Mass scanning method using an ion trap mass spectrometer

An improved method of using an ion trap mass spectrometer is disclosed. According to the method an asymmetrical trapping field is applied to the trap. Preferably, the asymmetrical trapping field comprises a quadrupole field and a dipole field having the same frequency. In addition, higher order trapping field components, such as hexapole or octopole fields, may also be included, and the electrodes of the ion trap can be shaped to introduce such higher order field components. The effect of the asymmetrical trapping field of the present invention is to cause the center of the trapping field to be displaced from the mechanical center of the ion trap. A supplemental quadrupole field is then applied to the ion trap, the center of the supplemental quadrupole field being located at the mechanical center of the trap, i.e., it is displaced from the center of the trapping field. The supplement quadrupole field and the trapping field may be viewed as forming one combined field which acts upon the ions in the trap. The combined field is then scanned to cause ions of differing masses to be resonantly ejected from the ion trap in sequential mass order. Preferably, the combined field is scanned by scanning the voltage of the trapping field. Preferably, the supplemental field is set to have a frequency which is two-thirds of the trapping field frequency and is phase locked with the trapping field frequency.

FIELD OF THE INVENTION 
The present invention is related to improved methods of using quadrupole 
ion trap mass spectrometers, and is particularly related to improved 
methods of obtaining mass spectra of ions which have been isolated within 
ion trap spectrometers. 
BACKGROUND OF THE INVENTION 
The present invention relates to methods of using the three-dimensional ion 
trap mass spectrometer ("ion trap") which was initially described by Paul, 
et al.; see, U.S. Pat. No. 2,939,952. In recent years, use of the ion trap 
mass spectrometer has grown dramatically, in part due to its relatively 
low cost, ease of manufacture, and its unique ability to store ions over a 
large range of masses for relatively long periods of time. 
As is well known, the ion trap comprises a ring-shaped electrode and two 
end cap electrodes. In the ideal embodiment of Paul, et al., both the ring 
electrode and the end cap electrodes have hyperbolic surfaces that are 
coaxially aligned and symmetrically spaced. More recently it has been 
shown that by using non-hyperbolic surfaces, higher order field components 
can be deliberately introduced into the trapping field. By higher order 
field components it is meant field components greater than the normal 
quadrupole field, e.g., hexapolar or octopolar fields. (See, for example, 
U.S. Pat. No. 5,468,958 to Franzen, et al.) By placing a combination of RF 
and DC voltages (conventionally designated "V" and "U", respectively) on 
the trap electrodes, a trapping field is created. In the simplest case, a 
trapping field is simply created by applying a fixed frequency 
(conventionally designated ".function.") RF voltage between the ring 
electrode and the end caps to create a quadrupole trapping field. It is 
well known that by using an RF voltage of proper frequency and amplitude, 
a wide range of masses can be simultaneously trapped. 
In its basic mode of operation, sample ions are introduced in the ion trap 
(i.e., the volume defined by the ion trap electrodes) and are then scanned 
out of the trap for mass detection. Commonly, sample is introduced into 
the trap from the output of a gas chromatograph ("GC"), although other 
sources of sample molecules, such as the output from a liquid 
chromatograph ("LC"), are also well known. Sample ions are normally 
created from sample molecules that are present within the trap, as by 
electron impact ("EI") or chemical ionization ("CI"). However, sample ions 
could also be created outside the trap and thereafter transported to 
within the trap volume. Various methods of creating and, if applicable, 
transporting sample ions, including ions used in so-called MS/MS 
experiments, are well-known in the art and need not be explained in 
further detail. 
As noted, the ion trap is capable of storing sample ions over a large range 
of masses. After the sample ions are stored in the trap and, if 
applicable, any additional experimental manipulations are conducted (e.g., 
as in an MS/MS technique) the spectroscopist is generally interested in 
obtaining a mass spectrum of the contents of the trap in order to identify 
the ions that are present. While various detection techniques are known 
for obtaining the mass spectrum, most of the methods use some form of 
scanning of the ion trap. The present invention is directed to a new, high 
resolution method of scanning the contents of the ion trap to obtain a 
mass spectrum. A typical scanning method involves causing the trapped ions 
to leave the trap in consecutive mass order, and using an external 
detector to measure the quantity of ions leaving the trap as a function of 
time. Typically, ions are ejected through perforations in one of the end 
cap electrodes and are detected with an electron multiplier. More 
elaborate experiments, such as MS/MS, generally build upon this basic 
technique, and often require the isolation and/or manipulation of specific 
ion masses, or ranges of ion masses in the ion trap. 
(It is common in the field to speak of the "mass" of an ion as shorthand 
for its mass-to-charge ratio. As a practical matter, most of the ions in 
an ion trap are singly ionized, such that the mass-to-charge ratio is the 
same as the mass. For convenience, this specification adopts the common 
practice, and generally uses the term "mass" as shorthand to mean 
mass-to-charge ratio.) 
In U.S. Pat. No. 4,540,884, to Stafford, et al., there is disclosed a 
so-called "mass instability" scanning method whereby the contents of the 
ion trap are scanned out of the ion trap by changing the trapping field 
parameters, e.g., by raising the trapping voltage, such that ions of 
different masses become sequentially unstable and leave the trap. 
U.S. Pat. No. 4,736,101, to Syka, et al., discloses a scanning method which 
relies on the fact that each ion in the trapping field has a "secular" 
frequency which depends on the mass of the ion and on the trapping field 
parameters. As had been well known, it is possible to excite ions of a 
given mass that are stably held by the trapping field by applying a 
supplemental AC dipole voltage to the ion trap having a frequency equal to 
the secular frequency of the ion mass. Ions in the trap can be made to 
resonantly absorb energy in this manner. At sufficiently high voltages, 
sufficient energy is imparted by the supplemental dipole voltage to cause 
those ions having a secular frequency matching the frequency of the 
supplemental voltage to be ejected from the trap volume. This technique is 
now commonly used to scan the trap by resonantly ejecting ions from the 
trap for detection by an external detector. (In addition, this technique 
may be used to eliminate unwanted ions from the ion trap, or when the 
supplemental dipole voltage is relatively low, it can be used in an MS/MS 
experiment to cause ions of a specific mass to resonate within the trap, 
undergoing dissociating collisions with molecules of a background.) 
In practice, the scanning method of Syka, et al., is implemented by 
scanning the trapping voltage (thereby varying the secular frequency of 
the ions) using a fixed supplemental dipole voltage. The teachings of 
Syka, et al., are limited to dipole excitation fields since the 
supplemental voltage can only be applied out of phase where the "end caps 
are common mode grounded through coupling transformer 32 . . . to resonate 
trapped ions at their axial resonant frequencies." Syka, et al., discloses 
only the use of the fundamental (N=0) secular axial dipole resonance. 
In commercial embodiments of the ion trap using resonance ejection as 
taught by Syka, et al., as a scanning technique, the frequency of the 
supplemental AC voltage is set at approximately one half of the frequency 
of the RF trapping voltage. It can be shown that the relationship of the 
frequencies of the trapping voltage and the supplemental voltage 
determines the mass value of ions that are at resonance. To achieve good 
mass resolution under the method of Syka, et al., it is desirable to use 
as low a supplemental voltage as is possible, while still of sufficient 
value to cause ejection of the ions. However, the growth in amplitude of 
the excited ions is linear in time, and the use of a low voltage, 
therefore, results in a slow ejection time. In other words there is a 
trade-off between mass resolution and ejection time, both of which are 
determined by the magnitude of the supplemental dipole voltage. 
The teachings of Stafford, et al., and Syka, et al., are limited to a pure 
quadrupole trapping field in an ideal ion trap. In such systems the 
trapped ions orbit about the mechanical center of the ion trap, which is 
also the center of the trapping field. In virtually all commercial ion 
traps a damping gas is introduced into the system to "thermalize" the 
ions, i.e., to reduce the spread in the initial ion condition and thereby 
improve resolution. When using a symmetrical trapping field, damping of 
the ions causes their orbits to collapse to a small volume near the center 
of the trap. 
U.S. Pat. No. 5,381,007, to Kelley, discloses a scanning method which uses 
two quadrupole (or higher order) trapping fields having identical spatial 
form. (Each of the trapping fields is said to be capable of independently 
trapping ions in the ion trap.) The second quadrupole trapping field is 
used to resonantly excite trapped ions, and is said to have a frequency 
which is below one half of the fundamental trapping field frequency. As 
had been taught in U.S. Pat. No. 3,065,640 to Langmuir, et al., a 
quadrupole field can be used in the same manner as a dipole field to 
resonantly excite ions in a trap. (In fact, Langmuir, et al., and other 
references teach the use of both supplemental dipole and quadrupole fields 
for this purpose.) Langmuir, et al., further teach that while a 
supplemental dipole field causes the axial amplitude of the excited ions 
to increase linearly with time, a supplemental quadrupole field causes the 
ion motion to increase exponentially with time. The ability of a 
supplemental quadrupole field to cause ejection of the ions more rapidly 
suggests a clear advantage of using such a field. However, unlike a dipole 
field, a supplemental quadrupole field has no effect at the very center of 
the ion trap, which is where trapped ions tend to reside. 
A disadvantage of Kelley is the fact that it requires the use of two 
trapping fields. As noted above in respect to the method of Syka, et al., 
a resonant excitation that is too intense will cause poor mass resolution. 
Yet, in order for the supplemental quadrupole field to act as a trapping 
field it must be rather strong, thereby causing severe broadening of the 
mass peak during the ejection process. Thus, unless a technique is used to 
move the ions away from the center of the ion trap, the method of Kelley 
must rely on processes such as random ion scattering and space charge 
repulsion to move ions away from the center of the trap and into an area 
where they can be excited by the supplemental quadrupole field. These 
processes result in poor mass resolution due to the incoherence and 
randomness of the displacement mechanisms. 
U.S. Pat. No. 5,298,746, to Franzen, et al., teaches the use of a weak 
dipole field to move ions away from the center of the ion trap where they 
can then be resonantly excited by a supplemental quadrupole (or higher 
order) excitation field. Thus, this technique uses both a supplemental 
dipole field and a supplemental quadrupole field to excite ions. Each of 
these supplemental fields is set to resonantly excite ions of the same 
mass. 
When any of the foregoing methods are used to scan the trap, ions are 
equally likely to move in either direction along the trap axis. Thus, half 
of the ions will move in the axial direction away from the detector and 
the other half will move toward the detector. This significantly limits 
the detection efficiency of the device. In addition, each of these 
techniques results in the storage of positive and negative ions (of the 
same mass) together, which can result in the undesired detection of 
negative ions when scanning the positive ion spectrum. This is a 
particular problem at higher masses where the energy of the ions that are 
ejected can be on the order of several kilovolts. Such ions can exceed the 
potential at the entrance to the electron multiplier causing an unwanted 
response. 
In commonly assigned U.S. Pat. No. 5,291,017 to Wang, et al., the 
disclosure of which is incorporated by reference, it was recently shown 
that an asymmetrical trapping field, comprising quadrupole and dipole 
components, could be used to preferentially eject ions in a preferred 
direction. In the Wang, et al., patent a supplemental dipole field is used 
to eject ions in a scanning operation. It has been determined that the 
effect of the asymmetrical field used disclosed in Wang, et al., is to 
displace the center of the trapping field away from the mechanical center 
of the trap, and to separate positive and negative ions from each other. 
An additional disadvantage of the prior art resonance scanning technique 
using resonant ejection where the frequency of the supplemental voltage is 
approximately one-half of the trapping voltage is the fact that a 
substantial beat frequency is present which presents a noticeable 
distortion of the mass peaks. Typically, this is mitigated by averaging 
the mass spectra from several successive scans of the on trap. However, 
the flow from a GC is continuous, and a modern high resolution GC produces 
narrow peaks, sometimes lasting only a matter of seconds. In order to 
obtain a mass spectrum of narrow peaks, it is necessary to perform at 
least one complete scan of the ion trap per second. The need to perform 
rapid scanning of the trap adds constraints which may also affect mass 
resolution and reproducibility. Similar constraints exist when using the 
ion trap with an LC or other continuously flowing, variable sample stream. 
Averaging scans in order to obtain accurate mass peaks reduces the scan 
cycle time and hence the number of different masses that can be monitored 
per unit time across a chromatographic peak. It is noted that the time for 
a single scan is more than just the scan time itself, since it must also 
include the ionization and ion isolation time, both of which are generally 
longer than the scan itself. Therefore, scan averaging for purposes of 
peak smoothing is an inherently inefficient process. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved method of scanning the contents of an ion trap mass spectrometer 
to obtain a mass spectrum of the ions masses which have been isolated 
within the trap volume. 
A further object of the present invention is to improve the mass resolution 
of a scan of the ion trap without appreciably increasing the time required 
to conduct a scan. 
Another object of the present invention is to provide an asymmetrical 
trapping field to displace the center of ion orbits away from the 
mechanical center of the ion trap. 
Yet another object of the present invention is to reduce the time needed to 
obtain a smooth, accurately centered mass peak of an ion species which has 
been isolated in an ion trap. 
Still another object of the present invention is to provide a trapping 
field which separates positive ions from negative ions. 
Yet another object of the present invention is to increase the proportion 
of ions ejected from an ion trap which are subject to capture by an 
external detector such that substantially more than one half of the ions 
are detected. 
These and other objects which will be apparent to those skilled in the art 
upon reading the present specification in conjunction with the attached 
drawings and the appended claims, are realized in the present invention 
comprising a method of using an ion trap mass spectrometer comprising the 
steps of applying an asymmetrical trapping field to the trap so that ions 
having mass to charge ratios within a desired range will be stably trapped 
within an ion storage region within the ion trap, such that the center of 
the ion storage region is offset from the mechanical center of the ion 
trap; introducing a sample into the ion trap mass spectrometer, ionizing 
the sample and applying a supplemental quadrupole excitation field to the 
ion trap to form a combined field and scanning the combined field to cause 
sample ions to be resonantly ejected from the trap. Preferably, the 
asymmetrical trapping field comprises a quadrupole field, and a dipole 
field having the same frequency, and the end cap electrodes of said ion 
trap are "stretched." In the preferred embodiment the supplemental 
quadrupole field which causes ion ejection is too weak to trap ions in the 
ion trap. In a further embodiment, a supplemental dipole field is applied 
to the ion trap while the trap is being scanned, and the supplemental 
quadrupole field and the supplemental dipole field have a frequency which 
is 2/3 of the frequency of the trapping field. In yet a further 
embodiment, an additional supplemental excitation field having a frequency 
which is 1/2 of the supplemental quadrupole frequency is also applied to 
the ion trap. Preferably, the trapping field voltages and the supplemental 
voltages are phase locked.

DETAILED DESCRIPTION 
Apparatus of the type which may be used in performing the method of the 
present invention is shown in FIG. 1. Most of what is depicted in FIG. 1 
is well known in the art, and need not be explained in detail. Ion trap 
10, shown schematically in cross-section, comprises a ring electrode 20 
coaxially aligned with upper and lower end cap electrodes 30 and 35, 
respectively. These electrodes define an interior trapping volume. 
Preferably, end cap electrodes 30 and 35 have inner surfaces with a 
cross-sectional shape which is "stretched." As used herein the term 
"stretched," when referring to the end cap electrodes, means electrodes 
which have the ideal hyperbolic shape, as taught by Paul, et al., but 
which are displaced from their ideal separation along the z-axis to induce 
higher order field components. The z-axis displacement is equal for each 
electrode, such that only even order multipole (e.g., octopole, etc.) 
field components are introduced. Those skilled in the art will appreciate 
that other techniques may also be used to introduce higher order field 
components, such as changing the shape of the electrode surfaces to depart 
from the ideal hyperbolic. For example, shapes which are more convex than 
hyperbolic may be used. It is also known that shapes which are not ideal, 
for example, electrodes having a cross-section forming an arc of a circle, 
may also be used to create trapping fields that are adequate for many 
purposes. Moreover, by using end caps which are the same, but which are 
not equally displaced, or which have different shapes, one can introduce 
odd order (e.g., hexapole) field components will be added. As described, 
the preferred stretched end cap electrodes introduce only even order 
higher order field components. The design and construction of ion trap 
mass spectrometers are well-known to those skilled in the art and need not 
be described in detail. A commercial model ion trap of the type described 
herein is sold by the assignee hereof under the model designation "Saturn. 
" 
Sample, for example from gas chromatograph ("GC") 40, is introduced into 
the ion trap 10. Since GCs typically operate at atmospheric pressure while 
ion traps operate at greatly reduced pressures, pressure reducing means 
(e.g., a vacuum pump and appropriate valves, etc., not shown) are 
required. Such pressure reducing means are conventional and well known to 
those skilled in the art. While the present invention is described using a 
GC as a sample source, the source of the sample is not considered a part 
of the invention and there is no intent to limit the invention to use with 
gas chromatographs. Other sample sources, such as, for example, liquid 
chromatographs ("LCs") with specialized interfaces, may also be used. For 
some applications, no sample separation is required, and sample gas may be 
introduced directly into the ion trap. 
A source of reagent gas (not shown) may also be connected to the ion trap 
for conducting chemical ionization ("CI") experiments. Sample (and 
optionally reagent) gas that is introduced into the interior of ion trap 
10 may be ionized by using a beam of electrons, such as from a thermionic 
filament 60 powered by filament power supply 65, and controlled by a gate 
electrode 70, which, in turn is controlled by the master computer 
controller 120. The center of upper end cap electrode 30 is perforated 
(not shown) to allow the electron beam generated by filament 60 to enter 
the interior of the trap. When gated "on" the electron beam enters the 
trap where it collides with sample and, if applicable, reagent molecules 
within the trap, thereby ionizing them. Electron impact ionization ("EI") 
of sample and reagent gases is also a well-known process that need not be 
described in greater detail. Of course, the method of the present 
invention is not limited to the use of electron beam ionization within the 
trap volume. Numerous other ionization methods are also well known in the 
art. For purposes of the present invention, the ionization technique used 
to introduce sample ions into the trap is generally unimportant. 
Although not shown, more than one source of reagent gas may be connected to 
the ion trap to allow experiments using different reagent ions, or to use 
one reagent gas as a source of precursor ions to chemically ionize another 
reagent gas. In addition, a background gas is typically introduced into 
the ion trap to dampen oscillations of trapped ions. Such a gas may also 
be used for collisionally induced dissociation of ions, and preferably 
comprises a species, such as helium, with a high ionization potential, 
i.e., above the energy of the electron beam or other ionizing source. When 
using an ion trap with a GC, helium is preferably also used as the GC 
carrier gas. 
A trapping field is created by the application of an RF voltage having a 
desired frequency and amplitude to stably trap ions within a desired range 
of masses. RF generator 80 is used to create this field, and is applied to 
ring electrode 20. The operation of RF generator 80 is, preferably, under 
the control of computer controller 120. A DC voltage source 250 (shown in 
FIG. 2) may also be used to apply a DC component to the trapping field as 
is well known in the art. However, in the preferred embodiment, no DC 
component is used in the trapping field. 
Computer controller 120 may comprise a computer system including standard 
features such as a central processing unit, volatile and non-volatile 
memory, input/output (I/O) devices, digital-to-analog and 
analog-to-digital converters (DACs and ADCs), digital signal processors 
and the like. In addition, system software for implementing the control 
functions and the instructions from the system operator may be 
incorporated into non-volatile memory and loaded into the system during 
operation. These features are all considered to be standard and do not 
require further discussion as they are not considered to be central to the 
present invention. 
As is explained in greater detail hereinafter, periodically ions are 
scanned out of ion trap 10 to produce a mass spectrum of the contents of 
the trap. Such scanning may be performed routinely, for example, to 
continuously monitor the substances present in the outflow from GC 40, or 
may be performed after an experiment is conducted in the ion trap, such as 
an MS/MS manipulation. According to the present invention, ions are 
scanned out of the trap in sequential mass order and are detected by an 
external detector such as electron multiplier 90, which is also subject to 
the control of computer controller 120. The output from electron 
multiplier 90 is amplified by amplifier 130, and the signal from amplifier 
130 is stored and processed by signal output store and sum circuitry 140. 
Data from signal output store and sum circuitry 140 is, in turn, processed 
by I/O process control card 150. As noted above, I/O card 150 is 
controlled by computer controller 120. The details of how components 90, 
130, 140 and 150 operate are well known and need not be described in 
further detail. 
The supplemental dipole voltage(s) used in the ion trap may be created by a 
supplemental waveform generator 100, coupled to the end cap electrodes 30, 
35 by transformer 110. Supplemental waveform generator 100 is of the type 
which is not only capable of generating a single supplemental frequency 
component for dipolar resonance excitation of a single species, but is 
also capable of generating a voltage waveform comprising of a wide range 
of discrete frequency components. Any suitable arbitrary waveform 
generator, subject to the control of controller 120, may be used to create 
the supplemental waveforms used in the present invention. According to the 
present invention, a multifrequency supplemental waveform created by 
generator 100 is applied to the end cap electrodes of the ion trap, while 
the trapping field is modulated, so as to simultaneously resonantly eject 
multiple ion masses from the trap, as in an ion isolation procedure. A 
method of generating a supplemental signal for isolating selected ion 
species is described in detail below. Supplemental waveform generator 100 
may also be used to create a low-voltage resonance signal to fragment 
parent ions in the trap by CID, as is well known in the art. 
As with most any instrument of its type, it is known that the dynamic range 
of an ion trap is limited, and that the most accurate and useful results 
are attained when the trap is filled with the optimal number of ions. If 
too few ions are present in the trap, sensitivity is low and peaks may be 
overwhelmed by noise. If too many ions are present in the trap, space 
charge effects can significantly distort the trapping field, and peak 
resolution can suffer. The prior art has addressed this problem by using a 
so-called automatic gain control (AGC) technique which aims to keep the 
total charge in the trap at a constant level. In particular, prior art AGC 
techniques use a fast "prescan" of the trap to estimate the charge present 
in the trap, and then use this prescan to control a subsequent analytical 
scan. According to the present invention, a prescan may also be used to 
control space charge and optimize the contents of the trap for an 
analytical scan. Alternately, the technique described in co-assigned U.S. 
Pat. No. 5,479,012 may be used to control space charge. 
According to the present invention, an asymmetrical trapping field is 
employed. Preferably, the trapping field is constructed from a combination 
of dipole and quadrupole components all having the same frequency 
.function.. In addition, if stretched end cap electrodes are used, higher 
order field components (e.g., octopole) are inherently introduced into the 
trapping field. Further, as described below, the "dipole" component of the 
trapping field inherently causes higher order odd order field component to 
be present in the trapping field, the predominant one being a hexapolar 
component. The asymmetrical trapping field used in accordance with the 
present invention has a center which is displaced from the mechanical 
center of the ion trap, (as defined by the electrode geometry). This is 
described in greater detail in coassigned U.S. Pat. No. 5,291,017, to 
Wang, et al., the disclosure of which is incorporated by reference, As 
noted, a damping gas is used in the ion trap and the collisionally damped 
trapped ions become positioned near and orbit about the center of the 
trapping field after ionization is completed. The inventors have 
determined that the secular frequencies of the ions trapped in an 
asymmetrical field are substantially the same as if they were trapped in a 
symmetrical field, but that the centers of the orbits are displaced in the 
axial direction. 
As used herein, and as is common among those skilled in the art, the term 
"dipole voltage" refers to a AC voltage applied across the end cap 
electrodes of the ion trap, such that one end cap receives a positive 
potential while the opposing end cap receives a negative potential of 
equal magnitude, (the potentials being relative to each other). More 
precisely, however, since the end caps are not parallel plates, the 
resultant field is not a pure dipole field, and inherently has higher 
order field components. As described below, one of the higher order field 
components is a hexapolar field which is used, in accordance with a 
preferred embodiment, to help excite ions out of the trap during mass 
scanning. 
In the preferred embodiment, the dipole component of the asymmetrical rf 
trapping field is passively created by using unequal lumped parameter 
impedances 210, 220 as shown in FIG. 2. This technique for generating the 
different components of the trapping field results in the components all 
having the same relative phase. The dipole component must be considered as 
being part of the trapping field as it has the same frequency and relative 
phase as the quadrupole trapping voltage. It is further noted that none of 
the trapped ions have secular frequencies which are the same as the 
frequency .function. of the trapping voltage. Therefore, the additional 
dipole trapping field component does not contribute to the ejection of 
ions by resonant excitation. Alternatively, a supplemental dipole voltage 
generator 100 may be used to actively create a dipole component of the 
trapping field. In such an embodiment, the phase of the supplemental 
dipole should be controlled to be the same as the quadrupole component. In 
yet another variation, both passive and active dipole components may be 
added to the trapping field. These latter embodiments permit variation in 
the ratio between the voltage of the dipole and quadrupole components for 
both the trapping field and the excitation field. 
Briefly, the impedances which are used to create the dipole take into 
account the capacitances between the end cap and ring electrodes 
("C.sub.re "), the capacitances between the end electrodes and ground 
("C.sub.eg "), and impedances 210 and 220 as shown in FIG. 2. In 
commercial ion traps, with mirrored symmetry (i.e., the end cap electrodes 
are the same shape and same displacement along the z-axis); C.sub.re1 
=C.sub.re2 and C.sub.re &lt;&lt;C.sub.eg. The dipole is created by the large and 
equal current flowing from trapping field rf generator 80 through 
C.sub.re1 and C.sub.re2. This current also flows through impedances 210 
and 220 to create unequal voltage drops thereby causing different voltages 
to be applied to the two end caps, and thereby causing a dipole voltage 
across the end caps. The supplemental (excitation) field dipole is created 
by the voltage divider action of impedance 210 and C.sub.eg1 as to the 
first end cap electrode 30 and the voltage and by the voltage divider 
action of impedance 220 and C.sub.eg2 as to the second end cap electrode 
35. A dipole voltage is created when the two voltage divider ratios are 
unequal. Since the value of C.sub.eg is largely set by the mechanical 
design of the ion trap, additional impedances Z.sub.eg (not shown) may be 
added to provide an extra degree of freedom. The determination of the 
impedance values of Z.sub.eg, and 210 and 220 may be done by standard 
electrical engineering analysis and synthesis techniques known to those 
skilled in the art. According to the preferred embodiment of the present 
invention the quadrupole component of the trapping field is created by the 
ring electrode, whereas the quadrupole component of the excitation field 
is created by the end cap electrodes. In addition, the trapping and 
excitation fields operate at different frequencies. Thus, impedances in 
the system, discussed above, operate differently on the voltages used to 
create the various field components. By appropriately choosing the values 
of the impedances added to the system, one can vary the relative 
proportion of quadrupole and dipole components in the fields. For, 
example, by appropriate selection, it is possible to create a trapping 
field with a significant dipole component, while creating an excitation 
field with little or no dipole component. 
While the present invention is described using voltage generators applied 
either to the ring electrode and/or across the end cap electrodes, it will 
be apparent to those skilled in the art that independent voltage sources 
can be applied to each of the three electrodes in the trap. Such voltage 
sources could, for example, be arbitrary waveform generators under the 
controlled of computer controller 120. 
The effect of the using an asymmetrical trapping field of the present 
invention is to greatly increase the percentage of ions, ejected from the 
ion trap during a scanning operation, which are directed to the detector. 
As noted above, when scanning using prior art symmetrical trapping fields, 
approximately half of the ions leave the trap in each axial direction. In 
addition, it has recently been discovered that the asymmetrical trapping 
field of the present invention causes positive and negative ions to be 
separated from each other, thereby obviating peak artifacts associated 
with scanning negative ions with sufficient energy to overcome the bias 
voltage of the electron multiplier. Such unwanted peak artifacts due to 
negative ions are common when scanning using a symmetrical trapping field. 
In its basic form the present invention uses an excitation field for ion 
ejection comprising a weak supplemental quadrupole field which is centered 
at the mechanical center of the ion trap. As shown in FIGS. 1 and 2, the 
quadrupole excitation field is created by applying the signal from 
supplemental voltage generator 160 to the center tap of the secondary coil 
of transformer 110. In this manner, the supplemental quadrupole excitation 
field is applied to the end cap electrodes so that this voltage signal 
does not interfere with the high-Q circuit used to apply the quadrupole 
trapping voltage to the ring electrode. Therefore, the center of the 
trapping field and the center of the weak supplemental excitation field 
are displaced from each other. This enables the supplemental quadrupole 
field to act on the trapped ions, since the supplemental quadrupole field 
is non-zero at the center of the trapping field. As used in the present 
specification, the term "weak supplemental quadrupole field" means that 
the field is not strong enough to independently trap a measurable number 
of ions. According to the preferred embodiment of the present invention, 
the frequency .omega. of the supplemental quadrupole excitation field is 
set at two-thirds (2/3) of the trapping field frequency, 
.omega./.function.=2/3. 
It is sometimes helpful to consider that the asymmetrical trapping field 
and the supplemental excitation field (which may include additional 
components as described below) act on ions within the trap as a single 
combined field. According to the present invention, one of the 
characteristics of this combined field is then scanned to bring ions into 
resonance with the supplemental excitation field in sequential mass order, 
thereby ejecting them from the ion trap for detection. Preferably, the 
voltage of the quadrupole component of the trapping field is scanned 
(i.e., linearly increased) to perform the mass scan. Other techniques for 
scanning the combined field are known to those skilled in the art and 
could also be used. However, such techniques are often more complicated 
and, therefore, less preferred. In addition, it is preferred to maintain 
the two-thirds relationship between the frequency .function. of the 
trapping voltage and the frequency .omega. of the excitation voltage, and, 
therefore frequency scanning is also not preferred for this reason. 
In U.S. Pat. No. 3,065,640, Langmuir taught that a supplemental quadrupole 
field with a frequency .omega..sub.p will have quadrupole axial parametric 
resonances that are related to the axial secular frequencies .omega..sub.z 
by the equation .omega..sub.p =2.omega..sub.z /N where N is a positive 
integer. Thus, the parametric frequencies are always less than or equal to 
twice one of the secular frequencies. It was also shown that a quadrupole 
excitation field at these frequencies will result in the exponential 
growth of axial oscillation. However, in the past, a limitation on the use 
of quadrupole excitation has been the fact that a quadrupole (or higher 
order) excitation field is zero at the center of the field. In the prior 
art, use of a quadrupole excitation field has been limited to symmetrical 
trapping fields, such that the center of the trapping field and the center 
of the excitation field where both at the mechanical center of the ion 
trap. Various techniques have been proposed to overcome this limitation, 
including using a dipole excitation field to move ions away from the 
center of the trapping field where they can be acted upon by the 
quadrupole excitation field, or using a very strong quadrupole excitation 
field, i.e., a supplemental quadrupole field which is strong enough to act 
as a trapping field. These solutions have not been satisfactory. 
According to the present invention, the center of the quadrupole excitation 
field does not coincide with the center of the asymmetrical trapping 
field. Thus, a weak quadrupole excitation field is able to act directly on 
the ions trapped in the asymmetrical trapping field because the ions are 
trapped in a region of the excitation field which is non-zero. 
Accordingly, the ions will be ejected from the ion trap by resonant 
excitation without the need to use a supplemental dipole field. In the 
preferred embodiment, ions are sequentially brought into resonance with 
the supplemental excitation field by increasing the amplitude of the 
trapping field which, in turn, changes the respective resonant frequencies 
of the trapped ions. 
Preferably, the supplemental excitation voltage also includes a dipole 
component in addition to the quadrupole component. This additional dipole 
component should have the same frequency .omega. as the quadrupole 
excitation field, preferably two-thirds of the trapping field frequency. 
The supplemental dipole component of the excitation field can be created 
in the same manner as the corresponding component of the trapping field, 
e.g., using unequal lumped parameter impedances 210 and 220, and/or using 
a phase locked active dipole voltage generator 100. 
Again, the passive approach has the advantage of easily assuring that the 
various field components have the same relative phase and reduced hardware 
requirements. The supplemental dipole field may be weak, such that it 
would not, acting alone, be capable of ejecting ions from the ion trap. 
Mass resolution is enhanced by minimizing all of the excitation field 
components, including the dipole field. 
It is well-known that the axial secular frequencies of the trapped ions 
have values .omega..sub.N =(2N+.beta.).function./2 where N is an integer 
and .beta. is related to the operating point of the trap. Previously, 
spectroscopists have used N=0 because the coupling coefficient is greatest 
for this value of N. (As the absolute value of N increases, the coupling 
coefficient decreases.) Thus, previously, there has been no recognized 
advantage for using a value of N other than 0. The present invention uses 
N=-1 to gain a heretofore unrecognized advantage. By way of example, 
assume that .function.=1050 kHz and .omega..sub.p =700 kHz. If the 
fundamental secular frequency (i.e., N=0) is used to excite the parametric 
oscillation, then it would be at 350 kHz and would require an additional 
rf generator. However, if .beta.=2/3 is selected as the operating point, 
the N=-1 harmonic of the secular motion would be at 700 kHz and, thus, a 
quadrupole field at this frequency would also act to excite the parametric 
oscillation. Thus, the selection of this combination of operating points 
and frequencies eliminates the need for an additional rf generator. In 
addition, this combination permits phase locking of the trapping field and 
the excitation field in a simple manner since the frequencies of the two 
fields have an integer relationship. Likewise, the trapping field dipole 
and the supplementary excitation field dipole can easily be phase locked 
while still using passive components, as described in connection with FIG. 
2. Finally, the technique of the present invention allows a linear 
increase in the supplemental quadrupole strength and dipole strength, 
e.g., respective voltages applied to the end caps, while maintaining a 
constant ratio between them, as the amplitude of the trapping voltage is 
increased during a scan. It can be shown from the equations of motion that 
it is advantageous to maintain a constant ratio between the excitation 
voltage and the trapping voltage. Specifically, as recognized by the 
inventors hereof when an asymmetrical trapping field is used in 
conjunction with a quadrupole excitation field, such that trapped ions are 
displaced from the center of the ion trap, the degree of excitation of 
ions is mass dependent. Specifically, as taught herein in connection with 
the preferred embodiment, there should be a constant ratio maintained 
between the field strengths of the dipole and quadrupole components of the 
trapping field scanning the trap in order for ion displacement to be 
independent of mass. This is not recognized in the prior art. 
As described above, when a dipole voltage is applied to end caps 
electrodes, higher odd order field components are also created, the 
predominant added field component being a hexapolar field. It can be shown 
that when using an operating point of .beta.=2/3 ions are also in 
resonance with the hexapolar component of the trapping field. As will be 
appreciated, the magnitude of the hexapolar field is a function of the 
magnitude of the dipole component of the trapping field. When using low 
dipole voltages, e.g., less than about 5% relative to the quadrupole 
voltage, then the hexapole component is too small to significantly affect 
the ejection process. However, when using a stronger dipole trapping field 
component, greater than 5% or, preferably greater than 10% of the 
quadrupole trapping voltage, then the hexapole component is significant 
and contributes to ion ejection when .beta.=2/3. In accordance with the 
present invention, the assistance in ejecting ions caused by this added 
field component appreciably improves mass resolution when scanning the ion 
trap and increases the fraction of ions that are ejected in a desired 
direction. 
While the use of hexapole fields is known in the prior art, such prior art 
fields have been created by shaping the electrodes of the ion trap. These 
mechanical methods of creating hexapole fields have a number of 
limitations which are overcome by the electrical technique of the present 
invention. When mechanical means are used to form a hexapole field, the 
relative position or "polarity" of the field is fixed. In contrast, when 
the hexapole field component is created electrically, its polarity or 
relative position can be reversed or otherwise modified by changing the 
relative phase of the quadrupole and dipole components of the trapping 
field. This can be important since the behavior of positive and negative 
ions in the trapping field is affected differently by a trapping field 
having a hexapole component. Depending on whether one is experimenting on 
positive or negative ions, one may want to reverse the polarity of the 
hexapole field component. Moreover, according to the present invention, it 
is possible to employ a symmetrical trapping field during the ion 
formation stage of an experiment and then apply an asymmetrical trapping 
field afterwards. During ion formation, ions tend to be distributed 
throughout the entire volume of the ion trap, and ions which are not near 
the center are subject to ejection due to the resonance with the hexapole 
field. After the ions are thermalized or damped to the center of the ion 
trap they are no longer susceptible to unwanted resonant ejection in this 
manner. Finally, the relative proportion of the hexapole and quadrupole 
components of the trapping field is fixed in a mechanical system, whereas 
the proportion can be varied, if desired, when the hexapole field is 
generated electrically. 
By using a set integer ratio between .function. and .omega., as in the 
present invention, it is possible to assure phase locking between the 
trapping voltages and the excitation voltages, thereby eliminating the 
effects of frequency beating. It is particularly advantageous to utilize 
the smallest possible integer ratio between these frequencies (e.g., 2:3) 
consistent with the other objects of the invention, because the advantages 
of phase locking will occur (and be repeated) in the smallest number of 
cycles. Phase lock circuitry 170, of the type which is well known in the 
art, is used to lock the phases of the voltages created by the trapping 
field generator 80 and the supplemental excitation field generator 160. 
When using a supplemental dipole excitation source, e.g., voltage source 
100 in FIG. 1, an additional phase lock circuitry 175 is, preferably also 
used. 
For the case of a symmetrical trapping field of the prior art, ions having 
a center of oscillation at the geometric center of the trap initially 
experience very little effect from a substantially quadrupole excitation 
applied symmetrically from the end caps, because the thermalized ions are 
trapped in a region of approximately null field. It is known to apply an 
excitation field having both dipole and quadrupole components whereby the 
trapped ions are first affected by the dipole component. Power is promptly 
absorbed from the dipole resonance and the resonantly mass selected ions 
are subject to greater axial amplitude oscillation. As a result of the 
greater axial amplitude, these ions then absorb power from the mass 
selective resonant quadrupole field component. This sequential process, 
governing the symmetric arrangement of prior art is to be contrasted with 
the present invention wherein the mass independent center of oscillation 
of the trapping field is displaced from the central region of the mass 
selective combined dipole quadrupole excitation field. See U.S. Pat. No. 
5,347,127 to Franzen where the sequential nature of the prior art is 
deliberately emphasized. 
FIG. 3 compares the method of the present invention, i.e., using an 
asymmetrical trapping field, with the same method but using a symmetrical 
trapping field, as discussed above. The mass scan on the left side of FIG. 
3, curve 310, was acquired used the method of the present invention, while 
the mass scan on the right side of FIG. 3, curve 320, was acquired using a 
symmetrical trapping field. In both instances, the supplemental excitation 
field comprised a quadrupole voltage and a dipole voltage of the same 
phase. It is apparent that the asymmetrical trapping field of the present 
invention, combined with a excitation voltage comprising quadrupole and 
dipole components, produces a higher intrinsic rate of ion ejection with a 
resulting better resolution and peak intensity. From a qualitative point 
of view the present invention provides a concurrent effect of both 
quadrupole and dipole excitation components rather than the sequential 
effect of the prior art because the relative displacement of the center of 
ion density is achieved by the asymmetrical trapping field. Accordingly, 
the mass selected ions are ejected promptly in time. For a given scan rate 
this clearly results in a more precise mass resolution than would be 
achievable for a less rapid ejection rate. 
FIG. 4 compares various scanning techniques. The mass scan 410 is the prior 
art resonant ejection technique using a dipole excitation voltage in a 
symmetrical trapping field. As described above, the frequency of the 
excitation voltage (.omega..sub.s =485 kHz) is set at about one half of 
the trapping field frequency (.function.=1050 kHz) as taught in the prior 
art. Noticeable distortions in the mass peak may be observed due to 
frequency beating. Mass scan 420 is taken under identical conditions using 
the asymmetrical trapping field of Wang, et al. While the height of the 
peak is higher due to the fact that ions are preferentially ejected 
towards the detector, the mass resolution is substantially the same. The 
effects of frequency beating are, again, noticeable. Mass scan 430 uses a 
symmetrical trapping field and an excitation voltage comprising both 
quadrupole and dipole components at a frequency (.omega..sub.d 
=.omega..sub.q =700 kHz) which is set at two-thirds of the trapping field 
frequency, .function.=1050 kHz. In curve 430 there is no noticeable 
frequency beating, and the mass resolution is slightly improved over scans 
410 and 420. Finally, scan 440, according to the preferred embodiment of 
present invention, was taken under identical conditions as scan 430, but 
using an asymmetrical trapping field. Note that the mass resolution is 
greatly improved over any of the other scans, there is no noticeable 
frequency beating, and the peak height is far better than the other scans. 
It is specifically recognized that the displacement of the center of 
oscillation of ions by the trapping field from the central region of the 
excitation field facilitates manipulation of trapped ion populations 
generally. By way of example, ion isolation procedures yield improved 
result because the simultaneous absorption of power from dipole and 
quadrupole fields (in contrast to sequential resonant absorption) allows 
for a more rapid mass selected ion ejection. The time spent in exciting 
masses greater than, and less than a selected mass is therefore minimized. 
The selected mass, which may be inherently unstable or which is subject to 
dissociation, is therefore available for a greater time interval for 
isolated ion processes. 
References herein to the excitation field are not limited to an excitation 
field characterized by a single discrete frequency. Broadband excitation 
comprising a plurality of frequency components is well known for the 
purpose of providing excitation to a selected range, or ranges of ion 
mass. The selection and phasing of the frequency components of the broad 
band waveform are well known in the art. Each such frequency component 
herein contains quadrupolar and preferably both quadrupolar and dipolar 
multipolarity. 
While the present invention has been described in connection with the 
preferred embodiments thereof, those skilled in the art will recognize 
other variations and equivalents to the subject matter described. 
Therefore, it is intended that the scope of the invention be limited only 
by the appended claims.