In one embodiment, a primary amine group is attached to the 5'-end of the oligonucleotide using hydrazine or an alkyldiamine reactant in the following reaction scheme: ##STR13## wherein b is an integer equal to 0 or 2 to 20, preferably 0 or 2 to 6, and most preferably is 0, 2, or 6. (See, e.g., Ghosh et al. (1989) Anal.