The term TPH (Total Petroleum Hydrocarbons) is a confusing one, and often misunderstood by the non conversant as being a fixed definition of a petroleum product that can be measured absolutely , and hence be directly compared with other TPH values. Unfortunately this is not the case.
In short analysis of the total petroleum hydrocarbon (TPH) content of soil and water samples is complex and confusing. This web page will attempt to explain some of this subject, help clarify the use of ‘TPH’ as an analytical quantity and direct you to sources of further information.
Definition of TPH
A quick Internet search will yield numerous definitions of TPH some of which can be seen below.
Total petroleum hydrocarbon is the measure of the concentration or mass of petroleum hydrocarbon constituents present in a given amount of soil or water. The word "total" is a misnomer--few, if any, of the procedures for quantifying hydrocarbons can measure all of them in a given sample. Volatile ones are usually lost in the process and not quantified and non-petroleum hydrocarbons sometimes appear in the analysis. (http://www.caslab.com/Total_Petroleum_Hydrocarbons_TPH_Meaning/ )
Total Petroleum Hydrocarbons (TPH) is a term used to describe a broad family of several hundred chemical compounds that originally come from crude oil. (Toxicological Profile for Total Petroleum Hydrocarbons (TPH) (http://www.atsdr.cdc.gov/toxprofiles/tp123.pdf)
TPH is defined as the measurable amount of petroleum-based hydrocarbon in an environmental media. It is, thus, dependent on analysis of the medium in which it is found (Gustafson 1997).
Total Petroleum Hydrocarbons (TPH) is sometimes referred to as mineral oil, hydrocarbon oil, extractable hydrocarbons, and oil and grease. 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. (Total Petroleum Hydrocarbon Criteria Working Group Series, Volume 1. Analysis of Petroleum Hydrocarbons in Environmental Media)
Total petroleum hydrocarbons. The mass of hydrocarbon present in a sample according to a specific analytical method. Land Contamination: Technical Guidance on Special Sites: Petroleum Refineries R&D Technical Report P5-042/TR/05 D J Nancarrow et al. (http://publications.environment-agency.gov.uk/pdf/SP5-042-TR-5-e-p.pdf)
With such a loose definition (and with good reason) is it not surprising that TPH data presents all sorts of problems for contaminated land professionals who are tasked with making decisions or understand recommendations based upon TPH values.
The crux of the problem stems from the complicated nature of petroleum products which can be complex mixtures of hundreds of hydrocarbon compounds, with the exact composition varying due to a number of factors including the crude oil source and the subsequent refining process. To complicate this even further are the changes to the refined product/spilled material caused by weathering once exposed to environmental conditions.
It is also worth noting that here are a number of other terms that are often used in conjunction with TPH:
EPH = Extractable Petroleum Compound,
DRO=Diesel Range Organics,
GRO=Gasoline Range Organics
EDRO=Extended Diesel Range Organics,
SVOCs=Semi Volatile Organic Compounds
With TPH being the sum of the aliphatic and aromatic compounds in the GRO and DRO and mineral ranges.
These ranges are essentially determined by the hydrocarbon chain length of the predominant component of the contaminant and have implications on the acceptable limits for disposal and other criteria. For example limits for diesel may be at 10,000 parts per million whilst non diesel hydrocarbon contamination could be 1,000. To the right is a idealised image of a Gas chromatography (GC) trace showing the
component ranges in terms of their retention times which in turn tends to be consistent with the hydrocarbon chain length which is how the classification of hydrocarbon characterisation is determined.
Further information can be found above in a power point presentation on TPH or for further reading material on TPH the following publications comes highly recommended. (Total Petroleum Hydrocarbon Criteria Working Group Series Volume 1 Analysis of Petroleum Hydrocarbons in Environmental Media.pdf) or (TOXICOLOGICAL PROFILE FOR
TPH correlation between Laboratories
Given the above discussion it comes as no surprise, but a very under acknowledged fact that the values reported in TPH analysis are highly affected by the extraction technique and method utilised by the laboratory as well as the TPH standards by which the laboratory quantify the sample. Despite the methods used being MCerts accredited, the different methods will yield significantly different results. In some instances the TPH values reported will exceed threshold limits yet a different but still legally defendable data set will report values below site specific criteria. This renders the decision to remediate/dispose or leave in situ a rather fraught one.
The problematic nature of TPH determination is also implicitly recognised in the way that MCerts allows for a 30% +/- variation in its determination between laboratories for the certified reference material (CRM). Given that the CRM is a well characterised and thoroughly homogenised dried sample the variance between laboratories for ‘real’ fresh field samples can be far greater, and has been shown to be so on many occasions. This margin for error could be further interpreted to allow for a certain amount of leeway when making decisions influenced by TPH values - The laboratory report says 1000 mg/kg, but is your result really 1,300ppm or could it 700ppm?
As previously outlined numerous methods of TPH determination exist involving different sample preparation regimes, different extraction techniques (different solvents, temperatures, pressures, extraction times, etc), different purge and trap methods for volatiles, different analytical techniques (e.g. IR and GC) and combinations thereof. Even the method of sample collection and storage (as well as sample preparation) can have a profound bearing on the determined TPH as volatiles can be easily lost by unavoidable transfer losses or by careless or lax procedures. It is a recognised reality that there is no ‘gold standard’ method of TPH determination.
There is a reason for the availability of a large number of TPH measurement techniques. Because petroleum and petroleum-derived products are such complex mixtures, there is no single “best” method for measuring all types of petroleum contamination. (Total Petroleum Hydrocarbon Criteria Working Group Series, Volume 1. Analysis of Petroleum Hydrocarbons in Environmental Media)
Essentially the important concept to understand from this is that TPH determination of a soil or water sample is very method specific.
This means that it is difficult to compare data from one TPH technique to another without the full method information being provided. This view is succinctly expressed in the ATSDR report on Toxicological Profile for Total Petroleum Hydrocarbons on its web site.
The term “total petroleum hydrocarbons” (TPH) is generally used to describe the measurable amount of petroleum-based hydrocarbons in the environment; and thus the TPH information obtained depends on the analytical method used. One of the difficulties with TPH analysis is that the scope of the methods varies greatly. Interpretation of analytical results requires an understanding of how the determination was made.
(Toxicological Profile for Total Petroleum Hydrocarbons (TPH)- 3. Identity And Analysis of Total Petroleum Hydrocarbons, September 1999 ( http://www.atsdr.cdc.gov/toxprofiles/tp123-c3.pdf )
In summary the use of TPH as a measurement tends to be as a surrogate for want of a better readily available technique, but as we have tried to demonstrate TPH is as a measurement largely defined by the method used for its determination. Consequently unless the contamination is well defined, reliance on this parameter alone is not an appropriate basis for contaminated soil management. It therefore follows that the comparison of TPH results produced by several different methods on samples from the same site may not give comparable results unless the analytical methodology is fully understood.
Naturally Occurring Interference
In addition to the issues discussed above, non-petroleum derived compounds (e.g. naturally occurring humic and fulvic acids) are frequently inadvertently (and wrongly) quantified as TPH even by MCerts analytical techniques. The table below clearly shows the extent of possible influence from non petroleum sources that could be quite readily found in soil samples.
As a point of interest here it should be noted that TPH quantification by QROS’s hydrocarbon analyser QED, is not unduly influenced by non petroleum hydrocarbon content.
TPH Analysis with Banding Data
Some TPH analysis is carried out to give banding information. Banding is a way of assigning the proportion of sample hydrocarbon that falls within a specific carbon number band. The number of carbon atoms in the molecule sets the band parameters, banding is often set as <C5, C5-C10, >C10-C12, >C12-C16 , >C16-C20, >C20-C25, >C25-C35., where a C8 compound is any molecule containing 8 carbon atoms. A mixture of aliphatic hydrocarbons with known carbon atom numbers is used to set the bands before the analysis.
A more advanced banding method splits the sample into the aliphatic and aromatic components and using 2 separate analysis runs measures the banding for both types of component.
The use of banding is a useful advance, because it does allow a better understanding of the hydrocarbon type and can be used to identify if plant material is a problem. As with all techniques, there are variabilities between the methods that are used. The biggest difference is that some laboratories use just aliphatic markers for both the aliphatic and aromatic fraction. The aromatic compounds in this situation will be put into different bands, typically the next highest band for the same carbon number compared to a method that uses aromatic compounds to assign aromatic carbon numbers. The other difference is in the sample preparation used to split the fractions into aromatic and aliphatic. The splitting is a little hit and miss and is very dependent on the concentration of hydrocarbon in the extract and the relative proportions of aliphatic to aromatic. In some cases, the lighter aromatic compounds can be split into the aliphatic fraction and counted in the aliphatic band. Petroleum hydrocarbons also contain complex, non aromatic compounds as well as compounds containing Nitrogen, Sulphur and Oxygen (NSO compounds). These non aromatic compounds may be included in the aromatic fraction, depending on the separation technique used. Many NSO compounds are aromatic in nature, but do not fit into the typical carbon banding window for true carbon only aromatics. The presence of these compounds can bias the results in the aromatic carbon banding data. A good indicator of plant material is where the aromatic >C25 band has a high value and the corresponding >C25 aliphatic band has a much lower value. The compounds in the aromatic fraction are usually humic and fulvic acids, commonly associated with natural plant material.
Banding data is useful, but as with all analytical techniques, caution should be used when interpreting the data and comparing data from one lab compared to another. Banding can be used to identify reasonably pure compounds, such as diesel or petrol, but mixtures may be more difficult to identify.
|
Sample |
Grass |
Dried oak leaves |
No.6 Fuel oil
|
Household petroleum jelly |
|
TPH (mg kg- |
14,000 |
18,000 |
16,000 |
749,000 |
API (2001) Risk-based Methodologies for Evaluating Petroleum Hydrocarbon Impacts at Oil and Natural Gas E&P Sites, API Publication 4709 , API Publishing Services, Washington DC. (May 2003) (http://www.api.org/aboutoilgas/sectors/explore/upload/4709.pdf)
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