Gas Chromatographic Retention Data Research Articles

Transfer of gas chromatographic retention data among poly(siloxane) columns by quantitative structure-retention relationships based on molecular descriptors of both solutes and stationary phases

In the present study, computational molecular descriptors of 90 saturated esters and seven poly(siloxane) stationary phases with different polarity (SE-30, OV-7, DC-710, OV-25, XE-60, OV-225 and Silar-5CP) were combined into quantitative structure-retention relationship (QSRR) models aimed at predicting the Kováts retention indices (RIs) of the solutes. The molecular descriptors (174) of the stationary phases included in the models were computed using Dragon software from poly(siloxane) oligomers made of 20 siloxane units reflecting the nominal composition of the stationary phase, whereas 439 molecular descriptors were adopted to represent the esters. Different QSRR models were generated by means of Partial Least Squares (PLS) regression to assess the accuracy of this approach in predicting the RIs of unexplored solutes both in known and external stationary phases. After calibration of each PLS model, the descriptors were selected/discarded according to their relevance, evaluated by Covariance Selection (CovSel), and the PLS models were re-built, which resulted in a noticeable improvement of their predictive ability. Firstly, all the available data were equally divided into a training and a test set; the model built on the calibration set was used to predict the RIs of the validation observations. Successively, seven diverse PLS models were created following a “leave-one-column-out” fashion procedure, each one finalized to the estimation of the RIs of the 90 esters associated with a single stationary phase, whereas the calibration model was calculated on the remaining data. All the estimated models provided successful results on the external stationary phase, and predictive performance further increased after variable selection based on CovSel analysis. The final models provided a Root Mean Square Error in Cross Validation (RMSECV) in the range 12–20, a Root Mean Square Error in Prediction (RMSEP) in the range 11–26, and Mean Absolute Percentage Errors in Prediction (MAMEPs) in the range 0.7–1.5, revealing accurate cross-column prediction. Eventually, to test the robustness of the proposed approach, the 90 solutes were equally partitioned into a calibration and a test set and two further QSSR strategies were applied. The first PLS model was calibrated on all the seven stationary phases and the RIs of the 45 external solutes in the same seven columns were simultaneously predicted. The last QSRR approach followed a “leave-one-column-out” scheme and RI of 45 test solutes on an external stationary phase was predicted by a PLS model calibrated with the data of the 45 remaining solutes and the six left stationary phases. After selection of the significant molecular descriptors, PLS regression provided RMSECV values in the range 6–19, RMSEPs in the range 10–14, and MAPEPs in the range 0.9–2.4, revealing the suitability of the approach to deduce the RI of unknown solutes in uncharted stationary phases.

Long-chain syn-1-phenylalkane-1,3-diyl diacetates, related phenylalkane derivatives, and sec-alcohols, all possessing dominantly iso-branched chain termini, and 2/3-methyl-branched fatty acids from Primula veris L. (Primulaceae) wax

Herein, the results of the first study of non-flavonoid constituents of aboveground surface-wax washings of Primula veris L. (Primulaceae) are presented. Chromatography of the washings yielded a minor fraction composed of n-, iso-, and anteiso-series of long-chained syn-1-phenylalkane-1,3-diyl diacetates, 3-oxo-1-phenylalkan-1-yl acetates, 1-phenylalkane-1,3-diones, 1-hydroxy-1-phenylalkan-3-ones, sec-alcohols (2- to 10-alkanols), and n-, iso-, anteiso-, 2-methylalkanoic and 3-methylalkanoic acids; 118 of these constituents represent up to now unreported natural compounds. The structural/stereochemical elucidation was accomplished by the synthesis of authentic standards, derivatization reactions, the use of gas chromatographic retention data and detailed 1D and 2D-NMR analyses of the obtained complex chromatographic fraction. Primula veris produces unusually high amounts of branched long-chained metabolites (>60%) except for the fatty acids where the percentage of branched isomers is comparable to the ones with n-chains. Noteworthy is the fact that long-chained α- and β-methyl substituted fatty acids were detected herein for the first time in the kingdom Plantae.

Detection of a secondary organic aerosol tracer derived from personal care products

Decamethylcyclopentasiloxane (D5) is frequently used in personal care products (PCPs). In the gas phase, D5 is oxidized to form 1-hydroxynonamethylcyclopentasiloxane (D4TOH), which can partition to the particle phase. Numerous studies have reported secondary organic aerosol (SOA) formation via hydroxyl (OH) radical-initiated oxidation of D5. It is expected that PCPs have a significant impact on SOA, but the extent has not yet been investigated. To date, no studies have reported the occurrence of PCP-derived SOA in ambient particulate matter. This study examined fine particulate matter (PM2.5) collected in Atlanta, GA and Houston, TX and determined D4TOH was present in 28 of 29 and 33 of 46 ambient PM2.5 samples, respectively. Gas chromatographic retention data in the form of the Kováts index is reported for the first time to aid others in identifying this compound in order to assess the impact of PCPs on SOA formation. The estimated concentration of D4TOH ranged from 16 to 185 pg m-3 in Atlanta and 19–206 pg m-3 in Houston. Synthetic musks were also detected in Atlanta and Houston PM2.5 samples, which is consistent with PCPs impacting urban air quality. Because of its specificity and demonstrated detectability, D4TOH may be useful as a tracer of PCP-derived secondary organic aerosol.

Measurements of Activity Coefficients at Infinite Dilution for Organic Solutes in the Ionic Liquids N-Ethyl- and N-Octyl-N-methylmorpholinium Bis(trifluoromethanesulfonyl)imide. A Useful Tool for Solvent Selection.

In recent years, many papers describing ionic liquids (IL) as promising solvents in separation techniques have been published. The conscious choice of appropriate ionic liquid as absorption media in effective extraction of selected types of analytes requires deeper understanding of the analyte−IL interactions. Therefore, intensive research is conducted to determine the values of activity coefficient at infinite dilution, which allows us to characterize the nature of these interactions. Based on the inverse gas chromatography retention data, activity coefficients at infinite dilution of 48 different organic compounds in the ionic liquids N-ethyl-N-methylmorpholinium bis(trifluoromethanesulfonyl)imide [C2C1Mor][TFSI] and N-octyl-N-methylmorpholinium bis(trifluoromethanesulfonyl)imide [C8C1Mor][TFSI] were determined. The measurements covered a broad range of volatile organic compounds, including n-alkanes, n-alkenes, n-alkynes, alcohols, aldehydes, ketones, aromatic compounds and common polar solvents, representing different types of interactions. Activity coefficients at infinite dilution were measured in the temperature range from 313.15 to 363.15 K. The excess partial molar enthalpies and entropies at infinite dilution were determined. Selectivity at infinite dilution was also calculated for exemplary separation processes in the hexane/benzene system. The obtained results were analyzed and compared with literature data for ionic liquids containing the same anion [TFSI]¯ and different cations. The study results indicate that some potential applications of the investigated ionic liquids in separation problems exist.

Descriptors for terpene esters from chromatographic and partition measurements: Estimation of human odor detection thresholds

We have used gas chromatographic retention data together with other data to obtain Abraham descriptors for 30 terpene esters. These include the air-water partition coefficient, as log Kw, for which no experimental values are available for any terpene ester. The other descriptors are the ester dipolarity, S, the hydrogen bond basicity, B, (the ester hydrogen bond acidity is zero for the esters studied), and L the logarithm of the air-hexadecane partition coefficient. Both S and B are larger than those for simple aliphatic esters, as expected from the terpene ester structures that include ring systems and ethylenic double bonds. These descriptors can then be used to obtain a large number of physicochemical and environmental properties of terpene esters. We have analyzed experimental results on human odor detection thresholds and have constructed another equation for the calculation of these thresholds, to go with a previous equation that we have reported. Then the descriptors for terpene esters can be used to estimate the important odor detection thresholds.

Characterization of cyclitol glycosides by gas chromatography coupled to mass spectrometry

Several cyclitol glycosides have been characterised as trimethylsilyl derivatives by their gas chromatographic (GC) retention data (linear retention indices) and electron impact mass spectrometric (MS) profiles. Both GC–MS results have been related to cyclitol glycosides structural features. Abundance ratios of characteristic m/z ions 133/129 and 260/265 have been proposed to distinguish glycosyl-inositols from glycosyl-methyl-inositols. These ratios in combination with the presence or absence of m/z 375 ion allowed the unequivocal characterization of cyclitol glycosides. These criteria have been applied to the characterization of new cyclitol glycosides in chickpea (Cicer arietinum) and adzuki bean (Vigna angularis) and in leaves of Coriaria myrtifolia and Coriaria ruscifolia.

Correlation of normal boiling points of dialkylalkyl phosphonates with topological indices on the gas chromatographic retention data

The normal boiling point of twelve dialkylalkyl phosphonates has been determined using gas chromatographic technique. Dibutylhydrogen phosphonate has been used as reference for computing normal boiling point of dialkylalkyl phosphonates. Retention times of dialkylalkyl phosphonates have been measured by using temperature programmed technique. The topological indices namely, odd–even index, atom type index and steric effect index have been designed to capture surface interaction parameters. It was found to exhibit excellent correlation of the topological indices to the normal boiling point of dialkylalkyl phosphonates. Multiple linear regression analysis has been performed for development of quantitative structure property relationships discipline. It exhibited good predictive power (R2=0.998).

Characterization of trimethylsilyl ethers of iminosugars by gas chromatography–mass spectrometry

A combination of gas chromatographic retention data (linear retention indices) on two different stationary phases (100% methyl- and 50% phenylmethylsilicone) and electron impact mass spectrometric relative abundances for characteristic m/z ratios of 12 trimethylsilylated piperidine and pyrrolidine iminosugars are reported. These results have been related to their structural features and applied to the characterization of iminosugar composition of an Aglaonema treubii root extract. Seven iminosugars were detected in this extract, two of them were described for the first time.

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.

Determination of solvation descriptors for terpene hydrocarbons from chromatographic measurements

Gas chromatographic retention data on 107 terpene hydrocarbons from the literature together with other data have been used to obtain a set of Abraham descriptors for these 107 compounds. For 88 aliphatic cyclic terpene hydrocarbons, a fragmentation scheme was constructed that allows key descriptors to be estimated just from structure. The total set of descriptors, including those estimated by the fragmentation schemes, were then used to predict water–octanol partition coefficients for the 88 compounds, there being good agreement with values calculated from a number of well-known programs. For a small number of terpene hydrocarbons, there was good agreement between predicted and experimental values of nasal pungency thresholds, and predicted and experimental gas–blood, gas–oil, and gas–water partition coefficients. It is suggested that the descriptors obtained for the 107 terpene hydrocarbons can be used to predict water–solvent partition coefficients, gas–solvent partition coefficients, and partition coefficients in a number of biological systems.

The relation between molecular connectivity and gas chromatographic retention data.

The recently developed topological index, molecular connectivity (chi), has been correlated using multiple regression analysis with gas chromatographic retention time (Rt) for various series of compounds. For saturated and unsaturated aliphatic hydrocarbons, and aliphatic aldehydes, good correlation with log Rt was achieved by one-parameter linear equations in terms of the first-order conncectivity index. (1chi). The equation for aliphatic alcohols required an additional parameter, the valence connectivity (vchi) for satisfactory correlation. One-parameter equations using 1chi also good correlation with log Rt for three series of drug molecules, amphetamines, barbiturates and phenothiazines. In all cases the equations generated gave good agreement between calculated and observed log Rt values.

The use of GC×GC/TOF MS with multivariate analysis for the characterization of foodborne pathogen bacteria profiles

The cellular fatty acid profiles of eight strains of Bacillus, Staphylococcus, and Enterobacteriacae (Escherichia coli and Salmonella) were analyzed by comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry. A novel template method was developed to standardize the raw two-dimensional gas chromatography retention data through the use of a chemical indexing mixture. Analyte retention coordinates were normalized in the primary dimension with respect to a series of n-alkanes (Kovats index) and in the secondary dimension with respect to a series of aromatic hydrocarbons (Lee index). Fatty acid profiles extracted from the templates were compared by multidimensional scaling and principal component analysis. Differences in the profiles of Gram-positive and Gram-negative bacteria were observed, and a series of heterogeneous mixtures comprising different fractions (containing one Gram-positive and one Gram-negative bacteria strain) were also distinguished from their homogeneous constituents.

Prediction of gas chromatographic retention time via an additive thermodynamic model

A straightforward group contribution model based on thermodynamic parameters was developed to predict retention times for a series of alcohols and ketones on three different stationary phases. Thermodynamic parameters determined from gas chromatographic retention data for structurally similar compounds via a three-parameter model were used to predict the retention times of test molecules consisting of ketones and alcohols. The model worked well for the compounds tested with a root mean square error of prediction of 5.50 s across all compounds, phases, and temperature ranges studied. Considering just the alcohols, the error of prediction was 2.79 s across all phases and temperatures.

Application of histograms in evaluation of large collections of gas chromatographic retention indices

The effective use of gas chromatographic retention data presented in the form of retention indices (RI) requires the development of a comprehensive structure-based digital archive of retention parameters. Development of such an archive includes the collection of all available RI values for a variety of compounds including replicates measured under slightly different conditions. Review of retention data often shows a relatively wide range of RI values for certain well studied compounds that is larger than expected on the basis of the simple reproducibility of experimental measurements. The finding of unusual RI data distributions and their examination presents a possible way to detect and correct errors during the development of comprehensive RI libraries. Our approach involves the construction of histograms representing the distribution of data-points in various RI intervals. The observed shape of the distribution is compared to the expected and observed shapes for well-identified compounds. Significant systematic deviations represent anomalies in the sets of RI data. The occurrence of more than a single maximum on a histogram generally indicates the presence of erroneous data. For some compounds such multimode RI distributions may be caused by differences in experimental conditions of the RI determination. The construction and interpretation of histograms for compounds with multiple RI measurements is illustrated by several examples. Thus, the RI sub-set for the diterpene alcohol, isophytol, was separated from the RI data set for phytol and four additional sub-groups of published RI data for one of the sesquiterpenes, γ-elemene, were re-identified as α-, β-, δ-elemenes and germacrene B.

Linear retention indices in gas chromatographic analysis: a review

AbstractThe main purpose of any chromatographic analysis is to resolve mixtures of compounds into less complex mixtures or ultimately into pure components. In addition to this function, the chromatographic system can provide retention data which serve as complementary information for the positive identification of resolved components. The need to express gas chromatographic retention data in a standardized system has long been recognized and retention index values presented to be a valuable parameter. Those values are mainly calculated by applying the equations proposed by Kováts, for isothermal analysis, and van den Dool and Kratz, for programmed gas chromatographic runs. In general, these indices denote the retention behaviour of the compounds of interest according to a uniform scale determined by a series of closely related standard substances. The use of retention indices in the flavour and fragrance field is well‐documented, and they are widely applied for the comparison of results between laboratories, as well as to characterize stationary phases. Copyright © 2008 John Wiley & Sons, Ltd.

Gas chromatography retention data of environmentally relevant polybrominated compounds

Polybrominated organic compounds are ubiquitous throughout the environment. This generic term comprises several classes of brominated flame retardants (e.g., polybrominated diphenyl ethers, polybrominated biphenyls, hexabromocyclododecane, dibromopropyltribromophenyl ether, 1,2-bis(2,4,6-tribromophenoxy)ethane) as well as a range of marine halogenated natural products (HNPs). Here we present gas chromatography retention times and elution orders (on DB-5) of 122 polybrominated compounds that may be found in food and environmental samples. Organobromine compounds in fish samples determined with gas chromatography interfaced to electron-capture negative ion mass spectrometry (GC/ECNI-MS) are discussed. The environmental relevance and important mass spectrometric features of the compounds are described as well. Our database aims to support the closer inspection and identification of peaks in gas chromatograms and to initiate dedicated screening for less frequently studied organobromines in samples.

Detecting organic vapors using a conductometric sensor prepared from Ardel®D-100 and graphite

The composites of graphite with Ardel®D-100 which is a trademark of a polyester of bisphenol-A with terephthalic and isophthalic acid were used as a sensing material in a conductometric vapor sensor. The magnitudes of responses are increased in the order of benzene, isobutyl acetate, isoamyl acetate, ethyl benzene and chloro benzene. This suggests that Ardel®D-100 can be used as a sensing material for chlorobenzene, ethyl benzene and isoamyl acetate. On the other hand, specific retention volumes of the sensed vapors on Ardel®D-100 coated on graphite were determined by inverse gas chromatography at temperatures between 200 and 260 °C. The distribution coefficient, K s of the vapors between stationary and mobile phases in the column was obtained. It was revealed for the first time that the logarithm of K s of the solvents varies almost linearly with their responses based on conductometric resistance of the sensing polymer composite. Subsequently, the study suggests that gas chromatographic retention data can be used in prediction of the conductometric responses of a polymeric sensor to vapors.

Development of a database of gas chromatographic retention properties of organic compounds

A comprehensive database of gas chromatographic retention properties of chemical compounds has been developed using multiple literature sources. The National Institute of Standards and Technology (NIST) database of retention data for non-polar and polar stationary phases currently contains 292,924 data records for 42,888 compounds. The database includes data for Kováts indices, linear indices, Lee indices, retention times and retention volumes. The first release of this database for non-polar stationary phases is available with NIST/US Environmental Protection Agency (EPA)/National Institutes of Health (NIH) Mass Spectral Database (June 2005) and through the internet (NIST Chemistry WebBook). The paper describes the database and the process by which it has been compiled. The format of data presentation and the quality control procedures are described. Data sources of gas chromatographic retention data are also discussed.

Quantitative structure and retention relationships for gas chromatographic data: Application to alkyl pyridines on apolar and polar phases

Quantitative structure and retention relationships (QSRR) have been developed to model gas chromatographic retention data of alkyl pyridines on apolar (branched alkane) and polar (primary alcohol) stationary phases. The retention properties analyzed are Kovats retention index, I; partial molar enthalpy, Δ H; partial molar entropy, Δ S and partition coefficient, log K. Using the seven valence molecular connectivity indices ( χ) calculated for the 18 alkyl pyridines, regression models are generated to predict the retention properties. The best model (model A) obtained with the descriptors χ P V 1 , χ P V 3 and χ CH V 6 was unable to produce a satisfactory statistical performance and correct order of elution. The model has been modified (model B) by including steric parameter s, which has been empirically derived by considering the steric effects due to the presence of alkyl groups at the ortho and meta positions. The modified model predicts the correct order of elution for all the alkyl pyridines and good correlation coefficients, r. The r values obtained for I, Δ H, Δ S and log K are: r = 0.955, 0.975, 0.984 and 0.955 (model A) become r = 0.999, 0.996, 0.990 and 0.999 (model B) on the apolar stationary phase and r = 0.931, 0.926, 0.911 and 0.931 (model A) become r = 0.999, 0.992, 0.958 and 0.999 (model B) on the polar stationary phase.

Quantitative structure-retention relationship study of the gas chromatographic retention indices of saturated esters on different stationary phases using novel topological indices

The gas chromatographic retention data of 90 saturated esters obtained isothermally at 150 °C on seven stationary phases (SE-30, OV-7, DC-710, OV-25, XE-60, OV-225 and Silar-5CP) are well-correlated with recently introduced Lu index and novel distance-based atom-type DAI topological indices using multiple linear regression (MLR) method. For the retention indices (RI) based on each stationary phase, the correlation coefficient r of the final models is larger than 0.994, and particularly, the decrease in the standard error is within the range of 53.0–64.8% as compared with the simple linear models with Lu index alone. The results indicate the efficiency of these indices in the structure–retention index correlations of complex compounds. The role of each of the molecular size and individual groups in the molecules is illustrated by analyzing the relative or fraction contributions of individual indices. The results also indicate that the molecular size makes a dominant contribution to retention behaviors, while some atomic type or groups have smaller influence dependent on the polarity of the column. The leave-one-out cross-validation demonstrates the final models to be statistically significant and reliable.