JP2001185073A - Apparatus and method of analyzing inductively coupled plasma - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ICP-MS device and method which can control a position and a shape of plasma face and reduce a desensitizing or sensitizing by a matrix effect. SOLUTION: An interface 150 of ICP-MS device 100 includes a first output electrode 156 and a second output electrode arranged in order at the rear side of a skimmer cone 153. Voltage switched between positive and negative voltages of the same extent as plasma is applied to the first ion output electrode, and the negative voltage is applied to the second ion output electrode 157.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to inductively coupled plasma mass spectrometry (ICP-MS). Apparatus and method.

[0002]

2. Description of the Related Art ICP-MS is a technique that enables ultra-trace (ppt) level analysis of many inorganic elements, mainly metal elements. This technology has many features such as the ability to analyze multiple elements simultaneously, the measurement of isotope ratios, and the ability to perform analysis with high accuracy and speed. Widely applied in fields such as science and environment.

[0003] ICP-MS usually uses inductively coupled plasma (ICP) with argon, where a mass spectrometer (MS) is used to separate and measure analytes in the ionized sample.
Is used. Generally, a sample is made into a solution and injected into a nebulizer by a peristaltic pump to form a sample aerosol. Sample aerosol is passed through the spray chamber to IC
It is sent to P and ionized. The sample ions thus obtained are transported from the plasma at atmospheric pressure 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 placed in a vacuum chamber. At a mass analyzer, 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 conventional elemental analysis techniques such as atomic absorption spectrometry (AAS) and ICP atomic emission spectrometry (ICP-AES). However, there are some problems that still need to be solved. One of them is non-spectroscopic interference called the matrix effect. This is a problem that, as in other analysis methods, the high concentration of the matrix present in the sample causes an increase or decrease in sensitivity to the analyte, thereby causing an error and hindering the analysis. For example, when cesium is present as a matrix in the sample at about 100 to 1000 ppm, the relative sensitivity to analytes such as lithium, thallium, and yttrium varies in a range of about 0.6 to 1.25 as compared to the case where no matrix is present. In many applications, there is a strong demand to reduce or eliminate desensitization and sensitization due to such a matrix effect.

[0005] Solutions to these problems include diluting the sample solution or pretreating the sample to separate it from the matrix, but the dilution results in analyte concentrations below the detection limit. Other problems arise, such as the need for additional effort.
There is also a method of relatively reducing the matrix effect by increasing the plasma temperature. However, there is a problem that the sensitivity is considerably lowered as a whole, and it is necessary to make a trade-off between them.

[0006]

An object of the present invention is to provide an improved ICP-MS measuring apparatus capable of improving matrix desensitization and sensitization by adjusting the sensitivity to elements in a sample. And a method.

Another object of the present invention is to provide an improved IC capable of optimally controlling the extraction of ions from a plasma by switching the voltage applied to the interface.
An object of the present invention is to provide a P-MS measuring device and method.

It is another object of the present invention to provide an improved ICP-MS measuring apparatus and method capable of adjusting the position and shape of a plasma surface through control of a voltage applied to an interface.

Yet another object of the present invention is to provide a novel interface for an ICP-MS measuring device.

[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]

SUMMARY OF THE INVENTION The present invention provides an ICP-MS device.
Disclosed is a device for generating an atmospheric pressure plasma.
Means and ions of the analyte by introducing the sample into the plasma
Means for forming the analyte and the ions of the analyte
Interface means for taking in from
To separate and measure the ions of the inserted analyte
An ion lens means for transferring to a mass spectrometer.
Prepare. The pressure of the interface means is usually 200-400Pa
Range, while the mass analyzer stage pressure is
10 ^-2~Ten^-FourAt Pa level.

[0012] The interface means includes at least a skimmer cone, together with a sampling cone usually arranged in front of the skimmer cone, and further includes first and second ion extraction electrodes sequentially arranged behind or after the skimmer cone. According to the present invention, a voltage switchable between positive and negative is applied to the first ion extraction electrode, and
A negative voltage is applied to the 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 an unusually positive voltage to the first extraction electrode.

[0013] The interface means plays a role in extracting ions created in the atmospheric plasma to a high vacuum region. The sampling cone and the skimmer cone are grounded, and usually, for extracting ions from the plasma toward the mass spectrometer, the first extraction electrode has, for example, -160.
A negative voltage of about V and higher than that of the first extraction electrode, for example, about -70 V, is applied to the second extraction electrode. However, according to the invention, the voltage applied to the first extraction electrode can be switched positively, which gives surprising results.

According to a preferred embodiment of the present invention, the first extraction electrode is provided with means for switching between a negative voltage and applying a potential substantially equal to the potential of the plasma. The plasma has a positive potential on the order of 0 to 10 V, and by maintaining the first extraction electrode at a positive potential on the same or slightly different level, the plasma enters the interface means via the skimmer cone. It enters and is electrically integrated with the first extraction electrode. That is, conceptually, the plasma comes to adhere to the first extraction electrode. In other words, such a configuration allows the plasma to enter the interface means and adjust the position of the plasma surface or the ion extraction surface with respect to the first ion extraction electrode.

A negative voltage for the second extraction electrode;
Typically, a voltage in the range of about 0 to -200 V is applied. 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.

According to the present invention, in order to more effectively obtain the effect of the extraction electrode, 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 is frustoconical or cylindrical with a peripheral flange, and overlaps the inside of the first ion extraction electrode. It is preferable that they are arranged in a vertical direction. The peripheral flange is preferably located at the rear end of the cylinder, but may be further forward as long as the feature that the second extraction electrode projects toward the first extraction electrode can be maintained. For example, a configuration in which the first and second extraction electrodes are both frusto-conical and arranged in a nested manner is preferable. In these cases, it is highly preferable that the distance between the tip of the second ion extraction electrode and the tip of the skimmer cone be within 25 mm from the viewpoint of good control of plasma having a sufficient density.

The above description can be understood more easily by referring to the accompanying drawings together with the following detailed description of embodiments of the present invention.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 schematically illustrates an exemplary ICP-MS apparatus to which the present invention is applied, generally at 100. As described below, the ICP-MS uses an inductively coupled plasma (ICP) as an ion source, a mass analyzer (MS) for separating ionized samples with respect to mass, and an interface between the two. Having.

In the analyzer shown, the sample is liquefied and first supplied by a sampling system 110 to a spray stage 120 which leads to the ICP. That is, the liquid sample 112 is sucked up by the peristaltic pump 111 and the nebulizer 1
From high pressure argon (A) into spray chamber 122 from 21
r) Spray with gas to form a sample aerosol. This aerosol is blown into the ionization section 130 containing ICP. The ionization section including the ICP comprises an ICP torch 131 composed of several concentric quartz tubes, which is located inside a radio frequency (RF) coil 132. The high frequency magnetic field created by the RF coil is Ar passing through the torch
Atoms are ionized, forming and maintaining a hot plasma. The sample aerosol is blown into the plasma after passing through a desolvation device if necessary, and is ionized.

The interface 150 is maintained at a pressure of typically 200-400 Pa by a conventional rotary pump (RP) 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 then pass through the orifice 154 of the skimmer cone 153 and form the first extraction electrode 156 and the second extraction electrode 1 forming the later stage of the interface.
Via 57 it is transferred to the mass spectrometer. Reference numeral 158 denotes a vacuum gauge that measures the pressure of the interface.

The mass analyzer comprises an ion lens region 160, a mass filter region 170 and a detector region 180.
The ion lens area 160 includes a vacuum chamber containing a series of electrostatic ion lenses and focuses the ion beam toward the mass filter area. These ion lenses may comprise, for example, a series of focusing lenses 162 and a steering lens (Ω-polarized lens) 163 mounted off-axis from the skimmer orifice. The intermediate vacuum chamber is evacuated to a pressure of usually 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 separate vacuum chambers separated from this intermediate vacuum chamber by a differential aperture 172.
This vacuum chamber is powered by a second turbo-molecular pump.
Usually, it is exhausted to a pressure of about 10 ^-4 Pa. The mass filter region 170 comprises a quadrupole mass analyzer 171 composed of four parallel rods, to which high frequency and DC voltages are applied. For a particular combination of high frequency voltage and DC voltage applied, the 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 by using, for example, a detector that is the electron multiplier tube type detector 181. After the ion signals of each mass are amplified, they are measured using a multi-channel 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.

FIG. 2 shows a preferred embodiment of the interface according to the present 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 a variable positive potential within a range of about 10 V,
It can be switched by a switch SW between a negative potential that is variable up to about V and the potential applied to the second extraction electrode 157 is adjusted within a range up to about -200 V. Negative potential.

In the first mode, the first extraction electrode 156
Is applied with a negative voltage of, for example, about -160 V, and a negative voltage of, for example, about -70 V 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 the form of a beam 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 and 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. As a result, the position of the ion extraction surface PS ′ of the plasma P can be adjusted in the vicinity of the first extraction electrode 156.
Can actually enter the extraction electrode. On the other hand, in this mode, the second extraction electrode 157 is
Is always negative with respect to the extraction electrode 156, and is set to a suitable potential, for example, about -150 V, up to about -200 V.
By controlling the voltage applied to the second extraction electrode 157, it is possible to change the shape of the plasma surface or the ion extraction surface PS 'facing the second extraction electrode 157, and control the detection of analyte ions. become.

For example, although the sensitivity of the ICP-MS apparatus to the analyte can change depending on the value of the voltage applied to the second extraction electrode 157, the negative voltage applied to the second ion extraction electrode is set to a predetermined range. Of these, for example, the voltage is swept between 0 to -200 V, the change in sensitivity due to the sweep is measured, and the detection voltage can be determined accordingly. During this sweep, the effect of the sample matrix on the separation and detection of ions of the analyte is measured, and the detection voltage can be determined in accordance with this measurement, for example, at a place where the matrix effect is least.

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 have a donut shape shown in FIG. 3A, a cylindrical shape shown in FIG. 3C, or the like, in addition to the truncated cone shape shown in FIGS. For the second ion extraction electrode 157, the first ion extraction electrode
It is preferable to be disposed so as to overlap the inside of 156. In addition to the frusto-conical shape shown in FIG. 2, a cylindrical shape having a peripheral flange as shown in FIG. 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 it can maintain a forwardly convex shape. As mentioned above, in terms of good control over the plasma,
(2) The tip of the ion extraction electrode 157 and the skimmer cone 15
It is highly preferred that the spacing between the three tips is within 25 mm.

[0026]

[Example] ICP-MS model HP45 available from Yokogawa Analytical Systems, Inc. and Agilent Technologies, Inc. of Palo Alto, California, USA
As shown in FIG. 2, 00 was modified so that 2-3 V having almost the same potential as the plasma could be applied to the first extraction electrode by switching the switch SW. Further, the voltage applied to the second extraction electrode can be variably applied within the range of 0 to 200 V. Experimental Example 1 For a solution containing lithium, vanadium, yttrium, indium, cerium, thallium, bismuth and uranium, the matrix concentrations were 0 ppm and 0.01 pp, respectively, to measure the matrix effect due to cesium.
Sample solutions of m, 0.1 ppm, 1 ppm, 10 ppm, 100 ppm and 1000 ppm were prepared. In a normal mode (potential of the first extraction electrode -160 V, potential of the second extraction electrode -70 V), the relative sensitivity to a solution containing 0 ppm of cesium was measured, and the matrix concentration was 100 ppm and 1000 pp.
In the case of m, desensitization and sensitization are large, and the relative sensitivity is about 0.6 to 1.
It varied within the range of about 25. Table 1 shows the results.
Other operating conditions were as follows.

ICP high frequency power 1.6 kW Carrier gas flow 1.4 liter / min Sampling depth 8 mm

[0028]

【table 1】

Experimental Example 2 Next, with respect to a solution containing lithium, yttrium and thallium, the cesium matrix concentration was reduced to 0 ppm.
And a sample solution having a concentration of 1000 ppm was prepared, and the potential of the first extraction electrode was switched to 4 V to be substantially the same as the plasma potential, and the voltage applied to the second extraction electrode was set to 0.
Of the ICP-MS instrument in the absence of matrix, swept from -200 V to
The relative detection sensitivity for the ppm solution was determined. The results are shown in Tables 2 and 3.

[0030]

[Table 2]

[0031]

[Table 3]

As shown in Table 2, in this mode, the second
The highest sensitivity is obtained when a negative voltage of about -40 to -60 V is applied to the extraction electrode. However, as is clear from Table 3, due to the matrix effect of cesium,
When a negative voltage of about -20 to -60 V is applied to the second extraction electrode, the relative sensitivity is reduced to about 0.6 to 0.8. Experimental Example 3 Therefore, the voltage applied to the second extraction electrode was
150 V was selected, and the same measurement was performed for the same sample solution as in Experimental Example 1. Table 4 shows the results. In this case, even when the cesium matrix concentration is high, the relative sensitivity is within an amplitude of about 0.25, and a remarkable effect on desensitization and sensitization is seen from the case shown in Table 1. Also, with some exceptions, the change in relative sensitivity is almost constant for each element.

[0033]

[Table 4]

[0034]

As is apparent from the above description, the present invention provides a method for controlling the position and shape of the ion extraction surface by controlling the voltage applied to the interface of the ICP-MS. It provides a very scalable technique for improving measurement performance. The terms and expressions used herein are used as terms for description and not for purposes of limitation, and the use of such terms and expressions is not limited to the illustrated or described features or parts thereof. There is no intention to exclude anything equivalent. Rather, it will be appreciated that various modifications are possible within the scope of the claimed invention. For example, in embodiments, the present invention
Although cesium has been particularly specifically described as an example of a matrix ion, a person skilled in the art can appropriately select an appropriate matrix according to the purpose of performing the measurement, and all such application aspects are within the scope of the present invention. Is included.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram schematically showing an exemplary ICP-MS device to which the present invention can be applied.

FIG. 2 is an exemplary schematic diagram illustrating control of applied voltage to first and second extraction electrodes according to one exemplary embodiment of the present invention.

FIGS. 3A to 3D are cross-sectional views exemplarily showing various forms that can be taken by first and second extraction electrodes used in the present invention.

[Explanation of symbols]

 100 ICP-MS device 110 Sampling system 120 Spray stage 130 Ionization section 150 Interface 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

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kazuo Yamanaka 1-15-5 Nakamachi, Musashino-shi, Tokyo Mitaka Takagi Building Yokogawa Analytical Systems Co., Ltd. F-term (reference) 5C038 GG09 GH11 GH13 GH15 JJ02 JJ11

Claims (9)

[Claims] A means for generating a plasma; a means for introducing a sample into the plasma to form ions of an analyte; and a skimmer for introducing ions of the analyte from the plasma. Inductive coupling comprising interface means including a cone, ion lens means for transporting the introduced analyte ions, and measurement means for separating and detecting the transferred analyte ions. A plasma mass spectrometer, wherein the interface means includes at least first and second ion extraction electrodes sequentially arranged behind the skimmer cone, and switches between the positive and negative ion extraction electrodes. Inductively coupled plasma, wherein a possible voltage is applied and a negative voltage is applied to the second ion extraction electrode The amount analyzer. 2. The inductively coupled plasma mass spectrometer of claim 1, wherein the positively switched voltage is substantially the same as a potential of the plasma. 3. The inductively coupled plasma mass spectrometer according to claim 1, wherein the first ion extraction electrode has a donut disk shape, a cylindrical shape, or a truncated cone shape. 4. The method according to claim 1, wherein the second ion extraction electrode has a truncated conical shape or a cylindrical shape having a peripheral flange, and is disposed inside the first ion extraction electrode. Or 1 inductively coupled plasma mass spectrometer. 5. The inductively coupled plasma mass spectrometer according to claim 1, wherein a distance between a tip of the second ion extraction electrode and a tip of the skimmer cone is within 25 mm. 6. An inductively coupled plasma mass spectrometry method, comprising: generating a plasma; introducing a sample into the plasma to form ions of the analyte; and interfacing the ions of the analyte with the interface. Taking the ions of the analyte through the ion lens means, and separating and detecting the transferred ions of the analyte. Wherein the interface means includes at least a first and a second ion extraction electrode sequentially arranged, wherein applying a first voltage substantially equal to the plasma to the first ion extraction electrode; and Applying a second voltage lower than the first voltage to the second ion extraction electrode. Wherein, inductively coupled plasma mass spectrometry. 7. An inductively coupled plasma mass spectrometry method, comprising: generating a plasma; introducing a sample into the plasma to form ions of an analyte; and interfacing the ions of the analyte with ions. Taking the ions of the analyte through the ion lens means, and separating and detecting the transferred ions of the analyte. Wherein the interface means includes at least a first and a second ion extraction electrode arranged sequentially, applying a first voltage to the first ion extraction electrode,
Allowing the plasma to enter the interface means and adjusting its position with respect to the first ion extraction electrode, and applying a second voltage lower than the first voltage to the second ion extraction electrode. Adjusting the surface shape of said plasma facing said second ion extraction electrode inside said interface means. 8. The inductively coupled plasma mass spectrometry method according to claim 7, wherein the adjustment of the position is performed so as to electrically integrate the first ion extraction electrode with the plasma inside the interface means. 9. The method further comprising: sweeping the second voltage applied to the second ion extraction electrode within a predetermined range, and measuring a matrix effect on separation and detection of ions of the analyte, According to this measurement,
9. The inductively coupled plasma mass spectrometry method according to claim 7, wherein the voltage of (2) is determined.