where Ao is the radioactivity at reference date t is the time between reference date and the time you are cal culating for, Tis the half-life of the isotope(note that both t and T must have the same units of time) It is easy to use the aforementioned decay charts as shown in the following two examples. Say you had 250uCi of>>s methionine at a certain reference date and the radioactive concentration was 15mCi/ml. Now it is 25 days after that reference date You calculate your new radioac tive concentration and total activity in the vial by looking on the chart to locate the fraction under the column and row that corre- sponds to 25 days postreference. This number should be 0.820 Multiply your starting radioactive concentration by this fraction to obtain the new radioactive concentration: 15mCi/ml x0.820=12.3 mCi/ml The total amount of activity can be likewise calculated for S with a half-life of 87 4 days; namely A Age- 15exp(0.693×25/874)=123mCi/m For the second example you can find out how much activity you had before the reference date. Some decay charts only have postreference fractions, but if you have a l mCi vial of P dUTP at 10mCi/ml, and it is 5 days prior to the reference date, how do you figure out how much you have? Go to the column and row on the P decay chart corresponding to 5 days postreference There you will see the fraction 0.872. You will divide your ref- erence activity and radioactive concentration by this number to obtain the proper amount of activity present, or 1/0.872 1.15 mCi. Note that the values should be greater than the stated amounts of activity and the referenced radioactive concentration For the calculation method you are now looking for Ao. Therefore Ao= Aeos=10exp(0.693 x 5//25)=11.5mCi/ml, using a half-life of 25 days for> P How Long after the Reference Date Can You Use Your material? Radioactively labeled compounds do not suddenly go bad after the reference date. It isnt an expiration date. It is used as a benchmark by which you can anchor your decay calculations Working Safely with Radioactive Materials 149where A0 is the radioactivity at reference date, t is the time between reference date and the time you are cal￾culating for, T is the half-life of the isotope (note that both t and T must have the same units of time). It is easy to use the aforementioned decay charts as shown in the following two examples. Say you had 250mCi of 35S methionine at a certain reference date, and the radioactive concentration was 15mCi/ml. Now it is 25 days after that reference date. You calculate your new radioac￾tive concentration and total activity in the vial by looking on the chart to locate the fraction under the column and row that corre￾sponds to 25 days postreference. This number should be 0.820. Multiply your starting radioactive concentration by this fraction to obtain the new radioactive concentration: 15 mCi/ml ¥ 0.820 = 12.3 mCi/ml The total amount of activity can be likewise calculated for 35S with a half-life of 87.4 days; namely A = A0e-0.693t/T = 15 exp(-0.693 ¥ 25/87.4) = 12.3mCi/ml. For the second example you can find out how much activity you had before the reference date. Some decay charts only have postreference fractions, but if you have a 1 mCi vial of 33P dUTP at 10 mCi/ml, and it is 5 days prior to the reference date, how do you figure out how much you have? Go to the column and row on the 33P decay chart corresponding to 5 days postreference. There you will see the fraction 0.872. You will divide your ref￾erence activity and radioactive concentration by this number to obtain the proper amount of activity present, or 1/0.872 = 1.15 mCi. Note that the values should be greater than the stated amounts of activity and the referenced radioactive concentration. For the calculation method you are now looking for A0. Therefore A0 = Ae0.693t/T = 10exp(0.693 ¥ 5/25) = 11.5mCi/ml, using a half-life of 25 days for 33P. How Long after the Reference Date Can You Use Your Material? Radioactively labeled compounds do not suddenly go bad after the reference date. It isn’t an expiration date. It is used as a benchmark by which you can anchor your decay calculations as described above. Working Safely with Radioactive Materials 149