MOLECULAR ION-BEAM METHOD OF ISOTOPIC ANALYSIS - EFFECT OF ERROR PROPAGATION, A CASE-STUDY WITH LI2BO2+

The isotopic analysis of boron has been carried out using different pairs of isotopic Li2BO2+ ions as monitors. The precision of the mass spectrometric measurements of different molecular abundance ratios is roughly the same. However, the values of the B-10/B-11 abundance ratio calculated from the experimental ratios of different pairs of isotopic molecular ions show large variations. These variations are shown to be a logical consequence of the calculation step which leads to the magnification of measurement errors to different extents for different peak ratios. Similar arguments would apply for the isotopic analysis of any element employing the molecular ion beam technique. The effect of isotopic fractionation on the molecular abundances is also discussed. The important implication of the present investigation is that it sets the guidelines for selecting suitable monitor molecules to avoid such errors.