Thermal ionisation mass spectrometry of Li 2BO +2 ions: determination of the isotopic abundance ratio of lithium

Abstract

Thermal ionisation mass spectrometric investigations of Li 2BO + 2 ions have been carried out. The central idea is to study the feasibility of making an accurate isotopic analysis of lithium employing an experimentally measured intensity ratio(s) of Li 2BO + 2 ions of different specific charges ( m/ z). This paper presents the results along with a discussion on the merits and demerits of the method. The studies show that the intensity ratios of Li 2BO + 2 ions are definitely changing with time owing to a certain amount of fractionation of lithium isotopes, the extent of which could, however, be controlled, leading to the determination of the 6Li/ 7Li abundance ratio with a reasonable accuracy without applying any correction factor. Factors affecting the accuracy of this molecular ion beam method of isotopic analysis are discussed. It is shown that the accuracy is governed by error propagation rather than by the achievable precision and accuracy of mass spectrometric measurements. This particular point is illustrated with examples of varying isotopic abundance patterns of lithium and boron. As a logical consequence, it is pointed out that a mere selection of the most abundant Li 2BO 2 molecules as monitors for mass spectrometric measurements may not always ensure accurate isotopic analysis of a constituent element.