Analytical applications of membrane extraction in chromatography and electrophoresis

An overview of the analytical applications of membrane-based systems for sample enrichment in chromatography and capillary electrophoresis is presented. A brief introduction to the different types of membranes and the main forces related to the transport through them is also given.     

Principles, developments and applications of on-line coupling of extraction with chromatography

On-line coupling of extraction and chromatographic separation allows the whole analysis to be performed in a closed system. On-line systems are particularly useful when the analytes are labile, the amount of sample is limited, or very high sensitivity is required. Many on-line systems have been developed both for liquid and for solid samples. This review discusses the different instruments that have been constructed and the factors that need to be considered in the coupling. Selected illustrative applications are described to illustrate the potential of the on-line systems.     

Low-pressure gas chromatography: Recent trends and developments

In the past few years, low-pressure gas chromatography (LP-GC) has been applied for the fast analysis of various pollutants in different environmental and food matrices. A typical LP-GC set-up involves the use of a short microbore column (typically 0.5–1 m × 0.10 mm internal diameter) at the injector side connected with a zero dead-volume connector to a short megabore column (typically 10 m × 0.53 mm) to be used with higher gas velocities. This set-up maintains atmospheric injection conditions, while the analytical column is operated under low-pressure conditions that are compatible with mass-spectrometer analyzers. Although the use of LP-GC results in a loss of separation efficiency, it offers a 3–5-fold reduction in analysis time for organic compounds and thus increased sample throughput and enhancement of the signal-to-noise ratio leading to improved detection limits. Considering the significance of, and the potential interest in, this topic, this review briefly describes the concept of LP-GC. Furthermore, we explore recent developments and applications of LP-GC, with a focus on the use of various column systems and analyzers. Finally, we critically evaluate the prospects for, and the limitations of, LP-GC.     

Review of recent developments and applications in low-pressure (vacuum outlet) gas chromatography

The concept of low pressure (LP) vacuum outlet gas chromatography (GC) was introduced more than 50 years ago, but it was not until the 2000s that its theoretical applicability to fast analysis of GC-amenable chemicals was realized. In practice, LPGC is implemented by placing the outlet of a short, wide (typically 10–15 m, 0.53 mm inner diameter) analytical column under vacuum conditions, which speeds the separation by reducing viscosity of the carrier gas, thereby leading to a higher optimal flow rate for the most separation efficiency. To keep the inlet at normal operating pressures, the analytical column is commonly coupled to a short, narrow uncoated restriction capillary that also acts as a guard column. The faster separations in LPGC usually result in worse separation efficiency relative to conventional GC, but selective detection usually overcomes this drawback. Mass spectrometry (MS) provides highly selective and sensitive universal detection, and nearly all GC-MS instruments provide vacuum outlet conditions for implementation of LPGC-MS(/MS) without need for adaptations. In addition to higher sample throughput, LPGC provides other benefits, including lower detection limits, less chance of analyte degradation, reduced peak tailing, increased sample loadability, and more ruggedness without overly narrow peaks that would necessitate excessively fast data acquisition rates. This critical review summarizes recent developments in the application of LPGC with MS and other detectors in the analysis of pesticides, environmental contaminants, explosives, phytosterols, and other semi-volatile compounds.     

Recent developments in polyetherimide membrane for gas separation

Polyetherimide (PEI) is an extraordinary type of polyimide with excellent thermal and mechanical properties. The polymer is also gas permeable and is considered one of the best membranes for gas separation. Despite the high selectivity, PEI suffers from low permeability due to the trade‐off between phenomena in polymers. To overcome this limitation, fillers are added during the membrane preparation to create voids for better gas transport. In this paper, permeability and selectivity data of PEI membranes for the separation of oxygen, carbon dioxide, and helium are discussed. The paper also summarizes the reported studies for adding fillers to improve the membrane performance.     

气相色谱固定相研究新进展

发展高选择性固定相是实现气相色谱(GC)高效分离样品组分及其分析测定的关键。近年,材料科学的快速发展促进了新型色谱固定相的研究和应用。该文综述了近5年有关多孔材料、石墨烯及类似物、三聚茚类材料和蝶烯类材料等作为GC固定相的研究进展,并对GC固定相研究进行了总结和展望。     

Novel applications for headspace gas chromatography

Summary New application areas for headspace gas chromatography in agricultural and polymer degradation research are described. Specific examples are drawn from the various forms of headspace analysis with emphasis on the automated static equilibrium method.Presented at the 32nd Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 9–13, 1981, Atlantic City, NJ (Paper No. 598).     

微型气相色谱的研究进展

具有高性能、智能化、低功耗、低物耗的分析仪器微型化研究已经是分析仪器的主要发展方向。气相色谱(GC)作为一种应用十分广泛的分离分析仪器,其微型化的研究一直受到科学界和工业界的重视。本文综述了近十几年来微型气相色谱(micro GC)的研究进展。对微型气相色谱的3个主要组成部分,即进样/富集装置、色谱柱和检测器,分别进行了详细的阐述,并简单总结了微型气相色谱的应用现状,对其发展趋势进行了展望。     

Development of membrane extraction with a sorbent interface-micro gas chromatography system for field analysis

The commercially available portable gas chromatographs have a rather limited scope of applications, typically allowing analysis of gaseous samples only, and having relatively poor sensitivity. Combination of those instruments with modern sampling/sample preparation techniques can remedy these problems. A Chrompack micro-GC system equipped with a thermal conductivity detector has been coupled to membrane extraction with a sorbent interface (MESI). The sorbent trap has replaced the GC injector. The design of the trap was also modified in order to enhance the preconcentration of analytes. The use of a thin flat sheet membrane reduces the response time, and decreases the memory effect of the system. Rapid separation times were achieved, and the sensitivity was significantly improved. MESI enables semi-continuous monitoring of both gaseous and aqueous samples, owing to the selectivity of the membrane material. The system does not use moving parts, therefore being reliable. The sensitivity of the micro-GC system was increased by a factor of more than 100 by the addition of the MESI system, even with a preconcentration time as short as 1 min. Chloroform, having a concentration lower than 1 ppb, was detected in tap water. A cup system was used to allow headspace sampling of volatile organic compounds from aqueous matrices, keeping the membrane away from interfering species that could be present in water, and improving the mass transfer. A linear calibration line was obtained, and the estimated limit of detection was 60 ppt. This represents a great improvement for the sensitivity of the micro-GC system.     

Inverse gas chromatography applications: A review

Inverse gas chromatography (IGC) is a versatile, powerful, sensitive and relatively fast technique for characterizing the physicochemical properties of materials. Due to its applicability in determining surface properties of solids in any form such as films, fibres and powders of both crystalline and amorphous structures, IGC became a popular technique for surface characterization, used extensively soon after its development. One of the most appealing features of IGC that led to its popularity among analytical scientists in early years was its similarity in principle to analytical gas chromatography (GC). The main aspect which distinguishes IGC experiments from conventional GC is the role of mobile and stationary phases. Contrary to conventional GC, the material under investigation is placed in the chromatographic column and a known probe vapour is used to provide information on the surface.     

国内气相色谱近年的进展

本文对近三年国内学者在气相色谱方面的研究和应用作了综述 ,国内学者的研究基本和国际上气相色谱方面的研究类似 ,在全二维气相色谱、快速气相色谱 ,微型气相色谱仪、新型气相色谱固定相和色谱柱的溶胶 凝胶涂渍技术领域的研究方面作出了贡献。有关气相色谱的应用研究中 ,介绍了大量在药物分析、食品分析、环境分析、石油和石化分析和化工产品及高聚物分析等领域中应用的题目和摘要     

Low thermal mass gas chromatography: principles and applications

In gas chromatography (GC), temperature programming is often considered to be the second most important parameter to control, the first being column selectivity. A radically new GC technology to achieve ultrafast temperature programming with an unprecedented cool down time and low power consumption has recently become available. This technology is referred to as low thermal mass GC (LTMGC). Though the technology has its roots in resistive heating, which forms the basis of principle and design concept, the approach taken to achieve ultrafast heating and cool down time by LTMGC represents a significant break-through in GC. Despite some rectifiable shortcomings, LTMGC has proven to be an ideal methodology to deliver near/real time GC data, high precision, and high throughput applications. It is a new approach for modern high-speed GC. This paper documents the fundamental design principles behind LTMGC, performance data, and examples of applications investigated.     

Determination of organophosphorus pesticides in aqueous samples by on-line membrane disk extraction and capillary gas chromatography

Summary A fast and simple procedure for the analysis of aqueous samples by on-line membrane disk extraction and capillary gas chromatography (GC) is presented. As an example, organophosphorus pesticides are preconcentrated from aqueous samples on three 0.5 mm thick, 4.2 mm diameter extraction disks. The layers are dried by a stream of nitrogen (10–15 min; ambient temperature). Desorption of the analytes is carried out with ethyl acetate which is directly introduced into a retention gap under partially concurrent solvent evaporation conditions, using an early solvent vapour exit. The final analysis is carried out by GC with thermionic detection. The technique is applied to the determination of a series of organophosphorus pesticides in tap water and water from two European rivers. With a sample volume of only 2.5 ml, detection limits of 10–30 ppt are achieved in tap water and of 50–100 ppt in river water.     

Practical fast gas chromatography: methods,instrumentation and applications

Minimal time of operation in gas chromatography (GC) has been a research topic ever since the introduction of GC. Today, revived interest in fast GC is seen to be driven by applications, such as process control and high-throughput analysis, or by the desire to reduce the costs of operation and ownership in routine analysis. Numerous options exist for speeding up GC separations. Which option to select depends strongly on the application under study. The first step should always be to see if it is possible to trade resolution for time. If this approach fails, the concept of “resolution-normalised conditions” comes into play. An overview of options for faster chromatography and a system for classifying chromatograms are brought together to give guidelines for speeding up specific applications. The practical consequences of implementing these options are discussed and the instrumental requirements are outlined. Applications of the various methods for faster GC are given.     

Aromatic polysulfone copolymers for gas separation membrane applications

New polysulfone (PSF) copolymers from bis(4-fluorophenyl)sulfone and based on equimolar mixtures of the rigid/compact naphthalene moiety with bulky connectors from bisphenols: tetramethyl, hexafluoro, and tetramethyl hexafluoro, respectively, were synthesized to measure significant physical properties related to the gas separation field. The flexible and transparent polymer dense films TM-NPSF, HF-NPSF and TMHF-NPSF show high glass transition temperatures T_g  230 °C and high decomposition temperatures T_D  400 °C (10 wt.% loss, in air). Free volume cavity sizes, as determined by PALS, are in the range of 94–139 Å³. Their gas permeability and selectivity combinations of properties, measured at 35 °C and 2 atm, are very attractive since their selectivity for the pair of gases H₂/CH₄, O₂/N₂, and CO₂/CH₄ are higher than those for commercial PSF membranes, having similar or superior permeability coefficients for the most permeable gases H₂, O₂, and CO₂. Especially important is the tetramethyl naphthalene polysulfone TM-NPSF membrane which reports selectivities for H₂/CH₄, O₂/N₂ and CO₂/CH₄ of 122, 7.6 and 38 with corresponding permeability coefficients (in Barrers) of 17 for H₂, 1.2 for O₂, and 5.2 for CO₂. These results are interpreted in terms of free volume size and glass transition temperature together with the respective contribution of gas solubility and diffusivity to the overall selectivity coefficients.     

Combining membrane extraction with mobile gas chromatography for the field analysis of volatile organic compounds in contaminated waters

A mobile gas chromatographic device (Airmobtx HC 1000 monitor manufactured by Airmotec, Germany), originally designed for the analysis of benzene, toluene, ethylbenzene and xylenes (BTEX) in air, was connected to a flow cell for dynamic membrane extraction. Volatile organic compounds (VOCs) diffuse out of a water stream through a hollow fibre, are enriched onto sorption tubes integrated in the mobile device, and are then thermally desorbed and analysed by gas chromatography-flame ionisation detection. Battery operation of the device enables continuous on-site analysis of VOCs. Influences of the water flow-rate on system response and memory effects were investigated. The linear range of the method depends on the flow-rate of the water sample and did not exceed two orders of magnitude. The detection limits for trichloroethene, chlorobenzene and the BTEX compounds were found to be between 0.1 and 1.0 microg/l using a water flow-rate of 30 ml/min. Dynamic membrane extraction combined with the mobile gas chromatographic device was used for the on-site analysis of contaminated waters in the area of Leipzig.     

Recent developments in the application of comprehensive two-dimensional gas chromatography

The literature on comprehensive two-dimensional gas chromatography (GC x GC) is reviewed, with emphasis on application-oriented studies published in the period 2004-2006. The various strategies that can be used in such studies, the state-of-the-art analytical performance and the high potential of GC x GC combined with time-of-flight mass spectrometric detection are highlighted.     

On-line automated sample preparation for liquid chromatography using parallel supported liquid membrane extraction and microporous membrane liquid-liquid extraction

An automated system was developed for analysis of non-polar and polar ionisable compounds at trace levels in natural water. Sample work-up was performed in a flow system using two parallel membrane extraction units. This system was connected on-line to a reversed-phase HPLC system for final determination. One of the membrane units was used for supported liquid membrane (SLM) extraction, which is suitable for ionisable or permanently charged compounds. The other unit was used for microporous membrane liquid-liquid extraction (MMLLE) suitable for uncharged compounds. The fungicide thiophanate methyl and its polar metabolites carbendazim and 2-aminobenzimidazole were used as model compounds. The whole system was controlled by means of four syringe pumps. While extracting one part of the sample using the SLM technique. the extract from the MMLLE extraction was analysed and vice versa. This gave a total analysis time of 63 min for each sample resulting in a sample throughput of 22 samples per 24 h.