# Patent

## Title
Ion guide for mass spectrometers

## Abstract

Disclosed is an improved method and apparatus for transporting ions from a first pressure region in a mass spectrometer to a second pressure region therein. More specifically, the present invention provides a segmented ion funnel for more efficient use in mass spectrometry (particularly with ionization sources) to transport ions from the first pressure region to the second pressure region.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. application Ser. No. 10/849,730, filed May 20, 2004 now abandoned, which is a divisional application of U.S. application Ser. No. 10/407,860, filed Apr. 4, 2003 now abandoned.


FIELD OF THE INVENTION
The present invention generally relates to an improved method and apparatus for the injection of ions into a mass spectrometer for subsequent analysis. Specifically, the invention relates to an apparatus for use with an ion source that facilitate the transmission of ions from an elevated pressure ion production region to a reduced pressure ion analysis region of a mass spectrometer. A preferred embodiment of the present invention allows for improved efficiency in the transmission of ions from a relatively high pressure region, through a multitude of differential pumping stages, to a mass analyzer.
BACKGROUND OF THE INVENTION
The present invention relates to ion guides for use in mass spectrometry. The apparatus and methods for ionization described herein are enhancements of the techniques referred to in the literature relating to mass spectrometry—an important tool in the analysis of a wide range of chemical compounds. Specifically, mass spectrometers can be used to determine the molecular weight of sample compounds. The analysis of samples by mass spectrometry consists of three main steps—formation of gas phase ions from sample material, mass analysis of the ions to separate the ions from one another according to ion mass, and detection of the ions. A variety of means and methods exist in the field of mass spectrometry to perform each of these three functions. The particular combination of the means and methods used in a given mass spectrometer determine the characteristics of that instrument.
To mass analyze ions, for example, one might use magnetic (B) or electrostatic (E) analysis, wherein ions passing through a magnetic or electrostatic field will follow a curved path. In a magnetic field, the curvature of the path will be indicative of the momentum-to-charge ratio of the ion. In an electrostatic field, the curvature of the path will be indicative of the energy-to-charge ratio of the ion. If magnetic and electrostatic analyzers are used consecutively, then both the momentum-to-charge and energy-to-charge ratios of the ions will be known and the mass of the ion will thereby be determined. Other mass analyzers are the quadrupole (Q), the ion cyclotron resonance (ICR), the time-of-flight (TOF), and the quadrupole ion trap analyzers. The analyzer which accepts ions from the ion guide described here may be any of a variety of these.
Before mass analysis can begin, gas phase ions must be formed from a sample material. If the sample material is sufficiently volatile, ions may be formed by electron ionization (EI) or chemical ionization (CI) of the gas phase sample molecules. Alternatively, for solid samples (e.g., semiconductors, or crystallized materials), ions can be formed by desorption and ionization of sample molecules by bombardment with high energy particles. Further, Secondary Ion Mass Spectrometry (SIMS), for example, uses keV ions to desorb and ionize sample material. In the SIMS process a large amount of energy is deposited in the analyte molecules, resulting in the fragmentation of fragile molecules. This fragmentation is undesirable in that information regarding the original composition of the sample (e.g., the molecular weight of sample molecules) will be lost.
For more labile, fragile molecules, other ionization methods now exist. The plasma desorption (PD) technique was introduced by Macfarlane et al. (R. D. Macfarlane, R. P. Skowronski, D. F. Torgerson, Biochem. Biophys. Res Commoun. 60 (1974) 616)(“McFarlane”). Macfarlane discovered that the impact of high energy (MeV) ions on a surface, like SIMS would cause desorption and ionization of small analyte molecules. However, unlike SIMS, the PD process also results in the desorption of larger, more labile species (e.g., insulin and other protein molecules).
Additionally, lasers have been used in a similar manner to induce desorption of biological or other labile molecules. See, for example, Cotter et al. (R. B. VanBreeman, M. Snow, R. J. Cotter, Int. J. Mass Spectrom. Ion Phys. 49 (1983) 35; Tabet, J. C.; Cotter, R. J., Tabet, J. C., Anal. Chem. 56 (1984) 1662; or R. J. Cotter, P. Demirev, I. Lys, J. K. Olthoff, J. K.; Lys, I.: Demirev, P.: Cotter et al., R. J., Anal. Instrument. 16 (1987) 93). Cotter modified a CVC 2000 time-of-flight mass spectrometer for infrared laser desorption of non-volatile biomolecules, using a Tachisto (Needham, Mass.) model 215G pulsed carbon dioxide laser. The plasma or laser desorption and ionization of labile molecules relies on the deposition of little or no energy in the analyte molecules of interest. The use of lasers to desorb and ionize labile molecules intact was enhanced by the introduction of matrix assisted laser desorption ionization (MALDI) (K. Tanaka, H. Waki, Y. Ido, S. Akita, Y. Yoshida, T. Yoshica, Rapid Commun. Mass Spectrom. 2 (1988) 151 and M. Karas, F. Hillenkamp, Anal. Chem. 60 (1988) 2299). In the MALDI process, an analyte is dissolved in a solid, organic matrix. Laser light of a wavelength that is absorbed by the solid matrix but not by the analyte is used to excite the sample. Thus, the matrix is excited directly by the laser, and the excited matrix sublimes into the gas phase carrying with it the analyte molecules. The analyte molecules are then ionized by proton, electron, or cation transfer from the matrix molecules to the analyte molecules. This process (i.e., MALDI) is typically used in conjunction with time-of-flight mass spectrometry (TOFMS) and can be used to measure the molecular weights of proteins in excess of 100,000 daltons.
Further, Atmospheric Pressure Ionization (API) includes a number of ion production means and methods. Typically, analyte ions are produced from liquid solution at atmospheric pressure. One of the more widely used methods, known as electrospray ionization (ESI), was first suggested by Dole et al. (M. Dole, L. L. Mack, R. L. Hines, R. C. Mobley, L. D. Ferguson, M. B. Alice, J. Chem. Phys. 49, 2240, 1968). In the electrospray technique, analyte is dissolved in a liquid solution and sprayed from a needle. The spray is induced by the application of a potential difference between the needle and a counter electrode. The spray results in the formation of fine, charged droplets of solution containing analyte molecules. In the gas phase, the solvent evaporates leaving behind charged, gas phase, analyte ions. This method allows for very large ions to be formed. Ions as large as 1 MDa have been detected by ESI in conjunction with mass spectrometry (ESMS).
In addition to ESI, many other ion production methods might be used at atmospheric or elevated pressure. For example, MALDI has recently been adapted by Laiko et al. to work at atmospheric pressure (Victor Laiko and Alma Burlingame, “Atmospheric Pressure Matrix Assisted Laser Desorption”, U.S. Pat. No. 5,965,884, and Atmospheric Pressure Matrix Assisted Laser Desorption Ionization, poster #1121, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998) and by Standing et al. at elevated pressures (Time of Flight Mass Spectrometry of Biomolecules with Orthogonal Injection+Collisional Cooling, poster #1272, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998; and Orthogonal Injection TOFMS Anal. Chem. 71(13), 452A (1999)). The benefit of adapting ion sources in this manner is that the ion optics (i.e., the electrode structure and operation) in the mass analyzer and mass spectral results obtained are largely independent of the ion production method used.
The elevated pressure MALDI source disclosed by Standing differs from what is disclosed by Laiko et al. Specifically, Laiko et al. disclose a source intended to operate at substantially atmospheric pressure. In contrast, as depicted in FIG. 1, the source 1 disclosed by Standing et al. is intended to operate at a pressure of about 70 mtorr. In addition, as shown in FIG. 1, the MALDI sample resides on the tip 6 of a MALDI probe 2 in the second pumping stage 3 immediately in front of the first of two quadrupole ion guides 4. Using a laser 7, ions are desorbed from the MALDI sample directly into 70 mtorr of gas and are immediately drawn into the ion guides 4 by the application of an electrostatic field. Even though this approach requires that one insert the sample into the vacuum system, it has the advantage of improved ion transmission efficiency over that of the Laiko source. That is, the possible loss of ions during transmission from the elevated pressure source 1, operated at atmospheric pressure, to the third pumping region and the ion guide therein is avoided because the ions are generated directly in the second pumping stage.
Elevated pressure (i.e., elevated relative to the pressure of the mass analyzer) and atmospheric pressure ion sources always have an ion production region, wherein ions are produced, and an ion transfer region, wherein ions are transferred through differential pumping stages and into the mass analyzer. Generally, mass analyzers operate in a vacuum between 10−4 and 10−10 torr depending on the type of mass analyzer used. When using, for example, an ESI or elevated pressure MALDI source, ions are formed and initially reside in a high pressure region of “carrier” gas. In order for the gas phase ions to enter the mass analyzer, the ions must be separated from the carrier gas and transported through the single or multiple vacuum stages.
As a result, the use of multipole ion guides has been shown to be an effective means of transporting ions through a vacuum system. Publications by Olivers et al. (Anal. Chem, Vol. 59, p. 1230-1232, 1987), Smith et al. (Anal. Chem. Vol. 60, p. 436-441, 1988) and Douglas et al. (U.S. Pat. No. 4,963,736) have reported the use of AC-only quadrupole ion guides to transport ions from an API source to a mass analyzer.
In the prior art, according to Douglas et al., as depicted in FIG. 2, ionization chamber 17 is connected to curtain gas chamber 24 via opening 18 in curtain gas plate 23. Curtain gas chamber 24 is connected by orifice 25 of orifice plate 29 to first vacuum chamber 44 that is pumped by vacuum pump 31. Vacuum chamber 44 contains a set of four AC-only quadrupole mass spectrometer rods 33. Also, the vacuum chamber 44 is connected by interchamber orifice 35 in separator plate 37 to a second vacuum chamber 51 pumped by vacuum pump 39. Chamber 51 contains a set of four standard quadrupole mass spectrometer rods 41.
An inert curtain gas, such as nitrogen, argon or carbon dioxide, is supplied via a curtain gas source 43 and duct 45 to the curtain gas chamber 24. (Dry air may also be used in some cases.) The curtain gas flows through orifice 25 into the first vacuum chamber 44 and also flows into the ionization chamber 17 to prevent air and contaminants in chamber 17 from entering the vacuum system. Excess sample, and curtain gas, leave the ionization chamber 17 via outlet 47.
Ions produced in the ionization chamber 17 are drifted by appropriate DC potentials on plates 23 and 29 and on the AC-only rod set 33 through opening 18 and orifice 25, and then are guided through the AC-only rod set 33 and interchamber orifice 35 into the rod set 41. An AC RF voltage (typically at a frequency of about 1 Megahertz) is applied between the rods of rod set 33, as is well known, to permit rod set 33 to perform its guiding and focusing function. Both DC and AC RF voltages are applied between the rods of rod set 41, so that rod set 41 performs its normal function as a mass filter, allowing only ions of selected mass to charge ratio to pass therethrough for detection by ion detector 49.
Douglas et al. found that under appropriate operating conditions, an increase in the gas pressure in the first vacuum chamber 44 not only failed to cause a decrease in the ion signal transmitted through orifice 35, but in fact most unexpectedly caused a considerable increase in the transmitted ion signal. In addition, under appropriate operating conditions, it was found that the energy spread of the transmitted ions was substantially reduced, thereby greatly improving the ease of analysis of the transmitted ion signal. The particular “appropriate operating conditions” disclosed by Douglas et al. maintain the second vacuum chamber 51 at low pressure (e.g. 0.02 millitorr or less) but the product of the pressure in the first chamber 44 and the length of the AC-only rods 33 is held above 2.25×10−2 torr-cm, preferably between 6×10−2 and 15×10−2 torr-cm, and the DC voltage between the inlet plate 29 and the AC-only rods 33 is kept low (e.g., between 1 and 30 volts) preferably between 1 and 10 volts.
As shown in FIG. 3, mass spectrometers similar to that of Whitehouse et al. (“Multipole Ion Guide for Mass Spectrometry”, U.S. Pat. No. 5,652,427) use multipole RF ion guides 42 to transfer ions from one pressure region 30 to another 34 in a differentially pumped system. In this ion source, ions are produced by ESI or APCI at substantially atmospheric pressure. These ions are transferred from atmospheric pressure to a first differential pumping region by the gas flow through a glass capillary 60. Further, ions are transferred from this first pumping region 30 to a second pumping region 32 through a “skimmer” 56 by gas flow as well as an electric field present between these regions. Multipole ion guide 42 in the second differentially pumped region 32 accepts ions of a selected mass/charge (m/z) ratio and guides them through a restriction and into a third differentially pumped region 34 by applying AC and DC voltages to the individual poles of the ion guide 42.
Further, as depicted in FIG. 3, a four vacuum stage ESI-reflectron-TOF mass spectrometer, according to Whitehouse et al., incorporates a multipole ion guide 42 beginning in one vacuum pumping stage 32 and extending contiguously into an adjacent pumping stage 34. As shown here, ions are formed from sample solution by an electrospray process. Sample bearing liquid is introduced through the electrospray needle 26 and is electrosprayed or nebulization-assisted electrosprayed into chamber 28 as it exits the needle tip 27 producing charged droplets. The charged droplets evaporate and desorb gas phase ions both in chamber 28 and as they are swept into the vacuum system through the annulus 38 in capillary 60. According to the prior art system shown in FIG. 3, capillary 60 is used to transport ions from chamber 28, where the ions are formed, to first pumping region 30. A portion of the ions that enter the first vacuum stage 30 through the capillary exit 40 are focused through the orifice 58 in skimmer 56 with the help of lens 62 and the potential set on the capillary exit 40. Ions passing through orifice 58 enter the multipole ion guide 42, which begins in vacuum pumping stage 32 and extends unbroken into vacuum stage 34. According to Whitehouse et al. the RF only ion guide 42 is a hexapole. The electrode rods of such prior art multipole ion guides are positioned parallel and are equally spaced at a common radius from the centerline of the ion guide. A high voltage RF potential is applied to the electrode rods of the ion guide so as to push the ions toward the centerline of the ion guide. Ions with a m/z ratio that fall within the ion guide stability window established by the applied voltages have stable trajectories within the ion guide's internal volume bounded by the evenly-spaced, parallel rods. This is true for quadrupoles, hexapoles, octapoles, or any other multipole used to guide ions. As previously disclosed by Douglas et al., operating the ion guide in an appropriate pressure range results in improved ion transmission efficiency.
Whitehouse et al. further disclose that collisions with the gas reduce the ion kinetic energy to that of the gas (i.e., room temperature). This hexapole ion guide 42 is intended to provide for the efficient transport of ions from one location (i.e., the entrance 58 of skimmer 56) to a second location (i.e., orifice 50). Of particular note is that a single contiguous multipole 42 resides in more than one differential pumping stage and guides ions through the pumping restriction between them. Compared to other prior art designs, this offers improved ion transmission through pumping restrictions.
If the multipole ion guide AC and DC voltages are set to pass ions falling within a range of m/z then ions within that range that enter the multipole ion guide 42 will exit at 46 and be focused with exit lens 48 through the TOF analyzer entrance orifice 50. The primary ion beam 82 passes between electrostatic lenses 64 and 68 that are located in the fourth pumping stage 36. The relative voltages on lenses 64, 68 and 70 are pulsed so that a portion of the ion beam 82 falling in between lenses 64 and 68 is ejected as a packet through grid lens 70 and accelerated down flight tube 80. The ions are steered by x and y lens sets diagrammatically illustrated by 72 as they continue moving down flight tube 80. As shown in this illustrative configuration, the ion packet is reflected through a reflectron or ion mirror 78, steered again by x and y lens sets illustrated by 76 and detected at detector 74. As a pulsed ion packet proceeds down flight tube 80, ions with different m/z separate in space due to their velocity differences and arrive at the detector at different times. Moreover, the use of orthogonal pulsing in an API/TOF system helps to reduce the ion energy spread of the initial ion packet allowing for the achievement of higher resolution and sensitivity.
In U.S. Pat. No. 6,011,259 Whitehouse et al. also disclose trapping ions in a multipole ion guide and subsequently releasing them to a TOF mass analyzer. In addition, Whitehouse et al. disclose ion selection in such a multipole ion guide, collision induced dissociation of selected ions, and release of the fragment ions thus produced to the TOF mass analyzer. Further, the use of two or more ion guides in consecutive vacuum pumping stages allowing for different DC and RF values is also disclosed by Whitehouse et al. However, losses in ion transmission efficiency may occur in the region of static voltage lenses between ion guides. For example, a commercially available API/MS instrument manufactured by Hewlett Packard incorporates two skimmers and an ion guide. An interstage port (also called a drag stage port) is used to pump the region between the skimmers. That is, an additional pumping stage/region is added without the addition of an extra turbo pump, thereby improving pumping efficiency. In this dual skimmer design, there is no ion focusing device between skimmers, therefore ion losses may occur as the gases are pumped away. A second example is demonstrated by a commercially available API/MS instrument manufactured by Finnigan which applies an electrostatic lens between capillary and skimmer to focus the ion beam. Due to a narrow mass range of the static lens, the instrument may need to scan the voltage to optimize the ion transmission.
According to Thomson et al. (entitled “Quadrupole with Axial DC Field”, U.S. Pat. No. 6,111,250), a quadrupole mass spectrometer contains four rod sets, referred to as Q0, Q1, Q2 and Q3. A rod set is constructed to create an axial field (e.g., a DC axial field) thereon. The axial field can be created by tapering the rods, or arranging the rods at angles with respect to each other, or segmenting the rods as depicted in FIG. 4. When the axial field is applied to Q0 in a tandem quadrupole set, it speeds passage of ions through Q0 and reduces delay caused by the need to refill Q0 with ions when jumping from low to high mass in Q1. When used as collision cell Q2, the axial field reduces the delay needed for daughter ions to drain out of Q2. The axial field can also be used to help dissociate ions in Q2, either by driving the ions forwardly against the collision gas, or by oscillating the ions axially within the collision cell.
One such prior art device disclosed by Thomson et al. is depicted in FIG. 4, which shows a quadrupole rod set 96 consisting of two pair of parallel cylindrical rod sets 96A and 96B arranged in the usual fashion but divided longitudinally into six segments 96A-1 to 96A-6 and 96B-1 to 96B-6. The gap 98 between adjacent segments or sections is very small (e.g., about 0.5 mm). Each A section and each B section is supplied with the same RF voltage from RF generator 74, via isolating capacitors C3, but each is supplied with a different DC voltage V1 to V6 via resistors R1 to R6. Thus, sections 96A-1, 96B-1 receive voltage V1, sections 96A-2, 96B-2 receive voltage V2, and so on. This produces a stepped voltage along the central longitudinal axis 100 of the rod set 96. Connection of the R-C network and thus the voltage applied to sections 96B-1 to 96B-6 are not separately shown. The separate potentials can be generated by separate DC power supplies for each section or by one power supply with a resistive divider network to supply each section. The step wise potential produces an approximately constant axial field. While more sections over the same length will produce a finer step size and a closer approximation to a linear axial field, it is found that using six sections as shown produces good results.
For example, such a segmented quadrupole was used to transmit ions from an atmospheric pressure ion source into a downstream mass analyzer. The pressure in the quadrupole was 8.0 millitorr. Thomson et al. found that at high pressure without an axial field the ions of a normal RF quadrupole at high pressure without an axial field can require several tens of milliseconds to reach a steady state signal. However, with the use of an axial field that keeps the ions moving through the segmented quadrupole, the recovery or fill-up time of segmented quadrupoles, after a large change in RF voltage, is much shorter.
In a similar manner Wilcox et al. (B. E. Wilcox, J. P. Quinn, M. R. Emmett, C. L. Hendrickson, and A. Marshall, Proceedings of the 50th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, Fla., Jun. 2-6, 2002) demonstrated the use of a pulsed electric field to eject ions from an octapole ion guide. Wilcox et al. found that the axial electric field caused ions in the octapole to be ejected more quickly. This resulted in an increase in the effective efficiency of transfer of ions from the octapole to their mass analyzer by as much as a factor of 14.
Another type of prior art ion guide, depicted in FIG. 5, is disclosed by Franzen et al. in U.S. Pat. No. 5,572,035, entitled “Method and Device for the Reflection of Charged Particles on Surfaces”. According to Franzen et al., the ion guide 13 comprises a series of parallel rings 12, each ring having a phase opposite that of its two neighboring rings. Thus, along the axis there exists a slightly undulating structure of the pseudo potential, slightly obstructive for a good and smooth guidance of ions. On the other hand, the diffuse reflection of particles at the cylinder wall is favorable for a fast thermalization of the ion's kinetic energy if the ions are shot about axially into the cylinder. This arrangement generates, in each of the ring centers, the well-known potential distribution of ion traps with their characteristic equipotential surfaces crossing in the center with angles of α=2arctan(1⁄20.5). The quadrupole fields, however, are restricted to very small areas around each center. In the direction of the cylinder axis, the pseudo potential wells of the centers are shallow because the traps follow each other in narrow sequence. In general, the pseudo potential wells are less deep the closer the rings are together. Emptying this type of ion guide by simply letting the ions flow out leaves some ions behind in the shallow wells.
In this prior art ion guide according to Franzen, an axial DC field is used to drive the ions out, ensuring that the ion guide is completely emptied. The electric circuits needed to generate this DC field are shown in FIG. 5. As shown, the RF voltage is supplied to the ring electrodes 12 via condensers, and the rings are connected by a series of resistance chokes 14 forming a resistive voltage divider for the DC voltage, and hindering the RF from flowing through the voltage divider. The DC current is switchable, and the DC field helps to empty the device of any stored ions. With rings 12 being approximately five millimeters in diameter, resistance chokes 14 of 10 microhenries and 100 Ohms, and capacitors 16 of 100 picofarads build up the desired DC fields. Fields of a few volts per centimeter are sufficient.
A similar means for guiding ions at “near atmospheric” pressures (i.e., pressures between 10−1 millibar and 1 bar) is disclosed by Smith et al. in U.S. Pat. No. 6,107,628, entitled “Method and Apparatus for Directing Ions and Other Charged Particles Generated at Near Atmospheric Pressures into a Region Under Vacuum”. One embodiment, illustrated in FIG. 6, consists of a plurality of elements, or rings 13, each element having an aperture, defined by the ring inner surface 20. At some location in the series of elements, each adjacent aperture has a smaller diameter than the previous aperture, the aggregate of the apertures thus forming a “funnel” shape, otherwise known as an ion funnel. The ion funnel thus has an entry, corresponding with the largest aperture 21, and an exit, corresponding with the smallest aperture 22. According to Smith et al., the rings 13 containing apertures 20 may be formed of any sufficiently conducting material. Preferably, the apertures are formed as a series of conducting rings, each ring having an aperture smaller than the aperture of the previous ring. Further, an RF voltage is applied to each of the successive elements so that the RF voltages of each successive element are 180 degrees out of phase with the adjacent element(s), although other relationships for the applied RF field would likely be appropriate. Under this embodiment, a DC electrical field is created using a power supply and a resistor chain to supply the desired and sufficient voltage to each element to create the desired net motion of ions through the funnel.
Each of the ion guide devices mentioned above in the prior art have their own particular advantages and disadvantages. For example, the “ion funnel” disclosed by Smith et al. has the advantage that it can efficiently transmit ions through a relatively high pressure region (i.e., >0.1 mbar) of a vacuum system, whereas multipole ion guides perform poorly at such pressures. However, the ion funnel disclosed by Smith et al. performs poorly at lower pressures where multipole ion guides transmit ions efficiently. In addition, this ion funnel has a narrow range of effective geometries. That is, the thickness of the plates and the gap between the plates must be relatively small compared to the size of the aperture in the plate. Otherwise, ions may get trapped in electrodynamic “wells” in the funnel and therefore not be efficiently transmitted.
Similarly, the ion guide disclosed by Franzen et al. and shown in FIG. 5 must have apertures which are large relative to plate thickness and gap. Also while Franzen et al.'s ion guide can have an “axial” DC electric field to push the ions towards the exit, the DC field cannot be changed rapidly or switched on or off quickly. That is, the speed with which the DC field is switched must be much slower than that represented by the frequency of the RF potential applied to confine the ions. Similarly, the segmented quadrupole of Thomson et al. allows for an axial DC electric field. However, in Thomson et al., the field cannot be rapidly switched.
As discussed below, the ion guide according to the present invention overcomes many of the limitations of prior art ion guides. The ion guide disclosed herein provides a unique combination of attributes making it more suitable for use in the transport of ions from high pressure ion production regions to low pressure mass analyzers.
SUMMARY OF THE INVENTION
The present invention relates generally to mass spectrometry and the analysis of chemical samples, and more particularly to ion guides for use therein. The invention described herein comprises an improved method and apparatus for transporting ions from a first pressure region in a mass spectrometer to a second pressure region therein. More specifically, the present invention provides a segmented ion funnel for more efficient use in mass spectrometry, particularly with ionization sources, to transport ions from the first pressure region to a second pressure region.
In light of the above described inadequacies in the prior art, a primary aspect of the present invention is to provide a means and method for efficiently guiding ions in and through high (i.e., >=0.1 mbar) and low (i.e., <=0.1 mbar) pressure regions of a mass spectrometer. Whereas, some prior art devices function well at high pressures and other devices function well at low pressures, the ion guide according to the present invention functions efficiently at both high and low pressures. It is therefore also considered another aspect of the present invention to provide an ion funnel device which begins in one pumping region and ends in another pumping region and guides ions through a pumping restriction between the two regions. The first of said pumping regions may be a relatively high pressure (i.e., >0.1 mbar) region whereas subsequent pumping regions are lower pressure.
It is another aspect of the present invention to provide a means and method for rapidly ejecting ions from an ion guide. Ions may initially be trapped, for example in a stacked ring ion guide, and then ejected from the guide as a pulse of ions. Ejection is effected by applying a pulsed electric potential to “DC electrodes” so as to force ions towards the exit end of the ion guide. Ions might be ejected into a mass analyzer or into some other device—e.g. a collision cell.
It is yet a further aspect of the present invention to provide a means and method for performing tandem mass spectrometry experiments. Particularly, a device according to the present invention might be used as a “collision cell” as well as an ion guide. When used in combination with an upstream mass analyzer, selected ions can be caused to form fragment ions. Further, a “downstream” mass analyzer may be used to analyze fragment ions thus formed. Therefore in combination with appropriate mass analyzers a fragment ion (or MS/MS) spectrum can be obtained. Alternatively, as discussed by Hofstadler et al. (“Methods and Apparatus for External Accumulation and Photodissociation of Ions Prior to Mass Spectrometric Analysis”, U.S. Pat. No. 6,342,393) the ion guide might operate at a predetermined pressure such that ions in the guide can be irradiated with light and thereby caused to form fragment ions for subsequent mass analysis.
It is yet a further aspect of the present invention to provide a means and method for accepting and guiding ions from a multitude of ion production means. As described above, a number of means and methods for producing ions are known in the prior art. An ion guide according to the present invention may accept ions simultaneously from more than one such ion production means. For example, an elevated pressure MALDI ion production means may be used in combination with an ESI or other API ion production means to accept ions either simultaneously or consecutively. Importantly, the ion production means need not be physically exchanged in order to switch between them. That is, for example, one need not dismount the MALDI means and mount an ESI means in its place to switch from MALDI to ESI.
It is yet a further aspect of the present invention to provide a means and method to improve the calibration of a mass spectrometer and the calibration of individual of spectra produced via a mass spectrometer. According to the present invention, a first ion souce is used to produce known calibrant ions while simultaneously or in close succession a second independent ion source is used to produce analyte ions. Ions from bost sources are accepted by an ion guide according to the present invention and transported to the mass analyzer. The mass analysis results in a spectrum containing signals corresponding to both calibrant and analyte ions. The calibrant signals can then be used to better calibrate the spectrum and thereby more accurately determine the mass of the analyte ions.
Other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of the structure, and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following detailed description with reference to the accompanying drawings, all of which form a part of this specification.



BRIEF DESCRIPTION OF THE DRAWINGS
A further understanding of the present invention can be obtained by reference to a preferred embodiment set forth in the illustrations of the accompanying drawings. Although the illustrated embodiment is merely exemplary of systems for carrying out the present invention, both the organization and method of operation of the invention, in general, together with further objectives and advantages thereof, may be more easily understood by reference to the drawings and the following description. The drawings are not intended to limit the scope of this invention, which is set forth with particularity in the claims as appended or as subsequently amended, but merely to clarify and exemplify the invention.
For a more complete understanding of the present invention, reference is now made to the following drawings in which:
FIG. 1 shows an elevated pressure MALDI source according to Standing et al.;
FIG. 2 depicts a prior art ion guide according to Douglas et al.;
FIG. 3 depicts a prior art mass spectrometer according to Whitehouse et al., including an ion guide for transmitting ions across differential pumping stages;
FIG. 4 is a diagram of a prior art segmented multipole according to Thomson et al.;
FIG. 5 shows a prior art “stacked ring” ion guide according to Franzen et al.;
FIG. 6 depicts a prior art “ion funnel” guide according to Smith et al.;
FIG. 7A depicts a “segmented” electrode ring according to the present invention which, in this example, includes four electrically conducting segments;
FIG. 7B is a cross-sectional view of the segmented electrode of FIG. 7A formed at line A-A;
FIG. 7C is a cross-sectional view of the segmented electrode of FIG. 7A formed at line B-B;
FIG. 7D depicts a “segmented” electrode ring according to the present invention which, in this example, includes six electrically conducting segments;
FIG. 7E is a cross-sectional view of the segmented electrode of FIG. 7D formed at line A-A;
FIG. 7F is a cross-sectional view of the segmented electrode of FIG. 7D formed at line B-B;
FIG. 8A depicts an end view of a “segmented” funnel according to the present invention constructed from segmented electrodes of the type shown in FIG. 7A;
FIG. 8B is a cross-sectional view of the segmented funnel of FIG. 8A formed at line A-A;
FIG. 9A shows a cross-sectional view of the segmented funnel of FIG. 8A formed at line A-A with the preferred corresponding electrical connections;
FIG. 9B shows a cross-sectional view of the segmented funnel of FIG. 8A formed at line B-B with the preferred corresponding electrical connections;
FIG. 10A shows an end view of a segmented funnel according to the present invention, including a DC lens element at its outlet end;
FIG. 10B shows a cross-sectional view of the segmented funnel of FIG. 10A formed at line A-A;
FIG. 11 depicts the segmented ion funnel of FIG. 10 in a vacuum system of a mass spectrometer, including “downstream” multipole ion guides;
FIG. 12 is a cross-sectional view of a two-stage segmented ion funnel;
FIG. 13 depicts the two-stage segmented ion funnel of FIG. 12 in a vacuum system of a mass spectrometer, including a “downstream” multipole ion guide;
FIG. 14 shows a cross-sectional view of a “stacked ring” ion guide according to an alternative embodiment of the present invention, including “DC electrodes” interleaved with RF guide rings;
FIG. 15 is a plot of electric potential vs. position within the “stacked ring” ion guide shown in FIG. 14;
FIG. 16 depicts a cross-sectional view of an alternative embodiment of the ion guide according to the present invention comprising features of both the funnel and the stacked ring ion guides shown in FIGS. 8A-B and 14, respectively;
FIG. 17 is a plot of electric potential vs. position within the “funnel/stacked ring” ion guide shown in FIG. 16;
FIG. 18 depicts a cross-sectional view of a two-stage ion funnel and “funnel/stacked ring” ion guide in a vacuum system of a mass spectrometer;
FIG. 19A shows a first cross-sectional view of the electrical connections to the “funnel/stacked ring” ion guide shown in FIG. 18;
FIG. 19B is a second cross-sectional view, orthogonal to that of FIG. 19A, of the electrical connection to the “funnel/stacked ring” ion guide shown in FIG. 18;
FIG. 20 depicts a cross-sectional view of an alternate configuration of the “funnel/stacked ring” ion guide of the present invention comprising multipoles placed between a two-stage segmented funnel ion guide and a funnel/stacked ring ion guides;
FIG. 21 is a plot of electric potential vs. position within the “funnel/stacked ring” ion guide according to the present invention with forward and reverse biasing;
FIG. 22 depicts a cross-sectional view of a two-stage ion funnel and “funnel/stacked ring” ion guide in a system according to the present invention wherein the inlet orifice is oriented so as to introduce ions orthogonally into an ion guide;
FIG. 23 shows the system according to the present invention as depicted in FIG. 22 wherein the deflection plate is used as a sample carrier for a MALDI ion production means.
FIG. 24 depicts the system according to an alternate embodiment of the present invention wherein the sample being ionized by MALDI and the capillary exit are offset from the funnel axis;
FIG. 25 depicts the system according to an alternate embodiment of the present invention wherein a metal “deflection” plate is used such that the gas stream from the capillary exit is deflected along a path leading into the funnel;
FIG. 26 depicts the system according to an alternate embodiment of the present invention wherein a single sample flow is split and ionized simultaneously by two independent ionization means;
FIG. 27 depicts the system according to an alternate embodiment of the present invention wherein the MALDI ionization means is placed in a separate vacuum region from the funnel ion guide;
FIG. 28 shows a MALDI spectrum obtained from glu-fibrinopeptide;
FIG. 29 depicts the system according to an alternate embodiment of the present invention employings a an RF hexapole and multiple funnels wherein the axes of the ionization means and the funnels are perpendicular to one another;
FIG. 30 is a plot of the DC potentials applied to the various elements of the system shown in FIG. 29;
FIG. 31 is a fragment ion spectrum of Luteinizing Hormone Releasing Hormone (LHRH) produced by the fragmentation system and method described with respect to FIGS. 28 and 29;
FIG. 32 shows a mass spectrum of Bovine Serum Albumin (BSA) tryptic digest analyte ions and ACTH 18-39 calibrant ions;
FIG. 33A depicts a top plan view of a hexapolar “segmented” electrode according to the present invention;
FIG. 33B is a side view of the hexapolar segmented electrode of FIG. 33A;
FIG. 33C depicts a bottom plan view of the segmented electrode of FIG. 33A;
FIG. 33D is a cross-sectional view of the segmented electrode of FIG. 33A formed at A-A;
FIG. 34A depicts a top plan view of an alternate embodiment of a hexapolar segmented electrode in according to the present invention;
FIG. 34B depicts aside view of the alternate segmented electrode of FIG. 34A;
FIG. 34C depicts a bottom planview of the alternate segmented electrode of FIG. 34A;
FIG. 34D is a cross-sectional view of the alternate hexapolar segmented electrode of FIG. 34A formed at A-A;
FIG. 35A depicts a top plan view of yet another segmented electrode in accordance with the present invention;
FIG. 35B depicts a side view of the alternate segmented electrode of FIG. 35A;
FIG. 35C depicts a bottom plan view of the alternate segmented electrode of FIG. 35A;
FIG. 36 depicts a cross-sectional view of an alternate embodiment of an ion guide assembly according to the present invention, including a multipole collision cell and a hexapole trapping cell;
FIG. 37 depicts a cross-sectional view of the ion guide assembly of FIG. 36 as used in a system according to the present invention utilizing a MALDI target, glass capillary, and segmented plates; and
FIG. 38 depicts a cross-sectional view of an alternate embodiment of an ion guide assembly according to the present invention, including a collision cell and trapping cell.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
As required, a detailed illustrative embodiment of the present invention is disclosed herein. However, techniques, systems and operating structures in accordance with the present invention may be embodied in a wide variety of sizes, shapes, forms and modes, some of which may be quite different from those in the disclosed embodiment. Consequently, the specific structural and functional details disclosed herein are merely representative, yet in that regard, they are deemed to afford the best embodiment for purposes of disclosure and to provide a basis for the claims herein which define the scope of the present invention.
The following presents a detailed description of a preferred embodiment of the present invention, as well as some alternate embodiments of the invention. As discussed above, the present invention relates generally to the mass spectroscopic analysis of chemical samples and more particularly to mass spectrometry. Specifically, an apparatus and method are described for the transport of ions within and between pressure regions within a mass spectrometer. Reference is herein made to the figures, wherein the numerals representing particular parts are consistently used throughout the figures and accompanying discussion.
With reference first to FIGS. 7A-C, shown is a plain view of “segmented” electrode 101 according to the present invention. More particularly, FIG. 7B shows a cross-sectional view formed at line A-A in FIG. 7A. FIG. 7C shows a cross-sectional view formed at line B-B in FIG. 7A. In the preferred embodiment, segmented electrode 101 includes ring-shaped electrically insulating support 115 having aperture 119 through which ions may pass. Four separate electrically conducting elements 101a-101d are formed on support 115 by, for example, bonding metal foils to support 115. Importantly, elements 101a-101d cover the inner rim 119a of aperture 119 as well as the front and back surfaces of support 115 such that ions passing through aperture 119, will in no event encounter an electrically insulating surface. Notice also slots 151a-151d formed in support 115 between elements 101a-101d. Slots 151a-151d serve not only to separate elements 101a-101d but also removes insulating material of support 115 from the vicinity of ions passing through aperture 119. The diameter of aperture 119, the thickness of segmented electrode 101, and the width and depth of slots 151a-151d may all be varied for optimal performance. However, in this example, the diameter of aperture 119 is 26 mm, the thickness of electrode 101 is 1.6 mm, and the width and depth of slots 151 are 1.6 mm and 3.8 mm, respectively.
Further, while the segmented electrode 101 shown in FIGS. 7A-C depicts the preferred embodiment of segmented electrode 101 as comprising four conducting elements 101a-101d, alternate embodiments may be configured with any number of electrically conducting elements more than one, such as two, six, or eight elements. For example, as shown in FIGS. 7D-F, segmented electrode 101′ includes ring-shaped electrically insulating support 115′ having aperture 119′ through which ions may pass. Here, though, six separate electrically conducting elements 101a′-101f′ are formed on support 115′. Importantly, elements 101a′-101f′ cover the inner rim of aperture 119′ as well as the front and back surfaces of support 115′ such that ions passing through aperture 119′, will in no event encounter an electrically insulating surface. Here too, slots are provided in support 115′ between each of elements 101a′-101f′ to both separate elements 101a′-101f′ from each other, and remove insulating material of support 115′ from the vicinity of ions passing through aperture 119′. The diameter of aperture 119′, the thickness of segmented electrode 101′, and the width and depth of the slots may all be varied as discussed above.
Turning next to FIGS. 8A-B, shown is an end view of a set of segmented electrodes 101-111 assembled into ion guide 152 according to the preferred embodiment of the present invention. FIG. 8B shows a cross-sectional view formed at line A-A in FIG. 8A, which depicts segmented electrodes 101 through 111 assembled about a common axis 153. In the preferred embodiment of ion guide 152, the distance between adjacent electrodes 101-111 is approximately equal to the thickness of the electrodes—in this case 1.6 mm. Also, the diameter of the apertures in the electrodes 101-111 is a function of the position of the electrode in ion guide assembly 152. For example, as depicted in FIG. 8B, the segmented electrode having the largest aperture (in this example segmented electrode 101) is at the entrance end 165 of the ion guide assembly 152 and the segmented electrode having the smallest aperture (in this example segmented electrode 111) is at the exit end 167 of the ion guide assembly 152. The aperture diameter in the preferred embodiment is a linear function of the segmented electrode's position in ion guide assembly 152. However, in alternate embodiments this function may be non-linear. Further, in the preferred embodiment, the angle α formed between common axis 153 and the inner boundary (i.e., formed by the inner rims 119a of the segmented electrodes 101-111) of the ion guide assembly 152 is approximately 19°. However, alternatively, any angle between 0° and 90° may be used.
Further, each segmented electrode 101-111 in ion guide assembly 152 consists of four conducting elements a-d. Within any given segmented electrode 101-111, element a is in electrical contact with element c and element b is in electrical contact with element d. That is, element 101a is electrically connected to element 101c, element 101b is electrically connected to element 101d, element 102a is electrically connected to element 102c, and so forth.
As shown in FIGS. 9A-B, the preferred embodiment of ion guide 152 comprises resistor and capacitor networks (R-C networks) to provide the electrical connection of all the elements of segmented electrodes 101-111 to power sources. FIG. 9A depicts a cross-sectional view of assembly 152 as formed at line A-A in FIG. 8A. Similarly, FIG. 9B depicts a cross-sectional view of assembly 152 as formed at line B-B in FIG. 8A. In the preferred embodiment, potentials which vary in a sinusoidal manner with time are applied to the electrodes. A first such sinusoidally varying potential is applied at +RF and a second sinusoidally varying potential of the same amplitude and frequency, but 180° out of phase, is applied at −RF.
FIG. 9A, the electrical connections for the application of the +RF 250 and −RF 251 potentials to electrodes 101a-111a and 101c-111c through capacitors 154 is shown. Similarly, electrostatic potentials +DC 254 and −DC 255 are applied to electrodes 101a-111a and 101c-111c via resistor divider 157. Similarly, FIG. 9B depicts the electrical connections for the application of the +RF 252 and −RF 253 potentials to electrodes 101b-111b and 101d-111d through capacitors 155, and the electrical connections for the application of electrostatic potentials +DC 256 and −DC 257 to electrodes 101b-1111b and 101d-111d via resistor divider 159. In the preferred embodiment, capacitors 154 and 155 have the same values such that the amplitude of the RF potentials 250, 251, 252 and 253 applied to each of the electrodes 101a-111a, 101b-111b, 101c-111c, and 101d-111d of the segmented electrodes 101-111 in the ion guide assembly 152 is the same. Also, the resistor dividers 157 and 159 preferably have the same values such that the DC potential is the same on each element a-d of a given segmented electrode 101-111.
As an example, the amplitude of the RF potential applied to +RF and −RF may be 500 Vpp with a frequency of about 1 MHz. The DC potential applied between +DC and −DC may be 100 V. The capacitance of capacitors 154 and 155 may be 1 nF. And the resistance of the resistors in dividers 157 and 159 may be 10 Mohm each. Notice that for the ions being transmitted the DC potential most repulsive to the ions is applied to segmented electrode 101 (i.e., at the entrance end 165 of ion guide 152) while the most attractive DC potential is applied to segmented electrode 111 (i.e., at the exit end 167 of ion guide 152). Notice also that each electrically conducting element 101a-111a, 101b-111b, 101c-111c, and 101d-111d of the segmented electrodes 101-111 has an RF potential applied to it which is 180° out of phase with the RF potential applied to its immediately adjacent elements. For example, the RF potential applied to element 102a is 180° out of phase with elements 101a and 103a on the adjacent segmented electrodes 101 and 103. Similarly, the same RF potential applied to element 102a is 180° out of phase with elements 102b and 102d as adjacent electrically conducting elements on the same segmented electrode 102. Application of the RF potentials in this way prevents the creation of pseudopotential wells which thereby prevents or at least minimizes the trapping of ions. Pseudopotential wells, as discussed in the prior art designs of Smith et al. and of Franzen et al., can result in the loss of ion transmission efficiency or the m/z range within which ions are transmitted.
Turning next to FIGS. 10A-B depicted is two separate views of ion guide assembly 169, according to an alternate embodiment of the invention, in which DC lens element 161 is provided at outlet end 171 of ion guide assembly 169. FIG. 10B shows a cross-sectional view formed at line A-A in FIG. 10A. In the preferred embodiment, lens element 161 is composed of electrically conducting material. Alternatively, lens element 161 may comprise an insulator having an electrically conductive coating. Preferably, lens element 161 includes aperture 163 aligned with axis 153 of ion guide 152. It is also preferred that aperture 163 be round with a diameter of approximately 2 mm. However, in alternate embodiments, the aperture may take any desired shape or size. In practice the DC potential applied to lens element 161 should be more attractive to the transmitted ions than segmented electrode 111.
As an ion guide, the present invention has applicability in a variety of ways in a mass spectrometer system. FIG. 11 depicts the ion guide assembly 161 of FIG. 10 in the vacuum system of a mass spectrometer. The vacuum system of the mass spectrometer shown consists, for example, of four chambers 173, 175, 177 and 179. Although gas pressures in the chambers may vary widely, examples of gas pressures in a system such as this are  ̃1 mbar in chamber 173,  ̃5×10−2 mbar in chamber 175,  ̃5×10−3 mbar in chamber 177, and  ̃5×10−7 in chamber 179. To achieve and maintain the desired pressure levels in these chambers, each of chambers 173, 175, 177, and 179 include pumping ports 181, 183, 184, and 185, respectively, through which gas may be pumped away.
In the embodiment shown, capillary 186 transmits ions and gas from an atmospheric pressure ion production means 258 into chamber 173. As indicated previously, such ion production means may include any known API means including but not limited to ESI, atmospheric pressure chemical ionization, atmospheric pressure MALDI, and atmospheric pressure photoionization. Also, other known prior art devices might be used instead of capillary 186 to transmit ions from ion production means 258 into first chamber 173. Once the transmitted ions exit capillary 186 into first chamber 173, ion guide assembly 169, residing in first chamber 173, accepts the transmitted ions, while gas introduced via capillary 186 is pumped away via pumping port 181 to maintain the desired pressure therein. Through the appropriate application of electric potentials as discussed above with respect to FIGS. 9A-B and 10A-B, ion guide assembly 169 focuses the transmitted ions from the exit end of the capillary 186 toward and through aperture 163 of lens element 161 positioned at outlet end 171 of ion guide 152. In addition, lens element 161 preferably acts as a pumping restriction between first chamber 173 and second chamber 175.
Preferably, multipole ion guide 187 resides in second chamber 175 and multipole ion guide 188 resides in third chamber 177. Ion guide 187 serves to guide ions through chamber 175 toward and through lens 189, while ion guide 188 similarly serves to guide ions from lens 189 through chamber 177 toward and through lens 190. Lenses 189 and 190 may also serve as pumping restrictions between chambers 175 and 177 and between chambers 177 and 179, respectively. In addition, lenses 189 and 190 are shown as electrode plates having an aperture therethrough, but other known lenses such as skimmers, etc., may be used. Ions passing through lens 190 into fourth chamber 179 may subsequently be analyzed by any known type of mass analyzer (not shown) residing in chamber 179.
Although the potentials applied to the components of the system shown in FIG. 11 may be varied widely, an example of the DC electric potentials that may be applied to each component in operating such a system are:

















capillary 186
125
V



segmented electrode 1
120
V



segmented electrode 111
20
V



lens element 161
19
V



multipole 187
18
V



lens element 189
17
V



multipole 188
15
V



lens element 190
0
V.










In an alternate embodiment, lens element 161 might be replaced with a segmented electrode of essentially the same structure as segmented electrodes 101-111. In such an embodiment, lens element 161 would preferably be electrically driven in substantially the same manner as the electrodes 101-111—i.e. RF and DC potentials—but would additionally act as a pumping restriction.
In the preferred embodiment of FIG. 11, the multipoles 187 and 188 are hexapoles, however in alternate embodiments they might be any type of multipole ion guide—e.g. quadrupole, octapole, etc. The RF potential applied to the rods of multipoles 187 and 188 may also vary widely, however one might apply a sinusoidally varying potential having an amplitude of 600 Vpp and frequency of 5 MHz.
In an alternate embodiment, multipole 188 might be a quadrupole. Further, as is known in the prior art, one might use multipole 188 to select and fragment ions of interest before transmitting them to chamber 179.
Turning next to FIG. 12, a two-stage ion guide 199 according to yet another alternate embodiment of the invention is depicted. As shown, two-stage ion guide 199 incorporates ion guide assembly 169 of FIGS. 10A-B with a second ion guide 201 comprising additional segmented electrodes 191-195 and DC lens 197. In this embodiment, ion guide assembly 169 acts as the first stage of two-stage ion guide 199, with the additional segmented electrodes 191-195 and lens 197 forming second stage 201 of the two-stage ion guide 199. As depicted, all of the segmented electrodes 101-111 and 191-195 and lenses 161 and 197 are aligned on common axis 153. While the angle β formed between the common axis 153 and the inner boundary (i.e., formed by the inner rims of the segmented electrodes 191-195) of the second stage 201 of two-stage ion guide 199 is independent from angle α of first stage ion guide assembly 169 (the angle α is discussed above in regard to FIGS. 8A-B), these angles α and β are preferably the same. Similarly, the thickness and spacing between segmented electrodes 191-195 are preferably the same as the thickness of and spacing between segmented electrodes 101-111, as discussed above. Also, it is preferred that lens 197 is electrically conducting with a 2 mm diameter aperture aligned on axis 153. The RF potentials applied to the electrically conducting elements of segmented electrodes 191-195 are preferably of the same amplitude and frequency as that applied in first stage ion guide assembly 169. The DC potentials applied to segmented electrodes 191-195 are such that ions are repelled from lens 161 and attracted toward lens 197.
Like FIG. 11, FIG. 13 depicts an ion guide according to the invention as it may be used in a mass spectrometer. Specifically, FIG. 13 depicts the two-stage ion guide 199 of FIG. 12 positioned in the vacuum system of a mass spectrometer. The system depicted in FIG. 13 is the same as that of FIG. 11 with the exception that ion guide 187 and lens 189 shown in FIG. 11 are replaced with second stage ion guide 201 in FIG. 13 which includes ion lens 197. As depicted in FIG. 13, two stage ion guide 199 is capable of accepting and focusing ions even at a relatively high pressure (i.e.,  ̃1 mbar in first pumping chamber 173) and can efficiently transmit them through a second, relatively low pressure differential pumping stage (i.e.,  ̃5×10−2 mbar in second pumping chamber 175) and into a third pumping chamber 177. Notice that although lenses 161 and 197 are shown to be integrated into two-stage ion guide 199, they also act as pumping restrictions between chambers 173 and 175, and between 175 and 177, respectively. The ability of two-stage ion guide 199, as a single device, to efficiently guide and transmit ions over a wide range of pressure regions and through a plurality of pumping stages is one of the principle advantages of the present invention over prior art ion guides.
In an alternate embodiment, lens element 161 might be replaced with a segmented electrode of essentially the same structure as segmented electrodes 101-111. In such an embodiment, lens element 161 would preferably be electrically driven in substantially the same manner as the electrodes 101-111—i.e. RF and DC potentials, but would additionally act as a pumping restriction.
In a further alternate embodiment, lens element 197 might also be replaced with a segmented electrode of essentially the same structure as segmented electrodes 101-111 and 191-195. In such an embodiment, lens element 197 would preferably be electrically driven in substantially the same manner as the electrodes 101-111 and 191-195—i.e. RF and DC potentials—but would additionally act as a pumping restriction.
Referring now to FIG. 14, depicted is a “stacked ring” ion guide 202 according to yet another alternate embodiment of the present invention. As shown, stacked ring ion guide 202 includes “DC electrodes” 203 interleaved with RF guide rings 204a and 204b. Preferably, RF guide rings 204 are apertured plates preferably composed of electrically conducting material (e.g., metal). The dimensions and placement of RF guide rings 204 may vary widely. However, it is preferred that RF guide rings 204a and 204b be approximately 1.6 mm thick, have apertures 208 which are approximately 6 mm in diameter, and be positioned with spacing between adjacent RF guide rings 204a and 204b of 1.6 mm. Also, rings 204a and 204b are preferably aligned along common axis 205. As shown, this embodiment includes apertured lens elements 206 and 207 positioned at either end of stacked ring ion guide 202 and are also aligned along axis 205. Lenses 206 and 207 are preferably electrically conducting plates with approximately 2 mm diameter apertures.
Stacked ring ion guide 202 also comprises DC electrodes 203 which are thin (e.g.,  ̃0.1 mm) electrically conducting plates positioned midway between adjacent RF guide rings 204a and 204b and have apertures 209 with preferably the same diameter as apertures 208 in RF guide rings 204a and 204b. 
During operation, sinusoidally time-varying potentials RF3 are applied to RF guide rings 204. Preferably a first time-varying potential +RF3 is applied to ring 204a, and a second time-varying potential −RF3 is applied to rings RF guide 204b. Potentials +RF3 and −RF3 are preferably of the same amplitude and frequency but are 180° out of phase with one another. Also, the potentials +RF3 and −RF3 may have a non-zero reference potential such that the entire stacked ring ion guide 202 has a “DC offset” of, for example,  ̃15V. Potentials are applied to DC electrodes 203 via RC network 210. In the preferred method of operation, the inputs TNL1 and TNL2 to RC network 210 are maintained at the same electrostatic potential as the DC offset of stacked ring ion guide 202 as a whole. Alternatively, to trap ions in the ion guide, one can set the DC potentials on lenses 206 and 207 to some potential above the DC offset of the remainder of stacked ring ion guide 202.
FIG. 15 shows a plot of electric potential vs. position within stacked ring ion guide 202. In particular, trace 211 of FIG. 15 is a plot of the electrostatic potential on axis 205 of ion guide 202 when operated in the manner described above to trap ions. One may operate stacked ring ion guide 202 in this manner to accumulate ions within stacked ring ion guide 202. Ions may be introduced into stacked ring ion guide 202 from an ion production means via aperture 213 in lens 206 (see FIG. 14). Ions may then undergo collisions with a gas in stacked ring ion guide 202 thus losing kinetic energy and becoming trapped. The efficiency of trapping ions in this manner is dependent on the gas pressure and composition within stacked ring ion guide 202.
Once ions are trapped in stacked ring ion guide 202, the electrostatic potential along axis 205 may be changed so as to eject ions from stacked ring ion guide 202. Trace 212 of FIG. 15 shows the electrostatic potential as a function of position along axis 205 when the potential at TNL2 (see FIG. 14) is lowered to only a few volts and potential L2 (see FIG. 14) applied to lens 207 is lowered to 0V. The gradient in the electrostatic potential along axis 205 will tend to eject ions from guide 202 through aperture 214 in lens 207.
When operated in the preferred manner, the potential on the elements 203 of stacked ring ion guide 202 are maintained for a predetermined time so as to accumulate and trap ions from an ion production means in stacked ring ion guide 202. After this predetermined time, however, the potentials TNL2 and L2 are rapidly pulsed to lower potentials so as to quickly eject ions from stacked ring ion guide 202. In the preferred method, the transition of the potentials TNL2 and L2 is on the same order of or faster than the frequency of the RF potential applied at RF3. Notice that, unlike the prior art ion guide of Franzen et al. discussed above, the formation of an electrostatic field along the axis of stacked ring ion guide 202 does not require the application of a DC potential gradient to RF guide rings 204a and 204b. Rather, the electrostatic field is formed via DC electrodes 203 independent of RF guide rings 204a and 204b. As a result, the electrostatic gradient represented by trace 212 can be generated as rapidly as necessary without considering the frequency at which RF guide rings 204a and 204b are being driven. As an example, potentials +RF3 and −RF3 may be 500 Vpp at 1 MHz, ions may be accumulated for 10 msec from an ESI source. Thereafter, the potentials TNL2 and L2 can be lowered to 4 V and 0 V respectively in a pulsed manner with a fall time of 100 ns and a duration of 100 μsec. After the duration of 100 μsec, the potentials TNL2 and L2 can be raised to their trapping potentials of 15 V and 25 V, respectively, and the process may be repeated. The pulses of ions thus produced are injected into a mass analyzer residing “downstream” from stacked ring ion guide 202.
Turning next to FIG. 16, shown is yet another alternative embodiment of an ion guide according to the present invention. As shown, this embodiment comprises features of both ion funnel 152 (FIGS. 8A-B) and stacked ring ion guide 202 (FIG. 14). Specifically, ion guide 220 of FIG. 16 is the same as ion guide 202 with the addition of guide rings 216-219, capacitors 215, and resistor divider 221. In this embodiment, guide rings 216-219 act as a funnel-like ion guide as describe above. The thickness and spacing between guide rings 216-219 may vary widely. However, the thickness of electrodes 216-219 is preferably the same as that of rings 204a and 204b (e.g., 1.6 mm) and the spacing between electrodes 216-219 is preferably the same as that between electrodes 204a and 204b (e.g. 1.6 mm). Also, the angle γ formed between common axis 205 of ion guide 220 and the inner boundary ring electrodes 216-219 may vary widely. However, it is shown here to be 19°. The RF potential on guide rings 216-219 is set by +RF3 and −RF3 through capacitors 215 as described above. In the preferred method of operation, the RF potential applied to guide rings 216-219 is the same as that applied to RF rings 204a and 204b. However, in alternate embodiments, the RF potential applied to rings 216-219 might be of a different amplitude or frequency than that applied to rings 204a and 204b. The DC potentials on rings 216-219 are applied via resistor divider 221. Also in the preferred method of operation, the potentials FNL1 and FNL2 applied to resistor divider 221 are such that ions are accelerated along axis 205 toward the exit end of the ion guide 220 at lens 207. Also, in the preferred method of operation, the DC potential on ring 219 should be approximately the same or slightly higher than that on electrodes 204a and 204b, as represented in traces 222 and 223 in FIG. 17.
Similar to FIG. 15, FIG. 17 plots the electrostatic potential as a function of position in ion guide 220 on axis 205. First, trace 222 of FIG. 17 is a plot of the electrostatic potential on axis 205 of ion guide 220 when operated to trap ions. One may operate in this manner to accumulate ions in ion guide 220. Ions may be introduced into guide 220 from an ion production means via aperture 213 in lens 206 (see FIG. 16). Ions may then undergo collisions with a gas in guide 220 thus losing kinetic energy and becoming trapped. The efficiency of trapping ions in this manner is dependent on the gas pressure and composition in ion guide 220.
Once ions are trapped in ion guide 220, the electrostatic potential along axis 205 may be changed so as to eject ions from ion guide 220. Trace 223 of FIG. 17 shows the electrostatic potential as a function of position along axis 205 when the potential at TNL2 (see FIG. 16) is lowered to only a few volts and potential L2 (see FIG. 16) applied to lens 207 is lowered to 0V. The gradient in the electrostatic potential along axis 205 will tend to eject ions from guide 220 through aperture 214 in lens 207.
When operated in the preferred manner, the potential on the elements 203 of ion guide 220 are maintained for a predetermined time so as to accumulate and trap ions from an ion production means in ion guide 220. After this predetermined time, however, the potentials TNL2 and L2 are rapidly pulsed to lower potentials so as to quickly eject ions from ion guide 220. In the preferred method, the transition of the potentials TNL2 and L2 is on the same order of or faster than the frequency of the RF potential applied at RF3. Notice that, unlike the prior art ion guide of Franzen et al. discussed above, the formation of an electrostatic field along the axis of ion guide 220 does not require the application of a DC potential gradient to RF guide rings 204a and 204b. Rather, the electrostatic field is formed via DC electrodes 203 independent of RF guide rings 204a and 204b. As a result, the electrostatic gradient represented by trace 223 can be generated as rapidly as necessary without considering the frequency at which RF guide rings 204a and 204b are being driven. As an example, potentials +RF3 and −RF3 may be 500 Vpp at 1 MHz, and ions may be accumulated for 10 msec from an ESI source. Thereafter, the potentials TNL2 and L2 can be lowered to 4 V and 0 V respectively in a pulsed manner with a fall time of 100 ns and a duration of 100 μsec. After the duration of 100 μsec, the potentials TNL2 and L2 may be raised to their trapping potentials of 15 V and 25 V, respectively, and the process may be repeated. The pulses of ions thus produced are injected into a mass analyzer residing “downstream” from ion guide 220.
While electrodes 204a and 204b of ion guides 202 and 220 have been described as ring electrodes, in an alternative embodiment of those ion guides according to the invention, electrodes 204a and 204b may further be segmented electrodes as described with reference to FIG. 7. Such a stacked ring ion guide with segmented electrodes is depicted in FIG. 18.
FIG. 18 further depicts two-stage ion guide 199 used in conjunction with stacked ring ion guide 224, assembled together in the vacuum system of a mass spectrometer. The system depicted in FIG. 18 is identical to that of FIG. 13 with the exception of the replacement of ion guide 188 in FIG. 13 with stacked ring ion guide 224 in FIG. 18. As depicted in FIG. 18, two stage ion guide 199 can accept ions and focus them even at a relatively high pressure (i.e., in first pumping stage 173) and can efficiently transmit them through a second, relatively low pressure, differential pumping stage (i.e., chamber 175) to third chamber 177. With the addition of ion guide 224, the assembly has the advantage over prior art that ions can be trapped and rapidly ejected into chamber 179 and the mass analyzer residing therein. In alternate embodiments, ion guide 224 might extend through multiple pumping stages. In such a system, one or more of the electrodes 204 might also serve as pumping restrictions.
Referring to FIGS. 19A-B shown are the electrical connections for ion guide 225 of FIG. 18. Specifically, FIG. 19A shows a first cross-sectional depiction of the electrical connections to ion guide 225 according to the present invention as depicted in FIG. 18. Next, FIG. 19B shows a second cross-sectional depiction, orthogonal to that of FIG. 19A, of the electrical connection to ion guide 225. As shown, ion guide 225 is electrically connected in a manner similar to that described above with respect to FIGS. 9, 14, and 16. In this embodiment, capacitors 154, 155, 215, 226, 228, and 230 all preferably have the same capacitance. Alternatively, the capacitance of capacitors 154 and 155 may differ from the capacitance of capacitors 226 and 228, as well as from that of capacitors 215 and 230. Similarly, resistors 157, 159, 221, 227, 229, and 231 are all preferably identical. However, in alternate embodiments, the resistance of these resistors may differ from one another. Also, in this embodiment, it is preferred that the RF potentials applied at RF1, RF2, and RF3 be identical to one another. However, in alternate embodiments, the RF frequencies and/or amplitudes applied at inputs RF1, RF2, and RF3 may differ from one another. Finally, it is preferred that the various DC potentials applied to the electrodes are such that the ions being transmitted are attracted toward the exit end of ion guide 225 and analyzer chamber 179. As discussed above, however, the inputs TNL1 and TNL2 of RC network 210 may be biased such that ions are either trapped in or ejected from that portion of ion guide 225.
Yet another alternative embodiment of the present invention is shown in FIG. 20. In particular, shown are ion guides 199 and 224 positioned in the vacuum system of a mass spectrometer with two multipole ion guides 188 and 232 positioned there between. In the embodiment depicted in FIG. 20, the pressures in vacuum chambers 173, 175, and 177 and the operation of elements 186, 199, and 188 are substantially similar to that described with reference to FIG. 13. According to this embodiment, multipole ion guide 188 is a hexapole and multipole ion guide 232 is a quadrupole. As described above, an RF-only potential is applied to hexapole ion guide 188 so as to guide ions through chamber 177 and into chamber 179.
Preferably, chamber 179 is operated at a pressure of 10−5 mbar or less such that quadrupole 232 may be used to select ions of interest. It is also preferable that quadrupole 232 be used either to transmit substantially all ions or only selected ions through chamber 179 into chamber 233 and ion guide 224 positioned therein. As is well known from the prior art, substantially all ions will be transmitted through quadrupole 232 when an RF-only potential is applied to it. To select ions of interest, both RF and DC potentials must be applied.
Similar to that described above, selected ions are accelerated into chamber 233 and ion guide 224 via an electric field. The gas pressure of chamber 233 is preferably 10−3 mbar or greater. Typically the gas used is inert (e.g., Nitrogen or Argon) however, reactive species might also be introduced into the chamber. When the potential difference between ion guides 232 and 224 is low, for example 5 V, the ions are simply transmitted therethrough. That is, the ions will collide with the gas in ion guide 224, but the energy of the collisions will be low enough that the ions will not fragment. However, if the potential difference between ion guides 232 and 224 is high, for example 100 V, the collisions between the ions and gas may cause the ions to fragment.
In this manner ion guide 224 may act as a “collision cell”. However, unlike prior art collision cells, the funnel-like entrance of ion guide 224 allow for the more efficient capture of the selected “precursor” and “fragment” ions. Precursor and fragment ions may be trapped in the manner described above with reference to FIGS. 16 and 17. Through collisions with the gas, the ions may be cooled to the temperature of the collision gas, typically room temperature. These ions will eventually be ejected from ion guide 224 into chamber 234 where an additional mass analyzer (not shown) may be used to analyze both the precursor and fragment ions and produce precursor and fragment ion spectra. In alternate embodiments, any of the other ion guides disclosed herein, for example ion guide 169 shown in FIG. 10B, may be substituted for ion guide 224.
The mass analyzer in chamber 234 may be any type of mass analyzer including but not limited to a time-of-flight, ion cyclotron resonance, linear quadrupole or quadrupole ion trap mass analyzer. Further, any type of mass analyzer might be substituted for quadrupole 232. For example, a quadrupole ion trap (i.e., a Paul trap), a magnetic or electric sector, or a time-of-flight mass analyzer might be substituted for quadrupole 232.
Still referring to FIG. 20, while trapped in ion guide 224 the ions may be further manipulated. For example, as discussed by Hofstadler et al., an ion guide may operate at a predetermined pressure such that ions within such ion guide may be irradiated with light and thereby caused to form fragment ions for subsequent mass analysis. Selected ions are preferably collected in the ion guide 224 in a generally mass-inselective manner. This permits dissociation over a broad mass range, with efficient retention of fragment ions. In the embodiments of the present invention disclosed herein, it is preferred that the pressure in chamber 233 be relatively high (e.g., on the order of 103-106 mbar). Irradiating ions in such a high pressure region results in two distinct advantages over traditional Infrared Multiphoton Dissociation (IRMPD) as exemplified in Fourier Transform Ion Resonance (FTICR) and Quadrupole Ion Trap (QIT) mass spectrometry. Under high pressures, collisions with neutrals will dampen the ion cloud to the center of ion guide 224 and stabilize fragment ions, resulting in significantly improved fragment ion retention. In addition, the fragment ion coverage is significantly improved, providing more sequence information.
Alternatively, ions might be activated toward fragmentation by oscillating the potentials on TNL1 and TNL2 (see RC network shown and described in reference to FIG. 16). As depicted in FIG. 21, ions may be accelerated back and forth within ion guide 224. When the potential applied at TNL1 (i.e., at lens 206) is held high relative to the potential applied at TNL2 (i.e., at lens 207) ions will be accelerated toward the exit end of ion guide 224 (i.e., toward chamber 234). As indicated by trace 237, the ions are prevented from escaping ion guide 224 by the RF on electrodes 204a and 204b and the repelling DC potential on lens electrode 207. Reversing the potentials applied at TNL1 and TNL2 results in a potential along the common axis of ion guide 224 represented by trace 238. The ions are then accelerated away from the exit end of ion guide 224 (i.e., at lens 207). In this situation, the ions are prevented from escaping ion guide 224 again by the RF potential on electrodes 204a and 204b and the repelling DC potentials on lens electrode 206 and ring electrodes 216-219. By rapidly alternating the forward and reverse acceleration of ions in guide 224 (i.e., by reversing the potentials applied at TNL1 and TNL2), the ions are caused to repeatedly undergo collisions with gas within ion guide 224. This tends to activate the ions toward fragmentation. At some predetermined time, the potentials on guide 224 will be brought back to that represented by trace 222 (seen in FIG. 17). At that time the ions will be cooled via collisions with the gas to the temperature of the gas. Then the ions will be ejected from ion guide 224 by applying potentials represented by trace 223 (seen in FIG. 17).
Turning now to FIG. 22, depicted is a system according to another embodiment of the present invention wherein an ion guide according to one or more of the embodiments disclosed herein (e.g., ion guide 225 seen in FIG. 18) may be used with an orthogonal ion production means. That is, axis 240 of inlet orifice or capillary 186 is oriented so as to introduce ions orthogonal to axis 153 of ion guide 225. As discussed above, gas and ions are introduced from, for example, an elevated pressure ion production means (not shown) into chamber 173 via an inlet orifice or capillary 186. After exiting orifice or capillary 186 the directional flow of the ions and gas will tend to follow axis 240. Preferably, pumping port 181 is coaxial with inlet orifice or capillary 186 so that the gas, entrained particulates and droplets will tend to pass directly to port 181 and the corresponding pump. This is a significant advantage in that electrode 239 and ion guide 225 will not readily become contaminated with these particulates and droplets.
In this embodiment, electrode 239 is preferably a planar, electrically conducting electrode oriented perpendicular to axis 153. A repulsive potential is applied to electrode 239 so that ions exiting orifice or capillary 186 are directed toward and into the inlet of ion guide 225. The distances between potentials applied to elements 186, 239, and 225 may vary widely, however, as an example, the distance between axis 153 and orifice 186 in is preferably 13 mm, the lateral distance between axis 240 and the entrance of ion guide 225 is preferably 6 mm, and the distance between electrode 239 and the entrance of ion guide 225 is preferably 12 mm. The DC potentials on electrodes 101, 186, and 239 may be 100 V, 200 V, and 200 V respectively, when analyzing positive ions. As shown, angle α is 90° (i.e., orthogonal), but in alternate embodiments the angle α need not be 90° but may be any angle.
Referring to FIG. 23, shown is the system depicted in FIG. 22 wherein electrode 239 is used as a sample carrier for a Matrix-Assisted Laser Desorption/Ionization (MALDI) ion production means. In this embodiment, electrode 239 may be removable or partly removable from the system via, for example, a vacuum interlock (not shown) to allow replacement of the sample carrier without shutting down the entire vacuum system. At atmospheric pressure, separate from the rest of the system, MALDI samples are applied to the surface of electrode 239 according to well known prior art methods. Electrode 239 now with samples deposited thereon (not shown) is introduced into the system via the above-mentioned vacuum interlock so that it comes to rest in a predetermined position as depicted in FIG. 23. Electrode 239 may reside on a “stage” which moves electrode 239 in the plane perpendicular to axis 153.
In this embodiment, window 242 is incorporated into the wall of chamber 173 such that laser beam 241 from a laser positioned outside the vacuum system may be focused onto the surface of electrode 239 such that the sample thereon is desorbed and ionized. On the sample carrier electrode 239, the sample being analyzed will reside approximately at axis 153. However, a multitude of samples may be deposited on the electrode 239, and as each sample is analyzed, the position of electrode 239 is changed via the above-mentioned stage such that the next sample to be analyzed is moved onto axis 153. For this embodiment, any prior art laser, MALDI sample preparation method, and MALDI sample analysis method might be used. Further, any means of bringing the laser light onto the sample spot (e.g., fiber optics) can be used. In alternate embodiments, MALDI target 239 can be fixed and the laser beam moved to address each sample in an array of samples on MALDI target 239.
During the MALDI analysis as described above, inlet orifice or capillary 186 can be plugged so that no gas, or alternatively a reduced flow of gas, enters chamber 173. Alternatively, a user may produce ions simultaneously via a multitude of ion production means. For example, ions can be introduced from an electrospray ion production means via orifice 186 while simultaneously producing MALDI ions from samples on electrode 239. Though not shown, more than two ion production means can be used in this manner either consecutively or simultaneously to introduce ions into ion guide 225.
In another alternate embodiment, the sample being ionized by MALDI may be offset from funnel axis 153 as depicted in FIG. 24, such that inlet orifice 186 is offset from funnel axis 153. As discussed above, gas and ions are introduced from an elevated pressure ion production means (not shown) into chamber 173 via an inlet orifice or capillary 186. After exiting orifice or capillary 186 the directional flow of the ions and gas will tend to follow an axis identical to the axis of the capillary 186. As shown in FIG. 24, the offset position of the MALDI target 239 and capillary 186 are such that the axis of capillary 186 does not intersect with axis 153 nor the path of the MALDI ions generated from target 239. Such an embodiment substantially prevents the interaction of the stream of gas from capillary 186 with the MALDI ions from target 239. That is, as discussed above regarding the embodiment depicted in FIG. 23, the stream of gas exiting capillaryl 86 and the path of the MALDI ions generated from target 239 intersect axis 153. While the DC potential between target 239 and funnel 225 will tend to force ions into funnel 225, the directional flow of gas across this path will tend to push the MALDI ions into pumping orifice 181. Offsetting either one or both of the MALDI sample position and capillary 186 will prevent this effect.
In additional embodiments with capillary 186 and/or MALDI sample position, apertures 119 (see FIGS. 7A-F) at entrance end 165 of funnel 152 (see FIGS. 8A-B) can be elongated into a substantially oval shape in the same dimension that orifice 186 and/or MALDI sample position are offset. This elongated shape can be tapered back to a substantially circular aperture as a function of position along funnel 152 such that at exit end 167 of funnel 152, the aperture shape is circular. The oval shape allows the funnel to more effectively capture ions from the offset orifice and MALDI sample. Alternatively, the funnel design can be changed to compensate for offset capillary 186 and offset MALDI sample position by simply increasing the diameter of the aperture at entrance end 165 of the funnel. That is, the angle α (see FIG. 8B) can be increased such that the entrance diameter is the original entrance diameter plus the offset of orifice 186 and the offset of the MALDI sample position.
Of course, other conceivable means can be used to prevent the interaction between the gas stream from orifice 186 and the MALDI ions, including, for example, a flow disrupter. A flow disruptor is an object (e.g., a metal rod or disk) placed in the gas stream so as to disrupt the directional flow of gas along its axis. Preferably, the flow disruptor is placed between capillary/orifice 186 and the path of the MALDI ions between the target 239 and funnel 225 such that the directional flow of the gas and its influence on the MALDI ions is substantially reduced. Optionally, the flow disruptor may be fixed, removable, or otherwise adjustable with respect to position.
Alternatively, the gas stream can be deflected before it can interact with the MALDI ions. For example, metal deflection plate 260 can be placed on axis 240 at an angle as shown in FIG. 25 such that the gas stream from capillary 186 is deflected along path 261 so that the gas stream has no consequential interaction with ions produced at target 239. Flow deflector 260 can be fixed, removable, or otherwise adjustable. For example, deflector 260 can be rotated so as to deflect the gas stream through a different angle. Of course, any other well known means for preventing interaction between the gas stream from the orifice can be used without departing from the spirit of the invention.
It should be clear that neither the presence of a second ionization means nor capillary 186 are required to operate the MALDI ionization means. Indeed, the presence of a MALDI means is not required for the operation of an atmospheric pressure ionization means. In the operation of funnel 225, the different ionization means are substantially independent from one another. In alternate embodiments any combination of ionization means can be used including, but not limited to, MALDI, ESI, atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), electron ionization (EI), chemical ionization (CI), secondary ionization (SIMS), fast atom bombardment (FAB), or laser desorption ionization (LDI).
In further embodiments, one ionization means can be used to affect another. For example, ESI can be used to produce primary ions used for SIMS or FAB. In one embodiment, the SIMS target is positioned on axis 240 on the opposite side of axis 153 from orifice 186 such that ESI primary ions are accelerated into the SIMS target and so that secondary ions are accelerated away from the SIMS target.
Furthermore, more than one means of the same or similar type can be used in combination. For example, two ESI means can be used such that a first ESI means generates ions from a first sample while simultaneously a second ESI means generates ions from a second sample.
Alternatively, one ionization means can be used to produce analyte ions while a second ionization means is used to produce reagent ions. For example, a first ESI source can be used to produce multiply charged analyte ions from a sample while simultaneously, or nearly simultaneously, singly charged negative reagent ions are produced from, for example, a CI source. The reagent ions are injected into region 173 such that they cross the path of the analyte ions. The reagent ions are injected at a location having a more negative potential than capillary 186 or axis 240. The DC potentials applied to the electrode in region 173 causes the negative reagent ions to move in one direction along axis 153 while analyte ions move in substantially the opposite direction (i.e., into ion guide 225). As the reagent and analyte ion beams cross paths, some of the ions react with one another. In this example, the reagent ion transfers an electron to the analyte ion causing neutralization of one of its charges and possibly inducing fragmentation of the analyte ion. This reaction is well known as electron transfer dissociation (see, for example, John E. P. Syka; Joshua J. Coon; Jae C. Schwartz; Jeffery C. Shabanowitz; Donald F. Hunt, Proceedings of the 52nd American Society for Mass Spectrometry Conference on Mass Spectrometry and Allied Topics, WOBam 11:15, May 23-27, 2004.). Of course, any other known gas phase ion-ion reaction can be carried out in a similar manner.
Further, ion-neutral reactions can be performed. For example, analyte ions are first introduced into region 173 via capillary 186. Simultaneously, a reagent gas is introduced from reservoir 263 into region 173 via leak valve 259. Alternatively, reagent gas may be introduced with analyte ions via capillary 186. As the ions traverse region 173, they react with the reagent gas to produce product ions. Alternatively, the analyte species may be neutral, for example, having been laser desorbed from target 239. Reagent ions, for example from ESI or CI, may be used to ionize the analyte species to form an analyte ion. Such postionization reactions are well known (see, for example, B. H. Wang, K. Dreiswerd, U. Bahr, M. Karas, F. Hillenkamp, J. Am. Soc. Mass Spectrom. 4, 393(1993).). Importantly, however, no such postionization has been performed in combination with a funnel ion guide.
In still another further alternate embodiment, fractions of a single sample may be ionized simultaneously (or nearly simultaneously) by two ionization methods as depicted in FIG. 26. For example, a solution of analyte is caused to flow through a tubing (e.g., through PEEK tubing 265) as the effluent from an LC separation. This flow is split into two fractions of either equal or unequal flows. Preferably, T-fitting 267 is used to accept a single flow from single tubing 265 which splits it into separate flows. These two flows are introduced separately into two independent ionization means. In the example of FIG. 26, one flow is introduced into ESI means 268 whereas the second flow is introduced into independent APCI means 269. Notice the embodiment of FIG. 26 is also an example of two atmospheric pressure ionization means in a single source. In this embodiment capillary 186′ resides on axis 153 and transfers ions from APCI means 269 into region 173. Notice also that deflection electrode 239′ includes an aperture through which capillary 186′ can pass. In this embodiment, the exit end of capillary 186′ and deflection electrode 239′ are held at the same DC potential. Of course, this embodiment can be extended to include a multitude of sample fractions introduced into a multitude of ionization methods.
Turning next to FIG. 27, shown is an embodiment wherein the MALDI ionization means is placed in separate vacuum region 272 from region 173 where ions from capillary 186 are introduced. Region 272 can be maintained at any desired pressure. As described above, laser radiation 241 passes through window 242 to desorb and ionize sample material on target 239. A potential difference between MALDI target 239 and guide 274 forces ions toward ion guide 274. Ion guide 274 may be any type of ion guiding device including an RF multipole, an ion funnel, an ion tunnel, a stacked ring ion guide, one or more DC electrodes, or a simple aperture or capillary. Analyte ions are captured by ion guide 274 and are transported therethrough into vacuum region 173. At the outlet of guide 274, ions are accelerated along path 153 by an electrical potential difference between guide 274, deflection plate 270, and funnel 225. Of course, any ionization means other than or in addition to MALDI can be placed in chamber 272 without departing from the spirit of the invention.
Referring next to FIG. 28, shown is a MALDI spectrum obtained from a source substantially as depicted in FIG. 23. In obtaining this spectrum the laser power was increased to a level substantially above the threshold power needed to produce signal. As a result of the relatively high laser power, analyte ions were not only desorbed and ionized, but rather, some analyte ions were caused to dissociate into fragment ions. In this particular example a known sample (i.e., glu-fibrinopeptide) was used. As shown, corresponding glu-fibrinopeptide molecular ion peak 280 appears at m/z 1571, while y-series 282 and b-series 284 fragment ion peaks appear at lower m/z. Such a series of peaks can be used to deduce the original composition of the analyte. In this case the analyte is a peptide and the series of peaks allow the original amino acid sequence in the peptide to be determined. This method of ion fragmentation is well known (see, for example, R. S. Brown, B. L. Carr, and J. J. Lennon, J. Am. Soc. Mass Spectrom. 7, 225(1996).) as “in source decay” but until now has been observed only in conjunction with vacuum MALDI instruments—that is, instruments wherein the space-time origin of the ions (i.e., where and when the ions are formed) is substantially the same as the space-time origin of the mass analysis (i.e., where and when the TOF mass analysis begins).
FIG. 29 depicts an alternate embodiment of the invention employing RF hexapole 188 (e.g., at 5 MHz and 600 Vpp) and funnels 169 and 201 (e.g., at 1.2 MHz and 200 Vpp) in a source wherein axes 240 and 253 are perpendicular to one another. As depicted in a preferred embodiment, DC potential “IF1” of +/−200V is applied to entrance end 294 of funnel 169, DC potential “Sk1” of +/−200V is applied to both exit end 296 of ion funnel 169 and pumping restriction 161, DC potential “IF2” of +/−100V is applied to entrance end 298 of funnel 201, DC potential “Sk2” of +/−100V is applied to both the exit end of funnel 201 and pumping restriction 197, DC potential “HEXDC” of +/−100V is applied to hexapole 188, DC potential “Extract/Trap” of +/−0V is applied to exit electrode 190, DC potential “CapExit” of +/−400V is applied to capillary exit 186, and DC potential “Deflector” of +/−400V is applied to deflection plate 239. In such an embodiment, typical tuning values for positive ion mode are: IF1 at +120V, Sk1 at +16V, IF2 at +12V, Sk2 at +10V, HEXDC at +4.8V, Extract/Trap at +20V (fast pulsing rise time <100 μsec for 10V), CapExit at +280V, and Deflector at +260V.
Turning next to FIG. 30, shown is a plot of the voltages applied to the various elements of the ion surce shown in FIG. 29, and refers to a method of using the ion guide as depicted in FIG. 29 not only to transmit ions from the ion production means to the mass analyzer, but also to induce fragmentation. Specifically, FIG. 30 is a plot of the DC potentials applied to each of the elements of the source shown in FIG. 29 so as to simply transmit reserpine ions—trace 290—or to dissociate reserpine ions and to transmit remaining reserpine and fragment ions—trace 292. Importantly, the DC potential difference between exit end 296 of ion funnel 169 (Sk1) and multipole 188 (HexDC) is substantially larger when inducing dissociation than when transmitting ions (compare trace 292 with trace 290 from Sk1 to IF2). This relatively high potential difference accelerates the ions. The ions then collide with gas in this region of the source. Such energetic collisions excite the vibrational modes of the ions and lead to fragmentation. In alternate embodiments, any other combination of DC potentials in the source can be used to excite and fragment ions of interest. Referring to FIG. 31, shown is a fragment ion spectrum of the Luteinizing Hormone Releasing Hormone (LHRH) produced by the fragmentation method described with respect to FIGS. 28 and 29. As indicated, the major LHRH fragment ion peaks 302 appear at m/z=249 Th, 499 Th, 662 Th, 749 Th, and 935 Th.
In yet another embodiment, a first ionization means may be used to produce “calibrant” ions while a second ionization means may be used to produce analyte ions. The calibrant and analyte ions can appear in the same mass spectrum. Because the calibrant ions are produced from a known substance and are of a known mass, they can be used to calibrate the mass axis of the spectrum.
An example of such a spectrum is shown in FIG. 32. In this example, analyte ions are produced from a tryptic digest of bovine serum albumin by MALDI in a source as depicted in FIG. 23. In close succession, calibrant ions of ACTH 18-39 are produced by ESI. Signal from the analyte and calibrant ions are summed into the same data set resulting in the spectrum of FIG. 32. In FIG. 32, ACTH 18-39 peaks 304 and 306 appear at m/z=822 Th and 1233 Th, respectively. These peaks are subsequently used to calibrate the mass spectrum, which is then analyzed to determine the masses of the remaining peaks in the spectrum. These mass assignments are then compared to a mass spectral library so as to identify the biochemical origin of the peaks. The results of this analysis appear in TABLE 1 below, which lists the experimentally determined mass (Exptl Mass) of the peaks in the spectrum of FIG. 32, the theoretical mass (Theo. Mass) of the corresponding ions, the mass error (Error(ppm)) in parts per million (ppm) (i.e., the difference between the experimental mass and the theoretical mass divided by the theoretical mass multiplied by one million), and the amino acid sequence (sequence) of the corresponding peptide. As can be seen from TABLE 1, the use of the calibrant peaks results in good agreement between the experimental and theoretical masses (i.e., the mass error is observed over a broad mass range and is minimal over that range).










TABLE 1





Exptl. Mass
Theo. Mass
Error (ppm)
Sequence


















927.4941
927.4934
0.1155
YLYEIAR


1479.7988
1479.7954
1.9015
LGEYGFQNALIVR


1163.6294
1163.6307
−1.5603
LVNELTEFAK


1439.8128
1439.8118
0.3426
RHPEYAVSVLLR


1305.7162
1305.7161
−0.3641
HLVDEPQNLIK


1249.624
1249.6212
1.8324
FKDLGEEHFK


1639.9383
1639.9377
0.003
KVPQVSTPTLVEVSR


1420.676
1420.6777
−1.5786
SLHTLFGDELCK 11: Carboxymethyl (C)


1567.7475
1567.7427
2.6909
DAFLGSFLYEYSR


1168.4632
1168.4609
1.5007
CCTKPESER 1: Carboxymethyl (C) 2:





Carboxymethyl (C)


899.4684
899.4655
2.6011
LCVLHEK 2: Carboxymethyl (C)


1140.4707
1140.466
3.6561
CCTESLVNR 1: Carboxymethyl (C) 2:





Carboxymethyl (C)


974.4552
974.4578
−3.2205
DLGEEHFK


1881.9094
1881.9051
1.9804
RPCFSALTPDETYVPK 3: Carboxymethyl (C)


1534.7587
1534.7491
5.8738
LKECCDKPLLEK 4: Carboxymethyl (C) 5:





Carboxymethyl (C)


1283.7092
1283.7106
−1.5561
HPEYAVSVLLR


1444.6339
1444.626
5.0574
YICDNQDTISSK 3: Carboxymethyl (C)


847.5003
847.5036
−4.5636
LSQKFPK


1577.7554
1577.7516
2.0713
LKPDPNTLCDEFK 9: Carboxymethyl (C)









In alternate embodiments, calibrant ions and analyte ions may appear in successive spectra, may be produced truly simultaneously rather than in close succession, and can be produced using any ionization means. Further, any number of ionization means be used to produce analyte ions from any number of analytes.

Referring next to FIGS. 33A-D, shown is the preferred embodiment of a hexapolar segmented electrode according to the invention. FIGS. 33A, 33B, and 33C show a top planview, a side view, and bottom plan view, respectively, of hexapolar segmented electrode 310. With particular reference to FIG. 33B, the view shown is obtained by rotating segmented electrode 310, as depicted in FIG. 33A, by 90° about axis 312 at line A-A and FIG. 33C is obtained by rotating segmented electrode 310, as depicted in FIG. 33A, by 180° about axis 312 at line A-A, which is an axis of symmetry. FIG. 33D shows a cross-sectional view of segmented electrode 310 formed at axis 312 at line A-A.
Electrode segments 316 and 318 are formed from the deposition of electrically conducting material on the surface of electrically insulating support 320. Importantly, segments 316 and 318 cover the inner surface of aperture 322 as well as the front and back surfaces of support 320 such that ions passing through aperture 322 will not come into contact with an electrically insulating surface. As shown, segments 316 and 318 extend completely through the interior of aperture 322.
Slots 326 formed in support 320 between segments 316 and 318 serve not only to separate segments 316 and 318 but also to remove insulating material of support 320 from the vicinity of ions passing through aperture 322. Holes 324 are used for mounting electrode 310 in the mass spectrometer assembly and may be of any size, number or location necessary for proper mounting. The diameter of aperture 322, the thickness of segmented electrode 310, and the width and depth of slots 326 may all be varied for optimal performance. Preferably, the diameter of aperture 322 is 3 mm, the thickness of electrode 310 is 3.175 mm, and the width and depth of slots 326 are 0.7 mm and 1.3 mm, respectively.
During operation, an RF electrical potential is applied between electrodes 316 and 318 such that ions passing through aperture 322 are forced toward the center of aperture 322. The RF potential applied to segment 316 is preferably the same magnitude and frequency but 180° out of phase with the potential applied to segment 318. Also, a DC potential may be applied between segmented electrode 310 and other elements in the mass spectrometer. The DC potential and the frequency and amplitude of the RF potential can be selected for optimum performance. Preferably, an RF frequency of 2.5 MHz, an amplitude of 400 Vpp, and a DC potential of 15 V referenced to ground are used.
Optionally, electrode 310 can be rotated 180° about axis 314 at line B-B without changing the electrode arrangement in the interior of aperture 322. That is, segments 316 and 318 appear in the same location before and after the rotation. As a result, the same phase RF appears in the same location before and after the rotation. This is advantageous when assembling segmented electrode 310 into the mass spectrometer, because it gives the additional freedom of determining whether segment 316 appears on the front face or back face of support 320.
Referring next to FIGS. 34A-D, shown is an alternate embodiment of the hexapolar segmented electrode according to the invention. Similar to that described with respect to FIG. 33, FIGS. 34A, 34B, and 34C show a top plan view, a side view, and a bottom plan view respectively. The side view shown in FIG. 34B is obtained by rotating segmented electrode 330, as depicted in FIG. 34A, by 90° about axis 332 at line A-A. The bottom plan view shown in FIG. 34C is obtained by rotating segmented electrode 330, as depicted in FIG. 34A, by 180° about axis 332 at line A-A, which is an axis of symmetry.
Electrode segments 336 and 338 are formed from the deposition of electrically conducting material on the surface of electrically insulating support 340. Importantly, 336 and 338 cover the inner surface of aperture 342 as well as the front and back surfaces of support 340 such that ions passing through aperture 342, will not come into contact with an electrically insulating surface. FIG. 34D shows a cross sectional view of segmented electrode 330 formed at axis 332 at line A-A. As shown, segments 336 and 338 extend completely through the interior of aperture 342.
Slots 346 formed in support 340 between segments 336 and 338 serve not only to separate segments 336 and 338 but also to remove insulating material of support 340 from the vicinity of ions passing through aperture 342. Segmented electrode 330 differs from segmented electrode 310 in that slots 346 of segmented electrode 330 terminate in holes 348 having a diameter substantially larger than the width of the slot. Also, insulating support 340 is shaped like an H rather than a square. Holes 348 and cutaways 349 in support 340 have the effect of easing the movement of gas between aperture 342 and the exterior of segmented electrode 330. That is, it is easier to pump gas away from the interior of segmented electrode 330 than from that of segmented electrode 310.
Holes 344 are used for mounting electrode 330 into the mass spectrometer assembly and may be of any size, number or location necessary for proper mounting. The diameter of aperture 342, the thickness of segmented electrode 330, the width and depth of slots 346, the diameter of holes 348, and the width and depth of cutaway 349 can all be varied for optimal performance. Preferably, the diameter of aperture 342 is 3 mm, the thickness of electrode 330 is 3.175 mm, the width and depth of slots 346 are 0.7 mm and 0.5 mm, respectively, the diameter of holes 348 is 2 mm, and the depth and width of cutaway 349 is 10 mm and 18 mm respectively.
During operation, an RF electrical potential is applied between electrodes 336 and 338 such that ions passing through aperture 342 are forced toward the center of aperture 342. The RF potential applied to segment 336 is preferably the same magnitude and frequency but 180° out of phase with the potential applied to segment 338. Also, a DC potential may be applied between segmented electrode 330 and other elements in the mass spectrometer. The DC potential as well as the frequency and amplitude of the RF potential may be selected for optimum performance. Preferably, an RF frequency of 2.5 MHz, amplitude of 400 Vpp, and DC potential of 15 V referenced to ground are used.
Optionally, electrode 330 can be rotated 180° about axis 334 at line B-B without changing the electrode arrangement in the interior of aperture 342. That is, segments 336 and 338 appear in the same location before and after the rotation. As a result, the same phase RF appears in the same location before and after the rotation. This is advantageous when assembling segmented electrode 330 into the mass spectrometer because it provides the additional freedom of determining whether segment 336 appears on the front face or back face of support 340.
Referring next to FIG. 35, shown is still another alternate embodiment of the hexapolar segmented electrode according to the invention. FIGS. 35A, 35B, and 35C show a top plan view, a side view, and a bottom plan view, respectively, of segmented electrode 350. The side view shown in FIG. 35B is obtained by rotating segmented electrode 350, as depicted in FIG. 35A, by 90° about axis 352 at line A-A. The bottom plan view shown in FIG. 35C is obtained by rotating segmented electrode 350, as depicted in FIG. 34A, by 180° about axis 352 at line A-A, which is an axis of symmetry. That is, rotating electrode 350, 180° about axis 352 results in the original electrode and mechanical arrangement.
Electrode segments 356 and 358 are formed from the deposition of electrically conducting material on the surface of electrically insulating support 360. Importantly, 356 and 358 cover the inner surface of aperture 362 as well as the top and bottom surfaces of support 360 such that ions passing through aperture 362 will not come into contact with an electrically insulating surface.
Slots 366 formed in support 360 between segments 356 and 358 serve not only to separate segments 356 and 358 but also to remove insulating material of support 360 from the vicinity of ions passing through aperture 362. Holes 364 are used for mounting electrode 350 in the mass spectrometer assembly. Further, support 360 of segmented electrode 350 is circular, which eases the use of an o-ring to create a vacuum seal between support 360 and an opening in the housing of the mass spectrometer. This allows for the use of segmented electrode 350 as an ion optical device and as a restriction between two pumping regions. The diameter of aperture 362, the thickness of segmented electrode 350, the width and depth of slots 366, and the diameter of support 360 may all be varied for optimal performance. Preferably, the diameter of aperture 362 is 3 mm, the thickness of electrode 350 is 3.175 mm, the width and depth of slots 366 are 0.7 mm and 1.3 mm, respectively, and the diameter of support 360 is 58 mm.
During operation, an RF electrical potential is applied between electrodes 356 and 358 such that ions passing through aperture 362 are forced toward the center of aperture 362. The RF potential applied to segment 356 is preferably the same magnitude and frequency but 180° out of phase with the potential applied to segment 358. Also, a DC potential may be applied between segmented electrode 350 and other elements in the mass spectrometer. The DC potential as well as the frequency and amplitude of the RF potential may be selected for optimum performance. Preferably, an RF potential with a frequency of 2.5 MHz and amplitude of 400 Vpp, and a DC potential of 15 V (referenced to ground) are used.
Optionally, electrode 350 can be rotated 180° about axis 354 at line B-B without changing the electrode arrangement in the interior of aperture 362. That is, the aperture segments 356 and 358 appear in the same location before and after the rotation. As a result, the same phase RF appears in the same location before and after the rotation. This is advantageous when assembling segmented electrode 350 into the mass spectrometer because it provides the additional freedom of determining whether segment 356 appears on the top or bottom of support 360.
Referring now to FIG. 36, shown is a cross-sectional view of assembly 400 consisting of multipole collision cell 386 and hexapole trapping cell 384, which consists of a plurality of hexapolar segmented electrodes 310a-1, 330a-1 and 350a-b. Collision cell 386 consists of enclosure 390, RF multipole 388, and entrance electrode 392 with entrance aperture 394 therein. Multipole 388 is a conventional RF hexapole known in the prior art, having an inscribed diameter of 8.8 mm and aligned with the axis of assembly 400. Of course, any RF multipole of any inscribed diameter can be used without departing from the spirit of the invention.
During operation, ions enter collision cell 386 through aperture 394. A DC potential difference applied between electrode 392 and multipole 388 forces the ions into multipole 388. An RF potential is applied between adjacent rods of multipole 388, and the resulting electric field focuses ions toward the central axis of multipole 388. The pressure in the collision cell is preferably maintained at 10−3 mbar or higher by introduction of a selected gas, which is, typically N2 or Ar. Other pressures and other types of gases or mixture of gases can be used. Collisions with gas molecules in collision cell 386 reduce the kinetic energy of the ions. If a retarding potential is applied to electrode 382, the ions will be trapped in multipole 388. That is, the RF potential applied between the multipole rods contains the ions radially and the DC potentials applied between electrode 392 and multipole 388 and between electrode 382 and multipole 388 contain the ions axially. These potentials can be selected for optimum performance. Preferably, however, an RF frequency of 1.2 MHz and 300 Vpp is applied between rods of multipole 388, a potential difference of 3V DC is applied between electrode 392 and multipole 388, and a potential difference of 20V DC is applied between electrode 382 and multipole 388.
If the potential difference between electrode 382 and multipole 388 is lowered, ions in collision cell 386 pass through the aperture in electrode 382 into hexapole trapping cell 384. Preferably, ions are trapped in multipole 388 for a predetermined period of time and then released as a pulse of ions into trapping cell 384. During the trapping period, the potential difference between electrode 382 and multipole 388 is held at a repulsive potential. To release the ions from the collision cell the potential difference between electrode 382 and multipole 388 is temporarily pulsed to a neutral or attractive potential. The timing and potentials may be selected for optimum performance. For example, the duration of the period in which ions are trapped may be 1 millisecond (ms), the duration of the pulse releasing the ions may be 0.2 ms, and the potential difference between electrode 382 and multipole 388 used to trap and release the ions may be 3V and −2V respectively. Of course, other combinations can be used without departing from the spirit of the invention.
The kinetic energy of the ions injected into collision cell 386 may be high enough such that collisions between the injected “precursor” ions and the collision gas in cell 386 can cause the precursor ions to dissociate and form fragment ions. In this case, the fragment and surviving precursor ions will be trapped and released as described above.
Hexapole trapping cell 384 consists of segmented electrodes 310a-1, 330a-1, and 350a-b, as described above with reference to FIGS. 32-34. Electrodes 310a-1, 330a-1, and 350a-b are assembled into cell 384 as shown in FIG. 36, such that the center of the aperture in electrodes 310a-1, 330a-1, and 350a-b reside coaxially with multipole 388 on the central axis of assembly 400. As depicted in FIG. 36, the segments of electrodes 310, 330, and 350 are aligned with segments in adjacent electrodes having the same RF phase. For example, segment 316a of electrode 310a is aligned with segment 316b of electrode 310b, and so on. Thus, ion optically trapping cell 384 has the appearance and function of an RF hexapole, which has been divided into sections in a similar manner as described with reference to FIG. 4. The sections according to the embodiment of FIG. 36 are preferably 3.175 mm long and the gap between sections is 0.79 mm.
As shown in FIG. 36, electrodes 310a-1 all have a construction identical to segmented electrode 310 as described with respect to FIG. 33. Electrodes 310a-h are assembled adjacent to electrode 382 together with teflon gaskets 374. Teflon gaskets 374, the small diameter aperture 322, and the short, narrow slots 326 result in a low gas conductivity in this case  ̃0.1 L/s—through the length of electrodes 310a-h. 
Similarly, electrodes 330a-1 all have a construction identical to segmented electrode 330 as described with respect to FIGS. 33A-D, and electrodes 330a-e are assembled adjacent to electrode 310h as depicted in FIG. 36. The relatively open construction of electrodes 330a-1 results in higher gas conductivity through the length of hexapole trap cell 384 composed of electrodes 330a-1. Further, the absence of gaskets between electrodes 330a-1 and cutaway 349 (see FIG. 34A) results in a higher gas conductance between the interior (i.e., aperture 342) and exterior of assembly 384 in those regions constructed from electrodes 330a-1.
Electrodes 350a-b have a construction identical to segmented electrode 350 as depicted in FIGS. 34A-D. Electrode 350a is assembled adjacent to electrode 330e as depicted in FIG. 36. O-ring 370a and retaining ring 372a are assembled together with electrode 350a to form a seal with the mass spectrometer housing when inserted into the instrument. Electrode 350a together with gasket 373 and the seal formed by o-ring 370a between electrode 350a and the wall of the mass spectrometer housing (not shown) form a pumping restriction between that region containing electrodes 310a-e (i.e., pumping region 179) and that region containing electrodes 330f-1 (i.e., pumping region 402).
Electrodes 330f-1 are assembled between electrodes 350a-b as depicted in FIG. 36. As discussed above, the absence of gaskets between electrodes 330f-1 and cutaway 349 results in a higher gas conductance between the interior (i.e., aperture 342) and exterior of assembly 384 in the pumping region formed between electrodes 350a and 350b (i.e., pumping region 402). A pump is used to pump gas away from assembly 384 in pumping region 402 through the gaps between electrodes 330f-1 and 350b. 
End electrodes 376, 378, and 380 are preferably apertured metal plates whose apertures are coaxially aligned with the axis of assembly 400. These electrodes form an exit lens for trapping cell 384. The dimensions of electrodes 376, 378, and 380 may vary widely, but preferably, the thickness of these electrodes is 0.5 mm, the gap between these electrodes is 0.5 mm, and the diameter of the aperture in these electrodes is 2 mm. Together with electrodes 310i-1, gaskets 375, o-ring 370b, and electrode 350b, electrodes 376, 378, and 380 form a pumping restriction between pumping region 402 and pumping region 234.
In one mode of operation of assembly 400, all segmented electrodes 310, 330, and 350 are held at the same selected DC and RF potentials. Electrodes 382 and 376 are used to control the entrance and exit respectively of ions into and out of cell 384. By placing a DC potential on electrodes 382 and 376 that is more repulsive than the DC potential on segmented electrodes 310, 330, and 350, ions are trapped in cell 384. For example, the DC potential applied to electrodes 382 and 376 may be 18V and 40V respectively while the DC potential applied to segmented electrodes may be 15V and the RF frequency and amplitude applied between segments 316 and 318, segments 336 and 338, and segments 356 and 358 is 2.5 MHz and 300 V respectively. In such a case, ions are trapped axially by the repulsive potential on electrodes 382 and 376 and radially by the RF potential applied between segments 316 and 318, 336 and 338, and 356 and 358.
If the potential difference between electrode 376 and segmented electrodes 310, 330, and 350 is lowered, ions in cell 384 may pass through the aperture in electrode 376 and out of cell 384. Preferably, ions are trapped in cell 384 for a predetermined period of time and then released as a pulse of ions. During the trapping period, the potential difference between electrode 376 and electrodes 310, 330, and 350 is held at a repulsive potential. To release the ions from the collision cell the potential difference between electrode 376 and electrodes 310, 330, and 350 is temporarily pulsed to a neutral or attractive potential. The timing and potentials may be selected for optimum performance. For instance, the duration of the period in which ions are trapped may be 0.5 ms, the duration of the pulse releasing the ions may be 0.2 ms, and the potential difference between electrode 376 and electrodes 310, 330, and 350 used to trap and release the ions may be 25V and −2V respectively. Of course, any other combination can be used without departing from the spirit of the invention.
During the release of ions from trapping cell 384, it is useful to focus the ions. The ions are typically focused into a parallel beam for injection into a mass analyzer following trapping cell 384. Electrodes 376, 378, and 380 are used together for this purpose. As an example, when releasing ions from cell 384, electrodes 310, 330, and 350 are held at a DC potential of 15V and electrodes 376, 378, and 380 are held at 13V, −50V, and 0V respectively. This focuses the ions exiting trapping cell 384 into a parallel beam. Alternatively, electrodes 310, 330, 350, 376, 378, and 380 can be held at any selected DC potential consistent with the release of ions from cell 384.
An example of the operating potentials applied to assembly 400 is provided in TABLE 2 below, which provides the elements in assembly 400 and the corresponding DC potentials applied to the enumerated elements when the ions are trapped in collision cell 386, when the ions are being released from collision cell 386 into trapping cell 384, and when the ions are being released from trapping cell 384.









TABLE 2









DC Potentials (V)













Trapping in
Release from
Release from



Element
Cell 386
Cell 386
Cell 384
















392
23
23
23



388
20
20
20



382
40
18
18



310a
15
15
15



310b
15
15
15



310c
15
15
15



310d
15
15
15



310e
15
15
15



310f
15
15
15



310g
15
15
15



310h
15
15
15



330a
15
15
15



330b
15
15
15



330c
15
15
15



330d
15
15
15



330e
15
15
15



350a
15
15
15



330f
15
15
15



330g
15
15
15



330h
15
15
15



330i
15
15
15



330j
15
15
15



330k
15
15
15



330l
15
15
15



350b
15
15
15



310i
15
15
15



310j
15
15
15



310k
15
15
15



310l
15
15
15



376
40
40
13



378
−50
−50
−50



380
0
0
0










Alternatively, an axial DC field can be used in trapping cell 384 either during the trapping or release of ions to push the ions towards the exit end of cell 384. An example of such alternate operating potentials is shown in TABLE 3 below, which provides the elements in assembly 400 the corresponding DC potentials applied to the enumerated elements when the ions are being trapped in collision cell 386, when the ions are being released from collision cell 386 into trapping cell 384, and when the ions are being released from trapping cell 384.









TABLE 3









DC Potentials (V)













Trapping in
Release from
Release from



Element
Cell 386
Cell 386
Cell 384
















392
23
23
23



388
20
20
20



382
40
18
18



310a
15
15
17.5



310b
15
15
17.4



310c
15
15
17.3



310d
15
15
17.2



310e
15
15
17.1



310f
15
15
17



310g
15
15
16.9



310h
15
15
16.8



330a
15
15
16.7



330b
15
15
16.6



330c
15
15
16.5



330d
15
15
16.4



330e
15
15
16.3



350a
15
15
16.2



330f
15
15
16.1



330g
15
15
16



330h
15
15
15.9



330i
15
15
15.8



330j
15
15
15.7



330k
15
15
15.6



330l
15
15
15.5



350b
15
15
15.4



310i
15
15
15.3



310j
15
15
15.2



310k
15
15
15.1



310l
15
15
15



376
40
40
13



378
−50
−50
−50



380
0
0
0










In this example a 0.1V DC potential difference between adjacent segmented electrodes results in a 30V/m DC axial electric field that pushes ions toward exit electrode 376. Simultaneously, the potential on electrode 376 is dropped, which reduces the time required to empty ions out of cell 384. Of course, any desired set of potentials can be used to produce any desired axial DC field strength. In addition, the DC potentials applied to the segmented electrodes can be used to focus the ions in a selected region of cell 384, to move ions back and forth within cell 384, or fragment ions in cell 384.
Further, the amplitude of the RF signal applied to the segmented electrodes is a function of the electrode position within assembly 400. A variation in RF amplitude with respect to position is used to manipulate the ions in the same manner as described with respect to the DC potentials above. An additional advantage of varying the RF amplitude with respect to its position is that both positive and negative ions are manipulated simultaneously in the same way. For example, if the RF amplitude applied to segmented electrodes at either end of cell 384 is greater than that applied to segmented electrodes in the central portion of cell 384, then both positive ions and negative ions may be trapped in the central region of cell 384. This may be of particular advantage when performing, for example, electron transfer dissociation reactions. That is multiply charged positive analyte ions can be trapped in the same volume (i.e., in cell 384) with singly charged negative reagent ions. When these ions interact, an electron is transferred from the negative reagent ion to the positively charged analyte ion. The energy released causes the dissociation of the analyte ion into fragment ions.
Referring next to FIG. 37, shown is assembly 400, including collision cell 386 and trapping cell 384, assembled in a system with ion guide 199, MALDI target 270, orthogonal glass capillary 186 by which ESI ions may be introduced, multipole ion guide 188, and analyzer quadrupole 232. As described with respect to FIG. 23, either MALDI or ESI may be used to produce ions simultaneously, in close succession, or independently. Of course, any other well known ionization means can be used to produce ions.
As discussed with respect to FIG. 20, after passing through ion guides 199 and 188, the ions are mass analyzed by analyzer quadrupole 232. That is, ions of a selected mass-to-charge ratio are passed from ion guide 188 to collision cell 386 via analyzer quadrupole 232 while rejecting substantially all other ions. In the present embodiment, a DC potential is applied between all adjacent elements so as to force the ions through the system from upstream elements (e.g., funnel 199) toward downstream elements (e.g., cell 384)—that is, from left to right in FIG. 37.
Also, as discussed with respect to FIG. 20, the gas pressure in collision cell 386 is preferably 10−3 mbar or greater. Typically the gas is inert (e.g., Nitrogen or Argon) however, reactive species might also be introduced into the chamber. When the potential difference between quadrupole 232 and cell 386 is low, for example 5V, the ions are simply transmitted therethrough. That is, the energy of collisions between the ions and the gas in ion guide 386 is too low to cause the ions to fragment. However, if the potential difference between quadrupole 232 and cell 386 is high, for example 100 V, the collisions between the ions and gas may cause the ions to fragment.
As described above with reference to FIG. 36, precursor and fragment ions may be trapped for a predetermined period in collision cell 386 before being released to cell 384. From trapping cell 384 the ions are released into region 234 where the precursor and fragment ions may be analyzed by a mass analyzer (not shown). Quadrupole 232, collision cell 386, and part of trapping cell 384 all preferably reside in pumping region 179. As discussed above with reference to FIG. 20, pressure in analyzer quadrupole 232 should be maintained at 10−5 mbar or less. The pressure in collision cell 386 should be maintained at 10−3 mbar or more. In the embodiment of FIG. 37, a selected collision gas is introduced into collision cell 386 through a leak valve (not shown), which maintains pressure in collision cell 386 by balancing the rate at which gas is leaked through the leak valve and the rate at which gas is escapes via the apertures in elements 382 and 394. Gas escaping cell 386 via the aperture in element 394 flows into analyzer quadrupole 232 but is pumped away via pumping port 185. Gas escaping cell 386 via the aperture in element 382 enters trapping cell 384 but is substantially pumped away via the gaps between elements 330a-e and pumping port 185. Most of the remaining gas in ion trap 384 passes through the gaps between elements 330f-1 and is pumped away via pumping port 404. As a result, the pressure in region 402 is reduced to about 10−6 mbar. Gas in cell 384 not pumped away via pumping port 185 or 404 passes through the apertures in elements 376, 378, and 380 and enters pumping chamber 234. From there the gas is pumped away via pumping port 236, which maintains a pressure of about 10−8 mbar.
Referring finally to FIG. 38, shown is assembly 410 comprising collision cell 386 and trapping cell 414. Assembly 410 is similar to assembly 400 except that segmented plates 310a-d have been replaced with segmented electrodes 411a-g and gaskets 412. Segmented electrodes 411a-g are each substantially similar to segmented electrode 310 except that the diameter of the central aperture in electrodes 411a-g is larger than aperture 322 of electrode 310 and the thickness of electrodes 411a-g is smaller, for example, it is 1.58 mm rather than the 3.175 mm of electrode 310. Also, the gap between adjacent electrodes 411a-g is 0.79 mm. Preferably, the diameter of the apertures in electrodes 411a-g is 8.81 mm, 8.56 mm, 7.79 mm, 7.02 mm, 6.25 mm, 5.48 mm, and 4.71 mm, respectively.
Still referring to FIG. 38, the frequency of the RF applied to multipole 388 is preferably the same as the frequency of the RF applied to all segment electrodes 411, 310, 330, and 350. Also, electrodes 411 are assembled into assembly 410 such that segments in adjacent electrodes are in phase with each other and also in phase with adjacent rods in multipole 388. An example of the DC potentials and RF amplitudes applied to the elements of assembly 410 is shown below in TABLE 4, which provides the elements in assembly 410 and the corresponding DC potentials and RF amplitudes applied to these elements when the ions are being trapped in trapping cell 386.










TABLE 4









Trapping in Cell 384












Element
DC (V)
RF (Vpp)















392
23
NA



388
20
500



411a
19.38
462.50



411b
18.75
425.00



411c
18.13
387.50



411d
17.50
350.00



411e
16.88
312.50



411f
16.25
275.00



411g
15.63
237.50



310e
15
200



310f
15
200



310g
15
200



310h
15
200



330a
15
200



330b
15
200



330c
15
200



330d
15
200



330e
15
200



350a
15
200



330f
15
200



330g
15
200



330h
15
200



330i
15
200



330j
15
200



330k
15
200



330l
15
200



350b
15
200



310i
15
200



310j
15
200



310k
15
200



310l
15
200



376
40
NA



378
−50
NA



380
0
NA










In this example the RF amplitudes and DC potentials applied to electrodes 411a-g are a linear function of their position in assembly 410. As a result, a DC field is formed which forces ions from collision cell 386 through electrodes 411 and into trapping cell 414. Simultaneously, the RF potential applied to electrodes 411 focuses the ions radially onto the axis of assembly 410 such that ions can be transmitted, with high efficiency, into cell 414. Of course, different aperture dimensions, a different number of electrodes 411, and different potentials can all be used without departing from the spirit of the invention.

While the present invention has been described with reference to one or more preferred and alternate embodiments, such embodiments are merely exemplary and are not intended to be limiting or represent an exhaustive enumeration of all aspects of the invention. The scope of the invention, therefore, shall be defined solely by the following claims. Further, it will be apparent to those of skill in the art that numerous changes may be made in such details without departing from the spirit and the principles of the invention. It should be appreciated that the present invention is capable of being embodied in other forms without departing from its essential characteristics.

## Claims

1. An ion source comprising:
first and second ionization means for generating first ions in a direction along a first axis and second ions in a direction along a second axis, respectively; and
at least one ion funnel having an entrance end, an exit end and a central axis;
wherein neither said first axis nor said second axis intersect said central axis;
wherein said first ions are introduced into said entrance end of said at least one ion funnel; and
wherein said at least one ion funnel guides said ions from said entrance end to said exit end.

2. An ion source according to claim 1, wherein said first axis does not intersect said second axis.

3. An ion source according to claim 1, wherein said first and second ionization means are selected from the group consisting of matrix-assisted laser desorption/ionization (MALDI), electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), electron ionization (EI), chemical ionization (CI), secondary ion mass spectrometry (SIMS), fast atom bombardment (FAB), and laser desorption ionization (LDI).

4. An ion source according to claim 1, wherein at least one of said first or second ionization means resides in a first vacuum chamber with said at least one ion funnel.

5. An ion source according to claim 1, wherein at least one of said first or second ionization means resides in a different vacuum chamber from said at least one ion funnel.

6. An ion source according to claim 1, wherein said at least one ion funnel begins in a first vacuum chamber and ends in a second vacuum chamber.

7. An ion source according to claim 1, wherein said at least one ion funnel comprises a plurality of coaxially arranged segmented electrodes.

8. An ion source according to claim 1, wherein said at least one ion funnel guides said first ions or said second ions into an ion trap.

9. An ion source according to claim 1, wherein said at least one ion funnel guides said first ions or said second ions into a mass analyzer.

10. An ion source according to claim 1, wherein the ions from the first and second ionization means are introduced into or ionized with a chamber, and further where the entrance end of the ion funnel is in the chamber.

11. An ion source according to claim 10, wherein the chamber is a vacuum chamber.

12. A method for guiding sample ions from an ion source to a mass analyzer, said method comprising the steps of:
introducing first ions from a first ion production means into an ion funnel from a first direction; and
introducing second ions from a second ion production means into said funnel from a second direction.

13. A method according to claim 12, wherein said first ions and said second ions are introduced simultaneously.

14. A method according to claim 12, wherein said first ions and said second ions are introduced in close succession.

15. A method according to claim 12, wherein said first ion production means and said second ion production means are selected from the group consisting of matrix-assisted laser desorption/ionization (MALDI), electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), electron ionization (EI), chemical ionization (CI), secondary ion mass spectrometry (SIMS), fast atom bombardment (FAB), and laser desorption ionization (LDI).

16. A method according to claim 12, said method further comprising the steps of: guiding said first ions and said second ions through said ion funnel towards a mass analyzer; analyzing the mass of said first ions and said second ions using said mass analyzer; and using signals produced from said first ions to effect an improved mass assignment of said second ions.

17. A method according to claim 16, wherein the mass of said first ions is known to high accuracy.

18. A method according to claim 17, wherein the mass of said second ions is unknown.

19. A method according to claim 12, wherein at least one of said first or second ion production means resides in a different vacuum chamber from said at least one ion funnel.

20. A method according to claim 12, wherein said ion funnel begins in a first vacuum chamber and ends in a second vacuum chamber.

21. A method according to claim 12, wherein said ion funnel comprises a plurality of coaxially arranged segmented electrodes.

22. A method according to claim 12, wherein said ion funnel guides said first ions or said second ions into an ion trap.

23. A method according to claim 12, wherein the ions from the first and second ionization means are introduced into or ionized with a chamber, and further where the entrance end of the ion funnel is in the chamber.

24. A method according to claim 23, wherein the chamber is a vacuum chamber.