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Calibration data for the polar variant of Chemcatcher were obtained in laboratory experiments in a similar experimental set up as described in Section 9.5.1. Experiments were designed to determine sampling rates RS of a selected number of triazine and phenylurea herbicides for various combinations of temperature and hydrodynamic conditions. An example of sampling rates of the triazine herbicides is shown in Fig. 9.7.

The sampling rates increase with increasing temperature, and the activation energy for the triazine herbicides under investigation (simazine and atrazine) was 130kJmol-1. This would correspond to an increase in RS of nearly a factor 10 over the temperature range 6-18°C. Thus, the temperature dependence of sampling rate for devices fitted with PES membranes seems to be greater than for those fitted with LDPE membranes. On the other hand, the observed effect of hydrodynamic conditions on sampler performance was only moderate.

Temperature [°C] Stirring speed [rpm]

Fig. 9.7. Effect of temperature (A: measured in turbulent water) and water turbulence (B: expressed as rotation speed of a carousel device loaded with samplers; measured at 11°C) on the sampling rate of the polar Chemcatcher for triazines.

Temperature [°C] Stirring speed [rpm]

Fig. 9.7. Effect of temperature (A: measured in turbulent water) and water turbulence (B: expressed as rotation speed of a carousel device loaded with samplers; measured at 11°C) on the sampling rate of the polar Chemcatcher for triazines.

9.6.2 Short pollution event detector

Many pesticides, some of which are polar molecules, are released at high concentrations into streams and rivers in episodic events, such as field runoff after pesticide spraying, heavy rain and storm events, or during wastewater discharge. These events usually last only a few hours and in order for these compounds to be detected by passive samplers, a device with a short response time is required. However, the device fitted with a PES membrane, although ideal for long-term monitoring, has a lag phase of several hours that represents the time necessary for the analytes to diffuse through the membrane to reach the receiving phase. The lag phase of the device can be predicted using a theoretical model for the mass flux through a plane sheet with constant concentration on both sides of the sheet, as outlined in Chapter 7. Since the PES membrane is discarded before analysis (only the receiving phase is analysed), the lag time for passage through the membrane has to be taken into account.

Shaw and Muller [16] suggested the use of a device fitted with only

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