EP0112004A2 - Vorrichtung und Verfahren zur Probeentnahme eines Plasmas in einer Vakuumkammer - Google Patents

Vorrichtung und Verfahren zur Probeentnahme eines Plasmas in einer Vakuumkammer Download PDF

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Publication number
EP0112004A2
EP0112004A2 EP83306372A EP83306372A EP0112004A2 EP 0112004 A2 EP0112004 A2 EP 0112004A2 EP 83306372 A EP83306372 A EP 83306372A EP 83306372 A EP83306372 A EP 83306372A EP 0112004 A2 EP0112004 A2 EP 0112004A2
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EP
European Patent Office
Prior art keywords
plasma
orifice
coil
vacuum chamber
ions
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP83306372A
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English (en)
French (fr)
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EP0112004B1 (de
EP0112004A3 (en
Inventor
Donald James Douglas
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Nordion Inc
Original Assignee
MDS Inc
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Filing date
Publication date
Application filed by MDS Inc filed Critical MDS Inc
Publication of EP0112004A2 publication Critical patent/EP0112004A2/de
Publication of EP0112004A3 publication Critical patent/EP0112004A3/en
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Publication of EP0112004B1 publication Critical patent/EP0112004B1/de
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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J49/00—Particle spectrometers or separator tubes
    • H01J49/02—Details
    • H01J49/04—Arrangements for introducing or extracting samples to be analysed, e.g. vacuum locks; Arrangements for external adjustment of electron- or ion-optical components
    • H01J49/0422—Arrangements for introducing or extracting samples to be analysed, e.g. vacuum locks; Arrangements for external adjustment of electron- or ion-optical components for gaseous samples
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J49/00—Particle spectrometers or separator tubes
    • H01J49/02—Details
    • H01J49/10—Ion sources; Ion guns
    • H01J49/105—Ion sources; Ion guns using high-frequency excitation, e.g. microwave excitation, Inductively Coupled Plasma [ICP]
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H1/00—Generating plasma; Handling plasma
    • H05H1/24—Generating plasma
    • H05H1/46—Generating plasma using applied electromagnetic fields, e.g. high frequency or microwave energy

Definitions

  • This invention relates to method and apparatus for sampling an inductively generated plasma through an orifice into a vacuum chamber, and to method and apparatus for mass analysis using such sampling. The invention will be described with reference to mass analysis.
  • Mass analyzers for detecting and analyzing trace substances require that ions of the substance to be analyzed be introduced into a vacuum chamber containing the mass analyzer. It is often desired to perform elemental analysis, i.e. to detect and measure the relative quantities of individual elements in the trace substance.
  • the trace substance can be reduced to its individual elements by introducing the trace substance into a high temperature plasma, which produces predominantly singly charged ions of the elements.
  • the use of a high temperature plasma as an ion source has a number of well recognized advantages, including the fact that it produces mostly singly charged ions; interference by other elements to the element to be detected is reduced, isotopic information is obtained, and the ionization efficiency of the source is very high so that numerous ions are produced for analysis.
  • the plasma is normally operated at atmospheric pressure; the mass analyzer is located in a vacuum chamber, and therefore a sample of the plasma - must be extracted from the plasma and directed through a small orifice into the vacuum chamber.
  • the plasma is at very high temperature (typically 4,000 degrees K to 10,000 degrees K) and is a relatively good electrical conductor. It is found that when a portion of the hot plasma is directed through a small orifice, an arc-like breakdown occurs between the plasma and the edge of the orifice, destroying the orifice and producing ultraviolet noise which enters the mass analyzer and interferes with the detection of ions. The effect has been called the "pinch" effect by other workers and it greatly limits the utility of the plasma ion source approach.
  • the invention provides method and apparatus for sampling'a plasma through an orifice into a vacuum chamber in which the problem of arcing and generation of ultraviolet noise at the orifice is greatly reduced, and in which the problem of recombination and reaction of ions adjacent the orifice is also reduced.
  • the invention provides apparatus for sampling a plasma into a vacuum chamber comprising:
  • vacuum chamber is intended to mean a chamber in which the pressure is substantially less than atmospheric.
  • the invention provides a method of sampling a plasma into a vacuum chamber comprising:
  • Fig. 1 shows a plasma tube 10 around which is wrapped an electrical induction coil 12.
  • a carrier gas e.g. argon
  • a further stream of the carrier gas is directed from the source 13 through an inner tube 15 within the plasma tube 10 and exists via a flared end 16 just upstream of the coil 12.
  • a sample gas containing the trace substance to be analyzed is supplied in argon from source 17 and is fed into the plasma tube 10 through a thin tube 18 within and coaxial with the tube 15. Thus the sample gas is released into the centre of the plasma to be formed.
  • the coil 12 normally has only a small number of turns (four turns in the embodiment tested) and is supplied with electrical power from an RF power source 20 fed through an impedance matching network 22.
  • the power fed to the coil 12 varies depending on the nature of the plasma required and may range between 200 and 10,000 watts.
  • the energy supplied is at high frequency, typical- . ly 27 MHz.
  • the voltage across the coil 12 is believed to be up to several thousand volts, depending on operating conditions.
  • the plasma generated by this arrangement is indicated at 24 and is at atmospheric pressure.
  • the plasma tube 10 is located adjacent a first orifice plate 26 which defines one end wall of a vacuum chamber 28.
  • Plate 26 is water cooled, by means not shown. Gases from the plasma 24 are sampled through an orifice 30 in the plate 26 into a first vacuum chamber section 32 which is evacuated through duct 34 by a pump 36. The remaining gases from the plasma exit through the space 38 between the plasma tube 10 and the plate 26.
  • the first vacuum chamber section 32 is separated from a second vacuum chamber section 40 by a second orifice plate 42 containing a second orifice 44.
  • the second vacuum chamber section 40 is evacuated by a vacuum pump 46.
  • a mass analyzer indicated at 48.
  • the mass analyzer may be a quadrupole mass spectrometer having rods 50.
  • the plasma tube 10 in Fig. 1 has been shown greatly enlarged with respect to the vacuum chamber.
  • the first vacuum chamber section 32 is typically maintained at a pressure of about 1 torr, and a second vacuum chamber section 40 is typically maintained at a pressure of 10- 5 torr.
  • a portion of the plasma 24 is sampled through the first orifice 30 into the first vacuum chamber section 32. Ions in the plasma are drawn through the first orifice 30 into the first vacuum chamber section 32 by the gas flow through the first orifice 30. The ions are then drawn through the second orifice 44 again by the gas flow through the second orifice 44.
  • the plasma 24 tends to arc through or to the first orifice 30 and sometimes may even arc through or from the first orifice 30 to the second orifice 44.
  • the arcing destroys the orifices and also generates ultraviolet noise which interferes with the analysis of any ions which may enter the mass analyzer 48.
  • ions characteristic of the orifice material may appear in the mass spectrum and interfere with the analysis.
  • the undesired arcing is aggravated when (as in the present case) there is a vacuum chamber 28 on the side of-the-first orifice plate 26 remote from the plasma 24.
  • the increased arcing occurs because the increased flow of gas through the orifice 30 caused by the vacuum tends to remove the cooled layer of gas which would otherwise tend to collect against the outside of the orifice plate 26 and which would provide some electrical insulation against arcing. If the first orifice 30 is made sufficiently small, then the cooled layer 51 of gas overlying the first orifice plate 26 at the first orifice will tend to exist even with vacuum pumping, but with a very small orifice 30, only a small sample of the plasma 24 can be drawn into the first vacuum chamber section 32, reducing the ion signal.
  • first orifice 30 is made very small it more readily tends to melt or clog. If the first orifice 30 is made larger, then the cooled layer 51 of gas overlying the orifice plate 26 becomes thin or vanishes and arcing occurs as indicated.
  • the applicant has discovered after extensive research that the arcing appears to be caused by large peak to peak voltage swings in the plasma itself. Although it is difficult to measure voltages in the plasma generated by a high frequency electrical field (because the probe used for measurement tends to be melted by the plasma and because of undesirable RF pick-up produced by the generating field), a determination has been made that the peak to peak voltage swing in the plasma with the arrangement shown is very large (e.g. of the order of up to 1,000 volts). Having made this determination, the next problem was to determine how this voltage swing was being produced.
  • Fig. 2 shows a circuit for the typical tuning and impedance matching device 22 used to supply RF power to the plasma.
  • the impedance matching device 22 consists of two variable capacitors Cl, C2 connected in series at terminal 52 with the power source 20 connected across capacitor Cl at terminals 52, 54.
  • a terminal 56 at the free end of capacitor C2 is connected to terminal 58 at the upstream end of the coil 12 while the other end 60 of coil 12 is connected to terminal 54.
  • the direction of gas flow through the coil 12 is indicated by arrow 62.
  • the arrangement as shown in Fig. 2 produced the very large voltage swings discovered in the plasma 24.
  • the first test was to connect a ground to terminal 60 immediately at the downstream end of the coil, on the theory that the long lead used from 60 to 54 had inductance which was generating a voltage swing at terminal 60 and that this was contributing to the voltage swing in the plasma. This additional ground reduced the voltage swing to less than half of that originally detected, but a large voltage swing in the plasma remained and still produced arcing.
  • the impedance matching circuit was modified as shown in Fig. 3, so that the former connection between ground and terminal 54 was removed. Instead the coil 12 was tapped at 64 and the tap 64 was grounded. The tap 64 was then moved back and forth along the coil and the peak to peak voltage swings in the plasma 24 were measured for different positions of the tap 64 along the coil 12.
  • the measurements are plotted to form curve 66 in Fig. 4, where the absolute value of the plasma peak to peak voltage swing is shown on the vertical axis and the position of the tap 64 is shown on the horizontal axis.
  • the number "0" indicates the terminal 60 at the downstream or exit end of the coil 12, and the number "4" denotes the terminal 58 at the entrance or upstream end of the coil 12.
  • the numbers "1", "2” and “3” indicate turns 1, 2 and 3 respectively of the coil 12.
  • the center of the coil is located at "2" in Fig. 4.
  • the tap 64 is located downstream of terminal 60, between terminals 54 and 60. It will be seen in Fig. 4 that the absolute value of the peak to peak voltage swing 66 in the plasma decreases as the tap 64 is moved from the downstream end "0" of the coil toward the center "2" of the coil, reaching a minimum at the two turn location. The voltage swing then increases as the tap 64 is moved toward the upstream end "4" of the coil.
  • the voltage at the null point 70 is indicated as being about 13 volts, but it is difficult to measure the voltage accurately to within less than five volts absolute value because of RF pick-up difficulties.
  • Fig. 5 shows on the vertical axis the number of ions travelling through the orifices 30, 44 into the mass analyzer 48, and on the horizontal axis the energies of such ions in electron volts.
  • Curve 72 shown in solid lines and with solid measurement points was produced when the tap 64 was located one-quarter turn from the end "0" of the coil, and curve 74 shown in dotted lines and with outline measurement points resulted when the tap 64 was located at one and three-quarter turns from the end "0" of the coil (i.e. nearly at the center of the coil).
  • curve 72 considerable arcing occurred through the orifice and there was considerable scatter of the observed points, as shown, so a smoothed line was drawn through the points.
  • the energy spread of the ions at 10% height was about 44 electron volts and at 50% height was about 17 electron volts.
  • the maximum energy of a substantial number of the ions exceeded 30 electron volts.
  • Fig. 6 is similar to Fig. 5 but shows curve 76 produced when the tap 64 was located at three-quarters of a turn from the end "0" of the coil and curve 78 produced when the tap 64 was again located one and three-quarter turns from the end "0" of the Y coil.
  • the results are similar to those described previously, i.e. for the tap 64 near the center of the coil, both the energy spread of the ions and the average energy of the ions are much reduced.
  • Figs. 7 and 8 are mass spectra for a ten parts per million solution of the element strontium. The number of ion counts detected is shown on the vertical axis and the mass in atomic mass units (amu) is shown on the horizontal axis.
  • Fig. 7 shows the mass spectrum obtained with the tap 64 located three-quarters of a turn from the downstream end "0" of the coil (as shown for curve 76 in Fig. 6).
  • Fig. 8 shows the mass spectrum obtained when the tap 64 is located one and three-quarter turns from the downstream end "0" of the coil (as shown for curve 78 in Fig. 6).
  • FIG. 9 shows a first orifice plate 26a having a blunt conical orifice structure 88 defined by a conical side wall 89, a flat (i.e. blunt) top wall 90, and an orifice 30a in the top wall 90.
  • blunt top wall 90 tends to produce a cool boundary layer (as shown at 51 in Fig. 1) of gas over the orifice 30a, which boundary layer insulates the orifice from the plasma in order to reduce arcing.
  • Fig. 9 shows an alternative first orifice plate 26b having a sharp edge orifice structure 92 defined by a conical side wall 94 teminating at a sharpe edge 96.
  • the edge 96 defines the first orifice 30b.
  • the Fig. 9 orifice structure results in the reduction or elimination of a cool boundary layer over orifice 30b (even though the plate 26b itself may be cooled), because there is not flat surface adjacent the orifice over which a cooled boundary layer can readily form.
  • the plasma being sampled through orifice 30b is not greatly cooled until after it enters vacuum chamber section 32. Since the pressure in vacuum chamber section 32 is only about one torr (as compared with 760 torr on the outside of orifice plate 26b), the recombination rate is reduced by about 760 3 and the reaction rate by about 760 2 .
  • Figs. 11 and 12 show mass spectra obtained for a ten parts per million solution of cerium.
  • Fig. 11 shows the mass spectrum 98 obtained using the blunt orifice structure 88 of Fig. 9
  • Fig. 12 shows the mass spectrum 100 obtained using the sharp edge orifice structure 92 of Fig. 10.
  • full scale on the vertical axis was 10 6 counts per second. It will be seen that in Fig.
  • the peak at 140 amu (which is the mass of cerium) is extremely small, while a large peak is located at mass 156 (cerium oxide) and a smaller peak (but still larger than the cerium peak) is located at mass 158 (the oxide of an isotope of cerium).
  • Fig. 12 shows a large peak at mass 140 (cerium) and a substantial peak at mass 142 (an isotope of cerium). Only a small peak now appears at mass 156 (cerium oxide), and virtually no peak appears at mass 158.
  • the enormous increase in ion signal for the elemental ions and the corresponding reduction in the quantity of oxides produced greatly improve the ability to decipher the complex spectrum obtained when many elements are mixed together. (For Fig. 12 the resolution was deliberately reduced to ensure that there would be no mass discrimation against the higher mass oxides.)
  • a further advantage of the invention is that it improves the response to elements of high ionization potential.
  • nitric oxide NO
  • the ionization potential of NO is 9.25 electron volts.
  • Metal ions of higher ionization potential in the plasma tended to undergo change transfer reactions with the NO to produce NO+ and neutral metal atoms. The metal atoms, having become neutral, could not be detected by the mass analyzer.
  • ions of higher ionization potential do not lose their charge and hence can be seen by the mass analyzer.
  • Fig. 13 shows relative numbers of ions on the vertical axis on a log scale, and the ionization potential of the elements in electron volts (various elements are marked on the graph) on the horizontal axis.
  • the curve for the prior art method without the use of the invention is shown at 110 and the curve with the invention used is shown at 120.
  • the improvement in ion signal can be by a factor of fifty. For mercury the improvement is even greater.
  • tap 64 is shown as grounded, it may instead be clamped to a different fixed potential, depending on the circuit arrangements provided. Alternatively a variable voltage may be applied to tap 64, so long as the effect is to reduce sufficiently the peak to peak voltage swing in the plasma.
  • the tap 64 may be eliminated entirely and a circuit such as that shown in Fig. 14 may be used.
  • the power supply 20 is connected to terminals 54, 56, i.e. across the two capacitors now indicated as Cl', C2', and the terminal 52 between capacitors Cl', C2' is grounded.
  • Terminals 56, 58 are connected together as are terminals 54, 60, as before.
  • the circuit is carefully balanced so that the capacitance of Cl' and its leads is equal to the capacitance of C2' and its leads, the circuit will be symmetrical and will be equivalent electrically to having a ground centre tap in coil 12.
  • the RF voltage at the centre of coil 12 will remain at or near zero as before.
  • Impedance matching if needed for the Fig. 14 circuit, may be effected by a transformer or other means located between the RF power source 20 and the location in the circuit now shown for the source 20.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Electromagnetism (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Other Investigation Or Analysis Of Materials By Electrical Means (AREA)
  • Electron Tubes For Measurement (AREA)
  • Plasma Technology (AREA)
EP19830306372 1982-12-08 1983-10-20 Vorrichtung und Verfahren zur Probeentnahme eines Plasmas in einer Vakuumkammer Expired EP0112004B1 (de)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
CA000417227A CA1189201A (en) 1982-12-08 1982-12-08 Method and apparatus for sampling a plasma into a vacuum chamber
CA417227 1982-12-08

Publications (3)

Publication Number Publication Date
EP0112004A2 true EP0112004A2 (de) 1984-06-27
EP0112004A3 EP0112004A3 (en) 1985-11-06
EP0112004B1 EP0112004B1 (de) 1989-04-12

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EP19830306372 Expired EP0112004B1 (de) 1982-12-08 1983-10-20 Vorrichtung und Verfahren zur Probeentnahme eines Plasmas in einer Vakuumkammer

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EP (1) EP0112004B1 (de)
JP (1) JPS6016063B2 (de)
CA (1) CA1189201A (de)
DE (1) DE3379617D1 (de)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2577072A1 (fr) * 1985-02-07 1986-08-08 Sherritt Gordon Mines Ltd Spectrometre de masse quadripolaire
EP0199455A2 (de) * 1985-04-24 1986-10-29 MDS Health Group Limited Einführung einer Plasmaprobe in eine Vakuumkammer
US4760253A (en) * 1986-01-31 1988-07-26 Vg Instruments Group Limited Mass spectrometer
WO1991015029A1 (en) * 1990-03-23 1991-10-03 Fisons Plc Plasma mass spectrometer
US5068534A (en) * 1988-06-03 1991-11-26 Vg Instruments Group Limited High resolution plasma mass spectrometer
US5519215A (en) * 1993-03-05 1996-05-21 Anderson; Stephen E. Plasma mass spectrometry

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS62213056A (ja) * 1986-03-14 1987-09-18 Yokogawa Electric Corp 高周波誘導結合プラズマを用いた分析装置
JPS639761U (de) * 1986-07-07 1988-01-22
JPS6326471A (ja) * 1986-07-17 1988-02-04 Yanmar Diesel Engine Co Ltd トラクタ−の油圧制御弁
JPS6355361U (de) * 1986-09-29 1988-04-13

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2831134A (en) * 1953-04-10 1958-04-15 Philips Corp Extraction probe for ion source
FR1471211A (fr) * 1966-01-18 1967-03-03 Aquitaine Petrole Spectromètre de masse

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2831134A (en) * 1953-04-10 1958-04-15 Philips Corp Extraction probe for ion source
FR1471211A (fr) * 1966-01-18 1967-03-03 Aquitaine Petrole Spectromètre de masse

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2577072A1 (fr) * 1985-02-07 1986-08-08 Sherritt Gordon Mines Ltd Spectrometre de masse quadripolaire
EP0199455A2 (de) * 1985-04-24 1986-10-29 MDS Health Group Limited Einführung einer Plasmaprobe in eine Vakuumkammer
EP0199455A3 (en) * 1985-04-24 1987-05-13 Mds Health Group Limited Sampling plasma into a vacuum chamber
US4760253A (en) * 1986-01-31 1988-07-26 Vg Instruments Group Limited Mass spectrometer
US5068534A (en) * 1988-06-03 1991-11-26 Vg Instruments Group Limited High resolution plasma mass spectrometer
WO1991015029A1 (en) * 1990-03-23 1991-10-03 Fisons Plc Plasma mass spectrometer
US5519215A (en) * 1993-03-05 1996-05-21 Anderson; Stephen E. Plasma mass spectrometry

Also Published As

Publication number Publication date
CA1189201A (en) 1985-06-18
EP0112004B1 (de) 1989-04-12
EP0112004A3 (en) 1985-11-06
DE3379617D1 (en) 1989-05-18
JPS6016063B2 (ja) 1985-04-23
JPS59105257A (ja) 1984-06-18

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