Measurements of calcium isotopes and isotope ratios: a new method based on helium plasma source "off-cone" sampling time-of-flight mass spectrometry

Operation of a helium-based low-power microwave plasma under "off-cone" sampling mode can lead to significant background suppression without sacrificing the analyte signal intensity. The nearly clean background spectrum obtained with "off-cone" sampling provides a unique approach for isotope ratio measurements, especially for low-mass elements that are often problematic. In this work, calcium isotopes and isotope ratios are quantitatively measured using "off-cone" sampling with microwave plasma time-of-flight mass spectrometry. The viability of measurements for all calcium isotopes, from the most abundant isotope (Ca-40) to the least abundant isotope (Ca-46), has been established for this technique. Several calcium isotope ratios are measured and calculated through both elemental calcium ion peaks and molecular calcium ion peaks. The relative standard deviations (RSD) for isotope ratios measured are in a range of 1.2 to 2.5% for commonly used major calcium isotopes. The absolute measurement errors are around 0.8% or less in most cases for major isotopes.