Knowing your A/A Split from your Elbow

The term Total Petroleum Hydrocarbons (TPH) still remains poorly defined so that TPH data provided by laboratories is prone to disparity according to the analytical procedure undertaken. This data may be required for regulatory compliance, risk assessment, investigation or remediation. An appreciation of the methodology and terminology will therefore assist in scheduling the most appropriate analytical procedure.

Hydrocarbons are ubiquitous in the environment in the form of petroleum and coal derivatives together with other naturally occurring organic sources.

Petroleum hydrocarbons are derived from crude oils.  They comprise a complex mixture of straight and branched chain paraffinic (aliphatics), cycloparaffinic, aromatic and polynuclear aromatic hydrocarbons.  Various petroleum products are produced by the distillation of crude oils and these may be generally classified according to their boiling range.

  • Petroleum solvents have boiling ranges typically up to 300°C and include petroleum ethers, BTEX compounds (benzene, toluene, ethyl benzene and xylenes), gasoline (‘petrol’), white spirits, kerosene (‘paraffin’)
  • Diesel and fuel oils
  • Lubricating base oils, greases and waxes have boiling points in the range 300-700°C
  • Bitumen is the solid or semi-solid residue left after distillation and includes asphaltenes, resins and polynuclear aromatic hydrocarbons (PAHs)

Many different test procedures exist to identify and quantify these various classes of hydrocarbons and no single procedure is suitable for determining them all.  Furthermore these procedures do not necessarily differentiate between petroleum and non-petroleum derived hydrocarbons.  The term “total petroleum hydrocarbons” is therefore somewhat ambiguous in the context of environmental analysis.  The term “mineral oils” is similarly misleading since the petroleum industry and the environmental sector provide differing interpretations.

Such ambiguities, together with an abundance of available test procedures, may lead to misunderstanding and confusion when specifying a particular test for “TPH”.  Therefore, when scheduling your analysis, be aware that different laboratories still use different terminology so be sure to discuss your initial requirements with your preferred supplier.  Chemtest routinely receives requests to undertake various tests including the following:

Solvent extractable material, for example Toluene Extractable Material (TEM), is a crude screening technique which involves extracting the sample with a chosen solvent followed by weighing the residue left after the solvent has been evaporated.  Naturally occurring substances, such as humic acids and sulfur, may also be extracted and many of the lighter hydrocarbons are lost by evaporation with the solvent.

Total petroleum hydrocarbons by infra-red (IR).  Infra-red light is selectively absorbed by specific chemical bonds present in organic molecules (for example, carbon-hydrogen).  The amount of infra-red light absorbed is proportional to the concentration of these bonds in a solvent extract.  It is thus possible to quantify the total amount of organic material present.  Unfortunately no information is provided as to the type of organic material present.  The water industry has historically referred to TPHs by this method as ‘mineral oils’.

Total petroleum hydrocarbons by gas chromatography-flame ionisation detection (GC-FID).  A solvent extract is injected into a gas chromatograph where it is volatilised. It is then passed by a constant stream of gas, referred to as the mobile phase, through a long, coiled capillary tube (column) which is coated internally with a liquid ‘stationary phase’.  Partition occurs between the mobile and stationary phases resulting in the separation of the components in the solvent extract as they pass along the column.  Various detectors are available to detect the separated components as they emerge from the end of the capillary column.  The flame ionisation detector is a non selective detector and is especially suited to general hydrocarbon analysis.  The identification of components is dependent simply on the time taken to travel through and emerge from the column (the retention time).  Using this technique a total TPH concentration may be obtained together with carbon banding if required.  The chromatogram may provide useful qualitative information as to the type of hydrocarbons detected (fingerprinting).  A typical chromatogram is shown in Fig 1.

Alternative detectors may be used such as the photoionisation detector (PID), which is specific for aromatic compounds, and the mass spectrometer (MS). GC-MS can provide additional information as to a component’s chemical structure and hence identity. In the case of petroleum hydrocarbons, however, these structures are generally all straight chain, branched chain and ring structures, the components within each group being chemically very similar. In conventional (electron ionisation) mode a mass spectrometer has difficulty in distinguishing the structures within each group. By comparison with GC-FID, GC-MS is not ideal for quantifying TPH; the response of a FID detector is proportional to the mass of hydrocarbons present, regardless of structure. A MS detector may give very different responses for two different hydrocarbons of similar mass. If, however, aromatic compounds or oxygenates, such as MTBE, are of particular interest, or if the presence of additional unrelated contaminants is suspected, selective MS detection may prove to be most appropriate.

Reliable quantitation by solvent extraction techniques is typically limited to carbon ranges >~nC10 due to the loss of volatiles during sample preparation and to the fact that the extractant solvent itself begins to interfere with the more volatile components in the sample. Hydrocarbons in the >~nC10 range may also be referred to as extractable petroleum hydrocarbons (EPH).

Accurate quantitation of hydrocarbons <~nC10 is typically performed by purge and trap or headspace analysis. These techniques entail minimal sample manipulation and the absence of any added solvent. Hydrocarbons in this range may be referred to as volatile petroleum hydrocarbons (VPH) and include, for example, BTEX compounds and MTBE. The increasing demand for in depth risk based assessments of contaminated sites has lead to methods involving the fractionation of hydrocarbons into their aliphatic and aromatic constituents. These two fractions are further banded as a series of carbon ranges based on equivalent carbon numbers derived from n-alkane chromatographic retention times. This is illustrated by example in the following table:

Hydrocarbon Fractions

Aliphatic FractionAromatic Fraction
>EC– EC6>EC– EC7
>EC– EC8 >EC– EC8
>EC– EC10>EC– EC10
>EC10 – EC12>EC10 – EC12
>EC12 – EC16>EC12 – EC16
>EC16 – EC35 >EC16– EC21
>EC35 – EC44>EC21– EC35
 .>EC35 – EC44

EC- Equivalent carbon number 

This analytical procedure is popularly referred to as an aliphatic/aromatic split (A/A split).  For hydrocarbons of <nC10 separation may be achieved in a single instrument run where selective detection such as mass spectrometry is employed, for example, by headspace GC-MS.

For hydrocarbons >nC10 it used to be necessary to separate a solvent extract of the sample into an aliphatic and an aliphatic fraction by solid phase extraction.  Each fraction had then to be run on the instrument separately.  Two chromatographic runs were therefore required to produce the EPH bandings listed in the table above.  Chemtest has since developed an accredited two dimensional chromatographic procedure (GCxGC) in which the instrument resolves the aliphatic/aromatic separation simultaneously with the separation of the components by carbon number.  This newer procedure therefore negates the necessity to prepare two solvent extracts, whilst requiring just a single chromatographic run.

Aliphatic/aromatic carbon banding data may be incorporated into the Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG) risk-based corrective action (RBCA) approach to site remediation.

This article is intended only as an introductory text and readers are advised to consult the references cited below for a more in depth understanding of contemporary practices and regulatory requirements.  Chemtest is always pleased to provide project specific advice in line with current legislation.

Contact us if you wish to discuss the above in more detail.

 

References:
The Determination of Hydrocarbon Compounds in Soils and Associated Materials, Environment Agency, MEWAM, 2008 (Draft)
The UK Approach for Evaluating Human Health Risks from Petroleum Hydrocarbons in Soil, Environment Agency Science Report P5-080/TR3, 2005
Texas Natural Resource Conservation Commission, Total Petroleum Hydrocarbons, Method 1005, 2001
Selection of Representative TPH Fractions Based on Fate and Transport Considerations, TPHCWG, 1997

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