Abstract:
An electrospray ionization device incorporates a shaped thin film with a microfluidic channel. The device may be interfaced to a time-of-flight mass spectrometer (TFOMS). In one embodiment, the shaped thin film has a polygonal-shaped or triangle-shaped thin polymer tip formed by lithography and etching. The microfluidic channel is approximately 20 micrometer wide and 10 micrometers deep, and embossed in a substrate using a silicon master. The shaped thin film is aligned with the channel and bonded between the channel substrate and a flat plate to create a microfluidic channel with a wicking tip protruding from the end of the channel. Application of a high voltage at one end of the channel creates an electrospray from the tip, which is provided to the TFOMS.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/394,757, filed Mar. 21, 2003, which claims priority to U.S. Provisional Patent Application Ser. No. 60/342,625, filed Dec. 21, 2002, which is incorporated herein by reference. 
     
    
     GOVERNMENT FUNDING  
       [0002]     The invention described herein was made with U.S. Government support under agreement number ECS-9876771 awarded by National Science Foundation. The United States Government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention relates to electrospray emitters, and in particular to an electrospray emitter for a microfluidic channel.  
       BACKGROUND OF THE INVENTION  
       [0004]     Chip-based fluid channels are used for many different applications. Applications include zone electrophoresis separation of analytes and capillary electrophoresis performed on chip-based substrates. Various spectroscopic detectors are used do detect the analytes. Attempts to couple mass spectrometry with the chip-based fluid channels involve creating an electrospray of analytes, which is directed to an ion sampling orifice of the mass spectrometer.  
         [0005]     Several different attempts to deliver analyte from a microchannel in an electrospray include direct spraying from a flat edge of the chip, the use of a pulled glass capillary, a sleeve to support a sprayer capillary on the edge of the chip, and disposable emitters. Still further prior methods include gluing a pulled capillary sprayer on the flat, larger surface of the chip aligned with the channel. An alternative approach employs a microfabricated monolithic nozzle surrounded by an annular cavity on the surface of a silicon substrate. A still further approach involves an integrated miniaturized pneumatic nebulizer is coupled via a sub-atmospheric liquid junction electrospray interface.  
         [0006]     Alternatives to spraying from the chip involve the use of miniaturized ion spray devices that are not formed by microfabrication techniques.  
       SUMMARY OF THE INVENTION  
       [0007]     An electrospray ionization device incorporates a shaped thin film having a tip for coupling with a microfluidic system such as a microfluidic channel. The device may be interfaced to a time-of-flight mass spectrometer (TFOMS).  
         [0008]     In one embodiment, the tip is a triangle-shaped thin polymer tip formed by lithography and etching. The microfluidic channel is approximately 20 micrometer wide and 10 micrometers deep, and embossed in a substrate using a silicon master. The channel may be formed in many different types of materials, be many different sizes, and be formed using various processes suitable for the type of material.  
         [0009]     The shaped thin film is aligned with the channel and bonded between the channel substrate and a flat plate to create a microfluidic channel with a wicking tip protruding from the end of the channel. An apex of the tip is triangular, curved, trapezoidal, or any other shape that facilitates formation of a Taylor cone. In one embodiment, a stable Taylor cone at the apex of the tip is formed by application of a high potential across the channel, forming an electrospray ionization source. In further embodiments, an integrated array system with multiple channels and integrated tips is formed using the same process used to form a single channel. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is an exploded block view example of a channel with integrated electrospray emitter.  
         [0011]      FIG. 2  is a view of a triangular tip electrospray emitter of  FIG. 1 , taken at a 45 degree angle tilt.  
         [0012]      FIG. 3  is an example representation of Taylor cone formation from a triangle tip electrospray emitter.  
         [0013]      FIG. 4  is a representation of a trapezoidal shaped emitter film.  
         [0014]      FIG. 5  is a series of cross section examples showing formation of the channel with integrated electrospray emitter of  FIG. 1 .  
         [0015]      FIG. 6  is a block circuit diagram of an electrospray device coupled to a mass spectrometer.  
         [0016]      FIG. 7  is a block representation of a multi-tip electrospray emitter device. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims.  
         [0018]     An electrospray device is shown at  100  in  FIG. 1 . In one embodiment, a top chip  110  has a microchannel  115  embossed therein. The device further comprises an emitter film  120 , having a triangular or trapezoidal shaped tip  130 . The emitter comprises a larger body portion which is rectangular in one embodiment, with the tip  130  extending from the rectangular portion. A bottom chip  140  is thermally bonded with the top chip  110 , sandwiching a portion of the emitter film to hold it firmly between the chips. In one embodiment, the film covers a portion of the length of the channel at one end of the bonded chips as indicated at  150 . The tip  130  extends laterally from the channel at end  150 . A reservoir  160  is coupled to the other end of the channel  115 .  
         [0019]     In further embodiments, the bottom chip  140  and tip  130  are formed as a single integrated piece, such as by injection molding. The integrated piece is then joined with the top chip  110 . In still further embodiments, the electrospray device  100  is formed as a single piece by injection molding or other method to form a thin film tip extending from an edge of a microfluidic system containing structure.  
         [0020]      FIG. 2  shows a triangular tip  130  extending from a channel exit  210 . The view is taken at a 45 degree tilt angle. The triangle tip acts like a nozzle or wick that helps to form a Taylor cone by guiding the location of a liquid droplet and cone at an apex of the tip. In one embodiment, the tip has an apex with an approximately 90 degree angle, A. Angles B and C, adjacent the channel are approximately 45 degrees. The angle of the apex may be varied, such as between 40 and 120 degrees. At smaller apex angles, liquid may spread at the base of the triangle contacting the microchannel chip, as the wetting angle of solutions in the channel may be smaller than the angles the base of the triangle makes with the chip.  
         [0021]     Different apex angles may be optimal for solutions with different wetting angles. In one embodiment, the base of the triangular tip is approximately 100 micrometers, and the height is approximately 50 micrometers. Thus, the base extends well beyond both sides of the channel when centered approximately at the center of the channel. As seen in  FIG. 2 , the apex has a small radius of curvature. The apex may be sharp if desired, and in further embodiment, the radius of curvature may be varied significantly. Many different shapes that promote formation of a Taylor cone may be used.  
         [0022]     The shape of the tip  130  helps form and fix a position of a Taylor cone  310 , as shown in  FIG. 3 . When a difference in potential is applied to the device, a liquid droplet with a critical curvature for establishing a Taylor cone is formed at the apex of the tip. A liquid jet  320  is formed at the apex. Highly charged small liquid droplets are made at a liquid plume  330  extending toward a counter electrode  340 . Excess electrostatic force extracts liquid from the apex of the Taylor cone to establish the liquid jet. The liquid jet  320  is branched at the plume  340  region due to repelling force acting among positively charged small droplets.  
         [0023]      FIG. 4  is a representation of a trapezoidal shaped emitter film. In one embodiment, the trapezoidal shaped emitter film comprises a body portion bonded between the chips, and a trapezoidal portion extends laterally from the channel. A long edge of the trapezoidal portion adjacent the channel is approximately 140 micrometers, and extends approximately 100 micrometers from the edge to an apex comprising a shorter edge of the trapezoidal portion at which the Taylor cone is formed.  
         [0024]     Formation of the emitter uses standard photolithographic processing of a four inch silicon wafer. First, a 5 micrometer layer of parylene, such as parlyene C is formed on the wafer by one of many methods, such as by deposition. Photoresist is formed on top of the parylene layer and patterned in a shape of the desired emitter shape. The wafer is then etched in plasma therm PT 72 using oxygen plasma for 15 micrometer depth; 5 micrometers from parylene and 10 micrometers from photoresist. The photoresist is removed by acetone or other means, and the remaining parylene is peeled off to form one or more thin film emitters. In one embodiment, the film is peeled off in an isopropyl alcohol solution. Such films may exhibit hydrophilic properties.  
         [0025]      FIG. 5  shows several cross sections representing a fabrication process for the device  100 . A silicon master is formed at  501 , followed by embossing  502  to form a polymer base, which is bonded at  503 , sandwiching the emitter. In one embodiment, the polymer base comprises a cyclo olefin polymer plastic plate. A silicon wafer  510  has a photoresist formed on one side such as by spinning, or any other suitable method. A mask  520  is used block exposure of a portion of the photoresist to UV light, resulting in a patterned layer of photoresist  525 . An SF 6  plasma etch is then performed to create a silicon master  530  having an embossing pattern  535  corresponding to a desired channel. In one embodiment, a 2.5 centimeters, 20 micrometer wide, and 10 micrometer deep microfluidic channel is embossed in a polymer chip  540  using the silicon master. A reservoir hole is also formed in the polymer chip if desired, such as by drilling. The embossed polymer chip  540  is subjected to an O 2  plasma.  
         [0026]     In various embodiments, the channel width varies from about 20 um to about 60 um and the range of the channel depth is from about 10 um to about 20 um. The thickness of the emitter film is 3 um to 10 um. The apex angle of the emitter film is smaller than 90 degrees. The width of microfluidic channel is smaller than that of the bottom of the triangular tip or is approximately the same size as the boom of the triangular tip. Instead of polymeric materials, metal material or ceramics can be used for the tip. The above parameters are approximate, and may be varied significantly in further embodiments.  
         [0027]     The emitter is sandwiched between the embossed polymer chip  540  and a cover chip  550 , also formed of polymer in one embodiment. The emitter is aligned such that an edge of the rectangular portion lines up with the side of the chip at the channel exit side of the chip. The tip  130  extends from the edge of the rectangular portion and side of the chip from the channel. In one embodiment, the triangular portion is centered on a center axis of the channel. Pressure and heat are applied to bond the chips, with the emitter positioned at the end of the channel to serve as an electrospray tip. In one embodiment, the chips and emitter were subjected to pressure and heated to 85 degrees C. for 10 to 15 minutes using a mini test press machine.  
         [0028]      FIG. 6  illustrates integration of an electrospray device  610  having a triangle emitter  615  with a time of flight (TOF) mass spectrometer  620 . In one embodiment, the device  610  is mounted on an X, Y, Z stage  622  for adjustment to provide maximum ion current. The triangle emitter  615  is positioned at one end, an exit of a channel  625 . Another end of the channel  625  is coupled to a reservoir  630 . The reservoir has a capillary tube  635 , or other fluid transport mechanism that couples it to a pump  640  to provide fluid to the reservoir and hence to the channel. In one embodiment, the capillary tube is formed of silica and coupled to the channel via a pipet tip glued to the reservoir  630 . In further embodiments, reservoir  630  is representative of further reservoirs that are provided along the channel  625  to facilitate desired separation of molecules. Examples of such reservoirs include buffer reservoirs, waste reservoirs and sample reservoirs. Selected reservoirs may be coupled via a T junctions with the channel  625 .  
         [0029]     The reservoir  630 , in one embodiment also has a conductive wire  650 , such as a gold wire coupling it to a power supply  655  for electrospray ionization. Glue is one method used to couple the wire to the reservoir. In one embodiment, power supply  655  provides 2500 volts to the reservoir. An aluminum counter electrode  660  is positioned approximately 10 millimeters from the tip of triangle emitter  615 , and is also coupled to a power supply  665  providing approximately 600 volts. The X,Y,Z stage provides the ability to adjust the distance between the emitter  615  and an orifice  670  of the mass spectrometer  620 . Thus, the voltages need not be the same as those used in this example, as the distance may be adjusted to optimize total ionic current. In further embodiments, the device  610  is fixed with respect to the mass spectrometer, or the mass spectrometer is moved.  
         [0030]     In one example, the mass spectrometer is maintained at a temperature of approximately 80 degrees C. via internal or external heaters. The pump is a syringe pump, and provides a stable flow of approximately 300 nanoliters per minute to supply liquid to the channel outlet proximate emitter  615 . A voltage of between approximately 2500 to 3000 volts is applied between the wire  650  and the orifice  670  with the orifice between 8 to 12 millimeters from the tip of the emitter  615  to produce a suitable spray. The voltage required to produce an optimal Taylor cone varies at least with tip shape, fluid flow rates, and distances to the electrode.  
         [0031]      FIG. 7  is a block representation of a multi-tip electrospray emitter device. Four triangle emitters  710 ,  715 ,  720  and  725  are shown. Each emitter is coupled to a channel. The may be operated in parallel, or may be sequentially operated. When operated in parallel, the emitters are spaced sufficiently to minimize interference between the respective sprays. A multichannel system when operated in a multiplexed manner operates reliably with no significant cross contamination between the channels.  
       CONCLUSION  
       [0032]     A thin film tip is integrated with a microchannel to form an electrospray of fluid from the microchannel. Dimensions of the thin film tip and microchannel may be varied significantly from the described embodiments. Shapes of thin film tips may also be varied in different polygonal configurations as desired to provide the ability to form a Taylor cone when subjected to a large difference in potential. The potential may also be varied. Materials selected for formation of the tip and channel may also be varied. The materials described provide for ease of manufacture using microfabrication techniques. Such materials are also compatible with various fluids used in separation processes. Other materials may also be utilized with similar or different manufacturing processes. Many different plastics may be used, as well as silicon or other materials. Embossing may be used with various plastics, and semiconductor processing may be utilized with silicon based substrates.  
         [0033]     The microchannel and tip combination may be integrated with other microfluidic structures, and is also useful in conjunction with a time of flight mass spectrometer. In one embodiment, the combination is positioned such that an electrospray is provided to an orifice of the spectrometer to provide the spectrometer a suitable spray for spectroscopic analysis.