Destructive stationary phase gradients for reversed-phase/hydrophilic interaction liquid chromatography

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.

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.

On-column modification for the creation of temperature-responsive stationary phases

Temperature-responsive liquid chromatography (TRLC) offers an alternative for retention and selectivity optimisation in HPLC. This approach thereby exploits temperature gradients on stimuli-responsive stationary phases and forfeits the necessity for solvent gradients, allowing analyses to be performed using aqueous mobile phases. Consequently, it can be employed as a green alternative to reversed-phase separations. However, current production to obtain temperature-responsive columns inherently require dedicated column packing processes with polymer-modified particles. To facilitate the development of temperature-responsive phases, a flow-through modification procedure was developed allowing on-column modification of aminopropyl silica columns. Three columns were manufactured using this novel flow-through approach, which achieved identical column efficiencies compared to existing TRLC column. Demonstrating the possibility of bypassing the dedicated packing processes without losing efficiency. Additionally, it was observed that flow-through produced columns yielded higher retention at elevated temperatures despite their reduced carbon load. Further investigation of the carbon load revealed the presence of stationary phase gradients, whose influence was studied via novel developed retention experiments, which revealed a negligible change reduction in retention upon a change of polymer modification from 19.8% to 14.5%. However, further decrease from 14.5% to 12.3% resulted in a larger change. Interestingly, a further enhancement in apparent plate numbers was observed when operating the column under a reversed flow, yielding an increasing stationary phase gradient. This on-column modification procedure demonstrates the potential for modification of existing (commercial) packed columns to achieve temperature-responsive phases without loss of efficiency or retention. Thus, not only facilitating accessibility to temperature-responsive phases, but also aiding with development of further generations of temperature-responsive phases by removing the need for packing optimisation. Additionally, a novel experiment was set up to easily investigate the effect of inhomogeneous stationary phases retention.

Study of Efficiency of Capacity Gradient Ion-Exchange Stationary Phases

Highly efficient columns are necessary for the modern analytical applications of liquid chromatography. In this work, the separation efficiency of ion-exchange capacity gradient stationary phases combined with eluent concentration gradient was studied by a theoretical approach. In the course of our work three different scenarios of capacity gradients were used with different shapes (linear, convex and concave). The resolutions of different gradient columns were calculated for each scenario. As a reference, a uniform column was considered, which had the same analysis time as the non-uniform column. In the case of separation of ions with same charges, the gradient column offered only a marginal advantage compared to the uniform column due to the bandwidth compression caused by the capacity gradient. In the case of ions with different charges, however, the advantage of the gradient column was more significant. This was mainly due to the increased retention time difference of solutes. Ion-exchange capacity gradient columns may be a new way to separate ions more efficiently.

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.

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.

Renewing the performance of an expired particle packed column using active flow technology end fittings

The performance of a particle packed column will inevitably degrade through use or misadventure. 'Active flow technology' (AFT) is known to greatly improve the performance of pristine columns, but is as of yet untested when used on columns that have degraded significantly. In this study AFT was used to regenerate a degraded column, where the reduced plate height and asymmetry values were 3.5 and 1.25 respectively. Once the AFT fittings were fitted to the column outlet and the flow segmentation ratio adjusted to 28% from the radial central exit port, the reduced plate height decreased to 2.0, and the bands were almost perfectly symmetrical with asymmetry factors equal to 1.04. Subsequently, the performance of the degraded column with AFT fittings provided performance that was comparable to that of a new conventional column fitted with traditional end fittings. The separation power of the degraded conventional column and that of the same column fitted with the AFT end fittings was then tested using the separation of oligostyrenes. In AFT mode, detection was undertaken at both the radial central exit port of the column and the peripheral exit port. The resulting separation that was achieved from the radial central exit port was superior to that observed on the conventional column, whereas, the separation observed from the peripheral port was very poor. It was subsequently determined that the reason for the degraded performance of the conventional column was a result of increased heterogeneity associated with the packing material in the wall region of the column.