GB2545670A

GB2545670A - Mass spectrometers

Info

Publication number: GB2545670A
Application number: GB1522533.7A
Authority: GB
Inventors: Freedman Philip; Duncan Mills Mark
Original assignee: Nu Instruments Ltd
Current assignee: Nu Instruments Ltd
Priority date: 2015-12-21
Filing date: 2015-12-21
Publication date: 2017-06-28
Anticipated expiration: 2035-12-21
Also published as: DE102016014941A1; GB2545670B; GB201522533D0

Abstract

ICP-MS apparatus is described with a plasma ion source 1 and a multi-collector mass analyser containing an entrance slit 6, electrostatic analyser 7, analysing magnet 8 and multiple ion collectors 9. In accordance with the invention, an additional magnetic field 10 is applied after the plasma ion source but before the entrance slit of the multi-collector mass analyser. This additional magnetic field deflects ions in the ion beam according to their momentum and so for ions of the same nominal energy acts as a pre mass selector. Such apparatus provides a reduction of the low mass background ions entering the mass analyser in order to enhance the abundance sensitivity of the mass spectrometer for higher mass ions. Line of sight restrictions between the ICP source and the entrance slit also prevents neutrals from entering the analyser.

Description

MASS SPECTROMETERS

This invention relates to multi-collector mass spectrometers.

Inductively coupled plasma mass spectrometry (ICP-MS) is a technique employed for analysing inorganic elements, in particular metals, and is widely used in many fields including geological, nuclear and environmental industries.

In ICP-MS, an inductively coupled plasma is used as the ion source and a mass spectrometer apparatus is then used to separate and measure analyte ions formed in the ICP source. Normally the sample, in solution, is pumped through a nebuliser to generate a sample aerosol. This aerosol is then desolvated, atomised and ionised. The resulting analyte ions are then transferred from the plasma at near atmospheric pressure to a mass spectrometer which is within a vacuum chamber using a differentially pumped interface. The interface usually consists of a sampler and skimmer cone, the volume between them being evacuated to less than 1mbar, which allow the ions to pass through the aperture in the skimmer cone to the vacuum chamber. The ions are then focused into a mass spectrometer arrangement which separates the ions by their mass to charge ratio before measurement. Each elemental isotope appears at a different mass to charge ratio with the signal intensity proportional to the concentration of the isotope in the sample and thus elemental concentrations in the sample can be measured. Nominally all isotopes of an element will behave similarly during generation and extraction so the technique can also be used for precise isotope ratio measurements.

Multi-collector magnetic sector mass spectrometers can be coupled to an ICP source to give low detection limits and high resolving powers. These devices are described in WO 97/15944, and have been commercially available for over 20 years, for example the Nu Instruments Plasma II instrument. These mass spectrometers allow for simultaneous measurements of several isotopes permitting high precision isotope ratio measurements to be obtained. A key performance criteria for these instruments is the abundance sensitivity. The abundance sensitivity is a measure of the ability to measure small peaks adjacent to much larger ones, and can be defined as the inverse of the ratio of the ion current at mass M relative to the contribution of the peak tail of this peak at mass M ± 1. Figure 1 is a diagram of signal intensity versus mass for a typical intensity peak of mass M.

The abundance sensitivity can be defined as

where Im-i is the ion current for ions of mass M-1, which is much less than the intensity for the ion current of ions of mass M.

The abundance sensitivity is thus of paramount importance if it is desired to measure weak signals in the presence of large signals, here the weak signal from ions of mass M-1 compared to that of ions of mass M.

One of the main contributions to the peak tail, and thus the abundance sensitivity, is collisions of the ions with background molecules and ions. These collisions scatter the ion beam and lead to peak tailing. Due to the double focusing properties of the known multi-collector mass spectrometers only collisions after the entrance slit are relevant when considering the peak tails.

One of the inherent problems associated with the use of an ICP ion source is the large ion current arising from the plasma gas, usually argon. This ion current is often many orders of magnitude greater than that of the analyte.

Ion currents of greater than 1 μΑ are commonly measured after the extraction from the plasma. This large ion beam impinges on the walls and ion optical elements of the instrument giving rise to a background gas which causes collisions with the ions of interest leading to a significant peak tailing and is a key factor in the limitation on the abundance sensitivity.

As well as the argon ions from the plasma there is also a considerable amount of argon neutrals arising from the ICP source. This neutral beam again causes collisions with the ions of interest and adds another limitation on the abundance sensitivity.

It is clearly desirable to reduce or remove the argon ions and neutrals before entering the mass spectrometer to improve the abundance sensitivity.

One method for removing the argon ions is by using a radio frequency quadrupole as described in UK patent application 1519796.5. This device uses the mass selectivity of the quadrupole to reject the argon ions while transporting the ions of interest into the mass spectrometer.

An alternative method is described in US-B-6259091 which uses a hydrogen collision cell to perform a charge exchange reaction thus neutralising the argon ions.

The present invention provides a means for a reduction in the ion beam current due to the background ions to enhance the abundance sensitivity for multi-collector mass spectrometers.

According to the present invention, a mass spectrometer apparatus of the type described above is characterised by including between the plasma ion source and the entrance slit of the multi-collector mass analyser, a magnetic field ion deflector arranged to deflect ions in the ion beam by an amount dependent on the momentum of the ions.

Applying a magnetic field after the extraction but before the entrance to the multi-collector mass analyser enables rejection of low mass ions before the mass analyser’s entrance slit whereby the abundance sensitivity of the mass spectrometer may be improved.

In such apparatus, the magnetic field ion deflector acts as a low resolution momentum dispersive device. Magnetic fields have been used as mass selective devices since some of the very first mass spectrometers. The ions passing through a perpendicular magnetic field experience a force perpendicular to the ion velocity and the magnetic field such that F = mv/r = zvB which produces a circular orbit of radius r that is dependent on the momentum (mv) of the ion. This principle is used in all magnetic sector type spectrometers to select the mass of interest by picking the correct radius for the ions of the desired momentum.

In apparatus according to the present invention, a localised magnetic field is applied to the ion beam after the ICP source but before the defining slit at the entrance of the multi-collector mass analyser, thus enabling a preselection of the mass of interest while rejecting other masses.

While the apparatus of the present invention may be applied to a very wide variety of analytical areas, one example will illustrate the value of the techniques which can be practised using the apparatus.

Uranium isotope ratios are of great interest in nuclear science, and the apparatus of the present invention can be employed to deflect the uranium ions on to the axis of the mass spectrometer while deflecting the argon ions on to an alternative path that does not point to the mass analyser entrance. For example, a 10 millimetre long magnetic field of 300 milliTesla would deflect uranium 238 ions with 6keV of kinetic energy by ~17 milliRadians where as an argon ion of mass 40 would be deflected by ~42mRad. If the axis of the entrance to the mass analyser is set to be at 17m Rad to the direction of the original ion beam, the uranium ions of interest would pass into the mass analyser but the argon ions would be rejected.

However for isotope ratio measurements the other isotopes of uranium must also pass into the mass analyser. The uranium 235 isotope would be deflected 0.11 milliRadians more than the 238 isotope. Although these ions could be accepted into the mass analyser they would have a lower transmission and so lead to a mass bias effect.

This mass bias can be corrected for provided it is stable throughout the duration of the experiment. This correction can be achieved by running standard reference sample bracketed analysis that has become common place in these types of analysis and is described by Potter et al (International Journal of Mass Spectrometry 247 (2005) 10 -17).

To ensure this mass biasing is stable, the magnetic field needs to be stable over the time period of the experiment which can range from a few minutes to several hours. The simplest method for creating localised magnetic fields of these strengths and stabilities is to use permanent magnets. The most common of these are the rare earth magnets such as Neodymium and Samarium-Cobalt. These rare earth magnets are ideal for creating small permanent magnetic fields and, when used in conjunction with an iron yoke as is well known to those familiar with the art, to create high strength localised magnetic fields.

Alternatively electromagnets with magnetometer feedback can be employed to ensure a stable magnetic field. However these are difficult to produce for a small localised field due to the size of the coils required to produce the magnetic field. A further benefit of the apparatus according to the present invention is that there is no direct line of sight between the plasma and the multi-collector mass analyser entrance, so the neutral beam is stopped from entering the mass analyser.

The present invention is further illustrated with reference to the accompanying drawings, in which:

Figure 1 is a diagram of signal intensity versus mass, as referred to above;

Figure 2 is a diagrammatic representation of a known ICP-MS unit;

Figure 3 is a diagrammatic representation of a first embodiment of apparatus according to the present invention; and

Figure 4 is a diagrammatic representation of a preferred embodiment of apparatus according to the present invention.

Referring to Figure 2, this shows a classical high energy extraction arrangement for an ICP-MS system. A jet of gas is extracted from a plasma source 1 and consists of a beam of ions from the sample to be analysed together with the neutral beam. The ions pass through a sampler 2 and skimmer cone 3 and are then accelerated and focused through an extraction lens 4 to follow an ion path denoted path 1. The ions are then focused with electrostatic lenses 5 in to the entrance aperture 6 of a multi-collector mass spectrometer, in which the ions are then energy focused by an electrostatic analyser 7 and then mass selected by means of magnetic field deflector 8. The ions of different mass are then focused on to individual collectors 9 and the isotope ratios measured.

An ICP-MS system according to the present invention is shown in Figure 3. As in the case of the system shown in Figure 2, a jet of gas is extracted from a plasma source 1 and consists of a beam of ions from the sample to be analysed together with the neutral beam. The ions pass through the sampler 2 and skimmer cone 3 and are then accelerated and focused through the extraction lens 4 along an initial ion path. The ion beam then passes through a magnetic field 10 which deflects the ions according to their momentum.

The ion beam of the mass of interest is deflected on to an ion path 1 which is offset from the original ion beam. The lighter argon ions are deflected on to a path 2 while the neutrals continue along a path 3. The ions of interest are then focused by the electrostatic lenses 5 so as to direct them to the entrance aperture of the multi-collector mass spectrometer 6. The ions are then energy focused by the electrostatic analyser 7 and then mass selected through the magnetic field deflector 8. The ions of different mass are then focused on to individual collectors 9 and the isotope ratios measured. A preferred embodiment of the system according to the present invention is shown in Figure 4. As before, a jet of gas is extracted from a plasma source 1 and consists of a beam of ions from the sample to be analysed together with the neutral beam. The ions pass through the sampler 2 and skimmer cone 3 and are then accelerated and focused through the extraction lens 4 along the initial ion path. The ion beam is then dog-legged by electrostatic deflectors 12 and 13, so separating the ions from the neutrals, which continue along a path denoted 3. The ion beam then passes through a magnetic field 10 which deflects the ions according to their momentum. As shown in Figure 4, the ion beam of the mass of interest is deflected on to an ion Path 1 which is offset from the original ion beam, while the argon ions are deflected on to a path designated Path 2. The ions of interest on Path 1 are then deflected by electrostatic deflectors 11 on to the mass spectrometer axis and then focused with electrostatic lenses 5 so as to direct them to the entrance aperture of the multi-collector mass spectrometer 6. The ions are then energy focused by the electrostatic analyser 7 and then mass selected through the magnetic field 8. The ions of different mass are then focused on to individual collectors 9 and the isotope ratios measured.

In this preferred embodiment the ICP source and mass analyser entrance share a common axis which is mechanically favourable to reduce alignment issues and minimise manufacturing uncertainties, however there is no direct line of sight between them so the neutral beam is still removed.

These preferred arrangements allow for a reduction or removal of the argon ions and argon neutrals when analysing higher mass ions, thus lowering the background gas pressure within the multi collector mass spectrometer and thereby significantly improving the observed abundance sensitivity of the instrument.

The disclosed invention is not limited to the specific examples given and alternative configurations could be employed that are well known in the art.

Claims

1. A mass spectrometer apparatus including a plasma ion source and a multi-collector mass analyser, the mass analyser including successively an entrance slit for an ion beam, an electrostatic analyser, an analysing magnet and multiple ion collectors, and which is characterised by means for subjecting the ion beam to a magnetic field between the plasma ion source and the mass analyser entrance slit, whereby to deflect ions in the beam to an extent dependent on their momentum.

2. Apparatus according to Claim 1 wherein the mass analyser axis is offset from the ICP source axis, thus separating the neutral beam from the ion beam.

3. Apparatus according to any one of Claims 1 to 3 wherein the strength of the magnetic field between the plasma ion source and the entrance slit is between 100 and 500 milliTesla.

3. Apparatus according to Claim 1 wherein the ion beam is doglegged between the ICP source and the mass analyser entrance which share a common axis but have no direct line of sight between them, thus separating the neutral beam from the ion beam.