Mass Spectroscopy
M S. SA R I K A C H A N D O O
C A P E C H E M I S T RY U N I T 2
SPECIFIC OBJECTIVE 7
OBJECTIVES
INTRODUCTION
➢Main use of a mass spec is the identification of organic compounds.
➢A mass spectrum is produced by exciting an atom or molecule in the
gas phase with enough energy to cause it to ionize.
➢The gaseous ions formed are usually positively charged (+1)
➢The mass: charge ratio (m/z) and relative abundances expressed
as a percentage of each of the gaseous ions are displayed on the
horizontal and the vertical axes of the mass spectrum.
➢The unit on the x axis is atomic mass units divided by the charge on
the ion (z) which is usually +1.
➢The unit on the y axis of the spectrum is percentage relative
abundance. The strongest peak (base peak) is assigned the value of
100%.
➢The percentages of all other ions are quoted relative to that base peal.
➢Detection depends on the particle carrying an electric charge.
➢Neutral particles are no detected.
MASS SPECTRA OF ATOMS & MOLECULES
❖Mass spectra of a pure samples of an element
which has more than one stable isotope will
show peaks of mass: charge ratio
corresponding to the mass of each isotope.
❖Most abundant isotope: give the highest peak:
relative abundance of 100%
❖This is called the base peak
❖Electron is removed from a covalent molecule
(M) in the gas phase, the positively charged
gaseous ion formed is known as the molecular
ion M +; (parent ion): Molecule minus an
electron (radical cation); unpaired electron.
❖M+ is unstable and undergoes fragmentation
to form ions of lower mass (lower m/z) ratio
called fragment ions.
MASS SPECTROMETER
A mass spectrometer separates atoms and
molecules according to their masses and also
shows the relative numbers of the different
atoms and molecules present.
Sample is vaporised. When vapor enters the
machine, it is bombarded by high energy
electrons.
This knocks electrons from the molecules and
break covalent bonds, fragmenting the
molecule.
Before atoms can be detected in a mass
spectrometer they must be converted to
positive ions in the vapour or gaseous state.
Inside the mass spec, there is a vacuum that
allows for the particles from the chemical under
test to be studied without interreference from
particles in air.
5 stages in mass spectrometer:
Vaporisation, ionisation, acceleration, deflection
and detection.
ANIMATIONS
DEFLECTION DUE TO M/Z RATIO
➢For a given electric and magnetic field, only those ions with a particular charge
and mass hit the detector .
➢By gradually increasing the strength of the magnetic field, ions of increasing
mass/charge ration (m/z) hit the detector. (z is the charge in the ion, usually
+1).
➢An ion with a mass charge ration of 15, an ion heavier than this eg m/z 16 is
deflected less and an ion lighter than this eg m/z 14 is deflected more.
➢If the ion is doubly charge it is deflected twice as much.
➢A Pb + ion has a m/z ration of 208 and a Pb 2+ has a m/z ration of 104.
VIDEOS
https://www.youtube.com/watch?v=2oPUyIbPxLo&list=RDCMUCQZnA4etbO
M7_n55hBQ3LmA&index=6
https://www.youtube.com/watch?v=gYlUp6iJ02Y
https://www.youtube.com/watch?v=GSYueQzo2n8
https://www.youtube.com/watch?v=T34mkIRwQgg
https://www.youtube.com/watch?v=x2p5y1f0ajM
RELATIVE MOLECULAR MASS
Mass spectra can be used to identify the different isotopes present in elements.
Different isotopes are detected at particular whole-number ratios because each
proton and neutron has a relative mass of 1.
Mass spectra can be used to calculate relative atomic masses:
Step 1: Multiply each isotopic mass by its % abundance.
Step 2: Add the figures together.
Step 3: Divide by 100.
WORKED EXAMPLE
Step 1: (20.5 × 70) + (27.4 × 72) + (7.8 × 73) + (36.5 × 74) + (7.8 × 75)
Step 2: 1435 + 1972.8 + 569.4 + 2701 + 585 = 7263.2
Step 3: 7263.2 / 100 = 72.632
Notice that the relative atomic mass is the weighted mean of the masses
of all the mass numbers of the isotopes present.
MOLECULAR MASS FROM MASS SPECTRA
▪The mass spectrum of chlorine shows peaks due to
singly charged ions of the 35Cl and 37Cl isotopes.
The small peaks at 17.5 and 18.5 are due to the
doubly-charged ions 35Cl2+ and 37Cl2+
▪The spectrum will also show small peaks caused by
ionised chlorine molecules, Cl2+.
▪There are 3 peaks of these molecular ions:
▪m/z 70: due to 35Cl—35Cl m/z 72 due to 35Cl—37Cl
▪m/z 74 due to 37Cl—37Cl
▪These small peaks due to the molecular ions are
called the molecular ion peaks
FRAGMENTATION
Peak at highest mass to charge ratio is cause by the molecular ion (M+)
Fragmentation occurs where the bonds are weakest
More stable fragment, greater the abundance in mass spec.
Tertiary carbocations> secondary carbocations> Primary carbocations.
This ion is formed by the sample molecule with one electron knocked out.
It gives us the relative molecular mass of the sample.
We can assume that the ions detected carry a single positive charge so the
reading on horizontal axis gives us mass.
M+= mass of 58.0 CH3COCH3 +, with a mass of (3 x 12.0) + (1 x 16.0) + (6 x1.0).
Peaks at 15 and 43 are due to fragments when propanone is broken apart.
The electron bombardment has caused the C -C single bonds in the propanone
molecules to break. This has resulted in the fragments at m/e 15 and 43 that are
observed in Figure 29.22. The breaking of single bonds, such as C- C, -C O or CN, is the most common cause of fragmentation.
MORE ON FRAGMENTATION
When compound such as propanone is ionised, a single
electron may be removed from the molecule.
The peak arising from this ionisation is called the molecular
ion peak, M+. It gives the molecular mass of the molecule.
For propanone, the molecular ion peak will appear at a
mass/charge ratio (m/z) ratio of 58.
In many compounds the relative abundance of molecular ion
peak is usually very low.
This is because the molecule breaks up in the mass
spectrometer to form fragments having particular m/z
rations.
This process is called fragmentation.
Fragmentation occurs where the bonds are the weakest.
The more stable the fragment, the greater is its abundance
in the mass spectrum.
Tertiary carbocations are more stable then secondary and
secondary are more stable than primary.
DISTINGUISHING BETWEEN MOLECULES
Butane and methylpropane have the same molecular
mass, 58. We can use mass spectrometry to distinguish
between these molecules.
THE M+1 PEAK
There will always be a very small peak 1 m/z unit
beyond the molecular ion peak is called the M+1
peak.
The M+1 peaks arises because in any organic
compounds, 1.10% of the carbon atoms are of
the 13C isotope.
We can work out the number of carbon atoms, n
in a molecule by saying:
So if the molecular ion peak has an abundance of
49.3% and the M+1 peak has an abundance of
3.8%, the number of carbon atoms in the
compound can be found.
If an organic compound contains Cl atoms you can get a
M+2peak and a M+4 peak in the spectrum. This is because Cl
has two isotopes 35Cl and 37Cl. For example in the compound
CH2Cl2, an M+2 peak is due to 35ClCH237Cl+ and an M+4 peak is
due to 37ClCH2 37Cl+
QUESTION
An unknown compound has a molecular ion peak, M+, with a relative abundance of 54.5% and has an [M +
1]+ peak with a relative abundance of 3.6%. How many carbon atoms does the unknown compound
contain?
M+2 AND M+4 PEAKS
If sample contains chlorine or bromine atoms, we get peaks beyond the
molecular ion peak because of isotopes of chlorine and bromine.
CHLOROMETHANE/ BROMOMETHANE
DICHLOROMETHANE
CHLOROBENZENE
APPLICATIONS OF MASS SPECTROMETRY
❖Deducing the number of carbon atoms in an organic compound.
❖Recognising the presence of bromine and chlorine atoms in an
organic compound ( or other elements with two or more isotopes):
distinctive mass spectra produced.
❑ sulphur: 32-S 95%; 34-S 4.2%
❑ chlorine: 35-Cl 75.5%; 37-Cl 24.5%
❑ bromine: 79-Br 50.5%; 81-Br 49.5%
❑ silicon: 28-Si 92.2%; 29-Si 4.7%; 30-Si 3.1%
❖Determining the elemental composition of molecular and fragment
ions.
❖Forensics
❖Environmental monitoring of pollutants
❖Drug testing in sports
❖Geological and archaeological dating
❖Airport security.
FRAGMENTATION