ISO/FDIS 6974-4

• Abstract:

  This document gives guidance for obtaining the best analysis results possible from a Gas Chromatograph (GC) when analysing natural gas and natural gas substitutes for combined use with the most recent versions of ISO 6974’s part 1, 2 and 3. (Examples are given.)

• Status: Not Published
• ICS: 75.060 - Natural gas
• Technical Committee: ISO/TC 193/SC 1 - Analysis of natural gas
• Drafting Committee: ISO/TC 193/SC 1 - Analysis of natural gas
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 16-Sep-2026
• Completion Date: 16-Sep-2026

• Consolidates: FprEN ISO 6974-4 - Natural gas - Determination of composition and associated uncertainty by gas chromatography - Part 4: Guidelines and requirements for gas analysis (ISO/FDIS 6974-4:2026)
• Effective Date: 12-Feb-2026
• Consolidates: ISO 11795:2018/Amd 1:2022 - Agricultural tractor drive wheel tyres — Explanation of rolling circumference index (RCI) and speed radius index (SRI) and method of measuring tyre rolling circumference — Amendment 1
• Effective Date: 28-Jan-2023
• Revises: ISO 6975:1997 - Natural gas — Extended analysis — Gas-chromatographic method
• Effective Date: 09-Aug-2025
• Revises: ISO 6974-6:2002 - Natural gas — Determination of composition with defined uncertainty by gas chromatography — Part 6: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and C1 to C8 hydrocarbons using three capillary columns
• Effective Date: 09-Aug-2025
• Revises: ISO 6974-5:2014 - Natural gas — Determination of composition and associated uncertainty by gas chromatography — Part 5: Isothermal method for nitrogen, carbon dioxide, C1 to C5 hydrocarbons and C6+ hydrocarbons
• Effective Date: 09-Aug-2025
• Revises: ISO 6974-4:2000 - Natural gas — Determination of composition with defined uncertainty by gas chromatography — Part 4: Determination of nitrogen, carbon dioxide and C1 to C5 and C6+ hydrocarbons for a laboratory and on-line measuring system using two columns
• Effective Date: 06-Jun-2022

Overview

ISO/FDIS 6974-4:2026 is a key international standard developed by ISO, focusing on the determination of natural gas composition and associated uncertainty using gas chromatography. This fourth part of the ISO 6974 series provides comprehensive guidelines and requirements to ensure high-quality gas analysis, enabling precise and reliable characterization of natural gas and natural gas substitutes. The document is intended for use in conjunction with Parts 1, 2, and 3 of the ISO 6974 series, covering key aspects of sample collection, instrument setup, analysis methodology, and quality control for gas chromatographic analysis.

Gas chromatography remains the cornerstone technique for natural gas analysis, supporting various applications across the energy sector. The standard is maintained by ISO Technical Committee ISO/TC 193/SC 1, ensuring ongoing relevance to industry needs and best practices.

Key Topics

ISO/FDIS 6974-4 addresses a range of essential topics for gas analysis, including:

• Sampling Procedures: Guidance on collecting and preparing representative natural gas samples, including considerations on gas origin, sample phase, and pressure.
• Sample Introduction: Requirements for sample loops, injection methods (including vacuum injection), and sample handling to prevent contamination or loss.
• Separation Process: Selection and maintenance of chromatographic columns, carrier gases, temperature control, and correction for the presence of oxygen and argon.
• Detection Techniques: Recommendations for using typical detectors (e.g., TCD, FID), ensuring peak resolution, and accurate component identification.
• Data Processing: Advice on handling raw chromatographic data, peak integration, data format output, and result reporting.
• Calibration and Optimization: Approaches to calibration, assessing method repeatability, and optimizing analytical procedures for reliable compositional results.
• Quality Control: Use of control charts, assessment of precision, bias, and uncertainty related to natural gas analysis.
• Reporting Requirements: Standardized outline for test reports to ensure consistency and traceability in gas quality measurements.

Applications

ISO/FDIS 6974-4 has significant value in multiple sectors where accurate natural gas composition analysis is essential:

• Energy Trade and Custody Transfer: Gas composition analysis underpins quality specifications, energy content calculations, and contractual agreements for natural gas trading.
• Pipeline Operations: Ensures compliance with regulatory and safety requirements, supports process optimization, and helps detect contaminants or compositional anomalies.
• Environmental Monitoring: Enables precise tracking of greenhouse gases and other environmental parameters in natural gas streams.
• Research & Development: Supports development of alternative fuels and natural gas substitutes by providing reliable compositional data and uncertainty estimates.
• Laboratory and On-line Monitoring: Used for both laboratory-based and on-line analytical systems, ensuring continual gas quality assurance.

By following ISO/FDIS 6974-4, organizations can achieve improved accuracy, reproducibility, and transparency in natural gas analysis, supporting critical decisions across the energy value chain.

Related Standards

ISO/FDIS 6974-4 is part of a comprehensive series and complements other important standards, including:

• ISO 6974-1: General requirements and guidelines for natural gas composition analysis by gas chromatography.
• ISO 6974-2: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide, and hydrocarbons up to C8 using two packed columns.
• ISO 6974-3: Determination of nitrogen, carbon dioxide, and hydrocarbons for laboratory and on-line measuring systems.
• ISO 6975: Use of control charts in analytical quality assurance.
• Other Related ISO Standards: Additional ISO guidelines detailed in Annex G of the standard, addressing diverse aspects of natural gas analysis, sampling, and measurement.

Organizations committed to high-quality natural gas characterization should ensure alignment with the latest versions of these international standards for maximum technical and commercial benefit.

Keywords: ISO/FDIS 6974-4, natural gas analysis, gas chromatography, compositional analysis, analytical uncertainty, sampling, data processing, calibration, quality control, energy sector standards.

FINAL DRAFT
International
Standard
ISO/TC 193/SC 1
Natural gas — Determination
Secretariat: NEN
of composition and
Voting begins on:
associated uncertainty by gas
2026-09-16
chromatography —
Voting terminates on:
2026-11-11
Part 4:
Guidelines and requirements for
gas analysis
Gaz naturel — Détermination de la composition et de
l'incertitude associée par chromatographie en phase gazeuse —
Partie 4: Lignes directrices et exigences pour l'analyse des gaz
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
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TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
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MADE IN NATIONAL REGULATIONS.
Reference number
FINAL DRAFT
International
Standard
ISO/TC 193/SC 1
Natural gas — Determination
Secretariat: NEN
of composition and
Voting begins on:
associated uncertainty by gas
chromatography —
Voting terminates on:
Part 4:
Guidelines and requirements for
gas analysis
Gaz naturel — Détermination de la composition et de
l'incertitude associée par chromatographie en phase gazeuse —
Partie 4: Lignes directrices et exigences pour l'analyse des gaz
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
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AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2026
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ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 2
5 Overview . 2
6 Sample . 4
6.1 General .4
6.2 Gas origin . . .4
6.3 Phase of sample .5
6.4 Sample pressure .5
6.5 Sampling .5
7 Sample introduction . 5
7.1 General .5
7.2 Sample loop .5
7.2.1 General .5
7.2.2 Temperature of the sample loop .6
7.2.3 Pressure within the loop .6
7.2.4 Sample shut off before injection .6
7.2.5 Atmospheric pressure effect .6
7.3 Injection .7
7.4 Vacuum injection .8
8 Separation . 9
8.1 General .9
8.2 Columns .10
8.3 Carrier gas .10
8.3.1 General .10
8.3.2 Types of gases .10
8.3.3 Carrier gas flowrate .10
8.3.4 Purity of the carrier and auxiliary gas .11
8.4 Temperature .11
8.5 Back-flush . 13
8.6 Maintenance related to column performance . 13
8.7 Environmental conditions . . 13
8.8 General setup .14
8.9 Correction for the presence of oxygen and argon .14
8.9.1 General .14
8.9.2 Gas containing oxygen. .14
8.9.3 Gas containing argon . 15
8.9.4 Air contamination correction for natural gas spot samples . 15
8.9.5 Correction when the amount of argon has been determined . 15
8.9.6 Correction when the amount of argon has not been determined .16
9 Detection . 17
9.1 Typical detectors for natural gas analysis .17
9.2 Peak resolution .18
9.3 Detector . 20
10 Data processing .21
10.1 Chromatogram .21
10.1.1 General .21
10.1.2 File .21

iii
10.1.3 A/D conversion .21
10.1.4 Chromatogram sampling frequency .21
10.2 Raw data .21
10.2.1 General .21
10.2.2 Signal conversion . 22
10.2.3 Allocation or peak identification . 22
10.3 Peak integration . 22
10.3.1 General . 22
10.3.2 Principle . 22
10.4 Data file format output . 23
11 Calibration .24
12 Optimization.24
12.1 General .24
12.2 Method .24
12.3 Peak area repeatability . 25
13 Precision and bias .25
14 Use of control charts .25
15 Test report .27
Annex A (informative) Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide
and hydrocarbons up to C using two packed columns .28
Annex B (informative) Determination of nitrogen, carbon dioxide and C to C and C
1 5 6+
hydrocarbons for a laboratory and on-line measuring system using two columns .38
Annex C (informative) Isothermal method for nitrogen, carbon dioxide, C to C hydrocarbons
1 5
and C . .46
6+
Annex D (informative) Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide
and C to C hydrocarbons using three capillary columns . 67
1 8
Annex E (informative) Extended analysis: Gas-chromatographic method .81
Annex F (informative) Combining higher hydrocarbons .89
Annex G (informative) Additional standards for natural gas .91
Bibliography .93

iv
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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 193, Natural gas, Subcommittee SC 1,
Analysis of natural gas, in collaboration with the European Committee for Standardization (CEN) Technical
Committee CEN/TC 238, Test gases, test pressures, appliance categories and gas appliance types, in accordance
with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the previous editions of ISO 6974-4:2000, ISO 6974-5:2014,
ISO 6974-6:2002, ISO 6974-6:2002/Cor 1:2003 and ISO 6975:1997, which have been technically revised.
The main changes are as follows:
— the specifications of ISO 6974-3:2000 have been incorporated into Annex A;
— the specifications of ISO 6974-4:2000 have been incorporated into Annex B;
— the specifications of ISO 6974-5:2014 have been incorporated into Annex C;
— the specifications of ISO 6974-6:2002 have been incorporated into Annex D;
— the specifications of ISO 6975:1997 have been incorporated into Annex E.
A list of all parts in the ISO 6974 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
Introduction
The composition of natural gas varies immensely and the addition of biogas, biomethane, hydrogen, syngas
and other natural gas substitutes only adds to the chromatography spectrum. The gas chromatographic
system should be designed not only to separate the components of economic (short-term) interest, but also
for components considered as trace or of no interest. These trace components can be problematic for public
health, safety and assets.
A precise and stable analysis of the main and trace components of a gas can be obtained by an analyser that
is fit-for-purpose. Satisfactory performance of a natural gas analyser requires that the method has good
precision and response characteristics which allow component amount fractions over the range of interest
to be accurately compared with the equivalent components in the reference mixture (calibration).
This document elaborates on all steps involved with gas chromatography and is comprised of extracts from
previous editions of this document and other standards. Added to the extracts is guidance, originating from
common sense and experience, which can help the user and manufacturer to ascertain that their analyser is
fit-for-purpose and that optimum analysis results are obtained from the analyser.

vi
FINAL DRAFT International Standard ISO/FDIS 6974-4:2026(en)
Natural gas — Determination of composition and associated
uncertainty by gas chromatography —
Part 4:
Guidelines and requirements for gas analysis
1 Scope
This document provides guidelines and requirements for the analysis of natural gas and natural gas
substitutes using gas chromatographic methods. It is intended for use in conjunction with ISO 6974-1,
ISO 6974-2 and ISO 6974-3.
The document specifies procedures for the configuration, operation and performance assessment of gas
chromatographs (GCs) used to determine the composition of natural gas and related gaseous fuels. It includes
methods for correcting for air contamination and provides recommendations for ensuring the accuracy and
reliability of analytical results.
This document is applicable to both laboratory and field analyses and supports the determination of
component amount fractions required for the calculation of physical properties such as calorific value,
relative density and Wobbe index.
This document does not apply to the determination of oxygenated compounds (e.g. water vapour, methanol,
glycols), sulfur compounds or to samples in liquid or multiphase states.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements 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 6974-1:2012, Natural gas — Determination of composition and associated uncertainty by gas
chromatography — Part 1: General guidelines and calculation of composition
ISO 6974-2, Natural gas — Determination of composition and associated uncertainty by gas chromatography —
Part 2: Uncertainty calculations
ISO 7504, Gas analysis — Vocabulary
ISO 14532, Natural gas — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 14532, ISO 7504 and the following
apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/

3.1
main components
nitrogen, carbon dioxide and saturated hydrocarbons from methane to n-pentane present in a natural-gas
sample
3.2
associated components
helium, hydrogen, argon and oxygen present in a natural-gas sample
3.3
trace components
hydrocarbons or groups of hydrocarbons beyond n-pentane present in a natural-gas sample
4 Symbols
Table 1 lists the symbols used in this document.
Table 1 — Symbols
Symbol Explanation Unit
amount fraction of nitrogen after correcting the amount fraction for air contamination
x
mol/mol
NC,
amount fraction of nitrogen in the sample after normalization
x
mol/mol
N
amount fraction of oxygen in the sample after normalization
x
mol/mol
O
normalized amount fraction of component j in the sample
x
mol/mol
j,S
* non-normalized amount fraction, of component j in the sample
x mol/mol
j,S
n total number of components -
* amount fraction of argon after correcting the amount fraction for air contamination
mol/mol
x
Ar,C
amount fraction of argon in the sample after normalization
x mol/mol
Ar
normalized amount fraction of component j in the sample after air contamination
x
mol/mol
j,C
correction
Is the non-normalized amount fraction of component j in the sample after air contami-
*
x
mol/mol
j ,C
nation correction
peak resolution
R –
AB
retention distances of the eluted components A and B
ddBA,
  s
RR
widths of the respective peaks at their base
w BA, w
  s
5 Overview
This document covers a wide range of subjects in relation to the analysis of natural gas.
Figure 1 provides a schematic overview. The main clauses are indicated according to the process-sequence.

Figure 1 — Gas analysis overview
The technical content of former parts of the ISO 6974 series (ISO 6974-3 to ISO 6974-6) and ISO 6975
is incorporated in Annex A, Annex B, Annex C, Annex D and Annex E, which provide guidance on the
configuration of gas chromatographs for different component ranges. Annex F provides information on the
validity of inclusion of C components.
6+
6 Sample
6.1 General
All gas analysis starts with a sample. The steps taken in an analytical process depend on the origin and
physical state of the sample.
This clause describes the most common concerns in the first step of that process. More detailed methods of
sampling can be found in ISO 10715.
6.2 Gas origin
The gas production process and its implications for the presence and behaviour of trace components shall
be evaluated and documented. The analytical method and analyser configuration shall take into account
the expected trace components associated with the gas origin. A regular, in-depth investigation of these
components can be useful. However, out-of-specification occurrences are often missed.
A few examples from the field of biomethane:
— Periodic replacement of activated carbon filters is a common operating practice, but overloading of
filters leading to breakthrough of contaminants is dependent on the product of the contaminant amount
fraction, flow rate and time. A small investment in an extra separation column or back-flush with detector
option provides up-to-date information on the contaminant-level and leads to better prediction of the
filter-pack efficiency and proper replacement interval.
— Biomethane usually contains nitrogen, which often originates from the air added to the fermentation
process to decrease the production of hydrogen sulfide and other sulfur compounds. When analysing
for oxygen in the produced biomethane, argon should be sufficiently separated from oxygen because
argon is present in a 0,934 to 78,084 molar ratio to nitrogen, whereas the oxygen can be lower in amount
fraction, because it is used in the process. For instance, at 5 cmol/mol nitrogen this adds up to about
600 µmol/mol argon.
— Water can be identified in the chromatogram. If water is identified in the chromatogram, it shall be
assessed for co-elution and it shall be quantitatively calibrated; amount fractions exceeding 50 µmol/mol
are common and can reach up to 2 cmol/mol. Measurement of the water content with a water dewpoint
analyser provides a way to prevent a mismeasurement.
Syngas or other gases from synthetic sources can contain unsaturated hydrocarbons or other volatile
components that are low in amount fraction individually, but can amount to be significant when lumped
together, either in total amount fraction or in calorific content.
Natural gas has a wide range of amount fractions for all occurring components. In some cases, the gas has
a high amount of higher hydrocarbons and aromatic compounds but in others can be almost pure methane.
This wide range of component amount fractions has implications for the setup of the gas analyser. The
analyser configuration and analytical method shall therefore be appropriate for the expected composition
range of the gas, as described in the annexes. Examples of suitable analyser configurations for different gas
compositions are provided in the annexes:
— Annex A: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and hydrocarbons up to
C using two packed columns
— Annex B: Determination of nitrogen, carbon dioxide and C to C and C hydrocarbons for a laboratory
1 5 6+
and on-line measuring system using two columns
— Annex C: Isothermal method for nitrogen, carbon dioxide, C to C hydrocarbons and C
1 5 6+
— Annex D: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and C to C hydrocarbons
1 8
using three capillary columns
— Annex E: Extended analysis, gas-chromatographic method

— Annex F: Combining higher hydrocarbons.
6.3 Phase of sample
For most applications, the sample phase is gaseous (with low water dewpoint and low hydrocarbon dewpoint,
i.e. dry gas), but it is possible that the sample has a high hydrocarbon or water dewpoint or is liquified.
This document does not cover samples that are liquid, multiphase or have a propensity towards multiphase
behaviour. In this document it is assumed that the introduced sample is entirely gaseous. There are various
solutions available for such samples, ranging from heated sample container cabinets and heat-traced sample
lines up to very advanced evaporation systems.
6.4 Sample pressure
Although the pressure (and temperature) of the sample is closely related to the phase behaviour of the gas, it
is also of concern for the sampling part of the analyser:
— If the sample is at ambient, sub-atmospheric or high pressure, it should be decided if the sample is to be
compressed, pumped or expanded for sampling.
— The gas wetted valves and tubing used shall be assessed to ensure that they do not adversely affect the
composition of the gas sample.
— After filling, the sample loop pressure shall be allowed to stabilise before injection.
— Filling the sample loop, the sample line from the sample container to the sample loop shall be purged at
a sufficient flow rate and for a sufficient duration to ensure that a representative sample is introduced
into the sample loop.
— The sample pressure and flow should not overload the vent system (e.g. of the lab or of other analysers).
These items are treated in more detail in Clause 7.
6.5 Sampling
The sampling of the gas of interest at its source is not treated in this document but is described extensively
in ISO 10715, along with guidelines on the materials used for the sample container and the sample line.
In relation to the configuration of the gas analyser, two types of sampling occur:
— single, from a sample container;
— continuous, e.g. from a gas supply line or process.
See Clause 7 for more information on sampling.
7 Sample introduction
7.1 General
A consistent means of sample introduction is required so that equivalent amounts of sample and reference
mixture are compared.
7.2 Sample loop
7.2.1 General
In most applications, a switching valve is used for sample introduction. The sample is purged through a loop,
which defines the sample size. When the valve operates, the loop is switched into the carrier gas stream and

the loop contents are swept by the carrier gas onto the separation system, see Figure 2 a) and Figure 2 b). An
alternative for micro-GCs uses pressure switching for a selected short time to achieve the effect. There are
some viscosity influences, but this is not considered here.
The sample loop has a defined volume, but the amount of the injected sample is influenced by the temperature
and pressure of the contained gas. The sample loop is sufficiently flushed until the volume of the sample or
reference gas has fully purged the previous loop contents.
The following subclauses of 7.2 shall therefore be taken into consideration.
7.2.2 Temperature of the sample loop
The sample loop can be fitted in the column oven or in a separately controlled zone, e.g. a valve box or
oven. The size of the zone is not usually critical, but the stability is. Temperature variations cause different
effective sample amounts and hence poor repeatability.
Higher temperature stability can be achieved by adding masses of metal around the sample-loop and sample-
valve. Combination with insulation and shielding the analyser from airflows (e.g. air-conditioning vents and
analyser exhausts) further improves stability.
7.2.3 Pressure within the loop
The sample line usually purges to atmosphere (possibly through a vent line extending outside the laboratory
or analyser housing). If the sample flowrate and the resistance of the vent line create a backpressure, the
pressure in the sample loop varies with flowrate. Usually, the flows of calibration gas and sample gas are
controlled by different means and can be set to different values, so there can be a difference in effective
sample size between the two and hence the possibility of bias error.
When using pressure correction, it is recommended to connect the (high-accuracy) barometer as closely as
possible to the sample-loop, i.e. connect the barometer directly to the exit of sample shut-off valve outlet.
This prevents measuring the fluctuating under- or overpressure of the laboratory.
Also, automatic pressure correction is possible, installed by the manufacturer of the GC. It should either be
disabled, removed or have the capability to be externally calibrated.
7.2.4 Sample shut off before injection
Stopping the sample (or calibration gas) flow a few seconds before injection allows the loop pressure to
decay to atmospheric. As a result, it only varies according to atmospheric pressure variations; however,
these can still be significant.
The optimum sample shut-off time is influenced by the stability (or deterioration) of the sample. This is
caused by sample composition, the sample-loop material and sample-loop temperature.
7.2.5 Atmospheric pressure effect
The atmospheric pressure effect can be measured at the exact time of injection and corrected for or ignored.
If ignored and the method is a single operation one, normalization corrects it, since sample loop pressure has
the same relative influence on all components. In fact, for single operation, atmospheric pressure correction
followed by normalization (pressure correction alone never sets results to 100 %) gives the same result as
normalization alone.
However, in multipoint calibration, no component has the same calibration function. As a consequence, any
pressure different from standard gives a slightly different correction on the raw analyser output compared to
the other components. This effect occurs both during calibration and analysis, so, for multipoint calibration
and subsequent analysis, pressure correction should be applied.

7.3 Injection
When only a small sample volume is available, or the GC is not equipped to handle atmospheric samples, a
direct injection (e.g. by syringe) on the GC injector is a good alternative.
Most GC are equipped with a split or splitless injector, a small, heated chamber that either flushes the sample
on to the column (splitless mode) or flushes a portion of the sample into the column (split mode).
In split mode, a part of the mixture of sample and carrier gas in the injection chamber is exhausted through
the split vent. Split injection should be used when working with samples with high analyte amount fractions.
Splitless injection is best suited for trace analysis with low amounts of analytes. Figure 2 shows the valve
positions for the operation of a simple GC configuration using a switching valve. Figure 2 a) shows the
sampling position and Figure 2 b) shows the injection position. In splitless mode, the valve opens for a pre-
set amount of time to flush the sample on the separating column with carrier gas.
a) Sampling
b) Injection
Key
A sample in
B carrier gas regulator
C separation column
D detector
E sample out
F sample loop
Figure 2 — Operation of a simple GC configuration using a switching valve
Different types of injection can be specified:
— Manual injection: For a small number of samples, manual injection can be used. The injection should be
repeatable in order to have repeatable results.
— Auto-sampling: For a large number of samples, an auto-sampler should be used to produce more
repeatable injections and results. For gas samples, the auto-sampler can be more complicated than for
liquid samples.
— Sample-loop: The capacity of the sample loop should be chosen based on the detection limit wanted and
on the column dimension. The split flow also plays a role in this case.
— Time-based valve injection: This kind of valve can be used for a better repeatability of the injection and
so, for the analysis.
— Injection using a Deans type switch: This is an effluent switching device based on controlling flows of
carrier gas instead of mechanical valves in the analytical flow path. This technique offers high inertness
and a wear-free operation.
In process GCs, a combination of an autosampler with a time-based valve injection or Deans type switch is
most often seen.
7.4 Vacuum injection
If the sample pressure is very low, or the amount of sample available is very small, it can be necessary to
evacuate the sampling system prior to injection. By removing any previous sample, air or carrier gas from
the sample loop using an integrated or external vacuum pump prior to introducing the sample, this greatly
reduces the amount of sample required to get a representative sample of the gas. See Figure 3 for a schematic
overview of a vacuum injection system.

Key
A sample in
B instrument sampling system
C vacuum pump
D sample out
Figure 3 — Vacuum injection
The procedure is as follows:
1) Connect the sample to sample-in position A, vacuum pump and pressure gauge as per Figure 3, with all
three valves closed.
2) Evacuate the sampling system by opening valve V1.
1)
3) Once a suitable pressure (vacuum) is achieved (< 10 mbar ) with vacuum pump C, isolate the vacuum
pump using valve V1. Safely discharge the gas from connection D using a blow-off line.
4) Monitor the pressure indicator to ensure that ambient air is not leaking into the system. If the pressure
is steadily increasing, check all the fittings and return to step 2).
5) The sample should then be slowly introduced to the sample injection system B. Valve V3 should be
opened first, then valve V2 can be slowly opened and used to control the flow. Once the pressure within
the system reaches atmospheric pressure, close valves V2 and V3.
6) The sample can then be injected.
7) Repeat steps 2) to 6) until the required number of injections is achieved.
If there is sufficient sample, it can be beneficial to omit step 6) after the first evacuation and re-pressurization
cycle to ensure the sampling system is fully purged before injecting.
8 Separation
8.1 General
A separation system, consisting of one or more chromatographic columns, which allows consistent retention
times and adequately separates all components of interest, is required to obtain reliable results.
This involves at least one column, usually more, in a temperature-controlled oven with a supply of carrier
gas controlled either by pressure or by flow. Where multiple columns are used, the flow configuration is
changed through the analytical cycle. This can be accomplished either by switching valves or by a Deans
type pressure control. Either system uses timed events within the analyser controller to create the changes.
To obtain optimum separation and reliable results, the user can select from a wide range of separation
options. Different combinations of parts and operating conditions can be used to achieve an analysis that
produces results for components of interest with adequate accuracy or to make the results as detailed
as possible. Subclauses 8.2 to 8.5 highlight some of the system parts used for a gas analysis system
configuration.
1) 1 bar = 0,1 MPa = 105 Pa; 1 MPa = 1 N/mm .

8.2 Columns
The separation column is the functional part of the GC. Separation occurs through interactions between
the individual compounds in the sample and the stationary phase within the column. Several types can be
discerned:
— Packed or capillary phase: Packed columns are more common in process GCs or for the separation of
bigger quantities of gas. Capillary columns give a better separation of the components since they have
a higher
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ISO/TC 193/SC 1
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Date: 2026-04-2909-01
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Natural gas — Determination of composition and associated
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uncertainty by gas chromatography — —
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Part 4:
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Guidelines and requirements for gas analysis
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Gaz naturel — Détermination de la composition et de l'incertitude associée par chromatographie en phase
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Partie 4: Lignes directrices et exigences pour l'analyse des gaz
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FDIS stage
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ii
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Contents
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Foreword . x
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Not at 0.71 cm
Introduction . xi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 2
5 Overview . 2
6 Sample . 5
6.1 General. 5
6.2 Gas origin . 5
6.3 Phase of sample . 6
6.4 Sample pressure . 6
6.5 Sampling . 6
7 Sample introduction . 7
7.1 General. 7
7.2 Sample loop . 7
7.3 Injection . 8
7.4 Vacuum injection . 11
8 Separation . 13
8.1 General. 13
8.2 Columns . 13
8.3 Carrier gas . 14
8.4 Temperature. 15
8.5 Back-flush . 18
8.6 Maintenance related to column performance . 18
8.7 Environmental conditions . 18
8.8 General setup . 19
8.9 Correction for the presence of oxygen and argon . 19
9 Detection . 23
9.1 Typical detectors for natural gas analysis . 23
9.2 Peak resolution . 24
9.3 Detector . 29
10 Data processing . 29
10.1 Chromatogram . 29
10.2 Raw data . 30
10.3 Peak integration . 30
10.4 Data file format output . 33
11 Calibration . 33
12 Optimization . 34
12.1 General. 34
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12.2 Method . 34
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12.3 Peak area repeatability . 34
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13 Precision and bias . 35
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14 Use of control charts . 35
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© ISO 2026 – All rights reserved
iii
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15 Test report . 39
Annex A (informative) Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide
and hydrocarbons up to C8 using two packed columns. 40
Annex B (informative) Determination of nitrogen, carbon dioxide and C to C and C
1 5 6+
hydrocarbons for a laboratory and on-line measuring system using two columns . 53
Annex C (informative) Isothermal method for nitrogen, carbon dioxide, C to C hydrocarbons
1 5
and C6+ . 65
Annex D (informative) Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide
and C to C hydrocarbons using three capillary columns . 61
1 8
Annex E (informative) Extended analysis: Gas-chromatographic method . 82
Annex F (informative) Combining higher hydrocarbons . 91
Annex G (informative) Additional standards for natural gas . 96
Bibliography . 98

1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 1
5 Overview . 2
6 Sample . 3
6.1 General. 3
6.2 Gas origin . 3
6.3 Phase of sample . 4
6.4 Sample pressure . 4
6.5 Sampling . 4
7 Sample introduction . 5
7.1 General. 5
7.2 Sample loop . 5
7.2.1 General. 5
7.2.2 The temperature of the sample loop . 5
7.2.3 The pressure within the loop . 5
7.2.4 Sample shut-off before injection . 6
7.2.5 The atmospheric pressure effect . 6
7.3 Injection . 6
7.4 Vacuum injection . 7
8 Separation . 8
8.1 General. 8
8.2 Columns . 9
8.3 Carrier gas . 9
8.3.1 Types of gases. 9
8.3.2 Carrier gas flowrate . 9
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8.3.3 Purity of the carrier and auxiliary gas . 10
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8.4 Temperature. 10
8.5 Separation columns . 11
8.6 Back-Flush . 11
8.7 Maintenance related to column performance . 12
8.8 Environmental conditions . 12
8.9 General setup . 12
8.10 Correction for the presence of oxygen and argon . 13
8.10.1 General. 13
8.10.2 Gas containing oxygen. 13
8.10.3 Gas containing argon . 14
8.10.4 Air contamination correction for natural gas spot samples . 14
8.10.5 Correction when the amount of argon has been determined . 14
8.10.6 Correction when the amount of argon has not been determined . 15
9 Detection . 16
9.1 Typical detectors for natural gas analysis . 16
9.2 Peak resolution . 17
9.3 Detector . 20
10 Data processing . 20
10.1 Data . 20
10.1.1 General. 20
10.1.2 Conversion . 21
10.1.3 Allocation or peak identification . 21
10.1.4 Data file format . 21
10.2 Peak integration . 22
10.2.1 General. 22
10.2.2 Principle . 22
10.3 Chromatogram . 23
10.3.1 General. 23
10.3.2 File . 23
10.3.3 A/D Conversion . 23
10.3.4 Sampling frequency . 23
11 Calibration . 23
12 Optimization . 23
12.1 General. 23
12.2 Method . 23
12.3 Repeatability . 24
13 Precision and bias . 24
14 Use of control charts (from ISO 6975:1997) . 24
15 Test Report . 27
Annex A (informative) Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide
and hydrocarbons up to C8 using two packed columns . 28
A.1 Application Ranges . 28
A.2 Principle . 28
A.3 Materials . 29
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A.3.1 For the determination of helium, hydrogen, oxygen and nitrogen. 29
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A.3.2 For the determination of nitrogen, carbon dioxide and hydrocarbons from C to C
1 8
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(separation on Porapak column), . 29
A.4 Apparatus . 30
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A.4.1 Laboratory gas chromatographic (GC) system . 30
A.5 Procedure . 32
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A.5.1 Gas chromatographic operating conditions . 32
A.5.2 Performance requirements . 34
A.5.3 Determination . 34
A.6 Example: Single-oven gas-chromatographic system consisting of two columns . 35
Annex B (informative) Determination of nitrogen, carbon dioxide and C1 to C5 and C6+
hydrocarbons for a laboratory and on-line measuring system using two columns . 39
B.1 Application ranges . 39
B.2 Principle . 39
B.3 Materials . 40
B.3.1 Helium carrier gas, . 40
B.3.2 Working-reference gas mixtures (WRM), . 40
B.4 Apparatus . 40
B.4.1 Laboratory gas chromatographic (GC) system, . 40
B.5 Procedure . 41
B.5.1 Gas chromatographic operating conditions . 41
B.5.2 Performance requirements — Peak resolution . 45
B.5.3 Determination — Outline of the analysis . 46
B.6 Expression of results . 46
B.6.1 Calculation . 46
B.6.2 Precision and accuracy . 46
B.7 Procedure for setting valve timings and restriction setting . 46
B.8 Final time settings . 47
Annex C (informative) Isothermal method for nitrogen, carbon dioxide, C1 to C5 hydrocarbons
and C6+ . 48
C.1 Application ranges . 48
C.2 Principle . 48
C.3 Materials . 49
C.3.1 Carrier gas, . 49
C.3.2 Auxiliary gases, . 49
C.3.3 Reference materials . 49
C.3.4 Reference gases, . 49
C.3.5 Gas mixture containing n-Pentane and 2,2-Di-Me-butane, . 50
C.4 Apparatus . 50
C.4.1 Gas chromatograph, . 50
C.4.2 Column oven, . 50
C.4.3 Valve oven,. 50
C.4.4 Pressure regulator, . 50
C.4.5 Injection device, . 50
C.4.6 Backflush valve, . 50
C.4.7 Column isolation valve, . 50
C.4.8 Columns, . 50
C.4.9 Tube and packing. . 50
C.4.10 Method of packing, . 52
C.4.11 Thermal Conductivity Detector (TCD), . 52
C.4.12 Controller/Peak Measurement System, . 52
C.4.13 Auxiliary valves, tubing and other accessories, . 52
C.5 Scheme of the configuration . 52
C.6 Procedure . 54
C.6.1 Control of the apparatus. 54
C.6.2 Column Conditioning . 55
C.6.3 Operation of the apparatus . 55
C.7 Expression of results . 58
C.7.1 Uncertainty. 58
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C.8 Example of application . 58
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C.8.1 General considerations . 58
C.8.2 Calculation of mole fractions . 61
C.8.3 Calculation of uncertainties of mole fractions . 68
C.8.4 Comparison of mean normalization and run-by-run approaches . 69
C.8.5 Reporting of results . 69
C.8.6 Excel spreadsheet . 69
C.9 Procedure for Setting Valve timings and Restrictor Setting . 70
C.9.1 Initial Flow Settings . 70
C.9.2 Backflushing . 70
C.9.3 V3 Timing . 70
C.9.4 Final timings . 71
Annex D (informative) Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide
and C1 to C8 hydrocarbons using three capillary columns . 72
D.1 Application ranges . 72
D.2 Principle . 74
D.2.1 Analysis of natural gas sample . 74
D.2.2 Auxiliary gases . 74
D.3 Materials . 74
D.3.1 Carrier gases . 74
D.3.1.1 Argon (Ar) . 74
D.3.1.2 Nitrogen (N2) . 74
D.3.1.3 Helium (He) . 74
D.3.2 Auxiliary gases . 74
D.3.2.1 For FID detection: . 74
D.3.2.1.1 Nitrogen (N ) . 74
D.3.2.1.2 Air . 74
D.3.2.1.3 Hydrogen (H ) . 75
D.3.2.2 For methanizer . 75
D.3.2.2.1 Hydrogen, . 75
D.3.2.2.2 Pressurized laboratory air . 75
D.3.3 Reference materials . 75
D.3.3.1 Working reference gas mixture (WRM) . 75
D.3.3.2 Performance test gases. . 75
D.3.3.2.1 For methanizer operation . 75
D.3.3.2.2 Gas containing benzene and cyclohexane . 75
D.3.3.2.3 Gas containing hydrogen and helium . 76
D.4 Apparatus . 76
D.4.1 Gas chromatograph system(s) . 76
D.4.1.1 Two column ovens . 76
D.4.1.1.1 Instrument 1 oven . 76
D.4.1.1.2 Instrument 2 oven, . 76
D.4.1.2 Flow regulators . 78
D.4.1.3 Gas sampling valves (GSV) . 78
D.4.1.4 Valveless or micro-valve column-switching system . 78
D.4.1.5 Thermal conductivity detector (TCD) and flame ionization detector (FID) . 78
D.4.1.5.1 Instrument 1 detectors . 78
D.4.1.5.2 Instrument 2 detector . 78
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D.4.1.6 Data acquisition system. 78
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D.4.1.7 Methanizer . 78
D.4.2 Capillary columns . 79
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D.4.2.1 PLOT fused silica capillary precolumn, . 79
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D.4.2.2 Molecular sieve PLOT fused silica capillary column . 79
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D.4.2.3 Non-polar WCOT fused silica capillary column, . 79
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D.5 Procedure . 81
D.5.1 Operating conditions . 81
D.5.1.1 Gas chromatograph . 81
D.5.1.2 Column conditioning . 81
D.5.1.3 Sample introduction . 82
D.5.2 Performance requirements . 83
D.5.2.1 Column performance evaluation. 83
D.5.2.2 Relative response factors .
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