Speciation of nitrogen-containing compounds in diesel fuel by comprehensive two-dimensional gas chromatography

Nitrogen-containing compounds in diesel fuel have been speciated by comprehensive two-dimensional gas chromatography with nitrogen chemiluminescence detector (GC x GC-NCD). The speciation of nitrogen-containing compounds in diesel is difficult because of low concentration and complexity. The advantages of GC x GC are improved resolution and enhanced sensitivity. GC x GC-NCD can achieve the type and class separation of nitrogen-containing compounds with an appropriate separation column combination. Diesel contains both neutral (indoles and carbazoles) and basic (pyridines and quinolines) nitrogen-containing compounds. Relative concentrations of each class as well as each carbon number family can be quantified by integrating their peak volumes. This study demonstrates the capability of GC x GC-NCD for speciation of nitrogen-containing compound classes.     

Optimization of separation and detection conditions for the multiresidue analysis of pesticides in grapes by comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry

A comprehensive GCxGC-TOFMS method was optimized for multiresidue analysis of pesticides using a combination of a non-polar (RTX-5MS, 10 m x 0.18 mm x 0.2 microm) and a polar capillary column (TR-50MS, 1 m x 0.1 mm x 0.1 microm), connected in series through a dual stage thermal modulator. The method resolved the co-elution problems as observed in full scan one-dimensional GC-MS analysis and allowed chromatographic separation of 51 pesticides within 24 min run time with library-searchable mass spectrometric confirmation. Four pesticides, viz. chlorpyrifos-methyl, vinclozoline, parathion-methyl and heptachlor could be baseline separated on GCxGC, which were otherwise closely eluting and interfering each other's detection in 1D GC-MS run. Similarly, it could be possible to separate myclobutanil, buprofezin, flusilazole and oxyfluorfen on GCxGC. Although in 1D GC-MS, these closely eluting compounds could be identified through deconvolution algorithm and 'peak-find' option of the Chromatof software but the spectral purity significantly improved on GCxGC analysis. Thorough optimization was accomplished for the oven temperature programming, ion source temperature and GCxGC parameters like modulation period, duration of hot pulses, modulation-offset temperature, acquisition rate, etc. to achieve best possible separation of the test compounds. The limit of detection significantly improved by 2-12 times on GCxGC-TOFMS against GC-TOFMS because of sharper and narrower peak shapes. The method was tested for grape matrix after preparing the samples using previously described method and recoveries of the entire test pesticides were within 70-110% at 10 ng/g level of fortification. GCxGC-TOFMS was found to be an excellent technique for library-based screening of pesticides with high accuracy and sensitivity.     

Model for predicting comprehensive two-dimensional gas chromatography retention times

A model for approximating the relative retention of solutes in comprehensive two-dimensional gas chromatography (GCxGC) is presented. The model uses retention data from standard single-column temperature-programmed separations. The one-dimensional retention times are first converted into retention indices and then these indices are combined in a simple manner to generate a retention diagram. A retention diagram is an approximation of the two-dimensional chromatogram that has retention order and spacing in both dimensions similar to that found in the experimental chromatogram. If required, the retention diagram can be scaled to more closely resemble the two-dimensional chromatogram. The model has been tested by using retention time data from single-column gas chromatography-mass spectrometry and valve-based GCxGC. A total of 139 volatile organic compounds (VOCs) were examined. Approximately half of the VOCs had a single functional group and a linear alkyl chain (i.e., compounds with the structure Z-(CH(2))(n)-H). The retention diagrams had primary retention orders that were in excellent agreement with the GCxGC chromatograms. The relative secondary retention order for compounds with similar structures was also accurately predicted by the retention diagram. However, the relative secondary retention for compounds with dissimilar structures, such as acyclic alcohols and multi-substituted alkylbenzenes, were less accurately modeled. This study demonstrates how readily available single-column retention time data can be used to provide an a priori estimate of the relative retention of solutes in a GCxGC chromatogram. Such a capability is useful for screening possible combinations of stationary phases.     

Application of comprehensive two-dimensional gas chromatography with nitrogen-selective detection for the analysis of fungicide residues in vegetable samples

Comprehensive two-dimensional gas chromatography (GCxGC) with nitrogen-phosphorus detection (NPD) has been investigated for the separation and quantitation of fungicides in vegetable samples. The detector gas flows (H(2), N(2) and air) were adjusted to achieve maximum response of signal whilst minimizing peak width. The comparison of different column sets and selection of the temperature program were carried out with a mixture of nine N-containing standard fungicides, eight of which were chlorinated. The results from GCxGC-NPD and GCxGC with micro electron-capture detection (muECD) were compared. External calibrations of fungicides were performed over a concentration range from 1 to 1,000 microgL(-1). The peak area calibration curves generally had regression coefficients of R(2)>0.9980, however for iprodione which was observed to undergo on-column degradation, an R(2) of 0.990 was found. The limit of detection (LOD) and limit of quantitation (LOQ) were less than about 74 and 246 ng L(-1), respectively. The intra-day and inter-day RSD values were measured for solutions of concentration 0.100, 0.500 and 1.50 mg L(-1). For the 0.500 mg L(-1) solution, intra- and inter-day precision of peak area and peak height for most of the pesticides were about 2% and 8%, respectively. Excellent linearity was observed for these standards, from 0.001 to 25.00 mg L(-1). The standard mixture peak positions were identified by using GCxGC with quadrupole mass spectrometry (qMS). To illustrate the potential and the versatility of both GCxGC-NPD and GCxGC-muECD, the method was applied to determination of fungicides in a vegetable extract. Decomposition of one fungicide standard (iprodione) during chromatography elution was readily observed in the two-dimensional (2D) GCxGC plot as a diagonal ridge response in the 2D chromatogram between the degrading compound and the decomposition product.     

Low-molecular-weight model study of peroxide cross-linking of ethylene-propylene-diene rubber using gas chromatography and mass spectrometry II. Addition and combination reactions

The dicumyl-peroxide-initiated addition and combination reactions of mixtures of alkanes (n-octane, n-decane) and alkenes [5,6-dihydrodicyclopentadiene (DCPDH), 5-ethylidene-2-norbornane (ENBH) and 5-vinylidene-2-norbornane (VNBH)] were studied to mimic the peroxide cross-linking reactions of terpolymerised ethylene, propylene and a diene monomer (EPDM). The reaction products of the mixtures were separated by both gas chromatography (GC) and comprehensive two-dimensional gas chromatography (GCxGC). The separated compounds were identified from their mass spectra and their GC and GCxGC elution pattern. Quantification of the various alkyl/alkyl, alkyl/allyl and allyl/allyl combination products shows that allylic-radicals comprise approximately 60% of the substrate radicals formed. The total concentration of the products formed by combination is found to be independent of the concentration and the type of alkene. The total concentration of the products formed by addition to the alkene increases with increasing concentration of alkene. In addition, the total concentration of the formed addition products depends strongly on the type of the alkene used, viz. VNBH>ENBH approximately DCPDH, which is a consequence of differences in steric hindrance of the unsaturation. The peroxide curing efficiency, defined as the number of moles of cross-linked products formed per mol of peroxide, is 173% using 9% (w/w) 5-vinylidene-2-norbornane (VNBH). This indicates that the addition reaction is recurrent. All these findings are consistent with experimental studies on peroxide curing of EPDM rubber. In addition, the present results provide more-detailed structural information, increasing the understanding of the mechanism of peroxide curing of EPDM. The described approach to use low-molecular-weight model compounds followed by GC-mass spectrometry (MS) and GCxGC-MS analysis is proven to be a very powerful tool to study the cross-linking of EPDM.     

Programmed automation of modulator cold jet flow for comprehensive two-dimensional gas chromatographic analysis of vacuum gas oils

A method is described for automating the regulation of cold jet flow of a comprehensive two-dimensional gas chromatograph (GCxGC) configured with flame ionization detection. This new capability enables the routine automated separation, identification, and quantitation of hydrocarbon types in petroleum fractions extending into the vacuum gas oil (VGO) range (IBP-540 degrees C). Chromatographic data acquisition software is programmed to precisely change the rate of flow from the cold jet of a nitrogen cooled loop modulator of a GCxGC instrument during sample analysis. This provides for the proper modulation of sample compounds across a wider boiling range. The boiling point distribution of the GCxGC separation is shown to be consistent with high temperature simulated distillation results indicating recovery of higher boiling semi-volatile VGO sample components. GCxGC configured with time-of-flight mass spectrometry is used to determine the molecular identity of individual sample components and boundaries of different molecular types.     

A novel method for the calibration of Kováts retention indices using n-alkanes with a thermionic nitrogen-phosphorus specific detector

This paper describes a novel method for the detection of compounds that do not contain nitrogen or phosphorus by a thermionic nitrogen-phosphorus specific detector (NPD), which normally detects only nitrogen- or phosphorus-containing compounds. This method allows for the calibration of gas chromatographic columns with NPD detectors using n-alkanes instead of nitrogen-containing drug mixtures. This results in a more rapid and accurate calibration for the calculation of relative retention indices (RRI), such as Kováts indices, than was previously possible when employing an NPD detector. The proposed method describes the temporary conversion of the NPD detector into a detector with properties much like a flame ionization detector. After a deliberate increase in the hydrogen gas flow rate to the thermionic bead from 4 ml/min to 8 ml/min, the n-alkanes (containing no nitrogen) can be detected and used as RRI calibrators. Once the column has been calibrated, the hydrogen gas flow rate is lowered to the normal rate of 4 ml/min. The detector then behaves as a normal NPD, no longer detecting the n-alkanes.     

Selectivity differences between alkyl and polar‐modified alkyl phases in reversed phase high performance liquid chromatography

Compared to conventional C18 phases, polar‐modified phases have distinct differences with regards to chromatographic behavior. In the present study, ODS phases and polar‐modified phases were synthesized. The columns containing these new packings demonstrated satisfactory stability under both acidic (pH 1.5) and basic (pH 10) conditions. We evaluated the selectivity differences between alkyl and polar‐modified alkyl RP columns by using a range of neutral analytes. The polar‐modified alkyl phases showed excellent peak shapes for almost all compounds. We also compared the selectivity differences between them for separating nucleotides by using 100% aqueous mobile phase and tricyclic antidepressants in the intermediate pH mobile phases. The results demonstrated that polar‐modified phases display a significantly reduced hydrophobic nature and a significantly reduced silanol activity compared to the conventional C18 phases.     

Comprehensive two-dimensional gas chromatography with flame ionization and time-of-flight mass spectrometry detection: qualitative and quantitative analysis of West Australian sandalwood oil

The use of gas chromatography (GC)-mass spectrometry (MS), GC-time-of-flight MS (TOFMS), comprehensive two-dimensional GC (GCxGC)-flame ionization detection (FID), and GCxGC-TOFMS is discussed for the characterization of the eight important representative components, including Z-alpha-santalol, epi-alpha-bisabolol, Z-alpha-trans-bergamotol, epi-beta-santalol, Z-beta-santalol, E,E-farnesol, Z-nuciferol, and Z-lanceol, in the oil of west Australian sandalwood (Santalum spicatum). Single-column GC-MS lacks the resolving power to separate all of the listed components as pure peaks and allow precise analytical measurement of individual component abundances. With enhanced peak resolution capabilities in GCxGC, these components are sufficiently well resolved to be quantitated using flame ionization detection, following initial characterization of components by using GCxGC-TOFMS.     

Comprehensive two-dimensional gas chromatography: metrics,potentials, limits

Metrics for evaluation of separation performance of comprehensive two-dimensional gas chromatography (GCxGC) and for quantitative comparison of that performance with similar performance of its 1D (one-dimensional) counterparts are described. The performance improvement can be expressed via reduction in the saturation of a chromatogram or-in the case of the uniform distribution of peaks along the second dimension--via the peak capacity gain due to GCxGC. An order of magnitude peak capacity gain due to the comprehensive GCxGC is possible under optimal conditions. Optimal parameters of the second dimension column as well as the optimal operational conditions for that column and for the modulator in a comprehensive GCxGC are also presented.     

Elucidation of fatty acid profiles in vegetable oils exploiting group-type patterning and enhanced sensitivity of comprehensive two-dimensional gas chromatography

The present research is focused on the use of comprehensive 2-D GC (GCxGC) for the thorough elucidation of fatty acid (FA) profiles contained in vegetable oils; the samples analysed consisted of extra-virgin olive oil and refined hazelnut oil. The enhanced sensitivity and the formation of group-type patterns provided by GCxGC enabled the identification and quantification of both well-known and rather unexpected FAs contained in the lipid matrices. Peak assignment was, in most cases, supported by using pure standard compounds. Of particular interest was the identification of a series of odd-numbered FAs in both samples. The results attained to demonstrate the usefulness of GCxGC also for the analysis of supposedly low-complex samples.     

Comprehensive two-dimensional gas chromatography of volatile and semi-volatile components using a diaphragm valve-based instrument

A high-temperature configuration for a diaphragm valve-based gas chromatography (GCXGC) instrument is demonstrated. GCxGC is a powerful instrumental tool often used to analyze complex mixtures. Previously, the temperature limitations of valve-based GCxGC instruments were set by the maximum operating temperature of the valve, typically 175 degrees C. Thus, valve-based GCxGC was constrained to the analysis of mainly volatile components; however, many complex mixtures contain semi-volatile components as well. A new configuration is described that extends the working temperature range of diaphragm valve-based GCxGC instruments to significantly higher temperatures, so both volatile and semi-volatile compounds can be readily separated. In the current investigation, separations at temperatures up to 250 degrees C are demonstrated. This new design features both chromatographic columns in the same oven with the valve interfacing the two columns mounted in the side of the oven wall so the valve is both partially inside as well as outside the oven. The diaphragm and the sample ports in the valve are located inside the oven while the temperature-restrictive portion of the valve (containing the O-rings) is outside the oven. Temperature measurements on the surface of the valve indicate that even after a sustained oven temperature of 240 degrees C, the portions of the valve directly involved with the sampling from the first column to the second column track the oven temperature to within 1.2% while the portions of the valve that are temperature-restrictive remain well below the maximum temperature of 175 degrees C. A 26-component mixture of alkanes, ketones, and alcohols whose boiling points range from 65 degrees C (n-hexane) to 270 degrees C (n-pentadecane) is used to test the new design. Peak shapes along the first column axis suggest that sample condensation or carry-over in the valve is not a problem. Chemometric data analysis is performed to demonstrate that the resulting data have a bilinear structure. After over 6 months of use and temperature conditions up to 265 degrees C, no deterioration of the valve or its performance has been observed.     

Speciation of sulfur-containing compounds in diesel by comprehensive two-dimensional gas chromatography

Sulfur-containing compounds in diesel have been speciated by comprehensive two-dimensional gas chromatography (GCxGC) with a sulfur chemiluminescence detector (SCD). The advantages of GCxGC technique are higher resolution and greater sensitivity. GCxGC-SCD can achieve the class separation of sulfur-containing compounds with an appropriate separation column combination. The major classes of sulfur-containing compounds in diesel are benzothiophenes and dibenzothiophenes. Relative concentration of each class as well as each carbon number family can be quantitated by the summation of the integrated areas corresponding to the individual group(s) in the GCxGC space. In practical applications, GCxGC-SCD can be used to characterize different diesels and to reflect desulfurization process efficiency. In this study, GCxGC-SCD has demonstrated its value in speciation of sulfur-containing compounds classes, which is difficult to accomplish by any other single technique.     

Semi-rotating cryogenic modulator for comprehensive two-dimensional gas chromatography

A laboratory-made cryogenic modulator for comprehensive two-dimensional gas chromatography (GCxGC) was constructed and evaluated. Analytes were trapped with carbon dioxide and desorbed with heated GC oven air. The GCxGC system included a non-polar first-dimension column and a semi-polar second-dimension column connected to a flame ionisation detector. A laboratory-written Matlab-based program was used for the data analysis. Peak widths at half height for n-alkanes, obtained with use of new modulator, were at narrowest 60 ms.     

Novel RPLC stationary phases for lipophilicity measurement: solvatochromic analysis of retention mechanisms for neutral and basic compounds

An RPLC was developed to rapidly determine lipophilicity of neutral and basic compounds using three base deactivated RPLC stationary phases particularly designed for the analysis of basic compounds, namely, Supelcosil ABZ(+)Plus, Discovery RP Amide C16, and Zorbax Extend C18. The work consisted of three sets of experiments. In the first log kw values of neutral compounds were extrapolated using hydroorganic mobile phases at different compositions. Good correlation between log kw and log Poct indicated that the method was appropriate for these supports, without adding a silanol masking agent. In the second set of experiments, isocratic log k values of neutral and basic compounds were measured with three different mobile phases. The best estimation of lipophilicity was obtained for neutral and basic compounds when the secondary interactions were strongly reduced (i. e., when basic compounds were under their neutral form). In the third set of experiments, isocratic retention factors of basic compounds (in their neutral form) were measured with a high-pH mobile phase, on a chemically stable support (Zorbax Extend C18). Under these chromatographic conditions, correlation between the isocratic retention factors and log Poct (log D10.5) for basic compounds was similar to that for neutral compounds.     

Comprehensive two-dimensional gas chromatography for fast screening of wash oils

Fast screening of wash oils is demonstrated using comprehensive two-dimensional gas chromatography (GCxGC). Wash oils are used in ethylene production plants to minimize compressor fouling. The composition of a wash oil determines its effectiveness in solubilizing heavy hydrocarbons. In particular, the relative amount of 1- and 2-ring aromatics is important. The presence of oxygenates is undesirable because of adverse effects to the process. It is shown that GCxGC is well suited for this application. Species in wash oils are separated and grouped into three bands: a nonpolar aliphatics band, 1- and 2-ring aromatics band, and polyaromatics band. For a given polar secondary column, the spacing between bands in the second dimension can be adjusted in a broad range by selecting a primary column and an oven-temperature-programming rate. Integration of GCxGC peaks is evaluated using a standard GC integration program and a new GCxGC integration program. Consistent results are obtained using both programs for well-separated GCxGC peaks with relative differences for individual peak ranging from 0.04% to 1.6%. Peak responses are integrated by the GCxGC software, and the relative amounts of aromatics content and aliphatics content are estimated by peak response percent with relative standard deviations ranging from 0.15% to 2.8% (n = 3).     

Analysis of organic compounds of water-in-crude oil emulsions separated by microwave heating using comprehensive two-dimensional gas chromatography and time-of-flight mass spectrometry

In this work the higher peak capacity and resolution of comprehensive two-dimensional gas chromatography (GCxGC) has been successfully applied, for the first time, to tentatively identify several polar organic compounds of organic extracts of aqueous phases resulting from microwave demulsification process of water-in-crude oil emulsions. Results have shown that higher temperatures and longer exposure time to microwave irradiation produced water phases with a wider variety of polar organic compounds. The microwave process showed to be suitable for the extraction of several polar compounds classes of petroleum. The proposed microwave extraction method and GCxGC identification of polar compounds of petroleum samples are of practical interest for the petrochemical industry due to corrosion and related problems associated with these polar compounds in refinery equipments. The GCxGC/time-of-flight MS technique shows to be very important in the total separation of different classes of compounds and allows the identification of many compounds in these classes.     

气相色谱-表面电离检测器分析汽油中含氮化合物的分布

采用液液萃取的方法分别从90#、93#、97#汽油中提取了含氮化合物,并将气相色谱(GC)和作者所在研究组研制的表面电离检测器(SID)联用对含氮化合物进行了分析。结合GC-氢火焰离子化检测器(FID)、GC-氮磷检测器(NPD)和GC-质谱(MS)的分析结果,可鉴定出GC-SID谱图中的峰基本为含氮化合物,且大部分为NPD和FID未检出的峰,说明SID的选择性和灵敏度更好。分析结果表明,这3种汽油含氮化合物种类相似,含量有所差异;所提取的含氮化合物主要是苯胺类化合物;SID能从汽油样品中检出多种痕量的高沸点含氮组分,对于检测含氮组分而言,SID具有优于商品NPD的灵敏度和选择性。SID为GC分析提供了一种性能优异的选择性检测器。     

Compehensive two-dimensional gas chromatography (GC×GC) and its applicability to the characterization of complex (petrochemical) mixtures

In general, petrochemical products contain only a limited number of chemical classes of compounds (sample dimensionality). The enormous number of individual components within these classes, however, soon puts limitations upon a single chromatographic technique when it comes to adequate characterization of these products. Comprehensive two-dimensional gas chromatography (GC×GC) clearly opens the possibility of estimating the composition of hydrocarbon mixtures in a far more detailed fashion than hitherto possible. Although the emphasis of papers of GCxGC thus far almost exclusively applies to the unsurpassed peak-capacity, in the oil industry there is a need for characterization, rather than for analyzing all the individual compounds. In principle a GCxGC system can provide an almost perfect match between its intrinsic properties and the dimensionality of oil samples. To establish the applicability of GCxGC towards petrochemical analytical challenges, a commercially aavailable prototype instrument was subjected to an exhaustive characterization of a typical hydrocarbon precess stream and a fast characterization of a light gas oil. Although there are no fundamental limitations towards the quantitative aspects of a GCxGC system, this paper confines itself to qualitative results only. Quantitative aspects of GCxGC will be published in a forthcoming paper.     

Study of the surface ionization detector for gas chromatography

The structure of the surface ionization detector (SID) and the operation parameters of GC-SID were investigated to reduce peak tailing and to enhance sensitivity. The performances of the GC-SID, including its repeatability, linearity, sensitivity, selectivity, and tolerance towards water vapor, were evaluated systematically. Compared with nitrogen-phosphorus detector (NPD), the SID was able to detect fg level triethylamine, and selectively respond to alkylamines, some anilines, and some nitrogen heterocyclic compounds. Among alkylamines, the SID sensitivity to diisobutylamine was rather small. Even so, it was also still 10 times higher than that on NPD. The SID selectivity, defined as the sensitivity ratio between triethylamine and various tested non-nitrogen compounds, was higher than 10(6). It was found that the SID is highly tolerant towards water vapor, allowing direct injection of water sample. Finally, the GC-SID was applied to directly measure trace amines in headspace gases of rotted meat and trace simazine in tap water. The SID sensitivity to simazine was proven to be 5 times higher than that on flame ionization detector (FID). This study suggests that the SID is a promising GC detector.