Value assignment and uncertainty evaluation for anion and single-element reference solutions incorporating historical information.

The National Institute of Standards and Technology, which is the national metrology institute of the USA, assigns certified values to the mass fractions of individual elements in single-element solutions, and to the mass fractions of anions in anion solutions, based on gravimetric preparations and instrumental methods of analysis. The instrumental method currently is high-performance inductively coupled plasma optical emission spectroscopy for the single-element solutions, and ion chromatography for the anion solutions. The uncertainty associated with each certified value comprises method-specific components, a component reflecting potential long-term instability that may affect the certified mass fraction during the useful lifetime of the solutions, and a component from between-method differences. Lately, the latter has been evaluated based only on the measurement results for the reference material being certified. The new procedure described in this contribution blends historical information about between-method differences for similar solutions produced previously, with the between-method difference observed when a new material is characterized. This blending procedure is justified because, with only rare exceptions, the same preparation and measurement methods have been used historically: in the course of almost 40 years for the preparation methods, and of 20 years for the instrumental methods. Also, the certified values of mass fraction, and the associated uncertainties, have been very similar, and the chemistry of the solutions also is closely comparable within each series of materials. If the new procedure will be applied to future SRM lots of single-element or anion solutions routinely, then it is expected that it will yield relative expanded uncertainties that are about 20 % smaller than the procedure for uncertainty evaluation currently in use, and that it will do so for the large majority of the solutions. However, more consequential than any reduction in uncertainty, is the improvement in the quality of the uncertainty evaluations that derives from incorporating the rich historical information about between-method differences and about the stability of the solutions over their expected lifetimes. The particular values listed for several existing SRMs are given merely as retrospective illustrations of the application of the new method, not to suggest that the certified values or their associated uncertainties should be revised.

Assessing Arsenic Species in Foods Using Regularized Linear Regression of the Arsenic K-edge X-ray Absorption Near Edge Structure.

The toxicity and bioavailability of arsenic is heavily dependent on its speciation. Therefore, robust and accurate methods are needed to determine arsenic speciation profiles for materials related to public health initiatives, such as food safety. Here, X-ray spectroscopies are attractive candidates as they provide in situ, nondestructive analyses of solid samples without perturbation to the arsenic species therein. This work provides a speciation analysis for three certified reference materials for the food chemistry community, whose assigned values may be used to assess the merit of the X-ray spectroscopy results. Furthermore, extracts of SRM 3232 Kelp Powder, which is value-assigned for arsenic species, are measured to provide further evidence of its efficacy. These analyses are performed on the results of As K-edge X-ray Absorption Near Edge Structure (XANES) measurements collected on each sample. Notably, such analyses have traditionally relied on linear combination fitting of a minimal subset of empirical standards selected by stepwise regression. This is known to be problematic for compounds with meaningfully collinear spectra and can yield overestimates of the accuracy of the analysis. Therefore, the least absolute shrinkage and selection operator (lasso) regression method is used to reduce the risk of overfitting and increase the interpretability of statistical inferences. As this is a biased statistical method, results and uncertainties are estimated using a bootstrap method accounting for the dominant sources of variability. Finally, this method does not separate model and data selection from regression analysis. Indeed, a survey of many spectral influences is presented including changes in the: state of methylation, state of protonation, oxidation state, coordination geometry, and sample phase. These compounds were all included in the model's training set, preventing model over-simplification and enabling high-throughput and robust analyses.

Assessing the microscale heterogeneity in Standard Reference Material 4600 Surrogate Post-detonation Urban Debris.

Nondestructive microbeam X-ray fluorescence (µXRF) spectrometry has been used to investigate the elemental microheterogeneity in a nuclear forensics reference material (RM), NIST SRM 4600 Surrogate Post-detonation Urban Debris. Using a principal component analysis (PCA) model, results indicate the majority of elements appear homogeneous; however, zinc (Zn) exhibits microscale heterogeneity for this SRM. To minimize contributions to the measurement uncertainty from elemental microheterogeneity, a minimum sample mass of 24 mg is recommended for analysis.

Measurement and Modeling of the Ability of Crack Fillers to Prevent Chloride Ingress into Mortar.

A common repair procedures applied to damaged concrete is to fill cracks with an organic polymer. This operation is performed to increase the service life of the concrete by removing a preferential pathway for the ingress of water, chlorides, and other deleterious species. To effectively fulfill its mission of preventing chloride ingress, the polymer must not only fully fill the macro-crack, but must also intrude the damage zone surrounding the crack perimeter. Here, the performance of two commonly employed crack fillers, one epoxy, and one methacrylate, are investigated using a combined experimental and computer modeling approach. Neutron tomography and microbeam X-ray fluorescence spectrometry (µXRF) measurements are employed on pre-cracked and chloride-exposed specimens to quantify the crack filling and chloride ingress limiting abilities, respectively, of the two polymers. A two-dimensional model of chloride transport is derived from a mass balance and solved by the finite element method. Crack images provided by µXRF are used to generate the input microstructure for the simulations. When chloride binding and a time-dependent mortar diffusivity are both included in the computer model, good agreement with the experimental results is obtained. Both crack fillers significantly reduce chloride ingress during the 21 d period of the present experiments; however, the epoxy itself contains approximately 4 % by mass chlorine. Leaching studies were performed assess the epoxy as a source of deleterious ions for initiating corrosion of the steel reinforcement in concrete structures.

Crystal structures of human 3-hydroxyanthranilate 3,4-dioxygenase with native and non-native metals bound in the active site.

3-Hydroxyanthranilate 3,4-dioxygenase (3HAO) is an enzyme in the microglial branch of the kynurenine pathway of tryptophan degradation. 3HAO is a non-heme iron-containing, ring-cleaving extradiol dioxygenase that catalyzes the addition of both atoms of O2 to the kynurenine pathway metabolite 3-hydroxyanthranilic acid (3-HANA) to form quinolinic acid (QUIN). QUIN is a highly potent excitotoxin that has been implicated in a number of neurodegenerative conditions, making 3HAO a target for pharmacological downregulation. Here, the first crystal structure of human 3HAO with the native iron bound in its active site is presented, together with an additional structure with zinc (a known inhibitor of human 3HAO) bound in the active site. The metal-binding environment is examined both structurally and via inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence spectroscopy (XRF) and electron paramagnetic resonance spectroscopy (EPR). The studies identified Met35 as the source of potential new interactions with substrates and inhibitors, which may prove useful in future therapeutic efforts.

Isotope dilution mass spectrometry for quantitative elemental analysis of powdered samples by radiofrequency pulsed glow discharge time of flight mass spectrometry.

In recent years particular effort is being devoted to the development of pulsed glow discharges (PGDs) for mass spectrometry because this powering operation mode could offer important ionization analytical advantages. However, the capabilities of radiofrequency (RF) PGD coupled to a time of flight mass spectrometry (ToFMS) for accurate isotope ratio measurements have not been demonstrated yet. This work is focused on investigating different time positions along the pulse profile for the accurate measurement of isotope ratios. As a result, a method has been developed for the direct and simultaneous multielement determination of trace elements in powdered geological samples by RF-PGD-ToFMS in combination with isotope dilution mass spectrometry (IDMS) as an absolute measurement method directly traceable to the International System of Units. Optimized operating conditions were 70 W of applied radiofrequency power, 250 Pa of pressure, 2 ms of pulse width and 4 ms of pulse period, being argon the plasma gas used. To homogeneously distribute the added isotopically-enriched standards, lithium borate fusion of powdered solid samples was used as sample preparation approach. In this way, Cu, Zn, Ba and Pb were successfully determined by RF-PGD-ToF(IDMS) in two NIST Standard Reference Materials (SRM 2586 and SRM 2780) representing two different matrices of geological interest (soil and rock samples). Cu, Zn, Ba and Pb concentrations determined by RF-PGD-ToF(IDMS) were well in agreement with the certified values at 95% confidence interval and precisions below 12% relative standard deviation were observed for three independent analyses. Elemental concentrations investigated were in the range of 81-5770 mg/kg, demonstrating the potential of RF-PGD-ToF(IDMS) for a sensitive, accurate and robust analysis of powdered samples.

Elemental analyses of soil and sediment fused with lithium borate using isotope dilution laser ablation-inductively coupled plasma-mass spectrometry.

Quantitative analysis using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) remains challenging primarily due to the lack of appropriate reference materials available for the wide variety of samples of interest and to elemental fractionation effects. Isotopic dilution mass spectrometry (IDMS) is becoming the methodology of choice to address these issues because the different isotopes of an element represent near-perfect internal standards. In this work, we investigated the lithium borate fusion of powdered solid samples, including soils, sediments, rock mine waste and a meteorite, as a strategy to homogenously distribute, i.e. equilibrate the elements and the added isotopically enriched standards. A comparison of this methodology using two pulsed laser ablation systems (ArF* excimer and Nd:YAG) with different wavelengths as well as two ICP-MS instruments (quadrupole and double-focusing sector field) was performed. Emphasis was put on using standard equipment to show the potential of the proposed strategy for its application in routine laboratories. Cr, Zn, Ba, Sr and Pb were successfully determined by LA-ICP-IDMS in six Standard Reference Materials (SRMs) representing different matrices of environmental interest. Experimental results showed the SRM fused glasses exhibited a low level of heterogeneity (intra- and inter-sample) for both natural abundance and isotopically enriched samples (RSD <3%, n=3, 1σ). A good agreement between experimental results and the certified values was also observed.

Standard addition method for laser ablation ICPMS using a spinning platform.

A method has been developed for the fast and easy determination of Pb, Sr, Ba, Ni, Cu, and Zn, which are of geological and environmental interest, in solid samples by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) using a spinning sample platform. The platform, containing a sample and a standard, is spun during the ablation, allowing the quasi-simultaneous ablation of both materials. The aerosols resulting from the ablation of sample and standard were mixed in the ablation cell allowing quantification of analytes by standard additions. The proportion of standard versus sample of the mixing can be increased by performing the ablation further from the axis of rotation. The ablated masses have been determined using a new strategy based on isotope dilution analysis. This spinning laser ablation method has been applied to the Allende meteorite and four powdered standard reference materials (SRMs) fused in lithium borate glasses: two sediments as well as a soil and a rock material. SRM 612 (Trace Elements in Glass) was also analyzed despite having a matrix slightly different from the glass standard obtained by lithium borate fusion. The deviation from the certified values was found to be less than 15% for most of the mass fractions for all the elements and samples studied, with an average precision of 10%. These results demonstrate the validity of the proposed method for the direct and fast analysis of solid samples of different matrixes by standard additions, using a single standard sample.

Putting a spin on LA-ICP-MS analysis combined to isotope dilution.

The determination of Zn, Sr, Ba, and Pb in solid samples has been achieved by laser ablation inductively coupled plasma isotope dilution mass spectrometry using a spinning platform. The fast rotation of a sample and an isotopically enriched spike placed close together on a sample holder allowed performing the isotope dilution directly inside the ablation cell. The proportion of spike versus sample of the aerosol mixture obtained has been determined online by isotope dilution in order to correct for differences in ablation rate although both materials were placed on the axis of rotation of the motor. Homogeneous, time-stable, and reusable samples were prepared by lithium borate fusion. A unique isotopically enriched spike glass was used to analyze four Standard Reference Materials of different matrix (after a simple polishing): two sediments Standard Reference Material (SRM) 1944 and SRM 2702 and two soils SRM 2586 and SRM 2711a. The proposed method yielded mass fractions with a deviation from the certified value usually lower than 12% and a precision of less than 9% RSD (except for Zn in SRM 2586 and 2711a). Although direct spiking of the solid before fusion could presumably provide better isotopic mixing, the presented methodology allows the reuse of the spike glass (thus, decreasing drastically the cost of the analysis) and is relatively faster because the spike does not need to be weighted, added, and evaporated each time. These results demonstrate the potential of this newly developed method for fast analysis of solid samples using isotope dilution at a low cost.

Classification of microheterogeneity in solid samples using microXRF.

Micro X-ray fluorescence (microXRF) has been used nondestructively to investigate elemental heterogeneity by constructing two-dimensional maps of elemental concentrations in reference materials. microXRF probes sample sizes well below the 100 mg mass usually recommended for reference materials by NIST. Multivariate methods of analysis, such as principal-component analysis (PCA), show promise in identifying whether "nugget" effects exist within a material, where an element is enriched in small, isolated areas of the sample. The PCA model is built based on data taken in one location and compared with each elemental map. This methodology is shown for several reference materials including SRM 2702 and SRM 2703 to show how PCA treatment can be used to identify which elements exhibit nugget effects within the sub-mg mass range. A method of calculating the minimum recommended mass for solid samples is suggested using PCA iteratively on X-ray maps from which adjacent data points have been averaged. This is repeated until the mass sampled in a map is indistinguishable from data taken at a single location, suggesting no nugget effects can be detected. For SRM 1577c, a mass as low as 370 microg can be used without measurable nugget effects.

Development of saw palmetto (Serenoa repens) fruit and extract standard reference materials.

As part of a collaboration with the National Institutes of Health's Office of Dietary Supplements and the Food and Drug Administration's Center for Drug Evaluation and Research, the National Institute of Standards and Technology has developed two standard reference materials (SRMs) representing different forms of saw palmetto (Serenoa repens), SRM 3250 Serenoa repens fruit and SRM 3251 Serenoa repens extract. Both of these SRMs have been characterized for their fatty acid and phytosterol content. The fatty acid concentration values are based on results from gas chromatography with flame ionization detection (GC-FID) and mass spectrometry (GC/MS) analysis while the sterol concentration values are based on results from GC-FID and liquid chromatography with mass spectrometry analysis. In addition, SRM 3250 has been characterized for lead content, and SRM 3251 has been characterized for the content of beta-carotene and tocopherols. SRM 3250 (fruit) has certified concentration values for three phytosterols, 14 fatty acids as triglycerides, and lead along with reference concentration values for four fatty acids as triglycerides and 16 free fatty acids. SRM 3251 (extract) has certified concentration values for three phytosterols, 17 fatty acids as triglycerides, beta-carotene, and gamma-tocopherol along with reference concentration values for three fatty acids as triglycerides, 17 fatty acids as free fatty acids, beta-carotene isomers, and delta-tocopherol and information values for two phytosterols. These SRMs will complement other reference materials currently available with concentrations for similar analytes and are part of a series of SRMs being developed for dietary supplements.

Detection of palladium by cold atom solution atomic absorption.

One of the largest obstacles in miniaturizing traditional atomic spectroscopic sources is the need for a thermal/electrical source for free atom production. A single article in the literature has demonstrated atomic absorption detection of Ag, Cu, and Pd in solution at room temperature for atoms in the gas phase, which may ultimately permit miniaturization. Unfortunately, several laboratories have found that reproducing the phenomenon has been difficult. Without a sound fundamental explanation of the processes leading to the signal, one must conclude that it can be done, but some unsuspected and unknown design/methodological nuances are responsible for only a single reported success. Gas phase atoms could exist at room temperature "in solution" if the atoms were trapped in very small bubbles. In the current study, submicrometer-sized bubbles were created in a flow-through cell during the mixing of an alcohol-water solution containing a reducing agent with water containing the analyte. A repeatable atomic absorption signal was produced. Replacement of ethanol with 1-propanol and use of a surfactant increased the signal. Limits of detection of approximately 100 ppb in Pd were achieved, and it is estimated that approximately 0.4% of the Pd initially added is contained within the bubbles as gaseous atoms. The paper discusses the fundamental processes needed to achieve a repeatable signal.