QED background subtraction

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. All analytical methods, even accredited laboratory methods, suffer from this problem to a greater or lesser extent.

The following images show the how QED can minimise the interfering effects of plant derived organic compounds.

The black line from the QED fingerprints on the left and right is from Humic acids obtained by extracting 100% organic peat using the QROS on site extraction procedure. The Humic acid fingerprint is similar for a wide range of rotted plant material found on site. The Humic acid fingerprint shown is not to scale and typically gives a 500 times lower signal intensity than degraded fuel or diesel.

The purple line on the left side fingerprint is is the degraded fuel fingerprint. It can be seen that there is an overlap of the two types. The purple line on the right side fingerprint is diesel, showing a much lower potential for interference if Humic acids are present.

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 sample 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.

The QED fingerprints (black lines) to either side show the typical clay fingerprint compared to degraded fuel and diesel. The clay was taken from a clean site with no history of petroleum contamination and at a depth of 2 metres. The TPH concentration reported was 25 mg/kg

For applications where very low detection limits (<50 mg/kg of TPH) are required, the background signal can be removed using the powerful QED software. QED can also capture the background fingerprint from a site and use this to remove site specific interferences

The 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.

The black fingerprint on the right shows the original sample fingerprint after removal of the humic acid contribution using the unique QED background subtraction method. The red line shows the library diesel fingerprint.

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.

The red line shows that the diesel fingerprint is the closest match and the purple line is the residual petroleum hydrocarbon showing the presence of mono aromatics , indicating a degraded gasoline and diesel mixture was present. 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.

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.

Background Subtraction

The QED fingerprint will indicate if a sample contains a high concentration of naturally occurring compounds. The QED is intrinsically less responsive to natural compounds, because QED measures in the Ultra Violet region of the spectrum and fluorescence from natural compounds in this region is typically 500 times lower than for petroleum derived compounds. If the QED identifies these compounds are present, they will be present 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, because Gas Chromatography analysers have a similar or increased sensitivity to these compounds compared to the calibrators they use, but the clean up itself can also create less accurate results. Samples being analysed by the laboratory for validation of samples that have previously been analysed by QED that show the typical naturally occurring organic fingerprint can be flagged as requiring special consideration during sample preparation.