Calibration of a quadrupole ion trap for particle mass spectrometry

A quadrupole ion trap (QIT) is calibrated for microparticle mass spectrometry by confining ϕ=2.02μ m dye-labeled polystyrene microspheres and measuring their secular oscillation frequencies and fluorescence spectra. A particle’s absolute mass and charge are found by measuring its secular oscillation frequencies within the QIT while initiating charge steps through photo-ejection of electrons. The radius of the same microsphere is determined by analyzing the fluorescence emission spectrum, which is dominated by optical cavity resonances, employing Mie theory. The mass of the microsphere is calculated from the radius using the density of bulk polystyrene. For nine particles originating from the same stock sample, the masses obtained from the two methods agrees to within 3% with no systematic deviation. Analysis reveals that small uncertainties in the secular frequency measurements result in significant error in the absolute charges and masses. Nevertheless, excellent agreement between the average masses determined using the two techniques confirm that the value of the trap parameter (z0z0) obtained from computer modeling is appropriate and that effects of electrode misalignments are small.