Stefan D. Leigh

Determination of perfluorinated alkyl acid concentrations in human serum and milk standard reference materials

Standard Reference Materials (SRMs) are certified reference materials produced by the National Institute of Standards and Technology that are homogeneous materials well characterized with values for specified properties, such as environmental contaminant concentrations. They can be used to validate measurement methods and are critical in improving data quality. Disagreements in perfluorinated alkyl acid (PFAA) concentrations measured in environmental matrices during past interlaboratory comparisons emphasized the need for SRMs with values assigned for PFAAs. We performed a new interlaboratory comparison among six laboratories and provided, for the first time, value assignment of PFAAs in SRMs. Concentrations for perfluorooctane sulfonate (PFOS), perfluorooctanoate (PFOA), and other PFAAs in two human serum and two human milk SRMs are reported. PFAA concentration measurements agreed for serum SRM 1957 using different analytical methods in six laboratories and for milk SRM 1954 in three laboratories. The interlaboratory relative standard deviation for PFOS in SRM 1957 was 7%, which is an improvement over past interlaboratory studies. Matrix interferences are discussed, as well as temporal trends and the percentage of branched vs. linear isomers. The concentrations in these SRMs are similar to the present-day average concentrations measured in human serum and milk, resulting in representative and useful control materials for PFAA human monitoring studies.

Development of a Standard Reference Material for Metabolomics Research

The National Institute of Standards and Technology (NIST), in collaboration with the National Institutes of Health (NIH), has developed a Standard Reference Material (SRM) to support technology development in metabolomics research. SRM 1950 Metabolites in Human Plasma is intended to have metabolite concentrations that are representative of those found in adult human plasma. The plasma used in the preparation of SRM 1950 was collected from both male and female donors, and donor ethnicity targets were selected based upon the ethnic makeup of the U.S. population. Metabolomics research is diverse in terms of both instrumentation and scientific goals. This SRM was designed to apply broadly to the field, not toward specific applications. Therefore, concentrations of approximately 100 analytes, including amino acids, fatty acids, trace elements, vitamins, hormones, selenoproteins, clinical markers, and perfluorinated compounds (PFCs), were determined. Value assignment measurements were performed by NIST and the Centers for Disease Control and Prevention (CDC). SRM 1950 is the first reference material developed specifically for metabolomics research.

Relative Intensity Correction of Raman Spectrometers: NIST SRMs 2241 through 2243 for 785 nm, 532 nm, and 488 nm/514.5 nm Excitation

Standard Reference Materials® SRMs 2241 through 2243 are certified spectroscopic standards intended for the correction of the relative intensity of Raman spectra obtained with instruments employing laser excitation wavelengths of 785 nm, 532 nm, or 488 nm/514.5 nm. These SRMs each consist of an optical glass that emits a broadband luminescence spectrum when illuminated with the Raman excitation laser. The shape of the luminescence spectrum is described by a polynomial expression that relates the relative spectral intensity to the Raman shift with units in wavenumber (cm−1). This polynomial, together with a measurement of the luminescence spectrum of the standard, can be used to determine the spectral intensity-response correction, which is unique to each Raman system. The resulting instrument intensity-response correction may then be used to obtain Raman spectra that are corrected for a number of, but not all, instrument-dependent artifacts. Peak area ratios of the intensity-corrected Raman spectrum of cyclohexane are presented as an example of a methodology to validate the spectral intensity calibration process and to illustrate variations that can occur in this measurement.

Improving phase Doppler volume flux measurements in low data rate applications

Phase Doppler interferometry (PDI) measurements in low number density sprays necessitate a compromise between collecting a large number of samples for adequate statistics and practical data acquisition times. This paper investigates the effect of insufficient sample statistics on the calculated probe area, and the resultant uncertainty in the volume flux measurement. Several methods of improving the probe area calculation and volume flux measurement are investigated using experimental data obtained from water sprays produced by residential fire sprinklers. It is shown that the corrections result in statistically significant improvements in the volume flux measurements.

Polycyclic aromatic hydrocarbons (PAHs) in a coal tar standard reference material—SRM 1597a updated

SRM 1597 Complex Mixture of Polycyclic Aromatic Hydrocarbons from Coal Tar, originally issued in 1987, was recently reanalyzed and reissued as SRM 1597a with 34 certified, 46 reference, and 12 information concentrations (as mass fractions) for polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic sulfur heterocycles (PASHs) including methyl-substituted PAHs and PASHs. The certified and reference concentrations (as mass fractions) were based on results of analyses of the coal tar material using multiple analytical techniques including gas chromatography/mass spectrometry on four different stationary phases and reversed-phase liquid chromatography. SRM 1597a is currently the most extensively characterized SRM for PAHs and PASHs.

Modeling of the bremsstrahlung radiation produced in pure-element targets by 10-40 keV electrons

A new global relationship has been developed for predicting electron‐excited bremsstrahlung intensities over a wide range of accelerating voltages 10–40 keV, atomic numbers 4–92, and x‐ray energies 1.5–20 keV. The new relationship was determined empirically from the mathematical modeling of extensive data and is designed for calculating bremsstrahlung intensities in analytical procedures, such as those requiring peak‐to‐background measurements, where the direct measurement of the bremsstrahlung intensities is impracticable. The distribution of errors between the data and the model is symmetrical, centered around zero error with 63% of the values falling between ±10% relative error.

Effect of plastic strain on magnetic and mechanical properties of ultralow carbon sheet steel

We have investigated how plastic deformation changes the yield stress and the magnetic properties of an ultralow carbon sheet steel. Plastic strains up to 10% nearly doubled the yield stress from 150 to 290 MPa (22–42 ksi), and the coercive field from 0.17 to 0.31 kA/m (2.1–3.9 Oe). The effects on other magnetic properties, such as the Barkhausen signal and permeability, were also determined. It was found that a single magnetic parameter could be related to the yield stress with residual standard deviations as low as 1 MPa (0.2 ksi). Observations using a high-resolution interference–contrast colloid technique revealed a fine intragrain magnetic domain structure with the walls more effectively pinned in the 10% strained material.

Excitation—Emission Matrix Fluorescence Spectroscopy for Natural Organic Matter Characterization: A Quantitative Evaluation of Calibration and Spectral Correction Procedures:

The influence of different data collection procedures and of wavelength-dependent instrumental biases on fluorescence excitation–emission matrix (EEM) spectral analysis of aqueous organic matter samples was investigated. Particular attention was given to fluorescence contours (spectral shape) and peak fluorescence intensities. Instrumental bias was evaluated by independently applying excitation and emission correction factors to the raw excitation and emission data, respectively. The peak fluorescence intensities of representative natural organic matter and tryptophan were significantly influenced by the application of excitation and emission spectral correction factors and by the manner in which the raw data was collected. Humification and fluorescence indices were also influenced by emission correction factors but were independent of reference (excitation) intensity normalization or correction. EEM surface contours were dependent on normalization of the fluorescence intensity to the reference intensity but were not influenced by either excitation or emission spectral correction factors. Authors should be explicit in how excitation and emission spectral correction procedures are implemented in their investigations, which will help to facilitate intra-laboratory comparisons and data sharing.

Recent approaches to extreme value estimation with application to wind speeds. Part I: the pickands method

The last 15 years have seen the development of a new body of theory and of a new generation of statistical estimation procedures applicable to extreme data. Essentially these approaches emphasize the primacy of the information concerning the largest of the extremes (the tails), as opposed to information inherent in the bulk of the extreme data (tile body of the distribution). One potentially attractive feature of some of these approaches is that, in principle, they make it possible to ascertain whether the distribution thai best fits a given set of extreme data has a finite tail. Whether extreme ~ind speed distributions have finite tails is a matter of great interest in structural engineering since the answer to this question may affect significantly the estimation of safety factors for wind-sensitive structures. Failure probability estimates based on the assumption that wind speeds are described by the Gumbel distribution (which has infinite upper tail) were shown to be unrealistically high (Ellingwood et a1,1980; Simiu, Shaver, and Filliben, 1981). This may be the case because the probability of occurrence of very high speeds, rather than being finite as in the Gumbel model, is in fact zero, since it is obvious on physical grounds that extratropical wind speeds must be bounded. It is natural to ask whether modern statistical methods can estimate those bounds from available sets of extreme wind speed data, and if so, what is the minimum size of the data sample that would allow the estimates to be made with acceptable confidence. Tile present work is motivated by these questions. It is part of a long<erm project aimed at evaluating various estimation methods Z a the theory of extremes. We examine two such methods here: (1) a condi t iona l mean exeeedanee ( C M E ) slope estimation method (Davisson-Smith,1990); and (2) the P iekands method (Pickands,1975). Both are based on the Generalized Pareto Distribution (GPD) model for excesses over a threshold. The GPD was shown to arise as a limiting distribution for excesses over thresholds if and only if the parent distribution tends asymptotically to one of the extreme value

Elemental analysis of a single-wall carbon nanotube candidate reference material

A material containing single-wall carbon nanotubes (SWCNTs) with other carbon species, catalyst residues, and trace element contaminants has been prepared by the National Institute of Standards and Technology for characterization and distribution as Standard Reference Material SRM 2483 Carbon Nanotube Soot. Neutron activation analysis (NAA) and inductively coupled plasma mass spectrometry (ICP–MS) were selected to characterize the elemental composition. Catalyst residues at percentage mass fraction level were determined with independent NAA procedures and a number of trace elements, including selected rare earth elements, were determined with NAA and ICP–MS procedures. The results of the investigated materials agreed well among the NAA and ICP–MS procedures and good agreement of measured values with certified values was found in selected SRMs included in the analyses. Based on this work mass fraction values for catalyst and trace elements were assigned to the candidate SRM.