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

The simultaneous temperature and inlet pressure programming (TPP) in gas chromatographic analysis decreases the retention time and the maximum value of temperature required for the elution of high boiling substances. Therefore, compounds sensitive to thermal degradation can be better analysed and column ageing is reduced. However, the empirical choice of proper analysis conditions requires many preliminary runs; this paper describes a procedure for the theoretical prediction of retention times in TPP using few preliminary runs carried out in isobaric and isothermal conditions. The used program permits the prediction of the retention times of the compounds analysed with any different TPP run carried out within the temperature and pressure ranges investigated with the preliminary runs. The influence of various analytical parameters on the accuracy of the prediction values was investigated. The proposed model also predicts the relative position in the chromatogram of closely eluting peaks and the possible coincidence of retention times or inversion of the elution order with changing temperature. It is also possible to foresee the analytical conditions, which offer a baseline separation of all of the peaks of the sample.     

Effect of injected sample amount on the shape of chromatographic peaks under condition of linear partition isotherm

In a previous paper a model function was tested in order to approximate the peak shape obtained on non-polar column by injecting different compounds. The simulation of the symmetrical or non-symmetrical shape of gas chromatographic peaks was satisfactory. In this paper, the influence of the amount of injected substance was investigated at different values of inlet pressure and carrier gas velocity, in order to evaluate the relative contribution to the total peak area and shape of the symmetrical distribution due to partition phenomena and of the non-symmetrical and tailing distribution due to adsorption-desorption kinetics. The effect of the molecular mass and of the chain length of compounds belonging to the homologous series of 1-alcohols and n-alkanes on the adsorption phenomena was evaluated.     

C18柱串联冠醚手性柱高效分离3种芳香族氨基酸对映体

将C18柱与手性冠醚柱串联,建立了一种反相高效液相色谱法用于3种芳香族氨基酸对映体同时拆分的方法.考察了反相色谱流动相的组成、pH值、柱温、流速对对映体拆分的影响.实验结果表明,当流动相为HClO4-乙睛溶液(86:14,V/V,pH 2.0)、柱温20℃、流速0.4 mL/min时,3种氨基酸对映体可获得基线分离.进一步对比了C18柱、冠醚手性柱和串联顺序不同的4种分离模式,结果表明,C18柱不能拆分氨基酸对映体,仅能分离不同种类氨基酸;冠醚手性柱可分离氨基酸映体,但不同种类氨基酸色谱峰出现重叠;串联模式能实现3种氨基酸对映体的基线分离,实现双柱优势互补,而串联顺序对分离影响不大,仅影响色谱峰的峰形.     

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.     

Diffusion coefficients from stopped-flow gas chromatography

Summary Mutual diffusion coefficients of two gases A and B can be determined in an empty gas chromatographic column by letting component B enter at an intermediate position of the column and continuously flow through a part only of it, as a carrier gas. The other component A is injected in a small amount instantaneously at the closed end of the column with the detector placed at the other end. By repeatedly stopping and then restoring after a short time the flow of B, narrow extra peaks are produced on the chromatographic elution curve, owing to diffusion of A into B. An equation is derived giving the area under the curve of each stop-peak as a function of time of the corresponding stop. Plotting the experimental data according to this equation permits the determination of the diffusion coefficient of A into B. Some results obtained by this method show negligible variations with changes in the experimental parameters.     

The simultaneous use of solute vapor pressure and geometry in multidimensional capillary gas chromatographic separations of polychlorinated biphenyls

The separation of coeluting congeners of Aroclor 1242, 1254, and 1260 on a DB-1 (low polarity) capillary gas chromatographic (GC) column is achieved when cuts of those peaks are transferred onto a smectic liquid crystalline column, which is commercially available. This procedure requires the use of a gas chromatograph equipped with two independent ovens for optimizing the temperature conditions of each column. Excellent base line separations are achieved on multicuts, up to six, of the same injection. This unique multidimensional/multimodal capillary GC system, in which the two separation modes of vapor pressure and molecule geometry are employed, is a powerful analytical technique for the separation of complex organic mixtures.     

Comparison of different types of stationary phases for the analysis of soy isoflavones by HPLC

Nowadays, there are new technologies in high-performance liquid chromatography columns available enabling faster and more efficient separations. In this work, we compared three different types of columns for the analysis of main soy isoflavones. The evaluated columns were a conventional reverse phase particle column, a fused-core particle column, and a monolithic column. The comparison was in terms of chromatographic parameters such as resolution, asymmetry, number of theoretical plates, variability of retention time, and peak width. The lower column pressure was provided by the monolithic column, although lower chromatographic performance was achieved. Conventional and fused-core particle columns presented similar pressure. Results also indicate that direct transfer between particle and monolithic columns is not possible requiring adjustment of conditions and a different method optimization strategy. The best chromatographic performance and separation speed were observed for the fused-core particle column. Also, the effect of sample solvent on the separation and peak shape was evaluated and indicated that monolithic column is the most affected especially when using higher concentrations of acetonitrile or ethanol. Sample solvent that showed the lowest effect on the chromatographic performance of the columns was methanol. Overall evaluation of methanol and acetonitrile as mobile phase for the separation of isoflavones indicated higher chromatographic performance of acetonitrile, although methanol may be an attractive alternative. Using acetonitrile as mobile phase resulted in faster, higher resolution, narrower, and more symmetric peaks than methanol with all columns. It also generated the lower column pressure and flatter pressure profile due to mobile phase changes, and therefore, it presents a higher potential to be explored for the development of faster separation methods.     

Detection of an impurity (1%) at low resolution (0.25)

The “Distribution Function Method”, which makes possible the measurement of systematic variations in the shape of signals, has been experimentally tested on the detection of two visually confounded gas chromatographic peaks, irrespective of the estimation of the resolution or the concentration ratio. Two detection procedures are suggested. The first compares directly, under the same experimental conditions, the shape of a pure peak and that of a peak suspected of contamination. The second makes use of variations of column temperature and gas flow rates to distinguish the behaviour of a pure substance from that of a contaminated substance. A marking procedure for the shape of a pure peak is then set up to dispense with the need of a reference standard. In practice, the two procedures make possible the detection of heptane as an impurity in n-hexane at levels of one percent and under experimental conditions artificially chosen so as to make the resolution inferior to 0.25.     

Optimizing the gas chromatographic separation and detection of polychlorinated biphenyls by use of electronic pressure programming and experimental design

The applicability of electronic pressure control and Taguchi L₂₇ experimental design to the optimization of the gas chromatographic separation and detection of polychlorinated biphenyls has been evaluated. The influence of several experimental variables, column temperature program, carrier gas pressure program, on-column injector temperature program, and make-up gas pressure program, was studied using analysis of variance. Simultaneous optimization of sample introduction, column efficiency, and detector performance could be achieved without compromising system performance. The relationships between system performance and experimental variables were established using regression analysis. Agreement between the simulated and experimental results obtained using suggested optimum conditions demonstrated the applicability of the technique developed in this study. The improvement achieved in the chromatographic separation of PCBs is presented.     

Prediction of the plate height of capillary columns operated at any inlet pressure of the carrier gas by using few retention data measured under isobaric conditions

Programming inlet pressure in gas chromatography permits to decrease the analysis time without changing the elution order of compounds of different polarity whose relative retention changes with changing temperature. The choice of the best values of the inlet pressure and flow-rate of the carrier gas often requires many preliminary analyses with different parameters to be carried out. A method for the prediction of the separation by starting from few experimental data measured in isothermal and isobaric conditions decreases the time required for the optimisation of the analysis. The efficiency of the separation depends on the change of the theoretical plate height at various pressures and temperatures, due to pressure drop along the column. By calculation of the diffusion coefficients of the analysed compounds into the mobile and stationary phase it is possible to evaluate the column efficiency and predict the number of theoretical plates at any inlet pressure. A procedure for the prediction of the plate height of a capillary column at any inlet pressure of the carrier gas and column temperature by using retention data of polar and non-polar compounds (1-alcohols and linear alkanes) obtained in few isobaric runs is described.     

乙醇气体标准物质包装容器的选择及分析条件的优化

采用不同分装方法考察了低含量乙醇气体在国内现有气瓶中的吸附情况,通过对比选择出最适合的乙醇气体的气瓶,然后使用高浓度乙醇气体对其进行预饱和,最后再次通过分装实验来验证其吸附情况.此外,通过选择不同的色谱柱、柱温、柱流速和分流比等条件,对乙醇分析的色谱条件进行了优化.     

Prediction of theoretical plate number in isothermal gas chromatographic analysis on capillary columns

A method for the prediction of the efficiency of gas chromatographic analysis in isothermal conditions by using experimental data of 1-alcohols and n-alkanes measured on capillary columns filled with polar and non-polar stationary phases in isothermal and isobaric conditions is described. The theoretical plate height trend indicates the change of separation efficiency as a function of inlet pressure and column temperature. By evaluating the variation of the diffusion coefficients of the analysed compounds into the mobile and stationary phase it is possible to predict the column efficiency and the number of theoretical plates at any temperature.     

The effects of molecular collisions between the mobile phase and the solute in gas–solid chromatography

In chromatographic processes, molecular collisions between the mobile phase and the solute result in the transfer of kinetic energy. Based on these interactions, the relationship between the gauge pressure of the carrier gas at the column inlet and the partition frequency of the solute is derived; consequently, the relationship between the column temperature and partition frequency can be obtained. These relationships have been experimentally validated. The change in the peak shape described herein has been successfully explained using this relationship: the partition frequency was calculated from the theoretical plate number of a tailing peak. We propose a new mechanism for peak tailing using plate theory, which states that as the number of plates increases, the symmetry of the peak increases.     

Enhancement of Signal-to-Noise Ratios in Capillary Gas Chromatography by Using a Longitudinally Modulated Cryogenic System

A Longitudinally Modulated Cryogenic System (LMCS) was evaluated for its use in detection enhancement in capillary gas chromatography. The mechanism for chromatographic re-elution for the LMCS is substantially different to other cryogenic devices. The cooled region of the capillary column in which chromatographic bands may be focused is heated by the surrounding oven temperature either by moving the trap along the column, or by moving the column out of the trap. By continually modulating the LMCS at the detector end of the capillary column, signal-to-noise ratios of routine chromatograms can be readily increased by a factor of ten, thus enhancing chromatographic detection. Base widths of peaks, which are often about 2–3 s or more can be easily reduced to 0.3 s when the LMCS is employed in the detection enhancement mode, thus offering a simple avenue to improved peak height sensitivity in capillary gas chromatography.     

色谱柱选择对23种防腐剂测定结果的影响

以化妆品中23种防腐剂检测方法为例,探讨色谱柱选择对液相色谱方法测定结果的影响。参照《化妆品安全技术规范》甲基异噻唑啉酮等23个组分的检验方法,在2台不同的高效液相色谱仪上用15款不同品牌、型号的C₁₈色谱柱检测23种防腐剂,计算色谱峰的理论塔板数和分离度,对23种组分的分离效果进行分析,并应用USP (United States Pharmacopeia)数据库和PQRI (Product Quality Research Institute)数据库等2种等效色谱柱选择方法,对不同色谱柱的分离效果及等效性进行评价和预测。实验结果表明,15款色谱柱对23种防腐剂的分离效果差异显著,仅有2款色谱柱可以实现23种组分的完全分离。USP和PQRI数据库中2种等效色谱柱选择方法均无法预测出合适的等效色谱柱,对23种防腐剂的液相色谱分析参考价值均较小。色谱柱是影响23种防腐剂液相色谱法测定结果准确性的关键因素,有关实验室在应用该方法时,应考虑色谱柱选择性差异。化妆品基质复杂,如何在现有研究成果的基础上,开发色谱柱的筛选和预测评价体系,进而指导实际样品的分离是下一步研究的重点、难点。建议有关部门在制修订检测方法时,注重色谱柱的耐用性考察,完善系统适应性指标,细化色谱柱分类和增加描述信息,指导色谱柱的合理选择,从而规避由于色谱柱使用过程中选择依据缺失而导致测定结果不准确的风险。     

Configuration and optimization of semiconductor gas sensors as gas chromatographic detectors

A tin oxide, gas-sensitive semiconductor sensor was configured as a gas chromatographic detector and its performance was optimized. Two sensor housings were compared but little difference was found in performance. The flow rate and temperature of the column and the internal heater voltage of the sensor affected both the sensitivity and peak shape. The temperature of the sensor surface was the most critical parameter. Optimal conditions for the gas chromatographic detection of a mixture of alkanes (C₁–C₅) and hydrogen were identified by using the simplex technique. The detection limit for hydrogen was improved by a factor of five to 20 ppb (v/v), illustrating the value of optimization and the excellent sensitivity of the detector. It is concluded that semiconductor gas sensors offer significant advantages as gas chromatographic detectors for the determination of reducing gases.     

Applications of a new vapour-phase pyrolyser

The application of a device for the post-column vapour-phase pyrolysis of compounds after gas chromatographic separation is described. The system proposed permits a mixture to be separated, pyrolysis to be carried out and pyrograms to be obtained in the chromatograph with one flame ionization detector. The reproducibility of the retention time and the intensity of the pyrogram peaks were studied. Some examples of applications to the identification of unknown components in a single gas chromatographic analysis are given.     

Effect of the pressure of the carrier gas on the parameters of the Van Deemter equation for monolithic silica gel gas chromatography columns

The chromatographic properties of monolithic capillary silica gel columns for gas chromatography were examined with the use of four different variants of the Van Deemter model. The corresponding experimental curves were measured for the elution of light hydrocarbons with the helium carrier gas in the isothermal mode at 60°C. Despite the models tested are based on different mechanisms of the smearing of chromatographic peaks, the values of the Van Deemter equation parameters proved to be very close to each other for three of the four models. All models yielded negative values of the parameter A. Physically reasonable values of the parameters of the Van Deemter equation were obtained only for the Giddings model, which takes into account the pressure drop across the column. At the same time, this model overestimated the contribution from diffusional smearing (parameter B). It was concluded that none of the models tested adequately described the chromatographic properties of monolithic capillary columns for gas chromatography.     

Gas Chromatographic Separation of Carbonyl Compounds as Their 2,4-dinitrophenylhydrazones Using Glass Capillary Columns

The gas chromatographic separation of 22 carbonyl compounds as their 2,4-dinitrophenylhydrazones was investigated using glass capillary columns. Complete separation of the 2,4-dinitrophenylhydrazones of ten aliphatic aldehydes, eight aliphatic ketones and four aromatic aldehydes was obtained, except for the derivatives of n-valeraldehyde and isobutyl methyl ketone, whose peaks overlapped, and the o- and m-tolualdehyde derivatives, which were poorly separated. The optimum conditions were as follows: stationary phase, SF-96; column size, 20 m × 0.25 mm I.D. ; column temperature, 200-240°; injection and detector temperatures, 280-290°; carrier gas flow-rate, helium 1.0-1.2 ml/min or nitrogen 1.1-1.2 ml/min. The method was applied to the analysis of aliphatic carbonyl compounds in car exhaust fumes and cigarette smoke.     

Prediction of the separation number of capillary columns in programmed temperature gas chromatographic analysis

The efficiency of capillary columns in programmed temperature analysis can be evaluated by calculation of the separation number (“Trennzahl”). A procedure for the prediction of this parameter at various initial temperatures, carrier gas pressures and heating rates, by using as the starting data the retention times and the peak widths obtained in some isobaric and isothermal runs is described. An equation is proposed that permits to obtain the values of the peak width at half height in any isothermal and linearly programmed temperature gas chromatographic run and therefore to calculate the separation number value. The effect on this parameter of the column polarity was investigated by using polar and non-polar compounds (n-alkanes and 1-alcohols).