Lecture 7: Heavy metal detection Flashcards
What are heavy metals?
-High density
-Can’t be degraded or destroyed
-Persistent in the environment
What are examples of heavy metals?
-Mercury
-Lead
-Chromium
-Cadmium
-Cobalt
What is AAS?
Atomic absorption spectroscopy
What is AAS used for?
A technique used to detect metals and metalloids
What are the advantages of using AAS to detect metals?
-Reliable
-Sensitive (Can go to very low wavelengths)
-Quantitative
-Can analyse over 62 elements
What does AAS look at when analysing?
Atoms, rather than molecules
What is measured during AAS analysis?
Atoms absorb energy and we measure the emission (The degree they emit energy back)
What is the energy gap for emission?
The energy gap for emission is the same as the energy gap for absorption
What is Planck’s equation?
E=hf
What does the E in Planck’s equation stand for?
Energy
What does the h in Planck’s equation stand for?
Planck’s constant
What does the f in Planck’s equation stand for?
Frequency
What is the principle of AAS?
Free atoms that are generated in an atomiser can absorb radiation at a specific frequency;
Each element absorbs at a different frequency
What does AAS quantify?
Quantifies the absorption of ground state atoms in the gaseous state
What is the concentration of the analyte in AAS determined by?
The analyte concentration is determined by the amount of energy absorption
What law is AAS governed by?
Beer-Lambert Law:
Absorption=constant x conc
What is the AAS instrument calibrated with ?
Standards of a known concentration
What is the Beer-Lambert equation?
A=EbC
*For any wavelength
What is the A in the Beer-Lambert equation?
A=Absorbance
What is the b in the Beer-Lambert equation?
b=Path length
What is the E in the Beer-Lambert equation?
E=Molar absorbtivity
What is the C in the Beer-Lambert equation?
C=Conc (mol/L)
What is heavy metal detection using AAS based on?
The properties of specific metal atoms to absorb specific wavelengths of light
What does the electronics on the AAS instrument do?
The electronics will measure light attenuation (How much light is held by an atom) & convert to the metal concentration of the sample
What are the elements of the AAS instrument?
-Light source
-Sample cell (flame/furnace)
-Specific light measurement:
-Monochromator
-Detector
-Electronics
-Readout
What is the purpose of the furnace in the AAS instrument?
Heats up the sample to convert it to the gaseous state
What is the process of sample atomisation in AAS?
-Desolvation
-Vaporization
-Volatilization
What is desolvation in AAS?
The liquid sample evaporates and the dry sample remains
What is vaporization in AAS?
Solid sample vaporizes into gas
What is volatilization in AAS?
Compounds that make up the sample are broken into free atoms
What is the light source component of the AAS instrument?
-A hollow cathode lamp emits a specific spectrum, focused through the sample cell into the monochromator
What does the light source contain?
-A tungsten anode
-Hollow cylinder cathode made of the element of interest
Where is the light source sealed?
In a glass tube with an inert gas
What is the Nebuliser component of the AAS instrument?
-Aspires liquid at a controlled rate
-Creates a fine aerosol particle to be introduced to the flame
What is the Atomiser component of the AAS instrument?
-Separates particles into individual molecules & breaks molecules into atoms
-Occurs under high temp in a furnace
What are the 2 types of atomiser?
-Flame atomiser
-Graphite tube atomiser
What is a flame atomiser in AAS?
A mixture of oxidant gas and a fuel
e.g. Air-acetylene flame
What is a graphite tube atomiser in AAS?
Uses graphite coated furnace to vaporise sample.
Sample deposits in small graphite coated tube, heated to vaporise and analyte is atomised
What are the basic steps of atomisation?
-Nebulisation
-Desolvation
-Volatilisation
-Dissociation
-Ionisation
-Excitation
What is the monochromator component of the AAS instrument?
-Selects the specific wavelength of light absorbed by the sample and excludes other wavelengths
-Allows the detection of the specific elements
What is the detection component of the AAS instrument?
-Photomultiplier tubes are the most common detector;
-Converts a light signal to an electric
signal , proportional to the intensity of
light
What is XRF?
X-Ray emission spectroscopy
What is the principle of XFR?
-Deals with characteristic X-rays
-When a sample is exposed to X-rays an electron could be displaced
What is the energy associated withe these X-rays sufficient enough to do?
Displace electrons from inside the orbitals of atoms (K,L,M)
What do X-rays release energy as?
X-ray photons
What is electron transition?
The movement of electrons between energy levels
What is the energy of emitted X-ray photons equal to?
The differences in energy levels involved in electron transition
What is Ka electron transition?
Electron moves from L to K shell
What is Kb electron transition?
Electron moves from M to K shell
What is La electron transition?
Electron moves from M to L shell
What is emission intensity in XRF?
Proportional to the concentration of the atom in the sample
What happens when X-rays encounter matter?
-They can be absorbed or transmitted through the sample (medical)
-Can be Diffracted or scattered from an ordered crystal (Crystal structure)
-Can cause generation of X-rays in diff colours (Fluorescence)
What does an atom consist of?
-Nucleus (contains protons and neutrons)
-Electrons in shells
What is the atomic number of an element?
-Number of protons and electrons
-Rep by letter Z
What are the 4 electron shells?
-K
-L
-M
-N
What is the K electron shell?
-Closest to the nucleus
-Holds 2 electrons
-Electrons are bound more tightly and harder to remove
What is the L electrons shell?
-2nd from the nucleus
-Holds 8 electrons
What is the M electron shell?
-3rd from the nucleus
-Holds 18 electrons
What is the N electron shell?
-Furthest from the nucleus
-Holds 32 electrons
Which shells do X-rays typically only affect?
Inner K & L shells
Why are the electrons in the K shell more difficult to move?
-They have the highest binding energy
-Requires more energy X-rays to move electrons out of the shell
Why will N shell electrons release more energy when falling between shells?
-They have the highest potential energy
What is X-ray attenuation?
Initial electrons may be scattered
What are the 2 types of scattering?
-Coherent
-Incoherent
What is coherent X-ray scattering?
-The X-ray collides with an atom & deviates without a loss in energy
-An electron in an alternating electromagnetic field, will oscillate at the same frequency
What is incoherent X-ray scattering?
-Incident X-ray loses some of it’s energy to the scattering electron
What is the X-ray tube source on the XRF instrument?
-High energy electrons are fired at the anode
-Can vary excitation and current
-Can use filters to tailor source profile for lower detection limits
What are the components of the XFR instrument?
-X-ray tube source
-Silicon drift detector
-Digital pulse processor
-Analyser software
What is the analyser software of the XRF instrument?
-Converts spectral data to a direct readout of results
What is the energy of a photon related to?
-The type of element
What is the emission rate related to?
-The concentration of the element
What does the detector do on the XRF instrument?
-Monitors the energy & the number of photons over a preset time
