Development of a Branched Radio-Frequency Ion Trap for Electron Based Dissociation and Related Applications

Abstract

Collision-induced dissociation (CID) is the most common tool for molecular analysis in mass spectrometry to date. However, there are difficulties associated with many applications because CID does not provide sufficient information to permit details of the molecular structures to be elucidated, including post-translational-modifications in proteomics, as well as isomer differentiation in metabolomics and lipidomics. To face these challenges, we are developing fast electron-based dissociation devices using a novel radio-frequency ion trap (i.e., a branched ion trap). These devices have the ability to perform electron capture dissociation (ECD) on multiply protonated peptide/proteins; in addition, the electron impact excitation of ions from organics (EIEIO) can be also performed on singly charged molecules using such a device. In this article, we review the development of this technology, in particular on how reaction speed for EIEIO analyses on singly charged ions can be improved. We also overview some unique, recently reported applications in both lipidomics and glycoproteomics.

Keywords: ECD; EIEIO; lipidomics; proteomics.

Fig. 1. Branched ion guide reported by Thomson and coworkers.³¹⁾ Reproduced with permission of the TAYLOR & FRANCIS GROUP LLC via Copyright Clearance Center.

Fig. 2. (a), (b) Branched ion trap and (c) electron based dissociation device using a branched ion trap. (a) RF rod configuration. The RF phase is indicated by the different hatching. (b) DC lens configuration. The dashed lines represent the potential minimum of pseudo potential generated by the RF. Reproduced with permission from ref. (Copyright 2015 American Chemical Society).

Fig. 3. Simultaneous electron and ion trapping of triply protonated neurotensin in ECD. (a) Pure flow through conditions, or closed duration of L2 for 0 ms. (b) Closed duration of L2 for 10 ms, and (c) 19 ms. Reprinted with permission from ref. (Copyright, American Chemical Society).

Fig. 4. ECD spectra of triply protonated neurotensin. (a) N42 grade neodymium magnet. Maximum ECD efficiency was obtained when the extraction bias between the emitter and gate was 25V. (b) N52 grade neodymium magnet. Maximum ECD efficiency was given at extraction bias at 50V. 23% increase of the magnetic field improved ECD efficiency by 3 times.

Fig. 5. Comparison of ECD performance between (a) a tungsten filament and (b) an yttria-coated iridium disk.

Fig. 6. LC-hot ECD MS of a tryptic digest of bovine fetuin in simultaneous ion-electron trapping mode.

Fig. 7. Comparison between dissociation product ion spectra by EIEIO (A) and CID (B). The sample was sphingomyelin, SM(d18:1,12 : 0). EIEIO spectrum of a phosphatidylcholine (PC) lipid (PC 16 : 0/18 : 1(n-9: cis)) is shown in (C) as a typical spectrum with the same head group as SM. This research was originally published in ref. (Copyright, the American Society for Biochemistry and Molecular Biology).

Fig. 8. EIEIO spectrum of synthesized regioisomers, (a) OPPC: PC 18 : 1(n-9:cis)/16 : 0 and (b) POPC: PC 16 : 0/18 : 1(n-9:cis)). Reprinted with permission from ref. (Copyright, American Chemical Society).

Fig. 9. TGs in olive oil (a and b) and hazelnut oil (c and d). (a) and (b) show free fatty acid constituents. (b) and (d) show regioisomers at sn-2=C18:2 (linoleic acid). EIEIO does not separately identify acyl groups at the sn-1 and sn-3 positions because still impossible to determine the chirality at the C₂ carbon in the glycerol backbone. Horizontal shows the shorter acyl group and the vertical scale shows the longer acyl group in a TG molecule. Figure (c) was originally appeared in ref. (Copyright, the American Society for Biochemistry and Molecular Biology).