Porous layer open-tubular capillary columns: preparations, applications and future directions

A stamped aluminium gas chromatographic column disk employing directly grown anodic aluminium oxide stationary phase for the separation of aromatic and chlorinated compounds

In this study, mesoporous anodic aluminium oxide (AAO) with moderate polarity was used as a GC stationary phase to demonstrate the applicability to various compound species. The fluidic channel measured 6 meters in length and had a cross-section area of 0.127 mm2. The column disk measured 6.2 cm in diameter and was fabricated through a stamping process on an aluminium substrate. The AAO stationary phase was directly grown on the aluminium substrate through an anodization process using oxalic acid as the electrolyte. The pore size of the AAO stationary phase was approximately 50-70 nm, with film thicknesses ranging from 6-20 μm. AAO based on oxalic acid exhibited significantly reduced surface polarity, making it suitable for separating polarizable and slightly polar compounds. The theoretical plate number for benzene had reached 1800 plates per meter, and for n-butane, it had reached 2500 plates per meter. A complex mixture of 16 compounds spanning alkanes, olefins, aromatics, and chlorinated hydrocarbons was effectively separated in 8 minutes with the temperature programmed to 200 °C.

Retention performance of alumina porous layer open-tubular column coated with γ-alumina nanoparticles in the highly volatile hydrocarbons separation

An alumina porous layer open-tubular (Al₂O₃ PLOT) column coated with γ-alumina nanoparticles (20 nm) for highly volatile hydrocarbons (C1 to C5) separation was described. Relative to the coating of bulk alumina, this column was easily coated with dynamic method under 0.4 or 0.6 MPa for 0.53 mm or 0.32 mm capillary, respectively. And the thickness of coating layer could be tuned by repeating the coating process after column was dried and activated at 300 °C for 3 h. The effect of deactivation agents on the physicochemical properties of nano γ-alumina was characterized by X-ray diffraction (XRD) measurement and nitrogen adsorption-desorption isotherms. The influences of deactivation agents, film thickness, conditioning and column dimensions on the inertness, polarity, selectivity and elution order of C1 to C5 separation were investigated in detail. The crystallite structure and size of nano alumina were not affected by the deactivation agents and remained constant during the column making processes, whereas specific surface area, pore volume and average half pore width altered significantly. The specific surface area decreased to 125.4 m² g^-1 or 174.0 m² g^-1 and the average half pore size distributions decreased to 1.6-8.4 nm or 2.4-14.3 nm when it was deactivated with potassium chloride or sodium sulfate solution, respectively. The deactivation agents and its concentrations impacted significantly on the retention performance of column. The column deactivated with sodium sulfate solution exhibited stronger polarity and lower selectivity than which deactivated with potassium chloride solution although both columns showed good inertness. The length, internal diameter and film thickness of the column had less influence on the selectivity and resolution for C1 to C5 hydrocarbons separation, whereas the conditioning temperature and time had an obvious influence. The column had distinguished polarity and selectivity which was different from either bulk or commercial alumina columns. Typically, the hydrocarbons were baseline separated with resolutions ranging from 1.65 to 15.33 within 9 min under programmed temperature below 100 °C, and the tailing factors ranging from 1.02 to 1.07.

A novel column modification approach for capillary gas chromatography: combination with a triptycene-based stationary phase achieves high separation performance and inertness

Integration of the novel column modification approach with a triptycene-based stationary phase achieves high-resolution performance and inertness towards acids/bases and isomers for capillary GC analysis.

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.

Microfabricated porous layer open tubular (PLOT) column

Development of micro gas chromatography (μGC) is aimed at rapid and in situ analysis of volatile organic compounds (VOCs) for environmental protection, industrial monitoring, and toxicology. However, due to the lack of appropriate microcolumns and associated stationary phases, current μGC is unable to separate highly volatile chemicals such as methane, methanol, and formaldehyde, which are of great interest for their high toxicity and carcinogenicity. This inability has significantly limited μGC field applicability. To address this deficiency, this paper reports the development and characterization of a microfabricated porous layer open tubular (μPLOT) column with a divinylbenzene-based stationary phase. The separation capabilities of the μPLOT column are demonstrated by three distinct analyses of light alkanes, formaldehyde solution, and organic solvents, exhibiting its general utility for a wide range of highly volatile compounds. Further characterization shows the robust performance of the μPLOT column in the presence of high moisture and at high temperatures (up to 300 °C). The small footprint and the ability to separate highly volatile chemicals make the μPLOT column highly suitable for integration into μGC systems, thus significantly broadening μGC's applicability to rapid, field analysis of VOCs.

Hydrophobic polymerized ionic liquids for trace metal solid phase extraction: thallium transfer from hydrochloric acid media

To our knowledge, there are a few articles on the application of polymerized ionic liquids for metal extraction from aqueous solutions.

Recent advances in open tubular capillary liquid chromatography

This review covers advances and applications of open tubular capillary liquid chromatography (OT-LC) over the period 2007–2018.

Analysis of short-chain fatty acids in human feces: A scoping review

Short-chain fatty acids (SCFAs) play a crucial role in maintaining homeostasis in humans, therefore the importance of a good and reliable SCFAs analytical detection has raised a lot in the past few years. The aim of this scoping review is to show the trends in the development of different methods of SCFAs analysis in feces, based on the literature published in the last eleven years in all major indexing databases. The search criteria included analytical quantification techniques of SCFAs in different human clinical and in vivo studies. SCFAs analysis is still predominantly performed using gas chromatography (GC), followed by high performance liquid chromatography (HPLC), nuclear magnetic resonance (NMR) and capillary electrophoresis (CE). Performances, drawbacks and advantages of these methods are discussed, especially in the light of choosing a proper pretreatment, as feces is a complex biological material. Further optimization to develop a simple, cost effective and robust method for routine use is needed.

An overview of multidimensional liquid phase separations in food analysis

Food safety is a priority public health concern that demands analytical methods capable to detect low concentration level of contaminants (e.g. pesticides and antibiotics) in different food matrices. Due to the high complexity of these matrices, a sample preparation step is in most cases mandatory to achieve satisfactory results being usually tedious, lengthy, and prone to the introduction of errors. For this reason, many research groups have focused efforts on the development of online systems capable to do the cleanup, concentration, and separation steps at once through multidimensional separation techniques (MDS). Among several possible setups, the most popular are the multidimensional chromatographic techniques (MDC) that consist in combining more than one mobile and/or stationary phase to provide a satisfactory separation. In the present review, we selected a variety of multidimensional separation systems used for food contaminant analysis in order to discuss the instrumentation aspects, the concept of orthogonality, column approaches used in these systems, and new materials that can be used in these columns. Selected classes of contaminants present in food matrices are introduced and discussed as example of the potential applications of multidimensional liquid phase separation techniques in food safety.

Preparation and characterization - including in situ Small Angle X-Ray Scattering - of gas chromatographic capillary columns with mesoporous silica thin films as stationary phases

In this study the preparation of various mesoporous silica thin films as new stationary phases for gas chromatography (GC) columns is presented. The synthesis was performed inside capillaries via a sol-gel process using a templating route. The as-obtained columns were found to be highly efficient for the fast separation of light n-alkanes (C1-C5) mixture; these columns exhibiting a normalized retention 30 times higher than that of a commercially available silica column used as standard. A particular effort was directed towards the characterization of the stationary phase physical features: thin film inspection by Scanning Electron Microscopy and, for the first time to our knowledge, in situ SAXS characterization using synchrotron radiation were used to study the impact of the pore-network structuration on the GC properties. Worm-like, cubic and hexagonal phases were observed for specific preparation conditions. Unexpectedly, the normalized retention relative to film thickness appeared higher with disordering of the pores network.

Fabrication of a polymer nozzle array in a microstructured fibre as a nanoelectrospray emitter for mass spectrometry

We report a modified silica microstructured fibre (MSF) as a multiple electrospray (MES) emitter, with dimensional compatibility with conventional liquid chromatography and mass spectrometry equipment, to generate stable electrospray from a wide range of applied potentials and flow rates. An array of polymer nozzles is fabricated in the MSF by a procedure involving templated polymerization of microtubes and wet chemical etching of the silica at the tip. The structure of the emitting end of the MSF was optimized with respect to the etching process, and the morphology of the polymer nozzles was optimized with respect to polymerization conditions. The mechanisms of the etching and of the templated polymerization of the microtubes were explored. Optimization experiments were performed using commercially available MSF having 126 tubular air channels arranged in a hexagonal pattern with channel diameter of ∼5.6 μm. However, the flexibility and versatility in the pattern, shape, and size of channels in MSFs allowed a custom-designed MSF to be fabricated and tested for MES. In the new design, six channels were evenly spaced in a radial pattern, and when polymer nozzles were made, six stable electrosprays were observed over a wide range of electrospray conditions. Using these MES emitters, the spray current is enhanced by a factor related to the number of nozzles.

The properties of capillary columns with silica organic-inorganic MCM-41 type porous layer stationary phase

In this work, we report the method of capillary columns preparation for gas-solid chromatography with a porous layer of MCM-41 type silica sorbent. The porous layer was synthesized by the sol-gel method inside the column. Scanning electron microscopy (SEM) measurements were performed to obtain information about the porous layer. The loading capacity of the prepared columns was investigated. An adsorbent selectivity was changed by using different relative contents of organic-inorganic precursors: vinyltriethoxysilane (VTEOS) and tetraethoxysilane (TEOS). Properties of the columns prepared are discussed. Separating examples of C1-C4 hydrocarbons and some other compounds are presented.

In-line sampling with gas chromatography-mass spectrometry to monitor ambient volatile organic compounds

An existing GC-MS/FID method coupling with the cryogenic trapping was improved to perform continuous field monitoring of 106 VOCs, covering a wide range of volatilities and polarities (C(2)-C(11) NMHCs, ≥C(1) halocarbons, toxic chlorinated compounds, ethers, some esters and ketones). Cryogenic enrichment was employed from the standpoints of higher signal-to-noise ratio, less carry-over and better protection of thermally labile compounds than chemical sorbent enrichment. However, cryogen consumption is large and creates a great logistical burden for field deployment. As a result, a new in-line sampling manifold was designed and incorporated into the system to separate the sampling from trapping during enrichment of ambient VOCs, which gave rise to two major advantages: (1) the sampling is performed by a pre-evacuated flask, which does not need cryogen when filling a sample, so that the sampling time can be extended to yield better sample representation (approximately one hour was chosen for the sampling time for hourly data resolution in this study) and (2) because the cryo-trapping only takes a short time period (3 min in this study), the consumption of cryogen is greatly reduced (4 L liquid nitrogen per sample for conventional cryo-trapping vs. 0.6L for the new method). The robustness of the automated GC-MS/FID coupling with in-line sampling for the 106 target compounds was assessed with a set of quality assurance criteria of system blank, wall effect, precision, linearity, detection limit and field test to support the field applicability of the method. The configuration of the proposed in-line sampling apparatus is simple and rugged, which can be easily built and connected with any GC or GC-MS and readily deployed in the field to perform high-quality continuous measurements of more than 106 VOCs.

Less common applications of monoliths. III. Gas chromatography

Porous polymer monoliths emerged about two decades ago. Despite this short time, they are finding applications in a variety of fields. In addition to the most common and certainly best known use of this new category of porous media as stationary phases in liquid chromatography, monolithic materials also found their applications in other areas. This review article focuses on monoliths in capillaries designed for separations in gas chromatography.

Open-tubular capillary columns with a porous layer of monolithic polymer for highly efficient and fast separations in electrochromatography

Open-tubular columns for CEC separations having inner-wall coated with a thin layer of porous monolithic polymer have been studied. A two-step process including (i) UV-initiated polymerization leading to a layer of porous poly(butyl methacrylate-co-ethylene dimethacrylate), and (ii) UV-initiated grafting of ionizable monomers appear to be well suited for the preparation of these columns. The thickness of the porous polymer layer is controlled by the percentage of monomers in the polymerization mixture and/or length of the irradiation time. The layer thickness significantly affects retention, efficiency, and resolution in open-tubular CEC. Under optimized conditions, column efficiencies up to 400,000 plates/m can be achieved. Use of higher temperature and application of pressure enables a significant acceleration of the open-tubular CEC separations.