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.