Solvent and temperature gradients in separation of synthetic oxyethylene-oxypropylene block (co)polymers using high-temperature liquid chromatography

Chromatographic behavior of synthetic block (co)oligomer samples (EO)n(PO)m(EO)n and (PO)n(EO)m(PO)n with different distribution of propylene oxide (PO) and ethylene oxide (EO) monomer units was investigated on three types of stationary phases on zirconium dioxide support: Zr-PS (polystyrene), Zr-carbon, and Zr-carbon C18. The effects of the distribution and sequence of the oxyethylene and oxypropylene monomer units on the chromatographic retention depend on the type of the stationary phase, but are strongly affected by the organic modifier (methanol or ACN) in aqueous-organic mobile phase. Special attention was focused on the influence of the mobile-phase composition on the separation according to the EO and PO distribution. Zirconia-based columns are stable at elevated temperatures and can be used in high-temperature LC (HTLC); hence, we investigated the temperature effects on the chromatographic behavior up to 90 degrees C. The applications of solvent and temperature gradients were compared on the zirconia stationary phases in the RP mode.     

Determination of adrenolytic drugs by SPME–LC–MS

Five adrenolytic drugs have been analyzed by liquid chromatography–mass spectrometry (LC–MS). Samples were prepared by solid-phase microextraction (SPME) using polypyrrole fibers coated on stainless steel support as an adsorbent for the drugs. Adsorption efficiencies were 95% and were close for all the drugs investigated. Relative standard deviations (RSD), calculated for samples prepared in standard solutions, were in the range 2.5–13%, however RSD values for the drugs in human plasma were 2.5–4.5%. Using LC–MS the limit of detection (LOD) and the limit of quantification (LOQ) were in the ranges 0.11–0.18 and 0.39–0.54 ng mL⁻¹, respectively, for the five drugs.     

Single point imaging with suppressed sound pressure levels through gradient-shape adjustment

Acoustic noise produced during single point imaging (SPI) experiments was modulated by changes in the spatial encoding gradients. Parameters of both linear and sine-shaped gradient ramps were modified to minimize the acoustic noise levels. Acoustic noise measurements during SPI were measured on three different gradient systems and revealed that for small gradient-bore systems a considerable acoustic noise reduction of more than 20 dB can easily be achieved. SPI in conjunction with an optimized gradient waveform can be a superb alternative to the previously introduced single point ramped imaging with T(1) enhancement (SPRITE) method when sound levels and overheating of gradients are a concern.     

Effect of stationary phase structure on retention and selectivity tuning in the high-throughput separation of tocopherol isomers by HPLC

Four stationary phases containing different groups such as: C18, C30, alkylamide, and cholesterolic, were presented for simultaneous HPLC analysis of structural isomers of tocopherol. Especially, the influence of stationary phase structure and properties on tuning of the highly selective HPLC separation of beta- and gamma-tocopherol pair demonstrating, respectively, para- and ortho- arrangement of methyl substituents on the 6-chromanol ring, has been elucidated. It was pointed out that selectivity of each stationary phase has been a result of modulation in the mass transfer and set of unspecific interactions in the tertiary system comprising analyte <==> stationary phase <==> mobile phase. Differences in observed retention and specific selectivity of tocopherols together with the stationary phase structure investigations indicated that a spatial organization changing of chemically bonded ligands as predominantly a solvation consequence. Additional molecular modeling studies preliminary explained some of these complicated supramolecular phenomena which caused that cholesterolic stationary phase offered beneficial performance in screening of tocopherols by HPLC and biomimetic studies of not completely recognized interactions of tocopherol isomers and biological membranes.     

Frictional Phenomena in Dynamical Systemwith Two-Frequency Excitation

The paper deals with investigation of a self-excited vibrating system with dry friction. The system is composed of a mass connected by viscoelastic element with the referring frame and interacting with a moving belt by means of dry friction. An experimentally identified, multi-parametric dry friction model for the pair composed of soft and hard elements like steel–polyester pair, describing both the case of stick-slip and quasi-harmonic vibration, has been applied. Additionally, the system is influenced by external, two-frequency kinematic excitation. The results of computer simulation for different excitation conditions are submitted in the present paper.     

The strong decay to equilibrium for the stochastic dynamics of unbounded spin systems on a lattice

Using a method based on the application of hypercontractivity we prove the strong exponential decay to equilibrium for a stochastic dynamics of unbounded spin system on a lattice.     

Voltammetric studies of diffusional and migrational transport of ferrocene derivative of tripeptide glutathione

The transport of mono- and divalent anions of S-ferrocenylmethyl-L-glutathione (FcCH2SG) was investigated voltammetrically at a microelectrode in solutions of low ionic strength. The electrooxidation reactions of the ferrocenyl group attached to biologically active glutathione neutralized with a strong base in two consecutive steps can be represented as FcCH(2)SG(-) --> Fc(+)CH(2)SG(-) + e and FcCH(2)SG(2-) --> Fc(+)CH(2)SG(2-) + e for the mono- and divalent anions of FcCH(2)SG, respectively. The limiting currents due to these electrode processes were investigated under the conditions of varying content of supporting ions. The results obtained for the electrooxidation of the monovalent anion of FcCH(2)SG deviate significantly from the theoretical predictions derived for the charge cancellation electrode processes (i.e., processes producing uncharged species upon electron transfer). The differences observed are attributed to the specific migrational behavior of the generated dipole-like product which formally bears no net charge but in fact contains two oppositely charged moieties within a molecule. To interpret the data obtained for the divalent anion of FcCH(2)SG, a recent model of the theory of migrational voltammetry has been adapted and extended. The agreement between the experimental data and the theory is obtained only for the ratio of diffusion coefficients of the electrode process product (Fc(+)CH(2)SG(2-)) and the substrate (FcCH(2)SG(2-)) smaller than 1. This leads to a conclusion that upon oxidation this molecule undergoes a conformation change and winds up because of the formation of a coordination bond. The conclusion is supported by molecular-mechanics calculations. The presented methodology allows one to study quantitatively the changes in the concentration distribution of biologically active molecules driven by migration and diffusion and to diagnose their possible structural changes upon reduction/oxidation. Both factors are essential for the proper understanding of the functionality of biologically active systems.