Is it worthwhile to go beyond the local-density approximation in subsystem density functional theory?

Frozen density embedding (FDE) theory is one of the major techniques aiming to bring modeling of extended chemical systems into the realm of high accuracy calculations. To improve its accuracy it is of interest to develop kinetic energy density functional approximations specifically for FDE applications. In the study reported here we focused on optimizing parameters of a generalized gradient approximation-like kinetic energy functional with the purpose of better describing electron excitation energies. We found that our optimized parametrizations, named excPBE and excPBE-3 (as these are derived from a Perdew-Burke-Ernzerhof-like parametrization), could not yield improvements over available functionals when applied on a test set of systems designed to probe solvatochromic shifts. Moreover, as several different functionals yielded very similar errors to the simple local-density approximation (LDA), it is questionable whether it is worthwhile to go beyond the LDA in this context.     

The effect of chromatographic conditions on the separation of selected alkaloids on CN-silica layers

ABSTRACT

Selected alkaloids were chromatographed on cyanopropyl-silica thin layers using nonaqueous and aqueous eluents containing various free silanol blocker agents. Because of the strong retention of these basic compounds, nonaqueous eluents containing medium polar diluents, strongly polar modifiers, or aqueous eluents containing organic modifier, water, and silanol blockers (ammonia, diethylamine, and ionic liquid) were required for analysis. Mobile phases containing addition of acids were also tested for separation of investigated alkaloids. The most selective and efficient systems were used in two-dimensional separations of isoquinoline alkaloids’ mixture and plant extracts.     

Analytical QbD-based systematic bioanalytical HPLC method development for estimation of quercetin dihydrate

The current work entails development of rapid, sensitive, and inexpensive high-performance liquid chromatographic method of quercetin dihydrate using the quality by design approach. Quality target method profile was defined and critical analytical attributes (CAAs) were earmarked. Chromatographic separation was accomplished on a C₁₈ column using acetonitrile and ammonium acetate buffer (35:65) %v/v (containing 0.1% acetic acid, pH 3.5) as mobile phase at 0.7?mL/min flow rate with UV detector at 237?nm. Screening studies using fractional factorial design revealed that organic modifier, injection volume, column temperature, and buffer strength have significant influence on method CAAs, namely, peak area, retention time, and peak tailing. The critical method parameters were systematically optimized using Box–Behnken design. Response surface mapping was used along with numerical optimization and desirability function for identifying the optimal chromatographic conditions. Linearity was observed in the drug concentration ranging between 2 and 50?µg/mL. Accuracy analysis revealed mean % recovery between 93.6 and 96.2%, while precision study revealed mean % recovery between 93.7 and 96.5%. Limits of detection and quantification of the developed method were found to be 12.1 and 36.6?ng/mL. Overall, the studies construed successful development of chromatographic method of quercetin with enhanced method performance.     

Optimization of an electrolytic conductivity detector for determination of toxic nitrogen-containing food contaminants separated by open tubular column gas chromatography

The combination of open tubular column gas chromatography with electrolytic conductivity detection has been evaluated for the determination of nitrogen-containing pesticide residues in food extracts. Optimization of the column position at the column-detector interface was crucial to the successful operation of the detector. The signal-to-noise ratio and response stability of the detector are greatly influenced by the composition of the electrolyte solvent. Large volume splitless injections using retention gaps and optimized detector operating conditions enabled pesticide residues in food extracts to be determined at sub parts-per-million levels. Although the electrolytic conductivity detector is less sensitive than the thermionic ionization detector, its greater nitrogen selectivity can he crucial to the determination of nitrogen-containing contaminants in food extracts, particularly in complex mixtures where phosphorus-containing contaminants or matrix compounds are also present.     

Optimization of sampling frequency for routine river water quality monitoring

Sampling frequency is an important factor to be considered during the design of a water monitoring network,and the cost-effective selection of possible ways and means for the optimization of sampling frequency is still needed.This paper introduces water pollution index deviation ratio comparison(WPI DRC),a procedure for the optimization of sampling frequency for a routine river water quality monitoring system.Sampling frequency optimized using WPI DRC at monitoring station X5 in the mainstream of Xiangjiang River is compared with that established using the traditional Statistical Algorithm method.The result of comparison indicates that WPI DRC is more feasible than the traditional one.And then,the sampling frequencies for other 16 monitoring stations also have been optimized,and the results show the sampling frequencies of all the stations except that X4 are reduced,and there is no unacceptable difference between water quality evaluation results at 17 stations before and after the optimization.Therefore,it is concluded that WPI DRC is an effective optimization process with operable results,which can be used to fulfill the requirement of practical monitoring work.     

Statistical mixture design selective extraction of compounds with antioxidant activity and total polyphenol content from Trichilia catigua

Statistical design mixtures of water, methanol, acetone and ethanol were used to extract material from Trichilia catigua (Meliaceae) barks to study the effects of different solvents and their mixtures on its yield, total polyphenol content and antioxidant activity. The experimental results and their response surface models showed that quaternary mixtures with approximately equal proportions of all four solvents provided the highest yields, total polyphenol contents and antioxidant activities of the crude extracts followed by ternary design mixtures. Principal component and hierarchical clustering analysis of the HPLC–DAD spectra of the chromatographic peaks of 1:1:1:1 water–methanol–acetone–ethanol mixture extracts indicate the presence of cinchonains, gallic acid derivatives, natural polyphenols, flavanoids, catechins, and epicatechins.     

Simultaneous determination of methyl- and ethyl-mercury by solid-phase microextraction followed by gas chromatography atomic fluorescence detection

A method for trace level determination of organomercury species in different biota matrixes by using aqueous-phase propylation followed by headspace solid-phase microextraction (HS-SPME) and gas chromatography (GC) coupled to pyrolysis-atomic fluorescence spectrometry (Py-AFS) detection has been optimized. To maximize peak area and symmetry factors of methylmercury (MeHg) and ethylmercury (EtHg) analyzed as propyl derivatives, carrier and make-up flow rates were optimized by a user-defined experimental design. A multiple response simultaneous optimization was applied using the desirability function to achieve global optimal operating conditions. They were attained at 2 and 6 mL min⁻¹ as carrier and make-up gas flow rates, respectively. In addition, pyrolyser temperature was also optimized, yielding the best value at 750 °C. Limits of detection and quantification at the optimum conditions were 0.04 ng g⁻¹ and 0.13 ng g⁻¹ for both, MeHg and EtHg. The developed analytical procedure was validated with a certified reference material (DORM-2) and applied to the determination of organomercury incurred in waterfowl egg and fish samples.