Non-polar Stationary Phases Research Articles

A comparison of published in 2018-2024 general-purpose models for predicting gas chromatographic retention indices

Retention indices are widely used in gas chromatography and chromatography-mass spectrometry as an additional factor in tentative identification (along with the mass spectrum). Reference data on retention indices are available only for a limited number of molecules; in other cases, retention indices predicted by mathematical models can be used. Models for predicting retention indices developed prior to 2018 mostly have either very low accuracy or a very narrow domain of applicability. However, in recent years, starting from 2018, the situation has begun to change: the use of deep neural networks and large training sets (mainly different versions of the NIST database) made it possible to build both accurate and general-purpose models for predicting gas chromatographic retention indices, with the accuracy increasing over time. In recent years, at least 7 deep learning-based models for predicting gas chromatographic retention indices have been released in the public domain. The authors always declare that their model is more accurate than previous models, however, in all cases, there are no independent measurements of accuracy. This work aimed to objectively and critically compare retention index prediction models and corresponding software using the same retention data set that was guaranteed not to intersect with the training sets used by the authors of the models. Seven models and corresponding software were considered, including MetExpert (2018), DeepReI (2021), SVEKLA (2021), and AIRI (2024). It was shown that for the non-polar stationary phase (ZB-5MS), the accuracy of the newest models gradually approaches the accuracy of the reference libraries and is quite high. The newer models are indeed more accurate than the older ones. At the same time, for the polar stationary phase (SH-Stabilwax), the accuracy (independent data set) is very low and significantly lower than that stated in the original papers devoted to the predictive models. For users with limited experience, the process of compiling and running software can be challenging, particularly when attempting to do so several years after publication. This is often due to incompatibility issues between model files and newer versions of the frameworks. It is not uncommon for software authors to discontinue any support of the software after an article has been published in a journal.

Retention Mechanisms of Basic Compounds in Liquid Chromatography with Sodium Dodecyl Sulfate and 1-Hexyl-3-Methylimidazolium Chloride as Mobile Phase Reagents in Two C18 Columns

Reversed-phase liquid chromatography (RPLC) relies on a non-polar stationary phase and a more polar hydro-organic mobile phase, where compound retention is primarily governed by hydrophobicity, with more hydrophobic compounds being retained longer. The introduction of secondary equilibria in the chromatographic system through additives, such as anionic surfactants and ionic liquids (ILs), was proposed to mitigate ionic interactions between positively charged analytes and the anionic free silanol groups in non-endcapped stationary phases, thereby preventing increased retention and peak tailing. Additionally, the combined hydrophobic and ionic interactions between cationic analytes and the ions in these additives was demonstrated to create mixed retention mechanisms that influence retention and selectivity. In this regard, this study investigates aqueous chromatographic systems incorporating both the anionic surfactant sodium dodecyl sulfate (SDS) and the IL 1-hexyl-3-methylimidazolium chloride as mobile phase reagents. This combination of reagents modulates the retention, eliminating the need for organic solvents and resulting in highly sustainable HPLC procedures. The chromatographic behavior was assessed using two different C18 columns (Zorbax Eclipse and XTerra-MS). The strength of solute interactions was estimated by calculating equilibrium parameters and the contributions of hydrophobic and ionic interactions through simple mathematical models. Focusing on the retention of six basic drugs (β-adrenoceptor antagonists), the study highlighted the significant role of ionic interactions. The results demonstrate the feasibility of using aqueous systems combining SDS and an IL for the efficient separation of moderately polar basic compounds without the use of organic solvents.

Critical evaluation of the NIST retention index database reliability with specific examples.

The NIST gas chromatographic retention index database is widely used in gas chromatography-mass spectrometry analysis. For many compounds, the NIST database contains many entries that are presumably obtained independently of each other. We showed with specific examples that there are cases in the NIST database where several entries exist for the same compound, and all of them are equally erroneous (an error of more than 100 units). In particular, we demonstrated that all retention index values for such an important compound as imidazole for non-polar stationary phases in theNISTdatabase are erroneous. In addition to imidazole, a similar situation is observed for four more nitrogen-containing heterocyclic compounds. For certainty, measurements were performed under several conditions, using various temperature programs, and using two specimens of columns. The structures were confirmed using nuclear magnetic resonance and mass spectrometry. It was shown with specific examples that many values are not reliable: either data were obtained using standard samples of undescribed origin without confirmation (without even using mass spectrometry) or, in some cases, standard samples were not used at all, and the retention index was obtained for a mixture component identified using a mass spectral library search. Some "independent" values are not such but are repeated publications of the same data (secondary sources), or simply several values taken from the same source. In the work, an analysis was carried out and assumptions were made about how several equally incorrect retention index values could appear in the NIST database.

Fuel property modeling by high-speed gas chromatography coupled with partial least squares data analysis

Partial least squares (PLS) regression is a valuable chemometric tool for property prediction when coupled with gas chromatography (GC). Since the separation run time and stationary phase selection are crucial for effective PLS modeling, we study these GC parameters on the prediction of viscosity, density and hydrogen content for 50 aerospace fuels. Due to the diversity of compounds in the fuels (primarily alkanes, cycloalkanes, and aromatics), we explore both polar and non-polar stationary phase columns. The robustness for the PLS models was evaluated by their normalized root mean square error of cross-validation (NRMSECV). PLS models built for viscosity across 1-min, 3-min, 7-min, and 10-min time window (TW) high-speed GC separations produced nearly the same NRMSECV with the polar column data with an average (standard deviation) of 4.41 % (0.34 %) versus the non-polar column data of 4.69 % (0.15 %). In contrast, while the NRMSECV of density modeling with the polar column data varied more than the viscosity models, averaging 7.54 % (0.67 %), the non-polar column data produced a significantly higher average NRMSECV of 10.06 % (0.35 %). Similarly, for hydrogen content, the NRMSECV with the polar column data averaged 9.50 % (0.87 %), which was significantly lower than the NRMSECV with the non-polar column data averaging 12.10 % (0.88 %). We also investigated the impact of smoothing the GC data on the corresponding PLS models. By applying varying degrees of smoothing, we can effectively obtain similar chromatographic peak patterns in a shorter TW. For example, a 10-min smoothed chromatogram appears like the 1-min separation with no smoothing but resulted in nearly the same NRMSECV. Overall, the fast separation with a 1-min TW produced robust PLS models for viscosity with either stationary phase column, whereas for density and hydrogen content the polar stationary phase column produced superior PLS models, thus with proper stationary phase selection, a fast separation run time could be readily applied with optimal PLS property modeling results.

Formation of Micelles by Nonionic Detergent Molecules Leads to the Breakthrough Peak in Reversed-Phase Ultraperformance Liquid Chromatography (UPLC).

A peculiar phenomenon known as "breakthrough" occurs under reversed-phase ultraperformance liquid chromatography (UPLC) conditions and has been under scrutiny for decades. This effect takes place when a large volume of analyte solution, prepared in a solvent with an eluotropic strength significantly higher than that of the initial mobile phase solvent, is injected. According to the literature, under specific experimental conditions, a substantial portion of solutes is carried by the mobile phase and detected near the dead time of the chromatographic system. This phenomenon is typically observed when the injected volume of a particular analyte is sufficiently large. However, the underlying physicochemical principles governing this phenomenon have remained elusive. We present evidence demonstrating that breakthroughs can occur even when injecting a sample of a neat solvent devoid of any solute. By mass spectrometric analysis, we identified the breakthrough peak to represent the nonionic detergent Triton. When columns are equilibrated with water, Triton molecules, present as impurities in filtered water, accumulate on the nonpolar stationary phase. Upon the introduction of a solvent with a stronger elution strength, Triton molecules retained on the stationary phase are removed. As detergents, these Triton molecules aggregate into micelles featuring a hydrophobic inner core and a hydrophilic outer shell. These hydrophilic micelles are carried by the polar mobile phase and detected as the breakthrough peak at the dead time of the chromatographic system. When analytes are present, a portion of the injected solutes is captured by the micelles and transported with the breakthrough plug. This assertion was verified and confirmed by liquid chromatography-mass spectrometry (LC-MS) analysis of a methanolic solution of perfluorooctanoic acid (PFOA). The mass spectra corresponding to the breakthrough plug featured a peak for the PFOA anion (m/z 413) in addition to those for Triton.

A new leaf essential oil from the Andean species Gynoxys szyszylowiczii Hieron. of southern Ecuador: chemical and enantioselective analyses

The essential oil obtained from the dry leaves of Gynoxys szyszylowiczii Hieron. was described in this study for the first time. The chemical analysis, conducted on two stationary phases of different polarity, permitted to identify sixty-four compounds, that were quantified with at least one column. The main components, on a non-polar and polar stationary phase respectively, were germacrene D (21.6–19.2%), α-pinene (4.4–4.9%), n-tricosane (4.3% on both columns), (E)-β-caryophyllene (3.3–4.3%), 1-docosene (3.2–2.8%), α-cadinol (2.8–3.1%), and cis-β-guaiene (2.6–3.0%). This investigation was complemented by the enantioselective analysis of some major chiral compounds, carried out on two chiral selectors based on β-cyclodextrines. As a result, (S)-( +)-α-phellandrene, (S)-( +)-β-phellandrene, and (1S,2R,6R,7R,8R)-( +)-α-copaene appeared enantiomerically pure, whereas α-pinene, β-pinene, terpinen-4-ol, and germacrene D were detected as scalemic mixtures. Finally, linalool was practically racemic. The distillation yield, analytically calculated by weight of dry plant material, was 0.03%.

The role of the cation and anion in aqueous liquid chromatography with sodium dodecyl sulphate and imidazolium-based ionic liquids as mobile phase reagents

BackgroundIn reversed-phase liquid chromatography, solute retention is primarily influenced by interactions between a nonpolar stationary phase and a moderately polar hydro-organic mobile phase, based on the solute lipophilicity. However, challenges regarding retention and peak tailing can arise due to ionic interactions between positively charged analytes and free silanols present on silica-based stationary phases. To address these challenges, incorporating surfactants and ionic liquids (ILs) into the mobile phase offers an effective solution. These additives synergistically enhance chromatographic performance through electrostatic and lipophilic interactions, which enable fine-tuning of selectivity and improved separation efficiency. ResultsThis study explores the chromatographic behaviour of several basic compounds in aqueous mixtures containing the anionic surfactant sodium dodecyl sulphate (SDS), above its critical micellar concentration, combined with various 1-alkyl-3-methylimidazolium-based ionic liquids (ILs) featuring chloride, tetrafluoroborate, and hexafluorophosphate anions, all without the addition of organic solvents. Specifically, this research investigates the influence of different anion types within the ILs and considers the impact of the IL cations. Analysis of solute peak profiles reveals narrow and symmetrical peaks. By introducing tetrafluoroborate and hexafluorophosphate IL anions into a mobile phase that contains an anionic surfactant, the study sheds light on the interactions occurring within the chromatographic column. This enhanced understanding of the combined effects of surfactants and ILs contributes to refining chromatographic methodologies. SignificanceThis research highlights the importance of carefully selecting the appropriate IL when incorporating it into a micellar mobile phase alongside SDS. This combination results in practical retention times that surpass the performance achieved with either the surfactant or IL alone in the mobile phase. The study particularly emphasises the impact of the IL anion, especially in the absence of SDS and organic solvents. This unveils interactions that are otherwise obscured in micellar and hydro-organic media, providing new insights into chromatographic dynamics.

New Essential Oils from Ecuadorian Gynoxys cuicochensis Cuatrec. and Gynoxys sancti-antonii Cuatrec. Chemical Compositions and Enantioselective Analyses.

The present study belonged to an unfunded project, dealing on the systematic description of unprecedented essential oils (EOs), distilled from 12 species of genus Gynoxys Cuatrec. In this very case, the aim was the first chemical and enantiomeric analyses of two volatile fractions, obtained from the leaves of Gynoxys cuicochensis Cuatrec. and Gynoxys sancti-antonii Cuatrec. These EOs were analyzed by GC-MS (qualitatively) and GC-FID (quantitatively), detecting 89 and 60 components from G. cuicochensis and G. sancti-antonii, respectively. Major components for G. cuicochensis EO, on a nonpolar and polar stationary phase, were α-pinene (29.4-29.6%), p-vinylguaiacol (3.3-3.6%), and germacrene D (20.8-19.9%). In G. sancti-antonii EO, the main compounds were α-pinene (3.0-2.9%), β-pinene (12.9-12.1%), γ-curcumene (19.7-18.3%), germacrene D (9.0% on the polar phase), ar-curcumene (5.3% on the polar phase), δ-cadinene (4.1-4.6%), α-muurolol (3.3-2.4%), α-cadinol (3.0% on both columns), and an undetermined compound, of molecular weight 220. In addition to chemical composition, the enantioselective analysis of the main chiral compounds was carried out on two different chiral selectors. In G. cuicochensis EO, (1R,5R)-(+)-α-pinene, (S)-(+)-β-phellandrene, (R)-(-)-piperitone, and (S)-(-)-germacrene D were enantiomerically pure, whereas β-pinene, sabinene, α-phellandrene, limonene, linalool, and terpinen-4-ol were observed as scalemic mixtures. On the other hand, in G. sancti-antonii EO, the pure enantiomers were (1S,5S)-(-)-α-pinene, (1R,5R)-(+)-sabinene, (R)-(-)-β-phellandrene, (S)-(-)-limonene, (1S,2R,6R,7R,8R)-(+)-α-copaene, (R)-(-)-terpinen-4-ol, and (S)-(-)-germacrene D, whereas β-pinene, linalool, and α-terpineol were present as scalemic mixtures. The principal component analysis demonstrated that G. cuicochensis volatile fraction was quite similar to many of the other EOs of the same genus, whereas G. sancti-antonii produced the most dissimilar EO. Furthermore, the enantioselective analyses showed the usual variable enantiomeric distribution, with a greater presence of enantiomerically pure compounds in G. sancti-antonii EO.

Headspace – Solid phase microextraction vs liquid injection GC-MS analysis of essential oils: Prediction of linear retention indices by multiple linear regression

AbstractDue to the relative independence from the operational parameters, the linear retention indices (LRIs) are useful tool in gas chromatography-mass spectrometry (GC-MS) qualitative analysis. The aim of the current study was to develop a multiple linear regression (MLR) model for the prediction of LRIs as a function of selected molecular descriptors. Liquid injection GC-MS was used for the analysis of Essential oils (Rose, Lavender and Peppermint) separating the ingredients by a semi-standard non-polar stationary phase. As a result, a sum of 103 compounds were identified and their experimental LRIs were derived relying on reference measurements of a standard mixture of n-alkanes (from C8 to C20). As a next step, a set of molecular descriptors was generated for the distinguished chemical structures. Further, a stepwise MLR was applied for the selection of the significant descriptors (variables) which can be used to predict the LRIs. From an exploit set of over 2000 molecular descriptors, it was found that only 16 can be regarded as significant and independent variables. At this point split validation was applied: the identified compounds were randomly divided into training (85%) and validation (15%) sets. The training set (87 compounds) was used to derive two MLR models by applying i) the ‘enter’ algorithm (R2 = 0.9960, RMSЕ = 17) and ii) the ‘stepwise’ one (R2 = 0.9958, RMSЕ = 17). The predictive power was assessed by the validation set (16 compounds) as follows i) q2F1 = 0.9896, RMSE = 25 and ii) q2F1 = 0.9886, RMSE = 26, respectively. The adequateness of both regression approaches was further evaluated. Newly developed headspace-solid phase microextraction (HS-SPME) procedures in combination with GC-MS were used for an alternative analysis of the studied Essential oils. Twelve additional compounds, not previously detected by the liquid sample introduction mode of analysis, were identified for which the values of the significant descriptors were within the working range of the developed MLRs. For the last compounds, the LRIs were calculated and the experimental data was used as an external set for assessment of the regression models. The predictive power for both regression approaches was assessed as follows: Enter RMSE = 41, q2F2 = 0.9503 and Stepwise RMSE = 40, q2F2 = 0.9521.

A general procedure for finding potentially erroneous entries in the database of retention indices

BackgroundThe NIST retention index database is one the most widely used sources of retention indices. In both untargeted analysis and machine learning studies filtering for potential errors is rather lacking or nonexistent. According to our estimates about 80% of the compounds from both NIST 17 and NIST 20 retention index databases have only one RI value per stationary phase, which makes searching for erroneous values with statistical methods impossible. Manual inspection is also impractical because the database contains more than 300 000 entries. ResultsWe suggest a two-step procedure to find potentially erroneous retention indices based on machine learning. The first step is to use five predictive models to obtain predicted retention index values for the whole database. The second one is to compare these predicted values against the experimental ones. We consider a retention index erroneous if its accuracy (the difference between predicted and experimental value) is in the bottom 5% for each of the five models simultaneously. Using this method, we were able to detect 2093 outlier entries for standard and semi-standard non-polar stationary phases in the NIST 17 retention index database, 566 of those were corrected or removed by the developers in the NIST 20. SignificanceThis is a novel approach to find potentially erroneous entries in a large-scale database with mostly unique entries, which can be applied not only to retention indices. The procedure can help filter and report mishandled data to improve the quality of the dataset for machine learning applications and experimental use.

СРАВНИТЕЛЬНЫЙ АНАЛИЗ АКТИВНОСТИ И СЕЛЕКТИВНОСТИ КАТАЛИЗАТОРА LD-145, РЕГЕНЕРИРОВАННОГО МЕТОДОМ СВЕРХКРИТИЧЕСКОГО ФЛЮИДНОГО ЭКСТРАКЦИОННОГО ПРОЦЕССА С ИСПОЛЬЗОВАНИЕМ В КАЧЕСТВЕ ЭКСТРАГЕНТА СВЕРХКРИТИЧЕСКОЙ ПРОПАН/БУТАНОВОЙ СМЕСИ И СВЕРХКРИТИЧЕСКОГО ДИОКСИДА УГЛЕРОДА

A comparative analysis of activity and selectivity of nickel-molybdenum catalyst LD-14 5samples regenerated by supercritical fluid extraction process using supercritical propane/butane mixture and supercritical carbon dioxide as an extractant was carried out. The activity and selectivity of the catalysts were evaluated by steady-state method in a 200 ml autoclave. The process was carried out in hydrogen medium at a temperature of 573 K and a pressure of 3.0 MPa. The products obtained during the reactions were identified on an Agilent chromatography-mass spectrometer with a quadrupole mass analyzer. The separation was carried out on a capillary column with nonpolar stationary phase (DB&HP-5MS UI). The conversion of dibenzothiophene (DBT) was used to evaluate the activity of the catalysts in hydrodesulfination reactions, and the selectivity of direct desulfurization and hydrogenation was determined. The method of catalytic tests under steady-state conditions used in this work is a comparative method and allows to obtain relative data on the activity and selectivity of catalysts under proper selection of experimental conditions (average conversion at the level of 40-60 %). The results of the analysis show that the samples regenerated using supercritical carbon dioxide exhibit higher activity than those regenerated using supercritical propane/butane mixture. The highest hydrogenation capacity was achieved on the samples regenerated with supercritical carbon dioxide, modified with hexane and ethanol, they have 84% and 82.6% conversion of methylnaphthalene, respectively, and recovered activity in hydrodesulfination reactions (DBT conversion 64.6% and 69.1%). The obtained data are in full agreement with the results of studies carried out earlier on the flow unit. Thus, on the basis of catalytic tests it has been established that regeneration of spent nickel-molybdenum catalyst LD-145 should be carried out by supercritical fluid extraction process using supercritical carbon dioxide modified with hexane or ethanol.

Silica-Based Stationary Phase with Surface Bound N-Acetyl-glucosamine for Hydrophilic Interaction Liquid Chromatography.

A hydrophilic silica-based stationary phase with surface bound N-acetylglucosamine (GlcNAc-silica) was prepared in house and characterized physically via Fourier transform infrared (FTIR) analysis and thermogravimetric analysis (TGA) and chromatographically over a wide range of mobile phase compositions. While both FTIR and TGA confirmed the attachment of the GlcNAc ligands to the silica surface, the chromatographic evaluation of GlcNAc-silica with polar and slightly polar standard solutes (e.g., sugars, nucleic acid fragments, phenolic, and benzoic acid derivatives) yielded the typical hydrophilic interaction liquid chromatography (HILIC) behaviors in the sense that retention increased with increases in solute polarity and the organic content (i.e., acetonitrile) of the hydro-organic mobile phase (i.e., ACN-rich mobile phase). Sugars derivatized with 1-naphthylamine (1-NA) and 2-aminoanthrcene (2-AA) such as xylose, glucose, and short chains maltooligosaccharides constituted the most polar species for HILIC retention evaluation, and in addition, the maltooligosaccharides offered a polar homologous series for gauging the hydrophilicity of GlcNAc-silica in analogy with alkylbenzene homologous series and other nonpolar homologues for evaluating the hydrophobicity of non-polar stationary phases. On the other hand, the benzoic acid and phenolic acid derivatives were the probe solutes for evaluating the HILIC retention dependence of ionizable solutes on the pH of the mobile phase. Similarly, the nucleobase and nucleoside weak basic solutes as well as some typical cyclic nucleotide acidic solutes allowed for the examination of the dependence of solute retention on the pH of the mobile as well as the polarity of the species.

Development progress of stationary phase for supercritical fluid chromatography and related application in natural products

Supercritical fluid chromatography (SFC) is an environment-friendly and efficient column chromatography technology that was developed to expand the application range of high performance liquid chromatography (HPLC) using a supercritical fluid as the mobile phase. A supercritical fluid has a temperature and pressure that are above the critical values as well as relatively dynamic characteristics that are between those of a gas and liquid. Supercritical fluids combine the advantages of high solubility and diffusion, as their diffusion and viscosity coefficients are equivalent to those of a gas, while maintaining a density that is comparable with that of a liquid. Owing to the remarkable compressibility of supercritical fluids, analyte retention in SFC is significantly influenced by the density of the mobile phase. Thus, the column temperature and back pressure are crucial variables that regulate analyte retention in SFC. Increasing the back pressure can increase the density and solubility of the mobile phase, leading to reductions in retention time. The column temperature can affect selectivity and retention, and the degree to which different analytes are affected by this property varies. On the one hand, increasing the temperature reduces the density of the mobile phase, thereby extending the retention time of the analytes; on the other hand, it can also increase the energy of molecules, leading to a shorter retention time of the analyte on the stationary phase. CO2, the most widely employed supercritical fluid to date, presents moderate critical conditions and, more importantly, is miscible with a variety of polar organic solvents, including small quantities of water. In comparison with the mobile phases used in normal-phase liquid chromatography (NPLC) and reversed-phase liquid chromatography (RPLC), the mobile phase for SFC has a polarity that can be extended over a wide range on account of its extensive miscibility. The compatibility of the mobile phase determines the diversity of the stationary phase. Nearly all stationary phases for HPLC, including the nonpolar stationary phases commonly used for RPLC and the polar stationary phases commonly used for NPLC, can be applied to SFC. Because all stationary phases can use the same mobile-phase composition, chromatographic columns with completely different polarities can be employed in SFC. The selectivity of SFC has been effectively expanded, and the technique can be used for the separation of diverse analytes ranging from lipid compounds to polar compounds such as flavonoids, saponins, and peptides. The choice of stationary phase has a great impact on the separation effect of analytes in SFC. As new stationary phases for HPLC are constantly investigated, specialized stationary phases for SFC have also been continuously developed. Researchers have discovered that polar stationary phases containing nitrogen heterocycles such as 2-EP and PIC are highly suitable for SFC because they can effectively manage the peak shape of alkaline compounds and provide good selectivity in separating acidic and neutral compounds.The development of various stationary phases has promoted the applications of SFC in numerous fields such as pharmaceuticals, food production, environmental protection, and natural products. In particular, natural products have specific active skeletons, multiple active groups, and excellent biological activity; hence, these materials can provide many new opportunities for the discovery of novel drugs. According to reports, compounds related to natural products account for 80% of all commercial drugs. However, natural products are among the most challenging compounds to separate because of their complex composition and low concentration of active ingredients. Thus, superior chromatographic methods are required to enable the qualitative and quantitative analysis of natural products. Thanks to technological improvements and a good theoretical framework, the benefits of SFC are gradually becoming more apparent, and its use in separating natural products is expanding. Indeed, in the past 50 years, SFC has developed into a widely used and efficient separation technology. This article provides a brief overview of the characteristics, advantages, and development process of SFC; reviews the available SFC stationary phases and their applications in natural products over the last decade; and discusses prospects on the future development of SFC.

A New Essential Oil from the Native Andean Species Nectandra laurel Klotzsch ex Nees of Southern Ecuador: Chemical and Enantioselective Analyses.

The leaves of Nectandra laurel Klotzsch ex Nees, belonging to the family, Lauraceae, were collected in the province of Loja (Ecuador), dried, and analytically steam-distilled. An unprecedented essential oil was obtained, with a 0.03% yield by weight of dry plant material. The volatile fraction was submitted to qualitative (GC-MS) and quantitative (GC-FID) chemical analysis, on two orthogonal stationary phases. Seventy-eight compounds were detected and quantified on at least one column. The essential oil was dominated by sesquiterpene hydrocarbons (53.0-53.8% on the non-polar and polar stationary phase, respectively), followed by oxygenated sesquiterpenoids (18.9-19.0%). A third group was constituted by metabolites of other origins, mainly aliphatic compounds, apparently derived from the acetate pathway (11.7-8.5%). The major components of the EO (≥3.0% with at least one column) were δ-selinene (30.5-28.8%), δ-cadinene (5.4-6.4%), epi-α-cadinol (4.9-5.2%), an undetermined compound with a molecular weight of 204 (3.4-4.2%), α-pinene (3.3-2.9%), and α-cadinol (2.9-3.0%). Finally, the essential oil was submitted to enantioselective analysis, on two β-cyclodextrin-based chiral selectors, determining the enantiomeric distribution of seven chiral terpenes. Among them, (1R,5R)-(+)-α-pinene, (1R,5R)-(+)-β-pinene, and (R)-(-)-α-phellandrene were enantiomerically pure, whereas camphene, borneol, α-copaene, and α-terpineol were present as scalemic mixtures.

A New Leaf Essential Oil from Endemic Gynoxys laurifolia (Kunth) Cass. of Southern Ecuador: Chemical and Enantioselective Analyses.

The fresh leaves of Gynoxys laurifolia (Kunth) Cass. (Asteraceae), collected in the province of Loja (Ecuador), were submitted to steam distillation, producing an essential oil with a yield of 0.02% by weight. This volatile fraction, described here for the first time, was submitted to qualitative (GC-MS) and quantitative (GC-FID) chemical analyses, on two orthogonal columns (non-polar and polar stationary phase). A total of 90 components, corresponding to 95.9-95.0% by weight on the non-polar and polar stationary phase, respectively, were detected and quantified with at least one column. Major constituents (≥3%) were: germacrene D (18.9-18.0%), (E)-β-caryophyllene (13.2-15.0%), α-pinene (11.0-10.3%), β-pinene (4.5-4.4%), β-phellandrene (4.0-3.0%), bicyclogermacrene (4.0-3.0%), and bakkenolide A (3.2-3.4%). This essential oil was dominated by sesquiterpene hydrocarbons (about 45%), followed by monoterpene hydrocarbons (about 25-30%). This research was complemented with the enantioselective analysis of some common chiral terpenes, carried out through 2,3-diethyl-6-tert-butyldimethylsilyl-β-cyclodextrin and 2,3-diacetyl-6-tert-butyldimethylsilyl-β-cyclodextrin as stationary phase chiral selectors. As a result, (1S,5S)-(-)-β-pinene, (R)-(-)-α-phellandrene, (R)-(-)-β-phellandrene, (S)-(-)-limonene, (S)-(+)-linalyl acetate, and (S)-(-)-germacrene D were observed as enantiomerically pure compounds, whereas α-pinene, linalool, terpinene-4-ol, and α-terpineol were present as scalemic mixtures. Finally, sabinene was practically racemic. Due to plant wildness and the relatively low distillation yield, no industrial applications can be identified, in the first instance for this essential oil. The focus of the present study is therefore academic.

An attempt to apply a subtraction model for characterization of non-polar stationary phase in supercritical fluid chromatography

This study verified the feasibility of using a subtraction model to characterize the non-polar stationary phases (including C4, C8, and phenyl-type) in supercritical fluid chromatography (SFC). The model with 6 terms was expressed as log α = η'H + θ'P + β'A + α'B + κ'C + σ'S, where a term θ'P indicating dipole or induced dipole interaction was intentionally supplemented. Ethylbenzene and SunFire C8 were respectively defined as the reference solute and column. A 7-step modeling procedure was proposed: in the first 6 steps, except σ'S, by the use of a bidirectional fitting method, other parameters were calculated based on the equation: log α = log (ki/kref) ≈ η'H + θ'P + β'A + α'B + κ'C; and in the 7th step, residual analysis was employed to describe the σ'S term according to the equation: σ'S = log αexp. - log αpre. Furthermore, six columns that were not involved in modeling process and 12 compounds with unknown retention were used for methodology validation. It showed good predictions of log k, as demonstrated by adjusted determination coefficient (R2adj) from 0.9927 to 0.9998 (column) and from 0.9940 to 0.9999 (compound), respectively. The subtraction model emphasized the contribution of dipole or induced dipole interaction to the retention in SFC, and it obtained the σ'S term through residual analysis. Moreover, it made reasonable physical-chemical sense as the linear solvation energy relationship (LSER) model did, with the distinct advantages of better fitting and more accurate prediction. This study provided some new insights into the characterization of non-polar stationary phases in SFC.

A New Essential Oil from the Leaves of Gynoxys rugulosa Muschl. (Asteraceae) Growing in Southern Ecuador: Chemical and Enantioselective Analyses.

An essential oil, distilled from the leaves of the Andean species Gynoxys rugulosa Muschl., is described in the present study for the first time. The chemical composition was qualitatively and quantitatively determined by GC-MS and GC-FID, respectively. On the one hand, the qualitative composition was obtained by comparing the mass spectrum and the linear retention index of each component with data from literature. On the other hand, the quantitative composition was determined by calculating the relative response factor of each constituent, according to its combustion enthalpy. Both analyses were carried out with two orthogonal columns of nonpolar and polar stationary phases. A total of 112 compounds were detected and quantified with at least one column, corresponding to 87.3-93.0% of the whole oil mass. Among the 112 detected components, 103 were identified. The main constituents were α-pinene (5.3-6.0%), (E)-β-caryophyllene (2.4-2.8%), α-humulene (3.0-3.2%), germacrene D (4.9-6.5%), δ-cadinene (2.2-2.3%), caryophyllene oxide (1.6-2.2%), α-cadinol (3.8-4.4%), 1-nonadecanol (1.7-1.9%), 1-eicosanol (0.9-1.2%), n-tricosane (3.3-3.4%), 1-heneicosanol (4.5-5.8%), n-pentacosane (5.8-7.1%), 1-tricosanol (4.0-4.5%), and n-heptacosane (3.0-3.5%). Furthermore, an enantioselective analysis was carried out on the essential oil, by means of two cyclodextrin-based capillary columns. The enantiomers of α-pinene, β-pinene, sabinene, α-phellandrene, β-phellandrene, linalool, α-copaene, terpinen-4-ol, α-terpineol, and germacrene D were detected, and the respective enantiomeric excess was calculated.

Comparison of Gas-Chromatographic Retention Parameters of Aliphatic Enyne Alcohols with the Data for Their Structural Analogues

We studied the regularities of gas-chromatographic retention indices of aliphatic enyne alcohols containing С=С and С≡С bonds in the molecule (14 secondary and two tertiary homologues) on standard nonpolar polydimethylsiloxane stationary phases, which have not been characterized in sufficient detail earlier. Using homologous increments of retention indices iRI = RI – 100x, where х = int(M/14), М is the molecular weight number, and 14 is the mass number of the CH2 homological difference, one can compare data for structural analogs, including alkanols, alkenols, and alkynols. The features of comparing iRI values for compounds belonging to different homologous groups y ≡ M(mod14), for example, CnH2n+2O (y = 4), CnH2nO (y = 2), CnH2n–2O (y = 0), and CnH2n–4O (y = 12), are discussed. The main feature is that when the boundary value of y = 0 is passed, the relation between the parameter x and the number of carbon atoms in the molecule changes due to an increase in the formal unsaturation (the condition n = x – 1 is replaced by the condition n = x). Therefore, in the latter case, 100 should be subtracted from the iRI homologous increments to compare them with the data for structural analogs of other series. Taking this correction into account, we found that the iRI values for secondary enyne alcohols (12 ± 16) and previously characterized sec-alkynols (23 ± 21) virtually coincide, and for tertiary enyne alcohols, they are consistent with the data for all considered groups of structural analogues (alkanols, alkenols, and alkynols).

Does High Polarity Mean High Retention on Stationary Phases in Gas Chromatography?

Stationary phase chemistry, polarity, and selectivity have been of ongoing interest since the inception of gas chromatography (GC) in the 1950s. In the early days when most analyses were performed on packed columns, there were hundreds of stationary phase materials available. Today, with modern capillary columns, most GC analyses are performed with a few stationary phases, with a wide array of choices for specialty applications. Stationary phases are often classified using the broad term polarity, with polar stationary phases recommended for separating polar analytes and nonpolar stationary phases recommended for nonpolar analytes. In this installment, we examine the idea of stationary phase polarity. We examine the assumptions inherent in the most popular stationary phase polarity evaluating systems—McReynolds constants and the polarity scale. We see that high polarity does not always mean greater retention or higher selectivity.

GC-MS and GC-FID analysis of volatile secondary metabolites of the root of Anaphalis contorta Hook F. from India


 ABSTRACT. The chemical composition of hydro-distilled essential oil of dried roots of Anaphalis contorta Hook F. was analyzed for the first time by using gas chromatography equipped with flame ionization detector (GC-FID) and gas chromatography coupled with mass spectrometry (GC-MS). The compounds of essential oil were identified according to their mass spectra and their relative retention indices determined on a capillary GC column (non-polar stationary phase). Twenty-seven compounds were identified representing 94.8% of the total oil. The major constituents were 2,2-dimethyl-2[2,4,6-trimethylphenyl] acetic acid (12.1%), labda-8,14-dien-13-ol (8.4%), δ-cadinene (8.1%), labda-7,14-dien-13-ol (6.9%), α-gurjunene (6.7%), viridiflorene (5.1%), and caryophyllene oxide (4.9%). The root essential oil of A. contorta produced different chemotypes.
 
 KEY WORDS: Anaphalis contorta Hook F., Essential oil composition, 2,2-Dimethyl-2[2,4,6-trimethylphenyl] acetic acid. labda-8,14-dien-13-ol, GC/MS
 Bull. Chem. Soc. Ethiop. 2022, 36(1), 235-240. 
 DOI: https://dx.doi.org/10.4314/bcse.v36i1.19