Smart multi-channel two-dimensional micro-gas chromatography for rapid workplace hazardous volatile organic compounds measurement

Ionic Liquid-Based Hybrid Gel Microcolumns for Enhanced Narcotic Detection in Portable Micro-Gas Chromatography

As the societal issue of increasing global illicit drug usage emerges, there is a growing demand for more portable and versatile drug detectors. Traditional drug analysis techniques such as gas chromatography (GC), liquid chromatography (LC), and Fourier transform infrared spectroscopy (FTIR) face significant challenges in adapting to diverse real-world applications due to their size, cost, and power requirements. While advancements have been made in the development of on-site drug detection methods such as fluorescence, stereoresonance energy transfer (FRET), colorimetric, electrochemical sensing, and lateral flow assays (LFAs), their reliance on specific reactive materials poses limitations in effectively detecting a wide range of narcotics. Therefore, this study proposes the development of specialized microcolumns with optimized stationary phases for next-generation portable microfabricated GC-based narcotic detectors. The stationary phase consists of a hybrid gel incorporating the ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and OV-1. The stationary phase not only enhances interactions between drug analytes but also demonstrates improved separation characteristics among various narcotic substances. Additionally, the principles of the separation results were validated through density functional theory (DFT) analysis, and the effective separation of over seven types of narcotics was demonstrated through temperature optimization. This research lays the groundwork for the advancement of next-generation portable drug analyzers, offering significant potential in the field.

Identification of Different Classes of VOCs Based on Optical Emission Spectra Using a Dielectric Barrier Helium Plasma Coupled with a Mini Spectrometer

In this study, a micro helium dielectric barrier discharge (μHDBD) plasma device fabricated using 3D printing and molding techniques was coupled with a mini spectrometer to detect and identify different classes of volatile organic compounds (VOCs) using optical emission spectrometry (OES). We tested 11 VOCs belonging to three different classes (straight-chain alkanes, aromatics, and polar organic compounds). Our results clearly demonstrate that the optical emission spectra of different classes of VOCs show clear differences, and therefore, can be used for identification. Additionally, the emission spectra of VOCs with a similar structure (such as n-pentane, n-hexane, n-heptane, n-octane, and n-nonane) have similar optical emission spectrum shape. Acetone and ethanol also have similar emission wavelengths, but they show different line intensities for the same concentrations. We also found that the side-chain group of aromatics will also affect the emission spectra even though they have a similar structure (all have a benzene ring). Moreover, our μHDBD-OES system can also identify multiple compounds in VOC mixtures. Our work also demonstrates the possibility of identifying different classes of VOCs by the OES shape.

Metal-organic framework-based high-performance column chip for micro gas chromatography: hybrid function for simultaneous preconcentration and separation of volatile organic compounds

Numerous attempts have been made to replace commercial bulky gas chromatography (GC) systems with compact GC systems for monitoring volatile organic compounds in indoor air. However, recently developed compact GC systems are still too bulky in terms of user convenience, portability, and on-site analysis. Hence, an advanced miniaturization of compact GC systems is needed. Importantly, the small and high-performance gas pretreatment chip devices should be developed for compact GC systems. This paper reports the development of a metal-organic framework (MOF)-coated hybrid micro gas chromatography column chip (hybrid GC chip) capable of preconcentration and separation on harmful volatile organic compounds at low-concentration in one single chip device. The hybrid GC chip, 2 cm × 2 cm in size, was fabricated using a microelectromechanical systems process. The synthesized MOF-5 particles were coated on the inner wall of the fabricated hybrid GC chip and acted as an adsorbent and a stationary phase. The developed hybrid GC chip afforded high preconcentration factors with 1033-1237 and high separation resolutions with 3.8-13.3. The developed column showed good performance as a gas preconcentrator and separation column, and is the first device to perform both roles in one single chip device.

In-Line Detection with Microfluidic Bulk Acoustic Wave Resonator Gas Sensor for Gas Chromatography

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.

Progressive Cellular Architecture in Microscale Gas Chromatography for Broad Chemical Analyses

Gas chromatography is widely used to identify and quantify volatile organic compounds for applications ranging from environmental monitoring to homeland security. We investigate a new architecture for microfabricated gas chromatography systems that can significantly improve the range, speed, and efficiency of such systems. By using a cellular approach, it performs a partial separation of analytes even as the sampling is being performed. The subsequent separation step is then rapidly performed within each cell. The cells, each of which contains a preconcentrator and separation column, are arranged in progression of retentiveness. While accommodating a wide range of analytes, this progressive cellular architecture (PCA) also provides a pathway to improving energy efficiency and lifetime by reducing the need for heating the separation columns. As a proof of concept, a three-cell subsystem (PCA3mv) has been built; it incorporates a number of microfabricated components, including preconcentrators, separation columns, valves, connectors, and a carrier gas filter. The preconcentrator and separation column of each cell are monolithically implemented as a single chip that has a footprint of 1.8 × 5.2 cm2. This subsystem also incorporates two manifold arrays of microfabricated valves, each of which has a footprint of 1.3 × 1.4 cm2. Operated together with a commercial flame ionization detector, the subsystem has been tested against polar and nonpolar analytes (including alkanes, alcohols, aromatics, and phosphonate esters) over a molecular weight range of 32-212 g/mol and a vapor pressure range of 0.005-231 mmHg. The separations require an average column temperature of 63-68 °C within a duration of 12 min, and provide separation resolutions >2 for any two homologues that differ by one methyl group.

Advancements in Microfabricated Gas Sensors and Microanalytical Tools for the Sensitive and Selective Detection of Odors

In recent years, advancements in micromachining techniques and nanomaterials have enabled the fabrication of highly sensitive devices for the detection of odorous species. Recent efforts done in the miniaturization of gas sensors have contributed to obtain increasingly compact and portable devices. Besides, the implementation of new nanomaterials in the active layer of these devices is helping to optimize their performance and increase their sensitivity close to humans' olfactory system. Nonetheless, a common concern of general-purpose gas sensors is their lack of selectivity towards multiple analytes. In recent years, advancements in microfabrication techniques and microfluidics have contributed to create new microanalytical tools, which represent a very good alternative to conventional analytical devices and sensor-array systems for the selective detection of odors. Hence, this paper presents a general overview of the recent advancements in microfabricated gas sensors and microanalytical devices for the sensitive and selective detection of volatile organic compounds (VOCs). The working principle of these devices, design requirements, implementation techniques, and the key parameters to optimize their performance are evaluated in this paper. The authors of this work intend to show the potential of combining both solutions in the creation of highly compact, low-cost, and easy-to-deploy platforms for odor monitoring.

Real-time and non-destructive hydrocarbon gas sensing using mid-infrared integrated photonic circuits

A chip-scale mid-infrared (mid-IR) sensor was developed for hydrocarbon gas detection. The sensor consisted of amorphous Si (a-Si) optical ridge waveguides that were fabricated by complementary metal–oxide–semiconductor (CMOS) processes. The waveguide exhibited a sharp fundamental mode through λ = 2.70 to 3.50 μm. Its sensing performance was characterized by measuring methane and acetylene. From the spectral mode attenuation, the characteristic C–H absorption bands associated with methane and acetylene were found at λ = 3.29–3.33 μm and λ = 3.00–3.06 μm, respectively. In addition, real-time methane and acetylene concentration monitoring was demonstrated at λ = 3.02 and 3.32 μm. Hence, the mid-IR waveguide sensor enabled an accurate and instantaneous analysis of hydrocarbon gas mixtures.

A chip-scale mid-infrared (mid-IR) sensor was developed for hydrocarbon gas detection.

Development of a Novel Micro Photoionization Detector for Rapid Volatile Organic Compounds Measurement

The simulation of the gas flow field and electrostatic field in the photoionization detector by COMSOL was conducted based on principle investigation in the present study. Under the guidance of simulation results, structural optimization was carried out to significantly reduce the dead volume of the ionization chamber, and finally, the relationship between offset voltage and collection efficiency was obtained which led to a remarkable increase in the collection efficiency of charged ions in the photoionization detector. Then an ionization chamber with low interference and fast response was developed. Then experiment was performed with toluene as a VOCs gas under the condition of optimal gas flow rate of 50 ml, UV lamp ionization energy of 10.86 eV. The results showed that the ion collection efficiency reached 91% at a bias voltage of 150 V. Moreover, a preferred linearity of 99.99% was obtained, and a ppb level of LOD can be achieved. The determination results well-fitted the relationship between offset voltage and the response value obtained in the simulation.

Microchip gas chromatography columns, interfacing and performance

Almost four decades of investigations have opened up many avenues to explore the production and utilization of planar (i.e., microchip) gas chromatographic columns. However, there remain many practical constraints that limit their widespread commercialization and use. The main challenges arise from non-ideal column geometries, dead volume issues and inadequate interfacing technologies, which all affect both column performance and range of applications. This review reflects back over the years on the extensive developments in the field, with the goal to stimulate future creative approaches and increased efforts to accelerate microchip gas chromatography development toward reaching its full potential.

Real-Time Gas Mixture Analysis Using Mid-Infrared Membrane Microcavities

Real-time gas analysis on-a-chip was demonstrated using a mid-infrared (mid-IR) microcavity. Optical apertures for the microcavity were made of ultrathin silicate membranes embedded in a silicon chip using the complementary metal-oxide-semiconductor (CMOS) process. Fourier transform infrared spectroscopy (FTIR) shows that the silicate membrane is transparent in the range of 2.5-6.0 μm, a region that overlaps with multiple characteristic gas absorption lines and therefore enables gas detection applications. A test station integrating a mid-IR tunable laser, a microgas delivery system, and a mid-IR camera was assembled to evaluate the gas detection performance. CH₄, CO₂, and N₂O were selected as analytes due to their strong absorption bands at λ = 3.25-3.50, 4.20-4.35, and 4.40-4.65 μm, which correspond to C-H, C-O, and O-N stretching, respectively. A short subsecond response time and high gas identification accuracy were achieved. Therefore, our chip-scale mid-IR sensor provides a new platform for an in situ, remote, and embedded gas monitoring system.

A miniature closed-loop gas chromatography system

This paper reports the characterization of a miniaturized circulatory column system that is capable of magnifying the effective column length by forming a circulatory loop with chip-scale columns, thus ultimately achieving high-efficiency target separation. The circulatory column system is composed of a tandem of 25 cm microcolumns and six valves for fluidic flow control in order to enable chromatographic separation in circulatory motions while requiring only 5.5 kPa of pressure, which current micropumps are currently capable of supplying. The developed column system (1) successfully demonstrated 16 times elongation of a virtual column length up to 800 cm by only utilizing two 25 cm microcolumns, which is the longest column length reported by any MEMS-scale functioning GC column, (2) achieved a high theoretical plate number of 68,696 with pentane circulating after 15.5 circulatory cycles, which corresponds to the plate number per length-pressure of 1611 plate m(-1) kPa(-1), the highest record reported yet, and (3) demonstrated successful separation of target molecules during circulation by utilizing a pentane/hexane mixture, resulting in magnification of the two corresponding peaks via circulation.

A Microfluidic-Based Fabry-Pérot Gas Sensor

We developed a micro-gas detector based on a Fabry-Pérot (FP) cavity embedded in a microfluidic channel. The detector was fabricated in two steps: a silicon substrate was bonded to a glass slide curved with a micro-groove, forming a microfluidic FP cavity; then an optical fiber was inserted through a hole drilled at the center of the groove into the microfluidic FP cavity, forming an FP cavity. The light is partially reflected at the optical fiber endface and the silicon surface, respectively, generating an interference spectrum. The detection is implemented by monitoring the interference spectrum shift caused by the refractive index change of the FP cavity when a gas analyte passes through. This detection mechanism (1) enables detecting a wide range of analytes, including both organic and inorganic (inertia) gases, significantly enhancing its versatility; (2) does not disturb any gas flow so that it can collaborate with other detectors to improve sensing performances; and (3) ensures a fast sensing response for potential applications in gas chromatography systems. In the experiments, we used various gases to demonstrate the sensing capability of the detector and observed drastically different sensor responses. The estimated sensitivity of the detector is 812.5 nm/refractive index unit (RIU) with a detection limit of 1.2 × 10⁻⁶ RIU assuming a 1 pm minimum resolvable wavelength shift.

Polymer-coated micro-optofluidic ring resonator detector for a comprehensive two-dimensional gas chromatographic microsystem: μGC ×μGC-μOFRR

We describe first results from a micro-analytical subsystem that integrates a detector comprising a polymer-coated micro-optofluidic ring resonator (μOFRR) chip with a microfabricated separation module capable of performing thermally modulated comprehensive two-dimensional gas chromatographic separations (μGC ×μGC) of volatile organic compound (VOC) mixtures. The 2 × 2 cm μOFRR chip consists of a hollow, contoured SiO(x) cylinder (250 μm i.d.; 1.2 μm wall thickness) grown from a Si substrate, and integrated optical and fluidic interconnection features. By coupling to a 1550 nm tunable laser and photodetector via an optical fiber taper, whispering gallery mode (WGM) resonances were generated within the μOFRR wall, and shifts in the WGM wavelength caused by transient sorption of eluting vapors into the PDMS film lining the μOFRR cylinder were monitored. Isothermal separations of a simple alkane mixture using a PDMS coated 1st-dimension ((1)D) μcolumn and an OV-215-coated 2nd-dimension ((2)D) μcolumn confirmed that efficient μGC ×μGC-μOFRR analyses could be performed and that responses were dominated by film-swelling. Subsequent tests with more diverse VOC mixtures demonstrated that the modulated peak width and the VOC sensitivity were inversely proportional to the vapor pressure of the analyte. Modulated peaks as narrow as 120 ms and limits of detection in the low-ng range were achieved. Structured contour plots generated with the μOFRR and a reference FID were comparable.

Flow-through microfluidic photoionization detectors for rapid and highly sensitive vapor detection

A photoionization detector (PID) is well known for its high sensitivity, large dynamic range, and non-destructive vapor detection capability. However, due to its tardy response, which results from the relatively large ionization chamber and dead volume, the application of the PID in gas chromatography (GC) has been limited. Here, we developed a rapid, flow-through, and highly sensitive microfluidic PID that was microfabricated directly on a conductive silicon wafer. The microfluidic PID has a significantly reduced ionization chamber volume of only 1.3 μL, nearly 10 times smaller than that of state-of-the-art PIDs and over 100 times smaller than that of commercial PIDs. Moreover, it has virtually zero dead volume due to its flow-through design. Consequently, the response time of the microfluidic PID can be considerably shortened, ultimately limited by its residence time (7.8 ms for 10 mL min(-1) and 78 ms for 1 mL min(-1)). Experimentally, the response of the microfluidic PID was measured to be the same as that of the standard flame ionization detector with peak full-widths-at-half-maximum of 0.25 s and 0.085 s for flow rates of 2.3 mL min(-1) and 10 mL min(-1), respectively. Our studies further show that the microfluidic PID was able to detect analytes down to the picogram level (at 3σ of noise) and had a linear dynamic range of six orders of magnitude. Finally, because of the very short distance between the electrodes, low voltage (<10 VDC, over 10 times lower than that in a regular PID) can be used for microfluidic PID operation. This work will open a door to broad applications of PIDs in gas analyzers, in particular, micro-GC and multi-dimensional GC.

Through the years with on-a-chip gas chromatography: a review

In recent years, the need for measurement and detection of samples in situ or with very small volume and low concentration (low and sub-parts per billion) is a cause for miniaturizing systems via microelectromechanical system (MEMS) technology. Gas chromatography (GC) is a common technique that is widely used for separating and measuring semi-volatile and volatile compounds. Conventional GCs are bulky and cannot be used for in situ analysis, hence in the past 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 the beginning up to the present.

Breath analysis as a potential and non-invasive frontier in disease diagnosis: an overview

Currently, a small number of diseases, particularly cardiovascular (CVDs), oncologic (ODs), neurodegenerative (NDDs), chronic respiratory diseases, as well as diabetes, form a severe burden to most of the countries worldwide. Hence, there is an urgent need for development of efficient diagnostic tools, particularly those enabling reliable detection of diseases, at their early stages, preferably using non-invasive approaches. Breath analysis is a non-invasive approach relying only on the characterisation of volatile composition of the exhaled breath (EB) that in turn reflects the volatile composition of the bloodstream and airways and therefore the status and condition of the whole organism metabolism. Advanced sampling procedures (solid-phase and needle traps microextraction) coupled with modern analytical technologies (proton transfer reaction mass spectrometry, selected ion flow tube mass spectrometry, ion mobility spectrometry, e-noses, etc.) allow the characterisation of EB composition to an unprecedented level. However, a key challenge in EB analysis is the proper statistical analysis and interpretation of the large and heterogeneous datasets obtained from EB research. There is no standard statistical framework/protocol yet available in literature that can be used for EB data analysis towards discovery of biomarkers for use in a typical clinical setup. Nevertheless, EB analysis has immense potential towards development of biomarkers for the early disease diagnosis of diseases.

Non-labeling multiplex surface enhanced Raman scattering (SERS) detection of volatile organic compounds (VOCs)

In this paper, we report multiplex SERS based VOCs detection with a leaning nano-pillar substrate. The VOCs analyte molecules adsorbed at the tips of the nano-pillars produced SERS signal due to the field enhancement occurring at the localized surface plasmon hot spots between adjacent leaning nano-pillars. In this experiment, detections of acetone and ethanol vapor at different concentrations were demonstrated. The detection limits were found to be 0.0017 ng and 0.0037 ng for ethanol and acetone vapor molecules respectively. Our approach is a non-labeling method such that it does not require the incorporation of any chemical sensing layer for the enrichment of gas molecules on sensor surface. The leaning nano-pillar substrate also showed highly reproducible SERS signal in cyclic VOCs detection, which can reduce the detection cost in practical applications. Further, multiplex SERS detection on different combination of acetone and ethanol vapor was also successfully demonstrated. The vibrational fingerprints of molecular structures provide specific Raman peaks for different VOCs contents. To the best of our knowledge, this is the first multiplex VOCs detection using SERS. We believe that this work may lead to a portable device for multiplex, specific and highly sensitive detection of complex VOCs samples that can find potential applications in exhaled breath analysis, hazardous gas analysis, homeland security and environmental monitoring.

Microfabricated gas chromatograph for rapid, trace-level determinations of gas-phase explosive marker compounds

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 the limit 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), along with 2,6-dinitrotoluene (2,6-DNT, a less-prominent TNT impurity), which was also included in numerous tests. Selective preconcentration, on-column focusing, temperature-programmed chromatographic separation, and sensor array detection/recognition facilitated determinations of the primary markers in the presence of 20 (or more) interferences within ∼2 min under laptop control. Estimated LODs of 2.2, 0.48, and 0.86 ng were achieved 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.

Smart three-dimensional gas chromatography

We developed a complete computer-controlled smart 3-dimensional gas chromatography (3-D GC) system with an automation algorithm. This smart 3-D GC architecture enabled independent optimization of and control over each dimension of separation and allowed for much longer separation time for the second- and third-dimensional columns than the conventional comprehensive 3-D GC could normally achieve. Therefore, it can potentially be employed to construct a novel GC system that exploits the multidimensional separation capability to a greater extent. In this Article, we introduced the smart 3-D GC concept, described its operation, and demonstrated its feasibility by separating 22 vapor analytes.