with a CV of 32% indicating strong WBL control. Furthermore, differ- ences in the polarity of organic analytes accumulated in SPMDs and POCIS have no relevance to equations used for the diffusion of aqueous solutes through the WBLs of POCIS and SPMDs. To date, small SPMDs containing PRCs have been used by either placing the SPMD inside the POCIS deployment canister or by attach- ing a small SPMD holder to the outside of the POCIS canister. How- ever, by fixing PRC-spiked SPMDs between POCIS compression rings and mounting the resulting disk similarly to POCIS disks in deploy- ment canisters, the turbulence regime experienced by the SPMD disk should be about the same as adjacent POCIS disks. With the ex- ception of the differences in biofouling between the POCIS and SPMD membranes, environmentally induced changes in the rates of PRC loss from SPMDs should reflect the changes in POCIS sampling rates. PRC- SPMD fixed to the outside of POCIS canisters provides only a means of adjusting relative differences in flow and temperature among sites. Although attempts to use the classical PRC approach (i.e., spiking of PRCs into sorptive matrix of the sampler) for current POCIS config- urations have been unsuccessful so far (D.A. Alvarez, unpublished data), research will continue in this area. Initial experiments have re- vealed that a silicone membrane disk may be a suitable alternative as a sorbent material for POCIS PRC determinations. Seven-day trials in microcosms containing 1 L of stirred water indicated that silicone disks fortified with 14 C diazinon lost up to 50% of the chemical to the sur- rounding water. Concurrent trials with the two sorbents commonly used in the POCIS showed less than 1% loss of 14 C diazinon to the water. Evaluation of the PRC-loaded silicone disks contained within the PES membranes is on-going. 8.6.2 Determination of sampling rate and kinetic data for chemicals of interest As previously mentioned, the estimation of ambient water concentra- tions is dependent on the availability of R s values for the chemicals of interest. Calibration studies to determine these R s values and meas- urements of the membrane–water partition coefficient (K mw ) for chem- icals over a range of K ow values are necessary for an improved understanding of HpOC uptake by POCIS. Further understanding as to the transport of chemicals through the membrane, whether by parti- tioning into the polymer matrix or passage through the water-filled pore structure, will allow for models to be refined. The influence of D.A. Alvarez et al. 194 Chapter 9 Monitoring of priority pollutants in water using Chemcatcher passive sampling devices Richard Greenwood, Graham A. Mills, Branislav Vrana, Ian Allan, Rocı ´ o Aguilar-Martı ´ nez and Gregory Morrison 9.1 INTRODUCTION In recent years, a number of alternative methods of monitoring water quality has been developed to complement and/or replace spot sampling methods that provide only an instantaneous estimate of the concen- tration of pollutants at the time and point of sampling. Amongst these alternative technologies are passive sampling devices that use a diffu- sion membrane to separate a receiving phase (with a high affinity for the pollutants to be monitored) from the aqueous environment. Over the last decade, a range of low-cost passive sampling devices, incorporating a polymeric membrane and a sorbent receiving phase held in an inert plastic body, for monitoring polar contaminants (e.g. triazine pesticides), non-polar organic pollutants (e.g. polycyclic aro- matic hydrocarbons (PAHs) and organochlorine pesticides (OCPs)), or- ganometallic compounds (e.g. organotin compounds) and heavy metals (e.g. copper, lead, mercury and zinc) in aquatic environments has been developed in our laboratory. The performance of the sampling devices for the various groups of target analytes was optimised by an appro- priate selection of combinations of various sorbent receiving phases and polymeric membranes. 9.2 CONCEPT OF CHEMCATCHER The design of this passive sampling device was developed to provide a single low-cost sampler body that could house a range of combinations of receiving phases and diffusion membranes as appropriate for the Comprehensive Analytical Chemistry 48 R. Greenwood, G. Mills and B. Vrana (Editors) Volume 48 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)48009-0 r 2007 Elsevier B.V. All rights reserved. 199 . spot sampling methods that provide only an instantaneous estimate of the concen- tration of pollutants at the time and point of sampling. Amongst these alternative technologies are passive sampling. of D.A. Alvarez et al. 194 Chapter 9 Monitoring of priority pollutants in water using Chemcatcher passive sampling devices Richard Greenwood, Graham A. Mills, Branislav Vrana, Ian Allan, Rocı ´ o Aguilar-Martı ´ nez. pollutants to be monitored) from the aqueous environment. Over the last decade, a range of low-cost passive sampling devices, incorporating a polymeric membrane and a sorbent receiving phase held in an