Analysis of reaction by-products in ethylene oligomers by capillary gas chromatography

Summary A gas chromatographic method using temperature-programmed poly-dimethylsiloxane OV-101 and poly(pheny ether) (5 rings) capillary columns was developed to separate and quantify the reaction by-products in ethylene oligomers. It was found that the content of 2-alkyl-1-alkenes, 2-alkenes and n-alkanes in the C₈–C₁₈ -olefin fractions amounts to 29, 10 and 2.6% respectively.     

Theoretical background of multicomponent gas chromatography for high concentrations

Summary The paper presents a classification of various theoretical models for multicomponent gas chromatography at u=var for high concentrations of the mixture components with consideration of the major distinctive features of the interphase mass exchange inside the porous grains of the adsorbent and outside them within the porous medium of the chromatographic column. It has been shown that the conditions determining a given front behaviour of multicomponent gas mixture at u=var depend on: theoretical models for the kinetics and dynamics of adsorption; the values of mass exchange parameters inside the porous grains and in the porous medium. Formulae have been derived for calculating the values of the quantities characterising the frontal behaviour in the stage of adsorption and desorption of multicomponent mixture at u=var.     

Modelling of n-alkane retention and optimization of the conditions for analysis in gas adsorption chromatography with temperature programming

Summary The possibility of constructing a mathematical model of n-alkanes retention upon their separation by gas solid chromatography with temperature programming has been studied. The functional dependence between the number of the carbon atoms in n-alkane molecules, their retention in isothermal conditions and temperature of chromatographic column was used for constructing this mathematical model. It showed necessary to take into account the variance in the process temperature programming of both the carrier gas volume velocity and the column inlet pressure to obtain the adequate mathematical model of the chromatographic retention. With the use of the specific retention parameters of substances i.e. relative to the surface or the mass of sorbents the proposed model can be used not only for Silochrom C-80 but for the whole class of macroporous silica sorbents.     

Retention indices as identification tool in pyrolysis-capillary gas chromatography

Pyrolysis-capillary gas chromatography (Py-cGC) represents important method to identify the analytes in the mixture after thermal degradation. This combines high effective analyte separation on-line coupled with thermal degradation process that depends on analyte structure. System of retention indices has been used for identification of the analytes after on-line pyrolysis and chromatographic separation. The pyrolysate composition has been studied during thermal degradation of polymethylmethacrylate (PMMA) at different pyrolysis temperatures and chromatographic column conditions. Homologues series of n-alkanes have been used for calculation of pyrolysate Kováts retention indices (I) and compared with mass spectrometric (MS) data of pyrolysate model mixture. To identify PMMA thermal degradation products the high density polyethylene (HDPE) as additive standard producing triplets of the olefin homologous series during co-pyrolysis has been used. These homologous series enable to calculate programmed temperature retention indices (ITPGC) to identify the analytes present in the pyrolysate. Calculated I values were compared with published I values databases to identify analytes yielded at different pyrolysis temperatures.     

Identification of the chlorination products of aliphatic ketones by gas chromatography and gas chromatography/mass spectrometry

The differences in the gas chromatographic retention indices of the chlorination products of aliphatic ketones and parent carbonyl compounds (ΔRI) are constant, and their numeric values depend on the number and position of chlorine atoms in the molecule. A simplest version of an additive scheme for the evaluation of retention indexes is developed to identify the chloro derivatives of carbonyl compounds. The order of the chromatographic elution of diastereomeric α,α′-dichloro-k-alkanones (k > 2) is found.     

Semi-micro solvent extraction as a rapid and efficient preconcentration technique for the determination of n-alkanes in oil-contaiminated water and soil samples by capillary gas chromatography

Summary In this study, a rapid and efficient semi-micro extraction procedure is presented for the extraction of some higher n-alkanes from water and soil samples. In the case of water samples n-hexane was used as the organic phase in a phase volume ratio (volume of aqueous phase/volume of organic phase) higher than 285, while in the case of soil samples, extraction with n-hexane was carried out in the presence of an excess of 2 M NaCl solution. The extraction rate from soil samples is very high and is better than Soxhlet extraction, comparable with supercritical fluid extraction. High preconcentration factor in water samples allows the limits of detection to be in the ng.mL⁻¹ level with the use of gas chromatographic analysis. Flame ionization detector was used for monitoring the analytes. The obtained recoveries of all studied compounds from both water and soil samples are higher than 90%. This method was successfully used to determine some n-alkanes in municipal wastewater and contaminated soil.     

Solute-solvent interaction parameters by gas chromatography

Gas-liquid distribution coefficients at ideal dilution in non-volatile solvents can be measured by gas chromatography. The numerical value of a coefficient depends on the choice of the concentration unit in the solvent and in the gas phase. The relationships between different coefficients characterizing gas-liquid equilibria are discussed and summarized. Coefficients determined at several temperatures permit calculation of the standard chemical potential difference of the solute with the ideal gas phase as reference as a function of temperature, the g-SPOT. Following the proposal of Kirchhoff the latter can be formulated as an equation with three constants. As in the gas phase the molecules of the solute have no interacting partners, the three constants, deltaH, deltaS and deltaC, characterize the interaction between solvent and solute molecules. They will be called the "solute-solvent interaction parameters". In the same system the values of these parameters depend on the choice of the distribution coefficient. Five different distribution coefficients result five sets of interaction parameters. It is shown that conversion of a parameter set to another implies additive corrections independent of the nature of the solute. If g-SPOT-s are measured in a series of solvents, the data may be used to calculate the corresponding liquid-liquid partition coefficients by electing one of the solvents as reference (l-SPOT). The corresponding "relative interaction parameters" can be calculated by simple substraction. In a second chapter the precautions are summarized, necessary for gas chromatographic determination of distribution coefficients and examples are given for interaction parameters in different systems. It is concluded that there are significant differences between g-SPOT-s related to different distribution coefficients. On the other hand, differences between l-SPOT-s are negligible.     

Porous polymers coated with stationary phases. Contribution of the support to retention behaviour in gas chromatography

Summary The gas chromatographic behaviour of a series of compounds of different polarity on columns of Ethofat 60/25 on Chromosorb P HMDS (an inactive support) and Chromosorb 101 (an active support) was investigated. To observe how the support influences the chromatographic process, we separated the contribution to the retention volume of the interaction phenomena associated with the stationary phase, the solid support and the corresponding interfaces. In some cases the Chromosorb 101 exerts an influence on the process even with high percentages (>10%) of stationary phase. The values of the experimental partition constants of a series of compounds and the factors that contribute to adsorption on these columns are calculated.     

New liquid nitrogen cryogenic modulator for comprehensive two-dimensional gas chromatography

A new liquid nitrogen (LN2) jet-based thermal modulator for performing comprehensive two-dimensional (2D) gas chromatographic (GC x GC) separations has been designed and constructed. Temperature measurements of the trapping zone, a segment of uncoated fused silica capillary, show that it can be cooled to -196 degrees C in about 300 ms. A film of liquid nitrogen develops on the outside of the trapping capillary even when the oven temperature is in excess of 200 degrees C. Compounds as volatile as propane can be trapped by the modulator and held for periods of at least 1 min without breakthrough. The peak widths for n-alkanes are on the order of 80 ms at half height after passing through an 80cm second dimension column. Repeated analysis of gasoline demonstrated excellent run-to-run reproducibility of the system.     

Relationship between carbon number and peak width in gas chromatography

When the unadjusted width (wR) is divided into hold-up width (wM) and adjusted width (w'R), a linear relationship between the natural logarithm of the width factor (p') and the carbon number (n) is demonstrated and can be described by: In p' = a + bn where a and b are constants. The coefficients of variance (r2) for n-alkanes, fatty acid methyl esters and fatty alcohols are 0.9997, 0.9996 and 0.9998, respectively.     

A polar macroporous sorbent for gas chromatography

Summary The gas chromatographic behaviour of crosslinked macroporous 2-hydroxyethyl methacrylate grafted with 2-methyloxazoline was investigated and the range of applicability of the copolymer was assessed. Retention index values were measured on this copolymer and compared with those obtained on the terpolymer 2-hydroxyethyl methacrylate-ethylene dimethacrylate-acrylonitrile. Rohrschneider constants and efficiency data for selected sorbates are given. The use of this type of polymer in the separation of various types of compounds is documented by various examples.     

Dynamic interconversion of chiral oxime compounds in gas chromatography

A number of chiral oxime compounds have been synthesised and their gas chromatographic analysis on both a polyethelene glycol phase column and two chiral column phases was investigated. Of particular interest to this work is the observation of dynamic interconversion behaviour, both in a single dimensional analysis, and by using comprehensive two-dimensional gas chromatography (GC × GC). A number of non-chiral compounds were studied as a means to understand the nature of the behaviour observed. As expected, the achiral compound on both the wax column and the chiral column generated two isomeric compounds—the E and Z isomers. On the wax column, a characteristic interconversion zone representing the dynamic process was observed, with extent of interconversion dependent on the conditions used. For the chiral compounds, two isomers and the interconversion zone were exhibited on the wax column, however on the chiral column 4 isomeric peaks were found—the (R) and (S) enantiomers of each of the E and Z isomers. In the case of the chiral column, the extent of interconversion was negligible, and this appears to correlate with the use of low polarity columns. In order to encourage dynamic interconversion, a polyethylene glycol column was coupled to the chiral column, by placing it either before or after the chiral column. In this case a monitor detector was employed between the two columns in order to isolate the effects of the first column from the behaviour on the second. In a further study, the most appropriate column arrangement from the earlier study was placed into a comprehensive two-dimensional gas chromatography instrument, with a wax-phase column in the second dimension. The unique location of peaks for each of the molecules in 2D space and patterns for the interconversion processes is interpreted phenomenologically.     

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.     

Solvation enthalpies and heat capacities of n-alkanes in four polymer phases by capillary gas chromatography

Molar solvation enthalpy (deltasol H(o)298) and molar heat capacity changes (deltasol C(o)p) were determined by gas chromatography for the C6-C12 n-alkanes on four preferred stationary phases (100% polydimethyl siloxane, 50% diphenyl-50% dimethyl polysiloxane, 50% trifluoropropyl methylsiloxane, and polyethylene glycol) in commercial FSOT. Statistical evaluation indicated the temperature independence of deltasol C(o)p in the range 303-393 K. Deltasol H(o)298 depends linearly on the number of carbon atoms in the n-alkanes, but no linearity could be established for deltasol C(o)p of higher homologues on polar columns, which may be due to a more ordered state on the liquid phase. The homologues for which a linear temperature dependence exists demonstrated that deltasol C(o)p is related linearly to the van der Waals volume and the temperature derivative of the density of the stationary phase. The results are consistent with a simple physical explanation at the molecular level.     

国内气相色谱近年的进展

本文对近三年国内学者在气相色谱方面的研究和应用作了综述 ,国内学者的研究基本和国际上气相色谱方面的研究类似 ,在全二维气相色谱、快速气相色谱 ,微型气相色谱仪、新型气相色谱固定相和色谱柱的溶胶 凝胶涂渍技术领域的研究方面作出了贡献。有关气相色谱的应用研究中 ,介绍了大量在药物分析、食品分析、环境分析、石油和石化分析和化工产品及高聚物分析等领域中应用的题目和摘要     

顶空气相色谱法快速测定污水中氨氮含量

研究了一种基于化学反应型顶空气相色谱的快速检测污水氨氮含量新方法。首先在顶空瓶中用过量的甲醛与NH₄⁺反应产生等物质的量的强酸，再用NaHCO₃与新生成的酸反应产生CO₂，最后经过气相色谱定量CO₂，间接得出样品中氨氮的含量。结果表明，氨氮含量与气相色谱信号值之间存在良好的线性相关性，相关系数为0.999，定量限为0.786 mg/L（以N的质量浓度计），相对标准偏差小于2%，加标回收率在95%~105%之间。与国标纳氏试剂法相比，该方法测定结果的相对偏差在±5%以内。该方法检测效率高，适用于快速批量检测污水中氨氮的含量。     

Investigation of the surface heterogeneity of solids from reversed-flow inverse gas chromatography

In the present work, the novel methodology of the inverse gas chromatographic technique of reversed-flow gas chromatography (RF-GC) was applied to the well-studied catalytic oxidation of carbon monoxide over silica supported Pt, Rh and Pt-Rh alloy catalysts. Adsorption energies, local isotherms, local monolayer capacities, surface diffusion coefficients, lateral interaction energies and energy distribution functions are simultaneously determined in a single experiment. The variation of the determined physicochemical parameters against the nature of the studied catalysts (Pt content) is consistent with the observed catalytic activity. The energy distribution functions, estimated by means of RF-GC, give useful information about the "topography" and the nature of the active sites on the catalyst surface, similar to those of experimental techniques, such as Thermal Desorption Spectroscopy studies of the adsorption of CO on group VIII noble metal surfaces. The experimentally found results explain the superior activity of Pt0.25 + Rh0.75 alloy, in comparison to that of the pure Pt and Rh catalysts.     

Application of target factor analysis of gas chromatography. Reproduction,prediction and classification

Summary Using the method of target factor analysis (TFA) described by Malinowski and Howery a computer program has been developed to study different sets of gas chromatographic retention data. Physico-chemical, topological and uniqueness parameters have been found to be basic factors to describe solute behaviour problems. Factor analytical solutions have been used to reproduce the data matrices and to make predictions based on best sets of basic factors. The mean absolute error in the reproduction step is between 1.72 retention index units (i.u.) for a relatively simple matrix consisting of retention indices of alcohols and 7.36 i.u. for a combined data matrix of alcohol, aldehyde and ketone retention indices. TFA has also been used to classify solutes based on their retention behaviour. Alkanes have been classified from cycloalkanes, alkanes from alkenes, and alcohols from aldehydes and ketones using only their retention data and a special kind of uniqueness vector.