Technical Bulletin: Hydrocarbon Fingerprinting
Does your contaminated site show one or more of the following characteristics?
Analytical results for benzene, toluene, ethylbenzene, xylenes, and/or petroleum hydrocarbon fractions 1 to 4 are unexpected and consistently increasing as delineation is advanced.
A documented history of prior hydrocarbon product releases or site contamination
Contaminate plumes are potentially due to multiple sources.
If so, hydrocarbon fingerprinting may be a useful tool for you. This bulletin aims to assist in identifying certain hydrocarbon fingerprints using gas chromatography in order to determine which hydrocarbons exist within your sample.
Understanding Hydrocarbons
Figure 1: Structure of Representative Hydrocarbons.
Hydrocarbons are any class of organic compounds that are composed of only carbon and hydrogen. Hydrocarbons are the principal elements that makeup petroleums and natural gas. These compounds can then be used for fuel, lubricants, and raw materials.
The two main categories of hydrocarbons are Aliphatic & Aromatic:
Aliphatic hydrocarbons are compounds where the carbon atoms are in straight branches.
Aromatic hydrocarbons have an aromatic ring structure and are relatively stable due to their conjugated bond system. Most aromatic hydrocarbons contain at least 1 benzene ring in their structure.
Petroleum hydrocarbons (PHC) is a general term used to describe mixtures of organic compounds found in or derived from geological substances such as oil, bitumen, and coal. For the purposes of this CWS, PHC is considered to be comprised of 4 fractions:
F1 (C6-C10)
F2 (C10-C16)
F3 (C16-C34)
F4 (>C34)
The main classes of PHCs that are of environmental concern are aromatic hydrocabons.
BTEX refers to the chemicals benzene, toluene, ethylbenzene, and xylene. All of these compounds are aromatic hydrocarbons. These compounds occur naturally in crude oil and can also be found in coal tar, crude petroleum, and a wide range of other petroleum products.
Hydrocarbon Analysis
Chromatograms depict the raw, unaltered results produced from a chromatography analysis and can provide additional information that is often not seen in the lab reported concentration values. The basis of chromatography is the separation of a mixture into the various components by passing it in solution, suspension, or vapor through a medium in which the components move at different rates.
Hydrocarbon analyses are typically performed using gas chromatography (GC) due to the volatile nature of these analytes. Each peak or ‘signal’ on a GC chromatogram is associated with a specific compound within the sample. The height of the peak (more accurately the area under the signal) is associated with the concentration of the specific compound.
Although there may be variation in the exact positioning of peaks between different GC instruments, the general pattern of the peaks is unique and will remain consistent. The pattern of the GC chromatogram peak signals is often referred to as the ‘hydrocarbon fingerprint’ and can be used in differentiating the type of hydrocarbon contamination present within a sample.
Figure 2: GC chromatogram of a crude oil sample. As crude oil is composed of a complex mixture of hydrocarbon compounds, the chromatogram depicts a number of closely packed peaks. The y-axis represents the signal strength and the x-axis represents elution time. Lighter hydrocarbon compounds generally elute faster than heavier compounds.
Site Example:
During remediation of a transformer oil release, analytical results for two sample locations (SL03 and SL06) indicated a decrease followed by an increase in PHC F2 and F3 concentrations during vertical delineation.
Figure 3: Laboratory site analysis detailing a change in the PHC F2 and F3 Hydrocarbons
Further review indicated a number of historical releases that had occurred on the site. Therefore, it was hypothesized that the deepest F2 and F3 impacts could be the result of inadequate remediation of previous spills.
Reviewing the chromatograms for the Source, SL03:0.6 and SL06:0.9 samples:
Figure 4: GC chromatogram of the Source sample.
Figure 5: GC chromatogram of SL03:0.6
Figure 6: GC chromatogram of SL06:0.9
The hydrocarbon fingerprint for the Source sample and SL03:0.6 are comparable indicating that SL03:0.6 is likely contaminated with transformer oil. The chromatogram for sample SL06:0.9 shows some similarities with the fingerprint of the Source sample however a distinct difference is present as highlighted by the red circle in Figure 6. These additional peaks on the chromatogram of SL06:0.9 that are absent from the Source chromatogram indicate that SL06:0.9 is contaminated with a mixture of transformer oil and another hydrocarbon product.
Further interpretation suggests that crude oil contamination is present in this sample, likely from a historical release on site.
Due to the presence of transformer oil in both SL03:0.6 and SL06:0.9, the client had a responsibility to remediate the impacts in the co-mingled plume, and further vertical delineation and subsequent excavation were required before achieving site closure. However, if the PHC fingerprint conclusively indicated that the hydrocarbon impacts at the deeper depths were resulting solely from a different source of non-transformer oil products, this analytical technique could have potentially reduced the remedial responsibilities for the client and provided the defensible lines of evidence required to reduce our client’s obligation as the responsible party. As you can see, it is crucial when assessing and conducting remediation on a release area to gain a thorough understanding of the laboratory analytical results.
References
Britannica (2022). Hydrocarbon. https://www.britannica.com/science/hydrocarbon
Canadian Council of Ministers of the Environment (2001). Canadian Wide Standards for Petroleum Hydrocarbons in the Soil.
Ridgeline Canada Inc (2021). Release Report.
