U.S. patent number RE35,681 [Application Number 08/600,715] was granted by the patent office on 1997-12-02 for atmospheric pressure ionization mass spectrometer.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Yasuhiro Mitsui, Osami Okada.
United States Patent |
RE35,681 |
Mitsui , et al. |
December 2, 1997 |
Atmospheric pressure ionization mass spectrometer
Abstract
An atmospheric pressure ionization mass spectrometer which
comprises an ion source for ionizing a sample gas, a low pressure
region provided with a mass filter and a collector therein, a
differential pumping region provided between the ion source and the
low pressure region and with electrodes provided on the side of the
ion source and on the side of the low pressure region,
respectively, and a pressure-gradient electrode means for
dissociation and removal of cluster ions, as connected to the
electrode on the side of the ion source among the electrodes
provided in the differential pumping region is disclosed.
Inventors: |
Mitsui; Yasuhiro (Fuchu,
JP), Okada; Osami (Chofu, JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
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Family
ID: |
17073997 |
Appl.
No.: |
08/600,715 |
Filed: |
February 13, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
924640 |
Oct 30, 1986 |
04769540 |
Sep 6, 1988 |
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Foreign Application Priority Data
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Oct 30, 1985 [JP] |
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60-241418 |
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Current U.S.
Class: |
250/282;
250/281 |
Current CPC
Class: |
H01J
49/145 (20130101); H01J 49/24 (20130101) |
Current International
Class: |
H01J
49/02 (20060101); H01J 49/10 (20060101); H01J
49/14 (20060101); H01J 49/04 (20060101); H01J
049/04 () |
Field of
Search: |
;250/288,423,281 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1398167 |
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Jun 1975 |
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GB |
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2127212 |
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Apr 1984 |
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GB |
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Other References
Gray and Date, "Inductively Coupled Plasma Source Mass Spectrometry
Using Continuum Flow Ion Extraction", The Analyst, vol. 108, No.
1290, pp. 1033-1050, Sep. 1983..
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Primary Examiner: Berman; Jack I.
Attorney, Agent or Firm: Jenner & Block
Claims
What is claimed is:
1. An atmospheric pressure ionization mass spectrometer
comprising:
an ion source for ionizing a sample gas;
a low pressure region provided with a mass filter and a collector
therein;
a differential pumping region provided between said ion source and
said low pressure region;
first and second electrodes provided on one side of said ion source
and on one side of said low pressure region, respectively; and
a pressure-gradient electrode means for providing an
.[.increasing.]. .Iadd.increased .Iaddend.pressure .Iadd.having a
.Iaddend.gradient from said first electrode toward said second
electrode for dissociation and removal of cluster ions, said
pressure-gradient electrode means being connected to said first
electrode and being provided in said differential pumping
region.
2. An atmospheric pressure ionization mass spectrometer according
to claim 1, wherein said pressure-gradient electrode means
comprises a cylindrically-shaped electrode including a base end
connected to said first electrode and a tip end having a smaller
size than that of said base end.
3. An atmospheric pressure ionization mass spectrometer according
to claim 2, wherein said pressure-gradient electrode means is
provided with an operating means for adjusting the length of said
pressure-gradient electrode means by extending said
pressure-gradient electrode means substantially perpendicular to
said first and second electrodes.
4. An atmospheric pressure ionization meass spectrometer according
to claim 3, wherein said pressure-gradient electrode means
comprises an electrode having a bellows for adjusting the length of
said pressure-gradient electrode means.
Description
BACKGROUND OF THE INVENTION
This invention relates to an improvement in an atmospheric pressure
ionization mass spectrometer, and particularly to an apparatus for
removing cluster ions, suitable for efficient removal of clustor
ions giving rise to sensitivity lowering and spectrum
complication.
The atmospheric pressure ionization mass spectrometer is an
apparatus very sensitive to gaseous substances and has now been
practically utilized in the fields of pollution measurement,
semiconductor production process and metabolite analysis. The
atmospheric pressure ionization mass spectrometer is characterized
by its high sensitivity, and thus it is important to eliminate
factors inhibiting this high sensitivity.
A conventional atmospheric pressure ionization mass spectrometer is
shown in FIG. 1, where a sample gas 15 is introduced into an ion
source 3 through a sample inlet pipe 1, and a portion of the sample
gas 15 is ionized under an ion source pressure of 1 atm. The thus
formed ions are led to a low pressure region 9 through a
differential pumping region 6. There is a quadrupole mass analyzer
7 in the low pressure region 9, and the ions are separated
according to the masses, and reach an ion collector 8. The ion
current obtained at the collector 8 is output to a recorder 12 and
a computer 14 through an amplifier 13. The pressure in the low
pressure region 9 is kept at about 10.sup.-4 Pa by the working
pressure of the quadrupole mass analyzer 7. The differential
pumping region 6 is provided to connect the low pressure region 9
to the ion source 3 under 1 atm, and is partitioned from the ion
source 3 under 1 atm by a first aperture electrode 4 having an
aperture through which the ions can pass and by a second aperture
electrode 5 having an aperture through which the ions can pass.
Ionization of the atmospheric pressure ionization mass spectrometer
is initiated by corona discharge at the tip end of a needle
electrode 2 to which a high voltage is applied. Trace amounts of
oxygen, carbon dioxide, and organic compounds (M) are contained in
a nitrogen gas, through ionization as follows: ##STR1##
N.sub.2, which is a main component in the sample gas 15, is ionized
according to the reaction (1), but the ions formed according to the
equation (1) undergo the following reactions owing to the very
short mean free path because the ionization is carried out under 1
atm.
Since the ionization potential of N.sub.4 is higher than those of
O.sub.2, CO.sub.2, H.sub.2 O, etc., ions of trace components are
formed in the nitrogen gas according to the reactions (4), (5) and
(6). The main component ions which are not analytical objects are
converted to trace component ions, which are analytical objects, as
given by the reactions (4), (5) and (6), which also occur under 1
atm. Thus, there are many chances for the reactions, and a highly
efficient ionization of the analytical trace components as the
objects can be attained. Since these ions are detected in the
analytical region 9 through the differential pumping region 6, the
atmospheric pressure mass spectrometer can have a higher
sensitivity. However, the following reactions occur to inhibit the
higher sensitivity.
The ions formed according to the reactions (8) to (12) are called
cluster ions, which have the following disadvantages: the spectrum
will be complicated, because, for example, the proper peak of water
appears at m/z=18, whereas cluster ions develop peaks at other
values of m/z, for example, H.sub.2 O+.N.sub.2 (m/z=46), H.sub.2
O+,(N.sub.2).sub.2 (m/z=74), etc., and S/N ratio will be lowered,
because the properly single peak is divided into a plurality of
peaks. Particularly, the lowering of S/N ratio will reduce the
sensitivity, and thus the removal of the cluster ions is
indispensable for an atmospheric pressure ionization mass
spectrometer.
The prior art of removing the cluster ions, as disclosed in
Japanese Patent Application Kokai (Laid-open) No. 53-81289 proposes
to provide a drift electric field in the differential pumping
region 6 in FIG. 1 to make the clustor ions collide with neutral
molecules, thereby dissociating the cluster bonds. That is, a
voltage is applied between the electrode 4 and the electrode 5 to
accelerate the cluster ions and make them collide with the neutral
molecules. The kinetic energy of the cluster ion is converted to
the internal energy by the collision, and if the number of
collisions is enough, the cluster ions will be dissociated at the
weak bonds.
The cluster bond energy is generally smaller than the chemical bond
energy. Therefore the cluster bond is dissociated according to the
reactions (13)-(16) and molecular ions are produced. In the prior
art the pressure in the intermediate pumping region is constant
(the number of collisions is constant), and thus the control of
cluster bond dissociation has been so far carried out by
controlling the kinetic energy, that is, by controlling the voltage
applied to the electrodes 4 and 5 (drift voltage). However, when
the drift voltage is increased to dissociate M.H+.(H.sub.2 O).sub.n
clusters having a larger n in the prior art controlling method, the
optimum conditions for focusing the ion beams into the aperture of
the electrode 5 cannot be obtained, and thus the amount of ions to
be introduced into the analytical region 9 is reduced. When the
drift voltage is not to be increased, it is necessary to increase
the number of collisions. When the pressure of the differential
pumping region 6 is elevated to increase the number of collisions,
the pressure of the low pressure region 9 will be increased and the
aperture of the electrode 5 is liable to be fouled. This gives rise
to charge-up and a consequential reduction in the amount of ions to
be introduced into the low pressure region 9. These are serious
disadvantages of the prior art.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an atmospheric
pressure ionization mass spectrometer, where the pressure in the
ionization region is higher than that in the analytical region,
which can readily and efficiently dissociate cluster ions as a
cause for the lowering of sensitivity and the complication of
spectrum.
The object of the present invention can be attained by providing a
pressure-gradient electrode in a differential pumping region, the
pressure-gradient electrode being connected to an electrode
partitioning the differential pumping region from an atmospheric
pressure ion source, while providing a drift electric field in the
differential pumping region. The pressure in the differential
pumping region is higher toward the ion source and lower toward the
low pressure region.
With this characteristic structure of the present invention, the
number of collisions can be increased to obtain an energy high
enough to dissociate the cluster ions. This is because the pressure
is higher toward the ion source in the differential pumping region.
The drift voltage can be kept to optimum conditions for focusing
the beams. On the other hand, the pressure is lower toward the low
pressure region in the differential pumping region, and thus the
pressure in the low pressure region will not be increased.
Furthermore, the electrode partitioning the differential pumping
region from the low pressure region (second aperture electrode) is
less fouled. In the present invention, cluster ions inhibiting the
higher sensitivity can be efficiently removed in this manner.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic structural view of an atmospheric pressure
ionization mass spectrometer according to the prior art.
FIG. 2 is a schematic structural view of an atmospheric pressure
ionization mass spectrometer according to one embodiment of the
present invention.
FIG. 3 is a schematic structural view of an atmospheric pressure
ionization mass spectrometer according to another embodiment of the
present invention.
FIG. 4 is a spectral diagram of cluster ions in an undissociated
state.
FIG. 5 is a spectral diagram of cluster ions in a dissociated
state.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described in detail below, referring
to the accompanying drawings.
FIG. 2 shows a basic structure of an atmospheric pressure
ionization mass spectrometer, provided with a pressure-gradient
electrode in the differential pumping region.
In FIG. 2, a sample gas 15 containing trace components is
introduced into an ion source 3 through a sample inlet pipe 1. The
thus introduced sample gas is ionized (primary ionization) by
corona discharge at the tip end of a needle electrode 2 to which a
high voltage is applied. Then, charge transfer reaction from the
main component ions having a higher ionization potential to trace
component molecules having a lower ionization potential proceeds as
secondary ionization. The ion source under 1 atm. has a short mean
free path, so that one ion usually repeats 10.sup.5 to 16.sup.6
collisions within the ion source 3. Thus, even the trace components
take part in the collisions substantially 100%, and the ionization
can be carried out with a high efficiency. At that time cluster
ions, which inhibit the higher sensitivity of the atmospheric
pressure ionization mass spectrometer and complicate the spectrum,
thereby making the analysis disadvantageous, are also formed. The
ions produced in the ion source 3 are introduced into the
differential pumping region 6. Among the ions introduced into the
differential pumping region 6, the cluster ions are dissociated by
collisions with neutral molecules and the resulting excitation in
the differential pumping region 6, and turn into molecular ions or
quasi-molecular ions. That is, a drift electric field is formed in
the differential pumping region 6 by the voltage applied between
the electrodes 4 and 5. The ions travel through the drift electric
field from the electrode 4 toward the electrode 5, while converting
the kinetic energy to the internal energy through the collisions
with the neutral molecules. The internal energy is thoroughly
excited through numbers of the collisions, and the cluster bonds
are ultimately dissociated. A relatively large energy is required
for conversion of cluster ions such as M.H+.(H.sub.2 O).sub.n with
a higher n to M.H+. Such an energy can be given either by giving a
large kinetic energy, that is, applying a high drift voltage, or by
increasing the number of collisions, that is, increasing the
pressure of the differential pumping region. However, when too high
a drift voltage is applied to dissociate the cluster ions, the ion
beams will not be focused to one point, and thus will not
efficiently pass through the aperture of electrode 5. That is, an
ion loss occurs at the electrode 5, and the amount of ions to be
introduced into the low pressure region 9 is reduced, with a
failure to obtain a higher sensitivity. On the other hand, when the
pressure is much elevated in the differential pressure region 6,
the pressure in the low pressure region will be elevated at the
same time, and a vacuum pump 11 of higher evacuation capacity must
be used to adjust the elevated pressure in the low pressure region.
That is, there are problems in the cost and the portability of the
analyzer. Furthermore, the elevated pressure leads to fouling of
the aperture of electrode 5 and consequent charge-up, and thus the
amount of ions to be introduced into the low pressure region is
reduced, inhibiting the higher sensitivity.
According to one embodiment of the present invention, the
pressure-gradient electrode 16 is provided in the differential
pumping region 6, as connected to the electrode 4, and takes a
cylindrical or similar shape. The pressure-gradient electrode 16 is
connected to the electrode and is open at the end near the
electrode 5. That is, the evacuation resistance in the ion passage
area near the aperture of electrode 4 is large in the differential
pressure region 6, and the pressure is increased in that area. The
evacuation resistance in the area near the aperture of electrode 5
is not influenced thereby, and thus the pressure is not increased.
That is, a sharp pressure gradient is provided between the
electrode 4 and the electrode 5. Thus, the cluster ions introduced
from the ion source 3 are subjected to increased number of
collisions owing to the increased pressure in the area near the
electrode 4 in the differential pumping region 6, and can receive
enough energy to occasion the dissociation of clusters without
applying a higher drift voltage. Furthermore, since the pressure in
the area near the aperture of the electrode 5 in the differential
pumping region 6 is not increased, the pressure in the low pressure
region 9 is not influenced, either, and no fouling of the aperture
of the electrode 5 occurs due to the pressure increase.
The cluster ions dissociated through the foregoing cluster
dissociating mechanism turn into molecular ions or quasi-molecular
ions, which are introduced into the low pressure region 9 and
separated according to masses by a quadrupole mass analyzer 7 and
turn into ionic currents at a collector 8. The ionic currents are
output to a recorder 12 and a computer 14 through an amplifier 13.
In this embodiment a sensitivity about three times higher than that
of the prior art can be obtained.
FIG. 3 shows a moving mechanism provided in the pressure-gradient
electrode 16 of FIG. 2, though the cluster dissociation mechanism
is the same as shown in the embodiment of FIG. 2. In this
embodiment, the following effects can be obtained. That is, a
bellows 17 is provided on the pressure-gradient electrode 16, and
an expanding-contracting mechanism 18 of bellows 17 can be operated
from the outside of the vaccuum vessel, so that an optimum pressure
gradient can be set while actually measuring the ions.
FIG. 4 and FIG. 5 show spectra in the case that no cluster
dissociation is carried out when trace amounts of ammonium and
water are contained in a nitrogen gas and in the case that the
cluster dissociation is carried out, respectively. That is, FIG. 4
shows the case of no cluster dissociation and FIG. 5 the case of
dissociation according to the present invention.
As is obvious from FIG. 4, a peak of ammonirm (NH.sub.4 +) properly
as a single peak is distributed into a plurality of peaks owing to
the cluster formation, reducing the S/N ratio, whereas in the
present invention, as is obvious from FIG. 5, a substantially
single, proper ammonium peak can be obtained owing to the cluster
dissociation, improving the S/N ratio.
According to the present invention, the cluster ions inhibiting the
higher sensitivity of an atmospheric pressure ionization mass
spectrometer can be removed ty dissociation without loss in the
amount of ions, increase in the amount of a gas to be introduced
into the low pressure region or fouling of the aperture through
which the ions pass, as described above, and thus the higher
sensitivity, which is a most important object in the atmospheric
pressure ionization mass spectrometer, can be effectively
attained.
* * * * *