Shock wave model for sputtering biomolecules using massive cluster impacts

Abstract

A shock wave model is proposed to explain certain features of recently reported spectra obtained by massive duster impact (MCI) mass spectrometry. It is suggested that clusters that impact glycerol matrices with energies/nucleon in the range 0.01 eV/u < E/N < 1.0 eV/u provide an extremely soft method for sputtering intact biomolecules, Compared to the high energy/nucleon characteristic of atomic or molecular ion primary beams (typically < 50 eV/u), massive cluster primary beams possess much lower energies/nucleon, which are insufficient to cause appreciable ionization and radiation damage of matrix material. Moreover, fragmentation products of parent molecular ions are effectively lower. With these benefits, MCI spectra show lower chemical noise background and enhanced signalto-noise ratios. Rankine-Hugoniot analysis of the shock conditions is used to arrive at an estimate of the heat retained in the collision-affected matrix volume after bombardment by a characteristic cluster. For a cluster collision resulting in a 26.8 GPa shock pressure, by analogy with water data, rapid heating of the shocked volume to 1000 °C or more is plausible. In a beam consisting of clusters distributed in size and charge, an estimate is made for the range of cluster sizes over which hyrodynamic shock wave theory applies.

Fundamental aspects of electrospray droplet impact/SIMS

“…This value is reported to be just on the border between the shockwave formation regime (Ä1 eV/u) and the bond-breaking and ionization regime (½1 eV/u). 18 The total ion current of the electrospray charged droplets irradiated on the sample target is in the range of 10 11 -10 10 A; that is, the flux of the charged droplets is estimated to be in the range of 10 7 -10 8 cm 2 s 1 . The samples were dissolved in an appropriate solvent and an amount of 1 µl of the sample solution was deposited on the stainless steel substrate with a spot size of 3 mm diameter and dried in the ambient air.…”

Study on ion formation in electrospray droplet impact secondary ion mass spectrometry

“…desorption of only molecular-level superficial sample, little damage accumulation and self-cleaning effect, [12][13][14]19 contamination free by using evaporative projectiles, soft ionization although not fragment-free, no need to use a matrix, and extremely high tolerance to the presence of salts. Since the beam of electrospray droplet projectiles is $3 mm in diameter, the spatial resolution is practically non-existent.…”

“…Mahoney and co-workers carried out pioneering work in generating multiply charged massive glycerol clusters with $200 charges via electrohydrodynamic emission in vacuum using a 1.5 M solution of ammonium acetate in glycerol. [12][13][14][15] The masses of the projectile, 10 6 -10 7 u, may be the largest among the SIMS clusters developed so far. This method was found to afford extremely soft desorption conditions for peptide and protein ions.…”

“…This method was found to afford extremely soft desorption conditions for peptide and protein ions. [12][13][14][15] Unfortunately, however, stable ion currents can only be obtained for several hours until the lens electrodes become contaminated by accumulated glycerol at which time source cleaning is necessary. 16 Intrigued by the original work by Mahoney and co-workers, we developed a new cluster SIMS method, termed as electrospray droplet impact (EDI), that uses contamination-free water droplet projectiles, namely, use of the atmospheric-pressure electrospray as a cluster source.…”

Development of New Ionization Methods for GC/MS and LC/MS Interfaces

“…68) FAB/SIMS is a rather hard ionization method and not very suitable for biological samples. In contrast, MCI is a soft ionization method and is well suited for biopolymers.…”