MASS SPECTROMETRY A guide for A level students KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS

MASS MASS SPECTROMETRYSPECTROMETRY

A guide for A level studentsA guide for A level students

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING2008 2008

SPECIFICATIONSSPECIFICATIONS

INTRODUCTION

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MASS SPECTROMETRYMASS SPECTROMETRYKNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

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 SPECTROMETRYMASS SPECTROMETRY

Before you start it would be helpful to…

• know that atoms are made up of protons, neutrons and electrons

• know that like charges repel


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.


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 Aston

A mass spectrometer consists of ... an ion source, an analyser and a detector.

ION SOURCE

ANALYSER

DETECTOR

A MASS SPECTROMETERA MASS SPECTROMETER

PARTICLES MUST BE IONISED SO THEY CAN BE ACCELERATED AND DEFLECTEDPARTICLES MUST BE IONISED SO THEY

CAN BE ACCELERATED AND DEFLECTED

HOW DOES IT WORK?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


ION SOURCE

ANALYSER

DETECTOR


ACCELERATION• ions are charged so can be ACCELERATED by an electric field


ION SOURCE

ANALYSER

DETECTOR



DEFLECTION• charged particles will be DEFLECTED by a magnetic or electric field


ION SOURCE

ANALYSER

DETECTOR




DETECTION• by electric or photographic methods


ION SOURCE

ANALYSER

DETECTOR




DETECTION• by electric or photographic methods

HOW DOES IT WORK? - DeflectionHOW 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

20Ne21Ne

22Ne

HEAVIER ISOTOPES ARE DEFLECTED LESS

HOW DOES IT WORK? - DeflectionHOW 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

20Ne21Ne

22Ne

HEAVIER ISOTOPES ARE DEFLECTED LESS

0 4 8 12 16 20 m/z values

AB

UN

DA

NC

E

1+ ions2+ ions

20Ne

22Ne

Doubling the charge, halves the m/z valueAbundance stays the same

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

MASS SPECTRUM OF NEON

19 20 21 22 23

20Ne 90.92%

21Ne 0.26%

22Ne 8.82%

WHAT IS A MASS SPECTRUM?WHAT IS A MASS SPECTRUM?

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) = 20.179 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 ifit is based on heights of lines in a mass spectrum, add up the heightsof the lines and then divide by that number (see later).

EXAMPLE CALCULATION (1)EXAMPLE CALCULATION (1)

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 39K in every 100; there will then be (100-xx) of 41K.

so 39x + 41 (100-xx) = 39.1 therefore 39 xx + 4100 - 41xx = 3910

100

thus - 2xx = - 190 so xx = 95 Ans. 95% 39K and 5% 41K

EXAMPLE CALCULATION (2)EXAMPLE CALCULATION (2)

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 QUESTIONTEST QUESTION

0 10 20 30 40 m/z values

AB

UN

DA

NC

E

60%

40%

100%

ANSWERS ON NEXT PAGE

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 QUESTIONTEST QUESTION

0 10 20 30 40 m/z values

AB

UN

DA

NC

E

60%

40%

100%

0 10 20 30 40 m/z values

AB

UN

DA

NC

E

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.

IONISATION

FRAGMENTION

FRAGMENTIONRE-ARRANGEMENT

OTHER USES OF MASS SPECTROMETRYOTHER USES OF MASS SPECTROMETRY

- MOLECULAR MASS DETERMINATION -- MOLECULAR MASS DETERMINATION -

Nowadays, mass spectrometry is used to identify unknown or new compounds.

When a molecule is ionised it forms a MOLECULAR ION which can also undergo FRAGMENTATION or RE-ARRANGEMENT to produce particles of smaller mass.

Only particles with a positive charge will be deflected and detected.

The resulting spectrum has many peaks.

The final peak (M+) shows the molecular ion (highest m/z value) and indicates the molecular mass. The rest of the spectrum provides information about the structure.

MOLECULAR ION

MASSMASS SPECTRUMSPECTRUM OFOF CC22HH55BrBr

The final peak in a mass spectrum is due to the molecular ion. In this case there are two because Br has two main isotopes. As each is of equal abundance, the peaks are the same size.

molecular ion contains...79Br 81Br

IDENTIFY THE PEAKSIDENTIFY THE PEAKS



OTHER USES OF MASS SPECTROMETRYOTHER USES OF MASS SPECTROMETRY

- SPACE EXPLORATION -- SPACE EXPLORATION -

Mass spectrometry is used on space probes to identify elements and compounds on the surface of planets.

In August and September 1975 the USA launched Viking 1 and 2.

Both probes entered Mars orbit to map the planet, dropping landers that transmitted pictures, acted as weather and scientific stations and analysed the Martian soil.

VIKING LANDER THE MARTIAN SURFACE

REVISION CHECKREVISION 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

Understand how mass spectrometry can be used to calculate molecular mass

Recall that mass spectrometry can be used to in space exploration

Recall other uses of mass spectrometry

CAN YOU DO ALL OF THESE? CAN YOU DO ALL OF THESE? YES YES NONO

You need to go over the You need to go over the relevant topic(s) againrelevant topic(s) again

Click on the button toClick on the button toreturn to the menureturn to the menu

WELL DONE!WELL DONE!Try some past paper questionsTry some past paper questions

© 2008 KNOCKHARDY PUBLISHING

MASS MASS SPECTROMETRYSPECTROMETRY

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MASS SPECTROMETRY A guide for A level students KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS