Table

Chemcatcher configurations for integrative sampling of various pollutant classes

Pollutant class Receiving phase Diffusion membrane

Hydrophobic organic compounds (log KOW> 3)

Hydrophilic organic compounds (log KOW<3)

Metals Mercury

Organotin compounds

C18 Empore™ disk

C18 Empore™ disk

SDB-RPS Empore™ disk

Chelating Empore™ disk

Chelating EmporeTM disk

C18 EmporeTM disk

Non-porous low-density polyethylene (LDPE)

Microporous polysulfone (PS)

Microporous polyethersulfone (PES)

Microporous cellulose acetate (CA)

Microporous polyethersulfone (PES)

Microporous cellulose acetate (CA)

Empore™ extraction disks were selected as convenient receiving phases for use in the Chemcatcher samplers. They are available as standard 47-mm diameter sorbent particle loaded disks. The particles are held together within an inert matrix made of polytetrafluoro-ethylene (PTFE) (90% sorbent: 10% PTFE, by weight). The variety of sorbent materials used in the EmporeTM disk technology enabled the selection of suitable receiving phases for all classes of pollutants under investigation, including polar and non-polar organic analytes, organo-metallic compounds and metals (Table 9.1). A further advantage is the availability of published extraction protocols for a number of analytes and a simple analyte elution with consistent recoveries. Moreover, procedures enabling the disks to be loaded (using procedures developed for solid-phase extraction) in a reproducible manner with internal standards or performance reference compounds (PRCs) by filtering an aqueous standard solution through the disk were developed [1].

9.2.2 Diffusion membranes

Two types of polymeric membranes have been tested for construction of Chemcatcher samplers; non-porous membranes including LDPE and microporous membranes including glass fibre, nylon, polycarbonate, PTFE, polyvinylidenedifluoride (PVDF), cellulose acetate (CA), poly-sulfone (PS), polyethersulfone (PES) and regenerated cellulose. The membranes separate the sorption phase from the bulk water phase, and reduce the flux to the sorption phase. The membrane acts as a semipermeable barrier between the receiving phase and the aqueous environment. The dissolved analytes can pass through to the receiving phase, while particulates, microorganisms and macromolecules with a size greater than the exclusion limit cannot permeate. Without the protection of the membrane, there is a risk of deterioration of the receiving phase disks in the aqueous environment due to biofouling. The criteria for selecting an optimum membrane for sampling a specific group of analytes have been discussed in Chapter 7.

The physical strengths, handling properties and chemical resistance of membrane materials were assessed during the initial evaluation. These tests were followed by accumulation studies of test analytes in prototype devices fitted with different membranes in a flow-through system. The latter studies were designed to determine the conductivity to mass transfer of membranes for a broad range of organic and organometallic pollutants and metal ions. Differences in conductivity of various membrane materials are shown in Fig. 9.1. In this first evaluation stage, optimum combinations of diffusion membrane/receiving phase systems were selected for a comprehensive evaluation, including calibration in the laboratory and testing in the field (Table 9.1).

PS and PES membranes were selected for sampler devices designed to sample polar organic pollutants (log KOW<3) and mercury. These membranes have a high degree of physical strength and good antifoul-ing properties, due to their low surface energy that prevents adsorption of macromolecules to the surface. Polar molecules readily diffuse through the 0.2-mm wide water-filled pores. In contrast, more hydrophobic compounds sorb to the polymer matrix of the membrane. Due to low diffusivity in the polymer matrix, conductivity of the membrane decreases dramatically with increasing hydrophobicity of sampled compounds. CA was selected as a material suitable for construction of Chemcatcher samplers for inorganic ions and organotin compounds, due to their optimum diffusion through the water-filled membrane pores, combined with negligible adsorption to the membrane material.

The non-porous LDPE allows permeation of hydrophobic analytes (log KOW>3-4), due to the favourable combination of high membrane/ water partitioning coefficients and membrane diffusivities for those compounds (see Chapter 7). On the other hand, the membrane has a

Fig. 9.1. The effect of diffusion membrane materials on the patterns of uptake of seven organic compounds. The exposure was performed at constant analyte concentration in water at 11°C in a flow-through tank. A 47-mm C18 Empore M disk was used as receiving phase in all cases.

high resistance to mass transfer of more polar compounds and completely excludes the permeation of ions and molecules with effective diameter larger than 1 nm.This material was used in the Chemcatcher designed to sample non-polar organic pollutants.

9.2.3 Sampler body

9.2.3.1 Reusable sampler body prototype

The principles of Fickian diffusion state that the flux of a substance to the receiving phase is proportional to the surface area over which diffusion takes place and is inversely proportional to the diffusion path length. Therefore, if passive sampling obeys Fickian diffusion, the physical dimensions of the sampler body significantly affect the sampling rate for analytes. During the development phase, the design of the Chemcatcher body was optimised in terms of both construction materials and sampler geometry.

In the evaluation stage, PTFE was selected as a construction material for the sampler body. Its advantage is a low sorption capacity for most environmental pollutants. Moreover, PTFE is denser than water and is not buoyant in the sampled environment, making it easy to deploy this prototype in the field by suspending it from a wire or a string.

The system was constructed to fit a 47-mm EmporeTM disk as the receiving phase, with the chosen diffusion membrane material being laid directly on its surface. Both were supported by means of a 50-mm rigid PTFE backing plate (Fig. 9.2). The active surface area of the Chemcatcher sampler is 17.5 cm2. To seal the sampler, a sleeve open at the back was screwed into place to hold the individual body sections together. In addition, a sealing plate allowed the system to be filled with water and sealed during storage and transport. Thus, the sampler body also acts as a container for storage and transport. The PTFE body could be reused several times, but only after a thorough cleaning involving a multi-step washing procedure.

70 mm 50 mm

Fig. 9.2. Schematic diagram of the prototype Chemcatcher device, used during the sampler development. The PTFE body parts (components 1 and 4) support the receiving phase (component 2) and the diffusion membrane (component 3) and sealed them in place. The sampler is sealed by means of a screw cap (component 5) for storage and transport.

Fig. 9.2. Schematic diagram of the prototype Chemcatcher device, used during the sampler development. The PTFE body parts (components 1 and 4) support the receiving phase (component 2) and the diffusion membrane (component 3) and sealed them in place. The sampler is sealed by means of a screw cap (component 5) for storage and transport.

In the early stages of development [2], a protective steel mesh was used to protect the surface of the membrane. However, the use of a mesh was later abandoned, because it proved to accumulate particulate matter in the field and also to provide shelter for colonising organisms that cause fouling or degradation of the membrane.

9.2.3.2 Disposable sampler body prototype

In subsequent performance tests, the uptake kinetics of many analytes were shown to be controlled by diffusion in the aqueous boundary layer on the membrane surface. The resistance to mass transfer of the boundary layer depends on hydrodynamic conditions in the membrane vicinity. These are significantly affected by the construction geometry of the sampler body. The membrane and receiving phase of the firstgeneration Chemcatcher prototype were located inside a 20-mm deep depression in the sampler body. This sampler design effectively buffers the effect of fluctuating flow on the sampler performance. However, it also effectively reduces convective transport of analytes to the sampler membrane, causing reduced sampling rates (i.e. the rate at which the sampler accumulates chemicals). For an optimum sampler performance, high sampling rates are essential, especially for sampling nonpolar chemicals, due to their extremely low concentrations in the water column. In order to increase sampling rates, the geometry of the body was further refined in the latest version of Chemcatcher body prototype by reducing the depth of the cavity to a minimum (Fig. 9.3). In comparison with the first-generation prototype, the second-generation sampler collects analytes with increased sampling rates. Tests showed that the sampling rate for non-polar compounds (log KOW>3-4), which are accumulated under aqueous boundary layer control, was increased by a factor of 2. This provides improved sensitivity, but also increased variation of sampling rates in response to fluctuations in turbulence (water flow).

In the latest design, the Chemcatcher body is made of mouldable plastic materials. The body consists of three components (two body parts and a lid for storage and transport), which are clipped together (Fig. 9.3). This makes the sampler assembly and disassembly faster than it was in the first-generation prototype, where assembly was made using screw threads. This sampler body prototype was designed as a disposable device for a single field deployment. This removes difficulties connected with cleaning procedures and accompanying quality control measures required for use in trace analysis. The plastic material can be recycled.

Fig. 9.3. Views of the disposable Chemcatcher sampler.

Depending on the nature (temperature, turbulence, presence of suspended solids) of the environment to be sampled and on the target analyte properties, a sampler design can be selected to provide an optimum performance.

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