An instrument combining a mass spectrometer with a liquid chromatograph has a low-pressure chamber that is evacuated by an evacuating equipment, an exit nozzle for allowing the effluent from the chromatograph to be ejected as a jet into the low-pressure chamber, and a connecting passage for conveying the jet of the effluent from the nozzle into the ionization chamber of the mass spectrometer, to stably supply the effluent from the chromatograph as the sample for the ion source into the ionization chamber. The instrument is further provided with a means for producing a flow of gas which moves along the inner walls of the low-pressure chamber and the connecting passage in such a way that the jet of the effluent is enveloped in the gas flow.