Membrane Extraction With A Sorbent Interface Research Articles

Use of flat-sheet membrane extraction with a sorbent interface for solvent-free determination of BTEX in water

An analytical method for solvent-free determination of benzene, toluene, ethylbenzene, and xylenes (BTEX) in water using flat-sheet membrane extraction with a sorbent interface (MESI) coupled to GC–MS was established by optimizing the flow rates of the donor (20ml water) and acceptor (helium) phases and extraction temperature. BTEX compounds permeated through a nonporous silicone membrane and evaporated into the acceptor phase were purged into a cryofocusing trap (−30°C) with helium gas. Enriched compounds were thermally desorbed into a capillary gas chromatograph and detected with a mass spectrometer. The optimum flow rates of the donor and acceptor phases were set at 1.5 and 55mlmin−1, respectively, and the temperature of the membrane extraction module was maintained within the 28–30°C range. The method as established showed low method detection limits (MDLs:∼0.1μgl−1) and highly linear calibration curves (r2>0.998) for all of the four compounds. High repeatability (relative standard deviation <∼5%) and a reasonably high extraction recovery (62–78%), after a single pass of the sample through the extraction module, also were established. Further, the method's high compatibility with the purge and trap (P&T) method indicates its applicability to field measurement. Other advantages include rapidity, simplicity, and a ready extendibility to automated on-line monitoring.

Membrane Extraction With a Sorbent Interface and Gas Chromatography for the Characterization of Ethylene in Human Breath

Membrane extraction with a sorbent interface (MESI) is a sample preparation technique with a rugged and simple design allowing for solvent-free, on-line performance. When coupled to gas chromatography (GC), MESI is extremely promising for the analysis of volatile organic compounds (VOCs), as it is selective and sensitive enough to detect trace levels of analytes. A new calibration method for the MESI technique is presented herein. The aim of this study was to characterize and quantify the breath biomarker ethylene in human subjects. The dominant calibration method was proven true using standard ethylene to verify changes in mass transfer trends, with variations in flow and temperature. Finally, the dominant calibration method was used to determine ethylene levels in real human breath samples from healthy and smoking volunteers. Results were consistent with those reported in literature. These findings suggest that the dominant calibration technique is a useful tool for monitoring ethylene in human breath.

Analysis of human breath with micro extraction techniques and continuous monitoring of carbon dioxide concentration

The detection of volatile organic compounds (VOCs) in human breath can be useful for the clinical routine diagnosis of several diseases in a non-invasive manner. Traditional methods of breath analysis have some major technical problems and limitations. Membrane extraction with a sorbent interface (MESI), however, has many advantages over current methods, including good selectivity and sensitivity, and is well suited for breath analysis. The aim of this project was to develop a simple and reproducible sampling device and method based on the MESI system for breath analysis. The feasibility and validity of the MESI system was tested with real human breath samples. Internal standard calibration methods were used for the quantitative analysis of various breath samples. Calibration curves for some main components (target analytes such as acetone and pentane) were determined in the research. The optimized stripping-side and feeding-side gas velocities were determined. The use of breath CO2 as an internal standard for the analysis of breath VOCs is an effective method to solve the difficulties associated with variations in the target analyte concentrations in a sample, which are attributed to mass losses and different breathing patterns of different subjects. In this study, the concentration of breath acetone was successfully expressed normalized to CO2 as in the alveolar air. Breath acetone of healthy males and females profiled at different times of the day was plotted using the MESI system, and results were consistent with the literature. This technique can be used for monitoring breath acetone concentrations of diabetic patients and for applications with other biomarker monitoring.

Internal Calibrant in the Stripping Gas. An Approach to Calibration of Membrane Extraction with a Sorbent Interface

A new technique for calibration in membrane extraction processes by adding an analytically noninterfering internal calibrant in the stripping gas is described. Membrane extraction with a sorbent interface (MESI) system was used to evaluate this method. During the membrane extraction process, the internal calibrant present in the carrier (stripping) gas and the target analyte present in the sample matrix will permeate simultaneously through the membrane in opposite directions. The changes of accumulation amounts of internal calibrant in the microtrap can be used as a means of calibration to correct the variations of extraction rate due to the variation in environmental factors, such as the sample velocity and the membrane temperature. Thus, this approach should allow for more accurate estimates of the concentrations of target analytes at various sampling or monitoring conditions during field analysis. Finally, a group of selected compounds was employed to test this calibration strategy, and the results indicated the advantages of the proposed approach for on-site analysis.

On-site environmental analysis by membrane extraction with a sorbent interface combined with a portable gas chromatograph system

A novel device, membrane extraction with a sorbent interface (MESI) coupled with a portable gas chromatograph (GC) system, has been developed. The main components of this system include a membrane module, a microtrap, and a control unit for the heater and cooler. The membrane module, as an on-line sample-introduction device for this system, can be manipulated in different configurations, allowing for the selective permeation of analytes across the membrane into the carrier/stripping gas. The analytes are trapped and concentrated onto a microtrap, which serves as an injector for gas chromatography separation. A concentration pulse of the trapped analytes is generated through direct electrical heating of the microtrap. The characteristics of this system have been explored, and its applicability and effectiveness have been demonstrated in field monitoring applications including the analysis of toluene in wastewater, Volatile organic compounds (VOCs) in laboratory air, and chloroform in swimming-pool water. This system is very promising, as it is a simple, fast, and portable tool for on-site process environmental monitoring.

New Developments and Applications of Solvent-Free Sampling and Sample Preparation Technologies for the Investigation of Living Systems

In recent years there has been considerable interest in developing techniques to monitor levels of biologically active compounds in living systems in natural environments. These efforts represent a significant departure from conventional ‘sampling’ techniques, where a portion of the system under study is removed from its natural environment, and the compounds of interest extracted and analyzed in a laboratory environment. An in vivo sampling approach can eliminate errors and reduce the time associated with sample transport and storage, and can therefore result in the collection of more accurate, precise, and faster analytical data. An ideal in vivo sampling technique should be portable, solvent-free, and offer integration of the sampling, sample preparation and sample analysis steps. These requirements are met by two techniques based on coated fibre and membrane technologies, presently under development in our laboratory.[1] Fibre solid-phase microextraction (fibreSPME) involves exposing a polymer-coated fused silica fibre to a sample. The analytes partition into the fibre coating until an equilibrium is reached. The fibre is then removed from the solution and the analytes are desorbed in the injector or injection loop of an analytical instrument such as a gas chromatograph (GC). The fibre is contained in a syringe-like device to facilitate handling.[2] Fibre-SPME can be used for both spot and time-averaged sampling. For spot sampling, the fibre is typically exposed to a sample matrix until the partitioning equilibrium between sample matrix and the coating material is reached. In the time-averaged technique the fibre remains in the needle during exposure of the SPME device to the sample. The fibre coating works as a trap for analytes that diffuse into the needle. In membrane extraction with a sorbent interface (MESI) a polymeric hollow fibre or a flat sheet membrane, in contact with a sample, is fitted directly into the carrier gas line of a GC equipped with a sorbent trap.[3]Analytes partition into the polymeric phase of the membrane and, after diffusion through the membrane, are carried by the gas to the sorbent trap. The concentrated analytes are periodically delivered onto the front of the column by a thermal pulse. MESI is a dynamic system, where the rate of analyte intake is dependent on both the diffusion coefficients of analytes in the membrane material and the membrane/sample matrix distribution constant. Similar to fibre-SPME, MESI can be used for both spot and time averaged monitoring. Both fibre-SPME and MESI techniques integrate sampling, sample preparation, and sample introduction to the analytical instrument, into a simple procedure. In fibreSPME mechanical movement of the fibre is necessary, as the sampling and sample introduction steps are separated in space, allowing one instrument to analyse large numbers of fibres. MESI, on the other hand, requires a dedicated, permanently attached instrument to one or several membrane/sorbent systems, eliminating the need for mechanical movement and therefore reducing the possibility of failure. MESI is thus suitable for continuous operation, allowing conversion of the analytical separation and detection instrument into a sensor-like device suitable for monitoring operations. Calibration procedures can be made very simple in both methods. For example, in air monitoring, the air/coating distribution constant can be estimated using the linear temperature-programmed retention index system (LTPRI). This allows quantification, even without identification, as long as the stationary phase used in the analytical column is identical to the fibre coating. The diffusion coefficient can be calculated by knowing the molecular weight of the target compound. To facilitate analysis of very polar analytes, a derivatization procedure can be used. For example, the validation field measurement of formaldehyde in ambient air, using both spot and time average sampling, was conducted using several techniques. Similar results were obtained for this challenging analyte using fibre-SPME and other more established procedures.[4]

Membrane extraction with sorbent interface-gas chromatography as an effective and fast means for continuous monitoring of thermal degradation products of polyacrylonitrile.

A novel sample preparation technique, membrane extraction with a sorbent interface (MESI) has been optimized and used for continuous monitoring thermal degradation products in polyacrylonitrile (PAN) polymer headspace at different temperatures, followed by gas chromatography-mass spectrometry (GC-MS). MESI with a flat sheet poly(dimethyl siloxane)-polycarbonate (PDMS-PC) membrane and Tenax trap was used. The system is very simple, fast and reliable and allowed us to extract, enrich and continuously monitor major volatile compounds released from the polymer at different temperatures. The volatile and semi-volatile gaseous degradation products were identified. Sensitivity of the method depends on the length of time for trapping.

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.

Membrane extraction with a sorbent interface for headspace monitoring of aqueous samples using a cap sampling device

A cap-shaped device was employed for headspace sampling. This sampling device coupled to membrane extraction with a sorbent interface (MESI) is intended to perform on-site and on-line aqueous sample monitoring. A laboratory sampling testwas performed both at the water surface and under water, and it showed some advantages in underwater monitoring. A group of volatile organic compounds (VOCs), varying in PDMS/gas and gas/water distribution constants, benzene, toluene, ethylbenzene, o-xylene, and trichloroethylene (TCE), was used for the sampling study. Magnetic stirring of the sample and circulation of the headspace air with a microfan were used for the enhancement of mass transfer between sample matrix and membrane to obtain higher extraction rate and efficiency. The agitation approaches were investigated individually and compared. The results showed that simultaneous agitation in water and air could greatly improve the extraction efficiency. Good linearity and precision and low detection limits were obtained for water-surface monitoring. The study demonstrated that Cap-MESI is a useful tool for field headspace monitoring of volatile organic compounds.

Membrane extraction with a sorbent interface (MESI): An efficient and fast cleanup method for the hollow silicone membrane

Nitrogen was used for cleanup of the silicone membrane after the extraction of analytes from aqueous samples by the membrane extraction with sorbent interface (MESI) method. The test samples consisted of solutions of organochlorinated and aromatic compounds at the relatively high concentrations of ca. 50 mg⋅L−1. The cleanup of the membrane with nitrogen was compared to cleanup with pure water. It is shown that in comparison to pure water, the use of nitrogen for membrane cleanup is much faster and may totally eliminate the analytes retained by the membrane. It is also shown that the use of heated nitrogen may be advantageous for some types of analytes. It is concluded that membrane cleanup with nitrogen may improve the use of MESI for sample preparation for gas chromatographic analysis. ©1999 John Wiley & Sons, Inc. J Micro Sep 11: 29–35, 1999

Aqueous Sample Direct Extraction and Analysis by Membrane Extraction With a Sorbent Interface†

A mathematical model was developed for aqueous sample analysis by membrane extraction with a sorbent interface (MESI). The model used in this paper includes the consideration of boundary layers which are located inside and outside of the membrane. In this study, benzene, toluene, ethylbenzene, trichloroethylene and hexane were the standard test analytes. Distribution constants of these analytes between the membrane and water were measured. Some significant parameters were investigated, such as agitation, temperature and headspace effect. Good agreement was found between the model and the experimental results.

Kinetic Model of Membrane Extraction with a Sorbent Interface

Membrane extraction with a sorbent interface (MESI) is an unique sample preparation alternative for trace organic analysis. The main features of MESI include its solvent-free nature, the rugged and simple design with no moving parts for long-term reliable performance, the fact that it is a single-step process which ensures good precision, its easy automation, and its feasibility for on-site operation. Among the available membrane extraction modules designed for the MESI system, the headspace configuration has continued to show its superior durability and versatility in membrane applications. The headspace membrane extraction configuration effectively eliminates the need for a sampling pump and flow metering and hence prevents the extraction system from plugging and greatly simplifies the extraction process. The module can be used for extraction of VOCs from gaseous, aqueous, or solid samples. A mathematical model has been developed for headspace membrane extraction of an aqueous sample, based on the assumption that the aqueous phase is perfectly stirred. The model is in good agreement with the experimental benzene extraction results obtained with an efficient agitation method such as high-speed magnetic stirring or sonication. The model has also been used to study the effects of various extraction parameters with respect to the sensitivity and response time of the MESI system. Sample agitation facilities analyte mass transport and hence improves both the system sensitivity and the response time. The sensitivity of the extraction method also increases with an increase of the extraction temperature.

Multiplex gas chromatography: a practical approach for environmental monitoring

A new analytical method, the membrane extraction with a sorbent interface (MESI) technique, has been developed to allow practical applications of multiplex gas chromatography for environmental monitoring of volatile organic compounds (VOCs). The advantages of the multiplex-MESI system include simplicity, durability, low cost, time efficiency, elimination of solvents and easy automation. The background and principles of the system are reviewed and potential applications and limitations of the multiplex-MESI system are discussed.

Headspace membrane extraction combined with multiplex gas chromatography and mass selective detector for monitoring of volatile organic compounds

A new analytical method, the membrane extraction with a sorbent interface (MESI) technique in multiplex mode combined with a gas chromatograph/mass selective detector (GC/MSD), has been developed to allow rapid screening analysis and long-term on-line monitoring of volatile organic compounds (VOCs). The advantages of the multiplex MESI system include simplicity, durability, low cost, time efficiency, elimination of solvents, and easy automation. The headspace membrane extraction configuration effectively eliminates the need for a sampling pump and flow metering thus prevents the extraction system from plugging and greatly simplifies the extraction process. The sorbent interface minimizes the loss of analytes by directly connecting the membrane extraction module to the GC/MSD. The sensitivity of the system is significantly enhanced due to the multiplex and high sample throughput advantages. The multiplex technique also reduces the demand on the sorbent interface which allows the use of a simpler interface than the MESI batch technique. The use of a MSD in the selective ion monitoring (SIM) mode not only selectively detects minor components in a sample, but also leads to a significant improvement in the system sensitivity. The MESI program provides an easy-to-use graphical user interface for system automation from sample introduction to report preparation. At optimum experimental conditions, the limit of detection for trichloroethylene in water was 0.5 μg/L, and a RSD below 6% was achieved in the SIM mode. Parameters influencing the sensitivity and precision of the system were studied. Quantitative contaminant determination of a soil leachate sample was performed with the standards addition method. A fumehood air sample was also analyzed using the new method. © 1996 John Wiley & Sons, Inc.

Headspace membrane extraction combined with multiplex gas chromatography and mass selective detector for monitoring of volatile organic compounds

A new analytical method, the membrane extraction with a sorbent interface (MESI) technique in multiplex mode combined with a gas chromatograph/mass selective detector (GC/MSD), has been developed to allow rapid screening analysis and long-term on-line monitoring of volatile organic compounds (VOCs). The advantages of the multiplex MESI system include simplicity, durability, low cost, time efficiency, elimination of solvents, and easy automation. The headspace membrane extraction configuration effectively eliminates the need for a sampling pump and flow metering thus prevents the extraction system from plugging and greatly simplifies the extraction process. The sorbent interface minimizes the loss of analytes by directly connecting the membrane extraction module to the GC/MSD. The sensitivity of the system is significantly enhanced due to the multiplex and high sample throughput advantages. The multiplex technique also reduces the demand on the sorbent interface which allows the use of a simpler interface than the MESI batch technique. The use of a MSD in the selective ion monitoring (SIM) mode not only selectively detects minor components in a sample, but also leads to a significant improvement in the system sensitivity. The MESI program provides an easy-to-use graphical user interface for system automation from sample introduction to report preparation. At optimum experimental conditions, the limit of detection for trichloroethylene in water was 0.5 μg/L, and a RSD below 6% was achieved in the SIM mode. Parameters influencing the sensitivity and precision of the system were studied. Quantitative contaminant determination of a soil leachate sample was performed with the standards addition method. A fumehood air sample was also analyzed using the new method. © 1996 John Wiley & Sons, Inc.