Patent Publication Number: US-6909089-B2

Title: Methods and apparatus for reducing artifacts in mass spectrometers

Description:
This application claims priority to co-pending provisional patent application No. 60/384,655 filed May 30, 2002, incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The invention relates generally to the field of mass spectrometers, and more particularly to the art of reducing or eliminating artifacts such as “ghost peaks” from mass scans obtained by mass analyzing ions contained in ion traps. 
     BACKGROUND OF INVENTION 
     Quadrupole mass analyzers have conventionally been used as flow-through devices, i.e., a continuous stream of ions enter and then exit the quadrupoles. More recently, however, the same quadrupole mass analyzer has been used as a combined linear ion trap and mass analyzer. That is, the linear ion trap accumulates and constrains ions within the quadrupole volume. The linear ion trap is characterized by an elongate multi-pole rod set in which a two dimensional RF field is used to constrain ions radially and DC barrier or trapping fields are used to constrain the ions axially. After a suitable fill time, the trapped ions are then scanned out mass dependently, for example, using a radial or axial ejection technique. Examples of quadrupole mass analyzers which combine ion trapping and mass analysis functions are described, inter alia, in U.S. Pat. No. 5,420,425 to Bier at al.; U.S. Pat. No. 6,177,668 to Hager; or in co-pending U.S. patent application Ser. No. 10/310,000, filed Dec. 4, 2002 and assigned to the assignee of the instant application. Each of these documents is incorporated herein by reference. 
     In such quadrupole mass analyzers, the mass scan sometimes reveals ghost peaks, i.e., satellite peaks that appear adjacent to the main peak, making the mass scan questionable. An example of this is shown in  FIG. 1A , where a mass scan  78  features a main mass peak  82 . The satellite peak  80 , on the low side of the main peak  82 , is a ghost peak or artifact. The small peak  84 , on the high side of mass peak  82 , is a legitimate isotope peak. These spectrograms were taken using a commercially available standard solution manufactured by Agilent™, product no. ES Mix G2421A, diluted in acetonitrile and water. Artifacts of these types have been observed on a number of mass spectrometers when a quadrupole rod set has been operated as a combined ion trap and mass analyzer. As mass increased, the severity of the artifact peaks increased in that the mass separation increased with mass, i.e., the problem was worst at high mass. The problem was also much more evident at slow scan speeds (e.g., 250 Da/s) when the resolution is the best. The age of the equipment and the length of the rods was also a factor. Depending on the parametric conditions, primarily the barrier potential on an end section member such as an exit lens used to trap ions axially, the artifact peaks could be minimized but at the expense of the main peak intensities. Again depending on the instrument and how it is set up the artifact peak can be either on the high or low mass side of the main peak. 
     SUMMARY OF INVENTION 
     The invention reduces and in certain cases can eliminate this undesirable phenomenon. 
     It is postulated that artifacts arise as a result of randomly distributed voltage gradients distributed along the length of the trapping quadrupole rod set. This causes spatially distributed and isolated ion populations of differing kinetic energies to exist in the ion trap. As the ions exit the trap, the isolated ion populations with the same m/z values will appear at the exit end at different times. Since ions exiting the trap can originate from anywhere along the entire length of the trap, ions of the same m/z values may not behave identically, causing the ghost peaks. 
     The invention provides three potential solutions to the artifact problem. The first approach involves improving the metallurgical properties of the rod sets, especially the conduction characteristics. The second approach involves the application of at least one continuous axial DC field to the trapping quadrupole rod set in order to urge ions towards a pre-determined region of the trap from which ions are eventually ejected, thus eliminating isolated ion populations. The third approach compartmentalizes the ion trap by applying at least one discrete axial fields to create a potential barriers along the axial dimension of the trap (in addition to the barriers used to initially trap the ions). These barriers prevent the isolated ion populations along the trap from equilibrating with one another. 
     According to one aspect of the invention, there is provided a method of operating a mass spectrometer having an elongate rod set which has an entrance end, a longitudinal axis, and a distal end. The method includes: (a) admitting ions into said rod set via the entrance end; (b) trapping at least some of the ions introduced into the rod set by producing an RF field between the rods and a barrier field adjacent to the distal end; (c) after trapping ions, establishing at least one additional barrier field in the interior of the rod set to define at least two compartments of trapped ions; (d) ejecting at least some ions of a selected mass-to-charge ratio from selected, but not all, of the compartments; and (e) detecting at least some of the ejected ions. 
     In preferred embodiments, ions are detected from only one of the compartments. 
     This method can be implemented on mass spectrometers where ions are ejected axially, i.e., along the longitudinal axis, or radially, i.e., transverse to the longitudinal axis. In the case of an axially ejecting spectrometer, the distal end functions as an exit end for the trapped ions and one additional barrier field is preferably produced such that the selected compartment is defined between the additional barrier field and the barrier field adjacent the distal/exit end. In the case of a radially ejecting mass spectrometer, the selected compartment can be defined anywhere along the rod set, preferably provided a detector is configured to detect ions ejecting substantially only from the selected compartment. 
     According to another aspect of the invention, a mass spectrometer is provided comprising: a multipole rod set, which defines a volume; power supply means connected to the rod set for generating an RF field in the volume in order to constrain ions of a selected range of mass-to-charge ratios along first and second orthogonal dimensions; means for introducing and trapping ions in the volume along a third dimension substantially orthogonal to the first and second dimensions; means for defining at least two compartments of trapped ions; and means for detecting ions from selected, but not all, of the compartments. 
     According to another aspect of the invention, an improvement is provided for an ion trap which employs a two-dimensional RF field to constrain ions in two dimensions and at least one barrier potential to constrain ions in a direction substantially normal to these two dimensions. The improvement includes: means for defining at least two compartments of trapped ions; and means for ejecting and detecting ions from at least one, but not all, of the compartments. 
     According to another aspect of the invention, there is provided another method of operating a mass spectrometer having an elongate rod set which has an entrance end, a longitudinal axis, and a distal end. The method includes: (a) admitting ions into the rod set via the entrance end; (b) trapping at least some of the ions introduced into the rod set by producing an RF field between the rods and by producing a barrier field adjacent the distal end; (c) establishing at least one DC field along the longitudinal axis in order to urge said trapped ions towards a pre-determined region of the volume defined by the rod set; (d) ejecting at least some ions of a selected mass-to-charge ratio from the pre-determined region; and (e) detecting at least some of the ejected ions. 
     This method can be implemented on mass spectrometers where ions are ejected axially or radially. In the case of an axially ejecting spectrometer, the distal end functions as an exit end for the trapped ions the ions are urged towards the distal end of the rod set. In the case of a radially ejecting mass spectrometer, the predetermined region can be situated anywhere along the rod set, preferably provided a detector is configured to detect ions ejecting substantially only from that region. 
     In preferred embodiments, the DC field(s) is established by a biased set of electrodes disposed adjacent to the rod set. Each of these electrodes has a T-shaped cross section including a stem, the depth of which varies over the length of the rod set in order to provide a substantially uniform electric field along the longitudinal axis. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The foregoing and other aspects of the invention will become more apparent from the following description of specific embodiments thereof and the accompanying drawings which illustrate, by way of example only, the principles of the invention. In the drawings: 
         FIG. 1A  is a mass spectrogram showing the existence of artifact ghost peaks. 
         FIG. 1B  is a mass spectrogram, obtained under conditions similar to  FIG. 1A , without the artifact ghost peaks. This spectrogram was produced by employing the artifact-eliminating apparatus shown in FIG.  5 . 
         FIG. 2  is a schematic diagram of a triple-quadrupole mass spectrometer having a linear ion trap (Q 3 ) with which the invention may be used. 
         FIG. 3  is a timing diagram showing a variety of waveforms used to control the linear ion trap (Q 3 ) shown in FIG.  2 . 
         FIGS. 4A and 4B  respectively show radial and axial cross-sectional views of a modified quadrupole rod set/linear ion trap fitted with linacs (extra electrodes) for producing an axial DC field. 
         FIG. 5  is a perspective view of a modified quadrupole rod set/linear ion trap fitted with biased metalized rings for generating potential barriers along the axial dimension of the rod set. 
         FIG. 6  is a timing diagram showing a variety of waveforms used to control the modified linear ion trap illustrated in FIG.  5 . 
         FIG. 7A  is a schematic diagram of a modified quadrupole rod set/linear ion trap configured to detect ions ejected radially from the trap. The trap includes means for producing axial fields. 
         FIG. 7B  is a schematic diagram of a modified quadrupole rod set/linear ion trap configured to detect ions ejected radially from the trap. The trap is fitted with biased metalized rings for generating potential barriers along the axial dimension of the rod set. 
         FIG. 8  is a side view of two rods of a tapered rod set enabling the generation of an axial field for use in place of or in addition to one of the quadrupole rod sets of a linear ion trap. 
         FIG. 9  is an end view of the entrance end of the  FIG. 8  rod set. 
         FIG. 10  is a cross-sectional view at the center of the rod set of FIG.  8 . 
         FIG. 11  is an end view of the exit end of the  FIG. 8  rod set. 
         FIG. 12  is a side view of two rods of a modified rod set enabling the generation of an axial field for use in place of or in addition to one of the quadrupole rod sets of a linear ion trap. 
         FIG. 13  is an end view of the entrance end of the  FIG. 12  rod set. 
         FIG. 14  is a cross-sectional view at the center of the  FIG. 12  rod set. 
         FIG. 15  is an end view of the exit end of the  FIG. 12  rod set. 
         FIG. 16  is a side view of two rods of a modified rod set enabling the generation of an axial field for use in place of or in addition to one of the quadrupole rod sets of a linear ion trap. 
         FIG. 17  is an end view of the rod set of FIG.  16  and showing electrical connections thereto. 
         FIG. 18  is a side view of two rods of another modified rod set enabling the generation of an axial field for use in place of or in addition to one of the quadrupole rod sets of a linear ion trap. 
         FIG. 19  is an end view of the rod set of FIG.  18  and showing electrical connections thereto. 
         FIG. 20  is a side view of another modified rod set enabling the generation of an axial field for use in place of or in addition to on of the quadrupole rod sets of a linear ion trap. 
         FIG. 21  is a side view of another modified rod set enabling the generation of an axial field for use in place of or in addition to one of the quadrupole rod sets of a linear ion trap. 
         FIG. 22  is a cross-sectional view at the center of the rod of FIG.  21 . 
         FIG. 23  is a diagrammatic view of yet another modified rod set. 
         FIG. 24  is an end view of the rod set of FIG.  23 . 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The inventors have theorized that the artifact problem may be attributed to metallurgical properties of the rods employed in linear ion traps (“LIT”), in conjunction with the geometry thereof. It was observed initially that swapping in a new set of rods, which are typically constructed from stainless steel, could solve this problem. It was also observed that in many cases when new rod sets were installed that no artifact peaks existed but after a period of many hours or even days the artifacts could re-appear. 
       FIG. 2  illustrates a triple-quadrupole mass spectrometer apparatus  10  in which one of the quadrupole rod sets, Q 3 , is operated as a combined linear ion trap and mass analyzer. Experiments were conducted on such an apparatus, and the invention may be used with spectrometers such as, but not limited to, this type. 
     More particularly, the apparatus  10  includes an ion source  12 , which may be an electrospray, an ion spray, a corona discharge device or any other known ion source. Ions from the ion source  12  are directed through an aperture  14  in an aperture plate  16 . On the other side of the plate  16 , there is a curtain gas chamber  18 , which is supplied with curtain gas from a source (not shown). The curtain gas can be argon, nitrogen or other inert gas, such as described in U.S. Pat. No. 4,861,988, to Cornell Research Foundation Inc., which also discloses a suitable ion spray device. The contents of this patent are incorporated herein by reference. 
     The ions then pass through an orifice  19  in an orifice plate  20  into a differentially pumped vacuum chamber  21 . The ions then pass through aperture  22  in a skimmer plate  24  into a second differentially pumped chamber  26 . Typically, the pressure in the differentially pumped chamber  21  is of the order of 1 or 2 Torr and the second differentially pumped chamber  26 , often considered to be the first chamber of the mass spectrometer, is evacuated to a pressure of about 7 or 8 mTorr. 
     In the chamber  26 , there is a conventional RF-only multipole ion guide Q 0 . Its function is to cool and focus the ions, and it is assisted by the relatively high gas pressure present in chamber  26 . This chamber  26  also serves to provide an interface between the atmospheric pressure ion source  12  and the lower pressure vacuum chambers, thereby serving to remove more of the gas from the ion stream, before further processing. 
     An interquad aperture IQ 1  separates the chamber  26  from a second main vacuum chamber  30 . In the second chamber  30 , there are RF-only rods labeled ST (short for “stubbies”, to indicate rods of short axial extent), which serve as a Brubaker lens. A quadrupole rod set Q 1  is located in the vacuum chamber  30 , which is evacuated to approximately 1 to 3×10 −5  Torr. A second quadrupole rod set Q 2  is located in a collision cell  32 , supplied with collision gas at  34 . The collision cell  32  is designed to provide an axial field toward the exit end as taught by Thomson and Jolliffe in U.S. Pat. No. 6,111,250, the entire contents of which are incorporated herein by reference. The cell  32 , which is typically maintained at a pressure in the range 5×10 −4  to 10 −2  Torr, is within the chamber  30  and includes interquad apertures IQ 2 , IQ 3  at either end. Following Q 2  is located a third quadrupole rod set Q 3 , indicated at  35 , and an exit lens  40 . 
     Each rod in Q 3  has a radius of about 10 mm and a length of about 120 mm, although other sizes are contemplated and may be used in practice. It is desirable for the rods to be as close to ideal configuration as possible, e.g., perfectly circular or having perfect hyperbolic faces, in order to achieve the substantial quadrupole field required for mass analysis. Opposing rods in Q 3  are preferably spaced apart approximately 20 mm, although other spacings are contemplated and used in practice. The pressure in the Q 3  region is nominally the same as that for Q 1 , namely 1 to 3×10 −5  Torr. A detector  76  is provided for detecting ions exiting axially through the exit lens  40 . 
     Power supplies  37 , for RF,  36 , for RF/DC, and  38 , for RF/DC and auxiliary AC are provided, connected to the quadrupoles Q 0 , Q 1 , Q 2 , and Q 3 . Q 0  is operated as an RF-only multipole ion guide whose function is to cool and focus the ions as taught in U.S. Pat. No. 4,963,736, the contents of which are incorporated herein by reference. Q 1  is a standard resolving RF/DC quadrupole. The RF and DC voltages are chosen to transmit only precursor ions of interest or a range of ions into Q 2 . Q 2  is supplied with collision gas from source  34  to dissociate precursor ions to produce a fragment ions. Q 3  was operated as a linear ion trap, and used to trap the fragment ions as well as any un-dissociated precursor ions. Ions are then scanned out of Q 3  in a mass dependent manner using an axial ejection technique. Q 3  can also function as a standard resolving RF/DC quadrupole. 
     In the illustrated embodiment, ions from ion source  12  are directed into the vacuum chamber  30  where, if desired, a precursor ion of a selected m/z value (or range of mass-to-charge ratios) may be selected by Q 1  through manipulation of the RF+DC voltages applied to the quadrupole rod set as well known in the art. Following precursor ion selection, the ions are accelerated into Q 2  by a suitable voltage drop between Q 1  and Q 2 , thereby inducing fragmentation as taught by U.S. Pat. No. 5,248,875 the contents of which are hereby incorporated by reference. The degree of fragmentation can be controlled in part by the pressure in the collision cell, Q 2 , and the potential difference between Q 1  and Q 2 . In the illustrated embodiment, a DC voltage drop of approximately 40-80 volts is present between Q 1  Q 2 . 
     The fragment ions along with non-dissociated precursor ions are carried into Q 3  as a result of their momentum and the ambient pressure gradient between Q 2  and Q 3 . After a suitable fill time a blocking potential can be applied to IQ 3  in order to trap the precursor ions and its fragments in Q 3 . Once trapped in Q 3 , the precursor ions and its fragments can be mass selectively scanned out of the linear ion trap, thereby yielding an MS/MS or MS 2  spectrum. 
       FIG. 3  shows the timing diagrams of waveforms applied to the quadrupole Q 3  in greater detail. In an initial phase  50 , a DC blocking potential on IQ 3  is dropped so as to permit the linear ion trap to fill for a time preferably in the range of approximately 5-1000 ms, with 50 ms being preferred. 
     Next, a cooling phase  52  follows in which the ions in the trap are allowed to cool or thermalize for a period of approximately 10 ms in Q 3 . The cooling phase is optional, and may be omitted in practice. 
     A mass scan or mass analysis phase  54  follows the cooling phase, in which ions are axially scanned out of Q 3  in a mass dependent manner. In the illustrated embodiment, an auxiliary dipole AC voltage, superimposed over the RF voltage used to trap ions in Q 3 , is applied to one set of pole pairs, in the x or y direction (being orthogonal to the axial direction. The frequency of the auxiliary AC voltage, f aux , is preferably set to a predetermined frequency ω ejec  known to effectuate axial ejection. (Each linear ion trap may have a somewhat different frequency for optimal axial ejection based on its exact geometrical configuration.) Simultaneously, the amplitudes of the Q 3  RF voltage and the Q 3  auxiliary AC voltage are ramped or scanned. This particular technique enhances the resolution of axial ejection, as taught in co-pending U.S. patent application Ser. No. 10/159,766 filed May 30, 2002, assigned to the instant assignee. The contents of this document are incorporated herein in their entirety. 
     After mass scanning, in a next phase  56  Q 3  is emptied of all ions. In this phase, all of the voltages are lowered to allow the trap to empty. 
     In investigating the artifact phenomenon, which in the apparatus  10  arises from Q 3 , it is known that the ions which are scanned axially out of the Q 3  LIT can and do originate from anywhere along the length of the Q 3  rod set, but ions of the same m/z value may not necessarily exit the trap at the same time. As such, it is believed that there are populations of ions along the length of the Q 3  rod set that are isolated from one another by voltage gradients, i.e., different ion populations are energized to slightly varying voltage potentials, and thus have slightly differing kinetic energies. Experience has shown that different rod sets are likely to have different isolated ion populations, implying the existence of randomly distributed voltage gradients on the Q 3  rod sets. 
     As such, some ion populations in the LIT can have different kinetic energies than other ion populations. It is thus expected that discrete or different ion populations will reflect off the voltage gradients or barriers including IQ 3  and the exit lens at the opposing ends of the Q 3  LIT. There may also be other mechanisms at play which result in randomly distributed voltage gradients or barriers that manifest along the length or axial dimension of Q 3 . 
     The randomly distributed voltage barriers or gradients affecting the transmission properties are believed to arise from non-uniformities of the surface potentials of the rods, probably as a result of different surface compositions, either elemental or oxides. Oxidation likely explains why the artifact effect occurs gradually. It is postulated that these irregularities cause variations in the work function on the rod surface thus varying the effective RF voltage amplitude at different positions along the rods. See Gerlich, Dieter., ‘Inhomogeneous RF Fields: A Versatile Tool For The Study of Processes With Slow Ions’, Advance in Chemical Physics Series, Vol. 52, pages 75-81, 1992. 
     There are three potential solutions to the artifact problem in LITs. The first approach involves improving the metallurgical properties of the rod sets, especially the conduction characteristics. The second approach involves the application of a continuous axial field to the LIT quadrupole rod set in order to urge ions towards the exit end of the trap, thus eliminating isolated ion populations. The behavior of the LIT was investigated when Linacs were used for this purpose. The third approach involves the application of discrete axial fields to create one or more potential barriers along the axial dimension of the trap. These barriers prevent the isolated ion populations along the trap from interfering with one another. The behaviour of the LIT was investigated when potential barriers were created through the use of biased metallized rings surrounding the quadrupole rod set. The second and third approaches provide a means for precluding isolated ion populations in detected ions. The first approach provides a means for improving the random potential gradients that arise from the metallurgical properties of the rods. 
     I. Improved Metallurgical Properties 
     One approach to reducing the artifact problem is to improve the metallurgical properties of the rod sets to have better conduction characteristics and less of a tendency to oxidize. The rod sets have traditionally been constructed from stainless steel, and manufactured using conventional machining methods. These methods are not always capable of meeting tight tolerance levels beyond a specific rod length (the high tolerances being important for achieving the substantial quadrupole field required for mass analysis), and so other materials and manufacturing techniques have been developed for providing precision-tolerance rod sets. For example, the assignee has developed relatively long rod sets using gold-plated ceramic rods. The following experiments were conducted using gold-plated ceramic rods and gold-plated stainless steel rods for the Q 3  rods. 
     Using nine gold-coated rod sets, it was observed that 8 of 9 sets reduced artifact effects to acceptable levels in at least one orientation or the other (orientation being defined as the rods being disposed towards Q 2  or alternatively towards the detector). Only one rod set passed in both orientations. It is postulated that the gold layer provides an improved uniform conductive layer therefore reducing random voltage barriers or gradients along the rods. However, gold-coating the rod sets only assisted in reducing the severity of the artifact peaks. It did not completely eliminate the phenomenon. 
     Instead of gold, other metallic amorphous coatings will suffice. 
     II. Continuous Axial Fields 
     Another approach centers on creating or providing one or more axial fields in the Q 3  LIT. One type of axial field, termed herein as a “continuous” field, functions to push or urge the ions trapped along the entire length of Q 3  towards the exit end of the rod set. This has the effect of congregating the trapped ions and eliminating discrete ion populations. The axial field also ensures that substantially all ions of a given m/z value selected for axial ejection exit the trap at substantially the same time. 
       FIGS. 4A and 4B  respectively show radial and axial cross-sectional views of “Manitoba”-style linacs  100 , which are one example of an apparatus that can be used to apply a continuous axial field. The linacs include four extra electrodes  102  introduced between the main quadrupole rods  35  of Q 3 . While a variety of electrode shapes are possible, the preferred electrodes have T-shaped cross-sections. The linac electrodes are held at the same DC potential  104 , but the depth, d, of the stem section  106  is varied as seen best in  FIG. 4B  to provide an approximately uniform electric field along the axial dimension of Q 3 . See Loboda et al., “Novel Linac II Electrode Geometry for Creating an Axial Field in a Multipole Ion Guide”, Eur. J. Mass Spectrom., 6, 531-536 (2000), the contents of which are incorporated herein by reference, for more detailed information on this subject. The linacs  100  create a continuous DC axial field (symbolically represented by field lines  108 ) which applies a force that pushes the ions towards the exit end of the Q 3  rod set. The artifacts phenomenon can be substantially eliminated using this approach. 
     Referring to  FIG. 3 , note that the axial field is preferably off during the ion injection phase  50 , so the space charge characteristics of the trap are not affected. (If the axial field is on during fill time, then the fill time is reduced.) During ejection, as the ions exit, the space charge effects are insignificant and/o compensated for by the axial field. 
     It was found that different axial gradients were required for different rod sets to mitigate the ghost artifact peaks. Accordingly, different rod sets may have to be individually tuned. Experimentally, the an LIT length of about 20 mm required a potential gradient of 0.05 to 0.15 volts/cm. The value can be varied with application to compensate for variation between instruments. Also, axial fields of different polarity are required for positive and negative mode ions. 
     In employing the linacs  100 , it was noted that there was some interaction between the linac fields near IQ 3  that affect the transmission of ions into Q 3  during the ion injection phase  50 . This could be overcome by adjusting the position of the linacs  100  relative to the end of the rod set. More particularly, the DC field interacts with a fringing field created by IQ 3  and the end of the Q 3  rod set. This interaction has an affect on ions filling the trap in that it reduces the fill amount. In order to avoid this interaction, the end of the linac electrode is moved away from the end of the rod set by 1 to 4 mm. Typically, the fringing field penetrates into the rod set by a distance equivalent to about a ½ rod radius, or about 6 mm in the illustrated embodiment. So, about a 4 mm gap is sufficient to elevate this interaction. It also appears that normal RF/DC resolving mode of operation is not significantly affected by the presence of the linac hardware when appropriate voltages are applied. 
     A variety of other mechanisms can be used in the alternative to create a continuous axial field in a linear ion trap that will eliminate the artifact problem. A number of these are described in U.S. Pat. Nos. 5,847,386 or 6,111,250 to Thomson and Jollife, incorporated herein by reference. Although these patents describe the creation of an auxiliary axial field in a standard resolving quadrupole or a collision cell where ions are not trapped, nevertheless most of these can be used for an ion trap. 
     Briefly, as described in the patents above, axial fields can be created in one or more rod sets by: tapering the rods ( FIGS. 8  to  11 ); arranging the rods at angles with respect to each other ( FIGS. 12  to  15 ); segmenting the rods (FIGS.  16 - 17 ); providing a segmented case around the rods (FIGS.  18 - 19 ); providing resistively coated or segmented auxiliary rods (FIGS.  18 - 19 ); providing a set of conductive metal bands spaced along each rod with a resistive coating between the bands (FIG.  20 ); forming each rod as a tube with a resistive exterior coating and a conductive inner coating (FIGS.  21 - 22 ); a combination of any two or more of the above; or any other appropriate methods. 
     More particularly,  FIGS. 8  to  11  show a tapered rod set  262  that provides an axial field. The rod set  262  comprises two pairs of rods  262 A and  262 B, both equally tapered. One pair  262 A is oriented so that the wide ends  264 A of the rods are at the entrance  266  to the interior volume  268  of the rod set, and the narrow ends  270 A are at the exit end  272  of the rod set. The other pair  262 B is oriented so that its wide ends  264 B are at the exit end  272  of the interior volume  268  and so that its narrow ends  270 B are at the entrance  266 . The rods define a central longitudinal axis  267 . Each pair of rods  262 A,  262 B is electrically connected together, with an RF potential applied to each pair (through isolation capacitors C 2 ) by an RF generator  274  which forms part of power supply  248 . A separate DC voltage is applied to each pair, e.g. voltage VI to one pair  262 A and voltage V2 to the other pair  262 B, by DC sources  276 - 1  and  276 - 2 . The tapered rods  262 A,  262 B are located in an insulated holder or support (not shown) so that the centers of the rods are on the four corners of a square. Other spacing may also be used to provide the desired fields. For example the centers of the wide ends of the rods may be located closer to the central axis  267  than the centers of the narrow ends. 
       FIGS. 12  to  15  show a angled rod set  262  that provides an axial field, and in which primed reference numerals indicate parts corresponding to those of  FIGS. 8  to  11 . In  FIGS. 8  to  11 , the rods are of the same diameter but with the ends  264 A 1  of one pair  262 A 1  being located closer to the axis  267   1  of the quadrupole at one end and the ends  268 B 1  of the other pair  262 B 1  being located closer to the central axis  267   1  at the other end. In both cases described, the DC voltages provide an axial potential (i.e. a potential on the axis  267 ) which is different at one end from that at the other end. Preferably the difference is smooth, but it can also be a step-wise difference. In either case an axial field is created along the axis  267 . 
       FIGS. 16 and 17 , show a segmented rod set  296  that provides an axial field, consisting of two pairs of parallel cylindrical rods  296 A,  296 B arranged in the usual fashion but divided longitudinally into six segments  296 A- 1  to  296 A- 6  and  296 B- 1  to  296 B- 6  (sections  296 B- 1  to  6  are not separately shown). The gap  298  between adjacent segments or sections is very small, e.g. about 0.5 mm. Each A section and each B section is supplied with the same RF voltage from RF generator  274 , via isolating capacitors C 3 , but each is supplied with a different DC voltage V1 to V6 via resistors R 1  to R 6 . Thus sections  296 A- 1 ,  296 B- 1  receive voltage V1, sections  296 A- 2 ,  296 B- 2  receive voltage V2, etc. This produces a stepped voltage along the central longitudinal axis  300  of the rod set  296 , as shown at  302  in  FIG. 16  which plots axial voltage on the vertical axis and distance along the rod set on the horizontal axis. The separate potentials can be generated by separate DC power supplies for each section or by one power supply with a resistive divider network to supply each section. 
       FIGS. 18-19  show a segmented case around the rods providing an axial field. In this arrangement, the quadrupole rods  316 A,  316 B are conventional but are surrounded by a cylindrical metal case or shell  318  which is divided into six segments  318 - 1  to  318 - 6 , separated by insulating rings  320 . The field at the central axis  322  of the quadrupole depends on the potentials on the rods  316 A,  316 B and also on the potential on the case  318 . The exact contribution of the case depends on the distance from the central axis  322  to the case and can be determined by a suitable modeling program. With the case divided into segments, an axial field can be created in a fashion similar to that of  FIGS. 16-17 , i.e. in a step-wise fashion approximating a gradient. 
       FIG. 20  shows a set of conductive metal bands spaced along each rod with a resistive coating between the bands as a manner of providing an axial field.  FIG. 20  shows a single rod  356  of a quadrupole. Rod  356  has five encircling conductive metal bands  358 - 1  to  358 - 5  as shown, dividing the rod into four segments  360 . The rest of the rod surface, i.e. each segment  360  is coated with resistive material to have a surface resistivity of between 2.0 and 50 ohms per square. The choice of five bands is a compromise between complexity of design versus maximum axial field, one constraint being the heat generated at the resistive surfaces. RF is applied to the metal bands  358 - 1  to  358 - 5 . Separate DC potentials V1 to V5 are applied to each metal band  358 - 1  to  358 - 5  via RF blocking chokes L 1  to L 5  respectively. 
       FIGS. 21-24  show resistively coated or segmented auxiliary rods that provide an axial field. Rod  370  is formed as an insulating ceramic tube  372  having on its exterior surface a pair of end metal bands  374  which are highly conductive. Bands  374  are separated by an exterior resistive outer surface coating  376 . The inside of the tube  372  is coated with conductive metal  378 . The wall of tube  372  is relatively thin, e.g. about 0.5 mm to 1.0 mm. The surface resistivity of the exterior resistive surface  376  will normally be between 1.0 and 10 Mohm per square. A DC voltage difference indicated by V1 and V2 is connected to the resistive surface  376  by the two metal bands  374 , while the RF is connected to the interior conductive metal surface  378 . The high resistivity of outer surface  376  restricts the electrons in the outer surface from responding to the RF (which is at a frequency of about 1.0 MHz), and therefore the RF is able to pass through the resistive surface with little attenuation. A the same time voltage source V1 establishes a DC gradient along the length of the rod  370 , again establishing an axial DC field. In  FIGS. 23 ,  24  each quadrupole rod  379  is coated with a surface material of low resistivity, e.g. 300 ohms per square, and RF potentials are applied to the rods in a conventional way by RF source  380 . Separate DC voltages V1, V2 are applied to each end of all four rods through RF chokes  381 - 1  to  381 - 4 . The low resistance of the surface of rods  379  will not materially affect the RF field but will allow a DC voltage gradient along the length of the rods, establishing an axial field. The resistivity should not be too high or resistance heating may occur. (Alternatively external rods or a shell can be used with a resistive coating). 
     It should also be appreciated that a continuous axial field or fields can also be applied to an LIT in which the trapped ions are radially ejected for mass detection. An example of such an LIT  150  is shown in  FIG. 7A , and comprises three sections: an elongate central section  154 , an entrance end section  152  and an exit end section  156 . Each section includes two pairs of opposing electrodes. In the trapping mode, the end sections  152 ,  156  are held at a higher DC potential than the central section  154 . In order to fill the trap the DC potential on the entrance section  152  is lowered. After a suitable fill time, the DC potential is raised, causing a potential well to be formed in the central section  154  of the trap which constrains the ions axially. 
     Elongate apertures  160  are formed in the electrode structures of the central section  154  in order to allow the trapped ions to be mass-selectively ejected radially, in a direction orthogonal to the axial dimension of the trap. Select ions are made unstable in the quadrupolar fields through manipulation of the RF and DC voltages applied to the rods. Those ions situated along the length of the trap that have been rendered unstable leave the central section  154  through the elongate apertures  160 . Alternatively, the apertures can be omitted and ions can be ejected radially in the space between the rods by applying phase synchronized resonance ejection fields to both pairs of rods in the central section  154 . A detector, not shown, is positioned to receive the radially ejected ions. 
     The entrance end section  152  can be readily interchanged with a plate having a central aperture and the exit end section  156  can likewise be interchanged with a plate. 
     Instead of ejecting ions from the entire length of the rod set, two axial fields of opposing polarity (schematically illustrated by arrows  155   a  and  155   b ) can be established using any of the forgoing techniques to urge ions into a central region  180  of the central section  154 , or to a specific point or area between the rods. The detector (not shown) can be shaped, or shielded, to receive or count only those ions emanating from the selected region. Alternatively, one axial field can be established to urge ions towards the entrance or end section  152  or  156 , with an appropriately shaped or shielded detector employed to detect ions emanating only from such section. 
     III. Discrete Axial Fields 
     As shown in the schematic diagram of  FIG. 5 , the quadrupole rod set of Q 3  is supported near both ends by collars  118  made from a non-conductive material such as ceramic. Each collar  118  has a portion that can be metallized to form a conductive ring,  120   a  or  120   b , around the circumference of the rod set while remaining electrically isolated from the rods  122  of the quadrupole. With an appropriately biased DC potential on each ring  120   a ,  120   b , discrete voltage barriers can be created within the LIT volume because a small fraction of the radial electric field created by the rings  120   a ,  120   b  penetrates inside the quadrupole. See Thomson and Jollife, U.S. Pat. No. 5,847,386. By controlling the voltage barriers induced by the metal rings  120   a  and  120   b , the ion populations within the Q 3  LIT can be controlled. Preferably the IQ 3  lens is electrically tied to the first or upstream metallized ring  120   a  and the second or downstream metallized ring  120   b  is controlled by an independent DC power supply  128 . 
     As shown in the modified timing diagram of  FIG. 6 , during the mass scan out phase  56  the DC voltage on the IQ 3  lens is dropped below the DC offset voltage on Q 3  (not specifically shown) to prevent reflections of ions that were accelerated towards IQ 3 . Since the upstream metallized ring  120   a  is tied to IQ 3  there is no significant voltage barrier induced by this ring  120   a  into Q 3 . However, if the downstream metallized ring  120   b  is appropriately biased, ions will be trapped in the region  130  between this ring  120   b  and the exit lens  40 , whereby ions between ring  120   b  and IQ 3  are prevented from entering region  130 , which provides a trapped ion compartment. So, only those ions within the region  130  defined by ring  120   b  and the exit lens  40  will be axially ejected and recorded in the mass scan. This technique successfully eliminated the artifact problem, as shown in mass spectrum  90  of  FIG. 1B  which was taken under the same operating conditions as the mass scan of  FIG. 1A  but with the preferred metallized ring  120   b  installed and actuated. 
     It was found that the DC potential on the downstream ring  120   b  needed to be adjusted differently for different rod sets in order to eliminate ghost artifact peaks. The DC voltage applied to the downstream ring  120   b  varied from LIT to LIT. The voltage varied from as low as 200 V to as much as 1500 V. Note that if the potential on the metallized ring  120   b  was set too high, then peak tailing could occur on the high-mass side of the peaks. 
     A variety of other mechanisms can be employed in the alternative to produce discrete potential barriers along the axial dimension of Q 3 . These include: segmenting the rods (as shown, for example, in  FIGS. 16 and 17 ) and applying different DC offset voltages. Alternatively, as shown in  FIG. 8B , the diameter of the rods can be tapered such that they have a larger diameter at the center  263  that than the ends. 
     It should also be appreciated that these discrete axial field techniques can also be applied to an LIT in which the trapped ions are radially ejected for mass detection, as described above with reference to  FIG. 7A , and modified appropriately as shown in FIG.  7 B. 
     As shown in  FIG. 7B , the rods of the central section  154  can be supported by non-conductive collars  165  made from a material such as ceramic. Each collar  165  has a portion that can be metallized to form a conductive ring,  170   a  or  170   b , around the circumference of the rod set while remaining electrically isolated from the rods of the quadrupole. With an appropriately biased DC potential on each ring  170   a ,  170   b , discrete voltage barriers can be created within the central section  154  because a small fraction of the electric field created by the rings  170   a ,  170   b  penetrates inside the central section  154 . In operation, these barriers are applied after the trap has been filled in order to create a second potential well in a region  180  between the rings  170   a  and  170   b . Ions are now prevented from leaving and entering this region  180 , which provides a trapped ion compartment within the central section. The apertures  160  are shortened, or the detector is preferably shortened and/or shielded so as to count only those ions emanating from region  180 . In this manner, any isolated ion populations that arise from random voltage gradients along the length of the trap are prevented from interfering with the mass scan, thereby minimizing the artifact phenomenon. 
     It will be appreciated that the compartment from which the trapped ions are ejected can alternately be the region defined between the entrance section  152  and the upstream ring  170   a , or the region defined between the end section  156  and the downstream ring  170   b . It will also be appreciated that while a triple quadrupole instrument has been presented and described, the invention can be used in a system where the rod sets upstream of the ion trap are omitted and an ion source is directly coupled to the combined ion trap/mass analyzer rod set. Similarly, those skilled in the art will appreciate that many modifications and variations may be made to the embodiments described herein without departing from the spirit of the invention.