Abstract:
An electrochemical flow cell comprises a substrate having an insulated surface, a polymer gasket integrally disposed on the surface, and a top cover disposed on the gasket. The components define a fluidic channel when assembled. An electrode(s) on the substrate surface provides for electrochemical detection of analytes in the fluid flowing over the electrode in the fluidic channel. The electrode(s) can be also integrated to the substrate. The assembly can be packaged. The flow cell inexpensive, versatile, and disposable. Small dimensions can facilitate good sensitivity and selectivity. Applications include environmental, life sciences, pharmaceuticals, and proteomics. The cell can be adapted for both detector and electrospray ionization applications.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is related to U.S. Provisional Patent Application Ser. No. 60/715,354, filed on Sep. 9, 2005, which is incorporated herein by reference and to which priority is claimed pursuant to 35 USC 119. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to the field of microfluidic, electrochemical flow cells and their fabrication.  
         [0004]     2. Description of the Prior Art  
         [0005]     Conventional micromachining and surface micromachining which can be used in the practice of electrochemical flow cells include, for example, (1) M. Madou, Fundamentals of Microfabrication, 2nd Ed., 2002, which describes for example, (2) Koch et al., Microfluidics Technology and Applications, 2000, (3) Van Zant, Microchip Fabrication, 5th Ed., 2004, (4) Lacourse, Pulsed Electrochemical Detection in High-Performance Liquid Chromatography, 1997, (5) “Integrated Parylene LC-ESI on a Chip,” Thesis by Jun Xie, Ph.D., California Institute of Technology, 2005, (6) Bard et al., Electrochemical Methods: Fundamentals and Applications, 2nd ed., Wiley, 2001, (7) Meyer, Practical High-Performance Liquid Chromatography, 3rd Ed., Wiley, 1998, (8) Acworth et al., “An Introduction to HPLC-Based Electrochemical Detection: from Single Electrode to Multi- Electrode Arrays.” In Progress in HPLC, Vol. 6. Acworth, I. N., et al. (Eds), 1996.  
         [0006]     Conventional liquid chromatography developments and applications are described in, for example, Harris, Analytical Chemistry, Feb. 1, 2003 , 65A-69A (“Shrinking the LC Landscape”).  
         [0007]     Small scale chromatography systems and applications and electrochemical flow cells are generally known in the art and commercially available. For example, electrochemical flow cells are described in, for example, U.S. Pat. Nos. 4,413,505 and 4,552,013 to Matson, and U.S. Pat. No. 6,783,645 to Cheng noted above, each of which are incorporated herein by reference. Electrochemical flow cells can be used as detectors. In contrast, U.S. Pat. No. 6,784,439 incorporated herein by reference describes flow through electrospray ionization devices which require high voltage electrodes. In addition, the &#39;439 patent describes electrodes and gaskets which are engineered to be removed from the substrate. They are not integrated with their supporting substrate. Electrochemical flow cells are also described in, for example, U.S. Provisional Application Ser. No. ______ filed Jun. 17, 2005, to Xie et al. “On-Chip Electrochemical Flow Cell” including electrode geometry, flow modeling, and microfabrication methods, which is hereby incorporated by reference in its entirety. The electrochemical flow cell can be engineered to provide the best selectivity and sensitivity for a given application. Multiple forms of electrochemical detection can be used including conductivity, dc amperometry, integrated amperometry, pulsed amperometry, and coulometry.  
         [0008]     Electrochemical flow cells can be important in, for example, environmental studies and proteomic analysis. Electrochemical flow cells can be used as detectors for a variety of separation methods such as capillary electrophoresis and chromatography, including liquid chromatography, ion chromatography, and HPLC. Also, they can be used in electrospray ionization mass spectral (ESI-MS) applications.  
         [0009]     In general, a need exists to miniaturize separation and bioanalytical methods including proteomics and environmental research. Many of the electrochemical flow cell commercially available on the market, in general, comprise a gasket which defines a fluidic channel and an electrode (e.g. a working electrode) which is exposed to the fluidic channel and in contact with a fluid inside the fluidic channel. In many cases, the electrode is a metal wire embedded inside a plastic block with the tip of the wire exposed to the fluidic channel. This type of electrode needs routine cleaning and polishing which can be labor intensive, time consuming, and unreliable.  
         [0010]     U.S. Pat. No. 6,783,645 (Cheng et. al.) provides another approach. It describes how to construct a metal disposable, working electrode on a polymer substrate using sputtering. Due to the mass production capability of the thin film process, a disposable working electrode structure can be manufactured. However, the devices disclosed in U.S. Pat. No. 6,783,645 need a careful alignment of the plastic gasket and the working electrode structure. This can be a problem, in particular, when the gasket is very thin (e.g. less than 25 μm). Gasket thickness is important because flow cell volume is directly related to gasket thickness. To achieve small flow cell volume, the gasket needs to carefully machined, which adds more complexity in dealing with thin material.  
         [0011]     Hence, improved approaches are needed. For example, better cost-effective, disposable systems are needed which also provide good performance. Better versatility and combination of properties are needed.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     The illustrated embodiments are an electrochemical flow cell device or assembly and methods of making and using the same. Larger systems and applications are also described. The illustrated embodiments can be used and adapted for both detector and ESI applications. In preferred embodiments, an alternative design and method is provided to make a disposable electrode on a substrate that has an integrated thin polymer gasket. The present electrochemical flow cells do not require high voltage electrodes or high voltage power supplies when used as a detector. The present electrochemical flow cells can be engineered for detection, if desired, rather than for electrospray ionization. Microfabrication generally makes it easier to fabricate a very thin gasket (e.g. less than 50 μm). Because the gasket is integrated on the same substrate where the electrodes are deposited, alignment and handling become simple and reliable.  
         [0013]     Another important advantage is that precise dimensions can be achieved because of microfabrication. For example, the flow channel that is defined by the gasket can be in a range between about 10 microns to about 1,000 microns wide. This leads to a small volume flow cell design which is desirable for small flow rate analysis, such as capillary LC or nano LC.  
         [0014]     One embodiment provides an electrochemical flow cell comprising: a substrate comprising an electrically insulated surface; a polymer gasket integrally disposed on the electrically insulated surface; a cover comprising a fluidic inlet and a fluidic outlet, the cover being disposed on the polymer gasket; wherein the electrically insulated surface, the polymer gasket, and the cover form a fluidic channel, and the inlet and the outlet are fluidly coupled to the fluidic channel; and at least one electrode disposed on the insulated surface, wherein the electrode is at least partially exposed to the fluidic channel.  
         [0015]     The cell can comprise a plurality of electrodes exposed to the fluidic channel. The electrode can be integrated with or integrally disposed on the substrate surface. The cover can be removably secured to the polymer gasket. The substrate can comprise, for example, silicon, glass, quartz, an organic polymer, or a combination thereof.  
         [0016]     The insulated surface can comprise, for example, silicon oxide, silicon nitride, parylene, polyimide, fluorinated polymer, Teflon®, or a combination thereof. The polymer gasket can comprise, for example, parylene, polyimide, fluorinated polymer, Teflon®, polycarbonate, polyolefin, polymethylmethacrylate (PMMA), polyester, or a combination thereof. In particular, the polymer gasket can comprise parylene or polyimide or Teflon®.  
         [0017]     The gasket can have a thickness, for example, between about 0.1 microns to about 100 microns. More particularly, the gasket can have a thickness between about 1 micron to about 25 microns. The fluidic channel can have a width, for example, between about 10 microns to about 1,000 microns.  
         [0018]     The electrode or plurality of electrodes can comprise metals such as, for example, gold, platinum, palladium, copper, silver, titanium, chromium, aluminum, tungsten, carbon, carbonaceous material, or a combination thereof. The electrode or the plurality of electrodes can have a thickness between about 10 nm and about 5000 nm, or about 10 nm to about 1,000 nm. The cover can be a polymeric cover; the cover can be a plastic cover.  
         [0019]     In one embodiment, the substrate is silicon, the insulated surface is silicon oxide, the cover is a plastic cover (such as PEEK), the polymer gasket comprises parylene, and the electrodes are metal electrodes. In this embodiment, the flow cell is made so that the fluidic channel has a channel width of about 10 microns to about 1,000 microns, and the electrodes have a thickness of about 10 nm to about 1,000 nm, and the gasket has a thickness of about 1 micron to about 25 microns.  
         [0020]     Also provided are cells further assembled with packaging. Additional components are used to hold the components together to provide seal and avoid leaks despite pressurization. Another embodiment provides a disposable electrochemical flow cell comprising: (i) a substrate comprising an insulated surface; (ii) a gasket integrally disposed on the electrically insulated surface; (iii) a cover comprising a fluidic inlet and a fluidic outlet, the cover being removably secured on the gasket; wherein the insulated surface, the gasket, and the cover form a fluidic channel, and the inlet and the outlet are fluidly coupled to the fluidic channel; and at least one electrode integrally disposed on the insulated surface, wherein the electrode is at least partially exposed to the fluidic channel.  
         [0021]     Another embodiment provides an electrochemical flow cell comprising: (A) a substrate comprising an electrically insulated surface, (B) an electrode or a plurality of electrodes on the electrically insulated surface, wherein the electrode or plurality of electrodes comprises at least one working electrode, (C) a gasket integrated with the electrically insulated surface and adapted to have a cover disposed thereon, wherein the gasket has an opening defining a microchannel for a fluid flowing through the flow cell, and the opening exposes the working electrode to the fluid and is adapted to be covered by the cover disposed on the gasket. This electrochemical flow cell can then be fitted with the cover and compressed or clamped as needed to become leak free.  
         [0022]     Another embodiment is a method of fabricating an electrochemical flow cell comprising a combination of the following steps: providing a substrate; providing an electrically insulated surface on the substrate; forming an electrode or a plurality of electrodes on the insulated surface; forming a polymer gasket on the insulting surface; providing a top cover with an inlet and an outlet; assembling the top cover, the polymer gasket, and the insulated surface to form a fluidic channel, wherein fluid coupling is provided for the inlet and the outlet to the fluidic channel, and wherein at least one of the electrodes or plurality of electrodes is exposed within the fluidic channel.  
         [0023]     Also provided is a larger system such as, for example, a chromatography system comprising a pump, a solvent source, a sample source, a chromatographic column, a column inlet, a column outlet, and an electrochemical flow cell detector as described herein. The chromatographic system can further comprise packaging for the electrochemical flow cell detector.  
         [0024]     Unlike the manufacturing process in U.S. Pat. No. 6,783,645, where electrode was made using shadow mask process, a variety of methods can be used to make the electrodes as described herein, such as wet etching or lift-off. This process can easily produce electrode with smaller dimensions. For example, interdigitated electrodes can be made that have about 10 micron spacing and about 10 micron width. Another important advantage is that the detector can be disposable and inexpensive. Another important advantage, at least for some embodiments, is that the fluidic channel can be formed directly without use of indirect methods such as etching away photoresist. It also provides ways to clean the electrodes during manufacturing such as by, for example, plasma cleaning.  
         [0025]     While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of “means” or “steps” limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a diagram of a side cross sectional view in enlarged scale of one embodiment of the flow cell.  
         [0027]      FIG. 2  is a perspective view of one embodiment of the flow cell.  
         [0028]      FIGS. 3   a - 3   d  are diagrams illustrating the fabrication process of one embodiment.  
         [0029]      FIG. 4  is a photograph of a fabricated device and assembly of one embodiment.  
         [0030]      FIG. 5  is a block diagram of a chromatography system incorporating the flow cell of  FIGS. 1 .- 4  as a detector. 
     
    
       [0031]     The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims. It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]      FIG. 1  is a cross sectional side view of an example of an electrochemical flow cell generally denoted by reference numeral  10 .  FIG. 1  illustrates a substrate  12 , a polymer gasket  14 , a cover  16 , and an electrode  18 . These components can be assembled to form an electrochemical flow cell assembly  20 , which may include multiple flow cells  10  and other kinds of fluidic and electronic devices. The assembly  20  can be a flow-through electrochemical cell assembly. the components may be held in place by temporary compression using, for example, clamping methods, wing nuts, and other methods and devices known in the art. The elements may be combined to provide a sealed relationship with each other, which allows fluid to flow under pressure without leaking.  
         [0033]      FIG. 2  is a three dimensional perspective view of an embodiment for the electrochemical flow cell  10 . In particular, the illustrated embodiment provides an electrochemical flow cell  10  comprising: a substrate  12  with an electrically insulated surface  12   a ; a polymer gasket  14  integrally disposed on the electrically insulated surface  12   a ; a cover  16  having a fluidic inlet  22  and a fluidic outlet  24 . The cover  16  (not shown in  FIG. 2 ) is disposed on the polymer gasket  14 . The electrically insulated surface  12   a  of substrate  12 , the polymer gasket  14 , and the cover  16  define the walls of a fluidic channel  26 . The inlet  22  and the outlet  24  are fluidly communicated to the fluidic channel  26 . An electrode  18  or a plurality of electrodes  18  are disposed on the insulated surface  12   a  with at least one of the electrode or plurality of electrodes  18  at least partially exposed to or in the fluidic channel  26 .  
         [0034]     In another embodiment the electrochemical flow cell is comprised of: (A) a substrate  12  having an electrically insulated surface  12   a , and (B) an electrode  18  or a plurality of electrodes  18  on the electrically insulated surface  12   a  with at least one of which is a working electrode  18 ; (C) a gasket  14  integrated with the electrically insulated surface  12   a  with a cover  16  disposed thereon. The gasket  14  has an opening  28  defining a microchannel  26  for a fluid flowing through the flow cell  10 . The opening  28  exposes the working electrode  18  to the fluid and is adapted to be covered by the cover  16  disposed on the gasket  14 .  
         [0035]     The cell  10  can be further fitted with the cover  16 . The electrochemical flow cells  10  can be made to be disposable. They can be engineered to be used only once or for a short time before being disposed of. After manufacture and use, they generally can be engineered not to need cleaning, e.g., electrode cleaning.  
         [0036]     Another embodiment is an disposable electrochemical flow cell  10  comprising: (i) a substrate  12  having an insulated surface  12   a ; (ii) a gasket  14  integrally disposed on the electrically insulated surface  12   a ; (iii) a cover  16  with a fluidic inlet  22  and a fluidic outlet  24 . The cover  16  is removably secured on the gasket  14 . The insulated surface  12   a , the gasket  14 , and the cover  16  define the walls a fluidic channel  26 . The inlet  22  and the outlet  24  are communicated with the fluidic channel  26 . At least one electrode  18  is integrally disposed on the insulated surface. The electrode  18  is at least partially exposed to or in the fluidic channel  26 .  
         [0037]     The components in a larger assembly  20  are now further described. The substrate  12  and the substrate surface  12   a  are not particularly limited but can comprise a variety of solid materials. The surface  12   a  may be planar or at least a substantially planar. The surface  12   a  may be insulated, particularly in the area wherein the electrode  18  is deposited. The whole surface  12   a  of the substrate  12  can be insulated. For example, the substrate  12  can composed of silicon, glass, quartz, an organic polymer, or a combination thereof. The insulated surface  12   a  can comprise, for example, silicon dioxide, silicon nitride, parylene, polyimide, fluorinated polymer, poly(tetrafluoroethylene), Teflon®, or a combination thereof.  
         [0038]     The substrate  12  is engineered to allow the polymer gasket  14  and the electrode or plurality of electrodes  18  to be disposed on the substrate  12  surface  12   a . The gasket  14  can be integrated onto the substrate  12 , providing good bonding and functionally permanent fixation or adhesion to the substrate. In general, the gasket  14  can be engineered to be not removable from the substrate  12 . Also, the electrode or electrodes  18  can be integrated onto the substrate  12 , providing good bonding and functionally permanent fixation or adhesion to the substrate  12 . In general, the electrode or electrodes  18  can be engineered to be not removable from the substrate  12 .  
         [0039]     In most cases, the gasket  14  can be a polymer gasket  14 . The polymer gasket  14  is not particularly limited in the type or nature of its composition provided that it can be deposited and patterned on the substrate  12  so that it is integrated with, or integrally disposed on the substrate  12 . Good bonding, fixation, or adhesion is desired. The polymer gasket  14  can be permanently coupled to the substrate  12 . Good binding and adhesion between the substrate surface  12   a  and the gasket  14  can be achieved to form an integral structure.  
         [0040]     The polymer gasket  14  can be a flexible gasket  14 . The polymer gasket  14  can comprise, for example, synthetic polymers, including organic polymers or silicone polymers, as well as elastomeric or thermoplastic polymers. Examples include parylene, polyimide, fluorinated polymer, poly(tetrafluoroethylene), Teflon®, polycarbonate, polyolefin, poly(methy1methacrylate), and polyester. Other examples include perfluoroelastomer, Kalrez®, nylon, polyetherimide, and photoresist. Parylene, poly(para-xylylene) is the generic name for a unique family of thermoplastic polymers that are deposited by using the dimer of para-xylylene. Parylene deposition can be carried out by at lower temperatures including room temperature and by chemical vapor deposition (CVD). The thickness of the polymer gasket  14  is not particularly limited but can be, for example, about 0.1 microns to about 100 microns, or more particularly, about 1 micron to about 25 microns, or more particularly about 1 micron to about 10 microns or 12.5 microns.  
         [0041]     The thickness of the polymer gasket  14  also generally controls the thickness of the fluidic channel  26  although in some regions the thickness of the fluidic channel  26  in a particular plane may also include the thickness of the electrode  18  in addition to the thickness of the gasket  14  as shown diagrammatically in  FIG. 1 . The gasket  14  can provide an opening or cutout  28  which allows for formation of the fluidic channel  26  upon assembly.  
         [0042]     Covers  16  including top covers or cover layers are previously known in microfluidic systems. See, for example, U.S. Pat. No. 6,756,019 which is incorporated herein by reference. The nature or composition of the cover  16  is not particularly limited but can be, for example, a plastic cover  16  including for example an engineering plastic including for example poly(ether ether ketone) (PEEK). If desired, the cover  16  can be transparent, semitransparent, or opaque. The cover  16  can comprise a fluidic inlet  22  and a fluidic outlet  24 , which are openings which allow fluid to flow into and out of the fluidic channel  26  after electrochemical detection and interaction with the working electrode  18 . Fluidic inlets  22  and fluidic outlets  24  per se are well known (see for example U.S. Pat. No. 6,827,095 incorporated herein by reference).  
         [0043]     The cover  16  can be disposed on the polymer gasket  14 . A good seal is generally desired. The size of the inlet  22  and outlet  24  is not particularly limited but can be, for example, 100 microns to 500 microns. The shape of the inlet  22  and outlet  24  is generally round, and they are typically mechanically machined. The cover  16  can be designed not to have a reservoir chamber for storing fluid. In general, the electrochemical flow cell  10  can be designed not to store fluid. The cover  16  can be designed to be removably or temporarily secured to the polymer gasket  14 .  
         [0044]     The electrically insulated surface  12   a , the polymer gasket  14 , and the cover  16  can form a fluidic channel  26  which is generally designed for fluid to flow from one point, or one end, to another point, or another end. The fluidic inlet  22  and fluidic outlet  24  for the cover  16  are in fluidic communication with the fluidic channel  26 . Flow channel  26  can also be a sample flow channel. The electrodes  18  can be designed so that they at least partially are exposed to or in the fluidic channel  26 . The working electrode  18  can interact with the fluid passing over the electrode  18  and allow for electrochemical detection. The shape and dimensions of the fluidic channel  26  are not particularly limited, but it can have a width of, for example, about 10 microns to about 1,000 microns, or about 100 microns to about 1,000 microns, or about 250 microns to about 750 microns. The length can be, for example, 0.5 mm to about 20 mm, or about 1 mm to about 10 mm. The height of the fluidic channel  26  is not particularly limited but can be, for example, about 0.1 microns to about 100 microns, or more particularly, about one micron to about 25 microns. The height can be an average height in that some zones within the fluidic channel  26  may have a different height because of the electrodes  18 .  
         [0045]     The fluidic channel  26  can be designed for flow of water, mixtures including water, polar organic solvents, nonpolar solvent, and generally solvents used for liquid chromatography, and other types of inorganic and organic liquids. The channel  26  can be, for example, designed to provide a substantially linear flow path over the electrode  18 . The length of the fluidic channel  26  can be designed with the length and height of the electrode  18  to provide good electrochemical detection and good efficiency so that as much of the analyte as possible is detected. Diffusion effects of the analyte can be taken into account in designing these geometries.  
         [0046]     Working, reference, and counter or auxiliary electrodes  18  are known in the art. Thin films structures can be used to provide the electrodes  18 . The electrodes  18  can be generally rectangular or round in shape and connect to structures which allow for further connection with a control circuit. Voltages for the electrode  18  do not need to be high voltages as needed in, for example, an electrospray ionization system, as described in for example U.S. Pat. No. 6,784,439 to Van Berkel, incorporated herein by reference.  
         [0047]     The electrodes  18  can be disposed directly on a flat substrate surface  12   a . The substrate surface  12   a  does not need to comprise a recess or depression to accommodate the electrode  18 . Rather, the electrode  18  can extend into the fluidic channel  26  above the substrate surface  12   a . The electrodes  18  are not particularly limited but can be patterned and can comprise one or more metals, including noble metals, including, for example, gold, platinum, palladium, copper, silver, titanium, chromium, aluminum, tungsten, carbon, carbonaceous material, or combinations or alloys thereof. Carbonaceous materials can be used for the electrode  18  including, for example, carbonized parylene and photoresist. Carbon electrodes  18  can be used. Pt/Ti electrodes  18  can be used. Electrically conductive and electrochemically active materials can be used. Working electrodes  18  can be made so that electrode surface reactions are carried out on the working electrode  18 .  
         [0048]     Working electrodes  18  and counter or auxiliary electrodes  18  can be made using the same thin film layer. The electrodes  18  can have, for example, a thickness of about 10 nm to about 5,000 nm, or about 10 nm to about 1,000 nm, or about 100 nm to about one micron. Thinner electrodes  18  can help provide less blocking of flow. The electrodes  18  can be configured with an interdigitated designs. Comb-like patterns can be used for electrodes  18 . ( 0047 ) Known methods and devices can be used to pass current to the electrodes  18  from external devices and to control current pulses. High voltages are not generally needed or desired in a detector embodiment. U.S. patent application Ser. No. 11/040,116 filed Jan. 24, 2005 (“Pyrolyzed Thin Film Carbon”), incorporated herein by reference, describes the formation and use of thin carbon electrodes  18 . In a preferred embodiment, the electrodes  18  and the gasket  14  are well integrated with, integrally disposed on, or permanently fixed or adhered to the substrate  12 .  
         [0049]      FIG. 3  is a diagram which illustrates one embodiment for making an electrochemical flow cell  10 . The method comprises a combination of one or more steps of the following steps. Good bonding at the interfaces between the substrate  12  and the electrode  18  or electrodes  18 , and between the substrate  12  and the gasket  14  is undertaken in each step. Substrate  12  is fabricated and/or provided and the insulative surface  12   a  on the substrate  12  is fabricated and/or provided as shown in  FIG. 3   a . The plurality of electrodes  18  or electrode  18  can be patterned on the insulative surface  12   a . The electrode or electrodes  18  are integrally formed on the substrate  12  as shown in  FIG. 3   b . The gasket  14  is deposited and patterned as shown in  FIG. 3   c . The gasket  14  can be integrally formed on the substrate  12 . The cover  16  is fabricated and provided. Fluidic inlets and fluidic outlets can be fabricated in the cover  16 . The substrate  12 , gasket  14 , and the cover  16  are assembled, forming the fluidic channel  26 , with the electrode  18  (or electrodes  18 ) ready to function as working electrode  18  in the fluidic channel  26  and interact with fluid passing over the electrode  18 .  
         [0050]     When cells  10  are simultaneously made in multiple numbers then the entire assembly is diced as desired into single cells  10  or subsets of cells  10  as assemblies  20 . The assembly  20  is cleaned and packaged as needed in the application.  
         [0051]     Consider now the fabrication methodology in greater detail. In one embodiment the method of fabricating an electrochemical flow cell  10  comprises the steps of providing a substrate  12 ; providing or forming an electrically insulated surface  12   a  on the substrate  12 ; forming or disposing an electrode or plurality of electrodes  18  on the insulated surface  12   a ; forming or disposing a polymer gasket  14  on the insulated surface  12   a ; providing or disposing a top cover  16  with an inlet  22  and an outlet  24 . The top cover  16 , the polymer gasket  14 , and the insulated surface  12   a  are combined to form a fluidic channel  26 , with which fluid communication to the fluidic channel  26  is provided from the inlet  22  and to the outlet  24 . At least one of the electrodes or plurality of electrodes  18  is exposed within the fluidic channel  26 .  
         [0052]     Providing or forming the polymer gasket  14  can comprise, for example, using spin coating, vapor deposition, plasma coating, or photolithography. Providing or forming the electrode or plurality of electrodes  18  can comprise, for example, using electron beam evaporation, sputtering, electroplating, lift-off, photolithography, or chemical wet etching. Providing or forming the electrically insulated surface  12   a  can comprise, for example, using thermal oxidation, spin coating, or chemical vapor deposition on the substrate  12 .  
         [0053]     Another embodiment comprises a method of fabricating an electrochemical flow cell  10  comprising the steps of (A) providing a substrate  12  having an electrically insulated surface  12   a , (B) depositing an electrode or a plurality of electrodes  18  on the electrically insulated surface  12   a , wherein the electrode or plurality of electrodes  18  comprises at least one working electrode  18 ; (C) fabricating a gasket  14  on the electrically insulated surface  12   a . The gasket  14  is permanently coupled to the electrically insulated surface  12   a  and has an opening  28  defining a microchannel for a fluid flowing through the flow cell  10 . The opening  28  exposes the working electrode  18  to the fluid.  
         [0054]     The electrochemical flow cell  10  can be and should be rugged and easy to use. It can further comprise or be part of a packaging system  30  which allows the fluid inlet  22  and outlet  24  to be coupled with external systems including the packaging system. The packaging system  30  can also provide sealing and protection for actual use. The packaging system  30  can also integrate the cell  10  with other chips or microfluidic components such as columns or detectors. For example, the packaging system  30  can provide a mechanical structure to clamp the top cover  16  and the electrode  18  together. In addition, exemplary components including pogo pins and PCBs for electrical connection and larger packaging assemblies. If desired, the system  30  can also comprise additional components such as, for example, temperature detectors, sensors, thermal sensors, controllers, flow controllers, and the like. For example, temperature detectors and controllers are described in, for example, U.S. patent application Ser. No. 11/059,625 filed Feb. 17, 2005 (“On Chip Temperature Controller Methods and Devices”) incorporated herein by reference. U.S. application Ser. No. 11/192,434 filed Jul. 29, 2005 (“Modular Microfluidic Packaging System”), incorporated herein by reference, describes examples of packaging systems. Packaging of fluidic and microfluidic systems is generally known as described in, for example, U.S. Pat. Nos. 6,548,895, 6,443,179, and 6,821,819 to Benavides et al., each of which are incorporated herein by reference.  
         [0055]     Applications for the electrochemical flow cells  10  are numerous and generally include any application to which conventional electrochemical flow cells can be applied including analytical applications, detectors, and ESI devices. These include, for example, applications in the environment, life sciences, pharmaceutical, food beverage, chemical, petrochemical, electronics, and power industries. Applications can be those used for the Dionex ED40 and ED50 electrochemical cell  10  detectors (see U.S. Pat. No. 6,783,645 for example) incorporated herein by reference. The electrochemical flow cells  10  can be used in, for example, liquid chromatography and flow injection analysis (FIA) applications. They can be used for detection of amino acids, carbohydrates, sugars, amino sugars, amines, amino thiols, or the like. For example, phenolic compounds can be determined by employing reversed-phase separations with amperometric detection.  
         [0056]     Nano-liquid chromatography systems are described in, for example, US Patent publication 2005/0051489 to Tai et al., published Mar. 10, 2005, which is incorporated by reference, including Pt/Ti sensing electrodes  18  and packaging and HPLC methods. Additional chromatography microfluidic chip applications are described in, for example, US Patent publication 200410124085to Tai et al., published Jul. 1, 2004, which is hereby incorporated by reference in its entirety, including electrochemical pumping and actuation of microfluidic chips and HPLC methods (see also, Xie et al., Anal. Chem., 2004, 76, 3756-3763, which is incorporated by reference). Additional chromatography components for use on a chip are described in, for example, Xie et al., US Patent Publication 2004/0253123 published Dec. 16, 2004; Xie et al. 2004/0237657 published Dec. 2, 2004; Xie et al., 2004/01 88648 published Sep. 30, 2004, each of which is incorporated by reference. An integrated chromatography system on a chip is described in, for example, U.S. patent application Ser. No. 11/177,505 filed Jul. 11, 2005 (“Integrated LC-ESI on a Chip”) incorporated by reference.  
         [0057]     Additional chromatography applications are described in, for example, U.S. provisional application 60/671,309 filed Apr. 14,2005 to Xie et al. (“Integrated Chromatography Devices for Monitoring Analytes in Real Time”) incorporated by reference. In one application, a chromatography system  20  is provided comprising a chromatography column  34  in fluid communication with the electrochemical flow cell  10 . Pumps  36  can be used including pumps on a chip. Solvent can first pass from one or more solvent reservoirs  38  and be mixed with a sample  40 . Sample injectors  46  on a chip can be provided. The sample  40  can be then introduced onto the column  34  and separation achieved. Separated samples  40  can elute from the column  34  and pass into the electrochemical cell  10  which is used as the detector. Additional analysis can be carried out, if desired, using for example electrospray ionization methods (ESI). Gradient elution and reverse phase methods can be used. Separation mechanisms are not particularly limited but can be based on size, charge, hydrophobicity, specific interactions, and the like.  
         [0058]     A larger system or assembly  20  can be fabricated such as, for example as shown in the block diagram of  FIG. 5 , a chromatography system comprising a pump  36 , a solvent source  38 , a sample source  40 , a chromatographic column  34 , a column inlet  42 , a column outlet  44 , and an electrochemical flow cell  10  as described above. The chromatographic system  20  can further comprises packaging for the electrochemical flow cell  10 .  
         [0059]     As working example, and not by way of limitation of the scope of the invention, an embodiment is shown is illustrated in  FIG. 4 .  FIG. 4  provides a photograph of a fabricated assembly  20  made from an electrochemical flow cell  10  which was made with a PEEK top cover  16 , an electrode chip made from a silicon substrate with thermal oxide, Ti/Pt electrode  18 , and parylene gasket  14  deposited on it. The larger assembly  20 , in addition to comprising the electrochemical flow cell  10 , also includes a mechanical structure to clamp the top cover  16  and the electrode chip  18  together, and pogo pins and PCB for electrical connection with larger packaging or other components. Fluidic inlet  22  and fluidic outlet  24  were machined inside the PEEK top cover  16 , so that commonly used fitting and tubing (e.g. capillary tubing from Upchurch) could be coupled to the flow cell  10 . Flow channel  26  in this flow cell  10  was 500 microns wide, 6 mm long, and 10 microns high. The flow cell  10  also had a resistive temperature detector integrated on the electrode chip  18  which was a thin film metal resistor.  
         [0060]     The process of making the electrode chip  18  started with a 4 inch silicon wafer coated with 0.5 um thermal oxide. Then two layers of photoresist (LorB from Microchem and AZ 15 18 from Clariant) were spin coated and patterned for lift-off. Before e-beam evaporation of metal, the wafer was cleaned using oxygen plasma and buffered HF dipping. 20 nm Ti and 200 nm Pt were evaporated on the chip as electrode  18 . PG remover from Microchem was used to strip the lift-off photoresist away. The parylene gasket  14  was formed by a 10 μm Parylene layer. Before the parylene layer deposition, adhesion promoter (such as A-174) was applied. A 10 nm Ti and 100 nm Au layer was used deposited and patterned as etching mask for parylene patterning. Parylene etching was done in oxygen plasma. The Ti/Au etching mask was then etched away. The wafer was finally diced and cleaned in solvents (such as ST-22 stripper or acetone).  
         [0061]     Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following invention and its various embodiments.  
         [0062]     Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. A teaching that two elements are combined in a claimed combination is further to be understood as also allowing for a claimed combination in which the two elements are not combined with each other, but may be used alone or combined in other combinations. The excision of any disclosed element of the invention is explicitly contemplated as within the scope of the invention.  
         [0063]     The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.  
         [0064]     The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination.  
         [0065]     Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements.  
         [0066]     The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.