Detection of heavy metals Flashcards
Forensic Chemistry
How can heavy metals be detected?
-Atomic absorption spectroscopy.
What is AAS and how can heavy metals be detected from it?
It is an analytical technique and it is where atoms of a sample can absorb light that can be all different wavelengths.
What can the absorption of light from a heavy metal tell us?
- When an atom of the sample absorbs light energy the concentration can be measured and the sample can be identified.
What is the basic principle of AAS?
- 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.
What are the three techniques in AAS?
- 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.
What fuel oxidants are used in AAS?
- Oxygen
- Nitrous oxide
- Air
What type of flames are the best for atoms in AAS?
-Fuel rich flames.
What type of lamp is used in AAS?
-A hollow cathode lamp.
- Negatively charged.
What are the samples sealed in during AAS and that gases can be used?
- In a glass cylinder.
- The gases used could be neon or argon.
What does the nebuliser do?
- Turns the liquid particles into a fine aerosol which is then introduced to the flame.
What is the atomiser?
- 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
What are the six steps to atomisation?
- Nebulisation
- Desolation
- Volatilisation
- Dissociation
- Ionisation
- Excitation
What is a monochromator?
- Selects a specific wavelength of light that is absorbed by the sample.
What is the most common detector?
- Photomultiplier tubes.
- Converts a light signal to an electrical signal that is proportional to the intensity of the light.
What does ICP-MS mean?
- Inductively coupled plasma mass spectrometry
What are the instrument components in ICP-MS?
- Sample introduction.
- Plasma ion source.
- Ion lenses.
- Collisions/ reaction cell.
- Quadrupole MS.
- Detector.
What does the peristaltic pump ensure in ICP-MS?
- Constant flow of a liquid.
What are the samples pumped at in ICP-MS?
1 ml/min
What type of chamber is used in ICP-MS?
Double pass spray chamber.
What type of plasma is used in ICP-MS?
-Argon plasma that then converts the sample into ions.
What states are usually the samples in ICP-MS?
liquids
What is the role of the ion lenses in ICP-MS?
- Focuses on ions and removes photons and neutrals.
What is the role of the reaction cell in ICP-MS?
- It removes spectral interferences.
What is the role of the quadrupole MS in ICP-MS?
- Separates ions by m/z unit mass ratio.
What is the role of the multi collector mass analyser?
- Separates ions according to the m/z.
What does XRF stand for?
X-ray fluorescence.
What is the principle of XRF?
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.
What are the three types of electron transition in XRF when an electron moves to fill a shell?
- Alpha- electron moves from one shell away.
- Beta- electron moves from two shells away.
- Gamma- electron moves from three shells away.
What are the electron cells in XRF and how many electrons are in each shell?
K=2
L= 8
M= 18
N=32
Which shell has the highest binding energy in XRF and why?
- K shell as it is the closest shell to the nucleus and it takes more energy to remove the electron from the shell.
Which shell has the highest potential energy and why?
- 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.
What is Coherent scattering?
- 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.
