Comparison of Extraction Techniques for EPH Contaminated Soils and Sediments

ALS Environmental (Canada)
Keith Brooks, Jennifer Jang, Emily Harris, Aaron Crack, Mark Hugdahl
February 24, 2003

Abstract

Executive Summary
In British Columbia, regulatory application of analytical results for Extractable Petroleum Hydrocarbons (EPH) in solids must meet the requirements of the provincial Waste Management Act and the Contaminated Sites Regulation. One requirement of these regulations is that sample extraction must utilize a Soxhlet extraction system or its equivalent. The mechanism for demonstration of equivalence of alternate techniques is specified.

ALS evaluated three potential alternatives to Soxhlet for the extraction of EPH from solid samples. The three methods were:

  • Accelerated Solvent Extraction (ASE), a commercially available system that performs automated high temperature, high-pressure solvent extractions.
  • Tumbler Extraction Method, an extraction method employing sodium sulfate drying agent and cold solvent with and end-over-end tumbling mechanism.
  • Ultra-Sonication Method, a cold-solvent extraction method employing low-power ultrasonic energy to encourage EPH extractability.

This study evaluated the performance of these three methods against a sixteen-hour Soxhlet extraction. Two reference material samples and five homogenized environmental samples were evaluated by all four methods. Each sample was analyzed in replicate (n=7). Steps were taken to minimize as many systematic variables as possible to permit measurement of small differences between the methods.

Table 1 summarizes the average differences from Soxhlet for each method (for all samples tested), and the largest single differences found versus Soxhlet.

Table 1: Average & Largest Differences From Soxhlet for Alternate Methods
Alternate
Method		Average Difference From Soxhlet
(combined data for all samples, As RPD, n=7 for each)		Largest Difference From Soxhlet
(based on means of 7 replicates, as RPD)
 EPHS10-19EPHS19-32EPHS10-19EPHS19-32
ASE-0.2%-1.2%6.6%8.0%
Tumbler-8.7%-10.2%-20%-20%
Sonication-12.0%-17.1%-43%-29%

Of the three alternate methods, only ASE met all the equivalence requirements listed in the BC EPH method (differences of up to 15% RPD versus Soxhlet are permitted). On average, for all samples evaluated, ASE results differed by Soxhlet by <1% RPD. For the ASE method, the largest single difference versus Soxhlet was +8% RPD, well within the 15% requirement.

For each of the Tumbler and Ultra-Sonication Methods, five of a total of fourteen results (i.e. for EPHS10-19 and EPHS19-32) exceeded the 15% RPD requirement. For the Tumbler method, the most severe failure was at 20% RPD. For the Ultra-Sonication method, the most severe failure was at 43% RPD. Both these methods tended to have poorer recoveries for heavier hydrocarbons, likely because both employ a single-extraction step with cold solvent.