kSIST FprEN ISO 6974-4:2026

• 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
• Public Enquiry End Date: 02-May-2025
• ICS: 75.060 - Natural gas
• Technical Committee: DPL - Gas supply
• Current Stage: 5020 - Formal vote (FV) (Adopted Project)
• Start Date: 24-Sep-2026
• Due Date: 12-Nov-2026
• Ref Project: 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)

• Revises: SIST EN ISO 6974-4:2001 - 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 (ISO 6974-4:2000)
• Effective Date: 01-Apr-2025

Overview

kSIST FprEN ISO 6974-4:2026 outlines essential guidelines and requirements for the precise analysis of natural gas and natural gas substitutes using gas chromatography (GC). Issued by SIST in alignment with international ISO standards, this document provides a comprehensive framework for achieving accurate determination of natural gas composition and associated uncertainty. Designed to be used alongside the most recent editions of ISO 6974 Parts 1, 2, and 3, this standard supports both laboratory and continuous online analytical systems. It ensures reliable, repeatable results vital for regulatory compliance, safety, and commercial transactions within the energy sector.

Key Topics

• Sampling and Sample Introduction

  • Guidance on ensuring representative sampling from different sources, such as natural gas, biomethane, hydrogen blends, or synthetic gases.
  • Sample phase (gaseous) management, pressure considerations, and appropriate use of sample loops for accurate injection.
  • Discussion of direct injection vs. automated sampling methods and associated advantages for repeatability and accuracy.

• Separation and Detection

  • Criteria for selecting appropriate chromatographic columns and carrier gases.
  • Guidance on managing temperature control, carrier gas purity, and system stabilization to enable high-resolution component separation.
  • Overview of typical detection techniques suitable for natural gas analysis, including peak resolution and bias considerations.

• Data Processing and Calibration

  • Detailed requirements for data integration, peak allocation, chromatogram handling, and format of output (including XML as per ISO 23219).
  • Emphasis on calibration procedures, including verification of system performance and use of reference mixtures.
  • Guidance on maintaining precision and minimizing bias through statistical evaluation.

• Uncertainty Evaluation

  • Strategies for uncertainty estimation and reporting, supporting traceable and robust measurement results.
  • Recommendations for the application of normalization, pressure correction, and calibration function adjustments.

Applications

This standard is fundamental for a range of applications across the natural gas industry:

• Quality Control and Regulatory Compliance

  • Ensures gas quality conforms to international and local regulations.
  • Enables precise monitoring of trace and bulk components, vital for health, safety, and asset protection.

• Commercial Transactions

  • Underpins fair trade by providing a reliable basis for gas composition assessment, crucial for billing and energy content calculation.

• Production and Processing Facilities

  • Supports continual process optimization by delivering robust gas analysis suitable for tracking operational changes and adapting to varying gas compositions.
  • Particularly important for mixed gas streams including biomethane, hydrogen, and syngas.

• Research and Development

  • Facilitates high-precision gas characterization, which is critical for developing new analytical techniques and advancing fuel technologies.

Related Standards

kSIST FprEN ISO 6974-4:2026 is part of a broader family of standards for natural gas analysis:

• ISO 6974-1: General guidelines and calculation of composition - foundation for compositional analysis.
• ISO 6974-2: Focuses on uncertainty calculations, establishing a framework for measurement reliability.
• ISO 6974-3: Addresses methods to evaluate precision and bias within gas chromatography.
• ISO 10715: Provides detailed procedures for natural gas sampling, ensuring representative sample collection.
• ISO 23219: Describes the XML file format for transmitting natural gas chromatograph analyser data.
• ISO 7504 & ISO 14532: Offer essential terminology for gas analysis and the natural gas supply chain.

Practical Value

Adherence to kSIST FprEN ISO 6974-4:2026 maximizes the accuracy and reliability of natural gas analysis by gas chromatography. It empowers laboratories, plant operators, and regulatory bodies to minimize uncertainty, maintain consistency, and optimize GC system performance. By following its guidelines, stakeholders can ensure regulatory compliance, support operational efficiency, and build trust throughout the natural gas value chain.

SLOVENSKI STANDARD
oSIST prEN ISO 6974-4:2025
01-april-2025
Zemeljski plin - Določanje sestave in s tem povezane negotovosti s plinsko
kromatografijo - 4. del: Navodilo za analizo plina (ISO/DIS 6974-4:2025)
Natural gas - Determination of composition and associated uncertainty by gas
chromatography - Part 4: Guidance on gas analysis (ISO/DIS 6974-4:2025)
Erdgas - Bestimmung der Zusammensetzung und der damit verbundenen Unsicherheit
durch Gaschromatographie - Teil 4: Leitfaden für die Gasanalyse
Gaz naturel - Détermination de la composition avec une incertitude définie par
chromatographie en phase gazeuse - Partie 4: Détermination de l'azote, du dioxyde de
carbone et des hydrocarbures C1 à C5 et C6+ pour un système de mesurage en
laboratoire et en continu employant deux colonnes
Ta slovenski standard je istoveten z: prEN ISO 6974-4
ICS:
75.060 Zemeljski plin Natural gas
oSIST prEN ISO 6974-4:2025 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

oSIST prEN ISO 6974-4:2025
oSIST prEN ISO 6974-4:2025
DRAFT
International
Standard
ISO/DIS 6974-4
ISO/TC 193/SC 1
Natural gas — Determination
Secretariat: NEN
of composition and
Voting begins on:
associated uncertainty by gas
2025-02-10
chromatography —
Voting terminates on:
2025-05-05
Part 4:
Guidance on gas analysis
ICS: 75.060
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
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RECIPIENTS OF THIS DRAFT ARE INVITED
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NOTIFICATION OF ANY RELEVANT PATENT
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Reference number
ISO/DIS 6974-4:2025(en)
oSIST prEN ISO 6974-4:2025
DRAFT
ISO/DIS 6974-4:2025(en)
International
Standard
ISO/DIS 6974-4
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:
Guidance on gas analysis
ICS: 75.060
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2025
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland Reference number
ISO/DIS 6974-4:2025(en)
ii
oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
Contents Page
Foreword .v
Introduction .vi
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 .5
7.2.5 The atmospheric pressure effect .6
7.3 Injection .6
7.4 Vacuum injection .8
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
8.3.3 Purity of the carrier and auxiliary gas .10
8.4 Temperature .10
8.5 Separation columns .11
8.6 Back-Flush. 12
8.7 Maintenance related to column performance . 12
8.8 Environmental conditions . 12
8.9 General setup . 13
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 . 20

iii
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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 . 22
10.3.1 General . 22
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 .26
Annex A (informative) Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide
and hydrocarbons up to C8 using two packed columns .27
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 .37
Annex C (informative) Isothermal method for nitrogen, carbon dioxide, C1 to C5 hydrocarbons
and C6+ .46
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) Natural gas -Extended analysis - Gas-Chromatographic method .82
Annex F (informative) Natural gas -Extended analysis - Gas-Chromatographic method .91
Annex G (informative) GPA 2286-95 .93
Bibliography .94

iv
oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
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 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 the following URL:
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.
This edition cancels and replaces the previous editions of ISO 6974-3:2000, ISO 6974-4:2000, ISO 6974-5:2014,
ISO 6974-6:2002 and ISO 6975:1997, which in part have been technically revised.
The main changes compared to the previous edition(s) are extensive, as this document is the compilation of
the formentioned documents, added to a collection of knowledge on natural gas chromatography.
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
oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
Introduction
The composition of natural gasses varies immensely, and the addition of biogas, biomethane, hydrogen,
syngas and other natural gas substitutes only adds to 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 could be potentially
problematic for public health, safety or assets.
A precise and stable analysis of the main and trace components of the 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 concentrations 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 older parts of this standard, and other standards. Added to the extracts is guidance, originating from
common sense and experience, which may 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
oSIST prEN ISO 6974-4:2025
DRAFT International Standard ISO/DIS 6974-4:2025(en)
Natural gas — Determination of composition and associated
uncertainty by gas chromatography —
Part 4:
Guidance on gas analysis
1 Scope
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).
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, 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 6974-3:2018, Natural gas — Determination of composition and associated uncertainty by gas
chromatography — Part 3: Precision and bias
ISO 7504:2015, Gas analysis — Vocabulary
ISO 10715:2022, Natural gas — Gas sampling
ISO 14532:2014, Natural gas — Vocabulary
ISO 23219:2022, Natural gas — Format for data from gas chromatograph analysers for natural gas — XML
file format
3 Terms and definitions
For the purposes of this document, the terms and definition used are taken from ISO 14532 and ISO 7504.
ISO and IEC maintain terminological 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/
4 Symbols
The table below, Table 4-1, lists the symbols used in the main text of this document.

oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
Table 4-1 — Symbols
symbol explanation unit
x mole fraction of nitrogen after correcting the mole fraction for air contamination mol/mol
NC,
mole fraction of nitrogen in the sample after normalization; mol/mol
x
N
x mole fraction of oxygen in the sample after normalization; mol/mol
O
normalized mole fraction of component j in the sample; mol/mol
x
jS,
*
non-normalized mole fraction, of component j in the sample; mol/mol
x
jS,
n total number of components; -
*
mole fraction of argon after correcting the mole fraction for air contamination; mol/mol
x
Ar ,C
mole fraction of argon in the sample after normalization; mol/mol
x
Ar
normalized mole fraction of component j in the sample after air contamination correc- mol/mol
x
jc,
tion;
*
Is the non-normalized mole fraction of component j in the sample after air contamina- mol/mol
x
jC,
tion correction;
R peak resolution -
AB
retention distances of the eluted components A and B s
dB(),dA()
RR
wB ,wA widths of the respective peaks at their base s
() ()
5 Overview
This guidance document covers a wide range of subjects in relation to the analysis of natural gas.
Below, in Figure 1, a schematic overview is given, the main clauses are indicated according to the process-
sequence.
oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
Figure 1 — Gas Analysis Overview
6 Sample
6.1 General
All gas analysis starts with a sample. The origin and physical state of the sample are of importance to the
subsequent steps in the analytical process. This clause describes the most common concerns in the first step
of that process. More detailed methods of sampling can be found in ISO10715.
6.2 Gas origin
A good understanding of the gas production process and the consequences for possible trace components
should be consulted for. Although a periodic in-depth investigation of these components seems like a wise
way of operating, it has the innate possibility to miss the (near) out-of-spec occurrences.
A few examples from the field of biomethane:
— Periodic replacement of activated carbon filters is a clear and common way of operating, but the
flooding of a filter is primarily caused by contaminant concentration times flow, and not by time alone.
A small investment in an extra separation column or back-flush with detector option provides up-to-
date information of the contaminant-level and leads to better prediction of the filter-pack efficiency and
proper moment of replacement.
— Biomethane usually contains nitrogen, which, more often than not, originates from the air added to the
fermentation process to decrease the production of hydrogen sulfide and other sulphur compounds.
When analyzing for oxygen in the produced biomethane, argon should be sufficiently separated from
oxygen because argon will be present in a 0,934 0 to 78,084 ratio to nitrogen, whereas the oxygen might

oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
be lower in concentration, because it is used in the process. For instance, at 5 mol% nitrogen this adds up
to about 600 ppm argon.
— Water should be identified in the chromatogram to determine co-elution or even calibrated for, because
a high level (> 50 ppm) is quite common and could get as high as 1 vol%. Measurement of the water
concentration with a water dewpoint analyser provides a way to prevent a mismeasurement.
Syngas or other gasses from synthetic sources may contain unsaturated hydrocarbons or other volatile
components that are low in concentration individually, but could amount to be significant when lumped
together, either in total concentration or in calorific content.
Natural gas has a wide range of concentration for all occurring components. In some cases, the gas has quite
a high amount of higher hydrocarbons and aromatic compounds, but could also be almost pure methane.
This wide range of component concentrations has implications for the set-up of the gas analyser (and will be
detailed further on in this document).
6.3 Phase of sample
For most applications the sample phase will be gaseous (with low water dewpoint and low hydrocarbon
dewpoint, i.e. dry gas), but it may very well be that the sample has a high hydrocarbon or water dewpoint or
is liquified.
This document will not treat samples that are liquid, multiphase or have a propensity towards multiphase
behavior, and for 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 pressure (and temperature) of the sample is closely related to phase behavior of the gas, it also is
of concern for the sampling part of the analyser:
— If the sample is at ambient, sub-atmospheric or high pressure it needs to be decided if the sample is to be
compressed, pumped or reduced for sampling.
— The gas wetted valves and tubing used need to be assessed accordingly.
— The sample loop pressure needs to be able to equilibrate within the time between filling and injection.
— The sample flow needs to be enough to properly purge the sample line, from the sample container to the
sample loop, so a representative sample is taken.
— The sample pressure and flow should not overload the vent system (e.g. of the lab or of other analyzers).
These items are treated in more detail further in this document.
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 mainly two types of sampling occur:
— Single, from a sample container
— Continuous, from e.g. a gas supply line or process
Being closely related to the next step, sample introduction, this is treated in more detail in the next clause.

oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
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.
This defines the sample size and when the valve operates. The loop is switched into the carrier gas stream.
The loop contents are swept by the carrier gas onto the separation system, see Figure 2. An alternative for
micro-GCs uses pressure switching for a selected short time to achieve the effect. There are some viscosity
influences, and it is not considered here.
The sample loop has a defined volume, but the size of injected sample is influenced by the temperature and
pressure of the contained gas. The sample loop is sufficiently flushed until the volume of sample or reference
gas has fully purged the previous loop contents.
We therefore need to consider the following.
7.2.2 The temperature of the sample loop
The sample loop can be controlled separately or fitted in the column oven. The size is not usually critical,
but the stability is. Temperature variations will cause different effective sample amounts and hence poorer
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 analyzer from airflows (e.g. air-conditioning vents and
analyser exhausts) will further improve stability.
7.2.3 The pressure within the loop
The sample line usually purges to atmosphere (possibly through a vent line extending outside the laboratory
or analyzer housing). If the sample flowrate and the resistance of the vent line create a backpressure, the
pressure in the sample loop will vary with flowrate. Usually the flows of calibration gas and sample gas are
controlled by different means and may be set to different values, so there may be a difference in effective
sample size between the two and hence the possibility of bias error.
When using pressure correction it is advisable 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, be aware of the possibility of automatic pressure correction, 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
This is a common procedure. Stopping the sample (or calibration gas) flow a few seconds before injection
allows the loop pressure to decay to atmospheric. As a result, it will only vary according to atmospheric
pressure variations, which can 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.

oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
7.2.5 The 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 will correct 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 will give a slightly different correction on the raw analyzer 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/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 is preferred when working with samples with high analyte concentrations.
Splitless injection, see Figure 2, is best suited for trace analysis with low amounts of analytes. In splitless
mode the split valve opens for a pre-set amount of time to flush the sample on the separating column. This
pre-set time should be optimized, a shorter time ensures less tailing but loss in response, a longer time
increases tailing but also signal.

oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
Figure 2 — Operation of a simple GC configuration using a switching valve; (a) sampling and (b)
injection
Different types of Injection can be specified:
— Manual injection: if you don’t have to analyze too many samples, manual injection can be used. Be aware
that the injection should be repeatable in order to have repeatable results.
— Auto-sampling: in case of many samples, an auto-sampler can be a good option and also to have more
repeatable injections and results. For gas samples, the auto-sampler could 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.
oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
7.4 Vacuum injection
If the sample pressure is very low, or the amount of sample available is very small it may be necessary to first
evacuate the sampling system prior to injection. By first removing any previous sample, air or carrier gas
from the sample loop using a vacuum pump prior to introducing the sample will greatly reduce the amount
of sample required to get a representative sample of the gas. See Figure 3 for schematic overview.
Figure 3 — Vacuum injection
Procedure:
1. Connect the sample, vacuum pump and pressure gauge as per the above figure, with all three valves closed.
2. Evacuate the sampling system by opening valve V1.
3. Once a suitable pressure (vacuum) is achieved (< 10 mbar) isolate the vacuum pump using valve V1.
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 system; valve V1 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 2-6 until the required number of injections is achieved.
If there is sufficient sample it may 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 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 analyzer controller to create the changes.
To obtain optimum separation and reliable results the user has the possibility to select from a wide range
of separation options, and with a wide range of combinations it is possible to have an analysis that produces
the components of interest at adequate accuracy or analyses to give results as detailed as possible. The
following sub-clauses highlight some of the system parts available for a gas analysis system configuration.

oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
8.2 Columns
The gas chromatographic columns are a vital part of the analyser and can be chosen according to
specifications:
— type of column (packed, capillary)
— stationary phase specification:
— stationary phase thickness (capillary column)
— characterization of solid phase (packed column)
— column dimensions
All these have an influence on the separation, retention time (speed of analysis) and sensitivity.
8.3 Carrier gas
The carrier gas is essential to the gas chromatographic separation of the injected sample. The choice of
carrier gas depends on the analytes that are to be separated. In multi-column setup, different carrier gasses
can be selected in order to be able to analyze the complementary components, i.e. use helium as carrier gas
to analyze for nitrogen and use nitrogen as carrier gas to analyze for helium.
8.3.1 Types of gases
Gases, supplied from cylinder or generator, used in GC analyses are divided by their use for the separation
and detection. The most common carrier gases are:
— Helium
— Hydrogen
— Argon
— Nitrogen
— Air
The choice of gas can depend on the available gas supply, the detection method and safety concerns.
8.3.2 Carrier gas flowrate
Component retention times are a direct function of carrier gas flowrate, and so flow variations will have
a possible effect on peak identification. With the thermal conductivity detector, which is concentration
sensitive, increased flowrate will give smaller peak areas (but similar peak heights) and vice versa. With the
flame ionization detector, which is mass sensitive, flowrate change will have a secondary effect on peak area
measurement due to the fact that detector sensitivity varies with the carrier gas/hydrogen ratio.
Most common configurations of the carrier gas flowrate are:
— Constant velocity
— Constant pressure
— Programmed
oSIST prEN ISO 6974-4:2025
ISO/DIS 6974-4:2025(en)
8.3.3 Purity of the carrier and auxiliary gas
The purity of the carrier and auxiliary gas has an influence on the (long-term) performance of a
separation column:
— A carrier gas purity shall be at least 99,99 % (4,0), but 99,999 % (5,0) or even 99,999 9 % (6,0) purity is
preferred. The additio
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