Precision and accuracy in isotope ratio measurements by plasma source mass spectrometry

Abstract

Precise and accurate isotope ratio measurements are an important task in many applications such as isotope dilution mass spectrometry, bioavailability studies or the determination of isotope variations in geological and cosmic samples. There is much more interest in ICP-MS for isotope ratio determinations at present compared with GDMS, which is preferred for the direct measurement of the isotopic composition of metallic solid samples. Spectroscopic interferences and a limited abundance sensitivity can influence the accuracy of isotope ratio determinations by GDMS and ICP-MS. In addition, in ICP-MS the space charge effect always influences the accuracy and a nozzle separation effect may also contribute to the total mass bias. Using a quadrupole ICP-MS the mass discrimination per mass unit can be >10% for elements with mass numbers <10, about 1–5% for mass numbers in the range of 20–120 and only <1% for heavier elements. Mass discrimination is strongly dependent on the potential of the different lenses of the ion optics and on the nebulizer gas flow. Even in magnetic sector field ICP-MS instruments a distinct mass discrimination is observed. Different procedures such as calibration by substances of consistent natural isotopic composition of the same or a neighbouring element and by isotopic standard reference materials, respectively, in combination with various mathematical functions, linear and exponential ones, are used for mass bias correction. In addition to the ion counting statistics, stability of the ion current is one of the most important topics which influences the precision of isotope ratio determinations. Magnetic sector field instruments, producing flat topped peak shapes, coupled with a multi-collector system for simultaneous measurement of different isotopes achieve the best relative standard deviation in the range of typically 0.005–0.02%, which is only comparable with precisions obtained by TIMS. However, ICP-TOFMS also has the potential for similar results. Typical relative standard deviations for other types of plasma source mass spectrometers for isotope ratio determinations are as follows: GDMS 0.1–1, quadrupole ICP-MS 0.1–0.5, and high resolution ICP-MS 0.05–0.2%.