Evaluation of a retention model in comprehensive two-dimensional gas chromatography

Modeling the retention in comprehensive two-dimensional gas chromatography (GC x GC) was achieved using retention indices obtained in conventional GC. Predicted results were compared with experimental data obtained in the two-dimensional separation of a synthetic hydrocarbon mixture. This proved to be helpful in optimizing the operating conditions of GC x GC separation of a complex petroleum sample and in identifying chemical families.     

Simultaneous analysis of polychlorinated biphenyls and polychlorinated naphthalenes by isotope dilution comprehensive two-dimensional gas chromatography high-resolution time-of-flight mass spectrometry

Polychlorinated biphenyls (PCBs) and polychlorinated naphthalenes (PCNs) are listed as persistent organic pollutants (POPs) under the Stockholm Convention. Because they have similar physical and chemical properties, they are coeluted and are usually analyzed separately by different gas chromatography high-resolution mass spectrometry (GC-HRMS) methods. In this study, a novel method was developed for simultaneous analysis of six indicator PCBs, 12 dioxin-like PCBs, and 16 PCNs using isotope dilution comprehensive two-dimensional gas chromatography with high-resolution time-of-flight mass spectrometry (GC × GC-HRTOF-MS). The method parameters, including the type of GC column, oven temperature program, and modulation period, were systematically optimized. Complete separation of all target analytes and the matrix was achieved with a DB-XLB column in the first dimension and a BPX-70 column in the second dimension. The isotope dilution method was used for quantification of the PCBs and PCNs by GC × GC-HRTOF-MS. The method showed good linearity from 5 to 500 pg μL⁻¹ for all the target compounds. The instrumental limit of detection ranged from 0.03 to 0.3 pg μL⁻¹ for the 18 PCB congeners and from 0.09 to 0.6 pg μL⁻¹ for the 16 PCN congeners. Repeatability for triplicate injections was always lower than 20%. The method was successfully applied to the determination of 18 PCBs present at 0.9–2054 pg g⁻¹ and 16 PCNs present at 0.2–15.7 pg g⁻¹ in three species of fish. The GC × GC-HRTOF-MS results agreed with those obtained by GC-HRMS. The GC × GC-HRTOF-MS method proved to be a sensitive and accurate technique for simultaneous analysis of the selected PCBs and PCNs. With the excellent chromatographic separation offered by GC × GC and accurate mass measurements offered by HRTOF-MS, this method allowed identification of non-target contaminants in the fish samples, including organochlorine pesticides and polycyclic aromatic hydrocarbons.     

Chiral capillary electrophoretic separation of amino acids derivatized with 9-fluorenylmethylchloroformate using mixed chiral selectors of beta-cyclodextrin and sodium taurodeoxycholate

Chiral separation of 19 pairs of amino acid (AA) enantiomers derivatized with 9-fluorenylmethylchloroformate (FMOC) was successfully conducted by capillary electrophoresis using the mixture of beta-CD and sodium taurodeoxycholate (STDC) as selectors. Resolution was considerably superior to that obtained by using either beta-CD or STDC alone. After a systematic inspection, a buffer composed of 150 mM borate and 18% v/v isopropanol at pH 8.0, dissolved with 30 mM beta-CD and 30 mM STDC, was adopted and able to generate baseline resolution (>1.50) for 17 pairs of FMOC-AA enantiomers and somewhat lower resolution for arginine (1.36) and alanine (1.18), respectively. Experimental data revealed that the addition of the second selector did not increase the mobility difference between a pair of enantiomers (Delta mu = mu(D) - mu(L) and the number of theoretical plates (N), but decreased the summed apparent mobility of a pair of enantiomers (Sigma mu = mu(D) - mu(L)), which was mainly due to the decrease of the electroosmotic flow. The variation of Sigma mu was thus the major reason responsible for the improvement of chiral resolution in this study. The result demonstrated that not only the intrinsic selectivity of the selectors was the basis of the chiral separation, but also the non-chiral effect of the selectors, the change of the electroosmotic flow, was an important factor in enhancing the enantioseparation resolution. This study could probably help to explain the reasons for resolution improvement in some dual selectors systems, which are not very clear at present.     

Direct enantiomer separation of chiral γ-lactones from food and beverages by multidimensional gas chromatography

Chiral γ-lactones from the raw flavor extract of strawberries and some commercially available fruit-containing food and beverages were stereoanalyzed directly by multidimensional gas chromatography (MDGC) employing heart-cutting techniques from DB-1701 as the preseparation column onto heptakis (3-O-acetyl-2,6-di-O-pentyl)-β-cyclodextrin as the chiral stationary phase.     

Studies on thermionic ionisation detection in comprehensive two-dimensional gas chromatography

This study explores the application of specific thermionic ionisation detection in comprehensive 2-D GC (GC x GC) and represents the first report of GC x GC with nitrogen phosphorus detection (GC x GC-NPD). Of particular interest is the performance of the NPD with respect to peak parameters of asymmetry and sensitivity. Since GC x GC produces much narrower peaks than obtained with fast GC (e.g. 100 ms vs. <1 s) the effect of detector response time and any lack of symmetry arising from the detection step is important if peak separation (resolution) is to be maintained. It was observed that detector gas flows had a significant impact on peak asymmetry and peak magnitude, and that optimisation of the detector was critical, particularly for complex sample analysis by GC x GC-NPD. Peak asymmetries ranging from As = 1.8 to 8.0 were observed under different conditions of detector gas flows. Comparison of GC x GC-NPD with GC x GC-flame ionisation detection (FID) showed the former to be approximately 20 times more sensitive for the detection of nitrogen-containing methoxypyrazines analytes, and GC x GC-NPD had a larger linear detection range compared to GC x GC-FID. Furthermore, comparison of GC x GC-NPD and GC x GC-TOFMS chromatograms for the analysis of coffee head-space demonstrated the benefits of selective detection, ultimately realised in a comparatively simplified contour plot.     

Solid‐phase nanoextraction of polychlorinated biphenyls in water and their determination by gas chromatography with electron capture detector

A solid‐phase nanoextraction method has been developed for the extraction and preconcentration of polychlorinated biphenyls using carboxyl multiwalled carbon nanotubes as a solid nano‐sorbent. Parameters affecting extraction efficiency such as sorbent amount, desorption solvent type and volume, extraction time, pH, and salt content have been studied. Under optimized conditions, the correlation coefficient was up to 0.9989, the limits of detection was in the range of 1.4–3.5 ng/L, and limits of quantification was between 4.8 and 11.6 ng/L. The recoveries were in the range of 99–106% for different spiked analytes. The relative standard deviation for water samples spiked with two different spiking levels has been between 4 and 10%. The proposed sustainable method is rapid, easy to use, and small consumption of organic solvent for the detection and determination of trace levels of polychlorinated biphenyls in environmental waters.     

Enantiomer separation on monolithic silica HPLC columns using chemically bonded methylated and methylated/acetylated 6-O-tert-butyldimethylsilylated beta-cyclodextrin

A new 2,3-methylated 3*-monoacetylated 6-O-tert-butyldimethylsilylated beta-CD derivative was synthesized and chemically bonded onto aminopropyl derivatized monolithic silica HPLC columns. In this CD derivative, only one of seven methyl groups in 3-position was substituted by an acetyl group. Its applicability as a chiral stationary phase for HPLC was tested and compared with exclusively 2,3-methylated 6-O-tert-butyldimethylsilylated beta-CD immobilized onto aminopropyl-modified monoliths. Thirty-two chiral compounds from different chemical classes and different functionalities were tested under RP conditions. Fourteen compounds were resolved into their enantiomers by methylated 6-O-tert-butyldimethylsilylated beta-CD. By use of methylated/acetylated 6-O-tert-butyldimethylsilylated beta-CD as the chiral stationary phase 7 analytes were successfully stereodifferentiated.     

Chiral comprehensive two-dimensional gas chromatography with electron-capture detection applied to the analysis of chiral polychlorinated biphenyls in food samples

The feasibility of comprehensive two-dimensional gas chromatography with electron-capture detection (GC x GC-ECD) for the enantioseparation of chiral PCBs from other possible interfering compounds has been evaluated. Three commercially available enantioselective beta-cyclodextrin-based capillary columns (Chirasil-Dex, BGB-172 and BGB-176SE) have been tested as first-dimension columns. Three non-enantioselective stationary phases (HT-8, BPX-50 and Supelcowax-10) were combined with the enantioselective columns to allow the unambiguous determination of the enantiomers of the target chiral PCBs. Each enantioselective first-dimension column tested was able to separate into enantiomers different PCB congeners, but in all cases, the use of Supelcowax-10 as second-dimension column provided the most satisfactory results. The Chirasil-Dex x Supelcowax-10 column combination allowed the determination of the enantiomeric fraction (EF) of PCBs 84, 91, 95, 132, 136, 149, 174 and 176 in the working standard solution, while that of congener 135 was hindered. The BGB-172 x Supelcowax-10 column set allowed a proper EF determination of congeners 45, 84, 131, 132, 135, 171, 174 and 183, while that of PCB 91 was interfered with co-elutants. The column combination BGB-176SE x Supelcowax-10 allowed the determination of all congeners that this enantioselective stationary phase was able to separate into enantiomers, i.e. PCBs 45, 91, 95, 136, 149 and 176. These column combinations have also been evaluated for the simultaneous determination of the 12 congeners with a toxic equivalency factor assigned by the WHO (PCBs 77, 81, 105, 114, 118, 123, 126, 156, 157, 167, 169, 189) and the seven indicator congeners (PCBs 28, 52, 101, 118, 138, 153 and 180), and evaluated for the analysis of food samples.     

Quantification and determination of enantiomeric ratios of chiral PCB 95, PCB 132, and PCB 149 in shark liver samples (C. coelolepis) from the Atlantic Ocean

Chiral 2,2′,3,5′,6-pentachlorobiphenyl (PCB 95), 2,2′,3,3′,4,6′-hexachlorobiphnyl (PCB 132) and 2,2′,3,4′,5′,6-hexachlorobiphenyl (PCB 149) were quantified in Atlantic Ocean shark liver samples (C. coelolepis) by gas chromatography-mass spectrometry (GC-MS) and their enantiomeric ratios were studied by multidimensional gas chromatography (MDGC) with an achiral-chiral column combination and detected by ECD and MS/SIM. The concentration range of the chiral PCBs was from 2 to 6 ng/g (fresh weight), which represents 2–6 % of the total PCB mean concentration. The investigations revealed a small enantiomeric bias of PCB 132 in most of the samples studied (ER = 0.75–0.89, ee = 6–14%), while PCB 95 and PCB 149 were present in racemic or almost racemic form.     

Determination of 26 polychlorinated biphenyls congeners in soil samples using microwave-assisted extraction with open vessel and gas chromatography

A procedure focused on microwave-assisted extraction in open vessel (MAE-OV) and gas chromatography with electron capture detection (GC-ECD) was used for the determination of 26 congeners of polychlorinated biphenyls (PCBs) in soil samples. The limit of detection (LOD) and limit of quantification (LOQ) were evaluated for commercial PCBs mixture Aroclor1260. LOD and LOQ were calculated for each PCB congener, in the ranges (0.03–0.27?ng?g^?1) and (0.11–0.70?ng?g^?1), respectively. After optimization, 26 PCBs congeners were successfully extracted from soil samples with recovery amounts ranging between 84.7% and 117.3% for all PCBs congeners. The evaluated method of MAE-OV showed good separation and extraction of all PCBs congeners from soil samples. Extraction parameters such as solvent choice, power and extraction time were investigated. This study indicated that MAE-OV could be an interesting alternative method to extract PCBs from soils, since it is economical, easy, fast and requires low amounts of solvents.     

Solid phase extraction with silicon dioxide microspheres for the analysis of polychlorinated biphenyls in environmental water samples prior to gas chromatography with electron capture detector

This paper describes a simple, rapid and sensitive method for the determination of polychlorinated biphenyls (PCBs) including PCB-28, PCB-52, PCB-101, PCB-138, PCB-153 and PCB-180 using silicon dioxide microspheres solid phase extraction cartridge prior to gas chromatography with electron capture detector. The experimental results indicate that there were excellent linear relationships between the peak areas and the concentrations of polychlorinated biphenyls, and the linear ranges were in the concentration range of 0.1 to 30?µg?L^?1 with the correlation coefficients in the range of 0.9962 to 0.9998. The detection limits were in the range of 0.003 to 0.019?µg?L^?1. The precisions of the proposed method were below 10% (n?=?6). The enrichment factors, described as the ratio of the slope of the calibration curves with and without extraction, were in the range of 605.2 to 1414.6. The proposed method was used to analyse the real water samples, and satisfactory spiked recoveries in the range of 80 to 117% were achieved. These results demonstrated that silicon dioxide microspheres without modification were good adsorbent for the enrichment of polychlorinated biphenyls and the developed method were of great value and would be a good alternative for the routine analysis of polychlorinated biphenyls in the future.     

Quantification of nitrogen compounds in diesel fuel samples by comprehensive two‐dimensional gas chromatography coupled with quadrupole mass spectrometry

Although several methods for the analysis of nitrogen compounds in diesel fuel have been described in the literature, the demand for rapid, sensitive, and robust analyses has increased in recent years. In this study, a comprehensive two‐dimensional gas chromatographic method was developed for the identification and quantification of nitrogen compounds in diesel fuel samples. The quantification was performed using the standard addition method and the analysis was conducted using comprehensive two‐dimensional gas chromatography coupled with fast quadrupole mass spectrometry. This study is the first to report quantification of nitrogen compounds in diesel fuel samples using the standard addition method without fractionation. This type of analysis was previously performed using many laborious separation steps, which can lead to errors and losses. The proposed method shows good linearity for target nitrogen compounds evaluated (m‐toluidine, 4‐ethylaniline, indole, 7‐methylindole, 7‐ethylindole, carbazole, isoquinoline, 4‐methylquinoline, benzo[h]quinolone, and acridine) over a range from 0.05 to 2.0 mg/L, and limits of detection and quantification of <0.06 and 0.16 mg/L, respectively, for all nitrogen compounds studied.     

Characterization of crude oil biomarkers using comprehensive two‐dimensional gas chromatography coupled to tandem mass spectrometry

Oil samples from Recôncavo basin (NE Brazil), previously analyzed by traditional techniques such as gas chromatography coupled to tandem mass spectrometry, were evaluated using comprehensive two‐dimensional gas chromatography coupled to quadrupole mass spectrometry and comprehensive two‐dimensional gas chromatography coupled to tandem mass spectrometry along with simplified methods of samples preparation to evaluate the differences and advantages of these analytical techniques to better understand the development of the organic matter in this basin without altering the normal distribution of the compounds in the samples. As a result, the geochemical parameters calculated by comprehensive two‐dimensional gas chromatography coupled to tandem mass spectrometry described better the origin, maturity, and biodegradation of both samples probably by increased selectivity, resolution, and sensitivity inherent of the multidimensional technique. Additionally, the detection of the compounds such as, the C(14α‐) homo‐26‐nor‐17α‐hopane series, diamoretanes, nor‐spergulanes, C₁₉–C₂₆ A‐nor‐steranes and 4α‐methylsteranes resolved and detected by comprehensive two‐dimensional gas chromatography coupled to tandem mass spectrometry were key to classify and differentiate these lacustrine samples according to their maturity and deposition conditions.     

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.     

Cyclodextrins as chiral stationary phases in capillary gas chromatography. Part II: Heptakis(3-O-acetyl-2,6-di-O-pentyl)-ß-cyclodextrin

Heptakis (3-O-acetyl-2,6-di-O-pentyl)-ß-cyclodextrin is used as a chiral stationary phase in capillary gas chromatography. High enantioselectivity towards trifluoro-acetylated α and β-chiral amines, amino alcohols, α- and β-amino acid esters, and cyclic trans-diols is observed. In contrast to chiral polysiloxane phases, where hydrogen bonding interaction is essential for enantiomer separation, in cyclodextrins inclusion properties contribute to enantioselectivity. This can be concluded from the separation of N-alkylated amino compounds. The new chiral stationary phase exhibits a wide operating temperature range and is stable above 200°C.     

Identification of volatiles from pineapple (Ananas comosus L.) pulp by comprehensive two-dimensional gas chromatography and gas chromatography/mass spectrometry

Combining qualitative data from the chromatographic structure of 2-D gas chromatography with flame ionization detection (GC×GC-FID) and that from gas chromatography-mass spectrometry (GC/MS) should result in a more accurate assignment of the peak identities than the simple analysis by GC/MS, where coelution of analytes is unavoidable in highly complex samples (rendering spectra unsuitable for qualitative purposes) or for compounds in very low concentrations. Using data from GC×GC-FID combined with GC/MS can reveal coelutions that were not detected by mass spectra deconvolution software. In addition, some compounds can be identified according to the structure of the GC×GC-FID chromatogram. In this article, the volatile fractions of fresh and dehydrated pineapple pulp were evaluated. The extraction of the volatiles was performed by dynamic headspace extraction coupled to solid-phase microextraction (DHS-SPME), a technique appropriate for slurries or solid matrices. Extracted analytes were then analyzed by GC×GC-FID and GC/MS. The results obtained using both techniques were combined to improve compound identifications.     

Enantiomeric separation of chiral polycyclic musks by capillary electrophoresis: Application to the analysis of cosmetic samples

The enantiomeric separation of four chiral polycyclic musks (Galaxolide, Tonalide, Traseolide and Phantolide) using CE was achieved for the first time in this work. Two chiral methodologies were developed by CD-MEKC using SDS as surfactant in a CHES buffer (pH 9.0). One methodology enabled the fast enantiomeric separation of individual polycyclic musks with analysis times lower than 10 min for Tonalide, 13 min for Traseolide and Phantolide, and 17 min for Galaxolide. Enantiomeric resolutions obtained were higher than 1.5 using different separation media for each compound. A second methodology was also developed enabling the simultaneous enantioseparation of the four musks. In this case, the use of a dual CD system containing two neutral CDs was necessary to achieve the separation of all enantiomers from three out of four musks in 45 min. Although a coelution between Galaxolide and Phantolide was observed, the use of different UV absorption wavelengths allowed the simultaneous analysis of both musks. In addition, a sweeping strategy was performed in order to increase the sensitivity of the method. Appropriate analytical characteristics (linearity, LOD and LOQ, precision and absence of matrix interferences) were obtained for conventional and sweeping methodologies. Finally, the usefulness of the method was demonstrated in the determination of the enantiomers of the polycyclic musks in personal care products as perfumes.     

Rapid separation of beryllium and lanthanide derivatives by capillary gas chromatography

Previous studies describe derivatization of metal ions followed by analysis using gas chromatography, usually on packed columns. In many of these studies, stable and volatile derivatives were formed using fluorinated β‐diketonate reagents. This paper extends previous work by investigating separations of the derivatives on small‐diameter capillary gas chromatography columns and exploring on‐fiber, solid‐phase microextraction derivatization techniques for beryllium. The β‐diketonate used for these studies was 1,1,1,2,2,6,6,7,7,7‐decafluoro‐3,5‐heptanedione. Derivatization of lanthanides also required addition of a neutral donor, dibutyl sulfoxide, in addition to 1,1,1,2,2,6,6,7,7,7‐decafluoro‐3,5‐heptanedione. Unoptimized separations on a 100‐μm i.d. capillary column proved capable of rapid separations (within 15 min) of lanthanide derivatives that are adjacent to one another in the periodic table. Full‐scan mass spectra were obtained from derivatives containing 5 ng of each lanthanide. Studies also developed a simple on‐fiber solid‐phase microextraction derivatization of beryllium. Beryllium could be analyzed in the presence of other alkali earth elements (Ba(II) and Sr(II)) without interference. Extension of the general approach was demonstrated for several additional elements (i.e. Cu(II), Cr(III), and Ga(III)).