MPL Laboratories

 

Total Petroleum Hydrocarbons (TPH)is sometimes referred to as mineral oil, hydrocarbon oil, extractable hydrocarbons, and oil and grease.

There are many analytical techniques available that measure TPH concentrations in the environment. No single method measures the entire range of petroleum-derived hydrocarbons. Because the techniques vary in the way hydrocarbons are extracted, cleaned up, and detected, they each measure slightly different subsets of the petroleum-derived hydrocarbons present in a sample.

The chemical composition of petroleum products is complex and change over time following release into the environment. Because petroleum and petroleum-derived products are such complex mixtures, there is no single “best” method for measuring all types of petroleum contamination.

These factors make it difficult to select the most appropriate analytical methods for evaluating environmental samples. There are several analytical methods for quantifying petroleum fractions. These include gas chromatography, infrared spectrometry, gravimetry and immunoassay measurements.

Some methods are appropriate for gasoline-contaminated samples while others are designed for hydrocarbon contamination such as jet or diesel fuel. Some methods measure more compounds than other methods because they employ more rigorous extraction techniques or more efficient solvents.
Methods may be subject to interference from naturally occurring materials such as animal and vegetable oils, peat moss, dried grass, or humic material in topsoil, resulting in artificially high reported TPH concentrations.

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Total Petroleum Hydrocarbons

The definition of TPH depends on the analytical method used because the TPH measurement is the total concentration of the hydrocarbons extracted and measured by a particular method.
The same sample analyzed by different TPH methods will produce different TPH values. Even with the same Gas Chromatography method (the most common instrumental method), some laboratories may correct for matrix spike recovery (e.g. diesel spike), whilst many do not.
Interpretation of the results depends on understanding the capabilities and limitations of the selected method. If used indiscriminately, TPH data can be misleading and could lead to an inaccurate assessment of risk.
Material in many soils (gypsum, clay, organic matter) binds to the hydrocarbons and prevents them from being extracted efficiently. This leads to a difference for almost all soils between the total material present (or spiked onto the soil) and what can be extracted and measured.
To clarify what it is that is being reported, many laboratories including MPL have moved towards reporting Total Recoverable Hydrocarbons. As the name suggests it is the amount of hydrocarbons able to be extracted under standard conditions and measured versus standards.

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Summary of analytical methods

1. GravimetricGravimetric: A sample is extracted with organic solvent, filtered, and the solvent evaporated to leave the oil/grease residue. This method does not give information as to the carbon range and only measures the heavy hydrocarbon range (oils and greases). Due to the low solubility of the heavy hydrocarbons C30 and in common solvents, an efficient extraction method such as soxhlet or microwave extraction is required to extract all material from visibly contaminated soils.
2. Infrared: The sample is extracted using a suitable solvent, and the hydrocarbon concentration is measured by infrared spectrophotometry. The solvent is not evaporated, the light hydrocarbons are measured, although the carbon range is not indicated. This method requires the use of solvents such as CFC’s so is no longer used routinely due to the hazards associated with the use of CFC’s.
3. Immunoassay: A relatively new method which is largely used for field measurements of indicative concentrations or presence/absence of hydrocarbons. The method does not give information as to carbon range and is also prone to interferences from humic acids etc.
4. Gas Chromatography: This is the method most commonly used for the extract of TPH. Samples for diesel contamination are extracted with organic solvent using sonication, tumbling, soxhlet or microwave extraction. The extract is analysed by gas chromatography with the compounds present being detected in order of the boiling points of the compounds and quantified by comparison to standards.

The results can thus be expressed in relation to hydrocarbon ranges (C6-C9, C10-C15, C16-C28). Best sensitivity for volatile hydrocarbons (C6-C9 range, VOC’s, BTEX) is achieved by using GCMS headspace or ‘purge and trap’ techniques.

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Sample collection / preservation

All samples for hydrocarbon analysis should be taken and stored in glass bottles or jars with a tightly sealed lid. Water samples should be filled to the top of the bottle to minimise the headspace.
Following sampling, samples should be kept cold during storage and transportation to the lab. Samples for volatile analysis can be sampled directly into suitable vials (40 mL) and preservative added in the field.

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At MPL Laboratories, we have new instruments and dedicated analysts to help with the assessment of TPH.  Contact us to see how we can work together to give you the best solution.

TPH is analysed by GCFID (Gas Chromatography using a Flame Ionisation Detector). The TPH is extracted from the sample with solvent, and injected onto a GC column. A flow of inert gas sweeps the sample through a narrow column, separating the components. The effluent from the GC column passes through the flame of the non-selective detector, which breaks down organic molecules and produces ions.  The ions are collected and produce a measurable electrical signal used to confirm the identity and amount of the compounds of interest.

 

Organic Analysis	LQL mg/L	LQL mg/kg	Method Reference
	Water	Soil	In-House, based on the following:
Total Recoverable Hydrocarbons (TRH)	0.5	20	Solvent extraction USEPA 3510, 3550
C6-C9 Hydrocarbon Range	0.05	10	GCFID
C10-C14 Hydrocarbon Range	0.05	10	GCFID
C15-C28 Hydrocarbon Range	0.05	10	GCFID
C29-C36 Hydrocarbon Range	0.5	20	GCFID
BTEX	0.001	1	GCMS - USEPA 8260