We demonstrate a closed path ammonia gas sensor based on a tunable lead-salt diode laser. In order to measure NH3 concentrations, the laser is scanned over two absorption lines 992.68 and 992.45 cm-1 of the ν2 vibrational band. These lines have free interference patterns with other species in the atmosphere. The experiments are performed with an ammonia mixture in the air at room temperature in an optical multi-path White cell. In our preliminary experiments, a detection limit of ~4 ppb is achieved using direct absorption at 100 mbar pressure and 15 m path length. A detection limit of 0.2 ppb can be reached.