Transcript Mass Spectrometer

Mass Spectrometer
1. Vaporisation
3. Acceleration
Mass Spectrometer
4. Deflection
2. Ionisation
5. Detection
1. Vaporisation
Sample vaporised by oven
if liquid, then injected into
vacuum chamber at low
pressure
3. Acceleration
Negatively charged plates produce an electric field
which accelerates the positively charged ions. They are
focused into a narrow beam as they pass through slits
2. Ionisation
•Sample bombarded
by a stream of high
energy electrons
from an electron
“gun”.
•These high energy
electrons knock an
e- from a sample
atom. Producing a
positive ion.
•X(g)  X+(g) + e-
Usually only one e- is removed to form X+(g) ions but X2+(g) are
occasionally formed. Sometimes the particles are broken into
fragments by the high energy electrons - fragmentation
4. Deflection
•The ions are deflected
by a strong, variable
magnetic field
•The extent of
deflection is
determined by the
mass and charge of
the ions.
X+
By varying the strength of the
magnetic field ions with specific
m/z are focused through the
collector slit
m/z too small
For a given magnetic field
•The greater the mass,
m, the less the ions are
deflected
m/z too large
•The greater the positive
charge, z, the more they
are deflected
The larger the value of the mass to charge ratio,
m/z, the smaller the deflection
5. Detection
All the counts together produce a mass
spectrum. The peaks are proportional
to the abundance of the positive ios
Abundance
When a positive ion reaches the detector it produces a tiny current. This
signal is fed to a recorder which counts the number of signals for each
setting of the magnetic field.
X+
m/z
The Mass Spectrum of Cl2
35Cl -35Cl
Relative
Abundance
35Cl
35Cl -37Cl
Or
Relative
intensity
37Cl
37Cl -37Cl
35 37
Ratio 3 : 1
70 72 74
Ratio 9 : 6 : 1
m/z
The Relative Atomic Mass (Ar)
of an element
The relative atomic mass (Ar) of an element is the
average mass of one atom compared with one
twelfth of the mass of an atom of carbon-12
Ar
=
average mass of 1 atom of element
1/12 mass of one atom of carbon-12
Ar
=
average mass of 1 atom of element x 12
mass of one atom of carbon-12
Calculating the Relative Atomic Mass
(Ar) of an element
Mass to charge ratio (m\z)
35
37
Relative Abundance (%)
75
25
Calculate a weighted average
Ar of chlorine = (35 x 75) + (37 x 25)
(75 + 25)
Ar of chlorine = 2625 + 925
100
Ar of chlorine = 35.5
Calculating the Relative Atomic Mass
(Ar) of Lead
Mass to charge ratio (m\z)
204
206
207
208
Relative Abundance (%)
1.55
23.6
22.6
52.3
Ar of lead= (204 x 1.55) + (206 x 23.6) + (207 x 22.6) + 208 x 52.3)
(1.55 + 23.6 + 22.6 + 52.3)
Ar of lead = 316.2 + 4861.6 + 4678.2 + 10878.4
100.05
Ar of lead = 20734.4
100.05
Ar of lead = 207.2
The Relative Molecular Mass (Mr)
of an element or compound
• Relative atomic mass values (Ar) can be used to calculate the Relative
molecular mass (Mr) of an element or compound
• The relative molecular mass (Mr) of an element or compound is the
sum of the relative atomic masses of all the atoms in the molecule,
divided by one twelfth of the mass of an atom of carbon-12
• Mr
=
mass of 1 molecule
1/12 mass of one atom of carbon-12
rearranged to
• Mr
=
mass of 1 molecule x 12
mass of one atom of carbon-12
Relative Molecular Mass (Mr)
• Mass spectra can be used to find the Mr of
a compound