IP 391 Petroleum Products - Determination of Aromatic Hydrocarbon Species in Middle Distillates - High Performance Liquid Chromatography Method with Refractive Index Determination

The standard “IP 391 Petroleum products - Determination of aromatic hydrocarbon species in middle distillates - High-performance liquid chromatography method with refractive index determination” published by the Energy Institute among IP test methods describes a test method used to determine the content of mono-aromatic, di-aromatic and ternary aromatic hydrocarbons in diesel fuels, paraffinic diesel fuels and petroleum distillates.

This standard includes two procedures, A and B:

• Procedure A is applicable to diesel fuels that may contain up to 30 percent (v/v) fatty acid methyl esters and to petroleum distillates in the boiling range of 150 degrees to 400 degrees.
• Procedure B is applicable to paraffinic diesel fuels containing up to 7 percent (v/v) fatty acid methyl esters. This procedure does not involve dilution of the sample to determine low levels of aromatic compounds in these fuels.

The polycyclic aromatic hydrocarbon content is calculated from the sum of di-aromatic and ternary aromatic hydrocarbons and the total content of aromatic compounds is calculated from the sum of individual aromatic hydrocarbon types. Compounds containing sulphur, nitrogen and oxygen may interfere in the determination, monoalkenes do not interfere, but conjugated di-alkenes and poly-alkenes may interfere if present.

Traditionally, aromatic hydrocarbon types are identified by their elution properties from a specified liquid chromatography column relative to model aromatic compounds. These are quantified using an external calibration with a single aromatic compound for each, which may or may not be representative of the aromatics present in the sample. Alternative techniques and test methods may classify and quantify individual aromatic hydrocarbon types differently.

The use of this standard may involve hazardous materials, processes and equipment. This standard is not intended to address all safety issues associated with its use. It is the responsibility of users of this standard to take appropriate precautions to ensure the safety and health of personnel prior to application of the standard and to meet legal and regulatory requirements for this purpose.

For the purposes of this standard, the following terms and definitions apply:

• Non-aromatic hydrocarbon refers to the compound that has a shorter retention time than most single aromatic hydrocarbons.
• Single aromatic hydrocarbon refers to the compound that has a longer retention time than most non-single aromatic hydrocarbons, but a shorter retention time than most non-single aromatic hydrocarbons.
• Di-aromatic hydrocarbon refers to the compound that has a longer retention time than most single aromatic hydrocarbons, but a shorter retention time than most ternary aromatic hydrocarbons.
• Ternary aromatic hydrocarbon refers to the compound that has a longer retention time than most diaromatic hydrocarbons, including chrysene.
• Polycyclic aromatic hydrocarbon refers to the sum of di-aromatic hydrocarbons and ternary aromatic hydrocarbons.
• Total aromatic hydrocarbons refers to the sum of mono-aromatic hydrocarbons, di-aromatic hydrocarbons, and ternary aromatic hydrocarbons. Published and unpublished data indicate that the major constituents for each hydrocarbon type include:
  • Non-aromatic hydrocarbons: cyclic and acyclic alkanes (paraffins and naphthenes), mono-alkenes (if present)
  • Monoaromatic hydrocarbons: benzenes, tetralines, indanes and higher naphthenobenzenes (e.g. octahydrophenanthrenes), thiophenes, styrenes, conjugated polyalkenes
  • Di-aromatic hydrocarbons: naphthalenes, biphenyls, indenes, fluorenes, acenaphthenes, benzothiophenes and dibenzothiophenes
  • Ternary aromatic hydrocarbons: phenanthrenes, pyrenes, fluoranthenes, chrysenes, triphenylenes, benzanthracenes
• Fatty acid methyl ester, refers to the mixture of fatty acid methyl esters derived from vegetable oil or animal fats and complying with the specification defined in the EN 14214 standard.

In principle, when performing the tests, a known sample mass is taken and a fixed volume of this sample is injected into a high-performance liquid chromatograph equipped with a polar column. Diesel fuels and petroleum distillates having up to 30 percent fatty acid methyl ester should be diluted with heptane (Procedure A). Paraffinic diesel fuels are injected neat (Procedure B).

This column has a low affinity for non-aromatic hydrocarbons but shows strong selectivity for aromatic hydrocarbons. As a result of this selectivity, aromatic hydrocarbons are separated from non-aromatic hydrocarbons and separated into different bands according to their ring structures, i.e. monoaromatic hydrocarbons, di-aromatic hydrocarbons and ternary aromatic hydrocarbon compounds.

The column is connected to a refractive index detector that detects compounds as they elute from the column. The electronic signal from the detector is continuously monitored by a data processor. The amplitudes of the signals from the aromatics in the sample are compared with those obtained from calibration standards to calculate the mass fraction of monoaromatic hydrocarbons, diaromatic hydrocarbons, and ternary aromatic hydrocarbons in the sample.

The sum of the mass fractions of diaromatic hydrocarbons and ternary aromatic hydrocarbons is reported as the mass fraction of polycyclic aromatic hydrocarbons, and the sum of the mass fractions of monoaromatic hydrocarbons, diaromatic hydrocarbons, and ternary aromatic hydrocarbons is reported as the mass fraction of total aromatic hydrocarbons.

When following Procedure A, after the aromatics have eluted from the column, the column can be backwashed to allow the remaining components, such as fatty acid methyl ester, to elute in a backwash peak. This provides better column cleaning, but caution should be exercised as it can affect the life of the column.

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