Radiant energy – Ionic separation or analysis – With sample supply means
Reexamination Certificate
2003-01-09
2004-09-07
Wells, Nikita (Department: 2881)
Radiant energy
Ionic separation or analysis
With sample supply means
C250S286000, C250S281000
Reexamination Certificate
active
06787765
ABSTRACT:
FIELD OF THE INVENTION
The instant invention relates generally to high field asymmetric waveform ion mobility spectrometry (FAIMS), more particularly the instant invention relates to an apparatus and method for non-destructive detection of ions separated by 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 separated in the drift tube on the basis of differences in their drift velocities. The drift velocity of an ion is proportional to the applied 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 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, 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 electric field, and 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). 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 due to the compound dependent behavior of K
h
as a function of the applied electric field strength.
In general, a device for separating ions according to the FAIMS principle has an analyzer region that is defined by a space between first and second spaced-apart electrodes. Often, the first electrode is maintained at ground potential while the second electrode 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
h
, lasting for a short period of time t
h
and a lower voltage component, V
l
, of opposite polarity, lasting a longer period of time t
l
. The waveform is synthesized such that the integrated voltage-time product, and thus the field-time product, applied to the second electrode during each complete cycle of the waveform is zero, for instance V
h
t
h
+V
l
t
l
=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.
Generally, the ions that are to be separated are entrained in a stream of gas flowing through the FAIMS analyzer region, for example between a pair of horizontally oriented, spaced-apart electrodes. Accordingly, the net motion of an ion within the analyzer region is the sum of a horizontal x-axis component due to the stream of gas and a transverse y-axis component due to the applied electric field. During the high voltage portion of the waveform an ion moves with a y-axis velocity component given by v
h
=K
h
E
h
, where E
h
is the applied field, and K
h
is the high field ion mobility under operating electric field, pressure and temperature conditions. The distance traveled by the ion during the high voltage portion of the waveform is given by d
h
=v
h
t
l
=K
h
E
h
t
h
, where t
h
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
l
=KE
l
, where K is the low field ion mobility under ambient pressure and temperature conditions. The distance traveled is d
l
=v
l
t
l
=KE
l
t
l
. Since the asymmetric waveform ensures that (V
h
t
h
)+(V
l
t
l
)=0, the field-time products E
h
t
l
, and E
l
t
l
are equal in magnitude. Thus, if K
h
and K are identical, d
h
and d
l
are equal, and the ion is returned to its original position along the y-axis during the negative cycle of the waveform. If at E
h
the mobility K
h
>K, the ion experiences a net displacement from its original position relative to the y-axis. For example, if a positive ion travels farther during the positive portion of the waveform, for instance d
h
>d
l
, then the ion migrates away from the second electrode and eventually will be neutralized at the first electrode.
In order to reverse the transverse drift of the positive ion in the above example, a constant negative dc voltage called the “compensation voltage” or CV can be applied to the second electrode. This dc voltage prevents the ion from migrating toward either the second or the first electrode. If ions derived from two compounds respond differently to the applied high strength electric fields, the ratio of K
h
, to K may be different for each compound. Consequently, the magnitude of the CV that is necessary to prevent the drift of the ion toward either electrode is also different for each compound. Thus, when a mixture including several species of ions, each with a unique K
h
/K ratio, is being analyzed by FAIMS, only one species of ion is selectively transmitted to a detector for a given combination of CV and DV. In one type of FAIMS experiment, 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.
U.S. Pat. No. 5,420,424, issued to Carnahan and Tarassov on May 30, 1995, teaches a FAIMS device having cylindrical electrode geometry and electrometric ion detection, the contents of which are incorporated herein by reference. The FAIMS analyzer region is defined by an annular space between inner and outer cylindrical electrodes. In use, ions that are to be separated are entrained into a flow of a carrier gas and are carried into the analyzer region via an ion inlet orifice. Once inside the analyzer region, the ions become distributed all the way around the inner electrode as a result of the carrier gas flow and ion-ion repulsive forces. The ions are selectively transmitted within the analyzer region to an ion extraction region at an end of the analyzer region opposite the ion inlet end. In particular, a plurality of ion outlet orifices is provided around the circumference of the outer electrode for extracting the selectively transmitted ions from the ion extraction region for electrometric detection. Of course, the electrometric detectors provide a signal that is indicative of the total ion current arriving at the detector. Accordingly, the CV spectrum that is obtained using the Carnahan device does not include information relating to an identity of the selectively transmitted ions. It is a limitation of the Carnahan device that the peaks in the CV spectrum are highly susceptible to being assigned incorrectly. It is another limitation of the Carnahan device that the ions are consumed upon being detected at the electrometric detector. Accordingly, it is not possible to perform further analysis or separation of the ions, or to collect the ions for other uses.
Re
Barnett David
Guevremont Roger
Purves Randy
Freedman & Associates
Ionalytics Corporation
Quash Anthony
LandOfFree
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