Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer (LA-ICP-MS) in Geosciences: Further Improvement for Elemental Analysis

A quantitative reduction strategy consisting of Ratioing, Standardization, and Normalization (RSN) was proposed to process the LA-ICP-MS transient signal of NIST, MPI-DING, USGS and CGSG glass reference materials. The RSN strategy allows the quantitative reduction without knowing the concentration of internal standard prior to LA-ICP-MS analysis. The down-hole fractionations of the investigated glasses in initial 5-35s ablation period were insignificant and independent of the chosen internal standards under the given laser conditions. The analytical accuracy obtained from internal standardization was affected by the given value of the internal standard. Contrarily, accuracy obtained from RSN strategy was independent of the chosen internal standard. Matrix effects between NIST610 and geological glasses were negligible. Imprecise certified values of several elements were identified. The prefer values reported in this study are the better-estimated values for these elements. Laser spot size could be down to 15 um where the applicability of RSN strategy was still acceptable. The short- and long-term precision (359 repetition analysis over three years) illustrated that either instrument conditions or the capability of RSN strategy were approvingly acceptable. The simplicity and applicability of RNS strategy in comparison with internal standardization strategy make it suitable for quantitative reduction for silicate glasses. Quantification strategy as an essential issue for laser ablation ICP-MS plays an important role in the guarantee of analytical accuracy. In this study, the reference value uncertainties of current available glass reference materials (including NIST, MPI-DING and USGS) as well as the matrix effects were systematically evaluated. The results illustrated that NIST610 was better than other glass reference materials from aspect of reference value uncertainty. The matrix effects among NIST, MPI-DING and USGS glasses were insignificant under the experimental conditions. The quantification strategy based on two reference materials (NIST610 and StHs6/80-G) and bulk normalization as 100 % (wt) was proposed to reduce the LA-ICP-MS generated transient signals, which eliminates the deficiencies encountered with the quantification strategy using single reference material, such as the extreme low content or large uncertainty of some elements. The comparison of ML3B-G results obtained from three quantification strategies (single reference material NIST610, single reference material StHs6/80-G and two reference materials) illustrated that the proposed strategy improved the analytical accuracy. Three reference materials including BCR-2G, CGSG-2 and KL-2G were quantified using the proposed strategy, and almost all data matched well with reference values. The data reported in this study could supplement the reference value database for BCR-2G, CGSG-2 and KL2-G. A comparison of sample preparation methods including ultrafine powder pellet and flux-free fusion glass for LA-ICP-MS analysis of granitic rock samples was carried out. The ultrafine powder particles are characterized as d50= 1.2 um, d90= 5.5 um. Our results illustrate that the agate abrasion was around 2-3 % relative to the mass of the original rock powders, however, contaminations with elements other than SiO2 from agate abrasion are negligible due to its chemicals are two magnitudes lower relative to original powders. A flux-free fusion protocol with a melting sequence of fusion, grinding and re-fusion was proposed for producing granitic glasses. The results demonstrate that the prerequisite for obtaining homogenous granitic glasses is the grinding procedure. Depletion of highly volatile elements (e.g. Pb) was observed during fusion. Laser ablation rates are specified to the individual matrix. Analytical precisions of fusion glasses are comparable to MPI-DING glasses, while the precisions of powder pellets are slightly worse, which may be ascribed to the existence of remaining larger crystal fragments. Data accuracy demonstrates that preparation methods of ultrafine powder pellet and fusion glass are practical for LA-ICP-MS bulk analysis of granitic samples, which otherwise may suffer from incomplete dissolution in solution ICP-MS measurements. The signal enhancement effects using guard electrode and the addition of small amount nitrogen (N2) and hydrogen (H2) into the carrier gas flow (Ar + He) of the Ar plasma in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is presented. The results illustrate that signal intensity of 54 investigated elements is enhanced up to 6 folds by using guard electrode compared to without guard electrode, which is related to an increase in the ion density that was induced by the shrinkage of the whole plasma due to the guard electrode. Guard electrode shifts the ionization zone backward to sample cone that needs a larger carrier gas flow to compensate. A small amount of H2 decreases the signal sensitivity in GE-off mode (with guard electrode), while slightly enhances the sensitivity in GE-on mode (without guard electrode). A small amount of N2 shifts the ionization zone backward to the sample cone in both of GE-off and GE-on modes. The results illustrate that those six modes (GE-off, GE-off-N2, GE-off-H2, GE-on, GE-on-N2, GE-on-H2) produced very similar analytical data. The GE-on-N2 with (2 ml min-1) is the best instrument conditions for routine multiple trace element analysis. Limits of detection (LODs), mass load effect, downhole induced fractionation and matrix effect of 193 nm ArF excimer laser ablation system at high spatial resolution were systematically investigated. Trace elements in GSD-1G, StHs6/80-G, and NIST612 were analyzed at 10 um spot size. The results showed that LODs decreased with increasing ablation diameter. LODs of some trace elements were in a range of 1~10 ug/g at 7 um spot size. Mass load effect was negatively correlated with corresponding oxide melting temperature, while positively correlated with elemental 1st ionization potential. Downhole fractionation was negligible when the ratio of ablation depth versus spot size was smaller than 1:1. Matrix effect based on elemental pair method showed that there were no significant changes between spot sizes of 50 um and 10 um among investigated reference materials (NIST610, GSD-1G, ATHO-G, and StHs6/80-G). Based on NIAT610 as external standard and Ca as an internal standard, the analytical results of 36 trace elements in GSD-1G, StHs6/80-G and NIST612 at 10 um spot size matched well with the reference value. Generally, 10 um spatial resolution could satisfy the requirements of trace element analysis. Knowing laser ablation behaviors of different target materials is essential for selecting optimum laser parameters, choosing external reference materials, as well as for the assurance of data quality. In this study, ablation behaviors of 193 nm ArF excimer laser for silicate glasses, common minerals, and powder pellets were investigated. Ablation rates influenced by laser parameters (including spot size, energy density, and laser frequency) were evaluated. Topographic images of laser craters illustrate that glasses and most minerals have the controllable ablation behaviors, except quartz. The worse ablation behavior of quartz may be ascribed to the micro-fluid inclusions, which could result in the overheating effect in laser pits. Powder pellets have worse ablation behaviors compared to glass, while their ablation behaviors could be improved either by increasing the tableting pressure or by decreasing the particle grain size. Ablation rates gradually decrease when the ablation depth is 1.5 times larger than spot size. The maximum ablation depth could reach twice as much spot size at the experiment conditions (RESOlution M-50 laser system and energy density 3.0 J/cm2). Ablation rates increase with the growth of laser energy density, while are not affected by the laser frequency (2-20 Hz). Ablation rates are specified to the individual sample substrates. In general, ablation rates of powder pellets are larger than glasses and minerals, carbonates and sulfides are greater than silicate minerals, NIST glasses are higher than geological glasses. Ablation rate data of 43 different sample substrates were presented, and these data could provide the reference for other laboratories.