| Date | May 2017 | Marks available | 2 | Reference code | 17M.3.sl.TZ2.6 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | Explain | Question number | 6 | Adapted from | N/A |
Question
Liquid Crystal on Silicon, LCoS, uses liquid crystals to control pixel brightness. The degree of rotation of plane polarized light is controlled by the voltage received from the silicon chip.
Two important properties of a liquid crystal molecule are being a polar molecule and having a long alkyl chain. Explain why these are essential components of a liquid crystal molecule.
Metal impurities during the production of LCoS can be analysed using ICP-MS. Each metal has a detection limit below which the uncertainty of data is too high to be valid. Suggest one factor which might influence a detection limit in ICP-MS/ICP-OES.
Markscheme
Polar molecule:
«orientation of molecule» influenced by electric field/«applied» voltage/«applied» potential «difference»/«applied» current
OR
can be switched on and off
Long alkyl chain:
prevent close packing of molecules
OR
molecules can align
OR
reduces the melting point of the liquid crystal/LC «phase making liquid at room temperature»
Accept “makes molecule rod-shaped” for M2.
[2 marks]
inability to replicate calibrations below certain levels
OR
variation in methodology
OR
variation between machines calibrated with the same samples
OR
variation in plasma torches
OR
different detection limits for MS AND OES
OR
interference from solvents/other chemicals
OR
inability to produce pure standards
OR
chance that low signal AND blank are same
[1 mark]
