J.A. Rijks

Increased speed of analysis in isothermal and temperature-programmed capillary gas chromatography by reduction of the column inner diameter

It is shown theoretically that for an isothermal analysis the relationship of the retention time to the column diameter is . The exponent z is determined by the column pressure drop and varies between z  1 for high plate number columns and z  2 for situations of low pressure drop. Equations derived for temperature-programmed conditions also lead to the same conclusion. The validity of the expressions was confirmed by experiments with columns of 30 and 50 μm internal diameter, installed in standard chromatographic equipment. With these small-diameter glass and fused-silica columns coated with non-polar stationary phases, plate numbers between 105 and 106 were obtained. A temperature-programmed run typically can be accomplished within 8 min on an 8 m x 50 μm column having over 105 plates. Several examples of high-speed, high-resolution analysis of complex samples are given.

Preparation of glass capillary columns coated with polar phase for hogh-temperature gas chromatography. 1. Study of Roughening of the glass surface

Abstract Some methods of surface roughening of glass capillary columns have been studied ( viz. , HCl etching, controlled devitrification and deposition of fine particles). The reaction of HCl with soda-lime glass proceeds easily, but is a self-limiting process. The resultant NaCl initially forms globules, which become cubic after prolonged heating. The presence of excess of HCl (as in flowing methods) or water reduces the crystallization rate markedly. A method of surface crystallization based on controlled devitrification has been developed. Spiked surfaces are obtained, which can easily be deactivated by current methods. The deposition of fine particles on the column wall offers another approach. A method is described for the preparation and application of (sub)micron NaCl particles, which results in a surface comparable with an HCl-etched surface. Preliminary results are given for surface-roughened capillary columns at high temperatures coated with polar phases.

Capillary gas chromatography of alkylbenzenes. I. Some problems encountered with the precision of the retention indices of alkylbenzenes

A high-precision gas chromatographic system, in combination with high-resolution stainless-steel capillary columns and three stationary phases of different polarity, was used to study the separation and retention behaviour of 47 alkyl-benzenes up to C15, including all possible isomers between C6 and C10. The precision of the relative retention data is discussed in relation to the method of time measurement, non-ideality of the carrier gas, column temperature and ageing of the column. Taking into account these factors, the inter-laboratory agreement of the retention indices for aromatic hydrocarbons determined within and between laboratories can be improved considerably. Kovats retention indices and their temperature dependence were obtained with a precision corresponding to a standard deviation of about 0.1 index unit.

Factors determining flow rate in chromatographic columns

SummaryIt is shown that the flow in chromatography is nearly always laminar in nature. Starting from the Darcy equation, expressions are given for the flow rate in both gas and liquid chromatography columns. The concepts of specific permeability, chromatographic permeability and column resistance factor are discussed for packed as well as open tubular columns. The experimental determination of all these factoers is demonstrated. The influence of the shape and pore volume of porous and non-porous supports on the column resistance factor and the chromatographic permeability is discussed.

Solvent elimination rate in temperature-programmed injections of large sample volumes in capillary gas chromatography

Temperature-programmed sample introduction is a very useful approach for the injection of large sample volumes in capillary gas chromatography and also holds promise for liquid chromatography-gas chromatography coupling. The optimization of a temperature- programmed injector for both these applications depends on numerous factors such as sample volume, liner design and temperature, speed of sample introduction and purge gas flow-rate. The maximum allowable speed of introduction of large sample volumes with simultaneous elimination of the solvent is determined by the solvent elimination rate. A theoretical model is proposed to predict an optimum combination of the speed of sample introduction, the initial liner temperature and the purge gas flow-rate. The validity of the model is discussed and evaluated. The solvent elimination rate is shown to depend on, amongst others, the vapour pressure of the solvent, and can be increased by an increase in the purge gas flow-rate and/or by a decrease in the inlet pressure. The observed cooling effect and the effect of the design of the liner on the solvent elimination rate are emphasized.

Comparison of methods for the deactivation of glass open-tubular columns with PEG 20M

Abstract A new deactivation method for glass capillary columns is described. Vapours of PEG 20M are bled from a short pre-column on to the capillary column and act as the deactivating agent. Bare columns, as well as coated columns, can be deactivated and eventually be re-deactivated after deterioration. Columns deactivated with this new method compare favourably with columns deactivated by other methods involving PEG 20M, with respect to adsorption and catalytic activity. The infuence of the deactivating agent on the polarity of SE-30 coated columns is small. PEG 20M deactivated roughned surfaces are amenable to subsequent coating with polar phases. We have concluded that the gas-phase deactivation method has decisive advantages over other methods involving PEG 20M.

Interactive retention index database for compound identification in temperature-programmed capillary gas chromatography

Abstract A procedure is described and evaluated that allows the calculation of linear temperature-programmed retention indices from accurate Kovats retention indices on a given stationary phase and their temperature variation coefficients. The influence of experimental factors such as column film thickness, phase ratio and variation of Kovats retention indices, column dead time and carrier gas flow-rates are examined. The calculation accuracies are ≤ 0.5 retention index units in most instances. The applicability and limitations of the procedure are discussed.

Possibilities and limitations of steam distillation—extraction as a pre-concentration technique for trace analysis of organics by capillary gas chromatography

The quantitative performance of steam distillation—extraction was investigated for different types of organic substances at concentrations ranging from tens of parts per million to parts per billion (parts per 105 to 109). A theoretical model is introduced that describes the recovery of different classes of organic compounds as a function of the process time. The effect of variations of some process factors predicted theoretically are in good agreement with the experimental results. A 100 % recovery is obtained within 20 min for most substances. Enrichment factors between 50 and 200 and recoveries corresponding to a standard deviation of 10% can be achieved in many applications.

The application of precision gas chromatography to the identification of types of hydrocarbons

Abstract It appears that under precisely controlled conditions retention indices of apolar substances on apolar stationary phases can be reproduced to 0.03 units. This permits the accurate measurement of the temperature coefficients of the K ovats index for different classes of hydrocarbons. In this way classes of compounds having similar ΔI values ( I polar - I apolar ) may be distinguished. This is of particular importance because of the lower precision of the index data on polar stationary phases.

Syn-anti isomerisation of 2,4-dinitrophenylhydrazones of volatile carbonyl compounds in capillary gas chromatographic-mass spectrometric analyses

Dinitrophenylhydrazone derivatives of volatile aldehydes and ketones were investigated by high-performance liquid chromatography (HPLC) and by gas chromatography-mass spectrometry (GC-MS). HPLC separations were slightly better than those obtained by existing procedures, but high-resolution GC is the method of choice. The equilibrium of the syn-anti isomerization of the compounds is influenced by the nature of the solvent, the deactivation of the column and the injection and column temperatures. The GC profile can therefore be controlled by careful selection of the experimental conditions. It is shown that this phenomenon, in addition to the occurrence of characteristic values of the difference in the retention indices of syn and anti isomers for a given compound, can facilitate the identification. Chemical ionization (methane) mass spectra of the compounds were found to contain more information than conventional electron impact spectra. Characteristic fragment ions are tabulated.