The coupling of supercritical fluid chromatography and field ionization time-of-flight high-resolution mass spectrometry for rapid and quantitative analysis of petroleum middle distillates

We report the first coupling of supercritical fluid chromatography (SFC) with field ionization time-of-flight high-resolution mass spectrometry (FI-ToF HRMS), in parallel with ultraviolet (UV) detection and flame ionization detection (FID), for rapid and quantitative analysis of petroleum middle distillates. SFC separates petroleum middle distillates into saturates and 1- to 3-ring aromatics. FI generates molecular ions for hydrocarbon species eluted from the SFC. The high resolution and exact mass measurements by ToF mass spectrometry provide elemental compositions of the molecules in the petroleum product. The amounts of saturates and aromatic ring types were quantified using the parallel SFC-FID assisted by SFC-UV. With a proper carbon-number calibration, the detailed composition of the petroleum middle distillate was rapidly determined.     

Determination of low levels of methanol in crude oils by multi-dimensional gas chromatography (MDGC) using novel micro channel flow technology

The accurate and precise determination of methanol in crude oils at concentrations less than 10 ppm is of economic value to the petroleum industry. This report presents the optimization, results and long term performance of a flow switching device MDGC hardware, the Swafer?, for the rapid and precise analysis of methanol from approximately 0.4 ppm (w/w) to 1000 ppm. The use of low temperature injection and backflush technique decreases maintenance and increases sample throughput. The short term quantitative percent relative standard deviations at 1, 30 and 1000 ppm (w/w) methanol in crude oils are 5, 3 and 0.3, respectively. The MDGC procedure follows closely that described in ASTM D7059 for determination of crude oils; however, D7059 previously has not been evaluated at concentrations of less than 10 ppm. This work further extends the application of D7059 to concentrations of <10 ppm.     

Determination of total biodiesel fatty acid methyl, ethyl esters, and hydrocarbon types in diesel fuels by supercritical fluid chromatography-flame ionization detection

A method for determining total biodiesel methyl and ethyl ester content in diesel fuels by supercritical fluid chromatography-flame ionization detection (SFC-FID) is developed. A silica column typically used for determining aromatics in conventional diesel fuels by ASTM D5186 is back-flushed after separation of the hydrocarbons to allow elution of the various esters as a single "total biodiesel" distinct peak. The modification concurrently allows the determination of total aromatic hydrocarbons and their distribution as mono- and polynuclear compounds, as described in the current version of D5186. The instrument response is linear from 1.0% to 50% biodiesel esters with a signal-to-noise ratio of 25 at the 0.1% level. The short-term relative standard is 0.8%. Normalized percent quantitation using a hydrocarbon response factor of 1.00 and an ester response factor of 1.19 provide an average percentage error of 1.8% when measuring actual biodiesel/hydrocarbon fuel blends. The ester response factor is the average of the response factors of 10 pure ester compounds. These responses are calculated from respective solutions of each ester and the four compounds, hexadecane, toluene, tetralin, and naphthalene, as used for the D5186 response factor mixture.     

Ga掺杂ZnO的电子结构与电性能的研究

采用密度泛函理论广义梯度近似第一性原理计算的方法研究了n型Ga掺杂的纤锌矿结构氧化物ZnO的晶格结构、能带结构和态密度,在此基础上分析了其电性能.计算结果表明,掺杂ZnO氧化物晶格a,b轴增大,c轴略有减小;Ga掺杂ZnO氧化物两能带之间具有0.6eV的直接带隙,需要载流子(电子)跃迁的能隙宽度较未掺杂的ZnO氧化物减小;掺杂体系费米能级附近的态密度大大提高,其能带主要由Gas态、Zns态和Os态电子构成,且他们之间存在着强相互作用,其中Gas态电子对导带贡献最大.电输运性能分析结果表明,Ga掺杂ZnO氧化物导电机构由Znp-Op电子在价带与导带的跃迁转变为Gas-Znd-Os电子在价带与导带的跃迁,这也表明Gas态电子在导电过程中的重要作用;掺杂体系费米能级附近的载流子有效质量较未掺杂体系增大,且价带中的载流子有效质量较大,导带中的载流子有效质量较小.     

The mononuclear metal center of type-I dihydroorotase from aquifex aeolicus

Background

Dihydroorotase (DHO) is a zinc metalloenzyme, although the number of active site zinc ions has been controversial. E. coli DHO was initially thought to have a mononuclear metal center, but the subsequent X-ray structure clearly showed two zinc ions, α and β, at the catalytic site. Aquifex aeolicus DHO, is a dodecamer comprised of six DHO and six aspartate transcarbamoylase (ATC) subunits. The isolated DHO monomer, which lacks catalytic activity, has an intact α-site and conserved β-site ligands, but the geometry of the second metal binding site is completely disrupted. However, the putative β-site is restored when the complex with ATC is formed and DHO activity is regained. Nevertheless, the X-ray structure of the complex revealed a single zinc ion at the active site. The structure of DHO from the pathogenic organism, S. aureus showed that it also has a single active site metal ion.

Results

Zinc analysis showed that the enzyme has one zinc/DHO subunit and the addition of excess metal ion did not stimulate catalytic activity, nor alter the kinetic parameters. The metal free apoenzyme was inactive, but the full activity was restored upon the addition of one equivalent of Zn²⁺ or Co²⁺. Moreover, deletion of the β-site by replacing the His180 and His232 with alanine had no effect on catalysis in the presence or absence of excess zinc. The 2.2 Å structure of the double mutant confirmed that the β-site was eliminated but that the active site remained otherwise intact.

Conclusions

Thus, kinetically competent A. aeolicus DHO has a mononuclear metal center. In contrast, elimination of the putative second metal binding site in amidohydrolyases with a binuclear metal center, resulted in the abolition of catalytic activity. The number of active site metal ions may be a consideration in the design of inhibitors that selectively target either the mononuclear or binuclear enzymes.