Comparison of the elution curves of pure components and their mixture and prediction of elution curves by a theoretical plate model in preparative gas-liquid chromatography

Summary Two close-boiling materials, diethyl ether (DEE) and dichloromethane (DCM), were separated to investigate the effects of the pure components and the mixture on elution in preparative gas-liquid chromatography.Nitrogen was used as the carrier gas, and the chromatographic column (1 cm I.D. and 0.75m length) was packed with Chromosorb A with different quantities of dinonyl phthalate stationary phase and particle sizes. Below ca. 7% (by wt.) of feed concentration, the experimental elution curves of pure DEE and DCM were almost the same as those of the mixture, and the theoretical plate model can be used successfully to predict the elution curves.     

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.     

Resolution optimization in micellar electrokinetic chromatography using two different overlapping resolution mapping schemes

Summary An appropriate optimization strategy should be able to find good separation conditions in the shortest time with only a few experiments, to assure the baseline-separation of all components. In this paper, the use of two optimization schemes has been studied using 10 substituted benzene as model compounds. The triangular overlapping resolution mapping (ORM) scheme and cubic ORM scheme were compared to optimize the three relevant parameters-the concentration of surfactant, the concentration of HP-β-cyclodextrin and pH. By conducting experiments that span the maximum working range of the system, overall optimum conditions for separation can be obtained. To confirm the validity of the two ORM schemes, additional experiments using the predicted optimum conditions were performed. Finally, 10 substituted benzene compounds were completely separated within 20 min by both sets of optimized conditions.     

Investigation of the role of the carrier gas in capillary gas-solid chromatography

Nonideal interactions of the sorbate and the carrier gas and adsorption of the sorbate on the adsorbent surface in capillary gas-solid chromatography were studied. Chromatographic retention was found to be largely determined by adsorption processes. With respect to the retention coefficients (capacity factors) of a sorbate (k) with different carrier gases (P₁ and P₂), the correlation relationshipk(P₂) =A·k(P₁) +B (A, B are parameters of the equation) is closely obeyed. The advantages of carbon dioxide as the carrier gas were analyzed; the use of carbon dioxide allows the efficiency of the column to be enhanced.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 627–633, March, 1996.     

Temperature-dependent separation of bryostatin 18 and 10 by high performance liquid chromatography

Summary The temperature-dependent separation of bryostatins by HPLC was examined on an octadecyl bonded stationary phase, using column temperatures between 0 and 40°C and mobile phase temperatures from 0 to 25°C. The retention time and resolution of bryostatins changed drastically and separation improved with decreasing temperature. A column temperature of less than 5°C and a mobile phase temperature of less than 15°C is recommended for a good resolution of bryostatins for routine work.     

The computer-aided optimization of capillary columns for minimum time analysis and minimum detectabiliy

A model utilizing the extended Golay-Giddings equation, with an added term that expresses the instrumental contributions to the plate height, has been tested and found to accurately predict the chromatograph's output for a given set of operating conditions. The model forms the basis of a computer program that will recommend the length of column (of a specified inside diameter and film thickness) to be used (at a given temperature and input pressure) to obtain a minimum time analysis of the most difficult to separate pair of the mixture. Alternative solutions are available that specify the length of column, operating pressure, and the maximum permissible sample size required to provide a minimum analysis time for a system with specified minimum detection limits.     

Prediction of retention times and peak widths in temperature-programmed gas chromatography using the finite element method

Optimization of separations in gas chromatography is often a time-consuming task. However, computer simulations of chromatographic experiments may greatly reduce the time required. In this study, the finite element method was used to predict the retention times and peak widths of three analytes eluting from each of four columns during chromatographic separations with two temperature programs. The data acquired were displayed in predicted chromatograms that were then compared to experimentally acquired chromatograms. The differences between the predicted and measured retention times were typically less than 0.1%, although the experimental peak widths were typically 10% larger than expected from the idealized calculations. Input data for the retention and peak dispersion calculations were obtained from isothermal experiments, and converted to thermodynamic parameters.     

Diastereoselective and enantioselective chromatography of the pyrethroid insecticides allethrin and cypermethrin

Summary Liquid and gas chromatographic separations of the pyrethroid insecticides allethrin and cypermethrin have been investigated with various achiral and chiral stationary phases. Diastercomeric and enantiomeric selectivity was observed for cypermethrin on a Pirkle-type chiral LC stationary phase, but very strong interactions and therefore long retention times prevented the separation of allethrin on this phase. Trans-allethrin isomers were separated on a chiral -cyclodextrin RP-HPLC column while cypermethrin showed some difficulties on this phase due to isomerization. Diastereomeric but no enantiomeric selectivity by GC was achieved for cypermethrin with an apolar DB 5 capillary. GC separation of the diastereomers was used to study the selective photodegradation of cypermethrin isomers after forestry applications. Chiral -cyclodextrin-based GC phases showed some enantioselectivity for cis- and trans-allethrin isomers. A separation of the eight isomers into six partially resolved peaks was achieved by GC with a coupled column consisting of chiral permethylated -cyclodextrin and DB 1701 as stationary phases. This combination was used to characterize allethrin formulations intended for indoor use and to investigate allethrin products formed by ozonolysis of thin films of the insecticide.     

A comparative study of chiral separation of proton pump inhibitors by supercritical fluid chromatography and high-performance liquid chromatography

A comparative study of chiral separation of pantoprazole and rabeprazole is carried out using supercritical fluid chromatography and high-performance liquid chromatography. The columns used were Chiralpak IA and Chiralpak IE. The best mobile phase in supercritical fluid chromatography was carbon dioxide-0.2% triethylamine in methanol (60:40) and 0.1% triethylamine in n-hexane-ethanol (50:50) in high-performance liquid chromatography. For supercritical fluid chromatography, values of the retention factor of pantoprazole enantiomers were 3.97 and 4.88. These values for rabeprazole enantiomers were 6.10 and 7.52. The values of separation and resolution factor for pantoprazole and rabeprazole were 1.23 and 1.23 and 2.20 and 3.36, respectively. Similarly, for high-performance liquid chromatography, the values of retention factor for enantiomers of pantoprazole were 4.02 and 7.32. These values for rabeprazole enantiomers were 5.32 and 7.88, respectively. The values of separation and resolution factor for pantoprazole and rabeprazole were 1.82 and 1.48 and 9.22 and 6.58, respectively. A comparison was carried out, which confirmed supercritical fluid chromatography as the best method due to its fastness, eco-friendly, and inexpensiveness. The reported methods are effective, efficient, and reproducible and may be used to separate and identify pantoprazole and rabeprazole in any unknown samples.     

Optimization and identification in any kind of multi-step temperature programmed gas chromatography

The theory which predicts the retention time, retention temperature, and peak width for any kind of multi-step TPGC and the principle of optimization has been described. Software for optimization and identification in TPGC has been developed on the basis of this theory. It has also been proven that the relationship between peak width in TPGC and the derived or “invented” retention time is similar to that between peak width and retention time in isothermal processes. The validity of the software has been proved by using it to predict retention temperature, retention time, and peak width for any kind of temperature programming and to predict the optimum temperature program for separation of a multihomolog mixture of industrial alcohols and 15 enantiomeric pairs of amino acids.     

The influence of the carrier gas on retention in capillary gas-solid chromatography

In capillary gas-solid chromatography where interactions between solute and carrier gas and adsorption of the solute on the surface of the adsorbent are considered to be imperfect, it has been shown that chromatographic retention is determined largely by adsorption processes. It has been established that correlation relationships k(P₂)=A k(P₁) + B, where k is the retention factor, and A and B are equation constants, was valid for use of different carrier gases P₁ and P₂. Column efficiency could be improved by use of carbon dioxide. The advantages of using carbon dioxide as the carrier gas were investigated.     

Information theory of optimization in chromatography: Analytical structure of optimization

Summary The analytical roles of chromatographic variables (column length, etc.) can be soundly comprehended and compared in terms of the precision (Φ) of measurements and efficiency (ϑ) of analysis which are described as Shannon information and information flow, respectively. The φϑ plots of the optimization process and the information Φ transmitted by a single peak are useful to understand the analytical structure of optimization. Variables treated here are mobile phase composition (X), column length (L), mobile phase velocity rate (u), detection wavelength (λ) and plate number (N).     

Computer-assisted optimization of reversed-phase HPLC isocratic separation of neutral compounds

An appropriate optimization strategy should be used to find a desired resolution or selectivity with a minimum number of experiments in a limited time, which could assure the baseline separation of all target compounds. It was usually realized by means of a specialized computer program. In this paper, mapping optimization method and overlapping resolution mapping were compared for the optimization of a reversed-phase high-performance liquid chromatography (HPLC) isocratic separation of neutral compounds. The calculated resolutions and separation time of 7 to 10 experiments are fitted by different equations, which were used to build a contour plot with a minimum effective resolution and maximum retention time as a function of a mobile phase composition. The balance between resolution and analysis time could be easily realized by the overlapping of the final overlapping resolution mapping and analysis time mapping. The validity of the two methods was confirmed by some typical experiments. The models are simple, visual, and common without theoretical arithmetic.     

Obtimization of separation conditions in reversed phase liquid chromatography using complex solvent mixtures

Summary Four different methods of calculation of retention in ternary mobile phases were compared and it was found that the simple calculation based only on two values of the capacity factors (one for each binary system that composes the ternary mobile phase) provides the accuracy of prediction that is, at least, comparable to the other methods of calculation that require a large number of preliminary experiments. The deviations from ideal behaviour in ternary solvent mixtures are discussed; some sources of errors can be avoided, at least partially, using binary systems of adequate compositions for preparation of ternary mobile phases. Several examples show the comparison of calculated and experimental selectivities in ternary solvent systems. A simple calculation can be used for rapid selection of an adequate ternary (or more complex) mobile phase, if it is necessary to achieve the separation of a given sample mixture.Presented at the 14th International Symposium on Chromatography London, September, 1982     

An expert system for the optimization of columns,operating conditions and instrumentation for high-pressure liquid chromatography

Summary Given the mobile and the stationary phase and values for the physical parameters such as temperature and pH, a separation can be optimized by varying the so-calledchromatographic parameters. These include the column dimensions, particle size, operating conditions (e.g. flow rate, attenuation) and instrumentation (e.g. detector cell, time constant). Optimization of the chromatographic parameters implies finding the best possible set of values, which we define as yielding (i) sufficient separation and (ii) sufficient sensitivity in (iii) the shortest possible time. Finding the best possible conditions (the global optimum) is very difficult for chromatographers in practice.An expert system is described that allows chromatographic optimization to be performed for isocratic separations. An initial chromatogram is required to consult the system. In return, the system provides a complete set of chromatographic parameters, which represents the global optimum within the limits set by the required resolution and signal-to-noise ratio specified by the user. The tolerated flow and pressure ranges, the volume of the available detector cells and the time constant of the detection system are constraints during the optimization. A separate module of the system concerns the sample preparation for pharmaceutical formulations in solid dosages and aqueous solutions.Prototype expert systems have been successfully implemented in the expert-system shell Knowledge Craft on a MicroVAX workstation.     

A calculation method for the prediction of effective plate height in capillary gas chromatography

The effective plate height, h_eff, is considered to be a better measure of the efficiency of capillary column than the conventional plate height, h, in isothermal conditions. By using experimental data of 1-alcohols and n-alkanes, 2-ketones and 1-alkenes measured on capillary columns coated with non-polar stationary phases in isothermal and isobaric conditions, the peak width at half height is predicted with a function similar at that of adjusted retention time. The results obtained under different analytical conditions as the head pressure and the temperature of the column confirm the validity of the model, whose parameters are linear, and as a consequence a unique solution is obtained.     

Total chromatographic optimization (TOCO) I. Simultaneous optimization of many variables in liquid chromatography

Summary The following chromatographic variables are totally optimized based on the recently developed information theory of optimization: mobile phase composition, column length, flow rate, wavelength, and the amount of internal standard. The optimal internal standard is selected from among six candidates. Two types of optimal conditions (- and -optimals) are proposed: the -optimal is defined as the most precise analysis (the maximal ) while the -optimal is the most efficient (rapid) analysis (the maximal ). The observation times for the determination of an antipyretics mixture (three components) in liquid chromatography are ca. 50 s for the -optimal and ca. 8 min for the -optimal. The reliability of the - and -optimals is verified by experiments.     

Determination of amino acid enantiomers in orange juices by chiral phase capillary gas chromatography

Summary The configurations of free amino acids (AAs) in orange juice beverages (commercial products of satisfactory and unsatisfactory quality), an orange juice concentrate (bulk product suspected of being adulterated), and in an orange juice that has been contaminated by addition ofLactobacillus plantarum as a model for microbial spoilage, were determined, after derivatization, by means of gas-liquid chromatography (GC) using fused-silica capillary columns coated with Chirasil-L-Val or Chirasil-D-Val as stationary phases. AAs were isolated from juices by treatment with Dowex WX8 ion-exchanger and were investigated, by GC, as theirN(O)-pentafluoropropionylorN(O)-trifluoroacetyl 1-propyl esters. It was found that the high quality orange juice beverage contained L-AAs exclusively whereas this juice, after fermentation withLactobacillus, contained free D-Ala (32.7%), D-Val (62.3%), D-Phe (20.0%), D-Glu (24.3%), D-Ser (2.6%), D-Asp (0.8%), and significant amounts of D-Pro [% D=100 D/(D+L)]. D-Ala (8.8%) and D-Ser (4.2%) were found in a sensory and analytically unsatisfactory orange juice beverage, whilst D-Ala (27.5%) and D-Ser (14.3%) were detected in the orange juice concentrate suspected of being adulterated.Although capillary GC on chiral stationary phases is regarded as being highly suitable for the determination of AA enantiomers in fruit juice beverages, detection of D-AAs is currently not considered as conclusive proof of fruit juice adulteration caused by addition of AA racemates since a non-microbial origin of D-AAs in the respective juice, or an original occurrence of D-AAs, in either the free, substituted, or peptide-bonded form in the fruits, cannot be excluded with certainty.Presented in part at the Deutscher Lebensmittelchemikertag, Sept. 18–21, 1990, Frankfurt and at the 14. Jahrestagung Deutscher Lebensmitteltechnologen, Nov. 15–17, 1990, Düsseldorf.     

Comparison of optimization methods in planar chromatography

Summary The optimisation of chromatographic parameters is studied. The Window Diagram and Overlapping Resolution Map (ORM) methods are compared. The latter is developed to take into account the spot diameter and the average plate height variations with the development distance, in order to improve the reliability of the method.     

Optimization of module configuration in membrane gas separation

Mathematical models have been developed to optimize three configurations for membrane gas separation modules. The three systems include the single stage, the two stage, and the continuous membrane column (CMC). Analysis of the three systems is carried out for the case of enriching a binary mixture of methane and carbon dioxide, where the reject stream is the desired product. The cost optimization function includes the capital cost for compressors and membranes as well as the energy operating cost. The cost function is solved subject to a set of equality and inequality constraints. The equality constraints include the module balance equations and the permeation fluxes across the membrane. The inequality equations include constraints on mole fractions in permeate and reject streams, operating pressure, membrane area, and the amount of methane recovered in reject stream. Model equations for the three systems are solved using GINO, a program for nonlinear optimization. A quasi-Newton search method is selected and found quite efficient for solution of the equations. Over the range of parameters considered in the analysis, results show that the two stage configuration has a lower production cost than the other two systems. In addition, the operating cost for the CMC and the single stage systems are found to be comparable. Irrespective of this, the optimum amount of methane recovered is the highest for the CMC system. Although the optimum operating costs for the CMC and the single stage systems are higher than the two stage system, comparison should consider other factors including higher methane recoveries generated by the CMC system and the simplicity of design and operation for the single stage system.