Comparison of correlation equations for the prediction of gas chromatographic separation of alkylbenzenes

Summary A detailed statistical analysis, comparing the validity of the best literature recommended equations for prediction of retention indices of alkylbenzenes on squalane and methylsilicone stationary phases with the unified retention index concept, is presented. A comparison with literature sources shows that the unified retention index provides the smallest residual error. Hence, it can be applied for prediction purposes.     

Correlation of unified retention indices for OV-101 and squalane

Summary Gas chromatographic unified retention indices for 43 hydrocarbons (alkanes and cycloalkanes) are given for squalane and OV-101. Comparison of these values and unified retention index increments are presented as linear regression equations with high correlation coefficients and acceptable standard deviations.     

Prediction of retention times and efficiency in linear gradient programmed pressure analysis on capillary columns

A method for the prediction of the retention time and the resolution of chromatographic peaks in different experimental conditions by starting from few experimental data measured in isothermal and isobaric analyses was published previously. In this paper, the same mathematical model was implemented for calculating the retention times and the column efficiency in programmed pressure runs. Some models originated from the Golay equation and reported in the literature are compared, and a new modified equation for the calculation of the peak width at half height is proposed. The procedure for the prediction of the retention time and the peak width at half height at programmed pressure of the carrier gas and different column temperature and linear gradient by using retention data of different compounds obtained in few isobaric runs is described. The prediction of the retention time and the separation efficiency of compounds with different polarity gave good results for the programmed pressure runs with linear gradient. The effect of the variation of the initial parameters of the experimental analyses and of the mathematical model on the accuracy of the prediction has been evaluated.     

Semi-empirical topological method for the prediction of the chromatographic retention of cis- and trans-alkene isomers and alkanes.

A new index is proposed for the prediction of the chromatographic retention of the cis- and trans-n-alkene isomers and alkanes. This index is based on the hypothesis that the chromatographic retention of the molecule is due to the interaction of each carbon atom with the stationary phase, and consequently the index is reduced by its neighbours' steric effects. The topological values are obtained by a numerical approximation considering the general behaviour of the chromatographic retention of the compounds. The simple linear regressions between the chromatographic retention and the index proposed for all branched alkanes and also isomers of the studied straight-chain C5 and C14 alkenes (1-ene, cis- and trans-2-, 3-, 4-, 5-, 6- and 7-enes) is very good (the correlation coefficient is r = 0.9999), and the elution sequence is correct for most of them. The models have a high predictive ability, as established by cross-validation values (r2cv). Thus, this new method, different from those already existent, can be used as complementary tool for the elucidation of the molecular structure, or prediction of the chromatographic retention of the cis- and trans-alkene isomers and branched alkanes. It could be extended with success, in the future, to the other types of compounds.     

Using Bezier curves for the calculation of retention indices of polycyclic aromatic hydrocarbons in the so-called Lee's scale in temperature-programmed gas chromatography with mass spectrometry detection

The retention indices of some polycyclic aromatic hydrocarbons (PAHs) separated by temperature-programmed gas chromatography are computed by smoothing reference data with Bezier curves of orders 6 are more consistent with the scheme of this retention parameter, and they present standard deviations systematically smaller than those associated with classical retention indices. The Bezier curve possesses the property of local control, (i.e., their graphs are modified only in the neighbourhood of the displaced point). The values thus obtained were compared with the corresponding ones calculated in a classical way. Detailed statistical analysis is presented to describe the retention indices of PAHs expressed in the so-called Lee's scale as a function of retention temperatures (in degrees C). As a training set, experimental retention data of PAHs obtained on a PE-5 phase is used for correlation. As prediction sets, literature experimental retention indices expressed in the so-called Lee's scale obtained on a DB-5 slightly polar stationary phase are applied for comparison. The method developed is successfully used for estimating and predicting the capillary gas chromatography retention index of PAHs.     

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.     

The absolute retention index in chromatography. Part 1

A definition for an absolute retention index is given. This index is independent of whether the process is isothermal or temperature programmed. The relation between this index and Kovats's index is discussed. General equations are derived for the retention times of homologous series, from which the position of the air peak or missing members can be determined. Methods used in the literature for air peak determination are improved and extended to other cases of interest. The retention index in temperature programmed chromatography is reexamined in the light of recent publications on temperature programming.     

气相色谱与液相色谱保留值的换算方法

根据色谱热力学理论,在色谱保留值公式统一形式的基础上导出了气相色谱保留系数（I）与反相液相色谱保留公式参数a,c之间的关系式,证明结构类似化合物的a,c值与保留指数呈线性关系,同时存在氢键作用能、偶极矩作用的影响,从而提出了色相色谱与反相液相色谱保留值换算的方法;该理论为氯代芳烃的文献数据所证实。     

Influence of mobile phase composition on retention factors in different HPLC systems with chemically bonded stationary phases

The influence of mobile phase composition on the retention of selected test analytes in different normal- and reversed-phase chromatographic systems is studied. A novel adsorption model for an accurate prediction of the analyte retention in the column chromatography with binary mobile phase is proposed. Performance of the model is compared with the retention model reported in the literature. Both models are verified for different HPLC systems by use of three criteria: (a). the sum of squared differences between the experimental and theoretical data, (b). approximation of the standard deviation, and (c). the Fisher test.     

直链醇、烷基苯气相色谱保留指数之比与其正辛醇/水分配系数的相关性

通过研究直链醇、烷基苯等有机物气相色谱保留指数与其正辛醇／水分配系数的关系得出,在极性相异固定相上保留指数之比Im/n与其正辛醇／水分配系数的对数值logkow之间有较好的线性相关性。因而由保留指数能很好的预测分配系数,从而为直链醇、烷基苯分配系数的测定提供了一种简便准确的新方法。     

静态GM（0,6）灰色模型在预测气相色谱保留指数上的应用

本文从麦克雷诺（ＭｃＲｅｙｎｏｌｄｓ）相常数法出发推导了五种标准物质与任意溶质ｉ的色谱保留指数之间的关系符合灰色ＧＭ（０，６）模型，对它们进行了灰色建模。获得了很高的模型精度，并根据模型式预测了涂有不同固定液色谱柱中的溶质ｉ的保留指数。通过预测值与文献值的比较，得到了良好的预测精度。     

Prediction of retention indices. V. Influence of electronic effects and column polarity on retention index

The retention index increment for addition of a methylene group to an analyte molecule is shown for 1-halo-n-alkanes to be different from 100 i.u., a value that is customarily assigned according to the current convention in retention index prediction. In temperature-programmed gas chromatography using linearly interpolated retention index I, a linear regression equation, I=AZ+(GRF), with the number of atoms (Z) in the molecule as variable can describe the retention of 16 homologous series of organic compounds on non-polar and polar columns with characteristic A (linear regression coefficient) and (GRF) (group retention factor) values. A molecular model of retention on the basis of electron density and electron density distribution relative to that of n-alkane is proposed. This model brings out the inter- and intramolecular electronic effects in the analyte molecule and its dipole-dipole interaction with the stationary liquid phases, as variations in the A value. The (GRF) value varies with the connectivity ability of a functional group for extended conjugation, substitution, etc., but is most influenced by hydrogen bonding (H-bonding) with the stationary liquid phase. One can estimate the sequence of elution of a mixture of organic compounds from any two of the three parameters on the right-hand side of the above equation or retrieve the retention indexes of an entire homologous series from its A and (GRF) values. The fact that each analyte molecule has its own A value on different columns makes column difference (deltaI) compound-specific rather than column-specific, a departure from previous assumptions.     

Prediction of equivalent chain lengths from two-dimensional fatty acid retention indices

A recently introduced two-dimensional fatty acid retention index system (2D-FARI) was used as basis for prediction of equivalent chain lengths (ECL) of fatty acid methyl esters (FAME) on a BPX-70 stationary phase. Models for the relationship between 2D-FARI data and ECL values of a calibration sample with 30 common fatty acids were established by a simple multivariate regression. The models were thereafter applied on 2D-FARI data for other FAMEs and used to predict the ECLs for these compounds. The 2D-FARI values for the fatty acids in the calibration sample are given by definition. Thus, the only information necessary to calculate the ECL value for a compound run under identical conditions as the calibration sample is the 2D-FARI values for the compound, which can be acquired from literature data. The method was validated with test sets analysed with different temperature and flow programs. ECLs of various marine FAME and trans isomers of Eicosapentaenoic and Docosahexaenoic acid were predicted with root mean squared error of prediction from 0.002 to 0.012 ECL units.     

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     

用胶束液相色谱模拟生物膜预测药物的醇-水分配系数log P

通过比较药物在不同类型表面活性剂的胶束液相色谱中的容量因子与药物的醇-水分配系数(log P)的相关性, 发现药物在非离子型表面活性剂Brij35作为流动相的胶束液相色谱中的容量因子与其文献中报道的log P间有一定的相关性, 进一步考察了Brij35浓度对药物保留的影响, 并建立了药物在不同浓度的Brij35胶束液相色谱中的容量因子与文献中log P的相关关系, 该方法简单、快速, 将为药物的疏水性评价提供重要的参考价值.     

Unified retention indices of hydrocarbons on BP-1 dimethylsiloxane stationary phase

Summary Gas chromatographic retention indices for 23 hydrocarbons were determined on BP-1 dimethylsiloxane between 45 and 65°C. From the data measured unified retention indices were calculated. These values agree well with the corresponding experimental values. The values of unified retention indices obtained on BP-1 are compared with those determined on OV-101 and on squalane. The differences between the unified and the experimental retention indices on BP-1, OV-101 and squalane columns are shown.     

Retention Characteristics of Peptides in RP-LC: Peptide Retention Prediction

A review of recent results of the use of chromatographic retention data in peptide identification and in the development of procedures for peptide retention prediction is presented. In recent years, reversed phase LC (RP-LC) has become an important tool in the separation of peptides in MS analysis. A challenging problem in a further expansion of RP-LC applications is the use of already available retention information for the identification purposes simultaneously with MS–MS identification. This overview focuses on the retention characteristics suggested in LC. We will discuss the application of the retention index concept in LC, which is widely used in GC to characterize retention of organic compounds. The use of retention indices as retention characteristics of analytes in LC was first suggested at the end of 1970s, however the application of retention indices is still somewhat rare today. There are several reasons for this. One is the relatively high sensitivity and variability of retention indices to the change of parameters of chromatographic systems. Another is the chemical restrictions in the search of the universal set of reference compounds suitable for retention scaling. Several methods were suggested for the prediction of the retention times of peptides. A frequently used approach is based on the additivity scheme and calculation of the elution time through the summation of retention coefficients of amino acids constituting the peptide. Such an approach allows fairly accurate predictions of the retention time of peptides made up of not more then 15–20 amino acid residues. Additional correction factors were suggested to improve predictions including corrections for the peptide length, peptide hydrophobicity, sequence of amino acids, etc. Suggested procedures are discussed in detail. Application of predicted retention times in the identification of peptides is considered. Current status of LC retention data collections is presented.