Critical comparison between quadrupole and time-of-flight inductively coupled plasma mass spectrometers for isotope ratio measurements in elemental speciation

Abstract

An evaluation and critical comparison of the performance of a reference sequential mass analyzer (quadrupole) and a ICP-TOFMS instrument for ICP-MS multielemental analysis in different types of transient signals, mimicking those encountered in speciation analysis, is described. Several parameters including integration time, the number of isotopes “simultaneously” monitored, detection limits, precision and accuracy attainable for multiisotopic ratios measurements in transient signals are critically evaluated. Multielemental measurements in typical speciation analysis transient signals of different widths at half peak heights: 26 s (similar to those obtained with HPLC separations); 8 s (alike to those obtained in fast HPLC separations); and 3.5 s (mimicking those obtained in CE separations) have been performed. As expected, the analytical performance is bound to the concentration levels and number of isotopes to be measured. The best precisions were achieved using a TOF mass analyzer (≤ 1%) for more than 15 isotopes at concentration levels over 10 ppb to be measured in transient signals of less than 5 s. However, when fewer isotopes had to be measured (less than 7), even for very short transient signals (3.5 s of fwhm), the measured precision was always similar or even better using a quadrupole mass analyzer. The detection limits obtained (even for very short integration times) were always favourable to the quadrupole ICP-MS, while ICP-TOFMS demonstrated similar or better accuracy for isotope ratio measurements. Finally, the comparative speciation analysis of a mixture of organometallics of mercury, lead and tin by GC-ICP-MS (0.7s fwhm) is also investigated and applications to real speciation problems are discussed.