Leaves and other background organics are incorrectly reported as TPH by standard laboratory methods

Soil and water samples containing fresh or decomposed leaves when analysed by the standard laboratory GC-FID analysis (gas chromatography/flame ionization detection) method can give a significantly higher Total Petroleum Hydrocarbon value than the true value. This leads to incorrect classification of the soil waste category leading to unnecessary and expensive remediation or disposal. Analysis of road sweepings containing predominantly crushed leaves gives an apparent TPH value of between 2,000 – 4000 mg/kg of TPH even when there are no petroleum compounds present. This incorrectly classifies the material as hazardous under WM3 guidance. This can be a significant issue during autumn and winter  when there are significant amounts of fallen leaves. Site activity quickly crushes the leaves into the soil , worms pull leaves into the soil and increased rainfall carries leaves in the runoff.

If a sample contains biogenic organic compounds (BOCs) the sample extract should be cleaned before analysis using GC-FID, or the BOCs will contribute to the TPH concentration, giving false positive results. The most common cleanup method is a Flourisil (or other activated silica based compound) cleanup cartridge. The initial sample extract is put onto the cartridge and washed through with solvent. This method does not always remove all BOCs, and adds extra time and expense to the analysis.

BOCs mainly consist of humic and fulvic acids, complex organic molecules that mimic C21+ aromatic petroleum hydrocarbons. These compounds are attracted to the silica in the cleanup cartridge and are retained while petroleum compounds pass through.There are some potential problems with using this cleanup method. The cartridge has a finite capacity for capturing BOCs. To ensure effective BOC removal, the initial sample extract should be passed through at least 2 cleanup cartridges, with analysis of the initial extract and each of the extracts after each cleanup cartridge. The rate of reduction in the TPH results will indicate the cleanup efficiency. This procedure unfortunately increases the cost of analysis significantly because 3 individual analyses are required, plus the cost of the cleanup procedures .

Humic and fulvic acids are not the only major BOC. All leaves are covered in a waxy substance to make them waterproof and to limit dehydration. This wax (the most well known type is carnauba wax) consists of predominantly C21 – C35 alkanes. These identical alkanes are also found in petroleum derived compounds. The silica cleanup cannot remove these compounds, so they are still included in the results for TPH. 3 separate cleanup cartridges can remove the majority of the BOC, but the waxy coating compounds still remaine, contributing to a final TPH above 500 mg/kg, but below the critical 1000 mg/kg.

The QED hydrocarbon analyser recognises, but ignores these BOCs, giving a true picture of TPH levels in the sample. If significant concentrations of BOCs are detected, the results are flagged by the QED as containing BOCs.  This easy to use analyser will give accurate and reliable TPH results within five minutes of collecting the sample, either on site or in a simple laboratory, and at low cost. The results and hydrocarbon identification are accepted by the Environment Agency, waste management companies and contractors as sufficiently robust to be used to classify waste soil for WM3.

background organics

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