Nanoparticle Analysis in Biomaterials Using Laser Ablation-Single Particle-Inductively Coupled Plasma Mass Spectrometry.

In the past decade, the development of single particle-inductively coupled plasma mass spectrometry (SP-ICPMS) has revolutionized the field of nanometallomics. Besides differentiation between dissolved and particulate metal signals, SP-ICPMS can quantify the nanoparticle (NP) number concentration and size. Because SP-ICPMS is limited to characterization of NPs in solution, we show how solid sampling by laser ablation (LA) adds spatial-resolution characteristics for localized NP analysis in biomaterials. Using custom-made gelatin standards doped with dissolved gold and commercial or synthesized gold nanoparticles, LA-SP-ICPMS conditions such as laser fluence, beam size, and dwell time were optimized for NP analysis to minimize NP degradation, peak overlap, and interferences from dissolved gold. A data-processing algorithm to retrieve the NP number concentration and size was developed for this purpose. As a proof-of-concept, a sunflower-root-sample cross-section, originating from a sunflower plant exposed to gold NPs, was successfully imaged using the optimized LA-SP-ICPMS conditions for localized NP characterization.

Perceptual Image Quality Metrics Concept in Continuous Scanning 2D Laser Ablation-Inductively Coupled Plasma Mass Spectrometry Bioimaging.

This work focuses on the structural similarity (SSIM) index as a tool for optimization of the perceived visual image quality obtainable by continuous scanning 2D LA-ICPMS bioimaging, but also other mass spec imaging techniques may benefit from this approach. This index quantifies the differences between a distorted image and a reference image based on parameters associated with luminance, contrast, and noise. Since reference images are not normally available, a protocol was developed to virtually apply distortion-related information introduced by the LA-ICPMS imaging system to a reference image of one's choice. Distortion-related information in the form of blur and noise was experimentally retrieved from line scans across a laser milled knife edge on custom-prepared gelatin standards (mimicking proteinaceous biomatrixes). Distorted images were generated via computational procedures developed earlier, warranting objective image quality assessment via the SSIM indices. We illustrate the potential of this approach for image quality optimization for a suite of LA-ICPMS imaging conditions.

Imaging Artifacts in Continuous Scanning 2D LA-ICPMS Imaging Due to Nonsynchronization Issues.

Pulsed laser ablation (LA) devices in laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) imaging have become very advanced, delivering laser pulses with high temporal accuracy and stable energy density. However, unintentional imaging artifacts may be generated in 2D element maps when the LA repetition rate and the data acquisition parameters of ICPMS instruments with a sequential mass spectrometer (i.e., quadrupole filter or sector-field mass spectrometer) are desynchronized. This may potentially lead to interference patterns, visible as ripples in elemental images, and thus, compromised image quality. This paper describes the background of aliasing in continuous scanning mode through simulation experiments and ways to modulate the effect. The existence of this image degradation source is demonstrated experimentally via real-life imaging of a homogeneous glass standard.

Gelatin gels as multi-element calibration standards in LA-ICP-MS bioimaging: fabrication of homogeneous standards and microhomogeneity testing.

Highly homogeneous multi-element gelatin calibration standards were fabricated for quantitative LA-ICP-MS bioimaging. Heterogeneity issues caused by the so-called "coffee-stain" and/or "Marangoni" effects were found to be element-dependent but could be circumvented by careful selection of drying/setting conditions. A micro-homogeneity test was developed for certification of the standards.

Non-matrix-matched calibration in bulk multi-element laser ablation - Inductively coupled plasma - Mass spectrometry analysis of diverse materials.

Laser ablation inductively coupled plasma - mass spectrometry (LA-ICP-MS) is a frequently used microanalytical technique in elemental analysis of solid samples. In most instances the use of matrix-matched calibration standards is necessary for the accurate determination of elemental concentrations. However, the main drawback of this approach is the limited availability of certified reference materials. Here, we present a novel conceptual framework in LA-ICP-MS quantification without the use of matrix-matched calibration standards but instead employment of an ablation volume-normalization method (via measurement of post-ablation line scan volumes by optical profilometry) in combination with a matrix-adapted fluence (slightly above the ablation threshold). This method was validated by cross-matrix quantification of reference materials typically investigated by LA-ICP-MS, including geological and biological materials. This allows for more accurate and precise multi-element quantification, and enables quantification of previously unquantifiable elements/materials.

Quantitative multi-element mapping of ancient glass using a simple and robust LA-ICP-MS rastering procedure in combination with image analysis.

The surface of two glass artefacts in mosaic style, probably fragments of conglomerate glass bowls dating back two millennia, was investigated by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS). By rastering with the laser beam over a selected area of the surface of the glass artefacts, elemental oxide maps were generated. Quantification of the elemental oxides in the maps was achieved using a so-called sum normalization procedure, summating the elements-54 in total-as their oxides to 100% (w/w), without using an internal standard and applying only one external standard (NIST SRM glass 610). This results in a robust mapping procedure which automatically corrects for drift and defocusing issues. Sum normalization was applied to each pixel in the map separately and required a custom source code to process all the data in the tens of thousands of pixels to generate the elemental oxide concentration maps. The digital element maps generated upon rastering of the two glass artefacts are very compelling and are an excellent entry point to gain detailed insight into their fabrication and provenance using image analysis software for retrieval of localized elemental oxide concentrations and correlations.

Basic modeling approach to optimize elemental imaging by laser ablation ICPMS.

The quality of element image maps generated by laser ablation (LA) ICPMS rastering depends on the measurement conditions (laser fluence, repetition rate, beam diameter, scanning speed, flow rate, and acquisition time). Optimizing these conditions is often a matter of trial and error since the quality criteria for elemental imaging (sensitivity, spatial resolution, noise, and analysis time) are intricately linked. A simple mathematical model, and ensuing software, was developed to simulate the LA-ICPMS output upon virtual rastering of a digital image of a cross-section of a sample. Even though the LA-ICPMS map is not directly correlated with elemental imaging, element distributions are often related to visual features in the sample, allowing optimization of the LA-ICPMS settings for the desired quality criteria (samples without any visual features can not be optimized in this way). The virtual LA-ICPMS rastering software assumes that the ablation cell and tube interface act as a continuous stirred-tank reactor (i.e., exponential washout) and a plug flow reactor (i.e., zero dispersion), respectively, using an inert gas for transport of the aerosol particles. The software also incorporates a random noise generator which simulates the experimentally deduced signal-to-noise ratios as a function of the anticipated concentration and LA-ICPMS settings. The software was successfully validated and demonstrated by comparing the software output of selected patterns created in the emulsion of a black-and-white negative film with the experimental rastering output performed by measurement of (107)Ag in the emulsion.

Multi-element analysis of wines by ICP-MS and ICP-OES and their classification according to geographical origin in Slovenia.

Inductively coupled plasma mass spectrometry and optical emission were used to determine the multi-element composition of 272 bottled Slovenian wines. To achieve geographical classification of the wines by their elemental composition, principal component analysis (PCA) and counter-propagation artificial neural networks (CPANN) have been used. From 49 elements measured, 19 were used to build the final classification models. CPANN was used for the final predictions because of its superior results. The best model gave 82% correct predictions for external set of the white wine samples. Taking into account the small size of whole Slovenian wine growing regions, we consider the classification results were very good. For the red wines, which were mostly represented from one region, even-sub region classification was possible with great precision. From the level maps of the CPANN model, some of the most important elements for classification were identified.

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.

Multi-element quantification of ancient/historic glasses by laser ablation inductively coupled plasma mass spectrometry using sum normalization calibration.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for quantitative analysis of ancient/historic glasses is subject to calibration issues which have been addressed in this work. Since ancient/historic glasses have widely ranging matrix compositions, a complementary analysis by an alternative method is generally employed to determine at least one major element which can be used as an internal standard. We demonstrate that such a complementary analysis is unnecessary using a so-called sum normalization calibration technique (mathematically formulated) by simultaneous measurement of 54 elements and normalizing them to 100% [w/w] based on their corresponding oxide concentrations. The crux of this approach is that by assuming a random internal standard concentration of a particular major oxide, e.g. SiO2, the normalization algorithm varies the internal standard concentration until the cumulated concentrations of all 54 elemental oxides reach 100% [w/w]. The fact that 54 elements are measured simultaneously predetermines the laser ablation mode to rastering. Nine glass standards, some replicating historic compositions, were used for calibration. The linearity of the calibration graphs (forced through the origin) represented by the relative standard deviations in the slope were between 0.1 and 6.6% using SiO2 as an internal standard. This allows high-accuracy determination of elemental oxides as confirmed by good agreement between found and reported values for major and minor elemental oxides in some synthetic glasses with typical medieval composition (European Science Foundation 151 and 158). Also for trace elemental concentrations of lanthanides in a reference glass (P&H Developments Ltd. DLH7, a base glass composition with nominally 75 microg g(-1) elements added) accurate data were obtained. Interferences from polyatomic species and doubly charged species on the masses of trace elements are possible, depending on the base composition of the glass, with Ba and Sb glasses showing potential interferences on some lanthanides. We showed that they may be reduced to a great extent by using an Octopole Reaction System although the overall sensitivity decreases which may be a problem for some low-level determinations.