Modeling Compound Loss from Polydimethylsiloxane  Passive Samplers

Danny Reible,Courtney Thomas

doi:10.3390/chromatography2040611

Abstract

Volatile losses were measured from polydimethylsiloxane (PDMS) passive samplers during determination of contaminant porewater concentrations in sediments. Volatile losses could occur between the time of retrieval and processing of the passive sampler or in intertidal environments where the passive sampler could potentially be exposed above the water surface at low tide. A model was developed to predict losses of absorbed compounds as a function of sorbent geometry and the Henry’s Law Coefficient and PDMS-water partition coefficient of the compound of interest. The model suggests that thin layers of PDMS typically used to minimize equilibration times in passive sampling (≤30 µm) may not provide quantitative measurement of naphthalenes or other lighter volatile compounds without special efforts to reduce losses. The results suggest that the samplers should be processed rapidly onsite or kept at low temperatures after retrieval to maximize retention of more volatile compounds or designed with thick PDMS layers. The results also suggest that less volatile compounds, including phenanthrene, and higher molecular weight polynuclear aromatic hydrocarbons (PAHs) exhibit minimal evaporative losses with typical sample processing times.

Highlights

The emerging technology of passive sampling to measure contaminant concentrations in sediment porewater has many benefits over conventional sampling techniques including, but not limited to, cost, detection limits, and representativeness [1]
Polydimethylsiloxane (PDMS) has been extensively used for passive sampling, partly due to relative rapid uptake kinetics compared to polyethylene (PE) and polyoxymethylene (POM), two other commonly used passive sampler materials
Equation (1) represents the mass balance on a PDMS layer with an initial concentration of a volatile compound resulting from exposure to a sediment porewater or surface water that is exposed to the air during processing before extraction: d where Vf is the volume (m3) of the passive sampler sorbent layer, Cf is the concentration sorbed to the passive sampler sorbent layer, Cg,bulk is the concentration of the compound in the ambient air, kd is the desorption rate coefficient (m/day) associated with the air motion around the sampler, A is the surface area of the passive sampler layer (m2), H is the Henry’s Law Constant, and Kfw is the PDMS-water partition coefficient

Summary

Introduction

The emerging technology of passive sampling to measure contaminant concentrations in sediment porewater has many benefits over conventional sampling techniques including, but not limited to, cost, detection limits, and representativeness [1]. Conventional solvent extraction techniques for sediment samples strip the majority of the compounds from the sediment While this method is useful for gaining a measure of the total compound mass within the sediment, it does not provide a direct indication of the freely available concentration, which is both mobile and bioavailable [1]. Volatile losses of performance reference compounds (PRCs) would lead to an overestimation of the extent of steady state since PRC losses are used to estimate rates of uptake of target compounds [12]. These losses are possible during the processing of the passive sampler between retrieval from the sediment or surface water and extraction into a stable solvent. These may be important if the sampler is exposed to air during periods of sampling, for example during periods of low tide in samplers deployed into shallow surface water

Objectives

Methods

Results