U.S. patent application number 10/221480 was filed with the patent office on 2003-03-27 for parallel plate geometry faims apparatus and method.
Invention is credited to Guevremont, Roger, Purves, Randy.
Application Number | 20030057367 10/221480 |
Document ID | / |
Family ID | 26884759 |
Filed Date | 2003-03-27 |
United States Patent
Application |
20030057367 |
Kind Code |
A1 |
Guevremont, Roger ; et
al. |
March 27, 2003 |
Parallel plate geometry faims apparatus and method
Abstract
A method and an apparatus is disclosed for selectively
transmitting ions in which the ions are subjected to a focusing
effect such that the overall efficiency of ion transmission is
increased. The method relies on a FAIMS analyzer having at least an
electrode disposed approximately between two other electrode
surfaces for producing an electric field that is symmetric on
opposing sides of the at least an electrode. An ion exiting from
between the electrodes experiences a balanced electric field, such
that impact with one of the electrodes is prevented.
Inventors: |
Guevremont, Roger; (Ontario,
CA) ; Purves, Randy; (Ontario, CA) |
Correspondence
Address: |
Freedman & Associates
Suite 350
117 Centrepointe Drive
Nepean
K2G 5X3
CA
|
Family ID: |
26884759 |
Appl. No.: |
10/221480 |
Filed: |
September 13, 2002 |
PCT Filed: |
March 14, 2001 |
PCT NO: |
PCT/CA01/00308 |
Current U.S.
Class: |
250/281 |
Current CPC
Class: |
G01N 27/624
20130101 |
Class at
Publication: |
250/281 |
International
Class: |
H01J 049/00 |
Claims
What is claimed is:
1. An apparatus for separating ions, comprising a high field
asymmetric waveform ion mobility spectrometer, including: a) an
analyzer region comprising: a first electrode, a second electrode
and a third electrode in a spaced apart stacked arrangement for
allowing ion flow therebetween, the first electrode having a first
inner surface, the second electrode having a first and a second
surface on opposite sides thereof, the third electrode having a
second inner surface; and, at least a contact on at least one of
the first, second and third electrodes, for receiving a
compensation voltage potential between the second and first
electrodes and between the second and third electrodes, and for
applying an asymmetric waveform to at least one of the electrodes,
wherein, in use, ions exiting from between the electrodes are other
than attracted to the second electrode to collide therewith.
2. An apparatus according to claim 1 comprising a fourth electrode
and a fifth electrode in stacked arrangement, the fourth and fifth
electrodes on opposing sides of the first and third electrode from
the second electrode.
3. An apparatus according to claim 2 wherein a voltage is applied
to at lease one of the second electrode and the first, third,
fourth and fifth electrodes for forming a focussing/trapping region
proximate ends of the electrodes.
4. An apparatus according to claim 1, wherein the first surface and
the second surface of the second electrode are joined by a smooth
curved surface therebetween at two opposite edges of the second
electrode.
5. An apparatus according to claim 4, wherein the second electrode
is disposed substantially midway between the first and third
electrodes such that an approximately uniform spacing is maintained
between the first inner surface of the first electrode and the
first surface of the second electrode and a substantially same
approximately uniform spacing is maintained between the second
surface of the second electrode and the second inner surface of the
third electrode.
6. An apparatus according to claim 5, wherein the second electrode
has a concave curve in at least an end thereof.
7. An apparatus according to claim 6, wherein the second electrode
is electrically isolated from the first and third electrodes.
8. An apparatus according to claim 5, wherein the first inner
surface of the first electrode, the first and the second surfaces
of the second electrode and the second inner surface of the third
electrode are parallel flat surfaces.
9. An apparatus according to claim 5, wherein the first inner
surface of the first electrode, the first and the second surfaces
of the second electrode and the second inner surface of the third
electrode are curved surfaces.
10. An apparatus according to claim 9, wherein the first surface of
the second electrode is curved in a first direction toward the
first inner surface of the first electrode, and the second inner
surface of the third electrode is curved in a same first direction
toward the second surface of the second electrode, the curved
surfaces for providing a first electrical field for transmitting
selectively ions between the first electrode and the second
electrode, and for providing a second different electrical field
for transmitting selectively other different ions between the
second electrode and the third electrode.
11. An apparatus according to claim 9, wherein the first surface of
the second electrode is curved in a first direction toward the
first inner surface of the first electrode, and the second other
surface of the second electrode is curved in a second substantially
opposite direction toward the second inner surface of the third
electrode, the curved surfaces for providing an electrical field
for transmitting selectively an ion between the first electrode and
the second electrode, and for providing a substantially similar
electrical field for transmitting selectively a same ion between
the second electrode and the third electrode.
12. An apparatus according to claim 8, comprising at least one
ionization source for producing ions.
13. An apparatus according to claim 12, wherein the analyzer region
comprises a gas inlet and a gas outlet for providing a flow of gas
through the analyzer region; and, an ion inlet for introducing ions
produced by the ionization source into the analyzer region and an
ion outlet for extracting ions transmitted selectively through the
analyzer region.
14. An apparatus according to claim 13, wherein each of the ion
inlet and the ion outlet is aligned with the center of the at least
an end of the second electrode.
15. An apparatus according to claim 14, comprising an electrical
controller electrically couplable to the at least a contact on the
at least one of the first, second and third electrodes, the
electrical controller for supplying an asymmetric waveform and a
direct-current compensation voltage for selectively transmitting at
least a selected ion type in said analyzer region at a given
combination of asymmetric waveform and compensation voltage.
16. An apparatus according to claim 9, comprising at least one
ionization source for producing ions.
17. An apparatus according to claim 16, wherein the analyzer region
comprises a gas inlet and a gas outlet for providing a flow of gas
through the analyzer region; and, an ion inlet for introducing ions
produced by the ionization source into the analyzer region and an
ion outlet for extracting ions transmitted selectively through the
analyzer region.
18. An apparatus according to claim 17, wherein each of the ion
inlet and the ion outlet is aligned with the center of the at least
an end of the second electrode.
19. An apparatus according to claim 18, comprising an electrical
controller electrically couplable the at least a contact on the at
least one of the first, second and third electrodes, the electrical
controller for supplying an asymmetric waveform and a
direct-current compensation voltage for selectively maintaining at
least a selected ion type in said analyzer region within the gas
flow at a given combination of asymmetric waveform and compensation
voltage.
20. An apparatus for separating ions, comprising a high field
asymmetric waveform ion mobility spectrometer, including: a) an
analyzer region comprising: a first electrode, a second electrode
and a third electrode in a spaced apart stacked arrangement for
allowing ion flow therebetween, the first electrode having a first
inner surface, the second electrode having a continuous smoothly
curved surface, the third electrode having a second inner surface;
and, at least a contact on at least one of the first, second and
third electrodes, for receiving a compensation voltage potential
between the second and first electrodes and between the second and
third electrodes, and for applying an asymmetric waveform to at
least one of the electrodes.
21. An apparatus according to claim 20, wherein the second
electrode is disposed substantially midway between the first and
third electrodes such that an approximately uniform spacing is
maintained between the first inner surface of the first electrode
and the continuous surface of the second electrode and a
substantially same approximately uniform spacing is maintained
between the continuous surface of the second electrode and the
second inner surface of the third electrode.
22. An apparatus according to claim 21, wherein the first electrode
encircles a first region of the continuous surface of the second
electrode, and the second electrode encircles a second different
region of the continuous surface of the second electrode.
23. An apparatus according to claim 22, wherein the first and third
electrodes in cross section form a single contiguous non-continuous
surface.
24. An apparatus according to claim 22, wherein the first and third
electrodes form a contiguous surface that defines a cylinder having
openings in other than the circular sides thereof, for forming a
gas inlet, a gas outlet, an ion inlet and, an ion outlet.
25. An apparatus according to claim 22, wherein the second
electrode is substantially a cylinder.
26. An apparatus according to claim 25, wherein the second
electrode is electrically isolated from the first and third
electrodes.
27. An apparatus according to claim 26, comprising at least one
ionization source for producing ions.
28. An apparatus according to claim 27, wherein the analyzer region
comprises a gas inlet and a gas outlet for providing, in use, a
flow of gas through the analyzer region; and, an ion inlet for
introducing ions produced by the ionization source into the
analyzer region and an ion outlet for extracting ions transmitted
selectively through the analyzer region.
29. An apparatus according to claim 28, comprising an electrical
controller electrically couplable to the at least a contact on the
at least one of the first, second and third electrodes, the
electrical controller for supplying an asymmetric waveform and a
direct-current compensation voltage for selectively transmitting at
least a selected ion type in said analyzer region at a given
combination of asymmetric waveform and compensation voltage.
30. An apparatus for separating ions, comprising a high field
asymmetric waveform ion mobility spectrometer, including: a) an
analyzer region comprising: a first electrode having a first
surface and a second surface on opposite sides thereof; a second
electrode absent a cross section thereof forming a closed surface
about a non contiguous centre region, the second electrode shaped
such that a first region of the surface defines a first inner
surface of the second electrode that opposes the first surface of
the first electrode and a second other region of the surface
defines a second inner surface of the second electrode that opposes
the second surface of the first electrode; and, at least a contact
on at least one of the first and second electrodes, for applying a
compensation voltage potential between the second and first
electrodes, and for receiving an asymmetric waveform to at least
one of the electrodes, wherein, in use, ions exiting from between
the electrodes are other than attracted to the first electrode,
such that the ions other than collide therewith.
31. An apparatus according to claim 30, wherein the first electrode
is disposed substantially midway between the first inner surface of
the second electrode and the second inner surface of the second
electrode, such that an approximately uniform spacing is maintained
between the first inner surface of the second electrode and the
first surface of the first electrode and a substantially same
approximately uniform spacing is maintained between the second
inner surface of the second electrode and the second surface of the
first electrode.
32. An apparatus according to claim 31, wherein the first surface
and the second surface of the first electrode are joined by a
smooth curve therebetween at two opposite edges of the first
electrode.
33. An apparatus according to claim 32, wherein the first electrode
has a concave curve in at least an end thereof.
34. An apparatus according to claim 31, wherein the first electrode
is electrically isolated from the second electrode.
35. An apparatus according to claim 31, wherein the first region of
the surface of the second electrode, the second other region of the
surface of the second electrode, and the first and second surfaces
of the first electrode on opposite sides thereof, are parallel flat
surfaces.
36. An apparatus according to claim 35, comprising an electrical
controller electrically couplable to the at least a contact on the
at least one of the first and second electrodes, the electrical
controller capable of supplying an asymmetric waveform and a
direct-current compensation voltage for selectively maintaining at
least a selected ion type within said analyzer region between the
electrodes at a given combination of asymmetric waveform and
compensation voltage.
37. An apparatus according to claim 34, wherein the first region of
the surface of the second electrode, the second other region of the
surface of the second electrode, and the first and second surfaces
of the first electrode on opposite sides thereof are curved
surfaces.
38. An apparatus according to claim 37, comprising an electrical
controller electrically couplable to the at least a contact on the
at least one of the first and second electrodes, the electrical
controller capable of supplying an asymmetric waveform and a
direct-current compensation voltage for selectively maintaining at
least a selected ion type in said analyzer region at a given
combination of asymmetric waveform and compensation voltage.
39. An analyzer comprising: a first electrode having in cross
section an approximately continuous periphery; a second electrode
having in cross section an approximately continuous periphery
approximately equidistant from the first electrode over a region
thereof and having at least an inlet for introduction of ions and a
carrier gas and at least an outlet in the approximately continuous
periphery; and, a contact on at least one of the first and second
electrode for providing an asymmetric electric field between the
first and second electrode; wherein, in use, ions flow through the
at least an inlet about the approximately continuous periphery of
the first electrode and out the at least an outlet wherein a
similar electric field is present on opposing sides of the first
electrode at an end proximate the at least an outlet.
40. An analyzer according to claim 39, wherein the first electrode
has an approximately continuous periphery along any cross section
thereof.
41. An analyzer according to claim 40, wherein the first electrode
is cylindrical and the second electrode is a concentric cylinder
and wherein, in use, ions flow about the circular cross section of
the first electrode from an inlet on one side of the circular cross
section to an outlet on a second opposing side of the circular
cross section.
42. A method for separating ions comprising the steps of: a)
providing at least an ionization source for producing ions
including two ionic species; b) providing an analyzer region
defined by a first analyzer region between a first electrode
surface on a first electrode and an opposing second electrode
surface on a second electrode; c) providing an asymmetric waveform
and a direct-current compensation voltage, to at least one of said
electrode surfaces, to form an electric field between the opposing
pair of electrode surfaces; d) setting said asymmetric waveform in
order to effect a difference in net displacement between said two
ionic species in the time of one cycle of said applied asymmetric
waveform; and, e) setting said compensation voltage to a determined
value to support selective transmission of one of said two ionic
species within the analyzer region; f) providing ions form the
ionisation source into the analyzer region; g) conducting the ions
through the analyzer region toward an outlet therefrom; and, h)
providing an electric field on an side of the first electrode
opposing the first surface wherein ions exiting from the outlet
from the analyzer region are other than attracted to the first
electrode, such that the ions other than collide therewith.
43. A method according to claim 42 comprising the steps of:
providing a second different analyzer space between a third
electrode surface on the first electrode and an opposing fourth
electrode surface other than on the first electrode, the first
electrode surface and the third electrode surface disposed on
opposing sides of the first electrode.
44. A method according to claim 42, comprising the additional step
of focusing ions received from the ionisation source at an inlet
end of the first electrode and focusing ions provided to the ion
outlet at an outlet end of the same first electrode.
45. A method according to claim 44, comprising the additional step
of providing an approximately uniform space between the first
electrode surface and the opposing second electrode surface, and
providing a substantially same approximately uniform space between
the third electrode surface and the opposing fourth electrode
surface such that the field is one of approximately symmetrical and
approximately inversely symmetrical on both sides of the same first
electrode.
46. A method according to claim 43, comprising the additional step
of detecting the selectively focused ions from the analyzer region
and received at the ion outlet.
47. A method according to claim 43 wherein the step of conducting
the ions comprises the steps of providing a carrier gas flow within
the analyzer region.
48. A method for separating ions comprising the steps of: a)
providing at least an ionization source for producing ions
including two ionic species; b) providing two analyzer regions on
opposing sides of an electrode, said two analyzer regions being in
communication with a gas inlet, a gas outlet and an ion inlet, said
ions produced by said ionization source being introduced into said
two analyzer regions at said ion inlet; c) forming an electric
field on opposing sides of the electrode by providing an asymmetric
waveform and a direct-current compensation voltage to the
electrode; d) setting said asymmetric waveform in order to effect a
difference in net displacement between said two ionic species in
the time of one cycle of said applied asymmetric waveform; and, e)
setting said compensation voltage to a determined value to
selectively focus one of said two ionic species.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an apparatus and method for
separating ions, more particularly the present invention relates to
an apparatus and method for separating ions based on the ion
focusing principles of high field asymmetric waveform ion mobility
spectrometry (FAIMS).
BACKGROUND OF THE INVENTION
[0002] High sensitivity and amenability to miniaturization for
field-portable applications have helped to make ion mobility
spectrometry (IMS) an important technique for the detection of many
compounds, including narcotics, explosives, and chemical warfare
agents as described, for example, by G. Eiceman and Z. Karpas in
their book entitled "Ion Mobility Spectrometry" (CRC, Boca Raton,
1994). In IMS, gas-phase ion mobilities are determined using a
drift tube with a constant electric field. Ions are gated into the
drift tube and are subsequently separated in dependence upon
differences in their drift velocity. The ion drift velocity is
proportional to the electric field strength at low electric field
strength, for example 200 V/cm, and the mobility, K, which is
determined from experimentation, is independent of the applied
electric field. Additionally, in IMS the ions travel through a bath
gas that is at sufficiently high pressure such that the ions
rapidly reach constant velocity when driven by the force of an
electric field that is constant both in time and location. This is
to be clearly distinguished from those techniques, most of which
are related to mass spectrometry, in which the gas pressure is
sufficiently low that, if under the influence of a constant
electric field, the ions continue to accelerate.
[0003] E. A. Mason and E. W. McDaniel in their book entitled
"Transport Properties of Ions in Gases" (Wiley, New York, 1988)
teach that at high electric field strength, for instance fields
stronger than approximately 5,000 V/cm, the ion drift velocity is
no longer directly proportional to the applied field, and K becomes
dependent upon the applied electric field. At high electric field
strength, K is better represented by K.sub.h, a non-constant high
field mobility term. The dependence of K.sub.h on the applied
electric field has been the basis for the development of high field
asymmetric waveform ion mobility spectrometry (FAIMS), a term used
by the inventors throughout this disclosure, and also referred to
as transverse field compensation ion mobility spectrometry, or
field ion spectrometry. Ions are separated in FAIMS on the basis of
a difference in the mobility of an ion at high field strength,
K.sub.h, relative to the mobility of the ion at low field strength,
K. In other words, the ions are separated because of the compound
dependent behavior of K.sub.h as a function of the applied electric
field strength. FAIMS offers a new tool for atmospheric pressure
gas-phase ion studies since it is the change in ion mobility, and
not the absolute ion mobility, that is being monitored.
[0004] The principles of operation of FAIMS using flat plate
electrodes have been described by I. A. Buryakov, E. V. Krylov, E.
G. Nazarov and U. Kh. Rasulev in a paper published in the
International Journal of Mass Spectrometry and Ion Processes;
volume 128 (1993), pp. 143-148, the contents of which are herein
incorporated by reference. The mobility of a given ion under the
influence of an electric field is expressed by: K.sub.h=K(1+f(E)),
where K.sub.h is the mobility of an ion at high electrical field
strength, K is the coefficient of ion mobility at low electric
field strength and f(E) describes the functional dependence of the
ion mobility on the electric field strength. Ions are classified
into one of three broad categories on the basis of a change in ion
mobility as a function of the strength of an applied electric
field, specifically: the mobility of type A ions increases with
increasing electric field strength; the mobility of type C ions
decreases; and, the mobility of type B ions increases initially
before decreasing at yet higher field strength. The separation of
ions in FAIMS is based upon these changes in mobility at high
electric field strength. Consider an ion, for example a type A ion,
which is being carried by a gas stream between two spaced-apart
parallel plate electrodes of a FAIMS device. The space between the
plates defines an analyzer region in which the separation of ions
occurs. The net motion of the ion between the plates is the sum of
a horizontal x-axis component due to the flowing stream of gas and
a transverse y-axis component due to the electric field between the
parallel plate electrodes. The term "net motion" refers to the
overall translation that the ion, for instance said type A ion,
experiences, even when this translational motion has a more rapid
oscillation superimposed upon it. Often, a first plate is
maintained at ground potential while the second plate has an
asymmetric waveform, V(t), applied to it. The asymmetric waveform
V(t) is composed of a repeating pattern including a high voltage
component, V.sub.1, lasting for a short period of time t.sub.2 and
a lower voltage component, V.sub.2, of opposite polarity, lasting a
longer period of time t.sub.1. The waveform is synthesized such
that the integrated voltage-time product, and thus the field-time
product, applied to the plate during each complete cycle of the
waveform is zero, for instance V.sub.1t.sub.2+V.sub.2t.sub.1=0; for
example +2000 V for 10 .mu.s followed by -1000 V for 20 .mu.s. The
peak voltage during the shorter, high voltage portion of the
waveform is called the "dispersion voltage" or DV in this
disclosure.
[0005] During the high voltage portion of the waveform, the
electric field causes the ion to move with a transverse y-axis
velocity component v.sub.1=K.sub.hE.sub.high, where E.sub.high is
the applied field, and K.sub.h is the high field ion mobility under
ambient electric field, pressure and temperature conditions. The
distance traveled is
d.sub.1=v.sub.1t.sub.2=K.sub.hE.sub.hight.sub.2, where t.sub.2 is
the time period of the applied high voltage. During the longer
duration, opposite polarity, low voltage portion of the asymmetric
waveform, the y-axis velocity component of the ion is
v.sub.2=KE.sub.low, where K is the low field ion mobility under
ambient pressure and temperature conditions. The distance traveled
is d.sub.2=v.sub.2t.sub.1=KE.sub.lowt.s- ub.1. Since the asymmetric
waveform ensures that (V.sub.1t.sub.2)+(V.sub.2- t.sub.1)=0, the
field-time products E.sub.hight.sub.2 and E.sub.lowt.sub.1 are
equal in magnitude. Thus, if K.sub.h and K are identical, d.sub.1
and d.sub.2 are equal, and the ion is returned to its original
position along the y-axis during the negative cycle of the
waveform, as would be expected if both portions of the waveform
were low voltage. If at E.sub.high the mobility K.sub.h>K, the
ion experiences a net displacement from its original position
relative to the y-axis. For example, positive ions of type A travel
farther during the positive portion of the waveform, for instance
d.sub.1>d.sub.2, and the type A ion migrates away from the
second plate. Similarly, positive ions of type C migrate towards
the second plate.
[0006] If a positive ion of type A is migrating away from the
second plate, a constant negative dc voltage can be applied to the
second plate to reverse, or to "compensate" for, this transverse
drift. This dc voltage, called the "compensation voltage" or CV in
this disclosure, prevents the ion from migrating towards either the
second or the first plate. If ions derived from two compounds
respond differently to the applied high strength electric fields,
the ratio of K.sub.h to K may be different for each compound.
Consequently, the magnitude of the CV necessary to prevent the
drift of the ion toward either plate is also different for each
compound. Thus, when a mixture including several species of ions is
being analyzed by FAIMS, only one species of ion is selectively
transmitted for a given combination of CV and DV. The remaining
species of ions, for instance those ions that are other than
selectively transmitted through FAIMS, drift towards one of the
parallel plate electrodes of FAIMS and are neutralized. Of course,
the speed at which the remaining species of ions move towards the
electrodes of FAIMS depends upon the degree to which their high
field mobility properties differ from those of the ions that are
selectively transmitted under the prevailing conditions of CV and
DV.
[0007] An instrument operating according to the FAIMS principle as
described previously is an ion filter, capable of selective
transmission of only those ions with the appropriate ratio of
K.sub.h to K. In one type of experiment using FAIMS devices, the
applied CV is scanned with time, for instance the CV is slowly
ramped or optionally the CV is stepped from one voltage to a next
voltage, and a resulting intensity of transmitted ions is measured.
In this way a CV spectrum showing the total ion current as a
function of CV, is obtained. It is a significant limitation of
early FAIMS devices, which used electrometer detectors, that the
identity of peaks appearing in the CV spectrum are other than
unambiguously confirmed solely on the basis of the CV of
transmission of a species of ion. This limitation is due to the
unpredictable, compound-specific dependence of K.sub.h on the
electric field strength. In other words, a peak in the CV spectrum
is easily assigned to a compound erroneously, since there is no way
to predict or even to estimate in advance, for example from the
structure of an ion, where that ion should appear in a CV spectrum.
In other words, additional information is necessary in order to
improve the likelihood of assigning correctly each of the peaks in
the CV spectrum. For example, subsequent mass spectrometric
analysis of the selectively transmitted ions greatly improves the
accuracy of peak assignments of the CV spectrum.
[0008] In U.S. Pat. No. 5,420,424 which issued on May 30 1995, B.
L. Carnahan and A. S. Tarassove disclose an improved FAIMS
electrode geometry in which the flat plates that are used to
separate the ions are replaced with concentric cylinders, the
contents of which are herein incorporated by reference. The
concentric cylinder design has several advantages, including higher
sensitivity compared to the flat plate configuration, as was
discussed by R. W. Purves, R. Guevremont, S. Day, C. W. Pipich, and
M. S. Matyjaszczyk in a paper published in Reviews of Scientific
Instruments; volume 69 (1998), pp 4094-4105. The higher sensitivity
of the cylindrical FAIMS is due to a two-dimensional atmospheric
pressure ion focusing effect that occurs in the analyzer region
between the concentric cylindrical electrodes. When no electrical
voltages are applied to the cylinders, the radial distribution of
ions should be approximately uniform across the FAIMS analyzer.
During application of DV and CV, however, the radial distribution
of ions is not uniform across the annular space of the FAIMS
analyzer region. Advantageously, with the application of an
appropriate DV and CV for an ion of interest, those ions become
focused into a band between the electrodes and the rate of loss of
ions, as a result of collisions with the FAIMS electrodes, is
reduced. The efficiency of transmission of the ions of interest
through the analyzer region of FAIMS is thereby improved as a
result of this two-dimensional ion focusing effect.
[0009] The focussing of ions by the use of asymmetric waveforms has
been discussed above. For completeness, the behavior of those ions
that are not focussed within the analyzer region of a cylindrical
geometry FAIMS is described here, briefly. As discussed previously,
those ions having high field ion mobility properties that are other
than suitable for focussing under a given set of DV, CV and
geometric conditions will drift toward one or another wall of the
FAIMS device. The rapidity with which these ions move towards the
wall depends on the degree to which their K.sub.h/K ratio differs
from that of the ion that is transmitted selectively under the
prevailing conditions. At the very extreme, ions of completely the
wrong property, for instance a type A ion versus a type C ion, are
lost to the walls of the FAIMS device very rapidly.
[0010] The loss of ions in FAIMS devices should be considered one
more way. If an ion of type A is focussed, for example at DV 2500
volts, CV -11 volts in a given geometry, it would seem reasonable
to expect that the ion is also focussed if the polarity of DV and
CV are reversed, for instance DV of -2500 volts and CV of +11
volts. This, however, is not observed and in fact the reversal of
polarity in this manner creates a mirror image effect of the
ion-focussing behavior of FAIMS. The result of such polarity
reversal is that the ions are not focussed, but rather are
extremely rapidly rejected from the device. The mirror image of a
focussing valley, is a hill-shaped potential surface. The ions
slide to the center of the bottom of a focussing potential valley
(2 or 3-dimensions), but slide off of the top of a hill-shaped
surface, and hit the wall of an electrode. This is the reason for
the existence, in the cylindrical geometry FAIMS, of the
independent "modes" called 1 and 2. Such a FAIMS instrument is
operated in one of four possible modes: P1, P2, N1, and N2. The "P"
and "N" describe the ion polarity, positive (P) and negative (N).
The waveform with positive DV, where DV describes the peak voltage
of the high voltage portion of the asymmetric waveform, yields
spectra of type P1 and N2, whereas the reversed polarity negative
DV, waveform yields P2 and N1. The discussion thus far has
considered positive ions but, in general, the same principles apply
to negative ions equally.
[0011] A further improvement to the cylindrical FAIMS design is
realized by providing a curved surface terminus of the inner
electrode. The curved surface terminus is continuous with the
cylindrical shape of the inner electrode and is aligned co-axially
with an ion-outlet orifice of the FAIMS analyzer region. The
application of an asymmetric waveform to the inner electrode
results in the normal ion-focussing behavior described above,
except that the ion-focussing action extends around the generally
spherically shaped terminus of the inner electrode. This means that
the selectively transmitted ions cannot escape from the region
around the terminus of the inner electrode. This only occurs if the
voltages applied to the inner electrode are the appropriate
combination of CV and DV as described in the discussion above
relating to 2-dimensional focussing. If the CV and DV are suitable
for the focussing of an ion in the FAIMS analyzer region, and the
physical geometry of the inner surface of the outer electrode does
not disturb this balance, the ions will collect within a
three-dimensional region of space near the terminus. Several
contradictory forces are acting on the ions in this region near the
terminus of the inner electrode. The force of the carrier gas flow
tends to influence the ion cloud to travel towards the ion-outlet
orifice, which advantageously also prevents the trapped ions from
migrating in a reverse direction, back towards the ionization
source. Additionally, the ions that get too close to the inner
electrode are pushed back away from the inner electrode, and those
near the outer electrode migrate back towards the inner electrode,
due to the focusing action of the applied electric fields. When all
forces acting upon the ions are balanced, the ions are effectively
captured in every direction, either by forces of the flowing gas,
or by the focussing effect of the electric fields of the FAIMS
mechanism. This is an example of a three-dimensional atmospheric
pressure ion trap, as disclosed in a copending PCT application in
the name of R. Guevremont and R. Purves, the contents of which are
herein incorporated by reference.
[0012] Ion focusing and ion trapping requires electric fields that
are other than constant in space, normally occurring in a
geometrical configuration of FAIMS in which the electrodes are
curved, and/or are not parallel to each other. For example, a
non-constant in space electric field is created using electrodes
that are cylinders or a part thereof; electrodes that are spheres
or a part thereof; electrodes that are elliptical spheres or a part
thereof; and, electrodes that are conical or a part thereof.
Optionally, various combinations of these electrode shapes are
used.
[0013] As discussed above, one previous limitation of the
cylindrical FAIMS technology is that the identity of the peaks
appearing in the CV spectra are not unambiguously confirmed due to
the unpredictable changes in K.sub.h at high electric field
strengths. Thus, one way to extend the capability of instruments
based on the FAIMS concept is to provide a way to determine the
make-up of the CV spectra more accurately, such as by introducing
ions from the FAIMS device into a mass spectrometer for
mass-to-charge (m/z) analysis. Advantageously, the ion focusing
property of cylindrical FAIMS devices acts to enhance the
efficiency for transporting ions from the analyzer region of a
FAIMS device into an external sampling orifice, for instance an
inlet of a mass spectrometer. This improved efficiency of
transporting ions into the inlet of the mass spectrometer is
optionally maximized by using a 3-dimensional trapping version of
FAIMS operated in nearly trapping conditions. Under near-trapping
conditions, the ions that have accumulated in the three-dimensional
region of space near the spherical terminus of the inner electrode
are caused to leak from this region, being pulled by a flow of gas
towards the ion-outlet orifice. The ions that leak out from this
region do so as a narrow, approximately collimated beam, which is
pulled by the gas flow through the ion-outlet orifice and into a
small orifice leading into the vacuum system of a mass
spectrometer.
[0014] Note that, while the above discussion refers to the ions as
being "captured" or "trapped", in fact, the ions are subject to
continuous `diffusion`. Diffusion always acts contrary to focussing
and trapping. The ions always require an electrical, or gas flow
force to reverse the process of diffusion. Thus, although the ions
are focused into an imaginary cylindrical zone in space with almost
zero thickness, or within a 3-dimensional ion trap, in reality it
is well known that the ions are actually dispersed in the vicinity
of this idealized zone in space because of diffusion. This is
important, and should be recognized as a global feature
superimposed upon all of the ion motions discussed in this
disclosure. This means that, for example, a 3-dimensional ion trap
actually has real spatial width, and ions continuously leak from
the 3-dimensional ion trap, for several physical, and chemical
reasons. Of course, the ions occupy a smaller physical region of
space if the trapping potential well is deeper.
[0015] Additionally, the resolution of a FAIMS device is defined in
terms of the extent to which ions having similar mobility
properties are separated under a set of predetermined operating
conditions. Thus, a high-resolution FAIMS device transmits
selectively a relatively small range of ion types having similar
mobility properties, whereas a low-resolution FAIMS device
transmits selectively a relatively large range of ion types having
similar mobility properties. It is generally well known that the
resolution of FAIMS in a cylindrical geometry FAIMS is compromised
relative to the resolution in a parallel plate geometry FAIMS
because the cylindrical geometry FAIMS has the capability of
focusing and trapping ions. This focusing action means that ions of
a wider range of mobility characteristics are simultaneously
focused in the analyzer region of the cylindrical geometry FAIMS. A
cylindrical geometry FAIMS with narrow electrodes has the strongest
focusing action, but the lowest resolution for separation of ions.
As the radii of curvature are increased, the focusing action
becomes weaker, and the ability of FAIMS to simultaneously focus
ions of similar high-field mobility characteristics is similarly
decreased. This means that the resolution of FAIMS increases as the
radii of the electrodes are increased, with parallel plate geometry
FAIMS having the maximum attainable resolution. It is a limitation
of the parallel plate FAIMS devices that are described in the prior
art, however, that an ion transmitted through the analyzer region
experiences a rapid change in the electric fields due to the finite
size of the parallel plate electrodes. Typically, when an ion moves
past the edge of the parallel plate analyzer region, the electric
field that is established by the asymmetric waveform suddenly
changes strength, and the ion trajectory is influenced by the
applied dc potential only, causing the transmitted ion to impact
with an electrode surface of the FAIMS device. It would therefore
be advantageous to provide an apparatus having the high resolution
property of a parallel plate FAIMS and the focusing capability and
high sensitivity that are inherent in a cylindrical electrode
geometry FAIMS device.
OBJECT OF THE INVENTION
[0016] In order to overcome these and other limitations of the
prior art, it is an object of the present invention to provide a
high field asymmetric waveform ion mobility spectrometer for
separating ions in which a transmitted ion when it exits the
analyzer region experiences a balanced electric field, such that
impact with one of the electrodes is prevented.
[0017] In order to overcome these and other limitations of the
prior art, it is another object of the present invention to provide
a high field asymmetric waveform ion mobility spectrometer for
separating ions in which has a focusing effect such that the
overall efficiency of ion transmission is increased.
[0018] In order to overcome these and other limitations of the
prior art, it is still another object of the present invention to
provide a high field asymmetric waveform ion mobility spectrometer
for separating ions in which a first type of ion is selectively
transmitted through a first region of an analyzer region under the
influence of a first non-constant in space electric field and a
second other type of ion is selectively transmitted through a
second other region of the analyzer region under the influence of a
second other non-constant in space electric field.
SUMMARY OF THE INVENTION
[0019] In accordance with the invention there is provided an
apparatus for separating ions, comprising a high field asymmetric
waveform ion mobility spectrometer, including:
[0020] a) an analyzer region comprising:
[0021] a first electrode, a second electrode and a third electrode
in a spaced apart stacked arrangement for allowing ion flow
therebetween, the first electrode having a first inner surface, the
second electrode having a first and a second surface on opposite
sides thereof, the third electrode having a second inner surface;
and,
[0022] at least a contact on at least one of the first, second and
third electrodes, for receiving a compensation voltage potential
between the second and first electrodes and between the second and
third electrodes, and for applying an asymmetric waveform to at
least one of the electrodes,
[0023] wherein, in use, ions exiting from between the electrodes
are other than attracted to the second electrode to collide
therewith.
[0024] In accordance with the invention there is provided an
apparatus for separating ions, comprising a high field asymmetric
waveform ion mobility spectrometer, including:
[0025] a) an analyzer region comprising:
[0026] a first electrode, a second electrode and a third electrode
in a spaced apart stacked arrangement for allowing ion flow
therebetween, the first electrode having a first inner surface, the
second electrode having a continuous smoothly curved surface, the
third electrode having a second inner surface; and,
[0027] at least a contact on at least one of the first, second and
third electrodes, for receiving a compensation voltage potential
between the second and first electrodes and between the second and
third electrodes, and for applying an asymmetric waveform to at
least one of the electrodes,
[0028] wherein, in use, ions exiting from between the electrodes
are other than attracted to the second electrode to collide
therewith.
[0029] In accordance with the invention there is provided an
apparatus for separating ions, comprising a high field asymmetric
waveform ion mobility spectrometer, including:
[0030] a) an analyzer region comprising:
[0031] a first electrode having a first surface and a second
surface on opposite sides thereof;
[0032] a second electrode absent a cross section forming a closed
surface, the second electrode shaped such that a first region of
the surface defines a first inner surface of the second electrode
that opposes the first surface of the first electrode and a second
other region of the surface defines a second inner surface of the
second electrode that opposes the second surface of the first
electrode; and,
[0033] at least a contact on at least one of the first and second
electrodes, for applying a compensation voltage potential between
the second and first electrodes, and for receiving an asymmetric
waveform to at least one of the electrodes,
[0034] wherein, in use, ions exiting from between the electrodes
are other than attracted to the first electrode, such that the ions
other than collide therewith.
[0035] In accordance with the invention there is provided an
analyzer comprising:
[0036] a first electrode having in cross section an approximately
continuous periphery;
[0037] a second electrode having in cross section an approximately
continuous periphery approximately equidistant from the first
electrode over a region thereof and having at least an inlet for
introduction of ions and a carrier gas and at least an outlet in
the approximately continuous periphery; and,
[0038] a contact on at least one of the first and second electrode
for providing an asymmetric electric field between the first and
second electrode;
[0039] wherein, in use, ions flow through the at least an inlet
about the approximately continuous periphery of the first electrode
and out the at least an outlet wherein a similar electric field is
present on opposing sides of the first electrode at an end
proximate the at least an outlet.
[0040] In accordance with the invention there is provided a method
for separating ions comprising the steps of:
[0041] a) providing at least an ionization source for producing
ions including two ionic species;
[0042] b) providing an analyzer region defined by a first analyzer
space between a first electrode surface and an opposing second
electrode surface and a second different analyzer space between a
third electrode surface and an opposing fourth electrode surface,
said analyzer region being in communication with a gas inlet, a gas
outlet and an ion inlet, said ions produced by said ionization
source being introduced into said analyzer region at said ion
inlet;
[0043] c) providing an asymmetric waveform and a direct-current
compensation voltage, to at least one of said electrode surfaces,
to form an electric field between the opposing pairs of electrode
surfaces;
[0044] d) setting said asymmetric waveform in order to effect a
difference in net displacement between said two ionic species in
the time of one cycle of said applied asymmetric waveform; and,
[0045] e) setting said compensation voltage to a determined value
to selectively transmit one of said two ionic species,
[0046] wherein the second electrode surface and the third electrode
surface are disposed on opposing sides of a same first
electrode.
[0047] In accordance with the invention there is provided a method
for separating ions comprising the steps of:
[0048] a) providing at least an ionization source for producing
ions including two ionic species;
[0049] b) providing two analyzer regions on opposing sides of an
electrode, said two analyzer regions being in communication with a
gas inlet, a gas outlet and an ion inlet, said ions produced by
said ionization source being introduced into said two analyzer
regions at said ion inlet;
[0050] c) forming an electric field on opposing sides of the
electrode by providing an asymmetric waveform and a direct-current
compensation voltage to the electrode;
[0051] d) setting said asymmetric waveform in order to effect a
difference in net displacement between said two ionic species in
the time of one cycle of said applied asymmetric waveform; and,
[0052] a) setting said compensation voltage to a determined value
to selectively focus one of said two ionic species.
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] FIG. 1 shows three possible examples of changes in ion
mobility as a function of the applied electric field strength;
[0054] FIG. 2a illustrates the trajectory of an ion between two
parallel plate electrodes under the influence of the electrical
potential V(t);
[0055] FIG. 2b shows an asymmetric waveform described by V(t);
[0056] FIG. 3 shows a simplified block diagram of an analyzer
region of a parallel plate FAIMS device according to the prior
art;
[0057] FIG. 4 shows a simplified block diagram of an analyzer
region of an improved parallel plate FAIMS device according to a
first embodiment of the present invention;
[0058] FIG. 5a shows a simplified block diagram of an improved
parallel plate FAIMS device according to a second embodiment of the
present invention;
[0059] FIG. 5b shows a facing view of the outline shape of an
electrode plate;
[0060] FIG. 5c shows a top view of a simplified block diagram of
the plates mounting within an analyzer region of an improved
parallel plate FAIMS device according to the second embodiment of
the present invention;
[0061] FIG. 6a shows detail including a curved cross section of the
top edge of a parallel plate electrode with ion focusing towards a
center thereof;
[0062] FIG. 6b shows detail including a curved cross section of the
top edge of a parallel plate electrode with ions being directed to
a flat plate portion thereof;
[0063] FIG. 7a shows detail including the curved cross section of
the bottom edge of a parallel plate electrode with ions diffusing
over a flat plate portion thereof;
[0064] FIG. 7b shows detail including the curved cross section of
the bottom edge of a parallel plate electrode with ions exiting as
a substantially collimated beam therefrom;
[0065] FIG. 8 shows a top view of a simplified block diagram of the
plates mounting within an analyzer region of an improved parallel
plate FAIMS device according to a third embodiment of the present
invention;
[0066] FIG. 9a shows a top view of a simplified block diagram of
the electrode plates mounting within an analyzer region of a
multi-mode FAIMS device according to a fourth embodiment of the
present invention;
[0067] FIG. 9b shows a facing view of the outline shape of an
electrode plate;
[0068] FIG. 9c shows a top view of a simplified block diagram of
another electrode plate mounting within an analyzer region of a
multi-mode FAIMS device according to a fourth embodiment of the
present invention;
[0069] FIG. 10a shows a top view of a simplified block diagram of
the electrode plates mounting within an analyzer region of a
multi-mode FAIMS device according to a fifth embodiment of the
present invention;
[0070] FIG. 10b shows a facing view of the outline shape of an
electrode plate;
[0071] FIG. 10c shows a top view of a simplified block diagram of
another electrode plate mounting within an analyzer region of a
multi-mode FAIMS device according to a fifth embodiment of the
present invention;
[0072] FIG. 11a shows a simplified block diagram of an electrode
configuration of an analyzer region according to a sixth embodiment
of the present invention;
[0073] FIG. 11b shows a simplified cross sectional view of an
electrode configuration of an analyzer region according to a sixth
embodiment of the present invention;
[0074] FIG. 12a shows a simplified block diagram of an electrode
configuration of an analyzer region according to a seventh
embodiment of the present invention;
[0075] FIG. 12b shows a simplified cross sectional view of an
electrode configuration of an analyzer region according to a
seventh embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0076] Referring to FIG. 1, shown are three possible examples of
the change in ion mobility properties with increasing electric
field strength, as was discussed previously. The separation of ions
in FAIMS is based upon a difference in these mobility properties
for a first ion relative to a second ion. For instance, a first
type A ion having a low field mobility K.sub.1,low is other than
separated in a FAIMS device from a second type A ion having a
second different low field mobility K.sub.2,low, if under the
influence of high electric field strength, the ratio
K.sub.1,high/K.sub.1,low is equal to the ratio
K.sub.2,high/K.sub.2,low. Interestingly, however, this same
separation is achieved using conventional ion mobility
spectrometry, which is based on a difference in ion mobilities at
low applied electric field strength.
[0077] Referring to FIG. 2a, shown is a schematic diagram
illustrating the mechanism of ion separation according to the FAIMS
principle. An ion 1, for instance a positively charged type A ion,
is carried by a gas stream 2 flowing between two spaced apart
parallel plate electrodes 3 and 4. One of the plates 4 is
maintained at ground potential, while the other plate 3 has an
asymmetric waveform described by V(t), applied to it. The peak
voltage applied during the waveform is called the dispersion
voltage (DV), as is shown in FIG. 2b. Referring still to FIG. 2b,
the waveform is synthesized so that the electric fields during the
two periods of time t.sub.high and t.sub.low are not equal. If
K.sub.h and K are identical at high and low fields, the ion 1 is
returned to its original position at the end of one cycle of the
waveform. However, under conditions of sufficiently high electric
fields, K.sub.h is greater than K and the distances traveled during
t.sub.high and t.sub.low are no longer identical. Within an
analyzer region defined by a space 16 between the first and second
spaced apart electrode plates, 3 and 4, respectively, the ion 1
experiences a net displacement from its original position relative
to the plates 3 and 4 as illustrated by the dashed line 5 in FIG.
2a.
[0078] If a type A ion is migrating away from the upper plate 3, a
constant negative dc compensation voltage CV is applied to plate 3
to reverse or "compensate" for this offset drift. Thus, the ion 1
does not travel toward either plate. If two species of ions respond
differently to the applied high electric field, for instance the
ratios of K.sub.h to K are not identical, the compensation voltages
necessary to prevent their drift toward either plate are similarly
different. To analyze a mixture of ions, the compensation voltage
is, for example, scanned to transmit each of the components of a
mixture in turn. This produces a compensation voltage spectrum, or
CV spectrum.
[0079] Referring to FIG. 3, a simplified block diagram of a
parallel plate FAIMS device according to the prior art is shown
generally at 10. The analyzer region is defined by a space 16
between two flat, parallel plate electrodes 3 and 4, and between an
ion-inlet electrode 6 having an ion-inlet orifice 19 and an
ion-outlet electrode 8 having an ion-outlet orifice 21. The
electrodes 3 and 4 are connected to an electrical controller 7 such
that, in use, an asymmetric waveform and a superimposed dc
compensation voltage is applied to electrode 3. Typically, the
electrode 4 is maintained at a same dc voltage relative to each of
the ion-inlet electrode 6 and the ion-outlet electrode 8. In this
example, the asymmetric waveform and CV are set so that a
particular type of positively charged ion (not shown) is
transmitted through the analyzer region within space 16 between the
plates 3 and 4, for instance the CV is negative, and the waveform
has positive polarity. The "net" ion trajectory through the
analyzer region is indicated in FIG. 3 by dotted line 18. In
general, the powered electrode 3 is attracting the positive ion
toward itself due to the negative dc bias, as indicated in FIG. 3
by the arrowheads of the electric force lines that are directed
toward electrode 3. Fortunately, within the analyzer region 16 the
effect of the asymmetric waveform is to push the ion away from the
electrode 3, as is indicated in FIG. 3 by the arrowheads of the
electric force lines that are directed away from electrode 3. As
long as the electric fields are strong, and as long as the fields
stay constant in strength, a balanced condition that is necessary
to allow the ion to pass through the analyzer region 16 is
maintained. This balanced condition is shown schematically in FIG.
3 as a series of double-headed electric force lines, comprising a
DV and CV component, which are selected for transmitting ions
having specific high field mobility properties.
[0080] Of course, the fields are not strong everywhere around the
powered electrode 3. On a side 3a of the powered electrode 3 that
opposes the second electrode 4, and on the end edges of the
electrode facing one of the ion-inlet electrode 6 and the
ion-outlet electrode 8, the fields are strong and the balanced
condition exists. However, where the electric field strength
changes, such as occurs on a side 3b of the powered electrode 3 at
the end edges of the electrode facing one of the ion-inlet
electrode 6 and the ion-outlet electrode 8, the ion path change
rapidly, resulting in a dramatic redirection of the ion stream.
This redirection lacks the balanced conditions that the ion stream
experiences between the plates 3 and 4. This means that on the back
side 3b of the powered electrode 3 the ion will impact onto the
metal surface, pulled by the negative polarity of the applied CV.
Although the ion maintains a stable trajectory along side 3a where
the opposing electrical forces are balanced, upon exiting space 16
the ion follows a curved path towards the back side 3b of electrode
3. The negative dc bias applied to electrode 3 creates a potential
hillside for the ion to slide down. The carrier gas flow is other
than able to prevent this downward slide unless the CV is very low
or the gas flow is very high. Even if impact with the plate 3 is
avoided, many ion paths do not proceed toward the ion-outlet
orifice 21 of the device 10, the ions being lost to a collision
with a different part of the FAIMS apparatus.
[0081] Referring to FIG. 4, a simplified block diagram of an
analyzer region of an improved parallel plate FAIMS device
according to a first embodiment of the present invention is shown
generally at 20. Three parallel plate electrodes are shown with a
space 17a between the first electrode 11 and second electrode 12
and a second space 17b between the second electrode 12 and the
third electrode 13. In a preferred embodiment, the second electrode
12 is disposed substantially mid-way between the first electrode 11
and the third electrode 13, such that the first second and third
electrodes are in a substantially uniformly spaced-apart stacked
arrangement. Optionally, the spacing between the first electrode 11
and the second electrode 12, and the spacing between the second
electrode 12 and the third electrode 13 is other than uniform. Of
course, the electrodes are mounted in an insulating support, which
is omitted for clarity in FIG. 4. Each space 17a and 17b defines a
separate ion flow path that is closed on four sides such that it is
other than possible for ions to move from one space to the other
space. Ions that are introduced through an ion-inlet orifice 19 of
ion-inlet electrode 6 follow a trajectory through only one of space
17a and space 17b in dependence upon an ion initial position, a
flow of carrier gas, random diffusion and the effects of
space-charge induced ion-ion repulsive forces. For example, in FIG.
4 one ion trajectory through space 17b is shown at dotted line
9.
[0082] Typically, electrode 12 is connected to an electrical
controller 7 such that, in use, an asymmetric waveform and a
superimposed first dc voltage, wherein the superimposed first dc
voltage is other than the compensation voltage, is applied to the
electrode 12. The electrodes 11 and 13 are connected to at least a
dc voltage controller, for instance two separate dc voltage
controllers 7a and 7b, such that, in use, electrodes 11 and 13 are
maintained at a predetermined second dc voltage. The ion-inlet
electrode 6 and an ion-outlet electrode 8 are also maintained at
predetermined de voltages by power supplies (not shown). The
compensation voltage (CV) is the difference between the
superimposed first de voltage applied to the electrode 12 and the
second dc voltage applied to the electrodes 11 and 13. Those ions
having appropriate mobility properties for a particular combination
of waveform amplitude and CV are selectively transmitted through
the analyzer region 17a between the electrodes 11 and 12, and
through the analyzer region 17b between the electrodes 12 and 13.
For example, the selective transmission of an analyte ion through
the FAIMS analyzer region may require the electrode 12 to be biased
5 volts lower than electrodes 11 and 13, for instance the CV is
negative 5 volts, and for the waveform to be of positive polarity,
for example 2500 volts.
[0083] A `net` trajectory for a selectively transmitted ion through
the FAIMS analyzer region is shown diagrammatically in FIG. 4 at
dotted line 9. In general, the powered electrode 12 is attracting
the ion toward itself due to the negative dc bias relative to
electrodes 11 and 13, as indicated in FIG. 4 by the arrowheads of
the electric force lines that are directed toward electrode 12.
Fortunately, within the analyzer regions 17a and 17b the effect of
the asymmetric waveform is to push the ion away from the electrode
12, as is indicated in FIG. 4 by the arrowheads of the electric
force lines that are directed away from electrode 12. As long as
the electric fields are strong, and as long as the fields stay
constant in strength, a balanced condition that is necessary to
allow the ion to pass through the analyzer region is maintained.
This balanced condition is shown schematically in FIG. 4 as a
series of double-headed electric force lines, comprising a DV and
CV component that are selected for transmitting ions having
specific high-field mobility properties. This balanced condition
extends completely around the inlet end of the electrode 12 facing
the ion-inlet electrode 6 and completely around the outlet end of
the electrode 12 facing the ion-outlet electrode 8.
[0084] Advantageously, when these balanced condition extend around
the inlet edge of the electrode 12, collisions between the ions
entering through the orifice 19 and the leading edge of electrode
12 are minimized. The ions are prevented from approaching the
electrode 12 by the effective repelling force that is created by
the asymmetric waveform. Similarly, at the opposite end of the
electrode 12 the balanced condition tends to pull the ions towards
the electrode 12 as they pass by the outlet end of the electrode
12, giving the `near-trapping` conditions shown by the ion
trajectory shown at dotted line 9 in FIG. 4. The ions would
otherwise be trapped at the outlet edge of electrode 12, for
example the ions are unable to move in any direction, absent a gas
flow that is sufficiently strong to carry the ions to the ion-exit
orifice 21. Of course, the dc voltage applied to the ion-exit
electrode 8 is adjusted to help pull the ions away from the
trailing edge of electrode 12 in a controlled fashion.
[0085] Unlike the prior art parallel plate FAIMS, the electric
fields extend on both sides of the center electrode plate
symmetrically within the analyzer region, such that the ion
continues to "see" the same balancing electric forces and will
continue along a stable trajectory to exit the analyzer. The
electrical forces for selectively transmitting the ion remain
balanced beyond the physical limit of the electrodes because the
two sides of the powered electrode 12 are symmetrical. For instance
a metal conductive surface of electrodes 11 and 13 is located a
same distance from each surface 12b and 12a, respectively, of the
powered electrode 12. As a result of this symmetrical electrode
geometry, the transmitted ions do not see an electrical potential
hillside to slide into. Under these conditions, even slowly flowing
gas will tend to keep the ions positioned near the trailing edge of
the electrode, in a position close to the ion-outlet orifice
21.
[0086] Further, if the middle electrode 12 of the system shown
generally at 20 in FIG. 4 is narrow relative to the spaces 17a and
17b between the electrodes, then the specific shape of the corners
at the edges of the electrode plates will other than critically
influence the ion trajectory. For instance rounded or squared
corners behave more or less the same in terms of the resulting
fields that the ion will experience in this region. This is because
the electric fields tend to `smooth` themselves out over a distance
away from a corner of the electrode, such that effectively the
fields around the electrode look exactly the same as if it was
rounded once you move more than some distance away. If the
electrode is thick, for example more than approximately 20% of the
thickness of the spaces, then the shape is important. Also, if the
ion trajectory is very close to the central electrode, a contour at
an edge of the electrode has more influence on the path of travel
than when the ions are further away from the electrode.
[0087] Referring to FIG. 5a, an improved parallel plate FAIMS
device having three electrodes is shown generally at 50 according
to a second preferred embodiment of the present invention. Three
parallel plate electrodes are shown with a space 65a between the
first electrode 52 and second electrode 51 and a second space 65b
between the second electrode 51 and the third electrode 53. In a
preferred embodiment the second electrode 51 is disposed
substantially mid-way between the first electrode 52 and the third
electrode 53, such that the first second and third electrodes are
in a substantially uniformly spaced-apart stacked arrangement.
Optionally, the spacing between the first electrode 52 and the
second electrode 51, and the spacing between the second electrode
51 and the third electrode 53 is other than uniform. Each space 65a
and 65b defines a separate ion flow path that is closed on four
sides such that it is other than possible for ions to move from one
space to the other space. Ions that are introduced through
ion-inlet orifice 19 follow a trajectory through only one of space
65a and space 65b, in dependence upon an ion initial position, a
flow of carrier gas, random diffusion and the effects of
space-charge induced ion-ion repulsive forces. The electrode plates
are mounted in an insulating support 64 in order to maintain the
uniform spacing therebetween. As shown in FIG. 5a, the insulating
support 64 extends entirely around the periphery of the FAIMS
analyzer to provide a physical barrier, such that the flow of
carrier gas is directed through the spaces 65a and 65b, only.
Optionally, separately formed physical barriers are provided for
directing the carrier gas flow.
[0088] The electrode 51 is connected to an electrical controller 54
such that, in use, an asymmetric waveform and a superimposed dc
voltage, which voltage is other than the compensation voltage, is
applied to electrode 51. The electrodes 52 and 53 are connected to
at least a dc voltage controller, for instance two separate dc
voltage controllers 54a and 54b, such that, in use, electrodes 52
and 53 are maintained at a dc voltage with respect to electrode 51
so as to establish a voltage difference between electrode 51 and
electrodes 52 and 53 that corresponds the compensation voltage.
Each one of an ion-inlet electrode 6 and an ion-outlet electrode 8
are attached to a voltage controller (not shown). Of course, the dc
voltage applied to at least the ion-outlet electrode 8 is adjusted
to maximize efficiency of the ion transmission.
[0089] Additional elements of the apparatus are shown in FIG. 5a,
including: an ionization source 57 for providing ions through
ion-inlet orifice 19; a gas inlet 55 and a gas outlet 56 for
providing in use a flow of a carrier gas through the analyzer
region; a housing 81 for containing the flow of carrier gas and for
providing surfaces internal to the housing for mounting at least
the electrodes; and an ion collecting electrode 58, for instance
attached to an electrometer 68, for detecting ions received through
an ion-outlet orifice 21 in ion-outlet electrode 8. Of course,
optionally a mass spectrometer detector system (not shown) is
used.
[0090] In the second preferred embodiment of the present invention
shown generally at 50, a leading edge and a trailing edge, with
respect to the direction of ion flow through the analyzer region
when in use, of at least the second electrode 51 are rounded in
cross section. This is shown most clearly in the inset view of FIG.
5a for the leading edge of electrode 51, wherein a smooth curve 59
joins the surfaces 51a and 51b on opposing sides thereof. The
trailing edge of electrode 51 is similarly provided with a smooth
curve for joining the surfaces 51a and 51b at the other opposite
end of electrode 51. The radius of curvature of the smooth curve 59
shown in FIG. 5a will be appropriate to focus and trap the ions at
leading and trailing edges, and of course the plates will be thick
enough to accommodate the radius of curvature. As is shown in FIG.
5a and the inset view, the edges of the electrodes 51 and 53 are
also optionally rounded in cross section.
[0091] Optionally, as shown in FIG. 5b, at least one of the leading
edge 62 and the trailing edge 63 of the at least the second plate
51 is further shaped to move the trapped ions to the center of the
leading and trailing edges 62 and 63, respectively. In face view,
the electrode 51 has a concave inwardly curved edge at is leading
edge 62, and a convex outwardly curved edge at its trailing edge
63. Ions being carried along by the carrier gas tend to follow the
electric fields around the concave surface of the leading edge of
the powered electrode 51 and flow generally towards the central
axis of the device. This will minimize the spread of the ions along
the width of the plates, as will be discussed in greater detail
below.
[0092] Referring to FIG. 5c, the series of plates are mounted into
an insulating support 64, which has grooves to hold the plates at
fixed distances of separation between the plates. This top view,
for instance a view of the leading edge of the electrode plates,
provided by FIG. 5c also illustrates the uniform geometry of the
two ion flow paths 65a and 65b. Each flow path is bordered on two
opposing sides by electrode surfaces and on two different opposing
sides by the insulating support. Of course, both ion flow paths are
open in a direction into and out of the plane of the drawing.
Additionally, as shown in FIG. 5c a physical barrier 64 is extended
along the outside edges of electrodes 52 and 53, such that the
carrier gas flows through the analyzer region within spaces 65a and
65b, only. This provides greater control and flexibility for
controlling the trajectories of ion as they exit the analyzer
region by adjusting the flow rate of the carrier gas.
[0093] Referring to FIGS. 6a, 6b, 7a and 7b, the combination of
parallel plate separation, and focusing action, to give maximum
sensitivity is achieved with the design of the improved parallel
plate FAIMS. In these figures, only the powered electrode 51 of the
second preferred embodiment of the present invention is shown. In
FIG. 6a the ions are carried towards edge 62 of the powered plate
51 by a gas flow and the dc bias applied to the powered plate 51.
There is a natural tendency for the ions to disperse in space due
to diffusion and space-charge induced ion-ion repulsion. This loss
is minimized by the focusing action of the curved surface, both in
cross section and along the leading edge 62 of electrode plate 51
in FIG. 6a, which is curved away from the direction of arriving
ions. As was described above, the electrode cross section is
rounded to create the non-uniform in space electric fields that are
necessary to focus and trap ions. The ions will move toward the
focusing region, above the curved cross section anywhere along the
leading edge 62 of the plate 51. In this region the requirement for
ion separation does not exist, therefore the surfaces optionally
have small radii of curvature, in order to capture the ions as
effectively as possible. The ions then tend to move to the center
axis of the plate following the concave inwards dip of the leading
edge 62 of the plate, as indicated by the series of arrows
indicating ion travel in FIG. 6a. Of course, the ions do not focus
in a region proximate to either one of the curved surfaces of the
non-powered electrodes, 52 and 53, which have been omitted from
this discussion for clarity. It should be noted, however, that the
non-powered electrodes are necessary for establishing the constant
electric fields for separating ions within the flat plate region of
the analyzer.
[0094] Referring to FIG. 6b, the focused ions cascade over the
leading edge 62 of the powered electrode 51, carried along by the
gas flow. The ions enter the flat plate region in a narrow focused
beam, which is located between the electrodes. This focusing action
maximizes the ion transmission past the leading edge 62 to the
region of flat-plate separation. Ion losses during separation
within the flat plate region of the device are also minimized as
well. This is because the ions, including the ions that are to be
transmitted, are initially focused in a narrow beam located between
the electrodes instead of being uniformly distributed between the
electrodes. The ions in this beam are separated as they traverse
the region along the flat surfaces of the plates. As will be
obvious to one of skill in the art, when the ions are initially
uniformly distributed between the electrodes, for instance when ion
focusing does not occur at the leading edge 62 of the electrode 51,
those ions that enter the flat plate region closest to an electrode
wall may collide with that electrode as a result of even very small
changes in the instantaneous ion position.
[0095] Referring to FIG. 7a those ions, which are transmitted
through the flat plate region of the improved parallel plate FAIMS,
are preferably captured and focused as near as possible to the
outlet orifice to the detector as possible. This is done in a
manner essentially identical to that described with reference to
FIG. 6 for the inlet of the improved parallel plate FAIMS. The
ions, which are dispersed across the region between the parallel
plates, feel the focusing action of FAIMS when they approach the
curved trailing edge 63 of the electrode 51 facing the exit
orifice. As a result, as many of these ions as possible are
collected, and again moved close to the central axis of the plate,
such that further transmission loss to the walls of the device are
minimized. The ions leave the plate in a focused beam.
Additionally, by using curves of small radii the effect of FAIMS
trapping at the bottom edge of the electrode is maximized. Although
the separation of ions does not take place effectively at the
leading 62 and trailing 63 edges of electrode 51, it is assumed
that the separation has taken place in the flat plate regions of
the FAIMS.
[0096] Referring to FIG. 8, a simplified block diagram in top view
of the mounting of electrode plates within an analyzer region of an
improved parallel plate FAIMS device according to a third
embodiment of the present invention is shown generally at 80.
According to the third embodiment, the two outer electrodes 51 and
52 of the second embodiment are replaced with an enveloping
electrode 66 that is fabricated from a single piece of material and
shaped to wrap around the central electrode 51. The plates are
mounted into an insulating support 64, which has grooves to hold
the plates at fixed distances of separation between the plates, so
as to define two separate ion flow paths 67a and 67b. As shown in
FIG. 8, a physical barrier 64a for directing gas flow through the
analyzer region optionally is other than integrally formed with the
insulating support 64. The parallel plate FAIMS device according to
the third embodiment of the present invention otherwise is
identical to the parallel plate FAIMS device according to the
second embodiment of the present invention, and a full discussion
will be omitted.
[0097] As discussed in the early FAIMS publications, the resolution
of FAIMS in a cylindrical geometry FAIMS is compromised relative to
the resolution in a parallel plate FAIMS because the cylindrical
geometry has the capability of focusing and trapping ions. This
focusing action means that ions of a wider range of mobility
characteristics are simultaneously focused in the analyzer region
of the cylindrical geometry FAIMS. Cylindrical geometry FAIMS with
narrow electrodes have the strongest focusing action, but the
lowest resolution for separation of ions. As the radii of curvature
are increased, the focusing action becomes weaker, and the ability
of FAIMS to simultaneously focus ions of similar high-field
mobility characteristics is similarly decreased. This means the
resolution of FAIMS increases as the radii of the electrodes are
increased, with parallel plates having the maximum resolution, but
no focusing or trapping capability. The new version of parallel
plate FAIMS described with reference to FIGS. 4 to 8 provides the
benefit of the highest resolution achievable with FAIMS using
parallel plate electrodes, with the maximum sensitivity that is
achievable by collection of as many of the transmitted ions as
possible. The focusing and trapping of ions near the leading and
trailing edge of the powered electrode plate 51 serves to improve
the total sensitivity of the parallel plate assembly by
minimization of ion loss at the leading edge of the electrode 51,
and maximization of the ion collection efficiency at the trailing
edge of the electrode 51.
[0098] Advantageously, the improved parallel plate FAIMS devices
described with reference to FIGS. 5 to 8 are of compact
construction and are suitable for interfacing with existing
instruments, such as for example a mass spectrometer. For example
the distance between the ion inlet and ion outlet is approximately
20 mm. In the simplest application the improved parallel plate
FAIMS device is used as a filter for performing a preseparation or
desolvation of ions prior to their introduction into a mass
spectrometer. In this way a focused beam including at least an ion
of interest is enriched relative to the background ions for mass
spectral analysis. In a more elaborate experimental system a FAIMS
device is used to perform an additional separation that is other
than possible using any mass spectrometric techniques, for example
the selective transmission of one isomeric species or conformer of
a mixture of different isomers or conformers, respectively. This
ability is very useful in the studies of drugs or other compounds
of biological interest, since often it is the three-dimensional
structure of the compound and not the chemical empirical formula
that determines biological activity. Of course, mass spectrometric
methods are other than capable of separating isomeric species,
since both types of isomer have a same mass to charge ratio.
Similarly, mass spectrometric methods are other than capable of
separating different conformers of a same molecule.
[0099] FIG. 9a shows a top view of a simplified block diagram of
the mounting of the electrode plates within the analyzer region of
a multi-mode FAIMS (mmFAIMS) device according to a fourth
embodiment of the present invention. The new version of mmFAIMS has
significantly different properties from the parallel plate devices
described with reference to FIGS. 5 to 8. In a pure parallel plate
FAIMS, the electrical fields within the two spaces of the analyzer
region, for instance spaces 17a and 17b in FIG. 4, are identical.
This means that the ions that are focused at the top edge of the
plate will be carried by the uniform gas flow through the spaces on
either side of the powered electrode 51. Since the two individual
analyzer regions 17a and 17b are identical, the separation and the
selection of ions that arrive at the bottom of the plate will be
identical.
[0100] Still referring to FIG. 9a, a simplified block diagram of an
analyzer region of a multi-mode FAIMS device having three curved
electrode plates, according to a fourth preferred embodiment of the
present invention, is shown. Three parallel plate electrodes are
shown with a space 94a between the first electrode 92 and second
electrode 91 and a second space 94b between the second electrode 91
and the third electrode 93. Additionally, the second electrode 91
is disposed substantially mid-way between the first electrode 92
and the third electrode 93, such that the first second and third
electrodes are in a substantially uniformly spaced-apart stacked
arrangement. Of course, optionally the electrode spacing is other
than uniform. The series of electrodes are mounted into an
insulating support, which has grooves to hold the plates at fixed
distances of separation between the plates. This top view, for
instance a view of the leading edge of the electrode plates,
provided by FIG. 9a also illustrates the uniform geometry of the
two ion flow paths 94a and 94b. Each flow path is bordered on two
opposing sides by electrode surfaces and on two different opposing
sides by the insulating support. Of course, both ion flow paths are
open in a direction into and out of the plane of the drawing.
Additionally, a physical barrier (not shown) extends along the
outside edges of electrodes 92 and 93, such that the flow of a
carrier gas is directed only through the two ion flow paths 94a and
94b.
[0101] Each space 94a and 94b defines a separate ion flow path that
is closed on four sides such that it is other than possible for
ions to move from one space to the other space. Ions that are
introduced into the analyzer region with a carrier gas must follow
a trajectory through only one of the spaces 94a and 94b, in
dependence upon an ion initial position, a flow of carrier gas,
random diffusion and the effects of space-charge induced ion-ion
repulsive forces. The electrode 91 is connected to an electrical
controller (not shown) such that, in use, an asymmetric waveform
electric field and a superimposed dc voltage, wherein the
superimposed dc voltage is other than the compensation voltage, is
applied to electrode 91. The electrodes 92 and 93 are connected to
at least a dc voltage controller (not shown), such that, in use,
electrodes 92 and 93 are maintained at a dc voltage with respect to
electrode 91 so as to establish a voltage difference between
electrode 91 and electrodes 92 and 93 that corresponds the
compensation voltage. Each one of an ion-inlet electrode (not
shown) and an ion-outlet electrode (not shown) are attached to a
voltage controller (not shown). Of course, the dc voltage applied
to at least the ion-outlet electrode is adjusted to maximize
efficiency of the ion transmission.
[0102] Since the electrode plates 92, 91 and 93 are curved, a
focusing region exists not only at the curved leading and trailing
edges, as was the case with the pure parallel plate FAIMS, but also
within the two spaces between the electrode plates. However since
the asymmetric waveform is applied to only the second electrode 91,
the fields within the space 94a between the first electrode 92 and
the second electrode 91 are other than identical to the fields
within the space 94b between the second electrode 91 and the third
electrode 93. For instance, the fields in the space 94a between the
first and second electrodes are formed in a manner analogous to the
cylindrical geometry FAIMS wherein the surface of the second
electrode 91 is the inner cylinder of FAIMS. This means that if the
waveform polarity is positive and CV is negative, the positive ions
of type A are focused in this region, but other ions are rejected.
On the other hand, the electric fields within the second space 94b
between the second and third electrodes are not identical to the
fields within the first space 94a. This is because for the pair of
surfaces defining space 94b, the fields are formed in a manner
analogous to the cylindrical geometry FAIMS wherein the surface of
the second electrode 91 is the outer cylinder of FAIMS. The
positive ions of type A, which were focused within the first space,
will not be focused within the second space. At first appearance
this might be considered a disadvantage. On the other hand, there
are situations in which the simultaneous, parallel selection of two
different ion species is advantageous. For instance, by adjusting
the voltages that are applied to the first and third electrode
plates, positive ions of type A and negative ions of type A are
simultaneously transmitted in the region between the first and
second electrodes and in the region between the second and third
electrodes, respectively. These two beams are joined together at
the bottom of the powered plate 91, such that an ion neutralization
experiment, in which some negative ions and some positive ions are
mixed to give a degree of ion reaction, occurs.
[0103] Optionally, as shown in FIG. 9b, at least one of the leading
edge 95 and the trailing edge 96 of at least the second plate 91 is
further shaped to move the trapped ions to the center of the
leading and trailing edges 95 and 96, respectively. In face view,
the electrode 91 has a concave inwardly curved edge at is leading
edge 95, and a convex outwardly curved edge at its trailing edge
96. Ions being carried along by the carrier gas tend to follow the
electric fields around the concave surface of the leading edge of
the powered electrode 91 and flow generally towards the central
axis of the device. This will minimize the spread of the ions along
the width of the plates, as was discussed with reference to FIGS.
6a, 6b, 7a and 7b.
[0104] Referring to FIGS. 10a and 10b, a simplified block diagram
of an analyzer region of a multi-mode FAIMS device having two
curved electrode plates and a third lens shaped electrode,
according to a fifth preferred embodiment of the present invention
is shown. Three electrodes are shown with a space 104a between the
first electrode 102 and the second electrode 101 and a second space
104b between the second electrode 101 and the third electrode 103.
Additionally, the second electrode 101 is disposed substantially
mid-way between the first electrode 102 and the third electrode
103, such that the first, second and third electrodes are in a
substantially uniformly spaced-apart stacked arrangement. Of
course, optionally the spacing between the electrodes is other than
uniform. The series of electrodes are mounted into an insulating
support, which has grooves to hold the plates at fixed distances of
separation between the plates. A physical barrier (not shown) is
also provided along the outside edges of electrode plate 102 and
103 such that the flow of a carrier gas is directed only through
the spaces 104a and 104b. This top view, for instance a view of the
leading edge of the electrode plates, provided by FIG. 10a also
illustrates the mirror symmetry geometry that exists between the
two ion flow paths 104a and 104b. Each flow path is bordered on two
opposing sides by electrode surfaces and on two different opposing
sides by the insulating support. Of course, both ion flow paths are
open in a direction into and out of the plane of the drawing.
[0105] Each space 104a and 104b defines a separate ion flow path
that is closed on four sides such that it is other than possible
for ions to move from one space to the other space. Ions that are
introduced into the analyzer region with a carrier gas must follow
a trajectory through only one of the spaces 104a and 104b, in
dependence upon an ion initial position, a flow of carrier gas,
random diffusion and the effects of space-charge induced ion-ion
repulsive forces. The electrodes 102, 101 and 103 are connected to
an electrical controller (not shown) such that, in use, an
asymmetric waveform and a superimposed dc compensation voltage are
applied to electrode 101. The electrodes 102 and 103 are typically
maintained at a ground potential. Alternatively, the electrodes 102
and 103 are maintained at some other dc voltages.
[0106] Since the electrode plates 102, 101 and 103 are curved, a
focusing region exists not only at the curved leading and trailing
edges, as was the case with the pure parallel plate FAIMS, but also
within the two spaces 104a and 104b between the electrode plates.
The electric fields between the first and second and the second and
third plates of the analyzer region shown generally at 100 are
modified significantly compared with those in shown generally at 90
in FIG. 9. In FIG. 10, the central second electrode acts, from the
point of view of the electric fields, like the inner electrode in
the cylindrical FAIMS geometry. In other words, with a positive
polarity DV applied to the second plate, and a negative dc bias of
this plate (CV) relative to the first and third plates, a same type
of positively charged ions will be focused in both analyzer regions
104a and 104b. In this case, the multi-mode FAIMS (mmFAIMS) is
acting in only one mode.
[0107] Optionally, as shown in FIG. 10b, at least one of the
leading edge 105 and the trailing edge 106 of at least the second
plate 101 is further shaped to improve transmission of the ions at
the leading and trailing edges 105 and 106, respectively. In face
view, the electrode 101 has a concave inwardly curved edge, which
is in fact a saddle-shaped depression, at is leading edge 105, and
a convex outwardly curved edge at its trailing edge 106. The saddle
shaped depression is required to ensure that ions do not collect in
a "pool" at the leading edge 105 of the electrode 101, which is
substantially lens shaped in cross section. In other words, the
saddle shaped depression provides a curvature in two directions to
focus the ions inwardly toward the central axis of the electrode
101 and to allow the focussed ions to cascade over the electrode
edge and into one of the analyzer spaces 104a and 104b. Ions
entrained in the carrier gas flow tend to follow the electric
fields around the concave surface of the leading edge of the
powered electrode 101 and flow generally towards the central axis
of the device. This focussing minimize the spread of the ions along
the width of the plates, as was discussed in greater detail with
reference to FIGS. 6a, 6b, 7a and 7b. Optionally, the electrode 101
has an outwardly curved dome at leading edge 105 for influencing
ion trajectories.
[0108] Further optionally, the width between the surfaces 101a and
101b on opposite sides of the central lens shaped electrode 101 is
varied, wherein uniform spacing is maintained between the first
electrode 102 and surface 101a, and between the third electrode 103
and surface 101b. Making this width smaller results in a device
having substantially parallel plate FAIMS properties, for instance
higher resolution and lower sensitivity. Alternatively, the width
is made larger such that the shape of the second electrode 101
approaches that of a cylinder. Optionally, the leading and trailing
edges 105 and 106, respectively, are provided with a curved surface
terminus for focusing and trapping ions that are selectively
transmitted within the semi-annular spaces between the first
electrode 102 and surface 101a, and between the third electrode 103
and surface 101b.
[0109] The devices shown in FIGS. 9a, 9b, 10a and 10b permit a
maximum flexibility in establishing conditions for simultaneous
transmission of ions with different high electric field strength
mobility properties. Advantageously, these devices also permit
simultaneous transmission of positive and negative ions by
establishing focusing conditions between the first and second
electrode plates that are different, and controllable, than the
focusing conditions between the second and third electrode plates.
Further advantageously, these devices are of a compact
construction, making them very well suited for interfacing with
existing scientific instruments, such as mass spectrometers. These
devices are capable of performing the types of ion separations that
will greatly extend the ability to study gas phase ion chemistry,
for example parallel selective transmission of both positive and
negative ion types. Although the devices described with reference
to FIGS. 9 and 10 have all employed thin non-powered electrodes, it
is anticipated by the inventors that alternatively other electrode
structures are used, such as those shown in FIGS. 9c and 10c. In
FIGS. 9c and 10c the first and third, for instance non-powered
electrodes, are constructed of blocks of conducting material, and
only a surface opposing the powered electrode is shaped to maintain
a uniform spacing within the analyzer region.
[0110] Referring to FIGS. 11a and 11b, shown is a simplified block
diagram of an analyzer region of an improved FAIMS device according
to a sixth preferred embodiment of the present invention. The
analyzer region 110 is an example of a perpendicular-gas-glow-FAIMS
(pFAIMS), shown in FIG. 11a in side view and in FIG. 11b in
end-view. Three curved electrode bodies are shown with a space 114a
between the first electrode 111 and second electrode 112 and a
second space 114b between the first electrode 111 and the third
electrode 113. The series of electrodes are mounted into an
insulating support 64, which has grooves to hold the plates at
fixed distances of separation between the plates. Each space 114a
and 114b defines a separate ion flow path on opposite sides of the
first electrode 111. In a preferred embodiment, the first electrode
111 is disposed substantially mid-way between the second electrode
112 and the third electrode 113, such that the first and second
spaces 114a and 114b, respectively, on opposite sides of the first
electrode 111 are symmetrically uniform. The electrodes 111, 112
and 113 are connected to an electrical controller (not shown) such
that, in use, an asymmetric waveform and a superimposed dc
compensation voltage are applied to electrode 111. The electrodes
112 and 113 are typically maintained at ground potential.
Optionally, the electrodes 112 and 113 are maintained at some other
dc voltages.
[0111] Still referring to FIG. 11a, ions are produced in the gas
phase by an ionization source (not shown) and carried by a gas flow
through an ion-inlet orifice 116 having substantially slit-like
geometry. The ions are carried by the gas flow in either direction
around the first electrode 111 within one of the spaces 114a and
114b, to an ion-outlet orifice 117 having substantially slit-like
geometry. Ions exiting the ion-outlet orifice 117 are observed by
an ion detector (not shown). Optionally, ions exiting the
ion-outlet orifice 117 are collected for further processing. An
asymmetric waveform and a compensation voltage are applied to the
first electrode 111, and serve to cause a separation of ions in the
pFAIMS analyzer region defined by spaces 114a and 114b. Those ions
with high field mobility properties suitable for selective
transmission at the applied dispersion voltage amplitude and the
applied compensation voltage will arrive at the ion-outlet orifice
117 of the pFAIMS. Under the influence of gas flows, and additional
applied electric fields, the ions leave the pFAIMS analyzer region
defined by spaces 114a and 114b, and travel out of the ion-outlet
orifice 117.
[0112] Referring to FIG. 12a, a pFAIMS device according to a
seventh embodiment of the present invention is shown generally at
110. Device 110 includes an inner cylindrical electrode 111 which
is a solid cylinder with a highly polished outer surface finish,
and an outer cylindrical electrode 119 which is a pipe having an
ion-inlet orifice 118 therethrough. A slit-shaped ion-outlet
orifice 117 is disposed opposite ion-inlet orifice 118. Ion-outlet
orifice 117 is formed within a flat portion along an edge of
electrode 119 by removal of sufficient metal such that a narrow
orifice 117 is formed.
[0113] As shown in FIG. 12b, the pFAIMS apparatus 110 is closely
spaced to an orifice plate 84 mounted on an electrically isolating
support 85 which is in turn mounted onto the front vacuum housing
86 of a mass spectrometer. A small orifice 83 in the orifice plate
84 allows ions to be pulled into the low pressure region of the
interface of the a mass spectrometer (not shown). There is a small,
optional space 87 between device 110 and the orifice plate 84. This
permits the accommodation of different gas flow rates through the
analyzer region 114 between the electrodes 111 and 118. For
instance, if the flow through pFAIMS device 110 exceeds the flow
through the orifice 83, the excess gas will flow out through the
gap 87. If the flows through the pFAIMS and through the orifice 83
are substantially equal, the pFAIMS is optionally mounted directly
against the orifice plate 84.
[0114] Still referring to FIG. 12b, ions are produced in the gas
phase by an ionization source, for instance an electrospray
ionization ion source composed of a liquid delivery capillary 21
and a fine-tipped electrospray needle 22 that is held at high
voltage (power supply not shown). Of course, any other suitable
ionization source is used optionally in place of the electrospray
ionization ion source. The ions pass to pFAIMS through a curtain
gas assembly composed of a curtain plate 23 with orifice 24, a gap
25 between the curtain plate 23 and the outer electrode 119 of
FAIMS, and into an ion-inlet orifice 118 in the outer FAIMS
electrode 119. A curtain gas 28 enters the gap 25, and escapes in
part out through the orifice 24 in the curtain plate 23, and in
part travels into the FAIMS analyzer region 114 through orifice
118. The ions enter the pFAIMS through orifice 118 and are
separated as they are carried by a flow of gas along the analyzer
region 114. The analyzer region 114 is an annular space between a
cylindrical inner FAIMS electrode 111 and the outer FAIMS electrode
119. The asymmetric waveform and the compensation voltage are
applied to the inner FAIMS electrode 111. The outer FAIMS electrode
119 is maintained at a dc voltage by a power supply (not shown).
The ions that pass through the analyzer region 114 are carried by
gas flow and electric fields out of the orifice 117 of the pFAIMS
and through an orifice 83 in orifice plate 84 leading into the low
pressure region of the interface of the a mass spectrometer (not
shown).
[0115] Advantageously, pFAIMS reduces the minimum distance which
must be travelled by the ions to about half of the circumference of
the inner FAIMS electrode 111. This is accomplished with the added
benefit that some of the ions will travel in opposite directions
around the inner electrode 111 once they have entered pFAIMS, thus
reducing the effective ion density and reducing the ion-ion
repulsion space charge effect. Further advantageously, the
reduction of the minimum ion travel distance will have the added
benefit of improving the ion transmission efficiency. For example,
by keeping the time for travel short, the ions will tend not to
spread out due to diffusion and ion-ion repulsion forces. In
keeping distances small, the transit time of the ions is also
short.
[0116] Of course, numerous other embodiments could be envisioned,
without departing significantly from the teachings of the present
invention.
* * * * *