Ejection of ions from ion traps by combined electrical dipole and quadrupole fields

The invention relates to an improved method and an apparatus for the mass-sequential ejection of ions from an RF quadrupole ion trap by electrical alternating fields which are generated in addition to the quadrupolar RF storage field and with different frequencies to it. In contrast to the already known ejection by a pure dipole field, the ions are here essentially ejected by a quadrupole field. The ions leave the ion trap through a perforated end cap and can be detected outside it with conventional means. A weak dipole field undertakes only excitation of the secular oscillation at the center, the amplitude increasing in linear manner in the stationary case. The more intense quadrupole field undertakes further widening of the oscillations with exponential growth in amplitudes. The dipole field is generated by an alternating voltage between the two end caps, while the quadrupole field is generated by an alternating voltage between the end caps on the one hand and the ring electrode on the other. The method is of particular use for the ions of very high masses ranging from approximately 5,000 u to 50,000 u. With the same mass resolution, it permits mass spectra to be recorded considerably quicker than the hitherto conventional use of pure dipole fields.

FIELD OF THE INVENTION 
The invention relates to a method and an apparatus for mass-sequential 
ejection of ions from an RF quadrupole ion trap by electrical alternating 
fields which are generated in addition to the quadrupolar RF storage field 
and with different frequencies to it. 
BACKGROUND OF THE INVENTION 
It is known (R. E. Kaiser et al., Rapid Commun. Mass Spectrom. 3, 225 
(1989), R. E. Kaiser et al., Int. J. Mass Spectrom. Ion Processes 106, 79 
(1991)) how to eject the ions mass-sequentially by a fixed dipole 
alternating field while slowly increasing the amplitude of the storage 
radio frequency linearly. The dipole alternating field is generated by an 
alternating voltage applied to the two end caps of the ion trap. The ions 
leave the ion trap through a perforated end cap and can be detected 
outside the trap with conventional means. The method is particularly used 
for ions of very high masses in the range from approximately 5,000 u to 
50,000 u. 
If ions with different mass-to-charge ratios are stored in an RF quadrupole 
ion trap according to Wolfgang Paul and Helmut Steinwedel (U.S. Pat. No. 
2,939,952), they can, according to present knowledge, be ejected 
mass-selectively, i.e. temporally separated in the order of their 
mass-to-charge ratios, by three different methods in an axial direction 
through one of the two end caps and detected outside in the form of a mass 
spectrum. For reasons of simplicity, only masses and not mass-to-charge 
ratios are referred to in the following. Although, strictly speaking, this 
applies only to singly charged ions, it should not be understood in a 
restricted sense here. The three mass selective ejection methods are as 
follows: 
(I) The "mass selective instability scan" (U.S. Pat. No. 4,540,884) uses 
the stability limit .beta..sub.z =1 of the first stability region in 
Mathieu's stability diagram. (See the following relevant literature: P. H. 
Dawson, "Quadrupole Mass Spectrometry and its Applications", Elsevier, 
Amsterdam, 1976; and R. E. March and R. J. Hughes, "Quadrupole Storage 
Mass Spectrometry", John Wiley & Sons, New York 1989). The working points 
of the ions are shifted across the stability border .beta..sub.z =1 by a 
continuous change in the operating parameters of the ion trap. To do so, 
the RF voltage of the storage field, the so-called drive voltage of the 
ion trap, is preferably enlarged linearly, this operating method resulting 
in a linear mass scale. The ions becoming instable according to their 
order of mass enlarge their oscillation amplitude in the axial direction 
("z" direction) on the other side of the stability border by the 
absorption of energy from the storage RF field in a temporally exponential 
manner and are finally able to leave the storage space of the ion trap 
through perforations in one of the end caps. Given certain conditions for 
the precise form of the quadrupole field (U.S. Pat. No. 5,028,777), this 
method provides spectra with a good mass resolution, i.e. ions of one mass 
are fully ejected and can be completely measured before it is the turn of 
ions of the next mass. 
(II) The "scan by nonlinear resonances" (U.S. Pat. No. 4,818,869 and U.S. 
Pat. No. 4,975,577) uses the amplitude growth, which our findings show to 
be sharply hyperbolic, in the secular oscillations due to nonlinear 
resonance conditions which arise in the ion trap due to superposition of 
the quadrupole field with higher-order multipole fields. As a result of 
the hyperbolic amplitude growth in the nonlinear resonance, this method 
leads to particularly quick scanning with a good mass resolution. Since 
the multipole fields at the center of the ion trap disappear, ions resting 
at the center after cooling with a collision gas are unable to experience 
the nonlinear resonances. They therefore need to be pushed through a 
dipole alternating field, the frequency of which is the same as or a 
little lower than the resonance frequency. The mass flow is generated as 
in method (I) by changing the operating parameters of the ion trap, 
preferably by a linear change in its drive voltage. 
(III) In addition, the ions can be expelled from the ion trap by resonant 
dipolar excitation in the axial direction. The dipole field is generated 
by an alternating voltage which is applied between the two end caps. 
Initial applications of the method are known from as long ago as the 
1950s. A detailed description of the various ejection options is given in 
U.S. Pat. No. Re. 34,000 (reissue of U.S. Pat. No. 4,736,101). The most 
successful method is to leave the frequency of the alternating voltage 
applied at the end caps for generation of the dipole field constant and to 
linearly increase the drive voltage of the ion trap. This causes the ions 
to undergo a change in the frequency of their secular oscillations. If the 
secular oscillations of a mass's ions enter into resonance with the dipole 
alternating field in the z-direction, the ion oscillations absorb energy 
from the dipole alternating field and enlarge their oscillation amplitude, 
enabling them to leave the ion trap if the dipole alternating field is 
sufficiently strong. 
Method (I) cannot be used for the ions of very high masses exceeding 
approximately 5,000 atomic units of mass u since the RF voltage is limited 
to approximately 15 kV by practical ion trap requirements such as gas 
pressure in the ion trap and insulation distances. A collision gas 
pressure of approximately 10.sup.-3 millibars must normally be maintained 
in ion traps. With the limitation to approximately 15 kV and a minimum 
frequency of approximately 500 kHz, which depends on the required number 
of storable ions, conventional ion traps have a resulting upper limit of 
approximately 4,000 u for the practically usable mass range. 
The mass range of method (II) is only marginally higher since the effective 
nonlinear resonances are not very far away from the instability limit. The 
most effective resonance at the point .beta..sub.z =2/3 of the hexapole 
field is only approximately 12% higher in mass than the stability limit 
.beta..sub.z =1, related to the same RF voltage. All higher nonlinear 
resonances (from approximately .beta..sub.z &lt;1/2) cannot be used for this 
method since they are far too weak. 
For this reason, method (III) has so far been used for ions of very high 
masses in the range of some 10,000 unified atomic mass units u (R. E. 
Kaiser et al., Rapid Commun. Mass Spectrom. 3, 225 (1989), R. E. Kaiser et 
al., Int. J. Mass Spectrom. Ion Processes 106, 79 (1991)). The method has, 
however, a serious drawback: it is extremely slow. In the papers above, 
approximately 500 secular oscillations were required for ejection of the 
ions of a mass to achieve a single mass resolution just resulting in the 
separation of two adjacent masses. The scan speed for this single mass 
resolution (not a high resolution) must therefore not exceed one mass unit 
for every 500 secular oscillations. In this regard, it must be taken into 
account that the secular oscillations of the heavy ions are very slow. (In 
the .beta..sub.z &lt;0.6 range, the secular oscillation frequencies 
.omega..sub.z are approximately inversely proportional to the mass). In 
comparison, ions of one mass can be completely ejected in approximately 10 
secular oscillations with method (II), while commercial equipment working 
according to method (I) uses a scan speed of approximately one mass per 90 
secular oscillations. With a dipole alternating frequency of 25 kilohertz, 
method (III) provides a scan speed of only 50 mass units per second, while 
method (II) measures 30,000 mass units per second, though at 333 kilohertz 
in the lower mass range. 
Physical Principles 
Our most recent examinations have shown that the increase in amplitude of 
the secular oscillation in a resonant alternating field depends on the 
multipole ordinal number of the exciting alternating field. It can be 
shown that the following differential equation holds for the temporal 
increase in amplitude in the z-direction: 
EQU dz/dt=C.sub.n *z.sup.(n-1), n=multipole order. (1) 
Integration produces the following: 
EQU z.sub.1 (t)=C'.sub.1 *t a linear increase for the dipole (n=1), (2) 
EQU z.sub.2 (t)=C'.sub.2 *exp(t) an exponential increase for the quadrupole 
(n=2), (3) 
EQU z.sub.3 (t)=C'.sub.3 /(t-C".sub.3) a hyperbolic increase for the hexapole 
(n=3). (4) 
Equations (2), (3), and (4) have been verified by computer simulations. 
Equation (2) was simulated by an electrical voltage at the end caps, 
equation (3) tested by means of the increase in amplitude at a fixed 
working point in the instable range, and equation (4) at different 
nonlinear resonances of superposition with a hexapole field which was 
generated by the shape of the electrodes. FIGS. 1 to 3 show the results of 
the computer simulations. 
It can be expected from these examinations that equation (3) with an 
exponential rise in the secular oscillation amplitude also applies to the 
case of superposition with a resonant quadrupole alternating field 
generated by electrical means with an alternating voltage between the ring 
and end cap electrodes. 
Therefore, it is among the objects of the invention to specify a fast scan 
method for the spectra of ions in a quadrupole ion trap, which, in 
particular, can be used for ions of very high masses. 
SUMMARY OF THE INVENTION 
The invention comprises replacing pure dipole excitation of the ion 
oscillations of method (III) by combined dipole and quadrupole excitation, 
both being generated by electrical alternating voltages at the electrodes 
of the ion trap. 
Here, dipole excitation can be very much weaker than in method (III). Its 
sole purpose is to make the ions, which normally rest at the center of the 
ion trap due to cooling with a collision gas, slightly oscillating. Since 
the quadrupole field disappears precisely at the center, the ions at the 
center would not see resonant acceleration by the quadrupole field at all. 
The small cloud of ions of the same mass resting at the center begins to 
oscillate synchronously and in relatively closed form due to the dipole 
alternating field. 
As soon as the ions then reach positions well away from the center, they 
are caught by the quadrupolar acceleration which, as expected, enlarges 
their oscillation amplitude not only linearly, but exponentially. As a 
result, the oscillations are quickly extended to the end caps, causing the 
cloud's ions to be ejected through the perforations in the end caps, 
podion by portion in a few subsequent oscillations of the secular motion.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
FIG. 4 shows the linear increase in amplitude of the secular oscillation of 
a very heavy ion of 16,000 u mass under the effect of an applied dipole 
field, the frequency of which is in resonance with the secular 
oscillation. The dipole field is approximately generated by a 20 volt and 
28.5 kHz alternating voltage which is applied diagonally across the two 
end caps. Before the commencement of excitation by the dipole field, the 
ion was resting precisely at the center of the ion trap. Here, the 
stationary case of constant operating conditions for the ion trap is 
given, no scanning of masses therefore taking place. 
FIG. 5 shows the very weak linear increase in amplitude with a dipole 
voltage of only 1 volt. 
In FIG. 6, an additional quadrupole field is switched on, generating an 
exponential enlargement of the oscillation amplitude. Ejection is 
significantly sharper as a result. The quadrupolar alternating field is 
generated by an alternating voltage between the end caps on the one hand 
and the ring electrode on the other. Although the dipole voltage is only 1 
volt as in FIG. 5, the quadrupole voltage, on the other hand, is 500 
volts. The quadrupole frequency is twice the value of the dipole 
frequency. Despite the low dipole voltage of only 1 volt, it would not be 
possible for the ion to be picked up by the quadrupole acceleration 
without this voltage since the quadrupole field disappears precisely at 
the center. Ion ejection therefore corresponds to ejection as per method 
(II) by nonlinear resonances which also disappear at the center and need a 
push by a weak dipole voltage. As expected, the amplitude increases 
exponentially with correct setting of the frequencies and phases. With 
incorrect setting of the phases, there is a transition region for 
adaptation of the oscillation phases. 
The double frequency of the quadrupole field is particularly advantageous 
since the ion then undergoes an acceleration in every half phase. The 
single frequency can also be used, though an even higher voltage is then 
required. Even-numbered multiples of the frequency, such as a fourfold or 
sixfold frequency, can also be used, though the acceleration decreases as 
the frequency rises. 
FIGS. 4 to 6 consider only the stationary case of a constant RF drive 
voltage of the ion trap and not the scan procedure for the mass-sequential 
ejection of ions as is required for the recording of mass spectra. The 
results of scanning with very heavy ions are shown in FIGS. 7 and 8. 
FIG. 7 first shows the behavior of a heavy ion in a dipole alternating 
field of medium intensity in a slow scan over 1,000 units of mass, without 
switching on the quadrupole field. The dipole voltage is 10 volts and the 
dipole frequency approximately 28.5 kilohertz. Strong beats form well 
before the resonance point. The beat antinodes become wider and the beat 
periods longer, the closer the secular frequency of the ion gets to the 
resonance point. Here, the 10-volt dipole voltage is just sufficient to 
eject the ions, representing the optimum case. A voltage of 8 volts is 
just insufficient for ion ejection while a voltage higher than 10 volts 
leads to much greater loops of oscillation. Indeed, a dipole voltage of a 
little more than 8 volts is used by Kaiser et al. (see above) in practical 
experiments. 
If the dipole field were only slightly weaker than shown in FIG. 7, no 
ejection of ions would take place. After running over the resonance point 
with a beat antinode of maximum size, the increase in energy of the 
oscillation would cease. Since the energy is not, however, subsequently 
released again, the beat retains approximately the same maximum amplitude, 
even if the beat frequency changes and again becomes quicker. 
FIG. 8 shows the behavior of the ion with a much weaker dipole field with a 
dipole voltage of only 0.5 volts but an additionally connected quadrupole 
field of 50 volts and double frequency of approximately 57 kilohertz. Due 
to the low dipole voltage, the beat antinodes are considerably smaller. 
The weakly oscillating ion is picked up by the quadrupole alternating 
field at the resonance point and its oscillation amplitude enlarged 
exponentially until the ion reaches the end cap. The point of ion ejection 
is strictly defined with regard to the mass scale and ion ejection is much 
sharper, i.e. a few secular oscillations suffice for portion-wise ejection 
of a small cloud. Consequently, a better mass resolution is achieved than 
the dipole ejection as per FIG. 7 is able to reveal. 
The quality of quadrupole resonance at 57 kilohertz is better than that of 
dipole resonance at 28.5 kilohertz. Consequently, ion ejection is more 
strictly and more reproducibly bound to a point on the mass scale for this 
reason. 
Before recording the spectra, the ions must be cooled by a collision gas, 
condensing them in a very small cloud at the center of the ion trap. The 
collision gas remains in the ion trap, even during recording of the 
spectra, to counter any continuous reheating of the cloud by the 
alternating fields and the oscillating ions of other masses during the 
mass flow. 
These heating processes, which are considerable in the case of dipole 
ejection, are suppressed to a very great extent by the only very weak 
dipole field since the beat antinodes of the ions are only very small 
before reaching the resonance point. 
Furthermore, fewer deflective scattered collisions between the ions and the 
collision gas take place since the ions remain at rest much longer with 
the new ejection method. Consequently, far fewer vagabond scattered ions 
arise in the ion trap and the noise background generated by them in the 
spectrum remains low. 
The presently known methods (I) and (II) have shown that it is advantageous 
to excite the ions by a dipole voltage between the end caps before they 
reach instability in method (I) or nonlinear resonance in method (II). 
This is done by selecting a slightly lower dipole voltage frequency than 
that corresponding to the instability or nonlinear resonance frequency. 
This enables the dipole voltage to again be lowered, making ion ejection 
even sharper. For this reason, it is here analogously proposed that the 
dipole frequency be lowered a little below half the quadrupole frequency. 
Furthermore, it is known that the quadrupole field can also be generated by 
applying the full quadrupole voltage (peak-to-peak) only jointly to the 
two end caps and not to the ring electrode. Any added potential (even 
alternating potential) is not decisive for the field in the ion trap. The 
reference point for the voltage is then a joint ground point for the 
quadrupole voltage and drive voltage. 
The dipole field can also be generated if the dipole voltage is applied 
only to one end cap electrode. This results in a superposition comprising 
a dipole field and a quadrupole field, each with the same intensity. The 
quadrupole field can then be left out of consideration for the further 
examinations due to its low intensity. 
Superposition of the storage quadrupole field of the ion trap with a weak 
octopole field, which can be generated by a special shape of the 
electrodes, has a further sharpening effect on ion ejection. An additional 
hexapole field, also generated by the shape of the electrodes, causes the 
ions to always be ejected only through the same end cap, doubling the ion 
current to be detected outside. It is therefore proposed in an embodiment 
to superpose higher multipole fields by the shape of the electrodes. 
Single-sided ion ejection by combined octopole and hexapole fields is 
shown in FIG. 9. 
Furthermore, it is possible to generate the additionally required 
alternating voltages in a digital manner. Previously calculated and stored 
values are output to the end caps at a constant generation rate via 
digital-to-analog converters. In particular, this also enables the 
voltages required for the two end caps to be generated separately. In 
addition, this also makes it possible to also generate frequency bands 
with a mixture of weighted frequencies. 
The foregoing description has been limited to specific embodiments of this 
invention. It will be apparent, however, that variations and modifications 
may be made to the invention, with the attainment of some or all of its 
advantages. Therefore, it is an object of the appended claims to cover all 
such variations and modifications as come within the true spirit and scope 
of the invention.