Because two-isotope elements have no extra ratio to normalize in order to correct the instrumental mass fractionation effect, it has been difficult to measure their isotopic composition with high precision. We propose a new technique to overcome this by analyzing monooxide ions and normalizing to the oxygen isotopic ratios in order to correct the instrumental mass fractionation effect. Using LaO as an example, we were able to show that mass fractionation causes both (LaO/LaO)-La-139-O-18-La-138-O-16 and (LaO/LaO)-La-138-O-16-La-139-O-16 to vary almost linearly with (LaO/LaO)-La-139-O-18-La-139-O-16. By normalizing (LaO/LaO)-La-139-O-18-La-139-O-16 to O-18/O-16, we can detect the isotopic shift in La-138/La-139 from both (LaO/LaO)-La-139-O-18-La-138-O-16 and (LaO/LaO)-La-138-O-16-La139-O-16 with a 95 % confidence interval of +/-0.08 % while the uncorrected (LaO/LaO)-La-139-O-18-La-139-O-16 varies by more than +/-2 % and (LaO/LaO)-La-138-O-16-La-139-O-16 varies by +/-0.2 %. The ability to detect small isotopic shifts in two-isotope elements will have many applications for natural isotopic variation studies and artificial tracer work.