Low concentration aerosol generator

A low concentration aerosol generator capable of generating aerosol partis of a known particle size and count is disclosed. The device includes a substantially hollow drying chamber having a first end, a second end, and a gas circulation means for circulating a gas in a flow through the drying chamber via an inlet means and an outlet means. The first end of the drying chamber has an aerosol droplet generator capable of generating aerosol droplets of a predetermined size and which droplets contain a detectable sample within the drying chamber. The second end of the drying chamber includes a delivery tube having a first orifice arranged within the drying chamber for receiving the detectable sample and a second orifice for allowing the detectable sample to exit the drying chamber. The gas circulation means includes means for generating and regulating the flow of the gas through the drying chamber. The invention also includes a method of generating and delivering a sample particle in a low velocity air stream to an analyzing instrument using the aforementioned device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus useful in generating and 
counting aerosol particles. In particular, the invention relates to an 
apparatus which is capable of generating and counting low concentrations 
of individual aerosol particles. 
2. Description of the Prior Art 
Recently, governments have become increasingly interested in identifying 
and controlling developed biological warfare agents. These agents are 
highly dangerous and extreme caution must be used in the handling of these 
agents. In the event that these agents are deployed, part of the initial 
defense includes the rapid and accurate detection of the agents. 
Instruments which are used to detect biological warfare agents include 
apparatus which analyze aerosols such as the CB mass spectrometer or a UV 
fluorescence-based detector. The instruments must be properly calibrated 
before being employed. Calibration of these instruments requires the use 
of aerosol particles that are agglomerates of individual bacteria cells or 
spores. In the past, the aerosolized particles, nebulizers, sprayers 
fluidized beds, sonic nozzles, etc. have been used. These apparatus, 
however, produce copious amounts of aerosols which may have a more or less 
compact size distribution depending upon the specific technique used to 
generate the aerosol. Measuring the amount of individual aerosol particles 
generated is often difficult and usually only approximated. In fact, the 
aerosols are measured by the mass of the material consumed by the 
apparatus rather than the actual amount of aerosol particles generated. In 
addition, although there are currently available scientific droplet 
generators capable of forming droplets of a uniform size such as the 
Vibrating Orifice Aerosol Generator (TSI Inc, Minneapolis, Minn.), they 
are not, however, suitable for the calibration needs described above. Such 
devices are of a high speed free-running nature and do not allow exact 
counts of small numbers of aerosol particles made. 
In view of the need to more accurately calibrate biological warfare 
detection apparatus, there is a need for improved aerosol generating 
apparatus. In particular, there is a need for an apparatus which allows an 
investigator to control the amount, size and rate of generation of aerosol 
particles. There is also a need in this field to provide a means of 
delivering aerosol particles in a low velocity air-stream which is 
suitable for direct injection into analyzing instruments. Then present 
invention addresses these needs. 
SUMMARY OF THE INVENTION 
In view of the foregoing, it is therefore an object of the present 
invention to provide an improved aerosol particle generation apparatus 
useful in the calibration and profiling of aerosol analyzing instruments 
which are used to detect biological warfare agents. 
It is a further object of the invention to provide an aerosol generating 
apparatus which is capable of generating aerosol particles of a known 
particle size, having a known particle count, and in low concentrations. 
In one aspect of the invention, these and other objects of the invention 
are achieved by a sample conditioning device for conditioning an aerosol 
droplet containing a detectable sample. The device or apparatus comprises: 
a substantially hollow drying chamber having a first end, a second end, and 
a gas circulation means for circulating a gas in a flow through said 
drying chamber via an inlet means and an outlet means; 
said first end of said drying chamber having an aerosol droplet generator 
capable of generating aerosol droplets of a predetermined size containing 
a detectable sample within said drying chamber; 
said second end of said drying chamber including a delivery tube having a 
first orifice arranged within said drying chamber for receiving said 
detectable sample and a second orifice for allowing said detectable sample 
to exit said drying chamber; and 
said gas circulation means having means for generating and regulating said 
flow of said gas through said drying chamber. 
In a second embodiment there is provided a method of delivering a sample 
particle in a low velocity air stream to an analyzing instrument such as 
CB mass spectrometer. The method includes: 
a) generating a liquid droplet containing sample particles such as via a 
sample conditioning device as described above; 
b) drying the liquid droplets so that only the dried sample particles 
remain; and 
c) directing the sample particles to analyzing instruments, preferably at a 
low velocity. 
The advantages of the present invention include the fact that it provides 
an unique degree of control over the aerosol, allowing the investigator to 
choose the material of which the particles are to be made, the size of the 
particles, the rate of generation, the total number of individual 
particles generated, and it delivers the particles in a low velocity air 
stream suitable for direct injection into analyzing instruments. 
An additional benefit, particularly when aerosols of hazardous materials 
are required, is that very small quantities of aerosol are generated and 
the aerosol is confined to the generator and the analyzer; it is not 
necessary to fill large chambers with lethal materials. In certain 
preferred embodiments, the parts of the apparatus that hold the sample 
material, i.e. the droplet generator and/or drying chamber are disposable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In one aspect of the present invention, the present invention includes an 
apparatus capable of generating and optionally counting low concentrations 
of individual aerosol sample particles. 
The sample particles generated by the apparatus of the present invention 
have the properties that they are dry and of nearly uniform size; the size 
can be chosen from the range from about 2 to 10 microns. The generated 
sample particles result from the evaporation of uniformly sized water 
droplets containing the material of interest, either in solution or 
suspension. The final particles can accordingly be either single crystals 
or aggregates of smaller particles. The rate of generation may be varied 
from zero up to several thousand particles per second. The apparatus keeps 
an exact count of the number of particles generated, and may be configured 
to automatically stop after generating a predetermined number of sample 
particles. 
The invention includes a sample conditioning device for conditioning an 
aerosol droplet containing a detectable sample. The apparatus comprises: 
a substantially hollow drying chamber having a first end, a second end, and 
a gas circulation means for circulating a gas in a flow through the drying 
chamber via an inlet means and an outlet means; 
the first end of the drying chamber having an aerosol droplet generator 
capable of generating aerosol droplets of a predetermined size containing 
a detectable sample within the drying chamber; 
the second end of the drying chamber including a delivery tube having a 
first orifice arranged within the drying chamber for receiving the 
detectable sample and a second orifice for allowing the detectable sample 
to exit the drying chamber; and 
the gas circulation means having means for generating and regulating the 
flow of the gas through the drying chamber. 
In this aspect of the invention, the apparatus 1 comprises three sections: 
1) a liquid droplet generator, 2) a drying chamber with air control, and 
3) controlling electronics. 
1. Liquid Droplet Generator 
Referring now to FIGS. 1 and 2, a preferred aspect of the invention is 
described. The apparatus 1 includes a droplet generator 10 which is 
capable of emitting one liquid droplet, of a substantially fixed size. 
Preferably, the droplets are generated each time a single electrical pulse 
(of appropriate amplitude and duration) from the pulse generator 12 via a 
suitable pulse controller 14 and conductor ribbon 13 is applied to the 
droplet generator 10. 
For purposes of illustration and not limitation, the liquid droplet 
generator 10 includes a disposable ink-jet cartridge 15 such as those used 
in the Hewlett-Packard line of ThinkJet and QuietJet printers, sold as HP 
part number 92261A. The cartridge incorporates a pliable bladder 16 that 
holds approximately 4 ml of ink. Access holes can be drilled through the 
face 18 and the ink in the reservoir removed and replaced with a solution 
or slurry of water and the material of interest. Alternatively, and more 
simply, unfilled cartridges may be purchased from HP and loaded and sealed 
by the user. 
The solution droplets 19 are ejected from a plurality of nozzles 17 found 
as microscopic holes in the faceplate 18 on a line, which for example in 
the HP inkjet cartridge 15, is about 4 mm long. The nozzles 17 are served 
by separate electrical conductors and liquid feed channels (not shown), 
and can be fired independently of each other. A nozzle is fired by 
applying sufficient voltage, i.e about 24 volts to its circuit for about 6 
microseconds. The current heats a tiny resistance heater that causes a 
bubble to form and expand in the feed channel, forcing a droplet 19 of 
solution, about 60 microns in diameter, out through the nozzle 17. In one 
preferred aspect, the device fires the nozzles one at a time in sequential 
order at a rate set by the user. 
In FIG. 1, the modified ink-jet cartridge acting as a droplet generator 10 
is to be physically attached to the drying chamber 20 and electrically 
interfaced to a pulse controller 14 using a printer carriage assembly 21 
(available from Hewlett-Packard as a replacement part, part number 
02225-60914). The assembly includes a snap-in holder for the cartridge and 
a conductor ribbon 13 for electrical connections. 
It is to be understood that other droplet generators may be used in place 
of a ThinkJet cartridge. For example, newer designs of ink cartridges, 
both bubble and piezoelectric types can be modified in the manner 
described herein in order to be included in the apparatus 1. Another 
appropriate droplet generator is the microparticle generator made by 
Uniphoton Systems Inc, Brooklyn, N.Y. 
2. Drying Chamber 
In FIG. 1, the drying chamber 20 is shown as a closed plastic chamber 
having a central metal delivery tube 22 having a first orifice 23 and a 
second orifice 24 that allow dried sample particles 25 to pass through the 
drying chamber. The delivery tube 22 extends through the second end of the 
drying chamber 20 and into the interior of the drying chamber a distance 
which is sufficient to allow the delivery tube 22 to receive the droplets 
19 generated by the droplet generator. The carriage assembly 21 holding 
the droplet generator, i.e. ink-jet cartridge, 10 is mounted onto the 
first end of the drying chamber 20. Referring now to FIGS. 1 and 2, the 
droplet generator cartridge is shown snap-mounted into the carriage 
assembly 21; its nozzles 17 pointing downward through a hole 28 in the 
center of the first end of the drying chamber 27. A rubber gasket 29 forms 
a seal between the drying chamber first end cap 27 and the cartridge face 
plate 18 surrounding the nozzles 17. 
In a preferred embodiment, a constant stream of gas, preferably dry air, is 
pumped into the drying chamber 20 through an inlet means 30 found in the 
second end of the drying chamber 20. The first end of the drying chamber 
20 also includes and outlet means 32 for allowing the constant stream to 
circulate through the drying chamber. In operation, the air flow divides 
between two exit ports, i.e the outlet 32 and the first orifice of the 
delivery tube 23 to leave the drying chamber. Part of the air flows down 
the delivery tube to carry out drying and dried particles 25 and the 
remainder--the winnowing flow--flows across the drying chamber and leaves 
through the outlet 32 shown near the top of the drying chamber. A 
flowmeter with a metering valve 33 measures and allows adjustment of the 
winnowing flow. The dried particle exits the delivery tube at a low 
velocity (approximately 0.5 liters/min.) air stream which is suitable for 
direct injection into analyzing instruments which need to be calibrated in 
their use as detectors for biological warfare agents. Examples of these 
analyzers are the CB mass spectrometer and UV fluorescence based 
detectors. Each of these devices require aerosol particles that are 
agglomerates of individual bacteria cells or spores in order to detect the 
presence of the biological warfare agents. 
A diaphragm air pump 34 with an integral flowmeter powers the circulation, 
and allows one to set and measure the flow into the drying chamber. The 
air pulled into the diaphragm pump consists of the winnowing flow plus 
ambient air drawn through a flowmeter 35 at a rate equal to the delivery 
tube flow. A tube of desiccant 36 dries the air leaving the diaphragm 
pump. A filter 37 after the tube of desiccant insures that air entering 
the drying chamber is particle free. The flow from the outlet 32 back to 
the inlet 30 is accomplished by tubing 50. 
The distance from the droplet generator, depicted in FIG. 1 as a modified 
ink jet cartridge, to the first orifice (entrance) 23 of the delivery tube 
22 is important for proper operation of the device. For example, a 
particle projected into still air travels a distance known as the stop 
distance before losing its initial momentum. The stop distance for the 
principal droplets (e.g. .about.60 microns in diameter) ejected from the 
modified ThinkJet cartridge was measured to be about 2 inches. Thus, in 
one preferred embodiment the gap between the nozzles 17 and the delivery 
tube first orifice 23 is about two inches. It will be understood that the 
distance between these points can vary according to the needs of the 
artisan and will depend, to a certain extent upon the design of the drying 
chamber, particle samples desired, etc. The full size particle emitted 
from a nozzle with each electrical pulse thus reaches and enters the 
delivery tube. It will quickly thereafter decelerate to its terminal 
falling velocity and slowly travel down the tube, slowing further as its 
water evaporates and finally emerging out the bottom as a particle of 
residue material embedded in the delivery tube air flow. One or two small 
liquid fragments frequently accompany the ejection of the main inkjet 
particle. Being less massive, their stop distance is much shorter and they 
will come to rest in the air before reaching the delivery tube entrance. 
Caught in the winnowing flow, they are swept from the drying chamber; only 
the main particles, one per pulse, are transported down the delivery tube. 
One illustrative delivery tube is 0.5 inches in diameter and 11 inches long 
and has a delivery tube flow of about nominally 0.5 liters/minute. 
Therefore particles spend about four seconds in the tube. If this time is 
insufficient for drying the droplets at ambient temperature, the 
temperature of the drying chamber can be elevated up to about 200 degrees 
Fahrenheit using a heat source. In FIG. 1, the delivery tube 22 is shown 
wrapped with flexible electric heating tape 38. An electrical power 
controller 39, into which the heating tape is plugged, permits regulation 
of the temperature. 
3. Controlling Electronics 
Referring now to the flow chart of FIG. 3 in conjunction with FIGS. 1 and 
2, two conceptually separate units are employed together to cause the 
droplet generator 10, e.g. ink-jet cartridge, to emit droplets at the rate 
and number the user desires. A pulse generator 12 is used to set the 
frequency at which a pulse controller 14 distributes and sends voltage 
pulses suitable for firing the nozzles 17. The pulse generator can be an 
outboard-type pulse generator or a built-in pulse generator. One pulse 
generator (PG) suitable for this purpose is an Avtech model AV-1002-C, due 
to its low cost and built in manual push-button arrangement. The PG output 
is adjustable in terms of frequency and pulse width. A pulse width of 
about 500 microseconds was found to be suitable over the range of 1 to 
1000 pulses per second. The transistor to transistor logic or TTL (0-5 
volt) output of the PG was used to ensure that correct logic levels were 
maintained at all times, and that no baseline offset inadvertently 
occurred. 
Once the output of the PG enters the pulse controller, it initially passes 
to a counter 40 such as a Veeder-Root Model 7910 Predetermining Counter 
having a presettable counter function which the user sets from the front 
panel. The counter totals the number of pulses it has received. When the 
preset limit is reached, the control output goes to a high state, where it 
remains until the user resets the counter. When the control output goes 
high, no further pulses are able to pass to the system, therefore no 
droplets can be generated. This is accomplished by first inverting the 
logic of the control output, and then doing a logical AND (using a 2-input 
CD4081 AND gate) with the output of the PG. 
If the preset limit has not been reached, the output of the PG passes 
through the AND gate unhindered, where it then acts as the clock input for 
the rest of the system. It initially passes to a second counter 41 such as 
a Decade Counter (CD4017) which operates in tandem with two flip-flops 42 
(CD4013) to extend its counting capability to 12 bits. The counter chosen 
for this design can be a Johnson type counter, in which one of 12 outputs 
goes high for a full clock period, in sequential order, every time a clock 
pulse is received. In this design, each output from the counter 
corresponds to one of the nozzles on the ink-jet cartridge, and is a 
controlling mechanism for it. Therefore, the high output from the counter 
will determine which nozzle will produce a droplet. 
When a counter output goes high, it triggers a 2N2222! transistor 42, 
which powers an LED 43 on the front display panel to light up for one 
clock cycle, demonstrating for the user the active nozzle. The outputs of 
the counter are then logically ANDed with the output of a pulse generator 
44 such a One-Shot (CD4047), which produces a 6 microsecond pulse once 
every clock pulse. Because the CD4047 is triggered on the falling edge of 
the clock pulse, it does not produce a pulse until well after the output 
of the Johnson Counter (CD4017) has gone high, thereby alleviating a 
potential timing problem. This operation then results in a 6 microsecond 
pulse on only the output of the counter which is high (all other outputs 
will be low, or inactive). This output then passes through a buffer 45 
(CD4503) to increase its current driving capability, and then to the gate 
46 of a 2N6782 Transistor 47 where it turns on the transistor for 6 
microseconds, allowing 24 volts to be applied to the selected 70 ohm 
resistor pad 48 in the ink jet cartridge. This causes a small volume of 
test sample to vaporize, and the resulting increase in pressure causes a 
liquid droplet 19 to be forced out of the nozzle 17. 
The next clock pulse from the PG begins the cycle over again, where it 
again passes through the predetermining counter. If the limit has not been 
reached, the pulse passes to the Johnson counter where the next output in 
sequence goes high. This output then turns on the LED and transistor, and 
emits a droplet from the next nozzle. This sequence of events repeats 
until the counter has determined that the preset number of pulses (or 
droplets) has been reached, at which point the control output goes into a 
high state, and prevents any further pulses from reaching the Johnson 
counter. The foregoing is also described in a flow chart. See FIG. 4. 
In another aspect of the invention there is provided a method of generating 
low concentration aerosols having a known particle size. This method 
includes: 
a) generating liquid droplets which contain sample particles; 
b) drying the liquid droplets so that only the dried sample particles 
remain; and 
c) directing the sample particles to an analyzing instrument, preferably at 
a low velocity. 
As pointed out above, the sample of interest can be agglomerates of 
bacteria cells or spores which are often part of a biological warfare 
agent. The exiting particle is directed to a sample analyzer by 
facilitating communication of the exit orifice of the delivery tube with 
the sample entry port of the analyzer. It is to understood that the 
connection of the tube to the analyzer must be such that it does not 
destroy the velocity of the particle or otherwise alter the physical 
properties of the sample particle.