Fabrication and characterisation of capillary polymeric monoliths incorporating continuous stationary phase gradients

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.

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.

Poly(acrylamide-co-N,N'-methylenebisacrylamide) Monoliths for High-Peak-Capacity Hydrophilic-Interaction Chromatography-High-Resolution Mass Spectrometry of Intact Proteins at Low Trifluoroacetic Acid Content

In this study, we optimized a polymerization mixture to synthesize poly(acrylamide-co-N,N′-methylenebisacrylamide) monolithic stationary phases for hydrophilic-interaction chromatography (HILIC) of intact proteins. Thermal polymerization was performed, and the effects of varying the amount of cross-linker and the porogen composition on the separation performance of the resulting columns were studied. The homogeneity of the structure and the different porosities were examined through scanning electron microscopy (SEM). Further characterization of the monolithic structure revealed a permeable (K_f between 2.5 × 10^–15 and 1.40 × 10^–13 m²) and polar stationary phase suitable for HILIC. The HILIC separation performance of the different columns was assessed using gradient separation of a sample containing four intact proteins, with the best performing stationary phase exhibiting a peak capacity of 51 in a gradient of 25 min. Polyacrylamide-based materials were compared with a silica-based particulate amide phase (2.7 μm core–shell particles). The monolith has no residual silanol sites and, therefore, fewer sites for ion-exchange interactions with proteins. Thus, it required lower concentrations of ion-pair reagent in HILIC of intact proteins. When using 0.1% of trifluoroacetic acid (TFA), the peak capacities of the two columns were similar (30 and 34 for the monolithic and packed column, respectively). However, when decreasing the concentration of TFA to 0.005%, the monolithic column maintained similar separation performance and selectivity (peak capacity 23), whereas the packed column showed greatly reduced performance (peak capacity 12), lower selectivity, and inability to elute all four reference proteins. Finally, using a mobile phase containing 0.1% formic acid and 0.005% TFA, the HILIC separation on the monolithic column was successfully hyphenated with high-resolution mass spectrometry. Detection sensitivity for protein and glycoproteins was increased and the amount of adducts formed was decreased in comparison with separations performed at 0.1% TFA.

Confinement of Monolithic Stationary Phases in Targeted Regions of 3D-Printed Titanium Devices Using Thermal Polymerization

In this study, we have prepared thermally initiated polymeric monolithic stationary phases within discrete regions of 3D-printed titanium devices. The devices were created with controllable hot and cold regions. The monolithic stationary phases were first locally created in capillaries inserted into the channels of the titanium devices. The homogeneity of the monolith structure and the interface length were studied by scanning a capacitively coupled conductivity contactless detector (C4D) along the length of the capillary. Homogeneous monolithic structures could be obtained within a titanium device equipped with a hot and cold jacket connected to two water baths. The confinement method was optimized in capillaries. The sharpest interfaces (between monolith and empty channel) were obtained with the hot region maintained at 70 °C and the cold region at 4 or 10 °C, with the latter temperature yielding better repeatability. The optimized conditions were used to create monoliths bound directly to the walls of the titanium channels. The fabricated monoliths were successfully used to separate a mixture of four intact proteins using reversed-phase liquid chromatography. Further chromatographic characterization showed a permeability (Kf) of ∼4 × 10-15 m2 and a total porosity of 60%.

Multi-lumen capillary based trypsin micro-reactor for the rapid digestion of proteins

In this work we evaluated a novel microreactor prepared using a surface modified, high surface-to-volume ratio multi-lumen fused silica capillary (MLC). The MLC investigated contained 126 parallel channels, each of 4 μm internal diameter. The MLC, along with conventional fused silica capillaries of 25 μm and 50 μm internal diameter, were treated by (3-aminopropyl)triethoxysilane (APTES) and then modified with gold nanoparticles, of ∼20 nm in diameter, to ultimately provide immobilisation sites for the proteolytic enzyme, trypsin. The modified capillaries and MLCs were characterised and profiled using non-invasive scanning capacitively coupled contactless conductivity detection (sC4D). The sC4D profiles confirmed a significantly higher amount of enzyme was immobilised to the MLC when compared to the fused silica capillaries, attributable to the increased surface to volume ratio. The MLC was used for dynamic protein digestion, where peptide fragments were collected and subjected to off-line chromatographic evaluation. The digestion was achieved with the MLC reactor, using a residence time of just 1.26 min, following which the HPLC peak associated with the intact protein decreased by >70%. The MLC reactors behaved similarly to the classical in vitro or in-solution approach, but provided a reduction in digestion time, and fewer peaks associated with trypsin auto-digestion, which is common using in-solution digestion. The digestion of cytochrome C using both the MLC-IMER and the in-solution approach, resulted in a sequence coverage of ∼80%. The preparation of the MLC microreactor was reproducible with <2.5% RSD between reactors (n = 3) as determined by sC4D.

Monolithic stationary phases with a longitudinal gradient of porosity

The duration of the hypercrosslinking reaction has been used to control the extent of small pores formation in polymer-based monolithic stationary phases. Segments of five columns hypercrosslinked for 30-360 min were coupled via zero-volume unions to prepare columns with segmented porosity gradients. The steepness of the porosity gradient affected column efficiency, mass transfer resistance, and separation of both small-molecule alkylbenzenes and high-molar-mass polystyrene standards. In addition, the segmented column with the steepest porosity gradient was prepared as a single column with a continuous porosity gradient. The steepness of porosity gradient in this type column was tuned. Compared to a completely hypercrosslinked column, the column with the shallower gradient produced comparable size-exclusion separation of polystyrene standards but allowed higher column permeability. The completely hypercrosslinked column and the column with porosity gradient were successfully coupled in online two-dimensional liquid chromatography of polymers.

Fabrication of surface charge gradients in open-tubular capillaries and their characterization by spatially resolved pulsed streaming potential measurements

Surface charge gradients have been formed on the inside surface of 75 μm i.d. silica capillaries via controlled rate infusion using 3-aminopropyltriethoxysilane as the reactive precursor. These 400 mm length gradients have been characterized using spatially resolved streaming potential measurements, from which the zeta potential as a function of distance was determined. The gradient capillaries exhibited a gradual variation in zeta potential from top to bottom, whereas uniformly modified and as-received capillaries were relatively homogeneous along their length. For a gradient prepared with a relatively high concentration of aminosilane, the zeta potential changed over 60 mV from one end of the capillary to the other, yielding a variation in the magnitude of the apparent surface charge of ~7 fold. By changing the concentration of the aminoalkoxysilane and/or the rate of infusion, both the value of the zeta potential (and hence surface charge) and its spatial profile (i.e., rate of change with distance) could be manipulated.

Production of novel polymer monolithic columns, with stationary phase gradients, using cyclic olefin co-polymer (COC) optical filters

Polymer monolithic columns with controlled surface ligand density, providing stationary phase gradients within monolithic capillary columns, have been developed using photo-grafting through optical filters. Utilising commercially available cyclic olefin co-polymer (COC) films, the production of an optical filter capable of attenuating UV irradiation, in a tailored manner, was investigated. This novel optical filter was successfully applied to the surface modification of poly(BuMA-co-EDMA) monolithic columns in a multi-step grafting procedure. Fabricated columns were subjected to scanning capacitively coupled contactless conductivity (sC(4)D), to determine the distribution of the grafted functional groups, axially along the column. Further modification to produce a chelating stationary phase gradient of iminodiacetic acid (IDA) was demonstrated. To demonstrate the distribution of the IDA sites, a metal cation (Cu(2+)) was complexed to the IDA forming a chelate. Upon the formation of a complex of IDA with Cu(2+), an overall drop in conductive response was observed. The COC optical filter was also used in the fabrication of a grafted gradient of strong cation exchanger (SCX), sulphopropyl methacrylate (SPM) upon a polymer monolith, demonstrating the broader applicability of such a filter.