the ‘hybrid cell’: a new compensated infinity cell for larger radius ion excitation in fourier...

RAPID COMMUNICATIONS IN MASS SPECTROMETRY

Rapid Commun. Mass Spectrom. 2008; 22: 1423–1429

) DOI: 10.1002/rcm.3516
Published online in Wiley InterScience (www.interscience.wiley.com
The ‘hybrid cell’: a new compensated infinity cell for

larger radius ion excitation in Fourier transform ion

cyclotron resonance mass spectrometry

Sunghwan Kim1, Myoung Choul Choi1, Manhoi Hur1, Hyun Sik Kim1, Jong Shin Yoo1*,

Christopher L. Hendrickson2 and Alan G. Marshall2

1Korea Basic Science Institute, 804-1 Yangcheong-Ri, Ochang-Myun, Cheongwon-Gun, Chungcheongbuk-Do, 363-883, Republic of Korea2Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Drive,

Tallahassee, FL 32310-4005, USA

Received 14 December 2007; Revised 25 February 2008; Accepted 26 February 2008

*CorrespoYangcheocheongbuE-mail: joContract/FT-ICR inContract/SpectromContract/Contract/Laborato

A new ‘hybrid’ ion cyclotron resonance (ICR) trap is proposed and analyzed by computer simu-

lations. The trap is basically a hybrid of a segmented end cap (Infinity) and capacitively coupled

cylindrical cell, with additional electrodes placed at the center of each end cap. The new trap

produces an on-axis electric field z-profile similar to that of the Infinity cell or capacitively coupled

open cylindrical cell during ion excitation. Simion simulations demonstrate that, during detection,

appropriate changes of the potentials applied to the two new sets of electrodes produce a radial

electric field z-profile that more closely approaches that for an ideal axial three-dimensional

quadrupolar potential at high post-excitation ICR orbital radius, for improved signal-to-noise ratio

and resolving power, and minimal m/z-discrimination. Copyright # 2008 John Wiley & Sons, Ltd.

Since its inception,1 Fourier transform ion cyclotron reson-

ance mass spectrometry (FT-ICR MS) has become the

ultimate standard for high-resolution broadband mass

analysis.2 A resolving power of 3 300 000 for a �1 kDa

peptide has been reportedwith this technique.3 Furthermore,

the mass measurement accuracy is sufficiently high that for

molecules up to �500Da in mass (with appropriate

elemental constraints), a unique elemental composition for

molecules in even the most complex natural mixtures

(namely, petroleum crude oil) can be calculated.4 The

high-resolution capability of FT-ICR MS has had a high

impact on protein and natural organic mixture analysis.5–12

The resolving power in FT-ICR MS is limited by the

duration of the time domain ICR signal.2 The time duration is

critically related to ion motion. Ideally, a Penning trap13

confines and stores ions by combination of a spatially

uniform static magnetic field and a three-dimensional axial

quadrupolar electrostatic field. Ions in such a trap exhibit

three periodic motions (cyclotron rotation, magnetron

rotation, and axial ‘trapping’ oscillation).14 Ion stability

derives from these motions. Cyclotron rotation results from

the Lorentz force on an ion of mass,m, and charge, q, moving

ndence to: J. S. Yoo, Korea Basic Science Institute, 804-1ng-Ri, Ochang-Myun, Cheongwon-Gun, Chung-k-Do, 363-883, Republic of [email protected] sponsor: KBSI project ‘‘Development of anstrument’’.grant sponsor: NSF National High Field FT-ICR Massetry Facility; contract/grant number: DMR-06-54118.grant sponsor: Florida State University.grant sponsor: The National High Magnetic Fieldry in Tallahassee, FL, USA.

in a static magnetic field, B0, and prevents ions from escaping

in directions perpendicular to B0. In the absence of an electric

field the ion cyclotron angular frequency, vc, is given by:

vc ¼qB0

mð‘Unperturbed

;cyclotron frequency; S:I:unitsÞ (1)

In a spatially uniform direct current (dc), magnetic field,

and a three-dimensional quadrupolar electrostatic trapping

potential (e.g. near the center of an ICR ion trap), the ion

radial and axial motions are uncoupled in energy and the ion

cyclotron, magnetron, and axial frequencies are each

independent of ion position.15 Ions can be confined for a

long period of time without significant loss (the record is

10 months for a single electron in a Penning trap16). However,

collisions with neutrals,17 deviation from quadrupole

electrostatic trapping potential due to truncated, apertured,

or otherwise imperfect trapping electrodes, and Coulombic

charge interactions18 can destabilize ions radially and result

in damping and distortion of the time-domain ICR signal.

Mathematically, higher order terms must be added to the

quadrupolar electrostatic potential, and the three ion

motions then depend on ion position in the trap. Various

trap configurations have been designed to ‘shim’ the

radio-frequence (rf) and/or dc potentials toward their ideal

shape.19–27

Application of a spatially uniform transverse (to the

magnetic field) resonant rf electric field coherently excites

ions to a detectably large ion cyclotron orbital radius.2

However, the non-uniform rf excitation electric field in an

actual finite-size ICR cell can result in axial ejection of ions

(particularly at low m/z).28 Axial ejection may be reduced by

distributing the ion excitation voltage onto appropriately

Copyright # 2008 John Wiley & Sons, Ltd.

Figure 1. Sagittal cross-sections of a schematic hybrid

cylindrical ICR cell, showing capacitive coupling between

compensation and excitation electrodes.

1424 S. Kim et al.

segmented end cap electrodes (e.g. the Infinity cell19 or the

capacitively coupled open cell28) to flatten the rf excitation

electric field isopotential surfaces.

In a three-dimensional axial quadrupolar dc potential,

F(r,z), the radial electric field, dF/dr, increases linearly with

increasing r but is independent of z.29 A grounded screen

mesh30 or a trap with interlacing ‘comb’ wires as rf end cap

electrodes20 both effectively produce near-zero (and thus

constant) radial electric field as a function of z. Physically or

electronically segmented trap electrodes have been pro-

posed.22 Othermulti-electrode configurations can also flatten

the radial electric field.23–25,31, 32

It is known that ICR detection sensitivity, measurement

accuracy, and/or isotopic distributions can be improved by

optimizing post-excitation ICR orbital radius.2,33,34 More

Figure 2. Simulated potential (top), radial electric field (middle), and radial electric field

z-gradient (bottom) at 33% (left column) and 60% (right column) of the trap radius in a closed,

cylindrical ICR trap. The end cap potentials are 1V. Red dashes indicate the ideal (quad-

rupolar) distributions of dF/dr and d2F/drdz. This figure is available in color online at

www.interscience.wiley.com/journal/rcm.

Copyright # 2008 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2008; 22: 1423–1429

DOI: 10.1002/rcm

Hybrid FT-ICR MS cell for larger radius excitation 1425

recently, it has been shown that altering the dc electric

potential field during detection can lengthen ICR time-

domain signal duration, for improved FT-ICR mass spectral

resolving power and peak shape.29,35

Here, we propose a new ICR trap that adds two types of

electrodes to an Infinity19 trap, to modify the trapping

potential at a given ion post-excitation ICR orbital radius.

The new trap produces an on-axis electric field distribution

similar to that of the Infinity cell during rf excitation. After

excitation, the field distribution of the new trap ismodified to

approach more closely the ideal quadrupolar electric field at

the post-excitation ion cyclotron radius in the ICR trap.

EXPERIMENTAL

The proposed hybrid cell (Fig. 1) is similar to the Infinity

cell,19 with two types of additional electrodes. One set of

‘compensation’ electrodes is located between the excitation/

detection and end cap electrodes, and consists of four

Figure 3. Simulated potential (top), radial e

field z-gradient (bottom) at 33% (left column) a

in an open cylindrical ICR trap. The end cap po

ideal distributions of dF/dr and d2F/drdz. T

www.interscience.wiley.com/journal/rcm.


cylindrical electrode quadrants of the same radius as the

excitation/detection electrodes. The compensation electro-

des adjacent to the excitation electrodes are capacitively

coupled to the excitation electrodes. The compensation

electrodes are physically separated from the trap and

excitation/detection electrodes. The placement of the

compensation electrodes is similar to that for the compen-

sated trap.16,31 The electrostatic potential field was simulated

for compensation electrodes of different lengths. The choice

of length is not critical because the change in potential could

be compensated for by optimizing the applied voltages.

The hybrid cell has additional electrodes compared with

the compensated trap: namely, the ‘center compensation’

electrodes located in a circular hole in the middle of each end

cap. In the conventional Infinity cell, so-called ‘sidekick’

electrodes corresponding to the ‘center compensation’

electrode used here are installed at one end of the cell.

The Hybrid cell has center compensation electrodes at both

ends of the cell for geometrical symmetry. The end caps

lectric field (middle), and radial electric

nd 60% (right column) of the trap radius

tentials are 1V. Red dashes indicate the

his figure is available in color online at


DOI: 10.1002/rcm

1426 S. Kim et al.

themselves are the same as for the Infinity cell,19 namely, a

pattern of electrically isolated segments designed to achieve

the excitation profile of an infinitively long trap. The center

compensation electrode diameter was chosen to match that

of a typical transfer octopole (5mm i.d.).

Three types of cells were simulated. The open cylindrical

cell had 60mm length and 60mm i.d. of detection and end

trapping electrodes. The same length and i.d. detection elec-

trodes were used for the new ‘hybrid’ and closed cylindrical

cells. Model cells were divided into small grid spacings

(0.5mm/grid unit), and were refined to a convergence level

of 10�5. Use of smaller grid spacing and better convergence

levels did not significantly change the results. Computer

simulations were performed with SIMION 3D36 version 7

running on a 3.1GHz Pentium 4 PC with 2 GByte RAM.

Figure 4. Simulated potential (top), radial elec

z-gradient (bottom) at 33% (left column) and 6

proposed hybrid ICR trap. Potentials on the compe

electrodes are indicated. Red dashes indicate th

and d2F/drdz. This figure is available in color on

rcm.


RESULTS AND DISCUSSION

Electrostatic potential field during detection inconventional ICR trapsIn a three-dimensional quadrupolar electrostatic dc potential

and spatially uniform rf excitation and detection electric

field, the image charge induced on ICR cell detection

electrodes increases linearly from zero (ions on-axis) with

increasing ion cyclotron orbital radius. However, the

trapping potential deviates increasingly from quadrupolar

with increasing ICR radius.37 The magnitudes of ‘harmonic’

signals at odd-integer multiples of the ‘fundamental’ ICR

frequency also increase,37 and the magnetic field homogen-

eity decreases. In addition, there are complex effects due to

Coulomb repulsions between ions.38 As a result, optimal

tric field (middle), and radial electric field

0% (right column) of the trap radius in a

nsation, end cap, and center compensation

e ideal (quadrupolar) distributions of dF/dr

line at www.interscience.wiley.com/journal/


DOI: 10.1002/rcm


detection is typically achieved at an ICR orbital radius of

30–60% of the cell radius.34 For the present simulations, we

have chosen to evaluate the electrostatic potential inside the

ICR trap at a radial displacement corresponding to ions

excited to 33% and 60% of trap radius (Figs. 2–5) (We

calculate a typical excitation radius of 33% in our Bruker

Apex FT-ICR instrument.) Achieving optimal post-excitation

radius is important to improve sensitivity and accuracy in

representing isotopic distributions.33

In a perfectly quadrupolar electrostatic trapping potential,

the radial electric field increases linearly with r but is

independent of z, so that d2F/dzdr is zero throughout the

trap. Plots ofF, dF/dr, and d2F/dzdr as a function of z thus

provide a simple graphical basis for judging departures from

Figure 5. Simulated potential (top), radial elec

z-gradient (bottom) at 33% (left column) and 6

proposed Hybrid ICR trap. Potentials on the comp

electrodes are indicated. Red dashes indicate th

and d2F/drdz. This figure is available in color on

rcm.


the ideal quadrupolar trapping potential. The ideal distri-

butions of dF/dr and d2F/drdz are noted with red dashes to

make the point graphically. Figures 2 and 3 show such plots

for two conventional trap configurations (closed right

circular cylinder of 1:1 aspect ratio and three equal-length

open cylinders), for an experimentally typical end cap

potential of 1V. For both conventional traps, the radial

electric field deviates more from the ideal quadrupolar form

as the distance from the trap central axis increases. Note the

much larger maximal magnitude of d2F/dzdr at 60% of the

cell radius than at 33%. That deviation helps to account for

reduced ion stability (and thus greater FT-ICR mass spectral

peak width and lower resolving power) at higher post-

excitation ICR orbital radius.39,40

tric field (middle), and radial electric field

0% (right column) of the trap radius in a

ensation, end cap, and center compensation

e ideal (quadrupolar) distributions of dF/dr

line at www.interscience.wiley.com/journal/


DOI: 10.1002/rcm

1428 S. Kim et al.

Electrostatic potential field during detection inthe hybrid cellThe trap potential well depth of the open cylindrical cell

(Fig. 3) is different from that of the closed cell (Fig. 2). In

general, the trapping potential is smaller in the center of the

cell for the open cylindrical cell than for the closed cell.28

Therefore, for the most direct comparison between hybrid

and conventional traps, we adjusted the voltages applied to

the outer and center compensation electrodes so that the

depth of the potential well of the hybrid cell matches that of

the closed cell (Fig. 4) or the open cylindrical cell (Fig. 5), for

33% and 60% radius, while maintaining the end cap voltage

at 1V. Although the radial potential gradient (dF/dr) near

the trap axial center (�10mm< z <10mm) and 33% of the

cell radius is as flat as those of the conventional traps

(compare middle left panels in Figs. 2 and 4, or 3 and 5),

the radial electric field z-gradient at 60% of the cell

radius is much flatter for the hybrid trap than for the

conventional traps (compare middle right panels in Figs. 2

and 4, or 3 and 5). These simulations clearly show that

the radial potential distribution at large post-excitation

ICR orbital radius can be improved with the hybrid cell

configuration.

Figure 6. Isopotential contours (left) and radi

of maximal radius for a hybrid trap, for each of

center compensation, and compensation ele

60% trap diameter are presented to indicate ex

color online at www.interscience.wiley.com/jo


Radial electric field contribution for eachelectrode simulated from the principle ofsuperpositionBy the principle of superposition, the overall potential field

inside an ICR cell is the sum of the potential fields for each of

the individual electrodes. For example, for the hybrid cell,

the potential field at 60% of maximum cell radius (Fig. 5, top

right) is obtained as the sum of the potential fields for (a) 3V

on the compensation electrodes, 0V on the end cap

electrodes, and 0V on the center compensation electrodes

(Fig. 6, top left), (b) 0V on the compensation electrodes, 1V

on the end caps, and 0V on the center compensation

electrodes (Fig. 6, middle left), and (c) 0V on the

compensation electrodes, 0V on the end caps, and –70V

on the center compensation electrodes (Fig. 6, bottom left).

The arrows in the figure represent a 60% excitation radius.

Ideally, d2F/dzdr should be zero at all values of z inside

the trap. Separating the potential (and radial electric field and

radial electric field z-gradient) components for each paired

set of electrodes helps in deciding how to scale the various

electrode voltages so as to minimize d2F/dzdr at a given

post-excitation ICR orbital radius. The compensation elec-

trodes of the hybrid trap generate a radial electric field

al electric field z-gradients (right) at 60%

three indicated combinations of end cap,

ctrode voltages. The arrows indicating

citation radius. This figure is available in

urnal/rcm.


DOI: 10.1002/rcm


z-profile similar to that of the open cylindrical or closed cell

(compare top right in Fig. 6 with bottom left in Figs. 2 and 3).

The overall design of the end cap electrodes with grounded

compensation electrodes (middle left in Fig. 6) is similar to a

closed elongated cell.41 Therefore, it is not surprising that the

end cap electrodes provide a radial electric field z-profile

with the same sign as the closed cell (middle right in Fig. 6).

The negative voltage at the center compensation electrodes

plays a crucial role in reducing radial electric field

z-gradients by providing the z-profile with the opposite

sign (bottom in Fig. 6). It cancels out the combined radial

electric field z-gradients provided by other electrodes. The

role of negative voltage at the center compensation electrode

can partially explain what was observed in the previous

studies.29,35 A beam of electrons at the center of a trap or

negative voltage at the sidekick electrodes can reduce the

radial electric field z-gradients and thus idealize the trapping

potential as is presented in Fig. 6.

In this study, a relatively large negative voltage (�70V in

Fig. 6) at the center compensation electrodes is required to

idealize the trapping potential at 60% excitation radius. The

negative voltage can reduce the trapping potential well

depth, which could result in ion loss. The reduced trapping

field is in turn compensated for by a combination of

potentials at the compensation and end cap electrodes.

The combination of the three sets of electrodes changes the

overall potential field to more closely approach the desired

three-dimensional axial quadrupolar potential at large ion

cyclotron post-excitation radius, without reducing the depth

of the axial trapping potential well at that radius.

CONCLUSIONS

A new cell (‘hybrid cell’) for FT-ICR MS is presented and

characterized by Simion simulation. The hybrid cell

combines the designs of the Infinity,19 compensated,31 and

capacitively coupled open cylindrical cell28 to achieve a

uniform rf excitation electric field. Comparison with

conventional cells with the same end cap potential shows

that a more ideal trapping potential can be achieved with the

hybrid cell at larger excitation radius, because three degrees

of freedom are available in the hybrid cell to achieve ideal

trapping potential compared to just one for the conventional

cells. Analysis from the principle of superposition shows that

the extra degrees of freedom provided by compensation

electrodes are critical to achieve a better trapping potential

distribution. The voltages applied to the hybrid cell in

Figs. 4–6 were chosen empirically by trial and error.

However, a more systematic method could be developed

to further optimize the result. Future efforts will be directed

at building and testing the hybrid cell experimentally, with

particular emphasis on maximizing the post-excitation ICR

orbital and its impact on detection sensitivity, mass accuracy,

and m/z discrimination.

AcknowledgementsThis work was supported by KBSI project ‘‘Development of

an FT-ICR instrument’’, the NSF National High Field FT-ICR

Mass Spectrometry Facility (DMR-06-54118), Florida State

University, and the National High Magnetic Field Labora-

tory in Tallahassee, FL, USA.


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DOI: 10.1002/rcm

the ‘hybrid cell’: a new compensated infinity cell for larger radius ion excitation in fourier...

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