U.S. patent application number 10/177080 was filed with the patent office on 2002-11-28 for large capacity acid or base generator apparatus.
This patent application is currently assigned to Dionex Corporation. Invention is credited to Avdalovic, Nebojsa, Liu, Yan, Small, Hamish.
Application Number | 20020177233 10/177080 |
Document ID | / |
Family ID | 21780434 |
Filed Date | 2002-11-28 |
United States Patent
Application |
20020177233 |
Kind Code |
A1 |
Liu, Yan ; et al. |
November 28, 2002 |
Large capacity acid or base generator apparatus
Abstract
Method and apparatus for generating an acid or base, e.g. for
chromatographic analysis of anions. For generating a base the
method includes the steps of providing a cation source in a cation
source reservoir, flowing an aqueous liquid stream through a base
generation chamber separated from the cation source reservoir by a
barrier (e.g. a charged membrane) substantially preventing liquid
flow while providing a cation transport bridge, applying an
electric potential between an anode cation source reservoir and a
cathode in the base generation chamber to electrolytically generate
hydroxide ions therein and to cause cations in the cation source
reservoir to electromigrate and to be transported across the
barrier toward the cathode to combine with the transported cations
to form cation hydroxide, and removing the cation hydroxide in an
aqueous liquid stream as an effluent from the first base generation
chamber. Suitable cation sources include a salt solution, a cation
hydroxide solution or cation exchange resin.
Inventors: |
Liu, Yan; (Santa Clara,
CA) ; Small, Hamish; (Leland, MI) ; Avdalovic,
Nebojsa; (San Jose, CA) |
Correspondence
Address: |
DAVID J. BREZNER, ESQ.
DORSEY & WHITNEY LLP
Suite 3400
Four Embarcadero Center
San Francisco
CA
94111-4187
US
|
Assignee: |
Dionex Corporation
|
Family ID: |
21780434 |
Appl. No.: |
10/177080 |
Filed: |
June 21, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10177080 |
Jun 21, 2002 |
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09612074 |
Jul 7, 2000 |
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09612074 |
Jul 7, 2000 |
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09017050 |
Feb 2, 1998 |
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6225129 |
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Current U.S.
Class: |
436/161 ;
204/551; 204/647; 205/335; 205/628; 205/633; 205/637; 210/198.2;
210/656; 422/70; 422/82.02; 436/150; 436/174; 73/61.53; 73/61.55;
73/61.56 |
Current CPC
Class: |
C25B 1/16 20130101; G01N
30/96 20130101; G01N 30/34 20130101; G01N 30/02 20130101; C25B 1/22
20130101; G01N 2030/965 20130101; Y10T 436/25 20150115; G01N 30/02
20130101; B01D 15/36 20130101; G01N 30/02 20130101; B01D 15/426
20130101 |
Class at
Publication: |
436/161 ;
436/150; 436/174; 422/70; 422/82.02; 204/551; 204/647; 205/335;
205/628; 205/633; 205/637; 210/198.2; 210/656; 73/61.53; 73/61.55;
73/61.56 |
International
Class: |
G01N 030/02 |
Claims
What is claimed is:
1. A method of generating a base comprising the steps of: (a)
providing a cation source in a cation source reservoir, (b) flowing
an aqueous liquid stream through a first base generation chamber
separated from said cation source reservoir by a first barrier
substantially preventing liquid flow while providing a cation
transport bridge, (c) applying an electric potential between an
anode in electrical communication with said cation source reservoir
and a cathode in electrical communication with said first base
generation chamber to electrolytically generate hydroxide ions in
said first base generation chamber and to cause cations in said
cation source reservoir to electromigrate toward said first barrier
and to be transported across said first barrier toward said cathode
to combine with said transported cations to form cation hydroxide,
and (d) removing the cation hydroxide in an aqueous liquid stream
as an effluent from said first base generation chamber.
2. The method of claim 1 in which said cation source comprises a
cation-containing solution selected from the group consisting of a
salt solution and a cation hydroxide solution.
3. The method of claim 2 in which said cation-containing solution
is supplied to said cation source reservoir by pumping from a
remote reservoir.
4. The method of claim 3 in which a stream of said
cation-containing solution is recycled from said cation reservoir
to said remote reservoir.
5. The method of claim 1 in which the volume of said cation source
reservoir is at least about 5 times the volume of said first base
generation chamber.
6. The method of claim 1 in which said first base generation
chamber is pressurized and the pressure maintained in said first
base generation chamber is at least about 2 times any pressure
maintained in said cation source reservoir.
7. The method of claim 1 in which said cation source comprises a
cation exchange bed including exchangeable cations of the type
which form said cation hydroxide.
8. The method of claim 7 in which said cation exchange bed
comprises cation exchange resin particles in a stationary bed or
suspended in an aqueous liquid.
9. The method of claim 7 in which said cation exchange bed includes
a downstream weakly acidic bed section proximal to said barrier and
an upstream strongly acidic bed section distal to said first
barrier, said upstream and downstream sections being in fluid
communication, so that in the migration on the weakly acidic bed
section toward the cathode of hydronium ions generated at the anode
is slowed in comparison to migration of the cations.
10. The method of claim 7 in which a source of cation-containing
solution is supplied to said cation reservoir by continuously
pumping from a remote reservoir.
11. The method of claim 10 in which a stream of said
cation-containing solution is recycled to said remote
reservoir.
12. The method of claim 1 in which said cations in said cation
source reservoir also electromigrate through a second barrier to
said first base generation chamber.
13. The method of claim 1 including at least a second base
generation chamber, and a second barrier being disposed between
said cation source reservoir and said base generation chamber.
14. The method of claim 1 used to form a base eluent for an anion
analysis system further comprising the steps of: (e) flowing said
cation hydroxide generated in step (d) and a liquid sample
containing anions to be detected through a chromatographic
separator in which anions to be detected are chromatographically
separated, forming a chromatograph effluent, and (f) flowing said
chromatography effluent, with or without further treatment, past a
detector in which the separated ions in said chromatography
effluent are detected.
15. The method of claim 14 further comprising between steps (e) and
(f) the step of: (g) flowing said chromatography effluent through a
suppressor including a cation exchange bed to convert said cation
hydroxide to weakly ionized form, said chromatography effluent
existing as a suppressor effluent which flows past said
detector.
16. The method of claim 15 further comprising, prior to step (e)
the following step: (h) pumping through a gradient pump one or more
gradient eluents into said cation hydroxide eluent stream.
17. The method of claim 14 further comprising pressurizing said
chromatography effluent by flow through a pressure restrictor
downstream from said chromatography effluent.
18. A method of generating an acid comprising the steps of: (a)
providing an anion source in an anion source reservoir, (b) flowing
an aqueous liquid stream through a first acid generation chamber
separated from said anion source reservoir by a first barrier
substantially preventing liquid flow while providing an anion
transport bridge, (c) applying an electric potential between a
cathode in electrical communication with said anion source
reservoir and an anode in electrical communication with said first
acid generation chamber to electrolytically generate hydronium ions
in said first acid generation chamber and to cause anions in said
anion source reservoir to electromigrate toward said first barrier
and to be transported across said first barrier toward said anode
to combine with said transported anions to form an acid, and (d)
removing the acid in an aqueous liquid stream as an effluent from
said first acid generation chamber.
19. The method of claim 18 in which said anion source comprises an
anion-containing solution selected from the group consisting of a
salt solution and an acid solution.
20. The method of claim 18 in which said anion-containing solution
is supplied to said anion source reservoir by pumping from a remote
reservoir.
21. The method of claim 20 in which a stream of said
anion-containing solution is recycled from said anion reservoir to
said remote reservoir.
22. The method of claim 18 in which the volume of said anion source
reservoir is at least about 5 times the volume of said first acid
generation chamber.
23. The method of claim 18 in which said first acid generation
chamber is pressurized and the pressure maintained in said first
acid generation chamber is at least about 2 times any pressure
maintained in said anion source reservoir.
24. The method of claim 18 in which said anion source comprises an
anion exchange bed including exchangeable anions of the type which
form said acid.
25. The method of claim 24 in which said anion exchange bed
comprises an ion exchange resin particles in a stationary or
suspended in an aqueous liquid.
26. The method of claim 24 in which said anion exchange bed
includes a downstream weakly basic bed section proximal to said
barrier and an upstream strongly basic bed section distal to said
first barrier, said upstream and downstream sections being in fluid
communication, so that in the migration on the weakly basic bed
section toward the anode of hydroxide ions generated at the cathode
is slowed in comparison to migration of the anions.
27. The method of claim 24 in which a source of anion-containing
solution is supplied to said anion reservoir by continuously
pumping from a remote reservoir.
28. The method of claim 27 in which a stream of said
anion-containing solution is recycled to said remote reservoir.
29. The method of claim 18 in which said anions in said anion
source reservoir also electromigrate through a second barrier to
said first acid generation chamber.
30. The method of claim 18 including at least a second anion
generation chamber, and a second barrier being disposed between
said anion source reservoir and said anion generation chamber.
31. The method of claim 18 used to form an acid eluent for an
cation analysis system further comprising the steps of: (e) flowing
said acid generated in step (d) and a liquid sample containing
cations to be detected through a chromatographic separator in which
cations to be detected are chromatographically separated, forming a
chromatograph effluent, and (f) flowing said chromatography
effluent, with or without further treatment, past a detector in
which the separated cations in said chromatography effluent are
detected.
32. The method of claim 31 further comprising between steps (e) and
(f) the step of: (g) flowing said chromatography effluent through a
suppressor including an anion exchange bed to convert said acid to
weakly ionized form, said chromatography effluent existing as a
suppressor effluent which flows past said detector.
33. The method of claim 32 further comprising, prior to step (c)
the following step: (h) pumping through a gradient pump one or more
gradient eluents into said acid eluent stream.
34. The method of claim 31 further comprising pressurizing said
chromatography effluent by flow through a pressure restrictor
downstream from said chromatography effluent.
35. An apparatus for generating an acid or base comprising: (a) an
ion source reservoir containing a source of either anions or
cations, (b) an acid or base generation chamber having inlet and
outlet ports, (c) a charged first barrier disposed between said ion
source reservoir and said acid or base generation chamber, said
barrier substantially preventing liquid flow while providing an ion
transport bridge for only ions of one charge, positive or negative,
(d) a first electrode in electrical communication with said ion
source reservoir, (e) a second electrode in electrical
communication with said first acid or base generation chamber, and
(f) an aqueous liquid source in fluid communication with said acid
or base generation chamber inlet port.
36. The apparatus of claim 35 further comprising: (g) a power
supply for applying an electrical potential between said first and
second electrodes.
37. The apparatus of claim 36 in which said acid or base generated
in said acid or base generation chamber is used as an eluent stream
for analysis of ions of interest of one charge only, positive or
negative, said apparatus further comprising: (h) a sample injection
port for injecting a liquid sample stream of ions to be detected,
(i) a chromatographic separator for separating said ions of
interest, and having inlet and outlet ports, said inlet port being
in fluid communication with said sample injection port and said
acid or base generator outlet port, whereby a chromatography
effluent exits from said outlet port, and (j) a detector in fluid
communication with said chromatographic separator for detecting the
separated ions of interest in said chromatography effluent.
38. The apparatus of claim 37 further comprising: (k) a gradient
pump for pumping one or more gradient eluents into said
ion-containing solution generated in said first acid or base
generation chamber.
39. The apparatus of claim 37 further comprising: (k) a flow
restrictor in fluid communication with the outlet of said first
acid or base generation chamber outlet port.
40. The apparatus of claim 35 in which said ion source reservoir
has inlet and outlet ports, said apparatus further comprising: (g)
a remote reservoir for ion-containing solution having inlet and
outlet ports, and (h) a pump for pumping ion-containing solution
from said remote reservoir outlet port to said ion source reservoir
inlet port.
41. The apparatus of claim 40 further comprising: (i) a recycle
conduit connecting said ion source reservoir outlet port and said
remote reservoir inlet port.
42. The apparatus of claim 33 in which the volume of said ion
source reservoir is at least 5 times the volume of said acid or
base generator chamber.
43. The apparatus of claim 35 in which said cation source comprises
an ion exchange bed including exchangeable ions of the type which
form said acid or base.
44. The apparatus of claim 43 in which said ion exchange bed
comprises a stationary bed of ion exchange resin particles or resin
particles suspended in an aqueous liquid.
45. The apparatus of claim 43 in which said ion exchange bed
comprises a bed of ion exchange resin particles including a
downstream weakly acidic or basic section proximal to said first
barrier and an upstream strongly acidic or strongly basic section
of the same charge as said weakly acidic or weakly basic section
and in fluid communication therewith.
46. The apparatus of claim 35 further comprising a second barrier
of the same type as said first barrier disposed between said ion
source reservoir and said first acid or base generation
chamber.
47. The apparatus of claim 35 further comprising: (g) a bed of ion
exchange resin with exchangeable ions of the same charge as said
first barrier disposed in said generation chamber between said
first barrier and said second electrode and providing an ion path
therebetween.
48. The apparatus of claim 35 further comprising: (g) a charged
screen of the same charge as said first barrier disposed between
said first barrier and said second electrode in said generation
chamber and providing an ion path therebetween.
49. The apparatus of claim 35 further comprising an uncharged
screen between said first barrier and said second electrode in said
generation chamber.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] Reference is made to co-pending H. Small, et al. U.S. patent
application Ser. No. 08/783,317, filed Jan. 15, 1997.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a large capacity apparatus
for generating a high purity acid or base particularly for use as a
chromatography eluent, and to a method of using the apparatus.
[0003] In liquid chromatography, a sample containing a number of
components to be separated is directed through a chromatography
separator, typically an ion exchange resin bed. The components are
separated on elution from the bed in a solution of eluent. One
effective form of liquid chromatography is referred to as ion
chromatography. In this known technique, ions to be detected in a
sample solution are directed through the separator using an eluent
containing an acid or base and thereafter to a suppressor, followed
by detection, typically by an electrical conductivity detector. In
the suppressor, the electrical conductivity of the electrolyte is
suppressed but not that of the separated ions so the latter may be
detected by the conductivity detector. This technique is described
in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and
3,956,559.
[0004] There is a general need for a convenient source of high
purity acid or base for use as an eluent for liquid chromatography
and, particularly, for ion chromatography. In one technique,
described in U.S. Pat. No. 5,045,204, an impure acid or base is
purified in an eluent generator while flowing through a source
channel along a permselective ion exchange membrane which separates
the source channel from a product channel. The membrane allows
selective passage of cations or anions. An electrical potential is
applied between the source channel and the product channel so that
the anions or cations of the acid or base pass from the former to
the latter to generate therein a base or acid with electrolytically
generated hydroxide ions or hydronium ions, respectively. This
system requires an aqueous stream of acid or base as a starting
source or reservoir.
[0005] There is a particular need for a pure source of acid or base
which can be generated at selected concentrations solely from an
ion exchange bed without the necessity of an independent reservoir
of an acid or base starting aqueous stream. There is a further need
for such a system which can be continuously regenerated. Such need
exists in chromatography, and specifically ion chromatography, as
well as other analytical applications using acid or base such as in
titration, flow injection analysis and the like.
SUMMARY OF THE INVENTION
[0006] In copending application Ser. No. 08/783,317, filed Jan. 15,
1997, a method and apparatus is described for generating acid or
base in an aqueous stream, such as water alone or in combination
with additives (e.g., ones which react with the acid or base or
with the sample). The system provides an excellent source of high
purity acid or base for use as an eluent for chromatography and,
particularly, ion chromatography. The present system is an
improvement over the one described in the copending
application.
[0007] Referring first to the present system in which a base is
generated e.g. for chromatographic analysis of anions, the method
includes the steps of:
[0008] (a) providing a cation source in a cation source
reservoir,
[0009] (b) flowing an aqueous liquid stream through a base
generation chamber separated from the cation source reservoir by a
barrier substantially preventing liquid flow while providing a
cation transport bridge,
[0010] (c) applying an electric potential between an anode in
electrical communication with said cation source reservoir and a
cathode in electrical communication with the base generation
chamber to electrolytically generate hydroxide ions in the base
generation chamber and to cause cations in the cation source
reservoir to electromigrate toward said first barrier and to be
transported across the barrier toward the cathode to combine with
the transported cations to form cation hydroxide, and
[0011] (d) removing the cation hydroxide in an aqueous liquid
stream as an effluent from the first base generation chamber.
[0012] Suitable cation sources include a salt solution or a cation
hydroxide solution which can be supplied to the cation source
reservoir by pumping from a remote reservoir. The solution can be
recycled to the remote reservoir. Also, the cation source may
comprise a cation exchange bed, e.g., resin particles in a
stationary bed or suspended in an aqueous liquid, alone or in
combination with the salt solution.
[0013] The method may also be used for generating an acid, e.g. for
use as an eluent for chromatographic analysis of cations by
reversing the charges of the ion source, the barrier, the
electrical potential and any other charged components of the
system.
[0014] Another embodiment of the invention is an apparatus for
generating an acid or base including:
[0015] (a) an ion source reservoir of either anions or cations,
[0016] (b) an acid or base generation chamber having inlet and
outlet ports,
[0017] (c) a first barrier between the ion source reservoir and the
acid or base generation chamber, substantially preventing liquid
flow while providing an ion transport bridge for only ions of one
charge, positive or negative,
[0018] (d) a first electrode in electrical communication with the
ion source reservoir,
[0019] (e) a second electrode in electrical communication with the
first acid or base generation chamber, and
[0020] (f) an aqueous liquid source in fluid communication with the
acid or base generation chamber inlet port.
[0021] The apparatus can be used to supply the generated acid or
base to a chromatography system or any other analytical system
which uses a high purity acid or base.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIGS. 1-8 and 10-12 are schematic representations of
apparatus according to the present invention.
[0023] FIG. 9 is an on-line high pressure gas removal device for
use in the present invention.
[0024] FIGS. 13-29 are graphical representations of experimental
results using the present base or acid generator system.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0025] The system is applicable to the generation of eluent for
liquid chromatography forms other than ion chromatography. For
example, it is applicable to liquid chromatography using an
ultraviolet (UV) detector. The eluent may be in a form (e.g. salt)
other than a pure acid or base. Thus, the term "aqueous stream"
includes pure water or water with such additives. Also, the terms
"eluent comprising a base", "eluent comprising an acid", an "acid"
or a "base" mean an aqueous stream including acid or base generated
according to the invention regardless of the form it takes on
mixing with other reagents present in the aqueous stream. As used
herein, the term "cation" excludes hydronium ion and the term
"anion" excludes hydroxide ion. The system is also applicable to
other non-chromatographic analytical systems which use a high
purity acid or base.
[0026] The copending application uses some of the same principles
as the present invention and its disclosure is incorporated by
reference. Such disclosure includes a high purity solution of acid
or base electrochemically generated by passing deionized water
through an electrically polarized bed of ion exchange resin in the
desired ionic form placed between two electrodes. For example, in
the generation of a KOH solution, deionized water is pumped through
a column packed with a cation exchange resin in K.sup.+ form, and a
DC voltage is applied between the anode at the column inlet and the
cathode at the column outlet. The electrochemical reaction at the
anode generates H.sup.+ ions by splitting water. Under the
influence of the electrical field, H.sup.+ ions electromigrate into
the resin bed to displace K.sup.+ ions, which in turn migrate
downstream through the resin bed and combine with OH.sup.- ions
generated at the cathode to produce KOH. The concentration of KOH
generated is determined by the electrical current applied and the
flow rate of the deionized water through the column. Similarly, a
high purity acid (e.g., methanesulfonic acid) solution can be
generated using a generation column containing an anion exchange
resin in the desired ionic form.
[0027] The acid or base generation column described above is an
attractive source of high purity eluent for ion and liquid
chromatography for a number of reasons. For example,
chromatographic separations can be conveniently performed using
only deionized water as the carrier. Since acid or base is
generated on-line, the need of often-tedious, off-line preparation
of eluents can be eliminated. Second, the eluent strength (the
concentration of acid or base) can be controlled precisely and
conveniently by controlling the electrical current applied to the
acid or base generation column and the flow rate. Third, gradient
chromatographic separations can be accomplished with current
gradients and a less costly isocratic pump instead of using a more
expensive gradient pump. Fourth, the use of an acid or base
generation column can improve the performance of chromatographic
methods, since the eluent generated on-line can be free of
contaminants that are often introduced if it is prepared off-line
by conventional means. For example, the presence of carbonate in
hydroxide eluent due to sorption of carbon dioxide from air often
seriously compromises the performance of an ion chromatography
method; this problem will be eliminated by using the high purity
hydroxide eluent generated on-line. Fifth, the reliability of the
chromatography pumping system can be improved, the lifetime of pump
seal can be extended significantly since the pump is used to pump
deionized water instead of more corrosive acid or base solution.
These same advantages and principles apply to the present
invention. In addition, the present invention retains the
advantages of the acid or base generation column, and provides a
significant improvement in the generation of high purity acid or
base solutions for an extended period of time for ion and liquid
chromatography, and other applications.
[0028] The method and apparatus for generation of acid or base
according to the present invention will first be described to
supply eluent, e.g., for ion chromatography. Although applicable to
anion or cation analysis, the system will be described for
generation of a base suitable for use as an eluent in the analysis
of anions on an ion exchange resin packed bed form. In this
instance, the cation exchange bed generates a base such as an
alkali metal hydroxide, typically sodium or potassium. For analysis
of cations, the eluent generated is an acid such as methanesulfonic
acid. The system will first be described for the generation of KOH
as the base.
[0029] FIG. 1 schematically illustrates a general form of a large
capacity base (KOH) generator form according to the present
invention. The apparatus includes cation (K.sup.+) ion source
reservoir 10. As will be explained in more detail below, the cation
source may be a cation-containing solution such as a salt solution
or a cation hydroxide solution. Alternatively, the cation source
may be a cation exchange bed including exchangeable cations of the
type which form a cation hydroxide. The bed may be formed of ion
exchange resin particles in a fixed or stationary bed or suspended
particles in an aqueous liquid. A gas vent may be provided in
reservoir 10 to vent oxygen generated therein as described
hereinafter.
[0030] Base generation chamber 12 is separated from the ion source
reservoir 10 by a barrier 14, suitably in the form of a charged
perm-selective membrane described below. Charged barrier 14
substantially prevents liquid flow while providing an ion transport
bridge for cations from the ion source reservoir 10 to base
generation chamber 12. As used herein, the term "barrier" refers to
the charged material (e.g. membrane) separating reservoir 10 and
chamber 12 which permits ion flow but blocks liquid flow, alone or
in combination with an appropriate flow-through housing in which
the barrier is mounted transverse to flow across the entire flow
path.
[0031] The charged barrier 14 should be of sufficient thickness to
withstand the pressures in chamber 12. For example, if chamber 12
is on line with a chromatography system, such pressures may be on
the order of 1,000 to 3,000 psi. When using a membrane as barrier
14, it is suitably configured of circular cross-section within a
cylindrical external short column. Typical dimensions for the
membrane are about 4-6 mm diameter and 1-3 mm in length. The
barrier can be fabricated by stacking multiple disks of cation
membranes together within the cylindrical column. Alternatively,
barrier 14 can be prepared from a single ion exchange membrane of
appropriate thickness or a block or rod of appropriate ion exchange
material which permits passage of the potassium but not of the
liquid.
[0032] An anode 16 is disposed in electrical contact with, and
preferably within, cation source reservoir 10 and a cathode 18 is
disposed in electrical contact with, and preferably within, base
generation chamber 12. A suitable DC power supply 20 connects the
anode and the cathode. Also, there is a continuous electrical path
from anode 16 through barrier 14 to cathode 18. Aqueous stream 20,
suitably deionize water, flows through an inlet port, not shown, in
base generation chamber 12. KOH is generated in base generation
chamber 12 and flows out of outlet port, not shown. A cation
exchange resin bed 19 (e.g. in K.sup.+ form) can be packed in
chamber 12 in contact with barrier 14 and cathode 18 to provide
good electrical contact therebetween. As illustrated, the flow of
aqueous stream 20 is toward cathode 18. However, if desired, the
flow may be in the opposite direction.
[0033] For the production of pure base (e.g. KOH), high-purity
deionized water from source 21 is pumped to generation chamber 12.
Water splitting takes place at both electrodes. The anode reaction
in reservoir 10 is as follows:
H.sub.2O-2e.sup.-.fwdarw.2H.sup.++1/2O.sub.2 (1)
[0034] During this reaction, hydronium ions are produced in
reservoir 10 for the resin form of the invention, the hydronium
ions pass into the cation exchange resin by electromigration
displacing the exchangeable cations (e.g. K.sup.+ ions) ahead of
them. This displacement takes place along the length of the bed and
the K.sup.+ ions pass through barrier 14 into chamber 12 eventually
leading to production of base (KOH) in the flowing aqueous stream
in generation chamber 12. The hydroxide ions are produced in the
following cathodic reaction.
2H.sub.2O+2e.sup.-.fwdarw.2OH.sup.-+H.sub.2 (2)
[0035] In one form of reservoir 10, the cation source is a
cation-containing solution, suitably either a salt solution or a
cation hydroxide solution (e.g. KOH). If a salt solution is used,
it is preferably of a weakly acidic anion salt such as
K.sub.2HPO.sub.4 to bind the hydronium ions produced at the anode.
In this manner, K.sup.+ is the primary ion passing through barrier
14, thereby minimizing the flow of H.sup.+ ions. The hydronium ion
generation in the reservoir provides electrical neutrality to the
solution in the reservoir as the K.sup.+ ions are driven across the
barrier.
[0036] Another embodiment of the invention is illustrated in FIG.
2. This device is specifically adapted for use with an ion exchange
resin form of cation source in reservoir 10. Because of the similar
components in FIGS. 1 and 2, like parts will be designated with
like numbers. The illustrated reservoir 10 is suitably in the form
of a solid horizontal hollow cylinder 10a with inlet and outlet
walls 10b and 10c, respectively, and packed with cation exchange
resin in K.sup.+ form. Alternative shapes, e.g. rectangular, of
reservoir 10 may be used. An aqueous stream, suitably dionized
water, is pumped through an inlet port, not shown, into reservoir
10. Similarly, a preferred housing for chamber 12 is a cylindrical
column defining a cylindrical chamber. Thus, the terms "chamber"
and "column" will be used interchangeably for chamber 12. Anode 16
is illustrated as a perforate disk disposed at the inlet side of
reservoir 10 adjacent inlet wall 10b. Flow-through cation exchange
resin bed 24 is suitably of similar ion exchange and flow
characteristics to a chromatographic separation bed.
[0037] A preferred form of ion exchange resin bed in reservoir 10
is a "dual-bed" including a long section 24a of a strongly acidic
cation exchange resin (e.g. a sulfonated resin such as sold under
the trademarks Dowex 50WX8 resin or Dionex ASC resin) in K.sup.+
form adjacent at the line X-X to a shorter section of a weakly
acidic cation exchange resin (e.g. a carboxylate resin such as sold
under the trademarks Dionex CS12A resin or Bio-Rex 70 resin) in
K.sup.+ form downstream at its outlet end. As used herein, "weakly
acidic" anion means an anion with an acid dissociation constant
(pKa) of greater than 3.0 and "strongly acidic anion" means an
anion with a pKa less than about 3.0. Preferably the strongly
acidic section 24a is at least about 10 percent of the length or
volume of reservoir 10 and more preferably at least about 90
percent of the length or volume. Alternatively, if desired, the
entire bed 24 may be formed of strongly acidic cation exchange
resin.
[0038] The dual-bed approach increases the useful capacity of a KOH
generator column. Once H.sup.+ ions reach the bed of the weakly
acidic resin, migration of H.sup.+ through the resin bed is
significantly slowed down because of its higher affinity to the
weakly acidic functional groups. On the other hand, the migration
of K.sup.+ ions through the resin bed is not significantly
reduced.
[0039] Therefore, more K.sup.+ ions are able to reach the cathode
to form KOH before the arrival of H.sup.+ ions at the cathode, and
thus the useful capacity of the KOH generator column is increased.
In the dual-bed once H.sup.+ ions reach the weakly acidic resin
bed, the applied voltage needed to maintain the constant current
will increase due to the development of the less conductive
protonated zone in the weakly acidic resin bed.
[0040] One function of barrier 14 is to permit use of a very large
reservoir 10 (e.g. 1-2 liters) supplying K.sup.+ ions to generation
chamber 12. This large capacity reservoir permits a long term
supply of K.sup.+ ions. By way of example, a typical KOH generation
chamber may have a volume on the order of less than 100 .mu.L and
more typically from 100 .mu.L to 1,000 .mu.L. Suitable dimensions
for a cylindrical shape are 4-7 mm ID and 10-50 mm in length. This
facilitates use on line in a chromatography system. In contrast,
reservoir 10 may be many times larger than the volume of the
generation chamber 12. For example, the ratio between reservoir 10
and chamber 12 may be at least 5:1 to 10:1 or 20:1 or even
higher.
[0041] Another function of barrier 14 is that it provides a high
pressure physical barrier that insulates the relatively low
pressure K.sup.+ ion supply reservoir 10 from the generation
chamber 12 which is of substantially high pressure when it is on
line with a high pressure chromatography system. For example, even
a very low pressure chromatography system would be pressurized to
at least about 50 psi. Assuming the reservoir's atmospheric
pressure (14.7 psi) the pressure maintained in the base generation
chamber 12 is at least about three times the pressure maintained in
reservoir 10. This isolation is particularly useful when that
pressure ratio is at least about 2:1 and is even more so when the
ratio is much higher, for example at least about 5:1 to at least
about 10:1 to 100:1 or higher.
[0042] Because it is operated under low pressure, a large K.sup.+
ion supply column can be prepared and operated safely without
demanding pressure constraint. A large K.sup.+ ion supply column
can contain a sufficient amount of cation exchange resin in K.sup.+
form to generate KOH over an extended period of time. For example,
a 10-cm ID.times.20-cm length K.sup.+ ion supply column has an
internal volume of 1570 mL and can contain 2670 meq of K.sup.+ ions
(calculated using the resin capacity of 1.7 meg/mL). If the KOH
generator column is used to generate 20 mM KOH at 1.0 mL/min, its
theoretical capacity is 2225 hours, and an actual useful time is
expected to be more than 1300 hours, assuming 60 percent of the
total K.sup.+ ion capacity is ultimately utilized for the
generation of KOH.
[0043] To step down from the large volume reservoir 10 to the
smaller size base generation chamber 12, an adapter section in the
form of hollow cylindrical column 26 packed with cation exchange
resin 28 may be disposed in open communication with column 10
through an opening in the end wall 10c of reservoir 10. Barrier 14
is disposed between cylinder 26 and generation chamber 12. A
suitable configuration of barrier 14 is a hollow cylinder
transverse to cylinder 26 with a barrier disk (e.g. permselective
membrane) across the flow path therebetween. Generation chamber 12
also is suitably is in the form of a hollow cylinder.
[0044] Barrier 14 is suitably in the form of a stack of cation
exchange membranes or a plug which prevents any significant liquid
flow but permits transport of the K.sup.+ ions into chamber 12. A
suitable form of membrane is supplied by Membrane International of
Glenrock, N.J. (designated CMI-7000 cation exchange membrane). As
illustrated, cathode 18 is a porous disk disposed adjacent to and
coextensive with the end wall at the exit of chamber 14. As in the
embodiment of FIG. 1, water is supplied to an inlet port of chamber
12. The KOH generated near cathode 18 exits from the outlet of
chamber 12. This is advantageous as the H.sub.2 gas generated at
the cathode is readily swept out of chamber 12.
[0045] Anode 16 and cathode 18 disposed in reservoir 10 and
generation chamber 12, respectively, can take the different forms
such as porous disks, frits, rings, screens, sheets, and probes so
long as they provide good contact (preferably direct contact) with
the ion source or ion exchange resin. For example, the anode is
preferably in direct contact with the ion exchange resin, if used,
or with the solution in the reservoir if no ion exchange resin is
used. Similarly, the cathode should be in direct contact with the
ion exchange resin when used in the generation chamber. The
electrode may also be formed by crumpling and forming a length of
fine platinum wire to form a roughly disk-shaped object that allows
easy flow through the structure. The electrodes are preferably made
of inert material, such as platinum. In the embodiments described
above, it is preferable that the electrodes be placed in a region
near the outlet of generation chamber 12, although other locations
may be used as well.
[0046] In another form of the electrodes, not shown, a thin inert
electrically conductive screen is wrapped partially or totally
around a bed of ion exchange resin in chamber 12 in a case-like
configuration. This electrode design provides good contact between
the cation exchange resin and the electrode surface, thus lowering
the device operating voltage. Thus, higher currents can be applied
to generate higher concentrations without being limited by possible
excessive heating.
[0047] In general, the method of the present invention using the
embodiment of FIG. 2 is performed as follows. The cation source is
provided by the combination of cation exchange resin 24 in
reservoir 12 and cation exchange resin 28 in column 26. The H.sup.+
ion formed near anode 12 drives the K.sup.+ ions through the resin
until they transport across barrier 14. The H.sup.+ ions produce
electrical neutrality to reservoir 10. The K.sup.+ ions travel
across barrier 14 into chamber 12 towards cathode 18 and combines
with the hydroxide ions formed at the cathode to form KOH. The
aqueous stream flowing through base generation chamber 12 carries
the KOH in solution for subsequent use in the analytical
system.
[0048] When using a packed ion exchange bed in reservoir 10 or
generation chamber 12, the higher the cross-linking of a resin the
higher its capacity (expressed as milliequivalents per ml. of
column); therefore, higher cross-linked resins give more compact
generators. This is desirable. However, the higher the
cross-linking of a resin, the less it deforms when packed in a
column. Some deformation is desirable in that it improves the area
of contact between resin beads thus lowering the electrical
resistance of the packed bed. Lower resistance means that a
particular level of current may be attained at a lower applied
voltage; this, in turn, leads to less heating of the bed while
carrying current, a desirable feature.
[0049] Bead deformation is favored by lowering the degree of cross
linking. But, resin of very low cross-linking (say 1 to 2%) is so
deformable that at certain flow rates the deformation can lead to
undesirably high pressure across the bed. In summary, a wide range
of cross-linking can be used. Resins of moderate cross-linkage are
to be preferred, typically in the range of 4 to 16% divinyl benzene
for styrene divinyl benzene polymer beads.
[0050] Other forms of ion exchange beds can be used such as a
porous continuous structure with sufficient porosity to permit flow
of an aqueous stream at a sufficient rate for use as an eluent for
chromatography without undue pressure drop and with sufficient ion
exchange capacity to form a conductive bridge of cations or anions
between the electrodes. One form of structure is a porous matrix or
a sponge-like material with a porosity of about 10 to 50%
permitting a flow rate of about 0.1 to 3 ml/min without excessive
pressure drop. Another suitable form is a roll of ion exchange film
(e.g. in a configuration of such a roll on a spindle disposed
parallel to liquid flow). Electrodes would be placed at each end of
the roll which could be textured to provide an adequate void
channel.
[0051] The aqueous stream flowing through chamber 12 may be
high-purity deionized water. However, for use in some forms of
chromatography, it may be desirable to modify the source with an
additive which reacts with the base generated in electrode chamber
12 to produce eluents of varying potency. For the production of
base, some well known additives include a source of carbonic acid,
phenol, cyanophenol, and the like. (For the production of acid,
such additives include m-phenylene diamine, pyridine, lysine and
amino propionic acid.)
[0052] It is preferable to control the concentration of base
produced in base generation chamber 12. To do so, the current,
directly related to concentration, is controlled. A feed-back loop
may be provided to assure sufficient voltage to deliver the
predetermined current. Thus, the current is monitored when the
resistance changes, and the potential is correspondingly changed by
the feed-back loop. Therefore, the voltage is a slave to the
reading of the current. Thus, it is preferable to supply a variable
output potential system of this type (e.g., sold under the
designation Electrophoresis Power Supply EPS 600 by Pharmacia
Biotech and Model 220 Programmable Current Source by Keithley).
[0053] The current (voltage) requirements of a generator depend on
(a) the eluent strength required; (b) the diameter of the column;
(c) the length of the column; (d) the electrical resistance of the
resin; and (e) the flow rate of the aqueous phase.
[0054] FIG. 3 illustrates another embodiment of the invention. In
this instance, no ion exchange resin is used in reservoir 10.
Instead, a solution of a potassium salt such as K.sub.2HPO.sub.4 is
employed. Alternatively, for specific applications, KOH may be
used. The potassium salt solution may be used in combination with a
cation exchange resin in K.sup.+ form either in a fixed resin bed
or in a bed in which the resin particles are suspended in the
solution. The concentration of K.sup.+ ions in solution is
preferable about 1 to 2 M or higher so that there is a sufficient
amount of K.sup.+ ions for the generation of KOH over an extended
time. However, if desired, the potassium salt solution containing
K.sup.+ ions at lower concentrations (e.g. 0.1 to 0.5 M) can be
used for specific applications. It is preferable that the anion of
the potassium salt not be oxidized by the anode. It is preferable
to use a potassium weakly acidic anion (e.g., HPO.sub.4.sup.2- or
CO.sub.3.sup.2-) with an acid dissociation constant (pK.sub.a) of 5
or higher so that the concentration of free H.sup.+ ions in the
solution is kept lower than 0.1 mM. H.sup.+ ions, like K.sup.+
ions, can migrate across barrier 14 into generation chamber 12. If
such H.sup.+ migration occurs in significant amounts, the direct
linear relationship between the applied current and the
concentration of KOH generated can be lost because H.sup.+ ions can
be combined with OH.sup.- ions generated at the cathode to form
water and thus the performance of the system can be compromised. By
using the K.sub.2HPO.sub.4 salt, the following reaction occurs
using H.sup.+ generated at anode 16 in equation (1) above.
2H.sup.++2HPO.sub.4.sup.2-=2H.sub.2PO.sub.4.sup.- (3)
[0055] As in the embodiments of FIGS. 1 and 2, an aqueous stream is
pumped through the generation chamber at 12 and a DC voltage is
applied between anode 16 and cathode 18. K.sup.+ ions migrate from
reservoir 10 into generation chamber 12 through barrier 14 in the
same manner described above. Also, as set out above, barrier 14
provides a high-pressure physical barrier that prevents liquid
leakage and diffusion of any ions from reservoir 10 into generation
chamber 12.
[0056] One advantage of this embodiment in which a solution without
resin is used in reservoir 10 is that the potassium salt (e.g.,
K.sub.2HPO.sub.4) is a less expensive source of K.sup.+ ion than
ion exchange resin with exchangeable K.sup.+ ions. Also, it is
easier to replenish the reservoir with a fresh source of potassium
salt. By way of example, in the embodiment of FIG. 3 using a one
liter reservoir filled with 2.0 M K.sub.2HPO.sub.4 as a theoretical
capacity of 4,000 meq K.sup.+ ions to generate 20 mM KOH at 1.0
mL/min, the device will have a useful lifetime of 2500 hours,
assuming a 75 percent consumption of K.sup.+ ions in its K.sup.+
ion supply reservoir before replacing the salt solution.
[0057] FIG. 4 illustrates a flow-through strongly acidic cation
exchange resin bed 30 in K.sup.+ form disposed in reservoir 10.
Anode 12 is suitably in the form of a perforated platinum electrode
at its outlet and adjacent an outlet port, not shown. Generation
chamber 12 is separated from reservoir 10 by barrier 14 of the type
described above. In this instance, cation solution in the form of
the potassium salt (e.g., 2.0 M K.sub.2HPO.sub.4) is continuously
pumped by a pump 34 to a reservoir 10 at a desired rate (e.g. about
0.1 to 2.0 mL/min). The same principles described above with
respect to concentration of the potassium salt and the type of salt
applied to this embodiment as well. Similarly, the same flows and
reactions occur in generator 12.
[0058] Continuous pumping of the potassium salt solution leads to a
continuous supply of K.sup.+ ions until the solution of salt in the
remote reservoir is consumed.
[0059] In one embodiment illustrated in FIG. 4, the potassium salt
solution is recycled in recycle line 36 from the outlet of
reservoir 10 to the inlet of remote reservoir 32. The system can be
operated until the concentration of K.sup.+ ions in remote
reservoir 32 has been decreased to a level insufficient to
consistently generate KOH at the desired concentration. Then the
device can be replenished by replacing the potassium salt solution
in the remote reservoir 32. Alternatively, in the non-recycle mode,
the solution exiting reservoir 10 flows to waste as illustrated by
dotted line 38. The flow rate of the potassium salt solution can be
slightly adjusted (e.g., about 0.005 to 0.050 mL/min) to provide a
sufficient supply of K.sup.+ ions to generate KOH at the desired
concentration. Similarly, the device is replenished by filling the
remote reservoir with potassium salt solution when the
concentration has dropped below the desired level.
[0060] In another embodiment of the invention, not shown, ion
exchange resin 30 may be eliminated from reservoir 10 so reservoir
10 is filled with salt solution flowing from a remote reservoir 32.
Otherwise the system is identical to the one described above.
[0061] Referring to FIG. 5, another embodiment of the invention is
illustrated including multiple generation chambers 12a, 12b, and
12c connected in series, each one including its own cathodes 18a,
18b, and 18c. Generation chambers 12a, 12b, and 12c are connected
to reservoir 10 by barriers 14a, 14b, and 14c as described above.
The difference is that there are smaller generation chambers and
smaller barriers. By way of example, if each generation chamber is
applied with a current of 80 mA to generate 25 mM of KOH at 2.0
mL/min the KOH generator with three generation chambers is capable
of producing about 75 mM of KOH at 2.0 mL/min. Additional KOH
generation chambers may also be employed. An advantage of using two
or more generation chambers is that the operating voltage of the
system may be lowered because the applied current used to generate
KOHs distributed among the generation chambers. Thus higher
currents may be applied to generate the base of higher
concentrations without being limited by potentially excessive
heating.
[0062] In another embodiment, not shown, two or more cathodes may
be disposed in a generation chamber 12, preferably spaced along the
length of the chamber in the direction of aqueous liquid flow, e.g.
near the inlet and outlet. This can serve to lower the electrical
resistance of the chamber and thus the operating voltage of the
system.
[0063] Referring to FIG. 6, another embodiment of the invention is
illustrated using a single generation chamber 12 and two barriers
14a and 14b interconnecting chamber 12 and reservoir 10. Use of
multiple barriers can reduce the device operating voltage.
Therefore the generation chamber 12 can be supplied with higher
currents to generate KOH at higher concentrations without being
limited by potentially excessive heating. Another advantage in the
use of multiple barriers is that flexible membranes of smaller
areas have better resistance to bursting than larger area
membranes.
[0064] Referring to FIG. 7, use of the KOH generator of the present
invention is schematically illustrated on-line in an ion
chromatography or liquid chromatography system. Water from source
40 is pumped by pump 42 through the generation chamber of the large
capacity KOH generator 44 with an anode in the cation source
reservoir and a cathode in the generation chamber connected to a
power supply 45, as described above. Generator 44 is on-line with a
conventional simplified ion chromatography system. Pump 42 is a
conventional chromatography pump which pumps the KOH output from
generator 44 through sample injection valve 48 into chromatographic
separator 50 packed with a chromatographic separation medium,
typically an ion exchange resin packed bed column. Alternatively,
other forms of separation medium may be used such as porous
hydrophobic chromatographic resin with essentially no permanently
attached ion exchange sites.
[0065] In ion chromatography, the effluent from the separation
column 50 flows through suppressor 52 serving to suppress the
conductivity of the base and the effluent from separator 50, but
not the conductivity of the ions injected through sample injector
48. Then, the effluent from suppressor 52 is directed through a
flow through detector 54, e.g. a conductivity detector, for
detecting the resolved ions in the effluent from suppressor 52. A
suitable data system, not shown as provided in the form of a
conventional conductivity detector for measuring the suppressor
effluent in the conductivity cell in which the presence of an ionic
species produces an electrical signal proportional to its
concentration. With the exception of generator 44, such ion
chromatography systems are well known as illustrated in U.S. Pat.
Nos. 3,897,213; 3,920,397; 3,925,019; and 3,956,559 incorporated
herein by reference.
[0066] Other forms of detectors 54 may also be employed and the
suppressor may be eliminated. Such other forms of detection include
UV, fluorescence and electrochemical.
[0067] In the large capacity 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 downstream chromatography
process. The potential problem of hydrogen gas can be eliminated by
application of Boyle's law. A flow restrictor can be placed after
the detector flow cell to elevate the pressure of the entire
chromatography system. Under high pressure (e.g., 1000 psi or
higher pressures), hydrogen gas is compressed to an insignificant
volume compared to the eluent flow so that it does not interfere
with the downstream chromatography process. This approach requires
the use of a detector flow cell capable of withstanding a pressure
of 1000 psi or more. In an ion chromatography system using
suppressed conductivity detection, the above approach also requires
the use of a suppressor that is capable of withstanding a pressure
of 1000 psi or more. The necessary pressure to accomplish this
depends on the volume of gasses produced. However, for a typical
system, a pressure of at least 250 to 500 psi is sufficient. One
mode of elevating the pressure is to connect a flow restrictor 56
such as a fine bore coiled tubing downstream of the detector (e.g.
three meters of 0.005 in I.D.). This elevates the pressure
throughout the chromatography system upstream of the detector.
[0068] Another approach to eliminate the potential problem
associated with hydrogen gas is to use an on-line pressure gas
removal device to remove hydrogen gas from the KOH solution. FIG. 8
illustrates a schematic outline of an ion chromatography system
employing a large capacity KOH generator and an on-line high
pressure gas removal device 60 instead of flow restrictor 56 in
FIG. 7. In this implementation, a high pressure gas removal device
60 is placed downstream of the outlet of the large capacity KOH
generator 44, suitably between it and sample injector 48. 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. One advantage
of this system is that a conventional detector flow cell and ion
chromatography suppressor can be used.
[0069] One preferred embodiment of the on-line high pressure gas
removal device is shown in FIG. 9. In this embodiment, gas
permeable polymeric tubing 62 is used to remove hydrogen gas in the
KOH product solution under high pressure. Aqueous solution 67 flows
in an annular space 64 outside of the gas permeable tubing 62
defined between tubing 64 and protective tubing 66. The released
hydrogen gas is removed from the device by in the flowing aqueous
liquid stream in space 64 which also serves to prevent absorption
of carbon dioxide from the ambient air into the KOH product stream.
One source of the aqueous liquid in space 64 is the detector
effluent.
[0070] Preferably, the polymeric tubing 62 is inert and has high
burst pressure and high gas permeability. The inner volume of the
gas permeable tubing should be small so that it does not have large
dead volume and thus does not compromise the gradient performance
of the large capacity eluent generator. It is preferred to use a
gas permeable tubing with inside diameter less than 0.015 inch so
that the gas removal device has low dead volume and high burst
pressure.
[0071] The polymeric tubing prepared from a number of polymers
including polymethylpentene, polypropylene, and fluoropolymers such
as PTFE, ETFE, PFA, and FEP is gas permeable under high pressure
and may be used as the gas removal tubing for the eluent
generator.
[0072] The on-line high pressure gas removal device shown in FIG. 8
can also be used to remove oxygen gas generated along with the acid
solution in a large capacity acid generator.
[0073] In another embodiment of the invention, not shown, the
system of FIG. 7 can be used in gradient ion or liquid
chromatography where eluent components in addition to KOH are
required. A gradient pump, e.g. a Dionex GP-40 pump type, can be
used to deliver a prescribed mixture of one or more eluent
components from separate reservoirs to the high pressure KOH
generation column. The eluent is modified with KOH which is
generated on-line at the exit end of the KOH generation column. The
concentration of KOH in the final eluent delivered to the
separation column can be controlled by controlling the applied
current to the large capacity KOH generator. The gradient system
using the large capacity KOH generator is especially beneficial to
applications that require the use of highly pure base hydroxide
solution.
[0074] Referring to FIG. 10, another form of the present invention
is illustrated. Here reservoir 10 includes a solution of cation
salt solution (e.g. one liter of K.sub.2HPO.sub.4 at 2 M
concentration). Barrier 14 extends substantially along the entire
length of the mating side generation chamber 16 in open
communication with the interior of the chamber. Cathode 18 is in
the form of a perforated platinum cathode which extends along the
flow path of the aqueous stream through chamber 12 in direct
contact with beds of ion exchange resin 19 in K.sup.+ form on both
sides of cathode 18. Water flows through an inlet port, not shown,
on the upstream side of the chamber. The KOH produced in chamber 12
exits at an outlet port, not shown, at the downstream side of the
chamber. The perforated platinum cathode is in the form of a screen
suitably extending along the entire length of resin bed and is
perforated to permit passage of solution through the cathode to
ensure an efficient removal of KOH generated.
[0075] Another form of generation chamber 12 is illustrated in FIG.
11. This embodiment differs from that of FIG. 9 in the use of a
cation exchange screen 70 in contact with perforated cathode 18 on
one side and with barrier 14 on the other side. The electrical path
between anode 16 and cathode 18 extends through barrier 14, cation
exchange screen 10 and perforated cathode 18. The aqueous stream
flows through the chamber 12 inlet port, through perforated cathode
18 into cation exchange screen 70 where it flows adjacent to the
cathode and out the chamber 12 outlet on the downstream side of
screen 70.
[0076] In another embodiment of the generation chamber, not shown,
the only structural eluent within chamber 12 is cathode 18 in the
form of a perforated platinum electrode screen in direct contact
with barrier 14. The aqueous stream flows through the perforated
platinum cathode screen. The screen uses openings of a size
suitably on the order of 50-100 .mu.m to permit the flow of the
aqueous stream through the platinum screen without undue pressure
drops. A suitable screen has a size of 1 to 5 cm.sup.2.
[0077] Another embodiment of the base generation chamber design is
illustrated in FIG. 12. As in the embodiment of FIG. 10, barrier 14
extends along the entire length of chamber 12. In this instance,
the perforated platinum cathode 18 is sandwiched between
non-charged screens 72 and 74 suitably formed of a non-charged
polymer such as a polypropylene which forms the fluid pathway in
the generation chamber 12. Screens 72 and 74 may be of the same
size as the screen cathode in the embodiment of FIG. 11. An inert
lead, e.g. platinum wire 76, provides electrical contact with
platinum cathode 18 and in direct contact with barrier 14. Upon the
application of electrical current a small amount of KOH is formed
in situ. The KOH serves as the ion transport bridge between barrier
12 and platinum electrode 18. Screens 72 and 74 have sufficient
porosity to permit the flow of water through the screen without
undue pressure drop.
[0078] The system of FIG. 12 can be operated by first filling
chamber 12 with KOH solution prepared externally which serves as
the ion transport bridge between barrier 14 and cathode 18. Then
current is applied. Good contact between the perforated
disk-cathode 18 and barrier 14 may be maintained by pressing one
against the other. The electrode can extend across all or part of
the aqueous liquid flow path through the chamber 12 to permit
intimate contact with the flowing aqueous stream.
[0079] Other embodiments of the interior configuration of the base
generation chamber may be employed so long as there is sufficient
electrical path between the anode and the cathode to permit the
cations to transport across the barrier and with the aqueous stream
flowing through the chamber to permit the efficient generation of
KOH. It has been found that systems in which the cathode and a
barrier in the form of a charged membrane extends substantially
along the entire flow path of the aqueous stream through the base
generation chamber is very efficient.
[0080] The system has been described with respect to generating a
base and specifically KOH. However, the system is also applicable
to the generation of an acid by reversal of the polarity of the ion
exchange beds, barrier and the electrodes. In this instance, anion
exchange beds, rather than cation exchange beds are employed. Also
the barriers are of a type which pass anions but not cations and
block the flow of liquid. Suitable barriers for use in the
production of acid can be prepared from a single or multiple ion
exchange membrane of appropriate thickness or a block or rod of ion
exchange material. A suitable form of membrane is supplied by
Membrane International of Glen Rock, N.J. (designated AMI-7000
anion exchange membrane).
[0081] The cations or anions for use as the source in reservoir 10
must also be sufficiently water soluble in base or acid form to be
used at the desired concentrations. Suitable cations are metals,
preferably alkali metals such as sodium, potassium, lithium and
cesium. Known packing for high capacity ion exchange resin beds
provide such cations or anion for use in the embodiment where resin
is used as the source of cations or anions. Typically, the resin
support particles would be in the potassium or sodium form.
Potassium is a particularly effective exchangeable cation because
of its high conductance. Suitable other cations are tetramethyl
ammonium and tetraethyl ammonium. Analogously, suitable
exchangeable anions for cation analysis include chloride, sulfate
and methane sulfonate.
[0082] Using the concept described above, a large capacity acid
generator can also be implemented. For example, a large capacity
methanesulfonic acid (MSA) generator employing a
CH.sub.3SO.sub.3.sup.- ion supply reservoir is described here as an
example. MSA generation chamber 12 is packed with a strongly basic
anion exchange resin in CH.sub.3SO.sub.3.sup.- form and equipped
with a Pt screen electrode (anode) which is in direct contact with
the anion exchange resin. The MSA generation chamber 12 is
connected to the CH.sub.3SO.sub.3.sup.- ion supply reservoir 10
using one or more anion ion exchange barriers of the same general
type as barrier 14. Barrier 14 permits the passage of
CH.sub.3SO.sub.3.sup.- ions from the supply reservoir into the
resin bed in the MSA generation column, while precluding the
passage of cations from the CH.sub.3SO.sub.3.sup.- ion supply
reservoir into the MSA generation column. Barrier 14 also serves
the role of a high pressure physical barrier that insulates the low
pressure CH.sub.3SO.sub.3.sup.- ion supply compartment from the
high pressure MSA generation chamber 12.
[0083] Analogous to the cation-source reservoir, the anion-source
(CH.sub.3SO.sub.3.sup.-) reservoir 10 is equipped with a cathode
and a gas vent hole. The reservoir (1 to 2 liters in volume) is
filled with a solution of a MSA salt such as
NH.sub.4CH.sub.3SO.sub.3. The concentration of
CH.sub.3SO.sub.3.sup.- ions in the solution is preferably 1 to 2 M
or higher so that there is a sufficient amount of
CH.sub.3SO.sub.3.sup.- ions in the CH.sub.3SO.sub.3.sup.- ion
supply reservoir for the generation of MSA over an extended period
of time; however, the MSA salt solution containing
CH.sub.3SO.sub.3.sup.- ions at lower concentrations can be used. It
is preferred that the cation of the MSA salt used can not be
reduced by the cathode int he CH.sub.3SO.sub.3.sup.- ion supply
reservoir. It is also preferred to use a "weakly basic cation"
(e.g., NH.sub.4.sup.+) defined to have a base dissociation constant
(pK.sub.b) of 4.5 or higher so that the concentration of free
OH.sup.- ions in the solution is kept lower than 0.1 mM. A
"strongly basic cation" is defined to have a base dissociation
constant (pK.sub.b) of less than 4.5. OH.sup.- ions, like
CH.sub.3SO.sub.3.sup.- ions, can migrate across the anion exchange
connector into the MSA generation column. If OH.sup.- ions migrate
across the anion exchange connector into the MSA generation column
in significant amounts, the direct linear relationship between the
applied current and the concentration of MSA generated is lost
because OH.sup.- ions can combine with H.sup.+ ions generated at
the anode to form water, and thus the performance of the MSA
generator is compromised.
[0084] To operate the large capacity MSA system, deionized water is
pumped through the MSA generation chamber 12, and a DC voltage is
applied between the anode is and cathode 18. Under the applied
field, the electrolysis of water occurs at the anode and cathode.
Water is reduced to form OH.sup.- ions and hydrogen at the
cathode:
2H.sub.2O+2e.sup.-.fwdarw.2OH.sup.-+H.sub.2.Arrow-up bold. (4)
[0085] and oxidized to form H.sup.+ ions and oxygen at the
anode:
H.sub.2O+2e.sup.-.fwdarw.2H.sup.++1/2O.sub.2.Arrow-up bold. (5)
[0086] CH.sub.3SO.sub.3.sup.- ions migrate through barrier 14 into
the resin bed in the MSA generation chamber 12, and eventually
combine with H.sup.+ ions generated at the anode to produce a MSA
solution suitable for use as a high purity eluent for ion or liquid
chromatography.
[0087] The large capacity acid or base generator can also be
implemented to generate high purity ion pairing reagents such as
octanesulfonic acid (OSA) and tetrabutylammonium hydroxide (TBAOH)
for use as eluents in mobile phase ion chromatography (MPIC) or
reversed-phase ion pair chromatography (RPIPC).
[0088] Although much of the above discussion relates to use of the
generated base or acid in ion and liquid chromatography, such use
can also be applied to other areas such as titration, flow
injection analysis and post-column reactors.
[0089] Specifically the generated base can be used in combination
with (a) conventional titration analyses, e.g. described in Douglas
A. Skoog and Donald M. West, Fundamentals of Analytical Chemistry,
4th Edition, Saunders College Publishing, San Francisco, 1982,
Chapter 8 Theory of Neutralization, p. 195 or Douglas A. Skoog,
Principles of Instrumental Analysis, 3rd Edition, Saunders College
Publishing, San Francisco, 1985, Chapter 20 Potentiometric Methods,
p. 638; (b) flow injection analysis, e.g., described in Theory and
Automation, Skoog, Chapter 29, p. 858-859; and (c) post-column
reactors, e.g. described in Paul R. Haddad and Peter E. Jackson,
Ion Chromatography, Elsevier, N.Y., 1988, p. 387 and R. W. Frei
Editor and K. Zech, Selective Sample Handling and Detection in
High-Performance Liquid Chromatography, Elsevier, N.Y., 1988, p.
396.
[0090] The following examples are provided in order to further
illustrate the present invention.
EXAMPLE 1
Generation of KOH Using a KOH Generator Employing a Large Capacity
K.sup.+ Ion Supply Reservoir (as Illustrated in FIG. 2)
[0091] A large capacity KOH generator consisting of a K.sup.+ ion
supply reservoir 10 in the form of column (18-mm ID.times.185-mm
length) and a KOH generation chamber in the form of column 12 (4-mm
ID.times.30-mm length) was constructed.
[0092] The KOH generation chamber was packed with an 18-.mu.m, 8%
cross-link sulfonated styrene/divinyl benzene resin in K.sup.+
form. The K.sup.+ ion supply column consisted of a 175 mm length
bed of an 18 .mu.m, 8% cross-link sulfonated styrene/divinyl
benzene resin in K.sup.+ form and a 10 mm length bed of a 50 .mu.m
polyacrylate resin in K.sup.+ form. The device was tested under an
applied current of 30 mA and a carrier flow rate of 1.0 mL/min for
48 hours. The conductance of the KOH solution generated and the
operating voltage of the KOH generator were monitored over the
testing period. The exhaustion profile (the conductance of the KOH
solution generated vs. time) and the operating voltage data are
shown on FIG. 13. The device produced a constant output of KOH
(18.7 mM KOH at the carrier flow rate of 1.0 mL/min) for 44.4
hours, or a useful capacity of 49.7 meq. After 44.4 hours of
operation, the operating voltage increased to 275 V (the operating
voltage limit of the power supply used in the experiment) due to
the development of a less conductive neutralized zone in the weakly
acidic carboxylated resin bed inside the K.sup.+ ion supply column,
and decreases in the operating current and concentration of KOH
generated were observed. These results indicate the feasibility of
using the large capacity KOH generator employing the large K.sup.+
ion supply column to generate the KOH solution over an extended
period of time.
EXAMPLE 2
Generation of KOH Using a Large Capacity KOH Generator Employing a
Flow-Through K.sup.+ Ions Supply Column (as Illustrated in FIG.
4)
[0093] A large capacity KOH generator employing the flow-through
K.sup.+ ion supply column was constructed to evaluate this
embodiment of the invention (FIG. 4). Both the flow-through K.sup.+
ion source reservoir 10 in the form of column (4-mm ID.times.25-mm
length) and the KOH generation chamber (4-mm ID.times.25-mm length)
were packed with an 18 .mu.m, 8% cross-link sulfonated
styrene/divinyl benzene resin in K.sup.+ form and equipped with
porous Pt frit electrodes at their outlets. A 100-mM KCI solution
in a remote reservoir was pumped continuously through the
flow-through K.sup.+ ion supply column at a flow rate of 1.0
mL/min. The large capacity KOH generator was tested under applied
currents of 10.5, 21, and 30.5 mA for about 23 hours. The operating
voltage ranged from 40 to 60 V during the experiment. FIG. 14 shows
the conductance profiles of the KOH solutions generated at a
carrier flow rate of 1.0 mL/min and applied currents of 10.5, 21,
and 30.5 mA. The concentration of KOH generated was directly
proportional to the applied current. The results indicate that it
is feasible to use a large capacity KOH generator employing a
flow-through K.sup.+ ion supply column to generate the KOH solution
over an extended period of time.
EXAMPLE 3
Generation of KOH Using a Large Capacity Generator Employing a
K.sup.+ Ion Supply Reservoir (as Illustrated in FIG. 3)
[0094] A large capacity KOH generator employing a K.sup.+ ion
source reservoir 10 was constructed to evaluate this preferred
embodiment of the invention (FIG. 3). The KOH generation chamber
(5.2-mm ID.times.37-mm length) was packed with an 18-.mu.tm, 8%
cross-link sulfonated styrene/divinyl benzene resin in K.sup.+ form
and equipped with a porous Pt frit electrode at its outlet. The
K.sup.+ ion source reservoir 10 was filled with a 2.0 M
K.sub.2HPO.sub.4 solution. The large capacity KOH generator was
operated continuously under a constant current of 30 mA and a
carrier flow rate of 1.0 mL/min for a total of 832 hours. The
operating voltage was about 60 V during the test. The KOH solutions
generated using the device were periodically collected and titrated
using a 10-mM nitric acid standard to determine the concentration
of KOH generated. FIG. 15 shows the determined concentration of KOH
in the solutions collected. Over the period of 744 hours, the
average determined KOH concentration was 17.7 mM (n=18 and
RSD=2.2%), corresponding to 95% of the theoretical concentration of
18.7 mM. The results indicate that it is feasible to use a large
capacity KOH generator employing a large capacity K.sup.+ ion
supply reservoir to generate the KOH solution over an extended
period of time.
EXAMPLE 4
Generation of KOH Using a Large Capacity Generator Employing a
K.sup.+ Ion Supply Reservoir and Three KOH Generation Chambers (as
Illustrated in FIG. 5)
[0095] A large capacity KOH generator employing a K.sup.+ ion
supply reservoir and three KOH generation chambers, as illustrated
in FIG. 5, was constructed. Each KOH generation chamber (5.2-mm
ID.times.10-mm length) was packed with an 18-.mu.m, 8% cross-link
sulfonated styrene/divinyl benzene resin in K.sup.+ form and
equipped with a porous Pt frit electrode at its outlet. The K.sup.+
ion supply reservoir was filled with a 2.0 M K.sub.2HPO.sub.4
solution. The large capacity KOH generator was used to generate KOH
solutions under applied currents ranging from 10 to 160 mA and
carrier flow rates of 1.0 or 2.0 mL/min. The operating voltage for
the KOH generator was 45 V when an applied current of 160 mA was
maintained to generate 50 mM KOH at 2.0 mL/minute.
[0096] The concentrations of KOH generated at different applied
currents using the KOH generator were determined by titration using
a 10-mM nitric acid standard. The results are summarized in Table
1. In this KOH generator, the KOH solution generated in the first
KOH generation chamber flows through the second and third KOH
generation chambers. The presence of KOH solution in the second and
third KOH generation chambers did not affect the KOH generation in
the second and third chamber. The percent electrolytic yield of
this KOH generator was very close to the theoretical limit, ranging
from 96.8 percent at 10 mA to 99.0 percent at 100 mA, as shown in
Table 1. There was also excellent correlation (R.sup.2=0.9998)
between the applied current and the determined concentration of KOH
generated (FIG. 16).
1TABLE 1 Calculated and Determined Concentrations of KOH Generated
Using a Large Capacity KOH Generator with Three KOH Generation
Chambers Calculated Determined Applied Flow rate, Concentration,
Concentration.sup.a, mM Percent Yield.sup.b Percent RSD Current
mL/min mM (n = 3) (n = 3) (n = 3) 10 mA 2.0 3.1 3.0 96.8 0.2 50 mA
2.0 15.5 15.1 97.4 0.4 100 mA 2.0 31.1 30.8 99.0 0.9 100 mA 1.0
62.2 61.2 98.4 0.5 30 mA + 10 mM 2.0 19.3 19.2 98.9 0.9 Na0H 60 mA
+ 10 mM 2.0 28.7 28.4 98.4 1.3 NaOH .sup.aThe number of
determinations was three. .sup.bPercent yield was calculated using
the following definition: Percent yield = (Determined concentration
- Calculated concentration)/Calculated concentration * 100
[0097] The above results indicate that connecting multiple KOH
generation chambers in series is a viable approach to boost the
concentration of KOH generated. The results also demonstrate that
KOH at relatively high concentrations can be accurately generated
using a large capacity KOH generator with multiple KOH generation
chambers without being limited by excessive heating.
EXAMPLE 5
Evaluation of a Large Capacity KOH Generator Employing a KOH
Generation Chamber with Multiple Ion Exchange Connectors (as
Illustrated in FIG. 6)
[0098] A large capacity KOH generator employing a K.sup.+ ion
source reservoir and a KOH generation chamber in the form of column
with two multiple ion exchange connectors, as illustrated in FIG.
7, was constructed. The K.sup.+ ion supply reservoir was filled
with a 2.0 M K.sub.2HPO.sub.4 solution. The KOH generation chamber
12 in the form of column ((5.2-mm ID).times.10-mm length) was
packed with an 18-.mu.m, 8% cross-link sulfonated styrene/divinyl
benzene resin in K.sup.+ form and equipped with a porous Pt frit
electrode at its outlet. The KOH generation column was connected to
the K.sup.+ ion supply reservoir using either one or two ion
exchange connectors (each with a 5 mm in contact diameter) during
the experiment. The applied current was varied from 10 to 90 mA and
the operating voltage was monitored. The carrier flow rate was
maintained at 2.0 mL/minute.
[0099] The dependence of the operating voltage on the applied
current determined for the KOH generator is shown in FIG. 17. For a
given applied current, the operating voltages required for the
generator using two ion exchange connectors were about 30 percent
lower than those required for the generator using one ion exchange
connector. The use of multiple ion exchange connectors in a single
KOH generation column clearly increases the pathway for the
transport of K.sup.+ ions from the K.sup.+ ion supply reservoir
into the KOH generation column and thus reduces the device
operating voltage. The results suggest that the use of multiple ion
exchange connectors in a single KOH generation column is a viable
approach to facilitate the generation of KOH at relatively high
concentrations.
EXAMPLE 6
Evaluation of Different Cathode Configurations for the Large
Capacity KOH Generator
[0100] A large capacity KOH generator employing a K.sup.+ ion
source reservoir, as illustrated in FIG. 3, was constructed. The
K.sup.+ ion supply reservoir was filled with a 2.0 M
K.sub.2HPO.sub.4 solution. The KOH generation chamber in the form
of column (5.2-mm ID.times.10-mm length) was packed with an 18
.mu.m, 8% cross-link sulfonated styrene/divinyl benzene resin in
K.sup.+ form. The KOH generation column was connected to the
K.sup.+ ion supply reservoir using one ion exchange connector (5 mm
in contact diameter). Three cathode configurations were tested for
the KOH generation column: one porous Pt frit (4 mm diameter)
placed at the outlet of the generation column, two porous Pt frits
(4 mm diameter) placed at the inlet and outlet of the generation
column, and a Pt screen that is formed to wrap around the resin bed
in the KOH generation column. The applied current was varied from
1.0 to 70 mA and the operating voltage was monitored. The carrier
flow rate was maintained at 2.0 mL/minute.
[0101] The dependence of the operating voltage on the applied
current determined for the KOH generator operated in three cathode
configurations is shown in FIG. 18. At an applied current of 60 mA,
the operating voltage was 45 V when one porous Pt frit was used as
the cathode, 40 V when two porous Pt frits were used as the
cathodes, and 29 V when the cathode was made of a Pt screen formed
to wrap around the resin bed. The results indicate that the
operating voltage of the KOH generator can be decreased
significantly by increasing the contact area between the ion
exchange resin and the electrode, so that KOH at relatively high
concentrations can be generated without being limited by excessive
heating.
EXAMPLE 7
On-Line High Pressure Removal of Hydrogen Gas
[0102] An on-line high pressure gas permeable removal device was
constructed according to the design shown in FIG. 9. A polymeric
tubing (0.020-inch OD.times.0.010-inch ID.times.1.0 meter length)
obtained from Biogeneral Inc. (San Diego, Calif.) was used as the
gas permeable tubing in the device. The device was tested for
removing hydrogen gas in the KOH solution generated at applied
currents up to 160 mA using the large capacity KOH generator
described in Example 4. The carrier flow rate for the generator was
2.0 mL/minute. In some experiments, the outlet of the device was
connected to a piece of 0.005-inch ID PEEK tubing that generated a
pressure drop of 1400 psi at 2.0 mL/min; the PEEK tubing outlet was
immersed in the deionized water in a small, clear glass vial, and
the presence of hydrogen gas in the KOH solution was visually
monitored (by observing the formation of gas bubbles). In some
experiments, the KOH generator and gas removal device were
installed in an ion chromatography system as shown in FIG. 10, the
baseline noise of the conductivity detector was monitored, and the
flow of chromatography system effluent was used to shield the
outside of the gas permeable tubing to remove the released hydrogen
gas and prevent the readsorption of carbon dioxide from the ambient
air, as shown in FIG. 9.
[0103] The on-line high pressure gas removal device was highly
effective in removing the hydrogen gas. No hydrogen gas bubbles
could be visually observed in the KOH solution generated at applied
currents up to 160 mA. FIG. 19 shows the baseline peak-to-peak
noises measured at different currents obtained using the device;
they are similar to those obtained with the conventional ion
chromatography system. At the applied current of 160 mA, hydrogen
gas is generated at a rate of about 1.1 mL/min (gas volume at 14.7
psi). Therefore, the gas removal efficiency of the device was quite
remarkable, especially considering the fact that the length of
tubing used was only 1.0 meter and its internal volume was only 51
mL.
EXAMPLE 8
Use of a Large Capacity KOH Generator in Isocratic and Gradient
Separation of Common Anions by Ion Chromatography
[0104] An ion chromatography system consisting of a large capacity
KOH generator, an on-line high pressure gas removal device, and
common Dionex ion chromatography system components was assembled as
shown in FIG. 10. The large capacity KOH generator used was similar
to the one described in Example 3. The on-line high pressure gas
removal device described in Example 7 was used. A Dionex AS 11
column (4-mm ID.times.250-mm length) was used as the analytical
separation column. In isocratic separation experiments, the large
capacity KOH generator was applied with a constant current of 40 mA
to generate 12.4 mM KOH at 2.0 mL/minute. In gradient separation
experiments, the current applied to the large capacity KOH
generator was changed from 2.0 to 50 mA in steps of 0.5 mA per 20
seconds to generate a gradient of KOH from 0.6 to 15.5 mM at 2.0
mL/minute.
[0105] FIGS. 20 and 21 show, respectively, the representative
isocratic and gradient separation of fluoride, chloride, nitrate,
sulfate, and phosphate. FIG. 22 shows the reproducible overlay of
16 consecutive KOH gradients generated using the large capacity KOH
generator. It is worthy to point out that the chromatographic
baseline shift during the KOH gradient was less than 50 nS in the
chromatogram shown in FIG. 21. If the same hydroxide gradient is
generated using a conventional gradient pump, the baseline shift is
usually about 500 to 1500 nS. These results demonstrate that the
high purity KOH solutions can be generated reproducibly using the
large capacity KOH generator, and used effectively as eluents in
ion chromatography. The results also suggest that the performance
of an ion chromatography method can be enhanced because the use of
high purity hydroxide solution generated on-line results in minimal
baseline shifts during gradient separation, as illustrated in the
next example.
EXAMPLE 9
Use of a Large Capacity KOH Generator in Determination of Trace
Anions in High Purity Water by Ion Chromatography
[0106] Dionex Application Note 113 describes a method for
determination of trace anions in high purity waters. In this
method, the large volume direction injection technique is used
(sample loop is 750 .mu.L), target anions are separated on a Dionex
microbore AS 11 column (2-mm ID.times.250-mm length) using a NaOH
gradient. FIG. 23 shows the typical chromatogram obtained when the
NaOH gradient (0.5 to 26 mM NaOH) was generated using a gradient
pump and NaOH solutions prepared by conventional means. The
baseline shift is about 500 nS during the gradient. The baseline
shift occurs because NaOH solutions are easily contaminated with
carbon dioxide in the ambient air during the solution preparation
and use, even with precautions.
[0107] To demonstrate the benefits of using high purity KOH eluent
generated by the large capacity KOH generator, an ion
chromatography system similar to the one used in Example 8 was
assembled. A Dionex microbore AS-11 column was used as the
analytical separation column. The current applied to the large
capacity KOH generator was changed from 0.4 to 21 mA in steps of
0.4 mA per 17 seconds to generate a gradient of KOH from 0.5 to 26
mM at 0.5 mL/minute.
[0108] FIG. 24 shows a representative chromatogram obtained for a
sample of deionized water spiked with 10 anions at levels of 0.9 to
3.0 ppb. Since the KOH solution generated with the large capacity
KOH generator was essentially free of carbonate contamination, the
observed baseline shift was less than 80 nS during the gradient.
The significantly smaller baseline shift during the gradient
achieved using the KOH generator leads to improvements in the
method performance. These results suggest that the performance of
an ion chromatography method can be enhanced by using a large
capacity KOH generator.
EXAMPLE 10
Generation of Methanesulfonic Acid (MSA) Using a Large Capacity MSA
Generator Employing a Large Capacity CH.sub.3SO.sub.3.sup.- Ion
Supply Reservoir
[0109] A large capacity MSA generator employing a
CH.sub.3SO.sub.3.sup.- ion supply reservoir was constructed to
evaluate this preferred embodiment of the invention. The MSA
generation column (7-mm ID.times.10-mm length) was packed with a
20-.mu.m, 8% cross-link strongly basic (quaternary amine functional
groups) styrene/divinyl benzene resin in CH.sub.3SO.sub.3.sup.-
form and equipped with a Pt screen cathode. The
CH.sub.3SO.sub.3.sup.- ion supply reservoir was filled with a 2.0 M
NH.sub.4CH.sub.3SO.sub.3 solution. The large capacity MSA generator
was used to generate MSA solutions at applied currents ranging from
10 to 100 mA and a carrier flow rate of 1.0 or 2.0 mL/min. The
operating voltage for the large capacity MSA generator was 9.5 V at
10 mA, 30 V at 50 mA, and 38.5 V at 100 mA. The concentrations of
MSA generated at 10, 40, and 80 mA were determined by titration
using a 10-mM NaOH standard. FIG. 25 shows that there was excellent
correlation (R.sup.2=0.9997) between the applied current and the
determined concentration of MSA generated. In some experiments, the
current applied to the large capacity MSA generator was changed
from 28.5 mA to 70 mA in steps of 1.0 mA per 5 seconds to generate
a gradient of MSA from 17.7 mM to 43.5 mM at 1.0 mL/min. FIG. 26
shows the reproducible overlay of 16 consecutive MSA gradients
generated using the large capacity MSA generator. These results
indicate that the large capacity MSA generator can be used to
generate MSA at desired concentrations accurately and
reproducibly.
EXAMPLE 11
Use of the Large Capacity MSA Generator in the Separation of
Cations by Ion Chromatography
[0110] An ion chromatography system consisting of a large capacity
MSA generator, an on-line high pressure gas removal device, and
common Dionex ion chromatography system components was assembled.
The large capacity MSA generator described in Example 10 was used.
The on-line high pressure gas removal device described in Example 7
was used. A Dionex CS 12A column (4-mm ID.times.250-mm length) was
used as the analytical separation column. The current applied to
the large capacity MSA generator was changed from 28.5 mA to 70 mA
in steps of 1.0 mA per 5 seconds to generate a gradient of MSA from
17.7 mM to 43.5 mM at 1.0 mL/min. In some experiments, MSA
gradients from 17.7 mM to 43.5 mM at 1.0 mL/min were generated by
using a Dionex GP40 gradient pump with deionized water and a 100 mM
MSA solution prepared from reagent grade MSA.
[0111] FIG. 27 shows a representative gradient separation of 10
cations using the MSA gradient generated using the large capacity
MSA generator. FIG. 28 shows the overlay of two representative
chromatograms obtained for a high purity water sample spiked with
10 cations at sub to low .mu.g/L levels, using identical MSA
gradients generated with either the large capacity MSA generator or
the GP40 gradient pump. The results show that the MSA generator
gradient yielded lower detector background and smaller baseline
shift during the gradient than the GP40 pump gradient. These
improvements can be attributed to the fact that the MSA solution
generated using the large capacity MSA generated is of high purity
and free of contaminants that may be present in the reagent grade
MSA.
[0112] The results also show that the elution of calcium,
strontium, and barium were delayed about one minute in the
chromatogram obtained using the GP40 pump gradient when compared to
the chromatogram obtained using the MSA generator gradient. In the
ion chromatography system employing the large capacity MSA
generator and the on-line high pressure gas removal device, the
total dead volume of the two device was less than 0.1 mL. On the
other hand, the GP40 gradient pump used had a total dead volume
(consisted of dead volumes in proportioning valves and pump heads)
of about 1.0 mL. FIG. 29 shows the comparison of MSA gradients
generated using the large capacity MSA generator and the GP40
gradient pump. The results show that the profile of the MSA
generator gradient had minimal delay in the MSA gradient while
noticeable gradient delay was observed when the GP40 gradient pump
was used.
* * * * *