Biological and biomedical applications of accelerator mass spectrometry (AMS) use isotope ratio mass spectrometry to quantify minute amounts of long-lived radioisotopes such as 14C. AMS target preparation involves first the oxidation of carbon (in sample of interest) to CO2 and second the reduction of CO2 to filamentous, fluffy, fuzzy, or firm graphite-like substances that coat a −400-mesh spherical iron powder (−400MSIP) catalyst. Until now, the quality of AMS targets has been variable; consequently, they often failed to produce robust ion currents that are required for reliable, accurate, precise, and high-throughput AMS for biological/biomedical applications. Therefore, we described our optimized method for reduction of CO2 to high-quality uniform AMS targets whose morphology we visualized using scanning electron microscope pictures. Key features of our optimized method were to reduce CO2 (from a sample of interest that provided 1 mg of C) using 100 ± 1.3 mg of Zn dust, 5 ± 0.4 mg of −400MSIP, and a reduction temperature of 500 °C for 3 h. The thermodynamics of our optimized method were more favorable for production of graphite-coated iron powders (GCIP) than those of previous methods. All AMS targets from our optimized method were of 100% GCIP, the graphitization yield exceeded 90%, and δ13C was −17.9 ± 0.3‰. The GCIP reliably produced strong 12C− currents and accurate and precise Fm values. The observed Fm value for oxalic acid II NIST SRM deviated from its accepted Fm value of 1.3407 by only 0.0003 ± 0.0027 (mean ± SE, n = 32), limit of detection of 14C was 0.04 amol, and limit of quantification was 0.07 amol, and a skilled analyst can prepare as many as 270 AMS targets per day. More information on the physical (hardness/color), morphological (SEMs), and structural (FT-IR, Raman, XRD spectra) characteristics of our AMS targets that determine accurate, precise, and high-hroughput AMS measurement are in the companion paper.

Isotope tracer studies, particularly radiocarbon measurements, play a key role in biological, nutritional, and environmental research. Accelerator mass spectrometry (AMS) is now the most sensitive detection method for radiocarbon, but AMS is not widely used in kinetic studies of humans. Part of the reason is the expense, but costs would decrease if AMS were used more widely. One component in the cost is sample preparation for AMS. Biological and environmental samples are commonly reduced to graphite before they are analyzed by AMS. Improvements and mechanization of this multi-step procedure is slowed by a lack of organized educational materials for AMS sample preparation that would allow new investigators to work with the technique without a substantial outlay of time and effort. We present a detailed sample preparation protocol for graphitizing biological samples for AMS and include examples of nutrition studies that have used this procedure.

43) Analogous to those in the ' n r * states of ketones, studied via the circular polarization of the fluorescence, see: Schippers, P. H.; Dekkers, H. P. J. M. Chem. Phys. Lett. 1982.88, 512-516. Dekkers, H. P. J. M.; Moraal, P. F. InAbstract: The first Raman spectra of the allyl radical have been obtained. The intensities of the observed Raman spectra indicate excited-state dynamics consistent with a disrotary photoisomerization of the allyl radical to form a cyclopropyl radical. Prior to this work, direct examination of the photoisomerization pathway was not possible due to limitations of the techniques applied. The ground-state vibrational frequencies observed are found to be in excellent agreement with recent theoretical calculations suggesting a reassignment of the literature infrared frequencies. This work demonstrates that resonance Raman spectroscopy is a powerful method for examination of gas-phase free radicals.

The number of biological/biomedical applications that require AMS to achieve their goals is increasing, and so is the need for a better understanding of the physical, morphological, and structural traits of high quality of AMS targets. The metrics of quality included color, hardness/texture, and appearance (photo and SEM), along with FT-IR, Raman, and powder X-ray diffraction spectra that correlate positively with reliable and intense ion currents and accuracy, precision, and sensitivity of fraction modern (Fm). Our previous method produced AMS targets of gray-colored iron−carbon materials (ICM) 20% of the time and of graphite-coated iron (GCI) 80% of the time. The ICM was hard, its FT-IR spectra lacked the sp2 bond, its Raman spectra had no detectable G′ band at 2700 cm−1, and it had more iron carbide (Fe3C) crystal than nanocrystalline graphite or graphitizable carbon (g-C). ICM produced low and variable ion current whereas the opposite was true for the graphitic GCI. Our optimized method produced AMS targets of graphite-coated iron powder (GCIP) 100% of the time. The GCIP shared some of the same properties as GCI in that both were black in color, both produced robust ion current consistently, their FT-IR spectra had the sp2 bond, their Raman spectra had matching D, G, G′, D+G, and D′′ bands, and their XRD spectra showed matching crystal size. GCIP was a powder that was easy to tamp into AMS target holders that also facilitated high throughput. We concluded that AMS targets of GCIP were a mix of graphitizable carbon and Fe3C crystal, because none of their spectra, FT-IR, Raman, or XRD, matched exactly those of the graphite standard. Nevertheless, AMS targets of GCIP consistently produced the strong, reliable, and reproducible ion currents for high-throughput AMS analysis (270 targets per skilled analyst/day) along with accurate and precise Fm values.

Oxygen transfer associated with natural convection in lakes and reservoirs was examined in a series of laboratory experiments. A thin, cool surface water layer (2-3 mm in thickness) was formed by chilling the air overlying a tank of surface area 0.6 m 2 and depth 0.6 m. The surface water layer became gravitationally unstable, resulting in the formation of negatively buoyant thermal plumes, which penetrated through the total depth of the water column. The spatial distribution of oxygen concentration at the air-water interface in the tank was visualized using a fluorescence imaging technique to quantify the oxygen transfer driven by only natural convection. Pyrenebutyric acid (PBA) at a concentration of 3.0 ϫ 10 Ϫ6 mole L Ϫ1 was used as the fluorophore, and the quenching of the fluorescence by oxygen was used to produce a spatial distribution of dissolved oxygen. A light plane was generated across the tank by the refraction of a laser light beam, and two-dimensional images were continuously acquired with an intensified charge coupled-device (ICCD) camera. Analysis of these images revealed the sinking of cooled water to transport oxygen, and the experiments enabled the quantification of the oxygen transferred from the air into water at a range of heat fluxes. The results confirm that vertical penetration of cold-dense water can be a significant source of oxygen for lakes and reservoirs.''Oxygen is the most fundamental parameter of lakes, aside from water itself. . . '' (Wetzel 1975), and an improved understanding of water quality issues is dependent on understanding how dissolved oxygen (DO) enters and is distributed through these systems. Oxygen and other gases enter and leave a water body across the air-water interface. The rate at which this occurs is frequently parameterized as being a function of the wind speed across the surface and on the gas concentration in the water, relative to its saturation level (Broecker et al. 1978;O'Connor 1983). At times of high wind speed and low DO concentration, the transfer rate into the aqueous phase will be relatively high.The study of gas transfer across an air-water interface has generally been conducted with reference to the ocean (for example, Watson et al. 1991;Wanninkhof 1992). Here, sus-

We emphasize two points: (l) the properties and mechanisms of very low-fluence ablation of copper surfaces and (2) the sensitivity and selectivity of resonant laser ablation (RLA). We present results for ablation of bulk copper and copper thin films; spot-size effects; the effects of surface-sample preparation and beam polarization; and an accurate measurement of material removal rates, typically ≤ 10(-3) Å at 35 mJ/cm(2). Velocity distributions were Maxwellian, with peak velocities ≈ 1-2 × 10(5) cm/s. In addition, we discuss the production of diffractionlike surface features, and the probable participation of nonthermal desorption mechanisms. RLA is shown to be a sensitive and useful diagnostic for studies of low-fluence laser-material interactions.

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