Apparatus and method for elemental mass spectrometry

A mass spectrometer and method of mass spectrometry in which polyatomic and doubly charged ion interferences are attenuated by establishing an electron population through which a beam of particles containing elemental sample ions and the interfering ions is passed such that the interfering ions preferentially undergo ion-electron recombination and thus dissociation to remove a significant number of the interfering ions. Means (30 or 32) for providing a population of electrons (34 or 36) in an ICP-MS (22) may comprise a magnetic field means such as an electric coil, or an electron generating device. The population of electrons has an electron number density (>1011 cm−3 to 1014 cm−3), a free electron energy (>0.01 eV to <5 eV) in a region at a low pressure (<10 Torr), such that for a predetermined path length (1–4 cm) of the ions through the electron population, the interfering ions will preferentially be attenuated by the dissociative recombination process. The ion beam (40) then passes to a mass analyser (42) and ions which have been separated according to their mass-to-charge ratio are detected by ion detector (44).

TECHNICAL FIELD

The present invention relates to an apparatus and method for mass spectrometry, in particular for elemental or isotopic analysis of a sample by mass spectrometry.

BACKGROUND

Elemental or isotopic analysis by mass spectrometry is known to be subject to interference from polyatomic and doubly charged ions. Interference arises when an ion has a mass-to-charge ratio that, within the limits of resolution of a mass spectrometer being used, is the same as that of an isotope of analytical interest. Such interferences can compromise the detection limits and dynamic range of the analysis, and can be particularly troublesome when an element of interest has but one isotope. It is known that an inductively coupled plasma (ICP) ion source is capable of generating many oxide, hydroxide, and doubly charged ion interferences. Other types of sources for atomising and ionizing a sample for elemental analysis by mass spectrometry such as for example Microwave Induced Plasmas, Laser Induced Plasmas, and Glow Discharges also produce interfering ions.

An object of the present invention is to provide an apparatus and method for mass spectrometry in which such polyatomic and doubly charged ion interferences are attenuated.

SUMMARY OF THE INVENTION

The present invention involves establishing conditions during mass spectrometry that cause polyatomic or doubly charged ions to preferentially undergo ion-electron recombination and thus disassociation in the presence of free electrons thereby removing a significant number of such interfering ions. A significant number of the interfering ions is such as will result in detectable improvements in the limit of detection of a mass spectrometer for trace amounts of an isotope. Usually this will involve removal of a substantial number of the interfering ions.

Accordingly, in a first aspect, the present invention provides a mass spectrometer for elemental analysis of a sample including

source means for atomising a portion of the sample,

means for extracting a beam of particles from the source means, wherein the beam contains elemental sample ions and interfering polyatomic or doubly charged ions,

means for providing a population of electrons in a region through which the beam of particles is passed and which defines a predetermined path length for the particles through the electron population, said region being located within an evacuable chamber of the mass spectrometer whereby a low pressure is establishable in said region, the population of electrons having an electron number density and free electron energy which, together with said predetermined path length and low pressure, provide for interfering polyatomic or doubly charged ions preferentially to undergo ion-electron recombination and thus dissociation thereby removing a significant number of the interfering ions from the beam of particles,

a mass analyser and an ion detector for receiving ions from the beam of particles after it has passed through the population of electrons for spectrometric analysis whereby concentrations of different elements in the sample are determinable.

In a mass spectrometer in which an inductively coupled plasma (ICP) is used to atomise a portion of the sample, the means for providing the population of electrons may be a device for providing a magnetic field for temporarily confining electrons from the plasma to a region defined by the magnetic field. Such a magnetic field may be provided by one or more electric coils, magnets or any other means of creating a suitable magnetic field. Indeed any “magnetic mirror” device, that is a device capable of creating a non-uniform (electron confining) co-axial magnetic field, may be used to confine electrons and ions along the axis of the magnetic field. Such a device, be it an electric coil or otherwise, may be placed behind a sampler cone or behind a skimmer cone, or such devices could be provided behind both the sampler and skimmer cones. This is applicable to any known plasma ion source for elemental analysis (ICP, Microwave Induced Plasma, Laser Induced Plasma, Glow Discharge Plasma), where free electrons already exist due to the ion-electron balance in the original plasma.

Alternatively the means for providing a population of electrons includes a reaction cell through which the beam of particles is passed, the reaction cell being located within said evacuable chamber of the mass spectrometer and having a plasma generating means associated with it for supplying a plasma into the reaction cell whereby the plasma electrons constitute said population of electrons.

In the first aspect of the invention, the means for providing the population of electrons, for example the plasma ion source in an ICP-Mass Spectrometer, or a separately supplied plasma to a reaction cell, does not provide for control over at least the free electron energy, nor to an extent over the electron number density, beyond the values for these parameters that derive from the plasma as such. In alternative apparatus, electrons may be created separately in which case the electron number density and free electron energy of such electrons may be established as required.

Thus according to a second aspect, the present invention provides a mass spectrometer for elemental analysis of a sample including

source means for atomising a portion of the sample,

means for extracting a beam of particles from the source means, wherein the beam contains elemental sample ions and interfering polyatomic or doubly charged ions,

means for providing a population of electrons in a region through which the beam of particles is passed and which defines a predetermined path length for the particles through the electron population, said region being located within an evacuable chamber of the mass spectrometer whereby a low pressure is establishable in said region, said means for providing the population of electrons also allowing establishment of an electron number density and free electron energy for the population of electrons which, together with said predetermined path length and low pressure, provide for interfering polyatomic or doubly charged ions preferentially to undergo ion-electron recombination and thus dissociation thereby removing a significant number of the interfering ions from the beam of particles,

a mass analyser and an ion detector for receiving ions from the beam of particles after it has passed through the population of electrons for spectrometric analysis whereby concentrations of different elements in the sample are determinable.

In the second aspect of the invention the means for providing the population of electrons is preferably an electron generating device by means of which the required electron number density and free electron energy for the population of electrons can be established. This electron generating device is preferably configured and operated to confine the so-created electrons thus establishing an electron population through which the beam of particles is passed.

The electron generating device may comprise a tubular electron emitting cathode within which is located a tubular mesh electrode that is operable as an electron attracting anode, whereby a required electron number density can be established. The electron generating device may furthermore include a second tubular mesh electrode located within the first described tubular mesh electrode (that is, the anode), which is operable via application of a suitable potential thereto to establish a suitable free electron energy for the population of electrons within the device.

As an alternative to configuring the electron generating device to confine the generated electrons, the arrangement may be such that the generated electrons are magnetically confined to provide the population thereof.

An electron generating device as in embodiments of the second aspect of the invention may be used with plasma source mass spectrometers for elemental analysis such as ICP-MS, Microwave Induced Plasma MS, Laser Induced Plasma MS, Glow Discharge Plasma MS.

According to a third aspect, the present invention provides a method for elemental mass spectrometry of a sample including removing polyatomic or doubly charged ion interferences, the method including

atomising a portion of the sample and creating a beam of particles therefrom, wherein the beam contains elemental sample ions and interfering polyatomic or doubly charged ions,

establishing a population of electrons having an electron number density and free electron energy in a region at a predetermined low pressure,

passing the beam of particles through the population of electrons, the beam of particles having a predetermined path length through the population of electrons,

wherein said electron number density, free electron energy, low pressure and path length are such that interfering polyatomic or doubly charged ions contained in the beam preferentially undergo ion-electron recombination and thus disassociation thereby removing a significant quantity of such ions from the beam, and

spectrometrically analysing the masses of ions in the resultant beam to determine the elemental composition of the sample.

The step of establishing the population of electrons may involve generating a plasma by which the portion of a sample is atomised and providing a magnetic field to establish the population of electrons, the magnetic field being located and shaped to confine electrons from the plasma to a region.

Alternatively, the population of electrons may be established by supplying a plasma into the region, for example into a reaction cell through which the beam of particles is passed, whereby the plasma electrons constitute the population of electrons.

Alternatively the population of electrons may be established by creating electrons using an electron generating device, and confining the so created electrons to establish the population of electrons.

Values for the electron number density (ne), free electron energy (Ee), pressure (P) and path length are—

The invention includes magnetic confinement of electrons from the plasma together with use of an electron generating device. The electrons from the electron generating device may be magnetically confined to form a population thereof, or the device may be configured and operable to confine the generated electrons and thus form a population thereof, or both. The invention includes use of a plurality of electron generating devices.

Theoretical Basis for the Invention

Theoretical considerations to support the invention will now be described.

The idea underlying the invention is that interfering polyatomic and doubly-charged ions can be removed by preferential ion-electron recombination in the presence of free electrons.

The theory of ion-electron recombination will now be presented, to provide a basis for understanding the invention.

Ion-electron recombination is one of the known electron loss mechanisms in plasmas.

The characteristic plasma decay time tris given by:
tr=1/(βne0)
where ne0is the initial electron density (number of electrons per unit volume), and β is the ion-electron recombination coefficient (unit volume times the number of ion-electron recombinations per unit time). Values of β for several gaseous ions are shown in Table 1.

In Table 1, P is the gas pressure in millimeters of mercury (mm Hg). Teis the temperature of the plasma electrons. The unit of measurement is electron-Volts (eV).

Dissociative Recombination of Polyatomic Ions

The dissociative recombination of a polyatomic ion A2+is described by:
A2++e=A+A+E
where e is an electron, A is a neutral atom and E is the energy balance.

The energy of creation of two neutral argon atoms from dissociative recombination of an argon dimer ion (14.4 eV) is well above the energy of creation of a metastable argon atom (Ar*: 11.55 eV, 11.61 eV, or 11.72 eV). That is why dissociative recombination of Ar2+usually produces a metastable atom (Ar*) and a stable neutral atom (Ar).

The dissociative recombination coefficient for electrons and gaseous diatomic argon ions (Ar2+) is of the order of 10−7cm3/s (reference: ‘Physics of Gas Discharge’, Y. P. Raizer, Science, Moscow, 1987, p. 139)

Conversion Reaction Generating A2+

The reaction
A+A+(kinetic or excitation energy)=A2++e
generates polyatomic ions. It involves a third particle, usually another atom. The rate of conversion is given by:
d(nA2+)/dt=knA
where k is the conversion rate constant (in units of volume to the sixth power per unit time) and n denotes the number of species per unit volume. Some measured values of k are given in Table 2.

First, conditions have to be chosen to favour the dissociation of polyatomic ions over the reverse reaction. Secondly, possible mechanisms for the loss of analyte ions have to be considered.

Generation of Polyatomic Ions by Means of an Associative Conversion Reaction

The associative conversion reaction
B++A=AB+
can happen in regions of relatively high pressure and small electron density. The lifetime, τconv, of the monatomic ion, is given by
τconv=1/(k·(nA)2)

For example, consider the formation of the diatomic argon ion Ar2+by this process. If the Ar gas pressure is 10 Torr (nAr=3.3×1017cm−3). The lifetime of an Ar+ion before it converts to a Ar2+ion by associative conversion is
1/(k·nAr2)=1/(10−31cm6/s·1035cm−6)=10−4s.

Compare this with the rate of dissociative recombination at the same pressure, with an electron concentration of ne=1011cm−3and β=10−7cm3/s [typical values]. The recombination time tris given by
tr=1/(βne0)=1/(10−7cm3/s. 1011cm−3)=10−4s.

In this case τconv=trec=10−4s. Therefore a pressure of 10 Torr and a plasma electron density of ne=1011cm−3is enough to have the molecular dissociative recombination process balanced by the associative conversion process. This implies that with pressures lower than 10 Torr and with nehigher than 1011cm−3dissociative recombination must prevail over associative conversion.

Radiative Recombination

This process is represented by
A++e=A+hν
where hν represents electromagnetic radiation (light) that carries away the energy released in the recombination. The radiative recombination mechanism does not represent any danger (at least theoretically) for significant loss of analyte ions.
Radiative Recombination in a Three-Body Collision

This process is represented by
A++e+e=A+e+hν

In this case the energy released in the recombination is distributed between electromagnetic radiation (hν) and the increased kinetic energy of the second electron. Theoretically this may represent another mechanism for the loss of analyte ions, but it can be considered negligible.

Dissociative Electron Attachment to Molecular Polyatomic Ions

In this reaction an electron attaches itself to a polyatomic ion, and the energy of the collision breaks the bond between the atoms making up the ion.
AB++e=A+B

This mechanism favours the loss of polyatomic ions. The reaction has a coefficient of attachment βda=3.4·10−8cm3/s, which can favour the dissociation of polyatomic ions.

The free electron energy Eeshould be ˜1 eV. On one hand Eeshould not be very small, that is, not less then 0.01 eV, to avoid enhancing the rate of three-body radiative recombination relative to dissociative recombination. On the other hand, Eeshould be less then 5 eV because this avoids additional electron impact ionisation of the neutrals and metastables.

The number density of free electrons neis ˜1013–1014cm−3.

The volume V where free electrons are generated is 1–4 cm3.

The ion current I+in a typical ICP-MS instrument is 0.1–1 μA.

The ion velocity is ˜2 mm/μs. This is the speed of Ar2+at the ion energy ˜10 eV.

Theoretical Estimates of Polyatomic Ion Attenuation

In this section it is assumed that a population of electrons has been generated in an electron-generating device of the invention which is described hereinbelow, called an Electron Reaction Cell (ERC). A plasma ion beam is assumed to pass through the electron population, which is assumed to fill the ERC.

Gas, preferably hydrogen may be injected into the ERC using a separate injection port. This gas at a pressure of preferably 10−3–10−1Torr, may be used to generate sufficient ion density r to compensate possible electron space charge effect. The ion density may be generated by means of electron-neutral impact mechanism or any other known phenomena. In some specific cases the pressure may be much higher, for example 1 Torr, in which case the ERC dimensions may be significantly reduced down to a length L=0.5–1 cm.

The electron-ion recombination coefficient is of the order of β=10−7cm3/s for most polyatomic ions in ICP-MS. This value is used in the following calculations. ERC lengths of 1 cm, 2 cm and 4 cm and different electron densities ne=1013cm−3, ne=2·1013cm−3and ne=1014cm−3will be considered. It is assumed the electron energy is 1 eV and the gas pressure in the ERC volume is 10−2–10−4Torr. The pressure of a gas, preferably hydrogen, supplied into the ERC can be adjusted in order to generate sufficient electron density though the electron-neutral impact mechanism to prevent possible electron space charge effect.

From tr=1/(βne) the speed of polyatomic recombination to the 50% level equals 1 μs.

The time an argon dimer ion Ar2+spends inside a 1 cm long ERC at a speed of 2 mm/μs is t=5 μs or 5τr. Polyatomic attenuation αAr2+=25=32.

The time an Ar2+ion spends inside a 2 cm long ERC is t=20 μs or 10τr. Polyatomic attenuation αAr2+in a 2 cm ERC using the above conditions can be αAr2+=210=1024.

ERC 4 cm Long

The time an Ar2+ion spends inside a 4 cm long ERC is t=20 μs or 20τr. Polyatomic attenuation αAr2=220=1048576, i.e. ˜1 million.

Analyte Ion Loss Due to Recombination Inside the ERC

For the calculations we have chosen Cs as the analyte:
βCs˜10−10cm3/sec
and applied the formula (1) tr=1(βne0),

Results of calculations such as that illustrated in the previous section are summarised in Table 4.

TABLE 4Attenuation of Cs+ and Ar2+ ions in electron reactioncells of various lengths and with various electron densities.ne, cm−3αCs+αAr2+αAr2/αCs+1 cm long ERC10131.0053232/1.0052 × 10131.0110001000/1.0110141.02510151015/1.0252 cm long ERC10131.00510001000/1.0052 × 10131.01106106/1.0110141.0510301030/1.054 cm long ERC10131.01106106/1.012 × 10131.0210121012/1.0510141.110601060/1.1αCs+= caesium (i.e. an analyte ion) signal attenuation,αAr2+= argon dimer (ie a polyatomic ion) attenuation
Conclusions Drawn from the Theoretical BackgroundPolyatomic ion attenuation of 1×1060with only 10% loss of analyte ion intensity is at least theoretically possible (that is, with ne=1014cm−3, Ee˜1 eV, ERC=4 cm long, Pressure (P)=10−4–10−2Torr)It is noteworthy that a 4 cm long ERC could be capable of significant attenuation of interferences with about the same density of electrons (˜1013cm−3) as the density of free electrons in the argon plasma commonly used for ICP-MS. Thus, by simply preserving the plasma electrons, it is theoretically possible to achieve a polyatomic ion attenuation factor of around 1 million with only ˜1% loss of analyte ions.

For a better understanding of the invention and to show how it may be performed, embodiments thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With reference toFIG. 1an electron generating device10(herein termed an “Electron Reaction Cell”, or ERC) for use in mass spectrometers according to embodiments of the invention is shown in cross-section. It comprises a cylindrical cathode12(the axis for which is labelled13) preferably made of tungsten and preferably having a diameter of approximately 14 mm and a wall thickness of approximately 0.1 mm. Such a cathode would require approximately 3 amps current and a 0.5–1 volt voltage drop to reach the required electron-emitting surface temperature of about 2500–3000 K. The potential of cathode12should be approximately −10 V relative to ground. A first cylindrical mesh grid14(diameter approximately 12 mm) is located inside the cathode cylinder12and is used as an electron-attracting electrode. The potential of mesh grid14can be any positive voltage from approximately +90 V up to approximately +200V relative to ground. This allows use of the so-called Shottky emission saturation region where the electron space charge is negligible. In this case the cathode 12 temperature, provided the first mesh 14 voltage is constant, can control the electron density. There is a limitation to that voltage of approximately +300V because of the danger of melting the electrodes by the bombardment of emitted electrons. A second mesh grid16(diameter approximately 10 mm, approximately 1 mm from the cathode12surface) is located inside the first cylindrical mesh grid14and is used to establish the electron energy in the Electron Reaction Cell10. Mesh grid16is shown as including radially extending end portions17, but these may be omitted. The electron energy is defined by the difference of the potential of cathode12and the potential of the second mesh grid16. For 5 eV electron energy the potential of mesh grid16must be −5 V if the potential of cathode12is −10V. The mesh grid16optical transparency is approximately 70%. The ERC10includes end plates18which define entrance and exit apertures20. These must be set at negative voltages to trap the electron population inside the cell10. The spacing of end plates18provides a predetermined path length for the beam of particles to pass through the population of electrons.

Considering, for example, operation of the cell10with the first mesh grid14at +90V., the second mesh grid16at −5 V and the cathode12at −10V. Using Child-Langmuir law, the maximum current Iedrawn from the cathode12is Ie=˜250 mA/cm2. Taking into consideration the emitting surface of cathode12is approximately 4 cm2per 1 cm3of volume and the transparency of the mesh grid is 0.7, the electron current into the middle part of the cell10could be up to 1 A/cm3·0.72=0.5 A/cm3. It means 3·×1019electrons enter the middle part of the cell every second. If the electron residence time in the middle part is 1 ms this gives at least 3×1013electrons per cm3. If the ERC10were to be surrounded by a co-axial magnetic field, the ERC would be able to hold electrons inside for a relatively long time.

An ERC10can be located anywhere behind the skimmer cone, i.e. in the second or third chamber of a conventional ICP-MS instrument. However use of a “low internal background mass-analyser” would be necessary, because the metastable atoms produced by the ERC10would otherwise lead to excessive continuous background. If the ERC10is in the third chamber it would be positioned slightly away from the entrance aperture to allow the residual gas pressure to drop to less than 10−4Torr.

With reference toFIG. 2, an embodiment of an ICP-MS22according to an embodiment of the invention is shown which employs magnetic fields to confine plasma ions and electrons to provide the electron population without the use of an ERC10. Such an embodiment is referred to as a Magnetohydrodynamic Magnetic mirror system. It preserves original plasma electrons for ion-electron dissociative combination to attenuate polyatomic and doubly charged ion interferences. The ICP-MS22has a source means24, that is an inductively coupled plasma, for atomising a portion of a sample which is entrained into the plasma24. The plasma and atomised sample24impinges on a sampler cone26, which in combination with a skimmer cone28forms an interface between the atmospheric pressure plasma24and a mass spectrometer. Such an interface is known in the art. Means for providing confined populations of electrons in the form of coils30and32are shown located behind, respectively, the sampler cone26and the skimmer cone28. These coils are for creating an axial magnetic field that causes ions and electrons from plasma24to be at least temporarily confined in regions34and36, and thereby favour dissociative recombination of polyatomic ions and doubly charged ions and electrons according to the invention. Region34is contained in evacuable chamber35(that is, the first chamber) of the mass spectrometer22and region36is contained in the second evacuable chamber37of the mass spectrometer22. On emerging from region36, all ions that have not undergone recombination with electrons are focussed by ion optics system38within chamber37to form an ion beam40. Ion beam40then enters mass analyser42contained in a third evacuable chamber41of the mass spectrometer22and ions are separated according to their mass-to-charge ratio and are subsequently detected by an ion detector44. The output45of the ion detector44is then processed to produce a mass spectrum as is known in the art.

According to the invention, either coil30or coil32alone may be provided in the ICP-MS22.

In the various following embodiments, the same reference numerals as are used inFIGS. 1 and 2are used to indicate corresponding components. Also, depiction of the chambers35,37and41has been omitted for clarity.

FIG. 3schematically illustrates another ICP-MS46in which the coil32of theFIG. 2embodiment22is replaced with an extraction electrode48followed by an ERC10as inFIG. 1. The extraction electrode48is operated at a selectable potential in the range 0 to −1000V to direct positive ions into the cell10. Otherwise the components are the same as in theFIG. 2embodiment.

FIG. 4schematically illustrates a modification of the embodiment ofFIG. 3. In this embodiment, an ICP-MS50includes, in addition to the components of theFIG. 3embodiment46, a coil52to establish an axial magnetic field inside the ERC10. This has the effect of increasing residence time of atoms and ions in the ERC10.

The embodiment of an ICP MS54shown inFIG. 5is similar to theFIG. 3embodiment, except that the ERC10is located after the ion optics system38and in front of the mass analyser42. An ERC10may be placed at any convenient location in the ion path between a sampler cone26and mass analyser42. Furthermore, coils such as30,32and/or52(as inFIGS. 2 and 4) for establishing axial magnetic fields may be used in the ICP-MS54.

FIG. 6illustrates an ICP-MS embodiment56which employs two ERC's10, respectively labelled10aand10b, in the ion path. ERC10ais located directly behind extraction electrode48and ERC10bis located directly in front of mass analyser42. Coils such as30,32and/or52(as inFIGS. 2 and 4) for establishing axial magnetic fields may be used in the ICP-MS56.

FIG. 7shows an ICP-MS58that is similar to theFIG. 6embodiment56except it includes a third ERC10cdirectly after the sampler cone26. As in previous embodiments, coils such as30,32and/or52for establishing axial magnetic fields may be used in the ICP-MS58.

FIG. 8schematically shows an ICP-MS60which utilises a reflective ion optics system62(instead of a transmissive system38as in the previous embodiments) to cause ion beam40to bend through 90°. A first ERC10ais located directly behind sampler cone26a second ERC10bis located directly behind extraction electrode48after skimmer cone28and a third ERC10cis located directly in front of the mass-analyser42.

FIG. 9schematically illustrates another reaction cell64which may be included in a mass spectrometer22as inFIG. 2in place of the coils30and32. Reaction cell64may be located, for example, in chamber37following the skimmer cone28for a beam of particles66therefrom, containing elemental sample ions and interfering polyatomic or doubly charged ions, to pass through the cell64. Alternatively respective reaction cells64may be located in place of the cells10a,10b,10cinFIG. 8. Associated with reaction cell64is a plasma generating means68for supplying plasma into the reaction cell64whereby the plasma electrons provide the required population of electrons for the interfering polyatomic or doubly charged ions preferentially to undergo ion-electron recombination and thus dissociation thereby removing a significant number of them from the beam66.

FIG. 10schematically illustrates a modification of an ERC10as inFIG. 1which may be used in the embodiments ofFIGS. 3–8in place of the ERCs therein. The same reference numerals as inFIG. 1have been used to indicate the corresponding parts. The modification is that an inlet70is provided for supplying an ionisable gas72, preferably hydrogen, into the ERC10. Gas72undergoes electron impact ionisation by electrons emitted from electrode12in the region between electrodes14and16. Ions so produced reduce possible electron space charge effects which might occur in the central part of the ERC10due to excessive electron density. Using hydrogen as the ionisable gas is preferred. First, because of its low mass and therefore it causes low scattering losses of analyte ions. Secondly, hydrogen neutrals have high reactivity with argon ions. This brings about reaction of argon ions with hydrogen forming hydrogen-argon ions. Formed hydrogen-argon ions can be removed effectively later by electron—molecular ion reactions in the ERC10.

From the above description, for the ion-electron recombination and thus dissociation process to prevail over the reverse associative conversion process, the electron number density (ne) needs to be greater than 1011cm−3and the low pressure (P) less than 10 Torr. Also, the free electron energy (Ee) needs to be greater than 0.01 eV to avoid enhancing three body radiative recombination relative to the desired dissociative recombination process, and less than 5 eV to avoid additional electron impact ionisation of neutral and metastable particles. Ideally, a free electron energy (Ee) of approximately 1 eV is established for the population of electrons. Given the means for providing a population of electrons (for example a coil such as30or32, or an ERC such as10) is contained in an evacuable chamber35or37of a mass spectrometer, the low pressure establishable in the region containing the population of electrons will be the typical pressure at which the relevant chamber is maintained, for example 1–10 Torr for first chamber35of an ICP-MS22, 10−3–10−4Torr for second chamber37and 10−5–10−6Torr for third chamber41. For an ERC10into which gas is supplied (as inFIG. 10), the pressure will be higher as determined by the size of apertures20, but must be maintained below 10 Torr, and ideally is about 10−2Torr. Likewise the pressure within a reaction cell64(FIG. 9) is establishable to be lower than 10 Torr via the pressure within the pumped chamber which contains the cell64, the pressure of the supplied plasma, and the size of entry and exit apertures of the cell64.

It is furthermore shown above that at electron population of electron number density (ne) of approximately 1013cm−3at a free electron energy of approximately 1 eV, a path length of 1 cm through the electron population could attenuate interferences by a factor of 32 for a signal attenuation of 0.5% (αAr2/αCs+=32/1.005) whereas for an electron number density of 1014cm−3, the interferences attenuation is possibly1015for a signal attenuation of 2.5% (αAr2/αCs+=1015/1.025). For an electron number density of 1013cm−3at a path length of 4 cm, with free electron energy of approximately 1 eV, the interferences attenuation could be 106for a signal attenuation of 1% (αAr2/αCs+=106/1.01). For an electron number density of 1014cm−3at a 4 cm path length, the interferences attenuation could be 1060for a signal attenuation of 10%.

Based on the above and particularly the figures in Table 4, it is considered that the viable outer limits for the four parameters involved are:

II. free electron energy (Ee)>0.01 eV to <5 eV.

Preferably the free electron energy (Ee) is approximately 1 eV and the pressure P is <10−3Torr.

Preferably the electron number density (ne) is between 1012–1014cm−3, more preferably it is 1013–1014cm3.

Preferably the path length is between 2 to 4 cm, more preferably it is between 3 to 4 cm.

The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the scope of the following claims.