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71.
Degner R 《Analytical and bioanalytical chemistry》2002,374(5):831-834
A measured value without even an approximate knowledge of the uncertainty is worthless. The uncertainty is part of every measured value and specification of the uncertainty is part of every analytical procedure. The uncertainty makes the value independent of its origin. The basis for estimation of the uncertainty is the "Guide to the Expression of Uncertainty in Measurement ". For some procedures, however, for example pH measurement, several problems arise in practice. This article describes a practical and inexpensive way of calculating the uncertainty of pH values. 相似文献
72.
Schützhold R 《Physical review letters》2002,89(8):081302
According to recent astrophysical observations the large scale mean pressure of our present Universe is negative suggesting a positive cosmological constant-like term. The issue of whether nonperturbative effects of self-interacting quantum fields in curved space-times may yield a significant contribution is addressed. Focusing on the trace anomaly of quantum chromodynamics, a preliminary estimate of the expected order of magnitude yields a remarkable coincidence with the empirical data, indicating the potential relevance of this effect. 相似文献
73.
Gain-guided diode lasers usually have emission wavelengths determined by the manufacturing process, with typically 0.5-1-nm bandwidth. Furthermore, their beam quality is rather poor. We show that external cavities allow for tunable narrow-bandwidth operation of gain-guided diode lasers. At the same time the beam quality is drastically improved; almost diffraction-limited light of more than 200 mW has been achieved over the whole tuning range from 910 to 942 nm with narrow bandwidth. 相似文献
74.
Rainer Malisch Ralf Lippold Martin Metschies William A. Moats 《Fresenius' Journal of Analytical Chemistry》1991,341(7-8):449-456
Summary Centrifugal vacuum concentrators can be applied in residue analysis successfully: They prevent the samples from bumping and foaming. Even methanol/water- or acetonitrile/water-mixtures as extracts from food or other samples can be concentrated without separation of water prior to the evaporation process. Sulfonamides and other drugs can be concentrated with 100% recovery. Extracts with some light-sensitive substances (nitrofuranes) were evaporated without losses, as well as some hydrolysis-sensitive substances (penicillins) solved in water, whereas tetracyclines decompose in aqueous solution considerably. For more volatile substances (such as organochlorine pesticides or polychlorinated biphenyls) centrifugal vacuum concentrators can be used for preconcentration e.g. of the gelchromatographic eluate. Further, centrifugal vacuum concentrators can automate the evaporation process. In comparison to rotary evaporators, they save time for the laboratory staff when running series of 4 or more samples. It is very important to supply permanently sufficient energy for the evaporation process, especially when concentrating volumes of 10 ml or more. If the only heated part is the wall of the centrifuge, after couple of minutes the samples cool down to very low temperatures, and the evaporation process slows down. A more efficient way to supply evaporation energy is by means of IR radiation. A very modern device offers an electronically controlled IR radiation by means of temperature control on the centrifugal tubes. This in combination with vapour pressure control ensures an optimal control of the most important factors of the evaporation process. 相似文献
75.
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77.
Liquid crystalline side group polymers with azo-chromophores and fluorinated tails of varying length
Ralf Ruhmann Thomas Thiele Dietmar Wolff Dietrich Prescher Jü rgen Springer 《Liquid crystals》1996,21(3):307-312
A series of side group liquid crystalline polymethacrylates with 4'-(1H,1H-perfluoroalkoxy)-azobenzene mesogenic units was synthesized and characterized by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction methods depending on the tail length. The phase behaviour is discussed as a function of the length of the tail groups. Very high clearing temperatures up to 290°C were observed for the polymers with long tail groups. 相似文献
78.
79.
Roy Nilsson Ralf Nyholm Anders Berndtsson Jan Hedman Carl Nordling 《Journal of Electron Spectroscopy and Related Phenomena》1976,9(4):337-353
A new source for producing YMξ radiation in an ESCA spectrometer is described. The system makes use of continuous evaporation of yttrium on a rotating anode. It is demonstrated that this scheme allows YMξ-excited electron spectra to be recorded without gradual loss of resolution and intensity due to anode oxidation. Electron spectra of Ar, Hg and N2 excited by YMξ radiation are studied. Relative photoelectric cross-sections of the 5d, 5d and 6s orbitals in Hg and the four valence orbitals in N2 are measured. The valence electron shake-up spectrum of N2 is discussed. 相似文献
80.