CD - Identifying Materials Used in a Painting

Question 1

What determines the properties of a paint?

Answer 1

The pigment dispersed in a binding medium (such as oil or water)
Both effect the paint properties

Question 2

Give 4 methods of identifying materials used in paints

Answer 2

• Gas Liquid Chromatography
• Mass Spectroscopy
• Visible Spectroscopy
• Atomic Emission Spectroscopy

Question 3

Are you gay?

Answer 3

yes (but only on a Thursday)

Question 4

Describe the stages of gas-liquid chromatography

Answer 4

1) The stationary phase is a viscous liquid to coat the tube
2) The mobile phase is an unreactive carrier gas
3) The sample is injected into the stream of carrier gas
4) The components of the mixture constantly dissolve in the stationary phase, evaporate in the mobile phase and redissolve as they travel through the tube
5) The solubility of each component depends on how long it spends dissolved in the stationary phase. A substance with high solubility will spend more time dissolved so will take longer to travel through the tube in the mobile phase.

Question 5

State the 2 phases of gas-liquid chromatography

Answer 5

1) A mobile phase

2) A stationary phase

Question 6

What do the peaks on gas-liquid chromatography chromatograms display

Answer 6

The times when the detector senses something other than the carrier gas leaving the tube (retention time)

Question 7

Define retention time

Answer 7

The time take for the substance to reach the detector

Question 8

What does the area under the peaks on a gas-liquid chromatography chromatogram display

Answer 8

The area under each peak is proportional to the relative amount of each substance in the original mixture.

Question 9

What does GLC-MS stand for?

Describe GLC-MS and give its benifits

Answer 9

• Gas-liquid chromatography-mass spectroscopy
• The sample is separated using GLC but instead of going to a detector, the separated components are fed into a mass spectrometer, which produces a spectrum for each component
• GLC-MS combines the ability to separate a mixture into its components (GLC) and identifying the unknown compounds (MS)

Question 10

How can you get a monochromatic light from a white light source?

Answer 10

A filter

Question 11

Give the 2 variants of visible spectroscopy

Answer 11

• Visible absorption spectroscopy

- Visible reflection spectroscopy

Question 12

Describe visible absorption spectroscopy

Answer 12

1) A beam of monochromatic light is passed through a dilute solution of the pigment. A detector measures the intensity of light before and after its passed through the solution. Absorbance = how much light the pigment absorbed
2) Different frequencies of light are passed through the solution to produce a visible absorption spectrum (a graph of frequency vs. absorbance). Peaks = most absorbed frequency
3) Every pigment has a unique colour, so produces a unique visible absorption spectrum

Question 13

Describe visible reflection spectroscopy

Answer 13

1) a beam of monochromatic light is shone onto the surface of a solid. The intensity of the beam before and after reflection from this surface is measured and the reflectance is calculated
2) Reflectance is a measure of what percentage of light falling on to the solid is reflected back. Low values of reflectance mean that lots of light is being absorbed. A visible reflectance spectra is produced by changing the frequency of light and calculating the reflectance values

Question 14

Explain why visual reflection spectroscopy might be used instead of visible absorption spectroscopy

Answer 14

It is not always possible to make a solution of a pigment because:

• It might not dissolve easily
• You may not want to remove it from the painting

Question 15

What does atomic emission spectroscopy identify?

Answer 15

The elements in a pigment

Question 16

Explain how atomic emission spectroscopy works

Answer 16

1) When an electron absorbs the right amount of energy, it is excited to a higher energy orbital. It falls back to its ground state by emitting its excess energy as UV, visible or infra-red light
2) Each energy level is discrete. Electrons emit a fixed amount of energy to move between energy levels. The frequency of light depends on the energy gap between energy levels
3) There’s more than one energy level that an electron can be excited to - as a result, different frequencies of light will be emitted by electrons returning to their ground states
4) The energy levels in every element are different, so each element will emit its own, unique set of frequencies (producing identifiable atomic emission spectra)

Question 17

Describe how atomic emission spectroscopy is carried out

Answer 17

1) A high-energy pulse of laser light is focused onto a tiny sample of paint (which vaporises the pigment)
2) The vapour is then passed between two electrodes - the energy from the electric current excites the electrons in the pigment of paint
3) When the electrons return to their ground state, they emit light with a particular set of frequencies. A spectrometer is used to separate out all the frequencies into an emission spectrum
4) The atoms present in the pigment are identified by comparing the spectrum produced to those from known elements. It’s also possible to find out the relative quantities of each element by measuring the intensities of certain frequencies of light

Question 18

For an emission spectrum for an element and for a pigment, describe the pattern on both of the spectra required for it to be concluded that element is in the pigment

Answer 18

• The spectrum pattern of the element must be present in the spectrum for the pigment for the element to be present.
• Other lines may be present, but this is because the pigment is a compound or mixture