Patent Grant
6870153
This invention relates to a novel analytical instrument, and to novel methods of measuring, inter alia, low concentrations of stable and radioisotopes and/or low abundance isotopes.
The determination of radionuclides at environmental levels using classical radiometric counting is well established and likely to remain the method of choice for short half-life species. However, innovations in analytical instrumentation in the last ten years have the potential to replace radiometric counting for a wide range of longer half-life species.
Elemental and isotopic analysis has advanced significantly with the introduction of plasma source mass spectrometry. A variety of plasmas have been used as ionization. sources, e.g., glow discharges, microwave induced plasmas, but the inductively coupled plasma (ICP) is the most widely accepted, and de facto, the preferred ion source for atomic mass spectrometry. The inductively coupled plasma is compatible with solid, liquid or gaseous sample introduction and is a robust and efficient ionization source for atomic mass spectrometry.
For some potential applications of plasma mass source spectrometry, e.g., environmental and biomedical monitoring of radioisotopes, current techniques may not possess the required detection limits or selectivity. Classical radiometric techniques may provide the required detection limits, but do so at the expense of protracted count times and extensive sample preparation and clean-up. For example, within a plutonium bioassay program, current radiometric methods offer detection limits of 500 μBq per litre, but require 1-2 days of sample preparation and radiometric count times of, e.g., four days with α-spectrometry and up to 28 days for α-track counting. there is a requirement to develop plasma source mass spectrometry to provide enhanced selectivity and improved detection limits without sacrificing the inherent flexibility, repidity and robustness of the technique.
The instrument of the invention is designed to measure isotopes at extremely low concentrations and isotopes of very low abundance. An example of this would be the ultra low level determination of the radionuclides. The increasing interest in the behaviour of radionuclides in the biosphere requires that new methods be developed that have detection limits equivalent to, or better than, that of the existing techniques, but combine this with superior speed and a reduced cost of analysis. Improvements in speed are essential to enable wider screening, plant and event management and to monitor illicit uses of nuclear materials. The recent OSPAR agreement has committed the UK to real reductions in levels of liquid effluent discharges. For many radionuclides, conventional radiochemical analysis will limit the ability to demonstrate that such reductions have been achieved.
To achieve the aim of improved detection limits in plasma source mass spectrometry, the factors that limit the selectivity and sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) were considered. The instrumental detection limits available from ICP-MS are, in most cases, limited by the background count and not the magnitude of the analytical signal derived from the ions of interest. The background is derived broadly from three distinct sources:
These observations are the key to the development of instrumentation with the superior detection limits required for determination of radionuclides at background environmental and biomedical concentrations by ICP-MS techniques.
A comparison of alternative techniques to plasma source mass spectrometry suggests that resonance ionisation mass spectrometry (RIMS) offers similar or better absolute detection limits than achieved with current generation ICP-MS instruments, e.g. about 4×106 atoms for 259 Pu. The singular advantage of RIMS over, for example, ICP-MS, is the greater isotopic selectivity derived from the laser induced ionisation process. However, the prior chemical separation, though less demanding that not required by radio-chemical methods, is nevertheless time consuming and requires specific recovery of the element, deposition onto a Ta foil and overplating with TL Accelerator mass spectrometry (AMS) offers absolute detection power of the order of 106 atoms. Selectivity is achieved through the use of high energy dissociation of molecular ions and avoidance of isobars through negative ion discrimination. Improved detection limits are obtained by high energy counting to discriminate against detector background. High abundance sensitivity is achieved by acceleration to high potentials thus minimizing the relative ion energy spread. However, AMS involves large, complex and costly instrumentation. Sample preparation is complex and time consuming, requiring preparation of the element in a pure form. For these reasons, AMS is restricted to highly specialized roles and cannot at this time be considered as a laboratory scale or general purpose instrument.
Thus, we have now developed analytical instrument and an analytical approach that overcomes or mitigates the problems with conventionally known instruments and techniques. As a technology demonstration, this new device is based upon an ICP-MS instrument, but is equally applicable to other forms of plasma mass spectrometry. Indeed, the range of applications includes all forms of atomic mass spectrometry and molecular mass spectrometry. This instrumentation also provides a flexible platform for spectroscopic studies of atoms and molecules to determine fundamental parameters.
Thus according to the invention, we provide an instrument comprising an Inductively Coupled Plasma Source Mass Spectrometer equipped with a multi-dimensional detector system wherein ions transmitted by the mass spectrometer are detected with high selectivity.
The instrument is provided preferably with detectors which are based upon specific detection of transmitted ions, e.g. via optical spectroscopy. The device is in principle an ICP-MS instrument operating in a multi-dimensional detection mode and including the following:
The detector device based upon optical spectroscopy provides:
Operation of the two detection systems as a single integrated coincidence detector that provides:
The descriptive term for this approach is Inductively Coupled Plasma Mass Spectrometry Coincidence Laser Spectroscopy (ICP-MS-CLS).
Thus, according to a preferred feature of the invention, we provide an ICP-MS-CLS instrument. We especially provide an ICP-MS-CLS instrument with a conventional non-specific ion detection device and a device based on optical spectroscopy as hereinbefore defined.
The instrument of the invention supplements the universal ion counting detector with one that has a high degree of species selectivity. The use of a detector based on resonance scattering from the ions to be detected, e.g., laser induced fluorescence (LIF), provides vastly improved selectivity thereby removing the problem of isobaric interference derived from either atomic or molecular ions, Additionally, by operating the optical detector in time correlation with a second detector, background count rates can be reduced by several orders of magnitude.
The instrumentation takes advantage of improved detector technology to achieve very high spatial and temporal resolution in the optical spectroscopy. This allows coincidence detection from single photons. This capability is important in that it allows the detection of ions in which there is a high probability of trapping in a metastable state. Ions in metastable states are transparent to the exciting laser and thus the overall photon multiplicity from these ions is low.
To allow for efficient interaction between the laser and ion beam, the ion beam must be defined accurately in space and be focussed to approximately the beam diameter of the laser. An imaging spectrometer provides an ideal solution and a sector mass spectrometer is one such device. A commercial, double focussing, sector ICP-MS provides the basic platform for development of ICP-MS-CLS.
A key feature of this instrument is the manipulation of the ion energies. To couple efficiently the energy from the laser into the ion to be detected, the optical bandwidths have to be matched. For example, an ion beam of energy of 5000±2.5 eV, has a Doppler spread of about 100 MHz for an ion of mass=240. This is in excess of the natural line width which is off the order of 15 MHz. The ion energies were manipulated by two devices. The first involves the introduction of a collision/reaction cell to act as an ion bridge between the sampler/skimmer plasma interface and the mass spectrometer. This thermalises the ions and reduces their energy spread to less than 1 eV. Additionally, it enables selective gas phase chemistry to dissociate interfering molecular ions. The second method involves acceleration of the ions to compress the optical bandwidth of the ions to be detected. For example, an ion beam of mass 240 but with a 40 000±5 eV energy range has a corresponding Doppler spread of about 37 MHz. In practice, by using a collision/reaction cell, lower standing voltages, e.g., 10 kV, can be employed. Assuming an ion energy spread of, e.g., 1 eV, at 10 kV, the Doppler spread is about 15 MHz which approximates natural line widths.
Programmed acceleration of the ions within the optical detector is important and ensures that the ions to be detected come into resonance with the exciting laser within the detection volume of the optical detector. This prevents optical trapping of the ions prior to their arrival in the detection volume of the optical detector.
The abundance sensitivity of the spectrometer can be improved by three methods:
Where optical trapping of the ions of interest becomes significant, this may be addressed via the use of two-colour excitation schemes in which the metastable state is in resonance with one of the laser frequencies. To provide maximum flexibility and elemental coverage, a two-colour CW laser system was employed. A twin laser system allows a variety of excitation schemes to be used, combining single color, two color, multiphoton excitation and combinations thereof.
A multi-slit assembly was included in the instrumentation for simultaneous detection of major isotopes, to be monitored via conventional detectors, to allow isotope ratio measurements. This will also provide reference beams so that the performance of the sample introduction system and ICP ion source can be monitored continuously and optimized.
The invention will now be illustrated, but in no way limited, with reference to the following examples and the accompanying drawings, in which,
Referring to
Referring to
Verification of Instrument Performance—Determination of Low Abundance Isotopes, e.g. 10Be
The operating characteristics of the system were established via an established CLS transition, e.g., the Be (II) line at 313 nm which is readily accessible to a CW tunable laser. Beryllium is an important element in its own right and its high mass isotope (10Be) is an important geochronometer. It is produced by nuclear spallation of oxygen by cosmic rays and reaches an equilibrium concentration in surface quartz of about 2 ×107 atoms per g−1. An isobaric interference with 10B exists, but this can be resolved in the optical detector. A reasonable measurement of 10Be was made by processing of a 5 g solution after removal of the major matrix elements. Other cosmogenic isotopes that might be amenable to detection include those of K, Cs, Ca, Mn, Ni, Pd, Al and the lanthenides depending on identifying suitable spectroscopic transitions.
Determination of Pu in Urine for Bioassay Purposes
An aliquot of urine was spiked with a Pu tracer, processed to remove the bulk of the matrix and yielded a final sample volume of 1 cm3. This sample was analyzed by ICP-MS-CLS using a low flow sample introduction system. The isotope ratios of isotope was monitored on a conventional detector whilst the isotopes of interest were determined using CLS detection. Isobaric interferences from, for example, 238U+, 238U1H+, 204Pb35Cl+, 241Am, were resolved optically in the CLS detector. A complete chemical separation of Pu from the matrix was not required and a simple, rapid, group separation of the actimides yielded a sample suitable for analysis by ICP-MS-CLS.
Determination of Fundamental Nuclear Parameters
Optical isotope shifts and fine structure can be used to probe nuclei for the purpose of deriving fundamental nuclear data. The ICP-MS-CLS instrumentation allows the precise measurement of optical isotope shifts using the voltage programming facilities to bring isotopes into resonance selectively with the tuneable laser operating in frequency locked mode.
| Number | Date | Country | Kind |
|---|---|---|---|
| 9904289 | Feb 1999 | GB | national |
This application claims priority and is a continuation of parent application Ser. No. 09/914,330 filed Feb. 12, 2002 now abandoned, which claims the benefit of PCT Application Serial No. PCT/GB00/00577 filed 18 Feb. 2000; the application was published in English under PCT Article 21(2). The international application claims priority from GB Application Serial No. 9904289.7 filed 25 Feb. 1999. The contents of these applications are hereby incorporated by reference as if recited in full herein.
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| Number | Date | Country | |
|---|---|---|---|
| 20040099802 A1 | May 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09914330 | US | |
| Child | 10716963 | US |
