Applications of inductively coupled plasma mass spectrometry and laser ablation inductively coupled plasma mass spectrometry in materials science

Inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) have been applied as the most important inorganic mass spectrometric techniques having multielemental capability for the characterization of solid samples in materials science. ICP-MS is used for the sensitive determination of trace and ultratrace elements in digested solutions of solid samples or of process chemicals (ultrapure water, acids and organic solutions) for the semiconductor industry with detection limits down to sub-picogram per liter levels. Whereas ICP-MS on solid samples (e.g. high-purity ceramics) sometimes requires time-consuming sample preparation for its application in materials science, and the risk of contamination is a serious drawback, a fast, direct determination of trace elements in solid materials without any sample preparation by LA-ICP-MS is possible. The detection limits for the direct analysis of solid samples by LA-ICP-MS have been determined for many elements down to the nanogram per gram range. A deterioration of detection limits was observed for elements where interferences with polyatomic ions occur. The inherent interference problem can often be solved by applying a double-focusing sector field mass spectrometer at higher mass resolution or by collision-induced reactions of polyatomic ions with a collision gas using an ICP-MS fitted with collision cell. The main problem of LA-ICP-MS is quantification if no suitable standard reference materials with a similar matrix composition are available. The calibration problem in LA-ICP-MS can be solved using on-line solution-based calibration, and different procedures, such as external calibration and standard addition, have been discussed with respect to their application in materials science. The application of isotope dilution in solution-based calibration for trace metal determination in small amounts of noble metals has been developed as a new calibration strategy. This review discusses new analytical developments and possible applications of ICP-MS and LA-ICP-MS for the quantitative determination of trace elements and in surface analysis for materials science.     

Double-pulse laser ablation inductively coupled plasma mass spectrometry

This paper describes the use of double-pulse laser ablation to improve ICP-MS internal (temporal relative standard deviation, %TRSD) and external (%RSD) precision. The first laser pulse is used to ablate a large quantity of mass from the sample surface. The second pulse is applied with a variable time delay after the first pulse to break the ablated mass into a finer aerosol, which is more readily transported to and digested in the ICP-MS. A factor of two improvement in %TRSD and factor of five in %RSD are demonstrated.     

Low pressure laser ablation coupled to inductively coupled plasma mass spectrometry

The particle size distribution in laser ablation inductively coupled plasma mass spectrometry is known to be a critical parameter for complete vaporization of particles. Any strategy to reduce the particle size distribution of laser generated aerosols has the potential to increase the ion signal intensity and to reduce fractionation effects. Due to the fact that vapor generation, nucleation, condensation, and agglomeration take place within an extremely short period of time, ablation under atmospheric pressure might not allow influencing these processes while under reduced pressure condition the cooling of the aerosol and therefore the condensation is expected to be slower. In this study, a low pressure laser ablation cell for the generation of laser aerosols was coupled to an ICP-MS. In contrast to the previously developed trapped ablation mode, the newly designed cell allows the adjustment of the pressure in the ablation cell between 20 and 1400 mbar prior to the ablation.Ablation experiments carried out using this configuration showed a dependence of the aerosol properties (size distribution and particle structure) on the ablation cell pressure. The intensity ratio U/Th measured as a figure of merit for complete vaporization within the ICP indicated a change in the aerosol structure at approximately 500 mbar toward smaller particle size. A significant difference between low pressure and at ambient pressure ablated aerosol was observed. The intensity ratios (U/Th) of the ablated sample moves closer to the bulk composition at lower pressures at the expense of sensitivity. Therefore the decrease in the ICP-MS signal intensity in the low pressure cell can be attributed to vapor deposition within the ablation cell walls.Moreover, scanning electron microscope images of aerosols collected on filters after the low pressure ablation cell suggest the possibility of a slower cooling velocity of the aerosol, which was observed in the condensed material on the surface of ejected spherical particles. The expansion of the laser aerosol was also investigated using polished brass substrates in the expansion path-way for particle collection.     

Elemental fractionation in laser ablation inductively coupled plasma mass spectrometry

The major challenge to the use of laser ablation sample introduction, combined with inductively coupled plasma mass spectrometry, is the problem of calibration. In the geological analysis of minerals, calibration is complicated by the extraordinarily wide variety of sample matrices which may be encountered. While there is a lack of mineral standards with well characterized concentrations near 1 g/g, the NIST glass reference materials (SRM 610–617) have been demonstrated to be very useful for the analysis of a wide variety of lithophile elements in silicate samples. An internal reference element, for which the concentration is known in the sample, has been widely used to make corrections for the multiplicative effects of volume (or weight) of the sample ablated, instrument drift, and matrix effects. This procedure works extremely well where elements being determined and the internal reference element being used share similar ablation behaviours; i.e., they do not fractionate progressively during the ablation and transport process. In this study, it is demonstrated that, in terms of ablation behaviour, elements fall into several distinct clusters and that the elements within these clusters correlate well with each other during a period of ablation. Thus, elements within a cluster can be determined using an internal reference element from within the same cluster. While a combination of periodic varying properties typifies the clusters, the geochemical classification of elements into lithophile (silicate loving), and chalcophile (sulphide loving) appears to offer the best characterization of the major groups.     

Analyte response in laser ablation inductively coupled plasma mass spectrometry

The dependence of analyte sensitivity and vaporization efficiency on the operating parameters of an inductively coupled plasma mass spectrometer (ICPMS) was investigated for a wide range of elements in aerosols, produced by laser ablation of silicate glass. The ion signals were recorded for different carrier gas flow rates at different plasma power for two different laser ablation systems and carrier gases. Differences in atomization efficiency and analyte sensitivity are significant for the two gases and the particle size distribution of the aerosol. Vaporization of the aerosol is enhanced when helium is used, which is attributed to a better energy-transfer from the plasma to the central channel of the ICP and a higher diffusion rate of the vaporized material. This minimizes elemental fractionation caused by sequential evaporation and reduces diffusion losses in the ICP. The sensitivity change with carrier gas flow variation is dependent on m/z of the analyte ion and the chemical properties of the element. Elements with high vaporization temperatures reach a maximum at lower gas flow rates than easily vaporized elements. The sensitivity change is furthermore dependent on m/z of the analyte ion, due to the mass dependence of the ion kinetic energies. The mass response curve of the ICPMS is thus not only a result of space charge effects in the ion optics but is also affected by radial diffusion of analyte ions and the mismatch between their kinetic energy after expansion in the vacuum interface and the ion optic settings.     

Elemental fractionation in laser ablation inductively coupled plasma mass spectrometry

The major challenge to the use of laser ablation sample introduction, combined with inductively coupled plasma mass spectrometry, is the problem of calibration. In the geological analysis of minerals, calibration is complicated by the extraordinarily wide variety of sample matrices which may be encountered. While there is a lack of mineral standards with well characterized concentrations near 1 microg/g, the NIST glass reference materials (SRM 610-617) have been demonstrated to be very useful for the analysis of a wide variety of lithophile elements in silicate samples. An internal reference element, for which the concentration is known in the sample, has been widely used to make corrections for the multiplicative effects of volume (or weight) of the sample ablated, instrument drift, and matrix effects. This procedure works extremely well where elements being determined and the internal reference element being used share similar ablation behaviours; i.e., they do not fractionate progressively during the ablation and transport process. In this study, it is demonstrated that, in terms of ablation behaviour, elements fall into several distinct clusters and that the elements within these clusters correlate well with each other during a period of ablation. Thus, elements within a cluster can be determined using an internal reference element from within the same cluster. While a combination of periodic varying properties typifies the clusters, the geochemical classification of elements into lithophile (silicate loving), and chalcophile (sulphide loving) appears to offer the best characterization of the major groups.     

On-line double isotope dilution laser ablation inductively coupled plasma mass spectrometry for the quantitative analysis of solid materials

We report on the determination of trace elements in solid samples by the combination of on-line double isotope dilution and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The proposed method requires the sequential analysis of the sample and a certified natural abundance standard by on-line IDMS using the same isotopically-enriched spike solution. In this way, the mass fraction of the analyte in the sample can be directly referred to the certified standard so the previous characterization of the spike solution is not required. To validate the procedure, Sr, Rb and Pb were determined in certified reference materials with different matrices, including silicate glasses (SRM 610, 612 and 614) and powdered samples (PACS-2, SRM 2710a, SRM 1944, SRM 2702 and SRM 2780). The analysis of powdered samples was carried out both by the preparation of pressed pellets and by lithium borate fusion. Experimental results for the analysis of powdered samples were in agreement with the certified values for all materials. Relative standard deviations in the range of 6–21% for pressed pellets and 3–21% for fused solids were obtained from n = 3 independent measurements. Minimal sample preparation, data treatment and consumption of the isotopically-enriched isotopes are the main advantages of the method over previously reported approaches.     

Automated in situ trace element analysis of silicate materials by laser ablation inductively coupled plasma mass spectrometry

This paper describes the automated in situ trace element analysis of solid materials by laser ablation (LA) inductively coupled plasma mass spectrometry (ICP-MS). A compact computer-controlled solid state Nd:YAG Merchantek EO UV laser ablation (LA) system has been coupled with the high sensitivity VG PQII S ICP-MS. A two-directional communication was interfaced in-house between the ICP-MS and the LA via serial RS-232 port. Each LA-ICP-MS analysis at a defined point includes a 60 s pre-ablation delay, a 60 s ablation, and a 90 s flush delay. The execution of each defined time setting by LA was corresponding to the ICP-MS data acquisition allowing samples to be run in automated cycle sequences like solution auto-sampler ICP-MS analysis. Each analytical cycle consists of four standards, one control reference material, and 15 samples, and requires about 70 min. Data produced by Time Resolved Analysis (TRA) from ICP-MS were later reduced off-line by in-house written software. Twenty-two trace elements from four reference materials (NIST SRM 613, and fused glass chips of BCR-2, SY-4, and G-2) were determined by the automated LA-ICP-MS method. NIST SRM 610 or NIST SRM 613 was used as an external calibration standard, and Ca as an internal standard to correct for drift, differences in transport efficiency and sampling yield. Except for Zr and Hf in G-2, relative standard deviations for all other elements are less than 10%. Results compare well with the data reported from literature with average limits of detection from 1 ng x g(-1) to 455 ng x g(-1) and less than 100 ng x g(-1) for most trace elements.     

Imaging mass spectrometry in biological tissues by laser ablation inductively coupled plasma mass spectrometry

Of all the inorganic mass spectrometric techniques, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) plays a key role as a powerful and sensitive microanalytical technique enabling multi- element trace analysis and isotope ratio measurements at trace and ultratrace level. LA-ICP-MS was used to produce images of detailed regionally-specific element distribution in 20 microm thin sections of different parts of the human brain. The quantitative determination of copper, zinc, lead and uranium distribution in thin slices of human brain samples was performed using matrix-matched laboratory standards via external calibration procedures. Imaging mass spectrometry provides new information on the spatially inhomogeneous element distribution in thin sections of human tissues, for example, of different brain regions (the insular region) or brain tumor tissues. The detection limits obtained for Cu, Zn, Pb and U were in the ng g(-1) range. Possible strategies of LA-ICP-MS in brain research and life sciences include the elemental imaging of thin slices of brain tissue or applications in proteome analysis by combination with matrix-assisted laser desorption/ionization MS to study phospho- and metal- containing proteins will be discussed.     

Rapid multielemental analysis of Chinese reference soils by laser ablation inductively coupled plasma-source mass spectrometry

Summary Laser ablation inductively coupled plasma-source mass spectrometry has been used to determine thirty elements (Na, Mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, As, Rb, Sb, Cs, Ba, La, Ce, Nd, Sm, Eu, Dy, Ho, Yb, Hf, Ta, W, Th, U) in seven Chinese reference soils. The Surrey prototype spectrometer was employed with sample ablation by a free-running ruby laser. Concentrations in the soils (GSS-2 to GSS-8) were calculated from elemental responses and sensitivities derived from another soil in the series, namely GSS-1. Comparisons with previous neutron activation analyses are made. Rapid semiquantitative analyses are proved feasible. About eighty percent of the LA-ICP-MS determined concentrations were within a factor of two of the concentrations measured by INAA, and many were considerably closer than this. Precisions were typically in the range 2–10% RSD, but some were considerably poorer for elements present at trace levels.     

Nuclear forensic analysis with laser ablation inductively coupled plasma mass spectrometry in CMX-6

Journal of Radioanalytical and Nuclear Chemistry - The 6th Collaborative Materials Exercise, CMX-6, was organized by the Nuclear Forensic International Technical Working Group in 2018 and 2019. Two...     

Design analysis of a laser ablation cell for inductively coupled plasma mass spectrometry by numerical simulation

A laser ablation setup including outer chamber, sample tube, sample holder and transport tubing was modelled and optimized using advanced computational fluid dynamics techniques. The different components of the setup were coupled and the whole device was modelled at once. The mass transport efficiency and transit times of near infrared femtosecond (fs) laser generated brass aerosols in pure argon and helium–argon mixtures were calculated at experimentally optimized conditions and a transient signal was constructed. The use of helium or argon did not influence the mass transport efficiency, but the signal structure changed. The signal fine structure was retrieved and experimentally validated. Bimodal peak structures were observed that seemed to originate from turbulent effects in the tubing connecting a Y-connector and the injector.     

Determination of copper, molybdenum and selenium in biological reference materials by inductively coupled plasma mass spectrometry

Summary In a contribution to the elemental characterization of 10 new reference materials, Bovine Muscle Powder (136), Corn Starch (162), Hard Red Spring Wheat Flour (165), Soft Winter Wheat Flour (166), Whole Milk Powder (183), Wheat Gluten (184), Corn Bran (186), Durum Wheat Flour (187), Whole Egg Powder (188) and Microcrystalline Cellulose (189), the total concentrations of Cu, Mo and Se were determined by the application of an analytical method based on isotope dilution inductively coupled plasma mass spectrometry. Cu and Mo contents were quantified by measurement of ⁶⁵Cu/⁶³Cu and ⁹⁷Mo/¹⁰⁰Mo isotopic ratios following spiking with ⁶⁵Cu and ⁹⁷Mo and digestion with nitric acid. Selenium was separated as hydrogen selenide from the matrix using sodium borohydride after spiking with ⁸²Se and acid digestion-dry ashing and quantified by measurement of the ⁸²Se/⁷⁸Se isotopic ratio. Comparison of these results with those from a variety of other methods and assessment of the procedures using certified reference materials indicated that the determinations of Cu, Mo and Se were performed without analytical bias.     

Laser-generated aerosols in laser ablation for inductively coupled plasma spectrometry

This paper is a summary of the current knowledge of laser-generated aerosols under atmospheric conditions. It is restricted to typical laser sampling conditions as they are used in LA-ICP spectrometry. Published experimental evidence and proposed models are reviewed and critical summarized. The collected works show that a certain agreement exists that independently of the sample two size fractions with different chemical composition are found. The mechanism generating the different particle fractions are currently not clear. Possible sources of particle generation are described and critically reviewed. Fundamentally three distinguishable modes (gas, liquid, solid) can be described that can appear: gas-to-particle conversion, hydrodynamic sputtering, mechanical spallation/exfoliation. More recently explosive boiling as a mechanism of liquid expulsion has been discussed as a further possible source under certain conditions. Particle conditioning during transport is discussed as a source for agglomeration. The correlation between size distribution and laser parameters is discussed.     

Multielemental analysis in small amounts of environmental reference materials with inductively coupled plasma mass spectrometry

The lowest possible sample weight for performing multielemental trace element analysis on environmental and biological samples by ICP-MS has been investigated. The certified reference materials Bovine Liver NIST SRM 1577b, Human Hair NCS DC 73347 and Oriental Tobacco Leaves CTA-OTL-1 were applied at sample weights (1, 5, 20 and 50 mg aliquots, n = 10) which were significantly lower than those recommended with most recoveries in the range of 95-110%. Samples were digested in a mixture of nitric acid, hydrogen peroxide and hydrogen fluoride by closed-vessel microwave digestion. Multielemental analysis was performed with an optimized ICP-QMS method. Aqueous standard solutions were applied for external calibration with rhodium as the internal standard element. The detection limits varied between 0.02-0.38 microg/g for Li, Na, Cr, Mn, Ni, Cu, Zn, Sr, Cd, Ba and Pb, and up to 1.92 microg/g for Mg, Al, Ca, Fe and Ni. Digested human plasma samples were spiked with multielemental solution (0.5-10 microg/L) to test the analytical method and the recoveries were 95-105% for most analytes. Our results show that in the case of homogeneous SRMs it is possible to use them in very low amounts (1-5 mg) for method development and quality control.     

Multielemental analysis in small amounts of environmental reference materials with inductively coupled plasma mass spectrometry

The lowest possible sample weight for performing multielemental trace element analysis on environmental and biological samples by ICP-MS has been investigated. The certified reference materials Bovine Liver NIST SRM 1577b, Human Hair NCS DC 73347 and Oriental Tobacco Leaves CTA-OTL-1 were applied at sample weights (1, 5, 20 and 50 mg aliquots, n = 10) which were significantly lower than those recommended with most recoveries in the range of 95–110%. Samples were digested in a mixture of nitric acid, hydrogen peroxide and hydrogen fluoride by closed-vessel microwave digestion. Multielemental analysis was performed with an optimized ICP-QMS method. Aqueous standard solutions were applied for external calibration with rhodium as the internal standard element. The detection limits varied between 0.02–¶0.38 μg/g for Li, Na, Cr, Mn, Ni, Cu, Zn, Sr, Cd, Ba and Pb, and up to 1.92 μg/g for Mg, Al, Ca, Fe and Ni. Digested human plasma samples were spiked with multielemental solution (0.5–10 μg/L) to test the analytical method and the recoveries were 95–105% for most analytes. Our results show that in the case of homogeneous SRMs it is possible to use them in very low amounts (1–5 mg) for method development and quality control.     

Analysis of liquid samples using dried-droplet laser ablation inductively coupled plasma mass spectrometry

In this study we developed a dried-droplet method for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The proposed method provides accurate and precise results when building calibration curves and determining elements of interest in real liquid samples. After placing just 1 μL of a liquid standard solution or a real sample onto the filter surface and then converting the solution into a very small, thin dry spot, the sample could be applied as an analytical subject for LA. To demonstrate the feasibility of this proposed method, we used LA-ICP-MS and conventional ICP-MS to determine the levels of 13 elements (Li, V, Mn, Co, Ni, Cu, Zn, As, Mo, Cd, Sb, Tl, and Pb) in five water samples. The correlation coefficients obtained from the various calibration curves ranged from 0.9920 (²⁰⁵Tl) to 0.9998 (⁵¹V), sufficient to allow the determination of a wide range of elements in the samples. We also investigated the effects of Methylene Blue (MB) and the NaCl concentration on the elemental analyses. MB could be used as an indicator during the ablation process; its presence in the samples only negligibly influenced the intensities of the signals of most of the tested elements. Notably, high NaCl contents led to signal suppression for some of the elements. In comparison with the established sample introduction by nebulization, our developed technique abrogates the need for time-consuming sample preparation and reduces the possibility of sample contamination.     

An auto-focus system for reproducible focusing in laser ablation inductively coupled plasma mass spectrometry

A method of controlling the focus position of the ablating laser was evaluated. This system consists of a displacement measuring unit based on the triangulation displacement measurement principle. A simple preliminary experimental setup with a light source (LED, light emitting diode) and a displacement-sensor (PSD, position sensing detector) with optics and a electronic circuit, resulted in a reproducibility of a given lateral position of ±5 μm. A fine-tuning of the components, as well as the use of a laser diode as a light source will certainly improve this value.     

Application of laser ablation inductively coupled plasma mass spectrometry for the isotopic analysis of single uranium particles

The paper describes the application of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for the isotopic analysis of individual uranium-oxide particles. The procedure developed is suitable for the accurate measurement of ²³⁴U, ²³⁵U, ²³⁶U and ²³⁸U isotopes in single actinide particles with lateral dimensions down to 10 μm. The ²³⁵U/²³⁸U isotope ratios can be obtained with a precision of a few percent relative standard deviation using a single collector ICP-MS instrument. The precision could be improved by the use of slow ablation and by taking several LA-ICP-MS replicate spectra on the same particle investigated. For the minor isotopes use of higher mass resolution (R = 4000) was necessary in some cases to avoid spectral interferences. The technique developed offers a rapid and accurate possibility for the isotopic composition determination of uranium-containing individual particles in environmental and safeguards samples.