A transfer capillary and interface are provided that allow samples from multiple fluid streams to be introduced into a single mass analyzer. The transfer capillary has two or more channels and is placed between a first chamber, in which ions from the multiple fluid streams are generated, and a second chamber that is in fluid communication with a mass analyzer. The interface includes ionizers that generate ionized sample from each fluid stream and that direct the ionized sample toward the channels of the transfer capillary. The transfer capillary includes a multiplex selector that allows ions to flow through a selected subset of channels and sequentially changes which of the channels are included in the subset of channels through which ions flow.