Microchip gas chromatography columns, interfacing and performance

A microchip gas chromatography column assembly with a 3D metal printing micro column oven and a flexible stainless-steel column

In this work, a microchip gas chromatography (GC) column assembly utilizing a three-dimensional (3D) printed micro oven and a flexible stainless steel capillary column was developed. The assembly's performance and separation capabilities were characterized. The key components include a 3D printed aluminum plate (7.50 × 7.50 × 0.16 cm) with a 3-meter-long circular spiral channel, serving as the oven, and the column coiled on the channel with an inner diameter of 320 μm and a stationary phase of OV-1. A heating ceramic plate was affixed on the opposite side of the plate. The assembly weighed 40.3 g. The design allows for easy disassembly, or stacking of heating devices and columns, enabling flexibility in adjusting column length. When using n-C13 as the test analyte at 140 °C, a retention factor (k) was 8.5, and 7797 plates (2599 plates/m) were obtained. The assembly, employing resistance heating, demonstrated effective separation performance for samples containing alkanes, aromatics, alcohols and ketones, with good reproducibility. The reduction in theoretical plates compared to oven heating was only 2.95 %. In the boiling point range of C6 to C18, rapid temperature programming (120 °C/min) was achieved with a power consumption of 119.512 W. The assembly was successfully employed to separate benzene series compounds, gasoline and volatile organic compounds (VOCs), demonstrating excellent separation performance. This innovative design addresses the challenges of the complexity and low repeatability of the fabrication process and the high cost associated with microchip columns. Furthermore, its versatility makes it suitable for outdoor analysis applications.

Pillar Array Mixer for Postcolumn Derivatization Integrated into Liquid Chromatography-Based Microfluidic Device

The chemical derivatization of target analytes can enhance the sensitivity and selectivity of separation-based methods for metabolite analysis using microfluidic devices. However, the development of chromatography-based microfluidic devices with integrated derivatization units is challenging. In this study, a novel derivatization unit with a pillar array (PA)-based mixing channel was developed for postcolumn derivatization during on-chip liquid chromatography (LC). The PA mixer enhanced mixing between the derivatization reagents and analytes in the transverse direction, while preventing analyte dispersion in the flow direction. After the concept was confirmed using computational fluid dynamics analysis, microfluidic devices with a LC column and PA mixer were fabricated on a 20 × 20 mm silicon plate. Fluid experiments were performed using a PA mixer with a pillar size of 5 or 10 μm or a hollow-channel mixer, which revealed that the PA mixer enhanced transverse mixing without increasing the width of the analyte peak. Moreover, the developed device enabled the analysis of three amino acids within 40 s by separation via hydrophilic interaction chromatography followed by postcolumn fluorogenic derivatization with naphthalene-2,3-dicarboxaldehyde and fluorescence detection. Our results demonstrate the potential of integrated derivatization units for the development of micrototal analysis systems for use in bioanalysis.

Characterization of semi-packed columns with different cross section in high-pressure gas chromatography

Hydrodynamics, efficiency, and loading capacity of two semi-packed columns with different cross sections (NANO 315 µm x 18 µm; CAP 1000 µm x 28 µm) and similar pillar diameter and pillar-pillar distance (respectively 5 µm and 2.5 µm) have been compared in high-pressure gas chromatography. A flow prediction tool has been first designed to determine pressure variations and hold-up time across the chromatographic system taking into account the rectangular geometry of the ducts into the semi-packed columns. Intrinsic values of Height Equivalent to Theoretical Plate were determined for NANO and CAP columns using helium as carrier gas and similar values have been obtained (30 µm) for the two columns. Loading capacity of semi-packed columns were determined for decane at 70 °C using helium, and the highest value was obtained from CAP column (larger cross section and stationary phase content). Finally, significant HETP improvement (down to 15 µm) and peak shape were observed when carbon dioxide was used as carrier gas, suggesting mobile phase adsorption on stationary phase in high pressure conditions.

Evaluation of Gas Chromatography Columns with Radially Elongated Pillars as Second-Dimension Columns in Comprehensive Two-Dimensional Gas Chromatography

The present study investigated the use of a dedicated gas chromatography (GC) column (L = 70 cm, 75 μm deep, and 6.195 mm wide) with radially elongated pillars (REPs) as the second column in a comprehensive two-dimensional gas chromatography (GC × μGC) system. Three stationary phases [apolar polydimethylsiloxane (PDMS), medium polar room-temperature ionic liquid (RTIL) based on monocationic phosphonium, and polar polyethylene glycol (PEG-1000)] have been coated using the static method at constant pressure or using an original vacuum pressure program (VPP) from 400 to 4 mbar. The best efficiency reached up to N = 62,000 theoretical plates for a film thickness of 47 nm at 100 °C for an iso-octane peak (k = 0.16) at an optimal flow rate of 4.8 mL/min. The use of the VPP improved the efficiency by approximately 15%. Efficiencies up to 28,000 and 47,000 were obtained for PEG-1000 and RTIL, respectively. A temperature-programmed separation of a mixture of 11 volatile compounds on a PDMS-coated chip was obtained in less than 36 s. The PDMS-, PEG-1000-, and RTIL-coated chips were tested as the second column using a microfluidic reverse fill/flush flow modulator in a GC × μGC system. The REP columns were highly compatible with the operating conditions in terms of flow rate and with more than 30,000 plates for the iso-octane peak. Moreover, a commercial solvent called white spirit containing alkanes and aromatic compounds was injected in three sets of columns in normal and reverse modes, demonstrating the great potential of the chip as a second-dimension separation column.

Selectivity in trace gas sensing: recent developments, challenges, and future perspectives

Selectivity is one of the most crucial figures of merit in trace gas sensing, and thus a comprehensive assessment is necessary to have a clear picture of sensitivity, selectivity, and their interrelations in terms of quantitative and qualitative views.

Gas chromatography of essential oil: State-of-the-art, recent advances, and perspectives

This review is an overview of the recent advances of gas chromatography in essential oil analysis; in particular, it focuses on both the new stationary phases and the advanced analytical methods and instrumentations. A paragraph is dedicated to ionic liquids as gas chromatography stationary phases, showing that, thanks to their peculiar selectivity, they can offer a complementary contribution to conventional stationary phases for the analysis of complex essential oils and the separation of critical pairs of components. Strategies to speed-up the analysis time, thus answering to the ever increasing request for routine essential oils quality control, are also discussed. Last but not least, a paragraph is dedicated to recent developments in column miniaturization in particular that based on microelectromechanical-system technology in a perspective of developing micro-gas chromatographic systems to optimize the energy consumption as well as the instrumentation dimensions. A number of applications in the essential oil field is also included.

Behavior of micro pillar array column in high pressure gas chromatography

Micro pillar array column with interpillar distance of 2.5 µm for pillars diameter of 5 µm has been introduced in high pressure gas chromatographic systems for online industrial analysis. Separation of gas mixtures have been performed under carrier gas pressure as high as 60 bar using rotating valve for gas injection without sample decompression stage prior to injection. A very low intrinsic height equivalent to a theoretical plate value of 14 µm has been obtained in few seconds. Instead of conventional gas chromatography, carrier gas nature such as helium, argon and carbon dioxide and pressure can be used to tune the selectivity. Liquid hydrocarbon samples have been successfully introduced in the column using a septum based split/splitless injector modified to work up to 40 bar. Separations of VOCs and gasoline samples have been successfully performed.

Emerging Separation Techniques in Supercritical Fluid Chromatography

Supercritical fluid chromatography (SFC) has unique separative characteristics distinguished from those of HPLC and gas chromatography. At present, SFC is widely used and there are many applications in various biological, medical, and pharmaceutical fields. In this review, we focus on recently developed novel techniques related to SFC separation including: new column stationary phases, microfluidics, two-dimensional separation, and gas-liquid separation. In addition, we discuss the application of SFC using a water-containing modifier to biological molecules such as amino acids, peptides, and small proteins that had been challenging analytes.

A Portable Micro-Gas Chromatography with Integrated Photonic Crystal Slab Sensors on Chip

The miniaturization of gas chromatography (GC) systems has made it possible to utilize the analytical technique in various on-site applications to rapidly analyze complex gas samples. Various types of miniaturized sensors have been developed for micro-gas chromatography (µGC). However, the integration of an appropriate detector in µGC systems still faces a significant challenge. We present a solution to the problem through integration of µGC with photonic crystal slab (PCS) sensors using transfer printing technology. This integration offers an opportunity to utilize the advantages of optical sensors, such as high sensitivity and rapid response time, and at the same time, compensate for the lack of detection specificity from which label-free optical sensors suffer. We transfer printed a 2D defect free PCS on a borofloat glass, bonded it to a silicon microfluidic gas cell or directly to a microfabricated GC column, and then coated it with a gas responsive polymer. Realtime spectral shift in Fano resonance of the PCS sensor was used to quantitatively detect analytes over a mass range of three orders. The integrated µGC–PCS system was used to demonstrate separation and detection of a complex mixture of 10 chemicals. Fast separation and detection (4 min) and a low detection limit (ng) was demonstrated.

Numerical investigation of the solute dispersion in finite-length microchannels with the interphase transport

This paper utilizes a combined approach of the convection-diffusion theory and the moment analysis to conduct a comprehensive investigation of the solute dispersion under the influence of the interphase transport in finitely long inner coated microchannels. The present work has threefold novel contributions: (1) The 2D solute concentration contours in the stationary phase are calculated for the first time to facilitate the understanding the role of the interphase transport in the solute dispersion in the mobile phase. (2) The skewness of the elution curves is investigated to guide the control of solute band shape at the channel outlet. (3) The 2D diffusion-convection theory and zero-dimensional (0D) moment analysis complement each other to present a characterization of the solute dispersion behaviors more comprehensive than that by either of the two methods alone. Parametric studies are performed to clarify the effects of four major parameters related to the interphase transport (i.e., stationary phase Péclet number, interphase transport rate, partition coefficient, and stationary phase thickness) on the solute dispersion characteristics. The results from this study provide a straightforward understanding of the effects of interphase transport on the solute dispersion in finitely long microchannels and are of potential relevance to the design and operation of the microfluidics-based analytical devices.

Isomer separation enabled by a micro circulatory gas chromatography system

Isomers, holding similar chemical and physical properties, are difficult to separate especially by utilizing a microfabricated gas chromatography system due to limited column lengths mainly imposed by low-pressure (<20 kPa) micropump capability. In this paper, we demonstrated the separation of a pair of structural isomers, isopentane and pentane, in a micro-scale gas chromatography system with a circulatory loop of two 25-cm micro open tubular columns, while operating under a minimal pressure requirement of <10 kPa. The developed micro circulatory gas chromatography (MCGC) system achieved an effective column length of 12.5 meters by circulating the isomer gases for 25 cycles, the longest micro open tubular column length ever reported by any microfabricated GC systems yet.

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.

A 3D-printed metal column for micro gas chromatography

In this work, a 3D-printed metal column was developed for micro gas chromatography (GC) applications and its properties and gas separation performances were characterized. By using a Ti6Al4V grade 23 powder, a square spiral one meter-long column (3D-column) was 3D-printed on a planar substrate of 3.4 × 3.3 × 0.2 cm and then perhydropolysilazane (PHPS) was deposited as a pre-treatment agent, followed by a coating of stationary phase (OV-1) onto the inner wall of the micro-channel. The 500 μm-diameter circular channel and two 800 μm-wide ports of the 3D-column were confirmed to be uniform by 3D X-ray microscopy without any distortion. The physical and thermal properties of the 3D-column were found to be very similar to that of the standard Ti6Al4V grade 23 alloy with near zero porosity (∼0.07%). The 3D-column with pre-treatment and stationary coating demonstrated efficient separation performance of gas mixtures containing alkanes, aromatics, alcohols, and ketones compared to a bare or only pretreated 3D-column in terms of the peak shape, broadening, and resolution (R > 1) within 2-3 min. The well-matched thermal responses to the target temperatures were demonstrated at the ramping rates of 10-20 °C min^-1 upto 200 °C with uniform heat distribution over the 3D-column. In addition, the column bleed profiles showed that the 3D-column with PHPS had a 71% lower baseline intensity at 350 °C than that without PHPS. The 3D-column was then employed to separate a gas mixture of twelve alkanes (C9-C18, C22, C24) without any significant column bleeding and peak tailing. Therefore, the thermal responses and stability of the 3D-column promise its applicability in high temperature GC applications.

Parallel Ionic Liquid Semi-Packed Microfabricated Columns for Complex Gas Analysis

The paper presents a parallel micro gas chromatography approach using three ionic liquid semipacked columns. Switching from single column to multiple parallel columns with different selectivity enhances the power of compound identification without increasing the analysis time. The columns are fabricated using microelectromechanical systems (MEMS) technology containing an array of microfabricated pillars. The columns are 1 m-long and 240 μm-deep with four pillars per row. All columns were functionalized with ionic liquid stationary phases using a modified static coating technique and demonstrated the number of theoretical plates between 5000 and 8300 per meter. The chip performance was investigated with four different samples: (1) a mixture of C7-C30 saturated alkanes, (2) a multianalyte mixture consisting of 20 compounds ranging from 80 to 238 °C in boiling point, (3) a mixture of five organic chemicals with varying degrees of polarity, and (4) 46-compounds mixture containing all the chemicals in the first three samples. The individual columns separated 75%-100% of the first three samples but failed to distinguish all 46 compounds due to coeluting analytes; however, the parallel configuration provided more retention time information by which all the compounds in all samples were fully determined.

Metal-Organic Framework Stationary Phases for One- and Two-Dimensional Micro-Gas Chromatographic Separations of Light Alkanes and Polar Toxic Industrial Chemicals

Despite promising advances with metal-organic frameworks (MOFs) as stationary phases for chromatography, the application of MOFs for one- and two-dimensional micro-gas chromatography (μGC and μGC × μGC) applications has yet to be shown. We demonstrate for the first time, μGC columns coated with two different MOFs, HKUST-1 and ZIF-8, for the rapid separation of high volatility light alkane hydrocarbons (natural gas) and determined the partition coefficients for toxic industrial chemicals, using μGC and μGC × μGC systems. Complete separation of natural gas components, methane through pentane, was completed within 1 min, with sufficient resolution to discriminate n-butane from i-butane. Layer-by-layer controlled deposition cycles of the MOFs were accomplished to establish the optimal film thickness, which was validated using GC (sorption thermodynamics), quartz-crystal microbalance gravimetric analysis and scanning electron microscopy. Complete surface coverage was not observed until after ~17 deposition cycles. Propane retention factors with HKUST-1-coated μGC and a state-of-the-art polar, porous-layer open-tubular (PLOT) stationary phase were approximately the same at ~7.5. However, with polar methanol, retention factors with these two stationary phases were 748 and 59, respectively, yielding methanol-to-propane selectivity factors of ~100 and ~8, respectively, a 13-fold increase in polarity with HKUST-1. These studies advance the applications of MOFs as μGC stationary phase.

Evolution and Evaluation of GC Columns

A chromatographic column is the fundamental element required for gas-chromatographic analysis. The separation of components coming from complex mixtures, prior to their detection was leading to a prominent revolution in different areas of science. Moreover, current advances in gas chromatographic (GC) columns technology and development have been providing almost unlimited possibilities for analysis employing diverse matrices. We aim through this review article to describe the evolution of chromatographic columns, by pointing the most important stages, as well as the new trends and future perspectives predicted for the new generation of GC columns. Furthermore, it was in our scope to present the main fundamentals regarding the theoretical relationships that describe the chromatographic separation, to introduce concepts related to columns selection in accordance with the required application as well as to discuss the available evaluation parameters for columns efficiency. Consequently, the early stages of first columns preparation up to the development of GC capillary columns used nowadays, together with examples of their applications are also reported and described in detail.

Metal 3D-printed catalytic jet and flame ionization detection for in situ trace carbon oxides analysis by gas chromatography

A gas chromatographic approach for the determination and quantification of trace levels of carbon oxides in gas phase matrices for in situ or near-line/at-line analysis has been successfully developed. Catalytic conversion of the target compounds to methane via the methanation process was conducted inside a metal 3D-printed jet that also acted as a hydrogen burner for the flame ionization detector. Modifications made to a field transportable gas chromatograph enabled the leveraging of advantaged microfluidic-enhanced chromatography capability for improved chromatographic performance and serviceability. The compatibility with adsorption chromatography technology was demonstrated with in-house constructed columns. Sustained reliable conversion efficiencies of greater than 99% with respectable peak symmetries were attained at 400°C. Quantification of carbon monoxide and carbon dioxide at a parts-per-million level over a range from 0.2 ppm to 5% v/v for both compounds with a respectable precision of less than 3% relative standard deviation for peak area (n = 10) and a detection limit of 0.1 ppm v/v was achieved. Linearity with correlation coefficients of R² greater than 0.9995 and measured recoveries of >99% for spike tests were achieved. The 3D-printed steel jet was found to be reliable and resilient against potential contamination from the matrices owing to the in situ backflushing capability.

A high-speed, high-performance, microfabricated comprehensive two-dimensional gas chromatograph

A small, consumable-free, low-power, ultra-high-speed comprehensive GC×GC system consisting of microfabricated columns, nanoelectromechanical system (NEMS) cantilever resonators for detection, and a valve-based stop-flow modulator is demonstrated. The separation of a highly polar 29-component mixture covering a boiling point range of 46 to 253 °C on a pair of microfabricated columns using a Staiger valve manifold in less than 7 seconds, and just over 4 seconds after the ensemble holdup time is demonstrated with a downstream FID. The analysis time of the second dimension was 160 ms, and peak widths in the second dimension range from 10-60 ms. A peak capacity of just over 300 was calculated for a separation of just over 6 s. Data from a continuous operation testing over 40 days and 20 000 runs of the GC×GC columns with the NEMS resonators using a 4-component test set is presented. The GC×GC-NEMS resonator system generated second-dimension peak widths as narrow as 8 ms with no discernable peak distortion due to under-sampling from the detector.