Homework 6

Question 1

How do gaseous and desorption sources differ?

Answer 1

With gas, the sample is first volatilized (by heat if necessary) then transmitted to the ionization area for ionizing. In desorption, a probe is used and ionization takes place directly from the condensed phase

Question 2

Advantages of gaseous sources

Answer 2

Simplicity and speed (no need to use probe and wait for probed area to be pumped out)

Question 3

Advantages desorption sources

Answer 3

Can be applied to high MW and thermally unstable samples

Question 4

How do the spectra for electron impact, field ionization, and chemical ionization differ

Answer 4

Electron impact has the most fragmentation and thus the most complex spectra. Field ionization has the simplest spectra. Chemical and electron result in higher sensitivity than field.

Question 5

Describe the difference between gaseous field ionization sources and field desorption sources

Answer 5

Both are performed at anodes containing numerous sharp tips so that very high electrical fields are required. In field ionization, the sample is volatilizated before ionization whereas field desorption takes place in an anode that has been coated with the sample. Desorption requires used of a sample probe.

Question 6

Why do double focusing mass spectrometers give narrower peaks and higher resolutions than single focusing instruments?

Answer 6

Resolution of single is limited by initial KE spread of the sample molecules. This spread is minimized in a double by accelerating the sample through an electrostatic analyzer which limits the range of KE of ions introduced into the magnetic sector analyzer. Significantly narrower peaks are the result.

Question 7

Differences between quadruple ion trap mass spectrometers and Fourier transform ICR

Answer 7

Quadruple: by combination of fields, ions temporarily stores in a trap. Then they release sequentially by increasing the radio frequency voltage applied. The ejected ions strike a detector. Plot of detector signal vs radio frequency voltage related to m/z is the spectrum.
In FTICR: ions trapped in a cell by electric trapping voltage and a magnetic field. Each ion assumes circular motion in plane perpendicular to direction of the field. Cyclotron frequency depends on inverse of the m/z value. In modern instruments a radio request pulse that increases linearly in frequency is used. Time domain image current is generated after termination of pulse. Mass spec yielded by Fourier transformation of time decay signal