Nuclear Chemistry (determing masses from radioactive decay)

[Quickman08]

I have been trying for quite some while but I can't seem to get this problem off the ground.1)Suppose that you hydrolyze 4.644 grams of a protein to form a mixture of different amino acids. To this is added a 2.80 mg sample of 14C-labeled threonine (one of the amino acids present).The activity of this small sample is 1950 dpm. A chromatographic separation of the amino acids is carried out, and a small sample of pure threonine is separated. This sample has an activity of 550 dpm. What fraction of the threonine present was separated? What is the total amount of threonine in the sample?

2)
I know that 1950dpm= 8.783E-10Ci and 550dpm=2.477E-10Ci
I am not sure if you need it but the molar mass of Threonine is 119.12 g mol−1
Also A=Lambda(N)
m=(N/Ao)*(MM)
A=radioactive mass units
Lambda=decay constant
N=number of atoms
Ao=avogadro's number
MM=molar mass3.
I have found the decay constant for the small sample of 2.80mg to be 1.375E-16
So if you take the 550dpm sample you can obtain a mass of .789g
Unfortunately once this mass is obtained I cannot figure out where to go from this point.

 

[Borek]

Don't worry about decay, assume sample activity is constant. It never is, but if half lives are long enough, that's a valid assumption.

You started with 1950 dpm, you have 550 dpm. What part of the radioactive threonine was recovered?