Radiant energy – Ionic separation or analysis – Methods
Reexamination Certificate
2001-03-23
2002-11-12
Anderson, Bruce (Department: 2881)
Radiant energy
Ionic separation or analysis
Methods
Reexamination Certificate
active
06479814
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ion source for an ion attachment mass spectrometry apparatus, and more particularly, to an ion source used for an ion attachment mass spectrometry apparatus which attaches metal ions emitted from an emitter to a detected gas to ionize it and analyze the mass of the detected gas.
2. Description of the Related Art
In mass analysis of gas molecules, it has been necessary to give a positive or negative charge to the gas molecules in order to make use of the fact that the motion of charged particles in an electromagnetic field differs depending on the ratio between the charge and the mass. As methods for ionizing the gas molecules, there are the electron impact ionization method, the chemical ionization method, the atmospheric pressure ionization method, and the ion attachment ionization method, etc. Among these, the ion attachment ionization method enables ionization without dissociation (splitting) of the gas molecules including weak bonds since the excess energy arising in the process of ionization of a detected gas is extremely small. Therefore, in a mass spectrometry apparatus, it is possible to measure the correct molecular weight of a detected gas from the molecular ion peaks according to the ion attachment ionization method. This is effective for mass analysis of easily dissociating organic samples.
The ion attachment ionization method uses the phenomenon that when a metal oxide (insulator) is heated and metal atoms contained are emitted as ions, these metal ions gently deposit at locations where the charges of the gas molecules concentrate. In particular, if an oxide containing an alkali metal is heated, it is known that positive charge metal ions are easily emitted from the surface thereof. Attaching the alkali metal ions to other gas molecules to ionize them has been reported in
Analytical Chemistry,
vol. 48, no. 6, p. 825 (1976) as the Hodges system, in
Analytical Chemistry,
vol. 56, no. 3, p. 396 (1984) as the Bombick system, and in
Journal of Applied Physics,
vol. 82, no. 5, p. 2056 (1997) as the Fujii system.
Next, an explanation will be given of a conventional ion source used in a mass spectrometry apparatus employing the ion attachment ionization method with reference to
FIG. 10
to FIG.
12
.
FIG. 10
is a schematic view of the configuration of the ion source,
FIG. 11
is an enlarged sectional view of the emitter, and
FIG. 12
is an equivalent circuit diagram of the emitter.
As shown in
FIG. 10
, the ion source employing the ion attachment ionization method is comprised of a conductive casing (container)
101
forming an ion attachment region inside it and having one end completely open, an aperture
102
attached to the right open end of the casing
101
, a voltage-impressed portion
103
passing through a part of the casing
101
while electrically insulated from the same, a spherical emitter
104
comprised of a metal oxide attached to a suitable position of the voltage-impressed portion
103
, and a gas inlet
105
for introducing a detected gas and other gases into the ion attachment region. The aperture
102
has an opening
106
for passing the ionized detected gas. By providing an insulator
107
at the connecting portion with the open end of the casing
101
, it is electrically insulated from the casing
101
. Further, the voltage-impressed portion
103
is connected to a heating power source
108
and a bias power source
109
.
The spherical emitter
104
, as shown in
FIG. 11
, is fixed by sintering for example to a wire-shaped voltage-impressed portion
103
. The diameter of the emitter
104
is about 2 to 3 mm, for example. The portion of the voltage-impressed portion
103
in contact with the emitter
104
will be particularly referred to as a reference-voltage-impressed portion
103
a.
The emitter
104
is a mixture of an alumina silicate comprised of Al
2
O
3
or SiO
2
and an oxide (compound) containing Li, that is, Li
2
O, when the metal ions to be emitted from the emitter are Li
+
ions. These are all oxides, so form insulators overall. The specific resistance is also at least 10
12
&OHgr;·m. At least the reference-voltage-impressed portion is a wire-shaped structure of a high melting point metal such as Ir (iridium) or W (tungsten). In the reference-voltage-impressed portion, Joule heat is generated by the flow of current.
In the above ion source, the aperture
102
is held at the ground voltage and a mixed gas of the detected gas and another gas is introduced through the gas inlet
105
into the ion attachment region evacuated to a vacuum state. The inside is evacuated to a reduced pressure atmosphere of about 100 Pa. The other gas is a gas such as N
2
to which metal ions do not easily attach. This is introduced so as to rob the excess energy produced when the metal ions are attached to the detected gas. The voltage-impressed portion
103
is supplied with a bias voltage by the bias voltage source
109
so that the reference-voltage-impressed portion
103
a
becomes 10V, for example. Further, the heat source
108
lets a current flow at the reference-voltage-impressed portion
103
a
and thereby the emitter
104
is heated to about 600° C. Due to the above operation, metal ions (Li
+
) are generated on the surface of the emitter
104
. These metal ions are attracted by the electric field formed in the space
110
between the emitter
104
and the ground potential aperture
102
, dissociated (emitted) from the surface of the emitter, and transported in the direction of the aperture
102
. Next, the metal ions attach to the detected gas introduced into the ion source so as to ionize the detected gas.
In the above-described conventional ion source, the emitter is produced from an insulating metal oxide, so there was the problem that a potential difference between the reference-voltage-impressed portion
103
a
and the ion emission point on the surface of the emitter
104
cyclically changes. Since the emitter is an insulator, a large electrical resistor is interposed between the reference-voltage-impressed portion and the ion emission point. The above problem is caused by the fact that there is a voltage drop at the insulator.
FIG. 12
shows the portion between the reference-voltage-impressed portion and the ion emission point by an equivalent circuit. An electrical resistor
112
is interposed between the reference-voltage-impressed portion
103
a
and the ion emission point
111
. In
FIG. 12
, when ions are emitted as shown by the arrows
113
from the emitter
104
, a current flows through the electrical resistor
112
having a large resistance value. A voltage drop occurs here and the potential at the ion emission point
111
falls. The relation of the voltage drop is expressed as
Vb=Va−I·R
(1)
where the potential of the reference-voltage-impressed portion
103
a
is Va, the resistance of the emitter
104
is R, the current flowing through the emitter
104
is I, and the potential of the ion emission point
111
is Vb. Based on this relation, if the potential Vb at the ion emission point
111
falls, the electric field between the ion emission point
111
and the aperture
102
becomes weak, the amount of ion emission falls, and the current (I) flowing through the emitter
104
falls. If the current (I) falls, the voltage drop becomes smaller and the potential of Vb rises, so the amount of ion emission again increases. In this way, the process of “Vb drop→I fall→Vb rise→I rise→Vb drop” is repeated and an unstable cyclical change of the amount of ion emission and the electric field continues. In the ion attachment mass spectrometry apparatus, to accurately detect the number of molecules of the ionized detected gas as an electrical signal, that is, to correctly analyze the mass, the amount of ion emission has to be stable. Therefore, if such a cyclic state of change arises, it is not possible to correctly analyze the mass of the detected gas.
As a means for solving the above problems, i
Fujii Toshihiro
Nakamura Megumi
Shiokawa Yoshiro
Anderson Bruce
Anelva Corporation
Hashmi Zia R.
Oliff & Berridg,e PLC
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