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.