WO2002065506A1

WO2002065506A1 - Mass spectrometer

Info

Publication number: WO2002065506A1
Application number: PCT/GB2002/000569
Authority: WO
Inventors: Ian Christopher Lyon
Original assignee: The Victoria University Of Manchester
Priority date: 2001-02-13
Filing date: 2002-02-11
Publication date: 2002-08-22
Also published as: GB0103593D0

Abstract

A mass spectrometer comprising means for directing a stream of analyte species from a sample to an energy resolving detector, the analyte species travelling at a known speed, wherein the energy resolving detector is arranged to measure the kinetic energy of the species, thereby allowing the masses of the species to be determined.

Description

MASS SPECTROMETER

The present invention relates to a mass spectrometer.

Mass spectrometers are well known, and are used to determine the mass/charge ratio of ionized species (ionized atoms, molecules or clusters). In all conventional mass spectrometers it is necessary to ionize a desired species for analysis and then accelerate the ions through a potential difference (typically between several kN to several 10s of kN) to give all those ions of the same charge, the same kinetic energy. The applied kinetic energy is arranged to be much larger than any initial energy spread of the ionized species. Ionized species which are multiply charged will gain that multiple of the kinetic energy gained by a singly charged species.

There are several methods of mass dispersing the ionized species, the most usual being: magnetic dispersion, time-of-flight or quadrupole separation. An ion detector is located after the mass dispersing element to register those ions which have undergone a specific path deflection or arrived at the detector after a time interval following ion formation specific to the mass/charge of the species. Conventional mass spectrometers of all types measure the mass to charge ratio of an ionized species from which, if the charge is known, the mass may be calculated.

Known mass spectrometers all use an applied potential difference to accelerate charged species and include a long total beam path along which the species must travel. Typically the long beam path is a single curved trajectory, although the path may be cyclic and closed such that the species is required to orbit within a trapping magnetic or electric field. The desired species for analysis must be ionized to allow the potential difference to impart acceleration to the species.

Known conventional mass spectrometers suffer from several disadvantages. Firstly, only the charge to mass ratio of a species may be measured. This is a disadvantage because uncertainty about the charge-state of the species leads to uncertainty in its calculated mass. Secondly, only ions may be measured. Neutral species cannot be measured because they are not accelerated by the applied potential difference. An ionisation step is often required to ionise neutral atoms, clusters or molecules. Typically the ionisation step is inefficient and may fragment delicate species. The applied potential difference itself is disadvantageous because it requires the use of a high voltage source. The long beam path is a disadvantage because it can make the mass spectrometer large.

It is an object of the present invention to provide a mass spectrometer which overcomes at least one of the above disadvantages.

According to a first aspect of the invention there is provided a mass spectrometer comprising means for directing a stream of analyte species from a sample to an energy resolving detector, the analyte species travelling at a known speed, wherein the energy resolving detector is arranged to measure the kinetic energy of the species, thereby allowing the masses of the species to be determined.

The invention is advantageous because it provides a measurement of the mass of an analyte species without applying a potential difference to accelerate the species, without having a long beam path along which the species must travel, and without requiring ionisation of the species (although ionisation may still be advantageous for certain applications). The invention provides a measurement the mass of the species irrespective of its charge state.

A mass spectrometer which includes a detector capable of resolving energy is described in US 5,994,694. However, the spectrometer described therein is a conventional time of flight spectrometer, and the detector is used simply to discriminate between singly and doubly charged molecules. The spectrometer described in US 5,994,694 includes an accelerator grid across which ionised molecules are accelerated using a potential difference, and a long flight tube which provides a beam path along which the molecules travel following acceleration. The inventor has realised that an energy resolving detector can be combined with a source of analyte species having a known speed (the speed may be selected or measured) to provide a mass spectrometer which does not require a potential difference or a long beam path.

Preferably, the energy resolving detector is a cryogenic detector.

Preferably, the known speed is substantially a single value. In some instances the known speed may not be a single speed but may be a known range of speeds. Where this is the case, means may be provided for measuring the speed of the analyte species and/or selecting analyte species of a required speed.

Suitably, the speed measurement and/or selection means comprises means for measuring the time of flight of the analyte species.

Suitably, when analyte species are released as ions from the sample with a distribution of speeds, the means for selecting analyte species of a required speed comprises a Wien filter.

Suitably, the Wien filter is arranged to direct ions with different speeds upon different parts of an array of energy resolving detectors. Here a Wien filter refers to an arrangement of opposed electric and magnetic fields in which an ion of a selected velocity moves on an un- deflected trajectory through the Wien filter.

Suitably, the analyte species generating means comprises a matrix containing analyte species of interest, and volatising means arranged to volatise the matrix such that the analyte species are ejected with substantially equal speeds.

Preferably, the volatising means is a pulse of laser light having sufficient energy to volatise the matrix. Preferably, the pulse of laser light is ultraviolet light. Alternatively, any other suitable wavelength of light may be used, for example visible light or infrared light.

Preferably, the matrix is constructed from a material which volatises under laser irradiation. The material may be for example sinapinic or nicotinic acid. Suitably, the analyte species generating means comprises an ion beam or laser directed at a surface of the sample such that atoms or molecules are sputtered from the sample.

Preferably, the ion beam and sample are selected such that the atoms or molecules are sputtered from the sample with a known speed.

Suitably, the analyte species are sputtered from the sample or with a speed distribution in which the speed of each individual analyte species may be determined by additional means. The additional means may be a measurement of the flight time between sputtering of the species from the sample by pulsed ion beam or laser to detection of the species at the energy resolving detector.

Suitably, the ion beam directed at the surface of the sample comprises alpha particles.

Suitably, the detector is located less than 50cm from the sample. The detector may be located less than 10cm from the sample.

According to a second aspect of the invention there is provided a method of measuring the mass of an analyte species, the method comprising directing a stream of analyte species with a known speed from a sample, measuring the energy of the species using an energy resolving detector, and determining the mass of the species using the known speed and the measured energy.

A specific embodiment of the invention will now be described by way of example only, with reference to the accompanying figures in which:

Figure 1 is a schematic illustration of a first embodiment of the invention; Figure 2 is a schematic illustration of a second embodiment of the invention; and Figure 3 is a schematic illustration of a third embodiment of the invention. Referring to figure 1, in a first embodiment of the invention a laser 1 directs a pulse of light at a sample 2 (the light may be for example ultraviolet light). The sample 2 comprises analyte species (typically large molecules or clusters) whose mass is to be analysed, dispersed within a matrix (for example sinapinic or nicotinic acid) in a ratio of about 1 to 10⁴. These could be for example peptides, proteins or DNA fragments.

When the laser pulse is incident upon the matrix, it volatilises the matrix (the matrix dissociates). The matrix is located in a vacuum, and the volatised matrix expands into this vacuum carrying with it the analyte species. The volatilised matrix expands as a plume which carries the desired analyte species in speed equilibrium with the matrix molecules until the density drops to a point where the analyte species has a speed characteristic of the expansion and independent of its mass (the speed is typically around 500 metres per second). The speed distribution of the analyte species is generally small, but in some cases may be as much as ±lOOmetres per second.

The species may pick up or lose electrons during the ejection process, thereby becoming ionised. The ejection and the ionisation are sufficiently gentle that the analyte species are usually not fragmented.

A portion of the species emitted from the sample are incident upon a detector 3 spaced a small distance from the sample 2 (the detector 3 is typically between a few centimetres and tens of centimetres away from the sample). The detector 3 measures the energy of each species that is incident on it. Since the velocity of that species is known, and the energy of the species has been measured by the detector, this allows the mass of the species to be calculated.

The detector 3 is a cryogenic detector of the type described in B.S. Karasik et al., Journal of Applied Physics, 87, 7586 (2000). The detector 3 is a superconductor, cooled using liquid helium, which is held at the upper temperature limit of its superconductivity. When a species is incident upon the detector 3 energy is absorbed by the detector 3, causing its temperature to rise. This temperature rise is proportional to the energy of the species, and by measuring the temperature rise the energy of the species may be determined. Current state-of-the-art capabilities of energy resolving detectors 3 provide sufficient energy resolution to allow the mass of the species to be determined with high resolution. There are not believed to be any fundamental physical limitations against further significant improvements to the capabilities of such detectors in the near future. Improvements in time and energy resolution and detection limits will allow significant improvement to the capability of such detectors for the purposes described above. Several different cryogenic energy resolving detectors are described in the prior art. Any of these may be used in place of the detector 3, provided that they provide sufficient energy resolution. A suitable non- cryogenic energy resolving detector may be used.

The invention is advantageous because it does not require the use of an applied potential difference, thereby avoiding the need for a high voltage source. Furthermore, the invention does not require a flight tube or other long beam path. This allows the mass spectrometer to be much more compact than most known mass spectrometers.

A further advantage of the invention is that it provides a measurement of the mass of a species irrespective of whether the species is ionised or not. In the expanding plume following volatisation, as the analyte molecules are being accelerated by the expansion some molecules pick up electrons or lose electrons thereby becoming ionised. However, in general the majority of the molecules are not ionised and remain neutral. The invention allows the mass of these neutral molecules to be measured (if required, ionised molecules may be deflected away from the detector using charged plates).

The arrangement described in relation to figure 1 provides a quick measurement which will give a reasonable mass determination, sufficient for rapid, imprecise surveying. In order to obtain a precise mass measurement it may be necessary to determine the speed of the ions that are incident on the detector. This can be done by determining the distance between the sample 2 and detector 3, and measuring the time taken from volatisation until each species strikes the detector 3. A sample comprising molecules held in a matrix which is volatised using an ultraviolet pulse have previously been used in the known matrix assisted laser desorption and ionisation (MALDI) technique. In a conventional MALDI arrangement, molecular ions ejected from the matrix are accelerated using an applied potential difference of typically 20- 30 kN, and then pass along a path of known length before being incident upon a detector. The time of flight of the molecules is determined, using the laser pulse as time zero. Thus, the MALDI technique has previously always been used as part of a conventional time of flight mass spectrometer which suffers from the above noted disadvantages of conventional mass spectrometers.

A further disadvantage of the conventional MALDI technique is that the analyte molecules formed within the expanding plume of vaporised matrix molecules become ionized at different times, whilst the mean free path of such analyte molecules is still very much smaller than the dimensions of the expanding plume. The expanding plume is within an electric field so that analyte ions are therefore accelerated whilst still within the plume and suffer higher energy collisions with matrix molecules. Similarly, ionized matrix molecules are accelerated, some of which collide with the analyte species. Such collisions cause a larger energy spread of the analyte ions, fragmentation of the analyte and the population of internal energy states (vibrational and rotational) of the analyte molecules and clusters. The invention avoids this disadvantage because no electric field is applied to the sample.

In a second embodiment of the invention a pulsed primary ion beam 11 sputters atoms or molecules from a sample 12 with high spatial resolution. Ions and neutral species are ejected from the sample into 2π steradians around the normal to the sample surface. The distribution of the ejected ions and neutral species is approximately Cosθ, and most species are ejected at an angle within approximately 60° to the normal to the surface. The species are generated with a known speed distribution. By pulsing the sputtering beam and measuring the time delay between sputtering and collection of the sputtered species, its speed may be determined. An array of cryogenic detectors 13 is spaced away from the sample 12, and measures the energy of each species that is incident on it. Since the speed and energy of the species are known, their mass may be calculated. The invention provides a measurement of the mass of a species irrespective of whether the species is ionised or not and relies only on the initial sputtered energy of the species. In addition, the ion sputtering will induce X-ray emission and characteristic X-ray energies which will be detected by the cryogenic particle detector and which will yield complementary information on chemical composition.

The absolute resolution of the cryogenic particle detector is independent of mass. In other words, the separation between two species which are 1 mass unit apart is measured as an equal increment of energy by the detector irrespective of whether the species are atoms at the low end of the periodic table or are very large molecules (provided that the working range of the detector is not exceeded). This stems from the fact that the kinetic energy is proportional to mass for species which are travelling at the same speed. In terms of percentage accuracy in relation to a mass being measured, the accuracy may be said to increase linearly with mass. This is advantageous when compared to conventional time of flight mass spectrometers or magnetic sector mass spectrometers, as the mass resolution provided by these decreases as the mass increases.

The arrangement described in relation to figure 2 provides a quick measurement which will give a reasonable mass determination, sufficient for rapid, imprecise surveying.

In order to obtain a precise mass measurement it may be necessary to determine the speed of the species that are incident on the detector. This can be done by determining the distance between the sample 12 and detector 13, and measuring the time taken from sputtering (by the pulsed primary ion beam) until each species strikes the detector 3.

In a third embodiment of the invention, illustrated in figure 3, ions are sputtered from a sample 21 using a radioactive alpha particle sputtering ion beam 22 (any other suitable ion beam may be used). The ions pass through a Wien filter 23. The Wien filter 23 comprises a pair of conducting plates 24 which lie substantially parallel with the direction of travel of the ions, a voltage being applied to the plates to that an electric field E is applied transverse to the direction of travel of the ions. The Wien filter further comprises a magnetic field B applied transverse to the direction of travel of the ions. The net effect of the to the electric field E and magnetic field B is that only ions having a particular velocity (given by V=E/B) will pass through the Wien filter without being deflected.

A cryogenic detector 25 is located in the path of the undeflected ions. Those ions which are incident upon the detector 25 have a known velocity (=E/B). The energy of the ions is measured by the cryogenic detector 25, and this allows the mass of the ions to be determined. Different portions of the speed spectrum of the sputtered ions may be sampled by varying the strength of the electric field E. Typically the magnetic field B is fixed and is provided by a permanent magnet, although the magnetic field may be varied if necessary. As an alternative to varying the strength of the electric field E or magnetic field B, an array of detectors may be arranged to detect ions which have been deflected by the Wien filter, thereby measuring a wide range of ion energies simultaneously.

The invention is of particular utility in applications where the mass spectrometer must be of limited size and have a low power requirement (for example a space probe). The radioactive alpha particle sputtering ion source is advantageous because it has no power requirement.

A Wien filter, or time of flight arrangement may be used in conjunction with the embodiment of the invention illustrated in figure 1.

The analyte species when they hit the detector have internal energy states (rotational, vibrational). This energy will cause an additional temperature rise and hence loss of resolution of the mass measurement. However, the energies of these states will be relatively small and due to the lack of an accelerating electric field, excitation of the analyte species in this manner will be very limited. Pre-calibration using species of known mass may be used to determine any internal energy distribution and remove this effect from a measurement of an unknown analyte species. When the species strikes the detector, sputtered atoms or fragments of the analyte species which are not captured by the detector remove energy from the detector thus degrading its resolution. However, the kinetic energy of the analyte molecules is sufficiently low that significant sputtering of material from the detector does not occur. Similarly, fragmentation and losses at the detector are small, and by suitable design of the detector surface (eg by etching), these losses may be made minimal.

The invention has several advantages. It can measure the mass of neutral atoms or molecules, since acceleration of ions in an electric field is not required. The invention avoids the problem of sputtering from the detector, where molecules which hit a detector with high speed knock molecules out of the detector, cooling down the surface of the detector and reducing the energy resolution of the detector. Sputtering from the detector is avoided because the speed of the molecules is slow, and molecules therefore do not significantly affect the surface of the detector.

In prior art mass spectrometers, molecules may become fragmented as they are accelerated by an electric field. The invention imparts no acceleration to molecules, thereby avoiding this problem.

Claims

1. A mass spectrometer comprising means for directing a stream of analyte species from a sample to an energy resolving detector, the analyte species travelling at a known speed, wherein the energy resolving detector is arranged to measure the kinetic energy of the species, thereby allowing the masses of the species to be determined.

2. A mass spectrometer according to claim 1, wherein the energy resolving detector is a cryogenic detector.

3. A mass spectrometer according to claim 1 or claim 2, wherein the known speed is substantially a single value.

4. A mass spectrometer according to claim 1 or claim 2, wherein the known speed is not a single speed but is a known range of speeds.

5. A mass spectrometer according to any preceding claim, wherein means is provided for measuring the speed of the analyte species and/or selecting analyte species of a required speed.

6. A mass spectrometer according to claim 5, wherein the speed measurement and/or selection means comprises means for measuring the time of flight of the analyte species.

7. A mass spectrometer according to any preceding claim, wherein the analyte species comprises ions and the means for selecting analyte species of a required speed comprises a Wien filter.

8. A mass spectrometer according to claim 7, wherein the Wien filter is arranged to direct ions with different speeds upon different parts of an array of energy resolving detectors.

9. A mass spectrometer according to claim 7, wherein the energy resolving detector is positioned to detect those ions which are not deflected by the Wien filter.

10. A mass spectrometer according to any preceding claim, wherein the analyte species generating means comprises a matrix containing analyte species of interest, and volatising means arranged to volatise the matrix such that the analyte species are ejected.

11. A mass spectrometer according to claim 10, wherein the analyte species are ejected with substantially equal speeds.

12. A mass spectrometer according to claim 10, wherein the analyte species are ejected with a known range of speeds.

13. A mass spectrometer according to any of claims 10 to 12, wherein the volatising means is a pulse of laser light having sufficient energy to volatise the matrix.

14. A mass spectrometer according to claim 13, wherein the pulse of laser light is ultraviolet light.

15. A mass spectrometer according to any of claims 10 to 14, wherein the matrix is constructed from a material which volatilises under laser irradiation.

16. A mass spectrometer according to claim 15, wherein the material is sinapinic or nicotinic acid.

17. A mass spectrometer according to any of claims 1 to 9, wherein the analyte species generating means comprises an ion beam or laser beam directed at a surface of the sample such that analyte species are sputtered from the sample.

18. A mass spectrometer according to claim 17, wherein the ion beam and sample are selected such that the analyte species are sputtered from the sample with a known speed.

19. A mass spectrometer according to claim 1, wherein the analyte species are sputtered from the sample or with a speed distribution in which the speed of each individual analyte species may be determined by additional means.

20. A mass spectrometer according to any of claims 17 to 19, wherein the ion beam directed at the surface of the sample comprises alpha particles.

21. A mass spectrometer according to any preceding claim, wherein the detector is located less than 50cm from the sample.

22. A mass spectrometer according to claim 21, wherein the detector is located less than 10cm from the sample.

23. A method of measuring the mass of an analyte species, the method comprising directing a stream of atoms or molecules with a known speed from a sample, measuring the energy of the species using an energy resolving detector, and determining the mass of the species using the known speed and the measured energy.

24. A mass spectrometer substantially as hereinbefore described with reference to the accompanying figures.

25. A method of measuring the mass of an atom or molecule substantially as hereinbefore described with reference to the accompanying figures.