Forensic Chemistry

Infrared and X-ray scans from the day – download IR & XRF scans.

Answers to Resource Questions

1. Plant pigments

1. Cut the spots out, dissolve the substance in a solvent and run a spectrum (infrared).
2. Different polarities.
3. Chlorophyll A & B
4. Anthocyanins

2. Lipsticks

1. A similar Rf value, even for the same coloured substance, does not definitely mean they are the same compound. It does mean that they could be.
2. (a) & (b) Both no. Extra tests, eg infared spectrum

3. Oil spills

1. Likely variations are peak height (different volumes) and different retention times (different speed in pressing the “start” indicator). The time variation is the main one that is a problem in helping to indentify; volume diffferences affect all peaks equally, and are only a problem for working out how much of a given substance is present.
2. Not really – if you know that the spill can only have come from one of these ships, then a match of profiles is sufficient.

4. Pesticides & perfumes

The peak in the chromatogram caused by the illegal substance is what is being masked by a compound chosen for its ability to come out at about the same time.

5. Electrophoresis

Fragments of the DNA chains

6. Distinguishing tests

1(a) A false positive is when a substance that is not part of the target group that the test is designed to show, gives a positive reaction.
(b) A false negative is when a substance that is part of the target group that the test is designed to show, gives a negative reaction.
2. The substance you are looking for could be present.

7. Pellets and paint

Very little difference in the spectra – not enough to tell them apart. The technique is not sensitive enough to detect the low concentrations of trace elements that could help distinguish them.

8. Paint and powders

We didn;t do the paints, but the powders definitely differ well enough to distinguish them.

9. Metals and flames

The colour would be the same, but of lower intensity.

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