TPH 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 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 of 300-700°C. Bitumen is the solid or semi-solid residue left after distillation and includes asphaltenes, resins and polynuclear aromatic hydrocarbons.
Many different test procedures exist to identify and quantify these various classes of hydrocarbons and no single procedure is suitable for determining them all. 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’, so be sure to discuss your initial requirements with your supplier, as different laboratories will differ in the terminology used. We routinely undertake various tests including the following:
Solvent extractable material, for example Toluene Extractable Material, 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. 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’.