IMPROVEMENTS IN OR RELATING TO THE PREPARATION OF SAMPLES FOR PALYNOLOGICAL ANALYSIS

Abstract: A method for the preparation of kerogen for palynological analysis, in which a kerogen-containing particulate rock sample is treated by hydrofluoric acid digestion, which comprises adding an inhibitor which is soluble in the hydrofluoric acid reaction medium and which inhibits the formation of an insoluble fluoride layer on the particles of the sample.

Description: IMPROVEMENTS IN OR RELATING TO THE PREPARATION OF SAMPLES FOR PALYNOLOGICAL ANALYSIS This invention relates to the preparation of kerogen samples for palynological analysis. Chemical preparation of siliclastic rock samples for palynology involves mainly dissolution of silicate and carbonate minerals followed by preparative treatment of the organic residue or kerogen, for example, for microscopic analysis. Dissolution of silicates, achieved using hydrofluoric acid (HF), is a time-consuming, expensive and extremely hazardous part of this process requiring the use of protective clothing and ventilated laboratory space and stringent waste disposal control. Industrial applications of palynology (monitoring of drilling progress, identification of casing points, identification of stratigraphic T.Ds) often require rapid preparation of rock samples for analysis. Increased speed of sample preparation in such circumstances may be achieved by rapid silicate digestion using hot HF. The use of microwave heating of samples in rapid HF digestion procedures is described by Ellin & McLean in Palynology, 18 (1994): 23-31. In order for HF digestion to be effective, however, it is necessary to remove cations such as calcium and magnesium which can give rise to an insoluble coating on the particles of rock comprising the sample which substantially inhibits further reaction with HF. Thus, in conventional palynological analysis, if, for example, calcium carbonate or magnesium carbonate is present, it is necessary to treat the sample with 35% hydrochloric acid (HC1), allow the insoluble material to settle, decant the liquid containing the cations and sieve the sample. This time-consuming step, which may have to be repeated in some cases in order to solubilise the calcium and magnesium present, makes it difficult to put in place an automated regime for the treatment of palynological samples. The process is also, in some cases, not able to remove all of those cations which may produce insoluble fluorides. A further problem which arises in the conventional treatment process is the necessity to dilute the reaction medium after HF digestion, neutralise excess HF, and sieve the sample to remove any fines and/or insoluble fluorides. This provides a further time-consuming step which impedes the automation of the procedure. It will be appreciated, therefore, that there is a pressing need for a method of treatment of palynological samples which can eliminate or reduce the need to sieve the sample. It would also be advantageous to provide a method which can use smaller quantities of reagents, which can use safer reagents, and which can be carried out more quickly and safely. These and other advantages can be fulfilled by preferred embodiments of the present invention which provides a method for the preparation of palynological samples by HF digestion in which the formation of an insoluble non-reactive layer on the sample can be substantially obviated. According to a first aspect of the invention, there is provided a method for the preparation of kerogen for palynological analysis in which a kerogen-containing particulate rock sample is treated by HF digestion, which comprises adding an inhibitor which is soluble in the HF reaction medium and inhibits the formation of an insoluble non-reactive layer on the particles of the sample. The inhibitor can be, for example, a metal salt having a cation that is soluble in hydrochloric acid, and in either hydrofluoric acid, or aqueous hydrofluoric acid, or both. Suitable soluble metal salts include, for example, salts of Group I and Group II metals, and transition metals, more particularly salts of alkali metals such as, for example, sodium and lithium. The anion of the metal salt is preferably a halogen anion and more preferably a fluoride. Other anions which may be used include chloride, bromide, nitrate and sulphate. Without wishing to be bound by any particular theory, it is believed that the soluble metal cations interfere with the deposition of insoluble fluorides on the particles of the sample by disrupting the lattice formation and thereby exposing the surfaces of the particles to further HF digestion. Excellent results have been obtained using sodium fluoride and sodium chloride, and these are the preferred inhibitors for use in the present invention. In a further embodiment of the invention, the inhibitor may be a fluoroboric acid, for example, tetrafluoroboric acid HBF 4 , which may be used in addition to, or instead of, the soluble metal salt inhibitor. Without wishing to be bound by any particular theory, it is believed that the action of the fluoroboric acid is to form soluble complex fluoroborates, with, for example, calcium ions, and thereby interfering with the deposition of insoluble calcium fluoride on the particles of the sample. In a preferred method according to the invention, the sample is washed, crushed and sieved to remove larger particles and small fines, leaving particles having an average diameter of from typically 1 to 3mm and preferably from about 0.71 to 1.70mm. This may optimise acid percolation and allow a larger surface area for acid attack. The particles of the sample generally contain clasts (which may also contain palynomorphs) and palynomorphs held in a silicate/carbonate cement matrix of variable composition. To the sample is added from 25 to 50% by weight, based on the weight of the sample, of a soluble metal salt, preferably sodium fluoride, or sodium chloride. The sample is then treated with HC1, of concentration preferably up to approximately 35 - 40%, (in a preferred embodiment 37% is used) at a temperature of from 20°C up to the boiling point of the reaction medium for a time of from 1 to 60 minutes in order to break up the cement and release the clasts and the palynomorphs. If desired, tetrafluoroboric acid HBF 4 may be used instead of hydrochloric acid in an amount of from, for example, 20 to 50% by weight, based on the weight of the sample. The HBF 4 may also function as an inhibitor. In either case, the sample is then preferably evaporated to reduce the volume so that further acid may be added, and in one embodiment it may be evaporated to near dryness, and subjected to HF digestion using hydrofluoric acid, for example, at a concentration of up to about 60% but preferably in the range 40-60%, at a temperature of from 20°C to 120°C and for a time of from 1 to 220 minutes. If desired, it may be advantageous to add a selective oxidising agent to further aid the digestion of carbonate and silicate materials such as, for example, sodium permanganate. The HF digestion is preferably carried out in a microwave heating apparatus. A suitable pressurised system microwave oven is described in Ellin & McLean, Palynology, 18 (1994): 23-31, the entire disclosure of which is incorporated herein by reference for all purposes. More preferably, however, there is used a focused microwave system in which a waveguide directs the microwave onto the reaction medium. In this method the reaction medium is preferably contained in a microwave transparent vessel and the microwaves are directed toward a lower part of the vessel, allowing a reflux action to take place in an upper part thereof. A focused microwave system of this type operates at atmospheric pressure, thus eliminating the need for a pressure relief vessel to contain the reaction medium. Suitable focused microwave heating systems are available from Prolabo and sold under the trade marks Microdigest 301 and Microdigest 401. The heating may be carried out under reflux for about 5 to 120 minutes at about 10-99%, and preferably 10 to 40%, power of a 400 watt focused microwave heating apparatus. Preferably the sample is situated at a position at which the focused microwave energy is greatest and, in a still further aspect of the invention, the sample is raised above the floor of the microwave transparent reaction vessel on a novel support. In one embodiment, the support may comprise a platform which may comprise a horizontal grating which has holes of a size sufficient to allow palynomorphs released from the sample to fall through. The grating may, for example, be supported by a surrounding microwave transparent support member, for example, a hollow tube having a foot, or feet, adapted to rest on the floor of the reaction vessel, and a shoulder or projection by means of which the platform may be raised or lowered into the vessel. In a further embodiment, the sample support may comprise a bag or container, having a permeable wall permitting ingress of the reaction medium and retention of the palynomorphs released from the sample. Such a bag may comprise, for example, a flexible "sock" formed from PTFE of mesh size 12 microns or less, preferably 10 microns or less, with a diameter of about 40mm and length 50 to 150mm, preferably about 90 to 100mm. The bag may be formed from one piece of PTFE mesh folded to shape, but is preferably formed from two sheets of PTFE mesh cut to size and stitched, or preferably glued or welded around their edges. The adhesive or welded joint is preferably able to withstand a temperature of at least 120°C. More preferably, however, the support is a container or capsule having a permeable wall, for example, a permeable-walled tube, such as, for example, a sintered PTFE tube. The tube can, for example, have a permeable wall with a pore size of from 5 to 20 microns, preferably about 10 microns. The bag or container may be supported, for example, by a support ring or lid secured to the top edge of the bag or container. Embodiments of sample supports according to this further aspect of the invention are illustrated, by way of example only, in Figures 1 and 2 of the accompanying Drawings, which show, in side elevation, partly in section, respectively a platform and a container, the latter being illustrated in position in a microwave vessel. Figure 3 shows a side cross-sectional view of a reaction vessel, a sample support in the form of a sock or bag and a tube for providing gas flow near the sample suppor . Referring firstly to Figure 1, the platform, illustrated generally at 1, comprises a horizontal grating 2 of 2mm thickness, having a plurality of vertically directed holes 3 of 0.5mm diameter therein. The grating 2 is supported around its periphery by a tube 4, of length 6cm, internal diameter 29mm and wall thickness 6mm. The tube 4 has a base 5 which rests on the floor of the reaction vessel and a shoulder 6 by which it can be raised or lowered into the vessel. Both the grating 2 and the tube 4 are preferably made from a suitable microwave transparent material, for example, Teflon. In use, the sample is placed on the grating 2, and the reaction medium heated by focused microwaves. As the palynomorphs are released by the HF digestion, they fall through the holes 3 to the floor of the reaction vessel. After HF digestion, the reaction medium may be diluted, neutralised, and the sample sieved and treated in accordance with further procedures as detailed below. Referring now to Figure 2, there is shown diagrammatically the relevant portion of a microwave digestion apparatus, illustrated generally at 11, which comprises a digestion vessel 12, and a reflux head 13. The digestion vessel 12 and reflux head 13 are connected by a tubular passage 14 which terminates in an opening 15 in the lid 16 of the vessel. Passing through the passage 14 and into the vessel 12 are reagent feed tube 17, and exhaust tube 18, connected respectively to a reagent entry hole 19 and a reflux head component 20. The reagent feed tube 17 is the smaller of the two tubes, and the exhaust tube 18 is the larger. The reagent feed tube 17 passes through and supports the top location block 21 of the digestion capsule 22, the body of which comprises a permeable PTFE tube 23. The tube 23 is formed from sintered PTFE and has an average pore size of 10 microns. In use, the sample 24 is placed in the capsule 22, and the focused microwaves applied. As the reaction proceeds the palynomorphs are released and retained in the tube 23. After HF digestion, the sample can be treated as described hereinafter. When preparing samples for palynological and other analysis it is often necessary to treat the kerogen liberated from rock samples by the hydrofluoric acid digestion further in order to make dark or thick-walled palynomorphs sufficiently translucent for transmitted light microscopic examination. When processing samples which contain unwanted kerogen components, for example, amorphous organic matter (AOM), such as those obtained from Kimmeridge Clay type facies of north west Europe, it is also necessary to minimise the levels of AOM present in order to concentrate and easily distinguish for identification any palynomorphs which may be present. AOM occurs in two forms, dispersed and clumped. Dispersed AOM can be simply removed by washing it through a suitable sized sieve, while clumped AOM cannot and often incorporates palynomorphs. In order to remove clumped AOM and release any palynomorphs which may be present, extended and frequently repeated treatment is usually required. This can take up to four days in some cases to obtain clean samples which can be identified using a microscope. The treatment is conventionally carried out using strong oxidising agents since oxidative treatment enables unwanted AOM to be selectively removed by virtue of the fact that the majority of AOM is more susceptible to oxidation than sporopollenin which forms the walls of many palynomorphs. Preferably, however, the kerogen liberated by HF digestion is treated by a method which involves subjecting the kerogen to microwave heating in the presence of a suitable reaction medium for a period of time sufficient to bleach the palynomorphs and/or to reduce the level of any unwanted kerogen present. The reaction medium can, for example, comprise nitric acid, and can be an oxidising medium if the nitric acid is present in a strength of greater than 50% concentration. Preferably there is used a method as described and claimed in our copending UK Patent Application No. 9403163.0, and/or in our copending International Patent Application No. PCT/EP95/00558. In a preferred method step, which comprises a further and separate aspect of the invention, after HF digestion, and before further treatment of the kerogen, a complexing agent can be added to the reaction medium to complex any free HF still present. The complexing agent solution may then be used as the reaction medium or solvent for the subsequent treatment step with nitric acid, or with an oxidising agent, or both. Examples of suitable complexing agents include boric acid H 3 BO 3 and aluminium chloride A1C1 3 .6H 2 0. Preferably the complexing agent is added as a saturated or near saturated solution. In the case of boric acid, a 5% solution or greater is preferred. The preferred complexing agent is, however, aluminium chloride, which has a higher solubility (greater than 25%) and can also dissolve insoluble fluorides. Preferred procedures for further treatment of kerogen liberated by HF digestion are fully described in the above mentioned patent applications, the entire disclosures of which are incorporated herein by reference for all purposes. Finally, the treated kerogen sample may be quenched, diluted with water, and neutralised, for example, with potassium hydroxide KOH, before being stained and mounted on microscope slides. By combining the preferred method steps of the invention it is possible to obtain a fully automated process for preparing palynological samples in which the appropriate reagents are added at timed intervals and there is no need to dilute the reaction medium and sieve the sample between the method steps. In an example of such an automated process, the steps can be as follows: 1. HC1 of strength up to 35% with, as an inhibitor, NaCl, NaF, and/or HBF 4 of strength up to 50%, is added to the sample, which has been manually sieved to give a particle diameter range of from 0.71mm to 1.70mm, and the reaction medium heated to an elevated temperature for a period of up to 60 minutes; 2. The reaction medium is evaporated to near dryness; 3. Hydrofluoric acid at a concentration of up to 60% is added and the reaction medium heated at an elevated temperature for up to 120 minutes; 4. Saturated boric acid or aluminium chloride is added to complex any free HF present; 5. Nitric acid is added to give a nitric acid concentration in the reaction medium of 50% or greater, and the sample heated at an elevated temperature for a period of up to 30 minutes; 6. The reaction medium is quenched, diluted with water and neutralised with 2% KOH; 8. The insoluble kerogen residue is stained and samples prepared for slide mounting. All the above method steps can, and preferably are, carried out in a focused microwave heating system equipped with, for example, a carousel for handling a plurality of samples, and with automated reagent dispensing and handling equipment. The microwave system can be enclosed in a suitable fume extraction enclosure thereby reducing the risk of exposure of the operator to the fumes or to the reagents involved in the procedure. The invention is illustrated by the following Examples: Example 1 Carbonate rich bituminous mudstone (carboniferous in age) is scrubbed, crushed and sieved to a particle size of 0.71 to 1.70mm average diameter. To lOg of the sample, placed on the digestion platform as shown in Figure 1, are added 50ml of 35% HC1 and 5g sodium fluoride, or 50ml of 50% HBF 4 , and the reactants heated for 15 minutes in a Prolabo A301 focused microwave system at a power setting of 20 - 40%. The sample is evaporated to near dryness at less than 60% power for 2 minutes. 50ml of 40% HF are added and the reactants heated for 15 minutes at 40% power setting, followed by a further evaporation stage as above. 50ml of saturated boric acid (40 - 50g/litre water) are added and the reactants heated for 2 minutes at 40% power setting. The amount of oxidation required can then be checked by removing a small sample for microscopic analysis. 25ml of fuming nitric acid are drip fed into the reaction medium and heated for 15 minutes at 40% power setting. 50ml of H 2 0 or 2% KOH solution are added to cool and dilute the reaction medium. After a final rinse with water the solution is sieved to obtain the insoluble palynomorphs for microscopic analysis. Example 2 The procedure of Example 1 is repeated to prepare the sample for HF digestion, except that in this case only 5g of sample is required, and the digestion vessel of Figure 2 is used. 50ml of 40% HF are added, together with, optionally, 5g of ammonium fluoride NH 4 F as a boiling point elevator, and/or optionally lg of aluminium fluoride A1F 3 as a catalyst and the reactants heated by focused microwaves for 15 minutes at 40% power setting, followed by further evaporation. 7ml of 4.1M aluminium chloride A1C1 3 6H 2 0 are added and the reactants further heated for 2 minutes at 40% power setting. The amount of oxidation that will be required is then checked by removing a small sample for microscopic analysis. 25ml of fuming nitric acid, containing optionally about 5ml of hydrogen peroxide, (as an accelerator and to reduce emission of oxides of nitrogen), are drip fed into the reaction medium and heated by focused microwaves for 15 minutes at 40% power setting. 50ml of H 2 0 or 2% KOH solution are added to cool and dilute the reaction medium. The insoluble palynomorphs, still contained in the PTFE sock, are rinsed with water, removed and subjected to microscopic analysis. Example 3 The rocks to be tested are crushed and sieved to a particle size of l-3mm in diameter. Whilst samples of up to about 1cm in particle size can be examined, it may be preferable to restrict the average diameter to 0.71 to 1.7mm average diameter. Typically particles of this size are produced during drilling through rock for oil exploration at the required depth of most interest such that palynology can be used to determine the age. Sieving eliminates fines, optimises acid percolation and provides a large surface area for acid attack. The sample may be weighed prior to testing to four decimal places and analyzed under the microscope to determine how many polynomorphs per gram are present. The sample is pre-treated with HCl and an inhibitor such as NaCl and/or HBF 4 . This may be carried out by: a) placing the sample in an acid-proof micron rated sieve mesh (eg 5-20 microns in size) tied with, for example, PTFE thread such as that seen in Figure 3; or b) placing the sample in a sample support such as those illustrated in Figures 1 or 2, for example, a sintered PTFE tube with a stopper. For a 5g rock sample, which may contain from 100% quartz to 100% CaC0 3 , typically 10ml to 50ml of HCl and/or HBF 4 are added. Typically, 20ml of HCl or HBF 4 , or a combination of 10ml of HCl and 10ml of HBF 4 are used in combination. Preferably, sufficient acid in the form of HCl or HBF 4 is provided to ensure substantial removal of C0 3 minerals (eg CaC0 3 , MgC0 3 , FeC0 3 ) . Though not essential, this is a preferred feature of the method of the invention. In a typical sample, of say 5g of rock, 10ml of 12 molar (37% concentration) HCl and 10ml of HBF 4 also 12 molar, are provided. The HBF 4 forms a complex with the C0 3 minerals and retains them in solution for subsequent removal. HBF 4 can be added first onto a sample and HCl acts as a buffer to prevent fluorides from precipitating out. Preferably sufficient acid is provided such that, once the HCl/ HBF 4 , digestion has begun, the concentration of acid, whilst falling, is still sufficient to enable the reaction to be completed in a reasonable time. Typically, 20ml of HCl and/or HBF 4 for 5g of rock will be sufficient. Focused microwave heating will further reduce the reaction time and is typically provided for up to an hour. The acid solution may be stirred by an air bubble stream as illustrated in Figure 3. Here, a digestion vessel 30 is provided. A bag or sock 31 containing the sample is placed in vessel 30. Bag 31, typically an acid-proof micron rated sieve mesh having a sieve dimension of 5-20 micrometres, is tied with a PTFE thread 32. The reactive mixture 35 is stirred by a flow of air bubbles 34 which are introduced into the reaction medium towards the base of vessel 30 by tube 33. The reagent medium 35 is mixed by the air flow 34, this also serves to disturb the contents of bag or sock 31 thus releasing fines from same into the surrounding reagent mixture. Typically, the air flow is controlled and is relatively slow. Excess acid mixture can be decanted from the tubes using a peristaltic pump, thus allowing for acid recycling. The reaction vessel may be topped up with water and decanted to remove the unwanted Ca * ion or by the use of backwashing using the micromesh bag or sock to retain the sample whilst eliminating unwanted ions, which may be partially complexed following treatment with HBF 4 . During the initial backwash with warm water, ions likely to precipitate with HF are removed in order to prepare the sample for treatment with HF. Alternatively, as in the previous examples, the sample may be evaporated to reduce the volume and/or to near dryness by heating. If the HCl has been removed or sufficient HCl does not remain in the reagent medium the sample, then further HCl, typically of 37% concentration (or 12 molar) is added to the sample to solubilise the fluorides formed by HF treatment. Such fluorides may be A1F 3 , NaF. Typically, for a 5g rock sample, 5-50ml of HCl will be added along with 50ml to 95ml of HF to form approximately 100ml of HF digestion mixture. For a 5g sample and HF of about 40-60% strength, about 25-100ml of HF may be added to digest any silicates present. Optionally, the salt NH 4 F may be added to the HF during the above process as this may increase the HF concentration upon heating. A stream of air bubbles is substantially continuously flowed through the reaction vessel as described in the preceding paragraphs to facilitate mixing and further digestion of the sample. The mixture is heated in a microwave under reflux for 5 - 120 mins at 10 - 99% power, and preferably 10 - 40% of power of 400w microwave focused in the lower 4cm of the microwave chamber. The acid mixture is then decanted from the reaction vessel using a peristaltic pump. This again may allow for acid recycling. The peristaltic pump may be adapted to accommodate, for example, 16 channels of acid removal at any one time. If the acid contains complexing agents such as HBF 4 then it may not be possible to recycle the acid mixture. Otherwise, the acid mixture may be mixed with fresh acid in the proportion 1:1 or 1:2 for later reuse. The sample is then back-washed with warm water and, if the sample is not within a bag or sock 31 for backwashing within the sample, a holder having an acid resistant monofilament nylon sieve and running hot water may be used, though this is less preferred. As alternative to back-washing the sample in acid mixture to cool off the reaction, saturated boric acid H 3 B0 3 may be added at a concentration of 50g per litre at room temperature. Alternatively, A1C1 3 .6H 2 0 may be added at a strength of 2 molar to complex the remaining HF and any insoluble fluorides. This acts in a molar proportion of 1:6 with respect to the HF. Any other suitable complexing agent to partially or substantially neutralise or remove any HF remaining in the sample may be used. A small portion of the sample is checked under the microscope in the lab to determine whether the polynomorphs are free of amorphus organic matter (AOM), otherwise this can be removed by an oxidising agent as described in the previous examples. Other sample determination can now be carried out. For example, once the fossils have been released from the rock and examined under a microscope, it is possible to grade them by colour and thereby determine against a known scale the likely temperature and/or depth of the rock and therefore an estimate of the likelihood of the rock containing substances such as oil and gas. Example 4 In a preferred embodiment, the steps of a 'one shot process' include the following: Water is used to wet the rock sample within a suitable container within a reaction vessel. Fluoroboric acid in the form of 54% HBF 4 is added as an inhibitor and diluted with water. It is believed, though the invention is not limited to any particular theory, that the fluoroboric acid acts as an inhibitor by forming a complex with the calcium ions. A tube such as tube 33 in Figure 3 is used to provide a gas flow, typically air, in a slow moving stream of air bubbles adjacent or below the rock sample. Acetone is also added to the reaction mixture to subdue the formation of foam caused by the air flow. The mixture is heated for 5 minutes to 1 hour within a focused microwave heating apparatus until the carbonate reaction has subsided. The reaction mixture is then topped up with 10-15ml of HCl and 60ml of HF and subsequently heated for up to 2 hours within a microwave chamber to facilitate HF digestion. Once HF digestion has been completed the sample is washed, for example, by 800ml water over a 10-15 micron sieve, or otherwise treated for further analysis as hereinbefore described. It will be apparent from the information disclosed herein that the method of the invention provides the basis for a simple and, in a preferred embodiment, almost fully automated process for preparing palynological sample in a relative quick and safe manner. Alternative embodiments of the invention can be envisaged by those skilled in the art from the information herein and all such alternative embodiments are intended to lie within the scope of this application. The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment. This invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims: CLAIMS 1. A method for the preparation of kerogen for palynological analysis, in which a kerogen- containing particulate rock sample is treated by hydrofluoric acid digestion, which comprises adding an inhibitor which is soluble in the hydrofluoric acid reaction medium and which inhibits the formation of an insoluble non-reactive layer on the particles of the sample. 2. A method according to Claim 1, in which the inhibitor is a metal salt having a cation that is soluble in hydrochloric acid, and in hydrofluoric acid and/or aqueous hydrofluoric acid. 3. A method according to Claim 1 or 2, in which the cation is a cation of a Group I or Group II metal, or a transition metal cation. 4. A method according to Claim 2 or 3, in which the cation is an alkali metal cation. 5. A method according to Claim 4, in which the metal salt is a sodium or a lithium salt. 6. A method according to any of Claims 2 to 5, in which the anion of the metal salt is a halogen anion. 7. A method according to Claim 6, in which the halogen anion is a chloride anion or a fluoride anion. 8. A method according to any of the preceding claims, in which the inhibitor is sodium fluoride or sodium chloride. 9. A method according to Claim 1, in which the inhibitor is a fluoroboric acid. 10. A method according to Claim 9, in which the fluoroboric acid is tetrafluoroboric acid. 11. A method according to any of Claims 2 to 9, in which there is used from 25 to 50% by weight, based upon the weight of the sample, of the metal salt. 12. A method according to any of the preceding claims, in which, prior to hydrofluoric acid digestion, the sample is treated with hydrochloric acid. 13. A method according to any of the preceding claims, in which the hydrofluoric acid digestion is carried out using hydrofluoric acid at a concentration of up to 60% (35 to 36M), at a temperature of from 20° to 120°, and for a time of from 1 to 220 minutes. 14. A method according to any of the preceding claims, in which there is added to the hydrofluoric acid digestion medium a selective oxidising agent. 15. A method according to Claim 14, in which the oxidising agent is sodium permanganate and/or nitric acid. 16. A method according to any of the preceding claims, in which the hydrofluoric acid digestion is carried out in a microwave heating apparatus. 17. A method according to Claim 16, in which the hydrofluoric acid digestion is carried out in a focused microwave heating system. 18. A method according to Claims 12 to 17, in which the treatment with hydrochloric acid is carried out in a microwave heating apparatus. 19. A method according to Claim 18 in which the treatment with hydrochloric acid is carried out in a focused microwave heating apparatus. 20 A method according to any preceding Claim in which the reaction components are substantially continuously mixed. 21. A method according to Claim 20 in which said mixing is a result of a flow of gas through the reaction medium. 22. A method according to Claim 21 in which the gas is released in the form of a stream of bubbles. 23. A method according to Claim 22 in which the gas is air. 24. A method according to Claims 20 to 23 wherein an agent is added to reduce foam formation during mixing. 25. A method according to Claims 20 to 24 wherein acetone is added. 26. A method according to any of the preceding claims, in which the kerogen liberated by hydrofluoric acid digestion is subjected to microwave heating in the presence of a suitable reaction medium for a period of time sufficient to bleach the palynomorphs and/or to reduce the level of any unwanted kerogen present. 27. A method according to Claim 26, in which the suitable reaction medium comprises nitric acid. 28. A method according to Claim 26 or 27, in which the kerogen liberated by hydrofluoric acid digestion is treated using a method as described in UK Patent Application No. 9403163.0. 29. A method according to any of the preceding claims, in which, after hydrofluoric acid digestion, a complexing agent is added to the reaction medium to complex any free hydrofluoric acid present. 30. A method according to Claim 29, in which the complexing agent solution is used as the reaction medium or solvent for the subsequent treatment step to reduce the level of unwanted kerogen present. 31. A method according to Claim 29 or 30, in which the complexing agent is boric acid. 32. A method according to Claim 29 or 30, in which the complexing agent is aluminium chloride. 33. A method for preparing palynological samples according to any of the preceding claims, which is carried out as an automated process in which the appropriate reagents are added at timed intervals without sieving the sample between the method steps. 34. A method according to Claims 17 - 33, in which the sample is placed in a microwave transparent reaction vessel in or on a support to raise it above the floor level of the vessel. 35. A method according to Claim 34, in which the support comprises a horizontal grating which is provided with holes of a size sufficient to allow palynomorphs released from the sample to fall therethrough. 36. A method according to Claim 34, in which the support comprises a bag or container having a permeable wall permitting ingress of the reaction medium and retention of the palynomorphs released from the sample. 37. A method according to Claim 36, in which the bag or container comprises a flexible "sock" formed from a PTFE mesh having a mesh size of 12 microns or less. 38. A method according to Claim 37, in which the PTFE "sock" is formed by welding sheets of PTFE mesh at their edges to one another. 39. A method according to Claim 36, in which the container comprises a permeable-walled tube. 40. A method according to any of Claims 34 to 39, in which a tube is provided for introducing a stream of gas into the reaction medium below the surface of the reaction medium. 41. A method according to Claim 40, in which the tube is arranged to introduce gas adjacent to or below the level of the sample in the reaction vessel. 42. A method according to any of the preceding claims substantially as described in the Examples. 43. A method according to any of the preceding claims substantially as hereinbefore described. 44. A method for the hydrofluoric acid digestion of kerogen-containing samples, in which, after reaction, boric acid and/or aluminium chloride is added to the reaction medium to complex any free hydrofluoric acid present. 45. A support for positioning a sample within a focused microwave reaction vessel which comprises a horizontal grating formed from a microwave transparent material which has holes of a size sufficiently small to retain the sample thereon, but large enough to allow any insoluble products of the reaction to pass therethrough. 46. A support for positioning a sample within a focused microwave reaction vessel which comprises a bag or container for the sample having a permeable wall permitting ingress of the reaction medium and retention of insoluble products of the reaction. 47. A support according to Claim 46, in which the bag or container is formed from a PTFE mesh. 48. A support according to Claim 46, in which the container comprises a permeable-walled tube. 49. A support for a focused microwave reaction vessel substantially as described.

Inventors: JONES RAE ANTHONY, JONES, RAE, ANTHONY
Applicants: JONES RAE ANTHONY, UNIV SHEFFIELD, JONES, RAE, ANTHONY, THE UNIVERSITY OF SHEFFIELD
Application year: 1996
Main classification: G01N1
IPC codes: C09K8, G01N1, G01N33