A corona discharge ionization source is disclosed in which neutral species are removed by directing a flow of oxygen containing gas in a direction substantially different to that of the ion flow. The surfaces may be, for example, opposed, substantially parallel surfaces, e. An apparatus is also provided for calibrating a sensor which may be used for measuring color of a display in one or more of these methods. The clamp is positioned around tubes, and the fastener is tightened to tighten the clamp around the connection.