## 2 Results and Discussion
### Matrix Effects  
Initial evaluation of treated wastewater effluent and raw wastewater (0.1 mL sample extract diluted to 1 mL prior to analysis such that 50/50 _v/v_% acetonitrile/methanol) showed moderate to severe MS suppression. No significant difference in LOQs or linearity of the calibration curves were found when the matrix was present. Matrix interferences and baseline shifts were observed for the raw and treated wastewater samples, and more severe for the raw wastewater matrix. Recoveries for SPE and pre-concentration steps were \(>\)85% as assessed from the deuterated surrogate (4 NP-d\({}_{4}\) and EQ). For surface water samples, it is expected that the presence of humic and fulvic acids is the main attribute to the matrix suppression effect, and no chemical additives were added during the water treatment process. To evaluate the influence of the matrix of the sample on the MS signal, the percentage matrix effect (%ME) was determined by comparison of the slope of the standard addition curve (which represents solutions where the standard is directly added to the sample extract and consequently matrix is present) with the slope of the calibration curve from solvent-based standards where  
\[\%\text{ME}=\frac{(\textit{Slope of the standard addition curve}-\textit{slope of solvent}-\textit{based standards})}{\textit{Slope of solvent}-\textit{based standards calibration curve}}\ \times\ 100. \tag{1}\]  
For the purpose of the %ME calculation, an internal standard was not used in the calibration curves as its response might also be altered by the sample matrix to a different extent than the target analyte. Herein, we did not use a matrix-matched calibration curve as the matrix of the water samples from the wastewater treatment plant (WWTP)s was highly variable in location and time of sampling such that a representative matrix for the WWTPs was not feasible. A negative %ME value represented the MS signal suppression, and positive %ME values indicated MS signal enhancement. The categorization of matrix effect could be interpreted as soft (0 to \(\pm\)20%), moderate (>\(\pm\)20 to \(\pm\)50%), and severe (>\(\pm\)50%) [47]. If matrix effects were soft, then solvent-based calibration standards with internal standard calibration could be used for quantitative analyses (herein referred to as solvent-based standards). For moderate matrix effects, the matrix-matched standards or standard addition calibration were typically used, while for severe matrix effects, the standard addition was used if good linearity in the calibration curve and the required method limits of the quantitation could be obtained. Figure 3 represents an example of the calibration curves (solvent-based and standard addition calibration) for BPA using the matrix from samples obtained at one WWTP. For the standard addition calibration, a calibration curve was completed for every sample and consequently, there was a standard addition calibration curve for the upstream, downstream, raw, and treated wastewater for each location. As can be seen, good linearity was obtained for the analytes for all calibration curves; however, the MS signal suppression was evident by the reduction in the slope of the standard addition calibration curves relative to the solvent-based (solvent-only) calibration. The calibration curves for BPA were typical of most analytes where the MS signal suppression was observed for all sample types (upstream, downstream, raw, and treated wastewater). All the water samples from the WWTPs showed a similar magnitude of matrix effect with the highest effect (either suppression or enhancement) in the raw wastewater collected at WWTP1, 4, and 5, and in treated wastewater collected at WWTP2 and 3. Figure 3: Calibration curves for BPA in the presence or absence of matrix from samples collected at a Wastewater Treatment Plant. Figure 4 shows the average matrix effects observed for each target analyte determined from water samples collected at five wastewater treatment plants located in Saskatchewan.