MASS SPECTROMETRY. CONTENTS Prior knowledge Background information The basic parts of a mass spectrometer The four stages of obtaining a spectrum How

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

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 SPECTROMETRYMASS 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 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

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


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


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 value

Abundance 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.

EXAMPLE CALCULATION (1)EXAMPLE CALCULATION (1)

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;

(100 in this case)

* 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 (2)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 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

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

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

Documents

MASS SPECTROMETRY. CONTENTS Prior knowledge Background information The basic parts of a mass spectrometer The four stages of obtaining a spectrum How