Abstract:
Electrospray systems, electrospray structures, removable electrospray structures, methods of operating electrospray systems, and methods of fabricating electrospray systems, are disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Divisional Application of U.S. patent application having Ser. No. 11/594,489, filed on Nov. 8, 2006, now U.S. Pat. No. 7,557,342 entitled “ELECTROSPRAY SYSTEMS AND METHODS”, the entirety of which is hereby incorporated by reference; which claims priority to a Continuation Application of U.S. patent application having Ser. No. 10/930,197, filed on Aug. 31, 2004, now U.S. Pat. No. 7,208,727 entitled “ELECTROSPRAY SYSTEMS AND METHODS”, the entirety of which is hereby incorporated by reference, which is a continuation-in-part application, which claims priority to U.S. Utility patent application Ser. No. 10/756,915 entitled “INTEGRATED MICRO FUEL PROCESSOR AND FLOW DELIVERY INFRASTRUCTURE” filed on Jan. 13, 2004, now U.S. Pat. No. 7,312,440 which claims priority to U.S. Provisional Patent Application Ser. No. 60/440,012, entitled “INTEGRATED MICRO FUEL PROCESSOR FOR HYDROGEN PRODUCTION AND PORTABLE POWER GENERATION” filed on Jan. 14, 2003, the entirety of which is hereby incorporated by reference. In addition, U.S. Utility patent application Ser. No. 10/756,915 claims priority to U.S. Provisional Patent Application Ser. No. 60/499,547, entitled “Piezoelectrically Driven Micromachined Electrospray Source for Mass Spectroscopy” filed on Sep. 2, 2003, the entirety of which is hereby incorporated by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to ionization systems, and relates more particularly, to electrospray systems and methods. 
     BACKGROUND 
     As reflected in the recent Proteomics special feature article (“Automated NanoElectrospray: A New Advance for Proteomics Researchers”,  Laboratory News,  2002) Mass Spectrometry (MS) has become the technology of choice to meet today&#39;s unprecedented demand for accurate bioanalytical measurements, including protein identification. Although MS can be used to analyze biomolecules with very large molecular weights (up to several MegaDaltons (Mda)), these molecules must be first converted to gas-phase ions before they can be introduced into a mass spectrometer for analysis. Electrospray ionization (ESI) has proven to be an enormous breakthrough in structural biology because it provides a mechanism for transferring large biological molecules into the gas phase as intact charged ions. It is the creation of efficient conversion of a very small quantity of a liquid sample (proteins are very expensive and often very difficult to produce in sizable quantities) into gas-phase ions that is one of the main bottlenecks for using mass spectrometry in high throughput proteomics. 
     Conventional (micro and nano) capillary ESI sources, as well as the more recently developed MEMS-based electrospray devices, rely on application of strong electric field, which is used for focusing of the charged jet leading to jet tip instabilities and formation of small droplets of the analyte sample. As a result, the size and homogeneity of the formed droplets is determined by the magnitude and geometry of the applied electric field, thus requiring high voltages for generating sufficiently small micrometer or sub-micrometer droplets via the so-called Taylor cone nebulization. Reliance on the electrohydrodynamic Taylor cone focusing of the jet to form the mist of sufficiently small charged droplets leading to single ion formation imposes several fundamental and significant limitations on the capabilities of the conventional ESI interface. 
     On such problem is that a very large electric potential needs to be applied to the capillary tip (up to a few kilovolts relative to the ground electrode of the MS interface) to ensure formation of the stable Taylor cone, especially at higher flow rates and with poorly conducting organic solvents. 
     An additional problem is that the choice of suitable solvents is very much restricted to those featuring high electrical conductivity and sufficiently low surface tension. This restriction imposes severe limitations on the range of biological molecules that can be analyzed via ESI Mass Spectrometry. For example, use of pure water (the most natural environment for most biomolecules) as a solvent is difficult in conventional ESI since the required onset electrospray voltage is greater than that of the corona discharge, leading to an unstable Taylor cone, damage to the emitter and uncontrollable droplet/ion formation. 
     Since the conventional ESI relies on the disintegration of the continuous jet emanating from the Taylor cone into an aerosol of charged droplets, there is the limit to the lowest flow rate (and therefore the minimum sample size) that can be used during the analysis. For example, commercial products require the minimum sample volume to be about 3 μL. 
     Another problem is that sample utilization (i.e., fraction of the sample volume that is introduced and being used in MS analysis relative to the total volume of the electrosprayed sample) is very low due to uncontrollable nature of electrohydrodynamic atomization process that relies on the surface instabilities. Further, a significant dead volume (i.e., a fraction of the sample that cannot be pulled from the capillary by electrical forces) is unavoidable in any jet-based atomization process. 
     Still other problems are that commercially available ESI devices are very expensive because of the manufacturing difficulties, and have a limited usable lifetime because of the high voltage operation in a chemically-aggressive solvent environment. 
     Accordingly, an electrospray system is desired that addresses at least some of the problems of existing technologies. 
     SUMMARY 
     Briefly described, embodiments of this disclosure, among others, include electrospray systems, electrospray structures, removable electrospray structures, methods of operating electrospray systems, and methods of fabricating electrospray systems. One exemplary electrospray system, among others, includes: a first reservoir configured to store a first fluid including a first ionizable molecule; a first actuator disposed in communication with the first reservoir configured to generate an ultrasonic pressure wave through the first fluid; an ionization source configured to ionize the first ionizable molecule to form a ionized first molecule; and a first set of ejector structures including at least one ejector nozzle configured to eject the first fluid in response to the ultrasonic pressure wave, wherein each ejector structure is configured to focus the ultrasonic pressure wave at a tip of the ejector nozzle, and wherein the first reservoir is disposed between the first actuator and the first set of ejector structures. The first actuator and the ionization source are configured to form a plurality of ionized first molecules from the first fluid, where the ionized first molecules are ejected from the ejector nozzles of the first set of ejector structures upon activation of the first actuator and the ionization source. 
     One exemplary removable electrospray structure, among others, includes: a first reservoir; an ionization source; and a first set of ejector structures including at least one ejector nozzle, wherein each ejector structure is configured to focus an ultrasonic pressure wave at a tip of the ejector nozzle. The removable electrospray structure is adapted to reversibly couple with a first actuator, where the first actuator is positioned in communication with the first reservoir. Upon activation of the first actuator and upon activation of the ionization source a first fluid including a plurality of ionized first molecules disposed in the first reservoir are ejected from the ejector nozzle of the first set of ejector structures. 
     One exemplary removable electrospray structure, among others, includes: a first reservoir; an ionization source disposed in fluidic communication with the first fluid; and a first set of ejector structures adjacent the first reservoir, wherein the first set of ejector structures include at least one ejector nozzle, wherein each ejector structure is configured to focus an ultrasonic pressure wave at a tip of the ejector nozzle. 
     One exemplary method, among others, includes: providing an electrospray system as described above; ionizing the first molecule in the first fluid to produce the first ionized molecule; activating the first actuator to generate the ultrasonic pressure wave for forcing the first fluid through the ejector nozzle; and ejecting the first fluid including the first ionized molecule through the ejector nozzle. 
     One exemplary method of fabricating an electrospray structure, among others, includes: providing a structure; forming a first set of ejector structures within the structure, the first set of ejector structures including at least one ejector nozzle configured to eject a first fluid in response to the ultrasonic pressure wave, wherein each ejector structure is configured to focus the ultrasonic pressure wave at a tip of the ejector nozzle; and disposing a first actuator on the structure, wherein a first space between the first actuator and the first set of ejector structures forms a first reservoir, wherein the first actuator is in communication with the first reservoir, wherein the actuator is configured to generate an ultrasonic pressure wave through a first reservoir. A first ionization source is disposed on a surface selected from an inside wall of the ejector nozzle adjacent the first reservoir, the first actuator adjacent the first reservoir, and combinations thereof. 
     Other apparatuses, systems, methods, features, and advantages of this disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional apparatuses, systems, methods, features, and advantages be included within this description, be within the scope of this disclosure, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further aspects of the present disclosure will be more readily appreciated upon review of the detailed description of its various embodiments, described below, when taken in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic of a representative embodiment of a mass spectrometry system. 
         FIG. 2  is an illustration of a cross-section of an embodiment of an electrospray system, as shown in  FIG. 1 . 
         FIG. 3  is an illustration of a cross-section of another embodiment of an electrospray system, as shown in  FIG. 1 . 
         FIGS. 4A through 4J  are illustrations of cross-sections of a representative embodiment of a method of forming the electrospray system shown in  FIG. 3 . 
         FIG. 5  is an illustration of a cross-section of another embodiment of an electrospray system, as shown in  FIG. 1 . 
         FIG. 6  is an illustration of a cross-section of another embodiment of an electrospray system, as shown in  FIG. 1 .  
         FIG. 7  is an illustration of a cross-section of another embodiment of an electrospray system, as shown in  FIG. 1 . 
         FIGS. 8A through 8K  are illustrations of cross-sections of a representative embodiment of a method of forming the electrospray system shown in  FIG. 7 . 
         FIGS. 9A through 9D  are illustrations of top views of representative embodiments of an electrospray system.  FIG. 9B  illustrates an acoustically responsive fluid bubble in one section of the electrospray system, while  FIG. 9C  illustrates a fluid bubble in the other section of the electrospray system. 
         FIGS. 10A through 10F  are illustrations of top views of representative embodiments of an electrospray system.  FIGS. 10B through 10F  illustrate an acoustically responsive fluid bubble being positioned from one section of the electrospray system to another. 
         FIG. 11  is a schematic of a representative micro-machined ultrasonic droplet generator. 
         FIG. 12  is a schematic of a representative process for forming the micro-machined ultrasonic droplet generator illustrated in  FIG. 11 . 
         FIGS. 13A and 13B  illustrate scanning electron micrographs (SEMs) of a KOH-etched pyramid-shaped horn with an ICP etched nozzle at the apex ( FIG. 13A ) and an array of nozzles fabricated on a silicon wafer ( FIG. 13B ). 
         FIG. 14A  illustrates a droplet ejection from several nozzles of a prototype device. 
         FIG. 14B  illustrates a stroboscopic image of a jet of about 8 μm diameter droplets ejected by a representative electrospray system. 
         FIG. 14C  illustrates a stroboscopic image of a jet of 5 μm droplets ejected by a representative electrospray system. 
         FIG. 15  illustrates a schematic of a representative experimental setup for experimental characterization of the micro-machined ultrasonic electrospray array when interfaced with a mass spectrometer (MS). 
         FIG. 16  illustrates an MS spectra of the MeOH:H 2 O:Acetic Acid (50:49.9:0.1) solvent mixture containing a standard low molecular weight test compound reserpine (MW=609 Da, CAS# 50-55-5) ionized using the electrospray system. 
     
    
    
     DETAILED DESCRIPTION 
     Mass spectrometry systems, methods of use thereof, electrospray systems, methods of use thereof, and methods of fabrication thereof, are disclosed. The mass spectrometry systems can be operated in a high throughput (parallel) and/or a multiplexed (individually controlled) mode. The mass spectrometry systems described herein include embodiments of electrospray systems that are capable of independently forming a fluid aerosol (i.e., droplets) and ionizing the molecules present in the fluid. The droplets are formed by producing resonant ultrasonic waves (e.g., acoustical pressure waves) within a reservoir interfaced with a structure having shaped cavities (e.g., acoustic horns) that focus the ultrasonic waves and thus amplify the pressure and form a pressure gradient at an ejector nozzle for each shaped cavity. The high pressure gradient close to the ejector nozzle accelerates fluid droplets of size comparable to the ejector nozzle diameter (e.g., a few micrometers) out of the ejector nozzle, which are thus controllably generated (e.g., ejected) during every cycle of the drive signal (e.g., a sinusoidal signal) after an initial transient. In other words, the droplets are produced either discretely (e.g., drop-on-demand), or as a continuous jet-based approach. 
     Decoupling of the droplet generation and the molecular ionization reduces the energy required to ionize the molecules and also lowers the sample size required, minimizes the dead volume, and improves sample utilization. In addition, decoupling of the droplet generation and the molecular ionization enables the electrospray system to produce droplets including ionized molecules at low voltages (e.g., about 80 to a few hundred Volts (V)), in contrast to commonly used electrospray systems (e.g., 1 kV to several kV). In addition, relatively small volumes of fluids (e.g., about 100 nanoliters (nL) to a few hundred nL) can be used in contrast to commonly used electrospray systems (e.g., 3 μL or more). 
     Embodiments of the electrospray system can be used in a continuous flow online operation (e.g., continuous loading of samples) and/or in discrete off-line operation. In discrete off-line operation, embodiments of the electrospray system can include a disposable nozzle system (e.g., array of nozzle systems that can include one or more samples and standards) that can be charged with one or more fluids and inserted into the electrospray system. The disposable nozzle system can be removed and replaced with another disposable nozzle system. 
     Additional embodiments of the electrospray system can be used in a high throughput electrospray system (e.g., simultaneous use of nozzles) and/or in a multiplexed electrospray system (e.g., using an array of individually addressable nozzles or individually addressable groups of nozzles). Details describing each of these embodiments are described in more detail below. 
       FIG. 1  is a schematic of a representative embodiment of a mass spectrometry system  10 . The mass spectrometry system  10  includes an electrospray system  12  and a mass spectrometer  14 . The electrospray system  12  is interfaced with the mass spectrometer  14  so that the fluid sample (e.g., in the form of droplets) is communicated from the electrospray system  12  to the mass spectrometer system  14  using electrostatic lenses and the like under one or more different vacuum pressures. In addition, the electrospray system  12  can be also interfaced with a liquid chromatography system, a fluidic system for selective delivery of different samples, and automated fluid charging system such as a pump, for example. 
     The mass spectrometer  14  can include, but is not limited to, a mass analyzer and an ion detector. The mass analyzer can include, but is not limited to, a time-of-flight (TOF) mass analyzer, an ion trap mass analyzer (IT-MS), a quadrupole (Q) mass analyzer, a magnetic sector mass analyzer, or an ion cyclotron resonance (ICR) mass analyzer. In some embodiments, because it can be used to separate ions having very high masses, the mass analyzer is a TOF mass analyzer. 
     The ion detector is a device for recording the number of ions that are subjected to an arrival time or position in a mass spectrometry system  25 , as is known by one skilled in the art. Ion detectors can include, for example, a microchannel plate multiplier detector, an electron multiplier detector, or a combination thereof. In addition, the mass spectrometry system  10  includes vacuum system components and electronic system components, as are known by one skilled in the art. 
     In general, the electrospray system  12  is capable of independently forming a fluid aerosol (i.e., droplets) and ionizing the molecules present in the fluid. The ionized molecules are then mass analyzed by the mass spectrometer  14 , which can provide information about the types of molecules present in the fluid sample. 
       FIG. 2  is an illustration of a cross-section of an embodiment of an electrospray system  20   a , as shown in  FIG. 1 . The electrospray system  20   a  includes, but is not limited to, an array structure  22  including ejector structures  26 , a separating layer  28 , a reservoir  32 , an actuator  42 , and an ionization source  44 . A fluid can be disposed in the reservoir  32  and in the array  22  of ejector structures  26 . Upon actuation of the actuator  42 , a resonant ultrasonic wave  52  can be produced within the reservoir  32  and fluid. The resonant ultrasonic wave  52  couples to and transmits through the liquid and is focused by the ejector structures  26  to form a pressure gradient  54  within the ejector structure  26 . The high-pressure gradient  54  accelerates fluid out of the ejector structure  26  to produce droplets  56 . The cycle of the drive signal applied to the actuator  42  dictates, at least in part, the rate at which the droplets are discretely produced. 
     A drop-on-demand ejection can be achieved by modulation of the actuation signal in time domain. The actuator  42  generating ultrasonic waves can be excited by a finite duration signal with a number of sinusoidal cycles (a tone burst) at the desired frequency. Since a certain energy level is reached for droplet ejection, during the initial cycles of this signal, the standing acoustic wave pattern in the resonant cavity is established and the energy level is brought up to the ejection threshold. The number of cycles required to achieve the threshold depends on the amplitude of the signal input to the wave generation device and the quality factor of the cavity resonance. After the threshold is reached, one or more droplets can be ejected in a controlled manner by reducing the input signal amplitude after the desired number cycles. This signal can be used repetitively, to eject a large number of droplets. Another useful feature of this operation is to reduce the thermal effects of the ejection, since the device can cool off when the actuator  42  is turned off between consecutive ejections. The ejection speed and droplet size can also be controlled by the amplitude and duration of the input signal applied to the actuator  42 . 
     The array structure  22  can include, but is not limited to, an ejector nozzle  24  and an ejector structure  26 . In general, the material that the array structure  22  is made of has substantially higher acoustic impedance as compared to the fluid. The array structure  22  can be made of materials such as, but not limited to, single crystal silicon (e.g., oriented in the (100), (010), or (001) direction), metals (e.g., aluminum, copper, and/or brass), plastics, silicon oxide, silicone nitride, and combinations thereof. 
     The ejector structure  26  can have a shape such as, but not limited to, conical, pyramidal, or horn-shaped with different cross-sections. In general, the cross-sectional area is decreasing (e.g., linear, exponential, or some other functional form) from a base of the ejector nozzle  26  (broadest point adjacent the reservoir  32 ) to the ejector nozzle  24 . The cross sections can include, but are not limited to, a triangular cross-section (as depicted in  FIG. 2 ), and exponentially narrowing. In an embodiment, the ejector structure  26  is a pyramidal shape. 
     The ejector structure  26  has acoustic wave focusing properties in order to establish a highly-localized, pressure maximum substantially close to the ejector nozzle  24 . This results in a large pressure gradient at the ejector nozzle  24  since there is effectively an acoustic pressure release surface at the ejector nozzle  24 . Since the acoustic velocity is related to the pressure gradient through Euler&#39;s relation, a significant momentum is transferred to the fluid volume close to the ejector nozzle  24  during each cycle of the acoustic wave in the ejector structure  26 . When the energy coupled by the acoustic wave in the fluid volume is substantially larger than the restoring energy due to surface tension, viscous friction, and other sources, the fluid surface is raised from its equilibrium position. Furthermore, the frequency of the waves should be such that there is enough time for the droplet to break away from the surface due to instabilities. 
     The ejector structure  26  has a diameter (at the base) of about 50 micrometers to 5 millimeters, 300 micrometers to 1 millimeter, and 600 micrometers to 900 micrometers. The distance (height) from the ejector nozzle  24  to the broadest point in the ejector structure  26  is from about 20 micrometers to 4 millimeters, 200 micrometers to 1 millimeter, and 400 micrometers to 600 micrometers. 
     The ejector nozzle  24  size effectively determines the droplet size and the amount of pressure focusing along with the ejector structure  26  geometry (i.e., cavity geometry). The ejector nozzle  24  can be formed using various micromachining techniques as described below and can have a shape such as, but not limited to, circular, elliptic, rectangular, and rhombic. The ejector nozzle  24  has a diameter of about 50 nanometers to 50 micrometers, 200 nanometers to 30 micrometers, and 1 micrometer to 10 micrometers. 
     In one embodiment all of the ejector nozzles are positioned inline with a mass spectrometer inlet, while in another embodiment only select ejector nozzles (1 or more) are positioned inline with the mass spectrometer inlet. 
     The array structure  22  can include one ejector nozzle  24  (not shown), a (one-dimensional) array of ejector nozzles  24 , or a (two dimensional) matrix of parallel arrays of ejector nozzles  24 . As shown in  FIG. 2 , the ejector structure  26  can include one ejector nozzle  24  each or include a plurality of ejector nozzles  24  in a single ejector structure  26 . 
     The separating layer  28  is disposed between the array structure  22  and the actuator  46 . The separating layer  28  can be fabricated of a material such as, but not limited to, silicon, metal, and plastic. The separating layer  28  is from about 50 micrometers to 5 millimeters in height (i.e., the distance from the actuator  42  to the array structure  22 ), from about 200 micrometers to 3 millimeters in height, and from about 500 micrometers to 1 millimeter in height. 
     The reservoir  32  is substantially defined by the separating layer  28 , the array structure  22 , and the actuator  42 . In general, the reservoir  32  and the ejector structures  26  include the fluid. The reservoir  32  is an open area connected to the open area of the ejector structures  26  so that fluid flows between both areas. In addition, the reservoir  32  can also be in fluidic communication (not shown) with a liquid chromatography system or other microfluidic structures capable of flowing fluid into the reservoir  32 . 
     In general, the dimensions of the reservoir  32  and the ejector structure  26  can be selected to excite a cavity resonance in the electrospray system at a desired frequency. The structures may have cavity resonances of about 100 kHz to 100 MHz, depending, in part, on fluid type and dimensions and cavity shape, when excited by the actuator  42 . 
     The dimensions of the reservoir  32  are from 100 micrometers to 4 centimeters in width, 100 micrometers to 4 centimeters in length, and 100 nanometers to 5 centimeters in height. In addition, the dimensions of the reservoir  32  are from 100 micrometers to 2 centimeters in width, 100 micrometers to 2 centimeters in length, and 1 micrometer to 3 millimeter in height. Further, the dimensions of the reservoir  32  are from 200 micrometers to 1 centimeters in width, 200 micrometers to 1 centimeters in length, and 100 micrometers to 2 millimeters in height. 
     The fluid can include liquids having low ultrasonic attenuation (e.g., featuring energy loss less than 0.1 dB/cm around 1 MHz operation frequency). The fluid can be liquids such as, but not limited to, water, methanol, dielectric fluorocarbon fluid, organic solvent, other liquids having a low ultrasonic attenuation, and combinations thereof. The fluids can include one or more molecules that can be solvated and ionized. The molecules can include, but are not limited to, polynucleotides, polypeptides, and combinations thereof. 
     The actuator  42  produces a resonant ultrasonic wave  52  within the reservoir  32  and fluid. As mentioned above, the resonant ultrasonic wave  52  couples to and transmits through the liquid and is focused by the ejector structures  26  to form a pressure gradient  54  within the ejector structure  26 . The high-pressure gradient  54  accelerates fluid out of the ejector structure  26  to produce droplets. The droplets are produced discretely in a drop-on-demand manner. The frequency in which the droplet are formed is a function of the drive cycle applied to the actuator  42  as well as the fluid, reservoir  32 , ejector structure  26 , and the ejector nozzle  24 . 
     An alternating voltage is applied (not shown) to the actuator  42  to cause the actuator  42  to produce the resonant ultrasonic wave  52 . The actuator  42  can operate at about 100 kHz to 100 MHz, 500 kHz to 15 MHz, and 800 kHz to 5 MHz. A direct current (DC) bias voltage can also be applied to the actuator  42  in addition to the alternating voltage. In embodiments where the actuator  42  is piezoelectric, this bias voltage can be used to prevent depolarization of the actuator  42  and also to generate an optimum ambient pressure in the reservoir  32 . In embodiments where the actuator  42  is electrostatic, the bias voltage is needed for efficient and linear operation of the actuator  42 . Operation of the actuator  42  is optimized within these frequency ranges in order to match the cavity resonances, and depends on the dimensions of and the materials used for fabrication of the reservoirs  32  and the array structure  22  as well the acoustic properties of the fluids inside the ejector. 
     The actuator  42  can include, but is not limited to, a piezoelectric actuator and a capacitive actuator. The piezoelectric actuator and the capacitive actuator are described in X. C. Jin, I. Ladabaum, F. L. Degertekin, S. Calmes and B. T. Khuri-Yakub, “Fabrication and Characterization of Surface Micromachined Capacitive Ultrasonic Immersion Transducers”, IEEE/ASME Journal of Microelectromechanical Systems, 8, pp. 100-114, 1999 and Meacham, J. M., Ejimofor, C., Kumar, S., Degertekin F. L., and Fedorov, A., A micromachined ultrasonic droplet generator based on liquid horn structure,  Rev. Sci. Instrum.,  75 (5), 1347-1352 (2004), which are incorporated herein by reference. 
     The dimensions of the actuator  42  depend on the type of actuator used. For embodiments where the actuator  42  is a piezoelectric actuator, the thickness of the actuator  42  is determined, at least in part, by the frequency of operation and the type of the piezoelectric material. The thickness of the piezoelectric actuator is chosen such that the thickness of the actuator  42  is about half the wavelength of longitudinal waves in the piezoelectric material at the frequency of operation. Therefore, in case of a piezoelectric actuator, the dimensions of the actuator  42  are from 100 micrometers to 4 centimeters in width, 10 micrometers to 1 centimeter in thickness, and 100 micrometers to 4 centimeters in length. In addition, the dimensions of the actuator  42  are from 100 micrometers to 2 centimeters in width, 10 micrometers to 5 millimeters in thickness, and 100 micrometers to 2 centimeters in length. Further, the dimensions of the actuator  42  are from 100 micrometers to 1 centimeters in width, 10 micrometers to 2 millimeters in thickness, and 100 micrometers to 1 centimeters in length. 
     In embodiments where the actuator  42  is an electrostatic actuator, the actuator  42  is built on a wafer made of silicon, glass, quartz, or other substrates suitable for microfabrication, where these substrates determine the thickness of the actuator  42 . Therefore, in case of a microfabricated electrostatic actuator, the dimensions of the actuator  42  are from 100 micrometers to 4 centimeters in width, 10 micrometers to 2 millimeter in thickness, and 100 micrometers to 4 centimeters in length. In addition, the dimensions of the actuator  42  are from 100 micrometers to 2 centimeters in width, 10 micrometers to 1 millimeter in thickness, and 100 micrometers to 2 centimeters in length. Further, the dimensions of the actuator  42  are from 100 micrometers to 1 centimeters in width, 10 micrometers to 600 micrometers in thickness, and 100 micrometers to 1 centimeter in length. 
     In the embodiment illustrated in  FIG. 2 , the ionization source  44  is disposed on the surface of the actuator  42  adjacent the reservoir  32 . A direct current bias voltage can be applied to the ionization source  44  via one or more sources through line  46 . The voltage applied to the ionization source  44  is substantially lower than that applied in currently used electrospray systems. The voltage applied to the ionization source  44  should be sufficient enough to cause charge separation to ionize the molecules present in the fluid. In this regard, the voltage applied to the ionization source  44  should be capable to produce redox reactions within the fluid. Therefore, the voltage applied to the ionization source  44  will depend, at least in part, upon the fluid and molecules present in the fluid. The voltage applied to the ionization source depends, in part, on the electrochemical redox potential of the given sample analyte and is typically from about 0 to ±1000 V, ±20 to ±600V, and ±80 to ±300V. 
     The ionization source  44  can include, but is not limited to, a wire electrode, a conductive material disposed on the reservoir  32 , and an electrode of the actuator  42 , and combinations thereof. The material that the wire and/or the conductive material is made of can include, but is not limited to, metal (e.g., copper, gold, and/or platinum), conductive polymers, and combinations thereof. The ionization source  44  may cover a small fraction (1%) or an entire surface (100%) of the actuator  42 . The ionization source  44  has a thickness of about 1 nanometer to 100 micrometers, 10 nanometers to 10 micrometers, and 100 nanometers to 1 micrometer. 
       FIG. 3  is an illustration of a cross-section of another embodiment of an electrospray system  20   b , as shown in  FIG. 1 . In this embodiment, a second ionization source  62  is disposed on portions of the inside surfaces of ejector structures  26 . An electrical potential can be applied to the second ionization source  62  via one or more sources through a line  64 . As in the embodiment shown in  FIG. 2 , the second ionization source  62  can be made of similar materials and dimensions. The second ionization source  62  can cover a small fraction (about 1% or just a tip) or an entire surface (100%) of the nozzle inner surface. This ionization source may not only produce ionization of molecules in the fluid when operated in DC mode, but also can support formation of electrocapillary waves at the fluid interface near the nozzle tip when operated in the AC mode in order to facilitate formation the droplets whose size is even smaller than the nozzle tip opening. 
     The following fabrication process is not intended to be an exhaustive list that includes all steps required for fabricating the electrospray system  20   b . In addition, the fabrication process is flexible because the process steps may be performed in a different order than the order illustrated in  FIGS. 4A through 4J . 
       FIGS. 4A through 4J  are illustrations of cross-sections of a representative embodiment of a method of forming the electrospray system shown in  FIG. 3 .  FIG. 4A  illustrates an array substrate  72  having a first masking layer  74  disposed thereon and patterned using photolithographic techniques. The first masking layer  74  can be formed of materials such as, but not limited to, a silicon nitride mask (Si 3 N 4 ). The first mask layer  74  can be formed using techniques such as, but not limited to, plasma enhanced chemical vapor deposition, low pressure chemical vapor deposition, and combinations thereof. The patterning of the first masking layer  74  is done using standard photolithography techniques. 
       FIG. 4B  illustrates the array substrate  72  after being etched to form the array structure  22  having ejector structures  26  formed in areas where the mask  74  was not disposed. The etching of the array substrate  72  to form the ejector structures  26 . The etching technique can include, but is not limited to, a potassium hydroxide (KOH) anisotropic etch, reactive ion etching (RIE), and inductively coupled plasma etch (ICP), and focused ion beam (FIB) machining. It should also be noted that the array substrate  72  can be formed via stamping, molding, or other manufacturing technique. 
     An example of etching includes, but is not limited to, the formation of a pyramidal ejector structure having internal wall angles of about 54.74° using anisotropic KOH etch of a single crystal silicon wafer from the (100) surface. The KOH solution etches the exposed (100) planes more rapidly than the (111) planes to form the pyramidal ejector structure such as described in Madou, M. J. (2002).  Fundamentals of Microfabrication . Boca Raton, Fla., CRC Press. 
       FIG. 4C  illustrates the removal of the first masking layer  74  using a reactive ion etching (RIE) process or similar process, if necessary, while  FIG. 4D  illustrates the addition of a second masking  76 . The second masking layer  76  can be formed of materials such as, but not limited to, a photoresist mask, a silicon nitride (hard) mask (Si 3 N 4 ), and a silicon oxide (hard) mask (SiO 2 ) which is patterned using photolithography techniques. The second masking layer  76  can be formed using techniques such as, but not limited to photolithography etching, inductively coupled plasma (ICP) etching, and reactive ion etching (RIE), and combinations thereof. 
       FIG. 4E  illustrates the etching of the second mask layer  76  to form the ejector nozzle  24  in the array substrate  22 . The etching technique can include, but is not limited to, photolithography etching, inductively coupled plasma (ICP) etching, and reactive ion etching (RIE). Alternatively, depending on the size and geometry, the ejector nozzles  24   a  and  24   b  can be cut from the wafer, using a dicing saw or other similar device. Also, the ejector nozzles  24   a  and  24   b  can be machined using focused ion beam (FIB), and laser or electron beam (E-beam) drilling as opposed to using the second mask layer  76 . 
       FIG. 4F  illustrates the removal of the second mask layer  76  using a reactive ion etching (RIE) process or similar process.  FIG. 4G  illustrates the deposition of the second ionization source  62  on the inside wall of the ejector structure  26 . The deposition techniques can include, but are not limited to, evaporation, sputtering, chemical vapor deposition (CVD), and electroplating. 
       FIG. 4H  illustrates the placement of the separating layer  28  on portions of the array structure  22  to form the lower portion  82  of the electrospray system  20   b . The separating layer  28  can be made separately by etching silicon, machining of the metal, or stamping the polymer. Once fabricated, this separating layer  28  can be bonded to the array structure  22  using a polyimide layer (such as Kapton™ or other bonding material). This dry film can be laminated and patterned using laser micromachining or photolithography techniques. The separating layer  28  can then be affixed/bonded to the piezoelectric transducer to form the operational device. Alternatively, the separating layer  28  is bonded to the upper portion  84  using a polyimide layer, for example. Then the separating layer  28  is bonded to the array structure  22 . 
       FIG. 4I  illustrates the lower portion  82  of the electrospray system  20   b  and the upper portion  84  of the electrospray system  20   b , while  FIG. 4J  illustrates the formation of the electrospray system  20   b  by joining (e.g., bonding and/or adhering) the lower portion  82  and the upper portion  84 . It should be noted that the lower portion  82  could be produced separately and be used as a disposable cartridge that is replaced regularly on the electrospray system  20   b , while the upper portion  84  is reused. In another embodiment not shown, the lower portion  82  does not include the separating layer  28  and the separating layer  28  is disposed on the upper portion  84 , so that the upper portion  84  with the separating layer  28  disposed thereon is reused. In still another embodiment, the separating layer  28  can be removed separately from either the upper portion  84  and the lower portion  82 . 
       FIG. 5  is an illustration of a cross-section of another embodiment of an electrospray system  12 , as shown in  FIG. 1 . In this embodiment, the electrospray system  100  includes a first reservoir  32   a  and a second reservoir  32   b . In addition, the first reservoir  32   a  and the second reservoir  32   b  each are adjacent a first actuator  42   a  and a second actuator  42   b , respectively. Furthermore, the first reservoir  32   a  and the second reservoir  32   b  each are adjacent a first ejector structure  24   a  and a second ejector structure  24   b , respectively. 
     The first reservoir  32   a  and the second reservoir  32   b  are separated by a center separating layer  28   c . The first reservoir  32   a  is bound by the first separating layer  28   a , the center separating layer  28   c , the first actuator  42   a , and the first ejector structure  26   a . The second reservoir  32   b  is bound by the second separating layer  28   b , the center separating layer  28   c , the second actuator  42   b , and the second ejector structure  26   b . The same or a different fluid can be disposed in the first reservoir  32   a  and the second reservoir  32   b , chosen to match the acoustic properties of the sample loaded in the cavity of the ejector structures  26   a  and  26   b , respectively. This configuration allows one to generate electrosprays of different fluids by simply electronically choosing the first actuator  42   a , or the second actuator  42   b . The number of the reservoirs can be increased by replicating this structure in the lateral dimension. 
       FIG. 6  is an illustration of a cross-section of another embodiment of an electrospray system  12 , as shown in  FIG. 1 . Similar to the electrospray system  100  shown in  FIG. 5 , the electrospray system  120  shown in  FIG. 6  includes a first reservoir  32   a  and a second reservoir  32   b . The first reservoir  32   a  is bound by the first separating layer  28   a , the center separating layer  28   c , the first actuator  42   a , and the first ejector structure  22   a . The first reservoir  32   a  includes a gas bubble (not shown). The second reservoir  32   b  is bound by the second separating layer  28   b , the center separating layer  32   c , a second actuator  42   b , and the second ejector structure  22   b . The second reservoir  32   b  includes a fluid bubble  208 . 
     In addition, as shown in  FIG. 7 , the electrospray system  120  includes a first separating structure  132   a  and a second separating structure  132   b , each disposed on top of the first ejection structure  26   a  and the second ejection structure  26   b , respectively, separating the first reservoir  32   a  and the second reservoir  32   b  from the first array structure  22   a  and second array structure  22   b , respectively. As demonstrated later with respect to  FIGS. 8A through 8K , the first array structure  22   a  and second array structure  22   b  are filled with a first fluid  134   a  and a second fluid  134   b , respectively, and then the first separating structure  132   a  and the second separating structure  132   b  are disposed on top of the first ejection structure  26   a  and the second ejection structure  26   b . It should be noted that the electrospary system  120  does not include a first ionization source  44   a  and  44   b  since the first actuator  42   a  and the second actuator  42   b  are separated from the first fluid  134   a  and the second fluid  134   b . This allows for individually addressable ionization sources, whose potential can be individually controlled. 
     The first separating structure  132   a  and the second separating structure  132   b  can be one structure or two distinct structures, which show little impedance to propagation of acoustic waves at the operation frequency of the actuators  42   a  and  42   b . The first separating structure  132   a  and the second separating structure  132   b  can be made of materials such as, but not limited to polyimide layer (such as Kapton™), pyrolene, and other suitable materials. The first separating structure  132   a  and the second separating structure  132   b  can have a thickness of about 1 micrometers to 200 micrometers. The length and width of the first separating structure  132   a  and the second separating structure  132   b  will depend upon the dimensions of the first array structure  22   a  and second array structure  22   b.    
     The first fluid  134   a  can be ejected out of the first ejection structure  26   a  by controllably positioning the fluid bubble (not shown) substantially between the first separating structure  132   a  and the first actuator  42   a  to fill in the reservoir  32   a . Likewise, the second fluid  134   b  can be ejected out of the second ejection structure  26   b  by controllably positioning the fluid bubble  208  substantially between the second separating structure  132   b  and the second actuator  42   b  to fill in the reservoir  32   b.    
     The ejection of the first fluid  134   a  and second fluid  134   b  can be controlled in at least two ways for the electrospray system  120  shown in  FIG. 6 . First, the first actuator  42   a  and the second actuator  42   b  can be individually activated to cause ejection of the first fluid  134   a  and the second fluid  134   b  if the fluid bubble  208  is properly positioned. Second, a gas bubble (not shown) can be positioned substantially between the first separating structure  132   a  and the first actuator  42   a  and/or the second separating structure  132   b  and the second actuator  42   b . Since the gas bubble does not effectively couple to and transmit the ultrasonic pressure wave, the first fluid  134   a  and the second fluid  134   b  will not be ejected, even if the first actuator  42   a  and/or the second actuator  42   b  are activated. The process for selectively ejecting fluid from one or more ejector structures is described in further detail in  FIGS. 9A  though  9 D and  10 A through  10 F. 
       FIG. 7  is an illustration of a cross-section of another embodiment of an electrospray system  12 , as shown in  FIG. 1 . In contrast to the electrospray system  120  in  FIG. 6 , the electrospray system  150  shown in  FIG. 7  includes only a single actuator  42  in communication with the first reservoir  32   a  and the second reservoir  32   b . As in the electrospray system  120  in  FIG. 6 , the first fluid  134   a  can be ejected out of the first ejection structure  26   a  by controllably positioning the fluid bubble (not shown) substantially between the first separating structure  132   a  and the first actuator  42   a  to fill in the reservoir  32   a . Likewise, the second fluid  134   b  can be ejected out of the second ejection structure  26   b  by controllably positioning the fluid bubble  208  substantially between the second separating structure  132   b  and the second actuator  42   b  to fill in the reservoir  32   b.    
     In addition, the first fluid  134   a  can not be ejected out of the first ejection structure  26   a  when the gas bubble (not shown) is positioned substantially between the first separating structure  132   a  and the first actuator  42   a  to fill in the reservoir  32   a . Likewise, the second fluid  134   b  can not be ejected out of the second ejection structure  26   b  when the gas bubble (not shown) is positioned substantially between the second separating structure  132   b  and the second actuator  42   b  to fill in the reservoir  32   b.    
     Therefore, upon actuation of the actuator  42  and positioning of the fluid bubble  208  and the gas bubble, the ejection of the first fluid  134   a  and the second fluid  134   b  can be selectively controlled. For example, in the configuration in  FIG. 7 , actuation of the actuator  42  causes the second fluid  134   b  to be ejected, while the first fluid  134   a  is not ejected. The process for selectively ejecting fluid from one or more ejector structures is described in further detail in  FIGS. 9A  though  9 C and  10 A through  10 E. 
     The following fabrication process is not intended to be an exhaustive list that includes all steps required for fabricating the electrospray system  150 . In addition, the fabrication process is flexible because the process steps may be performed in a different order than the order illustrated in  FIGS. 8A through 8K . 
       FIGS. 8A through 8K  are illustrations of cross-sections of a representative embodiment of a method of forming the electrospray system shown in  FIG. 7 .  FIG. 8A  illustrates an array substrate  72  having a first masking layer  144  disposed thereon. The first masking layer  144  can be formed of materials such as, but not limited to, a silicon nitride mask (Si 3 N 4 ), silicon oxide (SiO 2 ) and patterned using standard photolithography techniques. The first mask  144  can be disposed using techniques such as, but not limited to, inductively coupled plasma (ICP) etch, reactive ion etch (RIE), or wet etching. 
       FIG. 8B  illustrates the array substrate  72  after being etched to form the first array structure  22   a  and the second array structure  22   b  having the first ejector structures  26   a  and the second ejector structure  26   b  formed in areas where the mask  144  was not disposed. The etching of the array substrate  72  to form the first ejector structures  26   a  and the second ejector structure  26   b . The etching technique can include, but is not limited to, a potassium hydroxide (KOH) anisotropic etch of (100) single crystal silicon and laser micro-machining. 
       FIG. 8C  illustrates the removal of the first mask  144  using a reactive ion etching (RIE) process or similar process, and  FIG. 8D  illustrates the addition of a second masking layer  152 . The second mask  152  can be formed of materials such as, but not limited to, a silicon nitride mask (Si 3 N 4 ), a silicon oxide mask (SiO 2 ), or a photoresist. 
       FIG. 8E  illustrates the etching of the second mask  152  to form the ejector nozzles  24   a  and  24   b  for the first ejector structure  26   a  and the second ejector structure  26   b , respectively. The etching technique can include, but is not limited to, photolithography etching, inductively coupled plasma (ICP) etching, reactive ion etching (RIE), and wet chemical etching. Alternatively, depending on the size and geometry, the ejector nozzles  24   a  and  24   b  may be cut from the wafer, using a dicing saw or other similar device, and can be machined using focused ion beam (FIB), and laser or electron beam (E-beam) drilling, as opposed to using the second mask  152 .  FIG. 8F  illustrates the removal of the second mask  152  using a reactive ion etching (RIE) process or similar process. 
       FIG. 8G  illustrates the deposition of the second ionization source  62   a  and  62   b  on the inside wall of the first ejector structure  26   a  and the second ejector structure  26   b , respectively. The deposition techniques can include, but are not limited to, evaporation, sputtering, chemical vapor deposition, and electroplating. 
       FIG. 8H  illustrates the formation of the first separating structure  132   a  and the second separating structure  132   b  (these structures can be the same or be two distinct structures). In addition, an ejector nozzle sealing structure  136  is disposed on top of the ejector nozzles  24   a  and  24   b  of the first ejector structure  26   a  and second ejector structure  26   b . The ejector nozzle sealing structure  136  can be made of materials such as, but not limited to, polyimide layer (such as Kapton) or some other inert layer such as parylene film. 
     Prior to the formation of the first separating structure  132   a  and the second separating structure  132   b , the first ejector structure  26   a  and second ejector structure  26   b  are filled with a first fluid  134   a  and a second fluid  134   b . The first fluid  134   a  and the second fluid  134   b  can be the same fluid or different fluids. 
       FIG. 8I  illustrates the placement of the first separating layer  28   a , the second separating layer  28   b , and a center separating layer  28   d  on portions of the first array structure  22   a  and the second array structure  22   b  to form the lower portion  152  of the electrospray system  150 . The first separating layer  28   a , the second separating layer  28   b , and a center separating layer  28   d  can each be made separately by etching silicon or simple machining of the metal or stamping the polymer. Once fabricated, the first separating layer  28   a , the second separating layer  28   b , and a center separating layer  28   d  each can be bonded to the nozzle array using a polyimide layer (such as Kapton). This dry film can be laminated and patterned using laser micro-machining or photolithography techniques. This spacer layer can then be affixed/bonded to the piezoelectric transducer to form the operational device. 
     It should be noted that the first separating layer  28   a , the second separating layer  28   b , and a center separating layer  28   d  can be disposed on portions of the first array structure  22   a  and the second array structure  22   b  prior to the formation of the first separating structure  132   a  and the second separating structure  132   b  and/or the ejector nozzle sealing structure  136 . In addition, the first fluid  134   a  and the second fluid  134   b  can be disposed in the first ejector structure  26   a  and second ejector structure  26   b  after the first separating layer  28   a , the second a separating layer  28   b , and the center separating layer  28   d  are formed. 
     In this regard, a structure including the first ejector structure  26   a  and the second ejector structure  26   b  and the first separating layer  28   a , the second separating layer  28   b , and the center separating layer  28   d  can be produced. Then in a separate process, the ejector nozzle sealing structure  136  can be positioned adjacent the first ejector nozzle  24   a  and the second ejector nozzle  24   b , respectively. Subsequently, the first fluid  134   a  and the second fluid  134   b  can be dispensed into the first ejector structure  26   a  and second ejector structure  26   b , respectively. Lastly, the first separating structure  132   a  and the second separating structure  132   b  can be disposed on the top of the first ejector nozzle  24   a  and the second ejector nozzle  24   b , respectively. 
     In another embodiment not shown, the lower portion  152  does not include the first separating layer  28   a , the second separating layer  28   b , and the center separating layer  28   d . The first separating layer  28   a , the second separating layer  28   b , and the center separating layer  28   d  are disposed on the upper portion  154 . Therefore, the upper portion  154  with the first separating layer  28   a , the second separating layer  28   b , and the center separating layer  28   d  disposed thereon can be reused. In still another embodiment, the first separating layer  28   a , the second separating layer  28   b , and the center separating layer  28   d  can be removed separately from either the upper portion  154  or the lower portion  152 . 
       FIG. 8J  illustrates the lower portion  152  of the electrospray system  150  and the upper portion  154  of the electrospray system  150 , and  FIG. 8K  illustrates the formation of the electrospray system  150  by joining (e.g., bonding and/or adhering) the lower portion  152  and the upper portion  154 . It should be noted that the lower portion  152  could be produced separately and be used as a disposable cartridge that is replaced regularly on the electrospray system  150 , while the upper portion  154  is reused. 
       FIGS. 9A through 9D  are illustrations of top views of representative embodiments of an electrospray system  200 .  FIG. 9B  illustrates a fluid bubble in one section of the electrospray system  200 , while  FIG. 9C  illustrates a fluid bubble in the other section of the electrospray system  200 . The electrospray system  200  has a single actuator (not shown) positioned in communication with a first reservoir  202   a  and a second reservoir  202   b . The first reservoir  202   a  and the second reservoir  202   b  are separated from each other by a separating layer  206 . The first reservoir  202   a  and the second reservoir  202   b  are separated from the array structure (not shown) having a first ejector structure  204   a  and a second ejector structure  204   b  by a first separating structure and a second separating structure (not shown). The first ejector structure  204   a  and the second ejector structure  204   b  each contain a fluid within their respective cavities. 
       FIG. 9A  illustrates the electrospray system  200  in a state where only gas bubbles (not shown) are positioned within the first reservoir  202   a  and the second reservoir  202   b . As mentioned above, a gas bubble does not effectively couple to and transmit the ultrasonic pressure wave, so upon actuation of the actuator substantially no fluid is ejected from the first ejector structure  204   a  and the second ejector structure  204   b.    
       FIG. 9B  illustrates an acoustically responsive fluid bubble  208  in the second reservoir  202   b  of the electrospray system  200 . Since the fluid bubble  208  can substantially couple to and transmit the ultrasonic pressure wave, actuation of the actuator causes the fluid within the second ejector structure  204   b  to be ejected through the ejectors nozzles of the second ejector structure  204   b , but substantially no fluid is ejected from the first ejector structure  204   a  since the gas bubble does not effectively couple to and transmit the ultrasonic pressure wave produced by the actuator. 
       FIG. 9C  illustrates an acoustically responsive fluid bubble  208  in the first reservoir  202   a  of the electrospray system  200 . Since the fluid bubble  208  can substantially couple to and transmit the ultrasonic pressure wave, actuation of the actuator causes the fluid within the first ejector structure  204   a  to be ejected through the ejectors nozzles of the first ejector structure  204   a , but substantially no fluid is ejected from the second ejector structure  204   b  since the gas bubble does not effectively couple to and transmit the ultrasonic pressure wave produced by the actuator. 
       FIG. 9D  illustrates acoustically responsive fluid bubbles  208  in the first reservoir  202   a  and the second reservoir  202   b  of the electrospray system  200 . Since the fluid bubble  208  can substantially couple to and transmit the ultrasonic pressure wave, actuation of the actuator causes the fluid within the first ejector structure  204   a  and the second ejector structure  204   b  to be ejected through the ejectors nozzles of the first ejector structure  204   a  and the second ejector structure  204   b.    
       FIGS. 10A through 10F  are illustrations of top views of representative embodiments of an electrospray system  220  that may be used in a multiplexing format and/or parallel analysis.  FIGS. 10B through 10E  illustrate an acoustically responsive fluid bubble  208  being positioned from one section of the electrospray system  220  to another. The electrospray system  220  has a single actuator (not shown) positioned in communication with a first reservoir  222   a , a second reservoir  222   b , a third reservoir  222   c , and a fourth reservoir  222   d . The first reservoir  222   a , the second reservoir  222   b , the third reservoir  222   c , and the fourth reservoir  222   d  are separated from each other by a first separating layer  226   a  and a second separating layer  226   b . The first reservoir  222   a , the second reservoir  222   b , the third reservoir  222   c , and the fourth reservoir  222   d  are separated from the array structure (not shown) having a first ejector structure  224   a , a second ejector structure  224   b , a third ejector structure  224   c , and a fourth ejector structure  224   d , by a first separating structure, a second separating structure, a third separating structure, and a fourth separating structure (not shown). The first reservoir  222   a , the second reservoir  222   b , the third reservoir  222   c , and the fourth reservoir  222   d , each contain a fluid within their respective cavities. 
       FIG. 10A  illustrates the electrospray system  220  in a state where only gas bubbles (not shown) are positioned within the first reservoir  222   a , the second reservoir  222   b , the third reservoir  222   c , and the fourth reservoir  222   d . As mentioned above, a gas bubble does not effectively couple to and transmit the ultrasonic pressure wave. Thus, upon actuation of the actuators substantially no fluid is ejected from the first ejector structure  224   a , the second ejector structure  224   b , the third ejector structure  224   c , and the fourth ejector structure  224   d.    
     Similar to  FIGS. 9A through 9D , an acoustically responsive fluid bubble  208  is controllably moved from the first reservoir  222   a  to the fourth reservoir  224   c  in a stepwise manner in  FIGS. 10B through 10E . Since the fluid bubble  208  can substantially couple to and transmit the ultrasonic pressure wave, actuation of the actuator causes the fluid within the ejector structure having the fluid bubble disposed in the corresponding reservoir to be ejected through the ejectors nozzles of the that ejector structure. However, substantially no fluid is ejected from the other ejector structures since the gas bubble does not effectively couple to and transmit the ultrasonic pressure wave produced by the actuator. 
       FIG. 10F  illustrates an acoustically responsive fluid bubble  208  in the first reservoir  222   a  and the fourth reservoir  224   c . Since the fluid bubble  208  can substantially couple to and transmit the ultrasonic pressure wave, actuation of the actuator causes the fluid within first ejector structure  224   a  and the fourth ejector structure  224   d  to be ejected through the ejectors nozzles of the each ejector structure. In other embodiments, the fluid bubble  208  can be positioned in one or more of the reservoirs so that one or more fluids within the ejector structures can be ejected simultaneously. 
     While embodiments of electrospray system are described in connection with Examples 1 and 2 and the corresponding text and figures, there is no intent to limit embodiments of the electrospray system to these descriptions. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of embodiments of the present disclosure. 
     EXAMPLE 1 
     On-Demand Droplet Formation and Ejection Using Micromachined Ultrasonic Atomizer: 
     While embodiments of electrospray system are described in connection with examples 1 and 2 and the corresponding text and figures, there is no intent to limit embodiments of the electrospray system to these descriptions. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of embodiments of the present disclosure. An exemplary embodiment of a representative electrospray system has been developed and tested on a mass spectrometer (MS). As shown in  FIG. 11 , it includes of a piezoelectric transducer, a fluid reservoir, and a silicon cover plate containing the micromachined ejector nozzles, similar to the design in  FIG. 1 . A PZT-8 ceramic is selected for the piezoelectric transducer. The device generates droplets by utilizing cavity resonances in the about 1 to 5 MHz range, along with the acoustic wave focusing properties of liquid horns formed by a silicon wet etching process. At resonance, a standing acoustic wave is formed in the fluid reservoir with the peak pressure gradient occurring at the tip of the nozzle leading to droplet ejection. Finite element analysis using ANSYS (2003) not only confirms the acoustic wave focusing by the horn structure shown in  FIG. 11 , but also accurately predicts the resonant frequencies at which the device provides stable droplet ejection. 
     Although a number of horn shapes are capable of focusing acoustic waves, a pyramidal shape was selected as it can be readily fabricated via, for example, a single step potassium hydroxide (KOH) wet etch of (100) oriented silicon. As shown in  FIG. 12 , when square patterns are opened in a mask layer material, such as silicon nitride ( FIG. 12 , steps  2  and  3 ), deposited on the surface of a (100) oriented silicon wafer, and the edges are aligned to the &lt;110&gt; directions, the KOH solution etches the exposed (100) planes more rapidly than the (111) planes yielding a pyramid shaped horn ( FIG. 12 , step  4 ) making a 54.74° angle with the plane of the wafer. The sizes of the square features representing the base of the pyramid are designed so that the tip of these focusing pyramidal horns terminate within about 1 to 20 μm of the opposite surface of the ejector plate. 
     As the last step of the process, the nozzles of the desired diameter (about 3 to 5 μm in this embodiment) are formed by exemplary dry etching the remaining silicon from the opposite side in inductively coupled plasma (ICP) using a patterned silicon oxide layer as the hard mask ( FIG. 12 , steps  6  and  7 ). As shown in the Scanning Electron Micrographs (SEMs) in  FIGS. 13A and 13B , this simple exemplary process, with only two masks and two etching steps, has been used to fabricate hundreds of pyramidal horns with nozzles on a single silicon wafer. 
       FIGS. 14A through 14C  illustrate the device in operation, where the clouds of generated aerosol are emanating from the device.  FIGS. 14B and 14C  show enhanced stroboscobic images of about 8 μm and about 5 μm diameter water droplets ejected from a single nozzle on different wafers, at a frequency of about 1.4 MHz and about 916 kHz, respectively. By making the nozzles even smaller or exploiting the instabilities of the liquid interface during droplet formation (e.g., by promotion formation of electrocapillary waves at the fluid interface), it may be possible to produce even smaller, sub-micron droplets using this droplet generation technology. 
     EXAMPLE 2 
     Electrospray Generation of Protein Ions at Low Applied Voltages and Ms Analysis: 
     Protein ions suitable for high sensitivity mass spectrometric analysis with an ionization voltage below 300 V (rather than kilovolts required by the conventional nanospray sources) have been produced using embodiments of the electrospray system.  FIG. 15  illustrates a schematic of the experimental setup in which an electrode of the piezoelectric transducer is also used for electrochemical charging of the fluid by applying DC bias voltage in addition to the AC signal used for sound waves generation.  FIG. 16  shows a strong peak of the 609 Da molecular weight compound (with signal-to-noise ratios of 3 or better) obtained in MS analysis of the mixture containing a standard low molecular weight test peptide, such as reserpine (MW=609 Da, CAS# 50-55-5), ionized using the embodiment of the electrospray system. 
     Although the best methodologies of this disclosure have been particularly described in the foregoing disclosure, it is to be understood that such descriptions have been provided for purposes of illustration only, and that other variations both in form and in detail can be made thereupon by those skilled in the art without departing from the spirit and scope of the present invention, which is defined solely by the appended claims.