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Hafnium isotope analysis of mixed standard solutions by multi-collector inductively coupled plasma mass spectrometry: an evaluation of isobaric interference corrections
Journal of Analytical Science and Technology volume 4, Article number: 1 (2013)
The Lu-Hf isotope system is widely used to decipher the crustal evolution and mantle differentiation of the Earth. The most critical point in obtaining accurate Hf isotope data is to correct the isobaric interferences of Yb and Lu imposed on the 176Hf peak. In this study, we tested the validity of within-run correction protocol using MC-ICP-MS analysis of Hf standard solutions doped with Yb and Lu.
We found that the use of carefully selected Yb isotopic composition in the literature resulted in more reliable 176Hf/177Hf ratio. The 176Hf/177Hf ratios analyzed for a series of mixed Hf+Yb+Lu standard solutions could be quite accurately corrected for the mass bias and isobaric interferences. The systematic decreasing trend in the corrected 176Hf/177Hf ratios with increasing Yb/Hf ratios, however, indicates that the mass bias effect cannot be completely removed by the exponential law for samples high in Yb.
A close correlation of the calculated 176Yb/177Hf and 176Lu/177Hf ratios with the gravimetric values sheds light on the direct determination of inter-elemental isotope ratios without chemical purification.
Out of six naturally occurring isotopes of hafnium (174Hf, 176Hf, 177Hf, 178Hf, 179Hf and 180Hf), radiogenic 176Hf is produced by the β- decay of 176Lu with a half-life of 37.2 billion years in terrestrial samples (decay constant λ = 1.865 × 10-11 y-1) (Scherer et al. 2001). Hafnium is more incompatible than lutetium during partial melting of mantle peridotite and thus long-term enrichment of the former relative to the latter in the continental crust has yielded unradiogenic 176Hf/177Hf ratios compared with those in the depleted mantle (Patchett et al. 1981). In this respect, the Lu-Hf system has been effectively used to trace crustal evolution and mantle differentiation of the Earth since the early 1980s (Patchett et al. 1981; Patchett & Tatsumoto 1980; Patchett 1983). Early Lu-Hf works were majorly undertaken by thermal ionization mass spectrometry but recent advances in multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) have revolutionized the analysis of Lu-Hf isotopes, especially when combined with laser-ablation micro-sampling techniques (Thirlwall & Walder 1995; Griffin et al. 2000; Hawkesworth & Kemp 2006).
Accurate 176Hf/177Hf ratio is obtained only after the contribution of isobaric interferences by rare earth elements Yb and Lu on the 176Hf signal is carefully corrected (Woodhead et al. 2004; Iizuka & Hirata 2005). This is particularly important where hafnium purification is unavailable prior to sample introduction to the ion source, as in the case of laser ablation analysis. The present study tests the validity of isobaric interference correction at mass 176 by using MC-ICP-MS analysis of Hf standard solutions doped with Yb and Lu. As the precise values of Yb isotope ratios selected for the correction of mass bias and isobaric contribution critically concern the reliability of corrected 176Hf/177Hf ratio, previous reports on Yb isotopic abundances will also be evaluated.
In this study, Hf, Yb and Lu isotopic signals were measured by using a Neptune MC-ICP-MS installed at the Korea Basic Science Institute (KBSI) in Ochang. This instrument is a double focusing high-resolution ICP-MS equipped with eight motorized Faraday collectors and one fixed axial channel where ion beam intensities can be measured with either a Faraday collector or an ion counting electron multiplier. The gain calibration biases of the amplifiers are canceled out with the virtual amplifier design in which all Faraday collectors in a certain measurement are sequentially connected to all amplifiers. The Faraday collectors were statically set to simultaneously detect the required isotopes: 172Yb (low 4), 173Yb (low 3), 175Lu (low 2), 176(Yb+Lu+Hf) (low 1), 177Hf (axial), 178Hf (high 1), 179Hf (high 2) and 180Hf (high 3), respectively. The ion beam intensities were optimized by adjusting the torch position, gas flows and ion focus settings. The sensitivity on 180Hf was typically around 25 V/Hf ppm (10+11 Ωresistors) in a low resolution (ca. 400) mode. Details of the other operational parameters are summarized in Table 1.
Measurement of standard solutions
The basic instrumental capability of the KBSI Neptune MC-ICP-MS was tested by using a JMC 475 Hf standard solution with a concentration of 200 ng ml-1. The exponential law (Russel et al. 1978) was applied for mass bias correction using 179Hf/177Hf = 0.7325 (Patchett et al. 1981). One run consists of 20 cycles, in which one cycle has an integration time of 4.194 s. The average 176Hf/177Hf ratio (0.282167±0.000005, n=5, 2σ S. E.) agrees well with previous recommended values (Blichert-Toft et al. 1997; Nowell et al. 1998; Vervoort & Blichert-Toft 1999) (Table 2). A range of shorter integration times (0.161, 0.262, 0.524 s) were tried with one block of 30 cycles (n=3). All results of 176Hf/177Hf ratio are quite reproducible and accurate (Figure 1) and thus it is concluded that the isotopic composition of a small quantity of hafnium (< 20 ng) could be analyzed with reasonable precision and accuracy in a short (< 1 minute) measurement time.
We also measured Hf isotope ratios of in-house standard solution JMC 14375, delivered from Alfa Aesar of Johnson Matthey Company (stock no. 14375, lot no. 83-084740F, plasma standard solution). The 176Hf/177Hf ratio of this standard solution (300 ng ml-1 Hf), measured with the same analytical design as that for the measurement of JMC 475 standard solution (20 cycles, 4.194 s integration) gave an average of 0.282228±0.000005 (n=10, 2σ S. E.) (Table 3).
Correction for isobaric interferences
Several analytical strategies were suggested to correct the isobaric interferences by Yb and Lu on 176Hf: (1) Yb is doped with Hf isotope standard solution, and then use revised Yb isotopic compositions that give correct 176Hf/177Hf ratio (Thirlwall & Walder 1995; Griffin et al. 2000), (2) Determine the relationship between the Hf and Yb mass bias factors (Chu et al. 2002), (3) Yb mass bias factor is directly obtained from the Yb isotope ratios simultaneously measured with the Hf analysis (Woodhead et al. 2004; Iizuka & Hirata 2005). The last protocol would be the most effective unless Yb signal intensities are so low that precise isotope ratios are unavailable, considering that the mass bias factor is not a constant value during the MC-ICP-MS measurement (Woodhead et al. 2004; Iizuka & Hirata 2005). In this study, the isobaric interferences of 176Lu and 176Yb on 176Hf were directly estimated by monitoring the intensities of interference-free Lu and Yb signals as the following:
where β(Lu) and β(Yb) are respective exponential mass bias factors for Lu and Yb, and “M” denotes the mass of the isotope. The β(Hf) and β(Yb) values were measured by monitoring 179Hf/177Hf and 172Yb/173Yb ratios for a mixed standard solution of which concentrations were 298.7 ng ml-1 for JMC 14375 Hf, 30.4 ng ml-1 for Accu-Trace Yb (lot no. B4035064-2B, reference standard) and 3.0 ng ml-1 for Accu-Trace Lu (lot no. B8045141, reference standard). For the calculation of β(Yb) and isobaric interference correction, an accurate Yb isotopic composition is needed but previous reports are not uniform (Chu et al. 2002; McCulloch et al. 1977; Segal et al. 2003; Thirlwall & Anczkiewicz 2004; Vervoort et al. 2004). The 176Hf/177Hf ratio of the mixed standard solution was calculated using different sets of Yb isotope ratios as the followings.
As depicted in Figure 2, the results of 11 measurements (20 cycles, integration time=4.194 s) indicate that reports of Yb isotope ratios in (Chu et al. 2002; McCulloch et al. 1977) yielded incorrectly high 176Hf/177Hf ratios. Comparable 176Hf/177Hf ratios with that of unspiked JMC 14375 Hf (0.282228±0.000005) could be obtained by using Yb isotope ratios in (Segal et al. 2003; Thirlwall & Anczkiewicz 2004; Vervoort et al. 2004), and thus we hereafter give 1.35823 and 0.78696 as the (172Yb/173Yb)true and (176Yb/173Yb)true values, respectively (Thirlwall & Anczkiewicz 2004) for correcting mass fractionation of Yb and calculating its isobaric contribution to 176Hf. Internal normalization of mass fractionation is not available for Lu, because it has only two natural isotopes (175Lu and 176Lu). In this study, the β(Lu) is assumed to be identical to the β(Hf), and (176Lu/175Lu)true of 0.026549 (Chu et al. 2002) is employed to calculate the signal intensities of 176Lu. Possible difference between the β(Lu) and β(Hf) values does not affect the corrected Hf isotope ratio significantly because the contribution of 176Lu to 176Hf is typically very small in the crustal materials (ca. 1%, (Rudnick & Fountain 1995)). The β(Yb) value of each cycle is plotted against the β(Hf) value in Figure 3. This diagram confirms that the two values are not identical, and should be measured independently during the run. They are positively correlated with each other but a distinct regression line is not identified.
We further tested the validity of isobaric interference correction described above by using Hf+Yb+Lu solutions mixed with different elemental proportions (Hf = 300 ng ml-1 JMC 14375, Hf:Yb:Lu ≈ 200:10:1, 100:10:1, 50:10:1, 30:9:1). The results with 10 blocks of 20 cycles (integration time = 4.194 s) (Table 4) show that the correction protocol works pretty well. There is, however, a systematic decreasing trend in the corrected 176Hf/177Hf ratio with increasing Yb/Hf ratios, indicating that mass bias is not perfectly corrected by the exponential law for samples high in Yb. The 176Yb/177Hf and 176Lu/177Hf ratios are calculated as the followings (Iizuka & Hirata 2005):
The calculated ratios are not identical to the gravimetric values (Table 4) due to differences in elemental sensitivity but the two values are quite perfectly correlated with each other ((176Lu/177Hf)calculated = 1.277 × (176Lu/177Hf)gravimetric; (176Yb/177Hf)calculated = 1.327 × (176Yb/177Hf)gravimetric, R2 > 0.98), leaving a possibility that these inter-elemental isotope ratios can be accurately measured directly from the sample solution without chemical purification.
We tested the capability of a Neptune MC-ICP-MS in obtaining accurate Hf isotope ratios of the mixed Hf+Yb+Lu standard solution. Careful selection of Yb isotope compositions was essential for the correction of mass bias and isobaric interferences from Yb and Lu on the 176Hf peak. The validity of within-run correction protocol described here was confirmed by analyzing a series of mixed standard solutions, although the systematic decreasing trend in the corrected 176Hf/177Hf ratio with increasing Yb/Hf ratios indicated that mass bias was not completely corrected by the exponential law for samples high in Yb. A quite perfect correlation of the calculated 176Yb/177Hf and 176Lu/177Hf ratios with the gravimetric values leaves a probability to determine the inter-elemental isotope ratios directly from the sample solution without chemical separation.
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This study was supported by Korea Basic Science Institute grants (G32221, G32210 and C32710). Valuable comments of two anonymous reviewers are acknowledged.
The authors declare that they have no competing interest.
CSC, JK and HSS designed the study. MSC prepared the sample solutions and carried out isotope measurements. CSC drafted the manuscript. All authors read and approved the final manuscript.
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Choi, M.S., Cheong, C., Kim, J. et al. Hafnium isotope analysis of mixed standard solutions by multi-collector inductively coupled plasma mass spectrometry: an evaluation of isobaric interference corrections. J Anal Sci Technol 4, 1 (2013) doi:10.1186/2093-3371-4-1
- Isobaric interference
- Mass bias