Method of operating an ion trap

Abstract: A simple and economical method of mass analyzing a sample by means of a quadrupole ion trap mass spectro­ meter in an MS/MS mode comprises the steps of forming ions within a trap structure, changing the RF and/or DC voltages in such a way that the ions with mass-to-charge ratios within a desired range will be and remain trapped within the trap structure, dissociating such ions into frag­ ments by collisions, and increasing the field intensity again so that the generated fragments will become unstable and exit the trap volume sequentially to be detected. A sup­ plementary AC field may be applied additionally to provide various scan modes as well as dissociate the ions.

Description: The present invention relates to a method of operating an ion trap as described in the first part of claim 1. Such a method is known from the article in the Int. Journ. of Mass. Spectromety which will be discussed later. Ion trap mass spectrometers, or quadrupole ion stores, have been known for many years and described by a number of authors. They are devices in which ions are formed and contained with a physical structure by means of electrostatic fields such as RF, DC or a combination thereof. In general, a quadrupole electric field provides an ion storage region by the use of a hyperbolic electrode structure or a spherical electrode structure which provides an equivalent quadrupole trapping field. Mass storage is generally achieved by operating trap electrodes with values of RF voltage (V) and its frequency (f), DC voltage (U) and device size (ro) such that ions having their mass-to-charge ratios within a finite range are stably trapped inside the device. The aforementioned parameters are sometimes referred to as scanning parameters and have a fixed relationship to the mass-to-charge ratios of the trapped ions. For trapped ions, there is a distinctive secular frequency for each value of mass-to-charge ratio. In one method for detection of the ions, these secular frequencies can be determined by a frequency tuned circuit which couples to the oscillating motion of the ions within the trap, and then the mass-to-charge ratio may be determined by use of an improved analyzing technique. In spite of the relative length of time during which ion trap mass spectrometers and methods of using them for mass analyzing a sample have been known they have not gained popularity until recently because these mass selection techniques are insufficient and difficult to implement and yield poor mass resolution and limited mass range. A new method of ion trap operation (U.S. Patent No. 2939952 and EP-A-0113207 has overcome most of the past limitations and is gaining popularity as a product called the lon Trap Detector. An ion trap mass spectrometer is described in a paper by R.F. Bonnet, G. Lawson and J.F.J. Todd ("Ion-Molecule Reaction Studies with a Quadrupole Ion Storage Trap", International Journal of Mass Spectrometry and Ion Physics Vol. 10 No. 2 December 1972, pages 197-203, Elsevier Publishing Co., Amsterdam NL) which mass analyses a sample by means of a quadrupole mass spectrometer by defining a trap volume within an electrode structure comprising a ring electrode and two end caps at both sides of the ring electrode to which a DC voltage and a fundamental RF voltage is applied to form a three-dimensional quadrupole field adapted to trap ions within a predetermined range of mass-to-charge ratio. Ions are formed or injected within the trap volume such that those within the predetermined mass-to-charge range are trapped within the trap volume. An RF generator coupled to the end caps is utilised to apply a supplementary AC field superimposing the three-dimensional quadrupole field to form combined fields. According to the invention as described in claim 1 there is provided a method of mass analysing a sample by means of a quadrupole mass spectrometer, comprising the steps of defining a trap volume within an electrode structure comprising a ring electrode and two end caps at both sides . of the ring electrode to which a DC voltage and a fundamental RF voltage are applied to form a three-dimensional quadrupole field adapted to trap ions within a predetermined range of mass-to-charge ratio; forming or injecting ions within said trap volume such that those within said predetermined mass-to-charge range are trapped within said trap volume; and utilising an RF generator coupled to end caps to apply a supplementary AC field super-posing said three-dimensional quadrupole field to form combined fields, characterised by the steps of scanning said combined fields with the supplementary field turned on to cause ions of all mass-to-charge ratios in said range to escape said trap volume in consecutive mass-to-charge ratio order for detection and analysis. This invention provides a new method of operating an ion trap, in a mode of operation called MS/MS, which method enables mass analysis of a sample by forming and storing ions in the ion trap, mass-selecting them by a mass analyser, and ejecting ions of consecutive mass-to-charge ratio for detection and analysis by scanning the quadupole field and/or supplementary field. Examples useful in understanding this invention will now be described with reference to the drawings, in which:- Fig. 1 is a simplified schematic of a quadupole ion trap along with a block diagram of associated electrical circuits adapted to be used according to a method embodying the present invention; Fig. 2 shows a stability envelope for an ion trap device of the type shown in Fig. 1; Figs. 3(A) and 3(B) are spectrograms obtained by a series of experiments with a nitrobenzene sample; Fig. 4 shows a program that may be used for a notch-filter scan mode with a supplementary voltage and is also useful for understanding the invention; Figs. 5(A) and 5(B) are spectrograms obtained with a xenon sample by using the method of Fig. 4; Fig. 6(A) through Fig. 6(D) are spectrograms obtained with a nitrobenzene sample using the method of Fig. 4; Fig. 7 shows another program for an ion scan mode; and Fig. 8(A) through Fig. 8(D) are spectrograms obtained with an n-heptane sample by a series of experiments in which both the methods of Figs. 4 and 7 are used. There is shown in Fig. 1 at 10 a three-dimensional ion trap which includes a ring electrode 11 and two end caps 12 and 13 facing each other. A radio frequency voltage generator 14 is connected to the ring electrode 11 to supply a radio frequency voltage V sin Wt (the fundamental voltage) between the end caps and the ring electrode which provides the quadrupole field for trapping ions within the ion storage region or volume 16 having a radius ro and a vertical dimension z 0 (z 0 2 = r 0 2 /2). The field required for trapping is formed by coupling the RF voltage between the ring electrode 11 and the two end cap electrodes 12 and 13 which are common mode grounded through coupling transformer 32 as shown. A supplementary RF generator 35 is coupled to the end caps 22, 23 to supply a radio frequency voltage V 2 sin W 2 t between the end caps to resonate trapped ions at their axial resonant frequencies. A filament 17 which is fed by a filament power supply 18 is disposed to provide an ionizing electron beam for ionizing the sample molecules introduced into the ion storage region 16. A cylindrical gate electrode and lens 19 is powered by a filament lens controller 21. The gate electrode provides control to gate the electron beam on and off as desired. End cap 12 includes an aperture through which the electron beam projects. The opposite end cap 13 is perforated 23 to allow unstable ions in the fields of the ion trap to exit and be detected by an electron multiplier 24 which generates an ion signal on line 26. An electrometer 27 converts the signal on line 26 from current to voltage. The signal is summed and stored by the unit 28 and processed in unit 29. Controller 31 is connected to the fundamental RF generator 14 to allow the magnitude and/or frequency of the fundamental RF voltage to be varied for providing mass selection. The controller 31 is also connected to the supplementary RF generator 35 to allow the magnitude and/or frequency of the supplementary RF voltage to be varied or gated. The controller on line 32 gates the filament lens controller 21 to provide an ionizing electron beam only at time periods other than the scanning interval. Mechanical details of ion traps have been shown, for example, U.S. Patent No. US-A-2,939,952 and more recently in EP-A-0113207. The symmetric fields in the ion trap 10 lead to the well known stability diagram shown in Fig. 2. The parameters a and q in Fig. 2 are defined as a = -8eU/mr 0 2 ω 2 q = 4eV/mr 0 2 ω 2 where e and m are respectively charge on and mass of charged particle. For any particular ion, the values of a and q must be within the stability envelope if it is to be trapped within the quadrupole fields of the ion trap device. The type of trajectory a charged particle has in a described three-dimensional quadrupole field depends on how the specific mass of the particle, m/e, and the applied field parameters, U, V, r o and ω combine to map onto the stability diagram. If the scanning parameters combine to map inside the stability envelope then the given particle has a stable trajectory in the defined field. A charged particle having a stable trajectory in a three-dimensional quadrupole field is constrained to a periodic orbit about the center of the field. Such particles can be thought of as trapped by the field. If for a particle m/e, U, V, r o and ω combine to map outside the stability envelope on the stability diagram, then the given particle has an unstable trajectory in the defined field. Particles having unstable trajectories in a three-dimensional quadrupole field obtain displacements from the center of the field which approach infinity over time. Such particles can be thought of escaping the field and are consequently considered untrappable. For a three-dimensional quadrupole field defined by U, V, r o and ω, the locus of all possible mass-to-charge ratios maps onto the stability diagram as a single straight line running through the origin with a slope equal to -2U/V. (This locus is also referred to as the scan line.) That portion of the loci of all possible mass-to-charge ratios that maps within the stability region defines the region of mass-to-charge ratios particles may have if they are to be trapped in the applied field. By properly choosing the magnitude of U and V, the range of specific masses to trappable particles can be selected. If the ratio of U to V is chosen so that the locus of possible specific masses maps through an apex of the stability region (line A of Fig. 2) then only particles within a very narrow range of specific masses will have stable trajectories. However, if the ratio of U to V is chosen so that the locus of possible specific masses maps through the middle of the stability region (line B of Fig. 2) then particles of a broad range of specific masses will have stable trajectories. The ion trap of the type described above is operated as follows: ions are formed within the trap volume 16 by gating a burst of electrons from the filament 17 into the trap. The DC and RF voltages are applied to the three-dimensional electrode structure such that ions of a desired mass or mass range will be stable while all others will be unstable and expelled from the trap structure. This step may be carried out by using only the RF potential so that the trapped ions will lie on a horizontal line through the origin in the stability diagram of Fig. 2 (a = 0). The electron beam is then shut off and the trapping voltages are reduced until U becomes 0 in such a way that the loci of all stably trapped ions will stay inside the stability region in the stability diagram throughout this process. The value of q must be reduced sufficiently low so that not only the ions of interest but any fragment ions which are formed therefrom in a subsequent dissociation process to be described below will also remain trapped (because a lower mass-to-charge ratio means a large q value). In the dissociation step, the ions of interest are caused to collide with a gas so as to become dissociated into fragments which will remain within the trap, or within the stability region of Fig. 2. Since the ions to be fragmented may or may not have sufficient energy to undergo fragmentation by colliding with a gas, it may be necessary to pump energy into the ions of interest or to cause them to collide with energetic or excited neutral species so that the system will contain enough energy to cause fragmentation of the ions of interest. The fragment ions are then swept from the trap by the RF voltage along the horizontal line a = 0 in Fig. 2 so as to be detected. Any of the known ways of producing energetic neutral species may be used in the preceding step. Excited neutrals of argon or xenon may be introduced from a gun, pulsed at a proper time. A discharge source may be used alternatively. A laser pulse may be used to pump energy into the system, either through the ions or through the neutral species. In what follows, there will be shown results of experiment for determining in the case of nitrobenzene ions (with molecular weight M = 123 and degree of ionization Z = 1) what fragment ions (daughter ions), what fragment ions of fragment ions (granddaughter ions), etc. will arise when dissociation of the parent ions is induced by collisions with a background gas such as argon and the resultant ions out of the ion trap are scanned to determine their mass spectrum. Fig. 3(A) is an electron ionization mass spectrogram of nitrobenzene. Line M/Z = 124 arises from an ion-molecule reaction which adds a proton to M/Z = 123. Operating in the mode with U=0 and with 1.333x10 -2 Pa (1x10 -4 ) torr) of Ar, the RF voltage was adjusted first such that only ions with M/Z greater than 120 would be stored in the ion trap at the end of sample ionization. The RF voltage was then lowered such that the cut-off value would be M/Z = 20 so that ions with M/Z above this value would be trapped or stable in the ion trap. Parent ions with M/Z = 123 which remained trapped in the ion trap after ionization collided with a background gas of argon and dissociated. Next the RF was scanned up and the mass spectrogram shown in Fig. 3(B) was obtained, representing the ions produced from the parent with M/Z = 123. A variety of new scan modes becomes possible with the superposition of an AC field such as an RF field. For any ion stored in the ion trap, the displacement in any space coordinate must be a composite of periodic function of time. If a supplementary RF potential is applied that matches any of the component frequencies of the motion for a particular ion species, that ion will begin to oscillate along the coordinate with increased amplitude. The ion may be ejected from the trap, strike an electrode, or in the presence of significant pressure of sample or inert damping gas may assume a stable trajectory within the trap of mean displacement greater than before the application of the supplementary RF potential. If the supplementary RF potential is applied for a limited time, the ion may assume a stable orbit, even under conditions of low pressure. Fig. 4 illustrates a program that may be used for a notch-filter mode. Reference being made to this figure, ions of the mass range of interest are produced and stored in period A, and then the fundamental RF voltage applied to the ring electrode is increased to eject all ions of M/Z less than a given value. The fundamental RF voltage is then maintained at a fixed level which will trap all ions of M/Z greater than another given value (period D). A supplementary RF voltage of appropriate frequency and magnitude is then applied between the end caps and all ions of a particular M/Z value are ejected from the trap. The supplementary voltage is then turned off and the fundamental RF voltage is scanned to obtain a mass spectrum of the ions that are still in the trap (period E). Fig. 5(A) shows a spectrum of xenon in which the fundamental RF voltage is scanned as in Fig. 4 but in which a supplementary voltage is not used. Fig. 5(R) shows a spectrum obtained under similar conditions but a supplementary voltage of appropriate frequency and magnitude is used to eject ions of M/Z = 131 during period D. Fig. 5(B) shows that these ions are largely removed from the trap. There are many ways of actually using the notch-filter mode. For example, the supplementary RF voltage might be turned on during the ionization period and turned off at all other times. An ion which is present in a large amount would be ejected to facilitate the study of ions which are present in lesser amounts. In an embodiment of the invention, a useful scan mode uses the supplementary field during periods in which the fundamental RF voltage or its associated DC component is scanned rather than maintained at a constant level. For example, if a supplementary voltage of sufficient amplitude and fixed frequency is turned on during period E (instead of during period D), ions will be successively ejected from the trap as the fundamental RF voltage successively produces a resonant frequency in each ion species which matches the frequency of the supplementary voltage. In this way, a mass spectrum over a specified range of M/Z values can be obtained with a reduced maximum magnitude of the fundamental RF voltage or a larger maximum M/Z value may be attained for a given maximum magnitude of the fundamental RF voltage. Since the maximum attainable value of the fundamental RF voltage limits the mass range in the ordinary scan mode, the supplementary RF voltage extends the mass range of the instrument. Useful scan modes embodying the invention are also possible in which the frequency of the supplementary voltage is scanned. For example, the frequency of the supplementary voltage may be scanned while the fundamental RF voltage is fixed. This would correspond to Fig. 4 with period E absent and the frequency of the supplementary RF voltage being scanned during period D. A mass spectrum is obtained as ions are successively brought into resonance. Increased mass resolution is possible in this mode of operation. Also, an extended mass range is attainable because the fundamental RF voltage is fixed. The presence of a supplementary RF voltage may induce fragmentation of ions at or near resonance. Fig. 6(A) shows a spectrum of nitrobenzene (with 0.1333Pa (1x10 -3 torr) He) acquired with the scan program of Fig. 4 but without a supplementary RF voltage. All ions of M/Z less than 118 are ejected before and during period B so that the small peak at M/Z = 93 must have been formed after period B and before the ejection of ions of M/Z = 93 during period E. Fig. 6(B) shows a spectrum obtained under the same conditions except that a supplementary RF voltage at the resonant frequency of M/Z = 123 was applied during interval D. The spectrum shows abundant fragment ions at M/Z = 93 and 65. Similarly, Fig. 6(C) was acquired as was Fig. 6(A), except that all ions of M/Z less than 88 are ejected before and during period B. Fig. 6(D) was acquired under the same conditions as Fig. 6(C), except that a supplementary RF voltage at the resonant frequency of M/Z = 93 was applied during interval D. This spectrum shows an abundant fragment at M/Z = 65. Sequential experiments are possible in which daughter ions are produced with the supplementary RF field and granddaughter ions are then produced from those daughter ions by adjusting the conditions such as voltage or frequency of the fundamental RF field or the supplementary RF field so that the daughter ions are brought into resonance. Fig. 7 shows a particular way in which daughter ions may be produced. The frequency of the supplementary RF voltage remains constant but the fundamental RF voltage is adjusted during period DA to bring a particular parent ion into resonance so that granddaughter ions are produced. During period DB, the fundamental RF voltage is adjusted to bring a particular daughter ion into resonance so that granddaughter ions will be produced. Fig. 8(A) shows a spectrum of n-heptane during the acquisition of which the scan program of Fig. 7 was used, except that no supplementary RF voltage was used. Since all ions of M/Z less than 95 were ejected before and during period B, the small peaks at M/Z = 70 and 71 must be due to ions that were formed after period B. Fig. 8(B) was obtained by using the scan program shown in Fig. 4 with a supplementary frequency at the resonant frequency of M/Z = 100. Abundant daughter ions at M/Z = 70 and 71 are seen, and less intense peaks at M/Z = 55; 56 and 57 are evident. Fig. 8-(C) was acquired with the scan program used for Fig. 8(A), except that a supplementary RF voltage was used. The fundamental RF voltage during periods DA and DB, and the frequency of the supplementary RF voltage were chosen so that M/Z = 100 was in resonance during period DA so that daughter ions were produced. A particular daughter with M/Z = 70 that was produced during period DA was brought into resonance during period DB so that granddaughter ions were produced. These granddaughter ions are evident in Fig. 8(C) as the increased intensities of the peaks at M/Z = 55, 56 and 57. Fig. 8(D) is similar to Fig. 8(A) except that M/Z = 100 was in resonance during DA, and M/Z = 71 was in resonance during DB. Many other schemes may be used to obtain sequential daughter scans. For example, the frequency of the supplemental RF field may be changed instead of changing the fundamental RF voltage. Also, the trap may be cleared of undesired ions after daughter ions have been produced but before granddaughter ions are produced. Of course, further fragmentation may be induced by sequentially changing the fundamental RF voltage or the frequency of the supplementary RF voltage to bring the products of successive fragmentations into resonance. Modifications may be made to the methods described above within the scope of the claims. For example, the applied RF voltage need not be sinusoidal but is required only to be periodic. A different stability diagram will result but its general characteristics are similar, including a scan line. In other words, the RF voltage could comprise square waves, triangular waves, etc. The quadrupole ion trap would nevertheless operate in substantially the same manner. The ion trap sides were described above as hyperbolic but the ion traps can be formed with cylindrical or circular trap sides. Any electrode structure that produces an approximate three-dimensional quadrupole field could be used.

Claims: A method of mass analysing a sample by means of a quadrupole mass spectrometer, comprising the steps of defining a trap volume (16) within an electrode structure comprising a ring electrode (11) and two end caps (12,13) at both sides of the ring electrode (11) to which a DC voltage and a fundamental RF voltage are applied to form a three-dimensional quadrupole field adapted to trap ions within a predetermined range of mass-to-charge ratio; forming or injecting ions within said trap volume (16) such that those within said predetermined mass-to-charge range are trapped within said trap volume (16); and utilising an RF generator (35) coupled to end caps (22,23) to apply a supplementary AC field superposing said three-dimensional quadrupole field to form combined fields, characterised by the steps of scanning said combined fields with the supplementary field turned on to cause ions of all mass-to-charge ratios in said range to escape said trap volume (16) in consecutive mass-to-charge ratio order for detection and analysis. A method as claimed in Claim 1, characterised in that the frequency of said supplementary field is scanned while the voltage of said quadrupole field is fixed. A method as claimed in Claim 1, characterised in that said supplementary field is turned on while the intensity of said trapping field is scanned. A method as claimed in Claim 1, characterised in that the frequency of said supplementary field is constant.

Inventors: KELLEY PAUL E, LOURIS JOHN NATHAN, REYNOLDS WALTER E, STAFFORD GEORGE C, SYKA JOHN E P, KELLEY, PAUL E., LOURIS, JOHN NATHAN, REYNOLDS, WALTER E., STAFFORD, GEORGE C., SYKA, JOHN E.P.
Applicants: THERMO FINNIGAN LLC
Application year: 1986
Main classification: H01J49
IPC codes: G21K1, H01J49, G01N27