Detlef Günther

Inter-laboratory note. Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) produces complex, time-dependent signals. These require significantly different treatment both during data acquisition and reduction from the more steady-state signals produced by solution sample introduction. This paper discusses, in detail, data acquisition and reduction considerations in LA-ICP-MS analysis. Optimum data acquisition parameters are suggested. Equations are derived for the calculation of sample concentrations and LOD when time-resolved data acquisition is employed, sensitivity calibration is obtained from reference materials with known analyte concentrations and naturally occurring internal standards are used to correct for the multiplicative correction factors of drift, matrix effects and the amount of material ablated and transported to the ICP.

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

Quantitative multi-element analysis of minerals, fluid and melt inclusions by laser-ablation inductively-coupled-plasma mass-spectrometry

Laser-ablation ICPMS has become widely accessible as a powerful and efficient multi-element microanalytical technique. One of its key strengths is the ability to analyse a wide concentration range from major (tens of wt.%) to trace (ng/g) levels in minerals and their microscopic inclusions. An ArF excimer laser system (λ = 193 nm) with imaging optics for controlled UV ablation and simultaneous petrographic viewing was designed specifically for representative sampling and quantitative multi-element analysis of microscopic fluid, melt and mineral inclusions beneath the sample surface. After a review of the requirements and recent technical developments, results are presented which together document the reliability and reproducibility of quantitative microanalysis of complex samples such as zoned crystals or fluid and melt inclusions in various host minerals. Analytical errors due to elemental fractionation are reduced to the typical precision achieved by quadrupole LA-ICPMS in multi-element mode (2–5% RSD). This progress is largely due to the small size of aerosol particles generated by the optimized UV optical system. Depth profiling yields representative and accurate concentration results at a resolution of ∼0.1 μm perpendicular to the ablation surface. Ablation is largely matrix-insensitive for different elements, such that silicate and borate glasses, silicates and oxide minerals, or direct liquid ablation can be used interchangeably for external standardization of any homogeneous or heterogeneous material. The absolute ablation rate is material dependent, however, so that quantitative LA-ICPMS analysis requires an internal standard (i.e., an independent constraint such as the absolute concentration of one element). Our approach to quantifying fluid and melt inclusion compositions is described in detail. Experiments with synthetic fluid inclusions show that accurate results are obtained by combining the LA-ICPMS analysis of element concentration ratios with a microthermometric measurement of the NaCl equivalent concentration and an empirical description of the effect of major cations on the final melting temperatures of ice, hydrohalite or halite. Expected calibration errors for NaCl-H2O-dominated fluids are smaller than the typical analytical scatter within an assemblage of simultaneously trapped fluid inclusions. Analytical precision is limited by representative ablation of all phases in heterogeneous inclusions and the integration of transient ICPMS signals, to typically ±10 to 20% RSD. Element concentrations in devitrified and even coarsely crystallized silicate melt inclusions can be reconstituted from LA-ICPMS signals. Deconvolution of inclusion and host signals with internal standardization automatically corrects for sidewall crystallization after melt entrapment at high temperature. A test using melt inclusions in a midocean ridge basalt, a summary of published geochemical studies and a new application to REE analysis of coexisting fluids and mineral phases in carbonatite-related veins illustrate the versatility and some of the strengths and limitations of LA-ICPMS, in comparison with other microanalytical techniques.

Capabilities of an Argon Fluoride 193 nm Excimer Laser for LaserAblation Inductively Coupled Plasma Mass Spectometry Microanalysis ofGeological Materials

Recent developments in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have demonstrated its potential for in situ microanalysis for major, minor and trace elements in solids, such as minerals. With the low backgrounds and high sensitivity of new ICP-MS instruments, limits of detection of 1–10 ng g -1 in a 40 µm ablation pit for many elements can be reached. Fractionation effects due to different ablation rates of various elements have prevented quantification without matrix-matched standards with 1064 nm Nd:YAG lasers. These effects have been reduced but not eliminated using shorter UV wavelengths ( e.g . a quadrupled Nd:YAG 266 nm). Excimer lasers with wavelengths below 200 nm are expected to reduce fractionation effects further, but they present a serious challenge to the design of optical systems, especially if high-resolution UV ablation needs to be combined with high quality visual observation, which is essential for the study of complex materials, such as geological samples. An LA system was developed using an homogenized UV laser beam (193 nm, Argon Fluoride excimer) with a common UV–visual objective on a modified petrographic microscope with reflected and transmitted light illumination, in combination with a Perkin-Elmer Elan 6000 ICP-MS instrument. The optical system allows imaging of both visible and UV laser light onto the sample surface at the same time. Laser operating parameters and their influence on the ablation process were investigated using NIST SRM 612/610. Fractionation effects due to differential ablation of various elements as a function of time can be reduced to interelement correlation coefficients of r =0.9 or better and have become insignificant within the precision of quadrupole ICP-MS using this new optical design. Energy densities and repetition rates need to be kept within limited ranges for accurate and reproducible determinations of trace elements such as Zn, U and Pb, which have previously presented strong fractionation problems. LA-ICP-MS determinations on natural hornblende, augite, and garnet, calibrated against NIST SRM 612 using any major element as an internal standard, agree well with independent literature data. These experiments with the Argon Fluoride 193 nm excimer system demonstrate a greatly reduced matrix dependence of the ablation process, which facilitates in situ analysis of unknown samples.

Highly multiplexed imaging of tumor tissues with subcellular resolution by mass cytometry

Mass cytometry enables high-dimensional, single-cell analysis of cell type and state. In mass cytometry, rare earth metals are used as reporters on antibodies. Analysis of metal abundances using the mass cytometer allows determination of marker expression in individual cells. Mass cytometry has previously been applied only to cell suspensions. To gain spatial information, we have coupled immunohistochemical and immunocytochemical methods with high-resolution laser ablation to CyTOF mass cytometry. This approach enables the simultaneous imaging of 32 proteins and protein modifications at subcellular resolution; with the availability of additional isotopes, measurement of over 100 markers will be possible. We applied imaging mass cytometry to human breast cancer samples, allowing delineation of cell subpopulations and cell-cell interactions and highlighting tumor heterogeneity. Imaging mass cytometry complements existing imaging approaches. It will enable basic studies of tissue heterogeneity and function and support the transition of medicine toward individualized molecularly targeted diagnosis and therapies.

Metal fractionation between magmatic brine and vapor, determined by microanalysis of fluid inclusions

The major and trace element compositions of individual fluid inclusions from a range of magmatic-hydrothermal ore deposits were analyzed by laser-ablation inductively coupled plasma-mass spectrometry, to explore the behavior of ore-forming components during fluid phase separation (“boiling”) in high-temperature saline fluid systems. Data from 13 samples showing unambiguous evidence for coeval trapping of a liquid brine and a coexisting vapor phase identify two groups of elements with drastically different geochemical behavior. Na, K, Fe, Mn, Zn, Rb, Cs, Ag, Sn, Pb, and Tl preferentially partition into the brine (probably as Cl complexes), whereas Cu, As, Au (probably as HS complexes), and B selectively partition into the vapor. Fluid phase separation is probably a major, previously underestimated process in the chemical differentiation that contributes to the extreme range of selective element enrichments in magmatic-hydrothermal systems, from deep plutons through porphyry-style and greisen deposits to epithermal mineralization and volcanic fumaroles.

Gold concentrations of magmatic brines and the metal budget of porphyry copper deposits

Porphyry copper–molybdenum–gold deposits are the most important metal resources formed by hydrothermal processes associated with magmatism. It remains controversial, however, whether the metal content of porphyry-style and other magmatic–hydrothermal deposits is dominantly controlled by metal partitioning between magma and an exsolving magmatic fluid phase, or by scavenging of metals from solid upper-crustal rocks by surface-derived fluids. It also remains unknown to what degree the metal content in such deposits is affected by selective mineral precipitation from the ore fluid. Extremely saline fluids, precipitating quartz and ore minerals in veins have been inferred to have a significant magma-derived component, on the basis of geological, isotopic, and experimental evidence,. Here we report gold and copper concentrations of single fluid inclusions in quartz, determined by laser-ablation inductively coupled plasma mass spectrometry. The results show that the Au/Cu ratio of primary high-temperature brines is identical to the bulk Au/Cu ratio in two of the worlds largest copper–gold ore bodies. This indicates that the bulk metal budget of such deposits is primarily controlled by the composition of the incoming fluid, which is, in turn, likely to be controlled by the crystallization process in an underlying magma chamber.

Enhanced sensitivity in laser ablation-ICP mass spectrometry using helium-argon mixtures as aerosol carrier

Laser ablation-ICP-MS is a sensitive and accurate technique for major to trace multi-element analysis at high spatial resolution on the scale of 10 µm. A wide variety of samples can be studied quantitatively, including minerals and their solid, liquid or melt inclusions as required for geochemical studies. As the desired spatial resolution increases, however, detection limits become severely constrained by the total amount of sample material reaching the ICP. Detection limits are therefore determined by the ablation rate and by the efficiency of removal of ablated aerosol particles from the ablation spot and their transport into the plasma. Properties of the carrier gas are known to affect the ablation process and the efficiency of particle transportation. This study explores the effects of different ablation-cell configurations and the use of helium, dry argon and argon moistened with water for the transport of aerosols into an ICP-MS, using a prototype 193 nm ArF excimer laser. Deposition of visible particles deposited around the ablation pit is significantly reduced when helium is used instead of argon. A moderate flux of helium through the chamber, mixed with moistened argon immediately downstream from the ablation chamber, leads to at least a 2-3-fold increase in the signal intensities across the entire mass range when compared with argon gas only. Background intensities above mass 85 are significantly reduced, but polyatomic interferences in the low mass region increase by an order of magnitude, owing to oxide formation caused by the water load. A high flux of helium, mixed just behind the ablation cell with dry argon, yields a 2-3-fold sensitivity enhancement, in addition to greatly reduced background intensity across the entire mass range. This results in one order of magnitude improvement in detection limits for most elements. These modifications permit the routine determination of minor concentrations of chlorine in microscopic fluid inclusions or the analysis of minerals, such as trace element concentrations in quartz (e.g., Na and Li down to 500 ng g –1 , using a 40 µ ablation pit). Furthermore, this improved sensitivity has recently yielded the first quantitative determination of gold concentrations (∼0.1 µg g –1 ) and full rare-earth element patterns in single 25 µm fluid inclusions.

Effect of particle size distribution on ICP-induced elemental fractionation in laser ablation-inductively coupled plasma-mass spectrometry

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has become one of the well-accepted analytical techniques for in situ trace element analysis and a large number of successful applications have shown its potential. Each commonly employed laser wavelength (1064, 266, 213, 193, 157 nm) leads to some degree of non-stoichiometric ablation, which makes quantification using non-matrix-matched calibration standards difficult for some elements. Time-dependent changes in elemental ratios (so-called elemental fractionation) have been ascribed mostly to processes occurring at the ablation site. Therefore, wavelengths and related irradiance are the major variables that have been used to study this phenomenon in detail. However, there are a large number of parameters that influence the ablation process, aerosol transport, and the excitation process within the ICP. Each process can contribute to elemental fractionation, making the effects of each difficult to separate and to study in detail. The influence of the ICP as one possible source has not been studied thoroughly. The aim of this study was the determination of the source of elemental fractionation using a 266 nm Nd∶YAG laser ablation system. The sample transport system was designed to keep gas flows and plasma conditions constant. Various ablation procedures (single hole drilling and scanning) were tested to investigate the influence of the particle size on the excitation process within the ICP. Mineral wool was used to filter various fractions of the laser-induced aerosol to study signal behaviour as a function of the mass load of the ICP. Uranium and thorium, two elements with very similar properties (ionisation potential and concentration) in the NIST 600 Glass standard series, were used in particular to study ICP processes. It is shown that the particle size distribution is dependent on the wavelength of the laser and the absorption behaviour of the sample. The 266 nm Nd∶YAG laser produces a particle size distribution which is significantly larger in comparison with aerosols produced using a laser wavelength of 193 nm. Signals related to the ablated volume show that the larger particle fractions are not completely vaporised and ionised in the ICP. Filtering certain particle fractions allows final stoichiometric excitation and ionisation, but is accompanied by a loss of 50–80% of the total signal. For single hole ablation, the particle size distribution becomes smaller with increasing depth of the crater. Therefore, scanning mode ablation (which takes place always at the surface) produces a constant supply of larger particles, which results in significantly higher matrix effects within the ICP, as shown by significant changes in the elemental ratio of U∶Th. These studies indicate that the secondary effect of incomplete aerosol or particle excitation within the ICP is the dominant process influencing elemental fractionation during LA-ICP-MS. The effect was observed to be different for individual ICP sources and, therefore, the requirement for matrix-matched quantification in LA-ICP-MS remains instrument-dependent.

Quantitative analysis of major, minor and trace elements in fluid inclusions using laser ablation–inductively coupled plasmamass spectrometry

Microscopic fluid inclusions in minerals are the main source of information about the chemical composition of fluids associated with large-scale material transport in the Earths interior. Hydrothermal transport processes are responsible for the natural enrichment of metal resources in many ore deposits. For the multi-element analysis of the microscopic fluid inclusions (typically 5–50 µm in diameter), LA-ICP-MS has become one of the most promising techniques owing to the recent progress in laser optics design and the development of high-sensitivity ICP mass spectrometers. The quantitative analyses of 19 major, minor and trace elements covering a concentration range of five orders of magnitude were carried out on 39 single natural fluid inclusions, together with a number of experiments to optimise controlled ablation and to test the calibration procedure. A modified commercial ICP-MS instrument was used together with a prototype ablation system based on a 193 nm excimer laser. In a stepwise opening procedure for complex polyphase inclusions, a small hole (4 µm pit) was first drilled for the partial release of liquid and vapor, followed by complete drilling out using a pit covering the entire inclusion. Controlled ablation improves the reproducibility of element ratios to less than 20% for most major, minor and trace elements measured in an assemblage of cogenetic inclusions (including elements that are initially present as solid precipitates within the inclusion), provided that the entire transient ICP-MS signal is integrated. Element ratios were calculated from integrated intensity ratios using an external standard, either a NIST SRM glass or an aqueous standard solution ablated directly through a plastic film. Absolute concentrations were calculated from the element ratiosviaan internal standard element, whose concentration was determined prior to ablation. Microthermometric measurements of phase transitions were used to determine total salinity from known phase diagrams, by measuring either the depression of ice-melting temperature, or the temperature of dissolution of NaCl crystals. Salinity can be related to the concentration of Na (or in some cases Cl), which serves as the internal standard element for the quantification of trace element concentrations. Calculated limits of detection are in the ng g–1to µg g–1region, depending on the volume of the inclusions. The accuracy of the overall analysis, including internal and external calibration, is typically between 5 and 20%, as demonstrated on alkali elements in synthetic fluid inclusions of known composition.