Glass beads by borate fusion

Transcription

1 XRF SAMPLE PREPARATION Glass beads by borate fusion James P. Willis The Analytical X-ray Company

2 XRF SAMPLE PREPARATION Glass beads by borate fusions James P. Willis

3 About James Willis Prof. Emeritus James Willis (1937) worked in the Department of Geochemistry and Geological Sciences at the University of Cape Town from 1960 to 2000, during which period he also gained his Masters (with distinction) and Doctoral degrees. He has a very broad experience in many analytical techniques, including DC arc optical emission spectrometry, atomic absorption spectrophotometry, electron microprobe analysis, SEM-EDX, instrumental neutron activation analysis, ICP-AES, ICP-MS, HPIC, particle size analysis, X-ray diffraction and XRFS. Copyright 2010 by PANalytical BV, The Netherlands. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form by any means electronic, mechanical, photocopying or otherwise first obtaining written permission of the copyright owner. Although diligent care has been used to ensure that the information herein is accurate, nothing contained herein can be construed to imply any representation or warrantee as to the accuracy, currency or completeness of this information. The content hereof is subject to change without further notice. Please contact us for the latest version of this document or further information. PANalytical B.V. [2009] PN7216 This edition published in 2010 by: PANalytical B.V. Lelyweg 1, 7602 EA Almelo P.O. Box 13, 7600 AA Almelo The Netherlands Tel: +31 (0) Fax: +31 (0) info@panalytical.com ISBN: st edition. Based on chapter 19 from Understanding XRF spectrometry vol. 2

4 PANalytical Contents 1 Introduction Undesirable effects of using powders in XRFS Advantages of using fusion techniques Preparation of glass disks General procedure Typical simplified procedure A limitation to borate fusion Fluxes and additives Fluxes Properties of borate fluxes Impurities in fluxes Additives Non-wetting (releasing) agents Preparation before fusion Fusion mixture Crucibles and molds Heating Heating requirements Heating steps Heating steps for oxidation Heating steps for fusion and homogenization Heating steps for conditioning before casting Casting and cooling Casting into molds Solidification Examples of fusions that require special techniques Sulfide minerals Copper compounds Carbon Metallic materials Carbonates Pyrosulfate fusions Procedure Automatic preparation Requirements Problems Fusions of sulfide concentrates Analysis of disks Conclusion Fluxers Fusion instruments

5 PANalytical Sample preparation XRF and glass beads by borate fusions 5 Care and use of platinum ware Why platinum Damage to platinum ware Crucibles Molds Prolonging platinum ware lifetime Cleaning of platinum ware Maintenance Polishing and re-shaping kits Miscellaneous Contamination by crucibles Contamination by W when using WC grinding vessels Blanks Loss or gain on ignition (LOI or GOI) Advantages and limitations of fusion Advantages Reproducibility Accuracy Preparation of standards Versatility Cost The Eagon 2 from PANalytical Limitations Environment Selection of flux References Index

6 Introduction PANalytical 1. Introduction The use of powdered samples in XRFS has a number of undesirable effects (Figures 1 and 2). The best, and in some instances the only, means of eliminating these effects is to fuse the sample with a suitable flux and cast it as a glass bead. Glass beads are the ideal sample form for XRFS (Figure 2). Emitted X-ray intensity Particle size (coarser) Figure 1. Effect of mineralogy on emitted X-ray intensity. Reducing the particle size can either decrease or increase emitted X-ray intensity. Corr. count rate (kcps) Pressed powders R 2 = Conc. SiO 2 (wt %) 100 Corr. count rate (kcps) Fused beads Conc. SiO 2 (wt %) R 2 = Figure 2. Powder pellets are subject to particle size and mineralogical effects, surface roughness, preferential orientation and segregation problems, which can give poor quality calibrations. Fused beads provide the ideal sample for XRF analysis. 5

7 PANalytical Sample preparation XRF and glass beads by borate fusions 1.1 Undesirable effects of using powders in XRFS Particle size problems Mineralogical problems Surface roughness Segregation problems Preferential orientation Statistics However, pressed powers are often, and very successfully, used in production control, especially if the calibration ranges are short (Figure 3). However, it is not always easy to obtain suitable standards that have the same particle size, mineralogy, surface roughness and segregation characteristics as the production samples. The role of fused beads is extremely important in setting up reference calibrations for determining in house standard compositions for use in production control calibrations (Figure 3). Count rate (kcps) Pressed powder, production control calibrations C B A SiO 2 (wt %) Count rate (kcps) Fused bead reference calibration A B C SiO 2 (wt %) Figure 3. The use of in-house pressed powder standards in setting up calibrations over small concentration ranges for production control. The in-house standards concentrations were obtained from fused bead calibrations based on CRMs. 1.2 Advantages of using fusion techniques No particle size effects ONLY inter-element matrix effects All standard methods applicable (internal standards; dilution; double dilution) Preparation of artificial standards is relatively easy, and very much easier than preparing artificial standards for pressed powders Accurate application of inter-element corrections (theoretical and empirical) possible Fusion is a general name for all kinds of chemical attacks on solid samples to transform them into compounds that are easily transformed into a solution. These compounds are an intermediate step between the original sample and the solution that is used later in an analytical process. 6

8 Introduction PANalytical Most fusion processes result in a product that is a mixture of several compounds which have as a common property that all are soluble in a given solvent. These processes are regular chemical reactions and the products are crystalline. Although a borate fusion is also a chemical reaction, its characteristics are different. At high temperature the flux melts and becomes a solvent for most oxide compounds. One product only is formed, a homogeneous solution. This solution may be cooled without crystallizing to yield an amorphous homogeneous solid which is a glass. The latter is ideal for XRF work. Alternately it may be cooled drastically by dropping into a solvent, in which case it shatters into fine crystals that dissolve to yield a solution, which is a fast way to prepare samples for ICP analysis. This concept of borate fusion helps to understand why high temperature refractories can be fused at temperatures as low as 1000 C, why materials cannot be mixed with fluxes in any proportion, why some elements are lost by volatilization if heated at too high temperatures, etc. Application of the borate fusion to make glass beads for X ray fluorescence is generally simple, but more complex than for the preparation of solutions for ICP. Borate fusions are discussed in this booklet. 7

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10 Preparation of glass disks PANalytical 2. Preparation of glass disks 2.1 General procedure The general procedure for preparing fused beads for XRF analysis is shown in Figure 4. The sample, undried, dried at 100, 105 or 110 C (105 C is the norm for geological samples) and/or roasted at ~1000 C, is weighed with flux into a Pt crucible, pretreated if necessary, melted, mixed, cast into a Pt mold and cooled to form a stable glass bead. It is important to note that sulfates are dried at 40 C to avoid the formation of hemihydrates. Accurate data Figure 4. The general procedure for preparing stable fused beads for XRF analysis Typical simplified procedure Essentially a fusion procedure consists of (Figure 5): a) b) c) d) heating a mixture of sample and borate flux until the flux melts; continuing heating until the sample dissolves into the molten flux, and agitating to homogenize the melt; pouring into a hot mold; cooling to obtain a solid glass bead, ready for X ray measurement with no further treatment. 9

11 PANalytical Sample preparation XRF and glass beads by borate fusions Temperature ( C) Time Figure 5. Temperature-time curve for a full fusion cycle during which oxidation, decomposition and fusion take place, and the melt is cooled to form a solid, stable glass bead. In discussing the fusion procedure for glass beads one must consider the ingredients, their preparation, the utensils, and the fusion process itself. All of these will be considered. Making a glass bead today is a simple process thanks to the availability of automatic fusion instruments, such as PANalytical s, compact, high-performance Eagon 2 (Figure 6). Figure 6. PANalytical s fully automatic, high-performance Eagon 2 fusion machine for XRF sample preparation It is true that fusion is sometimes perceived to be a complex problem. The reason for this is that one wants to make better glass beads for greater analytical accuracy and to apply the technique to a larger diversity of samples. This means that more 10

12 Preparation of glass disks PANalytical versatility is necessary in the parameters involved such as composition of flux, temperature of fusion and cooling rate. The cost-effective Eagon 2 provides all the necessary versatility and optimum fusion conditions combined with unparalleled safety, together with a pre-loaded set of basic fusion programs A limitation to borate fusion A molten borate is a good solvent for oxides, but for oxides only. This includes hydrates, carbonates, sulfates and nitrates that are also soluble because they are a combination of two oxides, one of which is either volatile (H 2 O, CO 2, N 2 O 5 ) or partly volatile (SO 3 ). SO 4 2 ( SO 3 ) is retained in the flux. Halides are not really soluble as shown by the volatility of the halogen atom that increases rapidly with the atomic number; fluorine is rather marginal, being slightly volatile. Metals, carbides, nitrides and sulfides are insoluble in borate but they can be dissolved after an oxidation treatment. Consequently, the fusion technique will be described as if the sample were in its highest oxidation state, but the modifications required when the sample is in a reduced condition will be added when appropriate. 2.2 Fluxes and additives Fluxes The more commonly used fluxes are lithium tetraborate, lithium metaborate and sodium tetraborate. Lithium tetraborate (LiT), Li 2 B 4 O 7, was the more commonly used flux in XRF analysis; it is transparent to light element X-rays, it is barely hygroscopic, but it has a tendency to crystallize. With a melting point of 930 C it is the more difficult flux to fuse among the common ones. It is a suitable flux for basic materials, e.g. Ca and Mg oxides. It is a very good flux for carbonates, because carbonates decompose before or when the flux starts to melt. Therefore, the CO 2 gas escapes easily thus minimizing the trapping of gas in the flux mixture which would cause foaming and loss of material. Lithium metaborate (LiM), LiBO 2, is very susceptible to crystallization, more than the tetraborate, but has a better solubility for several elements. Its melting point is 849 C and it has a low viscosity. It is the preferred flux for the preparation of solutions. Mixed with Li 2 B 4 O 7 it is a useful flux for acidic materials. Anhydrous sodium tetraborate, Na 2 B 4 O 7, would be the more desirable flux for glass beads if it were not so hygroscopic. The glass beads have almost no tendency to crystallize or to crack but they must be kept in a desiccator to prevent the formation of a white layer of hydrated borate. The melting point is 741 C. Using Borax (Na 2 B 4 O 7 10 H 2 O) for XRF fusions is not recommended due to its large content of water that yields such a large volume of gas when heated fast that part of the fusion mixture might be blown out of the crucible. Obviously, the large amount of sodium in the flux precludes analysis of sodium in the samples. 11

13 PANalytical Sample preparation XRF and glass beads by borate fusions A combination of borates is useful and many mixtures are now available commercially (Figure 7). Such fluxes are more fluid and have a lower melting point than pure Li tetraborate. The so called flux, melting point 825 C and strongly favoured in Australia, contains 12 parts LiT and 22 parts LiM and is recommended for high silica rocks. A 66:34 mixture of LiT:LiM, melting point 875 C, is the best general purpose flux. A 50:50 mix of LiT:LiM is also a good general purpose flux for most geological rock types, including silicate (acidic material) and carbonate (alkaline material) rocks. Figure 7. A typical set of pure and mixed borate fluxes with non-wetting agent and platinum ware available from PANalytical The Norrish and Hutton (1969) mixture, Li 2 B 4 O 7 + Li 2 CO 3 (or NaNO 3 ) + La 2 O 3, was commonly used for fusing silicate rocks in the 1970s and 1980s. The addition of La 2 O 3 as a heavy absorber reduces matrix effects and increases the range of concentrations over which linear calibration curves can be obtained. The fusion must be rapidly quenched after casting otherwise crystallization is likely to occur. The introduction of modern software packages for the accurate correction of interelement matrix effects (absorption and enhancement) using influence coefficients or fundamental parameters has made the use of heavy absorbers unnecessary, and their use is no longer recommended. Lithium and sodium hexametaphosphates are lesser known fluxes that can be used in the same way as borate fluxes. Melting points are in the 600 C range, they fuse easily, the beads separate easily from the molds, and they have a better solubility than borates for certain oxides such as chromium and copper oxides (Banerjee and Olsen, 1978). It has been reported that copper samples do not stick to the molds as is often the case when borate fluxes are used. Problems experienced with metaphosphate fluxes is that although the beads may be dry when humidity is low, they become sticky when humidity is high. 12

14 Preparation of glass disks PANalytical Properties of borate fluxes Their properties are as important as their composition; the effect of purity is explained in section Fluxes are available as pure compounds or mixtures, as fine powders, agglomerates of fine powders, vitreous granular or spherical particles. Powder mixtures are not recommended on account of the segregation that may occur while stored in containers. The risk is larger when the mixed components differ in density or in particle shape and size. Very fine powders have a large specific surface area (Figure 8), and, since most borate fluxes have some affinity for water, they absorb humidity from the atmosphere to a variable and sometimes significant extent. Sodium tetraborate is the worst case, being able to absorb nearly 100% of its weight. Lithium tetraborate has a low affinity for water and only a thin layer on the surface of the particles becomes hydrated. As a result the very fine powders may contain up to 4 or 5% water depending on their exposure to the atmosphere, but the (larger) vitreous particles usually do not contain more than 0.1% by weight adsorbed water. A major inconvenience of the absorbed water obviously is a loss of accuracy in the analysis, and a lesser inconvenience is the volatilization of the water that sometimes carries away a fraction of the flux and sample. Crystals Fluxes Powders 30 µm Figure 8. Weight P = C # Particles P = 1000 C 300 µm Surface area P = 100 C Water absorbed C = 0.05% P ~ 5% Specific surface area of fine and coarse fluxes An advantage of the granular, pre-fused fluxes (0.3 to 0.5 mm diameter) is their high apparent density of about 1.3 as compared to about 0.3 for very fine fluxes. As a result, a standard 25 ml crucible is filled to the top with 8 g of fine flux (only 80% of the quantity required to make a 40 mm fused disk) but is only half filled with 10 g of granular flux. Granular fluxes are most advantageous. Pre-fused fluxes are more homogeneous (especially mixed fluxes), are easier to weigh and tend to be more anhydrous and less hygroscopic than non pre-fused fluxes. Due to electrostatic forces, dusty fluxes stick to weighing pans, funnels and crucible walls, resulting in a small loss of flux and the formation of small droplets of flux on the 13

15 PANalytical Sample preparation XRF and glass beads by borate fusions wall of the crucible when fusing; granular fluxes do not stick to any surface and leave the crucible wall clean after fusion Impurities in fluxes Any impurity in the flux is interpreted as a much higher concentration in the sample, proportionately to the dilution factor. For a fusion mixture of 1 part sample and 9 parts flux, any impurity in the flux will be equivalent to a 10 concentration in the sample. For example, 10 ppm in the flux is the same as 100 ppm (or 0.01%) in a sample. Batch to batch impurity differences may need to be accounted for Additives Chemicals are sometimes added to fluxes to modify their properties, namely: Heavy absorbers, for example La 2 O 3, BaO 2 or SrO, are added to decrease the matrix effects by increasing the X ray absorption of the flux. It has been reported that La 2 O 3 in Li tetraborate at a concentration of the order of 16% as currently used, yields a homogeneous melt that separates in two liquid phases on cooling. Since, by definition, these phases have different compositions, the glass bead is not homogeneous. As stated earlier, the use of heavy absorbers is no longer necessary nor recommended. Fluidizers, for example LiF, is used to ensure a better transfer of the molten glass into the mold. A concentration of 10% seems to be a maximum. LiF is also a releasing agent (see below), but is much less efficient than other releasing agents so that it must be used in larger quantities. Internal standards are added as oxides if required in the analytical technique. Oxidizers, for example NH 4 NO 3, NaNO 3, LiNO 3 or SrNO 3, are added at a concentration of a few percent to oxidize small amounts of organic material or partially oxidized elements, occasionally present in a sample, that may corrode the crucible. LiNO 3 is a very good low-temperature oxidant. Li 2 CO 3 is a hightemperature oxidant, efficient with metallic particles. In practice the number of additives should be kept to a minimum to save time in the preparation procedure and to minimize errors. The analyst should keep in mind that some of them (oxidants) change the composition of the sample. Unfortunately most of the additives cannot be added during the process of manufacturing fluxes because they lose their particular property if they are fused; they must be added in each sample Non-wetting (releasing) agents Depending on their composition, borate glasses vary widely in their tendency to stick to the crucibles and molds in the liquid and solid states. In most cases, the use of a non-wetting or releasing agent is beneficial. Iodine and bromine are the only two elements that are known to be efficient for that purpose. They can be added into the fusion mixture before or during the fusion, usually in the form of one of 14

16 Preparation of glass disks PANalytical their salts, for example, NaBr, LiBr, KI, CsI, NH 4 I, etc; the crystals of iodine and even the acid HBr have been reported to be successful. Ammonium iodide cannot be used when sulfur is an analyte because ammonium and sulfur react and escape together. Sometimes releasing agents cannot be used with copper because in high Cu samples the cation combines with oxygen, and I or Br combine with Cu forming volatile compounds (Equation 1), so that both Cu and I or Br are lost. However, a small amount of releasing agent can be used just before casting. An oxidant should not be used with releasing agents because an oxidation reaction takes place and I or Br evaporates very rapidly. 2KBr + Cu + O 2KI + Cu + O K 2 O + CuBr 2 (1) K 2 O + CuI 2 Usually, the halogens are added in the crucible on top of the fusion mixture in a quantity of about 0.2% of the weight of the flux. Less than 5% of the iodine remains in the melt after a normal fusion (Figure 9), but this quantity is usually sufficient to ensure essentially complete transfer of the melt into the mold. Bromine volatilizes at a considerably slower rate than iodine but is slightly less efficient. To avoid weighing the small quantity of releasing agent, a solution containing about 250 g per liter can be prepared and a drop or two is added to the fusion mixture in the crucible. Preferably the releasing agent can be added in solid form as a tablet. The innovative design and exceptional flexibility of the fully automatic Eagon 2 fusion machine allows a pellet of NH 4 I to be automatically, repetitively, uniformly, reliably and safely dispensed into the molten fusion mixture at the optimum point in the fusion cycle. Concentration Br I Time (minutes) Figure 9. Volatilization of I and Br as a function of fusion time The optimal quantity of releasing agent depends on the sample, the flux and the sample:flux ratio. A convenient way to determine the right quantity is to look at the shape of the bead in the mold after cooling (Figure 10): if the top surface is flat 15

17 PANalytical Sample preparation XRF and glass beads by borate fusions or slightly convex, the quantity is right; if it is concave at the edge, the quantity is too small; if it is convex, the quantity may be too large. Ideal Insufficient Too much Excess Figure 10. Effect of the amount of releasing agent on the shape of fusion beads. The use of excess releasing agent makes ideal fusions for dissolution in acids for ICP analysis. Some fusion instruments need larger quantities of releasing agent to avoid the need to clean the crucibles after fusion. Also, some elements in the sample seem to increase the rate of volatilization of iodine and bromine, so that it is sometimes desirable to add a fraction of the releasing agent just before pouring the melt into the mold. Cracking of beads in the mold during cooling is often caused by the adherence of the glass to the mold, and this can be prevented by injecting the releasing agent just before casting. The negative ions of releasing agents are insoluble in borate glasses, so that they must remain on the surface, preventing the sample-flux mixture from coming in contact with the platinum. However, the positive ions can diffuse into the molten glass, and positive ions of the dissolved sample may replace them. Then, the halide film becomes nearly a film of sample that is a strong absorber of lighter element radiations. Variations in halide content affect the line intensities. Use the lowest possible quantity of releasing agent. The volatility of releasing agents has not been explained fully yet but some observations indicate that the evaporation rate of releasing agents depends on their oxidation state. For example, one analyst who used to measure the Br Kα line on the finished glass disks observed that the intensity of this line decreased by a factor of 10 when one gram of LiNO 3 was added in the fusion mixture before heating. The following chemical equations may represent what happens: 2KBr + 2LiNO 3 K 2 O + Li 2 O + 2NO 2 + Br 2 (2) 2KI + 2LiNO 3 K 2 O + Li 2 O + 2NO 2 + Br 2 Releasing agents may interfere with analyte lines of elements of the sample: the Br Kβ line interferes with the Rb Kα line, the Br Lα line interferes with the Al Kα line, and the Br Lη line interferes with Mg Kα when using a ~5 nm LSM (Layered Synthetic Microstructure) analyzing crystal and coarse collimator; similarly, the I Kβ line interferes with the Ba Kα line. However it is seldom possible to use the Ba Kα line for the analysis of Ba due to difficulties in achieving infinite thickness (critical depth) in fusion beads. It is essential to make corrections for spectral overlap when Br or I are used as releasing agents, because the amount of Br or I remaining after fusion is not constant, and in many instances is dependent on sample composition. 16

18 Preparation of glass disks PANalytical 2.3 Preparation before fusion Fusion mixture As mentioned above, the following procedure applies to fully oxidized specimens only; special procedures are described in section 2.6 for other types of specimens. 1. Take a representative sample from the material to be analyzed. To ensure the sample is representative, the material should be ground fine. Since a borate fusion is a dissolution, the finer the particles the faster the dissolution; grinding the material to less than 200 mesh (<75 μm) or finer is recommended. 2. Choose the most appropriate flux for the sample. Obviously, the flux composition cannot be changed for each sample because correlation with standards would no longer apply, but one flux composition can be chosen for a great variety of samples. For example, lithium tetraborate with or without 5% lithium fluoride is a good flux for cements and most geological materials; a 1:1 mixture of lithium tetraborate and metaborate works well for all low acidity oxides such as most of the transition metals; a 1:2 mixture of lithium tetraborate and metaborate is excellent for pure silica and alumina. The addition of lithium nitrate to a flux is excellent to get rid of carbon often found in cement raw mix. See Table 1 for recommended fluxes for selected materials. 3. Choose a convenient sample:flux weight ratio, taking into consideration the following observations: a low ratio, for example 1:100, results in line intensities directly proportional to concentrations, and calibration curves are nearly free of inter-element matrix effects (Figure 11). However, the line/background intensity ratio is low and the limit of detection is relatively high. Very high sample:flux ratio (1:3) Very low sample:flux ratio (1:100) Intensity Concentration Figure 11. Effect of dilution on fluorescence intensities and matrix effects: High sample:flux ratio; very low sample:flux ratio, e.g. 1:100 17

19 PANalytical Sample preparation XRF and glass beads by borate fusions at high sample:flux ratios, for example 1:5, the inter-element matrix effects are relatively high; it is necessary to make corrections for inter-element matrix effects. - the decrease in intensity due to dilution by the flux is not as serious as might be expected, due to the low MAC of the flux (assuming no heavy absorber has been added) (Figure 12). The gain in intensity from a sample:flux ratio of 1:5 to 1:3 (low dilution fusion) is usually not worth the extra problems encountered in preparing fusion disks, unless the extra intensity is really necessary, e.g. for trace element analysis. there is no lower limit on the sample:flux ratio except that lithium tetraborate tends to crystallize when the concentration of sample is low. Apart from lithium metaborate, or close to it, crystallization can easily be prevented. There is, however, an upper limit to which one should not get too close. Sample:flux ratios that are reasonably high, yet sufficiently under the solubility limit, are given in Table 1. Table 1. Recommended upper sample:flux ratio and flux composition for some materials in the preparation of glass beads. In the case of preparations of solutions, lithium metaborate flux is recommended because the solubility is greater. Sample material Flux* Sample/Flux/LiNO 3 LiT LiT:LiM (1:1) Cement** X 2-3 / 6 / 0 Raw mix X 2-3 / 6 / 1 Rocks X / 6 / 0 Bauxite X / 6 / 0 Slags X X / 6 / 0 Sulfide ores X / 6 / 2 Sulfide concentrates X / 6 / 2-3 Sulfate X 1 / 6 / 0 Coal ash X 0.6 / 6 / 1 Alumina X 0.6 / 6 / 0 Silica X 1 / 6 / 0 Magnesia X 0.6 / 6 / 0 Titania X / 6 / 0 Zirconia X / 6 / 0 Ferric oxide X 0.4 / 6 / 0 Ferrous oxide X 0.4 / 6 / 1 Chrome oxide *** X 0.1 / 10 / 0 * LiT = Li tetraborate LiM = Li metaborate ** Fréchette et al. (1979) *** Mintek (Randburg, South Africa) has found a flux mixture of 2:1 (LiT:LiM) and a sample:flux ratio of 1:9 to work well for chromites, which are notoriously difficult to fuse satisfactorily. No oxidant was used. Potassium hexametaphosphate is also efficient for chromites, but makes analysis of K and P impossible. However, Sear (1997) found the use of intimate mixtures of sodium nitrate or sodium peroxide and boric acid to be very beneficial for the dissolution of chromite and cassiterite, allowing the use of smaller dilution ratios than is normally possible. 18

20 Preparation of glass disks PANalytical I Fe 1.0 slope = 5 1/3 1/ /10 1/15 1/20 Different sample:flux ratios C Fe2O3 1.0 Figure 12. Intensity of Fe Kα in Li 2 B 4 O 7 fusion beads for different sample:flux ratios as mentioned in section 2.2.3, any impurity in the flux appears as if it is an element of the sample at a concentration equal to its concentration in the flux multiplied by the flux/sample ratio. As an example, an impurity of about 50 ppm in the flux yields an XRF intensity corresponding to 0.5% in the sample if the flux:sample ratio is 100:1 and to 0.045% if the ratio is 9:1. 4. Weigh the sample to 1 mg or better (0.1 mg for a 9:0.9 flux to sample mixture). Weighing the flux to ±5 mg is usually sufficient. It is acceptable to weigh close to a given predetermined value and make corrections to compensate for the differences from it. Some XRF software packages can make accurate corrections for quite large differences in weight, provided the actual sample and flux weights are specified. 5. Transfer the fusion mixture into a crucible on the fusion apparatus and add releasing agent. 6. Determine a program for the fusion process. Although many substances accommodate to a simple program that contains one or two heating steps followed by casting and a two-step cooling, more complex programs are necessary for some types of samples. For example: if the sample contains carbon or sulfide minerals in significant concentration then low-temperature heating combined with use of an oxidant is a necessary step before the so called fusion is started; a gradual heating might be necessary when dealing with a carbonate rich sample in combination with a finely ground flux. These special cases are considered in section

21 PANalytical Sample preparation XRF and glass beads by borate fusions Crucibles and molds The best material for crucibles and molds is a platinum alloy with 5% gold (Pt-5% Au); it is strong, stable and long lasting. However, a Pt/Au/Rh alloy crucible (Figure 13) available from PANalytical provides even greater durability. Carbon crucibles do not last long in a gas flame (high-temperature oxidizing environment), but do last slightly longer in a furnace. Pt-Au crucibles cost less per analysis than graphite crucibles. Zirconium oxidizes and dissolves in the flux at the temperatures prevailing during a borate fusion, but zirconium-stabilized crucibles can be used for other kinds of low-temperature fusion. Figure 13. A 40 g highly durable Pt/Au/Rh alloy crucible and long-lasting 100 g Pt-Au alloy casting dish (mold) available from PANalytical A mirror polished crucible or mold is only slightly, if at all, wetted by the molten glass, but, as the number of fusions increases, the surface becomes rough on account of corrosion and aging. Then the molten glass tends to stick more strongly, and a larger quantity of releasing agent is needed to compensate for that. Please refer to chapters 5 and 6 for care of platinum ware. With a granular flux, a 20 to 25 ml capacity crucible is large enough to accommodate the fusion mixture to make a 40 mm diameter glass disk. If a fusion apparatus is used the crucible must be held safely in its holder and a crucible with a flat rim or equivalent is necessary (Figure 14). With all fusion instruments, a crucible with a mixing enhancer hump in the bottom (Figure 15) enhances mixing and ensures homogeneity of the melt in a short time by forcing the molten glass to pass through a narrow gap (Figures 15, 20) which produces strong convection currents. 20

22 Preparation of glass disks PANalytical Figure 14. Crucible with a flat rim for safe holding in a fusion apparatus Figure 15. Crucible with a mixing-enhancer hump in the base 2.4 Heating The fusion process consists of four operations: heating the sample flux mixture, homogenization of the melt, pouring the molten glass into molds, and solidification of the glass. The first two operations are considered in this section Heating requirements Basically, any means of heating can be used to bring the flux above its melting point to dissolve the sample, but in practice some means are more apt to meet the requirements of a good fusion. Whether heating is manual or automatic, the desirable requirements are: temperature with a minimum in the 200 or 300 C range for oxidation of sulfides and metallic samples when necessary, and up to 1100 C for fusion. Higher temperatures may cause excessive volatilization of flux and also some elements such as sodium, potassium and sulfur (Figure 16), and particularly releasing agents; easy and fast adjustment to any level of heat; reproducibility of heating conditions for maximum precision and accuracy; oxidizing conditions to minimize reduction of some elements; availability of agitation in the melt for homogenization; availability of injection of releasing agent just before pouring, to clean the crucible and to release the disk from the mold. In addition to all of the above features the Eagon 2 has very sophisticated and accurate temperature control by means of thermocouples to within ±5 C between C. 21

23 PANalytical Sample preparation XRF and glass beads by borate fusions Rate of volatilization Flux melts I Br S (as SO 2 ) Na, Li (Flux) Temperature ( C) Figure 16. Rate of volatilization of some elements as a function of temperature (qualitative) Electric furnace heating is the best means of heating, providing thermal equilibrium of both crucible and mold during the full fusion and casting cycle, and ensuring that crucible and mold are always at the same temperature prior to casting. In the past electric furnace heating was slow, required frequent manual agitation of the crucibles and was not convenient when more than one level of heat was required in the process. Manual agitation while standing in front of an open furnace at only 1000 C is an uncomfortable and potentially dangerous procedure for any operator, and is obviously much worse at higher temperatures. The high level of operational flexibility built into the Eagon 2 allows automatic agitation to be carried out in the furnace behind closed furnace doors with complete safety, and with full control over crucible angle and agitation speed. Electric resistive or induction coil fusion instruments are also available. Induction heating is excellent if agitation is provided. Its main disadvantage is high cost.their advantages are close to those of propane gas instruments, and in some cases far superior. Propane gas burner is a simple and efficient means of heating: temperature range is optimal (Figure 17); very low heating is obtainable because the gas flow can be controlled even when the flow is small. The maximum temperature that can be obtained is more than sufficient for any type of sample, yet there is no risk of damage to the crucibles when empty; 22

24 Preparation of glass disks PANalytical Temperature ( C) in melt With O 2 booster With venturi open psi p Setting on controller Figure 17. Typical controller settings and temperatures for Fisher standard grid-top adjustable burners. Temperatures were obtained using thermocouples in the melt and are probably accurate. control of gas flow is easy and ensures reproducible heating conditions with temperature, independent of the number of burners used. However, the heating conditions are not as reproducible with gas burners as they are with muffle furnaces. Different burner settings, easily blocked jets, the effect of air pressure and of moisture in compressed air lines can all lead to different temperatures and temperature instability; temperature can be changed rapidly and set independently for each heating step which is particularly important when dealing with sulfides and metals (for complete oxidation) and carbonates (when foaming occurs); maintaining oxidizing conditions in an open gas flame is not easy and great care must be taken to avoid reducing conditions that will damage the crucible; several samples can be processed at the same time but achieving and maintaining identical temperature conditions in multiple flames is exceedingly difficult; cost is low, but can be expensive per fusion when using an oxygen-enriched flame. Propane oxygen gas burner has a workable range of temperatures a few hundred degrees higher than the propane gas burner (Figure 17), which is unnecessary for borate fusions and increases the risk of damage to the crucibles and loss of some elements from the molten glass. At higher altitudes, however, the propane oxygen burner with its extra oxygen might be essential to achieve optimal fusion temperatures. Additional benefits of adding oxygen to the flame are less gas usage, reduced flame volume and a very much reduced heat load into the laboratory, with the accompanying advantage of lower cooling costs. Temperature measurement during fusion is not really necessary. As a matter of fact, such measurements have very limited significance when one observes that in gas burner machines the color of a hot crucible may vary considerably from top to 23

25 PANalytical Sample preparation XRF and glass beads by borate fusions bottom. In practice what counts is to ensure that the temperature does not exceed a level at which the elements of interest (essentially sulfur, halogens and alkalis, including the flux) volatilize at a non acceptable rate. A correct temperature is difficult to define since volatilization increases exponentially with temperature, so that it always takes place (Figure 16). The object is to operate at reproducible conditions (difficult with gas burners), with the temperature not too high, so that the loss by evaporation is small and constant. Take note that releasing agents are particularly volatile and that variations in their concentration affects light elements significantly. As a rule of thumb: a long fusion at a low temperature is preferable to a short fusion at a high temperature. Apart from the lower loss of sample and flux, the halogen element of the releasing agent should evaporate less and lead to a cleaner crucible and easier releasing of the bead Heating steps A complex fusion procedure may involve several heating steps (Figure 4). Simpler procedures can be adopted but since versatile programmable fusion apparatuses are available why not strive for the best operating conditions. The following heating steps should be considered Heating steps for oxidation Oxidants are used as an additive to the flux to oxidize components of the sample in a reduced state, for example metal sulfides or sub oxides such as Cu 2 O, FeO or carbon bearing samples. Usually the oxidant is a nitrate, such as NaNO 3 or LiNO 3, that melts before the flux melts. If the temperature at the beginning of the process is increased slowly, the oxidant melts first, wets the reduced component and reacts with it to yield an oxide that can dissolve into the flux. The temperature should be increased slowly enough to allow the reaction to reach the center of each oxidizable particle. Depending on the concentration of the material to be oxidized, one or two or perhaps three heating steps should be considered. The first at the lowest temperature for 2 or 3 minutes which allows the material at the edges of the crucible to react; the second at a higher temperature to allow the inner material to react also; and a third step at still higher temperature to make sure that the reaction is complete. Incidentally, the flux will have started to melt at the edges of the crucible when the third heating step is reached. During the oxidation procedure agitation of the crucible is not recommended Heating steps for fusion and homogenization When oxidation is not part of the fusion procedure, the fusion itself may consist of two steps, one without or with very little agitation, so that no particle of the flux is projected onto the wall of the crucible where it would form fine droplets that may not join the rest of the fusion mixture. This first step lasts until about two thirds of the flux has melted and is usually at a temperature lower than that of the fusion itself. This step is obviously not necessary when fusion is preceded by oxidation involving a high temperature step as described above. 24

26 Preparation of glass disks PANalytical The second step is accomplished at the cruising temperature for fusion and is accompanied with agitation to hasten homogenization. Convection currents produced by thermal gradients during fusion should be sufficient for homogenization but the process would be slow. In addition, just after the flux has melted, the sample is still nearly intact and dissolves extremely slowly unless it is agitated vigorously. Although most fusion mixtures will dissolve without agitation, agitation (provided as a flexible option by the Eagon 2 fusion machine) can greatly increase the efficiency and speed of dissolution. The reason agitation is so effective is that agitation helps to remove the saturated layer of fusion mixture surrounding mineral grains, thus promoting further dissolution. An example of a similar procedure in everyday life is the stirring of coffee or tea to ensure that added sugar grains dissolve completely. Often, even after some time has elapsed, partially dissolved sugar grains remain at the bottom of a cup of tea or coffee that has not been properly stirred. All kinds of agitation that produce friction between adjacent layers in the melt should be efficient. Those that produce a mere rotation or displacement of the molten mass as a whole are not efficient. Here are two examples. The effect of rocking of a standard crucible containing a liquid is shown in Figure 18. Changing the inclination of the crucible from A to B results in the liquid not staying level; trying to become level again and considering that molten fluxes are somewhat viscous, the liquid mass rotates about its center instead of flowing from left to right, with the result that almost no convection occurs. The situation is very different when a crucible with a hump in the bottom is used instead of a standard crucible, as shown in Figure 19. When changing the inclination of the crucible the hump in the crucible prevents the liquid mass from moving as a whole; instead, the two halves of the liquid must separate and move rapidly through the narrow gaps on each side of the hump; these currents are very efficient for mixing. Figure 18. Motion of glass during rocking in a standard crucible. Dots represent the motion of a liquid element. 25

27 PANalytical Sample preparation XRF and glass beads by borate fusions Figure 19. Motion of glass during rocking in a crucible with a hump in the base. Dots represent the motion of a liquid element. A similar situation arises in systems where the motion of the crucible is a rotation about its axis (Figure 20). With a standard crucible the liquid essentially rolls about its center but with a hump in the crucible the same tendency to roll produces currents that move in opposite directions in the narrow pass because the liquid is squeezed between the hump and the wall of the crucible. Standard flat bottom Top view Hump Hump Standard flat bottom Figure 20. Motion of glass during rocking in a crucible with a hump in the base. Dots represent the motion of a liquid element. Continuous agitation, even in the partially molten content of the crucible, during fusion is much more efficient for mixing as compared to short occasional agitation periods. In addition, the samples may eventually contain small quantities of particles likely to corrode the crucible; keeping these particles floating minimizes the reaction with the platinum. Another reason for strong agitation is the elimination of bubbles that sometimes form in the melt. Occasionally one finds very small bubbles floating on the melt that are not eliminated after long agitation; after casting they cause cracking of the glass disk. These bubbles are sometimes caused by carbon particles present in the sample; they stick to fine bubbles and prevent them from agglomerating into larger bubbles that would burst and disappear. The easiest way to get rid of them is to burn off the carbon by adding an oxidant into the flux or by injecting air into the crucible during the fusion Heating steps for conditioning before casting When using gas burner type fusion machines two extra short heating steps may be desirable before casting, one for adding a releasing agent tablet and one for adjusting the temperature of the mold or molten glass. Such extra steps are unnecessary when using the Eagon 2. 26

28 Preparation of glass disks PANalytical Some elements, those too acidic, seem to be allergic to the bromine or iodine of the releasing agent and reject it fast during fusion, with consequent adherence of the glass to the mold and to the crucible. The injection of a small crystal or a tablet of the releasing agent followed by a mixing period of one to twenty seconds before casting can help prevent sticking. In some fusion instruments the mold is heated above the crucible and is partly shielded from the flame. If, as a result of the shielding, the temperature of the mold is too low, crystallization in the bead will sometimes occur. To prevent that, one may increase the temperature for a period of about 10 seconds immediately before casting; the mold will heat up fast because it is thin while the molten glass will heat up just a little because it is more massive and less conductive. Fusion instruments that provide separate heatings for the mold do not require this step. PANalytical s Eagon 2 fusion machine obviates the need for these extra heating steps and allows the releasing agent tablet to be added at any time that is appropriate for a particular fusion program. 2.5 Casting and cooling Casting into molds When the hot melt is ready for casting it is merely poured into the mold. To prevent crystallization during cooling a few conditions must be met: the mold should contain no solid residue from the preceding fusion; any particle that has not melted while the mold was heated during fusion, and still does not melt when the hot glass is poured into the mold, is a seed for crystallization; the mold should be hot enough to avoid sudden crystallization of the glass that would lead to the same problem as in the preceding case Solidification This operation is a critical step in the production of fusion beads on account of its influence on the quality of the beads. Glass is a supercooled liquid; hence it is an unstable material that tends to crystallize, and does, if it is given a chance. The time required for a glass specimen to crystallize varies as a function of temperature (Figure 21). At temperatures above the melting point of the stable crystalline state, the material remains liquid indefinitely. Just below the melting point, the tendency to crystallize is low and the viscosity of the glass slows down the atoms in their attempt to form crystals; crystallization is very slow. As the temperature decreases both viscosity and tendency to crystallize increase, but the latter predominates so that the time before onset of crystallization decreases. Eventually a temperature is reached where the viscosity becomes so high that the atoms have a lower and lower possibility to move and form crystals; then the glass state becomes the stable state, but in appearance only and for a limited time that, fortunately for the XRF analyst, is very long at room temperature. 27

29 PANalytical Sample preparation XRF and glass beads by borate fusions Temperature ( C) Glass Crystals Figure 21. Time The crystallization curve of borate glass In simple terms, the secret of a successful glass disk lies in working out a cooling curve that avoids crossing the crystallization curve of the glass. In practice, the situation is complex because the cooling curve varies with the overall composition of the glass and the position in the mold. The analyst has limited means to change the shape of that curve, and one must take into account that forced cooling produces stresses that may lead to cracking, the more so when the glass does not release easily from the mold. A few types of cooling curves are shown in Figure 22: Cooling curve A crosses the crystallization curve above the nose usually a only short time after pouring into the mold; crystallization starts at several points on the periphery of the bead where the temperature is lower, and moves rapidly towards the center. The heat generated by the reaction, the volume change and the viscosity all together produce bubbling at the solid liquid interface so that, after cooling, the aspect of the bead is similar to a mountainous area with scattered peaks. The surface of the bead in contact with the mold is flat and porous with clearly visible dendrites. Such a disk is likely a mixture of several phases and is not suitable for accurate XRF work. This situation is typical of fusions with lithium tetraborate. The remedy to this type of crystallization is to move the crystallization curve to the right by changing the composition of the glass, i.e. by using a higher sample:flux ratio, adding a compound to stabilize the glass, or using a more basic flux, but not pure lithium metaborate. The latter usually crystallizes at all times, except in rare cases. It only differs from the tetraborate in that crystallization appears at the beginning of cooling and is uniform all over the surface of the bead. It is avoided by using a less basic flux. This suggests another fusion rule: Avoid the stoichiometric compounds tetraborate and metaborate as fluxes, except with highly acidic or highly basic samples. Prefer intermediate compositions. Cooling curve B is typical of a glass composition associated with a crystallization curve more to the right compared to A. The cooling curve intersects the latter at a much lower temperature so that crystallization can only just start. It is usually observed as small whitish opaque spots that form either at the periphery or wherever the surface is in contact with the mold. The resulting 28

30 Preparation of glass disks PANalytical beads are not useful; if the crystallized area is small the influence on the emitted fluorescence intensity may be small but the stresses around the crystals may induce shattering. A small modification of the composition of the glass is often sufficient to avoid crystallization, for example adding a little LiF to the flux. Another possibility is to modify the cooling curve as described below for D. Temperature ( C) Liquid glass Supercooled liquid A High-temp. crystals (LiBO 2 ) Time Area of crystallization B Low-temp. crystals Still-air cool & high-t C annealing (no fans) Still-air cool & low-t D annealing (fans on) E Still-air cool (Na 2 B 4 O 7 ) Figure 22. Effect of cooling rate on the bead. In practice the cooling curve is rather constant and the crystallization curve is the variable. Cooling must be fast enough to keep the cooling curve of the bead below the crystallization curve. Curve E is typical of very stable glasses. Most beads based on Na tetraborate are of this type. Cooling in still air to room temperature is not too slow. It is not recommended to anneal these disks because they will behave as in B, i.e. they will crystallize and crack. Curve D is typical of the conditions necessary to avoid crystallization of most glass beads. It is obtained by letting the glass solidify in still air for a short time, then cooling faster to prevent the onset of crystallization. A cooling period of about one to two minutes in still air followed by cooling with fans has been found to be satisfactory. A few practical observations that may facilitate the work of the analyst are given here: the crystallization curve of Li metaborate is close to the origin of the time scale (Figure 23), that of Li tetraborate is more towards the right and that of Na tetraborate is still more to the right; this seems related to the viscosity of the flux: the more fluid it is, the easier it crystallizes; the dissolution of a sample into a flux usually displaces the crystallization curve to the right (Figure 24); this seems related to the more complex composition of the liquid phase, making it more difficult for the atoms to find their partners in forming a crystal; after the hot molten glass has been poured into a mold, cooling the latter immediately by means of an air jet is certainly efficient to avoid the nose of the crystallization curve but creates large stresses in the glass bead; such beads 29

31 PANalytical Sample preparation XRF and glass beads by borate fusions have been observed to shatter in the hands of the analyst and there is a risk of damage to the eyes. If the glass is not left to cool in still air at least for a fraction of a minute before applying the air jet, then it should be done when the bead has reached a lower temperature to minimize the residual stresses. A useful tip (Mark Ingham, pers. comm.) for checking for residual stress in a bead is as follows: two squares of polarized plastic film are hinged together so that the polarizing planes are perpendicular to one another. When a bead is sandwiched between the films and held up to the light a non-stressed bead gives a Maltese cross interference figure, while stressed beads have bent or wavey interference figures. Temperature ( C) LiM LiT NaT Cooling curve Crystallization curves Time Figure 23. Crystallization curves of three common fluxes (qualitative). The cooling curve crosses the lithium metaborate (LiM) curve but not that of lithium tetraborate (LiT). Crystals will form in LiM but not in LiT or sodium tetraborate. Temperature ( C) a b c Cooling curve Time Crystallization curves Figure 24. Crystallization curves of: a) pure flux - no sample; b) low sample:flux ratio; c) high sample:flux ratio. Note that because the cooling curve crosses the pure flux crystallization curve it is very difficult to make an uncrystallized bead of pure flux. heating the crucible and the mold to a higher temperature just before pouring the glass into the mold increases the cooling rate at the beginning of solidification and may prevent several cases of the so called high temperature crystallization. 30

32 Preparation of glass disks PANalytical annealing the fusion bead after cooling is normally not necessary if the initial period of solidification in still air has lasted about one minute or longer. Annealing for a few minutes in the 200 to 400 C range is not wrong but longer annealing may lead to beginning of crystallization with consequent shattering of the bead (cooling curve C in Figure 22). during cooling, while the temperature is still in the neighborhood of 200 C, a glass bead should not be touched with a metallic object such as tongs; the faster cooling at the point of contact may lead to cracking. a freshly made glass bead should not be polished. If streaks or scratches are found on its surface, the mold instead should be well polished. Polishing a bead provides a risk of contamination by Si or Al, two commonly used abrasives. The Eagon 2 has all the necessary cooling flexibility required to navigate the phase transition space and thereby produces good, stable glass beads on a regular basis (Figure 25). Temperature ( C) Critical stage Quenching - Viscosity increases faster than diffusion required for crystal formation - Molten liquid ðsolid liquid Too slow ð crystallization Too fast ð stressed beads Cooling trajectory depends on sample and flux type Cooling flexibility needed to navigate the phase transition space - Passsive cooling - Forced cooling (2 flow-rate options) Time Figure 25. The Eagon 2 has all the necessary cooling flexibility required to navigate the phase transition space and thereby produce good glass beads on a regular basis. 2.6 Examples of fusions that require special techniques As mentioned above, only oxides can be dissolved into molten borate glasses. All other compounds must be oxidized prior to fusion. Ideally the oxidation and the fusion operations should be combined in a single procedure. Materials frequently considered as more difficult cases are sulfides, carbon, metallic materials and copper bearing samples. These and other interesting cases will now be considered. 31

33 PANalytical Sample preparation XRF and glass beads by borate fusions Sulfide minerals Sulfide minerals are risky to process in platinum crucibles unless a safe technique is used; a crucible can be ruined in a single fusion. There are two problems to overcome in processing sulfides. The first is to make sure the sample oxidizes totally as a sulfate and the second that sulfur (from sulfide or sulfate) is not lost as a gas. Oxidation done by heating the sample in air or in oxygen does not retain SO 2 totally. Heating the sample with an oxidant as part of the fusion process is more efficient. Suitable oxidants are potassium pyrosulfate (if K and S are not analyzed) or a nitrate of lithium or sodium. To retain sulfur during a fusion, the oxidation must be done at a temperature as low as possible to transform the sulfide into a sulfate instead of forming an oxide and SO 2 gas. The oxidation temperature should not exceed about 300 C. It can be increased to 1000 C, but preferably lower, after oxidation. When sulfur has to be analysed, ammonium iodide cannot be used as releasing agent because sulfur evaporates with ammonium. However, other iodides can be used. A technique that meets the above conditions has been described by Norrish and Thompson (1990). It has been applied to the preparation of fusion beads for X ray fluorescence. It consists of heating a mixture of 0.25 to 0.66 g of sample, 6.8 g of flux and 1.0 g of NaNO 3 in a muffle furnace at 700 C for 10 minutes, then fusing as for oxides at a temperature not exceeding 1050 C. Another procedure for sulfide concentrates using a fusion machine is the following. Mix about 0.3 g of sample with one or two grams of Li or Na nitrate and 0.5 g Li tetraborate, and put into the crucible. Cover with 6 g Li tetraborate and the releasing agent. Heat at a low temperature with slow agitation for oxidation, then below 1000 C with stronger agitation for complete melting and homogenisation. Sodium nitrate is a more efficient oxidizing agent than lithium nitrate, but absorbs X rays somewhat more and prevents analysis of sodium Copper compounds If copper is present in a partially oxidized state, metallic copper precipitates during fusion and alloys with the crucible. It is a safer practice to calcine such samples and to always add some oxidant to the fusion mixture. Releasing agents sometimes react with samples containing copper and copper halides volatilize (Equation 1). Loss of copper with consequent loss of accuracy is only part of the problem; copper being an element that sticks to crucibles and molds, a thin layer of glass remains all over the surface of the crucible after pouring, and the glass disk is very difficult 32

34 Preparation of glass disks PANalytical to release from the mold without breaking it. The injection of a 10 mg crystal of KI into the molten glass one second before casting has been observed to improve the problem of sticking, but not to full satisfaction; the loss of Cu by volatilization is small due to the short time of reaction. A simple successful technique for the fusion for CuO, and even for Cu metal, has been described by Blanchette (2001). Pure CuO is mixed 1:10 in a flux consisting of 50:50 mixture of lithium tetraborate and metaborate and 0.5 % LiBr and fused as usual. It might be possible, depending on the fusion apparatus used, to add LiBr as a solution and mix it well into the unfused mixture Carbon Carbon is often found in small concentrations in raw mix cement, fly ash and limestone. The addition of 0.1 g NaNO 3 in the fusion mixture is usually sufficient to eliminate it. Ammonium nitrate can also be added in any unweighed quantity because the unused portion volatilizes completely on heating. Heat at the lowest possible temperature (ammonium nitrate melts at 170 C) and allow enough time for the reaction to take place. Injection of air into the crucible during fusion is also efficient Metallic materials As a general procedure, metallic materials can be processed similarly as sulfides except for a few differences. Metallic particles are usually compact and dense as compared to sulfides which are generally porous, so that the oxidation of metals is more difficult and slower. Nitrates are not efficient for oxidizing metals, but, surprisingly, Li carbonate is very efficient. Metallic particles should be very, very fine. A practical method with fusion machines is the following. Use 5 g of Li tetraborate in a crucible to make a protection coating all over its surface, by melting the flux and rotating the crucible in an inclined position. Then let cool while rotating until the coating solidifies. Put a mixture of 0.5 g metal powder and 1.25 g Li carbonate in the coated crucible. Level and add a layer of 1.25 g Li carbonate, and then add a layer of 1.8 g B 2 O 3 (boron oxide, not boric acid!). Heat at a medium temperature to make the oxidation and to melt the boron oxide that will prevent the CO 2 gas from ejecting material out of the crucible. When oxidation is finished, increase the temperature and continue as in a normal fusion. This technique works well even with ferroalloys, but it is important to follow the procedure very carefully with ferrosilicon because the reaction is a highly exothermic chain reaction, capable of piercing the crucible Carbonates Samples that contain a large amount of carbonate sometimes are reported to foam and run over the crucible during fusion. The reason is that the flux has a melting point below that of the carbonate, so that the gas from the decomposition of the carbonate escapes through a viscous liquid. In the case of calcium carbonate that decomposes at 900 C, only lithium tetraborate flux has a higher melting point. This 33

35 PANalytical Sample preparation XRF and glass beads by borate fusions allows the gas to escape through a porous solid without any foaming, provided that the flux is also a rather coarse powder; otherwise the flow of gas would blow the fine particles out of the crucible. 34

36 Pyrosulfate fusions PANalytical 3. Pyrosulfate fusions 3.1 Procedure Pyrosulfate is a good fluxing agent to break up most minerals by transforming them into sulfates that are soluble in water or mild acids. Silica is a major exception. Powdered rocks are mixed with about 10 times their weight of potassium pyrosulfate, and the mixture is heated over a gas burner at low temperature until the molten pyrosulfate has dissolved the minerals. At the start of heating the reaction is rapid and a large amount of gas is formed; the effervescence must be kept low by moving the crucible in and out of the flame. After about three minutes of heating the effervescence has vanished, the liquid product is poured into a room temperature container (a mortar, for example) in which it immediately solidifies. To make a solution for wet chemical analysis, the solid is ground and dissolved in a suitable solvent. For XRF analysis, the solid is ground, a binder is mixed with the powder and the mixture is pressed as a disk. Detailed procedures for making beads for analysis by XRF have been described by Cullen (1960, 1962), Sear (1997) and O Neill and Fitzsimons. The first author used the loose powder as sample and obtained quite satisfactory results. Sear (1997) discussed the fusion of difficult materials such as chromite, cassiterite and sulfides. He made use of intimate mixtures of sodium nitrate and boric acid resulting in a sodium borate flux of very high oxidizing power. The process of lowtemperature oxidation followed by fusion at normal high temperatures provided strong oxidation conditions with no damage to the crucible. It was possible to completely retain pyritic sulfur with lithium carbonate-tetraborate fluxes. The use of pure alumina crucibles allowed the use of sodium peroxide for the fusion of ferro-chrome and ferro-niobium. The third authors poured the molten samplepyrosulfate mixture into a mold to obtain a solid disk, but had serious problems of cracking that necessitated adding powdered silica into the melt, and making only one disk at a time in the furnace because the heat generated in one crucible influenced the temperature of the crucible next to it. 3.2 Automatic preparation Fusion machines can be adapted easily to make pyrosulfate fusions. The molds are the same as those for making borate glass beads. Although theoretically platinum crucibles could be used, they are not convenient because their heat conductivity is high so that the effervescence is difficult to control. Silica crucibles work very well and require only a modified crucible holder. At the end of the fusion process, the solidified bead separates easily from the mold, leaving the mold clean. The crucible retains a residue that is easily cleaned by washing in water. 35

37 PANalytical Sample preparation XRF and glass beads by borate fusions 3.3 Requirements Fusion and casting of pyrosulfate disks is done essentially in the same manner as borate beads, but the experimental conditions are rather different because the physical properties are different. 1. The reaction of the pyrosulfate with the specimen is accompanied by effervescence. The temperature must be kept low, just above the melting point of the pyrosulfate (~300 C), so that the burners must be set at their lower limit of heating and a low heat conductivity crucible must be used. 2. Molten pyrosulfate is very fluid, so that pouring must be done slowly, otherwise splashing occurs outside of the depressed part of the mold. 3. The solidified disks are not vitreous as are borate disks. Since they are crystallized, they are weaker and more prone to cracking. If they are cooled slowly, they are rather hard and solid. 3.4 Problems Pyrosulfate fusions suffer from two problems, but it seems that solutions to these have been found. The first problem is that of indissoluble silica. When silica originates from a silicate that has reacted, the particles are very fine and stay in the melt as a dispersion. During cooling they tend not to segregate. If silica is from original quartz crystals, it is usually coarse enough to segregate at the bottom of the mold before the melt has started to crystallize. This problem apparently has been solved by agitating the melt rapidly before pouring and by a short period of fast cooling in the mold to form a thin solid layer in the mold immediately on pouring, then letting the rest of the melt crystallize and cool more slowly. In this way the surface of the disk exposed to X-rays has the average composition, and the disk maintains its solidity. The second problem is similar. It is due to the fact that lead sulfate, if present in large concentration, apparently crystallizes first and settles at the bottom of the mold at the start of crystallization. The remedy is the same as that for quartz. 3.5 Fusions of sulfide concentrates The Norrish method using Li 2 B 4 O 7 uses 0.2 g LiNO g sulfide. This is not enough oxygen to completely oxidize the S 2. Adding too much LiNO 3, i.e. Li 2 O, changes Li 2 B 4 O 7 to LiBO 2 which gives problems of crystallization. Pyrosulfate, however, has excess oxygen which reacts well with the S 2 : (3) 36

38 Pyrosulfate fusions PANalytical Pyrosulfate fusion of sulfide ores is probably a better method than using borate fluxes with plenty of oxidant, provided that sulfur needs not to be analysed. 3.6 Analysis of disks During the fusion, some SO 3 escapes from the pyrosulfate, some SO 3 moves from the pyrosulfate to the metallic elements of the specimen, some elemental sulfur escapes because there is an excess, etc. As a result, the composition of the specimen has changed, the flux is no longer the same, the sample:flux ratio has changed. If one wants to apply the Lachance-Traill equation to calculate the concentrations, the problem seems extremely complex; for example, it is observed that the loss of weight during fusion is about equal to the weight of the sample, which is considerable. However, a closer look at the situation indicates that such calculations are actually very simple. In the meantime, concentrations can be obtained by regression analysis on a sufficient number of specimens, and the results are very encouraging. 3.7 Conclusion The above details are given for information only. There has been much less effort spent on studying the pyrosulfate fluxes compared to the borates. 37

39

40 Fluxers PANalytical 4. Fluxers Many different types of fluxers or fusion instruments are available commercially, and are either of the gas burning, induction heating or muffle furnace type. 4.1 Fusion instruments A modern fusion instrument is usually a fully automatic instrument with microprocessor control. It is capable of the preparation of glass disks for analysis by X-ray fluorescence, and can make borate, hexametaphosphate or pyrosulfate fusions without alteration of the equipment. The main features of a typical fusion apparatus are: simultaneous processing of 1 to 6 samples; several programs adaptable to all kinds of fusion, including several heating steps, pouring, cooling, stirring of solutions; programmable heat levels, agitation and time; agitation can be by rotation of the crucible about its axis at an angle of ~60 degrees with the vertical or rocking back and forth; may need only propane, but some machines need air and/or oxygen (gas burner type); stable reproducible gas flow, independent of the number of burners (gas burner type); very low temperatures obtainable for oxidation; uniform crucible temperature top to bottom; heating of molds either simultaneously with or separately from heating of the crucible; non-wetting agent injector to prevent sticking of glass to Pt ware; optional injection of oxygen into the crucibles; optional injection of oxygen into the flame (temperature booster, gas burner type). With the exception of optional injection of oxygen into the crucible, something which is hardly ever necessary and the need for which can be removed by the use of additional oxidant, and the additional requirements pertaining only to gas burner type fusion instruments, the Eagon 2 more than meets all of the above criteria. 39

41

42 Care and use of platinum ware PANalytical 5. Care and use of platinum ware 5.1 Why platinum Modern chemical analysis would not be possible without the wide range of platinum apparatus and laboratory ware. Platinum is used, because it is essentially inert, does not contaminate the sample for normal analytical purposes and shows no significant weight loss even over prolonged periods of heating at temperatures over 1000 ºC in air. The use of crucibles and molds of platinum and its alloys in the preparation of samples has become an essential feature of XRF analysis. Its broad application results from its near perfect oxidation resistance and its excellent resistance to corrosive attack by metal oxide melts such as XRF fluxing agents. Simple basic precautions and good laboratory practice can result in obtaining the full benefits of using platinum. Newly manufactured platinum ware Platinum ware cleaning and maintenance Distilled water rinse Re-polish crucible Re-polish mold Sample fusion Sample preparation Platinum maintenance Re-shape crucible Dented or mis-shaped crucible Re-press mold Mold scratched or damaged? Figure 26. Care and use of platinum labware 41

43 PANalytical Sample preparation XRF and glass beads by borate fusions 5.2 Damage to platinum ware The effect commonly referred to as platinum corrosion is not corrosion in the normal sense of aqueous corrosion and rusting. It is usually the reaction of platinum with another element to form a compound with a lower melting point and the subsequent formation of a eutectic with an even lower melting point between the compounds of platinum. Some of examples of poison elements for platinum ware are: Arsenic, phosphorus, boron, bismuth, silicon, sulfur and a number of heavy metals, e.g. lead, zinc, tin, antimony. Even if these elements are only present at low concentrations they can diffuse gradually into the grain boundaries of the platinum where they lead to a loss of high-temperature strength Crucibles Do not heat non fully oxidized materials in Pt-Au crucibles unless an efficient oxidant is mixed with the material and heated slowly from low temperature until the non oxidized material has been completely oxidized. The oxidizing process can take place in a chemical laboratory, outside the fusion instrument. One single mistake may be fatal for the crucible (Figure 27). The most frequent non oxidized materials are sulfide minerals, sub oxides such as FeO, MnO, Cu 2 O (instead of Fe 2 O 3, MnO 2, CuO), metallic elements and organic materials. When sulfides are present in low concentrations, add 0.1 to 0.3 g of Li or Na nitrate in the flux and start heating at very low temperatures for one minute before proceeding to a regular fusion. If the concentration is much higher, increase nitrate to 2 or 3 g and heat for a few minutes. Platinum reacts with sulfides forming platinum sulfide that makes the crucible very brittle. When metallic particles are present, proceed as for sulfides but take note that oxidation is slower and more difficult. A reaction of metals with platinum usually causes less damage than sulfides because they form an alloy that is less brittle. Organic materials are simply heated at very low temperatures; they burn easily. After fusion has been completed the dissolved elements do not react any more with the crucible while still at high temperature, unless the surrounding atmosphere is reducing which may cause a few elements, such as Fe and Cu, to partially reduce and produce some free metal atoms that coat the surface of the crucible. Do not scrape a residue from a crucible by means of a hard tool, for example a screwdriver or metal spatula. Scratches on the crucible retain more residue in subsequent fusions. 42

44 Care and use of platinum ware PANalytical Figure 27. Pt alloy crucibles are easily destroyed when trying to fuse reduced (not fully oxidized) substances, or by bad handling Molds Never tap hard on a mold to release a stuck glass disk, rather use the whack technique (Figure 28). Place the cold mold with stuck bead in the middle of a thick wad of loose sheets of paper. Close the sheets and whack them on the table, desk or bench top. The disk will usually come away from the mold which will be undamaged. If the whack technique does not succeed, heat the mold for a few seconds only so that the mold expands without heating the fused bead too much, then turn the mold upside down and let the disk fall. If that is not successful, put some KI on the disk, heat until the KI and the disk melt, then pour the molten glass. Clean the mold as for crucibles. The surface of the glass disk is an exact replica of the surface of the mold, so keep the mold flat, well polished and free from scratches. Polishing can be done as discussed in Figure 28. The whack technique for releasing fusion beads stuck in molds 43

45 PANalytical Sample preparation XRF and glass beads by borate fusions 5.3 Prolonging platinum ware lifetime Cleaning of platinum ware A crucible or casting mold should never be hit or tapped with any implement to remove residual fused sample stuck to the surface. Any residual material should be removed by one of the following methods: Place the laboratory platinum ware in a 25% citric acid solution in an ultrasonic bath and keep it at 60 ºC for up to 30 minutes until clean and rinse with distilled or de-ionized water; To shorten the cleaning cycle, use a 20% w/v citric acid in a beaker at 80 ºC; For a more aggressive method use 10% HCl. The use of orthophosphoric acid should be avoided; A simple way to clean the crucibles is the following: put about 2 g of flux (purity is not important, a chip of a broken glass disk is acceptable) and 0.2 g of KI in the crucible, heat until fused, then cast. After acid cleaning, wash thoroughly with distilled or de-ionized water and dry. If a residue is still present after both these alternatives of cleaning, it is likely that a reaction has taken place between the platinum and sample and that the crucible is damaged permanently. WARNING: Never use a mixture of nitric acid and hydrochloric acid Maintenance Damaged casting molds can have a direct effect on the analysis of the fused beads. The figure below illustrates how a mold appears when manufactured and at the end of its life. Perfect Damaged casting dish Figure 29. Difference between a perfect mold and a dish with damaged surface Microscope analysis of the casting mold can be useful in more effectively determining the conditions of the mold, than just visual inspection. 44

46 Care and use of platinum ware PANalytical Another method to check for damaged casting molds is illustrated in Figure 30; take two beads and place the to be measured sides of the beads together and check for the perfect fit. Beads - convex (common) Beads - concave Beads - perfect Figure 30. Checking the molds for damages In general the crucibles will be more difficult to clean and deformations in the casting mold may cause difficulty in releasing the bead and will increase errors in the reported result. The mold can also be re-shaped, this can also help to prolong the useful life of the casting mold. Dented and deformed crucibles typically pour the fused sample badly, increasing the probability of a less homogeneous or cracked bead. Using a dedicated crucible re-shaper for the specific crucible type ensures continued efficient bead production. Regular re-shaping can help to prolong the useful life of the crucible. The most effective form of maintenance after re-shaping crucibles and re-pressing molds is to re-polish. The life of platinum ware can be considerably extended by careful use and polishing but it should be noted that polishing will reduce the weight of the platinum, subsequently reducing the amount of credit on replacement time Polishing and re-shaping kits A range of platinum ware care products are available from PANalytical to prolong the useful lifetime of crucibles and casting molds. These include; plastic cleaning baskets to avoid scratching and abrasion during ultrasonic cleaning, re-shapers and a polishing lathe. 45

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48 Miscellaneous PANalytical 6. Miscellaneous 6.1 Contamination by crucibles Residues left in a crucible before a fusion do contaminate the next fusion. However, the contamination is often so small that it is not necessary to clean the crucible after each fusion, and it is better to leave the crucible untouched on the fluxing machine as long as possible. Remember that manipulation of a crucible is one of the two most efficient ways of shortening the useful life of a crucible; the other one is corrosion. The amount of contamination resulting from not cleaning a crucible can be estimated by the following equation: Contamination (absolute %) = (Weight of residue)/(weight of total fusion) (C 2 C 1 ) where (C 2 C 1 ) = difference of concentration of the component in the two samples Example: A sample with a calculated concentration of 26.35% Fe is fused after one that contains 17.84% Fe. The residue of the first sample in the crucible is about 10 mg in a total fusion of 7.5 g. The contamination is: 10/7500 (26.35% 17.84%) = 0.01% which means that the calculated concentration of 26.35% is 0.01% too low and the corrected concentration is 26.36%. Cleaning is obviously not necessary. However, if the residue was 12 mg, the total mass of fusion 7000 mg, and the concentrations in the two samples 30% and 10%, respectively, then the absolute contamination would be 12/7000 (30% 10%) = 0.03% The analyst would have to decide whether or not this degree of contamination is acceptable. For Al 2 O 3 at 15% it probably would be acceptable. For TiO 2 at 0.3% it would not be acceptable. 6.2 Contamination by W when using WC grinding vessels Tungsten carbide (WC) can be introduced into samples during the grinding process of sample preparation with WC mortars. In addition to W and C, the alloying component of the WC grinding vessel, usually about 10% Co, is also introduced into the sample. The amount of contaminant added is generally very small (approximately %), because of the hardness of WC and the relatively softer sample. However, when the accuracy of the final analysis is really important, then corrections have to be made for dilution of the sample and the effect on the measured loss on ignition (LOI). The mathematics of the corrections is quite 47

49 PANalytical Sample preparation XRF and glass beads by borate fusions complex, but Bennett and Oliver (1992, pp ) give an excellent description of the equations and correction procedures necessary to correct for contamination by WC in pure tungsten carbide mortars, and by WC, Co and Ni in Co and Ni bonded WC mortars. 6.3 Blanks Pure LiT and 66:34 LiT:LiM can be cast as a blank bead. Pure LiM and 12:22 LiT:LiM always crystallize. The addition of about 10 mg KI a few seconds before casting stabilizes the vitreous state but the bead will not be a blank for K. Other non wetting agents are also likely to be efficient. Alternately, add a small amount of Li carbonate. That will make the tetraborate not perfectly stoichiometric, and crystallization will be more difficult. 6.4 Loss or gain on ignition (LOI or GOI) Loss of sample by volatilization (H 2 O, CO 2 ) or gain of weight by oxidation changes the total weight of a fusion mixture and yields apparent concentrations that are too high or too low respectively. Methods are available to correct for this: 1. The change of weight is measured by heating the sample (without flux) and comparing its weight before and after heating. Then the change of weight is calculated as a percentage of the sample and is considered in the same way as any element in an algorithm such as the Lachance Traill algorithm. The product of the alpha factor and the concentration of LOI or GOI being known, the concentration of each element can thus be corrected. 2. Using an algorithm as above, the concentration of LOI or GOI is assumed to be zero at first. The concentration of each element is calculated and the difference between the sum and 100% is taken as a first approximation of the LOI or GOI. The latter value is used in the next iteration and so on, to eventually yield the correct concentration of each element and an approximate, but adequate, value of the LOI or GOI (LeHouiller et al., 1977; Norrish and Thompson, 1990). LOI and GOI values can be ignored totally if the XRF software uses a Fundamental Parameter (FP) algorithm for matrix correction (for example in PANalytical software). The fundamental parameter equation can be used in a manner such that the result is the concentration of each element in the flux (Claisse, 2007). 48

50 Advantages and limitations of fusion PANalytical 7. Advantages and limitations of fusion 7.1 Advantages The fusion technique has many advantages over most other techniques of sample preparation. Apart from the elimination of particle size effect and mineralogical effect, the main ones are the following Reproducibility The fact that a quality glass disk is close to perfection as a specimen can be, i.e. it has a flat surface, no porosity, and good homogeneity (LeHouiller et al., 1974), the fluorescent intensities should be constant for replicate preparations of the same sample. Table 2 shows compositional variations and errors for 10 glass disk replicates of a cement sample made with an Eagon 2 fusion machine. The preparations were made on successive days over a ten days period and the errors listed unclude errors on weighing, fusion reproducibility, XRF instrument stability and counting statistics. The glass disks were prepared using 1 g cement mixed with 10 g of a mixed Li 2 B 4 O 7 : LiBO 2 flux (66:34), and 65 mg NH 4 I. Table 2. Maximum compositional variations from mean values of ten fused glass disks Sample name Al 2 O 3 (wt %) CaO (wt %) Fe 2 O 3 (wt %) K 2 O (wt %) MgO (wt %) Na 2 O (wt %) P 2 O 5 (wt %) SO 3 (wt %) SiO 2 (wt %) Day Day Day Day Day Day Day Day Day Day Average over 10 beads made over 10 days Mean Min Max StDev RSD % sigma

51 PANalytical Sample preparation XRF and glass beads by borate fusions Accuracy On account of high homogeneity and ideal shape, fusion beads meet the requirements of the hypotheses made in the development of the theory of X ray fluorescence emission. As a consequence, the application of software packages based on fundamental parameter calculations or theoretical influence coefficients should yield the most accurate analytical results that XRF analysis can reach. Many such software packages are available commercially Preparation of standards Standards of any composition are easily prepared by mixing pure oxides in the desired proportions. Preparation of fusion beads is the same as for the unknown samples and ensures full reliability of calibration. Giles et al. (1995) recommended the use synthetic standards, for the easy calibration in fused glass beads of the 13 most commonly occurring oxides, plus six others, in oxides, silicates and carbonates, using a single sample for each calibration. PANalytical supplies primarily synthetic standards for the calibration of cement (CEMOXI) and wide-range oxide (WROXI) applications. These standards are easily traceable to the pure compounds used in their manufacture, such that resulting analytical accuracy is as good, if not better, than applications calibrated with certified reference materials (CRM). Figure 31. Synthetic standards, such as the sets of CEMOXI (cements) and WROXI (wide-range oxides) standards, are available from PANalytical. The standards are supplied as powders and are made of oxides traceable to % pure compounds. They are ideal for fused reference calibrations Versatility Although the more reliable method of analysis is using a few standards in combination with a fundamental parameter software package, all the other analytical techniques are still applicable using fusion beads instead of powders: internal standard, spiking, double dilution, etc. Since particle size effects are absent in fusion beads and since matrix effects are smaller than in powders, these techniques work better than usual. 50

52 Advantages and limitations of fusion PANalytical Cost The purchase of a fusion apparatus with platinum crucibles and molds may seem expensive at first sight, but there are several other aspects to consider when a comparison is made with other techniques: crucibles and molds are consumables but the scrap metal value should be subtracted from the cost of new ones; essentially the only other consumables are fluxes (and propane gas for a gas flame fusion apparatus or electricity for an induction heating fusion apparatus or a muffle furnace), and the quantity used per analysis is small; labor is reduced to a minimum if a fusion machine is used; labor is reduced to a minimum if LOI and GOI can be ignored, or if weights of ingredients of the fusion need not be constant; after the operating conditions have been set in the machine, no particular skill is required to operate it; installation costs of fusion machines vary; some are very low. 7.2 The Eagon 2 from PANalytical PANalytical s Eagon 2 (Figure 32) fusion machine for XRF sample preparation with an innovative design, high performance and safe operation provides the perfect solution to prepare fused beads for XRF analysis of a wide variety of materials. Figure 32. From start to finish of the fusion process PANalytical s Eagon 2 provides the perfect solution to obtaining in a safe, reproducible and cost-effective procedure, stable, homogeneous glass beads for accurate XRF analysis of a very wide variety of materials. 51