Ionic Geochemistry: Enhanced Soil Analysis for Mineral Exploration via High-Efficiency Ligand-Based Mobile Ion Extraction with Simultaneous Detection of 57 Elements in a Single Extraction

Twenty-six international geological certified reference materials (CRM) and two in-house soil control samples were gathered and submitted ‘blind’ in duplicate to five commercial geochemical laboratories for the determination of Au and the platinum group elements (PGEs). The methods employed comprise: Pb fire assay (PbFA) combined with inductively coupled plasma mass spectrometry (ICP-MS); NiS fire assay combined with ICP-MS or instrumental neutron activation analysis (INAA); and aqua regia ICP-MS, with and without prior roasting of the sample at 600 o C. The CRMs vary widely in their matrix and PGE concentrations, ranging from a background soil (e.g. GPt-1), sediment (e.g. GPt-2, JSd-2), and rock (e.g. WGB-1, CHR-Bkg) to altered rocks (e.g. WPR-1) and ore material (e.g. GPt-6, SARM-7b, WMS-1). The results of this ‘round-robin’ are provided and discussed in this paper. Results for Au showed the greatest variation across the laboratories, with one evidently encountering significant and spurious contamination. Comparison of the two fire assay techniques for Au, Pt and Pd was difficult as the number of data points was low and the variance within each technique across laboratories was too high. In general, PbFA-based methods for Au, Pt and Pd produced more accurate and precise results than those by NiS fusion and the data support PbFA detection limits for a 5–10 g sample of 1, 0.1 and 0.5 ppb for Au, Pt and Pd, respectively. A PbFA dataset for Rh demonstrated that this element is not recovered efficiently using an Ag inquart. Measurement of Rh by INAA rather than ICP-MS following NiS fusion facilitates detection below 1 ppb to c. 0.1–0.2 ppb. NiS/ICP-MS results for Ru, Os and Ir support detection limits of 1–2, 2–3 and 0.1 ppb, respectively; mean precision for these elements is in the range 10–15% RSD. Recovery of Os was very low by one laboratory, probably caused by its volatilization as OsO 4 during final digestion in the NiS procedure. As expected, recovery of the analytes by aqua regia was low and highly variable across the different matrices for Pt, Ru, Os and Ir but that for Au and Pd was often >80%; prior roasting of the samples had mixed effects.

In this work, a novel method of bismuth fire assay (Bi-FA) combined with inductively coupled plasma mass spectrometry (ICP-MS) simultaneous determination of ultratrace gold (Au), platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), and iridium (Ir) in geologic samples was established. Bismuth oxide (Bi2O3) was used as noble metal elements fire assay collector, and Bi-remaining protection cupellation was employed to generate Bi granule. After the Bi granule was microwave-digested by aqua regia (40%, v/v), 197Au, 195Pt, 106Pd, 101Ru, 103Rh, and 193Ir in the sample solution were determined by ICP-MS. Using Bi as cupellation protector, volatile Ru could be collected effectively and without volatilization loss during microwave digestion and decompression. Moreover, the toxicity of Bi was exceptionally low compared to toxic nickel oxide and lead oxide in nickel sulfide/lead fire assay; thus Bi-FA was a green environmental analysis method. The mineral composition and decomposition character of chromite, black shale, and polymetallic ore were investigated, and pretreatment procedures were optimized for such special samples. Besides, the influence of mass spectrum interference of coexisting elements was discussed. Under the optimal conditions, excellent curve fittings of Au, Pt, Pd, Ru, Rh, and Ir were obtained between 0.01 and 100 ng·mL−1, with the correlation coefficients exceeding 0.9995. The detection limits were from 0.002 ng·g−1 to 0.025 ng·g−1. The developed method was applied to analyze the Chinese Certified Reference Materials (CRMs) and the determined values were in good agreement with the certified values.

Background: Platinum (Pt), palladium (Pd), rhodium (Rh) and iridium (Ir) are platinum group elements (PGEs) and also important elements of geochemistry and environmental chemistry with the similar physic-chemical properties, which have been widely used in industry and laboratory. However, due to the low abundance and inhomogeneous distribution in natural ore as well as the nugget effect, the accurate determination of PGEs has been a challenge to analytical chemistry. Methods: In this work, a novel fire assay method was reported for the determination of ultra-trace Pt, Pd, Rh and Ir in geochemical samples. Tin powder (Sn) instead of stannic oxide (SnO2) was used as a fire assay collector to reduce the melting temperature from 1250°C to 1050°C, the escape of molten material caused by high temperature was successfully avoided. Tin bead was compressed into thin slice and dissolved by HCl. For the target Pt, Pd, Rh and Ir, HCl insoluble substance such as PtSn4, PdSn4, RhSn4 and Ir3Sn7 were formed and separated from matrix by filtering. The metal compounds precipitate together with filter paper were microwave-assisted completely digested by aqua regia (50%, v/v), thence the sample solution was determined by inductively coupled plasma mass spectrometry (ICP-MS). Results: Compared with nickel oxide and lead oxide in nickel sulfide /lead fire assay, the reagent blank of tin powder were relatively low and could be directly employed in tin fire assay to collect Pt, Pd, Rh and Ir without purifying. Moreover, the harm of nickel oxide and lead oxide to the analyst and environment was avoided by using the non-toxic tin powder. The decomposition method of chromite and black shale was investigated as well as the amount of tin powder and flour, microwave digestion program for the determination of Pt, Pd, Rh and Ir were optimized. Besides, the influence of mass spectrum interference of co-existing elements was discussed and the standard mode and kinetic energy discrimination collision pool mode were compared. Under the optimal conditions, excellent curve fitting of Pt, Pd, Rh and Ir were obtained between 0.01~100 ng mL-1, with the correlation coefficients exceeding 0.9996. The detection limits were from 0.003 ng g-1 to 0.057 ng g-1. Conclusion: The developed method was applied to analyze the Chinese Certified Reference Materials and the determined values were in good agreement with the certified values.

The development of a rapid analytical technique for the determination of the platinumgroup elements (PGE) and gold in geological samples is described. The technique is based on selective aqua regia acid leach followed by a selective extraction (using diphenylthiourea and 1,2-dichloroethane) to separate the PGE and Au as a group from concomitant matrix elements. Organic extracts and aqueous raffinates were analysed by graphite furnace atomic absorption spectrometry, for which a comprehensive assessment of matrix interference effects was undertaken. Direct analysis of the aqua regia acid leachates by inductively coupled plasma-mass spectrometry (ICP-MS) was also evaluated. A number of other techniques were used to evaluate recoveries following acid attack, including x-ray fluorescence spectrometry and instrumental neutron activation analysis of solid residues and beta-autoradiography of thin sections to characterise PGE mineral solubility. The results of these investigations identified optimum conditions for aqua regia extraction (20 ml of normal (3: 1) aqua regia for a 10 g sample, stirring for two hours at room temperature) and that quantitative recoveries can be expected for Au and semiquantitative for Pd with lower, but variable recoveries for Pt, Rh, Ru and Os and very low recovery of Ir. The solvent extraction procedure was effective in selectively extracting Au and Rh but was not quantitative in the extraction of Pt, Pd and Ru, the extraction of which appeared to be influenced by the sample matrix. Further studies on the solubility in aqua regia of individual PGE minerals indicated that the main control of aqua regia extraction efficiency was sample mineralogy. The aqua regia leach procedure was applied to a range of samples that had been independently analysed by NiS fire assay as well as appropriate reference materials. This extraction procedure, with direct analysis of leachates by ICP-MS, was also used as the primary technique for characterising the homogeneity of two new chromitite reference materials, CHR-Pt+ and CHR-Bkg.

This study examines field portable X-ray fluorescence (pXRF) data for unsieved B- and C-horizon soil samples from two contrasting Au exploration programs in northern Canada and compares them to laboratory results from sieved samples to determine if a minimalist approach to sample preparation, equipment calibration and analysis produces results that are adequate for exploration purposes. The collection of 14 651 residual soil samples on detailed grids at the Whiskey Au project in the Black Hills of the Yukon Territory, Canada during the 2011 field season allows a comparison of pXRF data from unsieved samples with inductively-coupled plasma mass spectrometry (ICP-MS) data from the <100-µm grain size fraction digested in aqua regia. XRF data were generated on two devices: an Innov-X Delta handheld and a Niton FXL desk-top unit. Data for selected elements from the pXRF units show trends in gridded percentile plots comparable to the ICP-MS data, although the continuity of these trends reflects the degree to which the pathfinder element concentrations exceed the lower orders of detection. Acceptable merged gridded images for As and Cu from the two devices were generated without data levelling, whereas the Pb, Mo and Ni data require leveling prior to gridding in order to provide a reasonable fit to the laboratory data. Data for elements that occur in concentrations close to the lower limit of detection by pXRF, such as Sb, may provide some useful information but must be used with caution. Data for Fe also show a poor correlation, possibly due to heterogeneity of Fe in the samples, as well as due to incomplete digestion of all Fe-bearing minerals in some samples using an aqua regia digestion. A total of 680 till samples collected from regional and detailed sampling grids on the Kiyuk Lake Au property in southern Nunavut, Canada during the 2012 field season were analysed by pXRF and also by ICP-MS following an aqua regia digestion. pXRF data were generated on an Innov-X Delta using unsieved samples for comparison with ICP-MS data from the <70-µm grain size fraction. The pathfinder element As shows good agreement with the laboratory data, whereas Ni, Mo and Sb data give poor matches due to proximity of the data to the lower orders of detection by pXRF. The use of pXRF data on soil samples with no sample preparation other than drying and no instrument calibration is a robust approach for Au exploration using particular elements, such As and Cu, provided the data are monitored for consistency. Even greater compatibility between pXRF and laboratory data would be expected where sample preparation and site-specific calibrations are implemented. Other pathfinder elements for Au, including W, Sb, Bi, Ag and Te, as well as Au itself, typically occur at levels in soil either close to or below the lower order of detection of the current generation of field-portable analytical devices. The use of pXRF in Au exploration allows for quick decision-making and provides near-real time sampling guidance in the field where ‘fit for purpose’ data for suitable pathfinder and lithologically-controlled elements can be obtained.

Quantification of germanium in organometallics compounds by total-reflection X-ray fluorescence

Determination of the platinum-group elements and gold in solid samples by slurry nebulisation ICP-MS

Comparison of inductively coupled plasma mass spectrometry and colorimetric determination of total and extractable phosphorus in soils

Objective To establish an effective and reliable method for analysis of ultratrace uranium in multi-stage atmospheric particles providing the monitoring and evaluation of the content of radioactive uranium in the atmosphere. Methods A large volume six-stage-impactor sampler of atmosphere particles was used to collect aerosol samples, and ultratrace uranium in particles was digested using microwave and measured by inductively coupled plasma mass spectrometry. The filter material, digestion conditions and microwave digestion system had been optimized. Results The background of uranium level on the cellulose filter was the lowest, and the samples were digested by using HNO3-HCl (aqua regia)-H2O2 solution. Reference material SRM2783 was used to validate the accuracy of the method, and the relative error of the 238U was 7%, The detection limit of the method was 2×10-4 ng/m3. The aerosol actual samples were analyzed using the established method. The mass concentrations of uranium in PM2.5 was in the range of 0.023-0.065 ng/m3. Conclusions The established method was effective and reliable to monitor the concentration level of ultratrace uranium in multi-stage atmospheric particles. Key words: Atmospheric particles; High resolution inductively coupled plasma mass spectrometry (HR-ICP-MS); Size distribution; Uranium

Nearly 99% of the total content of extraterrestrial metals is composed of Fe and Ni, but with greatly variable trace element contents. The accuracy obtained in the inductively coupled plasma mass spectrometry (ICP-MS) analysis of solutions of these samples can be significantly influenced by matrix contents, polyatomic ion interference, and the concentrations of external standard solutions. An ICP-MS instrument (X Series 2) was used to determine 30 standard solutions with different concentrations of trace elements, and different matrix contents. Based on these measurements, the matrix effects were determined. Three iron meteorites were dissolved separately in aqua regia and HNO3. Deviations due to variation of matrix contents in the external standard solutions were evaluated and the analysis results of the two digestion methods for iron meteorites were assessed. Our results show obvious deviations due to unmatched matrix contents in the external standard solutions. Furthermore, discrepancy in the measurement of some elements was found between the sample solutions prepared with aqua regia and HNO3, due to loss of chloride during sample preparation and/or incomplete digestion of highly siderophile elements in iron meteorites. An accurate ICP-MS analysis method for extraterrestrial metal samples has been established using external standard solutions with matched matrix contents and digesting the samples with HNO3 and aqua regia. Using the data from this work, the Mundrabilla iron meteorite previously classified as IAB-ung is reclassified as IAB-MG.

Determination of trace elements in biochar usually involves complex and inefficient sample preparation strategies due to the high carbon content and presence of silicates in the sample matrix, as well as to the variety of raw materials used in its production. Most methods are time-consuming, employ hazardous reagents (e.g. hydrofluoric acid), and are prone to analyte contamination and loss. Another issue is the lack of validation to ensure that these methods provide accurate results. In this study, we describe a sample preparation strategy to determine Al, As, Ba, Ca, Co, Cr, Cu, Fe, K, Mg, Mo, Mn, Na, Ni, P, Pb, S, Sr, Ti and Zn in biochar by inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). The method includes a dry ashing step, followed by microwave-assisted extraction with diluted nitric acid, and hydrogen peroxide. Initially, it was evaluated for efficacy using a hog waste biochar and compared to a similar and commonly reported extraction with aqua regia. The method’s accuracy was assessed by addition and recovery experiments, with analyte recoveries in the 89.6%-114% range. Limits of detection were in the 0.02-3000 and 0.006-0.02 mg kg-1 ranges for ICP-OES and ICP-MS, respectively, with lower values for HNO3 + H2O2 compared to aqua regia. Relative standard deviation (RSD) values using HNO3 + H2O2 were below 10% for all analytes, except As (15%), Cr (12%), and Pb (11%), while aqua regia values were in the 15%-63% range. The method was then applied to the analysis of five commercially available biochar samples.

Alkali dilution of blood samples for high throughput ICP-MS analysis—comparison with acid digestion

Analysis of aqua regia extracts have been made by both graphite furnace atomic absorption spectrometry (GFAAS) and inductively coupled plasma mass spectrometry (ICP-MS) with a view to developing a rapid, simple, and inexpensive technique for the determination of gold and the platinum-group elements (PGE) in exploration samples. Although selective extraction by aqua regia is expected to leave the sample matrix largely unattacked, resultant solutions still contain high levels of Fe, Ni, Cu, Zn, Mg, and As, which produce significant interferences when the solutions are analyzed directly by GFAAS. To overcome these difficulties a simple solvent extraction procedure was investigated in which Au and the PGE are selectively complexed with triphenyl phosphine, then extracted from the aqueous phase into 1,2-dichloroethane. Results will be presented to demonstrate residual interference effects, the optimization of the furnace heating programme for organic solutions and the recovery efficiency of the solvent extraction process. Routine direct analyses of diluted aqua regia solutions were found to be possible by ICP-MS. The procedures developed for the correction of drift effects and the calibration against aqueous standards will be described. These procedures have been applied to: a. a wide range of reference materials, b. a series of samples that had been preanalyzed by nickel sulphide-fire assay, c. two new chromitite reference samples, CHR-Pt+ and CHR-Bkg, in an appraisal of sample homogeneity. Results will be presented to characterize the overall recovery efficiency of the aqua regia leach procedure and to assess the realistic capabilities of the technique for samples of varying matrix and PGE mineralogy.

Inductively coupled plasma mass spectrometry (ICP-MS) has been an indispensable technique in the analysis of geological samples in the last few decades. This technique can determine a large number of trace elements, such as the precious metals and rare earth elements. However, the technique is limited by the spectral and non-spectral interferences, which can affect the results of analysis. The aim of this study was to reduce the non-spectral interferences by the method of sample dilution (1000 - 10000). A sample containing gold-bearing minerals was subjected to a digestion in aqua regia, and determination was performed on a NexION 1000 PerkinElmer ICP-MS instrument. The results showed that the differences in gold and silver concentrations in the standard and He modes were insignificant at a dilution of 10000 indicating that the matrix effect was reduced applying a higher dilution. The results of XRD analysis have showed that the sulphide minerals pyrite (FeS2 ) and covellite (CuS) were very abundant in the tested samples, so the digestion of these samples with aqua regia has yielded the solutions containing iron and copper as the most abundant components of the matrix.

This study was carried out to characterise the mineralogical forms of barium and the trace heavy metal impurities in commercial barytes of different origins using electron probe microanalysis (EPMA), X-ray diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS). Qualitative EPMA results show the presence of typically eight different minerals in commercial barytes including barite (BaSO4), barium feldspar, galena (PbS), pyrite (FeS2), sphalerite (ZnS), quartz (SiO2), and silicates, etc. Quantitative EPMA confirms that the barite crystals in the barytes contain some strontium and a little calcium, whereas trace heavy metals occur in the associated minerals. Analysis of aqua regia extracts of barytes samples by ICP-MS has shown the presence of a large number of elements in the associated minerals. Arsenic, copper and zinc concentrations correlate closely in all 10 samples. The findings suggest that barytes is not, as traditionally thought, an inert mineral, but is a potentially toxic substance due to its associated heavy metal impurities, which can be determined by an aqua regia digest without the need for complete dissolution of the barite itself. X-ray powder diffraction was not informative as the complex barite pattern masks the very weak lines from the small amounts of associated minerals.