Abstract:
A method of setting a fill time for a mass spectrometer including a linear ion is provided. The mass spectrometer is operated first in a transmission mode and ions are supplied to the mass spectrometer. Ions are detected as they pass through at least part of the mass spectrometer in a preset time period, to determine the ion current. From a desired maximum charge density for the ion trap and the ion current, a fill time for the ion trap is determined. The mass spectrometer is operated in a trapping mode to trap ions in the ion trap, and the ion trap is filled for the fill time, as just determined. This utilizes the ion trap to its maximum, while avoiding problems due to overfilling the trap, causing space charge effects.

Description:
FIELD OF THE INVENTION 
     This invention relates to ion trap mass spectrometers and more particularly to controlling and reducing space charge effects in such mass spectrometers. 
     BACKGROUND OF THE INVENTION 
     Conventional ion trap mass spectrometers, of the kind described in U.S. Pat. No. 2,939,952, are generally composed of three electrodes, namely a ring electrode, and a pair of end cap electrodes. Appropriate applied RF and DC voltages are applied to the electrodes to establish a three dimensional field which traps ions within a specified mass-to-charge range. Linear quadruples can also be configured as ion trap mass spectrometers where radial confinement is provided by an applied RF voltage and axial confinement by DC barriers at the ends of the rod array. Mass selective detection of ions trapped within a linear ion trap can be accomplished by ejecting the ions radially, as taught by U.S. Pat. No. 5,420,425, or by ejecting the ions axially, as taught by U.S. Pat. No. 6,177,668. Ions may also be detected in situ using Fourier Transform techniques, as taught by U.S. Pat. No. 4,755,670. 
     The performance of any ion trap mass spectrometer is strongly influenced by the trapped ion density. Whenever this ion density increases above a particular limit, the resolution and mass assignment accuracy degrade. In extreme cases the mass spectral peaks can be completely smeared out and little useful information obtained. Accordingly, it is desirable to provide a method for rapid determination of the ion current provided by the ion source so that the number of ions injected into a linear ion trap mass spectrometer can be adjusted for optimal mass spectrometry performance. 
     Linear ion trap mass spectrometers are variations of 2-dimensional quadruple mass spectrometers or other multiple devices, which allow ion trapping by means of a two-dimensional quadruple, or multiple, field applied in the radial dimension and DC barriers applied at the ends of the device. Such linear ion traps may be fabricated from straight or curved rod-type electrodes. Quadruple ion traps, at least, then permit mass selective ejection from the quadruple followed by ion detection. U.S. Pat. No. 6,177,668 teaches that the ion path of a standard triple quadruple mass spectrometer can be configured such that one of the quadruples can be operated as a linear ion trap mass spectrometer. Such an instrument offers the capabilities of both an ion trap operational mode with the associated high sensitivity and the conventional operation mode of a standard triple quadruple mass spectrometer on the same platform, which is an advantage. The present inventor found that by combining the capabilities of both standard triple quadruple and linear ion trap modes a very rapid method of space charge minimization can be obtained. The invention is, in general, applicable to any linear ion trap capable of operating in both a trapping mode and a continuous transmission mode. 
     SUMMARY OF THE PRESENT INVENTION 
     In accordance with the present invention, there is provided a method of setting a fill time for a mass spectrometer including a linear ion trap the method comprising: 
     (a) operating the mass spectrometer in a transmission mode; 
     (b) supplying ions to the mass spectrometer; 
     (c) detecting ions passing through at least part of the mass spectrometer in a preset time period to determine the ion current; 
     (d) from a desired maximum charge density for the ion trap and the ion current determining a fill time for the ion trap; 
     (e) operating the mass spectrometer in a trapping mode to trap ions in the ion trap, and filling the ion trap for the fill time determined in step (d); and 
     (f) obtaining an analytical spectrum from ions trapped in the ion trap. 
    
    
     DESCRIPTION OF DRAWING FIGURES 
     For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
     FIG. 1 is a schematic view of a conventional triple quadruple mass spectrometer; 
     FIG. 2 is a timing diagram for a conventional scan function carried out on the mass spectrometer of FIG. 1; 
     FIG. 3 is a timing diagram, in accordance with the present invention, for minimizing space charge effects; 
     FIG. 4 is a graph showing variation of ion intensity with time; and 
     FIGS. 5 a  and  5   b  show a trapped ion spectrum for different fill times. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1, there is shown a conventional triple quadruple 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. 
     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. 
     Following the orifice plate  16 , there is a skimmer plate  20 . An intermediate pressure chamber  22  is defined between the orifice plate  16  and the skimmer plate  20  and the pressure in this chamber is typically of the order of 2 Torr. 
     Ions pass through the skimmer plate  20  into the first chamber of the mass spectrometer, indicated at  24 . A quadruple 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 an interface into the analyzing sections of the mass spectrometer. 
     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 . 
     A first mass resolving quadruple 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 quadruple rod set Q 2 , and following the collision cell  30 , there is a third quadruple rod set Q 3  for effecting a second mass analysis step. 
     The final or third quadruple rod set Q 3  is located in the main quadruple chamber  26  and subjected to the pressure therein typically 1×10 −5  Torr. As indicated, the second quadruple 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 enclosure of the collision cell of  30 . 
     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. 
     As indicated, a power supply  36  is provided for supplying RF and DC resolving voltages to the first quadruple rod set Q 1 . Similarly, a second powersupply  38  is provided for supplying drive RF and auxiliary AC voltages to the third quadruple 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, and an RF supply would also be connected to Q 2  within the collision cell  30 . 
     The apparatus of FIG. 1 is based on an Applied Biosystems/MDS SCIEX API 2000 triple quadruple mass spectrometer. In accordance with the present invention, the third quadruple 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 utilizing an auxiliary dipolar AC voltage (not shown in FIG. 1) to effect ion ejection. The instrument retains the capability to be operated as a conventional triple quadruple mass spectrometer. 
     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 Ion ejection in the direction normal to the axis of the linear ion trap can also be effected as taught by U.S. Pat. No. 5,420,425. Trapped ions may also be ejected by means of an auxiliary voltage applied in a quadrupolar fashion or without any auxiliary voltage by utilizing the q˜0.907 stability boundary. Trapped ions may also be detected in situ as taught by U.S. Pat. No. 4,755,670. 
     The conventional timing diagram for the axial ejection 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. 
     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 quadruple rod set Q 3  are maintained at levels  56  and  58 . This cooling period lasts 10-50 milliseconds. 
     Once the ions have been cooled, 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. 
     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 quadruple 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 . 
     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. 
     Conventional 3-dimensional ion traps, including quadruple linear ion traps, are susceptible to the effect of space charge primarily due to their small volume and the relatively high pressures at which they operate. Many techniques have been developed to maintain the trapped ion current within pre-specified ranges to minimize the deleterious effects of space charge. Most of these techniques, such as those disclosed in U.S. Pat. No. 4,771,172, rely on rapid “pre-scans” in which the content of the 3-dimensional ion trap is interrogated via a rapid mass selective scan of the contents of the ion trap itself. Such fast pre-scans typically require 50-200 ms to complete, i.e., they do require a significant amount of time. The detected ion signal is then compared to some pre-specified limit, and the fill time of subsequent “analytical” scans adjusted to give optimum mass spectroscopic performance. U.S. Pat. No. 5,572,022 discloses a method of increasing the dynamic range of a conventional 3-dimensional ion trap by placement of a resolving quadruple mass spectrometer in front of the ion trap. However, the step of determining the appropriate ion trap fill time is still based on trapping and rapid mass selective scanning out of the trap contents prior to the analytical scan. The method of the present invention provides for determination of the ion beam intensity via measurements of the entire ion path in transmission, rather than trapping, mode. 
     The ion path of the current apparatus allows a much simpler and more rapid technique for determining the analyte intensity emitted from the ion source, and the analyte intensity, once determined, can be used to adjust the fill time of the Q 3  linear ion trap. The method described herein utilizes the fact that, in the triple quadruple instrument  10 , there exists a resolving RF/DC quadruple Q 1  in the ion path between the ion source  12  and the detector  34  and that the ion current passing through this RF/DC quadruple Q 1  can be directly measured by the ion detector  34  without having to trap the ions in the ion trap (available in Q 3 ) and performing a mass scan of the ion trap itself. The ion path, being derived from that of a standard triple quadruple mass spectrometer, is well suited to making ion intensity measurements in direct transmission mode with the quadruples in a combination of resolving RF/DC and fully transmitting RF-only modes. In one embodiment, the detected ion signal from the resolving Q 1  mass spectrometer is measured while the Q 3  linear ion trap is operated in RF-only transmission, or “ion pipe”, mode to obtain a very rapid measure of the ion flux emitted from the ion source at a particular m/z range that is used to adjust the fill time for subsequent Q 3  linear ion trap mass selective scans. The advantages of this technique are that the resolved Q 1  signal can be obtained very rapidly (in &lt;10 ms) and that the ion intensity is a direct measure of the number of ions that will be directed into the Q 3  linear ion trap in subsequent mass selective ion trap scans. 
     FIG. 3 displays the timing diagram for a series of mass spectrometry scans employed to minimize the effects of space charge, in accordance with the present invention. The first step  80  is to set the ion path to triple quadruple mode, i.e. with Q 1  configured as an RF/DC quadruple transmitting mass spectrometer and both Q 2  and Q 3  configured as RF-only quadrupoles. Q 1  is set to the m/z value of the ion to be measured with the desired resolution as is conventionally done with triple quadrupole mass spectrometers Next, at  82 , the number of ions at the ion detector is measured in a single 1 ms measurement period. Then, the ion path can be re-configured as a linear ion trap mass spectrometer. This can be done very quickly (&lt;1 ms) because it only involves resetting several of the DC and RF voltages. The optimum fill time of the Q 3  linear ion trap is determined at  84 , by comparing the number of ions detected in the previous RF/DC transmission mode of operation with a pre-selected value. The optimum ion trap fill time is calculated at  86 , and a Q 3  linear ion trap mass spectrum is generated at  88 . Thus, the optimum Q 3  linear ion trap fill time is determined very rapidly without having to trap ions in Q 3  and perform a mass scan. 
     An example of the method of the present invention will now be described. FIG. 4 shows the Q 1  ion intensity of a 10 picomoles/microliter solution of renin substrate tetradecapeptide measured at m/z  587  obtained by setting the resolution of the RF/DC Q 1  quadrupole mass spectrometer to approximately 3 amu and operating Q 2  and Q 3  in RF-only transmission mode. This m/z corresponds to the (M+3H) 3+  renin substrate ion. The measurement time has been chosen to be 10 ms and 10 scans separated by about 290 ms (the timing here being determined by the experimental equipment available) have been displayed for clarity. The intensity from a single scan of a few milliseconds would be sufficient. The peak ion intensity at the detector was measured to be about 3.8×10 6  counts/sec, which corresponds to 3.8×10 4  detected ions in the 10 ms measurement time. It has been found empirically that for a quadrupole of standard dimensions, the best performance is obtained with admission of &lt;10,000 ions into the Q 3  linear ion trap mass spectrometer. Thus an appropriate fill time based on the measured continuous ion beam intensity measured in FIG. 4 is &lt;2.5 ms. 
     FIG. 5 displays the trapped ion mass spectrum of the m/z  587  renin substrate ion using a fill time of 20 ms (upper trace, FIG. 5 a ) and 2 ms (lower trace, FIG. 5 b ). The longer fill time results in the degraded resolution and slight shift to higher value of the apparent mass, while FIG. 5 b  shows noticeably better resolution. These differences are symptomatic of space charge at the longer fill time. The pre-measurement of the resolved Q 1  ion intensity, however, allows the optimum fill time to be determined rapidly. 
     The total ion current in transmission mode can be measured with all of the quadrupoles comprising the ion path operated as RF-only quadruples. This can also provide useful information for determining the appropriate fill time for the Q 3  linear ion trap in subsequent experiments. This can be useful to determine the total ion current from a source, as compared to the ion current at a certain mass or narrow range of masses. 
     It is not necessary for a resolving quadrupole to be placed in front of the linear ion trap mass spectrometer as described above. The Q 3  linear ion trap itself can be used to make the appropriate intensity measurements of the incoming ion beam since it too can be operated as a conventional RF/DC quadrupole mass spectrometer. In this embodiment other upstream quadrupoles (e.g., Q 1 , Q 2 ) would be operated as RF-only transmission quadrupoles and the intensity of a chosen m/z range would be set by Q 3  in RF/DC mode with no ion trapping implemented. The timing sequence is the same as that shown in FIG. 3 with the exception of a brief Q 3  ion measurement cycle in place of the Q 1  measurement step  80 . 
     It is to be understood that this method is applicable with any mass spectrometer system that includes a linear ion trap mass spectrometer that has the capability of being operated as a conventional RF/DC quadruple mass spectrometer, such as a QqTOF mass spectrometer, which is similar to the triple quadrupole instrument shown but has a Time of Flight (TOF) section replacing the final quadrupole Q 3  and detector. 
     It will also be understood that where a mass spectrometer has a plurality of different elements or sections, e.g., the individual quadrupole sections of a triple quadrupole mass spectrometer, it is not always necessary to pass the ion current through the entire instrument in the transmission made. For some types of instruments, it may be possible or preferable, to detect ions part way through the instrument and even upstream from the ion trap. This should still give an accurate measure of the ion current that would be received by the ion trap.