Ionization of Submicrometer-Sized Particles by Laser-Induced Radiofrequency Plasma for Mass Spectrometric Analysis

A laser-induced rf plasma (LIRFP) ion source was developed to ionize submicrometer-sized particles for the first time. The LIRFP ion source can increase the charge of those particles to several thousand charges via charge exchange reactions so that those particles can be trapped and analyzed with a charge detection quadrupole ion trap-mass spectrometer (CD QIT-MS). Different reagent gases for charge exchange reaction were investigated, viz. argon, nitrogen, oxygen, methane, helium, krypton, xenon, argon/methane (with ratios of 10:1 and 2:1), argon/nitrogen (with a ratio of 1:1), nitrogen/oxygen (10:1), krypton/methane (10:1), and air. The average charge of 0.75 μm polystyrene particles could reach 1631 using an argon/methane mixture with a ratio of ∼10:1. The average charges for freeze-dried Escherichia coli EC11303, Escherichia coli strain W, and Staphylococcus aureus were 842, 1112, and 971, respectively, with a mass-to-charge ratio ( m/ z) range from 10⁷ to 10⁸; and the average masses were 3.5 × 10¹⁰ Da, 6.0 × 10¹⁰ Da, and 5.6 × 10¹⁰ Da, respectively. The average mass and charge of the vaccinia virus were ∼9.1 × 10⁹ Da and ∼708 with a m/ z of ∼10⁷. This LIRFP CD QIT-MS method was rapid with only 20 min for each sample measurement.