Radiant energy – Ionic separation or analysis – With sample supply means
Reexamination Certificate
2001-03-23
2003-10-21
Anderson, Bruce (Department: 2881)
Radiant energy
Ionic separation or analysis
With sample supply means
C250S292000
Reexamination Certificate
active
06635868
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mass spectrometry apparatus, and more particularly to a mass spectrometry apparatus able to simultaneously or separately measure the molecular weight and analyze the molecular structure of a detected gas with a sufficient sensitivity.
2. Description of the Related Art
Measurement of the mass of gas molecules, which are electrically neutral, requires ionization of the gas molecules. The apparatus for ionizing the gas is called an “ion source”. The ionized molecules (hereinafter referred to as “ions”) enter the mass spectrometry mechanism. In the mass spectrometry mechanism, the ions are led into a specific electric field or magnetic field and move along a path in accordance with the mass of the ions due to the electrical field or magnetic field applied to the ions. As a result, different paths arise for each ion and only ions of a specific mass are detected.
Various ionization systems have been proposed in the past for the above ion source. For example, (1) electron impact ionization, (2) chemical ionization, (3) ionization by a composite of electron impact and chemical ionization, (4) atmospheric pressure ionization, (5) ionization by a composite of electron impact and atmospheric pressure ionization, and (6) ionization by ion attachment have been proposed. These ionization methods will be explained in brief below.
(1) Electron Impact Ionization
The electron impact ionization method is the method used most often for the ion sources of mass spectrometry apparatuses. In the ion source of the electron impact ionization method, the detected gas is introduced in an amount of 10
−3
Pa and the molecules of the detected gas are impacted by hot electrons emitted from a hot filament to be accelerated to about 50 to 100 eV. The negative charge electrons are stripped from the gas molecules by the electron impact, whereby the gas molecules become positive charge ions. The electron impact ionization method is simple in terms of hardware and has the advantage of a small difference in ionization efficiencies resulting from the type of the molecules. The pressure in the ion source is usually 10
−2
Pa or less, so as not to limit the movement of the electrons and ions. Note that the pressure in the mass spectrometry mechanism is usually 10
−3
Pa or less. In the ion trap type, however, operation is possible even with 10
−2
Pa.
The above electron impact ionization method has the feature of splitting (dissociating) molecules along with the ionization due to the excess energy of the electron impact when applied to a detected gas comprised of molecules with small energy of atomic bonds. Therefore, in this case, there is the advantage of obtaining effective information on the molecular structure. On the reverse side, there is the defect that it is not possible to obtain effective information on the molecular weight.
(2) Chemical Ionization
In an ion source of the chemical ionization method, a reaction gas of about 100 Pa (methane: CH
4
etc.) and a detected gas of about 1 Pa are introduced. First, the reaction gas is ionized by the electron emission and impact from the hot filament. Next, an ion and molecular reaction occurs between the ionized reaction gas and the detected gas. The detected gas is ionized to a positive charge or negative charge. The mechanism of ionization is extremely complicated and includes the phenomena of 1) the hydrogen ions in the ions of the reaction gas bonding with the molecules of the specimen, 2) hydrogen ions conversely being stripped from the detected gas, 3) charge movement, etc. Even in the chemical ionization method, when the hydrogen ions bond with the specimen molecules, disassociation often occurs in a detected gas with a weak bond energy. The chemical ionization method has the defect of a poor stability and reproducibility of the measurement values due to the complicated ionization mechanism. Further, it has the defect of a low measurement sensitivity.
(3) Ionization by Composite of Electron Impact and Chemical Ionization
There are two types of ion sources in this ionization method as shown in FIG.
11
A and FIG.
11
B and in FIG.
12
. The configuration of FIG.
11
A and
FIG. 11B
is a switching type. The configuration of
FIG. 12
is a continuous type.
According to the switching type ion source shown in FIG.
11
A and
FIG. 11B
, one of the electron impact ionization method (
FIG. 11A
) and the chemical ionization method (
FIG. 11B
) is selected and used by mechanical and electrical switching.
FIG. 11A
shows the state of operation in the case of electron impact ionization. An ion source including a filament
101
and a region
102
for electron impact and a condensing lens
104
are arranged in the same space
105
. This space
105
is evacuated by a single vacuum pump
106
. A carrier gas of He and the detected gas (specimen) are introduced to give a pressure of about 10
−3
Pa. A partition
109
with an ion passage port
108
for passing the produced and condensed ions is provided at the front of the mass spectrometry mechanism
107
. The mass spectrometry mechanism
107
is evacuated by another vacuum pump
110
so as to maintain the pressure of 10
−4
Pa.
FIG. 11B
shows the state of operation in the case of chemical ionization. The filament
101
and the condensing lens
107
remain unchanged, but the electron impact region
102
where the electrons impact is generally surrounded by the partition
111
. The electron impact region
102
, however, also has an opening
112
such as the electron passage port. This does not mean that ports other than the ion passage port
113
are closed. The electron impact region
102
is evacuated by the vacuum pump
106
through the space
105
where the filament
101
and the condensing lens
104
are positioned. A reaction gas (CH
4
) and a detected gas (specimen) are introduced into the electron impact region
102
to a pressure of 100 Pa. The ratio of the detected gas, however, is about 1%. Note that the pressure in the space around the electron impact region becomes 10
−2
Pa. In the above composite method, there is the defect of the need for switching of the ion source itself and the introduced gas and poor operability. Therefore, while this system is possible in current GC/MS products, in almost all cases only the electron impact ionization method is used. The chemical ionization method is only used on a supplementary basis.
The continuous type ion source shown in
FIG. 12
has a special structure designed especially for research (
Analytical Chemistry,
vol. 43, no. 12 (1971), p. 1720). In this structure, it is possible to perform the electron impact ionization and the chemical ionization continuously or simultaneously. Exclusive filaments
201
and
202
for the ionization methods are provided. The electron impact regions
203
and
204
are also independent. There is however no condensing lens. The electron impact region
204
of the chemical ionization method is substantially surrounded by the partition
205
. The ion source of the electron impact ionization method is positioned in the space around the electron impact region
204
of the chemical ionization method. These are evacuated by a single vacuum pump
206
. A reaction gas and a 1% detected gas are introduced to give a pressure of 100 Pa in the electron impact region
205
of the chemical ionization method. The reaction gas and the detected gas are reduced in pressure 10
−4
fold, that is, to about 10
−2
Pa, while leaving the ratio the same, and flow to the ion source of the electron impact ionization method. Therefore, the partial pressure of the detected gas at the ion source of the electron impact ionization method becomes a low 10
−4
Pa or so. The above composite system has the defects that not only does it ionize the reaction gas by the electron impact ionization method, but also the concentration of the detected gas is low and the sensitivity is poor. Therefore, products of this system have st
Fujii Toshihiro
Nakamura Megumi
Shiokawa Yoshiro
Anderson Bruce
Anelva Corporation
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