The lower the mass to
charge ratio the more it will
be deflected
5) Detection
using computer methods
Time of Flight Mass Spectrometry (TOF-MS)
Used to separate and to measure
the relative masses of the ions
produced under high-vacuum
conditions
History
1918 by Francis W Aston, a
student of J J Thompson (the
man who discovered the
electron
Aston used the instrument to show that there were
different forms of the same element. We now call
these isotopes.
Uses
Initially used to
show the identity
of a isotopes
Now used to calculate
molecular masses and
characterise new compounds
A spectrometer
Infra-Red Spectroscopy
The Infra-red Spectrophotometer
A beam of Infra-red
radiation is passed
through the sample
A similar beam is passed through
the reference cell
The frequency of the
radiation is varied
The amount of radiation absorbed by
a substance is compared with the
radiation
The results are stored
and plotted
Key points
In IR spectroscopy molecules
absorb frequencies
corresponding to their
functional group. These
absorptions are resonant
frequencies i.e the frequency
of the absorbed radiation
matched the vibrational
frequency.
Energy quoted in
wavenumbers. Unit= reciprocal
centimetres cm-1
The amount of vibration
depends on...
1) Bond strength (and bond length)
2) the mass of each atom
For a molecule to absorb IR,
vibration with in the
molecule (stretching or
bending) must cause a net
change in the dipole
moment)
Peak position depends on 1)
Bond strength 2) masses of
the atoms joined by the
bond
Strong bonds and light atoms absorb at lower
wavenumbers. Weak bonds and heavy atoms absorb
at high wavenumbers.
The Fingerprint region
Called the fingerprint region
because it is specific to each one
The low wave spectrum of the graph
(usually between 600 cm-1 and 1400 cm-1)
is known as the fingerprint region
This region is characteristics
of each individual compound,
and can be compared to a
known database)