Novel Sample Preparation Method For Whole-rock Geological Samples And Its Application In Elemental And Silicon Isotopic Analysis

Knowledge of the chemical and isotopic composition of geological samples including rocks,sediments,soils and minerals provides essential geochemical information for understanding and exploring the evolution of earth,climate changes,mineralization processes,distribution of mineral resources and other geoscience issues.Determination of major and trace element concentrations and isotope ratios in whole-rock geological samples has become the dailiest and most fundamental operation in geochemical laboratory.However,conventional digestion procedure for geological sample is tedious and time-consuming as well as huge consumption of hazardous digestion solvents,such as acid and alkali reagents.Moreover,extra chemical purification procedures for isotope ratios analyses further complicates the sample preparation process,which is necessary for minimizing matrix effects by separate interest isotopes from sample matrices.According to preliminary estimates,tens of thousands of hazardous chemical materials?e.g.acids,alkalis and toxic reagents?will be consumed accompanying emissions of vapors,gases and wastes in nearly 10,000 laboratories every year in our country,which inevitably cause risk to the operator and result in pollution of the environment.It is exploration for new sample preparation strategies with lower or non-reagent consumption and risk of pollution that hold the key to solve these problems.Basic state policy of environmental protection should be complied during geological analysis processes,this is necessary for sustainable development of geological laboratories.Laser ablation-inductively coupled plasma-mass spectrometry?LA-ICP-MS?is a direct solid sampling and introduce technique,avoiding laborious chemical digestion procedure,which completely agrees with the principles of green analytical method.LA-ICP-MS is a micro-sampling technique with small sampling volume?less than 1?g?,and the analytical precision depends considerably on sample homogeneity.Unfortunately,geological materials such as rocks,sediments and soils are extremely heterogeneity with complex matrix,elemental and isotopic analysis results usually unreliable?accurate and precision up to 10-40%?by laser ablation sampling technique for whole-rock samples without preparation.And improving the homogeneity of geological samples is the most fundamental solution to obtain satisfied analytical results.Therefore,the need for preparation of stable,homogeneous,and mechanically resistant targets prior to the analysis of whole-rock powdered samples has been a major drawback hindering accurate and precise measurements using this technique.In addition,the analysis of the major and trace elements and isotopic composition in geological samples such as rocks,sediments,soils and minerals also have huge market demand and economic value.In this study,a novel sample preparation method has been developed for whole-rock geological samples,which is suitable for direct soil sampling technique?LA-ICP-MS?elemental and isotopic composition analysis.And this sample preparation method has been successfully applied to major trace elements determination of a variety of geological samples such as rocks,sediments,soil and bauxite.Based on our sample preparation technique and appropriate matrix effects suppression strategies,an accurate and precise silicon?Si?isotopic analysis method was established for whole-rock samples using laser ablation multi-collector inductively coupled plasma mass spectrometry?LA-MC-ICP-MS?.1.Developed a green,cost-efficient,and simplified direct fusion technique,overcoming the bottleneck that LA-ICP-MS cannot be directly applied for bulk analysis of geological samples.In this study,we creatively used a high energy infrared laser to achieve direct fusion of geological samples,this is the first successful experiment and practice applied to whole-rock geological samples.The laser fusion sample preparation technique is characterized by green,efficient,and simplified.Complete fusion of silicate rock samples,including felsic rocks that contain refractory minerals such as zircon,was achieved takes only tens of milliseconds for each sample,bypassing the use of any chemical reagent or melting container.Compared to pressed pellet sample preparation,the analytical precisions of the developed laser fusion technique are higher by an order of magnitude and 5.2 times for most elements in granodiorite GSP-2 and basalt BHVO-2,respectively.Analytical results obtained for five USGS reference materials?ranging from mafic to intermediate to felsic?by LA-ICP-MS using the laser fusion technique generally agree with recommended values with discrepancies of less than 10%for most elements,except for highly volatile elements such as Cu,Zn and Pb.The achieved precision is within 5%for major elements and within 15%for most trace elements.In addition,the basalt,andesite and rhyolite samples were proved to be homogenous on the scale of 24?m,which can obtain reliable major and trace elements composition data by LA-ICP-MS.The ease of use of this developed laser fusion method has greatly improved the analysis efficiency of geological samples and reduced environmental pollution,which provides a model for research of green geological analysis methods and will significantly accelerate the application of laser ablation ICP-MS for whole-rock sample analysis.2.Improve the laser fused glasses method,and applied to samples with highly volatile components,such as sediments,soil and bauxite.The matrices of sediments and soils are complex and variable,and bauxites often contains refractory minerals?such as zircon,rutile,monazite et al.?,these samples usually contains higher volatile components as well.And samples with higher volatile components cannot be fused into bright glasses using direct laser fusion technique.Therefore,the above laser fused glasses method was improved by adding volatile components remove step before fusion.The analysis performances were compared of pressed pellet,direct laser fusion and improved laser fusion technique on sediments and soils,the improved laser fusion technique managed to fuse samples into glasses just as the igneous rocks.Compared to pressed pellet sample preparation,the analytical precisions of the improved laser fusion technique are higher by 7.7 3.9 and 3.9 times on average in MSCS-3,MSCS-5 and GSS-8,respectively.The improved fusion technique was applied to five marine sediment samples,four soil samples and six bauxite samples.Analytical results obtained for marine sediments by LA-ICP-MS using the laser fusion technique generally agree with recommended values with discrepancies of less than 10%for most elements.The achieved precision is within 5%for major elements,within 15%for rare earth elements?REEs?and within 10%for other trace elements.Analytical results obtained for soils by LA-ICP-MS using the laser fusion technique generally agree with recommended values with discrepancies of less than 5%for most major elements and less than 15%for most trace element.The achieved precision is within 5%for major elements and within 10%for most trace elements.Forty-four major and elements including REEs in six bauxite samples were determined by LA-ICP-MS based on the laser fusion technique,measured value of element W is first reported.The achieved precisions of NIST 697,GBW07178 and GBW07181 are within 10%for most elements,and the measured values generally agree with values reported in literatures within a discrepancy of 10%for most elements.Analytical precisions of NIST 698,NIST 69b and GBW07177are pretty good,while measured values have relatively large deviations up to 15-40%.Fortunately,measured values obtained in our work agree better with another recently published study.3.New accurate and precise Si isotopic analysis method for whole-rock geological samples was established,which is the most efficient Si isotopic analysis method for bulk silicate rock samples compared to published studies domestic and overseas.Si is the second most abundant element in the Earth's crust after oxygen and distributes widely in nature,which render the studies of Si isotopic fractionation in different natural reservoirs quite fundamental to understand Earth processes.Significant advances on Si isotope geochemistry come into a boom after the development of instrumentations and analysis methodologies which provide high measurement precisions,especially the advent and development of MC-ICP-MS.However,as mentioned above,sample pretreatment process for the isotope analysis of the solution method is more complicated.And there are few studies on Si isotopic analysis method for whole-rock geological samples by laser ablation sampling technique.An accurate and precise method for Si isotope ratio measurements by fs-LA-MC-ICP-MS for bulk silicate rock samples was established based on the new laser fusion technique.Compared to the pressed pellet technique,the measurement precision for fused glasses was improved?5.7-fold.Matrix effects during Si isotopic analysis were observed by using no-matrix matched calibration protocol under normal instrument conditions with both ns-and fs-laser under raster ablation mode.The measured?²⁹Si and?³⁰Si by fs-LA-MC-ICP-MS were in agreement with previous results when the matrix-matched standard BHVO-2G was used as bracketing calibrator,and results calibrated against silica standard NBS-28 show significant lighter?²⁹Si and?³⁰Si values(the offset up to?0.24 for?²⁹Si and?0.49 for?³⁰Si).While?²⁹Si and?³⁰Si values couldn't be accurate measured when using standard with different element compositions as bracketing calibrator even though the samples and standards are matrix-matched.Matrix effects were eliminated by the addition of water vapor into the plasma,realizing accurate non-matrix matched calibration.In addition,fs-laser provided better measurement precisions,the internal and external precisions were improved?1.85 and?2.4 folds compared to ns-laser under nearly equal signal intensities.Six well characterized silicate rock reference materials were analyzed using fs-LA-MC-ICP-MS,and results obtained are in good agreement with previous published data.Good precisions of 0.06‰to 0.11‰?2SD?and 0.10‰to 0.17‰?2SD?in all samples were obtained for?²⁹Si and?³⁰Si,respectively,confirming the reliability of this proposed Si isotopic analysis method.The Si isotope ratio analysis procedure based on the new laser fusion sample preparation method,verified to be the most efficient Si isotopic analysis method for bulk silicate rock samples at present,which is greatly simplifying the analysis process and improving the analysis efficiency.