Multimode ion detector with wide dynamic range and automatic mode switching

A method for detection of positive and negative ions in a mass spectrometer, the mass spectrometer comprising one or more ion detectors, a signal processing circuit to measure and process the signal output from each ion detector, the signal processing circuit further having one or several signal processing modes, an image current processing circuit to amplify and measure an image current and compare it against one or more reference thresholds to automatically determine which said one or more ion detectors the ion beam is directed to and which the signal processing mode is used, comprising steps of:

• placing an aperture plate that is conductive and ungrounded on the path of an ion beam having a polarity and a current density that is emerging from a mass analyzer and can pass through the aperture plate, thereby inducing an image current into the aperture plate which is proportional to the current density and polarity of the ion beam;
• placing the one or more ion detectors that is a discrete or continuous dynode electron multiplier placed off-axis with respect to an incoming ion beam,
• placing a conversion dynode off-axis with respect to the incoming ion beam,
• applying a high potential with reverse polarity to that of the ion beam to the conversion dynode to attract the ion beam to hit the conversion dynode, wherein the ion beam striking the conversion dynode liberates secondary electrons when the polarity of the ion beam is positive or turns the negative ions into positive ions when the polarity of the ion beam is negative,
• accelerating the secondary electrons or positive ions toward the one or more ion detectors by a potential difference between the conversion dynode and the discrete or continuous dynode electron multiplier,
• using the image current to automatically switch the polarity of the conversion dynode based on the polarity of the ion beam, in which there is no need for switching the polarity on the electron multiplier for detection of positive or negative ions, thereby increasing the lifetime of the one or more ion detectors.