Environmental Engineering Reference
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10. Explain what are the differences between ''SCAN'' and ''SIM'' in GC-MS. Why ''SIM''
is needed to achieve high sensitivity and lower detection limit?
11. Explain molecular ions, fragment ions, isotopic clusters, and base peak in a molecular
mass spectrum.
12. What are the abundance ratio of
35
Cl to
37
Cl and
79
Br to
81
Br? How do these isotopic
ratio affect the mass spectra of compounds containing Cl and Br?
13. The mass spectrum of pentane from an electron ionization is shown below:
(a) Indicate which peak is the molecular ion peak? Which one is the base peak?
(b) What causes the line at m/z of 57? at m/z of 43? and at m/z of 29?
(c) Can you propose a fragmentation mechanism based on the above mass spectrum?
(d) Write all the reactions involved in the fragmentation of pentane.
14. Give a list of the major U.S. EPA methods for the GC-MS analysis of (a) VOCs and
(B) SVOCs in drinking water, wastewater, and wastes.
15. Explain: Why NMR resolution is increased with the increasing strength of the magnetic
field (B
0
)? Why
1
H-NMR is more sensitive than
13
C-NMR when an equal number of
nuclei is compared?
16. If a
1
H-NMR spectrometer is equipped with a magnet with a magnetic field (B
0
)of
7.05 T, calculate the required operating frequency of the spectrometer in megahertz
(MHz). Use the tabulated value in Table 12.1 for the gyromagnetic ratio (g).
17. Refer to the above question, what will be the required frequency in MHz if this is an
13
C-NMR spectrometer? If the operating frequency is fixed at 200, 300, 500, 600 MHz,
what will be the required magnetic field strengths (B
0
) at each of these frequencies?
18. Calculate the chemical shift (d) in ppm if a proton signal is 1150 Hz higher than that of
the TMS standard. The
1
H-NMR spectrometer was run at a frequency of 600 MHz.
19. Explain why will a nucleus with more shielding resonate at a lower radio frequency, and
why will such a nucleus be located in the upfield?
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