The Many Faces of A New Versatile Passive Sampler

Monitoring the concentration of chemicals in water is a statutory requirement across Europe (e.g. EU’s Water Framework Directive [WFD]) and in most other countries. Most monitoring programmes involve the periodic collection of low volume spot (bottle or grab) samples of water that are analysed subsequently in the laboratory. Often the effectiveness of monitoring pollutants using this approach is challenging, particularly where levels fluctuate over time and when chemicals are present at trace, yet toxicologically significant concentrations. One solution to this problem is the use of passive sampling devices (PSDs). PSDs can be deployed in the environment for extended periods (from days to months) where they continually sequester compounds. For some analytes this can significantly lower their analytical limits of detection and enable the identification of pollutants that are episodically present.

PSDs are frequently used to obtain time-weighted average (TWA) concentrations of pollutants in the water column over a deployment period (Fig. 1). A similar approach is used for determining TWA concentrations of chemicals in the atmosphere. This type of information gives a more representative picture of the chemical quality of the environment over time. To obtain TWA concentrations samplers must first be calibrated in the laboratory to ascertain the uptake rate (measured as volume of water cleared per unit time i.e. L/day) for the pollutant of interest. For some samplers the uptake rate can be estimated from mathematical models using physico-chemicals properties of the compound under investigation. The period in which samplers remain in the time integrative mode is compound dependant and needs to be determined prior to deployment. Once this period is exceeded samplers eventually come to equilibrium with the ambient medium. An ISO standard (ISO 5667-23: 2011) concerning the use of passive samplers for the determination of priority pollutants in surface waters is available as a guide for end users. 

A range of different PSDs is available both commercially and as research tools. Examples include the semi-permeable membrane device (SPMD), low-density polyethylene and silicone rubber membranes, diffusion gradients in thin films (DGT), polar organic compound integrative samplers (POCIS) and the Chemcatcher®. Here we describe how the Chemcatcher® is proving a robust and versatile monitoring device for measuring a number of different pollutants in the aquatic environment.

Chemcatcher®- a cost effective passive sampler
Chemcatcher® was developed at the University of Portsmouth, Portsmouth, UK. It comprises a low-cost, three component, watertight body manufactured from PTFE into which an appropriate 47 mm receiving phase disk is inserted (Fig. 2). Normally, this disk is covered with a thin 47 mm diffusion limiting membrane (e.g. low-density polyethylene, polyethersulphone or cellulose acetate) depending on the application. Various types of receiving phase can be used depending on the properties of pollutants being measured. Examples include 3M Empore™ disks (e.g. C8, C18, styrene-divinylbenzene (SDB), anion-exchange and chelating) and the Horizon Atlantic™ range of immobilised solid-phase extraction materials. The sampler sequesters the pollutants of interest and these are retained on the receiving phase disk. After retrieval pollutants are eluted from the disk using a solvent or acid and analysed using conventional instrumental methods (e.g. gas or liquid chromatography with mass spectrometry or by atomic absorption techniques). Samplers can be deployed in potable, surface, coastal and marine waters. Normally this design of sampler is not used for monitoring ground water.