## 3 Results and Discussion
### Extraction Parameter Optimization and Analysis Method  
Fiber desorption conditions in the GC injection port can affect the sensitivity of THM target compounds. Desorption temperature is limited by the stability of the fiber coating and by analyte liability. The 65 \(\upmu\)m PDMS/DVB fiber stability is a non issue since the high temperature limit for the fiber coating is 270 \({}^{\circ}\)C. However, since THMs have a range of polarities, significant thermal degradation can occur in their analysis at temperatures above 200 \({}^{\circ}\)C [25]. Therefore a desorption temperature of 190 \({}^{\circ}\)C was used in the GC injector port. To minimize other fiber contamination from the sample matrix, and/or analyte carry over, a fiber conditioning step was included, by heating the fiber at 220 \({}^{\circ}\)C for two minutes before and after an analysis run. A blank run acquired after showed no carry over or breakdown products. Following parameter optimization, the conditions: 750 rpm stirring rate, 8 min extraction time at 25 \({}^{\circ}\)C, with 10% salt addition and 2-min desorption time at 190\({}^{\circ}\), were found to give the best overall extraction conditions and peak areas for all six THMs for DI-SPME, HS-SPME extraction conditions. Slight differences in the time to extract were found for direct immersion and for headspace sampling. In headspace sampling, the equilibrium is reached faster than for the direct aqueous sampling (by about 4 min) presumably through a faster exchange rate in the gas phase. However, the extra time was left in this sampling method to keep the conditions the same for comparison with the direct immersion method. See Figure 5b for HS-SPME _versus_ Figure 5a for DI-SPME extractions. The extra time did not change the amount acquired peak areas and this suggests that mass transfer to the fiber had reached equilibrium. Hence, Figure 5b was used for analysis for both DI- and HS-SPME method. A temperature of 25 \({}^{\circ}\)C was chosen to enable temperature maintenance in the lab by using a water jacket around the vial. Since absorption onto the fiber is an exothermic process, temperature changes affect diffusion rates and equilibrium times [21]. In headspace sampling, the rate of diffusion is faster in the gaseous headspace than in aqueous solution. For both HS- and DI- sampling methods, the temperature was purposely kept low to minimize such differences. Hence a temperature of 25 \({}^{\circ}\)C \(\pm\) 1 \({}^{\circ}\)C was used for quantification and for linear calibration. For TWA-SPME methods, a home built stop flow system (Figure 3) that contained six septum-sealed sampling ports where the six SPME fibers could be inserted and tested was used. The water flow (held at 10.00 mL/min flow rate at 25 \({}^{\circ}\)C) was controlled using a Dynamax model SD-200 solvent delivery system. This device was used to optimize TWA extraction rate (1, 2, 3 and 5 days) and to test fiber retracted distance into the SPME needle casing (5, 10, 15, 20, 25 and 30 mm) for optimal diffusion sampling length for HS-TWA method. A mixture of the THM standards at 1 ppm concentration in water methanol (90%:10%) solution were used to assess these parameters. Figure 5b shows typical extraction profile variations achieved over the sampling time of 1, 2 and 3 days periods for four of the THM standards. From these analyses, a two days sampling period for HS-TWA extraction and with a fiber retraction distance of 20 mm into the needle casing was found to give optimal maximum extraction of all six THM standards. For water analyses, the expected combined THM concentrations would be far below 1-ppm concentrations, as indicated from previously analyzed THM data for theses ponds [4]. Hence, possible saturation of the fiber with analytes would not be an issue when using this 2 days extraction time. These conditions were used for both onsite and vial sampling HS-TWA methods. For HS-TWA on-site sampling, the SPME fiber was placed in a holder at the appropriate sampling depth in the pond.