Atomic Structure (Physical Chem)

define relative isotopic mass

• The relative isotopic mass of an isotope is the ratio of the mass of one atom of that isotope to 1/12th of the mass of one atom of carbon-12.

define relative atomic mass

• The relative atomic mass of an atom is the ratio of the average mass of one atom of that

  element to 1/12th of the mass of one atom of carbon-12.

1/12th of the mass of one atom of carbon-12 is ?

1

relative isotopic mass is the same as?

• mass number

Define the The time of flight (TOF) mass spectrometer

• An instrument used for measuring the masses of atoms and molecules.

• It can also be used to measure the relative abundance of different isotopes and to predict the structure of more complex molecules.

describe How the mass spectrometer works

1. Vaporisation

• The sample is vaporised.

• This allows the sample to travel through the mass spectrometer as a gas.

2. lonisation - Electrospray lonisation

• The sample is dissolved in a polar solvent.

• It is pushed through a fine nozzle at high pressure.

• A high voltage is applied.

• Particles lose an electron.

• This produces positive gaseous ions.

3. Acceleration

• The positive ions are accelerated by an electric field.

• lons with a lower m/z ratio accelerate to a higher speed.

• lons with a higher m/z ratio accelerate to a lower speed.

4. Ion Drift

• The ions leave the electric field and travel at a constant speed.

• lons with a lower m/z travel faster.

• lons with a lower m/z reach the detector sooner.

• lons with a higher m/z travel slower and take longer to reach the detector.

5. Detection

• lons hit the detector and produce an electrical current.

• The time taken to reach the detector is used to determine the m/z ratio.

• The size of the current indicates the relative abundance of the ion.

• Greater abundance → larger current → larger peak on the mass spectrum.

Q: How does the time of flight of an ion depend on its mass and charge?

.

The greater the mass, the longer the time of flight.

The greater the charge, the shorter the time of flight.

Therefore, time of flight is inversely proportional to the m/z ratio.

Q: What is fragmentation in mass spectrometry?

Fragmentation is when molecules break up into smaller pieces inside the mass spectrometer.

This happens because the conditions inside the mass spectrometer are very extreme.

what is the molecular ion peak.

• the peak with the highest m/z ratio which comes from the unbroken molecular ion

• the m/z ratio of this peak = molecular mass of the molecule.