Modern Analytical Techniques I
Mass spectrometry
Mass spectrometry can be used to identify compounds. It is done using a mass
spectrometer and involves charging (i.e. ionising) molecules which can then be
detected due to their charge.
Here is an example of a mass spectrum:
The x- axis is
labelled as ‘m/z’ (mass to charge ratio of the ions), and the y-axis is labelled as
relative abundance. The last significant peak (i.e. 116 in this example) is known as
the molecular ion, and gives the Mr of the compound.
The smaller ion peaks are fragment ions and can indicate possible structures of the
molecules based on the groups present in that molecule.
Infrared (IR) spectroscopy
An infrared spectrum used infrared radiation and can be used to deduce which
functional groups are present in a molecule. This is due to their wavenumber
(measured in cm-1) which corresponds to the stretching absorption for certain
function groups:
C-H for alkanes, alkenes and aldehydes
C=C for alkenes
O-H for alcohols
C=O for aldehydes and ketones
C=O and a broad O-H for carboxylic acids
N-H for amines
Data for wavenumber above 1500cm-1 gives information about the functional
groups present. Data below that point is in the fingerprint region.
Here is an example of an infrared spectrum:
Mass spectrometry
Mass spectrometry can be used to identify compounds. It is done using a mass
spectrometer and involves charging (i.e. ionising) molecules which can then be
detected due to their charge.
Here is an example of a mass spectrum:
The x- axis is
labelled as ‘m/z’ (mass to charge ratio of the ions), and the y-axis is labelled as
relative abundance. The last significant peak (i.e. 116 in this example) is known as
the molecular ion, and gives the Mr of the compound.
The smaller ion peaks are fragment ions and can indicate possible structures of the
molecules based on the groups present in that molecule.
Infrared (IR) spectroscopy
An infrared spectrum used infrared radiation and can be used to deduce which
functional groups are present in a molecule. This is due to their wavenumber
(measured in cm-1) which corresponds to the stretching absorption for certain
function groups:
C-H for alkanes, alkenes and aldehydes
C=C for alkenes
O-H for alcohols
C=O for aldehydes and ketones
C=O and a broad O-H for carboxylic acids
N-H for amines
Data for wavenumber above 1500cm-1 gives information about the functional
groups present. Data below that point is in the fingerprint region.
Here is an example of an infrared spectrum:
