## 2 Results and Discussion
### LC-ESI-MS/MS Conditions
BPA-d\({}_{16}\) and E1-d\({}_{4}\), were selected as internal standards as they had no response for the SRMs for the target analytes over the concentration standards range used. Deuterated and non-deuterated standards (BPA and BPAd\({}_{16}\), E1 and E1d\({}_{4}\), and 4 NP, and 4 NPd\({}_{4}\)) co-eluted with each other; however, the response of the SRMs could be isolated from each other. At the concentration of the deuterated standard used, there was no response from these deuterated compounds at the SRM selected for BPA, E1, or 4 NP. Other deuterated standards that were evaluated included 17\(\alpha\)-estradiol-2,4-d\({}_{2}\) and 17\(\beta\)-estradiol-2,4-d\({}_{2}\); however, they were excluded from this study as the co-elution with E2 occurred and the deuterated standards also gave a response for the SRM of E2 at the lowest concentrations feasible for detection. As shown in Table 1, each analyte had at least one confirmation SRM. In some cases, there was only one fragment of significant abundance with collision-induced dissociation such that \([\mathrm{M-H}]^{-}\rightarrow[\mathrm{M-H}]^{-}\) was selected for the confirmation with no collision energy (4 OP; 205 \(>\) 205 and 4 NP 219 \(>\) 219). For confirmation, the ratio of SRM1/SRM2 must be within the relative standard deviation (RSD) determined from the day of analysis and was within an acceptable relative standard deviation range (RSD 1.39-13.0%). The minimum concentration that showed a percent relative standard deviation (%RSD) of less than 25% from the regression line determined from least-square regression analysis for both the quantitative and confirmation SRM transition was established as the limit of quantitation (LOQs). LOQs were 1.0 to 10 ng/L. Linearity was from LOQ to 100 ng/L with regression coefficients \(>\)0.990 for all the analytes (see Supplementary Materials, Table S1). This concentration range was appropriate for the determination of the target analytes in the extracts from SPE. The analysis of blank samples completed before and after each set of samples or standards showed no carry-over problems. Various mobile phase additives such as formic acid and ammonia were evaluated for both influences on chromatographic separation conditions and MS sensitivity of target analytes. Formic acid (0.05-0.1 v%) caused significant signal suppression, particularly for BPA, OP, and NP such that the use of formic acid in the mobile phase was not feasible. Even when formic acid or acetic acid was only present in sample extracts and not removed completely during the drying steps, the signal suppression was observed. Also, traces of formic acid present in the LC-MS/MS system from prior analysis had to be flushed for a minimum of two days to avoid loss of MS sensitivity. The lack of flushing of the LC-MS/MS system is likely a significant source of the variable results observed in the literature, and it is critical to obtaining consistently low detection limits. Other common additives, including ammonia (1 v%), did not improve the MS sensitivity. Ammonium acetate (5-10 mM) slightly improved the LOQs for equilin (Eq) and PRNL, but increased the LOQs for the other compounds significantly, and thus was not used.