Fundamental aspects of electrospray droplet impact/SIMS

A new ionization method, electrospray droplet impact ionization (EDI), has been developed for matrix-free secondary-ion mass spectrometry (SIMS). The charged droplets formed by electrospraying 1 M acetic acid aqueous solution are sampled through an orifice with a diameter of 400 microm into the first vacuum chamber, transported into a quadrupole ion guide, and accelerated by 10 kV after exiting the ion guide. The droplets impact on a dry solid sample (no matrix used) deposited on a stainless steel substrate. The secondary ions formed by the impact are transported to a second quadrupole ion guide and mass-analyzed by an orthogonal time-of-flight mass spectrometer (TOF-MS). Ten pmol of gramicidin S could be detected with the presence of as much as 10 nmol of NaCl. The ion signal for arginine disappeared with decrease in the substrate temperature below 150 K owing to the formation of ice film over the sample surface. While 10 fmol of gramicidin S could be detected for 30 min, the ionization/desorption efficiency for EDI becomes smaller with an increase in the molecular weight (MW) of a biological sample. The largest protein samples detected to date are cytochrome c and lysozyme. The high sensitivity for EDI is due to the fact that samples only a few monolayers thick are subject to desorption/ionization by EDI, with little fragmentation. A coherent phonon excitation may be the main mechanism for the desorption/ionization of the solid sample.