## 4 Conclusions  
Three SPME-GC/MS methods (HS-, DI- and HS-TWA-) are compared for the simultaneous analysis of six TMHs from raw aqueous solutions. SPME linking to GC/MS makes the method highly selective toward THM quantitation for water analysis, as demonstrated by the lack of significant signal interferences in both the standards and real sample matrices. Under optimized conditions, which included fiber selection, fiber position, sampling rate, extraction parameters and salt conditions, real samples were analyzed for natural THMs occurring in untreated aqueous waters. Of the seven SPME fibers tested, both the 65 \(\upmu\)m DVB/PDMS and the 50-\(\upmu\)m CAR/PDMS/DVB fiber performed best towards the six targeted THMs analytes. For our work, the 65 \(\upmu\)m PDMS/DVB was selected because it was available and equally able to efficiently target all compounds over the range of molecular weights and volatilities required for this study.  
Detection enhancement by addition of salt to the sampling matrix vial cannot be used in on-site TWA sampling conditions. The utility of salt modification in quantification studies on natural waters may very well depend upon the natural conditions. In low conductivity conditions, large deviations can occur if calibration solutions are not corrected for by matrix matching the ionic strength.  
The SPME-GC/MS methods allow for simple, fast and simultaneous detection of THM compounds at low ppb levels and as such, may provide insight into the chemistry and the occurrence of halo-analytes in untreated waters. This is important as THMs have potential toxicity and impact to human health. SPME-TWA sampling is easy to perform and to analyze and with its ability to flatten episodic changes in dynamic aqueous systems, TWA methods may give better natural THM estimates and better representative data for determining natural background THM levels in surveys.  
The utility of SPME techniques are most profound when the method is automated. This can be easily achieved with current samplers when water samples are collected and put into standard sampling vials. Under these circumstances, total optimization of SPME sampling, extraction and GC/MS injection parameters and analysis can be controlled under current software. However, in the case of on-site analysis, once the fibers have been retrieved, the development of an automated TWA SPME holder carousel would be useful to deliver samples for unattended automated analysis run. Further work to investigate such methodology is ongoing.  
We acknowledge the Department of Environment, of the Province of Newfoundland and Labrador, Canada and for supplying provincial 2013 trihalomethanes (THMs) study results for various water bodies on the island of Newfoundland. We thank the Environmental Science Scholarship awarded to [PERSON] and to [PERSON] and for the Vice-President Research Award, Grenfell Campus, Memorial University of Newfoundland, Newfoundland and Labrador, Canada (MUN) for support of this work.  
[PERSON] conceived and designed the experiments, wrote the paper and provided oversight during data analysis. [PERSON] performed the initial experiments regarding the SPME-GC/MS and helped analyze the data. [PERSON] performed the initial experiments regarding the TWA-stop-flow and GC/MS analysis.  
The authors declare no conflict of interest.