Patent Application
20230335388
The present invention relates to a linear ion trap and a method for operating the same.
A mass spectrometer employing an ion trap which holds ions by an effect of an electric field has been known as one type of mass spectrometer. There are two major types of ion traps used in mass spectrometers: a three-dimensional quadrupole ion trap and a linear ion trap. Any ion trap has a plurality of electrodes surrounding a space within which ions are to be captured. Generally speaking, linear ion traps provide a larger space for capturing ions than three-dimensional quadrupole ion traps.
A linear ion trap includes four rod electrodes arranged substantially parallel to each other so as to surround the space for capturing ions, as well as a pair of end electrodes (end-cap electrodes) respectively arranged outside the two end faces of the rod electrodes (see Patent Literatures 1 and 2). Normally, two of the four rod electrodes are arranged so as to face each other in a specific direction (X-axis direction) across the central axis of the capturing space, while the two remaining rod electrodes are arranged so as to face each other across the same central axis, in another direction (Y-axis direction) orthogonal to the X-axis direction. An ion ejection port is formed in a rod electrode of one of the two sets of rod electrodes. An ion introduction port is formed in one or both of the pair of end electrodes.
When ions are to be captured within the inner space of the linear ion trap, a direct voltage having the same polarity as the ions is applied to the pair of end electrodes, while two radio-frequency voltages having a phase difference of 180 degrees are respectively applied to the two pairs of rod electrodes facing each other in the X-axis and Y-axis directions. The ions introduced into the space surrounded by the four rod electrodes through the ion introduction port or ports formed in one or both of the end electrodes are captured within the same space by the effect of those voltages.
When the ions captured within the inner space are to be sequentially separated and detected according to their mass-to-charge ratios, the frequency or amplitude of the radio-frequency voltages applied to the two pairs of rod electrodes is controlled, and an excitation voltage is superposed on the radio-frequency voltage applied to one pair of rod electrodes, to induce resonance excitation of an ion having a specific mass-to-charge ratio. The resonance-excited ion significantly oscillates in bilateral directions substantially orthogonal to the central axis of the linear ion trap, to be ultimately ejected to the outside through the ion ejection port formed in the rod electrode. An ion detector is placed on the outside of the ion ejection port. The ion detector produces a detection signal corresponding to the number of ions which have reached the detector.
Patent Literature 1: U.S. Pat. No. 6,797,950 B
Patent Literature 2: JP 2012-184975 A
The conventional linear ion trap configured in this manner requires a complex power circuit for applying radio-frequency voltages having a phase difference of 180 degrees to the two pairs of rod electrodes so as to capture ions, and for superposing the voltage for resonance excitation on the radio-frequency voltage applied to one of the two pairs of rod electrodes so as to make ions oscillate by resonance excitation.
The objective of the present invention is to simplify the power circuit for the linear ion trap.
The first mode of the present invention is a linear ion trap, including:
The second mode of the present invention is a method for operating a linear ion trap including: two first rod electrodes arranged so as to face each other across a central axis, each of the first rod electrodes having an opening; two second rod electrodes arranged so as to face each other across the central axis, in a direction different from the direction in which the two first rod electrodes face each other; and a pair of end electrodes respectively arranged outside of the two end faces of the two first rod electrodes and the two second rod electrodes, the method including:
In the linear ion trap according to the first mode of the present invention, an opening is formed in each of the two first rod electrodes. One of the two openings serves as an ion introduction port, and one or both of the openings serve as an ejection port or ports. No opening for introducing ions is formed in the pair of end electrodes.
When an ion or ions are to be captured within the inner space of the linear ion trap, the radio-frequency voltage for capturing ions is applied to the two second rod electrodes by the radio-frequency voltage supplier. When an ion captured within the inner space is to be ejected, the voltage for resonance excitation is applied to the two first rod electrodes by the excitation voltage supplier, while the radio-frequency voltage for capturing ions is applied to the two second rod electrodes by the radio-frequency voltage supplier. An ion having a mass-to-charge ratio corresponding to the frequency of the radio-frequency voltage is thereby made to oscillate in an orthogonal direction to the central axis, to be ejected from one or both of the openings in the two first rod electrodes. Since the rod electrodes to which the radio-frequency voltage for capturing ions is applied are separated from the rod electrodes to which the voltage for resonance excitation is applied, the power circuit in the linear ion trap according to the first mode can be simple in configuration.
An ion trap mass spectrometer including a linear ion trap as one embodiment of the present invention is hereinafter described with reference to the drawings.
The ionizer 1 employs a matrix-assisted laser desorption/ionization (MALDI) method and includes a laser irradiator 11 configured to emit pulsed laser light, a sample plate 12 to which a specimen S containing a target sample component is adhered, and an ion transport system 13 configured to extract ions released from specimen S due to the irradiation with the laser light and guide the ions in a predetermined direction, as well as other related components. In the following descriptions, the direction in which ions are guided by the ion transport system 13 is called the “X-axis” direction, along with the “Y-axis” and “Z-axis” directions both of which are orthogonal to the X-axis direction, where the Z-axis direction corresponds to the direction perpendicular to the drawing sheet of
The linear ion trap 2 includes four rod electrodes 21, 22, 23 and 24 arranged parallel to each other around the central axis C extending in the X-axis direction, with their inner surfaces shaped like hyperbolae in the cross section. Among the four rod electrodes 21, 22, 23 and 24, the two rod electrodes 21 and 22 face each other in the X-axis direction across the central axis C, while the two rod electrodes 23 and 24 face each other in the Y-axis direction across the central axis C. The linear ion trap 2 in
The two rod electrodes 21 and 22 have an introduction port 21a and an ejection port 22a, respectively, each of which is an elongated opening extending in the Z-axis direction. Accordingly, the rod electrodes 21 and 22 correspond to the first rod electrodes in the present invention, while the rod electrodes 23 and 24 correspond to the second rod electrodes. Outside the two ends of the rod electrodes 21, 22, 23 and 24, a pair of end electrodes 25 and 26 having a substantially circular shape are arranged so that the rod electrodes 21, 22, 23 and 24 are sandwiched in between. No opening is formed in these end electrodes 25 and 26.
The ion detector 3 includes a conversion dynode 31 configured to convert ions into electrons as well as a secondary electron multiplier tube 32 configured to multiply and detect electrons coming from the conversion dynode 31. The ion detector 3 produces a detection signal corresponding to the amount of ions it has received, and sends the signal to the data processor 8. The data processor 8 has the function of creating a mass spectrum based on the detection signals obtained in the ion detector 3 for various kinds of ions sequentially ejected from the linear ion trap 2 while being separated from each other by their mass-to-charge ratios.
The main power source 4, which correspond to the radio-frequency voltage supplier in the present invention, applies a high rectangular voltage for capturing ions to the rod electrodes 23 and 24 in the linear ion trap 2. As shown in
The timing signal generator 6 generates drive pulses for controlling the on/off state of the first and second switching elements 43 and 44, as well as gives those pulses to the main power source 4. Those pulses drive the first and second switching elements 43 and 44 to alternately turn to the on state. When the first switching element 43 is in the on state, the first voltage VH is sent to the output. When the second switching element 43 is in the on state, the second voltage VL is sent to the output. Accordingly, under ideal conditions, the output voltage Vout will be a rectangular voltage which alternates between the high level VH and the low level VL with a predetermined frequency f (period 1/f). Normally, VH and VL are high voltages identical in absolute value and opposite in polarity. Their absolute value is approximately within a range from several hundred volts to 1 kilovolt. The frequency f is normally within a range from several tens of kHz to several MHz. When the frequency of the pulses for driving the switching elements 43 and 44 is changed by the timing signal generator 6, the rectangular voltage changes its frequency while maintaining its amplitude (voltage level) at the same level.
The timing signal generator 6 feeds the auxiliary power source 5 with pulse signals formed by dividing the frequency of the drive pulses supplied to the main power source 4 with an appropriate ratio. Based on the signals fed from the timing signal generator 6, the auxiliary power source 5 generates a low rectangular voltage of frequency f/n (provided that the division ratio is 1/n) and pulse width d, with a low level of 0 V and a high level of +V1, as well as another low rectangular voltage whose polarity is opposite to that of the counterpart. The generated low rectangular voltages are applied to the rod electrodes 21 and 22 in the linear ion trap 2. Normally, the voltage value V1 of the low rectangular voltages is much lower than the voltage values VH and VL of the high rectangular voltage; for example, V1 is several volts. The auxiliary power source 5 corresponds to the excitation voltage supplier in the present invention. The low rectangular voltages generated in the auxiliary power source 5 correspond to the voltage for resonance excitation in the present invention.
The controller 7 includes a personal computer as its main component, with its functions realized by executing a controlling/processing program previously installed on the personal computer.
A mass spectrometric operation in the mass spectrometer according to the present embodiment is hereinafter described with reference to
In the ionizer 1, a beam of laser light is emitted from the laser irradiator 11 to irradiate specimen S with this light. Due to the irradiation with the laser light, the matrix in specimen S is rapidly heated and vaporized along with the target component, to be ultimately turned into ions. The generated ions are converged by the electrostatic field formed by an ion lens in the ion transport system 13 and introduced through the introduction port 21a into the inner space surrounded by the rod electrodes 21, 22, 23 and 24. In this stage, direct voltages of opposite polarities are respectively applied to the rod electrodes 21 and 22, while no voltage is applied to the rod electrodes 23 and 24. The end electrodes 25 and 26 are maintained at the ground potential (see
After a predetermined period of time (t1) has passed since the introduction of the ions into the linear ion trap 2, the timing signal generator 6 supplies drive pulses of a predetermined frequency to the switching elements 43 and 44 according to an instruction from the controller 7. A high rectangular voltage having the corresponding frequency is thereby generated in the main power source 4 and applied to the rod electrodes 23 and 24. Consequently, a radio-frequency electric field is formed within the inner space. Due to the effect of this radio-frequency electric field, ions having a predetermined range of mass-to-charge ratios are captured within the linear ion trap 2. The ions are also cooled by coming in contact with cooling gas which has been introduced into the inner space in advance of the introduction of the ions.
After a predetermined period of time (t2) has passed since the introduction of the ions into the linear ion trap 2, the auxiliary power source 5 discontinues the application of the direct voltages to the rod electrodes 21 and 22 according to an instruction from the controller 7. Under this condition, the ions within the linear ion trap 2 are captured in a stable manner.
After the cooling of the ions has been performed, when the ions captured within the linear ion trap 2 are to be detected, the frequency of the drive pulses supplied from the timing signal generator 6 to the switching elements 43 and 44 is continuously changed, as shown in
Consequently, an ion having a specific mass-to-charge ratio is selectively made to oscillate due to the resonance excitation and ejected through the ejection port 22a, to be detected by the ion detector 3.
The present invention is not limited to the previously described embodiment and can be appropriately changed or modified.
For example, in the previously described embodiment, the high rectangular voltage having the predetermined frequency begins to be applied to the rod electrodes 23 and 24 when the predetermined period of time t1 has passed since the introduction of the ions into the linear ion trap 2.
As another example, as shown in
In the linear ion trap according to the present invention, no opening is formed in the pair of end electrodes. This means that an opening which should act as an ion introduction port or ion ejection port is not formed. This does not exclude the possibility of forming openings in the end electrodes, for example, in a system in which multiple linear ion traps are arranged in series along the central axis, with the neighboring linear ion traps having their respective inner spaces connected to each other through those openings.
[Various Modes]
A person skilled in the art can understand that the previously described illustrative embodiment is a specific example of the following modes of the present invention.
(Clause 1) A linear ion trap according to Clause 1 includes:
The linear ion trap according to Clause 1 does not require applying radio-frequency voltages having a phase difference of 180 degrees to any electrode in order to capture ions. Since the rod electrodes to which the radio-frequency voltage for capturing ions is applied are separated from the rod electrodes to which the voltage for resonance excitation is applied, it is unnecessary to superpose the voltage for resonance excitation on the radio-frequency voltages for capturing ions, as in the conventional linear ion trap. Therefore, the present linear ion trap can be operated with a power circuit which is simple in configuration.
(Clause 2) In the linear ion trap described in Clause 1, the controller may be configured so that:
In the linear ion trap according to Clause 2, after an ion or ions have been introduced from the opening in one of the two first rod electrodes into the inner space, the radio-frequency voltage for capturing ions is applied to the two second rod electrodes by the radio-frequency voltage supplier. The ions are thereby captured within the inner space. While the application of the radio-frequency voltage for capturing ions to the two second rod electrodes by the radio-frequency voltage supplier is continued in this state, the voltage for resonance excitation is applied to the two second rod electrodes by the excitation voltage supplier. This causes an ion having a mass-to-charge ratio corresponding to the frequency of the radio-frequency voltage to oscillate in an orthogonal direction to the central axis by resonance excitation and be ejected from one or both of the openings in the two first rod electrodes.
(Clause 3) In the linear ion trap described in Clause 1 or 2, the radio-frequency voltage for capturing ions may be a rectangular voltage, and the voltage for resonance excitation may be a rectangular voltage provided by dividing the frequency of the radio-frequency voltage for capturing ions with a predetermined division ratio, the latter rectangular voltage being lower in voltage than the radio-frequency voltage.
In the linear ion trap according to Clause 3, the electrodes are operated by a digital-driving system. Therefore, the frequency and/or duty cycle of the rectangular voltages applied to the first and second rod electrodes can be easily varied.
(Clause 4) In the linear ion trap described in Clause 3, the radio-frequency voltage supplier may include a first voltage source configured to generate a direct voltage, a second voltage source configured to generate a direct voltage different from the direct voltage generated by the first voltage source, a first switching section configured to turn on or off the direct voltage outputted from the first voltage source, and a second switching section configured to turn on or off the direct voltage outputted from the second voltage source, and the radio-frequency voltage supplier may be configured to generate the rectangular voltage by alternately turning on or off the first switching section and the second switching section.
In the linear ion trap according to Clause 4, the frequency of the radio-frequency voltage for capturing ions can be easily varied by changing the switching frequency of the on/off state of switching elements in the first and second switching sections. The duty cycle can also be easily varied by changing the timing to switch between the on/off states while maintaining the switching frequency of the switching elements.
(Clause 5) A method according to Clause 5 is a method for operating a linear ion trap including: two first rod electrodes arranged so as to face each other across a central axis, each of the first rod electrodes having an opening; two second rod electrodes arranged so as to face each other across the central axis, in a direction different from the direction in which the two first rod electrodes face each other; and a pair of end electrodes respectively arranged outside the two end faces of the two first rod electrodes and the two second rod electrodes, the method including:
