4 mass spectrometry
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4. Mass Spectrometry. Objectives: 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! Extension:

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4. Mass Spectrometry

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4 mass spectrometry

4. Mass Spectrometry

Objectives:

  • Know what information they can provide

  • Interpret simple mass spectra graphs

  • Know some uses of mass spectrometry


A mass spectrometer

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 stages


4 mass spectrometry

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.

The results produce a mass spectrum which portrays the different ions in order of their m/z value.

Francis Aston

USES

Mass spectrometry was initially used to show the identity of isotopes.

It is now used to calculate molecular masses and characterise new compounds


4 mass spectrometry

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


4 mass spectrometry

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


4 mass spectrometry

2+ ions

1+ ions

20Ne

22Ne

ABUNDANCE

Doubling the charge, halves the m/z value

Abundance stays the same

0 4 8 12 16 20 m/z values

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

  • if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved


4 mass spectrometry

20Ne 90.92%

21Ne 0.26%

22Ne 8.82%

19 20 21 22 23

What is a Mass Spectrum?

MASS SPECTRUM OF NEON

  • 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


4 mass spectrometry

Example calculation 1

Calculate the average relative atomic mass of neon using data on the previous page.

Out of every 100 atoms... 90.92 are 20Ne , 0.26 are 21Ne and 8.82 are 22Ne

Average =(90.92 x 20) + (0.26 x 21) + (8.82 x 22) = 20.179Ans. = 20.18

100

TIPIn 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).


4 mass spectrometry

Test Question

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?

100%

60%

40%

ABUNDANCE

0 10 20 30 40 m/z values

ANSWERS ON NEXT PAGE


4 mass spectrometry

Test Question

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?

100%

60%

40%

ABUNDANCE

0 10 20 30 40 m/z values

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.

100%

100%

66.7%

ABUNDANCE

0 10 20 30 40 m/z values


Now try these

Now try these….

  • Now give the questions in the textbook a go on your own (page 14)


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