Abstract:
A High Field Asymmetric Waveform ion Mobility Spectrometry (FAIMS) apparatus comprises (a) a first and a second gas inlet; a)) an expansion chamber receiving ions from an ion source and the first and second gas flows from the first and second gas inlets, respectively; (c) an outer electrode having a generally concave inner surface and comprising: (i) an ion inlet operable to receive, from the expansion chamber, the ions and a combined gas flow comprising portions of the first and second gas flows; and (ii) an ion outlet; and (d) an inner electrode having a generally convex outer surface that is disposed in a spaced-apart and facing arrangement relative to the inner surface of the outer electrode for defining an ion separation region therebetween, wherein the combined gas flow and a portion of the ions travel through the ion separation region from the ion inlet to the ion outlet.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of the filing date, under 35 U.S.C. 119(e), of U.S. Provisional Application for Patent No. 61/648,940, tiled on May 18, 2012 and titled “Control of Gas Flow in High Field Asymmetric Waveform Ion Mobility Spectrometry”, said Provisional application assigned to the assignee of the present invention and incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to guiding ions in the presence of a gas and, more particularly, to simultaneously controlling the flow of gas and the flow of ions in field-asymmetric ion mobility spectrometers and mass spectrometers. 
       BACKGROUND OF THE INVENTION 
       [0003]    In ion mobility spectrometry devices, separation of gas-phase ions is accomplished by exploiting variations in ion drift velocities under an applied electric field arising from differences in ion mobility. One well-known type of ion mobility spectrometry device is the High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) cell, also known by the term Differential Ion Mobility Spectrometry (DMS) cell, which separates ions on the basis of a difference in the mobility of an ion at high field strength (commonly denoted as K h ) relative to the mobility of the ion at low field strength (commonly denoted as K). Briefly described, a FAIMS cell comprises a pair of spaced apart electrodes that define therebetween a separation region through which a stream of ions is directed. An asymmetric waveform comprising a high voltage component and a lower voltage component of opposite polarity, together with a DC voltage (referred to as the compensation voltage, or CV) is applied to one of the electrodes. When the ion stream contains several species of ions, only one ion species is selectively transmitted through the FAIMS cell for a given combination of asymmetric waveform peak voltage (referred to as the dispersion voltage, or DV) and CV. The remaining species of ions drift toward one of the electrode surfaces and are neutralized. The FAIMS cell may be operated in single ion detection mode, wherein the DV and CV are maintained at constant values, or alternatively the applied CV may be scanned with time to sequentially transmit ion species having different mobilities. FAIMS cells may be used for a variety of purposes, including providing separation or filtering of an ion stream prior to entry into a mass analyzer. 
         [0004]      FIG. 1  schematically depicts a first known system  100  for analyzing ions that includes a FAIMS device  155 . A solution of sample to be analyzed is introduced as a spray of liquid droplets into an ionization chamber  105  via atmospheric pressure ion source  110 . Ionization chamber  105  is maintained at a high pressure relative to the regions downstream in the ion path, typically at or near atmospheric pressure. Atmospheric pressure ion source  110  may be configured as an electrospray ionization (ESE) probe, wherein a high DC voltage (either positive or negative) is applied to the capillary or “needle” through which the sample solution flows. Other suitable ionization techniques may be utilized in place of ESI, including without limitation such well-known techniques as atmospheric pressure chemical ionization (APCI), heated electrospray ionization (HESI), and thermospray ionization. 
         [0005]    Ions produced by the ion source enter the FAIMS cell  155  through an aperture  117  in an entrance plate  120  and then through an inlet orifice  150  after passing through an expansion chamber  111 . The expansion chamber is provided with a gas, typically helium or other inert gas, which is introduced into the expansion chamber  111  via a gas conduit  113 . A portion of the gas flows back into the ionization chamber  105  through entrance plate aperture  117  in counter-flow to the ions and droplets and serves to desolvate charged droplets. Another portion of the gas combines with the analyte ions in chamber  111  and serves as a carrier gas through the FAIMS cell  155 . The combined ion/carrier gas flow then enters FAIMS cell  155  through inlet orifice  150 . The carrier gas flow may be carefully metered to maintain flow rates within predetermined limits which will depend on the FAIMS cell size, electrode geometry, and operational considerations. An electrical potential difference is maintained between the entrance plate  120  and the FAIMS cell  155  and, thus, physical separation is maintained between these components. Accordingly, a non-conducting sealing element  173 , such as a gasket or P-ring maintains the FAIMS gas within the apparatus and prevents contamination of this gas from outside air. Because of drawing-space limitations, this sealing element is not explicitly shown in some of the accompanying drawings. 
         [0006]    Generally speaking, the FAIMS cell  155  includes inner and outer electrodes  165  and  170  having radially opposed surfaces, which define therebetween an annular separation region  175  (an “analytical gap”) through which the ions are transported. The FAIMS cell geometry depicted in  FIG. 1 , as well as in other figures herein may be generally referred to as a “side-to-side FAIMS cell”, in which the longitudinal axes (axes of cylindrical surfaces, directed out of the page) of inner electrode  165  and outer electrode  170  are oriented transversely with respect to the overall direction of ion flow. The principles of the design and operation of FAIMS cells and other ion mobility spectrometry devices have been extensively described elsewhere in the art (see, for example, U.S. Pat. No. 6,639,212 to Guevremont et al., incorporated by reference herein in its entirety), and hence will not be described in detail herein. In brief, the carrier gas and ions flow through the separation region  175  from inlet orifice  150  to exit orifice  185 . Ion separation is effected within the separation region (analytical gap)  175  of the FAIMS cell  155  by applying an asymmetric waveform having a peak voltage (DV) and a compensation voltage (CV) to one of the inner or outer electrodes,  165 ,  170 . The values of CV and DV arc set to allow transmission of a selected on species through separation region  175 . Other ion species having different relative values of high field and low field mobilities will migrate to the surface of one of the electrodes and be neutralized. 
         [0007]    Still referring to  FIG. 1 , the selected ions emerge from the FAIMS cell  155  through exit orifice  185  and pass through a small gap  183  separating the FAIMS cell  155  from a mass spectrometer  157 . Whereas most of the carrier gas exhausts through the gap  183  at atmospheric pressure, ions are electrostatically guided into at least one reduced pressure chamber  188  of the mass spectrometer  157  through an orifice in the mass spectrometer or through an ion transfer tube  163 . The at least one reduced pressure chamber may be evacuated by a vacuum port  191 . At least a portion of ion transfer tube  163  may be surrounded by and in good thermal contact with a heat source, such as heater jacket  167 . The heater jacket  167 , which may take the form of a conventional resistance heater, is operable to raise the temperature of ion transfer tube  163  to promote further desolvation of droplets entering the ion transfer tube  163 . 
         [0008]    From the at least one reduced pressure chamber  188 , ions are transferred through an orifice  193  of a skimmer  194  into a high vacuum chamber  195  maintained at a low pressure (typically around 100 millitorr) relative to the reduced pressure chamber  188 . The high vacuum chamber  195  is typically evacuated by turbo or similar high-vacuum pumps via a vacuum port  197 . The skimmer  194  may be fabricated from an electrically conductive material, and an offset voltage may be applied to skimmer  194  to assist in the transport of ions through interface region and into skimmer orifice  193 . Ions passing through skimmer orifice  193  may be focused or guided through ion optical assembly  198 , which may include various electrodes forming ion lenses, ion guides, ion gates, quadrupole or octopole rod sets, etc. The ion optical assembly  198  may serve to transport ions to an analyzer  199  for mass analysis. Analyzer  199  may be implemented as any one or a combination of conventional mass analyzers, including (without limitation) a quadrupole mass analyzer, ion trap, or time-of-flight analyzer. 
         [0009]    Computational and experimental studies have shown that the gas stream carrying ions from the ion source enters the FAIMS separation region  175  with high velocity. This high velocity gas flow causes the gas stream (including a significant portion of the ions) to impinge onto the portion of the inner electrode  165  that directly faces the inlet orifice  150  prior to turning into the analyzer gap, thus discharging a significant percentage of the ion population of interest on the inner electrode. An angular gas stream flowing out of the entrance plate and into the source region may also skew and misalign the ion beam with FAIMS entrance, thereby partially steering the ion beam onto the entrance plate causing further ion loss. 
         [0010]    The computational and experimental studies reveal poor ion transmission from the ion source to the exit of FAIMS, e.g. a transmission of approximately 10% for bromochloroacetate anion [BCA, having a mass-to-charge ratio, m/z, of 173]. For example,  FIG. 2  shows the results of calculated simulations of ion trajectories of the BCA anion within the FAIMS  155 , with the ion-cloud region  127  indicating the region within which most of the ions flow. The simulations shown in  FIG. 2 , which include simulations of ion flow within a flowing gas, indicate that significant ion losses occur at the entrance plate  120  and on a portion of the inner electrode  165  that is exposed to the FMS inlet  150 . Curve  204   FIG. 3  illustrates the results of a simulated CV scan of the BCA anion through the known FAIMS apparatus of  FIG. 2 . This may be compared with curve  202 , which presents the results of a second simulation in which ions are hypothetically introduced between the FAIMS electrodes, without encountering the entrance plate. The overall transmission of ions from the ion source to the exit orifice is only approximately 10%. Although ions are lost near the FAIMS entrance, few ions are lost inside the analytical gap  175  because, owing to the cylindrical shape of the FAIMS electrode ( FIG. 1 ), ions within the separation region  175  that is away from the inlet orifice  150  experience a non-uniform electric field which causes spatial focusing of ions that are travelling in the gap. This ion focusing results in only minimal ion loss of the selected ion species during transport as a result of diffusion or the separation field. In accordance with the above considerations, there is a need in the art of ion transport and analysis for improved means for controlling the effects of gas flow on ion trajectories. 
       SUMMARY 
       [0011]    In a first aspect of the present teachings, a dual gas inlet is provided to maintain a pressure balance in a FAIMS apparatus. Such a dual gas inlet generates symmetrical flows inside the FAIMS gap as well as in an associated ion source region. In a related aspect, a new design of electrode assembly is proposed to facilitate a gentle flow of gas around the FAIMS entrance. 
         [0012]    In another aspect in accordance with the present teachings, a novel gas and ion orifice structure is provided for electrodes. The novel orifice structure applies the principles of the Coand{hacek over (a)} effect, which is the general tendency of a fluid jet to be drawn towards and follow the contour of a curved solid surface. When used as a gas and ion inlet of a FAIMS outer electrode, the novel orifice structure produces curved streamlines of a carrier gas flow such that the gas stream enters a FAIMS gap with a progressive turn away from the inner electrode so as to flow generally parallel to the inner walls of the outer electrode. The orifice is constructed such that the edges of the outer electrode orifice are smoothly curved such that the inner diameter of the orifice increases towards both extremities of the orifice. A gas stream flowing against this curvature will adhere closely to the surface and gradually flow along the curvature. When this curvature is incorporated into the electrode geometry, the as stream entering the FAIMS will flow gently inside the analyzer gap rather than impact the inner electrode. 
         [0013]    In various embodiments, a High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) apparatus is provided, the apparatus comprising: (a) an ion source; (b) a first and a second gas inlet; (c) an expansion chamber receiving first and second gas flows from the first and second gas inlets, respectively; (d) an outer electrode having a generally concave inner surface and comprising: (i) an ion inlet operable to receive ions from the ion source and to receive a combined gas flow comprising portions of the first and second gas flows from the expansion chamber; and (ii) an ion outlet; and (e) an inner electrode having a generally convex outer surface that is disposed in a spaced-apart and facing arrangement relative to the inner surface of the outer electrode for defining an ion separation region therebetween, wherein the combined gas flow and a portion of the ions are received into the ion separation region from the ion inlet and travel through the ion separation region from the ion inlet to the ion outlet. 
         [0014]    In various embodiments, a High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) apparatus is provided, the apparatus comprising: (a) an expansion chamber receiving ions from an ion source and a gas flow from a gas inlet; (b) an outer electrode having a generally concave inner surface and comprising: (i) an ion inlet orifice operable to receive ions from the ion source and a portion of the gas flow from the expansion chamber, the ion inlet orifice comprising an orifice wall, an orifice inlet end and an orifice outlet end, the orifice wall being convexly curved between the inlet end and the outlet end; and (ii) an ion outlet; and (c) an inner electrode having a generally convex outer surface that is disposed in a spaced-apart and facing arrangement relative to the inner surface of the outer electrode for defining an ion separation region therebetween, wherein the combined gas flow and a portion of the ions are received into the ion separation region from the ion inlet and travel through the ion separation region from the ion inlet to the ion outlet. 
         [0015]    In various other embodiments, A High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) apparatus is provided, the apparatus comprising: (a) an ion source; (b) at least one gas inlet; (c) an expansion chamber receiving ions from the ion source and a gas flow from the at least one gas inlet; (d) a first electrode having a convexly curved surface; and (e) a second electrode have a concavely curved surface that is disposed in a spaced-apart and facing arrangement relative to the convexly curved surface of the first electrode for defining an ion separation region therebetween, the ion separation region receiving the ions and a portion of the gas flow from the expansion chamber, wherein a deflection of an average ion trajectory upon entering the ion separation region is greater than zero degrees but substantially less then ninety degrees. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The above noted and various other aspects of the present invention will become apparent from the following description which is given by way of non-limiting example only and with reference to the accompanying drawings, not drawn to scale, in which: 
           [0017]      FIG. 1  is a schematic diagram depicting a first known system for analyzing ions including an ion mobility device; 
           [0018]      FIG. 2  shows a simulation of a region of ion flow from an ion source region into and through a standard-configuration side-to-side FAIMS apparatus similar to that depicted in  FIG. 1 ; 
           [0019]      FIG. 3  illustrates a simulated CV scan of bromochloroacetate (BCA) anion through the FAIMS apparatus of  FIG. 2 , in comparison to a second simulation in which ions are hypothetically introduced between the FAIMS electrodes; 
           [0020]      FIG. 4  shows a simulation of a region of ion flow from an ion source region into and through a side-to-side FAIMS apparatus having novel dual gas inlets in accordance with the present teachings; 
           [0021]      FIG. 5  shows a simulation of a region of ion flow from an ion source region into and through a side-to-side FAIMS apparatus having a novel FAIMS electrode assembly in accordance with the present teachings; 
           [0022]      FIG. 6  illustrates gas flow streamlines, as modeled by computational fluid dynamics, through flow channels of a FAIMS apparatus in accordance with the present teachings; 
           [0023]      FIG. 7  illustrates gas flow streamlines, as modeled by computational fluid dynamics, through flow channels of a FAIMS apparatus having entrance electrode that is modified so as to exploit the Coand{hacek over (a)} effect; 
           [0024]      FIG. 8  a simulation of a region of ion flow from an ion source region into and through a side-to-side FAIMS apparatus that is modified so as to exploit the Coand{hacek over (a)} effect; 
           [0025]      FIG. 9  illustrates a comparison between simulated CV scans corresponding to the FAIMS apparatuses of  FIG. 2  and  FIG. 8 ; 
           [0026]      FIG. 10A  illustrates a doubly-cutaway perspective view of of another FAIMS apparatus in accordance with the present teachings; 
           [0027]      FIG. 10B  illustrates a cross-section view (both upper and lower diagrams) of the entrance plate and a portion of the outer electrode of the FAIMS apparatus of  FIG. 10A  and schematically illustrates flow vectors (lower diagram) within the gas expansion chamber of the apparatus; 
           [0028]      FIG. 11  illustrates a contour plot of calculated gas flow velocities within the FAIMS apparatus of  FIG. 10 ; and 
           [0029]      FIG. 12  illustrates a view of the internal gas volume within the FAIMS apparatus of  FIG. 10 , illustrating a grid of finite elements used in the fluid dynamic computations used to generate the data illustrated in  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0030]    The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments and examples shown but is to be accorded the widest possible scope in accordance with the features and principles shown and described. The particular features and advantages of the invention will become more apparent with reference to the appended  FIGS. 1-9 , taken in conjunction with the following description. 
         [0031]      FIG. 4  schematically illustrates an embodiment of a FAIMS apparatus in that represents a first modification of the apparatus shown in  FIGS. 1-2 . In contrast the single gas conduit  113  employed in the FAIMS apparatus  155  ( FIG. 1 ), the FAIMS apparatus  101  ( FIG. 4 ) employs two gas conduits  113   a,    113   b  that simultaneously deliver gas flows to the expansion chamber  111  and that are symmetrically disposed about the inlet orifice  150 . This symmetric gas introduction within the apparatus  101  eliminates the tendency within the conventional apparatus for a portion of the ions to be pushed, by gas flow from the single conduit  113 , towards one side of the apparatus midline. The results of the combined electrostatic and fluid dynamic modeling, which are indicated as ion cloud  123 , show a symmetrical flow of ions from source to FAIMS inlet with little or no loss on the entrance plate  120 . It is here noted that all fluid dynamic computations referenced in this document were performed using COMSOL Multiphysics® engineering simulation software which is commercially available from COMSOL, Inc., 10850 Wilshire Boulevard, Suite 800 Los Angeles, Calif. 90024 USA. Electric fields and charged particle trajectories were calculated using SIMION® charged particle optics simulation software commercially available from Scientific Instrument Services of 1027 Old York Rd. Ringoes, N.J. 08551-1054 USA. Combined fluid dynamic and ion trajectory computations were performed by first calculating bulk gas flows using the COMSOL Multiphysics® software and then inputting the results into a collision model within the SIMION® package. 
         [0032]    Even though loss of ions to the entrance plate is minimized within the apparatus  101  ( FIG. 4 ), a nontrivial proportion of ions are still lost by neutralization at the ion inlet orifice of the outer electrode  170  and at a portion of the inner electrode  165  that faces the ion inlet orifice. Computational fluid dynamics calculations indicate that the use of two gas inlet conduits in the configuration shown in  FIG. 4  increases the velocity of the gas stream entering the analytical gap and thus accelerates the ions. The excessive ion speed causes collision with the inner electrode resulting in low ion transmission through the apparatus  101 . 
         [0033]    Further design modifications are illustrated in  FIG. 5  so as to further reduce ion loss near the FAIMS entrance. Accordingly, the FAIMS apparatus  103  illustrated in  FIG. 5  represents a further improvement relative to the FAIMS apparatus  101  of  FIG. 4 . In both the conventional side-to-side FAIMS apparatus  155  ( FIGS. 1-2 ) as well as the apparatus  101  ( FIG. 4 ), the cylindrical symmetry dictated by the use of an inner electrode  165  comprising a cylinder with circular cross section causes both the carrier gas and the ions of interest to diverge to two different pathways after passing through the ion inlet orifice  150 . The geometry is such that a non-trivial proportion of ions are neutralized against the inner electrode  165 . The FAIMS apparatus  103  shown in  FIG. 5  partially overcomes this problem by replacing the inner electrode  165  and Outer electrode  170  by a first electrode  166  and a second electrode  171 , respectively, wherein the first electrode  166  has a convexly curved surface  11  and the second electrode  171  has a concavely curved surface  13 . The first and second electrodes are disposed such that the surfaces  11 ,  13  are arranged in a spaced-apart facing arrangement so as to define an annular channel  186  between the surfaces. The FAIMS apparatus  103  retains the two gas conduits  113   a,    113   b  as previously described. However, the inventors have determined that performance is improved if the gas conduits deliver gas flow into the expansion chamber  111  with a flow component in the same direction as the flow of ions or, in other words, with the gas conduits delivering gas from the ion source side of the expansion chamber as shown in  FIG. 5 . 
         [0034]    When a dispersion voltage (DV) and compensation voltage (CV) are applied to at least one of the first and second electrodes  166 ,  171  of the FAIMS apparatus  103 , the annular channel  186  then serves as an analytical gap, as is well-known with regard to the conventional side-to-side FAIMS  155  shown in  FIG. 1  as well as with regard to other known FAIMS apparatus designs. However, because the FAIMS  103  does not comprise a cylindrical inner electrode and comprises only a single analytical gap  186 , the gas and ion flow, as indicated by the ion cloud  125 , does not separate into diverging pathways, as occurs for conventional side-to-side FAIMS apparatuses. 
         [0035]    The geometry of the electrode surfaces  11 ,  13  of the FAIMS apparatus  103  are such that, upon entering the analytical gap  186 , the average flow of ions is in a direction that is not directly toward and not perpendicular to either of the surfaces  11 ,  13  defining the channel  186 . Instead, the geometry is such that the inlet orifice  150  into the channel  186  intersects the expansion chamber  111  at an angle such that the average flow direction of ions changes by substantially less than ninety degrees upon their entrance into the channel. This is illustrated in inset  83  of  FIG. 5  which shows an enlarged portion of the MIMS apparatus  103 . In the inset, the average or overall trajectory of ions passing into the expansion chamber  111  towards the two electrodes is shown as trajectory  85 , whereas the average ion trajectory upon entering the channel (analytical gap)  185  is shown as trajectory  87 . The angle of deflection of average ion trajectory is thus greater than zero degrees but substantially less then ninety degrees. As a result, the combined electrostatic and fluid dynamic simulation shows very little ion loss to either electrode, thus suggesting that the novel FAIMS design illustrated in  FIG. 5  delivers ions into the FAIMS gap  186  with high efficiency. 
         [0036]      FIG. 6  provides another schematic cross-sectional view of a dual-gas-inlet FAIMS apparatus  107 , somewhat similar to the apparatus previously illustrated in  FIG. 4 . One difference between the apparatus  107  shown in  FIG. 6  and the apparatus  101  shown in  FIG. 4  is that, in the FAIMS the apparatus  107 , the entry of gas into the expansion chamber  111  from the gas conduits  113   a,    113   b  is directed from the entrance-plate side (or, equivalently, from the ion-source side) of the expansion chamber instead of from the side that is opposite to the entrance plate. Inletting the gas in the fashion shown in  FIG. 6  reduces the degree of gas counter-flow into the ion source region, thus producing greater flow of carrier gas into the analytical gap  175 . In  FIG. 6 , several flow vectors are provided so as to indicate calculated gas flow within the ion expansion chamber  111  and the analytical gap  175 . These vectors indicate only as flow, as calculated from fluid dynamics principles, and do not incorporate ion movement. The ion inlet orifice  150  is indicated in  FIG. 6  as a simple cylindrical bore hole through the outer electrode  170 . This shape of the ion inlet orifice shown in  FIG. 6  is similar to its shape in the conventional side-to-side FAIMS apparatus  155  ( FIG. 1 ). The resulting flow profile is high velocity and aligned directly onto the inner electrode  165  where this electrode faces the ion inlet orifice  150 . Experimental data also show this phenomenon in the form of a buildup of chemical material on the inner electrode. The fluid dynamics calculations graphically illustrated in  FIG. 6  also indicate the existence of some degree of recirculation flow within the analytical gap on either side of the ion inlet orifice  150 . Such recirculation could possibly cause unwanted neutralization of ions on both inner and outer electrodes. 
         [0037]      FIG. 7  shows the FAIMS gas flow into an electrode set that is modified so as to decrease the volume and rate of gas flow directly onto the inner electrode. The FAIMS apparatus  109  that is schematically illustrated in  FIG. 7  is identical to the FAIMS apparatus  107  shown in  FIG. 6  except with regard to the shape of the ion inlet orifice. Inset  300  of  FIG. 7  illustrates an enlarged view of the vicinity of the ion inlet orifice  151  of the FAIMS  109 . Note that the wall of the ion inlet orifice  151  of the outer electrode as shown in  FIG. 7  is slightly enlarged and rounded as compared to the squared off orifice  150  in the conventional electrode set (e.g.,  FIG. 6 ). The walls  301  of the ion inlet orifice  151  of the FAIMS apparatus  109  are convexly curved between the orifice inlet end  302  and the orifice outlet end  303 . Thus, the inner diameter of the ion inlet orifice is at a minimum value within the orifice. Because of the curvature, the inner diameter smoothly increases or flares outward in both directions (i.e., towards the two ends of the orifice) away from the region of minimum diameter. The gas flow in the vicinity of the rounded walls of the ion inlet orifice  151  demonstrates the so-called Coand{hacek over (a)} effect, which is the general tendency of a fluid jet to be drawn towards and follow the contour of a curved solid surface. By means of the Coand{hacek over (a)} effect, the carrier gas flow entering the analytical gap  175  of the FAIMS apparatus  109  ( FIG. 7 ) is kept closer to the curvature of the entrance orifice than would otherwise be the case. This behavior allows for incorporation of the gas stream into the gap and away from the inner electrode as is indicated in  FIG. 7  by the smooth divergence of gas flow vectors away from the center electrode  165  and into the analytical gap  175 . The smooth divergence of the carrier gas into away front the center electrode and into the analytical gap  175  is expected to urge ions along similar pathways, thereby reducing the proportion of ions that are lost as a result of collision with the center electrode. As indicated by the fluid dynamics calculations, the smooth divergence also leads to a larger zone of laminar flow within the analytical gap, with reduced recirculation flow near the entrance orifice. 
         [0038]      FIG. 8  shows the results of combined fluid dynamic and ion trajectory modeling, through the FAIMS apparatus  109 .  FIG. 9  shows a comparison between the transmission efficiency of the apparatus having the curved an inlet orifice  151  and shown in  FIG. 8  (curve  212 ) with that of the prior FAIMS apparatus shown in  FIGS. 1 and 2  (curve  214 ). It is evident from the shape of the ion cloud  129  in  FIG. 8  that the curved orifice design promotes a smoother bifurcation of ion flow prior around the center electrode and into the analytic gap then is indicated for non-curved orifice designs. The smoother flow bifurcation appears to have the effect of reducing gas recirculation flow with the analytical gap just after passing through the inlet orifice, thereby significantly reducing ion neutralization at both inner and outer electrodes (for example, compare  FIG. 8  with  FIG. 1 ). 
         [0039]    It should be noted that, in addition to the provision of a curved inlet orifice, the design of the FAIMS apparatus  109  illustrated in  FIG. 8  further differs from the design of the known FAIMS apparatus ( FIG. 1 ) through the provision of a wider aperture  117  in the entrance plate  120  as well as through slight widening of the inlet ion orifice  151  (measured at its point of minimum width). Specifically, the width of the entrance plate aperture  117  is approximately 60% greater in the FAIMS  109  and the width of the ion inlet orifice (measured at its minimum diameter) is approximately 26% greater than the respective aperture widths in the known FAIMS apparatus  155  ( FIGS. 1 and 2 ). These wider apertures are such as to maintain the width of the ion cloud  129  at its region of passage through the ion inlet orifice the novel FAIMS  109  ( FIG. 8 ) the same as its width at its region of passage through the ion inlet orifice of the known FAIMS ( FIG. 2 ) in which it nearly completely fills the orifice. 
         [0040]    The simple re-design of the cross-sectional shape of the ion inlet orifice—as indicated by comparison of  FIG. 7  with FIG.  6 —improves the uniformity of flow of carrier gas through the FAIMS apparatus. This smoother flow is such that there is highly reduced flow rate of the carrier gas (and entrained ions) directly onto the electrodes, relative to the conventional FAIMS apparatus  155  ( FIG. 2 ). This smoother flow is believed to have a major effect in yielding the results of  FIG. 9 , in which simulated CV scans through the FAIMS  109  and through the FAIMS  155  are shown as curves  212  and  214 , respectively. The calculated approximate 10-fold improvement in transmission is achieved with minimal re-design of the overall apparatus dimensions or shape, relative to the conventional side-to-side FAIMS. In some implementations, this simple modification of an existing FAIMS design may be more desirable than using a different form of FAIMS apparatus, such as the apparatus  103  of  FIG. 5 . 
         [0041]      FIG. 10A  illustrates a perspective sectional view (a quarter section view) of another FAIMS apparatus  201  in accordance with the present teachings. The illustration in this figure is doubly-cutaway view that is cut away in a plane (the x-z plane) that includes the cylindrical axis  177  of the FAIMS inner electrode  165  and is also cut away in a plane (the y-z plane) that is perpendicular to the cylindrical axis  177 . Each such sectional plane bisects the overall FAIMS apparatus  201  and, thus, the view shown in  FIG. 10A  is a quarter-section view.  FIG. 10B  illustrates a cross-section view (both upper and lower diagrams of  FIG. 10B ) of the entrance plate and a portion of the outer electrode of the LAMS apparatus of  FIG. 10A .  FIG. 10B  also schematically illustrates (lower diagram only) flow vectors within the gas expansion chamber  111  of the apparatus  201 . 
         [0042]    Many of the elements of the FAIMS apparatus  201  are similar to corresponding elements in other FAIMS apparatuses previously described herein. However, the FAIMS apparatus  201  differs from previously-described apparatus with regard to the configuration of the ion inlet orifice  152 , the gas expansion chamber  111  and the relationship between the inlet orifice  152  and the gas expansion chamber  111 . As shown, the apparatus  201  also includes a desolvation chamber  115  recessed into the entrance plate  120  and surrounding the entrance plate aperture  117 . 
         [0043]    In contrast to apparatuses previously described herein, the expansion chamber  111  of the apparatus  201  forms a recess within the entrance plate  120  in a fashion so as to circumferentially surround the ion inlet orifice  152 . Further, the expansion chamber recess is provided such that the a portion of the walls of the ion inlet orifice  152  protrude into the expansion chamber  111  so as to form a ring  119  that circumferentially surrounds a portion of the ion inlet orifice  152 . The space between the entrance plate  120  and the inlet end  302  of the ion inlet orifice comprises a narrow gap  126  between the entrance plate and the inlet end of the ion inlet orifice. The entrance plate  120  is configured such that an overlap portion  128  of a face of the entrance plate that bounds the expansion chamber  111  extends beyond the expansion chamber so as to also face the ring portion  119  of the wails of the ion inlet orifice  152 . 
         [0044]    As a result of the configuration shown in  FIGS. 10A-10B , gas that enters the expansion chamber  111  through was conduits  113  is caused to flow around the circumference of the ring  119  and then to flow into and through the gap  126 . The apparatus  201  is configured such that the width of the gap  126  is significantly less than the width of the expansion chamber  111 , wherein the width of the gap is measured between the entrance plate  120  and the inlet end  302  of the ion inlet orifice  152  and the width of the chamber  111  is measured between the facing surfaces of the entrance plate  120  and the outer electrode  170  that bound the chamber. Because of these different widths, the gas pressure and flow velocity are both caused to increase as the gas flows into the gap. The increased-velocity gas flow then enters the inlet end  302  of the ion inlet orifice  152  at through the entirety of the gap  126  that circumferentially surrounds the inlet end of the ion inlet orifice  152 . Ions pass through the aperture  117  in the entrance plate and then cross the gap  126  and pass into the ion inlet orifice  152  where they are entrained in the gas flow. The overlap portion  128  of the entrance plate confines the gas to the gap  126  and enables the increase in flow velocity. 
         [0045]    As is illustrated in the lower portion of  FIG. 10B , the increased gas flow velocity produced by the squeezing of the gas flow into the gap  126  causes a high-velocity boundary layer to form against the convexly curved interior walls of the ion inlet orifice  152 , thereby causing the high velocity gas to follow the curved surface of the walls in accordance with the Coand{hacek over (a)} effect. Thus, as previously discussed in regard to the apparatus  109  shown in  FIGS. 7-8 , the high velocity gas flow (and, consequently, the majority of the gas itself) is diverted into the analytical gap  175  so as to avoid impacting the inner electrode  165 . The majority of the entrained ions are carried along with the gas, thereby improving the ion throughput through the FAIMS apparatus. 
         [0046]    The curvature of the interior walls of the ion inlet orifice  152  of the FAIMS apparatus  201  ( FIG. 10A ) differs from the curvature of the walls of the inlet oriface  151  of the FAIMS apparatus  109  ( FIGS. 7-8 ) in that the walls of the inlet orifice  152  comprise different radii of curvature in different cross sections. In the x-z cross section taken parallel to the cylindrical axis  177  of the inner electrode, the radius of curvature is of the wall is r 1  whereas, in the y-z cross section oriented perpendicular to the axis  177 , the radius of curvature is r 2 , where r 2 &gt;r 1 . The reason for this difference in the wall curvature in different directions is that, in the y-z plane, the required deflection of the gas jet in they direction is greater than the amount of deflection that that is either necessary or desirable in the x-direction in the x-z plane. The greater degree of deflection required in the y-z plane is a simple result of the geometry of the annular analytical gap  175 . For example, consider gas that approaches the ion inlet orifice  152  along the negative-y direction in the expansion chamber  111 . In order to be completely diverted into the analytical gap  175 , the direction of flow must be diverted so as to have a component vector in the positive y-direction. No such requirement exists for gas that approaches the ion inlet orifice  152  along, for example, the positive-x direction in the expansion chamber  111 . In this latter case, the most important requirement is to maintain most of the gas flow within the analytical gap near an axis (not shown) that passes from the ion inlet orifice  152  to the ion exit orifice  185 . The greater radius r 2  in the y-z cross section allows the required greater amount of angular deflection of the gas jet to be accomplished sufficiently gradually such that the boundary layer does not detach from the curved wall. 
         [0047]      FIG. 11  illustrates a contour plot of calculated gas flow velocities within the annular analytical gap  175  of the FAIMS apparatus  201  of  FIG. 10 , shown along they-z plane. Regions with progressively greater flow velocities are shown with progressively darker shading. The calculations were performed using a computational fluid dynamics routing employing the finite element grid pattern shown in FIG  12 . The grid pattern in  FIG. 12  is superimposed upon the internal gas volume within the FAIMS apparatus of  FIG. 10 . Note that no apparatus elements are shown in  FIG. 12 . The grid or mesh spacing is not uniform. A more closely spaced mesh was used in the region around the curved orifice walls and within the vicinity of the axis that passes from the ion inlet orifice to the ion exit orifice so as to accurately model the gas flow in these regions in which the velocity and flow streamlines change rapidly. The average quality of the mesh cells around the curved wall surface was 0.85. 
         [0048]    As illustrated in  FIG. 1 , FAIMS apparatuses are often used in conjunction with mass spectrometers, so as to eliminate interfering background ions from an ion stream prior to mass spectrometry analysis. For practical reasons, there are size limitations to a FAIMS apparatus that is interfaced to a mass spectrometer in such a fashion. From a practical standpoint, the size of the FAIMS apparatus is limited such that the diameter of the inner electrode is limited to approximately 20 mm or less and the width of the analytical gap is limited to approximately 20 mm or less. Accordingly, the fluid dynamic calculations illustrated in the accompanying figures were performed so as to model a FAIMS apparatus within this size range. The fluid dynamics calculations indicate that decreasing the width of the gap  126  ( FIG. 10B ) yields greater gas jet velocities corresponding to better deflection around the curved inlet orifice walls and better overall throughput. If the gap width is greater than 0.75 mm, then the streamlines diverge and separate from the curved wall surface. However, for practical reasons, the width of the gap is preferably greater than about 0.25 mm. Also, increasing the gas flow rate gives higher throughput, provided that the gas flow does not become turbulent. The Reynolds number for flow in the modeled apparatus is approximately 70 for an internal flow rate of 0.5 m/s and is approximately 209 for an internal flow rate of 5.0 m/s, so the flow remains laminar. However, throughput needs to be balanced against FAIMS analytical resolution, which decreases as the flow rate increases. 
         [0049]    The discussion included in this application is intended to serve as a basic description. Although the present in has been described in accordance with the various embodiments shown and described, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments or combinations of features in the various illustrated embodiments and those variations or combinations of features would be within the spirit and scope of the present invention. The reader should thus be aware that the specific discussion may not explicitly describe all embodiments possible; many alternatives are implicit. As but one example,  FIG. 10  illustrates a FAIMS apparatus comprising both an ion inlet orifice  152  that has curved walls and that protrudes into a gas expansion chamber and also comprising multiple gas inlet conduits  113  providing FAIMS bath gas to the expansion chamber from opposing sides. Although both such features are beneficial, the benefits of the novel ion inlet orifice configuration could also be realized using a conventional single gas inlet conduit. As another example, although the FAIMS bath gas is illustrated as being provided through “conduits”, any form of gas inlet could suffice, such as for example, gas inlet “apertures”. Similarly, although the ion inlet is described herein as an “orifice”, it could equally take the form of or be described as an “aperture” or, in some embodiments, a “conduit”. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the scope and essence of the invention. Neither the description nor the terminology is intended to limit the scope of the invention—the invention is defined only by the claims. Any patents, patent applications or other publications mentioned herein are hereby explicitly incorporated herein by reference in their respective entirety.