An airflow through a tube is used to guide a charged particle through the tube. A detector may be used to detect charge passing through the tube on the particle. The movement of the particle through the tube may be used to both detect its charge and size

Fuerstenau, Stephen D.

NASA Technical Reports Server

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(12) United States Patent (io) Patent No.: US 6,781,118 B2 
Fuerstenau (45) Date of Patent: Aug. 24,2004 
(54) 
(75) 
(73) 
PARTICLE CHARGE SPECTROMETER 
Inventor: Stephen D. Fuerstenau, Montrose, CA 
(US) 
Assignee: California Institute of Technology, 
Pasadena, CA (US) 
Subject to any disclaimer, the term of this 
patent is extended or adjusted under 35 
U.S.C. 154(b) by 0 days. 
Notice: 
Appl. No.: 10/053,343 
Filed: Jan. 17, 2002 
Prior Publication Data 
US 200210125422 A1 Sep. 12, 2002 
Related U.S. Application Data 
Provisional application No. 601262,497, filed on Jan. 17, 
2001. 
Int. C1.7 .......................... HOlJ 39/34; GOlN 21/00 
U.S. C1. ........................................ 250/282; 2501281 
Field of Search .................................. 2501282, 281 
(56) References Cited 
U.S. PATENT DOCUMENTS 
4,010,366 A * 311977 Neukermans et al. ....... 2501282 
4,633,714 A 111987 Mazumder et al. 
4,917,494 A 411990 Poole et al. 
4,938,592 A 711990 Poole et al. 
5,641,919 A 611997 Dahneke 
OTHER PUBLICATIONS 
Gajewski, J.B. and A. Szaynok, Journal of Electrostatics, 
vol. 10 (1981) p. 229. 
* cited by examiner 
Primary Examiner-John R. Lee 
Assistant Examiner-James J. Leyourne 
(74) Attorney, Agent, or F i r m C i s h  & Richardson P.C. 
(57) ABSTRACT 
An airflow through a tube is used to guide a charged particle 
through the tube. A detector may be used to detect charge 
passing through the tube on the particle. The movement of 
the particle through the tube may be used to both detect its 
charge and size. 
21 Claims, 3 Drawing Sheets 
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US 6,781,118 B2 
1 
PARTICLE CHARGE SPECTROMETER 
APPLICATIONS 
CROSS-REFERENCE TO RELATED 
The present application claims priority from provisional 
application No. 601262,497 filed Jan. 17, 2001. 
STATEMENT AS TO FEDERALLY-SPONSORED 
RESEARCH 
The invention described herein was made in the perfor- 
mance of work under a NASA 7-1407 contract, and is 
subject to the provisions of Public Law 96-517 (U.S.C. 202) 
in which the contractor has elected to retain title. 
BACKGROUND 
Determination of particle charge may be important in 
certain problems related to monitoring indoor and outdoor 
environments, geological and other atmospheric processes, 
as well as the processing and handling of industrial powders. 
Instruments that measure charge on particles have typi- 
cally employed indirect measurement techniques in which 
the particle is exposed to an electrical field while the particle 
motion in response to the field is monitored optically or by 
other means. Electrical mobility analyzers are an example of 
an indirect measurement but particles having a size range 
greater than a few micrometers are not easily handled by 
electrical mobility analyzers, which require a relatively high 
electric fields. 
Furthermore, electrical mobility analysis generally bases 
the measurement on the assumption that the particle carries 
at most one or two electrons of charge in order to provide 
size information about the particle. Other indirect particle 
charge measurement devices may include single particle 
traps and balances. While these may be relatively sensitive, 
they are not easily automated, and it may be difficult to use 
these devices to obtain rapid measurements. 
The single particle aerodynamic relaxation time (SPART) 
analyzer is described in U.S. Pat. No. 4,633,714: Aerosol 
particle charge and size analyzer. This is another indirect 
approach device which acquires images of tracks of aerosol 
particles falling through an oscillating electric field, locates 
pixels in each image that form the individual tracks, and 
forms non linear curve fits of the tracks in order to determine 
track parameters. These parameters are used to estimate the 
size of each particle and the charge on each particle. This 
device may be physically very large and costly to build. 
Other aerosol instruments measure the particle aerody- 
namic diameter (but not charge) by monitoring the motion of 
particles due to an accelerating flow. The Aerosizer, made 
and marketed by TSI, Inc is described in U.S. Pat. Nos. 
4,938,592; 4,917,494; and 5,641,919. The Aerosizer instru- 
ment particles velocities are determined in the expansion 
region of a free jet after the particle acceleration is virtually 
complete. Furthermore, its velocity measurements do not 
make use of an induced electrical signal from individual 
particles. 
Published research has demonstrated the use of a cylin- 
drical electrode to sense the charge on particles in a carrier 
gas transported through a relatively large diameter conduit 
with glass walls (eg. Gajewski, J. B. and A. Szaynok, Journal 
of Electrostatics, vol. 10, (1981) page 229). However, that 
work is concerned strictly with the measurement of charge 
carried by ensembles of particles, not the measurement of 
charge on individual particles. 
SUMMARY 
The present application teaches a device which may 
measure the charge and size of a large number of airborne 
2 
particles. The charge on an individual particle may be 
detected by using a gas flow to draw a particle through a 
cylindrical electrode that acts as a Faraday cage. 
According to an embodiment, an imposed gas flow inside 
5 a dielectric capillary tube may be used to guide the particle 
through the charge sensing volume. The charge is sensed 
through the capillary tube. 
The gas stream to draw the particle through the cylindrical 
sensing region may accelerate particles of different sizes 
lo accelerate at different rates in the flow. This may allow 
measurement of particle size via velocity observation. 
One use of the present system is in determination of 
electrostatic charge on individual particles or droplets sus- 
15 pended in a gas. The system may also be used to simulta- 
neously determine the aerodynamic diameter of each 
charged particle that is measured. 
BRIEF DESCRIPTION OF THE DRAWINGS 
20 These and other aspects will now be described in detail 
FIG. 1 shows a basic layout of the particle sizing device; 
FIGS. 2a and 2b shows an oscilloscope trace of a particle 
FIG. 3 shows a graph of determination of particle size as 
FIG. 4 shows a block diagram of the electronics layout. 
with reference to the accompanying drawings, wherein: 
trace; 
compared with transition time; and 
25 
30 DETAILED DESCRIPTION 
The present application defines a device that may detect 
charge on individual particles by using gas flow to draw 
particles through a tube to the electrode. Use of the gas flow 
has a significant benefit, since the particles of different sizes 
35 accelerate at different rates within the flow. This makes it 
possible to determine size of the particles by investigating 
their velocity. The present system monitors the particle 
motion while it is in a gas stream with a constant velocity, 
i.e. inside a capillary, unlike systems such as the Aerosizer 
The present system uses a direct charge measurement 
approach. Acharge sensitive amplifier is used to measure the 
charge induced on an electrode by the arrival or passage of 
45 a charged particle. Since measurement of induced signals is 
carried out, the present system may be formed very simply. 
One embodiment that is disclosed herein monitors the 
charge induced on a conducting cylinder as the particle 
passes through a glass capillary within the cylinder. Instru- 
so ments for measuring charge and particle velocity based on 
conductive cylinders have been reported in the meteorologi- 
cal instruments community. See for example, Gum, R., and 
G. D. Kinzer, 1949: The terminal velocity of fall for water 
droplets in stagnant air. J. Meteor., 6, 243-248.) 
An example of this approach is described in U.S. Pat. No. 
5,770,857. This system measures charge and velocity of ions 
and particles in a molecular beam inside a vacuum chamber. 
This patent includes a discussion of the type of ultra- 
sensitive electronics that may be used in the present inven- 
60 tion. To summarize that discussion, the sensing electrode 
may include an electrically isolated conductor with a hollow 
geometry such that the conductor walls will intercept a 
majority of the electric field lines emanating from a point 
charge inside the electrode. A charge inside this volume will 
65 induce a voltage on the cylinder walls that can be detected. 
This arrangement is termed a Faraday cage. The electrode is 
connected to the high impedance input of a field effect 
40 that looks at the particles after acceleration. 
ss 
US 6,781,118 B2 
3 4 
transistor. Feedback forms the front end of a low-noise The trace shown in FIG. 2A represents the result of a 
charge sensitive preamplifier. The noise can be further particle transition. For example, the time that the particle 
reduced by the use of shaping electronics which function as takes to make its trace depends on the size of the particle. 
a noise rejecting band-pass filter tuned to the transition time Different sized particles may have different traces. 
of the particle arrival and exit. 5 Therefore, the shape of the curve may be compared with 
Many previous devices have relied on gravity, ambient air other shaped curves by detector 115. By making this 
currents, i.e. wind, or a ballistic launch of particles as in a comparison, the size of the particles may be detected. For 
molecular beam apparatus, in order for the particles to transit example, larger particles may accelerate slower and travel 
the conducting cylinder. The present invention describes less distance based on the airflow in capillary 100. Smaller 
using a forced and regulated carrier gas stream to bring particles may correspondingly accelerate faster. 
particles into and out of the conducting cylinder in a rapid The use of a forced gas stream may also produce addi- 
and controlled fashion. By using rapid particle motion, e.g., tional advantages, including the ability to measure particle 
-100 meters per second, reduced noise pulse processing size from the same signal used to determine particle charge. 
techniques may be used on the amplified signal. The gas The forced gas stream moves particles of different sizes with 
stream also assures that a large number of particles will be different rates of acceleration. 
sampled and made to transit a cylinder with millimeter size One technique of determining calculated motion of dif- 
dimensions. A small electrode dimension may also reduce ferent size particles is illustrated in FIG. 3. This figure 
the electronic noise in the system because of the reduced predicts the timing of particles in a 100 meter per second air 
stray capacitance. stream entering a 7 mm long tube electrode situated 2 cm 
an embodiment, the forced gas stream is directed downstream of a capillary entrance. It is therefore possible 
through the conducting cylinder by a capillary formed of a 2o to measure the size of particles by determining their rate of 
dielectric material such as glass or any other dielectric acceleration. This principle is used in commercially avail- 
material, e,g,, with a dielectric constant less than 10, able instruments that determine the so-called aerodynamic 
preferably less than 4. The capillary constrains the flow of Particle diameter. 
the particle in a precise and easily adjustable manner but not The amplitude of the curve trace is proportional to particle 
impeding the detection of charge through the capillary wall. 25 charge. The duration of the trace indicates the transit time of 
The electrostatic charge can be sensed across a dielectric the particle. There are a variety of signal processing tech- 
wall but not across the wall of a grounded conductor. niques that can be used to obtain amplitude and duration 
~n embodiment is shown in FIG, 1, A particle capillary information from pulses at rates of several hundred Hz or 
100 forms a device with inner surfaces that may constrain 3o more. 
particle movement along the path defined by the surfaces. As an alternative, a model of particle size as a function of 
The capillary may be a tube which is 3 cm long and of any the acceleration profile for the particle may be developed by 
desired diameter, e.g. on the order of mm, for example 129 reviewing different particles. This model may then be used 
mm. The capillary may be formed, for example, of glass. to determine the particle information. 
The particle capillary passes through a Faraday cage 102 35 FIG. 4 shows a block diagram of the electronics layout of 
cylindrical electrode that is associated with an input of an the present system. Power from a power supply 400 is used 
amplifier 106. The Faraday cage may be used to detect the to power all components of the system. The air pump 410 is 
passage of the charged particle through the capillary. An connected to the sampling head by a flexible hose 415. The 
airflow, which may be produced by air pump 112, is used to capillary and cylindrical sensing electrode are housed inside 
draw particles through the capillary 100. The embodiment 4o the sampling head, 420, The sampling head 420, also 
may use grounded shield tubes on the exterior of the contains the FET and feedback elements which form the 
capillary near the entrance and exit of the conducting remote front end of the charge sensitive preamplifier 425. 
cylinder. The signal from the preamp 425, is applied to the pulse 
Each time a charged particle passes through the capillary processing electronic board 430. This board may process the 
100, the charge on the faraday electrode 102 is changed. The 45 signals using conventional techniques. 
output of amplifier 106 is directed to detector 115, which Although only a few embodiments have been disclosed in 
may detect parameters of the particle. The system may be detail above, other modifications may be possible. For 
formed within a housing, which may also be sealed by example, while the above embodiment has described a 
screwing a cover into the screw holes with a gasket. For cylindrical electrode being used as the charge detector, any 
example, the housing may be maintained within a vacuum. item which is capable of detecting charge in a particle 
An advantage of this system is the simplicity with which passage may be used. The capillary tube may be formed of 
the hardware can be formed. The glass capillary can simply any material provided that it does not shield the signal from 
be inserted within the tube electrode 102, and passage of the sensing electrode. Moreover, the airflow through the 
charged particles is carried out through the dielectric mate- capillary to may be formed by any means, including passive 
rial. 55 airflow. Moreover, the detector may be any electronic or 
For example, FIGS. 2A-2B shows an oscilloscope trace optical Or any other type of device which is capable of 
record for the passage of a positively charged particle of reacting to a charge. 
about 4000 unit charges or 6 . 4 ~ 1 0 - l ~  Coulombs. In this 
embodiment, the noise level associated with the measure- 
ment is around 200 e-, but can be made lower. FIG. 2A 60 
shows the raw signal from the preamp. FIG. 2B shows the 
differentiated pulse from that raw signal. 
The size and electrostatic charge of airborne particles and 
droplets may be measured as long as the particles hold a 
charge of more than a specified amount, e.g. more than 65 
twenty ato-coulombs (10-18 coulombs) of charge, about 
100 electrons. 
What is claimed is: 
1. A device comprising: 
a particle position constraining part formed of a dielectric 
material, having inner surfaces; 
an airflow producing part, producing an airflow within 
said inner surfaces to guide a particle within said inner 
surfaces; and 
a detector, which detects a charge of a charged particle in 
said particle position constraining part, and produces a 
signal indicative of a charge and a size of a particle. 
US 6,781,118 B2 
5 6 
2. A device as in claim 1, wherein said detector produces 
an output signal indicative of a charge of said particle, and 
a movement of said particle, and determines size of said 
particle from said movement of said particle. 
3. A device as in claim 1, wherein said airflow producing s 
part includes an air pump. 
4. A device as in claim 1, wherein said particle position 
constraining part includes a capillary tube. 
5. A device as in claim 1, wherein said detector includes 
14. A method as in claim 10, wherein said sensing 
comprises using a Faraday cage to Sense charge of this 
charged particle as a function of time, 
15. A method as in claim 14 wherein said using a Faraday 
cage comprises using a cylindrical electrode Faraday cage. 
16. A method, comprising: 
forcing a charged particle to travel through a tube formed 
of a dielectric material; and 
dielectric material. 
a Faraday cage. 10 
6, A device as in claim 5,  wherein said detector includes detecting a charge On said charged particle through said 
a Faraday cage cylindrical electrode. 
7. A device as in claim 5, further comprising a transistor, 
connected to said Faraday cage, and driven by an output of 
said Faraday cage to produce said signal. 
8. A device as in claim 1, wherein said particle constrain- 
ing part is a glass capillary. 
9. A device as in claim 1, wherein said particle constrain- 
ing part is a capillary having a diameter less than mm, 
10. A method, comprising: 
using airflow to guide a charged particle, having a charge 
greater than a specified amount, along a path defined by 
a dielectric material; 
17. A method as in claim 16, wherein said forcing 
comprises applying a known airflow to said charged particle. 
18. Amethod as in claim 16, further comprising detecting 
a size of said charged particle based on a waveform detected 
by said detecting. 
19. A method as in claim 16, wherein said dielectric 
20. A method as in claim 19, wherein said dielectric 
1s 
2o capillary has a diameter less than one mm. 
capillary is formed of glass. 
sensing a charge of the charged particle along the path 
producing a signal indicative of particle charge and par- 
11. A method as in claim 10, wherein said producing 
comprises analyzing a signal produced by said sensing to 3o 
determine a size of the particle. 
12. Amethod as in claim 10, wherein said using comprises 
confining said charged particle within a dielectric capillary. 
13. Amethod as in claim 10, wherein said using comprises 
confining said charged particle within a capillary having a 
diameter less than ten mm and formed of glass. 
2s from within the dielectric; and 
ticle size based on said sensing. 
21. A method, comprising: 
sliding a first smaller diameter tube of a dielectric material 
into a second, larger diameter tube which is a cylin- 
drical sensing electrode; 
forming a known airflow through said first smaller diam- 
eter tube, and causing charged particles to pass through 
said first smaller diameter tube; and 
sensing passage of said charged particles using said 
second larger diameter tube, through said dielectric 
material. 
* * * * *  


Particle charge spectrometer

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