Method for determination of precious metals by isotope dilution-gold assay-LA-ICP-MS

Abstract

This invention relates to the field of analytical chemistry, and discloses an isotope dilution-bismuth reagent-LA-ICP-MS method for the determination of noble metals. The method involves adding geochemical samples to bismuth reagent kits... 194 Pt isotope diluent was mixed with a silver carrier, and then melted and blown to obtain silver alloy particles. The silver alloy particles were annealed and pressed into silver sheets. The content of Pt in the silver sheets was determined by LA-ICP-MS. 105 Pd, 195 Pt and 197 The mass spectrometry intensity of Au was determined by adding platinum isotope diluent. 194 Pt was used as an internal standard for signal correction; the contents of gold, platinum, and palladium in the sample were calculated based on the measurement results. This invention utilizes solid-state injection of prepared Ag sheets and multi-point ablation by LA-ICP-MS, improving the analysis speed and avoiding interference from polyatomic ions and dilution effects in the solution; the added platinum isotope diluent... 194 Pt, as an internal standard, is consistent with the distribution trend of the element to be measured, thus solving the problem of uneven element distribution in Ag tablets. Ag tablets prepared with standard substances serve as a standard series with consistent matrix. The standard curve is plotted using the empirical coefficient method, showing good linearity.

Description

Technical Field

[0001] This invention relates to the field of chemical analysis technology, and in particular to an isotope dilution-bismuth assay-LA-ICP-MS method for the determination of noble metals. Background Technology

[0002] Gold (Au), platinum (Pt), and palladium (Pd) are precious metal elements with extremely low average abundance in the Earth's crust, at 1 × 10⁻⁶ respectively. -6 %, 5×10 -7 % and 5×10 -7 These minerals possess excellent ductility, thermal conductivity, and electrical conductivity; they are chemically stable, corrosion-resistant, and oxidation-resistant; and they exhibit superior catalytic activity, finding wide application in industries such as chemical engineering, aerospace, pharmaceuticals, electronics manufacturing, and catalysts. Therefore, accurately determining the content of Pt, Pd, and Au in geochemical samples is of great significance for the exploration, evaluation, and comprehensive utilization of precious metal mineral resources.

[0003] However, Pt, Pd, and Au are present in very low abundance and unevenly distributed in geochemical samples, making them difficult to determine directly and accurately using conventional analytical methods. Therefore, fire assay is usually used to separate and enrich the samples, followed by appropriate instrumental analytical methods to determine their content.

[0004] Currently, commonly used fire assay methods include nickel matte assay, antimony assay, tin assay, and lead assay. Each fire assay method has its own characteristics and limitations. Nickel matte and lead reagents can quantitatively enrich Pt, Pd, and Au in geochemical samples, but the content of Pt, Pd, and Au in their collectors is high and variable, requiring complex purification processes before use, which increases operational difficulty and the risk of blank results. Lead reagents, in particular, use highly toxic lead and its oxides, which may pose hazards to laboratory personnel and the environment.

[0005] Antimony assays exhibit poor screening properties during enrichment, and their enrichment effect is unstable for geochemical samples with complex compositions, making them unsuitable for the analysis of complex matrix samples.

[0006] Tin powder, the collector in tin assay, contains gold, and its blank value is unstable, which affects the accuracy of ultra-trace gold determination.

[0007] Furthermore, the aforementioned traditional fire assay methods typically involve subsequent detection using solution-injection inductively coupled plasma mass spectrometry (ICP-MS) or graphite furnace atomic absorption spectrometry (GF-AAS). These methods require complex sample digestion and solution preparation processes, are prone to introducing contamination and dilution effects of the analyte, and suffer from polyatomic ion interference, thus affecting the accuracy and precision of the analytical results.

[0008] Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) allows for direct solid sample introduction and offers advantages such as in-situ, rapid, low detection limits, and avoidance of polyatomic ion interference found in solution methods, making it an important technique for trace element analysis. However, its application to the detection of samples enriched with fire assay gold still faces several challenges: uneven distribution of analytes in the enriched material (such as silver alloy grains), a lack of solid standard materials that match the sample matrix, and how to effectively correct for fractionation effects and instrument signal drift during the laser ablation process. These issues limit its widespread application in ultra-trace analysis of precious metals.

[0009] Therefore, developing a green and environmentally friendly method with low blank values ​​that can effectively solve matrix matching and signal correction problems, and can accurately and stably determine trace amounts of gold, platinum and palladium in geochemical samples, is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0010] To address the aforementioned shortcomings, this invention utilizes isotope dilution technology as the core calibration method and environmentally friendly bismuth assay as the separation and enrichment method. Combined with laser ablation inductively coupled plasma mass spectrometry, by optimizing the bismuth assay formulation, silver sheet preparation process, laser ablation parameters, and isotope internal standard calibration method, it simultaneously achieves efficient enrichment and accurate determination of ultra-trace gold, platinum, and palladium in geochemical samples. This solves the problems of high blank values, high toxicity, numerous interferences in the detection process, and difficult calibration in existing fire assay methods.

[0011] The specific technical solution of the present invention is as follows: In a first aspect, the present invention provides an isotope dilution-bismuth assay-LA-ICP-MS method for determining noble metals, comprising the following steps: S1. Dry, grind, sieve, dry and cool the geochemical sample to obtain sample powder; mix the sample powder with bismuth reagent to obtain a mixture; take a portion of the mixture and add platinum isotope diluent, add silver carrier dropwise, evaporate to dryness, add another portion of the mixture, then cover with a covering agent, melt and blow away ash to obtain silver granules containing the elements to be tested, gold, platinum and palladium. S2. Anneal the silver granules and press them into silver sheets; S3. The mass spectrometric intensities of gold, platinum and palladium in the silver sheet were determined by laser ablation inductively coupled plasma mass spectrometry, and a specific isotope in the platinum isotope diluent was used as an internal standard for signal correction. S4. Based on the determination results of step S3, combined with the amount of platinum isotope diluent added, the mass of the silver sheet and the mass of the original sample, the contents of gold, platinum and palladium in the geochemical sample are calculated.

[0012] As a preferred technical solution, in step S1, the platinum isotope diluent is... 194Pt isotope diluent; the silver carrier is a silver nitrate solution with a concentration of 40 g / L, and the amount added is 200-400 μL, so that the mass of the silver particles obtained after ash blowing is 5-10 mg.

[0013] As a preferred technical solution, in step S1, the bismuth reagent includes borax, glass powder, sodium carbonate, bismuth trioxide (Bi2O3), and flour; the covering agent is a mixture of borax, glass powder, sodium carbonate, bismuth trioxide, and flour.

[0014] The mass ratio of sodium carbonate, borax, glass powder, Bi2O3, and flour in the covering agent is 50:20:20:40:4.

[0015] As a preferred technical solution, in step S1, the ingredients include the following raw materials in parts by weight: 5–20 geochemical samples; 15-20 parts of borax; 15-25 parts glass powder; Sodium carbonate 50-55 parts; 40-100 parts of bismuth trioxide; 3-5 parts flour; The geochemical sample is one of the following: silicate rock, carbonate rock, sulfide mineral rock, soil, stream sediment, black shale, and polymetallic mineral.

[0016] Further optimization involves preparing bismuth reagents for the geochemical samples according to the formulations in Table 1, based on the sample type: Table 1

[0017] Note: *The sample was calcined at 600℃ for 2 hours.

[0018] As a preferred technical solution, in step S1, the melting temperature is 1050-1100℃, the holding time is 30-50 min, and the ash blowing temperature is 930-950℃.

[0019] As a preferred technical solution, in step S2, the annealing temperature is 650-750℃, the annealing time is 20-40 min, the pressing pressure is 30-40 tons, and the pressing thickness is 0.15-0.25 mm.

[0020] As a preferred technical solution, a specific isotope in the platinum isotope diluent is used as an internal standard for signal correction. Specifically, Formula 4 is derived from Formulas 2 and 3, and Formula 4 is used to calculate the signal in the platinum isotope diluent. 194 The mass spectrometry intensity of Pt was used as an internal standard to calibrate the mass spectrometry intensities of the analytes gold, platinum, and palladium.

[0021] Formula 2 is as follows: (Formula 2), Formula 3 is as follows: (Formula 3), Formula 4 is as follows: (Formula 4), In the formula, It is the mass spectrometry intensity of the isotope diluent. These are geochemical samples 194 Mass spectrum intensity of Pt; This is the total mass spectrum intensity measured by ICP-MS at a mass number of 194. This is the total mass spectrum intensity measured by ICP-MS at a mass number of 195; A N194 In geochemical samples 194 The natural abundance of Pt, A N195 In geochemical samples 195 Natural abundance of Pt; A S194 It is an isotope diluent 194 Pt abundance, A S195 It is an isotope diluent 195 Pt abundance.

[0022] As a preferred technical solution, the determination conditions for laser ablation inductively coupled plasma mass spectrometry in step 3 are as follows: The laser ablation system has a wavelength of 193 nm, an energy density of 5–7 J / cm², and a He carrier gas flow rate of 0.28–0.35 L / min. The quantitative measurement conditions in the laser ablation system are a frequency of 12–16 Hz and a spot diameter of 135–175 μm. The surface scanning conditions in the laser ablation system are a frequency of 280–320 Hz and a spot diameter of 5–10 μm. The power of the inductively coupled plasma mass spectrometer is 1500–1550 W, the cooling gas flow rate is 13–14.5 L / min, the auxiliary gas flow rate is 0.70–1.00 L / min, the nebulizing gas flow rate is 0.80–1.20 L / min, the residence time is 60–90 ms, and the number of scans is 20–50.

[0023] As a preferred technical solution, step S3 further includes the step of establishing a calibration curve: selecting multiple national first-class standard materials, processing them using the same steps S1 and S2 as the sample to be tested, and preparing a series of standard silver sheets; using the empirical coefficient method, plotting the calibration curve with the mass fraction of the element to be tested in the standard silver sheet as the abscissa and the product of the ratio of the mass spectral intensity of the element to be tested in the standard silver sheet to the mass spectral intensity of the internal standard element as the ordinate.

[0024] As a preferred technical solution, in step S4, the contents of gold, platinum and palladium in the geochemical sample are calculated using formula 1; Formula 1 is as follows:

[0025] In the formula, It represents the mass fractions of Pd, Pt, and Au in a geochemical sample, expressed in ng / g. This represents the mass fractions of Pd, Pt, and Au in the blank silver sample, in μg / g. This is the mass of the blank sample silver sheet, in grams. It represents the mass fractions of Pd, Pt, and Au in the silver sample, in μg / g. This refers to the mass of the sample silver sheet, in grams. This is the mass of the geochemical sample, expressed in grams (g).

[0026] Secondly, the present invention also provides an application of the gold, platinum and palladium content in geochemical samples obtained by the above method.

[0027] Compared with the prior art, the present invention has the following beneficial effects: Green and environmentally friendly: It uses low-toxicity Bi2O3 to replace the traditional toxic lead assay method, reducing environmental pollution and health hazards to operators.

[0028] Low background values ​​and good stability: The background values ​​of gold, platinum and palladium in Bi2O3 are low and stable, eliminating the need for complex purification steps and ensuring the accuracy of ultra-trace analysis.

[0029] Matrix matching ensures reliable calibration: By treating standard materials with different matrices and the test samples with the same bismuth assay process, silver sheets with completely consistent matrices are produced, solving the problem of lacking matrix-matched standard materials in LA-ICP-MS analysis. Combined with the empirical coefficient method, this significantly improves the linearity of the calibration curve and the accuracy of the analytical results.

[0030] Isotope dilution internal standard correction is accurate: This innovative method utilizes internal standard correction added during the enrichment stage. 194Pt isotope diluent is used as an internal standard. This internal standard behaves completely identically to the analytes (Pt, Pd, Au) throughout the entire process of fire assay enrichment, laser ablation, ion transport, and ionization, and its distribution trend is the same as that of the analytes. This effectively corrects for uneven element distribution, instrument signal drift, and matrix effects. In particular, it solves the problem that Ag as an internal standard cannot correct Au, and significantly improves the precision and accuracy of the determination.

[0031] High analytical efficiency: The solid sample introduction method eliminates the complex sample digestion and solution preparation process. Combined with multi-point ablation technology, the analysis speed is fast and avoids the interference and dilution effect of polyatomic ions that may exist in solution methods.

[0032] Wide applicability: Through optimization of the bismuth assay formulation, this method is applicable to a variety of geochemical samples, including rocks, soils, stream sediments, black shale, and polymetallic minerals. Attached Figure Description

[0033] Figure 1 This is a graph showing the experimental results of optimizing the amount of silver added in Experiment Example 1 of this invention; Figure 2 This is a graph showing the results of the laser energy density optimization experiment in Experiment Example 2 of this invention; Figure 3 This is a graph showing the results of the laser frequency optimization experiment in Experiment Example 2 of this invention; Figure 4 This is a diagram showing the experimental results of laser spot size optimization in Experiment Example 2 of this invention; Figure 5 This is a graph showing the experimental results of ICP-MS residence time optimization in Experimental Example 2 of this invention; Figure 6 In the verification of the internal standard correction effect in Experiment Example 3 of this invention, the silver sheet... 194 Pt, 195 Pt, 105 Pd and 197 Au isotope surface imaging. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased.

[0035] Unless otherwise stated, all percentages in this invention represent mass fractions. Ratios refer to mass proportions.

[0036] Unless otherwise specified, all materials, instruments, and equipment used below are conventional materials, instruments, and equipment or obtained through commercial channels; all testing methods used are existing methods unless otherwise specified.

[0037] The instruments and equipment used in this embodiment and the experimental example are as follows: Inductively coupled plasma mass spectrometer: ICAP Q model, Thermo Scientific, USA. Its operating conditions are shown in Table 2.

[0038] Laser ablation system: IRIDIA type, CETAC Corporation, USA. Its operating conditions are shown in Table 3.

[0039] One part per million electronic balance: Model ME5, Sartorius GmbH, Germany.

[0040] Pipettes: 100–1000 μL, Brand GmbH, Germany.

[0041] The main standard solutions and reagents used in this embodiment and experimental example are as follows: Pt isotope diluent: purchased from Oak Ridge National Laboratory, USA. The abundance of each Pt isotope is based on the standard certificate values ​​provided by the manufacturer.

[0042] Standard silver solution (40g / L): Weigh 15.7485g of standard silver nitrate into a 250mL glass beaker, add 50mL of water and 20mL of nitric acid, stir until completely dissolved, then transfer to a 250mL brown volumetric flask. Wash the beaker with water, and transfer the washing solution into the volumetric flask as well. Make up to the mark and shake well.

[0043] Covering agent: Mix the following ingredients in a mass ratio of sodium carbonate + borax + glass powder + Bi2O3 + edible flour = 50 + 20 + 20 + 40 + 4.

[0044] Experimental reagents: Bi2O3 (analytical grade), glass powder (pretreated by heating and filtering with aqua regia), powdered sodium carbonate (analytical grade), borax (analytical grade), calcium oxide (analytical grade), sodium peroxide (analytical grade), edible flour, and powdered SiO2 (analytical grade).

[0045] In existing technologies, the determination of trace precious metals in geochemical samples mainly relies on fire assay methods such as lead assay for enrichment, followed by detection using GF-AAS or ICP-MS. However, lead assay uses toxic lead and its oxides, posing significant hazards to the environment and laboratory personnel; methods such as nickel matte assay, antimony assay, and tin assay suffer from problems such as high blank values ​​of the collecting agents, instability, poor selectivity, or complex procedures.

[0046] Existing fire assay methods often employ solution-based sampling for subsequent detection, requiring complex sample digestion processes that are prone to contamination and dilution effects, and are susceptible to polyatomic ion interference. While LA-ICP-MS allows for direct solid-state sampling, its application to the detection of samples enriched by fire assays faces challenges such as uneven distribution of analytes in the enriched material, lack of matrix-matched standard materials, and difficulties in signal correction.

[0047] To address the issue of uneven distribution of analytes in fire assay concentrates, previous studies have attempted to use silver as an internal standard for correction. However, silver may be partially lost during the ash blowing process, and gold cannot be determined using silver as an internal standard, presenting significant drawbacks. Furthermore, existing techniques lack systematic optimization of the bismuth assay-LA-ICP-MS coupling method, particularly in isotope internal standard correction and standard curve construction.

[0048] Furthermore, this invention proposes an isotope dilution-bismuth assay-LA-ICP-MS method for the determination of precious metals. By adding a platinum isotope diluent as an internal standard and combining it with a matrix-matched standard series and an empirical coefficient method calibration curve, the accurate determination of ultra-trace gold, platinum, and palladium in geochemical samples is achieved.

[0049] This invention uses low-toxicity Bi₂O₃ as a trapping agent to replace the traditional toxic lead assay method, reducing environmental pollution and health hazards to operators. The background values ​​of gold, platinum, and palladium in Bi₂O₃ are low and stable, eliminating the need for complex purification steps and ensuring the accuracy of ultra-trace analysis.

[0050] This invention solves the problem of lacking matrix-matching standard materials in LA-ICP-MS analysis by processing standard materials with different matrices and the test samples through the same bismuth assay process, thus producing silver sheets with completely consistent matrices. Furthermore, by combining the empirical coefficient method, it significantly improves the linearity of the calibration curve and the accuracy of the analytical results.

[0051] This invention innovatively employs the addition of [a specific ingredient] during the enrichment stage. 194 Pt isotope diluent is used as an internal standard. This internal standard behaves completely identically to the analytes (Pt, Pd, Au) throughout the entire process of fire assay enrichment, laser ablation, ion transport, and ionization, and its distribution trend is the same as that of the analytes. This effectively corrects for uneven element distribution, instrument signal drift, and matrix effects. In particular, it solves the problem that Ag as an internal standard cannot correct Au, and significantly improves the precision and accuracy of the determination.

[0052] Example 1: This embodiment provides an isotope dilution-bismuth assay-LA-ICP-MS method for determining trace amounts of gold, platinum, and palladium in geochemical samples, including the following steps: (1): Sample pretreatment and bismuth assay enrichment The silicate rock sample was crushed, ground, and passed through a 200-mesh nylon sieve. It was then dried in an oven at 105°C for 2 hours and placed in a desiccator to cool for later use, thus obtaining sample powder.

[0053] Weigh the following bismuth reagents according to the silicate rock formula in Table 1: 20g borax, 20g glass powder, 50g Na2CO3, 40g Bi2O3, and 4g flour. Put them together with 15g of sample powder into a 500mL mixing bottle and shake well.

[0054] First, pour about half of the ingredients into the bottom of a 400mL high-purity quartz crucible, then use a pipette to transfer the corresponding concentration... 194 Pt isotope diluent (the amount added is equivalent to the Pt content in the sample, and the total volume does not exceed 1000 μL) is evenly dropped onto the powder. Then, 200 μL of Ag standard solution (40 g / L) is pipetted and evenly dropped onto the powder. The mixture is then placed in an oven at 60–80 °C to evaporate to dryness. The remaining half of the mixture is then poured into a crucible to cover the groove, and 20 g of covering agent is evenly covered on top of the mixture.

[0055] Place the crucible into a high-temperature box furnace preheated to 1080℃, close the furnace door tightly, and hold at 1080℃ for 40 minutes. After cooling, pour off the slag and remove the bismuth clasp.

[0056] The bismuth coin was placed in a magnesium abrasive ash dish that had been preheated at 940℃ for more than 20 minutes. Ash blowing was carried out at 940℃±10℃. Ash blowing ended when the aggregates were observed to agglomerate into spheres that did not flow and produced flashes. The ash dish was immediately removed and cooled to obtain silver aggregates enriched with Pd, Pt and Au from the sample.

[0057] The silver alloy particles were placed in water and ultrasonically cleaned for 30 minutes. They were then removed and dried in an oven at 105°C. The mass of the silver alloy particles was measured using a 1 / 1,000,000 electronic balance.

[0058] (2) Preparation of silver sheet The silver alloy granules were annealed in a muffle furnace at 700℃ for 30 minutes, and then pressed into silver sheets with a thickness of about 0.2 mm using a 35-ton press.

[0059] (3) LA-ICP-MS determination The operating conditions for the laser ablation system and the inductively coupled plasma mass spectrometer are shown in Tables 2 and 3.

[0060] Table 2

[0061] Table 3

[0062] Based on the principles of high abundance and freedom from interference from isotopes, the following were selected.105 Pd, 195 Pt and 197 Au is used as an analytical isotope (see Table 4 below). Ag sheets prepared from geochemical samples after bismuth assay pretreatment generally contain trace amounts of Bi, Pt, Pd, Au, Ru, Rh, and Ir.

[0063] Table 4. Isotopes of Pt, Pd, and Au

[0064] Because LA-ICP-MS uses laser ablation of Ag plates for sample introduction, it avoids the introduction of large amounts of elements such as N, H, O, and Cl found in traditional solutions. 195 Pt, 105 Pd and 197 Au was not affected by polyatomic molecular ions of other elements coexisting in the Ag film (see Table 5).

[0065] Table 5 194 Pt, 195 Pt, 105 Pd and 197 Au polyatomic ion interference

[0066] A multi-point etching method was used, with 12 points randomly selected at different locations on each silver sheet for measurement. 105 Pd, 194 Pt, 195 Pt and 197 Mass spectrometric intensity of Au.

[0067] With the added platinum isotope diluent 194 Using Pt as an internal standard, the concentration of platinum isotopes in the diluent is calculated using the following formula. 194 Mass spectral intensity of Pt: (Formula 2) (Formula 3) (Formula 4) In the formula, It is an isotope diluent 194 Mass spectrum intensity of Pt, These are geochemical samples 194 Mass spectrum intensity of Pt; This is the total mass spectrum intensity measured by ICP-MS at a mass number of 194. This is the total mass spectral intensity measured by ICP-MS at a mass number of 195. A N194 In geochemical samples 194 The natural abundance of Pt, A N195In geochemical samples 195 Natural abundance of Pt; A S194 It is an isotope diluent 194 Pt abundance, A S195 It is an isotope diluent 195 Pt abundance.

[0068] Establishing calibration curves: National primary standard materials GBW07288, GBW07289, GBW07291, GBW07293, GBW07194, GBW07196, GBW07198, GBW07203, and GBW07342 were selected and processed using the same steps (1) and (2) as the samples to be tested, to obtain a series of standard silver sheets. Using the empirical coefficient method, the calibration curve was plotted with the mass fraction of the analyte in the standard silver sheet as the abscissa and the product of the ratio of the mass spectral intensity of the analyte in the standard silver sheet to the mass spectral intensity of the internal standard element as the ordinate.

[0069] Plot the calibration curve using the following formula: (5) In the formula, and These are the contents of the elements being measured in Ag slices from actual geochemical samples and Ag slices from standard geochemical samples, respectively. and These are the contents of internal standard elements in Ag slices from actual geochemical samples and Ag slices from standard geochemical samples, respectively. and These are the mass spectrometry intensities of the elements being measured in Ag slices from actual geochemical samples and Ag slices from standard geochemical samples, respectively. and These are the mass spectrometry intensities of the internal standard elements in Ag slices of actual geochemical samples and Ag slices of standard geochemical samples, respectively.

[0070] (4) Calculation results The mass fractions of Pd, Pt, and Au in the silver sample were calculated based on the calibration curve. The contents of Pd, Pt and Au in the sample Ag tablets are calculated according to formula (1):

[0071] In the formula, It represents the mass fractions of Pd, Pt, and Au in a geochemical sample, expressed in ng / g. This represents the mass fractions of Pd, Pt, and Au in the blank silver sample, in μg / g. This is the mass of the blank sample silver sheet, in grams. It represents the mass fractions of Pd, Pt, and Au in the silver sample, in μg / g. This refers to the mass of the sample silver sheet, in grams. This is the mass of the geochemical sample, expressed in grams (g).

[0072] Example 2 (Soil Sample) This embodiment is basically the same as Embodiment 1, except that the geochemical sample is soil, the sample weight is 15g, and the soil formula in Table 1 is as follows: 20g borax, 15g glass powder, 50g Na2CO3, 40g Bi2O3, and 4g flour. The remaining steps are the same as in Embodiment 1.

[0073] Example 3 (Aqueous Sediment Samples) This embodiment is basically the same as Embodiment 1, except that the geochemical sample is a stream sediment, the sample weight is 15g, and the formula for stream sediments in Table 1 is as follows: 20g borax, 15g glass powder, 50g Na2CO3, 40g Bi2O3, and 4g flour. The remaining steps are the same as in Embodiment 1.

[0074] Example 4 (Black Shale Sample) This embodiment is basically the same as Embodiment 1, except that the geochemical sample is black shale, the sample weight is 5g, and the black shale formula in Table 1 is as follows: 20g borax, 20g glass powder, 50g Na2CO3, 100g Bi2O3, and 3g flour. The remaining steps are the same as in Embodiment 1.

[0075] Example 5 (Polymetallic mineral sample) This embodiment is basically the same as Embodiment 1, except that the geochemical sample is a polymetallic mineral, the sample weight is 10g, and the polymetallic mineral formula in Table 1 is as follows: 15g borax, 20g glass powder, 50g Na2CO3, 100g Bi2O3, and 5g flour. The remaining steps are the same as in Embodiment 1.

[0076] Example 6 (Different laser parameters) This embodiment is basically the same as Embodiment 1, except for the laser ablation parameters: energy density of 6 J / cm², frequency of 14 Hz, and spot diameter of 160 μm. The remaining steps are the same as in Embodiment 1.

[0077] Example 7 (Different ICP-MS residence times) This embodiment is basically the same as Embodiment 1, except that the ICP-MS dwell time is 80ms. The remaining steps are the same as in Embodiment 1.

[0078] Effect Experiment Experimental Example 1 Ag tablets were prepared according to the method in Example 1. Chinese standard reference material GBW07198 was used as the experimental object. The only difference was the amount of Ag standard solution added in step (1) (the amount of Ag added was 5 mg, 10 mg, 15 mg, and 20 mg of silver granules, respectively), to investigate the effect of the amount of Ag added on the measurement results. The measured value and relative standard deviation (RSD) were used as evaluation indicators.

[0079] Experimental results are as follows Figure 1 As shown, with increasing Ag addition, the measured values ​​of the analytes Pd, Pt, and Au remained relatively similar, but the RSD tended to increase gradually. Furthermore, the RSD was almost identical when Ag addition was 5 mg and 10 mg. Additionally, when Ag addition was 5 mg, the measured values ​​of Pd, Pt, and Au were slightly lower, and the Ag granules were too small for proper handling; therefore, an Ag addition of 10 mg was chosen.

[0080] Experimental Example 2 Ag tablets were prepared according to the method in Example 1 (Ag addition was fixed at 10 mg). Using Ag tablets prepared with Chinese standard material GBW07198 as the experimental subject, the laser energy density, frequency, spot size, and ICP-MS residence time were varied during LA-ICP-MS measurement. Mass spectrometry intensity and relative standard deviation (RSD) were used as evaluation indicators, and parameters with high mass spectrometry intensity and low RSD were selected as the optimal conditions.

[0081] like Figure 2 As shown, with the increase of laser energy density, the mass spectrum intensity gradually increases, and the RSD decreases accordingly; however, when the energy density exceeds 7 J / cm², the RSD rises again, and the mass spectrum intensity decreases. Experiments show that the energy density performs well in the range of 5–7 J / cm², with 5 J / cm² being the optimal value.

[0082] like Figure 3 As shown, with the increase of laser frequency, the mass spectrum intensity gradually increases, and the RSD decreases accordingly; however, when the frequency exceeds 16 Hz, the RSD rebounds. Experiments show that the performance is good in the frequency range of 12–16 Hz, with 12 Hz being the optimal value.

[0083] like Figure 4 As shown, the mass spectrum intensity gradually increases with the increase of the laser spot size, and the RSD decreases accordingly; however, when the spot size exceeds 175 μm, the RSD rebounds. Experiments show that the laser performs well in the range of 135–175 μm, with 150 μm being the optimal size.

[0084] like Figure 5As shown, with the extension of ICP-MS residence time, the mass spectrometry intensity gradually increases, and the RSD decreases accordingly; however, when the residence time exceeds 90 ms, the signal intensity decreases instead of increasing, and the RSD increases. Experiments show that residence times in the range of 60–90 ms exhibit good performance, with 80 ms being the optimal value.

[0085] Experiment Example 3: Verification of Internal Standard Correction Effect Ag tablets were prepared according to the method in Example 1 (Ag addition was fixed at 10 mg). Using the national primary standard material GBW07198 as the experimental material, a Pt isotope diluent with approximately the same Pt ​​content as the sample was added during the bismuth assay process to prepare the Ag tablets. LA-ICP-MS surface imaging technology was used for detection. 194 Pt, 195 Pt, 105 Pd and 197 Distribution of Au in Ag tablets.

[0086] like Figure 6 As shown, 194 Pt, 195 Pt, 105 Pd and 197 Au is unevenly distributed in Ag films, but the trend of uneven distribution is consistent. Therefore, in Pt isotope diluents... 194 Pt, as an internal standard, can effectively correct errors caused by uneven element distribution.

[0087] Table 6 shows the Pt isotope diluent and the Pt isotope content in nature. The Pt isotope diluent contains... 194 Pt has the highest content and is the most abundant natural resource. 194 Pt abundance and 195 Pt is close.

[0088] Table 6. Pt isotopic abundance and relative atomic mass (A) of Pt in nature and Pt isotope diluents

[0089] Ag tablets (made with GBW07198 and UMT-1, with 5232 ng of Pt isotope diluent added) were tested by LA-ICP-MS. 194 Pt, 195 Pt, 105 Pd and 197 The mass spectrometry intensity of Au was calculated using formulas (2), (3), and (4) to determine the concentration of the isotope in the diluent. 194 The mass spectrometry intensity of Pt was used as an internal standard, and the results are shown in Table 7.

[0090] Table 7 Standard Reference Materials 195 Pt, 105Pd, 197 Au and isotope diluent 19 Mass spectral intensity and intensity ratio of 4Pt

[0091] Note: s I 194Pt This refers to Pt isotope diluents 194 Mass spectrum intensity of Pt, n I 194Pt Refers to geochemical samples 194 Mass spectrum intensity of Pt, n I 195Pt Refers to geochemical samples 195 Mass spectral intensity of Pt.

[0092] As shown in Table 7, Ag tablets contain 195 Pt (in standard reference) 195 Pt) , 105 Pd, 197 Au and internal standard (in isotope diluent) 194 The RSDs of the mass spectral intensity ratio of Pt are much lower than the RSDs of the mass spectral intensity of these elements themselves, especially 195 Pt (in standard reference) 195 Pt) and isotope diluent 194 The mass spectrometry intensity ratio of Pt is less than 1%. (This is in the context of Pt isotope diluents.) 194 Using Pt as an internal standard, compared to using Ag with an almost 100% concentration, avoids detector oversaturation caused by detecting excessively high Ag concentrations. The residence time is increased from 20 ms to 80 ms, reducing the instrument's signal-to-noise ratio and improving the precision of various target elements, especially Au. This overcomes the limitation of using Ag as an internal standard in determining Au. Furthermore... 194 The amount of Pt added can be controlled within the range of 1 to 2 times the Pt content in the sample. The mass spectrometry intensity is similar to that of the isotope of the element to be measured, which reduces the instability and systematic error of LA-ICP-MS test parameters and further improves the accuracy and precision of the measurement.

[0093] Experiment Example 4: Calibration Curve and Detection Limit A standard series of silver tablets (with a fixed Ag addition of 10 mg) was prepared according to the method in Example 1. In this experiment, standard Ag tablets prepared using the standard substances GBW07288, GBW07289, GBW07291, GBW07293, GBW07194, GBW07196, GBW07198, GBW07203, and GBW07342 were used as the standard series for LA-ICP-MS. According to formula (5), the amount of Ag in the standard Ag tablets... 195Pt, 105 Pd and 197 mass fraction of Au ( () is the x-axis, in standard Ag tablets 195 Pt (in standard reference) 195 Pt), 105 Pd and 197 The mass spectrometry intensity of Au and its content in the isotope diluent 194 The mass fraction of Pt and the content of isotope diluent 194 The product of the mass spectrometric intensity ratios of Pt (internal standard) ( The calibration curve was plotted with the vertical axis as the ordinate, and the results are shown in Table 8.

[0094] Table 8. Determination of Ag in Ag tablets by LA-ICP-MS 195 Pt, 105 Pd and 197 Linear equation of Au

[0095] Example 5: Sample Accuracy Verification The contents of Pt, Pd, and Au in several national primary standard reference materials and actual geochemical samples were determined using the methods described in Examples 1-5 of this invention. The results are shown in Table 9. As can be seen from the results, the measured values ​​of Pt, Pd, and Au in the standard reference materials are consistent with the certified values, and the measured values ​​of Pt, Pd, and Au in the actual geochemical samples are consistent with the measured values ​​of lead assay-GF-AAS.

[0096] Table 9. Bi-FA-LA-ICP-MS determination of Pt, Pd and Au values ​​(ng / g, n=6) in standard and actual samples.

[0097] Table 9

[0098] Note: * The reference values ​​for Sample 1 - Sample 10 were determined using lead assay-GF-AAS.

[0099] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. An isotope dilution-bismuth assay-LA-ICP-MS method for determining noble metals, characterized in that, Includes the following steps: S1. Dry, grind, sieve, dry and cool the geochemical sample to obtain sample powder; mix the sample powder with bismuth reagent to obtain a mixture; take a portion of the mixture and add platinum isotope diluent, add silver carrier dropwise, evaporate to dryness, add another portion of the mixture, then cover with a covering agent, melt and blow away ash to obtain silver granules containing the elements to be tested, gold, platinum and palladium. S2. Anneal the silver granules and press them into silver sheets; S3. The mass spectrometric intensities of gold, platinum and palladium in the silver sheet were determined by laser ablation inductively coupled plasma mass spectrometry, and a specific isotope in the platinum isotope diluent was used as an internal standard for signal correction. S4. Based on the determination results of step S3, combined with the amount of platinum isotope diluent added, the mass of the silver sheet and the mass of the original sample, the contents of gold, platinum and palladium in the geochemical sample are calculated.

2. The isotope dilution-bismuth assay-LA-ICP-MS method for determining noble metals according to claim 1, characterized in that: In step S1, the platinum isotope diluent is... 194 Pt isotope diluent; the silver carrier is a silver nitrate solution with a concentration of 40 g / L, and the amount added is 200-400 μL, so that the mass of the silver particles obtained after ash blowing is 5-10 mg.

3. The isotope dilution-bismuth assay-LA-ICP-MS method for determining noble metals according to claim 1, characterized in that: In step S1, the bismuth reagent includes borax, glass powder, sodium carbonate, bismuth trioxide, and flour; the covering agent is a mixture of borax, glass powder, sodium carbonate, bismuth trioxide, and flour.

4. The isotope dilution-bismuth assay-LA-ICP-MS method for determining noble metals according to claim 1, characterized in that, In step S1, the ingredients include the following raw materials in parts by weight: 5–20 geochemical samples; Borax 15-20 parts; 15-25 parts glass powder; Sodium carbonate 50-55 parts; 40-100 parts of bismuth trioxide; 3-5 parts flour; The geochemical sample is one of silicate rocks, carbonate rocks, sulfide rocks, soil, stream sediments, black shale, and polymetallic minerals; the sulfide rocks are sulfide rocks roasted at 600℃ for 2 hours.

5. The isotope dilution-bismuth assay-LA-ICP-MS method for determining noble metals according to claim 1, characterized in that: In step S1, the melting temperature is 1050–1100℃, the holding time is 30–50 min, and the ash blowing temperature is 930–950℃.

6. The isotope dilution-bismuth assay-LA-ICP-MS method for determining noble metals according to claim 1, characterized in that: In step S2, the annealing temperature is 650–750°C, the annealing time is 20–40 min, the pressing pressure is 30–40 tons, and the pressing thickness is 0.15–0.25 mm.

7. The isotope dilution-bismuth assay-LA-ICP-MS method for determining noble metals according to claim 1, characterized in that, In step S3, a specific isotope in the platinum isotope diluent is used as an internal standard for signal correction. Specifically, formula 4 is derived from formulas 2 and 3, and formula 4 is used to calculate the value of the platinum isotope diluent. 194 The mass spectrometry intensity of Pt was used as an internal standard to calibrate the mass spectrometry intensities of the analytes gold, platinum, and palladium. Formula 2 is as follows: (Official 2) Formula 3 is as follows: (Official 3) Formula 4 is as follows: (Official 4) In the formula, It is the mass spectrometry intensity of the isotope diluent. These are geochemical samples 194 Mass spectrum intensity of Pt; This is the total mass spectrum intensity measured by ICP-MS at a mass number of 194. This is the total mass spectrum intensity measured by ICP-MS at a mass number of 195; A N194 In geochemical samples 194 The natural abundance of Pt, A N195 In geochemical samples 195 Natural abundance of Pt; A S194 It is an isotope diluent 194 Pt abundance, A S195 It is an isotope diluent 195 Pt abundance.

8. The isotope dilution-bismuth assay-LA-ICP-MS method for determining noble metals according to claim 1, characterized in that, In step 3, the determination conditions for laser ablation inductively coupled plasma mass spectrometry are as follows: The laser ablation system has a wavelength of 193 nm, an energy density of 5–7 J / cm², and a He carrier gas flow rate of 0.28–0.35 L / min. The quantitative measurement conditions in the laser ablation system are a frequency of 12–16 Hz and a spot diameter of 135–175 μm. The surface scanning conditions in the laser ablation system are a frequency of 280–320 Hz and a spot diameter of 5–10 μm. The power of the inductively coupled plasma mass spectrometer is 1500–1550 W, the cooling gas flow rate is 13–14.5 L / min, the auxiliary gas flow rate is 0.70–1.00 L / min, the nebulizing gas flow rate is 0.80–1.20 L / min, the residence time is 60–90 ms, and the number of scans is 20–50.

9. The isotope dilution-bismuth assay-LA-ICP-MS method for determining noble metals according to claim 1, characterized in that: The S3 step also includes the step of establishing a calibration curve: select multiple national first-level standard materials, process them using the same S1 and S2 steps as the sample to be tested, and prepare a series of standard silver sheets; use the empirical coefficient method to plot the calibration curve with the mass fraction of the element to be tested in the standard silver sheet as the abscissa and the product of the ratio of the mass spectral intensity of the element to be tested in the standard silver sheet to the mass spectral intensity of the internal standard element as the ordinate.

10. The isotope dilution-bismuth assay-LA-ICP-MS method for determining noble metals according to claim 1, characterized in that: In step S4, the contents of gold, platinum and palladium in the geochemical sample are calculated using formula 1; Formula 1 is as follows: In the formula, It represents the mass fractions of Pd, Pt, and Au in a geochemical sample, expressed in ng / g. This represents the mass fractions of Pd, Pt, and Au in the blank silver sample, in μg / g. This is the mass of the blank sample silver sheet, in grams. It represents the mass fractions of Pd, Pt, and Au in the silver sample, in μg / g. This refers to the mass of the sample silver sheet, expressed in grams. This is the mass of the geochemical sample, expressed in grams (g).

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