Retention time, in gas chromatography

Products were quantitatively determined with H PLC analyses. The analytical conditions adopted for benzonitrile, benzamide and benzoic acid determination are fiilly described in [34], and those for nicotinic acid and nicotinamide in [35]. Propionitrile and propionamide were identified by their retention time in gas chromatography analyses [33]. 

Retention time in gas chromatography is related to a combination of retention and volatility, similar to solubility in liquid chromatography. Predicting volatility is as difficult as predicting solubility. Volatility has been explained as the enthalpy of vaporization (Avap f), and a method for predicting volatility has been proposed." If the A apH values are available, it may be possible to predict retention time. Unknown A apH values have been calculated from the relationship between the van der Waals volume and reference AyapH values. The values are summarized with the corresponding reference values in Table 1 of the Appendix (p. 278). Values of A apH have also been related to capacity ratios. The correlation coefficients were 0.896 and 0.852 ( = 48) for DBl and CPSilS columns, respectively, which appear to be acceptable correlation coefficients, except that the relationship for allq l alcohols deviated from those of other compounds as seen Figure 4.4. 

O. G. Piringer suggested a simple contribution method for the direct calculation of the partition coefficient a. A detailed explanation can be found in (Piringer 1993). The method is based on the comparison of the retention times in gas chromatography and is denoted as the Retention-index-method. The polyolefin will be simulated here by the non-polar ma-... 

Kende, A., Z. Csizmazia, T. Rikker, et al. 2006. Combination of stir bar sorptive extraction—retention time locked gas chromatography-mass spectrometry and automated mass spectral deconvolution for pesticide identification in fruits and vegetables. Microchem. J. 84 63-69. 

The most commonly used retention parameter in gas chromatography is the Kovats index. When the adjusted retention times are used to calculate Kovats indices, retention parameters are obtained which depend only on the column temperature and the stationaiy phase used. Kovats indices are highly reproducible, and with a well designed experimental technique and an accurate timing mechanism, an inter-laboratory reproducibility of one unit for larger values of Kovats indices and two units for indices below 400 is possible [14]. Instead of Kovats indices, sometimes in QSRR studies the logarithms of retention volumes of solutes are used. 

The separated hydrocarbon fraction is then analyzed by capillary column gas chromatography, which simultaneously separates the mixture of hydrocarbons into its components and measures how much of each compound is present. Individual n-alkanes can usually be identified by their relative retention times in gas chromatograms, although sometimes branched hydrocarbon molecules will share the same retention time as an n-alkane. For this reason, positive identification of biomarker hydrocarbons require the combination of gas chromatography and mass spectrometry using verified mass spectra. 

In special cases, polar gases such as ammonia, formic acid and water are doped into the carrier gas to improve the analyte s solubility in the carrier gas. In both supercritical fluid and liquid chromatography, the analyte solubility in the carrier liquid affects the retention time. The carrier liquid is called the eluent and/or the mobile phase. The prediction of retention times in liquid chromatography is very difficult due to the lack of a solubility prediction method. However, the retention can be predicted by computational chemical methods using model phases. ... 

Gas Chromatography (gc). A principal advantage of gas chromatography has been the faciUty with which it can be combined with mass spectrometry for amino acid identification and confirmation of purity. The gc-mass spectrometry combination offers the advantage of obtaining stmctural information rather than the identification by retention time in hplc. 

Retention is usually measured in units of time for convenience. Voliime units are more exact. Table 1.1, after suitable corrections have been applied (26). Under average chromatographic conditions liquids can be considered incompressible, but not so for gases, and in gas chromatography elution volumes are corrected to a mean column pressure by multiplying them by the gas compressibility factor, j, equation (1.2)... 

Davis, J.M. (2004). Assessment by Monte Carlo simulation of thermodynamic correlation of retention times in dual-column temperature programmed comprehensive two-dimensional gas chromatography. J. Sep. Sci. 27, 417. 

The same is true for the chiral polysiloxanes described here. Their use as stationary phases in gas chromatography allows the calculation of the differences in enthalpy and entropy for the formation of the diaste-reomeric association complexes between chiral receptor and two enantiomers from relative retention time over a wide temperature range. Only the minute amounts of the polysiloxanes required for coating of a glas capillary are necessary for such determinations. From these numbers further conclusions are drawn on the stereochemical and environmental properties required for designing systems of high enantio-selectivity in condensed liquid systems. 

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