WO2000077824A1

WO2000077824A1 - Mass spectrometer for molecular structural analysis using surface induced dissociation

Info

Publication number: WO2000077824A1
Application number: PCT/US2000/016179
Authority: WO
Inventors: Charles D. Martin; Gary L. Samuelson; Edward E. Owen; Vicki H. Wysocki; Arpad Somogyi; Eugene N. Nikolaev; Chungang Gu
Original assignee: Jeol Usa, Inc.; The University Of Arizona
Priority date: 1999-06-14
Filing date: 2000-06-13
Publication date: 2000-12-21

Abstract

An MS or MS/MS surface induced dissociation method comprises the steps of gating or pulsing a packet of precursor ions to a decelerating field, allowing the packet of precursor ions to pass out of the decelerating field and to strike a surface to induce dissociation of the precursor ions and the formation of product ions, applying a pulsed electric field to direct the product ions away from the surface and into an accelerating field, and displaying a time of flight mass spectrum from the signals.

Description

MASS SPECTROMETER FOR MOLECULAR STRUCTURAL ANALYSIS USING SURFACE INDUCED DISSOCIATION

BACKGROUND OF THE INVENTION It is useful for those skilled in mass spectrometry to be able to select and then dissociate molecular ions, and then to identify the mass of the constituent product ions. This is referred to as tandem mass spectrometry or MS/MS. A first spectrometer (MSI) produces and selects precursor ions. Ion selection is followed by ion activation, which causes ion dissociation. A second spectrometer (MS2) then analyzes the resulting product ions. Through dissociation and product ion mass analysis, it is possible to identify the parent molecule and to determine its structure, e.g. the amino acid sequence of oligopeptides.

Tandem mass spectrometry plays an essential role in structural analysis of a wide variety of compounds including biomolecules, such as peptides, proteins and oligonucletides . Surface induced dissociation (SID) is a method of forming product ions that has a dissociation efficiency of many tens of percent. The use of SID is an important improvement in MS/MS performance. However, previous experiments with Time of Flight (TOF) SID have been limited by the lack of production of high mass product ions on many samples. Moreover, resolution has been poor.

It has been observed by Cornish et al. in U.S. Patent No. 5,464,985 that a curved voltage field within a reflectron of a TOF mass analyzer reduces energy dependency in the reflected flight time. In fact, one type of curve shape, a parabolic curve of the form V = kx², is totally independent of all energy variations. However, there is no free-field drift region allowed in the focusing property of the parabolic reflector. SID Tandem mass spectrometry (MS/MS) using a time of flight (TOF) spectrometer for the separation of product ions is described in Williams et al. U.S. Patent No. 5,144,127. The method and apparatus described resulted in poor resolution and dissociation efficiency only somewhat better than achievable with collision induced dissociation (CID) methods. Pulsed ion sources have been used in single mass spectrometry experiments as early as 1955, W.C. Wiley and I.H. McLaren, Rev. Sci. Instrum.. 26 (1955) p. 1150, and are common today such as in LD-TOF, MALDI-TOF and ESI-TOF.

SUMMARY OF THE INVENTION This invention relates to methods and apparatus for surface induced dissociation (SID) MS or MS/MS experiments using pulsed extraction from the SID surface. MS refers to a single mass spec analysis, and MS/MS refers to a tandem mass spec analysis. Briefly, according to this invention, there is provided an MS and/or MS/MS surface induced dissociation method comprising the steps of: a) using a first mass spectrometer, or some combination of ion optics devices, to generate and select precursor ions, b) gating or pulsing a packet of precursor ions to a decelerating field, c) allowing the packet of precursor ions to pass out of the decelerating field and to strike a surface to induce dissociation of the precursor ions and the formation of product ions, d) applying a pulsed electric field to direct the product ions away from the SID surface and into an accelerating field, e) optionally further permitting the product ions passed out of the accelerating field into a field free region, f) detecting the arrival of product ions dispersed by mass at the end of the field free region and generating a signal corresponding thereto, or g₎ detecting the arrival of product ions dispersed by mass at the end of the accelerating field and generating a signal corresponding thereto, and h₎ displaying a time of flight mass spectrum from the signals.

The pulsed electric field may be timed to immediately follow the arrival of the packet of precursor ions at the SID surface. According to one embodiment, the pulsed electric field is delayed for a period of time following the arrival of the pulse of precursor ions at the SID surface to permit additional spontaneous dissociation of precursor and product ions.

According to a preferred embodiment, the same electric field is used for both deceleration of the precursor ions and acceleration of the product ions, for example, by use of a reflectron. Still further, according to this invention, there is provided an MS and/or MS/MS surface induced dissociation system comprising: a) a first mass spectrometer, or other combination of ion optics devices, for generating and selecting precursor ions of a selected mass, b) a timing device for control of the timing and duration of the required electric field pulsing, c) electrodes and power supply therefor gating a packet of precursor ions to the decelerating field, d) electrodes and power supply therefor defining a decelerating field, e) a surface at the end of the decelerating field for inducing dissociation of the precursor ions and the formation of product ions, f) electrodes and power supply therefor defining an accelerating field, g₎ a power supply for applying a pulsed electric field to direct the product ions away from the surface and into the accelerating field, h) optionally, a field free region beyond the accelerating field, i₎ a detector for detecting the arrival of product ions dispersed by mass in the accelerating and optional free field regions generating signals corresponding thereto, and j₎ a monitor, computer or printer for displaying a time of flight mass spectrum from the signals.

A timing device may time the pulsed electric field to immediately follow the arrival of the packet of precursor ions at the surface. According to a preferred embodiment, the timing device may delay the pulsed electric field for a period of time following the arrival of the pulse of precursor ions at said surface to permit additional spontaneous dissociation of precursor and product ions. Preferably, the decelerating and accelerating fields are superimposed upon one another as, e.g., by use of a reflectron in a TOF system, or by use of other suitable ion optic devices.

Many of the elements of an SID system, according to this invention, are present in a CID-TOF system. In a preferred embodiment this is a system that can perform both

SID and CID experiments with only minor adjustments. Both

CID and SID are powerful methods for structural analysis because they provide "fragment rich" tandem mass spectra A common feature of both methods is that the selected precursor ions are activated by collisions either with gaseous targets (CID) or solid surfaces (SID) , for example, self-assembled onolayers prepared on gold surfaces. There are, however, some noticeable differences between CID and SID. High energy CID is used under approximately single collision conditions at a fixed laboratory collision energy that is determined by the acceleration voltage of, for example, a sector instrument. Practically, the only parameter that can be changed to alter the relative abundance of fragment ions in high energy CID is the collision gas used (e.g., He, Ar, Xe, C0₂, and N₂ ) . On the other hand, SID is a very flexible ion activation method with respect to both the laboratory collision energy and the target (surface type) . In addition to the linear relationship between the internal energy and SID collision energy, the internal energy distribution provided by SID is narrow relative to that produced by high and low energy CID. The easy and controllable change of the SID internal energy allows determination of sharp fragmentation efficiency curves that have been used to reveal mechanisms of peptide fragmentation.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plan view of a sector hybrid TOF instrument suitable for practicing the methods of this invention;

Fig. 2 is a schematic diagram for explaining the difference in the operation of the TOF section during CID and SID modes;

Fig. 3 is a diagram illustrating voltage distribution during the CID mode; Fig. 4 is a CID TOF spectrum of Cs⁺(CsI)₆ clusters;

Fig. 5 is a CID TOF spectrum of singly protonated Angiotensin II; Fig. 6 is a graph illustrating resolution as a function of product ion mass for the spectrum displayed in Fig. 5;

Fig. 7 is an SID TOF spectrum of Cs⁺(CsI)₆ clusters; Figs. 8a and 8b are SID TOF spectra of leucine enkephalin at different extraction delay times;

Fig. 9 is a SID TOF spectrum focusing on the b4/y3 doublet of leucine enkephalin;

Fig. 10a is an SID TOF spectrum of singly charged Angiotensin II;

Fig. 10b is an SID TOF spectrum of singly charged Angiotensin II made with an Extrel double quadrupole MSI; Fig. 11 is a fragmentation efficiency curve for single charged leucine enkephalin obtained by SID TOF; Figs. 12a and 12a' show 5 keV SIMS spectra of a freshly prepare fluorinated alkanethiolate self-assembled monolayer (SAM) surface; nF.sr.RTPTION OF A PREFERRED EMBODIMENT

Referring to Fig. 1, a sector-time-of-flight instrument is capable of both CID and SID experiments. A more detailed schematic diagram of the TOF system with a

CID cell and SID surface as well as an ion gate, buncher, reflectron and lenses is shown in Fig. 2.

The sector portion of the instrument comprises an ion source, main slit, two quadrupole lenses, an electric field sector, another quadrupole lens, beta slit, magnetic field selector, and collector slit. For traditional sector experiments, a point detector may be used. The particular configuration of the sector instrument, or a preference for the use of a sector instrument for MSI, is not a feature of this invention. The TOF (MS2) instrument is comprised of dual quadrupole lenses, an ion gate, an ion buncher, a collision cell, a coaxial detector, a reflectron, and an SID surface. The buncher and the CID collision cell are only used for CID experiments. The SID surface may be coated with a thin film such as a fluorinated self-assembled monolayer (SAM), to improve dissociation efficiency and reproducibility.

In the case where a sector instrument is used for MSI the ion beam, upon entering the TOF vacuum chamber, is focused in space by the pair of quadrupole lenses in such a way that a rectangular (slit) type beam leaving the sector instrument is "squeezed" into a circle type beam prior to the CID collision cell region. The space focusing is the same for both CID and SID. An advantage of the invention is that the same ion optics arrangement is used for both CID and SID experiments.

The reflectron is constructed of multiple, closely spaced rings or plates. Between plates resistors are electrically attached for biasing the assembly with the desired voltage curve. Each plate is a 2.5 inch diameter disc with a 1.25 inch diameter center hole. The plates are spaced 0.250 inches apart. In a dual mode system design, when SID experiments are being conducted, it is not necessary to use a non-linear reflectron field. A linear reflectron field is suitable with some loss of resolution in the SID mode. A linear reflectron can be used in the CID mode by stepping the reflectron voltages in multiple experiments to cover the needed mass range, with some loss in sensitivity. A reflectron is not an essential part of the invention for SID experiments as described in this invention. To those practiced in the science it should be obvious that any suitable ion optics device can be used to decelerate the precursor ions and to accelerate the product ions. In a system design of this invention providing for dual use for CID and SID experiments an offset parabolic reflectron is a useful tool to optimize resolution and sensitivity in both modes. The SID surface is placed at a point just beyond the end of the reflectron. When performing SID experiments the precursor ions are decelerated and permitted to pass through the reflectron and to strike a surface placed just beyond the end of the reflectron, with collision energies of a few tens of eV. A large percentage of the ions that strike the surface will dissociate. After a short delay to allow dissociation, the ions are pulsed away from the surface and are accelerated back through the reflectron, coming into temporal focus at the detector located in front of the reflectron. In the case of a perfect parabolic reflectron the temporal focus point is the entrance electrode of the reflectron. Ions can also be brought into focus with a simple linear acceleration region, at a point determined by the acceleration voltage, the SID region pulse voltage, and the geometry of the ion optics.

By placing the TOF chamber, and thus the TOF ion flight path, in line with the magnetic sector ion flight path, it is possible to provide for high energy CID as well as low and high energy surface collisions. For CID experiments the reflectron voltage is increased to a value slightly greater than the acceleration voltage so as to cause ions to be turned back before passing entirely through the reflectron. For SID experiments the reflector voltage is set to a value slightly less than the acceleration voltage, allowing the ions to pass completely through the reflectron and into the SID region. SID collision energy is controlled by adjustment of the reflectron voltage and the SID surface potential.

The SID-TOF instrument described herein can be utilized with other types of mass spectrometers, thus allowing conversions of other MS systems to MS/MS systems. One preferred embodiment for SID experiments is presented in Fig. 2. In this particular design, the SID surface is positioned right after the last electrode of the reflectron. The surface and reflectron are electrically isolated from each other as shown in Fig. 2. The reflectron voltage is controlled by PS1. The voltage applied to the SID surface is controlled by high voltage power supply PS2 in Fig. 2, which is connected to the surface through a 1 Megohm resistor. For SID experiments the potential difference between the last electrode of the reflector and the SID surface is adjustable to be a few tens of volts to several tens of volts. Another power supply (PS3 in Fig. 2) is connected to the surface through a high voltage isolated capacitor and a fast switch ₍Fig. 2) . This pulsed power supply is used to sweep out SID product ions from the vicinity of the surface back through the reflectron to the detector. The voltage of this configuration profile is shown in Fig. 3.

The continuous primary ion beam coming from the sector instrument is interrupted by the ion gate G which creates an ion packet spread both in space and time (t_gate ⁾ .

This ion packet is not modulated by the buncher for SID experiments as it is desirable to keep the ion packet's ions monoenergetic for SID ,experiments . After a mass dependent time of flight and deceleration, this ion packet strikes the SID surface. As a result of the collision with the surface, the precursor ions lose kinetic energy. As a consequence both primary and product ions are trapped for some time between the SID surface and last electrode of the reflectron. Potentials may be tuned to trap these ions. To obtain SID product ion spectra the ions are swept out of the SID region using a pulsed potential of a few hundreds of volts applied to the SID surface. Optimal temporal focus at the detector is achieved if the electric field profile between the last electrode of the reflectron and the surface extends the reflectron potential profile. This may be accomplished by the use of shaped electric field electrodes, or by the use of multiple electrodes with either voltage or spatial biasing. Under this condition all ions positioned between the surface and the last reflectron electrode will arrive in temporal focus at the detector. If the ions initial velocity is not zero (i.e., if they leave the surface with a given velocity) the temporal focusing at the detector will be deteriorated. Simple estimation shows that for SID ions with a mass of around 500, a resolution of 1,000 could be reached if the kinetic energy distribution is less than 0.1 eV (with a reflectron working potential of 5 kV) . The pulse out potential is applied with a time delay which must be large enough to allow all ions to hit the surface. The correct pulse out potential will energy modulate the ions so that they will arrive at the detector in temporal focus. It is also observed that SID experiments are possible without the use of an ion gate.

An advantage of the described embodiment is that the trapping time (i.e., the time allowed for dissociation) can be varied so processes with different activation energies can be recognized. Another advantage of the described embodiment is that the collision energy can be controlled by adjustment of the high voltage power supplies, PS1 and PS2.

In high energy CID experiments, the collision cell is filled with a collision gas (which, in most cases, was He, and occasionally N₂ or Xe) . After leaving the quadrupole lenses, the continuous beam is modulated both in space and energy. For space modulation, an ion gate is used which creates a packet of ions. It should be noted that a ^Λpacket' may contain one ion, several ions, or no ions. The concept of packets of ions is realized only over many thousands of cycles of the ion gate which is switched at rates of up to approximately 10 kHz. The ion gate is normally on, and is pulsed off for the correct time period to allow the ion beam to fill the buncher region with ions. This time period is determined by the length of the buncher and the velocity of the ions. (For example, under certain conditions, a 800 ns gate pulse, and a 1400 ns buncher pulse were used for singly protonated leucine enkephalin (m/z 556).) The maximum rate at which the gate is switched is limited by the TOF of a single experiment. In our experiments typical TOFs were tens of microseconds to several tens of microseconds. To modulate the energy of the ions in the packet, the voltage gradient applied to the buncher is pulsed to zero volts while the ion packet is in the buncher electric field. This electric field is ideally a non-linear field. The kinetic energy of the ion packet leaving the buncher is thus modulated, the velocities of the projectile ions are also modulated accordingly. As a consequence, after some period of time the ions will be temporally focused in a relatively sharp point in space at a given time but will have varying kinetic energies. The reflectron is used to compensate for the energy distribution created by the gate-buncher combination. A parabolic field reflectron may be used which permits the detection of fragment ions without degradation of resolution (i.e., the focusing is not dependent on the mass or energy of the fragment) . Ideally a parabolic field reflectron in combination with an ion buncher will allow for a free field region between the reflectron and the detector and the reflectron and the CID collision cell.

The ions energy-focused by the reflectron are detected by an MCP detector positioned coaxially with the primary ion beam. The MCP detector assembly has an aperture in the center to allow the primary ion beam to pass through it. When reflected by the parabolic reflectron the ion beam will be radially disbursed and will strike the MCP surface that is concentric with the ion beam. For CID, typical voltages applied to the TOF elements are as follows: i) the voltage applied to the last element of the multi-element reflectron is slightly higher than the actual acceleration voltage (e.g., in our experiments we used 12 kV for the reflectron at 10 kV acceleration voltage), ii) gate voltage: +/- 250 V, iii) buncher voltage: 1150V) . The time during which the gate is open and the delay until the start of the buncher pulse were tuned for optimum resolution and maximum signal/noise according to the mass range of the projectile ions.

Fig. 4 shows the high energy CID spectrum of Csl cluster at m/z 1693. In this particular experiment, the peak widths for CID product ions were in the range of 15-100 ns, which provided at least unit resolution in the entire mass range. As can be seen in the insert of Fig. 4, the peak width for the fragment of m/z 912 is 32 ns, which corresponds to a resolution of 1050.

Fig. 5 shows the high energy CID spectrum of 5 singly protonated Angiotensin II generated by FAB ionization. The [M+H]⁺ parent ion was selected by the sector instrument at a sector resolution of 3,000 and collided at 10 keV with He gas at a pressure of 7.5 x 10"⁷ torr. In Fig. 6, the TOF resolution is plotted as a

10 function of CID product ion masses. Resolution is defined as the mass of the peak divided by the half height mass width of the peak. This plot demonstrates that unit resolution is achievable in the entire product ion mass range.

15_. The SID spectrum of the Csl cluster at m/z 1693 is shown in Fig. 7. The resolution for the fragment ions is 740 which is slightly lower than in CID (Fig. 4) . This is a significant improvement over the TOF-SID resolution reported previously. The 30 eV SID spectra of leucine

20 enkephalin obtained at different extraction delay times are shown in Figs. 8a and 8b. The delay time in Fig. 8a is 4 μs, and it is 24 μs in Fig. 8b. There are characteristic differences between the two spectra. By increasing the extraction delay time, the relative intensities of lighter

25 fragment ions are smaller than those of heavier fragments. This is believed to be in part due to the differences in kinetic energy of SID product ions of different masses: the lower the mass, the higher is the ion kinetic energy, so lighter ions can escape from the surface region faster than

30 heavier ions.

Initially, sputtered peaks from the fluorinated alkanethiolate SID surface (m/z 31, and 69) were seen in the spectra. The sputtered ions were proved to be the result of surface sputtering by high energy neutrals formed in the last free field region of the sector (between the magnet and the entrance slit of the TOF chamber) . Neutrals will not be modulated by the ion gate of the TOF system. As a consequence, they continuously hit the surface and product ions are accumulated in the SID surface/reflectron region and swept out, together with "real" SID ions. The intensities of these neutral product ions are dependent on the reflectron and surface voltages, and are proportional to the primary ion beam intensity. The neutral-generated product ions were eliminated by turning the TOF chamber axes by about 2° with reference to the last field free region of the sector. To demonstrate resolution in the SID mode of . operation on a peptide sample, the b4/y3 doublet of leucine enkephalin (at m/z 278 and 279, respectively) is presented in Fig. 9. The calculated resolution is 760. Note that the monoisotopic parent ion was selected by the sector instrument, so no ¹³C contamination is present in m/z 279.

SID spectra of singly charged Angiotensin II are shown in Figs. 10a and 10b. The SID spectrum of Figure 10a was obtained without changes in the hardware which was also used to acquire the CID spectrum in Fig. 5. Only voltage settings are changed. In the SID experiment the surface potential was pulsed by 600 volts. The comparison of the high energy CID spectrum in Fig. 5 and the SID spectrum of Fig. 10a indicates that there is no high mass discrimination in the SID setup. The fragmentation efficiency curves for singly charged leucine enkephalin obtained on a bare' gold and on a fluorinated alkanethiolate SID surface are shown in Fig. 11. The results obtained indicate that the SID instrument can be used to obtain fragmentation efficiency curves that are of particular importance to study energetics of peptide fragmentation. The efficiency of the SID process enables the use of much smaller quantities of samples than needed for the CID experiment to obtain the same signal-to-noise ratio.

Another advantage of the invention is that it can be used to characterize SID surfaces by secondary ion mass spectrometry (SIMS) . The characterization of self-assembly monolayers (SAMs) by ion-surface collision have been proved to be a useful tool. In these experiments, a projectile ion generated in the sector ion source is selected and collided with the surface with relatively high kinetic energy. For example, the sector acceleration voltage is set at lOkV and surface potential at 5 kV to achieve a 5 kV collision (for singly charged ions) . Primary ions modulated by the gate strike the surface and produce secondary sputtered ions. Secondary ions are extracted from the region between the surface and. the first reflectron electrode by elevating the surface potential in exactly the same way as in the case of SID fragment ions. Figs. 12a and 12a' show the SIMS spectrum of a fluorinated SAM surface sputtered by protonated leucine enkephalin generated by FAB ionization in the source of the sector instrument.

An in-line sector-TOF tandem mass spectrometer is disclosed herein. The unique features of this embodiment of the invention are that it makes possible both high energy CID experiments and SID experiments without changes in the hardware. In addition, it permits surface characterization by SIMS measurements. It has been demonstrated that the resolution is more than unit resolution over a relatively wide mass range for both CID and SID fragment ions. CID and SID spectra have been obtained for a variety of precursor ions generated by various ionization methods.

To those practiced in the science it will be obvious that the described invention can be applied to other types of mass spectrometers, for example, ESI-TOF and MALDI-TOF. Comparative experiments have shown that, for protonated peptides, the SID sensitivity (on SAM surfaces) is higher by about an order of magnitude than that of the CID sensitivity. The comparison of CID and SID spectra has also indicated that there is no discrimination of the high mass SID product ions. This is an important advantage of the invention, especially for peptide sequencing.

According to a preferred embodiment, the shape of the voltage curve of the reflectron is a modified parabolic function which represents plate voltage versus plate distance from the reflector entrance. This function is obtained by defining a new origin that is offset from the initial origin of a pure parabolic function.

Having thus described our invention with the detail and particularity required by the Patent Laws, what is desired to be protected by Letters Patent is set forth in the following claims.

Claims

WHAT IS CLAIMED IS:

1. An MS or MS/MS surface induced dissociation method comprising the steps of: a) using a first mass spectrometer, or other combination of ion optic devices, to generate and/or select precursor ions of a selected mass, b) gating or pulsing a packet of precursor ions to a decelerating field, c) allowing the packet of precursor ions to pass out of the decelerating field and to strike a surface to induce dissociation of the precursor ions and the formation of product ions, d) applying a pulsed electric field to direct the product ions away from said surface and into an accelerating field, e) optionally permitting the product ions passed out of the accelerating field into a field free region, f) detecting the arrival of product ions dispersed by mass at the end of the field free region, generating a signal corresponding thereto, or g) detecting the arrival of product ions dispersed by mass at the end of the accelerating field, generating a signal corresponding thereto, and h) displaying a time of flight mass spectrum from said signals.

2. The method according to claim 1 wherein the pulsed electric field is timed to immediately follow the arrival of the pulse of precursor ions at said surface.

3. The method according to claim 1 wherein the pulsed electric field is delayed for a period of time following the arrival of the pulse of precursor ions at said surface to permit additional spontaneous dissociation of precursor and product ions.

4. The method according to claim 1 wherein the same field is used for both deceleration of the precursor ions and acceleration of the product ions.

5. The method according to claim 4 wherein the field is produced by a reflectron.

6. The method according to claims 4 and 5 wherein the field is linear.

7. The method according to claims 4 and 5 wherein the field is nonlinear.

8. The method according to claims 4 and 5 wherein the field is an offset parabolic field and the arrival of the product ions is detected at the temporal focus of the field.

9. An MS or MS/MS surface induced dissociation system comprising: a) a first mass spectrometer, or other combination of ion optics devices for generating and/or selecting precursor ions, b) means for defining a decelerating field, c) means for gating a pulse of precursor ions to the decelerating field, d) a surface at the end of the decelerating field for inducing dissociation of the precursor ions and the formation of product ions, e) means for defining an accelerating field, f) means for applying a pulsed electric field to direct the product ions away from said surface and into the accelerating field, g) means for defining a field free region beyond the accelerating field, h) means for detecting the arrival of product ions dispersed by mass in the accelerating field and field free region, at the end of the accelerating region, or at the end of the free field region, and generating signals corresponding thereto, and i) means for displaying a time of flight mass spectrum from said signals.

10. The system according to claim 9 wherein the pulsed electric field is timed to immediately follow the arrival of the pulse of precursor ions at said surface.

11. The system according to claim 9 wherein the pulsed electric field is delayed for a period of time following the arrival of the pulse of precursor ions at said surface to permit additional spontaneous dissociation of precursor and product ions.

12. The system according to claim 9 wherein the decelerating and accelerating fields are superimposed upon one another.

13. The system according to claim 12 wherein the decelerating and the accelerating is the same field.

14. The system according to claims 12 or 13 wherein the means for defining the decelerating and accelerating fields is a reflectron.

15. The system according to claim 13 wherein the field is linear.

16. The system according to claim 13 wherein the field is nonlinear.

17. The system according to claim 13 wherein the field is an offset parabolic field and the means for detecting is positioned at the focuses of the field.

18. A method of high efficiency SID fragmentation for product ion analysis using a pulsed or gated ion source of precursor ions and a pulsed SID region.

19. A method of using the same ion optics components for both CID and SID experiments ^'" in a single MS/MS system.