On the influence of the solute sample size on temperature-programmed retention indices

The effect of the solute sample size on the measurement of temperature-programmed retention indices (I_TP) has been studied on glass capillary columns with different stationary phases, film thicknesses, and temperature programs. The sample capacity of a solute is little affected by the parameters of temperature program, i. e. initial temperature and heating rate. However, it has been found that in a temperature programmed run, allowable sample sizes for high I_TP values are lower than for low I_TP values. About 10 ng of each compound is recommended as the sample size for accurate measurement of I_TP values on commonly used capillary columns.     

Comparison of mathemical methods for the calculation of retention indices at high temperature in gas chromatography

The retention indices of three homologous series (2-alkanones, 1-alkanols, cycloalksanones) have been determined at high temperature by the application of two new adaptation methods: A multiparametric least-squares regressions iterative method based on the dertermination of the adjusted retention times and a cubic interpolation directly using the uncorrected retention times without dead time correction. The two methods were applied to two types of columns. The first group includes eight packed columns (seven OV polymethylphenylsiloxane and Apolane-87 stationary phases), while the second includes five glass capillary columns (four methyl-silicons with different film thicknesses and Apolane-87 stationary phases). The retention indices obtained with a multiparametric and a cubic interpolation methods were compared with each other and with those calculated by Grobler's, Guardino's, Kaiser's and Kovàts' methods. The influence of coating, film thickness, and temperature on them was investigated.     

Effect of temperature on the mechanism of retention of fullerenes in liquid chromatography using various alkyl bonded stationary phases

Summary The temperature-dependency of the separation of fullerenes in liquid chromatography (LC) has been examined using various alkyl bonded stationary phases. It has been found that a maximum retention temperature exists with long alkyl bonded stationary phases, whereas there is no similar effect with the newly synthesized alkyl bonded phases which have two phenyl groups at the base of the bonded phase. The interpretation of the retention behavior of fullerenes in the low temperature region on alkyl bonded stationary phases is discussed using information obtained by CP-MAS solid-state NMR spectroscopy and LC.     

Sensitivity of the methylbenzenes and chlorobenzenes retention index to column temperature, stationary phase polarity, and-number and chemical nature of substituents

Retention indices of methylbenzenes and chlorobenzenes on two fused silica capillary columns, HP-5 (diphenylsiloxane 5% diphenyldimethylsiloxane) and ZB-WAX (polyethylene glycol), have been calculated at various isothermal temperatures and compared with literature data. The retention index temperature effect was studied for each solute, finding greater retention index the higher the column temperature. A comparison between the straight line fit and the fit to the recently proposed equation I = A + B/T +C ln T was carried out. The effect of the stationary phase polarity on the retention index was checked. In general, a greater retention index was found for the more polar stationary phase. The retention indices of the chlorobenzenes are greater than the retention indices of the methylbenzenes, irrespective of the stationary phase and the column temperature. In addition, the influence of the methyl/chlorine substitution on the benzene molecule was investigated at each temperature. The retention indices increased as the number of substituents (methyl/chlorine) increased. The retention index increments of methyl and chloro derivatives are also discussed, which permits to compare the effect of both, methyl or chlorine, chemical functions, for a fixed substituent number in the benzene molecule.     

HPLC enantioseparation of β2‐homoamino acids using crown ether‐based chiral stationary phase

RP high‐performance liquid chromatographic methods were developed for the enantioseparation of eleven unusual β²‐homoamino acids. The underivatized analytes were separated on a chiral stationary phase containing (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid as chiral selector. The effects of organic (alcoholic) and acidic modifiers, the mobile phase composition and temperature on the separation were investigated. The structures of the substituents in the α‐position of the analytes substantially influenced the retention and resolution. The elution sequence was determined in some cases: the S enantiomers eluted before the R enantiomers.     

Joint use of cyclodextrin additives in chiral discrimination by reversed-phase high-performance liquid chromatography: temperature effects

The temperature dependence of chiral separations was investigated in combined system of reversed-phase (RP) liquid chromatography using two chiral additives: single or β native cyclodextrins and their permethylated derivatives. The model tested compounds of pharmaceutical interest were: methylphenobarbital, mephenytoin, morsuximide and camphor. Taking the localization of a complexation process as a criterion – the combined system with two selectors has been rationalized as occurring in three stages. The influence of temperature (in narrow range of 20°C) on retention and enantioselectivity was studied in; System I (complexation occurs in the mobile phase), in System II (complexation on the stationary phase) and in System III (complexation in both phases together). In System III (as for System I) it has been found that the model compounds could be classified into three groups based on their retention dependence on temperature: retention decrease with temperature decrease, retention increase with temperature decrease or no influence of temperature on retention. For all the compounds investigated, decrease in temperature increases the selectivity. Standard enthalpy (ΔH⁰) and entropy (ΔS⁰) changes of solute transfer between the mobile and the stationary phase and standard enthalpy (ΔH⁰_CD) and entropy (ΔS⁰_CD) changes of complex formation were also calculated. In Systems I and III, if the complexation in the mobile phase is favored process compared with interaction with the stationary phases (RP or covered by permethylated cyclodextrin), the shortest retention time and the best selectivity is observed at low temperature.     

Prediction of Peptide Retention in RP-LC

The retention of small peptides can be predicted by summing the hydrophobic contribution to retention of each amino acid of peptides. But the retention time of peptides larger than 10–15 residues are less than that predicted by summing the retention coefficients of each constitutive residue. A new prediction model, considering the effects of the peptide length and contact area of each amino acid with the stationary phase for larger peptides was proposed. The model was validated by 136 peptides identified by nano-flow 2-D-LC-ESI-MS-MS platform and other retention data observed from literature. The high degree of correlation between the observed and predicted retention time by using the new model is not only good evidence for the accuracy of our predictive method, but supports the supposition that the peptide length and contact area of each amino acid with stationary phase are important factors affecting peptide retention time for larger peptide. In addition it is found that the range of peptide length is wider, the accuracy of prediction is better. The ratio coefficient of surface area of non-polar, polar and charged amino residues contacting with the stationary phase were all calculated to be less than one. Revised: 30 June and 11 August 2005     

Factors affecting the reversed-phase liquid chromatographic separation of polycyclic aromatic hydrocarbon isomers

Reversed-phase liquid chromatography (LC) on C₁₈ stationary phases provides excellent selectivity for the separation of polycyclic aromatic hydrocarbons (PAH). Recent studies have shown that several factors affect selectivity for the LC separation of PAH including phase type (monomeric or polymeric), pore diameter and surface area of the silica substrate, and surface density of the C₁₈ ligands. In this paper the separation of eleven PAH isomers of molecular weight 278 is used to further illustrate the effect of stationary phase characteristics and shape of the solute (length-to-breadth ratio, L/B) on retention and selectivity. Only polymeric C₁₈ phases with a high C₁₈ surface coverage provided separation of all eleven isomers and the elution order of these isomers generally followed increasing L/B values. The effect of solute nonplanarity on reversed-phase LC retention was investigated on both monomeric and polymeric phases using a series of planar and nonplanar PAH pairs. For each solute pair, the nonplanar solute eluted earlier than the planar solute, the largest selectivity factors being observed on the C₁₈ phase with the highest percent carbon load. Based on these studies, a model is proposed to describe the retention of PAH on polymeric C₁₈ phases.     

Prediction of retention times of polycyclic aromatic hydrocarbons and <Emphasis Type="Italic">n</Emphasis>-alkanes in temperature-programmed gas chromatography

We have developed an iterative procedure for predicting the retention times of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes during separations by temperature-programmed gas chromatography. The procedure is based on estimates of two thermodynamic properties for each analyte (the differences in enthalpy and entropy associated with movements between the stationary and mobile phases) derived from data acquired experimentally in separations under isothermal conditions at temperatures spanning the range covered by the temperature programs in ten-degree increments. The columns used for this purpose were capillary columns containing polydimethylsiloxane-based stationary phases with three degrees of phenyl substitution (0%, 5%, and 50%). Predicted values were mostly within 1% of experimentally determined values, implying that the method is stable and precise. Figure Predicted values were mostly within 1 % of experimentally determined values, thus implying that the method is stable and precise     

Lattice-fluid model for gas-liquid chromatography

Lattice-fluid models describe molecular ensembles in terms of the number of lattice sites occupied by molecular species (r-mers) and the interactions between neighboring molecules. The lattice-fluid model proposed by Sanchez and Lacombe (Macromolecules, 1978;11:1145-1156) was used to model specific retention volume data for a series of n-alkane solutes with n-alkane, polystyrene, and poly(dimethylsiloxane) stationary liquid phases. Theoretical equations were derived for the specific retention volume and also for the temperature dependence and limiting (high temperature) values for the specific retention volume. The model was used to predict retention volumes within 10% for the n-alkanes phases; 22% for polystyrene; and from 20 to 70% for PDMS using no adjustable parameters. The temperature derivative (enthalpy) could be calculated within 5% for all of the solutes in nine stationary liquid phases. The limiting value for the specific retention volume at high temperature (entropy controlled state) could be calculated within 10% for all of the systems. The limiting data also provided a new chromatographic method to measure the size parameter, r, for any chromatographic solute using characteristic and size parameters for the stationary phase only. The calculated size parameters of the solutes were consistent, i.e. independent of the stationary phase and agreed within experimental error with the size parameters previously reported from saturated vapor pressure, latent heat of vaporization or density data.     

Evaluation of the Linear-Elution-Strength Model for the Prediction of Retention and Selectivity in Isothermal Gas Chromatography

Linear-elution strength theory and temperature-programmed gas chromatography is evaluated as a rapid method for predicting isothermal retention factors and column selectivity. Retention times for a wide range of compounds are determined at the program rates of 3 and 12 °C/min for the temperature range 60 to 160 °C on three open-tubular columns (DB-1701, DB-210 and EC-Wax) and used to predict isothermal retention factors for each column over the temperature range 60 to 140 °C. The temperature-program predicted isothermal retention factors are compared with experimental values using linear regression and the solvation parameter model. It is shown that isothermal retention factors predicted by the linear-elution-strength model only approximately represents the experimental data. The model fails to predict the slight curvature that exists in most plots of the experimental retention factor (log k) as a function of temperature. In addition, regression of the temperature-program predicted isothermal retention factors against the experimental values indicates that the slopes and intercepts deviate significantly from their target values of one and zero, respectively, in a manner which is temperature dependent. The temperature-program predicted isothermal retention factors result in system constants for the solvation parameter model that are different to those obtained from the experimental retention factors. These results are interpreted as indicating that linear-elution-strength theory predicts retention factors that fail to accurately model stationary phase interactions over a wide temperature range. It is concluded that temperature-program methods using linear-elution-strength theory are unsuitable for constructing system maps for isothermal separations.     

Fullerenes separation with monomeric type C30 stationary phase in high-performance liquid chromatography

The temperature effect on the separation of fullerenes in LC was examined using monomeric type C30, C18 and C8 alkyl bonded stationary phases. It appears that the C30 phase exhibits superior separation ability for fullerenes. It is observed that the maximum retention temperature of fullerenes on the C30 phase is around 20 degrees C. A strong correlation between the changes in NMR spectra and the retention behavior of the solutes was found. The interpretation of the retention behavior of fullerenes on the alkyl bonded stationary phases, including the behavior in subambient temperature, is discussed using the information obtained by CP-MAS solid-state NMR spectroscopy and LC.     

Solvation parameter models for retention on perfluorinated and fluorinated low temperature glassy carbon stationary phases in reversed-phase liquid chromatography

The retention of solutes on two fluorinated low temperature glassy carbon (F-LTGC) stationary phases under reversed-phase liquid chromatographic conditions was studied by employing the solvation parameter model. The two fluorinated glassy carbon stationary phases were produced by slowly heating zirconia particles that were encapsulated with oligo[1,3-dibutadiyne-1,3-(tetrafluoro)phenylene] precursor polymer to two different final temperatures (200 and 400 degrees C). The resulting carbon particles had different amounts of fluorine after thermal processing. The solvation parameter models indicated that different intermolecular interactions are important in describing retention on the two stationary phases. The interactions that are important for describing retention on the 200 degrees C processed F-LTGC stationary phase are hydrogen bond basicity> or =dispersion>hydrogen bond acidity>dipolarity/polarizability. The interactions that describe the retention on the 400 degrees C processed F-LTGC are hydrogen bond basicity>dispersion>excess molar refraction> or =hydrogen bond acidity. The solvation parameter model for the 200 degrees C processed F-LTGC showed similar trends in the relative importance of intermolecular interactions as previously found for octadecyl-polysiloxane stationary phases, while the 400 degrees C processed F-LTGC had similar intermolecular interactions with solutes as found with porous glassy carbon in that pi-pi interactions with the carbon surface contribute more so to the retention.     

重整生成汽油样品中烃的类别分析

用气相色谱以程序升温方式分析了重整生成汽油,并将各组分升温保留时间转换为恒温保留指数。以各组分在ＯＶ－１和ＳＥ－５４固定相上,同一柱温下的保留指数差及在各柱上的温度系数为三因素进行斜交因子分析和本征矢量旋转,给出了重整生成汽油样品中烷烃、烯烃、环烷烃、芳烃值,经气相色谱－质谱分析验证了结果的正确性,为重整成成汽油样品中烃的类别分析提供了一种新方法。     

GC behaviour of symmetrical n-dialkyl sulphides under isothermal and temperature programming conditions

Retention indices were determined for a homologous series of n-dialkyl sulphides on three stationary phases (SE-30, OV-17 and XE-60) under isothermal and linear temperature programming conditions. Under these two different GC conditions, equations were derived for each of the three stationary phases which showed the dependence of retention index on the number of carbon atoms and the boiling points for a homologous series of n-dialkyl sulphides. The equation for the correlation isothermal retention index was shown to be applicable to the identification of n-dialkyl sulphides using linear temperature programming. It was found that the GC behaviour of n-dialkyl sulphides makes these compounds suitable for use as a standard series instead of n-alkanes for the calculation of retention indices in GC analysis in which detectors insensitive to n-alkanes are employed. The use of the homologous series of n-dialkyl sulphides for the calculation of sulphide retention indices can be great practical importance in the microanalysis of natural compounds. We have used this method successfully in the analysis of pesticides containing S-atoms.     

System constants of synthesized poly(methyl-3,3,3-trifluoropropyl) siloxanes

The method of solvation model has been applied to five poly (methyl-trifluoropropyl) siloxanes (TFPSXX) prepared in our laboratories, at five trifluoropropyl (TFP) group contents, XX = 0, 11.5, 26.3, 35.5 and 50.0%, at 80, 100, 120 and 140 degrees C. Previously, specific retention volumes of 60-odd solutes of varied polarities were measured upon each of these stationary phases within the above temperature range. Constant s prevails over all other constants, TFPSXX stationary phases showing strong dipole/induced dipole forces with the solutes, moderate acidity and no basicity at all. Constant e is zero in the stationary phase without TFP groups, but has negative low-medium values for the other fluorine contents, XX from 11.5 to 50.0%, hinting at repulsive forces, as expected. Normal values for constant l, decreasing from the less cohesive TFPS00 to the more cohesive TFPS50, were found. For each TFP content constants s, a and l show a negative temperature dependence, while constant e increases as temperature increases. Constant c also decreases with increasing temperature. At each temperature, constants s and a increase with increasing %TFP (or increasing stationary phase polarity), whereas constants e and l show the opposite trend, diminishing with increasing polarity of the stationary phase. Principal component analysis shows that the five stationary phases presented in this work conform a group with other earlier synthesized trifluoropropyl siloxanes and other fluorinated stationary phases taken from literature: VB-210, QF-1, DB-200, DB-210 and PFS6, showing the same selectivity which only the fluorine atom confers. A dendrogram of 38 stationary phases supports these results.     

Disengaging solutes in shale- and petroleum-derived jet fuels by altering GC programmed temperature rates

Overlapping chromatographic peaks of components from different hydrocarbon classes can be disengaged by exploiting their shifts in relative retention behavior with changes in linear rates of programmed temperature. Many co-eluting species in complex chromatograms of shale- and petroleum-derived jet fuels can be resolved without varying stationary phase, column length, or initial column temperature. Retention indices were simultaneously determined on two bonded-phase, fused silica capillary columns of slightly different polarities at three different linear programmed temperature rates. For certain hydrocarbon types, no change in index values was observed with an alteration in programming rate. However, the indices of other hydrocarbon classes shifted uniformly with programmed temperature rates on each of the two stationary phases. When applied, this phenomenon could help resolve coeluting members of different or even the same hydrocarbon type and elucidate their probable structure. The overall precision of the retention indices, i.e., the mean standard deviation at the 95% confidence levels, was less than ±0.13 for either column at any of the three programming rates. Since the above technique is automated, it could also be a useful screening tool to search for specific hydrocarbons in a myriad of unknown components of a complex hydrocarbon mixture.     

Retention mechanism of poly(ethylene oxide) in reversed-phase and normal-phase liquid chromatography

The retention behavior of low- and high-molecular-mass poly(ethylene oxide) (PEO) in reversed-phase (RP) and normal-phase (NP) liquid chromatography was investigated. In RPLC using a C18 bonded silica stationary phase and an acetonitrile-water mixture mobile phase, the sorption process of PEO to the stationary phase showed deltaH(o) > 0 and deltaS(o) > 0. Therefore, PEO retention in RPLC separation is an energetically unfavorable, entropy-driven process, which results in an increase of PEO retention as the temperature increases. In addition, at the enthalpy-entropy compensation point the elution volume of PEO was very different from the column void volume. These observations are quite different from the RPLC retention behavior of many organic polymers. The peculiar retention behavior of PEO in RPLC separation can be understood in terms of the hydrophobic interaction of this class of typical amphiphilic compounds with the non-polar stationary phase, on the one hand, and with the aqueous mobile phase, on the other. The entropy gain due to the release of the solvated water molecules from the PEO chain and the stationary phase is believed to be responsible for the entropy-driven separation process. On the other hand, in NPLC using an amino-bonded silica stationary phase and an acetonitrile-water mixture mobile phase, PEO showed normal enthalpy-driven retention behavior: deltaH(o) < 0 and deltaS(o) < 0, with the retention decreasing with increasing temperature and PEO eluting near the column void volume at the enthalpy-entropy compensation point. Therefore, high-resolution temperature gradient NPLC separation of high-molecular-mass PEO samples can be achieved with relative ease. The molecular mass distribution of high-molecular-mass PEO was found to be much narrower than that measured by size-exclusion chromatography.