MASS
SPECTROMETRY
Demonstrate an understanding of the basic principles of a mass spectrometer to:i) Deduce the isotopic composition of a sample of an element, e.g. polonium. ii) Deduce the RAM of an element. iii) Measure the RMM of a compound.
Describe some uses of mass spectrometers, e.g. in radioactive dating, in space research, in sport to detect use of anabolic steroids, in the pharmaceutical industry to provide an identifier for compounds synthesised for possible identification as 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).
Particles with a large m/z value are deflected least those with a low m/z value are deflected most.
Francis Aston
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
A MASS SPECTROMETER
DETECTOR
ION SOURCE
ANALYSER
A mass spectrometer consists of ... an ion source, an analyser and a detector.
PARTICLES
PARTICLES MUST
MUST BE
BE IONISED
IONISED SO
SO THEY
THEY
CAN
CAN BE
BE ACCELERATED
ACCELERATED AND
AND DEFLECTED
DEFLECTED
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
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
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
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
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
HOW DOES IT WORK? - Deflection
20
Ne
Ne
21
22
Ne
HEAVIER ISOTOPES ARE
DEFLECTED LESS
•
the radius of the path depends on the value of the mass/charge ratio (m/z)
•
ions of heavier isotopes have larger m/z values so follow a larger radius curve
•
as most ions are 1+charged, the amount of separation depends on their mass
HOW DOES IT WORK? - Deflection
20
Ne
2+ ions
Ne
1+ ions
21
Ne
20
ABUNDANCE
22
Ne
Ne
22
HEAVIER ISOTOPES ARE
DEFLECTED LESS
0
4
8
12
16
20
m/z values
Doubling the charge, halves the m/z value
Abundance stays the same
•
the radius of the path depends on the value of the mass/charge ratio (m/z)
•
ions of heavier isotopes have larger m/z values so follow a larger radius curve
