Patent Publication Number: US-7709785-B2

Title: Method and apparatus for mass selective axial transport using quadrupolar DC

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefits of U.S. Provisional Application No. 60/681,947 filed May 18, 2005, and U.S. Provisional Application No. 60/721,072 filed Sep. 28, 2005. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to mass spectrometry, and more particularly relates to a method and apparatus for mass selective axial transport using quadrupolar DC. 
     BACKGROUND OF THE INVENTION 
     Many types of mass spectrometers are known, and are widely used for trace analysis to determine the structure of ions. These spectrometers typically separate ions based on the mass-to-charge ratio (“m/z”) of the ions. One such mass spectrometer system involves mass-selective axial ejection—see, for example, U.S. Pat. No. 6,177,668 (Hager), issued Jan. 23, 2001. This patent describes a linear ion trap including an elongated rod set in which ions of a selected mass-to-charge ratio are trapped. These trapped ions may be ejected axially in a mass selective way as described by Londry and Hager in “Mass Selective Axial Ejection from a Linear Quadrupole Ion Trap,” J Am Soc Mass Spectrom 2003, 14, 1130-1147. In mass selective axial ejection, as well as in other types of mass spectrometry systems, it will sometimes be advantageous to control the axial location of different ions. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, there is provided a method of operating a mass spectrometer having an elongated rod set, the rod set having an entrance end, an exit end, a plurality of rods and a central longitudinal axis. The method comprises: a) admitting ions into the entrance end of the rod set; b) producing an RF field between the plurality of rods to radially confine the ions in the rod set, the RF field having a resolving DC component field; and, c) varying the resolving DC component field along at least a portion of a length of the rod set to provide a DC axial force acting on the ions. 
     In accordance with a second aspect of the present invention, there is provided a mass spectrometer system comprising: (a) an ion source; (b) a rod set, the rod set having a plurality of rods extending along a longitudinal axis, an entrance end for admitting ions from the ion source, and an exit end for ejecting ions traversing the longitudinal axis of the rod set; and, (c) a voltage supply module for producing an RF field between the plurality of rods of the rod set, the RF field having a resolving DC component field. The voltage supply module is coupled to the rod set to vary the resolving DC component field along at least a portion of a length of the rod set to provide a DC axial force acting on the ions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A detailed description of preferred aspects of the present invention is provided herein below with reference to the following drawings, in which: 
         FIG. 1 , in a schematic view, illustrates a quadrupole rod set in which a dipolar auxiliary signal is provided to one of the rod pairs; 
         FIG. 2 , in a schematic view, illustrates an ion guide in accordance with a first aspect of the present invention; 
         FIG. 3 , in a schematic view, illustrates an ion guide in accordance with a second aspect of the present invention; 
         FIG. 4  is a stability diagram illustrating how a derived axial field of the ion guides of  FIG. 2  or  FIG. 3  can improve the efficiency of mass-selective axial ejection; 
         FIG. 5  is a graph illustrating a simulation of axial position of thermalized ions when a resolving DC quadrupolar voltage is applied to a rod set in accordance with aspects of the invention; and, 
         FIG. 6  is a graph illustrating the axial component of a trajectory of an ion when a resolving DC quadrupolar voltage is applied to the rods of a rod set in accordance with aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED ASPECTS OF THE PRESENT INVENTION 
     Referring to  FIG. 1 , there is illustrated in a schematic view a quadrupole rod set  20  in which a dipolar auxiliary AC signal is provided to one of the rod pairs. Specifically, the quadrupole rod set  20  comprises a pair of X-rods  22  and a pair of Y-rods  24  with RF voltage applied to them (in a known manner) by RF voltage source  26  to provide radial confinement of ions. The exit end of the quadrupole rod set  20  can be blocked by supplying an appropriate voltage to an exit electrode at the exit end. 
     In addition to the RF voltage that is applied to all of the rods by RF voltage source  26 , an auxiliary dipolar signal is provided to X-rods  22 , but not to Y-rods  24 , by AC voltage source  28  (in a known manner). 
     According to aspects of the invention, the RF voltage supplied to X-rods  22  and Y-rods  24  includes a quadrupolar or resolving DC component. The quadrupolar DC component applied to the X-rods  22  is opposite in polarity to the quadrupolar DC component applied to the Y-rods  24 . As will be described in more detail below in connection with  FIGS. 2 and 3 , the quadrupolar DC applied to the X-rods  22  and Y-rods  24  is applied in such a way that its magnitude changes along the lengths of the rods. According to one aspect of the present invention, illustrated in  FIG. 2  and described below, the quadrupolar DC profile along the rod set diminishes linearly from a maximum at the entrance end of the rod set to a minimum at the exit end of the rod set. According to another aspect of the invention described below in connection with  FIG. 3 , the quadrupolar DC profile along the rod set diminishes from a maximum near to the entrance end of the rod set to a minimum near the exit end of the rod set. In the description that follows, the charge carried by the ions is assumed to be positive, the quadrupolar resolving DC applied to the X-rods is assumed to be positive, and the quadrupolar resolving DC applied to the Y-rods is assumed to be negative. More generally, the quadrupolar resolving DC applied to the X-rods is assumed to be of the same polarity as the ions. 
     The derived axial force resulting from the variation in the DC quadrupolar voltage applied to the rods can be calculated, for the two-dimensional mid-section of a linear quadrupole rod set by considering the contribution to the potential of the resolving quadrupolar DC. In the central portion of a linear ion trap where end effects are negligible, the two-dimensional quadrupole potential can be written as 
                       O   ¨       2   ⁢           ⁢   D       =       φ   0     ⁢         x   2     -     y   2         r   0   2                 (   1   )               
where 2r 0  is the shortest distance between opposing rods and φ 0  is the electric potential, measured with respect to ground, applied with opposite polarity to each of the two poles. Traditionally, φ 0  has been written as a linear combination of DC and RF components as
 φ 0 =U—V cosΩt  (2) 
where Ù is the angular frequency of the RF drive.
 
     In this instance, we may disregard the alternating RF term and write the DC contribution as a linear function of the axial coordinate z, measured from the axial position at which the quadrupolar DC is a maximum, as 
                       O   ¨     DC     =         U   0     ⁡     (     1   -     z     z   0         )       ⁢         x   2     -     y   2         r   0   2                 (   3   )               
where, U 0  is the level of the resolving DC applied to the entrance end of the rods and z 0  is the axial dimension over which the quadrupolar DC is applied. The axial component of the electric field can be obtained by differentiating Eq. 3 with respect to the axial coordinate z to yield the following:
 
     
       
         
           
             
               
                 
                   
                     E 
                     z 
                   
                   = 
                   
                     
                       
                         U 
                         0 
                       
                       
                         
                           z 
                           0 
                         
                         ⁢ 
                         
                           r 
                           0 
                           2 
                         
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           x 
                           2 
                         
                         - 
                         
                           y 
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Consideration of Eq. 4 yields three significant features. First, the force is axially uniform. Second, axial field strength depends quadratically on radial displacement. Finally, the sign of the derived axial force is positive in the x-z plane but negative in the y-z plane. 
     To facilitate discussion, assume that the ions are positive and the polarity of the quadrupole DC applied to the X-pole rods is also positive. The discussion would apply equally well if the polarity of the ions was negative and the polarity of the quadrupolar DC applied to the X-pole rods was negative. One consequence of this arrangement is that thermal ions tend to congregate near the entrance end of the rod set, or where the derived axial force first begins. This occurs because the quadrupolar resolving DC is positive on the X-pole. Repelled by the positive potential on the X-rods, and attracted by the negative potential on the Y-rods, positive ions will tend to have somewhat higher radial amplitudes in the y-z plane than in the x-z plane. Thus, on average, the net field experienced by thermal ions is slightly negative, resulting in a higher ion density towards the entrance end of the rod set. As the derived axial force scales quadratically with radial amplitude, the net force felt by thermal ions is very weak: sufficient to reduce dramatically the amount of charge near the exit where it would perturb mass-selective axial ejection, but not so strong that ions would not be distributed over a significant length of the rod assembly. 
     The foregoing description deals with positive ions. In general, the dipolar auxiliary voltage signal should be provided to the rod pair that receives the quadrupolar resolving DC of the same polarity as the ions in the rod array. Thus, in the case where a quadrupolar rod set contains negative ions, and the quadrupolar resolving DC of negative polarity is provided to the X-rods, then the dipolar auxiliary voltage signal should be provided to the X-rods, as before. 
     Referring to  FIG. 2 , there is illustrated in a schematic diagram, an ion guide  118  in accordance with a first aspect of the present invention. For brevity, the description of  FIG. 1  will not be repeated with respect to  FIG. 2 , Instead, and for clarity, elements analogous to those described above in connection with  FIG. 1  will be designated using the same reference numerals, plus  100 . 
     As shown in  FIG. 2 , both the X-rods  122  and Y-rods  124  are coated with a high-dielectric insulating layer  132 . Preferably, this insulating layer  132  is capable of isolating a minimum of 200 V DC. This insulating layer  132  is, in turn, coated with a thin resistive coating  130 . Preferably, this thin resistive film  130  offers an end-to-end resistance on each rod of 10 to 20 MΩ. Preferably, both the resistive coating  130  and insulating layer  132  should be as thin as possible. 
     As shown in  FIG. 2 , quadrupolar DC is applied at one end of the X-rods  122  and Y-rods  124  by variable DC quadrupolar voltage sources  128   a  and  128   b  respectively. The DC quadrupolar voltage provided by variable DC quadrupolar voltage sources  128   a  and  128   b  are opposite in polarity. 
     Rod sets as described in  FIG. 2  may be constructed in any number of different ways. For example, a stainless steel rod 0.003″ smaller in radius than the desired final radius may be coated with a layer of alumina approximately 0.010″ thick. Subsequently, the rod may be machined to the desired radius, resulting in a layer of alumina of thickness 0.003″. The alumina-coated rod would then be masked, and the resistive coating  130  applied. As resistive coating  130  can be very thin, perhaps having a thickness of 10 microns or less, the thickness of resistive coating  130  need not significantly affect the radial dimension of the rods. Finally, metal bands may be applied to each end of the rods  122  and  124  to facilitate good ohmic contact with lead wires from variable DC quadrupolar voltage sources  128   a  and  128   b  at one end, and with lead wires  129  at the other end. 
     Alternatively, and more simply, ordinary stainless steel rods  122  and  124 , already machined to normal specifications, may be coated with a high-dielectric polymer (the resistive coating  130 ), which is sufficiently resistive such that a 10 micron layer suffices to withstand 200 V DC. Subsequently, ions are implanted in the polymer layer to a depth of only a few microns to create the resistive coating  130 . As described above, metal bands at the ends insure good ohmic contact between the resistive coating  130  and, at one end, lead wires from variable DC quadrupolar voltage sources  128   a  and  128   b , and, at the other end, lead wires  129 . 
     A third method of making the rod set of  FIG. 2  involves chemical vapour deposition (CVD) of an insulating layer from [2,2]-para-cyclophane paralyne to an average depth of 23 μm, followed by CVD of a resistive coating of hydrogenated amorphous silicon (a-Si:H) film of estimated thickness ˜0.5 μm. 
     Under normal RF/DC operation, quadrupolar, resolving DC is applied to both ends of the resistive coating  130 , to minimize variation in the quadrupolar DC over the length of the rods. However, in aspects of the present invention, the quadrupolar resolving DC, U DC  &lt;0.01×|V RF |, is applied to the resistive coating  130 , via the circumferential metal bands or other suitable means, at one end, preferably the entrance-end, of the rod set  120  only. At the exit end, as shown in  FIG. 2 , rods  122  and Y-rods  124 , which are of opposite polarity in terms of the quadrupolar DC applied to them, are connected to each other, by lead wires  129 . Lead wires  129  are connected to one another through variable resistors  131  that have sufficient resistance to compensate for variations in the end-to-end resistances of each rod so that the quadrupolar DC can be nulled, or reduced to some suitable minimum, at the exit-end of the ion guide  118 . 
     Referring to  FIG. 3 , there is illustrated in a schematic diagram, an ion guide  218  in accordance with a second aspect of the present invention. For brevity, the description of  FIG. 1  will not be repeated with respect to  FIG. 3 . Instead, and for clarity, elements analogous to those described above in connection with  FIG. 1  are designated using the same reference numerals, plus  200 . 
     As shown in  FIG. 3 , both the X-rods  222  and the Y-rods  224  are divided into segments, numbered S 1  to S 9  (it will, of course, be appreciated by those of skill in the art that the rods may be divided into a different number of segments). Variable resolving DC voltage sources  228   a  and  228   b  provide quadrupole resolving DC voltages of opposite polarity to X-rods  222  and Y-rods  224 . 
     As shown in  FIG. 3 , each of the segments of the X-rods  222  and Y-rods  224  are coupled along an RF path  242  by capacitive dividers  234 , and the RF voltage supplied by RF voltage source  226  is supplied to the individual segments via these capacitive dividers  234 . The capacitance of these capacitive dividers  234  define the RF voltage profile along the length of the ion guide  218 . Ideally, these would be chosen sufficiently small that the RF voltage will not drop appreciably over the length of the rods. However, in some applications, it may be desirable to vary the magnitude of quadrupolar RF along the length of the rods by this means. 
     In the embodiment of  FIG. 3 , resolving quadrupolar DC is provided to all segments, but the low resistance DC connections between segments S 1  and S 2 , and between segments S 2  and S 3 , of X-rods  222  and Y-rods  224 , provide a means of maintaining a constant quadrupolar DC level across segments S 1 , S 2 , and S 3 . Similarly, the low resistance DC connections between segments S 8  and S 9  of X-rods  222  and Y-rods  224 , provide a means of maintaining a constant quadrupolar DC level across segments S 8  and S 9  of X-rods  222  and Y-rods  224 . Consequently, the quadrupolar resolving DC provided by DC voltage sources  228   a  and  228   b  via DC path  244  to X-rods  222  and Y-rods  224  will remain constant between segments S 1 , S 2  and S 3 , vary between segments S 3  and S 4 , S 4  and S 5 , S 5  and S 6 , S 6  and S 7 , and S 7  and S 8 , and remain constant between segments S 8  and S 9 . In this way, the values of the resistances, which make DC electrical connections between adjacent segments along DC path  244 , define DC voltage profile along the ion guide  218 . 
     In the embodiment of  FIG. 3 , unlike the embodiment of  FIG. 2 , the derived axial force is negligible between segments S 1  and S 2 , between segments S 2  and S 3 , and between segments S 8  and S 9 . That is, the quadrupolar resolving DC field, from which the derived axial force is derived, remains constant until it begins to diminish between segments S 3  and S 4 . Consequently, the derived axial force from quadrupolar resolving DC will begin in the vicinity of segment S 3 . 
     Similarly, the derived axial force is negligible at segment S 9 . 
     Quadrupolar resolving DC path  244  is separate from RF path  242 ; however, as both of these paths are connected to the rod set, they must be electrically isolated from each other. For this reason, blocking inductors  238  are provided along quadrupolar resolving DC path  244  to isolate DC voltage sources  228   a  and  228   b , as well as variable resistors  231 , from RF current received via X-rods  222  and Y-rods  224 . Blocking capacitors  240  serve to isolate RF voltage source  226  from the quadrupole DC provided to segment S 9 . 
     Mass-Selective Axial Transport 
     The operation of the ion guides  118  and  218  of  FIGS. 2 and 3  respectively for mass-selective axial transport, in which ions are introduced to the ion guides from an ion source (not shown), and then accelerated axially by the axial gradient of the quadrupolar DC potential, will be explained with reference to  FIG. 4 .  FIG. 4  is a stability diagram, which illustrates how the derived axial field can be used to improve the efficiency of mass-selective axial ejection wherein the RF amplitude is ramped at a constant rate to bring ions of successively higher mass into resonance with the low-amplitude, dipolar, auxiliary signal provided as described above in connection with  FIG. 1 . In addition, it is important that the dipolar auxiliary AC signal be applied between the rods of the pole on which the polarity of the quadrupolar DC matches the polarity of the ion. In the discussion that follows, the polarity of the ion is positive and the positive pole of the quadrupolar resolving DC and the dipolar auxiliary signal are both applied to the X-rods. 
     In the stability diagram of  FIG. 4 , the U/V ratio is 0.01 at z=0.0, and drops to zero at z=127 mm. Consequently, the slope of the scan line is also a function of axial position. This relationship has been portrayed in  FIG. 4  by superposing the axial scale on the ordinate, indicating that the Mathieu parameter a is a function of axial position, but q is not. For any specific mass, q increases linearly in time as the RF amplitude is ramped. The frequency of the auxiliary signal is 380 kHz, corresponding to the iso-β line on which β=0.76 in a 1.0 MHz system. This corresponds to q eject =0.8433 for mass-selective axial ejection and both of these features are represented in  FIG. 4 . 
     Now consider the ion in  FIG. 4  located on the scan line at (a, q)=(0.0118, 0.8320), z=38 mm, whose path through stability-space, from higher to lower a, is shown with a solid line. By virtue of increasing RF amplitude, this ion has moved along the scan line until it comes into resonance with the auxiliary signal at the intersection of the scan line with β=0.76. Recall that the ion is always on the scan line, so that the slope of the scan line, and its intersection with the line β=0.76, changes with the axial position of the ion. In consequence of its increased X amplitude, the ion experiences an increased positive axial force and is accelerated towards the exit lens. As a result, its a-value is reduced and the ion comes off resonance. Whether its radial motion is damped through a collision with the low-pressure buffer gas, or the change in phase relationship between the auxiliary signal and the ion&#39;s secular motion, its acceleration towards the exit-lens slows. Alternatively, the ion may be reflected by the exit-lens potential; in this case, as indicated by the dashed line, the ion&#39;s path in the stability-space could approach the q-axis, if it moves sufficiently close to the exit end before being reflected back to higher a-values. In either case, in response to linearly increasing q, the ion&#39;s position on its scan line intersects with β=0.76 once again at lower a (and higher q), and the ion suffers additional resonant excitation. This cycle, or variations thereof, repeat until the ion either is ejected axially, or is lost on the rods, where the line β=0.76 intersects the q axis. By this means, ions of successfully higher mass can be combed toward the exit end of the rod set just prior to mass-selective axial ejection. 
     Simulation Results 
     The response of ions to the above-described derived axial force was studied using three-dimensional computer simulations of ion trajectories in a quadrupole linear ion trap (LIT). To that end, specific models were developed in which the quadrupolar DC applied to the rods varied with axial position. In the two-dimensional midsection of the LIT, the derived axial force was calculated analytically from two-dimensional numeric potentials. However, in the fringing regions at the ends of the rod set, it was necessary to solve the Laplace equation for electrode configurations where the quadrupolar DC voltage varied linearly with axial position on the rods. A few sample results are presented below. 
     As discussed above, ions tend to congregate near the entrance end of the ion guide in which the derived axial force is provided. Referring to  FIG. 5 , a graph plots data that illustrates this behavior. Specifically,  FIG. 5  shows the axial distribution of 1000 ions that were allowed to thermalize with a buffer gas while the derived axial force was provided. These data were obtained by cooling 1,000 ions of m/z 609 in 6 mtorr N 2  for 1 ms at q=0.84 with a U 0 /V ratio of 0.01. During the cooling period, +390 V was applied to the lenses of a rod set 127 mm in length. Each lens was located 3 mm distant from the ends of the rods. 
     The graph of  FIG. 6  shows the axial component of the trajectory of an ion with greater X than Y amplitude as it is reflected alternately by the exit lens and the derived axial force in a collision-free environment. 
     Other variations and modifications of the invention are possible. For example, other means of providing a variable quadrupolar resolving DC along the rods of an ion guide may be provided. All such modifications or variations are believed to be within the sphere and scope of the invention as defined by the claims appended hereto.