An ion transfer device for transferring ions emerging from an electrospray ion source at atmosphere to a vacuum chamber includes an inner surface in the shape of a diverging conical duct. The ion transfer device has an entrance aperture for positioning proximate the exit port of the electrospray ion source emitter, the entrance aperture receiving the electrosprayed ions from the exit port of the electrospray ion source emitter at atmosphere, the diverging conical duct being an electrode toward which the ions migrate and having an exit aperture with an inner diameter larger than an inner diameter of its entrance aperture, the exit aperture enclosed in the vacuum chamber, the diverging conical duct transporting the ions from atmosphere to vacuum. The vacuum chamber can be a chamber of a vacuum housing enclosing a mass analyzer.