Transcript PP Mass spectrometer and atoms

MASS
SPECTROMETRY
MASS SPECTROMETRY
CONTENTS
• Prior knowledge
• Background information
• The basic parts of a mass spectrometer
• The four stages of obtaining a spectrum
• How different ions are deflected
• Calculating molecular masses using mass spectra
• Example questions
• Test questions
• Other uses of mass spectrometry
• Check list
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
Francis Aston
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
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
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
20Ne
21Ne
22Ne
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
20Ne
2+ ions
22Ne
1+ ions
20Ne
ABUNDANCE
21Ne
22Ne
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
• as most ions are 1+charged, the amount of separation depends on their mass
• if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved
WHAT IS A MASS SPECTRUM?
20Ne
MASS
SPECTRUM
OF NEON
90.92%
21Ne
0.26%
22Ne
19
20
21
22
8.82%
23
In early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated
that naturally occurring neon consisted of three isotopes ... 20Ne, 21Ne and 22Ne.
• positions of the peaks gives atomic mass
• peak intensity gives the relative abundance
• highest abundance is scaled to 100% and other values are adjusted accordingly
EXAMPLE CALCULATION (1)
Calculate the average relative atomic mass of neon using data on the previous page.
Out of every 100 atoms...
Average =
TIP
90.92 are
20Ne
, 0.26 are
21Ne
and 8.82 are
(90.92 x 20) + (0.26 x 21) + (8.82 x 22) = 20.179
100
In calculations of this type...
22Ne
Ans. = 20.18
multiply each relative mass by its abundance
add up the total of these values
divide the result by the sum of the abundances;
(100 in this case)
* if the question is based on percentage abundance, divide by 100 but if
it is based on heights of lines in a mass spectrum, add up the heights
of the lines and then divide by that number (see later).
EXAMPLE CALCULATION (2)
Naturally occurring potassium consists of potassium-39 and potassium-41.
Calculate the percentage of each isotope present if the average is 39.1.
Assume that there are x nuclei of
there will then be (100-x) of 41K.
so
thus
39x + 41 (100-x) = 39.1
100
- 2x = - 190
39K
in every 100;
therefore
39 x + 4100 - 41x
= 3910
so x = 95
Ans.
95% 39K and 5%
41K
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
CAN YOU DO ALL OF THESE?
YES
NO