ISOTOPIC FRACTIONATION IN SECONDARY IONIZATION MASS-SPECTROMETRY

We have studied isotopic fractionation effects which occur during the analysis of oxygen isotope ratios by secondary ionization mass spectrometry. Variable instrumental isotopic fractionation has been well documented in the past as making reliable ion microprobe analyses of oxygen isotopes elusive. We report here techniques for minimizing these effects by careful optimization of an Isolab 54 ion microprobe and a method of integrating the secondary-ion beam to eliminate a major source of fractionation caused by unequal focusing of isotopes on the source slit of the mass spectrometer. A further improvement has been the installation of new ion detectors for measuring O-18- and O-17- secondary ion beams in which the ions strike a conversion dynode (Conversion dynode system-CDS) to release electrons into a channeltron. The CDS detectors have a uniform, high and stable gain which, coupled with the beam integration techniques referred to above, yield reproducible oxygen isotope ratios on both conducting and insulating mineral standards (less-than-or-equal-to 1.2 parts per thousand on O-18/O-16 during a day and < 2 parts per thousand over periods of days and weeks).