Amine Gradient Stationary Phases on In-House Built Monolithic Columns for Liquid Chromatography

Continuous stationary phase gradient preparation on planar chromatographic media using vapor phase deposition of silane

A new, versatile, and straightforward vapor phase deposition (VPD) approach was used to prepare continuous stationary phase gradients (cSPGs) on silica thin-layer chromatography (TLC) plates using phenyldimethylchlorosilane (PDCS) as a precursor. A mixture of paraffin oil and PDCS was placed at the bottom of an open-ended rectangular chamber, allowing the reactive silanes to evaporate and freely diffuse under a controlled atmosphere. As the volatile silane diffused across the length of the TLC plate, it reacted with the surface silanol groups thus functionalizing the surface in a gradient fashion. Characterization of the gradient TLC plates was done through UV visualization and diffuse reflectance spectroscopy (DRS). Visualizing the fluorescent gradient plates under UV radiation shows the clear presence of a gradient with the side closest to the vapor source undergoing the most modification. More quantitative characterization of the shape of the gradient was provided by DRS. The DRS showed that the degree of modification and shape of the gradient was dependent on the concentration of silane, VPD time, and relative humidity. To evaluate the chromatographic performance, a mixture of three aromatic compounds (acetaminophen (A), aspirin (As), and 3-hydroxy-2-naphthoic acid (3H)) was spotted on the high (GHP) and low phenyl (GLP) ends of the gradient TLC plates and the results compared to the separations carried out on unmodified and uniformly modified plates. The GHP TLC plates showed retention factors (Rf) of 0.060 ± 0.006, 0.391 ± 0.006, and 0.544 ± 0.006, whereas the unmodified plate displayed Rf values of 0.059 ± 0.006, 0.092 ± 0.003, and 0.037 ± 0.002 for the analytes A, As, and 3H, respectively. From the Rf values, it was observed that each modified plate exhibited different selectivity for the analytes. The GHP TLC plates exhibited better separation performance, and improved resolution compared to the GLP, unmodified, and uniformly modified plates. Overall, VPD is a new, cost-effective method for creating a gradient on the stationary phase which has the potential to advance chromatographic separation capabilities.

Preparation and characterization of stationary phase gradients on C8 liquid chromatography columns

Continuous C8 stationary phase gradients are created on commercial Waters Symmetry Shield RP8 columns by strategically cleaving the C8 moieties in a time-dependent fashion. The method relies on the controlled infusion of a trifluoroacetic acid/water/acetonitrile solution through the column to cleave the organic functionality (e.g., C8) from the siloxane framework. The bond cleavage solution is reactive enough to cleave the functional groups, even with polar groups embedded within the stationary phase to protect the silica. Both the longitudinal and radial heterogeneity were evaluated by extruding the silica powder into polyethylene tubing and evaluating the percent carbon content in the different sections using thermogravimetric analysis (TGA). TGA analysis shows the presence of a stationary phase gradient in the longitudinal direction but not in the radial direction. Two different gradient profiles were formed with good reproducibility by modifying the infusion method: one exhibited an 'S'-shaped gradient while the other exhibited a steep exponential-like gradient. The gradients were characterized chromatographically using test mixtures, and the results showed varied retention characteristics and an enhanced ability to resolve nicotine analytes.

Considering the selectivity of pore size gradient size exclusion chromatography columns

One of the most significant performance determining variables of a size exclusion column is the pore size of its packing material. This is most definitely the case for assigning the suitability of a given column for differently sized analytes. As technologies for particle and column manufacturing continue to advance, it is worth contemplating the value of more finely controlled manipulation of this parameter. The change in a packing material's pores across the length of a size exclusion column was thus explored. A change in average pore diameter and pore size distribution was studied by means of theoretical modeling. These parameters were investigated for independent and combinatorial effects. From our predictions, versus tandem column chromatography, a gradient column apparatus does not yield sizable increases in monomer to dimer selectivity of any given critical pair. Instead, our modeling suggests it can yield more universally effective separations of multiple pairs of species at once, as is sometimes necessary when analyzing the high molecular weight components of highly aggregated drug substances.

Studying the possibilities of dual stationary phase gradients to explore alternative selectivities in liquid chromatography

Previous studies on stationary phase gradients have inspired a new phase of theoretical work and an expansion of the concept to include multiple retention mechanisms. The theoretical work presented here corroborates previous reports that a stationary phase gradient can produce selectivity in a separation as effectively as careful adjustments to a mobile phase gradient program, especially when such column is operated in mobile phase gradient elution mode. In reality, no column is singularly based on one type of interaction mechanism, and many columns are nowadays designed to solicit multiple solute to stationary phase interactions. These mixed-mode columns inspired us to give more careful consideration to the idea of dual stationary phase gradients. With the theory applied here, it is suggested that the modulation of two unique interaction mechanisms across a chromatographic column has the potential to open up previously unseen selectivities. With the increasing prevalence of mixed-mode columns, we believe there will be ample opportunity to explore these new concepts in experimental work.

Preparation of ion exchange columns with longitudinal stationary phase gradients

Preparation of columns using electrostatic attachment of anion exchange latex particles with charge density gradients is demonstrated. When such columns are oriented with the highest charge density at the column outlet, the chromatographic performance at low linear velocity is enhanced. When multiple successive charge density gradients are prepared along the length of the column with the highest capacity oriented at the inlet end of the column, significant improvement in chromatographic performance is observed during gradient elution chromatography.

Advancements in the preparation and application of monolithic silica columns for efficient separation in liquid chromatography

Fast and efficient separation remains a big challenge in high performance liquid chromatography (HPLC). The need for higher efficiency and resolution in separation is constantly in demand. To achieve that, columns developed are rapidly moving towards having smaller particle sizes and internal diameters (i.d.). However, these parameters will lead to high back-pressure in the system and will burden the pumps of the HPLC instrument. To address this limitation, monolithic columns, especially silica-based monolithic columns have been introduced. These columns are being widely investigated for fast and efficient separation of a wide range of molecules. The present article describes the current methods developed to enhance the column efficiency of particle packed columns and how silica monolithic columns can act as an alternative in overcoming the low permeability of particle packed columns. The fundamental processes behind the fabrication of the monolith including the starting materials and the silica sol-gel process will be discussed. Different monolith derivatization and end-capping processes will be further elaborated and followed by highlights of the performance such monolithic columns in key applications in different fields with various types of matrices.

Amine/phenyl gradient derived base layer as a comprehensive extractive phase for headspace cooled in-tube microextraction of volatile organic compounds in saliva

A gradient derived base layer extractive phase was synthesized and applied for the determination of volatile organic compounds (VOCs) in saliva samples using the headspace cooled in-tube microextraction (HS-CITME) method. The base layers from three different sols of phenyltriethoxysilane (PTES), octyltrimethoxysilane (OTMS) and methyltrimethoxysilane (MTMS) as nonpolar precursors were individually dip coated on the stainless steel wires (SSW). Then, the hydrolyzed polar precursor aminopropyltriethoxysilane (APTES) reacted with the silanol groups already formed on the surface of SSWs via controlled rate infusion (CRI) method. The presence of polar and non-polar functional groups on the surface of substrate was evaluated by Fourier-transform infrared spectroscopy (FTIR) while the morphology and thickness of the most suitable gradient coating (amine/phenyl) were also investigated by scanning electron microscopy (SEM). Assessment of the gradient extractive phase efficiency was carried out determining a group of VOCs with different polarities coupled with gas chromatography-mass spectrometry (GCMS) and the improved performance of the synthesized base layer coatings was observed. Furthermore, a cooling device was designed and implemented to the extracting system to improve the efficiency by influencing the exothermic nature of process. The data were analyzed by principal component analysis (PCA), and hierarchical cluster analysis (HCA) and the results were interpreted by polarities of analytes. Finally, under the optimized conditions, the limits of detection (LOD) and limits of quantification (LOQ) were 0.15 and 0.50 ng L^-1, respectively. The intra-day and inter-day relative standard deviations (RSDs) at 5 and 50 ng L^-1 (n = 3) using a single extractive phase were 2-6 and 10-17, respectively. The data associated with RSDs% for three extractive phases were between 16 and 19 %. Eventually, the method was conveniently applied to the extraction of VOCs from saliva samples of smokers and satisfactory relative recoveries (RR%) (95-108 %) were achieved and low quantities of VOCs were detected.

Recent applications of chemometrics in one- and two-dimensional chromatography

The proliferation of increasingly more sophisticated analytical separation systems, often incorporating increasingly more powerful detection techniques, such as high-resolution mass spectrometry, causes an urgent need for highly efficient data-analysis and optimization strategies. This is especially true for comprehensive two-dimensional chromatography applied to the separation of very complex samples. In this contribution, the requirement for chemometric tools is explained and the latest developments in approaches for (pre-)processing and analyzing data arising from one- and two-dimensional chromatography systems are reviewed. The final part of this review focuses on the application of chemometrics for method development and optimization.

Experimental- and simulation-based investigations of coupling a mobile phase gradient with a continuous stationary phase gradient

This work seeks to explore and understand the effects of column orientation and degree of modification of continuous stationary phase gradient columns under a mobile phase gradient using both simulations and experiments. Peak parameters such as retention times, peak widths and resolution are obtained for five phenolic compounds on a C₁₈-silica gradient stationary phase. Simulations show that peak widths for the solutes are dependent upon the fractional composition of C₁₈ and orientation of the stationary phase gradient when coupled to a mobile phase gradient. Also, when compared to a simulated uniform mixed-mode column, peak widths reach a minimum on the gradient column with a coverage higher than 50% C₁₈ where the column is oriented to have the C₁₈ dense region at the end. Experimentally, continuous stationary phase gradients were fabricated to have a total C₁₈ composition of 78% of the original uniform column with an exponential profile using a previously described destructive controlled rate infusion method. Under gradient mobile phase conditions, experimental retention times for the gradient column showed a significant increase compared to the original 100% C₁₈ column. Simulations with a similar C₁₈ composition, however, predicted decreased retention times from the original C₁₈ column. A statistical increase in the retention time of protocatechuic acid and decrease in the peak width of tyrosol, caffeic acid, and coumaric acid were noted when the gradient column was oriented to have the C₁₈ dense region located near the detector. Collectively, combining gradients in both the mobile and stationary phases can yield interesting neighboring ligand effects and peak broadening/focusing effects.

Gradient extractive phase prepared by controlled rate infusion method: An applicable approach in solid phase microextraction for non-targeted analysis

The aim of this study is to introduce an extractive phase based on gradient concept by continuous changing in chemical functional groups for non-targeted analysis. For this purpose, three different two-component coatings containing (3-aminopropyl)trimethoxysilane (APTES) as polar and either phenyltriethoxysilane (PTES), octyl-trimethoxysilane (OTMS) or methyltrimethoxysilane (MTMS) as nonpolar precursors were formed on the modified stainless steel wires using controlled rate infusion (CRI) method. The presence of polar and/or non-polar functional groups on the surface of substrate was evaluated by Fourier-transform infrared spectroscopy (FTIR) together with contact angles determined alongside the gradient surface. The morphology and thickness of the prepared fibers were also investigated by scanning electron microscopy (SEM). Furthermore, uniform single-component fibers from polar (APTES) and nonpolar (PTES) coatings were fabricated in order to be compared with the gradient sorbent. The gradient phase was implemented as a fiber coating in headspace- or immersed-solid phase microextraction of various compounds including chlorobenzenes, polycyclic aromatic hydrocarbons, chlorophenols and volatile organic compounds (Log K_ow range: -0.77 to 4.64). Under the optimized condition, the limits of detection and quantification were obtained in the range of 0.01-0.5 μg L^-1 and 0.05-1.5 μg L^-1, respectively. The intra-day and inter-day relative standard deviations of 2-10% and 11-17% were achieved, respectively. The method was successfully applied to the extraction of VOCs from real water sample and relative recoveries were between 89 and 105%. The capability and efficiency of the gradient coating appears to be quite appropriate for non-targeted analysis.

Organosilane Chemical Gradients: Progress, Properties, and Promise

Chemical gradients play an important role in nature, driving many different phenomena critical to life, including the transport of chemical species across membranes and the transport, attachment, and assembly of cells. Taking a cue from these natural processes, scientists and engineers are now working to develop synthetic chemical gradients for use in a broad range of applications, such as in high-throughput investigations of surface properties, as means to guide the motions and/or assembly of liquid droplets, vesicles, nanoparticles, and cells and as new media for stationary-phase-gradient chemical separations. Our groups have been working to develop new methods for preparing chemical gradients from organoalkoxysilane and organochlorosilane precursors and to obtain a better understanding of their properties on macroscopic to microscopic length scales. This review highlights our recent work on the development of controlled-rate infusion and infusion-withdrawal dip-coating methods for the preparation of gradients on planar glass and silicon substrates, on thin-layer chromatography plates, and in capillaries and monoliths for liquid chromatography. We also cover the new knowledge gained from the characterization of our gradients using sessile drop and Wilhelmy plate dynamic water contact angle measurements, X-ray photoelectron spectroscopy mapping, and single-molecule tracking and spectroscopy. Our studies reveal important evidence of phase separation and cooperative interactions occurring along multicomponent gradients. Emerging concepts and new directions in the preparation and characterization of organosilane-based chemical gradients are also discussed.

Base Layer Influence on Protonated Aminosilane Gradient Wettability

Protonated amine gradients have been prepared on silicon wafers via programmed controlled rate infusion (CRI) with varying degrees of hydrophobicity and characterized by X-ray photoelectron spectroscopy (XPS) and static and Wilhelmy plate dynamic contact angle measurements. Initially, base layers were spin coated from sols containing tetramethoxysilane (TMOS) and either phenyltrimethoxysilane (PTMOS), dimethyldimethoxysilane (DMDMOS), or octyltrimethoxysilane (OTMOS, C8). Amine gradients were then prepared from 3-aminopropyltriethoxysilane (APTEOS) via CRI. Gradients were exposed to concentrated HCl vapor for amine protonation. XPS showed that NH₂ functional groups were distributed in a gradient fashion as a result of CRI controlling the time of exposure to APTEOS. Interestingly, the overall extent of N modification depended on the type of base layer used for gradient formation. The C8-derived base layer had about half the amount of nitrogen on the surface as compared to those prepared from TMOS, which was attributed to a reduction in the number and accessibility of surface silanol groups. The wettability and contact angle (CA) hysteresis were also dependent on the base layer and varied along the length of the gradient. The greatest CA change across the length of the gradient was observed on the gradient formed on the C8-derived base layer. Likewise, the CA hysteresis was approximately 2 times larger on the C8-modified surfaces, indicative of greater chemical inhomogeneity. In contrast to uniformly modified substrates, Wilhelmy plate CA analysis that involves the immersion of samples gave a unique S-shaped CA distance curve for the gradients. The three curve segments correspond to hydrophilic, hydrophobic, and a middle connecting region. Importantly, these curves give precise CAs along the gradient that reflect the surface chemistry and coverage defined by programmed CRI processing.