starter: spec links 1.06–1.10. green pens out! spec links 1.06–1.10

Starter:

Spec links 1.06–1.10

Green pens out!


Learning objectives

• analyse and interpret data from mass spectrometry to calculate the relative atomic mass from the relative abundance of isotopes and vice versa

• predict the mass spectra for diatomic molecules, for example chlorine

• understand how mass spectrometry can be used to determine the relative molecular mass of a molecule


Mass spectrometry?

• What did you learn?


WHAT IS A MASS SPECTROMETER

An instrument to accurately determine the relative atomic mass

THE MASS SPECTROMETER

• Relative atomic mass (Ar) is the average mass of atoms of an element relative to an atom of carbon

– 12

Separates atoms or molecules according to their charge and mass.

This can be used to identify substances e.g. illegal drugs

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 Aston


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

ION SOURCE

ANALYSER

DETECTOR

A 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?

ION SOURCE

ANALYSER

DETECTOR

IONISATION and VAPOURISATION• gaseous atoms are bombarded by electrons from an electron gun and are

IONISED• sufficient energy is given to form ions of 1+ charge


HOW DOES IT WORK?

ION SOURCE

ANALYSER

DETECTOR

IONISATION• gaseous atoms are bombarded by electrons from an electron gun and are


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


HOW DOES IT WORK?

ION SOURCE

ANALYSER

DETECTOR




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


HOW DOES IT WORK?

ION SOURCE

ANALYSER

DETECTOR




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

DETECTION• by electric or photographic methodsSpec links 1.06–1.10

THE LAYOUT


4 key stages:

• Ionisation

• Acceleration

• Deflection

• Detection

---- = heavy ions

---- = ions reaching detector

---- = light ions

SUMMARISING WHAT HAPPENS:

1. Ionisation: Atoms are converted to ions

2. Acceleration: Ions are accelerated

3. Deflected: Deflected according to their mass & charge

4. Detection: They arrive at a detector


CONDITIONS:

a)Vacuum so ions do not collide with air molecules (might stop them reaching the detector)

b)Gaseous State solids are vaporised before being injected (Gas chromatography)

LOOK IN MORE DETAIL:

Stage 1: Ionisation

Beam of electrons knocks electrons from atoms or molecules in the sample.

This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all

(argon, for example).

Nearly all lose just one electron (~5% will lose two)

Mass spectrometers always work with positive ions!!


Stage 2: Acceleration

The ions are accelerated so that they all have the same kinetic energy.

Stage 3: Deflection

The ions are then deflected by a magnetic field according to the ratio of their mass to charge (m/z), where z is the charge

(usually +1)

Heavier ions are deflected less than light ones

2+ ions are deflected twice as much as 1+ ions


Stage 4: Detection

Magnetic field is gradually increased increases deflection

This allows ions of increasing mass to enter the detector

On striking the detector ions accept electrons, lose their charge and create a current

Current created is proportional to the abundance of each ion


THE LAYOUT


4 key stages:

• Ionisation

• Acceleration

• Deflection

• Detection

---- = heavy ions

---- = ions reaching detector

---- = light ions

Mass spectrometry cut and stick


Mass spectrometry guide for dummies

Your task is to create a how to guide about mass spectrometry

You must include:• A labelled diagram• The name of each stage• What happens at each stage


MASS SPECTRA OF ELEMENTS

From the strength of the magnetic field at which a particular ion hits the detector the value of the mass to charge ratio (called

m/z) is calculated

A graph is produced (mass spectra) showing the relative abundances of each ion type


Mass spectra of zirconium positions of the peaks

gives atomic mass

MASS SPECTRA OF ELEMENTS

We can use the mass spectrometer to identify the different isotopes making up an element

Each isotope is detected separately because they have different masses

To calculate an elements relative atomic mass (which is given in the periodic table) you must take account of the relative abundances of each isotope


Zirconium has 5 isotopes!

CALCULATING RAM OF ELEMENTS

This is the mass spectra for chlorine

We have 2 isotopes with relative isotopic masses of 35 and 37,

detected in a ratio of 3:1 (or 75%:25%)

To calculate the relative atomic mass of chlorine:

(35 x 75) + (37 x 25) 100

= 35.5


Check the Ar of Chlorine in your periodic table


RAM of Cl = 35.5

Notice there is no line at 35.5 on the mass spectra. No atoms of Cl actually have this mass. It is the average of all the isotopes and their abundances!


STEPS:

1. Multiply the m/z value by the relative abundance % for each peak

2. Add results for each peak together

3. Divide by total relative abundance


Calculate the RAM of the element from its mass spectra:


23

100

Boron

51.5

17.4 2.811.

2

17.1

Zirconium

Most abundant assigned 100

OR Use the percentage detected



23

100

Boron(10 x 23) + (100 x

11)123

RAM = 10.8

(90 x 51.5) + (91 x 11.2) + (92 x 17.1) + (94 x 17.4) + (96 x 2.8)100

RAM = 91.3



51.5

17.4 2.811.

2

17.1

Mass spectrometry guide for dummies

To your guide add:• What a mass spectra shows• How you work out RAM using mass

spectrum


Mass Spec of a compound

When the molecule passes through the mass spectrometer it is broken into fragments which form ions and are detected. This process

is called fragmentation. The pattern of peaks is called a fragmentation pattern.

The last peak

is an ion

made from

the complete

molecule. It

is called the

molecular

ion peak

The biggest most stable peak is

called the base peak

Joint Chemistry Sixth form 3.1.1.2

MASS SPECTRUM OF C2H5Br

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


Past paper questions…


REVISION 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

CAN YOU DO ALL OF THESE? YES NO


starter: spec links 1.06–1.10. green pens out! spec links 1.06–1.10

Documents

mass spectrum

relative molecular mass

average mass of atoms

mass spectra of molecules

mass spectra of elements

charge spec links

electric field spec

form ions