Abstract
A negative chlorine ion source has been designed and constructed. The source utilizes direct surface ionization of chloromethane gas on a hot metal filament. Four different alloys for the filament material were tested: W99Th1, W75Re25, Hf97.5Zr2.5 and Mo52.5Re47.5. We conclude that the best filament material is the MoRe alloy, for which the signal-to-noise ratio is optimal. The ion source is used for chlorine isotope ratio measurements with higher precision and sensitivity than the positive ionization source used previously. Inasmuch as only negative ions of the two isotopes of interest are observed, no corrections to the measured isotope ratio are necessary, and less rigously purified samples may be analyzed. The negative ion currents are considerably larger than positive ion currents obtained with an electron ionization source. This implies higher analytical precision (typically 0.005 permil) and sensitivity.
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