This invention relates to emitters and in particular electrospray emitters that are useable on a micro scale. The electrospray emitter of the present invention is for use with mass spectrometers, coating systems, colloidal thruster systems, ion mobility spectrometers and the like.
The behavior of fluid-air interfaces in a strong electric field has been of interest since Zeleny first observed the deformation of a liquid interface under the influence of a large applied voltage. He reported the formation of a cone with a fine thread of liquid coming from the apex and the disintegration of the thread into small droplets after a short distance. Taylor in 1964 was the first to propose a concise analytical model for the formation and structure of this electrified cone, and it is to him the name ‘Taylor Cone’ is attributed. When Taylor applied a field on the order of thousands of volts normal to the surface of the liquid, he also observed the formation of a conical liquid interface where a narrow jet of liquid droplets was emitted from the apex. This phenomenon has since become referred to as ‘electrospray’.
Using a cone as the equilibrium shape, Taylor recognized that both surface tension and electric stress must vary with the inverse of the radius of the cone. Using the potential for a cone as determined by Hall, Taylor reported an equilibrium expression for the electrified cone and calculated only one possible angle where equilibrium exists.
Sujatha et al. later approached the equilibrium of an electrified interface using the variational principle. Their paper was critical of Taylor's equilibrium model, noting that the excess pressure term is omitted in his formulation. Sujatha et al. found that there was no cone of any angle that satisfied their equilibrium expressions.
Deviations between measured cone angles and Taylor's predicted angle are addressed by Fernandez de la Mora, who accounts for the space charge in the emitted jet when predicting the shape of the interface. Fernandez de la Mora and Loscertales and Ganan-Calvo et al. report a study of the spray current and emitted droplet size of a conical electrified interface, and introduced scaling laws to predict these two quantities. Cloupeau and Prunet-Foch investigated different spraying modes (interface shapes) of a charged interface and Suvorov and Zubarev studied the evolution of Taylor cone formation for a liquid metal ion source. The latter predicted that the free surface approaches a conical shape with a semi-angle nearly identical to that calculated by Taylor.
Understanding the equilibrium of an electrified interface and the conditions required for: 1) the onset of an electrospray and 2) maintaining a steady electrospray once it is formed have important applications in a number of areas. Most notably, the use of electrospray revolutionized the field of mass spectrometry; a result of the seminal work presented by Fenn et al. Other applications of electrosprays include formation of thin films and colloid thrusters for propulsion.
Accordingly, it would advantageous to provide an electrospray emitter that can be easily manufactured and easily used. Further it would be advantageous to provide an electrospray emitter that can be used in the field to collect samples.
The present invention relates to electrospray emitters that have a rigid substrate layer, a second layer, a channel formed in at least one of the rigid substrate layer the second layer, and an exit orifice in flow communication with the channel. The second layer is attached to the first layer. The exit orifice is capable of holding an electric charge.
In another aspect of the invention there is provided an ion mobility spectrometer. The ion mobility spectrometer includes a first and a second spaced apart ion mobility substrate, at least two spacers, a gate electrode, a field electrode and a detection electrode. The spacers are positioned between the first and second ion mobility substrates whereby the first and second ion mobility substrates and two of the at least two spacers define a drift chamber having an entrance and an exit. The gate electrode is positioned at the entrance of the drift chamber. The field electrode is positioned in the drift chamber, downstream of the gate electrode. The detection electrode is positioned in the drift chamber, downstream of the electrode.
In a further aspect of the invention there is provided a method of creating an electrospray using an electrospray emitter having a fluid channel, an exit orifice in flow communication with the fluid channel, a counter electrode spaced from the exit orifice and whereby there is exit orifice is capable of holding and electric charge and the exit orifice is capable of containing fluid with the perimeter of the orifice, comprising the steps of: applying a pressure to the exit orifice in a predetermined range; applying a pressure and maintaining the pressure to the fluid channel in a predetermined range; applying voltage in a predetermined range between the exit orifice and the counter electrode; and determining a separation distance between the exit orifice and the counter electrode in a predetermined range.
Further features of the invention will be described or will become apparent in the course of the following detailed description.
The invention will now be described by way of example only, with reference to the accompanying drawings, in which:
The microscale electrospray emitter of the present invention is fabricated and used to investigate an electrified air-fluid interface and the formation of quasi equilibrium states (i.e. electrospray). The emitter is designed to be compatible with traditional microfluidic device fabrication and is demonstrated to be compatible with on-chip sample processing. This design is less complicated to fabricate compared to other proposed concepts, and the fact that it is a closed system means it is less susceptible to solvent evaporation and channel contamination compared to other open channel emitters.
Referring to
In the embodiment shown in
It will be appreciated by those skilled in the art that there are many uses for the electrospray emitter of the present invention. The emitter 10 may be taken into the field to collect samples that are thereafter tested. Specifically, the emitter shown in
The channel 16 may have a variety of different configurations depending on the intended use of the electrospray emitter. For example, the channel may be a simple reservoir 24 formed in the top layer 18 as shown in
The fabrication procedure starts by cutting the glass substrate layer 12 to the appropriate size and then drilling a 2 millimeter fluid inlet 23 for fluidic access. The rigid glass substrate layer 12 is then cleaned in a hot Piranha (3:1 H2SO4:H2O2) solution for 10 minutes. A metal layer can be incorporated on top of the glass layer and used for upstream processing (i.e. CE) of the sample. Metal (chromium and/or gold) can be evaporated to a thickness of 400 nm and patterned to the desired shape.
The internal geometry of the PDMS emitter is formed by making a negative relief of the reservoir and channel network. The negative relief is made of patterned SU8™ by MicroChem. Corp. on a silicon wafer substrate. First, the silicon wafer is cleaned in a hot Piranha (3:1 H2SO4:H2O2) solution for 10 minutes and then in a dilute hydrofluoric acid solution (10:1 HF) for 5 minutes. SU8 2100™ is spun on to the wafer to create a thickness of 140 μm, however spin speeds can be controlled to create a range of thicknesses. The wafer is baked for 5 mins at 65° C. and 35 mins at 90° C. and then exposed to UV light using a mask aligner (Karl Suss) to transfer the desired pattern. The wafer is again baked for 15 mins at 90° C. and developed in SU8™ developer leaving only the desired pattern. This pattern will be used to form the channel network in the PDMS. The emitter in this design uses a reservoir that is 5 mm in diameter and a winding channel network with channel widths of 300 μm. The height of the channel is 140 μm.
PDMS is prepared by mixing the polymer solution with the curing agent in a ratio of 10:1. The mixed PDMS is then poured over the SU8™ relief structure and silicon wafer that is stored in a Petri dish (this will be the channel layer) and into a flat Petri dish containing no wafer (this will be the intermediate layer). The mixed solution is then placed in a vacuum chamber for 30 mins to remove any air bubbles trapped in the mixture. The PDMS filled dishes are then transfer to a convection oven at 80° C. for 2.5 hours. The thickness of the PDMS layers can be controlled by carefully measuring the dispensed mass of the polymer solution and curing agent. The emitter in this study has a channel layer that is ˜1 mm thick and an intermediate layer that is ˜200 μm thick. The intermediate layer can have holes punched in it for access to the metal layer on the surface of the glass wafer.
After curing, the PDMS layers are peeled off the silicon wafer/SU8™ relief structure and the flat Petri dish. The relief structure has now been formed in the channel layer of the PDMS. The three layers are bonded together by exposing the bonding surfaces to an oxygen plasma at 65 mT and 70 W for 15 seconds using an RIE/ICP etcher. After plasma exposure, the intermediate layer is aligned with the glass substrate and the two surfaces are contacted—forming a spontaneous bond. A hole is punched in the intermediate layer over the drilled hole. After again exposing the surfaces, the channel layer is aligned and contacted with the intermediate layer, again forming a spontaneous bond. An enclosed channel network has now been formed. A cross sectional layout of the prototype emitter is shown in
The metal tubing 22 is positioned such that there is flow communication between the metal tubing 22 and the channel 16. The tubing is where electrical connections are made and the electrospray is formed from its edge. In this embodiment, tubing with an internal diameter (ID) of 140 μm and an outer diameter (OD) of 300 μm is used. The metal tubing is aligned with the top of the intermediate layer and with the edge of the channel using a microscope. Preferably the metal tubing 22 has a sharp edge 25 than can easily penetrate PDMS. Using a mechanical stage, the emitter chip is slowly advanced until the metal tubing 22 contacts the edge of the channel 16. The correct positioning is again checked visually using a microscope.
The compliance of the PDMS ensures that the needle is held firmly in place and that no leakage occurs around the edge. In this context, compliance or compliant means a material that is not rigid, can deform, does not form cracks, and is malleable.
In this case, the metal tubing 22 can easily penetrate the top PDMS layer 18. It does not form cracks (it is soft)—again for the metal tubing. It is able to penetrate, for the metal tubing but once the tubing is in contact with the channel, the outside of the needle is held firmly by the surrounding PDMS—so the PDMS is ‘hugging’ the metal tubing. This is to prevent leakage. If the PDMS is too rigid, it will not ‘hug’ the tubing. However, to ensure a tight seal, the edge of the emitter is clamped overnight in the vicinity of the needle. At the low flow rates used in this study (on the order of ˜1 μL/min), we have found no leakage to occur around the edge of the needle. The compliance of the PDMS also helps to reduce the formation of dead volumes that often occur for similar concepts at the end of a channel network.
The final step in the fabrication of the PDMS emitter is to evaporate a layer of parylene over the entire device. A parylene coater (Specialty Coating Systems) is used to coat the device—but most importantly the metal tubing—with a parylene layer 1-2 μm thick. Parylene is hydrophobic, and it ensures that the droplet/Taylor cone is well isolated at the edge of the metal tubing.
Fluid flow and pressure can be supplied to the emitter by a pump. One possible pump is a syringe pump, where the connection to the device is made using a Nanoport™ (Upchurch Scientific). This method is useful for characterizing the performance of the emitter and evaluating the interfacial behaviour. Another possible pump is using mechanical pressure supplied by a clamp whose separation can be accurately controlled. In this approach, a hole is not drilled in the glass layer. The sample to be electrosprayed is injected into the reservoir chamber using a small gauge needle. The compliance of the PDMS tends to seal the hole after the needle is removed, preventing leakage (the hole can also be covered with epoxy). Pressure from the clamp deflects the PDMS over the reservoir and forces the fluid into the channel network and towards the end of the metal tube. Using this source of pressure, a stable electrospray can be formed for a short duration of time.
One advantages of this embodiment of the electrospray emitter of the present invention it that it is compatible and easily integrated with other microfluidic components used in upstream processing, it is uncomplicated to fabricate, it has limited dead volumes, and, since it is a closed system, it is not susceptible to solvent evaporation and channel contamination.
In another embodiment of the present invention, the electrospray emitter is used for capillary electrophoresis. An example of such an electrospray emitter is shown generally at 30 in
Electrospray emitter 30 has an outer or first channel 34 and an inner or second channel 36. Outer channel 34 and inner channel 36 are in flow communication and merge proximate to outlet 38. Outer channel 34 includes an outer fluid inlet or Nanoport 40. Inner channel 36 has a serpentine configuration and has three ports 42, 44 and 46 respectively. Port 42, 44 and 46 each have a separately controllable electrode 48, 50, and 52 respectively operably connected thereto. An exit electrode 54 is operably connected to the metal tubing 22.
In use voltages are applied at the ports 42, 44 and 46 and outlet 38. Pressure driven flow is supplied into the outer channel 34 through the Nanoport at the fluid inlet 40. This fluid (buffer solution) travels through the outer channel 34 network connected to the fluid inlet port 40 and is the supporting sheath flow. The second channel network or inner channel 36 bounded by the ports 42, 44 and 46 and outlet 38 is where capillary electrophoresis takes place under the action of the applied voltages. The separated sample (using CE) and the sheath flow merge near outlet 38. The flow proceeds to the outlet 38 and then into the metal tubing 22 for electrospray.
In the first step of capillary electrophoresis, port 42 is where the sample of interest in a buffer solution is loaded. This sample will be undergoing CE and electrospray. Port 44 is a waste port. A voltage on the order of 1000 V is applied to loading port 42 and ground potential (0 V) is applied to waste port 44. Port 46 and outlet 38 have no applied voltage. The sample will migrate under the influence of the electric field and will fill the portion of the channel network 36 between loading port 42 and waste port 44. This step will run for several seconds.
In the second step of capillary electrophoresis, the voltage is removed from port 42 and port 44. A voltage on the order of 1000 to 5000 volts is supplied to port 46 and ground (0 V) potential is applied to outlet 38. A ‘plug’ of sample will be injected into the separation channel. Under the influence of the electric field, the sample will be fractionated. The sample migrates to the ground potential at outlet 38 where it is merged with the sheath flow and carried to the outlet 38 for electrospray. During this process, the sheath flow can be run continuously.
For the purpose of fractionation using a stationary solid phase, a slurry of microbeads is forced—using pressure driven flow—into the straight channel or winding channel (non-CE) configuration. The channel is made narrow at the end of the network so that the microbeads remain trapped inside the channel. The width of the narrow section should be just smaller than the size of the microbeads. This step is the last fabrication step for the SPE configuration.
Once the channel is loaded with microbeads, the sample of interest can be driven through the channel network using pressure driven flow towards the metal tubing for electrospray.
The following are ranges of operational parameters as determined from an equilibrium model of an electrified interface. The range of surface tension coefficients for buffer solutions are 22.5 mN/m to 72.0 mN/m. The range of metal tubing radius is from 25 microns to 150 microns. The range of pressures at the interface for the onset of electrospray is from nearly 0 Pa to 2880 Pa (relative to atmospheric) and in cone-jet mode. The range of pressure at the interface for maintaining electrospray is from −8423 Pa to 1000 Pa (relative to atmospheric) and in cone-jet mode. The range of separation distances is from 2 millimeters to 15 millimeters. The range of applied voltages is from 1000 volts to 3000 volts (if possible, 0 volts to 3000 volts should be claimed).
The microscale electrospray emitter 10, 30 of the present invention may be used with a variety of different devices. For example it may be used with a mass spectrometer or an extractor electrode of a colloidal thrusters 54 as shown in
In microscale ion mobility spectrometry (μIMS), ions are generated using the electrospray emitter and are injected into a drift chamber. The ions can be simple metal salts, peptides and proteins, or various toxins. The drift chamber is a straight enclosed channel with an electric field applied along its length. Under the influence of the electric field, the ions are transported along the channel towards a detector at the end of the channel. The detector is a metal electrode where the ions are neutralized and an electrical current is produced.
The ion's mobility along the drift chamber is a function of the ion electric charge, mass, and size. Therefore, different ions will be transported at different velocities and each individual ion type will have its own signature mobility. By knowing the mobility of an ion species, the identity of the ion can be determined by comparing the measured results with a pre-determined database of values. Mobility refers to the speed of the ions in a given electric field—and speed is determined by measuring the time between ion injection and response at the detector.
Referring to
The following are the steps used to fabricate the μIMS (as shown in
The drift chamber 66 is formed in the enclosed space between the glass 62 and PDMS 64 sections. The height of the drift chamber is controlled by modifying the thickness of the spacers 64 or PDMS layer and the width of the drift chamber is controlled by modifying the spacing of the spacers 64 or PDMS sections. The height of the drift chamber 66 is a maximum of 5 mm and the maximum width of the drift chamber is 10 mm. The PDMS section width is between 3-5 mm, the glass substrate is 25×25 mm, and the electrode widths are between 0.5-5 mm.
Ions are produced using the electrospray emitter 10, 30 and they are injected into the drift chamber 66. Note that the μIMS in
The μIMS is highly scalable and multiple drift chambers can be incorporated on chip. As shown in
High potential is applied between the emitter 10, 30 and gate electrode 76. This field is used to produce a Taylor cone as described previously. An electric field (the IMS field) is also applied along the drift channel between the field electrode 78 and detection electrode 80 (otherwise known as the Faraday plate). The magnitude of the IMS field is between 100-500 V/cm. A commercially available current amplifier (Keithley) and oscilloscope (Agilent) measures the current (i.e. the ions) at the detection electrode.
The drift chamber 66 needs to be normally free of ions (i.e. ions need to be blocked from entering the drift chamber). When a measurement is to be performed, a packet or swarm of ions must be injected into the drift chamber using a gating technique. Note that the Taylor cone is normally operated in steady mode—continuously emitting a stream of ions. Therefore the voltage applied at the emitter, the field electrode(s), and detection electrode remains unchanged during device operation.
To block ions from entering the drift region 66, different potentials (referred to as ‘high’ and ‘low’) are applied to the upper 82 and lower 84 substrate gate electrode 76. The ‘high’ potential electrode is set to create the field necessary to produce a stable electrospray. The ‘low’ potential electrode is set to 0 V (ground). In this configuration, an asymmetry is created in the field and ions are blocked from entering the drift region. To inject an ion packet into the drift region, the ‘low’ potential electrode is set to be exactly equal to the ‘high’ potential electrode. This creates symmetry in the electric field and ions are allowed to enter the drift region and migrate to the detection electrode under the influence of the IMS field. The ‘low’ potential electrode is then rapidly switched back to 0 V (ground). The entire switching operation lasts for 0.1-1 ms and the operation.
The drift time of an ion species is the time separation between the injection operation and the measurement of an ion current peak. Up to 50 ion injections and time measurements are made and averaged to remove noise in the signal. The drift velocity is then calculated using the drift length (the distance between the gate 76 electrode and the detector electrode 80). Reduced mobility coefficients—the standard technique for representing ion mobility—are determined using the ion velocity and IMS electric field. The measured reduced mobility values are compared to a database of values to determine the identity of the ions.
It will be appreciated by those skilled in the art that different materials and configurations may be used for the electrospray emitter. Referring to
Generally speaking, the systems described herein are directed to electrospray emitters. As required, embodiments of the present invention are disclosed herein. However, the disclosed embodiments are merely exemplary, and it should be understood that the invention may be embodied in many various and alternative forms. The Figures are not to scale and some features may be exaggerated or minimized to show details of particular elements while related elements may have been eliminated to prevent obscuring novel aspects. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention. For purposes of teaching and not limitation, the illustrated embodiments are directed to electrospray emitters.
As used herein, the terms “comprises” and “comprising” are to be construed as being inclusive and opened rather than exclusive. Specifically, when used in this specification including the claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or components are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.
This patent application relates to U.S. Provisional Patent Application Ser. No. 60/924,725 filed on May 29, 2007 entitled ELECTROSPRAY EMITTER AND METHOD OF USING SAME which is incorporated herein by reference in its entirety.
Number | Date | Country | |
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60924725 | May 2007 | US |