Your job is to determine the concentration of leucine in a 5.0 mL solution containing 14C-leucine. You remove 5.0 L of the solution for counting and obtain 6.2 x 105 CPM. Assume the counting efficiency is 40.0%. The specific radioactivity of the 14C-leucine is 0.600 mCi/mgram. Calculate the concentration of leucine in mmolar units.
The given data is
The specific radioactivity of the 14C-leucine = 0.6 mCi/mg
Counting efficiency =40 %
The counts obtained for 5.0 L of the solution (please note that there is a printing mistake, hence I consider 5.0 L as 5 mL) = 6.2 x105 CPM
Counting efficiency = Number of Photons counted by detector/Number of Photons emitted by the radiation source
From above equation, the number of disintegrations per minute (dpm) can be = 6.2 x105 /0.4 =
6.2 x2.5x105 dpm = 6.2 x2.5x105x60 dps= 9.3x107 dps (Note dps = disintegrations per second)
So the 14C-leucine present in 5 ml gives the 9.3x107 disintegrations per second ........(I)
The specific radioactivity can be defined as activity per quantity of a radio nuclide.
From the data, specific radioactivity = 0.6 mCi/mg
We know that one curie (Ci) = 3.7x1010 disintegrations per second (dps)
So above specific radioactivity can be converted into disintegrations per second (dps)as follows
specific radioactivity = 0.6 x10-3x3.7x1010 = 2.22x107 dps/mg
Means one mg of 14C-leucine gives 2.22x107 disintegrations per second .......(II)
From (I) and (II), the weight of 14C-leucine present in 5 mL solution = 9.3x107 /2.22x107 = 4.189 mg.
Hence the concentration of leucine = 4.189 mg/5 mL = 0.8378 g/L
Molecular weight of leucine = 131.175 g/mol
So the concentration of leucine = 0.006386 mol/L = 6.386 mmol/L = 6.386 mM
The concentration of leucine is 6.386 in mmolar units
Your job is to determine the concentration of leucine in a 5.0 mL solution containing 14C-leucine....
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