Mass Spectrometry

Question 1

Explain ‘electron impact ionisation’

Answer 1

High energy electrons are fired at the gaseous sample by an electron gun. The electrons knock an electron off the atoms in the sample, forming 1+ ions
X(g) + e- —> X+(g) + 2e-

Question 2

Explain ‘electrospray’

Answer 2

Sample is dissolved in a volatile solvent, then turned into a fine spray at high-voltage via a charged needle. H+ gained from solvent increases m/z value by 1.
X(g) + H+ —> XH+(g)

Question 3

Explain ‘acceleration’

Answer 3

A negatively charged plate attracts the positive ions so their velocity increases.

Question 4

Explain ‘ion drift’

Answer 4

The ions travel along the flight tube, larger m/z ions take longer to reach the detector.

Question 5

Explain ‘detection’

Answer 5

Positive ions hit the negatively charged plate and electrons flow to the positive ions, causing them to gain an electron which produces an electrical current .
X+ + e- —> X

Question 6

Explain ‘data analysis’

Answer 6

Time of flight is converted to m/z and the size of the current produced is converted to abundance (size of current is proportional to abundance). Data is displayed as a spectrum

Question 7

What is the reasoning for a peak at half m/z on a spectrum?

Answer 7

Electron impact ionisation can form 2+ ions, therefore mass will be divided by 2 rather than 1, halving the m/z