ASTM D6591-19

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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: D6591 − 18 D6591 − 19
Designation: 548/06
Standard Test Method for
Determination of Aromatic Hydrocarbon Types in Middle
Distillates—High Performance Liquid Chromatography
Method with Refractive Index Detection
This standard is issued under the fixed designation D6591; 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.
INTRODUCTION
This test method has the same title as IP 548-06 and is intended to be technically equivalent. The
ASTM format for test methods has been used, and where possible, equivalent ASTM test methods
have replaced the IP or ISO standards.
The test method is intended to be used as one of several possible alternative instrumental test
methods that are aimed at quantitative determination of hydrocarbon types in fuels. This does not
imply that a correlation necessarily exists between this and any other test method intended to give this
information, and it is the responsibility of the user to determine such correlation if necessary.
1. Scope*
1.1 This test method covers a high performance liquid chromatographic test method for the determination of mono-aromatic,
di-aromatic, tri+-aromatic, and polycyclic aromatic hydrocarbon contents in diesel fuels and petroleum distillates boiling in the
range from 150 °C to 400 °C. The total aromatic content in % m/m is calculated from the sum of the corresponding individual
aromatic hydrocarbon types.
NOTE 1—Aviation fuels and petroleum distillates with a boiling point range from 50 °C to 300 °C are not determined by this test method and should
be analyzed by Test Method,Method D6379 or other suitable equivalent test methods.
1.2 The precision of this test method has been established for diesel fuels and their blending components, containing from 4 %
to 40 % (m/m) mono-aromatic hydrocarbons, 0 % to 20 % (m/m) di-aromatic hydrocarbons, 0 % to 6 % (m/m) tri+-aromatic
hydrocarbons, 0 % to 26 % (m/m) polycyclic aromatic hydrocarbons, and 4 % to 65 % (m ⁄m) total aromatic hydrocarbons.
1.3 Compounds containing sulfur, nitrogen, and oxygen are possible interferents. Mono-alkenes do not interfere, but conjugated
di- and poly-alkenes, if present, are possible interferents.
1.4 By convention, this standard defines the aromatic hydrocarbon types on the basis of their elution characteristics from the
specified liquid chromatography column relative to model aromatic compounds. Quantification is by external calibration using a
single aromatic compound, which may or may not be representative of the aromatics in the sample, for each aromatic hydrocarbon
type. Alternative techniques and methods may classify and quantify individual aromatic hydrocarbon types differently.
1.5 Fatty Acid Methyl Esters (FAME), if present, interfere with tri+-aromatic hydrocarbons. If this method is used for diesel
containing FAME, the amount of tri+-aromatics will be over estimated.
1.6 This test method includes a Relative Bias section for Test Method D6591 versus Test Method D1319 and Test Method
D5186 versus Test Method D6591 for diesel fuels only. The applicable concentration ranges of the correlations are presented in
the Relative Bias section. The correlations are applicable only in the stated ranges.
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
D02.04.0C on Liquid Chromatography.
This test method is based on material published in IP Standard Methods for Analysis and Testing of Petroleum and Related Products and British Standard 2000 Parts,
copyright The Institute of Petroleum, 61 New Cavendish Street, London W1M 8AR. Adapted with permission of The Institute of Petroleum.
Current edition approved Oct. 1, 2018June 1, 2019. Published December 2018August 2019. Originally approved in 2000. Last previous edition approved in 20172018 as
D6591 – 11 (2017).D6591 – 18. DOI: 10.1520/D6591-18.10.1520/D6591-19.
*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
D6591 − 19
1.7 This test method and correlations were developed for diesel samples not containing biodiesel; the presence of biodiesel will
interfere with the results. The correlation equations are only applicable between these concentration ranges and to diesel fuels that
do not contain biodiesel.
1.8 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.
1.9 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:
D1319 Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption
D2425 Test Method for Hydrocarbon Types in Middle Distillates by Mass Spectrometry
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D5186 Test Method for Determination of the Aromatic Content and Polynuclear Aromatic Content of Diesel Fuels By
Supercritical Fluid Chromatography
D6379 Test Method for Determination of Aromatic Hydrocarbon Types in Aviation Fuels and Petroleum Distillates—High
Performance Liquid Chromatography Method with Refractive Index Detection
D6708 Practice for Statistical Assessment and Improvement of Expected Agreement Between Two Test Methods that Purport
to Measure the Same Property of a Material
2.2 Energy Institute Standard:
IP 548 Test Method for Determination of Aromatic Hydrocarbon Types in Middle Distillates – High Performance Liquid
Chromatography Method with Refractive Index Detection
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 di-aromatic hydrocarbons (DAHs), n—in this test method, compounds that have a longer retention time on the specified
polar column than the majority of mono-aromatic hydrocarbons, but a shorter retention time than the majority of tri+-aromatic
hydrocarbons.
3.1.2 mono-aromatic hydrocarbons (MAHs), n—in this test method, compounds that have a longer retention time on the
specified polar column than the majority of non-aromatic hydrocarbons but a shorter retention time than the majority of DAHs.
3.1.3 non-aromatic hydrocarbons, n—in this test method, compounds that have a shorter retention time on the specified polar
column than the majority of mono-aromatic hydrocarbons.
3.1.4 polycyclic aromatic hydrocarbons (POLY-AHs), n—in this test method, sum of the di-aromatic hydrocarbons and
tri+-aromatic hydrocarbons.
3.1.5 total aromatic hydrocarbons, n—in this test method, sum of the MAHs, DAHs, and T+AHs.
3.1.6 tri+-aromatic hydrocarbons (T+AHs), n—in this test method, compounds that have a longer retention time on the
specified polar column than the majority of DAHs.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from Energy Institute, 61 New Cavendish St., London, WIG 7AR, U.K., http://www.energyinst.org.
3.1.6.1 Discussion—
The elution characteristics of aromatic and non-aromatic compounds on the specified polar column have not been specifically
determined for this test method. Published and unpublished data indicate the major constituents for each hydrocarbon type as
follows: (1) non-aromatic hydrocarbons: acyclic and cyclic alkanes (paraffins and naphthenes), mono-alkenes (if present), (2)
MAHs: benzenes, tetralins, indanes, thiophenes, and conjugated poly-alkenes, (3) DAHs: naphthalenes, biphenyls, indenes,
fluorenes, acenaphthenes, and benzothiophenes and dibenzothiophenes, (4) T+AHs: phenanthrenes, pyrenes, fluoranthenes,
chrysenes, triphenylenes, and benzanthracenes.
4. Summary of Test Method
4.1 A known mass of sample is diluted in the mobile phase, and a fixed volume of this solution is injected into a high
performance liquid chromatograph, fitted with a polar column. This column has little affinity for the non-aromatic hydrocarbons
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while exhibiting a pronounced selectivity for aromatic hydrocarbons. As a result of this selectivity, the aromatic hydrocarbons are
separated from the non-aromatic hydrocarbons into distinct bands in accordance with their ring structure, that is, MAHs, DAHs,
and T+AHs. At a predetermined time, after the elution of the DAHs, the column is backflushed to elute the T+AHs as a single sharp
band.
4.2 The column is connected to a refractive index detector that detects the components as they elute from the column. The
electronic signal from the detector is continually monitored by a data processor. The amplitudes of the signals (peak areas) from
the sample aromatics are compared with those obtained from previously measured calibration standards in order to calculate
percent m/m MAHs, DAHs, and T+AHs in the sample. The sum of the percentages by mass of DAHs and T+AHs is reported as
the percent m/m POLY-AH. The sum of MAHs, DAHs, and T+AHs is reported as the total aromatic content (percent m/m) of the
sample.
5. Significance and Use
5.1 The aromatic hydrocarbon content of motor diesel fuel is a factor that can affect exhaust emissions and fuel combustion
characteristics, as measured by cetane number.
5.2 The United States Environmental Protection Agency (US EPA) regulates the aromatic content of diesel fuels. California Air
Resources Board (CARB) regulations place limits on the total aromatics content and polynuclear aromatic hydrocarbon content
of motor diesel fuel, thus requiring an appropriate analytical determination to ensure compliance with the regulations.
5.3 This test method is applicable to materials in the same boiling range as motor diesel fuels and is unaffected by fuel
coloration. Test Method D1319, which has been mandated by the US EPA for the determination of aromatics in motor diesel fuel,
excludes materials with final boiling points greater than 315 °C (600 °F) from its scope. Test Method D2425 is applicable to the
determination of both total aromatics and polynuclear aromatic hydrocarbons in diesel fuel, but is much more costly and
time-consuming time consuming to perform. Test Method D5186, currently specified by CARB, is also applicable to the
determination of both total aromatics and polynuclear aromatic hydrocarbons in diesel fuel. Test Method D5186, however, specifies
the use of supercritical fluid chromatography equipment that may not be readily available.
NOTE 2—Test Method D5186 was previously specified by CARB as an alternative to Test Method D1319.
6. Apparatus
6.1 High Performance Liquid Chromatograph (HPLC)—Any HPLC capable of pumping the mobile phase at flow rates between
0.5 mL ⁄min and 1.5 mL ⁄min, with a precision better than 0.5 % and a pulsation of <1 % full scale full-scale deflection under the
test conditions described in Section 9.
6.2 Sample Injection System, capable of injecting 10 μL (nominal) of sample solution with a repeatability better than 1 %.
6.2.1 An equal and constant volume of the calibration and sample solutions shall be injected into the chromatograph. Both
manual and automatic sample injection systems (using either complete or partial filling of the sample loop) will, when used
correctly, meet the repeatability requirements laid down in 6.2. When using the partial loop-filling mode, it is recommended that
the injection volume should be less than half the total loop volume. For complete filling of the loop, best results are obtained by
overfilling the loop at least six times.
NOTE 3—The repeatability of the injection system may be checked by comparing peak areas from at least four injections of the system performance
standard (see 9.3).
6.2.2 Sample and calibration injection volumes other than 10 μL (typically in the range from 3 μL to 20 μL) may be used,
provided they meet the requirements laid down for injection repeatability (see 6.2), refractive index sensitivity and linearity (see
9.4.2 and 10.1.5), and column resolution (see 9.4.3).
6.3 Sample Filter, if required (see 10.2.1)—A microfilter of porosity 0.45 μm or less, which is chemically-inert towards
hydrocarbon solvents, is recommended for the removal of particulate matter from the sample solutions.
6.4 Column System—Any stainless steel HPLC column(s) packed with an approved amino-bonded (or polar amino/cyano-
bonded) silica stationary phase is suitable, provided it meets the resolution requirements laid down in 9.4.3. See Appendix X1 for
guidance on the selection and use of suitable column systems.
6.5 HPLC Column Oven—Any suitable HPLC column oven (block heating or air circulating) capable of maintaining a constant
temperature (61 °C) within the range from 20 °C to 40 °C.
NOTE 4—The refractive index detector is sensitive to both sudden and gradual changes in the temperature of the eluent. All necessary precautions
should be taken to establish constant temperature conditions throughout the liquid chromatograph system. The temperature should be optimized depending
on the stationary phase.
NOTE 5—Alternative forms of temperature control, for example, temperature-controlled laboratories, are permitted.
6.6 Backflush Valve—Any manual or automatic (air or electrically actuated) flow-switching valve designed for use in HPLC
systems that is capable of operating at pressures up to 2 × 10 kPa.
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6.7 Refractive Index Detector—Any refractive index detector may be used provided it is capable of being operated over the
refractive index range from 1.3 to 1.6, meets the sensitivity requirement specified in 9.4.2, gives a linear response over the
calibration range, and has a suitable output signal for the data system.
NOTE 6—If the refractive index detector has a facility for independent temperature control, it is recommended that this be set at the same temperature
as the column oven.
6.8 Computer or Computing Integrator—Any data system can be used provided it is compatible with the refractive index
detector, has a minimum sampling rate of 1 Hz, and is capable of peak area and retention time measurement. The data system
should also have minimum facilities for post-analysis data processing, such as baseline correction and reintegration. The ability
to perform automatic peak detection and identification and to calculate sample concentrations from peak area measurements is
recommended but not essential.
6.9 Volumetric Flasks, Grade B or better, of 10 mL and 100 mL capacity.
6.10 Analytical Balance, accurate to 60.0001 g.
7. Reagents
7.1 Cyclohexane, > 99 % >99 % pure.
NOTE 7—Cyclohexane may contain benzene as an impurity.
7.2 Heptane, HPLC Grade. For use as HPLC mobile phase. (Warning—Heptane is highly flammable and may cause irritation
by inhalation, ingestion, or skin contact.)
NOTE 8—Batch to batch Batch-to-batch variation of the solvent quality in terms of water content, viscosity, refractive index, and purity could cause
unpredictable column behavior. Drying and filtering the mobile phase could help to reduce the effect of the trace impurities in the solvent.
NOTE 9—It is recommended practice to degasde-gas the HPLC mobile phase before use; this can be done conveniently, on-line, or off-line by helium
sparging, vacuum degassingde-gassing, or ultrasonic agitation. A failure to de-gas the mobile phase may lead to negative peaks.
7.3 o-Xylene (1,2-Dimethylbenzene), ≥ 98 % ≥98 % pure.
7.4 1-Methylnaphthalene, ≥ 98 % ≥98 % pure.
7.5 Phenanthrene, ≥98 % pure.
7.6 Dibenzothiophene, ≥ 95 % ≥95 % pure.
7.7 9-Methylanthracene, ≥ 95 % ≥95 % pure. (Warning—Gloves should be worn when handling aromatic compounds (for
example, disposable vinyl gloves).)
NOTE 10—Purity is determined by gas chromatography with flame ionization detection. The highest purity standards available should be used.
8. Sampling
8.1 Unless otherwise specified in the commodity specification, samples are taken by following Practice D4057 or D4177, or a
similar standard. In certain situations, sampling is done in accordance with the requirements of national standards or regulations
for the sampling of the product under test.
9. Apparatus Preparation
9.1 Set up the chromatograph, injection system, column, backflush valve, column oven, refractive index detector, and
computing integrator in accordance with the appropriate equipment manuals. Install the HPLC column and backflush valve in the
column oven. Insert the backflush valve so that the detector is always connected independently of the direction of flow through
the column (see Fig. 1). Maintain the sample injection valve at the same temperature as the sample solution; in most cases this
will be at room temperature.
NOTE 11—The column oven is optional if alternative arrangements are made to maintain a constant temperature environment, for example, a
temperature-controlled laboratory (see 6.5). It is recommended to install the backflush valve in the column oven and to install the apparatus away from
drafts (that is, not near air-conditioning unit or fume cupboard). Pipework and/or valving which is not temperature controlled should be insulated.
NOTE 12—Regular maintenance of the liquid chromatograph and its components is important to ensure consistent performance. Leakages and partial
blockage of filters, frits, injector needles, and valve rotors can produce flow rate inconsistencies and poor injector performance.
9.2 Adjust the flow rate of the mobile phase to a constant 1.0 mL ⁄min 6 0.2 mL ⁄min, and ensure the reference cell of the
refractive index detector is full of mobile phase. Allow the temperature of the column oven (and refractive index detector, if
equipped with temperature control) to stabilize.
9.2.1 To minimize drift, it is essential to make sure the reference cell is full of solvent. The best way to accomplish this is either
(1) to flush the mobile phase through the reference cell (then isolate the reference cell to prevent evaporation of the solvent)
immediately prior to analysis, or (2) to continuously make up for solvent evaporation by supplying a steady flow through the
reference cell. The make-up flow is optimized so that reference and analytical cell miss-matchmis-match due to drying-out,
temperature, or pressure gradients are minimized. Typically, this can be accomplished with a make-up flow set at one tenth of the
analytical flow.
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FIG. 1 Diagrammatic Representation of Liquid Chromatograph
NOTE 13—The flow rate may be adjusted (typically within the range from 0.8 mL ⁄min to 1.2 mL ⁄min) to an optimum value in order to meet the
resolution requirements specified in 9.4.3.
9.3 Prepare a system performance standard (SPS) by weighing cyclohexane (1.0 g 6 0.1 g), o-xylene (0.5 g 6 0.05 g),
dibenzothiophene (0.05 g 6 0.005 g)0.005 g), and 9-methylanthracene (0.05 g 6 0.005 g) into a 100 mL volumetric flask and
making up to the mark with heptane. Ensure that the dibenzothiophene and 9-methylanthracene are dissolved in the
o-xylene-cyclohexane mixture (for example, by using an ultrasonic bath) before adding heptane.
NOTE 14—The SPS may be kept for up to one year if stored in a tightly stoppered bottle in a dark place between 5 °C and 25 °C.
9.4 When operating conditions are steady, as indicated by a stable horizontal baseline, inject 10 μL of the SPS (see 9.3) and
record the chromatogram, using the data system. Ensure the baseline drift over the period of the HPLC analysis run is less than
0.5 % of the peak height for cyclohexane.
NOTE 15—A baseline drift greater than this indicates problems with the temperature control of the column/refractive index detector or polar material
eluting from the column, or both. A period of up to 1 h may be required before the liquid chromatograph reaches steady state steady-state conditions.
9.4.1 Ensure that baseline separation is obtained between all four components of the SPS (see Fig. 2).
9.4.2 Ensure that the data system can accurately measure the peak areas of dibenzothiophene and 9-methylanthracene.
NOTE 16—The S/N (signal to noise) (signal-to-noise) ratio for dibenzothiophene and 9-methylanthracene should be 3:1 or greater.
9.4.3 Ensure that the resolution between cyc
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