Transcript Mass spectrometry powerpoint

Starter:
Spec links 1.06–1.10
Green pens out!
Spec links 1.06–1.10
Learning objectives
• analyse and interpret data from mass spectrometry to
calculate the relative atomic mass from the relative
abundance of isotopes and vice versa
• predict the mass spectra for diatomic molecules, for
example chlorine
• understand how mass spectrometry can be used to
determine the relative molecular mass of a molecule
Spec links 1.06–1.10
Mass spectrometry?
• What did you learn?
Spec links 1.06–1.10
THE MASS SPECTROMETER
WHAT IS A MASS SPECTROMETER
An instrument to accurately determine the relative
atomic mass
• Relative atomic mass (Ar) is the average mass of atoms
of an element relative to an atom of carbon – 12
Separates atoms or molecules according
to their charge and mass.
This can be used to identify substances
 e.g. illegal drugs
MASS SPECTROMETRY
The first mass spectrometer was built in 1918 by Francis W Aston,
a student of J J Thomson, 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.
In a mass spectrometer, particles are turned into positive ions,
accelerated and then deflected by an electric or magnetic field. The
resulting path of ions depends on their ‘mass to charge’ ratio (m/z).
Francis Aston
Particles with a large m/z value are deflected least
those with a low m/z value are deflected most.
The results produce a mass spectrum which portrays the different
ions in order of their m/z value.
USES
Mass spectrometry was initially used to show the identity of isotopes.
It is now used to calculate molecular masses and characterise new compounds
Spec links 1.06–1.10
A MASS SPECTROMETER
DETECTOR
ION SOURCE
ANALYSER
A mass spectrometer consists of ... an ion source, an analyser and a detector.
PARTICLES MUST BE IONISED SO THEY
CAN BE ACCELERATED AND DEFLECTED
Spec links 1.06–1.10
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
IONISATION and VAPOURISATION
• gaseous atoms are bombarded by electrons from an electron gun and are IONISED
• sufficient energy is given to form ions of 1+ charge
Spec links 1.06–1.10
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
IONISATION
• gaseous atoms are bombarded by electrons from an electron gun and are IONISED
• sufficient energy is given to form ions of 1+ charge
ACCELERATION
• ions are charged so can be ACCELERATED by an electric field
Spec links 1.06–1.10
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
IONISATION
• gaseous atoms are bombarded by electrons from an electron gun and are IONISED
• sufficient energy is given to form ions of 1+ charge
ACCELERATION
• ions are charged so can be ACCELERATED by an electric field
DEFLECTION
• charged particles will be DEFLECTED by a magnetic or electric field
Spec links 1.06–1.10
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
IONISATION
• gaseous atoms are bombarded by electrons from an electron gun and are IONISED
• sufficient energy is given to form ions of 1+ charge
ACCELERATION
• ions are charged so can be ACCELERATED by an electric field
DEFLECTION
• charged particles will be DEFLECTED by a magnetic or electric field
DETECTION
• by electric or photographic methods
Spec links 1.06–1.10
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
IONISATION
• gaseous atoms are bombarded by electrons from an electron gun and are IONISED
• sufficient energy is given to form ions of 1+ charge
ACCELERATION
• ions are charged so can be ACCELERATED by an electric field
DEFLECTION
• charged particles will be DEFLECTED by a magnetic or electric field
DETECTION
• by electric or photographic methods
Spec links 1.06–1.10
THE MASS SPECTROMETER
THE LAYOUT
4 key stages:
• Ionisation
• Acceleration
• Deflection
• Detection
---- = heavy ions
---- = ions reaching detector
---- = light ions
THE MASS SPECTROMETER
SUMMARISING WHAT HAPPENS:
1. Ionisation: Atoms are converted to ions
2. Acceleration: Ions are accelerated
3. Deflected: Deflected according to their mass & charge
4. Detection: They arrive at a detector
CONDITIONS:
a) Vacuum  so ions do not collide with air molecules
(might stop them reaching the detector)
b) Gaseous State  solids are vaporised before being
injected (Gas chromatography)
THE MASS SPECTROMETER
LOOK IN MORE DETAIL:
Stage 1: Ionisation
Beam of electrons knocks electrons from atoms or
molecules in the sample.
This is true even for things which you would normally
expect to form negative ions (chlorine, for
example) or never form ions at all (argon,
for example).
Nearly all lose just one electron (~5% will lose two)
Mass spectrometers always work with positive ions!!
THE MASS SPECTROMETER
Stage 2: Acceleration
The ions are accelerated so that they all have the same
kinetic energy.
Stage 3: Deflection
The ions are then deflected by a magnetic field according
to the ratio of their mass to charge (m/z),
where z is the charge (usually +1)
Heavier ions are deflected less than light ones
2+ ions are deflected twice as much as 1+ ions
THE MASS SPECTROMETER
Stage 4: Detection
Magnetic field is gradually increased  increases
deflection
This allows ions of increasing mass to enter the detector
On striking the detector ions accept electrons, lose their
charge and create a current
Current created is proportional to the abundance of each
ion
THE MASS SPECTROMETER
THE LAYOUT
4 key stages:
• Ionisation
• Acceleration
• Deflection
• Detection
---- = heavy ions
---- = ions reaching detector
---- = light ions
Mass spectrometry cut and
stick
Spec links 1.06–1.10
Mass spectrometry guide for
dummies
Your task is to create a how to guide about
mass spectrometry
You must include:
• A labelled diagram
• The name of each stage
• What happens at each stage
Spec links 1.06–1.10
THE MASS SPECTROMETER
MASS SPECTRA OF ELEMENTS
From the strength of the magnetic field at
which a particular ion hits the detector the
value of the mass to charge ratio (called m/z)
is calculated
A graph is produced
(mass spectra)
showing the relative
abundances of each
ion type
Mass spectra
of zirconium
positions of the peaks
gives atomic mass
THE MASS SPECTROMETER
MASS SPECTRA OF ELEMENTS
We can use the mass spectrometer to identify the
different isotopes making up an element
Each isotope is detected separately because they have
different masses
To calculate an
elements relative
atomic mass (which
is given in the periodic
table) you must take
account of the relative
abundances of each isotope
Zirconium has 5
isotopes!
THE MASS SPECTROMETER
CALCULATING RAM OF ELEMENTS
This is the mass spectra for chlorine
We have 2 isotopes with relative
isotopic masses of 35 and 37,
detected in a ratio of 3:1
(or 75%:25%)
To calculate the relative atomic
mass of chlorine:
(35 x 75) + (37 x 25)
100
= 35.5
Check the Ar of Chlorine
in your periodic table
THE MASS SPECTROMETER
CALCULATING RAM OF ELEMENTS
RAM of Cl = 35.5
Notice there is no line at 35.5 on the
mass spectra. No atoms of Cl actually
have this mass. It is the average of all
the isotopes and their abundances!
STEPS:
1. Multiply the m/z value by the relative abundance % for
each peak
2. Add results for each peak together
3. Divide by total relative abundance
THE MASS SPECTROMETER
CALCULATING RAM OF ELEMENTS
Calculate the RAM of the element from its mass spectra:
Boron
Zirconium
100
51.
5 17.1
11.2 17. 2.8
4
23
Most abundant
assigned 100
OR
Use the percentage
detected
THE MASS SPECTROMETER
CALCULATING RAM OF ELEMENTS
(10 x 23) + (100 x 11)
123
Boron
100
RAM = 10.8
23
THE MASS SPECTROMETER
CALCULATING RAM OF ELEMENTS
(90 x 51.5) + (91 x 11.2) + (92 x 17.1) + (94 x 17.4) + (96 x 2.8)
100
RAM = 91.3
51.
5 17.1
11.2 17. 2.8
4
Mass spectrometry guide for
dummies
To your guide add:
• What a mass spectra shows
• How you work out RAM using mass
spectrum
Spec links 1.06–1.10
Mass Spec of a compound
The biggest most
stable peak is called
the base peak
The last
peak is an
ion made
from the
complete
molecule.
It is called
the
molecular
ion peak
When the molecule passes through the mass spectrometer it is broken into
fragments which form ions and are detected. This process is called
fragmentation. The pattern of peaks is called a fragmentation pattern.
Joint Chemistry Sixth form 3.1.1.2
MASS SPECTRUM OF C2H5Br
The final peak in a mass spectrum is due to the molecular ion. In
this case there are two because Br has two main isotopes. As each
is of equal abundance, the peaks are the same size.
molecular ion contains...79Br
Spec links 1.06–1.10
81Br
Past paper questions…
Spec links 1.06–1.10
REVISION CHECK
What should you be able to do?
Recall the four basic stages in obtaining a mass spectrum
Understand what happens during each of the above four stages
Understand why particles need to be in the form of ions
Recall the the meaning of mass to charge ratio (m/z)
Explain how the mass/charge value affects the path of a deflected ion
Interpret a simple mass spectrum and calculate the average atomic mass
Understand how mass spectrometry can be used to calculate molecular mass
CAN YOU DO ALL OF THESE?
Spec links 1.06–1.10
YES
NO