The performance and application of a multidimensional system for gas chromatography

We have previously described [1] the design of a simple heart-cuttings device based on the Deans' principle of remote pressure switching [2]. Since that time, further work has been conducted to refine the design of the heart-cutting device and to develop complementary backflushing and cryofocusing units. This paper describes the results obtained from an intensive evaluation of the three modules in various combinations and considers parameters critical to the practical application of multidimensional gas chromatography (MDGC).     

Multidimensional gas chromatography for the characterization of permanent gases and light hydrocarbons in catalytic cracking process

An integrated gas chromatographic system has been successfully developed and implemented for the measurement of oxygen, nitrogen, carbon monoxide, carbon dioxide and light hydrocarbons in one single analysis. These analytes are frequently encountered in critical industrial petrochemical and chemical processes like catalytic cracking of naphtha or diesel fuel to lighter components used in gasoline. The system employs a practical, effective configuration consisting of two three-port planar microfluidic devices in series with each other, having built-in fluidic gates, and a mid-point pressure source. The use of planar microfluidic devices offers intangible advantages like in-oven switching with no mechanical moving parts, an inert sample flow path, and a leak-free operation even with multiple thermal cycles. In this way, necessary features such as selectivity enhancement, column isolation, column back-flushing, and improved system cleanliness were realized. Porous layer open tubular capillary columns were employed for the separation of hydrocarbons followed by flame ionization detection. After separation has occurred, carbon monoxide and carbon dioxide were converted to methane with the use of a nickel-based methanizer for detection with flame ionization. Flow modulated thermal conductivity detection was employed to measure oxygen and nitrogen. Separation of all the target analytes was achieved in one single analysis of less than 12 min. Reproducibility of retention times for all compounds were found to be less than 0.1% (n = 20). Reproducibility of area counts at two levels, namely 100 ppm_v and 1000 ppm_v over a period of two days were found to be less than 5.5% (n = 20). Oxygen and nitrogen were found to be linear over a range from 20 ppm_v to 10,000 ppm_v with correlation coefficients of at least 0.998 and detection limits of less than 10 ppm_v. Hydrocarbons of interest were found to be linear over a range from 200 ppb_v to 1000 ppm_v with correlation coefficients of greater than 0.999 and detection limits of less than 100 ppb_v.     

中心切割式多维气相色谱/质谱系统中中点限流器的改进及其在卷烟烟气中苯并[a]芘检测中的应用

对多维气相色谱的核心部件中点限流器进行了改进,用一段0.25mm i.d.毛细管柱取代石墨限流管,Y形玻璃压合连接器取代两孔石墨垫,显著提高了一维色谱保留时间的稳定性,保证了中心切割范围的准确性,且消除了石墨对苯并[a]芘的吸附残留。同时,将本系统应用于卷烟烟气中苯并[a]芘的检测。采用剑桥滤片捕集烟气粒相物,环己烷萃取,以氘代苯并[a]芘为内标,直接用中心切割式多维气相色谱/质谱(MDGC/MS)法测定。该法线性关系良好,其线性范围为1.47～29.4μg/L(r2=0.999 9);重复性好,测得实际样品中苯并[a]芘含量的相对标准偏差(RSD)为1.94%;回收率为90.74%～101.86%;检出限和定量限分别为0.31ng/支及1.03ng/支,可以满足检测要求。应用本系统对2R4F参比卷烟烟气中苯并[a]芘进行了检测,结果与文献报道值基本一致。     

Integrated multidimensional and comprehensive 2D GC analysis of fatty acid methyl esters

Fatty acid methyl ester (FAME) profiling in complex fish oil and milk fat samples was studied using integrated comprehensive 2D GC (GC × GC) and multidimensional GC (MDGC). Using GC × GC, FAME compounds – cis‐ and trans‐isomers, and essential fatty acid isomers – ranging from C18 to C22 in fish oil and C18 in milk fat were clearly displayed in contour plot format according to structural properties and patterns, further identified based on authentic standards. Incompletely resolved regions were subjected to MDGC, with Cn (n = 18, 20) zones transferred to a ²D column. Elution behavior of C18 FAME on various ²D column phases (ionic liquids IL111, IL100, IL76, and modified PEG) was evaluated. Individual isolated Cn zones demonstrated about four‐fold increased peak capacities. The IL100 provided superior separation, good peak shape, and utilization of elution space. For milk fat‐derived FAME, the ²D chromatogram revealed at least three peaks corresponding to C18:1, more than six peaks for cis/trans‐C18:2 isomers, and two peaks for C18:3. More than 17 peaks were obtained for the C20 region of fish oil‐derived FAMEs using MDGC, compared with ten peaks using GC × GC. The MDGC strategy is useful for improved FAME isomer separation and confirmation.     

Optimization of a heart-cutting multidimensional gas chromatography-based method for the assessment of enantiomeric fractions of o,p′-DDT in environmental samples

The enantiomeric fractions present in soil samples may provide information useful in distinguishing recent inputs of DDT from past DDT pollution. In this study, a chromatographic procedure for the determination of the enantiomeric fractions of o,p′-DDT based on heart-cutting multidimensional gas chromatography was developed. The optimization carried out achieved low ratios of DDT degradation (<15%) in the chromatographic system. High selectivity and sensitivity in the detection of the target compounds, with a limit of detection as low as 2.1 pg μL⁻¹, was reached. In addition, high degrees of repeatability (RSD < 2.0%) and reproducibility (RSD < 3.2%) were obtained for the enantiomeric fractions measured in analytical standards and soil samples.     

Enantiodifferentiation of α-ketols in sherry by one- and two-dimensional HRGC techniques

The enantiomeric distrubution of solerone (5-oxo-4-hexanolide) 1, ethyl 4-hydroxy- 5-oxohexanoate 2, ethyl 5-hydroxy-4-oxohexanoate 3, 4-oxo-5-hexanolide 4, and solerol (5-hydroxy-4-hexanolide) 5 in sherry wines was determined by several HRGC techniques. While gas chromatography-mass spectrometry on chiral cyclodextrin phases (chiral GC-MS) prevented any racemization of α-ketols 2 and 3 caused by keto-enol tautomerization during analysis, multidimensional gas chromatography MDGC (coupled either by “live-T” switching or by “moving column stream switching” MCSS) led to rearranged constitutional-and stereoisomers. The stereochemical results are discussed regarding the biogenesis of sherry constituents 1–5.     

Two multidimensional chromatographic methods for enantiomeric analysis of o,p′-DDT and o,p′-DDD in contaminated soil and air in a malaria area of South Africa

In rural parts of South Africa the organochlorine insecticide DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) is still used for malaria vector control where traditional dwellings are sprayed on the inside with small quantities of technical DDT. Since o,p′-DDT may show enantioselective oestrogenicity and biodegradability, it is important to analyse enantiomers of o,p′-DDT and its chiral degradation product, o,p′-DDD, for both health and environmental-forensic considerations. Generally, chiral analysis is performed using heart-cut multidimensional gas chromatography (MDGC) and, more recently, comprehensive two-dimensional gas chromatography (GC × GC). We developed an off-line gas chromatographic fraction collection (heart-cut) procedure for the selective capturing of the appropriate isomers from a first apolar column, followed by reinjection and separation on a second chiral column. Only the o,p′-isomers of DDT and DDD fractions from the first dimension complex chromatogram (achiral apolar GC column separation) were selectively collected onto a polydimethylsiloxane (PDMS) multichannel open tubular silicone rubber trap by simply placing the latter device on the flame tip of an inactivated flame ionisation detector (FID). The multichannel trap containing the o,p′-heart-cuts was then thermally desorbed into a GC with time-of-flight mass spectrometry detection (GC–TOFMS) for second dimension enantioselective separation on a chiral column (β-cyclodextrin-based). By selectively capturing only the o,p′-isomers from the complex sample chromatogram, ¹D separation of ultra-trace level enantiomers could be achieved on the second chiral column without matrix interference. Here, we present solventless concentration techniques for extraction of DDT from contaminated soil and air, and report enantiomeric fraction (EF) values of o,p′-DDT and o,p′-DDD obtained by a new multidimensional approach for heart-cut gas chromatographic fraction collection for off-line second dimension enantiomeric separation by ¹D GC–TOFMS of selected isomers. This multidimensional method is compared to the complementary technique of comprehensive GC × GC–TOFMS using the same enantioselective column, this time as the first dimension of separation.     

Evaluation of tea tree oil quality and ascaridole: A deep study by means of chiral and multi heart-cuts multidimensional gas chromatography system coupled to mass spectrometry detection

The natural-like assessment of essential oils is a demanding task due to the growing trend toward adulterations. Usually chiral chromatography was used for this purpose due to the capability of assessing stereospecificity which is directly related to the enzymatic pathways of each plant species. On the other hand, the quality of an essential oil involves also the evaluation of its oxidative state, mainly connected with the age and storage conditions. In fact, some modifications in the chemical profile of the oil can occur if not properly preserved. Alterations of the components due to oxidative reactions lead to the formation of peroxides, endoperoxides and epoxides, such as ascaridole and 1,2,4-trihydroxymenthane, usually present in very low amount, formed by the oxidation of terpinen-4-ol and α-terpinene, respectively. Therefore, in the present research, the quality of Australian Tea Tree oil (Melaleuca alternifolia (Maiden & Betche) Cheel, Myrtaceae) was investigated by means of a multi heart-cut multidimensional gas chromatographic system coupled to a mass spectrometer detector and by conventional enantio-GC. The MDGC system allowed the complete separation of the compounds of interest transferred from the first column to a second dimension based on a different separation mechanism. The MS detector at the end of the second column provided the identification of the peaks with high similarity values because of their high purities after the multidimensional separation. Method validation was carried out, in order to use this procedure for routine application, monitoring the repeatability of 1D retention times and 2D peak areas, LoD and LoQ. Finally, enantiomeric ratios for chiral compounds were established to support quality data obtained.     

Multiple component isolation in preparative multidimensional gas chromatography with characterisation by mass spectrometry and nuclear magnetic resonance spectroscopy

The preparative scale isolation of multiple components from an essential oil matrix is described using multidimensional gas chromatography (prep-MDGC) which allows their further characterisation by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. Menthol, linalyl acetate, carvone and geraniol were isolated individually, and were also collected in various combinations. It was demonstrated to be possible to collect multiple selected components from numerous repeat injections of the sample, to permit increased mass recovery from an external cryotrap collection device. Peak retention times remained reproducible (<0.3 s) over the repeated injections and switching events. This methodology may be utilised to confirm peak identity or to produce unique mixed-component reference standards, for instance to allow their identification in other samples using GC/MS, or identify them in comprehensive two-dimensional gas chromatography (GC × GC) analysis.