atomic structure : the nucleus notes Flashcards
basic principles of ToF mass spectrometer
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
What is the mass spectrometer able to do
- Separates atoms of different isotopes of an element
- Measures the abundance and mass of each isotope
- allows relative atomic mass to be calculated
What is stage 1 of the mass spectrometer and what happens in this stage
- Vaporisation
- sample is turned into a gas
What is stage 2 of the mass spectrometer and what happens in this stage
- 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)
What is stage 3 of the mass spectrometer and what happens in this stage
- Acceleration
- a negative plate (electric field) attracts the positive ions
- velocity increases
What is stage 4 of the mass spectrometer and what happens in this stage
- Ion drift
- Ions travel along the flight tube
- Larger m/z ions take longer to reach the detector
What is stage 5 of the mass spectrometer and what happens in this stage
- Detection
- Positive ion hits negative plate
- X+ + e- → X
What is stage 6 of the mass spectrometer and what happens in this stage
- 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
Describe how electron impact ionisation process occurs
high energy electrons from an electron gun hit an atom and knock off an electron off
Describe how electrospray ionisation process occurs
- sample is dissolved in volatile solvent is injected through a fine needle at a high voltage
- each molecule gains a proton
What type of substance is electrospray ionisation used for
molecules with higher Mr including molecules e.g. protein
- soft ionisation technique and fragmentation rarely takes place
What type of substance is electron impact ionisation used for
for the analysis of low-mass, volatile, thermally stable organic compounds which are difficult to ionise by other techniques.
Which ionisation method is most likely to break covalent bonds in the molecule
Electron impact
what is the breaking of bonds during ionisation called
fragtmentation
word equation to calculate the average mass of one atom
total mass of all atoms / total number of atoms (total relative abundance)
