A magnetic powder composed primarily of Fe that has been surface-treated with a silane coupling agent, which magnetic powder is characterized in that it contains Co such that CoFe expressed in atomic percent is 20-50 at. The method produces a stable mono-pegylated IL-10, which retains IL-10 activity, where pegylation is selective for the N-terminus on one subunit of IL-10 with little or no formation of monomeric IL-10. If an organism is: homozygous then the labels will be present in the ratio 1:1 or 1:0; or if heterozygous then the labels will be present in the ratio 1:2.