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101.
Despite the high nucleophilicity of sulfur atoms, thiacalixarenes have been alkylated only on oxygen atoms thus far. Using strong alkylating agents (triflates, trialkyloxonium salts), the substitution of the sulfur bridges has been successfully accomplished. The corresponding sulfonium salts of thiacalix[4]arene are formed regio- and stereoselectively as a completely new type of substitution pattern in thiacalixarene chemistry. These compounds possess interesting conformational behavior and could be used as unusual alkylating agents with uncommon selectivity. 相似文献
102.
The method is based on catalysed ignition of a sample portion in a flow of oxygen, capture of mercury by an amalgamator and
measurement of the mercury vapour's absorbance after thermic release from the amalgamator. Three powdered food samples, a
certified reference material (CRM) human hair and a reference material (RM) urine (liquid) were measured in the first range
of the instrument (the possible contents determined according to our measurement procedure were 0.0003–0.5 ppm). The calibration
function used was a line passing through the origin. The combined standard uncertainties of the mercury determinations were
computed from uncertainty components of five quantities: absorbance of the sample, absorbance of the sample blank, slope of
the calibration line, correction factor of the abscissa axis, and mass or volume of the sample. The most important uncertainty
component is the uncertainty of the sample absorbance measurement which amounts to 52% of the determination uncertainty at
the minimum (RM urine) and about 90% at the maximum (in our laboratory homogenized powdered food samples; analysis of variance
showed their homogeneity to be insufficient). The results of the CRM and RM analyses do not indicate a significant systematic
error for this determination. The relative expanded uncertainty (coefficient was 2) of the determination increases from 9
to 13% for the insufficiently homogenized samples with decreasing mercury content (range of 0.004–0.03 ppm); higher homogeneity
of samples results in a decrease of the expanded uncertainty, e.g. 4.6% for the liquid sample (RM urine).
Received: 26 April 1999 / Accepted: 30 August 1999 相似文献