Gas Chromatographic Retention Times Research Articles

A simple, fast, and accurate thermodynamic-based approach for transfer and prediction of gas chromatography retention times between columns and instruments Part I: Estimation of reference column geometry and thermodynamic parameters.

The transfer of retention times based on thermodynamic models between columns can aid in separation optimization and compound identification in gas chromatography. Although earlier investigations have been reported, this problem remains unsuccessfully addressed. One barrier is poor predictive accuracy when moving from a reference column or system to a new target column or system. This is attributed to challenges associated with the accurate determination of the effective geometric parameters of the columns. To overcome this, we designed least squares-based models that account for geometric parameters of the columns and thermodynamic parameters of compounds as they partition between mobile and stationary phases. Quasi-Newton-based algorithms were then used to perform the numerical optimization. In this first of three parts, the model used to determine the geometric parameters of the reference column and the thermodynamic parameters of compounds subjected to separation is introduced. As will be shown, the overall approach significantly improves the predictive accuracy and transferability of thermodynamic data (and retention times) between columns of the same stationary phase chemistry. The data required for the determination of the thermodynamic parameters and retention time prediction are obtained from fast and simple experiments. The proposed model and optimization algorithms were tested and validated using simulated and experimental data.

A simple, fast, and accurate thermodynamic-based approach for transfer and prediction of gas chromatography retention times between columns and instruments Part III: Retention time prediction on target column.

This is the third part of a three-part series of papers. In Part I, we presented a method for determining the actual effective geometry of a reference column as well as the thermodynamic-based parameters of a set of probe compounds in an in-house mixture. Part II introduced an approach for estimating the actual effective geometry of a target column by collecting retention data of the same mixture of probe compounds on the target column and using their thermodynamic parameters, acquired on the reference column, as a bridge between both systems. Part III, presented here, demonstrates the retention time transfer and prediction from the reference column to the target column using experimental data for a separate mixture of compounds. To predict the retention time of a new compound, we first estimate its thermodynamic-based parameters on the reference column (using geometric parameters determined previously). The compound's retention time on a second column (of previously determined geometry) is then predicted. The models and the associated optimization algorithms were tested using simulated and experimental data. The accuracy of predicted retention times shows that the proposed approach is simple, fast, and accurate for retention time transfer and prediction between gas chromatography columns.

Aggregation Pheromone of the Bearded Weevil, Rhinostomus barbirostris (Coleoptera: Curculionidae): Identification, Synthesis, Absolute Configuration and Bioactivity.

The bearded weevil, Rhinostomus barbirostris (Coleoptera: Curculionidae: Dryophthorinae), attacks coconut trees, oil palms and other species of Arecaceae. Besides direct damage, R. barbirostris may be a vector of diseases in coconut and oil palms, such as stem bleeding (resinosis) and red ring disease. Currently, the only method to control this weevil is by visual observation of damage and removal of infected plants. Semiochemical-based trapping could improve the effectiveness of monitoring and management of R. barbirostris. In comparisons of volatiles released by R. barbirostris males and females by gas chromatography (GC) two male-specific compounds were observed. GC-mass spectrometry (MS) and GC-Fourier transform-infrared (FTIR) analyses of the natural compounds suggested these were diastereoisomers of 5-hydroxy-4-methylheptan-3-one, also known as sitophilure, a pheromone component of other dryophthorine species. Synthesis of the mixture of all four stereoisomers of sitophilure was performed in two steps, and the chemical structures were confirmed by comparing GC retention times and MS and FTIR spectra of natural and synthetic compounds. The absolute configurations of the two male-specific compounds were elucidated by enantioselective GC; the major component was the (4S,5R)-isomer, and the minor component (4S,5S)-sitophilure. In analyses by GC-electroantennography (EAG) the antennae of male and female R. barbirostris only responded to the (4S,5R)-isomer of the synthetic sitophilure. The stereoisomeric mixture of sitophilure was attractive to both sexes of R. barbirostris in laboratory experiments in the presence of sugar cane volatiles, and a similar result was obtained in a preliminary field trapping test.

Application of Genetic Programming (GP) in Prediction of Gas Chromatographic Retention Time of some Pesticides

Application of Genetic Programming (GP) in Prediction of Gas Chromatographic Retention Time of some Pesticides

Extracting Vapor Pressure Data from GLC Retention Times. Part 1: Analysis of Single Reference Approach

The vapor pressures of 52 compounds including n-alkanes and their monosubstituted derivatives, as well as chloro- and alkylbenzenes were determined exploring the gas chromatographic relative retention time (GLC-RT) technique in its simplest form, that is, using a single reference standard and neglecting the activity coefficient effect (GLC-RT1S). The selection of compounds was limited to those which are in the liquid state under the measurement conditions and for which high-quality vapor pressures obtained by direct methods are available at temperatures of GLC measurements. This approach should eliminate possible secondary error sources (such as data extrapolation or recalculation from supercooled liquid to solid phase or vice versa). On the basis of a thorough comparison of the GLC-RT1S-based and directly measured vapor pressures for the group of (functionalized) n-alkanes and by exploiting n-alkanes as reference solutes it is clear that deviations from direct data are significant (in many tens of percen...

A Chiral Metal-Organic Material that Enables Enantiomeric Identification and Purification

Summary We show that CMOM-3S, a previously unreported porous crystalline metal-organic material that exhibits intrinsic homochirality, serves as a general-purpose chiral crystalline sponge (CCS) and a chiral stationary phase (CSP) for gas chromatography (GC). The properties of CMOM-3S are enabled by nano-sized channels connected to adaptable molecular recognition sites that mimic enzyme-binding sites. Further, CMOM-3S is composed of inexpensive components, facile to prepare, and requires only trace amounts of analyte. When coupled with the thermal and hydrolytic stability of CMOM-3S, these features mean that a coated fused silica capillary column in which CMOM-3S serves as a CSP is both more versatile and more robust than three benchmark commercial columns. That the enantiomer with the longer GC retention time is consistently captured in CCS experiments enables CMOM-3S to serve as a powerful tool to enable both chiral purification and enantiomer identification.

Determination and prediction of octanol-air partition coefficients for organophosphate flame retardants

Organophosphate flame retardants (OPFRs) have attracted wide concerns due to their toxicities and ubiquitous occurrence in the environment. In this work, Octanol-air partition coefficient (KOA) for 14 OPFRs including 4 halogenated alkyl-, 5 aryl- and 5 alkyl-OPFRs, were estimated as a function of temperature using a gas chromatographic retention time (GC-RT) method. Their log KOA-GC values and internal energies of phase transfer (ΔOAU/kJmol−1) ranged from 8.03 to 13.0 and from 69.7 to 149, respectively. Substitution pattern and molar volume (VM) were found to be capable of influencing log KOA-GC values of OPFRs. The halogenated alkyl-OPFRs had higher log KOA-GC values than aryl- or alkyl-OPFRs. The bigger the molar volume was, the greater the log KOA-GC values increased. In addition, a predicted model of log KOA-GC versus different relative retention times (RRTs) was developed with a high cross-validated value (Q2(cum)) of 0.951, indicating a good predictive ability and stability. Therefore, the log KOA-GC values of the remaining OPFRs can be predicted by using their RRTs on different GC columns.

A Reproducible Capillary Gas Chromatographic Technique for the Determination of Specific Terpenes in Grape Juice and Wine

A reliable and highly reproducible technique for the Freon 11 extraction of 16 terpenes from grape juice and wine and their gas chromatographic analyses is presented. Terpene concentrations were calculated as relative values by means of an internal standard. Standard deviations of the terpenes analysed varied between 1,2% and 13,9% for non-settled grape juice and between 2,0% and 5,2% for wine. Variation in gas chromatographic retention times was between 0,04 and 0,1 minutes. The freon extracts of grape juice and wine could be stored at minus 12°C for up to six days without marked changes occurring in the relevant terpene concentrations.

Predicting Partitioning and Diffusion Properties of Nonpolar Chemicals in Biotic Media and Passive Sampler Phases by GC × GC.

The chemical parameters needed to explain and predict bioavailability, biodynamics, and baseline toxicity are not readily available for most nonpolar chemicals detected in the environment. Here, we demonstrate that comprehensive two-dimensional gas chromatography (GC × GC) retention times can be used to predict 26 relevant properties for nonpolar chemicals, specifically: partition coefficients for diverse biotic media and passive sampler phases; aquatic baseline toxicity; and relevant diffusion coefficients. The considered biotic and passive sampler phases include membrane and storage lipids, serum and muscle proteins, carbohydrates, algae, mussels, polydimethylsiloxane, polyethylene, polyoxymethylene, polyacrylate, polyurethane, and semipermeable membrane devices. GC × GC-based chemical property predictions are validated with a compilation of 1038 experimental property data collected from the literature. As an example application, we overlay a map of baseline toxicity to fathead minnows onto the separated analyte signal of a polychlorinated alkanes (chlorinated paraffins) technical mixture that contains 7820 congeners. In a second application, GC × GC-estimated properties are used to parametrize multiphase partitioning models for mammalian tissues and organs. In a third example, we estimate chemical depuration kinetics for mussels. Finally, we illustrate an approach to screen the GC × GC chromatogram for nonpolar chemicals of potentially high concern, defined based on their GC × GC-estimated biopartitioning properties, diffusion properties, and baseline toxicity.

Gibbs energy additivity approaches to QSRR in generating gas chromatographic retention time for identification of fatty acid methyl ester.

The Gibbs energy additivity method was used to correlate the retention time (t R) of common fatty acid methyl esters (FAMEs) to their chemical structures. The t R of 20 standard FAMEs eluted from three capillary columns of different polarities (ZB-WAXplus, BPX70, and SLB-IL111) under both isothermal gas chromatography and temperature-programmed gas chromatography (TPGC) conditions were accurately predicted. Also, the predicted t R of FAMEs prepared from flowering pak choi seed oil obtained by multistep TPGC with the BPX70 column were within 1.0% of the experimental t R. The predicted t R or mathematical t R (t R(math)) values could possibly be used as references in identification of common FAMEs. Hence, FAMEs prepared from horse mussel and fish oil capsules were chromatographed on the BPX70 and ZB-WAXplus columns in single-step and multistep TPGC. Identification was done by comparison of t R with the t R of standard FAMEs and with t R(math). Both showed correct identifications. The proposed model has six numeric constants. Five of six could be directly transferred to other columns of the same stationary phase. The first numeric constant (a), which contained the column phase ratio, could also be transferred with the adjustment of the column phase ratio to the actual phase ratio of the transferred column. Additionally, the numeric constants could be transferred across laboratories, with similar correction of the first numeric constant. The TPGC t R predicted with the transferred column constants were in good agreement with the reported experimental t R of FAMEs. Moreover, hexane was used in place of the conventional t M marker in the calculation. Hence, the experimental methods were much simplified and practically feasible. The proposed method for using t R(math) as the references would provide an alternative to the uses of real FAMEs as the references. It is simple and rapid and with good accuracy compared with the use of experimental t R as references.

Identification of a novel fatty acid in the cell membrane of Chryseobacterium frigidisoli PB4T isolated from an East Antarctic glacier forefield

The cell membrane phospholipid fatty acid (PLFA) inventory of Chryseobacterium frigidisoli PB4T, a psychrotolerant, non-motile, gram-negative, aerobic bacterium isolated from ice free permafrost deposits in continental Antarctica contains a FA with a novel methyl ester mass spectrum. In a temperature cultivation experiment of C. frigidisoli at 0°C this unknown FA was the dominant PLFA. The molecular mass indicates a C17 FA with two double bond equivalents (DBEs). A 3-pyridylcarbinol derivatization experiment as well as hydrogenation of the double bonds allowed structural assignment of the respective FA as 14-methyl-hexadeca-9,13-dienoic acid (anteiso-heptadeca-9,13-dienoic acid or ai-17:2Δ9,13) an unusual anteiso-ω3-FA with bis-methylene interrupted double bonds (ai-17:2ω3,7 or ai-17:2n-3,7). The iso-counterpart of this unsaturated FA also occurred in trace amount. The close vicinity of the ω3 double bond to the anteiso branch and also to the iso branch seems to be responsible for a shift in the gas chromatography retention times of these compounds, resulting in elution shortly after the saturated counterparts. The dominance of the ai-17:2ω3,7 at 0°C cultivation temperature suggests its importance for cell membrane adaptation of C. frigidisoli with respect to low ambient temperature conditions.

Retention modeling and retention time prediction in gas chromatography and flow-modulation comprehensive two-dimensional gas chromatography: The contribution of pressure on solute partition

This study aims at modelling and predicting solute retention in capillary Gas Chromatography (GC) and Flow Modulation comprehensive GC (FM-GCxGC). A new thermodynamic model, taking into account the effects of temperature and pressure, is proposed to describe the variation of the equilibrium partition constant of a solute during its elution. This retention model was challenged with the classical one, and both were applied to: (i) stationary phase film thickness indirect estimation; (ii) retention time (RT) prediction of a set of 11 model polycyclic aromatic hydrocarbons (PAHs) on the SLB-IL60 and DB-35MS columns, in temperature-programmed mode. Film thickness determination led to values about 2 times lower than those indicated by column nominal dimensions, whatever the employed model. Prediction of retention times in GC led to 0.84 and 0.26% mean errors using the classical and the extended models, respectively. Prediction in GCxGC gave 5.5 and 0.44% mean errors in 1st dimension RTs, and 7.3 and 2.2% mean errors in 2nd dimension RTs, using the classical and the extended models, respectively.

Vapor pressures and vaporization enthalpy of (−) α-bisabolol and (dl) menthol by correlation gas chromatography

The vapor pressures and vaporization enthalpies of (−) α-bisabolol and (dl)-menthol, two GRAS chemicals (generally recognized as safe) are evaluated by correlation gas chromatography using a series of saturated primary alcohols as standards. Vaporization enthalpies of (96.6±2.4) and (74.2±2.8)kJmol−1 and vapor pressures of p/Pa=(0.020±0.003) and (4.5±0.44) were evaluated at T=298.15K for (−) α-bisabolol and (dl)-menthol, respectively, and compared to literature values. The vapor pressures of both compounds from T=(298.15 to 500)K have been derived from correlations using vapor pressures of a series of 1-alkanols and corresponding gas chromatographic retention times at 10K intervals. The results were fit to a second order polynomial. Calculated normal boiling temperatures of TB=(574.8 and 492.7)K are calculated for (−) α-bisabolol and (dl)-menthol, respectively. A normal boiling temperature of TB=(485.2, and 489.7)K has previously been reported for (dl)-menthol. Vapor pressures for both (l)-menthol and (dl)-menthol from a previous study and (dl)-menthol from this study are compared with literature values.

Radial basis function neural networks based on projection pursuit approach and solvatochromic descriptors: single and full column prediction of gas chromatography retention behavior of polychlorinated biphenyls

The ultimate goal of projection methods is to search “interesting” projections in a low‐dimensional subspace that can uncover the natural structure of the data. The aim of this work is to compare the ability of projection pursuit (PP) and principal component analysis (PCA) in dimension reduction. For this purpose to be achieved, the scores of PP and PCA, by a different number of factors, were used as inputs of radial basis function (RBF) neural network. RBF neural network was used as a nonlinear regression method in a quantitative structure‐retention relationships study of 209 polychlorinated biphenyls (PCBs). The dependent variable was the high‐resolution gas chromatographic relative retention times of PCBs on 18 different stationary phases, and independent variables were solvatochromic solute descriptors. The results demonstrate that the dimension reduction ability of the PP is better than that of the PCA for both single and full column retention models.

A procedure for comprehensive two-dimensional gas chromatography retention time locked dual detection

In this paper, a novel, and easy to perform, retention time locking procedure for locking primary and secondary retention times of detector signals in comprehensive two-dimensional gas chromatography (GCxGC) dual-detection is proposed and its advantages are demonstrated and discussed. The dual detection retention time locking procedure is a 2-step process for a GCxGC system in which the effluent of the primary column is split, by using a pressure regulated splitter, towards the GCxGC modulator using two identical secondary GC columns of which one is installed in the main GC oven and the other is installed in a secondary GC oven. The first step of the locking procedure is to minimize the secondary retention time difference between both detectors of a compound, which has a retention factor (k) close to 0. This is done by stepwise altering the effective secondary column length, simply by sliding the secondary column, which is installed in the main oven, forwards or backwards through the modulator. The second step is to minimize the secondary retention time difference of a compound which has a significant retention in both dimensions. This is done by stepwise altering the secondary oven temperature rate. This locking procedure was successfully demonstrated for the analysis of a diesel sample by GCxGC coupled to a time of flight mass spectrometer (TOFMS) and a nitrogen chemiluminescence detector (NCD) and by GCxGC coupled to a TOFMS and a flame ionization detector (FID). For all compounds the average absolute secondary retention time differences between the NCD or the FID and the TOFMS detectors were 0.03, and 0.07s, respectively, which are significantly less than the average peak widths at half heights, which was 0.2s.

Application of a quantitative structure retention relationship approach for the prediction of the two-dimensional gas chromatography retention times of polycyclic aromatic sulfur heterocycle compounds

Information on the sulfur classes present in petroleum is a key factor in determining the value of refined products and processing behavior in the refinery. A large part of the sulfur present is included in polycyclic aromatic sulfur heterocycles (PASHs), which in turn are difficult to desulfurize. Furthermore, some PASHs are potentially more mutagenic and carcinogenic than polycyclic aromatic hydrocarbons, PAHs. All of this calls for improved methods for the identification and quantification of individual sulfur species. Recent advances in analytical techniques such as comprehensive two-dimensional gas chromatography (GC×GC) have enabled the identification of many individual sulfur species. However, full identification of individual components, particularly in virgin oil fractions, is still out of reach as standards for numerous compounds are unavailable. In this work, a method for accurately predicting retention times in GC×GC using a QSRR (quantitative structure retention relationship) method was very helpful for the identification of individual sulfur compounds. Retention times for 89 saturated, aromatic, and polyaromatic sulfur-containing heterocyclic compounds were determined using two-dimensional gas chromatography. These retention data were correlated with molecular descriptors generated with CODESSA software. Two independent QSRR relationships were derived for the primary as well as the secondary retention characteristics. The predictive ability of the relationships was tested by using both independent sets of compounds and a cross-validation technique. When the corresponding chemical standards are unavailable, the equations developed for predicting retention times can be used to identify unknown chromatographic peaks by matching their retention times with those of sulfur compounds of known molecular structure.

Computational Model for Chromatographic Relative Retention Time of Polychlorinated Biphenyls Using Sub-structural Molecular Fragments

Quantitative structure-retention relationship (QSRR) analysis is a useful technique capable of relating chromato- graphic retention time to the chemical structure of a solute. Using the sub-structural molecular fragments (SMF) derived directly from the molecular structures, the gas chromatographic relative retention times (RRTs) of 209 polychlorinated biphenyls (PCBs) on the SE-54 stationary phase were calculated. An eight-variable regression equation with the correlation coefficient of 0.9945 and the root mean square errors of 0.0134 was developed. Forward and backward stepwise regression variable selection and multi-linear regression analysis (MLRA) are combined to describe the effect of molecular structure on the RRT of PCB according to the QSRR method. To quantitatively relate RRT with the molecular structure MLR analysis is performed on the set of 163 sub-structural molecular fragments (SMF) provided by the ISIDA software. The eight fragments selected by variable subset selection, all belonging to the sub-fragments, adequately represent the structural factors influencing the affinity of PCB to SE-54 stationary phase in the separation process. Finally, a QSRR model is selected based on leave-one-out cross-validation and its prediction ability is further tested on 42 representative compounds excluded from model calibration. The prediction results from the MLR model are in good agreement with the experimental values. By applying the MLR method we can predict the test set with squared cross validated correlation coefficient (Q 2) of 0.9913 and root mean square error (RMSE) of 0.0169.

A Comparison of Results by Correlation Gas Chromatography with Another Gas Chromatographic Retention Time Technique. The Effects of Retention Time Coincidence on Vaporization Enthalpy and Vapor Pressure

The vaporization enthalpies of a mixture of n-alkanes, aliphatic esters, and dialkyl phthalates are examined both by correlation gas chromatography and another commonly used gas chromatographic retention time technique according to the protocol described by Hamilton. The resulting vaporization enthalpies and vapor pressures at T/K = 298.15 are compared with respect to the functional groups present. Additionally, two compounds, docosane and ethyl octadecanoate, exhibited retention time coincidence over a T/K = 30 temperature range. These compounds have been examined by correlation gas chromatography to determine how this coincidence affects the thermochemical properties evaluated.

Novel saturated hexacyclic C34 and C35 hopanes in lacustrine oils and source rocks

Novel C34 and C35 hexacyclic hopanes having side chains with a pentacyclic or hexacyclic ring at C-22 are abundant in lacustrine oils and source rocks but almost absent from marine oils. The corresponding C34 and C35 hexacyclic hop-17(21)-enes were identified in thermally immature mudstones from Vietnam cored in the 500m deep ENRECA-3 well. All the novel hexacyclic compounds were unambiguously identified by comparison with synthetic standards. Traces of heptacyclic hopanes were detected in a lacustrine oil from Thailand. The C34 and C35 hexacyclic hopanes have much longer gas chromatography (GC) retention times than the corresponding pentacyclic hopanes. They are isomerized faster at C-22 than the pentacyclic hopanes and their 22S/(22S+22R) ratio values at equilibrium are much higher (0.8 for C34). Precursors could be penta- and hexafunctionalised hopanoids in cyanobacteria and methanotrophs.

Thermal isomerizations of monothiolated carboranes (HS)C2B10H11 and the solid-state investigation of 9-(HS)-1,2-C2B10H11 and 9-(HS)-1,7-C2B10H11

At 300–500 °C, three C-thiolated closo-dicarbadodecaborane isomers 1-(HS)-1,2-C2B10H11 (1-o), 1-(HS)-1,7-C2B10H11 (1-m), and 1-(HS)-1,12-C2B10H11 (1-p), and two B-thiolated isomers 9-(HS)-1,7-C2B10H11 (9-m) and 9-(HS)-1,2-C2B10H11 (9-o) show two types of reaction: first, removal of an SH group from the closo-dicarbadodecaborane skeleton, and second, skeletal isomerizations from ortho to meta to para that lead to new isomers. A previously unreported SH skip from carbon-to-boron is also observed. The effect of the thiol group on the skeletal rearrangement is discussed. The isomerisation products are assigned on the basis of correlation of their computationally obtained dipole moments with their gas-chromatographic retention times. Computational results on molecular energies for the mono-thiolated species show good agreement between the calculated relative stabilities and the incidence and relative quantities of the isomerization products. Two of the starting B-thiolated isomers, 9-o and 9-m, were characterized using single-crystal X-ray diffraction analyses and their crystallographic packings as well as some selected structural parameters are discussed. All starting compounds were characterized using multinuclear NMR spectroscopy.