《ISO-GUIDE-32-1997.pdf》由会员分享，可在线阅读，更多相关《ISO-GUIDE-32-1997.pdf（12页珍藏版）》请在三一文库上搜索。
1、STD-IS0 GUIDE 32-ENGL 1997 4851903 0735093 979 GUIDE 32 Calibration in analytical chemistry and use of certified reference ma ter i a Is This material is reproduced from IS0 documents under International Organization for Standardization (ISO) Copyright License number IHSIICC11996. Not for resale. No
2、 part of these IS0 documents may be reproduced in any form, electronic retrieval system or otherwise, except as allowed in the copyright law of the country of use, or with the prior written consent of IS0 (Case postale 56,121 i Geneva 20, Switzerland, Fax +41 22 734 10 79), IHS or the IS0 Licensors
3、members. First edition 1997 -,-,- STD-IS0 GUIDE 32-ENGL 1997 9851703 0735094 805 IS0 GUIDE 32:1997(E) Foreword IS0 (the International Organization for Standardization) and IEC (the International Electrotechnical Commission) form the specialized system for worldwide standardization. National bodies t
4、hat are members of IS0 or IEC participate in the development of International Standards through technical committees established by the respective organization to deal with particular fields of technical activity. IS0 and IEC technical committees collaborate in fields of mutual interest. Other inter
5、national organizations, governmental and non-governmental, in liaison with IS0 and IEC, also take part in the work. Draft Guides adopted by the responsible Committee or Group are circu- lated to national bodies for voting. Publication as a Guide requires approval by at least 75 % of the national bod
6、ies casting a vote. IS0 Guide 32 was prepared by the IS0 Committee on Reference Materials (REMCO). O IS0 1997 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and m
7、icrofilm, without permission in writing from the publisher. International Organization for Standardization Case postale 56 CH-1 21 1 Genve 20 Switzerland internet central Q iso.ch X.400 c=ch; a400net; p=iso; o=isocs; s=central Printed in Switzerland ii -,-,- STDmISO GUIDE 32-ENGL 1997 H 4853703 0735
8、075 743 W 0 IS0 IS0 GUIDE 32:1997(E) Introduction Within the framework of the International Laboratory Accreditation Conference (ILAC), held in Torino (Italy) in October 1990, a one-day technical seminar was devoted to Metrology in testing where the various aspects of metrological traceability in di
9、fferent fields of testing were addressed. In order to clarify the implementation of the principles of metrological traceability in chemistry, it was then requested that an ILAC Guide be established on Calibration in analytical chemistry and use o f certified reference materiais. A draft was establis
10、hed which, after amendments by an ILAC Working Group, was approved by the 1992 ILAC Conference in Ottawa (Canada). Moreover, due to a worldwide need for such information, it was further recommended to transmit this draft to ISO/REMCO to serve as basis for development of an IS0 Guide. This proposal w
11、as accepted by ISO/REMCO. Finalization of this Guide was carried out by ISO/REMCO Task Group 2, Calibration. iii -,-,- Calibration in analytical chemstry and use of certified reference materials 1 Scope Quality assurance in a testing laboratory, particularly in the case of its assessment (see ISO/IE
12、C Guide 25), highlights the need to consider closely the question of the accuracy of its measurements and analytical results, and to ensure that the principles necessary to establish demonstrated accuracy have not been omitted. The calibration of the parameters associated with chemical analyses and
13、material testing deserves particular attention, because major errors can be made by neglecting or ignoring the basic principles of metrology which also apply to these areas. This Guide identifies a number of general recommendations for those who, either in laboratories or as assessors, are faced wit
14、h this problem. 2 References ISO/IEC Guide 25:1990, General requirements for the competence of calibration and testing laboratores. IS0 Guide 30:1992, Terms and definitions used in connection with reference materials. IS0 Guide 31 :1981, Contents of certificates of reference materials. IS0 Guide 33:
15、1989, Uses of certified reference materials. IS0 Guide 34:1996, Qualify system guidelines for the production of reference materials. IS0 Guide 35:-, Certification of reference materials - General and statistical principles. VIM:1993, International vocabulary of basic and general terms in metrology.
16、BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML. Guide to the expression of uncertainty in measurement. 1 st edition, 1993 (corrected and reprinted 1995). 3 Basic considerations Any measurement, particularly any quantitative chemical analysis, shall employ reference elements to ensure demonstrated traceabi
17、lity to the relevant basic quantities. This is an essential condition for the accuracy of the results. The metrological quality of the calibration performed depends on: To be published. (Revision of IS0 Guide 35:1989) -,-,- STD-IS0 GUIDE 32-ENGL L977 ir851703 0735077 514 IS0 GUIDE 32:1997(E) IS0 o t
18、he uncertainty of the reference used (set of calibration masses, titrated solutions, gas mixtures, composition CRMS, etc.), the appropriateness (or fitness for purpose) of this reference under the practical conditions of use, also taking into account the analytical method used and the tested samples
19、. The uncertainty of calibration results from these two components, and it has to be optimized without ignoring either of them. The analyst should compare the uncertainty of calibration with the required uncertainty of the analysis (which, normally, should be agreed between the customer and the oper
20、ator). This comparison provides a useful guide for choosing between different available calibration procedures and, in the longer term, for improvements to the methods and procedures. In tests based on measurements of physical quantities, the principle of traceability of the standards and/or measuri
21、ng instruments of the accredited laboratory to a national primary standard through a national calibration system, is generally applied. Relevant principles for ensuring the traceability of chemical analyses are presented later in this document: the use of CRMs for that purpose has been gaining impor
22、tance in the last decades and may be expected to develop even more, if and when they are available. 4 Selection of calibration procedures i n chemical analysis 4.1 Types of analytical procedures The first step consists in classifying the analytical procedure used into one of the following categories
23、 : 0 Type I 0 Type II 0 Type III each of these categories being associated with : a basic principle a number of basic prerequisites. When the user classifies a method, it should be done by means of a detailed and close examination of all the parameters of the analytical procedure. He/she must never
24、be satisfied with simplifications which would only be applicable to the detection principle applied under idealized conditions. This approach generally results in underestimating the necessary conditions for a reliable calibration and of generating systematic errors. Calibration does not make an ina
25、ccurate method true (e.g. presence of major interferences). The variability of the influencing factors should only cause a negligible variation in the analytical signal. * Remarks: The definitions of RMs and CRMs can be found in IS0 Guide 30. RMs can also be used to validate methods (see IS0 Guide 3
26、3). They may also be used to check the drift with time, and possibly to correct an instrumental drift. They also serve as a basis for a conventional scale (e.g. octane index). These aspects of lhe use of RMs are not dealt with here, apart from a few remarks, and the reader can refer to IS0 Guide 33.
27、 One can refer also to more general documentation, such as the VIM (international vocabulary of basic and general terms in metrology). The analytical chemist is often a user of analytical materials or reagents. These products shall not be confused with CRMs. In fact, a CRM corresponds to an identifi
28、ed batch of material of which the certified characteristics have been determined with an optimised and defined accuracy. An analytical reagent is only characterised by a nominal value, determined with a large uncertainty. It is the users duty to observe all necessary precautions to ensure, when used
29、, that an analytical reagent meets hidher needs. 2 -,-,- STD-IS0 GUIDE 32-ENGL 1997 = 4853703 0735098 450 IS0 IS0 GUIDE 32:1997(E) The above classification is merely designed to identify the relevant calibration mode(s). It shall not be used as a scale of value of the methods. 4.1.1 Type I This meth
30、od type produces the anticipated result by performing a calculation defined on the basis of the laws governing the physical and chemical parameters involved, using measurements taken during the analysis, such as: 0 0 mass of the test sample, volume of titration reagent, mass of precipitate, volume o
31、f titration product generated. 4.1.2 Type II This method type compares the content of the sample to be analysed to a set of calibration samples of known content, using a detection system for which the response (ideally linear) is recognized in the relevant working area (without necessarily being cal
32、culable by theory). The value of the sample content is determined by interpolation of the sample signal with respect to the response curve of the calibration samples. This implies that any other difference in composition, form, etc., between the calibration set and the different samples analysed wil
33、l have no effect, or a negligible effect compared with the uncertainty, on the signal. For this condition to be satisfied, the analytical procedure could include: 0 means to reduce sensitivity to differences (e.9. spectral buffer, treatment of samples before analysis); 0 a procedure to give similar
34、form to the calibration set and the samples: - reduce the complex sample to a simpler sample, e.g. by acid digestion, the removal of major interferents, or selective extraction of analyte; - synthesize a more complex calibration set by multi-element matrix simulation or the use of a special medium (
35、e.9. oils). 0 limitation of the field of application. 4.1.3 Type 111 For this method type, the sample to be analysed is compared to a set of calibration samples, using a detection system which has to be recognised to be sensitive not only to the content of elements or molecules to be analysed, but a
36、lso to differences of matrix (of any type whatsoever). If this influence is ignored, it will generate a systematic error (bias). For this type of method to be really appropriate for use, it is essential: 0 to identify the type(s) of samples routinely analysed (type of matrix, type of structure, etc.
37、) and to draw up procedures to identify the introduction of “abnormal“ samples in comparison with the identified types, 0 to make up a set of CRMs suitable for each type of sample previously identified, to evaluate whether or not “intra-type differences are liable to generate an unacceptable bias in
38、 the analysis. 3 -,-,- STD-IS0 GUIDE 32-ENGL 3777 m 4853703 0735099 377 m IS0 GUIDE 32:1997(E) IS0 4.2 Other methods Any analytical method that fails to ensure traceability to the base units of the SI by one of these approaches is liable not to yield results of demonstrated uncertainty. Even if it o
39、ffers appreciable advantages of repeatibility and reproducibility, the results obtained are liable to be distorted by a systematic error. If such an analytical method is used by a single laboratory to analyse drift, or to transfer information within a restricted circle of users who are aware of the
40、limitations of the result, vigilance will be necessary to ensure that these results are not presented or used as accurate outside the circle. Assessors giving accreditation to such methods should take great care to check that the method is undertaken in such a way that appropriate accuracy is ensure
41、d through relevant procedures and means, and preferably that they are widely recognized as state-of-the-art methods. 5 Calibration procedures 5.1 Type I method The basic procedure is to identify every quantity whose measurement is necessary to establish the analytical result by calculation. It is re
42、commended that a “provisional list of uncertainties“ be drawn up which will evaluate the uncertainty of each measured quantity, keeping in mind the required uncertainty of the calibration. This will help to identify the main sources of uncertainty and to exercise special care in selecting the calibr
43、ation procedures. With this type of method, CRMs are used for validation (see IS0 Guide 33). Note that the CRM shall be analysed as presumed unknown, the result obtained being compared with the certified value. If an abnormal deviation is observed, the laboratory needs to identify the cause and corr
44、ect it. It is not recommended (except for very specific cases) to deduce a correction factor for the difference between the value found and the certified value. 5.2 Type II method For this type of method, the working standards generally consist of a determined quantity of analyte “diluted“ in a larg
45、er quantity of “diluent“. They are obtained by measuring the masses or volumes of the different pure, diluted and diluent materials. Depending on each case, metrological traceability implies the following. O Calibration of the mass measurements, by calibration or verification of the balances and/or
46、calibration of the volume measurement system. 0 Calibration of the system for measuring the correction parameters applied to the foregoing measurements (e.g. temperature, pressure, relative humidity). Since the uncertainties of these quantities are generally of the second order with respect to the t
47、otal analytical uncertainty, a simplified calibration procedure is often adequate. O Knowledge of the purity of the basic materials used, together with their uncertainties. For the substance which is diluted, it is necescary to ensure that : O O O the stoichiometry is correct. it is the compound of
48、interest, the nature of the impurities is identified (e.g. inorganics in an organic substance), 4 -,-,- STD-IS0 GUIDE 32-ENGL 1997 W 4851903 0735300 937 = 0 IS0 IS0 GUIDE 32:1997(E) For “diluents“, particular attention shall be paid to the residual level of impurities such as: 0 0 0 the substance to
49、 be diluted, any substance exhibiting a similar analytical response, any substance likely to react with the substance being diluted. For practical or economic reasons, laboratories may decide to use commercial standard solutions. If so, it is important to make sure that the uncertainty on their content is known, as require