JP2000323089A - Time-of-flight mass spectrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a TOFMS that avoids saturation of an MCP even if a strong ion pulse is incident on the MCP, to avoid lack of a mass spectrum accompanying a dead time and reduction of a service life of the MCP itself. SOLUTION: In this mass spectrometer, by previously detecting a current value of an ion pulse coming flying from an ion source with two ion detectors 13, 14 provided in two triple-convergence planes of an electrostatic sector electric field type TOFMS spectral part and feeding the detected value to a final ion detector 11, a gain of the final ion detector 11 is controlled to avoid saturation of the final ion detector 11. This technique can be also applied to a TOFMS using a pulse ionization method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a time of flight mass spectrometer (TOFMS).
er), and more particularly to a TOFMS that can avoid saturation of the ion detector.

[0002]

2. Description of the Related Art Vertical acceleration time-of-flight mass spectrometers (O
A-TOFMS; Orthogonal Acceleration Time of Fl
ight mass spectrometer) introduces an ion beam from an ion source that continuously generates ions into an ion reservoir, accelerates the ion beam in a direction that intersects the ion beam introduction direction, and accelerates the ion pulse. Is a device that measures the time until it is detected by the final ion detector and performs mass spectrometry.

FIG. 1 shows an electrostatic sector electric field type OA-TOF.
FIG. 3 is a schematic view of the configuration of the MS as viewed from the X-axis direction. In the figure, 1 is an electron impact (EI) for continuously generating positive ions,
Chemical ionization (CI), fast atom bombardment (FAB), electrospray (ESI), inductively coupled plasma (IC)
P) or an external ion source.

[0004] ion beam emitted from an external ion source 1 in the Y-axis direction is a converging lens 2 positive potential V _F is applied is converged in the Z-axis direction, the ion reservoir having an effective length of y ₀ 3 is introduced. Push-out near ion reservoir 3
A plate 4 is provided, and an ion extraction grid 5 having a ground potential and an exit grid 6 to which a negative potential V ₂ is applied are provided at a position facing the push-out plate 4. An electric field for pushing out ions is formed in a direction (Z-axis direction) crossing the introduction direction (Y-axis direction).

[0005] Push-out plate 4 and the ion extraction grid 5 is not face apart 2S ₀ together, the ion beam from an external ion source 1, Push-out plate 4
Is introduced into the ion reservoir 3 which is a space between the ion extraction grid 5 and the ion extraction grid 5. Further, the ion extraction grid 5 and the exit grid 6 face each other with a distance D therebetween.

A positive voltage (amplitude 2 V) is applied to the push-out plate 4.
_When a push-out pulse voltage 7 of _P ) is applied, an electric field gradient is instantaneously formed in the so-called two-stage acceleration section 8 between the push-out plate 4, the ion extraction grid 5, and the exit grid 6, and The ions are simultaneously accelerated in the Z-axis direction and ejected from the ion reservoir 3, pass through a first electrostatic sector electric field 9 and a second electrostatic sector electric field 10 provided at opposing positions, and then pass through the micro channel. -It reaches the final ion detector 11 composed of a plate (MCP) or the like.

Strictly speaking, ions have a velocity in the Y-axis direction when introduced into the ion reservoir 3, so that Pu
sh-out plate 4, ion extraction grid 5, and
Even if a force in the Z-axis direction is received between the exit grids 6, that is, by the electric field generated in the two-stage acceleration unit 8, the flight direction slightly shifts from the Z-axis direction in the Y-axis direction.

[0008] When receiving the above acceleration, push-out
Since constant energy corresponding to the potential difference between the plate 4 and the outlet grid 6 is given equally, at the end of the acceleration, ions with smaller mass have a higher velocity, and ions with larger mass have a lower velocity. As a result of such a velocity difference, while the ions fly through the TOF spectroscopic unit 12 composed of the two electrostatic sector electric fields, the mass dispersion of the ions is performed, and the final ion detector 1 is sequentially arranged from the lighter ions.
1 and the mass spectrum is measured.

In such a configuration, the MCP used for the final ion detector has a bundle of several millions of very thin conductive glass capillaries having a diameter of 10 to 25 μm and a length of 0.24 to 1.0 mm. , Each of which functions as a secondary electron multiplier. The traveling distance of secondary electrons is 1.0
mm or less, a high-speed response of 1 nanosecond (ns) is possible for a charged particle pulse of a pulse input. On the other hand, when a photomultiplier tube (photomultiplier) or a secondary electron multiplier tube (SEM tube) in which the traveling distance of secondary electrons is about several cm is used as a final ion detector, 5
A response time of around ns is required.

[0010] The mass resolution R of TOFMS is generally given by equation (1).

R = M / ΔM = t _TOF / 2 · Δt (1) where M is Dalton, ΔM is mass difference, t _TOF Is the flight time of the ion M ⁺ , and Δt is the ion pulse time width. The ion pulse width Δt depends on the place to be measured, but the narrower the width in the final ion detector, the smaller the mass resolution R
Can be higher. Therefore, it is ideal that the time width of the incident ion pulse and the output signal width after secondary electron conversion amplification in the final ion detector are the same, but the time spread in the final ion detector itself always occurs. (1)
The denominator is added to Δt of the denominator in the equation, and the denominator becomes large.

Normally, in the high-resolution TOFMS, Δt is about 5 ns at the time of incidence on the final ion detector. Since the time spread when using a photomultiplier or SEM tube is about 5 ns as described above, high-resolution TOF
The mass resolution R of MS has a significant effect. For example,
Δt = 5 ns when entering the final ion detector, and Δt ≒ 5 + 5 = 10 ns when outputting from the final ion detector
And the mass resolution R of the TOFMS is reduced by half. For this reason, in particular, the final ion detector for high-resolution TOFMS generally uses an MCP having a requirement of a response of 1 ns or less.

[0013]

However, in such a configuration, a problem when the MCP is used is that its input / output linearity is small. The linearity of the MCP is determined by the strip current value inherent to the MCP, and the input / output linearity of the MCP is three digits narrower than the five digits of the SEM tube. This strip current is MCP
Also functions to neutralize the charge of secondary electrons generated in
The saturation of P means that the average output current of the MCP is 5 times the strip current.
(Hamamatsu Photonics Co., Ltd., April 1995, technical data "MCP assembly technical data").

Of course, when the MCP gain is set high, secondary electron saturation of the MCP is likely to occur. Once this saturation occurs, the time required for neutralization by the strip current is on the order of microseconds (μs).
It takes longer as the amount of generated secondary electrons increases. In the period required for the neutralization, the MCP itself is in a dead time state, and the output of the peak (intensity) of the ions incident during this period is zero, and the spectrum of these ions is reduced from the mass spectrum. The problem of missing occurs. Furthermore, repeated saturation of the MCP
Accelerates self-deterioration and shortens life.

Here, as an example, let us consider how much mass range of an ion spectrum is lost when a dead time of 1 μs occurs. The time (t _TOF ) during which a monovalent ion having a mass of M Daltons is accelerated at V volts and flies over the length Lcm of free space is approximately given by the following equation (2).

T _TOF .0.72 · L√ (M / V) (2) where L is a flight Distance (cm), M is the mass of the ion (Da
lton), V is the acceleration voltage of the ions (volts).

For example, when the kinetic energy is V = 3000 electron volts (accelerating at 3000 volts) and the flight distance L = 100 cm, the flight time of ions of M = 99 and 100 daltons is t _TOF ≒ 13.08 μs and 13.14 μm.
s, the time difference of one dalton is about 60 ns. M = 2
Flight times of 99 and 300 dalton ions are t _TOF ≒ 2
At 2.73 μs and 22.77 μs, the time difference of 1 dalton is about 40 ns. In this quality region, 1 μs corresponds to a mass range of about 25 daltons, and the dead time on the order of microseconds due to saturation of the MCP has a problem that peaks in a considerably wide mass range are missing on the mass spectrum.

SUMMARY OF THE INVENTION In view of the foregoing, it is an object of the present invention to irradiate an MCP with a high-intensity ion pulse, saturate the MCP, cause a lack of mass spectrum due to the dead time, and reduce the MCP.
A TOF that can avoid such inconveniences even if it encounters measurement conditions that may cause its own life to be shortened
To provide MS.

[0019]

In order to achieve this object, a TOFMS according to the present invention comprises an ion source for emitting an ion pulse, an ion accelerator for accelerating the emitted ion pulse, and an ion accelerator for accelerating the ion pulse. A final ion detector in which the pulse enters after flying a predetermined distance, a time-of-flight spectroscopic unit that measures the time until the ion pulse emitted from the ion source reaches the final ion detector, and a time-of-flight spectrometer A plurality of electrostatic sector electric fields having triple convergence installed inside the unit, an ion intensity of an ion pulse which is provided at a plurality of locations inside the time-of-flight spectral unit, and is incident on the time-of-flight spectral unit, Measuring means for measuring the elapsed time after the emission of the ion pulse from the source section, and detecting the final ion pulse based on the time difference between the elapsed times measured at the plurality of locations. And a control means for controlling the gain of the final ion detector before the arrival of the ion pulse based on the measurement result of the ion intensity and the prediction result of the arrival time. And

The measuring means, which is provided at the plurality of locations and measures the ion intensity of the ion pulse incident on the time-of-flight spectroscopy unit and the elapsed time after the ion pulse is emitted from the ion source unit, comprises: This is characterized in that it is an intermediate ion detector provided on the triple focusing surface of the ion pulse in the type spectroscopic section.

Further, the triple converging surface of the ion pulse in the time-of-flight spectroscopic section is formed by the plurality of electrostatic sector electric fields having triple converging properties.

Further, in the time-of-flight spectroscopy unit, one of the triple converging surfaces of the ion pulses generated by the plurality of electrostatic sector electric fields coincides with the spatial converging surface of the ion accelerating unit. Features.

The mass number of ions forming an ion pulse based on ion pulse incident signals from a plurality of intermediate ion detectors provided on a plurality of triple converging surfaces of the time-of-flight spectroscopy unit. Is calculated, and the time at which the ion pulse reaches the final ion detector is predicted without calculating.

Further, the ion source section includes an external ion source that continuously emits ions, an ion reservoir into which an ion beam emitted from the ion source is introduced, and a pulse voltage applied from the ion reservoir. The ion source is a vertical acceleration type ion source comprising an ion acceleration section for pulsatingly accelerating the ion beam in a direction intersecting with the ion beam introduction direction.

[0025]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a schematic view of one embodiment of the electrostatic sector electric field type OA-TOFMS according to the present invention when viewed in the Y-axis direction. In the drawing, an ion beam emitted from an external ion source (not shown) is converged in the Z-axis direction by a converging lens (not shown) and introduced into the ion reservoir 3 along the central axis of the ion reservoir 3. You.

In the vicinity of the ion reservoir 3, a push-out plate 4
And a distance 2S from the push-out plate 4 in the Z-axis direction.
₀ Only ion extraction grid 5 at ground potential away, outlet grid 6 to which a negative potential V ₂ is applied at a distance D further from the ion extraction grid 5 is provided. The push-out plate 4, the ion extraction plate 5, and the exit plate 6 cooperate with each other to form a two-stage acceleration unit 8.

A positive voltage (amplitude 2 V) is applied to the push-out plate 4.
_When a push-out pulse voltage 7 of _P ) is applied, an electric field gradient is instantaneously formed in the so-called two-stage acceleration section 8 between the push-out plate 4, the ion extraction grid 5, and the exit grid 6, and The ions are simultaneously accelerated in the Z-axis direction and ejected from the ion reservoir 3, pass through a first electrostatic sector electric field 9 and a second electrostatic sector electric field 10 provided at opposing positions, and then pass through the micro channel. -It reaches the final ion detector 11 composed of a plate (MCP) or the like.

Strictly speaking, since the ions have a velocity in the Y-axis direction when introduced into the ion reservoir 3, Pu
sh-out plate 4, ion extraction grid 5, and
Even if a force in the Z-axis direction is received between the exit grids 6, that is, by the electric field generated in the two-stage acceleration unit 8, the flight direction slightly shifts from the Z-axis direction in the Y-axis direction.

When receiving the above acceleration, the ions are pushed out.
Since constant energy corresponding to the potential difference between the plate 4 and the outlet grid 6 is given equally, at the end of the acceleration, ions with smaller mass have a higher velocity, and ions with larger mass have a lower velocity. As a result of such a velocity difference, while the ions fly through the TOF spectroscopic unit 12 composed of the two electrostatic sector electric fields, the mass dispersion of the ions is performed, and the final ion detector 1 is sequentially arranged from the lighter ions.
1 and the mass spectrum is measured.

In such a configuration, the feature of the present invention is that the space convergence surface of the ions existing in the flight space of the ion pulse accelerated by the two-stage acceleration unit 8 is changed to the first static electricity constituting the TOF spectroscopic unit 12. The triple convergence surface F ₁ of the electric sector electric field 9 is made to coincide with the triple convergence surface on the opposite side of the first electrostatic sector electric field, and the triple convergence surface of the second electrostatic sector electric field 10 constituting the TOF spectroscopic unit 12 is formed. F ₂ to be matched, F ₁ and F
To each of _two, it is to put the intermediate ion detector 13 and 14 for monitoring the intensity and arrival time of the pulsed ions. Then, the final ion detector 11 is placed on the triple convergence surface F ₃ on the opposite side of the second electrostatic sector electric field 10.

The intermediate ion detectors 13 and 14
Has a rectangular hole at the center, and most of the incident ions can pass through the holes of the ion detectors 13 and 14 and reach the final ion detector 11 at the subsequent stage. It has become.

The three surfaces F ₁ , F ₂ and F ₃ are all triple convergence surfaces of the TOF spectroscopy unit 12, and among them,
F ₁ and F ₃ are planes perpendicular to the ion flight center orbit, F ₂
Is a plane having an inclination angle θ with respect to the ion flight center orbit. F ₁ corresponds to the object plane of the TOF spectroscopic section 12 and coincides with the spatial convergence plane of the two-stage acceleration section 8 as described above.

In this case, the ion accelerating unit does not necessarily have to be a two-stage accelerating unit, but may be a multi-stage accelerating unit. However, here, the two-stage accelerating unit is simple for explanation. Hereinafter, it is described as a two-stage acceleration unit.

Now, the two electrostatic sector electric fields 9 and 10 constituting the TOF spectroscopy section 12 have the same optical system dimension. That is each sector field central rotational radius r _e, the inner electrode radius of rotation r _a, the outer electrode rotation radius r _b, the electric field rotational angle phi _e, equal relationship free space L ₁ and L _2. Each of these two electrostatic sector electric fields has triple convergence (time, space and energy convergence) independently.

Here, the distance between the first acceleration section (between the push-out plate 4 and the ion extraction grid 5) in the two-stage acceleration section 8 and the distance between the second acceleration section (between the ion extraction grid 5 and the exit grid 6). distance, and the distance from the exit grid 6 is the end of the two-stage accelerating portion to the final ion detector 11, respectively 2S _0, D, L, and the center point S ₀ of 2S ₀
Monovalent ions (m ⁺ ) from to the final ion detector 11
Calculate the flight time t _TOF of.

[0036] The Z-axis direction of the kinetic energy of ions from an external ion source 1 are flying through the ion reservoir 3 in the Y-axis direction and initial energy ^{_{U i = mv i 2/2}} , repeller pulse voltage 2V in this ion The electric potential of the first acceleration unit and the electric potential of the second acceleration unit of the two-stage acceleration unit when _P is given are E _s and E _d , respectively.

Then, the kinetic energy U in the Z-axis direction of the ions after passing through the two-stage accelerating portion is given by equation (3).

U = U _i + e · S ₀ · E _s + e · D · E _d (3) The second step in the two-stage acceleration unit electric field E _s of first acceleration section, and the second acceleration portion of the electric field E and velocity of the ions at the time of exiting the _d each v _s, and v when _d, v _s and v _d is (4), and (5 )

V _s = √ (2 / m) √ (U _i + e ・ S ₀・ E _s ) (4) v _d = √ (2 / m)・ √U ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (5) In addition, we fly the acceleration fields E _s and E _d Ion flight time t
_s and t _d are given by equations (6) and (7).

[0040] _{t s = m (v s ±} v i) / e · E s = √ (2m) · {√ (U i + e · S 0 · E s) ± √U i} / e · E s ·· ..................... (6) t _{d = m (v d -v s} ) / e · E d = √ (2m) · {√U-√ (U i + e · S 0 · E s)} / e · E d ······· (7) Exit grid 6 and spatial convergence The free space distance to the plane F ₁ is L ₀ , the flight distance between two triple convergence planes associated with one electrostatic sector electric field is L _E , and the TOF from the exit grid 6 after being accelerated by the two-stage accelerator. Assuming that the flight time until reaching the final ion detector 11 via the spectroscopy unit is t _L , the flight time t _L is expressed by the equation (8). Given.

[0041] _{t L = √ (2m) ·} L / 2√U = √ (m / 2) · (L 0 + 2L E) / √ (U i + e · S 0 · E s + e · D · E d) · (8) Therefore, the total flight time t _TOF of the ions is the expression (9) obtained by adding the expression (8) to the expressions (6) and (7) during the two-stage acceleration section flight.

T _TOF = t _s + t _d + t _L (9) Equations (6), (7), and (8) are all proportional to the square root of the mass m of the flying ion, √m. Therefore, equation (9) is summarized by the apparatus constant K, and is expressed by equation (10). Given.

T _TOF = K√ (m / 2U) (10) where: If the focal plane F ₁ placed an ion detector 13, the flight time t _F1 of ions from the intermediate point S ₀ to the space focal plane F ₁ of the Push-out plate 4 and the ion extraction grid 5, (8) Is replaced by the distance L ₀ from the exit grid 6 to the space convergence plane F ₁ to arrange the free space distance (L ₀ + 2L _E ).

[0044] _{t F1 = t s + t d} + t L0 = K 0 · √ (m / 2U) · · · ························· (11) Here, t _L0 is the flight time of ions from the exit grid 6 to the space convergence plane F ₁ , and K ₀ is a device constant.

At present, a patent is pending (Japanese Patent Application No. 11-1210).
In the case of an ion detection system using a reflector-type TOF spectroscopy unit (see No. 16), when an ion pulse of each mass is detected by an intermediate ion detector, the ion intensity saturates the MCP of the final ion detector. First, for an ion pulse having an ion intensity that can saturate the MCP, a flight time t _F1 from the ion reservoir to the intermediate ion detector is obtained, and the value of this t _F1 is used as (11) ), The mass m of the ion is calculated, and using this m,
A process for determining the total flight time t _TOF of the ions by the equation (10) is required.

On the other hand, in this embodiment, the ion intensity for saturating the MCP of the final ion detector is provided by providing ion detectors at three places of the triple convergence surfaces F ₁ , F ₂ , and F ₃ of the TOF spectroscopic section. There is a merit that the process of calculating the mass m of the ions possessed each time can be omitted. Hereinafter, the operation of the present embodiment will be described.

The position indicated by S ₀ in FIG. 2 is the middle point of the first-stage acceleration unit 2S ₀ in the two-stage acceleration unit. The push-out plate 4 and the ion extraction grid 5 are kept at the ground potential (or the same potential) when ions are introduced into the ion reservoir 3 from an external ion source (not shown). All the ions introduced between the push-out plate 4 and the ion extraction grid 5 are actually dispersed in a space of ± S ₀ with the center point S ₀ as the center position.

In such a state, first, in order to accelerate and eject all the ions existing in the space of the first stage acceleration section 2S ₀ along the optical axis of the TOF spectroscopic section 12, the potential difference with respect to the ion extraction grid 5 Then, a push-out pulse voltage 7 (amplitude: 2 V _P ) for ion pushing is applied to the push-out plate 4. This application cycle follows the cycle of measuring the mass spectrum. Upon application of Push-out pulse voltage 7, in the first stage of the accelerating portion 2S _0, the electric field E _s (= V _P /
S ₀ ) occurs. The kinetic energy width of all ions existing in the space of 2S ₀ is 2V _P (= 2S ₀ / E _s ) electron volts (eV) at the maximum. Assuming that the kinetic energy width occupies about ± 10% of the kinetic energy U of the ions when the ions fly through the TOF spectroscopic unit 12, if U = 3 keV, a wide energy width of ± several hundred eV Will have.

Therefore, T other than the spatial convergence surface and the triple convergence surface
In the OF spectroscopic unit, the spread of the flight time is ± 10% of the flight time even for the ion group having the same mass.

On the other hand, in a portion of the distance D between the ion extraction grid 5 and the outlet grid 6 as a second-stage acceleration field of the two-stage accelerating portion 8 generally constant electric field E _d is given at all times . In the case where ions are unevenly distributed on the center point S ₀ and have no kinetic energy in the Z-axis direction (the optical axis direction of the TOF spectroscopy unit 12), this is an ideal case for TOFMS. When the push-out pulse voltage 7 is applied to the push-out plate 4, kinetic energy is obtained in the Z-axis direction (the optical axis direction of the TOF spectroscopy unit 12) by the two-stage acceleration unit and passes through the exit grid 6. After this, the kinetic energy U becomes U = e · S ₀ · E _s + e · D · E _d .

Therefore, when the ions are scattered in the space of 2S ₀ , which is the first stage of the two-stage accelerator, and do not have kinetic energy in the Z-axis direction (the optical axis direction of the TOF spectroscopy unit 12), the width of the kinetic energy U of all ions ± e · _{S 0} · E _s
Despite this, the ions nevertheless remain in the free space L
When ₀ approaches the space focal plane F ₁ to fly, ions convergence time, the same mass of the ion groups as ions pulses very narrow time width, to reach approximately the same time the space focal plane F ₁ .

[0052] Mass different group of ions are mass distribution is based on the flight time difference according to equation (10), sequentially, flying to the space focal plane F _1. Since the ion group is an ion pulse having a narrow time width for each mass, the arrival time of the ions m ⁺ of each mass to the space convergence plane F ₁ is accurately captured and monitored.

Before the push-out pulse voltage 7 for pushing the ions is applied to the push-out plate 7, the ions drifting in the space 2S ₀ of the first stage of the second stage acceleration unit 8 are: In the case of having U _i as the Z-axis direction component of the initial kinetic energy, despite the ion group having the same mass,
Not eliminated the influence of U _i even spatial focusing plane F _1, the ion pulse yields a temporal spread, the time width of about several ns. This phenomenon is called Turn Round Effect.

FIG. 3 shows an ion detector provided on _three triple converging surfaces F ₁ , F ₂ , and F ₃ of the TOF spectroscopy unit 12 and M
3 is a block diagram of a CP gain control circuit. In FIG. 3, reference numeral 8 denotes a two-stage acceleration unit, and reference numeral 12 denotes a TOF spectroscopy unit. Also, 13, 14, and 11 are TOF spectroscopy units 12
Are three ion detectors provided on the triple convergence surfaces F ₁ , F ₂ , and F ₃ . The last triple provided focal plane F ₃ ion detector 11, the final ion detector for measuring the mass spectrum, a MCP.

In such a configuration, the electrostatic sector power
Not only the objective surface of the field-type TOF spectroscopy unit, but also
Certain first triple convergent surface F₁When a rectangular slit is placed in
The image of the on-pulse can be made rectangular. for that reason,
A rectangular slit is provided here to fly the TOF spectroscopy unit 12.
The shape of the ion pulse is restricted to a rectangle. Also the same
For the same purpose, the second triple convergence surface F_TwoAlso a rectangular slit
Provide. These F₁And F _TwoThe two restriction slits
A part of the ion pulse to be cut by the ion detector 13
Incident ion pulse by receiving light at
Intensity and elapsed time t after ion pulse emission_F1as well as
t_F2Is measured.

It is important that the ion detectors 13 and 14 provided on the triple convergence planes F ₁ and F ₂ have the same response, so that the ion pulses between F _{1 and} F ₂ can be changed. Although the measurement error of the time-of-flight difference can be minimized, even if the responses are different, it is possible to correct the difference between the responses of the two detectors by performing calibration using known ions in advance. .

The circuit 15 and 16 generate a pulse train indicating that an ion pulse group having an ion intensity capable of saturating the final ion detector 11 has reached the triple convergence planes F ₁ and F _2. It is. In this circuit, the input of ion pulses to the ion detectors 13 and 14 placed on the triple convergence planes F ₁ and F ₂ is amplified by amplifiers 17 and 18, and this ion pulse is applied to the final ion placed at F _3. Whether or not the detector 11 has an ion intensity that saturates the MCP is determined based on the set voltage levels of the discriminators 19 and 20.

Incidentally, the ion intensity that saturates the MCP of the final ion detector 11 placed on the triple convergence plane F ₃ is as follows:
The back calculation can be easily performed from a preset MCP gain. Therefore, the relative ratio of the amount of ions received by the three ion detectors 13, 14, and 11 placed at F ₁ , F ₂ , and F ₃ is determined based on the result of the back calculation, and the appropriate ratio (for example, F ₁ : F ₂ : F ₃ = 5: 5: 100 or F ₁ :
F ₂ : F ₃ = 1: 1: 10, etc.).

This ratio is determined by the ion detectors 13 and 1
By the choice of the set voltage level of the gain and discriminators 19 and 20 of the amplifier 17 and 18 placed downstream of the 4, is monitored by cutting regulations slit provided the required minimum amount ions F ₁ and F ₂ It can be set as follows.

Reference numerals 21 and 22 incorporated in the pulse train generation circuits 15 and 16 are inversion circuits. Triple convergence surface F
_{On the} basis of the ion pulse incident signal from the ion detector 13 placed at ₁ , if the ion pulse has an ion intensity sufficient to generate a pulse signal from the inversion circuit 21 at time t _F1 , the triple convergence surface F Ion detector 1 placed in ₂
4, the sensitivity difference between the ion detectors 13 and 14, the gain difference between the amplifiers 17 and 18, and the setting between the discriminators 19 and 20 so that the pulse signal is always output from the inversion circuit 22 at the time t _F2. Eliminate voltage level differences.

In the figure, reference numeral 23 denotes a pulse train generation circuit 15 and 1
6, the M of the final ion detector 11
An MCP power control circuit for generating a timing pulse train 24 for controlling the CP gain. In synchronization with the timing pulse train 24, the output voltage V _MCP 2 of the MCP power supply 25
6, the detection sensitivity of the final ion detector 11 is lowered to control the final ion detector 11 not to be saturated even if an ion pulse is incident on the final ion detector 11.

FIG. 4 shows that the push-out pulse voltage 7 is
6 is a time chart illustrating operation timings of respective units after application to the plate 4. In the figure, the time of flight of the pulsed ions, the application timing of Push-out pulse voltage for ion extrusion, the timing of the appearance of intense peaks of ion pulse, is output by pulsed ions incident on the ion detector of F ₁ side The signal timing is the timing of the output signal due to the ion pulse incident on the ion detector on the F ₂ plane, the switching pulse train of the MCP gain,
Is the change in the MCP gain.

Now, time differences until the ion pulse emitted from the emission point S ₀ reaches the three triple converging surfaces F ₁ , F ₂ and F ₃ are represented by t _F1 , t ₁₂ and t ₂₃ , respectively. ,
t _TOF is as shown in equation (11) as shown by.

T _TOF = t _F3 = t _F1 + t ₁₂ + t ₂₃ (11) where: t _F1 is the flight time of the ion pulse from the emission point S ₀ to the first triple convergence surface F ₁ , and t ₁₂ is the first triple convergence surface F
Ion pulse time of flight from ₁ to triple focal plane F ₂ of the second, t ₂₃ is the time of flight of the pulsed ions from triplicate focal plane F ₂ of the second to the third triple focal plane F _3.

In the TOFMS of this embodiment, the flight time t ₁₂ between F _{1 and} F ₂ and the flight time t between F _{2 and} F ₃ are used because the TOF spectroscopic section uses two electrostatic sector electric fields of the same dimension. Since t ₂₃ matches (ie, t ₁₂ = t ₂₃ ), t
_If the value of ₁₂ is actually measured as the time difference between t _F1 and t _F2 , not only the value of t ₁₂ but also the value of t ₂₃ can be immediately known, and the value of t _TOF can be easily predicted.

T _TOF = t _F1 + t ₁₂ + t ₂₃ = t _F1 + 2t ₁₂ = t _F1 +2 (t _F2 −t _F1 ) = 2t _F2 −t _F1 .................. (12) after all, (12) as shown in equation subtracting the measured value of t _F1 actual values of t _F2 from twice the value Is used, the time t _{TOF at} which the ion pulse reaches the final ion detector can be obtained without calculating the mass m of the ions.

The switching of the MCP gain is controlled by the output timing of the MCP gain switching pulse and the time width of the output pulse. That is, MC in
The Dwell time (Δt _D ) of the P gain switching pulse is
The ions are sorted Δt _D / 2 back and forth around the ion flight time t _TOF . The standard of the time width of Δt _D is 50 ns or less even if the time width of Δt in Expression (1) is about five times,
Considering the response time of the high-speed switching switch of the MCP voltage power supply 21, even if 10 ns of Δt is taken and 100 ns, if the mass m of this ion pulse is several hundred daltons or less, the mass spectrum in the range of several daltons To the extent that is lost, or to the extent that mass spectra in the range of a few daltons are measured at low gain. In short, Δt _D
May be set to an optimum value according to the TOFMS apparatus.

Also, in the above, M which is initially set in advance
The CP gain is G _A , and the MCP gain when the MCP voltage is lowered by predicting the incidence of a strong ion pulse is G _B (= k
If G _A , k≪1), then M t before the time when the ion pulse is expected to be incident on the MCP by Δt _D / 2
CP gain is lowered from G _A to G _B, after pulsed ions incident, after only Delta] t _D / 2, MCP gain is returned from the G _B to G _A. Drop rate during this period of the MCP gain k (= G _B /
G _A ) may be set to an appropriate value in the range of 1/10 to 1/1000.

If the initial gain G _A of the MCP and the gain G _B reduced to avoid saturation of the MCP are stored as data, a spectrum portion measured at a low gain G _B can be stored. By multiplying the reciprocal 1 / k of the gain reduction rate k after the measurement is completed, a spectrum close to a mass spectrum obtained with the same gain can be restored.

[0070] Figure 5 shows, as an example, is a timing chart pulsed ions showed the time of flight of each ion species after exiting from the exit point S ₀ containing ionic species of different three types of mass. In the figure, three lines P ₁ , P ₂ , and P ₃ indicate ion pulses derived from three types of ion species having different masses. Is a push-out pulse voltage for ion extrusion, is an ion pulse immediately after being generated at the emission point S ₀ ,
The first triple focal plane ion pulse group flying to the F ₁ are mass dispersion, the pulse train generation circuit based on the incident ion pulse group to the ion detector placed in triplicate focal plane F ₁ of the first pulse train output, more mass dispersed by the second triple focal plane F ₂ ion pulse group flying in, the incidence of ions pulse group to the ion detector placed in triplicate focal plane F ₂ of the second pulse train output from the pulse train generation circuit based, more mass dispersed in the third triple focal plane ion pulse group flying to the F _3, the third triple focal plane F ₃ to put the final ion detector MCP
4 is a pulse train output from the MCP power supply control circuit to the MCP power supply in order to change the MCP gain in preparation for the incidence of an ion pulse group to the MCP power supply.

Now, the ion pulse group is represented by P ₁ , P ₂ , P ₃ ,.
, P _n , _{assuming that there} are n mass peaks of high ion intensity, the times t _Pi1 , t _Pi2 , and t _{Pi3 at} which the respective mass peaks reach the _three triple convergence surfaces F ₁ , F ₂ , and F ₃ Is (1
Based on the algorithm of the expression 2), the result is as shown in FIG. 6 (where i = 1, 2, 3,..., N).

[0072] Thus, from FIG. 6, a timing pulse train 24 shown in FIG. 3, the the pulse train .SIGMA.t _Pi1 5, and a pulse train .SIGMA.t _Pi2 of the FIG. 5, sigma (2t _Pi2 -
t _Pi1 ) (where i =
1, 2, 3,..., N).

A high-speed and high-pressure electric circuit is required to reduce and restore the MCP gain at high speed before and after the time when the ion pulse is incident on the MCP as the final ion detector. The MCP voltage is changed to 500 to 1 ns by using a semiconductor element such as a high-voltage MOSFET switch for high withstand voltage at a speed of 5 ns to 10 ns without using a relay or a Q switch.
It is also possible to change at a high speed with a width of kV.

In this embodiment, the OA-TOF using two electrostatic sector electric fields having the same dimension is used.
Although the description has been given by taking the MS as an example, if two intermediate ion detectors are to be provided, one at each of the two triple focusing surfaces before the final ion detector, the number of electrostatic sector electric fields is three or more. May be a combination of electrostatic sector electric fields having the same triple convergence. Further, these electrostatic sector electric fields may be cylindrical, spherical, or toroidal. In the manufacture of TOFMS, it is advantageous to use an electrostatic sector electric field of the same shape, but if the shape of the electrostatic sector electric field follows a similarity rule, the similarity ratio κ (κ If it is used by multiplying by (≠ 0), that is fine. That is, the pulse train for controlling the MCP is represented by Σ
{(1 + κ) t _Pi2 −t _Pi1 } (where i = 1, 2, 3,..., N).

The present invention is not limited to OA-TOFMS. The TOF for the space convergence surface of the ion accelerator
The ion accelerating unit is not limited to the two-stage accelerating type but may be a multi-stage accelerating type. The TOF using the pulse ionization method for the ion source is limited to the one that matches the object plane of the spectroscopic unit.
It is also applicable to MS.

As the final ion detector, MCP
In addition, microspheres that are more inexpensive than MCP and can amplify secondary electrons by diffusely reflecting electrons in the gaps between a large number of spherical particles without using a bundle of thin glass capillaries like MCP -A plate (MSP) or the like can also be used.

[0077]

As described above, according to the electrostatic sector electric field type TOFMS of the present invention, the ions fly from the ion source using the two ion detectors provided on the two triple focusing surfaces of the TOF spectroscopy unit. By detecting the current value of the ion pulse in advance and feeding the detected amount to the final ion detector, the gain of the final ion detector is controlled before the ion pulse reaches the final ion detector, and the final ion detection is performed. Since the detector is configured to avoid the saturation of the detector, it is possible to prevent partial loss of the mass spectrum due to the dead time due to the saturation of the final ion detector, and to shorten the life of the final ion detector itself. be able to.

[Brief description of the drawings]

FIG. 1 is a diagram showing a conventional electrostatic sector electric field type vertical acceleration type time-of-flight mass spectrometer.

FIG. 2 is a diagram showing one embodiment of an electrostatic sector electric field type vertical acceleration type time-of-flight mass spectrometer according to the present invention.

FIG. 3 is a diagram showing an example of a block diagram of an MCP gain control circuit of the electrostatic sector electric field type vertical acceleration time-of-flight mass spectrometer according to the present invention.

FIG. 4 is a diagram showing an example of a timing chart of each section of an MCP gain control circuit of the electrostatic sector electric field type vertical acceleration time-of-flight mass spectrometer according to the present invention.

FIG. 5 is a diagram showing an example of a timing chart of each part of an MCP gain control circuit of the electrostatic sector electric field type vertical acceleration type time-of-flight mass spectrometer according to the present invention.

FIG. 6 is a table summarizing an example of a flight time of an ion pulse group observed by the electrostatic sector electric field type vertical acceleration time-of-flight mass spectrometer according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... External ion source, 2 ... Converging lens, 3 ... Ion pool, 4 ... Push-out plate, 5 ... Ion extraction grid, 6 ... Exit grid, 7 ... Push-out pulse, 8 ...
A two-stage accelerating unit, 9 ... the first electrostatic sector electric field, 10 ...
Second electrostatic sector electric field, 11 ... final ion detector,
12: TOF spectroscopy unit, 13: ion detector, 14 ion detector, 15: pulse train generation circuit, 16: pulse train generation circuit, 17: amplifier, 18: amplifier, 19 ... Discriminator, 20 ... Discriminator,
21: inverting circuit, 22: inverting circuit, 23: MCP power control circuit, 24: timing pulse train, 25: MC
P power supply, 26... Output voltage V _MCP .

Claims (6)

[Claims] 1. An ion source for emitting an ion pulse, an ion accelerator for accelerating the emitted ion pulse, a final ion detector on which the accelerated ion pulse enters after flying a predetermined distance, and an ion source Time-of-flight spectroscopy unit that measures the time required for the ion pulse emitted from the unit to reach the final ion detector, and a plurality of triple-convergent electrostatic sector electric fields installed inside the time-of-flight spectroscopy unit When,
Measuring means provided at a plurality of locations inside the time-of-flight spectroscopic unit, for measuring the ion intensity of the ion pulse incident on the time-of-flight spectroscopic unit, and the elapsed time after the ion pulse is emitted from the ion source unit, Based on the time difference between the elapsed times measured at the plurality of locations, a prediction unit that predicts the time at which the ion pulse reaches the final ion detector, based on the measurement result of the ion intensity and the prediction result of the arrival time, Control means for controlling the gain of the final ion detector before the arrival of the ion pulse. And measuring means for measuring the ion intensity of the ion pulse incident on the time-of-flight spectroscopy unit and the elapsed time after the ion pulse is emitted from the ion source unit, provided at the plurality of locations. The time-of-flight mass spectrometer according to claim 1, wherein the mass spectrometer is an intermediate ion detector provided on a triple focusing surface of the ion pulse in the spectroscopic section. 3. The time of flight according to claim 2, wherein the triple convergence surface of the ion pulse in the time-of-flight spectroscopy unit is formed by the plurality of electrostatic sector electric fields having triple convergence. Type mass spectrometer. 4. In the time-of-flight spectroscopic unit, one of the triple converging surfaces of ion pulses generated by a plurality of electrostatic sector electric fields coincides with the spatial converging surface of the ion accelerating unit. The time-of-flight mass spectrometer according to claim 2 or 3, wherein: 5. A mass number of ions forming an ion pulse based on ion pulse incident signals from a plurality of intermediate ion detectors provided on a plurality of triple converging surfaces of the time-of-flight spectroscopy unit. 5. The time-of-flight mass spectrometer according to claim 2, wherein the time at which the ion pulse reaches the final ion detector is calculated without calculating the time. 6. An ion source for continuously outputting ions, an ion reservoir into which an ion beam emitted from the ion source is introduced, and a pulse voltage applied from the ion reservoir. 6. A vertical acceleration type ion source unit comprising: an ion acceleration unit for pulsatingly accelerating the ion beam in a direction intersecting with the ion beam introduction direction. The time-of-flight mass spectrometer described in the above.