Authors: Graham Corban, Hill Laboratories; Terry Cooney, Analytica Laboratories
Date: April 2022

Abstract

Positive detections of Natural Organic Matter (NOM) – also known as biogenic hydrocarbons – in the Total Petroleum Hydrocarbon (TPH) analytical method in soil and water samples are not necessarily due to anthropogenic (human-derived) petroleum hydrocarbon contamination, but instead maybe compounds extracted from NOM such as plant-based resins, oils, and natural hydrocarbons.

The TPH method involves a crude solvent extraction, which will extract both anthropogenic contaminants as
well as NOM. The method of quantitation involves a relatively non-specific gas chromatographic method with Flame Ionisation Detection (FID).

The FID detection method does not allow speciation of anthropogenic hydrocarbons from NOM. Examination of the chromatogram in combination with further analysis of the extract will allow an experienced Analyst to differentiate the source of the contamination, which will help to reduce the instances of productive soil being unnecessarily sent to landfill.

Introduction

The TPH method analytical method name is somewhat of a misnomer. The results are usually reported as carbon bands between C7 (n-pentane) to C36 (n-hexatriacontane). Therefore, it cannot be considered a true total because hydrocarbons lighter than C7 and heavier than C36 are excluded.

In addition, NOM compounds such as plant-based resins, oils, and natural hydrocarbons pinene are often detected, meaning that the test is not restricted to anthropogenic sources of petroleum compounds.

Finally, non-hydrocarbon synthetic organic compounds such as pesticides, phthalates, pentachlorophenol (PCP), and polychlorinated byphenyls (PCB) can also be detected if present at a high enough level in the soil or water sample.

While this may be seen as a weakness in terms of the poor specificity of the test, it can also be harnessed as a strength to screen for other unforeseen contaminants.

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