I need to make a specific concentration of radioactivity for small volumes (a few ml or less). The quick answer: Don’t! Instead of making calculations on and trying to manipulate small volumes or numbers of uCi, base your calculations on making large(r) volumes. So, if you need something like 0.8 uCi in 0.5 ml (1.6 uCi/ml), try making up 5 or 10 ml instead.To minimize radiation exposure caused by trying to get “exact” numbers of uCi, draw up some (somewhat arbitrary) larger number. Let’s say that it happens to be 76.8 uCi.To get our target concentration of 1.6 uCi/ml, we need to add the 76.8 uCi of radioactivity to a total of 48 ml of water. If you have a small volume of radioactivity, let’s say 0.1 ml, drawn up from the dose vial, then you might want to just add the 2 volumes and have a very small error in concentration: ((48.1 – 48.0) / 48.0) * 100 = 0.21 % Error For more exacting applications, q.s. the radioactive volume drawn up from the dose vial (0.1 ml) up to 48.0 ml. In other words, add the 0.1 ml of radioactivity to 47.9 ml of water.