A new approach to the determination of relative retention in thin-layer liquid chromatography

The relative retention values in chromatography depend on the experimental conditions to a lesser degree than the absolute values. Unfortunately, according to IUPAC recommendations, the relative retention values in planar and column chromatography are different. Therefore, a new equation is proposed to determine relative retention in TLC and a new method and new value are introduced to determine it experimentally. The equation is identical in principle to that used in column chromatography. Using the proposed equation, the relative retention value can be determined with a smaller error.     

Scintillation phase method: A new approach for studying surfactant behavior at liquid/liquid interface

Radiochemical approach for the investigation of partitioning and adsorption of surfactants at water-oil interfaces has been developed. The so-called “scintillation phase method” is based on the introduction of tritium labeled surfactant in an aqueous phase of the water-oil system followed by measuring radioactivity of the whole system. Partition coefficients and the value of adsorption at water-non-polar liquid interfaces of homologous series of alkyltrimethylammonium bromides (DTAB, TTAB and CTAB) have been experimentally determined by means of scintillation phase method. The influence of non-ionic surfactants on TTAB adsorption has also been studied.     

A new approach to prostanoids

A new asymmetric approach to 11-deoxyprostanoids is presented.     

A new approach to isochromanes

Some isochromane derivatives exhibit hypotensive,¹ antitumor,² and growth-regulating³ activities; some others have a specific effect on the dopaminergic system.⁴     

A new approach to the determination of column overload characteristics in reversed-phase liquid chromatography

Column overloading is very common during the separations of basic analytes in analytical scale reversed-phase liquid chromatography (RPLC). Due to the complex interactions of ionic analytes with stationary and mobile phases, only a very small amount of ionized sample compared to the amount of nonpolar solute can be injected before the peak shape is distorted by non-linear chromatographic processes. Often the amount that can be injected before overload is observed is so small that the signal is quite noisy, thereby making the measured plate count imprecise and possibly inaccurate. The purpose of the present study was to develop a practical method for the precise measurement of the plate count and a column overload parameter using a simple but mathematically rigorous model of Langmuirian non-linear chromatography. An “overload profile”, i.e. a plot of apparent plate count versus amount injected, is characterized by two parameters: the limiting plate count (N₀) and the column sample loading capacity (ω_0.5). The limiting plate count is the plate count that should be observed when the amount of sample injected is so small that a linear isotherm pertains. The column sample loading capacity, which is taken as the sample load that leads to a plate count equal to half of the limiting plate count, is a measure of the maximum amount of sample that can be injected into that column. The approach was tested by applying it to the study of cationic analytes in RPLC. We show that N₀ under constant conditions (column length, flow rate, mobile phase composition, etc.) is almost independent of column type (manufacturer); however, different column types (at the same length, diameter and flow rate) exhibit clear differences in their sample loading capacity (ω_0.5). We believe that for most well packed type B columns, the column sample loading capacity and not the limiting plate count is the more important property that accounts for most of the apparent differences in peak width when different types of columns are examined.     

A facile and specific approach to new liquid chromatography adsorbents obtained by ionic self-assembly

A new ionic-liquid monomer, 1-vinyl-3-octadecylimidazolium bromide ([C(18)VIm]Br), was prepared and polymerized on porous silica particles by means of a surface-initiated radical chain-transfer reaction. Further modification for functionalization was performed through the exchange of counteranions from bromide to methyl orange (MO). Two new silica-poly(octadecylimidazolium) (Sil-PImC(18)) hybrid materials (Sil-PImC(18)-Br and Sil-PImC(18)-MO) were synthesized and characterized by elemental analysis, thermogravimetric analysis, diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), and solid-state (13)C CP/MAS NMR spectroscopy. Sil-PImC(18)-MO presented ultra-high shape selectivity for constrained isomers of polycyclic aromatic hydrocarbons (PAHs) both in reversed- and normal-phase HPLC when used as the stationary phase. Fundamental aspects of the molecular shape selectivity were evaluated by using Standard Reference Material (SRM) 869b; the column selectivity test mixture for liquid chromatography. The impact of this phase was also demonstrated by the separation of SRM 1647e (16 priority pollutant PAHs) and several steroid isomers. Enhanced selectivity could be explained by the highly oriented arrangement between the octadecylimidazolium chain and a rigid segment of MO. These findings may open a new window of research for the design of materials used in chromatographic supports, solid extraction, catalysis, and electrolytes by simple modifications of the counterions in the poly(ionic liquid) analogous phase.     

Dimethylsubstituted pyrroles: A new approach to protective polymers

The development of sacrificial or protective films based on dimethylpyrrole analogues is described. The design considerations required for successful electropolymerisation of the monomer and the subsequent, selective dissolution of the film is described. A general synthetic strategy is presented for the formation of a diverse range of bifunctional monomers which, when polymerised, can be subsequently removed after exposure to hydroxylamine under neutral aqueous conditions at room temperature. Film formation and removal has been studied using cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) studies.     

RigFit: A new approach to superimposing ligand molecules

If structural knowledge of a receptor under consideration is lacking, drug design approaches focus on similarity or dissimilarity analysis of putative ligands. In this context the mutual ligand superposition is of utmost importance. Methods that are rapid enough to facilitate interactive usage, that allow to process sets of conformers and that enable database screening are of special interest here. The ability to superpose molecular fragments instead of entire molecules has proven to be helpful too. The RigFit approach meets these requirements and has several additional advantages. In three distinct test applications, we evaluated how closely we can approximate the observed relative orientation for a set of known crystal structures, we employed RigFit as a fragment placement procedure, and we performed a fragment-based database screening. The run time of RigFit can be traded off against its accuracy. To be competitive in accuracy with another state-of-the-art alignment tool, with which we compare our method explicitly, computing times of about 6s per superposition on a common day workstation are required. If longer run times can be afforded the accuracy increases significantly. RigFit is part of the flexible superposition software FlexS which can be accessed on the WWW [http://cartan.gmd.de/FlexS].     

Cyclopiperazines: A new approach to chiral macrocyclic receptors

The synthesis and preliminary substrate binding properties of a series of piperazine-containing macrocycles are described.     

Subcritical water chromatography: A green approach to high-temperature liquid chromatography

At temperatures and pressures lower than 374 degrees C and 218 atm, subcritical water has widely tunable properties such as dielectric constant, surface tension, viscosity, and dissociation constant achieved by simply adjusting the temperature with a moderate pressure to keep water in the liquid state. At elevated temperatures, water acts like a weak polar organic solvent. Thus, subcritical water has been used as a green eluent to replace hazardous solvents commonly used as organic modifiers in RPLC. Subcritical water chromatography (SBWC) is capable of separating polar, moderately polar, and even some nonpolar analytes. Most of these low molecular weight solutes are stable at elevated temperatures during a chromatographic run. Some new packing materials are also quite stable and robust at mild temperatures ranging from 80 to 150 degrees C. Advantages of SBWC include the elimination of hazardous organic solvents used in traditional RPLC, rapid analysis time, improved selectivity, temperature-dependent separation efficiency, temperature-programmed elution, and compatibility with both gas- and liquid-phase detectors. In this paper, the technical aspects as well as the applications of SBWC are reviewed. Topics addressed in this review include the unique characteristics of subcritical water, analytes separated by SBWC, packing materials tested for SBWC, the application of GC and LC detection techniques in SBWC, SBWC instrumentation development, temperature effects on SBWC separation, and models developed for separation in SBWC.     

A new synthetic approach to phosphatidylethanolamine

A new synthetic method for phosphatidylethanolamine head group was developed via ring-opening of cyclic dioxaphospholane 2 with sodium azide and subsequent hydrogenation.The advantage of this strategy included short reaction steps,readily available materials and good yields.     

A new approach to (+)-anisomycin

An efficient approach to enantiomerically pure (+)-deacetylanisomycin 2a and a formal synthesis of (+)-anisomycin 2 (11% overall yield in 10 steps) have been achieved through simple and good yielding reactions, starting from 1,2:3,4:5,6-tri-O-isopropylidene-d-mannitol 3. Grignard reaction and intramolecular cyclisation reactions are key steps in the strategy.     

A new approach to sulfenylated enolates

Bissulfenylation followed by monodesulfenylation with a Grignard reagent cleanly provides magnesium enolates of α-phenylthiocarbonyl compounds for aldol condensation.     

A new approach to canonical orthonormalisation

The canonical orthornormalisation procedure is derived through an integral equation with a finite rank kernel. Its optimal properties in connection with the problem of approximate linear dependence are established.     

A new catalytic approach to germasiloxanes

A new ruthenium(0) complex catalyzed coupling reaction of silanols with vinylgermanes leading to SiO-Ge bond formation with the evolution of ethylene is described. Under optimum conditions the catalytic O-germylation of silanols gives exclusively germasiloxanes.     

A new synthetic approach to benzomorphans

A new approach to the synthesis of benzomorphan derivatives is exemplified by the obtention of 3,6-dimethyl-benzomorphan from the readily available 1-methyl-2,3-benzobicyclo-[3,2,l]octane-4,6-dione.     

A new approach to local hardness

The applicability of the local hardness as defined by the derivative of the chemical potential with respect to the electron density is undermined by an essential ambiguity arising from this definition. Further, the local quantity defined in this way does not integrate to the (global) hardness-in contrast with the local softness, which integrates to the softness. It has also been shown recently that with the conventional formulae, the largest values of local hardness do not necessarily correspond to the hardest regions of a molecule. Here, in an attempt to fix these drawbacks, we propose a new approach to define and evaluate the local hardness. We define a local chemical potential, utilizing the fact that the chemical potential emerges as the additive constant term in the number-conserving functional derivative of the energy density functional. Then, differentiation of this local chemical potential with respect to the number of electrons leads to a local hardness that integrates to the hardness, and possesses a favourable property; namely, within any given electron system, it is in a local inverse relation with the Fukui function, which is known to be a proper indicator of local softness in the case of soft systems. Numerical tests for a few selected molecules and a detailed analysis, comparing the new definition of local hardness with the previous ones, show promising results.