U.S. patent application number 10/404352 was filed with the patent office on 2003-10-30 for process and apparatus for adjusting an aerosol charge by using a corona discharge.
Invention is credited to Dorman, Frank D., Kaufman, Stanley L..
Application Number | 20030202920 10/404352 |
Document ID | / |
Family ID | 22489428 |
Filed Date | 2003-10-30 |
United States Patent
Application |
20030202920 |
Kind Code |
A1 |
Kaufman, Stanley L. ; et
al. |
October 30, 2003 |
Process and apparatus for adjusting an aerosol charge by using a
corona discharge
Abstract
A system for analyzing aerosols incorporates a corona discharge
ion generator with a positively or negatively charged corona
discharge needle formed of platinum or a platinum alloy. A high
speed (40-210 meter per second) air flow sweeps the ions away from
the corona discharge, and propels the ions into a mixing chamber in
a turbulent jet that encounters an aerosol, also provided to the
mixing chamber. In one version of the ion generator, the ions are
carried into the mixing chamber through an orifice formed in a
positively or negatively biased plate. In another alternative, the
aerosol droplets are electrostatically generated, and propelled
into the mixing chamber as an aerosol jet that confronts the ion
jet to enhance a mixing of the charged droplets and the ions. In
this version the droplets are advantageously neutralized to leave
predominantly singly charged positive and negative particles, to
provide a neutralized aerosol particularly well suited for analysis
with a mass spectrometer.
Inventors: |
Kaufman, Stanley L.; (New
Brighton, MN) ; Dorman, Frank D.; (Minneapolis,
MN) |
Correspondence
Address: |
PATENT DEPARTMENT
LARKIN, HOFFMAN, DALY & LINDGREN, LTD.
1500 WELLS FARGO PLAZA
7900 XERXES AVENUE SOUTH
BLOOMINGTON
MN
55431
US
|
Family ID: |
22489428 |
Appl. No.: |
10/404352 |
Filed: |
April 1, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10404352 |
Apr 1, 2003 |
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09596159 |
Jun 16, 2000 |
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6544484 |
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60140033 |
Jun 18, 1999 |
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Current U.S.
Class: |
422/186 |
Current CPC
Class: |
H01J 49/165 20130101;
H01J 49/168 20130101; G01N 15/0266 20130101; G01N 2015/0046
20130101 |
Class at
Publication: |
422/186 |
International
Class: |
B01J 019/08 |
Claims
What is claimed is: [Claims 1-17 are canceled.]
18. A process for characterizing a non-volatile material uniformly
dispersed throughout an electrically conductive liquid, including:
providing a liquid sample including an electrically conductive
liquid and a non-volatile material substantially uniformly
dispersed throughout the liquid; electrostatically generating
multiple electrically charged droplets of the liquid sample, and
entraining at least a portion of the charged droplets in a first
gas flow to provide an aerosol jet; generating multiple first ions,
and entraining at least a portion of the first ions in a second gas
flow to provide a first ion jet; and causing the aerosol jet and
the first ion jet to merge at a merger location, to promote a
mixing of the charged droplets and the first ions to alter the
droplet charges toward neutralization.
19. The process of claim 18 wherein: causing the aerosol jet and
the ion jet to merge comprises causing the jets to move in opposite
directions towards one another.
20. The process of claim 18 further including: evaporating the
charged droplets during the altering of the droplet charges.
21. The process of claim 20 wherein: evaporating of the droplets
includes substantially drying the droplets, whereby the aerosol
removed from the merger location consists essentially of particles
of the non-volatile material.
22. The process of claim 20 wherein: the altering includes
adjusting the charges of the droplets at a rate sufficient to
prevent the droplets from disintegrating due to repulsive Coulombic
forces as they evaporate.
23. The process of claim 18 further including: after removing the
aerosol, providing the aerosol to an aerosol characterizing
device.
24. The process of claim 23 wherein: providing of the aerosol to an
aerosol characterizing device comprises providing the aerosol to a
mass spectrometer.
25. The process of claim 18 wherein: the aerosol jet and the first
ion jet have linear velocities of at least about 40 meters per
second as they approach the merger location.
26. The process of claim 18 further including: generating multiple
second ions having electrical charges opposite to the charges of
the first ions, and entraining at least a portion of the second
ions in a third gas flow to provide a second ion jet; and causing
the second ion jet to merge with the aerosol jet and the first ion
jet at the merger location. [Claims 27-44 are canceled.]
45. A process for ionizing an aerosol, including: generating an
aerosol; using a corona discharge member formed of a nobel metal to
generate multiple first ions, and providing a first gas flow past
the discharge member to entrain at least a portion of the first
ions in the gas flow to provide a first ion carrying jet; and
causing the first ion carrying jet to merge with the aerosol at a
selected location, to promote a mixing of the ions with the aerosol
to alter an electrical charge distribution of the aerosol.
46. The process of claim 45 wherein: using the discharge member to
generate the ions comprises applying an ionizing current to the
discharge member in the range including from about 10 microamperes
to about 20 microamperes.
47. The process of claim 45 wherein: generating the aerosol
comprises electrostatically generating multiple electrically
charged droplets of a liquid sample that includes an electrically
conductive liquid and a non-volatile material dispersed
substantially uniformly throughout the liquid, and entraining at
least a portion of the charged droplets in a second gas flow to
provide an aerosol jet; and wherein causing the aerosol and the
first ion carrying jet to merge comprises causing the aerosol jet
and the first ion carrying jet to move in opposite directions
towards one another.
48. The process of claim 47 further including: evaporating the
electrically charged droplets during the altering of the charge
distribution.
49. The process of claim 48 wherein: evaporating the droplets
includes substantially drying the droplets, whereby the aerosol
after said altering consists essentially of particles of the
non-volatile material.
50. The process of claim 48 wherein: said altering includes
reducing respective charges of the droplets at a rate sufficient to
prevent the droplets from disintegrating due to repulsive Coulombic
forces.
51. The process of claim 45 further including: after said altering
the charge distribution, removing the aerosol from the selected
location.
52. The process of claim 51 further including: after removing the
aerosol, providing the aerosol to an aerosol characterizing
device.
53. The process of claim 52 wherein: providing the aerosol to an
aerosol characterizing device comprises providing the aerosol to a
mass spectrometer.
54. The process of claim 45 wherein: providing a first gas flow
past the corona discharge member includes causing the gas to travel
past the discharge member at a mean velocity at least about 40
meters per second.
55. The process of claim 45 further including: generating multiple
second ions having electrical charges opposite to the charges of
the first ions, and causing at least a portion of the second ions
to merge with the aerosol at the selected location.
56. The process of claim 55 wherein: said using the corona
discharge member to generate multiple first ions comprises biasing
the corona discharge member to a first electrical polarity; and
wherein said generating multiple second ions comprises biasing a
corona discharge element to a second electrical polarity opposite
to the first electrical polarity, and providing a second gas flow
past the discharge element to entrain at least a portion of the
second ions in the second gas flow to provide a second ion carrying
jet.
57. The process of claim 56 further including: selectively varying
the levels of the first and second electrical polarities to which
the corona discharge member and the corona discharge element,
respectively, are biased.
58. The process of claim 45 wherein: said altering the charge
distribution comprises altering the electrical charge distribution
towards neutralizing the aerosol.
59. The process of claim 18 further including: after altering the
droplet charges toward neutralization, removing the aerosol from
the merger location.
60. The process of claim 18 wherein: said providing a first ion jet
comprises causing the second gas flow and entrained first ions to
travel at a mean velocity of at least about 40 meters per
second.
61. A device for altering an electrical charge distribution of an
aerosol, including: an enclosure defining a mixing chamber; an
aerosol generator having a droplet discharge region proximate the
enclosure, and adapted to generate multiple droplets of a liquid
sample at the droplet discharge region; a first ion generator
spaced apart from the droplet discharge region, and electrically
biased to a first electrical polarity to generate multiple first
ions at a first corona discharge region thereof proximate the
enclosure; a first flow guide for guiding a first gas flow past the
droplet discharge region to entrain at least a portion of the
droplets and form an aerosol of the sample, and to carry the
entrained droplets into the mixing chamber; and a second flow guide
for guiding a second gas flow past the corona discharge region to
entrain at least a portion of the first ions having the first
electrical polarity and to carry the entrained ions toward a merger
with the aerosol in the mixing chamber, to intermix the aerosol and
the entrained first ions and thereby alter an electrical charge
distribution of the aerosol.
62. The device of claim 61 wherein: the enclosure further defines a
first orifice for admitting the aerosol into the mixing chamber,
and a second orifice for admitting the entrained first ions into
the mixing chamber.
63. The device of claim 62 wherein: the enclosure further defines
an exit orifice permitting the aerosol to exit the mixing chamber
after said altering of the charge distribution.
64. The device of claim 61 further including: a second ion
generator disposed proximate the enclosure and spaced apart from
the first ion generator and the droplet discharge region, biased to
a second electrical polarity opposite said first electrical
polarity to generate multiple second ions at a second corona
discharge region thereof; a third flow guide for guiding a third
gas flow past the second corona discharge region to entrain at
least a portion of the second ions having said second electrical
polarity and carry the entrained ions into the mixing chamber for a
merger at least with the aerosol.
65. The device of claim 61 wherein: said aerosol generator includes
an electrostatic droplet generator, electrically biased to generate
said multiple droplets electrically charged to a second electrical
polarity opposite said first electrical polarity, whereby said
altering of the electrical charge distribution tends to neutralize
the aerosol.
66. The device of claim 61 further including: an aerosol
characterizing device disposed to receive the aerosol after the
electrical charge distribution of the aerosol is so altered.
67. The device of claim 66 wherein: the aerosol characterizing
device is selected from the group of devices consisting of: a
differential mobility analyzer in combination with either a
condensation nucleus counter or an electrometer; and a mass
spectrometer.
68. The device of claim 62 wherein: the first gas flow is directed
into the enclosure through the first orifice at a mean velocity of
at least about 40 meters per second; and the second gas flow is
directed past the corona discharge region at a mean velocity of at
least about 40 meters per second.
69. The device of claim 61 wherein: the first gas flow includes a
combination of air and an electronegative gas selected from the
group consisting of: carbon dioxide and sulfur hexafluoride,
wherein the electronegative gas constitutes at least three percent
of the first gas flow.
70. A device for adjusting the electrical charge distribution of an
aerosol, including: an enclosure defining a chamber adapted to
receive an aerosol into the chamber from outside of the enclosure;
an ion generator electrically biased to generate multiple ions at a
corona discharge region thereof; a first flow guide for guiding the
aerosol into the chamber; and a second flow guide for guiding a gas
flow past the corona discharge region to entrain at least a portion
of the ions having a selected electrical polarity and carry the
entrained ions toward a turbulent merger with the aerosol in the
chamber, to rapidly mix the aerosol and the entrained ions and
thereby alter an electrical charge distribution of the aerosol.
71. The device of claim 70 wherein: the enclosure further defines a
first orifice to accommodate entry of the aerosol into the
chamber.
72. The device of claim 71 wherein: the enclosure further defines a
second orifice to accommodate entry of the entrained ions into the
chamber.
73. The device of claim 71 wherein: the aerosol flows through the
first orifice into the chamber at a mean velocity of at least about
40 meters per second.
74. The device of claim 70 further including: an aerosol
characterizing device disposed to receive the aerosol after the
electrical charge distribution is so altered.
75. The device of claim 74 wherein: the aerosol characterizing
device is selected from the group of devices consisting of: a
differential mobility analyzer in combination with either a
condensation nucleus counter or an electrometer; and a mass
spectrometer.
76. The device of claim 70 wherein: the aerosol includes a
plurality of droplets suspended in a carrier gas, and the carrier
gas includes air in combination with an electronegative gas
selected from the group consisting of: carbon dioxide and sulfur
hexafluoride, wherein the electronegative gas constitutes at least
three percent of the carrier gas.
Description
[0001] This is a divisional of copending application Ser. No.
09/596,159, filed Jun. 16, 2000.
[0002] This application claims the benefit of priority based on
Provisional Application No. 60/140,033 entitled "Aerosol Charge
Adjusting Apparatus Employing a Corona Discharge," filed Jun. 18,
1999.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to devices used to alter the
electrical charge distributions of aerosols, and more particularly
devices that utilize a corona discharge to generate ions, which
then are merged with an aerosol to either charge or neutralize the
aerosol.
[0004] The study of aerosols involves a variety of applications in
which it is desired to adjust the charges on the particles or
droplets of the aerosol. There are applications in which it is
advantageous to provide a charge distribution in which positive and
negatively charged particles counterbalance one another, i.e., an
equilibrium charge distribution. In other applications, it is
considered more important that a predominant number of the
particles carry no charge. In yet further applications, researchers
skew the charge distribution toward either the positive or the
negative side, and in a more specific application of this type
attempt to maximize the number of particles that carry a specific
non-zero charge. Corona discharge can be used in all of these
applications.
[0005] To produce a corona discharge, a non-uniform electrostatic
field is established between an electrically conductive needle and
a conductive structure proximate the needle, e.g., a plate or a
tube surrounding the needle. Given a sufficient field strength, air
near the needle experiences a breakdown and becomes conductive. In
the conductive corona region, accelerated electrons collide with
air molecules to create a dense cloud of free electrons and
positive ions. If the needle is biased to a positive voltage
relative to the surrounding structure, the electrons return to the
needle while the positive ions stream away from the needle toward
the adjacent structure. When the discharge needle is disposed
within a gas stream, many of the ions do not reach the adjacent
structure, but instead become entrained in the gas stream and are
transported by the gas stream toward the aerosol. When the
discharge needle is negatively biased, the free electrons leave the
needle, some of them attaching to molecules of the gas to form
negative ions, and are transported toward the aerosol by the gas
stream.
[0006] In an increasing number of aerosol studies, it is desired to
generate aerosols in which the particles are monodisperse, i.e.,
substantially uniform in size. For these applications, an
electrospray nebulizer is preferred, due to its ability to generate
small and uniform droplets. In an electrospray nebulizer, an
electrically conductive liquid is supplied at a controlled rate to
a capillary tube. A voltage differential between the capillary tube
and a surrounding conductive wall creates an electrostatic field
that induces a surface charge in the liquid emerging from the tube.
Electrostatic forces disperse the liquid into a fine spray of
charged droplets.
[0007] To produce the spray, the droplets are charged near the
"Rayleigh" limit, i.e., near the charge at which electrostatic
repulsion would overcome the surface tension that otherwise holds
the droplet together. As each electrospray droplet evaporates, the
charge density on its surface increases, eventually exceeding the
Rayleigh limit, causing a disintegration of the droplet into
smaller droplets. The droplet fragments in turn continue to
evaporate and can experience further disintegration. As a result,
the distribution of droplet sizes lacks the uniformity desired for
analysis of residue within the droplets.
[0008] To counteract this tendency, the droplets are charge
neutralized, beginning immediately or shortly after their
formation. In one approach, disclosed in U.S. Pat. No. 5,247,842
(Kaufman, et al.), radioactive Polonium is placed inside a chamber
through which the electrospray generated droplets travel as they
evaporate. The Polonium produces radiation that ionizes air
molecules, which in turn encounter the droplets and reduce their
charge. This enhances uniformity of the droplets by counteracting
their tendency to disintegrate due to electrostatic forces.
[0009] This approach yields a reproducible charge distribution by
exposing the aerosol particles or droplets to a bipolar plasma of
gas ions, both positive and negative, allowing the aerosol elements
to reach a steady state of charge distribution. This distribution
is useful because it is predictable and produces a large proportion
of particles having no charge. This approach has disadvantages,
however, in that the use of radioactive materials raises safety and
regulatory concerns. The cost of radioactive Polonium is relatively
high, and its half-life is relatively short. Further, although the
level of ion production can be varied by partially shielding the
radioactive material, the level of ionization cannot be precisely
controlled.
[0010] In view of the above, ion generation through use of a corona
discharge has been considered as an alternative method of
neutralizing electrospray droplets. The corona discharge can
generate unipolar (e.g., only negative) ions, and thus be
configured to counteract the charge of the electrospray droplets.
Alternatively, if both positive and negative ions are desired,
corona discharge devices can have oppositely charged corona
discharge needles, or a single corona discharge tip can be rapidly
alternated between positively and negatively charged states.
[0011] A disadvantage of corona discharge devices is their tendency
to generate aerosol particles. The problem is thought to arise from
the removal of material from the discharge needle, the creation of
highly reactive gaseous species at concentrations sufficient to
allow their aggregation into particles, or a combination of these
factors. In any event, aerosols generated by the corona discharge
needle interfere with attempts to measure the aerosol under study.
The tendency especially interferes with the analysis of extremely
fine particles, i.e., particles having diameters of about ten
nanometers or less.
[0012] Particle generation by corona discharge devices interferes
with their use in semiconductor manufacturing clean rooms, because
the particles can be large enough to contaminate silicone wafers
during their processing. In recognition of the problem, U.S. Pat.
No. 4,967,608 (Yost) describes a test chamber for measuring
particles larger than three nanometers in diameter emitted from a
corona discharge device. U.S. Pat. No. 5,447,763 and U.S. Pat. No.
5,650,203, both issued to Gehlke, recommend selecting certain
materials for corona discharge tips, e.g., titanium, aluminum, and
other metals that form protective oxide layers. Silicone coated
tips of these materials were favored. Platinum and tungsten also
were considered, but said to show substantial particle production,
and thus found unsatisfactory.
[0013] Recently, electrostatic generation of droplets has been
considered as a source of aerosols subject to analysis by mass
spectrometry, given the capability of generating aerosol droplets
that are small (submicron) and monodisperse. In addition, the
ability to rapidly and efficiently neutralize the aerosol,
preferably to the point where the aerosol consists predominantly of
singly charged particles, is a key factor when the aerosol is
provided to a mass spectrometer. Although the aforementioned
Kaufman patent discloses both the droplet generation and
neutralizing beneficial in this regard, a more efficient and more
controllable neutralizing of the aerosol would considerably enhance
the utility of electrospray-ionization mass spectrometry.
[0014] Therefore, it is an object of the present invention to
provide an aerosol system in which the charged droplets or charged
particles are neutralized more rapidly and in a manner that affords
more control over the degree of neutralizing.
[0015] Another object is to provide a corona discharge device
capable of selectively altering the charge distributions of
aerosols formed of extremely small droplets and particles, without
generating its own detectable particles and thereby interfering
with an analysis of the aerosol under study.
[0016] A further object is to provide an electrospray-ionization
mass spectrometry system in which the electrostatically generated
aerosol is effectively neutralized without requiring the use of
radioactive materials.
[0017] Yet another object is to provide a corona discharge device
particularly well suited for charging and neutralizing aerosols
consisting of submicron droplets or particles.
SUMMARY OF THE INVENTION
[0018] To achieve these and other objects, there is provided a
system for generating a charge-adjusted aerosol. The system
includes an enclosure defining a mixing chamber, a first orifice
for admitting an aerosol into the chamber, and a second orifice for
admitting corona discharge ions into the chamber. The system
further includes an electrostatic droplet generator having an
electrostatic discharge adapted to generate multiple electrically
charge droplets of a sample that includes an electrically
conductive liquid and a non-volatile material dispersed
substantially uniformly throughout the liquid. An ion generator of
the system has a corona discharge region electrically biased to
generate multiple ions. A fluid passage is adapted for a coupling
with a gas source to guide a first gas flow past the electrostatic
discharge region. This allows the gas flow to entrain at least a
portion of the charged droplets and form an aerosol of the sample,
and to carry the entrained droplets into the mixing chamber through
the first orifice to direct an aerosol jet into the chamber.
[0019] A second fluid passage is adapted for a coupling with a gas
source to guide a second gas flow past the corona discharge region.
This allows the second gas flow to entrain at least a portion of
the ions and carry the entrained ions into the mixing chamber
through a second orifice to direct an ion carrying jet into the
mixing chamber. The aerosol jet and the ion carrying jet merge
inside the mixing chamber in a turbulent flow that promotes the
mixing of the charged droplets and the ions, to alter the droplet
charges toward a neutralizing of the aerosol. The enclosure further
defines an exit orifice permitting the aerosol to exit the mixing
chamber after the altering of the droplet charges.
[0020] As used in this application, the term "neutralizing" refers
to a reduction--not the complete removal--of the charges in the
particles, droplets or other elements of the aerosol. In this
sense, a "neutralized" aerosol can include both charged and neutral
(uncharged) particles or droplets. An aerosol with an unbalanced
electrical charge distribution can be neutralized in the sense of
reducing the predominance of a positive (or negative) charge.
[0021] The degree of neutralization varies with the nature of the
analytical application. Some applications require neutralization
levels sufficient to prevent Coulomb disintegration of charged
droplets as they evaporate. In other applications, droplet
disintegration may be of no concern, but there may be a need to
insure that the number of particles carrying more than a single
charge is insignificant. Other applications might require a
balanced charge distribution, with or without any limit on the
number of charges carried by any given particle.
[0022] The preferred ion generator includes an electrically
conductive needle providing the corona discharge region. The needle
is mounted within an electrically conductive ion generating housing
and electrically biased with respect to the housing to provide a
corona current, preferably maintained within a range of 10-20
microamperes. The needle, at least along the corona discharge
region, is formed of a noble metal, in particular either platinum
or a platinum iridium alloy. Other metals of the platinum family
may be suitable, although less preferred.
[0023] Several factors are believed to contribute to the virtual
elimination of measurable particle generation by the corona
discharge needle. These include the needle material, the relatively
low corona current, and the relatively high velocity gas flow
(usually air) past the needle. The rapid air flow tends to cool the
discharge needle, which may be a key factor in preventing the
particle generation of a platinum needle discussed in the
aforementioned Gehlke patents. The airflow also may avoid or reduce
the bombardment of the discharge needle tip by corona ions, which
otherwise would tend to heat the tip, perhaps sufficiently to
evaporate material and thereby generate particles. Furthermore, the
active species formed in the corona discharge may be diluted by the
rapid airflow before they can aggregate into particles. Finally,
the lower corona current contributes to the reduced discharge
needle temperature by generating less heat in the needle.
[0024] Along with the virtual elimination of corona generated
particles, the present system provides for a more efficient and
more controllable neutralizing of charged droplets. The turbulence
caused by the merger of the aerosol and ion jets effectively mixes
the charged droplets and the ions, considerably reducing the time
required for a significant number of oppositely charged ions to
encounter and reduce the charges of the droplets. The ability to
adjust the degree of electrical charge or bias applied to the
discharge needle affords a degree of control not available when
radioactive ion sources are employed.
[0025] An additional advantage of the corona discharge needle in
neutralizing electrospray droplets is that it provides a unipolar
ion source. If desired, however, the corona discharge can provide a
bipolar source of ions, either by providing two oppositely charged
corona discharge needles in separate chambers, or by rapidly
switching between alternative positive and negative biasing sources
coupled to a single discharge needle.
[0026] When an aerosol jet and a single ion carrying jet are
directed into the mixing chamber, the two jets preferably confront
one another and travel in opposite directions towards one another
to maximize the mixing potential. Preferably, the jets travel into
the chamber at mean velocities of at least 40 meters per second, to
insure rapid mixing within a turbulent flow. In an alternative
embodiment, a second ion generator provides oppositely charged ions
entrained in a third gas flow, resulting in a merger of the aerosol
jet with two jets of ions, oppositely charged. In this arrangement,
the ion jets are advantageously arranged to confront one another
and thus travel in opposite directions while the aerosol jet is
perpendicular to the ion jets.
[0027] Further in accordance with the invention, there is provided
a device for adjusting the electrical charge distribution of an
aerosol. An enclosure of the device defines chamber, a first
orifice for entry of an aerosol flow, and a second orifice for
entry of ions. The device includes an ion generator having a corona
discharge region disposed proximate the second orifice and
electrically biased to generate multiple ions. A fluid passage is
adapted for a coupling with a gas source to guide a gas flow past
the corona discharge region. Thus the gas flow entrains some of the
first ions and carries the entrained ions into the chamber through
the second orifice, to merge with an aerosol flowing into the
chamber through the first orifice. When merging with the aerosol,
the ions alter the electrical charge distribution of the aerosol. A
conductive member is provided proximate the corona discharge region
and the second orifice. The member is electrically biased, and has
the same electrical polarity as the corona discharge region. The
enclosure further has an exit orifice to allow aerosol to exit the
enclosure after the electrical charge distribution is altered.
[0028] The preferred conductive member is a conductive plate,
through which the second orifice is formed. The plate is charged or
biased at a level considerably lower than that of the discharge
needle, e.g., several hundred volts as compared to the 2,000 volt
potential at the needle. Applying a negative charge to the plate,
when the corona discharge needle also is negatively charged, has a
significant impact. When the plate is negatively charged, the ions
are capable of depositing sufficient negative charges on the
aerosol particles to produce a larger peak for singly charged
negative particles than for singly charged positive particles. The
result is an increased fraction of the aerosol particles having a
single negative charge, to more than 25 percent, depending on
particle size, considerably higher than the fraction obtained by
any other charging method. At the same time, a doubly-charged peak
on the negative side is avoided, reducing the complexity of the
mass spectrum in a manner not possible in systems using either
radioactive source ionization, or negative-ion corona sources that
lack the biased orifice plate.
[0029] Thus, in accordance with the present invention, aerosol
analyzing systems can employ an improved corona discharge device
that affords a more rapid and more effective altering of the
electrical charge distribution of an aerosol, whether to charge or
to neutralize the aerosol. The ionizer requires no radioactive ion
source, and virtually eliminates the problem of small particle
generation found in conventional corona discharge devices. In an
electrospray-ionization mass spectrometry system utilizing the
ionizer in combination with electrospray generated aerosols,
extremely small particles with a charge distribution dominated by
neutral and singly charged particles can be provided to the mass
spectrometer for analysis.
IN THE DRAWINGS
[0030] For a further understanding of the above and other features
and advantages, reference is made to the following detailed
description and to the drawings, in which:
[0031] FIG. 1 is a schematic view of a system for analyzing
electrostatically generated aerosols in accordance with the present
invention;
[0032] FIG. 2 is an enlarged sectioned view of an aerosol
neutralizing device of the system;
[0033] FIG. 3 is another enlarged sectional view of the
neutralizing device;
[0034] FIGS. 4-6 are enlarged schematic views of different portions
of the neutralizing device;
[0035] FIG. 7 is a sectioned elevation illustrating an aerosol
charging/neutralizing device constructed according to the present
invention;
[0036] FIG. 8 is a sectioned elevation of an alternative embodiment
charging/neutralizing device;
[0037] FIG. 9 is a schematic view of an alternative embodiment
electrospray-ionization mass spectrometry system;
[0038] FIG. 10 is a schematic view of an alternative embodiment
system for neutralizing an electrospray generated aerosol;
[0039] FIGS. 11 and 12 illustrate electrical charge distributions
of aerosols generated by a aerosol neutralizing device of the type
shown in FIGS. 2 and 3;
[0040] FIG. 13 is a chart showing efficiency of delivering neutral
and singly charged positive particles as a function of ionization
current; and
[0041] FIG. 14 is a chart showing efficiency of delivering neutral
and singly charged positive particles as a function of the
electrospray flow rate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] Turning now to the drawings, there is shown in FIG. 1 a
system 16 for analyzing particles composed of non-volatile material
dispersed throughout a liquid in a particle-liquid solution or
sample. The sample contains a volatile additive, such as ammonium
acetate provided at about 20 millimoles per liter, to enhance its
electrical conductivity. The sample is held in a vial 18, inside a
chamber 20 of a container 22 surrounding and supporting the vial.
An electrically conductive lead, preferably a wire 24 formed of
platinum or another inert metal, has one end submerged in the
liquid sample. The other end of the wire is coupled to a high
positive voltage source +V1, typically in the range of 2,000-4,000
volts, to bias the sample at essentially the same voltage.
[0043] An electrospray capillary 26, submerged in the sample along
with wire 24, is used to supply the sample to an aerosol
neutralizing device 28 at a constant flow rate, typically in the
range of 50-100 nanoliters per minute. Capillary 26 typically has a
length of 25-30 centimeters, and an internal bore diameter of about
25 micrometers. The supply rate is controlled by controlling a
pressure differential between the submerged end of the capillary
and a spraying tip at the opposite end of the capillary. More
particularly, pressure at the spraying tip is sensed, and any
change is matched by varying the pressure in chamber 20 to maintain
the pressure differential at a predetermined level, depending on
the desired sample flow rate. The desired pressure differential
varying with the desired flow rate, to a maximum of about 4
psi.
[0044] The generation of aerosol droplets based on the liquid
sample requires a gas flow. To this end, a pressurized supply 30 of
air augmented with carbon dioxide is provided to device 28 via a
line 32. The air/CO.sub.2 flow rate is selected within a range of
1-3 liters per minute, with the flow of CO.sub.2 alone being about
0.1 liters per minute. Another electronegative gas, e.g. sulfur
hexafluoride, can be used in lieu of CO.sub.2
[0045] Neutralizing device 28 incorporates an ion generator
including a corona discharge needle 34. The discharge needle is
biased to a voltage of -V2, typically about 3,000 volts. A source
36 of clean, filtered air under pressure is coupled to device 28
via a line 38. The pressure of air supply 36 is predetermined to
provide a steady flow in the range of 0.5-2.5 liters per minute,
more preferably about 0.75 liters per minute.
[0046] The neutralized aerosol exits device 28 via a line 40 and
proceeds to a differential mobility analyzer (DMA) 42. The DMA
separates a portion of the aerosol particles from the remainder of
the particles, based on electrical mobility, and can be configured
to select either positively charged or negatively charged
particles.
[0047] The output of the DMA is provided to a condensation particle
counter (CPC) 44, also known as a condensation nucleus counter. In
the CPC, the selected aerosol particles travel through a gas stream
saturated with butyl alcohol or another volatile liquid, which
condenses on the particles to "grow" each particle to a larger
effective size for easier detection. U.S. Pat. No. 4,790,650
(Keady) describes a condensation particle counter. The output of
CPC 44 is provided to a microprocessor 46 which provides
information useful for analyzing the aerosol, e.g. concentration
values. Alternatively, an electrometer can receive the DMA output.
The aerosol is collected on a surface, and the resulting rate of
charge arrival is measured as a current. For singly charged
particles, this current is proportional to the aerosol
concentration and flow rate.
[0048] FIG. 2 illustrates several structural segments of
neutralizing device 28. With reference to FIGS. 2 and 3, these
include an ion generator housing segment 48, an ion orifice support
segment 50 adjacent segment 48, a droplet generator housing segment
52, an electrospray orifice support segment 54 adjacent segment 52,
and a medial segment 56. These segments are preferably formed of
aluminum or stainless steel. Two insulative neutralizer body
segments 58 and 60 are provided to electrically isolate the
conductive segments from one another. The segments cooperate to
define a mixing chamber 62 inside device 28.
[0049] A fitting 64 is mounted to segment 58, and cooperates with
segments 48, 50 and 58 to form a fluid passage 66, through which
air from source 36 and line 38 is guided into mixing chamber 62.
Similarly, a fitting 68 cooperates with segments 52, 54 and 60 to
form a fluid passage 70 for guiding the air/carbon dioxide mixture
from supply 30 to the mixing chamber. A fitting 69 at the exit
orifice is adapted for a coupling to a length of tubing--40 that
carries the exit aerosol to the DMA.
[0050] As seen in FIG. 3, a 90 degree rotation relative to FIG. 2,
the capillary tip is illuminated by a light emitting diod 71
through a passage 73 for viewing by an optical element 61 to form a
magnefied image of the spray tip. The sensed pressure near the tip
is provided to a controller (not shown) that adjusts the pressure
inside chamber 20 to maintain the desired pressure differential
between the intake and spraying ends of the capillary.
[0051] With reference to FIG. 4, an electrically conductive
electrospray orifice plate 72 is mounted to segment 54. An
electrospray orifice 74 is formed through plate 72, and has a
diameter of about 0.5 millimeters. A spray tip 76 of capillary 26
is axially spaced from orifice 74 by a distance of about 0.5
millimeters.
[0052] As noted above, the liquid sample is biased at a positive
voltage, selected within the range of 2,000-4,000 volts. Due to the
conductivity of the liquid, the voltage at electrospray tip 76 of
the capillary is essentially the same. Medial segment 56,
electrically isolated from the capillary and segments 52 and 54, is
maintained at ground. The result is an intense electrical field
between electrospray tip 76 and the medial segment.
[0053] Consequently, as the liquid sample reaches the capillary
tip, the liquid breaks up into small droplets (typically about 150
nanometers in diameter) that carry a relatively high charge, e.g.
at least about 2,000 (e) units of charge. If desired, a positive
bias +V3 can be applied to electrospray orifice plate 72, which is
electrically isolated from medial segment 56 and spray tip 76.
[0054] Arrows in FIG. 4 indicate the direction of the gas (air and
carbon dioxide) flow past the spray tip, through electrospray
orifice 74 and into mixing chamber 62. Given the 0.5 millimeter
diameter of orifice 74 and the gas flow rate of 1-3 liters per
minute, the average linear velocity of the gas entering the chamber
through the orifice is approximately 80-240 meters per second. The
air/carbon dioxide mixture entrains the charged droplets as it
flows rapidly past spray tip 76, thus forming an aerosol of the
liquid sample. The aerosol is propelled into chamber 62 as a
turbulent jet of the highly charged droplets.
[0055] As seen in FIG. 5, an electrically conductive ionization
plate 78 is mounted to segment 50. A corona discharge orifice 80 is
formed through plate 78, and has a diameter of about 0.5
millimeters. Needle 34 has a corona discharge tip 82 axially spaced
apart from orifice 80 by about 0.5 millimeters. The arrows in FIG.
5 indicate the flow of air past tip 82, and into the chamber
through orifice 80. As noted previously, discharge needle 34 is
biased to a negative 3000 volts. Needle 34 is isolated from segment
48 by an insulative jacket. Accordingly, an intense electrical
field is formed between discharge tip 82 and grounded medial
segment 56. Ionization plate 78, electrically isolated from
discharge tip 82 and segment 48, can be grounded or biased to a
negative voltage -V4 if desired.
[0056] When ionization plate 78 is biased, it is biased at the same
polarity as corona discharge needle 34, but at a considerably lower
level, e.g. several hundred volts. A negative bias on the ion
orifice plate, when the corona discharge needle also is negatively
charged, enhances the production of singly charged negative
particles.
[0057] The intense electrical field results in a corona discharge
at the discharge tip forming electrons and positive ions. Given the
negative bias of the discharge needle, the positive ions are drawn
to the needle while the electrons are repelled, eventually
attaching to molecules of the passing air flow to form negative
ions entrained in the air flow. Given the air flow rate of 0.5-2.5
liters per minute, the mean linear velocity of the air flow
(including the entrained ions) through the orifice into mixing
chamber 62 is in the range of 40-210 meters per second.
Accordingly, the negative ions are propelled into the chamber in
the form of an ion carrying jet.
[0058] The negative bias to corona discharge needle 34 is provided
through a high resistance 84 (100,000,000 ohms), to stabilize the
corona current. Preferably the corona current is maintained within
the range of 10-20 microamperes. The corona current level is low
compared to that in many conventional corona discharge ionizers,
and thus generates less heat in the corona discharge needle, a
factor contributing to the considerable reduction in particles
generated by the needle. Corona discharge tip 82 also remains
cooler due to the rapid air flow past the discharge tip. Given the
close proximity of the discharge tip to ionization orifice 80, the
air flow velocity about the tip is substantially the same as the
velocity through the orifice. Aside from the convective removal of
heat from the corona discharge tip, the air flow is believed to
further reduce the possibility of particle formation by sweeping
ions, both positive and negative, away from the corona discharge
before they are able to aggregate into particles.
[0059] With orifice 74 and orifice 80 at opposite ends of mixing
chamber 62, the ion carrying jet and the aerosol jet confront one
another, traveling in opposite directions toward one another to a
merger region within the mixing chamber. The jets intermingle with
one another in a turbulent flow, which promotes a mixing of the
positively charged aerosol droplets and the predominantly
negatively charged ions. The result is a rapid and effective
neutralization of the aerosol droplets. This result is seen from
FIG. 6, where arrows schematically illustrate the confrontation of
the aerosol and ion carrying jets and the resulting turbulence.
[0060] As the sample aerosol travels through mixing chamber 62, the
liquid evaporates, with the result that the aerosol exiting the
chamber through an exit orifice 86 (FIG. 2) consists primarily of
uniformly sized residue particles of the material originally
dispersed throughout the liquid. As the aerosol droplets evaporate,
negative ions from the corona discharge needle transfer electrons
to the droplets, reducing their positive charge. After evaporation
is substantially complete, the negative ions operate similarly to
further reduce the charges in the residue particles. Given a
sufficient level of ion generation and residence time in mixing
chamber 62, neutralizing progresses to the point that the residue
particles are left predominantly with zero charge or with one net
positive or negative charge. Neutralization based on the corona
discharge, as compared to neutralization based on radioactive ion
generating materials, is more efficient, perhaps largely due to the
degree of mixing occasioned by the confronting aerosol and ionized
air jets. The unipolar nature of the ionized air also may be a
contributing factor. In any event, when a solution containing
Ferritin (a protein) was analyzed using the corona discharge,
forced convection neutralization, and also analyzed based on
radioactive source neutralization, the number of counts in the peak
Ferritin particle at a concentration of five parts per million was
higher for the corona discharge neutralized sample, by a factor of
about 1.5, when the respective aerosol outputs were provided to the
same differential mobility analyzer.
[0061] FIG. 7 illustrates an alternative embodiment of the
invention, in the form of an aerosol charging device 88. The device
includes a primary body segment 90, an ion generating segment 92
and an ionization orifice segment 94 spaced apart from segment 92.
A corona discharge needle 96 is mounted within segment 92, and has
a corona discharge tip positioned near an ionization orifice,
formed through an ionization plate 98 substantially as before.
[0062] On the opposite side of a mixing chamber 100, a segment 102
provides an aerosol passageway 104 in lieu of an electrospray
capillary, with a narrow orifice 106 open to the mixing chamber. In
this embodiment, the aerosol is generated by an alternative device
(not shown) such as a pneumatic nebulizer or an ultrasonic
nebulizer, or from another source such as an engine exhaust or the
atmosphere. Accordingly, the aerosol is essentially neutral, or has
an equilibrium electrical charge distribution, as it enters the
mixing chamber. In this device, the corona discharge ions are
intended to apply a predetermined charge distribution to the
aerosol, rather than neutralize the aerosol.
[0063] In certain applications, it may be desired to impart a
balanced charge distribution to the aerosol, or at least provide
charges of both polarities. To this end, a first voltage source -V
and a second voltage source +V are coupled to the corona discharge
needle to a switch 108, operable to rapidly alternate the polarity
at which needle 96 is charged. As a result, the corona discharge
tip provides ions in alternating waves of positively and negatively
charged ions. The opposite-polarity sources +V and -V can, but need
not, have the same absolute voltage level.
[0064] FIG. 8 schematically illustrates an alternative embodiment
neutralizing/charging device 110 capable of simultaneously applying
both positively charged ions and negatively charged ions to an
aerosol. A body 112 of the device supports a positively biased
corona discharge needle 114 on one side of a mixing chamber 116,
and supports a negatively biased corona discharge needle 118 on the
opposite side in confronting relation to needle 114. The
neutralizer body also supports an aerosol source 120. On the
opposite side of the chamber from the aerosol source is an exit
orifice 122. Respective fluid flow passageways are provided in
connection with the aerosol source and each of the ion generators,
to provide two confronting ion containing jets, and an aerosol jet
perpendicular to the ion jets. To adjust the proportion of positive
ions to negative ions, the respective levels +V and -V can be
varied.
[0065] FIG. 9 illustrates an alternative embodiment in the form of
an electrospray-ionization mass spectrometry system 124. System 124
includes an electrospray nebulizer 126 that provides charged
electrospray droplets from a capillary 128 to the mixing chamber
130 of a neutralizing device 132. A corona discharge needle 134
provides negatively charged ions to the mixing chamber. The
negatively charged ions are generated at a level sufficient to
substantially neutralize the positively charged aerosol droplets,
in the sense of providing a predominance of singly charged positive
and negative droplets in the exit aerosol.
[0066] The exit aerosol is provided to a mass spectrometer 136.
Because the residue particles reaching mass spectrometer 136 are
singly charged, the resulting mass spectrum is simplified, due to
an avoidance of the auxiliary peaks generated by particles carrying
multiple charges.
[0067] FIG. 10 illustrates an alternative embodiment aerosol
neutralizing system 138, that differs from system 16 in that
voltages +V3 and -V4, to the electrospray orifice plate and to the
ionization plate respectively, are not provided through independent
voltage sources. Rather, a negative bias is applied to the
ionization plate by passing the corona current through a resistor
140, and a positive bias is applied to the electrospray plate by
passing the electrospray current through a resistor 142.
[0068] FIGS. 11 and 12 illustrate two runs on a scanning mobility
particle sizer, with positively charged and negatively charged
center rods, respectively. FIG. 11, associated with the negative
rod, shows a peak of about 6,000 counts. FIG. 12, associated with
the positive rod, shows a peak of only about 60 counts. Thus,
corona discharge neutralizing reduces the positive charge to one
net charge with a very narrow charge distribution. The residue is
left mostly with charged particles of one positive charge,
indicating a high efficiency in preventing highly charged droplets
from being lost as they pass through the orifice, and the ability
to control the ratio of positive and negative particles.
[0069] The chart in FIG. 13 illustrates an increase in the
efficiency of producing neutral particles as the ionization current
increases.
[0070] The chart in FIG. 14 illustrates an increase in efficiency
of producing neutral and singly charged positive particles as the
electrospray flow rate increases.
[0071] A salient feature of the present invention is the capability
of tailoring the manner in which the charged distribution of an
aerosol is altered, whether by charging or by neutralizing the
aerosol. In addition to the ionization current and the electrospray
flow rate, parameters that can be varied include the gas flow rates
through the electrospray orifice and the ionization orifice, the
proximity of the confronting ion and aerosol jets, the size of the
mixing chamber, and the magnitude of the electrical bias applied to
the ionization orifice plate and the droplet orifice plate.
[0072] Thus in accordance with the present invention, a more
controlled and more efficient level of charging and neutralization
are achieved, while the use of radioactive materials is avoided.
Particle generation by the corona discharge tip is essentially
eliminated, enabling a more accurate analysis of extremely fine
particles.
* * * * *