U.S. patent application number 10/234097 was filed with the patent office on 2003-03-20 for mass spectrometer.
This patent application is currently assigned to Shimadzu Corporation. Invention is credited to Itoi, Hiroto, Shimomura, Manabu.
Application Number | 20030052266 10/234097 |
Document ID | / |
Family ID | 19104783 |
Filed Date | 2003-03-20 |
United States Patent
Application |
20030052266 |
Kind Code |
A1 |
Shimomura, Manabu ; et
al. |
March 20, 2003 |
Mass spectrometer
Abstract
In a mass spectrometer, an ion of a sample gas is produced in an
ion source by colliding the gas with an electron beam, and is
introduced into a magnetic field or an electric field. The ion is
separated based on the mass number, and the sample is analyzed from
a mass spectrum. The ion source has an electrode made of stainless
steel, which is baked at a temperature in a range from 200.degree.
C. to 700.degree. C. in an air atmosphere.
Inventors: |
Shimomura, Manabu; (Kyoto,
JP) ; Itoi, Hiroto; (Kyoto, JP) |
Correspondence
Address: |
KANESAKA AND TAKEUCHI
1423 Powhatan Street
Alexandria
VA
22314
US
|
Assignee: |
Shimadzu Corporation
|
Family ID: |
19104783 |
Appl. No.: |
10/234097 |
Filed: |
September 5, 2002 |
Current U.S.
Class: |
250/281 |
Current CPC
Class: |
H01J 49/147
20130101 |
Class at
Publication: |
250/281 |
International
Class: |
H01J 049/26 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 17, 2001 |
JP |
2001-280862 |
Claims
What is claimed is:
1. A mass spectrometer for analyzing a sample gas, comprising:
means for producing an ion of the sample gas through collision of
the gas with an electron beam, said producing means having an
electrode made of stainless steel baked at a temperature in a range
of 200.degree. C. to 700.degree. C. in an air atmosphere, and means
for processing the sample ion, said processing means guiding the
sample ion into one of a magnetic field and an electric field for
separating the ion according to a mass number to obtain a mass
spectrum for analysis.
2. A mass spectrometer according to claim 1, wherein said producing
means is an ion source or an ion trap.
3. A mass spectrometer according to claim 1, wherein said electrode
made of stainless steel includes a chromium dioxide and chromium
oxide on an outer surface thereof.
4. A mass spectrometer according to claim 3, wherein said producing
means includes an ionization chamber having a sample inlet and an
ion exit, a filament formed at one side of the ionization chamber
for producing thermo electrons, and a trap electrode formed at a
side opposite to the filament for receiving the thermo electrons so
that the sample gas introduced into the ionization chamber through
the sample inlet is collided with the thermo electrons to produce
ions of the sample, said ions being ejected through the ion exit.
Description
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
[0001] This invention relates to a mass spectrometer for measuring
a quantity of ions produced by impacting a sample gas with an
electron beam to ionize, introducing the produced ions into a
magnetic field or electric field, and separating the ions according
to their mass numbers to determine their ion numbers. In
particular, this invention relates to a technique for preventing a
sample gas from being decomposed at an ion source or in a trap to
adhere and deposit on electrodes or the like.
[0002] A mass spectrometer is an analyzer in which a sample
molecule is collided with the electron beam with several tens of
electron volts (eV) to ionize, and the produced ions are introduced
into a magnetic field or an electric field to separate according to
the mass number. Then, a mass spectrum with the mass number in a
horizontal axis and the ion quantity in a vertical axis is created
to determine the sample molecules.
[0003] The mass spectrometer is classified into a magnetic field
type and an electric field type based on the mass separating
method. FIG. 3(a) shows a principle of the magnetic field type mass
spectrometer, and FIG. 3(b) shows a principle of the electric field
type mass spectrometer.
[0004] In the magnetic field type mass spectrometer, an instrument
is maintained under a high vacuum of 10.sup.-6 to 10.sup.-8 Torr. A
sample gas is introduced into an ion source 10 at a constant flow
rate, and the sample gas is subjected to impact of the electron
beam having energy of the order of 50 to 100 eV to ionize the
sample gas. Acceleration electrodes 2 accelerate the ions from the
ion source 10 to enter a magnetic field 3. A path of the ions
inside the magnetic field 3 is curved according to the Fleming's
left hand rule, and then a detector 5 detects the ions after
passing through a collector slit 4. Since the curve radius is
different depending on the mass number, a mass spectrum can be
obtained.
[0005] In the case of the electric field type mass spectrometer, a
sample gas is ionized at the ion source 10. Accelerating electrodes
2a accelerate the ions to introduce into an electric field created
by quadruple electrodes 3a. A direct current voltage and a high
frequency voltage, i.e. .+-.(U+V cos .omega.t), are applied to four
bar-shape electrodes disposed in parallel to each other. When the
ions enter the electric field under a specific frequency condition,
only the ions with a specific mass number pass through with
specific amplitude defined by the x-axis and y-axis. The ions with
other mass numbers have amplitude that exponentially increases with
time, and eventually collide with the electrodes. Therefore, only
the ions with the specific mass number satisfying the electric
field condition can pass through and reach a secondary electron
multiplier 5a to be detected. By sweeping the electric field to
sequentially change the electric field condition, the mass spectrum
is obtained.
[0006] A method of ionizing the sample in the ion source 10 of the
mass spectrometer includes an electron ionization method (EI
method) by an electron and a chemical ionization method (CI method)
by a reactive gas ion. The electron ionization method has been most
widely used. When an electron beam hits a molecule with energy more
than necessary to separate an electron from the outmost orbit of
the molecule, a molecular ion (a parent ion) without the electron
on the outmost orbit is produced in addition to various ions
(fragmented ions) with cut off internal bonds. In the electron
ionization method, it is possible to conduct analysis from a mass
spectrum of the fragmented ions produced by the fragmentation (ion
cleavage). As opposed to the electron ionization method, the
chemical ionization method uses an ionization method in a milder
condition. As the fragmentation is difficult to take place,
information regarding a molecular weight can be obtained.
[0007] FIG. 4 is a schematic view showing an ion source 10
according to the conventional electron ionization method. A sample
introduction pipe 19 is connected to an ionization chamber disposed
in a vacuum atmosphere. A gas sample is introduced into the
ionization chamber 20 through the pipe. A filament 11 for
generating thermoelectron is disposed outside a thermoelectron
irradiating opening 11a with an opening on a wall surface of the
ionization chamber 20. When a power current is supplied to the
filament 11 from a current source 11b, the temperature of the
filament 11 is increased to thereby discharge the
thermoelectrons.
[0008] The thermoelectrons (e- in FIG. 4) are attracted by a
potential difference between the filament 11 and the trap electrode
12 to enter the ionization chamber 20, and further accelerated
toward the trap electrode 12. When the thermoelectron beam collides
against the sample molecule, electrons are kicked out from the
sample molecules, so that the molecules become positive ions. The
generated ions jump out of the ionization chamber 20 through the
ion exit 21. Then, the acceleration electrodes 2 (or 2a) pull and
accelerate the ions as shown in FIGS. 3(a) and 3(b) to introduce
into the mass spectrometer system. Since the number of electrons
trapped in the trap electrode 12 depends on the number of electrons
discharged from the filament 11, a controlling portion 11c controls
the current source 11b so that an electric current of the
thermoelectrons trapped at the trap electrode 12 becomes a specific
value. Thus, the quantity of the thermoelectrons at the filament 11
becomes substantially constant, so that a stable ionization can be
attained in the ionization chamber 20.
[0009] The conventional mass spectrometer is structured as
described above. However, an inner surface of the analysis
instrument, especially at the ion source having the electrodes for
generating the electric field or the ion trap, is exposed to the
sample gas molecules. As a result, a specific sample gas is
decomposed and deposited on the surface, causing an unexpected
result due to an interaction with the ions. For example, a
catalytic reaction due to a chemical reaction may take place on the
surface, and an analysis result may be distorted. Also, the surface
tends to promote the sample molecules to be deposited and increases
a temperature.
[0010] The catalytic action of the deposited sample material inside
the instrument affects the measurement. To prevent the effect, the
following approaches have been proposed: a method in which chrome
or chromium oxide is coated on a surface of the electrodes of the
ion source and the ion trap; a method in which an organic silane
reagent is chemically bonded to the surface; a method in which an
inert fused silica is coated on the surface with a thickness of
0.02 to 0.1 .mu.m; and a method in which alumina, silicon nitride,
a selected semiconductor material or the like is coated on the
surface, or these materials are alternatively coated. In the
surface treatment of the inert fused silica, alumina, silicon
nitride and the like, an inert non-organic, nonmetallic material is
coated on the electrode with a minimum thickness to prevent
pin-holes, therefore taking advantage of insulation and the
electric field formation. However, it is not easy for an operator
to perform such surface treatments. Therefore, the ion source and
ion trap are difficult to be maintained by the operator.
[0011] In view of the above problems, the present invention has
been made and an object of the invention is to provide a mass
spectrometer wherein an analyst can easily carry out maintenance of
an ion source and ion trap.
[0012] Further objects and advantages of the invention will be
apparent from the following description of the invention.
SUMMARY OF THE INVENTION
[0013] To attain the above objects, according to a mass
spectrometer of the present invention, an ion of a sample gas is
produced in an ion source by impacting an electron beam, and is
introduced into a magnetic field or an electric field. The ion is
separated based on the mass number and the sample is analyzed from
a mass spectrum. The mass spectrometer includes the ion source
having an electrode made of a stainless steel, which is baked at a
temperature in a range of from 200.degree. C. to 700.degree. C. in
an air atmosphere.
[0014] Also, in a mass spectrometer according to the present
invention, an ion of a sample gas is produced by impacting an
electron beam, and is temporally held in a trap. Then, the produced
ion is introduced into the magnetic field or the electric field
from a trap. The ion is separated based on the mass number and the
sample is analyzed from a mass spectrum. The mass spectrometer
includes the trap having an electrode made of stainless steel baked
at a temperature in a range of 200.degree. C. to 700.degree. C. in
an air atmosphere.
[0015] The mass spectrometer according to the present invention is
structured as described above. The electrodes used for the ion
source or ion trap are made of stainless steel, and the electrodes
are baked at a temperature in a range of 200.degree. C. to
700.degree. C. in an open air when the maintenance is carried out.
The decomposed sample molecules do not adhere and deposit on the
surfaces of the stainless steel electrodes, and there is no
interaction with the ions and the like. Thus, stable analysis can
be carried out.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a flow chart of a conditioning method of an ion
source of a mass spectrometer according to the present
invention;
[0017] FIG. 2(a) is a chromatogram of the mass spectrometer when an
ion source is conditioned, and FIG. 2(b) is a chromatogram of the
mass spectrometer when an ion source is not conditioned;
[0018] FIG. 3(a) is a view showing a structure of a conventional
magnetic field type mass spectrometer, and FIG. 3(b) is a view
showing a structure of a conventional electric field type mass
spectrometer; and
[0019] FIG. 4 is a block diagram showing an ion source of the
conventional mass spectrometer.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0020] Hereunder, embodiments of the present invention will be
explained with reference to the accompanied drawings. FIG. 1 is a
flow chart of a conditioning method of an ion source in a mass
spectrometer according to the present invention.
[0021] A mass spectrometer of the invention includes an ion source
or a trap having an electrode made of stainless steel and baked at
a temperature in a range of 200.degree. C. to 700.degree. C. in an
air atmosphere; an accelerating electrode 2 (or 2a) for
accelerating the ions from the ion source or the trap to enter an
analyzing system as shown on the right side of FIGS. 3(a), 3(b); a
magnetic field 3 (or quadruple electrode 3a) for separating the
accelerated ions based on the mass number; and a detector (a
detector 5 or a secondary electron multiplier 5a) for detecting the
separated ions.
[0022] While the mass spectrometer of the invention has the same
structure as that of the prior art shown in FIGS. 3(a) to 4, a
material of the electrode used in the ion source 10 and the trap
and a treatment method thereof are different from those of the
prior art.
[0023] In the mass spectrometer according to the present invention,
a material of the electrode used for the ion source or trap is
stainless steel, and the electrode is baked at a temperature in a
range of 200.degree. C. to 700.degree. C. in an air atmosphere.
[0024] In the present invention, stainless steel is used as an
electrode material. In the conventional electrode, nickel,
molybdenum, pure iron plated with nickel, Monel, austenite
stainless steel and the like have been used. However, since there
is an electric adsorption in addition to physical and chemical
adsorption of gas, the ions produced by the electrolytic
dissociation are subjected to an interaction with a deposit on a
surface thereof by an image force, electrostatic polarization,
permanent dipole or the like, causing the catalytic reaction.
Therefore, in addition to a molecular ion (a parent ion) wherein an
electron is lost from a molecule, various ions (i.e., fragment
ions) with broken internal bonds are produced, so that the
fragmentation takes place strongly due to the surplus energy. As a
result, depending on a type of molecule, the parent ion may not be
detected.
[0025] The reason for using stainless steel as the electrode
material is that stainless steel contains, as prominent components
thereof, iron, nickel, chrome and the like, and chrome on the
surface of stainless steel has a strong affinity with oxygen. Thus,
when stainless steel is baked at an elevated temperature in an open
air, the surface of the electrode is oxidized to produce a chromium
dioxide and chromium oxide. Due to this oxidization, an alloy
composition in the vicinity thereof may be changed and slightly
magnetized. Since the material is softened at a temperature above
700.degree. C., the temperature treatment should be carried out at
a temperature in a range from 200.degree. C. to 700.degree. C.,
lower than the softening temperature.
[0026] The chromium dioxide and chromium oxide are formed on the
surface as a non-reactive thin oxide layer to reduce pin holes, so
that the ions and the like of the sample molecule are hard to
adhere thereto.
[0027] Next, a method with which an operator or analyst can easily
treat the electrode material of the ion source in the mass
spectrometer of the invention will be explained.
[0028] In a method for treating or conditioning, the electrode of
the ion source or the trap, as shown in FIG. 1, first, the ion
source or the trap (hereinafter simply referred to as "the ion
source") is disassembled. Then, the separated parts are divided
into metal parts requiring cleaning and the other parts. Only the
metal parts are polished by aluminum powder for a specific time.
Then, the metal parts are taken out and the aluminum powder is
blown off to remove. Thereafter, the metal parts are cleaned in an
organic solvent. Then, the metal parts are heated in an electric
furnace for one hour in an air atmosphere at a temperature of
400.degree. C. (or a temperature within a range of 200.degree. C.
to 700.degree. C. according to a shape and thickness of the
electrode). Thereafter, the parts are assembled into a conditioned
ion source.
[0029] FIG. 2(a) shows a chromatogram using the electrode of the
ion source with the conditioning; and FIG. 2(b) shows a
chromatogram using the electrode of the ion source without the
conditioning. The horizontal axis represents time and the vertical
axis represents an intensity of analysis signal. When using the ion
source with the conditioning, a sharp peak value is obtained, while
when using the ion source without the conditioning, a broad and
inseparable curve is obtained. With the treatment as described
above, the catalysis effect of the interaction with the ions on the
surface of the electrode is eliminated, thus the mass spectrometer
can have a high sensitivity.
[0030] Next, as shown in FIG. 4, in an electron ionization method,
a method for conditioning the electrode of the ion source 10 will
be specifically explained.
[0031] First, the ion source 10 is disassembled into a filament 11
and focusing electrode thereof, the sample introduction pipe 19,
metal parts of an ion exit 21, the trap electrode 12 and the
ionization chamber 20. Then, those parts are divided into metal
parts requiring cleaning and insulating materials. In the filament
11, the electrode is cleaned except the insulating materials, and a
new filament 11 is mounted after the cleaning. Also, the sample
introduction pipe 19, metal parts of the ion exit 21, trap
electrode 12 and ionization chamber 20 are polished by aluminum
powder. After polishing for a specific time, the aluminum powder is
removed from the respective parts, and the parts are washed in the
organic solvent. Then, the respective metal parts are heated in an
electric furnace for one hour at a temperature of 400.degree. C.
under the atmospheric pressure. Then, the respective parts are
assembled into the original structure.
[0032] In the above embodiment, while the explanation has been made
with respect to only the ion source, the same conditioning method
of the electrode treatment as described above may be performed for
the ion trap having the electrode made of stainless steel to obtain
the same effects as those of the ion source.
[0033] The mass spectrometer of the present invention is structured
as described above. When the mass spectrometer is used to analyze
various sample gases for a long period of time, a specific sample
is decomposed and deposited on the surfaces of the electrodes in
the ion source or the ion trap of the mass spectrometer. Therefore,
the operator or analyst has to periodically disassemble the ion
source or ion trap to separate the metal parts made of stainless
steel from the insulating materials. The metal parts are polished
with the aluminum powder, and cleaned in an organic solvent. Then,
they are conditioned for one hour at a temperature of 400.degree.
C., and thereafter, are assembled again. With the relatively simple
treating method as described above, it is possible to prevent the
specific component from adhering to the surfaces of the metal
parts, so that the mass spectrometer can perform the stable
analysis with the maximum performance, resulting in a high ratio of
effect to cost.
[0034] While the invention has been explained with reference to the
specific embodiments of the invention, the explanation is
illustrative and the invention is limited only by the appended
claims.
* * * * *