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forensic chemistry > lecture 7 - Heavy metal detection > Flashcards

lecture 7 - Heavy metal detection Flashcards

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1
Q

Name 5 common heavy metals

A

Mercury, cadmium, chromium, lead and arsenic

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2
Q

give 3 properties of heavy metals

A

high density, persistent in its environment, and cannot degrade or be destroyed

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3
Q

what are the three ways heavy metals can enter the body

A

food, drink and the air

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4
Q

what is the difference between deliberate exposure and accidental

A

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

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5
Q

which technique do we use to detect metals and metalloids

A

Atomic Absorption Spectroscopy

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6
Q

Name 4 reasons why we use AAS

A
  1. Reliable
  2. Sensitive - ppb levels
  3. Can analyse over 62 elements
  4. Quantitative
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7
Q

How does AAS work to detect metals

A

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

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8
Q

what is AAS based on

A

the properties of specific metal atoms to absorb specific wavelengths of light.

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9
Q

what are the 3 steps to atomise samples in AAS

A

desolvation, vaporisation, volatilisation

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10
Q

what fuel is used in AAS

A

Acetylene

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11
Q

what are the 3 different oxidants used in AAS and what temperatures do they require

A
  1. Air - 2100 °C – 2400 °C (most common)
  2. N20 - 2600 °C – 2800 °C
  3. O2 - 3050 °C – 3150 °C
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12
Q

why do we use fuel rich flames in AAS

A

to reduce the likelihood of oxidation of the atoms, as oxidation should occur in the secondary combustion zone

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13
Q

Name the 7 basic components in AAS

A
  1. Light source
  2. Nebuliser
  3. Atomiser
  4. Monochromator
  5. Detector
  6. Absorption cell
  7. A display
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14
Q

explain what the light source is and what the component does in AAS?

A

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

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15
Q

what does the nebuliser do

A

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

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16
Q

what is atomisation

A

the separation of particles into individual molecules and breaking the molecules into atoms

17
Q

what does the atomiser do in AAS

A

separates the particles by exposing them to high temperatures in a flame or furnace

18
Q

what are the two different atomisers

A

-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

19
Q

what does the monochromator do in AAS

A

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

20
Q

What is the detector for AAS and how does it work

A

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

21
Q

what is the absorption cell in AAS

A

where atoms of a sample are produced

22
Q

what is the display in AAS

A

it shows the the reading after it has been processed by the instrument electronics

23
Q

what do we clean the AAS machine with and why

A

nitric acid and deionised water - to remove all elements and ions

24
Q
A

forensic chemistry (7 decks)

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