igure 2 shows 2.5 min averaged spectra of ambient aerosol
at nominal m/z 208 obtained with the HR-AMS located at T0. Panel A compares V- and W-mode open, closed and dif- ference (open minus closed) spectra, during a period with a high lead signal (marked in Fig. 6). The difference between the sensitivity and the resolution of V- and W-modes is evi- dent in the figure; W-mode spectra have more resolution but a noisier signal. Typically, the difference mass spectra cor- respond to the sampled aerosol, once the background gases in the detection region are accounted for by subtracting the closed from the open mass spectra (Canagaratna et al., 2007). However, in the case of lead ions, a closed signal of the same order of magnitude as the open signal was observed which, as it will be explained later, indicates that there is a residual sig- nal caused by aerosol components that evaporate slowly from the vaporizer due to their relatively low volatility (Huffman et al., 2009). Because of this slow evaporation, the difference signal cannot be interpreted as usual in the case of lead ions and will not be discussed further. Panel B in Fig. 2 shows the same open raw signals as Panel A together with spec- tral fits obtained assuming the presence of several individual ions whose signals have a modified Gaussian shape (DeCarlo et al., 2006). The atomic weight of the most abundant lead isotope (208Pb) is marked. The other fragments marked were selected in order to allow a more accurate fitting of the raw MS signal. They are most likely organic ions that contain C, H, O and/or N; however, their exact identification is be- yond the scope of this paper. Panel C in Fig. 2 is similar to panel B, but shows the spectra during a period with very low Pb signal (marked in Fig. 6). Signals corresponding to ions of the other main lead isotopes (207 Pb+ and 206 Pb+ ), as well as to the doubly charged ions of the three main lead isotopes (208 Pb++ , 207 Pb++ and 206 Pb++ ), were also observed (see Figs. S2 and S3). No signal for 204Pb+ was observed, as ex- pected due to its low abundance (0.027 relative to 208 Pb+ , (deLaeter et al., 2003)) and the limited signal-to-noise of our measurements.
Why wasn't information about the mass of isotopes sufficient for identifying the isotopes? Provide an example to explain the answer.
Figure 2 shows 2.5 min averaged spectra of ambient aerosol
at nominal m/z 208 obtained with the HR-AMS located at T0. Panel A compares V- and W-mode open, closed and dif- ference (open minus closed) spectra, during a period with a high lead signal (marked in Fig. 6). The difference between the sensitivity and the resolution of V- and W-modes is evi- dent in the figure; W-mode spectra have more resolution but a noisier signal. Typically, the difference mass spectra cor- respond to the sampled aerosol, once the background gases in the detection region are accounted for by subtracting the closed from the open mass spectra (Canagaratna et al., 2007). However, in the case of lead ions, a closed signal of the same order of magnitude as the open signal was observed which, as it will be explained later, indicates that there is a residual sig- nal caused by aerosol components that evaporate slowly from the vaporizer due to their relatively low volatility (Huffman et al., 2009). Because of this slow evaporation, the difference signal cannot be interpreted as usual in the case of lead ions and will not be discussed further. Panel B in Fig. 2 shows the same open raw signals as Panel A together with spec- tral fits obtained assuming the presence of several individual ions whose signals have a modified Gaussian shape (DeCarlo et al., 2006). The atomic weight of the most abundant lead isotope (208Pb) is marked. The other fragments marked were selected in order to allow a more accurate fitting of the raw MS signal. They are most likely organic ions that contain C, H, O and/or N; however, their exact identification is be- yond the scope of this paper. Panel C in Fig. 2 is similar to panel B, but shows the spectra during a period with very low Pb signal (marked in Fig. 6). Signals corresponding to ions of the other main lead isotopes (207 Pb+ and 206 Pb+ ), as well as to the doubly charged ions of the three main lead isotopes (208 Pb++ , 207 Pb++ and 206 Pb++ ), were also observed (see Figs. S2 and S3). No signal for 204Pb+ was observed, as ex- pected due to its low abundance (0.027 relative to 208 Pb+ , (deLaeter et al., 2003)) and the limited signal-to-noise of our measurements.
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