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Ion motion in a Fourier transform ion cyclotron resonance wire ion guide cell
ELSEVIER
International Journal of Mass Spectrometry and Ion Processes 1571158 (1996) 129-147
Ion motion in a Fourier transform ion cyclotron resonance wire ion guide cell Kent J. Gillig, Brian K. Bluhm, David H. Russell* Laboratory
for Biological
Mass Spectrometry,
Department
of Chemistry,
Texas A&M
University,
College
Station,
TX 77843,
USA
Received 3 January 1996; accepted 1 August 1996
Abstract Ion motion in a novel Fourier transform ion cyclotron resonance (IT-ICR) cell design is investigated. The cell design consists of a cylindrical electrostatic trap and an inner wire used to create a potential well inside the ICR cell. The ion cell is modeled as a Kingdon trap in an axial magnetic field. A theoretical description is given and advantages to using this cell (wire ion guide cell) design are discussed. Experimental results obtained using both an elongated wire-ion guide (WIG) cell (aspect ratio = 3.3) and a small WIG cell (aspect ratio = 1) to determine parameters affecting trapping performance are presented.
Keywords:
FI-ICR
cell design; Ion motion
1. Introduction One of the major focuses of current Fourier transform ion cyclotron resonance (IT-ICR) mass spectrometry research is to extend the mass range while retaining the high resolution capabilities of the method [1,2]. A variety of approaches for trapping high mass ions and obtaining high resolution at high mass have been described. These approaches include the use of external ion sources with a radiofrequency-only quadrupole [3] and electrostatic ion guide [4]; electrostatic lenses [5,6]; pulsed electrostatic [7] and electron beam [S] trapping techniques; collisional cooling combined with axialization by rf excitation [9,10], collisional relaxation [l 11; stronger magnetic fields and * Corresponding author.
ionization techniques that produce cooler ions and/or multiple charging [12-181. The approach we have taken is predicated on the view that the limiting factor to trapping and detecting high m/z ions is the excesskinetic energies of ions formed by the ionization methods used and the spatial dispersion of the ions in the FT-ICR cell. Thus, our approach has been to modify the existing ICR cell geometry to create a favorable environment for trapping high mass ions. Our initial efforts were aimed at damping the ion motion by collisions with a buffer gas and this approach was successful for detecting bovine insulin (M, 5734) and cytochrome c (M, 12231 (bovine)) [ll]. More recently, we developed a wire-ion guide (WIG) cell that greatly improves trapping and detection of high molecular mass analytes such as bovine serum albumin (66 430) and transferrin dimer (157 370). The WIG cell consists of
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an electrostatic trap and an inner wire used to create a potential well inside the ICR cell [19]. At the time the WIG cell was introduced, a detailed analysis of the electrostatic well and the resulting ion trajectories were not presented, thus questions as to how this cell trapped such high mass ions were not addressed. The geometry of the WIG cell approximates that of a Kingdon trap [20], an electrostatic trap used by physicists for numerous applications in particle physics [22-251, and originally suggested and investigated as a potential cell for ICR by Wanczek [21]. This paper presents a detailed theoretical description of the WIG cell, including electrostatic potential surface equations, equations of motion and ion trajectory plots. Experimental data obtained using an elongated WIG cell (aspect ratio = 3.3) and a small WIG cell (aspect ratio = 1) to examine the effects of applied voltages on mass measurement accuracy and resolution are also presented. 2. Experimental All experiments were performed using a 7 tesla FT-ICR mass spectrometer. Details of this instrument have been described elsewhere [ll]. Modifications to the design include a Nd: YAG laser for ionization and desorption (New Wave Research, Model- MiniLase 20) operated at a wavelength of 355 nm. C& samples were deposited on the probe tip in 2 ul aliquots from a 10 mg/ml benzene solution. Frequency shift data were collected by averaging 100 spectra from three separateruns. Two laboratory-built cylindrical WIG cells were employed, one elongated (16 x 4.5 cm) and the other with its length equal to the diameter (1.5 x 1.5 cm). The small cell was used in the ideal Kingdon trap experiments. 3. Results and discussion In the original paper where we reported the use
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of the WIG cell to detect bovine serum albumin (M, 66430), transferrin (M, 78685) and the dimer ion of transferrin (M, = 157370) we showed SIMION plots of the electrostatic trapping fields and calculated ion trajectories. The SIMION plots show that trapping fields for the WIG cell are quite different from the standard cubic and rectangular cell geometries. Positioning the WIG in the cell deepens the electrostatic trapping well along the centerline of the ion cell, thus facilitating ion trapping. On the basis of these results,we speculatedthat the WIG servesto focus the ions to the center of the cell, thus improving the overall sensitivity of matrix assisted laser desorption ionization (MALDI) FT-ICR, and to extend the homogeneous region of the d.c. (trapping) electric field. The initial study, however, did not addressissuesrelated to accurate masscalibrations at high mass.For example, it was evident that the WIG changes the electric field experienced by the ion, consequently, the ions experienced frequency shifts uncharacteristic of conventional ICR ion traps. In the following sections of this paper, we present detailed descriptions of the trapping potentials of the WIG cell and evaluate the device in terms of frequency shifts, m/zcalibration and mass resolution. 4. Background material for electrostatic ion traps The ideal Penning Trap has an equipotential surface that assumes the shape of rotational hyperboloids, yielding an optimum trapping potential given by ;r2--i (1) ( > where r and z are the radial and axial coordinates and A is a constant determined by the cell’s geometry and dimensions [26]. Electrostatic potentials in a cubic or cylindrical ICR cell compromise the optimum trapping potential described by Eq. (1) for an approximated V(r,z)=
-A
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optimum trapping environment in the form of a quadrupolar potential [27] represented by V(x,y,z) = v, [y - $x2 +y2 - 24
(2)
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where constants CX,y, and a have been solved for many different cell dimensions [28]. Actual equipotential lines deviate somewhat from this approximation, due to inhomogeneous electric
a.
b.
Fig. 1. (a) SIMION plot of equipotential lines for an elongated cylindrical cell. VT = 10 V (trapping lines for typical Kingdon trap arrangement, wire at - 10 V, cylinder at ground.
plates).(b)
SIMION
plot of equipotential
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- v=o - “2 r = RZ = radius of cylinder R, = radius of wire V, = voltage on wire V, = voltage on cylinder plates V = 0 = boundary condition for variable end-plates L = length of cell, z = 0 to z = L along z-axis Ei= angular coordinate
v=o Fig. 2. Schematic
of wire ion guide cell used in FT-ICR
fields, as shown in Fig. la. Exact solutions for these potentials have been solved and presented elsewhere [29,30].
conditions V(r, z) =
experiments.
is given by i:
n=l,3,5,...
[A,Z,,(K,r)
+&&(K,r)]co&z (44
5. Electrostatic equations for a WIG cell The electrostatic potential within a long, closed cylinder containing a wire approximates a radial logarithmic potential near the center of the cell given by
V(r)=
[ ln(~~a)jln(
k)
7rn
K n- --
-&ax
Z max =
L -2
(49
(4c)
(3)
The exact potential is obtained through a solution of Laplace’s equation in three dimensions. The boundary conditions consist of a cell of length L (z = 0 to z = L) and radius r with an internal wire (rod) of radius RI as shown in Fig. 2. The voltages are set at V = VI for the wire of radius RI, V=O = V2 for the segmented cylinder of radius Rz and V = 0 at z = 0 and z = L (endplates). The exact solution for these boundary
(44
where V is the potential on the wire, and I0 and Ka are zeroth order modified Bessel functions. A,,
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and B, are constants that satisfy the given boundary conditions. An example of the resulting equipotential lines is shown in Fig. lb. Note that within the cell boundaries the radial electric field is purely inward directed. In a normal ICR cell, it is the outward directed radial field that limits high mass detection, but this limitation can be overcome in the WIG cell because the bounded volume of the trap is no longer hollow (the bounded volume is electrically neutral). A standard ET-ICR cell has end plates with variable voltages. The WIG cell also consists of segmented plates and is thus capable of variable voltages. This addition requires modifying the equations of the electrostatic potential for the cell. The exact equations for a Kingdon trap with variable end plates (WIG cell) are given bY [311 2.2
V(r, z) =
r
An = I, (nnR,/L)K,
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plates of the cylinder, the electric potential can be assumed as being purely logarithmic. Twodimensional ion motion in a logarithmic potential [35] with no external magnetic field is described by the following equations. Conservation of energy yields E = $z($
+ r2b2) +Peln
1. 0 r0
which leads to ;=(;)ikqln(k)-($)I’
(6b)
and
(64
/ n*r\
&J5 , , I... 1A,‘o(y)
(4/m)
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[V&
+B,&(
y)]sin(
3
(59
(n7rR1/L) - v&J (n;lrRz/L)]
(mrR,/L)
-1, (mrR1/L)K,,
6. Ion motion in a Kingdon trap
Therefore,
All of the aforementioned traps possess different equipotential surfaces, thus, ions assume markedly different trajectories in each trap. Equations of ion motion for a quadrupolar potential have been solved and published elsewhere [29,32-341. Similar equations are not so easily obtained for the trapping conditions created by the arrangement of the Kingdon trap, due to the dependence of potential on radial position. Neglecting the effect of the voltage on the end
E-Pain(i)
(5b)
(mrR2/L)
- ($)
=O,
dr dr when the condition - = - = 0 df9 dt
6-9
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(W where L is the angular momentum, m is the mass of the ion, r is the radial distance, E is the kinetic energy of the ion, 8 is the angle, P 0 = .G&)’ r. lS the radius where P(ro) = 0, 0 is the angular component of motion and T is the temporal component of motion. Since 0 and T cannot be written in terms of known functions, they must be evaluated numerically. Hooverman [35] and Lewis [36] have previously evaluated the ion motion in an electrostatic Kingdon trap presuming a near circular orbit with the assumption that circular orbits will result in the longest trapping times. Circular orbits are also easier to calculate because the kinetic energy is constant for all radii. Examples of ion motion in an electrostatic Kingdon trap are given in Fig. 3. The motion is dependent on the ion’s centrifugal force, due to its angular momentum with respect to the attractive electric force. The sum of these two forces determines the radial acceleration or deceleration of the ions.
7. Ion motion in a WIG cell A Kingdon trap confined in an external magnetic field has been adapted for use in FT-ICR [ 191. Ion motion in this arrangement is influenced by both the electrostatic and the magnetic fields (E x B). Fig. 3. (a) SIMION plot of circular orbit ion trajectory in x-y plane for a Kingdon trap operated in electrostatic mode only. Wire at -10 V, cylinder at ground; mass of ion = 10000 Da; kinetic energy of ion = 1 eV and initial Y angle = -45”.(b) SIMION plot of retrograde pericycloidal ion trajectory in the x-y plane for a Kingdon trap operated in electrostatic mode only. Wire at -10 V, cylinder at ground; mass of ion - 10000 Da, kinetic energy of ion - 1 eV and initial Y angle = O”.(c) SIMION plot of retrograde pericycloidal ion trajectory in the x-y plane for a Kingdon trap operated in electrostatic mode only. Wire at -10 V, cylinder at ground; mass of ion = loo00 Da, kinetic energy of ion = 1 eV and initial Y angle = 30”.
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The equations of motion for this arrangement are as follows: From conservation of energy, where E varies with r only 2eVo 2e r i2 + (re)2 = y +E(r)dr m s ro
(74
coupled with the Force Equations f2 - r(b)2 = eE -
f-$(r2b) =(-JfJBi
(74
Whereby defining f(r)=
;j;oB(r)rdr+
(74
;
Integrating Eq. (7) gives e= &j(r)
(79
Then, substituting Eq. (7) into Eq. (7) yields the equation with constant magnetic field di’ -= dr
+ -1
(B&2)
+ (C/r)
-r [hV, + hE&gr
- [(&r/2)
+ (C/r)] ‘1’ Vf 1
where E is the kinetic energy, m is the massof the ion, c is the speed of light, B is the magnetic field strength, V. is the initial kinetic energy of the ion, h = %, B,, is the constant magnetic field, E,, is the constant electric field and r? is a constant. Examples of ion motion in a WIG cell are Fig. 4. SIMION plots of ion trajectories as a function of different potentials on the wire in a WIG cell, as seen in PI-ICR experiments. The wire is located at the midpoint of a 100 x 100 2D array at coordinates 50,50, coaxially surrounding the wire is a grounded segmented cylinder with a 25 grid point radius. All ions are 10 000 Da, with 10 eV of initial kinetic energy, starting with zero angle in the y direction from coordinates 48,48. The cell is surrounded by a uniform magnetic field of 7 T.(a) Cyclotron and magnetron motion around wire at -2 V.(b) Prolate hypotrochoidal motion around wire at -50 V.(c) Ion motion approximating retrograde pericycloidal motion with wire at -150 V (distorted by magnetic field).
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shown in Fig. 4. With a strong magnetic field and a weak electric field present (4a), motion consists of cyclotron motion with a superimposed drift motion whose direction and amplitude is dependent on the central force direction and amplitude. As the strength of the electric field increases, the magnitude of the guiding center drift increases and the motion is best described as central force motion in a precessing coordinate frame (4~). Note that ion motion in these examples is during the initial part of a normal ICR experiment sequence (ionization and trapping delay). It is apparent that modest voltages have a large effect on the ion’s behavior and that, with a large enough voltage, ion motion due to the influence of the electric field is dominant over the magnetic field. For example, an ion moving at 800 m/s positioned 0.1 mm from the wire in the region of the trap where the potential is purely logarithmic experiences an attractive electric force of -2.3*10-15 N when a 10 V potential difference is applied to the wire and cylinder. This force clearly exceeds the Lorentz force of -9-10-i6 N that the same ion experiences in a 7 T magnetic field. Under these conditions, it is possible to trap ions with high kinetic energies or mass and increase the total number of ions present (larger energy spread). Once the ions are stable/cooled, the trapping parameters can be switched to conditions that are favorable for the excitation and detection event (wire at ground or slightly positive, e.g., equal to trapping plate potentials). 8. Collisional cooling effects Ions with high kinetic energy, such as those formed by MALDI, were until recently not amenable to high resolution FT-ICR. To obtain high resolution, the ion signal must be observed for extended periods of time (i.e. long transient). The most obvious choice for cooling the ions is collisional cooling. Collisional cooling has been shown to enhance trapping of MALDI formed ions in t-f-traps, but in ICR cells this technique
Fig. 5. (a) SIMION plot of ion trajectory during collisional cooling in a WIG cell, showing the ion collapsing towards the wire. The wire is at -10 V, coaxially surrounding the wire is a segmented cylinder that is grounded. Ions are 10 000 Da with 100 eV of initial kinetic energy, starting with zero angle in the y direction from coordinates 48,48. The cell is surrounded by a uniform magnetic field of 7 T.(b) SIMION plot of ion trajectory during coIIisiona1 cooling in a dynamic Kingdon trap, showing the ion assuming a trajectory in the form of a cylindrical sheath around the wire. The wire is at coordinates SO,50 and is at -10 V, coaxially surrounding the wire is a segmented cylinder that is grounded. The ions are 10000 Da, with 10 eV of initial kinetic energy, starting with zero angle in the y direction from coordinates 557.5. The cell is surrounded by a uniform magnetic field of 7 T.
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720.5 IT-ICR
mass spectra of Cm obtained
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end plates at 14 V and wi Ire at -1 V.
721.5
721.0 with an elongated
WIG cell.
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-15
n n
-20 -2
0
6
2
TRAP&G
(a)
;
c
c
200.. 2
L
Km.-
: g !I
r
o.-
h
WQ I
VOLTAGE
A
300..
t 2'
ri
8
-100..
:
-zoo.-
8
-5
-4
-3
-2
0
1
2
3
4
5
6
VBN
(4
Fig. 8. (a) Observed
-1
1 # a b
frequency
shifts for an elongated
WIG cell, wire at ground,
1 s delay.(b)
Observed
frequency
shifts for an elongated
WIG
-
OBSERVED .-
CYCLOTRON f
FREQUENCY
(MHz)
OBSERVED
CYCLOTRON
FREQUENCY
(MHz)
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is detrimental without special modifications or parameter changes due to the magnetron motion expanding rapidly, thus causing the ion to be ejected from the cell. This expansion of the magnetron motion is due to the radial electric field being directed outward from the center of the cell in the case of a quadrupolar potential, described by
(8) where Q is 1.3869 (a constant determined by trap dimensions) for a cubic cell and a is the cell length. To compensate for this problem, quadrupolar axialization has been used to rapidly interconvert the magnetron and cyclotron motions, enabling the ions to converge to the cell center when cooled [9,10]. However, in a WIG cell, the radial electric field is directed inward as is shown bY
vk)=[ln(lL?I)l (4)
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creates equipotential lines approximated by V(r, z) = A (2 - r2/2 + Blnr)
(10)
where A and B are constantsfor a given cell. The equipotential lines approximating those described by Eq. (10) can be created by shaping the electrodes to conform to these potentials. A more convenient method is developed here by simply adjusting the voltage applied to the end plates of a WIG cell to form the field lines described by Eq. (10) and shown in Fig. 6. The optimum equipotential lines are created by a superposition of quadrupolar and logarithmic potentials, creating a spindle-like trapping volume. A major advantage of the ideal Kingdon trap would be the fact that it gives rise to a harmonic motion in the z-direction, extending trapping times and stabilizing the ion motion, which is given by the frequency
c-9
Therefore, the magnetron motion, when subjected to collisional relaxation, will shrink to the center of the cell with a rate proportional to the voltage applied to the wire. A low voltage on the wire causesthe magnetron motion to collapse slowly, but if collisional cooling is applied for too long a time the ions strike the wire and are lost, as is shown in Fig. 5a. An obvious solution is to superimpose an a.c. voltage onto the existing d.c. voltage on the wire during the cooling event, stabilizing the ion trajectories in a pseudopotential minimum located between the wire and the cylinder, with the ions confined in a volume representative of a cylindrical sheath [37], shown in Fig. Sb. 9. Ideal WIG cell To improve the WIG cell for FT-ICR, the cell design can be changed to approximate an ideal Kingdon trap [38]. The ideal Kingdon trap
10. Evaluation of a WIG cell and an ideal Kingdon trap To further evaluate a cylindrical WIG cell and characterize a small WIG cell that could be easily optimized to create an ideal Kingdon trap for implementation into ET-ICR experiments, comparisons are made of the variation of observed ICR frequency with trapping voltage and wire voltage. Our initial approach was to determine the effect of the wire’s physical presence on the resolution capabilities of the WIG cell. Fig. 7 contains a high resolution (m/Am -300 000) LD/FTICR mass spectra of Cm obtained using the elongated WIG cell. The resolution is at least comparable to that obtained in the same cell without the wire, indicating that ions are not colliding with the wire and being lost from the cell, and
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VTR=
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VBN=O
a
VBB=-5,
55.6
VBN=3
55.8
m/z
56.0 m/z
56.2
56.4
Fig. 10. FT-ICR mass spectra of Fe’ obtained with a small WIG cell.(a) 1 V trapping voltage, wire at ground during delay (VBB), wire at ground during excitation and detection (VBN).(b) 1 V trapping voltage, wire at -5 V during delay, +3 V during excitation and detection.
that the wire has no deleterious effect during ion excitation and detection. Fig. 8 contains plots of the observed frequency shifts for Cso as a function of trapping voltage (Fig. 8a) and wire voltage (Fig. 8b) obtained using the elongated WIG cell. The trapping voltage frequency shifts with the wire at ground throughout the experimental sequence are low, being only 2-3 Hz/V. Under these conditions, the wire improves the homogeneity of the electric field over a large portion of the total cell volume; consequently the ions do not experience frequency shifts due to aberrations
in the E x B fields. If the wire potential is varied during the excitation and detection events, the observed frequency shifts are considerably larger, -100 Hz/V. Note, however, that because the ion motion is now strongly influenced by the WJG, the frequency is independent of the trapping voltage. The frequency shift is positive or negative depending on the polarity of the wire voltage, indicating that the ions see the center of the cell from a different rotating reference frame when the polarity of the wire voltage is switched. Fig. 9 contains plots of observed frequency
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-5
-4
SMALL
-3
Fig. 13. Frequency
-2
-1
0
shift as a function
VBN
WIG CELL (FREQUENCY
of wire voItage
1
in a small WIG
2
cell.
3
4
SHIFT FROM VBB=O AND VBNd)
5
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0 v-
mua
aNv 3213x3
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shifts for laser desorbed Fe’ ions (355 nm, directly desorbed from a stainless steel probe) in a small cylindrical cell (Fig. 9a) and a small WIG cell (Fig. 9b) versus trapping voltage (a = no wire; b = with wire). The frequency shifts for both cells are -100 Hz/V, which is typical for a small ICR cell and similar because the potential near the cell center is non-zero in both cases.We explain the data in Fig. 9b as meaning the wire is ineffective at reducing the influence of the trapping voltages becauseof the cells small size.This is in contrast to an elongated WIG cell in which the trapping voltages are effectively “screened” by the wire. Thus, to obtain ideal Kingdon trap equipotentials in which the effective trapping region of the cell volume is maximized without applying high trapping voltages/WIG voltages the cell length must be reduced. Fig. 10 contains massspectra of laser desorbed Fe* ions obtained using a small WIG cell for trapping and detection. The spectrum shown in Fig. 10a was obtained with the trapping plates at 1 V and the wire at ground throughout the experiment. Whereas, the spectrum shown in Fig. lob was obtained under the same conditions except that the wire voltage was biased to -5 V during the trapping delay and then switched to +3 V during the excitation and detection. An increase in both resolution and ion signal by a factor of four to five is obtained. Switching the wire polarity between the trapping delay and the excitation and detection event alternates the cell configuration between a Kingdon trap (in which ion trapping efficiency in increased) and a “screened” ICR cell (in which frequency shifts are reduced and resolution is increased). To optimize the cell response in terms of ion trapping efficiency, mass measurement and resolution, a series of spectra were obtained in which the trapping voltage-wire voltage ratio was varied. Fig. 11 shows a plot of ion abundance versus the WIG voltage during the excitation and detection event for several runs where the voltage on the wire during the trapping delay was also varied. The ion abundance is greatest when the
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voltage on the wire is negative during the trapping delay and positive during the excition and detection. Note that the ion signal is very low when the WIG voltage is positive throughout the experimental sequence. Similar trends are indicated for resolution. Fig. 12 contains a plot of resolution versus WIG voltage during the excitation and detection event while varying the voltage on the wire during the trapping delay. The resolution is greatest when +3 V is applied to the wire during the excition and detection event. The observed frequency shifts for Fe+ ions are plotted as a function of WIG voltage during the excitation and detection event in Fig. 13. Note that the frequency observed for Fe’ ions is a function of both the WIG voltage before and during the excitation and detection event. However, at a WIG potential of +3 V, the frequency is independent of the voltage during the trapping delay. Our explanation of this effect is based on data shown is Fig. 14. Fig. 14 contains a plot of the WIG voltage during the excitation and detection event and the trapping voltage required to maintain a constant observed cyclotron frequency. In effect, the WIG cell can be “tuned” to obtain a field free detection enviroment, where the deviation in observed cyclotron frequency and true cyclotron frequency is minimized, thus improving the mass measurement accuracy as smaller calibration corrections are needed. This also corresponds to the voltage combination at which the resolving power is highest for a given aquisition period, e.g., the smallest number of ions are lost from the cell during excitation and detection. 11. Conclusion In this paper, we provide a complete description of electrostatic trapping potentials, ion trajectories and performance of a novel cell design approximating a Kingdon trap for use in FTICR experiments. The cell consists of a wire
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surrounded by a segmented elongated cylinder. The advantages of the cell design are multifold: The wire creates a deep potential well in which to trap ions with high kinetic energies and mass as well as providing a radial force vector directed towards the center of the cell, helping to dampen magnetron motion. A further advantage of this cell can be obtained by using parameters that create the ideal Kingdon trap equipotential lines, consequently providing harmonic motion in the z-direction. Experiments performed with an elongated and a small WIG cell indicate that resolution and mass measurement accuracy may be improved by adjusting parameters during the experimental sequence to minimize trapping voltage-induced frequency shifts or even completely eliminate shifts while still maintaining z-axis harmonicity. We are continuing to investigate advantages of this cell design, which may further enhance the ability of FT-ICR to solve problems that require high resolution and precise mass measurement accuracy.
Acknowledgements This work was supported by the National Science Foundation (Grant #CHE-9223629) and The Robert A. Welch Foundation.
References [l] D.H. Russell, Mass Spectrom. Rev., 5 (1986) 167-189. [2] CL. Holliman, D.L. Rempel and M.L. Gross, Mass Spectrom. Rev., 13 (1994) 105-132. [3] R.T. Mclver, R.L. Hunter and W.D. Bowers, Int. J. Mass Spectrom. Ion Processes, 64 (1985) 67-77. [4] P.A. Limbach, A.G. Marshall and M. Wang, Int. J. Mass Spectrom. Ion Processes, 125 (1993) 13.5-143. [5] P. Kofel, M. AIlemann, Hp. Kellerhals and K.P. Wanczek, lnt. J. Mass Spectrom. Ion Processes, 65 (1985) 97-103. [6] J.T. Meek and G.W. Stockton, U.S. Patent 4,686,365 (1987). [7] M. Dey, J.A. Castor0 and CL. WiIkins, Proc. 42nd ASMS Conference on Mass Spectrometry and Allied Topics, Chicago, 1994, p. 246.
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[8] CL. Hendrickson, F. Hadjarab and D.A. Laude, Jr., lnt. J. Mass Spectrom. Ion Processes, 141 (1995) 161-170. [9] L. Schweikhard, S. Guan and A.G. MarshaIl, lnt. J. Mass Spectrom. Ion Processes, 120 (1992) 71-83. [lo] D. Wineland and H. Dehmelt, Int. J. Mass Spectrom. Ion Phys., 16 (1975) 338-342. [ll] T. Solo&i and D.H. Russell, Proc. Natl. Acad. Sci. USA, 89 (1992) 5701-5704. [12] J.B. Fenn, M. Mann, C.K. Meng, S.F. Wong and CM. Whitehouse, Mass Spectrom. Rev., 9 (1990) 37-70. [13] J.B. Fenn, J. Am. Sot. Mass Spcctrom., 4 (1993) 524-535. [14] B.E. Winger, S.A. Hofstadler, J.E. Bruce, H.R. Udseth and R.D. Smith, J. Am. Sot. Mass Spectrom., 4 (1993) 566577. [15] Z. Guan, S.A. Hofstadler and D.A. Laude, Jr., Anal. Chem., 65 (1993) 1588. [16] S.C. Beu, M.W. Senko, J.P. Quinn and F.W. McLafferty, J. Am. Sot. Mass Spectrom., 4 (1993) 190-192. [17] J.A. Castor0 and C.L. Wilkins, Anal. Chem., 65 (1993) 26212627. [IS] C. Koster, J.A. Castor0 and CL. Wilkins, J. Am. Chem. Sot., 114 (1992) 7572-7574. [19] T. Solouki, K.J. Gillig and D.H. Russell, Anal. Chem., 66 (1994) 1583-1587. [20] K.H. Kingdon, Phys. Rev., 21 (1923) 408-418. [21] K.P. Wanczek, lnt. J. Mass Spectrom. Ion Processes, 95 (1989) l-38. [22] A.G. Calamai and C.E. Johnson, Phys. Rev. A, 42 (1990) 5425-5432. [23] M.H. Prior, R. Marrus and C.R. Vane, Phys. Rev. A, 28 (1983) 141-150. [24] L. Yang and D.A. Church, Nucl. lnstrum. Methods, B56-57 (1991) 1185-1187. [25] C. 0, H.A. Scheussler, Rev. Phys. Appl., 16 (1981) 441-445. [26] J. Byrne and P.S. Farago, Proc. Phys. Sot., 86 (1965) 801815. [27] P.B. Grosshans and A.G. Marshall, Anal. Chem., 63 (1991) 215A-229A. [28] J.B. Jefffries, S.E. Barlow and G.H. Dunn, lnt. J. Mass Spectrom. Ion Processes, 54 (1983) 169-187. [29] P. Kofel, M. AIlemann, Hp. Kellerhals and K.P. Wanczek, Int. J. Mass Spcctrom. Ion Processes, 74 (1986) 1-12. [30] J.D. Jackson, Classical Electrodynamics, 2nd edn., Wiley, New York, 1975. [31] C.E. Johnson, J. Appl. Phys., 55 (1984) 3207-3214. [32] R.C. Dunbar, J.H. Chen and J.D. Hays, lnt. J. Mass Spectrom. Ion Processes, 57 (1984) 39-56. [33] T.E. Sharp, J.R. Eyler and E. Li, lnt. J. Mass Spectrom. Ion Phys., 9 (1972) 421-439. [34] M. Wang and A.G. Marshall, lnt. J. Mass Spectrom. Ion Processes, 100 (1990) 323-346. [35] R.H. Hooverman, J. Appl. Phys., 34 (1963) 3505-3508. [36] R.R. Lewis, J. Appl. Phys., 53 (1982) 3975-3980. [37] R. Blutiiel, Appl. Phys. B, 60 (1995) 119-122. [38] R.D. Knight, Appl. Phys. Iett., 38 (1981) 221-222.
International Journal of Mass Spectrometry and Ion Processes 1571158 (1996) 129-147
Ion motion in a Fourier transform ion cyclotron resonance wire ion guide cell Kent J. Gillig, Brian K. Bluhm, David H. Russell* Laboratory
for Biological
Mass Spectrometry,
Department
of Chemistry,
Texas A&M
University,
College
Station,
TX 77843,
USA
Received 3 January 1996; accepted 1 August 1996
Abstract Ion motion in a novel Fourier transform ion cyclotron resonance (IT-ICR) cell design is investigated. The cell design consists of a cylindrical electrostatic trap and an inner wire used to create a potential well inside the ICR cell. The ion cell is modeled as a Kingdon trap in an axial magnetic field. A theoretical description is given and advantages to using this cell (wire ion guide cell) design are discussed. Experimental results obtained using both an elongated wire-ion guide (WIG) cell (aspect ratio = 3.3) and a small WIG cell (aspect ratio = 1) to determine parameters affecting trapping performance are presented.
Keywords:
FI-ICR
cell design; Ion motion
1. Introduction One of the major focuses of current Fourier transform ion cyclotron resonance (IT-ICR) mass spectrometry research is to extend the mass range while retaining the high resolution capabilities of the method [1,2]. A variety of approaches for trapping high mass ions and obtaining high resolution at high mass have been described. These approaches include the use of external ion sources with a radiofrequency-only quadrupole [3] and electrostatic ion guide [4]; electrostatic lenses [5,6]; pulsed electrostatic [7] and electron beam [S] trapping techniques; collisional cooling combined with axialization by rf excitation [9,10], collisional relaxation [l 11; stronger magnetic fields and * Corresponding author.
ionization techniques that produce cooler ions and/or multiple charging [12-181. The approach we have taken is predicated on the view that the limiting factor to trapping and detecting high m/z ions is the excesskinetic energies of ions formed by the ionization methods used and the spatial dispersion of the ions in the FT-ICR cell. Thus, our approach has been to modify the existing ICR cell geometry to create a favorable environment for trapping high mass ions. Our initial efforts were aimed at damping the ion motion by collisions with a buffer gas and this approach was successful for detecting bovine insulin (M, 5734) and cytochrome c (M, 12231 (bovine)) [ll]. More recently, we developed a wire-ion guide (WIG) cell that greatly improves trapping and detection of high molecular mass analytes such as bovine serum albumin (66 430) and transferrin dimer (157 370). The WIG cell consists of
0168-1176/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved SO168-1176(96)04465-5
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an electrostatic trap and an inner wire used to create a potential well inside the ICR cell [19]. At the time the WIG cell was introduced, a detailed analysis of the electrostatic well and the resulting ion trajectories were not presented, thus questions as to how this cell trapped such high mass ions were not addressed. The geometry of the WIG cell approximates that of a Kingdon trap [20], an electrostatic trap used by physicists for numerous applications in particle physics [22-251, and originally suggested and investigated as a potential cell for ICR by Wanczek [21]. This paper presents a detailed theoretical description of the WIG cell, including electrostatic potential surface equations, equations of motion and ion trajectory plots. Experimental data obtained using an elongated WIG cell (aspect ratio = 3.3) and a small WIG cell (aspect ratio = 1) to examine the effects of applied voltages on mass measurement accuracy and resolution are also presented. 2. Experimental All experiments were performed using a 7 tesla FT-ICR mass spectrometer. Details of this instrument have been described elsewhere [ll]. Modifications to the design include a Nd: YAG laser for ionization and desorption (New Wave Research, Model- MiniLase 20) operated at a wavelength of 355 nm. C& samples were deposited on the probe tip in 2 ul aliquots from a 10 mg/ml benzene solution. Frequency shift data were collected by averaging 100 spectra from three separateruns. Two laboratory-built cylindrical WIG cells were employed, one elongated (16 x 4.5 cm) and the other with its length equal to the diameter (1.5 x 1.5 cm). The small cell was used in the ideal Kingdon trap experiments. 3. Results and discussion In the original paper where we reported the use
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of the WIG cell to detect bovine serum albumin (M, 66430), transferrin (M, 78685) and the dimer ion of transferrin (M, = 157370) we showed SIMION plots of the electrostatic trapping fields and calculated ion trajectories. The SIMION plots show that trapping fields for the WIG cell are quite different from the standard cubic and rectangular cell geometries. Positioning the WIG in the cell deepens the electrostatic trapping well along the centerline of the ion cell, thus facilitating ion trapping. On the basis of these results,we speculatedthat the WIG servesto focus the ions to the center of the cell, thus improving the overall sensitivity of matrix assisted laser desorption ionization (MALDI) FT-ICR, and to extend the homogeneous region of the d.c. (trapping) electric field. The initial study, however, did not addressissuesrelated to accurate masscalibrations at high mass.For example, it was evident that the WIG changes the electric field experienced by the ion, consequently, the ions experienced frequency shifts uncharacteristic of conventional ICR ion traps. In the following sections of this paper, we present detailed descriptions of the trapping potentials of the WIG cell and evaluate the device in terms of frequency shifts, m/zcalibration and mass resolution. 4. Background material for electrostatic ion traps The ideal Penning Trap has an equipotential surface that assumes the shape of rotational hyperboloids, yielding an optimum trapping potential given by ;r2--i (1) ( > where r and z are the radial and axial coordinates and A is a constant determined by the cell’s geometry and dimensions [26]. Electrostatic potentials in a cubic or cylindrical ICR cell compromise the optimum trapping potential described by Eq. (1) for an approximated V(r,z)=
-A
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optimum trapping environment in the form of a quadrupolar potential [27] represented by V(x,y,z) = v, [y - $x2 +y2 - 24
(2)
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131
where constants CX,y, and a have been solved for many different cell dimensions [28]. Actual equipotential lines deviate somewhat from this approximation, due to inhomogeneous electric
a.
b.
Fig. 1. (a) SIMION plot of equipotential lines for an elongated cylindrical cell. VT = 10 V (trapping lines for typical Kingdon trap arrangement, wire at - 10 V, cylinder at ground.
plates).(b)
SIMION
plot of equipotential
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- v=o - “2 r = RZ = radius of cylinder R, = radius of wire V, = voltage on wire V, = voltage on cylinder plates V = 0 = boundary condition for variable end-plates L = length of cell, z = 0 to z = L along z-axis Ei= angular coordinate
v=o Fig. 2. Schematic
of wire ion guide cell used in FT-ICR
fields, as shown in Fig. la. Exact solutions for these potentials have been solved and presented elsewhere [29,30].
conditions V(r, z) =
experiments.
is given by i:
n=l,3,5,...
[A,Z,,(K,r)
+&&(K,r)]co&z (44
5. Electrostatic equations for a WIG cell The electrostatic potential within a long, closed cylinder containing a wire approximates a radial logarithmic potential near the center of the cell given by
V(r)=
[ ln(~~a)jln(
k)
7rn
K n- --
-&ax
Z max =
L -2
(49
(4c)
(3)
The exact potential is obtained through a solution of Laplace’s equation in three dimensions. The boundary conditions consist of a cell of length L (z = 0 to z = L) and radius r with an internal wire (rod) of radius RI as shown in Fig. 2. The voltages are set at V = VI for the wire of radius RI, V=O = V2 for the segmented cylinder of radius Rz and V = 0 at z = 0 and z = L (endplates). The exact solution for these boundary
(44
where V is the potential on the wire, and I0 and Ka are zeroth order modified Bessel functions. A,,
K.J. Gillig
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and B, are constants that satisfy the given boundary conditions. An example of the resulting equipotential lines is shown in Fig. lb. Note that within the cell boundaries the radial electric field is purely inward directed. In a normal ICR cell, it is the outward directed radial field that limits high mass detection, but this limitation can be overcome in the WIG cell because the bounded volume of the trap is no longer hollow (the bounded volume is electrically neutral). A standard ET-ICR cell has end plates with variable voltages. The WIG cell also consists of segmented plates and is thus capable of variable voltages. This addition requires modifying the equations of the electrostatic potential for the cell. The exact equations for a Kingdon trap with variable end plates (WIG cell) are given bY [311 2.2
V(r, z) =
r
An = I, (nnR,/L)K,
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133
plates of the cylinder, the electric potential can be assumed as being purely logarithmic. Twodimensional ion motion in a logarithmic potential [35] with no external magnetic field is described by the following equations. Conservation of energy yields E = $z($
+ r2b2) +Peln
1. 0 r0
which leads to ;=(;)ikqln(k)-($)I’
(6b)
and
(64
/ n*r\
&J5 , , I... 1A,‘o(y)
(4/m)
and Ion Processes
[V&
+B,&(
y)]sin(
3
(59
(n7rR1/L) - v&J (n;lrRz/L)]
(mrR,/L)
-1, (mrR1/L)K,,
6. Ion motion in a Kingdon trap
Therefore,
All of the aforementioned traps possess different equipotential surfaces, thus, ions assume markedly different trajectories in each trap. Equations of ion motion for a quadrupolar potential have been solved and published elsewhere [29,32-341. Similar equations are not so easily obtained for the trapping conditions created by the arrangement of the Kingdon trap, due to the dependence of potential on radial position. Neglecting the effect of the voltage on the end
E-Pain(i)
(5b)
(mrR2/L)
- ($)
=O,
dr dr when the condition - = - = 0 df9 dt
6-9
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(W where L is the angular momentum, m is the mass of the ion, r is the radial distance, E is the kinetic energy of the ion, 8 is the angle, P 0 = .G&)’ r. lS the radius where P(ro) = 0, 0 is the angular component of motion and T is the temporal component of motion. Since 0 and T cannot be written in terms of known functions, they must be evaluated numerically. Hooverman [35] and Lewis [36] have previously evaluated the ion motion in an electrostatic Kingdon trap presuming a near circular orbit with the assumption that circular orbits will result in the longest trapping times. Circular orbits are also easier to calculate because the kinetic energy is constant for all radii. Examples of ion motion in an electrostatic Kingdon trap are given in Fig. 3. The motion is dependent on the ion’s centrifugal force, due to its angular momentum with respect to the attractive electric force. The sum of these two forces determines the radial acceleration or deceleration of the ions.
7. Ion motion in a WIG cell A Kingdon trap confined in an external magnetic field has been adapted for use in FT-ICR [ 191. Ion motion in this arrangement is influenced by both the electrostatic and the magnetic fields (E x B). Fig. 3. (a) SIMION plot of circular orbit ion trajectory in x-y plane for a Kingdon trap operated in electrostatic mode only. Wire at -10 V, cylinder at ground; mass of ion = 10000 Da; kinetic energy of ion = 1 eV and initial Y angle = -45”.(b) SIMION plot of retrograde pericycloidal ion trajectory in the x-y plane for a Kingdon trap operated in electrostatic mode only. Wire at -10 V, cylinder at ground; mass of ion - 10000 Da, kinetic energy of ion - 1 eV and initial Y angle = O”.(c) SIMION plot of retrograde pericycloidal ion trajectory in the x-y plane for a Kingdon trap operated in electrostatic mode only. Wire at -10 V, cylinder at ground; mass of ion = loo00 Da, kinetic energy of ion = 1 eV and initial Y angle = 30”.
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The equations of motion for this arrangement are as follows: From conservation of energy, where E varies with r only 2eVo 2e r i2 + (re)2 = y +E(r)dr m s ro
(74
coupled with the Force Equations f2 - r(b)2 = eE -
f-$(r2b) =(-JfJBi
(74
Whereby defining f(r)=
;j;oB(r)rdr+
(74
;
Integrating Eq. (7) gives e= &j(r)
(79
Then, substituting Eq. (7) into Eq. (7) yields the equation with constant magnetic field di’ -= dr
+ -1
(B&2)
+ (C/r)
-r [hV, + hE&gr
- [(&r/2)
+ (C/r)] ‘1’ Vf 1
where E is the kinetic energy, m is the massof the ion, c is the speed of light, B is the magnetic field strength, V. is the initial kinetic energy of the ion, h = %, B,, is the constant magnetic field, E,, is the constant electric field and r? is a constant. Examples of ion motion in a WIG cell are Fig. 4. SIMION plots of ion trajectories as a function of different potentials on the wire in a WIG cell, as seen in PI-ICR experiments. The wire is located at the midpoint of a 100 x 100 2D array at coordinates 50,50, coaxially surrounding the wire is a grounded segmented cylinder with a 25 grid point radius. All ions are 10 000 Da, with 10 eV of initial kinetic energy, starting with zero angle in the y direction from coordinates 48,48. The cell is surrounded by a uniform magnetic field of 7 T.(a) Cyclotron and magnetron motion around wire at -2 V.(b) Prolate hypotrochoidal motion around wire at -50 V.(c) Ion motion approximating retrograde pericycloidal motion with wire at -150 V (distorted by magnetic field).
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shown in Fig. 4. With a strong magnetic field and a weak electric field present (4a), motion consists of cyclotron motion with a superimposed drift motion whose direction and amplitude is dependent on the central force direction and amplitude. As the strength of the electric field increases, the magnitude of the guiding center drift increases and the motion is best described as central force motion in a precessing coordinate frame (4~). Note that ion motion in these examples is during the initial part of a normal ICR experiment sequence (ionization and trapping delay). It is apparent that modest voltages have a large effect on the ion’s behavior and that, with a large enough voltage, ion motion due to the influence of the electric field is dominant over the magnetic field. For example, an ion moving at 800 m/s positioned 0.1 mm from the wire in the region of the trap where the potential is purely logarithmic experiences an attractive electric force of -2.3*10-15 N when a 10 V potential difference is applied to the wire and cylinder. This force clearly exceeds the Lorentz force of -9-10-i6 N that the same ion experiences in a 7 T magnetic field. Under these conditions, it is possible to trap ions with high kinetic energies or mass and increase the total number of ions present (larger energy spread). Once the ions are stable/cooled, the trapping parameters can be switched to conditions that are favorable for the excitation and detection event (wire at ground or slightly positive, e.g., equal to trapping plate potentials). 8. Collisional cooling effects Ions with high kinetic energy, such as those formed by MALDI, were until recently not amenable to high resolution FT-ICR. To obtain high resolution, the ion signal must be observed for extended periods of time (i.e. long transient). The most obvious choice for cooling the ions is collisional cooling. Collisional cooling has been shown to enhance trapping of MALDI formed ions in t-f-traps, but in ICR cells this technique
Fig. 5. (a) SIMION plot of ion trajectory during collisional cooling in a WIG cell, showing the ion collapsing towards the wire. The wire is at -10 V, coaxially surrounding the wire is a segmented cylinder that is grounded. Ions are 10 000 Da with 100 eV of initial kinetic energy, starting with zero angle in the y direction from coordinates 48,48. The cell is surrounded by a uniform magnetic field of 7 T.(b) SIMION plot of ion trajectory during coIIisiona1 cooling in a dynamic Kingdon trap, showing the ion assuming a trajectory in the form of a cylindrical sheath around the wire. The wire is at coordinates SO,50 and is at -10 V, coaxially surrounding the wire is a segmented cylinder that is grounded. The ions are 10000 Da, with 10 eV of initial kinetic energy, starting with zero angle in the y direction from coordinates 557.5. The cell is surrounded by a uniform magnetic field of 7 T.
KJ.
Cillig
Fig. 6. (a) SIMION
et al.llnternational
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lines for ideal Kingdon
720.0 Fig. 7. High resolution
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trap parameters,
720.5 IT-ICR
mass spectra of Cm obtained
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end plates at 14 V and wi Ire at -1 V.
721.5
721.0 with an elongated
WIG cell.
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-15
n n
-20 -2
0
6
2
TRAP&G
(a)
;
c
c
200.. 2
L
Km.-
: g !I
r
o.-
h
WQ I
VOLTAGE
A
300..
t 2'
ri
8
-100..
:
-zoo.-
8
-5
-4
-3
-2
0
1
2
3
4
5
6
VBN
(4
Fig. 8. (a) Observed
-1
1 # a b
frequency
shifts for an elongated
WIG cell, wire at ground,
1 s delay.(b)
Observed
frequency
shifts for an elongated
WIG
-
OBSERVED .-
CYCLOTRON f
FREQUENCY
(MHz)
OBSERVED
CYCLOTRON
FREQUENCY
(MHz)
140
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is detrimental without special modifications or parameter changes due to the magnetron motion expanding rapidly, thus causing the ion to be ejected from the cell. This expansion of the magnetron motion is due to the radial electric field being directed outward from the center of the cell in the case of a quadrupolar potential, described by
(8) where Q is 1.3869 (a constant determined by trap dimensions) for a cubic cell and a is the cell length. To compensate for this problem, quadrupolar axialization has been used to rapidly interconvert the magnetron and cyclotron motions, enabling the ions to converge to the cell center when cooled [9,10]. However, in a WIG cell, the radial electric field is directed inward as is shown bY
vk)=[ln(lL?I)l (4)
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creates equipotential lines approximated by V(r, z) = A (2 - r2/2 + Blnr)
(10)
where A and B are constantsfor a given cell. The equipotential lines approximating those described by Eq. (10) can be created by shaping the electrodes to conform to these potentials. A more convenient method is developed here by simply adjusting the voltage applied to the end plates of a WIG cell to form the field lines described by Eq. (10) and shown in Fig. 6. The optimum equipotential lines are created by a superposition of quadrupolar and logarithmic potentials, creating a spindle-like trapping volume. A major advantage of the ideal Kingdon trap would be the fact that it gives rise to a harmonic motion in the z-direction, extending trapping times and stabilizing the ion motion, which is given by the frequency
c-9
Therefore, the magnetron motion, when subjected to collisional relaxation, will shrink to the center of the cell with a rate proportional to the voltage applied to the wire. A low voltage on the wire causesthe magnetron motion to collapse slowly, but if collisional cooling is applied for too long a time the ions strike the wire and are lost, as is shown in Fig. 5a. An obvious solution is to superimpose an a.c. voltage onto the existing d.c. voltage on the wire during the cooling event, stabilizing the ion trajectories in a pseudopotential minimum located between the wire and the cylinder, with the ions confined in a volume representative of a cylindrical sheath [37], shown in Fig. Sb. 9. Ideal WIG cell To improve the WIG cell for FT-ICR, the cell design can be changed to approximate an ideal Kingdon trap [38]. The ideal Kingdon trap
10. Evaluation of a WIG cell and an ideal Kingdon trap To further evaluate a cylindrical WIG cell and characterize a small WIG cell that could be easily optimized to create an ideal Kingdon trap for implementation into ET-ICR experiments, comparisons are made of the variation of observed ICR frequency with trapping voltage and wire voltage. Our initial approach was to determine the effect of the wire’s physical presence on the resolution capabilities of the WIG cell. Fig. 7 contains a high resolution (m/Am -300 000) LD/FTICR mass spectra of Cm obtained using the elongated WIG cell. The resolution is at least comparable to that obtained in the same cell without the wire, indicating that ions are not colliding with the wire and being lost from the cell, and
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VTR=
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141
VBN=O
a
VBB=-5,
55.6
VBN=3
55.8
m/z
56.0 m/z
56.2
56.4
Fig. 10. FT-ICR mass spectra of Fe’ obtained with a small WIG cell.(a) 1 V trapping voltage, wire at ground during delay (VBB), wire at ground during excitation and detection (VBN).(b) 1 V trapping voltage, wire at -5 V during delay, +3 V during excitation and detection.
that the wire has no deleterious effect during ion excitation and detection. Fig. 8 contains plots of the observed frequency shifts for Cso as a function of trapping voltage (Fig. 8a) and wire voltage (Fig. 8b) obtained using the elongated WIG cell. The trapping voltage frequency shifts with the wire at ground throughout the experimental sequence are low, being only 2-3 Hz/V. Under these conditions, the wire improves the homogeneity of the electric field over a large portion of the total cell volume; consequently the ions do not experience frequency shifts due to aberrations
in the E x B fields. If the wire potential is varied during the excitation and detection events, the observed frequency shifts are considerably larger, -100 Hz/V. Note, however, that because the ion motion is now strongly influenced by the WJG, the frequency is independent of the trapping voltage. The frequency shift is positive or negative depending on the polarity of the wire voltage, indicating that the ions see the center of the cell from a different rotating reference frame when the polarity of the wire voltage is switched. Fig. 9 contains plots of observed frequency
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-5
-4
SMALL
-3
Fig. 13. Frequency
-2
-1
0
shift as a function
VBN
WIG CELL (FREQUENCY
of wire voItage
1
in a small WIG
2
cell.
3
4
SHIFT FROM VBB=O AND VBNd)
5
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0 v-
mua
aNv 3213x3
DNIWIQ
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shifts for laser desorbed Fe’ ions (355 nm, directly desorbed from a stainless steel probe) in a small cylindrical cell (Fig. 9a) and a small WIG cell (Fig. 9b) versus trapping voltage (a = no wire; b = with wire). The frequency shifts for both cells are -100 Hz/V, which is typical for a small ICR cell and similar because the potential near the cell center is non-zero in both cases.We explain the data in Fig. 9b as meaning the wire is ineffective at reducing the influence of the trapping voltages becauseof the cells small size.This is in contrast to an elongated WIG cell in which the trapping voltages are effectively “screened” by the wire. Thus, to obtain ideal Kingdon trap equipotentials in which the effective trapping region of the cell volume is maximized without applying high trapping voltages/WIG voltages the cell length must be reduced. Fig. 10 contains massspectra of laser desorbed Fe* ions obtained using a small WIG cell for trapping and detection. The spectrum shown in Fig. 10a was obtained with the trapping plates at 1 V and the wire at ground throughout the experiment. Whereas, the spectrum shown in Fig. lob was obtained under the same conditions except that the wire voltage was biased to -5 V during the trapping delay and then switched to +3 V during the excitation and detection. An increase in both resolution and ion signal by a factor of four to five is obtained. Switching the wire polarity between the trapping delay and the excitation and detection event alternates the cell configuration between a Kingdon trap (in which ion trapping efficiency in increased) and a “screened” ICR cell (in which frequency shifts are reduced and resolution is increased). To optimize the cell response in terms of ion trapping efficiency, mass measurement and resolution, a series of spectra were obtained in which the trapping voltage-wire voltage ratio was varied. Fig. 11 shows a plot of ion abundance versus the WIG voltage during the excitation and detection event for several runs where the voltage on the wire during the trapping delay was also varied. The ion abundance is greatest when the
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voltage on the wire is negative during the trapping delay and positive during the excition and detection. Note that the ion signal is very low when the WIG voltage is positive throughout the experimental sequence. Similar trends are indicated for resolution. Fig. 12 contains a plot of resolution versus WIG voltage during the excitation and detection event while varying the voltage on the wire during the trapping delay. The resolution is greatest when +3 V is applied to the wire during the excition and detection event. The observed frequency shifts for Fe+ ions are plotted as a function of WIG voltage during the excitation and detection event in Fig. 13. Note that the frequency observed for Fe’ ions is a function of both the WIG voltage before and during the excitation and detection event. However, at a WIG potential of +3 V, the frequency is independent of the voltage during the trapping delay. Our explanation of this effect is based on data shown is Fig. 14. Fig. 14 contains a plot of the WIG voltage during the excitation and detection event and the trapping voltage required to maintain a constant observed cyclotron frequency. In effect, the WIG cell can be “tuned” to obtain a field free detection enviroment, where the deviation in observed cyclotron frequency and true cyclotron frequency is minimized, thus improving the mass measurement accuracy as smaller calibration corrections are needed. This also corresponds to the voltage combination at which the resolving power is highest for a given aquisition period, e.g., the smallest number of ions are lost from the cell during excitation and detection. 11. Conclusion In this paper, we provide a complete description of electrostatic trapping potentials, ion trajectories and performance of a novel cell design approximating a Kingdon trap for use in FTICR experiments. The cell consists of a wire
K.J. Gillig
et al.ilnternational
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surrounded by a segmented elongated cylinder. The advantages of the cell design are multifold: The wire creates a deep potential well in which to trap ions with high kinetic energies and mass as well as providing a radial force vector directed towards the center of the cell, helping to dampen magnetron motion. A further advantage of this cell can be obtained by using parameters that create the ideal Kingdon trap equipotential lines, consequently providing harmonic motion in the z-direction. Experiments performed with an elongated and a small WIG cell indicate that resolution and mass measurement accuracy may be improved by adjusting parameters during the experimental sequence to minimize trapping voltage-induced frequency shifts or even completely eliminate shifts while still maintaining z-axis harmonicity. We are continuing to investigate advantages of this cell design, which may further enhance the ability of FT-ICR to solve problems that require high resolution and precise mass measurement accuracy.
Acknowledgements This work was supported by the National Science Foundation (Grant #CHE-9223629) and The Robert A. Welch Foundation.
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