Food products - Determination of the total nitrogen content by combustion according to the Dumas principle and calculation of the crude protein content - Part 1: Oilseeds and animal feeding stuffs (ISO 16634-1:2008)

This part of ISO 16634 specifies a method for the determinatio n of the total nitrogen content and the calculation of crude p rotein content of oilseeds and animal feeding stuffs. This met hod, like the Kjeldahl method, does not distinguish between pr otein nitrogen and non-protein nitrogen. For the calculation o f protein content, various conversion factors are used (see An nex D).

Getreide, Hülsenfrüchte, gemahlene Getreideerzeugnisse, Ölsamen und Futtermittel - Bestimmung des Gehaltes an Gesamtstickstoff mit dem Verbrennungsverfahren nach Dumas und Berechnung des Gehaltes an Rohprotein (ISO 16634-1:2008)

Diese Internationale Norm legt ein Verfahren zur Bestimmung des Gesamtstickstoffgehaltes und die
Berechnung des Rohproteingehaltes von Getreide, Hülsenfrüchten, gemahlenen Getreideerzeugnissen,
Ölsamen und Futtermitteln fest.
Dieses Verfahren gilt nicht für Milch und Milcherzeugnisse, für welche ein Verfahren in der ISO 14891 I IDF
185 [10] festgelegt ist.

Produits alimentaires - Détermination de la teneur en azote total par combustion selon le principe Dumas et calcul de la teneur en protéines brutes - Partie 1 : Graines oléagineuses et aliments des animaux (ISO 16634-1:2008)

L'ISO 16634-1:2008 spécifie une méthode pour la détermination de la teneur en azote total et le calcul de la teneur en protéines brutes des graines oléagineuses et des aliments pour animaux.
Cette méthode, comme la méthode Kjeldahl, ne distingue pas l'azote protéique de l'azote non protéique. Divers facteurs de conversion sont utilisés pour le calcul de la teneur en protéines.
Cette méthode ne s'applique pas au lait et aux produits laitier.

Živila - Določevanje skupnega dušika s sežigom po Dumasu in izračunom deleža surovih beljakovin - 1. del: Oljna semena in krma (ISO 16634-1:2008)

General Information

Status
Published
Publication Date
07-Dec-2008
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
01-Dec-2008
Due Date
05-Feb-2009
Completion Date
08-Dec-2008

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Getreide, Hülsenfrüchte, gemahlene Getreideerzeugnisse, Ölsamen und Futtermittel - Bestimmung des Gehaltes an Gesamtstickstoff mit dem Verbrennungsverfahren nach Dumas und Berechnung des Gehaltes an Rohprotein (ISO 16634-1:2008)Produits alimentaires - Détermination de la teneur en azote total par combustion selon le principe Dumas et calcul de la teneur en protéines brutes - Partie 1 : Graines oléagineuses et aliments des animaux (ISO 16634-1:2008)Food products - Determination of the total nitrogen content by combustion according to the Dumas principle and calculation of the crude protein content - Part 1: Oilseeds and animal feeding stuffs (ISO 16634-1:2008)67.200.20OljniceOilseeds65.120KrmilaAnimal feeding stuffsICS:Ta slovenski standard je istoveten z:EN ISO 16634-1:2008SIST EN ISO 16634-1:2009en,fr,de01-januar-2009SIST EN ISO 16634-1:2009SLOVENSKI

STANDARD
SIST EN ISO 16634-1:2009

EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN ISO 16634-1November 2008ICS 65.120; 67.200; 67.050 English VersionFood products - Determination of the total nitrogen content bycombustion according to the Dumas principle and calculation ofthe crude protein content - Part 1: Oilseeds and animal feedingstuffs (ISO 16634-1:2008)Produits alimentaires - Détermination de la teneur en azotetotal par combustion selon le principe Dumas et calcul de lateneur en protéines brutes - Partie 1: Graines oléagineuseset aliments des animaux (ISO 16634-1:2008)Lebensmittelerzeugnisse - Bestimmung des Gehaltes anGesamtstickstoff mit dem Verbrennungsverfahren nachDumas und Berechnung des Gehaltes an Rohprotein - Teil1: Ölsaaten und Futtermittel (ISO 16634-1:2008)This European Standard was approved by CEN on 1 November 2008.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the CEN Management Centre or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as theofficial versions.CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36

B-1050 Brussels© 2008 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN ISO 16634-1:2008: ESIST EN ISO 16634-1:2009

EN ISO 16634-1:2008 (E) 2 Contents Page Foreword..............................................................................................................................................................3

SIST EN ISO 16634-1:2009

EN ISO 16634-1:2008 (E) 3 Foreword This document (EN ISO 16634-1:2008) has been prepared by Technical Committee ISO/TC 34 "Agricultural food products" in collaboration with Technical Committee CEN/TC 327 “Animal feeding stuffs - Methods of sampling and analysis” the secretariat of which is held by NEN. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by May 2009, and conflicting national standards shall be withdrawn at the latest by May 2009. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom. Endorsement notice The text of ISO 16634-1:2008 has been approved by CEN as a EN ISO 16634-1:2008 without any modification.

SIST EN ISO 16634-1:2009
SIST EN ISO 16634-1:2009
Reference numberISO 16634-1:2008(E)© ISO 2008

INTERNATIONAL STANDARD ISO16634-1First edition2008-11-01Food products — Determination of the total nitrogen content by combustion according to the Dumas principle and calculation of the crude protein content — Part 1: Oilseeds and animal feeding stuffs Produits alimentaires — Détermination de la teneur en azote total par combustion selon le principe Dumas et calcul de la teneur en protéines brutes — Partie 1: Graines oléagineuses et aliments des animaux

SIST EN ISO 16634-1:2009

ISO 16634-1:2008(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat accepts no liability in this area. Adobe is a trademark of Adobe Systems Incorporated. Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.

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ISO 2008 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 microfilm, without permission in writing from either ISO at the address below or ISO's member body in the country of the requester. ISO copyright office Case postale 56 • CH-1211 Geneva 20 Tel.

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ii © ISO 2008 – All rights reserved
SIST EN ISO 16634-1:2009

ISO 16634-1:2008(E) © ISO 2008 – All rights reserved iiiContents Page Foreword............................................................................................................................................................iv Introduction........................................................................................................................................................v 1 Scope.....................................................................................................................................................1 2 Normative references...........................................................................................................................1 3 Terms and definitions...........................................................................................................................1 4 Principle.................................................................................................................................................2 5 Reagents................................................................................................................................................2 6 Apparatus..............................................................................................................................................3 7 Sampling................................................................................................................................................4 8 Preparation of test sample...................................................................................................................4 9 Procedure..............................................................................................................................................4 9.1 General...................................................................................................................................................4 9.2 Test portion...........................................................................................................................................4 9.3 Control of oxygen demand..................................................................................................................5 9.4 Calibration.............................................................................................................................................5 9.5 Determination........................................................................................................................................5 9.6 Detection and integration....................................................................................................................6 10 Calculation and expression of results................................................................................................6 10.1 Calculation.............................................................................................................................................6 10.1.1 Nitrogen content...................................................................................................................................6 10.1.2 Crude protein content..........................................................................................................................6 10.2 Expression of results...........................................................................................................................7 11 Precision................................................................................................................................................7 11.1 Interlaboratory tests.............................................................................................................................7 11.2 Repeatability..........................................................................................................................................7 11.3 Reproducibility......................................................................................................................................7 12 Test report.............................................................................................................................................7 Annex A (informative)

Flowchart for the basic design of a Dumas apparatus............................................8 Annex B (informative)

Schemes of suitable types of Dumas apparatus......................................................9 Annex C (informative)

Equipment calibration...............................................................................................12 Annex D (informative)

Examples of factors for converting nitrogen content to protein content ...........14 Annex E (informative)

Result of collaborative studies.................................................................................15 Annex F (informative)

Relationship between Dumas nitrogen and Kjeldahl nitrogen..............................24 Bibliography.....................................................................................................................................................28

SIST EN ISO 16634-1:2009

ISO 16634-1:2008(E) iv © ISO 2008 – 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 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. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical 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 document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO 16634-1 was prepared by Technical Committee ISO/TC 34, Food products. ISO 16634 consists of the following parts, under the general title Food products — Determination of the total nitrogen content by combustion according to the Dumas principle and calculation of the crude protein content: ⎯ Part 1: Oilseeds and animal feeding stuffs A part 2 on cereals, pulses and milled cereal products is in preparation.

SIST EN ISO 16634-1:2009

ISO 16634-1:2008(E) © ISO 2008 – All rights reserved vIntroduction For a long time the Kjeldahl method has been the most frequently used method for the determination of protein content of food products. However, in recent years, the Kjeldahl method has increasingly been replaced by the Dumas method, which is faster and does not use dangerous chemicals. Although the principles of the two methods are different, both measure the nitrogen content of the product. Nitrogen can be converted into protein content by using an appropriate factor. The value of this factor varies with the relative amounts of different proteins and their amino-acid composition in the given product. Neither the Dumas nor the Kjeldahl method distinguishes between protein and non-protein nitrogen. In most cases, results obtained by the Dumas method are slightly higher than those of the Kjeldahl method. This is due to the fact that the Dumas method measures almost all of the non-protein nitrogen, whereas the Kjeldahl method measures only a part of it. Taking into consideration that the calculated protein content of a product by both methods only approximates the true value, it is a matter of discretion which one is accepted. The most appropriate solution should be the use of a second factor for the elimination of the systematic error caused by the non-protein nitrogen content of the different products. However, this second factor has to be determined for each product, like existing factors, which show the ratio of the protein to the nitrogen content.

SIST EN ISO 16634-1:2009
SIST EN ISO 16634-1:2009

INTERNATIONAL STANDARD ISO 16634-1:2008(E) © ISO 2008 – All rights reserved 1Food products — Determination of the total nitrogen content by combustion according to the Dumas principle and calculation of the crude protein content — Part 1: Oilseeds and animal feeding stuffs 1 Scope This part of ISO 16634 specifies a method for the determination of the total nitrogen content and the calculation of crude protein content of oilseeds and animal feeding stuffs. This method, like the Kjeldahl method, does not distinguish between protein nitrogen and non-protein nitrogen. For the calculation of protein content, various conversion factors are used (see Annex D). This method is not applicable to milk and milk products, for which a method is specified in ISO 14891⎪IDF 185[10]. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 664, Oilseeds — Reduction of laboratory sample to test sample ISO 665, Oilseeds — Determination of moisture and volatile matter content ISO 771, Oilseed residues — Determination of moisture and volatile matter content ISO 6496, Animal feeding stuffs — Determination of moisture and other volatile matter content ISO 6498, Animal feeding stuffs — Preparation of test samples 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1 nitrogen content mass fraction of the total nitrogen determined by the procedure specified in this part of ISO 16634 NOTE The mass fraction is expressed as a percentage. SIST EN ISO 16634-1:2009

ISO 16634-1:2008(E) 2 © ISO 2008 – All rights reserved 3.2 crude protein content nitrogen content (3.1) multiplied by a factor, usually 6,25 NOTE 1 A listing of other factors for possible use with various commodities is given in Annex D. NOTE 2 The factors for calculation of crude protein content from the total content of nitrogen are derived from the Kjeldahl method which is the reference method for the determination of total nitrogen content. As the method uses the same factors as the Kjeldahl method, the use of these factors has to be verified due to the slight difference in results between the Kjeldahl and Dumas methods. 4 Principle Samples are converted to gases by heating in a combustion tube which gasifies samples. Interfering components are removed from the resulting gas mixture. The nitrogen compounds in the gas mixture or a representative part of them are converted to molecular nitrogen, which is quantitatively determined by a thermal conductivity detector. The nitrogen content is calculated by a microprocessor. 5 Reagents Use only reagents of recognized analytical grade, or reagents of equivalent purity as specified by instrument manufacturers. Except for the reference materials (5.12), all reagents shall be free from nitrogen. 5.1 Carrier gas(es): use one of 5.1.1 and 5.1.2. 5.1.1 Carbon dioxide, as pure as possible and of minimum volume fraction, ϕ(CO2) W 99,99 %. 5.1.2 Helium, as pure as possible and of minimum volume fraction, ϕ(He) W 99,99 %. 5.2 Oxygen, as pure as possible and of minimum volume fraction, ϕ(O2) W 99,99 %. 5.3 Sulfur dioxide and halogen absorbent, to eliminate any sulfur from the sample [e.g. lead chromate (PbCrO4) or steel wool]. 5.4 Copper oxide platinum catalyst (filling material for the post-combustion tube). Platinum catalyst [5 % of Pt on alumina (Al2O3)] is blended with CuO at a ratio of 1:7 parts or 1:8 parts according to the manufacturer's recommendations. To prevent separation as a result of the different bulk densities of the two materials, it is recommended not to prepare the mixture 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 or copper wool. This should be disaggregated before being inserted in the post-combustion or reduction tube. 5.6 Silica (quartz) or glass wool or cotton wool, as recommended by the instrument manufacturer. 5.7 Copper (wire, cuttings, turnings or powder), or tungsten for the reduction tube. The use of copper wires can improve the precision of analytical results for samples with low nitrogen contents (about 1 % mass fraction). 5.8 Diphosphorus pentoxide (P2O5) or granulated magnesium perchlorate [Mg(ClO4)2], or another suitable support material, to fill the drying tubes. SIST EN ISO 16634-1:2009

ISO 16634-1:2008(E) © ISO 2008 – All rights reserved 35.9 Hollow corundum spheres or aluminium oxide pellets, for the combustion tube. 5.10 Copper oxide (CuO), as filling material for the combustion tube. 5.11 Sodium hydroxide (NaOH), on a support material. 5.12 Aspartic acid (C4H7NO4) or ethylenediaminetetraacetic acid (C10H16N2O8) or glutamic acid (C5H9NO4) or hippuric acid (C9H9NO3) standard, or other suitable reference materials with known, constant, certified nitrogen content. Minimum recovery should be 99 % mass fraction. 5.13 Light petroleum, with boiling range between 30 °C and 60 °C, or acetone or ethanol. 6 Apparatus Usual laboratory equipment and, in particular, the following. 6.1 Analytical balance, capable of weighing to the nearest 0,000 1 g. 6.2 Grinding device, appropriate to the nature of the sample. 6.3 Sieve, of nominal size of openings 800 µm or 1 mm, made of non-ferrous material. 6.4 Crucibles (e.g. made of stainless steel, quartz, ceramic or platinum) or tin capsules or nitrogen-free filter paper for pressing pellets, suitable for the Dumas apparatus used. NOTE 1 Several commercial instruments are provided with an automatic sampler. NOTE 2 Some solid samples (e.g. powders) can be pressed to form pellets. 6.5 Dumas apparatus1), fitted with a furnace able to maintain a given temperature greater than or equal to 850 °C, with a thermal conductivity detector and suitable device for signal integration. Suitable Dumas apparatus operates according to the general flowchart given in Annex A, although different arrangements and components may be used. NOTE Schemes of three available instruments are shown as examples in Figures B.1, B.2, and B.3. To avoid leaks, the sealing O-rings shall be slightly lubricated with high-vacuum grease prior to installation. Experience has shown that it is important to clean all pieces of silica and glassware carefully, and to remove fingerprints from the tubes using a suitable solvent (e.g. acetone) before inserting them in the furnace.

1) Elementar Analysensysteme, Sumika Chemical Analysis Service, and LECO Instruments produce suitable equipment available commercially. This information is given for the convenience of users of this International Standard and does not constitute an endorsement by ISO of this equipment. Equivalent products may be used if they can be shown to lead to the same results. SIST EN ISO 16634-1:2009

ISO 16634-1:2008(E) 4 © ISO 2008 – All rights reserved 7 Sampling A representative sample should have been sent to the laboratory. It should not have been damaged or changed during transport or storage. Sampling is not part of the method specified in this part of ISO 16634. Recommended sampling methods are given in ISO 542[1] for oilseeds, in ISO 5500[3] for oilseed residues, and in ISO 6498 for animal feeding stuffs. 8 Preparation of test sample The laboratory sample shall be prepared in such a way that a homogeneous test sample is obtained, which is representative of the oilseeds (see ISO 664) or animal feeding stuff (see ISO 6498). Using a suitable grinding device (6.2), grind the laboratory sample. Generally, pass the ground material through a sieve (6.3) of nominal size of openings 800 µm for small sample sizes (under 300 mg), or a sieve of nominal size of openings 1 mm, for larger sample sizes (300 mg or more)[15]. Mills that produce particle sizes meeting the specifications given in Table 1 will give acceptable results. Table 1 — Required particle size Nominal size of sieve openings µm Amount passing through sieve % mass fraction 710 500 200 100 95 to 100 85 or less Grinding may result in moisture loss and therefore the moisture content of the ground sample should also be analysed when reporting nitrogen or protein values to dry matter or a constant moisture basis. Determination of the moisture shall be carried out according to ISO 665, ISO 771 or ISO 6496. The grinder efficiency may be checked by replicate preparation of ground samples of a 2+1 mixture of corn and soya seeds. The expected coefficient of variation should be less than 2 % mass fraction. 9 Procedure 9.1 General Carefully follow the manufacturer's instructions for instrument set-up, optimization, calibration and operation. Switch the instrument on and allow it to stabilize as defined in local procedures. An instrument performance test should be made daily, using the reference material (5.12). The recovery of nitrogen should be > 99,0 % mass fraction. 9.2 Test portion Weigh, to the nearest 0,000 1 g, at least 0,1 g of the test sample into a crucible or tin capsule or nitrogen-free filter paper for pressing pellets (6.4). For samples low in protein (< 1 % mass fraction), the amount of the test portion may be increased up to 3,5 g, depending on both the type of the Dumas equipment used and the nature of the test portion. Depending on the type of equipment used, if the samples contain over 17 % mass fraction moisture, it may be necessary to dry them before analysis. SIST EN ISO 16634-1:2009

ISO 16634-1:2008(E) © ISO 2008 – All rights reserved 5Lower masses may be necessary for very high protein content samples or where only very small amounts of sample are available. In the case of masses less than 0,1 g, a validation shall be performed. 9.3 Control of oxygen demand Control oxygen demand, in particular the flow, according to the instructions of the material supplier. Conduct as many blanks as necessary to stabilize the equipment, each using an equivalent mass of sucrose in place of the sample, with each set of nitrogen or protein determinations to mimic the test sample run. The sucrose blank provides the amount of nitrogen that is introduced by the atmospheric gases and is trapped within a powdered organic material source. Use the mean value of the atmospheric blank determinations as an error correction in the calculation of the nitrogen or protein determination of each test sample. 9.4 Calibration Use pure compounds with known constant nitrogen content, e.g. aspartic acid (5.12), as standards for long-term instrument calibration. Analyse, in duplicate, three pure compounds, each with three different concentrations, chosen according to the measurement range of the actual samples. To prepare a calibration curve, choose the compound and the amount used to ensure that an absolute amount of nitrogen in connection with the matrices to be analysed can be detected. For calibration, five standard samples (minimum) should be used, according to the scope of the analysed matrices. Above 200 mg of nitrogen, the calibration curve is expected to be non-linear. In this non-linear section, several short segments may be used for calibration. To ensure the quality of calibration in this region, standard samples should be increased in steps of 1 mg to 5 mg of nitrogen. Calibration may also be performed using aqueous standard solutions. Check the calibration at least three times at the beginning of the series, and then every 15 to 25 samples, analysing either one of the replicate standards, or a sample of known value. The value obtained shall be less than 0,05 % mass fraction nitrogen of the expected value. Otherwise, analyse the samples again after checking instrument performance. 9.5 Determination With the instrument under operating conditions, introduce the test portion according to the manufacturer's instructions. During analysis, the following processes take place in the instrument (see Figure B.1, B.2 or B 3). The test portion is quantitatively combusted under standardized conditions at a temperature of 850 °C minimum, depending on the instrument and the material being tested. Volatile decomposition products (mainly molecular nitrogen, nitrogen oxides, carbon dioxide, and water vapour) are transported by the carrier gas (5.1) through the instrument. Nitrogen oxides are reduced to molecular nitrogen and the excess of oxygen is bound to the copper or tungsten in the reduction column (5.7). Water is removed by means of a condenser filled with magnesium perchlorate, diphosphorus pentoxide or other drying agents (5.8). Unless carbon dioxide is used as carrier gas (5.1.1), it is removed by being passed over a suitable absorbent, e.g. sodium hydroxide on a support material (5.11). Interfering compounds (e.g. volatile halogen and sulfur compounds) are removed by absorbents (5.3) or contact materials [e.g. silver wool (5.5) or sodium hydroxide on a suitable support material (5.11)]. SIST EN ISO 16634-1:2009

ISO 16634-1:2008(E) 6 © ISO 2008 – All rights reserved The nitrogen in the residual gas mixture, consisting of nitrogen and carrier gas, is passed through a thermal conductivity detector. 9.6 Detection and integration For quantitative nitrogen determination, the instrument uses a sensitive thermal conductivity cell that is optimized for the carrier gas employed and which may have automatic zero adjustment between the measurement of the test portions. After amplification and analog/digital conversion of the detector signal, data obtained are processed by peripheral microprocessor hardware. 10 Calculation and expression of results 10.1 Calculation 10.1.1 Nitrogen content Results for the total nitrogen content, wN, expressed as a percentage mass fraction, are usually available from instrument printouts. 10.1.2 Crude protein content The correction factor, Fc, is obtained by using Equation (1): 22HO,1cHO,2100100wFw−=− (1) where wH2O,1 is the moisture mass fraction, expressed as a percentage, before grinding; wH2O,2 is the moisture mass fraction, expressed as a percentage, after grinding. The crude protein content, wp, expressed as a percentage mass fraction, is obtained by using Equation (2): wp = wN F Fc (2) where wN is the numerical value of the nitrogen content, expressed as a percentage mass fraction, of the sample at its natural moisture content; F is the generally agreed ratio for the analysed product, equal to 6,25 for animal feeding stuffs (see Annex D). On request, the crude protein content, wpd, expressed as a percentage mass fraction of the dry matter, can be calculated by using Equation (3): 2p

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