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A new efficient method for the separation of Ba from REEs for the accurate determination of REE content in Ba-rich samples by ICP-MS.

A cation-exchange resin is commonly used to separate Sr from geological and environmental samples. AG50 resin is the most traditional cation-exchange resin and has the advantages of good reusability and low cost. Although AG50 resin can meet the demands of most conventional geological samples, eliminating 87Rb isobaric interference from samples with high Rb/Sr ratios (>30) is difficult during one-step separation. Separating Sr twice using AG50 resin is the general approach to Rb removal. The two-step separation method is time consuming and leads to a low Sr yield. This study aimed to improve the sample throughput and overcome the problem of 87Rb isobaric interference from imperfect separation. A new separation method involving hydrofluoric acid (HF acid) coprecipitation combined with a cation-exchange resin (i.e. AG50 resin) for samples with high Rb/Sr ratios was developed. HF acid was used as a coprecipitator to remove most Rb (>90%) and recover Sr before performing AG50 column chemistry. The reliability of this method was examined using the rhyolite standard JR-2, a typical sample with a high Rb/Sr ratio (Rb/Sr = 37.36). Results in the JR-2 case showed that approximately 92% of Rb and most of the matrix elements (K, Ti, Fe, Al, Mn and Na) could be removed effectively. Moreover, Sr recovery exceeded 91% during HF acid coprecipitation. After the removal of Rb by HF acid coprecipitation, the Sr fraction with high purity was easily obtained through one-step separation using the AG50 resin column. However, we found that the removal rates of Rb and some matrix elements (such as Al and Na) were strongly affected by the composition of Al–Ca–Mg in samples. Our separation method is more suitable for samples with high Rb/Sr ratios and low Ca and Mg contents. A series of silicate rock reference materials were selected to evaluate the applicability of our method, and the Sr isotopic results were consistent with previously reported values. Overall, this rapid, simple and low-cost method shows great potential for the separation of Sr from geological samples with high Rb/Sr ratios, such as alkaline feldspar granite, alkaline granite and alkaline rhyolite.

This study describes a simple and low-cost method that requires small samples (

Nd can be ionized to NdO+ about 5–10 fold more efficiently than to Nd+ by thermal ionization mass spectrometry (TIMS). However, with the NdO+ method, Ce and Pr must be separated from Nd as completely as possible to reduce CeO+ and PrO+ isobaric interferences on NdO+ (e.g.141Pr18O+ on 143Nd16O+). With previously reported methods, it is difficult to achieve an efficient Pr–Nd separation although a perfect Sm–Nd separation can be achieved. In this study, a rapid, single column scheme for the separation of Nd and Sm from geological samples, based on TODGA resins (1 mL resin bed volume, 50–100 μm particle size), was developed. The total column separation time, including column cleaning steps, was about 8 h. With this method, not only Nd and Sm could be efficiently separated from the sample matrix, but also a perfect separation of Nd from Ce, Pr and Sm could be obtained. The recovery yield was greater than 90% for Nd and 95% for Sm. Furthermore, since only a single TODGA column separation step was involved, low procedural blanks (Sm < 5 pg; Nd < 10 pg) could be achieved. Therefore, the method is particularly useful for the separation of low levels of Nd from geological samples for isotope analysis as NdO+ by TIMS. Finally, several rock reference materials, including BCR-2, BHVO-2, BIR-1a and UB-N, with different column loads, were analyzed to verify the reliability of the chemical procedure, and the analytical results show good agreement with the recommended values for the 143Nd/144Nd isotopic ratios and Sm and Nd concentrations.

This study has led to a new technique for the separation of Cr from small silicate samples (0.3–2 mg) and determination of stable Cr isotopes in silicates by double spike thermal ionization mass spectrometry (DS-TIMS). Based on our recent two-step exchange resin scheme (Li et al., JAAS, 2016, 31, 2375), we further significantly reduced the procedural blank and improved Cr recovery (94.7–97.5%) from a range of complex silicate rocks varying in composition from andesite to ultra-mafic, and carbonates as well. The key improvement is that KMnO4 was employed to replace conventional (NH4)2S2O8 as the oxidizing reagent, which reduces the procedural blank from 1–20 ng in previous studies to 0.15 ± 0.04 ng (n = 8). In addition, the inhibition effect on the Cr signal caused by residual SO42− in the Cr fraction using the conventional method has been completely eliminated during TIMS measurements. This permits the high precision determination of Cr isotopic ratios from geological samples as small as ∼46 ng. This chemical procedure was verified using a variety of silicate rocks in the size range of 0.3–2 mg. A series of analyses demonstrated that it is possible to attain internal precisions (2 SE) of ±0.03 to 0.06‰. Replicate digestions and analyses of basalt standard BIR-1 with a sample size of 0.3 mg (δ53Cr = −0.158 ± 0.052‰, 2 SD) demonstrated that good intermediate precision is obtainable for extremely small silicate samples.

Chromium isotopes have important applications in cosmochemistry, environmental science, geochemistry, and paleoceanography studies. In previous investigations, thermal ionization mass spectrometry (TIMS) showed excellent precision for Cr isotopic ratios analysis. However, relative large sample sizes (200–2000 ng) were required for TIMS measurements and therefore hampered its wider application in geoscience and environmental science. In this study, a highly sensitive Nb2O5 emitter is employed for the first time to measure Cr isotopes using TIMS. This Nb2O5 emitter produces at least a 10-fold enhancement in the sensitivity and can not only significantly reduce the sample size, but also obtain good external precision. In this study, an external precision of ±0.23‰ (2 SD, n = 10) on δ53Cr was obtained on a 5 ng sample of NIST SRM 3112a standard. In addition, an improved two-column chemical separation procedure was devised in order to separate Cr from complex geological materials. Compared to the classical anion resin purification method, the procedural blank and severe inhibition effect from SO42− is significantly reduced by carefully optimizing and evaluating the dosage of (NH4)2S2O8 oxidant. This method has been verified using one dolomite, three basalt and one peridotite reference materials. A range of analytical tests demonstrates that it is possible to attain internal precisions (2 SE) of ±0.017 to 0.037‰ on a single 30 min analysis. Long-term reproducibility (2 SD) for 53Cr/52Cr is 0.113454 ± 0.000004 (∼0.035‰) obtained by replicate analyses (n = 105) of NIST SRM 3112a over a 6 month period. Replicate digestions and analyses of the basalt standard BIR-1 (δ53Cr = −0.110 ± 0.048‰, 2 SD) and the carbonate standard JDo-1 (δ53Cr = 1.710 ± 0.054‰, 2 SD) demonstrate that good external reproducibility is obtainable on natural samples.

A straightforward tandem column separation procedure is presented for the separation of Sr, Nd, Pb, and Hf from silicate materials. It allows rapid purification, without any intervening evaporation, of these four elements of great interest in Earth science and cosmochemistry. After sample loading, the upper Sr Spec column adsorbs Sr and Pb, while the lower TODGA Spec column adsorbs Hf and Nd. Strontium-lead and hafnium–neodymium elements are then back-extracted from the Sr Spec and TODGA Spec columns, respectively. The whole separation procedure, including column setup, cleaning, and pre-conditioning, takes approximately eight hours for separating a batch of 25 samples. The proposed procedure offers significant improvement in separation efficiency of these often-used four elements, compared with conventional four column methods. Fractions of Sr, Nd and Pb are then measured by TIMS and the Hf fraction is determined by MC-ICP-MS. The stability of this procedure was demonstrated by replicate measurements of 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotope ratios of eight international silicate rock reference materials, spanning a wide range of bulk compositions. The analytical results obtained in this study agree well with published data. The external reproducibility (2RSD, n = 8) of standard BCR-2 was ±0.0026% for 87Sr/86Sr, ±0.0020% for 143Nd/144Nd, ±0.0049% for 176Hf/177Hf, and ±0.026–0.034% for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotope ratios.

Thermal ionization mass spectrometry (TIMS) is considered the most accurate technique for determining Sr–Nd–Pb isotopic ratios in geological samples. However, time-consuming and complex sample separation procedures greatly hinder the instrumental measurement efficiency. In this study, a single-column separation chemistry procedure for Sr–Nd–Pb from single rock dissolution was developed. The chemistry procedure was designed to minimize the number of evaporation steps and considerably shorten the separation time, enabling high throughput for TIMS. In contrast to conventional three-column separation procedures (∼3 days), this technique was characterized by high efficiency superiority in terms of separation time (∼8 hours), a 3-fold enhancement in the separation efficiency. The stability of our procedure was demonstrated by replicated TIMS measurements of 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios for six international silicate rock reference materials, spanning a wide range of bulk compositions. The analytical results obtained for these standards agreed well with published data. The external reproducibility (2 RSD, n = 10) of a BCR-2 standard sample was ±0.0020% for 87Sr/86Sr, ±0.0023% for 143Nd/144Nd, and ±0.021–0.033% for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios.

In view of the low initial abundance of 146Sm, 142Nd anomalies are expected to be extremely small (less than 40 ppm), and their detection requires ultra-precise 142Nd/144Nd measurements. A rapid solid-phase micro-extraction (SPME) technique, using HEHEHP resin as sorbent, is established to completely separate Ce from rare earth element (REE) mixtures. This technique is applied to ultra-high-precision 142Nd/144Nd measurements in geological materials. In contrast to the traditional liquid–liquid micro-extraction (LLME) technique, the benefits of the SPME tandem column are high Nd recovery, low residual Ce (Ce/Nd 3.0. Thus, 142Ce interferences on 142Nd never exceed 1.3 ppm. Ultra-high-precision thermal ionization mass spectrometry analyses of silicate standards show that the internal precision of all runs are better than 4 ppm (2 RSE) for 142Nd/144Nd values. 142Nd/144Nd values for JNdi-1, JR-3, and BCR-2 have external precisions of ±4.8, ±4.4, and ±3.9 ppm (2 RSD), respectively. The external reproducibility is sufficient to distinguish and resolve 5 ppm anomalies in 142Nd/144Nd values.

Thermal ionization mass spectrometry and multiple-collector inductively coupled plasma mass spectrometry are considered to be “gold standards” for the determination of the isotope ratios of Sr–Nd and Hf in geological samples because of the extremely high precision and accuracy of these methods. However, the sample throughputs are hindered by time-consuming and tedious chemical procedures. Three-step ion exchange resin separation is traditionally employed to purify Sr–Nd–Hf from matrix elements. In this study, a one-step Sr–Nd–Hf separation scheme was developed to process geological samples. The separation scheme is based on the combined use of conventional AG50W-X12 cation-exchange resin and LN Spec extraction chromatographic material without any intervening evaporation step. The protocol not only prevents cross-contamination during operation using multiple-stage ion exchange resins but also significantly improves the efficiency of sample preparation. The stability of our chemical procedure was demonstrated by replicate measurements of 87Sr/86Sr, 143Nd/144Nd, and 176Hf/177Hf ratios in six international reference materials of silicate rocks. The analytical results obtained for these standard rocks compare well with the published data. The external reproducibility (2 SD, n = 10) of a BCR-2 standard sample was ±0.000018 for 87Sr/86Sr, ±0.000010 for 143Nd/144Nd, and ±0.000014 for 176Hf/177Hf.

Sm–Nd isotopic system is a powerful tracer and dating tool in geochemistry and cosmochemistry. Thermal ionization mass spectrometry (TIMS) is the benchmark method for obtaining high precision Nd isotope ratios. Traditionally, a two-step separation was employed in order to obtain high purity Nd fraction, which makes the sample preparation time-consuming. In this study, we present a new method to directly obtain precise and accurate Nd isotope ratios of geological samples without Sm–Nd separation. The key point is to well correct isobaric interferences of 144Sm on 144Nd for this method. The accurate intensity of 144Sm can be obtained using 147Sm/144Sm ratio of 4.87090 and mass discrimination factor of Sm (βSm), which is achieved using 147Sm/149Sm ratio of 1.08583 with Russell equation. Consequently, the real intensity of 144Nd was achieved, then also raw 146Nd/144Nd, 143Nd/144Nd ratios. Finally, 143Nd/144Nd ratio is normalized using 146Nd/144Nd ratio of 0.7219 following exponential law. The accuracy of this method is validated by replicating TIMS measurements for thirteen international reference materials of silicate rock, spanning a wide range of Sm/Nd from 0.13 to 0.46 and bulk compositions. The measured 143Nd/144Nd ratios of the rock standards are in good agreement with the reported values of within error of ±0.004%. Our new method shortens the analytical procedure and significantly improves sample throughput which greatly contributes to those studies requiring a large dataset and quick analysis.