Electrospray droplet impact/secondary ion mass spectrometry: cluster ion formation

The electrospray droplets that are sampled through an orifice into the vacuum chamber are accelerated by 10kV and impact on the stainless steel substrate. The mass and the kinetic energy of electrospray droplets are roughly estimated to be a few 10(6) u and similar to 10(6) eV, respectively. The molecular ion M+, and the protonated molecule [M+H](+) are observed as secondary ions for chrysene and coronene deposited on the metal substrate (no matrix used). The ionization may take place in the shock wave generated by the high-momentum coherent collision between the droplet projectile and the solid sample. Cluster ions of H+(H2O)(n) and CF3COO-(H2O)(n), with n up to similar to 150, were observed as secondary ions formed by the electrospray droplet impact ionization (EDI) for 10(-2) M trifluoroacetic acid (TFA) aqueous solution. This indicates that the charged droplets that collide with the metal substrate with the kinetic energy of similar to 10(6) eV do not vaporize completely but are disintegrated into many tiny microdroplets. The ion signal intensity anomalies (i.e. magic numbers) were observed for the cluster ions of H3O+(H2O)(n) and CF3COO-(H2O)(n) for 10(-2) M TFA aqueous solution and of Cs+(H2O)(n), I-(H2O)(n), Cs+(CsI)(n), and I-(CsI)(n) for 10(-2) M CsI aqueous solution. Copyright (c) 2006 John Wiley & Sons, Ltd.