Abstract:
A tandem mass spectrometer system and method are described, where a composite voltage waveform is applied to so as to trap ion having selected m/z. The trapped ions may be subject to collision induced ionization dissociation (CID) by a selectable discrete frequency voltage waveform positioned so as to be in a notch in a broadband waveform. The resultant ion products may be trapped using a second notch having a center frequency corresponding to the ion product to be trapped. The process may be repeated so as to increase the amount of ions produced, or the process a first resultant ion product to yield a second resultant in product, which may be trapped.

Description:
[0001]    The present application claims the benefit of priority to U.S. provisional application Ser. No. 61/392,776, that was filed on Oct. 13, 2010, and which is incorporated herein in its entirety by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present application may relate to an apparatus and method for performing mass spectrometry. 
       BACKGROUND 
       [0003]    Tandem mass spectrometry, or mass spectrometry/mass spectrometry (MS 2 ) may be used for complex mixture analysis due to its high specificity, wide applicability, and good sensitivity. 
         [0004]    Tandem mass spectrometry is a technique in mass spectrometry (MS) to provide both qualitative and quantitative information on the analyte molecules and having a discrimination effectiveness against noise. Noise may be understood as both electronic noise for small signals, and detected signals associated with unwanted molecules. Analysis activities in proteomics, for example, rely heavily on the identification of unknown proteins in complex mixtures using tandem mass spectrometry. In the pharmaceutical industry, reaction monitoring (one type of MS 2 ) is extensively employed to monitor the change of quantity of a selected metabolite as a function of time. 
         [0005]    Tandem mass spectrometry comprises: precursor-ion isolation (first stage of MS); reactions that change the mass-to-charge ratio (m/z) of the precursor ions; and, product-ion mass analysis (second stage of MS). The structural information characterizing the analyte is deduced based on the measured product-ion masses, the formation of which is influenced by the method used to induce the change of the m/z of the precursor ions. 
         [0006]    A variety of chemical or physical techniques have been employed to alter the m/z of precursor ions. These techniques include collision induced dissociation (CID), ion/photon interaction, ion/electron interaction or reaction, ion/molecule reactions and ion/ion reaction. CID is the most widely used activation method available on a commercial tandem mass spectrometer. 
         [0007]    Tandem mass spectrometry techniques can be categorized as “tandem-in-space” or “tandem-in-time.” In the former mode, mass analysis and reactions are performed on a beam of ions during the flight of the ions through the analysis device. Instruments suitable for tandem-in space analysis are typically transmission-type instruments including sectors, triple quadrupole, quadrupole/time-of-flight (TOF), and TOF/TOF. In the tandem-in-time analysis, the steps occur in the same space but follow a time sequence. Such an analysis may be performed in ion trapping mass analyzers such as a quadrupole ion trap, an ion cyclotron ion trap or a hybrid mass spectrometer containing an ion trapping mass analyzer. 
         [0008]    Tandem mass spectrometry, or MS 2 , contains two stages of mass analysis. By adding additional stages of reaction and mass analysis, MS n , where n&gt;2, can be performed. MS n  (n&gt;2) is typically performed in tandem-in-time ion trapping instruments. Higher orders of tandem mass spectrometry may provide additional structural information, which has been used to study mechanisms of sequential reactions, deduce the structural motif from the analyte ion, or to differentiate structural isomers. MS n  (n=12) has been demonstrated on a quadrupole ion trap instrument. 
         [0009]    The number of MS stages that can be executed, however, is limited by the number of ions that remain after each ion isolation step. Often, the intensities of the fragment ions within the MS n  analysis chain are too low to allow for the performing of MS n  analysis. 
       SUMMARY 
       [0010]    An apparatus and method of performing tandem mass spectrometry is described. The apparatus and method may be arranged such that the ions involved in the MS n  analysis chain (e.g., m 1 , m 2 , . . . m n-1 , wherein m n  represents a particular ion mass), are isolated and fragmented, while other fragment ions are ejected from an ion trap. By performing a selection of preferred ion masses, the ion type(s) of interest can be accumulated to a higher intensity with a less detrimental effect from the space charge or other artifacts in an ion trap. 
         [0011]    The apparatus may be a linear ion trap, or a Paul trap, using a composite excitation voltage waveform. The composite waveform may be an alternating current potential applied to the apparatus, where the broadband waveform contains frequency-domain amplitude notches for isolating one or more of fragment ions of a desired m/z and discrete frequency components to selectively induce collisional activation of these ions. Not all of the masses may be present at the outset. That is, the mass of the ion to be isolated may be derived from the mass of a precursor ion. Once a suitable intensity of the desired isolated ions is reached, the step of tandem mass spectrometry can be performed. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0012]      FIG. 1  is a schematic view of a triple-quadrupole/linear ion trap mass spectrometer; and B, is a block diagram of a system using the mass spectrometer; 
           [0013]      FIG. 2  is a schematic view of a composite waveform for a MS 4  experiment; 
           [0014]      FIG. 3  is a MS 2  ion trap CID mass spectrum of a disaccharide; 
           [0015]      FIG. 4  is a graph showing characteristics of a composite waveform as used in the experimental example; 
           [0016]      FIG. 5  is a MS 2  ion trap CID mass spectrum of a disaccharide with the application of the composite waveform shown in  FIG. 4 ; and 
           [0017]      FIG. 6  is a graph comparing the ion intensity of a MS 2  product as a function of ion injection time with or without the application of a composite waveform. 
       
    
    
     DESCRIPTION 
       [0018]    Exemplary embodiments may be better understood with reference to the drawings, but these embodiments are not intended to be of a limiting nature. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention which, however, may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the description. 
         [0019]    The combination of hardware and software to accomplish the tasks described herein is termed a system. Where otherwise not specifically defined, acronyms are given their ordinary meaning in the art. 
         [0020]    The processing of a signal may be by either analog or digital circuits, or a combination thereof. The signal processing may be also performed by one or more computers with associated memory and computer code which performs mathematical operations and functions equivalent to that performed by the analog or digital circuits. Herein, there is not intended to be a restriction of the type of circuit which performs each function, or the combination of types of circuits which may be used, although the examples may mention a specific type of circuit in the description thereof. 
         [0021]    A software program product for implementing processes or functions of the system, may be provided on computer-readable storage media or memories, such as CD-ROM, hard disk, FLASH memory, or the like, that is a non-transient memory, and may be downloaded to a computer memory and executed by a processor, computer, or the like. The functions, acts or tasks illustrated in the figures or described herein are executed in response to one or more sets of instructions stored in or on computer readable storage media. The functions, acts or tasks are independent of the particular type of instruction set, storage media, processor or processing strategy and may be performed by software, hardware, integrated circuits, firmware, micro code and the like, operating alone or in combination. Likewise, processing strategies may include multiprocessing, multitasking, parallel processing and the like. In an embodiment, the instructions may be stored on a removable media device for reading by local or remote systems. In other embodiments, the instructions may be stored in a remote location for transfer through a computer network, a local or wide area network or over telephone lines. In yet other embodiments, the instructions are stored within a given computer or system. 
         [0022]    While the methods disclosed herein have been described and shown with reference to particular steps performed in a particular order, it will be understood that these steps may be combined, sub-divided, or reordered to from an equivalent method without departing from the teachings herein. Accordingly, unless specifically indicated herein, the order and grouping of steps of the method is not intended to be a limitation. 
         [0023]    The operation of a mass spectrometer is well known to persons of skill in the art and is not described in detail. However, in general, a source of ions is provided, such as an electrostatic ion generator (ESI) that introduces ions into the mass spectrometer. The initial quadrupole arrays may serve to perform mass selection on the ions, and to serve as stages for cooling the ions, reaction of the ions with analytes, and isolation between regions of differing pressure. In the example described herein, the proximal source of ions being acted on by Q3 may be reactions or selections made in Q2. 
         [0024]    A hybrid linear ion trap mass spectrometer shown schematically in  FIG. 1A , such as the commercially available 4000 QTRAP instrument (AB SCIEX Concord, Ontario, Canada) was used to acquire the data presented herein. The instrument has a triple quadrupole configuration Q1, Q2, Q3, where the last quadrupole array (Q3) can also function as a linear ion trap for mass analysis. In this example, the Q3 linear ion trap was used as a mass analyzer to perform tandem-in-time MS n  analysis A frequency-notched broadband waveform, or composite waveform, was applied to the Q3 quadrupole array and controlled by a controller, which may be computer executing a stored set of instructions. 
         [0025]    The overall system  1 , including the mass-spectrometer  10 , a control computer  20 , which may be a personal computer, workstation, or the like, and a waveform generator  30 , which may be, for example, a plurality of waveform generators, a computer synthesizing a waveform, or a stored pattern of amplitude data that is clocked into a digital to analog converter, is shown in  FIG. 1B . The components of the system  1  may be separate modules connected by cables or one or more of the components may be combined into a single unit. 
         [0026]    m 1 , m 2 , and m 3  ions may be isolated by a frequency-notched broadband waveform (shown in  FIG. 2 ), while selected m 1  ions undergo collisional activation due to the applied discrete frequency components. The resultant m 2  ions are accumulated where, in this example, a frequency notch exists, but there is no corresponding discrete frequency waveform. 
         [0027]    The m 2  ions may be isolated with higher resolution using the RF/DC mode of operation after the ions are cooled in the ion trap. Another stage of collisional ion dissociation (CID) can be applied to m 2  and MS 4  data can also be obtained, for example. 
         [0028]    The two types of waveforms represented in  FIG. 2  may be applied simultaneously or sequentially using a plurality of signal sources, or combined in a single composite waveform during a ion-fill period to accumulate only m 2 . Composite waveforms of arbitrary amplitude, frequency and phase characteristics may be generated using a computer controlled waveform generator. Such generators, for example, may comprise a memory having a generated or stored sequence of amplitude values, where the stored sequence of values are output through a digital-to analog converter. The amplitude of the excitation waveform should be chosen sufficiently low that m 1  and m 2  that are being dissociated are not lost by collision with the rods or ejection form the device. 
         [0029]    Where the term “broadband (or wideband) waveform” is used an alternating current (AC) signal is meant. This waveform may also be understood by a person of skill in the art to be a “radio frequency” signal. The AC (or RF) signal used to fragment an ion species m n  may be a single, or discrete, frequency, or may be a signal having a narrower band than the notch in the wideband waveform, or a lower amplitude of signal in the notch region. That is, the amplitude and other characteristics of the signal in the notch region is selected based on the fragmentation process requirements and the amplitude outside of the notch region may be selected on the basis that notch region is being used for isolation, and the remaining regions preferably expel other ions. A plurality of notches in the broadband waveform, and a plurality of discrete frequencies may be provided either simultaneously or sequentially, depending on the specific objectives of the measurements to be performed. 
         [0030]    Moreover, a person of skill in the art would understand that the term “broadband” means that the frequency and amplitude of the waveform are selected so as to encompass a range of m/z such that the ions of undesired m/z values are selectively eliminated from the ion trap. A notch in the broadband waveform would be understood to be a reduction in the amplitude of the waveform, so that the effect of the waveform on an ion of a desired m/z is to retain the ion in the ion trap, with or without further dissociation. The amplitude of the waveform in the notch region is selected to either provide further dissociation of the ion, or is sufficiently low as to have little or no effect on dissociation. The width of such a notch may be determined by, amongst other things, the presence of ions having similar m/z ratios, any thermal or other mass spectrum spreading, or the like. In other situations, while the broadband waveform may have a notch in the spectrum, so as to retain or trap, a particular ion, the source ion that is being dissociated may be in a different m/z regime, and a narrowband waveform of appropriate frequency may be synthesized, and the amplitude at that frequency independently adjusted. 
         [0031]    A notch may be narrowband, whether it has an amplitude or not, and an independent “discrete” frequency” may be narrowband. The bandwidth of a discrete frequency component may be quite narrow (typically a single frequency), or have a bandwidth sufficient to account for experimental error and convenience, while not affecting any known adjacent ions in the m/z space. The frequency notch may centered around the secular frequency of the ions with a notch width of between about 4 and about 8 kHz, where the notch is not situated at an end of the broadband waveform band. 
         [0032]    Herein, the source of the alternating current signal may be termed a “radio frequency generator,” regardless of whether the radio frequency generator is a plurality of signal generators whose outputs are combined, or a computer synthezizer generating a composite waveform, or other technique producing a similar excitation waveform. 
         [0033]    A method for using a system, such as that shown in  FIG. 1B , for accumulating low abundance MS 2  ions for MS 3  analysis may include the following steps:
       1. Fill Q3 with all types of ions from the source or reactions;   2. After the ions have been cooled, isolate m 1  (and m 2  when a multi-step isolation is performed) in Q3 using a RF/DC technique;   3. Allow the isolated ions to cool;   4. Apply a composite waveform which has a broadband waveform component that is notched to isolate m 1  and m 2  and having a single frequency waveform to fragment m 1  so as to produce m 2 .   5. Allow the ions to cool.   6. Isolate m 2  in Q3 using a RF/DC technique;   7. Perform CID of m 2  in Q3; and   8. Perform mass analysis.
 
Steps 1-6 may be repeated until m 2  has been accumulated in sufficient numbers for analysis if the ion intensity from a single ion injection event is low. Note that isolation in step 2 may be also achieved, for example, using RF/DC.
       
 
         [0042]      FIG. 3  shows the ion-trap MS 2 -CID of [M-H] −  (m/z 341, m 1 ) ions of a disaccharide, laminaribiose, after a 50 ms injection time. The product ion of interest is the ion at m/z 221 (m 2 ). The intensity of the m/z 221 ion is too low, in this example, to acquire good quality data for MS 3 .  FIG. 5  shows that, when a composite waveform ( FIG. 4 ) is applied, the intensities of the fragment ions, e.g., m/z 113, 161, 179, are greatly reduced, while the relative intensity of the m/z 221 ions increases The composite waveform has a discrete frequency (69 kHz) for CID of m/z 341, and a broadband waveform having a notch for m/z 221 The broadband waveform has an effective notch in the region of m/z 341, which may be rather broad as there are no other components of interest in the nearby m/z space. With the application of the composite waveform, there is no loss of m/z 221 ions while the relative intensity of m/z 221 is greatly increased and the absolute intensity may not be reduced either. The intensities of m/z 221 ions (after RC/DC isolation in Q3) are plotted as a function of the injection time ( FIG. 6 ) with and without the composite waveform processing. One may observe that the intensity of m/z 221 ions (squares) are much higher than the intensity that can be obtained with traditional ion-trap CID method (diamonds) at relatively long injection periods (&gt;300 ms), which leads to enhanced sensitivities for MS 3  experiments. 
         [0043]    The technique may be extended to MS n  (n&gt;3), by adding more notches to the isolation waveform, and more components to the fragmentation waveform. This technique may be more effective in the low-pressure environment of Q3 if ions were allowed to thermalize between periods of isolation and fragmentation. However, in a higher pressure environment, such as that of Q2, where ions thermalize more quickly, an n th  generation ion may be accumulated by applying both the notched and narrowband waveforms continuously during an extended fill period. This latter approach may be useful on instruments which may not have the ability to repeat a specified range of segments a large number of times. 
         [0044]    Although only a few examples of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible without materially departing from the novel teachings and advantages of the invention. The invention is limited only by the claims and equivalents thereof.