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Enrichment of rare gas isotopes using a quadrupole mass spectrometer
Vacuum/volume 34lnumber Printed in Great Britain
Enrichment quadrupole
5lpages 581 to 58411984
0042-207X/84$3.00 + .OO @ 1984 Pergamon Press Ltd
of rare gas isotopes mass spectrometer*
using a
C H Chen. I? D Willist and G S Hurst, Chemical Physics Research Section, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, 7ennessee 37830, USA received 12 July 199.5; in revised form 9 August 1993
A small quadrupole mass spectrometer was operated in the static mode to enrich selected rare gas isotopes. Memory effects in the apparatus were observed and attributed to the re-emission of atoms implanted by the electron-impact ion source. Studies of the pumping mechanism led us to a practical means for reducing the rate of noble gas pumping
1. Introduction The static magnetic deflection mass spectrometer has been a most powerful tool for detecting small quantities of rare gas isotopes. Such a capability is important, for example, in analyzing rare gases in lunar samples and meteoritesle3. The ultimate sensitivity of such instruments is limited by memory effects and detector dark current. In the late 197Os,Hurst et u14*’demonstrated the ability to detect single alkali atoms using the laser technique of Resonance Ionization Spectroscopy (RIS). More recently, several groups6 have used RIS in conjunction with a magnetic deflection mass spectrometer to detect low levels of various transition metal atoms and to remove isobaric interferences. Using a laser resonance ionization source and a quadNpole mass filter for mass selection, Chen et aI’*s were able to detect a small number of rare gas atoms. The use of laser resonance ionization sources eliminates the pumping effects generally associated with conventional electron-impact ionization sources. Pulsed laser sources for ionization have the added advantage that time correlation can be used to gate the detector output, thus minimizing the dark current contribution. Applications of resonance ionization mass spectroscopy include solar neutrino flux measurements (by counting *‘Kr), dating of the polar ice sheets and old groundwater (“Kr), and determining rates of ocean mixing and circulation (“Ar). Because the expected *lKr and “Ar concentrations are so low in these samples (*lKr/total Kr < lo-l2 and 3gAr/total Arc lo- “). a large amount of sample is required in order to obtain a few thousand *‘Kr or 39Ar atoms. Because of space-charge effects in the ionization region, however, the number of ions is limited to about 10’ per pulse. It is therefore necessary to pre-enrich these
l Research sponsored in part by the Office of Health and Environmental Research, U.S. Department of Energy under contract W-74OS-eng-26 with the Union Carbide Corporation; in part under contract EV10664 with the University of California, San Diego; and in part by the Swiss Nationale Genossenschaft fur die Lagerung radioaktiver Abfalle (NAGRA). t Scripps Institution of Oceanography, La Jolla, California.
samples before their introduction into the resonance ionization mass spectrometer. Memory effects due to implantation of fast ions, and the subsequent reemission as neutral atoms, are known to occur in deflection-type mass spectrometers which transport ions of several keV of energy. In order to reduce this source of memory, a quadrupole mass spectrometer (in which ion energies are typically 30 eV or less) was chosen for the pre-enrichment work. Some memory effects were, nevertheless, observed. We attempted to discern the cause of the memory effects and our conclusions are discussed below.
2. Experimental A commercial quadrupole mass spectrometer (Extranuclear Laboratories, Inc.) was employed for this work. The electronimpact ionizer shown in Figure 1 is of axial geometry with a S-ml tungsten filament surrounding a cylindrical basket-type grid. These are enclosed by a solid stainless steel shield normally connected to the negative filament lead. Ions are extracted from the source by a series of extraction and focusing lenses. Ions of the selected mass are transmitted through the mass filter and collected by a CuBe electron multiplier or stainless steel Faraday plate. By applying a negative potential of several kilovolts to either collector, one can selectively implant atoms of a desired isotope to achieve enrichment. The mass spectrometer is housed in an electropolished, stainless steel vacuum chamber. The system is roughed down to a few microns by a cryosorption pump, and post-bakeout pressures of lo-” torr are routinely obtained with a cryogenic pump. The system is typically baked at 250’ for 48 h prior to sample introduction. Samples to be enriched were admitted to the apparatus (with the cryogenic pump valve closed) through a precision leak valve. When the desired quantity of gas had been admitted (1O-g-1O-s torr in the 2 1 volume), the leak valve was closed and the mass spectrometer turned on. Vacuum was a
581
C H Chen. R D Willis and G S Hurst:
Enrichment of rare gas isotopes MASS 78 VS TIME
AND
MASS 80
SIGNALS
IN STATIC MASS SPECTROMETER
FILAME SHIELD
MASS 90,
NOT IMPLANTED
l
30 V SOURCE o 90 v SOURCE
\-~
FIL SHIELD a 8
IO*
MASS 78,
4.: _
o
IMPLANTED
AT 2.5
keV
. 30 V SOURCE o 9OV SOURCE
A
A A
A A .
Figure 1. Schematic of ton source (Extranuclear Laboratories). Ions are extracted from the source by an extraction voltage which is a few volts less than I’_,.
. . A A
maintained during static operation by means of an SAES getter pump which had no measurable pumping speed for noble gases.
3. Results and discussion Krypton from Matheson was chosen for convenience for the enrichment experiments. By tuning the mass spectrometer to mass 78 and implanting the transmitted ions into the Faraday collector, one can selectively reduce the population of “Kr atoms in the gas phase. Thus, the 78Kr/80Kr ratio in the gas phase is observed to decrease with time as seen in Figure 2. Note that the *OKr signal decreases somewhat even though the transmission of *‘Kr ions through the mass filter is negligible. No loss of krypton was observed when the ion source was turned off, indicating that physical adsorption of thermal krypton atoms on the vacuum walls is negligible at room temperature. The implantation of lowenergy “Kr ions in the quadrupole rods was also ruled out. Hence, the mechanism to account for the loss of ‘OKr atoms must be related to the production of low-energy ions in the source itself. The ion source shown in Figure 1 can pump noble gas atoms via two mechanisms. (1) Some fraction of those ions produced between the filament and grid basket can be accelerated towards the negative biased filament shield and implanted into the surface with energies of 100 eV or less. Data by Kornelsen9 and others” indicate small but finite trapping efficiencies for low-energy ions in metal surfaces. (2)The same ions, rather than going to the filament shield, might be more strongly attracted to the nearby filament. The majority of ions striking the filament will be reflected as energetic neutrals having a large fraction of the energy of the original ions”. Some of these reflected fast neutrals will subsequently be implanted into surrounding surfaces. In order to determine the dominant pumping mechanisms, we modified the source to allow the filament shield to be biased 582
I
too D
100
I 200
I
I
I
I
300 400 TlME (min)
500
600
Figure 2. Enrichment of a°Kr in the gas phase by implanting “Kr into a stainless steel plate biased at -2.5 kV. Note that some loss of “Kr is observed even though this isotope is not transmitted by the mass filter. Enrichments were done at electron energies of 30 and 90eV. The spectrometer throughputs for krypton were 3.7 x 10m3 A torr-’ and 1.6 x 10W3A torr-‘, respectively. The volume of the static spectrometer was approximately 2 1.
independently of the filament. The pumping speed of the source was then measured as a function of the shield potential k’,(while keeping the throughput of the mass spectrometer fixed) and was found to be independent of V,.Since V,determines the energy of the ions arriving at the shield, and since the trapping efficiency is a strongly increasing function of ion energy below 100 eVg, we conclude that low-energy ion implantation is not the dominant pumping mechanism in our source. hence, the loss of rare gas atoms is mainly due to the implantation of energetic neutrals produced by charge-exchange scattering of low-energy ions at the filament. Such a mechanism was first proposed by Jepsen” and was recently invoked to explain the pumping of rare gas atoms in an electron-impact source of much simpler design’ 3. The ability to enrich krypton samples was investigated at electron energies of 30 and 90 eV. (The filament and filament shield were both at the same potential and the electron energy was determined by the potential difference between the filament and the grid.) The results of the two trials are displayed in Figure 3 as the ratio of ‘*Kr/*‘Kr vs time where mass 78 is the implanted species. At 30 eV the isotope ratio was reduced a factor of loo0 after 24 h relative to the natural abundance ratio. At 90 eV the enrichment factor was about one order of magnitude smaller for
C H Chen, R D Willis and G S Hurst:
Enrichment of rare gas isotopes
ENRICHMENT: RATIO OF MASS 78 TO MASS 80 WHILE IMPLANTING MASS 78 1.0 t
I
I
I
I
-
0 90 VOLTS
-
0
I
the same time period, in agreement with the detailed mechanism discussed elsewhere”. Additional enrichment results are shown in Figure 4 for several rare gases. For each species the outgassing rates for the isotopes of interest were measured and the source pumping rates and collector implantation rates were estimated by solving the following pair of coupled differential equations:
I
30 VOLTS
dx 5
dy
0
z =
0 0
. l
0
200
300 400 TIME lminl
i
500
600
700
Figure3. The ratio ~f’~Kr, “‘Kr in thegas phase as function of time for the data shown in Figure 2. An additional data point measured after 24 h showed the ratio mass 78/mass 80 to be 10e4.
100 _
k;-k,y(r),
Values of k, deducted from the data in Figure 4 for 3 keV ions implanted into a stainless steel collector were 0.0077 min-r, 0.012 min- ‘, and 0.015 min- ’ for argon, krypton and xenon, respectively, in our 2.5 1 volume. These rates are consistent with near-unity trapping efficiency at the collector within the combined accuracy of our volume and throughput measurements ( f 30%). The source pumping rates for the data shown in Figure 4 were about 30% of the corresponding implantation rates but, in general, were very sensitive to the cleanliness of the source.
FACTOROF 403 ENRICHMENT AFTER 24ha \
400
(la)
k,x(t) - k,x(t)
x(t)=gas-phase population at time r of the isotope which is purposely implanted after mass analysis; y(t)=population at time t of the unimplanted isotope; k,,, k;=outgassing rates (atoms min-‘) of the implanted and unimplanted isotopes, respectively; k,= source pumping rate (min- r)-assumed same for all species; k, = implantation rate at the collector (min- 1).
.
9
k,-
where
l
o.oo,
=
I
I
I
I
F
.L
Xe-129/Xe-131 .
. t
.
.
.
Kr-84/Kr-86
0
0
O 0 0
0
0
0
.
0.1 0
0
0
Ar-36/A?-40 .
I
I
40
80
. I
I
. I
120
f60
200
TIME
. 240
I 280
(min)
Figure 4. Typical enrichment data showing selected isotope ratios (in the gas phase) vs time for argon. krypton and xenon samples. In each case the mass filter was tuned IO the lighter isotope.
.
C H Chen, R D Willis and G S Hurst: Enrichment
of rare gas isotopes
The re-emission of rare gas atoms pumped by the ionization source represents the major source of memory in our apparatus and ultimately limits the amount of enrichment that can be achieved. Baking the system at 250°C for 48 h reduced the net (rare gas) outgassing rate of the apparatus by a factor of 100 at best. Running the source with relatively high pressure of a different gas species can desorb previously pumped gases by sputter release. In this way we were able to obtain another factor of two reduction in the rare gas outgassing rate. A more drastic but more effective way to ‘erase’ the memory of the source is to electropolish the source between enrichments. Further reductions in memory are possible by redesigning the source to improve the extraction efficiency (approximately 5% for the Extranuclear source used in our enrichment studies). Finally, by using photoionization rather than electron-impact ionization one should be able to eliminate source memory effects altogether. 4. Summary The ability to enrich small (lo- ’ std cc) quantities of noble gases (with minimal losses for selected isotopes) is a prerequisite for detecting several isotopes of geophysical importance using resonance ionization mass spectroscopy. We have achieved enrichment factors of 1000 by means of a quadrupole mass spectrometer operated in the static mode. The apparatus employed an electron-input ionization source and a biased metal plate which trapped the output of the mass filter. Larger enrichment factors should be possible by recycling the implanted atoms back to the gas phase (after pumping out residual gas in the system) for additional passes through the mass
584
filter. Recycling procedures are presently under development in our laboratory.
We wish to thank S L Allman, R C Phillips, S D Kramer and M G Payne for valuable discussions and technical assistance.
Referewes ’ J H Renolds, Ra Sci Instrum, 27,928 (1954). s V Costa, M P Ferreira, R Macedo and J H Reynolds, Eorrh and PlanerarJ Sci Lets, 25, 131 (1975). 3 C M Hohenberg, Rer Sci Instrum, 51, 1075 (1980). ’ G S Hurst, M H Nayfeh and J P Young, Appl Phys Left, 30,229 (1977). 5 S D Kramer, C E Remis, Jr, J P Young and G S Hurst, Oprics Lerr, 3,16 (1978). 6 D L Donohue, J P Young and D H Smith, Inr J Mass Spectrom Ion Ph_ss, 43, 293 (1982); C M Miller, N S Nogar, A J Gancarz and W R Shields, Anal Chem, 54,2377 (1982); J D Fassett, J C Travis, L J Moore and F E Lytle, Anal Chem (in press). ’ C H Chen, G S Hurst and M G Payne, Progress in Atomic SpecrroscopyParr C. (Edited by H F Reyer and H Kleinpoppen). Plenum Press, New York (1984). s C H Chen, G S Hurst and M G Payne, Chem Ph~x Lert, 75,473 (1980). 9 E V Komelsen, Can J Phys, 42, 364 (1964). lo W A Grant and G Carter, Vacuum, 15, 477 (1965). I’ W Eckstein and H Verbeek, IPP Report No. 9/32, Max-Planck-lnstttut fur Plasma-physik (1979). I2 R L Jepsen Trons 4th Int Vacuum Congr, p 317. Institute of Physicsand Physical So&y, London (1968). I3 R D Willis, S L Allman, C H Chen, G D Alton and G S Hurst, to be published.
Enrichment quadrupole
5lpages 581 to 58411984
0042-207X/84$3.00 + .OO @ 1984 Pergamon Press Ltd
of rare gas isotopes mass spectrometer*
using a
C H Chen. I? D Willist and G S Hurst, Chemical Physics Research Section, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, 7ennessee 37830, USA received 12 July 199.5; in revised form 9 August 1993
A small quadrupole mass spectrometer was operated in the static mode to enrich selected rare gas isotopes. Memory effects in the apparatus were observed and attributed to the re-emission of atoms implanted by the electron-impact ion source. Studies of the pumping mechanism led us to a practical means for reducing the rate of noble gas pumping
1. Introduction The static magnetic deflection mass spectrometer has been a most powerful tool for detecting small quantities of rare gas isotopes. Such a capability is important, for example, in analyzing rare gases in lunar samples and meteoritesle3. The ultimate sensitivity of such instruments is limited by memory effects and detector dark current. In the late 197Os,Hurst et u14*’demonstrated the ability to detect single alkali atoms using the laser technique of Resonance Ionization Spectroscopy (RIS). More recently, several groups6 have used RIS in conjunction with a magnetic deflection mass spectrometer to detect low levels of various transition metal atoms and to remove isobaric interferences. Using a laser resonance ionization source and a quadNpole mass filter for mass selection, Chen et aI’*s were able to detect a small number of rare gas atoms. The use of laser resonance ionization sources eliminates the pumping effects generally associated with conventional electron-impact ionization sources. Pulsed laser sources for ionization have the added advantage that time correlation can be used to gate the detector output, thus minimizing the dark current contribution. Applications of resonance ionization mass spectroscopy include solar neutrino flux measurements (by counting *‘Kr), dating of the polar ice sheets and old groundwater (“Kr), and determining rates of ocean mixing and circulation (“Ar). Because the expected *lKr and “Ar concentrations are so low in these samples (*lKr/total Kr < lo-l2 and 3gAr/total Arc lo- “). a large amount of sample is required in order to obtain a few thousand *‘Kr or 39Ar atoms. Because of space-charge effects in the ionization region, however, the number of ions is limited to about 10’ per pulse. It is therefore necessary to pre-enrich these
l Research sponsored in part by the Office of Health and Environmental Research, U.S. Department of Energy under contract W-74OS-eng-26 with the Union Carbide Corporation; in part under contract EV10664 with the University of California, San Diego; and in part by the Swiss Nationale Genossenschaft fur die Lagerung radioaktiver Abfalle (NAGRA). t Scripps Institution of Oceanography, La Jolla, California.
samples before their introduction into the resonance ionization mass spectrometer. Memory effects due to implantation of fast ions, and the subsequent reemission as neutral atoms, are known to occur in deflection-type mass spectrometers which transport ions of several keV of energy. In order to reduce this source of memory, a quadrupole mass spectrometer (in which ion energies are typically 30 eV or less) was chosen for the pre-enrichment work. Some memory effects were, nevertheless, observed. We attempted to discern the cause of the memory effects and our conclusions are discussed below.
2. Experimental A commercial quadrupole mass spectrometer (Extranuclear Laboratories, Inc.) was employed for this work. The electronimpact ionizer shown in Figure 1 is of axial geometry with a S-ml tungsten filament surrounding a cylindrical basket-type grid. These are enclosed by a solid stainless steel shield normally connected to the negative filament lead. Ions are extracted from the source by a series of extraction and focusing lenses. Ions of the selected mass are transmitted through the mass filter and collected by a CuBe electron multiplier or stainless steel Faraday plate. By applying a negative potential of several kilovolts to either collector, one can selectively implant atoms of a desired isotope to achieve enrichment. The mass spectrometer is housed in an electropolished, stainless steel vacuum chamber. The system is roughed down to a few microns by a cryosorption pump, and post-bakeout pressures of lo-” torr are routinely obtained with a cryogenic pump. The system is typically baked at 250’ for 48 h prior to sample introduction. Samples to be enriched were admitted to the apparatus (with the cryogenic pump valve closed) through a precision leak valve. When the desired quantity of gas had been admitted (1O-g-1O-s torr in the 2 1 volume), the leak valve was closed and the mass spectrometer turned on. Vacuum was a
581
C H Chen. R D Willis and G S Hurst:
Enrichment of rare gas isotopes MASS 78 VS TIME
AND
MASS 80
SIGNALS
IN STATIC MASS SPECTROMETER
FILAME SHIELD
MASS 90,
NOT IMPLANTED
l
30 V SOURCE o 90 v SOURCE
\-~
FIL SHIELD a 8
IO*
MASS 78,
4.: _
o
IMPLANTED
AT 2.5
keV
. 30 V SOURCE o 9OV SOURCE
A
A A
A A .
Figure 1. Schematic of ton source (Extranuclear Laboratories). Ions are extracted from the source by an extraction voltage which is a few volts less than I’_,.
. . A A
maintained during static operation by means of an SAES getter pump which had no measurable pumping speed for noble gases.
3. Results and discussion Krypton from Matheson was chosen for convenience for the enrichment experiments. By tuning the mass spectrometer to mass 78 and implanting the transmitted ions into the Faraday collector, one can selectively reduce the population of “Kr atoms in the gas phase. Thus, the 78Kr/80Kr ratio in the gas phase is observed to decrease with time as seen in Figure 2. Note that the *OKr signal decreases somewhat even though the transmission of *‘Kr ions through the mass filter is negligible. No loss of krypton was observed when the ion source was turned off, indicating that physical adsorption of thermal krypton atoms on the vacuum walls is negligible at room temperature. The implantation of lowenergy “Kr ions in the quadrupole rods was also ruled out. Hence, the mechanism to account for the loss of ‘OKr atoms must be related to the production of low-energy ions in the source itself. The ion source shown in Figure 1 can pump noble gas atoms via two mechanisms. (1) Some fraction of those ions produced between the filament and grid basket can be accelerated towards the negative biased filament shield and implanted into the surface with energies of 100 eV or less. Data by Kornelsen9 and others” indicate small but finite trapping efficiencies for low-energy ions in metal surfaces. (2)The same ions, rather than going to the filament shield, might be more strongly attracted to the nearby filament. The majority of ions striking the filament will be reflected as energetic neutrals having a large fraction of the energy of the original ions”. Some of these reflected fast neutrals will subsequently be implanted into surrounding surfaces. In order to determine the dominant pumping mechanisms, we modified the source to allow the filament shield to be biased 582
I
too D
100
I 200
I
I
I
I
300 400 TlME (min)
500
600
Figure 2. Enrichment of a°Kr in the gas phase by implanting “Kr into a stainless steel plate biased at -2.5 kV. Note that some loss of “Kr is observed even though this isotope is not transmitted by the mass filter. Enrichments were done at electron energies of 30 and 90eV. The spectrometer throughputs for krypton were 3.7 x 10m3 A torr-’ and 1.6 x 10W3A torr-‘, respectively. The volume of the static spectrometer was approximately 2 1.
independently of the filament. The pumping speed of the source was then measured as a function of the shield potential k’,(while keeping the throughput of the mass spectrometer fixed) and was found to be independent of V,.Since V,determines the energy of the ions arriving at the shield, and since the trapping efficiency is a strongly increasing function of ion energy below 100 eVg, we conclude that low-energy ion implantation is not the dominant pumping mechanism in our source. hence, the loss of rare gas atoms is mainly due to the implantation of energetic neutrals produced by charge-exchange scattering of low-energy ions at the filament. Such a mechanism was first proposed by Jepsen” and was recently invoked to explain the pumping of rare gas atoms in an electron-impact source of much simpler design’ 3. The ability to enrich krypton samples was investigated at electron energies of 30 and 90 eV. (The filament and filament shield were both at the same potential and the electron energy was determined by the potential difference between the filament and the grid.) The results of the two trials are displayed in Figure 3 as the ratio of ‘*Kr/*‘Kr vs time where mass 78 is the implanted species. At 30 eV the isotope ratio was reduced a factor of loo0 after 24 h relative to the natural abundance ratio. At 90 eV the enrichment factor was about one order of magnitude smaller for
C H Chen, R D Willis and G S Hurst:
Enrichment of rare gas isotopes
ENRICHMENT: RATIO OF MASS 78 TO MASS 80 WHILE IMPLANTING MASS 78 1.0 t
I
I
I
I
-
0 90 VOLTS
-
0
I
the same time period, in agreement with the detailed mechanism discussed elsewhere”. Additional enrichment results are shown in Figure 4 for several rare gases. For each species the outgassing rates for the isotopes of interest were measured and the source pumping rates and collector implantation rates were estimated by solving the following pair of coupled differential equations:
I
30 VOLTS
dx 5
dy
0
z =
0 0
. l
0
200
300 400 TIME lminl
i
500
600
700
Figure3. The ratio ~f’~Kr, “‘Kr in thegas phase as function of time for the data shown in Figure 2. An additional data point measured after 24 h showed the ratio mass 78/mass 80 to be 10e4.
100 _
k;-k,y(r),
Values of k, deducted from the data in Figure 4 for 3 keV ions implanted into a stainless steel collector were 0.0077 min-r, 0.012 min- ‘, and 0.015 min- ’ for argon, krypton and xenon, respectively, in our 2.5 1 volume. These rates are consistent with near-unity trapping efficiency at the collector within the combined accuracy of our volume and throughput measurements ( f 30%). The source pumping rates for the data shown in Figure 4 were about 30% of the corresponding implantation rates but, in general, were very sensitive to the cleanliness of the source.
FACTOROF 403 ENRICHMENT AFTER 24ha \
400
(la)
k,x(t) - k,x(t)
x(t)=gas-phase population at time r of the isotope which is purposely implanted after mass analysis; y(t)=population at time t of the unimplanted isotope; k,,, k;=outgassing rates (atoms min-‘) of the implanted and unimplanted isotopes, respectively; k,= source pumping rate (min- r)-assumed same for all species; k, = implantation rate at the collector (min- 1).
.
9
k,-
where
l
o.oo,
=
I
I
I
I
F
.L
Xe-129/Xe-131 .
. t
.
.
.
Kr-84/Kr-86
0
0
O 0 0
0
0
0
.
0.1 0
0
0
Ar-36/A?-40 .
I
I
40
80
. I
I
. I
120
f60
200
TIME
. 240
I 280
(min)
Figure 4. Typical enrichment data showing selected isotope ratios (in the gas phase) vs time for argon. krypton and xenon samples. In each case the mass filter was tuned IO the lighter isotope.
.
C H Chen, R D Willis and G S Hurst: Enrichment
of rare gas isotopes
The re-emission of rare gas atoms pumped by the ionization source represents the major source of memory in our apparatus and ultimately limits the amount of enrichment that can be achieved. Baking the system at 250°C for 48 h reduced the net (rare gas) outgassing rate of the apparatus by a factor of 100 at best. Running the source with relatively high pressure of a different gas species can desorb previously pumped gases by sputter release. In this way we were able to obtain another factor of two reduction in the rare gas outgassing rate. A more drastic but more effective way to ‘erase’ the memory of the source is to electropolish the source between enrichments. Further reductions in memory are possible by redesigning the source to improve the extraction efficiency (approximately 5% for the Extranuclear source used in our enrichment studies). Finally, by using photoionization rather than electron-impact ionization one should be able to eliminate source memory effects altogether. 4. Summary The ability to enrich small (lo- ’ std cc) quantities of noble gases (with minimal losses for selected isotopes) is a prerequisite for detecting several isotopes of geophysical importance using resonance ionization mass spectroscopy. We have achieved enrichment factors of 1000 by means of a quadrupole mass spectrometer operated in the static mode. The apparatus employed an electron-input ionization source and a biased metal plate which trapped the output of the mass filter. Larger enrichment factors should be possible by recycling the implanted atoms back to the gas phase (after pumping out residual gas in the system) for additional passes through the mass
584
filter. Recycling procedures are presently under development in our laboratory.
We wish to thank S L Allman, R C Phillips, S D Kramer and M G Payne for valuable discussions and technical assistance.
Referewes ’ J H Renolds, Ra Sci Instrum, 27,928 (1954). s V Costa, M P Ferreira, R Macedo and J H Reynolds, Eorrh and PlanerarJ Sci Lets, 25, 131 (1975). 3 C M Hohenberg, Rer Sci Instrum, 51, 1075 (1980). ’ G S Hurst, M H Nayfeh and J P Young, Appl Phys Left, 30,229 (1977). 5 S D Kramer, C E Remis, Jr, J P Young and G S Hurst, Oprics Lerr, 3,16 (1978). 6 D L Donohue, J P Young and D H Smith, Inr J Mass Spectrom Ion Ph_ss, 43, 293 (1982); C M Miller, N S Nogar, A J Gancarz and W R Shields, Anal Chem, 54,2377 (1982); J D Fassett, J C Travis, L J Moore and F E Lytle, Anal Chem (in press). ’ C H Chen, G S Hurst and M G Payne, Progress in Atomic SpecrroscopyParr C. (Edited by H F Reyer and H Kleinpoppen). Plenum Press, New York (1984). s C H Chen, G S Hurst and M G Payne, Chem Ph~x Lert, 75,473 (1980). 9 E V Komelsen, Can J Phys, 42, 364 (1964). lo W A Grant and G Carter, Vacuum, 15, 477 (1965). I’ W Eckstein and H Verbeek, IPP Report No. 9/32, Max-Planck-lnstttut fur Plasma-physik (1979). I2 R L Jepsen Trons 4th Int Vacuum Congr, p 317. Institute of Physicsand Physical So&y, London (1968). I3 R D Willis, S L Allman, C H Chen, G D Alton and G S Hurst, to be published.