Time Of Flight Mass Spectromotry

Question 1

What is a mass spectroscopy used for ?

Answer 1

It is a very powerful instrumental technique used to find the relative mass of elements and compounds

Question 2

Stage 1: ionisation

Explain electron impact

Answer 2

• Electron impact is used for elements and low Mr compounds
• high energy electrons are fired at the sample from and electron gun
• this knocks off one outer electron from each atom/molecule to form a 1+ ion

Question 3

What state does the atom have to be in before ionisation ?

Answer 3

Gaseous state

Question 4

Stage 1: ionisation

Explain electrospray ionisation

Answer 4

• the sample is dissolved in a volatile (turns to gas easily ) solvent
• it is then injected through a fine hypodermic needle as a fine spray into a vaccine in the ionisation chamber
• a very high voltage is applied to the end of the positively charged needle where the spray emerges
• the particles gain a proton and become ions as a fine mist
• the solvent evaporates leaving 1+ ions

Question 5

Why does it have to be in a vaccume

Answer 5

As you only want to analyse your specific focus

Question 6

Stage 2: acceleration of ions

Answer 6

• the ions are accelerated using an electrical field so all the ions have the same kinetic energy

Question 7

Stage 3: separation of charged ions

Explain ion drift

Answer 7

• Ion drift is where the ions enter the flight tube
• ions with different masses have a different time of flight
• the lighter ions travel faster and take less time to reach the detector

Question 8

Stage 4: detection

Answer 8

• the detector is a negatively charged plate so a current is produced when the ions hit the plate
• the more ions that hit the detector the larger the current
• the mass of the ions can be calculated from the time of flight
• the mass spectrum shows the abundance of each mass that hit the detector

Question 9

What do the axis on the graph show ?

Answer 9

• horizontal : mass to charge ratio of the particles that hit the detector (effectively mass as charge is usually +1)

Question 10

Where is the main peak on the graph for electrospray ionisation ?

Answer 10

The main peak is usually at Mr + 1 so the Mr is one less than the molecular ion peak

Question 11

Equation for finding the relative atomic mass

Answer 11

Average mass of the atoms = total mass of all the atoms \ total number of atoms

Question 12

In organic molecules there is often a small peak at …….. due to a small number of molecules containing …. or …. atoms

Answer 12

Mr + 1
13C
2H

Question 13

If a molecule contains more than one atom of 13C or 2H atoms there will be small undetectable peaks

Give an example

Answer 13

Butanone :
Main peak at 72 but also small peak at 73 (due to one atom 13C or 2H )
Tiny peaks at 74 and 75 (due to more than one atom 13C or 2H)