Fundamental aspects of electrospray droplet impact/SIMS

• Published in: Journal of mass spectrometry. Vol. 41; no. 7; pp. 894 - 902
• Main Authors: Hiraoka, Kenzo, Mori, Kunihiko, Asakawa, Daiki
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.07.2006; Wiley
• Subjects: Analytical biochemistry: general aspects, technics, instrumentation; Analytical chemistry; Analytical, structural and metabolic biochemistry; Biological and medical sciences; Chemistry; electrospray droplet impact; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; massive cluster impact; SIMS; Spectrometric and optical methods; electrospray droplet impact; Peptides; massive cluster impact; Secondary ion mass spectrometry; Droplet; Electrospray; Operating conditions; Antibiotic; Gramicidin; Polypeptide; Ionization; SIMS; Antibacterial agent; Mass spectrometry
• ISSN: 1076-5174; 1096-9888
• DOI: 10.1002/jms.1048

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 µm 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. Copyright © 2006 John Wiley & Sons, Ltd.