Hi Group

Ruth Wilcox asked a very relevant question with "Are there any other test methods that could support any discussion of uncertainty in any lab results due to the presence (confirmed) of naturally occurring hydrocarbons? "

The answer is yes and most people get this data as a matter of course. Most samples sent to the lab are analysed for TPH and PAH. PAH (polyaromatic hydrocarbons) are the 2 - 6 ring hydrocarbons that for some have toxic/carcinogenic properties. It is actually these compounds that pose the greatest risk in the petroleum hydrocarbons group, with the exception of benzene, the single ring aromatic. Benzo a Pyrene is a 5 ring PAH, one of the more "toxic" but by no means the most toxic.

The PAH analysis looks for the 16 (sometimes 17) target PAHs. These PAHs are examples of the non alkylated forms of each ring number and the way the rings are attached to each other. These were selected by the EPA many years ago as being good indicators of potential toxicity and were reasonably easy to analyse for because these compounds were readily available as certified reference calibrators. It is essential to use a technique called GC-MS to detect the PAHs. MS (mass spectrometry) can positively identify the molecule being detected. A %match confidence is used, with greater than 80% confidence indicating it is definitely the expected PAH and that there are no other compounds mixed up with it. GC-FID methods for PAH are not very robust or accurate, but are available because they are a much lower cost. (always check the method when assessing data)

These 16 PAHs are a subset of the C10 - C22 aromatics described in the CWG bands. The C10 - C22 aromatics are themselves a subset of the petroleum hydrocarbons and are found in all petroleum hydrocarbons. They are also found in coal tars and things that have been burned.

If you have the PAH analysis data, you can compare this to the TPH banding data for the C10 - C22 group. A valid TPH result will show the PAHs concentration being approximately 2 - 10% of the C10 - C22 aromatic band if the sample contains a petroleum compound. The value will be between 10 - 40% if the sample contains coal tar. This is a rough rule of thumb and there are exceptions.

What is often seen is the PAH value is more than 50% of the TPH value, sometimes higher than the TPH value. This of course cannot be true, so one of the results must be incorrect. The reason for this discrepancy is down to the different methods used to extract the sample. TPH analysis uses a relatively weak solvent ideal for petroleum compounds, but not tars, heavy fuels, bitumens or creosotes. The PAH analysis uses a much more powerful solvent. This means for results that show a PAH higher than 50% of the TPH, it is likely the TPH result is significantly under estimating the hydrocarbon concentration in the sample because the solvent used cannot get all the hydrocarbons out. The sample is likely to contain coal tars at a higher concentration than that reported in the TPH result.

The banding data can also show another possible error in the TPH results totals. If the C21 + aromatics show a significantly higher value than the C21+ aliphatics, the chances are the C21+ aromatics value is actually down to humic and fulvic acids found in decayed plant matter. A flourisil cleanup or similar should be performed to remove these non PAH compounds. Sometimes you see a high C21+ aliphatic result and a high C21+ aromatic group. Fresh leaves are coated with a natural wax that contains C21+ aliphatic compounds. Carnuba wax contains a high concentration of them. Fluorosil cleanup will not remove these compounds and as they are essentially identical to the C21+ aliphatics found in petroleum compounds, they will still be included in the final TPH result.

So, in answer to Ruth's question, the data most people get can be used to check if the TPH results look right. 

The original TPHCWG set out the exact procedures that should be used to extract, separate and analyse the various fractions found in the sample. If anything varies within the method, the results are no longer comparable. TPH methods are not proscribed in the UK, so several variations on the original TPHCWG method are available. It is essential to understand the implications of these variations before interpreting the data.

Of course, there are other methods too...............

Regards

Colin

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