REDUCING THE ENERGY-DISTRIBUTION IN A PLASMA-SOURCE SECTOR-FIELD MASS-SPECTROMETER INTERFACE

Ion-beam kinetic-energy distributions were measured with several secto r-field plasma-source mass spectrometer interface configurations. The original interface design produced relative energy distributions of 23 % (with an ICP source) and 13% (with a GD source). These relative ener gy distributions mere independent of the interface potential. Accelera ting more evenly and over a shorter distance (with a grid installed) i n the second vacuum stage dramatically reduced the relative energy dis tribution but also the signal level. The addition of an ICP torch shie ld also helped to reduce the relative energy distribution, presumably by lowering the plasma offset voltage. Nea ion optics were designed wi th the distance between the apertures leading to the second and third vacuum stages decreased (to roughly 20 mm) and with the majority of th e ion-beam acceleration moved from the second to a short distance into the third vacuum region. This configuration reduced the relative ion- beam energy distribution to roughly 5% for both sources but also resul ted in some loss of signal.