Abstract:
A method of analysing ions provides for separating ions with different charge states. Ions are first thermalized to have substantially the same energy, preferably in an ion trap. Then a barrier height is set to enable ions having a lower charge to escape, while retaining ions with higher charge states. Having effected separation of the ions either or both groups of ions can be subjected to various conventional mass analysis or other processing steps.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a mass spectrometry method and apparatus. More particularly, this invention relates a mass spectrometry technique enabling, or at least enhancing, separation of ions with different charges.  
         BACKGROUND OF THE INVENTION  
         [0002]    Mass spectrometry is now a well-established technique for analyzing substances by separating ions due to their differing mass to change ratios. A wide variety of mass spectrometers and ionization techniques are known. The present invention is particularly, although not exclusively, concerned with electrospray-generated ions, and more particularly the use of this ionization technique with large organic molecules.  
           [0003]    Mass spectrometry of electrospray-generated ions is a very sensitive technique for identification and quantification of trace compounds at low concentrations. In particular, it is now known that electrospray ionization techniques generate multiply charged ions allowing analysis with mass spectrometers with limited mass ranges. Many organic compounds can be ionized so to have multiple charges. For example, multiply charged ions of peptides formed from protein digestion by the enzyme trypsin have been shown to be useful for sequence determination following product ion MS/MS scans, as is described by Covey et. al. in U.S. Pat. No. 5,952,653. A product ion scan is now a well known analysis technique in mass spectrometry, in which a precursor ion is selected, caused to fragment (usually by acceleration into a collision cell), and then the fragments are scanned to determine the fragments or products generated from the selected precursor, which can give information about the structure of the precursor. One difficulty however is that it can be a challenge to identify low concentration multiply charged peptides in the single MS survey scan due to the presence of singly charged chemical noise that is often present in such scans. MS/MS techniques such as precursor ion and neutral loss scanning can partly offset the chemical noise problem by introducing an additional degree of specificity to the survey scans (a precursor ion scan holds the selected product or fragment ion mass to charge ratio fixed and scans to identify precursor ions that generate such the selected product of fragment ion; a neutral ion scan maintains a fixed mass difference between a selected precursor ion and a selected product/fragment ion). The utility of these scans however requires some prior knowledge of the sample, which is not always the case. For example, to carry out a meaningful precursor scan, it is necessary to have some knowledge of fragment ions that might be generated. Thus, analysis of analytes that produce multiply charged fragment ions can generate some unique problems.  
           [0004]    Linear ion traps have been reported to discriminate against higher m/z ions under conditions in which the overall charge density is high. This is due to the fact that, at a given RF voltage or trapping q-value, the potential wells for higher m/z ions are shallower than those for ions with lower m/z values [Tolmachev et. al. Rapid Commun. Mass Spectrom. 14, 1907-1913(2000)]. This is true for both linear ion traps with two-dimensional radio frequency trapping fields and conventional ion traps with three-dimensional trapping fields. However, this does not address the problem of differentiating between multiply charged ions (often desired analyte ions) and singly charged ions (often unwanted chemical noise) with the same m/z. The inventor of the present invention has found that the population of multiply charged ions of a given m/z can be enhanced relative to the population of singly charged ions at the same m/z. This then makes it possible to identify low concentration multiply charged ions in what would normally be much more concentrated singly charged chemical noise.  
         SUMMARY OF THE INVENTION  
         [0005]    Accordingly it is desirable to provide a method that enables multiply charged and singly charged ions of the same m/z to be distinguished from one another.  
           [0006]    The present invention provides a method for enhancing the appearance of multiply charged ions in the single MS survey scan by first ensuring the ions have substantially similar energies, preferably by collisional cooling, and then differentiating between the different ions by an energy barrier. These steps are preferably carried out in an ion trap, most preferably when utilizing a linear ion trap. The technique involves first allowing the trapped ions to cool via collisions with a background gas to the point where singly and multiply charged ions have the similar kinetic energies. Subsequently a normally repulsive DC barrier voltage at one end of the linear ion trap, previously used to maintain the trap, is reduced to a level where the singly charged ions are allowed to escape while the multiply charged ions remain trapped. Experimental results detailed below, show a dramatic reduction in the number of trapped singly charged ions with little loss of the multiply charged ion population. This method allows rapid identification of multiply charged ion fragments or products that can then be further subjected to MS/MS scans, such as product ion, precursor or neutral loss scans, to allow, at least for peptides and proteins, sequence information to be obtained.  
           [0007]    In accordance with a first aspect of the present invention, there is provided a method of analyzing ions, whereby the method comprising:  
           [0008]    (1) providing a stream of ions; and  
           [0009]    (2) providing, in an ion processing section, an energy barrier, having a magnitude between at least a first group of ions having a first charge and a second group of ions having a second, higher charge, whereby said at least a first group of ions are emptied from the ion processing section and the second group of ions are retained in the ion processing section for subsequent processing.  
           [0010]    In the most general case, either one or both of the first and second groups of ions can be subject to a mass analysis step, or other processing, i.e. fragmentation followed by mass analysis. As the first group of ions are necessarily emptied from the ion trap, any further processing or mass analysis must be effected outside of the trap. The second group of ions can be further processed in the trap (i.e. by scanning out by axial ejection, to effect mass analysis) or transferred to other devices for further processing.  
           [0011]    It will also be understood that where there are a large number of different multiply charged ion species, the energy barrier can be set initially at any number of different levels. For example, it may be desired to eject singly and doubly charged ions and just retain triply and greater charged ions, instead of ejecting just the singly charged ions. In this situation a further alternative is to progressively eject or empty each group of ions with a different charge, e.g. first singly charged ions, then doubly charged ions etc., so that each group of ions can be subject to individual secondary processing.  
           [0012]    Outside of the linear ion trap, mass analysis can be effected using a quadrupole or other multipole-based mass analysis, a time of flight mass spectrometer, a Fourier transform mass spectrometer, a conventional 3-dimensional ion trap mass spectrometer, or any other suitable mass spectrometer.  
           [0013]    To achieve a high level of separation of the first and second groups of ions, it is necessary to ensure that the energy distribution amongst the ions is sufficiently low, so that energy barrier will retain the second group of ions while permitting the first group of ions to empty or to escape. Accordingly, between steps (1) and (2), the method preferably includes ensuring that this energy distribution is low enough, to provide this separation. More preferably, this is achieved by thermalizing the ions with by collision with a neutral gas. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    For better understanding the present invention and to show more clearly how it may be carried into fact, reference will now be made, by way of example, to the accompanying drawings in which:  
         [0015]    [0015]FIG. 1 is a schematic view of a triple quadrupole mass spectrometer for use with the present invention;  
         [0016]    [0016]FIG. 2 is a timing diagram showing variation of voltages at different locations within the mass spectrometer of FIG. 1, in conventional operation;  
         [0017]    [0017]FIG. 3 shows a single MS survey scan utilizing the mass spectrometer of FIG. 1 in a single MS mode.  
         [0018]    [0018]FIG. 4 shows a timing diagram for the voltages of the apparatus of FIG. 1, according to the present invention;  
         [0019]    [0019]FIG. 5 shows a single MS survey scan, similar to FIG. 3, but with the mass spectrometer operated in accordance with FIG. 4, separating multiply charged ions from singly charged ions;  
         [0020]    [0020]FIG. 6 shows an exemplary MS/MS scan in accordance with the present invention;  
         [0021]    [0021]FIG. 7 shows schematically a Qq-TOF mass spectrometer for use with the present invention;  
         [0022]    [0022]FIG. 8 shows the total ion signal of a Qq-TOF instrument obtained as the IQ 3  lens voltage is reduced from 9.7 to 8.5 volts.  
         [0023]    [0023]FIG. 9 shows the summed mass spectra comprising the total ion signal in FIG. 8, with the inset being an expanded view of m/z 535 to 595.  
         [0024]    [0024]FIG. 10 shows the summed mass spectra for the circled region of FIG. 8, with the inset being an expanded view of m/z 535 to 595 and showing that the singly charged ions have been discriminated against leaving only multiply charged ions. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    Referring first to FIG. 1, there is shown a conventional triple quadrupole mass spectrometer apparatus generally designated by reference  10 . An ion source  12 , for example an electrospray ion source, generates ions directed towards a curtain plate  14 . Behind the curtain plate  14 , there is an orifice plate  16 , defining an orifice, in known manner.  
         [0026]    A curtain chamber  18  is formed between the curtain plate  14  and the orifice plate  16 , and a flow of curtain gas reduces the flow of unwanted neutrals into the analyzing sections of the mass spectrometer.  
         [0027]    Following the orifice plate  16 , there is a skimmer plate  20 . An intermediate pressure chamber  22  is define between the orifice plate  16  and the skimmer plate  20  and the pressure in this chamber is typically of the order of 2 Torr.  
         [0028]    Ions pass through the skimmer plate  20  into the first chamber of the mass spectrometer, indicated at  24 . A quadrupole rod set Q 0  is provided in this chamber  24 , for collecting and focusing ions. This chamber  24  serves to extract further remains of the solvent from the ion stream, and typically operates under a pressure of 7 mTorr. It provides interface into the analyzing sections of the mass spectrometer.  
         [0029]    A first interquad barrier or lens IQ 1  separates the chamber  24  from the main mass spectrometer chamber  26  and has an aperture for ions. Adjacent the interquad barrier IQ 1 , there is a short “stubbies” rod set, or Brubaker lens  28 .  
         [0030]    A first mass resolving quadrupole rod set Q 1  is provided in the chamber  26  for mass selection of a precursor ion. Following the rod set Q 1 , there is a collision cell of  30  containing a second quadrupole rod set Q 2 , and following the collision cell  30 , there is a third quadrupole rod set Q 3  for effecting a second mass analysis step.  
         [0031]    The final or third quadrupole rod set Q 3  is located in the main quadrupole chamber  26  and subjected to the pressure therein typically 1×10 −5  Torr. As indicated, the second quadrupole rod set Q 2  is contained within an enclosure forming the collision cell  30 , so that it can be maintained at a higher pressure; in known manner, this pressure is analyte dependent and could be 5 mTorr. Interquad barriers or lens IQ 2  and IQ 3  are provided at either end of the collision cell of  30 .  
         [0032]    Ions leaving Q 3  pass through an exit lens  32  to a detector  34 . It will be understood by those skilled in the art that the representation of FIG. 1 is schematic, and various additional elements would be provided to complete the apparatus. For example, a variety of power supplies are required for delivering AC and DC voltages to different elements of the apparatus. In addition, a pumping arrangement or scheme is required to maintain the pressures at the desired levels mentioned.  
         [0033]    As indicated, a power supply  36  is provided for supplying RF and DC resolving voltages to the first quadrupole rod set Q 1 . Similarly, a second power supply  38  is provided for supplying drive RF and auxiliary AC voltages to the third quadrupole rod set Q 3 , for scanning ions axially out of the rod set Q 3 . A collision gas is supplied, as indicated at  40 , to the collision cell  30 , for maintaining the desired pressure therein.  
         [0034]    The apparatus of FIG. 1 is based on an Applied Biosystems/MDS SCIEX API 2000 triple quadrupole mass spectrometer. In accordance with the present invention, the third quadrupole rod set Q 3  is modified to act as a linear ion trap mass spectrometer with the ability to effect axial scanning and ejection as disclosed in U.S. Pat. No. 6,177,668.  
         [0035]    The standard scan function, detailed in U.S. Pat. No. 6,177,668 involves operating Q 3  as a linear ion trap. Analyte ions are admitted into Q 3 , trapped and cooled. Then, the ions are mass selectively scanned out through the exit lens  32  to the detector  34 . Ions are ejected when their radial secular frequency matches that of a dipolar auxiliary AC signal applied to the rod set Q 3  due to the coupling of the radial and axial ion motion in the exit fringing field of the linear ion trap.  
         [0036]    The conventional timing diagram for this scan function is displayed in FIG. 2. In an initial injection phase, the DC voltages at IQ 2  and IQ 3  are maintained low, as indicated at  50  and  52 , while simultaneously the exit lens  32  is maintained at a high DC voltage  54 . This allows ions passage through rod sets Q 1  and Q 2  into Q 3 , and Q 3  functions as an ion trap preventing ions leaving from Q 3 . At this time, the drive RF and auxiliary AC voltages applied to Q 3 , are maintained at low voltages indicated at  56  and  58  in FIG. 2. The injection period typically lasts for 5-25 milliseconds.  
         [0037]    Following this there is a cooling period, during which voltages IQ 2  and IQ 3  are raised to levels indicated at  60  and  62 , to prevent further passage of ions. The voltage of the exit lens  32  is maintained at the voltage  54 . Consequently, ions are completely trapped within Q 3 , and are prevented from exiting from Q 3  in either direction and also are radially confined by the quadrupolar field. The drive RF and auxiliary AC voltages applied to quadrupole rod set Q 3  are maintained at levels  56  and  58 . This cooling period lasts 10-50 milliseconds.  
         [0038]    Once the ions have been cooled to substantially the same energy, the ions are scanned out in a mass scan period, during which the DC voltages on the lens IQ 2  and IQ 3  are maintained at the high, blocking voltage levels  60 ,  62  and the exit lens  32  is maintained at the voltage level  54 . These voltages are normally sufficient to maintain the ions trapped.  
         [0039]    However, in accordance with U.S. Pat. No. 6,177,668, during this mass scan period, the drive RF and auxiliary AC voltages applied to the quadrupole rod set Q 3  are scanned as indicated at  64  and  66 . This causes ions to be scanned out in a mass selective fashion through the ion lens  32  to the detector  34 .  
         [0040]    At the end of the mass scanning period, the drive RF and auxiliary AC voltages are returned to zero, as indicated at  68  and  70 . Simultaneously, the DC potentials applied to the lens or barriers IQ 2  and IQ 3  are reduced to zero as indicated at  72  and  74 , and correspondingly the voltage on the exit lens  32  is reduced to zero as indicated at  76 . This serves to empty the ion trap, formed by Q 3 , of ions.  
         [0041]    In the cooling period, ions are trapped within the linear ion trap formed by Q 3 , by the radially applied RF voltage and the DC barriers applied to both ends of the device, i.e. at the lens or barrier IQ 3  and the exit lens  32 . Once ions are trapped in the linear ion trap they experience numerous energy dissipating collisions to the point where the kinetic energy of the trapped ions is determined by the temperature of the surrounding neutral gas in addition to energy from the RF field. The background gas density and the collision cross section of the ion with the background gas determine the time required for this thermalization process. Given enough time a trapped ion population will thermalize even at very low background gas pressures.  
         [0042]    Once a trapped ion population containing singly and multiply charged ions has thermalized, the effective DC barrier height at the ends of the linear ion trap depends on the charge state of the ion. Ions will escape if their kinetic energy is greater than their charge state multiplied by the applied repulsive DC voltage. That is, if 
         mv 2 /2 &gt;q V 
         [0043]    where, m is the ion mass, v is the ion velocity, q is the ion charge state, and V is the applied repulsive DC voltage.  
         [0044]    For example, a DC barrier height of 10 volts appears as a 10 volt repulsive barrier for a singly charged ion, a 20 volt repulsive barrier for a doubly charged ion, and a 30 volt barrier for a triply charged ion. If the DC voltage applied to one or both ends of the linear ion trap is reduced to the point at which it is similar to the kinetic energies of the thermalized trapped ion population, some ions will escape, but in a charge state dependent manner. For example, if the DC trapping voltage applied to one of IQ 3  and the exit lens  32  of the linear ion trap of Q 3  is reduced to 1 volt for a mixed charge state ion population that has been thermalized to a kinetic energy of 1.5 electron volt, the singly charged ions will preferentially escape from the linear ion trap enhancing the relative concentration of ions with higher charge states since the higher charge states see proportionately higher effective barriers due to the applied 1 volt repulsive DC voltage. Optimization of the repulsive barrier height can result in removal of most singly charged ions from an original ion population in which they were the dominant trapped species.  
         [0045]    It is understood that the trapped ion population will be characterized by an energy distribution rather than a single energy. If completely thermalized this energy distribution will be close to a Maxwell-Boltzmann distribution characterized by the temperature of the neutral gas within the linear ion trap in addition to energy from the RF field. The implication is that each trapped ion will have a slightly different kinetic energy. Thus, it is unlikely that complete elimination of lower charge state ions from the linear ion trap can be accomplished at room temperature. However, enhancement of higher charge state ions relative to singly charged ions will occur. The trapped ion population within the linear ion trap need not be completely thermalized to affect some degree of charge state separation. However, the relative enhancement of the population of multiply charged ions to singly charged ions will not be as great since the multiply charged ions will in general be more energetic than the singly charged ions.  
         [0046]    Referring now to FIG. 3, this shows a single MS survey scan of a tryptic digest of 10 fm/micro-liter of bovine serum albumin (BSA). This spectrum was obtained by operating the Q 1  quadrupole rod set in RF-only mode in order to transmit most of the ions from the ion source into the Q 3  ion trap. The q 2  collision cell was maintained at approximately 5 milli-Torr of nitrogen to enhance the trapping efficiency of Q 3 , and potentials along the mass spectrometer  10  were selected to give desired ion movement without any significant fragmentation. Thus, the DC voltage offset between Q 1  and q 2  was maintained at less than 10 volts in order to maximize the Q 3  trapping efficiency. The mass spectrum in FIG. 3 shows the presence of many singly charged ion species with no easily recognizable multiply charged peptide features.  
         [0047]    Reference will now be made to FIG. 4 which shows a timing diagram similar to FIG. 2, but modified according to the present invention. For simplicity and brevity, like elements of FIG. 4 are given the same reference numeral as in FIG. 2, and description of these time periods is not repeated.  
         [0048]    The timing scheme of FIG. 4 has the same four periods as in FIG. 2, namely an initial injection period during which ions are passed through Q 1  and Q 2  into Q 3 , a cooling period during which ions are trapped in Q 3  and caused to cool down to an approximate uniform level; at the end of the timing diagram, there is the mass scanning period and the emptying time period. What is additionally provided is the separation or partial emptying period indicated at  80 . During this period, the DC voltage applied to the IQ 3  lens or barrier is reduced to a point where the trapped singly charged ions are allowed to escape while retaining the multiply charged ions within the linear ion trap of Q 3 . As is explained above, because of the different charges of the ions and because the ions have been cooled to approximately the same energy, this enables unwanted singly charged ions to be ejected from the ion trap while retaining desired, multiply charged ions.  
         [0049]    Note that it is possible to eject ions from the ion trap at Q 3  by reducing the voltage on either IQ 3  or the exit lens  32 . It is preferred to reduce the potential barrier at IQ 3 , since this prevents the ions hitting the ion detector which shortens the ion detector lifetime.  
         [0050]    A multiply charged enhancement scan, in accordance with the present invention, was then carried out by again filling the Q 3  ion trap with ions from the electrospray ion source, allowing the trapped ion population within the Q 3  linear ion trap to thermalize, and then providing a “separation” or “partial empty” step in which the IQ 3  barrier was reduced as indicated at  80  in FIG. 4. Again, ions were admitted into the Q 3  linear ion trap by reducing the DC voltage applied to the IQ 3  lens while the Exit lens  32  was maintained at an appropriate repulsive voltage with respect to the incoming ion energies for a period of 100-1000 ms. The ions were trapped and cooled within the Q 3  linear ion trap as before, for a period in the range 10-50 milliseconds, by collision with the residual background gas. The separation step at  80  of FIG. 4 was accomplished by reducing the repulsive DC voltage applied to IQ 3  to the point at which the singly charged ions can escape while ions with higher charge states remain trapped, for a period of 1-50 milliseconds. Mass analysis of the trap contents was carried out for a period of 100-1000 ms. Again, the final step expelled or emptied any residual trapped ions from the linear ion trap in an empty step of duration 5 ms.  
         [0051]    Implementation of the multiply charged enhancement scan results in the survey mass spectrum shown in FIG. 5 for the same 10 fm/micro-liter BSA digest solution as to FIG. 3. In FIG. 5, all of the major mass peaks in the spectrum are due to doubly charged BSA peptides, which are easily distinguished from the very low level singly charged noise. Thus, the data obtained from the multiply charged enhancement scan mode displays significantly better signal-to-noise ratios than the conventional single MS survey scan of FIG. 3, allowing very easy identification of multiply charged peptides.  
         [0052]    Once the ions of interest have been identified, conventional product ion MS/MS scans can be conducted on selected peptides as is shown in FIG. 6. This is the product ion mass spectrum obtained by selecting the doubly charged BSA tryptic peptide located at m/z 464, fragmenting the m/z 464 precursor ions by acceleration between Q 1  and q 2 , trapping the fragment and residual precursor ions in the Q 3  ion trap, and finally mass selectively scanning the trapped ions toward the detector.  
         [0053]    The multiply charged enhancement scan mode or method of the present invention is not restricted to apparatus employing a mass selective linear ion trap. Any mass spectrometer system that has the capability of trapping ions in a linear or curved multipole ion trap can be used. A straightforward example of an alternative implementation of the present invention is the use of the Q 2  collision cell of a Q-q-time-of-flight (TOF) tandem mass spectrometer as is schematically displayed in FIG. 7 (Q designating a mass analysis section and q a collision cell). Ions may be trapped within the Q 2  linear ion trap by reducing the voltage applied to IQ 2  while maintaining IQ 3  at a sufficiently high repulsive DC voltage during a specified fill time. The voltage applied to IQ 2  is then increased to trap an ion population within Q 2 . The ions within the Q 2  linear ion trap are thermalized quickly due to the milli-torr pressures in a conventional Q 2  collision cell. Next, the repulsive DC barrier applied to IQ 2 , IQ 3  or both lenses is reduced to the point where the lower charge state ions are allowed to escape. The remaining trapped ion population within the Q 2  linear ion trap is then pulsed out toward the TOF mass spectrometer for conventional mass analysis resulting in a mass spectrum in which the appearance of higher charge state ions has been enhanced.  
         [0054]    Since the Q-q-TOF instrument provides very rapid full mass spectra the identities of all of the ions originally trapped within the Q 2  linear ion trap can be ascertained by reducing the repulsive DC barrier applied to IQ 3  in a step wise fashion. The first ions to escape will be singly charged followed by the doubly charged ions, multiply charged ions, etc. If the rate at which the repulsive DC voltage applied to IQ 3  is slower than the TOF scan time, mass spectra can be obtained at each value of the IQ 3  barrier height. Thus, none of the ions trapped within the Q 2  linear ion trap will have been wasted and charge state separation will have been accomplished.  
         [0055]    An example of the method for charge state separation using a Qq-TOF instrument is shown in FIG. 8. Here, electrosprayed ions from a tryptic digest of bovine serum albumin were trapped in Q 2  and then allowed to escape by a step-wise reduction of the voltage applied to IQ 3 . The IQ 3  voltage was reduced from 9.7 to 8.5 volts with a DC offset of 8.5 volts applied to Q 2 . Thus, the DC barrier height was reduced from 1.2 volts to 0 volts uniformly during the time taken for the experiment. An axial field had been applied to concentrate the trapped ion population toward IQ 3 . FIG. 8 shows the total ion signal as a function of the time over which the IQ 3  voltage was reduced.  
         [0056]    As shown, as the voltage or IQ 3  is progressively reduced, ions begin to leak out at an increasing rate, which peaks at approximately 0.27 seconds and declines down to a minimum at approximately 0.5 seconds, this being primarily singly charged ions escaping. After 0.50 seconds, as the barrier is deceased further, another small peak occurs, as indicated by the circled area, this being primarily the doubly charged ions escaping from the ion trap.  
         [0057]    [0057]FIG. 9 shows the summed TOF mass spectra for the entire ion population of FIG. 8. These mass spectra are comprised of singly and multiply charged ions. The FIG. 9 inset is an expanded view of the m/z 535 to 595 region illustrating the complicated nature of the mass spectra.  
         [0058]    [0058]FIG. 10 shows the mass spectra obtained from the circled portion of the total ion signal of FIG. 8. These spectra contain mostly multiply charged ions with very little contribution from singly charged ions. The inset of FIG. 10 more clearly shows the spectral simplification in the same m/z 535 to 595 mass range highlighted in FIG. 9. The only prominent ions in the FIG. 10 inset are multiply charged. These multiply charged ions would be difficult to identify in the FIG. 9 mass spectra.  
         [0059]    DC barriers over which the lower charge state ions are allowed to escape can be created with ion optical elements other than a simple aperture lens. DC barriers can be created by another multipole device such as a quadrupole or a Brubaker lens with a suitable DC barrier applied to it. DC barriers have also been created by cylindrical ring electrodes placed around linear multipole ion traps as demonstrated by Gerlich [D. Gerlich,  Advances in Chemical Physics, Vol. LXXXII, 1-176 (1992)]. These ion optical elements can be used in place of, or in addition to, simple aperture lenses.  
         [0060]    DC barriers can also be created using properly shaped rods used to define the linear ion trap itself or via auxiliary electrodes inserted between the linear ion trap rods as described by Thomson and Jolliffe U.S. Pat. No. 5,847,386. These techniques offer the opportunity to create a continuous DC barrier or field within the linear ion trap itself and may lead to more efficient charge state discrimination.  
         [0061]    It is also possible that for some applications, trapping may not be required. Trapping is provided here to ensure that there is sufficient time to thermalize or cool all the ions to substantially the same energy level. In certain mass spectrometer systems, it may be possible to achieve this in continuous flow through devices. This would require, for example, that transit time through a cooling section and the number of collisions be sufficient to ensure that all ions are substantially thermalized at the end of the cooling section where an energy barrier is provided.