Structural Characterisation
Elemental Analysis (EA) - gives the percentage by mass of carbon, hydrogen, and nitrogen. It
works by combusting the compound and separating and analysing the gases produced. Elemental
analysis gives us the empirical formulae. The empirical formulae gives the ratio of the elements
in the structure. The molecular formulae gives the actual number of atoms in each molecule.
Double Bond Equivalence (DBE); also called the index of hydrogen deficiency (IHD) is used to
determine the amount of double bonds or rings.
Mass Spectrometry
Mass spectrometry gives the mass of the molecule. It is possible to measure the mass of a
molecule to 4 decimal places so the difference between molecules with the same Mr can be
separated. The exact mass is calculated by using the mass of the most common isotope.
Molecules are bombarded by high energy electrons in ionisation and electrons are knocked
out of the molecule leaving them with a positive charge and a free radical. The positively
charged molecular ion has one unpaired electron as one of the electrons has been removed.
The ions are then accelerated by an electric field where they are separated based on their m/z
ratio. The peaks correlate to fragments and the peak with the highest m/z value is the
molecular ion peak giving information on the molecular mass of the compound.
Characteristic fragments form due to loss of small molecules. It is important in identifying
specific molecules.
Structural Characterisation 1
Elemental Analysis (EA) - gives the percentage by mass of carbon, hydrogen, and nitrogen. It
works by combusting the compound and separating and analysing the gases produced. Elemental
analysis gives us the empirical formulae. The empirical formulae gives the ratio of the elements
in the structure. The molecular formulae gives the actual number of atoms in each molecule.
Double Bond Equivalence (DBE); also called the index of hydrogen deficiency (IHD) is used to
determine the amount of double bonds or rings.
Mass Spectrometry
Mass spectrometry gives the mass of the molecule. It is possible to measure the mass of a
molecule to 4 decimal places so the difference between molecules with the same Mr can be
separated. The exact mass is calculated by using the mass of the most common isotope.
Molecules are bombarded by high energy electrons in ionisation and electrons are knocked
out of the molecule leaving them with a positive charge and a free radical. The positively
charged molecular ion has one unpaired electron as one of the electrons has been removed.
The ions are then accelerated by an electric field where they are separated based on their m/z
ratio. The peaks correlate to fragments and the peak with the highest m/z value is the
molecular ion peak giving information on the molecular mass of the compound.
Characteristic fragments form due to loss of small molecules. It is important in identifying
specific molecules.
Structural Characterisation 1
