共查询到20条相似文献,搜索用时 31 毫秒
1.
The influence of temperature on the retention and separation of C60 and C70 fullerenes was studied under HPLC conditions.
Particularly, chromatographic experiments were conducted using moderate carbon loaded octadecylsilica stationary phase and
homologous series of n-alkanes including n-pentane, n-hexane and n-heptane as the mobile phases. All studies were performed across wide range of subambient temperature from −80 to +20 °C.
From practical point of view the best chromatographic conditions for baseline separation of the components of interest were
selected. The retention of analytes was strongly affected by temperature and below minus 30 °C strong deviation from van't
Hoff behavior was observed. To explore this phenomenon selected thermodynamic parameters including changes of enthalpy (Δ Ho) and changes of entropy (Δ So) were estimated. Positive values of the Δ Ho and Δ So at low temperature region may indicate the lack of the interaction with the stationary phase ligands. A possible retention
mechanism at different temperatures for C60 and C70 molecules has been discussed. 相似文献
2.
Summary Results of studies on stationary phases containing copper(II), nickel(II) and cobalt(II) chloride chemically bonded via cyano—or
thiol groups are presented. The retention parameters—i.e. retention factor ( k), retention index ( I) and molecular retention index (Δ Me) and specific retention volume ( Vg)-enabled the characteristics of specific interaction between aromatic and cyclic hydrocarbons and metal complexes chemically
bonded to stationary phases to be determined.
Presented at Balaton Symposium on High Performance Separation Methods, Siófok, Hungary, September 1–3, 1999 相似文献
3.
The linear solvation energy relationship equation
developed by Abraham and coworkers was applied to the retention factors k of a series of 20 polar solutes on four chemically
different RP-HPLC phases. Three of them were specially synthesized and are functionalized with ether, phenylsulfide or phenylsulfoxide
groups. Their retention properties are compared with those of a nonpolar octadecylsiloxane (ODS) phase. The phase properties
r, the excess molar refraction; s, the dipolarity; a and b, the hydrogen-bond basicity and acidity; and v, the cavity factor
show significant differences on the four phases and are used here to suggest a classification of stationary phases based on
the type of interactions that are important for the retention. The hydrophilic system properties r, s, a and b are the reason
for different elution orders of a set of solutes on the four phases. The intrinsic hydrophobicity of the system, the v/A ratio
(A is the surface coverage in μmol/m 2), shows a dependence on the mobile phase composition as do the normalized phase properties r/v, s/v, a/v and b/v. Averaging
the constants over a large span of mobile phase composition should be done very carefully. The LSER model is used to predict
the elution order on the stationary phases for five phenols which show coelution on ODS. On the phenylsulfide phase they are
resolved.
Received: 3 December 1998 / Revised: 1 February 1999 / Accepted: 8 February 1999 相似文献
4.
Summary Reversed-phase liquid chromatographic retention characteristics for the sixteen acyclic C 1−C 5 N-alkylbenzamide congeners were measured on various branched and linear, alkyl bonded hydrocarbon stationary phases. Retention
factors, k′, were determined in acetonitrile-water mobile phase compositions on ethyl, n-octyl, n-dodecyl, n-octadecyl, 1-ethyladamantyl,
4-butyloctyl, and 2,4,4-trimethylpentyl stationary phases. Statistical analysis of the two main effects investigated — type
of stationary phase and percentage of organic modifier (acetonitrile) in the mobile phase — described greater than ninety
percent of the variability in the data for most of the comparisons. Selectivity effects due to variation in the mobile phase
dominated the results. 相似文献
5.
Summary An experimental design has been used to study the effect of column temperature on the gas chromatographic retention of eight para-hydroxy benzoic esters. A rapid procedure has been developed to reduce the number of experiments compared with traditional methods. Capacity factors were determined and the enthalpy and entropy of transfer from the mobile phase to the stationary phase, H o and S o, respectively, were calculated using the linear Van't Hoff equation (dependence of ln k on 1/T). A retention prediction system (RPS) for these compounds in GC was investigated. The molecular connectivity index was used to describe the quantitative structure relationships. Enthalpy — entropy compensation revealed that the mechanism was similar for all the compounds studied. 相似文献
6.
Calixarene‐bonded stationary phases received growing interest in HPLC as stationary phases with special retention characteristics and selectivity. The commercially available unsubstituted and p‐ tert‐butyl‐substituted Caltrex ® columns have been intensively studied and characterized in our workgroup. They can be used as reversed phases, yet they support additional interactions. Especially, their steric, polar and ionic properties differ from conventional alkyl‐bonded phases. However, also the hydrophobic interaction shows differences since adsorption and partition interactions on or in a bonded layer of calixarenes are not similar to those of alkyl‐bonded layers. The relative strength of the hydrophobic properties of the stationary phases has been found depending on the methanol concentration of the mobile phase. Generally, the dependencies of their interaction strengths on mobile‐phase conditions, e.g. the change of the intensity of the hydrogen‐bonding abilities with decreasing methanol content, are not similar from phase to phase either. This probably gives calixarene‐bonded stationary phases enhanced suitability for analyses at extreme compositions of the mobile phase. An overview about the synthesis, retention and selectivity properties of Caltrex ® columns is given here. 相似文献
7.
A series of 11 homemade octadecyl bonded phases with different coverage densities were tested to determine the influence of the stationary phase on the retention in highly aqueous mobile phases. The concentrations of the organic modifiers (methanol and ACN) were in the range of 0–20% v/v. The coverage density of bonded ligands and the presence of the end‐capping have strong influence on the solute retention. Amoxicillin (AMO) was chosen as the test compound. Dual properties of AMO, which contain hydrophobic skeleton and polar groups (amino, hydroxyl and carbonyl), cause irregular changes of the retention over the stationary phase hydrophobicity and silanol activity at given mobile phase composition. Presented data show that application of non‐standard low coverage density C18 phases allow to determine AMO in the RPLC condition with high retention. 相似文献
8.
Summary The retention behavior of phenylamine and naphthylamine sulphonic acid was evaluated in reversed-phase ion-pair liquid chromatography as a function of organic modifier concentration and column temperature. It has been observed that the logarithm of capacity factors decrease linearly with organic modifier concentration, and there is a good linear relationship between the intercept and slope for this relationship. Phenylamine and naphthylamine sulphonic acid retention decreases with increase in column temperature. A linear dependence of lnk
ip
on the reciprocal of the absolute temperature, the Van't Hoff plot, was observed over the column temperature range studied, and the standard enhalpic change (H o) for these sulphonic acid transfers from the mobile phaser to the stationary phase was determined. H o was dependent on the solute structure and in the range from 2.5 Kcal/mol to 5.5 Kcal/mol, which is close to that observed in RP-HPLC. The enthalpy/entropy compensation effect was evaluated by plotting lnk
ip(T)
vs. –H o, and the apparent differences in retention mechanisms between the analytes were observed, which may arise from the significant differences in their configuration, hydrophobicity and the charges of the solutes as well as the complex retention processes of RP-IPC. 相似文献
9.
Known enthalpies of formation (Δ f
H
o) of carbenes in the ground state in the gas phase were analyzed; the prospects for the theoretical evaluation of (Δ f
H
o) were considered. The (Δ f
H
o) values of carbenes were calculated by the group-addition method, developed previously for free radicals, as well as by the
AM1 and PM3 quantum-chemical methods; these methods were compared.
Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 57–63, January, 1997. 相似文献
10.
Retention and selectivity characteristics of different calixarene‐, resorcinarene‐ and alkyl‐bonded stationary phases are examined by analyzing a set of test solutes covering the main interactions (hydrophobic, steric, ionic, polar) that apply in HPLC. Therefore Dolan and Snyder's multiple term linear equation has been adapted to fit the properties of calixarene‐bonded columns. The obtained parameters are used to describe retention and selectivity of the novel Caltrex ® phases and to elucidate underlying mechanisms of retention. Here, differences of stationary phase characteristics at different methanol concentrations in the mobile phases are examined. Both selectivity and retention were found to depend on the methanol content. Differences of these dependencies were found for different stationary phases and interactions. The differences between common alkyl‐bonded and novel calixarene‐bonded phases increase with increasing methanol content. 相似文献
12.
Summary Two reversed-phase LC systems were investigated by frontal analysis for the determination of linear chromatographic conditions,
as defined according to the isotherm concept. The Partisil ODS-3 bonded silica and the PRP-1 polystyrene-divinylbenzene resin
were used as stationary phases together with trans-2-hexen-al as test solute and methanol-water mixtures as mobile phases. Particular attention was paid to the respective influence
of the two main parameters which may cause sorbent overloading, that is, the capacity factor (k′) and the solute concentration
in the mobile phase (C m). Provided that k′C m≤10 −2 M, linear chromatographic behaviour was observed for both sorbents, the maximum capacities of which were found greater than
1mmolg −1. 相似文献
13.
A non-hygroscopic pharmaceutical composition was obtained following a host-guest strategy that used the anticonvulsant drug
sodium valproate (VA) and α-cyclodextrin. The pharmaceutical composition was fully characterized by thermal analyses (TG/DTG,
DSC), X-ray powder diffraction and by 1H, 13C, hydrogen relaxation times ( T1) and 2D-ROESY NMR techniques. Isothermal titration calorimetry (ITC) was used to determine the VA:α-CD 1:1 stoichiometry
as well as to calculate the equilibrium constant ( K) and thermodynamic energies of interaction (ΔG o, ΔH o and TΔS o). 相似文献
14.
The retention of fifty structurally different compounds has been studied using linear solvation energy relationships. Investigations were performed with the use of six various stationary phases with two mobile phases (50/50 % v/v methanol/water and 50/50 % v/v acetonitrile/water). Packing materials were home-made and functionalized with octadecyl, alkylamide, cholesterol, alkyl-phosphate and phenyl molecules. This is the first attempt to compare all of these stationary phases synthesized on the same silica gel batch. Therefore, all of them may be compared in more complex and believable way, than it was performed earlier in former investigations. The phase properties (based on Abraham model) were used to the classification of stationary phases according to their interaction properties. The hydrophilic system properties s, a, b indicate stronger interactions between solute and mobile phase for most of the columns. Both e and v cause greater retention as a consequence of preferable interactions with stationary phase by electron pairs and cavity formation as well as hydrophobic bonds. However, alkyl-phosphate phase has different retention properties, as it was expressed by positive sign of s coefficient. It may be concluded that most important parameters influencing the retention of compounds are volume and hydrogen bond acceptor basicity. The LSER coefficients showed also the dependency on the type of organic modifier used as a mobile phase component. 相似文献
15.
The power-time curves for the micelle formation process were determined for two anionic surfactants, sodium laurate (SLA)
and sodium dodecyl sulfate (SDS), in mixed alcohol + N,N-dimethylacetamide (DMA) solvent using titration microcalorimetry.
From the data of the lowest point and the area of the power-time curves, their critical micelle concentration (CMC) and Δ H
mo were obtained. The other thermodynamic functions of the micellization process (Δ G
mo and Δ S
mo) were also calculated with thermodynamic equations. For both surfactants, the effects of the carbon number (chain length)
of the alcohol, the concentration of alcohol, and the temperature on the CMC and thermodynamic functions are discussed. For
systems containing identical concentrations of a different alcohol, values of the CMC, Δ H
mo and Δ S
mo increased whereas Δ G
mo decreased with increasing temperature. For systems containing an identical alcohol concentration at the same temperature,
values of the CMC, Δ H
mo,Δ G
mo and Δ S
mo decrease with increasing carbon number of alcohol. For systems containing the same alcohol at the same temperature, the CMC
and Δ G
mo values increase whereas Δ H
mo and Δ S
mo decrease with increasing alcohol concentration. 相似文献
16.
The kinetics of the solvolytic aquation of trans-[Ru (3-Me Py) 4Cl 2] was studied spectrophotometrically in water – isopropanol in the range (30–90% v/v), and water acetonitrile in the range
(10–70% v/v), and in the temperature range 50–65 °C. Plots of log k versus the reciprocal of the relative permittivity and Grunwald–Winstien gave non-linear plots. This non-linearity is derived from
a large differential effect of solvent structure between the initial and transition states. The plot of log k versus water concentration was also non linear; evidence for the presence of a S
N
1
mechanism. However, extrema in the variation of enthalpy Δ H* and entropy Δ S* of activation correlate well with the extrema in physical properties of the mixtures which are related to changes in solvent
structure. Linear plots of Δ H* versus Δ S* were obtained and the iso- kinetic temperature indicates that the reaction is entropy controlled. 相似文献
17.
The relative enthalpies, Δ Ho (0) and ΔH o (298.15), of stationary points (four minimum and three transition structures) on the •O 3H potential energy surface were calculated with the aid of the G3MP2B3 as well as the CCSD(T)–CBS (W1U) procedures from which
we earlier found mean absolute deviations (MAD) of 3.9 kJ mol −1 and 2.3 kJ mol −1, respectively, between experimental and calculated standard enthalpies of the formation of a set of 32 free radicals. For
CCSD(T)-CBS (W1U) the well depth from O 3 + H • to trans- •O 3H, Δ Howell(298.15) = −339.1 kJ mol −1, as well as the reaction enthalpy of the overall reaction O 3 + H •→O 2 + •OH, Δ rH o(298.15) = −333.7 kJ mol −1, and the barrier of bond dissociation of trans- •O 3H → O 2 + •OH, ΔH o(298.15) = 22.3 kJ mol −1, affirm the stable short-lived intermediate •O 3H. In addition, for radicals cis- •O 3H and trans- •O 3H, the thermodynamic functions heat capacity Cop(T), entropy S o (T), and thermal energy content H o(T) − H o(0) are tabulated in the range of 100 − 3000 K. The much debated calculated standard enthalpy of the formation of the trans- •O 3H resulted to be Δ fH o(298.15) = 31.1 kJ mol −1 and 32.9 kJ mol −1, at the G3MP2B3 and CCSD(T)-CBS (W1U) levels of theory, respectively. In addition, MR-ACPF-CBS calculations were applied
to consider possible multiconfiguration effects and yield Δ fH o(298.15) = 21.2 kJ mol −1. The discrepancy between calculated values and the experimental value of −4.2 ± 21 kJ mol −1 is still unresolved.
Note added in proof: Yu-Ran Luo and J. Alistair Kerr, based on the discussion in reference 12, recently presented an experimental
value of Δ fH o(298.15) = 29.7 ± 8.4 kJ mol −1 in the 85th edition of the CRC Handbook of Chemistry and Physics (in progress). 相似文献
18.
The energy of combustion of cobalt tetrakis(4-metoxyphenyl)porphin was determined in an isothermic-shell liquid calorimeter
with a stationary calorimetric bomb. The standard enthalpies of combustion and formation of the complex were calculated, −Δ
c
H
o = 27334.06 ± 50.98 kJ/mol and Δ f
H
o = 3062.90 ± 50.97 kJ/mol.
Original Russian Text ? R.P. Tarasov, A.V. Volkov, M.I. Bazanov, A.S. Semeikin, 2009, published in Zhurnal Fizicheskoi Khimii,
2009, Vol. 83, No. 5, pp. 993–995. 相似文献
19.
DTA and DSC were used to study the thermal behaviour of Ca(NO 3) 2·4H 2O, Cd(NO 3) 2·4H 2O, Mg(NO 3) 2·6H 2O and their deuterated analogues. Evidence was found concerning the process of melting of the initial hydrates and deuterates,
followed by a one-stage dehydration of the melt to vield the respective anhydrous salt.
T
m, Δ H
m
o
, Δ S
m
o
and Δ H
deh
o
were determined and the Δ H
f
o
values for the investigated hydrates were calculated from the Δ H
deh
o
data.
Zusammenfassung DTA und DSC wurden zur Untersuchung des thermischen Verhaltens von Ca(NO3)2·4H2O, Cd(NO3)2·4H2O, Mg(NO3)2·6H2O und ihrer deuterierten Analoge eingesetzt. Man fand Aussagen bezüglich des Schmelzvorganges der Ausgangshydrate und Deuterate,
gefolgt von einer Einschritt-Dehydratation der Schmelze unter Bildung der entsprechenden wasserfreien Salze.
T
m, ΔH
m
o
, ΔS
m
o
und ΔH
deh
o
wurden ermittelt und die ΔH
f
o
Werte für die untersuchten Hydrate wurden anhand der ΔH
deh
o
berechnet.
相似文献
20.
The solvation parameter model is used to characterize the retention properties of a 3-aminopropylsiloxane-bonded (Alltima amino), three 3-cyanopropylsiloxane-bonded (Ultrasphere CN, Ultremex-CN and Zorbax SB-CN), a spacer bonded propanediol (LiChrospher DIOL) and a multifunctional macrocyclic glycopeptide (Chirobiotic T) silica-based stationary phases with mobile phases containing 10 and 20% (v/v) methanol-water. The low retention on the polar chemically bonded stationary phases compared with alkylsiloxane-bonded silica stationary phases arises from the higher cohesion of the polar chemically bonded phases and an unfavorable phase ratio. The solvated polar chemically bonded stationary phases are considerably more hydrogen-bond acidic and dipolar/polarizable than solvated alkylsiloxane-bonded silica stationary phases. Selectivity differences are not as great among the polar chemically bonded stationary phases as they are between the polar chemically bonded phases and alkylsiloxane-bonded silica stationary phases. 相似文献
|