U.S. patent application number 10/636449 was filed with the patent office on 2004-02-12 for method of filtering a fluid and apparatus therefor.
Invention is credited to Olson, Jeffrey A..
Application Number | 20040026304 10/636449 |
Document ID | / |
Family ID | 25448881 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040026304 |
Kind Code |
A1 |
Olson, Jeffrey A. |
February 12, 2004 |
Method of filtering a fluid and apparatus therefor
Abstract
An apparatus and a method for dividing a fluid into desired
proportions by means of a filtering device. In one aspect, this
invention provides a method for dividing a fluid into desired
portions, the method comprising the steps of: (a) providing at
least one filtration device, the filtration device comprising a
filter; (b) adding the fluid to the at least one filtration device,
the fluid containing material dissolved or suspended therein; (c)
placing the at least one filtration device to which fluid has been
added in a pressure vessel, the pressure vessel capable of
withstanding a specified level of pressure relative to ambient
pressure; (d) forming a trapped volume downstream of the filter;
(e) increasing the pressure in the pressure vessel upstream of the
filter; (f) allowing a period of time to elapse, the period of time
being sufficient to allow the pressure downstream of the filter in
the trapped volume to be substantially equal to the pressure
upstream of the filter; (g) unsealing the filtration device; and
(h) venting the pressure vessel. One type of fluid that is
particularly amenable to the method of this invention is a solution
containing proteinaceous material dissolved therein. In this type
of fluid, the solvent is typically an aqueous solvent. In another
aspect, this invention provides an apparatus for dividing a fluid
into portions.
Inventors: |
Olson, Jeffrey A.;
(Libertyville, IL) |
Correspondence
Address: |
David L. Weinstein
Abbott Laboratories
Dept. 377 / AP6D-2
100 Abbott Park Road
Abbott Park
IL
60064-6050
US
|
Family ID: |
25448881 |
Appl. No.: |
10/636449 |
Filed: |
August 7, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10636449 |
Aug 7, 2003 |
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09923560 |
Aug 7, 2001 |
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6632371 |
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Current U.S.
Class: |
210/120 ;
210/322; 210/416.1; 210/435; 210/445 |
Current CPC
Class: |
B01D 61/22 20130101;
B01D 61/20 20130101; G01N 1/4077 20130101; B01L 2400/049 20130101;
B01L 2400/0487 20130101; B01D 61/147 20130101; B01L 3/50255
20130101; Y10T 436/25375 20150115; B01D 65/00 20130101; Y10T
436/255 20150115; B01D 61/18 20130101 |
Class at
Publication: |
210/120 ;
210/322; 210/445; 210/435; 210/416.1 |
International
Class: |
B01D 035/00 |
Claims
What is claimed is:
1. A method for dividing a fluid containing material dissolved or
suspended therein into desired proportions, said method comprising
the steps of: (a) providing at least one filtration device, said
filtration device comprising a filter; (b) adding said fluid to at
least one filtration device; (c) placing said at least one
filtration device to which said fluid has been added in a pressure
vessel, said pressure vessel capable of withstanding a specified
level of pressure relative to ambient pressure; (d) forming a
trapped volume downstream of said filter; (e) increasing the
pressure in said pressure vessel upstream of said filter; (f)
allowing a period of time to elapse, said period of time being
sufficient to allow the pressure downstream of said filter in said
trapped volume to be substantially equal to the pressure upstream
of said filter; (f) unsealing said trapped volume; and (g) venting
said pressure vessel.
2. The method of claim 1, wherein said filter is a porous
membrane.
3. The method of claim 1, wherein said material suspended or
dissolved in said fluid comprises a protein.
4. The method of claim 1, wherein said pressure in step (e) exceeds
the initial pressure in said trapped volume.
5. The method of claim 4, wherein the size of said trapped volume
and the levels of pressure and vacuum determine the proportion of
fluid that will be retained upstream of said filter.
6. The method of claim 1, wherein the pressure in said pressure
vessel is reduced prior to step (d).
7. An apparatus for dividing a fluid containing material dissolved
or suspended therein into desired proportions, said apparatus
comprising: (a) a pressure vessel, said pressure vessel capable of
withstanding a specified level of pressure relative to ambient
pressure; (b) means for supporting at least one filtration device
having a filter; (c) means for sealing said at least one filtration
device, whereby a trapped volume can be created downstream of said
filter of said at least one filtration device inserted into said
pressure vessel; (c) means for providing negative pressure or
positive pressure or negative and positive pressure relative to
ambient pressure within said pressure vessel; and (d) means for
venting said pressure vessel.
8. The apparatus of claim 7, wherein said pressure vessel comprises
a chamber and a cover.
9. The apparatus of claim 7, wherein said means for sealing said at
least one filtration device comprises a seal located between said
filtration device and a rack for holding said at least one
filtration device.
10. The apparatus of claim 9, wherein said means for sealing said
at least one filtration device further comprises a filtration
device clamp assembly.
11. The apparatus of claim 10, wherein said filtration device clamp
assembly comprises a filtration device clamp and means for biasing
the filtration device clamp toward said filter.
12. The apparatus of claim 11, wherein said means for biasing
comprises a spring.
13. The apparatus of claim 12, wherein variation of force of said
means for biasing provides variation in the proportion of fluid
retained upstream of said filter.
14. The apparatus of claim 10, wherein said means for sealing said
at least one filtration device further comprises an upper clamp
plate and a lower clamp plate.
15. The apparatus of claim 14, wherein said means for sealing said
at least one filtration device further comprises at least one
spacer for separating said upper clamp plate from said lower clamp
plate.
16. The apparatus of claim 7, wherein size of said trapped volume
can be varied to provide variation in the proportion of fluid
retained upstream of said filter.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This inventin relates to filtration of a fluid by means of a
filtration device; more particularly, the invention relates to a
method and an apparatus for controlling the proportion of a fluid
that passes through a filter in a filtration device.
[0003] 2. Discussion of the Art
[0004] All processes for filtering fluids involve three
components--the fluid itself, a filter, such as, for example, a
membrane having a specified porosity, and means to force the fluid
through the filter, such as, for example, a pump, gravity,
centrifugal force. In some cases, the entire volume of fluid to be
filtered is forced through the filter; in these cases, the desired
product may be the fuild that has been forced through the filter.
In other cases, the purpose of the filtering process is to
concentrate a substance present in the fluid; in these cases, the
desired product may be the portion of the fluid that has not been
forced through the filter. In the latter cases, it may be desirable
to control the volume of fluid retained in order to recover a
product having a specific concentration of the substance. For
example, it may be desired to concentrate, such as, for example, by
a factor of 10 to 1, a specific protein dissolved or suspended in a
small volume of an aqueous solution.
[0005] The conventional method for performing a concentrating
operation involves depositing a sample of a fluid in a filter
cartridge, such as, for example, a Microcon.RTM. centrifugal filter
device, commercially available from Millipore Corporation. The
loaded filter cartridge is then placed in a laboratory centrifuge
and rotated at a high number of revolutions per minute. The
orientation of the filter cartridge in the centrifuge would be such
that the "g" forces created by the centrifugation operation would
tend to drive the fluid through the filter and into a collection
container, which is positioned downstream of the filter. The
concentration factor is assumed to be a function of the duration of
the centrifugation operation; the time required to achieve the
desired concentration factor would be estimated by a trained
operator. If the desired concentration factor is 10 to 1, then the
desired volume retained upstream of the filter of the filter
cartridge would be {fraction (1/10)} of the volume initially loaded
into the filter cartridge. At the end of the estimated time, the
operator would stop the centrifuge and measure the proportion of
fluid remaining upstream of the filter of the filter cartridge. If
the proportion of fluid remaining upstream of the filter is
determined to be approximately correct, the process would be
complete. In most cases, however, it would be found that the volume
of fluid retained upstream of the filter of the filter cartridge
exceeded or fell short of the correct amount. If the volume
retained upstream of the filter were too little, correction would
be impractical because the concentration of the protein in the
protein-containing solution would have been too high. If the volume
retained upstream of the filter were too high, another cycle of
centrifugation would be performed, with the duration again
estimated by the operator in order to provide the desired
concentration factor. This method, while effective, is laborious
and-inexact. If the number of samples to be concentrated at any one
time is high, such as, for example, several dozen, the method
quickly becomes impractical in a laboratory setting.
[0006] Accordingly, it would be desirable to develop a method for
separating a solution or suspension into predetermined proportions
by a means other than centrifugation, so that the separation can be
controlled more accurately. In addition, it would be desirable to
develop an automated method for separating a solution or suspension
into predetermined proportions so that the proportioning process,
once begun, can be performed without the need for an operator. It
is further desired to develop a method such that a great number of
proportioning processes can be performed simultaneously.
SUMMARY OF THE INVENTION
[0007] This invention provides an apparatus and a method for
dividing a fluid into desired proportions by means of a filtering
device. In one aspect, this invention provides a method for
dividing a fluid into desired portions, the method comprising the
steps of:
[0008] (a) providing at least one filtration device, the filtration
device comprising a filter;
[0009] (b) adding the fluid to the at least one filtration device,
the fluid containing material dissolved or suspended therein;
[0010] (c) placing the at least one filtration device to which
fluid has been added in a pressure vessel, the pressure vessel
capable of withstanding a specified level of pressure relative to
ambient pressure;
[0011] (d) forming a trapped volume downstream of the filter;
[0012] (e) increasing the pressure in the pressure vessel upstream
of the filter;
[0013] (f) allowing a period of time to elapse, the period of time
being sufficient to allow the pressure downstream of the filter in
the trapped volume to be substantially equal to the pressure
upstream of the filter;
[0014] (g) unsealing the filtration device; and
[0015] (h) venting the pressure vessel.
[0016] Optionally, the pressure within the pressure vessel can be
reduced before the step (d), the step of forming the trapped
volume. The filtration device preferably contains a membrane or
barrier having a specified porosity, supported in a housing or in
an equivalent element that can be mounted within the pressure
vessel.
[0017] Many different types of fluids can be filtered by the method
of this invention. One type of fluid that is particularly amenable
to the method of this invention is a solution containing
proteinaceous material dissolved therein. In this type of fluid,
the solvent is typically an aqueous solvent.
[0018] The operating conditions of this method can be varied
widely. For example, the pressure in step (e) can be raised to any
pressure that can be withstood by the equipment. Pressures of as
high as 215 psia are common in the method of this invention. The
pressure in the optional step preceding step (d) can be reduced to
as low a level as 0 psia. The pressure can be controlled to allow
the division of the fluid into proportions ranging from about 100
to 1 to about 1 to 100. The size of the trapped volume can be
varied by various techniques, such as, for example, insertion of
plugs or inserts into the volume downstream of the filtration
device or removal of plugs or inserts from the volume downstream of
the filtration device.
[0019] In another aspect, this invention provides an apparatus for
dividing a fluid into portions, the apparatus comprising:
[0020] (a) a pressure vessel, the pressure vessel capable of
withstanding a specified level of pressure relative to ambient
pressure;
[0021] (b) means for supporting at least one filtration device
having a filter;
[0022] (c) means for sealing the at least one filtration device,
whereby a trapped volume can be created downstream of the filter of
the at least one filtration device inserted into the pressure
vessel;
[0023] (c) means for creating negative pressure or positive
pressure or negative and positive pressure relative to ambient
pressure within the pressure vessel; and
[0024] (d) means for venting the pressure vessel.
[0025] Means for sealing (b) include, but are not limited to,
rings, gaskets, and similar types of seals. Means for providing
negative pressure (c) include, but are not limited to, vacuum
pumps. Means for providing positive pressure (c) include, but are
not limited to, compressors, pressurized air lines, nitrogen
cylinders. Means for providing positive or negative pressure within
the pressure vessel further include, but are not limited to,
solenoid valves, pneumatic valves, and the like.
[0026] The apparatus and method of this invention provide numerous
benefits in the field of separating fluids into proportions. These
benefits include the following:
[0027] (a) providing greater accuracy and repeatability of
proportioning operations;
[0028] (b) simplifying control of the proportioning operation,
i.e., filters may be allowed to remain in the apparatus for an
indefinite period of time without any detrimental effect;
[0029] (c) providing the capability of performing proportioning
operations on a great number of samples simultaneously;
[0030] (d) increasing the rapidity of the proportioning operation
relative to a centrifugation operation, on account of a lower
number of iterations;
[0031] (e) enabling complete automation of the proportioning
operation after the operation has begun;
[0032] (f) providing the capability of varying proportions merely
by adjusting the level of vacuum and the level of pressure in the
pressure vessel; and
[0033] (g) simplifying the introduction of samples into the
apparatus and the removal of samples from the apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a schematic diagram illustrating a system for
performing the method of this invention.
[0035] FIG. 2 is a side elevational view of a cross-section of a
cartridge type filter device suitable for use in the method and
apparatus of this invention.
[0036] FIG. 3A is a side elevational view of a cross-section of the
apparatus of this invention. The view shows the configuration
inside the pressure vessel when the filtration devices are in an
unclamped state.
[0037] FIG. 3B is an enlarged view of the encircled portion of the
view shown in FIG. 3A.
[0038] FIG. 4A is a side elevational view of a cross-section of the
apparatus of this invention. The view shows the configuration
inside the pressure vessel when the filtration devices are in a
clamped state.
[0039] FIG. 4B is an enlarged view of the encircled portion of the
view shown in FIG. 4A.
[0040] FIG. 5 is a plan view of a filtration device rack suitable
for use in the method and apparatus of this invention.
[0041] FIG. 6 is a side elevational view of a cross section of the
filtration device rack shown in FIG. 5.
[0042] FIGS. 7A, 7B, 7C, 7D, 7E, and 7F are diagrams illustrating
the steps employed in the method of this invention.
DETAILED DESCRIPTION
[0043] As used herein, the expression "filtration device" means a
device that comprises a filter. The term "filter" means a porous
substance through which a liquid is passed in order to remove
constituents such as dissolved matter or suspended matter. An
example of a filter suitable for use in this invention is a porous
membrane, typically made of polymeric material. A filter can be
used to separate dissolved matter from a liquid if the size of the
pores in the filter is sufficiently small to prevent passage of the
solute. For example, if a protein is dissolved in an aqueous
solvent, a filter can be used to separate the protein from the
solvent if the size of the pores in the filter is smaller than the
size of the protein. The term "fluid" means any liquid. The
expression "trapped volume" refers to a sealed region into which
the fluid flows during the proportioning operation and in which
fluid that has flowed is allowed to remain after the proportioning
operation is completed.
[0044] The apparatus of this invention comprises:
[0045] (a) a pressure vessel, the pressure vessel capable of
withstanding a specified level of pressure relative to ambient
pressure;
[0046] (b) means for supporting at least one filtration device
having a filter;
[0047] (c) means for sealing the at least one filtration device,
whereby a trapped volume can be created downstream of the filter of
the at least one filtration device inserted into the pressure
vessel;
[0048] (c) means for creating negative pressure or positive
pressure or negative and positive pressure relative to ambient
pressure within the pressure vessel;
[0049] and
[0050] (d) means for venting the pressure vessel.
[0051] Referring now to FIG. 1, the apparatus 10 comprises a
pressure vessel 12, which is capable of containing at least one
filtration device 14, an actuator 16, a control unit 18, at least
one pneumatic line that connects the control unit 18 to the
pressure vessel 12, and at least one pneumatic line that connects
the control unit 18 to the actuator 16. The pneumatic line 19a is a
line for venting the pressure vessel 12 to the environment
surrounding the pressure vessel 12. The pneumatic line 19b is a
line for connecting a vacuum to the pressure vessel 12 to reduce
the pressure in the pressure vessel 12. The pneumatic line 19c is a
line for introducing compressed air or another gas into the
pressure vessel 12 to increase the pressure in the pressure vessel
12. The pneumatic line 19d is a line for providing compressed air
or another gas to cause the actuator 16 to operate. A pneumatic
line 20a also connects the control unit 18 to a source of vacuum
(not shown), and a pneumatic line 20b also connects the control
unit 18 to a source of pressure (not shown). The control unit 18
typically comprises valves for regulating the flow of air or gas
through the pneumatic lines 19a, 19,b, 19c, 19d, 20a, and 20b. The
control unit 18 also typically comprises a computer to control the
operation of the aforementioned valves. It is also preferred that
the assembly comprising the pressure vessel 12 and the actuator 16
be mounted on an agitator 21. The function of the agitator 21 is to
accelerate the rate of the flow of fluid through the filter of the
filtration device 14.
[0052] Referring now to FIGS. 3A and 4A, the pressure vessel 12
comprises a chamber 22 and a cover 24. The pressure vessel 12 must
be designed to withstand the pressure and vacuum expected to be
encountered during the operation of the method of this invention.
The pressure and vacuum expected to be encountered during the
operation of the method of this invention determine (1) the
thickness of the walls of the chamber 22 and the cover 24 and (2)
the materials of construction of the chamber 22 and the cover 24.
The thickness of the walls and the materials of construction can be
readily determined by one of ordinary skill in the art. As shown in
FIGS. 3A and 4A, the chamber 22 of the pressure vessel 12 is in the
shape of a cylinder, and the cover 24 of the pressure vessel 12 is
in the shape of a cylinder. However, other shapes of these
components are also suitable. For example, the pressure vessel 12
can be spherical in shape, with the chamber 22 being hemispherical
and the cover 24 being hemispherical. The cover 24 can be secured
to the chamber 22 by any means suitable for such a securing
operation, such as, for example, bolts, tie rods, breach locking,
and the like.
[0053] A passageway 26 formed in the chamber 22 of the pressure
vessel 12 allows the passage of a gas, e.g., compressed air,
nitrogen, into the pressure vessel 12 and out of the pressure
vessel 12. The pressure within the air space 28 of the pressure
vessel 12 can be varied by allowing a gas, preferably compressed
air, to enter and leave the air space 28. A seal 30, e.g., an
o-ring, is provided between the chamber 22 and the cover 24 to
ensure the maintenance of the proper level of pressure or vacuum in
the pressure vessel 12. The specifications of the seal 30 can be
determined by one of ordinary skill in the art. In the embodiment
shown in FIGS. 3A and 4A, the chamber 22 has a bottom wall 32 and a
side wall 34.
[0054] Referring now to FIG. 2, the filtration device 14 comprises
a housing 40, preferably having walls made from a plastic material.
The housing 40 is preferably cylindrical in shape. The upper
portion of the-housing comprises a chamber 42 for receiving the
fluid that is to undergo the proportioning operation. Immediately
below the chamber 42 is a support 44 for supporting a filter 46,
through which the fluid passes during the fluid proportioning
process. Filtration devices 14 are well known to those of ordinary
skill in the art. A representative example of a filtration device
14 is Microcon.RTM. centrifugal filter device, commercially
available from Millipore Corporation, Bedford, Mass.
[0055] A filtration device rack 50 can be placed near the bottom
wall 32 of the chamber 22 of the pressure vessel 12. The filtration
device rack 50 in the embodiment shown in FIG. 5 is capable of
holding at least one filtration device 14 and as many as eighty
(80) filtration devices. Filtration devices can be inserted into
recesses 51 machined into the filtration rack 50. A representative
example of a filtration device suitable for use in this invention
is shown in FIG. 2. Eight identical filtration devices, one of
which is designated by the reference numeral 14, are shown in the
filtration device rack 50 in FIGS. 3A and 4A.
[0056] The filtration device rack 50 rests on a support 52, which
in turn is attached to a connecting rod 54. The connecting rod 54
passes through an opening 56 in the bottom wall 32 of the chamber
22 of the pressure vessel 12. The connecting rod 54 is connected to
the actuator 16. The purpose of the actuator 16 is to raise and
lower the support 52 and the filtration device rack 50 a small
distance from the bottom wall 32 of the pressure vessel 12. The
actuator 16 raises or lowers the filtration device rack 50 relative
to a filtration device clamp assembly 58 at appropriate times
during the operation of the apparatus 10. The actuator 16 can be
driven by compressed air, by hydraulic pressure, by a screw, by
rack and pinion, or by any other means for this purpose known to
one of ordinary skill in the art. Another seal 60, e.g., an o-ring,
is used to seal the space between the connecting rod 54 and the
opening 56 in the bottom wall 32 of the chamber 22 of the pressure
vessel 12. The specifications of the seal 60 can be determined by
one of ordinary skill in the art. A cover plate 62 traps the seal
60 in a recess 64 machined into the bottom wall 32 of the chamber
22 of the pressure vessel 12.
[0057] The filtration device clamp assembly 58 is positioned above
the filtration device rack 50. Eight identical filtration device
clamp assemblies, one of which is designated by the reference
numeral 58, are shown in FIGS. 3A and 4A. The filtration device
clamp assembly 58 comprises at least one filtration device clamp
66. Eight identical filtration device clamps, one of which is
designated by the reference numeral 66, are shown in the embodiment
depicted in FIGS. 3A and 4A. The filtration device clamp assembly
58 further comprises at least one means 68 for biasing a filtration
device clamp towards the filtration device 14. The preferred
biasing means is a spring. A representative example of a spring
suitable for use in this invention is a stainless steel spring
having part number LC-035D-13-SS, commercially available from Lee
Spring Company. This spring provides a clamping force of
approximately five (5) pounds. Eight identical springs, one of
which is designated by the reference numeral 68, are shown in FIGS.
3A and 4A. The filtration device clamp assembly 58 further
comprises an upper plate 70, a lower plate 72, and at least one
spacer. Two spacers 74 and 76 are shown in FIGS. 3A and 4A. The
filtration device clamps 66 are biased downward (toward the
filtration device 14) by the springs 68 and guided (or directed) by
openings 70a formed in the upper plate 70 that are in register with
openings 72a formed in the lower plate 72 (see FIGS. 3B and 4B).
Spacers 74 and 76 maintain the proper spacing between the upper
plate 70 and the lower plate 72. A sufficient amount of clearance
is provided above the filtration device clamps 66 so that the
filtration device clamps 66 are capable of a small upward
displacement relative to the upper plate 70 and the lower plate 72.
In the embodiment shown in FIGS. 3A, 3B, 4A, and 4B, each
filtration device clamp 66 comprises a head 66a and a shaft 66b
(see FIGS. 3B and 4B). Each of the shafts 66b is capable of
vertical movement in the openings 70a formed in the upper plate 70
and the openings 72a formed in the lower plate 72. Another set of
spacers 78 and 80 provide proper spacing distance between the
filtration device clamp assembly 66 and the cover 24. A guide post
82 attached to the center of the filtration device rack 50 is
fitted through an opening 84 in the upper plate 70 and an opening
86 the lower plate 72. The guide post 82 constrains the motion of
the filtration device rack 50, thereby preventing it from tilting
during operation.
[0058] When the filtration device rack 50 is raised, the filtration
devices 14 are said to be in a clamped state. In this position, as
shown in FIGS. 4A and 4B, the filtration device clamps 66 push
downward on the filtration devices 14 with forces determined by the
springs 68. The purpose of this clamping action is to create a
trapped volume 88 downstream of the filters in the filtration
devices 14. That is, when the filtration device rack 50 is clamped,
the trapped volume 88 is hermetically sealed from the air space 28
in the rest of the pressure vessel 12. In contrast, the air space
28 upstream of the filters in the filtration devices 14 is
maintained at the same pressure as the air space 28 in the pressure
vessel 12. Maintenance of this equalization of pressures can be
accomplished by a channel 90, which is formed in the area 92 of
each filtration device clamp 66 facing the filtration device. In
order to maintain a hermetic seal, a seal 94, e.g., an elastomeric
washer, is used to seal the space between the filtration device 14
and the filtration device rack 50. The specifications of the seal
94 can be determined by one of ordinary skill in the art. A
representative example of a seal 94 suitable for use in this
invention is made by Apple Rubber products. This seal 94 has an
outside diameter of 0.375 inch and an inside diameter of 0.25 inch.
The thickness of the seal 94 is 0.032 inch. The material of the
seal is a silicone elastomer having a hardness of 40 durometer
(Shore A scale).
[0059] When the filtration device rack 50 is not in the clamped
state, the trapped volume 88 is not sealed from the air space 28 in
the rest of the pressure vessel 12. In this state, the pressure
level that exists in the trapped volume 88 will be the same as that
in the rest of the air space 28. A collar 66c prevents the
filtration device clamp 66 from falling out of the upper plate 70
and the lower plate 72 when the filtration device rack 50 is not in
the clamped state.
[0060] Operation
[0061] The method of this invention comprises the following
steps:
[0062] (a) providing at least one filtration device, the filtration
device comprising a filter;
[0063] (b) adding the fluid to the at least one filtration device,
the fluid containing material dissolved or suspended therein;
[0064] (c) placing the at least one filtration device to which
fluid has been added in a pressure vessel, the pressure vessel
capable of withstanding a specified level of pressure relative to
ambient pressure;
[0065] (d) forming a trapped volume downstream of the filter;
[0066] (e) increasing the pressure in the pressure vessel upstream
of the filter;
[0067] (f) allowing a period of time to elapse, the period of time
being sufficient to allow the pressure downstream of the filter in
the trapped volume to be substantially equal to the pressure
upstream of the filter;
[0068] (g) unsealing the filtration device; and
[0069] (h) venting the pressure vessel. In an alternative
embodiment, the optional step of reducing the pressure within the
pressure vessel to a level below ambient can be included subsequent
to step (c) and prior to step (d).
[0070] The operation of this invention involves a sequence of steps
in which the level of pressure within the pressure vessel 12 varies
as a function of the position of the filtration device rack 50. In
other words, the level of pressure within the pressure vessel 12
depends on whether or not the filtration device rack 50 is in the
clamped state. The timing of the steps in this sequence, as well as
the level of pressure, is controlled by the control unit 18. See
FIGS. 7A, 7B, 7C, 7D, 7E, and 7F.
[0071] In order to simplify the explanation of the operation of the
method and apparatus of this invention, the following set of
conditions is selected:
1 Initial volume of fluid to be proportioned = 400 .mu.L Volume of
fluid to be retained upstream of the filter = 30 .mu.L Pressure
level = 115 psia Vacuum level = 2 psia Size of trapped volume = 376
.mu.L
[0072] The filtration device is a Microcon.RTM. centrifugal filter
device (Millipore Corporation, 500 82 L capacity on the upstream
side of the filter). The values of the conditions selected herein
are arbitrary; other levels of pressure and vacuum can be used, if
so desired. Because the volume of fluid to be retained upstream of
the filter is 30 .mu.L, it is desired to force 370 .mu.L of the
initial volume of fluid through the filtration device and then
prevent additional flow of fluid thereafter. The following
calculations further demonstrate the operating principle of the
method and apparatus:
[0073] Vacuum=2 psia
[0074] Desired flow through volume (V.sub.ft)=400 .mu.L-30
.mu.L=370 .mu.L
[0075] P.sub.f=final pressure in trapped volume at equilibrium=115
psia
[0076] P.sub.i=initial pressure in trapped volume after vacuum has
been applied=2 psia
[0077] V.sub.i=initial volume of air in trapped volume
[0078] V.sub.f=final volume of air in trapped volume
[0079] Vr=volume of fluid retained upstream of the filter in the
filtration device
[0080] Assuming isothermal compression of the air trapped in the
trapped volume
P.sub.f/P.sub.i=V.sub.i/V.sub.f (1)
P.sub.f/P.sub.i=V.sub.i/(V.sub.l-V.sub.ft) (2)
115 psia/2 psia=V.sub.i/(V.sub.l-370 .mu.L)
V.sub.i=376.5 .mu.L
[0081] Therefore, the desired volume for the trapped volume is
376.5 .mu.L.
[0082] The method and apparatus of this invention can accomplish
this proportioning of a sample of fluid by means of the sequence of
steps depicted in FIGS. 7A, 7B, 7C, 7D, 7E, and 7F. In this series
of figures, for the sake of simplification, only three each of the
filtration device 14 and filtration device clamp assembly 58 are
shown. Referring now-to FIG. 7A, each filtration device 14 is
filled with liquid (400 .mu.L), designated by the letter "F", and
placed on top of the seal 94 in the filtration device rack 50. The
cover 24 of the pressure vessel 12 is closed and secured,
preferably by means of bolts, so that the pressure vessel 12 is
sealed to the environment that is outside of the pressure vessel
12. The filtration device rack 50 is set in the unclamped state so
that no clamping force is exerted on the filtration device 14. A
vacuum is connected to the inlet 26 of the pressure vessel 12 so
that the entire interior volume of the pressure vessel 12,
including the volume thereof downstream of the filter 46, is
reduced to a level of pressure of 2 psia. The pneumatic line used
for reducing the level of pressure is the pneumatic line 19b.
During this step no fluid is forced through the filter of the
filtration device 14 because no pressure differential exists across
the filter. An arrow and the letter "V" indicates that a vacuum is
being applied.
[0083] Referring now to FIG. 7B, while the vacuum is still being
applied, the filtration device rack 50 is moved a sufficient
distance by means of the actuator 16 to set the filtration device
rack 50 into the clamped state, wherein a downward force is exerted
on the filtration device 14. The pneumatic line used for supplying
compressed air or gas for moving the actuator 16 is the pneumatic
line 19d. This downward force compresses the seal 94 between the
filtration device 14 and the filtration device rack 50, thereby
sealing the trapped volume 88 from the rest of the environment
inside the pressure vessel 12. At this point, the pressure within
the pressure vessel 12, as well as within the trapped volume 88,
remains at a level of 2 psia. An arrow and the letter "D" indicates
that a downward force is being applied.
[0084] Referring to FIG. 7C, the inlet 26 to the pressure vessel 12
is switched away from the vacuum and connected to a source of
compressed air. After a sufficient period of time has elapsed, the
absolute pressure in the chamber rises to 115 psia, but the
pressure in the trapped volume 88 continues to be 2 psia, thereby
resulting in a pressure difference of 113 psia across the filter
46. The pneumatic line used for supplying compressed air to raise
the pressure in the pressure vessel 12 is the pneumatic line 19c.
This pressure difference exerts a net force on the fluid and the
fluid begins to flow through the filter 46 into the trapped volume
88. An arrow and the letter "P" designates an increase in pressure
brought about by the introduction of compressed air into the
pressure vessel 12.
[0085] Referring to FIG. 7D, after an initial period of time has
elapsed, e.g., 15 minutes, a portion of the fluid has flowed
through the filter 46 and into the trapped volume 88. Because the
trapped volume 88 is sealed, and the fluid occupies a portion of
this trapped volume 88, the trapped air, which was at a pressure of
2 psia at the beginning, is compressed, thereby resulting in a
higher pressure in the trapped volume 88, e.g., about 30 psia. It
should be noted that a pressure difference still exists across the
filter 46, which pressure difference continues to drive fluid
through the filter 46 into the trapped volume 88.
[0086] Referring to FIG. 7E, after a subsequent period of time has
elapsed, a condition of equilibrium is reached. By this time, most
of the fluid has flowed through the filter 46 and the pressure in
the trapped volume 88 is now 115 psia, the same pressure as that
within the air space 28 of the pressure vessel 12. Because the net
pressure across the filter 46 is now zero, all flow of fluid stops.
The size of the trapped volume 88 determines the volume of fluid
that will be retained upstream of the filter at the completion of
the process. If the size of the trapped volume 88 has been
specified carefully, the volume of fluid retained upstream of the
filter will be near the desired amount--in this case 30 .mu.L. The
duration of the filtration process is not critical; after the
system has attained the state of equilibrium, the filtration device
14 can be held in the pressure vessel 12 indefinitely and no
further flow of fluid will take place. The end point of the
filtration process is determined solely by the size of the trapped
volume 88 and the levels of pressure and vacuum used in the
process.
[0087] Referring now to FIG. 7F, in the final step, the filtration
device rack 50 is moved to the unclamped state, and the gas (air)
within the pressure vessel 12 is vented to the atmosphere. The
pneumatic line used for venting the gas to the in atmosphere is the
pneumatic line 19a. When the clamping pressure against the seal 94
is removed, the air trapped downstream of the filter is now free to
escape harmlessly. If the pressure vessel 12 were vented without
unclamping the filtration device 14, the filter 46 would rupture,
due to the force of the trapped air downstream of the filter. If
the filter were to rupture, the fluid retained upstream of the
filter would be free to mix with the fluid collected downstream of
the filter, thereby defeating the purpose of the-method. An arrow
and the abbreviation "Ve" designates the flow of air or gas in the
venting step.
[0088] It should be noted that the method and the apparatus of this
invention employ the principle that fluid flow must cease when
there is no pressure differential across the filter. By carefully
specifying the levels of pressure and vacuum and the size of the
trapped volume 88, the point at which there is no pressure
difference across the filter, and thus no more flow of fluid, can
be selected. In this way, any desired fraction of the fluid can be
retained upstream of the filter.
[0089] The apparatus also provides means for making modest
adjustments to the proportion of fluid retained upstream of the
filter by varying the clamping force. Because the seal 94 is
somewhat compressible, an increase to the clamping force increases
the compression of the seal 94 and reduces the size of the trapped
volume 88. This increase in clamping force can be brought about by,
for example, using a stronger spring, which provides a higher
compressive force. For a given level of pressure, a reduction in
the size of the trapped volume 88 will lead to a decrease in the
volume of fluid that can flow through the filter, V.sub.ft, and,
thus, an increase in the volume of fluid retained upstream of the
filter, V.sub.r. Conversely, for a given level of pressure, a
decrease in the clamping force has the effect of increasing the
size of the trapped volume 88. An increase in the size of the
trapped volume will lead to an increase in the volume of fluid that
can flow through the filter, V.sub.ft, and, thus, a decrease in the
volume of fluid retained upstream of the filter, V.sub.r.
Therefore, by careful manipulation of the clamping force, fine
adjustments can be made to the system.
[0090] As stated previously, some of the major benefits
attributable to the method and apparatus of this invention include
the following:
[0091] (a) greater accuracy and repeatability of proportioning
operations;
[0092] (b) simplified control of the proportioning operation;
[0093] (c) the capability of performing proportioning operations on
a great number of samples simultaneously;
[0094] (d) increased rapidity of the proportioning operation
relative to a centrifugation operation, on account of a lower
number of iterations;
[0095] (e) complete automation of the proportioning operation after
the operation has begun;
[0096] (f) the capability of varying proportions merely by
adjusting the level of vacuum and the level of pressure in the
pressure vessel; and
[0097] (g) simplified introduction of samples into the apparatus
and the removal of samples from the apparatus.
[0098] Various modifications and alterations of this invention will
become apparent to those skilled in the art without departing from
the scope and spirit of this invention, and it should be understood
that this invention is not to be unduly limited to the illustrative
embodiments set forth herein.
* * * * *