Non-polar Column Research Articles

Temperature-Dependent Vapor Pressure of Selected Cyclic and Linear Polydimethylsiloxane Oligomers

The isothermal retention times of seven cyclic (D4 to D10) and nine linear (L5 to L13) dimethylsiloxane oligomers on a nonpolar gas chromatographic column were determined at temperatures between (308.15 and 438.15) K. Using normal alkanes as standard reference and calibration compounds, these retention times were used to derive liquid state vapor pressures at 298.15 K as well as enthalpies of vaporization that allow for the estimation of vapor pressures over a wide range of temperatures. The data agree within 0.25 log units with vapor pressure values previously reported by Flaningam (1986) but extend those values to lower temperatures and larger oligomers. Cyclic dimethylsiloxanes are more volatile than linear dimethylsiloxanes of a similar size. Oligomers with 10 and more dimethylsiloxane units are expected to sorb appreciably to aerosol at environmentally relevant temperatures.

Hexadecane/air partitioning coefficients of multifunctional compounds: Experimental data and modeling

The logarithmic hexadecane–air partitioning constant ( L) is one of the most common measures for solute–solvent nonspecific interaction. It is thus an essential compound descriptor for Linear Solute Energy Relationships (LSERs) that are used for predicting partitioning of organic chemicals. In this work L values from 2.3 to 13.7 – thus covering 11 orders of magnitude of the linear partitioning coefficient – were measured with standard deviations of <0.28 (average = 0.10) using inverse gas chromatography with non-polar capillary columns. The 104 studied compounds were all multifunctional, 52 of them were environmentally relevant pesticides, 6 hormones, 6 drugs and 4 phthalates. In addition, the predictive performance of 3 software tools (ABSOLVE, SPARC, COSMOtherm) for the L value was evaluated. To this end the data set was divided into 2 subsets: one with compounds containing maximally 2 functional groups and the other with more than 2 functional groups. None of the software tools reached the desired accuracy, defined as root mean square error over all compared compounds, of ±0.3 units. ABSOLVE performed well for the bifunctional compounds but failed for many pesticides and drugs. SPARC had major problems with highly fluorinated and phosphate containing compounds, but showed a good performance for all other compounds. COSMOtherm worked best for the predictions of the pesticides and drugs while its performance for the bifunctional compounds was somewhat poorer than the other tested models. Lastly, the experimental L values from this work were used to refine the LSER descriptors S, A and B of a suite of multifunctional and mostly polar compounds including pesticides and pharmaceuticals. The performance of the refined descriptors was significantly better for the prediction of the water–air partitioning, equal for the heptane–methanol partitioning but slightly poorer for the octanol–water partitioning than that of the former values.

Evaluation of stationary phases and gas chromatographic detectors for determination of amines in water

For the first time, a systematic overview deals with the advantages and disadvantages of several stationary phases (polar and non-polar) and gas chromatographic detectors (flame ionization detector, nitrogen-phosphorus detector and MS) for the determination of 27 amines (aliphatic and aromatic amines and N-nitrosamines) in water samples. To increase sensitivity (250 mL of sample was eluted with 150 μL of solvent) and matrix elimination, an automatic SPE system was employed prior to GC determination. The best results in terms of resolution and retention times were achieved using a column coated with 5% phenyl-dimethylpolysiloxane (DB-5). Capacity factor (k) values for the 27 amines increased with the rise in the polarity of the stationary phase, ranging from 3.0-27.7 and 2.2-14.4 for polar (polyethylene glycol) and non-polar (DB-5) columns, respectively. The detection limits of the method were 0.9-9 μg/L for flame ionization detector, 8-95 ng/L for nitrogen-phosphorus detector and 0.2-6.3 ng/L for MS. The precision was similar for the three detectors (RSD, 3.7-6.0%). The GC-MS method was applied with a high degree of accuracy and precision to determine amines in real samples including tap, river, pond, well, swimming pool and wastewaters.

Use of a polar ionic liquid as second column for the comprehensive two-dimensional GC separation of PCBs

The orthogonality of three columns coupled in two series was studied for the congener specific comprehensive two-dimensional GC separation of polychlorinated biphenyls (PCBs). A non-polar capillary column coated with poly(5%-phenyl–95%-methyl)siloxane was used as the first ( 1D) column in both series. A polar capillary column coated with 70% cyanopropyl-polysilphenylene-siloxane or a capillary column coated with the ionic liquid 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethane-sulfonyl)imide were used as the second ( 2D) columns. Nine multi-congener standard PCB solutions containing subsets of all native 209 PCBs, a mixture of 209 PCBs as well as Aroclor 1242 and 1260 formulations were used to study the orthogonality of both column series. Retention times of the corresponding PCB congeners on 1D and 2D columns were used to construct retention time dependences (apex plots) for assessing orthogonality of both columns coupled in series. For a visual assessment of the peak density of PCBs congeners on a retention plane, 2D images were compared. The degree of orthogonality of both column series was, along the visual assessment of distribution of PCBs on the retention plane, evaluated also by Pearson's correlation coefficient, which was found by correlation of retention times t R,i,2D and t R,i,1D of corresponding PCB congeners on both column series. It was demonstrated that the apolar + ionic liquid column series is almost orthogonal both for the 2D separation of PCBs present in Aroclor 1242 and 1260 formulations as well as for the separation of all of 209 PCBs. All toxic, dioxin-like PCBs, with the exception of PCB 118 that overlaps with PCB 106, were resolved by the apolar/ionic liquid series while on the apolar/polar column series three toxic PCBs overlapped (105 + 127, 81 + 148 and 118 + 106).

Analysis of Cymenes in Essential Oils: the Case of Lepechinia meyeni (Walp.) Epling.

The similarity of the mass spectra of the o-, m- and p-cymenes together with the small differences of their retention indices make it difficult to unambiguously identify the three cymene isomers. Moreover, there are discrepancies in the various data banks used as a reference in the analysis of essential oils. The authors have taken advantage of the presence of these three compounds in the oil of Lepechinia meyeni to identify the right order of elution of these compounds on both non-polar and polar columns. The order of elution on both DB-5 and S-wax 10 capillary columns is: m-, followed closely by p- and finally at some distance, o-cymene. A fourth compound having a similar mass spectrum was observed. From the literature, it was tentatively identified as 3,7,7-trimethyl-1,3,5-cycloheptatriene.

Preparation of a neutral porous monolith and its evaluation in pressurized capillary electrochromatography with neutral and charged solutes

A neutral, nonpolar monolithic capillary column was evaluated as a hydrophobic stationary phase in pressurized CEC system for neutral, acidic and basic solutes. The monolith was prepared by in situ copolymerization of octadecyl methacrylate and ethylene dimethacrylate in a binary porogenic solvent consisting of cyclohexanol/1,4-butanediol. EOF in this hydrophobic monolithic column was poor; even the pH value of the mobile phase was high. Because of the absence of fixed charges, the monolithic capillary column was free of electrostatic interactions with charged solutes. Separations of neutral solutes were based on the hydrophobic mechanism with the pressure as the driving force. The acidic and basic solutes were separated under pressurized CEC mode with the pressure and electrophoretic mobility as the driving force. The separation selectivity of charged solutes were based on their differences in electrophoretic mobility and hydrophobic interaction with the stationary phase, and no obvious peak tailing for basic analytes was observed. Effects of the mobile phase compositions on the retention of acidic compounds were also investigated. Under optimized conditions, high plate counts reaching 82,000 plates/m for neutral compounds, 134,000 plates/m for acid compounds and 150,000 plates/m for basic compounds were readily obtained.

Chromatographic Behavior of Bupivacaine and Five of its Major Metabolites in Human Plasma, Utilizing Solid- Phase Extraction and Capillary Gas Chromatography

Bupivacaine and its five known metabolites were determined in underivatized form using capillary gas chromatography. Gas chromatography with mass spectrometry (GC-MS) and nitrogen-phosphorus detection was used for the determination of bupivacaine and its metabolites. A comparison between seven commercially available columns of differing polarity and from different manufacturers was made. Peak symmetries and retention factors varied significantly between the columns. The nonpolar columns (dimethylpolylsiloxane), Ultra1 and CP-Sil, gave good peak symmetries and an analysis time of 9 min with complete baseline separation for all of the analytes studied. The stability of the substances at different injector temperatures, 200-350 degrees C, was studied. The limit of detection was 0.13-0.16 pmol using GC-MS in positive chemical ionization mode. Different conditions for the extraction of analytes from plasma samples using solid-phase extraction (SPE) were investigated. Different sorbents (C(2), C(8), C(18), and ENV+, divinyl-polystyrene) were tested. The ENV+ phase gave recoveries of between 82-120% for all the analytes studied. Several attempts at varying the washing and elution steps in order to obtain a high recovery and clean extract were investigated.

Quantitative Structure-Retention Relationship Study of Some Phenol Derivatives in Gas Chromatography

A quantitative structure-retention relationship (QSRR) model has been developed for the gas chromatographic retention times of 37 phenolic derivatives in a DB-5 non-polar column (95% dimethyl and 5% diphenyl-polysiloxane). As a first step, multiple linear regression (MLR) was employed to gain informative descriptors that can predict the retention times of these compounds. Descriptors appearing in the MLR model are categorized as topological and geometric parameters that comply with the applied column. Furthermore, each molecular descriptor in this model was examined to unfold the relationship between molecular structures and their retention times. Then, a 4-4-1 neural network was developed using the descriptors selected by the MLR model. The comparison of the standard errors and correlation coefficients reveals the superiority of artificial neural networks (ANN) over the MLR model. This refers to the fact that the retention behaviors of molecules display non-linear characteristics. The consistency and reliability of ANN model was investigated using the L4O cross-validation technique. The obtained results are closely in compliance with the experiment. Moreover, the mean effect of descriptors shows that Kier symmetry index is the most important factor affecting the retention behavior of molecules.

Aromas From Quebec. II. Composition of the Essential Oil of the Rhizomes and Roots of Asarum canadense L.

The fresh rhizomes and roots of Asarum canadense were steam-distilled to yield oils in 1.30% to 1.96%, respectively. The oils were analyzed by a combination of GC-FID and GC/MS techniques. In the case of the rhizome oil, the total amount of phenyl ethers including methyl eugenol, elemicins, and asaricin was about 58%. In the root oil, (E)-isoelemicin was the major product (20%) accompanied by elemicin (4.9%). From retention indices on both non-polar and polar columns and MS considerations, it may be inferred that methoxy-elemicin and both (E)- and (Z)-methoxy-isoelemicin as well as the new hydroxyl derivatives: hydroxyl-elemicin and (E)- and (Z)-hydroxy-isoelemicins were characterized in the oils.

Characterization of military fog oil by comprehensive two-dimensional gas chromatography

The most commonly used military fog oil is characterized by comprehensive two-dimensional gas chromatography (GC×GC) coupled to either Flame Ionization Detection (FID) or Time-of-Flight Mass Spectrometric Detection (TOFMS) to advance the knowledge regarding the complete chemical makeup of this complex matrix. Two different GC×GC column sets were investigated, one employing a non-polar column combined with a shape selective column and the other an inverse column set (medium-polar/non-polar). The inverse set maximizes the use of the two-dimensional separation space and segregates aliphatic from aromatic fractions. The shape selective column best separates individual polycyclic aromatic hydrocarbons (PAHs) from the bulk oil. The results reveal that fog oil (FO) is composed mainly of aliphatic compounds ranging from C 10 to C 30, where naphthenes comprise the major fraction. Although many different species of aromatics are present, they constitute only a minor fraction in this oil, and no conjugated PAHs are found. The composition of chemically similar aliphatic constituents limits the analytical power of silica gel fractionation and GC–MS analysis to characterize FO. Among the aliphatic compounds identified are alkanes, cyclohexanes, hexahydroindanes, decalins, adamantanes, and bicyclohexane. The aromatic fraction is composed of alkylbenzene compounds, indanes, tetrahydronaphthalenes, partially hydrogenated PAHs, biphenyls, dibenzofurans and dibenzothiophenes. This work represents the best characterization of military fog oil to date. As the characterization process shows, information on such complex samples can only be parsed using a combination of sample preprocessing steps, multiple detection schemes, and an intelligent selection of column chemistries.

A quantitative structure–retention relationship study for prediction of chromatographic relative retention time of chlorinated monoterpenes

A novel quantitative structure–retention relationship model has been developed for the gas chromatographic relative retention times (tR) of 67 polychlorinated monoterpene congeners in a non-polar column. Modeling of the relative retention time of these compounds as a function of the theoretically derived descriptors was established by principal component and partial least squares regressions. The choice of optimal training sets is efficiently performed by Kohonen self-organizing map. The genetic algorithm was used for the selection of the variables resulted in the best-fitted models. Appropriate models with low standard errors and high correlation coefficients were obtained. Wiener index, Balaban index, and ideal gas thermal capacity are examples of the descriptors affected by the retention times of polychlorinated monoterpenes.

Rhodanineacetic Acid Derivatives as Potential Drugs: Preparation, Hydrophobic Properties and Antifungal Activity of (5-Arylalkylidene-4-oxo-2-thioxo-1,3-thiazolidin-3-yl)acetic Acids

Some [(5Z)-(5-arylalkylidene-4-oxo-2-thioxo-1,3-thiazolidin-3-yl)]acetic acids were prepared as potential antifungal compounds. The general synthetic approach to all synthesized compounds is presented. Lipophilicity of all the discussed rhodanine-3-acetic acid derivatives was analyzed using a reversed phase high performance liquid chromatography (RP-HPLC) method. The procedure was performed under isocratic conditions with methanol as an organic modifier in the mobile phase using an end-capped non-polar C18 stationary RP column. The RP-HPLC retention parameter log k (the logarithm of the capacity factor k) is compared with log P values calculated in silico. All compounds were evaluated for antifungal effects against selected fungal species. Most compounds exhibited no interesting activity, and only {(5Z)-[4-oxo-5-(pyridin-2-ylmethylidene)-2-thioxo-1,3-thiazolidin-3-yl]}acetic acid strongly inhibited the growth of Candida tropicalis 156, Candida krusei E 28, Candida glabrata 20/I and Trichosporon asahii 1188.

Separation of diisopropylnaphthalene isomers

The separation of diisopropylnaphthalenes was reinvestigated. The application of GC × GC appears to be a clear and necessary improvement over the use of single column techniques, with a polar (CP-Wax-52) column as reference technique, and a non-polar (CP-Sil-8) column as an alternative. Both qualitative and quantitative separations of DIPN isomers showed to be superior on GC × GC. The composition of both a DIPN mixture resulting from a typical experiment with a zeolite catalyst and a commercial one could be quantitatively determined in this way.

Determination of Isosorbide Dinitrate in Serum by Gas Chromatography with New Generation of Electron Capture Detector and its Application in Pharmacokinetic Study

Isosorbide dinitrate (ISDN) is an effective drug in treatment of angina pectoris. In this study a new generation of electron capture detector (non-radioactive) with a short, non-polar and wide-bore column was used for analysis of ISDN in human serum. ISDN was extracted from serum by a mixture of ether and ethyl acetate and concentrated at room temperature. The method was linear between 5-50 ng/mL. Recovery and accuracy were 99-108% and greater than 90%, respectively, and inter-day precision was lower than 13%. Pharmacokinetic parameters were analyzed after oral administrations of Isocor 40 mg sustained release tablet in comparison with Isoket Retard 40. The statistical results obtained from comparison of C max , T max and AUC

Comprehensive Two-Dimensional Gas Chromatography Combustion Isotope Ratio Mass Spectrometry

We report the first coupling of comprehensive two-dimensional gas chromatography (GC x GC) to online combustion isotope ratio mass spectrometry (C-IRMS). A GC x GC system, equipped with a longitudinally modulated cryogenic system (LMCS), was interfaced to an optimized low dead volume combustion interface to preserve <300 ms full width at half-maximum (fwhm) fast GC peaks generated on the second GC column (GC2). The IRMS detector amplifiers were modified by configuration of resistors and capacitors to enable fast response, and a home-built system acquired data at 25 Hz. Software was home-written to handle isotopic time shifts of less than one bin (40 ms) and to integrate peak slices to recover isotope ratios from cryogenically sliced peaks. The performance of the GC x GCC-IRMS system was evaluated by isotopic analysis of urinary steroid standards. Steroids were separated by a nonpolar GC1 column (30 m x 0.25 mm, 5% phenyl), modulated into multiple 4- or 8-s cryogenic slices by the LMCS, and then separated on a polar GC2 column (1 or 2 m x 0.1 mm, 50% phenyl). GC2 peak widths from a 1-m column averaged 276 ms fwhm. Steroid standard sliced peaks were successfully reconstructed to yield delta(13)C VPDB values with average precisions of SD(delta(13)C) = 0.30 per thousand and average accuracies within 0.34 per thousand, at 8 ng on column. Two steroids, coeluting in GC1, were baseline separated in GC2 and resulted in delta(13)C VPDB values with average precisions of SD(delta(13)C) = 0.86 per thousand and average accuracies within 0.26 per thousand, at 3 ng on column. Results from this prototype system demonstrate that the enhanced peak capacity and signal available in GC x GC is compatible with high-precision carbon isotope analysis.

FAST ANALYSIS OF PAH BY GC AND GC/MS USING ISOMER SPECIFIC GC CAPILLARY COLUMNS

The required separation of polycyclic aromatic hydrocarbons (PAH) by GC, to meet regulatory requirements, depends largely on column selectivity. Liquid crystal (LC) capillary columns are highly selective for PAH separation. In the current investigation analysis and separation of PAH on columns coated with only the LC stationary phase, the combination of LC and non-polar stationary phase and only non-polar stationary phase, have been studied. The complete separation of isomeric PAH in the mixture of 16 priority PAH is achieved on the LC column. The inherent advantage of higher efficiency for microbore columns compared to conventional 0.25 mm internal diameter (ID) columns of the same length, in combination with the selectivity of the LC stationary phase, resulted in the fast analysis of 16 PAH on the columns with the combination of LC and non-polar stationary phases. Using this column, designated as the ICB-PAH column, separation of the USEPA 16 priority PAH in addition to other isomeric PAH (except benzo[j]fluoranthene) is achieved. Complete separation of 16 priority PAH was achieved in less than 5 minutes on a 6 m × 0.10 mm ID column, and in less than 7 minutes on a 15 m × 0.25 mm ID column. Separations of 16 priority PAH plus five additional isomeric PAH on LC, non-polar, and a combination of LC and non-polar stationary phase columns are discussed.

Gas Chromatographic Retention Indices of Fentanyl and Analogues

Narcotic analgesics of the fentanyl class are characterized by high potency and relatively short duration of action. These compounds nowadays have become a substitute for heroine and are highly addictive for abusers. Herein, we report retention indices of fentanyl and its eighteen analogues relative to the homologous n-alkane series. These values are determined on a moderately polar BP-5 capillary column under programmed temperature and isothermal chromatographic conditions. The analogues differ in the substituent attached to the piperidine ring nitrogen, and retention indices are found to vary according to the nature of the substituent. The effects of chromatographic conditions like temperature programming rate, carrier gas flow rate, and oven temperature are studied. Retention indices are also determined on a non-polar BP-1 column to study the influence of stationary phase polarity. Standard deviation of all the RI values is less than one index unit.

Rapid Determination of Low-Level Sulfur Compounds in Beer by Headspace Gas Chromatography with a Pulsed Flame Photometric Detector

Off-flavors in beer are frequently due to the presence of sulfur compounds. A simple and sensitive method for the analysis of volatile sulfur compounds in beer at trace levels was developed using headspace gas chromatography with a pulsed flame photometric detector. Different capillary columns were tested, and a 60-m nonpolar DB-1 capillary column was found to be the most appropriate. Four sulfur compounds, including hydrogen sulfide, methanethiol, ethanethiol, and dimethyl sulfide, could be quantified simultaneously using ethyl methyl sulfide as the internal standard. Calibration curves were established in a 4% (vol/vol) ethanolic solution with pH adjusted to 4.0; linear correlation coefficients (r) were greater than 0.99 for all target compounds. This method enables four sulfur compounds in beer to be determined at trace levels, with recoveries greater than 96%, a repeatability error of less than 5%, and limits of detection between 0.1 and 2 μg/L. The overall method was successfully applied to the determination of sulfur compounds studied in several beers.

A Simple, Convenient, and Efficient Preparative GC System that Uses a Short Megabore Capillary Column as a Trap

A simple, convenient, and highly efficient preparative GC system has been developed that uses short sections of megabore capillary columns as sample collection (sorbent) traps. The performance of this system with various types of capillary column traps and under various collection conditions was systematically investigated with model compounds, including C4 to C20 normal alkanes, esters, and alcohols. The thickness and polarity of the sorptive stationary phase and the temperature of the collection trap affected trap performance. Each group of compounds was efficiently trapped above a critical Kovat's index, and the type of trap (deactivated, methyl polysiloxane, polyethylene glycol), film thickness, and whether or not the trap was cooled significantly shifted this threshold index. Above this critical index, recovery efficiencies of traps with methyl polysiloxane films were 80-100% for a wide range of injected sample mass. For example, a DB-1 collection trap with a film thickness of 1.5 microm methyl polysiloxane operated at ambient temperature trapped >84% of the mass of injected compounds of all three chemical classes with Kovat's index >1,100 (determined on a nonpolar column) with injected sample mass ranging from 10 to 1,000 ng of each compound. This preparative GC system is technically and economically feasible for most researchers. Furthermore, it is suitable for the preparation of NMR samples of volatile and semivolatile compounds, especially with sample sizes ranging from several nanograms to several micrograms.

Simplified determination of lipophilic metabolites of nonylphenol ethoxylates: method development and application in aqueous samples from Buenos Aires, Argentina

In the present work we have developed an analytical methodology for the determination of nonylphenol (NP) and nonylphenol mono- and di-ethoxylates (NP1EO and NP2EO) in water samples. The applicability of this methodology was proved by means of the analysis of environmentally relevant aqueous samples from Buenos Aires. This constitutes a starting point for a rigorous assessment of the incidence of NPnEO surfactants in Argentina, as only very few, qualitative or semi-quantitative data on the occurrence of these compounds in local systems were available up to this time. Enrichment of the analytes was carried out by solid-phase extraction on a C-18 sorbent, followed by elution with ethyl acetate. Normal-phase high performance liquid chromatography on an amino-silica column and fluorescence detection at excitation-emission wavelengths of 230-300 nm were employed for separation and quantification of the analytes. Confirmation of peak assignment in selected real samples was performed by off-line coupling HPLC with GC-MS analysis. A non-polar GC capillary column was used, and a characteristic peak pattern was obtained for the alkyl chain isomers of each ethoxylated homologue and NP. GC-MS analyses yielded in all cases purity levels higher than 80% for the HPLC collected fractions. The elevated concentrations found for the estrogenic metabolites of NPnEO are in accordance with an unrestricted use of this class of non-ionic surfactants in the country.