U.S. patent number 3,869,067 [Application Number 05/433,376] was granted by the patent office on 1975-03-04 for apparatus for gradient elution.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to Howard L. Ashmead, Sydnor H. Byrne, Jr., John P. Wolf, III.
United States Patent |
3,869,067 |
Ashmead , et al. |
March 4, 1975 |
APPARATUS FOR GRADIENT ELUTION
Abstract
The use of separate supply means for each of two liquids and a
proportioning means comprising a mixing region, valving means
connected to the mixing chamber to control the flow of each liquid
to the mixing region, and a means for periodically controlling the
operation of the valving means to control the amount of each liquid
supplied to the mixing region during each period of valve
operation, produces a supply of eluent having a precisely
controlled time varying concentration of each liquid.
Inventors: |
Ashmead; Howard L. (Newark,
DE), Byrne, Jr.; Sydnor H. (Newark, DE), Wolf, III; John
P. (Wilmington, DE) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
26951541 |
Appl.
No.: |
05/433,376 |
Filed: |
January 14, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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265994 |
Jun 26, 1972 |
|
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36633 |
May 12, 1970 |
3712513 |
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Current U.S.
Class: |
222/639;
137/624.18; 210/101; 210/198.2; 222/134 |
Current CPC
Class: |
G01N
30/34 (20130101); G05D 11/132 (20130101); Y10T
137/86445 (20150401) |
Current International
Class: |
G01N
30/00 (20060101); G01N 30/34 (20060101); G05D
11/00 (20060101); G05D 11/13 (20060101); G05d
011/02 () |
Field of
Search: |
;222/70,134,135,145
;137/101.11,607,624.11,624.18 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Reeves; Robert B.
Assistant Examiner: Kocovksky; Thomas E.
Parent Case Text
This application is a continuation of application Ser. No. 265,994,
filed June 26, 1972 now abandoned which is in turn a division of
application Ser. No. 36,633, filed May 12, 1970 now U.S. Pat. No.
3,712,513.
Claims
What is claimed is:
1. An elution system comprising:
a. a source of first liquid;
b. a source of second liquid;
c. first liquid supply means connected to said source of first
liquid;
d. second liquid supply means connected to said source of second
liquid;
e. proportioning means comprising a mixing region, valving means
connected to said mixing region, and programming means connected to
said valving means for continuously changing the concentration of
the first and second liquids in the mixing region, said valving
means having at least two operative portions, a first operative
portion connected between said first liquid supply means and said
mixing region in a manner such as to supply said mixing region with
said first liquid when said first operative portion is activated
and a second operative portion connected between at least a portion
of said second liquid supply means and said mixing region in a
manner such as to supply said mixing region with said second liquid
when said second operative portion is activated, said programming
means being adapted to separately and periodically control the time
during which each operative portion of said valving means is
activated and thereby change the concentration of the liquids in
the mixing region, said mixing region having a volume greater than
the maximum volume of liquid entering said mixing region during the
period when either operative portion of said valving means is
activated, and comprising means to allow a portion of any liquid
contained in said mixing region to be removed from said mixing
region while the liquids are being added to the mixing region.
2. The elution system of claim 1 wherein said valving means is a
three-way valve, one path therein being said first operative
portion and the second path therein being said second operative
portion.
3. The elution system of claim 1 wherein said valving means
comprises two separate on-off valves, the first valve being said
first operative portion and the second valve being said second
operative portion.
4. An elution system comprising:
a. a source of first liquid;
b. a source of second liquid;
c. a first pump connected to said source of first liquid;
d. a second pump connected to said source of second liquid; and
e. proportioning means comprising a mixing region, valving means
connected to said mixing region, and programming means connected to
said valving means for continuously changing the concentration of
the first and second liquids in the mixing region, said valving
means having two operative portions, a first operative portion
connected to said first pump in a manner such as to supply said
mixing region with said first liquid when said first operative
portion is activated, and a second operative portion connected to
said second pump in a manner such as to supply said mixing region
with said second liquid when said second operative portion is
activated, said programming means being adapted to separately and
periodically control the time during which each operative portion
of said valving means is activated and thereby change the
concentration of the liquids in the mixing region, said mixing
region having a volume greater than the maximum volume of liquid
entering said mixing region during the period when either operative
portion of said valving means is activated, and comprising means to
allow a portion of any liquid contained in said mixing region to be
removed from said mixing region while the liquids are being added
to the mixing region.
5. The elution system of claim 4 wherein said valving means is a
three-way valve, one path therein being said first operative
portion and the second path therein being said second operative
portion.
6. The elution system of claim 4 wherein said valving means
comprising two separate on-off valves, the first valve being said
first operative portion and the second valve being the second
operative portion.
7. The elution system of claim 4 wherein said pumps are constant
pressure pumps.
8. The elution system of claim 4 wherein said pumps are constant
flow pumps.
9. An elution system comprising:
a. a source of first liquid;
b. a source of second liquid;
c. proportioning means comprising a mixing region, a first on-off
valve connected to said mixing region, a second on-off valve
connected to said mixing region, and programming means connected to
both said first and second on-off valves for continuously changing
the concentration of the first and second liquids in the mixing
region, said programming means being adapted to separately and
periodically control the duration during which each of said on-off
valves is open and thereby change the concentration of the liquids
in the mixing region, said mixing region having a volume greater
than the maximum volume of liquid entering said mixing region
during the period when either operative portion of said valving
means is activated, and comprising means to continuously allow a
portion of any liquid contained in said mixing region to leave said
mixing region while the liquids are being added to the mixing
region;
d. first liquid supply means connected between said source of first
liquid and said proportioning means to supply said first on-off
valve with said first liquid at constant pressure; and
e. second liquid supply means connected between said source of
second liquid and said proportioning means to supply said second
on-off valve with said second liquid at constant pressure.
10. The elution system of claim 9 wherein said first liquid supply
means is a first pump and said second liquid supply means is a
second pump.
11. The elution system of claim 9 wherein said first and second
on-off valves are air actuated valves.
12. The elution system of claim 9 wherein said first and second
on-off valves are solenoid valves.
13. The elution system of claim 9 wherein said first and second
on-off valves are normally closed solenoid valves and wherein said
programming means is a means to generate an electric signal which
will activate either of said on-off solenoid valves and to apply
said signal to at least one of said on-off solenoid valves for a
predetermined period of time.
14. The elution system of claim 9 wherein said programming means is
a means to generate a time varying signal and to apply said signal
alternately to each of said on-off solenoid valves.
Description
BACKGROUND OF THE INVENTION
This invention relates to the field of elution. More specifically
it relates to a method and system for producing an eluent having a
known concentration of different liquids. More specifically still,
it relates to a method and system for producing an eluent having a
time varying concentration of different liquids.
The simplest elution system, used for example in a chromatographic
column, supplies an eluent comprising a single liquid to the
column. In many instances, however, this single liquid is not
effective in removing all of the desired material from the column,
and a second liquid must subsequently be supplied. Alternately, an
eluent having a set concentration of the two liquids is used. More
than two liquids can and have been used, but for convenience in
describing the prior art and our invention, we will limit the
discussion that follows to a system using two liquids which we will
label liquid A and liquid B. Switching liquids in mid-operation and
the use of a mixture of liquids have not been favored procedurally,
and there has been a recent tendency to gradient elution systems.
In gradient elution, the initial eluent contains a set
concentration of the liquids (usually 100% of A and none of B), and
this concentration is slowly altered, through intermediate
concentrations, to a second set concentration of the liquids
(usually none of A and 100% of B). The conventional way of doing
this is to use two constant flow pumps. One of these pumps pumps
liquid from a pot containing liquid having the initial
concentration of the two liquids (usually 100% of A and none of B)
to the column. The second pump pumps liquid having the desired
final concentration of the two liquids (usually none of A and 100%
of B) into the pot provided for the first pump so that the
concentration of liquid in the pot which is being pumped to the
column, gradually changed to the final concentration. This
procedure has several disadvantages. Two pumps are necessary, and
although the change in concentration can be fairly precisely
controlled, the system is not versatile in terms of the ability to
tailor changes in concentration, to meet the needs of individual
tests. In some instances, instead of using the two stage system
described above, two proportional flow valves are used in
combination with the two pumps, and the flow of liquid through the
valves is varied to achieve the desired concentration. This system
suffers from the fact that the flow through proportional valves is
difficult if not impossible to control with any precision,
especially at low flow, so that the concentration cannot be
controlled precisely.
It is an object of the present invention to provide a method and
system for providing an eluent in which the concentrations of the
liquid parts is precisely known. It is a further object of the
present invention to provide an eluent having precisely controlled
time varying concentrations of the liquid parts. It is a still
further object of the present invention to provide a system for
producing an eluent having time varying concentrations of liquid
parts in which the means for controlling the concentration of
liquids provides for flexible control over the concentrations of
the liquids in the eluent and over the rate of change of those
concentrations. It is another object of the present invention to
provide a method and system for producing an eluent having a time
varying concentration of liquids, in which a single pump is
used.
SUMMARY OF THE DISCLOSURE
These objects are accomplished by providing: separate sources of
liquids, in the case being discussed, a source of first liquid and
a source of second liquid; a proportioning means comprising a
mixing region, valving means connected to the mixing region and
programming means connected to and adapted to activate the valving
means; and separate liquid supply means, in this case a first
liquid supply means and a second liquid supply means to supply the
first and second liquids respectively to the valving means. The
mixing region is then connected directly to the column. The valving
means comprises at least two operative portions, a first operative
portion connected between the first liquid supply means and the
mixing region and a second operative portion connected between at
least a portion of the second liquid supply means and the mixing
region. The programming means periodically activates one or the
other of the operative portions of the valving means to allow the
mixing region to be periodically supplied with each liquid.
The mixing region can be any region adapted to produce the required
amount of mixing between the two fluids. In one embodiment the
mixing region can be a separate mixing chamber with a volume
substantially larger than the maximum volume of liquid entering the
mixing region during the period when either operative portion of
the valving means is activated, and the chamber can be provided
with some means to thoroughly mix the liquids in the chamber so
that the mixture is homogeneous throughout the chamber. In a
simpler but still effective embodiment, the mixing region is
provided by the connecting lines between the valving means and the
column alone. Depending upon how homogeneous the mixture must be
for the particular purpose involved, the volume of the mixing
region can be less than, equal to or greater than the maximum
volume entering the mixing region during the period when either
operative portion of the valving means is operative. While it might
be expected that either a very large mixing region or some
additional mixing device, such as frittered plug to provide highly
turbulent regions would have to be provided, we have found that
except in those applications where a high degree of uniformity is
required, a reasonably small volume of connecting tubing alone will
provide substantial mixing. As an example, when each operative
portion of the valving means passes 0.25 cc. of liquid, a mixing
region having a volume of 0.50 cc. will produce a final mixture at
the end of the mixing region having a periodic variation of no more
than 1.0%. If a variation of 10 to 20% were tolerable, the volume
of the mixing region could even be less than the volume of liquid
periodically passed through the valving means. It appears that in
this dynamic system, substantial mixing is produced by diffusion,
friction, eddy currents and the like.
The mixing region is also provided with a means to allow a portion
of any liquid contained in the mixing region to be removed from the
mixing region, thereby supplying an eluent having a concentration
of each liquid equivalent to the instantaneous concentration of
each liquid in the mixing region at the time the liquid is removed
from the mixing region. Preferably the liquid is removed from the
mixing region and supplied to the column continuously.
The valving means can be: a three-way valve in which case one of
the paths therein is the first operative position and the other
path is the second operative position; a pair of separate valves,
in which each valve is a separate operative portion of the valving
means, or any suitable on-off valving means which will allow liquid
to be switched from one fluid line to another fluid line. For
convenience in the discussion that follows, we will limit our
discussion to a system in which two separate valves are used. These
valves can be any valve, such as a solenoid valve or an air
actuated valve, which can be activated by an externally generated
signal. The programming means is, then, any means that will
generate and supply a signal to the valves which will activate the
valves. In the preferred embodiment the valves are solenoid valves;
and the programming means is an electronic signal generator adapted
to produce two electronic signals, one being a periodic signal and
the other being a monotonically increasing function. The
programming means is further adapted to control the valves by
opening one when the instantaneous value of one signal exceeds the
instantaneous value of the other signal, and then opening the other
when the reverse situation occurs.
The first and second liquid supply means can be two separate pumps,
either constant flow or constant pressure pumps as the situation
demands; but in the preferred embodiment, a single pump is used.
Instead of the second pump, a holding chamber is provided. The
single pump alone is used as the first liquid supply means to
supply the first liquid to the first valve. The second liquid
supply means comprises both the holding chamber, which is adapted
to be filled with the second liquid, and the pump. The pump is
connected to the holding chamber and the second valve in a manner
such that when the second valve is open, the first liquid supplied
by the pump is used to force the second liquid contained in the
holding chamber to the second valve. The valve can be located on
either the upstream or downstream side of the holding chamber, and
either a constant flow or constant pressure pump can be used
depending on the particular needs of the system, The holding
chamber must be constructed so that substantial mixing of the first
liquid with the second liquid does not occur in the true interval
while the system is in operation. A holding chamber in the form of
a long thin tube which holds substantially more liquid than is to
be used in a particular situation fills this criteria.
The detailed operation of the system described above can best be
described by reference to the following figures:
FIG. 1 is a schematic diagram of the simplest embodiment of the
present invention;
FIG. 2 is a schematic diagram of a first embodiment of an improved
version of the present invention which requires the use of only one
pump;
FIG. 3 is a schematic diagram of a second embodiment of the present
invention which requires the use of only one pump;
FIG. 4 is a schematic diagram of the preferred embodiment of the
present invention;
FIG. 5 is a schematic diagram of one embodiment of a device used to
measure the amount of liquid removed from the holding chamber;
FIG. 6 is a diagram of several of the possible wave functions that
can be used to control the valves, and the programming sequence
they produce; and
FIG. 7 is a graph of one possible programming sequence.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring to FIG. 1, a source of liquid A, 11, and a source of
liquid B, 12, are provided and connected respectively to a first
and second liquid supply system which in this embodiment are
constant pressure pumps 13 and 14. These pumps are each connected
to proportioning means 15 which comprises two on-off valves 16 and
17, a mixing region, and a programming means 66 which is connected
to the valves 16 and 17 by connectors 68 and 70, respectively, and
adapted to control the operation of the valves. For convenience,
the mixing region is illustrated in this and the other figures as a
separate mixing chamber 18 though, as discussed above, this is not
necessary. Pumps 13 and 14 are connected respectively to valves 16
and 17 which are each connected to mixing chamber 18. The mixing
chamber is in turn connected to a chromatographic column system 19
comprising an injection port 20, a column 21, and a detector 22.
The column system in turn is connected to or empties into a drain
23. The elution system described herein is useful in any
application where a well controlled or changing concentration of
liquids in a mixture is required, but for convenience, we are
discussing the system in its application to a system for supplying
the eluent used in liquid chromatography. Being included for merely
descriptive purposes, the details of the chromatographic system
will not be described.
The on-off valves 16 and 17 are normally closed on-off valves of a
type controlled by an external signal. Solenoid valves or air
actuated valves are typical examples of such valves. Alternately, a
signal actuated three-way valve could be used, or any valving means
adapted to switch positively from one liquid to the other at
controlled intervals. For convenience, we will limit the discussion
that follows to solenoid valves and electronic programming means.
It is to be understood, however, that any externally activated
valve and a programming means adapted to activate it can be used
instead.
In operation, the constant pressure pumps supply their liquids at
constant pressure to each of the normally closed on-off valves
respectively. The programming means then opens one of the valves
for a set period of time. We will assume in what follows that
liquid A is always the first fluid to flow and consequently, that
valve 16 is the one opened, but the reverse can be true. A set
volume of liquid A will enter the mixing chamber. Then the
programming means will deactivate valve 16, and activate valve 17
for a second period of time to allow a set volume of fluid B to
enter the mixing chamber. In the normal situation, the initial
mixture of liquids in the mixing chamber would be 100% of liquid A
and none of liquid B. To fill the chamber initially, then, valve 16
would open either periodically or for a long duration until the
mixing chamber was filled with liquid A, and valve 17 would remain
closed. The volume of the mixing chamber in this embodiment is
larger than the volume of liquid each valve is programmed to allow
to pass during each of its cycles when the valves are in periodic
operation. In those situations where a high degree of uniformity is
required, only a fraction of the volume of liquid needed to fill
the mixing chamber is allowed to pass through the valve. The eluent
supplied to a chromatographic column can be supplied at high
pressure or just allowed to flow through the column under the force
of gravity. We will limit our discussion to the former, more
difficult situation, when the mixing chamber must be completely
filled with the initial concentration. In this situation, a
bleed-off valve on the mixing chamber may be useful to bleed off
air trapped in the chamber or to assist in changing the fluids in
the chamber. Assuming now that the mixing chamber is completely
filled with liquid A, the programming means operates to change the
concentration in the mixing chamber while the liquid in the chamber
is being supplied under pressure, the pressure of pumps 13 and 14,
to the column. The simplest situation would be where the final
mixture is to contain 100% of liquid B and none of liquid A. One
way to achieve this concentration would be to have the programming
means activate valve 17 and not activate valve 16 at all. If
substantial homogeneity of the mixture at the end of the mixing
chamber next to the column is not achieved, then the precision and
reproducibility of the concentration supplied to the column is poor
and the resulting chromatographic determinations suffer.
In the operation just discussed, valve 17 is periodically or
continuously opened introducing small volumes of liquid B into the
liquid in the mixing chamber. Gradually the concentration of liquid
B in the mixing chamber increases until the mixing chamber finally
contains only liquid B. It is apparent that in some cases the
supply of liquid A contained in the mixing chamber will not be
sufficient to allow the change from the initial concentration to
the final concentration to take place in a manner which will suit
the needs of most eluting devices. In this case the programming
means will alternately activate valve 16 and valve 17 to maintain
the supply of liquid A in the mixing chamber. If valves 16 and 17
are alternately opened for the same set period of time, the final
concentration will be 50% of liquid A and 50% of liquid B. To
achieve a final concentration of 100% liquid B, the period during
which valve 16 is open must be gradually decreased to zero. This is
usually accompanied by an increase in the period during which valve
17 is open. The relative periods involved can be determined by the
needs of the system. In fact the periods need not vary. If a set
concentration is desired, each valve can be programmed to open for
a different set period of time which will fix the desired
concentration, and the concentration can be maintained by leaving
the valve periods fixed during the entire operation.
FIG. 7 illustrates the versatility of such a device. In the
situation where the experimentor does not know the most useful
concentration of liquids. The initial concentration can be set at a
reasonable level, and the results observed. If unsatisfactory, the
concentration can be changed gradually until the desired or final
concentration is achieved, at which point the concentration can be
maintained at the desired level. If it is determined that gradient
elution, a change from one concentration to another, is desirable,
but the optimum rate of change is not known, the rate can be varied
simply by programming the valves to open at a different frequency
relative to one another, until the optimum rate of change is
found.
FIG. 2 illustrates an improved version of the system shown in FIG.
1. The only change is that pump 14 has been replaced by a liquid
supply means comprising a holding chamber 25 coupled between valve
17 and pump 13. The holding chamber is initially filled with liquid
B from source 12 by opening valve 24 and allowing the chamber to
fill by gravity or pressure. Valve 24 is then closed, and constant
pressure pump 13 operates alternately to supply a portion of liquid
A directly to valve 16 and a portion of liquid A to the holding
chamber 25 to force liquid B, contained in the holding chamber to
valve 17. The advantage of this system is that a single pump can be
used which means that the system is less expensive and also that
both liquids are supplied to their respective valves at the same
constant pressure or flow rate. When two pumps are used it is
difficult to achieve this latter condition because it is difficult
to match the pressures or flow rates developed by different pumps.
The holding chamber must be constructed so that substantial mixing
of liquid A and liquid B do not occur while liquid A is being used
to force liquid B to valve 17. Also enough of liquid B must be
contained in the holding chamber to complete the elution run. A
long narrow holding chamber, such as a long thin tube fulfills
these conditions.
FIG. 3 illustrates another embodiment of the same system. In this
embodiment, the two valves 16 and 17 have been replaced by a single
three-way valve 26, and constant pressure pump 13 has been replaced
by constant flow pump 27. Valve 26 is located upstream of holding
chamber 25 rather than on the downstream side but it could just
have easily been located on the downstream side. Operation of the
system is the same as that discussed with respect to FIG. 2.
FIG. 4 illustrates the preferred embodiment of the elution system.
In this embodiment liquid sources 11 and 12 are provided with sight
tubes 30 and 31, respectively, so that the level of liquid in each
can be monitored. The holding chamber is in the form of a long thin
coil of tubing 34, which is still filled by gravity or pressure
through valve 24. Air or vacuum can be supplied to liquid source
container 12 through valves 32 and 33 respectively to assist in
filling holding chamber 34. The holding chamber is connected
directly to drain 23 through valve 36 which is normally closed, and
used to both drain the system and to aid in filling the holding
chamber. The upstream side (low end) of the holding chamber is
connected to pump 13 through valve 35 which is opened when the
liquids are to be supplied to their respective valves. The
operation of the system is similar to that discussed with reference
to FIG. 2. The holding chamber is filled and valves 24 and 36 are
closed. Valve 35 is open and pump 13 is used to both supply liquid
A to valve 16 and to force the liquid contained in the holding
chamber 34 to valve 17. The programmed mixture is forced through
the column to a drain 23. Care must be taken in the operation of
the system so that holding chamber 34 is not inadvertently emptied
of liquid B during protracted operation so that only liquid A is
being supplied to the mixing chamber. To prevent this, some means
may be provided to keep track of the amount of liquid, originally
contained in the holding chamber; that has been used. Control means
37 and trap 38 have been provided for this purpose. As illustrated
control means 37 is a three-way valve. The three-way valve is
adapted so that when valve 17 is open, the three-way valve is in
the position illustrated and a volume of liquid equal to the volume
of liquid that has passed through valve 17 is collected in trap 38;
valve 39 being closed. When valve 17 is closed, the three-way valve
rotates a quarter turn to the left and the liquid passes directly
to the drain. The liquid level in trap 38 reflects the amount of
liquid B remaining in the holding chamber so that it can be
replenished before it runs out. When the run is over, trap 38 is
drained through valve 39.
It is convenient to have control means 37 activated by the same
signal that activates valve 17. FIG. 5 illustrates a simple way in
which this can be done. Liquid leaving detector 22 is passed
through a flexible tube 53 which is attached to rod 54, which in
turn forms part of a solenoid system 55. Two traps are provided.
One, 52, leads directly to drain 23; the other, 50, leads to the
drain 23 through valve 39 which is usually closed. Trap 50 can be
provided with a sight glass 51. When valve 17 is activated by an
electrical signal, solenoid 55 is also activated, and flexible tube
53 is positioned over trap 50. When valve 17 is closed, solenoid 55
returns flexible tube 53 to its position over drain 23. This is a
convenient way to collect an amount of liquid equal to the volume
passing through valve 17, but any convenient way known to those
skilled in the art can be used.
One advantage to all three of the systems described above is that
when a high pressure elution system is being used, there is always
the problem of high pressure liquid leaks. Leaks from the inside to
the outside of the valves are not difficult to stop, but leaks
through the valve are difficult to stop and require expensive
valving. In the case of the constant pressure system described
above, the pressure on both sides of valve 16 and 17 is
substantially the same, so there is no pressure differential to
cause a leak.
One of the greatest advantages of the present system is its
versatility. The initial and final concentration of the eluent can
be set with ease, and the rate of change of this concentration can
also be set with ease. FIG. 6 illustrates a programming sequence
that can be used to change the concentration in the eluent. This
system uses two electronic signals; one signal, signal 60, is a
periodic saw tooth wave, the other signal, signal 62, is a gating
signal, which in the case illustrated is a linear ramp. The
programming means, being an electronic signal generator, generates
the two signals simultaneously, and is adapted to activate one
valve, when the instantaneous value of of the saw tooth wave is
greater than the instantaneous value of the gating function, and to
actuate the other valve when the instantaneous value of the saw
tooth wave is less than the instantaneous value of the gating
function. If valve 16 is the first valve, the shaded region of the
bar below the graph indicates the time interval that liquid A is
being supplied to the mixing chamber, and the unshaded regions
indicate the time intervals where liquid B is being supplied. The
relative periods will depend on the types of functions generated.
Utilizing the versatility of electronic function generating, almost
any programming sequence can be envisioned. The one illustrated is
a useful programming sequence. A saw tooth wave 60 combined with
any monotonically increasing gating signal of arbitrary function,
such as signal 63 is another useful sequence. A linear gating
signal 62, combined with any periodic signal of arbitrary function
would also be useful.
The above discussion has been for the purpose of illustrating the
usefulness and operation of our invention. It is claimed that more
than two liquids can be handled in this manner, and that there are
numerous modifications of the system disclosed that would occur to
those skilled in the art. The above discussion is, therefore, not
meant to limit the scope of our invention which is set forth in the
following claims.
* * * * *