1、BSI Standards Publication Iron ores Wavelength dispersive X-ray fluorescence spectrometers Determination of precision PD ISO/TR 18231:2016National foreword This Published Document is the UK implementation of ISO/TR 18231:2016. The UK participation in its preparation was entrusted to Technical Commit
2、tee ISE/58, Iron ores. A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. The British Standards Institution 2016
3、. Published by BSI Standards Limited 2016 ISBN 978 0 580 80818 0 ICS 73.060.10 Compliance with a British Standard cannot confer immunity from legal obligations. This Published Document was published under the authority of the Standards Policy and Strategy Committee on 31 May 2016. Amendments/corrige
4、nda issued since publication Date Text affected PUBLISHED DOCUMENT PD ISO/TR 18231:2016 ISO 2016 Iron ores Wavelength dispersive X-ray fluorescence spectrometers Determination of precision Minerais de fer Spectromtres fluorescence rayons X longueur donde dispersive Dtermination de la prcision TECHNI
5、CAL REPORT ISO/TR 18231 Reference number ISO/TR 18231:2016(E) First edition 2016-05-01 PD ISO/TR 18231:2016 ISO/TR 18231:2016(E)ii ISO 2016 All rights reserved COPYRIGHT PROTECTED DOCUMENT ISO 2016, Published in Switzerland All rights reserved. Unless otherwise specified, no part of this publication
6、 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 ISOs member body in the country of
7、the requester. ISO copyright office Ch. de Blandonnet 8 CP 401 CH-1214 Vernier, Geneva, Switzerland Tel. +41 22 749 01 11 Fax +41 22 749 09 47 copyrightiso.org www.iso.org PD ISO/TR 18231:2016 ISO/TR 18231:2016(E)Foreword iv Introduction v 1 Scope . 1 2 Frequency of testing 1 3 Counter tests . 2 3.1
8、 Counter resolution . 2 3.1.1 General 2 3.1.2 Procedure . 4 3.1.3 Assessment of results 6 3.2 Conductivity of the gas flow proportional counter window 6 3.2.1 General 6 3.2.2 Procedure . 7 3.2.3 Assessment of results 7 3.3 Pulse shift corrector . 7 3.3.1 General 7 3.3.2 Procedure . 8 4 Spectrometer
9、tests 8 4.1 General . 8 4.2 Precision . 9 4.2.1 General 9 4.2.2 Calculation of counting statistical error .10 4.3 Test specimen 11 4.3.1 General.11 4.3.2 Sequential spectrometers .11 4.3.3 Simultaneous spectrometers .11 4.4 Instrumental conditions 11 4.4.1 General.11 4.4.2 Sequential spectrometers .
10、12 4.4.3 Simultaneous spectrometers .12 4.5 Stability test 12 4.6 Specimen rotation test 13 4.7 Carousel reproducibility test .13 4.8 Mounting and loading reproducibility test 13 4.9 Comparison of sample holders 13 4.10 Comparison of carousel positions .14 4.11 Angular reproducibility 14 4.12 Collim
11、ator reproducibility (for sequential spectrometers fitted with an interchangeable collimator) .14 4.13 Detector changing reproducibility (for sequential spectrometers fitted with more than one detector).14 4.14 Crystal changing reproducibility 14 4.15 Other tests 15 4.16 Note on glass bead curvature
12、 15 5 Determination of the dead time and the maximum usable count rate of the equipment .15 5.1 General 15 5.2 Methods of determination of dead time .16 5.2.1 General.16 5.2.2 Recommended method for determining dead time .17 Annex A (informative) Calculation of the coefficient of variation of duplic
13、ates 24 Bibliography .26 ISO 2016 All rights reserved iii Contents Page PD ISO/TR 18231:2016 ISO/TR 18231:2016(E) Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standard
14、s 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
15、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 1. In part
16、icular 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 2 (see www.iso.org/directives). Attention is drawn to the possibility that some of the elements of
17、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 identified during the development of the document will be in the Introduction and/or on the ISO list of patent declarations received (see w
18、ww.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 specific terms and expressions related to conformity assessment, as well as information about ISOs adherence to
19、 the WTO principles in the Technical Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information The committee responsible for this document is ISO/TC 102, Iron ore and direct reduced iron, Subcommittee SC 2, Chemical analysis.iv ISO 2016 All rights reserved PD ISO/TR 18231:2
20、016 ISO/TR 18231:2016(E) Introduction If an X-ray fluorescence spectrometer is to be used for precise analyses, it needs to be functioning correctly to specification, that is, the errors associated with the various functions of the instrument have to be very small. It is important therefore that the
21、 spectrometer be tested to ensure that it is indeed functioning to deliver the required precision. The objective of this Technical Report is to set out tests that can be used to ascertain the extent of the errors and to suggest procedures for their rectification. These tests are not used to ascertai
22、n whether the instrument is operating optimally but to determine whether the instrument is capable of giving a preselected precision. ISO 2016 All rights reserved v PD ISO/TR 18231:2016 Iron ores Wavelength dispersive X-ray fluorescence spectrometers Determination of precision 1 Scope This Technical
23、 Report describes methods of test that can be applied to wavelength dispersive X-ray fluorescence (WD-XRF) spectrometers to ensure that the spectrometers are functioning in a manner that allows precise analyses to be made. The tests outlined are designed to measure the errors associated with the ope
24、ration of certain parts of 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 assumed that the performance of the instrument has been optimized according to the manufacturers instructions. For all tests,
25、 the two-theta angle should be carefully set for the line being measured. The pulse height window should be set according to the manufacturers instructions and should have a broad setting which may also include the escape peak for gas proportional counters. The instrument and detector gas environmen
26、t should be as specified by the manufacturer, as should the power supply to the instrument. NOTE Where no distinction has been made, it is assumed that a test is applicable to both sequential and simultaneous spectrometers. 2 Frequency of testing Testing is not required to be carried out with each b
27、atch of analyses. The frequency of testing varies depending on the test involved. Table 1 lists the suggested frequency with which each test should be carried out. Where specific problems are encountered, more frequent testing may be required and remediation work performed. TECHNICAL REPORT ISO/TR 1
28、8231:2016(E) ISO 2016 All rights reserved 1 PD ISO/TR 18231:2016 ISO/TR 18231:2016(E) Table 1 Suggested frequency of precision tests Frequency Test Monthly Resolution of the gas-flow proportional counter Resolution of the scintillation and sealed gas counters Operation of the pulse height shift corr
29、ector a Half yearly Conductivity of gas-flow proportional counter window General stability Collimator reproducibility Detector changing reproducibility Crystal changing reproducibility Angular reproducibility Yearly Carousel reproducibility Comparison of carousel positions Comparison of sample holde
30、rs Sample loading and unloading aThe 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 1 are carried out are s
31、uggested 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. 3 Counter tests 3.1 Counter resolution 3.1.1 General 3
32、.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 se
33、aled 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. The resolution (RES) of a counter is related to its energy distribution curve, and is gi
34、ven by the measured peak width at half height (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): (1)
35、 The theoretical resolution (RES th ), using the full width at half height of a Gaussian distribution, can be calculated using the following formulae: = , (2)2 ISO 2016 All rights reserved PD ISO/TR 18231:2016 ISO/TR 18231:2016(E) = n (3) Expressed as a percentage relative to n, Formula (3) becomes:
36、 n (4) where n 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): = (5) E x is the energy of the incident radiation, in kilo electron vol
37、ts (keV); V i is the effective ionization potential of Argon for a flow counter, in kilo electron volts (keV) = 0,026 4. Substituting Formula (5) into Formula (4), and Formula (4) into Formula (2) gives: , , (6) Hence, for Cu K (E = 8,04 keV), the theoretical resolution of an Ar gas counter is 13,5
38、%. 3.1.1.2 Scintillation counter For a scintillation counter: = (7) and for Cu K, the resolution should be approximately 45 %. 1 3.1.1.3 Practical resolution In practice, however, the measured resolution achieved (RES m ) is given in Formula (8): m = (8) where k is a factor that varies with the desi
39、gn 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, RES mshould be less than 15,6 % for Cu K radiat
40、ion. For the scintillation counter, this value should be less than 52 %. ISO 2016 All rights reserved 3 PD ISO/TR 18231:2016 ISO/TR 18231:2016(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 he
41、ight 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 K or Fe K radiation for both detectors. However, if these lines are measured using only the scintilla
42、tion 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 RES mthen the following procedure should be used. a) Select a sample containing the appropriate anal
43、yte 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 2 10 4cps (counts per second). b) Select a narrow pulse height window (2 % to 4 % of the peak voltage V of Figure 1) and decr
44、ease 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 w
45、idth, 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 1.4 ISO 2016 All rights reserved PD ISO/TR 18231:2016 ISO/TR 18231:2
46、016(E) W AVB Key 1 half peak height 2 peak width at half peak height Figure 1 Intensity as a function of lower level setting (arbitrary energy units) displayed for Fe K radiation measured on a flow counter The measured counter resolution RES m(in %) is obtained from the plot as follows: (9) where B
47、and A are the lower level settings at the half height positions on either side of the peak; V is the lower level setting at the maximum of the pulse height distribution. The pulse height distribution should be determined at a count rate indicated by the instrument manufacturer, or just below 2 10 4c
48、ps (see Figure 1). When determining the pulse height distribution automatically, A, B, V and W are not usually displayed, but the graph, peak position and detector resolution are shown together with instrument specific data. ISO 2016 All rights reserved 5 PD ISO/TR 18231:2016 ISO/TR 18231:2016(E) 3.
49、1.3 Assessment of results 3.1.3.1 Gas flow proportional counter Since the resolution of a proportional counter depends on design, an absolute resolution value cannot be given but the value should be near to that given in Formula (6). The optimum resolution of a particular counter may be specified by the manufacturer or it may be found with use. When RES mincreases by a fact
