Food products - Determination of the total nitrogen content by combustion according to the Dumas principle and calculation of the crude protein content - Part 2: Cereals, pulses and milled cereal products (ISO 16634-2:2016)

This part of ISO 16634 specifies a method for the determination of the total nitrogen content and the
calculation of the crude protein content of cereals, pulses and milled cereal products.
This method, like the Kjeldahl method (see References [1] and [6]), does not distinguish between protein
nitrogen and non-protein nitrogen. For the calculation of the protein content, various conversion factors
are used (see 3.2).

Lebensmittelerzeugnisse - Bestimmung des Gehaltes an Gesamtstickstoff mit dem Verbrennungsverfahren nach Dumas und Berechnung des Gehaltes an Rohprotein - Teil 2: Getreide, Hülsenfrüchte und gemahlene Getreideerzeugnisse (ISO 16634-2:2016)

Dieser Teil von ISO 16634 legt ein Verfahren zur Bestimmung des Gesamtstickstoffgehaltes und zur Berechnung des Rohproteingehaltes von Getreide, Hülsenfrüchten und gemahlenen Getreideerzeugnissen fest.
Dieses Verfahren unterscheidet, wie auch das Kjeldahl-Verfahren (siehe Literaturhinweise [1] und [6]), nicht zwischen Protein Stickstoff und Nicht Protein Stickstoff. Zur Berechnung des Proteingehaltes werden ver¬schiedene Umrechnungsfaktoren verwendet (siehe 3.2).

Produits alimentaires - Détermination de la teneur en azote total par combustion selon le principe Dumas et calcul de la teneur en protéines brutes - Partie 2: Céréales, légumineuses et produits céréaliers de mouture (ISO 16634-2:2016)

ISO 16634-2:2016 spécifie une méthode pour la détermination de la teneur en azote total et le calcul de la teneur en protéines brutes des céréales, légumineuses et produits céréaliers de mouture.
Cette méthode, comme la méthode de Kjeldahl (voir Références [1] et [6]), 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 (voir 3.2).

Živila - Določevanje celotnega dušika s sežigom po Dumasu in z izračunom deleža surovih beljakovin - 2. del: Žito, stročnice in žitni mlevski proizvodi (ISO 16634-2:2016)

Ta del standarda ISO 16634 določa metodo za določevanje celotnega dušika in izračun deleža surovih beljakovin v žitu, stročnicah in žitnih mlevskih proizvodih.
Ta metoda, kot metoda po Kjeldahlu (glej sklica [1] in [6]), ne razlikuje med beljakovinskim in nebeljakovinskim dušikom. Pri izračunu deleža beljakovin se uporabljajo različni faktorji pretvorbe (glej 3.2).

General Information

Status
Published
Public Enquiry End Date
30-Dec-2014
Publication Date
16-Aug-2016
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
19-Jul-2016
Due Date
23-Sep-2016
Completion Date
17-Aug-2016

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SLOVENSKI STANDARD
SIST EN ISO 16634-2:2016
01-september-2016
1DGRPHãþD
SIST-TS CEN ISO/TS 16634-2:2010
äLYLOD'RORþHYDQMHFHORWQHJDGXãLNDVVHåLJRPSR'XPDVXLQ]L]UDþXQRPGHOHåD

VXURYLKEHOMDNRYLQGHOäLWRVWURþQLFHLQåLWQLPOHYVNLSURL]YRGL ,62



Food products - Determination of the total nitrogen content by combustion according to

the Dumas principle and calculation of the crude protein content - Part 2: Cereals, pulses

and milled cereal products (ISO 16634-2:2016)
Lebensmittelerzeugnisse - Bestimmung des Gehaltes an Gesamtstickstoff mit dem

Verbrennungsverfahren nach Dumas und Berechnung des Gehaltes an Rohprotein - Teil

2: Getreide, Hülsenfrüchte und gemahlene Getreideerzeugnisse (ISO 16634-2:2016)

Produits alimentaires - Détermination de la teneur en azote total par combustion selon le

principe Dumas et calcul de la teneur en protéines brutes - Partie 2: Céréales,
légumineuses et produits céréaliers de mouture (ISO 16634-2:2016)
Ta slovenski standard je istoveten z: EN ISO 16634-2:2016
ICS:
67.050 Splošne preskusne in General methods of tests and
analizne metode za živilske analysis for food products
proizvode
67.060 äLWDVWURþQLFHLQSURL]YRGLL] Cereals, pulses and derived
QMLK products
SIST EN ISO 16634-2:2016 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 16634-2:2016
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SIST EN ISO 16634-2:2016
EN ISO 16634-2
EUROPEAN STANDARD
NORME EUROPÉENNE
June 2016
EUROPÄISCHE NORM
ICS 67.060; 67.050 Supersedes CEN ISO/TS 16634-2:2009
English Version
Food products - Determination of the total nitrogen
content by combustion according to the Dumas principle
and calculation of the crude protein content - Part 2:
Cereals, pulses and milled cereal products (ISO 16634-
2:2016)

Produits alimentaires - Détermination de la teneur en Lebensmittelerzeugnisse - Bestimmung des Gehaltes

azote total par combustion selon le principe Dumas et an Gesamtstickstoff mit dem Verbrennungsverfahren

calcul de la teneur en protéines brutes - Partie 2: nach Dumas und Berechnung des Gehaltes an

Céréales, légumineuses et produits céréaliers de Rohprotein - Teil 2: Getreide, Hülsenfrüchte und

mouture (ISO 16634-2:2016) gemahlene Getreideerzeugnisse (ISO 16634-2:2016)
This European Standard was approved by CEN on 12 May 2016.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this

European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references

concerning such national standards may be obtained on application to the CEN-CENELEC 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

translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management

Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,

Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,

Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and

United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels

© 2016 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 16634-2:2016 E

worldwide for CEN national Members.
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SIST EN ISO 16634-2:2016
EN ISO 16634-2:2016 (E)
Contents Page

European Foreword ...................................................................................................................................................... 3

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SIST EN ISO 16634-2:2016
EN ISO 16634-2:2016 (E)
European Foreword

This document (EN ISO 16634-2:2016) has been prepared by Technical Committee ISO/TC 34 "Food

products" in collaboration with Technical Committee CEN/TC 338 “Cereal and cereal products” the

secretariat of which is held by AFNOR.

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 December 2016, and conflicting national standards

shall be withdrawn at the latest by December 2016.

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.
This document supersedes CEN ISO/TS 16634-2:2009.

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,

Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,

France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,

Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and the United Kingdom.
Endorsement notice

The text of ISO 16634-2:2016 has been approved by CEN as EN ISO 16634-2:2016 without any

modification.
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SIST EN ISO 16634-2:2016
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SIST EN ISO 16634-2:2016
INTERNATIONAL ISO
STANDARD 16634-2
First edition
2016-05-15
Food products — Determination of the
total nitrogen content by combustion
according to the Dumas principle
and calculation of the crude protein
content —
Part 2:
Cereals, pulses and milled cereal
products
Produits alimentaires — Détermination de la teneur en azote total
par combustion selon le principe Dumas et calcul de la teneur en
protéines brutes —
Partie 2: Céréales, légumineuses et produits céréaliers de mouture
Reference number
ISO 16634-2:2016(E)
ISO 2016
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SIST EN ISO 16634-2:2016
ISO 16634-2:2016(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2016, Published in Switzerland

All rights reserved. Unless otherwise specified, 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 8 • 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
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SIST EN ISO 16634-2:2016
ISO 16634-2:2016(E)
Contents 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 ........................................................................................................................................................................................................................ 3

8 Preparation of the test sample ............................................................................................................................................................... 3

9 Procedure..................................................................................................................................................................................................................... 4

9.1 General ........................................................................................................................................................................................................... 4

9.2 Test portion ................................................................................................................................................................................................ 4

9.3 Control of oxygen supply ................................................................................................................................................................ 4

9.4 Calibration .................................................................................................................................................................................................. 5

9.5 Determination ......................................................................................................................................................................................... 5

9.6 Detection and data processing .................................................................................................................................................. 5

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 .......................................................................................................................................................................... 6

11 Precision ....................................................................................................................................................................................................................... 6

11.1 Interlaboratory tests .......................................................................................................................................................................... 6

11.2 Repeatability ............................................................................................................................................................................................. 7

11.3 Reproducibility ....................................................................................................................................................................................... 7

11.4 Critical difference.................................................................................................................................................................................. 7

11.4.1 Comparison of two groups of measurements in the same laboratory .............................. 7

11.4.2 Comparison of two groups of measurements in two different laboratories ................ 8

11.5 Uncertainty ................................................................................................................................................................................................. 8

12 Test report ................................................................................................................................................................................................................... 8

Annex A (informative) Flowchart for a basic Dumas apparatus .............................................................................................. 9

Annex B (informative) Schematic diagrams of suitable types of Dumas apparatus ........................................10

Annex C (informative) Equipment calibration ........................................................................................................................................13

Annex D (informative) Results of interlaboratory tests ................................................................................................................15

Bibliography .............................................................................................................................................................................................................................22

© ISO 2016 – All rights reserved iii
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SIST EN ISO 16634-2:2016
ISO 16634-2:2016(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International 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 1. 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 2 (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 identified 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 specific 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 (TBT) see the following URL: Foreword - Supplementary information

The committee responsible for this document is ISO/TC 34, Food products, Subcommittee SC 4, Cereals

and pulses.

This first edition cancels and replaces ISO/TS 16634-2:2009, which has been technically revised.

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
— Part 2: Cereals, pulses and milled cereal products
iv © ISO 2016 – All rights reserved
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SIST EN ISO 16634-2:2016
ISO 16634-2:2016(E)
Introduction

For a long time, the Kjeldahl method has been the most frequently used method for the determination

of the protein content of food products. In recent years, the Dumas method has gained importance

compared to the Kjeldahl method because it 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 content can be converted into protein content by using an appropriate factor. The value of this

factor varies depending on the relative amounts of different proteins and their amino-acid composition

in a 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 because 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 protein content of a product calculated by both methods only

approximates to the true value, it is a matter of discretion which one is accepted. The best solution is

to use 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 the existing factors which

indicate the ratio of the protein content to the nitrogen content.
© ISO 2016 – All rights reserved v
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SIST EN ISO 16634-2:2016
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SIST EN ISO 16634-2:2016
INTERNATIONAL STANDARD ISO 16634-2:2016(E)
Food products — Determination of the total nitrogen
content by combustion according to the Dumas principle
and calculation of the crude protein content —
Part 2:
Cereals, pulses and milled cereal products
1 Scope

This part of ISO 16634 specifies a method for the determination of the total nitrogen content and the

calculation of the crude protein content of cereals, pulses and milled cereal products.

This method, like the Kjeldahl method (see References [1] and [6]), does not distinguish between protein

nitrogen and non-protein nitrogen. For the calculation of the protein content, various conversion factors

are used (see 3.2).
2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and are

indispensable for its application. For dated references, only the edition cited applies. For undated

references, the latest edition of the referenced document (including any amendments) applies.

ISO 712, Cereals and cereal products — Determination of moisture content — Reference method

ISO 6540, Maize — Determination of moisture content (on milled grains and on whole grains)

ISO 24557, Pulses — Determination of moisture content — Air-oven method
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

Note 1 to entry: Determined by the procedure specified in this part of ISO 16634.

Note 2 to entry: The mass fraction is expressed as a percentage.
3.2
crude protein content
nitrogen content (3.1) multiplied by a factor

Note 1 to entry: A 5,7 factor is generally used for cereals for human food (such as wheat, rye and their milled

products) and 6,25 for malting barley and cereals for feed and other products falling within the scope of this part

of ISO 16634.

Note 2 to entry: The factors for calculation of the crude protein content from the total nitrogen content are

derived from the Kjeldahl method, which is the reference method for the determination of total nitrogen content.

© ISO 2016 – All rights reserved 1
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SIST EN ISO 16634-2:2016
ISO 16634-2:2016(E)
4 Principle

Samples are converted into gases by heating in a combustion tube. 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 either 5.1.1 or 5.1.2.

5.1.1 Carbon dioxide, as pure as possible, but with a minimum CO volume fraction of 99,99 %.

5.1.2 Helium, as pure as possible, but with a minimum He volume fraction of 99,99 %.

5.2 Oxygen, as pure as possible, but with a minimum O volume fraction of 99,99 %.

5.3 Sulfur dioxide and halogen absorbent, to eliminate any sulfur from the sample [e.g. lead

chromate (PbCrO ) or steel wool].
5.4 Copper oxide/platinum catalyst, for the post-combustion tube.

Platinum catalyst [5 % of Pt on alumina (Al O )] is blended with CuO in the ratio 1 part:7 parts or

2 3
1 part:8 parts in accordance with 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, but 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 shall be disaggregated before being inserted into 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 or tungsten (wire, cuttings, turnings or powder), for the reduction tube.

The use of copper or tungsten in one of these forms can improve the precision of analytical results for

samples with low nitrogen contents (about 1 % mass fraction).

5.8 Diphosphorus pentoxide (P O ) or granulated magnesium perchlorate [Mg(ClO ) ], or

2 5 4 2
another suitable drying agent, to fill the drying tubes.
5.9 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.
2 © ISO 2016 – All rights reserved
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SIST EN ISO 16634-2:2016
ISO 16634-2:2016(E)

5.12 Aspartic acid (C H NO ) or ethylenediaminetetraacetic acid (C H N O ) or glutamic acid

4 7 4 10 16 2 8

(C H NO ) or hippuric acid (C H NO ) standard, or other suitable reference materials with a known,

5 9 4 9 9 3
constant, certified nitrogen content.
The minimum recovery should preferably be 99 % mass fraction.

5.13 Light petroleum, with a 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 opening size 800 µm or 1 mm, made of non-ferrous material.

6.4 Crucibles (e.g. made of stainless steel, quartz, ceramic material or platinum) or tin capsules

or tin foils or nitrogen-free filter paper, suitable for the Dumas apparatus used.

NOTE 1 Several instruments provided with an automatic sampler are commercially available.

NOTE 2 Some solid samples (e.g. powders) can be pressed to form pellets.

6.5 Dumas apparatus, 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 Schematic diagrams of three commercially available instruments are shown as examples in

Figures B.1 to 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 silicaware and glassware carefully

and to remove fingerprints from tubes, using a suitable solvent (5.13), before inserting them into the

furnace.
7 Sampling

A representative sample should have been sent to the laboratory. This sample 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 24333 for cereals and cereal products.
8 Preparation of the test sample

The test sample shall be prepared from the laboratory sample in such a way that a homogeneous test

sample is obtained.

Using a suitable grinding device (6.2), grind the laboratory sample. Generally, pass the ground material

through a sieve (6.3) of nominal opening size 800 µm for small sample sizes (under 300 mg) or a sieve of

© ISO 2016 – All rights reserved 3
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SIST EN ISO 16634-2:2016
ISO 16634-2:2016(E)

nominal opening size 1 mm for larger sample sizes (300 mg or more). 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 Amount passing through sieve
µm % mass fraction
710 100
500 95 to 100
200 85 or less

Grinding may result in moisture loss and, therefore, the moisture content of the ground sample should

preferably also be determined when reporting nitrogen or protein contents on a dry-matter or constant-

moisture basis. Determination of the moisture content shall be carried out in accordance with ISO 712

for cereals other than maize, ISO 6540 for maize and ISO 24557 for pulses.

The grinder efficiency can be checked by replicate preparation of ground samples of a 2 + 1 mixture of

maize 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 carried out 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 (6.4). For samples low in protein (<1 % mass fraction), the amount of the test portion

can be increased up to 3,5 g, depending on the type of Dumas equipment being used and on the nature

of the sample.

Depending on the type of equipment used, if the sample contains over 17 % mass fraction of moisture,

drying may be necessary before analysis.

Lower test portions may be necessary for very high protein content samples or when only very small

amounts of sample are available. In the case of portions below 0,1 g, a second (validation) determination

shall be performed.
9.3 Control of oxygen supply

Control the oxygen supply, in particular the flow, in accordance with the instructions of the material

supplier.

With each series of nitrogen content determinations, conduct as many blank runs as necessary to

stabilize the equipment, using for each run an equivalent mass of sucrose in place of the test portion.

The sucrose blank provides the amount of nitrogen that is introduced in the form of atmospheric air

trapped within a powdered organic material. Use the mean value of the blank determinations as an

error correction in the calculation of the nitrogen content of each test sample.
4 © ISO 2016 – All rights reserved
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SIST EN ISO 16634-2:2016
ISO 16634-2:2016(E)
9.4 Calibration

For instrument calibration, use pure compounds with a known, constant nitrogen content, e.g. aspartic

acid (5.12), as standards. Analyse in duplicate, three pure compounds, each in three different amounts

chosen as a function of the measurement range for the actual samples.

To prepare a calibration curve, carry out at least five determinations with different amounts of the

same compound, choosing the compound and the amounts used in such a way that the curve obtained

will cover the range of nitrogen contents in the samples to be analysed.

If the test portion contains more than 200 mg of nitrogen, the calibration curve is likely to be nonlinear.

In the nonlinear section, short segments can, nevertheless, be used for calibration purposes. To ensure

the reliability of the curve in these segments, the amount of standard used shall

...

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