A pulsed-molecular-beam Fabry-Perot cavity Fourier-transform microwave spectrometer developed at NIST has demonstrated sensitivities for many polar gas- phase molecular species in the low parts per million (ppm) to parts per billion (ppb) range. The highest sensitivity is obtained using neon or argon carrier gas but nitrogen or air can also be used, with some loss in sensitivity (up to 100 times) due to the less efficient rotational and vibrational cooling in the molecular beam with diatomic gases. The minimum detectable concentrations for several representative compounds are provided. These include acetaldehyde, acrolein, propionaldhyde, benzaldehyde, p- tolualdehyde, methanol, SO2, propene, methyl t-butyl ether, ethyl t-butyl ether, and others. Considerable attention has been given to making the instrument versatile and user friendly. The instrument is computer controlled using standard GPIB interfaces and several graphical interfaces under the CPLUPLU operating system.