Mass Spectrometer

Abstract

An isotope ratio mass spectrometer has an ion source, a static field mass filter, a reaction cell to induce a mass shift reaction, and a sector field mass analyser for spatially separating ions from the reaction cell according to their m/z. A detector platform detects a plurality of different ion species separated by the sector field mass analyser. The static field mass filter has a first Wien filter that deflects ions away from a longitudinal symmetry axis of the spectrometer in accordance with the ions' m/z, and a second Wien filter that deflects ions back towards the longitudinal symmetry axis in accordance with the ions' m/z. An inverting lens is positioned along the longitudinal axis between the Wien filters to invert the direction of deflection of the ions from the first Wien filter. The static field mass filter provides high transmission and improved spectrometer sensitivity. The first and second Wien filters permit simple tuning.

1 . A static field mass filter for a mass spectrometer comprising:
 an entrance aperture configured to receive an ion beam from an ion source;   an exit aperture configured to eject ions, the static field mass filter defining a longitudinal symmetry axis between the entrance and exit apertures;   a first Wien filter configured to deflect ions away from the longitudinal symmetry axis in accordance with their m/z;   a second Wien filter configured to deflect the ions back towards the longitudinal symmetry axis in accordance with their m/z; and   an inverting lens positioned along the longitudinal symmetry axis between the first and second Wien filters, to invert the direction of deflection of the ions from the first Wien filter.   
     
     
         2 . A mass spectrometer comprising the static field mass filter of  claim 1 , an ion source arranged upstream of the static field mass filter and configured to direct ions into the entrance aperture thereof, and a mass analyser arranged downstream of the static field mass filter, configured to mass analyse either (a) ions ejected from the exit aperture thereof, or (b) product ions derived from the ions ejected from the exit aperture of the static field mass filter. 
     
     
         3 . A mass spectrometer comprising the static field mass filter of  claim 1 , an ion source arranged upstream of the static field mass filter and configured to direct ions into the entrance aperture thereof, a reaction cell located downstream of the exit aperture of the static field mass filter, and a mass analyser downstream of the reaction cell, the reaction cell being configured to cause the ions received from the static field mass filter to undergo a change in their properties, and to eject the reacted ions to the mass analyser. 
     
     
         4 . A mass spectrometer according to  claim 2  for analysing isotope ratios of atoms or molecules contained in a sample wherein
 the ion source is configured to generate a beam of ions from the sample; 
 the mass analyser is a sector field mass analyser configured to spatially separate the analysed ions in accordance with their m/z; and further wherein the mass spectrometer comprises an ion multicollector having either (i) a plurality of ion detectors each arranged downstream of the sector field mass analyser or (ii) a single ion detector arranged downstream of the sector field mass analyser , for detecting a plurality of different ion species each separated by the sector field mass analyser. 
 
     
     
         5 . The mass spectrometer of  claim 2 , wherein the static field mass filter further comprises a diaphragm located between the first and the second Wien filters, the diaphragm having an aperture configured to allow passage of at least some of the ion species exiting the first Wien filter. 
     
     
         6 . The mass spectrometer of  claim 5 , wherein the aperture of the diaphragm is of a dimension such that ions having an m/z higher than a threshold m/z are deflected by the first Wien filter a relatively smaller distance in a direction orthogonal to the longitudinal symmetry axis so that they are able to pass through the aperture, and ions having an m/z lower than a threshold m/z are deflected by the first Wien filter a relatively greater distance in a direction orthogonal to the longitudinal symmetry axis so that they are unable to pass through the aperture. 
     
     
         7 . The mass spectrometer of  claim 5 , wherein the aperture of the diaphragm is of a dimension such that ions having an m/z lower than a threshold m/z are deflected by the first Wien filter a relatively smaller distance in a direction orthogonal to the longitudinal symmetry axis so that they are able to pass through the aperture, and ions having an m/z higher than a threshold m/z are deflected by the first Wien filter a relatively greater distance in a direction orthogonal to the longitudinal symmetry axis so that they are unable to pass through the aperture. 
     
     
         8 . The mass spectrometer of  claim 5 , wherein the aperture of the diaphragm is of a dimension such that ions having an m/z in a m/z window are deflected by the first Wien filter a relatively smaller distance in a direction orthogonal to the longitudinal symmetry axis so that they are able to pass through the aperture, and ions having an m/z outside m/z window are deflected by the first Wien filter a relatively greater distance in a direction orthogonal to the longitudinal symmetry axis so that they are unable to pass through the aperture. 
     
     
         9 . The mass spectrometer of  claim 2 , wherein the inverting lens is asymmetric. 
     
     
         10 . The mass spectrometer of  claim 9 , wherein the inverting lens is a multipole lens. 
     
     
         11 . The mass spectrometer of  claim 10 , wherein the multipole lens comprises a plurality of crossed slit aperture lenses. 
     
     
         12 . The mass spectrometer of  claim 10 , wherein the multipole lens is an Einzel lens having first and second circumferentially opposed lens segments to which a first voltage is applied in use, and having third and fourth circumferentially opposed lens segments interposed between the first and second circumferentially opposed lens segments, the third and fourth circumferentially opposed lens segments having a second voltage, different from the first voltage, applied to them in use. 
     
     
         13 . The mass spectrometer of  claim 2 , further comprising a first magnetic shield between the first Wien filter and the inverting lens, and a second magnetic shield between the inverting lens and the second Wien filter, to reduce or remove the effect of stray magnetic fields of the first and second Wien filters, upon the inverting lens. 
     
     
         14 . The mass spectrometer of  claim 2 , further comprising an entrance lens positioned between the entrance aperture of the static field mass filter and the first Wien filter. 
     
     
         15 . The mass spectrometer of  claim 14 , wherein the entrance lens is formed as an Einzel lens. 
     
     
         16 . The mass spectrometer of  claim 14 , further comprising a third magnetic shield between the entrance lens and the first Wien filter, to reduce or remove the effect of stray magnetic fields of the first and/or second Wien filters, upon the entrance lens. 
     
     
         17 . The mass spectrometer of  claim 2 , further comprising an exit lens positioned between the second Wien filter and the exit aperture of the static field mass filter. 
     
     
         18 . The mass spectrometer of  claim 17 , wherein the exit lens is formed as an Einzel lens. 
     
     
         19 . The mass spectrometer of  claim 17 , further comprising a fourth magnetic shield between the second Wien filter and the exit lens, to reduce or remove the effect of stray magnetic fields of the first and/or second Wien filters, upon the exit electrode. 
     
     
         20 . The mass spectrometer of  claim 15 , wherein the Einzel lens has a central electrode which is divided into a plurality of segments, preferably  4  segments. 
     
     
         21 . The mass spectrometer of  claim 2 , further comprising an electrostatic controller. 
     
     
         22 . The isotope ratio mass spectrometer of  claim 21 , wherein the electrostatic controller is connected to both the first and the second Wien filters so as to control the electric field and/or magnetic flux density of each. 
     
     
         23 . The mass spectrometer of  claim 2 , wherein the first Wien filter comprises a central channel through which ions in the ion beam can pass wherein the central channel is bounded by at least one baffle for catching ions deflected away from the said longitudinal symmetry axis. 
     
     
         24 . The mass spectrometer of  claim 23 , wherein the central channel is bounded on first and second sides by a plurality of baffles. 
     
     
         25 . The mass spectrometer of  claim 24 , wherein the plurality of baffles are positioned at an acute angle to the longitudinal symmetry axis in a herringbone pattern. 
     
     
         26 . The mass spectrometer of  claim 2 , wherein the first Wien filter includes a plurality of electrical conductors extending in a direction parallel with the longitudinal symmetry axis, for generating an electric field within a central channel of the first Wien filter through which ions in the ion beam can pass. 
     
     
         27 . The mass spectrometer of  claim 26 , wherein the first Wien filter includes a first electrode having a first group of the plurality of electrical conductors, and a second electrode having a second group of the plurality of electrical conductors, the first and second electrodes being spatially separated in a direction perpendicular to the said longitudinal symmetry axis so as at least partially to define between them the said central channel. 
     
     
         28 . The mass spectrometer of  claim 2 , wherein the second Wien filter includes a plurality of electrical conductors extending in a direction parallel with the longitudinal symmetry axis, for generating an electric field within a central channel of the second Wien filter through which ions in the ion beam can pass. 
     
     
         29 . The mass spectrometer of  claim 28 , wherein the second Wien filter includes a first electrode having a first group of the plurality of electrical conductors, and a second electrode having a second group of the plurality of electrical conductors, the first and second electrodes being spatially separated in a direction perpendicular to the said longitudinal symmetry axis so as at least partially to define between them the said central channel. 
     
     
         30 . The mass spectrometer of  claim 27 , wherein the first electrode comprises an electrically insulating substrate upon or within which is formed the said first group of the plurality of electrical conductors as an array, each conductor extending in a direction parallel with the said longitudinal symmetry axis, and further wherein the second electrode comprises an electrically insulating substrate upon or within which is formed the said second group of the plurality of electrical conductors as an array, each conductor extending in a direction parallel with the said longitudinal symmetry axis. 
     
     
         31 . The mass spectrometer of  claim 26 , wherein each of the electrical conductors in the first group is connected to a first voltage divider, and wherein each of the electrical conductors in the second group is connected to a second voltage divider. 
     
     
         32 . The isotope ratio mass spectrometer of  claim 2 , in which the ion source is an inductively coupled plasma (ICP) ion source. 
     
     
         33 . The isotope ratio mass spectrometer of  claim 3 , in which the reaction cell is a collision cell. 
     
     
         34 . A method of mass spectrometry, comprising the steps of:
 (a) generating a beam of ions from a sample;   (b) selecting ions from the beam in accordance with their mass to charge ratio, using a static field mass filter;   (c) spatially separating the ions received from the static field mass filter in a sector field analyser, based upon their mass to charge ratio; and   (d) detecting a plurality of different ion species from the separated ions in a multicollector.   
     
     
         35 . The method of mass spectrometry of  claim 34 , further comprising the step of inducing a mass shift reaction in ions received from the static field mass filter using a reaction cell before spatially separating the ions 
     
     
         36 . The method of  claim 34 , wherein the step (b) of selecting ions from the beam in accordance with their mass to charge ratio, comprises
 injecting ions from the ion source into the static field mass filter, through an entrance aperture thereof;   deflecting ions away from a longitudinal symmetry axis of the static field mass filter using a first Wien filter, so as to separate ions in a direction orthogonal to the longitudinal symmetry axis, in accordance with their m/z;   deflecting ions downstream of the first Wien filter back towards the longitudinal symmetry axis, using an inversion lens; and   focusing ions towards an exit aperture of the static field mass filter using a second Wien filter.   
     
     
         37 . The method of  claim 34 , wherein the step (a) comprises generating ions using an inductively coupled plasma (ICP) ion source. 
     
     
         38 . The method of  claim 34 , further comprising determining at least one isotopic ratio of atoms or molecules contained in the sample on the basis of the plurality of different ion species detected by the multicollector.