Nanosecond Laser Ablation Behavior Affecting The Physical Mechanism Of Matrix Effect And Element Fractionation Effect During Ablation

Laser ablation inductively coupled plasma mass spectrometry（LA-ICP-MS）is a technology widely used in the abundance determination of major and trace elements in solid geological samples,but the matrix effect and element fractionation effect in the analysis process seriously restrict the accuracy of LA-ICP-MS.The matrix effect refers to the suppressing or enhancing effect of the analytical signals caused by the matrix,and the element fractionation effect is the sum of all the non-stoichiometric sampling during the LA-ICP-MS analysis.Matrix effect and element fractionation effect occur in the ablation process,transport process,and ICP ionization process during LA-ICP-MS analysis.Because the particle deposition in the transport process and particle ionization in ICP largely depend on the mass and particle size distribution of primary aerosol produced in the laser ablation process,therefore,the matrix effect and element fractionation effect caused by transport process and ICP ionization process are strongly correlated with the laser ablation process.In order to clarify the generation mechanism of matrix effect and element fractionation effect in the laser ablation process of geological samples,the dynamic evolution process of plasma,shock wave（SW）,and ejected material produced in the ablation process of geological standard samples（BCR-2G,BHVO-2G,BIR-1G,NIST610,NIST612,and NIST614）is accurately recorded by high time-resolved pump-probe shadowgraph technique.In order to observe the ultrafast dynamic process inside the sample,high transparent calcium fluoride（Ca F₂）poly-crystal and fused quartz glass（amorphous）are used to research.These two kinds of samples are selected to represent the crystal and amorphous in geological samples respectively.At the same time,the matrix effect and element fractionation effect in the 8 geological standard samples（BCR-2G,BHVO-2G,BIR-1G,GSD-1G,GSE-1G,NIST610,NIST612,and NIST614）are analyzed by 193 nm Ar F ns-LA-ICP-MS,and the matrix-matched method is selected to quantify the content of trace elements in geological standard samples.The main conclusions and understandings are as follows:1.The difference of physical properties（crystal and amorphous）and chemical properties of samples will lead to different ultrafast dynamic behavior in the ablation process,which will affect the ablation behavior of samples,produce ablation particles of different sizes and quantities,and induce matrix effect.In LA-ICP-MS,the external standard matching with the sample matrix can be selected by analyzing the ultrafast dynamic behavior of laser ablation process of different samples,so as to realize matrix-matched correction.2.The size of particles induced by SW is larger than those directly generated by laser,indicating that the ablation particle morphology is related to the particle generation mechanism.While the element fractionation effect in LA-ICP-MS is closely related to the particle morphology,compared with small-size particles,large-size particles are more likely to induce element fractionation effect.3.The pressure,density,and velocity of each layer inside the ablation plume are different,resulting in different condensation and volatilization behaviors of the same element in different layers,thus inducing element fractionation effect.Laser energy has an effect on the multilayer structure inside the ablation plume.Therefore,during laser sampling,the properties of each layer can be close by changing the laser energy,so as to reduce the element fractionation effect in LA-ICP-MS.4.During laser ablation,the generation of large-size particles is related to the secondary SW outside the sample,which can be avoided in the following ways:during laser sampling,low-pressure environment and low viscosity gas（such as helium）are used to expand the SW front and reduce the collision and aggregation of ablation particles,so as to inhibit or control the generation of large particles or clusters.5.In ns-LA-ICP-MS analysis,when Ca is an internal standard,it can effectively reduce the matrix effect between the same series of standard samples（USGS series（BCR-2G,BHVO-2G,BIR-1G,GSD-1G,and GSE-1G）and NIST series（NIST610,NIST612,and NIST614）respectively）and the element fractionation effect in the standard samples.The laser fluence（4-12 J/cm²）selected in this paper has little effect on the matrix effect between the 8 standard samples and the element fractionation effect in GSE-1G and NIST610.6.LA-ICP-MS assisted matrix-matched total sum normalization method can accurately quantify the content of trace elements in silicate samples.