Yes. A nanogram is 10
-9 grams. A picogram is 10
-12 grams. And there are smaller units still. It might come down to whether or not we can measure them or whether we can just estimate a mass smaller than a nanogram. The mass of a proton, for exampe, is one that is not derived by placing a proton on a scale.
(The mass of a proton, by the way, is about 1.67×10−24 grams or so.)