atomic structure : the nucleus notes

Question 1

basic principles of ToF mass spectrometer

Answer 1

particles of the substance to form 1+ ions are accelerated so they all have same ke
- time taken to travel a fixed distance is used to find the mass of each ion in the sample

Question 2

What is the mass spectrometer able to do

Answer 2

• Separates atoms of different isotopes of an element
• Measures the abundance and mass of each isotope
• allows relative atomic mass to be calculated

Question 3

What is stage 1 of the mass spectrometer and what happens in this stage

Answer 3

• Vaporisation
• sample is turned into a gas

Question 4

What is stage 2 of the mass spectrometer and what happens in this stage

Answer 4

• Ionisation
• High energy electrons from an electron gun hit an atom and knock off 1 electron from the atoms
• X (g) → X+ (g) + e- or X (g) + H+ → XH+ (g)

Question 5

What is stage 3 of the mass spectrometer and what happens in this stage

Answer 5

• Acceleration
• a negative plate (electric field) attracts the positive ions
• velocity increases

Question 6

What is stage 4 of the mass spectrometer and what happens in this stage

Answer 6

• Ion drift
• Ions travel along the flight tube
• Larger m/z ions take longer to reach the detector

Question 7

What is stage 5 of the mass spectrometer and what happens in this stage

Answer 7

• Detection
• Positive ion hits negative plate
• X+ + e- → X

Question 8

What is stage 6 of the mass spectrometer and what happens in this stage

Answer 8

• Data analysis
• abundance measured as an electron flows from the plate to the ion
• size of current is proportional to the abundance
• ToF converted into m/z

Question 9

Describe how electron impact ionisation process occurs

Answer 9

high energy electrons from an electron gun hit an atom and knock off an electron off

Question 10

Describe how electrospray ionisation process occurs

Answer 10

• sample is dissolved in volatile solvent is injected through a fine needle at a high voltage
• each molecule gains a proton

Question 11

What type of substance is electrospray ionisation used for

Answer 11

molecules with higher Mr including molecules e.g. protein
- soft ionisation technique and fragmentation rarely takes place

Question 12

What type of substance is electron impact ionisation used for

Answer 12

for the analysis of low-mass, volatile, thermally stable organic compounds which are difficult to ionise by other techniques.

Question 13

Which ionisation method is most likely to break covalent bonds in the molecule

Answer 13

Electron impact

Question 14

what is the breaking of bonds during ionisation called

Answer 14

fragtmentation

Question 15

word equation to calculate the average mass of one atom

Answer 15

total mass of all atoms / total number of atoms (total relative abundance)

Question 16

what affects the velocity in mass spectrometer

Answer 16

the charge on the ion
reach detector in less time