A device for switching small accurate potentials at high voltage

A device is described as that which provides highly precise, switched potentials for the Manitoba II high-resolution, deflection mass spectrometer. In the spectrometer, mass differences (DeltaM) of doublets are determined with a precision of deltaM approximate to 3 parts in 10(9) of M, the mass at which the doublet occurs, by means of measurements of voltages applied to the plates of the electrostatic analyser. The device described is effective as an RF-based electronic chopper supply, by which these analyser potentials are changed by small (4 mV DC less than or equal to DeltaV less than or equal to 2 V DC) amounts, where the two analyser plates operate at nominal values of + 400 V DC and - 400 V DC with respect to ground. The changes in potential, DeltaV, must settle to highly reproducible and precisely determined values (better than 1 muV) within a few milliseconds. For our use in mass comparisons, these potentials are switched cyclically, with equal dwell times of order 40 ms. Since the system uses RF coupling to communicate these values, almost arbitrarily high voltages can be modulated by this method. (C) 2000 Elsevier Science B.V. All rights reserved.