Gas-chromatographic retention indices in dependence on the ratio of analytes to reference components

One of the main factors determining the interlaboratory reproducibility of gas-chromatographic retention indices is their linear dependence on the logarithms of the ratios of the amounts of analytes and reference components, which is rarely taken into account. It is found that the effect of this factor is much stronger for packed columns in comparison to that for more efficient capillary columns; it decreases with the sample dilution and with the increase in the temperature of gas-chromatographic separation. It was shown that the effect of relative amounts of components on their retention indices agrees with the main principles of the chromatographic plate theory and is associated with neither the overloading of chromatographic columns nor the nonlinearity of the adsorption isotherms. To improve the reproducibility of indices, it is recommended to use retention parameters measured at certain points in the front edges of the chromatographic peaks corresponding to similar intensities of the analytical signal rather than at the peak maxima.     

Identification of the chlorination products of aliphatic ketones by gas chromatography and gas chromatography/mass spectrometry

The differences in the gas chromatographic retention indices of the chlorination products of aliphatic ketones and parent carbonyl compounds (ΔRI) are constant, and their numeric values depend on the number and position of chlorine atoms in the molecule. A simplest version of an additive scheme for the evaluation of retention indexes is developed to identify the chloro derivatives of carbonyl compounds. The order of the chromatographic elution of diastereomeric α,α′-dichloro-k-alkanones (k > 2) is found.     

Identification of O-Isobutyl S-(2-Diethylaminoethyl) Methylthiophosphonate Chemical Neutralization Products in Bitumen–Salt Matrices

Products of the chemical neutralization of O-isobutyl S-(2-diethylaminoethyl) methylthiophosphonate, a VX Group warfare agent, in bitumen–salt matrices were studied by gas chromatography–mass spectrometry. Forty-four compounds were characterized by their electron impact ionization mass spectra and gas chromatographic retention indices on standard nonpolar polydimethylsiloxane phases; seven of these compounds were determined as derivatives. The structures of 26 compounds were identified, including structures of 24 O-isobutyl S-(2-diethylaminoethyl) methylthiophosphonate neutralization products. To solve this problem, chromatographic retention indices of unknown compounds were evaluated using reference data for their structural analogues. Partition coefficients in the binary heterophase hexane–octanol system were determined for the major components.     

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.     

Temperature changes of sorption parameters of di-n-alkylketones and methylcyclohexanones in capillary gas chromatography

The gas chromatographic behavior of di-n-alkylketones and isomeric methylcyclohexanones at variable temperatures in the isothermal runs has been studied using three capillary columns coated with SE-30, OV-225, and PEG-40M/KF stationary phases. The retention indices and their temperature increments were determined, and the partial molar free energies of the compounds under study were calculated. A linear dependence of the values of the partial molar free energy of sorption on the homologue number in di-n-alkylketones is not observed for the first member on the nonpolar SE-30 phase and for the third member on the polar OV-225 and PEG-40M/KF phases. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 328–332, February, 1997.     

Peculiar features of sorption of positional isomers of formyl-, acetyl-, and aminopyridines in capillary gas-liquid chromatography

The GLC retention parameters of α-, β-, and γ-isomers of formyl-, acetyl-, and aminopyridines were determined on two columns with stationary phases of different polarities. The contributions of formyl, acetyl, methyl, and amino groups located in the α-, β-, and γ-positions of the ring to the retention indices were calculated. The contributions of these groups to the retention were found to depend on their positions with respect to the N atom of the ring and are the smallest for α-substituents. The difference between pyridines containing electron-withdrawing substituents and those containing electron-donating substituents is manifested as different orders of elution of α-, β-, and γ-derivatives. The distinctions between the GLC behaviors of pyridines, benzenes, and furans with the same substituents were identified. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 91–94, January, 1997.     

Gas chromatographic system for the identification of halogenated pesticides by retention indices using n-alkanes as standards

A gas chromatographic system for the evaluation of linear temperature-programmed retention indices allowing n-alkanes to be adopted as the reference retention markers for any type of analyte, irrespective of the atoms present in their molecules, is described. It is based on the simultaneous use of two different detectors (a flame ionization detector and a specific detector suitable for the sample components), both connected (in parallel) to the same column outlet. The performance of this system has been tested by measuring the retention indices of fifteen chlorinated pesticides under conditions of linear programming temperature, by adopting an electron-capture detector as the specific detector. The reliability of the retention indices thus determined has been proven by verifying that they can be reproduced under different chromatographic conditions.     

Dangerous tendency in the presentation of GC-MS identification results

A dangerous tendency in the presentation of the results of the GC-MS identification of complex mixture components is discussed. This is an increasing number of publications where unacceptable results are presented. The main signs of them are the absence of a correspondence between the lists of identified compounds and the general principles of chromatographic retention, the indication of erroneous retention indices and chemical names, the unreliable chemical origin of analytes, etc. The main criteria for revealing such errors are the control of the chromatographic elution order of analytes and the absolute values of their retention indices.     

Chromatographic and chromatospectral characteristic of dicarboxylic acid monoesters

Monoalkyl esters of benzene-1,2-dicarboxylic (phthalic) acid have not been reliably characterized by analytical parameters for their chromatographic and chromatospectral identification. Mass spectra of a series of monoalkyl phthalates (R = C₁–C₈) are discussed; their gas chromatographic retention indices on the RTX-5 polydimethylsiloxane stationary phase (5% phenyl groups) are determined. A linear dependence of indices on the number of carbon atoms in n-alkyl fragments of molecules and a correlation between the indices of any monoalkyl phthalates and corresponding diesters were used for the control of the data obtained. Using the last correlation, we found that most part of index values of mono (2-ethylhexyl) phthalate given in literature is wrong and, therefore, the results of identification of this compound in various samples must be revised. It was found that simplest monoalkyl phthalates are unstable during gas chromatographic separation, which may result in distortions of their mass spectra. To compare with monoalkyl phthalates, a similar series of monoesters of (Z)-butenedioic (maleic) acid was characterized. Retention indices of monoalkyl phthalates in reversed phase HPLC were determined and discussed.     

Unified retention indices of hydrocarbons on BP-1 dimethylsiloxane stationary phase

Summary Gas chromatographic retention indices for 23 hydrocarbons were determined on BP-1 dimethylsiloxane between 45 and 65°C. From the data measured unified retention indices were calculated. These values agree well with the corresponding experimental values. The values of unified retention indices obtained on BP-1 are compared with those determined on OV-101 and on squalane. The differences between the unified and the experimental retention indices on BP-1, OV-101 and squalane columns are shown.     

Zur Vorausberechnung von Bruttoretentionszeiten bei temperaturprogrammierter Gaschromatographie mit Hilfe isotherm bestimmter Retentionsindices und einer Anpassung an experimentelle Retentionszeiten

Zusammenfassung Durch mathematische Modellierung und Parameterschätzung lassen sich aus isotherm bestimmten Retentionsindices und ihrem Temperaturgang sowie aus wenigstens drei Stützwerten einer aktuellen temperaturprogrammierten Ánalyse Bruttoretentionszeiten weiterer Komponenten vorausberechnen. Am Beispiel einer mit Methylsilikonöl OV-1 imprägnierten Glaskapillarsäule wird gezeigt, daß für 19 Parametersätze mit Variation von Trägergasvordruck, isothermer Vorlaufzeit, Temperaturanstieg und isothermer Nachlauftemperatur die Bruttoretentionszeiten von 28 Komponenten einer Benzinprobe mit mittleren relativen Gesamtfehlern zwischen 1,3% und 3,8% je Analyse erhalten werden konnten.

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Prediction of retention times in temperature-programmed gas chromatography from isothermally determined retention indices and adjustment using experimental values of retention times

Summary Mathematical modelling and parameter estimation from three or more reference values in an actual temperature-programmed gas chromatographic analysis allows the prediction of retention times of the other components from a knowledge of their isothermally determined retention indices and the temperature profile. It is shown using a glass capillary column coated with OV-1 for measuring 19 sets of parameters obtained by varying the carrier gas inlet pressure, pre-program time, rate of temperature rise and past-program time, that the retention times of 28 components of a gasoline sample can be predicted with a mean error lying between 1,3% and 3.8% for each of these 19 analyses.

     

Capillary chromatography of polyfluorinated carboxylic esters

Retention indices and dI/dT values were obtained for members of six homologous and six pseudohomologous series of polyfluorinated saturated carboxylic esters on capillary columns with OV-101 or XE-60 as stationary phase. There was a nonlinear dependence of retention indices of fluorinated fatty acid esters on carbon chain length of homologs with m, n=1–6. A universal correlation equation was shown to describe the chromatographic behavior of polyfluorinated carboxylic esters.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1740–1745, August, 1991.     

Dependence of molecular polarizability on the orientational order in nematic liquid crystals

The dispersion dependences of refractive indices in the visible range were used to obtain experimental values of the Lorentz tensor components L _j and the mean molecular polarizability $ \bar \gamma $ \bar \gamma for five nematic liquid crystals belonging to two homological series. The dependence of L _j components on the homologue number, mesophase temperature, birefringence value, and the orientational order of molecules in the nematic phase and upon a nematic-smectic A phase transition was revealed. The effect of the isotropization of the Lorentz tensors and the local field tensor with decreasing birefringence and molecular polarizability anisotropy Δγ was confirmed. The quadratic dependence $ \bar \gamma $ \bar \gamma (S) on the molecular orientational order parameter S in the nematic phase was found. It was invariant with respect to the nematic-smectic A transition. The dependences $ \bar \gamma $ \bar \gamma (S) and Δγ(S) are explained within molecular statistical theory as consequences of the correlation between orientational and conformational degrees of freedom of molecules. These conformational degrees of freedom are related to the internal rotation of molecular fragments, which affects the electronic conjugation of the fragments and the oscillator strengths of molecular transitions.     

Capillary gas chromatography of monosubstituted cyclopentenes and cyclohexenes

Summary Regularities in the variation of retention indices (I) and index increments (δI/°C, H, ΔI) of monosubstituted cyclopentenes and cyclohexenes with molecular structure on various liquid phases are discussed. An equation for the linear dependence of I on the carbon number is given. Differences in gas chromatographic characteristics between 1- and 3-isomers are more pronounced in the n-alkylcyclopentene series. In the n-alkylcyclohexene series extension of the side chain and increase of column temperature lead to the inversion of the elution order of 1-and 3-isomers. The temperature increments of I for cyclenes are markedly greater than those for open chain hydrocarbons (n-alkenes, n-alkynes).     

Capillary gas chromatography of aromatic bicyclic and tricyclic spiro ketones

Summary The separation of some aromatic bicyclic and tricyclic spiro ketones on fused silica capillary columns coated with polydimethylsiloxane, cyanopropylmethylsiloxane and poly(ethylene glycol) stationary phases was investigated. Retention indices were determined at two temperatures in order to enable understanding of the compounds’ chromatographic behaviour. The respective standard deviation values were 0.8, 0.5 and 0.3 index units. The influence of the polarity of the stationary phases on the chromatographic retention of these cyclic spiro ketones is discussed.     

Gas chromatographic retention indices for C6 alkanols on OV-101 and Carbowax 20M capillary columns

Summary Gas chromatographic retention indices for 16 C₆ alkanols measured on capillary columns coated with OV-101 methylsilicone and Carbowax 20M propylethyleneglycol liquid phases at 80° and 90°C were determined. The respective standard deviation values were 0.5 and 0.8 i.u. The correlation between the retention indices measured on non-polar and polar stationary phases and the boiling points of the alkanols was studied.     

Identification of organic reaction products in the absence of additivity of chromatographic retention indices. Chloro derivatives of methyl-tert-butyl ketone

As a result of a joint interpretation of mass spectra and gas chromatographic retention indices more than ten products of free radical chlorination of methyl-tert-butyl ketone are identified. They contain from one to six chlorine atoms in the molecule and were not previously characterized either by mass spectrometric or chromatographic reference data. It is found that retention index increments corresponding to a gradual increase in the number of chlorine atoms in the molecule per unit are non-additive and vary in wide ranges (from 53 i.u. to 219 i.u.), which does not hinder the use of separate elements of additive schemes for the estimation of the indices of products of such non-regioselective reactions.     

Reproducibility of retention indices measurement of alkylbenzenes on crosslinked methyl silicone phase fused silica capillaries

Summary Reproducibility of the Kovats retention indices of alkylbenzenes was studied on conventional “PONA” fused-silica capillary columns coated with crosslinked dimethyl silicone phase, with a film thickness of 0.5 μm. The data were compared with those obtained on conventional OV-101 columns. The tested PONA columns showed reproducible chromatographic properties, capacity ratios and high efficiencies. Having a standard deviation of s=0.03 i.u. of the retention indices on a single column, the column-to-columns differences were found to be up to 0.3 i.u. Higher indices were determined for crosslinked columns, the difference being in most cases about one i.u.     

Use of molecular connectivity indices to predict LC retention of dansylamides in six different eluent systems

Summary Molecular connectivity indices (x) through the sixth order were calculated and compared with measured reversed-phase liquid chromatographic retention data of sixteen dansyl derivatives of amines (dansylamides). Retention measurements were performed in the capacity factor, k′, range 1–30 with organic solvent-water eluents using acetone, acetonitrile, ethanol, methanol, 1-propanol and 2-propanol. Mainly valence level indices were selected to describe retention. Indices of different order were selected in the different eluents, suggesting that there are differences in the retention mechanism in these eluents. Retention can be predicted with great accuracy by the molecular connectivity indices, as the high correlations between the calculated and observed retention indicate. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Structural regularities governing sorption and gas chromatographic retention of aromatic nitrogen-containing heterocyclic compounds

The regularities governing differences in the gas chromatographic retention indices of aromatic nitrogen-containing heterocyclic compounds were studied during gas chromatographic analysis on a nonpolar capillary column. The retention indices of pyrrole, pyrazole, imidazole, 1,2,4-triazole, oxazole, thiazole, isoxazole, pyridine, pyridazine, pyrimidine, pyrazine, ands-triazine and their alkyl derivatives depend on the number, nature, and arrangement of heteroatoms and alkyl groups in the cycles. The majority of homologous series ofn-alkyl-substituted azoles and azines andn-alkylbenzenes is characterized by anomalously high differences between the retnetion indices of ethyl and methyl homologs. A scheme of calculation of retention indices of aromatic heterocyclic compounds from increments of heteroatoms was proposed.