JP4585069B2 - Inductively coupled plasma mass spectrometry apparatus and method - Google Patents

Description

【００２９】
実験例2
次いで、リチウム、イットリウム及びタリウムを含有する溶液について、セシウムのマトリックス濃度を0 ppm及び1000 ppmとしたサンプル溶液を調製し、第1の引き出し電極の電位を4 Vに切り換えてプラズマ電位と実質的に同じとし、第2の引き出し電極に印加される電圧を0から-200 Vの範囲で掃引して、マトリックスがない場合のICP-MS装置の感度と、これに対するマトリックス1000 ppmの溶液についての相対的な検出感度を測定した。結果を表2及び表3に示す。
【００３０】
【表２】

【００３１】
【表３】

【００３２】
表2に示されるように、このモードでは第2の引き出し電極に対して-40〜-60 V程度の負電圧を印加した場合が感度が最も高い。しかしながら、表3から明らかなように、セシウムのマトリックス効果により、第2の引き出し電極に対して-20〜-60 V程度の負電圧を印加した場合、相対感度は0.6〜0.8程度に低下する。
実験例3
そこで第2の引き出し電極に対して印加する電圧として-150 Vを選び、実験例1と同様のサンプル溶液について同様の測定を行った。結果を表4に示す。この場合は、セシウムのマトリックス濃度が高い場合においても、相対感度は0.25程度の振幅内に収まっており、表1に示した場合に対して減感及び増感に対する顕著な効果が見られる。また、例外はあるものの、相対感度の推移も元素ごとにほぼ一定している。
【００３３】
【表４】

【００３４】
【発明の効果】
以上の説明から明らかなように、本発明はICP-MSのインタフェースに印加される電圧の制御を介して、イオン引き出し表面の位置及び形状を制御することを通じて、ICP-MSによる測定性能を改善するための非常に拡張性のある技術を提供するものである。本明細書において用いられた用語及び表現は記載のための術語として使用されたものであって限定のためのものではなく、こうした用語及び表現を用いるについては、図示し記載した特徴又はその部分に均等な何物をも排除する意図はない。むしろ、特許請求の範囲に記載された発明の範囲内において、種々の変更が可能であることが認識されるものである。例えば、実施例においては本発明を、マトリックスイオンとしてセシウムを例にとって特に具体的に説明したが、当業者であれば、測定を行う用途に応じて適切なマトリックスを適宜選択可能であり、このような応用態様は全て本発明の範囲内に包含されるものである。
【図面の簡単な説明】
【図１】本発明を適用可能な例示的なICP-MS装置を概略的に示す説明図である。
【図２】本発明の1つの例示的な実施形態による、第1及び第2の引き出し電極に対する印加電圧の制御を示す例示的な模式図である。
【図３】（a）から（d）は、本発明に用いられる第1及び第2の引き出し電極がとりうる、種々の形態を例示的に示す断面図である。
【符号の説明】
100 ICP-MS装置
110 試料採取システム
120 噴霧段
130 イオン化部
150 インタフェース
151 サンプリングコーン
153 スキマーコーン
156 第1の引き出し電極
157 第2の引き出し電極
160 イオンレンズ領域
170 質量フィルタ領域
180 検出器領域[0001]
BACKGROUND OF THE INVENTION
The present invention relates to inductively coupled plasma mass spectrometry (ICP-MS), and more particularly to an apparatus and method that have excellent controllability for plasma and ion uptake from the plasma and that can reduce fluctuations in analytical sensitivity due to matrix effects.
[0002]
[Prior art]
ICP-MS is a technology that enables analysis at the ultra-trace level (ppt) level for many inorganic elements, mainly metal elements. This technology is capable of analyzing multiple elements at the same time, isotope ratios can be measured, and has many features such as being able to carry out analysis with high accuracy and speed. Widely applied in fields such as science and environment.
[0003]
ICP-MS typically uses inductively coupled plasma (ICP) with argon, where a mass spectrometer (MS) is used to separate and measure the analyte in the ionized sample. Generally, a sample is made into a solution and injected into a nebulizer by a peristaltic pump to form a sample aerosol. The sample aerosol is fed into the ICP through the spray chamber and ionized. The sample ions thus obtained are transported from the atmospheric pressure plasma to the differentially evacuated interface. The interface typically consists of a sampling cone with a large orifice diameter that allows sampling at high temperatures, a skimmer cone and extraction electrodes for ion beam formation, and the formed ion beam is contained within a vacuum chamber. Focused on a mass analyzer located at, for example, a quadrupole mass analyzer. The analyzer separates sample non-analyte ions based on the mass / charge ratio, and the separated sample non-analyte ions are then measured by an electron multiplier detection system.
[0004]
ICP-MS has been found to have higher sensitivity and lower detection limits than traditional elemental analysis techniques such as atomic absorption spectrometry (AAS) and ICP atomic emission spectrometry (ICP-AES), but still There are several problems that must be solved. One of them is non-spectroscopic interference called matrix effect. As in other analysis methods, this is a problem in which the sensitivity to the analyte increases or decreases due to the high concentration of the matrix present in the sample, causing an error and disturbing the analysis. For example, when about 100 to 1000 ppm of cesium is present in the sample as a matrix, the relative sensitivity to an analyte such as lithium, thallium, and yttrium varies in the range of about 0.6 to 1.25 compared to when no matrix is present. In many applications, there is a strong demand to reduce or eliminate such desensitization and sensitization due to the matrix effect.
[0005]
Solutions to these problems include diluting the sample solution or pre-treating the sample to separate it from the matrix, but the analyte concentration may be below the detection limit as a result of dilution. Another problem arises, requiring additional effort. In addition, there is a method of increasing the plasma temperature to relatively reduce the matrix effect, but this causes a problem that the sensitivity is considerably lowered as a whole, and it is necessary to make a trade-off between them.
[0006]
[Problems to be solved by the invention]
One object of the present invention is to provide an improved ICP-MS measurement apparatus and method that can improve matrix desensitization and sensitization by making the sensitivity to elements in a sample adjustable.
[0007]
Another object of the present invention is to provide an improved ICP-MS measurement apparatus and method capable of optimally controlling ion extraction from a plasma by switching a voltage applied to an interface.
[0008]
Another object of the present invention is to provide an improved ICP-MS measurement apparatus and method capable of adjusting the position and shape of a plasma surface through control of a voltage applied to an interface.
[0009]
Still another object of the present invention is to provide a novel interface for an ICP-MS measurement apparatus.
[0010]
Other objects, features and advantages of the present invention will be understood from the following detailed description, and more particularly are set forth in the appended claims.
[0011]
[Means for Solving the Problems]
The present invention discloses an ICP-MS apparatus that includes means for generating a plasma at atmospheric pressure, means for introducing a sample into the plasma to form analyte ions, and the apparatus. Interface means for taking analyte ions from the plasma and ion lens means for transferring the incorporated analyte ions to a mass analyzer for separation and measurement. The pressure of the interface means is usually in the range of 200 to 400 Pa, while the pressure of the mass analyzer stage is at a level of 10 ⁻² to 10 ⁻⁴ Pa during normal operation.
[0012]
The interface means includes at least a skimmer cone, usually with a sampling cone disposed in front of the skimmer cone, and further includes first and second ion extraction electrodes sequentially disposed behind or behind the skimmer cone. According to the present invention, a voltage that can be switched between positive and negative is applied to the first ion extraction electrode, and a negative voltage is applied to the second ion extraction electrode. That is, according to the present invention, the ICP-MS device comprises means for enabling such voltage application and switching, such as positive and negative variable DC power supplies, changeover switches and the like. More specifically, the present invention proposes to apply a positive voltage to the first extraction electrode, which is different from usual.
[0013]
The interface means serves to extract ions created in the atmospheric plasma into a high vacuum region. The sampling cone and the skimmer cone are grounded. Usually, in order to extract ions from the plasma toward the mass analyzer, the first extraction electrode has, for example, about -160 V, and the second extraction electrode has the first A negative voltage higher than the extraction electrode, for example, about -70 V is applied. However, according to the present invention, the voltage applied to the first extraction electrode can be switched positively, which results in surprising results.
[0014]
According to a preferred embodiment of the present invention, means are provided for applying a potential substantially the same as the plasma potential that can be switched between a negative voltage to the first extraction electrode. The plasma has a positive potential on the order of 0 to 10 V. By maintaining the first extraction electrode at a positive potential that is the same as or slightly different from this, the plasma is passed through the skimmer cone into the interface means. It penetrates and is integrated with the first extraction electrode in terms of potential. That is, conceptually, the plasma sticks to the first extraction electrode. In other words, with this configuration, the plasma can enter the interface means, and the position of the plasma surface or the ion extraction surface can be adjusted with respect to the first ion extraction electrode.
[0015]
A negative voltage, typically in the range of about 0 to -200 V, is applied to the second extraction electrode. By adjusting the voltage applied to the second extraction electrode within such a range, the surface shape of the plasma facing the second ion extraction electrode inside the interface means can be adjusted. This makes it possible to control the detection sensitivity for the analyte and to control desensitization or sensitization due to the matrix effect.
[0016]
According to the present invention, in order to obtain the effect of the extraction electrode more effectively, it is preferable that the first ion extraction electrode has a donut disk shape, a cylindrical shape, or a truncated cone shape. In order to more effectively control the plasma surface or the ion extraction surface, the second ion extraction electrode has a frustoconical shape or a cylindrical shape having a peripheral flange and overlaps the inside of the first ion extraction electrode. Are preferably arranged. The peripheral flange is preferably arranged at the rear end of the cylinder, but it may be provided in front of it as long as the second lead electrode can maintain the feature of projecting toward the first lead electrode. For example, the first and second extraction electrodes are both frustoconical and have a good configuration in which they are arranged in a nested manner. In these cases, the distance between the tip of the second ion extraction electrode and the tip of the skimmer cone is preferably within 25 mm from the viewpoint of good control over plasma having a sufficient density.
[0017]
The foregoing description can be understood more readily by reference to the accompanying drawings in conjunction with the following detailed description of embodiments of the invention.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1, an exemplary ICP-MS apparatus to which the present invention is applied is shown schematically generally at 100. As described below, this ICP-MS includes an inductively coupled plasma (ICP) as an ion source, a mass analyzer (MS) for separating an ionized sample with respect to mass, and an interface between the two. Have
[0019]
In the illustrated analyzer, the sample is made into a solution and first supplied by the sampling system 110 to the spray stage 120 that leads to the ICP. That is, the liquid sample 112 is sucked up by the peristaltic pump 111 and sprayed from the nebulizer 121 into the spray chamber 122 using high-pressure argon (Ar) gas to form a sample aerosol. This aerosol is blown into the ionization unit 130 containing ICP. The ionization section including the ICP includes an ICP torch 131 composed of several concentric quartz tubes, and this torch is disposed inside a radio frequency (RF) coil 132. The high frequency magnetic field produced by the RF coil ionizes Ar atoms passing through the torch, forming and maintaining a high temperature plasma. The sample aerosol is blown into the plasma and ionized after passing through a desolvation device if necessary.
[0020]
The interface 150 is maintained at a pressure of typically 200-400 Pa by a conventional rotary pump (RP) so as to separate the atmospheric plasma from the subsequent high vacuum stage containing the mass analyzer. Sample ions are extracted from the plasma through the orifice 152 of the sampling cone 151 to the interface. The sample ions are then transported through the orifice 154 of the skimmer cone 153 to the mass analyzer via the first extraction electrode 156 and the second extraction electrode 157 that form the latter stage of the interface. Reference numeral 158 denotes a vacuum gauge for measuring the interface pressure.
[0021]
The mass analyzer includes an ion lens region 160, a mass filter region 170, and a detector region 180. The ion lens region 160 includes a vacuum chamber containing a series of electrostatic ion lenses and focuses the ion beam toward the mass filter region. These ion lenses can comprise, for example, a series of converging lenses 162 and a steering lens (Ω-type polarizing lens) 163 mounted off the axis with the skimmer orifice. This intermediate vacuum chamber is usually evacuated to a pressure of about 10 ⁻² Pa by a turbo molecular pump (TMP) and a rotary pump. Both the mass filter region 170 and the detector region 180 are provided in a separate vacuum chamber that is separated from this intermediate vacuum chamber by a differential aperture 172. This vacuum chamber is usually evacuated to a pressure of about 10 ⁻⁴ Pa by the second turbomolecular pump. The mass filter region 170 includes a quadrupole mass analyzer 171 composed of four parallel rods, and high frequency and DC voltage are applied to these rods. For a particular combination of applied high frequency voltage and direct current voltage, analyzer 171 passes only ions of a fixed mass / charge ratio to the detector. This makes it possible to separate and measure ions of different elements with a detector, for example, an electron multiplier type detector 181. The ion signal for each mass is amplified and then measured using a multichannel counter. The signal intensity for a given mass (and therefore element) will be directly proportional to the concentration of that element in the sample solution.
[0022]
FIG. 2 shows a preferred embodiment of the interface according to the invention. In this example, the interface includes a sampling cone 151, a skimmer cone 153, a first extraction electrode 156, and a second extraction electrode 157. The sampling cone 151 and the skimmer cone 153 are grounded. The potential applied to the first extraction electrode 156 is switched by a switch SW between a positive potential variable within a range up to about 10 V and a negative potential variable within a range up to about -200 V. The potential applied to the second extraction electrode 157 is a negative potential adjusted in a range of up to about −200 V.
[0023]
In the first mode, a negative voltage of about −160 V, for example, is applied to the first extraction electrode 156, and a negative voltage of about −70 V, for example, is applied to the second extraction electrode 157. In this case, the ion extraction surface PS of the plasma P is located between the orifice of the skimmer cone 153 and the first extraction electrode, and only ions having a positive charge are extracted in a beam shape toward the ion extraction electrodes 156 and 157. To go. Such a mode is conventionally known. In the second mode, which can be switched between the first mode, a positive voltage substantially equal to the potential of the plasma P, usually about 2 to 3 V, is applied to the first extraction electrode 156. Is done. Thereby, the position of the ion extraction surface PS ′ of the plasma P can be adjusted in the vicinity of the first extraction electrode 156, and the plasma P can actually enter the first extraction electrode. On the other hand, in this mode, the second extraction electrode 157 is always negative with respect to the first extraction electrode 156 and is set to an appropriate potential between about -200 V, for example, about -150 V. By controlling the voltage applied to the second extraction electrode 157, the shape of the plasma surface or the ion extraction surface PS ′ that faces the second extraction electrode 157 can be changed to control the detection of the analyte ions. become.
[0024]
For example, the sensitivity of the ICP-MS device to the analyte may vary depending on the value of the voltage applied to the second extraction electrode 157, but the negative voltage applied to the second ion extraction electrode is within a predetermined range, for example, It is possible to sweep between 0 to −200 V, measure the change in sensitivity, and determine the detection voltage accordingly. As this sweep is performed, the effect of the sample matrix on the separation and detection of ions of the analyte is measured, and according to this measurement, for example, the detection voltage can be determined at the place where the matrix effect is least.
[0025]
FIG. 3 is a cross-sectional view showing various electrode shapes that can be used for each of the first and second extraction electrodes. The first ion extraction electrode 156 can take the donut shape of FIG. 3A in addition to the frustoconical shape shown in FIGS. 3B and 2. The second ion extraction electrode 157 is preferably disposed so as to overlap the inside of the first ion extraction electrode 156. In addition to the frustoconical shape of FIG. 2, a peripheral flange as shown in FIG. 3 (d) A cylindrical shape having can be preferably used. However, in the case of FIG. 3 (d), the peripheral flange may be located at a position other than the rear end of the cylinder as long as the forward convex shape can be maintained. As described above, it is extremely preferable that the distance between the tip of the second ion extraction electrode 157 and the tip of the skimmer cone 153 is within 25 mm from the viewpoint of good control over plasma.
[0026]
【Example】
The ICP-MS device model HP4500 available from Yokogawa Analytical Systems, Inc. and Agilent Technologies, Inc. of Palo Alto, California, USA was modified, and as shown in Fig. 2, the switch SW was switched to the first lead electrode. On the other hand, it was made possible to apply 2 to 3 V having the same potential as that of the plasma. The voltage applied to the second extraction electrode can be variably applied within the range of 0 to 200 V.
Example 1
To measure the matrix effect of cesium for solutions containing lithium, vanadium, yttrium, indium, cerium, thallium, bismuth and uranium, the matrix concentrations were 0 ppm, 0.01 ppm, 0.1 ppm, 1 ppm, 10 ppm, respectively. 100 ppm and 1000 ppm sample solutions were prepared. In the normal mode (first extraction electrode potential -160 V, second extraction electrode potential -70 V), relative sensitivity to a 0 ppm solution of cesium was measured, and the matrix concentrations were 100 ppm and 1000 ppm. In the case of, desensitization and sensitization were large, and the relative sensitivity varied within a range of about 0.6 to 1.25. The results are shown in Table 1. Other operating conditions were as follows.
[0027]
ICP high frequency power 1.6 kW
Carrier gas flow rate 1.4 l / min Sampling depth 8 mm
[0028]
[Table 1]

[0029]
Experimental example 2
Next, for a solution containing lithium, yttrium, and thallium, prepare sample solutions with cesium matrix concentrations of 0 ppm and 1000 ppm, and switch the potential of the first extraction electrode to 4 V to substantially equal the plasma potential. The same, sweeping the voltage applied to the second extraction electrode in the range of 0 to -200 V, the sensitivity of the ICP-MS instrument in the absence of matrix, and relative to the 1000 ppm solution of the matrix The detection sensitivity was measured. The results are shown in Table 2 and Table 3.
[0030]
[Table 2]

[0031]
[Table 3]

[0032]
As shown in Table 2, in this mode, the sensitivity is highest when a negative voltage of about −40 to −60 V is applied to the second extraction electrode. However, as is clear from Table 3, when a negative voltage of about −20 to −60 V is applied to the second extraction electrode due to the cesium matrix effect, the relative sensitivity is reduced to about 0.6 to 0.8.
Experimental Example 3
Therefore, −150 V was selected as the voltage to be applied to the second extraction electrode, and the same measurement was performed on the same sample solution as in Experimental Example 1. The results are shown in Table 4. In this case, even when the matrix concentration of cesium is high, the relative sensitivity is within an amplitude of about 0.25, and a remarkable effect on desensitization and sensitization is seen compared to the case shown in Table 1. In addition, although there are exceptions, the transition of relative sensitivity is almost constant for each element.
[0033]
[Table 4]

[0034]
【The invention's effect】
As is apparent from the above description, the present invention improves the measurement performance by ICP-MS by controlling the position and shape of the ion extraction surface through control of the voltage applied to the interface of ICP-MS. It provides a very scalable technology for. The terms and expressions used in this specification are used as descriptive terms and are not intended to be limiting, and the use of such terms and expressions should be noted in the illustrated or described features or portions thereof. There is no intention to exclude anything that is equal. Rather, it will be appreciated that various modifications can be made within the scope of the invention as set forth in the claims. For example, in the examples, the present invention has been specifically described by taking cesium as an example of a matrix ion. All such applications are intended to be included within the scope of the present invention.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram schematically showing an exemplary ICP-MS apparatus to which the present invention is applicable.
FIG. 2 is an exemplary schematic diagram illustrating control of the applied voltage to the first and second extraction electrodes, according to one exemplary embodiment of the present invention.
FIGS. 3A to 3D are cross-sectional views exemplifying various forms that the first and second extraction electrodes used in the present invention can take;
[Explanation of symbols]
100 ICP-MS equipment
110 Sampling system
120 spray stages
130 Ionization section
150 interfaces
151 Sampling cone
153 Skimmer Cone
156 First extraction electrode
157 Second extraction electrode
160 Ion lens area
170 Mass filter area
180 Detector area

Claims (5)

Means for generating a plasma;
Means for introducing a sample into the plasma to form analyte ions;
Interface means including a skimmer cone for taking in ions of the analyte from the plasma;
Ion lens means for transporting incorporated analyte ions; and
Means for separating ions of the transferred analyte;
An inductively coupled plasma mass spectrometer comprising measuring means for detecting separated ions of the analyte,
The interface means includes at least frustoconical first and second ion extraction electrodes sequentially disposed behind the skimmer cone;
A positive voltage is applied to the first ion extraction electrode;
An inductively coupled plasma mass spectrometer, wherein a negative voltage is applied to the second ion extraction electrode. The means for separating the transferred ions of the analyte is a quadrupole mass analyzing means for separating the ions of the transferred analyte,
Voltage before KiTadashi is inductively coupled plasma mass spectrometer according to claim 1, characterized in that any of the following values: 1 0V. An inductively coupled plasma mass spectrometry method comprising:
Generating a plasma; and
Introducing a sample into the plasma to form analyte ions;
Incorporating ions of the analyte from the plasma via interface means;
Transferring the incorporated ions of the analyte through ion lens means;
Separating the transferred ions of the analyte;
Detecting the separated ions of the analyte,
The interface means includes at least frustoconical first and second ion extraction electrodes arranged sequentially,
Applying a first voltage which is a positive voltage to the first ion extraction electrode;
Applying a second voltage lower than the first voltage to the second ion extraction electrode;
A method for inductively coupled plasma mass spectrometry, further comprising: Separating the transferred ions of the analyte using a quadrupole mass spectrometry means to separate the transferred ions of the analyte;
Inductively coupled plasma mass spectrometry method of claim 3, before the voltage of KiTadashi, characterized in that any of the following values: 1 0V. An inductively coupled plasma mass spectrometry method comprising:
Generating a plasma; and
Introducing a sample into the plasma to form analyte ions;
Incorporating ions of the analyte from the plasma via interface means;
Transferring the incorporated ions of the analyte through ion lens means;
Separating the transferred ions of the analyte;
Detecting the separated ions of the analyte,
The interface means includes at least first and second ion extraction electrodes arranged sequentially,
Applying a first voltage to the first ion extraction electrode to cause the plasma to enter the interface means;
An ion extraction surface of the plasma facing the second ion extraction electrode inside the interface means by applying a second negative voltage lower than the first voltage to the second ion extraction electrode Adjusting the shape of the
A method for inductively coupled plasma mass spectrometry, further comprising:

Images