Homework Solutions for Chem 422

Chapter 20

1. In a gaseous ionization source, the sample is first volatilized and then transmitted to the ionization area for ion formation. In a desorption source, a probe supports the sample and ionization takes place directly from the condensed form of the sample. The advantages of gaseous sources are their simplicity and speed (no need to use probe and wait for probe area to be pumped out). The advantage of desorption ionization is that is can be applied to high molecular mass and thermally unstable samples.

2. Electron impact ionization produces the most fragmentation and the most complex spectra. Field ionization produces the simplest spectra. Chemical and electron ionization produce greater sensitivities than does field ionization.

3. Both field ionization and field desorption ionization are performed at anodes containing numerous sharp tips that produce very high electrical fields. In field ionization, the sample is volatilized before it is ionized, whereas field desorption ionization takes place at an anode that has been coated with the sample. The latter method uses a sample probe.

8. A negative potential in the yz plane causes positive ions to move toward the rods where they are annihilated. In the presence of an added ac potential, this movement is inhibited during the positive half of the cycle with the lighter ions being more affected than the heavier ions. Thus, the yz plane acts as a low-pass filter removing heavier ions.

9. The resolution of a single-focusing mass spectrometer is limited by the initial kinetic energy spread of the sample molecules. This spread is minimized in a double-focusing instrument by accelerating the sample ions through an electrostatic analyzer, which limits the range of kinetic energies of ions being introduced into the magnetic analyzer. Significantly narrower peaks result.

+ 12 35 + 16. The M peak (m =84) is due to CH2 Cl2 .

13 35 + The m = 85 peak is due to CH2 Cl2 .

12 35 37 + The m = 86 peak is due to CH2 Cl Cl .

13 35 37 + The m = 87 peak is due to CH2 Cl Cl .

12 37 + The m = 88 peak is due to CH2 Cl2 .