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 storage trap as a pulsed extraction source and sample preconcentrator has been developed for air monitoring. This new instrument configuration is designed for real-time monitoring of changes in trace volatile and semivolatile compound concentrations in air. Typically, detection limits 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 drawn directly into the ion trap cavity, creating a local pressure that exceeds 10-4 Torr. Operating the ion trap at this pressure increases the extent of chemical ionization with the abundant N2 + and O2 + that are formed by electron impact. The air background also serves as a suitable collisional damping matrix, helping to compress the ion cloud to the center of the trap, which is ideal for TOF-MS analysis. Ions are typically accumulated in the trap for 400 ms, at which point they are extracted through the exit end cap and analyzed within 100 μs, thus yielding a duty cycle near 100%. The necessary hardware and electronics configuration needed to obtain optimum resolution (routinely 2000 m/Δm, fwhm), mass accuracy and sensitivity is described.