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Abstract

Micro gas chromatography (μGC) has been continuously gaining attention since the last century owing to multiple favorable characteristics, such as its small size, low power consumption and minimal production and maintenance costs. μGC has the potential to provide practical solutions to emerging analytical challenges in security, health, and environment. In this review, we summarize recent advances in micro detectors for μGC, including the study of the miniaturization of conventional detectors and the development of novel detectors for μGC chromatography.

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... Significant improvements have been occurring, driven by the need for analytical tools that can analyze target and non-target components from complex samples, from a sensitive and/or selective point of view. Thus, several advances have been performed, namely the development of new stationary phases of GC columns, improvements of chromatographic equipment (e.g., development of pneumatics, microfluidic devices, and modulators, among others) and detection systems (selective and/or high-sensitivity detectors, with increasingly compact configuration and more user-friendly maintenance), improvements in the hardware and software (Wong et al., 2013), and also the recent advances in micro-GC systems (Qu and Duan, 2019). Consequently, the improvement on resolution and limits of detection (LOD), and reduction on the time of instrumental analysis and data processing have been contributing to the deeper characterization of samples. ...
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The human senses shape the life in several aspects, namely well-being, socialization, health status, and diet, among others. However, only recently, the understanding of this highly sophisticated sensory neuronal pathway has gained new advances. Also, it is known that each olfactory receptor cell expresses only one type of odorant receptor, and each receptor can detect a limited number of odorant substances. Odorant substances are typically volatile or semi-volatile in nature, exhibit low relative molecular weight, and represent a wide variety of chemical families. These molecules may be released from foods, constituting clouds surrounding them, and are responsible for their aroma properties. A single natural aroma may contain a huge number of volatile components, and some of them are present in trace amounts, which make their study especially difficult. Understanding the components of food aromas has become more important than ever with the transformation of food systems and the increased innovation in the food industry. Two-dimensional gas chromatography and time-of-flight mass spectrometry (GC × GC-ToFMS) seems to be a powerful technique for the analytical coverage of the food aromas. Thus, the main purpose of this review is to critically discuss the potential of the GC × GC-based methodologies, combined with a headspace solvent-free microextraction technique, in tandem with data processing and data analysis, as a useful tool to the analysis of the chemical aroma clouds of foods. Due to the broad and complex nature of the aroma chemistry subject, some concepts and challenges related to the characterization of volatile molecules and the perception of aromas will be presented in advance. All topics covered in this review will be elucidated, as much as possible, with examples reported in recent publications, to make the interpretation of the fascinating world of food aroma chemistry more attractive and perceptive.
... If the gas composition varies during usage, output deviations and false alarms will occur, which severely restricts its application. For thermal conductivity detectors (TCD) in gas chromatography (GC) [19][20][21], a typical application of gas thermal conductivity detection, the flow rate variation will increase the noise, decrease the detection limit performance, present inaccurate quantitative test results, and even present false signal peaks. At the same time, miniaturized GC calls for a higher stability of flow rate and temperature [22,23], which requires a better TCD with flow-independence characteristics. ...
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In this article, novel thermal gas sensors with newly designed diffusion gas channels are proposed to reduce the flow-rate disturbance. Simulation studies suggest that by lowering the gas flow velocity near the hot film, the maximum normalized temperature changes caused by flow-rate variations in the two new designs (Type-H and Type-U) are decreased to only 1.22% and 0.02%, which is much smaller than in the traditional straight design (Type-I) of 20.16%. Experiment results are in agreement with the simulations that the maximum normalized flow-rate interferences in Type-H and Type-U are only 1.51% and 1.65%, compared to 24.91% in Type-I. As the introduced CO2 flow varied from 1 to 20 sccm, the normalized output deviations in Type-H and Type-U are 0.38% and 0.02%, respectively, which are 2 and 3 orders of magnitude lower than in Type-I of 10.20%. In addition, the recovery time is almost the same in all these sensors. These results indicate that the principle of decreasing the flow velocity near the hot film caused by the two novel diffusion designs can enhance the flow-rate independence and improve the accuracy of the thermal conductivity as well as the gas detection.
... Recent advances in chromatography have shown potential for the use of commercial micro-GC for VOC detection (Ho et al., 2001). However, its application in situ is still under development and thus has not been widely used for in-process monitoring (Qu and Duan, 2019). Rapid detection using commercially available sensors for detection of VOCs has been evaluated for in-process control as an alternative to bench-top based conventional laboratory methods (Szulczyński and Gę). ...
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Organic solvents are widely used to facilitate ink and coating applications during the lamination of packaging substrates using adhesives. These solvents may remain sequestered between the laminated layers due to their slow diffusion through the laminate structure. This diffusion increases the potential for subsequent migration into food. The residual solvents (also referred to as volatile organic compounds (VOCs)) in laminated films are a significant concern due to heightened regulatory, consumer, and environmental pressure in domestic and global markets. There is a need to develop in-process detection systems for packaging materials as conventional benchtop laboratory analyses are time-consuming, costly, and do not allow for 100% inspection. This work demonstrates an in-process detection system for monitoring VOCs on a commercial food grade film lamination line. This in-process detection system consists of portable sensors including photoionization detector, semiconducting metal oxide sensor, infrared sensor, and electrochemical sensor. Correlations were found between the sensor signals generated from VOC detection during the lamination process and post-conversion benchtop gas chromatography-flame ionization detector (GC-FID) analyses for several solventless and solvent based adhesives. The correlation coefficients between the readings of the electrochemical sensor and GC data were 0.75 and 0.91 for the solventless and Matte Lacquer Ethyl Acetate based adhesive, respectively. Strong positive correlations were also found among the response signals from most sensors for each type of adhesive. Linear regression modeling was completed to predict GC results from the sensor data measured in-process during the lamination process. Results showed that the sensor readings can predict the total amount of VOCs detected by GC-FID with high coefficients of determination (R² > 0.90) for the solventless adhesive, ethyl alcohol based adhesive and ethyl acetate based adhesive formulation two. This study is among the first that utilized portable sensors for the detection of VOCs during the film lamination process in the food packaging industry and compared the sensor readings with the results of laboratory GC-FID. Findings of this study demonstrate potential to decrease cost of laboratory analysis, improve solvent and adhesive application control, and increase traceability with nearly 100% inspection of packaging substrates utilizing in-process monitoring technologies. This study also demonstrated viability of in-process regulatory compliance for laminated packaging.
... The detectors for μGC need to be small in size and fitted to the small flow dimension within the microfluidic channel. Recently, an interesting review on miniaturization of conventional GC detectors and the developments of novel detectors for μGC chromatography has been published [57]. The basic principle of several microdetectors based on silicon technology is briefly discussed in this section. ...
... The further miniaturisation is possible by using nanoelectromechanical (NEMS) technology. Recently, an interesting review on miniaturisation of conventional GC detectors and the developments of novel detectors for micro GC chromatography has been published [3]. ...
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... The demand for optical waveguide passive devices, such as dense wavelength division multiplexers and demultiplexers, optical switches, and adjustable attenuators is growing fast [1][2][3]. Among them, silicon-based optical waveguides have the advantages of high integration, low transmission loss, and compatibility with existing silicon processes, which makes them the primary unit of integrated optical devices [4][5][6]. However, limited by the top-down processing method, controlling the section shape of the waveguide using planar technology is difficult [7]. ...
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A sensitive GC–MS method is reported for the determination of twelve polycyclic aromatic hydrocarbons (PAHs) in baby food. The sample preparation involves QuEChERS extraction combined with low-density solvent dispersive liquid–liquid microextraction (LDS-DLLME) and ultra-low temperature (−80 °C). Plackett–Burman screening design was employed to identify the main sample preparation variables that affect the extraction efficiency, such as the volume of toluene used in LDS-DLLME. The suitability of proposed method was verified by analytical selectivity, linearity in solvent and matrix-matched calibration curves and adequate recoveries (72–112%) and precision (RSD values ≤11%), under repeatability and within-laboratory reproducibility conditions. High analytical sensitivity was achieved for the monitoring of PAHs at the strict limit of 1 µg kg⁻¹ fixed by the European Commission for baby foods. The validated method was applied to thirty-two commercial baby food samples, and the investigated PAHs were not detected in any sample.
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Conducting polymers (CPs) have been widely investigated due to their extraordinary advantages over the traditional materials, including wide and tunable electrical conductivity, facile production approach, high mechanical stability, light weight, low cost and ease in material processing. Compared with bulk CPs, nanostructured CPs possess higher electrical conductivity, larger surface area, superior electrochemical activity, which make them suitable for various applications. Hybridization of CPs with other nanomaterials has obtained promising functional nanocomposites and achieved improved performance in different areas, such as energy storage, sensors, energy harvesting and protection applications. In this review, recent progress on nanostructured CPs and their composites is summarized from research all over the world in more than 400 references, especially from the last three years. The relevant synthesizing experiences are outlined and abundant application examples are illustrated. The approaches of production of nanostructured CPs are discussed and the efficacy and benefits of newest trends for the preparation of multifunctional nanomaterials/nanocomposites are presented. Mechanism of their electrical conductivity and the ways to tailor their properties are investigated. The remaining challenges in developing better CPs based nanomaterials are also elaborated.
Article
In this work, a novel detector of gaseous analytes for monitoring of low levels of BTEX (benzene, toluene, ethylbenzene and o-xylene) is discussed. The detector is based on SnO2 thin (30nm) films functionalized with gold-palladium (9:1 molar ratio) bimetal nanoparticles. The detector consists of four independent sensing elements thermally activated by an internal heater. The performance of Au/[email protected]2 sensing elements was evaluated in comparison with some other gas sensitive materials, such as: pure SnO2, SnO2 decorated with gold (Au), and SnO2 decorated with palladium (Pd) nanoparticles. The superiority of the gas sensing performance of the Au/[email protected]2 nanocomposite structure was demonstrated based on its response to low concentrations (12.5-500 ppb) of benzene or toluene. A more complex analysis of gaseous mixtures with different concentrations (0.3-20ppb) of BTEX mixture was accomplished by incorporating the novel detector into the portable gas chromatograph (GC) PAC R1120. High sensitivity of the detector in combination with fast response and recovery time allowed us to obtain fine gas chromatograms and to perform a complete analysis of a gas sample in less than 11min. Ultra-low concentrations of BTEX components at sub-ppb level (0.3 ppb) were identified and detected by the GC analysis. Besides the experimental data, the theoretical validation of the detector's high performance was provided based on high catalytic activity of Au/Pd nanoparticles and electronic interaction of bimetal nanoclusters with SnO2 support.
Article
Flexible and wearable sensors have drawn extensive concern due to their wide potential applications in wearable electronics and intelligent robots. Flexible sensors with high sensitivity, good flexibility, and excellent stability are highly desirable for monitoring human biomedical signals, movements and the environment. The active materials and the device structures are the keys to achieve high performance. Carbon nanomaterials, including carbon nanotubes (CNTs), graphene, carbon black and carbon nanofibers, are one of the most commonly used active materials for the fabrication of high-performance flexible sensors due to their superior properties. Especially, CNTs and graphene can be assembled into various multi-scaled macroscopic structures, including one dimensional fibers, two dimensional films and three dimensional architectures, endowing the facile design of flexible sensors for wide practical applications. In addition, the hybrid structured carbon materials derived from natural bio-materials also showed a bright prospect for applications in flexible sensors. This review provides a comprehensive presentation of flexible and wearable sensors based on the above various carbon materials. Following a brief introduction of flexible sensors and carbon materials, the fundamentals of typical flexible sensors, such as strain sensors, pressure sensors, temperature sensors and humidity sensors, are presented. Then, the latest progress of flexible sensors based on carbon materials, including the fabrication processes, performance and applications, are summarized. Finally, the remaining major challenges of carbon-based flexible electronics are discussed and the future research directions are proposed.
Article
The analysis of exhaled volatile organic compounds (VOCs) related to lung cancer is a very promising way in medical diagnosis because it is non-invasive and much less expensive than traditional medical analysis used so far. In that sense, a silicon micro-analytical platform consisting of a micro-preconcentrator coupled to a silicon spiral gas chromatographic micro‐column was built, and a metal oxide-based gas sensor was used as a miniaturized gas detector. This micro-fabricated device was successfully tested to selectively detect low concentrations of VOCs considered as lung cancer biomarkers, within a few minutes even in presence of high concentrations of water vapor and carbon dioxide.
Book
The bible of gas chromatography-offering everything the professional and the novice need to know about running, maintaining, and interpreting the results from GCAnalytical chemists, technicians, and scientists in allied disciplines have come to regard Modern Practice of Gas Chomatography as the standard reference in gas chromatography. In addition to serving as an invaluable reference for the experienced practitioner, this bestselling work provides the beginner with a solid understanding of gas chromatographic theory and basic techniques.This new Fourth Edition incorporates the most recent developments in the field, including entirely new chapters on gas chromatography/mass spectrometry (GC/MS); optimization of separations and computer assistance; high speed or fast gas chromatography; mobile phase requirements: gas system requirements and sample preparation techniques; qualitative and quantitative analysis by GC; updated information on detectors; validation and QA/QC of chromatographic methods; and useful hints for good gas chromatography.As in previous editions, contributing authors have been chosen for their expertise and active participation in their respective areas. Modern Practice of Gas Chromatography, Fourth Edition presents a well-rounded and comprehensive overview of the current state of this important technology, providing a practical reference that will greatly appeal to both experienced chomatographers and novices.
Article
Cannabis products have recently regained much attention due to the high pharmacological potential of their cannabinoid content. In this review, the most widely used sample preparation strategies for the extraction of cannabinoids are described for the specific application to either plant materials or biological matrices. Several analytical techniques are described pointing out their respective advantages and drawbacks. In particular, chromatographic methods, such as TLC, GC and HPLC, are discussed and compared in terms of selectivity and sensitivity. Various detection methods are also presented based on the specific aim of the cannabinoids analysis. Lastly, critical considerations are mentioned with the aim to deliver useful suggestions for the selection of the optimal and most suitable method of analysis of cannabinoids in either biomedical or cannabis derived samples.
Article
Lung cancer (LC) is the leading cause of cancer death in men and the second leading cause in women worldwide. The use of low-dose computed tomography in early diagnosis was shown to reduce mortality by 20% with a median follow-up time of 6.5 years. In order to increase profitability and reduce radiation risks and costs, exhaled biomarkers could serve to help establish narrower inclusion criteria. The aim of this study was to identify new, well-founded volatile organic compounds in exhaled breath which distinguish LC patients from chronic obstructive pulmonary disease (COPD) patients and healthy subjects. There were 210 subjects enrolled and divided into three groups: control group (n = 89), COPD group (n = 40 stable COPD patients) and LC group (n = 81 with histological confirmation). Exhaled breath samples were collected using BioVOC® breath sampler devices. The analytical technique used was thermal desorption-gas chromatography-mass spectrometry. The compounds studied were hexanal, heptanal, octanal, nonanal, propanoic and nonanoic acids. Nonanoic acid showed statistically significant differences between the LC group and the other groups. It is 2.5 times and almost 9 times more likely to be found in the LC group than in the control group or COPD group, respectively. It is independent of histology but depends on tumour stage.
Article
We developed a highly sensitive humidity sensor based on the combination of ultrahigh-frequency film bulk acoustic resonator (FBAR) and nano-assembled polyelectrolyte (PET) thin films. The water molecule absorption efficiency was optimized by forming loosely-packed PET nanostructures. Then, the humidity sensing characteristics were analyzed in terms of sensitivity, linearity, reversibility, stability and detection limit. As a result, PET-coated FBAR exhibits excellent humidity sensitivity of 2202.20 Hz/ppm, which is five orders of magnitude higher than quartz crystal microbalance (QCM). Additionally, temperature dependence was investigated with the result that PET-coated FBAR possessed a higher sensitivity at low temperature. Furthermore, we realized the selective detection of water vapor from volatile organic compounds (VOCs) with respect to the polarity property. Owing to the high sensitivity, miniaturized size and ultrahigh operating frequency, PET-coated FBAR is uniquely favorable as a wireless humidity sensor node to integrate into wireless sensor networks (WSNs).
Article
A portable chromatography device and a method were developed to analyze a gas mixture. The device comprises a chromatographic column for separating components of a sample of the gas mixture. The device further comprises an air pump coupled to the inlet of a chromatographic column for pumping an air to the chromatographic column and an injector coupled to the inlet of chromatographic column for feeding the sample through the chromatographic column via the air as a carrier gas. A detector is arranged downstream from the chromatographic column and coupled to the outlet of the chromatographic column. The detector is a nanostructure semi-conductive microfiber. The device further comprises an evaluation unit arranged and configured to evaluate each detected component so as to determine a concentration of each detected component. Designed portable system was used for simultaneous detection of amines. The possibility of applying dispersive liquid–liquid microextraction for the determination of analytes in trace levels is demonstrated. The reproducibility of this method is acceptable, and good standard deviations were obtained. The relative standard deviation value is less than 6% for all analytes. The method was successfully applied to the extraction and determination of analytes in water samples. This article is protected by copyright. All rights reserved
Article
We developed a fully automated portable 2-dimensional (2-D) gas chromatography (GC x GC) device, which had a dimension of 60 cm × 50 cm × 10 cm and weight less than 5 kg. The device incorporated a micropreconcentrator/injector, commercial columns, micro-Deans switches, microthermal injectors, microphotoionization detectors, data acquisition cards, and power supplies, as well as computer control and user interface. It employed multiple channels (4 channels) in the second dimension ((2)D) to increase the (2)D separation time (up to 32 s) and hence (2)D peak capacity. In addition, a nondestructive flow-through vapor detector was installed at the end of the (1)D column to monitor the eluent from (1)D and assist in reconstructing (1)D elution peaks. With the information obtained jointly from the (1)D and (2)D detectors, (1)D elution peaks could be reconstructed with significantly improved (1)D resolution. In this Article, we first discuss the details of the system operating principle and the algorithm to reconstruct (1)D elution peaks, followed by the description and characterization of each component. Finally, 2-D separation of 50 analytes, including alkane (C6-C12), alkene, alcohol, aldehyde, ketone, cycloalkane, and aromatic hydrocarbon, in 14 min is demonstrated, showing the peak capacity of 430-530 and the peak capacity production of 40-80/min.
Article
This paper presents the design, fabrication, and characterization of a microhelium dielectric barrier discharge photoionization detector (μHDBD-PID) on chip with dimensions of only ∼15 mm × ∼10 mm × ∼0.7 mm and weight of only ∼0.25 g. It offers low power consumption (<400 mW), low helium consumption (5.8 mL/min), rapid response (up to ∼60 ms at a flow rate of 1.5 mL/min), quick warm-up time (∼5 min), an excellent detection limit (a few picograms), a large linear dynamic range (>4 orders of magnitude), and maintenance-free operation. Furthermore, the μHDBD-PID can be driven with a miniaturized (∼5 cm × ∼2.5 cm × ∼2.5 cm), light (22 g), and low cost (∼$2) power supply with only 1.5 VDC input. The dependence of the μHDBD-PID performance on bias voltage, auxiliary helium flow rate, carrier gas flow rate, and temperature was also systematically investigated. Finally, the μHDBD-PID was employed to detect permanent gases and a sublist of the EPA 8260 standard reagents that include 51 analytes. The μHDBD-PID developed here can have a broad range of applications in portable and microgas chromatography systems for in situ, real-time, and sensitive gas analysis.
Article
A generic method to reduce the in-line flow dependence of thermal conductivity detectors (TCDs) is presented. The principle is based on a dual-MEMS device configuration. Two thin-film sensors on membranes in parallel in the gas stream on the same chip are differentially operated. Both micro-TCDs are designed to be identical in terms of contact with the main gas flow, however a different depth of the detection chamber results in a different response to the thermal conductivity of the sample gas. Static and dynamic simulations have been performed to characterize the design of the fabricated structures. Devices have been fabricated in a MEMS process using a combined surface- and bulk micromachining process. The devices have been characterized statically and dynamically. Measurements on prototypes show that depending on the range of gases, device size and flow range device the effect of flow on the thermal conductivity can be reduced by a factor 4-15.
Article
A photoionization detector (PID) is widely used as a gas chromatography (GC) detector. By virtue of its non-destructive nature, multiple PIDs can be used in multi-dimensional GC. However, different PIDs have different responsivities towards the same chemical compound with the same concentration or mass due to different aging conditions of the PID lamps and windows. Here, we carried out a systematic study regarding the response of 5 Krypton μPIDs in a 1 × 4-channel 2-dimensional μGC system to 7 different volatile organic compounds (VOCs) with the ionization potential ranging from 8.45 eV to 10.08 eV and the concentration ranging from ∼1 ng to ∼2000 ng. We used one of the PIDs as the reference detector and calculated the calibration factor for each of the remaining 4 PIDs against the first PID, which we found is quite uniform regardless of the analyte, its concentration, or chromatographic peak width. Based on the above observation, we were able to quantitatively reconstruct the coeluted peaks in the first dimension using the signal obtained with a PID array in the second dimension. Our work will enable rapid and in situ calibration of PIDs in a GC system using a single analyte at a single concentration. It will also lead to the development of multi-channel multi-dimensional GC where multiple PIDs are employed.
Article
With the help of micro-electromechanical systems (MEMS) and complementary metal-oxide-semiconductor (CMOS) technology, a portable micro gas chromatography ([Formula: see text]) system for lung cancer associated volatile organic compounds (VOCs) detection is realized for the first time. The system is composed of an MEMS preconcentrator, an MEMS separation column, and a CMOS system-on-chip (SoC). The preconcentrator provides a concentration ratio of 2170. The separation column can separate more than seven types of lung cancer associated VOCs. The SoC is fabricated by a TSMC [Formula: see text] 2P4M process including the CMOS VOCs detector, sensor calibration circuit, low-noise chopper instrumentation amplifier (IA), 10 bit analog to digital converter, and the microcontrol unit (MCU). Experimental results show that the system is able to detect seven types of lung cancer associated VOCs (acetone, 2-butanone, benzene, heptane, toluene, m-xylene, 1,3,5-trimethylbenzene). The concentration linearity is [Formula: see text] and the detection sensitivity is up to 15 ppb with 1,3,5-trimethylbenzene.
Article
To establish adequate on-site solvent trapping of volatile chemical warfare agents (CWAs) from air samples, we measured the breakthrough volumes of CWAs on three adsorbent resins by an elution technique using direct electron ionization mass spectrometry. The trapping characteristics of Tenax(®) TA were better than those of Tenax(®) GR and Carboxen(®) 1016. The latter two adsorbents showed non-reproducible breakthrough behavior and low VX recovery. The specific breakthrough values were more than 44 (sarin) L/g Tenax(®) TA resin at 20°C. Logarithmic values of specific breakthrough volume for four nerve agents (sarin, soman, tabun, and VX) showed a nearly linear correlation with the reciprocals of their boiling points, but the data point of sulfur mustard deviated from this linear curve. Next, we developed a method to determine volatile CWAs in ambient air by thermal desorption-gas chromatography (TD-GC/MS). CWA solutions that were spiked into the Tenax TA(®) adsorbent tubes were analyzed by a two-stage TD-GC/MS using a Tenax(®) TA-packed cold trap tube. Linear calibration curves for CWAs retained in the resin tubes were obtained in the range between 0.2pL and 100pL for sarin, soman, tabun, cyclohexylsarin, and sulfur mustard; and between 2pL and 100pL for VX and Russian VX. We also examined the stability of CWAs in Tenax(®) TA tubes purged with either dry or 50% relative humidity air under storage conditions at room temperature or 4°C. More than 80% sarin, soman, tabun, cyclohexylsarin, and sulfur mustard were recovered from the tubes within 2 weeks. In contrast, the recoveries of VX and Russian VX drastically reduced with storage time at room temperature, resulting in a drop to 10-30% after 2 weeks. Moreover, we examined the trapping efficiency of Tenax TA(®) adsorbent tubes for vaporized CWA samples (100mL) prepared in a 500mL gas sampling cylinder. In the concentration range of 0.2-2.5mg/m(3), >50% of sarin, soman, tabun, cyclohexylsarin, and HD were recovered, whereas <1% of VX and Russian VX were recovered in the same concentration range. The results indicate that CWA vapors, with the exception of VX and Russian VX, can be measured by an on-site collection procedure using the Tenax(®) TA resin tubes, followed by a subsequent TD-GC/MS analysis. Copyright © 2015 Elsevier B.V. All rights reserved.
Article
This paper describes the detection of volatile organic compounds (VOCs) using an e-nose type integrated microfabricated sensor array, in which each resonator is coated with different supramolecular monolayers: p-tert-butyl calix[8]arene (Calix[8]arene), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (Porphyrin), beta-cyclodextrin (β-CD) and cucurbit[8]uril (CB[8]). Supramolecular monolayers fabricated by Langmuir-Blodgett techniques work as specific sensing interface for different VOCs recognition which increase the sensor selectivity. Microfabricated ultra-high working frequency transducers (4.4 GHz) enable their high sensitivity towards monolayer sensing which facilitate the analyses of VOCs adsorption isotherms and kinetics. Two affinity constants (K1, K2) are obtained for each VOC, which indicate the gas molecule adsorption happen inside and outside of the supramolecular cavities. Additional kinetic information (adsorption/desorption rate constants (ka, kd)) are obtained, thus enrich the sensing matrix (△f, K1, K2, ka, kd) which can be used as fingerprint patterns for highly specific detection and discrimination of VOCs.
Article
A highly sensitive and fast responsive semiconductor metal oxide detector based on In2O3 nanoparticle film was developed and evaluated. The mechanical structure of the detector was properly designed to obtain a small chamber volume in order to well fit to a commercial gas chromatographic capillary column. The detector was constructed by coating In2O3 nanoparticle film onto a small ceramic plate. Results indicate that the proposed semiconductor metal oxide detector in this study shows a fast response (several seconds) and a high sensitivity. Moreover, the detector can be operated at ambient condition, which largely simplifies the design of portable gas chromatograph. The fabricated detector was also used to detect gas mixtures with a detection limit down to several ppb. It is concluded that the semiconductor metal oxide detector based on nanostructured materials possesses great potential for constructing portable gas chromatograph.
Article
In recent years, the need of measurement and detection of samples in situ or with very small volume and low concentration (low sub parts per billion) is a cause for going to miniaturize systems via micro electromechanical system (MEMS) technology. Gas chromatography (GC) is a common technique that is widely used for separating and measuring the semi volatile and volatile compounds. Conventional GCs are bulky and cannot be used for in situ analysis, hence in the last decades many studies have been reported with the aim of designing and developing chip-based GC. The focus of this review is to follow and investigate the development and the achievements in the field of chip-based GC and its components from beginning up to now.
Article
This paper reports a unique GC-on-chip module comprising a monolithically integrated semi-packed micro separation column (μSC) and a highly sensitive micro helium discharge photoionization detector (μDPID). While semi-packed μSC with atomic layer deposited (ALD) alumina as a stationary phase provides high separation performance, the μDPID implemented for the first time in a silicon-glass architecture inherits the desirable features of being universal, non-destructive, low power consumption (1.4 mW), and responsive. The integrated chip is 1.5 cm × 3 cm in size and requires a two-mask fabrication process. Monolithic integration alleviates the need for transfer lines between the column and the detector which improves the performance of the individual components with overall reduced fabrication and implementation costs. The chip is capable of operating under the isothermal as well as temperature and flow programming conditions to achieve rapid chromatographic analysis. The chip performance was investigated with two samples: 1) a multi-analyte gas mixture consisting of eight compounds ranging from 98 °C to 174 °C in boiling point and 2) a mixture containing higher alkanes (C9-C12). Our experiments indicate that the chip is capable of providing rapid chromatographic separation and detection of these compounds (<1 min) through the optimization of flow and temperature programming conditions. The GC-on-chip demonstrated a minimum detection limit of ~10 pg which is on a par with the widely used destructive flame ionization detector (FID).
Article
The characterization of a miniaturized helium discharge ionization detector (μHDID) for micro gas chromatography through a number of parameterized experimental measurements is presented. The response of the detector is directly related to the He discharge voltage, bias electrode-to-discharge distance, and collector-to-bias distance by a simple mathematical expression. The effect of the bias voltage and the bias and collector electrode spacings relative to the He discharge were found to improve the detector response as much as 12-fold depending on the design and various operational parameters. The detection of octane from a headspace injection was performed over 24 h of continuous operation with no noticeable degradation. Finally, a sensitivity test for octane in air was conducted using the design and parameters with the best response to obtain an absolute limit of detection of 60 pg for octane in air at 3.3 mW.
Article
We report on a small (20 × 10 mm) micromachined device for the detection of gases in micro-gas chromatography (GC). It incorporates a micro-discharge across a 20-μm gap, and a remote electrode in the micro cavity that generates an electrical signal corresponding to the photo-ionization of gaseous analytes in a stream of carrier gas. Multi-component mixtures were detected and the results compared to those obtained with a flame ionization detector. The minimum detectable limit is 350 pg.μL−1 of n-octane in air when applying a 1.4 mW discharge. The combination of wet etching of glass (as used for microfluidic channels) with a lift-off process for detector electrodes by a robust batch process results in a universal, non-destructive, and sensitive microdetector for micro-GC. Figure ᅟ
Article
Nearly all existing nanoelectronic sensors are based on charge detection, where molecular binding changes the charge density of the sensor and leads to sensing signal. However, intrinsically slow dynamics of interface-trapped charges and defect-mediated charge-transfer processes significantly limit those sensors' response to tens to hundreds of seconds, which has long been known as a bottleneck for studying the dynamics of molecule-nanomaterial interaction and for many applications requiring rapid and sensitive response. Here we report a fundamentally different sensing mechanism based on molecular dipole detection enabled by a pioneering graphene nanoelectronic heterodyne sensor. The dipole detection mechanism is confirmed by a plethora of experiments with vapour molecules of various dipole moments, particularly, with cis- and trans-isomers that have different polarities. Rapid (down to ~\n0.1 s) and sensitive (down to ~\n1 ppb) detection of a wide range of vapour analytes is achieved, representing orders of magnitude improvement over state-of-the-art nanoelectronics sensors.
Article
This paper reports a microfabricated 2×4 cm gas chromatography chip to separate and detect gases in a two-port structure by embedding a microthermal conductivity detector (μTCD) within a separation column. A circular on-chip heater is placed on the backside of the monolithic device enabling temperature programming and consequently faster analysis of the heavier components. A unique process enhanced by reactive ion etching lag (RIE lag) is used to achieve multiple etch depths in silicon and restrict the process flow to just three masks. The silicon substrate contains the separation column, the heater, and the tunnels for the TCD electric feed throughs. A Pyrex wafer containing the TCD elements is anodically bonded to the silicon substrate to seal the structure. Performance of a standalone μTCD fabricated in the same process and integrated in a hybrid fashion is also described. The single-chip design demonstrates successful separation and identification of multi-component gas mixtures with a performance comparable to that obtained through a flame ionization detector connected in series. Further, on-chip temperature programming capability was utilized to elevate the column temperature to 75°C to exhibit analysis in less than a minute.
Article
We developed a monolithic subsystem that integrates a microgas chromatography (μGC) separation column and on-column, nondestructive Fabry-Pérot (FP) vapor sensors on a single silicon chip. The device was fabricated using deep reactive ion etching of silicon to create fluidic channels and polymers were deposited on the same silicon chip to act as a stationary phase or an FP sensor, thus avoiding dead volumes caused by the interconnects between the column and sensor traditionally used in μGC. Two integration designs were studied. In the first design, a 25-cm long μGC column was coated with a layer of polymer that served as both the stationary phase and the FP sensor, which has the greatest level of integration. This design was capable of sub-second response times and detection limits under 10 ng. In the second design, an FP sensor array spray coated with different vapor sensing polymers was integrated with a 30-cm long μGC column, which significantly improves the system flexibility and detection sensitivity. With this design, we show that the FP sensors have a detection limit on the order of tens of picograms or ~500 ppb with a sub-second response time. Furthermore, the FP sensor array are shown to respond to a mixture of analytes separated by the integrated separation channel, allowing for the construction of response patterns, which, along with retention time, can be used as a basis of analyte identification.
Article
In this work, a comprehensive, predictive and quantitative model of a whole gas analyzer is provided in order to facilitate the design of such high performance devices. All the pre-analytical (gas separation) and analytical (detection and readout) stages have been modeled and experimentally calibrated. Heterogeneous simulations have been used to quantify the impact of the whole architecture on the output characteristic of the gas analyzer. Finally, the model of the NEMS sensor and the chromatography micro-column assembly has been experimentally validated with TEOX (toluene, ethylbenzene, octane, and xylene) gases.
Article
A prototype microfabricated gas chromatograph (μGC) adapted specifically for the rapid determination of selected gas-phase marker compounds of the explosive 2,4,6-trinitrotoluene (TNT) at sub-parts-per-billion (ppb) concentrations in complex mixtures is described. Si-microfabricated focuser, separation column, and sensor array components are integrated with a high-volume sampler of conventional construction to reduce analysis time and limits of detection (LOD). The primary markers selected as target analytes were 2,4-dinitrotoluene (2,4-DNT; a persistent impurity of TNT) and 2,3-dimethyl-2,3-dinitrobutane (DMNB; a taggant), with 2,6-dinitrotoluene (2,6-DNT; a less prominent impurity) also included in numerous tests. Selective preconcentration, on-column focusing, temperature-programmed chromatographic separation, and sensor array detection/recognition facilitate determinations of the primary markers in the presence of 20 (or more) interferences in 2 min under laptop control. Estimated LODs are 2.2, 0.48, and 0.86 ng for DMNB, 2,6-DNT, and 2,4-DNT, respectively, which correspond to 0.30, 0.067, and 0.12 ppb in each 1-L air sample collected.
Article
A micro-flame ionization detector (micro-FID) design is presented that is targeted for use in a portable gas sensor. Our micro-FID is based on a diffusion flame and features a folded flame structure that is more sensitive than a counter-flow flame designs. Unlike conventional FIDs that use a premixed or open diffusion flame, an air–hydrogen diffusion flame is employed and tested in an encapsulated structure of Quartz–Macor–Quartz layers. Diffusion flames are generally known to be more controllable and stable than premixed flames, where the stability of the micro-FID plays an important role for portable gas sensors. Various channel designs for oxidant and fuel flows meeting with different angles at the burner cavity are tested to obtain a stable flame and high output sensitivity over methane test samples. To verify the empirically designed microchannel, we simulate the temperature distribution in the microchannel by using computational fluid dynamics (CFD) software. To gauge the sensitivity of the device, the collected electric charges per mole (C/mol) is calculated and taken as a reference value of ionization efficiency. The result of the folded flame design is 1.959 × 10−2 C/mol for methane that is about 34 times higher than the result using a counter-flow flame, which is 5.73 × 10−4 C/mol for methane, while one of the commercial macro FIDs’ is 10−1 C/mol. This result shows that the micro-FID using the folded flame structure has higher ionization efficiency with less leakage of the analytes than of the classical counter-flow flame design.
Article
In this paper the combination and application of a silicon based miniaturised gas chromatography (GC) column packed with Carboxen 1000 and a gas detector based on a commercial SnO2 metal oxide sensor are described. Target analytes are gas mixtures containing hydrocarbons with low molecular weight from C1 and C2 and ethylene as main target gas. The detector achieved a resolution of at least 1 ppmv related to ethylene gas. The combination of the detector with the miniaturised GC-column provides a linear behaviour between the injected ethylene gas volume and the related peak area in the chromatogram with a resolution of 2 nl. Examination of the GC-performance gave HEPT-values of about 0.7 mm and chromatographic resolutions of 100% in most cases using a gas mixture containing CO2, nitrogen, acetylene, ethylene, methane and ethane with synthetic air as carrier gas.
Article
We report the instrumentation of a μ-GC system that was capable of performing real-time analysis of sub-ppb-levels of organic mixture vapors. This system consists of a multi-stage preconcentrator/injector, a capillary column with at-column heater configuration and a photo ionization detector (PID). A tablet computer was embedded inside the instrument case to form a stand-alone system that can provide both instrument control and chromatogram data handling without an external computer. Through the compact design of fluidic system, this fully functional GC measured 30 (l) × 17(w) × 8(h) cm and the weight was less than 3 kg. This system could be powered by either a 12 V DC adapter or batteries. Mixtures of 10 organic compounds were tested to demonstrate the performance of this system. Separation of the 10 compounds took only 2 min due to the rapid temperature programming ability of at-column heater. The detection limit ranged from 0.02 to 0.36 ppb can be achieved with 1.0 L sample volume. The analytical cycle including sampling, separation and cooling required only 15 min. The stability of this μ-GC was evaluated by analyzing VOCs at ~ 3 ppb vapor concentration through continuous operation over 24 h. The retention time varied less than 1.2% (RSD, n = 120). The variation in peak areas ranged from 2.2% (benzene) to 5.2% (m-xylene).
Article
We fabricated and characterized on-chip Fabry–Pérot (FP) vapor sensors for the development of on-column micro-gas chromatography (μGC) detectors. The FP sensors were made by coating a thin layer of polymer on a silicon wafer. The air–polymer and polymer–silicon interfaces form an FP cavity, whose resonance wavelengths change in response to the vapor absorption/desorption, thus allowing for rapid detection and quantification of vapors. For proof-of-concept, two polymers (PDMS and SU-8) were used independently and placed in an array in a microfluidic channel, and showed different sensitivities for different vapors. A sub-nano-gram detection limit and sub-second response time were achieved, representing orders of magnitude improvement over those previously reported. This on-chip design will enable the unprecedented integration of optical vapor sensors with μGC systems.
Article
This paper describes the design, modeling, fabrication, and characterization of a novel micro thermal conductivity detector (μTCD). To maximize the detection response, highly isolated thermistors and a four-filament Wheatstone bridge circuit were designed. To enhance the reliability, these thermistors were supported by a multi-layer structure beam formed by a diffusion of silicon layer, a silicon oxidation layer, and a silicon nitride layer. A polydimethylsiloxane (PDMS) membrane instead of a glass was used to seal the μTCD at the room temperature with a good sealing effect. Moreover this method could avoid the resistance variation of thermistors affected by the high temperature during the bonding step. From the pressure and the hermetic tests, the μTCD could withstand 0.5MPa pressure without leakage and destroying the chip. Then the μTCD was used to detect the CH4 with a detection response of 500ppm and a short response time of 30s.