A cavity ring-down spectroscopy apparatus employing a pulsed quantum cascade laser operating near 1027 cm-1 (9.7 μm) is investigated for trace ammonia detection. The trace ammonia in human breath and clean room atmosphere are measured and analyzed. A sensitivity of 10 ppb has been demonstrated for ammonia detection. The effects of the spectral line-width on the measurement accuracy and sensitivity of the system are analyzed. The measurement accuracy and sensitivity of the apparatus decrease with the increasing laser spectral line-width. The measurement results can be corrected by taking into account of the line-width effect in data analysis.