For the past few years we have been making isotopic ratio measurements on a variety of metals using a surface ionization type source. In certain applications, it is desirable to search for isotopes which are present in very small amounts. A specific difficulty has been encountered in searching for an isotope adjacent to another isotope of the same element in which the ratio of the large to small isotope is of the order of 10<sup>4</sup>. As is well known, the resolution of a mass spectrometer is a function of a number of parameters. The first is the radius of curvature of the analyzing magnet; this factor determines the dispersion. Secondly, the width of the source and detector slits is a limitation. That is, if the slits are very small, the beam will be attenuated; on the other hand, as the slit widths approach the dispersion distance, it becomes increasingly difficult to resolve adjacent isotopes. A third, and also important factor, is the energy spread of the ions leaving the source. This energy spread arises from the I. R. drop in voltage across the surface ionization filament as well as from the kinetic energy distribution of ions produced by the surface ionization mechanism.

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