Stationary phase optimized selectivity liquid chromatography: Basic possibilities of serially connected columns using the “PRISMA” principle

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.

Selectivity optimization in liquid chromatography via stationary phase tuning

In striving for the best possible separation, the selectivity of stationary phases as an optimization parameter is often underestimated although there are many ways to influence this powerful tool. This review serves to provide an insight into the various ways of adapting the selectivity of a separation in liquid chromatography. Approaches via temperature and flow rate tuning are discussed as a basis followed by focusing on the stationary phase as the superior optimization parameter. Highly selective stationary phases hereby provide an advantage for groups of similar analytes. For more complex mixtures, separations can be improved using mixed-mode technologies where different retention mechanisms are combined. Serial coupling, mixed-bed columns, and stationary phase optimized selectivity liquid chromatography provide solutions to various degrees. Finally, the advantages of stationary phase tuning over adaption of mobile phase and/or temperature are presented in terms of optimum application range.

Serially coupled column liquid chromatography: An alternative separation tool

Despite the rapid development of liquid chromatography (LC) in recent decades, it remains a challenge to achieve the desired chromatographic separation of complex matrices using a single column. Multi-column LC techniques, particularly serially coupled column LC (SCC-LC), have emerged as a promising solution to overcome this challenge. While more attention has been focused on heart-cutting or comprehensive two-dimensional LC, reviews specifically focusing on SCC-LC, which offers advantages in terms of precision and facile instrumentation, are scarce. Here, our concerns are devoted to the progress summary regarding the instrumentation and applications of SCC-LC. Emphasis is placed on column selection aiming to enlarge peak capacity, selectivity, or both through the optimization of combination types (e.g. RPLC-RPLC, -RPLC-HILIC, and achiral-chiral LC), connection devices (e.g. zero dead volume connector, tubing, and T-type connector), elution program (i.e. isocratic or gradient) and detectors (e.g. mass spectrometer, ultraviolet detector, and fluorescence detector). The application of SCC-LC in pharmaceutical, biological, environmental, and food fields is also reviewed, and future perspectives and potential directions for SCC-LC are discussed. We envision that the review can give meaningful information to analytical scientists when facing heavy chromatographic separation tasks for complicated matrices.

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.

Retention-directed and selectivity controlled chromatographic resolution: Rapid post-hoc analysis of DMPK samples to achieve high-throughput LC-MS separation

High quality chromatographic separation underpins robustness in LC-MS, frequently the analytical method of choice for pharmaceutical drug discovery work. The potential improvements in chromatographic selectivity afforded by serial column coupling (SCC), provide a useful means to enhance the resolution of complex samples. In this work, we present a revised high-throughput form of SCC, in which just two individual mixed phase columns were coupled together and combined with a gradient-optimised, retention-directed ultra-high pressure method to achieve rapid separations, with no further method optimisation necessary. The overall performance was evaluated from an open access DMPK analytical working environment perspective; where in anticipation of bioanalytical or metabolite identification chromatography challenges, or with the knowledge that stronger resolution was required for in-vitro sample analysis, the methodology could be immediately implemented by the analyst. Retention-directed selection of a shallow SCC gradient method was successful in separating peaks throughout the chromatographic window, resulting in a runtime still congruent to high-throughput analyses (3.5 min). In-vitro assay sample interferences were resolved 44-72% of the time, and the overall resolving power for isomeric separations significantly improved against single column comparisons (1.7-fold mean R_S improvement). Over a sustained period of time in our laboratory, SCC methods have been used for metabolite identification and bioanalytical samples, where both convenience and effectiveness in solving analytical challenges has been consistently demonstrated. Examples that highlight SCC chromatography, and a guided discussion of the main high-throughput considerations, are included. The technique offers wide applicability, and we would recommend it as a toolbox consideration to the laboratory analyst.

Algorithms to optimize multi-column chromatographic separations of proteins

The goal of this work was to provide a technical solution for the automated optimization of multi-column systems for protein separation and fractionation. Both algorithm and a software that can be downloaded are provided. In this algorithm, the length and order of the individual column segments can be considered. Various solutions are provided by the algorithm, including i) to obtain uniform peak distribution, ii) to park the different species at the inlet of the individual column segments, and iii) to elute all species as a single peak. Two representative examples are presented, showing the possibility to obtain uniform selectivity between monoclonal antibody (mAb) sub-units, and the on-column fractioning of intact mAbs.

Chiral stationary phase optimized selectivity liquid chromatography: A strategy for the separation of chiral isomers

Chiral Stationary-Phase Optimized Selectivity Liquid Chromatography (SOSLC) is proposed as a tool to optimally separate mixtures of enantiomers on a set of commercially available coupled chiral columns. This approach allows for the prediction of the separation profiles on any possible combination of the chiral stationary phases based on a limited number of preliminary analyses, followed by automated selection of the optimal column combination. Both the isocratic and gradient SOSLC approach were implemented for prediction of the retention times for a mixture of 4 chiral pairs on all possible combinations of the 5 commercial chiral columns. Predictions in isocratic and gradient mode were performed with a commercially available and with an in-house developed Microsoft visual basic algorithm, respectively. Optimal predictions in the isocratic mode required the coupling of 4 columns whereby relative deviations between the predicted and experimental retention times ranged between 2 and 7%. Gradient predictions led to the coupling of 3 chiral columns allowing baseline separation of all solutes, whereby differences between predictions and experiments ranged between 0 and 12%. The methodology is a novel tool allowing optimizing the separation of mixtures of optical isomers.

Two-dimensional heart-cut LC-LC improves accuracy of exact-matching double isotope dilution mass spectrometry measurements of aflatoxin B1 in cereal-based baby food, maize, and maize-based feed

Aflatoxins, mycotoxins of fungi of the Aspergillus sp., pose a risk to consumer health and are, therefore, regulated by more than 100 countries. To facilitate method development and validation as well as assessment of measurement capabilities, availability of certified reference materials and proficiency testing schemes is important. For these purposes, highly accurate determinations of the aflatoxin content in the materials used are necessary. We describe here the use of two-dimensional heart-cut LC-LC in combination with exact-matching double isotope dilution mass spectrometry to determine the content of aflatoxin B₁ in three materials used in a proficiency testing scheme. The serious reduction in ionization suppression afforded by the two-dimensional heart-cut LC-LC had a positive effect on the precision of the measured isotope ratios of the exact-matching double isotope dilution mass spectrometry. This is evidenced by the expanded measurement uncertainty (k = 2) of 0.017 μg/kg or 8.9 % relative to a mass fraction of aflatoxin B₁ in a cereal-based baby food of 0.197 μg/kg. This value is in perfect agreement with the consensus value of this material from a proficiency test (PT) scheme for National Reference Laboratories executed by the European Reference Laboratory for Mycotoxins. The effort necessary to perform the described methodology precludes its frequent use but for specific applications we see it as a valuable tool.

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.

Serial coupling of reversed-phase and hydrophilic interaction liquid chromatography to broaden the elution window for the analysis of pharmaceutical compounds

It is presently a common practice in drug discovery to analyse samples by reversed-phase liquid chromatography (RPLC) and hydrophilic interaction chromatography (HILIC). To increase throughput, HILIC was connected in series to RPLC by means of a T-piece with make-up flow. The first column is a 2mm I.D. column having an optimal flow between 0.1 and 0.2mL/min. Via the T-piece, the flow for the second dimension column with an I.D. of 4.6mm is adjusted to 1.5-2.0mL/min with a high acetonitrile content (i.e. >/=80%) mobile phase. Therefore, even in gradient RPLC analysis starting with a mobile phase with high water content, the HILIC column is always operated at high acetonitrile concentration which is required to obtain retention on the HILIC column. The performance of the hyphenated RPLC/HILIC set-up is illustrated with the analysis of two model samples of pharmaceutical interest. Optimization of the conditions in the HILIC dimension is discussed.