ASTM D2268-21

Standard Test Method for Analysis of High-Purity <emph type="ital">n</emph>-Heptane and <emph type="ital">Iso</emph>octane by Capillary Gas Chromatography

SIGNIFICANCE AND USE
5.1 This test method is used for specification analysis of high-purity n-heptane and  isooctane, which are used as ASTM Knock Test Reference Fuels. Hydrocarbon impurities or contaminants, which can adversely affect the octane number of these fuels, are precisely determined by this method.
SCOPE
1.1 This test method covers and provides for the analysis of high-purity (greater than 99.5 % by volume) n-heptane and isooctane (2,2,4-trimethylpentane), which are used as primary reference standards in determining the octane number of a fuel. Individual compounds present in concentrations of less than 0.01 % can be detected. Columns specified by this test method may not allow separation of all impurities in reference fuels.  
1.2 The values stated in SI units are to be regarded as the standard.  
1.2.1 Exception—The values given in parentheses are for information only.  
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Nov-2021
• ICS: 71.080.10 - Aliphatic hydrocarbons
• Technical Committee: D02 - Petroleum Products, Liquid Fuels, and Lubricants
• Drafting Committee: D02.04.0L - Gas Chromatography Methods

Relations

• Refers: ASTM D4175-23a - Standard Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
• Effective Date: 15-Dec-2023
• Refers: ASTM D4175-23e1 - Standard Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
• Effective Date: 01-Jul-2023
• Refers: ASTM E594-96(2019) - Standard Practice for Testing Flame Ionization Detectors Used in Gas or Supercritical Fluid Chromatography
• Effective Date: 01-Sep-2019
• Refers: ASTM E594-96(2011) - Standard Practice for Testing Flame Ionization Detectors Used in Gas or Supercritical Fluid Chromatography
• Effective Date: 01-Nov-2011
• Refers: ASTM E355-96(2007) - Standard Practice for Gas Chromatography Terms and Relationships
• Effective Date: 01-Mar-2007
• Refers: ASTM E594-96(2006) - Standard Practice for Testing Flame Ionization Detectors Used in Gas or Supercritical Fluid Chromatography
• Effective Date: 01-Mar-2006
• Refers: ASTM E594-96(2001) - Standard Practice for Testing Flame Ionization Detectors Used in Gas or Supercritical Fluid Chromatography
• Effective Date: 01-Jan-2001
• Refers: ASTM E355-96(2001) - Standard Practice for Gas Chromatography Terms and Relationships
• Effective Date: 01-Jan-2001
• Refers: ASTM E355-96 - Standard Practice for Gas Chromatography Terms and Relationships
• Effective Date: 01-Jan-2001
• Refers: ASTM E594-96 - Standard Practice for Testing Flame Ionization Detectors Used in Gas or Supercritical Fluid Chromatography
• Effective Date: 01-Jan-2001

Overview

ASTM D2268-21 is the internationally recognized standard test method for the analysis of high-purity n-heptane and isooctane (2,2,4-trimethylpentane) using capillary gas chromatography. Developed by ASTM International, this method is primarily used to determine the presence and concentration of hydrocarbon impurities in n-heptane and isooctane reference fuels, which are critical in the calibration of ASTM Knock Test engines for octane rating determination. The method ensures fuel purity by accurately detecting impurities at very low concentrations (less than 0.01%), which could otherwise negatively impact octane numbers and engine performance.

Key Topics

• Purpose and Scope

  • Analyzes high-purity n-heptane and isooctane (over 99.5% by volume) for quality control and specification compliance.
  • Identifies and quantifies trace hydrocarbon impurities that can affect octane number reliability.

• Analytical Method

  • Utilizes capillary gas chromatography with at least 61 meters of stainless steel tubing (0.25 mm internal diameter).
  • Employs a hydrogen flame ionization detector for precise measurement.
  • Internal standard cyclohexane is added to aid in impurity quantification.

• Significance

  • Ensures that reference fuels used in knock testing are uncontaminated and meet stringent purity specifications.
  • Directly supports accurate and consistent octane number determination, reducing the risk of test engine miscalibration.

Applications

• Reference Fuel Analysis

  • Used extensively in fuel laboratories, oil refineries, and research centers to analyze and verify the purity of n-heptane and isooctane before their use as ASTM Knock Test Reference Fuels.

• Quality Assurance

  • Offers highly sensitive impurity detection and quantification, supporting compliance with international fuel quality regulations.
  • Helps manufacturers certify their products for use in critical applications, ensuring reliability and consistency in engine testing.

• Industrial and Regulatory Settings

  • Essential for organizations involved in the production, supply, and certification of fuel reference standards.
  • Aids in research and development of fuels requiring stringent quality control processes.

Related Standards

• ASTM D4175 - Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
  Supports uniform understanding of the terminology used in petroleum and fuel testing.

• ASTM E355 - Practice for Gas Chromatography Terms and Relationships
  Provides definitions and explanations regarding the gas chromatography method.

• ASTM E594 - Practice for Testing Flame Ionization Detectors
  Outlines procedures for ensuring detector performance in chromatographic analyses.

• Other Reference Methods
  D2268-21 is part of a broader suite of ASTM standards for the analysis and certification of reference fuels used in octane rating systems globally.

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Keywords: high-purity n-heptane, isooctane, capillary gas chromatography, ASTM D2268-21, octane number, fuel analysis, reference fuel, hydrocarbon impurities, knock test, petroleum standards.

This standard enables accurate assessment and certification of fuels used as primary reference materials, guaranteeing the quality and reliability needed for modern engine and fuel research. For manufacturers, laboratories, and regulators seeking to ensure precision and compliance, ASTM D2268-21 serves as an essential tool for the analysis of fuel purity by capillary gas chromatography.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D2268 − 21
Standard Test Method for
Analysis of High-Purity n-Heptane and Isooctane by
Capillary Gas Chromatography
This standard is issued under the fixed designation D2268; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope* E594 Practice for Testing Flame Ionization Detectors Used
in Gas or Supercritical Fluid Chromatography
1.1 This test method covers and provides for the analysis of
high-purity (greater than 99.5 % by volume) n-heptane and
3. Terminology
isooctane (2,2,4-trimethylpentane), which are used as primary
3.1 Definitions:
referencestandardsindeterminingtheoctanenumberofafuel.
3.1.1 This test method makes reference to common gas
Individual compounds present in concentrations of less than
chromatographicprocedures,terms,andrelationships.Detailed
0.01 % can be detected. Columns specified by this test method
definitions of these can be found in Practices E355 and E594,
may not allow separation of all impurities in reference fuels.
and Terminology D4175.
1.2 The values stated in SI units are to be regarded as the
standard.
4. Summary of Test Method
1.2.1 Exception—The values given in parentheses are for
4.1 The sample is injected into a capillary gas chromato-
information only.
graphic column consisting of at least 61 m (200 ft) of stainless
1.3 This standard does not purport to address all of the
steel tubing (0.25 mm (0.010 in.) inside diameter), the inner
safety concerns, if any, associated with its use. It is the
walls of which are coated with a thin film of stationary liquid.
responsibility of the user of this standard to establish appro-
An inert gas transports the sample through the column, in
priate safety, health, and environmental practices and deter-
which it is partitioned into its individual components. As each
mine the applicability of regulatory limitations prior to use.
component is eluted from the column, it is detected with a
1.4 This international standard was developed in accor-
hydrogen flame ionization detector and recorded on a conven-
dance with internationally recognized principles on standard-
tional strip-chart recording potentiometer. The detector re-
ization established in the Decision on Principles for the
sponse from each impurity is then compared with that of a
Development of International Standards, Guides and Recom-
known quantity of an internal standard. After determining the
mendations issued by the World Trade Organization Technical
total impurity concentration, the n-heptane, or isooctane purity
Barriers to Trade (TBT) Committee.
is obtained by difference.
2. Referenced Documents
5. Significance and Use
2.1 ASTM Standards:
5.1 This test method is used for specification analysis of
D4175 Terminology Relating to Petroleum Products, Liquid
high-purity n-heptane and isooctane, which are used asASTM
Fuels, and Lubricants
Knock Test Reference Fuels. Hydrocarbon impurities or
E355 Practice for Gas Chromatography Terms and Relation-
contaminants, which can adversely affect the octane number of
ships
these fuels, are precisely determined by this method.
6. Apparatus
This test method is under the jurisdiction of ASTM Committee D02 on
6.1 Chromatograph—Gas chromatograph should be
Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of
equipped with a split-stream inlet device for introducing
Subcommittee D02.04.0L on Gas Chromatography Methods.
minute quantities of sample without fractionation, a capillary
Current edition approved Dec. 1, 2021. Published December 2021. Originally
column, and a hydrogen flame ionization detector. An elec-
approved in 1964. Last previous edition approved in 2017 as D2268 – 93 (2017).
DOI: 10.1520/D2268-21.
trometertoamplifythelowoutputsignalofthehydrogenflame
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
ionization detector, and a strip-chart recorder for recording the
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
detector signal are needed. The time constant of neither the
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. electrometernortherecordershouldexceed1 s.Aballanddisk
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2268 − 21
integrator or electronic integrator for peak area measurements 8.3 Prepare a solution containing 6 % by volume of
should be used. The detection system must have sufficient squalane in n-pentane. Fill the reservoir tube with the coating
sensitivity to produce a recorder deflection for cyclohexane of solution and promptly connect to the gas cylinder. Pass the
at least 8 divisions on a standard 0–100 scale chart using coating solution through the column at 500 psig (3.5 MPa
0.10 % by volume of cyclohexane in n-heptane as defined in gage) until the solution begins issuing from the end of the
9.1. capillary tubing; gradually reduce the inlet pressure in order to
keep the flow of the solution at a relatively even rate of
6.2 Microsyringe—A microsyringe is needed for injecting
40 drops⁄min to 60 drops⁄min. When the coating solution has
the sample into the split-stream inlet device.
been expelled from the column, reduce the inlet pressure to
6.3 Volumetric Pipet, 0.1 mL capacity.
345 kPa (50 psig gage) and allow gas to pass through the
column for 1 h to 2 h. Disconnect the 9 m to 12 m (30 ft to
6.4 Analytical Balance, 200 g capacity.
40 ft) tail section and then mount the column in the chromato-
graph.
7. Reagents and Materials
8.4 To test column resolution use
7.1 Carrier Gas—Argon, Nitrogen, or Helium; 99.99 % or Fig. 1 and calculate R,
from the distance between the cyclohexane and n-heptane
greater purity. (Warning—Compressed gases under high pres-
peaks at the peak maxima, d, and the widths of the peaks at the
sure.)
baseline, Y and Y .
1 2
7.2 Fuel Gas—Hydrogen; 99.99 % or greater purity.
R 5 2 d 2 d / Y 1Y (1)
(Warning—Compressed gas under high pressure. Extremely ~ ! ~ !
1 2 1 2
flammable gas.)
Resolution (R), using the above equation, must exceed a
7.3 Oxidant Gas—Air; 99.99 % or greater purity.
value of 10.
(Warning—Compressed gases under high pressure.)
9. Sample Preparation
7.4 Cyclohexane—At least 99 mol % pure, to be used as
internalstandard.(Warning—Flammableliquidandharmfulif 9.1 Place 20 mL to 30 mL of the reference fuel (n-heptane
or isooctane) into a 100 mL volumetric flask which has been
ingested or inhaled.)
previously weighed.
7.5 n-Pentane—Commercial grade. (Warning—Volatile
9.2 Weigh the sample. Using a 0.10 mL volumetric pipet,
and flammable liquid, and harmful if ingested or inhaled.)
add 0.10 mL of the internal standard cyclohexane (99 mol %,
7.6 Isooctane (2,2,4-trimethylpentane)—(Warning—
min) and reweigh. Dilute to the mark with the n-heptane or
Flammable liquid and harmful if ingested or inhaled.)
isooctane sample and weigh. Use a 200 g analytical balance
7.7 Squalane—Liquid phase for gas chromatographic col-
accurate to 60.0002 g. From these weights (masses) and the
umns.
relative density (specific gravities) of cyclohexane and
n-heptane or isooctane, calculate the volume percent of the
7.8 Tubing—Type 316, 321, or 347 stainless steel; 0.25 mm
cyclohexane internal standard to the nearest 0.001 % by
(0.010 in.) inside diameter.
volume. (Relative density (specific gravity) of cyclohexane at
20 °C = 0.7786; n-heptane = 0.6838, and 2,2,4-trimethylpen-
8. Preparation of Resolving Column
NOTE 1—There are many different procedures for coating capillary tane = 0.6919.)
columns.Asuitable procedure is given in 8.1 through 8.3. Other columns
Cyclohexane, volume% 5 wt cyclohexane/rel dens cyclohexane (2)
may be used provided they meet resolution and repeatability requirements
of the method.
÷ ~wt reference fuel/rel dens reference fuel! 3100
8.1 Connect a 229 mm (9 in.) section of stainless steel
tubing 6.4 mm ( ⁄4 in.) outside diameter, total volume of
10. Procedure
approximately 5 mL) to a high-pressure cylinder of argon,
10.1 Adjust the operating variables to optimum conditions.
helium, or nitrogen through a pressure regulator. Connect at
Temperatures should be as follows: Injection port and splitter
least 61 m (200 ft) of Type 316, 321, or 347 stainless steel
150 °Cto250 °C,columnatoptimumtemperatureanddetector
tubing (0.25 mm (0.
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D2268 − 93 (Reapproved 2017) D2268 − 21
Standard Test Method for
Analysis of High-Purity n-Heptane and Isooctane by
Capillary Gas Chromatography
This standard is issued under the fixed designation D2268; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope Scope*
1.1 This test method covers and provides for the analysis of high-purity (greater than 99.5 % by volume) n-heptane and isooctane
(2,2,4-trimethylpentane), which are used as primary reference standards in determining the octane number of a fuel. Individual
compounds present in concentrations of less than 0.01 % can be detected. Columns specified by this test method may not allow
separation of all impurities in reference fuels.
1.2 The values stated in SI units are to be regarded as the standard.
1.2.1 Exception—The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D4175 Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
E355 Practice for Gas Chromatography Terms and Relationships
E594 Practice for Testing Flame Ionization Detectors Used in Gas or Supercritical Fluid Chromatography
3. Terminology
3.1 Definitions:
3.1.1 This test method makes reference to common gas chromatographic procedures, terms, and relationships. Detailed definitions
of these can be found in Practices E355 and E594, and Terminology D4175.
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.04.0L on Gas Chromatography Methods.
Current edition approved Oct. 1, 2017Dec. 1, 2021. Published November 2017December 2021. Originally approved in 1964. Last previous edition approved in 20132017
as D2268 – 93 (2013).(2017). DOI: 10.1520/D2268-93R17.10.1520/D2268-21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2268 − 21
4. Summary of Test Method
4.1 The sample is injected into a capillary gas chromatographic column consisting of at least 61 m (200 ft) of stainless steel tubing
(0.25 mm (0.010 in.) inside diameter), the inner walls of which are coated with a thin film of stationary liquid. An inert gas
transports the sample through the column, in which it is partitioned into its individual components. As each component is eluted
from the column, it is detected with a hydrogen flame ionization detector and recorded on a conventional strip-chart recording
potentiometer. The detector response from each impurity is then compared with that of a known quantity of an internal standard.
After determining the total impurity concentration, the n-heptane, or isooctane purity is obtained by difference.
5. Significance and Use
5.1 This test method is used for specification analysis of high-purity n-heptane and isooctane, which are used as ASTM Knock
Test Reference Fuels. Hydrocarbon impurities or contaminants, which can adversely affect the octane number of these fuels, are
precisely determined by this method.
6. Apparatus
6.1 Chromatograph—Gas chromatograph should be equipped with a split-stream inlet device for introducing minute quantities of
sample without fractionation, a capillary column, and a hydrogen flame ionization detector. An electrometer to amplify the low
output signal of the hydrogen flame ionization detector, and a strip-chart recorder for recording the detector signal are needed. The
time constant of neither the electrometer nor the recorder should exceed 1 s. A ball and disk integrator or electronic integrator for
peak area measurements should be used. The detection system must have sufficient sensitivity to produce a recorder deflection for
cyclohexane of at least 8 divisions on a standard 0–100 scale chart using 0.10 % by volume of cyclohexane in n-heptane as defined
in 7.19.1.
6.2 Microsyringe—A microsyringe is needed for injecting the sample into the split-stream inlet device.
6.3 Volumetric Pipet, 0.1 mL capacity.
6.4 Analytical Balance, 200 g capacity.
7. Reagents and Materials
7.1 Carrier Gas—Argon, Nitrogen, or Helium; 99.99 % or greater purity. (Warning—Compressed gases under high pressure.)
7.2 Fuel Gas—Hydrogen; 99.99 % or greater purity. (Warning—Compressed gas under high pressure. Extremely flammable gas.)
7.3 Oxidant Gas—Air; 99.99 % or greater purity. (Warning—Compressed gases under high pressure.)
7.4 Cyclohexane—At least 99 mol % pure, to be used as internal standard. (Warning—Flammable liquid and harmful if ingested
or inhaled.)
7.5 n-Pentane—Commercial grade. (Warning—Volatile and flammable liquid, and harmful if ingested or inhaled.)
7.6 Isooctane (2,2,4-trimethylpentane)—(Warning—Flammable liquid and harmful if ingested or inhaled.)
7.7 Squalane—Liquid phase for gas chromatographic columns.
7.8 Tubing—Type 316, 321, or 347 stainless steel; 0.25 mm (0.010 in.) inside diameter.
8. Preparation of Resolving Column
NOTE 1—There are many different procedures for coating capillary columns. A suitable procedure is given in 6.18.1 through 6.38.3. Other columns may
be used provided they meet resolution and repeatability requirements of the method.
8.1 Connect a 229 mm (9 in.) section of stainless steel tubing 6.4 mm ( ⁄4 in.) outside diameter, total volume of approximately
D2268 − 21
5 mL) to a high-pressure cylinder of argon, helium, or nitrogen through a pressure regulator. Connect at least 61 m (200 ft) of Type
316, 321, or 347 stainless steel tubing (0.25 mm (0.010 in.) inside diameter) to the 229 mm section of 64 mm tubing which is to
be used as a reservoir for the coating solution. The capillary column is generally coiled on a suitable mandrel before coating. To
the other end of the capillary column, connect an additional 30 m to 9 m to 12 m (40 ft) of capillary tubing through a 1.6 mm
( ⁄16 in.) Swagelok union.
8.2 Clean the tubing by passing 25 mL to 30 mL (5 to 6 reservoir volumes) of n-pentane through the tubing with about 1.7 MPa
to 2.1 MPa (250 psig to 300 psig gage) of inert gas. After the column has been cleaned, disconnect the upstream end of the
reservoir tube and allow the pressure in the tubing to return to atmospheric.
8.3 Prepare a solution containing 6 6 % by volume percent of squalane in n-pentane. Fill the reservoir tube with the coating
solution and promptly connect to the gas cylinder. Pass the coating solution through the column at 500 psig (3.5 MPa gage) until
the solution begins issuing from the end of the capillary tubing; gradually reduce the inlet pressure in order to keep the flow of
the solution at a relatively even rate of 40 drops ⁄min to 60 drops ⁄min. When the coating solution has been expelled from the
column, reduce the inlet pressure to 345 kPa (50 psig gage) and allow gas to pass through the column for 1 h to 2 h. Disconnect
the 9 m to 12 m (30 ft to 40 ft) tail section and then mount the column in the chromatograph.
8.4 To test column resolution use Fig. 1 and calculate R, from the distance between the cyclohexane and n-heptane peaks at the
peak maxima, d, and the widths of the peaks at the baseline, Y and Y .
1 2
R 5 2 d 2 d / Y 1Y (1)
~ ! ~ !
1 2 1 2
Resolution (R), using the above equation, must exceed a value of 10.
9. Sample Preparation
9.1 Place 20 mL to 30 mL of the reference fuel (n-heptane or isooctane) into a 100 mL volumetric flask which has been previously
weighed.
9.2 Weigh the sample. Using a 0.10 mL volumetric pipet, add 0.10 mL of the internal standard cyclohexane (99 mol %, min)
...