A method of analyzing a molecule in a nanopore is disclosed. A voltage is applied across a nanopore that is inserted in a membrane by coupling the nanopore to a voltage source. The nanopore is decoupled from the voltage source. After the decoupling, a rate of decay of the voltage across the nanopore is determined. A molecule in the nanopore is distinguished from other possible molecules based on the determined rate of decay of the voltage across the nanopore.