Patent Application
20050199795
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.
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.
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
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
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
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—
Electron number density (ne)>1011 cm−3 to 1014 cm−3, preferably 1012 to 1014 cm−3, more preferably 1013 to 1014 cm−3.
Free electron energy (Ee)>0.01 eV to <5 eV, preferably approximately 1 eV.
Pressure (P)<10 Torr, preferably <10−3 Torr.
Path length 1 to 4 cm, preferably 2 to 4 cm, more preferably 3 to 4 cm.
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
Ion-electron recombination is one of the known electron loss mechanisms in plasmas.
The characteristic plasma decay time tr is given by:
tr=1/(βne0)
where ne0 is 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 millimetres of mercury (mm Hg). Te is 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.
For example, for the argon dimer ion:
Ar2++1.4 eV=Ar++Ar;
Ar+15.8 eV=Ar++e
Ar2++e=Ar+Ar+14.4 eV.
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 10−7 cm3/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.
Application of Electron-Ion Recombination to Interference Removal in Elemental Mass Spectrometry
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=3.3×1017 cm−3). The lifetime of an Ar+ ion before it converts to a Ar2+ ion by associative conversion is
1/(k·nAr2)=1/(10−31 cm6/s·1035 cm−6)=10−4 s.
Compare this with the rate of dissociative recombination at the same pressure, with an electron concentration of ne=1011 cm−3 and β=10−7 cm3/s [typical values]. The recombination time tr is given by
tr=1/(βne0)=1/(10−7 cm3/s. 1011 cm−3)=10−4 s.
In this case τconv=trec=10−4 s. Therefore a pressure of 10 Torr and a plasma electron density of ne=1011 cm−3 is 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 ne higher than 1011 cm−3 dissociative 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=3.4·10−8 cm3/s, which can favour the dissociation of polyatomic ions.
Electron Characteristics Favouring Dissociative Ion-Electron Recombination
The free electron energy Ee should be ˜1 eV. On one hand Ee should 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, Ee should be less then 5 eV because this avoids additional electron impact ionisation of the neutrals and metastables.
The number density of free electrons ne is ˜1013-1014 cm−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 10−3-10−1 Torr, 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=10−7 cm3/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=1013 cm−3, ne=2·1013 cm−3 and ne=1014 cm−3 will be considered. It is assumed the electron energy is 1 eV and the gas pressure in the ERC volume is 10−2-10−4 Torr. 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.
1) ne˜1013 cm−3
From tr=1/(βne) the speed of polyatomic recombination to the 50% level equals 1 μs.
ERC 1 cm long.
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.
ERC 2 cm long.
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˜10−10 cm3/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.
αCs+ = caesium (i.e. an analyte ion) signal attenuation,
αAr2+ = argon dimer (ie a polyatomic ion) attenuation
Conclusions Drawn from the Theoretical Background
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.
FIGS. 4 to 8 schematically illustrate further embodiments using various combinations of magnetic fields and electron generating devices to establish electron populations for attenuating interfering ions.
With reference to
Considering, for example, operation of the cell 10 with the first mesh grid 14 at +90V., the second mesh grid 16 at −5 V and the cathode 12 at −10V. Using Child-Langmuir law, the maximum current Ie drawn from the cathode 12 is Ie=˜250 mA/cm2. Taking into consideration the emitting surface of cathode 12 is approximately 4 cm2 per 1 cm3 of volume and the transparency of the mesh grid is 0.7, the electron current into the middle part of the cell 10 could be up to 1 A/cm3·0.72=0.5 A/cm3. It means 3·×1019 electrons 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 3×1013 electrons per cm3. If the ERC 10 were to be surrounded by a co-axial magnetic field, the ERC would be able to hold electrons inside for a relatively long time.
An ERC 10 can 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 ERC 10 would otherwise lead to excessive continuous background. If the ERC 10 is 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 10−4 Torr.
With reference to
According to the invention, either coil 30 or coil 32 alone may be provided in the ICP-MS 22.
In the various following embodiments, the same reference numerals as are used in
The embodiment of an ICP MS 54 shown in
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 1011 cm−3 and 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 as 30 or 32, or an ERC such as 10) is contained in an evacuable chamber 35 or 37 of 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 chamber 35 of an ICP-MS 22, 10−3-10−4 Torr for second chamber 37 and 10−5-10−6 Torr for third chamber 41. For an ERC 10 into which gas is supplied (as in
It is furthermore shown above that at electron population of electron number density (ne) of approximately 1013 cm−3 at 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 1014 cm−3, the interferences attenuation is possibly 1015 for a signal attenuation of 2.5% (αAr2/αCs+=1015/1.025). For an electron number density of 1013 cm−3 at a path length of 4 cm, with free electron energy of approximately 1 eV, the interferences attenuation could be 106 for a signal attenuation of 1% (αAr2/αCs+=106/1.01). For an electron number density of 1014 cm−3 at a 4 cm path length, the interferences attenuation could be 1060 for 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:
Preferably the free electron energy (Ee) is approximately 1 eV and the pressure P is <10−3 Torr.
Preferably the electron number density (ne) is between 1012-1014 cm−3, more preferably it is 1013-1014 cm3.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| PR 7593 | Sep 2001 | AU | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/AU02/01239 | Sep 2002 | US |
| Child | 10489215 | Mar 2004 | US |
