Patent Publication Number: US-9431228-B2

Title: Ion lens for reducing contaminant effects in an ion guide of a mass spectrometer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The application is a National Stage filing under 35 U.S.C. §371 of PCT/CA/000543 filed on May 10, 2011, which designated the U.S., and which claims the benefit of U.S. Provisional Application Ser. No. 61/333,333 filed on May 11, 2010, the contents of which are incorporated herein by reference in their entireties. 
    
    
     FIELD 
     The specification relates generally to mass spectrometers, and specifically to an ion lens for reducing contaminant effects in an ion guide of a mass spectrometer. 
     BACKGROUND 
     In mass spectrometers, ion guides typically have an ion lens at an exit end comprising a plate having an aperture for ions from the ion guide to pass through. The ion lens can act as an element in a differential pumping system. However, such ion lenses are prone to contamination and hence are generally deficient. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       Implementations are described with reference to the following figures, in which: 
         FIG. 1  depicts a block diagram of a mass spectrometer with flat ion lenses, according to the prior art; 
         FIG. 2  depicts a block diagram of a mass spectrometer with ion lenses for reducing contaminant effects in an ion guide, according to non-limiting implementations; 
         FIG. 3  depicts a perspective view of an ion guide side of an ion lens for reducing contaminant effects in an ion guide, according to non-limiting implementations; 
         FIG. 4  depicts a perspective view of an ion exit side of the ion lens of  FIG. 4 , according to non-limiting implementations; 
         FIG. 5  depicts a cross-section of the ion lens of  FIG. 4 , according to non-limiting implementations; 
         FIG. 6  depicts a block diagram of the ion lens of  FIG. 4  in place at an exit region of an ion guide, according to non-limiting implementations; 
         FIGS. 7 and 8  depict cross-section of ion lens for reducing contaminant effects in an ion guide, according to non-limiting implementations; 
         FIG. 9  depicts a block diagram of the ion guide of  FIG. 4  in place at an exit region of an ion guide having a bevelled exit region, according to non-limiting implementations; 
         FIG. 10  depicts detail of elements  FIG. 9 , according to non-limiting implementations; and, 
         FIG. 11  depicts a graph showing results of testing a successful prototype of the ion lens of  FIG. 4 , according to non-limiting implementations. 
     
    
    
     DETAILED DESCRIPTION 
     A first aspect of the specification provides an ion lens for reducing contaminant effects in an ion guide of a mass spectrometer. The ion lens comprises a structural member comprising an orifice of a given radius, the structural member for supporting the ion lens at an exit region of the ion guide. The ion lens further comprises a conical member extending from the structural member, the conical member being hollow and comprising a given cone angle, and a base of the given radius, a perimeter of the base connected to a perimeter of the orifice, the conical member further comprising an aperture through an apex of the conical member, the aperture for receiving ions there through from the ion guide. 
     The given radius, and the given cone angle can enable at least a portion of the conical member, including the apex, to reside within the exit region of the ion guide 
     The orifice can be located in a centre portion of the structural member and the conical member can extend from the centre portion. 
     The cone angle can be at least one of: between 10° and 80°; between 40° and 50°; and 45°. 
     The conical member can comprise at least one of: a cone; a convex cone; and a concave cone. 
     The conical member can be complimentary to an exit region of the ion guide, and the exit region of the ion guide can comprise a shape that is an inverse of the conical member. The exit region of the ion guide can be bevelled. 
     The structural member can be at least one of: complimentary to an end face of the ion guide; planar; a cylindrical section; and a spherical section. 
     A second aspect of the specification provides a mass spectrometer. The mass spectrometer comprises an ion source. The mass spectrometer further comprises a plurality of ion guides for receiving ions from the ion source, each of the plurality of ion guides comprising an entrance region, an exit region and a passage there between for ions from the ion source to pass there through. The mass spectrometer further comprises at least one ion lens located at an end face of at least one of the plurality of ion guides. The at least one ion lens comprises a structural member comprising an orifice of a given radius, the structural member for supporting the ion lens at an exit region of the at least one of the plurality of ion guides. The at least one ion lens comprises a conical member extending from the structural member, the conical member being hollow and comprising a given cone angle, and a base of the given radius, a perimeter of the base connected to a perimeter of the orifice, the conical member further comprising an aperture through an apex of the conical member, the aperture for receiving ions there through from the at least one of the plurality of ion guides. The mass spectrometer further comprises a detector located after the plurality of ion guides and the at least one ion lens for detecting the ions. 
     The given radius, and the given cone angle can enable at least a portion of the conical member, including the apex, to reside within the exit region of the at least one of the plurality of ion guides. 
     The orifice can be located in a centre portion of the at least one of the plurality of ion guides and the conical member can extend from the centre portion. 
     The cone angle can be at least one of: between 10° and 80°; between 40° and 50°; and 45°. 
     The conical member can comprise at least one of: a cone; a convex cone; and a concave cone. 
     The conical member can be complimentary to the exit region of the at least one of the plurality of ion guides. The exit region can comprise a shape that is an inverse of the conical member. The exit region of the at least one of the plurality of ion guides can be bevelled. 
     The structural member can be at least one of: complimentary to an end face of the at least one of the plurality of ion guides; planar; a cylindrical section; and a spherical section. 
     The aperture of the at least one ion lens can be aligned with the exit region of the at least one of the plurality of ion guides. The structural member can be substantially parallel to the end face of the at least one of the plurality of ion guides. 
     The conical member and the exit region can form at least one channel for gas exiting the at least one of the plurality of ion guides to pass there through. The mass spectrometer can further comprise a sleeve surrounding the at least one of the plurality of ion guides for containing the gas until the gas reaches the at least one channel. 
     When the conical member becomes contaminated with the ions, an angle of a resulting electrical field and a longitudinal axis of the at least one of the plurality of ion guides can be greater than zero. 
     Contamination of optical elements of mass spectrometer, for example an ion guide, due to contaminant ions and particles (such as clusters and/or droplets) is problematic as it reduces the transmission efficiency of the ion guide which impacts sensitivity of the mass spectrometer and introduces irreproducibility due to charging of contaminated surfaces. This is a common problem for virtually all ion optical elements in a mass spectrometer. In the case of ion guides that employ collisional cooling, the area most sensitive to contamination is generally the area near the exit of the ion guide. In collisional focusing, ions are slowed down and focussed by collisions with buffer gas molecules in the ion guide. Thus, when the ions reach the exit end of the ion guide their velocities are nearly thermal. In some ion guides, the pressure is high enough that gas dynamics plays a significant role. A typical ion guide setup is depicted in  FIG. 1 , according to the prior art, which depicts a mass spectrometer  100  comprising a first ion guide  120 , a second ion guide  130 , a quadrupole  140 , a collision cell  150  (e.g. a fragmentation module) and a detector  160  (comprising any suitable detector, including but not limited to a ToF (Time of Flight) detector). Note that the quadrupole  140  and collision cell  150  can also be configured as ion guides. Mass spectrometer  100  is enabled to transmit an ion beam  165  from ion source  110  through to detector  160 . It is appreciated that each of first ion guide  120 , second ion guide  130 , quadrupole  140  and collision cell  150  act as an ion guide for ions to pass there through. Ion lenses  170   a ,  170   b ,  170   c ,  170   d  (collectively ion lenses  170  and generically an ion lens  170 ) are located at the exits of one or more of first ion guide  110 , second ion guide  130 , quadrupole  140  and collision cell  150 . It is appreciated that the pressure in some ion guides, for example the first ion guide  120 , can be high enough so that gas dynamics can play a significant role which can exacerbate contamination issues. 
     In the prior art, each ion lenses  170  comprises a flat plate with an orifice for ion beam  165  to pass through as depicted in  FIG. 1 . The flat plate and the corresponding orifice often acts as an element of a differential pumping system allowing ion beam  165  to pass into the next chamber with a different pressure while the flow of gas into the next chamber is restricted. In some cases the pressure in an adjacent chamber can be lower, while in other cases the pressure can be higher depending on the application. Collision cell  150  is an example of a chamber where ions from the previous ion guide (i.e. quadrupole  140 ) enter the next chamber (collision cell  150 ) which contains higher pressure of gas. Various interfaces for Atmospheric Pressure Ionization (API) sources represent cases where a following chamber is at lower pressure than a previous one. In any case, when ions exiting an ion guide approach the aperture of an ion lens  170 , they generally have relatively low kinetic energy, for example on the order of a Volt per unit charge. Any contaminated surface near the aperture that develops an electric potential on the order of one Volt or higher can significantly alter trajectories of ions and lead to the loss of transmission or undesired blocking of the ion beam  165 . Therefore, the region near the exit of an ion guide (such as first ion guide  120 , second ion guide  130 , quadrupole  140  and collision cell  150 ), and the area near the aperture of each ion lens  170 , become the most sensitive areas for contamination. The situation can be further complicated as some ion sources generate droplets and clusters in addition to the ions of interest. Such droplets and clusters can be accelerated by gas dynamic flow, for example in the area of ion source  110 , and fly straight into the area near the exit region of an ion guide. Thus, the area near the ion guide can be bombarded and eventually coated by the droplets and clusters containing analyte material. This effect produces thin films that can be non-conductive and charge up leading to the problem with transmission and ion blocking, as described above. 
     These contaminant problems are addressed in a mass spectrometer  200  as depicted in  FIG. 2 , according to non-limiting implementations. Mass spectrometer  200  is similar to mass spectrometer  100  and comprises a first ion guide  220 , a second ion guide  230 , a quadrupole  240 , a collision cell  250  (e.g. a fragmentation module) and a detector  260  (comprising any suitable detector, including but not limited to a ToF (Time of Flight) detector; it is appreciated that detector  260  is not to be considered particularly limiting). Mass spectrometer  200  is enabled to transmit an ion beam  265  from ion source  210  through to detector  260 . It is appreciated that each of first ion guide  220 , second ion guide  230 , quadrupole  240  and collision cell  250  act as an ion guide for ions to pass there through. In contrast to mass spectrometer  100 , mass spectrometer  200  comprises ion lenses  270   a ,  270   b ,  270   c ,  270   d  (collectively ion lens  270  and generically an ion lens  270 ) each of which comprise a structural member and a conical member, the conical member located at the exit of a respective ion guide (e.g. first ion guide  220 , second ion guide  230 , quadrupole  240  or collision cell  250 ). Ion lenses  270 , and alternatives thereof, will be described in detail below with respect to  FIGS. 3 to 11   
     In some implementations, mass spectrometer  200  can further comprise a processor  285  for controlling operation of mass spectrometer  200 , including but not limited to controlling ion source  210  to ionise the ionisable materials, and controlling transfer of ions between modules of mass spectrometer  200 . In operation, ionisable materials are introduced into ion source  210 . Ion source  210  generally ionises the ionisable materials to produce ion beam  265 , which is transferred to first ion guide  220  (also identified as QJet). Ion beam  265  is transferred to second ion guide  230  (also identified as Q 0 ) through ion lens  270   a . Ion beam  265  is transferred from second ion guide  230 , though ion lens  270   b , to quadrupole  240  (also identified as Q 1 ), which can operate as a mass filter. Ion beam  265 , filtered or unfiltered, exit quadrupole  240 , via ion lens  270   c , and enter collision cell  250  (also identified as q 2 ). In some implementations, ions in ion beam  265  can be fragmented in collision cell  250 . It is understood that collision cell  250  as well as first ion guide  220  and second ion guide  230  can comprise any suitable multipole, including but not limited to a quadrupole, a hexapole, an octopole, or any other suitable ion guide such as a ring guide, an ion funnel or the like. In some implementations, collision cell  250  comprises a quadrupole, mechanically similar to quadrupole  240 . Ion beam  265  is then transferred to detector  260 , via ion lens  270   d , for production of mass spectra. 
     Furthermore, while also not depicted, mass spectrometer  200  can further comprise any suitable number of connectors, power sources, RF (radio-frequency) power sources, DC (direct current) power sources, gas sources (e.g. for ion source  210  and/or collision cell  250 ), and any other suitable components for enabling operation of mass spectrometer  200 . While not depicted, mass spectrometer  200  can comprise any suitable number of vacuum pumps to provide a suitable vacuum in ion source  210 , first ion guide  220 , second ion guide  230 , quadrupole  240 , collision cell  250  and/or detector  260 . It is understood that in some implementations a vacuum differential can be created between certain elements of mass spectrometer  200 : for example a vacuum differential is generally applied between ion source  210 , first ion guide  220 , and second ion guide  230 , such that ion source  210  is at atmospheric pressure, second ion guide  230  is under vacuum (e.g. approximately 10 mTorr or any other suitable pressure), and first ion guide  220  has a pressure there between (e.g. approximately 1 Torr or any other suitable pressure). Each ion lens  270  can assist in creating a vacuum differential between elements of mass spectrometer  200 . 
     Furthermore, each ion lens  270  assists in reducing contamination effects in each of their respective ions guides (e.g. first ion guide  220 , second ion guide  230 , quadrupole  240  and collision cell  250 ), as described below. Furthermore, in the following description it is appreciated that the term ion guide can refer to one or more of ion guide  220 , second ion guide  230 , quadrupole  240  and collision cell  250 , unless otherwise noted. 
     Attention is directed to  FIGS. 3, 4 and 5 , which respectively depict a perspective front view of ion lens  270 , a perspective rear view of ion lens  270 , and a cross-sectional view of ion lens  270 , according to non-limiting implementations. Ion lens  270  comprises a structural member  305 . In some implementations, structural member  305  can be complimentary to an end face of an ion guide. In some of these implementations, the end face of each ion guide is generally flat, as depicted in  FIG. 2 , and hence structural member  305  is generally planar, as depicted. However structural member  305  can comprise a section a cylindrical section, a spherical section, or any other suitable shape. As can be seen in the rear perspective view of ion guide  270  in  FIG. 4 , and in  FIG. 5 , structural member comprises an orifice  410  of a given radius r. It is appreciated that orifice  410  can be substantially circular, but is not limited to circular openings. Indeed, orifice  410  can be of any suitable shape, including but not limited to an ellipse. 
     Ion lens  270  further comprises a conical member  320  extending from structural member  305 . It is appreciated that conical member  320  is hollow. It is further appreciated that conical member  320  can be defined by a cone angle θ (as depicted in  FIG. 5 ), and the radius of the base of the conical member  320  is of the same given radius r as orifice  410  of structural member  305 . The perimeter of the base of conical member  320  is connected to a perimeter of orifice  410  such that conical member  320  and structural member  305  form an integrated structure. Conical member  320  further comprises an aperture  330  through an apex of conical member  320  of a radius r a , aperture  330  for receiving ions there through from an ion guide. 
     It is further appreciated that ion lens  270  is of a size that is commensurate with an end face of an ion guide in mass spectrometer  200 . For example, attention is directed to  FIG. 6 , which depicts a cross-section of ion lens  270  in place at an exit region  635  of an ion guide  640 , (which can be similar to first ion guide  220 , second ion guide  230 , quadrupole  240  and/or collision cell  250 ), exit region  635  having a radius R. Exit region  635  is appreciated to be an end region of ion guide  640  where ions passing there through exit ion guide  640 . Furthermore, it is appreciated that radius R can also be referred to as the inscribed radius of ion guide  640 . 
     For example, a length, width and breadth of structural member  305  can be of any suitable size that enables structural member  305  to be installed at exit region  635  of ion guide  640  (and in mass spectrometer  200 ). A distance between elements of ion guide  640  and elements of ion lens  270  can be chosen so as to avoid electrical breakdown at operating voltages. However, the distance between elements of ion guide  640  and elements of ion lens  270  can also be chosen to avoid ion losses. In a successful non-limiting prototype, the distance between ion guide  640  and ion lens  270  can be on the order of a few millimeters. 
     Furthermore, it is appreciated that a size of conical member  320  is commensurate with exit region  635 . In non-limiting implementations, the given radius r can be similar to the radius R of exit region  635  of ion guide  640 , though given radius r can be smaller than R or greater than R. Furthermore, radius r and cone angle θ can enable at least a portion of conical member  320 , including the apex, to reside within exit region  635 . Cone angle θ can be approximately 45°. However, in some implementations, cone angle θ can be between approximately 40° and approximately 50°. In yet further implementations, cone angle θ can be between approximately 10° and approximately 80°. It is appreciated that when cone angle θ is smaller, conical member  270  can penetrate deeper into exit region  635 . 
     It is further appreciated that radius r a  of aperture  330  is of a size for accepting an ion beam exiting ion guide  640 . Radius r a  of aperture  330  can be chosen to provide efficient transmission of ion beam  265 . In some implementations, the ratio of radius r a  to radius R, r a /R, is approximately 20%, however it is appreciated that a ratio of r a /R of approximately 0.2 is not to be considered unduly limiting and that any suitable ratio of r a /R is within the scope of present implementations. In general, however, it is appreciated that when ratio r a /R is too small, losses of ion beam  265  can occur; and when ratio r a /R is too large, too much gas will be transferred to the next stage of differential pumping through aperture  330 . In a successful non-limiting successful prototype, aperture  330  has a radius r a  of approximately 0.75 mm (or 1.5 mm in diameter 2r a ). 
     It is further appreciated that an end face  645  of ion guide  640  is substantially parallel to structural member  305 . In addition, exit region  635  and conical member  320  form at least one channel  650  for gas exiting ion guide  640  to pass there through. It is further appreciated that ion guide  640  can be encased in a suitable sleeve (not depicted) that prevents gas from escaping prior to encountering at least one channel  650 ; in these implementations the sleeve can be enabled to direct gas glow towards end region  635 . 
     It is appreciated that in implementations depicted in  FIGS. 2 to 6  that conical member  320  has straight sides extending from aperture  330  to structural member  305 . However,  FIG. 7  depicts alternative non-limiting implementations of an ion lens  270   a , depicted in cross section. Ion lens  270   a  is similar to ion lens  270 , ion lens  270   a  comprising a structural member  305   a , and a conical member  320   a  extending from structural member  305   a , with an aperture  330   a  there through at an apex. Each of structural member  305   a , conical member  320   a  and aperture  330   a  are similar to structural member  305 , conical member  320 , and aperture  330 , respectively, however conical member  320   a  has concave walls extending from an aperture  330   a  to structural member  305   a . Hence, in these implementations, conical member  320   a  comprises a concave cone. The curvature of the walls of the concave cone can be any suitable curvature. 
     Similarly,  FIG. 8  depicts alternative non-limiting implementations of an ion lens  270   b , depicted in cross section. Ion lens  270   b  is similar to ion lens  270 , ion lens  270   b  comprising a structural member  305   b , and a conical member  320   b  extending from structural member  305   b , with an aperture  330   b  there through at an apex. Each of structural member  305   b , conical member  320   b  and aperture  330   b  are similar to structural member  305 , conical member  320 , and aperture  330   b , respectively, however conical member  320   b  has convex walls extending from an aperture  330   b  to structural member  305   b . Hence, in these implementations, conical member  320   b  comprises a convex cone. The curvature of the walls of the convex cone can be any suitable curvature. 
     Attention is now directed to  FIG. 9 , which depicts ion lens  270  installed at an exit region  635   a  of an ion guide  640   a , according to non-limiting implementations.  FIG. 9  is similar to  FIG. 6 , however ion guide  640  has been replaced with ion guide  640   a . Ion guide  640   a  is similar to ion guide  640 , however exit region  635   a  of ion guide  640  has a cross section similar to conical member  320 , so that conical member  320  can fit therein. In other words, exit region  635   a  comprises a shape that is approximately an inverse conical member  320 . Hence, in some implementations, the walls of conical member  320  and the walls of exit region  635   a  are substantially parallel to one another; further it is appreciated that an end face  645   a  of ion guide  640   a  is substantially parallel to structural member  305 . It is yet further appreciated that exit region  635   a  of ion guide  640   a  is bevelled. 
     Hence, exit region  635   a  and conical member  320  form at least one channel  650   a  for gas exiting ion guide  640   a  to pass there through. 
     Attention is now directed to  FIG. 10 , which depicts a portion of  FIG. 9 , including an upper portion of channel  650   a , a portion of ion guide  640   a  and a portion of ion lens  270 , in more detail, with like elements having like numbers. However,  FIG. 10  also schematically depicts contaminant  1001  on an ion guide facing side  1003  of conical member  320 . Contaminant  1001  can, in some implementations, be carried into channel  650   a  via a buffer gas exiting ion guide  640   a  via channel  650   a . Furthermore, when contaminant  1001  is charged, a resulting electric filed E forms an angle φ with a longitudinal axis of ion guide  640   a , angle φ being greater than 0°. Indeed, it is appreciated that in these implementations, in the area of channel  650   a  where the walls of conical member  320  are parallel to walls of exit region  635   a , that angle φ is similar to cone angle θ. 
     It is further appreciated that a similar electric field can form in the arrangement depicted in  FIG. 6 , with such an electric field pointing between conical member  320  and walls of exit region  635 . 
     In any event, in either arrangement (i.e. the arrangement of  FIG. 6  or the arrangement of  FIGS. 9 and 10 ), the electric field that forms due to contaminants will have less effect on an ion beam passing through the respective ion guide, than an electric field that forms due to contaminant on ion lens  170  of  FIG. 1 . Indeed, it is appreciated that in  FIG. 1 , as ion lens  170  comprises a flat plate, an electric field that forms due to contaminant will be parallel to a longitudinal axis of a respective ion guide. Hence, electric fields that form due to contaminant on conical member  320  will have less effect on an ion beam as the electric field is directed away from the respective longitudinal axis. 
     Attention is now directed to  FIG. 11 , which depicts results of testing a successful prototype of ion lens  270 , with a cone angle θ of 45° as compared to flat ion lens  170 .  FIG. 11  depicts variation of normalized ion current intensity, over time, of an ion beam passing through respective similar ion guides with ion lens  270  and ion lens  170  in place after the ion guides as described above, with voltages of 45V and 35V applied as a DC (direct current) offset to the ion guides and voltage of 40 V applied to the respective ion lens. The ion intensities are normalized to the intensities recorded when the ion guide offset and the lens voltage are set to be the same (40 V/40 V for each of the ion guide and the respective ion lens) for each configuration. Hence the ion current density over time was measured under four different test conditions, in addition to the 40V/40V normalization: 
     1. Ion lens  170  at 40 V with an ion guide offset of 45 volts (a difference of +5 volts with respect to the exit region the ion guide), as represented by the open circles in  FIG. 11 , and labelled “Std 45/40”. 
     2. Ion lens  170  at 40 V with an ion guide offset of 35 volts (a difference of −5 volts with respect to the exit region of the ion guide), as represented by the closed circles in  FIG. 11 , and labelled “Std 35/40”. 
     3. Ion lens  270  at 40 V with an ion guide offset of 45 volts (a difference of +5 volts with respect to the exit region of the ion guide), as represented by the closed diamonds in  FIG. 11 , and labelled “Cone 45/40”. 
     4. Ion lens  170  at 40 V with an ion guide offset of 35 volts (a difference of −5 volts with respect to the exit region of the ion guide), as represented by the open diamonds in  FIG. 11 , and labelled “Cone 35/40”. 
     It is appreciated that a normalized ion current is provided in  FIG. 11 . 
     It is yet further appreciated that from 0 to 120 hours, the normalized ion current intensity for ion lens  170  (for either test condition of 35 V or 45 V applied to the ion guide), changes over time as contaminant builds up on ion lens  170 ; at 120 hours a cleaning of ion lens  170  occurred. Hence, the last point on the graph of  FIG. 1  for each curve associated with ion lens  170  (i.e. labelled “Std 45/40” and “Std 35/40”) represents the normalized ion current density after cleaning: performance has returned to the level observed at 5-10 hours. 
     It is further appreciated that the normalized ion current for ion lens  270  (for either test condition of 35 V or 45 V applied to the ion lens) is generally constant over time, indicating that contaminant effects have been reduced relative to lens  170 . Furthermore, time between cleaning cycles is significantly longer for ion lens  270  than for ion lens  170 . 
     Hence there can be at least several advantages that result from using an ion guide with an ion lens  270  comprising conical member  320 , as compared to a flat ion lens  170 : 
     Due to the conical shape of conical member  270 , aperture  330  can be placed within the exit region of an ion guide before an ion beam passing there through has a chance to spread out as naturally occurs when an ion beam exits an ion guide (e.g. between an ion guide and a flat ion lens  170 ). Hence, ion lens  270  can be more efficient at sampling an ion beam than is ion lens  170 , when conical member  320  is placed within the exit region of the ion guide. When ion guide is bevelled at the exit region, as in  FIGS. 9 and 10 , aperture  330  can be placed further into an ion guide than when the ion guide is not bevelled as in  FIG. 6 . 
     When the ion guide is operated at a high pressure, gas dynamics can play a role in the rate of contamination. The conical member  320  can enable smooth gas flow between conical member  320  and the end of the ion guide, which carries contaminants away with the flow (as opposes to impinging on a surface of a flat ion lens  170 ). Therefore, the rate at which contaminating particles will be depositing on the surface can be reduced. Further, when ion guide is bevelled, as in  FIGS. 9 and 10 , gas flowing through channels formed between ion lens  270  and the ion guide changes direction and velocity less abruptly and hence continues to carry contaminant rather then disturb contaminant out of the gas flow and precipitate onto either the exit region of the ion guide or onto ion lens  270 , as occurs with ion lens  170 . 
     Furthermore, deposition of droplets and clusters flying as projectiles along the longitudinal axis of the ion guide can be less efficient for the conical surface of conical member  320 . For example, conical member  320  presents a larger surface area over which contaminant can be deposited, as compared to the flat surface of ion lens  170 . Thus, it can take longer for a contamination coating to develop on conical member  270  as compared to ion lens  170 . 
     Moreover, due to the conical shape, less contaminant is deposited on the conical member  320  near aperture  330 , which can reduce the influence of contaminants ion motion near the exit region of the ion guide. Hence, the net electric field for the same voltage (developed due to charging) can be lower. 
     In addition, an electric field that develops due to contamination will be pointing away from the longitudinal axis of the ion guide (i.e. at angle φ) rather than along the longitudinal axis: an electric field pointing along the longitudinal axis blocks the ion motion along the longitudinal axis while a field pointing away from the longitudinal axis can have a reduced effect on the motion of the ion beam near the longitudinal axis. 
     Persons skilled in the art will appreciate that there are yet more alternative implementations and modifications possible for implementing the implementations, and that the above implementations and examples are only illustrations of one or more implementations. The scope, therefore, is only to be limited by the claims appended hereto.