Prediction of gas chromatographic retention indices of linear,branched, and cyclic alkanes from their physicochemical properties

Kováts retention indices for a series of linear, branched, and cyclic alkanes on squalane at any temperature, and on other stationary phases of different polarity at a given temperature, are related to physicochemical properties of the solutes, such as boiling point and molar refraction, by multiple regression analysis. The equations found permit calculation of the Kováts retention index for all alkanes, with standard deviations close to experimental error. The same equations can also be used for calculating the physicochemical parameters they contain.     

Prediction of the Retention Indices of Benzene and Methylbenzenes Based on their Retention Data-Physico Chemical Properties Relationship

Retention indices of benzene and 12 methylbenzenes were determined on two capillary columns coated with HP-5 and ZB-WAX at 100 °C. Comparison with the retention indices published by other researchers yields a good accordance. The relationship between the retention index values and several physico-chemical properties of these solutes (freezing point, boiling point, molar volume, van der Waals volume, molar refraction, refraction index, connectivity index, dipole moment and vapor pressure) was investigated by multiple linear regression analysis (MLRA). MLRA equations of seven and eight variables predicted retention indices with mean deviations of 0.66–0.97 index-units in squalane and Carbowax 20 M, and 0.76–1.25 index-units in HP-5 and ZB-WAX. PCA was applied to the system of solutes retention data and stationary phases used, giving results consistent with those of MLR analysis. Samples are scattered or joined by curves. An arrangement of the stationary phases according to the polarity is observed. TCEP was one outlier since it is much more polar than any others.Two groups of variables can be envisaged, one of which is close to the origin of the coordinates, the physicochemical properties that depend linearly. Solute vapor pressure and molar volumes were outliers.     

Applicability of linear and nonlinear retention‐time models for reversed‐phase liquid chromatography separations of small molecules,peptides, and intact proteins

The applicability and predictive properties of the linear solvent strength model and two nonlinear retention‐time models, i.e., the quadratic model and the Neue model, were assessed for the separation of small molecules (phenol derivatives), peptides, and intact proteins. Retention‐time measurements were conducted in isocratic mode and gradient mode applying different gradient times and elution‐strength combinations. The quadratic model provided the most accurate retention‐factor predictions for small molecules (average absolute prediction error of 1.5%) and peptides separations (with a prediction error of 2.3%). An advantage of the Neue model is that it can provide accurate predictions based on only three gradient scouting runs, making tedious isocratic retention‐time measurements obsolete. For peptides, the use of gradient scouting runs in combination with the Neue model resulted in better prediction errors (<2.2%) compared to the use of isocratic runs. The applicability of the quadratic model is limited due to a complex combination of error and exponential functions. For protein separations, only a small elution window could be applied, which is due to the strong effect of the content of organic modifier on retention. Hence, the linear retention‐time behavior of intact proteins is well described by the linear solvent strength model. Prediction errors using gradient scouting runs were significantly lower (2.2%) than when using isocratic scouting runs (3.2%).     

利用HCI程序确定儿茶酚化合物的反相高效液相色谱最佳分离条件

有效地确定了反相高效液相色谱分离儿茶酚化合物的最佳条件。在水和甲醇的二元流动相里分别加入乙酸缓冲液,利用基于ln k=ln kw +SF, k=A+B/F, ln k=L+MF+NF2 (F是流动相中有机物甲醇的体积分数)等保留因子的一次或二次方程式的塔板理论得到色谱分离结果;利用保留原理得到等度和梯度洗脱的最佳条件。得出最佳初始流动相是含0.1%乙酸的水和含0.1%乙酸的甲醇(体积比为75∶25)的混合溶液;梯度洗脱条件：初始流动相保持15 min,然后用10 min的时间将上述二元流动相的体积比线性变换成50∶50,直到完成全部分离。通过实验证实该计算结果与实验值相近。     

Prediction of retention indices. VI: Isothermal and temperature-programmed retention indices,methylene value,functionality constant,electronic and steric effects

The Kováts retention index system with n-alkanes as reference standards has properties not fully explored when single, isolated or stand-alone analytes are analyzed by isothermal gas chromatography. When a homologous series of analytes are analyzed by either linear or non-linear temperature-programmed gas chromatography, the retention data of the entire series can be treated systematically to produce an I vs. Z plot that is linear, thereby giving insight into the relationship between chemical structure and retention index. Dead time t_M is both instrument and temperature dependent. With no dead time t_M adjustment, the retention indices of analytes calculated from experimental retention times by the method of either linear or logarithmic interpolation give statistically identical values. Linear regression analysis of the data shows the slope as methylene value (A) and intercept as functionality constant or group retention factor (GRF) of the homologous series. The A and (GRF) values vary with chemical structures, intermolecular electronic and steric interactions, and polarity of column liquid phases, and can link gas chromatographic retention index to chemical structure. Examples of the influence of molecular electronic effects and steric effects on retention index are given and discussed.     

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.     

Retention index,connectivity index and Van der Waals' volume of alkanes (GLC)

Summary The relationships between retention index and Van der Waals' volume and between retention index and connectivity index have been studied for 58 different alkanes (C₁–C₉) on squalane. The correlation coefficient for the former is higher than for the latter. From these equations a linear relationship between Van der Waals' volume and connectivity index is obtained which indicates that the two parameters are equivalent. A simple method for calculating the Van der Waals' volume of alkanes is proposed.     

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.     

Contribution to linearly programmed temperature gas chromatography. Further application of the Van den Dool-Kratz equation, and a new utilization of the Sadtler retention index library

Programmed temperature retention indices (PTRIs) calculated according to the equations of Van den Dool and Kratz, Golovnya and Uraletz, and Erdey et al. (also referred to as Antoine's integrated equation) are used in this work. Precalculation of isotherm retention indices from the results of a linearly programmed temperature GC is also presented. Deviations between experimental and calculated isothermal retention indices are below 2 retention index units. A relative "volatility" retention index is defined, as a function of the "volatilities"of the solute and the bracket reference n-alkanes. The comparison of the "volatility" retention indices with the PTRIs obtained with the other above equations shows absolute deviations of up to 4 retention index units. Based on an earlier "equivalent" temperature concept and on Tekler's proviso, a novel way for the utilization of Sadtler's retention index database, which takes advantage of the 3 data supplied by the library, is proposed.     

Gas chromatographic retention indices for alkenes on OV-101 and squalane capillary columns

Summary Gas chromatographic retention indices for 47 C₅ to C₈ alkenes on OV-101 dimethylsilicone at 50 and 70 °C were determined within a standard deviation of 0.3i.u. Data obtained on OV-101 are compared with those measured on squalane. The comparison of the retention index and dl/dT values from both columns is presented as linear regression equations with correlation coefficients greater than 0.98.     

饱和链烃图特征和热力学性质

本文讨论饱和链烃分子异构化和热力学性质之间的关系.首先利用图论方法引进描写分子支化度的α_N指数,该指数能分辨仅有微小结构差异的饱和链烃分子,给出支化结构的合理描述.在此基础上,得到了利用分子图特征参量计算饱和链烃热力学量(如:沸点、密度、生成焓、原子生成热、折射率和色谱保留指数等)的统一表达式.     

Comparison of linear and non-linear equations for univariate calibration

Univariate data accumulated for the purpose of calibration of chromatographic and spectroscopic methods often exhibit slight but definite curvature. In this paper the performance of a non-linear calibration equation with the capacity to account empirically for the curvature, y = a + bx(m), (m not equal to 1) is compared with the commonly used linear equation, y = a + bx, as well as the quadratic equation, y = a + bx + cx2. All equations were applied to high quality HPLC calibration data using unweighted least squares. Parameter estimates and their standard errors were calculated for each equation. Standard errors and 95% prediction intervals in analyte concentrations were estimated with the aid of the fitted equations and their respective covariance matrices. Results indicate that the non-linear and quadratic equations each provide a better fit than the linear equation to the data considered here, as judged by the Akaikes information criterion (AIC), the adjusted coefficient of multiple determination, the magnitude and scatter of residuals, standard errors in estimated analyte concentrations and lack of fit analysis of variance (ANOVA). While the difference between the equations y = a + bx + cx2 and y = a + bx(m) as judged by the same criteria is more marginal, this work suggests that the non-linear calibration equation should be considered when a curve is required to be fitted to low noise calibration data which exhibit slight curvature.     

The temperature dependence of retention indices in gas chromatography

Summary A linear dependence of (T–T₁)/[1(T)–1(T₁)] on temperature (considering the retention index 1(T₁) at temperature T₁ as a standard value) is derived. Both ther retention index at an assigned temperature and the temperature dependence of the retention index can be calculated from retention data measured at two temperature-programing rates.     

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。