One of the biggest problems with hydrocarbon analysis is the effect naturally occurring compounds can have. Organic compounds from plants, algae and peats can give rise to a higher TPH value being reported than is actually present in the sample, leading to a far greater volume of soil being removed than is necessary, with all the extra expense that entails.All analytical methods, including accredited laboratory methods, suffer from this problem to a greater or lesser extent.
The QED can overcome this problem. The QED fingerprint will indicate if a sample contains a high concentration of naturally occurring compounds. If these compounds are found to be present, they will be at high enough concentrations to potentially cause false positives to be reported by the conventional laboratory Gas Chromatography method. Laboratories use a clean up procedure to try and remove these compounds, but this in itself can also create less accurate results.
The following images show how the QED uses background subtraction to minimise the interfering effects of plant derived organic compounds.
Other potentially interfering compounds are also found in clays and other soils. These compounds are likely to be organic in origin, but some types of mineral do fluoresce when exposed to Ultra Violet light. The extraction of the sample does however prevent the transfer of most mineral based interference into the extract solution. UV fluorescence has been used for many years on push probes that expose the soil to UV light as the probe is pushed through the soil. (LIF and ROST techniques) The background fluorescence of some minerals does make this direct read technique prone to false positives however. QED does not suffer from this effect because the minerals are not extracted.
A sample fingerprint is shown on the left. The black line is the sample, the red line is the Humic acid fingerprint and shows the sample contains Humic acids. The yellow line is the contribution of petroleum hydrocarbon after the Humic acid signal has been subtracted, using the unique QED background subtraction method.
The black fingerprint on the right shows the original sample fingerprint after removal of the humic acid contribution.Here it is compared with the library diesel fingerprint, which is the red line.. The purple line is the residual petroleum hydrocarbon showing the presence of mono aromatics , indicating that a degraded gasoline and diesel mixture was present.
The red line shows that the diesel fingerprint is the closest match, with a residual of degarded gasoline and diesel. The TPH concentration is calculated for the diesel and residual hydrocarbon, excluding the humic acid contribution which would have more than doubled the reported TPH value had it not been subtracted.
A useful application of this background subtraction technique is in bioremediation monitoring. Many biopiles have nutrients and other mixtures such as chicken litter or plant material added to help the process, Subsequent laboratory analysis may detect these compounds and be erroneously added to the TPH result, indicating the bioremediation has stalled. QED has a sensitivity some 500 times lower for natural compounds than petroleum hydrocarbons, but standard laboratory methods have a similar or higher response to naturally occuring compounds compared to petroleum hydrocarbons. QED analysis can indicate if natural organics are present, reducing costly re-working of biopiles. Efficient monitoring of the biopile will allow better biopile management and usually faster completion. The laboratory running the confirmation samples can be warned if high naturally occurring organic compounds are detected and modify their sample preparation technique accordingly to generate results that exclude natural compounds.
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The QED analyser software can be used to capture background fingerprints from any site. A soil sample from a clean area of the site containing the soil type to be analysed is extracted and the fingerprint captured. This can then be subtracted from the subsequent sample fingerprint to reveal any underlying petroleum hydrocarbon. The QED system has as standard the Humic Acid, Clay and Loam background fingerprints stored in the internal library. These background fingerprints are remarkably constant for each type, even at different sites and between countries.