The gravimetric method is the most popular method for preparing reference gas mixtures with high accuracy. We have designed and manufactured novel mass measurement equipment for gravimetric preparation of reference gas mixtures. This equipment consists of an electronic mass-comparator with a maximum capacity of 15 kg and readability of 1 mg and an automatic cylinder exchanger. The structure of this equipment is simpler and the cost is much lower than a conventional mechanical knife-edge type large balance used for gravimetric preparation of primary gas mixtures in Japan. This cylinder exchanger can mount two cylinders alternatively on the weighing pan of the comparator. In this study, the performance of the equipment has been evaluated. At first, the linearity and repeatability of the mass measurement were evaluated using standard mass pieces. Then, binary gas mixtures of propane and nitrogen were prepared and compared with those prepared with the conventional knife-edge type balance. The comparison resulted in good consistency at the compatibility criterion described in ISO6143:2001.
Cholesterol and cholesteryl esters in human serum were determined by supercritical fluid chromatography on an inert ODS-silica gel column using supercritical carbon dioxide as a mobile phase without a modifier. Chromatograms were obtained by monitoring the eluent simultaneously with an FID and UV detector at the wavelength of 190 nm. The retention behavior of cholesterol and cholesteryl esters was investigated in terms of the density of CO2 mobile phase. The separation mode was found to be reversed phase, as in liquid chromatography. The amounts of cholesterol and cholesteryl esters extracted from human serum reference material (NIST SRM 909) were determined individually using cholesteryl laurate as an internal standard to give good agreement of total cholesterol with the value certified by NIST.
A method for endcapping of octadecylsilyl (ODS)-silica gels was developed using supercritical carbon dioxide as a silylation medium. The effects of temperature and pressure of carbon dioxide on the deactivation of the gels were investigated under the silylation conditions at 100 -220˚C and 11.8 -24.5 MPa. High-performance liquid chromatographic evaluation using caffeine and phenol as test compounds indicated that the most inert gels were obtained from the endcapping under 180˚C and 24.5 MPa. Endcapped ODS-silica gels, which were commercially available, could be further deactivated under these conditions. The inertness of the gels was also evaluated by supercritical fluid chromatography (SFC) using pyridine and phenol as test compounds. The effectiveness of the proposed method was clearly observed in the SFC evaluation.
A method of supercritical fluid extraction (SFE) of triazine herbicides from soil has been developed. The recoveries of triazine herbicides increased with increase in both extraction pressure and temperature when supercritical carbon dioxide was used as the extraction medium alone. Because the recovery by SFE was influenced by the concentration of triazine herbicides in the soil matrix, the addition of methanol to carbon dioxide as a polar modifier was required for the satisfactory extraction of trace levels of the content of the herbicides. The moisture content of the soil also affected the recoveries by SFE. SFE was applied to the analysis of real samples. The results by SFE were almost the same as those from soxhlet extraction.
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