Airborne nanoparticle characterization with a digital ion trap–reflectron time of flight mass spectrometer

A digital ion trap–reflectron time of flight mass spectrometer is described for airborne nanoparticle characterization. Charged particles sampled into this nanoaerosol mass spectrometer (NAMS) are captured in the ion trap and ablated with a high fluence laser pulse to reach the “complete ionization limit”. Atomic ions produced from the trapped particle(s) are mass analyzed by time of flight, and the elemental composition is determined from the relative signal intensities in the mass spectrum. The particle size range captured in the ion trap is selected by the frequency applied to the ring electrode. Size selection is based on the mass normalized particle diameter, defined as the diameter of a spherical particle with unit density that has the same mass as the particle being analyzed. For the current instrument configuration, ring electrode frequencies between 5 and 140 kHz allow selective trapping of particles with a mass normalized diameter between 7 and 25 nm with a geometric standard deviation of about 1.1. The particle detection efficiency, defined as the fraction of charged particles entering the mass spectrometer that are subsequently captured and analyzed, is between l × l0−4 and 3 × l0−4 over this size range. The effective particle density can be determined from simultaneous measurement of the mobility and mass normalized diameters. Test nanoparticles composed of sucrose, polyethylene glycol, polypropylene glycol, sodium chloride, ammonium sulfate and copper(II) chloride are investigated. In most cases, the measured elemental compositions match the expected elemental compositions within ±5% or less and the measured compositions do not change with particle size. The one exception is copper chloride, which does not yield a well-developed plasma when it is irradiated by the laser pulse.