Linear temperature programmed retention indices of gasoline range hydrocarbons and chlorinated hydrocarbons on cross-linked polydimethylsiloxane

Separation orthogonality in temperature-programmed comprehensive two-dimensional gas chromatography

In a comprehensive two-dimensional gas chromatograph, a thermal modulator serially couples two columns containing dissimilar stationary phases. The secondary column generates a series of high-speed secondary chromatograms from the sample stream formed by the chromatogram eluting from the primary column. This series of secondary chromatograms forms a two-dimensional gas chromatogram with peaks dispersed over a retention plane rather than along a line. The method is comprehensive because the entire primary column chromatogram is transmitted through the secondary column with fidelity. One might expect that a two-dimensional separation in which both dimensions are basically the same technique, gas chromatography, would be inefficient because the two dimensions would behave similarly, generating peaks whose retentions correlate across dimensions. Applying a temperature program to the two columns, however, can tune the separation to eliminate this inefficiency. The temperature program reduces the retentive power of the secondary column as a function of progress of the primary chromatogram such that the retention mechanism of the primary column is eliminated from the second dimension. Retention of a substance in the second dimension is then determined by the difference in its interaction with the two stationary phases. Retention times in the second dimension then fall within a fixed range, and the whole retention plane is accessible. In a properly tuned comprehensive two-dimensional chromatogram, retention times in the two dimensions are independent of each other, and the two-dimensional chromatogram is orthogonal. Orthogonality is important for two reasons. First, an orthogonal separation efficiently uses the separation space and so has either greater speed or peak capacity than nonorthogonal separations. Second, retention in the two dimensions of an orthogonal chromatogram is determined by two different and independent mechanisms and so provides two independent measures of molecular properties.

Using Comprehensive Two-Dimensional Gas Chromatography Retention Indices To Estimate Environmental Partitioning Properties for a Complete Set of Diesel Fuel Hydrocarbons

Comprehensive two-dimensional gas chromatography (GCxGC) provides nearly complete composition data for some complex mixtures such as petroleum hydrocarbons. However, the potential wealth of physical property information contained in the corresponding two-dimensional chromatograms is largely untapped. We developed a simple but robust method to estimate GCxGC retention indices for diesel-range hydrocarbons. By exploiting n-alkanes as reference solutes in both dimensions, calculated retention indices were insensitive to uncertainty in the enthalpy of gas-stationary-phase transfer for a suite of representative diesel components. We used the resulting two-dimensional retention indices to estimate the liquid vapor pressures, aqueous solubilities, air-water partition coefficients, octanol-water partition coefficients, and vaporization enthalpies of a nearly complete set of diesel fuel hydrocarbons. Partitioning properties were typically estimated within a factor of 2; this is not as accurate as some previous estimation or measurement methods. However, these relationships may allow powerful and incisive analysis of phase-transfer processes affecting petroleum hydrocarbon mixtures in the environment. For example, GCxGC retention data might be used to quantitatively deconvolve the effects of water washing and evaporation on environmentally released diesel fuels.

Capillary gas chromatography–mass spectrometry of all 93 acyclic octenes and their identification in fluid catalytic cracked gasoline

The Kováts retention indices of all 93 acyclic octenes on polydimethylsiloxane and squalane as stationary phases as well as their mass spectra were measured. The means of gas chromatography-mass spectrometry (GC-MS) were used for confirmation of GC identification as well as for mass spectrometric deconvolution of the majority of gas chromatographic unseparated isomeric octene peaks. The distinction between corresponding E and Z acyclic octenes, that is either difficult or even impossible by means of GC-MS, was obtained on the basis of larger temperature coefficients of retention indices for Z isomeric octenes than for corresponding E isomers. The retention data expressed as homomorphy factors were correlated with the degree of branching, position of double bond, and position of alkyl group with respect to the double bond of acyclic octenes, and the structure-retention relationships were formulated. The 81 acyclic octenes were identified in FCC gasoline.

Mosaic increments for predicting the gas chromatographic retention data of the chlorobenzenes

The chlorinated organic compounds are very important from the point of view of the chemical industry and environmental protection, and therefore the gas chromatographic analysis of these compounds is very interesting for analytical chemists. In this paper we studied the relationship between the molecular structure and gas chromatographic retention on several stationary phases having different polarity and at several temperatures of benzene and 12 chlorobenzene compounds as model compounds. A coding system involving primary (mosaic increments) and secondary (bond increments)calculation methods was developed. The retention indices of benzene and the chlorobenzenes calculated on HP-5 at 120 degrees C shows a better performance of the mosaic increments (average absolute deviation delta of 1.7 retention index units) compared with the bond increments (delta = 11.7 retention index units). Retention factors, k, calculated with mosaic increments for chlorobenzenes on SPB-1 and WAX-10, at 140 degrees C, yield average relative errors of epsilon = 0.9 and 3.5%, respectively. Therefore, the presented paper provides a new possibility for precalculation of the retention data.

Gas chromatographic-mass spectrometric characterization of all acyclic C5-C7 alkenes from fluid catalytic cracked gasoline using polydimethylsiloxane and squalane stationary phases

Published retention indices of acyclic alkenes C5-C7 on squalane and polydimethylsiloxane as stationary phases were investigated, and reliable retention indices of alkenes from various sources were converted to separation systems used in a laboratory. Retention indices measured on available authentic commercial alkenes and on alkenic fraction of gasoline, published retention indices as well as means of GC-MS were used for verification of calculated retention indices. Retention of some gas chromatographic unseparated isomer pairs was obtained by mass spectrometric deconvolution using a specific single-ion monitoring. On the basis of these retention data, C5-C7 alkenes were identified and analyzed in the gasoline from fluid catalytic cracking. In the gasoline all 59 acyclic C5-C7 isomeric alkenes were determined at significantly different concentration levels.