| Popis: |
Fourier transform ion cyclotron resonance mass spectrometry has undergone significant advances in development of instrumentation, excitation, and detection modes since its introduction over 15 years ago. Relatively little effort has been focused, however, on development of quantitative relative isotopic abundance measurements. This paper attempts to provide an overview of the major experimental parameters which affect the accuracy and precision of isotope ratio measurements for both electron ionization (EI) of a gas phase and laser ionization (LI) of solids, and summarizes currently attainable levels of precision. Ion number dependent damping (controlled by ion density in the analysis cell), excite signal power, laser induced fractionation at the sample surface, and variations in oxide/ion ratios and singly- to doubly-charged ions may all affect both the absolute value and precision of measured isotope ratios. More precise measurements result from increasing the number of time domain transients summed prior to the Fourier transformation, optimizing the number of ions in the analysis cell such that the signal-to-noise ratio is high but space charge and magnetron effects are avoided, and including peak heights of oxide species and doubly-charged ions when present. Isotope ratios for EI experiments on Kr indicate that, under controlled conditions, precisions of better than ±0.4% are attainable. Precision for LI experiments is significantly lower at ±1–6% for abundant elements and ±9–12% for trace elements. Decreased precision for LI is likely due to fluctuations in ion density from shot to shot, large spreads in kinetic energies, and mass dependent fractionation at the sample surface. |
