Before he could begin his experiment, Nirenberg needed both a means to separate the complex from unbound components and a method to detect tRNA binding to the ribosome. To isolate the complex he exploited the ability of nylon filters to bind large RNA molecules, such as ribosomes, but not the smaller tRNA molecules. He used a nylon filter to separate ribosomes (and anything bound to the ribosomes) from unbound tRNA. To detect the tRNA bound to the ribosomes, Nirenberg used tRNA charged with amino acids that contained a radioactive label, 14C.