3.2 Optimization of mass spectrometric conditions
The PFAs were difficult to be ionized in positive mode due
2‒to their SO3. In the study, good responses for most of the PFAs were obtained in negative mode. 100 μg L‒1 PFAs standard solution was injected at flow rate of 20 μL min‒1 and scan in the range of m/z 200‒1000 of primary mass spectrometry in negative ion mode. The results showed that the peak of [M ‒ H]‒ was stronger and the most of other peaks were given by losing the hydrogen atoms in carboxyl from PFAs after ionization. Meanwhile, part of PFAs had few fragment ions, and only one ion was observed. After determining the parent ion, its daughter ion was scanned. The fragment ions with the maximum abundance were used for quantitative analysis, and the other ions were used for qualitative analysis. The optimum parameters of negative ionization mode, capillary voltage, the cone voltage and collision energy were displayed in Table 1.