Patent Publication Number: US-8541737-B2

Title: System and method for collisional activation of charged particles

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Provisional application No. 61/265,278 filed 30 Nov. 2009, which application is incorporated in its entirety herein. 
    
    
     STATEMENT REGARDING RIGHTS TO INVENTION MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention was made with Government support under Contract DE-AC06-76RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Identification of biomolecules is routine in biopharmaceutical and proteomics research. Current commercial mass spectrometers can be equipped with collision cells that employ quadrupoles or multipoles in which ion fragmentation occurs by a process known as collision-induced dissociation (CID). Conventional CID is a process in which ions are accelerated by an electric field to increase the ion kinetic energy. Upon collision with a buffer gas, the ions fragment. In these conventional devices, CID occurs at the entrance of the quadrupole, where ion scattering at the ends of the quadrupole rods and strength of the DC field gradient are the greatest. Fragmentation can also take place inside the quadrupole, but fragmentation efficiency strongly depends on the quadrupole pressure, as kinetic energy dampening due to collisions is significant while the electric field inside the quadrupole is zero. CID in conventional quadrupoles often suffers from poor fragmentation and poor collection efficiencies because of: 1) a relatively low operation pressure (typical pressures are 1-5 mTorr), 2) few collisions per unit length, 3) a low collision energy in the center-of-mass frame that limits activation of larger molecules, and 4) because fragment ions produced in RF-fringing fields at the quadrupole entrance can be easily lost due to scattering. For example, collection efficiencies for multiple-charged species in triple-quadrupole instruments have typical best values between 10% and 17%. 
     The primary purpose of RF fields in conventional devices is to radially confine ions. In some applications, RF fields can be used to cause ion instability that result in increased radial oscillations of precursor ions. In this case, all ions with an m/z below that of the precursor become unstable, meaning one can only detect fragments with an m/z above that of the precursor. For multiply-charged ions, this means that up to a full half of useful structural information can be lost in a mass spectrum. As a result, poor fragmentation patterns occur, and insufficient structural information is obtained to ascertain required sequencing information by which to unambiguously identify molecules of interest. If the internal energy content of the parent (primary precursor) ions is high, some fraction of the parent ions will gain sufficient energy to fragment further, producing secondary fragments from the primary fragments, which proves to be of little value for structural determination of complex ions. For example, in conventional devices, precursor ions typically dissociate in close proximity to the quadrupole entrance, resulting in fragment ions that impart additional activation energy further downstream in the quadrupole, which results in secondary fragments that provide little structural information or that gives rise to uninformative spectra. In addition, in conventional MS/MS, activation of singly-charged precursor ions requires higher electric, fields, which also results in secondary fragmentation of fragments produced by multiply-charged ions of the same species, which again provides little useful information for structural determination of ions. While conventional activation methodologies and devices provide some fragmentation data, ultimately, in excess of 25% of large bio-molecules including, e.g., proteins and peptides, are estimated to remain unidentified in conventional tandem MS/MS experiments using, e.g., conventional triple-quadrupole instruments. Triple-quadrupole instruments can fail to characterize and identify complex molecules due to an inability to provide sufficient structure-specific fragments for the molecules of interest. Accordingly, new systems and methods are needed that increase the fragmentation efficiencies necessary to producing an abundance of structurally-rich fragment ions by which to identify complex molecules. 
     SUMMARY OF THE INVENTION 
     The invention includes an IMS TOF-MS system and method for enhanced fragmentation of ions. The system is characterized by: an ion channel that defines an axis traversed by precursor ions in a buffer gas at a pressure greater than 20 mTorr, the collision dell having substantially orthogonal focusing RF-field and an axial DC-field along the axis; and a plurality of high-intensity, structurally-rich fragment ions inside the ion channel. The axial DC-field determines the collision energy of precursor ions interacting with a buffer gas in the RF-focusing field that provides radial confinement of both precursor and fragment ions, and also contributes to fragmentation of precursor ions, inside the ion channel. The ion channel it defined by a preselected number (N) of circumvolving elongate members including; but not limited to, e.g., rods, plates, and poles, where (N) is an even-numbered integer greater than or equal to 2. The elongate members each comprise at least two operably coupled linear segments that deliver a preselected potential of like or different kind. The linear segments are each insulated from another segment with a resistor chain or network that controls the axial DC-field applied to the elongate members. In another implementation, the axial, DC and radial RF fields are spatially decoupled, so that two 180° phase-shifted RF waveforms are applied to two pairs of solid rods, while an axial DC field is generated with a linear assembly of the segmented thin plates, or vanes, inserted between the rods in such a manner to remain on the zero RF potential line. Each vane assembly has the length of the collision cell. For a quadrupole, there are four sets of the segmented vane assemblies. To enable ion packet displacement in the radial direction, two adjacent sets of segmented vanes are coupled and biased with respect to the other two coupled sets, while the axial DC gradient is maintained, the same for all vane assemblies. The term “bias” means an applied potential with respect to an earth ground. The axial DC field is achieved by biasing segments with respect to each other in a single vane assembly. Radial displacement generated in the entrance region of the collision cell is removed in the exit region to ensure ion packet relaxation to the collision cell axis and efficient ion transmission to the downstream ion optics. The radial DC field can be constant or pulsed. Amplitudes of pulsed radial DC field voltages are preferably selected in the range from about 10 V (volts) to about 50 V (volts). Amplitudes of constant DC field voltages are preferably selected in the range from about 10 V to about 50 V. In One embodiment, the radial DC field is synchronized with an IMS gate to enable radial displacement of a species of interest previously separated in the drift tube IMS. In various embodiments, precursor ion activation in the collision cell is achieved by: i) an increased axial DC field alone (no radial displacement); ii) RF-heating due to radial displacement of ions with respect to the collision cell axis, with a minimum of ion activation due to the axial DC field; and iii) a combined RF-heating and axial DC-field. In another embodiment, the distribution of the radial DC field is symmetric about the ion channel axis. In yet another embodiment, the distribution of the radial DC field is asymmetric about the ion channel axis. In still yet another embodiment, a DC pulse generates a radial DC field that provides radial displacement of precursor ions from the axis inside the collision cell. Fragment ions are radially confined within the RF-focusing field. In another embodiment, the collision cell is coupled at the interface between a drift tube IMS stage and a TOF-MS instrument stage, but is not limited thereto. The system can include one or more operatively coupled stages including, but not limited to, e.g., drift tube ion mobility spectrometry, (DT IMS) stages; differential mobility analysis (DMA) stages; mass spectrometry (MS) stages; ion funnel trap stages; ion funnel stages; and combinations thereof. The method includes applying an axial DC-field and a substantially orthogonal RF-focusing field with respect to the ion channel axis of the collision cell; flowing a plurality of precursor ions at a pressure greater than 20 mTorr through the ion channel filled with a buffer gas; and fragmenting the precursor ions by collision with the buffer gas in the RF-focusing field, thereby generating a plurality of high-intensity, structurally-rich fragment ions inside the ion channel. The method includes applying a locally increased DC-field to accelerate the precursor ions along the ion channel axis. In axial collision induced dissociation (CID) mode, fragmenting the precursor ions includes accelerating the precursor ions axially in the DC-electric field to increase the impact velocity of the ions with the buffer gas inside the ion channel along the ion channel axis. The step of fragment ion refocusing includes collisionally cooling the fragment ions inside the ion channel to maximize the distribution and quantity of structurally-rich fragment ions inside the ion channel. The step of fragmenting includes use of a collision voltage preferably in the range from about 10 electron volts to about 100 electron volts, but voltages are not intended to be limited thereto. Fragment ions are radially confined within the RF-focusing field providing increased collection efficiency for same. Focusing the fragment ions along the axis of the ion channel using the radial RF field maximizes transmission of the ions to a subsequent analytical stage, e.g., an MS stage. The process of the invention provides a CID efficiency (E CID ) in the range from about 60% to about 90%. In RF-heating mode, the step of fragmenting includes radially displacing the precursor ions from the ion channel axis inside the collision cell by application of a DC-displacement pulse to a single quadrupole rod. The DC-displacement pulse produces a high-frequency radial RF-field that is uncompensated (i.e., not matched) by a field from an opposite rod. Precursor ions displaced from the ion channel axis have increased amplitudes of oscillation that induce fragmentation as the ions impact the buffer gas molecules. Radially-displaced fragment ions are focused back to the ion channel axis by removing the DC-displacement pulse. Relaxation of ions back to the ion channel axis results in efficient transmission of fragment ions to a subsequent instrument stage with minimum ion losses. The method can also include applying an axial DC-electric field along a center longitudinal axis of a segmented N-pole device that accelerates a beam of charged precursor ions introduced axially along the center longitudinal axis in the axial DC-electric field inside the segmented N-pole device; applying a radial DC-field that results in radial displacement of the precursor ions to a preselected region in the collision cell where uncompensated RF fields cause ion heating upon impact with the buffer gas; and fragmenting the precursor ions using both the axial DC field and RF heating by collision with neutral gas molecules in a stream of gas, producing fragment ions. 
     The purpose of the foregoing abstract is to enable the United States Patent and Trademark Office and the public generally, especially the scientists, engineers, and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. 
     Various advantages and novel features of the present invention are described herein and will become further readily apparent to those skilled in this art from the following detailed description. In the preceding and following descriptions, the various embodiments, including the preferred embodiments, have been shown and described. Included herein is a description of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of modification in various respects without departing from the invention. Accordingly, the drawings and description of the preferred embodiments set forth hereafter are to be regarded as illustrative in nature, and not as restrictive. Embodiments of the invention are described below with reference to the following accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an IMS-TOF-MS system that includes one embodiment of the invention. 
         FIG. 2   a  shows one embodiment of the invention. 
         FIG. 2   b  shows another embodiment of the invention. 
         FIG. 2   c  is a schematic of a “High-Q” RF-head drive used in conjunction with the invention. 
         FIG. 3   a  shows another embodiment of the invention. 
         FIG. 3   b  shows another view of the embodiment of  FIG. 3   a.    
         FIG. 3   c  is a wiring diagram for the embodiment of  FIG. 3   b.    
         FIG. 4  shows RF-phases applied in CID mode, according to another embodiment of the process of the invention. 
         FIG. 5   a  shows a dipolar DC-displacement pulse applied in conjunction with the invention. 
         FIG. 5   b  shows the off-axis radial displacement of ions achieved with an embodiment of the invention. 
         FIG. 5   c  shows another view of the off-axis radial displacement of ions achieved with an embodiment the invention. 
         FIG. 6  is a mass spectrum of fragments obtained for [Fibrinopeptide-A] 2+  ions (SEQ. ID. NO.: 1) in CID mode. 
         FIG. 7  is a mass spectrum of fragments obtained for [Neurotensin] 3+  ions (SEQ. ID. NO.: 2) in CID mode. 
         FIG. 8  is a mass spectrum of fragments obtained from [Angiotensin] 4+  ions (SEQ. ID. NO.: 3) in RF-heating mode. 
     
    
    
     DETAILED DESCRIPTION 
     The following description includes the preferred best mode of one embodiment of the present invention. It will be clear from this description of the invention that the invention is not limited to these illustrated embodiments but that the invention also includes a variety of modifications and embodiments thereto. Therefore the present description should be seen as illustrative and not limiting. While the invention is susceptible of various modifications and alternative constructions, it should be understood that there it no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention covers all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims. The invention provides analytical benefits in analysis of complex molecules not achieved with conventional processes and conventional devices including, but not limited to, e.g., higher sensitivity, and efficient activation. In particular, the invention fragments ions inside a collision cell in an RF-focusing field at increased collection efficiency. The invention further permits operation at a higher pressure, which can be combined seamlessly with various ion mobility mass spectrometry stages. Pressures employed within the collision cell are &gt;20 mTorr, and typically operate at pressures of 100 mTorr and higher. As compared to conventional fragmentation approaches at 1 mTorr, the invention provides softer ion activation, and yields ˜100-fold less energy per collision and 100-fold greater collisions per unit length at the same axial DC field strength, which minimizes over-fragmentation. As a result, the invention provides significantly more structurally-informative MS/MS spectra for complex ions. As used herein, the term “ion fragmentation” is used synonymously with the terms “ion activation” and “ion dissociation”. The term “fragment ion” means a product ion resulting from dissociation or fragmentation Of a precursor ion. The term “residue” refers to amino acids of a peptide chain according to standard conventions: alanine (A or Ala), cysteine (C or Cys), aspartic acid (D or Asp), glutamic acid (E or Glu), phenylalanine (F or Phe), glycine (G or Gly), histidine (H or His), isoleucine (I or Ile), lysine (K or Lys), leucine (L or Leu), methionine (M or Met), asparagine (N or Asn), proline (P or Pro), glutamine (Q or Gln), arginine (R or Arg), serine (S or Ser), threonine (T or Thr), valine (V or Val), tryptophan (W or Trp), and tyrosine (Y or Tyr). A fragment ion is considered to be structurally valuable (structurally-rich) if the identity of residues in the fragment provides Sequencing information useful in the identification of a precursor (primary or parent) ion. In contrast, fragments including, but not limited to, e.g., H 2 O and NH 3  do not provide any structural information by which to identify the precursor ions. Fragments of a peptide are denoted herein by reference to charged species including, but not limited to, e.g., a n  (“a-fragment”), b n  (“b-fragment”), y n  (“y-fragment”), and z n  (“z-fragment”) generated during dissociation of the peptide, where “n” denotes the residue position in the intact peptide. The fragment ion is designated as an “a” fragment (i.e., cleavage of a peptide bond behind a carbonyl residue between adjacent amino acids), “b” fragment (i.e., cleavage in front of a carbonyl residue), or “c” fragment (i.e., cleavage of in front of an N-H residue) when charge is retained on the N-terminus. By convention, residues in a “b” fragment are counted from the left-most residue to the right-most residue. Fragmentation of “b” fragment ions results in formation of “a” fragment ions. While many potential mechanisms exist for forming “a” fragment ions directly from a parent or precursor ion, it is generally accepted that “b” fragment ions lose a carbonyl (C═O) moiety (28 mass units) to form “a” fragment ions, where a n =b n =28. “X” fragment ions are generated by cleavage of a C—C α  bond. “Y” fragment ions result from cleavage in front of a carbonyl residue. “Z” fragment ions result from cleavage in front of an N—C α  bond, with charge retained on the C-terminus. By convention, “X” fragment and “Y” fragment residues are counted from the right-most residue to the left-most residue. Other common fragments include ions with masses corresponding to multiple losses of water or losses of NH 3 , e.g., b n  minus H 2 O. Internal fragments formed by cleavage of two backbone bonds are also typical in CID and include both b-type and a-type (“b” minus 28) fragments. Internal a-type ions composed of only one amino acid are called “immonium” ions. 
       FIG. 1  is a schematic diagram of an IMS-TOF-MS instrument  500  of an exemplary configuration that incorporates a collision cell  100 , according to one embodiment of the invention. Collision cell  100  is positioned between IMS stage  10  and TOF-MS stage  25 , but location is not limited thereto. IMS stage  10  includes an ESI source  2 , an ion funnel trap  4  described, e.g., by Clowers et al. [ Anal. Chem.  2008, 80 (3), 612-623] that is further coupled to an IMS drift cell  6 . IMS drift cell  6  interfaces to a (rear) ion funnel 8 described, e.g., by Belov et al. [ J. Am. Soc. Mass Spectrom.  2000, 11 (1), 19-23] that is coupled to a conventional time-of-flight (TOF) mass spectrometer (MS) instrument (i.e., TOF-MS) 25 (e.g., an Agilent 6210 TOF-MS) available commercially (Agilent Technologies, Santa Clara, Calif., USA). In one embodiment, collision cell  100  is of a segmented quadrupole (SQ) design that incorporates segmented rods  30 . In the figure, TOF-MS  25  (Agilent TechnologieS, Santa Clara, Calif.) includes two differentially-pumped multipoles (e.g., octopoles) ( 12 ,  14 ) that are coupled in series to a DC-quadrupole  15 , an ion extractor  18 , a reflection  20 , and a detector  22 , but components are not limited thereto. 
       FIG. 2   a  shows one embodiment of collision cell  100  of the invention. Collision cell  100  includes a preselected number (N) of segmented rods, where (N) is an even-numbered integer greater than or equal to two. In the exemplary configuration, collision cell  100  includes four 4) segregated rods ( 32 ,  34 ,  36 ,  38 ), described further in reference to  FIG. 2   b . In  FIG. 2   a  two, rods ( 32 ,  34 ) are shown, but number is not limited. An ion channel  40  is located between, and surrounded by, segmented rods ( 32 ,  34 ). A center axis  42  defined through the center of ion channel  40  provides transmission of ions to a subsequent instrument stage, e.g., an MS stage  25 . Rods ( 32 ,  34 ) each include a fragmentation section  46  and a focusing section  50 . In the exemplary embodiment, fragmentation section  46  of each rod ( 32 ,  34 ) includes a preselected number (e.g., 4) of electrically-coupled rod segments  44 . Focusing section  50  of each rod ( 32 ,  34 ) also includes a preselected number (e.g., 2) of electrically-coupled rod segments  48 . Segments ( 44 ,  48 ) individually and/or collectively deliver a preselected potential of like or different kind at preselected locations along rods ( 32 ,  34 ). In the exemplary configuration, each rod ( 32 ,  34 ) has a diameter of ˜6.4 mm, with an inscribed radius (r) of 2.8 mm. Segments ( 44 ,  48 ) have a combined length of ˜11.7 mm. Segments ( 44 ,  48 ) are separated by 0.5-mm polymer washers [e.g., polyetheretherketone (PEEK) washers (not shown)] that are nested between the segments ( 44 ,  48 ) so as not to be exposed to ions introduced to ion channel  40 . 
     Two operationally independent resistor chains ( 55 ,  57 ) couple to respective segments ( 44 ,  48 ) of fragmentation section  46  and focusing section  50  of each rod ( 32 ,  34 ), e.g., as shown. Resistor chain  55  applies an axial DC gradient  64  to any individual or Collective segments  44  of section  46  for each rod ( 32 ,  34 ). Resistor chain  57  applies an axial DC gradient  66  to any individual or collective segments  48  of section  50  for each rod ( 32 ,  34 ). A DC-power supply  68  (described further in reference to  FIG. 2   b ) delivers power to resistor chains ( 55 ,  57 ) independent of the other, providing independent operational control of DC gradients ( 64 ,  66 ) applied to any individual or collective segments ( 44 ,  48 ) of Sections ( 46 ,  50 ), respectively. 
     An RF-power drive  106  (described further in reference to  FIG. 2   c ) constructed in-house of high-Q (high-quality) components delivers power for generating respective RF-fields ( 70 ,  72 ) that can be independently applied to individual or collective segments ( 44 ,  48 ) in Sections ( 46 ,  50 ) of collision cell  100 , respectively. RF-fields ( 70 ,  72 ) are defined by capacitances selected for independent capacitor chains ( 61 ,  63 ), and by tuning of resonant frequencies for RF-fields ( 70 ,  72 ) applied to any individual, or groups of, segments ( 44 ,  48 ) in Sections ( 46 ,  50 ) for each rod ( 32 ,  34 ) of collision cell  100 , respectively. 
     Coupling wires  73  link segments ( 44 ,  48 ) along rods ( 32 ,  34 ) allowing for selective and/or collective operation thereof. For example, in one exemplary operation, one or more rods (e.g.  32 ,  34 ) can be electrically coupled together such that a single RF-field  70  (e.g., a positive RF-field, RF+) is applied to segments  44  and a single RF-field  72  (e.g., a positive. RF-field, RF+) is applied to segments  48  of Section  50  of the coupled rods ( 32 ,  34 ) of collision cell  100 , respectively. In an alternate mode, an independent DC-dipolar displacement pulse  102  (described further in reference to  FIG. 2   b ) can be independently applied to a single rod (e.g., rod  38  described further in reference to  FIG. 2   b ) in order to generate a radial. DC-displacement field, as described further herein. In addition, RF-voltages and frequencies that define RF fields ( 70 ,  72 ) for segments ( 44 ,  48 ) may be alike or different. Thus, no, limitations are intended by the exemplary descriptions. RE voltages [peak-to-peak/voltage (V pp )] are preferably selected in the range from about 100 V pp  to about 2000 V pp . RF frequencies are preferably selected in the range from about 500 kHz to about 2 MHz. In one exemplary test, an RF-focusing field  72  having a peak-to-peak voltage (V pp ) of 220 V and an RF-frequency of 800 kHz was applied to segments  48  of Section  50  of segmented rods ( 32 ,  34 ), but operating parameters are not limited thereto. In the exemplary configuration, a DC-only conductance limiting orifice (electrode)  59  (2.2 mm I.D.) was positioned in front (i.e., 1.4 mm ahead) of segmented rods ( 32 ,  34 ). Another DC-only conductance limiting orifice (electrode)  60  was positioned at the rear of (i.e., 1.4 mm after) segmented rods ( 32 ,  34 ) of collision cell  100  in front of octopole  12 , interfacing an IMS stage  10  to time-of-flight mass spectrometer (TOF-MS)  25  (see  FIG. 1 ). 
       FIG. 2   b  shows an exemplary RF- and DC-wiring diagram for collision cell  100  that, in RF-heating mode, applies a dipolar DC-displacement pulse  102  to induce RF-heating of ions. Four rods ( 32 ,  34 ,  36 , and  38 ) of collision cell  100  are shown and described. Rods ( 32 ,  34 ,  36 ,  38 ) of collision cell  100  each include four (4) rod segments  44  in a first fragmentation section  46  and two (2) rod segments  48  in a second focusing section  50 , but number of segments is not limited thereto. RF fields ( 70 ,  72 ) are defined by independent capacitor chains ( 61 ,  63 ) that link to respective segments ( 44 ,  48 ) of Section  46  and Section  50 , allowing RF-fields ( 70 ,  72 ) to be independently applied to respective segments ( 44 ,  48 ). Two rods ( 32 ,  34 ) of collision cell  100 , positioned opposite one another, are electrically coupled such, that RE fields ( 70 ,  72 ) applied to respective segments ( 44 ,  48 ) by RF-power drive  106  have an identical RF-phase (e.g., RF+). Phasing of RF-waveforms that define RF-fields ( 70 ,  72 ) is provided by RF-amplifiers described further herein in reference to  FIG. 2   c . Segments ( 44 ,  48 ) of coupled rods ( 32 ,  34 ) are linked by coupling wires  73 , e.g., as shown. In the figure, a resistor chain  55  couples to individual segments  44  of Section  46 , and delivers an axial DC-gradient  64  that can be selectively applied to any of these coupled segments  44 , DC gradient  64  is defined by the voltage difference between the DC (IN) terminal  95  and the DC (OUT) terminal  96  delivered by DC-power supply  68 . In the instant embodiment, segments  48  of section  50  for all rods ( 32 ,  34 ,  36 ,  38 ) are linked via coupling wires  73 . A separate resistor chain  57  couples to these segments  48 , which provides a DC-gradient  66  that can be independently applied to individual or collective segments  48  of Section  50  for all rods ( 32 ,  34 ,  36 ,  38 ). DC gradient  66  is defined by the voltage difference between the DC (IN) terminal  97  and the DC (OUT) terminal  98  delivered by DC-power supply  68 . A third segmented rod  36  has DC gradients ( 64 ,  66 ) selectively applied in concert with resistor chains ( 55 ,  57 ) that couple to individual segments ( 44 ,  48 ) of Sections ( 46 ,  50 ), respectively, as described above. A fourth rod  38  is wired to a separate resistor chain  58  independent, of those coupled to rods ( 32 ,  34 , or  36 ) of collision cell  100 . This configuration permits a specific dipolar DC-displacement pulse  102  to be applied to any individual or collective segments  44  of Section  46  of this fourth rod  38  independent of any other rods ( 32 ,  34 , or  36 ) of collision cell  100 . DC-displacement pulse  102  is defined by the voltage, difference between DC (IN) terminal  99  and DC (OUT) terminal  101  to this selected rod  38  relative to the DC gradient applied to rod  36  as delivered from DC-power supply  68 . DC-displacement pulse  102 , when applied, selectively displaces ions from center axis  42  within ion channel  40  inside collision cell  100 . This displacement induces RF-heating of ions locally at any of a variety of predetermined locations, e.g., between selected segments ( 44 ,  48 ) of sections  46  and  50 ; or, between any individual or collective segments  44  within section  46 , for example. This ability to selectively apply RF-heating of ions induces ion dissociation at these selected locations inside collision cell  100 , as described further herein. The third rod  36  and fourth rod  38  are each wired to receive an RF-phase [e.g., (RF−) and, (RF−′)] that is independent of that (e.g., RF+) applied to coupled rods ( 32 ,  34 ) by RF High-Q Drive  106  (described further herein in reference to  FIG. 2   c ). Although generated independently, magnitudes of RF-phases [e.g., (RF−) and (RF−′)] applied to third rod  30  and fourth rod  38  are essentially identical. 
     The RF- and DC-wiring configuration of the present embodiment allows RF fields ( 70 ,  72 ) to be independently decoupled from DC-displacement pulses  102  and from axial DC gradients ( 64 ,  66 ) that are applied locally to individual, or groups of, segments ( 44 ,  48 ) of each rod  30 , respectively. Configuration of the exemplary embodiment described herein improves ion fragmentation and enhances collection efficiency inside collision cell  100 . RF-phases (e.g., RF+, RF−, and RF−′) generated by RF High-Q Drive  106  will now be further described. 
       FIG. 2   c  shows a schematic of an RF High-Q head drive  106  built in-house that delivers current for driving collision cell  100 . While RF head drive  106  is described in reference to segmented quadrupole rods ( 32 ,  34 ,  36 ,  38 ), the drive is not limited thereto and may be used for both segmented and non-segmented components described further herein. Thus, no limitations are intended. In the figure, head drive  106  is configured to deliver current to three (3) drive circuits ( 74 ,  75 ,  76 ) but number is not limited thereto. Drive circuits ( 74 ,  75 ,  76 ) are preferably of an LC resonant-circuit, or a “tank circuit”, design. Drive circuits ( 74 ,  75 ,  76 ) provide radial displacement of ions, e.g., in combination with RF-fields  70  applied to any individual or collective rod segments  44  of Section  46 ; and further provide focusing of ions, e.g., in combination with RF-fields  72  applied to segments  48  of Section  50 , described previously herein in reference to  FIG. 2   b . RF head drive  106  includes an RF-source  77  of a low current design. RF-source  77  delivers a characteristic (i.e., resonant) rise in voltage of from about 100 V to about 2000 V within each (High-“Q”) drive circuit ( 74 ,  75 ,  76 ). RF sources include, but are not limited to, e.g., RF pulsed sources, RF modulators, RF signal generators, RF waveform generators, and other RF devices, as well as combinations of these devices. Drive circuits ( 74 ,  75 ,  76 ) include respective RF-amplifiers ( 78 ,  79 ,  80 ) that deliver RF-waveforms ( 81 ,  84 ,  87 ) of a preselected phase (e.g., RF+, RF−, RF−′). In each circuit ( 74 ,  75 ,  76 ), RF-amplifiers ( 78 ,  79 ,  80 ) couple to an inductor (L)  90 : Typical inductor values are in the range from about 10 μH to about 50 μH. Inductor  90  in each circuit ( 74 ,  75 ,  76 ), couples to a variable capacitor  92  that provides typical capacitances from about 100 pF to about 300 pF. Inductor  90  in each circuit ( 74 ,  75 ,  76 ) further couples to a capacitor (C)  91  that provides typical capacitances from about 1 pF to about 10 pF, but capacitances are not intended to be limited thereto. 
     In RF-heating mode, drive circuit  74  provides a first RF waveform  81  (e.g., RF+) to two coupled rods ( 32 ,  34 ) described previously in reference to  FIG. 2   b . In drive: circuit  74 , RF-waveform  81  (e.g., RF+) is 180 degrees out of phase with RF-waveforms  84  [e.g., (RF−)] and  87  [e.g., (RF−′)] delivered from drive circuits ( 75 ,  76 ), respectively. Frequencies and amplitudes of RF-waveforms applied to selected segmented rods can be of the same magnitude or of a different magnitude, as will be understood by those of ordinary skill in the art. No limitations are intended. Waveform  84  [e.g., (RF−)] and waveform  87  [e.g., (RF−′)] delivered from drive circuits ( 75 ,  76 ), respectively, are in-phase and decoupled from the other. Use of decoupled waveforms enables a specific dipolar DC-displacement pulse ( FIG. 2   b ) to be applied to a single selected rod (e.g. rod  38 ), e.g., in conjunction with a bias source or DC power supply (described previously in reference to  FIG. 2   b ). Decoupling allows an RF-displacement field  70  (e.g., RF−′) to be applied to any selected rod (e.g., rod  38 ) that is uncompensated by RF-fields  70  [e.g., (RF+) and (RF−)] applied to other rods [e.g., to coupled rods ( 32 ,  34 ) and to rod  36 , respectively]. This configuration allows, e.g., positively-charged ions to be radially displaced in the direction of the segmented rod having the lower DC bias voltage. For example, the uncompensated RF-field  70  applied, e.g., to selected segments ( 44 ,  48 ) of rod  38  provides radial displacement of ions from quadrupole axis  62 , which increases the ion energy and enhances ion fragmentation. 
     Different DC-gradients ( FIG. 2   b ) can be applied to selected rods simultaneously in combination with RF-fields described herein. RF-fields applied to each rod ( 32 ,  34 ,  36 ,  38 ) are defined by RF waveforms that are applied, e.g., as sine waves. In collision cell  100 , each rod ( 32 ,  34 ,  36 ,  38 ) at any point of time has an RF field defined by the sine wave that is applied. For example, coupled rods ( 32 ,  34 ) can have a positive RF-field (e.g., RF+) applied, while the remaining pair of rods ( 36 ,  38 ) can have a negative RF-field applied (e.g., RF− and RF−′, respectively). The negative RF-fields (e.g., RF− and RF−′) are defined by negative sine waves (waveforms) that can be matched with the magnitude of the amplitude of the positive sine wave applied to the positive pair of rods, e.g., as described further in reference to  FIG. 5   a.    
     A “positive” sine wave as used herein means the waveform is 180° out-of-phase or phase-shifted by 180° relative to a “negative” sine wave (waveform). Positive ions thus experience a positive RF-field as a repulsive field on one pair of rods, while the same ions experience a negative RF field as an attractive field on another pair of rods. Due to the high frequencies of RF waveforms that define RF-fields, ions experience the oscillating, and alternating phases of the RF-fields on rods ( 32 ,  34 ,  36 ,  38 ) of the collision cell  100  as a potential well. In normal axial operation, the potential well has a minimum located at the center of the quadrupole. Thus, ions traverse the center axis ( FIG. 2   a ) of quadrupole  100 . In short, for coupled rods ( 32 ,  34 ) the RF amplitude is of the same magnitude. If one pair of rods is de-coupled, e.g., when two RF sine waves of the same sign and phase are applied, but have, e.g., different amplitudes, the balance between positive and negative RF-fields can affect the ion motion. In such a case, for example, ions can drift from the center axis towards one of the rods, thereby experiencing a greater oscillation due to the closeness to the rod to which the alternate RF-field is selectively applied. Such is the case in RF-heating mode described herein. In one typical approach, applying two sine waves (waveforms) of different amplitudes and the same (e.g., negative) sign and phase are applied by adding a (pulsed or continuous) DC component to the RF component that is then applied into one of the rods (e.g., rod  38 ) as an RF-displacement field. As used herein, the notations (RF−) and (RF−′) denote an RF-field that at one moment of time is defined by a negative sine waveform having the same phase but that can be of a different amplitude. As used herein, the notation (RF+) denotes an RF-field that at one moment of time is defined by a positive sine waveform (RF+) having the same phase and same amplitude on a coupled pair of rods at the same moment. It will be understood by those of ordinary in the art that the same rods that, at one moment of time, have an (RF−) or (RF−′) applied, can subsequently have an RF+) or an (RF+′) waveform applied at another moment in time while the contrasting pair of rods will have an (RF−) waveform applied. No limitations are intended. All configurations as will be undertaken by those of ordinary skill in the art in view of the disclosure are within the scope of the invention. 
     Presence of a collisional cooling gas can further dampen the ion energy in collision cell  100 . In embodiments of the invention described herein, the last two segments  48  of each rod ( 32 ,  34 ,  36 ,  38 ) act as RF-focusing segments. The term “RF focusing” refers to the process whereby ion motion collapses to the center axis. In RF-focusing mode, ions stay near center axis  42 , while the first four segments  44  of each rod ( 32 ,  34 ,  36 ,  38 ) can act either as RF focusing segments (e.g., in CID mode) or as RF-displacement segments where ions are displaced from center axis  42  (e.g., in RF heating mode). 
     Segmented Vane Quadrupole 
       FIG. 3   a  is, a perspective view of another embodiment of collision cell  200  of a segmented vane design. Four vane assemblies  110  are nested between four (4) non-segmented quadrupole rods ( 32 ,  34 ,  36 ,  38 ) [e.g., radius (R)=3.18 mm (0.125″); inscribed radius (r)=2.79 mm (0.11″)]. Each rod ( 32 ,  34 ,  36 ,  38 ) is adjacent two vane assemblies  110  (e.g., stainless steel, 0.5 mm-thick). Each assembly  110  includes a preselected number (e.g., 6) of vane segments ( 112 ,  116 ). In the exemplary embodiment, four (4) vane segments  112  define Fragmentation Section  114  and two vane segments  116  define Focusing Section  118 . Each vane segment ( 112 ,  116 ) has a length of 11.68 mm (0.46 inches). Spacing between individual vane segments ( 112 ,  116 ) is ˜0.5 mm (e.g., ˜0.51-mm (0.02″). Vane assemblies  110  atm the decoupling of RF fields ( FIG. 2   a ) from dipolar DC-displacement pulses ( FIG. 2   b ) and/or from axial DC gradients ( FIG. 2   a ) that are locally applied to individual segments ( 112 ,  116 ) along rods ( 32 ,  34 ,  36 ,  38 ). Each vane assembly  110  is electrically decoupled from an adjacent quadrupole rod  30 , and is electrically coupled to a second vane assembly  110  using a resistor network described further herein in reference to  FIG. 3   c.    
       FIG. 3   b  shows an end-oh (front) view of segmented vane quadrupole  200  of  FIG. 3   a . In the figure, vanes  110  are positioned such that the potential between rods ( 32 ,  34 ,  36 ,  38 ) is zero (i.e., the so-called “zero RF-Potential plane”). Vanes  110  are positioned using a non-conducting positioning element  120 . 
       FIG. 3   c  shows a wiring diagram for RF- and DC-operation of the segmented vane collision cell  200  embodiment of  FIG. 3   a  in RF-heating mode. In the figure, two opposite rods ( 32 ,  34 ) are electrically coupled, as described previously herein in reference to  FIG. 2   b , giving them identical RF-fields [e.g., (RF+) and (RF+)], phasing, and amplitudes. While RF-fields ( FIG. 2   a ) applied to rods ( 32 ,  34 ) are shown to be positive (i.e., RF+), potentials of specific individual cods ( 32 ,  34 ,  36 ,  38 ) and vanes  110  are not limited thereto. In the figure, opposed vane segments  112  in Section  114  between two vane assemblies  110  of each coupled rod ( 32 ,  34 ) are electrically linked via coupling wires  73 . A resistor chain  55  is coupled to individual vane segments  112  of Section  114  of rods ( 32 ,  34 ), respectively, which allows an axial DC field gradient  64  to be independently applied to individual vane segments  112  in Section  114  for each rod ( 32 ,  34 ). This wiring arrangement further allows a dipolar DC-displacement pulse  102  to be superimposed over axial DC-gradient  64  applied to vane assemblies  114  surrounding rod  32 , while maintaining a static axial DC-gradient  64  to vane assemblies,  110  surrounding second rod  34 . Voltages applied between DC IN and DC OUT terminals (described previously in reference to  FIG. 2   b ) for vane segments  112  in Section  114  of first rod  32  provides a DC-displacement pulse  102  that can be applied to selected vane segments  112  in Section  114 . As described previously herein, DC-displacement pulse  102  displaces ions to a constant radial position within Section  114 . Vane segments  116  in Focusing Section  118  are all coupled together via coupling wires  73 . Thus, one axial DC gradient  66  is applied to all segments  116  of Section  118 . A single independent resistor chain  57  establishes the axial DC gradient  66  for all segments  116  (e.g., fifth and sixth segments) of Section  118 . 
     Fragmentation 
     Ion fragmentation in segmented collision cell  100  results as ions are accelerated during CID mode or during RF-heating mode as a consequence, of ion oscillation, displacement, and/or collision with gas molecules/atoms. In embodiments described previously in  FIGS. 2   a - 2   c , the invention provides ion fragmentation in a (narrow) localized region, e.g., between two adjacent segments ( 44 ,  48 ) of rods ( 32 ,  34 ,  36 ,  38 ) [e.g., between a 4 th  segment  44  and 5 th  segment  48 , or another localized region inside collision cell  100 . In embodiments described previously in  FIGS. 3   a - 3   c , ion fragmentation can also be effected in a localized (narrow) region between two adjacent segments ( 112 ,  116 ) of vanes  110  [e.g., between a 4 th  segment  112  and a 5 th  segment  116 , or another localized region inside vane collision cell  100 . DC- and RF-fields applied to rod segments ( 44 ,  48 ) or vane segments ( 112 ,  116 ) vary locally in time and ensure efficient decomposition of analyte precursor ions that exit IMS drift cell stage  10 . This localized fragmentation provided by the invention provides two primary advantages. First, a limited region of ion activation is maintained in quadrupole  100 , which ensures: 1) that numerous collisions are obtained, and 2) that excessive fragmentation is mitigated such that both multiply-charged and singly-charged ions decompose to give structurally-rich, informative fragment ions. Optimum collision energies can be selected for precursor ions of interest because fields can be applied to specific and individual segments. Ions accelerated by the axial electric field are first activated inside ion channel  40  of collision cell  100 , which leads to an abundance of primary fragments. 
     Fragmentation is followed by collisional cooling of fragment ions and any remaining parent (precursor) ions, which results in a narrowing of the internal energy distribution of both fragment ions and remaining precursor ions. Thus, all ions dispersed during the collision process are subsequently re-collimated to ion channel axis  42  by a radially confining RF-focusing field  72 . Experiments described hereafter were performed at a pressure of 200 mTorr inside collision cell (segmented quadrupole)  100 , but pressure is not limited thereto. In experiments deploying a drift tube IMS stage  10 , a voltage drop of ˜5 V was applied to each segment  44  in Section  46  of segmented quadrupole rods ( 32 ,  34 ,  36 ,  38 ) to reduce residence time of ions in collision cell  100 , thus minimizing peak dispersion in the drift time domain. DC voltages applied to exit segments  48  of Section  50  and conductance limiting orifice  60  were kept within ˜5 Volts of the DC-bias (˜32 V) applied to octopole  12  of TOF-MS stage  25  in order to optimize sensitivity, but parameters are not limited thereto. 
     Three fragmentation modes will now be described: 1) CID mode, 2) RF-heating Mode, and 3) combined axial CID and RF-heating mode. 
     Axial CID Mode 
       FIG. 4  is an end-on view of segmented rods ( 32 ,  34 ,  36 ,  38 ) that shows RF-field phases applied to collision cell  100  for operation in axial CID mode. In the figure, two quadrupole rods ( 32 ,  34 ) each have an RF-field  70  applied with a first RF-phase (e.g., RF+). Another two quadrupole rods ( 36 ,  38 ) each have an RF-field  70  applied, both with an opposite RF phase (e.g., RF−), e.g., as, shown, but operation is, not limited thereto, as detailed herein. In preferred operation, [i.e., in collision-induced-dissociation (CID) mode], a locally and selectively positioned voltage differential is applied, e.g., between two selected segments ( 44 ,  48 ) of each rod ( 32 ,  34 ,  36 ,  38 ) in collision cell  100 , as described previously herein that provides ion dissociation and fragmentation inside ion channel  40 . In collision cell  100 , the buffer gas used is a neutral gas including, but not limited to, e.g., nitrogen and argon used at high pressure, e.g., a pressure greater than about 20 mTorr. In an exemplary embodiment of the process, ion dissociation by CID was effected, and demonstrated, between the 4 th  segment  44  of Section  46  and the 5 th  segment  48  of Section  50 . In this mode, an axial DC-field ( 64 ,  66 ) voltage of equal magnitude is first established across all segments ( 44 ,  48 ) of rods ( 32 ,  34 ,  36 ,  38 ), which does not lead to ion fragmentation. Ion fragmentation by CID is then effected by increasing the axial DC-electric field (gradient)  64  between, e.g., the 4 th  segment  44  and the 5 th  segment  48  inside collision cell  100  ( FIG. 2   a ) while keeping axial DC-electric field  66  unchanged for the rest (i.e., remaining length) of each rod ( 32 ,  34 ,  36 ,  38 ). In the exemplary case, for example, axial DC-field  64  (bias) voltage applied to the four segments  44  of Section  46  can be increased by, e.g., up to 200 V relative to the (bias) voltage of the axial DC-field  66  applied to segments  48  of Section  50 , but is not limited thereto. This localized increase in axial DC-gradient  64  between selected segments ( 44 ,  48 ) accelerates ions in the selected region (i.e., proportional to the applied DC-field), increasing their ion kinetic energy. This increase in kinetic energy induces on-axis collisions between the precursor ions and buffer gas molecules leading into Section  50  within RF-focusing field  70 , resulting in fragmentation of the precursor ions. In the exemplary case, increasing the voltage difference between the 4 th  segment  44  of Section  46  and the 5 th  segment  48  of Section  50  accelerates ions in the selected region, increasing the velocity of impact between the precursor ions and the collision gas molecules between selected segments where locally selected voltage differences are applied, thus facilitating productive ion fragmentation in that localized and selected region. Alternate, segments can also be selected for CID fragmentation, as will be understood by those of ordinary skill in the art. Thus, the invention is not intended to be limited by the description to: the exemplary operation. After exiting the selected region between, e.g., segments  4  and  5 , (or another localized region) where locally elevated DC-gradients ( 64 ,  66 ) are selectively positioned, fragment ions are then collisionally cooled by the buffer gas. Ions are then focused to ion axis  42  using radial RF-focusing field  72 , which facilitates efficient transmission of fragment ions to mass spectrometer stage  25  for detection. 
     RF-Heating Mode 
       FIG. 5   a  is an end-on (front) view of collision cell  100  in RF-heating mode. Two opposite rods ( 32 ,  34 ) are coupled such that RF-fields  68  applied to these rods are identical in phase type (e.g., RF+) and magnitude. The voltage applied to rod  32  or segment ( 44 ,  48 ) of that rod is experienced by the opposite rod  34  or segment ( 44 ,  48 ), and vice versa. In the figure, remaining rods ( 36 ,  38 ) are not coupled so that voltages applied to one rod  36 , or individual segments ( 44 ,  48 ) of that rod  36 , are applied independently of voltages applied to the other rod  38 . This allows rod  38  to Which an (RF−′) phase is applied to be pulsed, although the selection of rod is not limited, thereto. Rods ( 36 ,  38 ) have the same polarity (e.g., RF− and RF−′) and essentially an equal magnitude, initially. Thus, ion energy is minimized (e.g., at the bottom of the pseudopotential energy well) due to fully compensated RF-fields (e.g., RF− and RF−′) from opposing rods ( 32 ,  34 ) and ( 36 ,  38 ) of collision cell  100 . In RF-heating mode, a DC-displacement pulse  102  is superimposed (applied) to one of the uncoupled rods, e.g., rod  38 , or segments ( 44 ,  48 ) of rod  38 . DC-displacement pulse  102 , generates a high RF-field  70  (e.g., RF−′) that is uncompensated (i.e., not matched) by the opposite rod  36  or segments ( 44 ,  48 ) of rod  36  because the rod  36  is not physically coupled to is independent of) the opposite rod  38 . For example, pulsing selected segments  44  of a single rod  38  with dipolar DC-displacement pulse  102  displaces ions radially from center axis  42  in a localized area selected, e.g., within, or between, segments (e.g., between segments  1 ,  2 ,  3 , and  4  of Section  46 ) inside collision cell  100 . 
       FIG. 5   b  is an, end-on (front) view of collision cell  100  (segmented quadrupole design) that shows the radial displacement of ions achieved by the dipolar DC-displacement pulse  102 . In the figure, ions are shifted off the center axis  42  away from rod  38  closer to opposed rod  36 . 
       FIG. 5C  shows a horizontal cross-sectional view through collision cell  100  in RF-heating mode. In the figure, a SIMION simulation shows the radial off-axis displacement of ions achieved: by dipolar DC-displacement pulse  102  when applied, e.g., between the 1 st  and 4 th  segments  44  of, Section  46 . While an exemplary localized area is shown, area to which displacement pulse  102  is applied inside collision cell  100  is not limited. In the figure, ions are displaced (i.e., off-axis) from ion channel (center) axis  42 . The radial displacement of ions from the center axis using the uncompensated RF-field  70  increases the ion energy (i.e., up the pseudopotential energy well), given that the RF-field  70  on rod  38  is not compensated by an opposite rod  36 . The uncompensated high-frequency RF-field  70  in Section  46  increases the amplitudes of ion oscillation, resulting in higher energy collisions with the buffer gas, and an increase in the ion temperature. As ions are pushed by the radial DC-displacement pulse  102  from ion channel axis  42  from one rod  38  toward an opposite rod  36 , associated ion temperatures increase, which induces fragmentation of the precursor ions as the ions impact with buffer gas molecules. All of these factors: increased amplitudes of ion oscillation; higher energy collisions; and increased ion temperatures can individually or collectively effect ion dissociation. Temperature of the ions is controlled by the extent of radial displacement, which in turn is a function of the magnitude of the applied dipolar DC-pulse, and the amplitude and frequency of the RF field. Ion activation in RF-heating mode has been shown to be broadband, meaning the process causes no m/z discrimination. The term “m/z discrimination” refers to the suppression of signals of certain m/z ions. Displaced fragment ions are subsequently focused (re-collimated) back to ion channel axis  42  by applying an RF-focusing (confinement) field described: previously herein  72 , e.g., along the last two (e.g., 5th and 6th) segments  48  of Section  50 . This refocusing results in collisional-cooling of ions with the buffer gas, and associated relaxation of the fragment ions back to the center axis  42 . Radial confinement of fragment ions back into the ion channel axis  42  with RF-focusing, field  72  minimizes ion losses, which provides effective coupling of collision cell  100  for high and efficient transmission of fragment ions, e.g., to a subsequent instrument stage  25 . In the figure, fragment ions are transmitted through limit (CL) interface  60  into mass spectrometer ( FIG. 2   a ). 
     Combined Axial CID and RF-Heating Mode 
     While ion dissociation has been described in reference to individual modes, e.g., CID mode and RF-heating mode, respectively, the invention is not limited thereto, as described hereafter. For example, off-axis RF-heating in conjunction with RF-fields ( 70 ,  72 ) can also be combined with collision-induced dissociation in conjunction with axial DC fields ( 64 ,  66 ) to attain higher fragmentation efficiency for larger molecules. For example, in other embodiments of the invention, ion dissociation can be induced using a combination of both RF-heating and CID. In the combined mode, precursor ions are first displaced from center axis  42  of collision cell  100  with a DC displacement pulse  102 , applied to one of the segmented rods ( 32 ,  34 ,  36 ,  38 ), as described previously in reference to  FIG. 5   a . This subjects them to a high-frequency RF field  70  and localized heating (i.e., by RF-heating). Displaced ions are then subjected to a localized drop in voltage between two selected segments, e.g., between a 4 th  segment  44  of Section  46  and a 5 th  segment  48  of Section  50  in segmented collision, cell  100 . This localized drop in voltage subjects the ions to axial collision with the buffer gas, inducing ion fragmentation by CID. This process thus combines the effects of both: 1) RF heating in RF-heating mode that increases ion energy, and 2) CID resulting from collisions with the buffet gas, which enhances fragmentation of the precursor ions. Axial DC-field  64  is increased, e.g., in the region between the 4 th  segment  44  of Section  46  and the 5 th  segment  48  of Section  50 . Once these selected quadrupole segments are energized at different potentials, axial DC field (gradient)  64  is concurrently generated parallel to ion axis  42  of collision cell  100 . Axial DC-field  64  decreases to zero on the surface of segments  44  and remains high between segments  44 . Axial DC-field  66  also decreases to zero on the surface of segments  48  and remains high between segments  48 . Therefore, if an ion is radially displaced off-axis  42 , and then traverses collision cell  100  near the surface of segments  44  (e.g., between the 4 th  segment  44  and the 5 th  segment  48 , the ion accelerates between segments ( 54 ,  55 ) just as it would along quadrupole axis  42 . Thus, axial DC-field  64  is generated not only on the quadrupole axis  42 , but also near the surface of between selected segments ( 44 ,  48 ). However, for ions approaching rods  30  in the radial direction, axial DC-field  64  gets lower along the segment surface (i.e., at the extreme, field  64  is zero on the surface), and remains high between segments ( 44 ,  48 ). In this fashion, precursor ions gain energy from both DC-fields ( 64 ,  66 ) and RF-fields ( 70 ,  72 ) and combined. Contributions to internal ion energy from each of the individual or combined fields can be varied by adjusting: 1) amplitude of the radial-displacement pulse  102 , 2) RF amplitude and frequency, and 3) axial DC-gradients ( 64 ,  66 ). When axial DC gradient  64  is reduced in Section  48  ( FIG. 2 ) and radial-displacement field  70  is removed, ions are re-collimated back to ion channel axis  42  of collision cell  100 , and collisional cooling of internal degrees of ion freedom occurs. 
     Fragmentation Efficiency and Collection Efficiency 
     Collection Efficiency (E c ) is defined as the ratio of the sum of intensities of all fragments (f i ) and remaining precursor ions (P) to the initial (MS-only) precursor ion (P 0 ) intensity, as given by Equation [1]: 
     
       
         
           
             
               
                 
                   
                     Collection 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Efficiency 
                     ⁢ 
                     
                       : 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       E 
                       c 
                     
                   
                   = 
                   
                     
                       P 
                       + 
                       
                         ∑ 
                         
                           f 
                           i 
                         
                       
                     
                     
                       P 
                       0 
                     
                   
                 
               
               
                 
                   [ 
                   1 
                   ] 
                 
               
             
           
         
       
     
     Fragmentation Efficiency (E f ) is defined as the ratio of intensities of all fragments (f i ) to the sum of intensities of both the remaining precursor ions (P) and all fragments (f i ), as given by Equation [2]: 
     
       
         
           
             
               
                 
                   
                     Fragmentation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Efficiency 
                     ⁢ 
                     
                       : 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       E 
                       f 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         f 
                         i 
                       
                     
                     
                       P 
                       + 
                       
                         ∑ 
                         
                           f 
                           i 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   2 
                   ] 
                 
               
             
           
         
       
     
     Collision Induced Dissociation (CID) Efficiency (E CID ) is defined as the ratio of intensities of all fragments (f i ) to the initial (MS-only) precursor ion (P 0 ) intensity. It is also determined as the product of the collection and fragmentation efficiencies, as given by Equation [3]: 
     
       
         
           
             
               
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     I 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     D 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Efficiency 
                     ⁢ 
                     
                       : 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       E 
                       CID 
                     
                   
                   = 
                   
                     
                       
                         E 
                         c 
                       
                       × 
                       
                         E 
                         f 
                       
                     
                     = 
                     
                       
                         ∑ 
                         
                           f 
                           i 
                         
                       
                       
                         P 
                         0 
                       
                     
                   
                 
               
               
                 
                   [ 
                   3 
                   ] 
                 
               
             
           
         
       
     
     Here, (P 0 ) is the intensity of the precursor ion, (P) is the surviving precursor ion intensity in the CID spectrum, (Σf i ) is the sum of all fragment intensities in the CID spectrum. (E c ) accounts for losses due to ion scattering/defocusing during the collision process. (E f ) reflects the efficiency of producing fragment ions. (E CID ) is the Overall CID efficiency, which incorporates both the fragmentation and collection efficiency. 
     The effective potential (V*) is given by Equation [4], as follows: 
     
       
         
           
             
               
                 
                   
                     
                       V 
                       * 
                     
                     ⁡ 
                     
                       ( 
                       
                         r 
                         , 
                         z 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         q 
                         2 
                       
                       ⁢ 
                       
                         
                           E 
                           rf 
                           2 
                         
                         ⁡ 
                         
                           ( 
                           
                             r 
                             , 
                             z 
                           
                           ) 
                         
                       
                     
                     
                       4 
                       ⁢ 
                       m 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ω 
                         2 
                       
                     
                   
                 
               
               
                 
                   [ 
                   4 
                   ] 
                 
               
             
           
         
       
     
     Here, q=ze is the ion charge; [E rf (r,z)] is the amplitude of the RF electric field; (m) is the ion mass, and (ω) is the angular frequency of the RF field. The DC gradient is superimposed on V* to generate a full effective potential. 
     Fragmentation Results 
     CID Mode 
     The invention system and process were assessed using various CID efficiency values, and other factors, including, e.g., collection and fragmentation efficiencies. CID efficiencies were assessed by examining CID spectra for a variety of peptides. 
       FIG. 6  shows a typical CID mass spectrum for [Fibrinopeptide-A] 2+  precursor ions (768.8498 m/z) having the sequence set forth in [SEQ ID NO.: 1] that were collisionally activated (fragmented) by the process of the invention inside the segmented quadrupole (SQ) collision cell at a collision voltage of 45 V. Inducing fragmentation inside the SQ resulted in detection of 21 high-intensity, structure-revealing fragments [i.e., a4, a5, b3 (quantity 2), b4, b5, b6 (quantity 2), b9, b11, y4, y5, y8, y9, y10, y11, y12, y13, y14, and y15] including the precursor ion (M). Optimum CID efficiency was determined by adjusting the electric field strength (i.e., collision energy) in the region between the 4 th  and 5 th  segments ( 44 ,  48 ) while maintaining a constant DC-gradient in other regions of collision cell  100 , as described previously in reference to  FIG. 2   a.    
       FIG. 7  is a CID mass spectrum obtained by the process of the invention for [Neurotensin] 3+  precursor ions (having the sequence set forth in [SEQ ID NO.: 2]) that shows ion fragments obtained from dissociation inside collision cell  100 . Precursor ions for [Neurotensin] 3+  (558.3105 m/z) were collisionally-activated inside the segmented quadrupole (SQ) at an exemplary collision voltage of 40 V, which is not limited. In the figure, the CID spectrum obtained with the CID approach shows a total of 22 high-intensity, structurally-revealing fragments, including, e.g., a11, a12, b2, b3, y6, y7, y8, y9, y10, y11, y12, z7, z9, z10, z11, and the precursor ion (M). Fragment ions were confidently identified using, a mass accuracy of ±10 ppm, but is not limited thereto. 
     TABLE 1 lists Collection Efficiency (E c ), Fragmentation Efficiency (E f ), and CID Efficiency (E CID ) data for the segmented quadrupole (SQ) in accordance with the invention at different voltage settings in tests performed on [Fibrinopeptide-A] 2+  (SEQ. ID. NO.: 1) and [Neurotensin] 3+  (SEQ. ID. NO.: 2) precursor ions. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Collection Efficiency (E c ), Fragmentation Efficiency (E f ), and CID 
               
               
                 Efficiency (E CID ) for [Fibrinopeptide-A] 2+  and [Neurotensin] 3+   
               
               
                 precursor ions inside the segmented quadrupole (SQ). 
               
            
           
           
               
               
               
               
               
            
               
                   
                 V 
                 E c   
                 E f   
                 E CID   
               
               
                   
                   
               
            
           
           
               
            
               
                 [Fibrinopeptide-A] 2+  (SEQ. ID. NO.: 1) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 37 
                 0.75 
                 0.37 
                 0.27 
               
               
                   
                 39 
                 0.74 
                 0.47 
                 0.35 
               
               
                   
                 40 
                 0.73 
                 0.55 
                 0.40 
               
               
                   
                 42 
                 0.67 
                 0.64 
                 0.43 
               
               
                   
                 45 
                 0.62 
                 0.84 
                 0.52 
               
               
                   
                 47 
                 0.64 
                 0.92 
                 0.59 
               
               
                   
                 50 
                 0.63 
                 0.97 
                 0.61 
               
               
                   
                 55 
                 0.60 
                 1.00 
                 0.60 
               
            
           
           
               
            
               
                 [Neurotensin] 3+  (SEQ. ID. NO.: 2) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 80 
                 0.64 
                 0.94 
                 0.60 
               
               
                   
                   
               
               
                   
                 V = collision voltage (volts). 
               
               
                   
                 E c  = collection efficiency. 
               
               
                   
                 E f  = fragmentation efficiency. 
               
               
                   
                 E CID  = CID efficiency. 
               
            
           
         
       
     
     As Shown in the table, fragmentation efficiencies (E f ) increased from 0.37 to 1.00 (37% to 100%) as collision voltages increased. Data also show that the segmented quadrupole (SQ) collision cell of the invention demonstrated a high collection efficiency, which is ascribed to better ion confinement in the segmented quadrupole following collision-induced fragmentation of the ions. At low collision voltages and low collision energies, ion loss due to ion defocusing and scattering is low, which leads to a high collection efficiency observed for the CID approach (i.e., 75%). At a higher acceleration voltage of 55 V (volts), the collection efficiency decreases to 0.60 (60%). From Equation [4], the effective potential of the segmented, quadrupole (SQ) collision cell was calculated under simulated conditions for the CID of [Neurotensin] 3+  ions (SEQ. ID. NO.: 2) using an exemplary collision voltage of 35 V (volts) applied between the 4 th  and 5 th  segments. Data indicate that inducing CID inside the RF-focusing field minimizes ions losses by confining the fragment ions. Inducing CID inside the quadrupole also allows collision products to be refocused into the axis of the quadrupole, which leads to effective transmission of product ions downstream through downstream ion optics into the mass spectrometer stage. 
     CID efficiency trends have also been observed for other peptides, including, but not limited to, e.g., Angiotensin-I (SEQ. ID. NO. 3) (Sigma-Aldrich, St. Louis, Mo., USA), Leucine Enkephalin (SEQ. ID. NO. 4), Methionine Enkephalin (SEQ. ID. NO. 5), Bradykinin (SEQ. ID. NO. 6), and tryptic digests of different proteins including, e.g., Bovine Serum Albumin (SEQ. ID. NO. 7) (Pierce Biotechnology, Rockford, Ill., USA): CID efficiencies for singly-charged Species in the IMS-CID-TOF instrument in accordance with the invention were comparable to those obtained from a conventional triple-quadrupole instrument. In particular, CID efficiency for singly-charged Leucine Enkephalin (SEQ. ID. NO. 4) (m/z 556) in the triple-quadrupole instrument was measured at 36%; a CID of 36% was also obtained in the IMS-CID-TOF instrument. The CID efficiency obtained for Methionine Enkephalin (SEQ. ID. NO. 5) in the conventional triple-quadrupole instrument was 39%. 
     The difference in E CID  values obtained for the triple-quadrupole instrument and IMS-CID-TOF approach in accordance with the invention becomes more pronounced when comparing multiply charged species such as double-charged. Fibrinopeptide-A ions (SEQ. ID. NO. 1) and triple-charged [Neurotensin] 3+  ions (SEQ. ID. NO. 2). E CID  values of 17% and 10% were obtained for the double-charged Fibrinopeptide-A ions (SEQ. ID. NO. 1) and triple-charged [Neurotensin] 3+  ions (SEQ. ID. NO. 2), respectively, in the conventional triple-quadrupole instrument. These. E CID  values are lower than those obtained in the IMS-CID-TOF instrument in accordance with the invention by factors of 3.6 and 6, respectively, (See TABLE 1). 
     The (m/z) distribution Of CID products for all studied peptides obtained in conjunction with the invention was broad. The broad range of in fragments produces a rich informational content by which to assess the structure of precursor ions. The content-rich MS spectra were attributed to precursor ions that were properly thermalized and that had a narrow internal energy distribution. Even at high collision energies sufficient to completely fragment all precursor ions [e.g., at collision energies greater than 60 V times (×) charge], typically, only a few fragments were observed at m/z values &lt;200 amu. This finding is significant because the region below 200 amu typically contains secondary fragments (i.e., fragments produced from primary fragments within the same, collision cell) and small fragments such as immonium ions. 
     These data demonstrate the advantages of inducing ion fragmentation at a higher pressure (e.g., 200 mTorr) inside the segmented quadrupole. The invention approach is characterized by more effective radial confinement of both precursor and fragment ions. Use of the higher pressure inside the segmented quadrupole also helps to collisionally cool: 1) the precursor ions before dissociation and before being accelerated and fragmented, and 2) fragmentation products following dissociation. Collisional cooling requires, at a minimum, a number [N] of collisions to occur along the length of the focusing device, as defined by Equation [5]: 
     
       
         
           
             
               
                 
                   N 
                   = 
                   
                     
                       M 
                       ion 
                     
                     
                       m 
                       gas 
                     
                   
                 
               
               
                 
                   [ 
                   5 
                   ] 
                 
               
             
           
         
       
     
     The length of the focusing device (L) Should be greater than ion relaxation length (λ), as given by Equation [6]: 
     
       
         
           
             
               
                 
                   λ 
                   = 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         M 
                         ion 
                       
                       
                         m 
                         gas 
                       
                     
                     ⁢ 
                     
                       1 
                       
                         n 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         σ 
                       
                     
                   
                 
               
               
                 
                   [ 
                   6 
                   ] 
                 
               
             
           
         
       
     
     Here, (C) is the proportionality coefficient (˜¾); (M ion ) is the ion mass; (M gas ) is the mass of the gas molecules; (n) is the gas number density; and (σ) is the ion collisional cross section. Values Selected for pressure and collision energy are sufficient to reproduce, the results obtained. CID efficiency (calculated as the ratio of the summed intensity of all fragment ions to the initial intensity of the precursor ion) is independent of the mode of ion activation (e.g., CID mode, RF-heating mode, or combined RF-beating and CID mode]. That is, it is decoupled from the efficiency values obtained in the experiment. Data shown in TABLE 1 demonstrate the superior performance of the CID approach inside the segmented quadrupole. In particular, results show an (E CID ) of 0.60 (i.e., 60%) under optimum conditions. The high CID efficiencies obtained are attributed to the ability of the segmented quadrupole to capture CID products at a high efficiency. 
     Fragmentation Results 
     RF-Heating Mode 
       FIG. 8  shows a typical fragmentation mass spectrum obtained by a process of the invention in RF-heating mode for [Angiotensin] 4+  precursor ions (SEQ. ID. NO. 3) by application of a DC-displacement voltage. The spectrum was generated by applying an exemplary DC-displacement voltage of 12.8 V, which is not limited. In the figure, the mass spectrum contains a rich distribution of major, high intensity fragments. Results further show a CID efficiency of ˜90%. 
     Example 
     CID Mode 
     Collision Induced Dissociation (CID) in accordance with the invention has been demonstrated in the interface between an ion mobility spectrometer (IMS) and a time-of-flight mass spectrometer (TOF MS). To deconvolute the IMS-multiplexed CID-TOF MS raw data, informatics approaches effectively using information on the precursor and fragment drift profiles and mass measurement accuracy (MMA) were developed. It was shown that radial confinement of ion packets inside an RF-only segmented quadrupole operating at a pressure of ˜200 mTorr and, having an axial DC-electric field minimizes ion losses due to defocusing and scattering, resulting in high abundance fragment ions which span a broad m/z range. Efficient dissociation at high pressure (˜200 mTorr) and high ion collection efficiency inside the segmented quadrupole resulted in CID efficiencies of singly-charged ions comparable to those reported with triple quadrupole mass spectrometers. The modulation of the axial DC-electric field strength inside the segmented quadrupole can be used either to induce or to prevent multiplexed ion fragmentation. In addition, the axial electric field ensures ion transmission through the quadrupole at velocities which do not affect the quality of IMS separation. Importantly, both the precursor and fragment ions were acquired at good MMA (&lt;20 ppm). The IMS-multiplexed CID TOF-MS approach was validated using a mixture of peptides and a tryptic digest of BSA. By aligning the precursor and fragment ion drift time profiles, an MMA of ±15 ppm for precursors and fragments, and the requirement of having greater than 3 unique fragments per unique precursor, 20 unique BSA tryptic peptides were confidently identified in a single IMS separation. On average, each peptide sequence was corroborated with 14 unique fragments. The peptide level false discovery rate of &lt;1% was determined when matching IMS-multiplexed CID-TOFMS features against a decoy database composed of tryptic peptides of glycogen phosphorylase (PYGM) without use of liquid phase separation (e.g., LC). Incorporating IMS information for precursors and fragments and a high MMA for fragments decreased the FDR by a factor of &gt;35 as compared to that obtained using the MMA information only. The developed IMS-multiplexed CID-TOF-MS approach provides high throughput, high confidence identifications of peptides from complex mixtures and, will be applied to identification of LC-IMS-TOF-MS features, which can only be detected due to separation in the IMS drift time domain. 
     CONCLUSIONS 
     Results demonstrate, that precursor ions activated inside an collision cell that combines an axial DC-electric field and RF-focusing produces abundant fragment ions which are radially confined within the RF-focusing field. In RF-heating mode, a dipolar DC-displacement pulse applied into one pair of the segmented quadrupole rods provides radial displacement of ions from the center ion channel axis. When radially displaced, ions gain energy from the RF-field, which increases the temperature of the ions and leads to dissociation of the ions. In collision-induced dissociation (CID) mode, precursor ions are collisionally activated in a locally increased axial DC field inside the focusing RF field. After collision and fragmentation, ions are collisionally cooled at high pressure and focused into the quadrupole axis, resulting in high transmission of fragmented products through the spectrometer interface to the mass spectrometer. In another variation of the approach, ion dissociation can be induced by a combination of collision-induced dissociation and RF-heating. 
     While a number of embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The appended claims, therefore, are intended to cover all such changes and modifications as they fall within the true spirit and scope of the invention.