Front-end electron transfer dissociation: a new ionization source

Abstract

Electron transfer dissociation (ETD), a technique that provides efficient fragmentation while depositing little energy into vibrational modes, has been widely integrated into proteomics workflows. Current implementations of this technique, as well as other ion-ion reactions like proton transfer, involve sophisticated hardware, lack robustness, and place severe design limitations on the instruments to which they are attached. Described herein is a novel, electrical discharge-based reagent ion source that is located in the first differentially pumped region of the mass spectrometer. The reagent source was found to produce intense reagent ion signals over extended periods of time while having no measurable impact on precursor ion signal. Further, the source is simple to construct and enables implementation of ETD on any instrument without modification to footprint. Finally, in the context of hybrid mass spectrometers, relocation of the reagent ion source to the front of the mass spectrometer enables new approaches to gas phase interrogation of intact proteins.

Figure 1

SolidWorks renderings of the reagent ion sources used with (A) tube lens/skimmer and (B) S-lens atmospheric interfaces.

Figure 2

Schematic of the reagent inlet system. Two mass flow controllers and a temperature controlled reagent vial deliver a stable stream of vaporized reagent to the discharge ionization source through an independent capillary.

Figure 3

Voltage changes occurring on a linear ion trap during an ion–ion reaction when reagent ions enter from the front of the ion trap. (A) A 3-section linear ion trap. (B–D) The voltages used to sequester ions and prevent mixing prior to charge sign independent trapping (E) and scan out (F).

Figure 4

Proposed mechanism for the formation of a stable anionic species that is observed experimentally.

Figure 5

Paschen's curve for air on a nickel cathode. The minimum voltage required to light a discharge with proper spacing of 6 mm at 1 Torr is slightly over 200 V.

Figure 6

Reagent ion spectra of both azulene (left) and fluoranthene (right) and a plot of reagent stability using fluoranthene.

Figure 7

Front-end ETD spectrum of ubiquitin (A) compared with front-end ETD/IIPT of ubiquitin (B). Sequence coverage for the ETD/IIPT spectrum is presented in panel C.