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1、 Reference numbers ISO 14891:2002(E) IDF 185:2002(E) ISO and IDF 2002 INTERNATIONAL STANDARD ISO 14891 IDF 185 First edition 2002-03-01 Milk and milk products Determination of nitrogen content Routine method using combustion according to the Dumas principle Lait et produits laitiers Dtermination de
2、la teneur en azote Mthode pratique par combustion selon le principe de Dumas ISO 14891:2002(E) IDF 185:2002(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces whic
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5、 IDF national committees. In the unlikely event that a problem relating to it is found, please inform the ISO Central Secretariat at the address given below. ISO and IDF 2002 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by
6、any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO or IDF at the respective address below. ISO copyright office International Dairy Federation Case postale 56 CH-1211 Geneva 20 41 Square Vergote B-1030 Brussels Tel. + 41 22 749 01
7、 11 Tel. + 32 2 733 98 88 Fax + 41 22 749 09 47 Fax + 32 2 733 04 13 E-mail copyrightiso.ch E-mail infofil-idf.org Web www.iso.ch Web www.fil-idf.org Printed in Switzerland ii ISO and IDF 2002 All rights reserved ISO 14891:2002(E) IDF 185:2002(E) ISO and IDF 2002 All rights reserved iii Contents Pag
8、e Forewordiv, vi 1 Scope1 2 Normative reference1 3 Term and definition .1 4 Principle1 5 Reagents.1 6 Apparatus.2 7 Sampling.3 8 Preparation of test sample3 8.1 General3 8.2 Liquid test samples.3 9 Procedure.4 9.1 General4 9.2 Test portion4 9.3 Control of oxygen demand.4 9.4 Calibration4 9.5 Determi
9、nation.4 9.6 Detection and integration .5 10 Calculation and expression of results.5 10.1 Calculation .5 10.2 Expression of results5 11 Precision.6 11.1 Interlaboratory test6 11.2 Repeatability 6 11.3 Reproducibility.7 12 Test report7 Annex A (informative) Flowchart for the basic design of a Dumas a
10、pparatus.8 Annex B (informative) Schemes of suitable types of Dumas apparatus.9 Annex C (informative) Equipment calibration11 Annex D (informative) Comparison of results using the Dumas method with those obtained by the Kjeldahl method.13 Bibliography14 ISO 14891:2002(E) IDF 185:2002(E) iv ISO and I
11、DF 2002 All rights reserved 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 intereste
12、d 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 Com
13、mission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the techni
14、cal committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of patent
15、rights. ISO shall not be held responsible for identifying any or all such patent rights. International Standard ISO 14891IDF 185 was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 5, Milk and milk products, and the International Dairy Federation (IDF), in collaboration wit
16、h AOAC International. It is being published jointly by ISO and IDF and separately by AOAC International. Annexes A, B, C and D are for information only. ISO 14891:2002(E) IDF 185:2002(E) ISO and IDF 2002 All rights reserved v Foreword IDF (the International Dairy Federation) is a worldwide federatio
17、n of the dairy sector with a National Committee in every member country. Every National Committee has the right to be represented on the IDF Standing Committees carrying out the technical work. IDF collaborates with ISO and AOAC International in the development of standard methods of analysis and sa
18、mpling for milk and milk products. Draft International Standards adopted by the Action Teams and Standing Committees are circulated to the National Committees for voting. Publication as an International Standard requires approval by at least 50 % of National Committees casting a vote. International
19、Standard ISO 14891/IDF 185 was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 5, Milk and milk products, and the International Dairy Federation (IDF), in collaboration with AOAC International. It is being published jointly by ISO and IDF and separately by AOAC Internationa
20、l. All work was carried out by the Joint ISO/IDF/AOAC Action Team, Nitrogen compounds, of the Standing Committee, Main compounds of milk, under the aegis of its project leader, Prof. H. Frister (DE). -,-,- INTERNATIONAL STANDARD ISO 14891:2002(E) IDF 185:2002(E) ISO and IDF 2002 All rights reserved
21、1 Milk and milk products Determination of nitrogen content Routine method using combustion according to the Dumas principle 1 Scope This International Standard specifies a routine method for the determination of the total nitrogen content of milk and milk products. 2 Normative reference The followin
22、g normative document contains provisions which, through reference in this text, constitute provisions of this International Standard. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this International St
23、andard are encouraged to investigate the possibility of applying the most recent edition of the normative document indicated below. For undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC maintain registers of currently valid International Sta
24、ndards. ISO 8968-1IDF 20-1, Milk Determination of nitrogen content Part 1: Kjeldahl method 3 Term and definition For the purposes of this International Standard, the following term and definition apply. 3.1 nitrogen content mass fraction of the total nitrogen determined by the procedure specified in
25、 this International Standard NOTE The nitrogen content is expressed as a percentage by mass. 4 Principle A test portion is heated to destruction in a combustion tube at high temperature (900 C to 1 200 C) in an oxygen atmosphere according to the Dumas principle. All interfering components are remove
26、d from the resulting gas mixture. The nitrogen compounds in the test portion are converted to molecular nitrogen, followed by quantitative determination using a thermal conductivity detector. The nitrogen content is calculated using a microprocessor. 5 Reagents Use only reagents of recognized analyt
27、ical grade, or reagents of equivalent purity as specified by instrument manufacturers. 5.1 Carrier gases: use one of the following. ISO 14891:2002(E) IDF 185:2002(E) 2 ISO and IDF 2002 All rights reserved 5.1.1 Carbon dioxide (CO2), purity 99,9 % (volume fraction) for samples with moderate nitrogen
28、contents; 99,995 % (volume fraction) for samples with low nitrogen contents. 5.1.2 Helium (He), purity 99,996 % (volume fraction). 5.2 Oxygen (O2), purity 99,995 % (volume fraction). 5.3 Absorbent for SO2, e.g. lead chromate to eliminate any sulfur compounds from the combustion products. 5.4 Copper
29、oxide platinum catalyst (as filling material for the post-combustion tube). Platinum catalyst (5 % of Pt on Al2O3) is blended with CuO at a ratio of 1 to 7, 8 or 9 parts. To prevent separation as result of the different bulk densities of the two materials, it is not recommended to prepare the mixtur
30、e before filling the tube. It is advisable to pour the platinum catalyst and copper oxide simultaneously into the post-combustion tube using a suitable funnel. 5.5 Silver wool. Silver wool should be desegregated before being inserted in the post-combustion or reduction tube. 5.6 Silica (quartz) or g
31、lass wool. 5.7 Copper or tungsten (wire, cuttings or powder), suitable for filling the reduction tube. To improve the precision of analytical results for low nitrogen contents, the use of copper or tungsten wire is recommended. 5.8 Phosphorus pentoxide (P2O5) or granulated magnesium perchlorate Mg(C
32、lO4)2, or another suitable support material, to fill the drying tubes. 5.9 Hollow corundum spheres or aluminium oxide pellets, suitable for filling the combustion tube. 5.10 Copper oxide (CuO), suitable as a filling material for the combustion tube. 5.11 Sodium hydroxide (NaOH), on a support materia
33、l. 5.12 Standard nitrogen compounds, e.g. aspartic acid (C4H7NO4), ethylenediaminetetraacetic acid (C10H16N2O8) (see annex C), or other suitable reference reagents with known constant nitrogen content (e.g. urea, tyrosine, phenylalanine); minimum assay should be 99 %. For calibration purposes (see 9
34、.4), use a standard nitrogen compound to prepare a standard series with a nitrogen content of 4 mg to 100 mg in equal steps of about 5 mg nitrogen. Prepare a second standard series with a nitrogen content of 10 mg to 200 mg in equal steps of about 10 mg nitrogen. 6 Apparatus Usual laboratory equipme
35、nt and, in particular, the following. 6.1 Analytical balance, capable of weighing to 0,000 1 g. 6.2 Drying oven, capable of maintaining a temperature of between 105 C and 110 C. 6.3 Crucibles, e.g. made of stainless steel or ceramic, or tin capsules, suitable for the Dumas apparatus used. NOTE 1 Sev
36、eral commercial instruments are provided with an automatic sampler. NOTE 2 Some solid samples (e.g. powders) can be pressed to form pellets. -,-,- ISO 14891:2002(E) IDF 185:2002(E) ISO and IDF 2002 All rights reserved 3 6.4 Dumas apparatus, with a thermal conductivity detector and a suitable device
37、for signal integration. Suitable types of Dumas apparatus available on the market operate according to the general scheme as given in Figure A.1, although they may be different in arrangement, both in composition and function. Flow diagrams of two available instruments are shown as examples in annex
38、 B (Figures B.1 and B.2). NOTE Instruments from the manufacturer Elementar Analysensysteme GmbH and LECO Instruments1) have been found to be suitable. To avoid leaks, O-rings used for sealing shall be slightly lubricated prior to installation using a high-vacuum grease. Experience has shown that it
39、is important to clean all pieces of quartz and glassware carefully and to remove fingerprints from the tubes using a suitable solvent (e.g. acetone) before inserting them in the furnace. 7 Sampling Sampling is not part of the method specified in this International Standard. A recommended sampling me
40、thod is given in ISO 707. It is important that the laboratory receive a sample which is truly representative and has not been damaged or changed during transport or storage. 8 Preparation of test sample 8.1 General Prepare the test sample in such a way that a homogeneous sample is obtained, which is
41、 representative of the product. The preparation of the test samples may be carried out as given in subclause 8.1 of ISO 6732:1985. NOTE In the future, however, the preparation of the test sample could be done automatically. 8.2 Liquid test samples For liquid samples, such as milk and whey, the follo
42、wing pre-drying procedure is recommended when the instruments make use of metal sample boats or crucibles. Weigh, to the nearest 0,000 1 g, 0,5 g to 3,5 g (depending on the nitrogen content and the type of instrument) of the prepared test sample into a boat or crucible and dry for 1 h in an oven (6.
43、2) set at between 105 C and 110 C. If a rack loaded with up to 50 test samples is placed in the oven (6.2), a longer drying time could be necessary. Check whether the contents of the boats or crucibles are dry before taking them out of the oven. 1) This information is given for the convenience of us
44、ers of this International Standard and does not constitute an endorsement by ISO of these products. ISO 14891:2002(E) IDF 185:2002(E) 4 ISO and IDF 2002 All rights reserved 9 Procedure 9.1 General Since various Dumas instruments are available, different in design and handling, the operator shall car
45、efully follow the instrument manufacturers instructions for instrument setting up, optimization, calibration and operation. Switch the instrument on and allow its operating conditions to stabilize for about 1 h. To assure that equivalent results are obtained, reference materials should be regularly
46、used for method and instrument performance testing, as far as such materials are available. A matrix/instrument specific correction factor derived from those measurements may be used, if necessary. If reference materials are not available, a regular comparison using the Kjeldahl method described in
47、ISO 8968-1IDF 20-1 is recommended. 9.2 Test portion Weigh, to the nearest 0,000 1 g, at least 0,2 g of the test sample (8.1 or 8.2) into a crucible or tin capsule (6.3). For samples with a protein content of less then 1 % (mass fraction), increase the amount of the test portion up to 3,5 g, dependin
48、g on both the type of the Dumas equipment used and the nature of the test sample. 9.3 Control of oxygen demand Some types of Dumas equipment require an estimation of the oxygen demand of the test portion. The calculated oxygen demand of some compounds used for calibration is given in annex C, Table
49、C.1. For instruments with a self-optimizing oxygen control, the mass fraction of the residual oxygen content shall be between 2 % and 8 %. 9.4 Calibration Pure standard nitrogen compounds (5.12) with a known constant nitrogen content are used for long-term instrument calibration. To prepare a calibration graph, the compound and its mass should be chosen in such a way that an absolute mass of nitrogen between 4 mg and 100 mg could