Hold-up time in gas chromatography. V. Dependence of the retention of n-alkanes on the chromatographic variables in isothermal gas chromatography

Determination and evaluation of gas holdup time with the quadratic equation model and comparison with nonlinear models for isothermal gas chromatography

Gas holdup time (tM) is a basic parameter in isothermal gas chromatography (GC). Determination and evaluation of tM and retention behaviors of n-alkanes under isothermal GC conditions have been extensively studied since the 1950s, but still remains unresolved. The difference equation (DE) model [J. Chromatogr. A 1260: 215-223] reveals retention behaviors of n-alkanes excluding tM, while the quadratic equation (QE) model [J. Chromatogr. A 1260: 224-231] including tM is suitable for applications. In the present study, tM values were calculated with the QE model, which is referred to as tMT, evaluated and compared with other three typical nonlinear models. The QE model gives an accurate estimation of tM in isothermal GC. The tMT values are highly accurate, stable, and easy to calculate and use. There is only one tMT value at each GC condition. The proper classification of tM values can clarify their disagreement and facilitate GC retention data standardization for which tMT values are promising reference tM values.

Isothermal retention indices on poly(3,3,3-trifluoropropylmethylsiloxane) stationary phases

Isothermal retention indices (I) of 56 solutes of varied chemical nature and polarity were determined by a well-proven accurate method on poly(3,3,3-trifluoropropylmethylsiloxanes) with trifluoropropyl group percentages between 0 and 50% within the 353-413 K temperature range. The column temperature effect on the retention index for a representative set of solutes was studied. The linear model and the recent expression I = a + bT(-1) + c ln T were discussed. The retention index dependence on the polymer polarity was also investigated. Principal component analysis and multiple linear regression (MLR) were applied to obtain retention models relating the I values to physicochemical and molecular properties of the solutes chromatographed. The best model of prediction of the retention in these fluorinated stationary phases was obtained by using the Abraham's descriptors.

An accurate and easy procedure to obtain isothermal Kováts retention indices in gas chromatography

Isothermal Kováts retention indices may be used in GC for identification purposes, but they are also useful in characterisation of stationary phases and for studying structural and physico-chemical properties of both the analyte and the stationary phase. They are currently reported as whole numbers with an accuracy of one index unit for non-polar stationary phases. The method recommended for their calculation uses a linear regression model, although non-linear models have also been applied with good results. In both cases, a computer programme and the retention times of a series of n-alkanes that elute correctly and resolved from the other compounds are needed, conditions which cannot always be fulfilled. However, it is possible to calculate retention indices with an accuracy of 0.1 retention index units (0.2 units for packed columns) with the help of only three n-alkanes using just a pocket calculator or a computer spreadsheet. The main requirement is that at least one of the n-alkanes has a retention index differing by no more than one hundred retention index units from that of the analyte being investigated. Examples are given for different stationary phases.

Resolution prediction and optimization of temperature programme in comprehensive two-dimensional gas chromatography

A model is developed for predicting the resolution of interested component pair and calculating the optimum temperature programming condition in the comprehensive two-dimensional gas chromatography (GC x GC). Based on at least three isothermal runs, retention times and the peak widths at half-height on both dimensions are predicted for any kind of linear temperature-programmed run on the first dimension and isothermal runs on the second dimension. The calculation of the optimum temperature programming condition is based on the prediction of the resolution of "difficult-to-separate components" in a given mixture. The resolution of all the neighboring peaks on the first dimension is obtained by the predicted retention time and peak width on the first dimension, the resolution on the second dimension is calculated only for the adjacent components with un-enough resolution on the first dimension and eluted within a same modulation period on the second dimension. The optimum temperature programming condition is acquired when the resolutions of all components of interest by GC x GC separation meet the analytical requirement and the analysis time is the shortest. The validity of the model has been proven by using it to predict and optimize GC x GC temperature programming condition of an alkylpyridine mixture.

Improving the accuracy of Kováts' retention indices in isothermal gas chromatography

Isothermal Kováts' retention indices are currently reported as whole numbers, and are frequently deduced from a linear least mean squares fitting of the logarithms of adjusted retention times of a number of n-alkanes versus carbon number, following an iterative method that minimises errors. The currently accepted accuracy is about one retention index unit for apolar stationary phases, and lower for polar stationary phases. This paper presents results that show how the accuracy of the retention index may be safely reported to one-tenth of a retention index unit by the use of a non-linear equation, with present day gas chromatographs without electronic flow controllers. Results are presented that prove the correctness of the retention indices found for several substances on one particular capillary column. Hints on the minimum retention times needed to achieve the 0.1 retention index accuracy are mentioned, for retention times recorded in minutes and in seconds. According to results of this paper, two chromatograms, run under the appropriate conditions, are sufficient to obtain the desired accuracy. The method proposed in this paper does not require knowledge of the hold-up time of the chromatogram.