CA2131942C - Small system for precision ultra-sensitive trace mass spectroscipy of carbon-14 and other selected nuclides - Google Patents

Abstract

A method of measuring in small samples the ultra-trace amounts of C-14 relative to C-12,13 in a background of N-14, comprises the following steps: the sample isphysically and chemically processed depending upon the final sensitivity desired;
the sample is injected into an ICP, which functions as a source of positive ions with energies up to a few tens of KeV, and as a molecular dissociator of order parts-per-trillion; the ions then analyzed with conventional Electrostatic and Magnetic spectrometers, which monitor the ion beams with adjacent masses and transmit the ion species of interest through defining apertures; the transmitted ions are re-foccused and accelerated to an energy of order tens of KeV; the positive ion beam strikes a thin foil or supersonic gas-jet canal where some of the beam intensityemerges with negative charge, eliminating atomic isobar backgrounds that cannot form negative ions, and as an additional molecular ion dissociator; the transmitted Negative ions are refoccused and accelerated by a few more tens of KeV; the ionsare then analyzed with conventional Electrostatic and Magnetic spectrometers, through a final defining aperture and onto a detector. Similar configurations may be used to measure ratios of other long-lived radio- to stable-isotopes whose com-peting stable atomic mass-isobars cannot form negative ions, such as: 26Al/27Al in a background with 26Mg; 55Fe/54Fe in a background with 55Mn; 129I/127I in a background with 129Xe, Similar configurations may be used for direct measure of platinum group elements (PGE), actinides (ACT), etc., when atomic-isobars need not be distinguished.

Description

DISCLOSURE of the INVENTION
The process and apparatus of the present invention, described below, provides a relatively small, inexpensive, accurate, and precise device to detect the concentra-tions of the radionuclides'4C and/or 26A1 and/or SSFe and/or'Z9I relative to their stable isotopes over a dynamic range of 1:10' to 1:10'2to's, using samples as small as lOp,g. Separation from interfering molecular and atomic mass isobars employs a completely difFerent approach than that used by traditional AMS, untraditional AMS, ICP-MS, or other methods. The process may also be used to increase the efficacy of the assay of ultra-trace concentrations of nuclides when atomic mass-isomers need not be distinguished, compared with standard ICP-MS and other methods.
BACKGROUND of the INVENTION
There exist two main categories of techniques to assay the quantity of a radionul-cide in a sample: Radioactive Decay Counting, whereby the nuclear radiations emitted by a sample are detected and analyzed, and Direct Atom Counting, where the individual atoms are separated analyzed by Mass Spectroscopy. The former is generally used when activities are relatively large, the latter for small activities, where halflives are long and / or abundances are small and / or the number of samples to be processed is great, and individual size of the sample is small.
For ultra-trace assay of certain radionulcides, Accelerator Mass Spectroscopy (AMS) is used to isolate the radionulcides under study from the overwhelmingly greater quantities of competing molecular and atomic mass isobars, which is usually im-possible even v~rith conventional conventional Mass Spectroscopy (MS, ICP-MS, etc.). The principles behind AMS originated in 1977 (1,2~ and are summarized a.s follows:
~ chemical processing to make graphite samples (or other chemical forms) of a few mg in mass ~ using a negative ion source to produce '9C- ions because the stable element mass isobar'4N can not be negatively ionized ~ low-resolution magnetic analysis of the ions ~ accelerating from 1 to tens of MeV and stripping the ions to charge q>2+ to eliminate molecular-ion mass isobars in post analyzed beam ~ using a high resolution magnetic spectrograph and a combination of medium resolution electrostatic, or Wien (E/M velocity) filter ~ detection of final ions using special detectors to measure the energy loss, range, velocity, and total energy to identify the final individual particles.
Useful Papers and summaries from several AMS workshops and symposiums may be found in the literature (3-9,12. Some relevant patents are: US 4,973,841 11/1990 Purser~US 4,037,100 07/1977 PurseryUS 4,489,237 12/1984 Litherland et al.
Other novel alternatives to AMS may also be found in the literature, such as use of a small, low energy cyclotron to phase separate 14C from 14N (18~;
using a laser source to generate q=+3 ions, analyzing them using time-of-flight, charge-exchanging to q=-1 using thin foils or gas-jets and reanalyzing with TOF or mass spectrometer (19~; using a tuned laser to selectively detach unwanted negative ions (eg 36S- from 36C1- (20~; etc. However, to date these alternatives still suffer from poor efficiency and /or discrimination are not yet viable.
Examples of conventional applications of ultra-trace radionuclide assay include (but by no means limited to) radiocarbon dating for archeology, geology, etc., where the ratio of C-14, v~~hich has a half-life of 5700 years, to C-12 ranges from 10-12 for modern samples to 10-IS for samples 62,000 years old (RDC requires samples up to several kg in size, bulk chemical processing and counting times up to days; AMS needs only O.lmg to lOmg samples, with scaled down processing, and actual measurement lasting minutes to hours); monitoring of nuclear waste, geological prospecting, etc.; "tagging" of chemical compounds using radionuclides as labels for studies in biomedicine, pharmacology, environment, etc. (with con-siderably higher doses and associated risks with RDC than AMS).
However, facilities needed to perform AMS are either very large and very expensive high energy (up to hundreds of MeV) laboratories that share beam time with AMS, or smaller (1-3MV), expensive (3-5M$) dedicated laboratories (eg ISOTRACE).
Furthermore, analyzing 100 samples per day stretches the limit of their capacity, especially for carbon samples older than 20Iia. There is an obvious need for an apparatus that is able to assay certain radionuclides in samples with sufficient ac-curacy, precision, sensitivity, efficiency and cost-effectiveness; hence the impetus behind the present invenvtion.

DETAILS of the INVENTION
The key details of the apparatus and methodology behind the of the present in-vention are described as follows:
~ an Inductively Coupled Plasma source (ICP) is used as a source of ions and as the primary molecular dissociator at the part-per-trillion level;
~ ions are accelerated only to relatively modest energies (few tens of KV) using simple high voltage DC potentials ~ high resolution energy and momentum analyzers are used to filter out and monitor ion species of extraneous mass ~ a thin foil or gas/jet canal is used as an electron adder to convert some of the ions to a negative charge state as well as act as a secondary molecular dissociator ~ high resolution energy (E/q) and momentum (p/q) analyzers are used to analyze the negative ions ~ a simple ion detector is used to count the final individual ions and measure their energy (E) A block diagram of the essential elements of the invention is shown in Figure-1.
For simplicity, the discussion below concentrates on ions relevant to C-14, with the understanding that similar techniques may be employed for the other ions of interest.
1. A sample (of order lmg) is introduced to the system after initial prepara-tion. Depending upon the final desired sensitivity of measurment, the sample may be in the form of a. gas (C02 purified from the raw sample), a liquid (Hydrocarbon, etc.), solid (graphite purified from the raw sample), colloidial suspension (graphite in water or other liquid carrier), or the raw sample it-self. Standard chemical techniques as employed by conventional AMS are used to prepare and purify the sample in the form of C02, graphite, etc.
Several options are available to volatalize the sample in the gas carrier (usu-ally argon) of the ICP (2) source:
~ the gas form of the sample (eg C02) may be injected directly into the carrier stream (this is a standard ICP technique) ~ a liquid form may be injected into the gas-carrier stream as an areosol using a suitable nozzle (this is a standard ICP technique) ~ a solid form suspended in a carrier liquid (usually water, but other carries liquids may be used) may be injected into the gas-carrier stream as an areosol (this is a standard ICP technique) ~ a solid or liquid sample may be completely volatalized by laser ablation within a chamber containing the gas-carrier stream (this is more recent, but now standard ICP technique) ~ other standard ICP techniques, such as spark, glow discharge, hot fila-ment, graphite (or other material) crucible furnace, etc. may be used, if necessary and appropriate (eg. a graphite crucible furnace would not be used in the measure of C-14 in a sample because of obvious carbon contamination) The preferred sample most suitable for radiocarbon dating (where 14C/1zC
ratios are well under lppt) is in the form of C02 using standard chemical techniques to prepare and purify the sample into this form. This provides maximum final sensitivity in the measurement of 14C/'ZC by reducing ini-tial levels of nitrogen, hydrogen, etc. that form molecular and atomic mass isobars, (eg 14N,'ZCHZ, ~2CD, ~3CH, etc.) as well as minimize accidental con-tamination from other sources of 14C during handling. Also, C02 is the most suitable form when raw samples are physically too small (eg under lmg) to physically manipulate in a practical manner. (Alternatively, a graphite form of the sample may be prepared; however, this requires initial transformation into C02 before graphitization, with a slightly greater risk of contamination due to greater handling.) 2. The ICP is used to generate positive ions with energy of order a few to tens of KeV and to dissociate molecular ions to the 1 part per trillion level. The ICP source electrically "floats" at a few to tens of KV from the remaining system, which is at ground potential; this potential difference is used to accelerate the ions.

3. Standard ICP vacuum baffles and electrostatic lenses are used to isolate the relatively high pressure in the region of the ICP from the high vacuum of the remaining system and to transfer the ions of interest from the source into the remaining system 4. 1 or 2 Einzel lenses, with finai focus at infinity (not critical) 5. an electrostatic analyzer (<30cm central radius of curvature, 90°
bend, but these values are not critical) 6. slit system 7. drift length (<30cm, dimensions not critical, except to match analyzers) 8. a magnetic analyzer (<30cm central radius of curvature, 90° bend, but these values are not critical) 9. slit system, including:
~ positionable Faraday cups to monitor mass 12-16 beams ~ slit and removable FC system to pass only central (mass 14) beam 10. An Electrostatic quadrupole doublet or triplet is used to reshape the beam to give a point focus simultaneously in x and y directions at the center of the "adder" foil or gas canal 11. A standard acceleration tube (aperture with insulator) is used to acceler-ate beam to a few KeV/nucleon and to electrically isolate the "adder-foil"
(below). (The accelerator "tube" may even be in the form of a simple drift space in vacuum, with acceleration potential provided by the "adder-foil".) 12. A 0.5 - 2 ~g self-supporting carbon foil ( "adder-foil" ) is used to:
~ charge exchange mass 14 beam from positive to negative with 5-10%
efficiency ~13-16) ~ remove r4N from further analysis as nitrogen can not form negative ions ~ further reduce abundance of mass 14 molecular isobars (by factor of 1000) Alternatively, a. supersonic gas jet or even a differentially pumped gas canal ( "adder gas-jet" ) may be employed for the charge exchange process.
The increase in x*B phase space by straggling a.nd beam divergence during charge exchange is minimized by focusing the beam impinging upon the "adder-foil" as tightly as possible.

13. Standard acceleration tube (aperture with insulator) to accelerate beam an additional few ICeV/nucleon as well as electrically isolate the "adder foil"
or "adder gas-jet" ) The accelerator "tube" be may even be in the form of a simple drift space in vacuum, with acceleration potential provided by the "adder-foil" (above). The additional acceleration is also used to reduce the effective "temperature" of the beam caused by the straggling and angular spread introduced by the previous charge exchange process, thereby facili-tating subsequent beam analysis by the following E- and B- analyzsers.

14. An electrostatic Einzel lens or quadrupole doublet or triplet is used to re-shape beam to give final focus at infinity (not critical) 15. a cylindrical electrostatic analyzer (<30cm radius, 90° bend, but these values are not critical) 16. slit system 17. drift length (<30cm, dimensions not critical) 18. a magnetic analyzer (<30cm radius, 90° bend; values are not critical) 19. slit system 20. An ion detectors) to count the final (mass-14) ions, or an ion detectors) to count the final ions and measure their energy (E) and / or energy loss (dE);
the detectors) may also include passive regions (i.e. absorbers to range out unwanted species, when applicable) 21. A reference scale length of 30cm is shown, which corresponds to the preferred dimensions of the energy and momentum analyzers.(Note that for the lighter isotopes, 3H and 14C, that the pair of analyzers upstream from the adder foil may even be reduced by well over 50%.) 22. Trajectory of the central ions, from initial source (2) to detector (20) 23. accelerating high voltage (up to a few tens of kVDC) applied to ICP source 24. accelerating high voltage (up to a few tens of kVDC) applied to electron adder (12), via an insulated feedthrough. Items 23) and 24) illustrate the low values of ion acceleration voltages and energies compared with conventional AMS
Standard high vacuum pumping systems are used.
Computer control, data acquisition, and data analysis systems are used to control the physical selection of samples into the ICP, power supplies, analyze the data and provide the final results. (The computer systems are similar to those em-ployed elsewhere in AMS, ICP and other facilities worldwide and their particular configurations are not critical. ) The ratio of radio to stable isotopes (eg 14C/'2C) is obtained by comparing the number of ions counted at the final detector with the integrated current from the first (Low Energy) magnetic analyzer and correcting with suitable transmission efficiency factors. Because the concentration of the radionuclide is measured rela-tive to its stable isotope, the final result is relatively independent of initial source intensity, sample matrix, and other initial conditios. The efficiency correction fac-tors are determined at regular intervals by using sample "standards" of known iaC concentration a.s well as by retuning both magnetic analyzers for transmis-sion of 12C to the final detector (this may include reducing the beam intensity using a mechanical chopping wheel, insertable grid-aperture, electrostatic or mag-netic "bouncing" (i.e. transient retuning between different parameters and back), etc.). Because several high resolution mass filtering stages are used, only simple detection and data analysis is necessary (c.~ conventional AMS which uses low to medium resolution filtering with complex detection (energy, energy loss, range) and particle identification algorithms).
Other features of the invention include:
~ The ion source ca.n easily handle solid, liquid or gas samples ~ The ion source can easily switch samples to over 1000 samples per day 21319%2 ~ The ability to use COZ instead of graphite saves valuable time and money and reduces risk of contamination, especially for "very old" (ie 14C/12C«lE-12) samples ~ The ability to analyze "young" or enriched samples directly without chemical preparation Only small sample sizes, from O.lmg to lOmg, are necessary, with the smaller sizes well suited for COZ extracted from difficult environments (such as in glacial and arctic ice pack studies); small sizes also are useful in reducing un-necessary radiation exposure to patients (animal as well as human) undergo-ing radiocarbon diagnostics and to radiation workers handling radio-tagged compounds or radio waste.
~ There is an obvious potential as a microprobe using laser ablation ~ Overall detection efficiency is comparable or slightly greater than conven-tional AMS
~ Final ion count rate is comparable or greater than conventional AMS
Sample char~geover time is considerably shorter than for conventional AMS;
there is also no lengthly sample "burn-in" period, which can be a major bottleneck for conventional AMS.
~ Short total distance reduces probability of residual gas scattering ~ Low energy reduces probability of slit scattering ~ Although molecular isobar rejection is not 100% exactly, the amount is more than adequate. (Note that for conventional AMS, which is tuned to se-lect charge states q>2+, molecular and atomic mass isobars still need to be discriminated in the final detectors due to secondary processes: eg, 14N
may be accelerated as '9NH-, stripped to 14N+a slit or gas scatter t0 19N+3 and appear as a background under 14C+3 that must be removed using range, energy-loss; ~2CH+2 may scatter to end up with the same momentum/charge ~ iaC+3 and rescatter after momentum analysis to give same energy/charge, etc.; so ineffect, stripping is not completely 100% either.) The use of foils and 2 pairs of E- and B- analyzers in addition to chemical preparations and an ICP source results in much greater sensitivity, and reduction and/or elim-ination of mass isobar backgrounds (by several orders of magnitude) than can be achieved with simple ICP-MS.
The preferred embodiments of the invention have been illustrated above by way of example. However, it is apparent that modifications and adaptations of those em-bodiments will occurr to those skilled in the art. Such modifications may include (but are not limited to) use of different ion analyzers (eg, WIEN (crossed mag-netic and electric field), time-of-flight, Radio-Frequency (~uadrupole(s) (RFC), etc.) and/or combinations with and/or without the above; beam optics;
different _ 2131942 charge-exchange mediums (thin films, gas canals, supersonic gas jets, etc.);
changes in the acceleration voltages; changes in dimensions; changes in sample chemical preparation; changes in sample volatilization and / or ionization; changes in ICP
configurations; etc. It. is to be expressly understood that such modifcations and adaptations are within the spirit and scope of the present invention.

References ( 1J K.H. Purser, R.B. Liebert, A.E. Litherland, R.P. Beukens, H.E. Gove, C.L.
Bennett, M.R. Clover and E. Sondheim, Rev. Phys. Appl. 12 (1977) 1487 ( 2J D.E. Nelson, R.G. Korteling and W.R. Stott, Science 198 (1977) 507 3J Proc. First Conference on Radiocarbon Dating with accelerators, ed., H.E.
Gove, Univ. of Rochester, USA (1978) 4J Proc. Symp. on Accelerator Mass Spectroscopy, eds., W. Henning, W.Kutschera and J.L. Yntema, Argonne National Lab, Argonne, USA, Report ANL/PHY-81-1 (1981) ( 5J Proc. Third Int. Symp. on Accelerator Mass Spectroscopy, eds., W. Woelfi, H.A. Polach and H.H.Anderson, Zuerich, Switzerland (1984) Nucl. Instr. and Meth. B5 ( 1984) 91-448 ( 6J Proc. Workshop on Techniques in Accelerator Mass Spectrometry, eds., R.E.M.
Hedges and E.T. Hall, Oxford, UK (1986) ( 7J Proc. Fourth Int. Symp. on Accelerator Mass Spectroscopy, ed., W. Woelfi, Niagara-on-the-Lake, Canada Nucl. Instr. and Meth. B29 (1987) 1 - 445 ( 8J Proc. Fifth Int. Symp. on Accelerator Mass Spectroscopy, Nucl. Instr. and Meth. B52 (1990) 211- 634 ( 9J Workshop on AMS Requirements in Canada, ed., D.B. Carlisle, Canada Center for Inland Waters, 15-16 April, 1991, Burlington, Canada NSERC & Environment Canada, Ottawa, Canada K1Y OH3 ISBN: 0772769508 (lOJ Low-energy fusion cross sections of d -f- d and d ~- 3He reactions ", A.
Itrasss, H.W. Becker, H.P. Trautvetter, C. Rolfs, and IC. Brand, Nucl. Phys. A465 (1987) p150 (12J "Biomolecular tracing through A.M.S.", J.S. Vogel & K.W. Turteltaub, Appl.
Radioat. Isot. vol 43 #1/2, pp61-68, 1992 (13J "Electronic stopping powers of low-velocity ions", D.J. Land and J.G.
Bren-nan, Atomic Data and Nuclear Data Tables v22#3 (1978) p236 (14J "Range and Stopping Powers of heavy ions", L.C. Northcliffe and R.F.
Sclulling, Nuclear Data Tables A7 ( 1970) p233 (15J "Angula.r Distributions of ions scattered in thin carbon foils", G.
Hoegberg and H. Norden, Nucl. Instr. and Meth. 90 (1970) p283 ~1 3194 2 ~16~ "Equalibrium Charge State Distributions", K.Shinxa, T. Mikumo and H.Tawara, Atomic and Nuclear Data Tables 34 (1986) p3~ 7 ~18~ "Ion motion in a small low energy cyclotron" ; Kirk J. Bertsche, Nucl.
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and Meth. A301 (1991) p171 ~19~ "The possibilities of cosmogenic isotope invcatigation by mean of mass -spec-trometrical methods ", G.E. Kocharov, V.T. Kogan, A.N. Konstantinov and A.K.
Pavlov, NIM B52 (1990) p384 "A Tandem Mass-spectrometric method of cosmogenic isotope analysis", A.K.
Pavlov, V.T. Kogan and G.Y. Gladkov, Radiocarbon 34 v2 (1992 p271 ~20~ "New developments and challenges in accelerator mass spectroscopy'', Michael Paul, Nucl. Instr. and Methods A328 (1993) p330 Relevant patents are:
US4973841. 111990, Purser:" Precision ultra-sensitive trace detector for carbon 14 when it is at concentration close to that present in recent organic materials"
US4037100, 71977, Purser: "Ultra-sensitive spectrometer for making mass and elemental analyses"
US4489237, 121984, Litherland et al.: "Method of broad band mass spectrometry and apparatus therefor"
1z

Claims (8)

1) A process of measuring the ultra-trace amounts of C-14 relative to C-12,13 in a small sample comprises the following steps:
- physical and chemical pre-processing of the sample to suitable form (solid, liquid, gas), depending upon the final sensitiveity desired;
- injection of the sample into an Inductively Coupled Plasma source (ICP) to volatalize the sample, create a positive ion beam, and to dissociate molecular ions in the ion beam;
- analyzing the ion beam with an electrostatic and magnetic spectrometer and through a defining aperture;
- electrostatically accelerating the ion beam to an energy of order 10 to 100 KeV;
- using a thin foil to add electrons to the ions in the ion beam to form a negative ion beam, and to further dissociate molecular ions in the negative ion beam ;
- electrostatically accelerating the negative ion beam to an energy of order 10 to 100 KeV;
- analyzing the negative ion beam with an electrostatic and magnetic spectrometer through a defining aperture and onto a detector.

2) A process as defined in claim 1) for detecting the ultra-trace amount of aluminum-26 relative to aluminum.

3) A process as defined in claim 1) for detecting the ultra-trace amount of iron-55 relative to iron.

4) A process as defined in claim 1) for detecting the ultra-trace amount of iodine-129 relative to iodine.

5) A process as defined in claim 1) for detecting the ultra-trace amount of nuclides, such as platinum group elements (PGE), or actinides (ACT), when the discrimination of atomic isobars is not required.

6) A process as defined in claims 1-5) whereby a Wien (a.k.a. velocity, a.k.a.
ExB) analyzer is used as one of the ion mass analyzing devices.

7) A process as defined in claims 1-6) whereby a radiofrequency quadrupole mass analyzer is used as one of the ion mass analyzing devices.

8) A process as defined in claims 1-7) whereby a gas-filled region is used in place of the thin foil for the step involving the addition of electrons to the ions in the beam and further dissociation of molecular ions.