Atomic Structure-Mass spectrometry

Question 1

One of the methods of ionising samples in time of flight mass
spectrometry is by electron impact. How is this ionisation done?
[4 marks]

Answer 1

-high energy electrons
-from hot cathode / electron gun
-fired at sample
-knocks off one electron

Question 2

A second method of ionising samples in time of flight mass spectrometry is
by electrospray ionisation. How is this ionisation done?
[4 marks]

Answer 2

-sample dissolved in volatile solvent
-injected through a fine hypodermic needle –giving a fine mist / aerosol
-tip of needle has high voltage
-each gains a proton as it leaves the needle

Question 3

Why is a negatively charged plate used?
[2 marks]

Answer 3

as the positively charged ions are attracted to the negative plate

Question 4

Explain why different ions take different times to travel through the flight
tube.
[2 marks]

Answer 4

-time of flight depends on mass of ions
-lighter particles travel faster

Question 5

What is Fragmentation?

Answer 5

The molecular ions are energetically unstable, and some of them will break up into smaller pieces. The simplest case is that a molecular ion breaks into two parts - one of which is another positive ion, and the other is an uncharged free radical.

Question 6

Detection

Answer 6

The positive ions hit a negatively charged electric plate. When they hit the detector plate, the
positive ions are discharged by gaining electrons from the plate. This generates a movement
of electrons and hence an electric current that is measured. The size of the current gives a
measure of the number of ions hitting the plate