Question

A solution contains 1 millicurie of $L$-phenylalanine ${}^{14}\text{C}$ (uniformly labelled) in 2.0 mL solution. The specific activity of the labelled sample is given as 150 millicuries/mmol. The activity of the solution in terms of counting per minute/mL at a counting efficiency of 80% is :

• A. $88.8\times {10}^{7}cpm/L$
• B. $88.8\times {10}^{6}cpm/mL$
• C. $88.8\times {10}^{5}cpm/mL$
• D. $88.8\times {10}^{7}cpm/mL$

Answer 1

The correct option is D $88.8\times {10}^{7}cpm/mL$

(a) $1mmol=150mcurie$
$1mcurie=1/150mmol$
$Now,concentration=\frac{mmol}{VinmL}=\frac{1}{150\times 2}$
$=3.33\times {10}^{-3}M$
(b) $1curie=3.7\times {10}^{10}dps$
$=3.7\times {10}^{10}\times 60dpm$
$=3.7\times {10}^{10}\times 60\times \left(80/100\right)countingperminute$
$\therefore 1millicurie=3.7\times {10}^{10}\times 60\times \left(80/100\right)\times {10}^{-3}cpm$
$\therefore cpm/mL=3.7\times {10}^{10}\times 60\times \left(80/100\right)\times \left({10}^{-3}/2\right)$
$=88.8\times {10}^{7}cpm/mL$