A method for determining the sequence of a target nucleic acid, including steps of contacting a target nucleic acid with a polymerase to sequentially remove nucleotide triphosphates from the target nucleic acid, wherein the nucleotide triphosphates that are removed have a variety of different base moieties; and distinguishing the different base moieties for the nucleotide triphosphates that are removed. Also provided is a apparatus including a nanopore positioned in a fluid impermeable barrier to form a passage through which a nucleotide triphosphate can pass from a first fluid reservoir to a second fluid reservoir, and a reaction mix in the first fluid reservoir that includes a polymerase, target nucleic acid having two strands, and pyrophosphorolytic concentration of pyrophosphate.