[Unit 1.4] | The Mass Spectrometer

Question 1

Explain the process of Electrospray Ionisation.

Answer 1

• Dissolve a sample in a volatile solvent and pump it through a hollow needle connected to the positive terminal of a high voltage supply.
• Molecules pick up a H+ ion and emerge as a fine mist.
• The solvent evaporates as the sample enters the Mass Spectrometer, leaving only the XH+ ions.

Question 2

Explain the process of Electron Impact Ionisation.

Answer 2

• A hot wire filament fires high energy electrons at the sample.
• This usually knocks a single electron off of the atom but can knock off multiple or cause fragmentation.

Question 3

Create an equation for element X showing Electron Impact Ionisation. Include state symbols.

Answer 3

X(g) + e- –> X+(g) + 2e-

Question 4

Describe the acceleration stage of TOF Mass Spectrometry.

Answer 4

• The positive ions are attracted to a negatively charged plate.
• Lighter ions move towards the plate faster: ions are separated based on their differing velocities.

Question 5

Describe the Ion Drift stage of TOF Mass Spectrometry.

Answer 5

• Ions pass through a hole in the negative plate.
• Each ion travels at a constant speed.

Question 6

Describe the Detection stage of TOF Mass Spectrometry.

Answer 6

• The ions pick up electrons from the detector which causes a current to flow.
• The current is detected and passed to a computer to generate a mass spectrum.
• A greater charge/abundance produces a greater current.

Question 7

How does the M/Z peak differ when using electrospray ionisation instead of Electron Impact?

Answer 7

• The M/Z peak is +1 higher due to the presence of the H+ ion attached to the sample.
• 1 must be subtracted from the Mr value to account for the H+ ion.

Question 8

Which 3 equations are associated with TOF Mass Spectrometry calculations?

Answer 8

• KE = ½mv² (m is in kg)
• v = d/t
• n = m/Mr (m is in g)