Patent Publication Number: US-6992299-B2

Title: Method and apparatus for aerodynamic ion focusing

Description:
PRIORITY CLAIM 
     The present invention claims priority to previously filed provisional application Ser. No. 60/433,993, filed on Dec. 18, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the delivery of ions to ion detection devices. More specifically, the invention describes a method and apparatus for improving the ability to focus ions after they are formed by using a front-end device so that a greater number of ions can be directed to an ion detection device for detection or further analysis. 
     2. Description of Related Art 
     The prior art is replete with improvements in systems that enable the formation of ions, and in the detection and analysis thereof. However, one of the difficulties of performing ion detection and analysis is the task of delivering a large quantity of ions to an ion detection or analysis device. The ions are difficult to direct to an appropriate orifice of an ion detection device for various reasons that are known to those skilled in the art. Nevertheless, the more ions that can be delivered to the ion detection device, the more “sensitive” or accurate the results will be. Such devices include an electron multiplier, Faraday plate, ion mobility spectrometer, and a time-of-flight mass spectrometer. In general, the present invention should be considered to apply to any device that needs to perform ion detection and/or analysis, whatever that device might be. But all of these devices should be considered to fall within the single descriptive term of “ion detection device”. 
     An important technique referred to as “electrospray ionization” was developed in order to improve the process of delivering ions to an ion detection device. For example, in electrospray ionization, a liquid sample is directed through a free end of a capillary tube or orifice, wherein the tube is coupled to a high voltage source. In general, the free end of the capillary or electrospray sprayer tip is spaced apart from an orifice plate or capillary that has a sampling orifice that leads to a vacuum chamber of the ion detection device. The orifice plate is also coupled to the high voltage source. The electric field generates a spray of charged droplets, and the droplets evaporate to produce ions. 
     Electrospray ionization has grown to be one of the most commonly used ionization techniques for mass spectrometry, and efforts continue to improve its performance. Typically, the electrospray tip must be very close to the orifice of the ion detection device in order to maximize the conduction of ions from the electrospray tip into the ion detection device. However, due to space-charge repulsion, most ions never reach the sampling orifice. 
     Nevertheless, the significance of electrospray ionization for mass spectrometry has recently been emphasized by the rewarding of the Nobel Prize for work in this area. Electrospray ionization is most recognized today for its application to biomolecules where high “sensitivity is of paramount importance.” It should be remembered that throughout this document, sensitivity more accurately refers to the total number of ions that can be delivered to an ion detection device. Electrospray ionization is known for its high sensitivity; however, the present invention will demonstrate that this process has the potential of becoming even more sensitive. 
     It is now known that a major limitation in sensitivity when using electrospray ionization and with ion detection devices is due to low ion transmission from the electrospray ionization source through the atmosphere-to-vacuum sampling orifice into an inner chamber of an ion detection device such as a mass spectrometer. Although the ionization efficiency approaches 100%, the typical ion transmission efficiency from the electrospray ionization source to the extraction region of the ion detection device is only 0.01-0.1%. 
     When dealing with ion detection devices, it is important to look closely at the process of ion delivery. During the process of electrospray ionization, analyte ions are generated at atmospheric pressure and transferred into a low-pressure extraction region of the mass spectrometer via a conductance-limiting aperture located in a high pressure region. Gas-phase collisions and Coulombic repulsion that are inevitably involved result in expansion of the ion cloud, directing ions away from the extraction region of the mass spectrometer, thus decreasing the sensitivity. Although conventional ion optic devices based on Coulombic effects can effectively focus ions in vacuum, they are largely ineffective in avoiding or reversing ion-cloud expansion generated by gas-phase collisions and Coulombic repulsion at high pressures. 
     To assist in desolvation and transmission of ions from the electrospray sprayer tip to the sampling capillary inlet, Henion et al. taught an “ion spray” device in which a high velocity sheath flow nebulizing gas was directed past the electrospray sprayer tip. By optimizing the flow rate for focusing and desolvating the electrosprayed ions, an approximately 30% increase in ion signal intensity was obtained as compared to a relatively low flow rate. However, no indication was given as to ion signal improvement compared to a conventional electrospray ionization source without the nebulizer-assisted device. 
     The prior art as taught by Covey et al. teaches that an electrospray ionization source in which a heated gas flow was directed at an angle toward the flow axis of a nebulizer-assisted electrospray source, and intersected the droplet flow at a region upstream of the sampling orifice of the mass spectrometer. The intersecting gas flows mixed with the droplet flow in a turbulent fashion, and helped to desolvate ions in the droplets and move them toward the sampling orifice. The intersecting flow device reportedly provided an increase in sensitivity of over 10 times, and significantly lowered the background in the resulting mass spectra. However, it was necessary to carefully control the two flows and the angle between them for stability of the electrospray. Thus, better performance may be difficult to obtain and maintain. 
     The prior art as taught by Smith et al. has improved the sensitivity of electrospray ionization by designing a so-called “ion funnel” in the first vacuum stage of the mass spectrometer between the sampling capillary inlet, and a skimmer that is internal to a mass spectrometer. This ion funnel consists of a series of cylindrical ring electrodes of progressively smaller internal diameters. By co-applying radio frequency (RF) and direct current (DC) electric fields on the electrode series, the ion cloud is more effectively focused and the Coulombic-driven ion cloud expansion is reduced under pressures of 10 −4  up to 9 Torr. Thus, ions are more efficiently captured, focused and transmitted as a collimated ion beam from the sampling orifice to the skimmer. Over an order of magnitude increase in ion signal intensity was reported as compared to a conventional electrospray ionization source. 
     A recent improvement to this ion funnel is the use of a multi-capillary inlet. With the combination of multi-capillary inlet and ion funnel, Kim et al. reported ion transmission efficiencies that are 23 times greater than can be obtained with conventional electrospray ionization ion optics. However, the ion funnel improves ion transport only at reduced pressures and cannot be applied at atmospheric pressure conditions between the electrospray tip and sampling nozzle where most ion losses occur. 
     Until now, few users have reported effective methods of improving ion signals in the high pressure region between the sprayer and the sampling orifice of the mass spectrometer. One group of users placed a ring electrode downstream from an electrospray ionization sprayer to focus ions into the mass spectrometer. Another group of users employed a focusing ring at atmospheric pressure on the inner wall of a heated glass capillary interface for electrospray ionization. Still other groups reported similar designs of a heated silica capillary to assist in desolvation in which end plate and cylindrical lenses were mentioned. These lenses were located at substantial distances from both the sprayer and the inlet orifice of the heated capillary. According to these users, the electrode rings were useful; however, no mention was made concerning how much they improved ion signal intensities. Finally, another group of users described the use of an oblong-shaped stainless steel electrode ring that was connected to a high voltage power supply, and placed near the electrospray tip at a potential less than that of the sprayer. It was reported that this lens produced a 2-fold increase in ion signal intensity and a 2-fold reduction in the signal relative standard deviation (RSD). Other advances included an increase in formation of multiple charged ions, less critical positioning of the sprayer for optimum performance, and more ease in use compared to the ion funnel and intersecting flow devices. 
     An alternative to focusing the electrospray ion beam toward the sampling orifice is to place the electrospray tip as close to the sampling nozzle as possible so that a larger portion of the spray enters the vacuum region. Low flow rates from small-bore electrospray ionization tips are desirable for stability of the “Taylor cone” and production of fine electrospray droplets. This combination has been accomplished using microspray and, especially, nanospray sources. The improvement in response can be explained by the fact that sprayed droplets are already small enough to produce gas-phase ions directly. Analyte concentrations down to low picomolar can be easily sprayed without sheath flow or pneumatic assistance for mass spectrometer detection. 
     For this reason, microspray and nanospray sources can be operated with the electrospray tip very close to the sampling orifice of the mass spectrometer. However, the closeness is limited by the electrical discharge threshold between the high voltage sprayer and the nozzle counter electrode, which is dependent on the voltage applied to the electrospray ionization sprayer tip. In order to overcome these limitations, different groups of users reported low-pressure electrospray devices, in which analyte solutions were electrosprayed inside the vacuum chamber at reduced pressures. Unfortunately, incomplete desolvation largely offset any improvement in increased sample introduction. Moreover, when the electrospray device was positioned in a very-low-pressure region, one group of users reported significant loss of analytes and fine droplets on the walls of the vacuum chamber and heated transfer line, thus, seriously decreasing the sensitivity. 
     It is generally believed that electrospray ionization technology has reached the point where modifications produce only minor gains in ion transmission in atmospheric pressure regions of electrospray ionization sources. Accordingly, what is needed are new approaches that will take advantage of the high sensitivity that is potentially available but not yet exploited by state of the art techniques in ion delivery. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved method and apparatus for delivering ions to a sampling orifice of an ion detection device to thereby improve sensitivity by increasing the total number of ions that are delivered thereto. 
     In a preferred embodiment, the present invention is a method and apparatus for focusing ions for delivery to an ion detection device using an aerodynamic ion focusing system that uses a high-velocity converging gas flow to focus an ion plume by reducing spreading and increasing desolvation of ions, and wherein a voltage is applied to at least a portion of the aerodynamic ion focusing system to assist in the focusing and delivery of ions to the ion detection device. 
     In a first aspect of the invention, a voltage gradient is created in the aerodynamic ion focusing device to thereby assist in focusing and conduction of ions. 
     In a second aspect of the invention, non-diverging gas flow reduces spreading of an electrospray plume of ions. 
     In a third aspect of the invention, converging gas flow reduces spreading of an electrospray plume of ions. 
     In a fourth aspect of the invention, concentric gas flow reduces spreading of an electrospray plume of ions. 
     These and other objects, features, advantages and alternative aspects of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a perspective diagram of the elements of a first embodiment made in accordance with the principles of the present invention. 
         FIG. 2  is a cut-away profile view of the aerodynamic ion focusing device of the present invention. 
         FIG. 3  is a cut-away profile view of the aerodynamic ion focusing device that illustrates desired air flow that is used to create a trajectory for ions that concentrates them for delivery to an ion detection device. 
         FIG. 4  is a cut-away profile view of the aerodynamic ion focusing device of  FIG. 3  with more detail regarding a portion that has been modified to enable application of an electrical potential so as to thereby create a voltage gradient. 
         FIG. 5A  is a mass spectra obtained without the aerodynamic ion focusing device. 
         FIG. 5B  is a mass spectra obtained with the aerodynamic ion focusing device without convergent gas flow but with applied voltage. 
         FIG. 5C  is a mass spectra obtained with the aerodynamic ion focusing device with convergent gas flow but without applied voltage. 
         FIG. 5D  is a mass spectra obtained with the aerodynamic ion focusing device with convergent gas flow and with applied voltage. 
         FIG. 6A  is a graph showing the base peak intensity as a function of distance between the electrospray tip and the capillary inlet. 
         FIG. 6B  is a graph showing the base peak intensity as a function of distance between the electrospray tip and the capillary inlet. 
         FIG. 6C  is a graph showing the base peak intensity as a function of distance between the electrospray tip and the capillary inlet, but without the aerodynamic ion focusing device. 
         FIG. 7A  is a graph of ion intensity when the electrospray tip was moved off-axis by +/−2 mm while the capillary inlet was axially fixed. 
         FIG. 7B  is a graph of ion intensity when the capillary inlet was moved off-axis by +/−2 mm while the electrospray tip was axially fixed. 
         FIG. 8  is a graph of the base peak intensity as plotted against concentration with the aerodynamic ion focusing device in its optimum position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made to the drawings in which the various elements of the present invention will be given numerical designations and in which the invention will be discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that the following description is only exemplary of the principles of the present invention, and should not be viewed as narrowing the claims which follow. 
       FIG. 1  is a provided as an overview of the method and apparatus taught by the present invention for the focusing and delivery of ions to an ion detection device. The improvements in the system result in substantial gains in the number of ions that are capable of being delivered to an ionic detection device. 
       FIG. 1  is a perspective view of the present invention. An aerodynamic ion focusing device  10  is shown having an entrance aperture  12 , a main body  14 , and an exit aperture  16 . A power supply  18  is indicated as applying a voltage. Note that an electrospray tip  8  is shown as being partially inserted into the entrance aperture  12 . An ion detection device  20 , such as a time-of-flight mass spectrometer, is shown as having a sampling orifice  22  at a junction between a vacuum chamber  24  of the ion detection device  20  and a nozzle or capillary inlet  26  that extends outwards from the ion detection device and towards the aerodynamic ion focusing device  10 . This document also discusses an electrospray tip. An electrospray tip  8  creates ions that are “sprayed” near or into the entrance aperture  12  of the aerodynamic ion focusing device  10 . 
     The electrospray tip  8  is not considered an element of the apparatus of the present invention, but is important because of the plume of ions that it generates and delivers to the aerodynamic ion focusing device  10 . Other sources of ions would include atmospheric pressure chemical ionization (APCI), and photoionization. These are examples only, and should not be considered a limiting factor. 
     It should be noted at the outset that the sampling orifice  22  of the ion detection device  20  does not need to have a capillary inlet  26 . The sampling orifice  22  may have any configuration of shaped walls around it to assist in directing ions into the ion detection device  20 . Accordingly, the presence of the capillary inlet  26  should not be considered a limiting factor, but is simply an illustration of one possible embodiment. 
     The critical aspects of the invention relate to the ability to use the flow of gas into the aerodynamic ion focusing device  10  to focus an ion plume from an electrospray tip or other source of ions near the entrance aperture  12 . A second critical aspect of the invention is the ability to apply a voltage to the aerodynamic ion focusing device  10  and thereby generate a voltage gradient along a portion of the length thereof that can also be used to focus the ion plume. 
       FIG. 2  is provided as a cut-away perspective view of the internal structure of one possible configuration of the aerodynamic ion focusing device  10 . Significant features include the entrance aperture  12 , the exit aperture  16 , a nitrogen gas supply inlet  30 , an annular chamber  32 , an annular gap  34 , induced input airflow lines  36 , and resulting output airflow lines  38 . These features illustrate the aspect of the aerodynamic ion focusing device to provide improved performance only because of gas flow. 
     It is significant to consider that electrospray ionization has grown to be one of the most commonly used ionization techniques for ion detection. Typically, an electrospray tip must be very close to a sampling orifice of an ion detection device in order to maximize the conduction of ions from the electrospray tip into the ion detection device. However, because of space-charge repulsion, most ions never reach the sampling orifice. The aerodynamic ion focusing device  10  shown in  FIG. 2  is a device based at least upon venturi and coanda effects. As a front-end for an ion detection device, the present invention improves upon the number of ions that are delivered thereto. Thus, the sensitivity of the ion detection is considered to be improved. 
     For example, when a series of reserpine solutions were monitored using mass spectrometry, an over 5-fold increase in ion intensity was measured for a separation distance of 14 mm between the electrospray tip and the capillary inlet, as compared to when the electrospray tip was in its normal position 1 mm in front of the capillary inlet without the aerodynamic ion focusing device. When a voltage was applied to the aerodynamic ion focusing device to further assist in focusing electrosprayed ions, approximately an 18-fold increase in ion intensity was obtained. In addition, a 34-fold improvement in method detection limit was observed. 
     While the aerodynamic ion focusing device  10  of  FIG. 2  is based upon the principles of venture and coanda effects, it should be explained that the present invention does not need to use either of these principles in order to operate. A gas flow that can be made to perform the function of drawing ions into a desired trajectory for delivery to an ion detection device can be created using other means. 
     For example, consider a device that creates suction at the exit aperture  16  to draw the ions into the aerodynamic ion focusing device  10 . For that matter, even a mildly diverging gas flow into the entrance aperture  12  of the aerodynamic ion focusing device  10  could also perform the desired function. This is because a stream of gas that is rapidly moving past the electrospray tip is sufficient to reduce the divergence of the ion plume. The moving gas pulls the ion plume into a long and thin ion plume. This can be true even if the gas flow is mildly diverging. The important point to understand is that the gas stream causes the ion plume to be drawn into a thin plume, which reduces space-charge and the subsequent expansion of the ion plume. 
     The nature of the trajectory has not been specifically addressed. However, if the trajectory is not linear, one useful purpose of such a trajectory would be to separate spray droplets from the ion plume. Therefore, the trajectory should not be considered to be limited to only a linear one, as there are advantages to non-linear trajectories. 
       FIG. 3  is provided to explain the improved operational aspects of the aerodynamic ion focusing device  10  because of the creation of a desired gas flow. The inert gas nitrogen is used to create the desired flow of gases into and through the aerodynamic ion focusing device  10 . The desired flow of gases is any flow that will result in a confinement of an electrospray ion plume at the entrance aperture  12  of the aerodynamic ion focusing device  10 . Increased confinement of the electrospray ion plume is more likely to result in a larger number of ions that are deliverable and delivered to the ion detection device  20 . 
     While nitrogen gas has been used, other gases can also be used, including helium, argon, and air. What is important is the function being performed by the gas, and that is to create a gas flow that drives an ion plume into a desired trajectory so that a larger number of ions can ultimately reach an ion detection device. 
     In the present embodiment of the invention, the desired flow of gases that result in increased confinement of the electrospray ion plume is created by the shape of the aerodynamic ion focusing device  10 , and the nature of the gas flow therethrough. Thus for example, a coanda effect on the nitrogen gas being introduced through the annular gap  34  is demonstrated when the gas immediately changes a direction of flow so as to stay relatively flush against and therefore to generally follow the contours of the inner surface of the aerodynamic ion focusing device  10 . This feature of the gas is indicated by nitrogen gas flow lines  40  in FIG.  3 . The flow of the nitrogen gas will thus cause the electrospray ion plume at the entrance aperture  12  to be concentrated along a trajectory that is shaped and determined by the gas. 
     For example, in this embodiment, the electrospray ion plume is likely to travel along a center or midpoint of the nitrogen gas flow, as shown by the trajectory indicated at  42 . In this embodiment, trajectory  42  should generally be considered to be coaxial with the entrance and exit apertures  12 ,  16  because of the symmetry of the aerodynamic ion focusing device  10  and the resulting gas flow therethrough that is induced by the flow of the nitrogen gas. 
     For example, the ion plume will be restricted because of the convergence of the air that is being pulled into the aerodynamic ion focusing device  10  at the entrance aperture  12  because of the flow of the nitrogen gas. In addition, the nitrogen gas flow can also be used to restrict the ion plume so as to be output in a planar structure. This feature of the present invention is thus determinable by the shape of the aerodynamic ion focusing device  10 . 
     Ideally, an entrance to the capillary inlet  26  extending from the ion detection device  20  will be positioned along trajectory  42  in order to take advantage of the ions that have been confined to this trajectory. Experimental results have shown approximately a 100-fold increase in concentration of ions that can be delivered to the ion detection device  20 . 
     It is a critical aspect of the invention to observe that the desired air flow into the entrance aperture  12  of the aerodynamic ion focusing device  10  can be characterized as a converging gas flow. This desired characteristic may also be classified more broadly as simply a non-diverging gas flow. As mentioned previously, even a mildly diverging gas flow, if properly directed, can create the desired effect on the ion plume. Another term that can be used to describe this desired gas flow is a concentric gas flow. 
     More specifically, the action of the high velocity nitrogen gas streaming down the exit aperture  16  of the aerodynamic ion focusing device  10  causes a pressure drop that induces a large flow of ambient air into the entrance aperture  12  of the aerodynamic ion focusing device  10 . The net effect is that the aerodynamic ion focusing device  10  uses the energy from a small volume of compressed nitrogen gas to produce a large volume, large velocity, and low-pressure outlet gas flow  38 . The volume of the outlet gas flow  38  can be as high as 100 times the supply flow, that is, 400 to 600 L min −1 . However, it should be remembered that these volumes are typical only for this particular aerodynamic ion focusing device shown here and may be different for different configurations of aerodynamic ion focusing devices  10 , and should therefore not be considered a limiting factor. 
     Not only can increased concentration of the ion plume along trajectory  42  be obtained by creating a desired gas flow into the aerodynamic ion focusing device  10 , it can be further increased through application of another aspect of the invention. Specifically, the use of a voltage gradient and the resulting electric field lines within the aerodynamic ion focusing device  10  can be used to enhance concentration of the electrospray ion plume at the entrance aperture  12 . The presently preferred embodiment of the invention is thus an aerodynamic ion focusing device  10  that is capable of generating a voltage gradient along at least a portion thereof. 
     It should be noted that an increasing voltage gradient is defined herein as a voltage gradient that drives the ions towards a desired trajectory through the device, whatever the actual voltage applied may be. 
     Thus, the present invention includes the means for applying an electrical potential to at least a portion of the aerodynamic ion focusing device  10 .  FIG. 4  is provided as a cut-away schematic illustration of one embodiment of the aerodynamic ion focusing device  10  that is capable of having a voltage applied thereto. In this figure, the entrance aperture  12  is shown disposed within a portion  50  that has been modified so as to be at least slightly electrically conductive. The electrical conductivity is made possible by the introduction of conductive materials, such as carbon, that enable the application of an electrical potential across the portion  50 . 
     It should be understood that a voltage gradient can be created within the aerodynamic ion focusing device  10  in various ways, and many may be appropriate in the present invention. For example, the conductivity of the materials used can be varied in order to obtain a voltage gradient. In addition, separate segments or rings could be disposed along a portion of the length of the aerodynamic ion focusing device  10 . Conductive inks or other types of electrode traces might also be disposed at various intervals. What is important to the present invention is that a voltage gradient can be formed by producing a gradation in the resistivity of the material and/or a change in the cross sectional area of the material. Thus, all of these methods can be considered to be within the scope of the present invention. 
     There are many materials that are suitable for use as the slightly electrically conductive portion of the aerodynamic ion focusing device  10 . These materials include PolyEtherImide and PolyAmide-Imide. These materials are relatively highly resistive, but are sufficiently conductive to enable application of a voltage that results in creation of a voltage gradient. The voltage gradient was modeled in software to predict its characteristics, but this is not required in order to obtain a desired voltage gradient. Generally, the voltage gradient functions so as to further focus the electrospray ion plume being introduced into the aerodynamic ion focusing device  10 . 
     The power supply  18  is used to apply the electrical potential across the portion  50 . The size of the electrical potential applied to the aerodynamic ion focusing device  10  is easily determined through experimentation. 
     It is therefore useful to look at some experimental results that demonstrate the effectiveness of the present invention to perform as desired. To test the electrospray ion plume focusing aspect of the present invention, ion signal enhancements were studied. 
     A series of reserpine concentrations were analyzed under the conditions of (1) no aerodynamic ion focusing device  10 , (2) with the aerodynamic ion focusing device  10  and applied voltage (1.9-2.0 kV), but no venturi-induced gas flow, (3) with the aerodynamic ion focusing device  10  and venturi-induced gas flow, but no applied voltage; and (4) with the aerodynamic ion focusing device  10 , venturi-induced gas flow, and applied voltage. Ten determinations of each measurement were made for statistical considerations. The capillary interface was heated to 75° C. 
     In these experiments, a JAGUAR™ time-of-flight mass spectrometer with a homemade heated capillary inlet was used to test the ion focusing of the present invention. An aluminum air amplifier was re-machined out of stainless steel and disposed between an electrospray tip and capillary inlet of a mass spectrometer. Two high-voltage power supplies were connected to the electrospray tip source, aerodynamic ion focusing device  10 , capillary inlet  26 , and skimmer and set at 2.8 to 4.0 kV, 0.0 to 3.0 kV, 300 V, and 65 V, respectively. The aerodynamic ion focusing device  10  was grounded, except when a voltage was applied to the entrance aperture  12 . The various reserpine solutions were introduced at an infusion rate of 1.5 μL min −1 . 
       FIGS. 5A-5D  are a series of graphs that show examples of mass spectra obtained without the aerodynamic ion focusing device ( 5 A), with the aerodynamic ion focusing device  10  without convergent gas flow but with applied voltage ( 5 B), with the aerodynamic ion focusing device  10  with convergent gas flow but without applied voltage ( 5 C), and with the aerodynamic ion focusing device  10  with convergent gas flow and with applied voltage ( 5 D). 
     These experiments were performed with the electrospray tip axially disposed 6 mm inside the entrance aperture  12  of the aerodynamic ion focusing device  10 , and a capillary inlet  26  was positioned 22.5 mm inside the exit aperture  16  of the aerodynamic ion focusing device (i.e., the electrospray tip was 14 mm from the capillary inlet  26  along the axial direction). 
     The greatest enhancement in ion signal intensity was observed when desired ambient air flow and applied voltage were used together in the aerodynamic ion focusing device  10 . Using the aerodynamic ion focusing device  10  without convergent gas flow but with applied voltage, the ion signal intensity increased by over 50% as compared to when no aerodynamic ion focusing device was used at all. With gas flow through the aerodynamic ion focusing device  10  and no voltage applied, over a 5-fold increase (amplification factor) was obtained. When 1.9-2.0 kV was applied to the aerodynamic ion focusing device  10  with venturi-induced gas flow, an 18-fold increase in ion signal intensity was obtained. 
     To find the optimum positions of an electrospray tip and capillary inlet  26  as described above, the electrospray tip, aerodynamic ion focusing device  10 , and capillary inlet  26  positions were axially modified relative to each other until the measured ion intensity was at a maximum. This was accomplished by moving the electrospray tip from 12 mm inside the entrance aperture  12  to 20 mm outside the entrance aperture at 1 mm increments and, at each increment, moving the electrospray tip and the aerodynamic ion focusing device  10  axially together so that the capillary inlet  26  was axially positioned from 25.5 mm inside the exit aperture  16  to 8.5 mm outside the exit aperture. It was discovered that that when the electrospray tip was axially positioned 6 mm inside the entrance aperture  12  of the aerodynamic ion focusing device  10  and the capillary inlet  26  was axially disposed 22.5 mm inside the exit aperture  16 , the ion intensity reached its peak value. 
     The results of these two experiments are illustrated in  FIGS. 6A and 6B .  FIG. 6A  shows the base peak intensity as a function of distance between the electrospray tip and the capillary inlet  26  when the nozzle was axially fixed 22.5 mm inside the exit aperture  16  of the aerodynamic ion focusing device  10 .  FIG. 6B  illustrates the base peak intensity as a function of distance between the electrospray tip and the capillary inlet  26  when the electrospray tip was axially fixed 6 mm inside the entrance aperture  12 . 
     Furthermore, a relatively broad range of electrospray tip and capillary inlet  26  positions was found for maintaining strong ion signal intensities. Even when the distance between the electrospray tip and the capillary inlet  26  was 20 mm, the ion intensity was still higher when the electrospray tip was disposed 1 mm in front of the sampling orifice  22  without the aerodynamic ion focusing device  10 , as shown in FIG.  6 C. 
     To evaluate the relationship between ion intensity and off-axis distance of the electrospray tip or the capillary inlet from their optimum positions, each was moved off-axis while the other was axially fixed in its optimum position. When the electrospray tip was moved off-axis by +/−2 mm while the capillary inlet was axially fixed, the ion intensity decreased by 40% as shown in FIG.  7 A. When the capillary inlet was moved off-axis by +/−2 mm while the electrospray tip was axially fixed, the ion intensity decreased by 19% as shown in FIG.  7 B. Very little loss in ion signal intensity was observed when the electrospray tip or the capillary inlet was moved +/−1 mm off axis. 
     Finally, the base peak intensity was plotted against concentration with the aerodynamic ion focusing device  10  in its optimum position as illustrated in FIG.  8 . After linear regression, the method detection limits were calculated on the basis of concentrations corresponding to three times the signal-to-noise ratio. A 34-fold improvement in method detection limit was obtained. In addition to enhancing analyte ion intensity, the aerodynamic ion focusing device  10  also suppresses background chemical noise. 
     Any gain in ion signal intensity is attributed to the ability of the aerodynamic ion focusing device  10  to stabilize the electrospray and improve conduction of ions into the ion detection device  20 . The electrospray tip can be located farther from the sampling orifice  22  than for conventional electrospray to produce better desolvation and less possibility of discharge. Another advantage of the aerodynamic ion focusing device  10  is that the electrospray can be positioned along the axial direction straight toward the capillary inlet  26 . Complex devices with off-axis orientation of the electrospray tip with respect to the capillary inlet  26  for separating ions from neutrals and improving desolvation are not necessary. 
     It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention. The appended claims are intended to cover such modifications and arrangements.