Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/26—Mass spectrometers or separator tubes
  • H01J49/34—Dynamic spectrometers
  • H01J49/40—Time-of-flight spectrometers
  • H01J49/405—Time-of-flight spectrometers characterised by the reflectron, e.g. curved field, electrode shapes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/004—Combinations of spectrometers, tandem spectrometers, e.g. MS/MS, MSn
  • H01J49/0045—Combinations of spectrometers, tandem spectrometers, e.g. MS/MS, MSn characterised by the fragmentation or other specific reaction
  • H01J49/005—Combinations of spectrometers, tandem spectrometers, e.g. MS/MS, MSn characterised by the fragmentation or other specific reaction by collision with gas, e.g. by introducing gas or by accelerating ions with an electric field

Definitions

• Mass spectrometers are commonly used for the determination of the mass of analyte molecules.
• ionized molecules are typically either created or introduced into a high vacuum chamber and accelerated to a known kenetic energy. Magnetic fields and electric fields are then used in various methods and fashions for mass selection, mass filtering, and thereby mass determination of the ionized molecules.
• magnetic sector, time-of-flight (TOF) ion trap, quadrupole, and ion cyclotron resonance instruments.
• TOF time-of-flight
• ion trap ion trap
• quadrupole quadrupole
• ion cyclotron resonance instruments There are also available instruments that are combinations of the various techniques of mass analysis.
• a magnetic field (magnetic sector) mass analyzer is scanned over a mass range of interest causing an ion beam output spectrum of mass versus magnetic field intensity.
• ESA electrostatic analyzer
• Scanning of the magnetic fields and electric fields is a relatively slow process resulting in low efficiency as the ionization is typically, although not always, a continuous process.
• Time-of-flight instruments have a significant duty cycle advantage over scanning instruments which require a much longer time period to scan the selected mass range.
• MSI mass analyzer wherein the analyte precursor molecule is mass analyzed and selected, a dissociation region wherein the mass selected precursor ion is collided with a gas, photons, or a surface, thereby causing dissociation of the precursor ions, and an MS2 mass analyzer wherein the resulting product ions are mass analyzed.
• MS/MS spectrometry or tandem mass spectrometry. Tandem mass spectrometry plays an essential role in the structural analysis of a wide variety of compounds including biomolecules, such as peptides, proteins, and oligonucleotides .
• CID Collision Induced Dissociation
• MS/MS instruments have until recently been high performance tandem sector instruments. These instruments tend to be large and expensive, and, due to the scanning nature of sector instruments, the product ion collection efficiency has been very low.
• This type of reflectron can correct for very large KE differences in the temporal focusing of ions.
• a disadvantage in using a parabolic field reflectron is that the spatial focal point of such a reflectron is located exactly at the entrance to the reflectron.
• an offset parabolic field reflectron is introduced. Use of an offset parabolic field moves the reflectron spatial focal point beyond the entrance of the reflectron, thereby providing for field free regions to exist between the reflectron and its focal point.
• Cotter, Cornish, and Musselman proposed the use of a curved field reflectron in a tandem sector/TOF instrument, however, a method of selection and focusing of the analyte precursor ions was not considered.
• field free regions may be defined in front of the reflectron. In TOF systems, these field free regions are commonly referred to as LI and L2.
• ion flight times for a given mass are not completely energy independent.
• ion flight times of a given mass are completely energy independent, an important feature of this invention.
• an offset parabolic field reflectron is used to achieve very high mass accuracy and resolution over the entire MS2 product ion mass range, above a low energy threshold determined by the offset value.
• mass spectrometers e.g., MALDI-TOF (Matrix Assisted Laser Desorption) , or ESI-TOF (Electro Spray Ionization) instruments could, and have been, substituted for the sector instrument as MSI.
• MALDI-TOF Microx Assisted Laser Desorption
• ESI-TOF Electro Spray Ionization
• a tandem mass spectrometry method with collision induced dissociation comprising the steps for: a) using a first mass spectrometer to select precursor ions of a selected mass, b) forming a packet of precursor ions, c) assigning a focusing energy to each packet of precursor ions in the ion buncher so as to bring the ions into temporal focus at some point in space, d) fragmenting the selected precursor ions near the spatial focus point to form product ions, e) passing the precursor and product ions into an offset parabolic field ion mirror (reflectron) for providing TOF dispersion among product ions of differing mass-to-charge ratios while maintaining near zero flight time dispersion (at the focal point) among product ions of the same mass-to-charge ratios but having large energy differences, and f) detecting the arrival times of the precursor and product ions.
• CID collision induced dissociation
• the precursor and product ions pass through a field free region.
• the packets of precursor ions are formed by assigning a focusing energy pulse to eject ions from an ion source region in MSI or gating a pulse of near mono-energetic precursor ions to an ion buncher.
• a tandem mass spectrometry apparatus comprising: a) a first mass spectrometer for selecting precursor ions of a predefined mass, b) apparatus for forming ion packets, preferably, a device for assigning a focusing energy pulse in the ion source, or an ion gate for forming a packet of precursor ions, alternatively, an ion buncher for applying a focusing pulse, c) a collision chamber for fragmenting the bunched precursor ions near the spatial focus point so as to form product ions, d) an ion mirror (reflectron) for providing TOF dispersion among product ions of differing mass-to-charge ratios while maintaining near zero flight time dispersion at the detector among product ions of the same mass-to-charge ratio and having large energy differences, and e) a detector for detecting the arrival times of the precursor and product ions.
• an ion buncher is provided for spatially focusing a mono-energetic pulse of precursor ions.
• a field free region is provided through which the precursor and product ions pass.
• the bunching device is capable of precisely focusing relatively long ion beam pulses or clusters, thereby increasing the duty cycle, and therefore sensitivity, of a measured signal. This requires special means for providing velocity compensation across the ion path region. This type of buncher is referred to herein as a long buncher.
• the ion mirror is provided with a uniquely-shaped voltage distribution which permits significant flight time dispersion between product ions of differing mass-to-charge ratios while maintaining near zero flight time dispersion among fragments of the same mass-to- charge ratios having large energy differences.
• This energy independence is necessary due to the large energy distribution imparted by the focusing pulse to the precursor ions.
• Ions generated in an ESI-TOF or MALDI-TOF source may be analyzed in a similar fashion without the use of a an ion buncher where the focusing voltage pulse is applied to the precursor ions in the source.
• Fig. 1 is a schematic diagram of a hybrid sector-TOF mass spectrometer according to the present invention
• Fig. 2 is a schematic diagram for showing the TOF portion of the instrument according to the present invention
• Fig. 3 is a diagram for illustrating the focus of ions of different mass in the ion mirror and in the field free region as a function of the focusing pulse voltage;
• Fig. 4(a) is a SIMION electric field potential diagram of the offset parabolic field reflectron according to the present invention
• Fig. 4(b) is an expansion of the offset region of the electric field potential diagram of the offset parabolic field reflectron according to the present invention
• Fig. 5 is a section view of a shaped field two-plate ion buncher according to the present invention.
• Fig. 6 shows the high resolution (15,000) spectrum of the Csl cluster at 3510 amu, a precursor ion
• Fig. 7 shows the CID spectrum of Csl 652 amu cluster, with a mass accuracy of +/- 0.04 amu and showing high resolution precursor and fragment ions acquired with the present invention
• Fig. 8 shows the high energy CID spectrum of Leucine Enkephalin (556.3 amu) generated by fast atom bombardment (FAB) ionization acquired with the present invention.
• Figs. 1 and 2 illustrate the schematic diagrams of the in-line sector/TOF instrument.
• Fig. 1 illustrates a hybrid sector-TOF mass spectrometer.
• the ion source 29 passes ions to quadrupole lens 26 with inlet and alpha slits 25 and 27 at each end thereof.
• the ions next pass through magnetic sector 24.
• the ions proceed on through lens 23 and slit 22 before entering the electric sector 21 and pass out through collector slit 20.
• the details of the sector instrument are not part of this invention.
• the precursor ions, mass, and energy selected by the sector instrument enter the TOF analyzer through the exit slit 1 (see Fig. 2) of the sector instrument.
• Methods of ionization employed include fast atom bombardment (FAB) , electrospray ionization (ESI) , and electron impact (El) .
• FAB fast atom bombardment
• ESI electrospray ionization
• El electron impact
• the ion beam is spatially focused (shaped) by a pair of quadrupole lenses 5.
• the ion beam is gated by a bipolar ion gating device 3 into short packets.
• the ion gate is normally biased so that all ions are deflected, which is referred to herein as the ion gate ON mode.
• the ion gate voltage provided by power supply 7 HV pulser
• HV pulser is rapidly pulsed off for a short period of time sufficient in duration to allow the ion buncher 6 to fill with precursor ions.
• the time required to fill the buncher is dependent on the velocity of the precursor ions and the length of the buncher and is, therefore, dependent on the acceleration voltage and the mass-to-charge (m/z) of the precursor ions .
• a long buncher consists of two main parts. As shown in Fig. 5, one electrode 51 is a totally enclosed, grounded can or cylinder. A cup-shaped exit electrode 52 is fixed by spacers 54 within the can. The inner electrode positioned within the can and the dimensions create the required field shape. Grounding screens are attached to the entrance and exit holes of the can thus totally isolating the inner field from the external environment. Field simulations confirm that the field shape provided by this design corresponds to the mathematically derived shape required for a linear velocity spread within the ion cluster.
• the shaped field two-plate buncher modulates the ion energy in such a way that the ions arriving first lose more energy than those arriving later.
• the entrance electrode of the buncher is at ground potential and the exit electrode of the buncher is held at a fixed voltage during the period of time that the buncher is filling with ions, thus decelerating the ions by a value determined by the voltage potential applied to the exit electrode and the penetration depth of the ion into the buncher.
• the exit electrode is pulsed to zero volts, thus assigning a modulation value to the ions within the buncher that is determined by each ion's position within the buncher.
• the ions in the lead of the ion packet will then have a velocity less than the ions in the back of the packet.
• the point at which the trailing ions overtake the lead ions will be the temporal focal point, and is determined by the magnitude of the modulating potential.
• this focal point should be the location of the coaxial micro channel plate (MCP) detector 18 (see Figs. 1 and 2) , which is also the spatial focal point (virtual source) of the reflectron 17.
• MCP coaxial micro channel plate
• the buncher focal point is independent of the mass of the ions.
• the buncher could be loaded while both electrodes were biased at zero volts, and then a back electrode could be pulsed up so as to modulate the energies of the ions inside the buncher.
• modulating energy assignments achieved in such a fashion are similar to the methods described by Wiley et al .
• U.S. Patent No. 2,839,687 wherein the modulating energy assignments are imposed by an electric field pulse applied in the ion source .
• the collision cell 13 is a small volume of a few millimeters in length and is located in a differentially pumped section of the vacuum chamber 4. In high energy CID experiments, the collision cell 13 is filled with a collision gas from source 14.
• the position of the CID cell is, in practice, not critical. Not regarding a very small kinetic energy release during dissociation, the velocity of the fragment ions will be very nearly the same as that of the parent ion. Therefore, the position of the temporal focusing point for both the parent and fragment ions is the same. It is required to locate the CID cell far enough away from the reflectron entrance so as to reduce ion dissociation in the reflectron region. Only ions that dissociate in the field free region before the reflectron will be correctly focused.
• the reflectron is constructed of multiple closely spaced discs, each with center apertures to allow ion transmission. Between each disc, resistors are electrically attached for biasing the assembly with the desired voltage curve.
• 100 discs of 3 inch diameter were spaced 0.250 inches apart, and each disc had a center aperture of 1.5 inches.
• the first and last disc apertures were covered with a conductive fine mesh screen material so as to minimize electric field distortions at the entrance and exit electrodes of the reflectron.
• the offset voltage bias is applied between the first and second reflectron disc, with the remaining discs biased to describe a parabolic rise in voltage.
• V 0 and d 0 allow a field free region to exist in the front of the reflectron such that ions of a given mass-to-charge ratio traveling through both the field free region and the reflectron will arrive at the spatial focal point in a time substantially independent of energy. Without the offset parameters, the energy independent focal point would be at the reflectron entrance .
• the offset parabolic field reflectron 17 is used to separate the product ions by mass and to compensate for the considerable energy distribution of the precursor and parent ions created by the gate-buncher combination.
• the offset parabolic field reflectron permits the detection of fragment ions over a large mass range without degradation of resolution because the focal length of the reflectron is not dependent on the mass of the fragment.
• Product ions of a very wide mass range will all have the same spatial focal point .
• the offset parabolic reflectron design provides the unique ability to focus the product ions without regard to the modulation energy spread introduced by the ion buncher or pulsed ion source, while at the same time providing for a field free region between the reflectron and the detector.
• the inclusion of a field free region in the temporal focal length of the offset parabolic reflectron allows for the space required to ideally locate the CID cell and the coaxial MCP detector.
• LI is defined as the distance between the ion source and the reflectron entrance
• L2 is defined as the distance between the reflectron entrance and the plane of detection.
• An example ion source would be a collision cell where fragmentation takes place.
• V 0 and d 0 There is a compromise associated with the selection of values V 0 and d 0 .
• V 0 and d 0 As the sum of LI plus L2 becomes longer, not all of the ion energy range remains time independent. A threshold occurs wherein ions of an energy level below this threshold return sooner than the bulk of energy independent ions. It is desirable, therefore, to keep the LI and L2 distances as short as possible and still allow space for the source and detection devices.
• V 0 and d 0 may be determined by an ion flight time simulation routine.
• Figs. 4(a) and 4(b) illustrate the reflectron focus versus buncher focus for selected masses.
• the ions temporally focused by the reflectron are detected by an MCP (micro channel plate) detector located at the spatial focal point of the reflectron and positioned coaxially with the primary ion beam.
• MCP micro channel plate
• the coaxial MCP detector assembly has a center aperture to allow the primary ion beam to pass through it.
• This flat anode detector design is capable of producing precursor ion sub-nanosecond fwhm peaks in the instrument.
• the acquisition system included a 1 GHz, multiple stop time to digital converter (TDC) 11.
• TDC multiple stop time to digital converter
• the minimum fwhm peak detectable by the model TDC used is 3.0 ns .
• the parent ion fwhm peaks from the flat anode detector were typically on the order of 1.5 ns fwhm, thus requiring the O 00/36633
• Low noise amplifiers and low pass filters were used to provide the 100 mV threshold, 3.0 ns fwhm signal required by the TDC.
• Experiments using an 8 GHz Digital Storage Oscilloscope (DSO) revealed consistent precursor ion peaks of less than 2 ns fwhm, and precursor mass resolutions greater than 10,000 (fwhm) were routinely obtainable.
• spectra were comprised of the summed average of several thousand individual acquisitions. As the TDC operates on the principle of a start and stop signal, it has virtually zero noise, and it is therefore possible to sum many hundreds of thousands of acquisitions without the accumulation of systematic noise.
• the spectrum acquisition rate was limited only by the TOF of the precursor ion, and was typically 7-10 kHz.
• Pulse timing for the ion gate and buncher was controlled with a digital delay generator 8.
• the ion gate and buncher electrodes were held at a constant voltage and then rapidly pulsed down to zero volts through HV pulsers 7, 9.
• a PC 12 and custom TOF analysis software were used for spectra acquisition and mass analysis.
• the mass axis was calibrated externally with a simple two-point calibration using a standard, which in most experiments was Csl.
• instrument calibration e.g., with Csl
• the analyte sample was introduced into the ion source of the spectrometer and spectra was acquired.
• a one-point calibration on the analyte precursor was then used in the analyte spectra to shift the calibration constants.
• An in-line sector-TOF tandem mass spectrometer has been disclosed which the sector (MSI) instrument's continuous primary ion beam (El, FAB, ESI) is chopped by an ion gate which creates short packets of ions that are introduced into the TOF analyzer (MS2) .
• An ion buncher is used to modulate the velocities of the ions contained within the ion packet so as to bring the ions into spatial focus at the focal point of an offset parabolic field reflectron. The reflectron is then used to separate the product ions in time according to their masses, without regard to the energy spread created in the ion buncher.
• the offset parabolic field reflectron permits the mass analysis of fragment ions without degradation of resolution (the focal point of the reflectron is not dependent on the mass of the fragment) , while allowing field free regions to exist between the reflectron entrance and a coaxial detector.
• the ions, focused by the buncher-reflectron combination, are detected by a detector positioned coaxially with the primary ion beam and located at the spatial focal point of the reflectron.

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• 1999
  • 1999-12-17 JP JP2000588791A patent/JP2002532845A/ja active Pending
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