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 iso tope ratio measurements and accuracy of isotope dilution was assessed by me asurements of isotopic reference materials for Li, Mg, Rb, Pt and Pb. Becau se 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 o f < 0.05% for isotope ratios were obtained at high signal levels measured i n the analog detection mode. Effects on isotope ratios derived from changes of instrumental parameters, such as detector voltage and transverse reject ion ion pulse settings, were evaluated. Isotope ratios for different concen trations changed when operating at too low detector voltages. An appropriat e detector voltage setting is important but optimum voltages differ dependi ng on the age and previous history of the detector. Results obtained for ID measurements of magnesium and rubidium amount content compared well with r esults from quadrupole ICP-MS and thermal ionization mass spectrometry. Mas s 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, meas ured discontinuously over several hours, is moderate; therefore measurement s of an isotopic reference material at regular intervals is necessary to co rrect for small variations in mass bias over time.