GB2217159A - Sample heating and analysis - Google Patents

Abstract

An apparatus (1) for heating a sample (2) under vacuum conditions, especially for extracting gas (105) from the sample for isotope ratio studies, the apparatus comprises: a first vacuum enclosure (18) for containing the sample disposed within a crucible (4); and a resistively heated electric heating element (26), disposed outside of the first vacuum enclosure and positioned around at least part of the crucible, the heating element being disposed within a second vacuum enclosure (20) and comprising a graphite carbon resistor (25). Preferably the heating element has cooled input and output terminals at least partly electroformed onto the resistor, and a gas purification system (6) and a mass spectrometer (8) are provided for isotope analysis of the gas and sample. <IMAGE>

Description

Sample Heating and Analysis The invention relates to an apparatus and method for heating a sample under vacuum conditions, and particularly though not exclusively to isotope ratio mass spectrometry, such as for studying mineral samples by the analysis of inert gases extracted from heated samples under high or ultra-high vacuum conditions.

Mineral rock samples may be dated or otherwise characterised by analysing the isotopic composition of inert gases extracted therefrom. Characterisation may comprise determining from the isotopic composition whether a sedimentary rock sample is of terrestrial or meteoric origin, or determining the history of fission processes in a sample. Examples of such studies are described by: E.W.Hennecke et al in the Journal of Inorganic Chemistry, volume 40, 1978, pages 1281 to 1284; F.A.Podosek et al in Geochimica et Cosmochimica Acta, volume 44, 1980, pages 1875 to 1884; and O.Eugster et al in Earth and Planetary Science Letters, volume 74, 1985, pages 27 to 34. In summary, the technique comprises: heating a sample in a crucible, extracting evolved gas, purifying the gas to remove non-inert material, and then analysing the isotopic composition of the remaining inert gas.The temperature of a sample may be raised in a step-wise manner to drive-off different fractions of gases evolved at characteristic temperatures from different structures within the mineral, eg from the surface region, or from various parts of the bulk of a sample. Apparatus and methods for inert gas studies of mineral samples are described for example by: V.Costa et al in Earth and Planetary Science Letters, volume 25, 1975, pages 131 to 141; C.R.Macedo et al in Earth and Planetary Science Letters, volume 34, 1977, pages 411 to 418; and T.Staudacher et al in the Journal of Physics E, Scientific Instruments, volume 11, 1978, pages 781 to 784.

The mineral samples may be heated by heat transfer from a resistively heated element, (ie an element through which an electric current is passed and which is heated by the Joule heating effect as a consequence of its electrical resistance), or by electron bombardment, or by radio-frequency heating. Radio-frequency heating has the disadvantage that eddy currents may be induced in, and so heat, parts of the structure other than the crucible.In T Staudacher et al's apparatus a crucible is heated up to 1700"C by radiation from a resisitively heated tantalum element disposed in an outer vacuum chamber, external to the crucible chamber Tantalum is one of few materials that can provide a reliable resistive heater at such temperatures, but a particular difficulty is that in order to get a suitably high resistance the material has to be slotted or otherwise cut-away, which leads to a weaker structure with a tendency to distort on heating.In the wider field of vacuum furnace design, graphite heating elements have been used to give temperatures reaching 2000 C, as described by: F.Remy in the Journal of Physics E, Scientific Instruments, volume 2, series 2, 1969 pages 287 to 288; A.E.B.Presland and J.R.White in the Journal of Physics E, volume 2, series 2, 1969, pages 67 to 68; M.Hastenrath et al in the Review of Scientific Instruments, volume 48, No 6, 1977, pages 605 to 609; United States Patent No. 3525795; and United Kingdom Patents Nos. 1302479 and 1318127.Graphite conductors are also used in the field of electrothermal atomisation, as reviewed for example by M.W.Routh et al in International Laboratory, May 1982, pages 101 to 118, although such apparatus generally operates under an atmosphere of inert carrier gas, for carrying atomised sample materials, and not under the stringent vacuum conditions required for isotope ratio studies.

Presland and White describe difficulties caused by evaporation of graphite when operating above 2000"C in vacuum, and preferred to use a furnace with continually flowing inert gas for operation above 2200"C. A further difficulty with graphite conductors is in making a high integrity electrical contact to the graphite. This is generally done using fixing screws or by compressive means such as clamping, as in the apparatus of J.Remy; the apparatus of A.E.B.Presland and J.R.White relies upon gravitational force to make an electrical contact.

It is an object of this invention to provide an improved apparatus for heating a sample maintained under high or ultra high vacuum conditions. It is a further object to provide an improved isotope ratio mass spectrometer adapted for the studying of mineral rock samples, and it is a yet further object to provide an improved method for analysing mineral samples.

According to one aspect of the invention there is provided an apparatus for heating a sample under vacuum conditions and comprising: a first vacuum enclosure for containing said sample disposed within a crucible; and a resistively heated electric heating element, disposed outside of said first vacuum enclosure and positioned around at least part of said crucible, said heating element being disposed within a second vacuum enclosure and comprising a graphite carbon resistor. Preferably the heating element also comprises a metal input terminal and a metal output terminal, wherein at least part of each terminal is electroformed onto said graphite resistor. Electroforming comprises the deposition during electrolysis of a metal onto a substrate electrode, which in this case is the graphite resistor.Preferably the metal is substantially copper, electrolytically deposited from a solution of a copper salt such as copper pyrophosphate. We have found that such electroformed terminals have improved durability, together with substantially homogeneous electrical contact, which provides improved uniformity of heating.

Preferably each terminal of the heating element comprises a first copper part electroformed onto the graphite resistor and a second copper part attached to, and in good electrical contact with, the first part; further electrical connections are then made to the second part of each terminal. Preferably the input and output terminals each have formed within them a cooling channel, and the apparatus comprises means for passing a cooling fluid, such as water, through the channels.

Preferably a cooling fluid channel is formed in the second part of each terminal. This provision of cooling fluid is especially advantageous for allowing the heating element to be operated substantially continuously for several hours, at a power output of typically 3.5 kW, without damage to the input and output terminals. Preferably the cooling fluid passes to the terminals through a conduit which is at least partially flexible, such as may be achieved by providing a loop in a pipe, or a bellows, for accommodating thermal expansion and contraction of the element. Preferably the second part of each terminal is brazed onto the first part; this gives good electrical and thermal contact, although other means of attachment, such as clamping, or by means of a screw thread formed in the first part, may be used.

Preferably the graphitic carbon is of the type known as high density carbon, and has a fine grain structure which makes it convenient to machine in manufacture.

The carbon may be shaped to aid adhesion of the metal terminals, for example with a concave depression or groove. In a preferred embodiment both the crucible and the carbon resistor of the heating element are elongate and tubular, and are preferably though not necessarily substantially cylindrical. Thus in an especially preferred embodiment the heating element comprises a high density carbon resistor in the form of a substantially cylindrical elongate tube having an input terminal at least a first part of which is electroformed onto one end, and an output terminal at least a first part of which is electroformed onto the other end. The tubular resistor may have an annular groove formed circumferentially around each end to aid adhesion of the electroformed metal.

It is convenient to provide a crucible liner, disposed substantially within the crucible, wherein the sample may be placed and supported; such a crucible liner may be replaced after the analysis of one or more samples.

In a further preferred embodiment the crucible forms part of a wall of the first vacuum enclosure, and projects, via a sealed port, into the second vacuum enclosure; in this case a removable crucible liner is convenient because the liner may be removed from within the crucible without breaking the seal at said port into the second vacuum enclosure.

In a preferred embodiment an elongate and substantially cylindrical crucible liner is disposed substantially within an elongate and substantially cylindrical crucible. Preferably each of the crucible liner, the crucible, and the heating element, are substantially concentric and co-axial, and may, together with a flange removably and sealably attachable at a port, constitute part of a conveniently removable sub-assembly. Heat shields may also be provided around the heating element and may also be part of the sub-assembly. The crucible liner and the crucible are each composed of a refractory material, preferably tantalum; alternatively molybdenum may be used, although that material has poorer vacuum compatibility.

The heat shields around the heating element are similarly composed of refractory material; also graphite cloth may be used in the heat shields.

The apparatus preferably comprises a first vacuum pumping system for evacuating the first vacuum enclosure and capable of establishing and maintaining a residual pressure of about leo 10 torr (1.33 x 10-8 Pa), or lower, therein. Such ultra-high vacuum conditions are important for maintaining the purity of gas extracted from the sample. Preferably the apparatus also has a second vacuum pumping system for evacuating the second vacuum enclosure, and while this may have the same performance as the first pumping system it is not necessary for it to have that performance; the pressure in the second vacuum enclosure is typically maintained at about 10 6 torr (1.33 x 10 4 Pa).

The apparatus also comprises means for energising the heating element, typically a power supply capable of supplying a current in the range of from about 300A to 600A, at a voltage of about 10V. Current is supplied to the heating element via a conductive path at least part of which comprises a flexible member, such as copper braid, for accommodating thermal expansion of the heating element. Means for monitoring temperature (of the heating element or crucible) may also be provided; for example a tungsten/rhenium thermocouple, preferably linked by feedback to the power supply. Thus the temperature may be raised in a step-wise manner and also selectively maintained at a chosen temperature.

Alternatively an optical pyrometer, serviced by an optical fibre link, may be provided.

The apparatus is particularly advantageous for heating, and eventually melting, rock mineral samples from which gases may be extracted and subsequently analysed; the gases of interest are particularly, though not exclusively, the inert gases: helium, neon, argon, krypton, and xenon. Thus in an especially preferred embodiment there is provided a mass spectrometer for the analysis of mineral samples, and comprising: heating apparatus as described above, a gas purification system, and a mass analyser. The gas purification system may comprise several stages for purifying extracted gas. Preferably the spectrometer is adapted for measuring the isotopic ratio of inert gases, and the mass analyser preferably comprises a magnetic sector analyser. Alternatively the spectrometer may comprise another form of analyser, such as a quadrupole mass analyser for example.

Also in a preferred embodiment the spectrometer comprises a sample loading chamber, into which several samples may be loaded for successive analysis without breaking into the vacuum seal, also an air lock may be provided for loading samples.

According to another aspect of the invention there is provided a method for the analysis of a mineral sample comprising: maintaining said sample in a vacuum in a first vacuum enclosure, heating said sample and thereby causing the release of gas therefrom, purifying said gas thereby selecting inert gas therefrom, and analysing the isotopic ratio of said inert gas; wherein said step of heating said sample comprises: passing an electric current through a heating element composed at least in part of graphitic carbon and disposed within a second vacuum enclosure, thereby resistively heating said heating element and consequently heating said sample with energy transferred therefrom. Preferably said heating element also comprises at least partially electroformed input and output terminals composed substantially of copper.Typically the method comprises heating the sample to in excess of 1500"C, we have found that temperatures can be used in excess of 2000 0C if required. The method may, though not necessarily, comprise melting the mineral sample. The invention thus conveniently provides a method for dating or otherwise characterising the mineral sample by isotopic ratio analysis.

A preferred embodiment of the invention will now be described in greater detail, by way of example, and with reference to the figures in which: Figure 1 schematically illustrates an isotope ratio mass spectrometer having a dual vacuum system and resistively heated graphite heating element according to the invention; and Figure 2 is a section through part of the apparatus showing the arrangements for sample heating in greater detail.

Referring first to figure 1 there is shown an isotope ratio mass spectrometer 1 in which a sample 2 is disposed within a tantalum crucible liner 3, within a tantalum crucible 4. The crucible 4 communicates, via a pipe 5 with a gas purification system 6 connected through a valve 7 to a mass analyser 8; mass analyser 8 comprises: an ion source 32, slits 33 and 34, a magnetic sector 35, and an ion detector 36. The gas purification system comprises a first (rough) stage 9 and a second stage 10, separated by a valve 11. Each of stages 9 and 10 may comprise several purifying agents, such as titanium-based getters for example, and each may be pumped by one or more pumps, represented schematically in figure 1 by pumps 12 and 13. The mass analyser 8 is also pumped by a pump 14 (which may be a turbomolecular pump or an ion pump for example).Sample 2 is introduced into crucible 4 from a sample loading chamber 15, which contains a carousel 16 on which several samples 17 awaiting analysis may be circumferentially mounted for successive analysis. Thus spectrometer 1 has a first vacuum enclosure 18 comprising: loading chamber 15, crucible 4, gas purification system 6, and mass analyser 8; the spectrometer 1 also has a first vacuum pumping system 19 comprising: pumps 12, 13 and 14, capable of establishing and maintaining a vacuum with a residual -10 -8 pressure of the order of 10 10 torr (1.33 x 10 8 Pa) or less within first vacuum enclosure 18.

Figure 1 also shows a second vacuum enclosure 20, comprising a chamber 21 and pumped by a second vacuum pumping system 22 to a vacuum of the order of 10 torr (1.33 x 10 4 Pa). In figure 1 chamber 21 is shown cut-away schematically to reveal crucible 4 entering via a sealed port 23 through a flange 24. A substantially cylindrical graphite resistor 25, being part of a heating element 26, is disposed around crucible 4 as shown. The interior of crucible 4 does not communicate with the pumped volume within the second vacuum enclosure 20, and thereby sample 2 is isolated from any contaminating material therein, which may be released from heating element 26 for example.

Heating element 26 is energised by a current from a power supply 27. The current passes, via feedthroughs 28 and 29, through pathways represented in figure 1 by conductors 30 and 31, which will be described in greater detail with reference to figure 2 below.

To operate the apparatus, samples 17 must first be loaded into chamber 15 (the isolating valves, such as valves 7 or 11, may be closed during this operation to avoid exposing all of the spectrometer 1 to atmospheric gas, additional valves may be provided for isolating the several components of the spectrometer, as required). Next, (with no sample 2 yet in crucible 4) at least chamber 15, crucible liner 3, and crucible 4 are baked to about 350"C for several hours. At least the first stage 9 of the gas purification system 6, and incidentally vacuum chamber 21, are baked at the same time. The baking is achieved by conventional baking ovens (not shown). Then the power supply 27 energises heating element 26, reaching a temperature of around 2000"C, for about 1 hour, thereby thoroughly degassing the crucible 4, and crucible liner 3.It will be appreciated that pumps, such as pump 12, need to be run during baking and degassing, to remove gas evolved at that time. Then the crucible is allowed to cool, and a sample 2 is dropped from carousel 16 into crucible liner 3. Next the heating element 26 is used to heat sample 2 as required to drive off gas 105 for analysis.

Typically gas 105 will first be collected in the first stage 9 of the gas purification system, then passed to the second stage 10, and finally to mass analyser 8 for analysis of the inert gas composition and isotopic ratio. After analysing each of the samples 17 in succession (one sample after another dropped into crucible liner 3) the loading chamber 15 may be re-opened and the solid residue from the analysed samples removed by removing liner 3. A new liner 3 may then be fitted and the procedure repeated. Thus, the provision of a removable crucible liner 3 means that the second vacuum enclosure 20 need not be broken into, which would be the case if the crucible had to be replaced.

Referring next to figure 2 there is shown, again: sample 2, crucible liner 3, crucible 4, heating element 26 with elongate substantially cylindrical tubular graphite resistor 25, feedthrough 28, and parts of pipe 5 and flange 24. The entry port 23 is shown in more detail and comprises a flange 37 affixed by bolts and a seal (not shown) to part of the flange 24, which in turn is affixed by bolts and a vacuum seal (not shown) to chamber 21 (figure 1). Pipe 5 is similarly sealed by a flange 38 and a copper sealing ring 39 to flange 37. Crucible 4 comprises a wall 40, and a base member 41 affixed to wall 40 by a weld 42.The end 101 of crucible 4 is shaped as shown and sealed by gold seals 43 and 44 in compression between flanges 24 and 37; gold is preferable to copper for this seal because it can be used without machining knife edges in the mating surfaces, which is difficult here because of the comparatively brittle nature of tantalum. The crucible 4 thus forms part of the first vacuum enclosure 18 described above with reference to figure 1. The crucible liner 3 is suspended from a ledge 45 formed in wall 40 of crucible 4, as shown. Also the crucible liner 3 has formed in its walls two holes 103 and 104 into which a pronged extractor (not shown) may be inserted for removing, or replacing, the liner.

In addition to graphite resistor 25, the heating element 26 also comprises a current input terminal 46 and an output terminal 47. Considering first the input terminal 46, this component comprises: a first part 48, which is manufactured by electroforming onto the graphite resistor 25; and a second part 49 which is brazed onto the first part 48. The second part 49 of terminal 46 has within it a cooling channel 51, connected to water pipes 52 and 53. Water flows as indicated by arrows 54 and 55 through pipe 52, channel 51 and pipe 53. Pipe 52 comprises a flange joint 56, and pipe 53 a flange joint 57, which joints are provided to facilitate servicing of the apparatus. Also both pipes have a loop such as a loop 58 in pipe 52 to allow movement resulting from thermal expansion during operation.

Similarly the output terminal 47 comprises: an electroformed first part 59; and a second part 60, brazed to the first part 59, and having a cooling channel 61 through which water flows from a pipe 62 to a pipe 63 as indicated by arrows 64 and 65. Output terminal 47 is brazed to a copper flange 66 which is affixed to flange 24 by three screws, of which one (a screw 67) is shown. A ceramic insulator 68 is provided for insulating flange 66 from flange 24.

Figure 2 also shows that feedthrough 28 comprises: a flange 69, sealed by a sealing ring 70, and affixed by bolts or screws (not shown) to flange 24; a stem 71; and a ceramic insulator 72 and a cap 73 which grip a hollow electrically conductive tube 74. Tube 74 is connected to the water pipe 52 by a coupling 75. A conducting bar 76 is connected to tube 74, and also to an annular (washer-shaped) ring conductor 77 by means of a screw, washer and nut assembly 78. Copper braids, such-as braids 79 and 80, carry current from ring conductor 77 to the input terminal 46 of the heater assembly, and accomodate thermal expansion of the heating element. The current passes in parallel along pipe 52 and bar 76. A second conducting bar 106 is fastened to the copper flange 66 at the output end of heating element 26.Bar 106 and the nearby water pipe 63 are connected to feedthrough 29 (figure 1) which is not shown in detail but is substantially the same as feedthrough 28. In addition to feedthroughs 28 and 29, the apparatus also has a further two similar feedthroughs (not shown) one connected to pipe 53 for removing water from channel 51, and the other connected to pipe 62 for providing cooling water to channel 61.

Each feedthrough is attached to flange 24 thus providing a conveniently removable sub-assembly. The water flows in one circuit, external to chamber 21 via a link (not shown) between the vacuum feedthroughs, thus taking the water out from pipe 53 back into pipe 62, before passing to an external sink.

Figure 2 further shows a number of heat shields. A tantalum shield 81, and a molybdenum shield 82 are mounted, with the aid of a spacer 83 and screws (such as screw 107) onto the first part 59 of output terminal 47. A further molybdenum shield 84 is mounted as shown by means of brackets 85 and 86, electrically insulating spacers 87 and 88, and screw assemblies 89 to 92. Also a graphite cloth 93 is packed inside shield 84.

Additional tantalum heat shields 94 to 97, are provided, along with spacers 98 and 99, fitted at the input terminal 46, as shown.

The tubular graphite resistor 25 is composed, for example, of the grade 'very fine grain structure - high density graphite' supplied by Le Carbone (GB) Ltd, of Portslade, Sussex, England. It is manufactured from a tube, approximately 200 mm long, of outer diameter 40 mm, and wall thickness 5 mm. The ends are shaped as shown, particularly incorporating annular grooves 50 and 100, seen in cross-section as concave depressions, which we have found aid adhesion of the electroformed copper metal. After shaping the ends, and forming the input and output terminals as described, the tube is machined on its external diameter to a final wall thickness of about 1 mm. In that configuration we have found that the graphite retains sufficient mechanical strength in operation (in terms of avoiding bending, sagging or breaking) while having an appropriate electrical resistance.Typically the electric current through resistor 25 is in a range of from 300 A to 600 A, the potential difference across it being about 10 V. A thermocouple 102 is provided for monitoring the temperature of the heater assembly which may be in excess of 2000 C.

The power supply 27 is programmable and, taking feedback signals from thermocouple 102, it can control the current to raise the temperature of element 26 in a controlled manner - for example to provide step-wise heating for extracting different noble gases released from different regions of a sample at different temperatures, or to maintain the temperature at a substantially constant level during gas extraction. We have found that the present invention, with more uniform and controllable sample heating than prior apparatus, gives significantly improved conditions for gas extraction and analysis. Moreover the heater assembly of the invention has improved reliability especially for operating temperatures in the region of 2000"C, and will tolerate faster thermal cycling than prior apparatus.

Claims (13)

CLAIMS:

1. An apparatus for heating a sample under vacuum conditions and comprising: a first vacuum enclosure for containing said sample disposed within a crucible; and a resistively heated electric heating element, disposed outside of said first vacuum enclosure and positioned around at least part of said crucible, said heating element being disposed within a second vacuum enclosure and comprising a graphite carbon resistor.

2. An apparatus as claimed in claim 1 in which said heating element also comprises a metal input terminal and a metal output terminal, wherein at least part of each of said terminals is electroformed onto said graphite carbon resistor.

3. An apparatus as claimed in claim 1 in which said graphite carbon resistor is in the form of a substantially cylindrical elongate tube having at least a first part of a metal input terminal electroformed onto one end, and at least a first part of a metal output terminal electroformed onto its other end; said tube also having an annular groove formed circumferentially around each said end, for aiding adhesion of each said electroformed part of said terminals.

4. An apparatus as claimed in claim 2 or claim 3 in which said metal of said metal input and output terminals is substantially copper.

5. An apparatus as claimed in any of claims 2 to 4, in which each of said terminals has a second part with a cooling channel formed therein, and said apparatus further comprises means for passing a cooling fluid through each said cooling channel.

6. An apparatus as claimed in any previous claim in which an at least partially flexible electrical conductor, and an at least partially flexible conduit of cooling fluid, are attached to said heating element.

7. An apparatus as claimed in any previous claim wherein said crucible and said graphite carbon resistor are both elongate and substantially cylindrical, and said crucible forms part of a wall of said first vacuum enclosure and projects through a seal into said second vacuum enclosure, and is substantially concentric and coaxial with said resistor.

8. An apparatus as claimed in any previous claim and further comprising a crucible liner disposed substantially within said crucible, wherein said sample is disposable within said crucible liner.

9. An apparatus as claimed in any previous claim and further comprising a mass analyzer for analyzing gases driven off from said sample by said heating.

10. A method for the analysis of a mineral sample comprising: maintaining said sample in a vacuum in a first vacuum enclosure, heating said sample and thereby causing the release of gas therefrom, purifying said gas thereby selecting inert gas therefrom, and analysing the isotopic ratio of said inert gas; wherein said step of heating said sample comprises: passing an electric current through a heating element composed at least in part of graphitic carbon and disposed within a second vacuum enclosure, thereby resistively heating said heating element and consequently heating said sample with energy transferred therefrom.

11. A method as claimed in claim 10 wherein said heating element also comprises at least partially electroformed input and output terminals composed substantially of copper, and said method further comprises cooling said terminals by passing a cooling fluid through channels formed therein.

12. A method as claimed in claim 10 comprising supporting said sample in a removable crucible liner within said first vacuum enclosure.

13. An apparatus substantially as described herein with reference to the accompanying drawings.