JP2001110354A - Quadrupole type plasma ion source mass spectroscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide quadrupole type plasma ion source mass spectroscope that secures detecting precision of trace elements by reducing background noise contained in the detected signal and enhances the detection sensitivity. SOLUTION: In this invention, post-rod of a quadrupole electrode is disposed at downstream of the main rod of the quadrupole electrode, applies a voltage mainly of a HF component with DC component kept less than 30 V, and the diameter of an aperture disposed between a detector and the post-rod is reduced to less than 1/2 of the distance between mutually facing electrodes of the main rod. Furthermore, the aperture is extended toward the post-rod side so as to make the length of the aperture to be larger than its diameter, and a negative potential is applied to the aperture.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plasma ion source mass spectrometer for identifying and quantifying trace impurities in a sample. The plasma ion source mass spectrometer includes an inductively coupled plasma mass spectrometer (referred to as ICP-MS) and a microwave induction plasma mass spectrometer (referred to as MIP-MS).

[0002]

2. Description of the Related Art The main structure of a conventional ICP mass spectrometer will be described with reference to FIG. In FIG. 6, reference numeral 3 denotes a plasma generator. The sample 1 to be analyzed is atomized by a nebulizer 2 and introduced into the plasma generated by the plasma generator 3 to be ionized. 4 is a sampling cone,
5 is a skimmer cone. The sampling cone 4 has a shape with a hole with a diameter of 0.8 to 1.2 mm at the tip of an obtuse cone, and the skimmer cone 5 has a diameter of 0.3 at the tip of an acute cone.
It has a shape with a hole of 0.6mm from. The sampling interface includes a sampling cone 4 and a skimmer cone 5. During the analysis, the space between the sampling cone 4 and the skimmer cone 5 is evacuated to about 1 Torr by a vacuum pump (not shown) (a rotary pump is generally used). 7 is an ion lens system, 8
Denotes a mass filter, 9 denotes a detector, and 12 denotes a data processing unit. The installed room detector 9 and the installed room ion lens system 7, another two vacuum pumps 6 _and 62 are evacuated by, ^10-4 in the installed room ion lens system 7 A vacuum of about 10 ⁻⁶ Torr is maintained at about 10 Torr in the room where the detector 9 is installed. Note these vacuum pumps _{61 and} 62 _2, a turbo molecular pump and an oil diffusion pump is generally used.

The sample ionized by the plasma P passes through the holes of the sampling cone 4 and the skimmer cone 5 together with the light of the plasma P and reaches the ion lens system 7. The ion lens system 7 functions to guide only ions of the arrived ions and light to the mass filter 8. The detector 9 at the subsequent stage uses a channeltron or a secondary electron multiplier, and has a function of shielding strong plasma light because it detects ultraviolet rays together with ions. Specifically, a stopper (shielding plate) is arranged on a central axis that penetrates the holes of the sampling cone 4 and the skimmer cone 5,
Bending the ion trajectory blocks the light component that goes straight. The mass filter 8 has a function of passing only ions having a predetermined mass out of the ions that have reached the mass filter 8. As the mass filter 8, for example, a quadrupole mass spectrometer is used. The detector 9 is a mass filter 8
Then, the detected ions are sent to the data processing unit 12 as electric signals. The data processing unit 12 calculates the mass of the ion from the value of the setting of the mass filter 8 when detected by the detector 9 and identifies the ion species. Then, the data processing unit 12 calculates the concentration of the ion identified from the detection intensity of the detector 9, that is, the impurity concentration in the sample.

In a plasma ion source mass spectrometer using a quadrupole mass spectrometer as the mass filter 8,
The ions generated by the plasma generator 3 are introduced into the room in which the ion lens system 7 is evacuated after passing through the holes of the sampling cone 4 and the skimmer cone 5, and further drawn into the quadrupole mass spectrometer. The ions are separated for each mass-to-charge ratio and reach the detector 9 and are detected. This quadrupole mass spectrometer is provided with four hyperbolic or cylindrical rod electrodes arranged parallel to each other as shown in FIG. 5, and ± (U + V
A superimposed DC voltage and high-frequency voltage (cos ωt) is applied. Electric fields in the x-axis direction and the y-axis direction are formed in the space between the electrodes by the voltages applied to the two pairs of orthogonal electrodes, and ions introduced from the ion source into this space from the z-axis direction are in the x-axis and y-axis directions. Receive the power of Voltage (U,
V) Only ions having a specific mass-to-charge ratio (m / e) under the conditions of the distance between the quadrupole electrodes and the frequency oscillate in the x-axis and y-axis directions inside the quadrupole. It can pass between the electrodes in the z-axis direction. Ions having other mass-to-charge ratios (m / e) increase in amplitude and are caught by the quadrupole or discharged outside from between the quadrupoles, and do not reach the detector 9. Since the mass filter 8 operates in this way, the spectrum can be obtained by detecting for each ion having the specified mass-to-charge ratio (m / e) by sequentially changing the applied voltage (U, V). However, some ions other than the specified mass-to-charge ratio (m / e) are included as noise slightly in the ions reaching the detector 9 and detected, and this is generally called background noise.
Incidentally, the number of ions drawn into this quadrupole mass spectrometer is generally on the order of 10 ¹² / sec or more, but ions other than the type specified in the detector reach about 10 / sec and are detected. That is the situation.

[0005]

SUMMARY OF THE INVENTION The object of the present invention is to provide:
An object of the present invention is to solve the above problems, that is, to provide a plasma ion source mass spectrometer that reduces background noise and ensures the accuracy of detection of trace elements.

[0006]

According to the present invention, a quadrupole post rod is provided downstream of a quadrupole main rod, and the post rod has a high frequency component mainly and a direct current component of 30 V or less. A voltage is applied, and the opening diameter of an aperture disposed between the detector and the post rod is reduced to １ ／ or less of the distance between opposed electrodes of the main rod. Further, the aperture length of the aperture is extended toward the post rod side so as to be equal to or longer than the aperture diameter, and a negative potential is applied to the aperture.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention detects only ions of mass and charge corresponding to the state of the mass filter 8 by sampling cations of a sample mixed in the plasma with a plasma torch of a plasma generator 3. In the plasma ion source mass spectrometer that obtains a mass spectrum by detecting ions of different masses and charges by sequentially changing the state of the mass filter 8, a phenomenon in a downstream region of a quadrupole main rod is focused. The phenomenon that is a problem of the present invention will be described with reference to FIG. It is well known that a quadrupole mass spectrometer has a phenomenon called background noise, and it is said that a signal three times its standard deviation is the limit of detection. Therefore, reducing the background noise is directly linked to improving the detection limit of the quadrupole mass spectrometer. The applied voltage (U,
V), only ions having a specific mass-to-charge ratio (m / e) under the conditions of the distance between the quadrupole electrodes and the frequency, vibrate in the x-axis and y-axis directions, and In the z-axis direction, and is emitted to the detection unit via the opening hole of the exit aperture 83. The ions passing through the exit aperture 83 receive a repulsive force from the repeller electrode 91 having a positive potential, are led to a detector 9 such as a secondary electron multiplier, and are detected. Upon detection by the detector 9, there is a phenomenon that ions having a specific mass-to-charge ratio (m / e) are mixed in the detected ions, and the mixed ions are generally referred to as background noise. I have. The ions introduced into the space between the quadrupole electrodes 81 travel in the z-axis direction while vibrating in the x- and y-axis directions due to the electric field in the space, and have a specific mass-to-charge ratio (m / e). Ions that have not been increased in amplitude and are caught by the quadrupole electrode 81 or are emitted to the outside from the gap between the quadrupole electrodes 81, and should theoretically not reach the detection unit 9, but actually, they are extremely small. A small amount of the ions pass through the distance between the quadrupole electrodes and reach the detection unit 9. And this phenomenon gives a limit to the analysis accuracy. The present applicant focused on this phenomenon and repeated observations while changing various conditions.When ions entered between the quadrupole electrodes 81, the background component independent of the voltage / frequency value applied to the quadrupole was observed. Is always detected by the detector 9, and the amount thereof is found to be larger as the opening diameter of the exit aperture 83 is larger.

[0008] Based on the above findings, it is conceivable to first reduce the opening diameter of the exit aperture 83. If this is done, of the ions passing between the quadrupole electrodes 81 in the z-axis direction,
The number of ions that cannot pass through the opening and are shielded increases, resulting in a weak detection signal. That is, since the signal component is reduced together with the background noise, the S / N ratio is not necessarily improved. It is necessary to devise a technique for allowing the ions passing between the quadrupole electrodes 81 in the z-axis direction to pass through the small opening of the exit aperture 83. Therefore, the present applicant arranges a post rod 82 adjacent to the downstream side of the quadrupole main rod electrode, and holds down the DC component to the post rod 82 so that only the high frequency component is applied. We have invented that all cations are focused on the z-axis regardless of the mass-to-charge ratio (m / e). As a result, ions passing between the quadrupole electrodes 81 in the z-axis direction are converged to the vicinity of the z-axis, and efficiently pass through the exit aperture 83 having a small opening. As shown in FIG. 1, the post rod 82 has the same quadrupole structure as the main rod 81 and is arranged on the extension of the main rod 81, but a gap is provided between the two and the DC component is electrically insulated. ing.

As described above, a voltage whose main component is a high frequency component and whose DC component is 30 V or less is applied to the post rod 82. When such a voltage is applied to the quadrupole, the vibration amplitude of the ions is increased. Becomes smaller and the ion beam becomes z
The phenomenon of convergence near the axis will be described. In this type of quadrupole mass spectrometer, in principle, ions are applied to the quadrupole electrode by z
It is known that when incident from the axial direction, the movement of ions in the x-axis and y-axis directions is represented by an equation called Mathiew equation. As shown in FIG. 2, the region of the stable solution has a triangular shape in which the vertical axis is α and the horizontal axis is q, and the horizontal axis is the base. Where α = 8 eU / mr ₀ ·
ω ² , q = 4 eV / mr ₀ · ω ^2, where U is a DC voltage, V is the peak value of a high frequency, e is the charge of an electron, m is the mass of an ion, r ₀ is half the distance between opposing electrodes, and ω
Is the angular velocity of the high frequency. An ion with a certain mass is r
_When the values of ₀ , ω, U, and V are determined, one point on the (α, q) plane is determined. The relationship of α / 2q = U / V is given from the definition of α, q. This equation is a straight line that passes through the origin on the (α, q) plane and whose gradient is determined by the values of U and V. Is determined independently of Once this U / V ratio is determined, all ions of different masses will be on this line. This is called a mass scanning line, and among all the ion groups arranged on the mass scanning line, only ions entering the stable region can pass through the quadrupole electric field in the z-axis direction. The primary function of the mass filter is to allow only certain ions to pass and other ions to oscillate in the x-axis and y-axis directions and not pass in the z-direction. The value is increased and the slope of the mass scan line is increased to pass near the apex of the triangular stable region so as to cause separation from other ions. The ion passing through the axis is z
Since it is desired to converge near the axis, the voltage applied to the quadrupole electrode keeps the DC voltage value U low so that an electric field near the bottom of the stable region is created. That is, α
= 8 eU / mr ₀ · ω ² , q = 4 eV / mr ₀ · ω
^{From the} relationship ^{2, the} (α, q) plane is e / mr _{0 for} both axes.
Because it is common for · ω ² becomes parameter,
It can correspond to the (U, V) plane as it is, and lowering the U value means an electric field in which many different mass ions are included in the stable region. The amplitude of the vibration of the ions received by this electric field becomes small, and all the ions converge near the z-axis.

Therefore, in the present invention, as shown in FIG. 1, a post rod 82 for applying a DC voltage of 30 V or less is provided downstream of the main rod 81, and the opening diameter of the exit aperture 83 is adjusted to the distance between the opposed electrodes. half was conceived to be a r ₀ or less. Further, in order to allow ions separated by the main rod and passing in the z-axis direction to reach the detector efficiently, a negative potential is applied to the outlet aperture to positively extract positive ions. The draw-out effect is further improved by extending the opening toward the post rod and increasing the passage distance. Although the background noise phenomenon has not been completely elucidated at present, the effect of the present invention as a solution has not been sufficiently analyzed, but the effect has been confirmed experimentally. Thus, the configuration of the present invention has been achieved.

[0011]

Embodiment 1 As shown in FIG. 3, in a conventional quadrupole plasma ion source mass spectrometer, a quadrupole post rod 82 is disposed downstream of a quadrupole main rod 81 with a gap therebetween. The main rod and the post rod are connected by a capacitor C for each corresponding electrode, so that the DC component is cut and only the high frequency component is applied to the post rod 82. The post rod is connected to a low voltage DC power supply 20 via a 5 MΩ resistor R to apply a low DC voltage. Exit aperture 83 one third of the distance between the electrodes 2r ₀ opening diameter
(2 to 3 mm), the post rod side end face is formed in a conical shape, and the length of the opening is made larger than the opening diameter to be 5 to 6 mm. The outlet aperture 83 has a DC negative voltage source 21.
From -500 V is applied.

[0012]

According to the present invention, a quadrupole post rod is provided downstream of a quadrupole main rod, and a high-frequency component is mainly applied to the post rod, and a voltage having a DC component of 30 V or less is applied. Since the plasma ion source mass spectrometer is configured to be separated, ions separated by the quadrupole main rod and passed in the z-axis direction are converged to the vicinity of the z-axis by the action of the electric field by the quadrupole post rod. The ions can be efficiently introduced into the detection unit through the opening of the exit aperture. In addition, the background noise can be reduced by setting the opening diameter of the exit aperture arranged between the detector and the post rod to be の or less of the distance between the opposing electrodes of the main rod. Combined with the electric field effect of the pole post rod, the effect of the background noise, which determines the detection limit of the device, was reduced, and the detection sensitivity was improved. Thus, it was possible to detect lower-concentration impurities. In the prior art, the barium intensity of 1 ppb was 1 × 10 ⁵ cps and the background noise was about 10 cps. However, according to the present invention, the barium intensity was the same as the conventional one and the background noise was 3 cps.
It became less than cps. Further, by extending the opening length of the aperture toward the post rod side so as to be equal to or larger than the opening diameter, by applying a negative potential to the aperture,
The ion detection efficiency can be further improved, and the function of detecting low-concentration impurities has been improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration around a detector from a downstream part of a mass filter which is a main part of the present invention.

FIG. 2 is a mass scanning line and a stability diagram of a quadrupole mass spectrometer.

FIG. 3 is a diagram showing a configuration around a post rod in the embodiment.

FIG. 4 is a diagram showing a configuration around a detector from the downstream of a conventional mass filter.

FIG. 5 is a diagram showing a quadrupole electrode structure and an applied voltage.

FIG. 6 is a diagram showing a main configuration of a conventional ICP mass spectrometer.

[Explanation of symbols]

1 sample 8 mass filter 2 nebulizer 81 main rod 3 plasma generator 82 post rod 4 sampling cone 83 outlet aperture 5 skimmer cone 9 detector 6 _{1, 6 2} vacuum pump 91 repeller electrode 7 ion lens system 10 extraction electrode 20 a DC low voltage Source 12 Data processing unit 21 Negative voltage source

Claims (2)

[Claims] 1. A quadrupole post rod is disposed downstream of a quadrupole main rod, and a high-frequency component is mainly applied to the post rod, and a DC component having a voltage of 30 V or less is applied to the post rod. A mass spectrometer for a plasma ion source, wherein an opening diameter of an exit aperture arranged between the post rod and a detector is set to be equal to or less than half a distance between opposed electrodes of a main rod. 2. The plasma ion source mass spectrometer according to claim 1, wherein the length of the opening of the outlet aperture is extended toward the post rod so as to be longer than the opening diameter, and a negative potential is applied to the aperture.