Abstract:
Systems and methods for automatic gain control in mass spectrometers are disclosed. An exemplary system may include a mass spectrometer, comprising a lens configured to receive a supply of ions, and a mass analyzer. The mass analyzer may include an ion trap for trapping the supplied ions. The mass analyzer may also include an ion detector for detecting ions that exit the ion trap. The lens may focus the ions non-uniformly based on mass of the ions to compensate for space charge effects reflected in a measurement output of the mass spectrometer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of U.S. patent application Ser. No. 14/206,524, filed Mar. 12, 2014 (now U.S. Pat. No. 8,969,794), which is a non-provisional application claiming priority to U.S. Provisional Patent Application No. 61/799,158, filed Mar. 15, 2013 and titled “Mass Dependent Automatic Gain Control for Mass Spectrometer,” all of which are incorporated herein by reference. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates generally to mass spectrometry and, more particularly, to systems and methods of mass-dependent automatic gain control. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    Mass spectrometers are instruments used to analyze the mass and abundance of various chemical components in a sample. Mass spectrometers work by ionizing the molecules of a chemical sample, separating the resulting ions according to their mass-charge ratios (m/z), and then detecting the abundance of ions at each m/z. The resulting spectrum can be interpreted to reveal the relative amount of each chemical component in the sample based on the abundance of the mass fragments of these components. 
         [0004]    Various mass spectrometers generate ions from the sample utilizing various methods, for example, electrospray ionization, atmospheric pressure chemical ionization, matrix-assisted laser desorption/ionization, and inductively coupled plasmas. In some situations, the ion source that generates the ions is located external to a mass analyzer. The ions are guided from the ion source into a mass analyzer, where the ions are separated based on mass. The ions then arrive at a detector that detects charge and/or current. Information based on the detected charge and/or current is then used to determine the quantity of ions of various masses. 
         [0005]    One type of mass analyzer used for mass spectrometry is called a quadrupole ion trap. Quadrupole ion traps take several forms, including three-dimensional ion traps, linear ion traps, and cylindrical ion traps. The operation in all cases, however, remains essentially the same. DC and time-varying radio frequency (RF) electric signals are applied to the electrodes to create electric fields within the ion trap. These fields trap ions in a “cloud” within the central volume of the ion trap. By manipulating the amplitude and/or frequency of the electric fields, ions are selectively scanned out by being ejected from the ion trap in accordance with their m/z. A detector records the number of ejected ions at each m/z as they arrive. 
         [0006]    Ion traps are optimized for a combination of speed, sensitivity, and resolution depending on the particular application. For a given instrument, an improvement in one category is usually made at the expense of another. For example, sensitivity can generally be increased by using a slower scan, and in the reverse, a scan can be performed faster at the expense of sensitivity. Similarly, sensitivity—especially to less abundant components of a sample—can be increased by trapping and scanning a larger total number of ions in a single scan. However, as the quantity of ions in the trap increases, the coulombic forces and collisions between the like-charged ions in the ion cloud increases, resulting in space charge effects. Mass spectrometers achieve resolution by ejecting all ions of the same m/z at close to the exact same moment. However, when space charge effects become significant, ions are ejected from the trap at different times. The result is broadening of spectral peaks and loss of resolution. Also, detectors used in mass spectrometers typically have a limited dynamic range, the difference between the lowest and highest concentration that can be detected. Concentrations lower than the lower bound are undetectable due to, for example, noise; and concentrations above the upper bound may saturate the detector. Additionally, mass analyzers may trap ions preferentially based on their mass, thus for a sample with a range of masses, larger ions may not be trapped as efficiently as lower masses. 
         [0007]    There is a need for systems and methods for expanding the range of concentrations detectable by mass spectrometers. The present disclosure is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art. 
       SUMMARY OF THE DISCLOSURE 
       [0008]    Embodiments of the present disclosure relate to chemical analysis instruments, such as mass spectrometers, that utilize automatic gain control. Various embodiments of the disclosure may include one or more of the following aspects. 
         [0009]    In one aspect, the present disclosure is directed to a mass spectrometer. The mass spectrometer may include a lens configured to receive a supply of ions, and a mass analyzer downstream of the lens. The mass analyzer may include an ion trap and an ion detector. Furthermore, the lens may focus a beam of the ions non-uniformly based on the mass of the ions to compensate for space charge effects reflected in a measurement output of the mass spectrometer. 
         [0010]    In another aspect, the present disclosure is directed to a mass analyzing control system for analyzing the mass of a sample. The system may include one or more memories storing instructions. The system may also include one or more processor configured to execute the instructions to perform operations. The processor may obtain a mass spectrum of an ion beam generated from the sample and identify a space charge characteristic based on the mass spectrum. The processor may defocus the lens based on the mass spectrum or detector saturation, wherein defocusing the lens may correspond to preferentially defocusing away lighter ions. The processor may then obtain a mass spectrum of a defocused ion beam generated from the sample. 
         [0011]    In yet another aspect, the present disclosure is directed to a method for analyzing the mass fragments of a sample. The method may include focusing an ion beam into a mass analyzer. The method may include obtaining a mass spectrum of the ion beam and identifying a space charge characteristic, or other mass dependent phenomena, based on the mass spectrum. The method may also include defocusing the lens based on the identified space charge characteristic, or other mass dependent phenomena, wherein defocusing the lens corresponds to preferentially defocusing away lighter ions. The method may further include obtaining a mass spectrum of a defocused ion beam generated from the sample. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The drawings are not necessarily to scale or exhaustive. Instead, emphasis is generally placed upon illustrating the principles of the inventions described herein. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments consistent with the disclosure and together with the description, serve to explain the principles of the disclosure. In the drawings: 
           [0013]      FIG. 1  is a pictorial illustration of a mass spectrometer according to some embodiments of the invention; 
           [0014]      FIGS. 2A and 2B  depict exemplary spectra with and without space charge effects; and 
           [0015]      FIGS. 3A ,  3 B, and  3 C depict simplified flight paths of ions for various voltages applied to an ion lens. 
           [0016]      FIG. 4  depicts another view of simplified flight paths of ions defocused preferentially by mass. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0017]    Reference will now be made in detail to the embodiments of the present disclosure described below and illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to same or like parts. 
         [0018]      FIG. 1  is a schematic diagram of a mass spectrometer  100  according to an embodiment of the invention. Mass spectrometer  100  may include an ion source  105  for generating sample ions  107  from a sample and an ions lens  110  for focusing and defocusing ions  107 . Mass spectrometer  100  may also include a mass analyzer  115 . In some embodiments, mass analyzer  115  may be an ion trap-type mass analyzer. Mass analyzer  115  may receive ions  107  after they have been focused or defocused by ion lens  110 . Eventually, ions  107  are scanned out of mass analyzer  115 , detected by detector  128 , and then converted into usable data by various components, such as an A/D converter  130  and a field-programmable gate array (“FPGA”)  140 . 
         [0019]    In various embodiments, ion source  105  may be any apparatus that produces sample ions  107  by ionizing a sample that is introduced into mass spectrometer  100 . For example, ion source  105  may include an electron ionization device comprising an electron filament, which is heated to a high enough temperature such that it emits energetic electrons. Ion source  105  may include an electron lens that focuses and accelerates the electrons into the sample, resulting in ionization of the sample and generation of sample ions  107 . Alternatively, ion source  105  may be other types of devices that ionize samples by various methods, e.g., chemical ionization or inductively coupled plasma. In various embodiments, ion source  105  may generate ions  107  at a relatively high pressure, such as at around atmospheric pressure. In addition to ions  107 , ion source  105  may contain a background gas, such as nitrogen, to which most of the pressure is attributed. 
         [0020]    When ions  107  are emitted from ion source  105 , ions  107  may begin to disperse unless focused by ion lenses. Ion lenses may be biased at various potentials to activate. The resulting electric field may result in electric forces on ions  107  that accordingly define the path of ions  107 . In some embodiments, mass spectrometer  100  may include one or more ion lenses  109  that focus ions  107  into a beam. Mass spectrometer  100  may also include ion lens  110  that controls the degree to which the beam of ions  107  are focused or defocused before entering mass analyzer  115 . The direction and acceleration of ions  107  passing through an aperture  113  of ion lens  110  may be controlled based on the voltage applied to ion lens  110 . In addition, changing the voltage applied to lens  110  may affect the cross-sectional area of the ion beam. Accordingly, the proportion of ions  107  that pass through lens  110  into mass analyzer  115  may vary based partly on the voltage applied to lens  110 . Lens  110  may then act as a voltage-controlled gate for controlling the number of ions  107  that enter the mass analyzer  115 . 
         [0021]    Mass analyzer  115  may include a first end cap electrode  116 , a ring electrode  117 , and a second end cap electrode  118 . First end cap electrode  116  may have an aperture  119 , through which ions  107  are received by mass analyzer  115 . By applying voltages to end caps  116  and  118 , and a voltage to the ring electrode  117 , which may be DC, AC, or combination of AC and DC voltages, an electric field may be generated in mass analyzer  115 . By appropriately setting the field strength, shape, and frequency of the field, ions  107  that enter mass analyzer  115  may be trapped as an ion cloud within mass analyzer  115 . However, ions  107  are not trapped statically in the ion trap. That is, ions  107  may continue to move within the ion cloud, based on the generated RF fields, electrostatic interactions among ions  107 , and collisions with background gas particles. 
         [0022]    The strength of the RF field and/or the frequency of the RF field may then be adjusted to selectively scan out ions  107  based on the mass (more specifically, the mass-to-charge ratio) of the ions. Ions  107  may be scanned out through an aperture  121  in second end cap  118 , and received by ion detector  128 . In some embodiments, a focusing lens  126  may precede ion detector  128 . Focusing lens may include an aperture  127  that is covered with a screen or grate that shields mass analyzer  115  from strong electric fields generated by a high voltage on ion detector  128 . For example, ion detector  128  may be biased with a voltage on the order of −2,000 V. Ion detector  128  may receive ions  107  and generate a detection signal. The output of ion detector  128  may feed into an ion amplifier  129 , which may be positioned in close proximity to ion detector  128 . Ion amplifier  129  may serve to buffer the output of the ion detector  128 , and allow for transmission to A/D converter  130  via a low-impedance signal line that is less susceptible to electromagnetic interference than the output of ion detector  128 . An A/D converter  130  may translate the analog output of the ion amplifier  129  into a digital signal to be read by field-programmable gate array (“FPGA”)  140  and eventually processed into an output spectrum to be read by a user or stored for future use. The output spectrum may depict the number of ions  107  as a function of mass. In some embodiments, the A/D converter  130  and FPGA  140  may be combined into a single complex device such as a digital signal processor (“DSP”), microprocessor, or any combination of analog or digital components known in the art. 
         [0023]    In various embodiments, the resolution of the output spectrum may be affected by space charge or other effects that affect the resolution of the mass spectrometer  100 . For example, space charge effects are due to numerous like-charged ions  107  being confined to a limited space. In various situations, the electric fields generated within mass analyzer  115  may be working to keep ions  107  close together at the center. But due to the closeness of so many like-charged ions  107 , ions  107  may experience counteracting electrostatic repulsive forces. Such space charge effects may introduce irregularities to the motion of ions  107  within the ion cloud and subsequently alter the resulting mass spectrum measured by detector  128 . In addition, some effects may preferentially affect ions based upon their mass. For example, collisions with neutral species such as background gasses will affect the trajectory of smaller ions more significantly than larger ions. 
         [0024]      FIGS. 2A and 2B  show exemplary spectra generated by mass spectrometer  100  without space charge effects and with space charge effects. In  FIG. 2A , peaks  211  and  212  indicate the presence of two isotopes of a same ion. In the absence of space charge effects, the peaks are easily discernible. In various embodiments, as the quantity of ions trapped in mass analyzer  115  increases, space charge effects begin to manifest such that spectral peaks widen and isotopes blur together. For example, in  FIG. 2B , the midpoint between peaks  221  and  222 , which represent the same isotopes as peaks  211  and  212  in  FIG. 2A , no longer drops back to the baseline. 
         [0025]      FIG. 2B  also reveals that space charge effects are more pronounced at lower masses. The loss in resolution from peak  212  to  222  is not as severe as the loss of resolution from  213  to  223 , where identification of isotopes, and in fact the identity of the main peak, has become impossible. There are various possible reasons for space charge effects manifesting more heavily at lower masses. One reason may be due to the fact that ions are scanned out of mass analyzer  115  in order from low mass to high mass. Low mass ions are scanned out of mass analyzer  115  when the ion trap is still full. Accordingly, space charge effects are more severe due to the higher number of charged ions still in the ion trap contributing to space charge. By the time higher mass ions are scanned out of the ion trap towards the end of the scan, only higher mass ions are left in the ion trap. Because the number of charged particles has reduced, space charge effects may likewise be reduced. Another reason that space charge effects may manifesto a greater extent at the lower end of a mass spectrum may be due to greater deflection of lighter masses as compared to heavier masses. That is, as various ions move towards each other and then repel each other, due to the electrostatic repulsive forces, the heavier ions may displace a small distance from the center of the ion trap, while the lighter ions may displace a much larger distance from the center. A useful analogy may be to consider a ping pong ball and a bowling ball. If a ping pong ball and a bowling ball collide, the ping pong ball tends to ricochet off the bowling ball with substantial speed and large deflection. The bowling ball, on the other hand, barely moves as result of the interaction with the ping pong ball. Similarly, as all of the ions  107  in mass analyzer  115  move about within the center and experience near-collisions with each other, lighter ions may be deflected more from the center of mass analyzer  115  as compared with heavier ions. The more that a set of ions  107  of the same mass are dispersed within mass analyzer  115 , the less likely that all of the ions are successfully scanned out simultaneously. As a result, spectral broadening occurs in the measurement. On the other hand, the more that trajectory of the set of ions  107  are controlled by the electrical signals applied to mass analyzer  115  and less by space charge effects, the more likely that all of the ions are scanned out near simultaneously and that a clean spectral peak can be obtained. 
         [0026]      FIGS. 3A ,  3 B, and  3 C illustrate varying degrees of focusing by ion lens  310 . Such adjustments may be utilized to control the extent of space charge effects exhibited in a measured spectrum, according to some embodiments. In  FIG. 3A , ion source  305  may generate ions  307 , which then may be focused by intermediary ion lenses  309 . After emerging from ion lenses  309 , ions  307  may continue to travel towards first end cap  316  of a mass analyzer, passing through aperture  313  of ion lens  310  along the way. A voltage may be applied to ion lens  310  such that the beam of ions  307  is focused or defocused accordingly. In some embodiments, for positive ions  307 , the applied voltage may be a negative voltage that results in some of ions  307  passing through aperture  319  while others hit first end cap  316 . In  FIGS. 3B and 3C , the voltage applied to ion lens  310  may be adjusted such that the beam of ions  307  becomes relatively more or less focused. For example, in  FIG. 3B , the voltage applied to ion lens  310  may be adjusted to be more negative than in  FIG. 3A . As a result, ion lens  310  may focus ions  307  into a narrower beam, and subsequently, a higher proportion of ions  307  may pass through aperture  319 . In  FIG. 3C , the voltage applied to ion lens  310  may be adjusted to be less negative than in  FIG. 3A . As a result, ion lens  307  may defocus the beam of ions  307  such that a lower proportion of ions  307  pass through aperture  319 . The number of ions  307  that enter the ion trap may therefore be reduced. In various other embodiments, when ion lens  310  is adjusted to be more positively biased, the beam of ions is defocused, and when ions lens  310  is adjusted to be more negatively biased, the beam of ions is focused. 
         [0027]    Furthermore, the trajectory of the ion will be affected by the electric field created by lens  310  according to the vector force applied to the ion: 
         [0000]                =q           
         [0000]    where F is the vector force applied to the ion, q is the charge on the ion, and E is the vector electric field strength. The change in the trajectory of the ion will be defined by: 
         [0000]                =m           
         [0000]    where F is the vector force from the applied electric field, m is the mass of the ion, and a is the vector acceleration. Since the force applied to the ion is defined only by the electric field strength and the charge, which may be similar for like ions; and the change in trajectory is dependent only upon the mass and applied acceleration, the change in on trajectory will depend upon the mass of the ion, provided that the ions are travelling at relatively the same velocity. This dependence is shown in  FIG. 4 , which is a magnified view of ion beam  407  passing through ion lens  410  and arriving at aperture  419  of first end cap  416 .  FIG. 4  shows the trajectories of exemplary light, medium, and heavy ion masses, wherein ion lens  410  preferentially defocuses away ions based on mass. Thus, referring back to  FIG. 3 , ion lens  310  may defocus ions  307  preferentially based on the mass of ions  307 . That is, lighter ions may tend to be deflected away from the central axis of the beam of ions  307  arriving at aperture  319 . However, heavier ions may not be deflected as much. Therefore, in  FIG. 3C , ions  307  that arrive inside the ion trap may preferentially include heavier ions  307 . That is, lighter ions  307  may be deflected such that they are at the edge of the beam and hit the surface of first end cap  316  instead of passing through aperture  319 . In various embodiments, by preferentially defocusing the beam of ions  307 , the number of lighter ions, which are the ions that exhibit more space charge effects, is reduced in the ion trap. In such manner, the overall space charge effects exhibited by the measured spectrum may be improved. 
         [0028]    Another way to understand this improvement on space charge effects may be as follows. Lens  310  may preferentially focus and defocus lighter ions  307 . A plot of the response of the lens, such as attenuation for a given applied voltage as a function of mass, would have a negative slope. This negative slope is due to the fact that lighter ions are defocused and deflected more than the heavier ions. In addition, a plot of the on trap with respect to space charge, such as resilience to space charge effects as a function of mass, would have a positive slope. This positive slope is due to, as discussed above, space charge effects affecting lighter mass ions more than heavier mass ions. If these two plots are added, the mass-dependent space charge effects may cancel, to a first order approximation. 
         [0029]    In various embodiments, an exemplary method for reducing space charge effects exhibited in a measured spectrum may be as follows. The ion trap may be loaded with ions  307 . The resulting spectrum may exhibit space charge effects at the lower end of the mass spectrum. The voltage applied to ion lens  310  may then be adjusted such that the beam of ions  307  is defocused away from aperture  319 , preferentially for the fighter ions. Because the lighter ions have been preferentially defocused away, less of the lighter ions may enter the ion trap via aperture  319 . As a result, overall space charge effects may be reduced. 
         [0030]    In some embodiments, the resulting spectrum after the beam of ions  307  is defocused may show an improvement with respect to space charge effects. However, the proportion of masses trapped in the ion trap and subsequently detected by the detector may be skewed, since lighter ions  307  are preferentially defocused away. A compensation for such spectral skew may be performed by various methods and algorithms after the spectrum has been obtained. For example, a computing processor (not shown) may execute instructions stored in memory for computationally adjusting the measured spectrum. As another example, another run of measurements may be performed, where lighter ions are preferentially focused into the ion trap. The resulting mass spectrum may then be combined with the first mass spectrum to derive a new mass spectrum with spectral skew removed and reduced space charge effects. 
         [0031]    In some other embodiments, the beam of ions  307  may be defocused without preference based on mass. For example, ions  307  may be generated and/or manipulated to have uniform momentum. The momentum of each ion  307  is defined by: 
         [0000]                =m           
         [0000]    where p is the vector momentum of the ion, m is the mass of the ion, and v is the vector velocity of the ion. Because ions  307  may have different masses, different ions  307  will travel at different velocities in order for ions  307  to have uniform momentum. Heavier ions may move at a slower velocity while lighter ions may move at a faster velocity. As ions  307  pass through aperture  313  in ion lens  310 , the electrostatic force generated by ion lens  310  may focus or defocus ions  307 . In some embodiments, as ions  307  travel from ion lens  310  to end cap  316 , the lighter ions will be accelerated by ion lens  310  in the y-direction (perpendicular to the axis connecting aperture  313  and aperture  319 ) more than the heavier ions. As discussed above, in situations where ions  307  have uniform velocity, the larger acceleration causes larger deflection of the lighter ions. However, when ions  307  enter ion lens  310  with uniform momentum, the lighter ions may be traveling at a faster velocity than the heavier ions. Therefore, even if the lighter ions experience greater acceleration in the y-direction, the lighter ions also traverse the distance between ion lens  310  and end cap  316  more quickly. Accordingly, the lighter ions traverse this distance in less time, which results in smaller deflections in the y-direction before the lighter ions arrive at end cap  316 . The heavier ions, on the other hand, travel the distance between ion lens  310  and end cap  316  more slowly, allowing for more time during which the heavier ions are deflected in the y-direction. In some embodiments, the fact that lighter ions are accelerated in the y-direction more than the heavier ions, but the heavier ions take longer to arrive at end cap  316  than the lighter ions may result in lighter ions and heavier ions being deflected by relatively the same amount. Therefore, ions  307  of various masses may be focused and defocused by ion lens  310  without preference based on mass. 
         [0032]    In embodiments that utilize ions  307  with uniform momentum, ion lens  310  may focus and defocus the beam of ions  307  such that a greater or lesser proportion of ions  307  enter mass analyzer. The group of ions  307  that enter the mass analyzer may maintain the same proportion of the various masses of ions  307  that is originally present in the beam that is focused or defocused by ion lens  310 . By reducing the number of ions  307  that are trapped simultaneously in the mass analyzer, space charge effects may be reduced. 
         [0033]    It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed systems and methods. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed systems and methods. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.