Radiant energy – Ionic separation or analysis – With sample supply means
Reexamination Certificate
1999-08-23
2002-07-23
Berman, Jack (Department: 2881)
Radiant energy
Ionic separation or analysis
With sample supply means
C250S289000, C250S281000, C250S292000, C250S3960ML, C250S430000, C250S42300F
Reexamination Certificate
active
06423965
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to mass spectrometers, which are important in analytical field, in particular, to plasma ion source mass spectrometers and liquid chromatography/mass spectrometers.
A plasma ion source mass spectrometer, wherein a sample is introduced into plasma at a high temperature and ionized by heat of the plasma, has been used in the field of chemical element analysis. Any of inductively coupled plasma and microwave plasma can be utilized for the ion source.
A conventional plasma ion source mass spectrometer is explained briefly referring to
FIG. 21
, hereinafter.
A liquid sample in a sample bottle
1
is nebulized in a spray chamber
2
, and ionized by introducing into plasma
3
. The generated ions are introduced into a differential pumping region
34
(approximately one Torr), which is pumped by a rotary pump
124
, via a sampling cone
4
. The ions entered the differential pumping region
34
are introduced into a vacuum region
35
(lower than approximately 10
−4
Torr.), which is pumped by a turbo molecule pump
125
, via a skimmer cone
5
. Orbits of the ions passed through a gate valve
8
are collimated by ion lenses
22
a,
22
b,
22
c,
and an entrance aperture
19
in the vacuum region
35
. Subsequently, the ions are deflected by 90 degrees using a deflector composed of quarterly split cylindrical electrodes
10
a,
10
b,
10
c,
and
10
d
(hereinafter, this deflector is called Q-deflector) for separating the ion from photon, which is a main source of noises. Then, collimation of the ions is improved by correcting electrodes
16
a
and
16
b,
and the ions are introduced into the mass analyzer
14
for performing mass separation, and detected by a detector
15
.
In accordance with using the Q-deflector as explained above, the photons and neutral molecules generated by the plasma are prevented from reaching the detector, and mass spectrum having a preferable ratio of signals versus noises (hereinafter, it is called S/N ratio) can be obtained.
In order to obtain a stable intensity of ions, the pressure in the vacuum region, wherein the mass analyzer is arranged, must be stabilized. Because it takes approximately one hour for stabilizing the pressure in the pressure chamber of the vacuum region
35
from start of pumping, the turbo molecule pump
125
for pumping the vacuum region
35
is preferably not stopped.
On the other hand, the pressure in the differential pumping region
34
can be stabilized in approximately one minute, because its reached pressure of pumping is relatively high in comparison with the pressure of the vacuum region. Therefore, a leading electrode
23
provided with an aperture having a shielding function to prevent neutral particles from entering is arranged between the gate valve
8
and the skimmer cone
5
. In order to restrict a portion to be contaminated in the vacuum region, the major portion of the neutral particles generated in the ion source are adhered to the leading electrode
23
. If the electrode is contaminated, an insulating film is formed and a charging phenomenon is occurred, and an electric field for transmitting the ions effectively is disturbed. Maintenance of the apparatus can be performed with pumping the vacuum region continuously by shutting the gate valve and cleaning the leading electrode
23
. Therefore, the time necessary for re-start up of the apparatus can be shortened.
The conventional plasma ion source mass spectrometer described above was disclosed in JP-A-7-78590 (1995) and JP-A-9-306418 (1997).
Currently, an apparatus, wherein a plasma ion source and a quadrupole ion trap mass spectrometer (hereinafter, called ion trap) are combined, has been used. The apparatus disclosed in “RAPID COMMUNICATIONS IN MASS SPECTROMETRY” vol. 8, 71-76 (1994) is explained hereinafter referring to FIG.
22
.
Ions generated by plasma are led to the first differential pumping region
36
, which is being pumped by a rotary pump
124
, and subsequently, the ions are introduced into the second differential pumping region
37
by a leading electrode
26
. After passing through the gate valve
8
, the ion orbits, which have been scattered by a photon shielding electrode
27
called as a photon stopper, are collimated by ion lenses (
28
a,
28
b
), and introduced into the vacuum region
35
. The ions are collimated again with the ion lenses (
29
a,
29
b,
29
c
), and introduced into the quadrupole mass spectrometer
30
. In accordance with the quadrupole mass analyzer
30
, ions having a specified mass number can be transmitted selectively. Subsequently, the ions are collimated with the ion lenses (
31
a,
31
b,
31
c
), and introduced into the ion trap mass analyzer
14
. The ions are detected by the detector
15
after being separated depending on their masses.
In the field of organic substance analysis, a liquid chromatograph/mass spectrometer (hereinafter, called LC/MS), which is composed by combining a liquid chromatograph and a mass spectrometer, has been used frequently. In accordance with the LC/MS, ions are separated from neutral particles such as micro-droplets and the like for increasing a S/N ratio. A conventional example of the LC/MS has been disclosed in U.S. Pat. No. 5,481,107.
In accordance with the conventional plasma ion source mass spectrometer, noises can be decreased, because entering the neutral particles is prevented by reducing a part of the leading electrode
23
for decreasing a sighting angle, or using the photon stopper
27
on the axis of the skimmer cone. On the other hand, a problem to decrease ion transmission is generated. In some cases, reproducibility of the ion transmission can not be obtained, because of errors in assembling the lenses after cleaning.
Generally, the gate valve
8
is electrically grounded. Therefore, the ions move slowly at this region, and the ions are readily effected by unintentional seeping of electrical field (called fringing field) from the electrode arranged in the vicinity of the gate valve. Generally, the unintentional electrical field is formed asymmetrically to the central axis of the lens. Accordingly, the ion lenses
22
a,
22
b,
22
c,
which are axially symmetrical, and an entrance aperture
19
have a problem to decrease the ion transmission by collimating the ion orbit insufficiently.
Furthermore, the conventional plasma ion source mass spectrometer indicated in
FIG. 21
was composed of two pairs of flat plates, each of the pairs faced each other (
16
a,
16
b,
and a pair of plates arranged at above and beneath this paper, which are not indicated in the figure), as correcting lenses for introducing the ions into the mass analyzer after the ions were deflected. That is, a moving direction of the ions is corrected and collimated by giving a potential gradient in a direction perpendicular to the moving direction of the ions deflected by the deflector. Therefore, because the most optimum voltage of the correcting electrodes
16
a,
16
b
depends strongly on kinetic energy of the ions, collimating the ions is difficult with the plasma ion source mass spectrometer, which has a large fluctuation in the kinetic energy of the ions.
That is, it is an issue that increasing the efficiency of the ion transportation by how eliminating particles other than the ions during introducing the ions from the ion source to the mass spectrometer for analyzing the ions. Therefore, the ion transport region for introducing the ions into the ion mass spectrometer has a fundamental composition, wherein the ions are introduced into the analyzing region after the ion orbit is remarkably bent in the ion transport region.
In some cases, reproducibility of the ion transmission can not be obtained, because of errors in assembling the lenses after cleaning.
In accordance with the conventional mass spectrometer, it is a problem that molecule ions such as ArOH
+
and NOH
+
are generated from argon gas or nitrogen gas used as a carrier gas for sample or plasma gas, and these molecule ions disturb the measurement by overlapping with peaks
Hashimoto Yuichiro
Nabeshima Takayuki
Sakairi Minoru
Takada Yasuaki
Tsukada Masamichi
Berman Jack
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Wells Nikita
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