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 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. . 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, and even for the accredited laboratory analysers as they have to make an assumption about the type of hydrocarbon present.
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, leading to a more accurate value being reported.
The powerful algorithms within the QED software monitor the subtle differences in the sample signal relative to the calibrator series used and can estimate the degree of hydrocarbon degradation.
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 (black line) 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 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.
The fingerprints taken by law enforcement agencies or computer security systems can identify a particular individual. The simple visual picture shows the fine detail that differentiates one fingerprint from another and this detail can easily be recognised by just looking at the picture. This is why the fingerprint, presented as a picture, is still as useful today as it was when first used to identify a suspected criminal in Argentina at the end of 1892.The QED UVF fingerprint, by presenting as a picture also allows a rapid and accurate identification of the substance being analysed.
It is possible to digitise the human fingerprint into a series of numbers, but the resulting columns of numbers are difficult for a person to immediately assimilate. To accurately capture all of the detail in a fingerprint, many thousands of numbers would be needed, producing an incomprehensible jumble of digits, impossible to interpret at a glance.
A simplified digitisation is possible using a complex formula based on the Henry system. Fingerprints have 3 basic forms, the Loop, Whirl and Arch. Each type is assigned to each finger from the hand to generate a number. This number classifies the basic fingerprint type which allows investigators to narrow down the fingerprint search to a smaller subset. The final identification still requires a manual search through the sub set of selected fingerprints, so this simple digitisation cannot be used to provide a definitive identification.
The Henry system uses just 3 variables to help classify the human fingerprint into a sub set of possible matches. The QED UVF system is considerably more complex because there are at least 12 different basic types of petroleum hydrocarbon product. Using just 3 or 4 variables would only be good enough to indicate if there are a higher proportion of lighter compounds present to heavier compounds. This may be acceptable for a fresh fuel, but it would be impossible for such a basic system to reliably identify a weathered fuel type or differentiate between for example, kerosene and diesel or kerosene and JP-8 aviation fuel. With just 4 variables a weathered diesel would be very similar to a heavy fuel oil and a weathered gasoline would be very similar to fresh kerosene. The QED UVF fingerprint can also allow the identification and separation of 2 fuel types mixed in the same sample. Another very useful aspect of having the QED UVF visual fingerprint is the ability to identify if background organics are causing interference.
In real world situations the sample is most likely to contain a weathered petroleum hydrocarbon, often mixed with another type of petroleum product (gasoline and diesel on the same site) In this situation, the QED UVF fingerprint is the only fingerprint system available that can reliably and reproducibly generate a genuine visual fingerprint that can be used to positively identify the petroleum hydrocarbon type. Other UVF systems that use a simple ratio analysis based on 3 or 4 numbers are unlikely to meet the criteria set out in the NCDENR requirement for fuel identification, even for fresh fuels that are not mixed with a second fuel type. The guidance notes state that fuel identification by UVF method must be approved by the DWM before it can be used as a replacement for Method 8015 and the QED UVF method has been approved. (Click link to go to Guidance notes : Table NC-DENR)
The QED was the method used to obtain acceptance of the UVF method as a replacement for Method 8015 for the investigation and remediation of fuel releases from both underground storage tanks (UST) and non UST by the North Carolina Department of Environment and Natural Resources.
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