QED Fingerprinting

Petroleum based hydrocarbon contamination includes petrol (gasoline), diesel type fuels, heavy fuel oils, jet fuels, bitumens, crude, hydraulic and lubricating oils. Similar compounds that are often classified as Petroleum Hydrocarbons come from pyrogenic processes such as Manufactured Gas Plants and include creosotes and tars. The proportion of aliphatic, aromatic and other chemicals that make up these compounds varies considerably between the types. To obtain accurate quantification, the calibrator used to create the calibration curve should be of the same type of hydrocarbon as found in the sample. This is true for laboratory methods as well as on site methods.

The problem for on site test kits and most on site analysers is that they cannot identify the hydrocarbon in the sample. To calibrate these older systems, assumptions have to be made about the hydrocarbon type in the sample, typically based on desk study or initial SI data. Most test kits and analysers use a single compound, usually diesel, for the TPH calibrator and a conversion factor can be applied to estimate different hydrocarbon types. Unfortunately, for most hydrocarbon contaminated sites the hydrocarbon in the sample analysed is rarely the same as the hydrocarbon identified in the SI, or the hydrocarbon used to calibrate the analyser or test kit. This is due to weathering and co-mingling of fuel types and the problem of poor homogeneity across a site. Many sites have mixtures of hydrocarbon types meaning several different calibrations should be used. This mis-match of calibrator to sample can cause significant false positive and negative results for older on site analysers, reducing the confidence in on site analysis.

Even the accredited laboratory analysers make an assumption about the type of hydrocarbon present, and for TPH analysis typically calibrate using diesel and lubricating oil. If the hydrocarbon in the sample is of a heavily weathered fuel, tarry or bituminous nature, the calibration will be incorrect and a new analysis should be run using a more appropriate calibrator.

The above QED fingerprint is from a fuel storage facility. The black line is the sample and the red line is the diesel calibrator fingerprint. It can easily be seen that if just a diesel calibrator had been used, a significant error in the calculated concentration would be made. The purple line is the amount of very degraded fuel remaining after the diesel has been subtracted.

The fingerprint above is from the same site as the sample to the left, but located a few metres away. The close match of the sample and the diesel calibrator fingerprint (red line) confirms diesel is the main hydrocarbon type, but that it is more degraded than the calibrator. QED can factor in the degradation to give a more accurate result.

The fingerprint above shows how QED can pull apart the black sample signal and identify the relative proportions that are comprised of degraded fuel (red line), diesel (blue line) and residual aromatics (purple line). The sample TPH is derived by calculating the concentration of each hydrocarbon type identified by applying the specific calibration curve for that hydrocarbon type and adding the results together. QED is calibrated with diesel, degraded fuel and PAH calibrators allowing each hydrocarbon type to be quantified using the most appropriate calibrator type.

The above fingerprint shows un-degraded gasoline, also from the same site. The diesel calibrator fingerprint (red line) misses the substantial mono-aromatic content (BTEX and others, seen as the purple trace to the left). If only diesel was being used as the calibrator, a false result would be obtained. A diesel calibrator also significantly under estimates the C5 - C12 aliphatic content of petrol. BTEX is a more hazardous material and knowing the sample contains a high concentration of these compounds would be useful. QED by visualising the hydrocarbon present ensures the correct calibrations are used and that important contaminants are not missed.

Typical degraded fuel showing proportions of degraded fuel, diesel and residual hydrocarbons after deconvolution by QED

QED is unique for on site hydrocarbon analysis because it can identify the most likely hydrocarbon type in the sample. QED is also able to deconvolute the sample fingerprint and estimate the proportion of different hydrocarbon types that make up the total signal that are in the sample. The QED software then applies the correct calibrator to each proportion identified to generate a result. Identification of the proportions of each hydrocarbon type means that QED effectively applies a sample specific calibration to generate the result, leading to a more accurate value being reported.

This ability to recognise the hydrocarbon type also helps modify the calibration response when QED encounters fuel hydrocarbon that is more or less degraded than the calibrator. The powerful algorithms within the software monitor the subtle differences in the sample signal relative to the calibrator series used and can estimate the degree of hydrocarbon degradation.

The ability to identify the hydrocarbon type on site is also very useful for managing SI projects where identification of pollutant sources, plumes and migration pathways in real time can bring substantial cost savings and improve the conceptual site model.