Know what information they can provide Interpret simple mass spectra graphs Know some uses of mass spectrometry A mass spectrometer Draw the mass spectrometer Describe the 5 step process AS A CARTOON STORY BOARD ID: 661650
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Slide1
4. Mass Spectrometry
Objectives:
Know what information they can provide
Interpret simple mass spectra graphs
Know some uses of mass spectrometrySlide2
A mass spectrometer….
Draw the mass spectrometer
Describe the 5 step process
AS A CARTOON STORY BOARD!
Extension:
Use your own words to describe the stagesSlide3
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.The results produce a mass spectrum which portrays the different ions in order of their m/z value.
Mass Spectrometry
USES
Mass spectrometry was initially used to show the identity of isotopes.
It is now used to calculate molecular masses and characterise new compounds
Francis AstonSlide4
A mass spectrometer consists of ... an
ion source
, an
analyser
and a detector.ION SOURCE
ANALYSER
DETECTOR
Mass Spectrometer
PARTICLES MUST BE IONISED SO THEY CAN BE ACCELERATED AND DEFLECTEDSlide5
How does it work?
ION SOURCE
ANALYSER
DETECTOR
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 methodsSlide6
How does it work - Deflection
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
20Ne
21
Ne
22Ne
HEAVIER ISOTOPES ARE DEFLECTED LESS
0 4 8 12 16 20 m/z values
ABUNDANCE
1+
ions
2+ ions
20
Ne
22
Ne
Doubling the charge, halves the m/z value
Abundance stays the sameSlide7
In early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated that naturally occurring neon consisted of three isotopes ...
20
Ne,
21
Ne 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
MASS SPECTRUM OF NEON
19 20 21 22 23
20
Ne 90.92%
21
Ne 0.26%
22
Ne 8.82%
What is a Mass Spectrum?Slide8
Calculate the average relative atomic mass of neon using data on the previous page
.
Out of every 100 atoms... 90.92 are
20
Ne , 0.26 are 21Ne and 8.82 are 22Ne Average = (90.92 x 20) + (0.26 x 21) + (8.82 x 22) = 20.179 Ans. = 20.18 100
TIP In calculations of this type... multiply each relative mass by its abundance add up the total of these values divide the result by the sum of the abundances * 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 1Slide9
A mass spectrum shows the presence of two isotopes of m/z values 38 and 40. Both have been formed as unipositive ions.
Redraw the diagram with the most abundant isotope scaled up to 100%.
Calculate the average relative atomic mass.
What would be a) the m/z values and b) the abundance if 2+ ions had been formed?
Test Question
0 10 20 30 40 m/z values
ABUNDANCE
60%
40%
100%
ANSWERS ON NEXT PAGESlide10
A mass spectrum shows the presence of two isotopes of m/z values 38 and 40. Both have been formed as unipositive ions.
Redraw the diagram with the most abundant isotope scaled up to 100%.
Calculate the average relative atomic mass.
What would be a) the m/z values and b) the abundance if 2+ ions had been formed?
Test Question
0 10 20 30 40 m/z values
ABUNDANCE
60%
40%
100%
0 10 20 30 40 m/z values
ABUNDANCE
100%
66.7%
100%
The new values are 100 and 66.7 (see diagram)
New scale atoms of mass 38; abundance = 100
atoms of mass 40; abundance = 66.7
Average = (100 x 38) + (66.7 x 40) = 38.80
166.7
By doubling the charge to 2+, m/z value is halved; new peaks at 19 and 20. The abundance is the same.Slide11
Now try these….
Now give the questions in the
textbook a go on
your own (page 14)