Multi-residue analysis of PAH, PCB, and OCP optimized for organic matter of forest soil

Abstract

Analyzing organic pollutants in forest soil is challenging because they are strongly physical and chemical bound to soil organic matter (SOM). Within the framework of a forest soil inventory, an analytical protocol for the determination of polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), and organochlorine pesticides (OCP) should be established and validated using one and the same extraction and cleanup procedure. The protocol should be applicable for reliable analysis of a high number of samples in a short timeframe. Two different soil samples representative for the humic layer from a typical mixed and coniferous forest soil had been used for the analysis. Three solvents of different polarity, namely cyclohexane (CH), ethylacetate (EA)/CH (1/1, v/v), and acetone (AC)/CH (2/1, v/v), and the six standard extraction techniques (pressurized liquid extraction (PLE), soxhlet extraction, fluidized bed extraction, sonication, shaking, and one-step extraction recommended for analyzing agricultural soil in Germany (VDLUFA 2008)) were compared concerning their extraction efficiency. For additional matrix separation, two different cleanup procedures (gel permeation chromatography (GPC) and solid-phase extraction (SPE) with different sorbents) were tested. Quantification was carried out using gas chromatography combined with mass spectrometry (GC–MS) and two different injection systems (split/splitless injection and programmable temperature vaporizer (PTV) injection). Labeled internal standards, added prior to extraction, were used for method evaluation. For the simultaneous extraction of PAH, PCB, and OCP from organic forest soil PLE with acetone/cyclohexane (2/1, v/v) provided the highest extraction efficiency. A two-step cleanup procedure consisting of GPC followed by SPE with silica gel was entirely sufficient for the separation of humic substances without discrimination of analytes. Recovery rates for the different extraction and cleanup steps ranged between 89% and 106%. For quantification, a GC–MS method was developed using two different injection systems and two capillary columns of different selectivity. By comparing six standard extraction techniques for PAH, PCB, and OCP from forest soil, we obtained the highest extraction efficiency when using PLE with AC/CH (2/1). For sample injection, we achieved best results using an optimized PTV injection system as it highly reduced the breakdown of thermolabile pesticides. Using this combination of technical equipment, it is possible to determine a concentration of the analytes in the trace level range of 1–2 μg kg−1 in humic soil.