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.     

Prediction of programmed temperature retention times on linked capillary columns of different polarity

Systems formed by serial connection of capillary columns of different polarity were studied with methods previously used to predict the behavior of linked capillary columns under isothermal conditions and to obtain programmed temperature gas chromatography (PTGC) retention times of the individual columns starting from isothermal data. The two calculation methods were simultaneously applied in order to predict PTGC retention times of the series system starting from isothermal data obtained on the two individual columns. Experimental retention values measured using different temperature programs on the individual columns and on the series systems were found to agree with those calculated.     

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.     

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).     

Life-time studies with capillary electrochromatography columns operated under different conditions

A test system has been established to permit the monitoring of the life-time performance of several reversed- phase capillary electrochromatography (CEC) columns. The retention factors, k(cec), peak symmetry coefficients, lambda(sym), and column efficiencies, N, of three neutral n-alkylbenzene analytes, namely ethyl-, n-butyl- and n-pentylbenzenes, were determined for Hypersil 3 microm n-octylsilica and n-octadecylsilica packed into CEC capillary columns of 100 microm I.D., with a packed length of 250 mm, and a total length of 335 mm. The performances of these CEC capillary columns were examined for a variety of eluents with pH values ranging between pH 2.0 - 8.0, similar to those employed to study the retention behaviour of peptides that we have previously reported. The relative standard deviation (RSD) of the retention factors (k(cec) values) of these n-alkylbenzenes, acquired with an eluent of (25 mM Tris-HCl, pH 8.0,)-acetonitrile (1:4, v/v), when the CEC capillary columns were used for the first time (virgin values), were 4% (based on data acquired with 4 CEC capillary columns) for the n-octyl bonded silica capillary columns, and 6% (based on 8 columns) for n-octadecyl bonded silica capillary columns. The RSD values of the k(cec) values of the n-alkylbenzenes for one set of replicates (n=6) with one CEC capillary column was < 0.5%. The theoretical plate numbers, N, for the virgin CEC capillary columns were ca. 60,000, whilst the observed N values for all new CEC capillary columns were > or = 40,000 for n-octyl bonded silica capillary columns and > or = 50,000 for n-octadecyl bonded silica capillary columns. The peak symmetry coefficients, lambda(sym), of the n-alkylbenzenes for virgin CEC capillary columns and for CEC capillary columns used for more than 1,000 injections were always in the range 0.95-1.05. The experimental results clearly document that the life-time performance of the CEC capillary columns depends on the eluent composition, as well as the nature of the analytes to which the CEC capillary columns are exposed.     

Prediction of retention values in linear temperature programming of narrow and mega-bore capillary columns

Retention times in linear temperature programmed gas chromatography on narrow bore and megabore capillary columns have been calculated from experimental retention times measured at three isothermal temperatures: an iterative procedure performed by BASIC programs was used to obtain the values of the constants enabling the calculation of the programmed temperature retention times. Different methods of calculation have been compared.     

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.     

Determination of retention indices in constant inlet pressure mode and conversion among different column temperature conditions in comprehensive two-dimensional gas chromatography

A method to determine the second dimensional real retention time, dead times on both dimensions and retention indices in constant inlet pressure mode was developed in comprehensive two-dimensional gas chromatography. At the same time, the conversion of GC x GC retention indices among different column temperature conditions were also conducted based on some thermodynamics parameters. The calculation accuracies are better than 1.0 retention index unit. Furthermore, a retention index database was developed and used to identify the compounds in a cigarette essential oil sample. It showed that identification by the database was of close agreement with by time-of-flight mass spectrometry, and some isomers could also be distinguished based on the retention index database.     

Joule heating and determination of temperature in capillary electrophoresis and capillary electrochromatography columns

This article reviews the progress that has taken place in the past decade on the topic of estimation of Joule heating and temperature inside an open or packed capillary in electro-driven separation techniques of capillary electrophoresis (CE) and capillary electrochromatography (CEC), respectively. Developments in theoretical modeling of the heat transfer in the capillary systems have focused on attempts to apply the existing models on newer techniques such as CEC and chip-based CE. However, the advent of novel analytical tools such as pulsed magnetic field gradient nuclear magnetic resonance (NMR), NMR thermometry, and Raman spectroscopy, have led to a revolution in the area of experimental estimation of Joule heating and temperature inside the capillary via the various noninvasive techniques. This review attempts to capture the major findings that have been reported in the past decade.     

High temperature silylation of glass capillary columns

This paper describes the results of a study on the deactivation of the surface of glass capillary columns by high temperature silylation (HTS). The different steps, leaching, washing, dehydration and HTS were optimized. A practical procedure yielding a high percentage of very good columns is given. The influence of leaching and HTS on the temperature stability and coating efficiency of capillary columns coated with OV-1 was studied. The inertness of the glass surface after HTS is demonstrated by several examples.     

On-line hydrogenation of unsaturated and saturated sample components in capillary reactors coupled to either inlet or outlet of capillary columns

Summary A special coupling piece for the connection of fused silica or softglass capillary hydrogenation reactors with capillary separation columns using a double oven GC instrument is described. The temperatures of reactor and column can be independently adjusted thereby. This system for on-line hydrogenation also includes two injectors to be used either before the reactor or before the column and a device for trapping the hydrogenated material inside the column. Platinum and palladium catalysts of various activities were investigated, aromatic hydrocarbons and compounds containing polar functional groups can be hydrogenated easily.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Use of hydrolyzed chlorosilanes for the preparation of high resolution glass capillary columns

Summary A new procedure for the preparation of high resolution open tubular glass capillary columns is described. This procedure involves the preparation of polysiloxane polymers obtained by alkaline hydrolysis of alkyl chlorosilane. The mixture of polysiloxane polymers is then coated on the wall of previously HCl treated glass capillary columns using a dynamic method. A base-catalysed reaction using gaseous ammonia, applied to the coated polymers leads to a stable chemically bonded stationary phase, with non-polar characteristics. This type of column is easier to prepare than conventional ones and exhibits excellent chromatographic properties, both with regard to their resolution, stability and reproducibility. The flexibility of this method permits the use of other types of commercially available chlorosilanes (i.e. methylphenyl chlorosilane) to prepare polar polysiloxane polymers suitable for analysis of complex biochemical mixtures, such as steroid metabolites.     

High temperature gas chromatography on narrow bore capillary columns

The use of high-temperature-stable, medium polarity glass capillary columns coated with immobilized PS-090 (a 20 % diphenyl-substituted, CH₃O-terminated polydimethylsiloxane) has made it possible to analyze routinely, and with good separation efficiency, high molecular weight compounds such as triglycerides and free base porphyrins. Cold on-column injection was used throughout this work to avoid discrimination against involatile compounds, and disposable (fused silica) retention gaps were used to protect the column against contamination with involatile material. On-column injection into narrow bore glass columns was achieved by using glass-to-silica connections to attach wider bore (0.2 mm i.d.) deactivated fused silica tubing to the columns.     

Stainless steel capillary columns for high temperature gas chromatography

A layer of elemental silicon has been deposited on the surface of stainless steel tubing by means of chemical vapor deposition (CVD). Two kinds of capillary column were prepared from the deactivated tubing: cross-linked, silanol-terminated polydime-thylsiloxane wall coated open tubular (WCOT) columns and molecular sieve 13X porous layer open tubular (PLOT) columns. Unlike fused silica capillary columns, stainless steel WCOT and PLOT columns can be operated at temperatures in excess of 400°C. High temperature simulated distillation has been performed successfully with a macro bore WCOT column and rapid PNA (paraffin, naphthene, and aromatic) analysis with a multidimensional gas solid chromatographic (GSC) system using PLOT columns.     

Computer-assisted optimization of selectivity by tuning temperature and carrier gas pressure drop in two GC capillary columns coupled in series

The paper shows a computer-assisted procedure for the optimization of selectivity of two columns coupled in series by tuning the working temperature (using the isothermal mode) and columns coupling-point pressure at constant inlet and outlet carrier gas pressures. The optimization procedure validation was tested by the separation of 32 hydrocarbons in a column series with the aim to resolve the maximum number of components in the shortest possible analysis time.Dedicated to Professor J. F. K. Huber on the occasion of his 65th birthday