Evaluation of an electrostatically-collimated ion source in an isotopic-ratio mass spectrometer

Electrostatic collimation or focussing of the electron beam has been used in the ion source of an isotope-ratio mass spectrometer and its performance evaluated. The design details of the ion source and the mass spectrometer are given. The sensitivity is comparable to that in the magnetic collimation mode. The reproducibility of the sensitivity with electrostatic collimation for various filament assemblies is good. The output vs. pressure curves determined for electrostatic collimation, magnetic collimation and a combined electrostatic and magnetic collimation show good linearity. This combined field collimation is also found to give good sensitivity, comparable to the conventional source, but at a much lower electron emission.

The electrostatically-collimated ion source was used for precision isotope ratio determinations of nitrogen gas samples collected from a cylinder. The relative internal standard deviation on a single sample ratio was better than 0.025%. The relative external standard deviation for repeated analysis of 19 samples drawn from the same cylinder over an 8-hour period was 0.08%. The advantages of using such an ion source are listed.