Sm-Nd isotopic analysis of mixed standard solutions by multi-collector inductively coupled plasma mass spectrometry: evaluations on isobaric interference correction of Nd isotopic composition and external calibration of Sm/Nd ratio
© Ryu et al.; licensee Springer. 2013
Received: 12 March 2013
Accepted: 12 March 2013
Published: 18 April 2013
The Sm-Nd isotope system has long been used to provide information on the age and geochemical evolution of terrestrial rocks and extraterrestrial objects. Traditional thermal ionization mass spectrometry requires a refined chemical separation of Sm and Nd. Here, we present multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) Sm-Nd isotopic results for a series of mixed standard solutions with different Sm/Nd ratios to test the validity of isobaric interference corrections of Nd isotopic composition and external calibration of Sm/Nd inter-elemental ratio.
Reliable 143Nd/144Nd and 145Nd/144Nd ratios of the mixed solutions were obtained by using the exponential law and selected Sm isotopic compositions. The Sm/Nd ratios of the mixed solutions corrected by the standard bracketing method were consistent with the gravimetric values mostly within 1% difference.
This study provides a simple and high-throughput technique that can simultaneously measure Nd isotopic composition and Sm/Nd ratio without chemical separation between Sm and Nd.
Sm and Nd are rare earth elements presenting in only small amounts in most rock-forming minerals. Sm and Nd each have seven naturally occurring isotopes (144Sm, 147Sm, 148Sm, 149Sm, 150Sm, 152Sm, 154Sm; 142Nd, 143Nd, 144Nd, 145Nd, 146Nd, 148Nd, 150Nd). One isotope of Sm (147Sm) decays by α-emission to one isotope of Nd (143Nd) with a half-life of 106 Ga (Lugmair & Marti 1978; Begemann et al. 2001). The Sm-Nd decay system has been efficiently used for determining the timing of major events occurred during the chemical evolution of planets and probing into the earth's interior (DePaolo 1988). The application of this system demands highly accurate Sm-Nd isotope data because its half-life is very long and natural variations in Sm/Nd inter-elemental ratio are typically quite limited.
Although the traditional thermal ionization mass spectrometry (TIMS) is still regarded as the benchmark technique for Sm-Nd isotopic measurement (Chu et al. 2009; Harvey & Baxter 2009; Ali & Srinivasan 2011), more recent multi collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) has also become a routine technique with high sample throughput and comparable precision to TIMS (Walder et al. 1993; Vance & Thirlwall 2002; Yang et al. 2010; Yang et al. 2011). The classic Sm-Nd isotope analysis requires a separation of two elements from the sample matrix by refined chemical procedures. Recently, however, it was reported that the 143Nd/144Nd ratio of geological samples could be measured accurately by MC-ICP-MS without Sm and Nd separation (Yang et al. 2010). This study further evaluates the validity of Nd isotopic and Sm/Nd elemental ratio measurements for a series of Sm + Nd mixed standard solutions by MC-ICP-MS technique, and revisited various sets of reported Sm isotopic composition.
Instrumental setting and operational parameters
RF forward power
RF reflected power
< 2 W
0.75 - 0.80 L/min
0.985 - 0.990 L/min
Quartz dual cyclonic
ESI PFA MicroFlow
Sample uptake rate
Mass analyzer pressure
2.9 × 10-9 mbar
Measurement of Nd standard solutions
The basic performance of KBSI Neptune was tested by using the JNdi standard solution with Nd concentration of 100 μg/L. The mass bias was exponentially normalized to 146Nd/144Nd = 0.7219. One measurement consists of 9 blocks of 10 cycles with an integration time of 4.194 s. The average 143Nd/144Nd ratio was 0.512100 ± 0.000004 (n = 10, 2σ S.E.), in reasonable agreement with the recommended value (0.512115 ± 0.000007) (Tanaka et al. 2000).
The in-house Nd standard solution of 100 μg/L was prepared from the AccuTrace™ Nd Reference Standard (lot no. B9035110, plasma emission standard). It yielded an average 143Nd/144Nd ratio of 0.512204 ± 0.000005 (n = 9, 2σ S.E.) with the same analytical design as above (9 blocks of 10 cycles with an integration time of 4.194 s). Two diluted in-house Nd solutions of 50 and 10 μg/L also yielded comparable results of 143Nd/144Nd = 0.512212 ± 0.000006 (n = 5, 2σ S.E.) and 0.512207 ± 0.000004 (n = 5, 2σ S.E.), respectively.
Isobaric interference correction
where M denotes the mass of the isotope.
Finally, the 144Sm-corrected 143Nd/144Nd ratio was exponentially normalized to 146Nd/144Nd = 0.7219.
Sm isotopic compositions were iteratively solved to minimize the residual sum of squares between corrected 143Nd/144Nd ratio of Sm-doped Nd standard solution and the unspiked value using the Excel Solver. The result suggests that the ratios of 147Sm to 149Sm and 144Sm to 149Sm would be 1.0844 and 0.22233, respectively. Because these values are the closest to the recommended values in (Wasserburg et al. 1981), we hereafter use 1.0851 and 0.22249 as the (147Sm/149Sm)true and (144Sm/149Sm)true ratios for correcting mass fractionation of Sm and calculating its isobaric contribution to 144Nd.
Nd isotope ratios of the Sm-doped Nd standard solutions with the Sm/Nd ratio of 0.2
100 μg/L Nd
200 μg/L Nd
50 μg/L Nd
Sm-Nd isotopic data of the Sm-doped Nd standard solutions of 100 μg/L Nd
Calibration of Sm/Nd ratio
The instrumental mass bias on isotope measurements could be corrected by using either double-spike or standard bracketing methods. The latter method consists in interpolating the mass bias of an unknown sample between the biases inferred from two standard runs, one preceding and one following the sample analysis (Albarede & Beard 2004).
In order to correct the inter-elemental mass bias on the measurement, this study considered the Sm-doped Nd standard solution with the Sm/Nd of 0.2 as the bracketing standard and various mixed solutions with different Sm/Nd values (Sm/Nd = ca. 0.1, 0.3, 0.4, and 0.5) as unknown samples. During the measurements, the average correlation factor in the instrumental mass bias inferred from the standard runs was 0.943 ± 0.002 (n = 7, 2σ S.E.), which yielded consistent Sm/Nd ratios with gravimetric Sm/Nd values mostly within 1% difference (Table 3; Figure 3). This result indicates that the mass bias in the measurement of the Sm/Nd ratio can be reasonably corrected by the standard bracketing method.
The Nd concentrations of the mixed solutions were calculated based on the intensity of the bracketing standard of 100-101 μg/L. Calculated Nd concentrations were consistent with gravimetric values within 3% difference (Table 3).
We evaluated the capability of a Neptune MC-ICP-MS to obtain accurate Nd isotopic composition and Sm/Nd elemental ratio using a series of Sm + Nd mixed standard solutions with different Sm/Nd ratios. The isobaric interference correction using the exponential law and selected Sm isotopic composition yielded accurate 143Nd/144Nd and 145Nd/144Nd ratios for the mixed solutions, although there is a systematic increasing trend in the corrected 143Nd/144Nd ratios with increasing Sm/Nd ratios. The Sm/Nd ratios of the Sm-doped Nd standard solutions could be reliably calibrated by the standard bracketing method mostly within 1% difference from the gravimetric values. These results indicate that accurate Nd isotopic composition and Sm/Nd ratio can be simultaneously measured by a simple and high-throughput technique without chemical separation of Sm and Nd.
This study was supported by the KBSI grants (G33200 and C33710).
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