Electrospray using a solid needle probe for the analysis of liquid samples was developed. The needle was moved up and down a vertical axis by a motor-driven system. At the lowest position of the needle, the tip of the needle touched the surface of the liquid sample and a small amount of liquid was picked up by the needle. At the highest position, a high voltage of approximately 3 kV was applied to the needle and the sample loaded on the tip of the needle was electrosprayed. The ions formed were measured by an orthogonal time-of-flight mass spectrometer. A single sample loading by the needle gave reasonably strong ion signals for amino acids, peptides, proteins and polyethylene glycol (PEG) in aqueous solution. The addition of salts or acids to aqueous solution led to dramatic enhancement of the ion signals.

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.

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