Gaseous analyte molecules are ionized at atmospheric pressure and provided to an inlet capillary of an ion spectrometer vacuum system by passing the ions through a reaction tube that ends in a conical intermediate piece for a gastight and smooth transition into the inlet capillary. The reaction tube is shaped so that the atmospheric pressure gas stream passing therethrough form the entrance of the tune to the intermediate piece is stably laminar. Analyte molecules from gas chromatographs, spray devices or vaporization devices can be introduced into the entrance of the reaction tube and ionized within the tube by single- or multi-photon ionization, by chemical ionization, by reactant ions or by physical ionization. For single- or multi-photon ionization, a beam from a laser can be passed axially down the reaction tube. Reactant ions can be produced by any means outside of the reaction tube and mixed with the analyte molecules within the tube.