Gas Chromatographic Retention Times Research Articles

Top-Down Approach to Retention Time Prediction in Comprehensive Two-Dimensional Gas Chromatography-Mass Spectrometry.

In this contribution, we describe a novel modeling approach to predicting retention times (tr) in comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC × GC-ToF-MS) with a particular emphasis on the second-dimension (2D) retention time predictions (2tr). This approach is referred to as a "top-down" approach in that it breaks down the complete GC × GC separation into two independent one-dimensional gas chromatography separations (1D-GC). In this regard, both dimensions, that is, first dimension (1D) and second dimension (2D) are treated separately, and the cryogenic modulator is simply considered as a second consecutive injection device. Separate 1D-GC tr predictions are performed on both dimensions using the same flow rate as the one deployed in the conventional GC × GC system. The separate tr predictions are then combined to account for the two-dimensional separation. This model was applied to 24 analytes from 2 standard mixtures (Grob Test Mix and Fragrance Materials Test Mix) and assessed across 9 GC × GC chromatographic conditions. The experimental and predicted chromatographic retention space occupations were assessed by using the convex hull approach defined by the Delaunay triangulation. The predicted percentage of space occupation corresponded favorably with the experimental values. Furthermore, the top-down approach enabled an accurate prediction of the 2tr of all investigated analytes, providing an average 2tr modeling error of 0.26 ± 0.01 s.

Study on Gas Chromatography Retention Time Variation of Acetic Acid Combined with Quantum Chemical Calculation

Study on Gas Chromatography Retention Time Variation of Acetic Acid Combined with Quantum Chemical Calculation

Application of gas chromatographic retention times to determine physicochemical properties of nitrated, oxygenated, and parent polycyclic aromatic hydrocarbons

Nitrated and oxygenated polycyclic aromatic hydrocarbons (NPAHs and OPAHs) are receiving attention because of their high toxicity compared with parent PAHs. However, the experimental data of their physicochemical properties has been limited. This study proposed the gas chromatographic retention time (GC-RT) technique as an effective alternative one to determine octanol-air partition coefficients (KOA) and sub-cooled liquid vapor pressures (PL) for 11 NPAHs, 10 OPAHs, and 19 parent PAHs. The slopes and intercepts of the linear regressions between temperature versus KOA and PL were provided and can be used to estimate KOA and PL for the 40 targeted compounds at any temperature. The internal energies of phase transfer (ΔUOA) and enthalpies of vaporization (ΔHL) for all targeted compounds were also calculated using the GC-RT technique. High-molecular-weight compounds may release or absorb higher heat energy to transform between different phases. NPAHs and OPAHs had a non-ideal solution behavior with activity in octanol (γoct) in the range of 19–53 and 18–1,078, respectively, which is larger than the unity threshold. A comparison among four groups of PAH derivatives showed that a functional group (nitro-, oxygen-, chloro-, and bromo-) in PAH derivatives increased γoct for corresponding parent PAHs by tens (mono-group) to hundreds of times (di-group). This study suggests that the GC-RT method is applicable for indirectly measuring the physicochemical properties of various groups of organic compounds.

Quantification of isomer-resolved iodide chemical ionization mass spectrometry sensitivity and uncertainty using a voltage-scanning approach

Abstract. Chemical ionization mass spectrometry (CIMS) using iodide as a reagent ion has been widely used to classify organic compounds in the atmosphere by their elemental formula. Unfortunately, calibration of these instruments is challenging due to a lack of commercially available standards for many compounds, which has led to the development of methods for estimating CIMS sensitivity. By coupling a thermal desorption aerosol gas chromatograph (TAG) simultaneously to a flame ionization detector (FID) and an iodide CIMS, we use the individual particle-phase analytes, quantified by the FID, to examine the sensitivity of the CIMS and its variability between isomers of the same elemental formula. Iodide CIMS sensitivities of isomers within a formula are found to generally vary by 1 order of magnitude with a maximum deviation of 2 orders of magnitude. Furthermore, we compare directly measured sensitivity to a method of estimating sensitivity based on declustering voltage (i.e., “voltage scanning”). This approach is found to carry high uncertainties for individual analytes (0.5 to 1 order of magnitude) but represents a central tendency that can be used to estimate the sum of analytes with reasonable error (∼30 % differences between predicted and measured moles). Finally, gas chromatography (GC) retention time, which is associated with vapor pressure and chemical functionality of an analyte, is found to qualitatively correlate with iodide CIMS sensitivity, but the relationship is not close enough to be quantitatively useful and could be explored further in the future as a potential calibration approach.

Measurement of octanol–air partition coefficients for liquid crystals based on gas chromatography-retention time and its implication in predicting long-range transport potential

Liquid crystals (LCs) are widely used in the modern society, but their environmental fate and related human health effects remain inadequately recognized. To assist in better understanding the environmental fate of LCs, the octanol–air partition coefficients (KOA) of 21 target LCs were determined with a gas chromatography-retention time (GC-RT) approach. Four classes of traditional organic pollutants, including polycyclic aromatic hydrocarbons, organochlorides, polybrominated diphenyl ethers, and polychlorinated biphenyls were employed as reference or calibration compounds. Cluster analysis indicated that the reference and calibration compounds somewhat influenced the relative and absolute magnitudes of GC-RT results. A quantitative structure-property relationship (QSPR) model was constructed from the experimental results and outperformed a widely-used model, KOAWIN, in estimating log KOA of LCs. This model was used to predict log KOAs for 116 LCs with the same element compositions and similar structures as the target LCs. Overall persistence and long-range transport potential were predicted based on the measured and estimated log KOA values, yielding consistent results. Several LCs were shown to have comparable characteristic travel distances and transport efficiencies as the traditional organic pollutants, suggesting they are potential environmental pollutants and the QSPR model is applicable in predicting the environmental fate of LCs.

Modeling approaches for temperature-programmed gas chromatographic retention times under vacuum outlet conditions

This contribution evaluates the performance of two predictive approaches in calculating temperature-programmed gas chromatographic retention times under vacuum outlet conditions. In the first approach, the predictions are performed according to a thermodynamic-based model, while in the second approach the predictions are conducted by using the temperature-programmed retention time equation. These modeling approaches were evaluated on 47 test compounds belonging to different chemical classes, under different experimental conditions, namely, two modes of gas flow regulation (i.e., constant inlet pressure and constant flow rate), and different temperature programs (i.e., 7 °C/min, 5 °C/min, and 3 °C/min). Both modeling approaches gave satisfactory results and were able to accurately predict the elution profiles of the studied test compounds. The thermodynamic-based model provided more satisfying results under constant flow rate mode, with average modeling errors of 0.43%, 0.33%, and 0.15% across all the studied temperature programs. Nevertheless, under constant inlet pressure mode, lower modeling errors were achieved when using the temperature-programmed retention time equation, with average modeling errors of 0.18%, 0.18%, and 0.31% across the used temperature programs.

Fuzzy wavelet network based on extended Kalman filter training algorithm combined with least square weight estimation: Efficient and improved chromatographic QSRR/QSPR models

Inspired by the theory of multiresolution analysis (MRA) of wavelet transforms and fuzzy concepts, a fuzzy wavelet network (FWN) is proposed for predicting the gas chromatographic retention time of phenol derivatives on fused-silica, wide-bore, and open-tubular columns with different polarities in terms of quantitative structure-retention relationship (QSRR) studies. The FWN consists of a set of fuzzy rules. Each rule corresponding to a sub-wavelet neural network (WNN) consists of single-scaling wavelets. The proposed FWN follows a two-stage efficient learning algorithm scheme, which undertakes the extended Kalman filter (EKF) training algorithm for adjusting all translation parameters, dilation parameters, and weight coefficients of the WNNs and then least square estimation for updating all the weights. 4-3-1 FWNs were developed using the descriptors selected by the multiple linear regression (MLR) models as inputs. By learning the translation parameters of the wavelets and adjusting the shape of membership functions, the model accuracy was remarkably improved. The accuracy and robustness of the generated models were illustrated using leave-one-out and leave-multiple-out cross-validations and also by Y-randomization. Sensitivity analysis by sequential zeroing of weights revealed that the molecular geometry and electronic interactions play a major role in the retention behavior of these compounds on non-polar and semi-polar columns, respectively.

Female-Produced Sex Pheromone of Tetrastichus planipennisi, a Parasitoid Introduced for Biological Control of the Invasive Emerald Ash Borer, Agrilus planipennis.

The Asian eulophid wasp Tetrastichus planipennisi is being released in North America as a biocontrol agent for the emerald ash borer (Agrilus planipennis), a very destructive invasive buprestid beetle that is devastating ash trees (Fraxinus spp.). We identified, synthesized, and tested a female-produced sex pheromone for the wasp. The key component eliciting behavioral responses from male wasps in flight tunnel bioassays was identified as (6S,10S)-(2E,4E,8E)-4,6,8,10-tetramethyltrideca-2,4,8-triene. Female specificity was demonstrated by gas chromatographic (GC) comparison of male and female volatile emissions and whole body extracts. The identification was aided by coupled gas chromatography/mass spectrometry analysis, microchemical reactions, NMR, GC analyses with a chiral stationary phase column, and matching GC retention times and mass spectra with those of synthetic standards. The tetramethyl-triene hydrocarbon was synthesized as a mixture of two enantiomeric pairs of diastereomers, and as the pure insect-produced stereoisomer. In flight-tunnel bioassays, males responded to both the natural pheromone and the chiral synthetic material by upwind flight and landing on the source. In contrast, the mixture of four stereoisomers was not attractive, indicating that one or more of the "unnatural" stereoisomers antagonized attraction. Field trials, using yellow pan traps baited with natural pheromone, captured significantly more male wasps than control traps over a four week trial. The identified pheromone could increase the efficiency and specificity of the current detection methods for Tetrastichus planipennisi and aid in the determination of parasitoid establishment at release sites.

Determining sub-cooled liquid vapor pressures and octanol-air partition coefficients for chlorinated and brominated polycyclic aromatic hydrocarbons based on gas chromatographic retention times: Application for gas/particle partitioning in air

Abstract Halogenated polycyclic aromatic hydrocarbons (Halo-PAHs) are a new group of PAH derivatives and are reported to be more toxic than their parent PAHs. However, limited research has been undertaken on their physicochemical properties. In this study, sub-cooled liquid vapor pressures (PL) and octanol–air partition coefficients (KOA) of 24 chlorinated and 11 brominated PAHs (ClPAHs and BrPAHs) were determined using the gas chromatographic retention time (GC-RT) technique. The temperature dependences of PL and KOA for the Halo-PAHs were determined by the slopes and intercepts of the linear regressions. The GC-RT method also provided the enthalpies of vaporization (ΔHL) and internal energies of phase transfer (ΔUOA) for all interested compounds. The activity of Halo-PAHs in octanol (γoct) ranging from 18 to 87, indicated the non-ideal solution behavior. The number of the chlorine atoms in the PAH rings could influence the interaction of ClPAHs with the organic matter, whereas this interaction for BrPAHs could not be studied due to the limited number of target BrPAHs. The measured values of PL and KOA were applied to investigate the gas/particle partition behaviors of Halo-PAHs in Ulsan, South Korea. This study is so far the first one to report the linear regression coefficients, which can be used to estimate the PL and KOA values of Halo-PAHs at any temperature.

Gas Chromatographic Estimation of Vapor Pressures and Octanol–Air Partition Coefficients of Semivolatile Organic Compounds of Emerging Concern

The subcooled liquid-phase vapor pressures (pL298/Pa) and octanol–air partition coefficients (KOA298) at T/K = 298, enthalpies of vaporization (ΔVAPH/kJ·mol–1), and internal energies of phase transfer from octanol to air (ΔOAU/kJ·mol–1) were estimated for synthetic musks, novel brominated flame retardants (N-BFR), organophosphate esters, and ultraviolet filters using the capillary gas chromatographic retention time (GC-RT) method. These compounds, which spanned approximately six and three orders of magnitude for pL298/Pa and KOA298, respectively, were co-chromatographed with one of three reference compounds to give initial estimates of properties at T/K = 298. The initial GC-RT property estimates were subsequently calibrated using 18 compounds that spanned 6 log units for pL298/Pa and 13 compounds covering 4 log units for KOA298. The calibrated log10 pL298/Pa values estimated here ranged from 0.14 ± 0.19 to −9.19 ± 0.29 for cyclopentadecanone to syn-dechlorane plus (syn-DDC-CO), respectively, while the range of log10 KOA298 values was 6.59 ± 0.26 to 11.40 ± 0.23 for cyclopentadecanone to 2,2′,4,4′,5-pentabromodiphenyl ether (BDE-99), respectively. The calibrated GC-RT-derived values were highly correlated with, and were within an average of 0.70 log units of, the literature data for compounds with well-established pL298/Pa and KOA298 measured or derived using non-GC-RT methods. Nonpolar compounds were used in this study to estimate the target polar compound data, which may introduce systematic errors. However, the comparison of our GC-RT results against the literature non-GC-RT values shows that the GC-RT methods performed similarly well for estimating both polar and nonpolar target compounds studied in this work.

Accurate Prediction of Gas Chromatographic Retention Times via Density Functional Theory Calculations: A Case Study Using Brominated Flame Retardants

AbstractIdentification of structurally similar congeners of polybrominated diphenyl ether (PBDE) can be challenging without the use of analytical standards. Density Functional Theory (DFT) calculations paired with gas chromatographic (GC) retention times of reference standards were used to predict the retention times of PBDE congeners. This was accomplished by calculating polarizability of 31 PBDEs using the open‐sourced software package General Atomic and Molecular Electronic Structure System (GAMESS). Three PBDEs were randomly chosen from a training set of PBDEs and were treated as “unknowns” to test the accuracy of linear regressions constructed from calculated polarizabilities versus experimentally determined retention times. The top performing functionals predicted retention times of these “unknown” congeners to be within 1.3% and 3% for the DB‐5HT GC column and the ZB‐SemiVolatiles GC column, respectively. This study demonstrates that DFT calculations combined with experimental retention times can be used to aid in the identification of unknown analytes or, at the very least, reduce the number of plausible identities of analytes.

Volatile organic compounds fingerprinting in faeces and urine of Alzheimer's disease model SAMP8 mice by headspace-gas chromatography-ion mobility spectrometry and headspace-solid phase microextraction-gas chromatography-mass spectrometry

Volatile organic compounds fingerprinting in faeces and urine of Alzheimer's disease model SAMP8 mice by headspace-gas chromatography-ion mobility spectrometry and headspace-solid phase microextraction-gas chromatography-mass spectrometry

Quantitative structure-retention relationships of the chromatographic retentions of phthalic acid ester contaminants in foods.

The harmful health effects caused by phthalic acid esters have been supported from the increasing scientific evidence, developing the efficient methodologies to monitor the levels of phthalic acid esters in various foods become especially important from the aspects of human exposure assessment and their migration mechanistic understanding. In this study, quantitative structure-retention relationship studies on both the gas and liquid chromatographic retention times of 23 phthalic acid esters were performed by genetic function approximation, and the optimal quantitative structure-retention relationship models (r2 >0.980, r2 CV >0.960, and r2 pred >0.865) passed the statistical tests of cross-validation, randomization, external prediction, Roy' rm 2 metrics, Golbraikh-Tropsha' criteria and applicability domain. The established predictive models elucidate the structural requirements for the retention of phthalic acid esters over different chromatographic columns, which were finally used to predict the retention times of 11 new phthalic acid esters. Hopefully, this work could provide useful guidelines for better understanding and accurate prediction of the retention behavior of undetermined phthalic acid esters when lacking standard samples or under poor experimental conditions, and make the simultaneous identification and quantification of numerous phthalic acid esters possible.

Thermodynamics-based modelling of gas chromatography separations across column geometries and systems, including the prediction of peak widths.

Thermodynamics-based models have been demonstrated to be useful for predicting retention time and peak widths in gas chromatography and two-dimensional gas chromatography separations. However, the collection of data to train the models can be time consuming, which lessens the practical utility of the method. In this contribution, a method for obtaining thermodynamic-based data to predict peak widths in temperature-programmed gas chromatography is presented. Experimental work to collect data for peak width prediction is identical to that required to collect data for retention time prediction using approaches that we have presented previously. Using this combined approach, chromatograms including retention times and peak widths are predicted with very high accuracy. Typical errors in retention time are<0.5%, while errors in peak width are typically<5% as demonstrated using polycycic aromatic hydrocarbons and a mixture containing compounds with aldehyde, ketone, alkene, alkane, alcohol, and ester functionalities.

Stationary Phase for Gas Chromatography on the Basis of a Precursor of Macroheterocycles and a Nematic Liquid Crystal

The stationary phase for gas chromatography on the basis of the chiral R(+)-1′,7′,7′-trimethylbicyclo-[2.2.1]heptano[2′,3′-b]-2,3-dicyanopyrazine (10.4 wt %) and a liquid crystalline 4,4′-dihexyloxyazoxybenzene was studied. The phase transition temperatures of the binary mixture were measured by polarizing microscopy. It was shown that the chiral camphor-substituted 2,3-dicyanopyrazine induces formation of an enantiotropic chiral helically coiled nematic phase in the achiral liquid crystal over a wide temperature range. The gas chromatographic retention times of organic compounds on this sorbents were measured. The structural and chiral selectivity of the stationary phase on the basis of a liquid crystal and camphor-substituted 2,3-dicyanopyrazine was determined. The retention volumes, activity coefficients, and sorption enthalpies and entropies were calculated and analyzed.

In Silico Prediction of Gas Chromatographic Retention Time of Some Organic Compounds on the Modified Carbon Nanotube Capillary Column

<i>In Silico</i> Prediction of Gas Chromatographic Retention Time of Some Organic Compounds on the Modified Carbon Nanotube Capillary Column

Binding Affinities and Spectroscopy of Complexes Formed by Polysiloxanes with Aniline and Chlorophenol Acetates

The theoretical binding energies of the complexes formed by polysiloxanes with aniline and chlorophenol acetates were calculated at B3LYP/6-31G(d, p) level after the basis set superposition error (BSSE) based on B3LYP/6-31G(d) optimized geometries. The high binding energies of the complexes lead to the long experimental retention times in gas chromatography (GC). The energy gaps of the complexes are reduced owing to the formation of the hydrogen bonds. The chemical shifts of the α-C atoms connected to the amino groups in the NMR spectra of the complexes computed with B3LYP/6-31G(d) method are gradually changed downfield as the Si–O chain increases. The aromaticities of the benzene rings in the complexes are decreased due to the transfer of electron clouds from the benzene rings to the Si–O chains based on the nuclear independent chemical shifts (NICS) calculated at B3LYP/6-31G(d) level.

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.