Chapter 20: 20-1 (page 533)
How do gaseous and desorption sources differ? What are the advantages of each?
A gaseous source, the sample is first volatilized and then transmitted to form ion.
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Figure (page 536) shows the mass spectrum of the same compound from an EI source and a CI source.
a. Which mass spectrum would be best for determining the molecular mass of the compound? Why?
b. Which mass spectrum would be best for determining the chemical structure? Why?
c. The EI source was a pulsed source used with a TOF mass analyzer. If the flight tube were m long and the accelerating voltage were V, what would the flight time be for the ion at ?
d. For two ions of m/z values and localid="1646035174939" , derive an equation for the difference in flight times as a function of the two masses, the charges, and the accelerating voltage.
e. For the same TOF analyzer as in part (c), calculate the difference in flight times between ions of and
f. To get more structural information, the compound of Figure was subjected to tandem mass spectrometry. Which ionization source, EI or CI, would be most suitable for this purpose? Why?
g. Using the ionization source chosen in part (f), describe the types of mass spectra that could be obtained from an MS/MS experiment by:
1. holding the first mass analyzer constant and scanning the second analyzer.
2. scanning both analyzers with a small m/z offset between them.
3. scanning the first analyzer while holding the second analyzer constant.
4. scanning the second mass analyzer for every mass selected by the first analyzer.
In your answer, use features of the mass spectrum of Figure 20-29 to illustrate your description.
The following figure is a simplified diagram of a commercially available EI source.
(a) What voltage must be applied between the filament and target so that electrons interacting with molecules
at the point marked SS (sample source) will have 70 eV of kinetic energy?
(b) What will happen to a molecule that diffuses toward the filament and is ionized at point P?
What mass differences can just be resolved at m values of and if the mass spectrometer has a resolution of
(a) (b) (c) (d) ?
Calculate the accelerating voltage that would be required to direct singly charged ions of mass 5000 through an instrument that is identical to the one described in Example 20-4.
Calculate the accelerating voltage that would be required to direct singly charged ions of mass through an instrument that is identical to the one described in Example 20-4.
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