PubMed · 14306884
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H H RICHTER. 1965. [DIFFICULTIES AND PRESENT PROBLEMS IN STEROID ANALYSIS].. https://pubmed.ncbi.nlm.nih.gov/14306884/
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An on-line Hg reduction technique using stannous chloride as the reductant was applied for accurate and precise mercury isotope ratio determinations by multi-collector (MC)-ICP/MS. Special attention has been paid to ensure optimal conditions (such as acquisition time and mercury concentration) allowing precision measurements good enough to be able to significantly detect the anticipated small differences in Hg isotope ratios in nature. Typically, internal precision was better than 0.002% (1 RSE) on all Hg ratios investigated as long as approximately 20 ng of Hg was measured with a 10-min acquisition time. Introducing higher amounts of mercury (50 ng Hg) improved the internal precision to <0.001%. Instrumental mass bias was corrected using (205)Tl/(203)Tl correction coupled to a standard-sample bracketing approach. The large number of data acquired allowed us to validate the consistency of our measurements over a one-year period. On average, the short-term uncertainty determined by repeated runs of NIST SRM 1641d Hg standard during a single day was <0.006% (1 RSD) for all isotope pairs investigated ((202)Hg/(198)Hg, (202)Hg/(199)Hg, (202)Hg/(200)Hg, and (202)Hg/(201)Hg). The precision fell to <0.01% if the long-term reproducibility, taken over 11 months (over 100 measurements), was considered. The extent of fractionation has been investigated in a series of sediments subject to various Hg sources from different locations worldwide. The ratio (202)Hg/(198)Hg expressed as delta values (per mil deviations relative to NIST SRM 1641d Hg standard solution) displayed differences from +0.74 to -4.00 per thousand. The magnitude of the Hg fractionation per amu was constant within one type of sample and did not exceed 1.00 per thousand. Considering all results (the reproducibility of Hg standard solutions, reference sediment samples, and the examination of natural samples), the analytical error of our delta values for the overall method was within +/-0.28 per thousand (1 SD), which was an order of magnitude lower than the extent of fractionation (4.74 per thousand) observed in sediments. This study confirmed that analytical techniques have reached a level of long-term precision and accuracy that is sufficiently sensitive to detect even small differences in Hg isotope ratios that occur within one type of samples (e.g., between different sediments) and so far have unequivocally shown that Hg isotope ratios in sediments vary within approximately 5 per thousand.
Self-assembling properties of "natural" glycolipid biosurfactants, mannosyl-erythritol lipids A and B (MEL-A, MEL-B), which are abundantly produced from yeast strains, were investigated by using the fluorescence-probe method, dynamic light-scattering (DLS) analysis, freeze-fracture transmission electron microscopy (FF-TEM), and synchrotron small/wide-angle X-ray scattering (SAXS/WAXS) analysis, among other methods. Both MEL-A and MEL-B exhibit excellent self-assembly properties at extremely low concentrations; they self-assemble into large unilamellar vesicles (LUV) just above their critical-aggregation concentration (CAC). The CAC(I) value was found to be 4.0x10(-6) M for MEL-A and 6.0x10(-6) M for MEL-B. Moreover, the self-assembled structure of MEL-A above a CAC(II) value of 2.0x10(-5) M was found to drastically change into sponge structures (L3) composed of a network of randomly connected bilayers that are usually obtained from a complicated multicomponent "synthetic" surfactant system. Interestingly, the average water-channel diameter of the sponge structure was 100 nm. This is relatively large compared with those obtained from "synthetic" surfactant systems. In addition, MEL-B, which has a hydroxyl group at the C-4' position on mannose instead of an acetyl group, gives only one CAC; the self-assembled structure of MEL-B seems to gradually move from LUV to multilamellar vesicles (MLV) with lattice constants of 4.4 nm, depending on the concentration. Furthermore, the lyotropic-liquid-crystal-phase observation at high concentrations demonstrates the formation of an inverted hexagonal phase (H2) for MEL-A, together with a lamella phase (L(alpha)) for MEL-B, indicating a difference between MEL-A and MEL-B molecules in the spontaneous curvature of the assemblies. These results clearly show that the difference in spontaneous curvature caused by the single acetyl group on the head group probably decides the direction of self-assembly of glycolipid biosurfactants. The unique and complex molecular structures with several chiral centers that are molecularly engineered by microorganisms must have led to the sophisticated self-assembling properties of the glycolipid biosurfactants.
A photoacoustic multi-gas sensor using tuneable laser diodes in the near-infrared region is reported. An optimized resonant configuration based on an acoustic longitudinal mode is described. Automatic tracking of the acoustic resonance frequency using a piezo-electric transducer and a servo electronics is demonstrated. Water vapour, methane and hydrogen chloride have been measured at sub-ppm level in different buffer gas mixtures. The importance of the system calibration in presence of several diluting gases is discussed. Finally, trace gas measurements have been assessed and detection limits (signal-to-noise ratio=3) of 80 ppb at 1651.0 nm for CH(4), 24 ppb at 1368.6 nm for H(2)O and 30 ppb at 1737.9 for HCl have been demonstrated.