Andrei Izmer

Evidence of near-field laser ablation inductively coupled plasma mass spectrometry (NF-LA-ICP-MS) at nanometre scale for elemental and isotopic analysis on gels and biological samples

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been established as a powerful surface analytical method for local elemental analysis on metallic, ceramic, geological or biological sample surfaces. Here we show a new way of nanometre scale analysis of elements on sample surfaces by near-field LA-ICP-MS (NF-LA-ICP-MS). This technique uses the near-field enhancement effect on the tip of a thin silver needle in a laser beam (Nd:YAG laser, wavelength 532 nm) on the sample surface. The thin silver needle was etched electrolytically in an electrochemical cell using a droplet of citric acid as electrolyte. For nanolocal analysis by NF-LA-ICP-MS on soft matter (e.g., on 2-D gels and biological samples) a small volume transparent laser ablation chamber was constructed and coupled to a double-focusing sector field inductively coupled plasma mass spectrometer (ICP-MS). A small amount of soft sample material is ablated at atmospheric pressure by a single laser shot in the near-field of the silver tip in the defocused Nd:YAG laser beam. The ablated material is transported with argon as carrier gas into the inductively coupled plasma (ICP) ion source of the sensitive double-focusing sector field mass spectrometer with reverse Nier–Johnson geometry. By single-shot analysis on 2-D gels and biological surfaces doped with uranium in the μg g−1 range using NF-LA-ICP-MS an enhancement of ion intensities of transient signals in comparison with the background signal of up to factor 60 was observed. In gels doped with isotopically enriched 65Cu and 67Zn spikes by NF-LA-ICP-MS (single shot analysis) ion intensities up to the n × 105 cps range and isotope ratios (235U/238U, 65Cu/63Cu and 67Zn/64Zn) were measured at a lateral resolution in the nanometre scale. Using the near-field effect in LA-ICP-MS, it was demonstrated that nanolocal analysis is possible in single-shot measurements of elements on biological samples and on a gel surface with spatial resolution at the hundreds of nanometres range. This first experiment on near-field LA-ICP-MS opens up a new, challenging path for future applications in nanoimaging of elements in life science, biology and medicine, e.g., for analyses of single cells, cell organelles or biological structures at nanometre range in order to detect neurodegenerative diseases, but also in material science, nanotechnologies and nanoelectronics.

Comparison of laser ablation-inductively coupled plasma-mass spectrometry and micro-X-ray fluorescence spectrometry for elemental imaging in Daphnia magna

Visualization of elemental distributions in thin sections of biological tissue is gaining importance in many disciplines of biological and medical research. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and scanning micro-X-ray fluorescence spectrometry (micro-XRF) are two widely used microanalytical techniques for elemental mapping. This article compares the capabilities of the two techniques for imaging the distribution of selected elements in the model organism Daphnia magna in terms of detection power and spatial resolution. Sections with a thickness of 10 and 20 microm of the fresh water crustacean Daphnia magna were subjected to LA-ICP-MS and micro-XRF analysis. The elemental distributions obtained for Ca, P, S and Zn allow element-to-tissue correlation. LA-ICP-MS and micro-XRF offer similar limits of detection for the elements Ca and P and thus, allow a cross-validation of the imaging results. LA-ICP-MS was particularly sensitive for determining Zn (LOD 20 microg g(-1), 15 microm spot size) in Daphnia magna, while the detection power of micro-XRF was insufficient in this context. However, LA-ICP-MS was inadequate for the measurement of the S distributions, which could be better visualized with micro-XRF (LOD 160 microg g(-1), 5 s live time). Both techniques are thus complementary in providing an exhaustive chemical profiling of tissue samples.

3D laser ablation-ICP-mass spectrometry mapping for the study of surface layer phenomena – a case study for weathered glass

In this work, a multi-elemental 3D laser ablation-ICP-mass spectrometry mapping procedure for high-resolution depth information retrieval to investigate surface layer phenomena is presented. The procedure is based on laser drilling on a virtual grid on the surface, followed by extraction of depth maps along the z-axis (for each element monitored). Using a burst of 50 laser pulses at 1 Hz on each point of the grid, a penetration rate of ca. 150 nm per pulse (in glass) was obtained and a lateral resolution in the order of the laser beam diameter. By ultrafast ICP-MS monitoring of individual ablation pulses (58 ms for a set of 19 elements) using a laser ablation cell with fast signal washout (less than 0.5 s for whole laser pulse), the corresponding peak areas could be consistently integrated, resulting in spatial elemental data associated with individual pulses. The usual laser drilling limitations such as pulse mixing and signal tailing are avoided with this approach. After manipulation of the spatial elemental datasets and quantification, stacks of 50 2D depth maps (for each element monitored) were produced which could be visualized as volume images or time-lapse movies. As a proof of concept, this approach was successfully used to investigate the degradation mechanisms of a medieval, weathered glass artifact by colocalization analysis of selected cross-sectional 2D elemental images in arbitrary planes of the volume images. It was shown that degradation must have started as a result of dealkalinization leading to depletion of alkalis/earth alkalis in glass surface layers and enrichment of network formers (Si and Al), and subsequent worsening by cracking and formation of corrosion pits and so-called spatiotemporal Liesegang rings indicative of radial leaching.

A pilot study on the use of laser ablation-ICP-mass spectrometry for assessing/mapping the distribution of a drug and its metabolites across the body compartments of rats

Application of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has been studied as an alternative to radioluminography (RLG) for assessing the distribution of a novel anti-tuberculosis compound containing bromine as a “hetero-element” and its metabolites over the body compartments of rat, used as a test animal. In contrast to RLG, LA-ICP-MS does not require labeling of the drug compound with a radionuclide. After administration of the Br-containing drug and a sufficient delay time to allow drug uptake and distribution, the sacrificed animal was frozen and embedded in carboxymethyl cellulose (CMC), after which thin sections were obtained using a microtome. For quantification purposes, hardened gelatin films on a glass support were used as “matrix-matched” Br standards. The limit of detection (LOD) for Br estimated using one of these in-house made standards was 0.1 μg g−1, which is sufficiently low to allow visualization of Br in the main organ of interest (lung). LA-ICP-MS analysis via single spot drilling with a wider laser beam diameter on the organ of interest was performed as well. In this case, quantification was accomplished via external calibration versus droplets of standards containing the pure drug that were co-embedded with the rat. Figures of merit achievable with LA-ICP-MS were critically evaluated and compared to those typical for the traditional technique, based on the use of radionuclides.

Pretreatment with VEGF(R)-inhibitors reduces interstitial fluid pressure, increases intraperitoneal chemotherapy drug penetration, and impedes tumor growth in a mouse colorectal carcinomatosis model

Cytoreductive surgery combined with intraperitoneal chemotherapy (IPC) is currently the standard treatment for selected patients with peritoneal carcinomatosis of colorectal cancer. However, especially after incomplete cytoreduction, disease progression is common and this is likely due to limited tissue penetration and efficacy of intraperitoneal cytotoxic drugs. Tumor microenvironment-targeting drugs, such as VEGF(R) and PDGFR inhibitors, can lower the heightened interstitial fluid pressure in tumors, a barrier to drug delivery. Here, we investigated whether tumor microenvironment-targeting drugs enhance the effectiveness of intraperitoneal chemotherapy. A mouse xenograft model with two large peritoneal implants of colorectal cancer cells was developed to study drug distribution and tumor physiology during intraperitoneal Oxaliplatin perfusion. Mice were treated for six days with either Placebo, Imatinib (anti-PDGFR, daily), Bevacizumab (anti-VEGF, twice) or Pazopanib (anti-PDGFR, -VEGFR; daily) followed by intraperitoneal oxaliplatin chemotherapy. Bevacizumab and Pazopanib significantly lowered interstitial fluid pressure, increased Oxaliplatin penetration (assessed by laser ablation inductively coupled plasma mass spectrometry) and delayed tumor growth of peritoneal implants (assessed by MRI). Our findings suggest that VEGF(R)-inhibition may improve the efficacy of IPC, particularly for patients for whom a complete cytoreduction might not be feasible.

Critical evaluation of quantitative methods for the multi-elemental analysis of ancient glasses using laser ablation inductively coupled plasma mass spectrometry

Compositional analyses of ancient glasses have the potential to unravel their provenance and identify the raw materials employed in their manufacture. During the last few years, the analysis of archaeological glasses has increasingly benefited from the introduction and further development of micro-destructive techniques with trace element capability, such as laser ablation-inductively coupled plasma-mass spectrometry. However, while a variety of analytical standards can be employed for quantitative analyses of modern glass, the lack of dedicated certified reference materials represents a major obstacle in the analysis of archaeological specimens. Heterogeneity and the uncertainty in the characterisation of commercially available multi-elemental standards can also negatively influence the calculation of the response factors required to obtain fully quantitative data. Finally, fractionation and matrix effects can strongly affect the measurement, resulting in the possibility of over- or under-estimating the concentration of the analytes. This paper describes a fully quantitative method used for the investigation and characterization of a large set of Roman and late Iron Age glasses used in the making of Iron Age British beads, the results of which are published elsewhere (Bertini et al., J. Archaeol. Sci., 2011, 38, 2750–2766). In this work, the impact of the aforementioned issues and the different methods of quantification used on the precision (determined as the bias between different replicates), accuracy and repeatability of the measurement are evaluated. Particular emphasis is placed on assessing the influence of laser sampling procedures, integration of the transient signal, data reduction strategies, quantification approaches and the possibility of adapting the method to suit a large range of glass compositions.

Novel Image Metrics for Retrieval of the Lateral Resolution in Line Scan-Based 2D LA-ICPMS Imaging via an Experimental-Modeling Approach

The quality of elemental image maps obtained via line scan-based LA-ICPMS is a function of the temporal response of the entire system, governed by the design of the system and mapping and acquisition conditions used, next to the characteristics of the sample. To quantify image degradation, ablation targets with periodic gratings are required for the construction of a modulation transfer function (MTF) and subsequent determination of the lateral resolution as a function of image noise and contrast. Since such ablation targets, with suitable matrix composition, are not readily available, computer-generated periodic gratings were virtually ablated via a computational process based on a two-step discrete-time convolution procedure using empirical/experimental input data. This experimental-modeling procedure simulates LA-ICPMS imaging based on two consecutive processes, viz., LA sampling (via ablation crater profiles [ACP]) and aerosol washout/transfer/ICPMS measurement (via single pulse responses [SPR]). By random selection of experimental SPRs from a large database for each individual pulse during the simulation, the convolution procedure simulates an accurate elemental image map of the periodic gratings with realistic (proportional or flicker) noise. This facilitates indirect retrieval of the experimental lateral resolution for the matrix targeted without performing actual line scanning on periodic gratings.

Application of laser ablation-ICP-mass spectrometry for 2-dimensional mapping of element distributions in a Late Archean impact spherule layer

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) has been successfully applied to visualize the 2-dimensional distribution of various major (Mg, Mn, Fe) and trace (Cr, Co, Ni, Ir, Pt) elements in the Late Archaean Paraburdoo impact spherule layer (PB SL). As this spherule layer is highly enriched in meteoritic material, this study particularly focuses on the distribution of siderophile elements. Multi-elemental maps with a lateral resolution of 15 μm, covering surface areas of approximately 1.5 mm × 1.5 mm and multiple impact spherules, were acquired by LA-ICP-MS using two different ablation – standard (circular) and teardrop – cells. It was observed that with the teardrop cell, the elemental maps follow the mineralogy as displayed by the optical microscopy image more closely, with limited memory and washout effects in comparison to the standard cell. In order to acquire representative quantitative data, 100 μm diameter spot LA-ICP-MS analysis in a vertical transect through the SL was performed additionally. Quantification of element concentrations in the PB SL was carried out using external calibration procedures. The validity of this approach was evaluated by comparison of the averages of the determined transect spot concentrations to bulk rock geochemical data and it was found that the mean concentrations fall within the uncertainty of the bulk geochemical analysis, but the results show biases of 5.9% for Pt, 16% for Ir, 11% for Cr and 38% for Ni. In addition, the bulk rock Pt/Ir elemental ratio obtained upon transect LA-ICP-MS analysis was reported and it was observed that the slope of the regression line (1.06) and the mean value (1.23) for all data points measured with LA-ICP-MS equal the bulk rock Pt/Ir value of 1.08–1.43 measured by fire assay/solution ICP-MS for all PB SL outcrops at the Governor site.

Analysis of xenon gas inclusions in nuclear fuel using laser ablation ICP-MS

Analysis of fluid and gaseous inclusions in solids have been a major interest in various fields and have been carried out at different pressures, temperatures, and phase conditions. In nuclear fuel, approximately 20% of the fission products (FPs) are gaseous with isotopes of Xe contributing up to 90% to the product gases. However, previous to this work quantitative analysis of Xe inclusions in nuclear fuel samples have not been performed systematically. The method used must incorporate simple sample handling procedures in a shielded environment. This study is the development of a method for the direct determination of the fission gas (FG) products in micro inclusions contained in nuclear fuels using LA-ICP-MS. To determine the concentration of Xe in nuclear fuel, two calibration strategies were investigated. The first strategy was based on the direct injection of a known quantity of a reference gas into the LA-ICP-MS carrier gas system. Further, the ablation of a ‘matrix-matched’ standard of a non-irradiated UO2 sample, implanted with a known amount of 129Xe was also applied. Using these quantification methods, quantitative LA-ICP-MS measurements were performed on high burnup nuclear fuel. This study demonstrates that direct gas injection is most suitable for the quantification of fission gas in micron-sized inclusions. The direct gas addition is simple and linear calibration curves were obtained. Good reproducibility was obtained and matrix effects were within the uncertainty of the measurements. For the quantification of fission gases in nuclear fuel, aerosol particles were filtered before entering the ICP to remove interferences on the Xe isotopes from the solid FP matrix. The first quantitative determinations of the amount of gas in nuclear fuel using the direct injection method for calibration led to sample pressure calculations which were in good agreement with pressures estimated from computer simulations.

The role of arbuscular mycorrhiza in mercury and mineral nutrient uptake in maize

This work aimed to study the role of arbuscular mycorrhizal fungi (AMF) in Hg and major mineral nutrient uptake and tissue localization of these elements in the roots of maize plants. Maize plants were grown in pots filled with non- and Hg-contaminated substrate (50 μg Hg g-1 as HgCl2) and inoculated with two types of AMF inocula: a) Glomus sp. originating from Hg-polluted soil of a former Hg smelting site in Idrija, Slovenia, and b) commercial AM inoculum Symbivit. Controls were inoculated by corresponding bacterial extracts only. Tissue localization of Hg and major mineral nutrients was performed by laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) on cryofixed and freeze-dried root cross-sections. AMF colonization increased plant biomass in non-contaminated substrate, while this effect was not seen in Hg-contaminated substrate. Hg increased total plant biomass more than AMF inoculation, possibly through hormetic effects. AMF increased Hg uptake into the roots, as well as Hg transfer to the shoots. AMF affected plant mineral nutrient uptake, depending on the type of AMF inoculum and the presence of Hg. In the roots, Hg was mainly localized in rhizodermis and endodermis, followed by the cortex and the central cylinder. Higher Hg concentrations were detected in the central cylinder of AM plants than in that of the controls, pointing to a higher Hg mobility and potential bioavailability in AMF inoculated plants.