Ion mobility storage trap and method

An apparatus and method to separate and store ions by exploiting mobility characteristics of the ions. A sample is introduced into a trap volume and ionized. The ions are separated according to their mobility characteristics by applying an electric field to the trap volume. The ions thus migrate to equilibrium positions in the trap volume due to a difference in mobilities and to changes in the electric field. The ions may be sequentially scanned from the trap by changing the electric field. The identity of ions within the trap may then be determined.

FIELD OF THE INVENTION 
The invention relates to ion mobility spectrometry to separate and store 
ions in gases using an asymmetric AC potential. 
BACKGROUND OF THE INVENTION 
Several devices are known that ionize a gaseous sample and analyze the 
product ions for the molecular makeup of the sample. The devices fall into 
two categories: those that operate under vacuum and those that operate 
under pressure conditions. The devices that operate under vacuum are know 
as mass spectrometers and separate ions according to charge-to-mass ratios 
using a combination of electromagnetic fields. The devices that operate 
under pressure are known as ion mobility spectrometers and separate ions 
according to mobilities through a drift gas in a constant electric field. 
Generally, mass spectrometers require a vacuum better than 10.sup.-3 mm Hg 
to eliminate the adverse effects of collisions between ions and neutral 
gas molecules. This is unlike ion mobility spectrometry where pressures 
greater than 10.sup.-3 mm Hg are needed to assure that collisions between 
the ions and neutral gas molecules firmly establish mobility values. Due 
to the absence of collisions in mass spectrometry, the ions can gain 
considerable energy as they respond to the imposed electromagnetic fields. 
In ion mobility spectrometry, the energy gained by the ions is rapidly 
dissipated by collisions between the ions and neutral gas molecules. One 
consequence of this difference in ion energy between mass spectrometry and 
ion mobility spectrometry is that the energetic ions of mass spectrometry 
do not follow electric field lines, while the thermal ions of ion mobility 
spectrometry do. Because of this difference, attempts to separate ions 
using one technique in the pressure regime of the other is generally 
unsuccessful. On the other hand, there are enough similarities between 
mass spectrometry and ion mobility spectrometry to encourage exploitation 
of common features. 
The technique of ion mobility spectrometry (IMS) was first disclosed in 
U.S. Pat. No. 3,699,333 which issued on Oct. 17, 1972 to M. J. Cohen, D. 
I. Carroll, R. F. Wemlund and W. D. Kilpatrick. It was originally 
conceived as a method to analyze and detect organic vapors in a gas 
mixture. FIG. 1 shows a simplified IMS detector cell. It contains two 
regions: a reaction (or reactor) region where the ions are ionized, and a 
drift (or drift tube) region where the ions are separated. The ionization 
and separation processes occur under a wide range of pressure to, 
conditions, but the preferred operating pressure in U.S. Pat. No. 
3,699,333 was atmospheric pressure. In the reaction region, the sample is 
either ionized directly by using ultraviolet radiation from a 
photoionization source, electrospraying the ions as a mist into the 
ionizer, etc.; or indirectly by reacting with an intermediate set of 
reactant ions (designated by R.sup..+-. in FIG. 1). The indirect method 
of ionization is known as chemical ionization and the reactant ions are 
created by using a radioactive source (e.g., Ni.sup.63, Am.sup.241, 
tritium, etc.), a corona discharge source, a thermionic emitter of alkali 
ions, or another primary source of ions. 
The nature of the reactant ions generated by the ionization source depends 
on the composition of the carrier gas used to transport sample into the 
reactor of the ion mobility spectrometer. This dependency can be used to 
selectively ionize a specific component in a sample matrix by adjusting 
the composition of the carrier gas. This is accomplished by doping the 
carrier gas with a low level of a chemical reagent, such as acetone, a 
chlorinated solvent, methyl salicylate, etc. The reactant ions then become 
a protonated di,acetones a chloride anion or a protonated monomer of 
methyl salicylate, etc. that react differently the ample. 
While the reactant ions and product ions (designated by P.sup..+-. in FIG. 
1) can be positively or negatively charged, the polarity of the ions that 
are extracted from the reactor and analyzed by the drift tube depends upon 
the directionality of the electric field applied to the drift tube. If the 
ionization source is biased positive relative to the ion collector, 
positive ions are extracted from the reactor and analyzed by the drift 
tube for mobility. If the ionization source is biased negative relative to 
the ion collector, negative ions are extracted from the reactor and 
analyzed by the drift tube for mobility. If no electric field is applied, 
the positive and negative ions recombine, and are otherwise lost for 
analysis by the drift tube. 
A shutter grid positioned between the reactor and the drift tube provides a 
means whereby a localized concentration of ions is extracted from the 
reactor and introduced into the drift tube. Typically this shutter grid 
consists of a planar array of parallel wires with neighboring wires 
electrically independent. When the two sets of electrically independent 
wires are at the same potential, the ions pass freely through the grid and 
enter the drift tube. When the two sets of neighboring wires are at 
different potentials, the ions are captured by the grid and are denied 
entry into the drift tube. Ion injection into the drift tube is 
accomplished by momentarily removing the blocking potential from the 
shutter grid. Once inside the drift tube and exposed to the drift field 
applied to the drift tube, the ions migrate toward an ion collector (or 
Faraday plate) located at the other end of the drift tube. When the ions 
arrive at the collector, their drift time is recorded and correlated with 
the composition of the original sample delivered to the reactor. 
The IMS technique, as described above, has several limitations. These 
include: 
1. The basic limits of detection are restricted to about ten picograms or 
ten parts per trillion due to build up of space charge in the reactor. 
There is no capability of concentrating and storing ions. 
2. Ion mobilities are sensitive to the composition of the drift gas, and 
decrease as the ion clusters with water vapor or other polar compounds. 
Ions attached to contaminant gases have different mobilities, making it 
difficult to identify the ions. 
3. Miniature IMS sensors are plagued by low total ion currents (the ion 
current collected by the ion collector when the shutter grid is biased 
open continuously) that limit the dynamic range of the device. 
U.S. Pat. No. 5,200,614 which issued on Apr. 6, 1993 to A. Jenkins and W. 
J. McGann describes an "ion trap mobility spectrometer" that attempts to 
remove one of the above limitations and improves the limits of detection 
of IMS for electrophilic compounds (e.g., nitro-compounds used as 
explosives). A schematic representation of their device is shown in FIG. 
2. The two halves of the shutter grid are separated to create a field-free 
ion storage region within the device. When the two grids (E1 and E2 in 
FIG. 2) are at the same potential, the ions entering the ion storage 
region from the reactor become "trapped" (i.e., lie motionless). By 
momentarily applying a high potential between grids E1 and E2 (V.sub.3 in 
FIG. 2), the "trapped" ions are injected into the drift tube. The ion 
storage region, therefore, behaves like a pulsed reactor for the IMS. When 
compared to the reactor of a conventional IMS, this pulsed reactor has the 
advantage that it increases the reaction time for ionization; and in the 
case where electron capture processes are important, thermalizes the 
reacting electrons. On the other hand, the disadvantages are that space 
charge can more easily build up in the ion storage region. Like 
conventional IMS, the drift times for the ions in the affected ion trap 
mobility spetrometer are by the composition of the carrier gas flowing 
through the drift region, and the total ion current decreases when 
attempts are made to miniaturize the cell. 
Disclosed in Russian Inventor's Certificate No. 9666583 is another method 
for separating ions according to mobility. The parallel plate ion 
separator for this invention is shown in FIG. 3 where the ion flow 
(induced by a drift gas) is from left to right. A transverse asymmetric AC 
field is applied across the electrodes of the separator to excite a 
perpendicular micromotion in the ions as they move in the average 
direction of the flowing drift gas. The amplitude of the micromotion is 
proportional to the electric field strength through a mobility coefficient 
K. The relationship v.sub.d =KE is a vector relationship between the drift 
velocity v.sub.d of the ion and the electric field E. Unlike conventional 
IMS where ion separation is accomplished using relatively low electric 
field strengths (e.g., 150-250 volts/cm), higher field strengths are 
required to successfully separate the ions in the Russian invention. E. A. 
Mason and E. W. McDaniel in their book entitled "Transport Properties of 
Ions in Gases" (Wiley, New York, 1988) teach that the mobility coefficient 
K is a function of the electric field E. An approximate expression for the 
functional dependence is K(E)=K.sub.0 +K.sub.2 E.sup.2 +K.sub.4 E.sup.4 + 
. . . , where the K.sub.1 's are coefficients dependent on the ion species 
under consideration. Therefore when an ion is exposed to an asymmetric AC 
potential that is oscillating between adequately high and low values, the 
ion experiences different mobilities when traveling in one direction 
compared to the other. This causes; the ions to move more in one direction 
than another, and be neutralized when they collide with the electrodes of 
the Russian invention. By adding a DC component to the asymmetric 
potential, the path taken by the ions can be altered; and depending on the 
combination of the DC and asymmetric AC potentials applied, certain ions 
can be directed towards an ion collector. Since the difference in 
mobilities created by the asymmetric field is dependent on the type of ion 
being analyzed, ion separation is possible. 
In a paper published in the International Journal of Mass Spectrometry and 
Ion Processes; volume 128 (1993), pp. 143-148; I. A. Buryakov, E. V. 
Krylov, E. G. Nazarov and U. Kh. Rasulev further describe the method of 
Certificate No. 9666583. They state that the ion separation is performed 
in a dense gas (e.g., air at 760 mm Hg) using a 2 megahertz RF potential. 
The waveform for the RF potential is rectangular with a period of 
T=t.sub.1 +t.sub.2 (t.sub.1 &lt;&lt;t.sub.2); the absolute value for the 
positive semi-period, t.sub.1 (E.sub.max), being much less than the 
absolute value for the negative semi-period, t.sub.2 (E.sub.min), and the 
integrated areas for the waveform above and below zero being equal. An ion 
spectrum (sometimes called an ionogram) is obtained by superimposing the 
asymmetric potential on top of a DC potential and scanning the DC 
potential. As the DC potential is scanned, ions with different mobilities 
sequentially pass through the device. Buryakov, et al. showed that amines 
in a gas mixture can be selectively detected within 10 seconds. They 
further stated that because of its small size, the parallel plate ion 
separator can be incorporated into a portable gas analyzer. 
The ion separator of FIG. 3, however, has several disadvantages. The linear 
velocity of the drift gas must be kept constant across the diameter of the 
tube and also, preferably, along its length. Diffusers are required to 
establish laminar flow conditions. In addition, the device has a slow 
response because the velocity of the ions along the longitudinal direction 
of the drift tube is controlled by the relatively slow moving drift gas. 
In U.S. Pat. No. 5,420,424 which issued on May 30, 1995, B. L. Carnahan and 
A. S. Tarassov disclosed a modified version of the parallel plate ion 
separator which they called a "transverse field ion mobility spectrometer" 
(later concatenated to "field ion spectrometer (FIS)") This device is 
shown in FIG. 4 which is a cross-sectional view of cylindrical geometry. 
The cylindrical capacitor provides a more uniform field and a greater 
cross-sectional area for transmission of ions. Instead of using the 
rectangular waveform of Buryakov, et al., they used an oscillating 
potential is superimposed upon its second harmonic to generate the 
asymmetric field; i.e., V(t)=V.sub.0 +V.sub.1 [(1-.beta.) cos 
.omega.t+.beta. cos 2.omega.t], where V.sub.0 and V.sub.1 are constants 
and 0.1&lt;.beta.&lt;0.7. The field ion spectrometer has been used to collect 
spectra on various organo phosphorus and aromatic compounds with a total 
analysis time of 0.1 to 1.3 seconds. However, the device continues to 
suffer from the deficiencies noted above for the parallel plate ion 
separator of FIG. 3. 
Despite the fact that they work only under vacuum conditions, certain mass 
spectrometers Be are related to the devices of FIGS. 1-4. An important 
mass spectrometer for this purpose is the quadrupole mass filter first 
disclosed by W. Paul, et al. in U.S. Pat. Nos. 2,939,952 and 2,950,389 
which issued on Jun. 7, 1960 and Aug. 30, 1960, respectively. Such a mass 
filter is illustrated in FIG. 5. The vacuum allows the focusing lenses to 
accelerate the ions and direct them onto the entrance aperture of four 
quadrupole rods. Since the function of the quadrupole rods is to separate 
ions, they are sometimes collectively referred to as a quadrupole ion 
filter. Again due to the vacuum, the ions maintain their linear velocity 
as they pass through the quadrupole filter; and as they interact with an 
oscillating electromagnetic field applied across the rods, they oscillate 
perpendicular to their original direction of motion. The oscillating field 
is created by a symmetric RF and DC potential applied across neighboring 
rods. The magnitude of the ion oscillation is dependent on the 
mass-to-charge ratio of the ions; and because the oscillations are so 
great, most of the ions hit the quadrupole rods. Certain of the ions, 
however, do not hit the rods and survive until they reach the opposite end 
of the filter. A detector, or electron multiplier, registers the arrival 
of the surviving ions. 
The principle of operation for the quadrupole mass filter relies upon the 
electric field applying a restoring force to the ion so that it oscillates 
about some preferred position within the rods. To effectively perform this 
function, the electric field must satisfy certain spatial distribution 
requirements. In particular, the electric field must be quadrupolar. Such 
a field is created by carefully sculpting the internal surfaces of the 
quadrupole rods. Theoretically, the internal surfaces should define 
complementary hyperbolas. However, due to difficulties in machining 
hyperbolas, the hyperbolic rods are often replaced with round rods, (as 
shown in FIG. 5) that are carefully placed relative to each other to 
create the desired hyperbolic field. 
An ion separator related to the quadrupole mass filter is the monopole mass 
filter described by U. von Zahn in a paper published in the Review of 
Scientific Instruments, volume 34 (1963), pp. 1-4. Such a filter is shown 
in FIG. 6. The monopole mass filter is a rod and an angle electrode 
located relative to each other so that a quarter-section of the quadrupole 
mass filter is approximated. Ions are separated by applying a combination 
of RF and DC potentials across the two electrodes. Because the angle 
electrode occupies the path that t:he ions would normally travel in a 
quadrupole mass filter, the ions are injected with a transverse, as well 
as a longitudinal, velocity component into the monopole mass filter. After 
injection, the ions describe an arc; first moving toward the rod, and then 
away from the rod and toward the angle electrode. Like the quadrupole mass 
filter, the combination of the RF and DC potentials determines which ions 
pass through the electrode structure for detection by an electron 
multiplier. 
In addition to the quadrupole mass filter, W. Paul and H. Steinwedel al so 
disclosed a three-dimensional analogue of the quadrupole mass filter in 
U.S. Pat. No. 2,939,952. This variation eventually became know as the "ion 
trap mass spectrometer (ITMS)" further disclosed by G. C. Stafford, P. E. 
Kelley and D. R. Stephens in U.S. Pat. No. 4,540,884, dated Sep. 10, 1985, 
and shown in FIG. 7. Being a cylindrical analogue of the linear quadrupole 
filter, the electrode structure for the ITMS consists of a ring-electrode 
sandwiched between two end-caps with the internal surfaces defining 
revolutions of complementary hyperbolas. Although other shapes for the 
electrode structures have been studied, more attention has been given to 
fabricating ideal electrode shapes (albeit with known distortions) for the 
ITMS than for the linear quadrupole. 
When a symmetric RF and DC (optional) potential is applied across the ring 
and end-cap electrodes of the ITMS, a trapping field develops within its 
volume. The trapping field is characterized by a potential well (more 
specifically, a rotating saddle point) that causes the ions to migrate 
towards and oscillate around the center of the trap. This trapping field 
is considerably different from the trapping field described earlier for 
the field free region of the ion trap mobility spectrometer of FIG. 2. 
Unlike the ion trap mobility spectrometer, the restoring force invoked by 
the rotating saddle point causes the ions to be trapped for longer periods 
of time near the center of the trap. In fact, the times are so long that 
the ITMS is sometimes referred to as an "ion storage trap", or "ion store" 
for short. On the other hand, the ions can be ejected at will by changing 
the combination of the RF and DC potentials applied to the electrodes. 
This combination of storing and then releasing ions allows ion 
concentrations (see U.S. Pat. No. 4,650,999, dated Mar. 17, 1987 for 
handling space charge effects) to be enriched before they are delivered to 
a detector. It also allows the ITMS to work as a mass spectrometer. 
Since the original ion trap inventions, several investigators have 
disclosed that the trapping field does not have to be quadrupolar. For 
example, J. Franzen, et al. in U.S. Pat. Nos. 4,882,484; 4,975,577; 
5,028,777; 5,170,054; 5,283,436; 5,331,157; 5,386,113 and 5,468,958 state 
that multipolar fields can be added to the quadrupolar field to create 
nonlinear resonances that improve mass resolution, scan speed and ion 
storage stability. In chapter 3 of a book entitled "Practical Aspects of 
Ion Trap Mass Spectrometry: Volume I" (CRC Press: Boca Raton, Fla., 1995), 
it is disclosed that the nonlinear resonances can be unintentionally 
introduced by imperfect machining of the electrodes, or intentionally 
introduced to improve the performance of the trap. 
Examples of electrode structures that can be used to generate multipolar 
fields are shown in FIGS. 8A-8C. FIG. 8A is the conventional quadrupole 
structure, while FIGS. 8B and 8C are the hexapole and octapole structures, 
respectively. The analytical expressions for the electric field and 
potential created by each of the structures are also shown in FIG. 8. "z" 
represents the vertical longitudinal dimension and "r" represents the 
horizontal radial dimension in each case. E.sub.r and E.sub.z represent 
the electric field in the r and z directions respectively, and .phi. 
represents the potential. To create a nonlinear resonance, a weighted sum 
of two or more of these fields is necessary. 
Other electrode structures that have been investigated for vacuum-operated 
ion trap mass spectrometers are shown in FIG. 9 along with the 
equipotential lines they generate. Each trap is a cross-sectional view of 
cylindrical geometry where the electrodes are indicated by cross-hatched 
areas. FIG. 9A is the conventional quadrupole structure and FIGS. 9B and 
9C are two planar analogues. Brewer, DeVoe and Kallenbach have 
theoretically analyzed the planar traps in a paper published in the 
journal entitled Physical Review A, volume 46 (1992), pp. R6781-6784. In 
each case, the electrode(s) corresponding to the ring-electrode is/are 
labeled as 1, and the electrodes corresponding to the end-caps as 2. It is 
evident that the equipotential line profile is largely quadrupolar in each 
case. It is also evident that the planar analogues, have other multipolar 
contributions. 
To date, none of the mass spectrometer devices of FIGS. 5-9 have been 
successfully used to separate ions at pressures greater than about 
10.sup.-2 mm Hg. Any attempt to do so results in a loss of signal. The 
reason for the loss in signal is that the ions collide with, and are 
scattered by, the neutral gas molecules contained in the analyzer. The 
collision and scattering events prevent the ions from reaching the ion 
collector. 
SUMMARY OF THE INVENTION 
An apparatus and method is described for analyzing a sample matrix by 
ionizing its constituent components and separating the ions in an ion 
mobility storage trait (IMST) with the aide of asymmetric AC and variable 
DC potentials that are applied across the electrode structure defining the 
trap volume. In response to the asymmetric AC and DC potentials, the ions 
are gathered and temporarily stored in the trap volume as they oscillate 
about preferred equilibrium position(s). The ions are then scanned out of 
the trap by varying one or more parameters (e.g., magnitudes, phases, 
etc.) of the AC or DC potentials, and/or their ratios, between 
predetermined limits. Using this approach, ions with a specific mobility, 
or a range of mobilities, can be ejected from the trap. The ions exiting 
the trap are detected by an ion detector (e.g., a Faraday plate) and are 
identified by the scanning parameters required to expel them from the 
trap. The device operates in any gas with a pressure greater than 
10.sup.-2 mm Hg (including greater than atmospheric pressure). 
Additionally if a scan function is not desirable, all the ions can be 
expelled from the trap by temporarily applying an accelerating potential 
across the electrode structure of the trap. 
Lastly, further embodiments are directed to an ion storage trap used in 
combination with a variety of ionizers and ion analyzers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 10A is a simplified schematic for an ion mobility storage trap system 
according to one example of the invention. Element 6 is a general 
representation of an ion mobility storage trap (also referred to 
hereinafter as an IMST) that may have mechanical features very similar to 
the ion trap of mass spectrometry (ITMS). The details of several examples 
of the ion mobility storage trap will be described in more detail later. 
AC voltage generator 3 generates an oscillating asymmetric potential and 
DC power supply 4 generates a variable DC potential. The AC asymmetric 
potential is superimposed on the DC potential by mixer 5 and is applied 
across electrodes (not shown) of the ion mobility storage trap 6. Mixer 5 
may be an autotransformer or a fast high voltage solid state switch (in 
combination with a high voltage power supply) similar to that manufactured 
by Behlke, Frankfurt, Germany. The functions of the AC voltage generator 3 
and the mixer 5 can also be combined into a high voltage pulse generator 
similar to that manufactured by Directed Energy, Inc., Fort Collins, Colo. 
A sample is introduced into volume 7 of the trap 6 by flowing a carrier 
gas, provided by a carrier gas source 25, through a hole in one of the 
electrodes, or through a space between the electrodes. Restrictor 26 and 
27 regulate the flow of carrier gas and sample into and out of volume 7. A 
vacuum pump 28 may also be connected to trap 6 and in fluid communication 
with trap volume 7 (not necessary for high pressure operation). 
Restrictors 26 and 27, in combination with vacuum pump 28, operate to 
regulate the pressure inside the trap volume, 7. 
Ions are formed from the sample using an ultra-violet (UV) flashlamp 8 that 
is pulsed using a frequency derived from the asymmetric AC generator 3. 
Alternatively, the ions may be generated outside the trap volume 7 by 
other known means. Photoionization, however, avoids issues related to 
injecting ions into trap volume 7 under high pressure conditions. A 
particularly suited flashlamp for this purpose is a krypton flashlamp 
equipped with a magnesium fluoride window as supplied by EG&G 
Electro-Optics in Salem, Mass. G. E. Spangler, J. E. Roehl, G. B. Patel 
and A. Dorman have described the use of such a flashlamp as an ionization 
source for IMS in U.S. Pat. No. 5,338,931 which issued on Aug. 16, 1994. 
Flashlamp 8 can either directly photoionize a sample introduced into trap 
volume 7, or indirectly ionize the sample when a dopant (e.g., acetone or 
benzene) is added to the carrier gas. Other flashlamps are also available 
with different energy characteristics. 
A grid (not shown) allows the flashlamp to be inserted into the trap 
without affecting the field distribution within volume 7. 
Flashlamp controller 9 is an energy storage device that matches the low 
voltage bias circuitry with the high voltage requirements of flashlamp 8. 
Between flashes, a capacitor within the flashlamp controller 9 charges, 
and then discharges through the lamp whenever pulse 10 is applied to 
flashlamp controller 9. 
The phase adjuster 12 and frequency divider 11 provide a means whereby the 
pulse is synchronized with the ion motion in trap volume 7. Specifically, 
it is desirable that flashlamp 8 discharge (or flash) when the phase of 
the asymmetric potential is such that the newly formed ions are propelled 
away from the electrodes and into the main volume 7 of the trap. If the 
oscillatory motion of the ions is such to allow them to hit the 
electrodes, the ions are neutralized and are lost for subsequent analysis. 
Once the ions are trapped, they experience a variety of forces as they 
migrate through the gas contained in volume 7. Some of the forces 
originate from collisions between the ions and the neutral gas molecules 
contained in the trap, but others include interaction of the ions with the 
electric field. As explained in more detail below, the function of the 
asymmetric AC and DC potentials across the electrodes is to create an 
oscillating electric field that causes the ions to migrate towards an 
equilibrium position within the trap. This tendency for migration is 
related to the field dependence of the mobility for the ions. Because 
different ions have different mobility characteristics, a mixture of 
different ions eventually occupy different locations within the trap and 
are separated according to type. 
The location for the equilibrium positions is changed by changing the 
relative amplitudes for the asymmetric AC and DC potentials. Thus, the 
ions can be moved around in trap volume 7 by scanning the potentials with 
scan controller 13. An appropriate scan function might be to continuously 
increase the DC potential so that the ions vertically exit the trap 
towards ion collector 14. Ion collector 14 may be encapsulated in a 
Faraday cage that is AC coupled to ground. This, together with a high gain 
electrometer 15, filters out ripples created by the asymmetric AC 
potential that might otherwise produce noise in the ion signal. 
The ion current induced in ion collector 14 is amplifed by electrometer 15 
for processing by the data acquisition system 16. The data acquisition 
system 16 first determines the combination of AC and DC potentials that 
caused the ions to exit the trap, and then from that information 
determines the type of ion. A DC offset 17 is provided so that ion 
collector 14 can be maintained at a virtual ground equal to system ground. 
The DC offset establishes the potential for aperture grid 18 that serves 
as an end-cap electrode for trap 6. 
FIG. 10B illustrates another example of the ion mobility storage trap. 
Elements labeled with the same numerals have the same function as those 
illustrated in FIG. 10A. Because their operation is the same, their 
description is omitted here. Additionally included in FIG. 10B is a high 
voltage power supply 19, a pair of electronic (including, but not limited 
to, relay switches) switches 20 and 21 selectively connecting the high 
voltage power supply to electrodes within trap 6, a frequency divider 22 
and an extraction width adjuster 23 for controlling the operation of 
switches 20 and 21. 
Instead of removing the ions from volume 7 using scan controller 13, the 
high voltage power supply 19 and switches 20 and 21 remove the ions by 
applying an accelerating potential between the end-cap electrodes as 
described by Q. Ji, M. R. Davenport, C. G. Enke and J. F. Holland in the 
Journal of the American Society of Mass Spectrometry volume 7 (1996), pp. 
1009-1017. The switches are driven by an extraction trigger pulse 24 that 
is synchronized with the data acquisition system 16 through frequency 
divider 22. The effect of the high voltage power supply 19 is to propel 
the ions towards ion collector 14. Buttrill, Jr., et al. in U.S. Pat. No. 
5,569,917 which issued on Oct. 29, 1996; M. H. Studier in a paper 
published in the Review of Scientific Instruments, volume 34 (1963), pp. 
1336-1370; B. F. Bonner, et al. in a paper published in the International 
Journal of Mass Spectrometry and Ion Physics, volume 10 (1972/1973), pp. 
197-203; R. E. Mather, et al., in a paper published in the International 
Journal of Mass Spectrometry and Ion Physics, volume 28 (1978), pp. 
347-374; and J. E. Fulford, et al. in a paper published in the Journal of 
Vacuum Science and Technology, volume 17 (1980), pp. 829-835 discuss and 
critically evaluate other approaches to electronically 
pulsing/accelerating ions out of an ion trap mass spectrometer. These, and 
other techniques are all applicable to this invention with equal facility. 
Ion Oscillations in Electromagnetic Fields 
Before describing in detail the motion of the ions in trap volume 7, it is 
instructive to first describe the dependence of an ion's mobility on the 
strength of the electric field in which it exists. 
FIG. 11 shows ion i positioned between two parallel plates p within chamber 
c. A voltage source V applies a potential across plates p to generate an 
electric field between them. The axis connecting the two plates is the 
x-axis with the origin located midway between the plates. The electric 
field generated between plates p will be assumed uniform (so that the 
electric field lines are parallel and evenly spaced). 
FIGS. 12A to 12D illustrate the motion of ion i using four different 
operating conditions in chamber c. FIGS. 12A and 12B illustrate the 
movement of the ion i when a vacuum condition exists in chamber c and a 
potential having a sinusoidal waveform is applied across parallel plates 
p. Newton's Law states that ion i experiences a force given by 
EQU F=ma=qE (1) 
where m is the mass of the ion, a is its acceleration, q is its charge, and 
E is the electric field strength. The location of the ion in volume c is 
determined by the second integral of the acceleration along with some 
initial conditions. If it is assumed that the ion is introduced into the 
chamber at x=0 with an initial velocity of v=0, and the initial phase of 
the sinusoidal waveform when the ion was introduced into the trap is 
90.degree. (i.e., sin (.omega.t) having a value of one), FIG. 12A shows 
that the subsequent motion for the ion will be sinusoidal where the broken 
curve represents the ion's velocity and the solid curve the ion's 
position. When the right electrode of FIG. 11 is positively charged, a 
positive ion will initially move in the negative x-direction with an 
increasing velocity. As the sinusoidal potential changes polarity, the ion 
decelerates and begins to move in the positive x-direction (i.e., change 
direction). Because the electric field applied between the plates exerts 
an equal and opposite force on the ion for each polarity of the potential 
(i.e., for each half cycle of the sinusoidal waveform), the displacement 
of the ion in each direction is the same. Thus, the ion oscillates about a 
fixed location within volume c. In FIG. 12A, the ion is shown oscillating 
about x=0. 
FIG. 12B shows the motion of the ion of 12A when it is introduced into 
chamber c with the initial phase of the sinusoidal potential is equal to 
zero. The ion immediately experiences a negative acceleration and drifts 
in the negative x-direction ad infinitum. The reason for the continued 
drift is that any momentum added to the ion by the electric field can not 
be completely removed. Thus when an ion has an initial velocity, it has a 
tendency to drift in the direction dictated by that initial velocity. The 
net effect of FIGS. 12A and 12B is that if a large number of ions are 
randomly introduced into chamber c at x=0, the ion cloud expands as some 
of the ions oscillate around x=0, and others drift off in the positive 
and/or negative x-directions respectively. 
FIG. 12C illustrates the movement of the ion i when the chamber c contains 
a gas with a pressure of 10.sup.-2 mm Hg or greater. A potential having a 
sinusoidal waveform is again applied to parallel plates p. The motion of 
the ions is dependent on the ion's mobility. Specifically, collisions 
between the ion and molecules of the gas impede the motion of the ion and, 
unlike the vacuum condition of FIGS. 12A and 12B, the ion drifts with a 
drift velocity proportional to the strength of the electric field (to 
which the ion is subjected). The drift velocity V.sub.d is given by a 
mobility coefficient K times the electric field E: 
EQU v.sub.d =.+-.K.multidot.E (2) 
The mobility coefficient K is constant for low electric fields, but 
increases as the electric field strength increases. An approximate 
expression for the dependence of K on the electric field is: 
EQU K(E)=K.sub.0 +K.sub.2 .multidot.E.sup.2 +K.sub.4 .multidot.E.sup.4 + . . . 
(3) 
where the K.sub.1 's are coefficients dependent on the ion species under 
investigation. In a weak electric field, the solid curve in FIG. 12C 
illustrates that ion i oscillates about a fixed position within chamber c. 
The deviations are generally sinusoidal; but since the position is related 
lo the integral of the driving potential (or the drift velocity as 
represented by the broken curve in FIG. 12C), the maximum deviations in 
position are 90 degrees out of phase with the electric field. 
In a strong electric field, FIG. 12D shows that the dependence of the ion 
mobility on electric field causes the drift velocity to peak at the 
extremes of the oscillating potential. That is, the mobility, and hence 
the velocity, of the ion increases and maximizes at the extremes of the 
driving potential. In order for this maximization to occur, the ion must 
gain energy from the electric field in excess of the thermal energy that 
it otherwise has in a low electric field. Because of the energy gain, the 
ion of FIG. 12D migrates with a non-sinusoidal trajectory in volume c. The 
ion still oscillates about a fixed position within chamber c, however, 
because the waveform of the potential is symmetrical during the positive 
and negative phases of the applied potential. The ion experiences 
equivalent accelerations and decelerations. 
The situation changes when an asymmetric potential is applied across the 
plates p of FIG. 11. FIG. 13 illustrates an appropriate waveform. The 
waveform has a period of t.sub.1 +t.sub.2, is constant and positive over 
sub-period t.sub.1, and is constant and negative over sub-period t.sub.2. 
The waveform is also shown to have an absolute value in the positive going 
direction (V.sub.max) much greater than in the negative going direction 
(V.sub.min), and an area defined by the positive going portion of the 
waveform (shaded area a.sub.1) equaling the area defined by the negative 
going portion of the waveform (shaded area a.sub.2). 
Now if chamber c in FIG. 11 is filled with a gas and the waveform of FIG. 
13 is applied across its plates p, the ion may or may not oscillate about 
a fixed position within the device depending upon the absolute values of 
V.sub.max and V.sub.min and the ion type. If the absolute values; are 
below a certain threshold value where the mobility of ion i is independent 
of the electric field strength (i.e., the mobility is always constant), 
the ion i will return to its original position after each completed cycle. 
This will occur regardless of the mobility of the ion. In completing its 
cycle, ion i will travel with velocities v.sub.t1 and v.sub.t2 during time 
periods t.sub.1 and t.sub.2, respectively. The total displacement of the 
ion for each cycle is t.sub.1 .multidot.v.sub.t1 -t.sub.2 
.multidot.v.sub.t2, or t.sub.1 .multidot.K.sub.t1 .multidot.E.sub.t1 
-t.sub.2 .multidot.K.sub.t2 .multidot.E.sub.t2, where K.sub.tx and 
E.sub.tx are the mobility of the ion and the electric field, respectively. 
Because the areas a.sub.1 and a.sub.2 of FIG. 13 are equal, t.sub.1 
.multidot.E.sub.t1 =t.sub.2 .multidot.E.sub.t2, and the total displacement 
is zero. 
If the absolute value of V.sub.max, or both V.sub.max and V.sub.min, exceed 
a certain threshold value, the mobility of the ion becomes dependent on 
the strength of the electric field. That is, if chamber c of FIG. 11 is 
again filled with a gas and the waveform of FIG. 13 is applied to its 
plates p, the mobility of ion i will be different as it travels in one 
direction versus the other. Because of this difference in mobility (and 
hence velocity), the ion will experience a displacement as it responds to 
one complete cycle of the oscillating potential that is given by t.sub.1 
.multidot.K.sub.t1 .multidot.E.sub.t1 -t.sub.2 .multidot.K.sub.t2 
.multidot.E.sub.t2 .noteq.0. The net result is that the ion gradually 
drifts towards one or the other of the plates of FIG. 11. This drift is 
illustrated in FIG. 14 where the light curve is the ion's motion under low 
field conditions, and the dark curve is the ion's motion under high field 
conditions. Since FIG. 14 shows the ion slowly migrating in the negative 
x-direction, it is moving toward the left plate in FIG. 11. 
One Example of an Electrode Configuration for an Ion Mobility Storage Trap 
FIG. 15 illustrates a cross-section of one example for an ion mobility 
storage trap 6 according this invention. The ion mobility storage trap 6 
includes ring electrode 1 and two end-cap electrodes 2, 2' of cylindrical 
geometry. The z-coordinate extends vertically along the symmetry axis 
connecting the two end-caps (positive towards end-cap electrode 2', 
negative towards end-cap electrode 2), the r-coordinate extends radially 
from the z-axis towards ring electrode 1, and the origin is located at the 
center of the trap. The end-cap electrodes 2 and 2' are hyperbolically 
shaped, the apex of each hyperbola lying on the z-axis. As its name 
implies, the ring electrode 1 is ring-shaped with its axis of symmetry 
located along the z-axis. Note that in FIG. 15, the cross-section for the 
ring electrode appears as two opposing hyperbolas. 
The electrodes 1, 2, and 2' define trap volume 7. Aperture grid 18 allows 
ions to leave the trap volume 7 along the z-axis (in the negative 
direction, or downward in FIG. 15). Faraday plate 14 acts as an ion 
collector to collect the ions as they leave the trap. The electric current 
generated by the ions is amplified by a high-gain amplifier or 
electrometer 15. 
A functional ion mobility storage trap has a potential applied between its 
ring electrode 1 and end-caps 2 and 2'. Except where special pulsing 
procedures are used to eject ions, the two end-caps 2 and 2' are connected 
electrically with the same potential. In FIG. 15, the electronics applying 
the potential to the electrodes is simplistically represented by voltage 
generator V.sub.g that may include, for example, one or more of the 
components in FIG. 10. Those skilled in the art of ion trap spectrometry 
will recognize that many types of voltage generators can be applied to the 
trap 6 other than those illustrated in FIG. 10. 
The trap volume 7 contains a neutral gas. Since the operation of the trap 6 
depends on the collisions of ions with neutral gas molecules, gas 
pressures greater than 10.sup.-2 mm Hg are desirable within volume 7. For 
pressures less than atmospheric, a vacuum pump is required to help 
regulate the pressure. The vacuum pump requirement can be removed by 
operating the trap under atmospheric pressure conditions, or slightly 
above. 
The operation of the trap can be explained with assistance from the SIMION 
plot of FIG. 16A and FIG. 16B which is a plot of the equipotential lines 
created by potential V.sub.g within trap volume 7. SIMION is an 
electrostatic lens analysis and design computer program developed by D. C. 
McGilvery at LaTrobe University in Australia, and extensively redesigned 
by D. A. Dahl at Idaho National Engineering Laboratories, Idaho Falls, Id. 
The cross-hatched areas correspond to electrodes 1, 2, and 2' in FIG. 15. 
FIG. 16A illustrates the motion of ion i in a potential of single polarity 
that is applied across ring electrode 1 and end-cap electrodes 2 and 2' 
(the end-cap electrodes are at the same potential). 
Depending on the charge on the ion and whether the potential is positive or 
negative, the ion travels from the ring electrode towards the end-caps, or 
vice versa. In FIG. 16A, ion i is shown traveling from ring electrode 1 
towards end-cap electrode 2'. The trajectory that the ion follows is 
normally perpendicular to the equipotential lines, as indicated by path 
"a" in FIG. 16A. The ion will continue to follow this trajectory until 
either it arrives at end-cap electrode 2', or the polarity of the 
potential is reversed, whichever occurs first. If it arrives at end-cap 
electrode 2', its charge is lost to the electronic circuitry biasing the 
electrodes. 
If, on the other hand, the polarity of the potential is reversed before ion 
i arrives at end-cap 2', the ion will change its direction of motion and 
retrace its path. 
Now when voltage generator V.sub.g applies an asymmetric potential (as 
illustrated in FIG. 13) between ring electrode 1 and end-caps 2 and 2' of 
the trap in FIG. 16A, the motion of the ions is altered. First, the ions 
have a tendency to migrate towards the center of the trap as illustrated 
in FIG. 16B. This occurs because during period t.sub.1, the ions 
experience a high electric field that draws them towards the center of the 
trap. The strength of this electric field is greater than that applied 
during period t.sub.2 that draws the ions away from the center of the trap 
(or towards end-cap electrodes 2 and 2'). Second, the ions have a tendency 
to distribute along the z-axis as illustrated in FIG. 17A. This occurs 
because the strength of the electric field varies within trap volume 7 and 
the ions seek a location where their displacements during time periods 
t.sub.1 and t.sub.2 are equal. More details on this motion will be given 
later in connection with FIGS. 20 to 34. 
Voltage generator V.sub.g also generally superimposes a DC potential on. 
the asymmetric AC potential (by variable DC voltage generator 4 in FIG. 
10, e.g.) of the trap in FIG. 16A and applies the resulting potential 
across the ring electrode 1 and end-cap electrodes 2 and 2'. In this 
example, the DC component shifts the waveform illustrated in FIG. 13 
downward. Typically, the DC potential is selected to counteract the ion 
movement created by the asymmetric electric field. The DC component of the 
potential acts to attract the ions toward end-cap electrodes 2 and 2', the 
AC potential component, as explained above, acts to repel the ions from 
end-cap electrodes 2 and 2'. Depending on the mobility of the ions and the 
AC and DC ratio, the ions will migrate towards an equilibrium position 
within the trap, and oscillate about that equilibrium position. More 
specifically, depending on the change of mobility of the ions as the 
strength of the electric field (here, a function of z), the ions will move 
toward an associated equilibrium position. Thus, samples may be 
continuously introduced into the trap and ionized. The ions of the samples 
will localize along the z-axis, separate according to type and distribute 
about associated equilibrium positions. 
FIG. 17 illustrates how a mixture of ions (i1, i2, i3 and i4) can be 
separated using an ion mobility storage trap of type shown in FIG. 15. In 
FIG. 17A, ion group i1 has a greater difference in mobilities during 
periods t.sub.1 and t.sub.2 ; thus the movement of ion group i1 is 
influenced by the AC component of the electric field at an area where the 
electric field is relatively weak, close to the center of trap volume 7. 
In contrast, at ion group i1's equilibrium position, ion group i4 has a 
lesser difference in mobilities during periods t.sub.1 and t.sub.2. The DC 
component of the electric field functions to pull ions of ion group i4 
closer to the end-cap electrode 2'. As the ions near end-cap electrode 2', 
the electric field strengthens and the difference in the mobility of ions 
of the ion group i4 during periods t.sub.1 and t.sub.2 grows greater. At 
ion group i4's equilibrium position, the difference in the mobility of the 
ions during periods t.sub.1, and t.sub.2 counteract the DC component of 
the electric field, and the ions of ion group i4 oscillate about their 
equilibrium position. The mobility characteristic, of ions i2 and i3 are 
intermediate to those of i1 and i4. 
FIG. 17B illustrates the state of ion groups i1, i2, i3 and i4 after the DC 
component of the electric field is applied. The ion groups have shifted 
along the z-axis towards the end-cap electrode 2'. Ion group i4 has 
migrated out of the trap volume 7 through an opening in the end-cap 
electrode 2' (not shown). The opening may contain an ion collector 
protected by an aperture grid (14 and 18 in FIGS. 10 and 15, e.g.). By 
continuing to increase the DC potential, all the ions can be scanned out 
of trap volume 7. As the ion collector 14 collects the ions leaving trap 
volume 7, the ion current is amplified by an electrometer circuit 15 for 
delivery to a data acquisition system 16. In addition to the DC potential, 
the amplitude or phase relationships within the AC asymmetric potential 
can be used to scan the ions out of the trap. 
This combination of equipment can be used to identify the components of a 
sample matrix that has been injected and ionized (by ionizer 8 in FIG. 15, 
e.g.) in trap volume 7 by monitoring the AC and DC potentials required to 
expel the ions from the trap. Consequently, a sample can be introduced 
into an ion mobility storage trap, the components ionized into 
characteristic ions, and the products ions stored along an axis of the 
trap. The ions are then scanned out of the trap by using a variable DC 
potential, detected by an electrometer amplifier circuit, and analyzed by 
a data acquisition system for content of the original sample. 
The above description applies to ions of either positive or negative 
polarity. The only difference in operation of the trap is the polarity of 
the potentials applied to the trap. Ions of opposite polarity are analyzed 
by inverting the polarity of both the AC asymmetric and DC potentials 
applied across electrodes 1, 2, and 2'. Also because of the symmetry of 
the trap, the ions can be trapped along the r-axis and scanned out of the 
trap through the ring electrode. This is accomplished by changing the 
polarity of either the DC or AC asymmetric potential, but not both. 
To assist in the explanation, the above description describes first 
applying a voltage with only an AC component and then with both AC and DC 
components across the electrodes. However, this is not necessary; a 
voltage with both the AC and DC components may be applied from the start 
of operations. 
FIG. 18 illustrates such a method of operation for the trap. In step S2, a 
sample is introduced into the trap volume. The sample is ionized in step 
S4. The ions are separated according to their mobility characteristics in 
step S6 by applying the appropriate potentials across the electrodes and 
causing the ions to migrate to their equilibrium positions. A decision is 
made in step S8 whether another sample should be introduced (or continued 
to be introduced). If another sample is introduced, the process repeats 
step S2, S4, and S6. If no further samples are introduced, the process 
proceeds to step S10. In step S10, the ions are removed from the trap. The 
ions may be sequentially scanned from the trap by changing one of the DC 
potential components, the AC potential components, or a combination 
thereof. In step S12, the identity of the ions removed from the trap are 
determined and correlated with the sample. Alternatively, in step S12, 
single ion monitoring can be performed, for example, by reversing the DC 
potential, scanning the reversed DC potential to eliminate ions with low 
mobilities, returning the DC potential to normal polarity, and scanning 
the DC potential to a predetermined value to eject the ions of interest. 
As apparent from the above description, other waveforms for the asymmetric 
potential, other than the one displayed in FIG. 13, can be used to excite 
ion motion in the ion mobility storage trap. For example, the waveform 
shown in FIG. 19 is a sinusoidal waveform superimposed on its second 
harmonic and a DC potential. The purpose of the asymmetric waveform is to 
induce in the ions a velocity component that varies in a non-compensating 
manner throughout one complete cycle of the waveform. Thus, the best 
approach to selecting an asymmetric potential is to intuitively focus on 
the dynamic changes that occur in the ion's velocity as it migrates 
through one complete cycle of the waveform and the parameters that prevent 
it from returning to its original position. 
To more fully appreciate the other types of asymmetric potentials that can 
be used to separate ions using the current invention, it is necessary to 
develop a more detailed description for ion motion in an ion mobility 
storage trap. The following provides such a description. As ion i responds 
to the oscillating electromagnetic field, it experiences a variety of 
forces. In addition to the forces that originate from interaction of the 
ion with the electric field, others originate from collisions of the ion 
with neutral gas molecules. For each collision, Newton's Force law of 
classical physics applies and the net force acting on the ion and its 
collision partner satisfies: 
EQU .SIGMA.F=Ma.sub.cm (5) 
where .SIGMA.F is the vector sum of the forces acting on the center-of-mass 
of the ion and its collision partner, M is the total mass of the colliding 
pair, and a.sub.cm is the acceleration of the center-of-mass. If m.sub.1 
is the mass of the collision partner and m.sub.2 is the mass of the ion, 
then the internal force exerted on m.sub.1 by m.sub.2 is F.sub.12 , the 
internal force exerted on m.sub.2 by m.sub.1 is F.sub.21, the external 
force exerted on m.sub.1 is F.sub.ext,1, and the external force exerted on 
m.sub.2 is F.sub.ext,2. When Newton's Force Law is applied to each 
particle, 
EQU F.sub.ext,1 +F.sub.12 =m.sub.1 a.sub.1 (6) 
EQU F.sub.ext,2 +F.sub.21 =m.sub.2 a.sub.2 (7) 
where a.sub.1 and a.sub.2 are the accelerations of m.sub.1 and m.sub.2 in 
the laboratory frame of reference, and 
EQU F.sub.ext,1 +F.sub.ext,2 +F.sub.12 +F.sub.21 =m.sub.1 a.sub.1 +m.sub.2 
a.sub.2 =Ma.sub.cm (8) 
For elastic collisions in three dimensions, F.sub.12 =-F.sub.21 and 
EQU .SIGMA.F.sub.ext =m.sub.1 a.sub.1 +m.sub.2 a.sub.2 =Ma.sub.cm(9) 
But when an ion responds to an electric field (the situation for the 
present invention), motion in only one-direction is of importance. In that 
direction F.sub.12 is not equal to -F.sub.21 and 
##EQU1## 
where the right-hand differential represents a loss in directional 
momentum due to scattering. Mason and McDaniel in their book entitled 
Transport Properties of Ions in Gases (Wiley: New York, 1988) argue that 
this differential is equal to .mu..nu.(.di-elect cons.)v.sub.d (equation 
5-2-8 of Mason and McDaniel), where .mu. is the reduced mass, 
.nu.(.di-elect cons.) is the collision frequency, .di-elect cons. is the 
mean relative energy, and v.sub.d is the drift velocity for the ion. If 
the direction of motion is in the r-direction, equations 8 to 10 become 
(m.sub.2 =m.sub.ion) 
##EQU2## 
where v.sub.d has been written as 
##EQU3## 
and a.sub.cm has been written as 
##EQU4## 
A correction for the laboratory frame of reference has been made. When 
equation 11 is reorganized, it becomes 
##EQU5## 
which is a general relationship describing the motion for the ion. 
Equation 12 reduces to Newton's Force Law when the pressure, and hence 
.nu.(.di-elect cons.), goes to zero; and reduces to v.sub.d =KE (where K 
is the mobility constant) when there is no acceleration. 
According to the Chapman-Enscog theory, the ratio q/.mu..nu.(.di-elect 
cons.) is related to the mobility constant, K, through 
##EQU6## 
where N is the neutral gas density, k is the Boltzmann constant, 
.OMEGA..sup.(1,1) (T.sub.eff) is the collision cross section, T is the 
drift temperature, and T.sub.eff =T +Mv.sub.d.sup.2 /3k is the effective 
ion temperature. When equation 13 is combined with equation 12, the 
expanded equation of motion !becomes 
##EQU7## 
E. W. McDaniel in his book entitled Atomic Collisions: Electron and Photon 
Projectiles (Wiley, New York, 1989) states that the collision cross 
section, .OMEGA..sup.(1,1) (T.sub.eff) is related to the interaction 
potential that accompanies the collision between the ion and the neutral 
gas molecule (assumed to be in excess). J. O. Hirschfelder, C. F. Curtiss 
and R. B. Bird in chapter 3 of their book entitled Molecular Theory of 
Gases and Liquids (Wiley: New York, 1954) state that there are basically 
two types of interactions that can occur: (1) an hard core (or nearly hard 
core since angular momentum plays a role) interaction that is typically 
described by either an infinite potential or a Lennard-Jones (6-12) 
potential, and (2) an induced quadrupole interaction between the ionic 
charge and the neutral molecules. E. A. Mason and H. W. Schamp in a paper 
published in Annals of Physics, volume 4 (1958), pp. 233-270 note that 
when these two interactions are combined, a 12-6-4 potential can be 
defined such that the interaction potential becomes 
##EQU8## 
In equation 15, .di-elect cons.=(e.sup.2 .alpha..sub.p)/(3r.sub.m.sup.2) 
is the depth of the potential energy minimum for the interaction, r.sub.m 
is the ion-neutral separation for the minimum potential energy, 
.alpha..sub.p is the polarizability of the neutral gas, and .gamma. is a 
parameter used to adjust the relative strength of the hard core versus the 
induced quadrupole interaction. For an ion which resonates charge 
throughout its molecular structure, a correction must be made to equation 
15 to account for uncertainties in charge location. In a paper published 
in the Journal of Physics B: Atomic and Molecular Physics, volume 5 
(1972), pp. 169-176, E. A. Mason, H. O'Hara and F. J. Smith proposed a 
core model for this purpose. The core model states that 
##EQU9## 
where "a" is the core diameter (a parameter that may depend upon the 
isomer being investigated). To relate the interaction potentials of 
equations 15 and 16 with the collision cross section, collision theory 
normally breaks the collision cross section, .OMEGA..sup.(1,1) 
(T.sub.eff), into two parts: the hard core cross section, 
.pi.r.sub.m.sup.2, and a multiplicative dimensionless parameter, 
.OMEGA..sup.(1,1) *(T.sub.eff). That is, .OMEGA..sup.(1,1) 
(T.sub.eff)=.pi.r.sub.m.sup.2 .OMEGA..sup.(1,1) *(T.sub.eff). 
.OMEGA..sup.(1,1) *(T.sub.eff) is a function of the effective temperature, 
T.sub.eff, or energy of the ion, and in the case of the core model, the 
core diameter, "a". Values for .OMEGA..sup.(1,1) *(T.sub.eff) are 
available in the open scientific literature for both the 12-6-4 (equation 
15) and the core (equation 16) models described above. G. E. Spangler (the 
inventor assigned to the present invention) has observed that equation 12 
can be used to fit mobility data generated by a conventional linear IMS 
using either the core model of equation 16, or a hard core (defined by 
.OMEGA..sup.(1,1) *(T.sub.eff)=1) model (Scientific Conference on Chemical 
and Biological Defense Research, Aberdeen Proving Ground, Md. November, 
1995; and Fifth International Workshop on Ion Mobility Spectrometry, 
Jackson, Wyo., August 1996). A conventional linear IMS uses a constant 
electric field to separate ions, as opposed to an AC field as described in 
this invention. 
The electric field generated in the IMST of FIG. 15 when using the 
asymmetric potential of FIG. 19 
##EQU10## 
where U is the DC potential, V.sub.1 and V.sub.2 are the magnitudes for 
the superimposed AC potentials, .omega. is the AC frequency, and r.sub.0 
and z.sub.0 are the internal dimensions of the quadrupole trap. When these 
expressions for the electric fields are substituted into equation 14, the 
equation of motion becomes 
##EQU11## 
in the r- and z-directions, respectively. Since T.sub.eff depends on the 
square of the ion velocity, these equations are non-linear and require 
numerical methods for their evaluation. Rosenbrock's techniques can be 
used for this purpose as they are implemented in Mathcad Plus 6.0, a 
computational software package available from MathSoft, Inc., Cambridge, 
Mass. 
The first observation that can be made on equations 19 and 20 is that they 
reduce to the Mathieu equation when the pressure (and hence the number 
density, N) goes to zero and V.sub.2 is set equal to zero. This condition 
corresponds to the condition typically used to separate ions in a 
quadrupole ion trap mass spectrometer (ITMS). FIGS. 20, 21, 22 and 23 show 
solutions to equations 19 and 20 using this condition with the following 
operating parameters: 
EXAMPLES 1-4 
______________________________________ 
Sample Mesitylene 
Carrier Gas Purified Air (absolutely no impurities) 
Pressure 1 .times. 10.sup.-6 mm Hg (Example 1, FIG. 20), 
5 .times. 10.sup.-3 mm Hg (Examples 2, 3, and 4, FIGS. 21, 
22, and 23, respectively) 
Temperature 50.degree. C. 
r.sub.0 5 mm 
z.sub.0 5 mm 
U 0 volts 
V.sub.1 1 volt (Examples 1 and 2) 
2.75 volts (Example 3) 
8.0 volts (Example 4) 
V.sub.2 0 volts 
.omega. 500 kilocycles/sec 
initial conditions r.sub.i = 1 mm, dr.sub.i /dt = 0, z.sub.i = 5 mm, 
dz.sub.i /dt = 0 
collision model hard core 
______________________________________ 
Equation 19 was used to obtain the plots illustrated in FIGS. 20A, 21A, 
22A, acid 23A (showing the movement of the ionized sample in the 
r-direction) and equation 20 was used to obtain the plots illustrated in 
FIGS. 20B, 21B, 22B, and 23B (showing the movement of the ionized sample 
in the z-direction). The mesitylene ion oscillates in both the r- and 
z-directions about r=0 and z=0. The secular frequency for the oscillations 
is 
##EQU12## 
where .DELTA.t is the time difference between points 25 and 26, and points 
27 and 28 in FIGS. 20 and 21. The secular frequencies are approximately 29 
kilocycles/second and 63 kilocycles/second in the r- and z-directions, 
respectively. Similar data for the benzene cation shows that the secular 
frequencies are 126 kilocycles/second and 251 kilocycles/second, 
respectively. That is the secular frequency is mass dependent. 
Superimposed on the secular frequency is a much faster oscillation 
indicated by points 29, 30 and 31, 32 in FIG. 20. The time difference 
between these points corresponds to approximately 500 kilocycles/second, a 
residual of the drive frequency. The amplitude of the secular oscillations 
are fairly pronounced and constant below about 5.times.10.sup.-4 mm Hg. 
However as the pressure is raised, they are damped. This is evident in 
FIG. 21 that illustrates the results for Example 2 with a pressure of 
5.times.10.sup.-3 mm Hg and V.sub.1 =1 volt. 
When V.sub.1 is increased in Example 2 (FIG. 21), the secular frequency 
increases and approaches that for the drive frequency. FIG. 22 shows the 
result for Example 3 where V.sub.1 equals 2.75 volts and the secular 
frequency is about 250 kilocycles/second in the z-direction (FIG. 22A) and 
84 kilocycles/second in the redirection (FIG. 22B). FIG. 23 shows the 
result for Example 4 where V.sub.1 equals 8.0 volts and the secular 
frequency is about 250 kilocycles/second in both directions. Further to 
increases in potential lead only to increased amplitudes for the 
oscillations with no further changes in frequencies. This region of higher 
pressure, potentials and secular frequencies is the region of interest for 
the present invention. 
The ion separation capability for the present invention at higher pressure 
is illustrated in FIGS. 24 to 34. FIGS. 24 to 28 (Examples 5-9, 
respectively) show solutions to equations 19 and 20 using the following 
set of operating conditions: 
EXAMPLES 
______________________________________ 
Sample Mesitylene 
Carrier Gas Purified Air (absolutely no impurities) 
Pressure 200 mm Hg 
Temperature 50.degree. C. 
r.sub.0 5 mm 
z.sub.0 5 mm 
U 0 volts (Examples 5 and 7) 
40 Volts (Examples 6, 8, and 9) 
V.sub.1 1200 volts 
V.sub.2 0 volts (Examples 5 and 6) 
600 Volts (Examples 7-9) 
.omega. 800 kilocycles/sec 
initial conditions r.sub.i = 1 mm, dr.sub.i /dt = 0, z.sub.i = 5 mm, 
dz.sub.i /dt = 0 
(Except z.sub.i = 1 mm for Example 9) 
collision model hard core 
______________________________________ 
For this higher pressure, higher potentials are needed to induce 
oscillations in the ions. Example 5 (FIG. 24) shows that the mesitylene 
ion is not deflected in the trap when the ring-electrode is excited only 
with a symmetrical AC potential of the type V.sub.1 cos .omega.t. Example 
6 (FIG. 25) shows that the mesitylene ion leaves the trap along the z-axis 
if, in addition to the AC excitation of FIG. 24, a DC potential of 40 
volts is added to the ring-electrode. Example 7 (FIG. 26) shows that the 
mesitylene ion collapses toward the center of the trap when the 
ring-electrode is excited with an asymmetric AC potential of the type 
V.sub.1 cos .omega.t+V.sub.2 cos 2.omega.t. Example 8 (FIG. 27) shows that 
the 40 volts DC potential of Example 6 can be used to offset the effects 
of the asymmetric potential in Example 7. Example 9 (FIG. 28) shows that 
the mesitylene ion migrates to the same location regardless of where it is 
formed in the trap. That is, the ion migrates towards z=5 mm, whether it 
is injected at z=5 mm in Example 8 or at z=1 mm in Example 9. 
FIG. 29 shows that the energy (as calculated from Wannier's expression for 
the average ion energy) of the mesitylene ion in Example 8 is a periodic 
function of time with a maximum energy of just under 4 times thermal 
energy. This energy is gained from the electric field that accompanies the 
high potential applied to the trap. For an ion that clusters with 
neutrals, this energy may be sufficient to partially decluster the ion, 
causing the mobility to approach that for a bare ion. 
FIGS. 30 to 34 show solutions to equations 19 and 20 with different ions; 
injected into the trap under the operating conditions of Example 8 (and 
9). These examples show that molecular ions with different mass-to-charge 
ratios occupy different locations within the trap. They are distributed 
along the z-axis in the following order: 
______________________________________ 
MASS-TO-CHARGE INCREASING 
RATIO ION z 
______________________________________ 
18 Water (FIG. 30) .dwnarw. 
58 Acetone (FIG. 31) 
78 Benzene (FIG. 32) 
92 Toluene (FIG. 33) 
106 Xylene (FIG. 34) 
120 Mesitylene (FIG. 27) 
______________________________________ 
The fractionation occurs because the electric field in a hyperbolic trap is 
a function of z, and the degree of fractionation is related to the energy 
gained by the ion from the electric field. Because ions with higher 
molecular weights gain less energy from the electric field, they occupy 
higher values of z. This is consistent with the earlier observation made 
on FIG. 17A where ions with greater mobility differences occupy lower 
values of z. By changing the polarity of one or more of the potentials, 
the ions can also be distributed along the r-axis of the trap. 
Similar separations can also be accomplished under atmospheric pressure 
conditions by simply increasing the potentials involved, lowering AC 
frequencies, and/or reducing trap dimensions. These changes are needed to 
overcome the effects of the increased number of energy sapping collisions 
that occur in this pressure regime. An example of a good set of operating 
parameters for atmospheric pressure conditions is: 
______________________________________ 
Carrier Gas Purified Air (absolutely no impurities) 
Pressure 760 mm Hg 
Temperature 50.degree. C. 
r.sub.0 5 mm 
z.sub.0 5 mm 
U 1 Volt 
V.sub.1 900 Volts 
V.sub.2 450 Volts 
.omega. 200 kilocycles/sec 
______________________________________ 
The pressure utilized in the device may vary for each application. As shown 
by the above examples, it is preferable that the pressure be at least 
5.times.10.sup.-3 mm Hg. However, any pressure greater than this pressure 
can be utilized. The use of atmospheric pressure is particular 
advantageous in that vacuum pumps are not needed to support operation of 
the device. Pressures greater than atmospheric pressure are also possible. 
Returning to the question relating to the asymmetric potential needed to 
excite an ion mobility storage trap, any waveform that allows equation 12 
to be satisfied is satisfactory. Because of the linearity of equation 12 
in the electric field, a linear combination of potentials that 
individually satisfy equation 12 are also satisfactory. For example, a 
linear combination of the waveforms in FIGS. 13 and 19 will provide a 
suitable asymmetric excitation potential, as will a linear combination of 
waveforms derived from FIGS. 13 or 19 with different frequencies. The 
Fourier principle suggests that a modified sawtooth waveform, a modified 
triangular waveform, etc. can be constructed using a weighted sum of the 
waveforms of the type displayed in FIG. 19 with different frequencies. On 
the other hand, not all waveforms will be as easy to generate, or be as 
effective in separating ions, as the waveform examples described in this 
patent. 
Further Examples of Trap Structure 
As will be obvious to those skilled in the art, the trap structure is not 
limited to the configuration of FIG. 15. Any device that creates a 
electromagnetic field in which, at least in one direction (e.g., along an 
axis), the strength of the field changes with location is equally suited. 
Such an electric field allows ions to be distributed along an axis at 
characteristic locations. Additionally, it is preferable that the 
equipotential lines of the electric field be an electrostatic focusing 
field so that the ions can be collected or stored at specific locations 
within the trap. Such a focusing field is created by designing an the 
electrode structure that has two or more poles. Because the device in FIG. 
15 has four poles, it is known as a quadrupole ion trap. As discussed in 
connection with FIG. 9, a quadrupole field can be generated using a 
variety electrode structures. For purposes of this invention, each of 
these electrode structures are equally effective in separating ions in an 
ion mobility storage trap using the techniques just described. The traps 
illustrated in FIGS. 9B and 9C are advantageous from a manufacturing point 
of view in that their simple design would simplify fabrication. As 
discussed in connection with FIG. 8, other electrode structures can be 
used to produce higher order multipolar fields, or a linear combination 
thereof. Again for purposes of this invention, all these electrode 
structures are equally effective for trapping ions in an ion mobility 
storage trap. Because the functional dependence of the electric field upon 
location within the trap depends on the multipolarity of the field 
generated by the electrode structure, it is possible to adjust the range 
of ion mobilities stored in the trap by properly selecting and/or 
designing the electrode structure. This capability lies within the art 
associated with this invention. 
Finally, FIG. 35 illustrates still other designs for an ion trap that works 
for the present invention. The equipotential lines displayed in FIG. 35 
are again derived from SIMION, an electrostatic lens analysis and design 
program developed by D. C. McGilvery at LaTrobe University in Australia 
and extensively redesigned by D. A. Dahl at Idaho National Engineering 
Laboratories, Idaho Falls, Id. An analysis of the equipotential lines 
clearly shows that the traps produce a dipolar field in that only two 
poles are needed to describe the major features of the trap. A dipolar 
trap is not able to separate ions under vacuum conditions. However it is 
able to separate ions for the pressure conditions being considered for 
this invention. The results of FIGS. 16 and 24 to 34 show that only half 
of a quadrupole ion trap is needed to separate ions since the ion 
trajectories reside completely in the top half of the trap. Being half of 
a quadrupole ion trap, a dipolar trap may also be used to separate ions. 
Furthermore, the reduction in the number of parts to construct a dipolar 
trap simplifies its construction. 
FIG. 35A shows a hyperbolically shaped top electrode 33 separated from a 
bottom electrode 34. The bottom electrode 34 comprises a centrally located 
(symmetric al about the z-axis) rectangular well part 34a in a flange part 
34b. Flange part 34b extends outwardly and upwardly towards electrode 33 
from the edge of rectangular well part 34a. The dependence of the strength 
of the electric field on the z-coordinate is adjusted by adjusting the 
depth of the rectangular well part 34a. 
FIG. 35B shows a planar version of FIG. 35A. Replacing the hyperbolic top 
electrode is a flat plate 35 separated from the bottom electrode 36. The 
bottom electrode 36 includes a rectangular well part 36a and flange part 
36b. Flange part 36b extends perpendicularly from the edges of rectangular 
well part 36a such that the surface of the flange part 36b is parallel 
with the opposing surface of top electrode 35. A comparison of the 
equipotential line profiles between FIGS. 35A and 35B shows that the 
simpler construction of the dipolar trap of FIG. 35B can be used to 
provide the same desired z-dependence for the electric field otherwise 
obtained by the more complex electrode structure of FIG. 35A. 
FIG. 35C shows still another example of a dipolar trap constructed using 
three parallel plates 37, 38 and 39. Plate 36 is interposed between plates 
37 and 39. Top electrode is comprised of plate 37. Plates 38 and 39 
comprise the bottom electrode and may be electrically connected to be held 
at the same potential. Plate 38 includes a circular hole 38a symmetrical 
about the z-axis thereby allowing an electric field to extend between 
plates 37 and 39 through hole 38a. Fringe fields radiating horizontally 
between plates 38 and 39 may cause defocussing of the ion cloud. This can 
be corrected by adding other plates (or another structure) between 38 and 
39, if desired. 
In FIG. 35, the sample is introduced into the traps by flowing a carrier 
gas through a hole in either of the top or bottom electrodes or through 
the space between the electrodes. Ions are removed along the z-axis 
through the top electrodes. The ionization of sample and the electrical 
biasing of the top and bottom electrodes is accomplished as described 
above. 
FIG. 36 shows an embodiment for a dipolar ion mobility storage trap that is 
compatible with a variety of ionization sources. FIG. 36A illustrates a 
pulsed photoionization lamp 8 coupled to a quadrupolar storage trap 41. 
The trap is a planar trap sandwiched between two metal flanges, 43 and 44, 
and clamped together using two or more support rods, 45. Two ceramic 
(e.g., Macor) blocks 47 position ring electrode 1 (for example, 
corresponding to electrode 1 in FIG. 9C) relative to end-cap electrodes 2 
and 2' (for example, corresponding to electrodes 2 and 2' in FIG. 9C). 
Teflon sheet or ceramic paper/tape 40 is used to electrically insulate 
ring electrode 1 from the end-caps 2 and 2'. The end-cap electrodes are 
perforated to allow UV irradiation to enter the trap through end-cap 
electrode 2, and ions to leave the trap through end-cap electrode 2'. In 
this capacity, ring electrode 2' also serves as an aperture grid (for 
example, corresponding to 18 in FIG. 15). Ion collector 14 is mounted in 
ceramic (e.g., Macor) insulator 48 encapsulated in metal flange 44. 
Insulator 48 in combination with flange 44 also serves as a flow manifold 
for the sample 49 entering trap volume 41. After the sample is ionized in 
trap volume 41, it exits the trap by flowing around the exterior envelop 
of the photoionization flashlamp 8. Electrometer 15 is directly coupled to 
ion collector 14 to minimize pick-up of stray noise. Thick film resistor 
elements 50 are applied to the exterior surfaces of the ceramic trap for 
heating purposes. 
FIG. 36B is a modification of the ion mobility storage trap of FIG. 36A 
where the photoionization flashlamp source 8 is replaced with a discharge 
ionization source. The discharge is located between electrodes 51 and 52 
where the sample is ionized. Unlike the photoionization flashlamp source, 
the discharge between electrodes 51 and 52 does not irradiate volume 41 of 
quadrupole trap 41 of FIG. 36A. Consequently, trap 41 in FIG. 36B is shown 
as a dipolar trap. This allows ions to be injected through ring electrode 
1 (including a grid for that purpose) and their removal through end-cap 
2'. Electrodes 51 and 52 are biased relative to ring electrode 1 to cause 
the ions created in the discharge gap to migrate towards and enter trap 
volume 41 The ions enter trap volume 41 during at least a portion of the 
AC cycle applied to ring electrode 1. Ion injection may also be assisted 
by pulsing the potential applied to the discharge source or 1:he trap. 
Details for constructing the discharge ionization source are well-known to 
those skilled in the art. Electrode 51 is typically tungsten and electrode 
52 may, for example, be a stainless steel capillary tube. 
FIG. 36C is still another modification of the ion mobility storage trap of 
FIG. 36A where a radioactive source 53 is used for ionization instead of 
photoionization source 8. One example for a radioactive source is a 
Ni.sup.63 isotope electroplated on a nickel foil. The Ni.sup.63 isotope 
emits beta-particles that have sufficient energy to ionize gas contained 
in the irradiated source volume. Again because a radioactive source is 
unable to ionize gas in the full volume 41 of the quadrupole trap shown in 
FIG. 36A, a dipolar trap is shown in FIG. 36C. Shutter grid 54 is provided 
to assist in introducing ions into the trap. Sample can be introduced into 
the trap for ionization either through the collector manifold 49, or 
through orifice 55 leading directly into the irradiated volume containing 
radioactive source 53. 
Sample can be introduced into the ion mobility storage traps (IMST's) of 
FIG. 36 using a variety of techniques. These include, but are not limited 
to, syringe injection, flash evaporation or desorption, purge and 
trapping, and membrane permeation of the sample into ports 49 or 55. 
Additionally a gas chromatographic column can be used to introduce sample 
into the trap. Gas chromatography is a technique whereby a sample is 
pre-separated into its component parts before it is delivered to a 
detector for detection. The pre-separation is accomplished by passing the 
sample through a column whose internal surface is coated with a 
polymerized "liquid" phase. As the sample passes through the column, its 
various components dissolve in the liquid phase and migrate through the 
column as the dissolution process competes with re-evaporation. As the 
sample components exit the column, they are presented to a detector for 
detection. 
Such a gas chromatograph 56 is shown in FIG. 37 where gas chromatographic 
column 57 is shown delivering sample entrained in carrier gas 58 to one or 
more IMST's 59 (one is shown). The sample is introduced into the gas 
chromatographic column 57 by injector 60 at entrance 57a. As known to 
those skilled in the art of gas chromatography, the injector may be a 
syringe injector, a gas sampling loop, a purge-and-trap system, an 
aspirating inlet, or the like. After injection, the sample separates into 
its component parts as it partitions between the flowing carrier gas 60 
and the stationary liquid phase. In FIG. 37, the path followed by the 
sample through the column is shown to first spiral into the center of gas 
chromatograph 58, and after reaching the center, spiraling back out. This 
is a satisfactory flow pattern, but others are also possible depending on 
how the gas chromatograph is constructed. As the sample components migrate 
through gas chromatographic column 57, they eventually exit the column and 
enter ion trap 59 through column exit 57b. Again as known to those skilled 
in the art of gas chromatography, column exit 57b may include a splitter 
to accommodate more than one IMST. 
The gas chromatograph 56 of FIG. 37 may be a commercial chromatograph such 
as is manufactured by Hewlett Packard of Wilmington, Del.; Perkin Elmer of 
Norwalk, Conn.; Shimadzu Scientific Instruments of Columbia, Md.; Varian 
of Walnut Creek, Calif.; or SRI Instruments of Torrance, Calif. For these 
instruments, the IMST would be mounted on the heated detector block and 
the glass capillary column inserted into ports 49 or 55 of FIG. 36 for the 
IMST. Alternatively, the gas chromatograph 56 of FIG. 37 may be a 
micromachined column as described by S. C. Terry in a dissertation written 
in partial fulfillment for a Ph.D. degree from Stanford University in May, 
1975. Terry disclosed procedures for fabricating a gas chromatographic 
column 57 by etching a groove in a silicon wafer, encapsulating the groove 
with a cover plate bonded to the silicon wafer, and coating the 
encapsulated groove (or column) with conventional coating techniques. His 
work is further described in IEEE Transactions on Electron Devices, volume 
ED-26 (December, 1979), pp. 1880-1886 and Scientific American, volume 248 
(April, 1983), pp. 44-55; and was repeated by E. S. Kolesar, et al. in the 
Journal of Microelectromechanical Systems, volume 3 (1994), pp. 134-154. 
While Terry and Kolesar used wet chemical etching techniques to etch their 
column, those skilled in the art of microfabrication know that other 
approaches (such as reactive ion etching) to micromachining silicon are 
possible, particularly if deep etching is required. Sample is delivered to 
the IMST from a micromachined column by an capillary column extension or 
by including the miniature IMST on the micromachined silicon wafer. 
Another application for the IMST is as an ion storage device and 
electromagnetic lens to focus ions from an ionization source onto an ion 
separator. An embodiment for the invention demonstrating this flnction for 
the IMST is shown in FIG. 38. In FIG. 38. IMST 41 serves to desolvate ions 
generated by an electrospray ionization source and focuses the icons onto 
the drift tube 62 of a linear ion mobility spectrometer for additional 
analysis. Electrospray ionization mobility spectrometry is described by D. 
Wittmer, Y. H. Chen, B. K. Luckenbill, and H. H. Hill, Jr. in Analytical 
Chemistry, volume 66 (1994), pp. 2348-2355. An electrospray ionization 
source consists of a syringe needle 63 through which is passed an 
electrolyte, for example from a liquid chromatograph. As the electrolyte 
emerges from the syringe needle, it is exposed to a high electric field 
and the solvent stripping action of a curtain gas 65. In Wittner, et al.'s 
application, volume 41 is a buffer volume that provides additional time 
for desolvation before the ions are submitted to drift tube 62 for 
analysis. The present invention replaces buffer volume 41 with an IMST 
that helps desolvate the ions by oscillating them in asymmetric field. The 
dipolar IMST consists of ring electrode 1 (including grid 64) and end-cap 
electrode 2. Because a high potential is maintained between the 
electrospray ionization source 61 and grid 64, the ring electrode of the 
IMST is not the electrode excited with RF as in the other configurations 
for this invention. Rather the RF potential is applied to end-cap 
electrode 2 which additionally serves in combination with grid 67 as a 
shutter grid to introduce ions into drift tube 62. The ions are stored by 
using predetermined values for the AC and DC components of the potential 
applied across electrodes 1 and 2 by voltage generator Vg. The stored ions 
are pulsed out of the reactor by simultaneously applying an accelerating 
potential between grids 2 and 67, and increasing the DC component of 
voltage source Vg. By providing a delay between the time the DC component 
of voltage source Vg is increased and when the shutter grid 66 is opened, 
ions with a narrow range of mobilities can be introduced into linear drift 
tube. The linear drift tube may be, for example, a stacked-ring drift tube 
as initially described by J. H. Schummers, G. M. Thomson, D. R. James, I. 
R. Gatland E. W. McDaniel in Physical Review A, volume 7 (1973), pp. 
683-688, or a ceramic drift tube containing an inlaid thick film resistor 
as described in U.S. Pat. No. 4,390,784 which issued on Jun. 28, 1983 to 
D. R. Browning, et al. 
As the ions enter drift tube 62, they are attracted towards Faraday plate 
14 due to a potential applied to grid 18. Because the drift tube is filled 
with a drift gas, the speed at which the ions travel through the drift 
tube is determined by their mobilities. The electric field produced within 
drift tube 62 is such that the mobilities of the ions are relatively 
constant, as compared to the mobilities of the ions within volume 41 of 
the IMST. As the ions hit Faraday plate 14, an ion current is generated. 
Based upon the time lapse between the opening of shutter grid 66 and the 
detection of the ion current, the ions may be identified. 
Because the IMST can store ions, the solvated ions can be more effectively 
desolvated before they are introduced into the drift tube. Furthermore 
because the IMST accumulates ions, a greater number of ions can be 
introduced into drift tube 62. Additionally, ions with progressively 
increasing or decreasing mobilities can be delivered to drift tube 62 by 
the IMST to provide an greater resolution in determining mobilities. This 
resolution can be traded for a shorter drift tube that provides the same 
resolution as a conventional ion mobility spectrometer. 
The embodiment of FIG. 38 can also be used without a shutter grid. For that 
configuration, grid 67 is removed. The asymmetric AC and DC voltage 
generator Vg then focuses and stores the ions at specific locations within 
trap volume 41, depending upon the mobility characteristics of the 
specific ions. When the asymmetric AC potential is removed and an 
accelerating potential is applied across electrodes 1 and 2, the ions are 
injected into drift tube 62 for subsequent mobility analysis. 
A final embodiment for the current invention is shown in FIGS. 39 and 40 
where the dipolar IMST is coupled to a mass spectrometer as an atmospheric 
pressure ionizer. The atmospheric pressure ionizer 100 of FIG. 40 is 
similar to that disclosed by E. C. Horning, et al. in a paper published in 
Analytical Chemistry, volume 45 (1973), pp. 936-943. Ionizer 100 uses a 
radioactive source 101 (other ionization sources are possible) to ionize 
sample under atmospheric pressure conditions, and the ions are sampled 
through orifice 102 into the mass spectrometer 103. Because the mass 
spectrometer requires a hard vacuum for operation, such a system typically 
uses fast vacuum pumps to draw the ions through a 10 to 100 micron pinhole 
or tube into the mass spectrometer using flowing gas. With one exception, 
there is usually no provision to focusing the ions formed by ionizer 100 
onto the ion sampling orifice 102 on the atmospheric pressure side. The 
exception is a corona discharge that enriches the ion concentration in the 
vicinity of the pinhole leading into the mass spectrometer. The current 
invention provides such a capability where a dipolar IMST is used to focus 
the ions onto the pinhole. The dipolar IMST comprises an electrode 
structure placed within atmospheric pressure ionizer 100 and a power 
source to apply an asymmetric AC and DC potential across the electrodes. 
Specifically, a first electrode 110 may define a majority of the 
atmospheric pressure ionizer chamber, while a second electrode 108 may 
define one side of the atmospheric pressure ionizer 100. The second 
electrode 108 includes pinhole 102. Under the influence of the applied 
asymmetric AC and DC potentials 104, the ions migrate toward the center of 
the ionizer 100 (i.e., r=0 or the z-axis which extends from orifice 102 to 
sample inlet 105) where they are introduced into orifice 102 by simply 
increasing the DC component of potential 104. 
FIG. 41 shows an ionizer 100 similar that in FIG. 40 coupled to an ion trap 
mass spectrometer 106. The ion trap mass spectrometer consists of 
ring-electrode 107, end-car, electrodes 108, and electron multiplier 109. 
Applied to ring-electrode 107 is a symmetric AC plus DC potential that can 
be scanned to produce a mass spectrum as taught in U.S. Pat. No. 4,540,884 
which issued on Sep. 10, 1985 to G. C. Stafford, P. E. Kelley and D. R. 
Stephens. In addition, an auxiliary AC can be applied to ring-electrode 
107 to allow operation of the trap in MS/MS mode as taught by U.S. Pat. 
No. 4,736,101 which issued on Apr. 5, 1988 to J. E. P. Syka, J. N. Louris, 
P. E. Kelley, G. C. Stafford and W. E. Reynolds along with corrections 
describe by U.S. Pat. No. Revision 34,000 issued on Jul. 21, 1992. Ionizer 
100 is a planar dipolar IMST including ring-electrode 110 and end-cap 
electrode 108. Asymmetric AC and DC potentials 111 are applied to the 
ring-electrode 110 to cause the ions to migrate to the center of dipolar 
IMST 100. The ions are then scanned through aperture 112 for mass analysis 
by the ion trap mass spectrometer 106 by increasing the DC component of 
potential 111. 
Manufacturing of the above disclosed trap electrode structures may be 
performed by microfabrication. Microfabrication is advantageous in order 
that a lower voltage may be applied across the electrodes which is still 
able to produce an electric field of sufficient strength due to the 
proximity of the electrodes. The dipolar ion mobility storage traps 
illustrated in FIG. 35 are particularly suited for microfabrication due to 
their simple structure. As compared to other spectrometry devices, 
micromachining is especially easy as the electrode structure of the ion 
mobility storage traps does not need to be extremely precise. Chemical 
etching, plasma etching, laser etching and LIGA several examples of 
micromachining techniques that may be used to properly shape electrodes of 
the ion mobility storage traps. 
In order to compensate for the lower amount of ions generated by a 
microfabricated storage trap (due to its size, e.g.), several 
microfabricated storage traps may be used in combination as an array. For 
example, the device illustrated in FIG. 37 may include a plurality of 
storage traps manufactured on a silicon wafer. 
The above description of the examples of the invention describe :specific 
storage trap examples in combination with specific ion mobility storage 
trap systems (such as those illustrated in FIGS. 10A and 10B), in 
combination with specific ionization sources, and in combination with 
other spectrometry devices. Additional embodiments of the invention 
include the replacement of the described examples of the storage traps, 
the trap systems, the ionization sources and other spectrometry devices 
with any of the corresponding elements described elsewhere in the 
specification. 
Also, the above description is intended only to exemplify the invention. 
Modifications and variations of these examples will be obvious to those 
skilled in the art which still achieve the spirit of this invention. For 
example, the above description describes two specific examples of systems 
which can be used to provide the appropriate voltages to traps. However, 
these examples may be replaced by any voltage source which achieves the 
appropriate electric field within a trap volume. In addition, the 
description of the trap structures are only several detailed examples; 
many different trap structures will be apparent to those skilled in the 
art which fall within the scope of this invention. 
Also, the above description details the positioning or localizing of the 
ions along an axis (or along a line or curve), the separation of the ions 
and the storage of the ions with respect to several different examples. It 
is intended that any of the disclosed examples may be utilized to perform 
only one of these functions or any combination thereof. Similarly, it is 
intended that one or more of these functions may be encompassed by other 
devices which fall within the scope of this invention. 
It is intended that any of the disclosed storage trap examples may be used 
with any voltage generation source, and/or any ion mobility storage trap 
system, and/or in combination with any of the disclosed spectrometry 
devices (such as the gas chromatographic column, the drift tube or mass 
spectrometer) and/or other spectrometry devices, and/or with any type of 
ionization source. Many other modifications other than those specifically 
mentioned here will be obvious to those skilled in the art. 
The above detailed descriptions of the examples of the invention are for 
illustrative purposes. Modifications and variations of these examples will 
be obvious to those skilled in the art which still achieve the spirit and 
scope of the present invention.