Detection of heavy metals

Question 1

How can heavy metals be detected?

Answer 1

-Atomic absorption spectroscopy.

Question 2

What is AAS and how can heavy metals be detected from it?

Answer 2

It is an analytical technique and it is where atoms of a sample can absorb light that can be all different wavelengths.

Question 3

What can the absorption of light from a heavy metal tell us?

Answer 3

• When an atom of the sample absorbs light energy the concentration can be measured and the sample can be identified.

Question 4

What is the basic principle of AAS?

Answer 4

• Free atoms that are gas generated in an atomiser can absorb light energy at different wavelengths.
• As the free atoms absorb the light energy, electrons go from ground state to excited state, due to the absorption of energy.
• Electrons move different energy levels and when they are excited they become unstable.
• The amount of light absorption can determine the concentration of the sample.
• Instrument is calibrated with known concentration samples.

Question 5

What are the three techniques in AAS?

Answer 5

• Desolvation- liquid solvent is evaporated and dry solid remains.
• Vaporisation- solid sample vaporizes into a gas.
• Volatilisation- compounds that make up the sample are broken down into free atoms.

Question 6

What fuel oxidants are used in AAS?

Answer 6

• Oxygen
• Nitrous oxide
• Air

Question 7

What type of flames are the best for atoms in AAS?

Answer 7

-Fuel rich flames.

Question 8

What type of lamp is used in AAS?

Answer 8

-A hollow cathode lamp.
- Negatively charged.

Question 9

What are the samples sealed in during AAS and that gases can be used?

Answer 9

• In a glass cylinder.
• The gases used could be neon or argon.

Question 10

What does the nebuliser do?

Answer 10

• Turns the liquid particles into a fine aerosol which is then introduced to the flame.

Question 11

What is the atomiser?

Answer 11

• The separation of particles into individual molecules and then into atoms.
• The analyse is exposed to high temperatures in a flame.
• A graphite tube atomiser vaporises the sample

Question 12

What are the six steps to atomisation?

Answer 12

1. Nebulisation
2. Desolation
3. Volatilisation
4. Dissociation
5. Ionisation
6. Excitation

Question 13

What is a monochromator?

Answer 13

• Selects a specific wavelength of light that is absorbed by the sample.

Question 14

What is the most common detector?

Answer 14

• Photomultiplier tubes.
• Converts a light signal to an electrical signal that is proportional to the intensity of the light.

Question 15

What does ICP-MS mean?

Answer 15

• Inductively coupled plasma mass spectrometry

Question 16

What are the instrument components in ICP-MS?

Answer 16

1. Sample introduction.
2. Plasma ion source.
3. Ion lenses.
4. Collisions/ reaction cell.
5. Quadrupole MS.
6. Detector.

Question 17

What does the peristaltic pump ensure in ICP-MS?

Answer 17

• Constant flow of a liquid.

Question 18

What are the samples pumped at in ICP-MS?

Answer 18

1 ml/min

Question 19

What type of chamber is used in ICP-MS?

Answer 19

Double pass spray chamber.

Question 20

What type of plasma is used in ICP-MS?

Answer 20

-Argon plasma that then converts the sample into ions.

Question 21

What states are usually the samples in ICP-MS?

Answer 21

liquids

Question 22

What is the role of the ion lenses in ICP-MS?

Answer 22

• Focuses on ions and removes photons and neutrals.

Question 23

What is the role of the reaction cell in ICP-MS?

Answer 23

• It removes spectral interferences.

Question 24

What is the role of the quadrupole MS in ICP-MS?

Answer 24

• Separates ions by m/z unit mass ratio.

Question 25

What is the role of the multi collector mass analyser?

Answer 25

• Separates ions according to the m/z.

Question 26

What does XRF stand for?

Answer 26

X-ray fluorescence.

Question 27

What is the principle of XRF?

Answer 27

When a sample is exposed to an x-ray the electrons that surround the atomic nucleus can be ejected from the atom and outer electrons move inwards to occupy the ejected electrons.
When electrons move into an empty orbital energy is emitted in the form of a x-ray.
The x-ray that is emitted is of a lower energy compared to the x-ray the sample was exposed to in the first place.
As different electrons can be ejected from the atom, and electrons are moving into different shells, multiple wavelengths can be occurring with different atoms.

Question 28

What are the three types of electron transition in XRF when an electron moves to fill a shell?

Answer 28

• Alpha- electron moves from one shell away.
• Beta- electron moves from two shells away.
• Gamma- electron moves from three shells away.

Question 29

What are the electron cells in XRF and how many electrons are in each shell?

Answer 29

K=2
L= 8
M= 18
N=32

Question 30

Which shell has the highest binding energy in XRF and why?

Answer 30

• K shell as it is the closest shell to the nucleus and it takes more energy to remove the electron from the shell.

Question 31

Which shell has the highest potential energy and why?

Answer 31

• N shell has the highest potential energy and this is because it is the shell the furthest away from the nucleus and it if an electron from the N shell falls to the K shell a lot of energy will be released.

Question 32

What is Coherent scattering?

Answer 32

• It occurs when an x-ray collides with an atom and there is no loss of energy.
• A type of elastic scattering, so no loss of energy.