Gas chromatography of hydrocarbons using packed liquid chromatographic capillary columns

Capillary columns of 0.3-0.5 mm i.d. packed with 3- to 30-μm silica-based stationary phases for liquid chromatography were used for gas chromatographic separation of hydrocarbons. Column efficiencies were evaluated for various commercially available packing material. The best column efficiency was achieved with 5-μm octadecyl group bonded silica gel, the surface of which was coated with a poly (dimethylsiloxane) film. The 30-cm column produced 11,000 theoretical plates.     

Gas chromatographic separation of the stereoisomers of organophosphorus chemical warfare agents using cyclodextrin capillary columns

The synthesis of the organophosphorus nerve agents sarin, tabun, and cyclohexyl methylphosphonofluoridate (GF) produces a mixture of two stereoisomers except for soman where four stereoisomers are produced. Significant differences exist in the reported toxicity and AChE inhibition rates of the various stereoisomers. This makes the ability to distinguish between the different stereoisomers desirable. Five different derivatized cyclodextrin stationary phases developed for gas chromatography were tested for their ability to resolve the nerve agent stereoisomers using a gas chromatograph interfaced to an atomic emission detector. Of the five columns that we examined, only the 2,6-di-O-pentyl-3-O-trifluoroacetyl or 2,6-di-O-pentyl-3-O-butyryl γ-cyclodextrins were able to successfully resolve all four soman stereoisomers. The elution order for each column was determined using solutions of isolated soman stereoisomers. Enantiomers of sarin, tabun, and GF were resolved with varying degrees of success on the different cyclodextrin stationary phases. Only the butyryl γ-cyclodextrin was able to separate the enantiomers of all four of the nerve agents examined in this study. The capacity (k) and selectivity (α) factors were determined for each of the chemical warfare agents successfully separated. The TNO Prins Maurits Laboratory in the Netherlands has previously developed several different chromatographic methods to resolve the stereoisomers of soman, sarin, and tabun. The advantage of the method described here is that commercially available cyclodextrin gas chromatography columns were used to resolve the stereoisomers, thereby facilitating rapid and routine analysis of organophosphorus nerve agents.     

A four factor simplex optimization of a serially coupled open tublar columns gas chromatographic system

Optimization of a gas chromatographic system with two capillary columns coupled in series in a single oven for the linear temperature programmed separation of a mixture of C₁ C₁₀ hydrocarbons has been carried out using the simplex optimization method. The following four selectivity parameters were optimized: the initial temperature of the oven; the initial hold time; the temperature programmed rate; and the pressure at the mid-point of the system. The optimization procedure was monitored by the C_p criterion.     

Evaluation of chromatographic columns packed with semi‐ and fully porous particles for benzimidazoles separation

The behavior of 15 benzimidazoles, including their main metabolites, using several C₁₈ columns with standard or narrow‐bore diameters and different particle size and type were evaluated. These commercial columns were selected because their differences could affect separation of benzimidazoles, and so they can be used as alternative columns. A simple screening method for the analysis of benzimidazole residues and their main metabolites was developed. First, the separation of benzimidazoles was optimized using a Kinetex C₁₈ column; later, analytical performances of other columns using the above optimized conditions were compared and then individually re‐optimized. Critical pairs resolution, analysis run time, column type and characteristics, and selectivity were considered for chromatographic columns comparison. Kinetex XB was selected because it provides the shortest analysis time and the best resolution of critical pairs. Using this column, the separation conditions were re‐optimized using a factorial design. Separations obtained with the different columns tested can be applied to the analysis of specific benzimidazoles residues or other applications.     

Headspace gas chromatographic separation of toxic organic impurities from air

Summary To determine toxic organic compounds (methanol, ethanol, n-propanol, acetone, methyl ethyl ketone, methyl isobutyl ketone) in air a method was developed for their headspace gas chromatographic analysis with preconcentration in n-butanol (alcohols) and in n-pentanol (ketones). The distribution coefficients of analytes in the air-absorbent system have been measured: 1051 (MeOH), 5630 (EtOH), 6773 (n-PrOH), 307 (Me₂CO), 580 (MeCOEt), 1035 (MeCOBu-i). The minimum detectable level (mg m⁻³) was determined as low as 0.9 (MeOH), 4.0 (EtOH), 0.9 (n-PrOH), 0.2 (Me₂CO), 0.1 (MeCOEt), 0.4 (MeCOBu-i). The method was effectively used for gas effluent air control in the workplace and in the atmosphere. Presented at the 21^st ISC held in Stuttgart, Germany, 15^th–20^th September, 1996     

Gas chromatographic determination of volatile anaesthetic agents in blood. Part 1. Preparation of standard gas mixtures of volatile anaesthetic agents

A method for preparing standard gas mixtures of the volatile anaesthetics halothane, enflurane and isoflurane is described. Static mixtures of gases of known concentration can be prepared manometrically by measuring the required pressure of anaesthetic gas into a bulb and diluting to atmospheric pressure with air. Standard gas mixtures in the concentration range 0-4% V/V can be prepared with an accuracy of +/- 0.01% V/V, and the relative standard error of measurements of a single standard concentration is less than 0.8%. Significant adsorptive losses in the gas sampling valve were observed for gas standards prepared in the absence of any diluent gas. These losses were not detected for measurements of standards made up to atmospheric pressure in air. A comparison with calibration procedures currently in practice is presented.     

Chemometric studies of retention in capillary gas chromatographic separation of hydrocarbons in coupled columns

This paper describes how different multivariate analysis and classification methods can be used, to characterize the gas chromatographic separation of complex hydrocarbon mixtures in three columns coupled in series. Principal component analysis (PCA), correspondence factor analysis (CFA), and hierarchical ascending classification (HAC) were used as potential tools for evaluating the experiments on single columns and on column series. It has been demonstrated that: (1) multivariate analysis with PCA and CFA offers a powerful strategy to search for the main factors influencing the separation of hydrocarbons without a priori knowledge of the key factors of the separation. (2) With CFA the contribution of retention due to vapour pressure can be minimized. The use of retention indices, which use the n-alkanes as reference compounds, also helps to decrease the dominant focus on vapour pressure in favor of the more selectivity-based interaction forces. (3) CFA helps to analyze the degree of relevance of the chosen experimental design to the most important factors, controlling chromatographic selectivity.     

Membrane rectification columns for gas separation and determination of the operating lines using the mccabe—thiele diagram

A new continuously operating separating unit — a membrane rectification column — has been developed for the separation of gases by membranes. The membrane rectification column can be characterized as a “step-by-step separating process”. In this article the equations for the operating lines have been constructed to show the separating process with the new enrichment technique in the McCabe—Thiele diagram.     

The efficiency of open tubular gas chromatographic columns

Summary A new parameter, the mean specific plate number, is proposed for assessing the efficiency of support coated open tubular (SCOT) and wall coated open tubular (WCOT) columns and for comparing the efficiency of different open tubular columns. It has the advantage over most previously used parameters in that it has only a small dependence on the partition ratio and it allows for the column diameter.A graphical presentation is given of the maximum theoretical mean specific plate number as a function of the partition ratio for SCOT columns having a range of relative porous layer thicknesses and for WCOT columns with a range of phase ratios.This presentation permits ready visual comparison of the potential efficiency of different columns and enables a simple evaluation of the percentage utilization of theoretical efficiency from experimentally determined values of the maximum mean specific plate number. For a given column the percentage utilization of theoretical efficiency at optimum average gas velocity and that at optimum practical gas velocity or at a higher average gas velocity are shown to be equal provided that corrections for column pressure drop are made.     

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).     

Preparation and use of packed capillary columns in chromatographic and related techniques

In this paper general considerations related to the various approaches and parameters influencing the preparation of highly efficient and stable capillary columns for use in liquid chromatography and related techniques are presented and the column production process is discussed in some detail. The different packing methods available for delivering a packing material into a capillary column are discussed from a practical viewpoint. Packing with a gas (dry packing), packing with a liquid solvent or a mixture of solvents (slurry packing), packing with supercritical carbon dioxide, electrokinetic packing, and sol-gel packing technologies are introduced and discussed throughout the paper. Practical recommendations for obtaining highly efficient (high plate numbers) and stable capillary packed columns are also addressed and discussed.     

Evaluation of two commercial capillary columns for the enantioselective gas chromatographic separation of organophosphorus pesticides

The separation of the enantiomers of 13 organophosphorus pesticides (OPPs) has been investigated by gas chromatography (GC) with flame ionisation detection (FID) using two different commercially available chiral columns, Chirasil-Val (l-valine-tert-butylamide) and CP-Chirasil-Dex CB (heptakis (2,3,6-tri-O-metil)-β-cyclodextrin). Using the Chirasil-Val column no chiral resolution was obtained for the OPPs investigated under any tested experimental condition. The use of the CP-Chirasil-Dex CB stationary phase enabled good individual enantiomeric separation of two OPPs, ruelene and trichlorfon and partial separation of naled, chloretoxyphos, isophenphos and metamidophos. Also, the obtained chromatographic results showed that Chirasil-Dex could resolve enantiomers through the combination of different mechanism (e.g. formation of inclusion complexes and/or interactions outside the cyclodextrin cavity).

Under optimised conditions, precision, linearity range and detection limits were evaluated for the enantiomers of ruelene and trichlorfon using CP-Chirasil-Dex CB column and electron capture detection (ECD). By using the GC-ECD method the enantiomers of these OPPs could be satisfactorily detected at very low concentration levels. The detection limits observed were 1.5 ng mL⁻¹ and 11.5 ng mL⁻¹ for the enantiomers of trichlorfon and ruelene, respectively.     

Theoretical and experimental study of packed capillary columns in gas chromatography

Chromatographia - The coupling theories byGiddings andLittle-wood are reviewed and discussed. A practical method to derive the values of the constants involved in the plate height equation from...     

Phenomenological study of the bed--wall friction in axially compressed packed chromatographic columns

The properties of column beds prepared with slurries of Kromasil C8 in 12 different solvents, using the same axial compression skid, were investigated. The extent of the consolidation of the column beds, their permeabilities, and the friction shear stress of these beds against the column wall were determined, as well as the column efficiencies (for an unretained tracer). The results of this study illustrate the influence of the wall effect on the consolidation. The permeability of columns consolidated under a constant compression stress was found to increase with increasing bed length. The bed-wall friction shear stress increases rapidly with increasing bed length and varies widely with the nature of the solvent used. No correlation was found between this shear stress and any physico-chemical property of the solvent. The best efficiency was observed for a column consolidated from a slurry in ethanol.