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

Operation of a helium-based low-power microwave plasma under "off-cone" sam pling mode can lead to significant background suppression without sacrifici ng the analyte signal intensity. The nearly clean background spectrum obtai ned with "off-cone" sampling provides a unique approach for isotope ratio m easurements, especially for low-mass elements that are often problematic. I n this work, calcium isotopes and isotope ratios are quantitatively measure d using "off-cone" sampling with microwave plasma time-of-flight mass spect rometry. The viability of measurements for all calcium isotopes, from the m ost abundant isotope (Ca-40) to the least abundant isotope (Ca-46), has bee n established for this technique. Several calcium isotope ratios are measur ed and calculated through both elemental calcium ion peaks and molecular ca lcium 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 isot opes. The absolute measurement errors are around 0.8% or less in most cases for major isotopes.