This relates to filter columns for isolating nucleic acids, particularly at small elution volumes. Before selecting the channel, the method detects a channel which another radio network is using and a channel having high noise. A metal sublayer is then deposited over and adjacent the spacer layer, followed by one of an oxidation process, a nitridation process, and an oxynitridation process, to produce an insulator from the metal sublayer. Multiple limit stubs and multiple curved limit channels communicating with a corresponding notch with an inclined side-wall are respectively formed on the connector and a connecting chamber in a connecting tube of the needle hub. Information about the height of the probe from the sample surface is obtained by detecting a probe shadow appearing immediately before the probe contacts the sample, or based on a change in relative positions of a probe image and a sample image that are formed as an ion beam is irradiated diagonally.