Capacity factors and peak widths in the gradient elution reversed phase separation of high molar mass polystyrenes

Summary A simplex method for determining the constantsS andk _o from the equation lnk’=lnk _o−S ϕ (was developed and applied to the reversed phase separation of high molar mass polystyrenes using gradients of any curvature. Experimental retention times described using the equation had a standard deviation of 1.1%. The inclusion of a quadratic term in the equation was found to be unwarranted. BothS and lnk _o varied linearly with ln molar mass. The logarithm of peak width of an eluting polystyrene peak was found to be a linear function of the mobile phase composition at elution. The slope was equal toS.     

Quasielastic light scattering study of chemically crosslinked gelatin solutions and gels

Quasielastic light scattering measurements are reported for experiments performed on mixtures of gelatin and glutaraldehyde (GA) in the aqueous phase, where the gelatin concentration was fixed at 5 (w/v) and the GA concentration was varied from 1×10⁻⁵ to 1×10⁻³ (w/v). The dynamic structure factor, S(q,t), was deduced from the measured intensity autocorrelation function, g ₂(τ), with appropriate allowance for heterodyning detection in the gel phase. The S(q,t) data could be fitted to S(q,t)=Aexp(−D _f q ² t)+Bexp(−t/τ_c)^β, both in the sol (50 and 60 ^∘C) and gel states (25 and 40 ^∘C). The fast-mode diffusion coefficient, D _f showed almost negligible dependence on the concentration of the crosslinker GA; however, the resultant mesh size, ξ, of the crosslinked network exhibited strong temperature dependence, ξ∼(0.5−χ)^1/5exp(−A/RT) implying shrinkage of the network as the gel phase was approached. The slow-mode relaxation was characterized by the stretched exponential factor exp(−t/τ_c)^β. β was found to be independent of GA concentration but strongly dependent on the temperature as β=β₀+β₁ T+β₂ T ². The slow-mode relaxation time, τ_c, exhibited a maximum GA concentration dependence in the gel phase and at a given temperature we found τ_c(c)=τ₀+τ₁ c+τ₂ c ². Our results agree with the predictions of the Zimm model in the gel case but differ significantly for the sol state. Received: 25 May 1999 /Accepted in revised form: 27 July 1999     

Dielectric permittivity and relaxation of aqueous cesium iodide solutions

The high-frequency dielectric permittivity and losses of CsI solutions were studied at 288–323 K in the range of water dielectric permittivity dispersion (7–25 GHz). The low-frequency electrical conductivity of these solutions was measured, and ionic losses at high frequencies were calculated. The Debye or Cole-Cole relaxation model was used for describing the spectra. The low-frequency limits of these relaxation region were calculated, which are the static dielectric constants ɛ_S and well as dielectric relaxation times (τ) and activation enthalpies (ΔH _ɛ⁺⁺). The ɛ_S values decrease in going from water to a solution. In concentrated solutions, the slope of the plot of ɛ_S versus temperature become zero. The decrease in τ and gDH _ɛ⁺⁺ is evidence of the structure-breaking effect of ions on water. At elevated temperatures (313 K), the decrease in τ is minimal. At 323 K, τ slightly increases in going from water to a solution.     

Structure of water-glycine solutions in saturated and near-saturated regions according to compressibility data

Ultrasonic velocity and density values are measured for aqueous solutions containing 2.00 mol.%, 4.00 mol.%, and 5.00 mol.% glycine in a temperature range of 15–65°C, 5.50 mol.% glycine (20–65°C), and 6.00 mol.% glycine (25–65°C). Adiabatic compressibilities (κ_S) and molar adiabatic compressibilities (K_S) are calculated. The values of κ_S and K_S decrease monotonically with an increase in glycine concentrations up to saturation at all the temperatures. The temperature dependences of κ_S and κ_S have minima that are typical of water and aqueous solutions; the positions of the minima depend on the glycine concentration. The temperature coefficients of the molar compressibility, ∂K_S/∂T, change their signs from negative to positive at lower temperatures (by approximately 10 deg) than ∂κ_S/∂T.     

Melting behavior of poly(tetrahydrofuran)-S and their blends

Melting behavior of poly(tetrahydrofuran)-s (PTHF) and their blend with different molecular masses has been studied by TM-DSC. PTHF and their blend show two endothermic peaks on their curve. The melting peak temperatures T _m1 and T _m2, entropy of fusion ΔS _f1 and ΔS _f2, and mean relaxation time for melting τ_f1 and τ_f2 have been estimated, and their dependence on the molecular mass has been examined. Plots of Tm1 to the reciprocal of their molecular mass fit a simple equation (T _m=a-b/M _n). Plots of T _m2 to their molecular mass also fit the equation with different factors. There seems to be a boundary around molecular mass 1200 in the molecular mass dependence of ΔS _fand τ_f. Effect of blending appeared on the τ_f and the non-reversing heat flow. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental Densities, Speeds of Sound, Isentropic Compressibilties and Shear Relaxation Times of CaSO4·2H2O + CaCl2 + H2O and CaSO4·2H2O + NaCl + H2O Systems at Temperatures 30 and 35 °C

Speeds of sound, u, have been measured as a function of concentration for the systems, CaSO₄·2H₂O +CaCl₂ + H₂O and CaSO₄·2H₂O + NaCl + H₂O, at temperatures of 30 and 35 °C. Derived parameters such as the isentropic compressibility, κ _S , and the shear relaxation time,τ,were calculated using the experimental speed of sound data in combination with viscosity values from our earlier work. Results have been compared with those of the CaCl₂+ H₂Oand NaCl + H₂O systems reported in literature,to examine the effect of adding CaSO₄·2H₂O(s).Values of κ _S for the system,CaSO₄·2H₂O + CaCl₂+H₂O,are smaller compared with those for the CaCl₂+ H₂O system.Values of τ are lower at lower concentrations and then cross over in a narrow concentration region.Values of κ _S for the system,CaSO₄·2H₂O+NaCl+H₂O, are also smaller when compared with those forthe NaCl +H₂O system.For this system the τ values are higher. These τ values reach a minimum at a certain concentration of NaCl in the solution and then increase with further increases in concentration.The influenceof solvent-separated and/or solvent-shared ion pairs plays a dominant role at higher concentrations for both systems.Results have been interpreted and discussed in terms of the expansion and contraction of the primary hydration shell of the ionic species present in the studied systems.     

Temperature dependence of the free energy of sorption ofn-alkanes and energy contribution of their methylene groups on columns with fullerene C60

The partial molar free energy, enthalpy, and entropy of sorption of C₁₁−C₂₃ n-alkanes were calculated on the basis of the GC data obtained on the glass capillary column coated with fullerene C₆₀ (Ful-60) as stationary phase. The thermodynamic parameters ofn-alkane sorption on a column with Ful-60 and a fused silica capillary column with polydimethylsiloxane OV-1 were determined and compared. The enthalpy-entropy compensation effect for the sorption ofn-alkanes on Ful-60 and OV-1 was found. A linear dependence of the partial molar free energy ofn-alkane sorption on the temperature of analysis and carbon chain length was found. The free energy contributions of the methylene groups were calculated, and their temperature dependences were studied. The differences in the temperature dependences of the energy contributions of methylene groups ofn-alkanes on Ful-60 and OV-1 were revealed. The entropy contribution is 68–82% of the enthalpy contribution which indicates a substantial role of the number of contacts with Ful-60 in retention ofn-alkanes. The ability of Ful-60 for dispersive interactions is similar to those of nonpolar liquid phases and substantially differs from that for carbon adsorbents. Fullerene columns were shown to be convenient for analysis of highly boiling organic substances in aqueous and organic solutions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1490–1495, August, 1999.     

Molar heat capacity and thermodynamic properties of 1,2-cyclohexane dicarboxylic anhydride [C8H10O3]

The molar heat capacity C _p,m of 1,2-cyclohexane dicarboxylic anhydride was measured in the temperature range from T=80 to 390 K with a small sample automated adiabatic calorimeter. The melting point T _m, the molar enthalpy Δ_fus H _m and the entropy Δ_fus S _m of fusion for the compound were determined to be 303.80 K, 14.71 kJ mol⁻¹ and 48.43 J K⁻¹ mol⁻¹, respectively. The thermodynamic functions [H _T-H _273.15] and [S _T-S _273.15] were derived in the temperature range from T=80 to 385 K with temperature interval of 5 K. The thermal stability of the compound was investigated by differential scanning calorimeter (DSC) and thermogravimetry (TG), when the process of the mass-loss was due to the evaporation, instead of its thermal decomposition.     

Thermal field-flow fractionation of colloidal materials: Methylmethacrylate-styrene linear di-block copolymers

Summary Model methylmethacrylate-styrene, linear di-block, copolymers were used to investigate the respective influnnces of temperature, of molar mass and of chemical composition on their Soret coefficient, s_T, by means of thermal field-flow fractionation (thermal FFF) in toluene and in THF. A recently developed thermal FFF retention model, which takes into account the variation of the basic FFF parameter λ with temperature, is applied to investigate the dependence of the Soret coefficient on temperature. It is found that the coefficient decreases approximately linearly with increasing temperature. At constant chemical composition and temperature, s_T exhibits a power law dependence on molar mass with an exponent é ? 0.55. At constant molar mass and temperature, s_T decreases monotonously with increasing weight percent styrene in the copolymer composition. At 300 K, s_T values are slightly larger in THF than in toluene. Presented at the 21^st ISC held in Stuttgart, Germany, 15^th–20^th September, 1996.     

Molar heat capacities and standard molar enthalpy of formation of 2-amino-5-methylpyridine

The heat capacities (C _p,m) of 2-amino-5-methylpyridine (AMP) were measured by a precision automated adiabatic calorimeter over the temperature range from 80 to 398 K. A solid-liquid phase transition was found in the range from 336 to 351 K with the peak heat capacity at 350.426 K. The melting temperature (T _m), the molar enthalpy (Δ_fus H _m⁰), and the molar entropy (Δ_fus S _m⁰) of fusion were determined to be 350.431±0.018 K, 18.108 kJ mol⁻¹ and 51.676 J K⁻¹ mol⁻¹, respectively. The mole fraction purity of the sample used was determined to be 0.99734 through the Van’t Hoff equation. The thermodynamic functions (H _T-H _298.15 and S _T-S _298.15) were calculated. The molar energy of combustion and the standard molar enthalpy of combustion were determined, ΔU _c(C₆H₈N₂,cr)= −3500.15±1.51 kJ mol⁻¹ and Δ_c H _m⁰ (C₆H₈N₂,cr)= −3502.64±1.51 kJ mol⁻¹, by means of a precision oxygen-bomb combustion calorimeter at T=298.15 K. The standard molar enthalpy of formation of the crystalline compound was derived, Δ_r H _m⁰ (C₆H₈N₂,cr)= −1.74±0.57 kJ mol⁻¹.     

Thermodynamic studies of (R)-BINOL-menthyl dicarbonates

The (R)-BINOL-menthyl dicarbonates, one of the most important compounds in catalytic asymmetric synthesis, was synthesized by a convenient method. The molar heat capacities C _p,m of the compound were measured over the temperature range from 80 to 378 K with a small sample automated adiabatic calorimeter. Thermodynamic functions [H _T–H _298.15] and [S _T–S _298.15] were derived in the above temperature range with a temperature interval of 5 K. The thermal stability of the substance was investigated by differential scanning calorimeter (DSC) and a thermogravimetric (TG) technique.     

Molecular dynamics and phase transitions in phospholipid monolayers at liquid-liquid interfaces

Stablen-hexadecane/water andn-tetradecane/water macroemulsions containing monolayers of natural (egg yolk lecithin, EY) and synthetic (dimyristoylphosphatidylcholine, DMPC) phospholipids at liquid-liquid interfaces were prepared. The existence of the monolayers was proved by studying the reduction kinetics of a surface-active spin probe with ascorbate anions. Spin labeled derivatives of stearic acid in which the nitroxide group is locared at different distances from the polar head (5-, 12-, and 16-doxylstearic acids) were used to study the temperature dependences of the molecular ordering, rotational mobility, and local polarity in the monolayers in emulsions and also in bilayers in liposomes obtained from the same lipids. In the EY monolayers, the degree of spin probe solubilization is higher, while the order parameters (S) and rotational correlation times (τ) are lower than those in EY bilayers. The differences between these parameters for mono- and bilayers increase with an increase in the distance of the reporter group from the aqueous phase. In the DMPC monolayers, a first-order phase transition was detected by measuring the temperature dependences ofS and τ. The temperature region of the phase transition in monolayers is shifted to lower temperatures with respect to that for bilayers and depends on the nature of the oil phase. It was concluded that the phospholipid monolayers in emulsions incorporate hydrocarbon molecules, whose concentration in the DMPC monolayers increases on going from the low-temperature (gel) to the high-temperature (liquid crystal) phase. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 418–425, March, 1998.     

Heat Capacities and Thermodynamic Properties of a H2O + Li2B4O7 Solution in the Temperature Range from 80 to 356 K

The molar heat capacities of an aqueous Li₂B₄O₇ solution were measured with a precision automated adiabatic calorimeter in the temperature range from 80 to 356 K at a concentration of 0.3492 mol⋅kg⁻¹. The occurrence of a phase transition was determined based on the changes in the curve of the heat capacity with temperature. A phase transition was observed at 271.72 K corresponding to the solid-liquid phase transition; the enthalpy and entropy of the phase transition were evaluated to be Δ H _m = 4.110 kJ⋅mol⁻¹ and Δ S _m = 15.13 J⋅K⁻¹⋅mol⁻¹, respectively. Using polynomial equations and thermodynamic relationship, the thermodynamic functions [H _T −H _298.15] and [S _T −S _298.15] of the aqueous Li₂B₄O₇ solution relative to 298.15 K were calculated in temperature range 80 to 355 K at intervals of 5 K. Values of the relative apparent molar heat capacities of the aqueous Li₂B₄O₇ solution, C _p,φ, were calculated at every 5 K in temperature range from 80 to 355 K from the experimental heat capacities of the solution and the heat capacities of pure water.     

Separation of L-DOPA and four metabolites in plasma using a porous graphitic carbon column in capillary liquid chromatography

Summary A sensitive HPLC method with marbofloxacin (MAR) as internal standard and fluorescence detection is described for the analysis of ofloxacin (OFL) enantiomers in plasma samples. Plasma samples were prepared by adding phosphate buffer (pH 7.4, 0.1m), then extracted with trichloromethane.S-OFL,R-OFL, and the internal standard were separated on a reversed-phase column with water-methanol, 85.5∶14.5, as mobile phase. The concentrations ofS-OFL andR-OFL eluting from the column (retention times 7.5 and 8.7 min, respectively) were monitored by fluorescence detection withλ _ex = 331 andλ _em = 488 nm. The detection and quantitation limits were 10 and 20 ng mL⁻¹, respectively, forS-OFL and 11 and 21 ng mL⁻¹ forR-OFL. Response was linearly related to concentration in the range 10 to 2500 ng mL⁻¹. Recovery was close to 93% for both compounds. The method was applied to determination of the enantiomers of OFL in plasma samples collected during pharmacokinetic studies.     

Multiplex ELISA in a single microfluidic channel

In this article, we demonstrate a novel approach to implementing multiplex enzyme-linked immunosorbent assay (ELISA) in a single microfluidic channel by exploiting the slow diffusion of the soluble enzyme reaction product across the different assay segments. The functionality of the reported device is realized by creating an array of ELISA regions within a straight conduit that are selectively patterned with chosen antibodies/antigens via a flow-based method. The different analytes are then captured in their respective assay segments by incubating a 5-μL aliquot of sample in the analysis channel for an hour under flow conditions. Once the ELISA surfaces have been prepared and the enzyme substrate introduced into the analysis channel, it is observed that the concentration of the soluble enzyme reaction product (resorufin) at the center of each assay region grows linearly with time. Further, the rate of resorufin generation at these locations is found to be proportional to the concentration of the analyte being assayed in that segment provided that the ELISA reaction time in the system (τ _R ) is kept much shorter than that required by the resorufin molecules to diffuse across an assay segment (τ _D ). Under the operating condition τ _R  << τ _D , the reported device has been shown to have a 35% lower limit of detection for the target analyte concentration compared with that on a commercial microtiter plate using only a twentieth of the sample volume.     

Measuring the ignition delay time for hydrogen-oxygen mixtures behind the front of an incident shock wave

The delay time τ has been measured for the formation of the ^·OH radical in igniting hydrogenoxygen mixtures diluted with argon (79–97%). The experiments have been carried out under incident shock wave conditions at temperatures of 900–3000 K, pressures of 0.5–2.5 atm, and H₂/O₂ ratios of 0.2–20. The dependence of τ on the pressure P _s of the stoichiometric part of the combustible mixture (2H₂-O₂) has been investigated for different mixture compositions. Under the above conditions, τ depends practically linearly on 1/P _s at P _s = 0.02−0.1 atm, irrespective of the mixture composition. This allows the measured τ data to be converted to one quantity, τP _s. The temperature dependence of τP _s in the P _s range from 0.02 to 0.1 atm is Arrhenius-like. For the hydrogen-rich mixtures (H₂/O₂ = 2–20), this dependence appears as τP _s= 0.057 + 0.0256exp(7470/T) μs atm; for the lean mixtures (H₂/O₂ = 0.125–1), τP _s = 0.021 + 0.0069exp(7470/T) μs atm. The length of the shock-heated gas plug in the incident shock wave poses limitations on the ignition delay time measurements at T < 900 K.     

Determination of the molecular weight and other physicochemical properties of poly(bubylene glycol) 1000 by gradient RPHPLC and measurement of the ELSD, UV, and fluorescence responses

Summary Molecular weight and other physicochemical data for poly(butylene glycol) 1000 (PBG 1000) have been determined by use of a new, highly efficient, gradient reversed-phase high-performance liquid-chromatographic (RPHPLC) procedure. Separation of the native material or its α, ω-bis(1-naphthylurethane) derivative was achieved on a C₁₈ stationary phase with a ternary acetonitrile-water-tetrahydrofuran mobile phase. Detection was achieved by measurement of the signal response from evaporative light scattering (ELSD), UV, and fluorescence (FD) detection. Proof that all the oligomers contained in the sample had been separated by the method was obtained by liquid chromatography coupled with electrospray ionization time-of-flight mass spectrometry (LC-ESI-TOFMS). It was also confirmed by this technique that di(butylene glycol) is the lowest homologue in the sample. Although the dimer was also observable in the HPLC-UV trace of the PBG 1000 α, ω-bis(1-naphthylurethane) derivative, it was obviously too valatile to be seen in the ELSD trace; tri(butylene glycol) was, nevertheless, still recognizable with sufficient signal intensity. Because all the homologues were separated to baseline, the method was used to calculate the number- and weight-average molecular weights,M _n andM _w, both from peak areas and from peak heights. The best fit to data obtained from end-group titration were obtained from calculations based on the HPLC-UV response;M _n values of 948 and 937 were obtained from peak heights and areas, respectively.M _n andM _w values calculated from the ELSD trace obtained from native PBG 1000 were substantially (ca 10%) lower that those calculated from the UV trace obtained from the α, ω-bis(1-naphthylurethane) derivative. Similar differences were also discovered by comparing theM _n andM _w values obtained from UV and FD-values were approximately 20% higher for FD. When the retention times of individual oligomers, from either ELSD of the native sample or from UV-FD of the α,ω-bis(1-naphthylurethane) derivative, were plotted against the number of repeat units,P, the ELSD curve approaches the UV-FD curve at values ofP of approximately 60. This observation can be explained by the pronounced contribution of the hydrophobic end-groups to the overall retention of PBG 1000; the effect of this decreases with increasing chain length and becomes nearly independent ofP only for a very high degree of polymerization. Papers dedicated to Professor Dr. Heinz Engelhardt on the Occasion of his 65th birthday     

An equation for the calculation of retention indices in temperature-programmed gas chromatography with allowance for the nonlinear variation of the retention parameters ofn-alkanes

In view of the nonlinear variation of the temperature increments ofn-alkanes found previously, the accuracy of the calculations of the retention indices (I _pr) of substances in temperature-programmed capillary gas chromatography carried out in terms of six known equations was verified. A new four-parameter equation was proposed, and a general method for the calculation of its coefficients, suitable for all stationary phases, based on the adjusted retention times ofn-alkanes was suggested. The coefficients of the equation for 12 temperature variation programs were determined. Using the homologous series of methyl esters of fatty acids as an example, it was shown that the proposed equation ensures the minimum error of determination ofI _pr under various conditions. The equation also makes it possible to carry out interpolation and extrapolation calculations. The coefficients of the equation are found using the least-squares method based on data for any 4–5 referencen-alkanes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 323–327, February, 1997.     

Dielectric relaxation of sulfolane in benzene solution from microwave absorption data

The electric constant (ɛ′) and dielectric loss (ɛ″) for dilute solutions of sulfolane in benzene solution has been measured at 9.885 GHz at different temperatures (25, 30, 35, and 40°C) by using standard microwave techniques. Following the single frequency concentration variational method, the dielectric relaxation time (τ) and dipole moment (μ) have been calculated. It is found that dielectric relaxation process can be treated as the rate process, just like the viscous flow. Based on the above studies, monomer structure of sulfolane in benzene has been inferred. The presence of solute-solvent associations in benzene solution has been proposed. Energy parameters (ΔH _ɛ, ΔF _ɛ, ΔS _ɛ) for dielectric relaxation process of sulfolane in benzene at 25, 30, 35, and 40°C have been calculated and compared with the corresponding energy parameters (ΔH _η, ΔF _η, ΔS _η) for the viscous flow.