrylonitrile. The laser was excited with short current pulses ( $5–10\mathrm{ns}$ ), and the pulse amplitude was modulated with a linear subthreshold current ramp at $20\mathrm{Hz}$ resulting in a $\sim 2.5{\mathrm{cm}}^{-1}$ frequency scan. This allowed the measurement of spectroscopic features of acrylonitrile with absorption line widths of $\sim 1{\mathrm{cm}}^{-1}$ . A demodulation approach followed by numerical filtering was utilized to improve the signal-to-noise ratio. We then superimposed a $10\mathrm{kHz}$ sine wave current modulation on top of the $20\mathrm{Hz}$ current ramp. The resulting high frequency temperature modulation of the DFB structure results in wavelength modulation. A minimum detectable absorbance of $\sim {10}^{-5}$ , corresponding to the sub ${10}^9$ levels of acrylonitrile, was achieved with less than a minute averaging time.