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51.
A solid-phase microextraction (SPME) and gas chromatography-mass spectrometry method for determining polycyclic aromatic hydrocarbons (PAHs) in environmental solid matrices is developed. Investigated matrices include seaweed (Undaria pinnatifida and Himanthalia elongata), humic substances (isolated from a wetland out-flow and purchased from Aldrich), and soil. Optimal conditions for a good SPME efficiency of 16 hydrocarbon compounds are obtained using a 100- micro m polydimethylsiloxane fiber directly immersed in aqueous carrier medium. The method is remarkable for presenting short extraction times and considerably high sensitivities. The SPME results obtained by using internal calibration give the total analyte concentration based on the identical partitioning behavior of native and spiked pollutants. The detection limits range from 0.001 to 0.1 mg of PAH per kilogram of dry matrix. SPME external calibration provides information regarding freely dissolved analytes. The detection limits range from 0.001 to 0.05 micro g of PAH per liter of carrier medium. The SPME with external calibration procedure can be applied to measure free concentrations of a target compound spiked into a carrier medium and onto a matrix. Based on a comparison of results obtained for the two samples, the partitioning of the target analyte between the matrix and the carrier medium is calculated. 相似文献
52.
M. J. Guiomar H. M. Lito M. Filomena G. F. C. Camões Arthur K. Covington 《Journal of solution chemistry》1998,27(10):925-933
A new computer program has been developed for the calculation of pH, pOH, hydroxide ion concentration m
OH, species distribution coefficients i, ionic activity coefficients i ionic strength I, buffer capacity , solubility product K
s0, and the two dissociation constants, K
b1 and K
b2, corresponding, respectively, to first and second dissociation steps of Ca(OH)2 in aqueous solution. Previously developed methodology, for the calculation of pH, i, i
I, and parameters of pH buffer solutions, starting from the corresponding acidity constants, has been adapted for the case of aqueous Ca(OH)2 solutions, for which the pertinent stoichiometric relationships are different from those applicable to mixtures of acids and their salts. The results show that, contrary to what is currently assumed, the first dissociation is far from complete. Values are given for the concentrations and activities of species Ca(OH)2(aq), Ca(OH)+(aq), and Ca2+ (aq) in saturated calcium hydroxide solutions at 25°C. 相似文献
53.
In order to enhance the UV detection sensitivity, an application study of an on-line preconcentration technique for micellar electrokinetic chromatographic (MEKC) was carried out. The simultaneous determination of four test ecdysteroids, 20-hydroxyecdysone, ajugasterone C, polypodine B and ponasterone A has been investigated by using the normal stacking mode in MEKC with UV detection. The effects of anionic surfactant composition and concentration, the applied voltage, the pH buffer, the kind and the amount of organic solvent and the injection time on the analyte resolution were evaluated. The optimised conditions for the separation involved the use of a 50 mM borate as the running buffer containing 50 mM of a mixture of sodium dodecyl sulphate (SDS) and sodium cholate (SC) in the ratio of 1:1 together with a concentration of 10% (v/v) of 2-PrOH at pH 9.0. Hydrodynamic injection of 12 s at 50 mbar and separation voltage of 20 kV at temperature of 20 degrees C were employed. These conditions allowed a repeatability separation within 21 min. Concentration detection limit for the neutral analytes studied improve about an order of magnitude. The method was also applied to the determination of ecdysteroids in a real sample. 相似文献
54.
55.
This paper describes the design and performance of various millimeter-wave printed-circuit filters and diplexers. These components include microstrip/suspended stripline and finline lowpass filters, capacitive-gap and inductive-iris-coupled bandpass filters, spurline bandstop filters, and finline diplexers. State-of-the-art performances from Ka-band (26.5 to 40 GHz) to D-band (110 to 170 GHz) have been obtained. Agreement between the measured and calculated results was also achieved. 相似文献
56.
57.
Potentiometric cells from which the pH values of standard buffer solutions are computed rely on the silver–silver chloride
reference electrode which has proved to be convenient, reproducible and reliable.
Of the various methods of preparation of silver-silver chloride electrodes only one concerns us here: the thermal–electrolytic
type that has been used more extensively than any other form. Once prepared, the electrodes need to be equilibrated before
use and between experiments. The equilibration technique must ensure voltage stability and inter-electrode, or bias, potential
below 0.1 mV. In potentiometry the stability of a reference electrode is of utmost importance since an offset of 1 mV is equivalent
to a deviation of about 0.02 in the pH value. 相似文献
58.
59.
Differential scanning calorimetry (DSC) has been scarcely used in conservation field and in particular, in the study of painting
layers. However DSC is a micro-destructive technique (sample from .05 to 1 mg can be analysed) because no pre-treatment of
the sample is required and so it can give rapid and useful information. The aim of the present work is, in fact, to describe
useful applications of DSC on the study of materials used in art. The analyses have been conducted under oxygen flow in order
to register the thermo-oxidative reactions of the organic binder or protective. In the first part of the paper, materials
of different chemical composition have been analysed to build up a data base of DSC profiles. Standard painting layers artificially
degraded by treatment with ultraviolet light (365 nm) for 21 days and with NOx (5 ppm)have be en analysed. Comparing the behaviour of the treated to the reference sample, it is useful to study what kind
of changes the treatments have produced on the sample. Another possible application of DSC is the study of the formation of
the film; analysing the raw,liquid binders and the different naturally aged films an evolution of the chemical structure canbe
registered.
As a conclusion, DSC analyses on standard used in artistic field allow to investigate their thermal behaviour and to understand
the effect of natural and artificial ageing.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
60.
In this work, an experimental study of spray impact onto a horizontal flat and rigid surface is presented. The phase Doppler
technique has been used to characterize both the impacting and the secondary spray in terms of mass and number flux, size
distribution and velocities of the droplets above the target. A high-resolution CCD camera has been used to measure the average
liquid film thickness formed due to spray impact, whereas a high-speed CMOS camera has been used to characterize the splashing
droplets from the wall. This visualization of the splashing phenomenon and the knowledge about the liquid film thickness are
used to formulate a new physical model of the crown evolution. Furthermore, information about the incident-to-ejected mass
fraction and number fraction are novel contributions of this study. Considerable data are provided comparing the impact of
single drops onto a liquid film to impact of drops in a spray, and the significance of the observed differences for modelling
efforts is discussed. The measurements of this study are also shown to be rather sensitive to the placement of the phase Doppler
measurement volume above the surface and to the operating parameters of the instrument. These effects have been documented
and discussed for this particular measurement situation. 相似文献