Multi-element Accurate Analysis of Sulfide Minerals by Low-temperature Ablation LA-ICP-MS

Micro-geochemical information of sulfide minerals plays a crucial role in the field of geochemistry, allowing discovery of the formation mechanism and evolution process of sulfide minerals by analyzing their element composition characteristics. LA-ICP-MS is currently the most popular microanalysis technology used for sulfide analysis, having yielded successful results. Due to their unique physical and chemical properties, sulfide mineral samples show different laser ablation behavior to conventional geological samples. The most intuitive phenomenon is the melting of ablation carters caused by laser thermal effect and the deposition of a large number of material particles around the ablation carters, which is the main factor limiting the precision and accuracy of sulfide sample analysis.　　Walting et al¹³ found that direct quantitation of multi-elements in sulfide minerals by infrared laser (1064-nm Nd:YAG laser) was impossible, which was because the strong thermal effect generated by the infrared laser will lead to severe large particle aerosol redeposition. It is reported that ablation systems with shorter wavelengths, such as ultraviolet lasers, including the 266 and 213nm laser, can be used to obtain acceptable analytical accuracy by reducing the thermal effect and aerosol particle size, but a poor precision was still observed^16-17. Guillong et al¹⁹ conducted a comparative study of 266, 213 and 193nm lasers and found that there were finer particle sizes of the aerosols and the weaker thermal effect when using 193nm laser ablation, and the RSDs of all elements less than 20% were obtained. In other words, collisions between photons and matter intensify in deep ultraviolet laser ablation systems (193nm) with shorter wavelength^20-21 and can help reduce the melt zone and aerosol particle size. However, there is still a slight thermal effect during 193nm UV laser ablation. Fernández et al²² found that there is still a melting layer during 193nm laser ablation, and it leads to the formation of large particle aerosols. Different methods have been proposed to improve the thermal effect during laser ablation of sulfide. Muller et al²⁵ found that the precision of line scanning could be improved by 50% compared to spot ablation. Guillong’s results showed that adding a small amount of hydrogen to the analysis could increase the sensitivity of the 47 elements in the test by two to four times²⁶. Moreover, research has focused on improving the thermal effect of sulfide minerals from shorter pulse width lasers and aerosol particle transport^27-31. However, there are still some thermal effects in the process of deep ultraviolet and short wavelength laser ablation, and how to inhibit the thermal effect in the process of ablation to obtain effective analysis results is still a difficulty in the analysis of sulfide mineral elements. The LA-ICP-MS low temperature ablation cell is an ablation system developed in recent years, whose main function is to provide a low temperature ablation environment to realize the effective analysis of cells, blood and other samples. The low-temperature ablation cell may be a new approach to resolve the thermal effect during sulfide mineral ablation.

In order to establish a high precision and high accuracy multi-element analysis method for sulfide minerals.

The use of a designed cryogenic ablation cell suppressed the thermal effect and refined aerosol particle sizes, which improved analytical precision and accuracy significantly. To explore the mechanism of sulfide ablation at low temperature, the aerosols ablated at low temperature were collected using an aerosol collection setup consisting of a membrane with an aperture of 0.1μm, which was installed at the outlet of the ablation cell. According to the micro-analysis results, the laser ablation behavior under low temperature ablation environment was further discussed.

A precision and accuracy method for multi-elements analysis of sulfide minerals using CLA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry with a cryogenic ablation cell) was described. Ablation craters were investigated via scanning electron microscope (SEM) images to compare the amounts of melt produced. SEM measurements showed significant differences in melting between the low temperature (−30℃) and room temperature (20℃). The diameters and size distribution of particles were measured from nanometer particle potentiometer images of the collected ablated aerosol. Particles ablated using cryogenic ablation cell were smaller in average diameter (190nm and 400nm) and shorter in distribution range (570nm). Compared to the precision of time-resolved signal during laser ablation processes between the two temperatures, the precision was significantly improved and the RSD was reduced from 20.1%-34.4% to 11.5%-15.8% with a cryogenic ablation cell.　　A designed cryogenic ablation cell in sulfide sample analysis was utilized to minimize the thermal effect and improve analytical precision and signal intensity. In this study, the CRM (MASS-1) sample was analyzed with spot ablation mode at low (−30℃) and room (20℃) temperatures, respectively, and the RSDs of three times parallel analysis at these two temperatures were compared. At room temperature, the RSDs of elemental signals ranged from 20.1% to 34.4%. In contrast, the RSD of elemental signals was less than 15.8% when the sample was ablated at low temperature (Fig.2a). The significant improvements may be attributed to low ablation temperature, which suppress the thermal effect. Moreover, the signal intensities of elements improved by approximately 11% to 52% with the decrease in temperature of the cryogenic ablation cell (Fig.2b). Fig.2c shows the time-resolved signals of the MASS-1 sample at room temperature, the significant fluctuations and spikes could be observed, and the RSDs of all elemental signals was more than 20.1%. Interestingly, the signals at low temperature exhibited ideal stability, and the RSDs of elemental signals were less than 15.8%, as shown in Fig.2d.　　In order to explore the reasons for improving the analytical performance of low temperature, the morphology of ablation under different temperature conditions of two standard sulfide samples were discussed. SEM images of four ablation craters on chalcopyrite and pyrite were taken to investigate the effect of temperature on the ablation process (Fig.3). The sulfide samples were ablated using a 193nm excimer laser with a spot size of 60nm and a fluence of 8J/cm². The ablation craters on the chalcopyrite showed a two-layer cyclic structure, in which the inner layer was a light-colored melting zone, and the outer layer was a white aerosol vapor sediment. At low temperature, there were fewer melt layers and thinner grain sediment zone than those at room temperature (Fig.3a, Fig.3b). However, the melting zone around the ablation craters of pyrite were more irregular. More of the unwanted ablation was melted away at room temperature (Fig.3c, Fig.3d). The craters formed at room temperature (Fig.3a, Fig.3c) showed a more serious melting phenomenon than those formed at low temperature (Fig.3b, Fig.3d), as evidenced by the abundance of molten ejecta around the former, especially at high laser energy densities. In contrast, the low temperature craters showed no obvious melting phenomenon and had a flatter bottom with a reduced number of large molten spherical particles. The use of CLA-ICP-MS weakened the melting phenomenon, thereby generating smaller aerosol particles, which further improved the aerosol transport and ionization efficiency.　　A particle size collection experiment was conducted to explore the distribution of aerosol particles at different temperatures. SEM images were used to analyze the shapes and sizes of particles that were collected on a membrane with an aperture of 0.1μm at room temperature (20℃) and low temperature (−30℃). The same sample chamber and 1m of tubing were used to transport the particles, and the ablation pulses continuously for 2min. The SEM images showed that the particles produced at room temperature were larger and formed large agglomerates (Fig.4a), whereas the particles produced at −30℃ were smaller and there were fewer agglomerations (Fig.4c). The shape of the agglomerates and their connection by filaments suggested strong charge during particle formation, which was more prominent at room temperature. Additionally, there were more single large particles produced at room temperature (Fig.4b), while there were fewer particles at −30℃ (Fig.4d). Comparative measurements were conducted using 193nm laser to investigate the influence of temperatures on particle size distribution. Fig.4 shows a typical size distribution for LA under He atmosphere. The left part of Fig.5 shows a distribution of aerosols produced by ablation of 2min pulses at room temperature. The peak heights of mean diameter in this distribution were determined to be approximately 300nm and 700nm, respectively. Similarly, particle size distribution at −30℃ also presented a bimodal pattern, which was consistent with previous studies. The average diameters were 190nm and 400nm, both smaller than at room temperature, while the peak width was shorter. The chemical composition of fine particles produced at low temperature is closer to the sample body, improving the transport and ionization of aerosol in ICP, reducing element fractionation, and enhancing the signal strength and stability, thereby improving the analytical performance of ICP-MS.

A new high-precision and accuracy method for determination of trace elements in sulfide minerals has been developed using the CLA-ICP-MS system. This method reduces thermal effect and decreases particle size during the ablation process, improving precision by freezing sulfide samples with a designed cryogenic ablation cell. Low temperature results in better data because fewer large particles are produced; sedimentation around the ablation crater and during transport is reduced, while ionization efficiency in ICP is higher. The precision calculated for transient signals decreases obviously if the sample is kept at low temperature (−30℃) compared to room temperature (20℃), while the sensitivity improved slightly. The deviation of all elements between the test values and the standard values falls within 7% by CLA-ICP-MS. In future work, it will be necessary to investigate even lower temperatures, as low temperatures can increase aerosol viscosity and affect analysis results. It is also worth exploring whether the performance of a long pulse width laser can be improved by lowering the temperature to match that of a short pulse width laser.