Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
2002-09-13
2004-10-05
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
C250S288000
Reexamination Certificate
active
06799355
ABSTRACT:
FIELD OF THE INVENTION
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
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.
E. A. Mason and E. W. McDaniel in their book entitled “Transport Properties of Ions in Gases” (Wiley, N.Y., 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
h
, a non-constant high field mobility term. The dependence of K
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
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
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.
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
h
=K(1+f(E)), where K
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
1
, lasting for a short period of time t
2
and a lower voltage component, V
2
, of opposite polarity, lasting a longer period of time t
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
1
t
2
+V
2
t
1
=0; for example +2000 V for 10 &mgr;s followed by −1000 V for 20 &mgr;s. The peak voltage during the shorter, high voltage portion of the waveform is called the “dispersion voltage” or DV in this disclosure.
During the high voltage portion of the waveform, the electric field causes the ion to move with a transverse y-axis velocity component v
1
=K
h
E
high
, where E
high
is the applied field, and K
h
is the high field ion mobility under ambient electric field, pressure and temperature conditions. The distance traveled is d
1
=v
1
t
2
=K
h
E
high
t
2
, where t
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
2
=KE
low
, where K is the low field ion mobility under ambient pressure and temperature conditions. The distance traveled is d
2
=v
2
t
1
=KE
low
t
1
. Since the asymmetric waveform ensures that (V
1
t
2
)+(V
2
t
1
)=0, the field-time products E
high
t
2
and E
low
t
1
are equal in magnitude. Thus, if K
h
and K are identical, d
1
and d
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
high
the mobility K
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
1
>d
2
, and the type A ion migrates away from the second plate. Similarly, positive ions of type C migrate towards the second plate.
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
h
to K is similarly 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
Barnett David
Guevremont Roger
Javahery Gholamreza
Purves Randy
Stott William Ronald
Freedman & Associates
Lee John R.
Leybourne James J.
National Research Council Canada
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