Minimum Reduced Plate Height Research Articles

Simulation of the performance of pillar array columns using the pore-throat ratio as efficiency descriptor.

Traditional packed beds in chromatography suffer from increased band broadening due to the random nature of packing, leading non-ideal fluid flow and channeling. To address these challenges, pillar array columns have been developed, offering improved performance over random packing thanks to their homogenous fluid profiles. The study aims to i) evaluate fluid dynamics and chromatographic performance across different PAC morphologies, ii) establish the influence of column morphology on performance, and iii) assess the correlation between chromatographic performance and hydrodynamic parameters. Approximately 200 PAC morphologies with different pillar shapes, pillar sizes, rotations and radial stretching were simulated using the finite element method (FEM). The pore-throat ratio was introduced as a predictor for separation efficiency, showing a strong positive correlation between this ratio and the minimum reduced plate height hmin. It is suggested that a more homogenous fluid profile, indicated by a pore-throat ratio approaching 1, is associated with higher column efficiency and permeability, allowing for larger optimal velocities and reduced separation time. For PACs, the hmin is close to 0.25 as the pore throat ratio approaches 1. The simplicity of calculating the pore-throat ratio from the column structure without the need for chromatographic experiments or modeling makes it a practical tool for predicting column performance.

On the road toward highly efficient and large volume three-dimensional-printed liquid chromatography columns?

The current performance of commercially packed liquid chromatography columns is limited by the random structure of the packed bed and by the wall-to-center heterogeneity of its structure. The minimum reduced plate heights observed are not smaller than 1.4, whereas they could theoretically be as low as 0.1 for dense and perfectly ordered packings of spheres. To bridge this gap, a wide inner diameter column with an ordered macroporous structure is printed in three dimensions by stereolithography of poly(ethylene glycol diacrylate) resin. Feature sizes below 100μm are achieved by combining conventional polymer stereolithography with photolithography using photomasks. A layer-by-layer polymerization is performed by alternating two distinct photomasks having horizontally and vertically oriented patterns. Despite the inevitable printing imperfections, minimum reduced plate heights around unity are measured for nonretained analytes. The next challenges for the successful printing of highly efficient and large volume liquid chromatography columns are threefold: reducing the feature size down to below 10μm, keeping minimum the unevenness of the flow channel dimensions, and tackling additive manufacturing of silica aerogels at such small feature sizes for higher mechanical stability and broader range of retention/selectivity than those delivered by polymer materials.

Characterization of a highly stable zwitterionic hydrophilic interaction chromatography stationary phase based on hybrid organic-inorganic particles.

We have characterized a sulfobetaine stationary phase based on 1.7 μm ethylene‐bridged hybrid organic–inorganic particles, which is intended for use in hydrophilic interaction chromatography. The efficiency of a column packed with this material was determined as a function of flow rate, demonstrating a minimum reduced plate height of 2.4. The batch‐to‐batch reproducibility was assessed using the separation of a mixture of acids, bases, and neutrals. We compared the retention and selectivity of the hybrid sulfobetaine stationary phase to that of several benchmark materials. The hybrid sulfobetaine material gave strong retention for polar neutrals and high selectivity for methyl groups, hydroxy groups, and configurational isomers. Large differences in cation and anion retention were observed among the columns. We characterized the acid and base stability of the hybrid sulfobetaine stationary phase, using accelerated tests at pH 1.3 and 11.0, both at 70°C. The results support a recommended pH range of 2–10. We also investigated the performance of columns packed with this material for metal‐sensitive analytes, comparing conventional stainless steel column hardware to hardware that incorporates hybrid surface technology to mitigate interactions with metal surfaces. Compared to the conventional columns, the hybrid surface technology columns showed a greatly improved peak shape.

Ellipsoidal particles for liquid chromatography: Fluid mechanics, efficiency and wall effects

Ellipsoidal particles are investigated as packing media for liquid chromatography using high resolution fluid mechanics and Brownian dynamics simulations. The simulations are conducted with packed capillary columns, as well as beds with periodic boundary conditions (PBCs) to study transport in the absence of wall effects. The performance of ellipsoidal particles is evaluated over a range of aspect ratios. The definition of effective diameter used to compare sphere and ellipsoidal particle performance metrics is presented and discussed along with scaling relationships which are necessary to compare sphere and ellipsoidal particle packs.Ellipsoidal particle packs are found to be inferior to sphere packs using PBCs to study chromatographic dispersion. The separation impedance was calculated with PBCs and shown to be approximately the same with ellipsoidal particles as those of spheres. Efficiency of ellipsoidal packs, as measured by plate height, is lower than spherical particle packs and the pressure drop is higher than sphere packs when using PBCs.However, a smaller wall effect is shown for ellipsoidal particles when packing cylindrical capillaries. Radial variations in packing porosity and in flow within the wall region are smaller for ellipsoidal packings. The minimum reduced plate height and the separation impedance for the packed capillaries clearly demonstrate the advantages of ellipsoidal particles compared to spherical particles. This predicted performance advantage remains to be demonstrated in actual practice.

Preliminary kinetic evaluation of an immobilized polysaccharide sub-2 μm column using a low dispersion supercritical fluid chromatograph

The performance of a 3×50mm, 1.6μm dp column with an immobilized polysaccharide stationary phase (ChiralPak IA-U) was evaluated for efficiency, and pressure drop, with respect to flow rate and modifier concentration using supercritical fluid chromatography (SFC). This appears to be the first such report using such a column in SFC. A unique low dispersion (ultra-high performance) SFC was used for the evaluation. The minimum reduced plate height of 2.78, indicates that the maximum efficiency was similar to or better than coated polysaccharide columns. Selectivity was different from ChiralPak AD, with the same chiral selector, as reported by many others.At high flows and high methanol concentrations, pump pressures sometimes approached 600bar. With 5% methanol, pressure vs. flow rate was non-linear suggesting turbulent flow in the connector tubing. The optimum flow rate (Fopt) at 40% methanol was ≈0.8mL/min, where the column efficiency was highest. At 5% methanol, Fopt increased to ≈1.6mL/min, but efficiency degraded noticeably. The differences in Fopt suggests that the solute diffusion coefficients are a strong function of modifier concentration. Several sub–1min separations, including a 7.5s separation, are presented.

The Joule-Thomson coefficient as a criterion for efficient operating conditions in supercritical fluid chromatography

When an SFC column is operated in a traditional oven with forced air at low pressures near the critical temperature, severe efficiency losses can occur. The mobile phase cools as it expands along the column, forming axial and radial temperature gradients. In this study we present a simple model based on a virtual fluid to predict the conditions which lead to the onset of efficiency loss. The model shows that the Joule-Thomson coefficient is an important factor leading to efficiency loss in packed columns under forced air conditions. The model was tested experimentally for elution of n-alkylbenzenes on 250×4.6-mm ID columns packed with 5-μm Luna-C18 (fully porous) and Kinetex-C18 (superficially porous) particles at optimum flow rates in a forced air oven at 20–80°C and outlet pressures from 90 to 250bar, with CO2 mobile phase containing 5, 10 and 20% methanol (v/v). For simplicity, we used a formal J-T coefficient corresponding to the inlet temperature and the outlet pressure to characterize the chromatographic conditions. For 5% methanol, there was no significant loss of efficiency for elution of n-octadecylbenzene as long as the formal J-T coefficient was less than 0.11K/bar for Luna or 0.15K/bar for Kinetex, with minimum reduced plate heights equal to 1.82 and 1.55, respectively, at an average apparent retention factor of approximately 4.0 for both columns. The Kinetex column provided superior efficiency in general, and at 10–20bar lower outlet pressures relative to the Luna column due to the higher thermal conductivity of the packing. Results for 10 and 20% methanol showed similar trends but were less predictable.

Characterization of the efficiency of microbore liquid chromatography columns by van Deemter and kinetic plot analysis.

The efficiency of miniaturized liquid chromatography columns with inner diameters between 200 and 300μm has been investigated using a dedicated micro-liquid chromatography system. Fully porous, core-shell and monolithic commercially available stationary phases were compared applying van Deemter and kinetic plot analysis. The sub-2μm fully porous as well as the 2.7μm core-shell particle packed columns showed superior efficiency and similar values for the minimum reduced plate heights (2.56-2.69) before correction for extra-column contribution compared to normal-bore columns. Moreover, the influence of extra-column contribution was investigated to demonstrate the difference between apparent and intrinsic efficiency by replacing the column by a zero dead volume union to determine the band spreading caused by the system. It was demonstrated that 72% of the intrinsic efficiency could be reached. The results of the kinetic plot analysis indicate the superior performance of the sub-2μm fully porous particle packed column for ultra-fast liquid chromatography.

Prototype sphere-on-sphere silica particles for the separation of large biomolecules

The goal of this study was to evaluate the possibilities offered by a prototype HPLC column packed with ∼2.5μm narrow size distribution sphere-on-sphere (SOS) silica particles bonded with C4 alkyl chains, for the analytical characterization of large biomolecules. The kinetic performance of this material was evaluated in both isocratic and gradient modes using various model analytes. The data were compared to those obtained on other widepore state-of-the-art fully core–shell and fully porous materials commonly employed to separate proteins moreover to a reference 5μm wide pore material that is still often used in QC labs. In isocratic mode, minimum reduced plate height values of hmin=2.6, 3.3 and 3.3 were observed on butylparaben, decapeptide and glucagon, respectively. In gradient elution mode, the SOS column performs very high efficiency when working with fast gradients. This prototype column was also comparable (and sometimes superior) to other widepore stationary phases, whatever the gradient time and flow rate, when analyzing the largest model protein, namely BSA. These benefits may be attributed to the SOS particle morphology, minimizing the intra-particle mass transfer resistance.Finally, the SOS column was also applied for the analytical characterization of commercial monoclonal antibody (mAb) and antibody-drug conjugate (ADC) samples. With these classes of proteins, the performance of SOS column was similar to the best widepore stationary phases available on the market.

Highly efficient capillary columns packed with superficially porous particles via sequential column packing

Highly efficient capillary columns packed with superficially porous particles were created for use in ultrahigh pressure liquid chromatography. Superficially porous particles around 1.5μm in diameter were packed into fused silica capillary columns with 30, 50, and 75μm internal diameters. To create the columns, several capillary columns were serially packed from the same slurry, with packing progress plots being generated to follow the packing of each column. Characterization of these columns using hydroquinone yielded calculated minimum reduced plate heights as low as 1.24 for the most efficient 30μm internal diameter column, corresponding to over 500,000plates/m. At least one highly efficient column (minimum reduced plate height less than 2) was created for all three of the investigated column inner diameters, with the smallest diameter columns having the highest efficiency. This study proves that highly efficient capillary columns can be created using superficially porous particles and shows the efficiency potential of these particles.

Impact of the column hardware volume on resolution in very high pressure liquid chromatography non-invasive investigations

The impact of the column hardware volume (≃1.7μL) on the optimum reduced plate heights of a series of short 2.1mm×50mm columns (hold-up volume ≃80–90μL) packed with 1.8μm HSS-T3, 1.7μm BEH-C18, 1.7μm CSH-C18, 1.6μm CORTECS-C18+, and 1.7μm BEH-C4 particles was investigated. A rapid and non-invasive method based on the reduction of the system dispersion (to only 0.15μL2) of an I-class Acquity system and on the corrected plate heights (for system dispersion) of five weakly retained n-alkanophenones in RPLC was proposed. Evidence for sample dispersion through the column hardware volume was also revealed from the experimental plot of the peak capacities for smooth linear gradients versus the corrected efficiency of a weakly retained alkanophenone (isocratic runs). The plot is built for a constant gradient steepness irrespective of the applied flow rates (0.01–0.30mL/min) and column lengths (2, 3, 5, and 10cm). The volume variance caused by column endfittings and frits was estimated in between 0.1 and 0.7μL2 depending on the applied flow rate. After correction for system and hardware dispersion, the minimum reduced plate heights of short (5cm) and narrow-bore (2.1mm i.d.) beds packed with sub-2μm fully and superficially porous particles were found close to 1.5 and 0.7, respectively, instead of the classical h values of 2.0 and 1.4 for the whole column assembly.

High-speed isocratic and gradient liquid-chromatography separations at 1500 bar

The need to improve either sample throughput on separation efficiency has spurred the development of ultra-high-pressure LC instrumentation, allowing to operate up to column pressures of 1500bar. In the present study, the isocratic and gradient performance limits were assessed at UHPLC conditions applying columns packed with core–shell particles. First, the extra-column band broadening contributions were assessed and minimized. Using an optimized system configuration minimum reduced plate heights of 1.8 were recorded on 2.1×100 columns packed with 1.5μm core–shell particles. Increasing the pressure limit from 500 to 1500bar and at the same time reducing the particle size from 2.6 to 1.5μm has allowed the analysis time to be decreased by a factor of 1.5 in isocratic mode, while maintaining separation efficiency (N=54,000). The kinetic time-gain factor in isocratic mode was proportional to the ratio of the separation impedance of both columns multiplied with the pressure ratio applied. In addition, the effect of operating pressure on the time gain factor was assessed in gradient mode. Using optimized gradient steepness (tG/t0=12) and increasing the operating pressure from 500 to 1500bar a time gain factor of almost 13 was achieved for the separation of a mixture of waste-water pollutants without compromising peak capacity.

Sub-2-μm seeded growth mesoporous thin shell particles for high-performance liquid chromatography: Synthesis, functionalisation and characterisation

Nanometer control over the porous shell thickness of sub-2-μm-shell particles is investigated. Three seeded growth mesoporous thin shell particles for HPLC were prepared, with 0.05μm (or 50nm) porous shell layers: particle sizes 1.5μm (solid core diameters 1.4μm), 1.7μm (solid core diameter 1.6μm), 1.9μm (solid core diameter 1.8μm) and compared with a fourth 1.7μm particle (solid core diameter 1.4μm) surrounded by 0.15μm (or 150nm) porous shell thickness. The thin shell particles were functionalised using a mono-functional octadecyldimethylchlorosilane ligand (C20H43SiCl) under optimised reflux conditions and packed in-house in narrow bore columns (2.1 I.D.×50mm) denoted as TS1.5-50-C18, TS1.7-50-C18, and TS1.9-50-C18 respectively. The synthesised thin shell particles and bonded materials were comprehensively characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), zeta potential, BET analysis, elemental analysis (CHN), thermogravimetric analysis (TGA) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Experimental data from inverse size exclusion chromatography (ISEC) was used to measure external, internal and total column porosities. Five probe analytes (uracil, naphthalene, acetophenone, benzene and toluene) were chosen for the chromatographic performance analysis of these columns. Column evaluation and measurements of height equivalent to a theoretical plate (HETP) data were performed on naphthalene using 55% acetonitile in water. The retention coefficients for the thin shell particles (TS1.9-50-C18, TS1.7-50-C18, TS1.5-50-C18) were in the range 1.26–1.35 and 5.6 for the core–shell particle (EiS1.7-150-C18). The minimum reduced plate heights range from 3.89 to 4.26 for the thin shell particles and 2.03 for the core–shell particle.

Homogeneous edge-plane carbon as stationary phase for reversed-phase liquid chromatography.

Carbon stationary phases have been widely used in HPLC due to their unique selectivity and high stability. Amorphous carbon as a stationary phase has at least two sites of interaction with analytes: basal-plane and edge-plane carbon sites. The polarity and adsorptivity of the two sites are different. In this work, the edge-plane carbon stationary phase is prepared by surface-directed liquid crystal assembly. Specific precursor polymers form discotic liquid crystal phases during the pyrolysis process. By using silica as the substrate to align the discotic liquid crystal, edge-plane carbon surfaces were formed. Similar efficiencies as observed for Hypercarb were observed in chromatograms. The column efficiency was studied as a function of linear flow rate. A minimum reduced plate height of 6 was observed in these studies. To evaluate the performance of the homogeneous edge-plane carbon stationary phase, linear solvation energy relationships were used to compare these ordered carbon surfaces to commercially available carbon stationary phases, including Hypercarb. Reversed-phase separations of nucleosides, nucleotides, and amino acids and derivatives were demonstrated using the ordered carbon surfaces, respectively. The column batch-to-batch reproducibility was also evaluated. The retention times for the analytes were reproducible within 1-6% depending on the analyte.

The quantitative impact of the mesopore size on the mass transfer mechanism of the new 1.9 μm fully porous Titan-C18 particles. I: Analysis of small molecules

Previous data have shown that could deliver a minimum reduced plate height as small as 1.7. Additionally, the reduction of the mesopore size after C18 derivatization and the subsequent restriction for sample diffusivity across the Titan-C18 particles were found responsible for the unusually small value of the experimental optimum reduced velocity (5 versus 10 for conventional particles) and for the large values of the average reduced solid–liquid mass transfer resistance coefficients (0.032 versus 0.016) measured for a series of seven n-alkanophenones. The improvements in column efficiency made by increasing the average mesopore size of the Titan silica from 80 to 120Å are investigated from a quantitative viewpoint based on the accurate measurements of the reduced coefficients (longitudinal diffusion, trans-particle mass transfer resistance, and eddy diffusion) and of the intra-particle diffusivity, pore, and surface diffusion for the same series of n-alkanophenone compounds. The experimental results reveal an increase (from 0% to 30%) of the longitudinal diffusion coefficients for the same sample concentration distribution (from 0.25 to 4) between the particle volume and the external volume of the column, a 40% increase of the intra-particle diffusivity for the same sample distribution (from 1 to 7) between the particle skeleton volume and the bulk phase, and a 15–30% decrease of the solid–liquid mass transfer coefficient for the n-alkanophenone compounds. Pore and surface diffusion are increased by 60% and 20%, respectively. The eddy dispersion term and the maximum column efficiency (295000plates/m) remain virtually unchanged. The rate of increase of the total plate height with increasing the chromatographic speed is reduced by 20% and it is mostly controlled (75% and 70% for 80 and 120Å pore size) by the flow rate dependence of the eddy dispersion term.

Comparison of superficially porous and fully porous silica supports used for a cyclofructan 6 hydrophilic interaction liquid chromatographic stationary phase

A new HILIC stationary phase comprised of native cyclofructan-6 (CF6) bonded to superficially porous silica particles (2.7μm) was developed. Its performance was evaluated and compared to fully porous silica particles with 5μm (commercially available as FRULIC-N) and 3μm diameters. Faster and more efficient chromatography was achieved with the superficially porous particles (SPPs). The columns were also evaluated in the normal phase mode. The peak efficiency, analysis time, resolution, and overall separation capabilities in both HILIC and normal phase modes were compared. The analysis times using the superficially porous based column in HILIC mode were shorter and the theoretical plates/min were higher over the entire range of flow rates studied. The column containing the superficially porous particles demonstrated higher optimum flow rates than the fully porous particle packed columns. At higher flow rates, the advantages of the superficially porous particles was more pronounced in normal phase separations than in HILIC, clearly demonstrating the influence that the mode of chromatography has on band broadening. However, the minimum reduced plate heights (hmin) were typically lower in HILIC than in the normal phase mode. Overall, the superficially porous particle based CF6 column showed clear advantages over the fully porous particle columns, in terms of high throughput and efficient separations of polar compounds in the HILIC mode.

The rationale for the optimum efficiency of columns packed with new 1.9 μm fully porous Titan-C18 particles—A detailed investigation of the intra-particle diffusivity

In a previous report, it was reported that columns packed with fully porous 1.9μm Titan-C18 particles provided a minimum reduced plate height as small as 1.7 for the most retained compound (n-octanophenone) under RPLC conditions. These particles are characterized by a relatively narrow size distribution with a relative standard deviation (RSD) of only 10%. A column packed with classical 5μm Symmetry-C18 particles, used as a reference RPLC column, generated a minimum reduced plate height of 2.1 for the same retained compound. This work demonstrates that this was due to an unusually low intra-particle diffusivity across these particles, which leads to a small longitudinal diffusion coefficient along the column. The demonstration is based on the combination of accurate measurements of the height equivalent to a theoretical plate (HETP), inverse size exclusion chromatography (ISEC), peak parking (PP), and minor disturbance method (MDM) experiments.The experimental results show that the reduced eddy dispersion HETP term (A=0.8 for a reduced velocity of 5), the internal particle porosity (ϵp=0.35), and the enrichment of acetonitrile in the pore volume (75% acetonitrile in the bulk, 85% inside the mesoporous volume) are identical on both the Titan-C18 and Symmetry-C18 columns. The difference between the internal structures of these two brands of RPLC-C18 fully porous particles lies in the values of the internal obstruction factor γp, which is 0.42 for the Symmetry-C18 but only 0.26 for the Titan-C18 particles. This is in part related to the diffusion hindrance due to the small average pore size of the Titan-C18 particles, around 59Å versus 77Å for Symmetry-C18 particles. A simple model of constriction along diffusion paths having the shape of a truncated cone suggests that the width of the pore size distribution (RSD of 30% and 20% for Titan-C18 and Symmetry-C18 particles) is mostly responsible for the difference in their obstruction factors.

Maximizing kinetic performance in supercritical fluid chromatography using state-of-the-art instruments

Recently, there has been a renewed interest in supercritical fluid chromatography (SFC), due to the introduction of state-of-the-art instruments and dedicated columns packed with small particles. However, the achievable kinetic performance and practical possibilities of such modern SFC instruments and columns has not been described in details until now. The goal of the present contribution was to provide some information about the optimal column dimensions (i.e. length, diameter and particle size) suitable for such state-of the-art systems, with respect to extra-column band broadening and system upper pressure limit. In addition, the reliability of the kinetic plot methodology, successfully applied in RPLC, was also evaluated under SFC conditions. Taking into account the system variance, measured at ∼85μL2, on modern SFC instruments, a column of 3mm I.D. was ideally suited for the current technology, as the loss in efficiency remained reasonable (i.e. less than 10% decrease for k>6). Conversely, these systems struggle with 2.1mm I.D. columns (55% loss in N for k=5). Regarding particle size, columns packed with 5μm particles provided unexpectedly high minimum reduced plate height values (hmin=3.0–3.4), while the 3.5 and 1.7μm packing provided lower reduced plate heights hmin=2.2–2.4 and hmin=2.7–3.2, respectively. Considering the system upper pressure limit, it appears that columns packed with 1.7μm particles give the lowest analysis time for efficiencies up to 40,000–60,000 plates, if the mobile phase composition is in the range of 2–19% MeOH. The 3.5μm particles were attractive for higher efficiencies, particularly when the modifier percentage was above 20%, while 5μm was never kinetically relevant with modern SFC instruments, due to an obvious limitation in terms of upper flow rate value. The present work also confirms that the kinetic plot methodology could be successfully applied to SFC, without the need for isopycnic measurements, as the difference in plate count between predicted and experimental values obtained by coupling several columns in series (up to 400mm) was on average equal to 3–6% and with a maximum of 13%.

Speed-resolution properties of columns packed with new 4.6 μm Kinetex-C18 core–shell particles

The achievable separation speed and resolution of columns packed with the new 4.6μm Kinetex particles were characterized by their permeability and their plate heights, measured meticulously. Their specific permeabilities range between 1.81 and 1.95×10−10cm2 and their external porosities between 0.394 and 0.405. The efficiencies of the eluted peaks measured by numerical integration of the whole band concentration provided minimum reduced plate heights for uracil (non-retained) and naphthalene (retained) between 1.3 and 1.5 (H=6.0–6.9μm) for two 4.6mm×150mm replicate columns of Kinetex and between 1.6 and 2.0 (H=7.4–9.2μm) for two narrow-bore 2.1mm×150mm replicate columns. The most efficient 4.6mm×150mm Kinetex column shows a deviation of one reduced plate height h unit with respect to the infinite diameter column (no wall, no inlet, no outlet endfitting) at high speed. Eventually, the separation speed and the resolution of columns packed with 4.6μm core–shell Kinetex particles are better or equivalent to those of columns packed with 2.5μm fully porous particles for hold-up times larger than only 10s. These core–shell materials are virtually equivalent to the second generation of silica monolithic columns with the advantage of operating well at pressure drops larger than 200bar.

Ion separation in nanofluidics

Ionic species with a constant charge-to-size ratio (i.e. electrophoretic mobility) cannot be separated in electroosmotic or pressure-driven flow along microscale channels. In nanoscale channels, however, the enormous electric fields inside electrical double layers cause transverse ion distributions yielding charge-dependent mean ion speeds in the flow. Those ions with a constant charge-to-size ratio can thus be separated solely by charge (or equivalently, size) in nanofluidics. Here we develop an analytical model to optimize and compare the separation of such ions in nanochannel chromatography and nanochannel electrophoresis in terms of selectivity, plate height and resolution. Both planar and cylindrical geometries are considered. It is found that in nanoscale channels chromatography yields a larger selectivity and a larger minimum reduced plate height than electrophoresis does. The maximum resolution is, however, comparable between these two nanofluidic approaches, where the optimal channel half-height or tube radius is within the range of 1-10 times the Debye length. Our results also suggest that cations can be better separated in nanofluidics than can anions.

Monolayer-protected gold nanoparticles as an efficient stationary phase for open tubular gas chromatography using a square capillary: Model for chip-based gas chromatography in square cornered microfabricated channels

The application of a dodecanethiol monolayer-protected gold nanoparticle (MPN) stationary phase within a microchannel environment was explored using a square capillary column as a model for high-speed, microfabricated gas chromatography (μGC). Successful deposition and evaluation of a dodecanethiol MPN phase within a 1.3 m long, 100 μm×100 μm square capillary is reported. The thickness of the MPN phase was evaluated using SEM analysis. An average thickness of 15 nm along the capillary walls was determined. While the film depth along the walls was very uniform, the corner depths were greater with the largest observed depth being 430 nm. Overall, an efficient chromatographic system was obtained with a minimum reduced plate height, h min, of 1.2 for octane ( k=0.22). Characterization of the MPN column was completed using four compound classes (alkanes, alcohols, ketones, and aromatics) that were used to form a seven-component mixture with a 2-s separation. A mixture consisting of a nerve agent simulant in a sample containing analytes that may commonly interfere with detection was also separated in only 2 s, much faster than a similar separation previously reported using a μGC system requiring 50 s. A comparison of the MPN stationary phase to phases employed in previously reported μGC systems is also made. Application of the square capillary MPN column for a high-speed separation as the second column of a comprehensive 2-D gas chromatography system (GC×GC) was also explored.