Ultratrace Platinum Group Elements in Geological Samples by Inductively Coupled Plasma-Mass Spectrometry with Nickel Sulfide Fire Assay

GUO Jiafan; CHEN Xiaoyu; SUN Yong; ZHONG Weilu; ZHU Shaoxuan; WANG Lin

doi:10.15898/j.ykcs.202407180159

Rock and Mineral Analysis > 2024 > 43(5): 693-702. > DOI: 10.15898/j.ykcs.202407180159

GUO Jiafan，CHEN Xiaoyu，SUN Yong，et al. Ultratrace Platinum Group Elements in Geological Samples by Inductively Coupled Plasma-Mass Spectrometry with Nickel Sulfide Fire Assay[J]. Rock and Mineral Analysis，2024，43（5）：693−702. DOI: 10.15898/j.ykcs.202407180159

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Ultratrace Platinum Group Elements in Geological Samples by Inductively Coupled Plasma-Mass Spectrometry with Nickel Sulfide Fire Assay

GUO Jiafan^{1, 2,},
CHEN Xiaoyu^{1, 2},
SUN Yong^{1, 2},
ZHONG Weilu^{1, 2},
ZHU Shaoxuan^{1, 2},
WANG Lin^{1, 2, ,}

1.
The Prevention and Control Center for the Geological Disaster of Henan Geological Bureau, Zhengzhou 450012, China
2.
Key Laboratory of Precious Metals Analysis and Exploration Technology of Ministry of Natural Resources, Zhengzhou 450012, China

More Information

Received Date: July 17, 2024
Revised Date: August 25, 2024
Accepted Date: September 01, 2024
Available Online: September 26, 2024

Abstract

HIGHLIGHTS
(1) The source of the blank was identified by checking reagent blanks and the purification methods for reagents were given. Carbonyl nickel powder was used as a nickel collector, with the lower reagent blank, and the detection limits for PGEs required by geochemical exploration were achieved.

(2) Carbonyl iron powder was added to ensure the NiS bead could be smashed in deionized water. In this way, the analysis process was simplified, and the risk of contamination caused by mechanical breakage of NiS bead was avoided.

(3) Using ICP-MS to determine the solution in the kinetic energy discrimination model effectively eliminated the matrix effect of Ni. The results showed that the background equivalent concentration of ruthenium in kinetic energy discrimination mode was two orders of magnitude lower than that in the standard mode.

ABSTRACT

Pt, Pd, Rh, Ir, Os and Ru are platinum group elements (PGEs) with similar properties. Due to the low abundance as well as the nugget effect, the accurate determination of PGEs has been a challenge for rock and mineral analysis. Fire assay methods with large sample weights were developed to separate and preconcentrate PGEs, however, there are still difficulties to accurately determine ultratrace PGEs because of the high reagent blanks and the matrix effect. A method of nickel sulfide fire assay combined with ICP-MS simultaneous determination of ultratrace PGEs in samples was established. The results showed that the blank mainly comes from hydrochloric acid and nickel collector when using nickel sulfide fire assay to capture PGEs. The intensities of PGEs were detected by ICP-MS in standard mode and kinetic energy discrimination. In standard mode, the detection limits were 0.2ng/g for Pt and Pd, and 0.02ng/g for Rh, Ir and Os, but it couldn’t reach 0.1ng/g for Ru. In kinetic energy discrimination, the background equivalent concentration of Ru was two orders of magnitude lower than that in the standard mode. With the matrix effect of Ni effectively eliminated, the detection limits reached 0.005ng/g for Ru. The detection limits for PGEs required by geochemical exploration were achieved. The certified reference materials of soil (GBW07288, GBW07294) and stream sediment (GBW07289) were analyzed to test the method. The determined values were in good agreement with the certified values. The relative errors were between −10.9% and 11.8%, the relative standard deviations (RSD, n=12) were 3.88%−9.37%, and the spiked recoveries were 92%−110%. This method is simple, rapid and meets the requirements of ultratrace PGEs determination in large quantities of geological samples. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202407180159.
BRIEF REPORT
Significance: Platinum-group elements (PGEs), such as Pt, Pd, Rh, Ir, Os and Ru exhibit similar physico-chemical properties and significant features, and have been used widely in geochemistry and environmental chemistry. As a result, the accurate determination of the concentration of PGEs in geological samples is very important. Although various analytical methods for PGEs have been developed in the past, accurate and simultaneous determination of PGEs concentrations on the same sample digestion remains a significant challenge. This is mainly due to: (1) their extremely low abundance, sample heterogeneity and the nugget effect, making it necessary to analyze large sample sizes to obtain representative analyses; (2) During sample dissolving, Os and Ru are easy to form volatile OsO₄ and RuO₄, making it difficult to simultaneously measure the concentrations of PGEs accurately.

　　Improved Carius tube combined high-pressure asher (HPA-S) techniques have been widely used for the determination of PGEs to avoid the loss of volatile OsO₄ and RuO₄. However, the relatively complex procedure does not meet the requirements of ultratrace PGEs determination in large quantities of geological samples. Nickel sulfide (NiS) fire assay is a classical method to completely extract the PGEs from a large sample size into a nickel sulphide button and easily separate it from the slag. This method combined with highly sensitivity instrumental measurement, i.e., inductively coupled plasma-mass spectrometry (ICP-MS), can be used to simultaneously determine multi-elements with high sensitivity and effectively reduce the detection limit. However, it is still difficult to determine ultratrace PGEs because of the high procedural blank that mainly derives from the commercial nickel reagents and polyatomic ion interferences formed by interaction of a sample aerosol with components of the plasma-forming gas (such as C, Ar) and residual Ni and Cu ions in solution. Therefore, the improvement of the method for simultaneous determination of PGEs by ICP-MS combined with nickel sulfide fire assay preconcentration is important.

　　An analytical method for accurate and simultaneous determination of ultratrace PGEs in geological samples by ICP-MS combined with nickel sulfide fire assay is proposed. At the same time, it can effectively reduce the procedural blank and suppress/eliminate interferences. More importantly, it provides a practical method for the accurate determination of ultratrace PGE concentrations on the same sample digestion for surveying a large number of geological samples.

Methods: In the experiment, it is important to reduce the procedural blank and suppress/eliminate interferences. The data show that the high reagent blank is primarily from hydrochloric acid (Table 3) and nickel collector (Table 5). Therefore, the hydrochloric acid after purification or obtained from other manufacturers can be used in the experiment. At the same time, carbonyl nickel powder rather than other type of nickel powder is used as the fire assay collector.

　　Before analysis, the PGEs concentrations in the reagent (including hydrochloric acid and nickel powder) were measured. The reagent was not suitable to analyze the ultratrace PGEs, when the results exceeded 0.02ng/mL. (1) 50mL hydrochloric acid was evaporated at low temperature, and 5mL of aqua regia prepared from the same bottle of hydrochloric acid was added. After being extracted at low temperature and adding 20mL of deionized water, the solution was determined by ICP-MS. (2) 1.6g nickel power was mixed well with 2g sulfur, 4g carbonyl iron powder, 25g Na₂B₄O₇∙10H₂O, 25g Na₂CO₃, 4g SiO₂, and 1g of edible flour and transferred into a 500mL fire-clay crucible, and fused in a furnace at approximately 1000℃. The furnace door was opened to cool to 800℃. Then the door was closed, and temperature gradually rose to 1050℃ and maintained for 30min. The fluid in the crucible was poured into a cast iron mold. After cooling, the NiS bead was separated from the slag, and placed into a 200mL Triangle bottle with 20mL deionized water until smashed. Then 20mL concentrated HCl was added into the Triangle bottle and heated for 1h until no bubbles formed. Filtering out the sediment through a filter membrane, sediment was transferred into a 200mL Triangle bottle and 5mL aqua regia was added. Next the air-cooled tube was loaded, and the solution was heated to boiling. The cooling solution was diluted to 25mL by the addition of deionized water. Lu (50ng/mL) was added as an internal standard for the determination of Pt, Pd, Ru, Rh and Ir by ICP-MS (Table 1) in kinetic energy discrimination mode.

　　Ultratrace PGEs in geological samples were determined after the reagent blanks were checked. 20g sample was mixed well with the above-mentioned flux and collectors (proration shown in Table 2). The fusion procedure was also described above. The solution was determined by ICP-MS in kinetic energy discrimination mode, which can effectively eliminate the matrix effect.

Data and Results: Determination of ruthenium in geological samples by ICP-MS, showed that the background equivalent concentration of ruthenium in kinetic energy discrimination mode was two orders of magnitude lower than that in the standard mode (Fig.1). In kinetic energy discrimination mode, the detection limit of the method was 0.005ng/g (Table 8). The certified reference materials of soil (GBW07288, GBW07294) and stream sediment (GBW07289) were analyzed to test the method. The determined values were in good agreement with the certified values. The relative errors were between −10.9% and 11.8%, the relative standard deviations (RSD, n=12) were 3.85%−9.37%, the spiked recoveries were between 92% and 110%.

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