Radiant energy – Ionic separation or analysis – Cyclically varying ion selecting field means
Reexamination Certificate
2000-09-12
2003-04-01
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Cyclically varying ion selecting field means
C250S299000, C250S293000, C250S294000, C250S296000, C250S300000
Reexamination Certificate
active
06541769
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the field of analytic chemistry, and especially to a mass spectrometer including a quadruple ion-trap type mass spectrometer unit.
A plasma ionizing mass spectrometer method, in which a sample is analyzed by being put in plasma generated at atmospheric pressure, and having the ionized sample introduced into the plasma in a vacuum, is well known as a high-sensitivity elemental analysis method. The general apparatus most generally used for this method is an inductively coupled plasma-mass spectrometer (hereafter abbreviated to ICP-MS). Plasma is generated by using a high frequency radio-wave, and a sample is ionized by being put in the generated plasma. Then, the ionized sample is introduced into a vacuum through a pinhole, and is analyzed. The ICP-MS is disclosed, for example, in “BUNSEKI”, Journal of The Japan Society for Analytical Chemistry, p 342, vol. 5, 1996.
In the ICP-MS, since argon gas is used to generate plasma, a large quantity of matrix ions, such as Ar
+
ions, ArO
+
ions, etc., which are produced from argon gas used, is generated. Therefore, there has been a problem in that ions such as Ca
+
ions, Fe
+
ions, etc., whose mass is near that of the matrix ion, can just barely be detected. Thus, a microwave induced plasma-mass spectrometer (hereafter abbreviated to MIP-MS), in which a microwave is used to generate plasma, has been developed. In the MIP-PS, since the high energy density can be obtained by concentrating energy into a narrow space, nitrogen or helium gas can be used as plasma source gas, which in turn can prevent the generation of argon gas related matrix ions. Consequently, it has become possible to analyze elements such as calcium, iron, and so on, with a high sensitivity. The conventional technique of the MIP-PS is disclosed, for example, in Japanese Patent Application Laid-Open Hei 1-309300. Meanwhile, it is known that since the plasma temperature in the MIP-MS is lower than that in the ICP-MS, the analysis sensitivity of the ICP-MS is higher for analyzing elements with high ionization potential than that of the MIP-MS.
The plasma-mass spectrometer such as the ICP-MS, the MIP-MS, etc., is generally used in the field of the analysis for environmental evaluation.
Meanwhile, these spectrometers are equipped with a protection cover to prevent an electrical shock, a heat burn, an influence of a high-frequency radio-wave on a human body, etc.
Here, because particles such as neutral particles, which pass through the mass spectrometer, may reach a detector, and may become detection noises, the mass spectrometer is sometimes composed so that the detector is located in a direction, altered from that in which ions are ejected from the ion ejection hole of the mass spectrometer. For the purpose, a deflecting device is located between the mass spectrometer and the detector. Such a deflecting device is disclosed in Japanese Patent Application Laid-Open Hei 9-161719 and Japanese Patent Application Laid-Open Hei 9-190797. Since the generated neutral particles which become noise sources go straight, they cannot reach the detector. On the other hand, the selected and ejected ions are led by the deflecting device to the detector, and are so detected.
In the plasma ion source-mass spectrometer such as the ICP-MS, the MIP-MS, etc., although various types of mass spectrometers can be used, the use of a quadrupole ion-trap type (referred to simply as ion-trap type) mass spectrometer has recently been tried. It is known that argon gas related molecular ions (ArO
+
, Arcl
+
, etc.), or metal oxide ions (CaO
+
, etc.), which hinder the analysis, can be decomposed by collisions among them in the quadrupole ion-trap type mass spectrometer.
As mentioned above, it has become possible to provide the plasma ion source-mass spectrometer with a new function which can decompose molecular ions hindering the analysis, constructed by combining the plasma ion sources such as the ICP, the MIP, etc., and an ion-trap type mass spectrometer. However, in the ion-trap type mass spectrometer, since the plasma-confinement potential is distorted by the space-charge effect if a large quantity of ions is confined in the mass spectrometer, this may deteriorate the fundamental analysis performances such as the mass resolution. Also, in the plasma ion source-mass spectrometer, since there is a large quantity of plasma gas related ions, and they are confined in the mass spectrometer, the induced space-charge effect becomes great.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a mass spectrometer which can prevent the degradation of its fundamental analysis performances, such as mass resolution, by reducing the above space-charge effect, so as to suppress the distortion of the confinement potential due to this space-charge effect.
Further, another objective of the present invention is to realize a lasting stable analysis in a plasma ion source-mass spectrometer which uses an ion-trap type mass spectrometry unit, by protecting an ion detector from being over-exposed by particles.
The using of a comparatively large size mass spectrometry unit has been investigated for a plasma ion source-mass spectrometer which uses an ion-trap type mass spectrometry unit. The size increase of a mass spectrometer increases the volume of the mass spectrometer, which in turn will reduce the space-charge effect of plasma ions. On the other hand, if the size of the mass spectrometer is increased, this causes a disadvantage in that the measurable upper limit of mass number decreases. However, since the required upper limit of mass number is approximately 250 in the element analysis field in which the plasma ion source-mass spectrometer is mainly used, the size of a mass spectrometer can be increased within a range satisfying the above condition.
Thus, a plasma ion source-mass spectrometer which uses an ion-trap type mass spectrometry unit with a size larger than that of conventional type of plasma ion source-mass spectrometry units has been developed and evaluated. That is, the smallest inside diameter r
0
in the ring electrode of the developed mass spectrometry unit is 16 mm which is larger than 7 mm or 10 mm of the conventional type of plasma ion source-mass spectrometry units. From the evaluation results, it has been found that the developed mass spectrometry unit has a new subject.
In the ion-trap type mass spectrometry unit, a mass spectrum is obtained by alternately setting an ion-confining (or trapping ) period and a mass-analyzing period. An electrode called an ion-stopping electrode is usually located between a mass spectrometry unit and a detector. During the ion-stopping period, positive voltage (typically +300V) is applied to the ion-stopping electrode so that ions do not reach the detector. On the other hand, during the mass-analyzing period, negative voltage (typically −300V) is applied to the ion-stopping electrode so that ions can pass through this electrode and reach the detector. In this way, by switching the sign of the voltage applied to the ion-stopping electrode, the timing of when ions reach the detector is controlled.
However, it was found that the detector actually outputs a high-level signal even during the ion-stopping period. Thus, it was proved that there is a problem in which the detector may break down due to the influence of an over current, or, the lifetime of the detector may be remarkably decreased.
It should be noted that, the efficiency of confining plasma gas related ions is low under the conditions suitable for confining ions of a sample. Therefore, one cause of the above problem is that many of the plasma gas related ions, which have reached the mass spectrometry unit, possibly pass through it and reach the detector. Accordingly, since a large quantity of plasma gas related ions pass through the mass spectrometry unit and reach the detector despite the preventing of ion-ejection from the mass spectrometry unit by applying positive voltage to the ion
Nabeshima Takayuki
Sakairi Minoru
Takada Yasuaki
Terui Yasushi
Yoshinari Kiyomi
Lee John R.
Mattingly Stanger & Malur, P.C.
Souw Bernard E.
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