A quantum cascade (QC) laser-based spectrometer is developed to measure trace gases in air. The proposed spectrometer is tested for N2O, and the results presented in this letter. This system takes advantage of recent technology in QC lasers by utilizing intra-pulse scan spectroscopy, which allows high sensitive measurement. Without calibration gases, the gas concentration can be calculated with scan integration and the corresponding values from the HITRAN04 database. By analyzing the Allan variance, a detection limit of 2 ppb is obtained. Continuous measurement of N2O sampled from ambient air shows the applicability of the proposed system for the field measurements of gases of environmental concern.