U.S. patent application number 10/886876 was filed with the patent office on 2004-12-23 for disposable working electrode for an electrochemical cell.
Invention is credited to Avdalovic, Nebojsa, Cheng, Jun, Jandik, Petr.
Application Number | 20040256216 10/886876 |
Document ID | / |
Family ID | 26765837 |
Filed Date | 2004-12-23 |
United States Patent
Application |
20040256216 |
Kind Code |
A1 |
Cheng, Jun ; et al. |
December 23, 2004 |
Disposable working electrode for an electrochemical cell
Abstract
A flow-through electrochemical cell assembly with a disposable
working electrode structure, including (a) a perimeter wall
defining a sample flow channel including an inlet and an outlet,
(b) a sample inlet line in fluid communication with the sample flow
channel inlet, (c) a sample outlet line providing fluid
communication between the sample flow channel outlet and a remote
reference electrode, and (d) a disposable working electrode
structure comprising an electrically conductive and
electrochemically active working electrode region bound as a layer,
directly or indirectly, to an electrically insulating substrate
surface. The substrate surface is in fluid-sealing relationship
with the sample flow channel, and the working electrode region is
in fluid communication with said sample flow channel. The working
electrode is vapor deposited, directly or indirectly, onto the
organic polymer substrate through a mask, and a fluid seal is
formed between said working electrode region and perimeter
wall.
Inventors: |
Cheng, Jun; (San Jose,
CA) ; Jandik, Petr; (Los Gatos, CA) ;
Avdalovic, Nebojsa; (Cupertino, CA) |
Correspondence
Address: |
DORSEY & WHITNEY LLP
INTELLECTUAL PROPERTY DEPARTMENT
4 EMBARCADERO CENTER
SUITE 3400
SAN FRANCISCO
CA
94111
US
|
Family ID: |
26765837 |
Appl. No.: |
10/886876 |
Filed: |
July 7, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10886876 |
Jul 7, 2004 |
|
|
|
10081691 |
Feb 20, 2002 |
|
|
|
6783645 |
|
|
|
|
60342137 |
Dec 18, 2001 |
|
|
|
Current U.S.
Class: |
204/192.1 ;
204/192.12; 204/275.1; 204/298.02; 427/248.1 |
Current CPC
Class: |
H01M 4/9058 20130101;
H01M 4/8867 20130101; Y10T 29/49002 20150115; Y02E 60/50 20130101;
H01M 4/86 20130101; G01N 27/307 20130101 |
Class at
Publication: |
204/192.1 ;
204/275.1; 427/248.1; 204/192.12; 204/298.02 |
International
Class: |
C23C 014/00; C23C
014/32; C23C 016/00 |
Claims
1-15. Cancelled.
16. A method for making a disposable working electrode structure
and a sample flow channel for use in an electrochemical cell
assembly, said method comprising: (a) vapor depositing electrically
conductive and electrochemically active material, directly or
indirectly, onto an organic polymer substrate through a mask to
form a pattern of a working electrode region, and (b) forming a
fluid seal between said working electrode region and a perimeter
wall to define said fluid sample flow channel with said working
electrode region in direct fluid contact with said fluid sample
flow channel.
17. The method of claim 16 in which said vapor deposition is
through said mask which mask forms a pattern of an electrically
conductive lead interconnecting said working electrode and an
electrically conductive contact region forming said disposable
working electrode structure.
18. The method of claim 16 further comprising, before step (a),
vapor depositing an adhesion layer onto said organic polymer
substrate through a mask, wherein step (a) is performed by vapor
depositing said electrically conductive material and
electrochemically active onto said adhesion layer.
19. The method of claim 18 in which said adhesion layer is formed
of a material selected from the group consisting of titanium,
tungsten, chromium, and alloys thereof.
20. The method of claim 16 in which said organic polymer is
selected from the group consisting of polyester, polycarbonate,
polyolefin, polyimide and polyetherimide.
21. The method of claim 16 in which said vapor depositing step
includes forming said pattern of said working electrode region to
be approximately 100 .ANG. to 10,000 .ANG. thick.
22. The method of claim 16 in which said working electrode region
is bound to said substrate by an intermediate adhesion layer.
23. The method of claim 22 in which said intermediate adhesion
layer is approximately 50 .ANG. to 5000 .ANG. thick.
24. The method of claim 16 in which at least a surface portion of
said substrate is exposed to said sample flow channel.
25. A method for making a flow-through electrical cell assembly
comprising: vapor depositing electrically conductive and
electrochemically active material, directly or indirectly, onto an
organic polymer substrate through a mask to form a pattern of a
working electrode region; providing a reference electrode including
a wall having an inlet and an outlet spaced therefrom; mounting a
sealing member to said wall to define a sample flow channel fluidly
coupling said inlet and said outlet; and positioning said
substrate, and said working electrode region thereon, on said
sealing member such that said working electrode region is in fluid
communication with said sample flow channel.
26. The method of claim 25 in which said vapor deposition through
said mask forms a pattern of an electrically conductive lead
interconnecting said working electrode and an electrically
conductive contact region forming said disposable working electrode
structure.
27. The method of claim 25 further comprising vapor depositing an
adhesion layer onto said organic polymer substrate through a mask,
wherein said vapor depositing step is performed by vapor depositing
said electrically conductive material and electrochemically active
onto said adhesion layer.
28. The method of claim 27 in which said adhesion layer is formed
of a material selected from the group consisting of titanium,
tungsten, chromium, and alloys thereof.
29. The method of claim 25 in which said organic polymer is
selected from the group consisting of polyester, polycarbonate,
polyolefin, polyimide and polyetherimide.
30. The method of claim 25 in which said vapor depositing step
includes forming said pattern of said working electrode region to
be approximately 100 .ANG. to 10,000 .ANG. thick.
31. The method of claim 25 in which said working electrode region
is bound to said substrate by an intermediate adhesion layer.
32. The method of claim 31 in which said intermediate adhesion
layer is approximately 50 .ANG. to 5000 .ANG. thick.
33. The method of claim 25 in which at least a surface portion of
said substrate is exposed to said sample flow channel.
Description
BACKGROUND OF THE INVENTION
[0001] Flow-through electrochemical cells are used as detectors for
a variety of separation systems including chromatographic and ion
chromatographic systems. Dionex Corporation sells such
electrochemical cells under the trademarks ED40 and ED50 cells.
Such cells include an amperometric working electrode in the form of
a cylindrical wire embedded into a plastic block with the tip of
the wire exposed to a sample flow-through channel, typically
enclosed by a plastic gasket held in place under compression. These
working electrodes are somewhat complicated and expensive to
manufacture. After a period of use, the electrode must be replaced
or reconditioned by laborious polishing or other methods which can
lead to a lack of reproducibility of the detector output.
[0002] Thin film disposable electrodes have been used as in vitro
test electrodes and as in vivo implantable monitoring electrodes in
a variety of applications. See, for example, Michel, et al. U.S.
Pat. No. 5,694,932; Dahl, et al. U.S. Pat. No. 5,554,178; Saban, et
al. U.S. Pat. No. 6,110,354; Krause, et al. U.S. Pat. No.
4,710,403; Grill, Jr., et al. U.S. Pat. No. 5,324,322; Kurnik, et
al. U.S. Pat. No. 5,989,409; Diebold, et al. U.S. Pat. No.
5,437,999; Kuennecke, et al. WO 99/36786; Bozon, et al.,
Electroanalysis 13:911-916 (2001); Soper, et al., Analytical
Chemistry 72:642A-651 A (2000); Lindner, et al., Analytical
Chemistry 72:336A-345A (2000); Bagel, et al., Analytical Chemistry
69:4688-4694 (1997); Madaras, et al., Analytical Chemistry
68:3832-3839 (1996); and Marsouk, et al., Analytical Chemistry
69:2646-2652 (1997). However, none of the disposable electrodes
described in these references are suggested for use in a
flow-through electrochemical cell. Such cells have unique
requirements such as the requirement of minimal contribution to
peak broadening and reference potential being independent of sample
composition.
[0003] The minimal contribution to peak broadening is predominantly
determined by a low value of "chromatographic dead volume."
[0004] The independence of reference potential from solution
composition is realized only in "true" reference electrodes e.g.
calomel or Ag/AgCl equipped by a special type of electrolytic
connection known as "salt bridge." A typical salt bridge is a
cylindrical container filled with a 3 M KCl solution. The
conductive connection to the reference half cell on one side and to
the sample on the other side is realized using ion permeable
diaphragms.
[0005] All existing microfabricated cells employ either "pseudo"
reference electrodes (e.g. palladium) or reference half cells
without salt bridges. The latter types of reference electrodes rely
on a constant concentration of chloride ions in a measured sample.
Achieving such constant concentration of chloride ions is not
practical under chromatographic conditions.
[0006] There is a need to provide a disposable and readily
removable amperometric working electrode for a flow-through
electrochemical cell which is less expensive to construct and is
replaceable, thus avoiding the potential lack of reproducibility
incurred in reconditioning permanent working electrodes.
SUMMARY OF THE INVENTION
[0007] In one aspect of the present invention, a flow-through
electrochemical cell assembly is provided with a disposable working
electrode structure. The assembly includes (a) a perimeter wall
defining a sample flow channel including an inlet and an outlet,
(b) a sample inlet line in fluid communication with the sample flow
channel inlet, (c) a sample outlet line providing fluid
communication between the sample flow channel outlet and a remote
reference electrode, and (d) a disposable working electrode
structure comprising an electrically conductive and
electrochemically active working electrode region bound as a layer,
directly or indirectly, to an electrically insulating substrate
surface. The substrate surface is in fluid-sealing relationship
with the sample flow channel, and the working electrode region is
in fluid communication with said sample flow channel. The working
electrode structure is readily removable from said electrochemical
cell assembly.
[0008] In another aspect of the invention, a method is provided for
making a disposable electrode structure and sample flow channel for
such an assembly. The method comprises the steps of (a) vapor
depositing electrically conductive and electrochemically active
material, directly or indirectly, onto an organic polymer substrate
through a mask to form a pattern of a working electrode region, and
(b) forming a fluid seal between said working electrode region and
a perimeter wall to define a fluid sample flow channel with said
working electrode region in direct fluid contact with said fluid
sample flow channel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is an exploded schematic view of an electrochemical
cell assembly according to the invention including a disposable
electrode.
[0010] FIG. 2 is a top view of a masking for vapor deposition of
the electrode onto a substrate.
[0011] FIGS. 3a-3c are schematic representations of a method for
masking a disposable electrode of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] Referring to FIG. 1, a flow-through electrochemical cell
detector is illustrated including one embodiment of a disposable
working electrode according the present invention. Most of the
components of this cell can be similar to a conventional
electrochemical cell such as the ED40 cell of Dionex Corporation,
with the exception that the disposable working electrode structure
replaces a generally permanent electrode structure which is
periodically reconditioned as by polishing.
[0013] In general terms, the flow-through electrochemical cell
includes a sample flow channel in contact with a working electrode.
Sample analyte in a liquid eluent solution flows through the sample
flow channel and from there through a reference electrode chamber.
Electrode surface reactions are carried out on the working
electrode, typically including an electrically conductive and
electrochemically active material which is in direct contact with
the sample solution flowing through the sample flow channel.
[0014] Specifically referring to FIG. 1, in one embodiment of an
electrochemical cell assembly 10, a conventional reference
electrode block 12 defines a contained cylindrical reference
electrode chamber 14 through which the sample solution flows
passing through the sample flow channel. Another suitable
conventional electrode is in the form of a counter or auxiliary
electrode 16.
[0015] The basic function of the auxiliary electrode is to prevent
the electrical current from running through the reference
electrode. This is achieved by means of so-called three-electrode
potentiostats. See pages 47-48, 239-241, William R. LaCourse,
Pulsed Electrochemical Detection in High-Performance Liquid
Chromatography, John Wiley, New York 1997, pages 47-48 and
239-241.
[0016] If the passage of the current through the reference is not
minimized, oxidation or reduction of the reference material can
take place (e.g. AgCl reduced back to silver or Ag oxidized to
silver oxide) or change of chloride concentration in the junction
solution which may result in a poor constancy of the reference
potential. The three-electrode potentiostats were introduced in the
1950s. Prior to that only two-electrode cells were in general use
for voltammetry (i.e. measurement of current while controlling the
potential)
[0017] As illustrated in FIG. 1, the sample flow channel in contact
with the working electrode is defined by a gasket 18 which is
retained in sealing relationship between the lower wall of counter
electrode 16 and the upwardly facing wall of disposable working
electrode structure 20 to be described hereinafter. Gasket 18
defines an interior cut-out forming a perimeter wall around sample
flow channel 18a. The configuration of the sample flow channel 18a
is defined by the thickness of gasket 18, and the length and width
of the cut-out, preferably in the form of an elongated flow-through
slot. As illustrated, the working electrode structure 20 includes a
support substrate 20a, preferably formed of an organic polymer, and
includes an electrically conductive and electrochemically active
working electrode region 20b, preferable in the form of a thin
layer, in a circular shape as illustrated. As will be described
hereinafter, the working electrode region 20b is preferably formed
by vapor deposition of an electrically conductive and
electrochemically active material, directly or indirectly, onto
substrate 20a. As used herein, "electrochemically active" means
material suitable for facilitating the required electrochemical
reactions for detection in electrochemical cells.
[0018] In the embodiment of FIG. 1, working electrode structure 20
also includes an electrically conductive contact region 20c,
suitably also in the form of a circular disk, and an electrically
conductive lead 20d interconnecting working electrode region 20b
and contact region 20c. In a preferred embodiment, working
electrode region 20b, contact region 20c and lead 20d are formed by
vapor deposition of the same electrically conductive material
directly or indirectly onto substrate 20a through a mask. As will
be described hereinafter, an adhesion layer preferrably is first
deposited onto an organic substrate to facilitate binding of the
electrode material. Preferably the adhesion material is of the same
configuration as regions 20b and 20c and lead 20d and is also
formed by vapor deposition through a mask of substantially the same
shape. As in a conventional electrochemical cell, the assembly
includes a working electrode connection 22, suitably spring loaded
and in electrical communication at one end of a potentiostat,
including a voltage or current source, and at the other end in
electrical contact with region 20c to establish an electrical
connection with working electrode region 20b through lead 20d.
[0019] As illustrated, the working electrode region 20b is disposed
in the sample flow channel 18a in direct contact with sample
flowing therethrough. In an illustrated embodiment, connection pin
22 and contact region 20c are disposed to the exterior of sample
flow channel 18a out of fluid contact with liquid flowing through
the flow channel. This has the advantage of simplicity. The working
electrode, connector and contact pad are located in a planar
arrangement on the same side of the polymeric substrate. This makes
it possible to manufacture the entire working electrode in what is
essentially a two-step deposition (e.g. with Ti and Au).
[0020] In contrast, the manufacturing of permanent electrodes
requires many more steps: machining of a kel-F block, machining of
a steel support plate, covering of a gold wire by a suitable
insulating materials, machining of a Teflon ferrule for the liquid
seal between the gold wire and the Kel F material, machining of the
gold contact pad cylinder, insertion of the gold wire and of the
contact pad into the opening in the Kel F material. Curing of the
conductive polymer between the electrode wire and the contact pad
cylinder. Sanding down the gold wire to the level of the KelF
material. Machine lapping of the gold wire, hand-polishing of the
gold wire. Of these multiple steps, the hand polishing is very
person-dependent and notorious for its lack of reproducibility. The
components are suitably held in the assembly under compression
using a holder block 24 which maintains gasket 18 and electrode
structure 20 in fluid sealing relationship. As illustrated, the
compression is accomplished by the use of conventional wing nuts 26
or other clamping means. In one alternative form, not shown, gasket
18 can be formed integral with or adhered to substrate 20 as by an
adhesive bond therebetween forming an integral unit which can be
readily removed from the cell and replaced by another integral
unit. Alternatively, gasket 18 can be mounted to counter electrode
16 or other support structure. In each of these or other possible
configurations, a disposable electrode structure can be removed
from the assembly and replaced alone or in combination with a
gasket and support plate or holder block.
[0021] Gasket 18 typically is flexible with a thickness in the
range of about 0.01 to 0.0005 inch consists of a fluoro polymer
such as Teflon.RTM. or such polymeric materials as polyetherimide
or nylon.
[0022] A similar type of gasket can be used as is used in the ED40
electrochemical cell. Such a gasket suitably includes an elongate
slot for flow channel 18a, suitably 0.5 to 10.0 mm, preferably 0.8
to 5 mm long. The channel width is suitably 0.1 to 3 mm, preferably
0.5 to 1.5 mm. The gaskets are suitably 0.005 to 0.5 mm, preferably
0.013 to 0.1 mm thick.
[0023] As illustrated, the gasket can be held in place by bolts
passing through openings in the gasket material at both ends of the
gasket.
[0024] For use with disposable electrodes it is advantageous to
modify the outer shape of the ED40 cell gasket as illustrated in
FIG. 1. An elongated partial protrusion or tab covering the lead
between the electrode and the contact pad improves the liquid seal.
Also of advantage is to use thicker (>0.05 mm) and/or softer
materials (PTFE) for gasketing of disposable electrodes.
[0025] In one embodiment, the gasket can also be made an integral
part of the disposable electrode. The polymeric gasket can be
permanently attached to the disposable electrode. This can be done
either by oxygen plasma treatment of both surfaces followed by
pressing the gasket against the electrode at room temperature.
Alternatively, a permanent bonding of gaskets and electrodes can be
achieved by using polyethylene coated polyester material of
suitable thickness as a gasket. After cutting the material to the
proper gasketing shape, the gasket is pressed to the face of the
disposable electrode at a suitable elevated temperature, usually
about 140.degree. C.
[0026] Typically, the sample containing separated analytes in an
eluent solution flows through conventional fittings, not shown,
from a chromatographic separator, such as a packed bed
chromatography column upstream of the electrochemical cell to flow
channel 18a. The sample solution flows through inlet tubing
connected to a sample flow channel inlet, not shown, in the path
illustrated by arrows 28. As in the ED40 cell, the inlet can be
formed by a pin hole opening through counter electrode 16 in the
upstream end of flow channel 18a. The solution flows across flow
channel 18a and exits through a sample flow channel outlet in the
path illustrated schematically by arrows 30 and flows through a pin
hole size opening, not shown, in counter electrode 16 into chamber
14 and exits chamber 14 through a fitting, not shown, through
chamber outlet 32.
[0027] In another system, a conventional chemical or
electrochemical suppressor is disposed between the electrochemical
cell detector and the chromatography separator of an ion
chromatography system.
[0028] The working electrode region is disposed within flow channel
18a to contact the flowing sample in eluent solution therein. A
preferred way to accomplish this and to provide electrical contact
with connector pin 22 is to space contact region 20c from working
electrode region 20b and to interconnect them by lead 20d. This can
be accomplished by the use of a mask which includes these three
elements vapor deposited through the mask. In this configuration,
the three elements are preferably in the form of thin film bound
directly or indirectly to substrate 20a.
[0029] Referring to FIG. 2, a top view of a mask 40 designed for
vapor depositing multiple electrode region is illustrated. Mask
includes alignment holes 41 to hold the screen in place and fixing
screw 42 together with a fixing bar 44. In one embodiment, the mask
40 is prepared by wet etching of aluminum or stainless steel
sheets. The electrical pattern is defined by openings in the mask.
One way to vapor deposit the electrode region is by placing a sheet
of polymeric substrate between mask 40 and a stainless steel plate,
not shown. The mask includes working electrode opening 40b which
defines working electrode region 20b, larger contact region 40c
which defines contact region 20c and slot opening 40d which defines
lead 20d. Suitable mask materials include metal (e.g. stainless
steel, molybdenum), glass, quartz and silicon.
[0030] The metallic pattern may be prepared by conventional micro
fabrication techniques used in semi-conduction manufacture as
described, for example, in M. Madou, Fundamentals of
Microfabrication, CRC Press, New York, 1997. These methods include
but are not limited to physical vapor deposition (PVD) and chemical
vapor deposition (CVD).
[0031] Preferably, before depositing the electrode region, an
adhesion layer is deposited using the mask 40. This method is
illustrated schematically in FIGS. 3a-c. Referring to FIG. 3a, a
thin film of the adhesion layer 50, illustrated as the darkened
region 50 in FIG. 3b, is sputtered through opening 40b, 40c, 40d
and mask 40 as by sputtering using a high vacuum with Ar plasma.
Such a technique is illustrated in M. Madou, Chapter 2, p. 60, FIG.
2.8 of Fundamentals of Microfabrication (CRC, 1997). Thereafter, as
illustrated in FIG. 3c the mask is maintained in place. A suitable
electrode material for direct contact with the sample in flow
channel 18b is vapor deposited as a second thin film 52 onto a
surface of the adhesion layer 50. The advantage of the adhesion
layer is that it improves the cohesion between the electrode layer
and the underlying substrate for any substrate, preferably an
organic polymer material.
[0032] Suitably the adhesion layer is formed of a material such as
titanium, tungsten, chromium and alloys of these materials. A
titanium or tungsten titanium alloy adhesion layer is particularly
effective to improve an adhesion of a metallic working electrode
layer to the polymeric substrate. A typical thickness for the
adhesion layer 50 is about 50 .ANG. to 5,000 .ANG..
[0033] A suitable electrode material in region 20a is a metal,
preferably a noble metal such as gold, platinum, copper or silver,
or alloys thereof, although gold is the most frequently used one.
In addition, a non-metallic electrode may be used for region 20b
such as a carboneous material (e.g. glassy carbon, graphite or
carbon paste) in combination with an adhesion layer such as
titanium. Similar sputtering techniques would be employed. A
typical thickness for the electrode material of layer 52 is about
100 .ANG. to 10,000 .ANG..
[0034] A suitable top view configuration of working electrode
region 20b is circular with a diameter of about 0.1 to 3 mm, and
suitably about 0.5 to 2.0 mm, preferably about 1 mm. A suitable
contact region 20c is larger because of the need to accommodate
different types of useful contacting arrangements and to ensure
good contact with pin 22.
[0035] Substrate 28 is preferably of a polymeric material with a
thickness in the range of about 0.002 to 0.020 inches. It is
preferably flexible for forming a good seal with gasket 18.
Suitably, the polymeric material can be a polyester (such as
polyethylene, terephthalate or polyethylene naphthalate),
polycarbonate, polyolefin, polyimide or polyetherimide. Preferably,
the polymeric material is a polyester (PEN or PET-type) or a
polycarbonate.
[0036] Other alternative structures for the disposable working
electrodes include different geometrical shapes of the working
electrode area such as triangle, square or rectangle. Several
possible arrangements relative to the flow path are possible for
each of the non-circular geometries of the working electrodes. Also
possible are comb-like patterns of two or more "finger" shaped
electrodes connected to the same lead as the circular electrodes
but protruding into the flow path either in a parallel or in radial
fashion. Also feasible are intercalated electrodes or two comb-like
electrode patterns protruding into the flow path from the opposing
sides.
[0037] The electronics connecting the system can be the ones
conventionally used in a Dionex ED40 or 50 electrochemical cell. A
true reference electrode, e.g., Ag/AgCl wire immersed in a
reference solution enclosed by suitable diaphragm or a glass
membrane may be employed.
[0038] In one embodiment of the invention, microfabricated
electrodes are used in conjunction with a salt bridge-equipped true
reference electrode. The combination pH/Ag/AgCl electrode
represents an improvement even over a "true" reference electrode.
An integral part of the detection mechanism is a cyclical creation
of a catalytic gold oxide layer on the working electrode's surface.
The IPAD mode freshly creates and removes the amino
acid-detection-enabling gold oxide layer with a frequency of 1 Hz
or higher. The creation of gold oxide is pH dependent and in
consequence different levels of oxidation current are generated as
a detection background at different pH. With a Ag/AgCl reference
electrode alone, any change of eluent pH, such as during a
chromatographic mobile phase gradient, results in a strongly
sloping chromatographic baseline. With the glass-membrane equipped
true-reference electrode such as pH/Ag/AgCl the reference potential
changes with pH in an identical fashion as the rate of gold oxide
formation. The pH-connected change of the reference potential is
thus providing an automatic compensation of the change of the
oxidation current. The resulting baseline during a pH gradient is
then completely flat.
[0039] In one embodiment of the invention, microfabricated
electrodes are used with a pH compensated reference potential (i.e.
true reference electrode, salt bridge, glass membrane).
[0040] The electrochemical cell of the present invention can be
used in any application in which ED40 or ED50 cell is used. Thus,
it can be used to detect separated amino acids, sugars, amino
sugars, amines, amino thiols or the like. One of the advantages of
the working electrode and reference electrode of the present
invention is that they are capable of off-setting the change of pH
and thus to eliminate excessive base line shifts. This is because
of the built-in pH-related compensation of oxidation currents.
[0041] An important advantage of the disposable electrode is that
it can be readily replaced after a single day or multiple day use
at low expense before loss of performance of the cell.
[0042] The disposable electrodes of the present invention are
compatible with a commercial low dead volume electrochemical cell.
This enables use of a true reference or pH based reference
potential.
[0043] A variety of samples were analyzed with different protocols
using an electrochemical cell with a disposable electrode according
to the present invention. The chromatograms from such experiments
were very comparable to ones performed using the ED40 cell.
[0044] In order to more clearly illustrate the present invention,
the following examples of its practice are presented.
EXAMPLE 1
[0045] This illustrates a method for forming a sputtered thin film
of titanium and gold on a polymeric substrate according to the
invention.
[0046] 1. Assembly of Polymeric Substrate, Stainless Steel Base
Plate and Stainless Steel Masks for Coating
[0047] Polymeric film substrates obtained from Du Pont or GE were
cleaned of all particles on their surface by blowing off with air,
rinsed successively with water, alcohol and then dried in air.
After punching the holes required for mounting the masks on top of
the film, the polymeric substrates were put on top of a stainless
steel base plate. We then placed first a thinner stainless steel
mask and then a thicker stainless steel mask on the exposed side of
the polymeric film. The patterns of the thinner mask is shown in
FIG. 2. The thinner mask defines the shape of the electrode,
connection lead and contact pad. The thicker mask, not shown, is
used for keeping the thinner mask flat, completely co-planar and in
close contact with the polymeric film. At the same time, the
thicker mask has open cutout areas, thus providing the structural
integrity without interference with the plasma during the
sputtering of titanium and gold. The polymeric films are sandwiched
tightly between the two masks and the supporting base plate. The
whole assembly is being held together by bars and screws. The bars
are positioned on top of the two masks.
[0048] 2. Physical Vapor Deposition of Titanium and Gold
[0049] The polymeric substrates assembled with masks are placed in
the sputtering chamber. A suitable vacuum is applied for 12 hours
(overnight) to reach the vacuum required for sputtering (at least
40 mTorr). The water adsorbed inside the polymer is slowly removed
from the chamber during that time. To initiate the deposition, the
substrate remains enclosed in a low-pressure gas atmosphere (ca. 10
mTorr of argon). For RF plasma deposition the substrate is
connected as anode and the metal source for deposition (target) is
connected as cathode. A suitable RF frequency is within the range
of 12-14 mHz. The suitable range of RF power is in the range of 1
to 2 kW. The deposition rate is different for different metals. For
the same frequency and power of the RF field, titanium deposition
is ca. 4.7 times slower than the deposition of gold (see for
example Table 3.8, page 100, M. Madou, Fundamentals of
Micromachining). The RF field generated between the substrate and
target is the sole heating source during the metal deposition. The
temperature of the polymeric substrate never exceeds the range of
50-70.degree. C.
[0050] A titanium layer is sputtered first to promote adhesion of
gold films to polymeric substrates. A typical thickness of the
first metallic layer is 50 to 1000 .ANG.. The layer of titanium is
the only adhesion-promoting agent utilized in our process. There
are no other adhesives being utilized to promote adhesion of gold
layer to the polymeric substrate. The second layer (Au) is usually
100 to 5000 .ANG. thick. The sputtering time varies from system to
system because the coating rate depends on the power of the radio
frequency (plasma source), the distance between the polymeric film
and target (source of metal being deposited) and others.
EXAMPLE 2
[0051] Assembling a Suitable Cell
[0052] (1) Remove the ED40 cell body made of titanium from the
stainless steel box serving as a Faraday Cage/electrode mounting
container and unscrew the steel cylinder holder for the reference
electrode.
[0053] (2) Verify that a black O ring (Viton) is in place in the
lower part of the reference electrode chamber.
[0054] Insert a pH/Ag/AgCl reference electrode (glass cylinder)
into the reference electrode chamber of the cell body.
[0055] (3) Install the steel cylinder holding the reference
electrode in pre-defined position inside the reference electrode
chamber.
[0056] (4) Connect the lead wires of the reference electrode to the
"pH" and "Ag" pins of the pre-amplifier board.
[0057] (5) The white cable of the working electrode connection
remains connected to the two "WE" pins.
[0058] (6) Unscrew the two winged screws and remove the permanent
working electrode from the cell body.
[0059] (7) Remove the standard cell gasket and replace it by a cell
gasket for use with disposable electrodes.
[0060] (8) Match the two holes of the disposable electrode unit
(outside dimensions 2.5.times.3 cm) to the two posts protruding
from the cell body. The two openings of the disposable electrode
match the distance between the two posts (2 cm). Slide the
disposable electrode all the way to the bottom of the two alignment
posts. This positions the working electrode correctly inside the
flow path defined by the gasket cutout. Make sure that the
metallized side of the disposable electrode unit faces the
electrode cell body and the gasket. The correct position of the
working electrode can be verified through the transparent polyester
substrate of the disposable electrode. The correct orientation of
the disposable working electrode is indicated by the titanium color
(not gold) being visible through the polyester when the unit is in
the position close to the cell body.
[0061] (9) Slide the permanent electrode (or alternatively a less
expensive holder block) onto the two posts pressing the disposable
electrode against the cell body. Check visually the presence of the
cell gasket and the correct contact between contact pin and contact
pad.
[0062] (10) Mount the two winged nuts.
[0063] (11) Make liquid connections to and from the electrode
cell.
[0064] (12) Slide the steel mounting box/Faraday Cage over the
assembled cell.
[0065] (13) Connect the assembled cell to the electronic unit of
the ED40 detector.
[0066] (14) Start the pump and wait until you see the first drops
coming out of the outlet capillary.
[0067] (15) Check the pH readout on the screen of the ED40
electronic unit.
[0068] (16) Apply a suitable detection potential or detection
waveform.
* * * * *