Lowenergy electron injection system based on

1
High Rate Electron Capture Dissociation in
Fourier Transform Ion Cyclotron Resonance Mass
Spectrometry Youri O. Tsybin, Roman A. Zubarev,
Magnus Palmblad, Margareta Ramström, and Per
Håkansson Ion Physics Division, Ångström
Laboratory, Uppsala University, Box 534, SE 751
21, Uppsala, Sweden Department of Chemistry,
University of Southern Denmark, Campusvej 55,
DK-5230 Odense M, Denmark Phone [phone]
28, E-mail youri.tsybin_at_angstrom.uu.se
Overview
A low-energy electron injection system based on
an indirectly heated dispenser cathode
facilitates electron capture dissociation (ECD)
in FTICR mass spectrometry 1,2. Short
irradiation times, 1-10 ms, make high rate ECD
possible. The improvement is not only due to the
larger total electron current but the larger
emitting area as well. The increase in ECD rate
made it possible to combine liquid chromatography
(LC) and ECD in FTICR mass spectrometry. First
results of successful application of this method
for the analysis of peptide mixture and BSA
tryptic digest have been obtained.
Experimental electron source

• Low-energy electron injection system based on
• indirectly heated dispenser cathode vs. filament
• Larger overlap of electron and ion beams
• Lower voltage drop over the emitting surface
• Lower temperature of the emitting surface
• Higher electron flux

• Stronger space charge effects
• More power is required
• More expensive

• working parameters
• heater current 2-2.2 A
• required voltage7-9 V
• grid voltage 10-15 V

-
Experimental implementation on a 9.4 T Bruker
BioApex II

• Both the ICR cell and the electron source are in
  a constant magnetic field
• Ions and electrons enter the cell on-axis from
  the opposite directions
• Distance from the electron emitter to the cell
  entrance is 130 mm
• Either the cathode surface potential or the grid
  potential could be pulsed
• The left trapping plate potential (PV2) is 1.5 V
• Vacuum conditions when electron source is off
  410-10 Torr, when electron source is on 810-10
  Torr
• External power supplier is used for supplying
  the filament current and grid voltage

Electron pulse duration in high rate ECD
single scan ECD (substance P)
c5
Example ubiquitin (8.6 kDa)
External ion accumulation and ECD
Electron energy and ECD efficiency
2
Electron beam width effects
The trace of an electron beam on a copper surface
of a collector the focusing effect of the strong
magnetic field preserved the grid structure
despite the 100 mm distance between the cathode
and the collector
Electron energy spread Hendrikson et al. used
the potential depression created by the space
charge of an electron beam to transport the
trapped ions from one ICR cell to another 3.
The depression can be expressed as where
Ve is electron energy, Ie- electron current, a -
beam diameter, ? is the electron charge-to-mass
ratio and e0 is the permittivity of vacuum. The
value obtained for potential well is around 0.1
eV on the outer surface of an electron beam. This
additional energy will increase the energy spread
of electrons in the beam.
Radial potential well created within the space
charge of an electron beam. Parameters are 1
?A, 0.5 V, 6 mm diameter electron beam
Trapping of ions total charge of the ions should
not exceed the charge of the electrons. The
trapping capacity of the electron beam (the
maximum number of trapped ions) is given by
Where L - the length of the area of ion-electron
interaction in cm.
The approximation gives Nq gt 2105
Taking into account the charge capacity of the
cell (around 107 ions) the required current could
be more easy provided by the dispenser cathode
than by the conventional filament.
The created potential well, in addition to the
magnetic field, traps the ions in the radial
direction.
Combination of high rate ECD and Liquid
Chromatography
LC- ECD - FTICR MS of peptide mixture
Principle
Method

• peptide mixture 4
• BSA tryptic digest 4

tested for
Conclusions
A low-energy electron injection system based on
an indirectly heated, large-area dispenser
cathode produces in FTICR MS effective electron
capture dissociation. The critical requirement is
that the electron beam is broad enough to
essentially overlap with the ion cloud. The
space charge of the electron beam creates a
radially trapping potential well, ensuring
effective ion-electron interaction. The routine
ECD technique for reproducible everyday analysis
is now available. The ECD event with the new
electron emission system takes only 1-100 ms,
thus opening new possibilities of employing ECD
in different applications, noticeably LC/MS/MS.
First results of successful application of
LC-ECD-FTICRMS in the analysis of peptide
mixture and products of tryptic digestion have
been obtained. (see poster D3 for further
details)
References
1. Zubarev R.A., Fridriksson E.K., Horn D.M.,
Kelleher N.L., Kruger N.A., Carpenter B.K., F.W.
McLafferty, Anal. Chem. 72 (2000) 563. 2. Tsybin
Y.O., Håkansson P., Budnik B.A., Haselmann K.F.,
Kjeldsen F., Gorshkov M., and Zubarev R.A.,
Rapid. Commun. Mass Spectrom. 2001 15 1849-1854
3. Hendrickson C.L., Hadjarab F., Laude D.A.
Jr. Int. J. Mass Spectrom. Ion Processes, 141
(1995) 1161 4. Palmblad M., Tsybin Y.O., Ramström
M., Håkansson P, manuscript in preparation.