ISO 6868:2026

This document specifies three methods for quantitative measurement of crystalline silica (CS) major polymorphs (quartz and cristobalite) mass percentage content in bulk samples using X-ray powder diffraction (XRPD). This document also provides general information about the capabilities and limitations of relevance to laboratories working for routine testing. Only X-ray diffractometers with Bragg-Brentano geometry are considered. XRPD techniques are used to characterize specimens in the form of loose powders, where the median grain size is between 1 μm and 10 μm physical diameter. Block specimens are not considered. Although a number of methods of analysis are considered in this document, other XRPD methods of analysis can be considered if they are demonstrated to give equivalent results.
\nInternational
\nStandard
\nISO 6868
\nFirst edition
\nWorkplace air — Quantitative
\n2026-08
\ndetermination of quartz and
\ncristobalite in bulk materials by
\nX-ray powder diffraction methods
\nAir des lieux de travail — Détermination quantitative du quartz
\net de la cristobalite dans les matériaux en vrac par des méthodes
\nde diffraction des rayons X sur poudre
\nReference number
\n© ISO 2026
\nAll rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
\nbe reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
\nthe internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
\nor ISO’s member body in the country of the requester.
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\nPublished in Switzerland
\nii
\nContents Page
\nForeword .v
\nIntroduction .vi
\n1 Scope . 1
\n2 Normative references . 1
\n3\t Terms\tand\tdefinitions . 1
\n4 Principle . 2
\n5 Equipment . 3
\n5.1 Diffraction system .3
\n5.1.1 Diffractometer .3
\n5.1.2 Specimen holders .3
\n5.2 Balance .3
\n5.3 Reference materials .3
\n5.3.1 Quartz and cristobalite .3
\n5.3.2 Internal standard . .4
\n5.3.3 XRD drift correction disc .4
\n5.4 Equipment for the preparation of specimens of infinite thickness .4
\n5.4.1 Laboratory drying oven .4
\n5.4.2 Masonry hammer, jaw crusher or similar device .4
\n5.4.3 Laboratory mill .4
\n5.4.4 Other equipment . .5
\n5.5 Equipment for the preparation of thin thickness specimens .5
\n5.5.1 Filters for analysis .5
\n5.5.2 Other equipment . .6
\n5.6 Equipment for sample treatment and reagents .7
\n5.6.1 Furnace .7
\n5.6.2 Reagents .7
\n6 Sampling . 8
\n7 Specimen preparation . . 8
\n7.1 General .8
\n7.2 Crushing .10
\n7.3 Grinding .10
\n7.4 Drying . 12
\n7.5 Pre-concentration and removal of interfering phases . 12
\n7.6 Weighing and mounting of a specimen of infinite thickness . . 12
\n7.7 Weighing and deposition of thin layer specimen on filter .14
\n8 Analysis . .15
\n8.1 Symbols . 15
\n8.2 Parameters of a diffraction line profile . 15
\n8.3 Instrument conditions .16
\n8.4 Analytical methods .17
\n8.4.1 Choice of the analytical method .17
\n8.4.2 Calibration specimens, reference diffraction patterns and reference values .17
\n8.4.3 Quartz and cristobalite identification: Preliminary qualitative analysis .18
\n8.4.4 Quantitative analysis by the external standard method .19
\n8.4.5 Quantitative analysis by the internal standard method .21
\n8.4.6 Quantitative analysis by the spiking method .24
\n9 Performance characteristics of the methods .25
\n9.1 Limits of detection and quantification . 25
\n9.2 Uncertainty of measurement . 26
\n9.3 Equivalency of the methods and between laboratory precision . 26
\n10 Test report .26
\n
\niii
\n10.1 Minimum report requirements . 26
\n10.2 Data to be archived by the laboratory .27
\nAnnex A (informative) Expected line positions and relative intensities of quartz and cristobalite .28
\nAnnex B (informative) Interferences on quartz and cristobalite diffraction patterns .30
\nAnnex C (informative) Sample treatment strategies .34
\nAnnex D (informative) Examples of analyses .36
\nAnnex E (informative) Examples of equivalency of the methods and between laboratory
\nprecision . 51
\nBibliography .53
\n
\niv
\nForeword
\nISO (the International Organization for Standardization) is a worldwide federation of national standards
\nbodies (ISO member bodies). The work of preparing International Standards is normally carried out through
\nISO technical committees. Each member body interested in a subject for which a technical committee
\nhas been established has the right to be represented on that committee. International organizations,
\ngovernmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
\nwith the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
\nThe procedures used to develop this document and those intended for its further maintenance are described
\nin the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
\nof ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
\nISO/IEC Directives, Part 2 (see www.iso.org/directives).
\nISO draws attention to the possibility that the implementation of this document may involve the use of
\npatents. ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
\nin respect thereof. As of the date of publication of this document, ISO had not received notice of patents
\nwhich may be required to implement this document. However, implementers are cautioned that this
\nmay not represent the latest information, which may be obtained from the patent database available at
\nwww.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
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\nThis document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 2, Workplace
\natmospheres.
\nAny feedback or questions on this document should be directed to the user’s national standards body. A
\ncomplete listing of these bodies can be found at www.iso.org/members.html.
\n
\nv
\nIntroduction
\nExposure to crystalline silica (CS) in the respirable fraction of inhaled airborne dust is a hazard to the health
\nof workers in many industries. For this document, CS refers to the two most common polymorphs, quartz
\nand cristobalite. To assess the exposure of an individual, personal samples of air are taken during a work
\nactivity, and then the amount of respirable CS is measured. In addition to the procedures for determining
\nthe airborne concentration of respirable CS in a workplace atmosphere, it can be useful, or required, to
\nevaluate the CS content in bulk materials that have a potential risk of being respirable when they are ground
\nor manipulated.
\nThis document is appropriate for the analysis of material selected to be representative of a product, or
\nsoil, rock, mineral, waste, depending on the nature of the investigation. Bulk materials are generally in
\nthe solid state but can also be made by watery mixtures of insoluble matter (slurries, muds). A variety of
\norigins, compositions, sizes and shapes, such as solid blocks, fragments, slabs, sand, powders and flours, can
\nrepresent bulk materials.
\nThe knowledge of CS content in natural rocks and soils is useful when, for example, assessing potential
\n[1]
\nexposure by inhalation of airborne dust meeting the respirable convention of ISO 7708 in work activities
\nthat involve excavation, earth moving, and drilling plant operations. Elevated exposures to respirable CS
\nare often found in confined spaces, such as in mining and tunnelling. Knowing the mass fraction of CS in
\nconstruction materials and natural rocks or soils contributes to the characterization of potential sources of
\nairborne dust and allows planning of risk management measures in order to reduce potential exposures to
\nairborne dust.
\nThe methods in this document can also be used for the analysis of samples, which have been collected for
\nthe purpose of classification and labelling of chemicals according to the Globally Harmonized System (GHS),
\nwhen a CS phase is found as a “substance”, or in “mixtures” and “articles”, as defined in the GHS.
\nAs is necessary in any good routine type analysis, especially where the results can have legal implications,
\nit is useful to establish standard procedures for analysing the specimens. There are several methods
\nmentioned in the literature for the determination of quartz, and many procedures can be developed, but
\nno single procedure is applicable to all situations. Standardizing procedures for X-ray powder diffraction
\n(XRPD) analysis of bulk materials, which are applicable to all materials, is difficult because of the variability
\nof the matrices and their potential interferences. Therefore, the method chosen for the analysis should be
\ncarefully tested in the laboratory for the specific conditions.
\nThe problems of processing samples containing either quartz or cristobalite, or both is essentially the same
\nas processing samples of bulk materials in general, and the major problem is preparing a specimen that is
\nrepresentative of the bulk material. The potential accuracy obtainable from a sub-sample or specimen of
\nbulk material is related to the effective number of particles interrogated by the X-ray beam, which is related
\nto the distribution of particles sizes and their absorption coefficients.
\n
\nvi
\nInternational Standard ISO 6868:2026(en)
\nWorkplace air — Quantitative determination of quartz and
\ncristobalite in bulk materials by X-ray powder diffraction
\nmethods
\n1 Scope
\nThis document specifies three methods for quantitative measurement of crystalline silica (CS) major
\npolymorphs (quartz and cristobalite) mass percentage content in bulk samples using X-ray powder
\ndiffraction (XRPD). This document also provides general information about the capabilities and limitations
\nof relevance to laboratories working for routine testing.
\nOnly X-ray diffractometers with Bragg-Brentano geometry are considered. XRPD techniques are used to
\ncharacterize specimens in the form of loose powders, where the median grain size is between 1 μm and
\n10 μm physical diameter. Block specimens are not considered.
\nAlthough a number of methods of analysis are considered in this document, other XRPD methods of analysis
\ncan be considered if they are demonstrated to give equivalent results.
\n2 Normative references
\nThe following documents are referred to in the text in such a way that some or all of their content constitutes
\nrequirements of this document. For dated references, only the edition cited applies. For undated references,
\nthe latest edition of the referenced document (including any amendments) applies.
\nISO 14488, Particulate materials — Sampling and sample splitting for the determination of particulate
\nproperties
\nISO 18158, Workplace air — Terminology
\n3\t Terms\tand\tdefinitions
\nFor the purposes of this document, the terms and definitions given in ISO 18158 and the following apply.
\nISO and IEC maintain terminology databases for use in standardization at the following addresses:
\n— ISO Online browsing platform: available at https:// www .iso .org/ obp
\n— IEC Electropedia: available at https:// www .electropedia .org/
\n3.1
\nblock
\nself-adhering single piece of interconnected particles or crystallites
\n[2]
\n[SOURCE: EN 13925-1:2003, 3.2 ]
\n3.2
\nspecimen
\nportion of the sample in the specific form used in the diffraction instrument for a given data collection
\nprocess
\n[2]
\n[SOURCE: EN 13925-1:2003, 3.3 ]
\n
\n3.3
\nphase
\ncrystallographic phase
\nthermodynamic phase
\nportion of a physical system sharing a common molecular and inter-molecular structure irrespective of any
\nsubdivision by size distribution or shape
\n[2]
\n[SOURCE: EN 13925-1:2003, 3.4 ]
\n3.4
\ndiffraction line
\npeak
\nreflection
\nBragg reflection
\nregion of the diffraction pattern containing an intensity maximum and corresponding to diffraction from a
\nset of lattice planes
\n[2]
\n[SOURCE: EN 13925-1:2003, 3.5 ]
\n4 Principle
\nThe experimental technique used in this document to determine the diffraction pattern of a specimen is
\nthe “angular dispersive technique”, where the X-rays are monochromatic and the measurement is made
\nby scanning the diffraction angle. Diffractometers with Bragg-Brentano geometry working in reflection
\ngeometry are to be used with this document; transmission geometry is not considered.
\nTwo types of powder specimens are considered in relation to their thickness:
\n— specimens of “infinite thickness”;
\n— “thin layer” specimens deposited on filters.
\nNOTE “Infinite thickness” and “thin layer” are defined by the preparation steps in this document.
\nPhase identification is a preliminary step in every quantitative phase analysis and provides information
\nnecessary to make a decision regarding the selection of procedure for quantitative analysis. The identification
\nof both quartz and cristobalite in an unknown specimen by XRPD is based on visual or computer-assisted
\ncomparisons of portions of its X-ray powder pattern that should include at least two main peaks, compared
\nto the experimental patterns of quartz and cristobalite reference materials.
\nIn quantitative phase analysis, the percentages by mass of quartz and cristobalite are determined based
\non the integrated intensities of one or more individual diffraction lines of each phase. These intensities are
\ncompared to the corresponding values from calibration specimens. Three methods are considered:
\n— external standard method, for thin layer specimens deposited on filters (comparison of the response
\nobtained with the specimen to the response in a calibration line for quartz or cristobalite reference
\nmaterial, see 8.4.4);
\n— internal standard method, for specimens of infinite thickness (addition of a known amount of a crystalline
\nphase material that has been shown not present in the original sample, see 8.4.5);
\n— spiking method, also known as standard addition method, for specimens of infinite thickness and quartz
\ncontents below approximately 20 %, depending on the matrix (addition of a known amount of quartz or
\ncristobalite to the sample being studied, see 8.4.6).
\nMultiple methods applied to a single sample can be used to verify analyses. Whole powder pattern analysis
\nand standard free methods (e.g. the Rietveld method) are not considered in this document.
\n
\n5 Equipment
\n5.1 Diffraction system
\n5.1.1 Diffractometer
\nThis document has been prepared to be used with a diffractometer employing the Bragg-Brentano
\nparafocusing geometry, where the specimen and the receiving slit all lay on the focusing circle. The vertical
\nθ:θ or θ:2θ configurations are most advantageous for handling powder specimens. A diffractometer
\ngenerally comprises:
\n— goniometer;
\n— X-ray source that includes a line-voltage supply, a high-voltage generator and a X-ray tube; a conventional
\nX-ray sealed tube source is often used and the most commonly used tube anode material is copper;
\n— incident beam optics, that can include monochromatisation or filtering, collimation and focusing or
\nparallelism of the beam;
\n— specimen stage; the specimen is contained in a specimen holder, which is constructed to allow specimen
\nrotation around the appropriate axis;
\n— diffracted beam optics, that can include monochromatisation or filtering, collimation and focusing or
\nparallelism of the beam;
\n— detector;
\n— data collection system.
\nProcedures for instrument alignment and performance characterisation of Bragg-Brentano diffractometers
\n[3]
\nare given in EN 13925-3 .
\n5.1.2 Specimen holders
\nSeveral types of specimen holders can be used for diffraction analysis. The following standard circular
\nspecimen holders are recommended when the analytical methods described in this document are used.
\n— Specimen holder for loose powder with cavity mount; top, back or side loaded. For back-loading of powder,
\na specimen holder consists of a bottom plate, which supports the powder, and a ring. The diameter of the
\ncavity to be filled generally varies between 15 mm and 30 mm, and the powder thickness is a minimum
\nof 1 mm (for infinite thickness specimen).
\n— Specimen holder for 25-mm filter (for thin layer specimen), where the support base of the filter, or the
\nfilter itself, is a disc made of a metal (e.g. silver, zinc or aluminium).
\n5.2 Balance
\nA microbalance capable of weighing ±1 μg or better over the range 0 g to 5 g is required for the preparation
\nof specimens deposited on 25-mm filters (thin layer). For the preparation of specimens of infinite thickness,
\nby mixing of powders, the materials should be weighed in glass weighing bottles with lids; capacities of
\n5 ml to 15 ml are generally required, so that the total mass exceeds 5 g, therefore, the balance operational
\nrange should span up to 20 g, and an analytical sensitivity of 10 μg is sufficient. An electrostatic eliminator
\nis needed when weighing.
\n5.3 Reference materials
\n5.3.1 Quartz and cristobalite
\nPure quartz and cristobalite reference materials with a median size of particles in the range 1 μm to 10 μm,
\npreferably smaller than 5 μm, and similar to the median size of particles in the specimens, are to be used.
\n
\nReference materials can be prepared in the laboratory from pure quartz and cristobalite crystals, if the
\ncorrect particle size is achieved and phase purity is characterized.
\nNOTE United States National Institute of Science and Technology (NIST) have developed Standard Reference
\nMaterials (SRM’s) for respirable quartz (1878 series) and respirable cristobalite (1879 series) with certified phase
\n1)
\npurity.
\n5.3.2 Internal standard
\nFor the internal standard method, the chosen mineral phase should have a fully resolved diffraction line,
\nnot overlapping any lines of the CS phases, and free from microabsorption (i.e. be of very small particle size
\nor have an absorption similar to the specimen) and extinction. Hence, it is essential that internal standard
\npowder crystallite size be in the range 1 μm to 10 μm, preferably smaller than 5 μm, and similar to the
\nmedian size of particles in the specimens. Internal standard, quartz and cristobalite reference materials are
\ngenerally prepared in the laboratory from pure crystals milled until the optimum size range is reached. A
\nsufficient amount of each reference material should be prepared, tested, stored and added to the specimens
\nfor analyses.
\nNOTE 1 The definition of “fully resolved” line is beyond the scope of this document. The experience of the analyst
\ncan guide in assessing and addressing this aspect of the analysis.
\nNOTE 2 Grain size can be measured by using one of the techniques described in 7.3 NOTE 4, and purity characterized
\nby XRPD.
\nNOTE 3 Corundum is a mineral phase often used as an internal standard. NIST SRM 676a is a corundum structure-
\n2)
\nalumina powder intended primarily for use as an internal standard for quantitative determinations by XRPD.
\n5.3.3 XRD drift correction disc
\nAn aluminium or silicon polycrystalline disc or another suitable robust material, should be used to correct
\nfor the drift in XRD radiation intensity over time.
\n3)
\nNOTE The sintered alumina disc NIST SRM 1976b is frequently used.
\n5.4\t Equipment\tfor\tthe\tpreparation\tof\tspecimens\tof\tinfinite\tthickness
\n5.4.1 Laboratory drying oven
\nFor ovens of the forced-draft type, the air circulation should not be so strong that any transport of particles
\ncan take place.
\n5.4.2 Masonry hammer, jaw crusher or similar device
\nInitial processing is often needed to reduce the material to the required size for milling. Fractionation (e.g.
\nuse of a masonry hammer or jaw crusher) of the material into smaller pieces can be performed dry, although
\nprocessing involving a grinding rather than a crushing action should be performed ‘wet’ to reduce the
\npotential for a deterioration of crystalline structure.
\n5.4.3 Laboratory mill
\nSeveral types of mechanical grinding devices, driven by a motor, can produce the required grinding action by
\nfreely moving milling pieces (i.e. balls and rods) in a cylindrical grinding chamber completely encapsulated.
\n1) This information is given for the convenience of users of this document and does not constitute an endorsement by
\nISO of the products named. Equivalent products may be used if they can be shown to lead to the same results.
\n2) This information is given for the convenience of users of this document and does not constitute an endorsement by
\nISO of the product named. Equivalent products may be used if they can be shown to lead to the same results.
\n3) This information is given for the convenience of users of this document and does not constitute an endorsement by
\nISO of the product named. Equivalent products may be used if they can be shown to lead to the same results.
\n
\nThe device rotates at a given speed, and size reduction is achieved by impact and shear between the milling
\npieces, whose effectiveness is improved by vibration. Chamber and milling pieces made of the same material
\nshould be used. Hard materials such as tungsten carbide, silicon nitride, zirconium oxide and corundum are
\nrecommended, while agate should be avoided because of potential contamination. Grinding chambers can
\ntypically have volumes between 80 ml and 500 ml, and the material filling can generally range between 15 %
\nand 50 % of the volume (the instruction of the manufacturer should be followed). A so-called “micronizing”
\nmill can also be used. This mill generally uses corundum cylindrical grinding elements in a small airtight
\npolypropylene jar that handle a sample volume of 4 ml (or 10 g). Wet grinding (water, 2-propanol or
\ncyclohexane) is the preferred grinding method for a micronizing mill.
\nNOTE The minimum quantity of material that can be ground is an important parameter. For example, if a 250 ml
\ngrinding chamber is used, a minimum filling of 30 ml is generally suggested. This minimum volume corresponds to a
\n-3
\nmass of approximately 40 g for a fine sand sample of average density (2,6 g cm ). Therefore, if the sample mass is less
\nthan 40 g, it cannot be ground in the described chamber.
\n5.4.4 Other equipment
\n5.4.4.1 Mortar and pestle for hand grinding and mixing. They are manufactured out of many different
\nmaterials, but hard mortar types such as boron carbide, are recommended, while agate can introduce
\ncontamination and should be avoided.
\n5.4.4.2 Preparation kit for mounting the powder in the specimen holder. The kit generally comprises a
\nspecimen “preparation table” onto which a specimen holder ring is clamped, a powder press block, and a
\nspatula to remove the surplus powder (see 7.6).
\n5.4.4.3 Stainless steel test sieves used to verify fragments size before grinding them in the laboratory
\nmill; a receiving pan is used to collect the passing material.
\nNOTE Fragment sizes of less than 10 mm can generally be milled, but a smaller initial size improves the result of
\ngrinding. The use of a test sieve with aperture size between 2,0 mm to 2,8 mm is suggested when a ball or rod mill is
\nused. If a “micronizing” mill is used, the sample can be passed through a 0,4 mm aperture sieve prior to grinding.
\n5.5 Equipment for the preparation of thin thickness specimens
\n5.5.1 Filters for analysis
\nFilters shall be of a 25 mm diameter. The filter types generally used for the XRPD analysis of CS, and their
\nadvantages and disadvantages, are listed in Table 1. These filter materials generally do not interfere with
\nthe measurement of the major reflections of quartz (101), (100), and (112), and cristobalite (101), (200 and
\n112), and (102). In any case, batches of filters should be tested to detect potential interferences, because
\nimpurities can be introduced during the filter manufacturing process. These tests are also useful to
\nverify the background fluctuation levels, since they have an effect on the readability of diffraction peaks,
\nworsening the limit of detection for CS. Silver filters exhibit the least variability and lowest background
\nlevels and thus are useful in situations where low limits of detection are required. However, cristobalite
\ncombined reflections (200 and 112) can sometimes be difficult to measure because they are located in the
\ntail at the side of silver reflection (100). Silver and mixed esters of cellulose filters are rigid and easy to
\nhandle when weighing and loading in the sample holder. PVC and especially polycarbonate filters are flexible
\nand require careful handling. A silver filter used for analysis allows an effective correction for absorption
\n(see 8.4.4.3). When an organic filter with high or medium transparency to X-ray radiation (see Table 1) is
\nused for analysis, an effective correction for absorption can still be carried out by measuring the intensity of
\na reflection of the underlying metallic (e.g. silver, zinc or aluminium) support base.
\n
\nTable\t1\t—\tExamples\tof\tfilters\tfor\tCS\tanalysis
\nBackground
\nMass absorption
\nInterfering peak fluctuation
\ncorrection
\nb
\nindex
\na
\nFilter material and pore size
\nQuartz Cristobalite Quartz
\nFilter transparency
\nc
\nindex
\n101 100 112 101 200 102 101 100
\nSilver (Ag) 0,45 μm no no no no yes no 1 1 1
\nd
\nPolyvinyl chloride (PVC) 5 μm no no no no yes no 1,5 1 0,72
\nMixed esters of cellulose (MCE)
\nd
\nno no no no yes no 2,4 1,7 0,78
\n0,8 μm
\nd
\nPolycarbonate (PC) 0,4 μm no no no no yes no 1,3 2,2 0,98
\nPolytetrafluoroethylene (PTFE)
\nd
\nno yes no yes yes no 2,5 ND 0,74
\n0,5 μm
\na
\nThe analyst should be aware that for any filter pore size the recovery will possibly not be complete.
\nb
\nBackground fluctuation index IF= /F where F and F are the standard deviations of XRD
\nBF Bf,,ilterB Ag BA, g Bf, ilter
\nintensities measured in the ranges of primary (Qz1) and secondary (Qz2) quartz peak profiles (Cu anode) on a blank silver filter,
\nand on a blank filter of the material under testing placed on a silver filter, respectively. I =1 is used as a reference for silver
\nBF
\nfilters.
\nc
\nFilter transparency index II= / I , where I and I are the XRD intensities measured in
\nFT filter,,Ag filterAg Ag filterAg,Ag filter,Ag
\nthe range of silver primary peak profile (Cu anode) on a blank silver filter, and on a blank filter of the material under testing
\nplaced on a silver filter, respectively. I =1 is used as a reference for silver filters.
\nFT
\nd
\nInterference is due to the silver filter placed under the filter of the material under testing.
\n5.5.2 Other equipment
\n5.5.2.1 Fume cupboard, to contain dusts, vapours and gases.
\n5.5.2.2 Beakers, latex gloves, tongs and powder spatulas.
\n5.5.2.3 Ultrasonic bath sonicator for breaking up the agglomerated particles of samples contained in
\nbeakers.
\n5.5.2.4 Magnetic stirrer, with thermally insulated top, to ensure homogeneous distribution of particles
\nin the 2-propanol suspensions contained in volumetric flasks.
\n5.5.2.5 Volumetric pipettes, for the desired aliquots.
\n5.5.2.6\t Volumetric\tflasks, with polyethylene stoppers for the preparation of 2-propanol suspensions of
\ncalibration powders.
\n5.5.2.7 Pump, to generate vacuum and rapidly filter the suspension.
\n5.5.2.8\t Vacuum\tfiltration\tassembly. A typical assembly with a side arm vacuum flask, with a 25-mm
\nfilter holder is shown in Figure 1. When in use, the flask is clamped to a ring stand to prevent it from falling
\nover, and is connected to the pump with vacuum tubing.
\n
\nDimensions in centimetres
\nKey
\n1 glass funnel
\n2 spring clamp
\n3 25-mm filter
\n4 vacuum base with sintered disc
\n5 rubber stopper
\n6 receiver flask
\n7 to vacuum pump
\nFigure\t1\t—\tExample\tof\ta\tfiltration\tapparatus\tsuitable\tfor\tpowder\tdeposition\ton\tfilters
\n5.6 Equipment for sample treatment and reagents
\n5.6.1 Furnace
\nA furnace capable of operating up to 1 000 °C can be required to remove interfering substances (see
\nAnnex C). The material is placed into platinum or glazed ceramic crucibles with lids.
\n5.6.2 Reagents
\n5.6.2.1 Deionised water.
\n5.6.2.2 2-propanol.
\n5.6.2.3 2 M HCl,of a volume fraction of 16,4 % (carbonate dissolution).
\n5.6.2.4 Glacial acetic acid, of a volume fraction of 10 % solution (carbonate dissolution).
\n
\n6 Sampling
\nThe sample from bulk material submitted for analysis should be as far as possible representative of the whole
\nmaterial. Sampling of particulate materials shall be performed in accordance with ISO 14488. Depending on
\nits size and physical state, this sample shall be stored in an appropriate sample bag or container, and an
\nidentification label will be attached allowing proper tracking to the source.
\nThe minimum quantity of bulk material to be collected should be defined before the sampling is performed.
\nContact the analytical laboratory to determine the quantity needed, based on discussion of the method
\nchosen for the analysis (see NOTE in 5.4.3) and the purpose of the results.
\nAs much information on the sample as possible should be provided to the analytical laboratory.
\nNOTE For example, it can be indicated how the sample was collected, what silica polymorphs and possible
\ninterference it can contain, how the sample was formed, including temperature and pressure if applicable. This
\ninformation can be based on previous analyses or a knowledge of the sample and process involved. Whether the
\nsample contains natural, unreacted raw materials or whether the sample has been processed (chemically, thermally
\nor otherwise), this information is important as it suggests the phases that will possibly be present (see Annex B).
\nIf the results have legal implications, standard procedures for collecting and processing, the samples can
\nbe required, and the desired minimum levels (fraction percentages) for quartz and cristobalite quantitative
\ndeterminations should be specified to the laboratory.
\n7 Specimen preparation
\n7.1 General
\nThe preparation of the specimen is one of the more critical steps in quantitative XRPD analysis, and the
\nsuccess of the analysis largely depends on the correct preparation for the specimen being analysed. In the
\nanalytical laboratory, the bulk sample is reduced to a loose powder and a specimen is prepared for analysis.
\nCare must be taken to ensure that the physical state and composition of the prepared specimen be as close
\nas possible to that of the original sample. Steps taken to homogenize and sub-sample the sample further for
\nanalysis should follow procedures in ISO 14488 and should be documented.
\nAlthough it is desirable to obtain the required information with the least amount of sample treatment, the
\nspecimen preparation can involve a number of steps such as:
\n— crushing a block of material;
\n— grinding the crushed material;
\n— treating the powder to remove interferences;
\n— drying the powder if it is wet;
\n— depositing the powder on a filter (thin layer specimen) or mounting it in a specimen holder (specimen of
\ninfinite thickness).
\nTwo types of specimen preparations are included in this document:
\n— specimen of “infinite” thickness, mounted in a specimen holder;
\n— “thin” layer specimen, deposited on a filter.
\nA specimen is said to have infinite thickness when its thickness is greater than the beam penetration depth,
\n[4]
\ni.e. according to EN 13925-2, when the diffracted intensity is at least 99,9 % of the maximum attainable by
\n
\nincreasing the specimen thickness. This infinite thickness, t , can be calculated (expressed in
\ninfinite
\ncentimetres) as shown in Formula (1):
\n34, 5\u001esin\u001c
\nt \u001f (1)
\ninfinite
\n\u001b\u001a\u001d\u001d\u001e
\nwhere
\n-3
\n\u001f
\n\u001e is the specimen density (g cm );
\n\u001f
\n\u001e is the weighted sum of the mass attenuation coefficients (or mass absorption coefficients,
\n2 -1
\ncm g ) of the constituent elements of the material;
\nθ is the X-ray diffraction angle (°).
\nThe formula is valid for a “solid” specimen and the effective thickness can be obtained by dividing t by
\ninfinite
\nthe granular volume of the specimen (generally >70 %, porosity <30 %) when minimized by good specimen
\npacking. In practice, the specimen thickness equals the thickness of the specimen holder (see 5.1.2) in which
\nit is mounted, that is always much greater than the beam penetration depth.
\nNOTE Hence, the effective infinite thickness of a specimen is dependent on porosity and the mass attenuation
\ncoefficients of the phases, since absorption affects the beam penetration depth. Figure 2 a) shows the range of possible
\npenetration depths of CuKα at diffraction angles from 20° to 60° 2θ in a wide range of μ' values, for specimens with
\n-3 2 -1
\n30 % porosity. For a specimen made of a pure quartz powder (ρ'= 2,65 g cm , μ'= 34 cm g ), an effective infinite
\nthickness t = 0,012 61 cm is calculated for the diffraction primary peak (θ = 13,33°) if a Cu anode is used.
\nThe deposition of a powder on a filter to produce a “thin” layer specimen, and its analysis by XRPD according
\nto an external standard method, is a procedure derived from the analysis of airborne dusts collected on
\n[5]
\nmembrane filters (e.g. see ISO 16258-2 ). This methodology involves the construction of a calibration
\ncurve by measuring the intensity of a diffraction peak from specimens of pure quartz/cristobalite with
\nknown masses deposited on filters. The analytical approach assumes that the diffracted intensity is a linear
\nfunction of the irradiated specimen mass, after a correction for the X-ray absorption (due to the powder
\nload thickness). This correction is calculated by measuring the transmittance of the specimen (see D.1.1.3)
\nand is more accurate when a small increase in powder load produces a great decrease of transmittance, i.e.
\nwhen the powder load is low. This document considers acceptable a load on the filter that produces a change
\nin the slope of the curve transmittance versus load (mg) lower than 15 %. The field of acceptable loads
\nfor specimens within a wide range of mass attenuation coefficients is shown in Figure 2 b), for deposition
\ndiameters of 1,5 cm and 2,0 cm.
\n[6]
\nAccording to AFNOR NF X 43-295 , the maximum load of powder on a 25-mm filter achievable without
\nany losses from the filter due to its manipulation should not exceed 10 mg, but this is greater than the
\nrecommended maximum mass for quantification [see Figure 2 b)].
\n
\na)\tEffective\tinfinite\tthickness\t(porosity\tin- b) Thin specimens, variation of transmittance calcu-
\ncluded) calculated for a 5 % quartz – 95 % al- lated for the same mixtures, when the diameter d of
\n2 -1
\nbite mixture (μ'=33 cm g ) and a 5 % quartz the\tdeposition\tarea\ton\tthe\tfilter\tis\t1,5\tcm\tor\t2,0\tcm
\n2 -1
\n– 95 % hematite (μ'=177 cm g ), for 2θ angles
\nfrom 20° to 60°
\nKey
\n°2θ diffraction angle
\net effective “infinite” thickness (μm)
\ninfinite
\nT transmittance
\nl powde
\n ...