APi-TOF - Atmospheric Pressure Inlet Time of Flight Mass Spectrometer
Ions are constantly produced in the atmosphere through high energy particles colliding with air molecules. Sources of these high energy particles are galactic cosmic rays and the radio-active decay of radon. Typical atmospheric ion concentrations at ground level are 100-1000 ions cm-3. With life times on the order of minutes, electrons/protons will have time to transfer from one molecule to another, with the transfer only occurring if it is energetically favorable. Thus, negative ions are dominated by species with the lowest proton affinities (acids), whereas positive ions are the ones with the highest proton affinities (bases).
By measuring the naturally produced ions, it is possible to get very sensitive detection of several different acidic and basic trace compounds. The ion spectra show what neutral molecules must be present, but do not give direct quantitative information of their concentrations.
A recently developed atmospheric pressure ionization time-of-flight mass spectrometer (APi-TOF) is able to measure the ion spectra up to around 3000 Da. It has been optimized for sensitivity while keeping a high resolving power of around 3000-4000, with an hourly detection efficiency of around 0.1 ions cm-3. The working principle of the APi-TOF is simple: air is sampled through a critical orifice into the APi, where air is differentially pumped away while quadrupole ion guides direct the ions further into the TOF mass spectrometer where the mass-to-charge of the ions are measured.
The first applications of the instrument have shown it to be a powerful tool for studying the first steps of gas-to-particle conversion by detecting clusters of up to four sulfuric acid molecules with one ammonia during new particle formation events. The APi-TOF has recently been installed for nucleation experiments at the JPAC atmospheric simulation chamber and will participate in upcoming field studies such as PEGASOS.