Carbon Banding and its relevance to TPH analysis

What is Carbon Banding?

Petroleum hydrocarbons are a complex mixture of  hundreds or even thousands  of different chemicals covering aliphatic, aromatic, saturated and unsaturated groups. From a human health and remediation perspective, however, it is easier to group these compounds based on their general physiochemical properties and to this end equivalent carbon numbers are used, which are grouped into carbon bands. Carbon banding helps to identify the petroleum hydrocarbon type.

Carbon Banding with GC-FID

Carbon banding is a method that measures boiling point ranges. The laboratory method normally used is Gas chromatography-flame ionization detection (GC-FID). Samples are heated up until they become a gas so they can enter the gas chromatography column. This technique can give an approximation of carbon banding because boiling point is equated to molecular mass which is related to the number of carbons in the molecule. This only works if the molecule is a simple aliphatic or aromatic hydrocarbon because the GC must be calibrated with the compounds expected in the sample. Labs use a specific calibration mixture of simple aliphatic and aromatic hydrocarbons covering the C5 to C40 range. If a compound contains an oxygen or nitrogen (commonly found in petroleum compounds) the carbon band this compound is assigned to is very different from hydrocarbons with the same molecular mass.

Carbon Numbers

The type of hydrocarbon also affects the carbon number that is identified. The following four hydrocarbons contain 10 carbon atoms. Naphthalene, a simple 2 ring aromatic hydrocarbon, a straight chain aliphatic compound Decane, the branched chain aliphatic (also called a paraffin) 2 ethyl octane and a cyclic aliphatic 1,3 diethyl cyclohexane. All 4 have different boiling points and do not appear with the same retention times and therefore carbon band windows when analysed using typical GC-FID banding methods. This is why most labs have very wide carbon band windows because it is accepted that the method is not particularly accurate.

Interpretation

Interpretation of banding data can be misleading. If a well known baby oil is analysed by GC-FID, the approximate carbon banding data would identify it as an unknown hydrocarbon broadly falling in the diesel (C8 – C20) range. Under WM3 (EU waste management rules are almost identical), if this material is detected in soil above 1,000 mg/kg, the soil is classified as hazardous. This baby oil is however considered so safe it can be liberally applied to a new born baby as often as the parent wants, for as many years as the parent wants. This shows how poor banding data can be when applied to assessing risk.

Identifying Petroleum Hydrcarbons

The standard GC-FID method cannot reliably identify or confirm if a sample contains petroleum hydrocarbons, naturally occurring hydrocarbons or synthetic hydrocarbons. Without a positive
identification any risk or waste management assessment will be flawed. The QED will identify the hydrocarbon type if it is petroleum derived. Unknown hydrocarbons that do not have the characteristic fingerprint of a petroleum derived hydrocarbon is generally flagged as an unidentified hydrocarbon or that the % match confidence with a known petroleum hydrocarbon is low. This can be used to assess the validity of any risk or waste management assessment.

Background Organics

Background organics derived from plant matter frequently create false results when using GC-FID. In the US, the GC method is only accepted if suitable sample clean up has been performed that is designed to remove these compounds. Grass can give an effective TPH value well over 10,000 mg/kg. Soil that is petroleum hydrocarbon free, but contains peat above 5%, can show a TPH value well in excess of 1000 mg/kg. With natural plant based hydrocarbons, the C25+ aromatic bands have the highest proportion of the TPH concentration reported, despite there being no aromatic compounds of that carbon number present. Humic and fulvic acids from rotted/decayed plant matter appear in the C25+ carbon band. The majority of petroleum hydrocarbons contain very low proportions of C25+ aromatic hydrocarbons because they are solid at room temperature. They also create a large amount of soot and smoke when burned in an internal combustion engine. This is highly undesirable for a fuel. Aromatics are also a problem for lubricating oils, degrading performance and reducing life. They are essentially removed during manufacturing. This is why caution should be exercised when identifying a sample as being impacted with petroleum hydrocarbon when the majority of the detected hydrocarbon is in the C25+ aromatic band.

The QED also identifies chlorophyl, a useful marker for the presence of naturally occurring organics from leaves, algae and even plant roots. The QED can also detect humic and fulvic acids, another background organics marker. This can be a useful feature to confirm a high lab TPH is likely to be background organics

Polymer Analysis

Common polymers used in construction are polystyrene and Celotex (polyisocyanurate), used for insulation. Both are considered non hazardous. Polymers after being buried for many years break down. Crushing and mixing during typical reclamation processes also break them down. The typical lab extraction process dissolves these compounds and they appear in both the aromatic and aliphatic >C21 TPH bands while not being a true petroleum hydrocarbon. The material appearing in the C21 band is unlikely to contain 21 carbon atoms. If a high concentration of Celotex or polystyrene is present in the sample, this can damage or destroy the GC column used by the lab. At over £400 a column and at least a day to change it, condition it and recalibrate this is an expensive laboratory method.

The sensitivity of the QED to polystyrene and Celotex is very low. These compounds do give an identifiable fingerprint for these compounds, but it will only appear if these compounds are the dominant hydrocarbon present. Analysing pure polystyrene or Celotex gives an apparent TPH value in the low 100s mg/kg. This equates to approximately 0.02%. As most samples will not contain more than a few percent of these compounds if they have been sorted, the contribution of these compounds to the QED TPH value will be below detection limits.

Bitumen

Another construction material widely used is a bitumen based floor sealer and concrete slab jointing compound. Bitumen is a petroleum derived compound but is recognized as an inert material and is non hazardous. This material gives high values in the C21+ aromatic and aliphatic bands. Crushed concrete slabs coated with bitumen compounds are a common source, but road binder is also a source of bitumen.

The contribution from bitumens is significantly reduced using the QED when the standard methanol extraction procedure is used. Methanol is not an efficient bitumen solvent, but a very efficient petroleum derived fuel or oil solvent.

PAHs and Banding

The resulting GC-FID banding values are therefore not always a true reflection of the nature of the hydrocarbon mix. If no sample cleanup has been performed, the results may also be incorrect. It can however be used to flag a non typical TPH result.

The typical lab analysis suite also includes the 16 target PAHs. These are considered marker compounds for petroleum hydrocarbons and coal tars. Gas Chromatography with Mass Spectrometry is the best and most widely method used. This method can when performed correctly identify each individual PAH in this set of 16. The 16 PAH results and the banding results should be in agreement. If the 16 PAH total value is low, yet the C21+ aromatics band is high, it is usually due to background organics. If the 16 PAHs total value is higher than the total TPH value, the sample probably contains coal tar and there is no actual TPH.

Carbon Banding with QED

The QED hydrocarbon analyser does not rely on effective boiling point to generate the banding data it displays, but uses fluorescence to identify the carbon number in fluorescent compounds. The fluorescence energy emitted is directly related to carbon number, allowing the QED to calculate an approximate carbon number. Like the GC-FID method, it also is not optimized to split the aromatic, aliphatic, cycloalkanes or paraffinic compounds into discreet carbon bands, but attempts to provide users familiar with GC-FID banding data, broadly equivalent values they can understand. Neither method gives a completely accurate banding split, which is why the values derived may not add up to 100%. Some sample types can bias the results considerably if they are not genuine petroleum derived compounds.

The QED gives you the carbon banding on the results page, and also on the fingerprint created by the analysis.

Reliable Hydrocarbon Identification

An advantage of the QED hydrocarbon analyser is that it can reliably identify the petroleum hydrocarbon type, even when they are significantly degraded. The US EPA produced numerous risk, health and safety and environmental impact documents for the most commonly encountered petroleum hydrocarbons. If the QED identification and quantification data is compared using these documents, a more realistic risk assessment can be made.

The creation of an accurate risk assessment or waste classification is essential for cost effective projects that also have the lowest carbon footprint. Laboratory analysis can be useful to help but only if carried out correctly and the data generated is interpreted correctly. The QED is a more reliable analysis method that allows inexperienced operators to correctly identify the hydrocarbon type and to provide a more accurate TPH value. The QED can also be used on site and generate results in a few minutes from taking a sample. Real time results are known to reduce costs and increase project efficiency.