GB2440800A

GB2440800A - Mass spectrometer

Info

Publication number: GB2440800A
Application number: GB0710134A
Authority: GB
Inventors: Philip Antony Freedman
Original assignee: Nu Instruments Ltd
Current assignee: Nu Instruments Ltd
Priority date: 2006-06-07
Filing date: 2007-05-25
Publication date: 2008-02-13
Anticipated expiration: 2027-05-25
Also published as: GB2440800B; GB0611146D0; GB0710134D0; DE102007026441A1

Abstract

An instrument 12 to accurately monitor the isotopic and elemental composition of a sample is described. A multiple collector ICP (inductively coupled plasma) mass spectrometer 4 is modified so that ions are deflected from the main path into a secondary analyser 14, such as a time-of-flight analyser. The deflection is achieved by applying a pulsed signal to a deflector 11. As such, the majority of the ions still pass into the isotope analyser 4, whilst small proportions are available for elemental analysis. Since the gas stream is not physically split prior to ionisation, sharp discontinuities in the composition of solid samples, for example geological rock samples, can be accurately measured.

Description

MASS SPECTROMETERS

This invention relates to mass spectrometers, and in particular to instruments and methods which enable the simultaneous determination of isotopic and elemental mass.

Mass spectrometers have been described in numerous publications, both in the general and in the patent literature. As noted, for example, in the introductory portion of GB-A-2396960, they can be used for target compound trace analysis, accurate mass measurements, isotope ratio elements and fundamental ion chemistry studies. EP-A-0427532 discloses a method and apparatus for high resolution mass spectrometry of recoiled ions for isotopic and trace elemental analysis.

Since their launch in the early 1 990s, multiple collector, inductively coupled plasma mass spectrometers (MC-ICP-MS) have become the instruments of choice to obtain accurate and precise measurements of the isotopic composition of inorganic samples, for example rock samples. Details are set out in A.J. Walder and P.A. Freedman Journal of Analytical Atomic Spectrometry 7 (1992), pages 571 -595k As disclosed in this paper, by studying the ratios of the isotopes of an element, one of which can be formed from the radioactive decay from a different parent element, it is possible to calculate the age at which a rock sample formed. This technique is now widely employed by the geological community to study the history of terrestrial and extraterrestrial samples.

Similarly, isotope ratios measurements of samples from nuclear power plants and reprocessing systems are used as an essential tool for quality control and inventory management. Further, since the isotope fingerprint of samples can depend on their origin, accurate isotope measurements can be employed as an environmental monitoring tool. As indicated by the name, such instruments may achieve very high precision by simultaneously monitoring as many of the isotopes of interest as possible. By this means, any intensity noise from the ion source is cancelled, since the signal from each recorded isotope changes in unison, and precision approaching a few parts per million (ppm) is often achieved: Since the changes in isotopic ratios are often small, such precision measurement capability is a practical requirement for many such instruments. In contrast to such simultaneous monitoring, elemental mass spectrometers tend to record each mass sequentially. Any ion source noise cannot be removed, but this is not a problem, since precision to the percent level is normally adequate.

Many samples for analysis are introduced into the plasma ion source of a mass spectrometer in liquid form, using a nebuliser to produce micron size particles. This approach requires solid samples to be dissolved into solution prior to analysis, losing any spatial information. This loss of potential data about the sample may be overcome by ablating such solid samples using a laser in a gas stream which, if the power density is high enough, produces micron-sized particles directly, normally achieved by using a pulse operated at about 10 Hz. This allows direct, site-specific, analysis to be undertaken and permits information, for example on particle growth, to be inferred.

Because of zoning in such samples, it is found that the isotopic ratios can change with both position and depth, sometimes quite sharply.

To provide further data about any such non-homogeneous rock sample, so that, for example, information on the provenance of such samples is easier to extract, it is advantageous to monitor the elemental composition of the sample site. Because of zoning in the sample, it is necessary to ensure that the two analyses (isotopic and elemental) are taken simultaneously from the same sample position. Currently, to achieve this aim using the known art, either the sample is analysed for isotopic composition, followed by the elemental analysis, in separate experiments, or the gas stream from the laser cell is split into two and analysed using two separate instruments. At best the first approach, in practice, only samples adjacent areas, whilst in the second, since the transfer gas lines spread out the recorded sample pulse, it is difficult.to ensure the two signals come from the same laser shot.

Another problem with using two mass spectrometers with the split gas stream in this manner is that the accuracy of the isotopic measurement is compromised. To see why this occurs, it is necessary to consider the limits of the achievable precision. For isotopic measurements, where small variations in observed ratios are often important (to a few tens of parts per million levels), using instruments fitted with a multiple collector, the total number of ions recorded at each mass is often the ultimate limitation. In such a case, Poisson statistics normally apply, with the best precision of a measurement of N events being given by IN. Therefore it is necessary to try to ensure that as many of the sample particles as possible are transmitted to the source of the MC-ICP-MS. In the case of the elemental analyser, a much lower precision is often acceptable to percent or tenths of a percent levels, and this is not normally a constraint. However, highly asymmetric gas stream splitting is difficult to achieve, and thus this approach degrades the achievable isotopic accuracy.

The purpose of this invention is to overcome these problems, and achieve optimal isotopic analysis precision, whilst providing effectively simultaneous elemental and isotopic sampling, permitting highly zoned solid samples to be accurately analyzed.

According to a first feature of the present invention there is provided a spectrometer apparatus for measuring the isotope ratio of an element within, and the elemental composition of, a sample, the apparatus comprising means for ionising a sample in an inductively coupled plasma, means for passing the ions through transfer optics including a pulsed deflector, a multiple collector isotope mass spectrometer analyser having a source slit and an elemental ion analyser, and means for actuating the pulsed deflector to enable ions to be deflected from the path towards the source slit of the multiple collector isotope mass spectrometer to a path leading to the elemental on analyser. The elemental ion analyser may be a time of flight analyser or an ion trap analyser, in either case of known type.

A laser ablator is preferably used to ablate material from the sample which is then fed to a location where it is ionised by the plasma.

The present invention also provides a method of measuring the isotope ratio of an element within a sample and the elemental composition of the sample, which method comprises ionizing the sample in an inductively coupled plasma, passing the ions though transfer optics including a pulsed deflector and controlling the pulsed deflector so that the majority of the time ions pass on to a source slit of a multiple collector isotope mass spectrometer analyser, and during the remainder of the time ions deflected from the path to the isotopic analyser travel to and enter an elemental ion analyser.

The sample may be, for example, a mineral specimen, laser ablated, with the material ablated by each shot of the laser being passed to the plasma for ionisation and then subjecting the ions to intermittent deflection and analysis as just indicated.

Preferably, the control of the pulsed deflector is such as to deflect ions to the elemental ion analyser less than 50% of the time, most preferably less than 5% of the time. The pulse rate to which the pulsed deflector is subjected is preferably much higher than the pulse rate of the laser used for ablation, e.g. a kilohertz frequency compared to, say, 10 Hz for the ablator, so the material from each ablation is split into two streams providing effectively simultaneous analysis of elemental and isotopic make-up.

The invention is illustrated by way of example only, with reference to a diagrammatic arrangement, shown in the accompanying drawings in which: Figure 1 is a diagram of a preferred combined MC-ICP-MS/elemental mass spectrometer according to the present invention; and Figure 2 is a diagram of an alternative embodiment using a different type of pulsed deflector.

Referring to Figure 1, this shows for the most part a standard MC-ICP-MS arrangement, in Which a gas stream passes over a specimen located in a laser ablation cell 2, illuminated with a laser 14, and then along a pipe 1 to an inductively coupled plasma torch 3. The ions produced in the flame pass through a sampler 5 and skimmer 6 arrangement of known type, into a transfer region 17 of low gas pressure (1 o to i0 mbar). Here a series of lenses 7, 8 accelerates the ions and converts the circular symmetric source (as defined by the skimmer 6) to an elongated image focussed at a source slit 9 of a multiple collector (isotope) mass spectrometer 4 in which the ion beam is separated by mass by a magnet 15, and the individual isotopes simultaneously monitored with a multiple detector array 16.

The transfer region 17 is pumped with a series of vacuum pumps (not shown) to enable the pressure to be lowered from that of atmosphere (at the plasma flame) to about 1 0 mbar in the final section. A number of apertures are used to separate the region associated with each pump, the last of these apertures being denoted 10 in Figure 1. By the time the ion beam enters the section beyond aperture 10, the ions have been fully accelerated to, for

example, 6kv.

Within this last pumped section of the transfer section, region 17, in accordance with the present invention, are elements to eject some of the ions into a time-of-flight (TOF) analyser 12. The deflection is approximately orthogonal to the original direction of the ion beam. To enable this to occur, the ions are decelerated to 500v, using an ion optical deceleration element 18. Since the front of the plasma region is maintained at 6kv, this results in the actual potential being applied to this region to be 5.5kv (6000v less 500v). They then transverse a short section, held at this lower voltage, before entering an acceleration element 19, before being finally focussed on to the mass spectrometer source slit 9 by the lens elements 8.

Part of the short section held at 5.5kv is bounded by a grid 11. This grid is connected to a power supply capable of providing fast rise time, voltage pulses from this 5.5kv, down to zero volts. When thus pulsed, the ions within the region bounded by end of deceleration element 18 and acceleration element 19 experience a strong tangential force corresponding to 5.5kv. They are then drawn into the (grounded) TOF analyser 12 at an angle of ArcTan /(500/5500) (approximately 17') as indicated in Figure 1.

Extra grids may be installed to enable corrections to be applied due to the different starting points of the ion beam within the draw out region between deceleration element 18 and acceleration element 19. The deflected items then enter the short drift length TOF analyser 12 where their arrival time at a multi-channel plate (MCP) detector or other fast ion detector 13 is used to provide elemental composition of the beam.

In the preferred embodiment, the pulse "on' time is about 30 to 50 nsecs, and the pulse repetition rate is about 10kHz. Thus the ion beam entering the isotope analyser is diminished in intensity by only about 0.5%, which has a minimal effect on the theoretical limit of precision. Since the deflection occurs prior to the defining slit of the isotope mass spectrometer, beam quality in this analyser is also unaffected.

There is a small spread of energy of the original ion beam of approximately 10ev which results from the formation process of the ions within the region of the sampler 5. This effect is well-known. This results in a small spread in the angular distribution of the ion beam as it passes through the TOF analyser. However, as can be easily shown, this is a small effect and lateral spread of the final ion beam image resulting from this divergence can be easily accommodated within the entrance aperture of commercially available detectors.

Figure 2 shows an alternative embodiment using a different approach for deflecting part of the ion beam in accordance with the present invention.

The apparatus is analogous to that shown in Figure 1, but, in this case, deflection is achieved by the installation in transfer region 17 of a small, pulsed electrostatic deflector element 20. For the majority of the time, element 20 has no voltage applied to it, allowing the ions to pass to the isotope analyser, as normal. A pulsed power supply, capable of providing fast rise time, voltage pulses, is connected to deflector 20 and, when the pulse is applied, deflects the ions out of the straight through path, as shown in the figure. Preferably a deflection of about 100 is used. Adjacent the straight through path is the entry port of a small electrostatic analyser (ESA) 21 which deflects the ions further, for example, until they are travelling normal to their original path. They then enter a short drift length TOF analyser 12, where their arrival time at the standard multi-channel plate (MCP) detector 13 is used in known fashion to provide data about elemental composition of the beam, and accordingly of the sample.

Since the ions have been accelerated by part of the transfer optics lenses 7, prior to the deflector, ions of different mass experience different degrees of deflection, since the tighter ions travel at a faster velocity compared to the heavier ones. Thus, for example, if we consider a pair of ions differing in mass by a factor of nine starting at the entrance of the deflector, the lighter will travel three times further than the heavier in the period the deflector voltage is applied. As such, this lighter ion will experience a greater deflection than its neighbour, and miss the entrance of the ESA 21, and accordingly not pass into the elemental analyser. However, lighter ions originating further back than this, will not be deflected in the early part of their path during the pulse period, and can be selected to undertake the correct deflection. Thus by limiting the acceptance angle of the ESA 21, one can define the starting position of deflected ions into the TOF analyser 12, compensating for the rather long deflection time period, which would otherwise limit the resolution of the analyser.

In an alternative embodiment of the invention, ions may be deflected sideways away from their initial trajectory, using a simple pulsed parallel plate deflector. A deflection of about 10 can be easily obtained, which enables the ions to be diverted into a small electrostatic analyser to increase the total deflection away from the orignal ion beam path, thus simplifying the physical construction of the TOF analyser next to the isotope mass spectrometer.

In a further alternative embodiment of the invention, instead of using a TOF analyser the ions are deflected into a storage ion trap analyser, whence they are ejected sequentially onto an ion detector, during the doff" period of the deflection pulse, again providing a record of the elemental composition of the sample.

Claims

CLAIMS

1. Spectrometer apparatus for measuring the isotope ratio of an element within, and the elemental composition of, a sample, the apparatus comprising means for ionising a sample in an inductively coupled plasma, means for passing the ions through transfer optics including a pulsed deflector, a multiple collector isotope mass spectrometer analyser having a source slit and an elemental ion analyser, and means for actuating the pulsed deflector to enable ions to be deflected from the path towards the source slit of the multiple collector isotope mass spectrometer to a path leading to the elemental ion analyser.

2. Apparatus according to Claim 1 wherein the elemental ion analyser is a time of flight analyser.

3. Apparatus according to Claim 1 wherein the elemental ion analyser is an ion trap analyser.

4. Apparatus according to any one of Claims 1 to 3 and including laser ablation means to ablate material from the sample, and means to feed the ablated material to the location of the plasma.

5. Apparatus according to any one of Claims 1 to 4 wherein the control means is configured to deflect ions from the path towards the source slit of the multiple collector isotope mass spectrometer at a pulse rate of at least 1 kHz and a pulse width of 30 to 50 nanoseconds.

6. A method of measuring the isotope ratio of an element within a sample and the elemental composition of the sample simultaneously, which method comprises ionizing the sample in an inductively coupled plasma, passing the ions though transfer optics including a pulsed deflector and controlling the pulsed deflector so that the majority of the time ions pass on to a source slit of a multiple collector isotope mass spectrometer analyser, and during the remainder of the time ions deflected from the path to the isotopic analyser travel to and enter an elemental ion analyser.