A novel molecular beam spectrometer for the purpose of trace gas sensing is described. Sensitivity is greatly enhanced and absorption interference by atmospheric H<sub>2</sub>O and CO<sub>2</sub> is greatly reduced by using a molecular expansion. The expansion results in rotational cooling and population enhancement of low-lying energy levels. The instrument employs a tunable mid-infrared lead salt diode, which operates in single mode from 2348.5 to 2351.1 cm<sup>-1</sup>, and a 36 m Herriott multipass cell. The sample gas is injected axially through a coupling hole in one of the spherical mirrors. The result is an increase of the residence time of the molecular beam in the sampling region. Pulsed operation of the nozzle allows background subtracted spectra to be acquired. The spectrometer can either be operated in fast scan mode, in which the laser frequency is rapidly scanned over the absorption feature of interest, or in frequency modulation mode with 2f-detection. A special adaptation of frequency modulation to the axial sampling system is described. Sample data of CO<sub>2</sub> are presented.