1.3 Comparing the masses of atoms – mass spectrometry & 1.4 Isotopes and relative isotopic masses

What is an Isotope?

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

What are the typical properties of isotopes?

• same proton number

• same electron number if neutral

• different neutron number

• different mass number

What can you deduce about the physical and chemical properties of isotopes?

• Isotopes have the same chemical properties because they have the same electron arrangement.

• They can have different physical properties because they have different masses.

What is the relative isotopic mass?

the mass of an isotope compared with 1/12 of the mass of a carbon-12 atom.

-has no units

What is the relative atomic mass?

the weighted mean mass of the atoms of an element compared with 1/12 of the mass of a carbon-12 atom.

What is the formula for relative atomic mass?

∑(isotopic mass×relative abundance)​ / ∑ relative abundance

What is the purpose of a mass spectrometre?

• identify different isotopes/ions

• determine their relative abundances

• determine relative atomic mass

• determine relative molecular mass

• analyse fragments produced from molecules

Describe the whole mass spectrometer process

Ionisation

• The sample is converted into ions.

• High-energy electrons remove electrons from atoms/molecules.

• Positive ions are produced.

Acceleration

• An electric field accelerates the positive ions.

• The ions are given kinetic energy.

Separation/deflection

• A magnetic field causes the ions to follow curved paths.

• Ions with different m/z values are separated.

Detection

• Ions reach the detector.

• Their arrival produces an electrical signal.

• The signal is related to the relative abundance of the ions.