Evaluation of the isotope ratio performance of an axial time-of-flight ICP mass spectrometer

The isotope ratio performance of an axial time-of-flight ICP mass spectrometer (Renaissance TOF-ICPMS, LECO Corp.) was evaluated. The isotope ratio precision, expressed as the relative standard deviation (RSD) for 10 successive measurements, was evaluated using multielement standard solutions with analyte concentrations of 50-500 mu g/L. The influence of the acquisition time per replicate measurement was studied by varying it between 0.5 and 300 s. For an acquisition time of 30 s per replicate and an elemental concentration of 500 mu g/L, typical isotope ratio precisions of less than or equal to 0.05% RSD were obtained. The fact that this isotope ratio precision can be obtained for many ratios simultaneously is an especially attractive feature of TOF-ICPMS, In contrast to what was expected, increasing the acquisition time per replicate to values of > 30 s resulted in a slightly deteriorated isotope ratio precision. At short acquisition times (< 10 s), isotope ratio precisions similar to, or better than, the best values ever reported for quadrupole-based instruments were obtained. The latter observation remained valid when working with transient signals of corresponding duration. Mass discrimination was observed to be analogous to that observed with other types of ICPMS instrumentation (similar to 1% per mass unit at midmass), The accuracy attainable was evaluated by comparing Pb isotopic results for a "natural" Pb standard solution obtained by TOF-ICPMS with those obtained by thermal ionization mass spectrometry.