WO1993018540A1

WO1993018540A1 - Mass spectrometer

Info

Publication number: WO1993018540A1
Application number: PCT/GB1993/000434
Authority: WO
Inventors: Thomas Oliver Merren
Original assignee: Fisons Plc
Priority date: 1992-03-03
Filing date: 1993-03-03
Publication date: 1993-09-16
Also published as: GB9204524D0

Abstract

The invention provides a mass spectrometer having a quadrupole mass analyzer having an entrance aperture, an ion source disposed on the axis of said analyzer, particle intercepting means blocking the line-of-sight path between said ion source and said entrance aperture, and field generating means for directing ions from said source into said entrance aperture, said field generating means comprising first field generating means for deflecting ions from said source away from said axis to avoid said particle intercepting means and second field generating means for directing into said entrance ions so deflected by said first means. Using this arrangement, substantial improvement in signal:noise ratio is achieved, corresponding in some instances to a reduction in noise by a factor of over 100.

Description

MASS SPECTROMETER

This invention relates to mass spectrometers and in particular to mass spectrometers wherein mass analysis is accomplished by means of a quadrupole mass analyzer or mass filter.

In all mass spectrometers an ion beam from an ion source passes to a detector via an intermediately positioned mass selector.

In a quadrupole mass spectrometer the mass selector, filter or analyzer, selects the ions permitted to impinge on the detector on the basis of their mass to charge ratio by means of superimposed radio frequency and dc electric fields.

In contrast, a magnetic analyzer selects ions on the basis of their momentum and charge. This involves the application of a magnetic field perpendicular to the ion beam causing it to be deflected by an amount dependent on the mass to charge ratio of the ions. By use of a suitably placed detector, which accepts only a narrow segment of the deflected beam, ions of a particular mass to charge ratio can be detected by selecting the strength of the magnetic field or the energy with which the ions enter the magnetic field.

Conversely, a quadrupole mass analyzer consists of four electrically conductive electrodes arranged symmetrically about and very accurately parallel to the line joining the ion source and detector. Opposite pairs of electrodes are coupled together and an electrostatic potential oscillating at a radio frequency is applied between the pairs. Opposite pairs are biased with dc potentials - positive in the x direction, negative in the y direction with respect to the potential on the axis defined by the electrodes. Ions are injected along the axis defined by the electrodes towards the detector and as a result of subjection to a combination of electrical forces, a complex trajectory is described.

Thus for an ion with a given mass to charge ratio (m/e) , the trajectory is determined by the dc potential (U) , the radiofrequency zero-to-peak voltage (V) and the angular frequency of the oscillating potential (ω) . In simple mass filters, these values may be set to permit only ions having a specific m/e ratio to reach the detector, ions having different m/e ratios being lost by collision with the device itself. Alternatively, the device may be used as a mass analyzer, in which U and V may be varied whilst maintaining their ratio constant so that ions of differing mass may be collected in turn on the detector.

Quadrupole analyzers are advantageous in that they are generally smaller and comparatively cheaper than analyzers based on magnetic sectors. Furthermore, since U and V may be electronically swept, high scan speeds are possible. However, a disadvantage of such quadrupole analyzers is that the signal to noise ratio can be poor.

It is known that physically blocking the line-of- sight path between the ion source and the entrance aperture of a quadrupole analyzer can in some circumstances improve the signal-to-noise ratio, particularly in the case of sources for secondary ion mass spectroscopy. For example, US Patents 3,859,226 and 3,939,344 describe apparatus which receive only ions which are travelling at certain angles to the line-of- sight path and focus them on to the entrance aperture of a quadrupole analyzer while blocking the line-of-sight path. These prior systems have low transmission efficiencies, however, because firstly, they do not transmit ions leaving source at angles less than a certain minimum, and secondly, because the ions are focused at the entrance aperture, the angle at which they enter the quadrupole is dependent on the angle at which they enter the focusing arrangement. As ions which enter at low angles are not transmitted, those entering the analyzer necessarily do so at high angles and will not be efficiently transmitted by the quadrupole analyzer, which typically has a limited angular acceptance.

Other approaches to transmitting ions from an ion source to a quadrupole analyzer while blocking the line- of-sight path include injecting the ions along a trajectory steeply inclined to the quadrupole axis and deflecting them by an electrostatic field into the entrance aperture, and injecting the ions along an axis ^• parallel to but displaced from the quadrupole axis and providing electrostatic deflecting fields to cause the ions to pass through the entrance aperture. Such arrangements are disclosed in US Patent 3,922,544, Japanese Patent application publication JP-A-01/183051 and in International Patent application WO 92/21139 (which was only published on 26th November 1992) . Displacement or inclination of the source axis relative to the quadrupole axis introduces mechanical complications, however, and precludes the fitting of an ion source to a multi-purpose quadrupole analyzer in the conventional line-of-sight configuration.

It is the object of the present invention to provide a quadrupole mass spectrometer without a line- of-sight path between its ion source and its quadrupole analyzer in which the source is disposed substantially on the axis of the quadrupole analyzer and which has a higher transmission efficiency than prior "on-axis" arrangements.

Thus, viewed from one aspect, the present invention provides a mass spectrometer comprising a quadrupole mass analyzer having an entrance aperture, an ion source disposed on the axis of said analyzer, particle intercepting means blocking the line-of-sight path between said ion source and said entrance aperture, and field generating means for directing ions from said ion source into said entrance aperture, said field generating means comprising first field generating means for deflecting ions from said source away from said axis to avoid said particle intercepting means and second field generating means for directing into said entrance aperture ions so deflected by said first means.

By using this arrangement of quadrupole, ion source, and ion deflecting and particle intercepting means that interrupts the line-of-sight path from ion source to mass analyzer we have found that a very significant improvement in the signal to noise ratio is achieved, corresponding to a reduction in. noise in some circumstances by a factor of over 100. Moreover the deflecting and intercepting means can be made sufficiently compact that their incorporation into a spectrometer having ion source and mass analyzer in the conventional line-of-sight configuration is generally relatively facile.

The field for directing the ion beam towards the quadrupole entrance may be generated in a variety of ways and may, for example, comprise magnetic or electrostatic fields disposed so as to prevent an ion beam from travelling along a trajectory defined by the direct line-of-sight path from the ion source to the quadrupole analyzer entrance in such a way that charged particles are caused to travel in trajectories which avoid collision with the means provided to block the said line-of-sight path but which preferably do not prevent subsequent mass:charge separation of particles in the analyzer itself.

In one convenient embodiment, the field generating means operate to impose at least one pair, e.g. a matched pair, of opposing deflecting electrostatic fields on the ion beam such that it is deflected by the field of the first field generating means off the axis defined by the ion source and the entrance to the quadrupole analyzer, and is subsequently returned to the axis by means of the field of the second field generating means.

Particularly preferably the second field generating means comprises means for directing ions into the entrance aperture of the analyzer along trajectories substantially aligned or parallel to the analyzer axis.

The field generating means conveniently comprise a plurality of conducting plates disposed about the ion path and defining apertures through which the ion path passes. Preferably the first field generating means comprises at least one such plate which tapers in thickness towards the ion path. Also preferably the second field generating means comprises a pair of plates on opposite sides of the ion path defining between them an aperture angled to the line-of-sight path from ion source to mass analyzer and with its mid point displaced from the line-of-sight path.

The line-of-sight path between the ion source and the entrance to the quadrupole analyzer may be blocked by any suitable physical means arranged so that at least one barrier completely blocks the path defined by a straight line joining the ion source and quadrupole entrance, although barriers provided by planar or curved plates are preferred. These may be formed of any suitable material, however, conducting materials such as metals are preferred. Conveniently this particle intercepting means is provided by one of the conducting plates of the field generating means which extends into the line-of-sight path between the ion source and the mass analyzer.

Thus, in a particularly convenient embodiment, deflecting electrostatic fields may be applied to the ion beam by means of the barriers themselves. Suitable potentials may be in the range of -20 to +350v, for negative ion separation. - 6 -

Where the deflecting means are not provided by the barriers themselves, the barriers may be disposed relative to the deflecting means such that the action of the deflecting field causes selected charged particles within the ion beam to travel in trajectories which avoid collision with the physical barriers. The actual number of barriers disposed between the ion source and quadrupole is not critical provided that the line-of- sight path is effectively blocked.

The conducting plates of the field generating means conveniently have apertures therein through which the ion path passes.

Viewed from an alternative aspect the invention provides a mass spectrometer having an ion source, a quadrupo.le mass analyzer, field generating means for directing ions from said source into the entrance of said analyzer along a non-linear ion path, and particle intercepting means blocking the line-of-sight path from said source to said entrance, said field generating and particle intercepting means comprising a plurality of conducting plates disposed to the sides of said non¬ linear path substantially perpendicular to said line-of- sight path, at least one of said plates being arranged to direct said ions away from said line-of-sight path, at least one of said plates being arranged to direct said ions towards said line-of-sight path and at least one of said plates tapering in thickness towards said non-linear path.

Viewed from a still further aspect, the present invention provides a method of enhancing the sensitivity of mass analysis of a sample comprising the steps of: a) creating an ionised, vaporised sample in an ion source lying on the axis of a quadrupole mass analyzer, preferably by ion or neutral particle beam bombardment of a solid surface, e.g. one on which a sample for analysis has been deposited, b) deflecting ions from said ionized vaporized sample away from said axis to avoid a particle intercepting means disposed on the line-of-sight path between said ion source and said analyzer, c) subsequently deflecting ions into the entrance of said analyzer, and d) submitting said ions to mass analysis.

The sample may be ionised by any means appropriate for mass spectrometry, for example photoionisation, electron impact, fast ion bombardment, fast atom bombardment, etc. , but as indicated above ion beam or neutral particle beam bombardment are preferred. Using this method, we have found significant improvements in signal to noise ratios, of a factor of over 100 with consequent improvement in the sensitivity of sample analysis.

Preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 1 is a schematic drawing of a mass spectrometer according to the invention;

Figure 2 is a schematic drawing of the mass spectrometer of Figure 1 in greater detail;

Figure 3 is a drawing of a conductive plate assembly suitable for use in the spectrometer of Figure 2;

Figure 4 is a drawing of another conductive plate assembly suitable for use in the spectrometer of Figure 2;

Figure 5 is a schematic drawing of a further alternative version of a spectrometer according to the invnetion; and

Figure 6 is a schematic drawing of another alternative version of a spectrometer according to the invention.

Figure 1 shows a mass spectrometer of the present invention arranged for separation of negatively charged ions. Plates (2, 3, 4 and 5) are held at approximately zero potential. Ions emerging from the ion source (l) travel towards the quadrupole analyzer and are influenced by a series of electrostatic fields created by positively charged plates (6, 7, 8, 9, 10, 11), where plate 8 blocks the line-of-sight path from the ion source to the quadrupole. A potential approximately of +180V is applied to plates 6 and 8, +60V to plates 7 and 9 and +160V to plates 10 and 11. The resulting fields cause the charged particles in the ion beam to be deflected from their original trajectory and ultimately be refocused at the entrance to the quadrupole analyzer (12) . After mass analysis, the beam of ions pass to a detector (13) which may, for example, be a Faraday plate or an electron multiplier.

When the source is being used in positive ion mode, the polarity of the plates described above is reversed.

In the spectrometer shown in Figure 2 , an ion source generally indicated by 1 comprises a solid surface 14 on the end of an insertion probe 15 on which a sample to be analyzed may be deposited. A beam of neutral particles 16 or ions is directed at the surface 14 and bombards the sample deposited on it to release ions for analysis. Ion source 1 further comprises a source chamber 17 and a repeller block 18 which are mounted on a flange 19.

The path of the ions leaving the surface -14 is schematically represented by the trajectory 20 and is determined by the electrostatic fields generated by suitable potentials applied to the conductive plates 2- 11 which are disposed as shown in Figure 2. The ions emerge from the last of these to pass through an entrance aperture 21 into a quadrupole mass analyzer 12 wherein they undergo mass analysis. An ion detector 13, which may be an electron multiplier or a Faraday detector, receives those ions which are transmitted by the analyzer 12. The conducting plates 2-11 and the plate 22 in which the aperture 21 is formed are mounted on four insulating rod assemblies from a flange 19 and are spaced apart by insulating spacers (some of which are shown at 23 in Figure 2) , or on the insulated spacers 24 from the source chamber 17.

The ion source 1, surface 14 and the entrance aperture 21 of the quadrupole analyzer 12 are disposed on the axis of the analyzer as in most conventional quadrupole mass spectrometers. However, the line-of- sight path 25 between the ion source 1 and the analyzer 12 which would otherwise exist is blocked by one of the conductive plates (8 in Figures 1 and 2) .

In accordance with the invention, the first field generating means for changing the direction of at least some of the ions leaving the source 1 comprises the conductive plates 2 , 3, 4 and 5. Although plates 2 and 4 may be separate plates they preferably comprise a single plate comprising a large aperture 26, asymmetrically disposed about the line-of-sight path 25, through which both the primary particle or ion beam 16 and the ions to be analyzed may pass. Similarly, although the conductive plates 3 and 5 may be separate plates, they preferably comprise the single assembly shown in Figure 3. The plate 3 which forms the lower part of the assembly extends to support the tapered upper portion 5 which comprises a machined block through which a hole 28 is bored to allow the passage of the primary beam 16. The tapered portion 5 is attached to the plate 3 by screws. Plate 3 also comprises an aperture 27 through which the ions to be analyzed may pass. Aperture 27 is displaced from the line-of-sight path 25 which falls on the centre-line 29 of the plate 3.

The first field generating means produces an electrostatic field in the space between the plates 3,5 and 6,8 which can be represented by curved lines of force which cross the line-of-sight path 25 at an acute angle rather than perpendicularly, as they would if the tapered conductive plate 5 was omitted. As a consequence, the direction of travel of the ions passing through the apertures 26 and 27 is changed to that indicated by the trajectory 20 (Figure 2) .

Particle intercepting means for blocking the line- of-sight path 25 are provided by the conducting plates 6, 8. These may comprise separate plates but preferably are a single plate comprising an aperture 30 which is positioned so that the line-of-sight path 25 does not pass through it.

The ions whose direction has been changed by the first field generating means pass through the aperture 30 to the second field generating means which comprises the conductive plates 7, 9, 10 and 11 and which is . arranged to change the direction of the ions so that they pass through the entrance aperture 21 on trajectories substantially aligned with the axis of the quadrupole analyzer 12, thereby ensuring that maximum performance is obtained from the analyzer. The conductive plates 7 and 9 are displaced from one another along the axis of the quadrupole as shown in Figure 2. They are joined by two bent conductive strips 31, 32 (Figure 4) and define an aperture 33 whose centre is displaced from the line-of-sight path 25 which is located on the centre-line 34 of Figure 3 assembly. The conductive plates 10 and 11 are conveniently a single plate comprising an aperture 35 which is located on the line-of-sight path 25.

Conveniently, all the conductive plates in the apparatus above described may be made of stainless steel. For the analysis of negative ions, the potentials applied to the various plates may be as below:

Plates 2, 3, 4 and 5: 0 volts Plates 6 and 8: +180 volts Plates 7 and 9: + 60 volts Plates 10 and 11: +160 volts.

For positive ions, the polarities of the above potentials may be reversed.

Figure 5 illustrates an alternative method of constructing the first field generating means. The tapered conductive plate 5 may be replaced by a series of flat plate electrodes 35-40 which are mounted on two mounting pillars 41 secured to the plate 3. Each of electrodes 35-40 comprises a hole through which the primary beam 16 may pass. The electrodes 35-40 may conveniently be maintained at the same potential and are arranged so that their edges define a surface similar^' to that defined by the tapered plate 4 in the embodiment of Figures l and 2, so that a similar electrostatic field is generated.

A similar, but less preferred, embodiment is shown in Figure 6. In Figure 6, the conductive plates 3 and 5 are replaced by a pair of plate electrodes 42 and 43 which define an aperture 44 displaced from the line-of- sight path 25. In contrast to the embodiments of Figures 1, 2 and 4, a potential difference is maintained between electrodes 42 and 43, thereby causing the ion beam passing through the aperture 44 to be deflected away from the line-of-sight path 25. A hole 45 is provided in the electrode 42 to allow the passage of the primary beam 16. The electrodes 42 and 43 may also comprise portions which extend substantially parallel to the line-of-sight path 25 in the manner of_. conventional beam deflector plates.

Claims

Claims :

1. A mass spectrometer comprising a quadrupole mass analyzer (12) having an entrance aperture, an ion source (1) disposed on the axis of said analyzer, particle intercepting means (8) blocking the line-of- sight path between said ion source and said entrance aperture, and field generating means (2-11) for directing ions from said source into said entrance aperture, said field generating means comprising first field generating means (2-5) for deflecting ions from said source away from said axis to avoid said particle intercepting means and second field generating means (6- 11) for directing into said entrance ions so deflected by said first means.

2. A mass spectrometer as claimed in claim 1 wherein said second field generating means comprises means (6-11) "for directing ions into said entrance aperture on trajectories aligned with our substantially parallel to said axis.

3. A mass spectrometer as claimed in either of claims 1 and 2 wherein said first and second field generating means comprise a plurality of conducting plates (2-11) which define apertures through which said ions may pass, and said particle intercepting means (8) is provided by at least one of said plurality of conducting plates extending into said line-of-sight path.

4. A mass spectrometer as claimed in claim 3 wherein one of said conducting plates (5) of said first field generating means tapers in thickness towards the path of said ions creating an electrostatic field which deflects said ions away from said line of sight path.

5. A mass spectrometer as claimed in claim 4 wherein said conducting plate (3,5) which tapers comprises an aperture the centre whereof is displaced from said line-of-sight path and said particle intercepting means comprises at least one conducting plate (6,8) defining an aperture the centre whereof is displaced from said line-of-sight path such that said line-of-sight path does not pass therethrough.

6. A mass spectrometer as claimed in claim 3 wherein said first field generating means comprises an array of substantially parallel plates (35-41) disposed with edges successively further distanced from said line-of-sight path in the direction of said entrance aperture (21) .

7. A mass spectrometer as claimed in claim 6 wherein said array of plates (35-41) is conductively linked.

8. A mass spectrometer as claimed in either of claims 6 and 7 wherein said array of plates (3, 35-41) comprises an aperture the centre whereof is displaced from said line-of-sight path and said particle intercepting means comprises at least one conducting plate (8) defining an aperture the centre whereof is displaced from said line-of-sight path such that said line-of-sight path does not pass therethrough.

9. A mass spectrometer as claimed in claim 1 wherein said first field generating means comprises a pair of conductive plates (42,43) disposed on either side of said axis and maintained at different potentials whereby to deflect said ions away from said axis.

10. A mass spectrometer as claimed in any one of claims 1 to 9 wherein said second field deflecting means comprises two conducting plates (7,9) displaced from one another along said axis and defining an aperture whose centre is displaced from said axis.

11. A mass spectrometer as claimed in any of the preceding claims wherein said ion source (1) comprises a solid surface bombarded by a beam of neutral particles or ions to release ions for analysis therefrom.

12. A mass spectrometer as claimed in claim 11 wherein a sample to be analyzed is deposited on said surface so that said ions for analysis are released by bombardment of said sample.

13. A method of enhancing the sensitivity of mass analysis of a sample comprising the steps of: a) creating an ionised, vaporised sample in an ion source lying on the axis of a quadrupole mass analyzer, b) deflecting ions from said ionized vaporized sample away from said axis to avoid a particle intercepting means disposed on the line-of-sight path between said ion source and said analyzer, c) subsequently deflecting ions into the entrance of said analyzer, and d) submitting said ions to mass analysis.