WO2013155638A2

WO2013155638A2 - Reference materials

Info

Publication number: WO2013155638A2
Application number: PCT/CH2013/000043
Authority: WO
Inventors: Detlef Günther; Wendelin Jan Stark; Norman Albert LÜCHINGER; Samuel Claude HALIM
Original assignee: Eth Zurich; Nanograde Ag
Priority date: 2012-04-18
Filing date: 2013-03-14
Publication date: 2013-10-24
Also published as: WO2013155638A3

Abstract

The invention relates to the manufacturing of reference materials in the form of shaped articles, comprising the steps of providing a single solution comprising a predetermined amount of soluble metal precursors and soluble matrix precursors, subjecting this solution to a pyrolytic process to obtain a solid material in powder form, compressing the obtained powder into a shaped article. The invention further relates to improved reference materials, to improved analytical methods, particularly nanobeam techniques, using such materials.

Description

Reference Materials

The present invention relates to methods for manufacturing solid shaped articles with improved homogeneity; to reference materials in the form of shaped articles, such as pellets; to the use of shaped articles as improved reference materials; to improved analytical methods using such articles.

It is known that an increased interest in direct trace elemental analysis of solids at the micrometer scale using advanced techniques, such as Laser Ablation ICP-MS and Electron/Ion icroprobe (EDX, XRF, SIMS) analysis has brought a new set of challenges to the field of reference material development. Generally, the preparation of reference materials suitable for microanalytical methods follows two approaches, either the glass-type approach or the precipitation type approach. Both approaches provide reference materials with certain drawbacks. In the first approach, a powdered material is melted in a high-temperature furnace and then flash-cooled to prevent crystallization. The resulting glass-like material retains the same chemical composition as the starting material (e.g. NIST glass standards SRMs 6xx) .

The glass-type reference material shows rim depletion for specific elements (scale > 20 μΐτΐ ) and element segregation (scale < 10 μίτΐ) for specific element pairs. Further, due to its manufacturing process, it is not possible to establish reference materials for all elements in the desired relative amounts of specific elements, particularly Pt group elements.

Eggins et al ( Geostandards Newsletter - the journal of geostandards and geoanalysis, Vol. 26, Iss. 3, 269-286, Nov. 2002) studied one of the most frequently used reference materials from NIST SRM610 (glass standard) ; these studies indicate that rim enrichment up to 30% occurs. Eggins demonstrate that in NIST glasses at least twenty-five elements (e.g., Ag, As, Au, B, Bi, Cd, Cr, Cs, Cu, Mo, Pb, Pt, Re, Rh, Sb, Se, Te, Tl, W) are inhomogeneously distributed at the 30- 65 μπι scale. This inhomogeneity appears to affect all NIST glasses, where severe depletions of some elements (Tl, Au, Re, Bi) are found in the rim region of the wafers. However, the majority of elements, including Be, Mg, Ca, Sc, Ti, V, Co, Ni, Zn, Ga, Rb, Sr, Y, Zr, Nb, In, Sn, Ba, REE, Hf, Ta, Th and U shows no evidence of significant compositional inhomogeneity . Jochum et al . (Geostandards and Geoanalytical Research, 1-33, 2011) determined quantitatively possible element inhomogeneities in glass type reference materials using different beam sizes of 25, 40, 80 and 200 μιτι. Although avoiding the rim region of the glass wafers, for several trace elements gross inhomogeneities were found at small beam sizes (see e.g. Fig . 7 ) .

In the second approach, a chemical precipitation procedure is used to produce a synthetic material with a well-defined elemental composition at predetermined concentrations (e.g. MACS-x carbonate standards) .

It is generally accepted that precipitation-type reference material does not show rim depletion. Further, such material may be obtained in basically any composition of elements. Such material, however, exhibits low homogeneity at the micrometer range (e.g. limited by the particle size resulting from the multielemental precipitation process) ; making it unsuitable for microbeam techniques, since they provide information at a smaller scale.

Chen et al (Chemical Geology 284 (2011), 283-295) discloses a specific calibration strategy for LA-ICP-MS of trace elements in a carbonate matrix to address the issue of low homogeneity. Barats et al (Anal Bioanal Chem (2007), 1131-1140) disclose a calibration method based on reference materials obtained by co-precipitation. As discussed in that document, the trace elements show low homogenit ies : Table 3 reports on RSD ranging from 2 - 22%, determined by LA-ICPMS at 5 positions (beam size 80 μιτι) .

Sliwinski et al (Chemical Geology (2012) , 97-115) disclose a precipitation method for manufacturing trace element standards in pellet form. The document reports on pellets having a carbonate matrix and various trace elements with RSD below 2.5% at the 50 - 100 ppm level, determined by XRF (beam size not specified) . It is also known to use pyrolytic processes for manufacturing shaped articles.

Hilarius et al (DE4320836) discloses the manufacturing varistors based on ceramic zinc oxide. The method disclosed therein involves pyrolysis of an aqueous solution followed by a mixing step with further material. Although not explicitly disclosed, it is concluded that this method provides shaped articles having low homogeneity.

Bayya et al (US2011 /017 989 ) disclose the manufacturing ceramic bodies made of calcium lanthan sulfide for LWIR sensor windows. The method disclosed therein does not suggest adding trace elements and consequently does not address the issue of homogeneity . Thus, it is an object of the present invention to mitigate at least some of these drawbacks of the state of the art. In particular, there is a need for reference materials with improved elemental homogeneity and/or with a broad range of accessible concentrations of (trace) elements. Such reference material is considered suitable for calibration of analytical instruments, particularly analytical instruments using micro- beams. Specifically, the invention aims to provide reference materials and methods for its manufacturing where all trace elements present show improved homogeneity.

These objectives are achieved by the manufacturing method as defined in claim 1 and by the shaped article as defined in claim 9. Further aspects of the invention are disclosed in the specification and independent claims, preferred embodiments are disclosed in the specification and the dependent claims.

The present invention will be described in more detail below. It is understood that the various embodiments, preferences and ranges as provided / disclosed in this specification may be combined at will. Further, depending of the specific embodiment, selected definitions, embodiments or ranges may not apply.

In the present invention, the following definitions shall apply, unless otherwise specified. The term analytical method is known in the field and denotes a method for qualitatively and/or quantitatively determining one or more analytes representative and reproducible within a certain mass or volume. Analytical methods typically make use of reference materials.

The term Micro-Beam Technique is known in the field as a specific analytical method and includes direct trace elemental analysis of solids at the micrometer scale e.g. using Laser Ablation ICP-MS and Electron/Ion Microprobe (such as EDX, XRF, SIMS) . Micro-Beam Techniques use some manner of sharply focused incident beams of particles or beams of energy for elemental analysis (chemical or isotopic composition of a probe) in the microscopic scale (typically diameters 20 nm - 50 μιτι, and thus also includes "nano-beams") . Micro-beam techniques include non-destructive techniques (e.g. X-ray) and destructive techniques (e.g. laser-ablation ICP-MS, SIMS) . Micro-Beam techniques are typically used in process control, geology, biology and material sciences.

The term solid shaped article is known in the field. Shaped articles, in the present context, typically have a volume of 1- 1000 mm³ and/or a mass of 1-5000 mg. Shaped articles may be obtained, e.g. by compacting a powder or granules (granular- type articles) . Although considered homogeneous, a shaped article may exhibit different concentrations of a specific element (a trace element) when measured along its cross- section. Shaped articles may have any physical form, particularly suitable are pellets, typically in cylindrical form. Shaped articles may also have the form of tablets, wavers (if of larger size), and beads. Shaped articles according to the invention are synthetic materials, thus excluding shaped articles of naturally occurring materials, such as sea shells. This may be emphasized by using the term synthetic solid shaped articles.

The term trace element is known in the field and includes elements with concentrations of 10-1000 ppm. The term trace element distinguishes from impurities, as the former is included intentionally and reproducibly, while the latter is present by chance and non-reproducible.

The term matrix is known in the field. It denotes the chemical entities that form the basic material of a sample and thus particularly includes all elements in a sample (such as in a shaped article) ranging from 0.1 wt% (= 1000 ppm) to 100 wt% . In the context of this invention, matrices are of inorganic material, such as salts (including carbonates and sulfides), oxides (including pure oxides and mixed oxides), metals or alloys .

The term reference material is known in the field and includes a material, sufficiently homogeneous and stable with respect to one or more specified properties, which has been established to be fit for its intended use in an analytical method. The above properties may be quantitative or qualitative. The above use includes (i) the calibration of a measurement system, (ii) assessment of a measurement procedure, (iii) assigning values to other materials and (iv) quality control (v) or quantification of unknown compositions. Reference Materials (RM) are also termed Standard Reference Materials (SRM) or Certified Reference Materials (CRM) . The term homogeneity relates to the quality of spatial element distribution (particularly trace element distribution) in a sample (particularly in a shaped article or in a powder) . The homogeneity of a sample is strongly influenced by the synthesis procedure; especially synthesis time scales.

It is known that precipitated reference materials (e.g. carbonates) suffer from a low homogeneity resulting from macroscopic element segregation during the precipitation process .

It is also known that flash-cooled (glassy) reference materials (e.g. NIST 6xx) suffer from a low homogeneity due to (I) rim depletion and (II) element segregation (see chemical incompatibilities based on Goldschmidt classification) .

The term rim depletion denotes the fact that the concentration of an element, such as a trace element, is decreased in the rim region of a shaped article (e.g. a pellet) when compared to the core region of the shaped article (pellet) . Rim depletion occurs in flash-cooled (glassy) reference materials because they are produced from a glass melt. At the corresponding high temperatures (> 1400°C) diffusion processes are highly activated and certain elements can transfer either into the crucible (typically metal crucibles) or into the gas phase (elements with low vapor pressure) leading to depleted concentrations at the rim.

The term element segregation denotes the fact that the affinity of specific elements to a certain matrix can be highly different.

Element segregation may occur during the synthesis of precipitated reference materials. This happens because the simultaneous co-precipitation of several elements is highly thermodynamically and chemically influenced leading to different precipitation kinetics for different elements. Therefore, the final element homogeneity is dominated by the precipitate particle size, which typically is > 5 μιτι.

Element segregation may further occur during the synthesis of glassy reference materials because during solidification of the melt (flash-cooling) specific elements segregate and accumulate due to their chemical incompatibility with the matrix. The Goldschmidt classification, a generally accepted geochemical classification, groups the chemical elements according to their affinity to a specific matrix into

- Lithophile: (Li, Be, Na, Mg, K, Ca, Sc, Ti, V, Cr, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, W, B, Al, Si, P, 0, F, CI, Br, I, At, REE) ,

- Siderophile: (Mn, Fe, Co, i, Mo, Ru, Rh, Pd, Re, Os, Ir,

Pt, Au) ,

- Chalcophile: (Cu, Zn, Ga, Ge, As, Se, S, Ag, Cd, In, Sn, Sb, Te, Hg, Tl, Pb, Bi, Po) and

- Atmophile: (H, C, N, He, Ne, Ar, Kr, Xe, Rn) elements.

Siderophile or chalcophile elements therefore have a tendency to accumulate or segregate in a lithophile matrix (e.g. Pt group elements in silicate matrices) which leads to a low element homogeneity under equilibrium conditions. In both, glassy and precipitated materials, element segregation occurs due to the fact that the corresponding synthesis procedures take place on a time scale of seconds (flash-cooling) or even minutes (precipitation). Under the corresponding near-equilibrium conditions (chemically and thermodynamically) chemical incompatibilities therefore will always cause significant element segregation.

The term pyrolytic process is known in the field. It particularly includes liquid-based processes i.e. where liquid precursors/reactants are combusted or pyrolized yielding inorganic materials. Typical examples of such processes include flame spray synthesis, where combustion takes place in a flame, and spray pyrolysis, where combustion takes place on a hot substrate.

The term combustible liquid is known in the field and includes carboxylic acids (such as 2-ethylhexanoate ) , ethers (such as thf ) , amines (such as aniline or hexylamine) , aliphatic solvents (such as hexane, octane or dodecane) , aromatic solvents (such as xylene, toluene), alcohols (such as methanol, ethanol, propanol, butanol, pentanol, hexanol), esters (such as ethylacetate, butylacetate ) . The present invention will be better understood by reference to the figures. Fig. 1 shows a schematic illustration of typical concentration profiles simulated across 3 types of standard reference materials (assuming a spatial resolution of 1-5 μιχι) . The scale relations are not quantitative and only for relative comparison. The y-axis shows the signal ratio of trace element B (siderophile element) to trace element A (lithophile element), the x-axis a line scan through the cross section of said reference materials. The matrix elements are not visualized in the figure.

The signal on the left side represents the signal ratio of element B to element A in a reference material obtained according to the inventive method. As can be seen, the heterogeneity is very low and limited by the small particle size of below 50 nm. The relative standard deviation of the signal is below 5% (RSD indicated by dashed lines) and dominated by internal measurement errors. Accordingly, homogeneity is limited by the particle size which is below 100 nm and thus below the spatial resolution.

The signal in the center represents the signal ratio of element B to element A in a reference material obtained by mixed powders or precipitated materials (e.g. MACS carbonate standards). It can be seen that a high signal variation is caused by the large particle size which is well above a typical beam sizes of 1-5 μπι.

The signal on the right side represents the signal ratio of element B to element A in a reference material obtained by flash-cooled glassy materials (e.g. NIST 6xx SRMs). It can be seen that rim depletion and element segregation/accumulation is present which is caused by highly enhanced diffusion processes and the insufficient cooling rates during synthesis. This is within the typical beam size of modern micro-beam techniques .

In more general terms, in a first aspect, the invention relates to a method for manufacturing a solid shaped article; said shaped article comprising one or more, preferably two or more, trace elements embedded in a matrix, said trace elements each having a concentration homogeneity characterized by a concentration based relative standard deviation ("RSD") of less than 5%, preferably less than 2.5%, much preferably less than 1% ; said method comprising the steps of (a) providing a single solution comprising a predetermined amount of soluble precursors of trace element (s) and soluble precursors of a matrix, (b) subjecting this solution to a pyrolytic process to obtain a solid material in powder form, (c) compacting the obtained powder into a shaped article.

This aspect of the invention shall be explained in further detail below.

The inventive manufacturing method provides shaped articles where trace elements are homogeneously distributed at the sub- micrometer level (< 1 micrometer) within a solid matrix. The inventive method thus may be used in the synthesis of reference materials, particularly reference materials suitable for micro and nanobeam techniques. The inventive method also provides access to a broad range of elemental compositions, unmatched homogeneity and tunable concentrations in a broad variety of matrices. Accordingly, the inventive manufacturing method is suited (i) to provide an alternative route for manufacturing known shaped articles and (ii) to provide a new route for manufacturing novel shaped articles.

[Shaped article:] Broadly speaking, the shaped article may be of any type; it is basically limited by step (c) . In an advantageous embodiment, the shaped article is selected from the group consisting of pellets, beads, tablets, wavers. Further, the shaped article may be adapted to accommodate specific receiving sections of an analytical instrument. Such embodiment improves compliance.

[trace element M(t) :] Trace elements may be present in oxidation state +/-0 (i.e. as element or alloy) or in positive oxidation state (i.e. in the form of a chemical compound, such as a salt, oxide) . It is understood that one or more types / classes of trace elements may be present in the shaped article. It is a clear advantage of the inventive manufacturing method to provide a broad range of specific compositions of trace elements, specifically adapted to the users need, within one shaped article. In an advantageous embodiment, each trace element is present in an amount of 10 - 1000 ppm.

In a further advantageous embodiment, the trace element is selected from the group consisting of main group metals, transition metals, rare earth metals (REE) and actinides. Main group metals include Lithium, Beryllium, Sodium, Magnesium, Potassium, Calcium, Rubidium, Strontium, Caesium, Barium, Francium, Radium, Aluminum, Gallium, Indium, Thallium, Germanium, Tin, Lead, Arsenic, Antimony, Selenium, Bismuth, Tellurium, Polonium. Transition metals include Scandium, Yttrium, Lanthanum, Actinium, Titanium, Zirconium, Hafnium, Vanadium, Niobium, Tantalum, Chromium, Molybdenum, Tungsten, Manganese, Rhenium, Iron, Ruthenium, Osmium, Cobalt, Rhodium, Iridium Nickel, Palladium, Platinum, Copper, Silver, Gold, Zinc, Cadmium, Mercury. Rare earth metals (REE) include Lanthanum, Cerium, Praseodymium, Neodymium, Promethium, Samarium, Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, Lutetium. Actinides include Actinium, Thorium, Protactinium, Uranium, Neptunium, Plutonium, Americium, Curium, Berkelium, Californium, Einsteinium, Fermium, Mendelevium, Nobelium, Lawrencium.

In a further advantageous embodiment, the trace element is selected from the group consisting of noble metals, particularly Ag, Au, Pt, Pd, Rh, Ru.

In a further advantageous embodiment, the trace element is selected from the group consisting of siderophile elements, particularly Mn, Fe, Co, Ni, Mo, Ru, Rh, Pd, Re, Os, Ir, Pt, Au. In a further advantageous embodiment, the trace element is selected from the group consisting of chalcophile elements, particularly Cu, Zn, Ga, Ge, As, Se, S, Ag, Cd, In, Sn, Sb, Te, Hg, Tl, Pb, Bi, Po; very particularly Cu, Zn, Ga, Ge, As, Se, Ag, Cd, In, Sn, Sb, Te, Hg, Tl, Pb, Bi, Po.

In a further advantageous embodiment, the trace element is selected from the group consisting of lithophile elements, particularly Li, Be, Na, Mg, K, Ca, Sc, Ti, V, Cr, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, W, B, Al , Si, P, 0, F, Cl, Br, I, At, REE, very particularly Li, Be, Na, Mg, K, Ca, Sc, Ti, V, Cr, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, , Al, Si, REE.

In a further advantageous embodiment, the trace element is selected from the group consisting of isotopes, particularly enriched isotopes, e.g. ²⁰⁶Pb, ²⁰⁷Pb, ²³⁵U, ²³⁹Pu.

In a further advantageous embodiment, a plurality of trace elements, e.g. two or more trace elements, preferably 6 or more tace elements, such as 6 - 24 trace elements are present simultaneously in one shaped article. Advantageously, said plurality of trace elements is selected from one of the above groups. The presence of a multitude of trace elements in one single shaped article facilitates analytical methods, such as micro and nanobeam techniques .

[Matrix:] It is apparent from the above, that the matrix is of inorganic material. In an advantageous embodiment, matrix is selected from the group consisting of metal oxides, metal salts, metals or alloys. In a further advantageous embodiment, the matrix is selected from the group consisting of single metal oxides, particularly Fe₂0₃, Si0₂, ZnO, Ti0₂, A1₂0₃.

In a further advantageous embodiment, the matrix is selected from the group consisting of mixed metal oxides, particularly, Al_xSiO_y, Ca_xSiO_y, ZrSi0 , Perovskites (e.g. BaTi0₃, SrTi0₃) .

In a further advantageous embodiment, the matrix is selected from the group consisting of metal carbonates, particularly Calcium carbonate.

In a further advantageous embodiment, the matrix is selected from the group consisting of metal sulphates, particularly CaS0₄, MgS0₄, BaS0₄.

In a further advantageous embodiment, the matrix is selected from the group consisting of metal phosphates, particularly Calcium phosphates, LiFeP0 , LiMnP0 , LiCoP0₄.

In a further advantageous embodiment, the matrix is selected from the group consisting of metal chlorides, particularly NaCl, MgCl₂, LiCl .

In a further advantageous embodiment, the matrix is selected from the group consisting of metal fluorides, particularly CaF₂, MgF₂, NaF, KF, LiF, BaF₂.

In a further advantageous embodiment, the matrix is selected from the group consisting of metal sulfides, particularly, iron sulfide, zinc sulfide, copper sulfide, lead sulfide, nickel sulfide.

In a further advantageous embodiment, the matrix is selected from the group consisting of metal borates, particularly, LiMnB0₃, LiFeB0₃, LiCoB0₃, LiNiB0₃.

In a further advantageous embodiment, the matrix is selected from the group consisting of metals and alloys, particularly, copper, bismuth, nickel, copper-nickel alloys, nickel- molybdenum alloys, cobalt-nickel alloys.

[Homogeneity:] Homogeneity of elements, i.e. the special distribution of elements in a sample, may be measured by methods known per se. Independent from the analytical method, the concentrations and corresponding RSD are always independent of mass, as all elements are equally homogeneously - distributed in the sample. _n

12

The inventive reference materials may be identified and distinguished from known reference materials by the fact that the element concentration errors in the inventive materials are significantly lower than in known reference materials. For example, NIST 6xx standard reference materials have been analyzed with respect to trace element concentration errors by different authors. For example Jochum et al . or Gao et al. (Geostandards newsletter - the journal of geostandards and geoanalysis, Vol. 26-2, 2002) showed that RSD values well above 10% can be found for numerous trace elements in NIST 6xx standard reference materials when analyzed by liquid ICP-MS at sample sizes above 10 mg .

Advantageously, homogeneity is measured at a sample size of lOmg from said shaped article by liquid ICP-MS.

Accordingly, a homogeneity measurement for identifying an inventive shaped article may be performed as follows:

1. Analytical technique/device: liquid ICP-MS

2. Sample size: lOmg @ 5 replicate digestions each. In case the shaped article has a weight below the above given, more than one shaped article has to be used.

3. Elements to measure: all trace elements of the shaped article .

4. Relevant measure: The RSD is < 5%. This value is obtained for all trace elements of the shaped article.

Without being bound by theory, it is believed that pyrolytic processes (step b) are particularly suited for the inventive method. As element segregation denotes a problem related to chemical equilibrium, pyrolytic processes (particularly FSP) are suited to overcome the equilibrium problem of element segregation. Pyrolytic processes are also suited to combine chemically incompatible elements, as the synthesis takes place on a time scale of milliseconds. In contrast to flash-cooled glass melts (cooling rate > seconds), in flame spray synthesis a gaseous mixture of elements (2000°C; ideal homogeneity) is condensed into solid particles (<100°C) in milliseconds. This very high cooling rate leads to a highly homogeneous combination of different elements and also especially chemically incompatible elements (see incompatibilities by Goldschmidt classification) . At this short time scales thermodynamic segregation is inhibited or limited to the particle size (< 50nm) . The inventive method thus provides shaped articles exhibiting high homogeneity at very low spatial resolutions (e.g. below 1 pm) .

[Step (a):] As outlined above, one single solution comprising both, one or more trace element precursor (s) and one or more matrix precursor(s) are provided to the pyrolysis step (b) . Although it is known to supply suspensions or multiple feeds to a pyrolysis process, such alternatives typically result in materials, having lower homogeneity and are thus less preferred.

In an advantageous embodiment, the feedstock for step (b) is a single solution comprising all precursor materials (trace element precursor (s) and matrix precursor ( s ) ) dissolved in a solvent or mixture of solvents. It was also found that such solution is particularly suitable if comprising at least 50 wt% of a solvent selected from the group of combustible liquid (s) . A combustible liquid, as used herein, is a liquid or a mixture of liquids which is organic and self-combusting when ignited. Suitable solvents therefore include carboxylic acids (such as 2-ethylhexanoic acid, acetic acid, propionic acid) , ethers (such as thf ) , aliphatic solvents (such as hexane, octane or dodecane), aromatic solvents (such as xylene, toluene) , alcohols (such as methanol, ethanol, propanol, butanol, pentanol, hexanol or higher alcohols) , esters (such as ethylacetate , butylacetate ) and mixtures thereof. Accordingly, the solvent may also contain minor amounts of non-combustible solvents, such as water, as long as the mixture in total is self-combusting.

Metal organic precursors suitable for process step b are known per se. The selection of precursors depends on its solubility and potential incompatibilities preventing formation of a solution. In many cases, carboxylates (such as 2- ethylhexanoate or acetate) are suitable metal organic precursors for matrix elements and trace elements. Additional metal organic precursors for both matrix and trace elements include chlorides, acetylacetonates , alkoxides (such as isopropoxides , butoxides, ethoxides) or nitrates. [Step (b) : ] Pyrolysis processes are known and widely used in industry and research, see e.g. Madler et al. (J. Aerosol Sci., 33, 369, 2002) and Grass et al. (J. Mater. Chem. , 16, 1825, 2006) which are incorporated by reference. Such processes are capable to produce solid materials in powder form, typically in the form of nanoparticles, in a controlled way with precise and complex elemental compositions. Such Pyrolysis processes particularly include flame spray pyrolysis (FSP) . In this process step (b) , the selected precursors (and thus the corresponding trace elements) are mixed on an atomic level (perfect homogeneity) in a liquid precursor solution (as outlined above) which is transferred into a powder, typically nanoparticles, with a high micro-homogeneity. The latter is only limited by the particle size, which is typically below 100 nanometers.

[Step (c) :] The nanoparticles of step (b) are used as building blocks which are compacted to obtain a solid reference material in the form of a shaped article, such as a pellet. In one embodiment, such compacting step c) comprises compressing of the powder obtained in step b) . Compressing typically takes place in a standard equipment powder press applying a pressure of 1 - 10000 MPa, preferably 10 - 1000 MPa for a period of 0.1 sec - 1 hr at temperatures between 10 °C up to one quarter of the melting temperature or glass transition temperature of the matrix material in degree Celsius.

In one further embodiment, such compacting step comprises compressing of the powder obtained in step b) followed by sintering. Sintering typically takes place by heating the compacted pellet to a temperature which corresponds to 2/3 of the melting temperature (or glass transition temperature) of the matrix material for a period between 1 sec and 10 hrs, preferably 1 min and 30 min.

[Pellet manufacturing:] In a preferred embodiment, the invention provides a method for manufacturing a pellet; said pellet comprising one or more trace element M(t) as defined herein and an inorganic matrix material as defined herein; each of said trace elements M(t) show a homogeneity as defined herein; each of said trace elements being present in an amount of 10 - 1000 ppm, said method comprising the steps of (a) , (b) and (c) as defined herein. These pellets are suitable as reference material in analytical methods.

[isotope selective reference materials:] In a further embodiment, the present invention also provides for the production of isotope selective reference materials. The inventive manufacturing method further solves the problem on how to provide compositions that are thermodynamically labile. Certain compositions are only accessible by pyrolytic processes, since the cooling time is much shorter than e.g. for melt quenching methods. This is a clear advantage over the currently used glass-type reference materials which suffer from isotope fractionation due to segregation during the synthesis. The invention thus particularly provides a method as described herein wherein the trace elements are selected from the group consisting of e.g. ²⁰⁶Pb, ²⁰⁷Pb, ²³⁵U, ²³⁹Pu.

In a second aspect, the invention relates to a shaped article containing (i.e. comprising or consisting of) a matrix and one or more trace elements embedded therein, with improved homogeneity.

This aspect of the invention shall be explained in further detail below.

[Homogenity : ] In one embodiment, the inventive shaped article shows an improved homogeneity, as outlined above; particularly low rim depletion and low element segregation. [Matrix, trace elements:] In one embodiment, the inventive shaped article comprises a matrix as outlined above.

In an advantageous embodiment, the matrix of the inventive article is as outlined above, but excluding carbonates. In an advantageous embodiment, the matrix of the inventive article is as outlined above, but excluding the group of carbonates .

In an advantageous embodiment, the matrix of the inventive article is selected from the group of sulfides.

In a advantageous embodiment, the matrix of the inventive article is selected from the group of silicates.

In one embodiment, the inventive shaped article comprises trace elements as outlined above. Advantageously, the amount of trace elements in the inventive article is in the range of 10 - 1000 ppm, preferably 30 - 1000 ppm.

In an advantageous embodiment, the trace elements (s) of the inventive article are selected from Pt group metals.

In a further advantageous embodiment, the trace elements (s) of the inventive article are selected from the group of enriched isotopes, e.g. ²⁰⁶Pb, ²⁰⁷Pb, ²³⁵U, ²³⁹Pu.

In an advantageous embodiment, the inventive article contains a matrix selected from the group of metal sulfides and trace elements selected from the group of noble metals and rare earth metals.

In a further advantageous embodiment, the inventive article contains a matrix selected from the group of iron sulfide (FeS) and trace elements selected from the group of Ag, Au, Pt, Pd, Rh, and Ru.

In a further advantageous embodiment, the inventive shaped article contains a matrix selected from the group of iron sulfide (FeS) and trace elements selected from the group of Ag, Au, Pt, Pd, Rh, Ru, Ir, In, Mn, Tl, Th, U, Nb, and Bi .

In a further advantageous embodiment, the inventive shaped article contains a matrix selected from the group of iron sulfide (FeS) and trace elements selected from the group of Ag, Au, Pt, Pd, Rh, Ru, Ir, In, Mn, Tl, Th, U, Nb, Bi, and Hg . In a further advantageous embodiment, the inventive shaped article contains a matrix consisting of a multi elemental mixed oxides where the cation is selected from Fe, Si, Mg, S, Ni, Ca, Al, Mn, Na, Cr and trace elements selected from the group of Cu, Zn, Co, P, and Se.

In a further advantageous embodiment, the inventive shaped article contains a matrix selected from the group of a Si02 and trace elements selected from the group of siderophile and chalcophile elements.

In a further advantageous embodiment, the trace element (s) and matrix of the inventive article are as outlined above, but excluding CaLa2S4 as a matrix and Si as a trace element.

In a further advantageous embodiment, the trace element (s) and matrix of the inventive article are as outlined above, but excluding ZnO as a matrix and Sb, Bi, Co, Mn, Cr, B and Al as trace elements.

[Shaped article:] In one embodiment, the shaped article is selected from the group consisting of pellets, beads, tablets, wavers as outlined above. Further, the shaped article may be adapted to accommodate specific receiving sections of an analytical instrument.

[Starting Materials:] In selected embodiments, the starting materials, i.e. powders obtained according to step b, are novel and thus subject of the present invention. The invention therefore also provides for a material in powder form containing, particularly consisting of, a matrix and trace elements as described herein and having a homogeneity as defined herein (RSD < 5% for all trace elements when analysed by liquid ICP-MS) . In an advantageous embodiment, the matrix is selected from the group of silica. In a further advantageous embodiment, the trace elements are selected from the group of platinum group elements.

[Product-by-process:] In a further embodiment, the invention relates to a shaped article obtainable or obtained according to a method as described herein.

[Calibration set:] In a further embodiment, the invention provides a kit, particularly a kit adapted for an analytical device, said kit comprising at least one shaped article as desribed herein.

In an advantageous embodiment, said kit contains two or more shaped articles whereby one shaped article is free of trace elements (blank) , the other shaped articles each contain the same trace element (s) in concentrations differing from each other. For example, a kit contains three silica pellets having a concentration of Au corresponding to (i) 0 ppm (blank) , (ii) 50 ppm Au and (iii) 500 ppm Au .

In a further advantageous embodiment, said kit contains two or more shaped articles whereby one shaped article is free of trace elements (blank) , the other shaped articles each contain trace element (s) in the same concentration but of different type. For example, a kit contains three silica pellets having a (i) no trace element (blank) , (ii) 50ppm Ag, 50ppm Au, 50ppm Pt, 50ppm Pd, 50ppm Rh, and 50ppm Ru, (iii) 500ppm Ag, 500ppm Au, 500ppm Pt, 500ppm Pd, 500ppm Rh, and 500ppm Ru, .

In a further advantageous embodiment, the invention provides a kit, said kit contains to or more shaped articles and uncompacted nanopowder, said shaped articles are as outlined above, said uncompacted nanopowder (i.e. a powder material having a primary particle size between 5 - 100 nm) having the same elemental composition as in the shaped articles and having a primary particle size below 100 nm. Such kit is particularly suitable for calibrating and running an analytical device. For example, such kit contains three silica pellets having a concentration of Au corresponding to 0 ppm (blank) , 50 ppm Au and 500 ppm Au and three types of nanopowder each with a primary particle size below 100 nm, a silica matrix and Au corresponding to (i) 0 ppm (blank) , (ii) 50 ppm Au and (iii) 500 ppm Au .

In a third aspect, the invention relates to a method for determining [quantitatively] trace element concentrations in an analyte, said method comprising the steps of (a) quantitatively measuring the trace element concentrations of a reference material as defined herein by analyzing non- compacted nanopowder with the same elemental composition as in the shaped article by the use of a liquid analytical method; (b) measuring the signals of trace elements on a shaped article as described herein by an analytical micro- or nanobeam technique in order to set calibration points; (c) measuring the signals of trace elements on an analyte sample (d) comparing measuring signal of the analyte sample and calibrating points to obtain quantitative trace element concentrations of the analyte sample.

This aspect of the invention shall be explained in further detail below:

As used herein, the term determining quantitatively a trace element includes quantitative analysis of said trace element. In an advantageous embodiment, the analyte is selected from naturally occurring minerals or synthetic materials.

Generally speaking, any analytical method making use of reference materials is applicable. In an advantageous embodiment, the analytical method is selected from the group consisting of micro- and nanobeam techniques.

In a further embodiment, the invention relates to the use of a shaped article (particularly a pellet) as described herein as reference material in an analytical micro- or nanobeam technique .

Advantageously, said method being selected from the group consisting of non-destructive Micro-beam techniques, particularly XRF, EDX, DX and destructive techniques, particularly laser-ablation ICP-MS, SIMS, EPMA) .

In a further embodiment, the invention relates to an analytical method for determining quantitatively trace element concentrations in an analytical sample, comprising the steps of (a) measuring the quantitave trace element concentrations on non-compacted reference materials (nanopowders) by a liquid analytic method (b) measuring the signals of trace elements by a micro- or nanobeam technique on a shaped article as described herein (pellet with the same elemental composition as in the nanopowder) in order to set calibration points; (c) measuring the signals of trace elements on an analytical sample (d) comparing measuring signal of the analytical sample and calibrating points to obtain quantitative trace element concentrations of the analytical sample. In an advantageous embodiment, the analyte is selected from the group of ore minerals, such as metal sulfides. In an advantageous embodiment, the method is selected from the group consisting of non-destructive Micro-beam techniques as outlined above and destructive techniques, as outlined above.

To further illustrate the invention, the following examples are provided. These examples are provided with no intent to limit the scope of the invention.

[General procedure:] All starting materials used were of high purity quality. Nanoparticles were synthesized by flame spray synthesis using a metal organic precursor containing all the elements of the final reference material. Such metal organic precursors were filtered through a 0.45 urn PTFE syringe filter and fed (5 ml min^"1, HNP Mikrosysteme, micro annular gear pump mzr-2900) to a spray nozzle, dispersed by oxygen (7 1 min^"1, PanGas tech.) and ignited by a premixed methane-oxygen flame (CH₄: 1.2 1 min^-1, 0₂: 2.2 1 min^"1).

In example 4 (FeS), the spray nozzle was placed in a glove box with a nitrogen atmosphere and an oxygen partial pressure below 100 ppm. In example 4 the precursor flow rate was 6 ml min^"1 and the dispersing oxygen flow rate was 5 1 min^"1. This results in reducing conditions and in the formation of iron sulfide, instead of iron oxide.

For all the examples, the off-gas was filtered through a glass fiber filter ( Schleicher&Schuell ) by a vacuum pump (Busch, Seco SV1040CV) at about 20 m³ h^"1. The obtained nanopowder was collected from the glass fiber filter. The obtained nanopowder is directly pressed into a pellet with 12 mm diameter at a pressure of 5 tons. The obtained pellets are directly used as reference materials in micro-beam techniques.

[Example 1:] A carbonate standard reference material was prepared with the nominal composition below. For the preparation of the precursor, corresponding portions of the following metal organic solutions were used as indicated below. The final precursor was diluted with xylene 2:1 by weight in order to reduce the precursor viscosity.

The obtained nanopowder was pressed into a pellet with 12 mm diameter at 5 tons.

For the determination of the trace element homogeneity and the concentrations of the PGEs in the calcium carbonate material, the pressed pellet was crushed. Subsequently, amounts of 10 mg (5 replicates) were weighed and dissolved by conventional digestion methods using diluted Aqua regia mixtures. The Aqua regia was prepared by mixing 30% Trace element grade HC1 and sub boiled HN03 (3:1). This solution was diluted in water (1:10). Several times during the digestion procedures, the vials were placed in an ultrasonic bath in order to disaggregate the crushed material and render it more susceptible to acid attack. Holmium was added as a recovery standard. 100 yL of the resulting solution was diluted with 5 % aqua regia (1:20 dilution in water) to 10 g. Thereby, an overall dilution factor of 1:100000 was obtained. For internal standardization, 100 μΐ, standard solution containing 500 ]ig/q each of Ir and Tb was added to the sample solution before it was made up to the final volume.

For external calibration in SN-ICPMS, commercially available standard solutions were used for all elements to be determined. These standards were mixed and equivalent masses PGEs and Ca were added to simulate the same matrix as present in the Calcium carbonate. The concentration ranges were Ca= 0- 20000 pg/g, Ag, Au, Rh, Ru, Pd, Pt, Ho= 0-20 g/g. Finally, concentrations were recorded using a quadrupole-based Perkin Elmer Elan 6000 ICPMS. Following Isotopes used for quantification: Ca, Ru, ¹⁰³Rh, ¹⁰⁷Ag, ¹⁰⁵Pd, ¹⁶⁵Ho, ¹⁹⁵Pt and well as Tb and Ir for internal standardization

These results show that at a sample size of lOmg, the relative standard deviation is below 5% indicating a very high micro- homogeneity of the elemental distribution.

[Example 2:] A glass standard reference material was prepared with the nominal composition given below. For the preparation of the precursor, corresponding portions of the following metal organic solutions were used as indicated below.

The obtained nanopowder was pressed into a pellet with diameter at 5 tons.

LA-ICP-MS measurements showed highly homogeneous element distributions for all noble metals at nominal trace element concentrations of 500 ppm. This corroborates the successful and homogeneous doping of noble metals into a glass matrix at noble metal concentrations well above the noble metal concentrations in NIST 6xx standard reference materials. target Analyte ion (used Wt%-oxide stdev RSD (wt%) for quantication) (measured) (wt%) (%)

Si02 68 29Si+ 66.17 2.32 3.51

CaO 11 44Ca+ 11.89 0.22 1.89

Na20 14 23Na+ 15.16 0.41 2.70

A1203 2 27A1+ 2.25 0.04 1.74

Fe203 1.7 57Fe+ 1.76 0.04 2.40

Ti02 2 48Ti+ 1.95 0.04 1.93

MgO 1 24Mg+ 1.33 0.05 3.99 target ppm Metal stdev RSD (ppm) (measured) (ppm) (%)

Ag 500 107Ag+ 427 12.8 2.99

Au 500 197Au+ 460 13.9 3.03

Pt 500 195Pt+ 491 11.2 2.28

Rh 500 103Rh+ 530 13.7 2.59

Pd 500 105Pd+ 455 11.9 2.63 not quatifyable with NIST 610, but well

Ru 500 101RU+ present in the sample

LA-ICP-MS measurement on a synthetically synthesized noble metal doped glass reference material. Laser: Pit size 120 micron; 5 pits for each element, GeoLasC (193nm) ; Reprate 10 Hz; Scanrate 25 um/s; Fluence 21 J/cm²; LDHCLA-Cell. ICP: Elan DRC+; Aux 0.8 L/min; Neb 0.75 L/min; plasma 17.5 L/min; power 1360 W.

[Example 3:] A phosphate standard reference material was prepared with the nominal composition given below. For the preparation of the precursor, corresponding portions of the following metal organic solutions were used as indicated below. The final precursor was diluted with tetrahydrof ran (thf) 2:1 in order to reduce the precursor viscosity.

The obtained nanopowder was pressed into a pellet with 12 diameter at 5 tons.

LA-ICP-MS measurements showed comparable results to example [Example 4:] A sulfide standard reference material was prepared with the nominal composition below. For the preparation of the precursor, corresponding portions of the following metal organic solutions were used as indicated below. The final precursor was diluted with THF 2:1 by weight in order to reduce the precursor viscosity.

The obtained nanopowder was pressed into a pellet with 12 mm diameter at 5 tons. For the determination of the trace element homogeneity and the concentrations of the PGEs in the sulfide material, the pressed pellet was crushed. Subsequently, amounts of 10 mg (5 replicates) were weighed and dissolved by conventional digestion methods. For external calibration in SN-ICPMS, commercially available standard solutions were used for all elements to be determined. Finally, concentrations were recorded using a quadrupole-based Perkin Elmer Elan 6000 ICPMS .

Claims

A method for manufacturing a solid shaped article, said shaped article comprising two or more trace elements embedded in a matrix;

said trace elements being present in a concentration range of 10 - 1000 ppm;

said trace elements each having a concentration

homogeneity characterized by a concentration based

relative standard deviation ("RSD") of less than 5%, said method comprising the steps of:

a) providing a single solution comprising (i) a defined

amount of soluble precursors of said one or more trace element (s) (ii) a defined amount of soluble precursors of said matrix and (iii) solvents comprising at least 50 wt% of a solvent selected from the group of combustible liquid ( s ) ,

b) subjecting this solution to a pyrolytic process to

obtain a solid material in powder form,

c) compacting the obtained powder into a shaped article.

The method of claim 1, wherein said shaped article is selected from the group consisting of pellets, tablets, and wavers.

The method of claim 1 or 2, wherein the RSD is measured with a 10 mg sample by liquid mass spectroscopy,

preferably liquid ICP-MS.

The method according to any of the preceding claims, wherein in said method

« the solution of step a) comprises at least 50 wt% of a solvent selected from the group of combustible liquids and / or

• the pyrolytic process of step b) is a flame spray

pyrolysis process and / or

* the compacting step c) comprises compressing of the

powder obtained in step b) , followed by sintering.

5. The method according to any of the preceding claims,

wherein said trace element is selected from the group • main group metals,

• transition metals,

• rare earth metals and

• actinides.

The method according to any of the preceding claims, wherein said trace element is selected from the group consisting of

β noble metals, particularly Ru, Rh, Pd, Pt, Ag, Au;

• siderophile elements, particularly Mn, Fe, Co, Ni, Mo, Ru, Rh, Pd, Re, Os, Ir, Pt, Au;

• chalcophile elements, particularly Cu, Zn, Ga, Ge, As, Se, Ag, Cd, In, Sn, Sb, Te, Hg, Tl, Pb, Bi, Po;

• lithophile elements, particularly Li, Be, Na, Mg, K, Ca, Sc, Ti, V, Cr, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, W, Al, Si, REE, actinides; and

» isotopes, particularly enriched isotopes, e.g. ²⁰⁶Pb,

2⁰⁷Pb, ²³⁵U, ²³⁹Pu.

The method according to any of the preceding claims, wherein said matrix is selected from the group consisting of metal oxides, metal salts, metals, and alloys.

The method according to any of the preceding claims, wherein said matrix is selected from the group consisting of

• single metal oxides, particularly ZnO, Fe20₃, Si0₂,

AI2O3, T1O₂, manganese oxide;

» mixed metal oxides, particularly, ZrSi0₄, Ca₃Si0₅,

aluminosilicates , BaTi0₃, SrTi0₃;

• Metal sulfides, particularly, iron sulfide, zinc

sulfide, copper sulfide, lead sulfide, nickel sulfide;

• Metal borates, particularly, LiMnB0₃, LiFeB0₃, LiCoB0₃, LiNiB0₃;

• alkaline and earth-alkaline metal carbonates,

particularly CaC0₃, SrC0₃;

• alkaline and earth-alkaline metal sulfates, particularly CaS0₄;

• alkaline and earth-alkaline metal phosphates,

particularly calcium phosphates, LiFeP0₄, LiMnP0 ;

• alkaline and earth-alkaline metal chlorides,

particularly NaCl; and « alkaline and earth-alkaline metal fluorides,

particularly CaF₂, MgF₂, LiF, NaF, KF, BaF₂.

A shaped article containing a matrix and two or more trace elements embdedded therein, wherein

said trace element is selected from the group consisting of

• noble metals, particularly Ru, Rh, Pd, Pt, Ag, Au;

« lithophile elements, particularly Li, Be, Na, Mg, K, Ca, Sc, Ti, V, Cr, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, W, Al, Si, REE, actinides; and

« isotopes, particularly enriched isotopes, e.g. ²⁰⁶Pb, 2⁰⁷Pb, ²³⁵U, ²³⁹Pu; and said matrix is selected from the group consisting of

• mixed metal oxides, particularly, ZrSi0₄, Ca₃SiC>5,

aluminosilicates , BaTiC>3, SrTi0₃;

• Metal sulfides, particularly, iron sulfide, zinc

sulfide, copper sulfide, lead sulfide, nickel sulfide;

« Metal borates, particularly, LiMnB0₃, LiFeB0₃, LiCoB0₃, LiNiB0₃;

• alkaline and earth-alkaline metal sulphates,

particularly CaSC>4;

« alkaline and earth-alkaline metal phosphates,

particularly calcium phosphates, LiFeP0₄, LiMnPC>4;

• alkaline and earth-alkaline metal chlorides,

particularly NaCl; and

• alkaline and earth-alkaline metal fluorides,

particularly CaF₂, MgF₂ , LiF, NaF, KF, BaF₂; and each of said trace elements being homogeneously

distributed within said matrix characterized by a RSD < 5%.

A shaped article obtainable or obtained according to a method of any of claims 1 - 8.

11. Use of a shaped article according to claim 9 or 10 as a reference material in an analytical method.

12. The use according to claim 11 wherein said analytical

method is selected from the group consisting of

• non-destructive micro-beam techniques, particularly X- ray fluorescence, EDX, WDX and

• destructive techniques, particularly laser-ablation ICP- MS, SIMS, EPMA.

13. A method for determining trace element concentrations in an analyte, comprising the steps of

a) measuring quantitative element concentrations in non- compacted reference materials by liquid elemental analysis ;

b) calibrating a measuring device suitable for micro- or nanobeam techniques with one or more shaped articles as defined in any of claims 9 - 10 to obtain one or more calibrating points;

c) analyzing a corresponding analyte with said device to obtain a measuring signal;

d) comparing measuring signal and calibrating points to obtain a quantitative result for the analyte.

14. The method of claim 14 wherein the analyte is selected

from the group consisting of ore minerals.

15. The method of claim 13 or 14, said micro- or nanobeam

technique being selected from the group consisting of

• non-destructive techniques, particularly XRF, EDX, WDX; and

« destructive techniques, particularly LA-ICP-MS, SIMS, EPMA.