Temperature Dependence Of Retention Indices Research Articles

Anomalous temperature dependence of gas chromatographic retention indices of polar compounds on nonpolar phases

The character of the temperature dependences of the retention indices RI(Т) of polar sorbates on nonpolar stationary phases was found to depend on the dosed amounts of sorbates, but not on column overloading. A physicochemical model was suggested to explain the observed anomalies in RI(Т).

Anomalous temperature dependence of gas chromatographic retention indices of polar compounds on non-polar stationary phases

Increasing the reliability of both GC and GC-MS identification requires appropriate interlaboratory reproducibility of gas chromatographic retention indices (I). Known temperature dependence, I(T), is the main source of non-reproducibility of these parameters. It can be approximated with a simple linear function I(T). However, since mid-1990s-beginning of 2000s some examples of anomalous temperature dependence, I(T), preferably for polar analytes on non-polar stationary phases were revealed independently by different authors. The effect implies the variations in the sign of the temperature coefficients β=dI/dT for selected compounds and, hence, the appearance of the I-extrema (usually, minima). The current work provides evidence that the character of the anomalous I(Т) dependences (ascending, descending, or with extrema) is strongly influenced by the amounts of analytes injected into the chromatographic column, but these anomalies appeared not to be connected directly with the mass overloading of separation systems. The physicochemical model is proposed to describe the observed anomalies of I(T) dependence. This model is based on three previously known principles of chromatography, namely: The superposition of these objectives allows understanding both the unusual temperature dependence of retention indices, and the influence of the amounts of polar analytes injected into GC column on the parameters of this dependence.

Temperature dependence of gas chromatography retention indices as one of the main factors determining their interlaboratory reproducibility

Temperature dependence of gas chromatography retention indices should be considered one of the main sources of their interlaboratory irreproducibility. The regularities of this dependence for packed and capillary columns or individual compounds allow revealing and characterizing other factors affecting reproducibility of the indices. These include the ratio of amounts of target analytes and reference compounds, as well as their polarity. Considering the anomalies in temperature dependences of retention indices of polar compounds on nonpolar phases allows assuming that it can be one of the causes of the asymmetry of the sets of reference data.

Overloading control of gas chromatographic systems.

Increasing the interlaboratory reproducibility of gas chromatographic retention indices requires avoiding measurements distorted by overloading effects. Several criteria of evaluating the limits of the mass overloading of gas chromatographic systems are compared and reconsidered. The criteria mostly appropriate for practical purposes are based on (i) the dependences of factors of peak broadening (ratio of peak height and its width) vs. amount of analyte injected into the chromatographic column and (ii) the dependence of parameters characterizing the peak distortion (asymmetry factor) vs. the amount of analyte. Both these criteria provide mutually comparable evaluations of the overloading limits for analytes of different polarity. At the same time, the dependence of retention indices vs. amounts of analyte injected in the chromatographic column cannot be recommended for overloading control, because the parameters of the corresponding linear regressions indicate temperature dependence. The interpretation of certain gas chromatographic anomalies requires the correct evaluation of overloading limits. For example, the unusual temperature dependence of retention indices of polar analytes on non-polar stationary phases and the dependence of retention indices on ratio of amounts of target analytes and reference compounds.

Effect of film thickness on the retention index of nitro and cyano compounds. Interpretation of the logarithmic equation.

Recently, a minimum was observed on the temperature dependence of retention indices in case of polar solutes chromatographed on apolar stationary phases. Physical significance is attributed to the equation constants describing the minimum. The equation constants are explained using a thermodynamic and a kinetic approach. Statistical properties of the models for different film thicknesses are compared. Chemical potential for one methylene unit is calculated in different ways for various film thicknesses and compared with literature values. The activation enthalpy attributed to the solvation process decreases as the carbon atom number (molecular mass) of the compounds decreases. The activation enthalpies for cyanoalkanes are always higher than those for the respective nitroalkanes. The interpretation of the equation constants does not change if the film thickness of the stationary phase increases (it is varied). This fact eliminates adsorption on the surface of the column wall from the possible causes responsible for the minimum on the retention index versus temperature curve.

Determination of Ambient Temperature Vapor Pressures and Vaporization Enthalpies of Branched Ethers

The standard molar enthalpies of vaporization of 11 branched tert-butyl alkyl ethers, tert-amyl alkyl ethers, and tert-octyl alkyl ethers have been determined from the temperature dependence of the vapor pressure measured by the transpiration method. The temperature dependences of retention indices for a set of 31 ethers with branched molecular structures were measured using a nonpolar gas chromatographic column. Values of for 13 ethers obtained from the transpiration method in this work (as well as some values selected from the literature) together with data for these ethers obtained from the correlation gas-chromatography method were used to derive a correlation for the prediction of the standard molar enthalpies of vaporization for branched ethers at the temperature T = 298.15 K. Experimental values of for 19 branched ethers were obtained with the help of this correlation. The procedure of the correlation gas-chromatography method was modified and substantially simplified by introduction of the Kovat's index for the correlation of the retention indices with the experimental .

The temperature dependence of retention indices in gas chromatography

A linear dependence of (T−T1)/[1(T)−1(T1)] on temperature (considering the retention index 1(T1) at temperature T1 as a standard value) is derived. Both ther retention index at an assigned temperature and the temperature dependence of the retention index can be calculated from retention data measured at two temperature-programing rates.

Gas chromatographic investigation of organometallic compounds and their carbon analogues : VI. Determination, calculation and correlation of Kováts retention indices for alkylalkoxysilanes

Several equations for the calculation of the retention indices of alkylalkoxysilanes are given and discussed. It is shown that accurate values of retention indices can be calculated on the basis of group retention indices of alkyl and alkoxy groups bonded to silicon. Temperature dependences of retention indices and △ I values are given, and a method for their estimation is described. A linear relationship exists between the retention indices of homologous series of alkylalkoxysilanes and those of the corresponding tetraalkoxysilanes. When retention indices of alkylalkoxysilanes on Apiezon M are plotted against the corresponding values on XE-60, two main linear relationships are found, one referring to so-called homologues lines and the other to so-called structure lines.

Gas—liquid chromatography on phenyl-substituted polysiloxane stationary phases, alcohols and alcohol benzoyl derivatives

Retention indices of 54 alcohols and 55 alcohol benzoyl derivatives measured on short stainless steel columns (1 m × 2 mm i.d.) filled with Chromosorb G (AWDMCS (HP)) coated with 5% ( w w ) SE-30, OV-3, OV-7, OV-11, OV-17 or OV-25 stationary phases at 100°C, 120°C and 140°C, appear to be very accurate, provided that there is careful control of the Chromatographic process. Values of the temperature dependence of retention index of alcohols and benzoates, and variations of these values with the polarity of the phase used, are given.

Gas chromatographic investigation of organometallic compounds and their carbon analogues : II. Improved method for calculating retention indices of tetraalkoxysilanes

A modified equation for the calculation of retention indices of mixed tetraalkoxysilanes from retention indices of the symmetrical counterparts is given. This equation is compared with the corresponding equation reported in a previous paper, and found to be more accurate. It is shown that the temperature dependence of retention indices of mixed tetraalkoxysilanes can be derived from those of the symmetrical counterparts with fair accuracy. A comparison is made between retention indices obtained on different stationary phases and different types of columns. It is inferred that the equations derived are valid for any column.

Gas chromatographic investigation of organometallic compounds and their carbon analogues : IV. Determination, calculation and correlation of kováts retention indices for tetraalkylsilanes

Equations for the calculation of retention indices for mixed tetraalkylsilanes are given. The significance of the sign of the charge on the terminal methyl carbon atom of the alkyl groups bonded to silicon in determining the magnitude of the retention index and the Δ I value is discussed. Rules for the estimation of the temperature dependence of retention indices for tetraalkysilanes are given. Linear relationships are shown to exist between retention indices for homologous series of mixed tetraalkylsilanes on the one hand and retention indices of symmetrical tetraalkylsilanes or of various tetraalkoxysilanes on the other. The two-phase plot of I ApL 160 versus I XE 160 for tetraalkylsilanes is shown to be completely different from that for tetraalkoxysilanes.

Effect of base treatment of the support on the temperature dependence of retention indices of organic bases

1. The temperature dependence of the values of the retention indices for n-amyl-, isoamyl-, dipropyl-, and triethylamines, piperidine, and pyridine on a column of 10% Apiezon-L and 5% NaOH suspended on “Novator” detergent was studied. 2. The obtained data confirm the hypothesis voiced earlier about the donor-acceptor interaction of primary and secondary amines with base suspended on the surface of the absorbent.