Patent Number: 
Section: claims

1. A method for cleaning ion source electrodes in a mass spectrometer by plasma etching, the mass spectrometer having a desorption ion source with the ion source electrodes located in a space in which a vacuum is present during ion source operation, the method comprising:(a) constructing the ion source electrodes with an arrangement and a position that supports formation of a plasma in the space;(b) admitting a reactant gas is into the space around the ion source electrodes, and(c) igniting a gas discharge in the reactant gas by applying to at least one of the ion source electrodes an electric voltage having characteristics that differ from characteristics of a voltage used to operate the ion source. 2. The method according to claim 1, wherein the electric voltage is an RF alternating voltage. 3. The method according to claim 2, wherein the electric alternating voltage has a frequency of substantially 13.57 megahertz. 4. The method according to claim 1, wherein the reactant gas is maintained at a preselected pressure of between 1 and 1000 Pascal. 5. The method according to claim 1, wherein the reactant gas contains oxygen for forming ions that act on organic materials to reactively decompose them. 6. The method according to claim 1, wherein the reactant gas contains a proportion of a heavy gas for forming ions that act by sputtering. 7. The method according to claim 1, wherein a special cleaning plate is positioned adjacent to the ion source electrodes in order to clean the ion source electrodes, wherein the shape of the cleaning plate contributes to the formation of the gas discharge. 8. The method according to claim 7, wherein the cleaning plate incorporates a machine-readable code which can be read by a mechanism in the mass spectrometer. 9. The method according to claim 8, further comprising automatically starting the cleaning method when the mechanism reads the machine-readable code. 10. The method according to claim 1, further comprising using a video system associated with the mass spectrometer to visually monitor the cleaning process. 11. A mass spectrometer comprising:an ion source having electrodes surrounded by a chamber, in which a vacuum is present during ion source operation, the electrodes being constructed with an arrangement and a position that supports formation of a plasma in the chamber;a device to admit a reactant gas into the chamber surrounding the ion source electrodes, andan electrical power supply connected to at least one of the ion source electrodes for applying to the at least one electrode an electric voltage having characteristics that differ from characteristics of a voltage used to operate the ion source in order to generate a gas discharge wherein ions produced by the gas discharge clean the ion source electrodes by plasma etching.