JP5243977B2 - Vertical acceleration time-of-flight mass spectrometer - Google Patents

Description

  The present invention relates to a time-of-flight mass spectrometer used in the fields of quantitative analysis, qualitative simultaneous analysis of trace compounds, and structural analysis of sample ions.

[Time of Flight Mass Spectrometer (TOFMS)]
TOFMS is a mass spectrometer that determines the mass-to-charge ratio of ions from the time it takes to reach a detector by accelerating and flying ions with a certain amount of energy. In TOFMS, ions are accelerated with _a constant pulse voltage Va. At this time, the ion velocity v is calculated from the energy conservation law.
_{^{mv 2/2 = qeV a .........}} (1)
v = √ (2qeV / m) (2)
Where m: ion mass, q: ion charge, e: elementary charge.

  The ions reach the detector placed after a certain distance L at the time of flight T.

T = L / v = L√ (m / 2qeV) (3)
TOFMS is a device that separates masses by using the fact that the time of flight T varies depending on the mass m of ions according to equation (3). FIG. 1 shows an example of a linear TOFMS. Reflective TOFMS is also widely used, which can improve energy convergence and extend flight distance by placing a reflection field between the ion source and the detector. FIG. 2 shows an example of a reflective TOFMS.

  Reflective TOFMS can measure the m / z value of an unknown substance with an error of about several ppm from the m / z value calculated from the composition formula. known.

[Helix orbit TOFMS]
The mass resolution of TOFMS is T, where total flight time is T and peak width is ΔT.
Mass resolution = T / 2ΔT (4)
Defined by That is, if the peak width ΔT is kept constant and the total flight time T can be extended, the mass resolution can be improved. However, in the conventional linear and reflective TOFMS, extending the total flight time T, that is, extending the total flight distance directly leads to an increase in the size of the apparatus. A multi-circular TOFMS (Non-Patent Document 1) is an apparatus developed to avoid an increase in the size of the apparatus and achieve high mass resolution. This device can extend the total flight time T by using four toroidal electric fields in which a Mazda plate is combined with a cylindrical electric field and by making multiple rounds of an 8-shaped orbit. In this apparatus, the spatial extent and temporal extent on the detection surface due to the initial position, initial angle, and initial kinetic energy are successfully converged to the first order term.

  However, TOFMS that makes multiple rounds of closed orbits has the problem of “overtaking”. This occurs because light ions (high speed) overtake heavy ions (low speed) because they orbit around the closed orbit. For this reason, the basic concept of TOFMS, which arrives in order from light ions to the detection surface, does not work.

  The helical orbital TOFMS was devised to solve this problem. The helical trajectory type TOFMS is characterized in that the start point and the end point of the closed trajectory in the multi-circular type are shifted in the direction perpendicular to the closed trajectory plane. In order to realize this, a method in which ions are incident obliquely from the beginning (Patent Document 1), a method in which the start point and end point of a closed orbit are shifted in a vertical direction using a deflector (Patent Document 2), and a laminated toroidal electric field are There is a method used (Patent Document 3).

  As a similar concept, a TOFMS in which the trajectory of a multiple reflection type TOFMS in which overtaking occurs in a zigzag shape has also been devised (Patent Document 4). With such TOFMS, more accurate composition estimation is possible.

[Vertical acceleration time-of-flight mass spectrometer (OA-TOFMS)]
Since TOFMS analyzes the difference in mass-to-charge ratio of ions based on the elapsed time from a certain time starting point, the ions must be accelerated in a pulse manner in the ion accelerator. Therefore, the compatibility with the ionization method in which ionization is performed in a pulsed manner by laser irradiation or the like is very good.

  However, in the ionization method of mass spectrometry, ions are continuously generated such as EI (electron impact ionization method), CI (chemical ionization method), ESI (electrospray ionization method), and APCI (atmospheric pressure chemical ionization method). There are many ionization methods. Orthogonal Acceleration (OA) was developed to combine these continuous ionization methods with TOFMS.

  Fig. 3 shows a conceptual diagram of TOFMS (hereafter referred to as OA-TOFMS) using the vertical acceleration method. An ion beam generated from an ion source that continuously generates ions is continuously transported to the vertical acceleration unit with a kinetic energy of several tens of eV.

  In the vertical acceleration unit, a pulse voltage of about 10 kV is applied to the ions, and the ions are accelerated in the direction perpendicular to the transport direction from the ion source. Since the time from the start of application of the pulse voltage to the arrival of the ions at the detector varies depending on the mass of the ions, ion mass separation is performed.

JP 2000-243345 A.

JP2003-86129A.

JP 2006-12782 A.

JP-T-2007-526596.

[Problem 1 of the prior art]
In the case of OA-TOFMS, ions are continuously flowing to the ion acceleration part, and it can be considered that only the range that can be used for measurement is cut out and measured. This ion utilization efficiency is called Duty Cycle,

と定義する。

It is defined as

From a slightly different perspective, this can be considered as the ion beam length used for measurement out of the ion beam length that passes through the ion accelerator. In other words, if the ion length that can be used for measurement is L _oa , the incident energy to the ion accelerator is eV _in , and the measurement interval of TOFMS is T _d ,

と表現できる。式（６）から、以下の３点を実現することでイオンの利用効率が向上する。

Can be expressed. From equation (6), the ion utilization efficiency is improved by realizing the following three points.

(1) Decrease _Td .

(2) to reduce the eV _in.

(3) Extend L _oa .

However, there are some problems in improving the utilization efficiency. T _d is related to the time of flight. From equation (4), if the flight time is extended, the mass resolution is improved, so the duty cycle and the improvement of the mass resolution are in a trade-off relationship.

Although it is effective to reduce eVin, _in reality, the effect of space charge effect and electrode charging becomes stronger, and the ionic strength itself becomes unstable, so that ions can be moved over long distances with extremely low V _oa values. It is impossible to transport.

L _oa is related to the acceptance of TOFMS. In the case of reflective TOFMS, the size of the detector (usually several tens of mm), and in the case of helical orbit TOFMS, the effective size of the ion optical system (usually 5 to 10 mm).

[Problem 2 of the prior art]
In order to improve the problem of the above-described Duty Cycle, a method has been devised in which an ion trap mechanism capable of accumulating ions for a certain period and intermittently ejecting them is arranged before introducing the pulse accelerator. However, in this case, a spatial distribution of ions depending on the m / z value occurs with discharge at a distance from the emission position of the ion trap mechanism to the pulse acceleration unit.

Position after T from the time the group of ions are emitted from the ion trap mechanism, when the voltage of the pulse discharge to V _in,

となり、ｍ／ｚ値の平方根に反比例する。たとえば、イオントラップ機構からパルス加速部までの距離をＬ_in、パルス加速部の有効距離をＬ_oaとすれば、測定対象とする最小のｍ／ｚ値（ｍ／ｚ）_minと最大のｍ／ｚ値（ｍ／ｚ）_maxの関係は次の式で表わせる。

And is inversely proportional to the square root of the m / z value. For example, if the distance from the ion trap mechanism to the pulse accelerating portion is L _in and the effective distance of the pulse accelerating portion is L _oa , the minimum m / z value (m / z) _min to be measured and the maximum m / z The relationship of z value (m / z) _max can be expressed by the following equation.

例えば、Ｌ_oa／Ｌ_in＝４とすると、（ｍ／ｚ）_max／（ｍ／ｚ）_min＝２５となる。仮に（ｍ／ｚ）_min＝５０とすると、（ｍ／ｚ）_max＝１２５０となり、測定できるｍ／ｚ範囲が制限されることとなる。特にアクセプタンスの狭い扇形電場を利用した系では、Ｌ_oa／Ｌ_inが小さくなり、測定できるｍ／ｚ範囲が極端に制限される。

For example, if L _oa / L _in = 4, then (m / z) _max / (m / z) _min = 25. If (m / z) _min = 50, then (m / z) _max = 1250, which limits the m / z range that can be measured. In particular, in a system using a sectoral electric field with a narrow acceptance, L _oa / L _in becomes small, and the m / z range that can be measured is extremely limited.

  In view of the above-mentioned points, the present invention is to solve various problems of the vertical acceleration time-of-flight mass spectrometer and to improve ion utilization efficiency.

In order to achieve this object, a vertical acceleration time-of-flight mass spectrometer according to the present invention includes:
(1) an ion source for ionizing a sample;
(2) ion transport means for transporting ions generated by the ion source by a predetermined distance;
(3) placed at the transport destination of the ion transport means, (i) an incident slit for restricting ions incident from the ion transport means, and (ii) placed at a stage subsequent to the entrance slit and parallel to the transport direction of the passed ions. , first and second acceleration electrodes arranged so as to sandwich the ion, is placed parallel to the first and second accelerating electrode during (iii) said first and second accelerating electrodes, enters compression electrodes for generating an electric field gradient as acts deceleration acts on the ions brought into, become pressurized et al., the acceleration voltage to the first and second electrodes in synchronization with the start signal serving as the flight time measurement of pulsed ions manner An ion accelerating unit that accelerates ions in a direction perpendicular to the transport direction of the ion transport means,
(4) an ion optical system for flying ions accelerated by the ion acceleration unit and performing ion mass separation using the fact that the flight speed of ions differs according to the mass-to-charge ratio;
(5) an ion detector that detects ions flying through the ion optical system in synchronization with a signal serving as a starting point for accelerating ions;
Equipped with a,
A configuration in which an intermediate voltage between the first acceleration electrode and the second acceleration electrode is applied to the compression electrode when accelerating ions by applying an acceleration voltage to the first and second acceleration electrodes. It is characterized by being.

  In addition, an ion trap means capable of trapping ions is provided between the ion transport means and the ion acceleration section, and the ion groups accumulated in the ion trap means are discharged while being synchronized with the acceleration timing of the ion acceleration section, The ion group is incident on the ion acceleration section intermittently.

The compression electrode is disposed between the first and second acceleration electrodes so as to be symmetric with respect to an ion incident axis to the ion accelerating portion, and the distance from each other approaches as the distance from the incident slit increases. And at least one pair of electrodes.

  Further, the ion source is EI, CI, ESI, or APCI.

  The ion optical system may be a linear type, a reflector type, or a spiral orbit type.

According to the vertical acceleration time -of- flight mass spectrometer of the present invention,
(1) an ion source for ionizing a sample;
(2) ion transport means for transporting ions generated by the ion source by a predetermined distance;
(3) placed at the transport destination of the ion transport means, (i) an incident slit for restricting ions incident from the ion transport means, and (ii) placed at a stage subsequent to the entrance slit and parallel to the transport direction of the passed ions. , first and second acceleration electrodes arranged so as to sandwich the ion, is placed parallel to the first and second accelerating electrode during (iii) said first and second accelerating electrodes, enters compression electrodes for generating an electric field gradient as acts deceleration acts on the ions brought into, become pressurized et al., the acceleration voltage to the first and second electrodes in synchronization with the start signal serving as the flight time measurement of pulsed ions manner An ion accelerating unit that accelerates ions in a direction perpendicular to the transport direction of the ion transport means,
(4) an ion optical system for flying ions accelerated by the ion acceleration unit and performing ion mass separation using the fact that the flight speed of ions differs according to the mass-to-charge ratio;
(5) an ion detector that detects ions flying through the ion optical system in synchronization with a signal serving as a starting point for accelerating ions;
Equipped with a,
A configuration in which an intermediate voltage between the first acceleration electrode and the second acceleration electrode is applied to the compression electrode when accelerating ions by applying an acceleration voltage to the first and second acceleration electrodes. because it is,
Various problems with vertical acceleration time-of-flight mass spectrometers have been solved, and ion utilization efficiency has been improved.

It is a figure which shows an example of the conventional time-of-flight mass spectrometer. It is a figure which shows another example of the conventional time-of-flight mass spectrometer. It is a figure which shows another example of the conventional time-of-flight mass spectrometer. It is one Example of the vertical acceleration type | mold time-of-flight mass spectrometer concerning this invention. It is one Example of the vertical acceleration type | mold time-of-flight mass spectrometer concerning this invention, and the compression electrode used for it. It is a figure which shows an example of the electric potential applied to the pulse acceleration part of this invention. It is a figure which shows an example of the flight locus | trajectory of the ion packet accelerated by the pulse acceleration part of this invention. It is a figure which shows another Example of the compression electrode concerning this invention. It is another Example of the vertical acceleration type | mold time-of-flight mass spectrometer concerning this invention.

  Embodiments of the present invention will be described below with reference to the drawings. In all the following examples, it is considered to measure monovalent positive ions. In the case of measuring negative ions, the voltage polarity may be reversed. The invention described in the following embodiments can be applied to any type of TOFMS, but is particularly effective for a system using a sectoral electric field with a small acceptance as an ion optical system.

  4 to 5 show examples of the configuration, and FIG. 6 shows changes in applied voltage. Ions are generated by a continuous ion source, transported by an ion transport means such as an ion guide, and incident on the pulse accelerator.

  (I) an incident slit that restricts ions incident from the ion transport means; (ii) an ion slit that is placed after the entrance slit and parallel to the transport direction of the ions that have passed through the ion transport means; (Iii) a compression electrode that is placed parallel to the acceleration electrode and generates an electric field gradient that acts on the ions as a decelerating action as it enters. The part consisting of the two is called an ion acceleration part or a pulse acceleration part.

  An ion beam is incident along the central axis (X axis) of the pulse accelerating unit, and the ion beam is accelerated in a direction (Y axis) perpendicular to the X axis by a pulse voltage applied to the acceleration electrode. The direction orthogonal to the XY plane is the Z axis, and the origin is the incident position of the pulse accelerator.

  A compression electrode is disposed along the central axis of the pulse accelerating portion. An example of the compression electrode is shown on the right side of FIG. The compression electrode is configured to extend long in the positive direction of the X axis and gradually decrease the distance from the X axis in the extending direction. In the example of FIG. 5, the distance between the pair of compression electrodes is 1.25 L on the ion beam incident side and 0.75 L on the opposite side.

  In the example of FIG. 5, a pair of compression electrodes is placed in parallel with the acceleration electrode 1 at a position separated from the acceleration electrode 1 by L in the Y-axis direction, and is further separated from the compression electrode by L in the Y-axis direction. The acceleration electrode 2 is placed parallel to the compression electrode, and the acceleration electrode 3 is placed parallel to the acceleration electrode 2 at a position further 10 L away from the acceleration electrode 2 in the Y-axis direction.

  Note that the compression electrodes do not necessarily have to be a pair, and may be composed of a plurality of pairs of symmetrical electrodes.

  In such a configuration, when a potential difference having the same polarity as the polarity of ions is applied between the entrance slit and the compression electrode, a deceleration field of ions can be generated in the pulse acceleration portion along the X-axis direction. As a result, the ions that enter the pulse accelerating portion deeper are decelerated, and the ion beam in the pulse accelerating portion is compressed in the X-axis direction.

  The operation of this embodiment will be described. FIG. 6 shows voltage values used in the simulation.

1. Ions continuously generated by the ion source are introduced as a continuous ion beam from the ion transport system through the incident restricting slit to the pulse acceleration unit with kinetic energy U _in (30 eV).

2. At this time, the potential V _{c of the} compression electrode, the potential V _{1 of} the acceleration electrode ₁ , the potential V ₂ of the acceleration electrode 2, and the potential V _{s of the} entrance slit are adjusted so that an appropriate deceleration field is generated in the pulse acceleration unit. Specifically, the potential before switching shown in FIG. 6 is used.

3. At the timing when the ion groups enters the fully pulse acceleration unit, the potential V _c of the compression electrode, the potential V ₁ of the accelerating electrode 1, the potential V ₂ of the accelerating electrode 2, and switching the potential V _s of the entrance slit, the ions Y Start acceleration in the axial direction. At this time, the applied voltage to the entrance slit and the compression electrode is about half of the voltage applied to the acceleration electrodes 1 and 2 so as not to disturb the acceleration field. This is because in this embodiment, a compression electrode for compressing the ion beam is arranged between the acceleration electrodes 1 and 2. Specifically, the potential after switching shown in FIG. 6 is used.

  4). After passing through the acceleration electrode 3, the ion packet passes through an ion optical system having kinetic energy convergence and is detected by a detector.

In addition, since ions entering deeper into the pulse acceleration unit in step 2 are decelerated and the ion beam is compressed, the ion residence time in the pulse acceleration unit is longer than in the case where compression is not performed, and the ion utilization efficiency is increased. Will improve. The angle φ with respect to the Y axis after acceleration is
φ = tan ^-1 √ (U _in / U _acc ) (9)
Can be expressed. However, the acceleration energy U _acc = e (V ₁ −V ₂ ) / 2−eV ₃ . As the distance of ions entering the pulse acceleration section increases and the ions are decelerated more, φ decreases, so that the length of the ion packet in the X direction is shorter when flying a certain distance as the compression is performed. Become. In FIG. 7, the ion packet after flying through the pulse acceleration part (length 12L) and free space (length 80L) has a length of 2/3 in the X direction.

  Due to these two effects, ion utilization efficiency is improved. The improvement in ion utilization efficiency varies depending on the shape of the deceleration field, that is, the shape of the compression electrode. In addition to the shape shown on the right side of FIG. 5, variations as shown in FIG. 8 are possible.

When the spiral trajectory TOFMS is used in the subsequent stage, the value of φ is determined by the spiral trajectory shape. When the compression electrode is not used, φ = 3.6 degrees when U _in = 30 eV and U _acc = 7400 eV. In a normal spiral orbit, φ = 1 to 2 degrees is appropriate, and deflection means (deflector or the like) for adjusting the angle is necessary. However, since the incident ions can be decelerated in this embodiment, it is possible to reduce φ, and it is also possible to directly connect the pulse accelerating unit to the spiral trajectory.

  Although the method of gradually decelerating along the X axis as in the first embodiment can spatially converge at a certain position, an angular distribution is generated in the ion packet. This is not so harmful for a system using a fan-shaped electric field that can be expected to have a lens effect in a toroidal electric field having a small acceptance.

  However, in systems where the acceptance is not so small, as in the case of reflective TOFMS using a reflected electric field, it may be better to transport with a beam that is nearly parallel. In such a case, it is possible to adjust the shape of the compression electrode and apply deceleration at a relatively early stage from the position of the entrance slit, and thereafter to reduce the change in the field.

  FIG. 9 shows an example of the configuration of this embodiment. The configuration and operation are almost the same as in the first embodiment. However, a means for trapping ions is arranged before the pulse accelerating portion and after the ion transport means. Then, the ion group accumulated in the trap means is discharged while being synchronized with the timing of acceleration of the pulse accelerator, and is incident on the pulse accelerator intermittently.

In this case, ions having a higher velocity and a smaller m / z value enter the pulse accelerating portion earlier, but due to the deceleration electric field created by the compression electrode, the difference is not as significant as when there is no compression electrode. As a result, since the value of L _oa / L _in shown in Problem 2 of the prior art can be increased, the measurable m / z range can be expanded.

  Thus, if a compression electrode is arrange | positioned in a vertical acceleration part, the electric field gradient which decelerates in an ion incident direction can be generated, and the utilization efficiency of ion can be improved.

  It can be widely used for time-of-flight mass spectrometers equipped with a vertical acceleration unit.

Claims (5)

(1) an ion source for ionizing a sample;
(2) ion transport means for transporting ions generated by the ion source by a predetermined distance;
(3) placed at the transport destination of the ion transport means, (i) an incident slit for restricting ions incident from the ion transport means, and (ii) placed at a stage subsequent to the entrance slit and parallel to the transport direction of the passed ions. , first and second acceleration electrodes arranged so as to sandwich the ion, is placed parallel to the first and second accelerating electrode during (iii) said first and second accelerating electrodes, enters compression electrodes for generating an electric field gradient as acts deceleration acts on the ions brought into, become pressurized et al., the acceleration voltage to the first and second electrodes in synchronization with the start signal serving as the flight time measurement of pulsed ions manner An ion accelerating unit that accelerates ions in a direction perpendicular to the transport direction of the ion transport means,
(4) an ion optical system for flying ions accelerated by the ion acceleration unit and performing ion mass separation using the fact that the flight speed of ions differs according to the mass-to-charge ratio;
(5) an ion detector that detects ions flying through the ion optical system in synchronization with a signal serving as a starting point for accelerating ions;
Equipped with a,
A configuration in which an intermediate voltage between the first acceleration electrode and the second acceleration electrode is applied to the compression electrode when accelerating ions by applying an acceleration voltage to the first and second acceleration electrodes. vertical acceleration time-of-flight mass spectrometer, characterized in that it is. An ion trap means capable of trapping ions is provided between the ion transport means and the ion acceleration section, and the ions collected by the ion trap means are discharged while being synchronized with the acceleration timing of the ion acceleration section. The vertical acceleration time-of-flight mass spectrometer according to claim 1, wherein the ion group is incident on the ion accelerator. The compression electrode is disposed between the first and second acceleration electrodes so as to be symmetric with respect to an ion incident axis to the ion accelerating portion, and the distance from each other approaches as the distance from the incident slit increases. 3. The vertical acceleration time-of-flight mass spectrometer according to claim 1, wherein the vertical acceleration type time-of-flight mass spectrometer is at least one pair of electrodes. The vertical acceleration time-of-flight mass spectrometer according to claim 1, wherein the ion source is EI, CI, ESI, or APCI. The vertical acceleration time-of-flight mass spectrometer according to claim 1, wherein the ion optical system is a linear type, a reflector type, or a spiral orbit type.

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