From the table we can see that labelling DNA with 15N should have approximately . This was indeed confirmed experimentally showing that fully 15N-labeled DNA in CsCl shows a density gain of ∼ 0.016 g ml−1 compared to a density gain of ∼0.036 g ml−1 with 13C \cite{1978}. Similarly, RNA fully labelled with 15N  showed a density shift of ∼0.015 g ml−1 \cite{Angel_2017} compared to 0.035 for 13C \cite{Lueders2004}.  Surprisingly, while 18O labelling should theoretically increase the mass of DNA by 23% and of RNA by 47% compared to labelling with 13C, in practice the observed shifts in BD is not much different than 0.04 g ml-1 \cite{Aanderud_2011,Angel_2013}, indicating that not all positions could be replaced with a heavy isotope.