Isotope ratio and isotope dilution measurements using axial inductively coupled plasma time of flight mass spectrometry

Performance of axial ICP-TOF-MS in terms of precision and mass bias for isotope ratio measurements and accuracy of isotope dilution was assessed by measurements of isotopic reference materials for Li, Mg, Rb, Pt and Pb. Because of simultaneous ion extraction from the plasma, remaining flicker noise is minimized resulting in improved precision for isotope ratios compared to sequential mass spectrometers. Experimental relative standard deviations of < 0.05% for isotope ratios were obtained at high signal levels measured in the analog detection mode. Effects on isotope ratios derived from changes of instrumental parameters, such as detector voltage and transverse rejection ion pulse settings, were evaluated. Isotope ratios for different concentrations changed when operating at too low detector voltages. An appropriate detector voltage setting is important but optimum voltages differ depending on the age and previous history of the detector. Results obtained for ID measurements of magnesium and rubidium amount content compared well with results from quadrupole ICP-MS and thermal ionization mass spectrometry. Mass bias per mass unit is high in the low mass range (13% for Li-6/Li-7) but around 0.2% in the high mass range. The long-term stability of ratios, measured discontinuously over several hours, is moderate; therefore measurements of an isotopic reference material at regular intervals is necessary to correct for small variations in mass bias over time.