Accurate isotope ratio measurements of ytterbium by multiple collection inductively coupled plasma mass spectrometry applying erbium and hafnium in an improved double external normalization procedure

Abstract

Four series of experiments had been carried out for correct ytterbium isotope ratios measurements (IRM). (1) For erbium IRM, ¹⁶⁷Er/¹⁶⁶Er = 0.68260, derived from recently published thermal ionization mass spectrometry (TIMS) measurements, was applied as an internal normalization factor. (2) Ytterbium has no calibrated isotope ratio values. In an erbium–ytterbium–hafnium mixture solution we determined (¹⁷¹Yb/¹⁷²Yb)_Er-corr = 0.653436(4) using the above ¹⁶⁷Er/¹⁶⁶Er ratio as external normalization factor. In the same mixture we also determined (¹⁷¹Yb/¹⁷²Yb)_Hf-corr = 0.653297(25) using the well-known IUPAC value for the ratio ¹⁷⁹Hf/¹⁷⁷Hf = 0.73250 as external normalization factor. In these measurements no atomic isobaric interference exists at the corresponding mass numbers. (3) These two ¹⁷¹Yb/¹⁷²Yb ratios, derived with data at mass intervals 166–167 and ∼178, were interpolated to mass interval 171–172, applying linear and slightly curved (exponential) line interpolations. ¹⁷¹Yb/¹⁷²Yb = 0.653367 and 0.653344 are the linear and curved line interpolated values, respectively. Then, all ytterbium isotope ratios were measured in pure ytterbium solutions and internally corrected with each of the two ¹⁷¹Yb/¹⁷²Yb ratios. (4) Using the literature value for ¹⁷⁸Hf/¹⁷⁷Hf = 1.467290, we obtained ¹⁷⁹Hf/¹⁷⁷Hf = 0.73249(2), which is in very good agreement with the above mentioned IUPAC value. The exponential correction equation was applied in this work. The mean Er ratio values are in good agreement with literature, except for the ¹⁶²Er/¹⁶⁶Er ratio, which is 2.19% lower than the reported absolute TIMS value. The mean Yb ratio values obtained in this work are in fair agreement with data reported in the last 25 years. We believe that they are the closest to the true values. The precision (2σ) is at least 3–8 times better comparing to the measured TIMS precision.