Interpreting the gas chromatographic retention of n-alkanes

Non-linear regressions were applied to n-alkanes retention data for the determination of gas hold-up in a preceding paper. It was found that at temperatures over 100 degrees C the reduced partial molar free energy of solution, deltaG/RT, tends to be negligible for the solute methane in poly(dimethylsiloxane) stationary phases. A consequence of interest can be inferred from this fact. The C-H bonds from terminal methyl groups of n-alkane solute molecules should not contribute significantly to deltaG/RT in these conditions. The analysis of data confirms that, within the chromatographic experimental error, the contributions of n-alkane end C-H bonds are also negligible in this temperature range. Consequently, the regression parameter that contains the phase ratio of the column only includes the gas hold-up as the accompanying factor.     

A New Equation for Solute Retention in Liquid Chromatography

In consideration of the adsorption of solvent, diluent and solute molecules on the surface of a stationaryphase, a new equation for solute retention in liquid chromatography is presented. This equation includesthree parameters: the displacement equilibrium constant (Ksd) between the solvent and diluent molecules onthe surface of the stationary phase, the total number(N) of the solvent and diluent molecules released fromthe stationary phase after one solute molecule being adsorbed, and the parameter (I) related to the thermody-namic equilibrium constant for the solute adsorption on the stationary phase. Over the whole concentrationrange of the solvent in the mobile phase, the experimental retention data can be well described by this equa-tion, parameters K~, N and I can be obtained by the regression analysis of the experimental retention data,and consequently the number of the solvent and the diluent molecules displaced by one solute molecule fromthe stationary phase can also be derived at different solvent concentrations in the mobile phase,     

Theoretical aspects of retention in overloaded columns

Issues regarding the concentration dependent solute distributions in overloaded chromatographic columns are discussed. Geometric simplicity of wall-coated capillary columns is taken as a reference and the discussion is developed using the example of the absorption isotherm of a solute having more affinity for itself than for the stationary phase. A retention function is deduced solving the equation of motion for the peak maximum, making some approximations. By means of this expression, the applicability of capillary gas chromatography (GC) as a technique for obtaining information on the sorption isotherm is analyzed.     

Sample Solvent as Analyte in High Performance Liquid Chromatography

Abstract

Solvents vary in their behavior in high performance liquid chromatography (HPLC). Water and methanol, among others, are widely used in the mobile phase as well as solvents for the solute. Few reports indicate that the solvent used for the solute can behave as an analyte. Normally, it is generally accepted that the solute solvent, a non-constituent of the mobile phase will be the first eluent. However, a solvent which is a component of the sample can show up as an unexpected peak with its own identity. This solvent may show a similar retention time as some of the unknown components of the sample. This indicates that in some cases the quantitative results may be the sum of the absorptivity of the solute and solvent used for the sample. It is assumed that some solvents show no absorption in the ultraviolet region at which the analysis is being conducted. Depending on the mobile phase composition some solvents can be detected at the wavelengts or wavelengths used for analysis. Water, ethylacetate, and methanol showed absorption at 210 nm when present in the sample being analyzed with a mobile phase of acetonitrile-methanol using a C₁₈ column. These solvents overlapped or showed retention times the same as estriol and testosterone.     

一个描述液相色谱体系中溶质进样量与保留值关系的方程

通过考虑溶质和溶剂在液相色谱体系中的相互作用,从理论上给出了一个描述溶质在液相色谱体系中进样量与保留值之间关系的方程。由方程可以证明,当进样量趋于零时,溶质的保留值为一定值,当进样量无限大时,溶质的保留值趋于零,且随着进样量的增加,溶质的保留值必然减小。通过方程的线性形式,可以获得两个描述色谱体系特征的重要参数：一个是溶质与固定相相互作用的平衡常数Ｋ,另一个是假想的分布在固定相表面上的活性点总数。     

Investigation of Quantitative Relationship between Adsorbed Amount of Solute and Solvent Concentration at Relatively High Solute Concentration by Frontal Analysis in RPLC

IntroductionManyfactors ,includingTraube’srule ,temperature ,solubilityofsolute ,inorganicsalt,thestructureofadsor bentandthenatureandconcentrationofthesolvent,affectsoluteadsorptioninaliquid solidsystem .1Amongthemsolventeffectsareofsignificance .Dispersionsofsilicaarestabilizedbyethanol.Itisreportedthatsolventfilmthick nessisgreatestinmixturesof 5 0percentethanolwithwa ter.2 TherepulsionbetweendispersedparticlesisabovetheeffectsfromthedoublelayerandvanderWaalsforces3andisattributedtostruc…     

Investigation of Quantitative Relationship between Adsorbed Amount of Solute and Solvent Concentration at Relatively High Solute Con－centration by Frontal Analysis in RPLC

In previous paper a new adsorption isotherm which relates the amount of solute absorbed to the solvent concentration is pro-posed and simplified, and it only can be used at lower solute concentration. In this article the scope of the new adsorption isotherm is extended and the expressions with three and four parameters are obtained. The equations with multi-parameters are valid when the adsorbed mounts are larger and show non-linear logarithmic relationships. Tests with a homologue of are-matic alcohols by frontal analysis in reversed phase liquid chro-matography demonstrate that the experimental results fit those equations well. In addition, the predicted values by the multi-parameters were found to fit the experimental values well also. The parameters have physical meaning only for the two-param-eter equation for the aromatic alcohols.     

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.     

在脂肪酸-水流动相中小分子在反相高效液相色谱中的保留表征

在反相液相色谱(RPLC)中用Snyder经验议程和计量置换保留模型(SDM-R)中的参数对深质为脂肪醇同系物,流动相为脂肪酸同系物时深质的保留行为进行了研究,结果表明用SDM-R参数具有明显的优越性,另外,由于用Snyder经验公式中二参数之间的作园无法准确求得斜率,且其不具有明确的物理意义,而由SDM-R二参数作图,不但能准确求得斜率j(与1mol溶剂和固相结合能有关的常数),而且j具有明确的物理意义并符合碳数规律,所以,参数j有可能用于RPLC中表征深剂强度,由此得出,在RPLC中,对同一置换剂面言,随同系物溶质的Z(1mol深剂化深质被深剂化固定相吸附时,从二接解面释放出的置换剂分子数)和logI(与1mol深质和固定相亲和势有关的常数)值增大,它们的保留时间也增大,对同一深质而言,随着在同系物置换剂中碳链的增长,Z和logI值的减小,它的保留时间也缩短,同时还可得出,随着同系物置换剂j值的减小,它们的洗脱能力也增强.     

色谱灰色数模的建立及其应用研究(Ⅰ)──连续GM(1,N)模型的建立及色谱保留行为的预测

应用灰色系统理论和最小二乘原理，推导出连续GM（1，N）模型，并将该模型用于高效液相色谱中溶质保留行为的预测．研究了烷烃、烷基苯及对羟基苯甲酸酯等化合物的容量因子与流动相组成、容量因子与碳数、保留时间与温度、保留时间与碳数的关系，并建立了相应的数学模型，经验证，预测结果良好．     

液相色谱梯度洗脱中的谱带压缩效应

谱带压缩效应是梯度洗脱区别于等度洗脱的重要特征。经典的范德姆特(van Deemter)理论塔板高度方程基于等度洗脱推导得到,因此不能对谱带压缩效应进行描述。在梯度洗脱中,保留因子(k)会随流动相组成(φ)的改变而发生变化,这就使得对梯度洗脱机理的研究要比等度洗脱复杂许多。该文对近10年来谱带压缩效应的研究进展,特别是溶剂强度模型(即描述ln k与φ关系的数学表达式)的非线性特征对谱带压缩因子(G)的影响进行了述评,指出为了更好地认识谱带压缩效应需要将这种非线性因素考虑在内。     

Contribution of Steric Repulsion to Retention on an Octadecylsiloxane-Bonded Silica Stationary Phase in Reversed-Phase Liquid Chromatography

Plots of the retention factor against mobile phase composition were used to organize a varied group of solutes into three categories according to their retention mechanism on an octadecylsilioxane-bonded silica stationary phase, Ascentis ^TM C18, with acetonitrile-water and methanol-water mobile phase compositions containing 10–70% (v/v) organic solvent. The solutes in category 1 could be fit to a general retention model, Eq. (1), and exhibited normal retention behavior for the full composition range. The solutes in category 2 exhibited normal retention behavior at high organic solvent compositions with a discontinuity at low organic solvent mobile phase compositions. The solutes in category 3 exhibited a pronounced step or plateau in the middle region of the retention plots with a retention mechanism similar to category 1 at mobile phase compositions after the discontinuity and a different retention mechanism before the discontinuity. Selecting solutes and appropriate composition ranges from the three categories where a single retention mechanism was operative allowed modeling of the experimental retention factors using the solvation parameter model. These models were then used to predict retention factors for solutes excluded from the models. The overwhelming number of residual values, here defined as the difference between experimental and model predicted retention factors for the excluded solutes, could be explained by contributions from steric repulsion. The latter defined as the inability of solutes to fully insert themselves into the solvated stationary phase because of their size or conformation. Steric repulsion resulted in a systematic reduction in retention compared with predicted values for the fully inserted solute. The bonding density of the stationary phase; the type and composition of the mobile phase; and the size, conformation, type and number of functional groups on the solute are shown to affect the contribution of steric repulsion to the retention mechanism.     

有机酸取代基和溶剂效应的静电模型

本文根据气相实验结果和以往的静电理论, 建立了一个描述有机酸电离取代基和溶剂效应的静电模型。该模型不仅满足气相边界条件, 而且其参数B具有较明确的物理意义, 即与酸官能团的溶剂化半径以及取代基与溶剂分子之间特殊相互作用有关, 因而是描述溶质-溶剂相互作用的重要参数。     

Retention index calculation without n-alkanes--the virtual carbon number

For the fast gas chromatographic identification of separated components the retention index is still one of the most often used tools, although mass spectrometry is available in almost all analytical laboratories. For the calculation of the retention indices it is not necessary to use n-alkanes or any other homologous series. If the analyte contains some compounds, not necessarily belonging to a homologous series, with well-known retention indices those compounds can be used as index references and the index of the other compounds can be calculated as is done using n-alkanes. The only difference is that instead of the carbon number of the n-alkanes, virtual carbon numbers of the reference compounds should be used. The method of calculation, and the effect of this method of calculation on the reproducibility are discussed in this paper, and analyses of some halogenated compounds and marjoram oil are used as experimental examples.     

Comparative study on Np(V) sorption on oxides of aluminum and silicon: effects of humic substance and carbonate in solution

It has been proven that all isotherm equations which include the expression 1-Theta contradict the exact Gibbs thermodynamics. These contradictions have been discussed in detail in the case of the Langmuir (L) equation applied to gas/solid (G/S), solid/liquid (S/L), and gas/liquid (G/L) interfaces. In G/S adsorption the L equation can theoretically be applied only at low equilibrium pressures on condition that vg > vs . vg is the molar volume of the adsorbed amount in the gas phase and vs is the same in the Gibbs phase. In S/L and G/L adsorption the L equation is practically applicable only in the domain of very low concentrations. The cause of these contradictions (inconsistencies) is that Gibbs thermodynamics takes excess adsorbed amounts into account; however, the L and other isotherm equations calculate with the absolute adsorbed amount. The two amounts may be practically equal to each other when the limiting conditions mentioned above are fulfilled. It is also discussed how these inconsistent isotherm equations can be transformed into consistent ones.     

推导一个表面张力等温式

本文试图从二点假设出发推导一个表面张力等温式：（１）溶液的表面张力是溶剂的表面张力和溶质的表面张力按其所占表面比率的相加,（２）溶质在表面的量与在体相中的量存在一个动态平衡。由此可导出如下表面张力等温式：σ＝σ０１－ａｃ１＋ｂｃ,其中常数ａ、ｂ具有物理意义。经实验数据的验证,该式在大多数情况下比Ｓｚｙｓｚｋｏｗｓｋｉ经验式更好     

Insights into the retention mechanism on an octadecylsiloxane-bonded silica stationary phase (HyPURITY C18) in reversed-phase liquid chromatography

Plots of the retention factor against mobile phase composition were used to organize a varied group of solutes into three categories according to their retention mechanism on an octadecylsiloxane-bonded silica stationary phase HyPURITY C18 with methanol-water and acetonitrile-water mobile phase compositions containing 10-70% (v/v) organic solvent. The solutes in category 1 could be fit to a general retention model, Eq. (2), and exhibited normal retention behavior for the full composition range. The solutes in category 2 exhibited normal retention behavior at high organic solvent composition with a discontinuity at low organic solvent compositions. The solutes in category 3 exhibited a pronounced step or plateau in the middle region of the retention plots with a retention mechanism similar to category 1 solutes at mobile phase compositions after the discontinuity and a different retention mechanism before the discontinuity. Selecting solutes and appropriate composition ranges from the three categories where a single retention mechanism was operative allowed modeling of the experimental retention factors using the solvation parameter model. These models were then used to predict retention factors for solutes not included in the models. The overwhelming number of residual values [log k (experimental) - log k (model predicted)] were negative and could be explained by contributions from steric repulsion, defined as the inability of the solute to insert itself fully into the stationary phase because of its bulkiness (i.e., volume and/or shape). Steric repulsion is shown to strongly depend on the mobile phase composition and was more significant for mobile phases with a low volume fraction of organic solvent in general and for mobile phases containing methanol rather than acetonitrile. For mobile phases containing less than about 20 % (v/v) organic solvent the mobile phase was unable to completely wet the stationary phase resulting in a significant change in the phase ratio and for acetonitrile (but less so methanol) changes in the solvation environment indicated by a discontinuity in the system maps.     

Retention studies of sulphated glycosaminoglycan disaccharides on porous graphitic carbon capillary columns

Summary The retention behaviour of highly polar and charged disaccharide isomers has been studied on porous graphitic carbon columns and experimental parameters were varied over wide a range, including concentration and type of mobile phase constituents as well as temperature. The hydrophilic and anionic sugar analytes were highly retained on this stationary phase compared to the octadecyl-derivatized silica packings more commonly used. E.g., an increase in retention with polarity of a solute and with temperature was observed. By isotherm measurements and nonlinear fitting of Langmuirian expressions to the experimental data the graphite surface appeared homogeneous with only one kind of active adsorption site for these kinds of compound which was furthermore supported by the linear Van't Hoff plots obtained by varying the temperature. The gain in free energy was found to be entropically driven after determination of the ΔH° and ΔS° values. However, enthalpy-entropy compensation behavior was not met.