Savelas A. Rabb

Improved measurement results for the Avogadro constant using a 28Si-enriched crystal

New results are reported from an ongoing international research effort to accurately determine the Avogadro constant by counting the atoms in an isotopically enriched silicon crystal. The surfaces of two 28Si-enriched spheres were decontaminated and reworked in order to produce an outer surface without metal contamination and improved sphericity. New measurements were then made on these two reconditioned spheres using improved methods and apparatuses. When combined with other recently refined parameter measurements, the Avogadro constant derived from these new results has a value of

Factors Affecting Quantification of Total DNA by UV Spectroscopy and PicoGreen Fluorescence

N_A = 6.022 140 76(12) \times 10^{23}

Absolute silicon molar mass measurements, the Avogadro constant and the redefinition of the kilogram

mol

DNA damaging potential of photoactivated p25 titanium dioxide nanoparticles.

^{-1}

Heat-Assisted Argon Electrospray Interface for Low-Flow Rate Liquid Sample Introduction in Plasma Spectrometry

. The X-ray crystal density method has thus achieved the target relative standard uncertainty of

Multiple Method Analysis of TiO2 Nanoparticle Uptake in Rice (Oryza sativa L.) Plants

2.0 \times 10^{-8}

Characterization of SiGe films for use as a National Institute of Standards and Technology Microanalysis Reference Material (RM 8905).

necessary for the realization of the definition of the new kilogram.

Kinetics of Photodegradation and Nanoparticle Surface Accumulation of a Nanosilica/Epoxy Coating Exposed to UV Light

The total amount of DNA in a preparation extracted from tissues can be measured in several ways, each method offering advantages and disadvantages. For the sake of accuracy in quantitation, it is of interest to compare these methodologies and determine if good correlation can be achieved between them. Different answers can also be clues to the physical state of the DNA. In this study, we investigated the lack of correlation between ultraviolet (UV) absorbance and fluorescent (PicoGreen) measurements of the concentration of DNAs isolated from plant tissues. We found that quantitation based on the absorbance-based method correlated with quantitation based on phosphorus content, while the PicoGreen-based method did not. We also found evidence of the production of single-stranded DNA under conditions where the DNA was not fragmented into small pieces. The PicoGreen fluorescent signal was dependent on DNA fragment size but only if the DNA was in pure water, while DNA in buffer was much less sensitive. Finally, we document the high sensitivity of the PicoGreen assays to the detergent known as CTAB (cetyldimethylethylammonium bromide). The CTAB-based method is highly popular for low-cost DNA extraction with many published variations for plant and other tissues. The removal of residual CTAB is important for accurate quantitation of DNA using PicoGreen.

Accurate determinations of Ge atom fractions in SiGe semiconductor chips using high performance ICP-OES

The results of an absolute silicon molar mass determination of two independent sets of samples from the highly 28Si-enriched crystal (AVO28) produced by the International Avogadro Coordination are presented and compared with results published by the Physikalisch-Technische Bundesanstalt (PTB, Germany), the National Research Council (NRC, Canada) and the National Metrology Institute of Japan (NMIJ, Japan). This study developed and describes significant changes to the published protocols for producing absolute silicon isotope ratios. The measurements were made at very high resolution on a multi-collector inductively coupled plasma mass spectrometer using tetramethylammonium hydroxide (TMAH) to dissolve and dilute all samples. The various changes in the measurement protocol and the use of TMAH resulted in significant improvements to the silicon isotope ratio precision over previously reported measurements and in particular, the robustness of the 29Si/30Si ratio of the AVO28 material. These new results suggest that a limited isotopic variability is present in the AVO28 material. The presence of this variability is at present singular and therefore its significance is not well understood. Fortunately, its magnitude is small enough so as to have an insignificant effect on the overall uncertainty of an Avogadro constant derived from the average molar mass of all four AVO28 silicon samples measured in this study. The NIST results confirm the AVO28 molar mass values reported by PTB and NMIJ and confirm that the virtual element–isotope dilution mass spectrometry approach to calibrated absolute isotope ratio measurements developed by PTB is capable of very high precision as well as accuracy. The Avogadro constant NA and derived Planck constant h based on these measurements, together with their associated standard uncertainties, are 6.02214076(19) × 1023 mol−1 and 6.62607017(21) × 10−34 Js, respectively.

Quantification of nanoparticle release from polymer nanocomposite coatings due to environmental stressing

Titanium dioxide nanoparticles (TiO2 NPs) are found in numerous commercial and personal care products. Thus, it is necessary to understand and characterize their potential environmental health and safety risks. It is well-known that photoactivated TiO2 NPs in aerated aqueous solutions can generate highly reactive hydroxyl radicals ((•)OH), which can damage DNA. Surprisingly, recent in vitro studies utilizing the comet assay have shown that nonphotoactivated TiO2 NPs kept in the dark can also induce DNA damage. In this work, we utilize stable isotope-dilution gas chromatography/tandem mass spectrometry to quantitatively characterize the levels and types of oxidatively generated base lesions in genomic DNA exposed to NIST Standard Reference Material TiO2 NPs (Degussa P25) under precisely controlled illumination conditions. We show that DNA samples incubated in the dark for 24 h with TiO2 NPs (0.5-50 μg/mL) do not lead to the formation of base lesions. However, when the same DNA is exposed to either visible light from 400 to 800 nm (energy dose of ∼14.5 kJ/m(2)) for 24 h or UVA light at 370 nm for 30 min (energy dose of ∼10 kJ/m(2)), there is a significant formation of lesions at the 50 μg/mL dose for the visible light exposure and a significant formation of lesions at the 5 and 50 μg/mL doses for the UVA light exposure. These findings suggest that commercial P25 TiO2 NPs do not have an inherent capacity to oxidatively damage DNA bases in the absence of sufficient photoactivation; however, TiO2 NPs exposed to electromagnetic radiation within the visible portion of the light spectrum can induce the formation of DNA lesions. On the basis of these findings, comet assay processing of cells exposed to TiO2 should be performed in the dark to minimize potential artifacts from laboratory light.