3. Results3.1. Calibration curve and recoveryUltrapure water was always used as the background matrix, because it did not contain residual chlorine or THM, being < always LOD. Selective ion – MRM optimized transitions for THM are depicted in Table 1. Mass spectra of the four trihalomethanes contain characteristic patterns caused by chlorine and bromine isotopes. For example, the trichloromethane mass spectrum contains an ion cluster at m/z 83, 85, and 87 with known relative abundances of 35Cl and 37Cl isotopes, while the bromine mass spectrum contains an ion cluster at m/z 171, 173, and 175 with known relative abundances of 79Br and 81Br isotopes. Similar isotope clusters are produced by fragmentation of molecules containing chlorine and bromine. Total ion chromatogram showing each THM analyte, including the internal standard and surrogate ion are shown in Fig. 1. Linear calibration curves were observed for all THM in the range of 39–20,240 ng L−1, with an r2 value of 0.9998 or better ( Table 2). Accordingly, the LODs for all THM were in the range of 11–27 ng L−1 ( Table 2). Intra-, and inter-day variability of analytical measurements was always <3.5%, while average (for the 700 and 1000 ng L−1) THM recoveries in pooled urine matrix was 95–101% with an average relative standard deviation of 4%. The mean surrogate recovery was 84%.
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