JP2002245964A - Time-of-flight mass spectrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a time-of-flight mass spectrometer capable of avoiding a problem of narrowing a dynamic range of an ion detector without lowering an S/N ratio of spectrum even when ion cleaves in a time-of-flight spectrum part. SOLUTION: A deceleration electric field electric-potentially set to reflect the ion creaved in the time-of-flight spectrum part and to pass the ion not creaved in the time-of-flight spectrum part and a reacceleration electric field electric potentially set to accelerate the ion until the ion reaches speed exceeding speed at the time when the ion is incident on the deceleration electric field are provided immediately before the ion detector.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a time-of-flight mass spectrometer, and more particularly to a time-of-flight mass spectrometer capable of improving the S / N ratio of a mass spectrum.

[0002]

2. Description of the Related Art A mass spectrometer is a device that causes ions generated from a sample to fly in a vacuum, separates ions having different masses during the flight, and records them as a spectrum.
The mass spectrometer includes a magnetic field type mass spectrometer that disperses ions using a sector magnetic field, and a quadrupole mass spectrometer (QMS) that selects (filters) ions by mass using a quadrupole electrode. ), A time-of-flight mass spectrometer (TOFMS) that separates ions by utilizing the difference in the flight time of ions depending on the mass, and the like are known.

[0003] Of these mass spectrometers, the magnetic field type mass spectrometer and the QMS are suitable for an ion source of a type that continuously generates ions, whereas the TOFMS generates ions in a pulsed manner. Compatible with all types of ion sources. Therefore, if a continuous ion source is to be used for TOFMS, it is necessary to devise how to use the ion source. Vertical acceleration time-of-flight mass spectrometer (OA-TOF)
MS (orthogonal acceleration TOFMS) is an example of TOFMS designed so that pulsed ions can be ejected from a continuous ion source.

FIG. 1 shows the configuration of a typical OA-TOFMS. OA-TOFMS is an electron impact ion source (E
I), chemical ionization ion source (CI), field desorption ion source (FD), fast atom bombardment ion source (FAB), electrospray ion source (ESI), atmospheric pressure chemical ionization ion source (APCI), high frequency inductive coupling A continuous ion source 1 such as a plasma ion source (ICP), an intermediate chamber 3 in which an ion guide 2 is placed, a focusing lens 4, a plate electrode 5, an accelerating electric field 6, a reflector 7, an ion detector 8, and the like. A measurement chamber 9 in which components constituting the ion optical system are placed.

In such a configuration, ions generated from the sample in the ion source 1 are guided to a high vacuum measurement chamber 9 by the ion guide 2 placed in the intermediate chamber 3. An orifice 10 is provided in a partition separating the intermediate chamber 3 and the measurement chamber 9, and ions guided from the ion guide 2 are shaped into an ion beam having a constant diameter by the orifice 10, and the measurement chamber 9 is introduced.

At the entrance of the measuring chamber 9, a focusing lens 4 is provided. The ion beam that has entered the measurement chamber 9 is focused by the focusing lens 4 and enters an elongated space along the gap between the plate electrode 5 and the accelerating electric field 6 in parallel with the plate electrode 5 and the accelerating electric field 6. . Plate electrode 5
By applying a pulse-like pushing voltage to the plate electrode 5, the ion beam having a certain length moving in the vicinity of the accelerating electric field 6 is pulsed in a direction perpendicular to the ion beam entering axis direction. It is extruded and becomes an ion pulse, and starts flying toward the reflecting section 7 provided at a position facing the plate electrode 5 and the accelerating electric field 6.

The ions accelerated in the vertical direction enter the measuring chamber 9
And the velocity given by the plate electrode 5 and the accelerating electric field 6 in a direction perpendicular to the velocity, so that the airplane flies not vertically but slightly obliquely. Then, the light is reflected by the reflector 7 and reaches the ion detector 8.

In the process of accelerating ions, the lighter the ions, the faster the speed becomes, and the heavier the ions, the slower the speed becomes.
The difference in ion mass appears as a difference in arrival time until reaching the ion detector 8, and the difference in ion mass can be separated as a difference in ion flight time.

As described above, the ion beam generated from the continuous ion source 1 is applied to the plate electrode 5 and the acceleration lens 6.
By accelerating in a pulsed manner, a continuous ion source can be applied to TOFMS having compatibility with an ion source generated in a pulsed manner.

[0010] Further, the application range of TOFMS is not limited to the example of OA-TOFMS as described above. OA-T
SIMS, which is not OFMS, but ionizes a sample placed in an accelerating electric field or on a plate electrode by electron beam irradiation
In many cases, TOFMS is also applied to the MALDI method in which a sample placed in an accelerating electric field or on a plate electrode is ionized by laser light irradiation.

[0011]

However, TOFMS
Utilizes the fact that when the same kinetic energy E ₀ is given to ions, ions with a smaller mass-to-charge ratio m / e reach the ion detector in a shorter time, and the flight distance of the ions is L. , time of flight t _TOF, when the pulse width of the ion upon reaching the ion detector and Delta] t, the resolution R of the flight time t _{T oF} and mass spectrum of the ions,
It is given by equation (1) and equation (2). t _TOF = Lm / 2E ₀ (1) R = t _TOF / 2Δt (= m / Δm (2) If the flight distance is constant, the smaller the ion pulse width At, the higher the resolution R according to the equations (1) and (2). Get higher. Note that Δt is determined by the spatial spread of ions during ion acceleration, the dispersion of initial velocities, the aberration of the time-of-flight spectroscopic unit, and the like. The reflecting section is widely used to improve the aberration of the total time-of-flight spectroscopic section in consideration of the spatial spread of ions and the dispersion of the initial velocity, and to improve the resolution R of the mass spectrum. is there.

Among the ions generated by the ion source, ions having an excessive internal energy (metastable ions) are often cleaved during flight in the time-of-flight spectroscopic section. Further, even a stable ion having no excessive internal energy at the time of ionization may be cleaved during flight through the time-of-flight spectroscopic unit by inelastic collision with residual gas molecules in the spectroscopic unit.

For example, ions m having a mass number of m ₀ are generated.
₀ ⁺ is cleaved, consider a case where a neutral molecule ion _m ^{X +} and mass number of mass number _{m X (m} 0 -m _X) taken as an example. In the reflection type TOFMS having the reflection portion, almost all of the newly generated ions m _x ⁺ that have been cleaved in the entire flight space enter the ion detector. On the other hand, for neutral molecules (m ₀ −m _X ) that are newly generated by cleavage, when the ions are cleaved just before reaching the apex of the quadratic hyperbolic orbit in the reflection part, and neutral molecules are generated, Not all neutral molecules are incident on the ion detector. Also, when ions are cleaved after passing over the apex of the quadratic hyperbolic orbit in the reflection part to generate neutral molecules, neutral molecules whose flight direction at the time of cleavage is not on the extension of the ion detector are excluded. , All neutral molecules are incident on the ion detector.

The ion m _x ⁺ and the neutral molecule (m ₀ −m _x ) after the cleavage enter the ion detector at a flight time different from the flight time of the ion m ₀ ⁺ that has not been cleaved. It becomes noise on the baseline of the mass spectrum,
Lowering the S / N ratio of the mass peak, narrowing the effective dynamic range of the ion detector, or causing a ghost peak to cause mass assignment of the mass peak to be a hindrance to analysis.

FIG. 2 shows a state in which the flight trajectory of the ions changes with the cleavage of the ions. In the figure, reference numeral 11 denotes a first stage ion accelerating unit,
The acceleration units (d ₁ ) and 12 are the second stage ion acceleration unit, that is, the second acceleration unit (d ₂ ). Since the ion accelerator of this TOFMS is divided into two stages, it is generally called a two-stage accelerator. The two-stage acceleration unit includes a plate electrode 13, a first grid 14, and a second grid 15. Plate electrode 13 applies a pulse voltage to the ion m ₀ ^+, toward the time-of-flight spectrometer unit, to fly the ions m ₀ ^+.

Normally, voltages U ₁ (= α * U ₀ ) and U ₂ (= β * U ₀ ) having the same polarity as the ion to be analyzed are respectively applied to the plate electrode 13 and the first grid 14. The potential is applied to only the second grid 15 to the ground potential or the common potential of the device.

The ions m ₀ ⁺ generated by various ionization methods are accelerated by the first acceleration unit 11 and the second acceleration unit 12, and are given kinetic energy e * U ₀ (1 ± δ, δ≪1). . The ions m ₀ ⁺ that have passed through the two-stage acceleration unit are
After passing through the third grid 16 stretched at the entrance of the reflecting portion 18 through the free space (d ₃ ), and entering the inside (d ₄ ) of the reflecting portion 18, draw a trajectory on a predetermined parabola reflected Te, passes through the third grid 16 is stretched at the entrance of the reflecting part 18 again to reach the ion detector 19 via the second free space (d _5), is observed as a mass spectrum.

The third grid 16 at the entrance of the reflecting section
Is normally set to the ground potential or the common potential of the device. Also, the opposite side of the entrance of the reflection section 18 is the fourth
And serves to pass particles that are not reflected inside the reflecting portion 18 (d ₄ ). Normally, a high voltage U ₄ (= ρ * U ₀ ) having the same polarity as the ion to be analyzed is applied to the fourth grid 17.

In the drawing, a thick parabolic solid line shows the flight trajectory of ions that have not been cleaved in the entire flight space. The thin dashed lines show the flight trajectories of newly generated ions and neutral molecules that are cleaved during the flight.

As the ion detector 19, a secondary electron multiplier is generally used. In TOFMS, in particular, a micro channel plate (MCP) is widely used as the ion detector 19, but the MCP has a dynamic range (linearity as the ion detector) of 3 as the ion detector.
There are only digits. In addition, when the cleaved ions or neutral molecules generated by the cleaving enter the MCP, they cause baseline noise in the mass spectrum. In some cases, the dynamic range of the MCP ion detector can be up to two orders of magnitude. Sometimes it is narrowed.

A description will be given below as to which position in the flight space is cleaved to cause the noise to enter the ion detector 19 or cause a ghost peak.

First, the total flight time is T (k), the precursor
・ Mass of ion mass₀, The mass of the cleaved ion is m_X, Pre
The central acceleration voltage of Casa Ion is U₀, Precursor I
On the flight speed in free space v₀(= $ 2e * U₀
(K * α + β) / m₀｝), The acceleration voltage of the first acceleration unit is set to U
₁(= Α * U₀), The gap of the first accelerating part is d₁, First
Let z be the cleavage position of the ion at the accelerating part.₁, Acceleration of the second acceleration unit
U voltage₂(= Β * U ₀), The gap of the second accelerating section is d
₂, The cleavage position in the second acceleration part is z₂In the first acceleration unit
The ON center position is η (0 <η <1) (where U₀=
(Η * α + β) * U₀, Η * α = α₁, Α₁+ Β =
1) The spread coefficient of the ion beam position in the first acceleration unit is
k (0 <k <2), and the first free flight space distance is d₃, Second
Free flight space distance d₅, The length of the reflecting portion 18 is d₄, Anti
U₄(= Ρ * U₀), Deepest flight of ion reflection part
Row position d_ref(= D₄* (K * α + β) / ρ), anti
The cleavage position of the ions in the projecting portion 18 is z₄And z₅And
Good.

In the time-of-flight spectrometer having the above-described optical parameters, the time of flight of the cleaved ions to the ion detector, the kinetic energy of the cleaved ions, and the neutral molecules generated by the cleaving Consider the presence or absence of arrival at the ion detector and the flight time when it arrives.
As a precondition for the calculation, it is assumed that there is no release of kinetic energy and no change in flight speed when the ion is cleaved.

First, the flight time T ₀ (k) of an ion that does not cause cleavage in the entire flight space is expressed by equation (3).

[0025]

(Equation 1)

Next, the flight time T ₁ (k) of the ion m _X ⁺ when the ion m ₀ ⁺ is cleaved in the first acceleration section is represented by the following equation (4).

[0027]

(Equation 2)

Here, the ion cleavage position z ₁ (0 <
z ₁ <η * d ₁ ) represents a distance from the first grid 14. From this equation, the flight time T of the ion after cleavage is
₁ (k) _is found to be expressed by a function of the m X / _{m 0} and _{z 1.} Then, as the value of z ₁ is reduced, decreasing the amount of flight time T _{1 (k)} and the kinetic energy is can be seen that large. In addition, α ₁ is smaller than β. If α ₁ is one order of magnitude smaller than β, the final kinetic energy reduction rate will be at most 10% or less.

Next, the flight time T ₂ (k) of the ion m _X ⁺ when the ion m ₀ ⁺ is cleaved in the second acceleration section is expressed by the following equation (5).

[0030]

(Equation 3)

The ion cleavage position z ₂ (0 <z ₂ <d
₂ ) represents a distance from the second grid 15. From this equation, the flight time T ₂ (k) of the ion after cleavage is m _X
It can be seen that it can be represented by a function of / m ₀ and z ₂ . _Then, once the value of m X / _{m 0,} as _{z 2} approaches _{d 2,} the reduction rate of kinetic energy can be seen to increase.

Next, the flight time T ₃ (k) of the ion m _X ⁺ when the ion m ₀ ⁺ is cleaved in the first free space is expressed by equation (6).

[0033]

(Equation 4)

As is apparent from equation (6), the first free sky
Cleavage ion m between_X ⁺Flight time T ₃(K) is reflection
Only the flight time in the department is m_X/ M₀Function. Sand
In addition, the flight time of the first free space
There is no difference. Therefore, m in this space₀ ⁺Cleaved from
Generated ion m_X ⁺All at the same flight time
Reaches the detector 19 and is detected as a ghost peak
You.

Next, the flight time T ₄ (k) of the ion m _X ⁺ when the ion m ₀ ⁺ is cleaved in the first half of the reflection portion is expressed by the following equation (7).

[0036]

(Equation 5)

Here, the ion cleavage position z ₄ (0 ≦ z ₄
≦ d _ref ) represents the distance from the third grid 16. As is apparent from equation _(7), once the value of m X / _{m 0,} as _{z 4} increases (i.e., as the distance from the third grid 16 is away), shorter time _{m X} It can be seen that ⁺ reaches the ion detector 19.

Next, the flight time T ₅ (k) of the ion m _X ⁺ when the ion m ₀ ⁺ is cleaved in the latter half of the reflection portion is expressed by the following equation (8).

[0039]

(Equation 6)

Here, the ion cleavage position z ₅ (0 ≦ z ₅
≦ d _ref ) represents the distance from the third grid 16. As it is apparent from equation _(8), m once the value of X / _{m 0,} as is _{z 5} decreases (i.e., as the distance from the third grid 16 is closer), shorter time _{m X} It can be seen that ⁺ reaches the ion detector 19.

Next, when the ion m ₀ ⁺ is cleaved in the second free space, the flight speed of the ion m _X ⁺ is invariable.
Flight time T _{6 (k)} is expressed by equation (3), reach the ion detector 19 to the ion m ₀ ⁺ the same time not cleaved, is detected ion m ₀ ⁺ and with not cleaved.

As described above, the ion m₀ ⁺Different places to cleave
Every time, ion m_X ⁺Reaches the ion detector 19
We have considered how the time required to
_X/ M₀<1, so that in equations (3) to (8), T
₁(K)> T₂(K), T₃(K), T₄(K), T₅
(K), T₆(K). Also, the formula
From (4), (5), (7), and (8),
The ion m cleaved in the row space _X ⁺Is the cleavage position z₁,
z₂, Z₄, And z₅Flight time varies depending on the value of
Becomes m₀ ⁺Appear as noise on the lower mass side
I understand. These are the baseline spectra of mass spectra.
Is increasing.

[0043] On the other hand, in the first free space _{d 3,} ions _{m 0}
^{No matter} where ⁺ is cleaved, the generated ion m _X ⁺
Is calculated according to the equation (6) at the same flight time.
To reach. From this, it can be seen that the ions m _X ⁺ cleaved in the first free space d ₃ all appear as ghost peaks in the mass spectrum.

Next, when the kinetic energy when the cleaved ions enter the ion detector 19 is calculated from the equations (4) to (8), the following equation is obtained.

First, the kinetic energy of uncleaved ions is expressed by equation (9).

[0046]

(Equation 7)

On the other hand, the kinetic energy of the ions cleaved in the first and second acceleration sections, the second acceleration section, the first and second free spaces, and the first and second halves of the reflection section is given by the ratio to E _{0 in} equation (9). Expressions (10) to (14) are obtained.

The kinetic energy ratio E ₁ of the ions cleaved in the first acceleration part:

[0049]

(Equation 8)

[0050] From equation (10), when _{z 1} is reduced, reduction of kinetic energy is can be seen that large.

The kinetic energy ratio E ₂ of the ions cleaved in the second acceleration section:

[0052]

(Equation 9)

[0053] From equation (11), when _{z 2} is reduced, reducing the amount of kinetic energy is can be seen that large.

The ratio E ₃ of the kinetic energy of the ions cleaved in the first free space:

[0055]

(Equation 10)

From equation (12), the amount of decrease in the kinetic energy of ions cleaved in the first free space is m _x / m
_It only depends on ₀ .

The kinetic energy ratios E ₄ and E _{5 of} the ions cleaved in the first half and the second half of the reflecting portion:

[0058]

[Equation 11]

[0059]

(Equation 12)

From [0060] Equation (13) and (14), depending on whether the second half of the first half of the cleaved location reflection of ions, no difference in the ratio of the kinetic energy of the ions, E ₄ and E ₅ is both the same formula It can be seen that Then, it can be seen that as the values of z ₄ and z ₅ increase, the decrease amount of the kinetic energy decreases, and that when z ₄ = z ₅ , the kinetic energy becomes the same.

The kinetic energy ratio E ₆ of the ion cleaved in the second free space:

[0062]

(Equation 13)

From equation (15), the amount of reduction in the kinetic energy of the ions cleaved in the second free space is m _x / m
_It only depends on ₀ .

In the equations (13) and (14), z ₄
= Z ₅ = (d ₄ / ρ) √ (k * α + β) Except for the case of m _X <m ₀ , from formulas (10) to (15), the ion m ₀ ⁺ before cleavage is It can be seen that the ratio of the kinetic energies of the cleavage ions m _X ⁺ is less than 1.

Next, the behavior of neutral molecules generated after cleavage will be considered. First, when ions are cleaved in the flight space up to the first half of the reflector, that is, in the first accelerator, the second accelerator, the first free space, and the first half of the reflector, the generated neutral molecules leave the reflector as they are. Then, the light is discharged from the fourth grid 17 to the outside of the reflecting portion. Therefore, neutral molecules do not reach the ion detector 19 and do not become noise on the mass spectrum.

On the other hand, when the ions are cleaved in the flight space after the latter half of the reflection part, that is, in the latter half of the reflection part and in the second free space, the generated neutral molecules have a flight velocity v of m ₀ ⁺ ions at the time of cleavage. _The flight continues toward the ion detector 19 while maintaining _z . Among them, the relationship between the flight speed v _z having a cleavage position z ₅ and the neutral molecule when cleaved in the second half reflecting portion is represented by the formula (16).

[0067]

[Equation 14]

At this time, 0 ≦ z ₅ <d ₄ * (k * α ₁ +
β) / ρ, 0 <v _z <v ₀ * (k * α ₁ +
β). For this reason, if the flight direction of the ions at the cleavage position in the latter half of the reflecting portion is on the extension of the ion detector 19,
Due to the slower flight speed than m ₀ ⁺ , the neutral molecule is m ₀ ⁺
Incident on the ion detector 19 later, and becomes baseline noise on the high mass side.

[0069] Also, when the ions in the second free space has cleavage, because all neutral molecules flying at the same flight velocity and m ₀ ^+, neutral molecules, m ₀ ⁺ simultaneously ion detector 1
Reach 9

As described above, when the ions are cleaved in the time-of-flight spectroscopic unit, ions having a small mass and neutral molecules are newly generated, and they become ghost peaks or noise on the mass spectrum. , The S / N ratio is reduced, and in some cases, the dynamic range of the MCP ion detector is narrowed.

In view of the foregoing, it is an object of the present invention to prevent a decrease in the S / N ratio of a mass spectrum even when ions are cleaved in a time-of-flight spectroscopic unit, and furthermore, a dynamic range of an ion detector. An object of the present invention is to provide a TOFMS capable of avoiding the problem of narrowing.

[0072]

In order to achieve this object, a TOFMS according to the present invention provides a time-of-flight mass spectrometer provided with an ion source, a time-of-flight spectroscope, and an ion detector. A decelerating electric field, which is set at a potential so as to reflect ions cleaved in the time-type spectroscopic unit and pass ions not cleaved in the time-of-flight spectroscopic unit, is provided immediately before the ion detector. .

Further, an ion source, a time-of-flight spectroscopy unit,
In a time-of-flight mass spectrometer equipped with an ion detector, a decelerating electric field and then an accelerating electric field are provided immediately before the ion detector. In the accelerating electric field, the ions reach a speed exceeding the speed at which the ions were incident on the decelerating electric field. It is characterized by accelerating ions up to that point.

Further, the deceleration electric field is set at a potential so as to reflect ions cleaved in the time-of-flight spectroscopic unit and to pass ions not cleaved in the time-of-flight spectroscopic unit. .

[0075]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 3 shows a TOF according to the present invention.
9 shows an embodiment of MS. In the figure, 11 is 1
The first stage ion accelerating section (d ₁ ), 1
Reference numeral 2 denotes a second-stage ion acceleration unit, that is, a second acceleration unit (d ₂ ). The ion accelerator of this TOFMS has 2
Since it is divided into stages, it is generally called a two-stage accelerator.
The two-stage accelerating unit includes the plate electrode 13, the first grid 1
4 and the second grid 15. The plate electrode 13 applies a pulse voltage to the ion m ₀ ⁺ ,
The ions m ₀ ⁺ are made to fly toward the time-of-flight spectroscopy unit.

Normally, voltages U ₁ (= α * U ₀ ) and U ₂ (= β * U ₀ ) having the same polarity as the ion to be analyzed are applied to the plate electrode 13 and the first grid 14, respectively. The potential is applied to only the second grid 15 to the ground potential or the common potential of the device.

The ions m ₀ ⁺ generated by various ionization methods are accelerated by the first acceleration unit 11 and the second acceleration unit 12, and are given kinetic energy e * U ₀ (1 ± δ, δ≪1). . The ions m ₀ ⁺ that have passed through the two-stage acceleration unit are
After passing through the third grid 16 stretched at the entrance of the reflecting portion 18 through the free space (d ₃ ) of the above, and entering the inside (d ₄ ) of the reflecting portion 18, a trajectory on a predetermined parabola is drawn And passes through the third grid 16 stretched again at the entrance of the reflector 18 to form a second free space (d ₅ ), and
In this embodiment, the ion beam reaches the ion detector 19 via the deceleration electric field 20 and the re-acceleration electric field 21 which are newly provided, and is observed as a mass spectrum.

The third grid 16 is usually set to the ground potential or the common potential of the device. Also,
The other side of the entrance of the reflection part 18 is a fourth grid 17 and functions to pass particles not reflected inside the reflection part 18 (d ₄ ). Fourth grid 17
Is applied with a high voltage U ₄ (= ρ * U ₀ ) having the same polarity as the ion to be analyzed.

In this embodiment, immediately before the ion detector 19,
A deceleration electric field 20, which temporarily reduces the flight speed of ions,
Next, a re-acceleration electric field 21 for re-accelerating the flight speed of ions
Is provided. Deceleration electric field 20
Is divided by a fifth grid 22 and a sixth grid 23. On the other hand, the re-acceleration electric field 21 is partitioned by the sixth grid 23 and the ion incident surface of the ion detector 19.

Normally, the fifth grid 22 has a ground potential
Alternatively, it is set to the common potential of the device. Also, the sixth group
The lid 23 has an electrode having the same polarity as the ion to be analyzed.
Pressure U ₆(= Κ * U₀) Is applied and the ion detector 19
The ion incidence surface has a polarity opposite to the polarity of the analyte ion.
Voltage U₇(= Γ * U₀) Is applied.

These deceleration electric field 20 and re-acceleration electric field 21
Is disposed in front of the ion detector 19, the S / N ratio of the mass spectrum can be improved and the ghost peak can be removed. Therefore, the mechanism will be described.

First, in TOFMS having a reflection field, the potential U ₆ (= κ *) of the newly added deceleration electric field 20
U ₀ ) is U ₆ / U ₀ <1. Incidentally, _{U 6} is, 0 <U
₆ <(k * α + β) * U ₀ , but considering the case where the ions are spatially spread in the first acceleration unit 11, practically 0.5 * U ₀ <U ₆ ≦
It is limited to β * _{U 0.}

[0083] Equation (9) As is clear from to (15), in any flight space, ion _m ^{X +} is newly generated with the ion _m ^{0 +} cleavage, prior to dehiscence ion m
₀ ⁺ kinetic energy is decreasing than. for that reason,
The deceleration electric field 20, after cleavage ion m _X ⁺ becomes a potential barrier for the predetermined kinetic energy e * has only low kinetic energy than U ₆ ions m _X ⁺ is determined by the potential of the U _6, all deceleration electric field The light is reflected by 20 and cannot enter the ion detector 19.

Cleavage ions m by this deceleration electric field 20_X ⁺
Has a certain limit (e * U)₆)
The same effect as the electrostatic sector, the ion detector
Cleavage ion m to 19_X ⁺By blocking the incidence of
Cleavage ion m_X ⁺Reduced baseline noise
To improve the S / N ratio of the mass spectrum and the ion detector 1
9 can improve the effective dynamic range. Ma
Also, the cleavage ion m in the first free space_X ⁺Ghost by
・ The peak occurrence is (m_X/ M₀) <(U₆/ U ₀)so
If there is, it can be completely prevented.

Next, the effect of the re-acceleration electric field 21 will be described.
explain. M in the latter half of the reflector 18₀ ⁺Is formed by cleavage
Neutral molecules that are cleaved at a deep position
Wachiz₅The greater the value, the higher its speed and
Kinetic energy is reduced. This is from equation (16)
it is obvious. Such neutral molecules can be detected by the ion detector 1
Speed and kinetic energy at the time of incidence on
Therefore, the secondary electron multiplication efficiency on the MCP is low.

[0086 At this time, the polarity of the analyte ions in the re-accelerating electric field 21 applied a voltage U ₇ of opposite polarity, to the absolute value and gamma * U _0. Then, the neutral molecule becomes the re-acceleration electric field 21
Is not accelerated by the action of, but only the ions to be analyzed are accelerated by the action of the re-acceleration electric field 21, so that only the velocity and the kinetic energy of the ions to be analyzed when entering the ion detector 19 are increased. Therefore, the secondary electron multiplication efficiency on the MCP can be increased. As a result, the baseline noise derived from neutral molecules does not increase, but only the signal derived from the ion to be analyzed increases, and the S / N ratio of the mass spectrum can be improved.

The flight time T ₆ (k) of the stable ion m ₀ ⁺ that does not cause cleavage in the entire flight space is newly expressed by the following equation (17) when passing through the deceleration electric field 20 and the re-acceleration electric field 21. It is.

[0088]

(Equation 15)

In equation (17), the fifth term is the deceleration electric field 20
The sixth term gives the time of flight in the re-acceleration electric field 21. Here, d ₆ is the flight space of the deceleration electric field 20, d ₇
Is a flight space of the re-acceleration electric field 21. For the stable ion m ₀ ⁺ that is not cleaved in the entire flight space, even if deceleration or re-acceleration is performed, each optical equation in the equation (17) is obtained so that a high-resolution mass spectrum as before can be obtained. Needless to say, the parameters need to be optimized.

When the generation location of the ion beam generated by the first acceleration unit 11 can be limited to the vicinity of the plate electrode 13, for example, the sample placed on the surface of the plate electrode 13 is irradiated with pulsed laser light. In the case of the MALDI method for ionizing the sample or the SIMS method for irradiating the sample placed on the surface of the plate electrode 13 with a pulsed electron beam to ionize the sample, in particular, U ₆ = β As * U ₀ , the set voltage U ₆ of the sixth grid 23 is set to the same value as the set voltage U ₂ (= β * U ₀ ) of the first grid 14 for applying an electric field to the second acceleration unit 12, It is also possible to share one power supply for supplying voltage to both grids.

In the above embodiment, the deceleration electric field 20 and the re-acceleration electric field 21 are used integrally.
And the re-acceleration electric field 21 may be separately provided at separate places.

In the above embodiment, the decelerating electric field 20 and the re-acceleration electric field 21 are used together. However, even if only the decelerating electric field 20 is used alone, the reflection of the cleavage ions can be performed. It goes without saying that there is a certain effect on the improvement of the N ratio.

Further, in the above embodiment, the ion acceleration
TOF with single-stage or multi-stage acceleration unit
A deceleration electric field and a re-acceleration electric field may be applied to the MS.

In the above embodiment, the number of the reflecting portions is one. However, the number of the reflecting portions may be two or more, or an electric field gradient type may be used.

Further, in TOFMS other than the reflection type, application to a linear type TOFMS can be considered. However, in the case of the linear TOFMS, all the neutral molecules generated by the cleavage during the flight process go straight and enter the ion detector. It is considered that there is an effect of improving the S / N ratio by the deceleration / re-acceleration.

[0096]

As described above, the TOFMS of the present invention
According to the present invention, a deceleration electric field which is set to a potential so as to reflect ions cleaved in the time-of-flight spectroscopic unit and pass ions not cleaved in the time-of-flight spectroscopic unit, and when the ions enter the deceleration electric field A re-acceleration electric field was set just before the ion detector to accelerate the ions until they reached a speed exceeding the speed of the ion detector. It is possible to suppress the occurrence of a decrease in the S / N ratio of the mass spectrum and the appearance of a ghost peak, and it is possible to avoid the problem that the dynamic range of the ion detector is narrowed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a conventional time-of-flight mass spectrometer.

FIG. 2 is a diagram showing a conventional time-of-flight mass spectrometer.

FIG. 3 is a diagram showing one embodiment of a time-of-flight mass spectrometer according to the present invention.

[Explanation of symbols]

1 ... Ion source, 2 ... Ion guide, 3 ... Intermediate chamber, 4 ...
..Converging lens, 5 ... Plate electrode, 6 ... Acceleration electric field, 7
... Reflector, 8 ... Ion detector, 9 ... Measurement room, 10 ...
Orifice, 11: first acceleration unit, 12: second acceleration unit,
13 ... plate electrode, 14 ... first grid, 15 ...
-2nd grid, 16 ... 3rd grid, 17 ... 4th grid, 18 ... reflection part, 19 ... ion detector,
20: deceleration electric field, 21: re-acceleration electric field, 22: fifth grid, 23: sixth grid.

Claims (3)

[Claims] 1. A time-of-flight mass spectrometer provided with an ion source, a time-of-flight spectroscopic unit, and an ion detector, which reflects ions cleaved in the time-of-flight spectroscopic unit,
A time-of-flight mass spectrometer characterized in that a decelerating electric field whose potential is set so as to pass ions that have not been cleaved in a time-of-flight spectroscopic unit is provided immediately before an ion detector. 2. A time-of-flight mass spectrometer comprising an ion source, a time-of-flight spectroscopic unit, and an ion detector, wherein a deceleration electric field and then an acceleration electric field are provided immediately before the ion detector.
A time-of-flight mass spectrometer characterized in that ions are accelerated in an accelerating electric field until the ions reach a speed exceeding a speed at which the ions enter the decelerating electric field. 3. The electric field is set such that the decelerating electric field reflects ions cleaved in the time-of-flight spectroscopic unit and passes ions not cleaved in the time-of-flight spectroscopic unit. The time-of-flight mass spectrometer according to claim 2.