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- The active compound to be used is 14C-labelled n-hexadecane already mixed with liquid scintillation cocktail. You have been given 15 cm3of this in a plastic scintillation vial.
- Ask a demonstrator what the disintegration rate (d.p.m) of the 14C-labelled hexadecane is.
- 15 cm3 of the pure scintillation cocktail should be pipetted into a second vial as background.
- Put both vials in a counting rack and insert the Protocol 25 flag on the left hand side of the rack. Make sure the flag has been pushed to the left hand side. Place the rack on the right hand side of the instrument and press F2 to start counting.
- Count both sample and background. Note the counts per minute (c.p.m.) in both channels (regions) A and B, and note the energies of these channels, A = 50-160 keV and B = 50-80 keV.
- Results will be printed but make note of ongoing results in your lab book as the instrument is shared with other students doing other experiments.
- Remove rack and samples from the instrument. Before recounting any sample, make sure that the flag has been pushed to the left hand side again.
- Add 10 μl of CCl4 using the automatic pipette to the 14C-labelled hexadecane sample, shake well, and recount. It is not necessary to recount the background sample.
- Add another 10 μl of CCl4 to the same vial and recount.
- Then add successively 10, 20, 25, 25, 50 and 50 μl of CCl4 and recount each time.
- Be very careful to make sure there are no drops on the outside of the automatic pipette when using it. (The small overall volume changes caused by the addition of the CCl4 can be ignored).
- Measure the activity of the quenched unknown(s).
- Calculate the efficiencies (as percentages) of all the recorded c.p.m. measurements (corrected for background) in each channel, i.e. (100 x c.p.m./d.p.m.).
- Plot graph of efficiency against CCl4 amount for both channels on the same axes.
- Plot a graph of counting efficiency in channel A against the channel ratio (c.p.m. in channel A / c.p.m. in channel B).
- Using the plot produced, determine the true activity (d.p.m.) of the unknown sample).