lecture 7 - Heavy metal detection Flashcards
Name 5 common heavy metals
Mercury, cadmium, chromium, lead and arsenic
give 3 properties of heavy metals
high density, persistent in its environment, and cannot degrade or be destroyed
what are the three ways heavy metals can enter the body
food, drink and the air
what is the difference between deliberate exposure and accidental
Deliberate exposure is where the person chooses to work with that heavy metal, accidental is when a person is unaware there are heavy metals near them
which technique do we use to detect metals and metalloids
Atomic Absorption Spectroscopy
Name 4 reasons why we use AAS
- Reliable
- Sensitive - ppb levels
- Can analyse over 62 elements
- Quantitative
How does AAS work to detect metals
Free atoms that are gas generated in an atomiser, can absorb radiation at a specific frequency
AAS quantifies absorption of ground state atoms in the gaseous state
The atoms absorb UV or visible light and make transition to higher electronic energy levels
The analyte concentration is determined from the amount of absorption
what is AAS based on
the properties of specific metal atoms to absorb specific wavelengths of light.
what are the 3 steps to atomise samples in AAS
desolvation, vaporisation, volatilisation
what fuel is used in AAS
Acetylene
what are the 3 different oxidants used in AAS and what temperatures do they require
- Air - 2100 °C – 2400 °C (most common)
- N20 - 2600 °C – 2800 °C
- O2 - 3050 °C – 3150 °C
why do we use fuel rich flames in AAS
to reduce the likelihood of oxidation of the atoms, as oxidation should occur in the secondary combustion zone
Name the 7 basic components in AAS
- Light source
- Nebuliser
- Atomiser
- Monochromator
- Detector
- Absorption cell
- A display
explain what the light source is and what the component does in AAS?
It is a tungsten anode and a hollow cylinder cathode made of the element to be analysed, sealed in a glass tube filled with inert gas (neon or argon).
The hollow cathode lamp emits a light spectrum specific to the element of which it is made, which is focused through the sample cell into the monochromator
what does the nebuliser do
aspires the liquid sample at a controlled rate, which creates fine aerosol particles that can be introduced to the flame. It also mixes the aerosol, fuel and oxidant thoroughly
what is atomisation
the separation of particles into individual molecules and breaking the molecules into atoms
what does the atomiser do in AAS
separates the particles by exposing them to high temperatures in a flame or furnace
what are the two different atomisers
-flame atomiser - mixture of oxidant gas and a fuel (example air-acetylene flame or nitrous oxide acetylene flame)
-graphite tube atomiser - uses graphite coated furnace to vaporise the sample. Samples are deposited in a small graphite coated tube then heated to vaporise and atomise the analyte
what does the monochromator do in AAS
It selects the specific wavelength of light that is absorbed by the sample and removes all the others. It also isolates the absorption line from the background light due to interferences
What is the detector for AAS and how does it work
Photomultiplier tubes are the most common detectors for AAS. They convert a light signal to an electrical signal that is proportional to the intensity of light. A signal amplifier processes the electrical signal
what is the absorption cell in AAS
where atoms of a sample are produced
what is the display in AAS
it shows the the reading after it has been processed by the instrument electronics
what do we clean the AAS machine with and why
nitric acid and deionised water - to remove all elements and ions
