A portable, pulsed-molecular-beam, Fourier-transform microwave spectrometer designed for chemical analysis

M. D. Harmony, K. A. Beran, D. M. Angst, and K. L. Ratzlaff [Rev. Sci. Instrum. 66, 5196 (1995)] recently published some design specifications for a smaller version of a Fourier transform microwave (FTMW) spectrometer. In that work they used a nozzle arrangement which pulsed the molecular beam perpendicular to the axis of the Fabry-Perot cavity. They found that even though the size of the vacuum chamber and Fabry-Perot cavity mirrors had been reduced, the overall sensitivity of the instrument was nearly the same as one with a conventional sized resonator. In an effort to establish FTMW spectroscopy as a viable new technique for analytical chemists, we have constructed a miniaturized version of our laboratory instrument for use as an analytical instrument. The vacuum chamber of the instrument is based on a commercially available, multiport 30 cm (12 in. ! sphere. An integral end-flange mirror permits a coaxial nozzle injection of the molecular beam which greatly improves the sensitivity of the instrument. The movable cavity mirror rides on a fast motorized stage which allows tuning to any frequency within the range of the spectrometer in 1-2 s. The entire spectrometer is mounted on a mobile cart, allowing it to be easily transported to other laboratories or remote locations. The per-pulse sensitivity of this smaller instrument is slightly less than the larger laboratory instruments, however the smaller vacuum chamber allows the nozzle to be pulsed much faster without overloading the vacuum pumps. The new miniaturized FTMW spectrometer is only a factor of 2 less sensitive than the larger laboratory instrument. This instrument provides analytical chemists with a new tool that can unambiguously identify trace amounts of large organic compounds in gas streams. The instrument also permits real-time analysis which should be useful for monitoring and optimization of process gas streams. Lower detection limits are typically in the nanomol/mol regime. (C) 1999 American Institute of Physics. [S0034-6748(99)01204-6].