U.S. patent application number 11/328968 was filed with the patent office on 2007-07-12 for ion trap mobility spectrometer.
This patent application is currently assigned to GE Security, Inc.. Invention is credited to Paul E. Haigh.
Application Number | 20070158548 11/328968 |
Document ID | / |
Family ID | 38231890 |
Filed Date | 2007-07-12 |
United States Patent
Application |
20070158548 |
Kind Code |
A1 |
Haigh; Paul E. |
July 12, 2007 |
Ion trap mobility spectrometer
Abstract
An ITMS includes an inlet for receiving a sample that will be
tested for a substance of interest. The inlet communicates with an
ionization chamber and a drift chamber communicates with the
downstream end of the ionization chamber. A first grid electrode
extends across the downstream end of the ionization chamber and a
second grid electrode is slightly downstream from and parallel to
the first grid electrode. A slight potential bias is applied to the
first grid electrode to hold the ions in the potential well between
the first and second grid electrodes. However a pulse is applied
periodically to the first grid electrode to accelerate ions into
the drift chamber. The accumulation of the ions in the potential
well prior to generation of the pulse results in a thinner band of
ions ejected into the drift chamber and hence achieves higher
resolution.
Inventors: |
Haigh; Paul E.;
(Londonderry, NH) |
Correspondence
Address: |
CASELLA & HESPOS
274 MADISON AVENUE
NEW YORK
NY
10016
US
|
Assignee: |
GE Security, Inc.
Bradenton
FL
|
Family ID: |
38231890 |
Appl. No.: |
11/328968 |
Filed: |
January 9, 2006 |
Current U.S.
Class: |
250/287 ;
250/282 |
Current CPC
Class: |
G01N 33/22 20130101;
G01N 27/622 20130101 |
Class at
Publication: |
250/287 ;
250/282 |
International
Class: |
H01J 49/40 20060101
H01J049/40 |
Claims
1. An ion trap mobility spectrometer comprising: an inlet for
receiving a sample to be tested for the presence of at least one
substance of interest; an ionization chamber having an upstream end
communicating with the inlet and a downstream end; a drift chamber
having an upstream end communicating with the downstream end of the
ionization chamber and a downstream end opposed to the upstream end
thereof; a collector electrode in proximity to the downstream end
of the drift chamber for collecting ions passing through the drift
chamber; first and second grid electrodes aligned substantially
parallel to one another substantially at an interface between the
downstream end of the ionization chamber and the upstream end of
the drift chamber; and a controller for applying a slight potential
bias to the first grid electrode relative to the second grid
electrode during an ion accumulation phase for accumulating ions
between the first and second grid electrodes, the controller
further being operative for applying a pulse to the first grid
electrode for accelerating ions accumulated between the first and
second grid electrodes into the drift chamber and towards the
collector electrode.
2. The ion trap mobility spectrometer of claim 1, wherein the
second grid electrode and the upstream end of the drift chamber are
maintained at substantially a common potential when the pulse is
applied to the first grid electrode.
3. The ion trap mobility spectrometer of claim 2, wherein a voltage
of approximately 1,000 volts is applied to the second grid
electrode when the pulse is applied to the first grid electrode for
accelerating ions between the first and second grid electrodes into
the drift chamber.
4. The ion trap mobility spectrometer of claim 1, wherein the
ionization chamber includes a substantially cup-shaped chamber wall
formed from a conductive material.
5. The ion trap mobility spectrometer of claim 1, wherein the
controller is operative for applying a negative bias to the first
grid electrode for a positive ion mode of operation and for
applying a positive bias to the first grid electrode for negative
ion mode operation.
6. A method for operating an ion trap mobility spectrometer to
determine whether trace amounts of substances of interest are
present in a sample, the ion trap mobility spectrometer having an
inlet for receiving a sample to be tested for the trace amounts of
the substance of interest, an ionization chamber communicating with
the inlet, first and second substantially parallel grid electrode
disposed sequentially substantially at a downstream end of the
ionization chamber and a drift chamber downstream from the second
grid electrode, said method comprising: operating the ion trap
mobility spectrometer during an ion accumulation phase by applying
a potential bias to the first grid electrode relative to both
peripheral walls of the ionization chamber and the second grid
electrode for defining a potential well between the first and
second grid electrodes that accumulates a narrow band of ions
therein; applying a pulse to the first grid electrode while
maintaining the second grid electrode substantially at a potential
defined by the drift chamber for accelerating the narrow band of
ions from the potential well defined between the first and second
grid electrodes.
7. The method of claim 6, wherein the step of applying a potential
bias to the first grid electrode comprises applying a negative bias
to the first grid electrode for a positive ion mode of operation to
detect narcotics in the sample.
8. The method of claim 6, wherein the step of applying a potential
bias to the first grid electrode comprises providing a positive
bias to the first grid electrode for operating the ion trap
mobility spectrometer in a negative mode to test samples for the
presence of explosives.
9. The method of claim 6, wherein the step of applying a pulse
comprises a pulse for a duration of 0.1-0.2 mS.
10. The method of claim 9, further comprising adjusting the second
grid electrode to a voltage substantially corresponding to voltage
existing in adjacent portions of the drift chamber while the pulse
is applied to the first grid electrode.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a detector to test for trace
amounts of substances of interest.
[0003] 2. Description of the Related Art
[0004] Terrorism risks continue at transportation facilities,
government buildings, banks, restaurants, hotels and other
locations where there is a significant flow of pedestrian or
vehicular traffic. As a result, virtually all airports and many
other buildings now include apparatus for detecting trace amounts
of explosives.
[0005] Narcotics are illegal and insidious. Furthermore, it is
known that many terrorist organizations fund their terrorism
through the lucrative sale of narcotics. Accordingly, many airports
and other public buildings recognize the need to check for
narcotics.
[0006] Ion mobility spectrometers have been commercially available
since about 1970, and are used to test for the presence of at least
selected constituents in a stream of sample gas. An ion mobility
spectrometer can be used to detect the presence of explosives in
the sample gas. The typical prior art ion mobility spectrometer
includes an ionization region and a drift region. A sample of air
to be analyzed is fed into the ionization region on a stream of
carrier gas containing a halogenated compound. The carrier gas is
ionized by .beta. particles emitted from radioactive walls of the
ionization chamber to form positive ions and electrons. The
electrons are captured by gases, causing a series of reactions that
lead to the ionization of the halogen. Molecules of interest form
ions by interaction with these gas phase ions. An electric field is
established in the ionization region. The polarity of the field can
be set to direct the ions of interest towards the drift region of
the prior art detector. The ions travel through the drift region
and towards a collector electrode at the end of the drift region
opposite the ionization region. The drift time to the collector
electrode varies in accordance with the size-to-change ratio of the
ions. A current will be established at the collector electrode at
different times depending upon the arrival times of the ions. This
current is amplified and converted to a voltage for signal analysis
purposes. Specific substances of interest will have a unique drift
time. The detector can be calibrated to identify substances of
interest based on that drift time and produce an alarm signal to
the operator. Unfortunately, the conventional ion mobility
spectrometer allows ions to pass into the drift region for only a
short period of time. Ions arriving at the entry to the drift
region at all other times are discharged. As a result, most ions
are discharged in older ion mobility spectrometers and the
ionization and collection efficiency of older ion mobility
spectrometers is less than 0.01%. Accordingly, older ion mobility
spectrometers can not detect many substances of interest that might
be present in a sample.
[0007] U.S. Pat. No. 5,200,614 and U.S. Pat. No. 5,491,337 each
disclose an ion trap mobility spectrometer (ITMS) that provide
enhanced ability to detect trace amounts of substances of interest.
The ion trap mobility spectrometer shown in U.S. Pat. No.
5,200,614, carries a sample vapor into a detector inlet on a
carrier gas. The carrier gas may be doped with a low concentration
vapor employed as a charge transfer mediator. Sample molecules of
interest are fed through an inlet and a diffuser, and into an
ionization chamber. The prior art ionization chamber has a
cup-shaped metal wall and a radioactive material is disposed in the
chamber. An open grid electrode is at the downstream end of the
ionization chamber and normally is at the same potential as the
metal walls of the ionization chamber. Thus, a largely field-free
space is defined in the ionization chamber. Electrons and ion
charges build up this field-free space and interact with the sample
molecules under bombardment by beta-particles from the radioactive
walls. However, a field is established periodically across the
ionization region to sweep the ions into a drift region of the
ITMS. The ions in the drift region experience a constant electric
field maintained by annular electrodes, and are impelled along the
drift region towards a collector electrode for analysis on the
basis of their spectra. The field across the grid electrode and
metal cup of the ionization chamber is reduced again to zero after
about 0.2 mS and the ion population again is allowed to build up in
the chamber preparatory to the imposition of the next field. The
polarity of the fields of the prior art detector are chosen on the
basis of whether the detector is operated in a negative or positive
ion mode. A negative ion mode usually is preferred when detecting
explosives. U.S. Pat. No. 5,491,337 discloses an ion trap mobility
spectrometer with enhanced performance in a positive mode to test
for the presence of trace amounts of narcotics. U.S. Pat. No.
6,765,198 discloses an ion trap mobility spectrometer that can
analyze a single sample in a negative mode to test for explosives
and in a positive mode to test for narcotics. The disclosures of
U.S. Pat. No. 5,200,614, U.S. Pat. No. 5,491,337 and U.S. Pat. No.
6,765,198 are incorporated herein by reference.
[0008] Detectors that incorporate the teaching of U.S. Pat. No.
5,200,614, U.S. Pat. No. 5,491,337 and U.S. Pat. No. 6,765,198 are
marketed by GE Security, Inc. and have proved to be very effective
and commercially successful. However, a demand still exists for
detectors with improved resolution. In this regard, it has been
determined that the resolution of the peaks detected by the
above-described detectors are dependent on the width of the pulse
of ions introduced into the drift chamber as well as the broadening
that the clouds of ions experience in the drift chamber due to
diffusion, electronic repulsion and other factors. It also has been
determined that the area of the outer periphery of the ionization
chamber close to the grid electrode create a higher electric field
during the introduction of ions into the drift chamber. Ions in
these areas of higher electric fields would be introduced into the
drift region before ions in lower electric field areas of the
ionization chamber inwardly from the peripheral walls that define
the ionization chamber. This difference in ion introduction time
widens the pulse of ions and degrades the overall peak resolution
of the detector. Accordingly, an object of the invention is to
provide a detector with improved resolution.
SUMMARY OF THE INVENTION
[0009] The invention relates to a detector and preferably a
detector that includes an ion trap mobility spectrometer. The
detector has an inlet for receiving a sample that will be tested
for substances of interest. The sample may be received by: a flow
of air traveling past a human being or package; particles
transferred from a package or luggage to a sample trap; particles
transferred from a suspect's hand to a ticket or card; particles
transferred directly from the hand of the suspect to a sample
collection surface of the detector; or other optional ways for
receiving samples that may have substances of interest. The
detector includes a heater for heating the collected sample
sufficiently to vaporize the sample. The detector then includes a
supply of a transporting gas for transporting the vaporized sample
into the detector. The apparatus for collecting the sample,
vaporizing the sample and transporting the sample into the detector
can be a known apparatus, such as those disclosed in U.S. Pat. No.
5,200,614, U.S. Pat. No. 5,491,331, U.S. Pat. No. 6,073,499, U.S.
Patent Publication No. 2005/0019220 or pending U.S. patent
application Ser. No. 10/929,915, the disclosures of which are
incorporated herein by reference.
[0010] The detector has an inlet for receiving the gas stream that
includes the vaporized sample that will be tested for the presence
of at least one substance of interest. The detector further
includes an ionization chamber that communicates with the inlet and
a drift chamber that communicates with the ionization chamber.
[0011] The ionization chamber preferably is a generally cup-shaped
structure with an inlet and an outlet. The inlet to the ionization
chamber receives the gas that includes the vaporized sample that
will be tested for the presence of at least one substance of
interest. The outlet end of the ionization chamber preferably is
cross-sectionally larger than the inlet. The ionization chamber
preferably is formed from a metallic material and includes or
consists of a radioactive material. A preferred ionization chamber
is formed from a gold plated metal cup and a radioactive foil is
applied to an inner peripheral surface of the cup. A preferred
radioactive material is nickel.sup.63.
[0012] The drift chamber has an inlet end that communicates with
the outlet of the ionization chamber. The drift chamber also
includes an outlet end, and a collection electrode is disposed near
the outlet end of the drift chamber. Drift electrodes are disposed
between the inlet end of the drift chamber and the collector
electrode and function to maintain field conditions conducive to a
controlled downstream drift of ions from the ionization chamber to
the collector electrode.
[0013] The detector of the subject invention further includes a
first grid electrode at the outlet from the ionization chamber and
a second grid electrode downstream from the first grid electrode. A
slight potential bias is applied to the first grid electrode with
respect to the walls of the ionization chamber and with respect to
the second grid electrode during the ion-accumulation stage of the
operation of the detector. As a result, ions created in the
ionization chamber are attracted to the first grid electrode. More
specifically, a negative bias is applied for positive ion mode
operation (e.g., narcotics detection) and a positive bias is
applied for negative ion mode operation (e.g., explosive
detection). The potential difference between the first grid
electrode and the ionization chamber and the second grid electrode
is small, but effectively creates a "potential well". Ions
accumulate in the potential well between the first and second grid
electrodes during the ion accumulation stage of the operation.
[0014] The detector introduces ions into the drift chamber by
creating an electric field between the first and second grid
electrodes for a short duration. This distinguishes from the prior
art detector where the ions are introduced into the draft chamber
by creating an electric field between the cup-shaped walls of the
ionization chamber and the grid electrode. The first and second
grid electrodes of the subject invention are coplanar,
substantially parallel and close together. As a result, the
electric field generated between the first and second grid
electrodes is substantially uniform. Ions of a given type
accelerated in this potential well between the first and second
grid electrodes move substantially simultaneously and at
substantially the same speed towards the drift region. As a result,
a thinner band of ions is introduced into the drift chamber,
thereby improving resolution. Additionally, sensitivity is improved
because the ions are concentrated in the relatively small space
defined by the potential well between the first and second grid
electrodes.
BRIEF DESCRIPTION OF THE DRAWING
[0015] FIG. 1 is a schematic cross-sectional view of an ion trap
mobility spectrometer detector in accordance with the subject
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0016] An ion trap mobility spectrometer in accordance with the
subject invention is illustrated in FIG. 1. The ion trap mobility
spectrometer includes a generally cylindrical detector 20 having an
inlet 22 at one end for receiving a sample air of interest borne by
a carrier gas. The carrier gas may be doped with a low
concentration vapor employed as a charged transfer mediator, as
described in the above-referenced patents that have been
incorporated herein by reference. The sample air, carrier gas and
any dopant molecules that may be employed pass through the inlet 22
and are spread by a diffuser 24 into an ionization chamber 26. The
ionization chamber 26 includes a cup 28 formed from a gold-plated
conductive metal with an inlet end wall 30 that extends outwardly
from the inlet 22 and a cylindrical or flared sidewall 32 that
extends downstream from the inlet end wall 30. A foil 34 is applied
to the inner surface of the cylindrical sidewall 32. The foil 34 is
formed from a radioactive material such as nickel.sup.63 or tritium
that emits .beta. particles. A first grid electrode G1 extends
across the downstream end of the cylindrical sidewall 32. A second
grid electrode G2 is disposed slightly downstream from the first
grid electrode G1 and is aligned substantially parallel
thereto.
[0017] A drift chamber 40 communicates with the downstream end of
the ionization chamber 26. The drift chamber 40 includes a
plurality of open grid electrodes E.sup.1-E.sup.N aligned
substantially parallel to one another and downstream from the
second grid electrode G2. The collector electrode 42 is
substantially at the downstream end of the drift chamber 40 and
communicates with a processor and readout device. The downstream
end of the drift chamber 40 also includes an exhaust outlet for
permitting an outflow of gas from the ion trap mobility
spectrometer 20.
[0018] A slight potential bias is applied by a controller 48 to the
first grid electrode G1 with respect to both the cup 28 and the
second grid electrode G2 during the ion accumulation phase of an
operation cycle of the ion trap mobility spectrometer. In this
regard, a negative bias is applied to the first grid electrode G1
during the positive ion mode operation for detecting narcotics and
a positive bias is applied to the first grid electrode G1 during
the negative ion mode operation for detecting explosives. As a
result, ions created in the ionization chamber 26 are attracted to
the first grid electrode G1 during the ion accumulation phase of
the operation of the ion trap mobility spectrometer 20. This lower
potential of the first grid electrode G1 between the cup 28 and the
second grid electrode G2 creates a potential well 50, and ions tend
to accumulate in the area of the potential well 50.
[0019] The ion trap mobility spectrometer then is subjected to a
kick-out phase for introducing the ions into the drift chamber 40.
More particularly, a short duration (e.g., 0.1-0.2 mS) pulse is
applied to the first grid electrode G1 while adjusting the second
grid electrode G2 to approximately the highest potential in the
drift field of the drift chamber 40, which typically is about 1,000
volts. Thus, ions are introduced into the drift chamber 40 by the
creation of the electric field between first and second grid
electrodes G1 and G2 instead of by creating an electric field
merely between the cup 28 and the first grid electrode G1, as in
the prior art. The first and second grid electrodes G1 and G2 are
planar, parallel and closely spaced. This differs from the
non-planar configuration of the cup 28. As a result, the electric
field generated between the first and second grid electrodes G1 and
G2 is uniform. Ions of a given type accelerate into drift chamber
40 at approximately the same time and at the same speed. As a
result, thinner bans of ions move into the drift region to achieve
enhanced resolution. Additionally, sensitivity is improved because
the ions are concentrated in the relatively small space of the
potential well 50 defined between the first and second grid
electrodes G1 and G2.
[0020] While the invention has been described with respect to a
preferred embodiment, it is apparent that various changes can be
made without departing from the scope of the invention as defined
by the appended claims.
* * * * *