GB2535826A - Mass spectrometers - Google Patents

Abstract

Mass spectrometer with an ion source 1 and a mass analyser 17. The ion may be directed along one of two paths between the ion source 1 and the mass analyser 17. Path 1 contains at least one ion optical element 7 with a periodic oscillating electric field to enable mass rejection of ions to occur. Ions travelling along path 2 are subjected to purely electrostatic forces so no mass discrimination occurs. The apparatus includes means 4 to direct beams of the ions along one or other path as desired. Such apparatus provides a reduction in the background ions in order to enhance the abundance sensitivity of the mass spectrometer while providing an alternative electrostatic ion path enabling the same apparatus to be used for analysis of low mass ions with minimal mass biasing. The mass spectrometer may have and inductively coupled plasma as the ion source, and the analyser may be of a double focussing magnetic sector type.

Description

MASS SPECTROMETERS

This invention relates to mass spectrometers, in particular those which use an inductively coupled plasma as the source of ions which are then passed into a mass analyser arrangement to enable ions of different mass to be separated and their abundance measured or relative abundance analysed. Such is referred to as ICP-MS -inductively coupled plasma mass spectrometers.

ICP-MS is a technique employed for analysing inorganic elements, in particular metals, and is widely used in many fields including geological and environmental industries.

The ICP-MS uses an inductively coupled plasma as the ion source and the mass analyser is then used to separate and measure analyte ions formed in the ICP source. Normally the sample, in solution, is pumped through a nebulizer to generate a sample aerosol. This is then passed in to the ICP where it is desolvated, atomized and ionized. The resulting analyte ions are then transferred from the plasma at near atmospheric pressure to the mass analyser which is within a vacuum chamber using a differentially pumped interface. The interface usually consists of a sample and skimmer cone, the volume between them being evacuated to less than 1 mbar, which allow the ions to pass through to the vacuum chamber. The ions are then extracted from the skimmer cone and focused into the mass analyser. The mass analyser then 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 is also used for isotope ratio measurements.

High resolution mass analysers such as a double focusing magnetic sector type can be coupled with the ICP source to provide mass spectrometers having low detection limits and high resolving powers. A key performance criterion is high abundance sensitivity which is the ratio of the maximum ion current recorded at a given mass to the ion current arising from the same species recorded at an adjacent mass.

While ICP MS has a high sensitivity and low detection limits one of the inherent problems with the 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 pA are commonly measured after the extraction from the plasma which through scattering can lead to a significant background on the mass spectrum. This background is a key factor in the limitation on the abundance sensitivity.

It is clearly desirable to reduce or remove the argon background ions before entering the mass analyser.

The traditional extraction and transfer optics used in high resolution ICP-MS use zo electrostatic lenses with a continuous ion beam. These elements extract and focus the ion beam but, as for all electrostatic devices, the focusing is only dependent on the ion's kinetic energy and not its mass. Since the ions are usually accelerated to high energies, to a first order approximation the ion kinetic energy is independent of mass. This leads to minimal mass discrimination which is an important factor in isotope ratio measurements but does not enable removal or reduction of certain mass ranges.

Mass selection or removal is commonly achieved using periodic oscillating field devices that can trap or reject selected masses. Ions in these devices typically have between 1-100 electron volts of kinetic energy and a transit time through the optical elements of 0.1 to 1000 ps. The periodic oscillating fields used to constrain these ions are often in the frequency range 100 kHz to 10MHz and as such are commonly referred to as RF (Radio Frequency) devices since they sit within the radio frequency spectrum.

One device that can be used as a high pass filter is an RF only quadrupole which has the properties of passing all ions with a mass to charge ratio above the cut off value while rejecting all those below it. This low mass cut off can be calculated from the Mathieu constant as shown in Equation 1. eV

Equation I Where w is the angular frequency, V is the RF voltage and ro is the inscribed radius.

The limit of stability for an RF only quadrupole occurs when q 0.908 so from this the low mass cut off is given by Equation 2.

JA Y Equation 2 Figure 1 of the accompanying drawings shows the typical transmission curve of an RF only quadrupole. It can be seen from this example that the quadrupole acts as a high pass mass filter providing a sharp low mass cut off.

Alternatively higher order multipoles can be used as a high pass mass filter but they have a more complex cut off response due to their more complex trajectories. This can be shown by considering the motion of charged particles in a 2 dimensional RF only oscillating field. Equation 3 shows the equation of motion for charged particles in an RF only multipole having 2p poles.

ci2Z Equation 3 -1= Where the displacement in the complex xy plane, Z = X +iY = Ref° and the independent time variable T are unitless. It follows from this equation that only when p = 2 (quadrupole) that the X and Y coordinates are separate while all higher order multipoles have a dependence on the position in both axes. This can lead to a more complex and more undefined low mass cut off. This is shown in Figure 1 for the octopole where the rise of the low mass cut off extends over many tens of mass to charge units. This goes to explain why in practice quadrupoles are the only multipoles used as a mass filter rather than just a transport device.

The RF only quadrupole, which provides a solution for removing the low mass ions when analysing higher mass ions, is of no assistance when analysis of lower mass ions such as lithium is required.

One solution to this would be to use the quadrupole with a much lower cut off to pass all the ions above the masses of interest. However when quadrupoles are used with a very low mass cut off and hence small RF voltage, a significant mass bias arises near the low mass cut off. This mass bias is due to the properties of the quadrupole and the effective number of cycles of the RF voltage the ions experience. The ions in an RF only quadrupole can be seen to have two different oscillations as shown in Figure 2. These modes can be described as a micromode with a frequency that is only dependent on the RF frequency and a macromode that has a frequency dependence on the mass to [-2q cc -charge ratio (amongst other things). The frequency of the macromotion is given in Equation 4 and is dependent on the main RF frequency f, the low mass cut off mcutoff as well as the ion mass m.

21 Equation 4 The relative difference in mass of the isotopes of small ions such as lithium is large and hence the frequency of the oscillations is significantly different. Figure 2 of the accompanying drawings shows the variation in the oscillations for ions with a mass to charge ratio of 6 and of 7, with the same kinetic energy. The ions with a mass to charge ratio of 7 perform just over 7 periods of macromode oscillations while the ions with m/q = 6 perform nearly 8 periods of macromode oscillations. This leads to a mass discrimination for the lower masses which is extremely detrimental to accurate isotope ratio measurements. This is also true for all higher order multipoles due to the periodic oscillating fields.

This approach of using periodic oscillating field elements to remove or reduce unwanted masses is known. US Patent Specification 4963736 describes an ICP MS with an RF only quadrupole that precedes a mass resolving quadrupole mass spectrometer. This first quadrupole could also be used as a collision cell to thermally cool and focus the ions as well as mass rejection before the mass analyser. US-A-6140638 describes the use of a band pass quadrupole and reaction cell for a more selective removal of background ions. US-A- 2007/0228268 describes a first mass rejection using a quadrupole followed by a collision cell and finally the mass analyser. EP-A-0813228 proposes the use of an RF only hexapole as a collision cell with addition of an inert gas before the mass spectrometer. However all these devices as described previously have a large mass dependence so an alternative approach is needed for analysing low mass analyte ions.

The electrostatic fields commonly used in the transfer optics of mass spectrometers only discriminate by energy and so give minimal mass biasing, especially for the lower mass ions when compared to RF devices which, often intentionally, discriminate by mass. It is clearly desirable to maintain this property when analysing low mass analyte ions whilst being able to remove the background ions when looking at higher mass analyte ions, and this invention accordingly seeks to provide mass spectrometer arrangements which achieve this.

In accordance with the present invention, there is provided mass spectrometer apparatus including an ion source, a mass analyser, and means to direct ions from the ion source along one of two paths between the ion source and the mass analyser, wherein one path contains at least one ion optical element with a periodic oscillating electric field (thus enabling mass rejection) and wherein the other path applies purely electrostatic forces to the ions, thus operating without any mass discrimination, and wherein the apparatus includes means to direct the ion beam between the ion source and the mass analyser along one or other path as desired. Essentially the path taken by the ion beams is split and then recombined so that the ions emerge from the ion source, pass along one or other path, and then pass into the mass analyser.

By operating in this way, the mass spectrometer apparatus does not require the provision of multiple mass analysers, as disclosed in various prior art patent specifications, in particular US-A-2009/0090853, GB-A-2483201, WO 2007/133469, WO 2004/068523 and US-A-20040119012. Each of these disclosures deflecting or splitting an ion beam so as to direct ions selectively to one or more of a plurality of separate mass analysers or detectors.

Such an arrangement enables a reduction in the background ions to be achieved so as to enhance the abundance sensitivity of the mass spectrometer apparatus while providing an alternate electrostatic ion path to allow analysis of low mass ions with minimal mass biasing, and without the need to use separate mass analysers or detector systems. The invention is of particular value in ICP-MS arrangements.

One way of putting the invention into practice is illustrated by way of example with reference to Figures 3 and 4 of the accompanying drawings, which each show diagrammatically the construction of ICP-MS apparatus according to the present invention.

Referring first to Figure 3, by means of a standard ICP unit (not shown, but located to the left of the component shown in Figure 3), a jet of gas is extracted from a plasma source. The jet consists of a beam of ions from the sample to be analysed and neutrals. The ions pass through a sampler 1 and skimmer cone 2 and are then accelerated and focused by an extraction lens 3 so that they travel along a horizontal path (as shown in Figure 1 from left to right) toward a mass analyser unit 17 shown at the right in Figure 3.

For analysis of samples that require the reduction of the argon ion background, a deflector 4 is energised to deflect the ions along a path denoted "Path 1". The ions are then deflected again by a further deflector 5 and decelerated by using a conventional lens element 6 prior to passing into an RF quadrupole unit 7. Once in the RF only quadrupole unit, the ions with a mass to charge ratio lower than that of the low mass cut off of the quadrupole follow an unstable trajectory and are deflected from the main ion beam. The ions with a mass to charge ratio greater than that of the low mass cut off continue on a stable trajectory through the quadrupole. These ions, on leaving the quadrupole, are then re-accelerated by means of a standard lens element 8 and focused by a standard focussing lens arrangement 9 just upstream of an entrance aperture 10 of the mass analyser unit 17.

When analysis of samples containing lower mass ions or those that do not require the removal of the argon related ions is to be undertaken, deflector 4 is unenergised, allowing the ion beam to continue along a path denoted "Path 2". The ions are then focused by a lens arrangement 11 and deflected by a pair of deflectors 12 and 13 which are energised to deflect the ion beam upwards and then horizontally as shown in Figure 3 so that they are then directed along the same paths leading to the entrance aperture 10 of the mass analyser unit 17.

As can be seen from Figure 3, the ions reach the mass analyser 17 after being focussed by the lens arrangement 9 in front of the entrance aperture 10.

This preferred arrangement allows for a reduction or removal of the argon background when analysing higher mass ions using the ion Path 1 while retaining the ability to analyse low mass ions with minimal mass biasing by directing the ions along the alternate ion Path 2 between the ion source and the mass analyser 17.

This arrangement also has the benefit of having the main mass spectrometer ion path on a different axis to the source (note that, as seen in Figure 3, the ion beam path at the entrance to sample 1 is lower than the entance aperture 10) hence reducing the gas load in to the mass spectrometer, since undeflected neutrals will no longer enter the mass spectrometer slit. To further improve the vacuum in the mass spectrometer the vacuum chamber containing these elements can be split into two regions by means of a wall containing two apertures 14, 15 through which the ion beam passes dependent on whether it is directed along Path 1 or Path 2 respectively. Differential pumping may be provided by means of pumps 1, 2 and 3 which evacuate the three chambers illustrated to different degrees. By providing the central wall with apertures 14 and 15, the vacuum in the chamber to the right of the wall as shown in the drawings may be subjected to a greater vacuum than that prevailing in the central chamber, which reduces the probability of ion collisions and further improves abundance sensitivity.

Other elements may be placed in the path occupied by the quadrupole, to provide increased performance, as is well known in the art. Thus the simple quadrupole may be replaced by a collision cell, through which a buffer gas is passed, to enable charge exchange to occur between the argon ions and collision gas. This again can reduce the argon background, whilst also permitting the breakdown of any molecular ions present in the ion beam. This is of importance in cases such as (for example) the analysis of iron isotopes, where the 56Fe ion occurs at the same nominal mass as the 40Ar160 molecular species. To separate these two ions, in the absence of the collision cell, the mass spectrometer has to be used in a high resolution mode, with a concomitant reduction of transmission.

Multiple radio frequency elements can also be employed in the path as shown in Figure 4 to enable more complex ion clean-up/reactions to occur. Thus a collision cell 16 may be placed after the quadrupole filter, to enable more specific reactions to take place, and to remove the possibility of competing reactions (which could result in ions which interfered with those of interest) occurring.

In a further embodiment a mass resolving quadrupole could be employed to select a small range of masses to be transmitted. Alternatively, the quadrupole filter could be run in a "notch" transmission mode, in which ions of a selected mass are rejected while all others are transmitted.

Since the ion beam transmitted through the sampler/skimmer region is also accompanied by a large beam of neutral (un-ionised) species, it is normally sensible to ensure that the collision cell/quadrupole optics do not lie in the direct path of this neutral beam. This will ensure that any deposits on the rods of these elements is minimised. This is important since the ion energy used with such devices is quite low (to ensure that a reasonably large number of oscillations occur within the active field regions). As such, field distortions due to charge build up on the rods from the deposit of non conducting material need to be minimised, and it is customary to place these elements such that they are not exposed to the neutral species beam. This is shown in the arrangement in Figure 3, but other arrangements are possible, as is well known in the art. The simple configuration shown here, where the second path available for those species for which the extra reaction or conditioning prior to mass analysis is not required, contains the longer path with coincident neutral species which is not a disadvantage, since the ions which travel this route are not decelerated, and so are more immune to contamination problems. Other configurations for the split and then recombined path for the ion beam between the source and the analyser will be readily conceived by those familiar with the art.

This simple configuration can also be employed with mass spectrometers fitted with other ionisation sources, and is not limited to a plasma.

While the present invention is of particular value when applied to mass spectrometers using double focusing magnetic sector type mass analysers, it 25 can be applied to mass spectrometers using any other type of mass analyser well-known in the art.

Claims (9)

1. CLAIMS1. Mass spectrometer apparatus including an ion source, a mass analyser, and means to direct ions from the ion source along one of two paths between the ion source and the mass analyser, wherein one path contains at least one ion optical element with a periodic oscillating electric field (thus enabling mass rejection) and wherein the other path applies purely electrostatic forces to the ions, thus operating without any mass discrimination, and wherein the apparatus includes means to direct the ion beam between the ion source and the mass analyser along one or other path as desired. Essentially the path taken by the ion beams is split and then recombined so that the ions emerge from the ion source, pass along one or other path, and then pass into the mass analyser.
2. 2. Mass spectrometer apparatus according to Claim 1 wherein a high pass quadrupole is placed in the first path.
3. 3. Mass spectrometer apparatus according to Claim 1 wherein a radiofrequency controlled collision cell is placed in the first path.
4. 4. Mass spectrometer apparatus according to Claim 1 wherein a high pass quadrupole followed by a collision cell is placed in the first path.
5. 5. Mass spectrometer apparatus according to Claim 1 wherein a mass resolving quadrupole is placed in the first path.
6. 6. Mass spectrometer apparatus according to Claim 1 wherein a notch filter quadrupole is placed in the first path to remove selected mass ions.
7. 7. Mass spectrometer apparatus according to Claim 1 wherein a radio frequency hexapole or high order multipole is placed in the first path.
8. 8. Mass spectrometer apparatus according to any one of the preceding Claims wherein the ion source is an inductively coupled plasma.
9. 9. Mass spectrometer apparatus according to any one of the preceding Claims wherein the mass analyser is of a double focusing magnetic sector type.