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© ISO 2016 Iron ores — Wavelength dispersive X ray fluorescence spectrometers — Determination of precision Minerais de fer — Spectromètres à fluorescence à rayons X à longueur d’onde dispersive — Déte[.]
TECHNIC AL REPORT ISO/TR 18231 First edition 01 6-05-01 Iron ores — Wavelength dispersive X-ray fluorescence spectrometers — Determination of precision Minerais de fer — Spectromètres fluorescence rayons X longueur d’onde dispersive — Détermination de la précision Reference number ISO/TR 82 : 01 6(E) © ISO 01 ISO/TR 182 1:2 016(E) COPYRIGHT PROTECTED DOCUMENT © ISO 2016, Published in Switzerland All rights reserved Unless otherwise speci fied, no part of this publication may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior written permission Permission can be requested from either ISO at the address below or ISO’s member body in the country of the requester ISO copyright office Ch de Blandonnet • CP 401 CH-1214 Vernier, Geneva, Switzerland Tel +41 22 749 01 11 Fax +41 22 749 09 47 copyright@iso.org www.iso.org ii © ISO 2016 – All rights reserved ISO/TR 182 1:2 016(E) Contents Page Foreword iv Introduction v Scope Frequency of testing Counter tests 3.2 3 1 General Procedure 3 Assessment of results General 2 Procedure 3 Assessment of results Conductivity of the gas flow proportional counter window Pulse shift corrector 3 General 3 Procedure Spectrometer tests 4.1 General 4.2 Precision 4.3 4.4 4.2 General 4.2 Calculation of counting statistical error Test specimen 1 4.3 General 1 4.3 Sequential spectrometers 1 4.3 Simultaneous spectrometers 1 Instrumental conditions 1 4.4.1 General 1 4.4.2 Sequential spectrometers 4.4.3 Simultaneous spectrometers 4.5 Stability test 4.7 4.8 Carousel reproducibility test Mounting and loading reproducibility test 4.6 4.9 4.1 4.11 4.12 4.13 4.14 4.1 4.1 Counter resolution Specimen rotation test Comparison of sample holders Comparison of carousel positions Angular reproducibility Collimator reproducibility (for sequential spectrometers fitted with an interchangeable collimator) Detector changing reproducibility (for sequential spectrometers fitted with more than one detector) Crystal changing reproducibility Other tests Note on glass bead curvature Determination of the dead time and the maximum usable count rate of the equipment 15 General 5 Methods of determination of dead time 5.2 General 5.2 Recommended method for determining dead time Annex A (informative) Calculation of the coefficient of variation of duplicates Bibliography © ISO 01 – All rights reserved iii ISO/TR 182 1:2 016(E) Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies) The work of preparing International Standards is normally carried out through ISO technical committees Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part In particular the different approval criteria needed for the different types of ISO documents should be noted This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part (see www.iso.org/directives) Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights ISO shall not be held responsible for identifying any or all such patent rights Details of any patent rights identi fied during the development of the document will be in the Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents) Any trade name used in this document is information given for the convenience of users and does not constitute an endorsement For an explanation on the meaning of ISO speci fic terms and expressions related to conformity assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers to Trade (TB T) see the following URL: Foreword - Supplementary information The committee responsible for this document is ISO/TC 102 , Subcommittee SC , iv Chemical analysis Iron ore and direct reduced iron , © ISO 01 – All rights reserved ISO/TR 182 1:2 016(E) Introduction I f an X-ray f luorescence s p ec trometer is to b e used for precis e analyses , it needs to b e func tioning correc tly to s p eci fication, that is , the errors as sociated with the various func tions of the in s trument have to b e ver y s mal l I t is imp or tant therefore that the s p ec trometer b e tes ted to ens ure that it is indeed func tioning to deliver the required precis ion T he obj ec tive of this Technical Rep or t is to set o ut te s ts th at can be used to a s cer ta i n the e x te n t o f the e r ro r s a nd to s u g ge s t p ro c e du re s fo r the i r rec ti fication T hese tes ts are not used to ascer tain whether the ins trument is op erating op timal ly but to de te r m i ne whe the r the i n s tr u me n t i s c ap ab l e o f g i v i n g a p re s e l e c te d p re c i s io n © I S O – Al l ri gh ts re s e rve d v TECHNICAL REPORT ISO/TR 182 1:2 016(E) Iron ores — Wavelength dispersive X-ray fluorescence spectrometers — Determination of precision Scope This Technical Report describes methods of test that can be applied to wavelength dispersive X-ray f luorescence (WD-XRF) spectrometers to ensure that the spectrometers are functioning in a manner that allows precise analyses to be made T he te s ts o u tl i ne d a re de s i g ne d to me a s u re the e r ro r s a s s o c i ate d w i th the o p e ratio n o f ce r t a i n p a r ts o f the spectrometer They are not designed to check every part of the spectrometer but only those parts that may be the common sources of error It is a s s u me d th at the p e r fo r m a nc e of the i n s tr u me n t has b e en o p ti m i z e d ac co rd i n g to the i n s tr uc ti o n s a nd manufacturer’s instructions For all tests, the two-theta angle should be carefully set for the line being me a s u re d T he pulse he i ght w i ndo w s ho u ld be set acc o rd i n g to the m a nu fac tu re r ’s should have a broad setting which may also include the escape peak for gas proportional counters The instrument and detector gas environment should be as speci fied by the manufacturer, as should the power supply to the instrument NO TE W here no d i s ti n c ti o n has b e en m ade , it is a s s u me d th at a te s t is ap p l i c ab l e to b o th s e q ue n ti a l a nd s i mu l t a n e o u s s p e c tr o m e te r s Frequency of testing Testing is not required to be carried out with each batch of analyses The frequency of testing varies lists the suggested frequency with which each test should be carried out Where speci fic problems are encountered, more frequent testing may be required and de p e nd i n g o n the te s t i nvo l ve d Tab le re me d i atio n wo rk p e r fo r me d © I S O – Al l ri gh ts re s e rve d ISO/TR 182 1:2 016(E) Table — Suggested frequency of precision tests Frequency Monthly Test Resolution of the gas- flow proportional counter Resolution of the scintillation and sealed gas counters Operation of the pulse height shift corrector a Half yearly Yearly Conductivity of gas- flow proportional counter window General stability Collimator reproducibility Detector changing reproducibility Crystal changing reproducibility Angular reproducibility Carousel reproducibility Comparison of carousel positions Comparison of sample holders Sample loading and unloading a The position of the pulse height peak should also be checked after changing a bottle of detector gas since a variation in the methane content of the gas will change the position of the peak The frequencies with which the tests listed in Table are carried out are suggested on the basis that there have been no changes to the spectrometer If mechanical or electronic maintenance of a major nature is carried out, the appropriate tests should be made before the spectrometer is taken back into routine service Counter tests 3.1 Counter resolution 1.1 General 3.1.1.1 Theoretical resolution Impurities in the flow gas and contamination of the anode wire may cause gas flow proportional counters to gradually deteriorate, which will result in both a shift and a broadening of the energy distribution (pulse height) curve Similarly, scintillation counters and sealed gas counters may, for various reasons, exhibit the same gradual deterioration This can, ultimately, adversely affect the measurements Impurities in detector gas can be minimized by the use of gas filters RES) of a counter is related to its energy distribution curve, and is given by the measured W) expressed as a percentage of the maximum of the pulse amplitude distribution ( V) , using Formula (1) where the values of W and V are in terms of arbitrary units (which vary between instrument manufacturers) obtained from the X-axis (see Figure 1) : The resolution ( peak width at half height ( RES = W V × 100 (1) RESth) , using the full width at half height of a Gaussian distribution, can be The theoretical resolution ( calculated using the following formulae: RES th = 36 σ , (2) © ISO 01 – All rights reserved ISO/TR 182 1:2 016(E) σ (3) = n Expressed as a percentage relative to n , Formula (3) becomes: 100 σ ( in % ) = n where n (4) is the number of primary electrons per incident photon (gas counters) or number of photoelectrons collected by the first dynode of the photomultiplier tube (scintillation coun- ters), calculated using Formula (5): n= E V (5) x i Ex is the energy of the incident radiation, in kilo electron volts (keV); Vi is the effective ionization potential of Argon for a flow counter, in kilo electron volts (keV) = 0,026 Substituting Formula (5) into Formula (4), and Formula (4) into Formula (2) gives: RES th = 236 × 0, 026 Ex = 38, Ex (6) Hence, for Cu Kα (E = 8,04 keV), the theoretical resolution of an Ar gas counter is 13,5 % 3.1.1.2 Scintillation counter For a scintillation counter: RES th = 128 Ex (7) and for Cu Kα, the resolution should be approximately 45 % [1] 3.1.1.3 Practical resolution In practice, however, the measured resolution achieved (RESm) is given in Formula (8): RES m =k R (8) where k is a factor that varies with the design of the counter, phosphor efficiency (scintillation counters), diameter, cleanliness and composition of the anode wire (gas counters) For a well-designed and clean gas- flow proportional counter, k should be less than 1,15 Hence, for such a counter, RESm should be less than 15,6 % for Cu Kα radiation For the scintillation counter, this value should be less than 52 % © ISO 2016 – All rights reserved ISO/TR 182 1:2 016(E) 3.1.2 Procedure This test should be carried out on all counters used in the spectrometer Most modern instruments provide the facility to measure pulse height distributions and to print out the counter resolution and this facility should be used if available For sequential spectrometers, it is recommended that the test be carried out using either Cu Kα or Fe Kα radiation for both detectors However, if these lines are measured using only the scintillation counter in actual analysis, measure an X-ray line of a major element analysed with the gas proportional counter for testing If the spectrometer does not provide automatic functions to determine procedure should be used RESm then the following a) Select a sample containing the appropriate analyte and, using a lower level setting and the pulse height analyser (PHA) window set to “threshold” (no upper level), adjust the X-ray tube power to give a count rate of about × 10 cps (counts per second) b) Select a narrow pulse height window (2 % to % of the peak voltage V of Figure 1) and decrease the lower level setting until the count rate drops to essentially zero c) Increase the lower level stepwise, noting the count rate at each step, until the peak has been passed and the count rate drops again to a very low value Each step should be of the same width as the pulse height window width, i.e if the pulse height window width corresponds to 0,2 units, then each step of the lower level should be 0,2 units d) Plot the count rate obtained at each step against the lower level values An example is shown in Figure © ISO 2016 – All rights reserved ISO/TR 182 1:2 016(E) 4.10 Comparison of carousel positions To compare the combined radial and axial carousel positioning reproducibility, with the test specimen in the same sample holder, make 20 measurements in each carousel position If the coefficient of variation of all the results is excessive (see 2) , the measurements for the individual carousel positions should be inspected to determine which positions are giving excessively high or low results Where possible, errors arising from differences in carousel positions should be eliminated by adjustment of the carousel positions This adjustment will normally be carried out by the spectrometer manufacturer or a quali fied technician If empirical corrections are to be applied for each carousel position, the ratios should be determined for each wavelength where high precision is required This is necessary as the ratios can vary with collimator and may even vary with crystal and angle NOTE Radial misalignment of carousel positions (in addition to resulting in poor precision between positions) may result in increased background concentrations of trace elements In severe cases, elevated background concentrations are the result of the sample cup being measured as part of the analysis (giving an increase in the background concentration of sample cup constituents) 4.11 Angular reproducibility For the angular reproducibility test, the test specimen remains in the spectrometer but between each measurement, the 2θ angle is altered by 10° to either the high or low angle side alternately and then returned to its original value Twenty such measurements are required The results should be assessed as set out in If the measuring sequence involves angular changes between sample and monitor measurements, then high angular reproducibility is essential These measurements can also be made on a scanning channel in a simultaneous spectrometer 4.12 Collimator reproducibility (for sequential spectrometers fitted with an interchangeable collimator) In the collimator reproducibility test, the test specimen should remain in the spectrometer while alternate measurements are made for each collimator Twenty such sets of measurements are required for each collimator The results for each collimator should be assessed separately as set out in 4.13 Detector changing reproducibility (for sequential spectrometers fitted with more than one detector) In the detector changing reproducibility test, the test specimen should remain in the spectrometer while alternate measurements are made for each detector Twenty such sets of measurements are required The results for each detector should be assessed separately as set out in For some spectrometers, the goniometer also moves when the detector is changed For such spectrometers, consider the goniometer reproducibility when assessing the detector changing reproducibility test result 4.14 Crystal changing reproducibility In the crystal changing reproducibility test, the test specimen will remain in the spectrometer but between each measurement, the crystal is changed and then returned to its original position Twenty measurements are required The results should be assessed as set out in This test is also applicable to simultaneous instruments fitted with goniometer units which have programmable crystal changers Only one crystal needs to be checked when testing the crystal changing mechanism 14 © ISO 01 – All rights reserved