Selectivity control in the separation of aromatic amino acid enantiomers with sulphated beta-cyclodextrin

Control of selectivity in the enantiomeric separation of three aromatic amino acids (phenylalanine, tyrosine and tryptophan) is demonstrated by electrokinetic capillary chromatography utilising temperature variations coupled with the use of sulphated-beta-cyclodextrin (s-beta-CD) as a pseudostationary phase. The concentration of s-beta-CD and temperature were used as experimental variables to control the observed selectivity. A double-coated capillary was used and proved very robust with reproducibility of migration times being <2.0% R.S.D. between runs and <2.6% on using a new capillary. The system was modelled successfully using an artificial neural network (ANN) comprising one input layer, two hidden layers and one output layer. The model accurately described the observed separations with a correlation coefficient of 0.999 being observed between predicted and observed migration times. Selectivity optimisation was achieved using the normalised resolution product and minimum resolution criteria, with both providing optima at different experimental conditions. The selectivity changes observed also allowed the estimation of electrolyte temperatures within the capillary at high operating currents (>100 microA). Using a 50 microm i.d. capillary and an electrolyte comprising 20 mM phosphate and 15 mM s-beta-CD, a temperature of 52 degrees C was calculated within the capillary at an applied voltage of +30 kV.     

Mixed-mode electrokinetic chromatography of aromatic bases with two pseudo-stationary phases and pH control

The electrokinetic chromatographic (EKC) separation of a series of aromatic bases was achieved utilising an electrolyte system comprising an anionic soluble polymer (polyvinylsulfonic acid, PVS) and a neutral beta-cyclodextrin (beta-CD) as pseudo-stationary phases. The separation mechanism was based on a combination of electrophoresis, ion-exchange interactions with PVS, and hydrophobic interactions with beta-CD. The extent of each chromatographic interaction was independently variable, allowing for control of the separation selectivity of the system. The ion-exchange and the hydrophobic interactions could be varied by changing the PVS and the beta-CD concentrations, respectively. Additionally, mobilities of the bases could be controlled by varying pH, due to their large range of pKa values. The separation system was very robust with reproducibility of migration times being <2% RSD. The two-dimensional parameter space defined by the two variables, [beta-CD] and %PVS, was modelled using a physical model derived from first principles. This model gave very good correlation between predicted and observed mobilities (r2=0.999) for the 13 aromatic bases and parameters derived from the model agreed with the expected ion-exchange and hydrophobic character of each analyte. The complexity of the mathematical model was increased to include pH and this three-dimensional system was modelled successfully using an artificial neural network (ANN). Optimisation of both the two-dimensional and three-dimensional systems was achieved using the normalised resolution product and minimum resolution criteria. An example of using the ANN to predict conditions needed to obtain a separation with a desired migration order between two of the analytes is also shown.     

Use of reversed-phase high-performance liquid chromatographic analysis for the determination of provitamin A carotenes in tomatoes

The usual methods for provitamin A evaluation of foods convert the total pigment amount, determined spectrophotometrically, into vitamin A units. Since the totally inactive lycopene is the major carotenoid in the tomato, such readings result in erroneously high provitamin A values. In view of the recent development of chemically bonded, reversed-phase, microparticulate packings and their use in high-performance liquid chromatography which combines highly accurate and reproducible resolution with the speed and ease of operation, a new method using such a system was developed to isolate carotenoid pigments from tomato samples. A 15-min column separation was thus achieved, dramatically decreasing the analysis time of the classical open column chromatographic procedures, which often result in unresolved and altered fractions due to long-term exposure to oxygen, light, solvents and sometimes adsorbent. beta-Carotene and lycopene were determined and quantitated in six tomato samples. beta-Carotene, 100% vitamin A-active, was expressed in International Units of vitamin A. The newly developed method gives a more reliable evaluation of the fruit potency in vitamin A than the methods of the Association of Official Analytical Chemists currently used for food composition tables.     

Crystal structure of triethylentetraammonium bis monohydrogenmonophosphate dihydrate, [NH3(CH2)2NH2(CH2)2NH2(CH2)2NH3](HPO4)2 · 2H2O

The salt triethylentetraammonium bis monohydrogenmonophosphate dihydrate is monoclinic with the following unit cell dimensions: a = 8.462(1), b = 10.500(1), c = 9.520 Å, = 99.297(1)°, space group P2₁/n with Z = 2. The structure was solved by the Patterson method and refined to final R value of 0.043 for 1590 independent reflections. The structure consists of infinite parallel two-dimensional [ 01] planes built of mutually connected ions and water molecules by strong O—H···O and N—H···O hydrogen bonding.     

Adducts of hexa­methyl­ene­tetr­amine with ferrocene­carboxyl­ic acid and ferrocene‐1,1′‐di­carboxyl­ic acid: multiple disorder in space groups Fmm2 and Cmcm

Hexa­methyl­ene­tetr­amine, C₆H₁₂N₄, and ferrocene­carboxyl­ic acid, C₁₁H₁₀FeO₂, form a 1:2 adduct, (I), which is a salt, viz. hexa­methyl­ene­tetramin­ium(2+) bis(ferro­cene­car­box­yl­ate), (C₆H₁₄N₄)[Fe(C₅H₅)(C₆H₄O₂)]₂. The dication in (I) is disordered with two orientations at a site of mm2 symmetry in space group Fmm2, while the anion lies across a mirror plane with its unsubstituted cyclo­penta­dienyl ring disordered over two sets of sites. With ferrocene‐1,1′‐di­carboxyl­ic acid, C₁₂H₁₀FeO₄, hexa­methyl­enetetr­amine forms a 1:1 adduct, (II), in which both components are neutral, viz. hexa­methyl­ene­tetramine–ferro­cene‐1,1′‐di­car­box­ylic acid (1/1), [Fe(C₆H₅O₂)₂]·C₆H₁₂N₄. The amine component in (II) is disordered with two orientations at a site of mm2 symmetry in space group Cmcm, while the acid component is disordered with two orientations at a site of 2/m symmetry. The components in (I) are linked into a finite three‐ion aggregate by a single N—H⃛O hydrogen bond, while the components of (II) are linked into continuous chains by a single O—H⃛N hydrogen bond.     

Sodium 2‐nitro­ethanaloximate forms a layered‐framework structure

In the title compound, [Na(C₂H₃N₂O₃)], the Na⁺ cation lies on a centre of inversion in space group P2₁/m and all the atoms of the anion lie on a mirror plane. Na is octahedrally coordinated by four O and two N atoms from six different anions and each anion is coordinated to six different Na⁺ cations, forming chains of confacial octahedra which link the anion layers. Within these layers, the individual anions are linked by both O—H?O and C—H?O hydrogen bonds.     

Accuracy of the Adomian decomposition method applied to the Lorenz system

In this paper, the Adomian decomposition method (ADM) is applied to the famous Lorenz system. The ADM yields an analytical solution in terms of a rapidly convergent infinite power series with easily computable terms. Comparisons between the decomposition solutions and the fourth-order Runge–Kutta (RK4) numerical solutions are made for various time steps. In particular we look at the accuracy of the ADM as the Lorenz system changes from a non-chaotic system to a chaotic one.     

Ionic conductivity studies of chitosan-based polymer electrolytes doped with adipic acid

Chitosan acetate–adipic acid film polymer electrolytes have been prepared by the solution cast technique. The highest conductivity is 1.4 × 10⁻⁹ S cm⁻¹ for 35 wt.% of adipic acid at room temperature. The sample with highest conductivity has the lowest activation energy. Calculations using the Rice and Roth model provide number of mobile ions, η. The conductivity is dependent on the diffusion coefficient and mobility.