ION TRAP DESIGN METHOD AND ION TRAP MASS SPECTROMETER

Abstract

In a three-dimensional quadrupole-type ion trap, a shape and an arrangement of the ring electrode and the end cap electrodes 11 and 12 are shifted from an ideal state in which only a quadrupole electric field is formed, so that the polarities of the ratio of strength of an octupole electric field with respect to the strength of a quadrupole electric field and the ratio of strength of a dodecapole electric field with respect to the strength of the quadrupole electric field are different from each other, their absolute values are equal to or greater than 0.02, and the absolute value of the ratio of strength of the octupole electric field with respect to the strength of the dodecapole electric field is within the range of from 0.6 to 1.4.

Claims

1. An ion trap design method for designing an ion trap for capturing ions in a space in which a quadrupole electric field and a multipole electric field of order higher than that of the quadrupole electric field surrounded by equal to or greater than three electrodes by the voltage applied to each of those electrodes and for carrying out ion isolation by allowing ions with a specific mass-to-charge ratio or a specific range of mass-to-charge ratio to remain while eliminating other ions from the ions captured, the method comprising: determining a shape and an arrangement of the three or more electrodes so that the polarities of the ratio of strength of an octupole electric field with respect to the strength of a quadrupole electric field and the ratio of strength of a dodecapole electric field with respect to the strength of the quadrupole electric field are different from each other, the absolute values of the strength of the octupole electric field and the strength of the dodecapole electric field are equal to or greater than 0.02, and the absolute value of the ratio of the strength of the octupole electric field with respect to the strength of the dodecapole electric field is within the range of from 0.6 to 1.4.

2. The ion trap design method according to claim 1, wherein the ion trap is a three-dimensional quadrupole-type ion trap comprising one ring electrode and two end cap electrodes arranged so as to facing each other, in which the octupole electric field and the dodecapole electric field are superposed on the quadrupole electric field by reducing the inscribed radius of the ring electrode and shifting the two end cap electrodes in the direction close to the central point from the ideal state in which only the quadrupole electric field is formed in the ion trap.

3. An ion trap mass spectrometer, comprising: an ion source for generating ions originating from a sample, an ion trap comprising equal to or greater than three electrodes for capturing ions in a space by forming a quadrupole electric field and a multipole electric field of order higher than that of the quadrupole electric field in a space surrounded by these electrodes by a voltage applied to these electrodes, and an ion detector for detecting ions discharged from the ion trap; wherein the ion trap mass spectrometer is used for performing an ion isolation in which after the ions are captured by the ion trap, of those ions, the ions having a specific mass-to-charge ratio or included in a specific range of mass-to-charge ratio are allowed to remain while the other ions are eliminated, wherein the ion trap is configured to have a shape and an arrangement of the three or more electrodes determined so that the polarities of the ratio of strength of an octupole electric field with respect to the strength of a quadrupole electric field and the ratio of strength of a dodecapole electric field with respect to the strength of the quadrupole electric field are different from each other, the absolute values of the strength of the octupole electric field and the strength of the dodecapole electric field are equal to or greater than 0.02, and the absolute value of the ratio of strength of the octupole electric field with respect to the strength of the dodecapole electric field is within the range of from 0.6 to 1.4.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 is a schematic configuration drawing of an ion trap mass spectrometer according to one example of embodiment of the present invention.

[0028] FIG. 2 is a drawing showing the results of simulating the relationship between the amplitude of ions and the oscillation frequency when a signal of resonance frequency at which an octupole electric field having 2% of a ratio of strength with respect to a quadrupole electric field is superposed is applied to the end cap electrodes.

[0029] FIG. 3 is a drawing showing the results of simulating the relationship between the amplitude of ions and the oscillation frequency when a signal of the resonance frequency at which an octupole electric field having 4% of a ratio of strength with respect to a quadrupole electric field and a dodecapole electric field having −2% of a ratio of strength are superposed is applied to the end cap electrodes.

[0030] FIG. 4 is a drawing showing the results of simulating the relationship between the amplitude of ions and the oscillation frequency when a signal of the resonance frequency at which an octupole electric field, a dodecapole electric field, a hexadecapole electric field, and an icosapole electric field having 2% or −2% of a ratio of strength with respect to a quadrupole electric field are superposed is applied to the end cap electrodes.

[0031] FIG. 5 is a drawing illustrating a resonance curve showing the relationship between the oscillation frequency and the vibration amplitude.

[0032] FIG. 6 is a drawing illustrating the simulation results of the resonance curve when the high-frequency electric field is only a quadrupole electric field and when an octupole electric field having 2% of a ratio of strength with respect to a quadrupole electric field is superposed.

[0033] FIG. 7 is a drawing illustrating an example of the shape and arrangement of electrodes when the strengths of an octupole electric field and a dodecapole electric field superimposed on a quadrupole electric field are changed.

[0034] FIG. 8 is a drawing illustrating an example of the shape and arrangement of electrodes when the strengths of an octupole electric field and a dodecapole electric field superimposed on a quadrupole electric field are changed.

[0035] FIG. 9 is a drawing illustrating an example of the shape and arrangement of electrodes when the strengths of an octupole electric field and a dodecapole electric field superimposed on a quadrupole electric field are changed.

[0036] FIG. 10 is a drawing showing a ratio of strength of an octupole electric field and a dodecapole electric field with respect to a quadrupole electric field and the ratio of strength of an octupole electric field with respect to a dodecapole electric field in each model in which the shape and arrangement of electrodes have been changed.

[0037] FIG. 11 is a drawing showing the simulation results of the resonance curve in each model shown in FIG. 10.

[0038] FIG. 12 is an explanatory drawing of a shape of the resonance curve in model (D) shown in FIG. 11.

[0039] FIG. 13 at (a) is a cross sectional view illustrating a basic configuration of a three-dimensional quadrupole-type ion trap, which is a typical ion trap, and at (b) is a cross sectional view of an example of a configuration in which an ion trap is intentionally distorted from a theoretical shape.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0040] One example of embodiment of the method of designing an ion trap and the ion trap mass spectrometer using the ion trap designed by the method according to the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic configuration diagram of the ion trap mass spectrometer of the present example of embodiment.

[0041] The ion trap mass spectrometer according to the present example of embodiment is equipped with an ion source 2 for ionizing a target sample, an ion trap 1, which is of a three-dimensional quadrupole type, and an ion detector 3 for detecting ions discharged from the ion trap 1, and all of these are housed inside a vacuum chamber, not shown in the drawing.

[0042] The ion trap 1 comprises one ring electrode 10, an inlet-side end cap electrode 11 and an outlet-side end cap electrode 12 arranged facing one another so as to hold this [ring electrode] in between, and the space surrounded by these three electrodes 10, 11, and 12 becomes the ion capture area. On one hand, an ion incident aperture Ila is drilled nearly in the center of the inlet-side end cap electrode 11, and the ion exiting from the ion source 2 is introduced into the ion trap 1 through this ion incident aperture 11a. On the other hand, an ion exit aperture 12a is drilled nearly in the center of the outlet-side end cap electrode 12, the ion detector 3 is arranged on the outer side of this ion exit aperture 12a to detect the ions discharged passing through the ion exit aperture 12a.

[0043] A power supply unit 4 is used for applying a predetermined sinusoidal voltage to each of the electrodes 10, 11, 12 that constitute the ion trap 1. To be specific, the power supply unit 4 applies a sinusoidal voltage of Vcos Ωt to the ring electrode 10 for capturing ions in a capture area. That frequency Ω is adjusted depending on the range of mass-to-charge ratio of the ions captured. Meanwhile, the power supply unit 4 applies high-frequency voltage±Vec cos Ωec t of reversed polarity to both end cap electrodes 11 and 12 for eliminating unnecessary ions among the ions captured in the capture area or for discharging and detecting ions captured through the ion exit aperture 12a. The applicable ions are resonated by matching the frequency Ωec of applied voltage to these end cap electrodes 11 and 12 with the oscillation frequency of ions, making it possible to carry out ion isolation and discharge.

[0044] An ideal ion trap as described above has the ring electrode 10 and the end cap electrodes 11 and 12 having their inner surface in a rotating hyperboloid shape, and the distance z.sub.0 between the top of the end cap electrodes 11 and 12 and the center point of the ion trap 1 and the inscribed radius r.sub.0 of the ring electrode 10 fulfill the equation (1) above.

[0045] In the ion trap 1 as shown in FIG. 1, the potential distribution φ inside the surface including an ion optical axis (z-axis in this example) in an axisymmetric field can generally be realized by the following equation (2).

φ(ρ, θ)=VΣA.sub.n(ρ/z0).sup.nP.sub.n(cos θ)  (2)

Σ here is the total sum from n=0 until ∞. Furthermore, ρ is the distant from the origin (the center point of the ion trap 1) until the observation point, ρ=√(r.sup.2+z.sup.2), θ is the angle from z-axis of the observation point centering on the origin, V is an applied voltage, A.sub.n is a multipole electric field coefficient, A.sub.2 is a quadrupole, A.sub.3 is a hexapole, A.sub.4 is an octupole, A.sub.5 is a decapole, and A.sub.6 is a dodecapole. When the shape and the arrangement of the electrodes 10, 11, and 12 are axisymmetric surrounding the r-axis and z-axis, the top in which n is an odd number does not exist, only the top in which n is an even number exits. Distant z.sub.0 is used as a normalization constant. Pn is a Legendre polynomial.

[0046] In principle, a quadrupole field is dominant for the ion trap 1, and the potential distribution of the quadrupole field is expressed by the following equation (3).

φ=(V/z0.sup.2)A.sub.2(2z.sup.2−r.sup.2)  (3)

Although only this quadrupole field is the electric field formed in the ion trap of an ideal state, a multipole electric field of high order occurs when the shape and the arrangement of the electrodes are shifted from the ideal state. Here, the fact that the shape and the arrangement of the electrodes 10, 11, and 12 are axisymmetric surrounding the r-axis and z-axis is maintained, and odd higher-order terms are not taken into consideration. The potential distribution of the octupole electric field is expressed by the following equation (4).

φ=VA.sub.4((8z.sup.4−24z.sup.2r.sup.2+3r.sup.4)/8z0.sup.4)  (4)

Furthermore, the potential distribution of the dodecapole electric field is expressed by the following equation (5).

φVA.sub.6((16z.sup.6−120z.sup.4r.sup.2+90z.sup.2r.sup.4−5r.sup.6)/16z0.sup.6)  (5)

[0047] Now, the case of the existence where an octupole electric field superimposes a quadrupole electric field is considered. The potential distribution inside the ion trap 1 in such a case is expressed by the following equation (6).

φ=(V/z0.sup.2)A.sub.2(2z.sup.2−r.sup.2)+(V/8z0.sup.4)A.sub.4(8z.sup.4−24z.sup.2r.sup.2+3r.sup.4)  (6)

[0048] In this case, the ion confining potential φeff is expressed by the following equation (7).

φeff=(eEz.sup.2)/(4mQ.sup.2)=((qA.sub.2.sup.2V)/(4z0.sup.2))z.sup.2+((qA.sub.2A.sub.4V)/(z0.sup.4))z.sup.4  (7)

[0049] When ions are captured while being vibrated by this potential, such equation of motion is expressed by the following equation (8).

z+((eqA.sub.2.sup.2V)/(2z0.sup.2))z=−((4eqA.sub.2A.sub.4V)/(z0.sup.4))z.sup.3  (8)

[0050] The term z.sup.3 exists on the right side of equation (8). This is the equation of nonlinear oscillation called Duffing equation, and its solution is well known. When a forced oscillation by a forced oscillating electric field is added to the vibration based on such equation, the resonance curve that plots a vibration amplitude with respect to a forced oscillating frequency may be become the one as shown in FIG. 5 at (c) (refer to Non-Patent Literature 2). When the resonance curve is in the shape as shown in FIG. 5 at (c), for example, the amplitude increases according to slope f along with the change in the direction (the left direction along the horizontal axis in the drawing) in which the frequency becomes small, and the amplitude at the position of point d rapidly changes to point b. On the contrary, when it is with the change in the direction in which the frequency becomes large, the amplitude increase according to slope a along with that change, and the amplitude at the position of point c rapidly changes to point e Such discontinuous change is a jumping phenomenon, which will be explained later.

[0051] In the resonance curve as shown in FIG. 5, a deviation Aw of a resonance frequency is expressed by the following equation (9).

Δω=(A.sub.4/A.sub.2) (P.sup.2/(z0.sup.2))∞0  (9)

Where P is an amplitude value of the vibration. Equation (9) means that the resonance frequency shifts at a ratio of A.sub.4/A.sub.2 when the amplitude P is z.sub.0.

[0052] FIG. 2 is a drawing showing the results of simulating the relationship between the amplitude of ions (vertical axis) and the oscillation frequency (horizontal axis) when a signal of resonance frequency at which an octupole electric field having 2% of a ratio of strength with respect to a quadrupole electric field is superposed is applied to the end cap electrodes 11 and 12. According to FIG. 2, the following result was obtained: as the amplitude increases, the resonance frequency shifts 2%.

[0053] FIG. 4 is a drawing showing the results of simulating the relationship between the amplitude of ions and the oscillation frequency when a signal of a resonance frequency at which a dodecapole, a hexadecapole, an icosapole, and multipole electric fields of higher order are superposed is applied to the end cap electrodes 11 and 12. The ratio of strength of the multipole electric field with respect to the quadrupole electric field is +2% or −2%. As can be seen in FIG. 4, as the electric field becomes a higher order, the amplitude becomes large, initiating a deviation of the resonance frequency. In addition, it was also found that when the signs of the positive and negative electric fields superposed became in reverse, the deviation was observed in the direction where the resonance frequency became low.

[0054] Next, FIG. 3 a drawing showing the results of simulating the relationship between the oscillation frequency and the amplitude of ions when a signal of a resonance frequency at which an octupole electric field having 4% of a ratio of strength with respect to a quadrupole electric field is superposed on a dodecapole electric field having −2% of a ratio of strength is applied to the end cap electrodes 11 and 12. As can be seen from FIG. 3, the influence of the octupole electric field similarly as the one described in FIG. 2 is dominant while the amplitude is small, and a deviation is observed in the direction where the resonance frequency is high (that is, in the direction to the right); however, when the amplitude becomes large to some extent, the influence of the dodecapole electric field causes a deviation in the direction where the resonance frequency decreases (that is, in the direction to the left). This behavior is shown by a thick dotted arrow in FIG. 3.

[0055] The jumping phenomenon as shown in FIG. 5 at (c) has been known to occur in a nonlinear vibration as described above. FIG. 6 at (b) shows the result of calculating the resonance curve under the condition in which the octupole electric field having a ratio of strength of 2% with respect to the quadrupole electric field is superposed. As can be seen from FIG. 6 at (b), on the high frequency side, the slope of a peak is steep extending in an almost vertical manner. This can be conjectured to be due to the jumping phenomenon described above. When a conventional ion trap is used as a mass separator, the ion discharge from the ion trap by the steep slope on this high frequency side is rapidly carried out, and it has an effect of improving the mass resolution. FIG. 6 at (a) is a resonance curve in the case of only the quadrupole electric field, and when compared to this, the vibration amplitude of the peak top can be suppressed in FIG. 6 at (b). This means that the ability to confine ions is increasing, leading to the improvement in the ion capture efficiency.

[0056] On the other hand, the slope of the peak of the resonance curve shown in FIG. 6 at (b) on the low frequency side is quite gentle compared to the slope of the resonance curve shown in FIG. 6 at (a). This lowers the resolution of ion isolation on the low frequency side. That is, the slope of the resonance curve should be made as steep as possible on both the low and high frequency sides while suppressing the vibration amplitude of the peak of the resonance curve in order to achieve high resolution of ion isolation on both sides, the low frequency side and the high frequency side, while keeping the ion capture efficiency high.

[0057] As described above, simply superposing the octupole electric field on the quadrupole electric field results in a steep slope of the resonance curve peak on the high frequency side but a gentle slope on the low frequency side. On the contrary, it is expected from the results shown in FIG. 3 that superposing the octupole electric field on the quadrupole electric field and further superposing the dodecapole electric field, which has a reversed polarity as that of the octupole electric field, offsets the shift of the resonance curve peak.

[0058] The following three methods can be considered mainly as the methods of increasing the ratio of the multipole electric field superposed on the quadrupole electric field.

[0059] (1) As shown in FIG. 7, the inscribed radius r.sub.0 is made small while the shape of the ring electrode 10 of the ion trap 1 is kept to be in an ideal state. For example, when the inscribed radius r.sub.0 in an ideal state is 10 mm, setting this inscribed radius to 7 mm allows the generation of a multipole electric field having 4% in A.sub.4/A.sub.2 (the ratio of strength of the octupole electric field with respect to the strength of the quadrupole electric field) and −2.3% in A.sub.6/A.sub.2 (the ratio of strength of the dodecapole electric field with respect to the quadrupole electric field.

[0060] (2) As shown in FIG. 8, the surface shape of both end cap electrodes 11 and 12 surrounding z-axis is in substantially conical shape on the top side from the plane that is orthogonal to z-axis at a predetermined position on z-axis. By shifting the shape of the end cap electrodes 11 and 12 in this manner from the ideal state, the octupole electric field in which A.sub.4/A.sub.2 is positive and the dodecapole electric field in which A.sub.6/A.sub.2 is negative can be superposed on the quadrupole electric field.

[0061] (3) As shown in FIG. 9, the shapes of both end cap electrodes 11 and 12 are shifted inwardly at the same distant each while maintaining an ideal shape. Thereby, it is possible to reduce the octupole electric field while keeping the dodecapole electric field to some extent.

[0062] The change of strength of the multipole electric field according to the shape and the arrangement of electrodes as described above and the change of the resonance curve according to that [change of strength] were confirmed by the simulations. In the simulations, the following six models of ion traps, A-F, were assumed. In either case, the inscribed radius r.sub.0 of the ring electrode 10 was reduced from 10 mm, which is the ideal state, to 7 mm while maintaining the shape thereof. In addition, the opening aperture of the ion incident aperture 11a and the ion exit aperture 12a drilled in the center of the end cap electrodes 11 and 12 was 1.4 mm.

[0063] (A): The portions of both end cap electrodes 11 and 12 extended inwardly from the position of the inscribed radius of 4 mm were changed into a conical shape.

[0064] (B): The portions of both end cap electrodes 11 and 12 extended inwardly from the position of the inscribed radius of 1.25 mm were changed in to a conical shape.

[0065] (C): The positions of both end cap electrodes 11 and 12 were shifted 0.1 mm inwardly from the ideal state.

[0066] (D): The positions of both end cap electrodes 11 and 12 were shifted 0.2 mm inwardly from the ideal state.

[0067] (E): The positions of both end cap electrodes 11 and 12 were shifted 0.5 mm inwardly from the ideal state.

[0068] (F): The positions of both end cap electrodes 11 and 12 were shifted 0.6 mm inwardly from the ideal state.

[0069] FIG. 10 shows the results after calculating the quadrupole electric field component, the octupole electric field component, and the dodecapole electric field component in the six models of ion traps described above and calculating the ratio of the strength of the octupole electric field component and the strength of the dodecapole electric field component with respect to the strength of the quadrupole electric field component. FIG. 11 shows the results of drawing the resonance curve of these six models of ion traps. As can be seen from FIG. 10, the ratio of the octupole electric field with respect to the quadrupole electric field (A.sub.4/A.sub.2) decreases in the order of from (A) to (F), and the dodecapole electric field component increases relatively.

[0070] When the octupole electric field component is dominant when compared to the octupole electric field component and the dodecapole electric field component, the peak of the resonance curve shows strong asymmetric as shown in FIG. 11. And the peak of the resonance curve as the octupole electric field component decreases and the dodecapole electric field component relatively increases is close to a symmetrical shape. This can be conjectured to be due to the effect of elimination of peak shift of the resonance curve by the dodecapole electric field.

[0071] The resonance curve shown in FIG. 11 at (d) has its slopes in between the peak in a shape that stands almost vertically, and it is presumed that a jumping phenomenon also occurred not only on the high frequency side but also on the low frequency side. This is considered to be due to the occurrence of the jumping phenomenon caused by the octupole electric field component on the high frequency side and the jumping phenomenon caused by the dodecapole electric field as shown in FIG. 12 on the low frequency side. If the slopes can be made to be close to vertical on both high and low frequency sides by the coexistence of the octupole electric field component and the dodecapole electric field component having the same size with their positive and negative polarity being in reverse, the performance of the ion isolation can be improved. Furthermore, the vibration amplitude of the peak can be suppressed, so it is possible to also realize high ion capture efficiency.

[0072] On the other hand, in the resonance curve as shown in FIG. 11 at (f), since the octupole electric field component decreases too much, the rounding of the edge of the peak on the high frequency side becomes slightly larger. In this state, the performance of ion isolation on the high frequency side tends to deteriorate. As such, it can be seen that the requirements of the ratio of the octupole electric field component and the dodecapole electric field component that can achieve edge-like slopes that are almost vertical for both the high frequency and low frequency in the peak of the resonance curve are limited.

[0073] To be specific, as can be seen from the results above, to fulfill the requirements described above, the absolute values of the ratio of the octupole electric field component with respect to the quadrupole electric field component (A.sub.4/A.sub.2) and the ratio of the dodecapole electric field component with respect to the quadrupole electric field component (A.sub.6/A.sub.2) should be equal to or greater than 0.02, and the absolute value of the ratio of the octupole electric field component with respect to the dodecapole electric field component (A.sub.4/A.sub.6) should be in the range of from 0.6 to 1.4. Of the six models described above, models C-F fulfilled these requirements. That is, the inscribed radius r.sub.0 of the ring electrode 10 was reduced from 10 mm, which is an ideal state, to 7 mm while maintaining its shape, and the positions of both end cap electrodes 11 and 12 were shifted in the range of from 0.1 to 0.6 mm inwardly from their ideal state. Such configuration makes it possible to achieve sufficiently high ion isolation resolution while sufficiently maintaining high ion capture efficiency.

[0074] The example of embodiment described above employed a three-dimensional quadrupole-type ion trap as the ion trap; however, the present invention can also be applied to a linear-type ion trap that can capture ions by the same principle, and having the effects described above has been clarified.

EXPLANATION OF REFERENCES

[0075] 1 . . . an ion trap

[0076] 10 . . . a ring electrode

[0077] 11, 12 . . . end cap electrodes

[0078] 11a . . . an ion incident aperture

[0079] 12a . . . an ion exit aperture

[0080] 2 . . . an ion source

[0081] 3 . . . an ion detector

[0082] 4 . . . a power supply unit