Page 14 Questions

Mass Spectra

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Objectives: Interpret mass spectra of elements Calculate Relative Atomic Mass from isotopic abundance of mononuclear ions Know a significant use of mass spectometry

Mass Spectra

Mass Spectrometry Specification content - understand the principles of a simple mass spectrometer, limited to ionisation, acceleration, deflection and detection - know that the mass spectrometer gives accurate information about relative isotopic mass and also about the relative abundance of isotopes - be able to interpret simple mass spectra of elements and calculate relative atomic mass from isotopic abundance, limited to mononuclear ions - know that mass spectrometry can be used to identify elements (as used for example in planetary space probes) - know that mass spectrometry can be used to determine relative molecular mass Completed?

Mass Spectra

Mass Spectrometry Specification content - understand the principles of a simple mass spectrometer, limited to ionisation, acceleration, deflection and detection - know that the mass spectrometer gives accurate information about relative isotopic mass and also about the relative abundance of isotopes - be able to interpret simple mass spectra of elements and calculate relative atomic mass from isotopic abundance, limited to mononuclear ions - know that mass spectrometry can be used to identify elements (as used for example in planetary space probes) - know that mass spectrometry can be used to determine relative molecular mass Completed?

What does the mass spectrum tell us?

1. Relative abundance 2. Mass/charge ratio (m/z) -You can assume the charge is 1+, so m/z is just mass (relative isotopic mass)

What does the mass spectrum tell us?

Questions to try… 1. How many isotopes are present In this spectra?

2. Which element are these isotopes of?

3. What is the relative isotopic mass of each isotope?

4. What is the relative isotopic abundance of each isotope?

Calculating Relative Atomic mass from spectra Step 1: For each peak, multiply the relative atomic abundance by the relative atomic mass. This gives you the mass of each isotope Step 2: Add up the total mass of each isotope Step 3: (if % abundance) divide by 100 [NB: if not % abundance, then divide by the total relative abundance instead]

Step 1:

1 st isotope: 51.5 x 90 = 4635 2 nd isotope: 11.2 x 91 = 1019.2

3 rd isotope: 17.1 x 92 = 1573.2

4 th isotope: 17.4 x 94 = 1635.6

5 th isotope: 2.8 x 96 = 268.8

Step 2:

4635 + 1019.2 + 1573.2 + 1635.6 + 268.8 = 9131.8

Step 3:

9131.8 / 100 = 91.318

51.5

17.1

11.2

17.4

2.8

Calculating Relative Atomic mass from spectra Step 1: For each peak, multiply the relative atomic abundance by the relative atomic mass. This gives you the mass of each isotope Step 2: Add up the total mass of each isotope Step 3: (if % abundance) divide by 100 [NB: if not % abundance, then divide by the total relative abundance instead]

Step 1:

1 st isotope: 51.5 x 90 = 4635 2 nd isotope: 11.2 x 91 = 1019.2

3 rd isotope: 17.1 x 92 = 1573.2

4 th isotope: 17.4 x 94 = 1635.6

5 th isotope: 2.8 x 96 = 268.8

Step 2:

4635 + 1019.2 + 1573.2 + 1635.6 + 268.8 = 9131.8

Step 3:

9131.8 / 100 = 91.318

51.5

17.1

11.2

17.4

2.8

Calculating Relative Atomic mass from spectra Now try these… Krypton Boron 23 100

What do you think has happened in the graph on the right hand side?

Using mass spectra to determine molecular mass As well as elemental isotopes, a mass spectrometer can detect molecules too!

When a vapourised sample is bombarded with high energy electrons, a single electron can be removed from a molecule This creates a molecular ion, M+ (g), but also the molecule can break up and create a Fragmentation pattern, which then shows up on the spectrum as the lines to the LEFT of the Mr of the molecule…

Using mass spectra to determine molecular mass As well as elemental isotopes, a mass spectrometer can detect molecules too!

When a vapourised sample is bombarded with high energy electrons, a single electron can be removed from a molecule This creates a molecular ion, M+ (g), but also the molecule can break up and create a Fragmentation pattern, which then shows up on the spectrum as the lines to the LEFT of the Mr of the molecule…

Using mass spectra to determine molecular mass Now try the examples on page 17

PRACTICE!!!

Now try the Lesson 5-6 past paper questions and finish for homework

Long form practice exam question

Q1. (a) State the relative charge and relative mass of a proton, of a neutron and of an electron.

In terms of particles, explain the relationship between two isotopes of the same element.

Explain why these isotopes have identical chemical properties.

(7 marks)

(b) Define the term relative atomic mass. An element exists as a mixture of three isotopes.

Explain, in detail, how the relative atomic mass of this element can be calculated from data obtained from the mass spectrum of the element.

(7 marks) (Total 14 marks)

Long form practice exam question

M1. (a) Proton: mass 1, charge + 1 (1) Neutron: mass 1, charge 0 (1) Electron mass 1/1840, charge -1 (1) Allow mass = 0, or negligible, or 1/1800 to 1/2000 Isotopes have the same number of protons (1) OR atomic number different number of neutrons (1) Isotopes have the same electronic configuration (1) OR same number of electrons Chemical properties depend on electrons (1)

7 Marks

Long form practice exam question

(b)

OR

×12 (1) × 12 or in words Spectrum gives (relative) abundance (1) OR % or amount And m/z (1) Multiply m/z by relative abundance for each isotope (1) Allow instead of m/z mass no, Ar or actual value from example Sum these values (1) Divide by the sum of the relative abundances (1) only award this mark if previous 2 given Max 2 if e.g. has only 2 isotopes

7 Marks [14]

OTHER USES OF MASS SPECTROMETRY - 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 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