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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
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


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


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

  • 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


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


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


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).


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


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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