Temperature-dependent separation of bryostatin 18 and 10 by high performance liquid chromatography

Summary The temperature-dependent separation of bryostatins by HPLC was examined on an octadecyl bonded stationary phase, using column temperatures between 0 and 40°C and mobile phase temperatures from 0 to 25°C. The retention time and resolution of bryostatins changed drastically and separation improved with decreasing temperature. A column temperature of less than 5°C and a mobile phase temperature of less than 15°C is recommended for a good resolution of bryostatins for routine work.     

Effect of Temperature and Mobile Phase Composition on RP-HPLC Separation of Cephalosporins

Abstract

The effects of mobile phase composition and column temperature on resolution in reversed-phase high performance liquid chromatography (RP-HPLC) were used to separate the following cephalosporins: cefonicid, cefaclor, cephazolin, cefodizime, cephaloridine, cephamandole and cephalotin, from a single sample. The capacity factor (k') was described as a function of temperature and mobile phase composition. Semi-empirically estimated values of k' were determined using a small number of experimental data for different temperatures between 20°C and 60°C, and mobile phase compositions (acetate buffer/isopropanol). The capacity factor of each cephalosporin was observed to decrease with increasing temperature and the volume fraction (V) of isopropanol in the mobile phase; linear relationships were obtained for plots of In k' versus 1/T and log k' versus log V. The method developed, while simple, reveals the optimal isocratic elution conditions for column temperature and eluent concentration for the complete separation and rapid analysis of the cephalosporins studied.     

Regulatory Aspects in Development of Stability-Indicating Methods: A Review

Poly(N-vinylcaprolactam) (PVCL) connected to aminopropyl silica is a new stationary phase for temperature responsive liquid chromatography (TR-LC). PVCL shows a transition from hydrophilic to hydrophobic interaction between 30 and 40 °C. The synthesis is described in detail. The temperature responsive characteristic of the phase is illustrated with a mixture of steroids using pure water as mobile phase. An increase in retention is observed when raising the temperature. H–u plots at different temperatures were constructed. Below the lower critical solution temperature (LCST), no optimal velocity could be measured because of substantial resistance to mass transfer. Above the LCST, u _opt was ca. 0.3 mm s^?1 with reduced plate heights from 4 at 45 °C to 3 at 65 °C. The temperature responsive nature of the polymer is lost in green chromatography with ethanol as modifier in concentrations above 5%.     

Temperature effects in Microcolumn Size Exclusion Chromatography

Abstract

Effects of column temperature on the column efficiency, retention time and stability of analytes were studied in microcolumn size-exclusion chromatography. Larger theoretical plates were achieved at column temperatures betwen 70 and 100 °C. In the constant-flow mode the retention time of analytes decreased with increasing column temperature, which was due mainly to thermal expansion of the mobile phase. When the column temperature was around or higher than the critical temperature of the mobile phase, the retention times of the analytes observed under supercritical pressure conditions still indicated dominancy of the size-exclusion mechanism, while another retention mechanism was involved under subcritical pressure conditions. In the case of the analysis of saccharides, the column temperature should be lower than 100 °C because oligosaccharides were decomposed at higher temperatures.     

Phase transition temperatures of Sn–Zn–Al system and their comparison with calculated phase diagrams

The Sn?CZn?CAl system was studied in connection with the possible substitution of lead-based solders for temperatures up to 350?°C. Ternary alloys with up to 3?wt% of aluminium were prepared. The investigated alloys lie close to the monovariant line (eutectic valley) of the Sn?CZn?CAl system. The temperatures of phase transitions of six binary Sn?CZn reference alloys and fourteen ternary Sn?CZn?CAl alloys using DTA method were investigated in this paper. DTA experiments were performed at the heating/cooling rate of?4?°C?min^?1 using Setaram SETSYS 18_TM experimental equipment. The temperatures of phase transitions in the ternary Sn?CZn?CAl system were obtained, namely, the temperature of ternary eutectic reaction T _E1 (197.7?±?0.7?°C), temperature of ternary transition reaction T _U1 (278.6?±?0.7?°C), temperatures of liquidus and other transition temperatures for studied alloys. Temperatures obtained during DTA heating runs were used as authoritative. DTA curves obtained during cooling enabled realising better differentiation of the obtained overlapped heat effects (peaks) during heating. Theoretical isopleths of the Sn?CZn?CAl phase diagram were calculated using the Thermocalc software and MP0602 thermodynamic database. Experimental data were compared with the calculated temperatures, and a good agreement was obtained.     

Liquid chromatographic separation and thermodynamic investigation of stereoisomers of darunavir on Chiralpak AD‐Hcolumn

Liquid chromatographic separation of stereoisomers of darunavir on Chiralpak AD‐H, a column containing the stationary phase coated with amylose tris(3,5‐dimethylphenylcarbamate) as a chiral selector, was studied under normal‐phase conditions at different temperatures between 20 and 50°C. The effect of quality and quantity of different polar organic modifiers viz: methanol, ethanol, 1‐propanol, and 2‐propanol in the mobile phase as well as column temperature on retention, separation, and resolution was investigated and optimized. The optimum separation was accomplished using a mobile phase composed of n‐hexane/ethanol/diethyl amine (80:20:0.1 v/v/v) at 40°C. Apparent thermodynamic parameters ΔH⁰ and ΔS* were derived from the Van't Hoff plots (lnk′ versus 1/T) and used to explain the strength of interactions between the stereoisomers and amylose tris(3,5‐dimethylphenylcarbamate) coated chiral stationary phase.     

Ligand-exchange chromatography of amino acid enantiomers and its application to the biotransformation of DL-aspartic acid byPseudomonas dacunhae

Summary The separation of the D and L enantiomers of eighteen essential α amino acids has been investigated by ligand-exchange chromatography (LEC). The effect of column temperature on the retention times and resolution of individual amino acid enantiomers has been studied by varying the temperature from 25 to 50 °C for a mobile phase containing Cu²⁺ ions. By use of a temperature of 50 °C and Zn²⁺ in the mobile phase, eight of the eighteen amino acid enantiomers can be resolved sufficiently well for practical application. Only phenylalamine, tyrosine, and tryptophan can be separated by use of Ni²⁺ as complexation metal at 50 °C. LEC has been used to monitor the decarboxylation of racemic DL-aspartic acid byPseudomonas dacunhae. Analysis of DL amino acid enantiomers in different media was performed at column temperatures of 30 and 50°C by addition of 0.125 mM Cu²⁺ to the aqueous mobile phase. It was found that the analytical performance is most dependent on the identity of the metal used for complexation; the concentration of the metal was of secondary importance and the column temperature less important still.     

Improved Separation of Polyols and Carbohydrates by High Performance Liquid Chromatography

Abstract

A reliable High Performance Liquid Chromatographic analysis of glucose-fructose-sucrose-sorbitol and mannitol is developed. A Sugar Pak I column at 85°C is used employing water as the mobile phase. The separation is completed within 20 minutes, and the resolution is very acceptable.     

The Study of Separation and Thermodynamics of Adsorption of the Enantiomers of Ethofumasate on a Modified Cellulose

Abstract

Cellulose and cellulose derivatives are biopolymers that are often used as stationary phases for the separation of enantiomers. Describing the mechanism of such separations is a difficult task due to the complexity of these phases. In the present study, direct enantiomeric resolution of ethofumesate has been achieved, using hexane as the mobile phase with various alcoholic modifiers on cellulose tri(3,5‐dimethylphenylcarbamate) chiral stationary phase (CDMPC CSP). The influence of the mobile phase composition and the column temperature on the chiral separation was studied. It was found that at a constant temperature and within a certain range of alcohol modifier concentration, the conformation of the polymeric phase, and the selective adsorption sites were not affected by alcohol modifier concentration. The type and the concentration of the alcoholic modifiers influenced the retention factor and the separation factor. Ethofumesate gained the best enantioseparation using sec‐butanol as alcoholic modifier at 25°C with α‐value 1.70. And the separation factor decreased with the increase of the column temperature. The van't Hoff plots were linear (R ²>0.96) for ethofumesate from 25°C to 50°C. That showed the enantioselective interactions do not change over the temperature range studied. Furthermore the values of ΔH° and ΔS° were both negative, which indicated an enthalpy‐driven separation. And the possible chiral recognition mechanism of the analyte and CDMPC was discussed. It was found that hydrogen bonding plays an important role on enantioseparation of CDMPC CSP. The inclusion and fitness of solute shape in the chiral cavity significantly contributed to the enantioseparation of solute.     

Determination of Steroids in Human Urine by Micellar Liquid Chromatography

Abstract

A simple and sensitive method for the determination of steroids using micellar liquid chromatography is described. The steroids, including hydroxycorticosterone. corticosterone, northisterone, testosterone, mexdroprogesterone acetate and progesterone, were separated by reversed-phase using a micelles mobile phase following UV detection at 245 nm. The parameters affecting retention of the test solutes such as the concentration of sodium dodecyl sulfate (SDS) and n-butanol-1 in the mobile phase were investigated. It was found that the retention of the solutes was dependent on the composition of mobile phase. The linear calibration plots range from 0.1 to 10 μg ml^?1 in mobile phase containing 5.0 × 10^?2 mol l^?1 SDS/9 % n-butanol-1 at pH 6.0, and the detection limit in order of 0.1 μg ml^?1 was obtained. The proposed method was used for the determination of steroids in urine using direct injection of samples without previous treatment.     

Electrodeposition of silver from dense gaseous solutions of silver nitrate in ammonia

Abstract

Silver was electrodeposited on platinum electrodes from solutions of AgNO₃ in NH₃ at temperatures from ?70°C. to 140°C. The solutions were 0.036 molar in AgNO₃ at ? 78°C. In the dense gaseous region, the concentrations of AgNO₃ and NH₃ were 0.018 and 22 moles/liter respectively.

The deposit was observed to become more and more crystalline in appearance as the temperature approached the critical temperature of NH₃(133°C). All electrodeposits at temperatures above 133°C were characterized by either dendritic or needle-like growths on sharp edges and corners. These were not observed at lower temperatures. Both three- and four-sided symmetries were observed, suggesting that the deposits were growing in the [111] and [100] directions, respectively.     

In-situ observation of the nitrogenation of Sm2Fe17 by means of high temperature x-ray diffraction

The mechanism of nitrogenation of Sm₂Fe₁₇ was investigated with in-situ high temperature x-ray diffraction with Debye-Scherrer optic using a quartz capillary equipped with an additional buffer volume to supply sufficient nitrogen for the expected phase reaction. The formation of Sm₂Fe₁₇N_x with x ≈ 3 was observed at temperatures between 325?°C and 450?°C. During the phase transition period, taking 30–90 min at 450?°C and up to 300 min at 350?°C, two phases were observed simultaneously, a non-nitrided Sm₂Fe₁₇ phase and the already fully nitrided Sm₂Fe₁₇N₃ phase. At 325?°C no single-phase material could be obtained during 300 min. The resulting lattice parameters measured after the complete nitrogenation are the same as those, obtained from the externally nitrogenized samples.     

Thermal stability and dynamic mechanical thermal analysis of epoxy thermosets possessing N,N′-disubstituted pyromellitimide units

Abstract

In this work, three epoxy resins including diglycidyl ethers of N,N′-bis(2-hydroxyethyl)pyromellitimide (DIDGE), bisphenol-A (BADGE), and polyethylene glycol (PEDGE) were isothermally cured by an amine curing agent possessing N,N′-disubstituted pyromellitimide units (denoted by DIDAM). DIDGE resin was synthesized from the reaction of N,N′-bis(2-hydroxyethyl)pyromellitimide with an excess of epichlorohydrin. Also, DIDAM curing agent was prepared from the reaction of pyromellitic dianhydride with an excess of ethylene diamine. Completion of the isothermal curing processes was approved by both Fourier transform-infrared spectroscopy and non-isothermal differential scanning calorimetry (DSC). The DSC traces showed only the phase transitions related to the thermal degradation of the resulting thermosets. According to the thermogravimetric analyses, the DIDGE/DIDAM thermoset showed higher thermal stability at temperatures above 425?°C than the other two thermosets. While BADGE/DIDAM and PEDGE/DIDAM thermosets showed about 70% weight loss in the thermal range of 400–850?°C, DIDGE/DIDAM thermoset was encountered with only about 40% weight loss. The glass transition temperatures (T_g ) of the resulting thermosets were determined using tan δ vs temperature plots obtained from dynamic mechanical thermal analysis. The T_g values of BADGE/DIDAM, DIDGE/DIDAM, and PEDGE/DIDAM thermosets were found to be 211?°C, 189?°C, and 81?°C, respectively.     

Influence of Bonded-Phase Column Type,Mobile Phase Composition,Temperature and Flow-Rate in the Analysis of Triglycerides by Reverse-Phase High Performance Liquid Chromatography

Abstract

Two chromatographic systems for RP-HPLC analysis of triglycerides, operating under isocratic conditions using octadecylsilane and octylisilane bonded phases, are described.

The influence of such chromatographic factors as bonded phase column type, mobile phase composition, temperature and flow rate on retention, analysis selectivity and efficiency, and separation of mixtures of homogeneous triglycerides was assessed. Linear relationships were established for the logarithm of the capacity factor and selectivity for each triglyceride in relation to temperature, the proportion of certain mobile phase components and flow rate.

The octadecylsilane bonded phase was more selective when analyzing triglycerides with a partition number below 48, while octylsilane was appropiate for separating mixtures of long chain saturated triglycerides to the detriment of the resolution of triglycerides with low partition numbers. ACN/ACE/THF (58/38/4) was a suitable mobile phase for use with the octadecylsilane bonded phase, and ACN/THF /H₂O (60/40/1) for the octylsilane bonded phase. A column temperature of 30°C and a flow rate of 1.5 mL/min resulted in acceptable resolution and analysis time in both systems.     

Phosphorescent Sensor Approach for Non-Destructive Measurement of Oxygen in Packaged Foods: Optimisation of Disposable Oxygen Sensors and their Characterization Over a Wide Temperature Range

ABSTRACT

Phosphorescent oxygen sensors were evaluated for their suitability as a non-destructive method of measuring oxygen in packaged foods. Using phosphorescent phase measurements, characteristics of several types of disposable oxygen sensors were studied in order to optimize sensor chemistry, fabrication technology and performance. The optimal sensor was characterized in both the gas phase and in the liquid phase, over a temperature range of –17°C ? +30°C and oxygen concentrations between 0 and 21 kPa. Calibrations, analytical equations and temperature coefficients were obtained, which enabled accurate quantitation of oxygen and correction of optical measurements for sample temperature variations. For disposable sensor elements the resolution of the system at 22°C was about ±0.02 kPa and ±0.5 kPa at 0 and 21 kPa oxygen respectively, and in continuous monitoring mode - ±0.0054 kPa and ±0.081 kPa oxygen, respectively. Results of the use of the oxygen sensors in food packaging applications and practical recommendations are presented.     

Effect of thermal annealing on the crystalline structure of PVDF/PMMA-modified graphene nanocomposites

Abstract

Poly(vinylidene fluoride)-based nanocomposite films with different loadings of PMMA-modified graphene sheets were prepared by a solution-mixing and solvent casting method. The prepared films were annealed at three different temperatures and the crystalline structures of the samples were investigated. X-ray diffraction data confirmed the PMMA-modified graphene nanosheets enhanced the preferential β-crystalline structure when increasing the annealing temperature to 90?°C, while increasing the annealing temperature to 120?°C led to a β → γ phase conversion. The tendency of the graphene sheets to restack by increasing the annealing temperature was confirmed by XRD. The PMMA-modified graphene sheets, however, didn’t showa nucleating effect on the PVDF crystallizationduring DSC cooling scans.     

Determination of thermophysical properties of high temperature alloy IN713LC by thermal analysis

The presented paper deals with the study of thermophysical properties of cast and complex alloyed nickel based on superalloy Inconel 713LC (IN713LC). In this work, the technique of Differential Thermal Analysis was selected for determination of the phase transformation temperatures and for the study of the effect of varying heating/cooling rate at these temperatures. The samples taken from as-received state of superalloy were analysed at heating and cooling rates of 1, 5, 10, 20 and 50?°C?min^?1 with the help of the experimental system Setaram SETSYS 18_TM. Moreover, the transformation temperatures at zero heating/cooling rate were calculated. The recommended values for IN713LC after correcting to a zero heating rate, are 1,205?°C (T _?á?,solvus), 1,250?°C (solidus) and 1,349?°C (liquidus). Influence of heating/cooling rate on shift of almost all temperatures of phase transformations was established from the DTA curves. Undercooling was observed at the cooling process. The samples before and after DTA analysis were also subjected to the phase analysis by scanning electron microscopy using the microscope JEOL JSM-6490LV equipped with an energy dispersive analyser EDAX (EDS INCA x-act). Documentation of the microstructure was made in the mode of secondary (SEI) and backscattered (BEI) electron imaging. On the basis of DTA analysis and phase analysis it may be stated that development of phase transformations of the alloy IN713LC will probably correspond to the following scheme: melting?????? phase; melting???????+?MC; melting????eutectics ??/?á?; melting???????+?minority phases (e.g. borides); and matrix ???????á?.     

The formation of a series of barium tungsten bronzes

The phases occurring in samples of gross composition Ba_xWO₃ (0.01 < x < 0.33) heated at temperatures between 1073 and 1373°K have been determined using X-ray diffraction and electron microscopy. At all temperatures a tetragonal tungsten bronze phase with a narrow homogeneity range of x = 0.20?0.21 was observed to form. In addition, at temperatures up to 1273°K, a series of orthorhombic intergrowth bronzes forms within a restricted composition range around x = 0.04. The latter phases are unstable at higher temperatures and were not found in preparations made at 1323°K. Similarly a new type of bronze phase forms at x = 0.14?0.16 at temperatures up to 1323°K, but not at 1373°K. The structure of this phase is unknown. Aspects of the crystal chemistry of the barium bronzes and the relationships to other bronze phases are discussed.     

Preparation and Characterization of Temperature-responsive Porous Monoliths

A new temperature‐responsive porous monolith has been prepared by surface‐initiated activators generated by electron transfer atom transfer radical polymerization (AGET ATRP) grafting poly(N‐isopropylacrylamide) (PNIPAAm) within the pores of the porous polymer monolith. The grafting copolymerization was carried out by a method based on a continuous flow‐through technique without special deoxygenation procedure needed in the general ATRP. The addition of ascorbic acid could counteract the oxidation effect of oxygen diffusing into the reaction system. The resulting grafted monolith was characterized by a mercury intrusion method and the size of macropore was 3.65 µm, which was suitable for flow through the monolith for HPLC. The thermally responsive property of the grafted monolith was evaluated by HPLC using steroids with various hydrophobicities as probes. Through determination of retention factor of each steroid on the grafted monolith at different temperatures using water as mobile phase, it was found that the slope of the plot of retention factor of each steroid versus the temperature changed around the low critical solution temperature (LCST, 32°C) of PNIPAAm in water. It was relative to the grafted PNIPAAm temperature sensitivity that a hydrophobic and hydrophilic alternation would take place around its LCST. Based on this thermally responsive property, the grafted monolith was used as stationary phase for HPLC and to separate the steroids using water as mobile phase by changing the column temperature. As a mobile phase, water is much better than organic solvents concerning the environment.     

A Stability Indicating HPLC Assay for Prednisolone Sodium Phosphate in Implantable Infusion Pumps

Abstract

A stability indicting assay for prednisolone sodium phosphate (PSP) in solutions for implantable infusion pumps was developed. PSP and its major breakdown product, prednisolone, were separated from formulation excipients by reverse phase chromatography on a phenyl-bonded phase column using an acetonitrile-phosphate buffer mobile phase. Detection was by ultraviolet absorbance at 243 mm. Recovery from a synthetic formulation was 101.0 ± 0.4% (n=6). The method was used to monitor the stability of PSP solutions in implantable infusion pumps maintained at 37°C over a 21 day period.