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Forensic Chemistry > Lecture 7: Heavy metal detection > Flashcards

Lecture 7: Heavy metal detection Flashcards

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1
Q

What are heavy metals?

A

-High density
-Can’t be degraded or destroyed
-Persistent in the environment

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2
Q

What are examples of heavy metals?

A

-Mercury
-Lead
-Chromium
-Cadmium
-Cobalt

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3
Q

What is AAS?

A

Atomic absorption spectroscopy

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4
Q

What is AAS used for?

A

A technique used to detect metals and metalloids

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5
Q

What are the advantages of using AAS to detect metals?

A

-Reliable
-Sensitive (Can go to very low wavelengths)
-Quantitative
-Can analyse over 62 elements

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6
Q

What does AAS look at when analysing?

A

Atoms, rather than molecules

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7
Q

What is measured during AAS analysis?

A

Atoms absorb energy and we measure the emission (The degree they emit energy back)

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8
Q

What is the energy gap for emission?

A

The energy gap for emission is the same as the energy gap for absorption

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9
Q

What is Planck’s equation?

A

E=hf

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10
Q

What does the E in Planck’s equation stand for?

A

Energy

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11
Q

What does the h in Planck’s equation stand for?

A

Planck’s constant

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12
Q

What does the f in Planck’s equation stand for?

A

Frequency

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13
Q

What is the principle of AAS?

A

Free atoms that are generated in an atomiser can absorb radiation at a specific frequency;
Each element absorbs at a different frequency

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14
Q

What does AAS quantify?

A

Quantifies the absorption of ground state atoms in the gaseous state

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15
Q

What is the concentration of the analyte in AAS determined by?

A

The analyte concentration is determined by the amount of energy absorption

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16
Q

What law is AAS governed by?

A

Beer-Lambert Law:
Absorption=constant x conc

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17
Q

What is the AAS instrument calibrated with ?

A

Standards of a known concentration

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18
Q

What is the Beer-Lambert equation?

A

A=EbC
*For any wavelength

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19
Q

What is the A in the Beer-Lambert equation?

A

A=Absorbance

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20
Q

What is the b in the Beer-Lambert equation?

A

b=Path length

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21
Q

What is the E in the Beer-Lambert equation?

A

E=Molar absorbtivity

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22
Q

What is the C in the Beer-Lambert equation?

A

C=Conc (mol/L)

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23
Q

What is heavy metal detection using AAS based on?

A

The properties of specific metal atoms to absorb specific wavelengths of light

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24
Q

What does the electronics on the AAS instrument do?

A

The electronics will measure light attenuation (How much light is held by an atom) & convert to the metal concentration of the sample

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25
What are the elements of the AAS instrument?
-Light source -Sample cell (flame/furnace) -Specific light measurement: -Monochromator -Detector -Electronics -Readout
26
What is the purpose of the furnace in the AAS instrument?
Heats up the sample to convert it to the gaseous state
27
What is the process of sample atomisation in AAS?
-Desolvation -Vaporization -Volatilization
28
What is desolvation in AAS?
The liquid sample evaporates and the dry sample remains
29
What is vaporization in AAS?
Solid sample vaporizes into gas
30
What is volatilization in AAS?
Compounds that make up the sample are broken into free atoms
31
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
32
What does the light source contain?
-A tungsten anode -Hollow cylinder cathode made of the element of interest
33
Where is the light source sealed?
In a glass tube with an inert gas
34
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
35
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
36
What are the 2 types of atomiser?
-Flame atomiser -Graphite tube atomiser
37
What is a flame atomiser in AAS?
A mixture of oxidant gas and a fuel e.g. Air-acetylene flame
38
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
39
What are the basic steps of atomisation?
-Nebulisation -Desolvation -Volatilisation -Dissociation -Ionisation -Excitation
40
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
41
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
42
What is XRF?
X-Ray emission spectroscopy
43
What is the principle of XFR?
-Deals with characteristic X-rays -When a sample is exposed to X-rays an electron could be displaced
44
What is the energy associated withe these X-rays sufficient enough to do?
Displace electrons from inside the orbitals of atoms (K,L,M)
45
What do X-rays release energy as?
X-ray photons
46
What is electron transition?
The movement of electrons between energy levels
47
What is the energy of emitted X-ray photons equal to?
The differences in energy levels involved in electron transition
48
What is Ka electron transition?
Electron moves from L to K shell
49
What is Kb electron transition?
Electron moves from M to K shell
50
What is La electron transition?
Electron moves from M to L shell
51
What is emission intensity in XRF?
Proportional to the concentration of the atom in the sample
52
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)
53
What does an atom consist of?
-Nucleus (contains protons and neutrons) -Electrons in shells
54
What is the atomic number of an element?
-Number of protons and electrons -Rep by letter Z
55
What are the 4 electron shells?
-K -L -M -N
56
What is the K electron shell?
-Closest to the nucleus -Holds 2 electrons -Electrons are bound more tightly and harder to remove
57
What is the L electrons shell?
-2nd from the nucleus -Holds 8 electrons
58
What is the M electron shell?
-3rd from the nucleus -Holds 18 electrons
59
What is the N electron shell?
-Furthest from the nucleus -Holds 32 electrons
60
Which shells do X-rays typically only affect?
Inner K & L shells
61
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
62
Why will N shell electrons release more energy when falling between shells?
-They have the highest potential energy
63
What is X-ray attenuation?
Initial electrons may be scattered
64
What are the 2 types of scattering?
-Coherent -Incoherent
65
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
66
What is incoherent X-ray scattering?
-Incident X-ray loses some of it's energy to the scattering electron
67
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
68
What are the components of the XFR instrument?
-X-ray tube source -Silicon drift detector -Digital pulse processor -Analyser software
69
What is the analyser software of the XRF instrument?
-Converts spectral data to a direct readout of results
70
What is the energy of a photon related to?
-The type of element
71
What is the emission rate related to?
-The concentration of the element
72
What does the detector do on the XRF instrument?
-Monitors the energy & the number of photons over a preset time

Forensic Chemistry (7 decks)

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