DEVELOPMENT OF AN ION STORE TIME-OF-FLIGHT MASS-SPECTROMETER FOR THE ANALYSIS OF VOLATILE COMPOUNDS IN AIR

A time-of-flight mass spectrometer (TOF-MS) that incorporates an ion s torage trap as a pulsed extraction source and sample preconcentrator h as been developed for air monitoring, This new instrument configuratio n is designed for real-time monitoring of changes in trace volatile an d semivolatile compound concentrations in air. Typically, detection li mits are in the low parts per billion (v/v) range. This ion store/TOF- MS configuration permits. a relatively large volume air flux to be dra wn directly into the ion trap cavity, creating a local pressure that e xceeds 10(-4) Torr. Operating the ion trap at this pressure increases the extent of chemical ionization with the abundant N2- and O2+ that a re formed by electron impact The air background also serves as a suita ble collisional damping matrix, helping to compress the ion cloud to t he center of the trap, which is ideal for TOF-MS analysis. Ions are ty pically accumulated in the trap for 400 ms, at which point they are ex tracted through the exit end cap and analyzed within 100 ps, thus yiel ding a duty cycle near 100%. The necessary hardware and electronics co nfiguration needed to obtain optimum resolution (routinely 2000 m/Delt a m, fwhm), mass accuracy and sensitivity is described.