JP2000348669A - Plasma ion source mass spectrometer and ion holding mechanism - Google Patents

Abstract

(57) [Summary] To minimize the influence of molecular ions formed by combining ions originating from elements present in a sample introduced into plasma with other ions. Kind Code: A1 A plasma ion source mass spectrometer comprising a plasma ion source for ionizing an analysis sample by plasma, and a mass spectrometer for mass spectrometry analysis of the ionized sample. Ion holding means for holding the ions and decomposing the held ions. The sensitivity and detection accuracy of mass spectrometry can be improved, and qualitative analysis in mass spectrometry can be facilitated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a plasma ion source mass spectrometer using plasma generated by various methods as an ion source and a mass spectrometer as a detection unit.

[0002]

2. Description of the Related Art Conventional techniques are discussed in, for example, paragraphs 13 to 43 of "Spectroscopy Society of Japan Measurement Method Series 28" Plasma Ion Source Mass Spectrometry ", edited by Koji Kawaguchi and Taketoshi Nakahara.

[0003] That is, the plasma ion source mass spectrometer ionizes an element to be measured contained in a solution sample by plasma, draws the resulting ions into a vacuum through an interface section, and performs measurement by a mass spectrometer. Is what you do. The mass spectrometer can be a low-resolution quadrupole mass spectrometer or a high-resolution double-focusing mass spectrometer, but usually a low-cost quadrupole mass spectrometer is mainly used. Have been.

[0004]

However, in the above prior art, when the ion source is an inductively coupled plasma (hereinafter referred to as ICP), an argon gas is used as a plasma generating gas, and a microwave induced plasma (hereinafter referred to as MIP) is used as a plasma generating gas. Since nitrogen gas is used in some cases, when ions are drawn into the plasma or vacuum from the interface section, argon ions or argon atoms, or nitrogen ions or nitrogen atoms are replaced with other ions (oxygen ions, hydrogen ions). , Chlorine ions, etc.) and atoms (oxygen, hydrogen, chlorine, etc.) to form molecular ions. In addition, ions or atoms originating from the elements present in the sample introduced into the plasma are combined with other ions (oxygen ions, hydrogen ions, chlorine ions, etc.) or atoms (oxygen atoms, hydrogen atoms, chlorine atoms, etc.). Then, molecular ions are generated. For this reason, in a mass spectrometer using a low-resolution quadrupole mass spectrometer, the mass spectrum of the element to be measured overlaps with the mass spectrum of these molecular ions, which lowers the detection limit.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it has been proposed that ions or atoms derived from a plasma generating gas are introduced into molecular ions or plasma formed by combining with other ions or atoms. It is an object of the present invention to provide a plasma ion source mass spectrometer capable of minimizing the influence of molecular ions generated by ions or atoms originating from elements present in a sample combined with other ions or atoms.

It is still another object of the present invention to provide a plasma ion source mass spectrometer having an ion holding unit capable of holding more ions during mass analysis.

[0007]

SUMMARY OF THE INVENTION The present invention for solving the above-mentioned problems is characterized by a plasma ion source for ionizing an analysis sample by plasma, and a mass spectrometer for mass-analyzing the ionized sample. Wherein the mass spectrometry unit is provided with ion holding means for holding ions to be analyzed and decomposing the held ions.

Further, there is provided a plasma ion source mass spectrometer comprising a plasma ion source for ionizing an analysis sample by plasma, and a mass spectrometer for mass spectroscopy of the ionized sample, wherein the mass spectrometer comprises: An ion holding portion including a ring electrode and a pair of end cap electrodes is provided, and the inner diameter of the ring electrode is configured to be 24 mm to 40 mm.

[0009] Still further, an ion holding device provided in the mass spectrometer of the plasma ion source mass spectrometer having a plasma ion source for ionizing an analysis sample by plasma and a mass spectrometer for mass analyzing the ionized sample. The ion holding mechanism comprises a ring-shaped electrode and a pair of hemispherical electrodes, and the inner diameter of the ring-shaped electrode is 24 mm to 40 mm.

[0010] The present invention relates to a method for producing argon ions or argon atoms derived from plasma generation gas, or nitrogen ions or nitrogen atoms, and other ions (oxygen ions, hydrogen ions, chlorine ions, etc.) or atoms (oxygen atoms, hydrogen atoms, chlorine atoms, etc.). Molecular ions generated by the bond with atoms, atoms or atoms originating from elements present in the sample introduced into the plasma, and other ions (oxygen ions, hydrogen ions, chlorine ions, etc.) or atoms ( Oxygen atom,
In order to suppress the influence of molecular ions generated by bonding with hydrogen atoms, chlorine atoms, etc.), as described above, during the retention of ions conventionally used in liquid chromatography / mass spectrometer (LC / MS), The greatest feature is that an ion holding unit having a function of decomposing the held ions is provided in the plasma ion source mass spectrometer.

In this way, when performing mass spectrometry, background data other than the measurement result can be removed by decomposing the retained ions while retaining the ions, thereby improving the detection limit. Further, it is possible to realize a plasma ion source mass spectrometer capable of suppressing data variation. Therefore, it is possible to improve sensitivity and accuracy.

Further, in the plasma ion source mass spectrometer, since the object of measurement is mainly an element, the mass number of the object to be measured is at most about 250. Therefore, with the configuration of the present invention, it is possible to realize an ion holding unit that can hold more ions than LC / MS without increasing the applied amplitude voltage when performing mass spectrometry. Become.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIG.

FIG. 1 shows an overall configuration diagram of a trace element mass spectrometer using ICP as an ion source as an example.

Reference numeral 2 denotes a sample to be measured. Usually, the sample 2 is a liquid, and is passed through the capillary tube 4.
The sample 2 is supplied to an atomizer 6 such as a nebulizer that atomizes the sample 2 under appropriate conditions. In the atomizer 6, the sample is atomized. The sample 2 may be an aqueous solution as long as it is a liquid,
An organic solvent such as alcohol may be used.

Then, the atomized sample 2 is led to the torch 10. In the atomizer 6, a gas supply unit 50 is provided for guiding the atomized sample to the torch 10. The sample is transferred to the torch 10 by the carrier gas 8 supplied from the gas supply unit 50. Sent. The supply amount of the carrier gas 8 from the gas supply unit 50 is controlled by the control computer 52.

The torch 10 is supplied with the plasma gas 16 from the gas supply unit 50 and the high frequency from the plasma generation unit 12. 27 MHz (megahertz) or 40 MH by the high frequency power supply 54
A high frequency 14 of z is supplied. The high frequency 14 and the plasma gas 16 generate a plasma 18 in the atmosphere. The temperature of the plasma 18 is about 5000 ° C to 60 ° C.
Reach 00 ° C. High frequency power supply 54 and gas supply unit 50
Is controlled by the control computer 52 so that the plasma 18 becomes the optimum condition.

The sample 2 is supplied to a plasma 18 generated at atmospheric pressure, and dissociated, atomized,
Ionized and released into the atmosphere. Here, the plasma 18 may be generated by another ion source such as MIP.

Next, the sample ionized by the plasma 18 is guided to a mass spectrometer to perform mass spectrometry. Since the mass spectrometer 24 operates in a vacuum, the ionized sample is efficiently evacuated. An interface section (sampling cone and skimmer cone) 20 is provided for drawing in. This interface unit 2
The sample ionized in the plasma 18 from 0 is drawn toward the mass analyzer 24.

The mass spectrometer 24 is operated in a vacuum (about 10 ^-6 Tor
In order to operate in the environment of r), the present apparatus performs three stages of evacuation, that is, working exhaust, as shown in FIG. The evacuation is usually performed by a rotary pump 34, a combination of a turbo molecular pump and a rotary pump 30, 32. 3
0, 32 may be a combination of an oil diffusion pump and a rotary pump. These vacuum pumps are also controlled by the control computer 52, and the evacuation is performed automatically.

The ionized sample drawn into the vacuum through the interface section (sampling cone and skimmer cone) 20 is converged on the mass analysis section 24 by the ion lens 22 controlled by the control computer 52. The ion lens 22 is connected to the ion lens power supply 5.
6 is supplied with a voltage, and it is possible to control the behavior of the flow of ions having electrical properties.

In the mass spectrometer 24 of this embodiment, a three-dimensional quadrupole mass spectrometer (also called an ion trap type mass spectrometer) comprising a ring electrode and a pair of end cap electrodes is used. FIG. 4 shows a structural example of a three-dimensional quadrupole mass spectrometer.

The mass spectrometer 24 of the present invention is not limited to the above-described three-dimensional quadrupole mass spectrometer, but also has a function of temporarily holding incident ions and a function of decomposing and decomposing molecular ions. (So-called MS / MS (MS ² ) function)
Any type can be used as long as it is provided.

The ionized sample introduced into the mass spectrometer 24 is once held in the mass spectrometer 24 and then discharged to the deflection electrode 26 for each mass. The mass spectrometer 24 can take out an ionized sample having an arbitrary mass number by controlling the power supply 58 for driving the mass spectrometer by the control computer 52.

The mass spectrometer 24 contains He (helium) in order to efficiently keep the incident ions.
A gas supply device 60 is provided and is controlled by a control computer 52. The gas supplied here is H
In addition to e gas, Ar gas, nitrogen gas, oxygen gas and the like may be used.

In a normal measurement, the ionized sample separated by the mass analyzer 24 for each mass number is thereafter deflected by 90 degrees by the deflection electrode 26, guided to the ion detector 28 and detected. This signal is digitally amplified by the amplifier 60 using a method called pulse counting, which can detect each incident ion, thereby obtaining data. This amplifier 60 may be an amplifier that amplifies in an analog manner.

Hereinafter, specific measurement examples of the present invention will be described.

FIG. 2 shows the device shown in FIG.
3 shows a mass spectrum obtained when (iron) is measured.

FIG. 2A shows a mass spectrum when the measurement is performed without decomposition / cleavage in the mass spectrometer 24. Fe (iron) has an isotope near a mass number of 56, and several mass spectra can be obtained.

However, when an attempt is made to measure Fe contained in seawater as an example, since a large amount of Ca is contained in seawater, as shown in FIG. Even when only one is introduced, a mass spectrum appears at a mass number of 56. This is because CaO in which Ca having a mass number of 40 is combined with O having a mass number of 16 appears at a mass number of 56. That is, FIG.
The measured spectrum obtained in (a) is a synthetic spectrum of Fe and CaO.

FIG. 2C shows that the mass spectrometer 24
After holding a certain range of mass of ionized sample,
Decomposes and cleaves molecular ions of the retained ionized sample (M
(S / MS). According to this result, it can be seen that the ionic strength of the mass number 56 was reduced due to the decomposition of the CaO ion which is a molecular ion, and data of only Fe ion was obtained.

As described above, when the measurement is performed by ICP / MS, if the mass spectrometer is a general quadrupole mass spectrometer, an accurate value can be obtained due to the background generated by the molecular ion peak. It will be extremely difficult to get However, by having means for resolving and removing the generated molecular ion peak as in the present invention, the detection lower limit is improved and more accurate measurement can be performed.

Further, ICP / MS or MIP / MS
In a plasma ion source mass spectrometer such as
As long as is supplied to the plasma 18 of the torch 10, an ionized sample of the same component is continuously supplied to the mass spectrometer 24. Therefore, if the ionized sample is once held in the mass spectrometry unit 24, and then the detection is performed as it is, and if the background is observed based on the detection result,
The ionized sample is again held in the mass spectrometry unit 24, decomposed and cleaved in that state, and compared with the previous detection result, whereby a decrease in background can be confirmed. That is,
Even if measurement is performed a plurality of times in succession, mass analysis can be performed on a sample in the same state, so that optimal decomposition and cleavage can be performed. On the other hand, in LC / MS and GC (Gas Chromatography) / MS, the sample supplied to the ion source of the mass spectrometer is separated and eluted by LC and GC.
The successive components change. For this reason, the process of further decomposing and decomposing ions while retaining ions in the mass spectrometry unit cannot always be performed on ionized samples of the same component.

FIG. 3 schematically shows the process of decomposition of the molecular ion peak.

FIG. 3A shows, for example, taking FIG. 2 as an example.
This corresponds to the case where the mass number 56 is observed. That is, Fe ions having a mass number of 56, Ca having a mass number of 40, and a mass number of 16
It is impossible to measure only Fe ions because the CaO ions to which O is bonded have the same mass number of 56. On the other hand, in the present invention, the mass spectrometer 2 shown in FIG.
At 4, while holding the ions, the ions are energized by an electric or magnetic field to break molecular bonds. Specifically, the power supply 5 for driving the mass spectrometer
8 is performed by changing the applied voltage. Here, the energy given to the molecule needs to be set to a range that is sufficient to break the bond of the molecule and that does not cause the molecular ion to fly out of the mass spectrometer 24.

By applying energy to the molecular ions in this manner, the molecular ions (CaO ions) are separated into Ca and O as shown in FIG. 3B, and the observed mass numbers are 40 (Ca) and 16 (Ca). Move to (O). As a result, as shown in FIG. 3 (c), the molecular ion is cut off, the mass number is changed, and only the Fe ion having the target mass number of 56 can be measured.

In the above description, the case of CaO has been described.
And Ar bond (ArAr and Se overlap at a mass number of 80)
In any combination where a molecular ion peak occurs as a background, such as a bond between Ar and Cl (chlorine) (ArCl and As overlap at a mass number of 75), the measurement of a target element can be performed by applying the above-described example. It can be performed.

Here, the molecular ions to be cut are not limited to diatomic molecules as described above. In the case of a triatomic molecule, the bond of the molecular ion is cut twice (MS
/ MS / MS (MS ³ )), the same effect can be obtained. The same applies to the case of four or more atomic molecules. The means for eliminating molecular ions from the same mass number does not need to be limited to decomposition / cleavage.
For example, by combining molecular ions with each other or with molecular ions and atomic ions, a molecular ion peak serving as a background can be removed from a target mass number.

Further, the means for cutting molecular ions need not be limited to electric and magnetic fields. For example, a means for decomposing by collision with a neutral atom or a neutral molecule existing in the mass spectrometric unit may be used. Further, a collision reaction with a neutral atom or a neutral molecule existing in the mass spectrometer may cause a binding reaction to remove a background molecular ion peak from a target mass number.

FIG. 4 shows a specific example of the mass analyzer 24. Here, an example of a three-dimensional quadrupole mass spectrometer is shown. As shown in FIG. 4, the three-dimensional quadrupole mass spectrometer includes a pair of end cap electrodes and a ring electrode.

The relationship between the mass number of ions stored in the three-dimensional quadrupole mass spectrometer and the amplitude of the high-frequency voltage is approximately as follows.

M / Z = kV / (r ₀ ² + 2z ₀ ² ) where M is the mass of the ion, Z is the valence of the ion, k is a proportional constant, V is the amplitude of the high-frequency voltage, and r ₀ is three-dimensional. The distance from the center of the quadrupole mass spectrometer to the ring electrode, z ₀ , represents the distance from the center of the three-dimensional quadrupole mass spectrometer to the end cap electrode. Here, in order to normally drive the three-dimensional quadrupole mass spectrometer, the mass number range (M / M
When Z) is up to 2000 (corresponding to the case of a liquid chromatograph / mass spectrometer (LC / MS)), a high frequency having a frequency of 1 MHz and a maximum amplitude of about 7 kV is applied to the ring electrode. At this time, the size of the three-dimensional quadrupole mass spectrometer is such that r ₀ is about 7 mm and L is about 40 to 50 mm.

In the case of a gas chromatograph / mass spectrometer (GC / MS), since the measured mass number range (M / Z) is up to about 650, a gas having a size of r ₀ of about 10 mm is used. ing.

On the other hand, when the function of storing ions in the three-dimensional quadrupole mass spectrometer itself is considered, the larger the size of the three-dimensional quadrupole mass spectrometer, the smaller the amount of ions that can be stored inside. growing. However,
As shown in the above equation, in order to increase r ₀ while maintaining the size of the measured mass number range, the applied amplitude V must also be increased. However, when a high voltage having a maximum amplitude of 7 kV or more is applied, a discharge is generated in a narrow space inside the three-dimensional quadrupole mass spectrometer, so that it is usually practically impossible to apply a higher voltage. . Therefore, LC
It is extremely difficult to make r ₀ larger than about 7 mm for those requiring a measured mass number range of up to about 2000, such as / MS. Therefore, the dimension of r ₀ is 7 to
It is limited to about 8mm.

On the other hand, in the case of a plasma ion source mass spectrometer for mainly measuring elements, the required mass number range may be up to about 250 at the maximum. This means that the maximum mass number of naturally occurring elements does not exceed 250, and any molecules weighing more are decomposed in the plasma and introduced into the three-dimensional quadrupole mass spectrometer. Because it will never be. Thus, the plasma ion source mass spectrometer of the present invention, the maximum amplitude of the applied less 7 kV, and, about 12mm~20mm the r _0, the L from 1:60 to
Use a large ring electrode of about 00 mm. This is a size that sufficiently satisfies the above equation. With a ring electrode of such a size and operating conditions of 7 kV or less, the amount of ions stored in the three-dimensional quadrupole mass spectrometer can be increased, and the effects of improving sensitivity and expanding the dynamic range can be obtained. .

[0046]

As described above, according to the present invention,
The background can be easily removed due to the combination of various components in the measurement sample and the components of the plasma generating gas, and the detection sensitivity and detection accuracy of the element to be measured can be improved, Qualitative analysis in mass spectrometry can be facilitated.
Further, according to the configuration of the present invention, more ions can be stored, so that the sensitivity can be improved and the dynamic range can be expanded.

[Brief description of the drawings]

FIG. 1 is a diagram showing an outline of a device configuration of the present invention.

FIG. 2 is a diagram showing a mass spectrum at the time of iron measurement.

FIG. 3 is a diagram for explaining a state of ion separation.

FIG. 4 is a diagram showing a structure of a three-dimensional quadrupole mass spectrometer.

[Explanation of symbols]

2 sample, 10 torch, 18 plasma, 20 interface, 24 mass spectrometer, 26 deflection electrode,
28 ... Ion detector, 52 ... Control computer.

Continued on the front page (72) Inventor Yasushi Terui 882 Ma, Ichiki, Hitachinaka-shi, Ibaraki Pref.Measurement Division, Hitachi, Ltd. Inside the Measuring Instruments Division (72) Inventor Hidenori Nanba 882-Chair, Oaza-shi, Hitachinaka-shi, Ibaraki Prefecture Inside the Measuring Division, Hitachi Ltd. 5C038 GG09 HH16 JJ02 JJ06

Claims (8)

[Claims] 1. A plasma ion source mass spectrometer comprising: a plasma ion source for ionizing an analysis sample by plasma; and a mass spectrometer for mass spectrometry analysis of the ionized sample. 1. A plasma ion source mass spectrometer comprising an ion holding means for holding ions to be decomposed and decomposing the held ions. 2. The plasma ion source mass spectrometer according to claim 1, wherein the ions held in said ion holding means are decomposed at least twice. 3. The mass spectrometer according to claim 1, wherein said ion holding means comprises a ring-shaped electrode and a pair of hemispherical electrodes. 4. A plasma ion source mass spectrometer comprising: a plasma ion source for ionizing an analysis sample by plasma; and a mass analysis unit for mass analyzing the ionized sample, wherein the mass analysis unit includes a ring electrode. A plasma ion source mass spectrometer, comprising: an ion holding part comprising: a pair of end cap electrodes; and an inner diameter of the ring electrode is 24 mm to 40 mm. 5. The plasma ion source mass spectrometer according to claim 4, wherein the outer diameter of the ring electrode is 60 to 100 mm. 6. The plasma ion source mass spectrometer according to claim 4, wherein the amplitude of the voltage applied to said ring electrode is 7 kV or less. 7. A plasma ion source mass spectrometer having a plasma ion source for ionizing an analysis sample by plasma and a mass analysis unit for mass spectrometry analysis of the ionized sample is provided by an ion holding mechanism provided in the mass analysis unit. The ion holding mechanism includes a ring-shaped electrode and a pair of hemispherical electrodes, and the inner diameter of the ring-shaped electrode is 24 mm.
An ion holding mechanism characterized in that the ion holding mechanism has a size of about 40 mm. 8. The plasma ion source mass spectrometer according to claim 7, wherein a voltage of 7 kV or less is applied to said ring-shaped electrode.