No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Dual detection chromatographic method for fast characterization of nano-gravimetric detector
Abstract
Nano-gravimetric detector (NGD) has been recently introduced as miniaturized gas chromatography detector. The NGD response is based on an adsorption-desorption mechanism of compounds between the gaseous phase and the NGD porous oxide layer. The NGD response was characterized by hyphenating NGD in-line with FID detector and a chromatographic column. Such method led to the full adsorption-desorption isotherms of several compounds in a single run. Langmuir model was used to describe the experimental isotherms, and the initial slope of the isotherm (Mm.KT) obtained at low gas concentration was used to compare the NGD response for different compounds (good repeatability was demonstrated with a relative standard deviation lower than 3%). The column-NGD-FID hyphenated method was validated using alkane compounds according to the number of carbon atoms in the alkyl chain and to the NGD temperature (all results agreed with thermodynamic relations associated to partition coefficient). Furthermore, relative response factor to alkanes, for ketones, alkylbenzenes, and fatty acid methyl esters have been obtained. These relative response index values led to easier calibration of NGD. The established methodology can be used for any sensor characterization based on adsorption mechanism.
... Throughout the former years, numerous sorts of gas sensors have been introduced to the scientific society including versatile materials and operating platforms [5]. Multiple routes were introduced for the gas sensing platforms such as piezoelectric gas sensors [8,9], optical gas sensors [10,11], conductive gas sensors [12], chromatographic sensors [13], metal oxide gas sensors [14,15], and more [16][17][18]. The functionalization of carbon nanotubes (CNTs) is desirable in gas sensing applications. ...
Today, to get a full molecular characterization of gases, a gas chromatographic system hyphenated to both flame ionization (FID) and thermal conductivity (TCD) detectors is essential. However, for on-site, on-line analysis (mostly ATEX zone) FID are forbidden and TCD only allows quantification up to C7. A new universal detector (NGD for Nano Gravimetric Detector) covering a hydrocarbon range from C1 to C40+ has been developed and implemented in miniaturized and transportable system [1].
The first step is to evaluate the NGD response for different families of compounds present in the gas (synthetic and process gases) and develop different approaches to simplify the quantification. This one will be carried out in laboratories. To increase productivity and accuracy for PVT experiment, this detector will be implemented on a PVT unit. Once validated on fully controlled flashed gases, the system could be updated to work with batteries to be fully transportable for on-site or adapted for on-line analysis to be able to monitor any gas anywhere up to C11/ C12.
All compounds were well separated and detected with the chromatographic resolution required for this study. Different standards gases have been used: from nC1 to nC10 with BTEX (benzene, toluene, ethylbenzene, m-xylene, p-xylene, and o-xylene). To achieve an external quantification, two approaches were carried out. A first approach based on the linear relationship that exists between the NGD response (as the logarithm of the peak area normalized by concentration) and the number of carbon atoms. These linear relationships are different depending on the family of compounds analyzed. The second approach uses a hyperbolic method based on the Langmuir model by applying an inverted Langmuir equation. This approach allows calibration of the NGD detector using a single linear relationship for all compounds regardless the family to which they belong. A quantification on diluted standards gases was carried out (4 levels) and the relative error was less than 10%. Moreover, a hyperbolic Langmuir approach improved the accuracy of the results with errors inferior at 5%. During 2023, the aim is to calibrate the NGD detector for the quantification of all compounds present in a process gas from a PVT unit and to proceed to the on-line monitoring.
Today, only FID detector allows C5+ quantification on gaseous effluent, only present off-line. Tomorrow, thanks to the NGD detector, the gas analysis could be carried out on-line with a full molecular characterization of gases with a complete automatization of monitoring.
A microfluidic film bulk acoustic wave resonator gas sensor (mFBAR) adapted specifically as an in-line detector in gas chromatography was described. This miniaturized vapor sensor was a non-destructive detector with very low dead volume (0.02 μL). It was prepared by enclosing the resonator in a microfluidic channel on a chip with dimensions of only 15 mm × 15 mm × 1 mm. The device with polymer coating showed satisfactory performance in the detection of organophosphorus compound, demonstrating a very low detection limit (a dozen parts per billion) with relatively short response time (about fifteen seconds) toward the simulant of chemical warfare agent, dimethyl methylphosphonate. The in-line detection of the mFBAR sensor with FID was constructed and employed to directly measure the concentration profile on the solid surface by the mFBAR with the controlled concentration profile in the mobile phase at the same time. The difference of peak-maximum position between mobile phase and solid phase could be a convenient indicator to measure mass transfer rate. With the response of the mFBAR and FID obtained in one injection, an injection mass-independent parameter can be calculated and used to identify the analyte of interest.
Background:
There is an urgent need to rapidly distinguish COVID-19 from other respiratory conditions, including influenza, at first-presentation. Point-of-care tests not requiring laboratory- support will speed diagnosis and protect health-care staff. We studied the feasibility of using breath-analysis to distinguish these conditions with near-patient gas chromatography-ion mobility spectrometry (GC-IMS).
Methods:
Independent observational prevalence studies at Edinburgh, UK, and Dortmund, Germany, recruited adult patients with possible COVID-19 at hospital presentation. Participants gave a single breath-sample for VOC analysis by GC-IMS. COVID-19 infection was identified by transcription polymerase chain reaction (RT- qPCR) of oral/nasal swabs together with clinical-review. Following correction for environmental contaminants, potential COVID-19 breath-biomarkers were identified by multi-variate analysis and comparison to GC-IMS databases. A COVID-19 breath-score based on the relative abundance of a panel of volatile organic compounds was proposed and tested against the cohort data.
Findings:
Ninety-eight patients were recruited, of whom 21/33 (63.6%) and 10/65 (15.4%) had COVID-19 in Edinburgh and Dortmund, respectively. Other diagnoses included asthma, COPD, bacterial pneumonia, and cardiac conditions. Multivariate analysis identified aldehydes (ethanal, octanal), ketones (acetone, butanone), and methanol that discriminated COVID-19 from other conditions. An unidentified-feature with significant predictive power for severity/death was isolated in Edinburgh, while heptanal was identified in Dortmund. Differentiation of patients with definite diagnosis (25 and 65) of COVID-19 from non-COVID-19 was possible with 80% and 81.5% accuracy in Edinburgh and Dortmund respectively (sensitivity/specificity 82.4%/75%; area-under-the-receiver- operator-characteristic [AUROC] 0.87 95% CI 0.67 to 1) and Dortmund (sensitivity / specificity 90%/80%; AUROC 0.91 95% CI 0.87 to 1).
Interpretation:
These two studies independently indicate that patients with COVID-19 can be rapidly distinguished from patients with other conditions at first healthcare contact. The identity of the marker compounds is consistent with COVID-19 derangement of breath-biochemistry by ketosis, gastrointestinal effects, and inflammatory processes. Development and validation of this approach may allow rapid diagnosis of COVID-19 in the coming endemic flu seasons.
Funding:
MR was supported by an NHS Research Scotland Career Researcher Clinician award. DMR was supported by the University of Edinburgh ref COV_29.
Quartz Crystal Resonator (QCR) has been proved to be effective for CO2 gas sensing, however, the mechanism of this sensing process remains unclarified. This paper proposes an innovative modeling for a QCR-based CO2 gas sensor. The proposed modeling is based on the Langmuir Adsorption Theorem and the Sauerbrey equation of QCR. The Langmuir Adsorption Theorem assumes that a monolayer of gas molecules is formed at the maximum gas concentration while each adsorbate molecule is assumed to take only one active site on the adsorbent surface. Therefore, the Sauerbrey equation is modified by combining Langmuir Adsorption Theorem and then the effect of both surface concentration and Langmuir constant are investigated and analyzed. Different from traditional analysis, the proposed modeling works well on not only the linear relationship between CO2 concentration and frequency shift at low CO2 concentration but also the non-linear saturation stage at higher CO2 concentration. Moreover, a lab experiment is carried out to verify the Langmuir-Sauerbrey adsorption modeling. Surface concentration constant and Langmuir constant are retrieved from the experiment and considered as material properties for gas sensing capability. The proposed modeling could also be applied to most QCR-based gas sensor, in which physical adsorption is mainly involved. Additionally, determination of surface concentration constant and Langmuir constant will benefit the understanding of many other gas sensing process.
This paper reports a complete micro gas chromatography (μGC) system in which all the components are lithographically microfabricated and electronically interfaced. The components include a bi-directional Knudsen pump, a preconcentrator, separation columns and a pair of capacitive gas detectors; together, these form the iGC3.c2 system. All the fluidic components of the system are fabricated by a common three-mask lithographic process. The Knudsen pump is a thermomolecular pump that provides air flow to the μGC without any moving parts. The film heaters embedded in the separation columns permit temperature programming. The capacitive detectors provide complementary response patterns, enhancing vapor recognition and resolving co-eluting peaks. With the components assembled on printed circuit boards, the system has a footprint of 8×10 cm2. Using room air as the carrier gas, the system is used to experimentally demonstrate the analysis of 19 chemicals with concentration levels on the order of parts per million (p.p.m.) and parts per billion (p.p.b.). The tested chemicals include alkanes, aromatic hydrocarbons, aldehydes, halogenated hydrocarbons and terpenes. This set of chemicals represents a variety of common indoor air pollutants, among which benzene, toluene and xylenes (BTX) are of particular interest.
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).
Poor air quality, particularly in urban areas, causes various diseases and degrades living standards. Air quality could be affected by emissions of odor, Volatile Organic Compounds (VOCs), and other gases. Therefore, assessment and monitoring of odorous air quality using sensitive, simple, rapid, accurate and portable tools is very important for public health. This study aimed to characterize odor emissions to detect malfunctions in facilities and to prevent air pollution and olfactory nuisance in the environment. A gas chromatographic method, in conjunction with sensorial analysis were performed for odorous air samples analysis collected from neighborhood of Meknes city (Morocco). Advanced multivariate statistical approaches, such as Principal Components Analysis (PCA), Discriminant Function Analysis (DFA), Support Vector Machines (SVMs), and Hierarchical Cluster Analysis (HCA), were used to describe samples similarities. The electronic nose (e-nose) data processing exhibits a satisfactory discrimination between the odorous air samples. Twenty-four VOCs with known molecular formulas were identified with Thermal Desorption-Gas Chromatography-Mass Spectrometry (TD-GC-MS). A validated Partial Least Square (PLS) model foresees good calibration between e-nose measurement and TD-GC-MS analysis. The finding indicates that TD-GC–MS approach in conjunction with e-nose unit could be suitable tool for environmental measurement-based odor emissions.
A nano-gravimetric detector (NGD) for gas chromatography is based on a nanoelectromechanical array of adsorbent-coated resonating double clamped beams. NGD is a concentration-sensitive detector and its sensitivity is analyte-dependent based on the affinity of the analyte with the porous layer coated on the NEMS surface. This affinity is also strongly related to the NGD temperature (NGD working temperature can be dynamically set up from 40 to 220 °C), so the sensitivity can be tuned through temperature detector control. An adsorption-desorption model was set up to characterize the NGD response on a large set of n-alkanes from C10 to C22 at different NGD temperatures. For fast identification of petroleum mixture based on chromatogram fingerprint, a general strategy for NGD temperature program design was developed leading to a constant relative response factor between 0.96 and 1.03 for all the alkanes, and then chromatograms are very similar to those obtained with a flame ionization detector (FID). The analysis of a real petroleum fluid was also performed and compared to FID results: quantitative results obtained for all the analytes were satisfactory according to precision (<5%) and accuracy (average relative error = 4.3%). Based on such temperature control strategy, NGD sensitivity and the dynamic linear range can be adjusted and detection limits at a picogram level can be easily achieved for all n-alkanes.
Among a number of gas analyzers, portable gas chromatography (GC) systems created by the integration of microfabricated components are promising candidates for rapid and on-site analysis of a number of complex chemical mixtures. This Feature provides a snapshot of the progress made in developing micro gas chromatography (µGC) systems in the last four decades. In particular, we discuss the development of microfabricated preconcentrators, separation columns, and detectors. Furthermore, we review different stationary phase materials used to coat the separation columns and the major efforts toward the development of an integrated µGC.
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.
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
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.
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.
The physical properties of pure and doped (Ni, Os, Pd and Pt) SnO2 thin films, prepared by using a chloride-based inorganic sol–gel route, have been reported. These properties were investigated by using differential scanning calorimetry and thermogravimetric (DSC/TG), FTIR, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques. Microsensors for gas detection were also fabricated and tested in various atmospheres (nitrogen oxide, carbon monoxide, ethanol, methanol and methane). The dopants affected the coating properties depending on the formation of nanoparticles. Furthermore the gas-sensing properties towards reducing and oxidising gases were found to be dependent on the nature of doping element. An array consisting of the manufactured sensors has been arranged and examples of applications for “electronic nose” are given.
The volatile components of commercially milled head and broken rice samples and the effect of water washing on volatile components were investigated over 30 d of storage at 37°C and 70% relative humidity. Pentanal, pentanol, hexanal, pentylfuran, octanal, and nonanal were the major volatile components identified by gas chromatography-mass spectrometry (GC-MS) analysis in both head and broken rice samples. The concentration of volatile components was greater in broken rice than in head rice and increased significantly during the storage period for both rice types. Water washing was found to be effective in reducing the volatile components for both head and broken rice.
Quartz microbalance sensors coated with silicon-based polymers are found to be extremely reliable for the detection of CO2 around 70 °C. The optimization of our differently functionalized silicon-based polymers leads to excellent sensor performances with a low detection limit (<50 vpm). The response times in the order of 1 min can be optimized by adjusting optimum operation temperatures. The sensors have been used for more than nine months without significant loss in sensitivity, reversibility and reproducibility in the continuous monitoring of CO2 in air. Small effects due to interferences from other gases are discussed finally.
A complete field-deployable microfabricated gas chromatograph (μGC) is described, and its adaptation to the analysis of low- and subparts-per-billion (ppb) concentrations of trichloroethylene (TCE) vapors in complex mixtures is demonstrated through laboratory testing. The specific application being addressed concerns the problem of indoor air contamination by TCE vapor intrusion. The μGC prototype employs a microfabricated focuser, dual microfabricated separation columns, and a microsensor array. These are interfaced to a nonmicrofabricated front-end pretrap and high-volume sampler module to reduce analysis time and limits of detection (LOD). Selective preconcentration and focusing are coupled with rapid chromatographic separation and multisensor detection for the determination of TCE in the presence of up to 45 interferences. Autonomous operation is possible via a laptop computer. Preconcentration factors as high as 500 000 are achieved. Sensitivities are constant over the range of captured TCE masses tested (i.e., 9-390 ng), and TCE is measured in a test atmosphere at 120 parts-per-trillion (ppt), with a projected LOD of 40 ppt (4.2 ng captured, 20 L sample) and a maximum sampling + analytical cycle time of 36 min. Short- and medium-term (1 month) variations in retention time, absolute responses, and response patterns are within acceptable limits.