U.S. patent application number 13/846420 was filed with the patent office on 2013-08-22 for high pressure degas assembly for chromatography system and method.
The applicant listed for this patent is Milton LIU, Yan LIU, Zongqing LU, Michael J. MCADAMS, Khosro MOSHFEGH, Christopher A. POHL, Hamish SMALL. Invention is credited to Milton LIU, Yan LIU, Zongqing LU, Michael J. MCADAMS, Khosro MOSHFEGH, Christopher A. POHL, Hamish SMALL.
Application Number | 20130213225 13/846420 |
Document ID | / |
Family ID | 45021202 |
Filed Date | 2013-08-22 |
United States Patent
Application |
20130213225 |
Kind Code |
A1 |
LIU; Yan ; et al. |
August 22, 2013 |
HIGH PRESSURE DEGAS ASSEMBLY FOR CHROMATOGRAPHY SYSTEM AND
METHOD
Abstract
A degas assembly including a low pressure fluid channel for
carrying a wash fluid at a first pressure, a pressurized channel
for carrying eluent including a gas at a second pressure higher
than the first pressure, and a degas separator defining a fluid
barrier between the low pressure fluid channel and pressurized
fluid channel, the separator configured to retain liquid in the
pressurized fluid channel and allow gas to flow through the
separator to the low pressure fluid channel. The pressurized fluid
channel may extend along an outer periphery of the low pressure
fluid channel. The eluent may be received from an eluent generator
at a pressure of at least about 3300 psi, and in various
embodiments up to about 5000 psi. A liquid chromatography system
and method are also disclosed.
Inventors: |
LIU; Yan; (Palo Alto,
CA) ; POHL; Christopher A.; (Union City, CA) ;
MCADAMS; Michael J.; (Los Gatos, CA) ; SMALL;
Hamish; (Ashland, OR) ; LU; Zongqing;
(Fremont, CA) ; LIU; Milton; (Burlingame, CA)
; MOSHFEGH; Khosro; (Fremont, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LIU; Yan
POHL; Christopher A.
MCADAMS; Michael J.
SMALL; Hamish
LU; Zongqing
LIU; Milton
MOSHFEGH; Khosro |
Palo Alto
Union City
Los Gatos
Ashland
Fremont
Burlingame
Fremont |
CA
CA
CA
OR
CA
CA
CA |
US
US
US
US
US
US
US |
|
|
Family ID: |
45021202 |
Appl. No.: |
13/846420 |
Filed: |
March 18, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12791732 |
Jun 1, 2010 |
8414684 |
|
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13846420 |
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Current U.S.
Class: |
95/46 ; 96/6 |
Current CPC
Class: |
B01D 19/0073 20130101;
G01N 30/96 20130101; B01D 19/0031 20130101; B01D 19/0036 20130101;
G01N 30/32 20130101; G01N 30/32 20130101; G01N 2030/146
20130101 |
Class at
Publication: |
95/46 ; 96/6 |
International
Class: |
B01D 19/00 20060101
B01D019/00 |
Claims
1. A method of separating a gas from a liquid phase for liquid
chromatography, the method comprising: flowing a high pressure
liquid phase to a degas assembly, the degas assembly including an
inner flow channel and an outer flow channel with a degas separator
disposed therebetween, said degas separator being a gas permeable
membrane; flowing the liquid phase to the outer flow channel; and
separating the gas from the liquid phase into the inner flow
channel but retaining liquid from the liquid phase in the outer
flow channel via the degas separator; wherein the high pressure
liquid phase has a pressure of at least about 1000 psi.
2. The method according to claim 1, further comprising directing
the separated liquid phase from an outlet of the outer flow channel
to a liquid chromatography column.
3. The method according to claim 1, further comprising flowing
regenerant through the inner flow channel thereby flushing the
separated gas.
4. The method according to claim 1, wherein the liquid phase is an
eluent, the eluent being received from an eluent generator.
5. The method according to claim 1, wherein the liquid phase
inputted into the degas assembly is at a pressure of at least about
3300 psi.
6. The method according to claim 1, wherein the liquid phase
inputted into the degas assembly is at a pressure of between about
3300 psi and about 5000 psi.
7. The method according to claim 1, wherein the degas separator is
an amorphous fluoropolymers tube.
8. The method according to claim 1, the outer flow channel at least
partially enveloping the inner flow channel, wherein the flowing
liquid phase to the outer flow channel is performed so as to place
the inner flow channel in compression.
9. The method according to claim 1, in which a low pressure channel
member defining the low pressure fluid channel and a pressurized
channel member defining the pressurized fluid channel, each of the
channel members comprising inert polymer tubing, wherein the low
pressure channel member includes at least one flared end, and the
at least one flared end is configured to form a fluid seal when
engaged with a fluid inlet or outlet port.
10. The method according to claim 9, wherein the at least one
flared end is thermally formed.
11. The method according to claim 1, wherein the low pressure fluid
channel is capped at one end, the method further comprising
removing the separated gas from the inner flow channel.
12. A degas assembly comprising: a low pressure fluid channel for
carrying a wash fluid at a first pressure; a pressurized channel
for carrying eluent including a gas at a second pressure higher
than the first pressure; a degas separator defining a fluid barrier
between the low pressure fluid channel and pressurized fluid
channel, the separator configured to retain liquid in the
pressurized fluid channel and allow the gas to flow through the
separator to the low pressure fluid channel; wherein the
pressurized fluid channel extends along an outer periphery of the
low pressure fluid channel, wherein a low pressure channel member
defines the low pressure fluid channel and a pressurized channel
member defines the pressurized fluid channel, each of the channel
members comprising inert polymer tubing, wherein the low pressure
fluid member is capped at one end.
13. The degas assembly according to claim 12, wherein the low
pressure fluid channel member comprises amorphous fluoropolymer
tubing.
14. The degas assembly according to claim 12, further comprising:
an inlet housing including: an eluent inlet for connecting to an
eluent generator; and a wash inlet for connecting to a wash source;
and an outlet housing including: an eluent outlet; and a wash
outlet.
Description
FIELD OF THE INVENTION
[0001] This invention relates, in general, to a system for
high-performance ion or liquid chromatography and in various
aspects an apparatus and method for removal of gas from an
eluent.
BACKGROUND OF THE INVENTION
[0002] Ion chromatography is a common technique for analysis of
sample materials. Conventional ion chromatography typically
includes a chromatographic separation stage using an eluent
containing an electrolyte and an eluent suppression stage followed
by detection. In the chromatographic separation stage, analyte ions
of interest in an injected sample are eluted through a separation
column using an electrolyte as the eluent. In the suppression
stage, electrical conductivity of the electrolyte is suppressed
while not affecting the separated ions so that the ions may be
determined by a conductivity cell. This technique is described in
detail in U.S. Pat. Nos. 3,897,213; 3,920,397; 3,925,019 and
3,926,559.
[0003] Dilute solutions of acids, bases, or salts are commonly used
as chromatographic eluents. Traditionally, these eluents are
prepared off-line by dilution with reagent-grade chemicals.
Off-line preparation of chromatographic eluents can be tedious and
prone to operator errors, and often introduces contaminants. For
example, dilute sodium hydroxide (NaOH) solutions, widely used as
eluents in the ion chromatographic separation of anions, are easily
contaminated by carbonate. The preparation of carbonate-free NaOH
eluents is difficult because carbonate can be introduced as an
impurity from the reagents or by adsorption of carbon dioxide from
air. The presence of carbonate in NaOH eluents can compromise the
performance of an ion chromatographic method, and can cause an
undesirable chromatographic baseline drift during a hydroxide
gradient and even irreproducible retention times of target
analytes. In recent years, several approaches that utilize the
electrolysis of water and charge-selective electromigration of ions
through ion-exchange media have been investigated by researchers to
purify or generate high-purity ion chromatographic eluents. U.S.
Pat. Nos. 6,225,129, 6,682,701, and 6,955,922 describe electrolytic
devices that can be used to generate high purity acid and base
solutions by using water as the carrier. Using these devices, high
purity, contaminant-free acid or base solutions are automatically
generated on-line for use as eluents in chromatographic
separations. These devices simplify gradient separations that can
now be performed using electrical current gradients, with minimal
delay, instead of using a conventional mechanical gradient
pump.
[0004] With conventional electrolytic eluent generators, however,
gases can be introduced into the eluent during the electrolytic
reaction or at other stages in the analysis process. For example,
in a large capacity potassium hydroxide (KOH) generator,
electrolysis reactions produce hydrogen and oxygen gases. When used
in a chromatography system, the hydrogen gas, along with the KOH
solution, is carried forward into the chromatographic flow path. If
hydrogen gas is produced in a significant volume relative to the
liquid flow, its presence can be detrimental to the detection
process and other downstream chromatography processes.
[0005] One solution to the problem of a presence of gas in the
eluent is disclosed by U.S. Pat. No. 6,225,129 to Liu et al. ("Liu
patent"). The Liu patent discloses a method for addressing the
potential problem of hydrogen gas by application of Boyle's law. A
flow restrictor is placed after the detector flow cell to create
backpressure and elevate the pressure of the entire chromatography
system. Under elevated pressure (e.g., 1000 psi or higher),
hydrogen gas is compressed to an insignificant volume compared to
the eluent flow so that it does not interfere with the downstream
chromatography process. But this approach has several drawbacks.
Because of the elevated pressures, the detector flow cell must be
capable of withstanding a pressure of 1000 psi or more. In the case
of ion chromatography system using suppressed conductivity
detection, the suppressor must also be capable of withstanding an
elevated high pressure. Therefore, this approach limits the type of
components that can be used in an ion chromatography system
employing an electrolytic eluent generator.
[0006] Another approach involves using an on-line gas removal
device to remove hydrogen gas from the KOH solution. One way to
remove the gas from an effluent is to pass the effluent through a
gas removal device having a gas diffusion membrane prior to
reaching the detection cell. An exemplar of a gas removal device
used with a chromatography system is disclosed in U.S. Pat. No.
5,045,204 to Dasgupta et al. ("Dasgupta patent").
[0007] The Dasgupta system includes a device for removal of gas
(e.g. hydrogen) generated in the electrolytic cell from the product
stream (e.g. sodium hydroxide). In one embodiment, the gas removal
device is a gas diffusion cell including a plurality of blocks and
a gas diffusion membrane separating a degassed product channel from
a gas carrier channel. In another embodiment, gas-containing
product is directed into a porous hydrophobic tube that is
configured for the product to flow downwardly and then upwardly out
of an exit port. The tube is formed of hydrophobic materials (e.g.
as porous polytetraofluoroethylene (PTFE), (expanded) PTFE,
Accurel.RTM., or Celgard.RTM.) similar to the membrane. The
hydrogen gas flows outwardly through the tube to a gas vent. As the
KOH eluent stream passes through the tube under pressure, hydrogen
gas diffuses through the tube and is carried to waste. In this
manner gas is effectively removed from the eluent before it reaches
the sample injector of the chromatography system so that the
downstream chromatographic process is not affected. One advantage
of this system is that a conventional detector flow cell and ion
chromatography suppressor can be used.
[0008] The Liu patent discloses a similar gas removal device for
on-line removal of gas from the eluent solution. The gas removal
device includes a gas-permeable tubing coaxially aligned within a
protective tubing. The gas-permeable tubing functions like a
membrane. In operation, the KOH solution containing hydrogen gas is
pumped through the gas permeable tubing and the hydrogen gas
escapes through the tubing. A stream of aqueous solution flowing in
an annular space between the outside of the gas permeable tubing
and the protective tubing carries away the released gas.
[0009] One problem with such conventional gas removal devices is
that current gas diffusion materials can not withstand pressures
found in modern systems. Ion chromatography systems, in particular
high-performance liquid chromatography (HPLC) systems, experience
high in-line pressures. Conventional membrane materials have low
burst pressures by comparison. By example, typical systems can rise
above 1000 psi, and modern pumps can generate pressures in excess
of 3000 psi and even 5000 psi. Such pressure levels are above the
burst pressure of porous and gas-permeable tubing used for
conventional gas removal devices such as those of the Dasgupta and
Liu patents. Further, the low pressure threshold of such
conventional devices limits the capabilities of the overall system.
For example, systems making use of such gas removal devices are
limited to about 3000 psi in the separation column. High pressure
is desirable for greater efficiency and performance.
[0010] One solution to this has been to position the electrolytic
eluent generator and the gas removal device on the low pressure
side of the system, meaning in the pump intake line, or external
(off-line) to the system. However, these positioning solutions
limit the effectiveness of the devices and add to the volume of the
electrolytic eluent generation system, thus compromising the
overall performance of the ion chromatography system.
[0011] Thus, there is need to develop a degasser device that can be
used in conjunction with an electrolytic eluent generator in ion
chromatography and liquid chromatography systems over a wider range
of operational pressures. There is a continuing need for
chromatography systems with increased efficiency and
performance.
[0012] In light of the foregoing, it would be beneficial to have
methods and apparatuses which overcome the above and other
disadvantages of known gas removal devices and chromatography
systems.
BRIEF SUMMARY OF THE INVENTION
[0013] In summary, one aspect of the present invention is directed
to a degas assembly including a low pressure fluid channel for
carrying a wash fluid at a first pressure, a pressurized channel
for carrying eluent including a gas at a second pressure higher
than the first pressure, and a degas separator defining a fluid
barrier between the low pressure fluid channel and pressurized
fluid channel, the separator configured to retain liquid in the
pressurized fluid channel and allow gas to flow through the
separator to the low pressure fluid channel.
[0014] In various embodiments, the pressurized fluid channel
extends along an outer periphery of the low pressure fluid channel.
In various embodiments, the second pressure is at least about 3000
psi. In various embodiments, the second pressure is at least about
3300 psi. In various embodiments, the second pressure is between
about 3000 psi and about 5000 psi.
[0015] In various embodiments, the degas assembly includes a
central lumen extending within an outer tubing. The central lumen
forms the low pressure fluid channel, an annular space between the
central lumen and the tubing defines the pressurized channel, and a
wall of the central lumen defines the degas separator. In various
embodiments, the pressurized fluid channel extends along
substantially the entire outer periphery and substantially the
entire length of the low pressure fluid channel.
[0016] In various embodiments, the eluent is received from an
eluent generator and has a pressure of at least about 3300 psi. In
various embodiments, the eluent is received from an eluent
generator and has a pressure between about 3300 psi and about 5000
psi.
[0017] In various embodiments, the degas assembly further includes
a low pressure channel member defining the low pressure fluid
channel and a pressurized channel member defining the pressurized
fluid channel, each of the channel members comprising inert polymer
tubing. The low pressure fluid channel member may include amorphous
fluoropolymer tubing. The pressurized fluid channel member may
include reinforced polyetheretherketone (PEEK) tubing.
[0018] In various embodiments, the degas assembly further includes
an inlet housing and outlet housing. The inlet housing may include
an eluent inlet for connecting to an eluent generator and a wash
inlet for connecting to a wash source. The outlet housing may
include an eluent outlet and a wash outlet.
[0019] In various embodiments, the low pressure fluid channel is
fluidicly connected to the wash inlet and the wash outlet, and the
pressurized fluid channel is fluidicly connected to the eluent
inlet and the eluent outlet. In various embodiments, the low
pressure fluid channel and pressurized fluid channel are
substantially coaxial flexible tubes. At least one of the low
pressure and pressurized tubes may include a splined portion at an
outlet end and clamped within the outlet housing. The splined
portion may be configured to allow fluid to flow through the
splines when the splined portion is clamped. In various
embodiments, the splined portion extends along substantially the
entire length of the pressurized tube. In various embodiments, the
outer tubing has a non-circular cross-section. In various
embodiments, an outer surface of the central lumen is
non-cylindrical.
[0020] Various aspects of the inventions are directed to a liquid
chromatography system including a degas assembly and a pressurized
liquid chromatography column. In various embodiments, eluent from
the outlet housing flows to the column.
[0021] In various embodiments, the system further includes a pump,
and the degas assembly is positioned downstream from the pump. In
various embodiments, the system further includes a pump, and the
degas assembly is positioned upstream from an inlet of the
pump.
[0022] Various aspects of the inventions are directed to a method
of separating gas from an eluent for liquid chromatography. The
method includes flowing high pressure eluent to a degas assembly,
the degas assembly including an inner flow channel and outer flow
channel with a degas separator disposed therebetween, said degas
separator being a permeable membrane; flowing the eluent to the
outer flow channel; and separating gas from the eluent into the
inner flow channel but retaining liquid from the eluent in the
outer flow channel via the degas separator.
[0023] In various embodiments, the eluent includes gas resulting
from electrolysis.
[0024] In various embodiments, the method further includes
directing the separated eluent from an outlet of the outer flow
channel to a liquid chromatography column.
[0025] In various embodiments, the method further includes flowing
regenerant through the inner flow channel thereby flushing the
separated gas. In various embodiments, the eluent is received from
an eluent generator. In various embodiments, eluent from the eluent
generator is at a pressure of at least about 3300 psi. In various
embodiments, eluent from the eluent generator is at a pressure of
between about 3300 psi and about 5000 psi.
[0026] In various embodiments, the degas separator is a polymer
tube.
[0027] In various embodiments, the outer flow channel at least
partially envelops the inner flow channel, and the flowing eluent
to the outer flow channel is performed so as to place the inner
flow channel in compression.
[0028] The apparatus and method of the present invention(s) have
other features and advantages which will be apparent from or are
set forth in more detail in the accompanying drawings, which are
incorporated in and form a part of this specification, and the
following Detailed Description of the Invention, which together
serve to explain the principles of the present invention(s).
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic representation of an exemplary system
and gas removal apparatus in accordance with the present
invention.
[0030] FIG. 2 is a cross-sectional view of an exemplary gas removal
assembly used with the system of FIG. 1.
[0031] FIG. 3 is an enlarged view of the inlet end of the gas
removal assembly of FIG. 2.
[0032] FIG. 4 is an enlarged view of the outlet end of the gas
removal assembly of FIG. 2.
[0033] FIG. 5 is an enlarged view of a portion of the degas
assembly inlet of FIG. 4.
[0034] FIG. 6 is an enlarged perspective view of an end of the
outer protective jacket of the assembly of FIG. 2, illustrating a
splined pattern on an inner surface.
[0035] FIG. 7 is a front view of the end of the outer protective
jacket.
[0036] FIG. 8 is a cross-sectional view taken through the line 8-8
of FIG. 7.
[0037] FIG. 9 is an exploded view of one of half of the makeup
assembly of the gas removal assembly of FIG. 2.
[0038] FIG. 10 is a cross-sectional view of tubing for a gas
removal apparatus similar to that of FIG. 1, illustrating an inner
channel member having a non-circular periphery.
[0039] FIG. 11 is a schematic of tubing for a gas removal apparatus
similar to that of FIG. 1, illustrating the low pressure flow and
pressurized flow in opposite directions.
[0040] FIG. 12 is a schematic of tubing for a gas removal apparatus
similar to that of FIG. 1, illustrating a vacuum line in place of a
low pressure channel carrying an aqueous solution and a sleeve in
accordance with the invention.
[0041] FIG. 13 is an assembly view of a gas removal apparatus
similar to that of FIG. 1, illustrating alternative housings at
each end for attachment to the eluent and wash fluid lines.
[0042] FIG. 14 is a cross-sectional view of one of the housings of
FIG. 13 taken through the line 14-14.
[0043] FIG. 15 illustrates the results of separation of eight
common anions using the system of FIG. 1.
[0044] FIG. 16 illustrates the results of separation of six common
cations using the system of FIG. 1 with the carbonate removal
device removed.
[0045] FIG. 17A shows the conductance of KOH eluents using the
system of FIG. 1 with a conventional gas removal device. FIG. 17B
shows the conductance of KOH eluents using the system of FIG. 1
with a gas removal device in accordance with the present invention.
The gas removal flowpath of the system of FIG. 17A is about 60
inches in length, and the gas removal flowpath of the system of
FIG. 17B is about 15 inches in length.
[0046] FIG. 18 shows the conductance of KOH eluents using the
system of FIG. 1 without any gas removal device. FIG. 18 shows the
poor conductivity profile of the KOH eluents when there is
insufficient removal of hydrogen gas in conjunction with a KOH
electrolytic eluent generator.
DETAILED DESCRIPTION OF THE INVENTION
[0047] Reference will now be made in detail to the various
embodiments of the invention, examples of which are illustrated in
the accompanying drawings. While the invention will be described in
conjunction with the various embodiments, it will be understood
that they are not intended to limit the invention to those
embodiments. On the contrary, the invention is intended to cover
alternatives, modifications and equivalents, which may be included
within the spirit and scope of the invention as defined by the
appended claims.
[0048] Various aspects of the present invention are similar to the
devices and systems described in U.S. Pat. Nos. 7,390,386 B2;
6,682,701 B1; 6,225,129 B1; 5,569,365; and 5,045,204 and U.S.
Patent Application No. 2003/0127392 A1, the entire contents of
which are incorporated herein for all purposes by this
reference.
[0049] The gas removal assembly and method of the present invention
will first be broadly described in combination with an ion
chromatographic system. For the analysis of anions on a
chromatographic system generally, the eluent is an electrolyte,
typically a cation hydroxide such as sodium hydroxide (NaOH).
Conversely, for the analysis of cations, the eluent typically is an
acid such as methanesulfonic acid (MSA). The gas removal device and
method of the present invention, however, are also applicable to
liquid chromatography forms other than ion chromatography. The
present inventions are also applicable to other gas removal and
separation applications including, but not limited to, industrial
applications involving gassified fluid streams. An example of use
of a conventional gas separation device for gas clean-up and
purification of a liquid stream is disclosed by U.S. Pat. No.
6,350,297, which is incorporated herein for all purposes by this
reference.
[0050] By "gassified" it is meant that the liquid stream includes a
gas component, whether resulting from a prior process or occurring
naturally. "Degassed" refers to the resulting product after removal
of the gas in accordance with the invention. "Degassed", "degas",
and "degasser" are used interchangeably in various respects to
refer to the device and resulting product using the device. In
various respects, "separated solution" refers to degassed sample
solution.
[0051] FIG. 1 represents an ion or liquid chromatography system 30
in accordance with the present invention. The system feeds a
potassium hydroxide (KOH) eluent to a column for analysis of a
sample. A stream of deionized water from an eluent source or
reservoir 32 is drawn by one or more pumps 33. The pump delivers
the water stream to an eluent generator 35, which includes power
source 37. An eluent purifier may also be paired with the eluent
generator. The exemplary KOH eluent generator includes a high
pressure generation chamber containing a platinum (Pt) cathode and
a low pressure electrolyte reservoir containing a Pt anode. Under
the applied electrical field, the potassium ions migrate across an
ion exchange connector to combine with hydroxide ions generated at
the cathode to form the KOH eluent. The concentration of KOH
solution formed is proportional to the applied current and
inversely proportional to the flow rate of the deionized water
carrier stream.
[0052] In the exemplary system, the eluent flowing from the
generator includes a gas component (e.g. hydrogen) as a result of
the electrolytic reaction. From the generator, the eluent flows
through a degas assembly, generally designated 39, for removal of
the gas. The exemplary degas assembly is provided on-line in the
system and downstream from the pump, the outlet of the large
capacity KOH generator, and an optional continuously-regenerated
trap column (CR-TC) 40. The degassed sample is fed to a sample
injector 42 for injection into a separation column 44, such as a
chromatography column. Thus, hydrogen gas is effectively removed
from the KOH eluent before it reaches the sample injector of the
chromatography system so that the downstream chromatographic
process is not affected.
[0053] The exemplary system includes chromatographic separation
means in the form of chromatographic column 44 which is packed with
a chromatographic separation medium.
[0054] In one embodiment, the separation medium is an ion-exchange
resin. In one embodiment, the separation medium is a porous
hydrophobic chromatographic resin with essentially no permanently
attached ion-exchange sites. The resin system may be used for
mobile phase ion chromatography (MPIC) as described in U.S. Pat.
No. 4,265,634.
[0055] The eluate from column 44 is fed to a self-regenerating
suppressor 46 similar to those described in U.S. Pat. No. 5,352,360
and of the type sold by Dionex Corporation of Sunnyvale, Calif.
under the SRS.RTM. name. The suppressor serves to suppress the
conductivity of the electrolyte of the eluent from the column but
not the conductivity of the separated ions. The suppressor
generally converts the electrolyte of the eluent to a weakly
conducting form. The suppression process usually enhances the
conductivity of the separated ions.
[0056] With continued reference to FIG. 1, the effluent from
suppressor 46 is directed to a detector 47, such as a flow-through
conductivity cell, for detecting the resolved ionic species. In the
detector, the presence of ionic species produces an electrical
signal proportional to the amount of ionic material. The output
signal is typically directed from the detector to a conductivity
meter thereby permitting detection of the concentration of
separated ionic species.
[0057] Recycled aqueous liquid from detector 47 may be utilized as
a regenerant solution. In various embodiments, the regenerant from
the suppressor is used as a wash fluid to carry separated gas from
degas assembly 39 to a waste receptacle 49. The regenerant solution
from exemplary detector 47 flows in a fluid line, generally
designated 51, and serves as the solution to carry away the removed
gas from the degas assembly so that the system can be operated
continuously. The recycled liquid may be directed to the degas
assembly directly or via other components. An optional carbonate
removal device (CRD) 53 is positioned between the suppressor and
detector.
[0058] Turning now to FIGS. 2-4, the degas assembly and method of
removing gas in accordance with the present invention will now be
described in more detail.
[0059] Various aspects of the gas removal device are similar to the
device described in U.S. Pat. No. 5,045,204 to Dasgupta et al., the
entire contents of which are incorporated herein for all purposes
by this reference.
[0060] Degas assembly 39 includes an inlet end 54 and outlet end 56
connected by fluid channels. In various embodiments, the degas
assembly includes a low pressure fluid channel 58 for carrying a
wash fluid 60 at a first pressure and a pressurized or high
pressure channel 61 for carrying an eluent, or any gas-containing
product, at a second pressure. The second pressure is higher than
the first pressure.
[0061] "High pressure" and "low pressure" are to be understood as
used in the analytical, chemical, and mechanical arts and are
generally used with reference to each other under operational
conditions.
[0062] "High pressure" generally refers to an elevated pressure. In
various respects, "high pressure" refers to a pressure downstream
from the pump and above the "low pressure" or local system
pressure. In various respects, "high pressure" refers to a pressure
above about 1000 psi, in some embodiments above about 3000 psi, in
some embodiments above 3300 psi, in some embodiments between about
3300 psi and about 5000 psi, and in some embodiments above about
5000 psi.
[0063] In some respects, "high pressure" and "pressurized", "high
pressure channel" and "pressurized channel", and "high pressure
channel member" and "pressurized channel member" are used
essentially interchangeably.
[0064] "Low pressure" refers to an unaltered local pressure or
pressure below the "high pressure." In various respects, "low
pressure" refers to a decreased pressure level obtained by a
process or device of the invention.
[0065] "Wash fluid" refers generally to a fluid or other means for
washing or evacuating the removed gas species from the channel. In
various respects, "wash fluid" refers to a gas or void such as a
vacuum. "Liquid" and "aqueous" are used essentially
interchangeably. In various aspects, the wash fluid is essentially
100% organic solvents. In various aspects, the wash fluid is a
mixture of water and organic solvents.
[0066] "Channel" is to be understood as generally used in the
chemical and mechanical arts and generally refers to any of a
variety of passageways for carrying a liquid or gas and includes,
but is not limited to, grooves, conduits, and tubes. A "channel"
may be in an open (e.g. groove) or closed (e.g. conduit) form.
[0067] A degas separator 63 is positioned between low pressure
fluid channel 58 and pressurized fluid channel 61. The degas
separator defines a fluid barrier between the low pressure fluid
channel and pressurized fluid channel. The separator 63 is
configured to retain liquid in the pressurized fluid channel while
allowing gas to flow through the separator to the low pressure
fluid channel.
[0068] In various embodiments, the pressurized fluid channel
extends along an outer periphery of the low pressure fluid channel.
"Periphery" is to be understood as used in the art and refers to
the perimeter, outer surface, or exterior of the respective
structure. The pressurized channel may extend along a portion or
all of the outer periphery in the circumferential (transverse) or
longitudinal (lengthwise) directions.
[0069] In various embodiments, pressurized channel member 65 and
low pressure channel member 67 are tubes. In various embodiments,
the low pressure channel member is an inner lumen formed within the
outer pressurized channel member such that the pressurized channel
member envelopes or surrounds, in whole or in part, the low
pressure channel member. In various embodiments, the central lumen
forms the low pressure fluid channel, an annular space between the
central lumen and the outer tubing defines the pressurized channel,
and the degas separator is the wall of the central lumen.
[0070] Pressurized channel 61 is defined by a higher pressure
channel member 65, and low pressure channel 58 is defined by a low
pressure channel member 67. The second pressure in pressurized
channel 61 is higher pressure than the first pressure in low
pressure channel 58.
[0071] The exemplary gas separator is a diffusion membrane that
allows gas to permeate therethrough but retains liquid on one side.
In various embodiments, the degas separator blocks substantial bulk
liquid flow but allows gas to permeate. In other words, the
membrane functions to permit the ready transmembrane passage of gas
in the product flow while substantially preventing the
transmembrane passage of liquid. The gas of interest generally
permeates the gas separator in a conventional manner.
[0072] In the exemplary degas assembly, the wall of the low
pressure fluid channel member is the degas separator (e.g.
membrane). Accordingly, the material of the low pressure channel
member affects the function of the degas assembly.
[0073] In various embodiments, the low pressure channel member 67
is formed of a chemically inert material such as an inert polymer.
In various embodiments, the inner surfaces of the annular space
between the pressurized channel member and low pressure channel
member are chemically inert. The exemplary inner low pressure
channel member is a PTFE tube. Suitable materials for the degas
separator and/or low pressure channel member include, but are not
limited to, polymers including polymethylpentene and polypropylene,
and fluoropolymers such as polytetrafluoroethylene (PTFE), ethylene
tetrafluoroethylene (ETFE), expanded-PTFE (ePTFE), perfluoroalkoxy
(PFA), and fluorinated ethylene propylene (FEP). In various
embodiments, the degas separator is formed of a material that is
permeable by a gas under high pressure. In various embodiments, the
degas separator is a gas-permeable polymer. In various embodiments,
the degas separator is an amorphous fluoropolymer tube. The
exemplary degas separator is a gas-permeable tube fabricated from
Teflon.RTM. AF2400 amorphous polymer sold by DuPont. One will also
appreciate that the low pressure channel member and/or degas
separator may be treated with an additive or modified, such as by
conjugation with a molecule. In various embodiments, the degas
separator is a coating of an amorphous fluoropolymer on the
exterior surface of a porous substrate tube such as
Celgard.RTM..
[0074] In various aspects of the invention, the degas separator is
formed of a material selected to have one of a high gas
permeability, high compressibility, low thermal conductivity, high
creep resistance, and a combination thereof. The degas separator
can also be used in conjunction with other features to enhance or
promote separation based on ionic, chemical, and electrostatic
forces. In various embodiments, the degas separator includes ion
exchange sites.
[0075] The level of purity desired may depend on the application.
The amount of gas separation generally depends on the diffusion
properties of the gas separator, time, pressure, and diffusion
area. One of skill will understand from the description herein the
manner for configuring the gas separator to remove the gas and
obtain the desired purity of the resulting gas-free liquid product.
For example, the separation process may be configured as a
multi-step process using a plurality of separation devices or by
feeding product from outlet back into the degas assembly for
further separation. In various embodiments, the degas assembly
separates between about 50% and about 100% of the gas that enters
the inlet end. In various embodiments, the degas assembly separates
at least about 50% of the gas that enters the inlet end. In various
embodiments, the degas assembly separates at least about 60% of the
gas that enters the inlet end. In various embodiments, the degas
assembly separates at least about 70% of the gas that enters the
inlet end. In various embodiments, the degas assembly separates at
least about 80% of the gas that enters the inlet end. In various
embodiments, the degas assembly separates at least about 90% of the
gas that enters the inlet end.
[0076] In various embodiments, the pressurized channel member 65 is
formed of a chemically inert material, preferably an inert polymer.
The exemplary pressurized channel member is a reinforced
polyetheretherketone (PEEK) tube.
[0077] Suitable materials for the pressurized channel member
include, but are not limited to, polymers such as fluoropolymers,
stainless steel, and elastomers. In various embodiments, the
pressurized channel member is formed of a high tensile polymeric
material such as PEEK. One will appreciate, however, that the
selection of materials and configuration will likely depend on the
application and materials to be separated.
[0078] The exemplary degas assembly includes an inlet housing 68
and outlet housing 70. In the exemplary assembly, polymeric
interface tees are attached to each end of the assembly to achieve
fluid sealing of the high pressure channel. The two interface tees
function to direct the stream of KOH solution containing hydrogen
gas from the eluent generator into the exemplary high pressure
channel between the outer tubing wall of the inner low pressure
tubing and the inner wall of the high pressure outer tubing.
[0079] The inlet housing includes an eluent inlet 72 for connecting
to eluent generator 35 and a wash inlet 74 for connecting to a wash
source. In the exemplary system, recycled fluid serves as the wash
fluid and detector 47 serves as the wash source. Outlet housing 70
includes an eluent outlet 75 and a wash outlet 77.
[0080] Low pressure fluid channel 58 is fluidicly connected to wash
inlet 74 at one end and wash outlet 77 at an opposite end.
Pressurized fluid channel 61 is fluidicly connected to eluent inlet
72 at one end and eluent outlet 75 at an opposite end. Thus, the
pressurized eluent flow is directed through the pressurized channel
and the low pressure wash that flushes to waste is in the low
pressure channel. Eluent from the outlet housing flows to column
44. The wash fluid and the eluent flow may flow concurrently or in
opposite directions.
[0081] As used herein, "end" is to be understood broadly and refers
to a region, portion, or side adjacent the end point. "Opposite
end" generally refers to a different region or portion on an
opposite side. In various respects, "end" and "opposite end" refer
to points upstream and downstream from each other as would be
understood by one of skill in the art from the description
herein.
[0082] In exemplary degas assembly 39, low pressure fluid channel
58 and pressurized fluid channel 61 are substantially coaxial
tubes. The low pressure fluid channel member is a tube centrally
located in the outer pressurized channel member. The exemplary
pressurized channel member serves as a sheath or protective jacket
for the fluid flow therethrough and may include a protective outer
covering. As will be appreciated from the description herein, the
exemplary degas assembly thus is configured for a low pressure,
flow-through low pressure inner channel member and a higher
pressure, flow-through space between the inner low pressure channel
member and outer high pressure channel member.
[0083] The exemplary high pressure channel member has sufficient
wall thickness to withstand a wide range of pressure. In various
embodiments, the wall thickness is sufficient to withstand pressure
between 0 psi and about 5000 psi or higher. The gas-containing
eluent stream is exposed to the outer surface of the low pressure
channel tubing under pressure.
[0084] In view of the pressure differential between the high and
low pressure channels, a compressive force is exerted on the inner,
low pressure channel member. The degas assembly is able to
withstand high pressure in the pressurized channel for multiple
reasons. First, the degas separator (e.g. the wall of the inner
lumen) defining the low pressure channel is placed in compression.
Many materials of interest, such as amorphous fluoropolymers,
exhibit greater strength in compression than tension. Accordingly,
much higher pressures can be applied before the material fails.
Second, in the exemplary assembly, the inward pressure exerted by
the high pressure flow on the inner channel member is
counterbalanced by an opposing pressure inside the inner low
pressure channel. In the exemplary assembly which includes coaxial
flexible tubes, the fluid in the low pressure channel is compressed
and provides a countervailing force. In various embodiments, the
wash fluid is selected, or the wash fluid channel is pressurized,
to achieve a desired pressure in the wash fluid channel thereby
increasing the maximum pressure achievable in the high pressure
channel.
[0085] The method of making the degas assembly in accordance with
the present invention will now be described. The low and
pressurized channel members are formed by conventional techniques
such as extrusion. The channel members may be standard polymeric
tubing supplied by a vendor. The channel flow portion of the degas
assembly is formed by threading the low pressure tube through the
pressurized tube. In the exemplary assembly, the channel members
are loose and not fastened to each other except in the inlet and
outlet housings as described.
[0086] Referring to FIGS. 4-5, exemplary housings 68 and 70 are
configured as interface tees to attach to each end of the channel
members and form a tight seal to ensure gas and liquid do not leak.
The housings include inlets and outlets to port the flow path as
described herein.
[0087] The pressurized channel member and low pressure channel
members are fastened in place by attachment at each end to the
inlet and outlet housings using ferrules 79. The ferrules clamp
down on the channel member ends to secure them in place. In the
exemplary system, the channel members are made of flexible polymer
that deforms in response to the clamping force of the ferrules.
[0088] As shown in FIGS. 6-8, exemplary degas assembly 39 includes
a pressurized channel member (outer tubing) with a splined portion
81 including splines 82 (shown in FIG. 6). The splines are
relatively rigid such that they are not crushed when the end of the
high and low pressure channel members are clamped to the outlet
housing as shown in FIG. 4. Instead, the splined portion
essentially maintains its shape so fluid can pass through the
splines. The splined portion has a length of L.sub.S, and the
splines have a depth of D.sub.S (shown in FIGS. 7 and 8).
[0089] In various embodiments, the splined portion is positioned at
one end of the channel member and extends inwardly from the end
along a portion of channel member. The length of the spline feature
may be at least about 0.01 inch, at least about 0.375 inch, or
longer. The spline feature may extend from each end of the tubing.
One will appreciate that the splined portion may also extend along
essentially the entire length.
[0090] The spline feature allows the use of a ferrule fitting in
each to seal the high pressure inside the high pressure tubing
without sealing off the annular space between the outer surface of
the gas permeable inner tubing and inner surface of the high
pressure tubing where the KOH solution containing hydrogen gas
flows.
[0091] The exemplary outer pressurized tubing has a non-circular
starfish shape (best shown in FIG. 6). Alternatively, the inner low
pressure tubing may have an irregular shape as shown in FIG. 10.
The irregular fluting surface increases the diffusion surface area
without requiring greater wall thickness. One will appreciate from
the description herein that other shapes and configurations may be
employed to increase efficiency and performance of the device
accordingly. In various embodiments, the inner surface of the
tubing has a shape selected from the group consisting of square,
pentahedral, hexahedral, and octahedral. The exemplary non-circular
shape allows the use of a ferrule fitting to seal the high pressure
inside the tubing without sealing off the annular space between the
outer surface of the gas permeable tubing and inner surface of the
polymeric shield tubing where the KOH solution containing hydrogen
gas flows.
[0092] The spline feature of the high pressure outer tubing may be
prepared by using a heat stamping method with a heated
spline-forming tool made of stainless steel. An extrusion tool may
be used to produce the high pressure outer tubing with the spline
feature, which extends along the entire length of the tube.
Additionally, a special extrusion tool may be fabricated to produce
the low pressure inner tubing with a spline feature similar to that
shown in FIG. 10.
[0093] With reference back to FIGS. 3-5, the method of
manufacturing the exemplary tubing of the degas assembly will now
be described. The inlet ends of the low and pressurized channel
members are attached to the inlet housing first using conventional
techniques. The outlet ends are then attached using the following
technique.
[0094] At outlet end 56 of the degas assembly, the exemplary tubing
forming the low pressure channel member is threaded through the
wash fluid outlet port in the housing. The outer pressurized tubing
does not extend to the end of the inner low pressure tubing such
that the inner tubing can be pulled through the port past the end
of the housing. The end of the tube is pulled a sufficient distance
past the housing to allow easy access to the end. With the end
extending freely out of the back of the housing outlet, the end of
the tubing is flared to provide a backstop. Next the tubing is
pulled back towards the inlet end until the flared portion engages
the fluid outlet port to form a fluid seal.
[0095] As best seen in FIGS. 5 and 9, in the exemplary degas
assembly, each end of the gas permeable inner flow channel member
is thermally flared. The flared end tubing may be compressed
against the polymeric fitting components of the interfacing housing
so that the entire fluidic pathway can be leak-free and capable of
withstanding high pressure.
[0096] A support ring 84 is optionally provided to fix the tubing
in position and enhance the seal. The seal is sufficient to prevent
leaking under high pressure. With the end of the low pressure
tubing sealed to the housing, a gap is formed around the end of the
tubing, which corresponds to the distance past which the tube
extended during the flaring process.
[0097] The exemplary degas assembly includes a makeup assembly 86
for enveloping the low pressure tube and filling the gap left
around the low pressure tube. In the exemplary embodiment, the
makeup assembly is loosely placed around the low pressure channel
member and pinched between support ring 84 on one end and
pressurized channel member 65 on the opposite end. The makeup
assembly resides in the gap to minimize dead space in the system.
The exemplary makeup assembly is configured to provide a fluid
conduit between pressurized channel member 65 and eluent outlet 75
of outlet housing 70.
[0098] The makeup assembly is configured as a two-piece clamshell
(portion of makeup assembly best shown in FIG. 9). The two-piece
design allows the makeup assembly to be easily attached to the end
of the inner tube. The exemplary makeup assembly is about the same
length or slightly shorter than the gap formed by the take-up of
the inner tube after it is fastened by the flaring technique above.
As described above, this gap corresponds to the slack in the tube
when it is pulled through the housing.
[0099] In contrast to conventional devices and methods, the makeup
assembly of the present invention provides for easier and faster
assembly of the gas removal device of the present invention.
[0100] It is desirable to reduce voids and dead space in fluid
systems, and chromatography systems in particular. Consequently,
the makeup assembly beneficially reduces dead space that would
otherwise result around the inner tube.
[0101] In various respects, the method of using the degas assembly
and system in accordance with the present invention is similar to
that described in U.S. Pat. No. 6,225,129, which is incorporated
herein for all purposes by this reference.
[0102] With reference to FIGS. 1-2, in operation, high pressure
eluent from eluent generator 35 flows to degas assembly 39 via the
inner diameter (ID) of the system tubing and enters the first
interface housing 68. As described above, the inlet and outlet
housings have a low dead volume design. The inlet housing ports the
eluent flow to the exemplary annular gap (i.e. pressurized channel
61) between the outer diameter of low pressure inner channel member
67 (in the exemplary case, Teflon.RTM. AF2400 tubing) and the inner
diameter of pressurized channel member 65 (in the exemplary case, a
PEEK tubing jacket). The pressurized channel member wall thickness
is designed to withstand the high pressure of the eluent.
[0103] The eluent flow in the degas assembly is received from the
eluent generator at high pressure. In various embodiments, the
pressure is at least about 100 psi. In various embodiments, the
pressure is at least about 3000 psi, and in some aspects at least
about 3300 psi. In various embodiments, the pressure is at least
about 5000 psi. The maximum pressure or peak pressure in some cases
may be even higher.
[0104] With the high pressure eluent flow being on the outside of
the inner gas-permeable tubing, the gas-permeable tubing material
is in compression. In other words, the pressurized (high pressure)
side is on the outside of the gas-permeable tubing instead of the
inside. Thus, the burst pressure of the gas-permeable tubing is
generally inconsequential. As the eluent flows in the annular gap
corresponding to the pressurized channel, the gas is removed from
the eluent solution and diffuses into the inner diameter of the low
pressure channel member (e.g. the gas-permeable tubing).
[0105] With continued reference to FIGS. 1-2, the gas is removed
from the gas-containing product by diffusion through the degas
separator in a manner similar the degassing unit described in U.S.
Pat. No. 7,390,386. The gassified eluent flows through the degas
assembly where gas is separated from the liquid phase. As the
gassified eluent flows through the gas-permeable tubing, gas
diffuses through the member into the inner lumen and is removed by
the flowing aqueous liquid stream. At the outlet end, the gas and
liquid flow out of separate ports. Along the length of the
pressurized channel, therefore, the amount of gas in the eluent
solution decreases. In this way, the gas is separated and isolated
by the degas assembly. As will be understood by one of skill in the
art, the assembly can be dimensioned and configured to provide a
desired amount of separation to achieve a desired sample solution
purity.
[0106] The flowing aqueous liquid stream in the lumen also serves
to prevent adsorption of carbon dioxide from the ambient air into
the eluent stream. As described above, one source of the flowing
aqueous liquid is the detector effluent. At the end of pressurized
channel 61, the eluent flows into outlet housing 70, which mirrors
the inlet housing, and is ported back to inner low pressure channel
58 of the system for directing to injector 42. After the sample has
been injected into the separation column and thereafter detected at
detector 47, the eluent (also referred to as regenerant) is
directed back to the inlet housing 68 of the degas assembly. This
time it is ported to the inner diameter of the low pressure channel
member (e.g. 58) where the flow sweeps away gas that has diffused
through the degas separator. One will appreciate from the foregoing
that the exemplary system may generally be operated continuously.
In the degas assembly, high pressure eluent may flow in the
pressurized channel while recycled eluent flows through the low
pressure channel.
[0107] One will appreciate from the description herein that the gas
removal assembly may have other configurations in accordance with
the present invention. For example, the assembly may be
non-tubular.
[0108] Turning to FIG. 11, a degas assembly 39a similar to degas
assembly 39 is shown. Degas assembly 39a includes a gas permeable
tubing positioned in a housing or jacket 65a. The housing may be
made of high tensile strength polymer such as PEEK. The housing has
an inlet port 54a and outlet port 56a to direct a stream of KOH
solution containing hydrogen gas from the KOH eluent generator into
the space between the outer surface of the gas permeable tubing and
the inner surface of housing. Each end of the gas permeable tubing
is thermally flared. The flared end tubing is compressed against
the polymeric fitting components of the housing so that the entire
fluidic pathway can be leak-free and capable of withstanding high
pressure.
[0109] With reference to FIG. 12, a degas assembly 39b similar to
degas assembly 39 and degas assembly 39a is shown. Degas assembly
39b is placed at the inlet of a fluid pump used in the ion
chromatography or liquid chromatography system. The chromatography
eluent to be degassed is directed through the space or channel
between the outer surface of the gas permeable tubing 67b and the
inner surface of housing 65b. One end of the gas permeable tubing
is capped. The other end of the gas permeable tubing is connected
to a vacuum. Under the applied vacuum, the gases in the eluent
stream diffuse into the inner lumen of gas permeable tubing 67b and
are removed.
[0110] Turning to FIGS. 13-14, a degas assembly 39c similar to
degas assembly 39 is shown. Degas assembly 39c includes a piece of
gas permeable tubing (e.g., 0.033-inch OD.times.0.008 ID AF2400
tubing) positioned inside a piece of polymeric shield tubing 65c
capable of withstanding pressures up to 5000 psi or higher (e.g.,
1/16-inch OD.times.0.034 ID PEEK tubing). Degas assembly 39c is
configured and operates similarly to degas assembly 39 but includes
two tees 54c, 56c in place of inlet and outlet housings 54, 56. The
tees provide fluidic pathways to direct a stream of KOH solution
containing hydrogen gas from the KOH eluent generator into the
annular space between the outer surface of the gas permeable tubing
67c and the inner surface of housing 65c. In various embodiments,
the polymeric shield tubing has an internal spline feature similar
to that of degas assembly 39 (see FIGS. 6 and 10). In various
embodiments, the entire length of polymeric shield tubing contains
the spline feature. The gas permeable tubing at the degasser inlet
end 54c and outlet end 56c is placed inside a short piece of
polymeric tubing 88 which is used as a sleeve over the gas
permeable tubing. The ID of the sleeve tubing is dimensioned to
ensure that the gas permeable tubing is compressed against the
inner lumen of the polymeric tubing by interference fit so the
entire fluidic pathway is essentially leak-free and capable of
withstanding high pressure. Inlet 54c and outlet 56c are configured
similar to inlet 54 and outlet 56 except that ports are arranged in
perpendicular fashion. The inlet and outlet housings are formed of
a polymer.
[0111] The degas assembly and method of the invention has several
advantages. As described above, the eluent can be degassed at a
higher system pressure than conventional systems using electrolytic
eluent generators. In particular, conventional degas assemblies use
materials such as Teflon.RTM. AF2400 tubing with a theoretical
burst pressure of about 3300 psi. In practice, the system can only
achieve a maximum of about 3000 psi before failure. By contrast,
the system in accordance with the present inventions can operate at
a pressure approaching the maximum tensile strength of the
material. With other configurations such as a reinforced outer
jacket, the system pressure can be even higher. This allows use of
the degas assembly in a greater variety of systems and
configurations. For example, the degas assembly may be provided
in-line on the high pressure side of the system after the pump.
Conventional degas assemblies must be used off-line or in low
pressure systems.
[0112] The high pressure capability of the degas assembly in
accordance with the present invention also allows the system to be
used for other applications such as RFIC-EG systems at elevated
pressures. With the expanded range of system operating pressure, it
is possible to increase the separation speed by performing
separations at higher flow rates or using shorter separation
columns packed with stationary phases of smaller particle
sizes.
[0113] The exemplary annular degasser device offers improved degas
efficiency. In the exemplary system, the gas-containing eluent
stream is exposed to the outer surface of the polymeric gas
permeable tubing (e.g., Teflon AF2400 tubing). The outer surface
area of the degas tubing is significantly higher than the surface
area of the inner lumen of the degas tubing. For example, the
typical dimension of Teflon AF tubing used in the Dionex RFIC-EG
(reagent-free IC, eluent generation) systems is 0.032-inch
OD.times.0.008 inch ID. In this case, the outer surface area is
approximately 4 times the inner surface area. Since the degasser
efficiency depends on the surface area, the degas assembly provides
significantly improved degas efficiency.
[0114] The gas permeable tubing in the exemplary degas assembly can
withstand much higher pressures without bursting due to the fact
that the pressurized eluent stream flows outside of the outer
surface of the gas permeable tubing and the tubing is under
compression from the outer surface inwards. For applications of
given operating pressures, it is possible to use thinner-wall gas
permeable tubing to further improve the degas efficiency. The use
of thinner-wall version of the costly gas permeable tubing (e.g.,
Teflon AF2400 tubing) reduces the degasser cost.
[0115] The annular degasser devices of the present invention can be
constructed to have lower degasser volume since the degas assembly
offers improved degas efficiency. To achieve the required degassing
capability, degassers of lower dead volume can be prepared using
the degas assembly of the present invention and the degas assembly
may utilize shorter length degas tubing. The use of reduced length
of the costly gas permeable tubing reduces the degasser cost. For
example, Teflon AF2400 tubing currently costs about $50 per foot.
The lower volume provides shorter gradient delay, improved gradient
fidelity, and improved overall system performance in ion
chromatography systems using electrolytic eluent generators. The
degas assembly of the present invention can achieve significantly
higher gradient fidelity. The improved efficiency and performance
of the degas assembly in accordance with the invention also allows
for use of separation columns with smaller particle sizes.
[0116] The degas assembly of the present invention makes it
possible to operate an ion chromatography system using an
electrolytic eluent generator at lower system operating pressures.
To remove the gases from a gas-containing eluent stream using
conventional degassers such as those in the Liu patent, it is
necessary to add some backpressure after the degasser to "squeeze
out" the gases from the eluent stream. It is often necessary to
maintain a backpressure threshold of about 2000 psi to achieve the
desired degas efficiency. In a typical ion chromatography system
using the electrolytic eluent generator, a piece of PEEK tubing
with smaller ID (e.g., 0.003-inch) of appropriate length is often
used to add the pressure in addition to that generated by the
separation column. This approach adds to the system operation
complexity. Since the degas assembly of the present invention has
improved degassing efficiency, it is possible to reduce the
pressure threshold to lower pressures. The need to add an
additional backpressure device may be eliminated entirely. The
system reliability may also be consequently improved.
[0117] The degas assembly of the invention also allows the use of a
greater number and variety of lower-pressure-rating, gas-permeable
tubing materials at considerable cost savings. Some of these lower
tensile strength materials also offer better gas permeability. The
degas assembly may also provide better rejection of contaminants
coming from the suppressor such as hydrogen peroxide and ozone.
EXAMPLES
[0118] The invention is further illustrated by the Examples that
follow. The Examples are not intended to define or limit the scope
of the invention.
Example 1
Use of Degas Assembly in an Ion Chromatography System Containing an
IC Cube for Separation of Common Anions on a Capillary Anion
Exchange Separation Column
[0119] An ICS-5000 ion chromatography system (Dionex Coproration,
Sunnyvale, Calif.) was used. The system consisted of a pump module,
electrolytic eluent generator (EG) module, and a conventional-scale
chromatography compartment (DC) module. A Dionex Chromeleon 6.8
chromatography data system was used for instrument control, data
collection, and processing. The system was configured according to
FIG. 1. A Dionex IC (ion chromatography) cube was used. The IC cube
contained capillary-scale system components such as a degasser
assembly, a sample injector, a separation column, an electrolytic
suppressor, a carbonate removal device (CRD).
[0120] The degas assembly was constructed according to FIG. 2. The
IC cube was physically placed in the upper compartment of the
ICS-5000 DC module. The detection of analytes was accomplished
using an ICS-5000 capillary conductivity detector. A
capillary-scale electrolytic KOH eluent generator and a capillary
CR-ATC was installed in the ICS-5000 EG module and controlled by
the ICS-5000 EG module.
[0121] The exemplary degas assembly was formed using a PEEK tube
for the outer pressurized channel member. The PEEK tube had a 2 mm
outer diameter (OD) and 0.034'' inner diameter (ID). The degas
assembly had an outer diameter (OD) corresponding to the outer
surface of the PEEK tube.
[0122] The inner channel member was AF2400 tube having an ID of
0.008'' and OD of 0.033''. The inner AF tubing was placed inside
the PEEK tubing jacket. Two housings were attached to each end of
the tubes to seal both ends and port flow in the correct flow
path.
[0123] A flare seal design was used to seal the AF tubing against
leaking from high pressure flow. The pressurized tube included a
splined portion with LS=3/8'' on each end. The internal splines had
a depth, DS, equal to 0.375''. The splines were found to allow a
tight seal of the pressurized tubing under high pressure with a
conventional ferrule fitting and without occluding the annular
space for the eluent between the inner Teflon.RTM. AF2400 tube and
outer PEEK tube.
[0124] A makeup assembly was provided around the AF tubing and
between the end of the PEEK jacket and housing wall. The makeup
assembly had a 2 mm OD, 0.034'' ID, and length of 0.200''.
[0125] A capillary separation column (0.4 mm.times.250 mm) was
packed with the Dionex AS19 anion exchange resin. FIG. 15 shows the
separation of 8 common anions including fluoride, chlorite,
chloride, nitrite, chlorate, bromide, nitrate, and sulfate obtained
using the system under the eluting condition of 20 mM KOH at 10
.mu.L/min. In FIG. 15, the y-axis is micro-siemens and the x-axis
is minutes. FIG. 15 shows an overlay of 30 consecutive separations
of the target analytes. The results show highly reproducible
separation of the target anions with analyte retention percent
relative standard deviation (RDS) ranging from 0.047% for nitrite
to 0.078% for sulfate, and analyte peak area percent RSD ranging
from 0.28% for fluoride to 0.33% for bromate. These results
demonstrate that the capillary ion chromatography system fitted
with degas assembly of the invention can be used to provide
reliable capillary-scale ion chromatographic separation of target
anionic analytes using only deionized water as the carrier
stream.
Example 2
Use of Degas Assembly in an Ion Chromatography System Employing an
Electrolytic Methanesulfonic Acid Generator for Separation of
Common Cations on a Cation Exchange Separation Column
[0126] An ICS-2000 ion chromatography system (Dionex Coproration,
Sunnyvale, Calif.) was used. The system was configured according to
FIG. 1 except that the carbonate removal device was not used. A
Dionex Chromeleon 6.8 chromatography data system was used for
instrument control, data collection, and processing. The degas
assembly was constructed according to FIGS. 13-14. Dionex CS12A and
CG12 cation exchange columns were used. The separation was
performed using 20 mM methanesulfonic acid at 0.5 mL/min. FIG. 16
shows the separation of six common cations. The results show highly
reproducible separation of the target cations over three hundred
injections. These results demonstrate that the ion chromatography
system fitted with degas assembly of the invention can be used to
provide reliable ion chromatographic separation of target cationic
analytes using only deionized water as the carrier streams in
conjuction with electrolytic eluent generator.
Example 3
Generation of KOH Eluent Using an EGC KOH Cartridge and a Degas
Assembly
[0127] An ICS-2000 ion chromatography system (Dionex Coproration,
Sunnyvale, Calif.) was used. A Dionex Chromeleon 6.8 chromatography
data system was used for instrument control, data collection, and
processing. The ICS-2000 system was fitted with a Dionex EGC KOH
cartridge to generate KOH eluents electrolytically. The degas
assembly was constructed according to FIGS. 13-14. A piece of
Teflon AF2400 tubing (0.031-inch OD.times.0.008-inch
ID.times.1.25-ft length) was used in the degas assembly. FIG. 17B
shows the conductance of the KOH eluents generated at 1.0 mL/min.
The results show that annular degasser described in this invention
can be used to remove hydrogen gas produced by the electrolytic
eluent generation process. FIG. 17A shows the conductance of the
KOH eluents generated at 1.0 mL/min when the degas assembly was
replaced with a conventional gas separation device such as that
disclosed by the Liu patent. The results show that a good step-wise
conductance profile may be achieved with the gas removal device and
method of the present invention. By contrast, FIG. 18 shows the
conductance of the KOH eluents when the system is used with the
degas assembly removed. As shown in FIG. 18, the conductance
profile of the KOH eluents is very noisy due to the presence of
hydrogen gas bubbles in the eluent stream. In FIGS. 17A, 17B, and
18, the y-axis is micro-siemens and the x-axis is minutes.
[0128] For convenience in explanation and accurate definition in
the appended claims, the terms "up" or "upper", "down" or "lower",
"inside" or "inner", "outer" and "outside" are used to describe
features of the present invention with reference to the positions
of such features as displayed in the figures.
[0129] The foregoing descriptions of specific embodiments of the
present invention have been presented for purposes of illustration
and description. They are not intended to be exhaustive or to limit
the invention to the precise forms disclosed, and obviously many
modifications and variations are possible in light of the above
teaching. The embodiments were chosen and described in order to
best explain the principles of the invention and its practical
application, to thereby enable others skilled in the art to best
utilize the invention and various embodiments with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
Claims appended hereto and their equivalents.
* * * * *